1Pnmtojpeg User Manual(0)                              Pnmtojpeg User Manual(0)
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NAME

6       pnmtojpeg - convert PNM image to a JFIF ("JPEG") image
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SYNOPSIS

10       pnmtojpeg   [-exif=filespec]   [-quality=n]   [{-grayscale|-greyscale}]
11       [-density=nxn[dpi,dpcm]]  [-optimize|-optimise]  [-rgb]  [-progressive]
12       [-comment=text]   [-dct={int|fast|float}]   [-arithmetic]  [-restart=n]
13       [-smooth=n] [-maxmemory=n] [-verbose]  [-baseline]  [-qtables=filespec]
14       [-qslots=n[,...]]         [-sample=HxV[,...]]         [-scans=filespec]
15       [-tracelevel=N]
16
17       filename
18
19       Minimum unique abbreviation of option is acceptable.  You may use  dou‐
20       ble  hyphens  instead  of single hyphen to denote options.  You may use
21       white space in place of the equals sign to separate an option name from
22       its value.
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25

DESCRIPTION

27       This program is part of Netpbm(1).
28
29       pnmtojpeg  converts the named PBM, PGM, or PPM image file, or the stan‐
30       dard input if no file is named, to a JFIF file on the standard output.
31
32       pnmtojpeg uses the Independent JPEG Group's JPEG library to create  the
33       output  file.  See http://www.ijg.orghttp://www.ijg.orgfor infor‐
34       mation on the library.
35
36       "JFIF" is the correct name for  the  image  format  commonly  known  as
37       "JPEG."  Strictly speaking, JPEG is a method of compression.  The image
38       format using JPEG compression that is by far the most common  is  JFIF.
39       There is also a subformat of TIFF that uses JPEG compression.
40
41       EXIF  is  an  image  format that is a subformat of JFIF (to wit, a JFIF
42       file that contains an EXIF header as an APP1 marker).   pnmtojpeg  cre‐
43       ates an EXIF image when you specify the -exif option.
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45

OPTIONS

47       The basic options are:
48
49
50
51       -exif=filespec
52              This  option  specifies  that  the output image is to be EXIF (a
53              subformat of JFIF), i.e. it will have an EXIF header as  a  JFIF
54              APP1  marker.   The  contents of that marker are the contents of
55              the specified file.  The special value - means to read the  EXIF
56              header  contents  from standard input.  It is invalid to specify
57              standard input for both the EXIF header and the input image.
58
59              The EXIF file starts with a two byte field which is  the  length
60              of  the  file,  including the length field, in pure binary, most
61              significant byte first.  The  special  value  of  zero  for  the
62              length  field means there is to be no EXIF header, i.e. the same
63              as no -exif option.  This is useful for when you convert a  file
64              from  JFIF  to PNM using jpegtopnm, then transform it, then con‐
65              vert it back to JFIF with pnmtojpeg, and you don't know  whether
66              or  not  it  includes an EXIF header.  jpegtopnm creates an EXIF
67              file containing nothing but two bytes of  zero  when  the  input
68              JFIF  file  has no EXIF header.  Thus, you can transfer any EXIF
69              header from the input JFIF to the output JFIF  without  worrying
70              about whether an EXIF header actually exists.
71
72              The  contents  of  the  EXIF file after the length field are the
73              exact byte for byte contents of the APP1  marker,  not  counting
74              the length field, that constitutes the EXIF header.
75
76
77       -quality=n
78              Scale  quantization  tables  to  adjust  image  quality.  n is 0
79              (worst) to 100 (best); default is 75.  Below about 25  can  pro‐
80              duce  images  some interpreters won't be able to interpret.  See
81              below for more info.
82
83
84       -grayscale
85
86       -greyscale
87
88       -rgb   These options determine the color space used in the JFIF output.
89              -grayscale  (or  -greyscale)  means to create a gray scale JFIF,
90              converting from color PPM input if  necessary.   -rgb  means  to
91              create  an  RGB  JFIF, and the program fails if the input is not
92              PPM.
93
94              If you specify neither, The output file is in  YCbCr  format  if
95              the  input  is  PPM, and grayscale format if the input is PBM or
96              PGM.
97
98              YCbCr format (a color is represented by an intensity  value  and
99              two  chrominance values) usually compresses much better than RGB
100              (a color is represented by one red,  one  green,  and  one  blue
101              value).   RGB  is  rare.  But you may be able to convert between
102              JFIF and PPM faster with RGB, since it's the  same  color  space
103              PPM uses.
104
105              The  testimg.ppm file that comes with Netpbm is 2.3 times larger
106              with the -rgb option than with the YCbCr  default,  and  in  one
107              experiment  pnmtojpeg took 16% more CPU time to convert it.  The
108              extra CPU time probably indicates that  processing  of  all  the
109              extra  compressed  data  consumed  all the CPU time saved by not
110              having to convert the RGB inputs to YCbCr.
111
112              Grayscale format takes up a lot less space and takes  less  time
113              to  create and process than the color formats, even if the image
114              contains nothing but black, white, and gray.
115
116              The -rgb option was added in Netpbm 10.11 in October 2002.
117
118
119       -density=density
120              This option determines the density (aka resolution)  information
121              recorded  in  the  JFIF  output  image.   It does not affect the
122              raster in any way; it just tells whoever reads the JFIF  how  to
123              interpret the raster.
124
125              The  density  value  takes  the form xxy followed by an optional
126              unit specifier of dpi or dpcm.  Examples: 1x1, 3x2,  300x300dpi,
127              100x200dpcm.   The  first  number is the horizontal density; the
128              2nd number is the vertical density.  Each  may  be  any  integer
129              from 1 to 65535.  The unit specifier is dpi for dots per inch or
130              dpcm for dots per centimeter.  If you don't specify  the  units,
131              the  density  information  goes into the JFIF explicitly stating
132              "density unspecified" (also interpreted as "unknown").  This may
133              seem pointless, but note that even without specifying the units,
134              the density numbers tell the aspect ratio of the  pixels.   E.g.
135              1x1  tells  you the pixels are square.  3x2 tells you the pixels
136              are vertical rectangles.
137
138              Note that if you specify different horizontal and vertical  den‐
139              sities, the resulting JFIF image is not a true representation of
140              the input PNM  image,  because  pnmtojpeg  converts  the  raster
141              pixel-for-pixel  and the pixels of a PNM image are defined to be
142              square.  Thus, if you start with a square PNM image and  specify
143              -density=3x2,   the  resulting  JFIF  image  is  a  horizontally
144              squashed version of the original.  However, it is common to  use
145              an  input  image  which is a slight variation on PNM rather than
146              true PNM such that the pixels are not square.  In that case, the
147              appropriate  -density  option  yields a faithful reproduction of
148              the input pseudo-PNM image.
149
150              The default is 1x1 in unspecified units.
151
152              Before Netpbm 10.15 (April 2003), this option did not exist  and
153              the  pnmtojpeg  always  created  a JFIF with a density of 1x1 in
154              unspecified units.
155
156
157       -optimize
158               Perform optimization of entropy encoding  parameters.   Without
159              this,  pnmtojpeg  uses  default  encoding parameters.  -optimize
160              usually makes the JFIF file a little smaller, but pnmtojpeg runs
161              somewhat  slower  and needs much more memory.  Image quality and
162              speed of decompression are unaffected by -optimize.
163
164
165       -progressive
166              Create a progressive JPEG file (see below).
167
168       -comment=text
169              Include a comment marker in the JFIF output, with  comment  text
170              text.
171
172              Without this option, there are no comment markers in the output.
173
174
175
176       The  -quality  option  lets  you trade off compressed file size against
177       quality of the reconstructed image: the higher the quality setting, the
178       larger  the  JFIF  file, and the closer the output image will be to the
179       original input.  Normally you want to use the  lowest  quality  setting
180       (smallest  file)  that  decompresses  into something visually indistin‐
181       guishable from the original image.  For this purpose the  quality  set‐
182       ting should be between 50 and 95 for reasonable results; the default of
183       75 is often about right.  If you see defects at -quality=75, then go up
184       5  or  10  counts  at a time until you are happy with the output image.
185       (The optimal setting will vary from one image to another.)
186
187       -quality=100 generates a quantization table of all 1's, minimizing loss
188       in  the  quantization step (but there is still information loss in sub‐
189       sampling, as well as roundoff error).  This setting is mainly of inter‐
190       est  for  experimental purposes.  Quality values above about 95 are not
191       recommended for normal use; the compressed file size goes  up  dramati‐
192       cally for hardly any gain in output image quality.
193
194       In the other direction, quality values below 50 will produce very small
195       files of low image quality.  Settings around 5 to 10 might be useful in
196       preparing  an  index of a large image library, for example.  Try -qual‐
197       ity=2 (or so) for some amusing Cubist effects.  (Note:  quality  values
198       below  about  25 generate 2-byte quantization tables, which are consid‐
199       ered optional in the JFIF standard.  pnmtojpeg emits a warning  message
200       when  you  give  such a quality value, because some other JFIF programs
201       may be unable to decode the resulting file.  Use -baseline if you  need
202       to ensure compatibility at low quality values.)
203
204       The  -progressive  option  creates  a "progressive JPEG" file.  In this
205       type of JFIF file, the data is stored in multiple scans  of  increasing
206       quality.   If  the file is being transmitted over a slow communications
207       link, the decoder can use the first scan to display a low-quality image
208       very  quickly,  and  can  then improve the display with each subsequent
209       scan.  The final image is exactly equivalent to a standard JFIF file of
210       the  same quality setting, and the total file size is about the same --
211       often a little smaller.
212
213       Caution: progressive JPEG  is  not  yet  widely  implemented,  so  many
214       decoders will be unable to view a progressive JPEG file at all.
215
216       If  you're  trying to control the quality/file size tradeoff, you might
217       consider the JPEG2000 format instead.  See pamtojpeg2k(1).
218
219       Options for advanced users:
220
221
222
223       -dct=int
224              Use integer DCT method (default).
225
226
227       -dct=fast
228              Use fast integer DCT (less accurate).
229
230
231       -dct=float
232              Use  floating-point  DCT  method.   The  float  method  is  very
233              slightly  more  accurate than the int method, but is much slower
234              unless your machine has very fast floating-point hardware.  Also
235              note that results of the floating-point method may vary slightly
236              across machines, while the integer methods should give the  same
237              results  everywhere.  The fast integer method is much less accu‐
238              rate than the other two.
239
240
241       -arithmetic
242              Use arithmetic coding.  Default is Huffman encoding.  Arithmetic
243              coding tends to get you a smaller result.
244
245              You  may  need patent licenses to use this option.  According to
246              the JPEG FAQ ⟨http://www.faqs.org/faqs/jpeg-faq⟩ ,  This  method
247              is covered by patents owned by IBM, AT&T, and Mitsubishi.
248
249              The author of the FAQ recommends against using arithmetic coding
250              (and therefore this option) because the  space  savings  is  not
251              great enough to justify the legal hassles.
252
253              Most  JPEG  libraries, including any distributed by the Indepen‐
254              dent JPEG Group since about 1998 are not capable  of  arithmetic
255              encoding.   pnmtojpeg  uses  a  JPEG library (either bound to it
256              when the pnmtojpeg executable was built or accessed on your sys‐
257              tem  at  run time) to do the JPEG encoding.  If pnmtojpeg termi‐
258              nates with the message, "Sorry, there are legal restrictions  on
259              arithmetic  coding" or "Sorry, arithmetic coding not supported,"
260              this is the problem.
261
262
263       -restart=n
264              Emit a JPEG restart marker every n MCU  rows,  or  every  n  MCU
265              blocks  if you append B to the number.  -restart 0 (the default)
266              means no restart markers.
267
268
269       -smooth=n
270              Smooth the input image to eliminate dithering noise.  n, ranging
271              from  1  to  100,  indicates  the strength of smoothing.  0 (the
272              default) means no smoothing.
273
274
275       -maxmemory=n
276              Set a limit for amount of memory  to  use  in  processing  large
277              images.  Value is in thousands of bytes, or millions of bytes if
278              you append M  to  the  number.   For  example,  -max=4m  selects
279              4,000,000  bytes.   If  pnmtojpeg  needs more space, it will use
280              temporary files.
281
282
283       -verbose
284              Print to the Standard Error file messages about  the  conversion
285              process.  This can be helpful in debugging problems.
286
287
288       The -restart option tells pnmtojpeg  to insert extra markers that allow
289       a JPEG decoder to resynchronize after a  transmission  error.   Without
290       restart  markers, any damage to a compressed file will usually ruin the
291       image from the point of the error to the end of the image; with restart
292       markers,  the damage is usually confined to the portion of the image up
293       to the next restart marker.  Of  course,  the  restart  markers  occupy
294       extra space.  We recommend -restart=1 for images that will be transmit‐
295       ted across unreliable networks such as Usenet.
296
297       The -smooth option filters the input  to  eliminate  fine-scale  noise.
298       This  is often useful when converting dithered images to JFIF: a moder‐
299       ate smoothing factor of 10 to 50 gets rid of dithering patterns in  the
300       input  file,  resulting  in  a  smaller  JFIF file and a better-looking
301       image.  Too large a smoothing factor will visibly blur the image,  how‐
302       ever.
303
304       Options for wizards:
305
306
307
308       -baseline
309              Force  baseline-compatible  quantization tables to be generated.
310              This clamps quantization values to 8 bits even  at  low  quality
311              settings.   (This  switch  is  poorly  named,  since it does not
312              ensure that the output is actually baseline JPEG.  For  example,
313              you can use -baseline and -progressive together.)
314
315
316       -qtables=filespec
317              Use the quantization tables given in the specified text file.
318
319
320       -qslots=n[,...]
321              Select which quantization table to use for each color component.
322
323
324       -sample=HxV[,...]
325              Set JPEG sampling factors for each color component.
326
327
328       -scans=filespec
329              Use the scan script given in the specified text file.  See below
330              for information on scan scripts.
331
332
333       -tracelevel=N
334              This sets the level of debug tracing the program outputs  as  it
335              runs.   0  means none, and is the default.  This level primarily
336              controls tracing of the JPEG  library,  and  you  can  get  some
337              pretty interesting information about the compression process.
338
339
340
341       The  "wizard"  options  are intended for experimentation with JPEG.  If
342       you don't know what you are doing, don't use them.  These switches  are
343       documented  further in the file wizard.doc that comes with the Indepen‐
344       dent JPEG Group's JPEG library.
345
346

EXAMPLES

348       This example compresses the PPM file foo.ppm with a quality  factor  of
349       60 and saves the output as foo.jpg:
350
351           pnmtojpeg -quality=60 foo.ppm > foo.jpg
352
353       Here's a more typical example.  It converts from BMP to JFIF:
354
355           cat foo.bmp | bmptoppm | pnmtojpeg > foo.jpg
356
357

JPEG Loss

359       When you compress with JPEG, you lose information -- i.e. the resulting
360       image has somewhat lower quality than the original.  This is a  charac‐
361       teristic  of JPEG itself, not any particular program.  So if you do the
362       usual Netpbm thing and convert from JFIF to PNM, manipulate, then  con‐
363       vert back to JFIF, you will lose quality.  The more you do it, the more
364       you lose.  Drawings (charts, cartoons, line drawings, and such with few
365       colors and sharp edges) suffer the most.
366
367       To  avoid  this, you can use a compressed image format other than JPEG.
368       PNG and JPEG2000 are good choices, and Netpbm contains  converters  for
369       those.
370
371       If you need to use JFIF on a drawing, you should experiment with pnmto‐
372       jpeg's -quality and -smooth options to get a  satisfactory  conversion.
373       -smooth 10 or so is often helpful.
374
375       Because  of the loss, you should do all the manipulation you have to do
376       on the image in some other format and convert to JFIF as the last step.
377       And if you can keep a copy in the original format, so much the better.
378
379       The  -optimize option to pnmtojpeg is worth using when you are making a
380       "final" version for posting or archiving.  It's also a win when you are
381       using  low quality settings to make very small JFIF files; the percent‐
382       age improvement is often a lot more than it is on  larger  files.   (At
383       present,  -optimize mode is automatically in effect when you generate a
384       progressive JPEG file).
385
386       You can do flipping and rotating transformations  losslessly  with  the
387       program  jpegtran,  which is packaged with the Independent Jpeg Group's
388       JPEG library.  jpegtran exercises its intimate  knowledge  of  the  way
389       JPEG works to do the transformation without ever actually decompressing
390       the image.
391
392
393
394
395       Another program, cjpeg, is similar.  cjpeg is
396       maintained by the Independent JPEG Group and  packaged  with  the  JPEG
397       library  which  pnmtojpeg uses for all its JPEG work.  Because of that,
398       you may expect it to exploit more current JPEG features.   Also,  since
399       you  have  to  have  the  library to run pnmtojpeg, but not vice versa,
400       cjpeg may be more commonly available.
401
402       On the other hand, cjpeg does not use the NetPBM libraries  to  process
403       its input, as all the NetPBM tools such as pnmtojpeg do.  This means it
404       is less likely to be consistent with all the other programs  that  deal
405       with the NetPBM formats.  Also, the command syntax of pnmtojpeg is con‐
406       sistent with that of the other Netpbm tools, unlike cjpeg.
407
408

SCAN SCRIPTS

410       Use the -scan option to specify a scan script.  Or use the -progressive
411       option to specify a particular built-in scan script.
412
413       Just  what  a  scan  script is, and the basic format of the scan script
414       file, is covered in the wizard.doc file that comes with the Independent
415       JPEG  Group's JPEG library.  Scan scripts are same for pnmtojpeg as the
416       are for cjpeg.
417
418       This section contains additional information that isn't,  but  probably
419       should be, in that document.
420
421       First, there are many restrictions on what is a valid scan script.  The
422       JPEG library, and thus pnmtojpeg, checks thoroughly  for  any  lack  of
423       compliance with these restrictions, but does little to tell you how the
424       script fails to comply.  The messages are very  general  and  sometimes
425       untrue.
426
427       To  start with, the entries for the DC coefficient must come before any
428       entries for the AC coefficients.  The DC coefficient is Coefficient  0;
429       all the other coefficients are AC coefficients.  So in an entry for the
430       DC coefficient, the two numbers after the colon must be 0 and 0.  In an
431       entry for AC coefficients, the first number after the colon must not be
432       0.
433
434       In a DC entry, the color components must be in increasing order.   E.g.
435       "0,2,1" before the colon is wrong.  So is "0,0,0".
436
437       In an entry for an AC coefficient, you must specify only one color com‐
438       ponent.  I.e. there can be only one number before the colon.
439
440       In the first entry for a particular coefficient for a particular  color
441       component,  the  "Ah"  value  must be zero, but the Al value can be any
442       valid bit number.  In subsequent entries, Ah must be the Al value  from
443       the previous entry (for that coefficient for that color component), and
444       the Al value must be one less than the Ah value.
445
446       The script must ultimately specify at least some of the DC  coefficient
447       for  every  color  component.   Otherwise,  you  get  the error message
448       "Script does not transmit all the data."  You need not specify  all  of
449       the bits of the DC coefficient, or any of the AC coefficients.
450
451       There  is  a  standard option in building the JPEG library to omit scan
452       script capability.  If for some reason your library was built with this
453       option,  you  get the message "Requested feature was omitted at compile
454       time."
455
456

ENVIRONMENT

458       JPEGMEM
459              If this environment variable is set, its value  is  the  default
460              memory  limit.   The  value  is  specified  as described for the
461              -maxmemory option.  An explicit -maxmemory  option overrides any
462              JPEGMEM.
463
464
465
466

SEE ALSO

468       jpegtopnm(1),  pnm(1),  cjpeg  man  page,  djpeg man page, jpegtran man
469       page, rdjpgcom man page, wrjpgcom man page
470
471       Wallace, Gregory K.  "The JPEG  Still  Picture  Compression  Standard",
472       Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
473
474
475

AUTHOR

477       pnmtojpeg and this manual were derived in large part from cjpeg, by the
478       Independent JPEG Group.  The program is otherwise by Bryan Henderson on
479       March 07, 2000.
480

DOCUMENT SOURCE

482       This  manual  page was generated by the Netpbm tool 'makeman' from HTML
483       source.  The master documentation is at
484
485              http://netpbm.sourceforge.net/doc/pnmtojpeg.html
486
487netpbm documentation             23 April 2007        Pnmtojpeg User Manual(0)
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