distance

[Dan Ebberts]

Sounds like you’re on the right track. Expression results don’t persist from frame to frame. If you want to know what’s happened in the past, your expression generally has to loop through its calculations from frame 0 to the current frame. You can avoid this if you can somehow express your calculation as a function of time, but that’s not always possible.

In your case, you could write a loop that would accumulate the distance traveled from one frame to the next using something like this:

accum += length(position.valueAtTime(t), position.valueAtTime(t – thisComp.frameDuration);

Tricky, but not impossible.

Dan

[Aaron Martinez]

Hello Dan,

thanks for the tip. unfortunately i still havnt gotten very far. i have pasted the expression you gave to me…

accum += length(position.valueAtTime(t), position.valueAtTime(t – thisComp.frameDuration);

into position and i received an error. i also pasted it into a text layer to test her out and i got an error. i took the position word from the expression, highlighted it and pickwipped the position of my layer and i still get an error. so my question is where do i paste this line in to get a proper result? thanks for the help…

[Dan Ebberts]

Ah, OK. That was only meant to be part of the looping expression. Here’s a more complete version to calculate rotation based on distance traveled. It assumes that the diameter is the same as the layer’s width:

accum = 0;
for (t = 0; t <= time; t += thisComp.frameDuration){ accum += length(position.valueAtTime(t), position.valueAtTime(t - thisComp.frameDuration)); } 360*accum/(Math.PI*width) Dan