1Pgmtoppm User Manual(0) Pgmtoppm User Manual(0)
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6 pgmtoppm - colorize a PGM (grayscale) image into a PPM (color) image
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10 pgmtoppm colorspec [pgmfile] pgmtoppm colorspec1-colorspec2 [pgmfile]
11 pgmtoppm -map=mapfile [pgmfile]
12 Minimum unique abbreviation of option is acceptable. You may use dou‐
14 ble hyphens instead of single hyphen to denote options. You may use
15 white space in place of the equals sign to separate an option name from
16 its value.
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20 This program is part of Netpbm(1).
21 pgmtoppm reads a PGM as input and produces a PPM file as output with a
23 specific color assigned to each gray value in the input.
24 If you specify one color argument, black in the pgm file stays black
26 and white in the pgm file turns into the specified color in the ppm
27 file. Gray values in between are linearly mapped to differing intensi‐
28 ties of the specified color.
29 If you specify two color arguments (separated by a hyphen), then black
31 gets mapped to the first color and white gets mapped to the second and
32 gray values in between get mapped linearly (across a three dimensional
33 space) to colors in between.
34 Specify the color (color) as described for the argument of the
36 ppm_parsecolor() library routine ⟨libppm.html#colorname⟩ .
37 Also, you can specify an entire colormap with the -map option. The
39 mapfile is just a ppm file; it can be any shape, all that matters is
40 the colors in it and their order. In this case, black gets mapped into
41 the first color in the map file, and white gets mapped to the last and
42 gray values in between are mapped linearly onto the sequence of colors
43 in between. The maxval of the output image is the maxval of the map
44 image.
45 A more direct way to specify a particular color to replace each partic‐
47 ular gray level is to use pamlookup. You make an index file that
48 explicitly associates a color with each possible gray level.
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52 When you don't use -map, the 'maxval,' or depth, of the output image is
53 the same as that of the input image. The maxval affects the color res‐
54 olution, which may cause quantization errors you don't anticipate in
55 your output. For example, you have a simple black and white image as a
56 PGM with maxval 1. Run this image through pgmtoppm 0f/00/00 to try to
57 make the image black and faint red. Because the output image will also
58 have maxval 1, there is no such thing as faint red. It has to be
59 either full-on red or black. pgmtoppm rounds the color 0f/00/00 down
60 to black, and you get an output image that is nothing but black.
61 The fix is easy: Pass the input through pamdepth on the way into pgm‐
63 toppm to increase its depth to something that would give you the reso‐
64 lution you need to get your desired color. In this case, pamdepth 16
65 would do it. Or spare yourself the unnecessary thinking and just say
66 pamdepth 255.
67 PBM input is a special case. While you might think this would be
69 equivalent to a PGM with maxval 1 since only two gray levels are neces‐
70 sary to represent a PBM image, pgmtoppm, like all Netpbm programs, in
71 fact treats it as a maxval of 255.
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75 pamdepth(1), rgb3toppm(1), ppmtopgm(1), ppmtorgb3(1), ppm(1), pgm(1)
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79 Copyright (C) 1991 by Jef Poskanzer.
80netpbm documentation 10 December 2006 Pgmtoppm User Manual(0)