CCITT Group 3
CCITT Group 3 is a lossless compressed data format for bi-level images. It is also known as T4, T.4, Fax3, Group 3, G3, or ITU-T Fax Group 3. It is one of the formats used by fax machines. It is usually embedded in another image file format such as TIFF, but can be stored in a file by itself.
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 (MH) compression, and the 2-dimensional variety uses Modified READ (MR) compression.
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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.
Other formats that can use CCITT Group 3 compression include PDF and SPIFF.
See also
Specifications
- ITU-T Recommendation T.4
- TIFF 6.0 Specification, Section 10: Modified Huffman Compression
Software
- Netpbm (g3topbm, pbmtog3)
- ImageMagick (format named "FAX")
