jpegtopnm

JPEGTOPNM(1)                General Commands Manual               JPEGTOPNM(1)

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]  [-ver-
       bose] [-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  han-
       dles 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  li-
       braries  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 accu-
              rate 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 En-
              capsulated 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 ver-
              sion, since Photoshop and its emulators  seem  to  be  the  main
              source of CMYK and YCCK images.  But with experience of use, fu-
              ture 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 identi-
              cal 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.  -ver-
              bose 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 signifi-
       cant in practice.

       Another program, djpeg, is similar.  djpeg is maintained by  the  Inde-
       pendent  JPEG  Group and packaged with the JPEG library which jpegtopnm
       uses for all its JPEG work.  Because of that, you may expect it to  ex-
       ploit more current JPEG features.  Also, since you have to have the li-
       brary 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 Hender-
       son on March 19, 2000.

                                 19 March 2000                    JPEGTOPNM(1)