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Pammasksharpen User Manual(0)                    Pammasksharpen User Manual(0)

       pammasksharpen - Sharpen an image via an unsharp mask

       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.

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

       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: Cal-
       culate 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 obliter-
       ate 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.

              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.

              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).

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

       pammasksharpen was new in Netpbm 10.23 (July 2004).

netpbm documentation             14 June 2006    Pammasksharpen User Manual(0)