Questions tagged [brdf]
The brdf tag has no usage guidance.
127
questions
2
votes
1answer
54 views
How is the maximum value for alpha (roughness == 1) decided for microfacet models?
Recently I have been looking at pbrt, and was looking at their remapping from roughness to $\alpha$ values:
https://github.com/mmp/pbrt-v3/blob/master/src/core/microfacet.h#L86
I understand that it is ...
0
votes
1answer
44 views
PDF of BRDF respecting the spherical coordinates
I'm reading the article Sampling microfacet BRDF.
The GGX function is $D(h)$, the articles says the PDF respecting the spherical coordinates is $P_1(\theta)=D(h)Cos(\theta)Sin(\theta)$. But I think ...
0
votes
1answer
43 views
What is the terminology for the brightest point on a plane
Given a plane, assuming that view direction and light direction are both fixed, there must be point which is the brightest point on that plane(where half vector equals to the normal vector).What is ...
1
vote
1answer
46 views
Confusion of deriving the Jacobian of the reflection transformation in Walter et. al's 2007 paper
When deriving the Jacobian of the reflection transformation in Walter et. al's 2007 paper:
First we have a macrosurface BSDF which is:
$f_{s}(\mathbf{i}, \mathbf{o}, \mathbf{n})=\int\left|\frac{\...
1
vote
1answer
42 views
BTF Find Four Closest Sampling Directions
I am trying to use the techniques discussed in this paper to implement a BTF into PBRT. I am stuck on how to actually interpolate the weights of the four closest sampling directions.
At the BTF, I ...
1
vote
0answers
33 views
How to understand Beckmann is a Gaussian distribution in slope space
I'm recently looking into how slope space is used in microfacet BRDF. Can anyone help to explain how Beckmann is a Gaussian distribution in slope space? Thanks
1
vote
1answer
76 views
What is slope distribution
I was reading this slope space integral paper. Can anyone help to explain slope space and slope distribution mentioned in this paper, and why we can use normal and half vector to represent 2D ...
0
votes
1answer
56 views
How can I make the simplest physically based mixed specular-diffuse BRDF (and how to properly sample it)?
I am having troubles making a BRDF that has both specular and diffuse part. Each one is weighted by a coefficient and the sum of coefficients should be equal to one.
I want this layered BRDF to be the ...
1
vote
1answer
72 views
How to find the importance sampling function for a specified BRDF?
I am currently working on BRDF and their associated probability density functions (PDF) in order to importance-sample them. It is really hard for me to find the correct way to compute the PDF of a ...
2
votes
2answers
191 views
Why is the symbol for solid angle a small omega in the definition of the BRDF?
In many resources the BRDF is defined like $$f_{r}(\omega_{i},\omega_{r}) = \frac{\mathrm{d}L_{r}(\omega_{r})}{L_{i}(\omega_{i})\,\cos{\theta_{i}}\,\mathrm{d}\omega_{i}}.$$ The index $r$ might be ...
2
votes
4answers
171 views
Question about function for color
I would like a help to understand the following line of code:
return float3(pow(color.r, 2.2), pow(color.g, 2.2), pow(color.b, 2.2));
I've seen some places calling ...
2
votes
0answers
86 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 ...
0
votes
0answers
43 views
Path tracing: how to ensure the new direction vector is a valid direction vector with respect to a BSDF?
Given the BSDF function and the Normal vector of the intersection point in world space, how can I generate a new direction vector ...
0
votes
1answer
51 views
Path tracing: How to ensure we are sampling a direction vector within the visible hemisphere of a surface?
Given a surface Normal vector of an intersection point, how can one generate a random direction vector that is guaranteed to be in the visible hemisphere? The way I ...
3
votes
0answers
97 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 ...
0
votes
1answer
85 views
Pathtracing - many bounces reduce shadows
I'm creating DXR PathTracer highly influence by Matt Pettineo's one - https://github.com/TheRealMJP/DXRPathTracer ; Relevant HLSL code below:
...
1
vote
1answer
167 views
Importance sampling a cosine distribution
Let's take a look at this blog article - https://schuttejoe.github.io/post/ggximportancesamplingpart1/ and image presented as a result of implementing it:
Inside above link, you can see description ...
5
votes
1answer
82 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 ...
3
votes
1answer
184 views
What is 2D BRDF Slice/Lobe?
I was recently trying to read some rendering papers and textbooks. The word 2D BRDF slice/lobe is often used while no official definition is provided. I know that BRDF is a 4D function of input ...
0
votes
1answer
140 views
BRDF for point lights should not return values over 1
This is standard Cook-Torrance BRDF in formulation for a single point light. No intergrals over a hemisphere, just a single direction to a source of incoming energy.
$$
L_o = { \bigg ( (1-F) \frac{...
3
votes
2answers
200 views
BRDF that interpolates between diffuse, glossy and reflective?
Is there a way to obtain a parametrized BRDF that smoothly interpolates between diffuse, glossy and mirror? For example, $\lambda = 0$ would be perfectly diffuse, $\lambda=0.5$ glossy and $\lambda = 1$...
3
votes
1answer
139 views
How do I make sure my cosine-weighted sampling works fine?
I'm working on a pathtracer and while it's working mostly fine, I'm getting different outputs with cosine-weighted sampling and uniform sampling :
Uniform sampling is on the left, cosine-weighted is ...
2
votes
0answers
205 views
How to properly combine diffuse and rough specular BRDFs in a path tracer
Looking around here on various sources (including related questions on this site), if my understanding is correct, when implementing a path tracer with a material which has both a diffuse term and a ...
1
vote
1answer
153 views
BRDF sampling and evaluation of diffuse vs specular component
When I am sampling diffuse BRDF I am using cosine-weighted distribution and when I want to sample specular BRDF I use sampling by GGX distribution. I can decide how much light is reflected and how ...
0
votes
3answers
326 views
Correct way to think about Fresnel effect
I am using Schlick approximation to calculate the value of the Fresnel effect I found here
When I try to find corner cases for the formula I get the value of 1 when the angle between normal and ...
0
votes
2answers
166 views
Understanding of Microfacet monte-carlo path tracing
I am trying to implement BRDF microfacet path tracing and I think I need a little push.
I will explain what I understand so that you can correct my mistakes.
I start with simple rendering equation:
...
0
votes
0answers
120 views
Pdf division in BRDF sampling
While following scratchapixel's tutorial on indirect diffuse implementation in path tracing, the samples gets divided by the pdf, which in case of a uniform pdf results in $1/2\pi$ so we have that the ...
3
votes
0answers
159 views
Understanding BRDF sampling
I'm trying to understand BRDF importance sampling following the exemples of the lambertian/diffuse model and the reciprocal Phong model.
I would like to be able to extend the principle to other ...
1
vote
0answers
48 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 (...
0
votes
0answers
48 views
Formula for non area light BRDFs
I'm reading a graphics book and it talks about BRDFs being the ratio between differential outgoing radiance and differential irradiance. The formula it gives is
f(l, v) = dL(v)/dE(l)
Where l is the ...
3
votes
1answer
282 views
Implementing the example of Walter et al. (2007) paper
As the title suggests I'm trying to implement the last images of the "Microfacet Models for Refraction through Rough Surfaces" Paper by Walter et al. in OpenGL
I "think" (I had to ...
1
vote
2answers
219 views
Should ray throughput always decrease when bouncing?
In the accepted answer for the question What is Importance Sampling?, in the example code there is this line:
...
2
votes
0answers
144 views
Importance Sampling path tracer, Different pdfs result in different luminance?
I'm having a lot of trouble implementing Importance Sampling path tracer.
If I understand it correctly, what I need to evaluate for a path tracer is this:
$\int_\Omega L_{in}(L) * fr(L, V) * \cos(\...
1
vote
1answer
66 views
Value range of normal distribution function
Is the normal distribution term D in the Cook-Torrance BRDF supposed to be between 0 and 1? What does this number actually represent?
2
votes
1answer
224 views
Rendering equation for spherical lights
I am currently implementing spherical lights in my DirectX game engine. I decided to build the shading formula from the classical rendering equation. Here is what I get:
The simplifications show that ...
1
vote
0answers
52 views
Oren-Nayar Reflectance
I'm trying to implement Oren-Nayar reflectance and I don't quite understand the Wikipedia article on it.
In the article, it has this diagram:
I can't figure out what $(\theta_i, \phi_i)$ and $(\...
2
votes
0answers
271 views
Implementing GGX VNDF importance sampling with DXR
I am currently implementing the GGX specular BRDF in a DXR raytracer using the importance sampling method describe by Heitz
I am sampling the VNDF with:
...
1
vote
1answer
348 views
Roughness Value of (Close to) Zero in Physically Based Rendering
In my Direct Light Physically Based Renderer, when I set the roughness too low (ie. 0.0 or close) the specular reflection doesn't show, not even at high resolutions and at perfect angle with the ...
0
votes
1answer
84 views
Microfacet BRDF artifacts
I've been trying to implement a microfacet specular BRDF based on the equations in Real Shading in UE4, but I'm getting weird ring-shaped artifacts in the highlights of rough surfaces:
Here's the ...
0
votes
1answer
80 views
Physically realistic received irradiance values given some BRDF?
I am trying to simulate the actual response of a camera given some object that is reflecting light. I've written a ray tracer, and have a BRDF that I need to use, and I have a camera sensitivity in ...
1
vote
0answers
49 views
Where to find BRDF textures
I've read about measured BDRFs and models like cook-torrance have parameters that need to be set. It isn't hard to imagine that these parameters and the measured properties would vary from point to ...
2
votes
1answer
187 views
Strange dark spot when lighting in deferred rendering PBR shaders
It's probably something obvious but there seems to be a strange issue with lighting in my pipeline. I'm trying to keep everything in world space at the moment for simplicity but cannot figure out what'...
3
votes
0answers
182 views
Microfacet GGX not integrating as expected
I'm trying to complete a look-up table for an energy-conserving variation of microfacet GGX as implemented here: https://patapom.com/blog/BRDF/MSBRDFEnergyCompensation/
I have a (pretty standard) GGX ...
2
votes
1answer
73 views
Confusion about data types in Jensen's subsurface scattering paper
I am currently trying to implement Jensen's paper on subsurface scattering, but I am getting confused when I am trying to implement it in my pathtracer. I have questions about the data types of ...
4
votes
1answer
461 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 ...
1
vote
0answers
71 views
Questions about BRDF parameterization
Hi I'm trying to get a better understanding of the BRDF modeling. I only implemented naive diffuse(lambert), specular and microfacet BRDF before, but I don't have much experience in mixed BRDFs and ...
5
votes
0answers
402 views
Where do "fireflies" come from?
I'm using a renderer developed by other people, and I found there are fireflies. It is strange that in a completely diffuse scene, there are still fireflies presented.
To my understanding, I think it ...
1
vote
0answers
466 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:
...
0
votes
2answers
809 views
How to set equivalent PDFs for cosine-weighted and uniform-sampled hemispheres?
I'm trying to add BRDFs to a very basic path tracer. Starting out, I'd like to convert just the Lambertian material, with two different sampling methods, to ensure that everything is working right. ...
7
votes
1answer
2k 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 ...
