Re: Error with gravity expression

Also, I thought I'd take this space to mention that I wrote a little expression that will take keyframed values and add an exponential decay to the end of the keyframes... This is a derivative of something Dan did on his site, so credit is due, as usual, mostly to Dan =)


n = value.length;
if(numKeys > 1){
firstKey = key(1).time;
lastKey = key(numKeys).time;
v = velocityAtTime(lastKey - thisComp.frameDuration);
a = 20;
decay = 2;
f = 1/a;
t = Math.max(time - lastKey, 0);
x = a * Math.sin(v[0] * t * f) / Math.exp(decay * t);
y = a * Math.sin(v[1] * t * f) / Math.exp(decay * t);
z = a * Math.sin(v[2] * t * f) / Math.exp(decay * t);

value + [x,y,z];
}
else value;


This doesn't work for something bouncing off a hard surface like a floor, but it works well to simulate springy motions. In the above you may want your own values for 'a' and 'decay.' The above is for a 3 dimensional vector (such as 3D position or 3D scale). For a scalar value (1 dimensional like rotation) you would use:


if(numKeys > 1){
firstKey = key(1).time;
lastKey = key(numKeys).time;
v = velocityAtTime(lastKey - thisComp.frameDuration);
a = 2;
decay = 2;
f = 1/a;
t = Math.max(time - lastKey, 0);
x = a * Math.sin(v * t * f) / Math.exp(decay * t);

value + x;
}
else value;


Darby Edelen
Lead Designer
Left Coast Digital
Santa Cruz, CA