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 for-
       mat.   They  are informally called PPM files, but to be absolutely pre-
       cise, you should indicate the variation from true  PPM.   For  example,
       "PPM using the red, green, and blue colors that the scanner in question

       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

       - In the raster, the sample values are "nonlinear."  They  are  propor-
         tional  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  sam-
         ples,  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.

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