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NAME

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

SYNOPSIS

       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.

DESCRIPTION

       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
       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 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.

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 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.

       -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).

       -exif=filespec
              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.

       -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 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
              >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(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.

LIMITATIONS

       Arithmetic coding is not supported for legal reasons.

       The program could be much faster.

AUTHOR

       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                              JPEGTOPNM(1)