Brian Corrie (bcorrie++at++cs.anu.edu.au)
Tue, 28 Jan 1997 11:12:34 +1100
I have what may seem like a silly question, but I am a bit of a newbie to high
end SGI graphics systems and this seems like the place to clarify some things.
Sorry about the length of this email, but I thought it best to give as much
info as possible to avoid confusion. We are looking at purchasing an
Immersadesk VR system and an Onyx2 Reality Deskside system to drive it. The
Immersadesk is a drafting sized immersive VR system with stereo viewing and
both head and hand tracking. Up until recently, these devices were typically
driven by a Onyx RE or Onyx IR system, usually with 1 pipe. Given the
previous statement, it appears obvious that the Onyx2 Reality Deskside system
would be more than capable of driving the Immersadesk.
The problem (and my question) arises from the fact that the Onyx2 Reality is a
scaled down version of the Onyx2 IR and I am not familiar enough with high end
SGI hardware (nor familiar enough with the VR requirements imposed on these
machines) to determine whether or not this 'scaling down' is going to impact
drastically on opearting the Immersadesk... I am unfortunately in the
position that I have to advise the powers that be on whether the machine will
be capable of driving the Immeradesk successfully, so I need to find these
things out...
The main potential problem is the configuartion of the frame buffer. The
Onyx2 Reality has an RM8 with 40 MB of frame buffer as standard, which is half
the frame buffer of what the IR RM7s have (80 MB). To drive the Immersadesk,
we require 1280x1024 stereo, double buffered rendering. With 40 MB of frame
buffer at 1280x1024 pixels, this gives us 256 bits/pixel. My problem is that
I do not know enough about the frame buffer modes of the IR pipes to be sure
that we are not going to lose some capabability that we require due to having
"only" 256 bits/pixel. If we are using 12bit RGBA pixels with stereo double
buffering my calculations tell me that we need 4 x 48 bits (for left/front,
right/front, left/back, and right/back images) and we will say 23 bits for
depth. This leaves us with just over 40 bits/pxel remaining, which seems like
more than enough...
So my question is, am I missing anything here???? Our SGI reps here in
Canberra have expressed some concern that this may not be enough.
Specifically, they have stated that there will be no anti-aliasing available
(only 1 sample per pixel). I have the Onyx2 Technical Report and it mentions
the Accumulation Buffer, stencils, overlays, tags, and things like LA32, LA16,
and MS Color (see page 20 of the Onyx2 Tech Report) without really defining
what they are and how they are used. What I need to know in what
circumstances are they used and how many of my valuable bits/pixel are they
going to chew up? I am also interested in finding out if any of these are
essential in driving a system like the Immersadesk and if so, will my 40 MB of
frame buffer be enough... (Gee, I am really starting to sound possesive about
that good ol' Onyx2 Reality box, I wonder if they will let me have it in my
office 8-)
I am hoping that someone on this list is both an Onyx2 and an Immersadesk
user, as that is really what I need to answer my questions. Of course any
input from anyone would be HUGELY appreciated!!!!
Thanks in advance,
Brian Corrie.
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