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+\
+.\" This man page was generated by the Netpbm tool 'makeman' from HTML source.
+.\" Do not hand-hack it!  If you have bug fixes or improvements, please find
+.\" the corresponding HTML page on the Netpbm website, generate a patch
+.\" against that, and send it to the Netpbm maintainer.
+.TH "Pamtohdiff User Manual" 0 "15 April 2002" "netpbm documentation"
+
+.SH NAME
+
+pamtohdiff - convert PAM image to horizontal difference image
+
+.UN synopsis
+.SH SYNOPSIS
+
+\fBpamtohdiff\fP
+[\fIpamfile\fP]
+[\fB-verbose\fP]
+
+.UN description
+.SH DESCRIPTION
+.PP
+This program is part of
+.BR "Netpbm" (1)\c
+\&.
+.PP
+\fBpamtohdiff\fP 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.
+.PP
+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 \fBpamtohdiff\fP 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.
+.PP
+Before the modulus operation, the values \fBpamtohdiff\fP
+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 \fBpamtohdiff\fP 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
+\fBppmdim 50\fP on the original image.  To avoid losing information,
+though, do a \fBpamdepth\fP to double the maxval first.
+.PP
+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.
+.PP
+The output PAM has a tuple type of "hdiff".
+.PP
+You can use \fBhdifftopam\fP to recover the original image from a
+horizontal difference image PAM.
+
+
+.UN options
+.SH OPTIONS
+.PP
+In addition to the options common to all programs based on libnetpbm
+(most notably \fB-quiet\fP, see 
+.UR index.html#commonoptions
+ Common Options
+.UE
+\&), \fBpamtohdiff\fP recognizes the following
+command line option:
+
+
+.TP
+\fB-verbose\fP
+Currently no effect.  This may change in future versions.
+
+
+
+
+
+.UN seealso
+.SH SEE ALSO
+.BR "hdifftopam" (1)\c
+\&,
+.BR "pamdepth" (1)\c
+\&
+
+.UN author
+.SH AUTHOR
+
+Bryan Henderson
+.SH DOCUMENT SOURCE
+This manual page was generated by the Netpbm tool 'makeman' from HTML
+source.  The master documentation is at
+.IP
+.B http://netpbm.sourceforge.net/doc/pamtohdiff.html
+.PP
+\ No newline at end of file