the elusive constant frame rate

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Geoff Levner (g.levner++at++acsys.it)
Thu, 01 Jul 1999 13:24:40 +0200

I have a MultiGen scene which runs in perfly at 30-60 Hz in
PFPHASE_FREE_RUN, and I would like for it to run at a constant 30 Hz.
Sounds easy. But if I set the phase to PFPHASE_LOCK (or FLOAT, or
LIMIT) and set the target frame rate to 30, the actual frame rate varies
between 20 and 30 Hz, or sometimes even 15. And if I set the target
frame rate to 15, the actual frame rate can go as low as 7.5!

Can anyone tell me what is happening here?

On further investigation, I noticed that the actual frame rate is
slowest when the viewing frustum is the most uncluttered. That is, with
a target of 15 Hz, I get 15 Hz somewhere in the middle of the scene, but
7.5 Hz when looking up toward the sky (which is the default pfEarthSky,
no textures). In fact, anything I do to increase drawing time (such as
turning off culling) actually increases the frame rate (sic!). If you
look at the stats when the frame rate is lagging, everyone is waiting on
the draw process, which consists of a tiny solid line followed by a
looooong dotted line.

Even without my scene, I found a similar phenomenon in perfly. If I run
perfly with an empty scene and lock the frame rate to 15 Hz --

    perfly -P 1 -r 15 -E skyclear

-- and then turn on the statistics, the actual frame rate is 7.5....

All tests were done on an Onyx2 IR2 with 4 CPUs running IRIX64 6.5 and
Performer v2.2.2.

HELP!

-- 
Geoff Levner -- g.levner++at++acsys.it
ACS Studio, via Aurelia 58, 00165 Roma, Italy
tel. +39-063936331, fax +39-0639363317

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