• Adobe After Effects

  Add alpha channel to AE6.5

  Posted by Simon Burley on August 3, 2009 at 9:57 am

  Hi everyone,
  I only use AE once every few months, and every time I do it feels like I have to start from scratch…

  I need to add a simple luma key but pass through RGB untouched, so I can write an RGBA file for external processing.

  Performing the luma key isn’t the problem, my issue is that AE is then keying the image. I’ve tried using Set Channels to take RGB from a copy of the image, and whatever I try (solid backgrounds with alpha levels set to 0) AE is always keying the output image.

  Please tell me what I’m doing wrong!

  Simon

  Curious Turtle replied 17 years ago 2 Members · 6 Replies
• 6 Replies

• 

  Curious Turtle

  August 3, 2009 at 2:02 pm

  What is it you’re trying to achieve? I get you need an alpha channel, but for what purpose? As you seem a bit peturbed that the luma key is keying your footage :o)

  I *think* what you want is to have a file that contains unadulterated RGB channels and an alpha channel on top. This is no problem. Render out to Quicktime Animation with a Straight alpha and you’ve got it all! You can quickly check this by playing around with the Visual Settings in Quicktime Pro.

  Hope that helps,
  Ben

  Curious Turtle Pro Video
  Training | Editing | Support
• 

  Simon Burley

  August 3, 2009 at 3:02 pm

  I want to pass RGB unadulterated, with the luma key on the alpha channel.
  We have an external program that requires 3 channel images with alpha to guide it’s processing, which in this case will be shadow detail only, hence the luma key.

  I could write a plugin (or a standalone program) to do this, but that’s really overkill. My understand of the Set Channels effect is probably at fault.
• 

  Curious Turtle

  August 3, 2009 at 5:52 pm

  The method given in my previous post will give you that. Rendering out Straight instead of Premultiplied maintains your RGB channels. Using Animation codec allows you to also encode the alpha into the same file.

  Cheers,
  Ben

  Curious Turtle Pro Video
  Training | Editing | Support
• 

  Simon Burley

  August 5, 2009 at 5:56 pm

  Thanks Ben.

  We’re still not quite there – the luma key cuts holes in the layer (obviously) allowing the background of the comp to be seen through the holes. So, I put another copy of the image behind the one I’m luma keying, but then the alpha of the image behind comes through the holes of the keyed image resulting in a useless solid alpha channel.

  I thought I could solve this by using the Set Channels effect but seemingly not.

  I’m sure there’s a way to do this easily but I can’t figure it out.
• 

  Simon Burley

  August 6, 2009 at 6:48 am

  Sorry Dave, I tried my best to get my need across in my first message…

  I want an RGBA file, with the luma key on the alpha channel, and the RGB passed through from the input file to output files untouched.

  Performing the key isn’t the issue, my issue is in working out how to pass RGB through without alteration. The luma keyer (rightly) wants to key the layer to which it is applied. So I tried putting another copy of the input files below the keyed layer. This doesn’t work, and produces a solid alpha on the output.

  So, how to do it?
• 

  Curious Turtle

  August 6, 2009 at 6:12 pm

  Key your footage with a luma key. Have only that keyed layer in the comp.

  Export to a Quicktime file with the Animation codec with a RGB+Alpha. Set your Alpha to Straight. That’s it.

  Cheers,
  Ben

  Curious Turtle Pro Video
  Training | Editing | Support

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