Re: Question on I/O devices

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Jim Helman (jimh++at++surreal)
Mon, 20 Nov 95 13:22:06 -0800

> Two ways that I know of:
>
> 1. Use XtAppAddInput to receive X events representing the motion sensor - see
> man page/book on XtAppAddInput.
>
> 2. Use shared memory between pf process and a process which will collect
> data from the motion sensor and write into the shared mem.

Of these two, the shared memory approach is probably preferable in
terms of throughput and consistent latencies. One has a lot more
real-time control over processing when X isn't involved.

> I have been working with the Polhemus Fastrak in my Performer
> application. I set up the Fastrak so that it outputs a continuous
> stream of binary data and I sproc a lightweight process from the APP
> to process this stream and place that data in buffers that can be
> accessed by the APP. If this sounds like something you want to do, it
> can be no different from writing a device driver for a non-Performer
> application.

This is a good approach. Ideally, you'd like to sync the Polhemus to
video so that its frame rate doesn't beat with the rendering frame rate
and cause variations in latency.

rgds,

-jim helman

jimh++at++surreal.asd.sgi.com
415/933-1151

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