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View Full Version : Using red / gree/ blue lights then separate them in post?


Panupat
03-16-2012, 09:28 AM
I ran into a small studio that uses this workflow and wondering what's the pro/con of it? Their workflow goes like this

- Key lights are pure green
- fill lights are pure blue
- ambient occlusion lights are pure red.

They'd render those 3 colors out in a single pass, then use nuke to separate them into 3 passes.

Altho there's nothing spectacular about their renders, I'm curious if doing it that way has any pros? Are there any other studio that does this? Are there any render speed advantage for combining the 3 compare to split them into 3 passes?

DanGrover
03-16-2012, 02:37 PM
Well, it wouldn't reproduce colour correctly, one would imagine. For example, if you have a surface that's incredibly red facing the key light, it's going to be almost black to the key light (coloured green). Obviously this is not indicative of reality, where (in an RGB image for each light), only the green and blue channels would be very dark - the red channel would be very bright. But this information is absent from a render if the key light is 100% green.

playmesumch00ns
03-16-2012, 06:54 PM
This is what we did in the olden days - render R,G,B lights as mattes so that you could (kinda) control the lights' contributions in comp afterwards.

Nowadays we just render each light as a separate AOV. I think most renderers can do this now?

CHRiTTeR
03-16-2012, 07:06 PM
This is what we did in the olden days - render R,G,B lights as mattes so that you could (kinda) control the lights' contributions in comp afterwards.

Nowadays we just render each light as a separate AOV. I think most renderers can do this now?

Only the direct lighting pass as far as i know. Doing this with GI support still seems problematic for some (many?) renderers, dont ask me why though.

LowJacK
03-17-2012, 04:58 AM
I've worked at mutiple studios that have used this approach. The benefit is total control in post without the cost of re-rendering.

Depending on how you comp it, the results can be just as good as any other lighting technique.

rendermaniac
03-17-2012, 02:21 PM
You can get around the issue of material response being wrong by rendering out flat textures and untextured lighting separately. But then you have to multiply passes which causes edge artefacts (especially with motion blur, opacity, fur etc), unless you unpremultiply some passes etc.

If you are just doing some minor correction, such as making a light a bit warmer/cooler then using an RGB pass as a mask can sometimes work.

Rendering out each light into a separate pass (GI will be an issue) will give you less headaches.

Simon

Panupat
03-19-2012, 11:45 AM
Thanks for your input everyone. I'm quite surprised to hear that this approach is more commonly used than I expect.

DanGrover - What is AOV pass?

LowJacK - From what you've seen, do most studios do this more for realistic renders? Or stylized/cartoony?

LowJacK
03-22-2012, 05:08 AM
LowJacK - From what you've seen, do most studios do this more for realistic renders? Or stylized/cartoony?

The shows I've used it for had a more stylized cartoony render, although it was possible to get a pretty good realistic looking result with the exact same lighting.

When you seperate out each light into a controllable matte, you have a lot of room to adjust the gradation and color transition of every aspect of the scene. What you do with it is up to you and how you composite it.

There is definitely an art to using this approach as it requires some practice and experience setting up the lights to give you the best results.

ndeboar
03-23-2012, 05:35 AM
Vray/Mental Ray/ Maxwell can spit out each lights contribution (vray/mentalray dont include gi, not sure about maxwell). And it's pretty easy to add support for this in renderman/3delight. What renderer are you using?

Panupat
03-23-2012, 07:08 AM
My studio is using mainly 3delight for maya because of it's speed. And mentalray for some passes as needed. We have no renderman artists here however so we can't dive much into the RIB or SL : /

Bitter
03-23-2012, 08:40 PM
The easiest way when not built-in is to render a separate render layer for the sets of lights you want.

The catch is cost of basically re-rendering the frame for those lights.

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