Questions tagged [brdf]
The brdf tag has no usage guidance.
0
votes
2answers
64 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. ...
4
votes
0answers
64 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
35 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
votes
0answers
40 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
94 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
87 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
90 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
vote
0answers
39 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
158 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
128 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 ...
2
votes
1answer
115 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
99 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
107 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
95 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
230 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^\...
2
votes
1answer
218 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 ...
6
votes
3answers
845 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
74 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
223 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
156 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.
4
votes
2answers
193 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
511 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
217 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)$$
And this is how you choose an ...
5
votes
1answer
248 views
Intuitively understanding conservation of energy for the Lambertian BRDF
The Lambertian BRDF is defined as $R / \pi$, where the division by $\pi$ is there to maintain conservation of energy. The derivation is provided here.
The maths makes sense, and I can understand ...
2
votes
1answer
336 views
How can I implement this BSDF?
I am trying to implement a path tracer, but I have some problem. The class Spectrum is color. I implemented a mirror material like this:
...
4
votes
1answer
179 views
Where does the cosine factor comes from in the GGX PDF?
The GGX NDF, as it appears on the paper where it is presented is:
$$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 $...
7
votes
1answer
105 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
141 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. ...
7
votes
0answers
909 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 ...
3
votes
0answers
223 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
198 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 ...
5
votes
3answers
6k 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
263 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 ...
7
votes
1answer
473 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
vote
0answers
63 views
Mitsuba Bidirectional Framework
I'm confused by the Mitsuba bidirectional framework for quite a while now, especially by the different eval and connect ...
4
votes
0answers
235 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 ...
6
votes
1answer
77 views
How is measured BRDF data recorded?
How is measured BRDF data recorded? Like the MERL BRDF Database.
What kind of equipment is used and what are the key values that need to be recorded?
I know one place this data was used was in the ...
1
vote
0answers
88 views
Explain Radiance Estimate for photon mapping
I'm adding photon mapping to my current raytracer to add indirect lighting.
Can someone explain how the radiance estimate works in simple terms and with some simple math? I have read a lot of ...
1
vote
0answers
218 views
Is there a material table for the Cook Torrance BRDF?
Is there a material table that lists all the specific values to obtain a certain type of material (e.g: copper or asphalt) available?
I found this table in the paper: A Reflectance Model for ...
5
votes
2answers
267 views
What does “Bidirectional” in BRDF mean?
So Bidirectional essentially means "two directions", right? But what are these two direction (left-right? up-down? north-south?) we are talking about in regards to bidirectional reflectance ...
4
votes
1answer
125 views
Troubleshoot half vector sampling from a distribution (cook-torrence, trowbridge-reitz, etc)
I am learning ray tracing and the mathematics behind it. I have a working monte-carlo raytracer I am experimenting on. I have gotten past pure diffuse BRDFS, area lights, acceleration structures, ...
29
votes
2answers
4k views
What is Importance Sampling?
What is importance sampling? Every article I read about it mentions 'PDF' what is that as well?
From what I gather, importance sampling is a technique to only sample areas on a hemisphere that matter ...
6
votes
1answer
408 views
Fresnel and specular colour
I'm trying to implement Cook-Torrance BRDF and had previously bookmarked this question as it looked very well thought out. Going through it, I'm confused about the Fresnel equation. The author has the ...
5
votes
1answer
459 views
Energy conservation of BRDF
I'm reading Advanced Global Illumination.
Here is the part confusing me:
What do the second equation and $\delta$-function mean?
Why the third equation is a sufficient condition even though a reason ...
4
votes
1answer
346 views
Why the BRDF of specular reflection is infinite in the reflection direction?
I know the BRDF of specular reflection is nonzero only in the reflection direction.
But why it is infinite?
A paragraph on page 36 of Advanced Global Illumination:
6
votes
2answers
2k views
Refraction in a Ray Tracer: What do with an intersection within the medium?
I am currently following a tutorial at Scratchapixel.com on refraction
Here is the refract function:
...
4
votes
0answers
575 views
Better alternatives to the Lambert BRDF
After reading about the new Disney Diffuse BRDF, I have been wondering if this is currently the best Diffuse BRDF at the moment? How much more of a difference would this make compared to using a ...
6
votes
1answer
105 views
Trying to understand environmental cubemaps
I am trying to implement a physically based renderer, in OpenGL. I was planning to implement the Cook-Torrance BRDF. I read about radiance cubemaps (for reflections), like the one discussed here, for ...
5
votes
1answer
725 views
Volume Rendering (Path Tracing & Real-Time)
I have a few doubts and questions regarding volume rendering, I was hoping someone here could help me understand.
I commonly hear the terms 'Ray-Marching' & 'Volume Rendering' being used together ...
23
votes
2answers
3k views
Path tracing the Cook-Torrance BRDF
-- Sorry for the long post, but I prefer to do that way because "Devil is in the details." :)
I am writing a path tracer from the scratch and it is working nicely for perfectly diffuse (Lambertian) ...
