man > pnmremap(1)

Pnmremap User Manual(1)                General Commands Manual               Pnmremap User Manual(1)

NAME
       pnmremap - replace colors in a PNM image with colors from another set


SYNOPSIS
       pnmremap

       -mapfile=palettefile

       [-floyd|-fs|-nfloyd|-nofs]

       {[-norandom]|randomseed=n}

       [-firstisdefault]

       [-verbose]

       [-missingcolor=colorspec]

       [pnmfile]

       All  options can be abbreviated to their shortest unique prefix.  You may use two hyphens in‐
       stead of one to designate an option.  You may use either white space or an  equals  sign  be‐
       tween 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 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 these
       choices:



       •      Choose the closest color from the palette.


       •      Choose the first color from the palette.


       •      Use a color specified by a command option (-missing).


       •      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  ir‐
       relevant.  Multiple pixels 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 "colormap." But it  doesn't  re‐
       ally  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 proceeds 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  sup‐
       posed  to  know the 216 "web safe" colors which are essentially all the colors you can repre‐
       sent 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
       In addition to the options common to all programs based on libnetpbm  (most  notably  -quiet,
       see ]8;;index.html#commonoptions\ Common Options]8;;\ ), pnmremap recognizes the following command line 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, especially 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  ef‐
              fect if you aren't doing Floyd-Steinberg dithering.

              By default, pnmremap initializes the error propagation accumulator to random values to
              avoid  the  appearance  of  unwanted  patterns.   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  dif‐
              ferent output every time, which makes comparison difficult.

              With  -norandom, pnmremap initializes the error accumulators to zero and the output is
              completely predictable.

              Alternatively, you can use -randomseed to get  randomization  across  the  image,  but
              still have repeatable results.

              You cannot specify this along with -randomseed.

              -norandom was new in Netpbm 10.39 (June 2007).



       -randomseed=n
              This  option affects a detail of the Floyd-Steinberg dithering process.  It has no ef‐
              fect if you aren't doing Floyd-Steinberg dithering.

              This option supplies the seed for the random number generator used in  the  randomiza‐
              tion  process  described  in the explanation of the -norandom option.  If you run pnm‐
              remap twice with the same -randomseed value, you will get identical results.

              If you do not specify -randomseed, pnmremap chooses a seed at random,  adding  another
              level of randomness to the dithering.

              You cannot specify this along with -norandom.

              This option was new in Netpbm 10.82 (March 2018).



       -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 im‐
              age)

              See ]8;;#description\DESCRIPTION]8;;\ .

              If you specify -firstisdefault, the maxval of your input must match the maxval of your
              palette image.


       -missingcolor=colorspec
              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 regardless.

              colorspec is as described for the ]8;;libnetpbm_image.html#colorname\argument of the pnm_parsecolor() library routine]8;;\ .

              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),  pamlookup(1),  pnmquant(1),  ppmquantall(1),  pamdepth(1), ppmdither(1), pp‐
       mquant(1), pamseq(1), ppm(1)


HISTORY
       pnmremap first appeared in Netpbm 9.23 (January 2002).  Before that, its function was  avail‐
       able  only  as  part  of  the function of pnmquant (which was derived from the much older pp‐
       mquant).  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 pnmremap.


AUTHOR
       Copyright (C) 1989, 1991 by Jef Poskanzer.

DOCUMENT SOURCE
       This manual page was generated by the Netpbm tool 'makeman' from  HTML  source.   The  master
       documentation is at

              http://netpbm.sourceforge.net/doc/pnmremap.html

netpbm documentation                      13 November 2014                   Pnmremap User Manual(1)