• Adobe After Effects Expressions

  Rotational velocity scripting woes

  Posted by Ben Jaffe on March 5, 2008 at 8:05 pm

  Hi! I’m trying to get the rotational velocity of an object that is auto-orienting to a path. The object is a top-down view of a bicycle, and I’m trying to link the front wheel rotation to the bike’s rotation. (Once I get the rotation worked out, I’ll work the speed of the bike into the equation too.) I can’t get the bold section to work. I don’t know if I’m putting it in the wrong place, or if I need different syntax for this situation. The original code that I’m modifying gets the rotation of an object that is auto-oriented. Any help anyone can give would be very VERY appreciated.

  Mainly, I’m having the issue with getting the value of the object’s rotation a frame before, so I can determine how much the front wheel of the bike should be turned. Thanks so much, and here’s my code.

  L = comp(“Final Animation”).layer(“cyclist1”);
  u = L.toWorldVec([1,0]);
  uOld = L.toWorldVec([1,0]).valueAtTime(time-1); // <==here's where I'm getting my error r1 = radiansToDegrees(Math.atan2(u[1],u[0])); r2 = radiansToDegrees(Math.atan2(uOld[1],uOld[0])); r1 - r2

  Ben Jaffe replied 18 years, 2 months ago 3 Members · 3 Replies
• 3 Replies

• 

  Filip Vandueren

  March 5, 2008 at 9:27 pm

  Hi Ben,

  valueAtTime only works when applied to a property, you can’t append it to a mathod or function like smooth, ease, or any of the layer Transforms.

  Lucky enough, most functions have a time-factor as one of their parameters.

  In your case this should work:
  uOld = L.toWorldVec([1,0],(time-1));
• 

  Dan Ebberts

  March 5, 2008 at 9:41 pm

  Yup, and if you want the value from one frame ago you’ll want something like this:

  uOld = L.toWorldVec([1,0],time – thisComp.frameDuration);

  I’m curious to see how you’re going to use the result of this expression. Please let us know how it turns out.

  Dan
• 

  Ben Jaffe

  March 5, 2008 at 10:21 pm

  That worked perfectly! Thank you so much!

  By the way, to get the speed, I’m using
  comp(“Final Animation”).layer(“cyclist1”).transform.position.speed

  and it works! I’ll post the final formula I use for the bike wheel when I finish it.

  Thanks again for the help!

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