Questions tagged [importance-sampling]

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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) ...
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101 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, ...
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2 votes
1 answer
80 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 ...
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4 votes
1 answer
62 views

Relationship between probability densities

I'm reading section 8.2 of Eric Veach's thesis, in regards to the relationship between probability densities per solid angle vs projected solid angle: $$\frac{dP}{d\sigma^\perp}(w_o) = \frac{dP}{d\...
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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 ...
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143 views

Integrating BRDF using Importance Sampling

I read some articles about BRDF integration with and without importance sampling and I don't understand one thing in equations. If we integrate a BRDF over hemisphere with uniform sample direction ...
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36 views

How to calculate MIS weight for indirect light path?

I am trying to add MIS into my render. So, looking into some shadertoy examples. https://www.shadertoy.com/view/MsXfz4 https://www.shadertoy.com/view/4sSXWt Here comes the problem. All of them are ...
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  • 131
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108 views

How can I make the simplest physically based mixed specular-diffuse BRDF (and how to properly sample it)?

I am having troubles making a BRDF that has both specular and diffuse part. Each one is weighted by a coefficient and the sum of coefficients should be equal to one. I want this layered BRDF to be the ...
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Double sided area light produces much more fireflies than single sided, how to fix it?

I am testing area light with my render. I prefer double sided light, because it produces some glow around the light. But it produces much more fireflies which is annoying. While the single sided light ...
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  • 131
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1 answer
53 views

When sampling direct light, what to do if testing ray been blocked by transparent object?

As the title above. I am trying to sampling direct light and indirect light. But I have many transparent objects in scene, it's very likely to block the testing ray. To my understanding, it's not good ...
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  • 131
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2 answers
216 views

PBR : Understanding importance sampling

I have read this article On PBR and in it we have NDF function defined as follows Then I have read this article on random vector generation Here are what I feel are the important bits. Now jump to ...
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Hero wavelength multiple importance sampling

I want to add spectral rendering and dispersion to my pathtracer. I found the Hero Wavelength paper but I have trouble implementing this. This is how I understand the process and where I am stuck: ...
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Importance sampling GGX NDF - fireflies and bright final result

https://schuttejoe.github.io/post/ggximportancesamplingpart1/ - I have problem implementing this method. Somebody asked question about this before ( Can't understand the Importance sampling GGX ) ...
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2 votes
1 answer
424 views

Importance sampling a cosine distribution

Let's take a look at this blog article - https://schuttejoe.github.io/post/ggximportancesamplingpart1/ and image presented as a result of implementing it: Inside above link, you can see description ...
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1 vote
2 answers
145 views

issue with importance sampling

I was trying to do importance sampling on lambertian surface.At first I uniformly choose direction from unit sphere. ...
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2 votes
0 answers
302 views

How to properly combine diffuse and rough specular BRDFs in a path tracer

Looking around here on various sources (including related questions on this site), if my understanding is correct, when implementing a path tracer with a material which has both a diffuse term and a ...
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168 views

Understanding BRDF sampling

I'm trying to understand BRDF importance sampling following the exemples of the lambertian/diffuse model and the reciprocal Phong model. I would like to be able to extend the principle to other ...
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1 answer
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Confusion about light PDF

I have a doubt regarding shape lights pdfs, specifically various sources point at the fact that the pdf is a constant: $$ lightpdf = 1/A $$ Where $A$ is the area of the light surface. Expressed in ...
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Importance Sampling path tracer, Different pdfs result in different luminance?

I'm having a lot of trouble implementing Importance Sampling path tracer. If I understand it correctly, what I need to evaluate for a path tracer is this: $\int_\Omega L_{in}(L) * fr(L, V) * \cos(\...
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Drawing non-uniform samples from uniform density

To calculate the cartesian coordinates for scattering rays over a hemisphere based on cosine-weighted importance sampling, one method can be to obtain the marginal and conditional PDF for the polar ...
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How does multiple importance sampling work and how should I generate the samples?

Is my understanding of MIS algorithm true? I first generate a random number $x\in U(0,1)$ and $w_1=\frac{p_1(direct)}{p_1(direct)+p_2(hemisphere)}$, and then if $x < w_1$ generate the sample ...
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3 votes
1 answer
312 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. ...
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199 views

Pre-filtered environment map, deriving the equation

I'm reading through this article, and more specifically I'm trying to derive the equation that would explain the implementation the following shader (still in the same article): ...
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3 votes
1 answer
298 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}{...
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4 votes
0 answers
150 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-...
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1 vote
1 answer
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Can't understand the Importance sampling GGX

I've read many articles relating to importance sampling of GGX. However, I still feel confused. The formula of GGX is : And many article says we should use this formula to sample: In the formula, ...
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5 votes
2 answers
2k 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 ...
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4 votes
1 answer
666 views

BSDF sampling versus light sampling

I wish to understand the specific application of Multiple Importance Sampling (MIS) to weighting BSDF sampling and light sampling together. Refer to the following diagram from these Stanford course ...
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3 votes
1 answer
245 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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1 vote
0 answers
307 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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3 votes
1 answer
327 views

Importance Sampling in Unreal Engine Formula Derivation (Irradiance term)

I am currently trying to understand IBL in Unreal Engine, and there are so much things that I don't understand about the formula. Unreal approximate the specular term of shading equation by doing ...
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1 vote
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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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3 votes
0 answers
444 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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1 vote
1 answer
393 views

Direct illumination, specular surfaces, and MIS

Multiple Importance Sampling (MIS) is a technique used by Veach's VCM technique to balance responses of different surfaces to different types of importance sampling for direct illumination. My ...
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1 vote
0 answers
94 views

How to make Convolved Diffuse and Specular image in C#

I am trying to take either a HDR panoramic or HDR cubeMap and from that create a Lambert Irradiance environment map and a Cook-Torrence Specular environment map. I am stuck at the proper ...
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2 votes
1 answer
844 views

Power heuristic in multiple importance sampling

Both Mitsuba and Tungsten use the power heuristic to do multiple importance sampling (MIS). Mitsuba: ...
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1 vote
0 answers
127 views

An example of Combining Stratified Sampling and Importance Sampling in AGI

Being confused about the example of Combining Stratified Sampling and Importance Sampling in Advanced Global Illumination (P72,P73). I know how to do importance sampling and stratified sampling. But ...
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42 votes
2 answers
10k views

What is Importance Sampling?

What is importance sampling? Every article I read about it mentions 'PDF' what is that as well? From what I gather, importance sampling is a technique to only sample areas on a hemisphere that matter ...
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3 votes
1 answer
938 views

Efficiently sampling specific surfaces using MIS in path tracing

Apologies for posting two questions in one post, but as they are relevant I kept them in one. Question 1 In the definition of multi sample estimator that Veach has defined, the integrand is the same ...
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3 votes
2 answers
1k views

Generation of stratified cosine weighted samples on hemisphere

I have had luck with cosine weighted hemisphere samples, and I know how to generate stratified uniform samples, but I wanted to experiment with combining the two. However, how do I correctly stratify ...
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12 votes
1 answer
6k views

Does cosine weighted hemisphere sampling still require NdotL when calculating contribution for indirect light?

When converting from uniform hemisphere sampling to cosine weighted hemisphere sampling I am confused by a statement in an article. My current indirect contribution is calculated as: ...
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9 votes
1 answer
2k views

Path weight for direct light sampling

I originally wrote a pathtracer that just bounces by the BRDF and each sample is added to a buffer that divides by the current number of frames. Now I'd like to do next event estimation at each ...
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2 votes
0 answers
719 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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15 votes
3 answers
3k views

Importance Sampling of Environment Maps

What is the best currently known and ideally also production-verified approach for sampling environment maps (EM) in a MIS-based uni-directional path tracer and similar types of renderers? I would ...
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6 votes
1 answer
111 views

using importance sampling to reduce the error of outscattered radiance

http://renderman.pixar.com/view/implementing-a-skin-bssrdf In this paper on subsurface scattering, I'm trying to understand how importance sampling is used to compute single scattering. It says that ...
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11 votes
1 answer
1k views

What is the difference between importance sampling and mutiple importance sampling?

I can't understand math equations. I'm a graphic designer. What is "importance sampling"? What is "multiple importance sampling"? Could you explain easily, using illustrations and no math equations? ...
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  • 111
4 votes
1 answer
1k views

Multiple Importance Sampling with more than one light

(Note: This has been cross posted from my ompf2 post.) Recently I've implemented Multiple Importance Sampling for the sampling of surfaces in my ray tracer. I do this by, on each intersection, ...
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