Questions tagged [brdf]

The tag has no usage guidance.

Filter by
Sorted by
Tagged with
5
votes
1answer
57 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
28 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
71 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
99 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
73 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 ...
1
vote
0answers
46 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
0answers
33 views

How to find the BRDF of a rough specular surface

So I know that the BRDF for a diffuse material is `albedo/π', and I know that the BRDF for a perfectly specular surface is only non-zero on the reflected direction. How would I find the BRDF for a ...
1
vote
1answer
89 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
126 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
95 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
69 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
136 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
41 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
46 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 ...
0
votes
0answers
61 views

Understanding Oren-Nayar reflectance model

In the Oren–Nayar reflectance model, each facet is assumed to be Lambertian in reflectance. wiki My understanding : Law of reflection tells us that angle of incidence is equal to angle of ...
3
votes
1answer
204 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 ...
0
votes
1answer
146 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
78 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
59 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
172 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
46 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
106 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
123 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
72 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
63 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
48 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
95 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
142 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
69 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
297 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
66 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
273 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
364 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
438 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
1k 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
40 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?
4
votes
0answers
53 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 ...
5
votes
1answer
154 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 ...
0
votes
1answer
165 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 ...
1
vote
1answer
127 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)...
0
votes
0answers
55 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
227 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
412 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
228 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
114 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 ...
6
votes
1answer
217 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
103 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 ...
5
votes
1answer
902 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
654 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 ...
10
votes
3answers
9k 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 ...