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 ncolors|all
23 [pnmfile]
25
28 This program is part of Netpbm(1).
29 pnmcolormap reads a PNM or PAM image as input, chooses ncolors colors
31 to best represent the image and writes a PNM color map defining them as
32 output. A PAM image may actually contain tuples of any kind, but pnm‐
33 colormap's concept of the tuple values that best represent the ones
34 present in the image may not make sense if the tuple type isn't RGB or
35 GRAYSCALE. The design of the program, and the rest of this manual,
36 assumes the tuples represent colors.
37 You can use this map as input to pnmremap on the same input image to
39 quantize the colors in that image, I.e. produce a similar image with
40 fewer colors. pnmquant does both the pnmcolormap and pnmremap steps
41 for you.
42 A PNM colormap is a PNM image of any dimensions that contains at least
44 one pixel of each color in the set of colors it represents. The ones
45 pnmcolormap generates have exactly one pixel of each color, except
46 where padding is necessary with the -square option.
47 The quantization method is Heckbert's "median cut". See QUANTIZATION
49 METHOD ⟨#quant⟩ .
50 The output image is of the same format (PBM, PGM, PPM, PAM) as the
52 input image. Note that a colormap of a PBM image is not very interest‐
53 ing.
54 The colormap generally has the same maxval as the input image, but pnm‐
56 colormap may reduce it if there are too many colors in the input, as
57 part of its quantization algorithm.
58 pnmcolormap works on a multi-image input stream. In that case, it pro‐
60 duces one colormap that applies to all of the colors in all of the
61 input images. All the images must have the same format, depth, and
62 maxval (but may have different height and width). This is useful if
63 you need to quantize a bunch of images that will form a movie or other‐
64 wise be used together -- you generally want them all to draw from the
65 same palette, whereas computing a colormap separately from each image
66 would make the same color in two images map to different colors.
67 Before Netpbm 10.31 (December 2005), pnmcolormap ignored any image
68 after the first.
69 If you want to create a colormap without basing it on the colors in an
71 input image, pamseq, ppmmake, and pnmcat can be useful.
72
75 The single parameter, which is required, is the number of colors you
76 want in the output colormap. pnmcolormap may produce a color map with
77 slightly fewer colors than that. You may specify all to get a colormap
78 of every color in the input image (no quantization). When you specify
79 all, the function is essentially the same as that of ppmhist -map.
80 ppmhist is much older.
81
85 In addition to the options common to all programs based on libnetpbm
86 (most notably -quiet, see
87 Common Options ⟨index.html#commonoptions⟩ ), pnmcolormap recognizes
88 the following command line options:
89 All options can be abbreviated to their shortest unique prefix. You
91 may use two hyphens instead of one to designate an option. You may use
92 either white space or an equals sign between an option name and its
93 value.
94 -sort This option causes the output colormap to be sorted by the red
98 component intensity, then the green, then the blue in ascending
99 order. This is an insertion sort, so it is not very fast on
100 large colormaps. Sorting is useful because it allows you to
101 compare two sets of colors.
102 -square
105 By default, pnmcolormap produces as the color map a PPM image
106 with one row and with one column for each color in the colormap.
107 This option causes pnmcolormap instead to produce a PPM image
108 that is within one row or column of being square, with the last
109 pixel duplicated as necessary to create a number of pixels which
110 is such an almost-perfect square.
111 -verbose
114 This option causes pnmcolormap to display messages to Standard
115 Error about the quantization..TP -center
116 -meancolor
119 -meanpixel
122 -spreadbrightness
125 -spreadluminosity
128 -splitpixelct
131 -splitcolorct
134 -splitspread
137 These options control the quantization algorithm. See
138 QUANTIZATION METHOD ⟨#quant⟩ .
139
145 A quantization method is a way to choose which colors, being fewer in
146 number than in the input, you want in the output. pnmcolormap uses
147 Heckbert's "median cut" quantization method.
148 This method involves separating all the colors into "boxes," each hold‐
150 ing colors that represent about the same number of pixels. You start
151 with one box and split boxes in two until the number of boxes is the
152 same as the number of colors you want in the output, and choose one
153 color to represent each box.
154 There are three ways pnmcolormap can choose the box to split in
156 each step:
157 · Split the box containing the most pixels. This is the default,
160 and you can select it explicitly with option -splitpix‐
161 elct.
162 · Split the box containing the most colors. This appears to be
164 useful
165 for academic purposes only. Select this with option
166 -splitcolorct.
167 · Split the box containing the largest color spread. Select this
169 with option -splitspread. This can produce a better
170 result for
171 small details with colors not found elsewhere in the
172 image.
173 -splitpixelct, splitcolorct, and splitspread were new
176 in Netpbm 10.88 (September 2019). Before that, pnmcolormap always
177 splits the box containing the most pixels.
178 When you split a box, you do it so each sub-box has the same number of
181 pixels (except one sub-box has more if the full box has an odd number),
182 with the 'greatest' pixels in one sub-box and the 'least' pixels in the
183 other. "Greater," for a particular box, means it is brighter in the
184 color component (red, green, blue) which has the largest spread in that
185 box. pnmcolormap gives you two ways to define "largest spread.": 1)
186 largest spread of brightness; 2) largest spread of contribution to the
187 luminosity of the color. E.g. red is weighted much more than blue.
188 Select among these with the -spreadbrightness and -spreadluminosity
189 options. The default is -spreadbrightness. Where there are multiple
190 colors of the median magnitude, they are distributed arbitrarily among
191 between the subboxes. This arbitrary distribution depends upon what
192 the system's qsort function does with multiple equal values, so pnmcol‐
193 ormap may produce slightly different results on different systems.
194 pnmcolormap provides three ways of choosing a color to represent a box:
196 1) the center color - the color halfway between the greatest and least
197 colors in the box, using the above definition of "greater"; 2) the mean
198 of the colors (each component averaged separately by brightness) in the
199 box; 3) the mean weighted by the number of pixels of a color in the
200 image.
201 Select among these with the -center, -meancolor, and -meanpixel
203 options. The default is -center.
204 Note that in all three methods, there may be colors in the output which
206 do not appear in the input at all.
207 Also note that the color chosen to represent the colors in Box A the
209 best may also represent a color in Box B better than the color chosen
210 to represent the colors in Box B the best. This is true for various
211 measures of goodness of representation of one color by another. In
212 particular, if you use pnmremap to map the colors in the very image
213 that you used to create the color map to the colors in that colormap,
214 the colors in Box B will often map to the color pnmcolormap chose to
215 represent some other box and in fact the color pnmcolormap chose to
216 represent Box B may not appear in the pnmremap output at all.
217
221 "Color Image Quantization for Frame Buffer Display" by Paul Heckbert,
222 SIGGRAPH '82 Proceedings, page 297.
223
226 pnmremap(1), pnmquant(1), ppmquantall(1), pamgetcolor(1), pamdepth(1),
227 ppmdither(1), pamseq(1), ppmmake(1), pnmcat(1), ppm(1)
228
231 Before Netpbm 10.15 (April 2003), pnmcolormap used a lot more memory
232 for large images because it kept the entire input image in memory.
233 Now, it processes it a row at a time, but because it sometimes must
234 make multiple passes through the image, it first copies the input into
235 a temporary seekable file if it is not already in a seekable file.
236 pnmcolormap first appeared in Netpbm 9.23 (January 2002). Before that,
238 its function was available only as part of the function of pnmquant
239 (which was derived from the much older ppmquant). Color quantization
240 really has two main subfunctions, so Netpbm 9.23 split it out into two
241 separate programs: pnmcolormap and pnmremap and then Netpbm 9.24
242 replaced pnmquant with a program that simply calls pnmcolormap and pnm‐
243 remap.
244
247 Copyright (C) 1989, 1991 by Jef Poskanzer.
248
250 This manual page was generated by the Netpbm tool 'makeman' from HTML
251 source. The master documentation is at
252 http://netpbm.sourceforge.net/doc/pnmcolormap.html
254netpbm documentation 09 February 2019 Pnmcolormap User Manual(0)