to the mighty code senseis,

Here's the hypothetical setup:
I have a sketch with two rects() whizzing around the screen.
If I want the rects to draw a trail behind them, then I just let them move around.
If I want no trails to be drawn, then I put  background(0) at the beginning of the draw-cycle
That's easy enough.

But how do I manage to, simultaneously, draw one rect with a trail and the other without?

Is there a way to prevent one object from coloring the background pixels, while the other object does, but still have it appear on screen?
To give a more concrete example: Within a drawing tool, how can the paintbrush icon move over the canvas, while beneath it a stroke is drawn?
Or can toggable guidelines within a sketch exist without them recoloring the background pixels?

The only solution I can think of, is to save the pixel values from the background area the object is going to cover, then draw an object and when it moves away/is deleted redraw the background pixels using the presaved values. But this approach seems very unsexy and clunky.

I don't even know under which keywords I should start researching this problem. I hope I've managed to make my question clear enough without creating too much mass confusion. Any pointers to where I should start looking would be great.

cheers,
Greg

ok, I'll have to dig into this.
thanks!