• Adobe After Effects Expressions

  Getting layers real bounds using sampleImage()

  Posted by Arthur Yashin on January 28, 2016 at 8:05 pm

  Hi, forum! I am trying to write an expression that will return a position of the pixel that is furthest to the left side (as an example) and alpha of its color is not equal to zero.

  How I see the algorithm:
  We have a “point” that is located at [0, 0]. We “move” that point down until it hits thisLayer.height ( [0, 1080] for example) and then we add +1 to x value and change y value to 0, so it becomes [1, 0] and so on until the “point” meets the first pixel that has non-zero alpha.

  I suggest this to apply on a point control:
  x=[0, 0];

  do {

  if (x[1]>thisLayer.height) {
  x[0]=x[0]+1
  x[1]=0 }
  else x[1]++
  }
  while (thisLayer.sampleImage(x, [0.5, 0.5]/2, true, time)[3] == 0);

  x
  but it gives error – “Timeout while waiting for the engine”.

  So is it possible to make it work?

  Arthur Yashin replied 10 years, 7 months ago 2 Members · 2 Replies
• 2 Replies

• 

  Xavier Gomez

  January 29, 2016 at 1:07 am

  If you are on a Text layer or Shape layer, you can use the sourceRectAtTime method, which is way easier to manipulate and also faster.

  For a layer with source (like a solid with a mask etc), to find the left most pixel, you should scan the layer with a verical band that is as high as the layer (ie radius[1] = layer.height). Otherwise with a radius of 0.5 you will need ages to finish.

  Method 1 is to roll that a thin vertical band by increments of 1 until you hit something.

  Method 2 is to start with a sample as big as the layer (so it is not really vertical).
  If you hit, then:

  step0 : you divide the width (radius x) by 2,
  step1 : sample the left part of the image.
  step2 : if you hit, go to step 0 again, with left part of the image,
  else go to step 2, but with the right part.
  And so forth until the radius x becomes smaller than 0.5 (or more if you dont need pixel accuracy);
• 

  Arthur Yashin

  January 29, 2016 at 1:11 pm

  Thank you very much! Problem is solved

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