ppmchange red blue redimage.ppm >blueimage.ppm ppmchange red red -remainder=black myimage.ppm >redblack.ppm ppmchange -closeness=90 white white black black
This program is part of Netpbm.
ppmchange reads a PPM image as input and changes all pixels of colr oldcolor to color 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.
Specify the colors as described for the argument of the ppm_parsecolor() library routine.
If a pixel matches two different oldcolors, ppmchange replaces it with the newcolor of the leftmost specified one.
The maxval of the output image is the same as that of the input image. If a newcolor you specify cannot be exactly represented in that maxval, ppmchange assumes a color that is as close as possible to what you specified but can be represented with your maxval. Unless you specify the -closeok option, ppmchange issues a warning that it is using an approximation.
A common way that you can have this maxval problem, where the color you specify cannot be represented with your maxval, is that your input is a PBM (black and white) image that you are colorizing. The maxval in this case is 1, which severely limits the colors to which you can change. To avoid this problem, use pamdepth to make the maxval of your input something consistent with your colors. 255 is usually a good choice.
Before Netpbm 10.22 (April 2004), ppmchange always behaved as if the user specified -closeok and there was no -closeok option.
A pixel gets replaced if the distance in color between it and the color you specified is less than or equal to closeness per cent of the maxval.
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.
This option was new in Netpbm 10.22 (April 2004). Before that, ppmchange always behaved as if you specified this option.
An example application of this is
ppmchange -remainder=black red redto lift only the red portions from an image, or
ppmchange -remainder=black red white | ppmtopgmto create a mask file for the red portions of the image.