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Currently our rainbow has a circular shape, even though most of it is hidden below the ground plane. You can easily create a rainbow arc by using the arc_angle keyword with an angle below 360 degrees.
If you use arc_angle 120 for example you'll get a rainbow arc that abruptly vanishes at the arc's ends. This does not look good. To avoid this the falloff_angle keyword can be used to specify a region where the arc smoothly blends into the background.
As explained in the rainbow's reference section (see "Rainbow") the arc extends from -arc_angle/2 to arc_angle/2 while the blending takes place from -arc_angle/2 to -falloff_angle/2 and falloff_angle/2 to arc_angle/2. This is the reason why the falloff_angle has to be smaller or equal to the arc_angle.
In the following examples we use an 120 degrees arc with a 45 degree falloff region on both sides of the arc (rainbow3.pov).
rainbow {
angle 42.5
width 5
arc_angle 120
falloff_angle 30
distance 1.0e7
direction <-0.2, -0.2, 1>
jitter 0.01
color_map {
[0.000 color r_violet1 transmit 0.98]
[0.100 color r_violet2 transmit 0.96]
[0.214 color r_indigo transmit 0.94]
[0.328 color r_blue transmit 0.92]
[0.442 color r_cyan transmit 0.90]
[0.556 color r_green transmit 0.92]
[0.670 color r_yellow transmit 0.94]
[0.784 color r_orange transmit 0.96]
[0.900 color r_red1 transmit 0.98]
}
}
The arc angles are measured against the rainbows up direction which can be specified using the up keyword. By default the up direction is the y-axis.
A rainbow arc.
We finally have a realistic looking rainbow arc.
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