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)