[pygame] Drawing Transparent Sprites

[Kris Schnee <kschnee@xxxxxxxxxx>]

I built a system of sprites and a tiled, scrolling world, and am having 
trouble drawing the sprites when they move.

The world has a big Surface that's drawn once, with a clipping rectangle 
used to blit part of it to the screen in each frame. The sprites were 
using a system by which they drew themselves to the world's Surface and 
cleaned up after themselves, but (a) that was complicated, (b) it didn't 
work well when they moved, and (c) it didn't work well when they were 
near other sprites. I tried giving the world a "sprite_layer" Surface 
that could be drawn on top of the existing one and cleared each frame, 
but that didn't work because it's opaque and black, and I'm not sure how 
to make it transparent only in areas where sprites (which might be 
black) have been drawn. Suggestions?

Kris