Cousin Ricky <ric### [at] yahoocom> wrote:
> Many years ago, alphaQuad aka Melody issued a glory ping pong challenge,
> but was quite unsuccessful.

I had some very nice exchanges with the friendly Philip Laven, and we talked
about how simulating the optics was "fiendishly complicated", and how here on
the forum we'd like to merely simulate the effect as best we could without
approximating the underlying plane wave interference.

So, for the moment, I noticed that POV-Ray has an inbuilt rainbow atmospheric
effect.

https://wiki.povray.org/content/Reference:Rainbow

So I took the image from
http://www.philiplaven.com/Mie-r10-175-180-0p01-300col.jpg
(at http://www.philiplaven.com/p2c1.html)
Which already has the rgb values for the glory laid out in a linear form, and
plugged that (as a cropped version) as an image_map into a pigment and then into
a function, and used a loop to define my color map entries using the 0-1 index
as an argument to the function.
Then I used that color_map for the rainbow, tweaked some parameters, stuck a
plane with a cloud pigment behind it, and there we go.

Important note: the rainbow must make use of a mechanism very much like media,
since the plane needed to be hollow to see the rainbow. (One can also just flip
the plane normal using the inverse keyword)

I'm trying to figure out how to cast a shadow _through_ scattering media to
simulate the Brocken Spectre, but haven't figured that part out yet.

I might need to alter the color_map function to change the fading effect, but it
looks passable for now.

- BW

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