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 [-noclear]
23 [-nolzw]
25 [-aspect=fraction]
27 [-verbose] [netpbmfile]
29 All options can be abbreviated to their shortest unique prefix. You
31 may use two hyphens instead of one to designate an option. You may use
32 either white space or an equals sign between an option name and its
33 value.
34
37 This program is part of Netpbm(1).
38 pamtogif reads a Netpbm image as input and produces a GIF file as out‐
40 put.
41 This program creates only individual GIF images. To combine multiple
43 GIF images into an animated GIF, use gifsicle
44 ⟨http://www.lcdf.org/gifsicle/⟩ (not part of the Netpbm package).
45 pamtogif creates either an original GIF87 format GIF file or the newer
47 GIF89 format. It creates GIF89 when you request features that were new
48 with GIF89, to wit the -transparent or -comment options. Otherwise, it
49 creates GIF87. Really old GIF readers conceivably could not recognize
50 GIF89.
51 The GIF format is not capable of representing an image with more than
53 256 colors in it (it contains a color map with a maximum size of 256).
54 If the image you want to convert has more colors than that (ppmhist can
55 tell you), you can use pnmquant to reduce it to 256. Or use the more
56 complex but faster method described under the -mapfile option.
57 If your input image is a PAM with transparency information, ppmtogif
59 uses one entry in the GIF colormap specifically for the transparent
60 pixels, so you can have at most 255 opaque colors. In contrast, if you
61 use the -transparent option, one of the colors from the input becomes
62 transparent, so the limit is still 256.
63 pamtogif was new in Netpbm 10.37 (December 2006). In older Netpbm, use
65 ppmtogif.
66
69 -interlace
70 Produce an interlaced GIF file.
71 -sort Produce a GIF file with a color map sorted in a predictable
74 order.
75 This does not produce the sorted color map which is part of the
77 GIF format. That kind of sorted color map is one where the col‐
78 ors are sorted according to how important they are, and the GIF
79 header tells the viewer that it is sorted that way. Its purpose
80 is to allow the viewer to use fewer colors than are in the color
81 map if it is not capable of displaying all the colors.
82 What this option produces is a color map sorted by red value,
84 then green, then blue. That can be useful in analyzing GIF
85 images, particularly those made with two versions of the pro‐
86 gram, because it removes some of the variability.
87 -mapfile=mapfile
91 Use the colors found in the file mapfile to create the colormap
93 in the GIF file, instead of the colors from netpbmfile. mapfile
94 can be any PPM file; all that matters is the colors in it. If
95 the colors in netpbmfile do not match those in mapfile, pamtogif
96 matches them to a "best match." You can obtain a much better
97 result by using pnmremap to change the colors in the input to
98 those in the map file.
99 The mapfile file is not a palette file, just an image whose col‐
101 ors you want to use. The order of colors in the GIF palette
102 have nothing to do with where they appear in the mapfile image,
103 and duplication of colors in the image is irrelevant.
104 The map file's depth must match the number of color components
106 in the input (which is not necessarily the same as the input's
107 depth -- the input might have a transparency plane in addition).
108 If your map file does not, or it might not, run your input
109 through pnmremap using the same map file so that it does.
110 You can use -mapfile to speed up conversion of an image where
112 you already have a map file because of earlier processing of
113 your image. For example, it is common to start with an image
114 that has more than 256 colors and remap its colors to a set of
115 256 colors so that pamgtogif can convert it (a GIF can have only
116 256 colors; pamtogif without -mapfile fails on any image that
117 has more than that) with pnmquant. When you do this, pnmquant
118 generates a palette to do the color quantization, then pamtogif
119 generates an identical palette from the quantized image. You
120 can save computation by generating the palette once:
121 $ pnmcolormap 256 myimage.ppm >/tmp/colormap.ppm
123 $ pamtogif myimage.ppm -mapfile=/tmp/colormap.ppm >output.gif
124 -transparent=color
128 pamtogif marks the specified color as transparent in the GIF
129 image.
130 If you don't specify -transparent, pamtogif does not mark any
132 color transparent (except as indicated by the transparency
133 information in the input file).
134 Specify the color (color) as described for the argument of the
136 pnm_parsecolor() library routine
137 ⟨libnetpbm_image.html#colorname⟩ .
138 If the color you specify is not present in the image, pamtogif
140 selects instead the color in the image that is closest to the
141 one you specify. Closeness is measured as a Cartesian distance
142 between colors in RGB space. If multiple colors are equidis‐
143 tant, pamtogif chooses one of them arbitrarily.
144 However, if you prefix your color specification with "=", e.g.
146 -transparent==red, only the exact color you specify will be
147 transparent. If that color does not appear in the image, there
148 will be no transparency. pamtogif issues an information message
149 when this is the case.
150 When you specify -transparent, pamtogif ignores explicit trans‐
152 parency information (the "alpha channel") in the input image.
153 -alpha=pgmfile
156 There is no -alpha option. pamtogif's predecessor had such an
157 option because it was not capable of taking PAM input that con‐
158 tains a transparency (alpha) plane, so one used this option to
159 supply a transparency plane as a separate PGM file.
160 This option names a PGM file that contains a transparency mask
162 for the image. pamtogif creates fully transparent pixels wher‐
163 ever the transparency mask indicates transparency greater than
164 50%. The color of those pixels is that specified by the -alpha‐
165 color option, or black by default.
166 To do this, pamtogif creates an entry in the GIF colormap in
168 addition to the entries for colors that are actually in the
169 image. It marks that colormap entry as transparent and uses
170 that colormap index in the output image to create a transparent
171 pixel.
172 The transparency image must be the same dimensions as the input
174 image, but may have any maxval. White means opaque and black
175 means transparent.
176 You cannot specify both -transparent and -alpha.
178 -alphacolor=color
181 This specifies the foreground color for transparent pixels. A
182 viewer may use the foreground color for a transparent pixel if
183 it chooses not to have another color "show through.". The
184 default is black.
185 This applies only to pixels that are transparent in the GIF
187 because they are transparent in the Netpbm input. If a GIF
188 pixel is transparent because of the -transparent option, the
189 foreground color is the color indicated by that option.
190 Note that in GIF, all transparent pixels have the same fore‐
192 ground color. (There is only one entry in the GIF colormap for
193 transparent pixels).
194 Specify the color (color) as described for the argument of the
196 pnm_parsecolor() library routine
197 ⟨libnetpbm_image.html#colorname⟩ .
198 -comment=text
201 Include a comment in the GIF output with comment text text.
202 Without this option, there are no comments in the output.
204 Note that in a command shell, you'll have to use quotation marks
206 around text if it contains characters (e.g. space) that would
207 make the shell think it is multiple arguments:
208 $ pamtogif -comment "this is a comment" <xxx.ppm >xxx.gif
209 -noclear
212 This option causes the output not to contain any GIF clear
214 codes.
215 In GIF, the stream defines codes that represent strings of pix‐
217 els as it goes. The stream contains definitions of codes mixed
218 in with the references to those codes that describe the pixels
219 of the image. GIF specifies a maximum number of codes that can
220 be defined; when the stream has defined that many, the stream
221 can either just use those for the rest of the image or include a
222 clear code, deleting all the string codes so that the stream can
223 start over defining new ones.
224 By far the most common choice is the clear code. This usually
226 results in a smaller stream because the set of strings of pixels
227 that occur in an image vary over the parts of the image. Hardly
228 any GIF encoders produce streams that don't use the clear code.
229 But it is conceivable that a stream could be smaller without the
231 use of the clear code because it saves the stream having to
232 redefine the same string codes over and over. It could even
233 avoid a thrashing situation where the stream continually defines
234 a set of strings that never get used again before the maximum is
235 reached.
236 The default is to use the clear codes.
238 This option was new in Netpbm 10.82 (March 2018). Before that,
240 the program aways uses the clear codes.
241 -nolzw
244 This option is mainly of historical interest -- it involves use
246 of a patent that is now expired.
247 This option causes the GIF output, and thus pamtogif, not to use
249 LZW (Lempel-Ziv) compression. As a result, the image file is
250 larger and, before the patent expired, no royalties would be
251 owed to the holder of the patent on LZW. See the section
252 LICENSE below.
253 LZW is a method for combining the information from multiple pix‐
255 els into a single GIF code. With the -nolzw option, pamtogif
256 creates one GIF code per pixel, so it is not doing any compres‐
257 sion and not using LZW. However, any GIF decoder, whether it
258 uses an LZW decompressor or not, will correctly decode this
259 uncompressed format. An LZW decompressor would see this as a
260 particular case of LZW compression.
261 Note that if someone uses an LZW decompressor such as the one in
263 giftopnm or pretty much any graphics display program to process
264 the output of pamtogif -nolzw , he is then using the LZW patent.
265 But the patent holder expressed far less interest in enforcing
266 the patent on decoding than on encoding.
267 -aspect=fraction
270 This is the aspect ratio of the pixels of the image. Its only
271 effect is to record that information in the GIF for use by what‐
272 ever interprets the GIF. Note that this feature of GIF is
273 hardly ever used and most GIF decoders ignore this information
274 and assume pixels are square.
275 Pixels in a Netpbm image do not have aspect ratios; there is
277 always a one-one correspondence between GIF pixels and Netpbm
278 pixels.
279 The aspect ratio is the quotient of width divided by height.
281 GIF allows aspect ratios from 0.25 (1:4) to 4 (4:1) in incre‐
282 ments of 1/64. pamtogif implements a natural extension of GIF
283 that allows an aspect ratio up to 4 14/64. If you specify any‐
284 thing outside this range, pamtogif fails. pamtogif rounds frac‐
285 tion to the nearest 1/64.
286 The default is square (1.0).
288 This option was new in Netpbm 10.38 (March 2007). Before that,
290 the pixels are always square.
291 -verbose
295 This option causes pamtogif to display information about the
296 conversion process and the image it produces.
297
302 giftopnm(1), pnmremap(1), ppmtogif(1),
303 gifsicle http://www.lcdf.org/gifsicle ⟨http://www.lcdf.org/gifsicle⟩ ,
305 pnm(1), pam(1).
306
309 pamtogif was new in Netpbm 10.37 (December 2006). It replaced ppm‐
310 togif, which created GIF images for Pbmplus/Netpbm users since 1989.
311 The main outward change in the conversion from ppmtogif to pamtogif was
313 that pamtogif was able to use transparency information ("alpha chan‐
314 nel") in PAM input, whereas with ppmtogif, one had to supply the trans‐
315 parency mask in a separate pseudo-PGM image (via the -alpha option).
316 Jef Poskanzer wrote ppmtogif in 1989, and it has always been a corner‐
318 stone of Pbmplus/Netpbm because GIF is such a popular image format.
319 Jef based the LZW encoding on GIFENCOD by David Rowley <mgardi@watd‐
320 csu.waterloo.edu>. Jef included GIFENCOD's GIFCOMPR.C file pretty much
321 whole. Rowley, in turn, adapted the LZW compression code from classic
322 Unix compress, which used techniques described in IEEE Computer, June
323 1984.
324 Jef's ppmtogif notably lacked the ability to use a transparency mask
326 with it. You could create transparent pixels in a GIF, but only with
327 the -transparent option, which allowed one to specify that all pixels
328 of a certain color in the input were to be transparent. Bryan Hender‐
329 son added the -alpha option in July 2001 so you could supply a mask
330 image that indicates exactly which pixels are to be transparent, and
331 those pixels could have the same color as other opaque ones.
332 Bryan Henderson added another significant piece of code and function in
334 October 2001: the ability to generate a GIF without using the LZW
335 patent -- an uncompressed GIF. This was very important to many people
336 at the time because the GIF patent was still in force, and this allowed
337 them to make an image that any GIF viewer could display, royalty-free.
338 Bryan adapted code from the Independent JPEG Group's djpeg for that.
339 There is no code in pamtogif from Jef's original, but Jef may still
341 hold copyright over it because of the way in which it evolved. Virtu‐
342 ally all of the code in pamtogif was written by Bryan Henderson and
343 contributed to the public domain.
344
348 If you use pamtogif without the -nolzw option, you are using a patent
349 on the LZW compression method which is owned by Unisys. The patent has
350 expired (in 2003 in the US and in 2004 elsewhere), so it doesn't mat‐
351 ter. While the patent was in force, most people who used pamtogif and
352 similar programs did so without a license from Unisys to do so. Unisys
353 typically asked $5000 for a license for trivial use of the patent.
354 Unisys never enforced the patent against trivial users.
355 Rumor has it that IBM also owns or owned a patent covering pamtogif.
357 A replacement for the GIF format that never required any patents to use
359 is the PNG format.
360
362 This manual page was generated by the Netpbm tool 'makeman' from HTML
363 source. The master documentation is at
364 http://netpbm.sourceforge.net/doc/pamtogif.html
366netpbm documentation 22 March 2007 Pamtogif User Manual(0)