Questions tagged [brdf]
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131
questions
2
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1
answer
40
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Is the dichromatic model physically based?
lately I have been learning about photometry, and everywhere I look I see a BRDF written according to the "dichromatic model". Roughly speaking: $$\rho= \rho_D+\rho_S \cdot (...)$$
for some $...
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2
votes
1
answer
49
views
[Subsurface Scattering]Reflection from Layered Surfaces due to Subsurface Scattering
Recently, I am doing some research about subsurface scattering. i am a little confused about the backscattered radiance mentioned in this paper Reflection from Layered Surfaces due to Subsurface ...
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1
vote
0
answers
80
views
Cosine in Rendering Equation and BRDF cancel out: Why cosine at all?
I am somewhat confused with the following issue reading about the rendering equation and BRDFs. That is, the BRDF is usually defined as surface radiance $Lr(ω_r)$ over irradiance $L_i(ω_i)cosθ_i$ for ...
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1
vote
1
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103
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Conflicting definitions for the distribution of normals $D$ in microfacet BSDFs
Please do not confuse this question with this one. In Understanding the Masking-Shadowing Function in Microfacet-Based BRDFs, Eric Heitz defines the distribution of normals as.
There, the footnote. ...
2
votes
1
answer
143
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Integrating BRDF using Importance Sampling
I read some articles about BRDF integration with and without importance sampling and I don't understand one thing in equations.
If we integrate a BRDF over hemisphere with uniform sample direction ...
- 193
2
votes
1
answer
83
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 ...
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0
votes
1
answer
48
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
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1
answer
45
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
1
answer
78
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{\...
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1
vote
1
answer
50
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 ...
- 111
1
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0
answers
45
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
- 33
1
vote
1
answer
107
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 ...
- 33
0
votes
1
answer
108
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 ...
- 56
1
vote
1
answer
102
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 ...
- 56
2
votes
2
answers
211
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 ...
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2
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4
answers
199
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
0
answers
90
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 ...
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1
answer
90
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 ...
- 241
3
votes
0
answers
117
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 ...
- 117
0
votes
1
answer
93
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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:
...
2
votes
1
answer
419
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
1
answer
92
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
1
answer
302
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 ...
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1
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182
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{...
- 193
3
votes
2
answers
284
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 $\lambda = 1$...
- 67
3
votes
1
answer
171
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 ...
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2
votes
0
answers
301
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 ...
- 135
1
vote
1
answer
223
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 ...
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0
votes
3
answers
488
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 ...
- 45
0
votes
2
answers
219
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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:
...
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0
votes
0
answers
167
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 ...
- 135
3
votes
0
answers
168
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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 ...
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1
vote
0
answers
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 (...
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0
votes
0
answers
61
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 ...
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3
votes
1
answer
322
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 ...
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vote
2
answers
241
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:
...
- 133
2
votes
0
answers
194
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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(\...
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1
vote
1
answer
75
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?
- 133
2
votes
1
answer
275
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 ...
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1
vote
0
answers
58
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 $(\...
- 111
2
votes
0
answers
375
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:
...
- 21
1
vote
1
answer
475
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 ...
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0
votes
1
answer
90
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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 ...
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0
votes
1
answer
84
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
0
answers
51
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 ...
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3
votes
1
answer
226
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'...
- 51
3
votes
0
answers
197
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 ...
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2
votes
1
answer
77
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 ...
- 121
5
votes
1
answer
553
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 ...
- 51
1
vote
0
answers
74
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 ...
- 61