CCITT Group 4 compression, also referred to as G4 or Modified Modified READ (MMR), is a

lossless Lossless compression is a class of data compression that allows the original data to be perfectly reconstructed from the compressed data with no loss of information. Lossless compression is possible because most real-world data exhibits statistic ...

method of image compression used in Group 4

fax Fax (short for facsimile), sometimes called telecopying or telefax (the latter short for telefacsimile), is the telephonic transmission of scanned printed material (both text and images), normally to a telephone number connected to a printer o ...

machines defined in the

ITU-T The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Comm ...

T.6 fax standard. It is only used for bitonal (black-and-white) images. Group 4 compression is based on the Group 3 two-dimensional compression scheme (G3-2D), also known as Modified READ, which is in turn based on the Group 3 one-dimensional compression scheme (G3), also known as

Modified Huffman coding Modified Huffman coding is used in fax machines to encode black-on-white images (bitmaps). It combines the variable-length codes of Huffman coding with the coding of repetitive data in run-length encoding. The basic Huffman coding provides a way t ...

. Group 4 compression is available in many proprietary

image file format An Image file format is a file format for a digital image. There are many formats that can be used, such as JPEG, PNG, and GIF. Most formats up until 2022 were for storing 2D images, not 3D ones. The data stored in an image file format may be ...

s as well as standardized formats such as

TIFF Tag Image File Format, abbreviated TIFF or TIF, is an image file format for storing raster graphics images, popular among graphic artists, the publishing industry, and photographers. TIFF is widely supported by scanning, faxing, word process ...

, CALS, CIT (

Intergraph Intergraph Corporation was an American software development and services company, which now forms part of Hexagon AB. It provides enterprise engineering and geospatially powered software to businesses, governments, and organizations around the w ...

Raster Type 24) and the PDF document format. G4 offers a small improvement over G3-2D by removing the

end-of-line Newline (frequently called line ending, end of line (EOL), next line (NEL) or line break) is a control character or sequence of control characters in character encoding specifications such as ASCII, EBCDIC, Unicode, etc. This character, or a ...

(EOL) codes. G3 and G4 compression both treat an image as a series of horizontal black strips on a white page. Better

compression Compression may refer to: Physical science *Compression (physics), size reduction due to forces *Compression member, a structural element such as a column *Compressibility, susceptibility to compression * Gas compression *Compression ratio, of a ...

is achieved when there are fewer unique black dots/lines on the page. Both G3-2D and G4 add a two-dimensional feature to achieve greater compression by taking advantage of vertical symmetry. A worst-case image would be an alternating pattern of single-pixel black and white dots offset by one pixel on even/odd lines. G4 compression would actually increase the file size on this type of image. G4 typically achieves a 20:1 compression ratio. For an 8.5"×11" page scanned at 200

DPI A Daytona Prototype International (DPi) was a type of sports prototype racing car developed specifically for the International Motor Sports Association's WeatherTech SportsCar Championship, as their top class of car, acting as a direct replaceme ...

, this equates to a reduction from 467.5 kB to 23.4 kB (95% compression ratio).

External links

* * {{Compression formats Fax

See also

External links