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 [-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).
77
80 All options can be abbreviated to their shortest unique prefix. You
81 may use two hyphens instead of one to designate an option. You may use
82 either white space or an equals sign between an option name and its
83 value.
84 -sort This option causes the output colormap to be sorted by the red
88 component intensity, then the green, then the blue in ascending
89 order. This is an insertion sort, so it is not very fast on
90 large colormaps. Sorting is useful because it allows you to
91 compare two sets of colors.
92 -square
95 By default, pnmcolormap produces as the color map a PPM image
96 with one row and one column for each color in the colormap.
97 This option causes pnmcolormap instead to produce a PPM image
98 that is within one row or column of being square, with the last
99 pixel duplicated as necessary to create a number of pixels which
100 is such an almost-perfect square.
101 -verbose
104 This option causes pnmcolormap to display messages to Standard
105 Error about the quantization..TP -center
106 -meancolor
109 -meanpixel
112 -spreadbrightness
115 -spreadluminosity
118 These options control the quantization algorithm. See
119 QUANTIZATION METHOD ⟨#quant⟩ .
120
126 A quantization method is a way to choose which colors, being fewer in
127 number than in the input, you want in the output. pnmcolormap uses
128 Heckbert's 'median cut' quantization method.
129 This method involves separating all the colors into 'boxes,' each hold‐
131 ing colors that represent about the same number of pixels. You start
132 with one box and split boxes in two until the number of boxes is the
133 same as the number of colors you want in the output, and choose one
134 color to represent each box.
135 When you split a box, you do it so that all the colors in one sub-box
137 are 'greater' than all the colors in the other. 'Greater,' for a par‐
138 ticular box, means it is brighter in the color component (red, green,
139 blue) which has the largest spread in that box. pnmcolormap gives you
140 two ways to define 'largest spread.': 1) largest spread of brightness;
141 2) largest spread of contribution to the luminosity of the color. E.g.
142 red is weighted much more than blue. Select among these with the
143 -spreadbrightness and -spreadluminosity options. The default is
144 -spreadbrightness.
145 pnmcolormap provides three ways of choosing a color to represent a box:
147 1) the center color - the color halfway between the greatest and least
148 colors in the box, using the above definition of 'greater'; 2) the mean
149 of the colors (each component averaged separately by brightness) in the
150 box; 3) the mean weighted by the number of pixels of a color in the
151 image.
152 Note that in all three methods, there may be colors in the output which
154 do not appear in the input at all.
155 Select among these with the -center, -meancolor, and -meanpixel
157 options. The default is -center.
158
161 'Color Image Quantization for Frame Buffer Display' by Paul Heckbert,
162 SIGGRAPH '82 Proceedings, page 297.
163
166 pnmremap(1), pnmquant(1), ppmquantall(1), pamdepth(1), ppmdither(1),
167 pamseq(1), ppmmake(1), pnmcat(1), ppm(1)
168
171 Before Netpbm 10.15 (April 2003), pnmcolormap used a lot more memory
172 for large images because it kept the entire input image in memory.
173 Now, it processes it a row at a time, but because it sometimes must
174 make multiple passes through the image, it first copies the input into
175 a temporary seekable file if it is not already in a seekable file.
176 pnmcolormap first appeared in Netpbm 9.23 (January 2002). Before that,
178 its function was available only as part of the function of pnmquant
179 (which was derived from the much older ppmquant). Color quantization
180 really has two main subfunctions, so Netpbm 9.23 split it out into two
181 separate programs: pnmcolormap and pnmremap and then Netpbm 9.24
182 replaced pnmquant with a program that simply calls pnmcolormap and pnm‐
183 remap.
184
187 Copyright (C) 1989, 1991 by Jef Poskanzer.
188netpbm documentation 23 October 2005 Pnmcolormap User Manual(0)