3.6.2 Media

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. Previous versions of POV-Ray had two incompatible systems for generating such effects. One was halo for effects enclosed in a transparent or semi-transparent object. The other was atmosphere for effects that permeated the entire scene. This duplication of systems was complex and unnecessary. Both halo and atmosphere have been eliminated. See "Why are Interior and Media Necessary?" for further details on this change. See "Object Media" for details on how to use media with objects. See "Atmospheric Media" for details on using media for atmospheric effects outside of objects. This section and the sub-sections which follow explains the details of the various media options which are useful for either object media or atmospheric media.

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. POV-Ray provides three methods of sampling. When used in an object's interior statement, sampling only occurs inside the object. When used for atmospheric media, the samples run from the camera location until the ray strikes an object. Therefore for localized effects, it is best to use an enclosing object even though the density pattern might only produce results in a small area whether the media was enclosed or not.

The complete syntax for a media statement is as follows:

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 default values:

aa_level     : 4
aa_threshold : 0.1
absorption   : <0,0,0>
confidence   : 0.9
emission     : <0,0,0>
intervals    : 10
method       : 3
ratio        : 0.9
samples      : Min 1, Max 1
variance     : 1/128
SCATTERING
  COLOR        : <0,0,0>
  eccentricity : 0.0
  extinction   : 1.0

If a media identifier is specified, it must be the first item. All other media items may be specified in any order. All are optional. You may have multiple density statements in a single media statement. See "Multiple Density vs. Multiple Media" for details. Transformations apply only the density statements which have been already specified. Any density after a transformation is not affected. If the media has no density statements and none was specified in any media identifier, then the transformation has no effect. All other media items except for density and transformations override default values or any previously set values for this media statement.

Note: some media effects depend upon light sources. However the participation of a light source depends upon the media_interaction and media_attenuation keywords. See "Atmospheric Media Interaction" and "Atmospheric Attenuation" for details.

Note: In the POV-Ray 3.1 documentation it said: "Note a strange design side-effect was discovered during testing and it was too difficult to fix. If the enclosing object uses transmit rather than filter for transparency, then the media casts no shadows." This is not the case anymore since POV-Ray 3.5. Whether you specify transmit or filter to create a transparent container object, the media will always cast a shadow. If a shadow is not desired, use the no_shadow keyword for the container object.

3.6.2.1 Media Types

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:

  emission 0.75
  emission rgb<0.75,0.75,0.75>

3.6.2.1.1 Absorption

The absorption keyword specifies a color of light which is absorbed when looking through the media. For example 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 normally.

3.6.2.1.2 Emission

The 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 ambient values. The default value is rgb<0,0,0> which means no light is emitted.

3.6.2.1.3 Scattering

The syntax of a scattering statement is:

SCATTERING:
    scattering { 
        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 occurs.

The scattering effect is only visible when light is shining on the media from a light source. This is similar to diffuse 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. Using 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.

The Henyey-Greenstein scattering function for different eccentricity values.

3.6.2.2 Sampling Parameters & Methods

Media effects are calculated by sampling the media along the path of the ray. It uses a method called Monte Carlo integration. The intervals keyword may be used to specify the integer number of intervals used to sample the ray. The default number of intervals is 10. For object media the intervals are spread between the entry and exit points as the ray passes through the container object. For atmospheric media, the intervals spans the entire length of the ray from its start until it hits an object. For media types which interact with spotlights or cylinder lights, the intervals which are not illuminated by these light types are weighted differently than the illuminated intervals when distributing samples.

The ratio keyword distributes intervals differently between lit and unlit areas. The default value of ratio 0.9 means that lit intervals get more samples than unlit intervals. Note that the total number of intervals must exceed the number of illuminated intervals. If a ray passes in and out of 8 spotlights but you have only specified 5 intervals then an error occurs.

The samples Min, Max keyword specifies the minimum and maximum number of samples taken per interval. The default values are samples 1,1.

As each interval is sampled, the variance is computed. If the variance is below a threshold value, then no more samples are needed. The variance and confidence keywords specify the permitted variance allowed and the confidence that you are within that variance. The exact calculations are quite complex and involve chi-squared tests and other statistical principles too messy to describe here. The default values are variance 1.0/128 and confidence 0.9. For slower more accurate results, decrease the variance and increase the confidence.

Note: the maximum number of samples limits the calculations even if the proper variance and confidence are never reached.

The method keyword lets you specify what sampling method is used, POV-Ray provides three. Method 1 is the method described above.

Sample method 2 distributes samples evenly along the viewing ray or light ray. The latter can make things look smoother sometimes. If you specify a max samples higher than the minimum samples, POV will take additional samples, but they will be random, just like in method 1. Therefore, it is suggested you set the max samples equal to the minimum samples. jitter will cause method 2 to look similar to method 1. It should be followed by a float, and a value of 1 will stagger the samples in the full range between samples.

Sample method 3 uses adaptive sampling (similar to adaptive anti-aliasing) which is very much like the sampling method used in POV-Ray 3.0's atmosphere. This code was written from the ground-up to work with media, however. Adaptive sampling works by taking another sample between two existing samples if there is too much variance in the original two samples. This leads to fewer samples being taken in areas where the effect from the media remains constant. The adaptive sampling is only performed if the minimum samples are set to 3 or more.

You can specify the anti-aliasing recursion depth using the aa_level keyword followed by an integer. You can specify the anti-aliasing threshold by using the aa_threshold followed by a float. The default for aa_level is 4 and the default aa_threshold is 0.1. jitter also works with method 3. Sample method 3 ignores the maximum samples value. It is usually best to only use one interval with method 3. Too many intervals can lead to artefacts, and POV will create more intervals if it needs them.

3.6.2.3 Density

Particles of media are normally distributed in constant density throughout the media. However 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. More than one density may be specified per media statement. See "Multiple Density vs. Multiple Media". The syntax for density is:

DENSITY:
    density
    {
        [DENSITY_IDENTIFIER]
        [DENSITY_TYPE]
        [DENSITY_MODIFIER...]
    }
DENSITY_TYPE:
    PATTERN_TYPE | COLOR 
DENSITY_MODIFIER:
    PATTERN_MODIFIER | DENSITY_LIST | color_map { COLOR_MAP_BODY } |
    colour_map { COLOR_MAP_BODY } | density_map { DENSITY_MAP_BODY }

The density statement may begin with an optional density identifier. All subsequent values modify the defaults or the values in the identifier. The next item is a pattern type. This 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 which were previously available only for use with our discontinued halo feature. 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. See "Patterns" for details on particular pattern types. Although a solid COLOR pattern is legal, in general it is used only when the density statement is inside a density_map.

3.6.2.3.1 General Density Modifiers

A density statement may be modified by any of the general pattern modifiers such as transformations, turbulence and warp. See "Pattern Modifiers" for details. In addition there are several density-specific modifiers which can be used.

3.6.2.3.2 Density with color_map

Typically a media uses just one constant color throughout. Even if you vary the density, it is usually just one color which is specified by the absorption, emission, or scattering keywords. However when using emission to simulate fire or explosions, the center of the flame (high density area) is typically brighter and white or yellow. The outer edge of the flame (less density) fades to orange, red, or in some cases deep blue. To model the density-dependent change in color which is visible, you may specify a color_map. The pattern function returns a value from 0.0 to 1.0 and the value is passed to the color map to compute what color or blend of colors is used. See "Color Maps" for details on how pattern values work with color_map. This resulting color is multiplied by the absorption, emission and scattering color. Currently there is no way to specify different color maps for each media type within the same media statement.

Consider this example:

  media{
    emission 0.75
    scattering {1, 0.5}
    density { spherical
      color_map {
        [0.0 rgb <0,0,0.5>]
        [0.5 rgb <0.8, 0.8, 0.4>]
        [1.0 rgb <1,1,1>]
      }
    }
  }

The color map ranges from white at density 1.0 to bright yellow at density 0.5 to deep blue at density 0. Assume we sample a point at density 0.5. The emission is 0.75*<0.8,0.8,0.4> or <0.6,0.6,0.3>. Similarly the scattering color is 0.5*<0.8,0.8,0.4> or <0.4,0.4,0.2>.

For block pattern types checker, hexagon, and brick you may specify a color list such as this:

  density{
    checker 
    density {rgb<1,0,0>}
    density {rgb<0,0,0>}
  }

See "Color List Pigments" which describes how pigment uses a color list. The same principles apply when using them with density.

3.6.2.3.3 Density Maps and Density Lists

In addition to specifying blended colors with a color map you may create a blend of densities using a density_map. The syntax for a density map is identical to a color map except you specify a density in each map entry (and not a color).

The syntax for density_map is as follows:

DENSITY_MAP:
    density_map { DENSITY_MAP_BODY }
DENSITY_MAP_BODY:
    DENSITY_MAP_IDENTIFIER | DENSITY_MAP_ENTRY...
DENSITY_MAP_ENTRY:
    [ Value DENSITY_BODY ]

Where Value is a float value between 0.0 and 1.0 inclusive and each DENSITY_BODY is anything which can be inside a density{...} statement. The density keyword and {} braces need not be specified.

Note: the [] brackets are part of the actual DENSITY_MAP_ENTRY. They are not notational symbols denoting optional parts. The brackets surround each entry in the density map.

There may be from 2 to 256 entries in the map.

Density maps may be nested to any level of complexity you desire. The densities in a map may have color maps or density maps or any type of density you want.

Entire densities may also be used with the block patterns such as checker, hexagon and brick. For example...

  density {
    checker
    density { Flame scale .8 }
    density { Fire scale .5 }
  }

Note: in the case of block patterns the density wrapping is required around the density information.

A density map is also used with the average density type. See "Average" for details.

You may declare and use density map identifiers but the only way to declare a density block pattern list is to declare a density identifier for the entire density.

3.6.2.3.4 Multiple Density vs. Multiple Media

It is possible to have more than one media specified per object and it is legal to have more than one density per media. The effects are quite different. Consider this example:

  object {
    MyObject
    pigment { rgbf 1 }
    interior {
      media {
        density { Some_Density }
        density { Another_Density }
      }
    }
  }

As the media is sampled, calculations are performed for each density pattern at each sample point. The resulting samples are multiplied together. Suppose one density returned rgb<.8,.8,.4> and the other returned rgb<.25,.25,0>. The resulting color is rgb<.2,.2,0>.

Note: in areas where one density returns zero, it will wipe out the other density. The end result is that only density areas which overlap will be visible. This is similar to a CSG intersection operation. Now consider

  object { 
    MyObject
    pigment { rgbf 1 }
    interior {
      media {
        density { Some_Density }
      }
      media {
        density { Another_Density }
      }
    }
  }

In this case each media is computed independently. The resulting colors are added together. Suppose one density and media returned rgb<.8,.8,.4> and the other returned rgb<.25,.25,0>. The resulting color is rgb<1.05,1.05,.4>. The end result is that density areas which overlap will be especially bright and all areas will be visible. This is similar to a CSG union operation. See the sample scene scenes\interior\media\media4.pov for an example which illustrates this.