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Jpegtopnm User Manual(0)                              Jpegtopnm User Manual(0)



NAME
       jpegtopnm - convert JPEG/JFIF file to PPM or PGM image


SYNOPSIS
       jpegtopnm  [-dct  {int|fast|float}]  [-nosmooth]  [-maxmemory  N]  [{-adobe|-notadobe}] [-comments] [-dumpexif]
       [-exif=filespec] [-multiple] [-repair] [-verbose] [-tracelevel N] [filename]

       Minimum unique abbreviation of option is acceptable.  You may use double hyphens instead of  single  hyphen  to
       denote options.  You may use white space in place of the equals sign to separate an option name from its value.


DESCRIPTION
       This program is part of Netpbm(1).

       jpegtopnm converts JFIF images to PPM or PGM images.

       By default, jpegtopnm expects the input stream to contain one JFIF image and produces one PGM or PPM  image  as
       output.  It fails if the input stream is empty.

       But  with  the  -multiple option, jpegtopnm reads JFIF images sequentially from the input stream and writes one
       PPM or PGM image to the output stream for each JFIF input.  If the input stream is empty, so is the output.

       The input stream is the filename you specify or, if you don't specify filename,  Standard  Input.   The  output
       stream is Standard Output.

       If  a JFIF input image is of the grayscale variety, jpegtopnm generates a PGM image.  Otherwise, it generates a
       PPM image.

       Before Netpbm 10.11 (October 2002), jpegtopnm did not have the multiple image stream  capability.   From  10.11
       through  10.22,  Netpbm  always behaved as if you specified -multiple.  Starting with Netpbm 10.23 (July 2004),
       Netpbm's default behavior went back to the pre-10.11 behavior and the new -multiple option selected  the  10.12
       behavior.   The  reason  for  the reversion was that there were discovered in the world files that contain JFIF
       images followed by something other than another JFIF image.  The producers of these files expect them  to  work
       with any JFIF interpreter because most JFIF interpreters just stop reading the file after the first JFIF image.

       jpegtopnm uses the Independent JPEG Group's JPEG library to interpret the input file.  See http://www.ijg.org
       for information on the library.

       'JFIF'  is  the correct name for the image format commonly known as 'JPEG.' Strictly speaking, JPEG is a method
       of compression.  The image format using JPEG compression that is by far the most common is JFIF.  There is also
       a subformat of TIFF that uses JPEG compression.

       EXIF  is  an  image  format that is a subformat of JFIF (to wit, a JFIF file that contains an EXIF header as an
       APP1 marker).  jpegtopnm handles EXIF.

       JFIF files can have either 8 bits per sample or 12 bits per sample.  The 8 bit variety is by far the most  com-
       mon.   There  are two versions of the IJG JPEG library.  One reads only 8 bit files and the other reads only 12
       bit files.  You must link the appropriate one of these libraries with jpegtopnm.  Ordinarily,  this  means  the
       library is in your shared library search path when you run jpegtopnm.

       jpegtopnm generates output with either one byte or two bytes per sample depending on whether the JFIF input has
       either 8 bits or 12 bits per sample.  You can use pamdepth to reduce a two-byte-per-sample file to a  one-byte-
       per-sample file if you need to.

       If  the  JFIF file uses the CMYK or YCCK color space, the input does not actually contain enough information to
       know what color each pixel is.  To know what color a pixel is, one would have to know  the  properties  of  the
       inks  to  which  the color space refers.  jpegtopnm interprets the colors using the common transformation which
       assumes all the inks are simply subtractive and linear.

       See the jpegtopnmmanual(1) for information on how images lose quality when you convert to and from JFIF.


OPTIONS
       The options are only for advanced users:


       -dct int
              Use integer DCT method (default).


       -dct fast
              Use fast integer DCT (less accurate).


       -dct float
              Use floating-point DCT method.  The float method is very slightly more accurate than the int method, but
              is much slower unless your machine has very fast floating-point hardware.  Also note that results of the
              floating-point method may vary slightly across machines, while the integer methods should give the  same
              results everywhere.  The fast integer method is much less accurate than the other two.


       -nosmooth
              Use a faster, lower-quality upsampling routine.

       -maxmemory N
              Set  limit  on the amount of memory jpegtopnm uses in processing large images.  Value is in thousands of
              bytes, or millions of bytes if 'M' is suffixed to  the  number.   For  example,  -maxmemory  4m  selects
              4000000 bytes.  If jpegtopnm needs more space, it uses temporary files.


       -adobe

       -notadobe
              There are two variations on the CMYK (and likewise YCCK) color space that may be used in the JFIF input.
              In the normal one, a zero value for a color components indicates absence of ink.  In the other,  a  zero
              value means the maximum ink coverage.  The latter is used by Adobe Photoshop when it creates a bare JFIF
              output file (but not when it creates JFIF output as part of Encapsulated Postscript output).

              These options tell jpegtopnm which version of the CMYK or YCCK color space the image uses.  If you spec-
              ify  neither,  jpegtopnm tries to figure it out on its own.  In the present version, it doesn't try very
              hard at all: It just assumes the Photoshop version, since Photoshop and its emulators  seem  to  be  the
              main  source of CMYK and YCCK images.  But with experience of use, future versions might be more sophis-
              ticated.

              If the JFIF image does not indicate that it is CMYK or YCCK, these options have no effect.

              If you don't use the right one of these options, the symptom is output that looks like a negative.


       -dumpexif
              Print the interpreted contents of any Exif header in the input file to the Standard Error file.  Similar
              to the program jhead (not part of the Netpbm package).

              This option was added in Netpbm 9.19 (September 2001).


       -exif=filespec
              Extract  the  contents of the EXIF header from the input image and write it to the file filespec.  file-
              spec=- means write it to Standard Output.  When you write the EXIF header to Standard Output,  jpegtopnm
              does not output the converted image (which is what normally would go to Standard Output) at all.

              jpegtopnm  writes the contents of the EXIF header byte-for-byte, starting with the two byte length field
              (which length includes those two bytes).

              You can use this file as input to pnmtojpeg to insert an identical EXIF header into a new JFIF image.

              If there is no EXIF header, jpegtopnm writes two bytes of binary zero and nothing else.

              An EXIF header takes the form of a JFIF APP1 marker.  Only the first such marker within the JFIF  header
              counts.

              This option was added in Netpbm 9.19 (September 2001).


       -multiple
              Read multiple JFIF images sequentially from the input stream.  See Description section  for details.

              This option was new in Netpbm 10.23 (July 2004).


       -repair
              If the JFIF input is invalid, try to salvage whatever information is there and produce a valid PNM image
              as output.

              Without this option, some invalid input causes jpegtopnm to fail, and what output it produces  is  unde-
              fined.  With -repair such invalid input causes jpegtopnm to succeed instead.

              But  note  that there are some forms of invalid input that always cause jpegtopnm to fail and other than
              always cause it to salvage image information and succeed.

              One particular case where -repair makes a difference is the common case that the file is truncated some-
              where  after  the  JFIF  header.   Without -repair, that always causes a failure; with -repair it always
              causes success.  Because the image information is laid out generally top to bottom in the  JFIF  stream,
              the  image  jpegtopnm  produces in this case has the proper image contents at the top, but the bottom is
              padded with gray.

              This option was new in Netpbm 10.38.0 (March 2007).  Before that, jpegtopnm always fails in the cases in
              question.



       -comments
              Print any comments in the input file to the Standard Error file.


       -verbose
              Print details about the conversion to the Standard Error file.


       -tracelevel n
              Turn  on  the  JPEG  library's trace messages to the Standard Error file.  A higher value of n gets more
              trace information.  -verbose implies a trace level of at least 1.




EXAMPLES
       This example converts the color JFIF file foo.jpg to a PPM file named foo.ppm:

           jpegtopnm foo.jpg >foo.ppm


HINTS
       You can use pnmquant to color quantize the result, i.e. to reduce the number of distinct colors in  the  image.
       In  fact,  you may have to if you want to convert the PPM file to certain other formats.  ppmdither Does a more
       sophisticated quantization.

       Use pamscale to change the dimensions of the resulting image.

       Use ppmtopgm  to convert a color JFIF file to a grayscale PGM file.

       You can easily use these converters together.  E.g.:

           jpegtopnm foo.jpg | ppmtopgm | pamscale .25 >foo.pgm

       -dct fast and/or -nosmooth gain speed at a small sacrifice in quality.

       If you are fortunate enough to have very fast floating point hardware, -dct float may be even faster than  -dct
       fast.  But on most machines -dct float is slower than -dct int; in this case it is not worth using, because its
       theoretical accuracy advantage is too small to be significant in practice.

       Another program, djpeg, is similar.  djpeg is maintained by the Independent JPEG Group and  packaged  with  the
       JPEG  library  which  jpegtopnm uses for all its JPEG work.  Because of that, you may expect it to exploit more
       current JPEG features.  Also, since you have to have the library to run jpegtopnm, but not  vice  versa,  cjpeg
       may be more commonly available.

       On the other hand, djpeg does not use the NetPBM libraries to generate its output, as all the NetPBM tools such
       as jpegtopnm do.  This means it is less likely to be consistent with all the other programs that deal with  the
       NetPBM  formats.   Also,  the  command  syntax  of jpegtopnm is consistent with that of the other Netpbm tools,
       unlike djpeg.


ENVIRONMENT
       JPEGMEM
              If this environment variable is set, its value is the default memory limit.  The value is  specified  as
              described for the -maxmemory option.  An explicit -maxmemory  option overrides any JPEGMEM.




SEE ALSO
       ppm(1), pgm(1), pnmtojpeg(1), pnmquant(1), pamscale(1), ppmtopgm(1), ppmdither(1), pamdepth(1),

       djpeg man page, cjpeg man page, jpegtran man page, rdjpgcom man page, wrjpgcom man page, jhead man page

       Wallace, Gregory K.  'The JPEG Still Picture Compression Standard', Communications of the ACM, April 1991 (vol.
       34, no. 4), pp. 30-44.


AUTHOR
       jpegtopnm and this manual were derived in large part from djpeg, by the Independent JPEG Group.  The program is
       otherwise by Bryan Henderson on March 19, 2000.



netpbm documentation            13 October 2002       Jpegtopnm User Manual(0)