Computer Graphics Stack Exchange
Computer Graphics Stack Exchange

Questions tagged [ggx]

15 questions
7
votes
1 answer

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 of an isotropic GGX, then they should be equal when…
honzukka
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6
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2 answers

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 several articles, microfacet brdf can have zero…
Hyuk Kim
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6
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1 answer

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 in the pic below). However, with the smoothness…
russ
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5
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1 answer

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 directions that are generated below the hemisphere?…
Gregory Ginzburg
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4
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0 answers

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 explaining it? Smith's shadow masking function $G1$…
Arjan Singh
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4
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1 answer

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 integrate to 1 (I convert solid angle to spherical…
Nicholas Leon
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3
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0 answers

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 this article: Real Shading in Unreal Engine 4…
user1754322
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3
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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 sampling), and then sample the environment map.…
user1754322
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2
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0 answers

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 brdf times the cosines of the reflected…
kipgon
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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 (negative). These bas samples become more of an issue…
noName
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1
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0 answers

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: float BRDF(float3 viewDir, float roughness, float3 normal, float3 Li) { …
TIANLUN ZHU
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1
vote
1 answer

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, theta is the angle between the normal and the…
TIANLUN ZHU
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0
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0 answers

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. color = D(wh) * G(wo, wi) * F / (4 * wi.z * wo.z); pdf = D(wi) *…
iaomw
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0
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1 answer

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 online: shorturl.at/tvzGY The algorithm works as…
user1754322
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0
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2 answers

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. float ao = texAO.sample(textureSampler, hitRecord.uv).r; float3 albedo =…
iaomw
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