How to use alpha channel?

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  03 March 2011
How to use alpha channel?

Hi there guys!

So I have this targa image with two characters and a background, with RGB and alpha channels (the alpha is the characters). How do I use it's alpha to isolate the characters from the background? I've tried alot of things but I'm kinda new to Nuke, so I had no success!

  03 March 2011
Use a premult node.
  03 March 2011
Thank you for your help cgbeige, it worked

And since I'm on the subject, is there anyway to smooth the edges? I have some little contours around the characters, I suspect it's the background kind of "leaking" into them. In After Effects I know I could smooth or feather the edges. Is there anyway of doing this in Nuke?

Thank you again for your time.
  03 March 2011
You can use one of the erode filters.
  03 March 2011
Personally I would use a shuffle node, set this one to alpha, and then connect this via a copy node into the tree main comp tree. this way you have more control over the alpaha channel, you can add edge detect and so on on only the alpha channel.
  03 March 2011
how's that different from just erode and premult after that? Unless you have a bunch of merges in there, where you'd need to unpremult first, it's overkill for such a simple scene. The erode works on the alpha only by default.
  03 March 2011
yes, makes sense.
It's just the way I'm used to work, but most of the times we do indeed work with quite some passes.
Just said what I would do.
  04 April 2011
Thank you for your replies, you have helped me greatly.
  04 April 2011
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