Rotation Expression

[Demorae5C2F]

I saw a comment on the thread I have linked below, I will also attach a couple screenshots of the comment which describes the expression I need. I want to know how to make the rotation property increase every time the value of both channels goes up instead of returning to the original position when the audio levels drop.

https://community.adobe.com/t5/after-effects/how-can-i-associate-an-expression-to-move-a-rotating-wh...

[Dan Ebberts]

Your question could be interpreted more than one way, and it's not clear which one you're after, but I think this is one possible solution:

a = thisComp.layer("Audio Amplitude").effect("Both Channels")("Slider");
mult = 1.0;
f = timeToFrames(time);
accum = 0;
prev = 0;
for (i = 0; i <= f; i++){
  next = a.valueAtTime(framesToTime(i));
  if (next > prev){
    accum += (next - prev)*mult;
  }
  prev = next;
}
accum

[Demorae5C2F]

Hi Dan thanks for your response. The effect I am aiming to achieve is to have the rotation of the subject increase smoothly as the volume of the audio amplitude/audio keyframes increases and as the volume decreases I'd like the rotation to slow down while still rotating in the same direction (clockwise). I tried to use the expression you gave me however thte motion is really jagged and not smooth. Hope that made sense, if possible it would be great if we could have a zoom call so I can share my screen with you but if not please provide the appropriate expression.

Many Thanks!

[Dan Ebberts]

The key is coming up with the algorithm that defines the relationship between the audio that has occured in the past and the current rotation value. Maybe you could provide a sketch of what that would look like?

[Demorae5C2F]

Hi Dan,

I have made a video detailing what I want hope this helps. rotation expression explaination 

[Dan Ebberts · Accepted solution]

You could try playing around with this, but it'll get pretty slow if your comp is long:

a = thisComp.layer("Audio Amplitude").effect("Both Channels")("Slider");
constSpeed = 10;
mult = 10;
n = a.numKeys;
if (n > 0 && a.key(1).time < time){
  accum = a.key(1).value*(a.key(1).time - inPoint);
  for (i = 2; i <= n; i++){
    if (a.key(i).time > time) break;
    k1 = a.key(i-1);
    k2 = a.key(i);
    accum += (k1.value + k2.value)*(k2.time - k1.time)/2;
  }
  accum += (a.value + a.key(i-1).value)*(time - a.key(i-1).time)/2;
}else{
  accum = a.value*(time - inPoint);
}
value + accum*mult + constSpeed*(time-inPoint)

Adjust constSpeed to control how fast it rotates with low/no audio and adjust mult to control how much it responds to loud audio.

[Demorae5C2F]

Thank you this did the trick! Are there any other values within this expression I can manipulate and play with?

[Dan Ebberts]

No, not really.

[Demorae5C2F]

I'm just asking because as I adjust the const speed lower and the mult, the difference between slow and fast rotation isn't drastic enough. const speed has little effect on the because the mult also increases the minimum rotation so much.