View Full Version : Render passes in separate r g b channels for post

04 April 2009, 03:58 AM
i have seen in a company rendering passes in separate r g b channels and then compositing them back in fusion. It gives complete control for color correction. Can anyone tell me the procedure how to render those passes from maya and composite them...?

04 April 2009, 06:41 AM
I imagine you mean passes for light, in which case you can just use red, green and blue lights and a diffuse color pass. Otherwise, you need to add the different maps (using of course only r, g or b) you need (ambient occlusion, Z, etc.) in the shader. My maya knowledge is pretty rusty, but I'm quite sure there's a node in HyperShade for blending two maps additively.

04 April 2009, 10:32 PM
hmm thats quite interesting
am not sure about this but i guess u can extract pure shadow occlusion and specular passes by rendering out the r g b passes in comp
if anyone knows about it please let me know

05 May 2009, 09:51 AM
Some places do this, setting one light to pure red, green and blue. You then multiply by a colour in 2d.

If you go down this route do realise that you will get bad edges from doubling up anti-aliasing and motion blur. This is because you are multiplying the passes together.

Also any sort of semi-opaque and fine objects will not work - a good example is fur. This is because you need to premultiply the light colour with the surface opacity. In 2d you only have the alpha channel - which is not the same.

Also for anything which isn't an intensity or colour change - ie moving lights, or changing surface roughness, you will need to re-render and reload all the passes in your composit. Multiple channel exr files make this simpler.

Also your compositing script should mirror your shader materials. It can be quite a lot of work keeping these in sync.

Rendering different passes for each light is useful, but personally I don't see much benefit splitting things down further.

05 May 2009, 11:16 AM
thanks for ur quick reply, another quetion is
u know any seperate pass i can render from maya to grade my shadows?
like for example breaking the shadow into 3 tones according to how further it is from the object?
dark tones or bounced light from the object,midtones and less daker tones?
i have been looking all over and cant find it
am thinking along the lines of a normal map but for shadow....

05 May 2009, 03:45 PM
Some companies use R,G,B channels to hold separate passes, but only for masks and mattes that otherwise could be independent, such as if you are rendering mattes for 3 different groups of objects, or shadow passes for the shadows from 3 different lights. This is pretty much a lossless proposition, because the information that might have been put into the alpha channels of 3 different image files is just being packed into 1 file, but you have the same number of separate masks once you split them out. Here's a brief tutorial on a way to put 3 passes into your R, G, and B channels:
Or for object mattes you want in a 3-pack, just assign surface shaders to each object in the layer, and give some a pure red out color, some pure green, and some pure blue. You can use matte opacity to put some into the alpha with this approach too, if you want 4 mattes from a render pass.


05 May 2009, 11:08 AM
thanks for ur reply jeremy
i liked the link u gave me....would try it in my next pack shot commercial...and i wud give u the link to check it out
but am and want more information i saw a render pass once for shadows but dont remember where,
where the shadow has an rgb grading measuring how far away it is from the object
which later helps the compositor in grading the shadow....with bounced light for example darkening in the middle of the shadow according to distance and lifting the shadow pass at towards the end....
i guess a plus minus average node with light info connected to it can be used but i really dont know how
i have been trying to figure this out for a very loong time now but i cant get it right
any help wud be appreciated....


05 May 2009, 12:34 AM
To get different RGB information in the core vs. edges of the shadows, you could do the same technique with the different colored shadows from different lights, but instead of having them in different positions use 2 or more copies of the same light with harder shadows in the red channel, softer shadows in the green, very soft shadows into the blue if needed. In post, you could matte them all within the hard shadow if you need to maintain a crisp edge, but you'd also be able to pick one color channel for extra shadow density and darkening.


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