Subject: Re: trying something on for _size_

From: Dave Stampe-Psy+Eng <[email-masked]>
Date: Sun, 20 Oct 91 01:43:00 -0400

I'd like to try something on for _size_ here, having to do with the number
of polys in a low-end VR database versus rendering capacity.

This means that mail and postings are solicited, on opinions, facts,
or whatever.  I'm trying to get a feel for the whole subject, as the
VR graphics project seems to be of interest right now.

First, I'm looking at the followup to Fundamentals of Interactive Computer
Graphics, which is titled Computer Graphics: Principles and Practice by
James Foley et al.  It seems pretty decent, if you avoid the inevitable
IRIS bias.  It has good stuff on transformations and a whole chapter on
various visible-surface determination algorithms.  I think I see a fast way
way to do the scan-line algorith that would take less than 25% of the rendering
time, and would result in each screen pixel being written to once (i.e 10 
frames per second, 320x200x256 colors on a 386/25 IBM PC).  Have to check
further, though.

Now, the main event.  I am proposing a top-down clipping sequence for a
10,000 polygon database (world, that is), that goes as follows:

database:	10,000 polys, arranged as objects with some extra fields
clipping 1:	4,000 polys, by known hidden objects (i.e behind user)
clipping 2:	2,000 polys, by quick plane tests to see if in viewport
backside:	1,000 polys, by removing polys on back of objects.

All this clipping can be done BEFORE converting the polys to viewport
(X,Y,depth) coordinate system.  Most of this is easily done during the
transversal of the database (i.e. movong the world model to the renderer).

1000 polys is a lot to have to draw, but it's within a factor of 3 of what
my proposed renderer *may* be able to do at 10 frames/sec.

SO...  Any comments?  Is the 10,000 poly database unrealisticly big?
Is my clipping estimate overoptimistic?  If so, how many polys SHOULD I
count on getting thru to the renderer?

All suggestions welcome.

- Dave Stampe

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From: James Helman <[email-masked]>
Date: Sun, 20 Oct 91 11:27:27 -0700

1) Since Andy Van Dam was associated with Stellar and PHIGS/PHIGS+
(competitors to SGI & GL), I don't think that Foley+Van
Dam+Feiner+Hughes has an IRIS bias.  A high-end bias, perhaps.

2) Your proposed clipping is a first step, but if you want to maintain
anything resembling a constant frame rate in a complex world, consider
using multiple resolution models and switching between them depending
on their apparent size and scene complexity.

3) Texture mapping is the only way to make scenes with few polys look
halfway decent.  See Sense8's software running on ActionMedia cards in
a PC or on a vanilla SPARCStation GX.  Gullichsen has done a good job,
mostly in software.  Even the IRIS Indigo does texture mapping in
software.  The poly count / texture tradeoff is worth considering.

-jim

Jim Helman					Lab:  (415) 723-9127
Stanford University				FAX:  (415) 591-8165
([email-masked]) 			Home: (415) 593-1233

"The power of the computer is locked behind a door with no knob."
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