1Pnmcolormap User Manual(0)                          Pnmcolormap User Manual(0)
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NAME

6       pnmcolormap - create quantization color map for a Netpbm image
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SYNOPSIS

10       pnmcolormap
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12       [-center|-meancolor|-meanpixel]
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14       [-spreadbrightness|-spreadluminosity]
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16       [-sort]
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18       [-square]
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20       ncolors|all
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22       [pnmfile]
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DESCRIPTION

26       This program is part of Netpbm(1).
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28       pnmcolormap  reads  a PNM or PAM image as input, chooses ncolors colors
29       to best represent the image and writes a PNM color map defining them as
30       output.   A PAM image may actually contain tuples of any kind, but pnm‐
31       colormap's concept of the tuple values that  best  represent  the  ones
32       present  in the image may not make sense if the tuple type isn't RGB or
33       GRAYSCALE.  The design of the program, and the  rest  of  this  manual,
34       assumes the tuples represent colors.
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36       You  can  use  this map as input to pnmremap on the same input image to
37       quantize the colors in that image, I.e. produce a  similar  image  with
38       fewer  colors.   pnmquant  does both the pnmcolormap and pnmremap steps
39       for you.
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41       A PNM colormap is a PNM image of any dimensions that contains at  least
42       one  pixel  of each color in the set of colors it represents.  The ones
43       pnmcolormap generates have exactly one  pixel  of  each  color,  except
44       where padding is necessary with the -square option.
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46       The  quantization  method is Heckbert's "median cut".  See QUANTIZATION
47       METHOD ⟨#quant⟩ .
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49       The output image is of the same format (PBM,  PGM,  PPM,  PAM)  as  the
50       input image.  Note that a colormap of a PBM image is not very interest‐
51       ing.
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53       The colormap generally has the same maxval as the input image, but pnm‐
54       colormap  may  reduce  it if there are too many colors in the input, as
55       part of its quantization algorithm.
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57       pnmcolormap works on a multi-image input stream.  In that case, it pro‐
58       duces  one  colormap  that  applies  to all of the colors in all of the
59       input images.  All the images must have the  same  format,  depth,  and
60       maxval  (but  may  have different height and width).  This is useful if
61       you need to quantize a bunch of images that will form a movie or other‐
62       wise  be  used together -- you generally want them all to draw from the
63       same palette, whereas computing a colormap separately from  each  image
64       would  make  the  same  color  in  two  images map to different colors.
65       Before Netpbm 10.31 (December  2005),  pnmcolormap  ignored  any  image
66       after the first.
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68       If  you want to create a colormap without basing it on the colors in an
69       input image, pamseq, ppmmake, and pnmcat can be useful.
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PARAMETERS

73       The single parameter, which is required, is the number  of  colors  you
74       want  in the output colormap.  pnmcolormap may produce a color map with
75       slightly fewer colors than that.  You may specify all to get a colormap
76       of  every color in the input image (no quantization).  When you specify
77       all, the function is essentially the same  as  that  of  ppmhist  -map.
78       ppmhist is much older.
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OPTIONS

83       All  options  can  be abbreviated to their shortest unique prefix.  You
84       may use two hyphens instead of one to designate an option.  You may use
85       either  white  space  or  an equals sign between an option name and its
86       value.
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90       -sort  This option causes the output colormap to be sorted by  the  red
91              component  intensity, then the green, then the blue in ascending
92              order.  This is an insertion sort, so it is  not  very  fast  on
93              large  colormaps.   Sorting  is  useful because it allows you to
94              compare two sets of colors.
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97       -square
98              By default, pnmcolormap produces as the color map  a  PPM  image
99              with one row and with one column for each color in the colormap.
100              This option causes pnmcolormap instead to produce  a  PPM  image
101              that  is within one row or column of being square, with the last
102              pixel duplicated as necessary to create a number of pixels which
103              is such an almost-perfect square.
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106       -verbose
107              This  option  causes pnmcolormap to display messages to Standard
108              Error about the quantization..TP -center
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111       -meancolor
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114       -meanpixel
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117       -spreadbrightness
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120       -spreadluminosity
121              These  options  control   the   quantization   algorithm.    See
122              QUANTIZATION METHOD ⟨#quant⟩ .
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QUANTIZATION METHOD

129       A  quantization  method is a way to choose which colors, being fewer in
130       number than in the input, you want in  the  output.   pnmcolormap  uses
131       Heckbert's "median cut" quantization method.
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133       This method involves separating all the colors into "boxes," each hold‐
134       ing colors that represent about the same number of pixels.   You  start
135       with  one  box  and split boxes in two until the number of boxes is the
136       same as the number of colors you want in the  output,  and  choose  one
137       color to represent each box.
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139       When  you split a box, you do it so each sub-box has the same number of
140       pixels (except one sub-box has more if the full box has an odd number),
141       with the 'greatest' pixels in one sub-box and the 'least' pixels in the
142       other.  "Greater," for a particular box, means it is  brighter  in  the
143       color component (red, green, blue) which has the largest spread in that
144       box.  pnmcolormap gives you two ways to define  "largest  spread.":  1)
145       largest  spread of brightness; 2) largest spread of contribution to the
146       luminosity of the color.  E.g. red is weighted  much  more  than  blue.
147       Select  among  these  with  the -spreadbrightness and -spreadluminosity
148       options.  The default is -spreadbrightness.  Where there  are  multiple
149       colors  of the median magnitude, they are distributed arbitrarily among
150       between the subboxes.  This arbitrary distribution  depends  upon  what
151       the system's qsort function does with multiple equal values, so pnmcol‐
152       ormap may produce slightly different results on different systems.
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154       pnmcolormap provides three ways of choosing a color to represent a box:
155       1)  the center color - the color halfway between the greatest and least
156       colors in the box, using the above definition of "greater"; 2) the mean
157       of the colors (each component averaged separately by brightness) in the
158       box; 3) the mean weighted by the number of pixels of  a  color  in  the
159       image.
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161       Select  among  these  with  the  -center,  -meancolor,  and  -meanpixel
162       options.  The default is -center.
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164       Note that in all three methods, there may be colors in the output which
165       do not appear in the input at all.
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167       Also  note  that  the color chosen to represent the colors in Box A the
168       best may also represent a color in Box B better than the  color  chosen
169       to  represent  the  colors in Box B the best.  This is true for various
170       measures of goodness of representation of one  color  by  another.   In
171       particular,  if  you  use  pnmremap to map the colors in the very image
172       that you used to create the color map to the colors in  that  colormap,
173       the  colors  in  Box B will often map to the color pnmcolormap chose to
174       represent some other box and in fact the  color  pnmcolormap  chose  to
175       represent Box B may not appear in the pnmremap output at all.
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REFERENCES

180       "Color  Image  Quantization for Frame Buffer Display" by Paul Heckbert,
181       SIGGRAPH '82 Proceedings, page 297.
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SEE ALSO

185       pnmremap(1), pnmquant(1), ppmquantall(1), pamgetcolor(1),  pamdepth(1),
186       ppmdither(1), pamseq(1), ppmmake(1), pnmcat(1), ppm(1)
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HISTORY

190       Before  Netpbm  10.15  (April 2003), pnmcolormap used a lot more memory
191       for large images because it kept the  entire  input  image  in  memory.
192       Now,  it  processes  it  a row at a time, but because it sometimes must
193       make multiple passes through the image, it first copies the input  into
194       a temporary seekable file if it is not already in a seekable file.
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196       pnmcolormap first appeared in Netpbm 9.23 (January 2002).  Before that,
197       its function was available only as part of  the  function  of  pnmquant
198       (which  was  derived from the much older ppmquant).  Color quantization
199       really has two main subfunctions, so Netpbm 9.23 split it out into  two
200       separate  programs:  pnmcolormap  and  pnmremap  and  then  Netpbm 9.24
201       replaced pnmquant with a program that simply calls pnmcolormap and pnm‐
202       remap.
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AUTHOR

206       Copyright (C) 1989, 1991 by Jef Poskanzer.
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DOCUMENT SOURCE

209       This  manual  page was generated by the Netpbm tool 'makeman' from HTML
210       source.  The master documentation is at
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212              http://netpbm.sourceforge.net/doc/pnmcolormap.html
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214netpbm documentation           09 February 2019     Pnmcolormap User Manual(0)
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