search

mmp / pbrt-v3

Source code for pbrt, the renderer described in the third edition of "Physically Based Rendering: From Theory To Implementation", by Matt Pharr, Wenzel Jakob, and Greg Humphreys.
http://pbrt.org
BSD 2-Clause "Simplified" License
4.88k stars 1.19k forks source link

Problem about "Sw" normalization in DisneyBSSRDF #173

Closed Immocat closed 6 years ago

Immocat commented 6 years ago

Hi,

The problem is described as following:

PBD

The Separable BSSRDF is calculated by the equation before [11.7] on page 694 of pbrt v3 book. As the described in the last comment by @wjakob in issue #19, PBD tabulates the fresnel-scaled irradiance and the Fresnel term in "Sw" is to modulate the directional profile.

DisneyBSSRDF

However, as mentioned in [2015 Siggraph Course note](equation (5) and (6) on page 7) and [Pixar Memo], their profile is intrinsically normalized(Integrating Rd over hemisphere and area gets 1).

My thoughts:

So I think there should not be a c in "Sw" when DisneyBSSRDF is used. We only multiply two Fresnels, diffusion profile, and 1/pi. (As equation (5) in [Jensen01]). I am not sure if I am correct since there is a C in [Pixar Memo] without explanation.

Thanks for your attention.

Zejian Wang

mmp commented 6 years ago

Nice find. I believe you're right. Fixed!