• Adobe After Effects Expressions

  Incredibly slow looping sampleimage expression

  Posted by Paul Roper on February 8, 2016 at 5:36 pm

  Hello,
  I’m trying to determine the height of a wave (a shape layer with just a path and a stroke) at various points along the x-axis. I’ve made (adapted) an expression to do a sampleImage starting at y=0, moving down the comp a pixel at a time, until it finds a pixel whose alpha is greater than 0.

  But it’s unbelievably slow to evaluate, sometimes just timing out. Any suggestions why this is happening? The “for” loop should only be looping a few hundred times to find the pixel. the wave does not change shape, so the expression only needs to evaluate once.

  x=900;

  L=thisComp.layer("audioWave");

  for (i = 0; i <= 1080; i++){
  temp = L.sampleImage([x,i],[0.5,0.5],true,1);
  if (temp[3] > 0) break;
  }

  y=i;

  [x,y]

  Paul Roper replied 10 years, 3 months ago 1 Member · 1 Reply
• 1 Reply

• 

  Paul Roper

  February 8, 2016 at 5:41 pm

  Hmmmm….I seem to have fixed it, by making it evaluate every second pixel. I changed i++ to i=i+2 and now I can render a preview faster than realtime! Weird. I’d predict that doing this would make it evaluate twice as fast, not hundreds of times faster!

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