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Starting with V-Ray 5, the Irradiance Map GI method is deprecated in V-Ray for 3ds Max, Maya, Cinema 4D, HoudiniNuke.

Irradiance is a function defined for any point in the 3D space, representing the light arriving at a point and all the directions from which the light arrives. The irradiance map GI method creates a map of this lighting.

In general, irradiance is different at every point and can represent a large amount of information. However, there are two useful restrictions that can be made when using irradiance for rendering. The first is the restriction of looking only at surface irradiance, which is the irradiance arriving at points that 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 by their surfaces. The second restriction is that for diffuse surface irradiance (the total amount of light arriving at a given surface point) we can disregard the direction from which the light 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. 

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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 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. 

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