CGTalk - Shadow catcher alpha a.o

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dirkusrhombas

03-20-2004, 08:32 PM

Short version ... i want to render the shadows as an alpha without any geometry showing when I take it into photoshop!! .... i just want my shadows to be there ( and ambient occlusion ) its ok if they have to be done separately.

Main issue is that I have a 3d inteior scene - want to render a ambient occlusion pass and then comp over the top of the beauty pass then add spec and so on .... blah blah blah
Example
Ie at the moment when I render out a alpha of this scene,the geometry gets rendered in the alpha. I want to be able to render in passes for spec, diffuse, colour, A.O ect and have only the shadow or the spec visible in my alpha or just the colour and so forth and have everything else dissapear ... is that actually possible?.
I dont get it

Mental ray------------]
dirtmap----------------] I am usingthese
use background-----]

Dirtmap is working perfectly ... and so is use background but the geometry is still being rendered

Spent all day trying to figure this one out searched this forum
and checked the docs but I am finding it hard as nails.. :hmm:

lazzhar

03-20-2004, 09:40 PM

I'm understanding exactly what are you after, but if in general, passes are just a rendering of a modified version of the scene.

Let's say you want to add an AO pass over your regular rendering with lights, so you just create a new Surface Material and connect a dirtmap shader into the outColor then assign this Material to each geometry in the scene. then you'd get a nice AO pass that contains only black and white colors. Then in Photoshop you bring up this pass over the first one using Multiply mode and you're going to be happy after some tweaking.

Hope this helps.

dirkusrhombas

03-20-2004, 11:19 PM

What I am interested in is alpha channels .tga or .tiff perhaps.
For example jozvex uses this method Jozvex use background shader for final gather (http://www.cgtalk.com/showthread.php?s=&threadid=118924)
You know when you take that kind of an image into photoshop and
all the black areas are transparent ... that is what I want for the following.

1] Colour (Diffuse)
2] Bump
3] Specular
4] Shadows final gather (Thanks jozvex)
5] Shadows normal (Background shader works fine)
6] Reflectivity
7] Ambient occlusion (Multiply in photoshop works quite well thanks for that tip) I would prefer a alpha channell though.

So the idea is to have full controll over all the render layers .... there must be workflows for this already.
Edit
I came across a few of your renders while I was researching this (they were very nice) I dont doubt that the multiply method is a good one ... I just would like to think there was a better way to controll the different render elements more accuratly. (ie alpha channell seems the most logical)
When I render a cool reflection I want to be able to have full controll of just the reflection.

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