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 dou-
       ble 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  horizon-
       tal  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

       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 pnmdepth 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.


       hdifftopam(1), pnmdepth(1),


       Bryan Henderson

netpbm documentation             15 April 2002       Pamtohdiff User Manual(0)

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