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| author | giraffedata <giraffedata@9d0c8265-081b-0410-96cb-a4ca84ce46f8> | 2006-12-25 03:06:05 +0000 |
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| committer | giraffedata <giraffedata@9d0c8265-081b-0410-96cb-a4ca84ce46f8> | 2006-12-25 03:06:05 +0000 |
| commit | 1017cbebe5d5edd859e0fddad0a8600f509f4821 (patch) | |
| tree | 78bdf336648566f7a7d55f42837357dea3dd674c /pamtohdiff.html | |
| parent | 16f2ac126651015a376eba864a3a35f738b0b25a (diff) | |
| download | netpbm-mirror-1017cbebe5d5edd859e0fddad0a8600f509f4821.tar.gz netpbm-mirror-1017cbebe5d5edd859e0fddad0a8600f509f4821.tar.xz netpbm-mirror-1017cbebe5d5edd859e0fddad0a8600f509f4821.zip | |
Place user guide into Subversion repository
git-svn-id: http://svn.code.sf.net/p/netpbm/code/userguide@181 9d0c8265-081b-0410-96cb-a4ca84ce46f8
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1 files changed, 97 insertions, 0 deletions
diff --git a/pamtohdiff.html b/pamtohdiff.html new file mode 100644 index 00000000..8bd12dc6 --- /dev/null +++ b/pamtohdiff.html @@ -0,0 +1,97 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2//EN"> +<HTML><HEAD><TITLE>Pamtohdiff User Manual</TITLE></HEAD> +<BODY> +<H1>pamtohdiff</H1> +Updated: 15 April 2002 +<BR> +<A HREF="#index">Table Of Contents</A> + +<A NAME="lbAB"> </A> +<H2>NAME</H2> + +pamtohdiff - convert PAM image to horizontal difference image + +<A NAME="lbAC"> </A> +<H2>SYNOPSIS</H2> + +<B>pamtohdiff</B> +[<I>pamfile</I>] +[<B>-verbose</B>] + +<P> +Minimum unique abbreviation of option is acceptable. You may use double +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. + +<A NAME="lbAD"> </A> +<H2>DESCRIPTION</H2> + +<p>This program is part of <a href="index.html">Netpbm</a>. + +<P><B>pamtohdiff</B> takes a PAM (or PNM) image as input and produced 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. + +<P>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 <b>pamtohdiff</b> 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. + +<p>Before the modulus operation, the values <b>pamtohdiff</b> +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 <b>pamtohdiff</b> 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 +<b>ppmdim 50</b> on the original image. To avoid losing information, +though, do a <b>pamdepth</b> to double the maxval first. + +<p>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. + +<P>The output PAM has a tuple type of "hdiff". + +<P>You can use <B>hdifftopam</B> to recover the original image from a +horizontal difference image PAM. + + +<A NAME="lbAF"> </A> +<H2>SEE ALSO</H2> + +<B><A HREF="hdifftopam.html">hdifftopam</A></B>, +<B><A HREF="pamdepth.html">pamdepth</A></B>, + +<A NAME="lbAG"> </A> +<H2>AUTHOR</H2> + +Bryan Henderson + +<HR> +<A NAME="index"> </A> +<H2>Table Of Contents</H2> +<UL> +<LI><A HREF="#lbAB">NAME</A> +<LI><A HREF="#lbAC">SYNOPSIS</A> +<LI><A HREF="#lbAD">DESCRIPTION</A> +<LI><A HREF="#lbAF">SEE ALSO</A> +<LI><A HREF="#lbAG">AUTHOR</A> +</UL> +</BODY> +</HTML> |