NAME

       ppm - portable pixmap file format

DESCRIPTION

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

COMPATIBILITY

       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.

SEE ALSO

       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)

AUTHOR

       Copyright (C) 1989, 1991 by Jef Poskanzer.

                                                  08 April 2000                                           ppm(5)