On 5/25/20 2:33 AM, Thomas de Groot wrote:
> Op 24/05/2020 om 18:16 schreef Bald Eagle:
>> William F Pokorny <ano### [at] anonymousorg> wrote:
>>
> 
> [snip]
> 
>>> The finish irid feature ....
>>> ... This feature has some
>>> other issues like lack of ability to scale relative to the incoming
>>> coordinates which will make it tricky to get something which always
>>> works with respect to scene dimensions.
>>
> [snip]
> 
>>
>> Yeah - iridescence seems ... "special".
>> Using Iridescence
>> "Iridescence is what we see on the surface of an oil slick when the 
>> sun shines
>> on it. The rainbow effect is created by something called thin-film 
>> interference
>> (read section Iridescence for details). For now let's just try using it.
>> Iridescence is specified by the irid statement and three values: amount,
>> thickness and turbulence. The amount is the contribution to the 
>> overall surface
>> color. Usually 0.1 to 0.5 is sufficient here. The thickness affects 
>> the busyness
>> of the effect. Keep this between 0.25 and 1 for best results. The 
>> turbulence is
>> a little different from pigment or normal turbulence. We cannot set 
>> octaves,
>> lambda or omega but we can specify an amount which will affect the 
>> thickness in
>> a slightly different way from the thickness value. Values between 0.25 
>> and 1
>> work best here too."
>>
> 
> I am probably wrong, but I seem to recall that Christoph did some 
> massive work in overhauling iridescence a number of years ago...? Indeed 
> in 2011:
> 
> http://news.povray.org/povray.binaries.images/thread/%3C4ed6f24c@news.povray.org%3E/

> 
> 
> ...there should be more about this.
> 
> Or is this unrelated to the present problem?
> 

Thank you for the newsgroup pointer.

His updates - assuming they are the code I see - are tangle with this 
problem because Turbulence() wasn't - or isn't now in any case - 
returning the distribution and range assumed in the code.

Christoph's changes were - I believe made for the v37 release. The 
documentation notes changes to actual physical dimensions for v37(1). 
Which, on more thought, means I need to yank my fudge matching, magic 
factor back out of the code and update the scenes for the proper end 
thicknesses given the turbulence instead!

He mentions being unable to scale for the same effect - which I believe 
is correct with respect to the viewer frame. I'm suggesting when using 
turbulence adding some method to scale and/or shift the turbulence of 
the thickness a good idea given the difficulty matching up with changing 
scene scaling of some object. Like media and density, if you want the 
turbulence to be constant (maybe yes, maybe some, maybe no) you'd need 
to 'counter' adjust it in some way on scene scaling - we cannot do this.

(1) The documentation in r3_6.html#r3_6_1_3_7 has this 3.7 note:

"Note: The version 3.7 iridescence feature has had a major overhaul. The 
syntax remains the same, however, both the thickness and amount values 
are now specified in microns. Consequently, iridescence effects will 
vary from previous versions."

which emphasizes 'both' thickness and amount being in microns. The 
'thickness amount' is, and the 'turbulence amount' is, but the 
irid_amount is just a color intensity multiplier for the irid_wavelength
color specified in global_settings. We could likely help new to irid 
users by making this clearer.

The "See also the global setting Irid_Wavelength for additional 
information." sentence I think should probably be up in the paragraph 
with irid_amount because - indirectly - this is the 'amount' of color, 
color to which it's related.

Anyway, this rushed so apologies for the mistakes. Off to real life for 
a few days. Thanks again for the prompt. I need to yank that fudge 
factor in my povr code.

Bill P.

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