Questions tagged [pathtracing]
For questions about the path tracing Monte Carlo algorithm for physically accurate global illumination, or its variants.
167
questions
0
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1answer
43 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
votes
2answers
79 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$...
2
votes
0answers
50 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
votes
1answer
64 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
34 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
31 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 ...
0
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0answers
19 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 ...
1
vote
1answer
79 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
119 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
83 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:
...
3
votes
0answers
128 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 ...
0
votes
0answers
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....
1
vote
0answers
84 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 ...
0
votes
0answers
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 ...
0
votes
0answers
77 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 ...
0
votes
0answers
44 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, ...
1
vote
1answer
73 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 ...
1
vote
0answers
33 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 ...
0
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0answers
41 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 ...
1
vote
0answers
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 ...
2
votes
1answer
149 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 ...
0
votes
0answers
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/...
0
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0answers
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:
...
0
votes
0answers
33 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 ...
2
votes
0answers
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}=(\...
0
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0answers
34 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 ...
0
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0answers
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 ...
1
vote
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
votes
1answer
66 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 ...
4
votes
1answer
144 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 ...
0
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0answers
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 ...
0
votes
1answer
50 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:
...
0
votes
1answer
145 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
70 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(\...
2
votes
1answer
153 views
Why is Russian Roulette used in ray tracing?
Why should we use Russian Roulette? Does it reduce ray tracing artifacts?
1
vote
1answer
117 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 ...
1
vote
0answers
76 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?
0
votes
0answers
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 ...
0
votes
0answers
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 ...
1
vote
0answers
88 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:
<...
2
votes
1answer
95 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:
...
1
vote
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 ...
0
votes
0answers
76 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)$$...
0
votes
0answers
37 views
Probability of choosing the current strategy in the pbrt implementation of Multiplexed Metropolis Light Transport
In the description of the Multiplexed Metropolis Light Transport implementation of pbrt it is claimed that
"the inverse probability of choosing the current strategy $(s,t)$
[...] is equal to <...
1
vote
0answers
26 views
Image contribution function and reconstruction using filters
So I've been reading Physicall Based Rendering which in section 16.4.3 defines the "image contribution function" to be the following
$$I_j = \int_\Omega h_j(X)L(X)d\Omega$$
where the variables are ...
4
votes
0answers
218 views
Proper way of consuming stratified samples in path/ray tracing
I'm working on a (toy-level) path/ray-tracer, currently trying to implement (and understand) improved sampling strategies.
The obvious next choice after uniform random sampling would be stratified/...
2
votes
0answers
130 views
The termination probability of Russian roulette in PBRT
As described in PBRT 14.5, the path tracer stops bouncing rays at a probability q, which is determined by the throughput of the path.
...
4
votes
1answer
157 views
Rendering equation in terms of paths rather than directions
Path tracing seems to perform Monte Carlo integration where the space being integrated over is the space of paths (e.g. chains of rays) but the rendering equation is stated in terms of outgoing rays. ...
1
vote
0answers
45 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
134 views
Offline path tracer produces a darker image than expected
I've been going though Peter Shirley's beginner raytracing books. Upon building the Cornell box and rendering an image, I get a much darker / dimmer image than expected.
The color values are ...
