Math.sin – how to vary the speed smoothly

[Koby Goldberg]

What you’ve tried is indeed incorrect for such a motion.
It’s a bit too complicated to explain.
Try this instead:

sum = 0;
dt = thisComp.frameDuration;
f = thisComp.layer("Null 1").effect("time_variable")("Slider");
for (t=inPoint; t < time; t+=dt) {
sum += f.valueAtTime(t);
}
50*Math.sin(sum)*dt

Koby.

[Dan Ebberts]

It’s complicated. The problem is that when you change the frequency, the result you get from Math.sin() is the same as if the frequency had always been the new value. You need to perform an integration. If you’re using linear keyframes, you can do it like this:

myProp = thisComp.layer(“Null 1”).effect(“time_variable”)(“Slider”);
v1 = myProp.value;
t1 = time;
accum = 0;

n = 0;
if (myProp.numKeys > 0){
n = myProp.nearestKey(time).index;
if (myProp.key(n).time > time){
n–;
}
}

if (n > 0){

t0 = myProp.key(n).time;
v0 = myProp.key(n).value;
accum += ((v0 + v1)/2)*(t1-t0);
t1 = t0;
v1 = v0;

while (n > 1){

t0 = myProp.key(n-1).time;
v0 = myProp.key(n-1).value;

accum += ((v0 + v1)/2)*(t1-t0);
t1 = t0;
v1 = v0;
n–

}

}

accum += ((myProp.valueAtTime(0) + v1)/2)*t1;

50*Math.sin(accum)

Otherwise, you’ll need to do a frame-by-frame integration which would be less code, but could bog down if your comp is long.

Dan

[Paul Binns]

Dan and Koby

thankyou so much for the effort you put into your answers.
I will try them both later today.

I figured out (while lying in bed thinking about the problem) that it had something to do with the way the sin values are being created, but there is no way that I would have been able to find a solution like what you both suggested.
I am surprised that after effects doesn’t have a more elegant way to create modulation like this. Coming from a music synthesis background this kind of thing is incredibly easy to build in a modular synthesis environment.

[Raymond Rossell]

A million thanks!! I learned now how to integrate a value using keyframes, even though i didn’t use the “trapezoid” integration because i’m using hold keyframes, so i simplified the math and used only the value at v0 to calculate the accumulator. Again, thanks. And of course, this method avoids frame-by-frame integration.

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