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ppm(5)									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
       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
       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,
       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 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)
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