[racket] How does Universe.ss paint? How is it so optimized with almost little to no flickring?

The teachpack relies on the underlying graphics toolbox (Gracket, not Racket for now). The client program constructs an image and hands over the complete image to the draw function, which then inserts it into a drawing context, i.e., a software device that draws pixels on the screen. 

You can still get flickering if/when the client program floods the universe handler with large and/or lots of images. 

One day I am hoping to find serious time to re-design this piece. 

-- Matthias