Add gradfun filter, ported from MPlayer.

Patch by Nolan L nol888 <=> gmail >=< com.

See thread:
Subject: [FFmpeg-devel] [PATCH] Port gradfun to libavfilter (GCI)
Date: Mon, 29 Nov 2010 07:18:14 -0500

Originally committed as revision 25942 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/filters.texi

@@ -425,6 +425,35 @@ frei0r=perspective:0.2/0.2:0.8/0.2
 For more information see:
 @url{http://piksel.org/frei0r}
 
+@section gradfun
+
+Fix the banding artifacts that are sometimes introduced into nearly flat
+regions by truncation to 8bit colordepth.
+Interpolate the gradients that should go where the bands are, and
+dither them.
+
+The filter takes two optional parameters, separated by ':':
+@var{strength}:@var{radius}
+
+@var{strength} is the maximum amount by which the filter will change
+any one pixel. Also the threshold for detecting nearly flat
+regions. Acceptable values range from .51 to 255, default value is
+1.2, out-of-range values will be clipped to the valid range.
+
+@var{radius} is the neighborhood to fit the gradient to. A larger
+radius makes for smoother gradients, but also prevents the filter from
+modifying the pixels near detailed regions. Acceptable values are
+8-32, default value is 16, out-of-range values will be clipped to the
+valid range.
+
+@example
+# default parameters
+gradfun=1.2:16
+
+# omitting radius
+gradfun=1.2
+@end example
+
 @section hflip
 
 Flip the input video horizontally.