Questions tagged [brdf]
The brdf tag has no usage guidance.
93
questions
1
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0answers
32 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
47 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:
...
0
votes
1answer
45 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
49 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
43 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
38 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
78 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
99 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
58 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 ...
3
votes
1answer
152 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 ...
0
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0answers
24 views
Why need a $h$ parameter in the Geometry Function of a BRDF
Geometry Function in the specular BRDF term is defined as $G(l, v, h)$. Why need a $h$ since $h=\frac{l+v}2$.
$f_{spec}(l,v) = \frac{F(h,v)G_2(l,v,h)D(h)}{4|n\cdot l||n\cdot v|}$ from "Real Time ...
1
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0answers
49 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
147 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
219 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
153 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. ...
5
votes
1answer
527 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 ...
1
vote
0answers
38 views
confused on isotropic BRDF
I am reading a paper and there is a part I don't understand:
I am wondering why the isotropic BRDF only depends on the view direction and roughness? Why is the light direction not related?
5
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0answers
47 views
Ask for help on understanding a formula related to change of variable
Although I have tried for a long time, I cannot understand Eq.(11) of paper "Microfacet Models for Refraction through Rough Surfaces" written by Bruce Walter, Stephen Marschner, Hongsong Li and ...
6
votes
1answer
122 views
“Artist Friendly Metallic Fresnel” reversible?
While implementing the "Artist Friendly Metallic Fresnel" paper by Ole Gulbrandsen (http://jcgt.org/published/0003/04/03/paper.pdf), I read that it is fully reversible:
(section 2.3.2): for all r,g ...
1
vote
1answer
130 views
total internal reflection implementation in pbrt-v3
I've been implementing a path tracer, referring pbrt-v3. However, I'm a bit confused about its implementation. Code is here.
It seems when we sample to do transmission, it tests if it meets a total ...
2
votes
1answer
106 views
Ask for detailed derivation of a formula in “Advanced Global Illumination”
Attached is page 34 of "Advanced Global Illumination", 2nd edition, by Dutre et al. I don't understand how (2.22) is derived from (2.21) and the given incident radiance distribution $L(x\leftarrow\Psi)...
1
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0answers
48 views
Cook-Torrance Specular BRDF Artifact [duplicate]
I am currently experiencing a very obvious artifact that occurs when looking at objects on very grazing angles
Here is a picture of the artifact:
I noticed that the issue was coming from my ...
1
vote
0answers
188 views
Normal Artifacts On Grazing Angles
I am currently experiencing a very obvious artifact that occurs when looking at objects on very grazing angles
Here is a picture of the artifact:
I noticed that the issue was coming from my ...
2
votes
1answer
261 views
PBR - How to handle multiple BSDFs and material type
So I am in the midst of writing my own path traced renderer and right now I have only implemented the old Blinn-Phong models where we separately calculated the diffuse and specular components of the ...
3
votes
1answer
152 views
Importance Sampling in Unreal Engine Formula Derivation (Irradiance term)
I am currently trying to understand IBL in Unreal Engine, and there are so much things that I don't understand about the formula. Unreal approximate the specular term of shading equation by doing ...
5
votes
2answers
107 views
Refraction: given an incoming and transmitted direction, can I deduce the normal?
It's easy to find guides to calculating the transmitted direction of light given an incoming direction, the normal, and indexes of refraction. However, I can't find anything about determining the ...
5
votes
1answer
150 views
Radiative Transfer Equation for Photorealistic Rendering
I've recently become interested in photorealistic rendering, and I've been looking at the different rendering philosophies. I read this Disney Research bachelor's thesis, which states both the ...
1
vote
0answers
98 views
How to validate whether a reflection model is implemented correctly?
I think this is one of the most important part when you write a physically-based renderer.
The solution I came up with is to put a sphere to the scene with a constant background radiance. Solve the ...
3
votes
1answer
530 views
Phong and the Rendering Equation: What's with the cosine?
Ignoring emission and shadowing for simplicity, the rendering equation can be stripped down to:
$$L(x, \, \vec \omega) = \int_{\Omega}{f_r(x, \, \vec \omega^\prime, \, \vec \omega) \, (\vec \omega^\...
3
votes
1answer
402 views
BRDF normalization
I don't understand how to normalize the BRDF.
the constraint is that for each incoming ray, the exitant ray will be $1$ at most
ie integral of hemisphere of $f_r\cdot\cos(\theta)$ where $f_r$ is the ...
7
votes
3answers
3k views
What is the difference between radiance and irradiance in BRDF
Currently I am reading the BRDF section from Real Time Rendering and I am having a hard time to visually understand the definition of this function. BRDF is the ratio between the radiance in outgoing ...
4
votes
0answers
78 views
Predicting reflectance functions from complex surfaces
There are three principal ways to obtain a BRDF. We can
Derive an analytic formula using physical principles
Use simulation given an assumed or measured model of the surface
microgeometry
Measure ...
4
votes
0answers
287 views
Wrong output while implementing GGX importance sampling
I'm receiving a non-energy conserving output while trying to importance sample the GGX Distributionof the Microfacet model, which is generally 3-4 times bigger than the resulting PDF
I'm following ...
4
votes
2answers
284 views
What is a Gaussian Lobe concerning BRDF and NDF?
I was reading a paper about voxel cone tracing and came across this term. So, what is a Gaussian Lobe or a Lobe in general?
I am asking in the context of Computer Graphics, BRDF and NDF.
3
votes
2answers
338 views
Relationship between roughness and BRDF
In a BRDF $f_r$, assume Cook-Torrance model, we have a microfacets distribution $D$. This distribution essentially models the rougheness of the material as far as I've understood. Given a direction $v$...
5
votes
1answer
715 views
Microfacet shading for diffuse materials
I recently replaced the Lambertian BRDF in my path-tracer with Oren-Nayar, under the assumption that I could adjust it to use the GGX distribution model with appropriate masking/shadowing.
PBR ...
1
vote
1answer
409 views
Phong Modified BRDF Implementation
According to: https://www.cs.virginia.edu/~jdl/importance.doc
For a specular sample: (page 4 of the document) $$pdf(\theta_{i}) = \frac{n+1}{2\pi} \cos^n(a)$$
And this is how you choose an ...
5
votes
1answer
564 views
Intuitively understanding conservation of energy for the Lambertian BRDF
The Lambertian BRDF is defined as $R / \pi$, where the division by $\pi$ is there to maintain conservation of energy. The derivation is provided here.
The maths makes sense, and I can understand ...
2
votes
1answer
463 views
How can I implement this BSDF?
I am trying to implement a path tracer, but I have some problem. The class Spectrum is color. I implemented a mirror material like this:
...
4
votes
1answer
229 views
Where does the cosine factor comes from in the GGX PDF?
The GGX NDF, as it appears on the paper where it is presented is:
$$D(m)=\frac{\alpha_g^2\space\chi^+(m\cdot n)}{\pi\cos^4(\theta_m)(\alpha_g^2+\tan^2(\theta_m))^2}$$
It is equivalent, in the range $...
6
votes
1answer
123 views
fix over exposure in importance sampling in screen space reflections
I am having my first encounter with the rendering of indirect reflections in the form of screen space reflections in a game engine, but i am struggling to get correct looking results.
(patterns arise ...
5
votes
1answer
189 views
Roughness Remapping
I have been recently exploring physically-based rendering (PBR), but there is one part that still confuses me. I don't quite get roughness remapping, as described in the PBR section on Learn OpenGL. ...
6
votes
0answers
1k views
Diffuse BRDF component of the Disney BRDF
In the SIGGRAPH course:
BURLEY B.: Physically Based Shading at Disney, SIGGRAPH 2012 Course: Practical Physically Based Shading in Film and Game Production, 2012.
it is mentioned that some BRDF ...
2
votes
0answers
281 views
How to simulate specular reflection in Light Tracing?
I am implementing light tracing from this website.
The right wall on the below image is perfect mirror(specular reflection). Its BRDF returns zero that's why it's been rendered black. But how is it ...
5
votes
1answer
253 views
Phong BRDF in Mitsuba, Tungsten and AGI
In the book AGI (Advanced Global Illumination), the Phong BRDF is
$f_r(x,\Psi\leftrightarrow\Theta)=k_s\cfrac{(R\cdot\Theta)^n}{N\cdot\Psi}+k_d$
$k_d=\cfrac{albedo}\pi$
In Mitsuba and Tungsten, the ...
6
votes
3answers
9k views
What is the difference between glossy and specular reflection?
What is the difference between glossy and specular reflection?
What is their relations with BRDF?
6
votes
1answer
345 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 ...
8
votes
2answers
664 views
Why is the half vector not used in diffuse BRDF Fresnel calculations?
I am currently working with Earl Hammon Jr's Presentation PBR Diffuse Lighting for GGX+Smith Microsurfaces (now mentioned as [PBR, p.XYZ] and have read through (among others) Brent Burley's Physically-...
1
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0answers
66 views
Mitsuba Bidirectional Framework
I'm confused by the Mitsuba bidirectional framework for quite a while now, especially by the different eval and connect ...
3
votes
0answers
273 views
Mathematics of Sub Surface Scattering
Can somebody help explain the mathematics of Sub Surface Scattering in a Path Tracer based on the Diffuse Approximation in A Practical Model for Subsurface Light Transport ?
I would like someone to ...
