Questions tagged [importance-sampling]
The importance-sampling tag has no usage guidance.
60
questions
2
votes
1
answer
78
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 ...
1
vote
1
answer
33
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 ...
0
votes
0
answers
28
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 ...
1
vote
1
answer
129
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):
...
1
vote
1
answer
54
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 ...
1
vote
0
answers
42
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 ...
2
votes
0
answers
42
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 ...
0
votes
0
answers
93
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 ...
1
vote
0
answers
62
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)...
0
votes
1
answer
30
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 ...
0
votes
0
answers
29
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....
1
vote
1
answer
74
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 ...
1
vote
0
answers
96
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 ...
0
votes
0
answers
26
views
The initial sample for Metropolis Sampling
I just implemented metropolis sampling from PBRT 13.4.
The main loop was built with pseudo code:
...
1
vote
0
answers
138
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 ...
0
votes
0
answers
124
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 ...
2
votes
1
answer
175
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) ...
1
vote
0
answers
243
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, ...
3
votes
1
answer
243
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 ...
3
votes
1
answer
85
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\...
1
vote
1
answer
276
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 ...
1
vote
1
answer
397
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 ...
0
votes
1
answer
241
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 ...
1
vote
1
answer
62
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 ...
0
votes
2
answers
398
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 ...
3
votes
0
answers
344
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:
...
0
votes
1
answer
448
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 ) ...
2
votes
1
answer
920
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 ...
1
vote
2
answers
179
views
issue with importance sampling
I was trying to do importance sampling on lambertian surface.At first I uniformly choose direction from unit sphere.
...
2
votes
0
answers
542
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 ...
3
votes
0
answers
208
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 ...
0
votes
1
answer
208
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
$$
Where $A$ is the area of the light surface. Expressed in ...
2
votes
0
answers
217
views
Importance Sampling path tracer, Different pdfs result in different luminance?
I'm having a lot of trouble implementing Importance Sampling path tracer.
If I understand it correctly, what I need to evaluate for a path tracer is this:
$\int_\Omega L_{in}(L) * fr(L, V) * \cos(\...
1
vote
0
answers
134
views
How does multiple importance sampling work and how should I generate the samples?
Is my understanding of MIS algorithm true?
I first generate a random number $x\in U(0,1)$ and $w_1=\frac{p_1(direct)}{p_1(direct)+p_2(hemisphere)}$, and then if $x < w_1$ generate the sample ...
3
votes
1
answer
421
views
Rendering equation in terms of paths rather than directions
Path tracing seems to perform Monte Carlo integration where the space being integrated over is the space of paths (e.g. chains of rays) but the rendering equation is stated in terms of outgoing rays. ...
4
votes
0
answers
256
views
Pre-filtered environment map, deriving the equation
I'm reading through this article, and more specifically I'm trying to derive the equation that would explain the implementation the following shader (still in the same article):
...
3
votes
1
answer
407
views
How does Next Event Estimation work with transformed lights?
I am relatively new to path tracing, and I am trying to implement next event estimation. I am following the Monte Carlo integral used to compute the direct illumination from an area light:
$$\frac{1}{...
4
votes
0
answers
164
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-...
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, ...
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
...
6
votes
1
answer
866
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 ...
3
votes
1
answer
274
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
0
answers
336
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 ...
3
votes
1
answer
417
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 ...
1
vote
0
answers
61
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
0
answers
496
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 ...
1
vote
1
answer
485
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 ...
1
vote
0
answers
142
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 ...
2
votes
1
answer
1k
views
Power heuristic in multiple importance sampling
Both Mitsuba and Tungsten use the power heuristic to do multiple importance sampling (MIS).
Mitsuba:
...
1
vote
0
answers
166
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 ...