Light-mapping

Light-mapping is a technique for approximating the global illumination in a scene. It is very similar to photon mapping, but without many of its limitations.

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, very similar to the photon map. To some extent, this 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.

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

• It is easier 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-mapping 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 of standard omni lights without inverse-square falloff.
• The light map 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 map can be visualized directly.

Even with these advantages, light mapping is similar in speed to the photon map and can produce approximations to the global lighting in a scene very quickly. In addition, the lightmap can be used successfully for adding GI effects to animations. Here are some examples.

Example 1

Direct light only:

Unfiltered lightmap, 100 (one hundred) light bounces.

Filtered lightmap:

Irradiance map for the first diffuse bounce, and the light map for secondary bounces:

Example 2

Noite: the Sponza Atrium model is created by Marco Dabrovic (http://www.rna.hr)

Direct light only:

Unfiltered lightmap, 100 (one hundred) light bounces:

Filtered lightmap:

Irradiance map for the first diffuse bounce, and the light map for secondary bounces:

Example 3

This simple animation was rendered with a filtered lightmap only:

Example 4

Follow this link for some comparisons between lighting solutions produced by radiosity in 3dsmax and VRay.