SDL_Surface* temp_sprite = IMG_Load(SPRITEFILE); 
if(!temp_sprite)
	return false;
sprite = SDL_DisplayFormatAlpha(temp_sprite);
SDL_FreeSurface(temp_sprite);

SDL_SetAlpha(sprite, SDL_SRCALPHA|SDL_RLEACCEL, 128);
SDL_BlitSurface(sprite, NULL, screen, &dstrect);

Quote:Original post by Jack Sotac
If your surface has an alpha channel , the per-surface alpha value will be ignored. That is to say, you can't blit doing a per-pixel and per-surface alpha at the same time

Checkout the last line of the description section for
SDL_SetAlpha.

If you wish to use the per-pixel and per-surface alpha values to blit, then you have to write your own function.

Good Luck.

Quote:Original post by Jack Sotac

Quote:Original post by Iori Branford PNG transparency is color-key-based

I am sure PNG transparency is done through an 8-bit alpha channel. Although some png images may not include an alpha-channel(no transparency), that is rare.

You can blit an image with an alpha-channel using a colorkey if SDL_SRCALPHA is NOT set. But if SDL_SRCALPHA is not set, then you cannot use per-surface or per-pixel alpha, only the colorkey.

It is true that colorkey and per-surface alpha can be used together. But if a surface has an alpha-channel and SDL_SRCALPHA is set, then both colorkey and per-surface alpha will be ignore.

An SDL surface has an alpha-channel if (surface->format->Amask != 0) is true.

See the description of SDL_BlitSurface and specifically the psuedocode segment to see how per-surface,per-pixel alpha, and colorkeying works and doesn't work together.

PNG reference,see Colour depths.

Quote:Original post by Iori Branford
I've got it. The problem was twofold.

First, SDL_Image. For whatever reason, an image loaded with it could not be alpha blended. I made a BMP, switched to SDL_LoadBMP and voila.