1Giftopnm User Manual(0) Giftopnm User Manual(0)
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6 giftopnm - convert a GIF file into a PNM image
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10 giftopnm [--alphaout={alpha-filename,-}] [-verbose] [-comments]
11 [-image={N,all}] [-repair] [-quitearly] [GIFfile]
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.
17
20 This program is part of Netpbm(1).
21 This is a graphics format converter from the GIF format to the PNM
23 (i.e. PBM, PGM, or PPM) format.
24 If the image contains only black and maximally bright white, the output
26 is PBM. If the image contains more than those two colors, but only
27 grays, the output is PGM. If the image contains other colors, the out‐
28 put is PPM.
29 A GIF image contains rectangular pixels. They all have the same
31 aspect ratio, but may not be square (it's actually quite unusual for
32 them not to be square, but it could happen). The pixels of a Netpbm
33 image are always square. Because of the engineering complexity to do
34 otherwise, giftopnm converts a GIF image to a Netpbm image pixel-for-
35 pixel. This means if the GIF pixels are not square, the Netpbm output
36 image has the wrong aspect ratio. In this case, giftopnm issues an
37 informational message telling you to run pamscale to correct the out‐
38 put.
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42 --alphaout=alpha-filename
43 giftopnm creates a PGM (portable graymap) file containing the
44 alpha channel values in the input image. If the input image
45 doesn't contain an alpha channel, the alpha-filename file con‐
46 tains all zero (transparent) alpha values. If you don't specify
47 --alphaout, giftopnm does not generate an alpha file, and if the
48 input image has an alpha channel, giftopnm simply discards it.
49 If you specify - as the filename, giftopnm writes the alpha out‐
51 put to Standard Output and discards the image.
52 See pamcomp(1)foronewaytouse the alpha output file.
54 -verbose
57 Produce verbose output about the GIF file input.
58 -comments
61 Only output GIF89 comment fields.
62 -image={N,all}
65 This option identifies which image from the GIF stream you want.
66 You can select either one image or all the images. Select al
67 the images with all. Select one image by specifying its
68 sequence number in the stream: 1, 2, 3, etc.
69 The default is just Image 1.
71 A GIF stream normally contains only one image, so you don't need
73 this option. But some streams, including animated GIFs, have
74 multiple images.
75 When you select multiple GIF images, the output is a PNM stream
77 with multiple images.
78 If you specify a single image, giftopnm must read and partially
80 validate the images before that in the stream. It may or may
81 not do the same for the images after it; see -quitearly.
82 The all value was added in Netpbm 10.16 (June 2003). Earlier
84 giftopnm can extract only one image.
85 -repair
88 This option makes giftopnm try to salvage what it can from an
89 invalid GIF input.
90 In particular, when giftopnm detects that the GIF input is
92 invalid so that it is impossible to determine what the pixels
93 are intended to be, it produces a single arbitrary color for all
94 further pixels in the image. giftopnm processes the image from
95 top to bottom, left to right, so this means the bottommost pix‐
96 els will be this padding.
97 giftopnm issues warning messages when it salvages an image in
99 this way.
100 Without this option, giftopnm fails when it detects invalid GIF
102 input. Any output it produces is arbitrary, and typically is
103 not a valid PNM image.
104 It is fairly common for an image to be corrupted such that is
106 started off as a valid GIF, but had the end of the file cut off.
107 An interrupted network transfer tends to do this. In this case,
108 giftopnm's salvage operation will produce a valid PNM image of
109 the proper dimensions, but with a single arbitrary color for the
110 pixels that were left out of the file.
111 This option was new in Netpbm 10.38 (March 2007). From 10.32
113 through 10.37, giftopnm always fails if it detects invalid GIF
114 input. Before 10.32, it succeeds in the case of a truncated
115 image, and replaces the missing pixels with arbitrary colors,
116 not necessarily all the same (The pre-10.32 behavior wasn't
117 actually intended by the design).
118 -quitearly
122 This option makes giftopnm stop reading its input file as soon
123 as it has converted and output the images from the input that
124 you requested. By default, giftopnm reads until the end of the
125 GIF stream, ignoring any data after the images you requested.
126 Two reasons not to use this option:
128 · The input file is a pipe and the process that is filling that
131 pipe expects the pipe to take the entire stream and will fail or
132 get stuck if it doesn't.
133 · You want to validate the entire GIF stream.
136 Two reasons to use this option:
140 · It saves the time and other resources to read the end of the
144 stream.
145 · There are errors in the end of the stream that make giftopnm
147 fail.
148 This option has no effect if you also specify -image=all
151 This option was new in Netpbm 10.35 (August 2006). Before that,
153 giftopnm always reads the entire stream.
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159 This does not correctly handle the Plain Text Extension of the GIF89
160 standard, since I did not have any example input files containing them.
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164 pamtogif(1), ppmcolormask(1), pamcomp(1), http://www.lcdf.org/gifsicle
165 ⟨http://www.lcdf.org/gifsicle⟩ , ppm(1).
166
169 Copyright (c) 1993 by David Koblas (koblas@netcom.com)
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173 As a historical note, for a long time if you used giftopnm, you were
174 using a patent on the LZW compression method which was owned by Unisys,
175 and in all probability you did not have a license from Unisys to do so.
176 Unisys typically asked $5000 for a license for trivial use of the
177 patent. Unisys never enforced the patent against trivial users, and
178 made statements that it is much less concerned about people using the
179 patent for decompression (which is what giftopnm does than for compres‐
180 sion. The patent expired in 2003.
181 Rumor has it that IBM also owns a patent covering giftopnm.
183 A replacement for the GIF format that has never required any patent
185 license to use is the PNG format.
186netpbm documentation 29 January 2007 Giftopnm User Manual(0)