avfilter: add chorus filter

Signed-off-by: Paul B Mahol <onemda@gmail.com>

Changelog

@@ -16,6 +16,7 @@ version <next>:
 - unpack DivX-style packed B-frames in MPEG-4 bitstream filter
 - WebM Live Chunk Muxer
 - nvenc level and tier options
+- chorus filter
 
 
 version 2.6:

doc/filters.texi

@@ -1320,6 +1320,61 @@ front_center.wav -map '[LFE]' lfe.wav -map '[SL]' side_left.wav -map '[SR]'
 side_right.wav
 @end example
 
+@section chorus
+Add a chorus effect to the audio.
+
+Can make a single vocal sound like a chorus, but can also be applied to instrumentation.
+
+Chorus resembles an echo effect with a short delay, but whereas with echo the delay is
+constant, with chorus, it is varied using using sinusoidal or triangular modulation.
+The modulation depth defines the range the modulated delay is played before or after
+the delay. Hence the delayed sound will sound slower or faster, that is the delayed
+sound tuned around the original one, like in a chorus where some vocals are slightly
+off key.
+
+It accepts the following parameters:
+@table @option
+@item in_gain
+Set input gain. Default is 0.4.
+
+@item out_gain
+Set output gain. Default is 0.4.
+
+@item delays
+Set delays. A typical delay is around 40ms to 60ms.
+
+@item decays
+Set decays.
+
+@item speeds
+Set speeds.
+
+@item depths
+Set depths.
+@end table
+
+@subsection Examples
+
+@itemize
+@item
+A single delay:
+@example
+chorus=0.7:0.9:55:0.4:0.25:2
+@end example
+
+@item
+Two delays:
+@example
+chorus=0.6:0.9:50|60:0.4|0.32:0.25|0.4:2|1.3
+@end example
+
+@item
+Fuller sounding chorus with three delays:
+@example
+chorus=0.5:0.9:50|60|40:0.4|0.32|0.3:0.25|0.4|0.3:2|2.3|1.3
+@end example
+@end itemize
+
 @section compand
 Compress or expand the audio's dynamic range.
 

libavfilter/Makefile

@@ -64,6 +64,7 @@ OBJS-$(CONFIG_BIQUAD_FILTER)                 += af_biquads.o
 OBJS-$(CONFIG_BS2B_FILTER)                   += af_bs2b.o
 OBJS-$(CONFIG_CHANNELMAP_FILTER)             += af_channelmap.o
 OBJS-$(CONFIG_CHANNELSPLIT_FILTER)           += af_channelsplit.o
+OBJS-$(CONFIG_CHORUS_FILTER)                 += af_chorus.o generate_wave_table.o
 OBJS-$(CONFIG_COMPAND_FILTER)                += af_compand.o
 OBJS-$(CONFIG_DCSHIFT_FILTER)                += af_dcshift.o
 OBJS-$(CONFIG_EARWAX_FILTER)                 += af_earwax.o

libavfilter/allfilters.c

@@ -80,6 +80,7 @@ void avfilter_register_all(void)
     REGISTER_FILTER(BS2B,           bs2b,           af);
     REGISTER_FILTER(CHANNELMAP,     channelmap,     af);
     REGISTER_FILTER(CHANNELSPLIT,   channelsplit,   af);
+    REGISTER_FILTER(CHORUS,         chorus,         af);
     REGISTER_FILTER(COMPAND,        compand,        af);
     REGISTER_FILTER(DCSHIFT,        dcshift,        af);
     REGISTER_FILTER(EARWAX,         earwax,         af);

libavfilter/version.h

@@ -30,8 +30,8 @@
 #include "libavutil/version.h"
 
 #define LIBAVFILTER_VERSION_MAJOR  5
-#define LIBAVFILTER_VERSION_MINOR  13
-#define LIBAVFILTER_VERSION_MICRO 101
+#define LIBAVFILTER_VERSION_MINOR  14
+#define LIBAVFILTER_VERSION_MICRO 100
 
 #define LIBAVFILTER_VERSION_INT AV_VERSION_INT(LIBAVFILTER_VERSION_MAJOR, \
                                                LIBAVFILTER_VERSION_MINOR, \