View Full Version : Problem Compositing...

01 January 2009, 02:25 AM
Hey all... I'm don't know a whole bunch about compositing; I'm mainly into 3D (Maya).
I have rendered parts of my 3D character into many different layers:

The image HERE ( shows that I'm getting some unusual light that I don't know how to get
rid of on the character's lower torso...

I'm using a diffuse layer and 3 different skin layers (Epidermis, SubDermis, and BackScatter)
-which all have this light issuse, on the 'skin' portion of his body...

Any help? Thanks in Advance!
~Andrew R. Shelton Sr.

01 January 2009, 08:47 PM
Without knowing what compositor you use, You should be able to place the pants layer lower in the stack or order, to where the skin would cover the white outline. You could also try different layer blending modes ( multiply, add, screen, etc) and see which one knocks out that white edge.
In your keying controls you should find something that allows you choke the edges (very commonly used to choke that last little bit of green screen spill). Maybe called "edge matte" control. I use Combustion, so I'm not sure about After Effects or others. After Effects uses "Keylight" as one of it's keyers. There should be a slider for edge control

01 January 2009, 11:05 PM
unpremultiply all your passes,
'add' them together..
then premultiply the result.

If there is still an edge, the shaders do not handle transparency and holdouts properly when rendered separately will have to just dial the result in maya and just render one pass. That is a 'cleaner' more professional way to do it anyways

01 January 2009, 06:54 PM
[QUOTE=frizDog]unpremultiply all your passes,
'add' them together..
then premultiply the result.

exactly. This is a pre-multiplication problem. The last thing you want to do is mess around with edge matte controls, that's just wrong.

01 January 2009, 08:51 PM
Thanks for the correction...I don't remember having that sort of problem. Was just suggesting some things to try.

01 January 2009, 09:00 PM
It's actually much simpler than all that...
image matte needs to be set to "premultiplied", also, the alpha shade used in the image was a dark gray instead of solid black. That's why it didn't work with the default settings... After picking the correct color to premultiply in After Effects, it works like a champ.
Just needed to sample the correct color from the alpha in the unmatted image.

01 January 2009, 06:04 AM
Definitely check into pre-multiplication . . . it looks like you are getting edges around everything. As above, you shouldn't need to do edge treatments (choking, dilate/erode) on mattes generated in Maya. By definition they should be perfect.

It might not be the case here, but I have also had issues in the past with different renders dealing with anti-aliasing slightly differently, giving bad edges for mattes. (eg, using in 3ds Max, using Brazil for skin shaders on characters and V-Ray for sets).

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01 January 2009, 06:04 AM
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