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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 -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 com-
putation 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 usu-
ally 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: reduc-
ing one allows you to increase the other.
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(0)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
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