From: deering@deering.Eng.Sun.COM (Michael F. Deering)
Newsgroups: sci.virtual-worlds
Subject: Re: PAPER: Galilean Antialiasing for VR, Part 01/04
Date: 6 Nov 92 04:58:02 GMT
Article-I.D.: exodus.lfjuqqINNb62
Organization: Sun Microsystems, Mt. View, Ca.
Originator: hlab@stein.u.washington.edu


My general policy has not been to criticize every posting on
sci.virtual-worlds that I spot some flaw in (that would take too much
time (;-)), but due to the length of Costella's posting (92 pages) I
felt that other net readers would appriciate some observations from one
who indeed did plow through much of the document.

                            ONE LINE OPINON
Some good observations, but far to baroque of algorithm for pratice.

                                SUMMARY
The (92 page) paper describes several ideas.  Starting with our basic
assumptions about visual perception, alternate frame buffer display
technologies for 3D rendered images are derived, similar to motion estimation
algorithm based systems for image decompression (e.g. MPEG 2) (which the
author doesn't seem to be aware of).  After postulating some hardware
details, the author goes on to motivate the need for non-flat plane image
displays for very wide field of view head mounted displays, ending up
with more spherical image generation planes.  And then there's some
systems stuff for tieing all this together.

                                COMMENTS
One of the best descriptions of the distinction between traditional
computer graphics and VR that I like is: traditional computer graphics
mainly cares about emulating a television image, and details of human
perception in the real world are only indirectly addressed.  The first
part of this paper addresses this distinction, and points out how what
we know about perception can allow for different types of display
systems that our traditional ones.  Many of the points made are quite
valid, and not yet understood by many VR praticioners.  Many of the
other points are much more borderline.

The central idea is that one can save money by not always re-rendering
from scratch 3D Z-buffered images every frame, but by augmenting the
frame buffer with local motion vectors, a smart frame buffer can in-between
several sub-frames before the whole rendering system deliveres a complete
new frame.  (A variation of this approach is indeed used by pixar and
others for their motion blurring algorithms, with limitations.)

As a designer of digital circuits for rendering, I have a fairly good feel
for how much circuitry would be required to implement this sort of algorithm.
The problem is that it appears that it would cost *more* to support the smart
pixels that the current brute force rendering circuitry takes!  (In the
authors defence the relative amounts of circuits needed are not obvious.)
Furthermore slightly similar algorithms have been proposed in the past:
in the days when batch raytracing took too long to generate enough images
for motion sequences, people tried to do forms of automatic in-betweening.
The idea was similar: ``gee, all that changes are the object occlusion edges,
we can come close''.  Unfortunatly the edges is just where the human visual
system is looking, even small glitches here turn out to be quite percieveble.
Also the amount and complexity of bookeeping could be quite high.  Finially,
these sort of algorithms are not the sort of thing you have to build custom
hardware to test out, just modify a slow all-software rendering system,
batch compute a few dozen frames, and blit the results at 60Hz (or more)
on a conventional display.  (This would take some effort, but much less
than typing a 90 page paper.)

Rendering hardware and image displays *will* change when we start targeting
wide field of view head mounted displays (I know this because I'm changing
them now).  But some changes can be very expensive.  Most *all* current
cost-effective rendering hardware is based upon the simplifications
afforder by flat image displays, e.g. linear interpolation.  Changing to
non-flat display will in no way be cheep for a long time to come.
The numerical accuracy necessary to support even flat display is quite
high, to piece-wise support higher order curved display surfaces is
higher still.

I've always been a fan on Commonwealth (e.g. non-American english) humor,
this paper has some good passages, even when things run on for three
pages about the tram and problem with the turist not knowing about the
double jerk and thus the problem with the matron's lap and, um where was I?
Oh yes, I was saying that these deversions can tend to make one lose
track of the point being made, though making the document funnier to read.

--
Michael Deering             Email: Michael.Deering@Eng.Sun.COM
Sun Microsystems            Phone: (415) 336-3017
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