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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 /pnmconvol.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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diff --git a/pnmconvol.html b/pnmconvol.html new file mode 100644 index 00000000..cce61cde --- /dev/null +++ b/pnmconvol.html @@ -0,0 +1,163 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2//EN"> +<HTML><HEAD><TITLE>Pnmconvol User Manual</TITLE></HEAD> +<BODY> +<H1>pnmconvol</H1> +Updated: 29 June 2005 +<BR> +<A HREF="#index">Table Of Contents</A> + +<A NAME="lbAB"> </A> +<H2>NAME</H2> + +pnmconvol - general MxN convolution on a PNM image + +<A NAME="lbAC"> </A> +<H2>SYNOPSIS</H2> + +<B>pnmconvol</B> + +<I>convolution_matrix_file</I> +[-nooffset] +[<I>pnmfile</I>] + +<p>Minimum unique abbreviation of option is acceptable. You may use double +hyphens instead of single hyphen 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>pnmconvol</b> reads two PNM images as input, convolves the +second using the first, and writes a PNM image as output. + +<P>Convolution means replacing each pixel with a weighted average of +the nearby pixels. The weights and the area to average are determined +by the convolution matrix (sometimes called a convolution kernel), +which you supply by way of the PNM image in the file you identify with +the <i>convolution_matrix_file</i> argument. There are two ways +<b>pnmconvol</b> interprets the PNM convolution matrix image pixels as +weights: with offsets, and without offsets. + +<p>The simpler of the two is without offsets. That is what happens +when you specify the <b>-nooffset</b> option. In that case, +<b>pnmconvol</b> simply normalizes the sample values in the PNM image +by dividing by the maxval. + +<p>For example, here is a sample convolution file that causes an output pixel +to be a simple average of its corresponding input pixel and its 8 neighbors, +resulting in a smoothed image: + +<PRE> + P2 + 3 3 + 18 + 2 2 2 + 2 2 2 + 2 2 2 +</PRE> + +<p>(Note that the above text is an actual PGM file -- you can cut and paste +it. If you're not familiar with the plain PGM format, see +<a href="pgm.html">the PGM format specification</a>). + +<p><b>pnmconvol</b> divides each of the sample values (2) by the maxval +(18) so the weight of each of the 9 input pixels gets is 1/9, which is +exactly what you want to keep the overall brightness of the image the +same. <b>pnmconvol</b> creates an output pixel by multiplying the +values of each of 9 pixels by 1/9 and adding. + +<p>Note that with maxval 18, the range of possible values is 0 to 18. +After scaling, the range is 0 to 1. + +<p>For a normal convolution, where you're neither adding nor +subtracting total value from the image, but merely moving it around, +you'll want to make sure that all the scaled values in (each plane of) +your convolution PNM add up to 1, which means all the actual sample +values add up to the maxval. + +<p>When you <em>don't</em> specify <b>-nooffset</b>, <b>pnmconvol</b> +applies an offset, the purpose of which is to allow you to indicate +negative weights even though PNM sample values are never negative. In +this case, <b>pnmconvol</b> subtracts half the maxval from each sample +and then normalizes by dividing by half the maxval. So to get the +same result as we did above with <b>-nooffset</b>, the convolution +matrix PNM image would have to look like this: + +<PRE> + P2 + 3 3 + 18 + 10 10 10 + 10 10 10 + 10 10 10 +</PRE> + +<P>To see how this works, do the above-mentioned offset: 10 - 18/2 +gives 1. The normalization step divides by 18/2 = 9, which makes it +1/9 - exactly what you want. The equivalent matrix for 5x5 smoothing +would have maxval 50 and be filled with 26. + +<p>Note that with maxval 18, the range of possible values is 0 to 18. +After offset, that's -9 to 9, and after normalizing, the range is -1 to 1. + +<p>For a normal convolution, where you're neither adding nor +subtracting total value from the image, but merely moving it around, +you'll want to make sure that all the offset, scaled values in (each +plane of) your convolution PNM add up to 1. That means the actual +sample values, less half the maxval, add up to half the maxval as in +the example above. + +<P>The convolution file will usually be a PGM, so that the same +convolution gets applied to each color component. However, if you +want to use a PPM and do a different convolution to different +colors, you can certainly do that. + +<P>At the edges of the convolved image, where the convolution matrix +would extend over the edge of the image, <B>pnmconvol</B> just copies +the input pixels directly to the output. + +<p>The convolution computation can result in a value which is outside the +range representable in the output. When that happens, <b>pnmconvol</b> just +clips the output, which means brightness is not conserved. + +<A NAME="history"> </A> +<H2>HISTORY</H2> +<p>The <b>-nooffset</b> option was new in Netpbm 10.23 (July 2004). + + +<A NAME="lbAE"> </A> +<H2>SEE ALSO</H2> + +<B><A HREF="pnmsmooth.html">pnmsmooth</A></B>, +<B><A HREF="pgmmorphconv.html">pgmmorphconv</A></B>, +<B><A HREF="pnmnlfilt.html">pnmnlfilt</A></B>, +<B><A HREF="pgmkernel.html">pgmkernel</A></B>, +<B><A HREF="pamgauss.html">pamgauss</A></B>, +<B><A HREF="pnm.html">pnm</A></B> + +<A NAME="lbAF"> </A> +<H2>AUTHORS</H2> + +Copyright (C) 1989, 1991 by Jef Poskanzer. +<BR> + +Modified 26 November 1994 by Mike Burns, <A +HREF="mailto:burns@chem.psu.edu">burns@chem.psu.edu</A> + + +<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="#lbAE">SEE ALSO</A> +<LI><A HREF="#history">HISTORY</A> +<LI><A HREF="#lbAF">AUTHORS</A> +</UL> +</BODY> +</HTML> |