1Pamtogif User Manual(0) Pamtogif User Manual(0)
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6 pamtogif - convert a Netpbm image to a GIF image
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10 pamtogif
11 [-interlace]
13 [-sort]
15 [-mapfile=mapfile] [-transparent=[=]color]
17 [-alphacolor=color]
19 [-comment=text]
21 [-nolzw]
23 [-aspect=fraction]
25 [-verbose] [netpbmfile]
27 All options can be abbreviated to their shortest unique prefix. You
29 may use two hyphens instead of one to designate an option. You may use
30 either white space or an equals sign between an option name and its
31 value.
32
35 This program is part of Netpbm(1).
36 pamtogif reads a Netpbm image as input and produces a GIF file as out‐
38 put.
39 This program creates only individual GIF images. To combine multiple
41 GIF images into an animated GIF, use gifsicle
42 ⟨http://www.lcdf.org/gifsicle/⟩ (not part of the Netpbm package).
43 pamtogif creates either an original GIF87 format GIF file or the newer
45 GIF89 format. It creates GIF89 when you request features that were new
46 with GIF89, to wit the -transparent or -comment options. Otherwise, it
47 creates GIF87. Really old GIF readers conceivably could not recognize
48 GIF89.
49 The GIF format is not capable of representing an image with more than
51 256 colors in it (it contains a color map with a maximum size of 256).
52 If the image you want to convert has more colors than that (ppmhist can
53 tell you), you can use pnmquant to reduce it to 256.
54 If your input image is a PAM with transparency information, ppmtogif
56 uses one entry in the GIF colormap specifically for the transparent
57 pixels, so you can have at most 255 opaque colors. In contrast, if you
58 use the -transparent option, one of the colors from the input becomes
59 transparent, so the limit is still 256.
60 pamtogif was new in Netpbm 10.37 (December 2006). In older Netpbm, use
62 ppmtogif.
63
66 -interlace
67 Produce an interlaced GIF file.
68 -sort Produce a GIF file with a color map sorted in a predictable
71 order.
72 This does not produce the sorted color map which is part of the
74 GIF format. That kind of sorted color map is one where the col‐
75 ors are sorted according to how important they are, and the GIF
76 header tells the viewer that it is sorted that way. Its purpose
77 is to allow the viewer to use fewer colors than are in the color
78 map if it is not capable of displaying all the colors.
79 What this option produces is a color map sorted by red value,
81 then green, then blue. That can be useful in analyzing GIF
82 images, particularly those made with two versions of the pro‐
83 gram, because it removes some of the variability.
84 -mapfile=mapfile
88 Use the colors found in the file mapfile to create the colormap
90 in the GIF file, instead of the colors from netpbmfile. mapfile
91 can be any PPM file; all that matters is the colors in it. If
92 the colors in netpbmfile do not match those in mapfile, pamtogif
93 matches them to a 'best match.' You can obtain a much better
94 result by using pnmremap to change the colors in the input to
95 those in the map file.
96 The mapfile file is not a palette file, just an image whose col‐
98 ors you want to use. The order of colors in the GIF palette
99 have nothing to do with where they appear in the mapfile image,
100 and duplication of colors in the image is irrelevant.
101 The map file's depth must match the number of color components
103 in the input (which is not necessarily the same as the input's
104 depth -- the input might have an alpha plane in addition). If
105 your map file does not, or it might not, run your input through
106 pnmremap using the same map file so that it does.
107 -transparent=color
110 pamtogif marks the specified color as transparent in the GIF
111 image.
112 If you don't specify -transparent, pamtogif does not mark any
114 color transparent (except as indicated by the transparency
115 information in the input file).
116 Specify the color (color) as described for the argument of the
118 ppm_parsecolor() library routine ⟨libppm.html#colorname⟩ .
119 If the color you specify is not present in the image, pamtogif
121 selects instead the color in the image that is closest to the
122 one you specify. Closeness is measured as a cartesian distance
123 between colors in RGB space. If multiple colors are equidis‐
124 tant, pamtogif chooses one of them arbitrarily.
125 However, if you prefix your color specification with '=', e.g.
127 -transparent==red, only the exact color you specify will be
128 transparent. If that color does not appear in the image, there
129 will be no transparency. pamtogif issues an information message
130 when this is the case.
131 When you specify -transparent, pamtogif ignores explicit trans‐
133 parency information (the 'alpha channel') in the input image.
134 -alpha=pgmfile
137 There is no -alpha option. pamtogif's predecessor had such an
138 option because it was not capable of taking PAM input that con‐
139 tains a transparency (alpha) plane, so one used this option to
140 supply a transparency plane as a separate PGM file.
141 This option names a PGM file that contains an alpha mask for
143 the image. pamtogif creates fully transparent pixels wherever
144 the alpha mask indicates transparency greater than 50%. The
145 color of those pixels is that specified by the -alphacolor
146 option, or black by default.
147 To do this, pamtogif creates an entry in the GIF colormap in
149 addition to the entries for colors that are actually in the
150 image. It marks that colormap entry as transparent and uses
151 that colormap index in the output image to create a transparent
152 pixel.
153 The alpha image must be the same dimensions as the input image,
155 but may have any maxval. White means opaque and black means
156 transparent.
157 You cannot specify both -transparent and -alpha.
159 -alphacolor=color
162 This specifies the foreground color for transparent pixels. A
163 viewer may use the foreground color for a transparent pixel if
164 it chooses not to have another color 'show through.'. The
165 default is black.
166 This applies only to pixels that are transparent in the GIF
168 because they are transparent in the Netpbm input. If a GIF
169 pixel is transparent because of the -transparent option, the
170 foreground color is the color indicated by that option.
171 Note that in GIF, all transparent pixels have the same fore‐
173 ground color. (There is only one entry in the GIF colormap for
174 transparent pixels).
175 Specify the color (color) as described for the argument of the
177 ppm_parsecolor() library routine ⟨libppm.html#colorname⟩ .
178 -comment=text
181 Include a comment in the GIF output with comment text text.
182 Without this option, there are no comments in the output.
184 Note that in a command shell, you'll have to use quotation marks
186 around text if it contains characters (e.g. space) that would
187 make the shell think it is multiple arguments:
188 $ pamtogif -comment "this is a comment" <xxx.ppm >xxx.gif
189 -nolzw
192 This option is mainly of historical interest -- it involves use
194 of a patent that is now expired.
195 This option causes the GIF output, and thus pamtogif, not to use
197 LZW (Lempel-Ziv) compression. As a result, the image file is
198 larger and, before the patent expired, no royalties would be
199 owed to the holder of the patent on LZW. See the section
200 LICENSE below.
201 LZW is a method for combining the information from multiple pix‐
203 els into a single GIF code. With the -nolzw option, pamtogif
204 creates one GIF code per pixel, so it is not doing any compres‐
205 sion and not using LZW. However, any GIF decoder, whether it
206 uses an LZW decompressor or not, will correctly decode this
207 uncompressed format. An LZW decompressor would see this as a
208 particular case of LZW compression.
209 Note that if someone uses an LZW decompressor such as the one in
211 giftopnm or pretty much any graphics display program to process
212 the output of pamtogif -nolzw , he is then using the LZW patent.
213 But the patent holder expressed far less interest in enforcing
214 the patent on decoding than on encoding.
215 -aspect=fraction
218 This is the aspect ratio of the pixels of the image. Its only
219 effect is to record that information in the GIF for use by what‐
220 ever interprets the GIF. Note that this feature of GIF is
221 hardly ever used and most GIF decoders ignore this information
222 and assume pixels are square.
223 Pixels in a Netpbm image do not have aspect ratios; there is
225 always a one-one correspondence between GIF pixels and Netpbm
226 pixels.
227 The aspect ratio is the quotient of width divided by height.
229 GIF allows aspect ratios from 0.25 (1:4) to 4 (4:1) in incre‐
230 ments of 1/64. pamtogif implements a natural extension of GIF
231 that allows an aspect ratio up to 4 14/64. If you specify any‐
232 thing outside this range, pamtogif fails. pamtogif rounds frac‐
233 tion to the nearest 1/64.
234 The default is square (1.0).
236 This option was new in Netpbm 10.38 (March 2007). Before that,
238 the pixels are always square.
239 -verbose
243 This option causes pamtogif to display information about the
244 conversion process and the image it produces.
245
250 giftopnm(1), pnmremap(1), ppmtogif(1),
251 gifsicle http://www.lcdf.org/gifsicle ⟨http://www.lcdf.org/gifsicle⟩ ,
253 pnm(1), pam(1).
254
257 pamtogif was new in Netpbm 10.37 (December 2006). It replaced ppm‐
258 togif, which created GIF images for Pbmplus/Netpbm users since 1989.
259 The main outward change in the conversion from ppmtogif to pamtogif was
261 that pamtogif was able to use transparency information ('alpha chan‐
262 nel') in PAM input, whereas with ppmtogif, one had to supply the trans‐
263 parency mask in a separate pseudo-PGM image (via the -alpha option).
264 Jef Poskanzer wrote ppmtogif in 1989, and it has always been a corner‐
266 stone of Pbmplus/Netpbm because GIF is such a popular image format.
267 Jef based the LZW encoding on GIFENCOD by David Rowley <mgardi@watd‐
268 csu.waterloo.edu>. Jef included GIFENCOD's GIFCOMPR.C file pretty much
269 whole. Rowley, in turn, adapted the LZW compression code from classic
270 Unix compress, which used techniques described in IEEE Computer, June
271 1984.
272 Jef's ppmtogif notably lacked the ability to use a transparency mask
274 with it. You could create transparent pixels in a GIF, but only with
275 the -transparent option, which allowed one to specify that all pixels
276 of a certain color in the input were to be transparent. Bryan Hender‐
277 son added the -alpha option in July 2001 so you could supply a mask
278 image that indicates exactly which pixels are to be transparent, and
279 those pixels could have the same color as other opaque ones.
280 Bryan Henderson added another significant piece of code and function in
282 October 2001: the ability to generate a GIF without using the LZW
283 patent -- an uncompressed GIF. This was very important to many people
284 at the time because the GIF patent was still in force, and this allowed
285 them to make an image that any GIF viewer could display, royalty-free.
286 Bryan adapted code from the Independent Jpeg Group's djpeg for that.
287 There is no code in pamtogif from Jef's original, but Jef may still
289 hold copyright over it due to the way in which it evolved. Virtually
290 all of the code in pamtogif was written by Bryan Henderson and contrib‐
291 uted to the public domain.
292
296 If you use pamtogif without the -nolzw option, you are using a patent
297 on the LZW compression method which is owned by Unisys. The patent has
298 expired (in 2003 in the US and in 2004 elsewhere), so it doesn't mat‐
299 ter. While the patent was in force, most people who used pamtogif and
300 similar programs did so without a license from Unisys to do so. Unisys
301 typically asked $5000 for a license for trivial use of the patent.
302 Unisys never enforced the patent against trivial users.
303 Rumor has it that IBM also owns or owned a patent covering pamtogif.
305 A replacement for the GIF format that never required any patents to use
307 is the PNG format.
308netpbm documentation 22 March 2007 Pamtogif User Manual(0)