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       ppm - portable pixmap file format


       The portable pixmap format is a lowest common denominator color image file format.

       It  should  be noted that this format is egregiously inefficient.  It is highly redundant,
       while containing a lot of information that the human eye can't even discern.  Furthermore,
       the format allows very little information about the image besides basic color, which means
       you may have to couple a file in this format with other independent information to get any
       decent  use  out of it.  However, it is very easy to write and analyze programs to process
       this format, and that is the point.

       It should also be noted that files often conform to this format in  every  respect  except
       the precise semantics of the sample values.  These files are useful because of the way PPM
       is used as an intermediary format.  They are  informally  called  PPM  files,  but  to  be
       absolutely  precise,  you  should indicate the variation from true PPM.  For example, "PPM
       using the red, green, and blue colors that the scanner in question uses."

       The format definition is as follows.

       A PPM file consists of a  sequence  of  one  or  more  PPM  images.  There  are  no  data,
       delimiters, or padding before, after, or between images.

       Each PPM image consists of the following:

       - A  "magic  number" for identifying the file type.  A ppm image's magic number is the two
         characters "P6".

       - Whitespace (blanks, TABs, CRs, LFs).

       - A width, formatted as ASCII characters in decimal.

       - Whitespace.

       - A height, again in ASCII decimal.

       - Whitespace.

       - The maximum color value (Maxval), again in ASCII decimal.  Must be less than 65536.

       - Newline or other single whitespace character.

       - A raster of Width * Height pixels,  proceeding  through  the  image  in  normal  English
         reading  order.  Each pixel is a triplet of red, green, and blue samples, in that order.
         Each sample is represented in pure binary by either 1 or 2 bytes.  If the Maxval is less
         than 256, it is 1 byte.  Otherwise, it is 2 bytes.  The most significant byte is first.

       - In  the  raster,  the  sample  values  are  "nonlinear."   They  are proportional to the
         intensity of the CIE Rec. 709 red, green, and blue in the pixel,  adjusted  by  the  CIE
         Rec.  709  gamma transfer function.  (That transfer function specifies a gamma number of
         2.2 and has a linear section for small intensities).  A value of Maxval  for  all  three
         samples  represents  CIE  D65  white and the most intense color in the color universe of
         which the image is part (the color universe is all the colors in  all  images  to  which
         this image might be compared).

       - Note that a common variation on the PPM format is to have the sample values be "linear,"
         i.e. as specified above except without the gamma adjustment.  pnmgamma takes such a  PPM
         variant as input and produces a true PPM as output.

       - Characters  from a "#" to the next end-of-line, before the maxval line, are comments and
         are ignored.

       Note that you can use pnmdepth to convert between a the format with 1 byte per sample  and
       the one with 2 bytes per sample.

       There  is  actually  another  version  of  the PPM format that is fairly rare: "plain" PPM
       format.  The format above, which generally considered the normal  one,  is  known  as  the
       "raw"  PPM  format.  See pbm(5) for some commentary on how plain and raw formats relate to
       one another.

       The difference in the plain format is:

       - There is exactly one image in a file.

       - The magic number is P3 instead of P6.

       - Each sample in the raster is represented as an ASCII decimal number (of arbitrary size).

       - Each sample in the raster has white space before and after it.  There must be  at  least
         one character of white space between any two samples, but there is no maximum.  There is
         no particular separation of one pixel from  another  --  just  the  required  separation
         between the blue sample of one pixel from the red sample of the next pixel.

       - No line should be longer than 70 characters.

       Here is an example of a small pixmap in this format:
       # feep.ppm
       4 4
        0  0  0    0  0  0    0  0  0   15  0 15
        0  0  0    0 15  7    0  0  0    0  0  0
        0  0  0    0  0  0    0 15  7    0  0  0
       15  0 15    0  0  0    0  0  0    0  0  0

       Programs  that  read this format should be as lenient as possible, accepting anything that
       looks remotely like a pixmap.


       Before April 2000, a raw format PPM file could not have a maxval greater than 255.  Hence,
       it could not have more than one byte per sample.  Old programs may depend on this.

       Before July 2000, there could be at most one image in a PPM file.  As a result, most tools
       to process PPM files ignore (and don't read) any data after the first image.


       giftopnm(1),   gouldtoppm(1),   ilbmtoppm(1),   imgtoppm(1),   mtvtoppm(1),   pcxtoppm(1),
       pgmtoppm(1),    pi1toppm(1),    picttoppm(1),    pjtoppm(1),   qrttoppm(1),   rawtoppm(1),
       rgb3toppm(1),   sldtoppm(1),   spctoppm(1),   sputoppm(1),    tgatoppm(1),    ximtoppm(1),
       xpmtoppm(1),    yuvtoppm(1),   ppmtoacad(1),   ppmtogif(1),   ppmtoicr(1),   ppmtoilbm(1),
       ppmtopcx(1),   ppmtopgm(1),   ppmtopi1(1),   ppmtopict(1),    ppmtopj(1),    ppmtopuzz(1),
       ppmtorgb3(1),   ppmtosixel(1),   ppmtotga(1),   ppmtouil(1),   ppmtoxpm(1),   ppmtoyuv(1),
       ppmdither(1), ppmforge(1), ppmhist(1), ppmmake(1), ppmpat(1), ppmquant(1), ppmquantall(1),
       ppmrelief(1), pnm(5), pgm(5), pbm(5)


       Copyright (C) 1989, 1991 by Jef Poskanzer.

                                          08 April 2000                                    ppm(5)