Methods for generating shadows in vissims

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Evan John Gridley (ejg1++at++cornell.edu)
Wed, 25 Jan 1995 12:08:23 -0500 (EST)

   Greetings to all. We're trying to add shadows to our vissim of
(generally) aircraft departures/arrivals at various airports.
Unfortunately, most shadowing methods seem to be written in a general
form, for the app/system developer, and require treatment of the draw
process at the per-pixel level. Performer, while it provides many
advantages, seems to hide from us much of the process we want to reengineer
to compute shadows; Performer pre- and post-node callbacks only provide
us with per-node control over the rendering process, making life difficult.

   An approach we are considering, as we are only concerned with
shadowing the actual aircraft for depth-cueing information and buildings
are pretty static and serve as a background more than anything else, is to
find the aircraft's location on the ground, projected from the light
source (infinite, in our case, but it changes over time), and placing some
sort of semi-transparent (textured) polygon there. We'd prefer this
method since it prevents us from having to render the scene twice or do
some such calculation, and we are not interested in having 100% accurate
shadows. However, we weren't exactly sure how the logistics would work
out on keeping the polygon decaled to the correct ground polygons, etc.
and if it was even feasible.

   Well, apologies for not being brief with this, but I haven't seen
anyone discussing this topic on the mailing list since I joined (3 months
ago) and was wondering if anyone had tried it or had any good ideas.
Also, any discussion of relative performance issues of any (more classic,
maybe) shadow methods on a RE would be interesting and useful.

   Thanks a lot.

-Evan (Evan Gridley, ejg1++at++cornell.edu, egridley++at++
                                      ++at++maestro.mitre.org)
------------------------------------------------------------------------
                                        The MITRE Corporation, Mclean VA

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