Since I enjoy seeing sunset twice, I performed an experiment in
FlightGear: watching the sun set at the end of the runway, then taking
off and climbing to bring the sun back above the horizon to see it set
again.
The problem is that the sun came above the horizon too quickly, and
stayed above the horizon far too long from high altitude (say, thirty
thousand feet). It turns out since only visible scenery is rendered,
the scenery behind the visibility fog is simply not there to get in
the way of the sun; so until the sun passes under the (possibly quite
small) patch of visible ground below you, it will remain "up" well
into the night.
I am interested in incorporating a better horizon into FlightGear but
am not sure how to approach the problem. I imagine a dynamic flat
surface beneath the landscape that expands outwards to provide an
intersection between one's line of sight and the curve of the earth:
_
<_/ (aircraft)
/ \
/ \
/ \ (line of sight to horizon)
/ === \
/ ... ... \
... ...
----------------- (a simple flat surface
. . thrown in as a horizon)
In this rough diagram, the equals signs represent the surface that is
actually being rendered given the aircraft's height and the prevailing
fvisibility; but as we see, it covers only part of what would be the
visible earth beneath the plane. So I am suggesting the addition of
the dashed line, which forms the rough disc where the pilot's line of
sight would intersect the globe were we rendering it. This surface
would need no texture, and indeed would not be itself visible, but
would properly obscure the sun so that sunset for 747 pilots at 30,000
feet occurs just when it should.
I am not proposing that the horizon disk account for any surface
features - no need at this point to determine whether the Himalayas
lie in front of the distant sunset and require the horizon-disc to be
extended to account for them.
But others might object to modeling visibility in such a way as to
make surface features disappear from above; should an F-16 pilot at
50,000 feet really find himself looking down on a tiny patch of ground
surrounded by a white featureless soup? It is possible that instead
of my horizon-disc idea, we simply need a low-resolution version of
our landscape that can be extended all the way to the earth's limb as
viewed from a few miles up, providing F-16 pilots and other high
flyers with a more realistic view of the earth from on high.
--
Brandon Craig Rhodes [EMAIL PROTECTED] http://rhodesmill.org/brandon
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