[SDL] "layering" surfaces

[Ryan C. Gordon]

> My question is, does anything like this exist in SDL (either 1.2.x or 
> the forthcoming 1.3)?

You could just use OpenGL inside SDL...this is going to be your best bet.

If you need to do it with a 2D surface, you have your work cut out for you.

You can draw back-to-front (so the surface with a depth of -1 gets drawn 
before the surface with a depth of zero). This will naturally layer them 
as you wish, since the one "on top" will overwrite the parts of the 
things below it that overlap so the final result looks correct. If you 
have a lot of pixels to move, this can be very inefficient...ten 
surfaces that completely overlap will all get blitted when you could 
have just blitted one for the same result...but it will work and it's 
simple to write and maintain.

Faster: draw front-to-back, and only draw things that you know won't be 
covered. This can get REALLY complicated...you'll either want to walk 
through all the surfaces and build a list of rectangles, and then blit 
bits and pieces of each surface as it would be on the screen, or use 
something called a "span list" (google for that one). This code can get 
complicated and unreadable.

Extra credit for not blitting parts of the screen that didn't change 
between frames.

Scaling and rotation make it worse by an order of magnitude. SDL doesn't 
provide these functions and a lot of shortcuts you could take in 
building spanlists, etc, go away when you can't count on the size or 
shape of the surface.

But really, just use OpenGL. You might as well dump all the data to the 
GL without any consideration and let the hardware sort it out for 
you...and get the massive speed boost, too.

--ryan.