[SDL] Blitting through an Intermediary

[David Olofson]

On Monday 26 April 2004 14.20, Brian wrote:
[...alpha & intermediate surface...]

I'm doing that in Fixed Rate Pig (http://olofson.net/examples.html), 
in order to avoid some clipping overhead when using RLE acceleration. 
The basic procedure is as follows:

	1. Load the image. (Let's call it tmp.)

	2. Disable alpha blending;
		SDL_SetAlpha(tmp, 0, 0);

	3. For each target surface (tmp2), create an RGBA
	   surface:
		tmp2 = SDL_CreateRGBSurface(SDL_SWSURFACE,
				sw, sh, 32,
				0xff000000, 0x00ff0000,
				0x0000ff00, 0x000000ff);

	3b. Disable alpha blending for the target surface:
		SDL_SetAlpha(tmp2, 0, 0);

	3c. Blit the area(s) you want as usual:
		SDL_BlitSurface(tmp, &rect, tmp2, NULL);

	3d. Enable alpha blending for the target surface:
		SDL_SetAlpha(tmp2, SDL_SRCALPHA | SDL_RLEACCEL,
				SDL_ALPHA_OPAQUE);

	3e. (For performance) convert the surface to a suitable
	    format for blitting to the screen:
		sprite = SDL_DisplayFormatAlpha(tmp2);

And of course, don't forget to SDL_FreeSurface() the temporary 
surfaces. (Note that SDL_DisplayFormat*() creates a new surface 
without freeing the original surface.)


//David Olofson - Programmer, Composer, Open Source Advocate

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