On 2020-04-30 5:16 PM (-4), Norbert Kern wrote:
> 
> I got the sky values from a photograph and the light values are very old - in
> fact I derived them from radiosity newsgroups discussions back in 2000 or 2001
> (rgb <1,0.87,0.57>*7 IIRC).

The problem with photographs of the sky is that I cannot tell how bright 
the sky is in relation to sunlight.  The color that shows up in the 
photo also depends on the camera's white point.  This is why I've 
searched for solar spectral and absorption data to inform my colors.

I have found photographs useful in estimating the gradient; however, one 
must be careful that the photo does not exceed the RGB dynamic range. 
This happens with quite a few of my outdoor photos.  In such cases, 
sampling is useless.

One thing that has surprised me over the years is just how dark plants 
are.  For example, the darker green sphere, which is somewhat lighter 
(and bluer) than the _Cordia_ leaf I sampled, works out to rgb <0.032, 
0.080, 0.028>, with luminance a paltry 0.066.  But with sky blue turning 
out as dark as it is, plants have to be even darker.  I think the 
wildcard here is that we simply underestimate how freaking bright the 
Sun is--and this brightens up the whole landscape.

Another factor, though, is the non-linearity of both our perceptions and 
the way we pick colors.  The color rgb <0.032, 0.080, 0.028> at diffuse 
1 turns out to be srgb <0.257, 0.401, 0.239> at the default diffuse 0.6! 
  That certainly *seems* a lot brighter than the linear version.

(FYI, the values I got for the _Cordia_ leaf were rgb <0.046, 0.054, 
0.026> and <0.045, 0.060, 0.032>--using eval_pigment() in a loop, which 
is why I expressed the colors in linear terms from the outset.  No eye 
dropper tool for this operation!)

> This differs very much from your value - I should take a look in your render
> rig...

I have not published it.  When I started my latest rewrite, I wrote it 
in such a manner than it could be easily integrated with the Object 
Collection; but it's not ready yet.

I can say that the main sky implementation at this time is:

   fog
   { fog_type 2
     distance 165000 feet * user unit conversion
     color rgb <0.141, 0.293, 0.778>
     fog_offset 0
     fog_alt 18400 feet * user unit conversion
   }

There is also a light blue glow around the Sun, which also contributes 
to the sky brightness.

My sunlight and sky colors assume a reference white of D50.

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