Questions tagged [pathtracing]
For questions about the path tracing Monte Carlo algorithm for physically accurate global illumination, or its variants.
2
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28 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
64 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
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1answer
91 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 ...
3
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1answer
182 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
87 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 ...
3
votes
1answer
138 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
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0answers
36 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
77 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
61 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, ...
0
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1answer
85 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
vote
1answer
117 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
134 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
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1answer
200 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
104 views
smallpt's early clamping
Kevin Beason's smallpt estimates the pixel radiance by accumulating $2\times2$ subpixel radiance estimates using the following expression:
...
1
vote
1answer
100 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 ...
1
vote
1answer
280 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
vote
0answers
128 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
243 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
vote
0answers
103 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
118 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
166 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 ...
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0answers
74 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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0answers
43 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
600 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, ...
2
votes
1answer
241 views
Is the microfacet GGX BSDF normally implemented as separate BRDF and BTDFs?
I'm trying to build a toy path tracer using something similar to Disney's BSDF, where many materials can be represented as combinations of properties like color, metalness, roughness, transmission, ...
1
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0answers
95 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 ...
2
votes
1answer
77 views
Path Tracer - Model loading Libraries?
so I am in the process of making my own GPU path tracer based on OpenCL as part of my bachelor thesis. I have already made a CPU and a GPU ray tracer based on OpenGL compute shaders but all I have ray-...
3
votes
1answer
115 views
Can a glass plate act like a lens?
I am trying to implement rough dielectrics on my renderer. I compared the results of my renderer and Mitsuba renderer. However, a thin glass plate which is a cube scaled by (0.4, 0.4, 0.01) magnifies ...
4
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0answers
74 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
votes
0answers
223 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
votes
0answers
98 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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1answer
87 views
Light Falloff through a grid
I'm playing with a grid-based lighting system, where light is transported and accumulated through a grid. I initially tried attenuation based on ...
4
votes
2answers
148 views
Inconsistent shading in pathtraced image
I am currently building a pathtracer, which has support for vertex normals and face normals.
There seems to be an issue when it comes to rendering objects with triangles exported from Blender. ...
1
vote
1answer
217 views
Phong Modified BRDF Implementation
According to: https://www.cs.virginia.edu/~jdl/importance.doc
For a specular sample: (page 4 of the document) $$pdf(\theta_{i}) = \frac{n+1}{2\pi} \cos^n(a)$$
And this is how you choose an ...
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0answers
55 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 ...
1
vote
1answer
81 views
Ringlike artifacts produced by rays bouncing off a light source
I recently set up a (very basic) ray-marched path-tracer, and I'm getting strange surface patterns on my area light [link]. I'm using known-valid analytical normals for the light's surface, and the ...
1
vote
1answer
143 views
Structural artifacts in ray jitter
I recently set up temporal super-sampling in my path-tracer, and now I'm trying to improve early samples by adding random offsets between -[PIXEL_WIDTH] and +[PIXEL_WIDTH] to each initial ray position....
3
votes
1answer
500 views
Probability density function while using spherical coordinates
I'm following this tutorial where at somepoint the derived PDF for spherical coordinates for a Lambertian surface is
\begin{array}{l}
p(\theta, \phi) = \dfrac{\sin \theta}{2 \pi}.
\end{array}
But as ...
2
votes
1answer
105 views
Primary ray directions (MC Path tracer)
Do primary rays conform to the frustum? If they're randomly directed, is the randomness fitted to some distribution (as it would be for a diffuse BRDF? I'm building my existing raymarcher into a monte ...
5
votes
0answers
194 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 ...
3
votes
2answers
39 views
How to get the length of paths in an given font/character
I wan't create some LED-rope text, using some script font, like the following:
For this I need somewhat estimate the needed length of the LED-rope for the given letter (or combinations of letters).
...
4
votes
1answer
206 views
How to do heterogeneous volume absorption using path tracing?
I want to do volume rendering using path tracing with a absorption only model. And my data is in a 3D grid, which is turned into a voxel medium in my renderer.
However, I can't seem to be correct ...
7
votes
2answers
597 views
Path Tracing & Materials
I'm wondering about the technique used when sampling a layered material for the next event in a path tracer. I have a material with three layers: a base diffuse, specular and sheen lobes. How does one ...
2
votes
1answer
998 views
Path tracing with Next event estimation and area lights auto occlusion
I am working my way through my first path tracer and I am currently designing how to deal with area lights. I have two questions:
Each object in my scene can have a Light attached to it, this is how ...
7
votes
1answer
176 views
An Effective way to explain Ray Tracing?
In my CS class in high school I'm doing a presentation on Computer Graphics and I wanted to talk about Ray Tracing. I was wondering what would be the easiest way to do this without confusing my ...
3
votes
0answers
223 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 ...
1
vote
1answer
146 views
PBRT path tracing: negation of ray direction before next event estimation
I am very new to computer graphics, so I am sorry if this results in a stupid question.
For educational purposes I am looking through the pbrt code and I have noticed that before computing the ...
9
votes
0answers
294 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 ...
5
votes
1answer
198 views
Phong BRDF in Mitsuba, Tungsten and AGI
In the book AGI (Advanced Global Illumination), the Phong BRDF is
$f_r(x,\Psi\leftrightarrow\Theta)=k_s\cfrac{(R\cdot\Theta)^n}{N\cdot\Psi}+k_d$
$k_d=\cfrac{albedo}\pi$
In Mitsuba and Tungsten, the ...
4
votes
1answer
634 views
Compute sphere tangent for normal mapping
I'm implementing normal mapping on my toy pathtracer. I need to compute the tangent (and bitangent) of any sphere point in order to create the matrix that will transform tangent space to world space.
...
