• Adobe After Effects

  Using a sum of the Alpha Channel of all frames as a Mask

  Posted by Diego Mellado on March 11, 2014 at 4:31 pm

  Hello everybody!
  I am looking for a fast way finding out where is “something” in an animation using its alpha channel, after taking out a chroma.
  If I am not wrong, if I add (in a logic way of thinking) all the alpha channels in all frames (this means: for a pixel to be consider as transparent, that pixels has to be transparent in all the frames of the animation) I can use that as a way to find where there is animation and where not.
  Is there any way of doing this without or with expressions?

  Thanks everybody for your help!

  Best,
  Diego

  Diego Mellado replied 12 years, 4 months ago 3 Members · 3 Replies
• 3 Replies

• 

  Kevin Camp

  March 11, 2014 at 4:53 pm

  this may do what you need…

  create a new solid that is a color that will stand out against the background color of the comp — white may be just fine.

  make sure it is at the top of the comp, make sure ‘modes’ is visible in the time line and then click the checkbox under the ‘T’ mode. this will make that solid take on the transparency, or alpha, of all the layers bellow it, essentially adding all the alphas.

  you might also try switching the preview mode from rgb to rgb straight or alpha. to do this, at the bottom of the preview window, simply click the ‘show channels’ option (looks like 3 rgb circles) and choose ‘alpha’ or ‘rgb straight’

  Kevin Camp
  Art Director
  KCPQ, KZJO & KRCW
• 

  Darby Edelen

  March 11, 2014 at 5:20 pm

  I’d first shuffle the Alpha into the RGB channels using a Channel > Shift Channels effect with these values:

  Take Alpha From: Full On
  Take Red From: Alpha
  Take Green From: Alpha
  Take Blue From: Alpha

  Then I’d apply the Time > Echo effect with these expressions:

  Echo Time (seconds):

  framesToTime(1);

  Number of Echoes:

  timeToFrames(outPoint - inPoint);

  Echo Operator: Maximum

  The image on the first frame of the layer will be black wherever there was no alpha value throughout the entire layer’s animation.

  Note that this only works for the non-transform animations on a footage layer. If the position of the layer is animated then you’ll need to pre-compose before applying these effects or apply them to an Adjustment Layer.

  Darby Edelen
• 

  Diego Mellado

  March 13, 2014 at 7:59 pm

  Hi everybody!
  First of all, thanks for your ideas and sorry for taking so long in answering.
  Echo solution seems quite good, a bit resource consuming though but it works for what I needed so that’s a solution 🙂
  Thanks a lot again!
  Best,
  Diego

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