"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.

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