Re: Fix for 687337 Zerodivide in cie_cache_mult

Dan,

My old experiense is that exact zero is extremily rare with doubles.
I mean a result of an expression.
Therefore I mechanically apply it whenever performance isn't important -
it minimizes a headache in future.

Igor.

----- Original Message ----- 
From: "Dan Coby" <[email protected]>
To: "Igor V. Melichev" <[email protected]>; "Gs-Code-Review" <[email protected]>
Sent: Tuesday, March 16, 2004 5:20 AM
Subject: RE: [gs-code-review] Fix for 687337 Zerodivide in cie_cache_mult xefitra


> 
> Igor,
> 
> >Instead the comparison with zero,
> >I would recommend any_abs(delta) < 1e-30 .
> 
> I do no think that this is possible with real (i.e. not test)
> color rendering dictionary data.  However since the change
> that you suggest involves little additional CPU time and it is
> possible that I am wrong and this change might save one situation,
> I am willing add this modification.
> 
> 
> Dan
> 
> 
> -----Original Message-----
> From: Igor V. Melichev [mailto:[email protected]]
> Sent: Saturday, March 13, 2004 10:45 AM
> To: '[email protected]'; Gs-Code-Review
> Subject: Re: [gs-code-review] Fix for 687337 Zerodivide in
> cie_cache_mult xefitra
> 
> 
> Dan,
> 
> Instead the comparison with zero,
> I would recommend any_abs(delta) < 1e-30 .
> This threshold is near the precision of double.
> Such thing is significantly more stable, because <1e-30
> frequently happen due to subtraction of close doubles.
> 
> Scale=1 for the degenerate case is fine with me.
> 
> Igor.
> 
> ----- Original Message -----
> From: "Dan Coby" <[email protected]>
> To: "Gs-Code-Review" <[email protected]>
> Sent: Friday, March 12, 2004 10:25 AM
> Subject: [gs-code-review] Fix for 687337 Zerodivide in cie_cache_mult
> 
> 
> >
> > Fix for 687337 Zerodivide in cie_cache_mult.
> >
> > DETAILS:
> >
> > The direct cause of the problem, and also the reason that we
> > have not seen this problem in real world files is that the
> > 050-01.ps test file contains the following color rendering
> > dictionary.
> >
> > /RenderDict2
> > 12 dict begin
> >   /BlackPoint [ 0 0 0 ] def
> >   /ColorRenderingType 1 def
> >   /EncodeABC [ {} {} {} ] def
> >   /EncodeLMN [ {} {} {} ] def
> >   /MatrixABC [ 0 0 0  0 0 0  0 0 0 ] def
> >   /MatrixLMN [ 0 0 0  0 0 0  0 0 0 ] def
> >   /RangeABC [ 0 1  0 1  0 1 ] def
> >   /RangeLMN [ 0 1  0 1  0 1 ] def
> >   /RangePQR [ 0 1  0 1  0 1 ] def
> >   /TransformPQR [ {} {} {} ] def
> >   /WhitePoint [ 1 1 1 ] def
> >
> > Note:  The matrices full of zeroes do not represent real
> > world data.  The zero matrices will result in all output
> > component values being zero no matter what the inputs.
> >
> > Using this data, Ghostscript uses the given ranges and
> > matrices to calculate values for DomainABC and DomainLMN.
> > The results are [0, 0] for each domain.  The domain values
> > are later used to calculate a scaling factors which is
> > also 0.  Calculations, which use one of these values as
> > a divisor, then generate a divide by zero exception.
> >
> > One option to fix this problem would be to check for zero
> > before doing any calculations with the scaling factors.
> > However another and simpler option is to check for the zero
> > factors when they are crated and set the value to a value
> > which will not cause the divide problem.  Note:  Even though
> > we are using a dummy factor (1.0), the zero matrices still
> > result in zero output values from the calculations.
> >
> >
> > Index: src/gscie.c
> > ===================================================================
> > RCS file: /cvs/ghostscript/gs/src/gscie.c,v
> > retrieving revision 1.14
> > diff -u -r1.14 gscie.c
> > --- a/src/gscie.c       4 Dec 2003 12:35:35 -0000       1.14
> > +++ b/src/gscie.c       12 Mar 2004 07:21:25 -0000
> > @@ -821,7 +821,13 @@
> >  #else
> >      pcache->base = A - delta / 2;      /* so lookup will round */
> >  #endif
> > -    pcache->factor = (delta == 0 ? 0 : N / R);
> > +    /*
> > +     * If size of the domain is zero, then use 1.0 as the scaling
> > +     * factor.  This prevents divide by zero errors in later
> calculations.
> > +     * This should only occurs with zero matrices.  It does occur with
> > +     * Genoa test file 050-01.ps.
> > +     */
> > +    pcache->factor = (delta == 0.0 ? 1.0 : N / R);
> >      if_debug4('c', "[c]cache %s 0x%lx base=%g, factor=%g\n",
> >               (const char *)cname, (ulong) pcache,
> >               pcache->base, pcache->factor);
> > _______________________________________________
> > gs-code-review mailing list
> > [email protected]
> > http://www.ghostscript.com/mailman/listinfo/gs-code-review
> >
> 
>