Ubuntu Manpage: PPM - Netpbm color image format

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.

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