Re: [Flightgear-devel] why jittering,use external fdm

[Stuart B. <stu...@ya...>]

--- tangyong wrote:
> Hi,everybody.I use these command lines to record a flight,then I replay
> it.
>  
>  Recording: fgfs --generic=file,out,20,flight.out,playback
>  Playback:  fgfs --generic=file,in,20,flight.out,playback --fdm=external
> 
> but when I replay the flight record,I find the aircraft jittering,the
> flight is not smooth.why?And in the protocol "playback.xml",it does
> record the aircraft's veolocities and positions etc.Since the position
> is only record at intervals ,does the FG use the velocity value to
> display aircraft in the meantime, between records.  If FG does this,the
> flight and aircraft shuld not jitter.Can anybody tell me the reson?

Hi Tanyong,

As I recall (though I may be wrong), the playback.xml file doesn't include
acceleration information, so the interpolation between data points will
just be based on the velocities recorded, while the next data point will
have more accurate position information that will "reset" the aircraft
position. It is also the case that the FDM will cause subtle velocity and
position effects that cannot simply be reduced to a number recorded every
.05 of a second.

This is most obvious on the ground when you are accelerating for take-off
or decelerating for landing. You can reduce the effect of this by
increasing the rate at which you take data points. 

Another reason for this occurring is when you are unable to record the
data at the requested rate. For example, if you ask to record at 20Hz, but
in fact your machine is only recording at 19Hz, when you come to replay
your data at 20Hz there will be a discrepency between the position
recorded and that resulting from the velocity of the previous frame.

I don't know if we have a high-resolution timer in the properties tree,
but it would be useful to record that in the .xml file to check the actual
data recording rate.

When I've used the playback system, I've tended to record at 20Hz, then
play back at 40Hz. This seems to look more impressive from the outside. ;)

Hope this is of some help.

-Stuart


	
	
		
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