NAME

       pammasksharpen - Sharpen an image via an unsharp mask

SYNOPSIS

       pammasksharpen [-sharpness=realnum] [-threshold=realnum] maskfile [inputfile]

       All  options  can be abbreviated to their shortest unique prefix.  You may use two hyphens
       instead of one.  You may separate an option name and its value with white space instead of
       an equals sign.

EXAMPLES

          pamgauss 5 5 -sigma=.7 -tupletype=GRAYSCALE | pamtopnm > gauss.pgm
          pnmconvol -nooffset gauss.pgm myimage.ppm > blurred.ppm
          pammasksharpen blurred.ppm myimage.ppm > sharpened.ppm

DESCRIPTION

       This program is part of Netpbm(1).

       pammasksharpen  reads  a  Netpbm image as input and produces a sharpened version of it, in
       the same format, as output.  It does this via an unsharp mask, which you supply as another
       Netpbm image.

       An  unsharp  mask  is  generally  a blurred version of the original image.  The sharpening
       computation is this: Calculate the "edgeness" of a sample in the input image as the signed
       difference  between  the  sample  value  and the corresponding sample in the unsharp mask.
       This tells how different the pixel is from its neighbors.  Add a multiple of the  edgeness
       to  the  original sample to get the corresponding output sample.  Clip as necessary.  This
       causes pixels that are brighter than their neighbors to get even  brighter,  while  pixels
       that  are  dimmer  than their neighbors get even dimmer.  This makes edges -- places where
       pixel values change quickly in space -- stand out more.

       The unsharp mask must be the same dimensions and have the same maxval as the input image.

   The Unsharp Mask
       You usually create the unsharp mask as a Gaussian blur of the original  image,  which  you
       can  do  using pamgauss and pnmconvol as in the example above.  The convolution kernel you
       use with pnmconvol is normally a square with side length an odd number of pixels.

       When you create an unsharp mask like this, you will have to choose the side length of  the
       convolution  kernel.   That  length  implements  the  parameter of unsharp mask sharpening
       usually known as "radius."   In  particular,  a  radius  of  R  pixels  corresponds  to  a
       convolution kernel 2R+1 pixels on a side.

       Radius  is a very important parameter; you can ruin an image with a radius too large.  You
       can safely use the highest radius with an inanimate object, while a human face demands the
       least.   Landscapes  fall  in  between.  But it really depends on the size of the details.
       Fine detail needs a smaller radius, or else you may obliterate tiny  detail  of  the  same
       size  as  the Radius width.  A large image often has larger detail (more pixels involved),
       so can use a larger radius.   Radius  and  sharpness  (see  -sharpness  option)  interact:
       reducing one allows you to increase the other.

OPTIONS

       In addition to the options common to all programs based on libnetpbm (most notably -quiet,
       see
        Common Options ⟨index.html#commonoptions⟩  ),  pammasksharpen  recognizes  the  following
       command line options:

       -sharpness=realnum
              This  specifies  the  magnitude  of the sharpening.  It is the multiple of edgeness
              that gets added to each sample as described above.

              realnum is a nonnegative real decimal number.  Zero means no sharpening at all.

              This parameter is known as "amount" in ImageMagick.

              The default is 1.0.

              This option was new in Netpbm 10.30 (October 2005).  Before that, the sharpness was
              always 1.0.

       -threshold=realnum
              This  minimum  amount of edgeness that will be considered edgeness at all.  i.e. if
              the magnitude of the edgeness is less than  this,  pammasksharpen  will  treat  the
              edgeness as zero.

              A nonzero value may be necessary here to avoid speckling in smooth areas.

              This is a fraction of the maxval (so it is in the range [0, 1.0]).

              The default is 0.

              This option was new in Netpbm 10.34 (June 2006).

SEE ALSO

       pnmconvol(1), pamedge(1), pamsharpness(1), pamsharpmap(1), pamarith(1), pnm(1), pam(1)

HISTORY

       pammasksharpen was new in Netpbm 10.23 (July 2004).

DOCUMENT SOURCE

       This  manual page was generated by the Netpbm tool 'makeman' from HTML source.  The master
       documentation is at

              http://netpbm.sourceforge.net/doc/pammasksharpen.html