4
votes
Accepted
What does GGX stand for?
According to E. Heitz (Sampling the GGX Distribution of Visible Normals) it stands for "ground glass unknown". I am not sure what the word unknown means though.
By the way, it is equivalent ...
- 256
3
votes
Accepted
Lots of bad samples (below the hemisphere) when sampling the GGX VNDF
After some discusson with Enigmatisms, it turns out that:
I had an error in my renderer
Bad samples are expected from VNDF sampling.
The error in my renderer
I'm using VNDF sampling to sample a ...
- 249
3
votes
Conflicting definitions for the distribution of normals $D$ in microfacet BSDFs
The units of the NDF are tricky. For whatever it's worth, Heitz's convention of defining it relative to a 1 m² reference geometric surface is unusual, and although I can see why he would want to ...
- 25.3k
3
votes
Accepted
Why can I see the diffuse lighting effect on a perfect mirror?
Dielectric materials (which is what you get when metalness is 0) don't exhibit a mirror-like effect. Think of a sheet of smooth, non-transparent plastic. Real-life mirrors are panes of glass or ...
- 892
3
votes
Accepted
Can't understand the Importance sampling GGX
In microfacet BRDFs, the half-vector is the same as the microfacet normal. The half-vector is exactly the required normal for a microfacet to reflect light from the incident ray to the outgoing ray, ...
- 25.3k
3
votes
Accepted
Relationship between roughness and BRDF
Most normal distribution functions (NDFs) are parametrized by some variable (tipically $m$ or $\alpha$) that determines the "roughness" or "spikiness" of the NDF (this is often meant to be the rms ...
- 588
2
votes
Relationship between roughness and BRDF
for each point $p$ in theory we have a 3D function that tells us the orientation in a given direction
You have missunderstood this. $D$ is a Normal Distribution Function (or short NDF), so it doesn't ...
- 1,613
2
votes
Implementing the example of Walter et al. (2007) paper
Initially and still I'm not so sure how to get the color of the object behind the glass into the formulas.
Normal, this was written with path / ray tracers in mind where that part is easy. Your idea ...
- 1,585
2
votes
Can microfacet BRDF (including ggx) represent perfect mirror?
If this was correct, $\frac{D(H)G(L,V,H)}{4|N⋅V|}$ should be a dirac delta function. But I don't think so.
Actually, you're close to your answer - you just are trying to find out, what the assumption ...
- 1,613
2
votes
Accepted
Importance sampling microfacet BRDF and coordinate frame
The importance sampling methods for the two reflection models you mentioned are different - they pretty much reflect their definition.
Cook-Torrance model describes the surface as a collection of ...
- 256
1
vote
Microfacet-Based BRDFs general form
I am not sure whether 2 refers to the second line or the third line (since it is directly above it) so I will explain both derivations.
For line 2
(This part might ...
- 1,302
1
vote
Accepted
How is the distribution of normals constructed from the distribution of slopes in 'Understanding the masking-shadowing function' paper?
The "slope space" is a coordinate system that describes unit vectors in the upper hemisphere using their $x$ and $y$ slopes, i.e. $-x/z$ and $-y/z$. It's related to the usual polar ...
- 25.3k
1
vote
Accepted
How is the maximum value for alpha (roughness == 1) decided for microfacet models?
In truth, there is no mathematical maximum value for $\alpha$. As you noted, microfacet slope is unbounded, so in principle you could have arbitrarily large slope values and hence arbitrarily large $\...
- 25.3k
1
vote
Confusion of deriving the Jacobian of the reflection transformation in Walter et. al's 2007 paper
There is a functional relationship between $\mathbf{h}$ and $\mathbf{o}$ (and also $\mathbf{i}$) whose Jacobian is being taken. But there's no prior functional relationship between $\mathbf{m}$ and $\...
- 25.3k
1
vote
Accepted
Microfacet BRDF artifacts
It turns out that the ring was happening from a negative divided by a negative, so adding nom = max(nom, 0.0); fixed the problem. The new highlight, amplified:
- 198
1
vote
Can microfacet BRDF (including ggx) represent perfect mirror?
I actually have stumbled at this same exact problem. I think I have found the solution in the pbr book: https://www.pbr-book.org/3ed-2018/Reflection_Models/Microfacet_Models#...
- 11
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