[FFmpeg-devel] [PATCH] avfilter: avoid testing float == 0

[Michael Niedermayer]

On Thu, May 09, 2013 at 06:32:46PM +0200, Hendrik Leppkes wrote:
> On Thu, May 9, 2013 at 6:21 PM, Clément Bœsch <ubitux at gmail.com> wrote:
> >
> > On Thu, May 09, 2013 at 04:56:39PM +0200, Michael Niedermayer wrote:
> > > This fixes the hypothetical case of rounding errors causing
> > > incorrect values to be used.
> > >
> > > Signed-off-by: Michael Niedermayer <michaelni at gmx.at>
> > > ---
> > >  libavfilter/avfilter.c |    2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/libavfilter/avfilter.c b/libavfilter/avfilter.c
> > > index 3f94dde..6c3cb05 100644
> > > --- a/libavfilter/avfilter.c
> > > +++ b/libavfilter/avfilter.c
> > > @@ -994,7 +994,7 @@ static int ff_filter_frame_framed(AVFilterLink *link, AVFrame *frame)
> > >          dstctx->var_values[VAR_T] = pts == AV_NOPTS_VALUE ? NAN : pts * av_q2d(link->time_base);
> > >          dstctx->var_values[VAR_POS] = pos == -1 ? NAN : pos;
> > >
> > > -        dstctx->is_disabled = !av_expr_eval(dstctx->enable, dstctx->var_values, NULL);
> > > +        dstctx->is_disabled = fabs( av_expr_eval(dstctx->enable, dstctx->var_values, NULL) ) < 0.5;
> >
> > Why a so high value? isn't 0.001 more than enough?
> >
> > nit: remove spaces after ( and before )
> 
> 
> There is even a constant supposed to be used for such comparisons, if
> a float is an "inaccurate zero", FLT_EPSILON or DBL_EPSILON, depending
> on the type.

What i was trying to sugest with the patch was to change the API of
the expression to 0 is false, 1 is true, and the check is more or less
what is closer to either.

Another posibility would be positive vs negative but that might be
less intuitiv to use

The 0 / not 0 distinction is flawed in floats, epsilon is not solving
that. Theres nothing that keeps a rounding error of a complex
expression smaller than epsilon except sheer luck

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

The worst form of inequality is to try to make unequal things equal.
-- Aristotle
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