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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.
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.
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.
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.
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.
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