phpman > man > JPEGTOPNM(1)

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] [-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, jpeg‐‐
       topnm 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 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 in‐
       terprets 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 ex‐
              ample,  -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  lat‐
              ter  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 ef‐
              fect.

              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 col‐
       ors  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 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),  pn‐‐
       mdepth(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)