Questions tagged [pathtracing]

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

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Direct Light Sampling produces way too bright images compared to naive diffuse bounces only

it's me again! :D I have finally implemented area lights, but without modifying the emission value of the material, this is what it looks like with indirect light only, this is what it looks like with ...
Syrinxos's user avatar
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1 answer
87 views

What are some ways of modelling data for GPU side where e.g. OOP is not available?

I'm currently on a journey of writing a basic path-tracer. My current implementation uses fragment shaders to run the path tracing algorithm per pixel. I am at the point where I need to start ...
unusually_crispy_crisps's user avatar
2 votes
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53 views

Multiple importance sampling with Next Event Estimation gathers less energy compared to One-sample-model

I'm developing a webgpu pathtracer using multiple importance sampling with two similiar techniques, next event estimation and one-sample model. For some reason, next event estimation seems to gather ...
Domenico's user avatar
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1 answer
68 views

Point light source radiance

I was reading Deriving radiance from irradiance and intensity and the last comment said that by dividing $r^2$ we convert radiant intensity to irradiance. What confuses me then is the fact that for ...
Balu's user avatar
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Measure (or Jacobian) conversion for direction sampling

If in the path tracing use case, I have some sampling strategies to sample a scattering direction in a 2D space ($\theta, \phi$). Of course, I need to convert it to a 3D direction vector (with length ...
Enigmatisms's user avatar
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73 views

Forward path tracing versus backward path tracing. How may I achieve realistic shading in forward path tracing?

I have a forward path tracer. Essentially, it looks flat: This is the backward path tracer in comparison: My question is: How do I get pseudorandom lighting like with the backward path tracer, when ...
shawn_halayka's user avatar
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44 views

Understanding the BRDF Equation

I am studying the BRDF equation. From Wikipedia, the equation is as follows: I have one main question: Where does the $\cos{\theta_i}$ in the denominator come from? The denominator implies that $...
Christopher Miller's user avatar
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117 views

Evaluating the Rendering Equation by hand (simple case)

I'm trying to make intuitive sense of the rendering equation and so I'm trying to work out a trivial example by hand, though ultimately I'm just trying to understand how monte carlo importance ...
Chris Gnam's user avatar
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How does a path tracer capture light falloff if it deals exclusively with radiance?

My understanding is that path tracers deal with radiance because radiance is constant along a ray. You simply go through and evaluate the light transport equation: $$ L_o(p, \omega_o) = L_e(p,\...
Chris Gnam's user avatar
1 vote
0 answers
58 views

Vulkan (Hardware) Path Tracing Formulation

I've been implementing a path tracer in Vulkan using the VK_ray_tracing extension and am a bit confused on how to set up the pipeline. Currently I've been following the style of the Reference Path ...
sshashank124's user avatar
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What performance to expect from an optimised pathtracer?

I am currently writing a fairly basic pathtracer as a compute shader in C++/CUDA, which has been running on a GTX 1080, and I have noticed that when logging some statistics, I get around ~1 billion ...
Jack Freeth's user avatar
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1 answer
98 views

How to approach implementing Cook-Torrence microfacet brdf?

Trying to implement the Cook-Torrance BRDF for my path tracer. Confused about how to do it though. I've seen two approaches: 1: Sample the NDF, evaluate diffuse and specular components and add them ...
Ruben's user avatar
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Combining Path Tracing and Photon Mapping

I'm trying to implementing the paper "Deep Kernel Density Estimation for Photon Mapping". One part that has me confused is when the authors describe their rendering algorithm as hybrid ...
Maher Raess's user avatar
1 vote
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70 views

Confusion about different sampling strategies in Monte Carlo Path Tracing

I am studying Monte Carlo Path Tracing from different sources. However, I have some confusion about the sampling part and dare to post here for experts' ...
bim's user avatar
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1 answer
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Fixing bias in Specular lobe

Im currently making some modifications for a path tracer for Minecraft, specifically adding glossy refractions. The program currently uses for specular reflections this method, linear interpolation ...
Consuelo Ugarte's user avatar
2 votes
1 answer
206 views

Cosine weighted hemisphere sampling is a little bit darker and arguably noisier than reference

I'm writing a small path tracer that currently: Samples a random light source at each bounce (direct lighting) Bounces rays around multiple times (indirect lighting) The scene only contains ...
Tom Clabault's user avatar
1 vote
1 answer
305 views

Importance sampling of the blinn-phong brdf in pathtracing

tl;dr: How do you importance sample the blinn-phong-brdf? Recipe for importance sampling of the phong brdf as far as i understood it (pseudo-code): ...
Berti Krüger's user avatar
1 vote
1 answer
159 views

Importance sampling in a path tracer

So, I'm attempting to implement a monte carlo path tracer that uses the ggx brdf model. However, I'm rather confused about how to sample the light direction. I know that theta and phi can be sampled ...
RandomPigYT's user avatar
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1 answer
35 views

Theoretical speedup from sample number in path tracing algorithm

Suppose, in a Kajiya style path tracing algorithm, I have 100x100 pixels, and I am using 32 samples per pixel. So, the total ...
bim's user avatar
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Bright results when importance sampling GGX NDF

I've been trying to importance sample the GGX NDF of my Cook-Torrance BRDF for some time now but I can't get it right. I've been following this article. Here's my code for the importance sampling part....
Tom Clabault's user avatar
1 vote
1 answer
87 views

Importance sampling microfacet BRDF and coordinate frame

I'm trying to importance sample the Cook Torrance BRDF for my path tracer but I'm not sure about the part where you have to change the sample from one coordinate frame to another. This resource states ...
Tom Clabault's user avatar
2 votes
1 answer
173 views

How to properly use the BRDF from glTF 2.0 specification in path tracing while using metallic/roughness materials?

I've built a renderer which should render glTF materials, as they are defined in their specification. So I should be able to render satisfying (similar to Cycles and Ospray) results just from the base ...
AllenTeehrsie's user avatar
1 vote
1 answer
182 views

Confusion about PDF defined in solid angle / area measure

When using MIS, we always want to express the PDF value in a unified measure in order not to render the ratio between them meaningless. Yet when I read through chapter 16 of PBR-book, I got confused ...
Enigmatisms's user avatar
1 vote
1 answer
131 views

Can anyone summarize in simple terms, how PSSMLT/MLT algorithms work?

I understand backwards path tracing and bidirectional path tracing (BDPT). However I'm failing to understand how Metrpolis Light Transport methods are even remotely feasible to run. My understanding ...
Chris Gnam's user avatar
1 vote
1 answer
238 views

Path tracing: how to deal with ray hitting an emitter

I am writing my first path tracer and kind of feeling confused by some mechanisms. For example, when I trace a ray and the ray hits a area emitter, then how to determine the radiance of this direct ...
Enigmatisms's user avatar
4 votes
1 answer
147 views

Why do I need to remove the positive charactaristic function for the pdf of the outgoing direction to integrate to 1 with GGX?

Context: I am attempting to implement Trowbridge–Reitz (GGX) based on Microfacet Models for Refraction through Rough Surfaces in a pathtracer. I use numerical integration to check if the pdfs ...
Nicholas Leon's user avatar
2 votes
1 answer
203 views

Rendering equation integration variable change

I'm watching the TU Wien rendering lectures in the course we were presented with a rendering equation (first one on the screenshot) and then another, which results from a change of variables (if I ...
analysis1's user avatar
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88 views

Trouble getting a raytracer camera representation for different aspect ratios

I am trying to make my camera representation work for images whose aspect ratio isn't 1 (640x480, 1920x1080...), but I am having some trouble getting it to work. The camera model is pretty simple, as ...
Lightsong's user avatar
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1 vote
1 answer
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Path tracing - colors/shadows sharper than they should be

Problem description When comparing Cornell Box image generated by my path tracer I noticed that colors are much more sharp/saturated (don't know specific term) than they should be. Comparison (on the ...
RafalMaziejuk's user avatar
1 vote
0 answers
64 views

Question about reconstruction filter in ray tracing?

Suppose we have an 512x512 image, then the value of pixel j should be $I_j = \int_{0}^{512} \int_{0}^{512} h_j(u,v)L(u,v) \,dudv$, where $h_j(u,v)$ is the filter function for pixel j, $L(u,v)$ is the ...
Andy's user avatar
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Why does cos sampling directions over hemisphere use cos theta distribution instead of cos theta / pi distribution?

When choosing a probability density function for Monte Carlo evaluation of rendering integrals, I get how importance sampling suggests choosing e.g. $cos(\theta)$ for $p(x)$, and then, over say the ...
James B's user avatar
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1 vote
1 answer
54 views

How do I convert pathtraced distance to value that is usable by OpenGL's depthbuffer?

I was hoping someone could check my understanding. I've got a pipeline that does some path tracing, and then draws some debugging rays using standard rasterization. I'm trying to get depth to work ...
Cascades's user avatar
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3 votes
1 answer
162 views

(Lorenz-)Mie phase function instead of Henyey-Greenstein?

Is it practical to implement the Lorenz-Mie(LM) phase function in a renderer ? $p(\cos\theta)=\frac{|S_1(\theta)|^2+|S_2(\theta)|^2}{4\pi\sum_{n=1}^\infty (2n+1)(|a_n|^2+|b_n|^2)}$ I'll spare you the ...
JSBach49's user avatar
1 vote
0 answers
155 views

Applying Monte-Carlo methods in path tracing

I've noticed that I consistently get worse results in path-tracing when I try to do monte-carlo correctly compared to when I don't, and I think this might be because of a fundamental misunderstanding. ...
Kate's user avatar
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0 votes
1 answer
84 views

What is the rendering cost of photo realistic video today?

I can only find online infomation such as some scene need days to render a single frame, but I'm not sure what is the hardware based on, it may be based on CPU rendering farms in the 2000s. There is ...
jw_'s user avatar
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2 votes
1 answer
85 views

What does 'free-path' mean?

I've seen some phrases like 'free-path', 'free-path distributions', 'mean free path' in some computer graphics resources, but none of them explains what does these phrases mean, does anyone have an ...
Wenjian Zhou's user avatar
2 votes
1 answer
206 views

importance sampling rectangular light

I have been struggling very much to wrap my head around this part of Peter Shirley's book. There is no explanation what the angle Alpha represents and to make things worse in the code the cos(Alpha) ...
Sina Dasht's user avatar
2 votes
1 answer
751 views

Rendering equation - brdf / pdf > 1?

I am creating a path tracer and I got some strange results when calculating the strength of light. I am using a rendering equation inspired by this diagram: When I use the cosine everything gets ...
Vít Gardoň's user avatar
2 votes
0 answers
313 views

How are point and pure directional lights sampled in an unbiased path tracer?

To perform importance sampling on a light source, a probability density function must be provided for the sampling distribution. For most light sources, a valid probability distribution can be found, ...
SupaKoopaTroopa64's user avatar
3 votes
1 answer
325 views

Next Event Estimation for Triangle-based Light Sources

I have a question regarding the NEE for light sources that are composed from triangles. I implemented the next event estimation (explicit light source sampling) for spherical light sources but after ...
Metalian's user avatar
1 vote
0 answers
94 views

Why my bsdf can not pass Chi2test?

I implemented a MicrofacetTransmission in nori. But it can not pass all Chi2test when alpha is large(>= 0.4 or ?),part of chi2test is passed.And I can not found the problem.Can someone help me? ...
bin guo's user avatar
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0 votes
1 answer
824 views

What is meant by ‘NEE’ in computer graphics?

I am reading this paper [Practical Path Guiding for Efficient Light-Transport Simulation, Müller et al, 2017] and it contains the following sentence: In practice, however, we recommend combining path-...
Jonas De Schouwer's user avatar
1 vote
1 answer
341 views

PDF for sampling emissive meshes for NEE/MIS

I'm writing a Monte Carlo path tracer, and I'm trying to allow any mesh to be an emitter, but I'm not entirely sure about the probabilities to use when I sample them. Right now, my algorithm for light ...
uhwo's user avatar
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3 votes
0 answers
53 views

How much information should intersection routines return in a path tracer?

When designing/coding/"architecting" some geometry-ray intersection routines for a path tracer, there seems to be a spectrum of potential designs, with these two extremes: Return the ...
Simon's user avatar
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1 answer
117 views

HalfEdge: How to get list of all faces from one face

I need to, starting from one face, iterate over the rest of the faces until I reach the first facet. For this I can use twin, start, end or other info of halfedges, faces, vertex. That is, I need to ...
JoungCPPser's user avatar
0 votes
0 answers
119 views

Unity crashes when running path tracing

I'm studying path tracing, at the moment I created a compute shader that aims to calculate the intersection of rays with meshes. However, when I run the code the unity crashes, displaying a graphics ...
Matheus Markies's user avatar
0 votes
2 answers
364 views

Can someone explain why raytracing isn't able to make Global Illumination?

I've watched a few videos about raytracing vs pathtracing, and one thing that consistently pops up is the idea that pathtracing seems to be able to do global illumination, whilst raytracing is not. ...
LinguiniThePasta's user avatar
3 votes
0 answers
506 views

Path tracing : fireflies/hot pixels when directly sampling lights

I am implementing a pathtracer with both indirect and direct light sampling at each intersection (I believe the "direct light sampling" part is also referred to as "next event ...
Ewaren's user avatar
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0 answers
77 views

Unity path tracing makes the pc crash

Good Morning. I'm testing Path Tracing techniques with unity, following this tutorial. http://three-eyed-games.com/2019/03/18/gpu-path-tracing-in-unity-part-3/ But I'm in trouble. Whenever I try to ...
Matheus Markies's user avatar
0 votes
1 answer
111 views

Magic number meaning in smallpt's camera ray calculation

In smallpt's source at line 90 the direction is multiplied by 140 and I was wondering where that number is coming from. I was hoping for it to be explained in the ...
Alexandru Ica's user avatar

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