Re: Methods for generating shadows in vissims
Sean Halliday (halliday++at++BanffCentre.AB.CA)
Wed, 25 Jan 1995 12:24:20 -0700 (MST)
>
>
> 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
>
>
Hmm, I haven't tried this, but here's an idea.
1) Make the shadow polygon into a mesh (the finer it is the better it will look
going over uneven terrain, but will be more expensive to calculate.)
2) Assuming Y+ is towards the Sky, calculate the Y cooridinate of each vertex
of the shadow polygon by intersection tests between the ground and aircraft,
along the line between light and aircraft.
3) Draw the shadow polygon. You may have to turn of zbuffering because the
shadow will coincide with the ground.
These are just some thoughts. I sounds similar to the Projected Texture
Demo. (Does anyone know if it was done this way?)
--
Sean Halliday
Computer Graphics Software Developer
New Media Research, Banff Centre for the Arts.
halliday++at++BanffCentre.AB.CA
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