ppmchange - change all pixels of one color to another in a portable pixmap
ppmchange [ -closeness closeness_percent ] [ -remainder remainder_color ] [ oldcolor new-
color ] ... [ppmfile]
Reads a portable pixmap as input. Changes all pixels of oldcolor to newcolor. You may
specify up to 256 oldcolor/newcolor pairs on the command line. ppmchange leaves all col-
ors not mentioned unchanged, unless you specify the -remainder option, in which case they
are all changed to the single specified color.
You can specify that colors similar, but not identical, to the ones you specify get
replaced by specifying a "closeness" factor.
The colors can be specified in five ways:
o A name, assuming that a pointer to an X11-style color names file was compiled in.
o An X11-style hexadecimal specifier: rgb:r/g/b, where r g and b are each 1- to
4-digit hexadecimal numbers.
o An X11-style decimal specifier: rgbi:r/g/b, where r g and b are floating point num-
bers between 0 and 1.
o For backwards compatibility, an old-X11-style hexadecimal number: #rgb, #rrggbb,
#rrrgggbbb, or #rrrrggggbbbb.
o For backwards compatibility, a triplet of numbers separated by commas: r,g,b, where
r g and b are floating point numbers between 0 and 1. (This style was added before
MIT came up with the similar rgbi style.)
If a pixel matches two different oldcolors, ppmchange replaces it with the newcolor
of the leftmost specified one.
closeness is an integer per centage indicating how close to the color you specified
a pixel must be to get replaced. By default, it is 0, which means the pixel must
be the exact color you specified.
A pixel gets replaced if the distance in color between it and the color you speci-
fied is less than or equal to closeness.
The "distance" in color is defined as the cartesian sum of the individual differ-
ences in red, green, and blue intensities between the two pixels, normalized so
that the difference between black and white is 100%.
This is probably simpler than what you want most the time. You probably would like
to change colors that have similar chrominance, regardless of their intensity. So
if there's a red barn that is variously shadowed, you want the entire barn changed.
But because the shadowing significantly changes the color according to ppmchange's
distance formula, parts of the barn are probably about as distant in color from
other parts of the barn as they are from green grass next to the barn.
Maybe ppmchange will be enhanced some day to do chrominance analysis.
ppmchange changes all pixels which are not of a color for which you specify an
explicit replacement color on the command line to color color.
An example application of this is
ppmchange -remainder=black red red
to lift only the red portions from an image, or
ppmchange -remainder=black red white | ppmtopgm
to create a mask file for the red portions of the image.
pgmtoppm(1), ppmcolormask(1), ppm(5)
Wilson H. Bent. Jr. (firstname.lastname@example.org) with modifications by Alberto Accomazzi
07 January 2001 ppmchange(1)