Scale expression based on rotation

[Walter Soyka]

This is a deceptively complicated problem.

I borrowed some code from a typically brilliant Dan Ebberts’s post [link], and modified it a bit for your request:

// get the world position of three points;
// a = a point above the center of the clock, representing 12 o'clock
// b = the center of the clock
// c = this layer's anchor point
a = thisComp.layer("PARENT LAYER").toWorld(thisComp.layer("PARENT LAYER").anchorPoint) - [0,1];
b = thisComp.layer("PARENT LAYER").toWorld(thisComp.layer("PARENT LAYER").anchorPoint);
c = thisLayer.toWorld(thisLayer.anchorPoint);

// figure out the angle between vectors ab and bc
// this is the relative rotation between this layer and the center
// in other words, its location on the clock, represented in degrees
v1 = normalize(a-b);
v2 = normalize(c-b);
angle = radiansToDegrees(Math.acos(dot(v1,v2)));
if (cross(v1,v2)[2] < 0) angle = 360 - angle;

// adjust the scale according to the angle
angle = angle % 360;
if (angle < 180) s = linear(angle, 0, 180, 100, 85) else s = linear (angle, 180, 360, 85, 100);
[s,s]

Alt+click the Scale stopwatch for each layer you’d like to be affected, and paste that code in. You’ll need to either rename your rotation-controlling layer to “PARENT LAYER” or replace the references to “PARENT LAYER” in the code above with the name of your own layer.

Walter Soyka
Designer & Mad Scientist at Keen Live [link]
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
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[Darby Edelen]

Assuming that the layers are parented to the center rotator I think you could use something like this expression on the scale:

center = parent.toWorld(parent.anchorPoint);
p = toWorld(anchorPoint);
v = p - center;
linear(normalize(v)[1], -1, 1, value, 0.85 * value)

Darby Edelen