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Ppmchange User Manual(0) Ppmchange User Manual(0)
NAME
ppmchange - change all pixels of one color to another in a PPM image
SYNOPSIS
ppmchange
[-closeness=closeness_percent] [-remainder=remainder_color] [-closeok]
[oldcolor newcolor] ... [ppmfile]
EXAMPLES
ppmchange red blue redimage.ppm >blueimage.ppm
ppmchange red red -remainder=black myimage.ppm >redblack.ppm
ppmchange -closeness=90 white white black black
DESCRIPTION
This program is part of Netpbm(1)
ppmchange reads a PPM image as input and changes all pixels of color
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 <libppm.html#colorname> .
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.
OPTIONS
-closeness closeness_percent
closeness is an integer percentage 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 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 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.
This option was new in Netpbm 9.8 (September 2000).
-closeok
This option affects how ppmchange interprets a color you specify
in the arguments. When you specify this option, ppmchange may
use a color close to, but not the same as what you specify. See
the description section <#description> for details.
This option was new in Netpbm 10.22 (April 2004). Before that,
ppmchange always behaved as if you specified this option.
-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(1) , ppmcolormask(1) , ppm(5)
AUTHOR
Wilson H. Bent. Jr. (whb@usc.edu) with modifications by Alberto Acco-
mazzi (alberto@cfa.harvard.edu)
netpbm documentation September 2005 Ppmchange User Manual(0)
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