avfilter: add audio limiter filter

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

Changelog

@@ -5,6 +5,7 @@ version <next>:
 - DXV decoding
 - extrastereo filter
 - ocr filter
+- alimiter filter
 
 
 version 2.8:

doc/filters.texi

@@ -641,6 +641,41 @@ Force the output to either unsigned 8-bit or signed 16-bit stereo
 aformat=sample_fmts=u8|s16:channel_layouts=stereo
 @end example
 
+@section alimiter
+
+The limiter prevents input signal from raising over a desired threshold.
+This limiter uses lookahead technology to prevent your signal from distorting.
+It means that there is a small delay after signal is processed. Keep in mind
+that the delay it produces is the attack time you set.
+
+The filter accepts the following options:
+
+@table @option
+@item limit
+Don't let signals above this level pass the limiter. The removed amplitude is
+added automatically. Default is 1.
+
+@item attack
+The limiter will reach its attenuation level in this amount of time in
+milliseconds. Default is 5 milliseconds.
+
+@item release
+Come back from limiting to attenuation 1.0 in this amount of milliseconds.
+Default is 50 milliseconds.
+
+@item asc
+When gain reduction is always needed ASC takes care of releasing to an
+average reduction level rather than reaching a reduction of 0 in the release
+time.
+
+@item asc_level
+Select how much the release time is affected by ASC, 0 means nearly no changes
+in release time while 1 produces higher release times.
+@end table
+
+Depending on picked setting it is recommended to upsample input 2x or 4x times
+with @ref{aresample} before applying this filter.
+
 @section allpass
 
 Apply a two-pole all-pass filter with central frequency (in Hz)

libavfilter/Makefile

@@ -30,6 +30,7 @@ OBJS-$(CONFIG_AEVAL_FILTER)                  += aeval.o
 OBJS-$(CONFIG_AFADE_FILTER)                  += af_afade.o
 OBJS-$(CONFIG_AFORMAT_FILTER)                += af_aformat.o
 OBJS-$(CONFIG_AINTERLEAVE_FILTER)            += f_interleave.o
+OBJS-$(CONFIG_ALIMITER_FILTER)               += af_alimiter.o
 OBJS-$(CONFIG_ALLPASS_FILTER)                += af_biquads.o
 OBJS-$(CONFIG_AMERGE_FILTER)                 += af_amerge.o
 OBJS-$(CONFIG_AMIX_FILTER)                   += af_amix.o

libavfilter/allfilters.c

@@ -52,6 +52,7 @@ void avfilter_register_all(void)
     REGISTER_FILTER(AFADE,          afade,          af);
     REGISTER_FILTER(AFORMAT,        aformat,        af);
     REGISTER_FILTER(AINTERLEAVE,    ainterleave,    af);
+    REGISTER_FILTER(ALIMITER,       alimiter,       af);
     REGISTER_FILTER(ALLPASS,        allpass,        af);
     REGISTER_FILTER(AMERGE,         amerge,         af);
     REGISTER_FILTER(AMIX,           amix,           af);

libavfilter/version.h

@@ -30,7 +30,7 @@
 #include "libavutil/version.h"
 
 #define LIBAVFILTER_VERSION_MAJOR   6
-#define LIBAVFILTER_VERSION_MINOR   2
+#define LIBAVFILTER_VERSION_MINOR   3
 #define LIBAVFILTER_VERSION_MICRO 100
 
 #define LIBAVFILTER_VERSION_INT AV_VERSION_INT(LIBAVFILTER_VERSION_MAJOR, \