1Pnmremap User Manual(0) Pnmremap User Manual(0)
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6 pnmremap - replace colors in a PNM image with colors from another set
7
10 pnmremap
11 -mapfile=palettefile
13 [-floyd|-fs|-nfloyd|-nofs]
15 [-norandom]
17 [-firstisdefault]
19 [-verbose]
21 [-missingcolor=color]
23 [pnmfile]
25 All options can be abbreviated to their shortest unique prefix. You
27 may use two hyphens instead of one to designate an option. You may use
28 either white space or an equals sign between an option name and its
29 value.
30
33 This program is part of Netpbm(1).
34 pnmremap replaces the colors in an input image with those from a pal‐
36 ette you specify. Where colors in the input are present in the pal‐
37 ette, they just stay the same in the output. But where the input con‐
38 tains a color that is not in the palette, pnmremap gives you these
39 choices:
40 · Choose the closest color from the palette.
44 · Choose the first color from the palette.
47 · Use a color specified by a command option (-missing).
50 · Dither. This means rather than mapping pixel by pixel, pnmremap
53 uses colors from the palette to try to make multi-pixel regions
54 of the output have the same average color as the input (for
55 another kind of dithering, see ppmdither).
56 Two reasons to use this program are: 1) you want to reduce the number
60 of colors in the input image; and 2) you need to feed the image to
61 something that can handle only certain colors.
62 To reduce colors, you can generate the palette with pnmcolormap.
64 By default, pnmremap maps an input color that is not in the palette to
66 the closest color that is in the palette. Closest means with the
67 smallest cartesian distance in the red, green, blue brightness space
68 (smallest sum of the squares of the differences in red, green, and blue
69 ITU-R Recommedation BT.709 gamma-adjusted intensities).
70 You can instead specify a single default color for pnmremap to use for
72 any color in the input image that is not in the palette. Use the
73 -missing option for this.
74 You can also specify that the first color in the palette image is the
76 default. Use the -firstisdefault option for this.
77 The palette is simply a PNM image. The colors of the pixels in the
79 image are the colors in the palette. Where the pixels appear in the
80 image, and the dimensions of the image, are irrelevant. Multiple pix‐
81 els of the same color are fine. However, a palette image is typically
82 a single row with one pixel per color.
83 If you specify -missing, the color you so specify is in the palette in
85 addition to whatever is in the palette image.
86 For historical reasons, Netpbm sometimes calls the palette a 'col‐
88 ormap.' But it doesn't really map anything. pnmremap creates its own
89 map, based on the palette, to map colors from the input image to output
90 colors.
91 Palette/Image Type Mismatch
94 In the simple case, the palette image is of the same depth (number of
95 planes, i.e. number of components in each tuple (pixel)) as the input
96 image and pnmremap just does a straightforward search of the palette
97 for each input tuple (pixel). In fact, pnmremap doesn't even care if
98 the image is a visual image.
99 But what about when the depths differ? In that case, pnmremap converts
101 the input image (in its own memory) to match the palette and then pro‐
102 ceeds as above.
103 There are only two such cases in which pnmremap knows how to do the
105 conversion: when one of them is tuple type RGB, depth 3, and the other
106 is tuple type GRAYSCALE or BLACKANDWHITE, depth 1; and vice versa.
107 In any other case, pnmremap issues and error message and fails.
109 Note that as long as your input and palette images are PNM, they'll
111 always fall into one of the cases pnmremap can handle. There's an
112 issue only if you're using some exotic PAM image.
113 Before Netpbm 10.27 (March 2005), pnmremap could not handle the case of
115 a palette of greater depth than the input image. (It would issue an
116 error message and fail in that case). You can use ppmtoppm to increase
117 the depth of the input image to work around this limitation.
118 In any case, the output image has the same tuple type and depth as the
120 palette image.
121 Multiple Image Stream
124 pnmremap handles a multiple image input stream, producing a multiple
125 image output stream. The input images need not be similar in any way.
126 Before Netpbm 10.30 (October 2005), pnmremap ignored any image after
128 the first.
129 Examples
133 pnmcolormap testimg.ppm 256 >palette.ppm
134 pnmremap -map=palette.ppm testimg.ppm >reduced_testimg.ppm
136 To limit colors to a certain set, a typical example is to create an
138 image for posting on the World Wide Web, where different browsers know
139 different colors. But all browsers are supposed to know the 216 'web
140 safe' colors which are essentially all the colors you can represent in
141 a PPM image with a maxval of 5. So you can do this:
142 pamseq 3 5 >websafe.pam
144 pnmremap -map=websafe.pam testimg.ppm >websafe_testimg.ppm
146 Another useful palette is one for the 8 color IBM TTL color set, which
148 you can create with
149 pamseq 3 1 >ibmttl.pam
150 If you want to quantize one image to use the colors in another one,
152 just use the second one as the palette. You don't have to reduce it
153 down to only one pixel of each color, just use it as is.
154 The output image has the same type and maxval as the palette image.
156
159 There is one parameter, which is required: The file specification of
160 the input PNM file.
161
165 -mapfile=palettefilename
166 This names the file that contains the palette image.
167 This option is mandatory.
169 -floyd
172 -fs
174 -nofloyd
176 -nofs These options determine whether pnmremap does Floyd-Steinberg
178 dithering. Without Floyd-Steinberg, pnmremap selects the output
179 color of a pixel based on the color of only the corresponding
180 input pixel. With Floyd-Steinberg, pnmremap considers regions
181 of pixels such that the average color of a region is the same in
182 the output as in the input. The dithering effect appears as a
183 dot pattern up close, but from a distance, the dots blend so
184 that you see more colors than are present in the color map.
185 As an example, if your color map contains only black and white,
187 and the input image has 4 adjacent pixels of gray, pnmremap with
188 Floyd-Steinberg would generate output pixels black, white,
189 black, white, which from a distance looks gray. But without
190 Floyd-Steinberg, pnmremap would generate 4 white pixels, white
191 being the single-pixel approximation of gray.
192 Floyd-Steinberg gives vastly better results on images where
194 unmodified quantization has banding or other artifacts, espe‐
195 cially when going to a small number of colors such as the above
196 IBM set. However, it does take substantially more CPU time.
197 -fs is a synomym for -floyd. -nofs is a synonym for -nofloyd.
199 The default is -nofloyd.
201 Before Netpbm 10.46 (March 2009), dithering doesn't work quite
203 as you expect if the color map has a lower maxval than the
204 input. pnmremap reduces the color resolution to the color map's
205 maxval before doing any dithering, so the dithering does not
206 have the effect of making the image, at a distance, appear to
207 have the original maxval. In current Netpbm, it does.
208 -norandom
211 This option affects a detail of the Floyd-Steinberg dithering
212 process. It has no effect if you aren't doing Floyd-Steinberg
213 dithering.
214 By default, pnmremap initializes the error propagation accumula‐
216 tor to random values to avoid the appearance of unwanted pat‐
217 terns. This is an extension of the original Floyd-Steinberg
218 algorithm.
219 A drawback of this is that the same pnmremap on the same input
221 produces slightly different output every time, which makes com‐
222 parison difficult.
223 With -norandom, pnmremap initializes the error accumulators to
225 zero and the output is completely predictable.
226 -norandom was new in Netpbm 10.39 (June 2007).
228 -firstisdefault
232 This tells pnmremap to map any input color that is not in the
233 palette to the first color in the palette (the color of the
234 pixel in the top left corner of the palette image)
235 See DESCRIPTION ⟨#description⟩ .
237 If you specify -firstisdefault, the maxval of your input must
239 match the maxval of your palette image.
240 -missingcolor=color
243 This specifies the default color for pnmremap to map to a color
244 in the input image that isn't in the palette. color may or may
245 not be in the palette image; it is part of the palette regard‐
246 less.
247 If you specify -missingcolor, the maxval of your input must
249 match the maxval of your palette image.
250 -verbose
253 Display helpful messages about the mapping process.
254
260 pnmcolormap(1), pamseq(1), pnmquant(1), ppmquantall(1), pamdepth(1),
261 ppmdither(1), ppmquant(1), ppm(1)
262
265 pnmremap first appeared in Netpbm 9.23 (January 2002). Before that,
266 its function was available only as part of the function of pnmquant
267 (which was derived from the much older ppmquant). Color quantization
268 really has two main subfunctions, so Netpbm 9.23 split it out into two
269 separate programs: pnmcolormap and pnmremap and then Netpbm 9.24
270 replaced pnmquant with a program that simply calls pnmcolormap and pnm‐
271 remap.
272
275 Copyright (C) 1989, 1991 by Jef Poskanzer.
276netpbm documentation 03 June 2009 Pnmremap User Manual(0)