This is a faster version of the pnmconvol.c program that comes with netpbm.
There are no changes to the command line arguments, so this program can be
dropped in without affecting the way you currently run it.  An updated man
page is also included.

My original intention was to improve the running time of applying a
neighborhood averaging convolution matrix to an image by using a different
algorithm, but I also improved the run time of performing the general
convolution by optimizing that code.  The general convolution runs in 1/4 to
1/2 of the original time and neighborhood averaging runs in near constant
time for the convolution masks I tested (3x3, 5x5, 7x7, 9x9).

Sample times for two computers are below.  Times are in seconds as reported
by /bin/time for a 512x512 pgm image.

Matrix                  IBM RS6000      SUN IPC
Size & Type                220

3x3
original pnmconvol         6.3            18.4
new general case           3.1             6.0
new average case           1.8             2.6

5x5
original pnmconvol        11.9            44.4
new general case           5.6            11.9
new average case           1.8             2.6

7x7
original pnmconvol        20.3            82.9
new general case           9.4            20.7
new average case           1.8             2.6

9x9
original pnmconvol        30.9           132.4
new general case          14.4            31.8
new average case           1.8             2.6


Send all questions/comments/bugs to me at burns@chem.psu.edu.

- Mike