On Sun, Nov 15, 2015 at 11:59 AM, Ganesh Ajjanagadde <gajja...@mit.edu> wrote: > On Sun, Nov 15, 2015 at 11:34 AM, Michael Niedermayer > <mich...@niedermayer.cc> wrote: >> 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 > > Yes, I was extremely surprised myself, and you are right that it > defeats the NaN's purpose. Some day I may dig up the committee's > rationale for this, am curious. I doubt it was oversight, since they > are usually very careful about such things, and the defined behavior > is very specific suggesting deep thought. > > Anyway, I do not take this as a formal ack yet. Hopefully we don't run > into Carl's weird debian thing that forced disabling fmax, fmin > emulation.
Anyone willing to do a Windows build test to make sure that the compat hack works? I want to avoid build failures. Thanks. > >> >> [...] >> -- >> Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB >> >> Why not whip the teacher when the pupil misbehaves? -- Diogenes of Sinope >> >> _______________________________________________ >> ffmpeg-devel mailing list >> ffmpeg-devel@ffmpeg.org >> http://ffmpeg.org/mailman/listinfo/ffmpeg-devel >> _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
