On Sun, Nov 15, 2015 at 11:01:58AM -0500, Ganesh Ajjanagadde wrote: > On Sun, Nov 15, 2015 at 10:56 AM, Michael Niedermayer > <mich...@niedermayer.cc> wrote: > > On Sun, Nov 15, 2015 at 10:03:37AM -0500, Ganesh Ajjanagadde wrote: > >> It is known that the naive sqrt(x*x + y*y) approach for computing the > >> hypotenuse suffers from overflow and accuracy issues, see e.g > >> http://www.johndcook.com/blog/2010/06/02/whats-so-hard-about-finding-a-hypotenuse/. > >> This adds hypot support to FFmpeg, a C99 function. > >> > >> On platforms without hypot, this patch does a reaonable workaround, that > >> although not as accurate as GNU libm, is readable and does not suffer > >> from the overflow issue. Improvements can be made separately. > >> > >> Signed-off-by: Ganesh Ajjanagadde <gajjanaga...@gmail.com> > >> --- > >> configure | 2 ++ > >> libavutil/libm.h | 23 +++++++++++++++++++++++ > >> 2 files changed, 25 insertions(+) > >> > >> diff --git a/configure b/configure > >> index d518b21..45df724 100755 > >> --- a/configure > >> +++ b/configure > >> @@ -1774,6 +1774,7 @@ MATH_FUNCS=" > >> exp2 > >> exp2f > >> expf > >> + hypot > >> isinf > >> isnan > >> ldexpf > >> @@ -5309,6 +5310,7 @@ disabled crystalhd || check_lib > >> libcrystalhd/libcrystalhd_if.h DtsCrystalHDVersi > >> > >> atan2f_args=2 > >> copysign_args=2 > >> +hypot_args=2 > >> ldexpf_args=2 > >> powf_args=2 > >> > >> diff --git a/libavutil/libm.h b/libavutil/libm.h > >> index 6c17b28..f7a2b41 100644 > >> --- a/libavutil/libm.h > >> +++ b/libavutil/libm.h > >> @@ -102,6 +102,29 @@ static av_always_inline av_const int isnan(float x) > >> } > >> #endif /* HAVE_ISNAN */ > >> > >> +#if !HAVE_HYPOT > >> +#undef hypot > >> +static inline av_const double hypot(double x, double y) > >> +{ > >> + double ret, temp; > >> + x = fabs(x); > >> + y = fabs(y); > >> + > >> + if (isinf(x) || isinf(y)) > >> + return av_int2double(0x7ff0000000000000); > > > > if either is NaN the result should be NaN i think > > return x+y > > might achive this > > No, quoting the man page/standard: > If x or y is an infinity, positive infinity is returned. > > If x or y is a NaN, and the other argument is not an infinity, > a NaN is returned.
indeed, the spec says thats how it should be. this is not what i expected though and renders the function problematic in practice IMHO. For example a big use of NaN is to detect errors. One does a big complicated computation and if at the end the result is NaN or +-Inf then one knows there was something wrong in it. if NaN is infective then any computation that returns it can reliably be detected. These exceptions in C like hypot() break this. 1/hypot(x,y) should be NaN if either x or y was NaN also mathematically its wrong to ignore a NaN argument consider hypot(sqrt(-x), sqrt(x)) for x->infinite of course theres nothing we can or should do about hypot() its defined in C as it is defined but its something one should be aware of if one expects that NaNs can be used as a reliable means to detect NaNs from intermediate steps of a complicated calculation [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB Why not whip the teacher when the pupil misbehaves? -- Diogenes of Sinope
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