Indirect Illumination

This page introduces Indirect Illumination in V-Ray for 3ds Max.

Overview

Indirect illumination refers to illumination that results from the bounced light in a scene, as opposed to illumination directly from light sources. Global illumination (GI) refers to the computation of this effect through computer graphics.

V-Ray implements several approaches (called engines) for computing indirect illumination with different trade-offs between quality and speed. The engine selected as the primary is used to compute the first light bounce, while the secondary engine is used to compute any subsequent bounces. A primary diffuse bounce occurs when a shaded point is directly visible by the camera, or through specular reflective or refractive surfaces. A secondary bounce occurs when a shaded point is used in GI calculations. These engines can be combined in a number of ways to get the desired result in the minimum time.

The primary and secondary GI engines are selected in the Global Illumination rollout under the GI tab in the Render Setup window.

The following types of indirect illumination engines are supported in V-Ray for 3ds Max.

Brute Force GI - The simplest approach where indirect illumination is computed individually for each shaded surface point.
Irradiance Map - Indirect Illumination is computed for a few points in a scene, and remaining points use interpolated values. Built adaptively but is view-dependent.
Light Cache GI - Traces multiple paths from the camera. View-dependent, but handles details and corners better.

Irradiance Map GI engine is deprecated. It doesn't support some of the new V-Ray features and will be soon removed as an option.

Comparison of GI Engines

Here is a quick comparison of GI engines. See the topic for each engine for more information.

	Overall speed	Load/Save	Retains fine detail	Overall noise level	Handles bump maps well	Skylight Support	Requires Additional Memory	Distributed Rendering (DR) Support
Brute Force	Slow	No	Yes	High	Yes	Yes	No	Yes
Irradiance Map	Fast	Yes	Maybe	Low	Yes	Yes	Yes	Yes
Light Cache	Fast	Yes	Yes	Low	No	Yes	Yes	Yes

Brute Force

Brute force is the simplest approach; indirect illumination is computed independently for each shaded surface point by tracing a number of rays in different directions on the hemisphere above that point. For more information, see the Brute Force GI topic.

Advantages:

Preserves all the detail in the indirect lighting, for example with small and sharp shadows.
Free from defects like flickering in animations.
Does not require additional memory.
Indirect illumination for motion-blurred moving objects is computed correctly.
GI calculations can be distributed over multiple machines using Distributed Rendering (DR).

Disadvantages:

The approach is very slow for complex images.

Irradiance Map

This approach is based on irradiance caching; the basic idea is to compute the indirect illumination only at some points in the scene, and interpolate for the rest of the points. For more information, see the Irradiance Map topic.

Advantages:

The irradiance map is very fast compared to direct computation, especially for scenes with large flat areas.
The noise inherent to direct computation is greatly reduced.
The irradiance map can be saved and reused to speed up calculations of different views for the same scene and of fly-through animations.
The irradiance map can also be used to accelerate direct diffuse lighting from area light sources.
The GI calculations can be distributed over multiple machines using Distributed Rendering (DR).

Disadvantages:

Some details in indirect lighting can be lost or blurred due to the interpolation.
If low settings are used, flickering may occur when rendering animations.
The irradiance map requires additional memory.
Indirect illumination with motion-blurred moving objects is not entirely correct and may lead to noise (although in most cases this is not noticeable).

Light Cache

Light caching is a technique for approximating the global illumination in a scene. The light map is built by tracing many many eye paths from the camera. Each of the bounces in the path stores the illumination from the rest of the path into a 3d structure. The light map is a universal GI solution that can be used for both interior or exterior scenes, either directly or as a secondary bounce approximation when used with the irradiance map or the brute force GI method. For more information, see the Light Cache GI topic.

Advantages:

The light cache is easy to set up. We only have the camera to trace rays from.
The light-caching approach works efficiently with any lights - including skylight, self-illuminated objects, non-physical lights, photometric lights etc.
The light cache produces correct results in corners and around small objects.
In many cases the light cache can be visualized directly for very fast and smooth previews of the lighting in the scene.

Disadvantages:

Like the irradiance map, the light cache is view-dependent and is generated for a particular position of the camera. However, it generates an approximation for indirectly visible parts of the scene as well - for example, one light cache can approximate completely the GI in a closed room.
The light cache does not work very well with bump maps; use the irradiance map or brute force GI if you want to achieve better results with bump maps.
Lighting involving motion-blurred moving objects is not entirely correct, but is very smooth since the light cache blurs GI in time as well (as opposed to the irradiance map, where each sample is computed at a particular instant of time).

Example: Comparisons of Different GI Methods

These examples show the effect of the different primary and secondary engines. Note that by default Brute Force has 3 light bounces and Light Cache works with 100 light bounces. All the examples are set to their default values.