libavcodec/kbdwin.c

   1 /*
   2  * This file is part of Libav.
   3  *
   4  * Libav is free software; you can redistribute it and/or
   5  * modify it under the terms of the GNU Lesser General Public
   6  * License as published by the Free Software Foundation; either
   7  * version 2.1 of the License, or (at your option) any later version.
   8  *
   9  * Libav is distributed in the hope that it will be useful,
  10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12  * Lesser General Public License for more details.
  13  *
  14  * You should have received a copy of the GNU Lesser General Public
  15  * License along with Libav; if not, write to the Free Software
  16  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  17  */
  18
  19 #include <assert.h>
  20 #include "libavutil/mathematics.h"
  21 #include "libavutil/attributes.h"
  22 #include "kbdwin.h"
  23
  24 #define BESSEL_I0_ITER 50 // default: 50 iterations of Bessel I0 approximation
  25
  26 av_cold void ff_kbd_window_init(float *window, float alpha, int n)
  27 {
  28    int i, j;
  29    double sum = 0.0, bessel, tmp;
  30    double local_window[FF_KBD_WINDOW_MAX];
  31    double alpha2 = (alpha * M_PI / n) * (alpha * M_PI / n);
  32
  33    assert(n <= FF_KBD_WINDOW_MAX);
  34
  35    for (i = 0; i < n; i++) {
  36        tmp = i * (n - i) * alpha2;
  37        bessel = 1.0;
  38        for (j = BESSEL_I0_ITER; j > 0; j--)
  39            bessel = bessel * tmp / (j * j) + 1;
  40        sum += bessel;
  41        local_window[i] = sum;
  42    }
  43
  44    sum++;
  45    for (i = 0; i < n; i++)
  46        window[i] = sqrt(local_window[i] / sum);
  47 }