pammasksharpen
Updated: 14 June 2006
Table Of Contents
NAME
pammasksharpen - Sharpen an image via an unsharp mask
SYNOPSIS
pammasksharpen [-sharpness=realnum [-threshold=realnum mask-
file [inputfile]
All options can be abbreviated to their shortest unique pre-
fix. 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 gauss.pgm myimage.ppm >blurred.ppm
pammasksharpen blurred.ppm myimage.ppm >sharpened.ppm
DESCRIPTION
This program is part of Netpbm.
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 dif-
ferent 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 nor-
mally 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 particu-
lar, 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 inan-
imate object,
while a human face demands the least. Landscapes fall in be-
tween. 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 pix-
els involved),
so can use a larger radius. Radius and sharpness (see -sharp-
ness option)
interact: reducing 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). Be-
fore that, the
sharpness was always 1.0.
-sharpness=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 speck-
ling 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, pamedge, pamsharpness, pamsharpmap, pamarith, pnm,
pam
HISTORY
pammasksharpen was new in Netpbm 10.23 (July 2004).
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Table Of Contents
* NAME
* SYNOPSIS
* DESCRIPTION
* HISTORY
* SEE ALSO
