ppmchange Man page

ppmchange General Commands Manual ppmchange

NAME

ppmchange – change all pixels of one color to another in a portable
pixmap

SYNOPSIS

ppmchange [ -closeness closeness_percent ] [ -remainder remainder_color
] [ oldcolor newcolor ] … [ppmfile]

DESCRIPTION

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 colors 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 numbers between 0 and 1.

o For backwards compatibility, an old-X11-style hexadecimal num‐
ber: #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.

OPTIONS

-closeness closeness_percent
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 specified is less than or equal to closeness.

The “distance” in color is defined as the cartesian sum of the
individual differences 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 chromi‐
nance, 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.

-remainder color
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.

SEE ALSO

pgmtoppm, ppmcolormask, ppm(5)

AUTHOR

Wilson H. Bent. Jr. (whb@usc.edu) with modifications by Alberto Acco‐
mazzi (alberto@cfa.harvard.edu)

07 January 2001 ppmchange

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