| Realistic Illumination, IDL & OpenGL [message #37828] |
Fri, 30 January 2004 14:06  |
nasalmon
Messages: 33
Registered: August 2003
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Could someone please help me with problems of illumination in IDL.
For Object Graphics to emulate reality, illumination must be
accurately represented. Take for example an outdoor scene, where sky
background emission above and up-welling emission below bathes an
object in different intensities from all directions. Can this be
accurately described in IDL or OpenGL?
In IDL Object Graphics there is the possibility to bring in the
"light" by way of obj_new "IDLgrLight". However, this can only be:
type 0) Ambient; 1) point source; 2) collimated beam; 3) spot light
(apertured). The number of lights is limited to 8. Is this
illumination limited by IDL or the OpenGL on which IDL operates?
Gereral illumination in an outdoor environment has something of an
angular distribution, for an overcast day at least. This might be
represented in an alt-azimuth coordinate system; large amount of
emission at zenith, with some kind of distribution coming down to the
horizon, with some up-welling illumination from the ground. This could
be either simulated by a very large number of point sources or ambient
illumination with some angular distribution on it. Are there plans to
introduce this kind of "light" into IDL in future versions?
many thanks,
Neil
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| Re: Realistic Illumination, IDL & OpenGL [message #37893 is a reply to message #37828] |
Mon, 02 February 2004 11:19  |
Matt Feinstein
Messages: 33
Registered: July 2002
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Member |
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On 30 Jan 2004 14:06:09 -0800, nasalmon@onetel.net.uk (Neil) wrote:
> Could someone please help me with problems of illumination in IDL.
>
> For Object Graphics to emulate reality, illumination must be
> accurately represented. Take for example an outdoor scene, where sky
> background emission above and up-welling emission below bathes an
> object in different intensities from all directions. Can this be
> accurately described in IDL or OpenGL?
>
> In IDL Object Graphics there is the possibility to bring in the
> "light" by way of obj_new "IDLgrLight". However, this can only be:
> type 0) Ambient; 1) point source; 2) collimated beam; 3) spot light
> (apertured). The number of lights is limited to 8. Is this
> illumination limited by IDL or the OpenGL on which IDL operates?
For what it's worth: Each light in the OpenGL lighting model has three
components; specular, diffuse and ambient, . Specular obeys a
parameterized variant of the 'angle of reflection equals angle of
incidence' model, diffuse obeys a lambertian model, and ambient is,
well, ambient. The lights can be in any position, including infinity,
and they may have any angular width. The illumination from any given
source is a sum of these three models. The OpenGL spec allows a
lighting model of up to eight independent light sources. There is also
a 'true' ambient illumination that isn't tied to a light source.
This is combined with a general coloring model-- the color of every
facet is a combination of four components; specular, diffuse, ambient,
and emissive. The observed color of any component due to a given
source is the product of the color of that component of the source and
the color of that component of the material.
So, there are -lots- of parameters. In fact, considering that every
facet may have a different color and a different normal vector (the
normal vector is used in the specular lighting model), a rendered
scene could easily have many thousands of parameters. The difficulty
in modeling lighting is not a lack of parameters.
And, I've neglected to mention shadowing, which is a whole other
problem-- you have to figure out a way to compute shadows
(non-trivial) and -then- combine the shadows with your existing
lighting model.
Matt Feinstein
--
There is no virtue in believing something that can be proved to be true.
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