1Pamtilt User Manual(0)                                  Pamtilt User Manual(0)
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NAME

6       pamtilt - print the tilt angle of a PGM file
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SYNOPSIS

10       pamtilt  [-angle=maxangle]  [-fast]  [-quality=q] [-hstep=n] [-vstep=n]
11       [-dstep=n] [-astep=n] [-verbose] [pgmfile]
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EXAMPLES

15           scanimage --mode Gray --resolution 300 >crooked.pgm
16           pnmrotate -b white `pamtilt crooked.pgm` crooked.pgm >straight.pgm
17           (then crop, threshold, etc.)
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DESCRIPTION

21       This program is part of Netpbm(1).
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23       pamtilt tries to find the correct angle for  untilting  (de-skewing)  a
24       scanned  text  document.   The output is a single floating-point number
25       (the angle in degrees) for use as the argument to pnmrotate.
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27       'Document skew' is the name given to what happens when you feed a  page
28       into  an  image  scanner  at  an  angle: the resulting image is tilted.
29       pamtilt aims to correct that.
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31       pamtilt makes three iterations at successively finer increments,  test‐
32       ing  prospective  rotation  angles to find the best one.  pamtilt works
33       best for straightening images with strong  horizontal  lines  and  does
34       poorly  with  arbitrary  photos.   If  pamtilt has no confidence in its
35       results, it prints the special value 00.00; you can check for  this  or
36       just pass it as a legal argument to pnmrotate.
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38       pamtilt operates on the first plane of the input image, which is either
39       PNM or PAM, and ignores any other planes.  Ordinarily, the input is PGM
40       or GRAYSCALE PAM, so there is only one plane.
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42       pamtilt  works  on  bilevel  (PBM, BLACKANDWHITE PAM) images as well as
43       grayscale, but you will minimize artifacts if you scan  and  rotate  in
44       grayscale before you apply a threshold to make a bilevel image.
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OPTIONS

48       A few options have general utility:
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53       -angle=maxangle
54              Assume  a  maximum tilt angle of maxangle (measured in degrees).
55              The default value is sufficient  for  most  images,  even  those
56              scanned somewhat carelessly.
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58              The default is 10.0.
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61       -fast  Skip the third iteration for speed at the expense of accuracy.
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64       -verbose
65              Show  on Standard Error the measurements computed at each tested
66              angle.
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70       Here are some other options you  can  use  to  tune  the  operation  of
71       pamtilt  but  they're  seldom needed.  The default values accommodate a
72       wide variety of input documents.
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77       -quality=q
78              Require a signal-to-noise ratio of a least q on the first itera‐
79              tion to report a valid result.  Larger values reduce the chances
80              of obtaining a bogus result at the risk of obtaining  no  result
81              at all.
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83              The default is 1.0.
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86       -hstep=n
87              Set  the  horizontal  increment to check every nth column.  This
88              value affects both run time and memory requirements.
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90              The default is 11.
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93       -vstep=n
94              Set the vertical increment to check every nth row.  Larger  val‐
95              ues  usually work, reducing run time, but they increase the risk
96              of incorrect results.
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98              The default is 5.
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101       -dstep=n
102              Set the vertical distance used when checking pixels in a column.
103              The  default is intended to minimize the effect of noise along a
104              horizontal boundary.
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106              The default is 2.
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109       -astep=n
110              Set the angle increment of the first iteration, in degrees.
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112              The default is 1.0.
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REFERENCES

119       pamtilt implements a somewhat simplified algorithm inspired  by:  "Mea‐
120       suring  Document  Image Skew and Orientation", by Bloomberg, Kopec, and
121       Dasari.  In SPIE Volume 2422, Document Recognition II,  pages  302-316,
122       February 1995.
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SEE ALSO

126       ·
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128              pnmrotate(1)
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130       ·
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132              pgm(1)
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HISTORY

137       pamtilt was new in Netpbm 10.30 (October 2005).
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139       Gregg  Townsend wrote it and sent it to Bryan Henderson in August 2005.
140       Bryan recoded it to fit Netpbm conventions.
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144netpbm documentation            28 August 2005          Pamtilt User Manual(0)
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