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CCITT Group 4 is officially know as the CCITT T.6 Standard.

The G4 compression is two-dimensional by default. It is very similar to the G3 2D compression, but it can produce compressed images that are half the size of a G3-compressed file.

It is slower, however, and does not have the same error recovery built in that the G3 format has. The decrease in speed occurs because G4 was designed specifically for encoding disk data. For this reason, it may be advisable to use the G3-Fax compression scheme if final compression size is not crucial, but speed is.