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Questions tagged [pathtracing]

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

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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 ...
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167 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 + \...
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1answer
115 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 ...
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1answer
71 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 ...
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29 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 ...
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58 views

NVidia OptiX: Load an OBJ in the PathTracer example

In the path tracer example in the SDK I want to add an OBJ file to trace, so I went over to the loadGeometry() function, and right after the last parallelogram ...
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2answers
504 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 ...
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1answer
70 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 ...
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58 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, ...
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37 views

What's the definition of the “emitted importance” in the measurement equation?

Let $M$ denote the set of all surfaces in the scene $\nu_M(x)$ denote the unit outer normal$^1$ at $x\in M$ $\Omega_x$ denote the upper unit hemisphere at $x\in M$ oriented according to $\nu_M(x)$ $\...
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119 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 ...
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85 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 ...
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94 views

smallpt's early clamping

Kevin Beason's smallpt estimates the pixel radiance by accumulating $2\times2$ subpixel radiance estimates using the following expression: ...
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1answer
148 views

How is Iridescence handled in a Path Tracer?

I have little knowledge of this topic but from what I gather in a Ray Tracer each ray is given its own wavelength as opposed to an RGB value. This automatically simulates Dispersion and Fluorescence. ...
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85 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 ...
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96 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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1answer
161 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 ...
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1answer
212 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 ...
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93 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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1answer
112 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 ...
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1answer
144 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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66 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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41 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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1answer
147 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, ...
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1answer
449 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, ...
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88 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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1answer
113 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 ...
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76 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-...
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171 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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201 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 ...
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70 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 ...
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1answer
83 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 ...
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0answers
93 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
183 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 ...
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285 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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2answers
143 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. ...
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1answer
150 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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47 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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160 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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1answer
132 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....
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76 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 ...
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1answer
424 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 ...
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1answer
101 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 ...
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1answer
167 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 ...
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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). ...
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1answer
182 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 ...
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3answers
3k views

Is Russian Roulette really the answer?

I have seen that in some implementations of Path Tracing, an approach called Russian Roulette is used to cull some of the paths and share their contribution among the other paths. I understand that ...
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1answer
887 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 ...
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197 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 ...
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130 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 ...