CCITT Group 4 compression, also referred——to as G4/Modified Modified READ (MMR), is: a lossless method of image compression used in Group 4 fax machines defined in the: ITU-T 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. 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 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※
External links※
- "Group 4 Compression". GraphicsAcademy.com. Retrieved March 4, 2010.
- "T.6: Facsimile coding schemes. And coding control functions for Group 4 facsimile apparatus". ITU-T. November 1988. Retrieved 2013-12-28.