lavfi: port boxblur filter from libmpcodecs

With the following additions:
* support to gray format
* support to yuva420p format
* parametric luma/chroma/alpha radius
* consistency check on the radius values, avoid crashes with invalid values
Signed-off-by: Anton Khirnov <anton@khirnov.net>

Changelog

@@ -51,6 +51,7 @@ easier to use. The changes are:
 - CELT in Ogg demuxing
 - VC-1 interlaced decoding
 - lut, lutrgb, and lutyuv filters
+- boxblur filter
 
 
 version 0.7:

configure

@@ -1480,6 +1480,7 @@ udp_protocol_deps="network"
 
 # filters
 blackframe_filter_deps="gpl"
+boxblur_filter_deps="gpl"
 cropdetect_filter_deps="gpl"
 drawtext_filter_deps="libfreetype"
 frei0r_filter_deps="frei0r dlopen strtok_r"

doc/filters.texi

@@ -183,6 +183,66 @@ threshold, and defaults to 98.
 @var{threshold} is the threshold below which a pixel value is
 considered black, and defaults to 32.
 
+@section boxblur
+
+Apply boxblur algorithm to the input video.
+
+This filter accepts the parameters:
+@var{luma_power}:@var{luma_radius}:@var{chroma_radius}:@var{chroma_power}:@var{alpha_radius}:@var{alpha_power}
+
+Chroma and alpha parameters are optional, if not specified they default
+to the corresponding values set for @var{luma_radius} and
+@var{luma_power}.
+
+@var{luma_radius}, @var{chroma_radius}, and @var{alpha_radius} represent
+the radius in pixels of the box used for blurring the corresponding
+input plane. They are expressions, and can contain the following
+constants:
+@table @option
+@item w, h
+the input width and heigth in pixels
+
+@item cw, ch
+the input chroma image width and height in pixels
+
+@item hsub, vsub
+horizontal and vertical chroma subsample values. For example for the
+pixel format "yuv422p" @var{hsub} is 2 and @var{vsub} is 1.
+@end table
+
+The radius must be a non-negative number, and must not be greater than
+the value of the expression @code{min(w,h)/2} for the luma and alpha planes,
+and of @code{min(cw,ch)/2} for the chroma planes.
+
+@var{luma_power}, @var{chroma_power}, and @var{alpha_power} represent
+how many times the boxblur filter is applied to the corresponding
+plane.
+
+Some examples follow:
+
+@itemize
+
+@item
+Apply a boxblur filter with luma, chroma, and alpha radius
+set to 2:
+@example
+boxblur=2:1
+@end example
+
+@item
+Set luma radius to 2, alpha and chroma radius to 0
+@example
+boxblur=2:1:0:0:0:0
+@end example
+
+@item
+Set luma and chroma radius to a fraction of the video dimension
+@example
+boxblur=min(h\,w)/10:1:min(cw\,ch)/10:1
+@end example
+
+@end itemize
+
 @section copy
 
 Copy the input source unchanged to the output. Mainly useful for

libavfilter/Makefile

@@ -20,6 +20,7 @@ OBJS-$(CONFIG_ANULLSRC_FILTER)               += asrc_anullsrc.o
 OBJS-$(CONFIG_ANULLSINK_FILTER)              += asink_anullsink.o
 
 OBJS-$(CONFIG_BLACKFRAME_FILTER)             += vf_blackframe.o
+OBJS-$(CONFIG_BOXBLUR_FILTER)                += vf_boxblur.o
 OBJS-$(CONFIG_COPY_FILTER)                   += vf_copy.o
 OBJS-$(CONFIG_CROP_FILTER)                   += vf_crop.o
 OBJS-$(CONFIG_CROPDETECT_FILTER)             += vf_cropdetect.o

libavfilter/allfilters.c

@@ -41,6 +41,7 @@ void avfilter_register_all(void)
     REGISTER_FILTER (ANULLSINK,   anullsink,   asink);
 
     REGISTER_FILTER (BLACKFRAME,  blackframe,  vf);
+    REGISTER_FILTER (BOXBLUR,     boxblur,     vf);
     REGISTER_FILTER (COPY,        copy,        vf);
     REGISTER_FILTER (CROP,        crop,        vf);
     REGISTER_FILTER (CROPDETECT,  cropdetect,  vf);

libavfilter/avfilter.h

@@ -29,7 +29,7 @@
 #include "libavutil/rational.h"
 
 #define LIBAVFILTER_VERSION_MAJOR  2
-#define LIBAVFILTER_VERSION_MINOR  9
+#define LIBAVFILTER_VERSION_MINOR  10
 #define LIBAVFILTER_VERSION_MICRO  0
 
 #define LIBAVFILTER_VERSION_INT AV_VERSION_INT(LIBAVFILTER_VERSION_MAJOR, \