Manual: pamtohdiff

Pamtohdiff User Manual(1)			   General Commands Manual			   Pamtohdiff User Manual(1)

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	 im‐
       age.   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  (max‐
       val  + 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)