CCITT Compression G4

CCITT Group 4 compression, also known as G4 or Modified Modified Read (MMR), is a lossless compression method for image compression used on Group 4 fax machines defined in the fax standard T.6 from ITU-T. It is only used for bitonal images (black and white). The compression group 4 is based on the two-dimensional compression scheme Group 3 (G3-2D), also known as Modified Read, which in turn is based on the compression scheme of a known Group 3 (G3) dimension as well as Modified Huffman coding. Group 4 compression is available in many proprietary image file formats and in standard formats such as TIFF, CALS and PDF document format.

Group 4 offers a small improvement over G3-2D by eliminating end-of-line (EOL) codes. The compression G3 and G4 both treat the image as a series of horizontal stripes on a white page. Better compression is achieved when there are a few single points or black lines on the page. Both G3-2D and G4 add a two-dimensional feature to achieve more compression by taking advantage of vertical symmetry. G4 compression typically achieves a 20: 1 compression ratio. For a 8.5 "x11" page scanned at 200PPI, this equates to a size reduction of 456.54 kb at 22.82 kb.

Compression in PDF format

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