CCITT Group 4 compression, also referred to asG4 orModified Modified READ (MMR), is alossless method ofimage compression used in Group 4fax machines defined in theITU-T T.6 fax standard. It is only used forbitonal (black-and-white) images. Group 4 compression is based on theGroup 3 two-dimensional compression scheme (G3-2D), also known as Modified READ, which is in turn based on theGroup 3 one-dimensional compression scheme (G3), also known asModified Huffman coding. Group 4 compression is available in many proprietaryimage file formats as well as standardized formats such asTIFF,CALS, CIT (Intergraph Raster Type 24) and thePDF document format.
G4 offers a small improvement over G3-2D by removing theend-of-line (EOL) codes. G3 and G4 compression both treat an image as a series of horizontal black strips on a white page. Bettercompression 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.[citation needed] 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).