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, 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

       In  addition  to  the options common to all programs based on libnetpbm (most notably -quiet, see  Common
       Options ), 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 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(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)