1Jpegtopnm User Manual(0) Jpegtopnm User Manual(0)
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6 jpegtopnm - convert JPEG/JFIF file to PPM or PGM image
7
10 jpegtopnm [-dct {int|fast|float}] [-nosmooth] [-maxmemory N]
11 [{-adobe|-notadobe}] [-comments] [-dumpexif] [-exif=filespec] [-multi‐
12 ple] [-repair] [-verbose] [-tracelevel N] [filename]
13 Minimum unique abbreviation of option is acceptable. You may use dou‐
15 ble hyphens instead of single hyphen to denote options. You may use
16 white space in place of the equals sign to separate an option name from
17 its value.
18
21 This program is part of Netpbm(1).
22 jpegtopnm converts JFIF images to PPM or PGM images.
24 By default, jpegtopnm expects the input stream to contain one JFIF
26 image and produces one PGM or PPM image as output. It fails if the
27 input stream is empty.
28 But with the -multiple option, jpegtopnm reads JFIF images sequentially
30 from the input stream and writes one PPM or PGM image to the output
31 stream for each JFIF input. If the input stream is empty, so is the
32 output.
33 The input stream is the filename you specify or, if you don't specify
35 filename, Standard Input. The output stream is Standard Output.
36 If a JFIF input image is of the grayscale variety, jpegtopnm generates
38 a PGM image. Otherwise, it generates a PPM image.
39 Before Netpbm 10.11 (October 2002), jpegtopnm did not have the multiple
41 image stream capability. From 10.11 through 10.22, Netpbm always
42 behaved as if you specified -multiple. Starting with Netpbm 10.23
43 (July 2004), Netpbm's default behavior went back to the pre-10.11
44 behavior and the new -multiple option selected the 10.12 behavior. The
45 reason for the reversion was that there were discovered in the world
46 files that contain JFIF images followed by something other than another
47 JFIF image. The producers of these files expect them to work with any
48 JFIF interpreter because most JFIF interpreters just stop reading the
49 file after the first JFIF image.
50 jpegtopnm uses the Independent JPEG Group's JPEG library to interpret
52 the input file. See http://www.ijg.org ⟨http://www.ijg.org⟩ for
53 information on the library.
54 "JFIF" is the correct name for the image format commonly known as
56 "JPEG." Strictly speaking, JPEG is a method of compression. The image
57 format using JPEG compression that is by far the most common is JFIF.
58 There is also a subformat of TIFF that uses JPEG compression.
59 EXIF is an image format that is a subformat of JFIF (to wit, a JFIF
61 file that contains an EXIF header as an APP1 marker). jpegtopnm han‐
62 dles EXIF.
63 JFIF files can have either 8 bits per sample or 12 bits per sample.
65 The 8 bit variety is by far the most common. There are two versions of
66 the IJG JPEG library. One reads only 8 bit files and the other reads
67 only 12 bit files. You must link the appropriate one of these
68 libraries with jpegtopnm. Ordinarily, this means the library is in
69 your shared library search path when you run jpegtopnm.
70 jpegtopnm generates output with either one byte or two bytes per sample
72 depending on whether the JFIF input has either 8 bits or 12 bits per
73 sample. You can use pamdepth to reduce a two-byte-per-sample file to a
74 one-byte-per-sample file if you need to.
75 If the JFIF file uses the CMYK or YCCK color space, the input does not
77 actually contain enough information to know what color each pixel is.
78 To know what color a pixel is, one would have to know the properties of
79 the inks to which the color space refers. jpegtopnm interprets the
80 colors using the common transformation which assumes all the inks are
81 simply subtractive and linear.
82 See the jpegtopnm manual(1) for information on how images lose quality
84 when you convert to and from JFIF.
85
88 The options are only for advanced users:
89 -dct int
92 Use integer DCT method (default).
93 -dct fast
96 Use fast integer DCT (less accurate).
97 -dct float
100 Use floating-point DCT method. The float method is very
101 slightly more accurate than the int method, but is much slower
102 unless your machine has very fast floating-point hardware. Also
103 note that results of the floating-point method may vary slightly
104 across machines, while the integer methods should give the same
105 results everywhere. The fast integer method is much less accu‐
106 rate than the other two.
107 -nosmooth
110 Use a faster, lower-quality upsampling routine.
111 -maxmemory N
113 Set limit on the amount of memory jpegtopnm uses in processing
114 large images. Value is in thousands of bytes, or millions of
115 bytes if "M" is suffixed to the number. For example, -maxmemory
116 4m selects 4000000 bytes. If jpegtopnm needs more space, it
117 uses temporary files.
118 -adobe
121 -notadobe
123 There are two variations on the CMYK (and likewise YCCK) color
124 space that may be used in the JFIF input. In the normal one, a
125 zero value for a color components indicates absence of ink. In
126 the other, a zero value means the maximum ink coverage. The
127 latter is used by Adobe Photoshop when it creates a bare JFIF
128 output file (but not when it creates JFIF output as part of
129 Encapsulated Postscript output).
130 These options tell jpegtopnm which version of the CMYK or YCCK
132 color space the image uses. If you specify neither, jpegtopnm
133 tries to figure it out on its own. In the present version, it
134 doesn't try very hard at all: It just assumes the Photoshop ver‐
135 sion, since Photoshop and its emulators seem to be the main
136 source of CMYK and YCCK images. But with experience of use,
137 future versions might be more sophisticated.
138 If the JFIF image does not indicate that it is CMYK or YCCK,
140 these options have no effect.
141 If you don't use the right one of these options, the symptom is
143 output that looks like a negative.
144 -dumpexif
147 Print the interpreted contents of any Exif header in the input
148 file to the Standard Error file. Similar to the program jhead
149 (not part of the Netpbm package).
150 This option was added in Netpbm 9.19 (September 2001).
152 -exif=filespec
155 Extract the contents of the EXIF header from the input image and
156 write it to the file filespec. filespec=- means write it to
157 Standard Output. When you write the EXIF header to Standard
158 Output, jpegtopnm does not output the converted image (which is
159 what normally would go to Standard Output) at all.
160 jpegtopnm writes the contents of the EXIF header byte-for-byte,
162 starting with the two byte length field (which length includes
163 those two bytes).
164 You can use this file as input to pnmtojpeg to insert an identi‐
166 cal EXIF header into a new JFIF image.
167 If there is no EXIF header, jpegtopnm writes two bytes of binary
169 zero and nothing else.
170 An EXIF header takes the form of a JFIF APP1 marker. Only the
172 first such marker within the JFIF header counts.
173 This option was added in Netpbm 9.19 (September 2001).
175 -multiple
178 Read multiple JFIF images sequentially from the input stream.
179 See Description section ⟨#description⟩ for details.
180 This option was new in Netpbm 10.23 (July 2004).
182 -repair
185 If the JFIF input is invalid, try to salvage whatever informa‐
186 tion is there and produce a valid PNM image as output.
187 Without this option, some invalid input causes jpegtopnm to
189 fail, and what output it produces is undefined. With -repair
190 such invalid input causes jpegtopnm to succeed instead.
191 But note that there are some forms of invalid input that always
193 cause jpegtopnm to fail and others that always cause it to sal‐
194 vage image information and succeed.
195 One particular case where -repair makes a difference is the com‐
197 mon case that the file is truncated somewhere after the JFIF
198 header. Without -repair, that always causes a failure; with
199 -repair it always causes success. Because the image information
200 is laid out generally top to bottom in the JFIF stream, the
201 image jpegtopnm produces in this case has the proper image con‐
202 tents at the top, but the bottom is padded with gray.
203 This option was new in Netpbm 10.38.0 (March 2007). Before
205 that, jpegtopnm always fails in the cases in question.
206 -comments
210 Print any comments in the input file to the Standard Error file.
211 -verbose
214 Print details about the conversion to the Standard Error file.
215 -tracelevel n
218 Turn on the JPEG library's trace messages to the Standard Error
219 file. A higher value of n gets more trace information. -ver‐
220 bose implies a trace level of at least 1.
221
226 This example converts the color JFIF file foo.jpg to a PPM file named
227 foo.ppm:
228 jpegtopnm foo.jpg >foo.ppm
230
233 You can use pnmquant to color quantize the result, i.e. to reduce the
234 number of distinct colors in the image. In fact, you may have to if
235 you want to convert the PPM file to certain other formats. ppmdither
236 Does a more sophisticated quantization.
237 Use pamscale to change the dimensions of the resulting image.
239 Use ppmtopgm to convert a color JFIF file to a grayscale PGM file.
241 You can easily use these converters together. E.g.:
243 jpegtopnm foo.jpg | ppmtopgm | pamscale .25 >foo.pgm
245 -dct fast and/or -nosmooth gain speed at a small sacrifice in quality.
247 If you are fortunate enough to have very fast floating point hardware,
249 -dct float may be even faster than -dct fast. But on most machines
250 -dct float is slower than -dct int; in this case it is not worth using,
251 because its theoretical accuracy advantage is too small to be signifi‐
252 cant in practice.
253 Another program, djpeg, is similar. djpeg is maintained by the Inde‐
255 pendent JPEG Group and packaged with the JPEG library which jpegtopnm
256 uses for all its JPEG work. Because of that, you may expect it to
257 exploit more current JPEG features. Also, since you have to have the
258 library to run jpegtopnm, but not vice versa, cjpeg may be more com‐
259 monly available.
260 On the other hand, djpeg does not use the NetPBM libraries to generate
262 its output, as all the NetPBM tools such as jpegtopnm do. This means
263 it is less likely to be consistent with all the other programs that
264 deal with the NetPBM formats. Also, the command syntax of jpegtopnm is
265 consistent with that of the other Netpbm tools, unlike djpeg.
266
269 JPEGMEM
270 If this environment variable is set, its value is the default
271 memory limit. The value is specified as described for the
272 -maxmemory option. An explicit -maxmemory option overrides any
273 JPEGMEM.
274
279 ppm(1), pgm(1), pnmtojpeg(1), pnmquant(1), pamscale(1), ppmtopgm(1),
280 ppmdither(1), pamdepth(1),
281 djpeg man page, cjpeg man page, jpegtran man page, rdjpgcom man page,
283 wrjpgcom man page, jhead man page
284 Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
286 Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
287
290 jpegtopnm and this manual were derived in large part from djpeg, by the
291 Independent JPEG Group. The program is otherwise by Bryan Henderson on
292 March 19, 2000.
293
295 This manual page was generated by the Netpbm tool 'makeman' from HTML
296 source. The master documentation is at
297 http://netpbm.sourceforge.net/doc/jpegtopnm.html
299netpbm documentation 13 October 2002 Jpegtopnm User Manual(0)