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           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
           - 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.


           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 sam-
           ple.  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.
                                     08 April 2000                          ppm(5)

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