Shape layer expression query.

If you make some assumptions, like the shape layers are centered (so thier shape transform anchor points and positions are [0,0], you can probably get away with something like this for the red dots shape position expression:

r = thisComp.layer("Shape Layer 1").content("Ellipse 1").content("Ellipse Path 1").size[0]/2;
w = wiggle(2,r);
if (length(w) > r){
  normalize(w)*r;
}else
  w

I would start with Dan Ebbert's Collision Detection from motionscript.com

In the example, Dan is controlling opacity, but you could use the same basic math to control the position and force another move. I'm not saying that it would be simple, but it could be done.

Here's Dan's basic code:

function getMin(a, b, c, d){
  return Math.min(Math.min(a,b),Math.min(c,d));
}

function getMax(a, b, c, d){
  return Math.max(Math.max(a,b),Math.max(c,d));
}

function getBoundingBox(theLayer){
  bb = [];
  c1 = theLayer.toWorld([0,0]);
  c2 = theLayer.toWorld([theLayer.width,0]);
  c3 = theLayer.toWorld([theLayer.width,theLayer.height]);
  c4 = theLayer.toWorld([0,theLayer.height]);
  bb[0] = getMin(c1[0],c2[0],c3[0],c4[0]);
  bb[1] = getMin(c1[1],c2[1],c3[1],c4[1]);
  bb[2] = getMax(c1[0],c2[0],c3[0],c4[0]);
  bb[3] = getMax(c1[1],c2[1],c3[1],c4[1]);
  return bb;
}

function checkLayers(){
  cUL = [];
  cLR = [];
  for ( idx = 1; idx <= thisComp.numLayers; idx++){
    if (index == idx) continue;
    L = thisComp.layer(idx);
    if (! L.active) continue;
    BB = getBoundingBox(L);
    UL = [BB[0],BB[1]];
    LR = [BB[2],BB[3]];

    if (!(myLR[1] < UL[1] || LR[1] < myUL[1] ||
          myLR[0] < UL[0] || LR[0] < myUL[0])){

      cUL[1] = (myUL[1] < UL[1]) ? UL[1] : myUL[1];
      cUL[0] = (myUL[0] < UL[0]) ? UL[0] : myUL[0];
      cLR[1] = (myLR[1] < LR[1]) ? myLR[1] : LR[1];
      cLR[0] = (myLR[0] < LR[0]) ? myLR[0] : LR[0];

      for(i = cUL[0]; i <= cLR[0]; i++){
        for (j = cUL[1]; j <= cLR[1]; j++){
          if (sampleImage(fromWorld([i,j]))[3] > 0 &&
              L.sampleImage(L.fromWorld([i,j]))[3] > 0){

            return true;
          }
        }
      }
    }
  }
  return false;
}

myBB = getBoundingBox(thisLayer);
myUL = [myBB[0],myBB[1]];
myLR = [myBB[2],myBB[3]];

if (checkLayers()){ 100 }else{ 30 }

As I said, this is controlling opacity, but it could be set to trigger a change in direction.

The other option: Newton. If your time is worth $50/hr and it takes you 4 hours to figure out the code you have more than paid for Newton.

Is it doable? Yes, just not with a "dumb" wiggle. Since there is no simple collision function in the expression engine, you'd have to check the position against the parametric definition of a circle with sinus and cosinus and then it turns into quite some math. A simplification of this might be to wiggle along the radius of the circle, but then of course the motion looks different. So there you have it. It would take soem time to sit down and craft the code. Other than that if you really just want some round blobs to bounce off a mask in 2D give good old Particle Playground a try.

Mylenium