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 ), 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

netpbm documentation                              14 June 2006                     Pammasksharpen User Manual(1)