[Flightgear-devel] Moonlight

[Thorsten R. <tho...@sc...>]

I've used the opportunity to illuminate Earthview's clouds with 
moonlight as well, and try as I might, I can't make sense of the 
dynamical moonlight computation.


First, /environment/moonlight seems to be a tied property which prevents 
this from being used properly in the effect framework. I see the value 
this has at startup, but it's never updated in the graphics when I 
change the date and the moon phase changes as a result. This is sort of 
unfortunate as the effect framework is pretty much the only place where 
we use it.


Second, I don't get the correlation between moon phase and light. 
Currently I see phase 0.4 (consistent with the visuals), yet the 
moonlight brightness is 0.7, i.e. less than half of the area lit give 
70% of maximal illumination. In reality this ought to be much different 
and I'd expect less than 1/6 of maximal brightness.

http://home.earthlink.net/~kitathome/LunarLight/moonlight_gallery/technique/moonbright.gif


(the lunar surface reflectivity is strongly angle-dependent and peaks 
towards full moon). There should be perception and atmosphere 
corrections, but I still can't reproduce this using what I usually use 
for perception modeling.


Third, I see no change in /environment/moonlight when the moon is below 
the horizon. I also can't make any sense of /time/moon-angle-rad - the 
sun angle next to this is the angle the sun has with the local zenith - 
the moon angle displayed here seems to be something visually different, 
at least it does not take pi/2 when the moon is at the horizon like the 
sun angle does.


So unfortunately right now the system delivers wrong illumination for a 
number of situations and basically only gets it right when a near full 
moon is out and the user doesn't change date.

Edward, could you please look at this again and check whether it really 
does what it should (and un-tie the /environment/moonlight property)?


Thanks,


* Thorsten