Questions tagged [pathtracing]

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

36 questions with no upvoted or accepted answers
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375 views

Using Monte carlo on Rayleigh scattering

Update I am editing and posting this question in a different way; this time from the point of view of Nishita paper. Sunlight gets scattered at P and attenuated before reaching Pv. Therefore ...
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168 views

Help with start up bias in Metropolis Light Transport

I'm trying to implement metropolis light transport based on this paper, and I have the basic thing working. I run the algorithm multiple times with different starting points, add the results for each ...
8
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695 views

More accurate Fresnel approximation for a RGB-based Path Tracer

I'm writing a path tracer and, for the moment, spectra related data (spectral power distributions and spectral reflectance curves) are stored as RGB tuples. In the Cook-Torrance specular BRDF I am ...
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287 views

How does path tracing handle specular surfaces(Dirac BSDFs) such as water?

I'm trying to render water using path tracing. However, since in path tracing, the light sampling is skipped while the intersection point is perfect specular and thus no direct light would be taken ...
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136 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 ...
4
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0answers
57 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/...
4
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109 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-...
4
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45 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 ...
4
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78 views

Predicting reflectance functions from complex surfaces

There are three principal ways to obtain a BRDF. We can Derive an analytic formula using physical principles Use simulation given an assumed or measured model of the surface microgeometry Measure ...
4
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0answers
274 views

Wrong output while implementing GGX importance sampling

I'm receiving a non-energy conserving output while trying to importance sample the GGX Distributionof the Microfacet model, which is generally 3-4 times bigger than the resulting PDF I'm following ...
4
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0answers
104 views

What techniques are used to build ray intersection acceleration structures on the GPU?

What improvements and alternatives have been developed since HLBVH was introduced in the 2010 paper HLBVH: Hierarcihcal LBVH Construction for Real-Time Ray Tracing of Dynamic Geometry by J. Pantaleoni ...
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203 views

Beer's law in dielectric material

I am trying to implement Beer's law according to this page in dielectric material but I am not sure have done it correctly as the image does not suggest so. The formula is: I(s) = I(0) * pow(e, ln(A)*...
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100 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 ...
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139 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 ...
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267 views

Mathematics of Sub Surface Scattering

Can somebody help explain the mathematics of Sub Surface Scattering in a Path Tracer based on the Diffuse Approximation in A Practical Model for Subsurface Light Transport ? I would like someone to ...
3
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0answers
376 views

Problem with specular BRDF (Monte Carlo & Path Tracing)

Above on the left is my attempt and on the right is what I'm aiming for.As you can see my sphere with the specular brdf has a weird dark ring In my radiance function I am recursing through the ...
2
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71 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. ...
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79 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
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270 views

How to simulate specular reflection in Light Tracing?

I am implementing light tracing from this website. The right wall on the below image is perfect mirror(specular reflection). Its BRDF returns zero that's why it's been rendered black. But how is it ...
2
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507 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 ...
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14 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 ...
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37 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 ...
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39 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 ...
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82 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 ...
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199 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 ...
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132 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 ...
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94 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.
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52 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 ...
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97 views

How to validate whether a reflection model is implemented correctly?

I think this is one of the most important part when you write a physically-based renderer. The solution I came up with is to put a sphere to the scene with a constant background radiance. Solve the ...
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66 views

How should I understand path-space measurement equation?

I read the corresponding chapter of PBRT-v3, as well as Veach's thesis which introduced BDPT. However I didn't quite understand why we use the measurement equation. It seems that the measurement ...
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63 views

Sample Correlation and Random Variables — An Intuitive Explanation?

I'm mainly trying to understand what it means for Monte Carlo samples to be correlated. Can somebody fit this into the theory of what I know about random variables, covariance and correlation? And ...
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66 views

Mitsuba Bidirectional Framework

I'm confused by the Mitsuba bidirectional framework for quite a while now, especially by the different eval and connect ...
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330 views

Volume Rendering: Implementation

Can somebody explain how to implement volume rendering and scattering? I've been told that this is the easiest and most effective method to render things like smoke, milk and fog. Scratchapixel has ...
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150 views

Pathtracing specular reflections vs. specular highlights

In path-tracing, is a specular reflection model like Blinn-Phong useful? Is it more accurate to render a complete reflection dependent on the objects roughness?
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24 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 ...
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89 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 *...