The PBM Format

# pbm

Updated: 07 November 2025
Table Of Contents

## NAME

pbm - Netpbm bi-level image format

## DESCRIPTION

This program is part of Netpbm.

The PBM format is a lowest common denominator monochrome file format. It serves as the common language of a large family of bitmap image conversion filters. Because the format pays no heed to efficiency, it is simple and general enough that one can easily develop programs to convert to and from just about any other graphics format, or to manipulate the image.

The name "PBM" is an acronym derived from "Portable Bit Map."

This is not a format that one would normally use to store a file or to transmit it to someone -- it's too expensive and not expressive enough for that. It's just an intermediary format. In it's purest use, it lives only in a pipe between two other programs.

## THE LAYOUT

The format definition is as follows.

A PBM file consists of a sequence of one or more PBM images. There are no data, delimiters, or padding before, after, or between images.

Each PBM image consists of the following:

- A "magic number" for identifying the file type. A pbm image's magic number is the two characters "P4".
- Whitespace (blanks, TABs, CRs, LFs).
- The width in pixels of the image, formatted as ASCII characters in decimal.
- Whitespace.
- The height in pixels of the image, again in ASCII decimal.
- A single whitespace character (usually a newline).
- A raster of Height rows, in order from top to bottom. Each row is Width bits, packed 8 to a byte, with don't care bits to fill out the last byte in the row. Each bit represents a pixel: 1 is black, 0 is white. The order of the pixels is left to right. The order of their storage within each file byte is most significant bit to least significant bit. The order of the file bytes is from the beginning of the file toward the end of the file.

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

- Before the whitespace character that delimits the raster, any characters from a "#" to but not including the next carriage return or newline character, or end of file, is a comment and is ignored.

  (Before June 2024, the carriage return or line feed was specified to be part of the comment and ignored, but in the 22 years that comments had existed in the specification, Netpbm never implemented that).

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).

### Minimal Subset

The PBM format is so simple that many programmers write programs to process it directly, without using a library such as libnetpbm, reverse engineering a subset of the format from examples rather than using the specification. Therefore, if you want to produce an image that as many programs as possible can read, you should stick to the following subset.

The image consists of the following bytes:

1. "P4" followed by newline
2. width and height separated by a space (e.g. "50 100"), followed by newline
3. The raster

There is only one image in the file.

The program pnmtopnm produces this minimal subset, so if you have a more complex PBM image, you can pass it to to a less tolerant program through pnmtopnm.

### Plain PBM

There is actually another version of the PBM format, even more simplistic, more lavishly wasteful of space than PBM, called Plain PBM. Plain PBM actually came first, but even its inventor couldn't stand its recklessly squanderous use of resources after a while and switched to what we now know as the regular PBM format. But Plain PBM is so redundant -- so overstated -- that it's virtually impossible to break. You can send it through the most liberal mail system (which was the original purpose of the PBM format) and it will arrive still readable. You can flip a dozen random bits and easily piece back together the original image. And we hardly need to define the format here, because you can decode it by inspection.

Netpbm programs generate Raw PBM format instead of Plain PBM by default, but the common option -plain chooses Plain PBM.

The difference is:

- There is exactly one image in a file.
- The "magic number" is "P1" instead of "P4".
- Each pixel in the raster is represented by a byte containing ASCII '1' or '0', representing black and white respectively. There are no fill bits at the end of a row.
- White space in the raster section is ignored.
- You can put any junk you want after the raster, if it starts with a white space character.
- No line should be longer than 70 characters.

Here is an example of a small image in the plain PBM format.

    P1
    # feep.pbm
    24 7
    0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
    0 1 1 1 1 0 0 1 1 1 1 0 0 1 1 1 1 0 0 1 1 1 1 0
    0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 1 0
    0 1 1 1 0 0 0 1 1 1 0 0 0 1 1 1 0 0 0 1 1 1 1 0
    0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0
    0 1 0 0 0 0 0 1 1 1 1 0 0 1 1 1 1 0 0 1 0 0 0 0
    0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

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

You can generate the Plain PBM format from the regular PBM format (first image in the file only) with the pnmtoplainpnm program.

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

## INTERNET MEDIA TYPE

No Internet Media Type (aka MIME type, content type) for PBM has been registered with IANA, but the value `image/x-portable-bitmap` 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 PBM file, but the convention is to use the suffix ".pbm". "pnm" is also conventional, for cases where distinguishing between the particular subformats of PNM is not convenient.

## COMPATIBILITY

Before July 2000, there could be at most one image in a PBM file. As a result, most tools to process PBM files ignore (and don't read) any data after the first image.

## SEE ALSO

libnetpbm, pnm, pgm, ppm, pam, programs that process PBM

## Table Of Contents

- DESCRIPTION
- THE LAYOUT
  - Minimal Subset
  - Plain PBM
- INTERNET MEDIA TYPE
- FILE NAME
- COMPATIBILITY
- SEE ALSO