Feature Description

LEADTOOLS supports CCITT Group3 and Group4 industry standards to compress bitonal (1-bit) image data. Both Group3 and Group4 encode the source image line by line. The difference between the two algorithms is in the way the two standards handle successive scan lines.

In Group3-1D, each scan line is encoded independently, whereas in Group4, scan lines are encoded with reference to the previous one, resulting in improved compression ratios. In Group3-1D, a scan line is encoded as a set of runs, each representing a number of white or black pixels, with white runs alternating with black runs. Every run is encoded using a variable number of bits that can be uniquely identified upon decoding. This means that frequently occurring lengths of run may be encoded very efficiently, while infrequent ones cause the data to grow. For example, a white run of 2 or 3 pixels can be encoded using just 2 bits, while a run of 1000 pixels can be encoded in 25.

Group4 encoding is more complex as compared to Group3 encoding. Each scan line is compared with the previous one. Because of this, vertical features in the source image are used to achieve high compression ratios.

There is also a Group3-2D algorithm. Group3-2D is a combination of Group3-1D and Group4 compression schemes.

Overview of LEADTOOLS CCITT G3/G4 Image Compression SDK Technology

Technology Related to G3/G4 Image Compression