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Questions tagged [pbrt]

For questions related to techniques described in the book *Physically Based Rendering: From Theory to Implementation*

Physically Based Rendering: From Theory to Implementation

20 questions
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Plotting pbrt-v3 Gaussian filter

I want to plot the Gaussian filter of pbrt-v4 with a radius $(100,100)$ and a standard deviation ($\sigma$) of $0.5$. I would expect something like this: When I plot the pbrt-v4 Gaussian filter with radius $(100,100)$ and $\sigma=0.5$ I get: Is…
Vertexwahn
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Does the RGBSpectrum class in pbrt store RGB color values directly, not power?

I had a hard time to understand this. I thought *Spectrum classes (SampledSpectrum, specifically) store power (SI unit: W) in them and when I want RGB colors from them, I have to use some complicated method to convert power to XYZ then to RGB. I…
Cu2S
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Scaling of the final image in Metropolis Light Transport

I don't understand why the PBR implementation of Metorpolis Light Transport scales the final image by b / mutationsPerPixel. The authors write: Each Metropolis iteration within "Run nChains Markov chains in parallel" has splatted contributions with…
0xbadf00d
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How to decide which child of BVH to visit based on sign of ray's direction vector?

I'm reading about BVH from PBRT. According to the book, in the linear bvh part using morton encoding: An efficient way to perform a front-to-back traversal without incurring the expense of intersecting the ray with both child nodes and comparing…
Karthik
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How is the maximum value for alpha (roughness == 1) decided for microfacet models?

Recently I have been looking at pbrt, and was looking at their remapping from roughness to $\alpha$ values: https://github.com/mmp/pbrt-v3/blob/master/src/core/microfacet.h#L86 I understand that it is quite common to use some non-linear remapping…
Shane Simms
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BTF Find Four Closest Sampling Directions

I am trying to use the techniques discussed in this paper to implement a BTF into PBRT. I am stuck on how to actually interpolate the weights of the four closest sampling directions. At the BTF, I have converted $w_o$ and $w_i$ from cartesian to…
Dave
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PBRT: requests sample dimensions in the same order

PBRT 7.2.2 illustrates an example on requests sample dimensions in the same order: sampler->StartPixel(p); do { Float v = a(sampler->Get1D()); if (v > 0) v += b(sampler->Get1D()); v += c(sampler->Get1D()); } while…
Rahn
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Sampling Quad Shape Light Using Spherical Rectangles in Local Space

The paper "An Area-Preserving Parametrization for Spherical Rectangles" describes an efficient approach for sampling solid angles for rectangle light sources. I am wondering if The light source in world space is a quad shape (not necessarily a…
ali
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Conflicting definitions for the distribution of normals $D$ in microfacet BSDFs

Please do not confuse this question with this one. In Understanding the Masking-Shadowing Function in Microfacet-Based BRDFs, Eric Heitz defines the distribution of normals as. There, the footnote. Even the footnote description is confusing "per…
GroundGlassUnknown
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How to change a RGB value using a radiance value?

I am new to this awesome community, I am currently writing a ray tracer but I hit a problem... Let's say I have a surface(material doesn't matter) of colour [1, 0, 0] in RGB, and the radiance reaching a point on the surface is say 2000 units. So my…
The Light
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Is spectral response curve and spectral power distribution the same thing?

I'm reading section 5.2 of pbrt 3rd edition and came across the part where they explain the concept of RGB colors. When we display an RGB color on a display, the spectrum that is actually displayed is basically determined by the weighted sum…
Peter
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How can I account for specular interactions in shadow rays?

I've been working through PBRT, implementing my own version of it in Rust. Right now I'm working on shadow rays, and it looks like pbrt doesn't do anything to account for specular behavior, and treats materials like perfect glass as opaque with…
Calvin Godfrey
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Why is there a T factor in the definition of Shah given in Matt Pharr's Physically Based Rendering?

The specific chapter about this is here - Sampling Theory Unlike what I read anywhere else, it defines Shah as: $$Ш_T(x)=T\sum\nolimits_i{\delta{(x-Ti)}}$$ And the T is still present in the reconstructed function:…
Eugene
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An equation about PDFs and differentials in the book of pbrt-v3

Why does the equation at the bottom of the following page hold? http://www.pbr-book.org/3ed-2018/Monte_Carlo_Integration/Transforming_between_Distributions.html $p(\theta,\phi)d\theta d\phi=p(\omega)d\omega$ Is this a conclusion of probability and…
chaosink
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Considering a path generated by bidirectional path tracing under a different strategy

Say I have constructed a path, using bidirectional path tracing (BDPT), consisting of a light subpath $y$ with $s=2$ vertices and a camera subpath $z$ with $t=3$ camera vertices. Now I want to compute the contribution (measurement contribution over…
0xbadf00d
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