NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/comp/graphics/13264

Internet Message Format | 1992-12-30 | 1.7 KB

Path: sparky!uunet!munnari.oz.au!bruce.cs.monash.edu.au!merlin!iwsd01.itwol.bhp.com.au!wewila
From: [e-mail] (Alan Wilkie)
Newsgroups: comp.graphics
Subject: POV Arithmetic fault (on VAX)
Message-ID: <[e-mail]>
Date: 30 Dec 92 13:32:54 +1000
Organization: BHP Information Technology
Lines: 28

I don't know whether this problem is related to the POV problems being
discussed earlier in this group (black lines, etc), but I wonder if it
might be a bug worth reporting.

I compiled POV on a VAX and several of the sample scene descriptions crash
with arithmetic faults.  I haven't had time to figure out exactly why,
but I was able to get around it with a bit of code modification.  In VECT.C
(and elsewhere) there are tests like :

    if (lf == 0.0 || lf * f < 0)

Sometimes, lf * f seems to generate an arithmetic under/over flow (I'm not
sure which).  Anyway, all I did was replace the test with :

    if (lf == 0.0 || (lf > 0.0 && f < 0.0) || (lf < 0.0 && f > 0.0))

This achieves the same result without the multiplication (hence avoiding the
arithmetic fault).  It may also have the side effect of a slight speed up
(since multiplication is so much slower than subtraction) - don't know
for sure :-).
--
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