The following is the documentation for PPM format.

             NAME
                  ppm - portable pixmap file format

             DESCRIPTION
                  The portable pixmap format is a  lowest  common  denominator color image file format.  The definition is as follows:

                  - A "magic number" for identifying the  file  type.   A  ppm
                    file's magic number is the two characters "P3".

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

                  - A width, formatted as ASCII characters in decimal.

                  - Whitespace.

                  - A height, again in ASCII decimal.

                  - Whitespace.

                  - The maximum color-component value, again in ASCII decimal.

                  - Whitespace.

                  - Width * height pixels, each  three  ASCII  decimal  values
                    between 0 and the specified maximum value, starting at the
                    top-left  corner  of  the  pixmap,  proceeding  in  normal
                    English  reading  order.   The three values for each pixel
                    represent red, green, and blue, respectively; a value of 0
                    means  that color is off, and the maximum value means that
                    color is maxxed out.

                  - Characters from a "#" to the next end-of-line are  ignored
                    (comments).

                  - No line should be longer than 70 characters.

                  Here is an example of a small pixmap in this format:
                  P3
                  # feep.ppm
                  4 4
                  15
                   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 read this format should be as lenient as  possible, accepting anything that looks remotely like apixmap.

                  There is also a variant on the format, available by  setting the  RAWBITS  option  at  compile  time.   This  variant  is
                  different in the following ways:

                  - The "magic number" is "P6" instead of "P3".

                  - The pixel values are stored as  plain  bytes,  instead  of
                    ASCII decimal.

                  - Whitespace is not allowed in the pixels area, and  only  a
                    single  character  of  whitespace (typically a newline) is
                    allowed after the maxval.

                  - The files are smaller and many times faster  to  read  and
                    write.

                  Note that this raw format can only be used for maxvals  less than or equal to 255.  If you use the ppm library and try to
                  write a file with a larger  maxval,  it  will  automatically fall back on the slower but more general plain format.