Reference:User Defined Pattern

New in version 3.7.1 a special pigment pattern user_defined has been added to define the pigment color directly in terms of a set of functions.

The syntax is a follows:

pigment {
  user_defined {
    function { RED_FUNCTION },
    function { GREEN_FUNCTION },
    function { BLUE_FUNCTION },
    function { FILTER_FUNCTION },
    function { TRANSMIT_FUNCTION }
    }
  }

Each function is optional, in which case the corresponding component is set to 0. For example, the following sets the color to rgbft <x,y,0,0,z>:

pigment {
  user_defined {
    function { x },
    function { y },
    ,,
    function { z }
    }
  }

The pattern is similar to image_map in that it directly specifies colors rather than using a Color Map or Pigment Map.