Expression for Exponential Scale ?

[Crivello Laurent]

Instead of using Keyframe assistaant > Exponential Scale, I would like to use an expression on the Scale property. What would be the correect expression taking into account scale at inpoint, scale at outpoint and duration between inpoint and outpoint? (my picture is zooming out)

Thanks !

[Mylenium]

Something like this perhaps:

tStart=key(1).time;

tEnd=key(2).time;

tDiff=tEnd-tStart;

vStart=valueAtTime(tStart);

vEnd=valueAtTime(tEnd);

vDiff=vEnd-vStart;

vStart+Math.exp(tDiff)*vDiff;

Mylenium

[Crivello Laurent]

Thanks Mylenium. Sorry, I should have mentionned that current time should be taken into account to have the exponential scaling from tStart to tEnd. How would you fit the (time-inpoint) within your expression ? I tried to but was unable. Thanks !

[Rick Gerard]

If you want the animation to start at the in point of the layer:  tStart = inPoint;

If you want the animation to end 1 second before the out point of the layer: tEnd = outPoint - 1;

Check the expression I posted. 

[Crivello Laurent]

Thank you !

[Mylenium · Accepted solution]

tStart=key(1).time;
tEnd=key(2).time;
tDiff=tEnd-tStart;

vStart=valueAtTime(tStart);
vEnd=valueAtTime(tEnd);
vDiff=vEnd-vStart;

mStart=Math.exp(0);
mEnd=Math.exp(1);
mFac=linear(time,tStart,tEnd,0,1);
mMul=Math.exp(mFac);

vStart+linear(mMul,mStart,mEnd,0,1)*vDiff;

Mylenium

[Dan Ebberts]

Something like this maybe:

t1 = inPoint;
t2 = outPoint - thisComp.frameDuration;
v1 = valueAtTime(t1);
v2 = valueAtTime(t2);
if (time < t1){
  v1;
}else if (time > t2){
  v2;
}else{
  d = t2 - t1;
  dv = v2 - v1;
  t = (time - t1)/d;
  v1 + dv*Math.exp(10*(t-1));
}

[Rick Gerard]

If you want to work without keyframes, try this version of Mylenium's expression:

tStart=inPoint;
tEnd= inPoint + 2;// move time in seconds
tDiff=tEnd-tStart;
vStart=[10, 10];// starting Scale
vEnd=[70, 70];// ending Scale
vDiff=vEnd-vStart;
mStart=Math.exp(0);
mEnd=Math.exp(1);
mFac=linear(time,tStart,tEnd,0,1);
mMul=Math.exp(mFac);
vStart+linear(mMul,mStart,mEnd,0,1)*vDiff;

 The math does not match applying the Keyframe Assistants' Exponential Scale perfectly, but it looks pretty good.

Change line two to tEnd = outPoint, and the animation will last as long as the layer.