man > ppm(5)

PPM Format Specification(5)              File Formats Manual             PPM Format Specification(5)

NAME
       PPM - Netpbm color image format


DESCRIPTION
       This program is part of Netpbm(1).

       The PPM 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  name "PPM" is an acronym derived from "Portable Pixel Map."  Images in this format (or a
       precursor of it) were once also called "portable pixmaps."


THE FORMAT
       The format definition is as follows.  You can use the libnetpbm(1) C  subroutine  library  to
       read and interpret the format conveniently and accurately.

       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 and
              more than zero.


       •      A single whitespace character (usually a newline).


       •      A raster of Height rows, in order from top to bottom.  Each row consists of Width pix‐
              els, in order from left to right.  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.

              A  row  of an image is horizontal.  A column is vertical.  The pixels in the image are
              square and contiguous.

              In the raster, the sample values are "nonlinear." They are proportional to the  inten‐
              sity of the ITU-R Recommendation BT.709 red, green, and blue in the pixel, adjusted by
              the  BT.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 uni‐
              verse of which the image is part (the color universe is all the colors in  all  images
              to which this image might be compared).

              BT.709's range of channel values (16-240) is irrelevant to PPM.

              ITU-R Recommendation BT.709 is a renaming of the former CCIR Recommendation 709.  When
              CCIR  was  absorbed  into its parent organization, the ITU, ca. 2000, the standard was
              renamed.  This document once referred to the standard as CIE Rec. 709,  but  it  isn't
              clear now that CIE ever sponsored such a standard.

              Note  that another popular color space is the newer sRGB.  A common variation from PPM
              is to substitute this color space for the one specified.  You can use pnmgamma to con‐
              vert between this variation and true PPM.

              Note that a common variation from the PPM format is to have the sample values be "lin‐
              ear," 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.



       Strings starting with "#" may be comments, the same as with PBM(1).

       Note that you can use pamdepth to convert between a the format with 1 byte per sample and the
       one with 2 bytes per sample.

       All  characters  referred  to herein are encoded in ASCII.  "newline" refers to the character
       known in ASCII as Line Feed or LF.  A "white space" character is space, CR, LF, TAB,  VT,  or
       FF (I.e. what the ANSI standard C isspace() function calls white space).


   Plain PPM
       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(1) for some commentary on how plain and raw formats relate to one another and
       how to use them.

       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 image 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


       There is a newline character at the end of each of these lines.

       Programs  that  read  this  format  should be as lenient as possible, accepting anything that
       looks remotely like a PPM image.



INTERNET MEDIA TYPE
       No Internet Media Type (aka MIME type, content type) for PPM has been registered  with  IANA,
       but the value image/x-portable-pixmap is conventional.

       Note that the PNM Internet Media Type image/x-portable-anymap also applies.



FILE NAME
       There  are no requirements on the name of a PPM file, but the convention is to use the suffix
       ".ppm".  "pnm" is also conventional, for cases where distinguishing  between  the  particular
       subformats of PNM is not convenient.



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
       pnm(1), pgm(1), pbm(1), pam(1), programs that process PPM(1)

DOCUMENT SOURCE
       This  manual  page  was  generated by the Netpbm tool 'makeman' from HTML source.  The master
       documentation is at

              http://netpbm.sourceforge.net/doc/ppm.html

netpbm documentation                       09 October 2016               PPM Format Specification(5)