Databases Reference
In-Depth Information
T A B L E 7 . 5
Comparison of binary image coding schemes. Data from [
103
].
Source Description Original Size (pixels) MH (bytes) MR (bytes) MMR (bytes) JBIG (bytes)
Letter
4352
×
3072
20,605
14,290
8,531
6,682
Sparse text
4352
×
3072
26,155
16,676
9,956
7,696
Dense text
4352
×
3072
135,705
105,684
92,100
70,703
T.4 and T.6 is not valid for halftone images. The JBIG algorithm, with its adaptive model and
coder, suffers from no such drawbacks and performs well for halftone images [
103
].
7.7.5 JBIG2-T.88
The JBIG2 standard was approved in February 2000. Besides facsimile transmission, the
standard is also intended for document storage, archiving, wireless transmission, print spool-
ing, and coding of images on the Web. The standard provides specifications only for the
decoder, leaving the encoder design open. This means that the encoder design can be con-
stantly refined, subject only to compatibility with the decoder specifications. This situation
also allows for lossy compression, because the encoder can incorporate lossy transformations
to the data that enhance the level of compression.
The compression algorithm in JBIG provides excellent compression of a generic bi-
level image. The compression algorithm proposed for JBIG2 uses the same arithmetic cod-
ing scheme as JBIG. However, it takes advantage of the fact that a significant number of
bi-level images contain structure that can be used to enhance compression performance. A
large percentage of bi-level images consist of text on some background, while another signifi-
cant percentage of bi-level images are or contain halftone images. The JBIG2 approach allows
the encoder to select the compression technique that would provide the best performance for
the type of data. To do so, the encoder divides the page to be compressed into three types of
regions called
symbol regions
,
halftone regions
, and
generic regions
. The symbol regions are
those containing text data, the halftone regions are those containing halftone images, and the
generic regions are all the regions that do not fit into either category.
The partitioning information has to be supplied to the decoder. The decoder requires that
all information provided to it be organized into
segments
that are made up of a segment header,
a data header, and segment data. The page information segment contains information about
the page including the size and resolution. The decoder uses this information to set up the
page buffer. It then decodes the various regions using the appropriate decoding procedure and
places the different regions in the appropriate location.
Generic Decoding Procedures
There are two procedures used for decoding the generic regions: the generic region decoding
procedure and the generic refinement region decoding procedure. The generic region decoding
procedure uses one of two procedures; the MMR technique used in the Group 3 and Group
