ppm man page

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:

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

Referenced By

cjpeg(1), csepdjvu(1), dcraw(1), ddjvu(1), djpeg(1), mplayer(1), pnm2ppa(1), pnmtoy4m(1), ppmtoy4m(1), sng(1), vidwhacker(6), xcf2pnm(1), xwintoppm.1x(1), xzgv(1), y4mtopnm(1), y4mtoppm(1).

09 October 2016 netpbm documentation