Statistics: Posted by Paleos — Tue Apr 14, 2015 9:06 pm

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while reserving the costly Half Vector Space perturbation only for paths where it is necessary such as Specular Diffuse Specular(SDS) and the like.

What i would like input on, would be a path difficulty metric to determine probability of using ether MMLT mutations or Half Vector Space perturbations,

based on the frequency of the interactions, the order they come in as well as the size of both the aperture and the light source.

As a side note i would like input on a heuristic for determining for both mutations whether to mutate all vertices or keep a section of the path fixed.

Statistics: Posted by Paleos — Tue Apr 14, 2015 8:15 pm

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Flickering in animations

The remedy to this limitation is to mutate paths through both space and time, not just space.

Physically-based Animation Rendering with Markov Chain Monte Carlo

Poor SIMD efficiency

The remedy to this to have multiple proposals per iteration instead of one.

Coherent Metropolis Light Transport with Multiple-Try Mutations this paper uses multiple try metropolis and exploits the correlation between proposals to accelerate it with ray packets.

however since then more efficient multiple proposal metropolis algorithms such as the one i posted in the Useful Markov Chain Monte Carlo Papers to apply to Graphics

Post

Outliers(aka. Fireflies, Bright spots)

The unbiased remedy to this is to ensure that there is a way of finding a point that gets accepted within a few iterations. first of all use the right proposal distribution such as such as a robust mutation strategy(such as The Natural-Constraint Representation of the Path Space for Efficient Light Transport Simulation and A Simple and Robust Mutation Strategy for the Metropolis Light Transport Algorithm) and choose the right step size, an Adaptive Markov Chain Monte Carlo method(see the Useful Markov Chain Monte Carlo Papers to apply to Graphics

Post for papers on this topic) is an automatic way of doing this. second of all use multiple proposals to increase the chance that one of them gets accepted. and as a last resort use a form of parallel tempering between the actual importance function and one with flatter peaks(such as in Replica exchange light transport on relaxed distributions) or no peaks(such as in Robust Adaptive Photon Tracing using Photon Path Visibility).

Lack of Stratification

The remedies to this include using Markov Chain Quasi Monte Carlo methods(see the Useful Markov Chain Monte Carlo Papers to apply to Graphics

Post for papers on this topic),a good mutation strategy (such as in The Natural-Constraint Representation of the Path Space for Efficient Light Transport Simulation) , a importance function such as luminance relative to each individual pixel or variance(such as in Arbitrary Importance Functions for Metropolis Light Transport), having multiple proposals in each iteration placed in a stratified manner (such as in Acceleration of the Multiple-Try Metropolis algorithm using antithetic and stratified sampling).

Statistics: Posted by Paleos — Mon Apr 13, 2015 11:48 pm

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Statistics: Posted by Paleos — Mon Apr 13, 2015 9:06 pm

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http://www.cs.cornell.edu/Courses/cs663 ... olpath.pdf

if you are doing path tracing. But I don't see how you can "ignore" out-scattering, since it does not contribute in the spherical integration of the radiative transfer equation, but only works for computing transmittance.

BTW, as far as I can see ratio/residual tracking is something for evaluating transmittance, but does it help when it comes to sampling distance for indirect illumination? It's vaguely put in the second part of section 5.2.

Statistics: Posted by citadel — Thu Apr 09, 2015 4:52 pm

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shocker_0x15 wrote:

The paper for Residual Ratio Tracking will be a good reference for you.

http://drz.disneyresearch.com/~jnovak/p ... index.html

If you want to compute transmittance in in-homogeneous media in unbiased way, you can use Woodcock tracking. Furthermore, in the paper a better method RRTracking is proposed.

The paper for Residual Ratio Tracking will be a good reference for you.

http://drz.disneyresearch.com/~jnovak/p ... index.html

If you want to compute transmittance in in-homogeneous media in unbiased way, you can use Woodcock tracking. Furthermore, in the paper a better method RRTracking is proposed.

Thanks! It is exactly what I was looking for

Statistics: Posted by koiava — Tue Apr 07, 2015 11:42 am

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If you want to compute transmittance in in-homogeneous media in unbiased way, you can use Woodcock tracking. Furthermore, in the paper a better method RRTracking is proposed.

Statistics: Posted by shocker_0x15 — Tue Apr 07, 2015 10:00 am

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shocker_0x15 wrote:

Let's consider the eye point x0, the nearest surface point x1, the point sampled on a light source y0.

You mean your renderer samples a few points(xp) along x0x1 and connects xp and y0, add a contribution attenuated by the factors T(y0, xp) and T(xp, x0), followed by adding contribution from the surface scattering attenuated by the factors T(y0, x1) and T(x1, x0), right?

Out-scattering effects are included as T (which consists of absorption and out-scattering).

Single scattering only decreases energy gain (compared to full consideration of scattering) along a path, so I think it does not violate energy conservation.

Let's consider the eye point x0, the nearest surface point x1, the point sampled on a light source y0.

You mean your renderer samples a few points(xp) along x0x1 and connects xp and y0, add a contribution attenuated by the factors T(y0, xp) and T(xp, x0), followed by adding contribution from the surface scattering attenuated by the factors T(y0, x1) and T(x1, x0), right?

Out-scattering effects are included as T (which consists of absorption and out-scattering).

Single scattering only decreases energy gain (compared to full consideration of scattering) along a path, so I think it does not violate energy conservation.

energy conservation is definitely violated. Lets look at more clear example.

for example we have some plate illuminated from top and we have some dense non-homogeneous participating media(cloud) between light and plate, This case you must get shadow of this media on plate. Reason of shadow is actually out-scattering because light coming from emitter is scattered on volume particle in different directions and couldn't received on plate.

In my example volumetric and surface shadows are caused only by surfaces which occludes light.

One way to detect volumetric occluders is ray marching but it's dependent on marching step and is biased.

Dade wrote:

You can also find a quite detailed explanation of the topic in PBRT book.

Yeah, I will definitely look.

Statistics: Posted by koiava — Tue Apr 07, 2015 8:47 am

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Out-scattering effects are included as T (which consists of absorption and out-scattering).

Single scattering only decreases energy gain (compared to full consideration of scattering) along a path, so I think it does not violate energy conservation.

Statistics: Posted by shocker_0x15 — Tue Apr 07, 2015 12:39 am

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You are right saying that

int1.png

but the integral you need have to be this one:

int2.png

Statistics: Posted by raider — Mon Apr 06, 2015 9:11 pm

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When you look at the density of a uniform hemispherical distribution, it looks like a hemisphere. Thus its surface is 2pi.

If you look at the density of a cosine distribution, you will observe that it is, itself, a sphere of one unit diameter or 0,5 radius above the surface. Thus its surface is 4pi * 0.5^2 = pi.

Statistics: Posted by ypoissant — Mon Apr 06, 2015 7:54 pm

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I'm using inverse square law for light attenuation from point light sample, but this law is correct when light traced in vacuum, but when we have some media between scattering events and if this media scatters light, then we have both in-scattering and out-scattering. In my Single Scattering example I'm calculating scattering for nearest surface and in-scattering for volumetric particle, but in shadowing part I have included only surfaces, this means that I'm ignoring out-scattering phenomena and violating energy conservation. What is most common way to calculate out-scattering correctly?

Statistics: Posted by koiava — Mon Apr 06, 2015 7:27 am

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josh247 wrote:

The issue I find is that my images appear to get brighter when I use importance sampling due to the PDF always being less than one (well less than Pi) and therefore increasing the radiance when used for division. Surely when sampling the PDF the value should occasionally equal more than one so that the radiance is reduced, producing the same results as when I don't importance sample the hemisphere?

The issue I find is that my images appear to get brighter when I use importance sampling due to the PDF always being less than one (well less than Pi) and therefore increasing the radiance when used for division. Surely when sampling the PDF the value should occasionally equal more than one so that the radiance is reduced, producing the same results as when I don't importance sample the hemisphere?

Are you sure your results were correct *before* you introduced importance sampling? With uniform sampling of the hemisphere you still have a pdf; it just has a different shape (constant 1 / (2 pi) I would assume, since you're integrating over half a sphere, and a sphere has area 4 pi r^2).

Statistics: Posted by friedlinguini — Thu Apr 02, 2015 7:34 pm

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The issue I find is that my images appear to get brighter when I use importance sampling due to the PDF always being less than one (well less than Pi) and therefore increasing the radiance when used for division. Surely when sampling the PDF the value should occasionally equal more than one so that the radiance is reduced, producing the same results as when I don't importance sample the hemisphere?

Statistics: Posted by josh247 — Thu Apr 02, 2015 1:19 pm

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