Add biquad high-pass and low-pass IIR filters.

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavcodec/iirfilter.c

@@ -112,6 +112,51 @@ static int butterworth_init_coeffs(void *avc, struct FFIIRFilterCoeffs *c,
     return 0;
 }
 
+static int biquad_init_coeffs(void *avc, struct FFIIRFilterCoeffs *c,
+                              enum IIRFilterMode filt_mode, int order,
+                              float cutoff_ratio, float stopband)
+{
+    double cos_w0, sin_w0;
+    double a0, x0, x1;
+
+    if (filt_mode != FF_FILTER_MODE_HIGHPASS &&
+        filt_mode != FF_FILTER_MODE_LOWPASS) {
+        av_log(avc, AV_LOG_ERROR, "Biquad filter currently only supports "
+               "high-pass and low-pass filter modes\n");
+        return -1;
+    }
+    if (order != 2) {
+        av_log(avc, AV_LOG_ERROR, "Biquad filter must have order of 2\n");
+        return -1;
+    }
+
+    cos_w0 = cos(M_PI * cutoff_ratio);
+    sin_w0 = sin(M_PI * cutoff_ratio);
+
+    a0 = 1.0 + (sin_w0 / 2.0);
+
+    if (filt_mode == FF_FILTER_MODE_HIGHPASS) {
+        c->gain  =  ((1.0 + cos_w0) / 2.0)  / a0;
+        x0       = (-(1.0 + cos_w0))        / a0;
+        x1       =  ((1.0 + cos_w0) / 2.0)  / a0;
+    } else { // FF_FILTER_MODE_LOWPASS
+        c->gain  =  ((1.0 - cos_w0) / 2.0)  / a0;
+        x0       =   (1.0 - cos_w0)         / a0;
+        x1       =  ((1.0 - cos_w0) / 2.0)  / a0;
+    }
+    c->cy[0] =  (2.0 *  cos_w0)        / a0;
+    c->cy[1] = (-1.0 + (sin_w0 / 2.0)) / a0;
+
+    // divide by gain to make the x coeffs integers.
+    // during filtering, the delay state will include the gain multiplication
+    c->cx[0] = lrintf(x0 / c->gain);
+    c->cx[1] = lrintf(x1 / c->gain);
+    c->cy[0] /= c->gain;
+    c->cy[1] /= c->gain;
+
+    return 0;
+}
+
 av_cold struct FFIIRFilterCoeffs* ff_iir_filter_init_coeffs(void *avc,
                                                 enum IIRFilterType filt_type,
                                                 enum IIRFilterMode filt_mode,
@@ -136,6 +181,11 @@ av_cold struct FFIIRFilterCoeffs* ff_iir_filter_init_coeffs(void *avc,
             stopband)) {
             goto init_fail;
         }
+    } else if (filt_type == FF_FILTER_TYPE_BIQUAD) {
+        if (biquad_init_coeffs(avc, c, filt_mode, order, cutoff_ratio,
+            stopband)) {
+            goto init_fail;
+        }
     } else {
         av_log(avc, AV_LOG_ERROR, "filter type is not currently implemented\n");
         goto init_fail;

libavcodec/iirfilter.h

@@ -34,6 +34,7 @@ struct FFIIRFilterState;
 
 enum IIRFilterType{
     FF_FILTER_TYPE_BESSEL,
+    FF_FILTER_TYPE_BIQUAD,
     FF_FILTER_TYPE_BUTTERWORTH,
     FF_FILTER_TYPE_CHEBYSHEV,
     FF_FILTER_TYPE_ELLIPTIC,