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From: cdshaw@cs.UAlberta.CA (Chris Shaw)
Newsgroups: sci.virtual-worlds
Subject: Re: VR/Video
Message-ID: <1991Apr14.060259.13515@milton.u.washington.edu>
Date: 14 Apr 1991 07:01:00 GMT
Sender: hlab@milton.u.washington.edu (Human Int. Technology Lab)
Organization: University of Alberta, Edmonton, Canada
Lines: 56
Approved: cyberoid@milton.u.washington.edu


In article <1991Apr13.180518.1243@milton.u.washington.edu> lilj write:
>Chris Shaw writes:
>
>>While video may be a useful adjunct to a virtual reality system,
>>it lacks the fundamental property of arbitrary real-time view
>>position and orientation control.
>> . . . the simulation component doesn't exist because the view cannot 
>>be arbitrarily controlled . . .
>
>The NASA Mars fly-by film and several other projects demonstrate that you 
>can derive an accurate voxel set from a single stereo pair. 

Yes, and CAT scans indicate that you can do the same type of thing for
medicine. The task of generating a 3D model from sensor-based input is
called tomography. However, tomography isn't easy. I asked someone who has
studied tomography, and got an estimate of at least 10 billion pictures
needed to digitize a 20 foot by 20 foot room, given 5 degree intervals
on view direction, and one inch resolution on position, but this was
without making a model.

In any case, the stereo pair statement is a bit dubious. I seem to
remember reading a report on Mars sensing that said that the stereo
pair data was inaccurate, and that other sensor methods were being used
to sense altitude. The second thing to bear in mind is that there are
almost no hidden surfaces in the planetary sensing application, since
the surface is being sensed from above.

Thirdly, the process of turning video input into a movie is one in
which a polygonal mesh is created from the sensor data, and pictures 
of the surface are texture-mapped onto the mesh. "Mars: The Movie"
was made this way. Certainly, video is useful in this application,
but only at the front end. In any case, the bottom line is that
it ain't real time yet.

By the way, a slight clarification of the Aspen Movie Maps project:
There were only 4 views at each point: North, South, East & West (the
town grid, in any case).

>Given sufficient computer speed and power, we should ultimately be able to do 
>this (i.e., be able to extrapolate alternative viewpoints from a single 
>video stereopair) in realtime. 

I don't think so. The Mars movie was created from a subset of a database
of the ENTIRE PLANET. It's not like a space probe flew by one
afternoon and took a few pictures. That move was culled from TERABytes
of data. The flight simulator people have to do the same thing, grab
real data off a disk in time for the pilot to fly over it. But it's
not just raw video images they are grabbing, it's "video" that has
been processed in advance to extract altitude.

>lilj@well
-- 
Chris Shaw     University of Alberta
cdshaw@cs.UAlberta.ca           Now with new, minty Internet flavour!
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