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



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 -noffset 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 sam-
       ple 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 edge-
       ness  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 val-
       ues 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 inani-
       mate  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
       -sharpness=realnum
	      This  specifies the magnitude of the sharpening.	It is the mul-
	      tiple 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).



netpbm documentation		 14 June 2006	 Pammasksharpen User Manual(0)