avoiding fringe when rendering for composition with global illumination in mental ray

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  08 August 2012
Replying to myself again...

Originally Posted by chrishecker: The other alternative is to figure out how to have a black camera environment background color, but still get the nice and easy warmth to the scene with GI/FG without a zillion lights.

Hmm, it looks like leaving the maya camera -> environment -> background color black, but the adding a mib_illum_lambert as the maya camera -> mental ray -> environment shader might work, and then I can just do this in one pass. Need to test it more on real scenes.

Edit: Ugh, except it's 6x slower than just using the background color on a test scene. Needs more testing.

Edit2: And it doesn't actually do GI/FG right, it just looks like a regular light. So, no dice.


Last edited by chrishecker : 08 August 2012 at 08:50 PM.
  08 August 2012
Don't use a material shader as your environment. That's actually a scary idea.

Use an environment as your environment. . . :-)

The production shaders have a rayswitch node for the environment you can use to add a global one for lighting but leave the backplate black.
My opinions are always my own...and maybe a friend's, but never my employer's.
  08 August 2012
Bitter, could you elaborate on what shader/material issues you've had with plemult off in the frame buffer? Admittedly, I dont generally change it from the default as my preferred method for color correcting (in post) is to unpremult the layer first, do my changes and then premult it again. But from the times Ive test rendered with premult off in Maya, it appeared to create essentially the same footage as if I had unpremulted in in Nuke. Is it border/alpha issues or is it that the shader produced render errors?
  09 September 2012
Originally Posted by Bitter: Whatever reaches infinity calls the environment. If your environment has a color, then it gets filtered with the pixel. That's correct. All renderers do this.

The fact that all renderers do it doesn't make it "correct". The problem is if your environment color isn't black, you can't composite correctly (even to that same background color...I think, I'd have to write out the math to be sure), so if you're going to make a mask for compositing for rays that hit infinity, then you are certainly rendering for compositing, in which case you should use black as the color. I can't see why you'd ever want to do anything different than that in the case where you'd want transparent pixels for the environment.

Thanks for the note about the material shader vs. environment shader, that makes sense. I tried to use the first thing I found that let me set a simple color.

  09 September 2012
Quote: The fact that all renderers do it doesn't make it "correct".

The accepted workflow is based on the math behind the rendering. Math is pretty basic so the "correctness" isn't really up to interpretation. (2+2)

The compositing process gives you control without re-rendering but should follow the same principles or it may introduce artifacts.

Some shaders may rely on premultiplication to do their job. Turning that off in the renderer will cause this to fail. Since you can render and composite correctly with premultiply "on", it should be the default or you may have other problems with shaders and transparency etc.
My opinions are always my own...and maybe a friend's, but never my employer's.
  09 September 2012
I came across this thread after watching a decent video that was highlighted on the Lester Banks site. The video is called 'Understanding Premultiplication in Alpha Compositing'and is located here. Photoshop is not used but AE and a little Nuke are, might be worth a watch.

Anyways, after reading this thread I was wondering why use the Camera Environment Color to tweak the lighting? Maybe I am missing something here but why not use MR IBL? Or an Environment? Then couldn't you just render with the IBL Primary Visibility OFF in the Render stats?

Not an expert on this but after watching the video I am curious. Thanks,

  09 September 2012
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