Canon fax basic 2000 サービスマニュアル

5–29

ït

çi

G3 FACSIMILE COMMUNICATIONS

4.5

JBIG Image Compression Encoding Scheme

4.5.1 Outline of the JBIG image compression encoding

scheme

The JBIG Image Compression Encoding Scheme is recommended in ITU-

T T.82/T.85 as a new bi-level (bi-level: White and Black) image compres-

sion encoding scheme developed by JBIG (Joint Bi-level Image experts

Group).

The JBIG Image Compression Encoding Scheme has the following char-

acteristics with regards to text documents, quasi-gray scale images with lit-

tle continuous black and white, and gray scale images which use a

dithering scheme: a higher compression rate (1.1 ~ 30 times higher) than

the conventional MMR compression scheme, the encoded volume will not

exceed the volume of original image information after compression, and

when decoding, the image can be completely re-assembled to its original

condition in the same way as with conventional MR/MMR.

A coding scheme such as this that allows the original image to be com-

pletely restored is called a “reversible scheme,” and a coding scheme that

does not allow the original image to be completely restored an “irrevers-

ible scheme.”

There are two types of JBIG image compression coding sdhemes: Progres-

sive Bi-level Image Compression and Single Progression Sequential Bi-

level Image Compression.

The JBIG Image Compression Encoding Scheme contains Progressive Bi-

level Image Compression for searching image databases, recommended in

T.82, and Single Progression Sequential Bi-level Image Compres-

sion for facsimile, recommended in ITU-T T.82 and T.85.

Images will take on the form shown below.