[SDL] Alpha/Transparency from BMP files

[Phoenix Kokido]

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
kokido at postmark.net

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
>