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Pnmremap User Manual(0) Pnmremap User Manual(0)
NAME
pnmremap - replace colors in a PNM image with colors from another set
SYNOPSIS
pnmremap
-mapfile=palettefile
[-floyd|-fs|-nfloyd|-nofs]
[-norandom]
[-firstisdefault]
[-verbose]
[-missingcolor=color]
[pnmfile]
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
value.
DESCRIPTION
This program is part of Netpbm(1)
pnmremap replaces the colors in an input image with those from a pal-
ette you specify. Where colors in the input are present in the pal-
ette, they just stay the same in the output. But where the input con-
tains a color that is not in the palette, pnmremap gives you these
choices:
o Choose the closest color from the palette.
o Choose the first color from the palette.
o Use a color specified by a command option (-missing).
o Dither. This 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 (for
another kind of dithering, see ppmdither).
Two reasons to use this program 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 Recommendation 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 pix-
els of the same color are fine. However, a palette image is typically
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
colors.
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). You can use ppmtoppm to increase
the depth of the input image to work around this limitation.
In any case, the output image has the same tuple type and depth as the
palette image.
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
the first.
Examples
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.
PARAMETERS
There is one parameter, which is required: The file specification of
the input PNM file.
OPTIONS
-mapfile=palettefilename
This names the file that contains the palette image.
This option is mandatory.
-floyd
-fs
-nofloyd
-nofs These options determine whether pnmremap does Floyd-Steinberg
dithering. Without Floyd-Steinberg, pnmremap selects the output
color of a pixel based on the color of only the corresponding
input pixel. With Floyd-Steinberg, pnmremap considers regions
of pixels such that the average color of a region is the same in
the output as in the input. The dithering effect appears as a
dot pattern up close, but from a distance, the dots blend so
that you see more colors than are present in the color map.
As an example, if your color map contains only black and white,
and the input image has 4 adjacent pixels of gray, pnmremap with
Floyd-Steinberg would generate output pixels black, white,
black, white, which from a distance looks gray. But without
Floyd-Steinberg, pnmremap would generate 4 white pixels, white
being the single-pixel approximation of gray.
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 synonym for -floyd. -nofs is a synonym for -nofloyd.
The default is -nofloyd.
Before Netpbm 10.46 (March 2009), dithering doesn't work quite
as you expect if the color map has a lower maxval than the
input. pnmremap reduces the color resolution to the color map's
maxval before doing any dithering, so the dithering does not
have the effect of making the image, at a distance, appear to
have the original maxval. In current Netpbm, it does.
-norandom
This option affects a detail of the Floyd-Steinberg dithering
process. It has no effect if you aren't doing Floyd-Steinberg
dithering.
By default, pnmremap initializes the error propagation accumula-
tor to random values to avoid the appearance of unwanted pat-
terns. This is an extension of the original Floyd-Steinberg
algorithm.
A drawback of this is that the same pnmremap on the same input
produces slightly different output every time, which makes com-
parison difficult.
With -norandom, pnmremap initializes the error accumulators to
zero and the output is completely predictable.
-norandom was new in Netpbm 10.39 (June 2007).
-firstisdefault
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 <#description> .
If you specify -firstisdefault, the maxval of your input must
match the maxval of your palette image.
-missingcolor=color
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-
less.
If you specify -missingcolor, the maxval of your input must
match the maxval of your palette image.
-verbose
Display helpful messages about the mapping process.
SEE ALSO
pnmcolormap(1) , pamseq(1) , pnmquant(1) , ppmquantall(1) , pamdepth(1)
, ppmdither(1) , ppmquant(1) , ppm(5)
HISTORY
pnmremap first appeared in Netpbm 9.23 (January 2002). Before that,
its function was available only as part of the function of pnmquant
(which was derived from the much older ppmquant). Color quantization
really has two main subfunctions, so Netpbm 9.23 split it out into two
separate programs: pnmcolormap and pnmremap and then Netpbm 9.24
replaced pnmquant with a program that simply calls pnmcolormap and pnm-
remap.
AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer.
netpbm documentation 03 June 2009 Pnmremap User Manual(0)
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