[SDL] Alpha again

[Ilya Olevsky]

> Maybe I'm not understanding how to do this or even what alpha is about. 
> I've got a display surface on which I've 
> 
> 	SDL_SetAlpha (screen, SDL_SRCALPHA, 255);
> 
> The pixels I'm placing directly on the surface, after having blitted a
> solid white background, have been OR'd with 0x20000000.  Should I not
> see some indication of partial transparency.  I seem to be getting solid
> colors in this test.

It doesn't quite work that way. There's per-surface alpha, and per-pixel 
alpha, and they can't be combined. What you're setting with that call is 
the per-surface alpha. So you're basically setting the surface to to be 
100% opaque. What you want to do requires the surface to have an alpha 
channel (RGBA), which will cause per-pixel blending. Easiest way to do 
it is to make your image file store the alpha channel (use PNG or TGA). 
SDL_Image will create a properly set up surface for you when you load 
it.