avfilter/af_sidechaincompress: implement mode option

doc/filters.texi

@@ -390,6 +390,10 @@ The filter accepts the following options:
 @item level_in
 Set input gain. Default is 1. Range is between 0.015625 and 64.
 
+@item mode
+Set mode of compressor operation. Can be @code{upward} or @code{downward}.
+Default is @code{downward}.
+
 @item threshold
 If a signal of stream rises above this level it will affect the gain
 reduction.
@@ -4247,6 +4251,10 @@ The filter accepts the following options:
 @item level_in
 Set input gain. Default is 1. Range is between 0.015625 and 64.
 
+@item mode
+Set mode of compressor operation. Can be @code{upward} or @code{downward}.
+Default is @code{downward}.
+
 @item threshold
 If a signal of second stream raises above this level it will affect the gain
 reduction of first stream.

libavfilter/af_sidechaincompress.c

@@ -54,10 +54,14 @@ typedef struct SidechainCompressContext {
     double knee_start;
     double knee_stop;
     double lin_knee_start;
+    double lin_knee_stop;
     double adj_knee_start;
+    double adj_knee_stop;
+    double compressed_knee_start;
     double compressed_knee_stop;
     int link;
     int detection;
+    int mode;
 
     AVAudioFifo *fifo[2];
     int64_t pts;
@@ -69,6 +73,9 @@ typedef struct SidechainCompressContext {
 
 static const AVOption options[] = {
     { "level_in",  "set input gain",     OFFSET(level_in),  AV_OPT_TYPE_DOUBLE, {.dbl=1},        0.015625,   64, A|F },
+    { "mode",      "set mode",           OFFSET(mode),      AV_OPT_TYPE_INT,    {.i64=0},               0,    1, A|F, "mode" },
+    {   "downward",0,                    0,                 AV_OPT_TYPE_CONST,  {.i64=0},               0,    0, A|F, "mode" },
+    {   "upward",  0,                    0,                 AV_OPT_TYPE_CONST,  {.i64=1},               0,    0, A|F, "mode" },
     { "threshold", "set threshold",      OFFSET(threshold), AV_OPT_TYPE_DOUBLE, {.dbl=0.125}, 0.000976563,    1, A|F },
     { "ratio",     "set ratio",          OFFSET(ratio),     AV_OPT_TYPE_DOUBLE, {.dbl=2},               1,   20, A|F },
     { "attack",    "set attack",         OFFSET(attack),    AV_OPT_TYPE_DOUBLE, {.dbl=20},           0.01, 2000, A|F },
@@ -97,7 +104,9 @@ AVFILTER_DEFINE_CLASS(sidechaincompress);
 
 static double output_gain(double lin_slope, double ratio, double thres,
                           double knee, double knee_start, double knee_stop,
-                          double compressed_knee_stop, int detection)
+                          double compressed_knee_start,
+                          double compressed_knee_stop,
+                          int detection, int mode)
 {
     double slope = log(lin_slope);
     double gain = 0.0;
@@ -114,10 +123,17 @@ static double output_gain(double lin_slope, double ratio, double thres,
         delta = 1.0 / ratio;
     }
 
+    if (mode) {
+        if (knee > 1.0 && slope > knee_start)
+            gain = hermite_interpolation(slope, knee_stop, knee_start,
+                                         knee_stop, compressed_knee_start,
+                                         1.0, delta);
+    } else {
         if (knee > 1.0 && slope < knee_stop)
             gain = hermite_interpolation(slope, knee_start, knee_stop,
                                          knee_start, compressed_knee_stop,
                                          1.0, delta);
+    }
 
     return exp(gain - slope);
 }
@@ -129,9 +145,12 @@ static int compressor_config_output(AVFilterLink *outlink)
 
     s->thres = log(s->threshold);
     s->lin_knee_start = s->threshold / sqrt(s->knee);
+    s->lin_knee_stop = s->threshold * sqrt(s->knee);
     s->adj_knee_start = s->lin_knee_start * s->lin_knee_start;
+    s->adj_knee_stop = s->lin_knee_stop * s->lin_knee_stop;
     s->knee_start = log(s->lin_knee_start);
-    s->knee_stop = log(s->threshold * sqrt(s->knee));
+    s->knee_stop = log(s->lin_knee_stop);
+    s->compressed_knee_start = (s->knee_start - s->thres) / s->ratio + s->thres;
     s->compressed_knee_stop = (s->knee_stop - s->thres) / s->ratio + s->thres;
 
     s->attack_coeff = FFMIN(1., 1. / (s->attack * outlink->sample_rate / 4000.));
@@ -151,6 +170,8 @@ static void compressor(SidechainCompressContext *s,
 
     for (i = 0; i < nb_samples; i++) {
         double abs_sample, gain = 1.0;
+        double detector;
+        int detected;
 
         abs_sample = fabs(scsrc[0] * level_sc);
 
@@ -169,10 +190,20 @@ static void compressor(SidechainCompressContext *s,
 
         s->lin_slope += (abs_sample - s->lin_slope) * (abs_sample > s->lin_slope ? s->attack_coeff : s->release_coeff);
 
-        if (s->lin_slope > 0.0 && s->lin_slope > (s->detection ? s->adj_knee_start : s->lin_knee_start))
+        if (s->mode) {
+            detector = (s->detection ? s->adj_knee_stop : s->lin_knee_stop);
+            detected = s->lin_slope < detector;
+        } else {
+            detector = (s->detection ? s->adj_knee_start : s->lin_knee_start);
+            detected = s->lin_slope > detector;
+        }
+
+        if (s->lin_slope > 0.0 && detected)
             gain = output_gain(s->lin_slope, s->ratio, s->thres, s->knee,
                                s->knee_start, s->knee_stop,
-                               s->compressed_knee_stop, s->detection);
+                               s->compressed_knee_start,
+                               s->compressed_knee_stop,
+                               s->detection, s->mode);
 
         for (c = 0; c < inlink->channels; c++)
             dst[c] = src[c] * level_in * (gain * makeup * mix + (1. - mix));