shadow color mismatch with Gi

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  01 January 2013
shadow color mismatch with Gi

Hello everyone,

I've been trying to get the correct shadow color out of the framebuffer and into Nuke but I haven't had much luck. I setup a simple scene. A teapot, a ground plane, with a sphere light and a dome light. The sphere light is casting the shadow seen in the image below, and the dome light is adding a bluish tint to the scene which is giving the shadow a bluish tint.

The top image is straight from the full render (not comp) and is correct. The second image is the Nuke comp render which shows no bluish tint to the shadow. This is not correct. The shadow pass doesn't contain any colors.

The third image is the vray setting I used to get the shadow pass. The teapot is set "Visible to Camera" to off.

Does anyone have ideas or solutions that I can try to get a correct shadow pass??


  02 February 2013
You could render out a vray "MatteShadow" render element which includes the colour information for your shadow and GI spill/colourbleed (if using GI). Then invert the MatteShadow element in nuke and merge it over your background using a multiply. This will give you the result your after and should perfectly match your beauty render.
  02 February 2013
Are you using a linear workflow? You have to apply the monitors gamma, 2.2, after adding the layers together for it to be accurate.

  02 February 2013
Render an indirect pass.
  02 February 2013
Thanks for your responses.

@Druski: I believe the matte shadow that gets rendered is not the same as whats represented in render. It seems the gradient/falloff on the matte shadow is more opaque. The shadow rendered has a nice smooth falloff. I will check again to make sure again when I get home tonight.

@kanooshka : I rendered it out of vray with Linear and 2.2 gamma. The final output from nuke (comped) matches the input render.

@LowJack : The GI pass is the Indirect pass is it not? I don't remember seeing a indirect element, but I will check again tonight.

A friend of mine spoke to the Vray guys about this issue and they said that currently Vray doesn't support the calculation of Gi with the shadow pass. Meaning when it renders the shadow, it will calculate it as an element of the lighting pass which doesn't include Gi.

What I am able to do to solve this issue is grade the color from black to bluish to match. Its not accurate but no one noticed.

I did think of another idea. I could take the shadow pass and subtract it from the lighting pass then add that to the Gi. I'll give it a try and let you guys know if that works.

Thanks again and if you have ideas I am will to test it.
  02 February 2013
Do the output passes have the 2.2 gamma already applied before you bring them into Nuke? To composite the layers you can't apply the 2.2 gamma until you add each pass together in comp.
  02 February 2013
The output from 3dsmax is Linear. The input into Nuke is Linear. When I render out of Nuke, I render in srgb space. So....the 2.2 is added at the very end.

  02 February 2013
Its parobably not a bad idea to post this on the chaos group forums
  02 February 2013
Unfortunately my Chaos Group account doesn't allow for posting messages in the forums
I can only read. :(
  02 February 2013
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