Re: [Flightgear-devel] Moonlight

[Edward d'A. <tru...@gm...>]

On 24 May 2017 at 14:45, Thorsten Renk <tho...@sc...> wrote:
>
>> Actually log(lux) is a fundamental part of the physics of this
>> equation.
>
> lux isn't a physics unit in the first place - it's a perception-weighted
> unit.
>
> https://en.wikipedia.org/wiki/Lux
>
> "In photometry, this is used as a measure of the intensity, as perceived
> by the human eye, of light that hits or passes through a surface. It is
> analogous to the radiometric unit watt per square metre, but with the
> power at each wavelength weighted according to the luminosity function, a
> standardized model of human visual brightness perception. "

Well, it's the moon's power spectrum passed through the luminosity
function [1] and summed.  So the luminosity function takes the
biophysics of the human eye into account.  Biophysics is real physics,
even if physics will never accept the concept ;)  For those not
following, Thorsten is a physicist and I'm a biophysicist :P  In any
case, I still don't see why the simple log(lux) calculation could not
be part of the simgear ephemeris code as it currently is.  This value
is related to the brightness of the moon, which is an ephemeris
concept, and that could be additionally calculated and used for the
brightness of the moon disk.  We could take the log(lux) value and in
FGLight::update() convert it to a non-tied property
/ephemeris/moon/log_lux and the value scaled between 0 and 1 as
/environment/moonlight.  I'm really not getting the argument for why
this should be in FGData and not on the C++ side?


>> All I did was to take the
>> moon input data equivalent to that for the sun from the 20 year old
>> ephemeris code and pass it into FGLight::updateBodyPos().  So for a
>> solar eclipse, they should match.
>
> This has nothing to do with FG moon vs. real moon position. This is an
> inconsistency within FG itself: The position FG shows the moon in the sky
> is not consistent with the value the zenith angle property takes: For any
> celestial object on the horizon, the zenith angle should be pi/2
> (regardless of whether the object is at the horizon at that time in
> reality or not) - but for the FG Moon at the horizon, the angle can be
> nearly anything (I've seen values between 2.31 and 1.28)

"I've seen..." is a rather vague and non-reproducible bug description.
The sets of -start-date-lat, --lat, --lon, --altitude, and --heading
values would create 100% reproducible sets of data points for
debugging and checking all of the ephemeris and moon luminosity code,
to machine precision.  I.e. I could code this into system tests that
check the simgear ephemeris code, the flightgear environment Ephemeris
subsystem, the flightgear FGLight subsystem, and the position of the
moon disk relative to the local frame.

Regards,

Edward


[1]  https://en.wikipedia.org/wiki/Luminosity_function