• Adobe After Effects Expressions

  set initial x, y, and z with expression – subsequent frames hold frame 1 values

  Posted by David Ford on June 8, 2006 at 2:46 pm

  It seems afx re-runs expressions every frame without remembering previous frame’s variables values. How do you work around this?

  I would like to re-use the initial variables (x, y and z) set at frame 1, so that the position is not changing in subsequent frames. Code below.

  [code]
  if(time == 0){
  x = Math.floor(random(thisComp.width));
  y = Math.floor(random(thisComp.height));
  z = Math.floor(random(-300, 300));
  [x,y,z];
  } else{
  // of course this produces errors
  [x,y,z];
  }
  [/code]

  I’ve seen similar problems in other posts here, but can’t seem to work those into this scenario. If so, could you please send link?
  Thanks

  Mylenium replied 20 years, 3 months ago 3 Members · 5 Replies
• 5 Replies

• 

  Dan Ebberts

  June 8, 2006 at 3:13 pm

  Expressions execute at each frame and there is no way to pass data from one frame to the next. The work-arounds depend on what it is that you’re trying to do.

  If all you want to do is generate a static random position, you could do it this way:

  seedRandom(index,true);
  x = Math.floor(random(thisComp.width));
  y = Math.floor(random(thisComp.height));
  z = Math.floor(random(-300, 300));
  [x,y,z];

  Dan
• 

  David Ford

  June 8, 2006 at 3:31 pm

  Hi Dan,

  Thanks for the reply and the code!

  Guess you’ve said this a million times by now. I noticed in other forums you’ve answered this same question. Thanks for your patience!

  For some reason the code below causes the layer text to disappear after frame 1. Am I missing another step in using this correctly?
  Comp set up – 640×480 10 seconds

  seedRandom(index,true);
  x = Math.floor(random(thisComp.width));
  y = Math.floor(random(thisComp.height));
  z = Math.floor(random(-300, 300));
  [x,y,z];
• 

  Dan Ebberts

  June 8, 2006 at 3:37 pm

  I can’t repro the problem here. You have a text layer and you’ve applied the expression to the layer’s Position property, is that correct?

  Dan
• 

  David Ford

  June 8, 2006 at 3:43 pm

  Dan, thank you so much! I had another expression that was removing the text value. Works like a charm!

  I guess I would like to understand more about wha the seedRandom() method does. Is there any information on this anywhere? The help files are pretty sparse.
• 

  Mylenium

  June 8, 2006 at 6:00 pm

  [David Ford] “I guess I would like to understand more about wha the seedRandom() method does. Is there any information on this anywhere? The help files are pretty sparse.”

  seedRandom() assigns a continuous – erm – seed to random functions. Randomness in computers is not really random, it’s based on formulas. For instance a common method is Perlin Noise which itself is just a set of overlapping sinus/ cosinus functions with random phase and periods. If you calculate a time dependent sinus with Math.sin(time), your result is a wave. Multiple such overlapping functions with different multipliers (e.g. time*2) create pseudo-random peaks and grooves. By fixing the seed in random functions, you more or less freeze time – with the same input, the result is always the same.

  Mylenium

  [Pour Myl

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