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From: reuel@khaki.asd.sgi.com (Reuel Nash)
Newsgroups: comp.sys.sgi
Subject: Re: Interrupting long draw cycles
Summary: Don't interrupt gl!
Message-ID: <90500@sgi.sgi.com>
Date: 12 Mar 91 18:18:32 GMT
References: <9103112256.AA08202@karron.med.nyu.edu>
Sender: guest@sgi.sgi.com
Organization: Silicon Graphics, Inc., Mountain View, CA
Lines: 33

In article <9103112256.AA08202@karron.med.nyu.edu>, Dan Karron@UCBVAX.BERKELEY.EDU writes:
> I want
> to interrupt the drawing program with a signal, say SIGUSR1. The signal
> handler will then do a longjmp back to the main drawing loop with
> new eye position and viewing values and start drawing.
> 
> This will entail the gl calls being interrupted and aborted at any step.
> 
	Don't do it this way. Commands and data written by the gl must
	be complete or you can take the graphics system down.

> Is there a gracefull way to interrupt the gl subsystem to regain
> control ? Can I stop a long draw cycle and start another without
> dammage to the gl pipeline ?
> 
	The appropriate thing to do is to have your signal handler 
	set a flag that is checked syncronously by your main drawing
	loop. This can (and should) be at a higher level than every line
	or polygon. If data is organized as objects, check after drawing
	each object. If data is not organized as objects, check after 
	drawing every N objects, where N is set by your desired response
	time and drawing rate. The flag should be declared 'volatile'
	or the optimizer will think it can't change inside the loop
	and may optimize it away.
> 
> Dan.



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