[FFmpeg-devel] [PATCH] swresample/resample: speed up build_filter for Blackman-Nuttall filter

[Michael Niedermayer]

On Wed, Nov 04, 2015 at 10:08:27PM -0500, Ganesh Ajjanagadde wrote:
> This uses the trigonometric double and triple angle formulae to avoid
> repeated (expensive) evaluation of libc's cos().
> 
> Sample benchmark (x86-64, Haswell, GNU/Linux)
> old:
> 1104466600 decicycles in build_filter(loop 1000),     256 runs,      0 skips
> 1096765286 decicycles in build_filter(loop 1000),     512 runs,      0 skips
> 1070479590 decicycles in build_filter(loop 1000),    1024 runs,      0 skips
> 
> new:
> 588861423 decicycles in build_filter(loop 1000),     256 runs,      0 skips
> 591262754 decicycles in build_filter(loop 1000),     512 runs,      0 skips
> 577355145 decicycles in build_filter(loop 1000),    1024 runs,      0 skips
> 
> This results in small differences with the old expression:
> difference (worst case on [0, 2*M_PI]), argmax 0.008:
> max diff (relative): 0.000000000000157289807188
> blackman_old(0.008): 0.000363951585488813192382
> blackman_new(0.008): 0.000363951585488755946507
> 
> These are judged to be insignificant for the performance gain. PSNR to
> reference file is unchanged up to second decimal point for instance.
> 
> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
> ---
>  libswresample/resample.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/libswresample/resample.c b/libswresample/resample.c
> index a2cbb48..5d32cc2 100644
> --- a/libswresample/resample.c
> +++ b/libswresample/resample.c
> @@ -171,7 +171,7 @@ static int build_filter(ResampleContext *c, void *filter, double factor, int tap
>                  break;}
>              case SWR_FILTER_TYPE_BLACKMAN_NUTTALL:
>                  w = 2.0*x / (factor*tap_count) + M_PI;
> -                y *= 0.3635819 - 0.4891775 * cos(w) + 0.1365995 * cos(2*w) - 0.0106411 * cos(3*w);
> +                y *= 0.3635819 - 0.4891775 * cos(w) + 0.1365995 * (2*cos(w)*cos(w)-1) - 0.0106411 * (4*cos(w)*cos(w)*cos(w) - 3*cos(w));

i would use a temporary variable for cos(w)
either way LGTM

thx

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Avoid a single point of failure, be that a person or equipment.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 181 bytes
Desc: Digital signature
URL: <http://ffmpeg.org/pipermail/ffmpeg-devel/attachments/20151106/181d20f3/attachment.sig>