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 <gajjanaga...@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));
break;
case SWR_FILTER_TYPE_KAISER:
w = 2.0*x / (factor*tap_count*M_PI);
--
2.6.2
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