• Adobe After Effects Expressions

  Expression efficiency (sampleImage)

  Posted by Adam Cole on October 30, 2012 at 3:47 pm

  An RGB value from one layer needs to affect another layer’s Opacity and Scale. So there are two options – use the sampleImage function in both scale and opacity OR reference scale in opacity:

  Scale
  target = thisComp.layer(“Layer1”);
  value = target.sampleImage([x,y],[5,5])[0]*100;

  Opacity
  target = thisComp.layer(“Layer1”);
  target.sampleImage([x,y],[5,5])[0]*50;

  —–OR——

  Scale
  target = thisComp.layer(“Layer1”);
  value = target.sampleImage([x,y],[5,5])[0]*100;

  Opacity
  transform.scale[0]*0.5

  Is on of these options more efficient/faster/will cause me less trouble?

  Darby Edelen replied 13 years, 11 months ago 3 Members · 2 Replies
• 2 Replies

• 

  Dan Ebberts

  October 30, 2012 at 4:05 pm

  I’m not sure about performance–I suspect that sampleImage() gets run twice per frame either way, but I’d say try it both ways and find out.

  Style-wise, I like the second way because it lets you tweak the sampleImage() parameters in one place.

  Dan
• 

  Darby Edelen

  October 30, 2012 at 5:33 pm

  I agree with Dan. I’d also add that if one is going to be faster it’d be the 2nd one, but if there is a speedup it may not be noticeable.

  Reducing the sample size should add a little to any speed gains.

  Also note that sampleImage() expressions disable ‘Render Multiple Frames Simultaneously.’ For this reason I almost always convert the expression to keyframes before I kick out my final render. This can be a pain if you need to adjust the expression later, so I only recommend it once you’re happy with the results. This command is run with the properties you want to bake selected and is located here:

  Animation > Keyframe Assistant > Convert Expression to Keyframes

  Darby Edelen

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