On Sat, Nov 14, 2015 at 4:48 PM, Paul B Mahol <one...@gmail.com> wrote: > On 11/14/15, Ganesh Ajjanagadde <gajjanaga...@gmail.com> 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 for simplicity does the naive >> direct computation outlined above. Improvements can be made separately. >> >> Signed-off-by: Ganesh Ajjanagadde <gajjanaga...@gmail.com> >> --- >> configure | 2 ++ >> libavutil/libm.h | 8 ++++++++ >> 2 files changed, 10 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..ab5df1b 100644 >> --- a/libavutil/libm.h >> +++ b/libavutil/libm.h >> @@ -82,6 +82,14 @@ static av_always_inline float cbrtf(float x) >> #define exp2f(x) ((float)exp2(x)) >> #endif /* HAVE_EXP2F */ >> >> +#if !HAVE_HYPOT >> +#undef hypot >> +static av_always_inline av_const double hypot(double x, double y) >> +{ >> + return sqrt(x*x + y*y); >> +} >> +#endif /* HAVE_HYPOT */ >> + >> #if !HAVE_ISINF >> static av_always_inline av_const int isinf(float x) >> { >> -- >> 2.6.2 >> >> _______________________________________________ >> ffmpeg-devel mailing list >> ffmpeg-devel@ffmpeg.org >> http://ffmpeg.org/mailman/listinfo/ffmpeg-devel >> > > nonsense. either do the right thing or don't touch the code.
The "right thing" is also a very ambiguous term here. For instance, if one defines the "right thing" by a correctly rounded math library that meets the ISO C spec, essentially all libm's fail on certain inputs and functions. The best of the lmainstream lot in this respect is the GNU libm. Even the GNU libm sometimes fails to achieve the last bits of precision for some functions, do e.g man j0 and look at the "bugs" section. If one defines the "right thing" as a reasonable fallback, there are different gradations depending on how much accuracy and readability one wants. For instance, the backup log10 is definitely incorrect according to the "strict" criteria, recall the whole log episode. Nevertheless, I and I believe all other devs here call it a reasonable fallback. I can do an implementation that goes a bit beyond the naive sqrt. Ultimately, somewhere the line needs to be drawn. _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
