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-

       - 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.

       Copyright (C) 1989, 1991 by Jef Poskanzer.

                                 08 April 2000                          ppm(5)
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