CCITT Group 3

CCITT Group 3 is a lossless compression algorithm for bilevel images. It is also known as T.4, Fax 3, and ITU-T Fax Group 3. It is commonly used for storing scanned images, usually in TIFF format. It is notable in that it is sometimes used in fax transmission.

It comes in two main varieties: so-called 1-dimensional and 2-dimensional (though both are used on 2-dimensional images).

The 1-dimensional variety uses Modified Huffman compression (= RLE + Huffman coding, sometimes abbreviated "MH").

The 2-dimensional variety uses Modified READ compression (= delta + RLE + Huffman coding, sometimes abbreviated "MR").

Related formats
In a TIFF file, compression type 2 is a type of 1-dimensional CCITT Group 3. Compression type 3 without T4Options bit 0 set is a slightly different type of 1-dimensional CCITT Group 3. Compression type 3 with T4Options bit 0 set is 2-dimensional CCITT Group 3.

In theory, CCITT Group 3 can be used in SPIFF files (compression types 1 and 2).

Specifications

 * ITU-T Recommendation T.4
 * TIFF 6.0 Specification, Section 10: Modified Huffman Compression