Some benchmark testing of the system yesterday evening with pictures of aerial sunrises/sunsets I found in internet searches.
Horizon glow just pre-dawn
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Real:
And Flightgear:
All in all, not doing too bad here - the high altitude clouds should become more salmon-coloured instead of just yellow, but the light function is a tricky beast to tune (for performance reason, I am restricted to a generalized logistic function for all color channels). Also, we don't get the bright glow of clouds directly in the sun's path - that's Mie forward scattering of direct light, and far outside the range of what the cloud shader can be reasonably expected to do.
The terminator line from high altitude:
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ISS (?) from about 300 km altitude
And Flightgear from just 100 km (with 250 km max visibility, you wouldn't see any terrain from 300 km altitude... - but I couldn't find NASA pictures from 100 km, so...)
Focus on the lower half of the pics - the hue of terrain, haze and clouds is reasonably well captured. In the upper half, reality brightens up much more than the Flightgear light function does (this is actually not just something I did - I just fitted the Flightgear light function, and the fit gets to full light even before the real thing from the core does). This is, in fact, a variant of the 'Why is the snow so yellow' question - because the light takes an awfully long time to go to full strength in all three color channels, so for a long time after sunrise, ambient light is rather yellow. Needs to be changed apparently - neither is snow seen so yellow, nor is the beyond terminator region really so low intensity.
Dense brightly glowing morning ground haze:
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Reality:
And Flightgear:
Yep - works out well. We don't have clouds casting shades into the haze (pity, but technically an absolute nightmare to do) and we don't have enough self-shading of the clouds in these conditions (that can probably be improved somewhat by tuning the shader code), but otherwise it's coming out nicely.
Actually, what I am really missing is aerial shots of sunset/sunrise condition *away* from the sun - of course, no one takes those as it's much less spectacular, but as it stands I have only a very foggy idea of how the scene should look like when not seen from the ground. Fixing it from the ground isn't so difficult, but shots from high altitude are way more challenging to get right.