Transfer Functions

In order to render participating media, renderable properties like absorption, emission and scattering have to be available. A problem however is, that in general, absorption-, emission- and scattering-functions can not use scalar density values directly. Therefore, function specific parameters, like the color of emitted light for emission-functions has to be derived from scalar density functions. For this reason, so-called transfer functions are used to map scalar density values to values that can be used for rendering. I.e. in medical volume rendering systems, the density function often depends on the type of tissue - transfer functions are used to color different types of tissue differently.