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Sky and ALS shader needs tweaking during dawn and dusk

An exciting "new" option in FlightGear, that includes reflections, lightmaps, the particle system etc.. A lot is yet to be discovered/implemented!

Re: Sky and ALS shader needs tweaking during dawn and dusk

Postby Thorsten » Thu Jun 11, 2020 5:48 am

Regardless of setting, I get this band of brown haze at the horizon.


That's the 'Belt of Venus' - the shadow the Earth casts into its own atmosphere. In FG, it is somewhat darker than in most photographs for a couple of reasons:

* the O'Neil shader has a single scattering approximation, but air inside the shaded region is really illuminated by multiple scattering processes (which is complicated to implement) - the other shaders basically follow in brightness to blend with O'Neil part (anything else looks really silly). so they're also a bit darker than reality
* in photography, one typically brightens the scene by opening the aperture or prolonging the exposure time - many sunset photographs are not what you would see with bare eyes
* then there's the question of eye adaption of course - with the sun on one horizon and darkness on the other, it matters very much where you have looked before. If you've been looking into the sun for a moment, the rest of the scene will appear very dark, if you've been looking into darkness only, it will appear brightter. That's not in the simulation yet.

If you do a hue comparison between your screenshot and the real photo, you'll find that while your screenshot is darker (as explained above), the hue in FG is actually a bit less brown and more reddish than in reality. So despite this being dominated by multiple scattering processes (which is computationally really tough because you'd need another integral), the color hues are actually pretty good.
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Re: Sky and ALS shader needs tweaking during dawn and dusk

Postby danielHL » Sun Jun 14, 2020 6:43 pm

Well, today I learned something :) Thank you Thorsten.
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Re: Sky and ALS shader needs tweaking during dawn and dusk

Postby Mihajlo » Tue Jul 14, 2020 9:23 am

This may be unrelated, but last year I've had a chance to see the sunrise away from the sun and have a series of photos which I've sent to Thorsten. One thing I've noticed which seems different from FG is that the Belt of Venus seems to cut through the Earth's shadow, because It thins out and seems to expand from left to right while the shadow gets thicker ( In FG this does not happen and the Belt of Venus seems to appear in a different way (does not get thinner and does not expand from left to right). It would be appreciated if Thorsten or someone else can explain where the difference comes from. I could have wrongly interpreted some of the things I've seen as I'm not a physicist, but the scene away from the sun in my photo certainly appears different compared to FG, at least in this particular case.
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Re: Sky and ALS shader needs tweaking during dawn and dusk

Postby bugman » Tue Jul 14, 2020 11:29 am

Zac wrote in Wed Jun 10, 2020 10:59 am:Altered screenshot:

This is a very strong chartreuse green for me too! That reminds me of some old CRT monitors that would render such greens in a very, very different colour. It looks like nothing I've ever seen in the air, nor on the ground. Well, I have seen the green flash once before in my life, but it is so rare that I don't think it should be simulated in FG (then again Thorsten might have already implemented it ;)).

I'd suggest that the issue here is likely to be monitor colour calibration! I would highly recommend looking into this. For example this YouTube video will probably help a lot:


Edit: Note that screenshots of scenes that look realistic on your monitor will look never look realistic on an uncalibrated or badly calibrated monitor, so posting such screenshots will not support the argument that FG does a good job.

Edit 2: As a test, view the screenshot on a separate mobile device and put it and the monitor side-by-side. I can actually see quite a difference between my monitor, my mobile phone, and my tablet. They are nevertheless all different shades of chartreuse green (the Samsung tablet is, not surprisingly, very, very green).
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