Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!hellgate.utah.edu!cs.utah.edu!thomson
From: thomson@cs.utah.edu (Rich Thomson)
Newsgroups: comp.graphics
Subject: Re: immediate mode graphics versus display-list graphics
Message-ID: <1990Oct1.094440.15213@hellgate.utah.edu>
Date: 1 Oct 90 15:44:40 GMT
References: <1990Sep30.060606.213@morrow.stanford.edu>
Lines: 70

In article <1990Sep30.060606.213@morrow.stanford.edu> rick@pangea.Stanford.EDU
    (Rick Ottolini) writes:
>[...] one popular 3-D workstation vendor was knocking display-list based
>graphics languages such as Dore or PHIGS as being compartively inflexible
>and slow.

Well, I can guess that your ``popular 3-D workstation vendor'' was SGI
because SGI systems are immediate-mode based; naturally they are going
to knock display-list graphics.  The fact is that there are applications
in which display-list graphics have a natural fit and there are
applications in which immediate-mode graphics have a natural fit.  I
think that the world of graphics is going to see both of these styles
of graphics for quite a while.

>This seemed to be the case for equivalent demos under the two systems.

Again, it all depends on the application/demo you are looking at.
What you find is that people who use immediate-mode systems (I'm
speaking from the experience of looking at the inside of real-world
applications) like them because they can have their own structured
(i.e. display-list) graphics kernel that suits them.

This brings up some interesting points in the case of PEX (PHIGS+
Extensions to X), because the PEX protocol provides a way for
immediate-mode style graphics.  In a networking environment, it can be
a real advantage to have a structured graphics type approach because
it isn't necessary for the application to send the entire graphics
structure over the wire each time a refresh of the screen is needed.

On the other hand, as a previous poster pointed out, it can be painful
to deal with all the structure store of PHIGS when the entire geometry
changes every frame.  When this is the case the structure store isn't
buying you anything.  Interestingly enough, no one has decided on an
"immediate-mode" language binding to the PEX protocol, yet the
capability is there for those die-hards willing to talk directly to
the X server over the socket.

The fact of the matter is that there is no clear winner of either one:
structured graphics (i.e. PHIGS, PEX) can make the job of writing a
complex 3-D graphics program easier because it can do the work of
managing all the attribute inheritance and structure management for
you.  On the other hand, with immediate-mode graphics you can control
all of that yourself at the expense of writing your own "structure
walker" (the piece of code that traverses the hierarchy and outputs
individual graphics primitives).

Again, my experience is that applications that want to see an immediate-mode
interface already have some kind of structure-walker built into the
application and are really doing structured graphics anyway.  I find
it interesting that PEX provides a way of doing both within the same
framework; in that sense, I find it more flexible because I could
choose which one I want to use.  The sample implementation from Sun
will eventually support what is called "client-side structure traversal",
wherein the application will manage and traverse structure storage.
This will result in *alot* of communication to the server for
applications with complicated images.  There is alot of talk about
creating a language binding for the immediate-mode interface to PEX
(built around the concept of a "renderer"); whether or not this
language binding will look like PHIGS or not is still up to debate.
Considering that the PEX protocol itself is based on PHIGS, my guess
is that the immediate-mode interface (when and if it becomes
available) will have lots of PHIGSisms except when it comes to
managing hierarchy and display control.

						-- Rich

Disclaimer: I work for Evans & Sutherland, which makes a PEX box, but
    I don't represent them in any official manner.
Rich Thomson	thomson@cs.utah.edu  {bellcore,hplabs,uunet}!utah-cs!thomson
``If everybody is thinking the same thing, is anybody thinking?'' --Bob Johnson