Questions tagged [global-illumination]

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15
votes
1answer
5k views

Progressive Path Tracing with Explicit Light Sampling

I understood the logic behind the importance sampling for the BRDF part. However, when it comes to sampling light sources explicitly, all becomes confusing. For example, if I have one point light ...
10
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0answers
474 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 ...
9
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2answers
2k views

Rendering equation - why unsolvable directly?

Why is the rendering equation, introduced by Kajiya in 1986, not solvable directly/analytically?
8
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2answers
831 views

Selection of GI techniques working in real time

I'm considering topic of my thesis connected with computer graphics - I would compare some GI techniques which work completely in real time (no precomputation to lightmaps etc. and each frame ...
7
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2answers
3k views

A recent approach for subsurface scattering

I read "A Practical Model for Subsurface Light Transport" and "A Rapid Hierarchical Rendering Technique for Translucent Materials". If I understand correctly, the former does not consider the case ...
6
votes
2answers
110 views

Global illumination leading to image variation

I have a 3D scene I use to generate some images which I then process in Matlab. I noticed that when global illumination (GI) is not enabled, rendering always generates exactly the same pixel-for-pixel ...
6
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1answer
769 views

Confused about prefiltering environment maps (Manson 16)

I'm working my way through this paper about GGX-filtering environment maps: http://www.ppsloan.org/publications/ggx_filtering.pdf I find myself stuck on the basics. In particular, I am confused by ...
6
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1answer
2k views

How does voxel cone tracing work?

Assume that we have voxelized a 3D scene into a regular 3D grid of cubes, storing the average (diffuse) radiance and some flag indicating whether or not a voxel is empty. Next, the finer levels of the ...
6
votes
1answer
474 views

How Sunlight is added in Bidirectional Path tracing?

How is distant directional light source(like sun) added in BDPT? If this is an indoor scene that gets sunlight through windows, how is this handled in BDPT(other than to consider Sun as a big disc far ...
6
votes
1answer
2k views

What is Radiosity?

After reading a wikipedia article about Global Illumination It mentions a variety of techniques like Ray Tracing, Path Tracing and Ambient Occlusion. One technique it mentions is Radiosity which I don'...
5
votes
1answer
147 views

What is a “light source” in global illumination?

In the context of global illumination and the scattering equation, do we need to understand "light sources" as surfaces with a "purely emissive" material on which no scattering is present? In ...
5
votes
1answer
92 views

The variance for the Monte Carlo estimator

I was wondering about the variance for Monte Carlo. The book (Advanced Global Illumination 2ed, p.60) writes the equation as follows: $\sigma^2 = \frac{1}{N} \int(\frac{f(x)}{p(x)} - I)^2p(x)dx$ $\...
4
votes
2answers
620 views

Path tracer not rendering shadows

I am trying to implement for research purposes a gradient-domain path tracer. In order to achieve this step I first need a working Path tracer. I have been creating one so far but results are wrong ...
4
votes
1answer
73 views

Is there a material that contributes to GI, but isn't directly visible?

I am doing renders of the inside of a room. I would like to show a "bird's eye" view of the room, from above the ceiling (outside the room), but still see the interior of the room. An easy way of ...
4
votes
1answer
648 views

Modern Screen Space Ambient Occlusion Techniques

Down the road, I'm planning to implement one or another GPU based ambient occlusion technique. My goal will be to approximate a physically based offline renderer, so I was pretty happy when Wikipedia ...
4
votes
1answer
294 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 ...
4
votes
1answer
762 views

How to compensate low amount of rays reaching the light in a Path Tracer

I am trying to implement for research purposes a path tracer, but so far but results are not so good and I will explain you why. The general idea before getting to the code: I am working on paths ...
4
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0answers
222 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)*...
4
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0answers
109 views

Should I use nonsymmetric BSDF for refraction in BPT?

I'm implementing bidirectional path tracing. I want to simulate perfect refraction. I read in the Veach thesis that a perfect refraction BSDF is non-symmetric. Does it mean that I need to use ...
3
votes
1answer
135 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 ...
3
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1answer
341 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 ...
3
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2answers
882 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 ...
3
votes
1answer
2k 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 ...
3
votes
1answer
230 views

Real time image based lighting for arbitrary BRDFs coordinate alignment

I was trying to understand how real-time illumination happens with image-based lighting. I understood that importance of representing the Environment Map and BRDF in spherical harmonics. Because ...
3
votes
1answer
723 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 ...
3
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1answer
145 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 ...
3
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0answers
102 views

Irradiance field approximation with black probes

I'm posting here after some time searching the Internet and reading presentations and papers, to find out the best options to solve the problem I have. I'm implementing irradiance field with ...
3
votes
1answer
102 views

Multiplication of SH coefficient sets representing occlusion and color

Currently I got interested in implementation of Precomputed Radiance Transfer. I decided that occlusion representation (which is not trivial to compute) would be precomputed as CPU raytracing and ...
2
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1answer
169 views

Comprehensive explanation of conservative rasterization algorithm

I am trying to implement conservative voxelization as part of real time GI in my hobby rendering engine. I find this article by NVIDIA. I am stuck at understanding the second algorithm. The problem ...
2
votes
1answer
114 views

light probe (for global illumination) placement techniques?

I'm learning light probes used for dynamic global illumination. I have a question regarding the placement of light probes, as based on most of the pictures I have seen, they seem to be placed ...
2
votes
1answer
133 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, ...
2
votes
1answer
276 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 ...
2
votes
1answer
418 views

Radiosity normal map light baking

I'm trying to implement radiosity normal maps from the old Valve paper: Half-Life® 2 / Valve Source™ Shading.pdf | valvesoftware.com I've tried generating the light maps using Vray and 3dStudio max, ...
2
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0answers
134 views

Scaling of the final image in Metropolis Light Transport

I don't understand why the PBR implementation of Metorpolis Light Transport scales the final image by b / mutationsPerPixel. The authors write: Each Metropolis ...
2
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0answers
29 views

Quantify the variance of pixel measurements

Say $E_j$ is an estimator of the value $$I_j=\int f_j\:{\rm d}\lambda$$ of the $j$th pixel given by the path space integral of the corresponding measurement contribution function $f_j$. $f_j$ is of ...
2
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0answers
47 views

Chart swaps and path inversion in bidirectional path tracing

I've got a hard time to understand how we would implement the following in practice. I'd be great if someone could explain the details for the example below. Assume we're generating a path ${\rm x}=(\...
2
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0answers
123 views

How does Voxel Cone Tracing solve the rendering equation? [duplicate]

so I am not new to interactive graphics programming, but I wanted to try out advanced techniques. Currently I am struggling to understand how the author of the 2011 paper "Interactive Indirect ...
2
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0answers
327 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 ...
2
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0answers
156 views

Picking multiple light sources from a mixture densities

According to Peter Shirely paper, one pdf(probability density function) can be defined for the union of the light sources and pick only one using a random number from this density: $$p(x) = \alpha_1 ...
1
vote
1answer
911 views

Spectral path tracing - image color/brightness incorrect

I implemented a spectral path tracing using physically base BRDF models such as Oren-Nayar,Specular Reflection and Transmission, Lambertian. All calculation in the path tracer uses standard illuminant ...
1
vote
1answer
255 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 ...
1
vote
1answer
54 views

In bidirectional path tracing, is a camera subpath with $n$ vertices determined by a fixed number of random numbers?

In the Multiplexed Metropolis Light Transport implementation of the book Physically Based Rendering, the proposal samples are generated by the Primary Sample Space Sampler ...
1
vote
1answer
191 views

Understanding material .mtl files

I am reading this piece of documentation to understand mtl files So far I am stuck trying to understand the difference between ambient, diffuse and specular reflectivity (Ka, Kd, Ks). If I am ...
1
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0answers
271 views

Understanding bad shadows with voxel ray tracing

I have a voxel raytracer that is generating some weird shadows and I want to understand what is the source of the problem. The image is voxelized, resulting in: I then generate Hard shadows from the ...
1
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0answers
169 views

Voxel cone tracing artifacts (what causes them and how to solve them)?

I am trying to generate soft shadows using voxel cone tracing. Which gives me the next images: In the first image you can see an artifact of a sequence of horizontal shadows alternating with ...
1
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0answers
368 views

How exactly do soft shadows get generated using a voxel cone tracing rendering technique?

Let's assume we have voxelized a scene, and all of our materials have an opacity level of 1. The reuslt would then look like: If we raytraced the final volume that gets generated after rasterization. ...
1
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0answers
103 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 ...
1
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0answers
133 views

What is voxel based global illumination in a nutshell?

As title says. What are the broad, general ideas of voxel based global illumination and what exactly makes it efficient enough to produce real time dynamic shadows with so much photorealism?
1
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0answers
238 views

Octahedral mapping for Spherical functions

I guess most of you are familiar with environment maps, they represent distant lighting distribution, and are used to render objects given their BRDF or SVBRDF distribution. Many papers map the ...
1
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0answers
52 views

Intensity of pixel area light in reflective shadow map?

In reflective shadow map, there is definition about intensity of pixel area light: $$ I_p(\omega) = \Phi_p \max\{0, \langle n_p | \omega \rangle\} $$ where $ \langle | \rangle $ is the dot product, $...