Cousin Ricky <ric### [at] yahoocom> wrote:

> Unless someone can think of some clever way to simulate diffractive
> effects.  I won't say it can't be done; that would only ensure that it
> *won't* be done.  Perhaps some clever (ab)use of irid?  Or finding a way
> to accurately fake it with regular ior and dispersion?

I took an initial stab at this using the interference model, and that was
probably too complicated an approach (for now).

My current experiment is trying to implement this:

We know that the effect is angle-dependent.  "The rainbow isn't at any distance
from you - it's simply coming from a _direction."

So I played a bit with calculating the angle on an image plane, using only the
2-4 degree angle of the glory, and then trying to remap those angles to
0-(approx) 280 so that I could just apply an HSV color gradient across it.

Sleep and coffee let my brain see that using something *like* a sky_sphere would
be the way to go.  Then it's just a matter of daisy-chaining functions together
to get the rgb 1 emissive inner circle, the Alexander's dark band, and whatever
other higher order rainbows the user decides to include.

Maybe there's somehow something that can be done with an actual cone {} spanning
the desired angles, and some kind of intersection {} with a hollow sphere could
be used?  Media with a df3?

I like the irid idea - I'll have to take a look at the source for that to see if
something can be implemented using that code.

With regard to my earlier recognition that the glory resembled the Fresnel
diffraction pattern in shadows:  What if one used an emissive light source that
had the ROYGBIV HSV gradient, and it cast a penumbral shadow?

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