ppm

ppm(5)                        File Formats Manual                       ppm(5)

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 in-
       formation 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  ev-
       ery  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
       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  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:
       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,  ac-
       cepting 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 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.

SEE ALSO
       giftopnm(1),  gouldtoppm(1),  ilbmtoppm(1),  imgtoppm(1),  mtvtoppm(1),
       pcxtoppm(1), pgmtoppm(1), pi1toppm(1), picttoppm(1),  pjtoppm(1),  qrt-
       toppm(1),    rawtoppm(1),   rgb3toppm(1),   sldtoppm(1),   spctoppm(1),
       sputoppm(1), tgatoppm(1), ximtoppm(1), xpmtoppm(1),  yuvtoppm(1),  ppm-
       toacad(1),  ppmtogif(1),  ppmtoicr(1),  ppmtoilbm(1), ppmtopcx(1), ppm-
       topgm(1), ppmtopi1(1),  ppmtopict(1),  ppmtopj(1),  ppmtopuzz(1),  ppm-
       torgb3(1),  ppmtosixel(1),  ppmtotga(1), ppmtouil(1), ppmtoxpm(1), ppm-
       toyuv(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)