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Light scattering on hazes

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

Light scattering on hazes

Postby Thorsten » Fri Dec 04, 2015 11:50 am

I've just finished working through a book on atmosphere and weather (my first science textbook I've read in Finnish) and it turns out there's tons of stuff I haven't known.

One interesting chapter was about light phenomena associated with various kinds of hazes. Turns out ice crystal hazes are specifically interesting, because the crystals have a regular structure, that structure is non-trivial (there's all sorts of crystal shapes dependent on temperature), they're basically all the same, under certain conditions they're even arranged roughly the same way, and this gives rise to a whole zoo of halos, rings, arcs and other phenomena that can be observed (the book lists a good 30 different ones). In contrast, liquid droplets are always round, so the only parameter that can change is the size distribution, and dust is always irregular - so dust and water droplet scattering is much easier than ice.

I couldn't resist to persuade ALS to do some of the more common ones, so here we go:

That's the 22 deg ring generated on a high cloud layer (it doesn't look too ring-like here, but you can sort of see the upper right arc really far from the sun - it also doesn't always look perfectly ring-like in reality, most of these are subtle). It's one of the most common phenomena, since high hazes are almost always frozen and not really rare, and you get to see the ring against a dark sky background.

Image

That's a light pillar and a side-sun generated on low ice haze - they get cut off from below when the haze gets too dense, but there's a sweet spot where they appear most prominent. They're both fairly common phenomena as well, but they tend to occur for low sun only - which isn't so much of a problem in winter really.

Image

You can look forward to see the whole package integrated with the weather system for Christmas :-)
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Re: Light scattering on hazes

Postby Thorsten » Fri Dec 04, 2015 12:52 pm

I guess you know what these are... :-)

Image

Image
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Re: Light scattering on hazes

Postby Hooray » Fri Dec 04, 2015 1:18 pm

if you continue to extend FlightGear like this, one day, we won't be able to tell the difference between looking at our screens our out of the window ...
Please don't send support requests by PM, instead post your questions on the forum so that all users can contribute and benefit
Thanks & all the best,
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Re: Light scattering on hazes

Postby dg-505 » Fri Dec 04, 2015 1:40 pm

One day we'll wake up and realize that this isn't a flight simulator anymore but instead an Atmospheric-light-scattering-and-advanced-weather-simulator... :wink: Wondeful work, Thorsten. Keep it up!
It's not just a hobby. I'ts a passion: FLYING !!

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Re: Light scattering on hazes

Postby gsagostinho » Fri Dec 04, 2015 1:42 pm

This is simply fantastic, Thorsten!
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Re: Light scattering on hazes

Postby legoboyvdlp » Fri Dec 04, 2015 1:46 pm

I could not say it better than the above.... simply fantastif.
I particularly love the rainbow.
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Re: Light scattering on hazes

Postby MIG29pilot » Fri Dec 04, 2015 1:48 pm

Just as I read that it occurred to me it would be hilarious to have that show up when Terrasync doesn't work and you find yourself underwater in an endless sea.
Great stuff!
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Re: Light scattering on hazes

Postby bugman » Fri Dec 04, 2015 1:58 pm

Wow, this sounds like it's going to be amazing!!! I look forward to seeing sun dogs :D

Image

Cheers,
Edward
Last edited by bugman on Fri Dec 04, 2015 2:02 pm, edited 1 time in total.
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Re: Light scattering on hazes

Postby bugman » Fri Dec 04, 2015 2:00 pm

These things are pretty complicated though!

Image

Regards,
Edward
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Re: Light scattering on hazes

Postby bugman » Fri Dec 04, 2015 2:55 pm

I hope you also consider adding these effects to the moon as well :) That could make winter night flying quite spectacular!

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Image

Regards,
Edward

Edit: Oh, is the book you have called Halot, by Marko Riikonen?
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Re: Light scattering on hazes

Postby Thorsten » Fri Dec 04, 2015 3:22 pm

The book is 'Ilmakehä ja sää' by Karttunen, Koistinen, Saltikoff and Manner - it's an older one from the local library.

bugman wrote in Fri Dec 04, 2015 2:00 pm:These things are pretty complicated though!

Image


The problem with this is that if I'd actually render it, everyone will claim that this can never be real.

I think lots of the more spectacular ones are fairly common in Antarctica, but for instance the Parry occurs perhaps once per year in Finland. So I'll confine myself to the more simple ones. Perhaps I'll add circumhorizontal and circumzenithal rings - they also seem to be reasonably common (i.e. more than ten days a year here).

The sun dogs are actually already done, it's what I referred to above as 'side sun' (I translated 'sivuaurinko' directly - that's the problem when you read not in English - you tend to pick up odd nomenclature...)
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Re: Light scattering on hazes

Postby gsagostinho » Fri Dec 04, 2015 3:31 pm

By the way Thorsten, how does your rainbow look like from the air? Does it look like this?



According to wikipedia, "[...] every rainbow is a circle, but from the ground only its upper half can be seen. [...] Viewing the rainbow's lower half requires the presence of water droplets below the observer's horizon, as well as sunlight that is able to reach them. These requirements are not usually met when the viewer is at ground level, either because droplets are absent in the required position, or because the sunlight is obstructed by the landscape behind the observer. From a high viewpoint such as a high building or an aircraft, however, the requirements can be met and the full circle rainbow can be seen."
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Re: Light scattering on hazes

Postby Thorsten » Fri Dec 04, 2015 3:33 pm

if you manage to find a large enough rainfront and to get the sun right, you'll get to see a full circle, yes - it's not a texture, it checks the physical light scattering condition on the rain. Also if you manage to catch it at dawn, you'll get only the red part of it.
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Re: Light scattering on hazes

Postby gsagostinho » Fri Dec 04, 2015 3:34 pm

Fantastic! :D
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Re: Light scattering on hazes

Postby Thorsten » Fri Dec 04, 2015 3:35 pm

I hope you also consider adding these effects to the moon as well That could make winter night flying quite spectacular!


Considering as in 'Would be cool to have!' - definitely. Considering as in actually implementing - no. The renderer currently has no idea where the moon is, so it can't be done without touching the C++ side and introducing a new uniform vec3 with the information.
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