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Some notes and rules of thumb on using PBM, PGM, PPM, and PNM:
To make a full-screen version of an image, or as close to it as you can
get, compute the ratio x/y for your image and for your screen. If the
ratio is higher for your image, do a ppmscale -xsize <screenwidth>; if
your image's ratio is lower, do a ppmscale -ysize <screenheight>.
Even margins: an easy way to get nice equal-sized margins on all sides
of a non-full-screen image is to pnmcrop it, get the size and add 10%
or so, pbmmake a bitmap of that size, and pnmpaste the image into the
bitmap.
Smoothing when enlarging: if you pnmenlarge or ppmscale up by a factor
of three or more, you should add a ppmsmooth step. Otherwise you can
see the original pixels in the resulting image.
Rotating: as mentioned in their manual entries, pnmtrnspos and
pnmfliptb / pnmfliplr can be used for 90 degree rotations. Ppmrotate
can be used for other angles between -90 and 90. To go all the way
around the circle, combine these two techniques.
When using pbmpaste's logical operations to do masking, it is important
to remember that while the PBM file format represents black as 1 and
white as 0, pbmpaste acts as if the reverse is true: black is 0 and
white is 1. See the pbmmask manual entry for examples.
Enhancing contrast in a grayscale image: do a pgmhist, and look for
likely values of -bvalue and -wvalue to use with pgmnorm. If you just
want to enhance the contrast, then choose values at elbows in the
histogram; e.g. if value 29 represents 3% of the image but value 30
represents 20%, choose 30 for bvalue. If you want to lighten the
image, then set bvalue to 0 and just fiddle with wvalue; similarly, to
darken the image, set wvalue to maxval and play with bvalue.
Avoid odd-numbered widths. Displaying them causes problems on some machines.
To turn a Usenix FaceSaver image into PBM, first do an "fstopgm > /dev/null".
This will tell you whether you need to use ppmscale. Then use one of the
following pipelines:
fstopgm | pnmenlarge -3 | ppmtopgm | pgmnorm | pgmtopbm
fstopgm | pnmenlarge -3 | ppmscale -whatever | ppmtopgm | pgmnorm | pgmtopbm
Enlarging by more than 3 does not look good.
Re-halftoning with pbmreduce: Let's say you have a scanner that only
produces black&white, not grayscale, and it does a terrible job of
halftoning (most scanners fit this description). One way to fix the
halftoning is to scan at the highest possible resolution, say 300 dpi,
and then reduce by a factor of three or so using pbmreduce. Pbmreduce
duplicates a lot of the functionality of pgmtopbm -- you could do
something like ppmscale | ppmtopgm | pgmtopbm, but pbmreduce is a lot
faster and doesn't use as much memory. You can even correct the
brightness of an image with pbmreduce, by using the -value flag.
If you compiled with the RAWBITS option, you may sometimes want some
non-raw output. Maybe you want to use sed to change one color to
another, or something. Here's one way to get this conversion: unset
RAWBITS; choose one of the PNM programs that can be used like "cat",
for instance, pnmflip; compile the libraries, compile pnmflip; rename
your non-raw pnmflip to something like pnmunraw; set RAWBITS again, and
recompile things.