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  1. #1
    Join Date
    Apr 2012

    Default use inverse of the red channel as alpha HELLLLP!!!

    hi all!,

    this sounds really simple, but i cant get it to work without there being a white rim around my character.

    basically i need to use the inverse of the red channel as the alpha channel of my image. So i have used a shuffle node to make the red channel the alpha, then and invert node, then piped the result into the mask channel of a merge node (with the input being my original image.

    However the invert node causes the black space outside my image to become white. and then the very edge of the red channel mask then leaves a sliver of white rim (from the inverted alpha) in my resulting mask. Does anyone have a way of not getting the white rim??

    thanks, Sam

  2. #2


    I am confused by your description. Could you post some screenshots of your node tree and explain more clearly?

    Is the Merge node you mention merging your A and B inputs together, and then you are trying to mask that merge using the inverted alpha you created? Can you explain this more clearly?

    I\'m not sure whether the source of this white rim you mention is from your elements being composited incorrectly, or from something else. It would be hard to tell without a screenshot or a more complete description.

  3. #3
    Join Date
    Apr 2012


    Im just wondering; is your source material 8 bit? or higher? you may have values in your red channel that go above 1, so your alpha will be going above 1 or below 0 too... this could be causing the rim you are talking about, but as jedypod has said; it would be useful to see the tree and the effect.

    for now though just mouse over your alpha and see if the values are going above 1 or below 0.


  4. #4
    Join Date
    Feb 2011


    It would help seeing a screen shot, and I\'m assuming this is Nuke from the nodes you have listed...unless I have missed something. Make sure you are premult (ing) your alphas right and no other alpha channels are being passed into the the alpha stream.
    I would need a screenshot to understand this further.
    Hope it works out.

  5. #5
    Join Date
    May 2012


    Sounds like you need to unpremultiply the alpha mask by the object matte before you invert it. This is a common mistake in compositing. In nuke, here\'s a solution that takes 4 nodes:

    1. shuffle the alpha of your object into your matte
    2. unpremult
    3. shuffle the alpha to the red channel
    4. invert

    Let me know if you have any questions.

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