Re: Fix for 687337 Zerodivide in cie_cache_mult

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);
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