• Adobe After Effects

  Exporting Alpha Channel ONLY

  Posted by Ryan Risley on March 16, 2009 at 7:44 pm

  Hi,

  I have recently shot and keyed some people to be used in a CG environment created by a team of animators. They have requested that I provide footage with both a keyed, color clip and a clip that is only the alpha (black/white/gray).

  Viewing the Alpha channel is easy enough by clicking the Show Channel button and viewing alpha only. But when I render in this view I still get RGB and the alpha.

  The best work around I have found so far is to re-import the exported footage with alpha, then apply Effect>Channel>Alpha Levels. I changed Output Black Level from 0 to 255 and Output White Level from 255 to 0. This effectively “reversed” the alpha (keyed area is solid black and the keyed person is now gone leaving an empty silhouette). I then placed a white solid underneath the re-imported footage layer and I get what looks to be an alpha channel only composition (person is all white and background is all black).

  This seems to be a cumbersome work flow and I’m not 100% this will work (testing now). I think it should work in theory, since there isn’t any partial transparency needed (like a feathered edge)and the hard outlines are very sharp. But can anyone recommend a better method?

  Also, not exactly sure if this (the alpha only channel) should be exported Premult or Straight. My keyed footage looks great and very sharp and i obviously exported it Straight, RGB + Alpha, millions +, etc. But since this RGB only (just a black and white image), I wasn’t sure what to use. AE suggests using Premult when not rendering with an alpha channel included.

  Sorry about the novel – just trying to be thorough 🙂

  Any insight on this is greatly appreciated!!!!

  Sincerely,
  Ryan Risley

  Provis Media Group
  http://www.provismedia.com

  Ryan Risley replied 17 years, 2 months ago 27,225 Members · 1 Reply
• 1 Reply

• 

  Ryan Risley

  March 16, 2009 at 8:59 pm

  Hi Dave,

  Thanks for your reply. Just to clarify, I always use the Render Que and i never do File > Export. I’m not a total noob 🙂

  I somehow missed the “ALPHA” setting inbetween RGB and RGB + ALPHA under the channels selection, but I have now found it thanks to your help and a few more cups of coffee.

  Ryan

  Provis Media Group
  http://www.provismedia.com

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