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 ), 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

netpbm documentation                              15 April 2002                        Pamtohdiff User Manual(1)