Photon Mapping |
photon map provides a simple, yet effcient way of caching
illumination in a given scene.'
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
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.