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| author | giraffedata <giraffedata@9d0c8265-081b-0410-96cb-a4ca84ce46f8> | 2017-04-30 03:28:11 +0000 |
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| committer | giraffedata <giraffedata@9d0c8265-081b-0410-96cb-a4ca84ce46f8> | 2017-04-30 03:28:11 +0000 |
| commit | cbc0563b6390cc3e920dbf4c7d02bfded95143ec (patch) | |
| tree | c549f8436cd5d1587af3616b51a2d3d6666ff1b7 /pnmconvol.html | |
| parent | 2c074cfcc434e192acd0cdc2457f71b0bd7f33b4 (diff) | |
| download | netpbm-mirror-cbc0563b6390cc3e920dbf4c7d02bfded95143ec.tar.gz netpbm-mirror-cbc0563b6390cc3e920dbf4c7d02bfded95143ec.tar.xz netpbm-mirror-cbc0563b6390cc3e920dbf4c7d02bfded95143ec.zip | |
miscellaneous update
git-svn-id: http://svn.code.sf.net/p/netpbm/code/userguide@2969 9d0c8265-081b-0410-96cb-a4ca84ce46f8
Diffstat (limited to 'pnmconvol.html')
| -rw-r--r-- | pnmconvol.html | 29 |
1 files changed, 20 insertions, 9 deletions
diff --git a/pnmconvol.html b/pnmconvol.html index 6efdc197..b1d28164 100644 --- a/pnmconvol.html +++ b/pnmconvol.html @@ -2,7 +2,7 @@ <HTML><HEAD><TITLE>Pnmconvol User Manual</TITLE></HEAD> <BODY> <H1>pnmconvol</H1> -Updated: 06 August 2014 +Updated: 30 April 2017 <BR> <A HREF="#index">Table Of Contents</A> @@ -50,21 +50,30 @@ by the convolution matrix (sometimes called a convolution kernel), which you supply in one of several ways. See <a href="#convolutionmatrix"> Convolution Matrix</a>. -<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>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. It's often better to deal with the pixels near +an edge by assuming some blank or background color beyond the edge. To do +that, use <b>pnmpad</b> to add a margin all around whose size is half that of +your convolution matrix size, not counting its center, in the same dimension. +(E.g. if your convolution matrix is 5 wide by 3 high, use +<kbd>pnmpad -left=2 -right=2 -top=1 -bottom=1</kbd>). Feed that enlarged +image to <b>pnmconvol</b>, then use <b>pamcut</b> to chop the edges off the +convolved output, getting back to your original image dimensions. (E.g. +<kbd>pamcut -left=2 -right=-2 -top=1 -bottom=-1</kbd>). <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. -<p>To avoid clipping, you may want to scale your input. For example, if your -convolution matrix might produce an output value as much as double the maximum -value in the input, then make sure the maxval of the input (which is also the -maxval of the output) is at least twice the actual maximum value in the input. +<p>To avoid clipping, you may want to scale your input values. For example, +if your convolution matrix might produce an output value as much as double the +maximum value in the input, then make sure the maxval of the input (which is +also the maxval of the output) is at least twice the actual maximum value in +the input. <p>Clipping negative numbers deserves special consideration. If your -convolution matrix includes negative numbers, it is possible for +convolution matrix includes negative numbers, it is possible for <b>pnmconvol</b> to calculate an output pixel as a negative value, which <b>pnmconvol</b> would of course clip to zero, since Netpbm formats cannot represent negative numbers. @@ -404,6 +413,8 @@ the maxval. <B><A HREF="pgmkernel.html">pgmkernel</A></B>, <B><A HREF="pamgauss.html">pamgauss</A></B>, <B><A HREF="pammasksharpen.html">pammasksharpen</A></B>, +<B><A HREF="pnmpad.html">pnmpad</A></B>, +<B><A HREF="pamcut.html">pamcut</A></B>, <B><A HREF="pnm.html">pnm</A></B> <H2 id="authors">AUTHORS</H2> |