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## Overview

Some background for understanding how the Irradiance map works is necessary in order to grasp the meaning of these parameters.

Irradiance is a function defined for any point in the 3D space and represents the light arriving at this point from all possible directions. In general, Irradiance is different in every point and in every direction. However, there are two useful restrictions that can be made. The first is the surface Irradiance - which is the Irradiance arriving at points which lie on the surface of objects in the scene. This is a natural restriction since we are usually interested in the illumination of objects in the scene, and objects are usually defined through their surface. The second restriction is that of diffuse surface Irradiance - which is the total amount of light arriving at a given surface point, disregarding the direction from which it comes.

In more simple terms, one can think of the diffuse surface Irradiance as being the visible color of a surface, if we assume that its material is purely white and diffuse.

In V-Ray, the term Irradiance map refers to a method of efficiently computing the diffuse surface Irradiance for objects in the scene. Since not all parts of the scene have the same detail in indirect illumination, it makes sense to compute GI more accurately in the important parts (e.g. where objects are close to each other, or in places with sharp GI shadows), and less accurately in uninteresting parts (e.g. large uniformly lit areas). The Irradiance map is therefore built adaptively. This is done by rendering the image several times (each rendering is called a pass) with the rendering resolution being doubled with each pass. The idea is to start with a low resolution (say a quarter of the resolution of the final image) and work up to the final image resolution.

The Irradiance map is in fact a collection of points in 3D space (a point cloud) along with the computed indirect illumination at those points. When an object is hit during a GI pass, V-Ray looks into the Irradiance map to see if there are any points similar in position and orientation to the current one. From those already computed points, V-Ray can extract various information (i.e. if there are any objects close by, how fast the indirect illumination is varying etc.). Based on that information, V-Ray decides if the indirect illumination for the current point can be adequately interpolated from the points already in the Irradiance map, or not. If not, the indirect illumination for the current point is computed, and that point is stored in the Irradiance map. During the actual rendering, V-Ray uses a sophisticated interpolation method to derive an approximation of the Irradiance for all surfaces in the scene.

The following diagram shows the way the Irradiance map is generated. Since the method is view-dependent, the first rays (Black) are traced from the camera into the scene in order to determine the placement of the Irradiance samples. Once this is done, GI rays (Red) are traced from the samples into the scene in order to determine the illumination coming from the environment. The number of traced rays is determined by the HSph. subdivs parameter. The Irradiance map only traces one bounce of light. All additional bounces (Blue) are traced from the secondary engine. The Irradiance map is created on several passes - each pass adding more samples where this is needed. During rendering, for each rendered point, V-Ray takes several samples from the already complete Irradiance map and interpolates between them in order to create a smooth GI solution. The number of samples taken is determined by the Interpolation samples parameter.

## Notes

• Irradiance maps can be viewed, merged, and saved with the tool.
• For animated Irradiance maps, GI samples on different objects are not shared; this may lead to small objects to appear black in the final renders. To solve this issue, group those objects together - this will work as GI samples are shared for objects which are part of the same group.
• Тhe Irradiance Map calculations can be sped up by distributing them among several machines with the help of Distributed Rendering (DR).
• For more details on how the Irradiance Map GI render settings work, please see the Irradiance Map Settings page.

• 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.