ppm.c: Functions to manipulate PPM files in memory

link cfunc
November 17, 1997; Gregg M. Townsend
Requires: Dynamic loading
This file is in the public domain.

These functions manipulate raw (P6) PPM image files in memory.
The images must not contain comment strings.

ppmwidth(s) -- return width of PPM image.
ppmheight(s) -- return height of PPM image.
ppmmax(s) -- return maximum value in PPM header.
ppmdata(s) -- return data portion of PPM image.

ppmimage(s,p,f) -- quantify image s using palette p, with flags f.
   Returns an Icon image string.  Flag "o" selects ordered dithering.
   Defaults:  p="c6",  f="o"

ppmstretch(s,lo,hi,max) -- apply contrast stretch operation
   Returns a PPM string image that results from setting all
   values <= lo to zero, all values >= hi to max, with values
   between scaling linearly.  If hi = lo + 1, this becomes a
   simple threshold operation.  If lo=0 and hi=ppmmax(s), this
   simply scales an image to a new maximum.

   Requirements: 0 <= lo < hi <= ppmmax(s), 1 <= max <= 255.
   Defaults:   lo=0, hi=ppmmax(s), max=255.

ppm3x3(s,a,b,c,d,e,f,g,h,i) -- apply 3x3 convolution to PPM image.
   The matrix of real numbers [[a,b,c],[d,e,f],[g,h,i]] is used
   as a transformation matrix applied independently to the three
   color components of the image.