CCITT Group 4 compression, also referred to as G4 or Modified Modified READ (MMR), is a
lossless method of
image compression
Image compression is a type of data compression applied to digital images, to reduce their cost for storage or transmission. Algorithms may take advantage of visual perception and the statistical properties of image data to provide superior r ...
used in Group 4
fax machines defined in the
ITU-T T.6 fax standard. It is only used for
bitonal Polytonality (also polyharmony) is the musical use of more than one key simultaneously. Bitonality is the use of only two different keys at the same time. Polyvalence or polyvalency is the use of more than one harmonic function, from the same key, ...
(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. Group 4 compression is available in many proprietary
image file formats as well as standardized formats such as
TIFF,
CALS, CIT (
Intergraph Raster Type 24) and the
PDF
Portable Document Format (PDF), standardized as ISO 32000, is a file format developed by Adobe in 1992 to present documents, including text formatting and images, in a manner independent of application software, hardware, and operating systems. ...
document format.
G4 offers a small improvement over G3-2D by removing the
end-of-line (EOL) codes. G3 and G4 compression both treat an image as a series of horizontal black strips on a white page. Better
compression 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, this equates to a reduction from 467.5 kB to 23.4 kB (95% compression ratio).
See also
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JBIG2
JBIG2 is an image compression standard for bi-level images, developed by the Joint Bi-level Image Experts Group. It is suitable for both lossless and lossy compression. According to a press release from the Group, in its lossless mode JBIG2 ty ...
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Binary image
A binary image is one that consists of pixels that can have one of exactly two colors, usually black and white. Binary images are also called ''bi-level'' or ''two-level'', Pixelart made of two colours is often referred to as ''1-Bit'' or ''1b ...
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Digital fax compression
External links
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{{Compression formats
Fax