Importance Sampling, and Multiple Importance Sampling

mental ray Tutorial

Version 3.12

Copyright 1986, 2015 NVIDIA ARC GmbH. All rights reserved.

Table of Contents

• Introduction
• Light Importance Sampling
• Multiple Importance Sampling

This document is a short tutorial about two new features available in mental ray 3.11, called light importance sampling and multiple importance sampling.

Introduction

The scene provided as an example is called "scene.mi" (there is actually a second .mi file, loaded from the first, that only contains the objects definitions) and can be rendered directly, provided the user has the public "base" shaders installed. It consists of the following elements:

• a ground plane, that has a simple Lambertian base shader as a material;
• three spheres, with three different built-in BSDF shaders (the red sphere is a carpaint BSDF, the yellow one is an architectural BSDF and the blue one is an Ashikhmin BSDF)
• a sky dome (a hemisphere) that acts as an illumination source with an HDR texture. This is a built-in object light, that is an object with a built-in material attached that turns it into a light (this is also a new feature of mental ray 3.11)

This lighting scenario is quite hard for classical mental ray light sampling, which is uniform: the light source is very wide (it encompasses the whole scene) and it has a texture attached that has a high dynamic range and is a pretty high frequency image (it's the "sky_beach.hdr" image file, that is a panoramic view of a beach and a sunny sky. It is an edited version of an HDR texture courtesy of the www.openfootage.net website). A render with 2000 samples for the light (200 samples in reflections) yields a pretty noisy image:

Even raising the number of samples to 10000 (and 1000 in reflections) doesn't solve the noise issue completely, as it can be seen in the following image, and most importantly it takes quite a long time to render:

Light Importance Sampling

In a lighting scenario like this one, light importance sampling (abbreviated in IS from now on) can give a huge benefit. It is a light sampling mechanism that is new in mental ray 3.11 and can be enabled by setting to on the "light importance sampling" string option (or alternatively, by specifying "-light_is on" in the command line options. There are also other string options that allow a finer control over IS, please have a look at the documentation for those). When IS is enabled, the area lights are not sampled uniformly anymore: instead, more samples are placed on parts of the lights that are brighter relative to the current point being shaded (with a proper re-balancing of the light emission value according to the probability associated with the sample), thus reducing the amount of noise in the rendered image (or equivalently, reducing the number of samples for a similar amount of noise). Nothing needs to be changed in the code of the light and material shaders, the core takes care of everything. The supported light types are rectangular, disc, sphere, cylinder and object area lights; user area lights are not supported (and of course, non-area light types like spot, direction and point lights are not supported either).

In the given example scene, try setting to on the value of the "light importance sampling" string option in the options block at the beginning of the file, lower the number of samples for the light (look for the definition of "sky_dome_physical_light") and render the scene again: you will now see a much less noisy image (compared to the first render) and rendered in a fraction of the time (this is due to the fact that with less noise there is also less oversampling for anti-aliasing). The rendered image should look like the following (this has been rendered with 500 samples for the light, 64 in reflections):

The image above is a significant improvement over the first render produced with uniform sampling of the light, but it is still not perfect. The yellow sphere still presents some noisy pixels, and the reflection of the sky is just barely discernible. Moreover, the blue sphere is too noisy. One way of solving these problems is to raise the number of samples for the light, but in this case it is also possible to use multiple importance sampling, to exploit the fact that the materials are BSDFs.

Multiple Importance Sampling

Multiple importance sampling (abbreviated in MIS from now on) is a way of combining together different sampling techniques. Some of the samples are generated by sampling the light, and some are generated by sampling the BSDFs; these samples are then combined to produce the final result. This approach can obviously be used only when the materials have a BSDF attached in the BSDF slot of the material definition, like in this example (as it was mentioned above, the materials are actually built-in BSDFs). MIS is enabled by setting to on the "multiple importance sampling" string option (or by specifying "-mis on" in the command line options), if you do that in the scene (leaving IS enabled) and render it again you should get the following image:

The image above takes even less time to render, while being less noisy than the one produced with IS alone. Compared to the latter, the yellow sphere has now clean reflections and the blue sphere has virtually no noise. Please note that IS and MIS can be enabled separately, but they can also be used together (like in this case).

MIS is a mechanism that combines different sampling techniques, so it can be used only when there are materials that have BSDFs attached; when a material has no BSDF, mental ray defaults to light sampling only (whether with or without IS). In the last two renders, you can see that the ground plane looks identical: this is because it has a Lambertian base shader with no BSDF attached, and mental ray doesn't use MIS for it (but it uses light IS, that's why it looks much less noisy than in the first renders done with uniform light sampling).

Copyright © 1986, 2015 NVIDIA ARC GmbH. All rights reserved.