Cousin Ricky <ric### [at] yahoocom> wrote:
> 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.


Hi Richard,

all of this makes total sense to me.
I also recognized the darkness of many plants (back in 2001, when I worked on my
"warm_up" image). In fact many leaves only seem to be bright because of specular
highlights and reflections. In outdoor scenes like L&S with a max_trace_level of
255 there is no way to add reflections to the finish of all plants, but in
indoor scenes they are of great importance.

I will test your settings - and I've a question - what is the suncolor value at



Regards,
Norbert

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