Question

I want to load an SDL_Surface into an OpenGL texture with padding (so that NPOT->POT) and apply a color key on the surface afterwards. I either end up colorkeying all pixels, regardless of their color, or not colorkey anything at all. I have tried a lot of different things, but none of them seem to work.

Here’s the working snippet of my code. I use a custom color class for the colorkey (range [0-1]):

// Create an empty surface with the same settings as the original image
SDL_Surface* paddedImage = SDL_CreateRGBSurface(image->flags, width, height,
                                                    image->format->BitsPerPixel,
                                                    #if SDL_BYTEORDER == SDL_BIG_ENDIAN
                                                        0xff000000,
                                                        0x00ff0000,
                                                        0x0000ff00,
                                                        0x000000ff
                                                    #else
                                                        0x000000ff,
                                                        0x0000ff00,
                                                        0x00ff0000,
                                                        0xff000000
                                                    #endif
                                                    );
// Map RGBA color to pixel format value
Uint32 colorKeyPixelFormat = SDL_MapRGBA(paddedImage->format,
                                         static_cast<Uint8>(colorKey.R * 255),
                                         static_cast<Uint8>(colorKey.G * 255),
                                         static_cast<Uint8>(colorKey.B * 255),
                                         static_cast<Uint8>(colorKey.A * 255));

SDL_FillRect(paddedImage, NULL, colorKeyPixelFormat);

// Blit the image onto the padded image
SDL_BlitSurface(image, NULL, paddedImage, NULL);

SDL_SetColorKey(paddedImage, SDL_SRCCOLORKEY, colorKeyPixelFormat);

Afterwards, I generate an OpenGL texture from paddedImage using similar code to the SDL+OpenGL texture loading code found online (I’ll post if necessary). This code works if I just want the texture with or without padding, and is likely not the problem.

I realize that I set all pixels in paddedImage to have alpha zero which causes the first problem I mentioned, but I can’t seem to figure out how to do this. Should I just loop over the pixels and set the appropriate colors to have alpha zero?

PARTIAL SOLUTION:

1. Create paddedImage as above
2. SDL_FillRect the paddedImage with the colorkey
3. Generate the texture “as usual”
4. Manually copy the image (SDL_Surface*) pixels to the paddedImage (OGL texture)

This works almost always expect some cases where the image has 3 color components (i.e. no alpha channel). I’m trying to fix that now by converting them to 4 color components

Answer 1

I think that It could be used together with OpenGL if you can convert SDL_Surfaces into OGL textures, and then you could use the blit function to combine your textures, and manipulate thing using the SDL workflow.
I dont know what you want to achieve. You want to transfer one surface to an OGL texture and preserve the Colorkey, or just want to apply the colorkeyed surface to another surface which then you convert into an OGL texture.
Also you dont have to use Per-pixel alphas, as SDL gives you the ability to use per-surface alphas, but its quite complex as of what alphas and colorkeys can be combined and used together.

As this is a complex thing, please refer to the SDL reference, and this tutorial may be helpful too(tough it doesnt handle OGL stuff):
http://www.sdltutorials.com/the-ins-and-outs-and-overlays-of-alpha-blending