• Adobe After Effects

  Extract Alpha: Pre-Rendered Keys

  Posted by Francois Driessen on May 19, 2009 at 3:15 pm

  Hi there,
  Want to apply a LightWrap Plugin on Pre-Rendered Keyed footage. In order for this workflow to work, I need to be able to access the alpha channel of the PreRendered footage. What’s the best way to extract the alpha channel?
  (Duping the footage and applying layer TrkMat Alpha does not work for this workflow)
  thnx

  Francois

  >- e y e f l a m e .tv –< https://eyeflame.tv

  Francois Driessen replied 16 years, 11 months ago 2 Members · 5 Replies
• 5 Replies

• 

  Francois Driessen

  May 19, 2009 at 3:36 pm

  PreRenders from keys pulled in AE. Animation MoC+. PreMultiplied Embedded Alpha
• 

  Francois Driessen

  May 19, 2009 at 4:06 pm

  Yep. Tried that. But pulling a TrkMat with a dupe layer does not work.
  Coming full circle in thinking about it, it means I’m stuck with a limitation on the Plugin that can’t assign layers as alpha and only grabs the alpha created by masks & keyer on a layer (regardless of premultiplied transparency). So even if there was an Extract Alpha (Like Extract Z in 3D Channel Extract) it would not have made a difference since it involves more than one layer interacting.

  Thanks for bouncing thoughts

  So what I’d really need to do is get the premul alpha re-applied within the layer itself. Any tips among the herd for this one?

  .

  Francois

  >- e y e f l a m e .tv –< https://eyeflame.tv
• 

  Francois Driessen

  May 19, 2009 at 4:30 pm

  Hi, D

  Hehe. The Red Giant kit is exactly what I’m trying to use in this situation.
  KeyCorrect Pro : LightWrap
  Problem is its not set up for working with pre-mul footage, and we pulled keys already &pre rendered as part of a needed workflow to work with a complex 3D comp with multiple large keyed layers.

  thnx. aware of the”spill” article

  Francois

  >- e y e f l a m e .tv –< https://eyeflame.tv
• 

  Philippe Lessard

  May 19, 2009 at 6:38 pm

  At my job, light wrap from key correct pro work with premultiplied footage, check the footage interpretation.If not working, try to duplicate then use as track matte then precompose then aplly light wrap!
• 

  Francois Driessen

  May 20, 2009 at 3:01 pm

  Aaah. SOLVED.

  You are correct. It is in fact respecting the Alpha. What it does not do is scale the target footage and BackGround layer to match each other in final comp. To explain: I’m working with 1080×1920 (yes, portrait format) keyed footage comped onto a 1280×720 BG. Only the overlap of 1080×720 renders the LightWrap effect. And that was happening off-screen for the 3D cam comp which sent me on an egg-hunt.

  Pre-comping again. Sad. Since in this case that actually knocks down quality quite visibly.

  Francois

  >- e y e f l a m e .tv –< https://eyeflame.tv

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