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Questions tagged [brdf]

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7
votes
1answer
312 views

What is the most physically accurate model of surface materials possible in computer graphics?

I am looking for theoretical and implemented models to represent surface materials in software that strives for physical accuracy. Is BRDF really the best model? What alternatives are better? Why? ...
8
votes
1answer
648 views

Why normalise Lambertian BRDF by 1/pi?

Why is a Lambertian BRDF normalised by dividing by $\pi$? Since the area of a unit sphere is $4 \pi$, and the area of the half sphere above the surface is $2 \pi$, shouldn't it rather be $1/(2\pi)$?
7
votes
1answer
243 views

Derivation of Wikipedia's path tracing diffuse BRDF

Wikipedia's path tracing algorithm has the following line for the diffuse BRDF: Color BRDF = 2 * m.reflectance * cos_theta; I'm a little confused as I've been ...
6
votes
2answers
108 views

Problem with Basic Diffuse Lighting

So recently I have been learning how to implement Diffuse Lighting in DirectX and I have done so with the following code: ...
5
votes
0answers
95 views

Should I use nonsymmetric BSDF for refraction in BPT?

I'm implementing bidirectional path tracing. I want to simulate perfect refraction. I read in the Veach thesis that a perfect refraction BSDF is non-symmetric. Does it mean that I need to use ...
3
votes
1answer
192 views

Real time image based lighting for arbitrary BRDFs coordinate alignment

I was trying to understand how real-time illumination happens with image-based lighting. I understood that importance of representing the Environment Map and BRDF in spherical harmonics. Because ...
3
votes
1answer
252 views

Bug: Overly Sharp Highlights in PBR

I'm still working on my BRDF equations. Theoretically, all equations are correct, but the hilights are super sharp. I'm not sure if this is a result of me not using Image-Based Lighting (I currently ...
1
vote
1answer
272 views

Blinn Phong rendering

I am trying to render Blinn Phong as a BRDF model on a surface plane. I assume that every normal in every pixel is perpendicular with the plane. I know that to do a rendering we have to use 3d ...
10
votes
1answer
275 views

Why isn't a BRDF a ratio of radiances?

I'm Learning about BRDFs and wondering why the BRDF is defined as the ratio of outgoing radiance to a given direction and incoming irradiance from another direction. Why isn't the BRDF defined as ...
8
votes
2answers
1k views

Rendering equation - why unsolvable directly?

Why is the rendering equation, introduced by Kajiya in 1986, not solvable directly/analytically?
5
votes
1answer
414 views

How to correctly implement Lambertian BRDF with point light

I'm implementing a simple Lamberitan BRDF in GLSL, using a point light source. My fragment shader simply returns light_colour * 1.0/PI * cosine, which should model ...
4
votes
2answers
225 views

What is the effect of the BRDF denominator?

I was reading a paper on BRDF. I've come across this formula : $$ f(\omega_i, \omega_o ) = \frac{FDG}{4(N.V)(N.L)}$$ The (N.L) term can be cancelled by the cosine term which appears in the rendering ...
8
votes
2answers
953 views

In a physically based BRDF, what vector should be used to compute the Fresnel coefficient?

The well known Schlick approximation of the Fresnel coefficient gives the equation: $F=F_0+(1 - F_0)(1 - cos(\theta))^5$ And $cos(\theta)$ is equal to the dot product of the surface normal vector ...
9
votes
1answer
2k views

Correct form of the GGX geometry term

I'm trying to implement a microfacet BRDF in my raytracer but I'm running into some issues. A lot of the papers and articles I've read define the partial geometry term as a function of the view and ...
13
votes
1answer
2k views

Choosing Reflection or Refraction in Path Tracing

I am trying to implement refraction and transmission in my path tracer and I'm a bit unsure on how to implement it. First, some background: When light hits a surface, a portion of it will reflect, ...
3
votes
0answers
463 views

How to properly implement Phong material and importance sample it in a path tracer?

I'm recently implementing Phong materials in my path tracer. My implementation of a randomly sampled Phong material works and looks fine. But it doesn't work well if the shininess(aka power) is too ...
7
votes
5answers
308 views

Why does the 1/r² term appear with point sources?

I'm currently reading some books on radiometry. They mention that radiance is constant along a ray. It doesn't change with distance. However, I've seen some raytracer and they put the 1/r² factor when ...
3
votes
1answer
68 views

Data fitting SVBRDF with L-M

I am reading two shot svbrdf capture . I want to understand the data fitting. For example, I have my input image 3000x2000 pixels, and I divide it to create my source tiles of 192x192 pixels. The way ...
17
votes
2answers
4k views

How to properly combine the diffuse and specular terms?

As far as I understand, in a BRDF the Fresnel term is telling us the probability for a photon to be reflected or refracted when it hits a surface. The reflected photons will contribute to the ...
12
votes
1answer
488 views

Compensation for energy loss in single-scattering microfacet BSDF models

Single-scattering microfacet-based surface models like the original Torrance-Sparrow BRDF or derived models like the BSDF for rough dielectric surfaces by Walter et al. neglect inter-reflection of ...
6
votes
1answer
217 views

Ray tracing - BRDF using Cornell measured spectral data

I wrote a ray tracer that implements various BRDF models (Oren Nayar, Lamber, Torrance Sparrow and so on). Now I'm trying to implement a BRDF from measured data. I choose the Cornell database data ...
11
votes
1answer
142 views

Reasons of the assumptions for the microfacet distribution function?

The paper Microfacet Models for Refraction through Rough Surfaces (among others) reminds us the following assumptions about the microfacet distribution function D: Microfacet density is positive ...
6
votes
1answer
957 views

BRDF and Spherical coordinate in ray tracing

I developed a ray tracer that use standard phong/blinn phong lighting model. Now I'm modifying it to support physically based rendering, so I'm implementing various BRDF models. At the moment I'm ...
8
votes
1answer
344 views

How does Smith multiple scattering interact with diffuse subsurface scattering?

The Multiple Scattering Microfacet BSDFs with the Smith Model paper describes a statistical model for replacing the masking-shadowing functions in microfacet BSDFs (which account for paths with more ...
7
votes
1answer
1k views

Energy Conservation for Blinn-Phong BRDF

I am trying to sample from the Blinn-Phong BRDF. For testing, I use a spherical light source (Lambertian emission) sitting on a specular plane. Here's a reference image using energy-conserving Phong ...
13
votes
2answers
528 views

Fresnel reflectance of gold: red channel greater than 1?

I've been trying to understand some of the physical principles behind light and material interaction lately. In his talk Physics and Math of Shading, Naty Hoffman explains Fresnel reflectance and ...
4
votes
0answers
129 views

How to extract a BRDF from a shader [closed]

For normalization purposes, how to "extract" a BRDF function from an ad-hoc piece of pixel shading code ? If we have a shader function like: ...
11
votes
1answer
2k views

What is the accepted method of converting shininess to roughness, and vice versa?

Most modern renderers use physically-based materials and their models are often parameterized over roughness. Since this wasn't always the case with renderers, conventional assets often don't have a ...
34
votes
2answers
4k views

How physically-based is the diffuse and specular distinction?

The classical way of shading surfaces in real-time computer graphics is a combination of a (Lambertian) diffuse term and a specular term, most likely Phong or Blinn-Phong. Now with the trend going ...