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pnmconvol(1)                           General Commands Manual                          pnmconvol(1)



NAME
       pnmconvol - general MxN convolution on a portable anymap

SYNOPSIS
       pnmconvol convolutionfile [pnmfile]

DESCRIPTION
       Reads two portable anymaps as input.  Convolves the second using the first, and writes a por‐
       table anymap as output.

       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.  The unsigned num‐
       bers in the convolution file are offset by -maxval/2 to make signed numbers, and then normal‐
       ized, so the actual values in the convolution file are only relative.

       Here  is a sample convolution file; it does a simple average of the nine immediate neighbors,
       resulting in a smoothed image:
           P2
           3 3
           18
           10 10 10
           10 10 10
           10 10 10

       To see how this works, do the above-mentioned offset: 10 - 18/2 gives 1.  The possible  range
       of values is from 0 to 18, and after the offset that's -9 to 9.  The normalization step makes
       the range -1 to 1, and the values get scaled correspondingly so they  become  1/9  -  exactly
       what  you  want.   The equivalent matrix for 5x5 smoothing would have maxval 50 and be filled
       with 26.

       The convolution file will usually be a graymap, so that the same convolution gets applied  to
       each color component.  However, if you want to use a pixmap and do a different convolution to
       different colors, you can certainly do that.

       At the edges of the convolved image, where the convolution matrix would extend over the  edge
       of the image, pnmconvol just copies the input pixels directly to the output.


SEE ALSO
       pnmsmooth(1), pnm(5)


AUTHORS
       Copyright (C) 1989, 1991 by Jef Poskanzer.
       Modified 26 November 1994 by Mike Burns, burns AT chem.edu



                                          26 November 1994                              pnmconvol(1)