1Pnmcolormap User Manual(0) Pnmcolormap User Manual(0)
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6 pnmcolormap - create quantization color map for a Netpbm image
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10 pnmcolormap
11 [-center|-meancolor|-meanpixel]
13 [-spreadbrightness|-spreadluminosity]
15 [-sort]
17 [-square]
19 ncolors|all
21 [pnmfile]
23
26 This program is part of Netpbm(1).
27 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.
35 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.
40 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.
45 The quantization method is Heckbert's "median cut". See QUANTIZATION
47 METHOD ⟨#quant⟩ .
48 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.
52 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.
56 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.
67 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.
70
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.
79
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.
87 -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.
95 -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.
104 -verbose
107 This option causes pnmcolormap to display messages to Standard
108 Error about the quantization..TP -center
109 -meancolor
112 -meanpixel
115 -spreadbrightness
118 -spreadluminosity
121 These options control the quantization algorithm. See
122 QUANTIZATION METHOD ⟨#quant⟩ .
123
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.
132 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.
138 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.
153 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.
160 Select among these with the -center, -meancolor, and -meanpixel
162 options. The default is -center.
163 Note that in all three methods, there may be colors in the output which
165 do not appear in the input at all.
166 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.
176
180 "Color Image Quantization for Frame Buffer Display" by Paul Heckbert,
181 SIGGRAPH '82 Proceedings, page 297.
182
185 pnmremap(1), pnmquant(1), ppmquantall(1), pamgetcolor(1), pamdepth(1),
186 ppmdither(1), pamseq(1), ppmmake(1), pnmcat(1), ppm(1)
187
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.
195 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.
203
206 Copyright (C) 1989, 1991 by Jef Poskanzer.
207
209 This manual page was generated by the Netpbm tool 'makeman' from HTML
210 source. The master documentation is at
211 http://netpbm.sourceforge.net/doc/pnmcolormap.html
213netpbm documentation 09 February 2019 Pnmcolormap User Manual(0)