Questions tagged [pathtracing]

For questions about the path tracing Monte Carlo algorithm for physically accurate global illumination, or its variants.

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31 views

Why does my image has a square pattern in cuda path tracer?

I am writting a path tracer in cuda following the Shirley's book. I am having a square pattern as an output. The size of the block change if I change the block dimensions. For this image, the block ...
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1answer
34 views

Path tracing - cannot converge diffuse sampling + oversaturation

I'm in process of creating progressive path tracer using DX12 + DXR. I managed to create working raytracing code + pathtracing code with multiple bounces but I'm experiencing problems stated below (...
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1answer
44 views

Path Integral Form with Path Tracing

The description is going to be long and detailed, thanks in advance for your patience! I am learning the path integral form of light transport equation(LTE) to be able to follow recent rendering ...
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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 ...
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2answers
64 views

issue with importance sampling

I was trying to do importance sampling on lambertian surface.At first I uniformly choose direction from unit sphere. ...
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2answers
257 views

Why is eye-based ray tracing preferred over light-based ray tracing?

It seems that virtually all path tracers use eye-based or view-based path tracing. That is, the light rays originate from the camera rather than the light source. The reason given for this everywhere ...
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1answer
56 views

Probability density for explicit light sampling

When implementing explicit light sampling, I am separating the monte-carlo approximation to two parts $$\frac{1}{N+M}(\sum^N \text{direct} + \sum^M \text{indirect})$$ $$\text{direct} = \frac{\text{...
3
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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 $\lambda = 1$...
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0answers
52 views

Implementing monte carlo path traced refractions problem

I am writing a forward monte carlo path tracer with the added feature of specular refractions for transparent materials, I have managed to come this far but something is not quite right and I am ...
3
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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 ...
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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 ...
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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 ...
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21 views

Birectional path tracing - connection of paths

I have a few questions about setting up a bidirectional path tracer described by Veach here. I have created both an eye path and a light path. Both are using BRDF for calculating how much light can ...
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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 ...
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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 ...
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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: ...
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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 ...
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28 views

Wrong calculation of the balance heuristic weights in PBRT

Please consider section 16.3.4 of the PBR book, where the authors describe how the balance heuristic weights for the multiple importance sampling estimator in bidrectional path tracing can be computed....
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85 views

Visualize the variance of an image

Let $h_j$ denote the image reconstruction filter of pixel $j$. I'm estimating the color value $$I_j=\int h_jf\:{\rm d}\mu$$ of the $j$th pixel (see [Veach, Section 8.2]) by an asymptotically ...
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53 views

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 ...
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82 views

How to achieve higher FPS for my GPU pathtracer?

So I've written a GPU based pathtracer using OpenCL-GL interoperability. The system uses the Mega-Kernel Approach instead of a wavefront one as I was aiming it as an educational software for other ...
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45 views

How is the image reconstruction filter on the path space defined in terms of the filter on the raster space?

Say we have a image reconstruction filter $h:R\to\mathbb R$ defined on the raster space $[0,w]\times[0,h]$, where $w,h\in\mathbb N$ define the width and height of the image plane in pixels, ...
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1answer
77 views

BDPT - How to handle deterministic ray PDF?

I'm trying to implement a simple BDPT without MIS as described in Lafortune's paper. I've used the weighting scheme defined in the paper in which they suggest that the weight be based on the surface ...
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35 views

Terminology Question: Temporal Stability

I am reading the paper Blockwise Multi-Order Feature Regression for Real-Time Path Tracing Reconstruction and the authors use a term several times which I am not sure what it means. This is in the ...
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42 views

Case where ray hits corner of box

I am writing a path tracer and when wrting method for returning the normal for the plane that was hit, I am not quite sure what to do when the ray hits box in a corner, placing the intersection point ...
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37 views

How do I fix skipping ray-triangle intersections on concave edges/cavities of model?

I'm writing a path tracer and I have a problem with ray-triangle intersection tests. The problem happens most frequently with materials that implement random scattering of the ray. Here's a function ...
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1answer
172 views

Sampling the light vs. sampling the BRDF: difference in intensity

(possible duplicate) I'm trying to implement multiple importance sampling, as described in the PBRT book (no bouncing yet, I want to make MIS work first). What my problem is, that while the BRDF ...
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48 views

Subpath connection in Bidirectional Path Tracing for subpaths consisting of a single vertex

Consider the PBRT implementation of the subpath connection in bidirectional path tracing for the strategy with a single camera vertex, as described here: http://www.pbr-book.org/3ed-2018/...
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47 views

Distribution of the first vertex on a camera subpath in bidirectional path tracing

Suppose we are sampling camera rays of a perspective camera with a finite aperture. The PBRT implementation of the sampling scheme looks like this: ...
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34 views

Computing probabilties when connecting a camera subpath to a single light point

I'm struggling to understand how the connection step in bidirectional path tracing for the strategy with a single light vertex has to be implemented. The following code essentially shows the PBRT ...
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47 views

Chart swaps and path inversion in bidirectional path tracing

I've got a hard time to understand how we would implement the following in practice. I'd be great if someone could explain the details for the example below. Assume we're generating a path ${\rm x}=(\...
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35 views

Inverse path sampling in (bidirectional) path tracing

In Section 4.1 of the paper Fusing State Spaces for Markov Chain Monte Carlo Rendering, it is described that the CDF $P$ of the distribution of paths of length $k\in\mathbb N$ may be factored into a ...
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24 views

Computing inverses in Reversible-Jump Metropolis Light Transport

Say we are sampling a path $\rm x=(\rm x_0,\ldots,\rm x_{n-1})$ of length $n-1$ using bidirectional path tracing. Let $p_s$ denote the area product density corresponding to the used connection ...
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1answer
54 views

In bidirectional path tracing, is a camera subpath with $n$ vertices determined by a fixed number of random numbers?

In the Multiplexed Metropolis Light Transport implementation of the book Physically Based Rendering, the proposal samples are generated by the Primary Sample Space Sampler ...
3
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1answer
67 views

Intrepret path/light tracing by rewriting light transport equation

Given the rendering equation: $$ L_o(x, \theta_o) = L_e(x, \theta_o) + \int_{\Omega} \! f(x,\theta_i,\theta_o) L_i(x,\theta_i) \cos(\theta_i, \theta_o) \, \mathrm{d} \theta_i. $$ Rewriting it using ...
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1answer
147 views

What is a “light source” in global illumination?

In the context of global illumination and the scattering equation, do we need to understand "light sources" as surfaces with a "purely emissive" material on which no scattering is present? In ...
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50 views

Implementation of the Primary Sample Space Sampler in the book Primary Sample Space Sampler

In the Multiplexed Metropolis Light Transport implementation of the book Physically Based Rendering, the proposal samples are generated by the Primary Sample Space Sampler ...
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1answer
51 views

Trouble making path tracer multithreaded, read access violation

I'm trying to make the path tracer shown in the Ray Tracing in One Weekend series multithreaded. Here's the starting of my main function: ...
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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: ...
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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(\...
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1answer
165 views

Why is Russian Roulette used in ray tracing?

Why should we use Russian Roulette? Does it reduce ray tracing artifacts?
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1answer
127 views

Multiple samples per pixel and texture blurring

As far as I know path tracers (or super-sampling antialiasing) typically calculate the final color of a single pixel by averaging the result of all samples taken inside that pixel. This gives a nice ...
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79 views

Path tracing: How to do tone mapping and gamma correction?

I am not sure what is the correct way to do tone mapping and gamma correction in a Monte-Carlo Path Tracer. Should I apply tone mapping before or after averaging the samples?
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38 views

Questions in Veach path-space formulation

http://graphics.stanford.edu/papers/veach_thesis/chapter8.ps In chapter 8 of his thesis which he talks about the three-point form of the transport equations, equation (8.1) in particular, it seems ...
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36 views

Factorize probability densities in Bidirectional Path Tracing

Say we have fixed a length $k$ for paths we want to sample using bidirectional path tracing as described in the Physically Based Rendering book. Let $q_{s,\:t}$ denote the probability density of ...
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100 views

Why does the reflected component in Lambertian path tracing get multiplied by a factor of 2?

I would like to intuitively understand the Path Tracing equation/formula/pseudocode, but I'm not fully getting the last return statement. As per Wikipedia page: <...
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1answer
98 views

Strange artifacts while rendering a dielectric material

I am trying to render a dielectric sphere inside the Cornell Box. Some circles are generated on the sphere. Here is the photo generated by path tracer: And below is my main function: ...
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0answers
25 views

Report standard deviation in Rendering results

I noticed that a common metrics for image quality assessment (Monte Carlo path tracing) is MSE between the generated image and a reference one. My question is: why isn't the standard deviation ever ...
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81 views

Multiple Importance Sampling in Bidirectional Path Tracing

I'm trying to understand the computation of the path density described in the book Physically Based Rendering. They assume that a path $$x=(x_0,\ldots,x_{n-1})=(q_0,\ldots,q_{s-1},p_{t-1},\ldots,p_0)$$...
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27 views

Image contribution function and reconstruction using filters

So I've been reading Physicall Based Rendering which in section 16.4.3 defines the "image contribution function" to be the following $$I_j = \int_\Omega h_j(X)L(X)d\Omega$$ where the variables are ...