Category Archives: Tutorial

The tutorial category contains all the Free Pascal and SDL tutorial posts. The tutorial may treat different SDL versions, e.g. SDL 1.2 or SDL 2.0.

Keyboard State and Key States

Tutorial, , ,

Last updated on September 21st, 2019

In the next chapter we will see how so called events are processed to handle input of any kind (keyboard, mouse, joystick,…) Before that I’d like to introduce a simple, yet powerful way to handle keyboard input.

The keyboard state represents the state (pressed = 1 or unpressed = 0) of all the keyboard keys, hence the key states. By 

function SDL_GetKeyboardState(numkeys: PInt): PUInt8

we have easy access to this array.

Keyboard state demo.
The red rectangle can be moved by the WASD keys by reading out their key states.

The following code example will draw a red rectangle which can be moved by the WASD keys. Therefore we read out their key states on every cycle of the program loop.

program SDL_KeyboardState;

uses SDL2;

var
  sdlWindow1: PSDL_Window;
  sdlRenderer: PSDL_Renderer;
  sdlRectangle: TSDL_Rect;
  sdlKeyboardState: PUInt8;
  Running: Boolean = True;

begin

  //initilization of video subsystem
  if SDL_Init(SDL_INIT_VIDEO) < 0 then Halt;

  if SDL_CreateWindowAndRenderer(500, 500, SDL_WINDOW_SHOWN, @sdlWindow1, @sdlRenderer) <> 0
    then Halt;

  // prepare rectangle
  sdlRectangle.x := 250;
  sdlRectangle.y := 250;
  sdlRectangle.w := 10;
  sdlRectangle.h := 10;

  // program loop
  while Running = True do
  begin

    SDL_PumpEvents;
    sdlKeyboardState := SDL_GetKeyboardState(nil);

    // ESC pressed
    if sdlKeyboardState[SDL_SCANCODE_ESCAPE] = 1 then
      Running := False;

    // WASD keys pressed
    if sdlKeyboardState[SDL_SCANCODE_W] = 1 then
      sdlRectangle.y := sdlRectangle.y-1;
    if sdlKeyboardState[SDL_SCANCODE_A] = 1 then
      sdlRectangle.x := sdlRectangle.x-1;
    if sdlKeyboardState[SDL_SCANCODE_S] = 1 then
      sdlRectangle.y := sdlRectangle.y+1;
    if sdlKeyboardState[SDL_SCANCODE_D] = 1 then
      sdlRectangle.x := sdlRectangle.x+1;

    // black background
    SDL_SetRenderDrawColor(sdlRenderer, 0, 0, 0, SDL_ALPHA_OPAQUE);
    SDL_RenderClear(sdlRenderer);
    // draw red rectangle
    SDL_SetRenderDrawColor(sdlRenderer, 255, 0, 0, SDL_ALPHA_OPAQUE);
    SDL_RenderDrawRect(sdlRenderer, @sdlRectangle);

    SDL_RenderPresent(sdlRenderer);
    SDL_Delay(20);
  end;

  // clear memory
  SDL_DestroyRenderer(sdlRenderer);
  SDL_DestroyWindow (sdlWindow1);

  //closing SDL2
  SDL_Quit;

end.

To get the keyboard state, we define a unsigned 8 bit pointer variable sdlKeyboardState in the var clause. It points to the array of key states.

  // program loop
  while Running = True do
  begin

    SDL_PumpEvents;
    sdlKeyboardState := SDL_GetKeyboardState(nil);

After setting up a SDL2 window and and preparing a SDL2 rectangle, the program loop is entered. Here we need to update the event queue by procedure SDL_PumpEvents. After that we can grab the keyboard state by the former mentioned function SDL_GetKeyboardState. The argument should be nil. These actions have to performed on every cycle.

    // ESC pressed
    if sdlKeyboardState[SDL_SCANCODE_ESCAPE] = 1 then
      Running := False;

    // WASD keys pressed
    if sdlKeyboardState[SDL_SCANCODE_W] = 1 then
      sdlRectangle.y := sdlRectangle.y-1;
    if sdlKeyboardState[SDL_SCANCODE_A] = 1 then
      sdlRectangle.x := sdlRectangle.x-1;
    if sdlKeyboardState[SDL_SCANCODE_S] = 1 then
      sdlRectangle.y := sdlRectangle.y+1;
    if sdlKeyboardState[SDL_SCANCODE_D] = 1 then
      sdlRectangle.x := sdlRectangle.x+1;

We now can check for the of any key in the array by sdlKeyboardState[SDL_SCANCODE_…] using its
scancode as a handle (e.g. SDL_SCANCODE_ESCAPE for the escape key) and react as desired, e.g. exit the program loop or change the x/y coordinates of the rectangle. The scancode represents the position of the related key state in the array. A detailed description of scancodes in the next chapter. A list of all the scancodes shows all possible scancodes.

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Rectangles, Image Movement and Scaling

Tutorial,

Last updated on August 11th, 2018

Screens and Images are rectangular, so this shape has a special importance to SDL2 and graphics programming in particular.

Rectangles: TSDL_Rect and PSDL_Rect in SDL 2.0

Often functions require an argument of PSDL_Rect type. This is the pointer counterpart to TSDL_Rect. It is declared as follows.

PSDL_Rect = ^TSDL_Rect;
TSDL_Rect = record
  x,y: SInt32;
  w,h: SInt32;
end;

Simply, this record describes a rectangle, hence the name. The variables x and y correspond to the x/y coordinates of the left upper corner of the rectangle, related to the origin 0/0 which is the left upper corner of, e.g. a texture, window,… The variable w is the width and h the height of the rectangle. That’s it. The next step is to define the rectangle by assign some values for x, y, w and h.

If you use PSDL_Rect, you free the required memory by Pascal’s new procedure as you would for any simple record pointer.

Using Rectangles for Movement and Scaling

The following code demonstrates the basic principle how to achieve the impression of movement of images (sprites) and how scaling works. You will be impressed how simple it actually is.

program SDL_RectanglesScaling;

uses SDL2;

var
  sdlWindow1: PSDL_Window;
  sdlRenderer: PSDL_Renderer;
  sdlSurface1: PSDL_Surface;
  sdlTexture1: PSDL_Texture;
  sdlRectangle: TSDL_Rect;

begin

  //initilization of video subsystem
  if SDL_Init(SDL_INIT_VIDEO) < 0 then Halt;

  if SDL_CreateWindowAndRenderer(500, 500, SDL_WINDOW_SHOWN, @sdlWindow1, @sdlRenderer) <> 0
    then Halt;

  // set scaling quality
  SDL_SetHint(SDL_HINT_RENDER_SCALE_QUALITY, 'nearest');

  // create surface from file
  sdlSurface1 := SDL_LoadBMP('fpsdl.bmp');
  if sdlSurface1 = nil then
    Halt;

  // load image file
  sdlTexture1 := SDL_CreateTextureFromSurface(sdlRenderer, sdlSurface1);
  if sdlTexture1 = nil then
    Halt;

  // prepare rectangle
  sdlRectangle.x := 12;
  sdlRectangle.y := 25;
  sdlRectangle.w := 178;
  sdlRectangle.h := 116;

  // render texture
  SDL_RenderCopy(sdlRenderer, sdlTexture1, @sdlRectangle, nil);
  SDL_RenderCopy(sdlRenderer, sdlTexture1, nil, @sdlRectangle);

  // render to window for 2 seconds
  SDL_RenderPresent(sdlRenderer);
  SDL_Delay(2000);

  // clear memory
  SDL_DestroyTexture(sdlTexture1);
  SDL_FreeSurface(sdlSurface1);
  SDL_DestroyRenderer(sdlRenderer);
  SDL_DestroyWindow (sdlWindow1);

  //closing SDL2
  SDL_Quit;

end.

We will get this as a result.

Creative Commons License This image by https://www.freepascal-meets-sdl.net is licensed under a Creative Commons Attribution 4.0 International License.
The result of the example program (500×500 px window). A part of the full image is stretched in the background while the full image is also squeezed into an area above the word “Free”.

For comparison, here is the original 200×200 px image.

Free Pascal meets SDL sample image bmp format
Original image (200×200 px).

Let’s disect the code.

var
  sdlWindow1: PSDL_Window;
  sdlRenderer: PSDL_Renderer;
  sdlTexture1: PSDL_Texture;
  sdlRectangle: TSDL_Rect;

In the var clause we declare the known variables for window, renderer and a texture. Also we have a new variable of TSDL_Rect type.

  // prepare rectangle
  sdlRectangle.x := 12;
  sdlRectangle.y := 25;
  sdlRectangle.w := 178;
  sdlRectangle.h := 116;

After initializing SDL2 and setting up the window and renderer as known, the rectangle is getting some values. It just encloses the words “Free Pascal meets SDL” in the original image (see above).

 

Scaling in SDL2

Scaling Quality

Right before creating the surface and texture, there is this line in code.

  // set scaling quality
  SDL_SetHint(SDL_HINT_RENDER_SCALE_QUALITY, 'nearest');

It sets the render quality. It has to be set before creating the texture. The SDL_SetHint(hint name, hint value) function is no specific function for setting scaling quality, but here we use it for exactly that. Possible values are

  1. nearest or 0
    • nearest pixel sampling
  2. linear or 1
    •  linear filtering
    • support by OpenGL and Direct3D
  3. best or 2
    • anisotropic filtering
    • support by Direct3D.

All of the values have to be set as string values, so ‘nearest’ or ‘0’. Here is a comparision of the nearest- and the linear filter.

  • Result image of the code.
    Nearest pixel sampling filter for scaling.
  • Result of code with linear filter.
    Linear filter for scaling.

The anisotropic filter doesn’t do anything for me, even if I used Direct3D.

Scaling by using Rectangles

  // render texture
  SDL_RenderCopy(sdlRenderer, sdlTexture1, @sdlRectangle, nil);
  SDL_RenderCopy(sdlRenderer, sdlTexture1, nil, @sdlRectangle);

At this point happens the magic that leads to the resulting image. By the way, since the SDL_RenderCopy() function requires the rectangle arguments to be of PSDL_Rect, we use the @-operator (pointer operator) here. 

  SDL_RenderCopy(sdlRenderer, sdlTexture1, @sdlRectangle, nil);

This means, copy the area described by “sdlRectangle” from the source (“sdlTexture1” here) to the whole area (because of nil value) of the destination, hence the window.

Since the window has a width and height of 500 px each, the source rectangle just a width of 178 px and a height of 116 px, SDL2 automatically scales the image to fit into the larger (or smaller) dimensions of the destination.

  SDL_RenderCopy(sdlRenderer, sdlTexture1, nil, @sdlRectangle);

This means, copy the whole source (because of nil value) to the area described by “sdlRectangle”. The source is the 200×200 px image, which has to squeezed to the 178×116 px rectangle at position (12/25). This is just what you see in the resulting image (above) where the whole image is squeezed into this area.

Movement of Images (Sprites)

Although not covered directly by this code example, you get the picture how movement works. Every frame you adjust the (x/y) coordinates of the rectangle for the destination to bring the sprite about to move.

After cleaning up the memory the program finishes.

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Loading and Rendering a Bitmap File

Tutorial, , ,

Last updated on August 11th, 2018

Loading of bitmap image files (BMP files) is natively supported by SDL2. The way to go is as follows (from the flow diagram).

 

Creative Commons License This image by https://www.freepascal-meets-sdl.net is licensed under a Creative Commons Attribution 4.0 International License.
Flow diagram of the loading and rendering of a bitmap file in SDL2.

Let’s start on the left in the diagram. The easiest way to get a bitmap (BMP) image file for a game or application ready for usage is to create one in a drawing application. Or use the example bitmap “fpsdl.bmp” we used in the code.

Free Pascal meets SDL sample image bmp format
Creative Commons License This image by https://www.freepascal-meets-sdl.net is licensed under a Creative Commons Attribution 4.0 International License.
The original image is 200×200 pixels wide.

The bmp image file is stored on your hard drive and can be loaded by SDL_LoadBMP function to a SDL2 surface. This SDL2 surface is then transformed into a SDL2 texture by SDL_CreateTextureFromSurface function (whose name is just explaining what is does). And finally this texture is rendered by SDL_RenderPresent, this function we know already.

And now let’s see how it is done in code.

program SDL_LoadingRenderingBMP;

uses SDL2;

var
  sdlWindow1: PSDL_Window;
  sdlRenderer: PSDL_Renderer;
  sdlSurface1: PSDL_Surface;
  sdlTexture1: PSDL_Texture;

begin

  //initilization of video subsystem
  if SDL_Init(SDL_INIT_VIDEO) < 0 then Halt;

  if SDL_CreateWindowAndRenderer(500, 500, SDL_WINDOW_SHOWN, @sdlWindow1, @sdlRenderer) <> 0
    then Halt;

  // create surface from file
  sdlSurface1 := SDL_LoadBMP('fpsdl.bmp');
  if sdlSurface1 = nil then
    Halt;

  // create texture from surface
  sdlTexture1 := SDL_CreateTextureFromSurface(sdlRenderer, sdlSurface1);
  if sdlTexture1 = nil then
    Halt;

  // render texture
  if SDL_RenderCopy(sdlRenderer, sdlTexture1, nil, nil) <> 0 then
    Halt;

  // render to window for 2 seconds
  SDL_RenderPresent(sdlRenderer);
  SDL_Delay(2000);

  // clear memory
  SDL_DestroyTexture(sdlTexture1);
  SDL_FreeSurface(sdlSurface1);
  SDL_DestroyRenderer(sdlRenderer);
  SDL_DestroyWindow (sdlWindow1);

  //closing SDL2
  SDL_Quit;

end.

The result is this:

Result screenshot for chapter 4
Creative Commons License This image by https://www.freepascal-meets-sdl.net is licensed under a Creative Commons Attribution 4.0 International License.
This is the result of the code.

The var clause,

var
  sdlWindow1: PSDL_Window;
  sdlRenderer: PSDL_Renderer;
  sdlSurface1: PSDL_Surface;
  sdlTexture1: PSDL_Texture;

contains two new variables, namely “sdlSurface1” and “sdlTexture1” of the pointer types PSDL_Surface and PSDL_Texture, respecitvely.

After setting up SDL2, a window and a renderer as known, we find this.

Step 1: Loading the BMP file to a SDL2 Surface

  // create surface from file
  sdlSurface1 := SDL_LoadBMP('fpsdl.bmp');
  if sdlSurface1 = nil then
    Halt;

SDL_LoadBMP(name of bmp image file) does what you expect, it loads the image file and generates a SDL2 surface from it. Attention though, if you just give a file name, it is assumed that the file is found in the same folder as the executing application. Optionally you can also give a full file path, e.g. in Windows something like ‘C:\MyImages\fpsdl.bmp’. The function is declared as

 SDL_LoadBMP(_file: PAnsiChar): PSDL_Surface

and return nil on error, e.g. if the file is not found.

Step 2: Creating a SDL2 Texture from the SDL2 Surface

The next step is to get a SDL2 texture. That’s achieve as follows.


  // create texture from surface
  sdlTexture1 := SDL_CreateTextureFromSurface(sdlRenderer, sdlSurface1);
  if sdlTexture1 = nil then
    Halt;

The function to use is SDL_CreateTextureFromSurface(renderer, surface).  

SDL_CreateTextureFromSurface(renderer: PSDL_Renderer; surface: PSDL_Surface): PSDL_Texture

It just does what you expect and transforms the SDL2 surface into a SDL2 texture with the help of the given renderer.

Step 3: Prepare the SDL2 Texture to be Rendered

Before actually rendering the texture, we need to copy it to the rendering target (our window) by SDL_RenderCopy(renderer, texture, source rectangle (texture), destination rectangle (rendering target)).

// render texture
  if SDL_RenderCopy(sdlRenderer, sdlTexture1, nil, nil) <> 0 then
    Halt;

So the texture is copied to the rendering target (which is the window). The first nil argument means that we want to copy the whole rectangle. The second nil means that we want to copy to the whole dimensions of the rendering target. Let’s have a closer look at the function.

SDL_RenderCopy(renderer: PSDL_Renderer; texture: PSDL_Texture; srcrect: PSDL_Rect; dstrect: PSDL_Rect): SInt32

You see here, that you could use arguments of type PSDL_Rect, which basically describes rectangles.

Step 4: The Rendering

And finally the rendering is done by the known SDL_RenderPresent().

Step 5: Destroying Surfaces and Textures

It is important to free the memory occupied by the surface and texture by SDL_FreeSurface(surface) and SDL_DestroyTexture(texture) right after . 

  // clear memory
  SDL_DestroyTexture(sdlTexture1);
  SDL_FreeSurface(sdlSurface1);
  SDL_DestroyRenderer(sdlRenderer);
  SDL_DestroyWindow (sdlWindow1);

Now you know how to load and render a bmp file to a window. 🙂

Remark: DO NEVER combine SDL_CreateTextureFromSurface() and SDL_LoadIMG!

Do never combine step 1 and step 2 to avoid declaring and freeing a surface. DO NEVER do this:

sdlTexture1 := SDL_CreateTextureFromSurface(sdlRenderer, SDL_LoadBMP('fpsdl.bmp'));

This will run without any problem, though SDL_CreateTextureFromSurface() will not free the surface created by SDL_LoadBMP(). And you have no handle to free this surface. This creates a memory leak. 

← previous Chapter | next Chapter →

Surfaces and Textures

Tutorial, , ,

Last updated on August 8th, 2018

This chapter treats some basics you should know to understand the way SDL2 works.

Briefly: The Basics of Graphics Programming

Loading and the movement of images in a game (or other applications) is a major concept in (game) programming. These images are then refered to as sprites, usually. Let’s have a look at a simple example:

left: ball and paddle erased between each frame; right: ball and paddle not erased between frames
Left: Window is cleared between each new drawn frame. Right: Window is not cleared.

Here are two screenshots from a simple game. The player has to move the yellow-green paddle up- and downwards to prevent the blue ball from getting through to the right side. The game uses two sprites, the blue ball sprite and the yellow-green paddle sprite (see left screenshot). The background color is set to black. The left screenshot is how the game usually appears to the player, here between each frame that got drawn, the former frame has been cleared. The right screenshot demonstrates what happens if the former frame hasn’t been erased before the next one is drawn. – Now it is clearly visible that the sprites are redrawn again and again with sligthly different coordinates, and that is how (game) graphics work (even for the most sophisticated 3d games):

  1. Draw the frame
  2. Show the frame (in a window on screen)
  3. Clear the frame (and go back to step 1)

Briefly: The Relation between Graphic Objects (e.g. Sprites) and Hardware

Actually there are just three locations where these images are stored in your computer system. All images (photo images, drawings, sprites for 2d games, textures for 3d games) are stored on your harddrive somewhere. If you start a photo viewer, a paint program, a 2d game or a 3d game, in all cases the corresponding images need to be loaded from your harddrive to RAM (Random-Access Memory) since displaying and manipulation (e.g. rotation of a photo image by 90°) of images loaded to RAM is much, much faster. Especially for games a fast access to the image data is highly important! And finally there isn’t just one RAM but two, a CPU controlled one located on the motherboard used by every program/application that needs some RAM. The second RAM is located right at your graphic board and controlled by the so-called GPU (graphics processing unit). This is what we want to use if we develop games since it is dedicated, optimized and just hungry for tasks related to fast image processing.

Many games and applications do not only target at common computer systems, but for mobile devices, e.g. smart phones. The principles described are also true for these devices even though there may be differences in detail.

The SDL2 Surface

The SDL2 surface allows you to represent graphic objects like sprites. Every SDL2 surface has a width and height, a pixel format and other properties. Nevertheless, it is a concept which originates from the outdated SDL 1.2 and therefore should not be used anymore. Still, there are reasons why we need to introduce it here. This will be clear soon.

The SDL2 Texture

The SDL2 texture allows you to represent graphic objects just like the SDL2 surface does, although there is a major difference: It is hardware accalerated. So the graphic object is stored in the graphic board’s RAM and any manipulation is done by the graphic board’s GPU.

So as a rule,

always use SDL2 Textures to store your graphic objects (sprites) for SDL 2.0,

then you go for high performance!

Three ways to SDL_Texture

So, how to get a SDL_Texture? In principle there are three ways to create SDL2 textures. For way 2 and 3 the flow diagram may illustrate how it works.

Way 1: From Scratch

You create a SDL_Texture from scratch, so you set a pixel format and texture access format and have to fill in your texture data manually. This is the most sophisticated way and is usually not necessary, unless you work with raw pixel data.

Way 2: The path from the file to the surface, to the texture and to the screen. Way 3: The path rom the file to the texture and to the screen.

Way 2: From SDL2 Surface

2) You have or create a SDL_Surface from an image file first and then you create the SDL_Texture from the SDL_Surface. This way is shown in the diagram but it means two steps.

Way 3: Directly from Image File

3) You create a SDL_Texture from and image file directly. This is shown in the diagram, too. This is the simplest way to create a SDL_Texture.

← previous Chapter | next Chapter →

Window and Renderer

Tutorial, , , , ,

Last updated on August 7th, 2018

Every SDL2 program that shall show some graphic output has to have at least one SDL2 window and a SDL2 renderer. The window is the entity that is showing the graphic output and the renderer is the “machine” that is generating the output to be shown in the window. The code to set up a window and a renderer is as follows.

program SDL_WindowAndRenderer;

uses SDL2;

var
  sdlWindow1: PSDL_Window;
  sdlRenderer: PSDL_Renderer;

begin

  //initilization of video subsystem
  if SDL_Init(SDL_INIT_VIDEO) < 0 then Halt;

  // full set up
  sdlWindow1 := SDL_CreateWindow('Window1', 50, 50, 500, 500, SDL_WINDOW_SHOWN);
  if sdlWindow1 = nil then Halt;

  sdlRenderer := SDL_CreateRenderer(sdlWindow1, -1, 0);
  if sdlRenderer = nil then Halt;

  // quick set up
  {
  if SDL_CreateWindowAndRenderer(500, 500, SDL_WINDOW_SHOWN, @sdlWindow1, @sdlRenderer) <> 0
    then Halt;
  }

  // render to window for 2 seconds
  SDL_RenderPresent(sdlRenderer);
  SDL_Delay(2000);

  // clear memory
  SDL_DestroyRenderer(sdlRenderer);
  SDL_DestroyWindow (sdlWindow1);

  //closing SDL2
  SDL_Quit;

end.

Let’s have closer look at the var clause.

var
  sdlWindow1: PSDL_Window;
  sdlRenderer: PSDL_Renderer;

The SDL2 Window

In SDL 2.0 you can create as many windows as you like, and each window is adressed by its PSDL_Window variable. We just need one window for now, let’s call it “sdlWindow1”. It defines the window’s properties, e.g. size, appearance, border, title name and so on. And it holds the content it shows.

Creation of a Window

  // full set up
  sdlWindow1 := SDL_CreateWindow('Window1', 50, 50, 500, 500, SDL_WINDOW_SHOWN);
  if sdlWindow1 = nil then Halt;

The creation of a SDL2 window is simple as using the function SDL_CreateWindow(title, x, y, width, height, flags) or more specific:

SDL_CreateWindow(const title: PAnsiChar; x: SInt32; y: SInt32; w: SInt32; h: SInt32; flags: UInt32): PSDL_Window

In our example the window is titled “Window1”, it is located at position x = 50 and y = 50 pixels (relative to your screen). It has a width and height of 500 pixels respecitvly. And we have used the flag SDL_WINDOW_SHOWN. More about these flags later. First let’s get an understanding of the coordinate system in SDL2.

The Coordinate System in SDL 2.0

This rule applies:

The origin from where to count to place a window is always the left upper corner of your screen.

So if you choose (0/0) as coordinates the window’s left upper corner will be placed right at the left upper corner of your screen. The diagram below may help to understand this. You may try SDL_WINDOWPOS_CENTERED for each or both coordinates which will lead to a centered window with respect of the screen. If you choose SDL_WINDOWPOS_UNDEFINED you don’t care for the window’s position.

SDL2 window and coordinates diagram
Creative Commons License This image by https://www.freepascal-meets-sdl.net is licensed under a Creative Commons Attribution 4.0 International License.
The window is placed with respect to the left upper corner of the screen.

SDL 2.0 windows and their properties

Now let’s talk about the flags. They decide for the properties of the window. Look at the following table (source) of possible flags and you may get an idea what they do.

Flag
Description
SDL_WINDOW_FULLSCREENfullscreen window
SDL_WINDOW_FULLSCREEN_DESKTOPfullscreen window at the current desktop resolution
SDL_WINDOW_OPENGLwindow usable with OpenGL context
SDL_WINDOW_SHOWNwindow is visible
SDL_WINDOW_HIDDENwindow is not visible
SDL_WINDOW_BORDERLESSno window decoration
SDL_WINDOW_RESIZABLEwindow can be resized
SDL_WINDOW_MINIMIZEDwindow is minimized
SDL_WINDOW_MAXIMIZEDwindow is maximized
SDL_WINDOW_INPUT_GRABBEDwindow has grabbed input focus
SDL_WINDOW_INPUT_FOCUSwindow has input focus
SDL_WINDOW_MOUSE_FOCUSwindow has mouse focus
SDL_WINDOW_FOREIGNwindow not created by SDL
SDL_WINDOW_ALLOW_HIGHDPIwindow should be created in high-DPI mode if supported (available since SDL 2.0.1)

As you can see, these flags determine different properties of  the window. E.g. SDL_WINDOW_FULLSCREEN will create a fullscreen window and SDL_WINDOW_BORDERLESS will create a borderless window. You may combine several flags by OR (if appropriate). For our purpose SDL_WINDOW_SHOWN is a good choice because we just create a shown window without any further restrictions.

The SDL2 Renderer

In computer graphics rendering means the process of synthesizing the final image on your screen from the individual basic data structures. To draw some content to the window, we need therefore a renderer. The PSDL_Renderer (which we declared in the var clause) is responsible for synthesizing all the content in a window, be it some lines, a flat background, a texture, a 3d object, or whatever. We call our PSDL_Renderer “sdlRenderer”.

Creation of a Renderer

  sdlRenderer := SDL_CreateRenderer(sdlWindow1, -1, 0);
  if sdlRenderer = nil then Halt;

The creation of the renderer is as simple as one function call of SDL_CreateRenderer(window, index, flags) or

SDL_CreateRenderer(window: PSDL_Window; index: SInt32; flags: UInt32): PSDL_Renderer

First we need the renderer to know where to render the finished/rendered output. That will be “Window1” in our case. Next the shown function asks for a cryptic “index”. Well, each driver which is capable of rendering (e.g. OpenGL, Direct3d, Software,…) is indexed in SDL 2.0. In principle you could choose one specific driver here by choosing the corresponding index. Since we don’t know too much about the drivers at the moment the best choice is -1. -1 means that the first driver which is supporting the chosen flag(s) is chosen. Talking about flags, there are four flags you may choose:

  1. SDL_RENDERER_SOFTWARE
  2. SDL_RENDERER_ACCELERATED
  3. SDL_RENDERER_PRESENTVSYNC
  4. SDL_RENDERER_TARGETTEXTURE

You should always prefer SDL_RENDERER_ACCELERATED because this means the graphics board is responsible for rendering, SDL_RENDERER_SOFTWARE in contrast means, the CPU has to do the rendering. As discussed before for best performance the graphic board is the best choice for rendering/graphic related tasks. SDL_RENDERER_PRESENTVSYNC allows for so called vertical synchronization which means that the display of the rendered image is synchronized with the refresh rate of the monitor. SDL_RENDERER_TARGETTEXTURE allows for rendering to a texture. You may have noticed that none of these flags but 0 was used in the example code. This automatically gives priority to hardware accelerated renderers.

Quick Creation of a Window and a Renderer

 // quick set up
  {
  if SDL_CreateWindowAndRenderer(500, 500, SDL_WINDOW_SHOWN, @sdlWindow1, @sdlRenderer) <> 0
    then Halt;
  }

Instead of creating the window and the renderer separately as demonstrated, you may use SDL_CreateWindowAndRenderer(width, height, window flags, window pointer pointer, renderer pointer pointer). This has the advantage that you just need one line to set up a window and a renderer, though setting a window title, a window position or specific renderer flags have to be done afterwards if necessary.

Just remove the curly brackets and enclose the “full set up” -part to try it.

SDL_CreateWindowAndRenderer(width: SInt32; height: SInt32; window_flags: UInt32; window: PPSDL_Window; renderer: PPSDL_Renderer): SInt32

This function returns 0 on success and -1 on failure.

Rendering a SDL2 Scene

The actual rendering is achieved by SDL_RenderPresent(renderer). As a sidenote for people coming from SDL 1.2, this is what formerly has been achieved by SDL_Flip(). 

SDL_RenderPresent(renderer: PSDL_Renderer)

Freezing (delaying) a running program in SDL 2.0

SDL_Delay(time in milliseconds) is a simple, yet powerful and important procedure to stop the program running for a certain time in milliseconds. 2000 milliseconds are two seconds. This is kind of a twin of Pascal’s Delay procedure.

Clean up the memory in SDL 2.0

Now the final lines of code are discussed. One of the most important rules for sophisticated programming is followed here:

Always clean up the memory on program finish.

For nearly any pointer type generated by SDL 2.0, there is a destroy procedure to remove it from memory. These procedures are comparable to Pascal’s dispose procedure to remove pointer types from memory. Make sure to destroy the objects in the opposite sequence of their generation. We first created a window, then a renderer. So now we go the opposite way, first destroy the renderer and then the window by the procedures SDL_DestroyRenderer(renderer) and SDL_DestroyWindow(window) respectively.

Here we go:

  // clear memory
  SDL_DestroyRenderer(sdlRenderer);
  SDL_DestroyWindow (sdlWindow1);

Do not forget to quit SDL2 finally (which we don’t).

That’s it. And now things are going to get really interesting :-).

← previous Chapter | next Chapter →

Viewports

Tutorial, , ,

Last updated on August 11th, 2018

SDL2 allows for viewports. Have a look at the following screenshot of a SDL2 game (Battle for Wesnoth).

Modified screenshot of Battle of Wesnoth. (Original source image, as found in the Wesnoth Wiki by Wesnoth developers. Image license: GFDL.)

This is a classical situation to use viewports. The game screen is clearly parted into three distinguished areas. The main screen is the large part left with the mountains and the castles. Then there is the minimap in the right-upper corner. And a statistics overview under the minimap. These areas and the corresponding viewports are highlightened in the following screenshot.

Modified screenshot of Battle of Wesnoth. (Original source image, as found in the Wesnoth Wiki by Wesnoth developers. Image license: GFDL.)

The advantage of viewports is that each of them behaves like an own window, so if you draw to the right outside of viewport 1 in the screenshot above, the texture will just be clipped and there is no overlap into viewport 2 oder 3.

SDL2 window and viewport coordinates diagram
The relation of your screen, a SDL2 window and a viewport within this window are outlined here.

Let’s have a look at the code.

program SDL2_Viewport;

uses SDL2;

const
  Viewport1: TSDL_Rect = (x: 0;   y:   0; w: 400; h: 500);
  Viewport2: TSDL_Rect = (x: 400; y:   0; w: 100; h: 300);
  Viewport3: TSDL_Rect = (x: 400; y: 300; w: 100; h: 200);

  BlackDot:  TSDL_Rect = (x: 10;  y:  10; w:   3; h:   3);

var
  sdlWindow1: PSDL_Window;
  sdlRenderer: PSDL_Renderer;

begin

  //initilization of video subsystem
  if SDL_Init(SDL_INIT_VIDEO) < 0 then Halt;

  SDL_CreateWindowAndRenderer(500, 500, SDL_WINDOW_SHOWN, @sdlWindow1, @sdlRenderer);
  if (sdlWindow1 = nil) or (sdlRenderer = nil) then Halt;

  // fill every viewport with background color and draw a black dot into it
  SDL_RenderSetViewport(sdlRenderer, @Viewport1);
  SDL_SetRenderDrawColor(sdlRenderer, 255, 0, 0, SDL_ALPHA_OPAQUE);
  SDL_RenderFillRect(sdlRenderer, nil);
  SDL_SetRenderDrawColor(sdlRenderer, 0, 0, 0, SDL_ALPHA_OPAQUE);
  SDL_RenderFillRect(sdlRenderer, @BlackDot);

  SDL_RenderSetViewport(sdlRenderer, @Viewport2);
  SDL_SetRenderDrawColor(sdlRenderer, 255, 255, 0, SDL_ALPHA_OPAQUE);
  SDL_RenderFillRect(sdlRenderer, nil);
  SDL_SetRenderDrawColor(sdlRenderer, 0, 0, 0, SDL_ALPHA_OPAQUE);
  SDL_RenderFillRect(sdlRenderer, @BlackDot);

  SDL_RenderSetViewport(sdlRenderer, @Viewport3);
  SDL_SetRenderDrawColor(sdlRenderer, 0, 255, 0, SDL_ALPHA_OPAQUE);
  SDL_RenderFillRect(sdlRenderer, nil);
  SDL_SetRenderDrawColor(sdlRenderer, 0, 0, 0, SDL_ALPHA_OPAQUE);
  SDL_RenderFillRect(sdlRenderer, @BlackDot);

  // render to window for 2 seconds
  SDL_RenderPresent(sdlRenderer);
  SDL_Delay(2000);

  // clear memory
  SDL_DestroyRenderer(sdlRenderer);
  SDL_DestroyWindow (sdlWindow1);

  //shutting down video subsystem
  SDL_Quit;

end.

The result will look like this:

Creative Commons License This image by https://www.freepascal-meets-sdl.net is licensed under a Creative Commons Attribution 4.0 International License.
There are three viewports with different background colors and a black dot in the left upper corner.

First we set up some SDL2 rectangles by

const
  Viewport1: TSDL_Rect = (x: 0;   y:   0; w: 400; h: 500);
  Viewport2: TSDL_Rect = (x: 400; y:   0; w: 100; h: 300);
  Viewport3: TSDL_Rect = (x: 400; y: 300; w: 100; h: 200);

  BlackDot:  TSDL_Rect = (x: 10;  y:  10; w:   3; h:   3);

“Viewport1” represents the red viewport (left), “Viewport2” the yellow (upper-right) and “Viewport3” the green (lower-right) viewport in the result image.

Notice how we just prepare one “BlackDot” rectangle for a black dot of 3×3 px dimension at location (10/10).

After setting up SDL2, a renderer and a window as known, we start to set up the first (red, left) viewport.

  // fill every viewport with background color and draw a black dot into it
  SDL_RenderSetViewport(sdlRenderer, @Viewport1);
  SDL_SetRenderDrawColor(sdlRenderer, 255, 0, 0, SDL_ALPHA_OPAQUE);
  SDL_RenderFillRect(sdlRenderer, nil);
  SDL_SetRenderDrawColor(sdlRenderer, 0, 0, 0, SDL_ALPHA_OPAQUE);
  SDL_RenderFillRect(sdlRenderer, @BlackDot);

It is simple as that. Use the function SDL_RenderSetViewport(renderer, rectangle pointer) to set up a viewport. This function returns 0 on success or -1 on failure.

SDL_RenderSetViewport(renderer: PSDL_Renderer; const rect: PSDL_Rect)

After we set up the viewport, we set the draw color by SDL_SetRenderDrawColor(renderer, red, green, blue, alpha) to red (255/0/0/no transparency). Then we use SDL_RenderFillRect(renderer, rectangle pointer) to fill the whole viewport by not specifying a rectangle (nil). Both functions are known from a previous chapter.

Then the color is set to black and a tiny 3×3 rectangle is drawn at location (10/10).

This procedure is repeated for the other two viewports. Notice again, how we use the same rectangle for the black dot though and where it is shown in the result image. The black dot is always drawn at location (10/10) relative to the respective viewport’s location!

As general rule it applies:

The coordinates are always relative to the currently set viewport.

Well, the remaining parts of the code provides nothing new, just the rendering for 2 seconds and some clean up.

Let’s close with some helpful remarks.

No SDL_RenderClear for Viewports!

Do not use SDL_RenderClear(renderer). It will ignore the viewports and clear the whole window with the set drawing color.

Resetting the Viewport

The resetting is done simple by SDL_RenderSetViewport(renderer, nil) as one would expect.

← previous Chapter | next Chapter →

A Custom Mouse Cursor

Tutorial, , , , ,

Last updated on September 21st, 2019

Any good game has a custom mouse cursor. You may think it would be a good idea to have a SDL2 surface or SDL2 texture and render it as any other sprite right at the mouse position to simulate a mouse cursor. DO NOT do this! The mouse cursor is handled separatly from the other rendering to have it smooth and working in critical situations. 

The following code shows how to set up a custom mouse cursor with SDL2 the correct way.

program SDL_MouseCursor;

uses SDL2, SDL2_image;

var
  sdlWindow1: PSDL_Window;
  sdlRenderer: PSDL_Renderer;
  sdlSurface1: PSDL_Surface;
  sdlMouseCursor: PSDL_Cursor;
  sdlEvent: TSDL_Event;
  ExitLoop: Boolean = False;

begin

  //initilization of video subsystem
  if SDL_Init(SDL_INIT_VIDEO) < 0 then Halt;

  SDL_CreateWindowAndRenderer(500, 500, SDL_WINDOW_SHOWN, @sdlWindow1, @sdlRenderer);
  if (sdlWindow1 = nil) or (sdlRenderer = nil) then Halt;

  sdlSurface1 := IMG_Load('Cursor.png' );
  if sdlSurface1 = nil then Halt;

  // create and set new mouse cursor
  sdlMouseCursor := SDL_CreateColorCursor(sdlSurface1, 8, 8);
  if sdlMouseCursor = nil then Halt;

  SDL_SetCursor(sdlMouseCursor);

  while ExitLoop = False do
  begin

    // exit loop if mouse button pressed
    while SDL_PollEvent(@sdlEvent) = 1 do
      if sdlEvent.type_ = SDL_MOUSEBUTTONDOWN then
        ExitLoop := True;

    SDL_SetRenderDrawColor(sdlRenderer, 128, 128, 128, SDL_ALPHA_OPAQUE);
    SDL_RenderClear(sdlRenderer);

    SDL_RenderPresent(sdlRenderer);
    SDL_Delay( 20 );
  end;

  SDL_FreeCursor(sdlMouseCursor);

  SDL_FreeSurface(sdlSurface1);
  SDL_DestroyRenderer(sdlRenderer);
  SDL_DestroyWindow (sdlWindow1);

  //shutting down video subsystem
  SDL_Quit;

end.

To have a custom mouse cursor we need a variable of type PSDL_Cursor. We call it “sdlMouseCursor” here.

The result looks like this:

Custom Mouse Cursor in SDL2
Creative Commons License This image by https://www.freepascal-meets-sdl.net is licensed under a Creative Commons Attribution 4.0 International License.
The blue cross with the yellow outline is the mouse cursor on the grey canvas/window.  
  sdlSurface1 := IMG_Load('Cursor.png' );
  if sdlSurface1 = nil then Halt;

  // create and set new mouse cursor
  sdlMouseCursor := SDL_CreateColorCursor(sdlSurface1, 8, 8);
  if sdlMouseCursor = nil then Halt;

  SDL_SetCursor(sdlMouseCursor);

This is the interesting part of the code with regard to creating a custom mouse cursor. The cursor’s image is defined by a SDL surface. We create the SDL surface as known from a previous chapter from a png image file to “sdlSurface1” here.

The custom mouse cursor is created by the following function, which returns nil on error.

SDL_CreateColorCursor(surface: PSDL_Surface; hot_x: SInt32; hot_y: SInt32): PSDL_Cursor

It needs the surface to use as cursor image and two coordinates (hot_x/hot_y) as arguments. They determine where the actual hitting point for this cursor is. Since the example cursor image is of dimensions 16×16 px and represents a cross, the “hot” (hitting) coordiates are (8/8), hence the cross’ center is used for hitting a button or something. In contrast you may imagine a typical arrow shaped mouse cursor, where the hitting point has to be adjusted to be right on the tip of the arrow in the arrow’s image.

If the cursor creation has been successful, it is necessary to set it to be the actual cursor. You may have created many different cursors, so tell SDL which one to use by the following procedure.

SDL_SetCursor(cursor: PSDL_Cursor)

The remaining part of the code is just rendering a 500 by 500 pixels window with a grey (128, 128, 128) background that is updated as long as no mouse button has been pressed.

Finally do not forget to free the mouse cursor by SDL_FreeCursor(mouse cursor) as shown.

← previous Chapter | next Chapter →

Chapter 2 – Installation and Configuration (Linux version)

Tutorial, ,

Last updated on July 22nd, 2020

This chapter illustrates quickly how to set up a Free Pascal and SDL2 development environment within a few minutes under Linux.

Attention: The following instruction particularly work for many Debian and Ubuntu based Linux distributions (like Linux Mint used here). In general it outlines the way to go, though.

The distribution and software I used:

  • Linux Distribution: Linux Mint 19.3 (Ubuntu/Debian based)
  • Desktop: Cinnamon Desktop
  • Lazarus 2.0.6 (installed from .deb file)
  • FPC 3.0.4 (installed from .deb file)
  • FPC 3.0.4 Source Code (installed from .deb file)
  • Tim Blume’s SDL2 units (header translation)
  • SDL2, SDL2_image, SDL2_ttf shared object files (from distro’s package manager)

Download and install FPC, FPC sourc code and Lazarus

The first step is to install the Free Pascal compiler (version 3.0.4 or higher), the Compiler’s source code (same version as the compiler) and the Lazarurs IDE (version 2.0.6 or higher). To get the most recent, stable environment, download these three files from the official Lazarus website: https://www.lazarus-ide.org/index.php?page=downloads.

Important: Do not intermix FPC or Lazarus installs from the package manager. This will lead to troubles because these installs are not compatible. Purge any of these installs. Use, e.g.:

dpkg --list (show all installed packages)
sudo apt-get --purge fp-compiler.... [exact package name] (remove package incl. config files)

Choose one of the “Linux DEB Releases” according to your system (32 bit or 64 bit). Most probably your running on a 64 bit system, which is the standard case for Linux.

In case of a 64 bit system download these three files:

  • fpc-laz_3.0.4-1_amd64.deb
  • fpc-src_3.0.4-2_amd64.deb
  • lazarus-project_2.0.6-0_amd64.deb

The download page looks somewhat like this:

Install Packages for Linux FPC SDL2 environment with Lazarus
SourceForge download page for all three files necessary. The original description is kept in the image.

If you downloaded these three files successfully, you run them in the same order! First FPC, then FPC’s sources and finally Lazarus.

If everything went right, Lazarus can be started up by typing “startlazarus” in the terminal or by finding the program in the application menue.

Start up Lazarus the first time

On start up of Lazarus the directories for FPC and the FPC source code were found and set already. As a hint I show where these are located on my system:

  • FPC: /usr/bin/fpc
  • FPC Source code: /usr/share/fpcsrc/3.0.4 (because $(FPCVER) equals the version number, see screenshot)

The configurations screen may look somewhat different to the screen of the following screenshot, but that is because the screenshot is outdated a bit.

Path FPC and FPC Source code
Either detected automatically or can be manually added by Tools > Options …

Before proceeding, my suggestion is to simply compile the project (press F9 in Lazarus) which is presented to you. It should compile and show the form.

Get the SDL2 units

Get the latest version of the translated SDL2 units.

Download SDL2 units on GitHub
Choose the master branch (1), click on “Clone or download” (2) and click on Download ZIP (3).

Make sure you have the master branch chosen and then click on “Clone or download”, then “Download ZIP”.

After extracting the ZIP file I suggest to rename the new folder into “sdl2” or “SDL2” and place it at any location, perhaps your development folder, e.g.:

  • ~/projects/sdl2 (a.k.a /home/[username]/projects/sdl2)
  • (DO NOT use the suggested folder in the screenshot)
Path to SDL2 units
This folder is suggested as a place for the SDL2 units (DO NOT use this folder!). By the way, “Chap7” is just a random name for this project and you may have anything else there instead (I was trying out Chapter 7 tutorial code).

Get the SDL2 shared object files

If you are looking for the most recent pre-compiled SDL2 dynamic link library files (e.g. libSDL2.so) on the official SDL2 website, you just find a remark that reads like this:

Linux:
Please contact your distribution maintainer for updates.

Since SDL2 is very widespread it is very likely that your distribution maintainer already included the files.

Find SDL2 and all necessary libraries in your distribution’s package manager, here it is the synaptic package manager. Look for libsdl and install every package shown in the screenshot:

Synaptic package manager shows installed sdl2 shared object packages.

Find all necessary libraries and install them. These you should install:

  • libsdl2
  • libsdl2-dev
  • libsdl2-gfx
  • libsdl2-gfx-dev
  • libsdl2-image
  • libsdl2-image-dev
  • libsdl2-mixer
  • libsdl2-mixer-dev
  • libsdl2-net
  • libsdl2-net-dev
  • libsdl2-ttf
  • libsdl2-ttf-dev

The dev-packages are necessary to compile SDL2 applications. The other packages are necessary to run SDL2 applications.

The version of these libraries does not necessarily need to be the most recent unfortunately. If you really need the most recent versions here, you may try to contact the maintainer to ask to update the version.

Congratulation! After that, everything should run smoothly :-)!

← Chapter 1 | Chapter 3 →

Chapter 10 – SDL and modern OpenGL 3.0+

Tutorial, , , ,

This chapter will introduce you on how to combine the SDL library with the famous Open Graphics Library (OpenGL).

What is OpenGL?

OpenGL is the first choice when it comes to platform independent 2d and 3d graphics programming. The emphasis is on graphics programming only though!

Why and when to combine SDL and OpenGL?

SDL is an excellent choice if you need platform independent 2d graphics. OpenGL is capable of 2d graphics, too, but why using the more complicated library if you could use the easy to use SDL library? – And by the way, underneath SDL is actually using OpenGL (or similar libraries depending upon the system) to achieve its hardware accelerated 2d graphics.

However, if your project needs 3d graphics, which isn’t covered by SDL, you can set up your system for this quite easy with SDL. The setup of an OpenGL environment is very easy and platform independent with SDL. Without SDL you would’ve to write different code to set up OpenGL for each operating system. In fact, even professional developers use SDL to set up their OpenGL applications.

Furthermore, since OpenGL is a pure graphics library, any other task is further done by SDL (e.g. keyboard handling, sound,…).

At this point I’d like to quote Klaus Vor der Landwehr (professional developer) from Turtle-Games, who described the relation of SDL and OpenGL in a very clear way.

Although the graphics are often in the foreground, it is for me as a game programmer only one aspect of many with which I have to deal. And the graphics do not even require the most work. OpenAL for example costs much more time and effort if you want to build a 3D sound channel management. And there are many other interfaces. Here is a list of categories in which SDL has been a great help:

  • multiple displays
  • window management
  • Event handling
  • keyboard
  • mouse
  • joystick
  • game controller
  • force feedback
  • threads
  • timers

… for Windows, Mac and Linux.

Source: Pascal Game Development Community.

What exactly is modern OpenGL?

As of version 2.0 of OpenGL, the so-called fixed pipeline has been replaced by a programmable pipeline (modern OpenGL). In general, the pipeline makes your input data appear on the screen in a hardware accelerated manner by using your graphics card. For the fixed pipeline it was easy to draw something to the screen but, as the name suggests, it was quite fixed and unflexible. The programmable pipeline which is controlled by a newly introduced shader (script) language is far more flexible, though, the ease is gone :-D.

Anyway, some people refer to OpenGL version 3.0 and up as modern OpenGL. This is because a lot of typical functionality was deprecated as of this version. The backwards compatibility is gone.

In this chapter I will demonstrate how to use SDL 3.0 and up to set up a modern OpenGL environment using some basic shaders. I based the description heavily on an excellent C++ tutorial over at opengl-tutorial.org and their second chapter. You may look for further OpenGL stuff there or have a look at this WikiBook OpenGL Introduction (C++). I’m not aware of OpenGL tutorials for Free Pascal or Delphi treating modern OpenGL (let me know if you know).

OpenGL Pascal units (headers)

Similar to SDL 2.0, you need specific units which translate and connect your code to the OpenGL library. There is a native OpenGL unit GL which covers the main functionality of OpenGL. Additionally for modern OpenGL you need the unit GLext which covers the functionality up to OpenGL version 4.0. These units are shipped right along with the Free Pascal compiler.

In case you are interested in support of OpenGL version 4.4, you should look into the dglOpenGL.pas. This unit is not shipped natively along with the Free Pascal compiler.

Let the fun begin

Let’s have a look at the code:

program chap10_SDL2;
uses Classes, SysUtils, SDL2, GL, GLext;

const
  vertexShaderFile = 'VertexShader.txt';
  fragmentShaderFile = 'FragmentShader.txt';
  triangleData: array[0..8] of GLfloat = ( -1.0, -1.0, 0.0,
                                            1.0, -1.0, 0.0,
                                            0.0,  1.0, 0.0  );

var
sdlWindow1: PSDL_Window;
sdlGLContext1: TSDL_GLContext;
i: Word;
VertexArrayID: GLuint;
triangleVBO: GLuint;

VertexShaderID: GLuint;
VertexShaderCode: PGLchar;
FragmentShaderID: GLuint;
FragmentShaderCode: PGLchar;
ShaderCode: TStringList;
ProgramID: GLuint;
compilationResult: GLint = GL_FALSE;
InfoLogLength: GLint;
ErrorMessageArray: array of GLChar;

begin
  if SDL_Init( SDL_INIT_VIDEO ) < 0 then HALT;

  //get an OpenGL window and create OpenGL context
  sdlWindow1 := SDL_CreateWindow( 'OpenGL window', 50, 50, 500, 500, SDL_WINDOW_OPENGL );
  if sdlWindow1 = nil then HALT;

  sdlGLContext1 := SDL_GL_CreateContext( sdlWindow1 );
  if @sdlGLContext1 = nil then HALT;

  //init OpenGL and load extensions
  if Load_GL_VERSION_4_0 = false then
    if Load_GL_VERSION_3_3 = false then
      if Load_GL_VERSION_3_2 = false then
        if Load_GL_VERSION_3_0 = false then
        begin
          writeln(' ERROR: OpenGL 3.0 or higher needed. '); readln;
          HALT;
        end;

  //print out OpenGL vendor, version and shader version
  writeln( 'Vendor: ' + glGetString( GL_VENDOR ) );
  writeln( 'OpenGL Version: ' + glGetString( GL_VERSION ) );
  writeln( 'Shader Version: ' + glGetString( GL_SHADING_LANGUAGE_VERSION ) );

  //create Vertex Array Object (VAO)
  glGenVertexArrays( 1, @VertexArrayID );
  glBindVertexArray( VertexArrayID );

  //creating Vertex Buffer Object (VBO)
  glGenBuffers( 1, @triangleVBO );
  glBindBuffer( GL_ARRAY_BUFFER, triangleVBO );
  glBufferData( GL_ARRAY_BUFFER, SizeOf( triangleData ), @triangleData, GL_STATIC_DRAW );

  //creating shaders
  VertexShaderID := glCreateShader( GL_VERTEX_SHADER );
  FragmentShaderID := glCreateShader( GL_FRAGMENT_SHADER );

  //load shader code and get PChars
  ShaderCode := TStringList.Create;
  ShaderCode.LoadFromFile( VertexShaderFile );
  VertexShaderCode := ShaderCode.GetText;
  if VertexShaderCode = nil then HALT;
  ShaderCode.LoadFromFile( FragmentShaderFile );
  FragmentShaderCode := ShaderCode.GetText;
  if FragmentShaderCode = nil then HALT;
  ShaderCode.Free;

  //compiling and error checking vertex shader
  write('Compiling and error checking Vertex Shader... ' );
  glShaderSource( VertexShaderID, 1, @VertexShaderCode, nil );
  glCompileShader( VertexShaderID );

  glGetShaderiv( VertexShaderID, GL_COMPILE_STATUS, @compilationResult );
  glGetShaderiv( VertexShaderID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  //compiling and error checking fragment shader
  write('Compiling and error checking Fragment Shader... ' );
  glShaderSource( FragmentShaderID, 1, @FragmentShaderCode, nil );
  glCompileShader( FragmentShaderID );

  glGetShaderiv( FragmentShaderID, GL_COMPILE_STATUS, @compilationResult );
  glGetShaderiv( FragmentShaderID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  //creating and linking program
  write('Creating and linking program... ' );
  ProgramID := glCreateProgram();
  glAttachShader( ProgramID, VertexShaderID );
  glAttachShader( ProgramID, FragmentShaderID );
  glLinkProgram( ProgramID );

  glGetShaderiv( ProgramID, GL_LINK_STATUS, @compilationResult );
  glGetShaderiv( ProgramID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  for i := 0 to 400 do
  begin
    glClearColor( 0.0, 1.0-i/400, 0.0+i/400, 1.0 );
    glClear( GL_COLOR_BUFFER_BIT );
    glUseProgram( ProgramID );
    glEnableVertexAttribArray( 0 );
    glBindBuffer( GL_ARRAY_BUFFER, triangleVBO );
    glVertexAttribPointer( 0, 3, GL_FLOAT, GL_FALSE, 0, nil );
    glDrawArrays( GL_TRIANGLES, 0, 3 );
    glDisableVertexAttribArray( 0 );
    SDL_Delay( 20 );
    SDL_GL_SwapWindow( sdlWindow1 );
  end;

  //clean up
  glDetachShader( ProgramID, VertexShaderID );
  glDetachShader( ProgramID, FragmentShaderID );

  glDeleteShader( VertexShaderID );
  glDeleteShader( FragmentShaderID );
  glDeleteProgram( ProgramID );

  StrDispose( VertexShaderCode );
  StrDispose( FragmentShaderCode );

  glDeleteBuffers( 1, @triangleVBO );
  glDeleteVertexArrays( 1, @VertexArrayID );

  SDL_GL_DeleteContext( sdlGLContext1 );
  SDL_DestroyWindow( sdlWindow1 );

  SDL_Quit;
end.

Wow that is a lot of code. What you will get is this:

Result for chapter 10

The background will change slowly from green to blue. And you will get this:

Command result for chapter 10

The vendor, OpenGL version and shader version information will be different according to your system. Also, if your system doen’t support the needed OpenGL version you’ll not have “success” but rather “failure” after the compiling and linking processes. Additional information may be shown then.

program chap10_SDL2;
uses Classes, SysUtils, SDL2, GL, GLext;

const
  vertexShaderFile = 'VertexShader.txt';
  fragmentShaderFile = 'FragmentShader.txt';
  triangleData: array[0..8] of GLfloat = ( -1.0, -1.0, 0.0,
                                            1.0, -1.0, 0.0,
                                            0.0,  1.0, 0.0  );

var
sdlWindow1: PSDL_Window;
sdlGLContext1: TSDL_GLContext;
i: Word;
VertexArrayID: GLuint;
triangleVBO: GLuint;

VertexShaderID: GLuint;
VertexShaderCode: PGLchar;
FragmentShaderID: GLuint;
FragmentShaderCode: PGLchar;
ShaderCode: TStringList;
ProgramID: GLuint;
compilationResult: GLint = GL_FALSE;
InfoLogLength: GLint;
ErrorMessageArray: array of GLChar;

begin
  if SDL_Init( SDL_INIT_VIDEO ) < 0 then HALT;

  //get an OpenGL window and create OpenGL context
  sdlWindow1 := SDL_CreateWindow( 'OpenGL window', 50, 50, 500, 500, SDL_WINDOW_OPENGL );
  if sdlWindow1 = nil then HALT;

  sdlGLContext1 := SDL_GL_CreateContext( sdlWindow1 );
  if @sdlGLContext1 = nil then HALT;

  //init OpenGL and load extensions
  if Load_GL_VERSION_4_0 = false then
    if Load_GL_VERSION_3_3 = false then
      if Load_GL_VERSION_3_2 = false then
        if Load_GL_VERSION_3_0 = false then
        begin
          writeln(' ERROR: OpenGL 3.0 or higher needed. '); readln;
          HALT;
        end;

  //print out OpenGL vendor, version and shader version
  writeln( 'Vendor: ' + glGetString( GL_VENDOR ) );
  writeln( 'OpenGL Version: ' + glGetString( GL_VERSION ) );
  writeln( 'Shader Version: ' + glGetString( GL_SHADING_LANGUAGE_VERSION ) );

  //create Vertex Array Object (VAO)
  glGenVertexArrays( 1, @VertexArrayID );
  glBindVertexArray( VertexArrayID );

  //creating Vertex Buffer Object (VBO)
  glGenBuffers( 1, @triangleVBO );
  glBindBuffer( GL_ARRAY_BUFFER, triangleVBO );
  glBufferData( GL_ARRAY_BUFFER, SizeOf( triangleData ), @triangleData, GL_STATIC_DRAW );

  //creating shaders
  VertexShaderID := glCreateShader( GL_VERTEX_SHADER );
  FragmentShaderID := glCreateShader( GL_FRAGMENT_SHADER );

  //load shader code and get PChars
  ShaderCode := TStringList.Create;
  ShaderCode.LoadFromFile( VertexShaderFile );
  VertexShaderCode := ShaderCode.GetText;
  if VertexShaderCode = nil then HALT;
  ShaderCode.LoadFromFile( FragmentShaderFile );
  FragmentShaderCode := ShaderCode.GetText;
  if FragmentShaderCode = nil then HALT;
  ShaderCode.Free;

  //compiling and error checking vertex shader
  write('Compiling and error checking Vertex Shader... ' );
  glShaderSource( VertexShaderID, 1, @VertexShaderCode, nil );
  glCompileShader( VertexShaderID );

  glGetShaderiv( VertexShaderID, GL_COMPILE_STATUS, @compilationResult );
  glGetShaderiv( VertexShaderID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  //compiling and error checking fragment shader
  write('Compiling and error checking Fragment Shader... ' );
  glShaderSource( FragmentShaderID, 1, @FragmentShaderCode, nil );
  glCompileShader( FragmentShaderID );

  glGetShaderiv( FragmentShaderID, GL_COMPILE_STATUS, @compilationResult );
  glGetShaderiv( FragmentShaderID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  //creating and linking program
  write('Creating and linking program... ' );
  ProgramID := glCreateProgram();
  glAttachShader( ProgramID, VertexShaderID );
  glAttachShader( ProgramID, FragmentShaderID );
  glLinkProgram( ProgramID );

  glGetShaderiv( ProgramID, GL_LINK_STATUS, @compilationResult );
  glGetShaderiv( ProgramID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  for i := 0 to 400 do
  begin
    glClearColor( 0.0, 1.0-i/400, 0.0+i/400, 1.0 );
    glClear( GL_COLOR_BUFFER_BIT );
    glUseProgram( ProgramID );
    glEnableVertexAttribArray( 0 );
    glBindBuffer( GL_ARRAY_BUFFER, triangleVBO );
    glVertexAttribPointer( 0, 3, GL_FLOAT, GL_FALSE, 0, nil );
    glDrawArrays( GL_TRIANGLES, 0, 3 );
    glDisableVertexAttribArray( 0 );
    SDL_Delay( 20 );
    SDL_GL_SwapWindow( sdlWindow1 );
  end;

  //clean up
  glDetachShader( ProgramID, VertexShaderID );
  glDetachShader( ProgramID, FragmentShaderID );

  glDeleteShader( VertexShaderID );
  glDeleteShader( FragmentShaderID );
  glDeleteProgram( ProgramID );

  StrDispose( VertexShaderCode );
  StrDispose( FragmentShaderCode );

  glDeleteBuffers( 1, @triangleVBO );
  glDeleteVertexArrays( 1, @VertexArrayID );

  SDL_GL_DeleteContext( sdlGLContext1 );
  SDL_DestroyWindow( sdlWindow1 );

  SDL_Quit;
end.

The program is called “chap10_SDL2” for obvious reason.

Additionally to the SDL2 unit we load the native FPC units Classes (for TStringList support), SysUtils (for PChar functions) and GL and GLext for OpenGL support.

Thre are three constants declared. The first two are defined as the filenames of the so-called shader source files.  Basically they are simple text files which contain a script. More about shaders and the script later. The third is an array of nine GLfloat values. GLfloat is the OpenGL float variable type which in fact is translated as Pascal’s Single type. In short, these nine values describe three points in 3d space which, if connected, form a triangle. More about this later.

The first variable “sdlWindow1” is well known from previous chapters. Any variable to follow is new though. Most of them are related to OpenGL.

“sdlGLContext1” is of type TSDL_GLContext needed to create a so-called OpenGL context. In fact, this variable type is provided by SDL and a key type to set up an OpenGL conext in a simple and cross-platform manner.

The variable “i” is a simple Word variable for counting purposes.

OpenGL’s Integers and Strings

Most of the following variables are either of type GLuint or of type PGLchar. The last variable is an dynamic array of GLchars. Their specific meaning will be discussed later but GLuint is the OpenGL unsigned integer type (no negative values) which translates to Pascal’s Cardinal/Longword type. Text handling in OpenGL works by null-terminated strings of type PGLchar which translate to Pascal’s PChar. GLchar translates to Char then, obviously.

At this point you may wonder why as for SDL the null-terminated strings are used instead of simple strings (see Chapter 7 for the PAnsiChar variable type discussion). The answer again is that OpenGL is based upon C which handles strings this way. PChar equals PAnsiChar by the way.

The remaining variables “ShaderCode” of type TStringList will be used to handle the shader text files. “compilationResult” and “InfoLogLength” are of GLint type. In contrast to GLuint they allow for negative values.

program chap10_SDL2;
uses Classes, SysUtils, SDL2, GL, GLext;

const
  vertexShaderFile = 'VertexShader.txt';
  fragmentShaderFile = 'FragmentShader.txt';
  triangleData: array[0..8] of GLfloat = ( -1.0, -1.0, 0.0,
                                            1.0, -1.0, 0.0,
                                            0.0,  1.0, 0.0  );

var
sdlWindow1: PSDL_Window;
sdlGLContext1: TSDL_GLContext;
i: Word;
VertexArrayID: GLuint;
triangleVBO: GLuint;

VertexShaderID: GLuint;
VertexShaderCode: PGLchar;
FragmentShaderID: GLuint;
FragmentShaderCode: PGLchar;
ShaderCode: TStringList;
ProgramID: GLuint;
compilationResult: GLint = GL_FALSE;
InfoLogLength: GLint;
ErrorMessageArray: array of GLChar;

begin
  if SDL_Init( SDL_INIT_VIDEO ) < 0 then HALT;

  //get an OpenGL window and create OpenGL context
  sdlWindow1 := SDL_CreateWindow( 'OpenGL window', 50, 50, 500, 500, SDL_WINDOW_OPENGL );
  if sdlWindow1 = nil then HALT;

  sdlGLContext1 := SDL_GL_CreateContext( sdlWindow1 );
  if @sdlGLContext1 = nil then HALT;

  //init OpenGL and load extensions
  if Load_GL_VERSION_4_0 = false then
    if Load_GL_VERSION_3_3 = false then
      if Load_GL_VERSION_3_2 = false then
        if Load_GL_VERSION_3_0 = false then
        begin
          writeln(' ERROR: OpenGL 3.0 or higher needed. '); readln;
          HALT;
        end;

  //print out OpenGL vendor, version and shader version
  writeln( 'Vendor: ' + glGetString( GL_VENDOR ) );
  writeln( 'OpenGL Version: ' + glGetString( GL_VERSION ) );
  writeln( 'Shader Version: ' + glGetString( GL_SHADING_LANGUAGE_VERSION ) );

  //create Vertex Array Object (VAO)
  glGenVertexArrays( 1, @VertexArrayID );
  glBindVertexArray( VertexArrayID );

  //creating Vertex Buffer Object (VBO)
  glGenBuffers( 1, @triangleVBO );
  glBindBuffer( GL_ARRAY_BUFFER, triangleVBO );
  glBufferData( GL_ARRAY_BUFFER, SizeOf( triangleData ), @triangleData, GL_STATIC_DRAW );

  //creating shaders
  VertexShaderID := glCreateShader( GL_VERTEX_SHADER );
  FragmentShaderID := glCreateShader( GL_FRAGMENT_SHADER );

  //load shader code and get PChars
  ShaderCode := TStringList.Create;
  ShaderCode.LoadFromFile( VertexShaderFile );
  VertexShaderCode := ShaderCode.GetText;
  if VertexShaderCode = nil then HALT;
  ShaderCode.LoadFromFile( FragmentShaderFile );
  FragmentShaderCode := ShaderCode.GetText;
  if FragmentShaderCode = nil then HALT;
  ShaderCode.Free;

  //compiling and error checking vertex shader
  write('Compiling and error checking Vertex Shader... ' );
  glShaderSource( VertexShaderID, 1, @VertexShaderCode, nil );
  glCompileShader( VertexShaderID );

  glGetShaderiv( VertexShaderID, GL_COMPILE_STATUS, @compilationResult );
  glGetShaderiv( VertexShaderID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  //compiling and error checking fragment shader
  write('Compiling and error checking Fragment Shader... ' );
  glShaderSource( FragmentShaderID, 1, @FragmentShaderCode, nil );
  glCompileShader( FragmentShaderID );

  glGetShaderiv( FragmentShaderID, GL_COMPILE_STATUS, @compilationResult );
  glGetShaderiv( FragmentShaderID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  //creating and linking program
  write('Creating and linking program... ' );
  ProgramID := glCreateProgram();
  glAttachShader( ProgramID, VertexShaderID );
  glAttachShader( ProgramID, FragmentShaderID );
  glLinkProgram( ProgramID );

  glGetShaderiv( ProgramID, GL_LINK_STATUS, @compilationResult );
  glGetShaderiv( ProgramID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  for i := 0 to 400 do
  begin
    glClearColor( 0.0, 1.0-i/400, 0.0+i/400, 1.0 );
    glClear( GL_COLOR_BUFFER_BIT );
    glUseProgram( ProgramID );
    glEnableVertexAttribArray( 0 );
    glBindBuffer( GL_ARRAY_BUFFER, triangleVBO );
    glVertexAttribPointer( 0, 3, GL_FLOAT, GL_FALSE, 0, nil );
    glDrawArrays( GL_TRIANGLES, 0, 3 );
    glDisableVertexAttribArray( 0 );
    SDL_Delay( 20 );
    SDL_GL_SwapWindow( sdlWindow1 );
  end;

  //clean up
  glDetachShader( ProgramID, VertexShaderID );
  glDetachShader( ProgramID, FragmentShaderID );

  glDeleteShader( VertexShaderID );
  glDeleteShader( FragmentShaderID );
  glDeleteProgram( ProgramID );

  StrDispose( VertexShaderCode );
  StrDispose( FragmentShaderCode );

  glDeleteBuffers( 1, @triangleVBO );
  glDeleteVertexArrays( 1, @VertexArrayID );

  SDL_GL_DeleteContext( sdlGLContext1 );
  SDL_DestroyWindow( sdlWindow1 );

  SDL_Quit;
end.

First SDL2 is initilized as known. “sdlWindow1” is created as known by SDL_CreateWindow. Be careful though, in order to work with OpenGL the flag SDL_WINDOW_OPENGL has to be set!

SDL 2.0 and the OpenGL context

An OpenGL context is kind of an abstract name. It doesn’t represents just a window, even though it is created from a SDL2 window, but rather it contains everything (including the window information) that is related to this OpenGL context. The OpenGL context is therefore kind of “broader” than just a window, that is why it is called context rather than just a OpenGL window.

The function to create an OpenGL context from a SDL2 window is:

function SDL_GL_CreateContext(window: PSDL_Window): TSDL_GLContext

So, it is simple as that, just use the SDL2 window as argument and voila, you’ll get a OpenGL context, platform-independent. That is why everybody loves SDL2 to work with OpenGL. Note that the returned Context isn’t a pointer but an actual instance. So to error check against nil you need to refer to the instance’s addresse by the @ operator.

OpenGL version check and initialization

The nested if-then-statements check if at least version 3.0 of OpenGL is installed. If so, the highest available version is loaded. If not, the program is stopped and returns a text message.

If your hardware doen’t support OpenGL 3.0 or higher you should try to update your graphics driver. There is a good chance that you are able to use OpenGL 3.0 or higher then. Anyway, if the upgrade doesn’t work out or you wouldn’t want to update, you may have a look into the JEDI-SDL Chapter about OpenGL, there the old OpenGL is treated (although that chapter treats SDL 1.2, it shouldn’t be too hard to make it work with SDL 2.0 with minor changes).

Next there are three text messages printed out. These present the Vendor, the OpenGL version and the Shading language version. To get them in a readable form you need to transform the constants into strings by function glGetString. Let’s have a look at the command window again:

Command result for chapter 10

Have a look at the first three lines and you see what it could look like.

Vertex Array Object and Vertex Buffer Object

program chap10_SDL2;
uses Classes, SysUtils, SDL2, GL, GLext;

const
  vertexShaderFile = 'VertexShader.txt';
  fragmentShaderFile = 'FragmentShader.txt';
  triangleData: array[0..8] of GLfloat = ( -1.0, -1.0, 0.0,
                                            1.0, -1.0, 0.0,
                                            0.0,  1.0, 0.0  );

var
sdlWindow1: PSDL_Window;
sdlGLContext1: TSDL_GLContext;
i: Word;
VertexArrayID: GLuint;
triangleVBO: GLuint;

VertexShaderID: GLuint;
VertexShaderCode: PGLchar;
FragmentShaderID: GLuint;
FragmentShaderCode: PGLchar;
ShaderCode: TStringList;
ProgramID: GLuint;
compilationResult: GLint = GL_FALSE;
InfoLogLength: GLint;
ErrorMessageArray: array of GLChar;

begin
  if SDL_Init( SDL_INIT_VIDEO ) < 0 then HALT;

  //get an OpenGL window and create OpenGL context
  sdlWindow1 := SDL_CreateWindow( 'OpenGL window', 50, 50, 500, 500, SDL_WINDOW_OPENGL );
  if sdlWindow1 = nil then HALT;

  sdlGLContext1 := SDL_GL_CreateContext( sdlWindow1 );
  if @sdlGLContext1 = nil then HALT;

  //init OpenGL and load extensions
  if Load_GL_VERSION_4_0 = false then
    if Load_GL_VERSION_3_3 = false then
      if Load_GL_VERSION_3_2 = false then
        if Load_GL_VERSION_3_0 = false then
        begin
          writeln(' ERROR: OpenGL 3.0 or higher needed. '); readln;
          HALT;
        end;

  //print out OpenGL vendor, version and shader version
  writeln( 'Vendor: ' + glGetString( GL_VENDOR ) );
  writeln( 'OpenGL Version: ' + glGetString( GL_VERSION ) );
  writeln( 'Shader Version: ' + glGetString( GL_SHADING_LANGUAGE_VERSION ) );

  //create Vertex Array Object (VAO)
  glGenVertexArrays( 1, @VertexArrayID );
  glBindVertexArray( VertexArrayID );

  //creating Vertex Buffer Object (VBO)
  glGenBuffers( 1, @triangleVBO );
  glBindBuffer( GL_ARRAY_BUFFER, triangleVBO );
  glBufferData( GL_ARRAY_BUFFER, SizeOf( triangleData ), @triangleData, GL_STATIC_DRAW );

  //creating shaders
  VertexShaderID := glCreateShader( GL_VERTEX_SHADER );
  FragmentShaderID := glCreateShader( GL_FRAGMENT_SHADER );

  //load shader code and get PChars
  ShaderCode := TStringList.Create;
  ShaderCode.LoadFromFile( VertexShaderFile );
  VertexShaderCode := ShaderCode.GetText;
  if VertexShaderCode = nil then HALT;
  ShaderCode.LoadFromFile( FragmentShaderFile );
  FragmentShaderCode := ShaderCode.GetText;
  if FragmentShaderCode = nil then HALT;
  ShaderCode.Free;

  //compiling and error checking vertex shader
  write('Compiling and error checking Vertex Shader... ' );
  glShaderSource( VertexShaderID, 1, @VertexShaderCode, nil );
  glCompileShader( VertexShaderID );

  glGetShaderiv( VertexShaderID, GL_COMPILE_STATUS, @compilationResult );
  glGetShaderiv( VertexShaderID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  //compiling and error checking fragment shader
  write('Compiling and error checking Fragment Shader... ' );
  glShaderSource( FragmentShaderID, 1, @FragmentShaderCode, nil );
  glCompileShader( FragmentShaderID );

  glGetShaderiv( FragmentShaderID, GL_COMPILE_STATUS, @compilationResult );
  glGetShaderiv( FragmentShaderID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  //creating and linking program
  write('Creating and linking program... ' );
  ProgramID := glCreateProgram();
  glAttachShader( ProgramID, VertexShaderID );
  glAttachShader( ProgramID, FragmentShaderID );
  glLinkProgram( ProgramID );

  glGetShaderiv( ProgramID, GL_LINK_STATUS, @compilationResult );
  glGetShaderiv( ProgramID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  for i := 0 to 400 do
  begin
    glClearColor( 0.0, 1.0-i/400, 0.0+i/400, 1.0 );
    glClear( GL_COLOR_BUFFER_BIT );
    glUseProgram( ProgramID );
    glEnableVertexAttribArray( 0 );
    glBindBuffer( GL_ARRAY_BUFFER, triangleVBO );
    glVertexAttribPointer( 0, 3, GL_FLOAT, GL_FALSE, 0, nil );
    glDrawArrays( GL_TRIANGLES, 0, 3 );
    glDisableVertexAttribArray( 0 );
    SDL_Delay( 20 );
    SDL_GL_SwapWindow( sdlWindow1 );
  end;

  //clean up
  glDetachShader( ProgramID, VertexShaderID );
  glDetachShader( ProgramID, FragmentShaderID );

  glDeleteShader( VertexShaderID );
  glDeleteShader( FragmentShaderID );
  glDeleteProgram( ProgramID );

  StrDispose( VertexShaderCode );
  StrDispose( FragmentShaderCode );

  glDeleteBuffers( 1, @triangleVBO );
  glDeleteVertexArrays( 1, @VertexArrayID );

  SDL_GL_DeleteContext( sdlGLContext1 );
  SDL_DestroyWindow( sdlWindow1 );

  SDL_Quit;
end.

If you are new to OpenGL,  OpenGL as kind of machine with many switches. Each switch

Briefly, a Vertex Array Object (VAO) is a specific OpenGL object which contains important settings (e.g. format of vertex data) and references to other objects, including Vertex Buffer Objects (VBO). Notice, it doesn’t store the object’s data (content) itself, it just stores the reference to these objects.

The Vertex Buffer Object (VBO) contains the actual data (content). In the example case these are three vertices, each described by three float point values in cartesian space.

OpenGL Object name or ID

Because it is important to understand, in contrast to SDL where objects usually are directly submitted to a function by its pointer reference, in OpenGL you have a so-called OpenGL Object name, which actually is an integer value of GLuint type. Therefore ID is a suitable name, too. Let’s see how this works:

The VAO is created by function glGenVertexArrays( number of VAO names, pointer to VAO names array ). The first parameter determines how many VAO names I’d like to create. We just need 1. The second parameter asks for a pointer to an array of VAO names. Since VAO names are just simple GLuints, it is a simple array of GLuints. Anyway, since we just need one, a pointer to a simple GLuint variable will be suitable, too. In our case that is “VertexArrayID”. To bind (“activate”) the corresponding VAO to the OpenGL context, the function glBindVertexArray( name of VAO ) is used. The argument is the name of the VAO we just created in “VertexArrayID”.

Similar to the VAO, the VBO is created by function glGenBuffers( number of VBO names, pointer to VBO names array ). Again, we just need 1 VBO whose name should be returned to “triangleVBO”. This variable just stores an ID (object name) of GLuint type.

From the naming of “triangleVBO” it is clear to us what we intent here (representing a triangle by three vertices), anyway, how should OpenGL know? – We explain the meaning of this buffer object to OpenGL by using glBindBuffer ( target, VBO name ). There are numerous options as target but GL_VERTEX_BUFFER is the right choice here.

The actual VBO is created by glBufferData( target, size of object’s data store in bytes, pointer to data to be copied into VBO, expected usage ). This functions takes four arguments. The target is GL_VERTEX_BUFFER again. The size of the VBO’s data store in bytes is determined by Pascal’s SizeOf function applied to “triangleData”. The “triangleData” constant also holds the data to be copied into the VBO, so its pointer is suitable as the third argument. Since we are not going to change the data a lot, we should use GL_STATIC_DRAW as fourth argument.

If you are a newcomer to OpenGL, don’t worry if you are confused the first time. Most people are. And now it may even get worse :-(.

Shaders and OpenGL Shading Language

When starting with modern OpenGL the so-called Shaders are talked about a lot. Shaders are scripts written in a C-like script language called OpenGL Shading Language (GLSL). These scripts are compiled at runtime and influence the way how the graphics data is processed at certain steps in the so-called rendering pipeline of OpenGL. In fact, you can create rather complex and special effects with shaders without even changing one line of code of your source code.

Vertex Shader and Fragment Shader

There are two Shaders that are crucial and have to be set up to work with modern OpenGL. They are called  Vertex Shader and Fragment Shader. There are more Shaders not covered here, though. Each type of Shader influences different aspects of the rendering.

The Vertex Shader is the first Shader program executed in the rendering pipeline. Every vertex is “put through” the Vertex Shader program and processed accordingly, hence the name. Often this Shader is used to perform transformation operations. The Shader script used in this tutorial is shown next:

#version 330 core

layout(location = 0) in vec3 vertexPosition_modelspace;

void main(void) {
	gl_Position.xyz = vertexPosition_modelspace;
        gl_Position.w = 1.0;
}

This GLSL source code is saved into a file VertexShader.txt and located in the same directory as the source code of this chapter’s example source code. I’m not going to explain this GLSL code in detail here, but a detailed explanation is found over at opengl-tutorial.org Chapter 2 where I got this Shader code from, by the way.

The Frgament Shader is the last Shader program executed in the renderin pipeline. The so-called rasterization process produces fragments. Every fragment is “put through” the Fragment Shader program and processed accordingly. The Shader script used for the Fragment Shader is:

#version 330 core

out vec3 color;

void main(){
   color = vec3(1,0,0);
 }

This code is in file FragmentShader.txt and located in the same directory as the VertexShader.txt. The detailed explanation is found over at opengl-tutorial.org Chapter 2 again. Anyway, you’ll notice that there is a “color” variable (three component vector). As you see, it sets the (red,green,blue) values for the fragments to (1,0,0) which means red should be the result, red = 100%, green and blue = 0%. You may play around with these values.

program chap10_SDL2;
uses Classes, SysUtils, SDL2, GL, GLext;

const
  vertexShaderFile = 'VertexShader.txt';
  fragmentShaderFile = 'FragmentShader.txt';
  triangleData: array[0..8] of GLfloat = ( -1.0, -1.0, 0.0,
                                            1.0, -1.0, 0.0,
                                            0.0,  1.0, 0.0  );

var
sdlWindow1: PSDL_Window;
sdlGLContext1: TSDL_GLContext;
i: Word;
VertexArrayID: GLuint;
triangleVBO: GLuint;

VertexShaderID: GLuint;
VertexShaderCode: PGLchar;
FragmentShaderID: GLuint;
FragmentShaderCode: PGLchar;
ShaderCode: TStringList;
ProgramID: GLuint;
compilationResult: GLint = GL_FALSE;
InfoLogLength: GLint;
ErrorMessageArray: array of GLChar;

begin
  if SDL_Init( SDL_INIT_VIDEO ) < 0 then HALT;

  //get an OpenGL window and create OpenGL context
  sdlWindow1 := SDL_CreateWindow( 'OpenGL window', 50, 50, 500, 500, SDL_WINDOW_OPENGL );
  if sdlWindow1 = nil then HALT;

  sdlGLContext1 := SDL_GL_CreateContext( sdlWindow1 );
  if @sdlGLContext1 = nil then HALT;

  //init OpenGL and load extensions
  if Load_GL_VERSION_4_0 = false then
    if Load_GL_VERSION_3_3 = false then
      if Load_GL_VERSION_3_2 = false then
        if Load_GL_VERSION_3_0 = false then
        begin
          writeln(' ERROR: OpenGL 3.0 or higher needed. '); readln;
          HALT;
        end;

  //print out OpenGL vendor, version and shader version
  writeln( 'Vendor: ' + glGetString( GL_VENDOR ) );
  writeln( 'OpenGL Version: ' + glGetString( GL_VERSION ) );
  writeln( 'Shader Version: ' + glGetString( GL_SHADING_LANGUAGE_VERSION ) );

  //create Vertex Array Object (VAO)
  glGenVertexArrays( 1, @VertexArrayID );
  glBindVertexArray( VertexArrayID );

  //creating Vertex Buffer Object (VBO)
  glGenBuffers( 1, @triangleVBO );
  glBindBuffer( GL_ARRAY_BUFFER, triangleVBO );
  glBufferData( GL_ARRAY_BUFFER, SizeOf( triangleData ), @triangleData, GL_STATIC_DRAW );

  //creating shaders
  VertexShaderID := glCreateShader( GL_VERTEX_SHADER );
  FragmentShaderID := glCreateShader( GL_FRAGMENT_SHADER );

  //load shader code and get PChars
  ShaderCode := TStringList.Create;
  ShaderCode.LoadFromFile( VertexShaderFile );
  VertexShaderCode := ShaderCode.GetText;
  if VertexShaderCode = nil then HALT;
  ShaderCode.LoadFromFile( FragmentShaderFile );
  FragmentShaderCode := ShaderCode.GetText;
  if FragmentShaderCode = nil then HALT;
  ShaderCode.Free;

  //compiling and error checking vertex shader
  write('Compiling and error checking Vertex Shader... ' );
  glShaderSource( VertexShaderID, 1, @VertexShaderCode, nil );
  glCompileShader( VertexShaderID );

  glGetShaderiv( VertexShaderID, GL_COMPILE_STATUS, @compilationResult );
  glGetShaderiv( VertexShaderID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  //compiling and error checking fragment shader
  write('Compiling and error checking Fragment Shader... ' );
  glShaderSource( FragmentShaderID, 1, @FragmentShaderCode, nil );
  glCompileShader( FragmentShaderID );

  glGetShaderiv( FragmentShaderID, GL_COMPILE_STATUS, @compilationResult );
  glGetShaderiv( FragmentShaderID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  //creating and linking program
  write('Creating and linking program... ' );
  ProgramID := glCreateProgram();
  glAttachShader( ProgramID, VertexShaderID );
  glAttachShader( ProgramID, FragmentShaderID );
  glLinkProgram( ProgramID );

  glGetShaderiv( ProgramID, GL_LINK_STATUS, @compilationResult );
  glGetShaderiv( ProgramID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  for i := 0 to 400 do
  begin
    glClearColor( 0.0, 1.0-i/400, 0.0+i/400, 1.0 );
    glClear( GL_COLOR_BUFFER_BIT );
    glUseProgram( ProgramID );
    glEnableVertexAttribArray( 0 );
    glBindBuffer( GL_ARRAY_BUFFER, triangleVBO );
    glVertexAttribPointer( 0, 3, GL_FLOAT, GL_FALSE, 0, nil );
    glDrawArrays( GL_TRIANGLES, 0, 3 );
    glDisableVertexAttribArray( 0 );
    SDL_Delay( 20 );
    SDL_GL_SwapWindow( sdlWindow1 );
  end;

  //clean up
  glDetachShader( ProgramID, VertexShaderID );
  glDetachShader( ProgramID, FragmentShaderID );

  glDeleteShader( VertexShaderID );
  glDeleteShader( FragmentShaderID );
  glDeleteProgram( ProgramID );

  StrDispose( VertexShaderCode );
  StrDispose( FragmentShaderCode );

  glDeleteBuffers( 1, @triangleVBO );
  glDeleteVertexArrays( 1, @VertexArrayID );

  SDL_GL_DeleteContext( sdlGLContext1 );
  SDL_DestroyWindow( sdlWindow1 );

  SDL_Quit;
end.

Both Shaders are created by function glCreateShader( Shader type ). It returns the reference (or name) as GLuint as seen before for the VAO and VBO. We store them in the VertexShaderID and FragmenShaderID, respecitvely.

The next part is about loading the source code from the two Shader files (VertexShader.txt, FragmentShader.txt) and converting them to be used with OpenGL. First a “ShaderCode” variable of TStringList type is created. Its LoadFromFile method let us load the file contents into the variable conveniently. First for the Vertex Shader, whose file name is stored in constant “VertexShaderFile”. The variable “VertexShaderCode” is of type PGLchar, which is the way OpenGL handles strings. Since PGLchar is of type PChar anyway, the method GetText is perfectly suitable here to convert the source code string into a null-terminated array of chars. Finally, there is a simple check if the PGLchars are empty (nil), which shouldn’t be the case if the source code is pointed to as expected.

Exactly the same is done for the FragmentShader and the source code associated with “FragmentShaderCode”.

Finally, the dummy variable “ShaderCode” is free’d.

program chap10_SDL2;
uses Classes, SysUtils, SDL2, GL, GLext;

const
  vertexShaderFile = 'VertexShader.txt';
  fragmentShaderFile = 'FragmentShader.txt';
  triangleData: array[0..8] of GLfloat = ( -1.0, -1.0, 0.0,
                                            1.0, -1.0, 0.0,
                                            0.0,  1.0, 0.0  );

var
sdlWindow1: PSDL_Window;
sdlGLContext1: TSDL_GLContext;
i: Word;
VertexArrayID: GLuint;
triangleVBO: GLuint;

VertexShaderID: GLuint;
VertexShaderCode: PGLchar;
FragmentShaderID: GLuint;
FragmentShaderCode: PGLchar;
ShaderCode: TStringList;
ProgramID: GLuint;
compilationResult: GLint = GL_FALSE;
InfoLogLength: GLint;
ErrorMessageArray: array of GLChar;

begin
  if SDL_Init( SDL_INIT_VIDEO ) < 0 then HALT;

  //get an OpenGL window and create OpenGL context
  sdlWindow1 := SDL_CreateWindow( 'OpenGL window', 50, 50, 500, 500, SDL_WINDOW_OPENGL );
  if sdlWindow1 = nil then HALT;

  sdlGLContext1 := SDL_GL_CreateContext( sdlWindow1 );
  if @sdlGLContext1 = nil then HALT;

  //init OpenGL and load extensions
  if Load_GL_VERSION_4_0 = false then
    if Load_GL_VERSION_3_3 = false then
      if Load_GL_VERSION_3_2 = false then
        if Load_GL_VERSION_3_0 = false then
        begin
          writeln(' ERROR: OpenGL 3.0 or higher needed. '); readln;
          HALT;
        end;

  //print out OpenGL vendor, version and shader version
  writeln( 'Vendor: ' + glGetString( GL_VENDOR ) );
  writeln( 'OpenGL Version: ' + glGetString( GL_VERSION ) );
  writeln( 'Shader Version: ' + glGetString( GL_SHADING_LANGUAGE_VERSION ) );

  //create Vertex Array Object (VAO)
  glGenVertexArrays( 1, @VertexArrayID );
  glBindVertexArray( VertexArrayID );

  //creating Vertex Buffer Object (VBO)
  glGenBuffers( 1, @triangleVBO );
  glBindBuffer( GL_ARRAY_BUFFER, triangleVBO );
  glBufferData( GL_ARRAY_BUFFER, SizeOf( triangleData ), @triangleData, GL_STATIC_DRAW );

  //creating shaders
  VertexShaderID := glCreateShader( GL_VERTEX_SHADER );
  FragmentShaderID := glCreateShader( GL_FRAGMENT_SHADER );

  //load shader code and get PChars
  ShaderCode := TStringList.Create;
  ShaderCode.LoadFromFile( VertexShaderFile );
  VertexShaderCode := ShaderCode.GetText;
  if VertexShaderCode = nil then HALT;
  ShaderCode.LoadFromFile( FragmentShaderFile );
  FragmentShaderCode := ShaderCode.GetText;
  if FragmentShaderCode = nil then HALT;
  ShaderCode.Free;

  //compiling and error checking vertex shader
  write('Compiling and error checking Vertex Shader... ' );
  glShaderSource( VertexShaderID, 1, @VertexShaderCode, nil );
  glCompileShader( VertexShaderID );

  glGetShaderiv( VertexShaderID, GL_COMPILE_STATUS, @compilationResult );
  glGetShaderiv( VertexShaderID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  //compiling and error checking fragment shader
  write('Compiling and error checking Fragment Shader... ' );
  glShaderSource( FragmentShaderID, 1, @FragmentShaderCode, nil );
  glCompileShader( FragmentShaderID );

  glGetShaderiv( FragmentShaderID, GL_COMPILE_STATUS, @compilationResult );
  glGetShaderiv( FragmentShaderID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  //creating and linking program
  write('Creating and linking program... ' );
  ProgramID := glCreateProgram();
  glAttachShader( ProgramID, VertexShaderID );
  glAttachShader( ProgramID, FragmentShaderID );
  glLinkProgram( ProgramID );

  glGetShaderiv( ProgramID, GL_LINK_STATUS, @compilationResult );
  glGetShaderiv( ProgramID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  for i := 0 to 400 do
  begin
    glClearColor( 0.0, 1.0-i/400, 0.0+i/400, 1.0 );
    glClear( GL_COLOR_BUFFER_BIT );
    glUseProgram( ProgramID );
    glEnableVertexAttribArray( 0 );
    glBindBuffer( GL_ARRAY_BUFFER, triangleVBO );
    glVertexAttribPointer( 0, 3, GL_FLOAT, GL_FALSE, 0, nil );
    glDrawArrays( GL_TRIANGLES, 0, 3 );
    glDisableVertexAttribArray( 0 );
    SDL_Delay( 20 );
    SDL_GL_SwapWindow( sdlWindow1 );
  end;

  //clean up
  glDetachShader( ProgramID, VertexShaderID );
  glDetachShader( ProgramID, FragmentShaderID );

  glDeleteShader( VertexShaderID );
  glDeleteShader( FragmentShaderID );
  glDeleteProgram( ProgramID );

  StrDispose( VertexShaderCode );
  StrDispose( FragmentShaderCode );

  glDeleteBuffers( 1, @triangleVBO );
  glDeleteVertexArrays( 1, @VertexArrayID );

  SDL_GL_DeleteContext( sdlGLContext1 );
  SDL_DestroyWindow( sdlWindow1 );

  SDL_Quit;
end.

To associate the source code we stored in “VertexSourceCode” to “VertexShaderID” of GLuint type, the function glShaderSource( Shader reference, number of array elements, pointer to array of source code PGLchars, pointer to array of lengths ). The Vertex Shader reference is stored in “VertexShaderID” which is the first argument. We just have one source code, so the second argument is 1. The source code is stored VertexShaderCode, and its pointer is addressed by @VertexShaderCode as the third argument. As seen before, since we just have one element here, it is not necessary to have really an array. The fourth parameter allows for some length specification, but if set to nil it expects null-terminated arrays of chars.

The compilation is straight forward done by glCompileShader( Shader reference ). It is really advised to to error checking here, that is why it is shown how to do that. The function glGetShaderiv( Shader reference, object parameter, pointer of correct type for return value ) is used to request information about objects. First we like to know if the compilation was successful. The Shader reference is stored in “VertexShaderID”, the object parameter is GL_COMPILE_STATUS. This will return a GLint value, which can be interpreted as GL_FALSE or GL_TRUE. The result is stored in “compilationResult” by using its pointer (@compilationResult) as argument.

Right after that we request the length of the information log by GL_INFO_LOG_LENGTH. It will be greater than 0 if some information were logged (probably an error occured on compilation then). The result is returned to “InfoLogLength” by its pointer @InfoLogLength.

If an error occurs, “compilationResult” is GL_FALSE. In this case “failure” along with more specific information is printed out. I’m not going into detail here, since this shouldn’t happen. Otherwise (and that should be the case), “success” is printed out.

The very same way the Fragment Shader is compiled and checked.

program chap10_SDL2;
uses Classes, SysUtils, SDL2, GL, GLext;

const
  vertexShaderFile = 'VertexShader.txt';
  fragmentShaderFile = 'FragmentShader.txt';
  triangleData: array[0..8] of GLfloat = ( -1.0, -1.0, 0.0,
                                            1.0, -1.0, 0.0,
                                            0.0,  1.0, 0.0  );

var
sdlWindow1: PSDL_Window;
sdlGLContext1: TSDL_GLContext;
i: Word;
VertexArrayID: GLuint;
triangleVBO: GLuint;

VertexShaderID: GLuint;
VertexShaderCode: PGLchar;
FragmentShaderID: GLuint;
FragmentShaderCode: PGLchar;
ShaderCode: TStringList;
ProgramID: GLuint;
compilationResult: GLint = GL_FALSE;
InfoLogLength: GLint;
ErrorMessageArray: array of GLChar;

begin
  if SDL_Init( SDL_INIT_VIDEO ) < 0 then HALT;

  //get an OpenGL window and create OpenGL context
  sdlWindow1 := SDL_CreateWindow( 'OpenGL window', 50, 50, 500, 500, SDL_WINDOW_OPENGL );
  if sdlWindow1 = nil then HALT;

  sdlGLContext1 := SDL_GL_CreateContext( sdlWindow1 );
  if @sdlGLContext1 = nil then HALT;

  //init OpenGL and load extensions
  if Load_GL_VERSION_4_0 = false then
    if Load_GL_VERSION_3_3 = false then
      if Load_GL_VERSION_3_2 = false then
        if Load_GL_VERSION_3_0 = false then
        begin
          writeln(' ERROR: OpenGL 3.0 or higher needed. '); readln;
          HALT;
        end;

  //print out OpenGL vendor, version and shader version
  writeln( 'Vendor: ' + glGetString( GL_VENDOR ) );
  writeln( 'OpenGL Version: ' + glGetString( GL_VERSION ) );
  writeln( 'Shader Version: ' + glGetString( GL_SHADING_LANGUAGE_VERSION ) );

  //create Vertex Array Object (VAO)
  glGenVertexArrays( 1, @VertexArrayID );
  glBindVertexArray( VertexArrayID );

  //creating Vertex Buffer Object (VBO)
  glGenBuffers( 1, @triangleVBO );
  glBindBuffer( GL_ARRAY_BUFFER, triangleVBO );
  glBufferData( GL_ARRAY_BUFFER, SizeOf( triangleData ), @triangleData, GL_STATIC_DRAW );

  //creating shaders
  VertexShaderID := glCreateShader( GL_VERTEX_SHADER );
  FragmentShaderID := glCreateShader( GL_FRAGMENT_SHADER );

  //load shader code and get PChars
  ShaderCode := TStringList.Create;
  ShaderCode.LoadFromFile( VertexShaderFile );
  VertexShaderCode := ShaderCode.GetText;
  if VertexShaderCode = nil then HALT;
  ShaderCode.LoadFromFile( FragmentShaderFile );
  FragmentShaderCode := ShaderCode.GetText;
  if FragmentShaderCode = nil then HALT;
  ShaderCode.Free;

  //compiling and error checking vertex shader
  write('Compiling and error checking Vertex Shader... ' );
  glShaderSource( VertexShaderID, 1, @VertexShaderCode, nil );
  glCompileShader( VertexShaderID );

  glGetShaderiv( VertexShaderID, GL_COMPILE_STATUS, @compilationResult );
  glGetShaderiv( VertexShaderID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  //compiling and error checking fragment shader
  write('Compiling and error checking Fragment Shader... ' );
  glShaderSource( FragmentShaderID, 1, @FragmentShaderCode, nil );
  glCompileShader( FragmentShaderID );

  glGetShaderiv( FragmentShaderID, GL_COMPILE_STATUS, @compilationResult );
  glGetShaderiv( FragmentShaderID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  //creating and linking program
  write('Creating and linking program... ' );
  ProgramID := glCreateProgram();
  glAttachShader( ProgramID, VertexShaderID );
  glAttachShader( ProgramID, FragmentShaderID );
  glLinkProgram( ProgramID );

  glGetShaderiv( ProgramID, GL_LINK_STATUS, @compilationResult );
  glGetShaderiv( ProgramID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  for i := 0 to 400 do
  begin
    glClearColor( 0.0, 1.0-i/400, 0.0+i/400, 1.0 );
    glClear( GL_COLOR_BUFFER_BIT );
    glUseProgram( ProgramID );
    glEnableVertexAttribArray( 0 );
    glBindBuffer( GL_ARRAY_BUFFER, triangleVBO );
    glVertexAttribPointer( 0, 3, GL_FLOAT, GL_FALSE, 0, nil );
    glDrawArrays( GL_TRIANGLES, 0, 3 );
    glDisableVertexAttribArray( 0 );
    SDL_Delay( 20 );
    SDL_GL_SwapWindow( sdlWindow1 );
  end;

  //clean up
  glDetachShader( ProgramID, VertexShaderID );
  glDetachShader( ProgramID, FragmentShaderID );

  glDeleteShader( VertexShaderID );
  glDeleteShader( FragmentShaderID );
  glDeleteProgram( ProgramID );

  StrDispose( VertexShaderCode );
  StrDispose( FragmentShaderCode );

  glDeleteBuffers( 1, @triangleVBO );
  glDeleteVertexArrays( 1, @VertexArrayID );

  SDL_GL_DeleteContext( sdlGLContext1 );
  SDL_DestroyWindow( sdlWindow1 );

  SDL_Quit;
end.

The shaders have to be attached and linked by a Shader program. A Shader program is created by glCreateProgram(). The parenthesis are important here. It returns an reference of GLuint type which is stored in ProgramID.

The Shaders are attached to this Shader program by glAttacheShader( Program reference, Shader reference ). The program is linked by glLinkProgram( Program reference ). The reference for the Shader program is “ProgramID”. The references for the Shaders are “VertexShaderID” and “FragmentShaderID”, respectively.

By complete analogy to the error checking for the Shader compilation, the Shader program linking is checked. Anyway, instead of GL_COMPILE_STATUS, GL_LINK_STATUS is used.

program chap10_SDL2;
uses Classes, SysUtils, SDL2, GL, GLext;

const
  vertexShaderFile = 'VertexShader.txt';
  fragmentShaderFile = 'FragmentShader.txt';
  triangleData: array[0..8] of GLfloat = ( -1.0, -1.0, 0.0,
                                            1.0, -1.0, 0.0,
                                            0.0,  1.0, 0.0  );

var
sdlWindow1: PSDL_Window;
sdlGLContext1: TSDL_GLContext;
i: Word;
VertexArrayID: GLuint;
triangleVBO: GLuint;

VertexShaderID: GLuint;
VertexShaderCode: PGLchar;
FragmentShaderID: GLuint;
FragmentShaderCode: PGLchar;
ShaderCode: TStringList;
ProgramID: GLuint;
compilationResult: GLint = GL_FALSE;
InfoLogLength: GLint;
ErrorMessageArray: array of GLChar;

begin
  if SDL_Init( SDL_INIT_VIDEO ) < 0 then HALT;

  //get an OpenGL window and create OpenGL context
  sdlWindow1 := SDL_CreateWindow( 'OpenGL window', 50, 50, 500, 500, SDL_WINDOW_OPENGL );
  if sdlWindow1 = nil then HALT;

  sdlGLContext1 := SDL_GL_CreateContext( sdlWindow1 );
  if @sdlGLContext1 = nil then HALT;

  //init OpenGL and load extensions
  if Load_GL_VERSION_4_0 = false then
    if Load_GL_VERSION_3_3 = false then
      if Load_GL_VERSION_3_2 = false then
        if Load_GL_VERSION_3_0 = false then
        begin
          writeln(' ERROR: OpenGL 3.0 or higher needed. '); readln;
          HALT;
        end;

  //print out OpenGL vendor, version and shader version
  writeln( 'Vendor: ' + glGetString( GL_VENDOR ) );
  writeln( 'OpenGL Version: ' + glGetString( GL_VERSION ) );
  writeln( 'Shader Version: ' + glGetString( GL_SHADING_LANGUAGE_VERSION ) );

  //create Vertex Array Object (VAO)
  glGenVertexArrays( 1, @VertexArrayID );
  glBindVertexArray( VertexArrayID );

  //creating Vertex Buffer Object (VBO)
  glGenBuffers( 1, @triangleVBO );
  glBindBuffer( GL_ARRAY_BUFFER, triangleVBO );
  glBufferData( GL_ARRAY_BUFFER, SizeOf( triangleData ), @triangleData, GL_STATIC_DRAW );

  //creating shaders
  VertexShaderID := glCreateShader( GL_VERTEX_SHADER );
  FragmentShaderID := glCreateShader( GL_FRAGMENT_SHADER );

  //load shader code and get PChars
  ShaderCode := TStringList.Create;
  ShaderCode.LoadFromFile( VertexShaderFile );
  VertexShaderCode := ShaderCode.GetText;
  if VertexShaderCode = nil then HALT;
  ShaderCode.LoadFromFile( FragmentShaderFile );
  FragmentShaderCode := ShaderCode.GetText;
  if FragmentShaderCode = nil then HALT;
  ShaderCode.Free;

  //compiling and error checking vertex shader
  write('Compiling and error checking Vertex Shader... ' );
  glShaderSource( VertexShaderID, 1, @VertexShaderCode, nil );
  glCompileShader( VertexShaderID );

  glGetShaderiv( VertexShaderID, GL_COMPILE_STATUS, @compilationResult );
  glGetShaderiv( VertexShaderID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  //compiling and error checking fragment shader
  write('Compiling and error checking Fragment Shader... ' );
  glShaderSource( FragmentShaderID, 1, @FragmentShaderCode, nil );
  glCompileShader( FragmentShaderID );

  glGetShaderiv( FragmentShaderID, GL_COMPILE_STATUS, @compilationResult );
  glGetShaderiv( FragmentShaderID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  //creating and linking program
  write('Creating and linking program... ' );
  ProgramID := glCreateProgram();
  glAttachShader( ProgramID, VertexShaderID );
  glAttachShader( ProgramID, FragmentShaderID );
  glLinkProgram( ProgramID );

  glGetShaderiv( ProgramID, GL_LINK_STATUS, @compilationResult );
  glGetShaderiv( ProgramID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  for i := 0 to 400 do
  begin
    glClearColor( 0.0, 1.0-i/400, 0.0+i/400, 1.0 );
    glClear( GL_COLOR_BUFFER_BIT );
    glUseProgram( ProgramID );
    glEnableVertexAttribArray( 0 );
    glBindBuffer( GL_ARRAY_BUFFER, triangleVBO );
    glVertexAttribPointer( 0, 3, GL_FLOAT, GL_FALSE, 0, nil );
    glDrawArrays( GL_TRIANGLES, 0, 3 );
    glDisableVertexAttribArray( 0 );
    SDL_Delay( 20 );
    SDL_GL_SwapWindow( sdlWindow1 );
  end;

  //clean up
  glDetachShader( ProgramID, VertexShaderID );
  glDetachShader( ProgramID, FragmentShaderID );

  glDeleteShader( VertexShaderID );
  glDeleteShader( FragmentShaderID );
  glDeleteProgram( ProgramID );

  StrDispose( VertexShaderCode );
  StrDispose( FragmentShaderCode );

  glDeleteBuffers( 1, @triangleVBO );
  glDeleteVertexArrays( 1, @VertexArrayID );

  SDL_GL_DeleteContext( sdlGLContext1 );
  SDL_DestroyWindow( sdlWindow1 );

  SDL_Quit;
end.

The for-loop counts from 0 to 400. Within each cycle it first changes the background color by glClearColor( red, green, blue, alpha ). Red and alpha are constant, green and blue are varied each cycle dependend upon variable i. This makes the background slowly changing from green to blue, feel free to play around with the rgba values. To actually clear the color buffer glClear( buffer ) with GL_COLOR_BUFFER_BIT as argument is used.

glUseProgram( Shader program reference ) is used to apply the Shader program to the rendering state. “ProgramID” is the Shader program reference in the example code.

glEnableVertexAttribArray( array index ) is used in order to make the attribute array available for rendering by glDrawArrays. The index is 0 here. The “triangleVBO” is bound by glBindBuffer( target, buffer ) to the GL_BUFFER_ARRAY target to change attribute data of said VBO. Latter is done by glVertexAttribPointer( index, size, type, normalized, stride, offset of first component ) with the given arguments. Hence, the index is 0, 3 components per generic vertex attribute, each of float point type (thus, GL_FLOAT), not normalized (thus GL_FALSE), no strides between vertex attributes, no offset for the first component.

The rendering is done by glDrawArrays( type of primitive, starting index, number of elements ). The type of primitive is a triangle, hence GL_TRIANGLES is the first argument. We start at the very beginning, so index is 0. We have 3 sequential elements (vertices).

glDisableVertexAttribArray( array index ) is the counter function to glEnableVertexAttribArray( array index ), obviously. It disables the vertex attribute array.

SDL_Delay delays the loop by 20 milliseconds.

The procedure

procedure SDL_GL_SwapWindow(window: PSDL_Window)

is used to actually display the rendering result to the the window “sdlWindow1”. Keep in mind that this window has to be initialized as an OpenGL window. This procedure is comparable to SDL’s SDL_RenderPresent.

After i matching 400, the for-loop is left.

program chap10_SDL2;
uses Classes, SysUtils, SDL2, GL, GLext;

const
  vertexShaderFile = 'VertexShader.txt';
  fragmentShaderFile = 'FragmentShader.txt';
  triangleData: array[0..8] of GLfloat = ( -1.0, -1.0, 0.0,
                                            1.0, -1.0, 0.0,
                                            0.0,  1.0, 0.0  );

var
sdlWindow1: PSDL_Window;
sdlGLContext1: TSDL_GLContext;
i: Word;
VertexArrayID: GLuint;
triangleVBO: GLuint;

VertexShaderID: GLuint;
VertexShaderCode: PGLchar;
FragmentShaderID: GLuint;
FragmentShaderCode: PGLchar;
ShaderCode: TStringList;
ProgramID: GLuint;
compilationResult: GLint = GL_FALSE;
InfoLogLength: GLint;
ErrorMessageArray: array of GLChar;

begin
  if SDL_Init( SDL_INIT_VIDEO ) < 0 then HALT;

  //get an OpenGL window and create OpenGL context
  sdlWindow1 := SDL_CreateWindow( 'OpenGL window', 50, 50, 500, 500, SDL_WINDOW_OPENGL );
  if sdlWindow1 = nil then HALT;

  sdlGLContext1 := SDL_GL_CreateContext( sdlWindow1 );
  if @sdlGLContext1 = nil then HALT;

  //init OpenGL and load extensions
  if Load_GL_VERSION_4_0 = false then
    if Load_GL_VERSION_3_3 = false then
      if Load_GL_VERSION_3_2 = false then
        if Load_GL_VERSION_3_0 = false then
        begin
          writeln(' ERROR: OpenGL 3.0 or higher needed. '); readln;
          HALT;
        end;

  //print out OpenGL vendor, version and shader version
  writeln( 'Vendor: ' + glGetString( GL_VENDOR ) );
  writeln( 'OpenGL Version: ' + glGetString( GL_VERSION ) );
  writeln( 'Shader Version: ' + glGetString( GL_SHADING_LANGUAGE_VERSION ) );

  //create Vertex Array Object (VAO)
  glGenVertexArrays( 1, @VertexArrayID );
  glBindVertexArray( VertexArrayID );

  //creating Vertex Buffer Object (VBO)
  glGenBuffers( 1, @triangleVBO );
  glBindBuffer( GL_ARRAY_BUFFER, triangleVBO );
  glBufferData( GL_ARRAY_BUFFER, SizeOf( triangleData ), @triangleData, GL_STATIC_DRAW );

  //creating shaders
  VertexShaderID := glCreateShader( GL_VERTEX_SHADER );
  FragmentShaderID := glCreateShader( GL_FRAGMENT_SHADER );

  //load shader code and get PChars
  ShaderCode := TStringList.Create;
  ShaderCode.LoadFromFile( VertexShaderFile );
  VertexShaderCode := ShaderCode.GetText;
  if VertexShaderCode = nil then HALT;
  ShaderCode.LoadFromFile( FragmentShaderFile );
  FragmentShaderCode := ShaderCode.GetText;
  if FragmentShaderCode = nil then HALT;
  ShaderCode.Free;

  //compiling and error checking vertex shader
  write('Compiling and error checking Vertex Shader... ' );
  glShaderSource( VertexShaderID, 1, @VertexShaderCode, nil );
  glCompileShader( VertexShaderID );

  glGetShaderiv( VertexShaderID, GL_COMPILE_STATUS, @compilationResult );
  glGetShaderiv( VertexShaderID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  //compiling and error checking fragment shader
  write('Compiling and error checking Fragment Shader... ' );
  glShaderSource( FragmentShaderID, 1, @FragmentShaderCode, nil );
  glCompileShader( FragmentShaderID );

  glGetShaderiv( FragmentShaderID, GL_COMPILE_STATUS, @compilationResult );
  glGetShaderiv( FragmentShaderID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  //creating and linking program
  write('Creating and linking program... ' );
  ProgramID := glCreateProgram();
  glAttachShader( ProgramID, VertexShaderID );
  glAttachShader( ProgramID, FragmentShaderID );
  glLinkProgram( ProgramID );

  glGetShaderiv( ProgramID, GL_LINK_STATUS, @compilationResult );
  glGetShaderiv( ProgramID, GL_INFO_LOG_LENGTH, @InfoLogLength );
  if compilationResult = GL_FALSE then
  begin
    writeln( 'failure' );
    SetLength( ErrorMessageArray, InfoLogLength+1 );
    glGetShaderInfoLog( VertexShaderID, InfoLogLength, nil, @ErrorMessageArray[0] );
    for i := 0 to InfoLogLength do write( String( ErrorMessageArray[i] ) );
    writeln;
  end else writeln( 'success' );

  for i := 0 to 400 do
  begin
    glClearColor( 0.0, 1.0-i/400, 0.0+i/400, 1.0 );
    glClear( GL_COLOR_BUFFER_BIT );
    glUseProgram( ProgramID );
    glEnableVertexAttribArray( 0 );
    glBindBuffer( GL_ARRAY_BUFFER, triangleVBO );
    glVertexAttribPointer( 0, 3, GL_FLOAT, GL_FALSE, 0, nil );
    glDrawArrays( GL_TRIANGLES, 0, 3 );
    glDisableVertexAttribArray( 0 );
    SDL_Delay( 20 );
    SDL_GL_SwapWindow( sdlWindow1 );
  end;

  //clean up
  glDetachShader( ProgramID, VertexShaderID );
  glDetachShader( ProgramID, FragmentShaderID );

  glDeleteShader( VertexShaderID );
  glDeleteShader( FragmentShaderID );
  glDeleteProgram( ProgramID );

  StrDispose( VertexShaderCode );
  StrDispose( FragmentShaderCode );

  glDeleteBuffers( 1, @triangleVBO );
  glDeleteVertexArrays( 1, @VertexArrayID );

  SDL_GL_DeleteContext( sdlGLContext1 );
  SDL_DestroyWindow( sdlWindow1 );

  SDL_Quit;
end.

For the clean up, the shaders have to be detached from the shader program by glDetachShader( program, shader ). After that they can be deleted by glDeleteShader( shader ), and the program by glDeleteProgram( program ).

The shader script PChars are disposed by StrDispose( PChar ).

The VBO and the vertex array have to be free’d by glDeleteBuffers( number of buffer objects, pointer to array of buffers ) and glDeleteVertexArrays( number of VAOs, pointer to array of VAOs ) respectively. The first parameter is the number of objects to be deleted, which is 1 in both our cases.

To resolve the OpenGL context

procedure SDL_GL_DeleteContext(context: TSDL_GLContext)

is used.

The SDL Window is destroyed as known. Finally SDL is shut down as known by SDL_Quit.

← Chapter 9 | Chapter 11 (not existent yet) →

Chapter 9 – Music and Sound

Tutorial, , ,

ATTENTION: You need the most recent Pascal translations by Tim Blume  for this chapter to work properly. Version 1.72 of Tim Blume’s Pascal translations will not work! Check step 2) in Chapter 2 for details.

This chapter will introduce you on how to load music and sounds since these are key features of every game and many applications.

SDL_mixer 2.0 for easy music and sound support

Although SDL 2.0 supports music and sound handling natively, there is an easier way to play music and sound files. The official unit SDL_mixer 2.0 (unit’s name SDL2_mixer) has been created exactly for this purpose and is maintained by the same authors (Sam Lantinga, Stephane Peter, Ryan Gordon) as SDL 2.0 itself. The Pascal translation is fortunately available, too in Tim Blume’s header translations.

Supported music and sound file formats in SDL 2.0

According to the official SDL2_mixer documentation the following music and sound formats are supported:

  • WAVE/RIFF (.wav)
  • AIFF (.aiff)
  • VOC (.voc)
  • MOD (.mod .xm .s3m .669 .it .med and more) requiring libmikmod on system
  • MIDI (.mid) using timidity or native midi hardware
  • OggVorbis (.ogg) requiring ogg/vorbis libraries on system
  • MP3 (.mp3) requiring SMPEG or MAD library on system
  • FLAC (.flac) requiring the FLAC library on system

You’ll need these files:

Software
Version		 File name
Link		 Description
SDL2_mixer dynamic link library
2.0.1		 SDL2_mixer-2.0.1-win32-x86.zip (32-bit Windows)
SDL2_mixer-2.0.1-win32-x64.zip (64-bit Windows)
https://www.libsdl.org/projects/SDL_mixer/		 This is the corresponding dynamic link library file for the unit for Windows. Note: Mac OS X and Linux versions are also available.
dial.wav sound file dial.wav
dial.wav		 A simple telephone dial sound. Source: pdsounds. License: Public domain.
In my mind.ogg music file In my mind.ogg
In my mind.ogg		 A nice music sample. Source/Creator: First. License: CC BY-ND 3.0.

You should extract the zip-file and get several files. These are four license text files and a readme text file, furthermore you have eight dll files including the important SDL2_mixer.dll. The other dll files are necessary to play the different sound and music formats. Copy all the files to the Windows system32-folder (or the corresponding place). If you forget this and run the examples below you will get an error with exitcode = 309.

Now that you are prepared, let’s have an overview of the code.

program chap9_SDL2;
uses SDL2, SDL2_mixer;

var
sdlWindow1: PSDL_Window;
sdlEvent: PSDL_Event;
music: PMix_Music;
sound: PMix_Chunk;
exitloop: boolean = false;

begin
  if SDL_Init( SDL_INIT_VIDEO or SDL_INIT_AUDIO ) < 0 then HALT;

  //get a window
  sdlWindow1 := SDL_CreateWindow( 'Music and Sound window', 50, 50, 500, 500, SDL_WINDOW_SHOWN );
  if sdlWindow1 = nil then HALT;

  //prepare mixer
  if Mix_OpenAudio( MIX_DEFAULT_FREQUENCY, MIX_DEFAULT_FORMAT,
                    MIX_DEFAULT_CHANNELS, 4096 ) < 0 then HALT;

  //load music
  music := Mix_LoadMUS( 'In my mind.ogg' );
  if music = nil then HALT;
  Mix_VolumeMusic( MIX_MAX_VOLUME );

  //load sound
  sound := Mix_LoadWAV( 'dial.wav' );
  if sound = nil then HALT;
  Mix_VolumeChunk( sound, MIX_MAX_VOLUME );

  //create a menu
  writeln( '(1) Play music once' );
  writeln( '(2) Pause music' );
  writeln( '(3) Resume music' );
  writeln( '(4) Rewind music' );
  writeln( '(5) Play sound once on channel 1' );
  writeln( '(6) Play sound once on free channel' );
  writeln( '(7) Pause sound on all channels' );
  writeln( '(8) Resume sound on all channels' );
  writeln( '(A) Fade in music within 3 seconds' );
  writeln( '(S) Fade out music within 3 seconds from now' );
  writeln( '(D) Fade in sound on channel 1 within 2 seconds' );
  writeln( '(F) Fade out sound on channel 1 within 1 second from now' );
  writeln;
  writeln( '=== Effects on channel 1 ===' );
  writeln( '(G) Panning: left: 255, right: 32' );
  writeln( '(H) Stop panning' );
  writeln( '(J) Distance: very far ');
  writeln( '(K) Unregister distance effect' );
  writeln( '(L) Position 45ø to the right, in front, middle distance' );
  writeln( '(M) Unregister position effect' );
  writeln;
  writeln( '(ESC) Exit program' );

  new( sdlEvent );

  while exitloop = false do
  begin
    while SDL_PollEvent( sdlEvent ) = 1 do
    begin

      case sdlEvent^.type_ of
        SDL_KEYDOWN: begin
                       case sdlEvent^.key.keysym.sym of
                         SDLK_1: if Mix_PlayMusic( music, 0 ) < 0 then HALT;
                         SDLK_2: Mix_PauseMusic;
                         SDLK_3: Mix_ResumeMusic;
                         SDLK_4: Mix_RewindMusic;
                         SDLK_5: if Mix_PlayChannel( 1, sound, 0 ) < 0 then HALT;
                         SDLK_6: if Mix_PlayChannel( -1, sound, 0 ) < 0 then HALT;
                         SDLK_7: Mix_Pause( -1 );
                         SDLK_8: Mix_Resume( -1 );
                         SDLK_A: if Mix_FadeInMusic( music, 1, 3000 ) < 0 then HALT;
                         SDLK_S: if Mix_FadeOutMusic( 3000 ) = 0 then HALT;
                         SDLK_D: if Mix_FadeInChannel( 1, sound, 0, 2000 ) < 0 then HALT;
                         SDLK_F: if Mix_FadeOutChannel( 1, 1000 ) < 0 then HALT;
                         SDLK_G: if Mix_SetPanning( 1, 255, 32 ) = 0 then HALT;
                         SDLK_H: if Mix_SetPanning( 1, 255, 255 ) = 0 then HALT; //unregister panning
                         SDLK_J: if Mix_SetDistance( 1, 223 ) = 0 then HALT;
                         SDLK_K: if Mix_SetDistance( 1, 0 ) = 0 then HALT; //unregister distance effect
                         SDLK_L: if Mix_SetPosition( 1, 45, 127 ) = 0 then HALT;
                         SDLK_M: if Mix_SetPosition( 1, 0, 0 ) = 0 then HALT; //unregister position effect
                         SDLK_ESCAPE: exitloop := true;
                       end;
                     end;
      end;

    end;
    SDL_Delay( 5 );
  end;

  //clean up
  dispose( sdlEvent );
  SDL_DestroyWindow( sdlWindow1 );

  Mix_FreeMusic( music );
  Mix_FreeChunk( sound );

  Mix_CloseAudio;
  SDL_Quit;
end.

When running this program, you’ll get two windows. A simple SDL 2.0 window without any content (white) and the title “Music and Sound window” and a console window showing a simple menu (list) of possible actions to play around with the music and sound. Keep in mind that the focus has to be on the SDL 2.0 window for the key recognition to work.

Chapter 9 application preview

Let’s start with the first part of the code.

program chap9_SDL2;
uses SDL2, SDL2_mixer;

var
sdlWindow1: PSDL_Window;
sdlEvent: PSDL_Event;
music: PMix_Music;
sound: PMix_Chunk;
exitloop: boolean = false;

begin
  if SDL_Init( SDL_INIT_VIDEO or SDL_INIT_AUDIO ) < 0 then HALT;

  //get a window
  sdlWindow1 := SDL_CreateWindow( 'Music and Sound window', 50, 50, 500, 500, SDL_WINDOW_SHOWN );
  if sdlWindow1 = nil then HALT;

  //prepare mixer
  if Mix_OpenAudio( MIX_DEFAULT_FREQUENCY, MIX_DEFAULT_FORMAT,
                    MIX_DEFAULT_CHANNELS, 4096 ) < 0 then HALT;

  //load music
  music := Mix_LoadMUS( 'In my mind.ogg' );
  if music = nil then HALT;
  Mix_VolumeMusic( MIX_MAX_VOLUME );

  //load sound
  sound := Mix_LoadWAV( 'dial.wav' );
  if sound = nil then HALT;
  Mix_VolumeChunk( sound, MIX_MAX_VOLUME );

  //create a menu
  writeln( '(1) Play music once' );
  writeln( '(2) Pause music' );
  writeln( '(3) Resume music' );
  writeln( '(4) Rewind music' );
  writeln( '(5) Play sound once on channel 1' );
  writeln( '(6) Play sound once on free channel' );
  writeln( '(7) Pause sound on all channels' );
  writeln( '(8) Resume sound on all channels' );
  writeln( '(A) Fade in music within 3 seconds' );
  writeln( '(S) Fade out music within 3 seconds from now' );
  writeln( '(D) Fade in sound on channel 1 within 2 seconds' );
  writeln( '(F) Fade out sound on channel 1 within 1 second from now' );
  writeln;
  writeln( '=== Effects on channel 1 ===' );
  writeln( '(G) Panning: left: 255, right: 32' );
  writeln( '(H) Stop panning' );
  writeln( '(J) Distance: very far ');
  writeln( '(K) Unregister distance effect' );
  writeln( '(L) Position 45ø to the right, in front, middle distance' );
  writeln( '(M) Unregister position effect' );
  writeln;
  writeln( '(ESC) Exit program' );

  new( sdlEvent );

  while exitloop = false do
  begin
    while SDL_PollEvent( sdlEvent ) = 1 do
    begin

      case sdlEvent^.type_ of
        SDL_KEYDOWN: begin
                       case sdlEvent^.key.keysym.sym of
                         SDLK_1: if Mix_PlayMusic( music, 0 ) < 0 then HALT;
                         SDLK_2: Mix_PauseMusic;
                         SDLK_3: Mix_ResumeMusic;
                         SDLK_4: Mix_RewindMusic;
                         SDLK_5: if Mix_PlayChannel( 1, sound, 0 ) < 0 then HALT;
                         SDLK_6: if Mix_PlayChannel( -1, sound, 0 ) < 0 then HALT;
                         SDLK_7: Mix_Pause( -1 );
                         SDLK_8: Mix_Resume( -1 );
                         SDLK_A: if Mix_FadeInMusic( music, 1, 3000 ) < 0 then HALT;
                         SDLK_S: if Mix_FadeOutMusic( 3000 ) = 0 then HALT;
                         SDLK_D: if Mix_FadeInChannel( 1, sound, 0, 2000 ) < 0 then HALT;
                         SDLK_F: if Mix_FadeOutChannel( 1, 1000 ) < 0 then HALT;
                         SDLK_G: if Mix_SetPanning( 1, 255, 32 ) = 0 then HALT;
                         SDLK_H: if Mix_SetPanning( 1, 255, 255 ) = 0 then HALT; //unregister panning
                         SDLK_J: if Mix_SetDistance( 1, 223 ) = 0 then HALT;
                         SDLK_K: if Mix_SetDistance( 1, 0 ) = 0 then HALT; //unregister distance effect
                         SDLK_L: if Mix_SetPosition( 1, 45, 127 ) = 0 then HALT;
                         SDLK_M: if Mix_SetPosition( 1, 0, 0 ) = 0 then HALT; //unregister position effect
                         SDLK_ESCAPE: exitloop := true;
                       end;
                     end;
      end;

    end;
    SDL_Delay( 5 );
  end;

  //clean up
  dispose( sdlEvent );
  SDL_DestroyWindow( sdlWindow1 );
  
  Mix_FreeMusic( music );
  Mix_FreeChunk( sound );

  Mix_CloseAudio;
  SDL_Quit;
end.

The program is called “chap9_SDL2”. It uses the SDL2 and the new SDL2_mixer unit.

We’re going to use event handling as discussed in Chapter 8 to check what keys have been pressed. Consequently we need a variable “sdlWindow1” for the SDL 2.0 application window (for the focus) and an event variable “sdlEvent”.

The pointer variable “music” is of the new type PMix_Music and points at the loaded music file data later. The “sound” pointer variable is of type PMix_Chunk and points at the loaded sound file data later. Note: Each song is referenced by an own PMix_Music pointer and each sound effect (e.g. explosions, shots, …) is referenced by an own PMix_Chunk pointer.

What is the difference between music and sound?

Music is associated with PMix_Music pointers and sounds are associated with PMix_Chunk pointers, but why distinguish them at all? – Well, it is convenient to have them separated since they have rather different properties. Music is usually several minutes long, sounds are usually just a few seconds short. It is usually just one song played at a time, sounds need to mix up if necessary. For music it usually doesn’t plays a role if it is sligthly delayed to when it should start, for sounds a larger delay usually means a strange feeling for the player. So, for music there is exactly one channel reserved where the music is played. For sounds eight sound channels are available to make it possible for them to mix up. Now let’s get back to the code.

The variable “exitloop” is a boolean variable to determine when the main loop of the program should be exited.

The program is initialized by SDL_Init as known. Note though that not only SDL_INIT_VIDEO but additionally SDL_INIT_AUDIO is set. They are combined by the or operator (not the and operator :-)!).  The window is set up afterwards by SDL_CreateWindow as known.

program chap9_SDL2;
uses SDL2, SDL2_mixer;

var
sdlWindow1: PSDL_Window;
sdlEvent: PSDL_Event;
music: PMix_Music;
sound: PMix_Chunk;
exitloop: boolean = false;

begin
  if SDL_Init( SDL_INIT_VIDEO or SDL_INIT_AUDIO ) < 0 then HALT;

  //get a window
  sdlWindow1 := SDL_CreateWindow( 'Music and Sound window', 50, 50, 500, 500, SDL_WINDOW_SHOWN );
  if sdlWindow1 = nil then HALT;

  //prepare mixer
  if Mix_OpenAudio( MIX_DEFAULT_FREQUENCY, MIX_DEFAULT_FORMAT,
                    MIX_DEFAULT_CHANNELS, 4096 ) < 0 then HALT;

  //load music
  music := Mix_LoadMUS( 'In my mind.ogg' );
  if music = nil then HALT;
  Mix_VolumeMusic( MIX_MAX_VOLUME );

  //load sound
  sound := Mix_LoadWAV( 'dial.wav' );
  if sound = nil then HALT;
  Mix_VolumeChunk( sound, MIX_MAX_VOLUME );

  //create a menu
  writeln( '(1) Play music once' );
  writeln( '(2) Pause music' );
  writeln( '(3) Resume music' );
  writeln( '(4) Rewind music' );
  writeln( '(5) Play sound once on channel 1' );
  writeln( '(6) Play sound once on free channel' );
  writeln( '(7) Pause sound on all channels' );
  writeln( '(8) Resume sound on all channels' );
  writeln( '(A) Fade in music within 3 seconds' );
  writeln( '(S) Fade out music within 3 seconds from now' );
  writeln( '(D) Fade in sound on channel 1 within 2 seconds' );
  writeln( '(F) Fade out sound on channel 1 within 1 second from now' );
  writeln;
  writeln( '=== Effects on channel 1 ===' );
  writeln( '(G) Panning: left: 255, right: 32' );
  writeln( '(H) Stop panning' );
  writeln( '(J) Distance: very far ');
  writeln( '(K) Unregister distance effect' );
  writeln( '(L) Position 45ø to the right, in front, middle distance' );
  writeln( '(M) Unregister position effect' );
  writeln;
  writeln( '(ESC) Exit program' );

  new( sdlEvent );

  while exitloop = false do
  begin
    while SDL_PollEvent( sdlEvent ) = 1 do
    begin

      case sdlEvent^.type_ of
        SDL_KEYDOWN: begin
                       case sdlEvent^.key.keysym.sym of
                         SDLK_1: if Mix_PlayMusic( music, 0 ) < 0 then HALT;
                         SDLK_2: Mix_PauseMusic;
                         SDLK_3: Mix_ResumeMusic;
                         SDLK_4: Mix_RewindMusic;
                         SDLK_5: if Mix_PlayChannel( 1, sound, 0 ) < 0 then HALT;
                         SDLK_6: if Mix_PlayChannel( -1, sound, 0 ) < 0 then HALT;
                         SDLK_7: Mix_Pause( -1 );
                         SDLK_8: Mix_Resume( -1 );
                         SDLK_A: if Mix_FadeInMusic( music, 1, 3000 ) < 0 then HALT;
                         SDLK_S: if Mix_FadeOutMusic( 3000 ) = 0 then HALT;
                         SDLK_D: if Mix_FadeInChannel( 1, sound, 0, 2000 ) < 0 then HALT;
                         SDLK_F: if Mix_FadeOutChannel( 1, 1000 ) < 0 then HALT;
                         SDLK_G: if Mix_SetPanning( 1, 255, 32 ) = 0 then HALT;
                         SDLK_H: if Mix_SetPanning( 1, 255, 255 ) = 0 then HALT; //unregister panning
                         SDLK_J: if Mix_SetDistance( 1, 223 ) = 0 then HALT;
                         SDLK_K: if Mix_SetDistance( 1, 0 ) = 0 then HALT; //unregister distance effect
                         SDLK_L: if Mix_SetPosition( 1, 45, 127 ) = 0 then HALT;
                         SDLK_M: if Mix_SetPosition( 1, 0, 0 ) = 0 then HALT; //unregister position effect
                         SDLK_ESCAPE: exitloop := true;
                       end;
                     end;
      end;

    end;
    SDL_Delay( 5 );
  end;

  //clean up
  dispose( sdlEvent );
  SDL_DestroyWindow( sdlWindow1 );

  Mix_FreeMusic( music );
  Mix_FreeChunk( sound );

  Mix_CloseAudio;
  SDL_Quit;
end.

To initialize SDL2_mixer the function

function Mix_OpenAudio(frequency: Integer; format: UInt16; channels: Integer; chunksize: Integer): Integer

has to be called. It requests four parameters. These are the frequency, the audio format, the channels (mono or audio) and the chunksize. All these parameters are of Integer type. Function Mix_OpenAudio returns 0 on success and -1 on errors.

The frequency in Hertz (1/s) usually is 22050 Hz or MIX_DEFAULT_FREQUENCY in games but double as high for CD quality sound (44100 Hz). The higher the frequency, the more CPU power is needed.

Next we have to define the audio format. It determines how the audio data is stored. MIX_DEFAULT_FORMAT equals AUDIO_S16SYS. Different values are given in the following list (from official SDL2_mixer documentation):

  • Format constant name Description
  • AUDIO_U8 Unsigned 8-bit samples
  • AUDIO_S8 Signed 8-bit samples
  • AUDIO_U16LSB Unsigned 16-bit samples, in little-endian byte order
  • AUDIO_S16LSB Signed 16-bit samples, in little-endian byte order
  • AUDIO_U16MSB Unsigned 16-bit samples, in big-endian byte order
  • AUDIO_S16MSB Signed 16-bit samples, in big-endian byte order
  • AUDIO_U16 same as AUDIO_U16LSB (for backwards compatability probably)
  • AUDIO_S16 same as AUDIO_S16LSB (for backwards compatability probably)
  • AUDIO_U16SYS Unsigned 16-bit samples, in system byte order
  • AUDIO_S16SYS Signed 16-bit samples, in system byte order

Then we decide for a sound channel type, which means either stereo or mono. For stereo sound we choose 2 and for mono we choose 1 as value. MIX_DEFAULT_CHANNELS equals stereo output.

Finally the chunksize has to be set, where 4096 bytes per sample is a good and default value. Too low values may lead to skipping samples. Too high values may lead to delayed playing.

Loading music and sounds from files in SDL 2.0

We will load the music by

function Mix_LoadMUS(_file: PAnsiChar): PMix_Music.

_file can be the absolute path. If the Pascal file and the music file are in the same folder the file name is sufficient (as shown in the example). If the loading was unsuccessful, the function returns nil. The volume can be set by

function Mix_VolumeMusic(volume: Integer): Integer.

The volume can be set between 0 (silence) to 128 (maximum volume). The latter equals MIX_MAX_VOLUME. By the way, if you set -1 as argument, the return value corresponds to the set volume.

Quite similar sound files are loaded by

function Mix_LoadWAV(_file: PAnsiChar): PMix_Chunk.

Although the function’s name doesn’t indicate this, use this function to load the sound files of any format (Wave, Aiff, Riff, Ogg, Voc), not only for .wav sound files. It return nil on error. There is alittle difference when setting the volume by

function Mix_VolumeChunk(chunk: PMix_Chunk; volume: Integer): Integer.

Instead of just setting a general volume, the volume is bound to a certain sound. In the example case the sound is in variable “sound”. Just to mention it here, there is a third possibility, to set the volume for a certain channel:

function Mix_Volume(channel: Integer; volume: Integer): Integer.

This can be quite handy, anyway, channels are discussed a little bit later below. If your argument is -1 the average volume will be returned.

program chap9_SDL2;
uses SDL2, SDL2_mixer;

var
sdlWindow1: PSDL_Window;
sdlEvent: PSDL_Event;
music: PMix_Music;
sound: PMix_Chunk;
exitloop: boolean = false;

begin
  if SDL_Init( SDL_INIT_VIDEO or SDL_INIT_AUDIO ) < 0 then HALT;

  //get a window
  sdlWindow1 := SDL_CreateWindow( 'Music and Sound window', 50, 50, 500, 500, SDL_WINDOW_SHOWN );
  if sdlWindow1 = nil then HALT;

  //prepare mixer
  if Mix_OpenAudio( MIX_DEFAULT_FREQUENCY, MIX_DEFAULT_FORMAT,
                    MIX_DEFAULT_CHANNELS, 4096 ) < 0 then HALT;

  //load music
  music := Mix_LoadMUS( 'In my mind.ogg' );
  if music = nil then HALT;
  Mix_VolumeMusic( MIX_MAX_VOLUME );

  //load sound
  sound := Mix_LoadWAV( 'dial.wav' );
  if sound = nil then HALT;
  Mix_VolumeChunk( sound, MIX_MAX_VOLUME );

  //create a menu
  writeln( '(1) Play music once' );
  writeln( '(2) Pause music' );
  writeln( '(3) Resume music' );
  writeln( '(4) Rewind music' );
  writeln( '(5) Play sound once on channel 1' );
  writeln( '(6) Play sound once on free channel' );
  writeln( '(7) Pause sound on all channels' );
  writeln( '(8) Resume sound on all channels' );
  writeln( '(A) Fade in music within 3 seconds' );
  writeln( '(S) Fade out music within 3 seconds from now' );
  writeln( '(D) Fade in sound on channel 1 within 2 seconds' );
  writeln( '(F) Fade out sound on channel 1 within 1 second from now' );
  writeln;
  writeln( '=== Effects on channel 1 ===' );
  writeln( '(G) Panning: left: 255, right: 32' );
  writeln( '(H) Stop panning' );
  writeln( '(J) Distance: very far ');
  writeln( '(K) Unregister distance effect' );
  writeln( '(L) Position 45ø to the right, in front, middle distance' );
  writeln( '(M) Unregister position effect' );
  writeln;
  writeln( '(ESC) Exit program' );

  new( sdlEvent );

  while exitloop = false do
  begin
    while SDL_PollEvent( sdlEvent ) = 1 do
    begin

      case sdlEvent^.type_ of
        SDL_KEYDOWN: begin
                       case sdlEvent^.key.keysym.sym of
                         SDLK_1: if Mix_PlayMusic( music, 0 ) < 0 then HALT;
                         SDLK_2: Mix_PauseMusic;
                         SDLK_3: Mix_ResumeMusic;
                         SDLK_4: Mix_RewindMusic;
                         SDLK_5: if Mix_PlayChannel( 1, sound, 0 ) < 0 then HALT;
                         SDLK_6: if Mix_PlayChannel( -1, sound, 0 ) < 0 then HALT;
                         SDLK_7: Mix_Pause( -1 );
                         SDLK_8: Mix_Resume( -1 );
                         SDLK_A: if Mix_FadeInMusic( music, 1, 3000 ) < 0 then HALT;
                         SDLK_S: if Mix_FadeOutMusic( 3000 ) = 0 then HALT;
                         SDLK_D: if Mix_FadeInChannel( 1, sound, 0, 2000 ) < 0 then HALT;
                         SDLK_F: if Mix_FadeOutChannel( 1, 1000 ) < 0 then HALT;
                         SDLK_G: if Mix_SetPanning( 1, 255, 32 ) = 0 then HALT;
                         SDLK_H: if Mix_SetPanning( 1, 255, 255 ) = 0 then HALT; //unregister panning
                         SDLK_J: if Mix_SetDistance( 1, 223 ) = 0 then HALT;
                         SDLK_K: if Mix_SetDistance( 1, 0 ) = 0 then HALT; //unregister distance effect
                         SDLK_L: if Mix_SetPosition( 1, 45, 127 ) = 0 then HALT;
                         SDLK_M: if Mix_SetPosition( 1, 0, 0 ) = 0 then HALT; //unregister position effect
                         SDLK_ESCAPE: exitloop := true;
                       end;
                     end;
      end;

    end;
    SDL_Delay( 5 );
  end;

  //clean up
  dispose( sdlEvent );
  SDL_DestroyWindow( sdlWindow1 );

  Mix_FreeMusic( music );
  Mix_FreeChunk( sound );

  Mix_CloseAudio;
  SDL_Quit;
end.

Here nothing new can be found. The menu is printed out and some memory is allocated to the event pointer. Let’s proceed.

Playing, pausing, resuming music and sounds

program chap9_SDL2;
uses SDL2, SDL2_mixer;

var
sdlWindow1: PSDL_Window;
sdlEvent: PSDL_Event;
music: PMix_Music;
sound: PMix_Chunk;
exitloop: boolean = false;

begin
  if SDL_Init( SDL_INIT_VIDEO or SDL_INIT_AUDIO ) < 0 then HALT;

  //get a window
  sdlWindow1 := SDL_CreateWindow( 'Music and Sound window', 50, 50, 500, 500, SDL_WINDOW_SHOWN );
  if sdlWindow1 = nil then HALT;

  //prepare mixer
  if Mix_OpenAudio( MIX_DEFAULT_FREQUENCY, MIX_DEFAULT_FORMAT,
                    MIX_DEFAULT_CHANNELS, 4096 ) < 0 then HALT;

  //load music
  music := Mix_LoadMUS( 'In my mind.ogg' );
  if music = nil then HALT;
  Mix_VolumeMusic( MIX_MAX_VOLUME );

  //load sound
  sound := Mix_LoadWAV( 'dial.wav' );
  if sound = nil then HALT;
  Mix_VolumeChunk( sound, MIX_MAX_VOLUME );

  //create a menu
  writeln( '(1) Play music once' );
  writeln( '(2) Pause music' );
  writeln( '(3) Resume music' );
  writeln( '(4) Rewind music' );
  writeln( '(5) Play sound once on channel 1' );
  writeln( '(6) Play sound once on free channel' );
  writeln( '(7) Pause sound on all channels' );
  writeln( '(8) Resume sound on all channels' );
  writeln( '(A) Fade in music within 3 seconds' );
  writeln( '(S) Fade out music within 3 seconds from now' );
  writeln( '(D) Fade in sound on channel 1 within 2 seconds' );
  writeln( '(F) Fade out sound on channel 1 within 1 second from now' );
  writeln;
  writeln( '=== Effects on channel 1 ===' );
  writeln( '(G) Panning: left: 255, right: 32' );
  writeln( '(H) Stop panning' );
  writeln( '(J) Distance: very far ');
  writeln( '(K) Unregister distance effect' );
  writeln( '(L) Position 45ø to the right, in front, middle distance' );
  writeln( '(M) Unregister position effect' );
  writeln;
  writeln( '(ESC) Exit program' );

  new( sdlEvent );

  while exitloop = false do
  begin
    while SDL_PollEvent( sdlEvent ) = 1 do
    begin

      case sdlEvent^.type_ of
        SDL_KEYDOWN: begin
                       case sdlEvent^.key.keysym.sym of
                         SDLK_1: if Mix_PlayMusic( music, 0 ) < 0 then HALT;
                         SDLK_2: Mix_PauseMusic;
                         SDLK_3: Mix_ResumeMusic;
                         SDLK_4: Mix_RewindMusic;
                         SDLK_5: if Mix_PlayChannel( 1, sound, 0 ) < 0 then HALT;
                         SDLK_6: if Mix_PlayChannel( -1, sound, 0 ) < 0 then HALT;
                         SDLK_7: Mix_Pause( -1 );
                         SDLK_8: Mix_Resume( -1 );
                         SDLK_A: if Mix_FadeInMusic( music, 1, 3000 ) < 0 then HALT;
                         SDLK_S: if Mix_FadeOutMusic( 3000 ) = 0 then HALT;
                         SDLK_D: if Mix_FadeInChannel( 1, sound, 0, 2000 ) < 0 then HALT;
                         SDLK_F: if Mix_FadeOutChannel( 1, 1000 ) < 0 then HALT;
                         SDLK_G: if Mix_SetPanning( 1, 255, 32 ) = 0 then HALT;
                         SDLK_H: if Mix_SetPanning( 1, 255, 255 ) = 0 then HALT; //unregister panning
                         SDLK_J: if Mix_SetDistance( 1, 223 ) = 0 then HALT;
                         SDLK_K: if Mix_SetDistance( 1, 0 ) = 0 then HALT; //unregister distance effect
                         SDLK_L: if Mix_SetPosition( 1, 45, 127 ) = 0 then HALT;
                         SDLK_M: if Mix_SetPosition( 1, 0, 0 ) = 0 then HALT; //unregister position effect
                         SDLK_ESCAPE: exitloop := true;
                       end;
                     end;
      end;

    end;
    SDL_Delay( 5 );
  end;

  //clean up
  dispose( sdlEvent );
  SDL_DestroyWindow( sdlWindow1 );

  Mix_FreeMusic( music );
  Mix_FreeChunk( sound );

  Mix_CloseAudio;
  SDL_Quit;
end.

The main application loop is entered and should be left if variable “exitloop” is true.

As known from previous Chapter 8 about event handling, we firstly poll for events by SDL_PollEvent() and enter a further loop. Secondly, if there are events found waiting to be processed, we check by a case statement what exact key has been pressed.

For each key a different music or sound related function is used to do something. Let’s discuss them now in detail. We start with the functions used for keys 1-4:

function Mix_PlayMusic(music: PMix_Music; loops: Integer): Integer

procedure Mix_PauseMusic

procedure Mix_ResumeMusic

procedure Mix_RewindMusic

Mix_PlayMusic plays music. The first argument is the music itself which we previously got from the example .ogg music file and associated with the “music” pointer of PMix_Music type. The second argument determines how often the music is played. Additionally -1 means infinite repetition. This function returns 0 on success and -1 on error.

The meaning of the three procedures Mix_PauseMusic, Mix_ResumeMusic and Mix_RewindMusic ist obvious from their names. They are used to pause, resume (if paused before) and rewind the played music.

The analogous function and procedures for sound channels are shown now:

function Mix_PlayChannel(channel: Integer; chunk: PMix_Chunk; loops: Integer): Integer

procedure Mix_Pause(channel: Integer)

procedure Mix_Resume(channel: Integer)

As you can see the function Mix_PlayChannel has three parameters. The first parameter asks for a channel with which the sound should be associated. SDL2_mixer provides eight sound channels. Usually you should use -1 which means the first free channel will be used to play the sound. Anyway, you could specifiy a certain channel here if you like. The second parameter asks for a sound pointer of PMix_Chunk type. The  third parameter is the same as for music, -1 means an infinite loop. Attention though for specific repetitions. The value for loops is increased by 1. So, if you want a sound to be played once, you need the argument to be 0. The return value corresponds to the sound channel the sound is played on, it is -1 on error.

The procedures Mix_Pause and Mix_Resume are self-explanatory. Use them to pause or resume a certain channel, which is the argument. If the argument is -1 (as in the example code) you pause or resume all channels. By the way, you may have noticed that there is not a rewind analogue, because sounds are not expected to be rewound.

Fading and distance effects

program chap9_SDL2;
uses SDL2, SDL2_mixer;

var
sdlWindow1: PSDL_Window;
sdlEvent: PSDL_Event;
music: PMix_Music;
sound: PMix_Chunk;
exitloop: boolean = false;

begin
  if SDL_Init( SDL_INIT_VIDEO or SDL_INIT_AUDIO ) < 0 then HALT;

  //get a window
  sdlWindow1 := SDL_CreateWindow( 'Music and Sound window', 50, 50, 500, 500, SDL_WINDOW_SHOWN );
  if sdlWindow1 = nil then HALT;

  //prepare mixer
  if Mix_OpenAudio( MIX_DEFAULT_FREQUENCY, MIX_DEFAULT_FORMAT,
                    MIX_DEFAULT_CHANNELS, 4096 ) < 0 then HALT;

  //load music
  music := Mix_LoadMUS( 'In my mind.ogg' );
  if music = nil then HALT;
  Mix_VolumeMusic( MIX_MAX_VOLUME );

  //load sound
  sound := Mix_LoadWAV( 'dial.wav' );
  if sound = nil then HALT;
  Mix_VolumeChunk( sound, MIX_MAX_VOLUME );

  //create a menu
  writeln( '(1) Play music once' );
  writeln( '(2) Pause music' );
  writeln( '(3) Resume music' );
  writeln( '(4) Rewind music' );
  writeln( '(5) Play sound once on channel 1' );
  writeln( '(6) Play sound once on free channel' );
  writeln( '(7) Pause sound on all channels' );
  writeln( '(8) Resume sound on all channels' );
  writeln( '(A) Fade in music within 3 seconds' );
  writeln( '(S) Fade out music within 3 seconds from now' );
  writeln( '(D) Fade in sound on channel 1 within 2 seconds' );
  writeln( '(F) Fade out sound on channel 1 within 1 second from now' );
  writeln;
  writeln( '=== Effects on channel 1 ===' );
  writeln( '(G) Panning: left: 255, right: 32' );
  writeln( '(H) Stop panning' );
  writeln( '(J) Distance: very far ');
  writeln( '(K) Unregister distance effect' );
  writeln( '(L) Position 45ø to the right, in front, middle distance' );
  writeln( '(M) Unregister position effect' );
  writeln;
  writeln( '(ESC) Exit program' );

  new( sdlEvent );

  while exitloop = false do
  begin
    while SDL_PollEvent( sdlEvent ) = 1 do
    begin

      case sdlEvent^.type_ of
        SDL_KEYDOWN: begin
                       case sdlEvent^.key.keysym.sym of
                         SDLK_1: if Mix_PlayMusic( music, 0 ) < 0 then HALT;
                         SDLK_2: Mix_PauseMusic;
                         SDLK_3: Mix_ResumeMusic;
                         SDLK_4: Mix_RewindMusic;
                         SDLK_5: if Mix_PlayChannel( 1, sound, 0 ) < 0 then HALT;
                         SDLK_6: if Mix_PlayChannel( -1, sound, 0 ) < 0 then HALT;
                         SDLK_7: Mix_Pause( -1 );
                         SDLK_8: Mix_Resume( -1 );
                         SDLK_A: if Mix_FadeInMusic( music, 1, 3000 ) < 0 then HALT;
                         SDLK_S: if Mix_FadeOutMusic( 3000 ) = 0 then HALT;
                         SDLK_D: if Mix_FadeInChannel( 1, sound, 0, 2000 ) < 0 then HALT;
                         SDLK_F: if Mix_FadeOutChannel( 1, 1000 ) < 0 then HALT;
                         SDLK_G: if Mix_SetPanning( 1, 255, 32 ) = 0 then HALT;
                         SDLK_H: if Mix_SetPanning( 1, 255, 255 ) = 0 then HALT; //unregister panning
                         SDLK_J: if Mix_SetDistance( 1, 223 ) = 0 then HALT;
                         SDLK_K: if Mix_SetDistance( 1, 0 ) = 0 then HALT; //unregister distance effect
                         SDLK_L: if Mix_SetPosition( 1, 45, 127 ) = 0 then HALT;
                         SDLK_M: if Mix_SetPosition( 1, 0, 0 ) = 0 then HALT; //unregister position effect
                         SDLK_ESCAPE: exitloop := true;
                       end;
                     end;
      end;

    end;
    SDL_Delay( 5 );
  end;

  //clean up
  dispose( sdlEvent );
  SDL_DestroyWindow( sdlWindow1 );

  Mix_FreeMusic( music );
  Mix_FreeChunk( sound );

  Mix_CloseAudio;
  SDL_Quit;
end.

Important to know about effects like fading. They are performed on the fly to your data, so the more effects you use, the more computational power they require. It is important to unregister effects you don’t need anymore.

These are the four fade functions triggered by the next keys (A-F):

function Mix_FadeInMusic(music: PMix_Music; loops: Integer; ms: Integer): Integer

function Mix_FadeOutMusic(ms: Integer): Integer

function Mix_FadeInChannel(channel: Integer; chunk: PMix_Chunk; loops: Integer; ms: Integer): Integer

function Mix_FadeOutChannel(which: Integer; ms: Integer): Integer

The function Mix_FadeInMusic works the very same way as Mix_PlayMusic above, but there is a further parameter which allows for a fade in effect in milliseconds. 3000 milliseconds equals 3 seconds. It returns 0 on success and -1 on error.

Mix_FadeOutMusic fades out the playing music within the given time in milliseconds in the moment the function is called (hence the key is pressed in the example program). Attention, it returns 1 on success and 0 on error.

Quite similar the functions Mix_FadeInChannel and Mix_FadeOutChannel work. Instead of the music pointer, the sound pointer has to be given. Additionally the channel has to be given. By the way, also here the number of times the sound is played is increased by 1, so if you want to play a sound with fade in effect once, the argument has to be 0. Both functions return 0 on success and -1 on error.

The functions triggered by the keys G-M are:

function Mix_SetPanning(channel: Integer; left: UInt8; right: UInt8): Integer

For setting the panning you need the channel to which panning should be applied and the volume values which may vary from 0 (quiet) to 255 (loudest). For a consistent panning effect you may substract the left value from the right value (left: volume, right: 255-volume). To unregister this effect, you should set both values to 255, which is recommended to be done if you don’t need panning anymore. The function returns 0 on errors.

function Mix_SetDistance(channel: Integer; distance: UInt8): Integer

The higher the distance the quieter the sound appears. The value can range between 0 (near) and 255 (far). If distance is set to 0 the effect is unregistered. It returns 0 on errors.

function Mix_SetPosition(channel: Integer; angle: SInt16; distance: UInt8): Integer

This function adjusts volumes according to an angle and a distance value, hence it combines the effects of the two functions discussed before for panning and distance. The distance value works the same way as discussed for Mix_SetDistance. Angle of 0 (degree) means in front of you, 90 means directly to the right, 180 means directly behind, 270 means directly to the left.

Finally, if the SDLK_ESCAPE has been pressed, the loop stopping variable “exitloop” is set to true, which will stop the main loop.

program chap9_SDL2;
uses SDL2, SDL2_mixer;

var
sdlWindow1: PSDL_Window;
sdlEvent: PSDL_Event;
music: PMix_Music;
sound: PMix_Chunk;
exitloop: boolean = false;

begin
  if SDL_Init( SDL_INIT_VIDEO or SDL_INIT_AUDIO ) < 0 then HALT;

  //get a window
  sdlWindow1 := SDL_CreateWindow( 'Music and Sound window', 50, 50, 500, 500, SDL_WINDOW_SHOWN );
  if sdlWindow1 = nil then HALT;

  //prepare mixer
  if Mix_OpenAudio( MIX_DEFAULT_FREQUENCY, MIX_DEFAULT_FORMAT,
                    MIX_DEFAULT_CHANNELS, 4096 ) < 0 then HALT;

  //load music
  music := Mix_LoadMUS( 'In my mind.ogg' );
  if music = nil then HALT;
  Mix_VolumeMusic( MIX_MAX_VOLUME );

  //load sound
  sound := Mix_LoadWAV( 'dial.wav' );
  if sound = nil then HALT;
  Mix_VolumeChunk( sound, MIX_MAX_VOLUME );

  //create a menu
  writeln( '(1) Play music once' );
  writeln( '(2) Pause music' );
  writeln( '(3) Resume music' );
  writeln( '(4) Rewind music' );
  writeln( '(5) Play sound once on channel 1' );
  writeln( '(6) Play sound once on free channel' );
  writeln( '(7) Pause sound on all channels' );
  writeln( '(8) Resume sound on all channels' );
  writeln( '(A) Fade in music within 3 seconds' );
  writeln( '(S) Fade out music within 3 seconds from now' );
  writeln( '(D) Fade in sound on channel 1 within 2 seconds' );
  writeln( '(F) Fade out sound on channel 1 within 1 second from now' );
  writeln;
  writeln( '=== Effects on channel 1 ===' );
  writeln( '(G) Panning: left: 255, right: 32' );
  writeln( '(H) Stop panning' );
  writeln( '(J) Distance: very far ');
  writeln( '(K) Unregister distance effect' );
  writeln( '(L) Position 45ø to the right, in front, middle distance' );
  writeln( '(M) Unregister position effect' );
  writeln;
  writeln( '(ESC) Exit program' );

  new( sdlEvent );

  while exitloop = false do
  begin
    while SDL_PollEvent( sdlEvent ) = 1 do
    begin

      case sdlEvent^.type_ of
        SDL_KEYDOWN: begin
                       case sdlEvent^.key.keysym.sym of
                         SDLK_1: if Mix_PlayMusic( music, 0 ) < 0 then HALT;
                         SDLK_2: Mix_PauseMusic;
                         SDLK_3: Mix_ResumeMusic;
                         SDLK_4: Mix_RewindMusic;
                         SDLK_5: if Mix_PlayChannel( 1, sound, 0 ) < 0 then HALT;
                         SDLK_6: if Mix_PlayChannel( -1, sound, 0 ) < 0 then HALT;
                         SDLK_7: Mix_Pause( -1 );
                         SDLK_8: Mix_Resume( -1 );
                         SDLK_A: if Mix_FadeInMusic( music, 1, 3000 ) < 0 then HALT;
                         SDLK_S: if Mix_FadeOutMusic( 3000 ) = 0 then HALT;
                         SDLK_D: if Mix_FadeInChannel( 1, sound, 0, 2000 ) < 0 then HALT;
                         SDLK_F: if Mix_FadeOutChannel( 1, 1000 ) < 0 then HALT;
                         SDLK_G: if Mix_SetPanning( 1, 255, 32 ) = 0 then HALT;
                         SDLK_H: if Mix_SetPanning( 1, 255, 255 ) = 0 then HALT; //unregister panning
                         SDLK_J: if Mix_SetDistance( 1, 223 ) = 0 then HALT;
                         SDLK_K: if Mix_SetDistance( 1, 0 ) = 0 then HALT; //unregister distance effect
                         SDLK_L: if Mix_SetPosition( 1, 45, 127 ) = 0 then HALT;
                         SDLK_M: if Mix_SetPosition( 1, 0, 0 ) = 0 then HALT; //unregister position effect
                         SDLK_ESCAPE: exitloop := true;
                       end;
                     end;
      end;

    end;
    SDL_Delay( 5 );
  end;

  //clean up
  dispose( sdlEvent );
  SDL_DestroyWindow( sdlWindow1 );

  Mix_FreeMusic( music );
  Mix_FreeChunk( sound );

  Mix_CloseAudio;
  SDL_Quit;
end.

Within the main loop a short delay of 5 milliseconds is used to prevent the application from running an empty loop no events are waiting.

The clean up means freeing sdlEvent, and use the procedures Mix_FreeMusic and Mix_FreeChunk to free the music and sound data.

procedure Mix_FreeMusic(music: PMix_Music)

procedure Mix_FreeChunk(chunk: PMix_Chunk)

The arguments have to be the music and sound pointer respectively.

SDL_mixer 2.0 has to be closed by

procedure Mix_CloseAudio.

Finally SDL 2.0 and the application are shut down as known.

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