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Rainbows are implemented using fog-like, circular arcs. Their syntax is:
RAINBOW:
rainbow { [RAINBOW_IDENTIFIER] [RAINBOW_ITEMS...] }
RAINBOW_ITEM:
direction <Dir> | angle Angle | width Width |
distance Distance | COLOR_MAP | jitter Jitter | up <Up> |
arc_angle Arc_Angle | falloff_angle Falloff_Angle
Rainbow default values:
arc_angle : 180.0
falloff_angle : 180.0
jitter : 0.0
up : y
The required direction vector determines the direction of the (virtual) light that is responsible for
the rainbow. Ideally this is an infinitely far away light source like the sun that emits parallel light rays. The
position and size of the rainbow are specified by the required angle and width keywords. To
understand how they work you should first know how the rainbow is calculated.
For each ray the angle between the rainbow's direction vector and the ray's direction vector is calculated. If this
angle lies in the interval from Angle-Width/2 to Angle+Width/2 the
rainbow is hit by the ray. The color is then determined by using the angle as an index into the rainbow's color_map.
After the color has been determined it will be mixed with the background color in the same way like it is done for
fogs.
Thus the angle and width parameters determine the angles under which the rainbow will
be seen. The optional jitter keyword can be used to add random noise to the index. This adds some
irregularity to the rainbow that makes it look more realistic.
The required distance keyword is the same like the one used with fogs.
Since the rainbow is a fog-like effect it is possible that the rainbow is noticeable on objects. If this effect is not
wanted it can be avoided by using a large distance value. By default a sufficiently large value is used to make sure
that this effect does not occur.
The color_map statement is used to assign a color map that will be mapped
onto the rainbow. To be able to create realistic rainbows it is important to know that the index into the color map
increases with the angle between the ray's and rainbow's direction vector. The index is zero at the innermost ring and
one at the outermost ring. The filter and transmittance values of the colors in the color map have the same meaning as
the ones used with fogs (see section "Fog").
The default rainbow is a 360 degree arc that looks like a circle. This is no problem as long as you have a ground
plane that hides the lower, non-visible part of the rainbow. If this is not the case or if you do not want the full
arc to be visible you can use the optional keywords up, arc_angle and falloff_angle
to specify a smaller arc.
The arc_angle keyword determines the size of
the arc in degrees (from 0 to 360 degrees). A value smaller than 360 degrees results in an arc that abruptly vanishes.
Since this does not look nice you can use the falloff_angle keyword to specify a region in which the
rainbow will smoothly blend into the background making it vanish softly. The falloff angle has to be smaller or equal
to the arc angle.
The up keyword determines were the zero angle position is. By changing
this vector you can rotate the rainbow about its direction. You should note that the arc goes from -Arc_Angle/2
to +Arc_Angle/2. The soft regions go from -Arc_Angle/2 to -Falloff_Angle/2 and from +Falloff_Angle/2
to +Arc_Angle/2.
The following example generates a 120 degrees rainbow arc that has a falloff region of 30 degrees at both ends:
rainbow {
direction <0, 0, 1>
angle 42.5
width 5
distance 1000
jitter 0.01
color_map { Rainbow_Color_Map }
up <0, 1, 0>
arc_angle 120
falloff_angle 30
}
It is possible to use any number of rainbows and to combine them with other atmospheric effects.
More about "Fog"
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