Questions tagged [importance-sampling]
The importance-sampling tag has no usage guidance.
67
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How to importance sample emissive textures on meshes?
I have a mesh in my scene. That mesh has an emissive texture mapped on it.
For my direct lighting integration, I'll need to sample that mesh. I could sample it uniformly and read the emissive texture ...
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Lots of bad samples (below the hemisphere) when sampling the GGX VNDF
I've noticed that my implementation of GGX VNDF sampling produces quite a lot of bad samples (below the hemisphere). When such a sample occurs, I terminate the ray, leading to an immediate black pixel....
- 217
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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 $x$...
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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 ...
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"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 ...
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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 ...
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151
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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,\...
- 229
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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 ...
- 23
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91
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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' ...
- 135
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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 ...
- 217
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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 ...
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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 ...
- 1,197
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443
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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):
...
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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 ...
- 1,197
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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 ...
- 1,197
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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 ...
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212
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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 ...
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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)...
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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 ...
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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 ...
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138
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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 ...
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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 ...
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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 ...
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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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370
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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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342
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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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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\...
- 67
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401
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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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601
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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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380
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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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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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503
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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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414
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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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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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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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657
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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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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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241
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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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142
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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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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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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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450
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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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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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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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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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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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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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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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