yading@10: /*
yading@10:  * This file is part of FFmpeg.
yading@10:  *
yading@10:  * FFmpeg is free software; you can redistribute it and/or
yading@10:  * modify it under the terms of the GNU Lesser General Public
yading@10:  * License as published by the Free Software Foundation; either
yading@10:  * version 2.1 of the License, or (at your option) any later version.
yading@10:  *
yading@10:  * FFmpeg is distributed in the hope that it will be useful,
yading@10:  * but WITHOUT ANY WARRANTY; without even the implied warranty of
yading@10:  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
yading@10:  * Lesser General Public License for more details.
yading@10:  *
yading@10:  * You should have received a copy of the GNU Lesser General Public
yading@10:  * License along with FFmpeg; if not, write to the Free Software
yading@10:  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
yading@10:  */
yading@10: 
yading@10: #include "libavutil/avassert.h"
yading@10: #include "libavutil/mathematics.h"
yading@10: #include "libavutil/attributes.h"
yading@10: #include "kbdwin.h"
yading@10: 
yading@10: #define BESSEL_I0_ITER 50 // default: 50 iterations of Bessel I0 approximation
yading@10: 
yading@10: av_cold void ff_kbd_window_init(float *window, float alpha, int n)
yading@10: {
yading@10:    int i, j;
yading@10:    double sum = 0.0, bessel, tmp;
yading@10:    double local_window[FF_KBD_WINDOW_MAX];
yading@10:    double alpha2 = (alpha * M_PI / n) * (alpha * M_PI / n);
yading@10: 
yading@10:    av_assert0(n <= FF_KBD_WINDOW_MAX);
yading@10: 
yading@10:    for (i = 0; i < n; i++) {
yading@10:        tmp = i * (n - i) * alpha2;
yading@10:        bessel = 1.0;
yading@10:        for (j = BESSEL_I0_ITER; j > 0; j--)
yading@10:            bessel = bessel * tmp / (j * j) + 1;
yading@10:        sum += bessel;
yading@10:        local_window[i] = sum;
yading@10:    }
yading@10: 
yading@10:    sum++;
yading@10:    for (i = 0; i < n; i++)
yading@10:        window[i] = sqrt(local_window[i] / sum);
yading@10: }