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Photons

Photons are a based on the physical concept of light and are a divisible packet of energy emitted from a light source. Photons are traced in a scene in a similar fashion to rays however a value for the light energy is calculated which is more physically accurate than the concept of light color used in raytracing.

Caustic effects are calculated during rendering using photon mapping a technique developed to simulate indirect illumination using photons. A photon is emitted into a scene and can can be reflected, transmitted or absorbed by the surfaces it comes into contact with. Photons are emitted into the scene from a light source and adjustments to this process are described in the lighting section.

Photons are stored in shaders attached to various objects in the scene. Photon behaviour at each surface is described in detail in the objects section. If a photon does not hit an object it disappears into infinity and no further information is stored. A photon is absorbed when it has hit a surface the maximum number of times set in the photon depth limit option. Caustics can be calculated by following the path of certain photons. If a photon is emitted, specularly reflected or refracted then diffusely reflected onto the camera by a diffuse surface this path will produce a caustic effect.

Photons traced along paths in the scene deposite energy data which is stored in a photon map. Further description of this process is contained in the photon mapping section. Photons which are transmitted through coloured objects retain this colour in their effect.

Photon shaders store photons only if the surface has a diffuse component. Indirect illumination completely specular surfaces with no diffuse component is computed with ray tracing so no photons are stored. The photon can then be reflected or transmitted by the photon shader in a direction determined by the photon shader.
individual photons transmitted through coloured glass.
Further Information
Susan Windeatt