Quote - RE:Additional parameters called aconst, bconst, cconst, dconst, econst allow you to fine tune the color of the sky to simulate various atmospheric conditions.

Does anyone have and insight on what these relate to colorwise?
i.e RGB etc etc...................?
I could try numerous combinations to come up with conclusions.
Just seems easier for all, if someone has sorted this out.

Thanks
 

I can only find really abstract info on this. It is an implementation of the "Perez All-Weather Sky Model". This model is not actually based on physics, but is an empirical model. Empirical means "we are able to imitate a real phenomenon to a pretty good degree even though the math we're using has nothing whatsoever to do with how the phenomenon actually happens."

I don't know why Lux, which is in all other ways physically based, uses this empirical model instead of a physical one. It is old - was published in 1993.

As with many empirical models, there are a bunch of constants that have very little to do with any sort of intuitive understanding of what to do with them. We have six; a, b, c, d, e, and turbidity. Of these, only turbidity has any sort of intuitive value. The others are used as follows:

sky pixel luminance = [1 + a * exp(b / cos theta)] * [1 + c * exp(d * gamma) + e cos^2 gamma]

where theta is the zenith angle of the sky pixel being rendered and gamma is the angle between the sky pixel and the sun.

Does this help? Not even me. And it doesn't even begin to explain how colors are produced - it only talks about luminance.

When I have time, I will experiment with these coefficients and supply a bunch of named presets. Eventually I may come to understand what they actually do instead of randomly trying them and construct some simpler UI in front of these six values. Something that would let you use buttons like "more/less blue, red, or yellow", "more/less hazy", "more/less variation from straight up to horizon", etc.

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