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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] [-repair] [-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  ⟨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 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  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 ⟨#description⟩  for details.

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

       -repair
	      If  the  JFIF input is invalid, try to salvage whatever informa‐
	      tion 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 other than always cause it to sal‐
	      vage image information and succeed.

	      One particular case where -repair makes a difference is the com‐
	      mon  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 con‐
	      tents 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.	 -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), 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.

netpbm documentation		13 October 2002	      Jpegtopnm User Manual(0)
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