Alpha/Transparency from BMP files

SDL Development
1

After using SDL_LoadBMP to load a small bitmap, I want to blit it to a
hardware surface with transparent sections.

  • I create the destination surface with
    Screen = SDL_SetVideoMode(XRes, YRes, Bpp,
    SDL_HWSURFACE|SDL_HWPALETTE|SDL_DOUBLEBUF);

  • Load the bitmap
    image = SDL_LoadBMP(imagename);

  • Set ColorKey and Alpha information
    SDL_SetColorKey(image, 0, SDL_MapRGB(Screen->format, 0xFF, 0x00, 0xFF));
    SDL_SetAlpha(image, SDL_SRCALPHA, 255); //tried with and without this line

  • Then blit it and flip it.
    SDL_BlitSurface(image, NULL, Screen, &dest);
    SDL_Flip(Screen);

The ugly purple pixels show, when I think they shouldn’t. I’m using NT 4.0,
Visual Studio 6.

Am I doing something wrong?
Does LoadBMP set up a surface that doesn’t support transparency or Alpha?
Do I need to do something to my Screen before blitting?
Is SDL broke? I hope not - I really like it so far.

Also, can someone lay out the procedure or point to a reference for using
ImageLib with SDL on the NT and Linux platforms?

  • Scott Russell
2

Do all the ugly purple pixels show, colorkeys are very exact and when
images are antialiased if one channel is even one off it will be
blitted.

Phoenix Kokido
members.xoom.com/kokido
@Wes_Poole

Scott Russell wrote:

After using SDL_LoadBMP to load a small bitmap, I want to blit it to a
hardware surface with transparent sections.

  • I create the destination surface with
    Screen = SDL_SetVideoMode(XRes, YRes, Bpp,
    SDL_HWSURFACE|SDL_HWPALETTE|SDL_DOUBLEBUF);

  • Load the bitmap
    image = SDL_LoadBMP(imagename);

  • Set ColorKey and Alpha information
    SDL_SetColorKey(image, 0, SDL_MapRGB(Screen->format, 0xFF, 0x00, 0xFF));
    SDL_SetAlpha(image, SDL_SRCALPHA, 255); //tried with and without this line

  • Then blit it and flip it.
    SDL_BlitSurface(image, NULL, Screen, &dest);
    SDL_Flip(Screen);

The ugly purple pixels show, when I think they shouldn’t. I’m using NT
4.0,> Visual Studio 6.

Am I doing something wrong?
Does LoadBMP set up a surface that doesn’t support transparency or Alpha?
Do I need to do something to my Screen before blitting?
Is SDL broke? I hope not - I really like it so far.

Also, can someone lay out the procedure or point to a reference for using
ImageLib with SDL on the NT and Linux platforms?

  • Scott Russell
3

Yes - I assumed this was the problem at first, and used a non-aliasing tool
to check the bitmaps - the pixels are all 0xFF00FF.

  • Scott
4

Fixed it, I think. It required an SDL_ConvertSurface after the load. Seems
to improve performance, too.

  • Scott