SDL Surface Pixel Format Conversion

Question

I want to convert an SDL_Surface, which was loaded by IMG_Load() to an other pixel format (rgba8) for an OpenGL Texture. How can I do that? I've read about SDL_ConvertSurface() in the documentation, but I can't figure out, how to put it together.

Answer 1

Give "How To Load an OpenGL Texture from an SDL_Surface" a shot:

GLuint texture;         // This is a handle to our texture object
SDL_Surface *surface;   // This surface will tell us the details of the image
GLenum texture_format;
GLint  nOfColors;

if( (surface = SDL_LoadBMP("image.bmp")) )
{ 
    // Check that the image's width is a power of 2
    if( (surface->w & (surface->w - 1)) != 0 )
    {
        printf("warning: image.bmp's width is not a power of 2\n");
    }

    // Also check if the height is a power of 2
    if( (surface->h & (surface->h - 1)) != 0 )
    {
        printf("warning: image.bmp's height is not a power of 2\n");
    }

    // get the number of channels in the SDL surface
    nOfColors = surface->format->BytesPerPixel;
    if( nOfColors == 4 )     // contains an alpha channel
    {
        if(surface->format->Rmask == 0x000000ff)
            texture_format = GL_RGBA;
        else
            texture_format = GL_BGRA;
    }
    else if( nOfColors == 3 )     // no alpha channel
    {
        if(surface->format->Rmask == 0x000000ff)
            texture_format = GL_RGB;
        else
            texture_format = GL_BGR;
    }
    else
    {
        printf("warning: the image is not truecolor..  this will probably break\n");
        // this error should not go unhandled
    }

    // Have OpenGL generate a texture object handle for us
    glGenTextures( 1, &texture );

    // Bind the texture object
    glBindTexture( GL_TEXTURE_2D, texture );

    // Set the texture's stretching properties
    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR );
    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );

    // Edit the texture object's image data using the information SDL_Surface gives us
    glTexImage2D
        (
        GL_TEXTURE_2D,
        0,
        nOfColors,
        surface->w,
        surface->h,
        0,
        texture_format,
        GL_UNSIGNED_BYTE,
        surface->pixels
        );
} 
else
{
    printf("SDL could not load image.bmp: %s\n", SDL_GetError());
    SDL_Quit();
    return 1;
}    

// Free the SDL_Surface only if it was successfully created
if( surface )
{ 
    SDL_FreeSurface( surface );
}