ppm(5) - man - phpMan

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 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 sam‐
         ple 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 lin‐
         ear 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 for‐
       mat.  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), pp‐
       mquantall(1), ppmrelief(1), pnm(5), pgm(5), pbm(5)

AUTHOR
       Copyright (C) 1989, 1991 by Jef Poskanzer.



                                            08 April 2000                                     ppm(5)