[SDL] Re: Transparency not that transparent.

[Mike Battersby]

> In principle, one ought to be able to do it by simply setting the alpha
> value of every pixel to either fully transparent or fully opaque based
> on whether the RGB values match the color key.  This seems fairly
> tricky, though, especially since both RGB and RGBA are represented
> as Uint32.  I assume the color key is probably just RGB?

The easiest way to do this I found was to simply copy the Surface into
a similar-sized Surface filled with alpha = 0.  This way the color-keyed
pixels won't be copied and will retain their 0 alpha value, and the
non-color-keyed pixels will get the correct colors and the default alpha.

C code something like this (may not be optimal, suggestions for improvement
gratefully accepted).

	SDL_Surface *orig, *rgba;

	orig = IMG_Load(filename);

	rgba = SDL_CreateRGBSurface(SDL_SWSURFACE, orig->w, orig->h,
				    orig->format->BitsPerPixel,
				    orig->format->rmask, orig->format->bmask,
				    orig->format->gmask, orig->format->amask);

	/* May need this if your image loader hasn't set the color key */
	/* SDL_SetColorKey(orig, SDL_SRCCOLORKEY, colorkeyvalue); */

	SDL_FillRect(rgba, NULL, SDL_MapRGBA(rgba->format, 0, 0, 0, 0));

	SDL_BlitSurface(orig, NULL, rgba, NULL);
	SDL_FreeSurface(orig);

This might be coloured by the fact that I needed to change the image
format anyway for OpenGL (my version of this code uses new depth and
Xmask params).

I guess you could do something like this if your Surface is already in the
right format:

	foreach pixel
		SDL_GetRGB(pixel, format, &r, &g, &b);
		if (SDL_MapRGB(format, r, g, b) == format->colorkey)
			pixel = SDL_MapRGBA(format, r, g, b, 0);

Cheers,

 - Mike