There are three types of particle interaction in
media: absorbing, emitting, and scattering. All three
activities may occur in a single media. Each of these three specifications requires a color. Only the red, green, and
blue components of the color are used. The filter and transmit values are ignored. For this reason it is permissible
to use one float value to specify an intensity of white color. For example the following two lines are legal and
produce the same results:
absorption keyword specifies a color of light which is absorbed when looking through the media.
absorption rgb<0,1,0> blocks the green light but permits red and blue to get through.
Therefore a white object behind the media will appear magenta.
The default value is
rgb<0,0,0> which means no light is absorbed -- all light passes through
emission keyword specifies a color of the light emitted from the particles. Although we say they
"emit" light, this only means that they are visible without any illumination shining on them. They do not
really emit light that is cast on to nearby objects. This is similar to an object with high
values. The default value is
rgb<0,0,0> which means no light is emitted.
The syntax of a
scattering statement is:
Type, COLOR [ eccentricity Value ] [ extinction Value ]
The first float value specifies the type of scattering. This is followed by the color of the scattered light. The
default value if no
scattering statement is given is
rgb<0,0,0> which means no
scattering effect is only visible when light is shining on the media from a light source. This is similar to
reflection off of an object. In addition to reflecting light, a scattering media also absorbs light like an
absorption media. The balance between how much absorption occurs for a given amount of scattering is controlled
by the optional
extinction keyword and a single float value. The default value of 1.0 gives an
extinction effect that matches the scattering. Values such as
extinction 0.25 give 25% the normal amount.
extinction 0.0 turns it off completely. Any value other than the 1.0 default is contrary to the
real physical model but decreasing extinction can give you more artistic flexibility.
The integer value
Type specifies one of five different scattering phase functions
representing the different models: isotropic, Mie (haze and murky atmosphere), Rayleigh, and Henyey-Greenstein.
Type 1, isotropic scattering is the simplest form of scattering because it is independent of direction.
The amount of light scattered by particles in the atmosphere does not depend on the angle between the viewing
direction and the incoming light.
Types 2 and 3 are Mie haze and Mie murky scattering which are used for relatively small particles
such as minuscule water droplets of fog, cloud particles, and particles responsible for the polluted sky. In this
model the scattering is extremely directional in the forward direction i.e. the amount of scattered light is largest
when the incident light is anti-parallel to the viewing direction (the light goes directly to the viewer). It is
smallest when the incident light is parallel to the viewing direction. The haze and murky atmosphere models differ in
their scattering characteristics. The murky model is much more directional than the haze model.
Type 4 Rayleigh scattering models the scattering for extremely small particles such as molecules of the
air. The amount of scattered light depends on the incident light angle. It is largest when the incident light is
parallel or anti-parallel to the viewing direction and smallest when the incident light is perpendicular to the
viewing direction. You should note that the Rayleigh model used in POV-Ray does not take the dependency of scattering
on the wavelength into account.
Type 5 is the Henyey-Greenstein scattering model. It is based on an analytical function and can be used to
model a large variety of different scattering types. The function models an ellipse with a given eccentricity e. This
eccentricity is specified by the optional keyword
eccentricity which is only used for scattering type
five. The default eccentricity value of zero defines isotropic scattering while positive values lead to scattering in
the direction of the light and negative values lead to scattering in the opposite direction of the light. Larger
values of e (or smaller values in the negative case) increase the directional property of the scattering.