specific angles in canvas

[Wendy Davidson]

Hello all,
I was hoping someone could help me with my latest problem!

I have managed to use the canvas object to design an experiment in which a 
dot travels along the screen, 'disappears' then reappears following a delay. 
The subject then indicates whether the dot is following the same trajectory. 
I want to alter the trajectory by varying levels of difficulty (i.e 30% 
difference in angular trajectory to 5% difference).

So far I can only figure out how to change the x and y coordinates. Is it 
possible to convert this to angular differences?

As usual any help would be gratefully appreciated.

Wendy Davidson
University of Aberdeen