Direct and Indirect Illumination

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Direct lighting is considered to be the first time a light beam interacts with a surface from a given light source. Indirect lighting is the reflection of light from another surface other than the light source.

A simple example that comes to mind is what might happen if you pushed someone. If the person falls on another person who is standing nearby, the person nearby will probably fall down as well. The reason why the third person fell down is because of you pushing the second person. The person you pushed indirectly falls on the third person. This simple analogy can be applied to lighting.

If light interacts with a surface and some of the light is reflected on another surface nearby, you can say the surface that received the reflected light is being indirectly lit. A shadow is a perfect example of indirect lighting. Sometimes indirect light from a surface is actually reflected onto itself by another close-by surface. It can therefore get a bit tricky and it can also become a nightmare for a recursive function. The direct lighting hits the first surface as it comes directly from the light source. The indirect lighting on the second surface comes from (or is reflected by) the first surface.

Direct lighting is a must in all applications. The first rendering applications were capable of only simulating direct lighting. For example an algorithm named ray trac ing was the first to incorporate direct lighting. Direct lighting is also known as local lighting . Indirect illumination is a very important effect that we must simulate using pho-ton mapping. The reason for this is indirect illumination is poor in other rendering algorithms. Our goal is to fix this problem using photon mapping.

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TIP

Indirect illumination is a very impor tant element of a global illumination algorithm. It helps to contribute to the direct lighting that intersects surfaces from the light sources. The combination of direct and indirect illumination helps in adding great realism to an environment.

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