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Sunday
Oct102010

Deferred Physical Shading

This year's SIGGRAPH had an interesting segment covering Physically-based BRDFs, that offered some sobering insights into the problems of traditional realtime lighting models. Particularly, how current models not only produce less believable results, but force artists to waste time and effort compensating for their definciencies. Naturally, they explained how to derive better lighting models that behave more realistically, and offered some insights into the behavior of real-world materials.

 

Links:

[1] SIGGRAPH '10: Physically-Based Shading Models in Films and Games

[2] Practical Implementation of Physically-Based Shading Models

[3] Crafting Physically Motivated Shading Models for Games

 

Implications for Deferred Shading:

The difference that concerns me most is that the Physically-Based BRDFs rely on Fresnel terms for both their diffuse & specular calculations. Specifically, how these Fresnel terms require the per-light Light Direction & Half Angle vectors. So in a Deferred Renderer, each object's material's Fresnel coefficients must be stored in the G-Buffer so that they can be available for the lighting phase. Also, for materials that use per-RGB Fresnel coefficients, I would need to be able to accumulate colored specular lighting.

 

These conditions present problems for trying to integrate such a system into a Light Pre-Pass (LPP) renderer. At a minimum, I would need to add single or per-RGB Fresnel coefficients to the already overcrowded G-Buffer. So I really have no choice but to add another render target to the G-Buffer. Then per-RGB fresnel would force me to have a dedicated specular light accumulation buffer, rather than using the approximation trick.

 

Conclusions:

Ultimately, these changes would make the LPP renderer more bloated than a standard Deferred Renderer. So if I wanted to start using a Physically-based BRDF, I would have to make the switch. Now I'm at an impasse, because my initial tests have demonstrated enough of a visual improvement to peak my interest. This has left me with much to consider before I can proceed.

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Reader Comments (2)

Have you tried the method mentioned in this link:
http://www.realtimerendering.com/blog/deferred-lighting-approaches/

It basically boils down to moving the fresnel stage into the final stage of LPP instead of the lighting stage. This is done by replacing L.H in the fresnel equation with N.V, which is constant across all lights. I haven't tried this yet, but they say it is a reasonable approximation.
October 30, 2010 | Unregistered CommenterAmanieu
The problem is that that approximation isn't physically correct, as it will make highlights at edges appear too dark. This approximation would only really work if all objects in a given scene were receiving most of their lighting from an environment map.
November 2, 2010 | Registered Commentern00body

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