The dynamic distance modifier allows you to dynamically (once per frame)
change the source pixels for each ring of pixels out from the center.
In the 'pixel' code section, 'd' represents the distance in pixels
the current ring is from the center, and code can modify it to
change the distance from the center where the source pixels for
that ring would be read. This is a terrible explanation, and if
you want to make a better one send it to me.

Examples:
Zoom in: 'd=d*0.9'
Zoom out: 'd=d*1.1'
Back and forth: pixel='d=d*(1.0+0.1*cos(t));', frame='t=t+0.1'