Russ Moulton, Jr (russ.jrm++at++eosoft.com)
Wed, 28 Oct 1998 13:12:42 -0500
Balaji,
Assuming you want an OpenGL-compatible, time-dynamic database,
you are generally stuck with a set of properties (ambient,
diffuse, specular, emissive) for modeling the reflected and
self-emitted signature components in a particular spectral band
(like visible, near, mid or longwave-IR).
OpenGL (and thus Performer) handles dynamic reflected signature
phenomenology very well, like visible-band light sources
interacting with visible-band reflective surfaces for example. Indeed,
the OpenGL lighting model (often implemented in hardware) computes
the total reflected component "on-the-fly". You assign the appropriate
position/in-band intensity light source, and assign the
appropriate in-band reflective properties (ambient, diffuse, specular)
on the vertices of your database and it does the rest.
Unfortunately, you are "on your own" for the emissive component -- you
have to assign that yourself.
For the visible (e.g. human eyeball, TV cameras) and near-IR band
(e.g. Night Vision Goggles), you can ignore the neglible emitted
component
for terrestrial and man-made scene objects (provided
they don't get too hot, like bulb-filaments). You must of course,
appropriately assign the time-dependent source intensities (sun, moon,
skyshine, earthshine, etc) and reflectivities for the band you want.
But in the mid-IR and longwave-IR, you must have a means to compute
the dominant emissive component. This component is driven by the
object's temperature and in-band emissivity. In general, to get a
dynamic
temperature, you need to solve the heat diffusion equation,
which takes into account the thermophysical and surface optical
properties of the object, as well as the incident irradiance.
(thermal conductivity, specific heat, density, coefficient of
convection, solar absorptivity, longwave emissivity, solar insolence,
longwave irradiance, internal heat sources/sinks, and evaporative/
condensating heat transfer).
There are a myriad of methods and shortcuts to address the
emitted component, depending on your application's real needs.
One technique, for example, adapts the lighting model and unique
OpenGL properties you would assign to produce dynamic (pre-computed)
IR signatures.
Hope this is helpful in understanding the issues involved.
-- Russ Moulton, Jr. JRM Enterprises, Inc. Voice:540-785-4585 Fax:540-785-4586 russ.jrm++at++eosoft.com
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