If you define the center and the radius, something like this should work:

 

c = [thisComp.width,thisComp.height]/2; // center

r = 200; // radius

 

d = length(value,c);

if (d > r){

  v = value - c;

  a = Math.atan2(v[1],v[0]);

  c + [Math.cos(a),Math.sin(a)]*r

}else

  value

 

 

Dan

I was hoping to enlist your guidance in a way to constrain a layers position to the perimeter of a circle. So I can freely move a little square inside the circle, but when trying to drag it outside the circle it would stop at the edge. My brain is fried from the long weekend I had and the math is slipping me!