Add variable alpha and size of half window for Kaiser-Bessel Derived window

generation. Hard code Bessel I0 approximation iterations to 50.

See thread for discussion:
[FFmpeg-devel] [PATCH] Move Kaiser-Bessel Derived window to mdct.c
Started on the 2008/01/10

Originally committed as revision 11520 to svn://svn.ffmpeg.org/ffmpeg/trunk

1 files changed, 8 insertions, 7 deletions

diff --git a/libavcodec/mdct.c b/libavcodec/mdct.c
index c7008df4f1..e809fcdcbc 100644
--- a/libavcodec/mdct.c
+++ b/libavcodec/mdct.c
@@ -26,24 +26,25 @@
  */
 
 // Generate a Kaiser-Bessel Derived Window.
-void ff_kbd_window_init(float *window)
+#define BESSEL_I0_ITER 50 // default: 50 iterations of Bessel I0 approximation
+void ff_kbd_window_init(float *window, float alpha, int n)
 {
    int i, j;
    double sum = 0.0, bessel, tmp;
-   double local_window[256];
-   double alpha2 = (5.0 * M_PI / 256.0) * (5.0 * M_PI / 256.0);
+   double local_window[n];
+   double alpha2 = (alpha * M_PI / n) * (alpha * M_PI / n);
 
-   for (i = 0; i < 256; i++) {
-       tmp = i * (256 - i) * alpha2;
+   for (i = 0; i < n; i++) {
+       tmp = i * (n - i) * alpha2;
        bessel = 1.0;
-       for (j = 100; j > 0; j--) /* default to 100 iterations */
+       for (j = BESSEL_I0_ITER; j > 0; j--)
            bessel = bessel * tmp / (j * j) + 1;
        sum += bessel;
        local_window[i] = sum;
    }
 
    sum++;
-   for (i = 0; i < 256; i++)
+   for (i = 0; i < n; i++)
        window[i] = sqrt(local_window[i] / sum);
 }