jpegtopnm



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]  [-multi-
       ple] [-verbose] [-tracelevel N] [filename]

       Minimum  unique abbreviation of option is acceptable.  You may use dou-
       ble 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  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
       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.

       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  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
              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 ver-
              sion,  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).

              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.  filespec=-  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 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.

              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).


       -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 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  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
       exploit more current JPEG features.  Also, since you have to  have  the
       library  to  run  jpegtopnm, but not vice versa, cjpeg may be more com-
       monly 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), pnmdepth(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.



LIMITATIONS

       Arithmetic coding is not offered for legal reasons.  The program  could
       be much faster.



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)

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