Ray Length "Depth Pass" -- DOF in Reflections

Date : Thu, 10 Aug 2006 00:18:44 +0200
To : xsi(at)Softimage.COM
From : Christian Götzinger <cg(at)pension-goetzinger.de>
Subject : Ray Length "Depth Pass" -- DOF in Reflections
Hi List,

I hate the fact that there's no proper way to calculate true depth of field in mental ray. Mental ray's native DOF shader is absolutely ridiculous. Way too slow and it doesn't even simulate the aperture shape.
Also, I had to find out recently that even X-DOF is fake because reflections are not blurred correctly based on ray length - instead the z buffer is used. For reflective objects this ALWAYS looks totally wrong and fake.


Now, I was thinking that if you rendered a diffuse pass and a reflection pass and applied DOF in post independently to both passes then all you'd need is an additional "depth pass" that's based on ray length. You could apply DOF to the reflection pass based on this ray length pass and add the result on top of the diffuse (which receives independent DOF).
Of course this is still not ideal because you have to guess the depth of field amount for every shot instead of basing it on real world values, but doing depth of field in post seems like the only solution to getting good depth of field in reflections when dealing with mental ray.


Is this thinking flawed? Is there any way to do this? I'd appreciate any help on this because lack of realistic depth of field is driving me crazy.

Cheers,
Chris


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