Standards, Environments, and Macros                        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  ineffi-
     cient.   It  is  highly redundant, while containing a lot of
     information that the human eye can't even discern.  Further-
     more,  the  format  allows very little information about the
     image besides basic color, which means you may have to  cou-
     ple 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  pgm
       file'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.

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Standards, Environments, and Macros                        ppm(5)

     - 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 proportional  to  the
       intensity  of the CIE Rec. 709 red, green, and blue in the
       pixel.  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).

     - 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 for-
     mat with 1 byte per sample and the one with 2 bytes per sam-
     ple.

     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

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Standards, Environments, and Macros                        ppm(5)

      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  pos-
     sible, 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.

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