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So I'm adding spherical area lights to my application, and comparing my results with mitsuba, I am getting some differences (left is my approach, right is mitsuba - a pathtraced reference):

Comparison

What I am mainly noticing is that:

  1. The specular is way too bright for roughness values over 0.5

  2. The specular shape gets "flat" (Left-mine, Right-mitsuba, noise caused by low sample count):

Flat Specular

I am using the closest point approximation and the normalization term proposed in Real Shading in UE4, that is: (radius / (distance * 2.0) + alpha)²

I believe issue 1) could be an issue with my normalization term, however I think issue 2) might probably be an issue with the closest point approximation?

Is this kind of inaccuracy expected with the approximations I use, and if so, are there better approximations?

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    \$\begingroup\$ For context could you show us the rest of your Implementation please. \$\endgroup\$ Commented Oct 29, 2017 at 10:42
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    \$\begingroup\$ you haven't provided us enough code for us to actually help you. \$\endgroup\$ Commented Mar 23, 2018 at 17:19

1 Answer 1

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Is this kind of inaccuracy expected with the approximations...

Yes, any approximation is just that an approximation.

Your path-traced reference is also an incorrect but better approximation.

True color values require a perfect lighting mathematical model, not possible for three reasons: rays are infinite in number, the exact quantum state is a truly random distribution, and finite precision disallows irrational number operations.

The physical-based shader included in UE4 are only designed to provide decent approximations for real-time or very near real-time(a few hours maximum) rendering as used in games, and short videos.

This infinite restriction is also present in the physics system; Good but not perfect.

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