Pammasksharpen User Manual(1General Commands ManuPammasksharpen User Manual(1)
NAME
pammasksharpen - Sharpen an image via an unsharp mask
SYNOPSIS
pammasksharpen [-sharpness=realnum] [-threshold=realnum] maskfile [in-
putfile]
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 un-
sharp 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 pa-
rameter of unsharp mask sharpening usually known as "radius." In par-
ticular, 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 ra-
dius 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 de-
tail 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 de-
tail (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 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).
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)
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