Great image of the "bike".
As it stands the program allows a moving slit set the following:
boolean scanR = true;
boolean scanL = true;
You can control the speed of scan with:
int scanSlow = 3;
e.g. move slit 1 pixel every 3 frames.
This is a bit basic, and I guess it wouldn't be too difficult program to allow fractions of pixel movement. Is that what you need?