pamtilt(1) - phpMan

Pamtilt User Manual(0) Pamtilt User Manual(0)

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 docu-
ment. 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 scan-
ner at an angle: the resulting image is tilted. pamtilt aims to help correct that.

pamtilt makes three iterations at successively finer increments, testing prospective rota-
tion 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 sel-
dom 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.