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Light caching (called "light mapping" in older versions of V-Ray) is a technique for approximating the global illumination in a scene. This method was developed originally by Chaos Group specifically for the V-Ray renderer. It is very similar to photon mapping, but without many of its limitations. 

The light cache 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, very similar to the photon map. On the other hand, in a sense, it is the exact opposite of the photon map, which traces paths from the lights, and stores the accumulated energy from the beginning of the path into the photon map.

For more details on how to adjust the Light cache GI render settings, please see the Light Cache Settings page.

Although very simple, the light-caching approach has many advantages over the photon map:

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  • It is easier easy to set up. We only have the camera to trace rays from, as opposed to the photon map, which must process each light in the scene and usually requires separate setup for each light.
  • The light-caching approach works efficiently with any lights - including skylight, self-illuminated objects, non-physical lights, photometric lights, etc. In contrast, the photon map is limited in the lighting effects it can reproduce - for example, the photon map cannot reproduce the illumination from skylight or from standard omni lights without inverse-square falloff.
  • The light cache produces correct results in corners and around small objects. The photon map, on the other hand, relies on tricky density estimation schemes, which often produce wrong results in these cases, either darkening or brightening those areas.
  • In many cases the light cache can be visualized directly for very fast and smooth previews of the lighting in the scene.

Even with these advantages, light caching is similar in speed to the photon map and can produce approximations to the global lighting in a scene very quickly. In addition, the light The light cache can be used successfully for adding GI effects to animations.

Of course, the The light cache has some limitations:

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  • Like the irradiance map, it is view-dependent and is generated for a particular position of the camera.
  • Like the photon map, the The light cache is not adaptive. The illumination is computed at a fixed resolution, which is determined by the user.
  • The light cache does not work very well with bump maps.

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Notes

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  • Similar to the photon map, you You can get "light leaks" with the light cache around very thin surfaces with substantially different illumination on both sides. Sometimes it may be possible to reduce this effect by assigning different GI Surface ID's to the objects on both sides of the thin surface (see the Object settings  dialogue); the effect can also be reduced by decreasing the Sample sizeand/or the filtering.
  • Using Light Cache with V-Ray GPU is possible only when V-Ray GPU is set as Production render, or with From file mode (already computed Light Cache .vrlmap) when using ActiveShade.
  • The Light Cache calculation cannot be distributed among several machines with the help of Distributed Rendering(DR).
  • For more details on how the Light cache GI render settings work, please see the Light Cache  Settings page.