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Questions tagged [importance-sampling]

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Distance sampling with unbiased transmittance estimators

I'm currently trying some things with transmittance estimators in PBRT and I have implemented the following paper: Unbiased Ray-Marching Transmittance Estimator. This method assumes that two points $x$...
Matthias K.'s user avatar
2 votes
1 answer
49 views

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
1 vote
0 answers
102 views

"Charlie" Sheen BRDF's directional albedo integrates to values higher than 1 at low roughnesses. Does this mean it is not energy conserving?

I am implementing the BRDF from the 2017 paper "Production Friendly Microfacet Sheen BRDF". This BRDF is modelled as a coat layer stacked on top of a base specular. In order to do so in an ...
avilapa's user avatar
  • 43
0 votes
1 answer
122 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
0 votes
1 answer
126 views

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
2 votes
1 answer
136 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
  • 23
1 vote
0 answers
74 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
  • 135
2 votes
1 answer
267 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
37 views

Sampling scattering direction around directions other than the last ray direction

In a path tracer, if we want to sample a ray direction in scattering medium, we can use phase function sampling, which actually use the direction of the ray before sampling as a sort of anchor to ...
Enigmatisms's user avatar
1 vote
0 answers
63 views

Questions about equi-angular sampling

Equiangular sampling is a sampling method for point (sphere) emitter in scattering medium. While it claims to be very efficient when dealing with point (sphere) emitter in scattering medium (see this ...
Enigmatisms's user avatar
1 vote
1 answer
349 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
58 views

Confusion about the bias during Monte Carlo integration

I recently stumbled into this question: Say, for example, we are doing mean-free-path sampling (distance sampling) in a scattering medium and to keep it simple, let's only consider the single ...
Enigmatisms's user avatar
1 vote
0 answers
52 views

Measure for volumetric rendering equation

Recently I've been digging deeper into the volumetric rendering (volumetric path tracing, to be specific). I already know that for surface rendering, there are two commonly used measures (reference ...
Enigmatisms's user avatar
2 votes
0 answers
46 views

How to properly use scattering coefficient during volumetric path tracing?

The increased radiance during sampling is known by: $$ L_s = L_e + \sigma_s \int p(w, w')f_p(w, w')dw' $$ The latter part, which accounts for in-scattering radiance bears a $\sigma_s$ in it. The ...
Enigmatisms's user avatar
0 votes
0 answers
138 views

One-sample model Multiple Importance Sampling with balance heuristic

Recently I feel a bit confused about one-sample model MIS. One sample model MIS can be found here in Veach 1997. 9.2.4 The one-sample model, and this one-sample model is widely used in the opensource ...
Enigmatisms's user avatar
1 vote
0 answers
65 views

Motivation for densities wrt the same measure in MIS

In Veach's thesis the MIS weights (e.g. using the balance heuristic) for two strategies with densities $p_1, p_2$ are given as $w_1(x) = \frac{p_1(x)}{p_1(x)+p_2(x)}$ and $w_2(x) = \frac{p_2(x)}{p_1(x)...
lightxbulb's user avatar
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0 votes
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
  • 135
0 votes
0 answers
40 views

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
90 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
1 vote
0 answers
120 views

Directly Sampling an Area Light

So in my path tracer I sample both some bounce ray given an importance function for the current BRDF, and if applicable (reflection is non-specular) I directly sample all light sources (for my ...
Chris Gnam's user avatar
2 votes
0 answers
160 views

MIS path tracing got darker image than brute force path tracing

I'm trying to implement the multiple importance sampling path tracing, the MIS path tracing uses the MIS to combine the direct light sampling and brdf sampling to evaluate the direct lighting. The ...
yys_c's user avatar
  • 21
0 votes
0 answers
170 views

Derivation of Sampling 2d Tent Distribution

I am trying to do the derivation of transforming uniformly distributed 2D points on the unit square into the 2D "tent" distribution below. i.e the standard method of getting the cdf and ...
ali's user avatar
  • 740
2 votes
1 answer
212 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
0 answers
323 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
329 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
3 votes
1 answer
88 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\...
James B's user avatar
  • 67
1 vote
1 answer
362 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
  • 55
1 vote
1 answer
517 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 ...
Emil Kabirov's user avatar
0 votes
1 answer
329 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 ...
Balfar's user avatar
  • 76
1 vote
1 answer
63 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 ...
iaomw's user avatar
  • 131
0 votes
2 answers
473 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 ...
Sync it's user avatar
  • 179
3 votes
0 answers
389 views

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: ...
lyinch's user avatar
  • 203
1 vote
1 answer
531 views

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 ) ...
DirectX_Programmer's user avatar
2 votes
1 answer
1k 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 ...
DirectX_Programmer's user avatar
1 vote
2 answers
190 views

issue with importance sampling

I was trying to do importance sampling on lambertian surface.At first I uniformly choose direction from unit sphere. ...
dsukrect's user avatar
3 votes
0 answers
626 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 ...
yggdrasil's user avatar
  • 145
3 votes
0 answers
219 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 ...
kipgon's user avatar
  • 151
0 votes
1 answer
228 views

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 ...
yggdrasil's user avatar
  • 145
2 votes
0 answers
222 views

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(\...
arctiq's user avatar
  • 31
1 vote
0 answers
139 views

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 ...
bitWise's user avatar
  • 233
3 votes
1 answer
474 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. ...
Jake's user avatar
  • 205
4 votes
0 answers
285 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): ...
user8469759's user avatar
3 votes
1 answer
431 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}{...
Andrew Farm's user avatar
4 votes
0 answers
165 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-...
maurock's user avatar
  • 189
1 vote
1 answer
2k views

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, ...
TIANLUN ZHU's user avatar
5 votes
2 answers
3k 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 ...
gallickgunner's user avatar
6 votes
1 answer
947 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 ...
SomeHumanBeing's user avatar
3 votes
1 answer
294 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 ...
gallickgunner's user avatar
1 vote
0 answers
343 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 ...
Paul Ferris's user avatar
3 votes
1 answer
467 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 ...
kevinyu's user avatar
  • 493