man > jpegtopnm(1)

Jpegtopnm User Manual(1)               General Commands Manual              Jpegtopnm User Manual(1)

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]  [-multiple]  [-repair] [-verbose] [-tracelevel N] [-traceexif]
       [filename]

       Minimum unique abbreviation of option is acceptable.  You may use double hyphens  instead  of
       single hyphen to denote options.  You may use white space in place of the equals sign to sep‐
       arate 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  In‐
       put.  The output stream is Standard Output.

       If  a  JFIF input image is of the grayscale variety, jpegtopnm generates a PGM image.  Other‐
       wise, it generates a PPM image.

       Before Netpbm 10.11 (October 2002), jpegtopnm did not have the multiple image stream capabil‐
       ity.  From 10.11 through 10.22, Netpbm always behaved as if you specified -multiple.   Start‐
       ing  with  Netpbm 10.23 (July 2004), Netpbm's default behavior went back to the pre-10.11 be‐
       havior and the new -multiple option selected the 10.12 behavior.  The reason for  the  rever‐
       sion  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
       ]8;;http://www.ijg.org\http://www.ijg.org]8;;\  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  pamdepth  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.

       See the jpegtopnm manual(1) for information on how images lose quality when  you  convert  to
       and from JFIF.


OPTIONS
       In  addition  to  the options common to all programs based on libnetpbm (most notably -quiet,
       see ]8;;index.html#commonoptions\ Common Options]8;;\ ), jpegtopnm recognizes the following command line 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).

              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 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.

              This option was added in Netpbm 9.19 (September 2001).


       -multiple
              Read multiple JFIF images sequentially from the input stream.  See ]8;;#description\Description section]8;;\
               for details.

              This option was new in Netpbm 10.23 (July 2004).


       -repair
              If the JFIF input is invalid, try to salvage whatever information is there and produce
              a valid PNM image as output.

              Without  this  option, some invalid input causes jpegtopnm to fail, and what output it
              produces is undefined.  With -repair such invalid input causes  jpegtopnm  to  succeed
              instead.

              But  note  that  there  are some forms of invalid input that always cause jpegtopnm to
              fail and others that always cause it to salvage image information and succeed.

              One particular case where -repair makes a difference is the common case that the  file
              is  truncated  somewhere after the JFIF header.  Without -repair, that always causes a
              failure; with -repair it always causes success.  Because the image information is laid
              out generally top to bottom in the JFIF stream, the image jpegtopnm produces  in  this
              case has the proper image contents at the top, but the bottom is padded with gray.

              This  option  was  new  in Netpbm 10.38.0 (March 2007).  Before that, jpegtopnm always
              fails in the cases in question.



       -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.


       -traceexif
              Print trace information about the processing of EXIF header information.

              This option was new in Netpbm 11.02 (March 2023).




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 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 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 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(1),  pgm(1),   pnmtojpeg(1),   pnmquant(1),   pamscale(1),   ppmtopgm(1),   ppmdither(1),
       pamdepth(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.


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.

DOCUMENT SOURCE
       This manual page was generated by the Netpbm tool 'makeman' from  HTML  source.   The  master
       documentation is at

              http://netpbm.sourceforge.net/doc/jpegtopnm.html

netpbm documentation                        20 March 2023                   Jpegtopnm User Manual(1)