[comp.sys.dec] DECstation 5000 graphics slowness

cullip@pooh.cs.unc.edu (Timothy Cullip) (10/05/90)

Can someone explain to me why a 2D graphics accelerated DEC5000 (this
is the PX model) doesn't handle animated sequences well?

Specifically, I have a display program that creates a sequence of pixmaps
(one for each frame of a precomputed animation).  Once they are all
created I do a simple loop which copies each successive pixmap into
the onscreen window.

On a DEC 3100 this works fine for large sequences (for example 50 frames
of 256x256 pixmaps).  I can easily get over 10 frames/second.

On a DEC 5000 PX up to about 10 frames can go blazingly fast (looks
like about 30 frames/second or more), but anything over a certain
total size and number (usually in the range of about 1Mbyte of total
pixmap memory) and suddenly things slow to a craw.  For example the
first 10 frames may go at the 30 frame/sec rate, but then the others
go at about 2 frames/sec.  When it loops back to those first 10, they
go fast again, and the others still go slow.  This is rather annoying
to see part of the animation go lightning fast and the rest of it
to crawl along slowing.

This is not a disk swapping problem! The program happily gets everything
into memory.

My best uneducated guess is that the PX model (and most likely all the
more expensive versions above it) separate frame buffer memory (of which
it has 2 screens worth) from main memory and the CPU.  If you have only
a few windows or pixmaps, they can be held in the off screen frame buffer
memory and copied into an onscreen window rapidly.  If you have more
than can be held in the frame buffer, then the extras reside in the
CPU's main memory space and probably have to be suffled through the
accelerators (misnomer in this situation) hardware to get to the
frame buffer.  

Anyways, I'm looking for people that have had similar experiences that
might be able to shed some light on the situation.  Especially if you
know if there is a fix to this problem.  I'd hate to have to drop back
to a CX model just because a PX wasn't designed to do large numbers of
pixmap to window copies.

   Tim Cullip
   cullip@cs.unc.edu


   Tim Cullip
   cullip@cs.unc.edu