Scene Creation
Photon Mapping

'A photon map provides a simple, yet effcient way of caching illumination in a given scene.'
Henrik Wann Jensen

Photon Mapping is a technique used in XSI and Mental Ray to calculate indirect lighting in a scene and can produce the caustic effects on objects. During the rendering process information on photon storage is combined into a three dimensional database called a photon map. This photon map is used by Mental Ray to compute illumination information such as caustic effects.

Photon paths are traced around a scene and at each location where a photon is stored it deposites energy. A photon is stored on every diffuse or partially diffuse surface it hits including the surface at which it is absorbed. Photons are stored by photon shaders attached to the material of all objects specified as transmitting or receiving photons. Photon mapping is only used in situations that cannot be handled by direct illumination therefore photons are not stored at the first reflection or transmission location.

The data in a photon map is stored in a seperate file and is therefore relatively independent of the scene geometery whcih means that it is not limited to less complex scenes and the rendering time is not increased for more complex scenes. Photon mapping can be used with any scene which can be raytraced.

Softimage XSI stores the photon map data into a file with extension .pmap which can be specified in the render options photon tab. Once the file is saved it can be reloaded and used for every frame if the scene is not changed significantly. If the option to rebuild is ticked mental ray will rebuild the photon map everyframe instead of reloading the specified filename. Recomputing the photon map at each frame in this way can produce interactive animated lighting effects.

The number of stored photons is generally in the range 10 000 and 1 000 000 and the sharpness of the caustic effect is increased with higher numbers. Unstored photons are not considered during the rendering process and do not affect the photon map.

'Photon maps work because the computer does not analyze every element in the scene,
but instead computes the flow of light necessary to render realistic images.'

Henrik Wann Jensen
Henrik Wann Jensen's Stanford Page

Further Information
Susan Windeatt