1Pamtohdiff User Manual(0) Pamtohdiff User Manual(0)
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6 pamtohdiff - convert PAM image to horizontal difference image
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10 pamtohdiff [pamfile] [-verbose]
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14 This program is part of Netpbm(1).
15 pamtohdiff takes a PAM (or PNM) image as input and produces a horizon‐
17 tal difference image version of it as output. A horizontal difference
18 image is one where the samples in each row indicate the difference
19 between the sample value in the corresponding sample of the input image
20 and the sample directly above it (in the previous row) in the input
21 image. The horizontal difference image has the property that if a row
22 of the original image is identical to the row above it over a long
23 extent, the corresponding row in the horizontal difference image will
24 contain all zeroes. That makes it compress better than the original
25 image.
26 Because the horizontal difference samples can be positive or negative,
28 but PAM samples are unsigned integers, the samples in the horizontal
29 difference image PAM are defined to be the difference modulus the range
30 of the input (maxval + 1). This doesn't lose any information, as it
31 might seem, because: of the two differences that could result in the
32 same pamtohdiff output value (e.g. if maxval is 99, +20 and -80 would
33 both result in "20" in the output), only one is possible in context and
34 the other would result, when reconstructing the original image, in a
35 value less than 0 or greater than maxval.
36 Before the modulus operation, the values pamtohdiff computes are also
38 biased by half the maxval. This is to make the results easier to
39 inspect visually. Because of the bias, you can display the pamtohdiff
40 output as if it were a PNM image. As long as none of your differences
41 are more than half the maxval, large negative differences show up as
42 dark spots, smaller negative differences are lighter, zero differences
43 are medium intensity, and positive differences are light. If you want
44 this to work even for images that have differences that exceed half the
45 maxval, just use ppmdim 50 on the original image. To avoid losing
46 information, though, do a pamdepth to double the maxval first.
47 Note that because of the transfer function just described, a difference
49 of zero, which is most common, is represented by a PAM sample value in
50 the output of one half the maxval.
51 The output PAM has a tuple type of "hdiff".
53 You can use hdifftopam to recover the original image from a horizontal
55 difference image PAM.
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60 In addition to the options common to all programs based on libnetpbm
61 (most notably -quiet, see
62 Common Options ⟨index.html#commonoptions⟩ ), pamtohdiff recognizes the
63 following command line option:
64 -verbose
68 Currently no effect. This may change in future versions.
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76 hdifftopam(1), pamdepth(1)
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80 Bryan Henderson
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83 This manual page was generated by the Netpbm tool 'makeman' from HTML
84 source. The master documentation is at
85 http://netpbm.sourceforge.net/doc/pamtohdiff.html
87netpbm documentation 15 April 2002 Pamtohdiff User Manual(0)