On 2022-03-23 16:22 (-4), Kenneth wrote:
> 
> That's a masterful analysis; thanks for taking the time to describe the details
> so clearly.

You're welcome.

> 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.

It's an effect called opposition surge.  Because the Sun is illuminating
the Moon head on from our point of view, we are not seeing the shadows
cast by craters and mountains.

> 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. [snip]

So my estimate was very close!  :)

>
https://physics.stackexchange.com/questions/244922/why-does-moonlight-have-a-lower-color-temperature

The simplest way to summarize ProfRob's answer is that the Moon is not a
neutral gray; it is slightly brownish.

> 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?)

For your "viewing environment," think about where you want "white" to
be.  To give you some context, warm LEDs are 2700K; incandescent lights
are around 2800K; cool fluorescent lights are more or less around 4200K;
and daylight fluorescents and LEDs are 5000K.  Think about how bluish
your computer monitor looks, and you can see that D65 is way too high
for typical scenes; everything will look reddish.

I read somewhere that outdoor photographic film is calibrated to 5600K.

This post shows an outdoor sunny scene under various white points:
  https://news.povray.org/617229c5%40news.povray.org
This might give you an idea of what white point to choose.

Alas, I do not have a simple explanation for how this works; I don't
completely understand it myself.  I just examined test_lightsys.pov, and
used the technique on a solar spectrum to get my light_source colors.
Lightsys IV can do white point adjustments without requiring a spectal
curve, but I haven't figured out how to do that yet.  Have a look at
scenes im_test_day.pov and im_test_inc.pov to see if that helps.

> 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!

I know nothing about the psychology of low light, but I do know that we
lose color sensitivity in the dark.  So regardless the actual color of
moonlight, you might want to gray it down.  I'm thinking this would be
best done in post-processing, though.

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