Hi Curt,
Just as you and Stuart said,the main reason of jittering is the inconsistent
frame rate.But I find that if several FG clients connect to a local server
''fgms'',each client can see each other correctly and the flight is smooth,no
jittering.Is there no fps varying when we use multiplayer function just like
this:
Player1
--multiplay=out,10,serveraddress,5002 --multiplay=in,10,myaddress,5002
--callsign=player1
Player2:
--multiplay=out,10,serveraddress,5002 --multiplay=in,10,myaddress,5002
--callsign=player2
Then I modify the server to let it don't transmit Player2's velocity and
acceleration data to Player1.I find that even I use a very low in/out
frequency(1hz) in Player2 while the Player1's in/out =10hz,the Player2's
flight still looks very smooth in Player1,no jittering.I don't see much
changing than I use Player2's in/out=10hz,and transmit the velocity and
acceleration data.why?what the difference bewteen them?
Best Regards,
-Tangyong
I've never tried to play back data recorded with the "generic" protocol so I
don't know all the issues, but Stuart makes some good points. Timing is very
critical. Even though you specify an output rate, FlightGear isn't doing this
in a separate independent thread (not that a thread would directly help the
situation anyway.) FlightGear can only output data once at the end of drawing
each frame of graphics.
So as Stuart points out, if you are running at a somewhat inconsistent frame
rate (let's say varying between 35-45 fps) and you can only output at the end
of each frame, then the actual data intervals in your data file will jitter
around substantially. Include sim time in your output and look at your "dt"
between each record.
I'm certain that the "generic" protocol makes no attempt to interpolate the
incoming data in time. It just just plucks out the next record every 20 hz (or
as close to that as it can get) and updates the view.
So hear again if your playback frame rates are varying and FlightGear waits
until at least 50ms has elapsed before fetching the next data packet, then any
jitter at the recording stage is only going to be magnified at the playback
stage.
When dealing with external animation sources or playback, you *really* need to
worry about locking FlightGear into a consistent frame rate (either with sync
to vblank ... which is always a good idea ... and/or with the built in frame
rate throttler code.)
Note that all these mechanism can limit frame rates, but nothing can make
FlightGear go faster (short of simplifying the graphics load or improving your
hardware.)
Take care to ensure you are running with exact, consistent frame rates, and
your life will become a lot happier!
Curt.
--
Curtis Olson - University of Minnesota - FlightGear Project
http://baron.flightgear.org/~curt/ http://www.humanfirst.umn.edu/
http://www.flightgear.org
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