maya, mental ray Bad color alpha: p_Hair + mip_matteshadow pass

#1

Hi,

I'm at my wits end here. I'm trying to render a geometry pass, a hair pass, and a shadow pass. Pardon my poor example. 

I think i've made some progress using mip_matteshadow from here 
[http://www.djx.com.au/blog/2008/04/08/mip_matteshadow-and-how-i-have-used-it/](http://www.djx.com.au/blog/2008/04/08/mip_matteshadow-and-how-i-have-used-it/)

I rendered one image with scanline, raytracing, and final gather and it looks like this.
[img]http://mikefudge.com/wip/hairball_beaut.jpg[/img]

When render three separate passes, I'm having trouble with a dark outline on my alpha. Here are my passes.
Background
[img]http://mikefudge.com/wip/hairball_Bg.jpg[/img]

Hair only (the black white background is there only to show the dark border alpha)
[img]http://mikefudge.com/wip/hairball_badAlpha.jpg[/img]
And shadow pass
[img]http://mikefudge.com/wip/hairballShadowPass.jpg[/img]

When comped together it looks like this, nothing special, just each image on top of each other.
[img]http://mikefudge.com/wip/hairballcomp.jpg[/img]
which is not the same as the single pass render, again here..
[img]http://mikefudge.com/wip/hairball_beaut.jpg[/img]

It would be great if these passes would work well together, but im  having better results rendering a single pass with fg and raytracing,  but it takes longer. Does anyone know why i have a black border from my  alpha? It shouldn't be coming from p_hair, I just don't know how to get  rid of it. Any help is greatly appreciated.



Thanks.
#2

what is this comped with? if AE you need to click on interpret footage and click guess for alpha, looks like its premultiplying it with black for you.

also, when I comp shadows, i generally put them under the beauty pass as a multiply.

#3

#1, turn off Scanline (select raytrace, don’t use scanline anymore)
#2, your passes should be additive and in linear colorspace. AE and linear compositing with float images is generally a pain, sorry.

Multiplying in a composite should be strictly avoided. Mathematically it will generate edges and fringe. You can try shadows as a subtractive process (similar math to add)

You’re compositing on the pixel level and since these sub-pixel samples are filtered against the background (black) you will end up with darkening that doesn’t match the beauty.

Render your shadow with the diffuse pass to resolve that easiest.

You will need: direct lighting, indirect lighting, spec, reflection, refraction, additional color (emissive) and SSS. ADD these passes. This should match the beauty mathematically. (old bug in Maya might still exist where their automatic framebuffers are goofy, but manual connections work fine)

#4

This thread has been automatically closed as it remained inactive for 12 months. If you wish to continue the discussion, please create a new thread in the appropriate forum.