Questions tagged [pathtracing]
For questions about the path tracing Monte Carlo algorithm for physically accurate global illumination, or its variants.
133
questions
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2answers
110 views
Why is Russian Roulette used in ray tracing?
Why should we use Russian Roulette? Does it reduce ray tracing artifacts?
1
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1answer
85 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
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0answers
36 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
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0answers
35 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
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0answers
28 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
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0answers
56 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
84 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
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0answers
23 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
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0answers
52 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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30 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 <...
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0answers
23 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
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0answers
101 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
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0answers
96 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.
...
3
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1answer
104 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
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0answers
42 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
75 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 ...
1
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0answers
40 views
Path tracing best materials to learn
i want some of your advices. To begin with i ve never studied computer graphics and i want to do a project with Path tracing which i didnt find the right materials to begin with. Is it recomeneded to ...
0
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0answers
25 views
How to calculate rotational irradiance gradient vector?
I am reading the irradiance gradient on Greg Ward Radiance book and also this web page:
Quoting from this web page:
For normalized spherical coordinates the gradient is:
The irradiance ...
3
votes
0answers
104 views
Can I use Path tracing together with Ray Marching?
So I'm trying to render participating media like clouds/sky/fog (Sky is the first step) etc. I had already implemented a normal pathtracer and I was thinking of extending it. But I heard terms like ...
2
votes
1answer
63 views
How does Next Event Estimation work with transformed lights?
I am relatively new to path tracing, and I am trying to implement next event estimation. I am following the Monte Carlo integral used to compute the direct illumination from an area light:
$$\frac{1}{...
4
votes
0answers
124 views
Learning light transport using Q-Learning
I am trying to reproduce the results obtained by Dahm et al. in the paper Learning Light Transport the Reinforced Way.
This method takes advantage of the similarity between the Bellman equation (Q-...
0
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1answer
147 views
smallpt: ray scattering and Importance Sampling
Introduction
For those familiar with smallpt by Kevin Beason:
In path tracing, when rays hit a surface, they are scattered and the theta angle (the angle between the normal to the surface and the ...
1
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0answers
99 views
Should I expect differences with explicit light sampling other that the noise?
I am trying to understand why my explicit light sampling implemetation does not produce the same results as my naive path tracer. Here are my results :
Naive Version :
Explicit light sampling ...
1
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1answer
94 views
smallpt: when to stop diffuse ray scattering
For those familiar with smallpt, the ray tracer in 99 lines: I am checking the code, and there is something that is not clear to me in the DIFFUSE ray scattering.
After the rays are shot, they keep ...
5
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0answers
155 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 ...
0
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0answers
105 views
Path tracing - Better images without Gamma correction?
So I read about tonemapping and Gamma Correction recently and have implemented it in my path tracer.
For the time being I'm using Reinhard tonemapping operator as described in his paper
$L_d = L_w *...
3
votes
0answers
191 views
How to implement explicit light sampling when implementing path tracing?
I implemented a path tracer and got a similar result like this one.
Sample: 4096, time used: 537.05s
The algorithm is exactly the same as the one in wikipedia, where only the Gathering Path is ...
0
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2answers
191 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. ...
2
votes
1answer
204 views
Naive Path tracer produces much darker images than Explicit light sampling
So I just thought of comparing the results from a Naive Path tracer and one using Next Event Estimation aka Explicit Direct Light Sampling. However the results from the Naive PT are very dark.
Is ...
5
votes
2answers
799 views
Multiple Importance Sampling in Path tracer produces Dark Images
So I recently implemented Multiple Importance Sampling in my path tracer which was based on next event estimation.
The problem is without MIS I get images like,
This is obtained by setting
...
1
vote
1answer
138 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 ...
4
votes
1answer
211 views
High variance in path tracing specular objects
I'm testing my path tracer for specular surfaces. Images rendered with my own code have very noisy appearance, and I think they look like variance.
I use point light and the bunny is of mirror ...
4
votes
0answers
49 views
Does MRG32k3a compare well with PCG and Xorshift256*?
I've been using standard 32-bit Xorshift in my GPU path-tracer for a while, following Nathan Reed's approach with hashed seeds and decorrelated state. I (finally) discovered today that Xorshift is ...
1
vote
1answer
85 views
How to handle GUI in a path traced renderer?
This isn't a pure CG question but it's more of a programming one related to CG.
The main problem that occurs is both the GUI and the rendered scene need to be drawn regularly by swapping buffers. If ...
2
votes
1answer
102 views
How do we approximate the integral of the measurement contribution function in Metropolis Light Transport?
In Metropolis Light Transport, we need to approximate the integral of the measurement contribution function. I've read that this can be done using a traditional approach like path tracing. However, ...
1
vote
1answer
198 views
How does Primary Sample Space Metropolis Light Transport work?
For simplicity, assume we're only dealing with surfaces which have either a Lambertian or perfectly specular material. Morever, assume that the only type of lights are area lights (i.e. surfaces with ...
1
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1answer
195 views
Path tracer isn't converging
So I just completed a naive path tracer that uses explicit direct light sampling.
Problem is the path tracer isn't converging. It's like a raytracer, showing a single image on each pass. More info on ...
3
votes
1answer
275 views
Is there ongoing research on Markov Chain Monte Carlo algorithms (other than Metropolis-Hastings) for use in ray tracing?
Metropolis Light Transport is an application of the Metropolis-Hastings algorithm. Its variants differ only in the used "mutation strategy".
Is there ongoing research on other Markov Chain Monte ...
4
votes
1answer
355 views
PDF when sampling from Rectangular Area Lights
So I was reading up on how to sample Rectangular Area lights, and the procedure is pretty straightforward. That is, I can get random points using the equation.
$ p = X_o + \epsilon_1V_1 + \...
1
vote
2answers
179 views
smallpt's early clamping
Kevin Beason's smallpt estimates the pixel radiance by accumulating $2\times2$ subpixel radiance estimates using the following expression:
...
2
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1answer
149 views
Path Tracer - sampling dimensions confusion
Suppose my path tracer is shooting 64 rays per pixel (in an 8x8 grid). The confusion is, when accounting for Global illumination, should I shoot additional X number ...
2
votes
1answer
892 views
How does a path tracer with next-event estimation work?
I am trying to implement a simple path tracer with next-event estimation in Java. The general idea is to trace a ray through the scene as usual (using a cosine distribution to determine the next ...
1
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0answers
235 views
Interpreting Cornell Box data for path tracer
So I want to path trace a Cornel Box scene, but I don't understand the values given here in section 2 and 3.
Cornell Box Data | Cornell University Program of Computer Graphics
No.2 Lists surface ...
3
votes
1answer
352 views
Should direct illumination and path tracers render the same scene equally bright?
I am using the book Ray Tracing from the Ground Up by Kevin Suffern to build my first ray tracers in Java. Based on the theory, I expected that direct illumination and simple path tracers render the ...
1
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0answers
145 views
Handling shadow acne for dynamic surfaces
My path-tracer is generally fine and gives good output for regular test scenes, but I've noticed dense fireflies and dark speckles over fractal surfaces. The noise disappears if I render without light ...
0
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1answer
132 views
BRDF ray directions
The BRDF takes two directions, $\omega_{o}$ and $\omega_{i}$. I've heard that both directions should point away from the surface (one towards the camera, one towards the light), but iq's Oren-Nayar ...
4
votes
1answer
240 views
Path Tracer - implementation questions
so I am currently in the middle of implementing a path tracer for my bachelor thesis. And so far I have understood and mapped the general flow of the program except for 2 things. Note that I'm using ...
1
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0answers
105 views
How to calculate solid angle subtended by a pixel?
How is the solid angle subtended by a pixel calculated?
both in perspective and fish eye lens.
1
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0answers
53 views
Reverse PDF updates for s-2 or t-2 vertices in MIS for BDPT
MIS weights for BDPT are calculated with ratios of PDFs for each vertex under different types of light transport (radiance vs. importance). These probabilities are regenerated for frontier vertices ...
3
votes
1answer
1k views
Importance sampling microfacet GGX
I have a renderer where the BxDF interface is Sample(), PDF(), and Eval(). The Lambertian BRDF is working well, and I believe I have properly implemented Eval for GGX based on another user's question, ...
