avfilter: add normalize filter

Changelog

@@ -19,6 +19,7 @@ version <next>:
 - acontrast audio filter
 - OpenCL overlay filter
 - video mix filter
+- video normalize filter
 
 
 version 3.4:

doc/filters.texi

@@ -10867,6 +10867,86 @@ Add temporal and uniform noise to input video:
 noise=alls=20:allf=t+u
 @end example
 
+@section normalize
+
+Normalize RGB video (aka histogram stretching, contrast stretching).
+See: https://en.wikipedia.org/wiki/Normalization_(image_processing)
+
+For each channel of each frame, the filter computes the input range and maps
+it linearly to the user-specified output range. The output range defaults
+to the full dynamic range from pure black to pure white.
+
+Temporal smoothing can be used on the input range to reduce flickering (rapid
+changes in brightness) caused when small dark or bright objects enter or leave
+the scene. This is similar to the auto-exposure (automatic gain control) on a
+video camera, and, like a video camera, it may cause a period of over- or
+under-exposure of the video.
+
+The R,G,B channels can be normalized independently, which may cause some
+color shifting, or linked together as a single channel, which prevents
+color shifting. Linked normalization preserves hue. Independent normalization
+does not, so it can be used to remove some color casts. Independent and linked
+normalization can be combined in any ratio.
+
+The normalize filter accepts the following options:
+
+@table @option
+@item blackpt
+@item whitept
+Colors which define the output range. The minimum input value is mapped to
+the @var{blackpt}. The maximum input value is mapped to the @var{whitept}.
+The defaults are black and white respectively. Specifying white for
+@var{blackpt} and black for @var{whitept} will give color-inverted,
+normalized video. Shades of grey can be used to reduce the dynamic range
+(contrast). Specifying saturated colors here can create some interesting
+effects.
+
+@item smoothing
+The number of previous frames to use for temporal smoothing. The input range
+of each channel is smoothed using a rolling average over the current frame
+and the @var{smoothing} previous frames. The default is 0 (no temporal
+smoothing).
+
+@item independence
+Controls the ratio of independent (color shifting) channel normalization to
+linked (color preserving) normalization. 0.0 is fully linked, 1.0 is fully
+independent. Defaults to 1.0 (fully independent).
+
+@item strength
+Overall strength of the filter. 1.0 is full strength. 0.0 is a rather
+expensive no-op. Defaults to 1.0 (full strength).
+
+@end table
+
+@subsection Examples
+
+Stretch video contrast to use the full dynamic range, with no temporal
+smoothing; may flicker depending on the source content:
+@example
+normalize=blackpt=black:whitept=white:smoothing=0
+@end example
+
+As above, but with 50 frames of temporal smoothing; flicker should be
+reduced, depending on the source content:
+@example
+normalize=blackpt=black:whitept=white:smoothing=50
+@end example
+
+As above, but with hue-preserving linked channel normalization:
+@example
+normalize=blackpt=black:whitept=white:smoothing=50:independence=0
+@end example
+
+As above, but with half strength:
+@example
+normalize=blackpt=black:whitept=white:smoothing=50:independence=0:strength=0.5
+@end example
+
+Map the darkest input color to red, the brightest input color to cyan:
+@example
+normalize=blackpt=red:whitept=cyan
+@end example
+
 @section null
 
 Pass the video source unchanged to the output.

libavfilter/Makefile

@@ -247,6 +247,7 @@ OBJS-$(CONFIG_NLMEANS_FILTER)                += vf_nlmeans.o
 OBJS-$(CONFIG_NNEDI_FILTER)                  += vf_nnedi.o
 OBJS-$(CONFIG_NOFORMAT_FILTER)               += vf_format.o
 OBJS-$(CONFIG_NOISE_FILTER)                  += vf_noise.o
+OBJS-$(CONFIG_NORMALIZE_FILTER)              += vf_normalize.o
 OBJS-$(CONFIG_NULL_FILTER)                   += vf_null.o
 OBJS-$(CONFIG_OCR_FILTER)                    += vf_ocr.o
 OBJS-$(CONFIG_OCV_FILTER)                    += vf_libopencv.o

libavfilter/allfilters.c

@@ -257,6 +257,7 @@ static void register_all(void)
     REGISTER_FILTER(NNEDI,          nnedi,          vf);
     REGISTER_FILTER(NOFORMAT,       noformat,       vf);
     REGISTER_FILTER(NOISE,          noise,          vf);
+    REGISTER_FILTER(NORMALIZE,      normalize,      vf);
     REGISTER_FILTER(NULL,           null,           vf);
     REGISTER_FILTER(OCR,            ocr,            vf);
     REGISTER_FILTER(OCV,            ocv,            vf);

libavfilter/version.h

@@ -30,7 +30,7 @@
 #include "libavutil/version.h"
 
 #define LIBAVFILTER_VERSION_MAJOR   7
-#define LIBAVFILTER_VERSION_MINOR   3
+#define LIBAVFILTER_VERSION_MINOR   4
 #define LIBAVFILTER_VERSION_MICRO 100
 
 #define LIBAVFILTER_VERSION_INT AV_VERSION_INT(LIBAVFILTER_VERSION_MAJOR, \