av_cold struct FFIIRFilterCoeffs* ff_iir_filter_init_coeffs(enum IIRFilterType filt_type, enum IIRFilterMode filt_mode, int order, float cutoff_ratio, float stopband, float ripple) { int i, j; FFIIRFilterCoeffs *c; double wa; double p[MAXORDER + 1][2]; if(filt_type != FF_FILTER_TYPE_BUTTERWORTH || filt_mode != FF_FILTER_MODE_LOWPASS) return NULL; if(order <= 1 || (order & 1) || order > MAXORDER || cutoff_ratio >= 1.0) return NULL; c = av_malloc(sizeof(FFIIRFilterCoeffs)); c->cx = av_malloc(sizeof(c->cx[0]) * ((order >> 1) + 1)); c->cy = av_malloc(sizeof(c->cy[0]) * order); c->order = order; wa = 2 * tan(M_PI * 0.5 * cutoff_ratio); c->cx[0] = 1; for(i = 1; i < (order >> 1) + 1; i++) c->cx[i] = c->cx[i - 1] * (order - i + 1LL) / i; p[0][0] = 1.0; p[0][1] = 0.0; for(i = 1; i <= order; i++) p[i][0] = p[i][1] = 0.0; for(i = 0; i < order; i++){ double zp[2]; double th = (i + (order >> 1) + 0.5) * M_PI / order; double a_re, a_im, c_re, c_im; zp[0] = cos(th) * wa; zp[1] = sin(th) * wa; a_re = zp[0] + 2.0; c_re = zp[0] - 2.0; a_im = c_im = zp[1]; zp[0] = (a_re * c_re + a_im * c_im) / (c_re * c_re + c_im * c_im); zp[1] = (a_im * c_re - a_re * c_im) / (c_re * c_re + c_im * c_im); for(j = order; j >= 1; j--) { a_re = p[j][0]; a_im = p[j][1]; p[j][0] = a_re*zp[0] - a_im*zp[1] + p[j-1][0]; p[j][1] = a_re*zp[1] + a_im*zp[0] + p[j-1][1]; } a_re = p[0][0]*zp[0] - p[0][1]*zp[1]; p[0][1] = p[0][0]*zp[1] + p[0][1]*zp[0]; p[0][0] = a_re; } c->gain = p[order][0]; for(i = 0; i < order; i++){ c->gain += p[i][0]; c->cy[i] = (-p[i][0] * p[order][0] + -p[i][1] * p[order][1]) / (p[order][0] * p[order][0] + p[order][1] * p[order][1]); } c->gain /= 1 << order; return c; }