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