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Pamtohdiff User Manual(0)                            Pamtohdiff User Manual(0)

       pamtohdiff - convert PAM image to horizontal difference image

       pamtohdiff [pamfile] [-verbose]

       Minimum  unique  abbreviation of option is acceptable.  You may use double hyphens instead of single hyphens to
       denote options.  You may use white space in place of the equals sign to separate an option name from its value.

       This program is part of Netpbm(1).

       pamtohdiff takes a PAM (or PNM) image as input and produced a horizontal difference image version of it as out-
       put.  A horizontal difference image is one where the samples in each row indicate the  difference  between  the
       sample  value  in the corresponding sample of the input image and the sample directly above it (in the previous
       row) in the input image.  The horizontal difference image has the property that if a row of the original  image
       is  identical  to the row above it over a long extent, the corresponding row in the horizontal difference image
       will contain all zeroes.  That makes it compress better than the original image.

       Because the horizontal difference samples can be positive or negative, but PAM samples are  unsigned  integers,
       the  samples  in  the horizontal difference image PAM are defined to be the difference modulus the range of the
       input (maxval + 1).  This doesn't lose any information, as it might seem, because: of the two differences  that
       could  result  in the same pamtohdiff output value (e.g. if maxval is 99, +20 and -80 would both result in "20"
       in the output), only one is possible in context and the other would result, when  reconstructing  the  original
       image, in a value less than 0 or greater than maxval.

       Before  the  modulus  operation, the values pamtohdiff computes are also biased by half the maxval.  This is to
       make the results easier to inspect visually.  Because of the bias, you can display the pamtohdiff output as  if
       it were a PNM image.  As long as none of your differences are more than half the maxval, large negative differ-
       ences show up as dark spots, smaller negative differences are lighter, zero differences are  medium  intensity,
       and positive differences are light.  If you want this to work even for images that have differences that exceed
       half the maxval, just use ppmdim 50 on the original image.  To avoid losing information, though, do a  pamdepth
       to double the maxval first.

       Note  that because of the transfer function just described, a difference of zero, which is most common, is rep-
       resented by a PAM sample value in the output of one half the maxval.

       The output PAM has a tuple type of "hdiff".

       You can use hdifftopam to recover the original image from a horizontal difference image PAM.

       hdifftopam(1), pamdepth(1),

       Bryan Henderson

netpbm documentation             15 April 2002       Pamtohdiff User Manual(0)