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What is the PBM format?

Portable bitmap format (black and white)

The PBM (Portable Bitmap) format is one of the simplest and earliest graphics file formats used for storing monochrome images. It is part of the Netpbm suite, which also includes PGM (Portable GrayMap) for grayscale images and PPM (Portable PixMap) for color images. The PBM format is designed to be extremely easy to read and write in a program, and to be clear and unambiguous. It is not intended to be a stand-alone format, but rather a lowest common denominator for converting between different image formats.

The PBM format supports only black and white (1-bit) images. Each pixel in the image is represented by a single bit – 0 for white and 1 for black. The simplicity of the format makes it straightforward to manipulate using basic text editing tools or programming languages without the need for specialized image processing libraries. However, this simplicity also means that PBM files can be larger than more sophisticated formats like JPEG or PNG, which use compression algorithms to reduce file size.

There are two variations of the PBM format: the ASCII (plain) format, known as P1, and the binary (raw) format, known as P4. The ASCII format is human-readable and can be created or edited with a simple text editor. The binary format is not human-readable but is more space-efficient and faster for programs to read and write. Despite the differences in storage, both formats represent the same type of image data and can be converted between each other without loss of information.

The structure of a PBM file in ASCII format begins with a two-byte magic number that identifies the file type. For PBM ASCII format, this is 'P1'. Following the magic number, there is whitespace (blanks, TABs, CRs, LFs), and then a width specification, which is the number of columns in the image, followed by more whitespace, and then a height specification, which is the number of rows in the image. After the height specification, there is more whitespace, and then the pixel data begins.

The pixel data in an ASCII PBM file consists of a series of '0's and '1's, with each '0' representing a white pixel and each '1' representing a black pixel. The pixels are arranged in rows, with each row of pixels on a new line. Whitespace is allowed anywhere in the pixel data except within a two-character sequence (it is not allowed between the two characters of the sequence). The end of the file is reached after reading width*height bits.

In contrast, the binary PBM format starts with a magic number of 'P4' instead of 'P1'. After the magic number, the format of the file is the same as the ASCII version until the pixel data begins. The binary pixel data is packed into bytes, with the most significant bit (MSB) of each byte representing the leftmost pixel, and each row of pixels padded as necessary to fill out the last byte. The padding bits are not significant and their values are ignored.

The binary format is more space-efficient because it uses a full byte to represent eight pixels, as opposed to the ASCII format which uses at least eight bytes (one character per pixel plus whitespace). However, the binary format is not human-readable and requires a program that understands the PBM format to display or edit the image.

Creating a PBM file programmatically is relatively simple. In a programming language like C, one would open a file in write mode, output the appropriate magic number, write the width and height as ASCII numbers separated by whitespace, and then output the pixel data. For an ASCII PBM, the pixel data can be written as a series of '0's and '1's with appropriate line breaks. For a binary PBM, the pixel data must be packed into bytes and written to the file in binary mode.

Reading a PBM file is also straightforward. A program would read the magic number to determine the format, skip the whitespace, read the width and height, skip more whitespace, and then read the pixel data. For an ASCII PBM, the program can read characters one at a time and interpret them as pixel values. For a binary PBM, the program must read bytes and unpack them into individual bits to get the pixel values.

The PBM format does not support any form of compression or encoding, which means that the file size is directly proportional to the number of pixels in the image. This can result in very large files for high-resolution images. However, the simplicity of the format makes it ideal for learning about image processing, for use in situations where image fidelity is more important than file size, or for use as an intermediary format in image conversion processes.

One of the advantages of the PBM format is its simplicity and the ease with which it can be manipulated. For example, to invert a PBM image (turn all black pixels white and vice versa), one can simply replace all '0's with '1's and all '1's with '0's in the pixel data. This can be done with a simple text processing script or program. Similarly, other basic image operations like rotation or mirroring can be implemented with simple algorithms.

Despite its simplicity, the PBM format is not widely used for general image storage or exchange. This is primarily due to its lack of compression, which makes it inefficient for storing large images or for use over the internet where bandwidth may be a concern. More modern formats like JPEG, PNG, and GIF offer various forms of compression and are better suited for these purposes. However, the PBM format is still used in some contexts, particularly for simple graphics in software development, and as a teaching tool for image processing concepts.

The Netpbm suite, which includes the PBM format, provides a collection of tools for manipulating PBM, PGM, and PPM files. These tools allow for conversion between the Netpbm formats and other popular image formats, as well as basic image processing operations like scaling, cropping, and color manipulation. The suite is designed to be easily extensible, with a simple interface for adding new functionality.

In conclusion, the PBM image format is a simple, no-frills file format for storing monochrome bitmap images. Its simplicity makes it easy to understand and manipulate, which can be advantageous for educational purposes or for simple image processing tasks. While it is not suitable for all applications due to its lack of compression and resulting large file sizes, it remains a useful format within the specific contexts where its strengths are most beneficial. The PBM format, along with the rest of the Netpbm suite, continues to be a valuable tool for those working with basic image processing and format conversion.

Supported formats

AAI.aai

AAI Dune image

AI.ai

Adobe Illustrator CS2

AVIF.avif

AV1 Image File Format

AVS.avs

AVS X image

BAYER.bayer

Raw Bayer Image

BMP.bmp

Microsoft Windows bitmap image

CIN.cin

Cineon Image File

CLIP.clip

Image Clip Mask

CMYK.cmyk

Raw cyan, magenta, yellow, and black samples

CMYKA.cmyka

Raw cyan, magenta, yellow, black, and alpha samples

CUR.cur

Microsoft icon

DCX.dcx

ZSoft IBM PC multi-page Paintbrush

DDS.dds

Microsoft DirectDraw Surface

DPX.dpx

SMTPE 268M-2003 (DPX 2.0) image

DXT1.dxt1

Microsoft DirectDraw Surface

EPDF.epdf

Encapsulated Portable Document Format

EPI.epi

Adobe Encapsulated PostScript Interchange format

EPS.eps

Adobe Encapsulated PostScript

EPSF.epsf

Adobe Encapsulated PostScript

EPSI.epsi

Adobe Encapsulated PostScript Interchange format

EPT.ept

Encapsulated PostScript with TIFF preview

EPT2.ept2

Encapsulated PostScript Level II with TIFF preview

EXR.exr

High dynamic-range (HDR) image

FARBFELD.ff

Farbfeld

FF.ff

Farbfeld

FITS.fits

Flexible Image Transport System

GIF.gif

CompuServe graphics interchange format

GIF87.gif87

CompuServe graphics interchange format (version 87a)

GROUP4.group4

Raw CCITT Group4

HDR.hdr

High Dynamic Range image

HRZ.hrz

Slow Scan TeleVision

ICO.ico

Microsoft icon

ICON.icon

Microsoft icon

IPL.ipl

IP2 Location Image

J2C.j2c

JPEG-2000 codestream

J2K.j2k

JPEG-2000 codestream

JNG.jng

JPEG Network Graphics

JP2.jp2

JPEG-2000 File Format Syntax

JPC.jpc

JPEG-2000 codestream

JPE.jpe

Joint Photographic Experts Group JFIF format

JPEG.jpeg

Joint Photographic Experts Group JFIF format

JPG.jpg

Joint Photographic Experts Group JFIF format

JPM.jpm

JPEG-2000 File Format Syntax

JPS.jps

Joint Photographic Experts Group JPS format

JPT.jpt

JPEG-2000 File Format Syntax

JXL.jxl

JPEG XL image

MAP.map

Multi-resolution Seamless Image Database (MrSID)

MAT.mat

MATLAB level 5 image format

PAL.pal

Palm pixmap

PALM.palm

Palm pixmap

PAM.pam

Common 2-dimensional bitmap format

PBM.pbm

Portable bitmap format (black and white)

PCD.pcd

Photo CD

PCDS.pcds

Photo CD

PCT.pct

Apple Macintosh QuickDraw/PICT

PCX.pcx

ZSoft IBM PC Paintbrush

PDB.pdb

Palm Database ImageViewer Format

PDF.pdf

Portable Document Format

PDFA.pdfa

Portable Document Archive Format

PFM.pfm

Portable float format

PGM.pgm

Portable graymap format (gray scale)

PGX.pgx

JPEG 2000 uncompressed format

PICON.picon

Personal Icon

PICT.pict

Apple Macintosh QuickDraw/PICT

PJPEG.pjpeg

Joint Photographic Experts Group JFIF format

PNG.png

Portable Network Graphics

PNG00.png00

PNG inheriting bit-depth, color-type from original image

PNG24.png24

Opaque or binary transparent 24-bit RGB (zlib 1.2.11)

PNG32.png32

Opaque or binary transparent 32-bit RGBA

PNG48.png48

Opaque or binary transparent 48-bit RGB

PNG64.png64

Opaque or binary transparent 64-bit RGBA

PNG8.png8

Opaque or binary transparent 8-bit indexed

PNM.pnm

Portable anymap

PPM.ppm

Portable pixmap format (color)

PS.ps

Adobe PostScript file

PSB.psb

Adobe Large Document Format

PSD.psd

Adobe Photoshop bitmap

RGB.rgb

Raw red, green, and blue samples

RGBA.rgba

Raw red, green, blue, and alpha samples

RGBO.rgbo

Raw red, green, blue, and opacity samples

SIX.six

DEC SIXEL Graphics Format

SUN.sun

Sun Rasterfile

SVG.svg

Scalable Vector Graphics

SVGZ.svgz

Compressed Scalable Vector Graphics

TIFF.tiff

Tagged Image File Format

VDA.vda

Truevision Targa image

VIPS.vips

VIPS image

WBMP.wbmp

Wireless Bitmap (level 0) image

WEBP.webp

WebP Image Format

YUV.yuv

CCIR 601 4:1:1 or 4:2:2

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What file types can I convert?

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