1Pamtogif User Manual(0) Pamtogif User Manual(0)
2
6 pamtogif - convert a Netpbm image to a GIF image
7
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 In addition to the options common to all programs based on libnetpbm
70 (most notably -quiet, see
71 Common Options ⟨index.html#commonoptions⟩ ), pamtogif recognizes the
72 following command line options:
73 -interlace
78 Produce an interlaced GIF file.
79 -sort Produce a GIF file with a color map sorted in a predictable
82 order.
83 This does not produce the sorted color map which is part of the
85 GIF format. That kind of sorted color map is one where the col‐
86 ors are sorted according to how important they are, and the GIF
87 header tells the viewer that it is sorted that way. Its purpose
88 is to allow the viewer to use fewer colors than are in the color
89 map if it is not capable of displaying all the colors.
90 What this option produces is a color map sorted by red value,
92 then green, then blue. That can be useful in analyzing GIF
93 images, particularly those made with two versions of the pro‐
94 gram, because it removes some of the variability.
95 -mapfile=mapfile
99 Use the colors found in the file mapfile to create the colormap
101 in the GIF file, instead of the colors from netpbmfile. mapfile
102 can be any PPM file; all that matters is the colors in it. If
103 the colors in netpbmfile do not match those in mapfile, pamtogif
104 matches them to a "best match." You can obtain a much better
105 result by using pnmremap to change the colors in the input to
106 those in the map file.
107 The mapfile file is not a palette file, just an image whose col‐
109 ors you want to use. The order of colors in the GIF palette
110 have nothing to do with where they appear in the mapfile image,
111 and duplication of colors in the image is irrelevant.
112 The map file's depth must match the number of color components
114 in the input (which is not necessarily the same as the input's
115 depth -- the input might have a transparency plane in addition).
116 If your map file does not, or it might not, run your input
117 through pnmremap using the same map file so that it does.
118 You can use -mapfile to speed up conversion of an image where
120 you already have a map file because of earlier processing of
121 your image. For example, it is common to start with an image
122 that has more than 256 colors and remap its colors to a set of
123 256 colors so that pamgtogif can convert it (a GIF can have only
124 256 colors; pamtogif without -mapfile fails on any image that
125 has more than that) with pnmquant. When you do this, pnmquant
126 generates a palette to do the color quantization, then pamtogif
127 generates an identical palette from the quantized image. You
128 can save computation by generating the palette once:
129 $ pnmcolormap 256 myimage.ppm >/tmp/colormap.ppm
131 $ pamtogif myimage.ppm -mapfile=/tmp/colormap.ppm >output.gif
132 -transparent=color
136 pamtogif marks the specified color as transparent in the GIF
137 image.
138 If you don't specify -transparent, pamtogif does not mark any
140 color transparent (except as indicated by the transparency
141 information in the input file).
142 Specify the color (color) as described for the argument of the
144 pnm_parsecolor() library routine
145 ⟨libnetpbm_image.html#colorname⟩ .
146 If the color you specify is not present in the image, pamtogif
148 selects instead the color in the image that is closest to the
149 one you specify. Closeness is measured as a Cartesian distance
150 between colors in RGB space. If multiple colors are equidis‐
151 tant, pamtogif chooses one of them arbitrarily.
152 However, if you prefix your color specification with "=", e.g.
154 -transparent==red, only the exact color you specify will be
155 transparent. If that color does not appear in the image, there
156 will be no transparency. pamtogif issues an information message
157 when this is the case.
158 When you specify -transparent, pamtogif ignores explicit trans‐
160 parency information (the "alpha channel") in the input image.
161 -alpha=pgmfile
164 There is no -alpha option. pamtogif's predecessor had such an
165 option because it was not capable of taking PAM input that con‐
166 tains a transparency (alpha) plane, so one used this option to
167 supply a transparency plane as a separate PGM file.
168 This option names a PGM file that contains a transparency mask
170 for the image. pamtogif creates fully transparent pixels wher‐
171 ever the transparency mask indicates transparency greater than
172 50%. The color of those pixels is that specified by the -alpha‐
173 color option, or black by default.
174 To do this, pamtogif creates an entry in the GIF colormap in
176 addition to the entries for colors that are actually in the
177 image. It marks that colormap entry as transparent and uses
178 that colormap index in the output image to create a transparent
179 pixel.
180 The transparency image must be the same dimensions as the input
182 image, but may have any maxval. White means opaque and black
183 means transparent.
184 You cannot specify both -transparent and -alpha.
186 -alphacolor=color
189 This specifies the foreground color for transparent pixels. A
190 viewer may use the foreground color for a transparent pixel if
191 it chooses not to have another color "show through.". The
192 default is black.
193 This applies only to pixels that are transparent in the GIF
195 because they are transparent in the Netpbm input. If a GIF
196 pixel is transparent because of the -transparent option, the
197 foreground color is the color indicated by that option.
198 Note that in GIF, all transparent pixels have the same fore‐
200 ground color. (There is only one entry in the GIF colormap for
201 transparent pixels).
202 Specify the color (color) as described for the argument of the
204 pnm_parsecolor() library routine
205 ⟨libnetpbm_image.html#colorname⟩ .
206 -comment=text
209 Include a comment in the GIF output with comment text text.
210 Without this option, there are no comments in the output.
212 Note that in a command shell, you'll have to use quotation marks
214 around text if it contains characters (e.g. space) that would
215 make the shell think it is multiple arguments:
216 $ pamtogif -comment "this is a comment" <xxx.ppm >xxx.gif
217 -noclear
220 This option causes the output not to contain any GIF clear
222 codes.
223 In GIF, the stream defines codes that represent strings of pix‐
225 els as it goes. The stream contains definitions of codes mixed
226 in with the references to those codes that describe the pixels
227 of the image. GIF specifies a maximum number of codes that can
228 be defined; when the stream has defined that many, the stream
229 can either just use those for the rest of the image or include a
230 clear code, deleting all the string codes so that the stream can
231 start over defining new ones.
232 By far the most common choice is the clear code. This usually
234 results in a smaller stream because the set of strings of pixels
235 that occur in an image vary over the parts of the image. Hardly
236 any GIF encoders produce streams that don't use the clear code.
237 But it is conceivable that a stream could be smaller without the
239 use of the clear code because it saves the stream having to
240 redefine the same string codes over and over. It could even
241 avoid a thrashing situation where the stream continually defines
242 a set of strings that never get used again before the maximum is
243 reached.
244 The default is to use the clear codes.
246 This option was new in Netpbm 10.82 (March 2018). Before that,
248 the program aways uses the clear codes.
249 -nolzw
252 This option is mainly of historical interest -- it involves use
254 of a patent that is now expired.
255 This option causes the GIF output, and thus pamtogif, not to use
257 LZW (Lempel-Ziv) compression. As a result, the image file is
258 larger and, before the patent expired, no royalties would be
259 owed to the holder of the patent on LZW. See the section
260 LICENSE below.
261 LZW is a method for combining the information from multiple pix‐
263 els into a single GIF code. With the -nolzw option, pamtogif
264 creates one GIF code per pixel, so it is not doing any compres‐
265 sion and not using LZW. However, any GIF decoder, whether it
266 uses an LZW decompressor or not, will correctly decode this
267 uncompressed format. An LZW decompressor would see this as a
268 particular case of LZW compression.
269 Note that if someone uses an LZW decompressor such as the one in
271 giftopnm or pretty much any graphics display program to process
272 the output of pamtogif -nolzw , he is then using the LZW patent.
273 But the patent holder expressed far less interest in enforcing
274 the patent on decoding than on encoding.
275 -aspect=fraction
278 This is the aspect ratio of the pixels of the image. Its only
279 effect is to record that information in the GIF for use by what‐
280 ever interprets the GIF. Note that this feature of GIF is
281 hardly ever used and most GIF decoders ignore this information
282 and assume pixels are square.
283 Pixels in a Netpbm image do not have aspect ratios; there is
285 always a one-one correspondence between GIF pixels and Netpbm
286 pixels.
287 The aspect ratio is the quotient of width divided by height.
289 GIF allows aspect ratios from 0.25 (1:4) to 4 (4:1) in incre‐
290 ments of 1/64. pamtogif implements a natural extension of GIF
291 that allows an aspect ratio up to 4 14/64. If you specify any‐
292 thing outside this range, pamtogif fails. pamtogif rounds frac‐
293 tion to the nearest 1/64.
294 The default is square (1.0).
296 This option was new in Netpbm 10.38 (March 2007). Before that,
298 the pixels are always square.
299 -verbose
303 This option causes pamtogif to display information about the
304 conversion process and the image it produces.
305
310 giftopnm(1), pnmremap(1), ppmtogif(1),
311 gifsicle http://www.lcdf.org/gifsicle ⟨http://www.lcdf.org/gifsicle⟩ ,
313 pnm(1), pam(1).
314
317 pamtogif was new in Netpbm 10.37 (December 2006). It replaced ppm‐
318 togif, which created GIF images for Pbmplus/Netpbm users since 1989.
319 The main outward change in the conversion from ppmtogif to pamtogif was
321 that pamtogif was able to use transparency information ("alpha chan‐
322 nel") in PAM input, whereas with ppmtogif, one had to supply the trans‐
323 parency mask in a separate pseudo-PGM image (via the -alpha option).
324 Jef Poskanzer wrote ppmtogif in 1989, and it has always been a corner‐
326 stone of Pbmplus/Netpbm because GIF is such a popular image format.
327 Jef based the LZW encoding on GIFENCOD by David Rowley <mgardi@watd‐
328 csu.waterloo.edu>. Jef included GIFENCOD's GIFCOMPR.C file pretty much
329 whole. Rowley, in turn, adapted the LZW compression code from classic
330 Unix compress, which used techniques described in IEEE Computer, June
331 1984.
332 Jef's ppmtogif notably lacked the ability to use a transparency mask
334 with it. You could create transparent pixels in a GIF, but only with
335 the -transparent option, which allowed one to specify that all pixels
336 of a certain color in the input were to be transparent. Bryan Hender‐
337 son added the -alpha option in July 2001 so you could supply a mask
338 image that indicates exactly which pixels are to be transparent, and
339 those pixels could have the same color as other opaque ones.
340 Bryan Henderson added another significant piece of code and function in
342 October 2001: the ability to generate a GIF without using the LZW
343 patent -- an uncompressed GIF. This was very important to many people
344 at the time because the GIF patent was still in force, and this allowed
345 them to make an image that any GIF viewer could display, royalty-free.
346 Bryan adapted code from the Independent JPEG Group's djpeg for that.
347 There is no code in pamtogif from Jef's original, but Jef may still
349 hold copyright over it because of the way in which it evolved. Virtu‐
350 ally all of the code in pamtogif was written by Bryan Henderson and
351 contributed to the public domain.
352
356 If you use pamtogif without the -nolzw option, you are using a patent
357 on the LZW compression method which is owned by Unisys. The patent has
358 expired (in 2003 in the US and in 2004 elsewhere), so it doesn't mat‐
359 ter. While the patent was in force, most people who used pamtogif and
360 similar programs did so without a license from Unisys to do so. Unisys
361 typically asked $5000 for a license for trivial use of the patent.
362 Unisys never enforced the patent against trivial users.
363 Rumor has it that IBM also owns or owned a patent covering pamtogif.
365 A replacement for the GIF format that never required any patents to use
367 is the PNG format.
368
370 This manual page was generated by the Netpbm tool 'makeman' from HTML
371 source. The master documentation is at
372 http://netpbm.sourceforge.net/doc/pamtogif.html
374netpbm documentation 22 March 2007 Pamtogif User Manual(0)