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pamtilt(1) |
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pamtilt
Updated: 28 August 2005
Table Of Contents
NAME
pamtilt - print the tilt angle of a PGM file
SYNOPSIS
pamtilt [-angle=maxangle]
[-fast] [-quality=q [-hstep=n] [-vstep=n]
[-dstep=n] [-astep=n] [-verbose] [pgmfile]
EXAMPLES
scanimage --mode Gray
--resolution 300 >crooked.pgm
pnmrotate -b white ‘pamtilt crooked.pgm‘
crooked.pgm >straight.pgm
(then crop, threshold, etc.)
DESCRIPTION
This program is part of Netpbm.
pamtilt tries to find the
correct angle for untilting (de-skewing) a scanned
text document. The output is a single floating-point number
(the angle in
degrees) for use as the argument to pnmrotate.
"Document skew" is the
name given to what happens when you feed a page into
an image scanner at an angle: the resulting image is tilted.
pamtilt aims to
correct that.
pamtilt makes three iterations
at successively finer increments, testing
prospective rotation angles to find the best one. pamtilt
works best for
straightening images with strong horizontal lines and does
poorly with
arbitrary photos. If pamtilt has no confidence in its
results, it prints the
special value 00.00; you can check for this or just pass it
as a legal
argument to pnmrotate.
pamtilt operates on the first
plane of the input image, which is either PNM
or PAM, and ignores any other planes. Ordinarily, the input
is PGM or
GRAYSCALE PAM, so there is only one plane.
pamtilt works on bilevel (PBM,
BLACKANDWHITE PAM) images as well as
grayscale, but you will minimize artifacts if you scan and
rotate in
grayscale before you apply a threshold to make a bilevel
image.
OPTIONS
A few options have general
utility:
-angle=maxangle
Assume a maximum tilt angle of maxangle (measured in
degrees). The
default value is sufficient for most images, even those
scanned
somewhat carelessly.
The default is 10.0.
-fast
Skip the third iteration for speed at the expense of
accuracy.
-verbose
Show on Standard Error the measurements computed at each
tested
angle.
Here are some other options you
can use to tune the operation of pamtilt but
they’re seldom needed. The default values accommodate
a wide variety of
input documents.
-quality=q
Require a signal-to-noise ratio of a least q on the first
iteration
to report a valid result. Larger values reduce the chances
of
obtaining a bogus result at the risk of obtaining no result
at all.
The default is 1.0.
-hstep=n
Set the horizontal increment to check every nth column. This
value
affects both run time and memory requirements.
The default is 11.
-vstep=n
Set the vertical increment to check every nth row. Larger
values
usually work, reducing run time, but they increase the risk
of
incorrect results.
The default is 5.
-dstep=n
Set the vertical distance used when checking pixels in a
column. The
default is intended to minimize the effect of noise along a
horizontal boundary.
The default is 2.
-astep=n
Set the angle increment of the first iteration, in
degrees.
The default is 1.0.
REFERENCES
pamtilt implements a somewhat
simplified algorithm inspired by: "Measuring
Document Image Skew and Orientation", by Bloomberg,
Kopec, and Dasari. In
SPIE Volume 2422, Document Recognition II, pages 302-316,
February 1995.
SEE ALSO
* pnmrotate
* pgm
HISTORY
pamtilt was new in Netpbm 10.30 (October 2005).
Gregg Townsend wrote it and sent
it to Bryan Henderson in August 2005. Bryan
recoded it to fit Netpbm conventions.
_________________________________________________________________
Table Of Contents
* NAME
* SYNOPSIS
* EXAMPLES
* DESCRIPTION
* OPTIONS
* SEE ALSO
* HISTORY
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pamtilt(1) | |