"Kari Kivisalo" <kar### [at] kivisalonet> wrote in message
news:39587BE1.162461B9@kivisalo.net...
| Mark Gordon wrote:
| > remember that the sky is typically brighter closer to the sun, even at
| > noon.  It's not realistic for the sky to have uniform brightness unless
| > it's either uniformally dark or overcast.
|
| I was planning to make an image map for the sky and calculate
| it's average brightness. If the ratio is 5 I would use color*5
| for sun and ambient 1/average_brightness for sky.
|
| All I need is a rough figure for the ratio.
| Is it 1,2,5,10 or 20?

I'm not sure of the actual idea but if you could go by magnitudes then it
might be plausible.  The Sun is around -25 mag. while the clear blue sky (not
near the sun anyway, but daylight sky) should be around -2 mag.  I'm guessing
this right now from what the planet Venus mag. can be while still seeing it in
the daytime, meaning the sky would be about the same or a little less.
So, looking up astronomical magnitudes might yield some useful info.
Hmm, I just looked up what magnitude the Sun has and I was close, -26.7.
Since Venus varies in brightness it averages probably about -3 (I must've been
thinking of Jupiter before :-) Sure I was...) but I think it's daytime showing
is only when at around -4 or greater mag., which I looked up as well (the
magnitude, not the specifics).  So daylight sky must average about that, -4.
Now, in case you don't know this already, a magnitude of +1 is 2.5 times
dimmer than magnitude 0 and each preceding or proceeding magnitude is likewise
2.5X the adjacent one.
Maybe that will help you, but if I were you I'd look it up someplace.

Bob

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