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
              pixels, 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
              intensity  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 universe 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 convert between this variation and true PPM.

              Note  that  a  common variation from 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.

       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 format.  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