Media and Particle Systems

Particle systems are commonly used in computer graphics to model various types of special effects such as fire, fluids, and flocking behavior of birds. Particle systems are collections of particles, typically point masses, in which the dynamic behavior of the particles can be determined by the solution of sets of coupled differential equations.

One can think of a particle system as dozens, hundreds, or even thousands of balls bouncing around and interacting with the light and objects in the scene. In general, particles are a kind of material, so they are created, applied, adjusted, and removed like a material. Particles are emitted from surfaces, and they can realistically collide with other surfaces while showing the effects of gravity and other phenomena based on natural physics. The parameters of a particle system can also be changed over time.

See the following links for examples:

Media in Povray

The notes below are excerpts from the POV-Ray tutorial.

Particle systems in Povray/Moray are generated using the media statement. Media can be global (atmosphere) or local to an object. In the latter case, the media is attached to the object as a material.

The media statement is used to specify particulate matter suspended in a medium such air or water. It can be used to specify

smoke, haze, fog, gas, fire, dust etc.

Media works by sampling the density of particles at some specified number of points along the ray's path. Sub-samples are also taken until the results reach a specified confidence level.

Media can be used in 2 ways::

Object Media: associated with the interior of an object. Used in an object's interior statement. Sampling only occurs inside the object.
Atmospheric Media: Globally applied. The samples run from the camera location until the ray strikes an object.

Syntax:

MEDIA:
    media { [MEDIA_IDENTIFIER] [MEDIA_ITEMS...] }
MEDIA_ITEMS:
    method Number | intervals Number | samples Min, Max |
    confidence Value  | variance Value | ratio Value |
    absorption COLOR | emission COLOR | aa_threshold Value |
    aa_level Value | 
    scattering { 
       Type, COLOR [ eccentricity Value ] [ extinction Value ]
    }  | 
    density { 
       [DENSITY_IDENTIFIER] [PATTERN_TYPE] [DENSITY_MODIFIER...]
    }   | 
    TRANSFORMATIONS
DENSITY_MODIFIER:
    PATTERN_MODIFIER | DENSITY_LIST | COLOR_LIST |
    color_map { COLOR_MAP_BODY } | colour_map { COLOR_MAP_BODY } |
    density_map { DENSITY_MAP_BODY }

Media Types

There are three types of particle interaction in media:

absorbing - specifies a color of light which is absorbed when looking through the media. The default value is rgb<0,0,0> which means no light is absorbed -- all light passes through normally.
emitting - specifies a color of the light emitted from the particles.
scattering - only visible when light is shining on the media from a light source. See details below.

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 (no filter and transmit). The following two lines are legal and produce the same results:

emission 0.75
emission rgb<0.75,0.75,0.75>

Scattering

scattering { Type, COLOR [ eccentricity Value ] [ extinction Value ] }

Type: specifies one of five different scattering phase functions
- isotropic (type 1): is independent of direction.
- Mie (haze (type 2) and murky (type 3)): used for relatively small particles such as minuscule water droplets of fog. Scattering is in the forward direction. 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). Murky is much more directional than haze.
- Rayleigh (type 4) scattering for extremely small particles such as molecules of the air. The amount of scattered light is largest when the incident light is parallel or anti-parallel to the viewing direction.
- Henyey-Greenstein (type 5): models an ellipse with a given eccentricity e.
eccentricity: (only used in type 5) default 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.
extinction: balance between how much absorption occurs for a given amount of scattering. The default value of 1.0 gives an extinction effect that matches the scattering, 0.0 turns it off completely.

Density

The density statement allows you to vary the density across space using any of POV-Ray's pattern functions such as those used in textures. If no density statement is given then the density remains a constant value of 1.0 throughout the media.

The pattern type is any one of POV-Ray's pattern functions such as bozo, wood, gradient, waves, etc. Of particular usefulness are the spherical, planar, cylindrical, and boxed patterns. All patterns return a value from 0.0 to 1.0. This value is interpreted as the density of the media at that particular point.

Examples

Very Simple Povray Examples

Absorbing - Code	Scattering - Code	Emmision - Code
Atmospheric Media with Spotlight - Code	Refraction - Code

More Complex Povray Examples

	Povray Code: Uses atmospheric media with spotlight. Particles are scattering with bozo density.
	Uses atmospheric media with spotlight. Particles are scattering with wave density. For the moray code, see vase.mdl in our lab folder L:\classes\cs140.
	Povray Code: Uses object media with spotlight. Particles are scattering (green), emissive (blue), and absorptive (white) with bozo density.

See POV-Ray Tutorial for many more details!

A number of examples came with Povray. For what the images look like, click here. To find the code, look in the folder
C:\Program Files\POV-Ray for Windows v3.6\scenes\interior\media

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