Section: MJPEG tools manual (1)
Updated: 11 November 2004
y4munsharp - Unsharp filter for YUV4MPEG2 streams
is an implementation of the GIMP unsharp filter for
YUV4MPEG2 streams. The usual 3 parameters for unshap mask processing
(radius, amount and threshold) are specified on the command line. By default
only the Y' (luma) component is processed although processing of the CbCr
(chroma) may be requested by a commandline option.
Output must be either to a pipe or a file, leaving stdout as a terminal
will elicit an error and the program will exit.
accepts the following options:
- -v num
Set verbosity level (0 = quiet, 1 = normal, 2 = debug)
Print a suage summary and exit
- -L radius,amount,threshold
Y' (luma) unsharp parameters.
The radius and amount" arguments are floating point numbers. The
threshold argument is an integer.
CbCr (chroma) unsharp parameters. The chroma is not processed unless this
option is given. Arguments are the same type as for the -L option above.
Filtering can produce output outside the 16 to 240 range for Y' and 16 to
235 for CbCr. By default values outside the legal range are clipped/cored
(values over 240 for Y' are set to 240 for example). Using -N the
limits 0 and 255 are used instead.
A mild setting:
y4munsharp -L 2.0,0.3,0
An aggressive setting:
y4munsharp -L 5.0,0.5,0
The radius, amount, threshold parameters have the same meaning
as the GIMP's unsharp plugin. This means you can use the GIMP on selected
frame still images to determine suitable settings for y4munsharp
key thing to remember is to be subtle - you just barely want to notice the
sharpening effect. If the effect is obvious ("jumps out at you") then the
parameters are set too aggressively.
The chroma is not processed for a several reasons: 1) The eye
is much more sensitive to changes in brightness, 2) the chroma is usually
subsampled and doesn't contribute much to the perceived
sharpness of an image - so not processing the chroma provides a boost in
speed of processing the image, and 3) possible artifacting - changes in Y'
just make an image brighter/darker but changes in Cb or Cr can change colors
and possibly lead to shifts in hue. Use the -C option to enable
processing of the chroma planes.
Only the 3 plane YUV4MPEG2 formats are supported.
Only progressive, top field first or bottom field first interlaced streams
are handled. The mixed interlaced stream format is not supported.