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