• Adobe After Effects Expressions

  Single script to control multiple properties

  Posted by Touko Maksimainen on August 31, 2009 at 6:01 am

  Hello everyone.

  I’ve got a swarm of layers that need to be positioned and rotation controlled in a specific way. Now I know that I could just make a single layer, put all my expressions in relevant properties and then duplicate it as required. The real problem though is that if I have to change anything, I need to paste the new script to all layers again, which is doable but shouldn’t there be a more elegant way of doing this?

  I know that an expression can’t control any other property than the one it’s been applied to (why not?), but I think I saw an example somewhere that used external javascript functions to do this. It was something like putting “function={“myfunc”}; to the desired property and then have your script sit somewhere and have it control all properties it has been applied to. Can anyone confirm this?

  Filip Vandueren replied 16 years, 8 months ago 2 Members · 1 Reply
• 1 Reply

• 

  Filip Vandueren

  August 31, 2009 at 1:41 pm

  Why not?

  Because an expression is a calculation that is substituted for or added to the keyframes of the property it is applied to.
  Just like you can’t set a keyframe for position that changes the rotation, this applies to an expression.

  About your request, in after effects there are no shared functions or global variables, you can’t declare a function in one expression, and call it somewhere else, which is a shame.

  Workarounds are:
  storing a variable that is needed in several expressions, in a slider, or point-control
  storing a text-string in marker-comments
  storing a text-string in an (invisible) text-layer

  The nice thing about Javascript is that you can take a string, and evaluate that string as if it was actuakl source code in the expression.

  So you could have a text-layer that has as text:

  textlayer: “expressionTxt”
  
x=Math.cos(time*2+index)*500;
value+[x,0]

  (or whatever)

  and then from all your swarm layers have position-expression like this:

  
expr=thisComp.layer("expressionTxt").text.sourceText.value; // don't forget the .value at the end !
eval(expr);


  you can use this to “import” functions too. Just eval() the function first, then add the rest of your code calling that imported function.

  Amazing bonus: you can keyframe the sourcetext value of your expressionTxt layer, so you can in effect keyframe what expression is applied to all your layers.

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