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Questions tagged [brdf]

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18 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 ...
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31 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
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0answers
111 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 ...
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0answers
61 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
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2answers
89 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
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0answers
128 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
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0answers
37 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
44 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
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1answer
101 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
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1answer
96 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
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1answer
97 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)...
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0answers
42 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 ...
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0answers
166 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 ...
3
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1answer
166 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
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1answer
127 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
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2answers
100 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
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1answer
116 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
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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
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1answer
301 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
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1answer
267 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
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3answers
2k 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
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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
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0answers
246 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
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2answers
199 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
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2answers
228 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
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1answer
573 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
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1answer
266 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
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1answer
333 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
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1answer
390 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
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1answer
191 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
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1answer
112 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 ...
6
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1answer
150 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
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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 ...
3
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0answers
246 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 ...
6
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1answer
214 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 ...
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3answers
7k 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?
7
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1answer
290 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
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1answer
510 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-...
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0answers
65 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
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0answers
243 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
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1answer
82 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
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0answers
91 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
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0answers
236 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
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2answers
303 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
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1answer
135 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, ...
31
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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 ...
7
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1answer
441 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
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1answer
536 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
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1answer
369 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:
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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: ...