Pnmremap User Manual(0) Pnmremap User Manual(0)
pnmremap - replace colors in a PNM image with colors from another set
All options can be abbreviated to their shortest unique prefix. You
may use two hyphens instead of one to designate an option. You may use
either white space or an equals sign between an option name and its
This program is part of Netpbm(1).
pnmremap replaces the colors in an input image with those from a
palette you specify. Where colors in the input are present in the
palette, they just stay the same in the output. But where the input
contains a color that is not in the palette, pnmremap gives you three
choices: 1) choose the closest color from the palette; 2) choose the
first color from the palette; 3) use a color specified by a command
You can also dither, which means rather than mapping pixel by pixel,
pnmremap uses colors from the palette to try to make multi-pixel
regions of the output have the same average color as the input.
Two reasons to do this are: 1) you want to reduce the number of colors
in the input image; and 2) you need to feed the image to something that
can handle only certain colors.
To reduce colors, you can generate the palette with pnmcolormap.
By default, pnmremap maps an input color that is not in the palette to
the closest color that is in the palette. Closest means with the
smallest cartesian distance in the red, green, blue brightness space
(smallest sum of the squares of the differences in red, greeN, and blue
ITU-R Recommedation BT.709 gamma-adjusted intensities).
You can instead specify a single default color for pnmremap to use for
any color in the input image that is not in the palette. Use the
-missing option for this.
You can also specify that the first color in the palette image is the
default. Use the -firstisdefault option for this.
The palette is simply a PNM image. The colors of the pixels in the
image are the colors in the palette. Where the pixels appear in the
image, and the dimensions of the image, are irrelevant. Multiple
pixels of the same color are fine. However, a palette image is typi-
cally a single row with one pixel per color.
If you specify -missing, the color you so specify is in the palette in
addition to whatever is in the palette image.
For historical reasons, Netpbm sometimes calls the palette a ’col-
ormap.’ But it doesn’t really map anything. pnmremap creates its own
map, based on the palette, to map colors from the input image to output
Palette/Image Type Mismatch
In the simple case, the palette image is of the same depth (number of
planes, i.e. number of components in each tuple (pixel)) as the input
image and pnmremap just does a straightforward search of the palette
for each input tuple (pixel). In fact, pnmremap doesn’t even care if
the image is a visual image.
But what about when the depths differ? In that case, pnmremap converts
the input image (in its own memory) to match the palette and then pro-
ceeds as above.
There are only two such cases in which pnmremap knows how to do the
conversion: when one of them is tuple type RGB, depth 3, and the other
is tuple type GRAYSCALE or BLACKANDWHITE, depth 1; and vice versa.
In any other case, pnmremap issues and error message and fails.
Note that as long as your input and palette images are PNM, they’ll
always fall into one of the cases pnmremap can handle. There’s an
issue only if you’re using some exotic PAM image.
Before Netpbm 10.27 (March 2005), pnmremap could not handle the case of
a palette of greater depth than the input image. (It would issue an
error message and fail in that case).
In any case, the output image has the same tuple type and depth as the
Multiple Image Stream
pnmremap handles a multiple image input stream, producing a multiple
image output stream. The input images need not be similar in any way.
Before Netpbm 10.30 (October 2005), pnmremap ignored any image after
pnmcolormap testimg.ppm 256 >palette.ppm
pnmremap -map=palette.ppm testimg.ppm >reduced_testimg.ppm
To limit colors to a certain set, a typical example is to create an
image for posting on the World Wide Web, where different browsers know
different colors. But all browsers are supposed to know the 216 ’web
safe’ colors which are essentially all the colors you can represent in
a PPM image with a maxval of 5. So you can do this:
pamseq 3 5 >websafe.pam
pnmremap -map=websafe.pam testimg.ppm >websafe_testimg.ppm
Another useful palette is one for the 8 color IBM TTL color set, which
you can create with
pamseq 3 1 >ibmttl.pam
If you want to quantize one image to use the colors in another one,
just use the second one as the palette. You don’t have to reduce it
down to only one pixel of each color, just use it as is.
The output image has the same type and maxval as the palette image.
There is one parameter, which is required: The file specification of
the input PNM file.
This names the file that contains the palette image.
This option is mandatory.
-nofs These options determine whether Floyd-Steinberg dithering is
done. Without Floyd-Steinberg, the selection of output color of
a pixel is based on the color of only the corresponding input
pixel. With Floyd-Steinberg, multiple input pixels are consid-
ered so that the average color of an area tends to stay more the
same than without Floyd-Steinberg. For example, if you map an
image with a black, gray, gray, and white pixel adjacent, to a
palette that contains only black and white, it might result in
an output of black, black, white, white. Pixel-by-pixel mapping
would instead map both the gray pixels to the same color.
Floyd-Steinberg gives vastly better results on images where
unmodified quantization has banding or other artifacts, espe-
cially when going to a small number of colors such as the above
IBM set. However, it does take substantially more CPU time.
-fs is a synomym for -floyd. -nofs is a synonym for -nofloyd.
The default is -nofloyd.
This tells pnmremap to map any input color that is not in the
palette to the first color in the palette (the color of the
pixel in the top left corner of the palette image)
See DESCRIPTION .
If you specify -firstisdefault, the maxval of your input must
match the maxval of your palette image.
This specifies the default color for pnmremap to map to a color
in the input image that isn’t in the palette. color may or may
not be in the palette image; it is part of the palette regard-
If you specify -missingcolor, the maxval of your input must
match the maxval of your palette image.
Display helpful messages about the mapping process.
pnmcolormap(1), pamseq(1), pnmquant(1), ppmquantall(1), pnmdepth(1),
ppmdither(1), ppmquant(1), ppm(1)
Copyright (C) 1989, 1991 by Jef Poskanzer.
netpbm documentation 01 January 2002 Pnmremap User Manual(0)
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