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Mitch

Dark Outline Around Objects When Rendering Alpha Channel

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  • I'm getting a small dark line around my objects when I render out the alpha channel. I'm rendering out png sequences. I do separate renders for the reflection, shadow and objects. I then combine them in AE. I am also using ambient occlusion. The line shouldn't be there and its making my renders look cartoonish.

    post-89758-0-10208500-1345699756_thumb.j

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    Guest dataflow

    a guess would be its a reflection issue.

    try lowering the phong tag angle to about 14 or 15

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    Looks like a premultiplication issue. How are you interpreting the footage and setting the mask(s) in AE?

    Mark

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    Also, if you are using multipasses to composite the final shot, Multiply is not your friend.

    Another reason why RGBA with shadows pass would be a life saver.

    Unpremult - premult might indeed be your problem.

    The solution I've found is using Nuke's blending operation called disjoint-over (A+B(1-a)/ b, A+B if a+b<1) and here's what the user guide says about it:

    Similar to the over operation, except that if a pixel is par- tially covered by both a and b, disjoint-over assumes the two objects do not overlap. For instance, two polygons that touch and share an edge. Nor- mal over will produce a slightly transparent seam here.

    This can be useful if you want to merge element a over ele- ment b, and element a has element b already held out. For example, you may have a CG character whose hair, skin, and clothing are rendered separately so that each object has the other objects held out of the render.

    In this case, using the over operation would produce dark lines around the comped objects. This is because over does a hold-out of the background image, meaning the back- ground will be held out twice.

    For those examples you've shown, I would try to get away with the beauty pass, and adding your shadows under with the Multiply option and adding a Light Wrap to make your CG comp better over the footage. There's many ways to create OK lightwraps, and maltaannon also has a few tricks for this http://maltaannon.com/articles/after-effects/ce-lightwrap/

    EDIT:

    Just to clarify, Nuke's OVER operation in AE means basically reading the alpha information and slapping the layer over your footage.

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  • Hey thanks so much guys! Happyrender: thanks for the info. I'm checking out the nuke's stuff now. Hula boy: when I interpreted the footage as Pre multiplied it got rid of almost all the out lines! Thanks for pointing me in that direction. I love this site

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