1Pnmcolormap User Manual(0) Pnmcolormap User Manual(0)
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6 pnmcolormap - create quantization color map for a Netpbm image
7
10 pnmcolormap
11 [-center|-meancolor|-meanpixel]
13 [-spreadbrightness|-spreadluminosity]
15 [-splitpixelct|-splitcolorct|-splitspread]
17 [-sort]
19 [-square]
21 [-verbose]
23 [-debug]
25 ncolors|all
27 [pnmfile]
29
32 This program is part of Netpbm(1).
33 pnmcolormap reads a PNM or PAM image as input, chooses ncolors colors
35 to best represent the image and writes a PNM color map defining them as
36 output. A PAM image may actually contain tuples of any kind, but pnm‐
37 colormap's concept of the tuple values that best represent the ones
38 present in the image may not make sense if the tuple type isn't RGB or
39 GRAYSCALE. The design of the program, and the rest of this manual, as‐
40 sumes the tuples represent colors.
41 You can use this map as input to pnmremap on the same input image to
43 quantize the colors in that image, I.e. produce a similar image with
44 fewer colors. pnmquant does both the pnmcolormap and pnmremap steps
45 for you.
46 A PNM colormap is a PNM image of any dimensions that contains at least
48 one pixel of each color in the set of colors it represents. The ones
49 pnmcolormap generates have exactly one pixel of each color, except
50 where padding is necessary with the -square option.
51 The quantization method is Heckbert's "median cut". See QUANTIZATION
53 METHOD ⟨#quant⟩ .
54 The output image is of the same format (PBM, PGM, PPM, PAM) as the in‐
56 put image. Note that a colormap of a PBM image is not very interest‐
57 ing.
58 The colormap generally has the same maxval as the input image, but pnm‐
60 colormap may reduce it if there are too many colors in the input, as
61 part of its quantization algorithm.
62 pnmcolormap works on a multi-image input stream. In that case, it pro‐
64 duces one colormap that applies to all of the colors in all of the in‐
65 put images. All the images must have the same format, depth, and max‐
66 val (but may have different height and width). This is useful if you
67 need to quantize a bunch of images that will form a movie or otherwise
68 be used together -- you generally want them all to draw from the same
69 palette, whereas computing a colormap separately from each image would
70 make the same color in two images map to different colors. Before
71 Netpbm 10.31 (December 2005), pnmcolormap ignored any image after the
72 first.
73 If you want to create a colormap without basing it on the colors in an
75 input image, pamseq, ppmmake, and pnmcat can be useful.
76
79 The single parameter, which is required, is the number of colors you
80 want in the output colormap. pnmcolormap may produce a color map with
81 slightly fewer colors than that. You may specify all to get a colormap
82 of every color in the input image (no quantization). When you specify
83 all, the function is essentially the same as that of ppmhist -map.
84 ppmhist is much older.
85
89 In addition to the options common to all programs based on libnetpbm
90 (most notably -quiet, see
91 Common Options ⟨index.html#commonoptions⟩ ), pnmcolormap recognizes
92 the following command line options:
93 All options can be abbreviated to their shortest unique prefix. You
95 may use two hyphens instead of one to designate an option. You may use
96 either white space or an equals sign between an option name and its
97 value.
98 -sort This option causes the output colormap to be sorted by the red
102 component intensity, then the green, then the blue in ascending
103 order. This is an insertion sort, so it is not very fast on
104 large colormaps. Sorting is useful because it allows you to
105 compare two sets of colors.
106 -square
109 By default, pnmcolormap produces as the color map a PPM image
110 with one row and with one column for each color in the colormap.
111 This option causes pnmcolormap instead to produce a PPM image
112 that is within one row or column of being square, with the last
113 pixel duplicated as necessary to create a number of pixels which
114 is such an almost-perfect square.
115 -verbose
118 This option causes pnmcolormap to display messages to Standard
119 Error about the quantization..TP -center
120 -debug This option causes pnmcolormap to display messages to Standard
123 Error about internal workings.
124 -center
127 -meancolor
130 -meanpixel
133 -spreadbrightness
136 -spreadluminosity
139 -splitpixelct
142 -splitcolorct
145 -splitspread
148 These options control the quantization algorithm. See
149 QUANTIZATION METHOD ⟨#quant⟩ .
150
156 A quantization method is a way to choose which colors, being fewer in
157 number than in the input, you want in the output. pnmcolormap uses
158 Heckbert's "median cut" quantization method.
159 This method involves separating all the colors into "boxes," each hold‐
161 ing colors that represent about the same number of pixels. You start
162 with one box and split boxes in two until the number of boxes is the
163 same as the number of colors you want in the output, and choose one
164 color to represent each box.
165 There are three ways pnmcolormap can choose the box to split in
167 each step:
168 • Split the box containing the most pixels. This is the default,
171 and you can select it explicitly with option -splitpix‐
172 elct.
173 • Split the box containing the most colors. This appears to be
175 useful
176 for academic purposes only. Select this with option
177 -splitcolorct.
178 • Split the box containing the largest color spread. Select this
180 with option -splitspread. This can produce a better re‐
181 sult for
182 small details with colors not found elsewhere in the im‐
183 age.
184 -splitpixelct, splitcolorct, and splitspread were new
187 in Netpbm 10.88 (September 2019). Before that, pnmcolormap always
188 splits the box containing the most pixels.
189 When you split a box, you do it so each sub-box has the same number of
192 pixels (except one sub-box has more if the full box has an odd number),
193 with the 'greatest' pixels in one sub-box and the 'least' pixels in the
194 other. "Greater," for a particular box, means it is brighter in the
195 color component (red, green, blue) which has the largest spread in that
196 box. pnmcolormap gives you two ways to define "largest spread.": 1)
197 largest spread of brightness; 2) largest spread of contribution to the
198 luminosity of the color. E.g. red is weighted much more than blue.
199 Select among these with the -spreadbrightness and -spreadluminosity op‐
200 tions. The default is -spreadbrightness. Where there are multiple
201 colors of the median magnitude, they are distributed arbitrarily among
202 between the subboxes. This arbitrary distribution depends upon what
203 the system's qsort function does with multiple equal values, so pnmcol‐
204 ormap may produce slightly different results on different systems.
205 pnmcolormap provides three ways of choosing a color to represent a box:
207 1) the center color - the color halfway between the greatest and least
208 colors in the box, using the above definition of "greater"; 2) the mean
209 of the colors (each component averaged separately by brightness) in the
210 box; 3) the mean weighted by the number of pixels of a color in the im‐
211 age.
212 Select among these with the -center, -meancolor, and -meanpixel op‐
214 tions. The default is -center.
215 Note that in all three methods, there may be colors in the output which
217 do not appear in the input at all.
218 Also note that the color chosen to represent the colors in Box A the
220 best may also represent a color in Box B better than the color chosen
221 to represent the colors in Box B the best. This is true for various
222 measures of goodness of representation of one color by another. In
223 particular, if you use pnmremap to map the colors in the very image
224 that you used to create the color map to the colors in that colormap,
225 the colors in Box B will often map to the color pnmcolormap chose to
226 represent some other box and in fact the color pnmcolormap chose to
227 represent Box B may not appear in the pnmremap output at all.
228
232 "Color Image Quantization for Frame Buffer Display" by Paul Heckbert,
233 SIGGRAPH '82 Proceedings, page 297.
234
237 pnmremap(1), pnmquant(1), ppmquantall(1), pamgetcolor(1), pamdepth(1),
238 ppmdither(1), pamseq(1), ppmmake(1), pnmcat(1), ppm(1)
239
242 Before Netpbm 10.15 (April 2003), pnmcolormap used a lot more memory
243 for large images because it kept the entire input image in memory.
244 Now, it processes it a row at a time, but because it sometimes must
245 make multiple passes through the image, it first copies the input into
246 a temporary seekable file if it is not already in a seekable file.
247 pnmcolormap first appeared in Netpbm 9.23 (January 2002). Before that,
249 its function was available only as part of the function of pnmquant
250 (which was derived from the much older ppmquant). Color quantization
251 really has two main subfunctions, so Netpbm 9.23 split it out into two
252 separate programs: pnmcolormap and pnmremap and then Netpbm 9.24 re‐
253 placed pnmquant with a program that simply calls pnmcolormap and pnm‐
254 remap.
255
258 Copyright (C) 1989, 1991 by Jef Poskanzer.
259
261 This manual page was generated by the Netpbm tool 'makeman' from HTML
262 source. The master documentation is at
263 http://netpbm.sourceforge.net/doc/pnmcolormap.html
265netpbm documentation 21 February 2023 Pnmcolormap User Manual(0)