1Pamtogif User Manual(0) Pamtogif User Manual(0)
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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
47 newer GIF89 format. It creates GIF89 when the output needs to have
48 features
49 that were new with GIF89, to wit transparency or comments. Other‐
50 wise, it
51 creates GIF87. Really old GIF readers conceivably could not recog‐
52 nize
53 GIF89. The output needs to have transparency when either the input
54 has a
55 transparency information or you specify the -transparent option. It
56 needs to have comments when you specify the
57 -comment option.
58 pamtogif generates a GIF image with a single image block, which means
60 the image cannot have more than 256 colors in it (it contains a single
61 color map with a maximum size of 256). If the image you want to con‐
62 vert has more colors than that (ppmhist can tell you), you can use pn‐
63 mquant to reduce it to 256. Or use the more complex but faster method
64 described under the -mapfile option.
65 If your input image is a PAM with transparency information, pamtogif
67 uses one entry in the GIF colormap specifically for the transparent
68 pixels, so you can have at most 255 opaque colors. In contrast, if you
69 use the -transparent option, one of the colors from the input becomes
70 transparent, so the limit is still 256.
71 pamtogif recognizes transparency information in the input by the
73 tuple type being RGB_ALPHA, GRAYSCALE_ALPHA, or
74 BLACKANDWHITE_ALPHA. This is the case for any image that has
75 transparency information and was created by a Netpbm program that
76 manipulates visual images. If, on the other hand, you have a PAM
77 generated
78 some other way, but you know the planes have the same meaning as im‐
79 plied by
80 these tuple types, you can make pamtogif process the transparency
81 information by changing the tuple type accordingly before you pass it
82 to pamtogif. You can use pamstack to change the tuple type.
83 pamtogif was new in Netpbm 10.37 (December 2006). In older Netpbm, use
85 ppmtogif.
86
89 In addition to the options common to all programs based on libnetpbm
90 (most notably -quiet, see
91 Common Options ⟨index.html#commonoptions⟩ ), pamtogif recognizes the
92 following command line options:
93 -interlace
98 Produce an interlaced GIF file.
99 -sort Produce a GIF file with a color map sorted in a predictable or‐
102 der.
103 This does not produce the sorted color map which is part of the
105 GIF format. That kind of sorted color map is one where the col‐
106 ors are sorted according to how important they are, and the GIF
107 header tells the viewer that it is sorted that way. Its purpose
108 is to allow the viewer to use fewer colors than are in the color
109 map if it is not capable of displaying all the colors.
110 What this option produces is a color map sorted by red value,
112 then green, then blue. That can be useful in analyzing GIF im‐
113 ages, particularly those made with two versions of the program,
114 because it removes some of the variability.
115 -mapfile=mapfile
119 Use the colors found in the file mapfile to create the colormap
121 in the GIF file, instead of the colors from netpbmfile. mapfile
122 can be any PPM file; all that matters is the colors in it. If
123 the colors in netpbmfile do not match those in mapfile, pamtogif
124 matches them to a "best match." You can obtain a much better re‐
125 sult by using pnmremap to change the colors in the input to
126 those in the map file.
127 The mapfile file is not a palette file, just an image whose col‐
129 ors you want to use. The order of colors in the GIF palette
130 have nothing to do with where they appear in the mapfile image,
131 and duplication of colors in the image is irrelevant.
132 The map file's depth must match the number of color components
134 in the input (which is not necessarily the same as the input's
135 depth -- the input might have a transparency plane in addition).
136 If your map file does not, or it might not, run your input
137 through pnmremap using the same map file so that it does.
138 You can use -mapfile to speed up conversion of an image where
140 you already have a map file because of earlier processing of
141 your image. For example, it is common to start with an image
142 that has more than 256 colors and remap its colors to a set of
143 256 colors so that pamgtogif can convert it (a GIF can have only
144 256 colors; pamtogif without -mapfile fails on any image that
145 has more than that) with pnmquant. When you do this, pnmquant
146 generates a palette to do the color quantization, then pamtogif
147 generates an identical palette from the quantized image. You
148 can save computation by generating the palette once:
149 $ pnmcolormap 256 myimage.ppm >/tmp/colormap.ppm
151 $ pamtogif myimage.ppm -mapfile=/tmp/colormap.ppm >output.gif
152 -transparent=color
157 pamtogif marks the specified color as transparent in the GIF im‐
158 age.
159 If you don't specify -transparent, pamtogif does not mark any
161 color transparent (except as indicated by the transparency in‐
162 formation in the input file).
163 Specify the color (color) as described for the argument of the
165 pnm_parsecolor() library routine
166 ⟨libnetpbm_image.html#colorname⟩ .
167 If the color you specify is not present in the image, pamtogif
169 selects instead the color in the image that is closest to the
170 one you specify. Closeness is measured as a Cartesian distance
171 between colors in RGB space. If multiple colors are equidis‐
172 tant, pamtogif chooses one of them arbitrarily.
173 However, if you prefix your color specification with "=", e.g.
175 -transparent==red, only the exact color you specify will be
176 transparent. If that color does not appear in the image, there
177 will be no transparency. pamtogif issues an information message
178 when this is the case.
179 When you specify -transparent, pamtogif ignores explicit trans‐
181 parency information (the "alpha channel") in the input image.
182 -alphacolor=color
185 This specifies the foreground color for transparent pixels. A
186 viewer may use the foreground color for a transparent pixel if
187 it chooses not to have another color "show through.". The de‐
188 fault is black.
189 This applies only to pixels that are transparent in the GIF be‐
191 cause they are transparent in the Netpbm input. If a GIF pixel
192 is transparent because of the -transparent option, the fore‐
193 ground color is the color indicated by that option.
194 Note that in GIF, all transparent pixels have the same fore‐
196 ground color. (There is only one entry in the GIF colormap for
197 transparent pixels).
198 Specify the color (color) as described for the argument of the
200 pnm_parsecolor() library routine
201 ⟨libnetpbm_image.html#colorname⟩ .
202 -comment=text
205 Include a comment in the GIF output with comment text text.
206 Without this option, there are no comments in the output.
208 Note that in a command shell, you'll have to use quotation marks
210 around text if it contains characters (e.g. space) that would
211 make the shell think it is multiple arguments:
212 $ pamtogif -comment "this is a comment" <xxx.ppm >xxx.gif
213 -noclear
217 This option causes the output not to contain any GIF clear
219 codes.
220 In GIF, the stream defines codes that represent strings of pix‐
222 els as it goes. The stream contains definitions of codes mixed
223 in with the references to those codes that describe the pixels
224 of the image. GIF specifies a maximum number of codes that can
225 be defined; when the stream has defined that many, the stream
226 can either just use those for the rest of the image or include a
227 clear code, deleting all the string codes so that the stream can
228 start over defining new ones.
229 By far the most common choice is the clear code. This usually
231 results in a smaller stream because the set of strings of pixels
232 that occur in an image vary over the parts of the image. Hardly
233 any GIF encoders produce streams that don't use the clear code.
234 But it is conceivable that a stream could be smaller without the
236 use of the clear code because it saves the stream having to re‐
237 define the same string codes over and over. It could even avoid
238 a thrashing situation where the stream continually defines a set
239 of strings that never get used again before the maximum is
240 reached.
241 The default is to use the clear codes.
243 This option was new in Netpbm 10.82 (March 2018). Before that,
245 the program aways uses the clear codes.
246 -nolzw
249 This option is mainly of historical interest -- it involves use
251 of a patent that is now expired.
252 This option causes the GIF output, and thus pamtogif, not to use
254 LZW (Lempel-Ziv) compression. As a result, the image file is
255 larger and, before the patent expired, no royalties would be
256 owed to the holder of the patent on LZW. See the section LI‐
257 CENSE below.
258 LZW is a method for combining the information from multiple pix‐
260 els into a single GIF code. With the -nolzw option, pamtogif
261 creates one GIF code per pixel, so it is not doing any compres‐
262 sion and not using LZW. However, any GIF decoder, whether it
263 uses an LZW decompressor or not, will correctly decode this un‐
264 compressed format. An LZW decompressor would see this as a par‐
265 ticular case of LZW compression.
266 Note that if someone uses an LZW decompressor such as the one in
268 giftopnm or pretty much any graphics display program to process
269 the output of pamtogif -nolzw , he is then using the LZW patent.
270 But the patent holder expressed far less interest in enforcing
271 the patent on decoding than on encoding.
272 -aspect=fraction
275 This is the aspect ratio of the pixels of the image. Its only
276 effect is to record that information in the GIF for use by what‐
277 ever interprets the GIF. Note that this feature of GIF is
278 hardly ever used and most GIF decoders ignore this information
279 and assume pixels are square.
280 Pixels in a Netpbm image do not have aspect ratios; there is al‐
282 ways a one-one correspondence between GIF pixels and Netpbm pix‐
283 els.
284 The aspect ratio is the quotient of width divided by height.
286 GIF allows aspect ratios from 0.25 (1:4) to 4 (4:1) in incre‐
287 ments of 1/64. pamtogif implements a natural extension of GIF
288 that allows an aspect ratio up to 4 14/64. If you specify any‐
289 thing outside this range, pamtogif fails. pamtogif rounds frac‐
290 tion to the nearest 1/64.
291 The default is square (1.0).
293 This option was new in Netpbm 10.38 (March 2007). Before that,
295 the pixels are always square.
296 -verbose
300 This option causes pamtogif to display information about the
301 conversion process and the image it produces.
302
307 giftopnm(1), pnmremap(1), ppmtogif(1),
308 gifsicle http://www.lcdf.org/gifsicle ⟨http://www.lcdf.org/gifsicle⟩ ,
310 pnm(1), pam(1).
311
314 pamtogif was new in Netpbm 10.37 (December 2006). It replaced ppm‐
315 togif, which created GIF images for Pbmplus/Netpbm users since 1989.
316 The main outward change in the conversion from ppmtogif to pamtogif was
318 that pamtogif was able to use transparency information ("alpha chan‐
319 nel") in PAM input, whereas with ppmtogif, one had to supply the trans‐
320 parency mask in a separate pseudo-PGM image (via the -alpha option).
321 Jef Poskanzer wrote ppmtogif in 1989, and it has always been a corner‐
323 stone of Pbmplus/Netpbm because GIF is such a popular image format.
324 Jef based the LZW encoding on GIFENCOD by David Rowley <mgardi@watd‐
325 csu.waterloo.edu>. Jef included GIFENCOD's GIFCOMPR.C file pretty much
326 whole. Rowley, in turn, adapted the LZW compression code from classic
327 Unix compress, which used techniques described in IEEE Computer, June
328 1984.
329 Jef's ppmtogif notably lacked the ability to use a transparency mask
331 with it. You could create transparent pixels in a GIF, but only with
332 the -transparent option, which allowed one to specify that all pixels
333 of a certain color in the input were to be transparent. Bryan Hender‐
334 son added the -alpha option in July 2001 so you could supply a mask im‐
335 age that indicates exactly which pixels are to be transparent, and
336 those pixels could have the same color as other opaque ones.
337 Bryan Henderson added another significant piece of code and function in
339 October 2001: the ability to generate a GIF without using the LZW
340 patent -- an uncompressed GIF. This was very important to many people
341 at the time because the GIF patent was still in force, and this allowed
342 them to make an image that any GIF viewer could display, royalty-free.
343 Bryan adapted code from the Independent JPEG Group's djpeg for that.
344 There is no code in pamtogif from Jef's original, but Jef may still
346 hold copyright over it because of the way in which it evolved. Virtu‐
347 ally all of the code in pamtogif was written by Bryan Henderson and
348 contributed to the public domain.
349
353 If you use pamtogif without the -nolzw option, you are using a patent
354 on the LZW compression method which is owned by Unisys. The patent has
355 expired (in 2003 in the US and in 2004 elsewhere), so it doesn't mat‐
356 ter. While the patent was in force, most people who used pamtogif and
357 similar programs did so without a license from Unisys to do so. Unisys
358 typically asked $5000 for a license for trivial use of the patent.
359 Unisys never enforced the patent against trivial users.
360 Rumor has it that IBM also owns or owned a patent covering pamtogif.
362 A replacement for the GIF format that never required any patents to use
364 is the PNG format.
365
367 This manual page was generated by the Netpbm tool 'makeman' from HTML
368 source. The master documentation is at
369 http://netpbm.sourceforge.net/doc/pamtogif.html
371netpbm documentation 09 June 2021 Pamtogif User Manual(0)