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       jpegtopnm - convert JPEG/JFIF file to portable pixmap or graymap


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

       All options may be abbreviated to their shortest unique prefix.


       jpegtopnm  converts  the  named  JFIF file, or the standard input if no
       file is named to a PPM or PGM image file on the  standard  output.   If
       the  JFIF  file  is of the grayscale variety, jpegtopnm generates a PGM
       (Portable Graymap) file.   Otherwise,  it  generates  a  PPM  (Portable
       Pixmap) file.

       jpegtopnm  uses  the Independent JPEG Group’s JPEG library to interpret
       the input file.  See 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 common.  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 pnmdepth 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.


       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.

              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.


              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 specify  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 sophisticated.

              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.

              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).

              Extract the contents of the EXIF header from the input image and
              write it to the file filespec.  filespec = - means write  it  to
              Standard  Output.   In  this case, jpegtopnm does not output the
              converted image 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  ppmtojpeg  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.

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

              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.


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

              jpegtopnm foo.jpg >foo.ppm


       You  can  use ppmquant 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 pnmscale 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 | pnmscale .25

       -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.


              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


       ppm(5),  pgm(5),  ppmtojpeg(1),  ppmquant(1), pnmscale(1), ppmtopgm(1),
       ppmdither(1), pnmdepth(1),
       djpeg(1), cjpeg(1), jpegtran(1), rdjpgcom(1), wrjpgcom(1), jhead(1)
       Wallace, Gregory K.  "The JPEG  Still  Picture  Compression  Standard",
       Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.


       Arithmetic coding is not supported for legal reasons.

       The program could be much faster.


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

                                 19 March 2000