NAME

       pamtohdiff - convert PAM image to horizontal difference image

SYNOPSIS

       pamtohdiff [pamfile] [-verbose]

DESCRIPTION

       This program is part of Netpbm(1).

       pamtohdiff  takes a PAM (or PNM) image as input and produces a horizontal difference image
       version of it as output.  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 differences 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 represented 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.

OPTIONS

       In addition to the options common to all programs based on libnetpbm (most notably -quiet,
       see
        Common Options ⟨index.html#commonoptions⟩ ), pamtohdiff recognizes the following  command
       line option:

       -verbose
              Currently no effect.  This may change in future versions.

SEE ALSO

       hdifftopam(1), pamdepth(1)

AUTHOR

       Bryan Henderson

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/pamtohdiff.html