Pamtilt User Manual(1) General Commands Manual Pamtilt User Manual(1)
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(1).
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 help correct that.
pamtilt makes three iterations at successively finer increments, test-
ing 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 re-
sults, 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
In addition to the options common to all programs based on libnetpbm
(most notably -quiet, see
Common Options ⟨index.html#commonoptions⟩ ), pamtilt recognizes the
following command line options:
-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 itera-
tion 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 val-
ues 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: "Mea-
suring 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(1)
•
pgm(1)
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.
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/pamtilt.html
netpbm documentation 28 August 2005 Pamtilt User Manual(1)
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