"Chris R" <car### [at] comcastnet> wrote:
> "Kenneth" <kdw### [at] gmailcom> wrote:
> > Cousin Ricky <ric### [at] yahoocom> wrote:
> > > On 2022-03-21 10:58 (-4), Chris R wrote:
> > > >
> > > > ...I am currently using a made-up color of <0.45, 0.5, 0.75>
> > > > and an intensity of 1.0 with no fading...Has anyone done any spectrum
> > > > analysis for moonlight to suggest better values?
> > >
> > > Lumens is not what you want for moonlight; this is a measure of the
> > > total amount of light emitted (or reflected, in this case).  Lux is the
> > > proper unit for a light source that is effectively at infinity...
> > > [clip]
> >
> > That's a masterful analysis; thanks for taking the time to describe the details
> > so clearly.
> >
> > I find it interesting that the full Moon as seen at zenith (with just our eyes,
> > not through a telescope) *appears* to be a Lambertian 'disc', not a sphere--
> > that is, it seems to have equal illuminance from all points on its surface...
> > even though the Sun's rays are hitting the sphere at progressively shallower
> > angles, out towards the rim. I'm guessing that this is a result of our automatic
> >  eye/brain adjustments having to do with 'local contrast' against a dark sky,
> > and because of the behavior of our eyes' receptors in *relatively* dim light.
> >
> > The color temperature of Moonlight, as reflected from objects, seem to be
> > somewhat subjective from what I've read so far. We think of it as being slightly
> > blue-ish-- but the Moon itself, reflecting Sunlight, has a LOWER color
> > temperature than the Sun, something like 4100-deg Kelvin. Taken by itself, that
> > would mean that Moonlight should be slightly *yellowish*(!) in comparison. But
> > other things come into play-- the Earth's atmospheric scattering, our eyes'
> > reaction to dim light, and probably other mysteries that I don't know about...
> > all of which combine to create a slightly blue-ish result.  It's also possible
> > that we have been 'trained' to think of Moonlight as being blue-- from centuries
> > of artists' interpretations, how it is usually reproduced in movies, etc. No one
> > wants to see yellow-ish Moonlight, it just looks... wrong! ;-)
> >
> > Take a look at this discussion (although I don't think it comes to any definite
> > conclusion)...
> >
> >
https://physics.stackexchange.com/questions/244922/why-does-moonlight-have-a-lower-color-temperature
> >
> > Here's another little monkey-wrench to throw into the mix: the D65 'white point'
> > of computer monitors and some color standards. That's a 6500-deg Kelvin color
> > temperature, which is meant to match 'daylight'-- meaning, the Sun's light AND
> > the surrounding blue sky. That's more blue-ish than the Sun by itself (which is
> > around 5400 to 5700K?) But does this viewing 'environment' still hold true when
> > trying to create a 'low-light' Moonlit scene in POV-ray (or any other graphics
> > program)? By nature, such a scene should be reproduced very dim and dark to look
> > 'natural'-- compared to a 'Sun-lit' scene. But that's not how we would usually
> > render it, because we want it to look...normal. Meaning, colors and brightnesses
> > rendered from 0 to 1.0 (0 to 255) or thereabouts-- just like a typical scene in
> > 'daylight'. This...discrepancy(?) in how we render low-light scenes and their
> > associated 'color temperatures' seems like an interesting conundrum. Do I have a
> > solution to it? No! (ha). I just thought I would bring it up as a philosophical
> > discussion point. Maybe we should just follow the great artists of the past, and
> > color the Moonlight blue regardless!
>
> So, I'm feeling less bad about my arbitrary adjustments to the moon's light
> color now.  I found a whole slew of information about the eye's dark adaptation
> ability, and what it can do to our perception of color and brightness,
> (specifically the Purkinje effect causing a shift towards blue sensitivity,
> which provides another explanation for our perception of moonlight as blue.)
>
> So, to make my macros look more technically astute, I have just added a "Dark
> Adaptation" color value when computing the color of the moon's light before
> applying the lux value and atmospheric attenuation.  It still means I'm mostly
> just adjusting things until it looks right, but I can pretend there's some math
> and science behind it.  :-)
>
> -- Chris R.

I just posted some renderings of the scene with the moonlight as light sources
in p.b.i:

http://news.povray.org/povray.binaries.images/thread/%3Cweb.623cb1d52227bb25cc71334a5cc1b6e@news.povray.org%3E//povray.
binaries.images/thread/%3Cweb.623cb1d52227bb25cc71334a5cc1b6e@news.povray.org%3E/


-- Chris R.

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