From: mark@cs.UAlberta.CA (Mark Green)
Subject: Re: VR/Video
Date: Sat, 13 Apr 1991 22:43:46 GMT
Organization: University of Alberta, Edmonton, Canada


In article <1991Apr13.180518.1243@milton.u.washington.edu>, decwrl!well.sf.ca.us
!well!lilj@uunet.UU.NET (Joshua Neil Rubin) writes:
> 
> 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 . . .
> 
>            *           *          * 
> 
> >Chris Shaw     University of Alberta
> >cdshaw@cs.UAlberta.ca           Now with new, minty Internet flavour!
> >CatchPhrase: Bogus as HELL !
> 
> 
> The NASA Mars fly-by film and several other projects demonstrate that you 
> can derive an accurate voxel set from a single stereo pair.  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.  Of course, the hidden surfaces must be 
> interpolated.  But hey.
> 
> 
> lilj@well
> 

What??? The NASA Mars fly-by film and similar works are computer
generated graphics produced from a collection of different data sources (ie.
there are no stereo pairs involved).  The basic procedure is to
determine a height field for the planet.  This can be done by different
forms of range finding or multiple views of the planet.  Lew Hitchner
of NASA Ames has mentioned this type of data for Mars in this news
group and we have used the same data for an interactive fly-by of Mars.
Once the height field is available a polgonal mesh can be constructed
that represents the planet's surface.  Now you have a basic geometrical
model of the planet.  To get the surface detail, pictures of the planet
(at the appropriate locations) are texture mapped onto the polygons
when they are displayed.  Unless you are using a high-end Silicon
Graphics machine this is not a real-time operation.  Note that this
is a purely geometrical model, there is no video, stereo-pairs,
voxels or anything like it involved.  From a single stereo pair there
is no way of deducing the geometry of an arbitrary object so it can
be displayed from any view point.  This only requires a little thought,
after all how can you see the back of the object from only one view??

- Mark Green
  University of Alberta  mark@cs.ualberta.ca