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openglcolorstexturessdl

Identify pixel data format in SDL

Running on OS X, I've loaded a texture in OpenGL using SDL_Image library (using IMG_LOAD() which returns SDL_Surface*). It appeared that the color channels have been swapped, i.e. I had to set GL_BGRA as a pixel format parameter in glTexImage2D().

Is there a way to determine the correct data format (BGRA or RGBA or etc...) without just simply compiling and checking the texture? And what is the reason that SDL swaps these color channels?

Solution

  • Yes. Following link has code examples of how to determine the channel shift for each component: http://wiki.libsdl.org/SDL_PixelFormat#Code_Examples

    From the site:

    SDL_PixelFormat *fmt;
SDL_Surface *surface;
Uint32 temp, pixel;
Uint8 red, green, blue, alpha;
.
.
.
fmt = surface->format;
SDL_LockSurface(surface);
pixel = *((Uint32*)surface->pixels);
SDL_UnlockSurface(surface);

/* Get Red component */
temp = pixel & fmt->Rmask;  /* Isolate red component */
temp = temp >> fmt->Rshift; /* Shift it down to 8-bit */
temp = temp << fmt->Rloss;  /* Expand to a full 8-bit number */
red = (Uint8)temp;

    You should be able to sort the Xmasks by value. Then you may determine if its RGBA or BGRA. If Xmask == 0 then the color channel does not exist.

    I have no idea why the swaps occur.

    Edit: Changed from Xshift to Xmask as the latter may be used to determine both location AND existance of color channels.