Section: User Commands (1)
Updated: 07 January 2001
ppmchange [ -closeness closeness_percent ] [ -remainder remainder_color ] [ oldcolor newcolor ] ... [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 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.
If a pixel matches two different oldcolors, ppmchange replaces it with the newcolor of the leftmost specified one.
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 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.
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.
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