documentation: add setdar and setsar description to filters.texi

Signed-off-by: Anton Khirnov <anton@khirnov.net>

doc/filters.texi

@@ -789,6 +789,36 @@ ratio of the input image.
 
 The default value of @var{width} and @var{height} is 0.
 
+@anchor{setdar}
+@section setdar
+
+Set the Display Aspect Ratio for the filter output video.
+
+This is done by changing the specified Sample (aka Pixel) Aspect
+Ratio, according to the following equation:
+@math{DAR = HORIZONTAL_RESOLUTION / VERTICAL_RESOLUTION * SAR}
+
+Keep in mind that this filter does not modify the pixel dimensions of
+the video frame. Also the display aspect ratio set by this filter may
+be changed by later filters in the filterchain, e.g. in case of
+scaling or if another "setdar" or a "setsar" filter is applied.
+
+The filter accepts a parameter string which represents the wanted
+display aspect ratio.
+The parameter can be a floating point number string, or an expression
+of the form @var{num}:@var{den}, where @var{num} and @var{den} are the
+numerator and denominator of the aspect ratio.
+If the parameter is not specified, it is assumed the value "0:1".
+
+For example to change the display aspect ratio to 16:9, specify:
+@example
+setdar=16:9
+# the above is equivalent to
+setdar=1.77777
+@end example
+
+See also the "setsar" filter documentation (@pxref{setsar}).
+
 @section setpts
 
 Change the PTS (presentation timestamp) of the input video frames.
@@ -849,6 +879,32 @@ setpts=N/(25*TB)
 setpts='1/(25*TB) * (N + 0.05 * sin(N*2*PI/25))'
 @end example
 
+@anchor{setsar}
+@section setsar
+
+Set the Sample (aka Pixel) Aspect Ratio for the filter output video.
+
+Note that as a consequence of the application of this filter, the
+output display aspect ratio will change according to the following
+equation:
+@math{DAR = HORIZONTAL_RESOLUTION / VERTICAL_RESOLUTION * SAR}
+
+Keep in mind that the sample aspect ratio set by this filter may be
+changed by later filters in the filterchain, e.g. if another "setsar"
+or a "setdar" filter is applied.
+
+The filter accepts a parameter string which represents the wanted
+sample aspect ratio.
+The parameter can be a floating point number string, or an expression
+of the form @var{num}:@var{den}, where @var{num} and @var{den} are the
+numerator and denominator of the aspect ratio.
+If the parameter is not specified, it is assumed the value "0:1".
+
+For example to change the sample aspect ratio to 10:11, specify:
+@example
+setsar=10:11
+@end example
+
 @section settb
 
 Set the timebase to use for the output frames timestamps.