Questions tagged [ggx]

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2 votes
1 answer
98 views

How to approach implementing Cook-Torrence microfacet brdf?

Trying to implement the Cook-Torrance BRDF for my path tracer. Confused about how to do it though. I've seen two approaches: 1: Sample the NDF, evaluate diffuse and specular components and add them ...
Ruben's user avatar
  • 23
0 votes
0 answers
44 views

Does sampling GGX VNDF take into account the cosine incidene of outgoing ray?

Could sampling GGX VNDF sampling, based on: "Sampling Visible GGX Normals with Spherical Caps" and "Sampling the GGX Distribution of Visible Normals" produce a normal that (based ...
ali's user avatar
  • 740
1 vote
0 answers
34 views

How to solve GGX roughness from existing maps

I have a problem that I cannot really wrap my head around. I have a minimal light stage setup to scan heads: I have cross polarized cameras and parallel polarized cameras. I have parallel polarized ...
Schaggo Mikatis's user avatar
0 votes
0 answers
40 views

Bright results when importance sampling GGX NDF

I've been trying to importance sample the GGX NDF of my Cook-Torrance BRDF for some time now but I can't get it right. I've been following this article. Here's my code for the importance sampling part....
Tom Clabault's user avatar
4 votes
1 answer
147 views

Why do I need to remove the positive charactaristic function for the pdf of the outgoing direction to integrate to 1 with GGX?

Context: I am attempting to implement Trowbridge–Reitz (GGX) based on Microfacet Models for Refraction through Rough Surfaces in a pathtracer. I use numerical integration to check if the pdfs ...
Nicholas Leon's user avatar
0 votes
0 answers
169 views

Raytracing, divide by PDF neutralized the effect of D and G terms, what is the problem?

Following some documents on internet, I divide my ray tracing result by PDF. Then it seem the D and G terms disappeared. Only F term is still visible, it looks like Fresnel only. ...
iaomw's user avatar
  • 131
2 votes
0 answers
108 views

Do all microfacet brdf models diverge at grazing angle?

My question follows directly this one. Reading the famous paper of Walter, it comes that the plotted curves of figure 13 (in question in the related topic) are not directly the brdf but rather the ...
kipgon's user avatar
  • 151
3 votes
0 answers
623 views

Comparing different sampling methods for GGX IBL

I have made a demo that shades a sphere with an environment map. I wanted to compare 3 different ways of sampling the environment. Uniform sampling. Importance sampling with the NDF as suggested in ...
user1754322's user avatar
3 votes
0 answers
170 views

IBL looks too dark with uniform sampling

I'm making a program that illuminates a sphere using an environment map. For each fragment in the sphere, I compute a many random directions in the hemisphere along the surface normal (no importance ...
user1754322's user avatar
0 votes
1 answer
103 views

Problem sampling uniformly envmap with GGX

I have made a program that samples uniformly an environment map and computes the lighting using the GGX BRDF. I have made a small example that reproduces the issue I'm having. You can check it out ...
user1754322's user avatar
6 votes
1 answer
163 views

What should I do when a new sampled direction is below a normal oriented hemisphere?

I generate random directions from the pdf of D term in GGX BRDF. On the picture, white dots are new directions and the cross is the light direction, with roughness = 1. What should I do with the ...
Gregory Ginzburg's user avatar
0 votes
2 answers
468 views

Ray Tracing, why does the GGX make my rendering result darker and darker for each frame?

I am trying to add GGX and PBR textures to my GPU ray tracer. The result is very strange, it's getting darker for each frame. ...
iaomw's user avatar
  • 131
1 vote
0 answers
66 views

How to handle negative reflection directions in raytracing?

I got a function that generates samples from a GGX visible normal distribution function. When I use it to calculate reflection directions obviously some of these intersect with the surface agin (...
noName's user avatar
  • 11
6 votes
2 answers
879 views

Can microfacet BRDF (including ggx) represent perfect mirror?

Perfect mirror's brdf is simple, $F\frac{\delta(\omega_i-\omega_r)}{|\cos(\theta_r)|}$ as in http://www.pbr-book.org/3ed-2018/Reflection_Models/Specular_Reflection_and_Transmission.html As shown in ...
Hyuk Kim's user avatar
1 vote
0 answers
850 views

GGX BRDF is greater than 1?

I am implementing a GGX BRDF, this is the formula I used is from:https://schuttejoe.github.io/post/ggximportancesamplingpart1/ And this is my implementation: ...
TIANLUN ZHU's user avatar
8 votes
1 answer
3k views

Anisotropic GGX BRDF implementation. How is it related to isotropic GGX BRDF?

Introduction I am implementing anisotropic GGX BRDF and have encountered strange behaviour of my implementation. I thought that if I compare the microfacet distribution function I have with the one ...
honzukka's user avatar
  • 121
1 vote
1 answer
2k views

Can't understand the Importance sampling GGX

I've read many articles relating to importance sampling of GGX. However, I still feel confused. The formula of GGX is : And many article says we should use this formula to sample: In the formula, ...
TIANLUN ZHU's user avatar
4 votes
0 answers
2k views

Explanation of math of the Smith G1 term

In appendix A of the paper Microfacet Models for Refraction through Rough Surfaces there is a derivation provided for $\Lambda(w)$ but the mathematics is very confusing. Could somebody help me by ...
Arjan Singh's user avatar
  • 2,511
6 votes
1 answer
776 views

Dealing with blown-out highlights in PBR

I've been writing some custom shaders based around Unity's Standard BRDF, which is uses GGX distribution term and Smith visibility term. It looks great at either end of the smoothness scale (1 and 2 ...
russ's user avatar
  • 2,392