cximage702_full - (VS2010编译通过)

Using the JBIG-KIT library

--------------------------

Markus Kuhn -- 2004-06-10

This text explains how to use the functions provided by the JBIG-

KIT

portable image compression library in your application software.

1 Introduction to JBIG

We start with a short introduction to JBIG1. More detailed

planes,

layers, and stripes. Each plane contains one bit per pixel. The

number

of planes stored in a JBIG data stream is the number of bits per

pixel. Resolution layers are numbered from 0 to D with 0 being

the

layer with the lowest resolution and D the one with the highest.

Each

next higher resolution layer has twice the number of rows and

columns.

into

several horizontal stripes. All stripes of one resolution layer

have

equal size, except perhaps the final one. The number of stripes

of an

image is equal in all resolution layers and in all bit planes.

The compressed data stream specified by the JBIG standard is

called a

bi-level image entity (BIE). A BIE consists of a 20-byte header,

followed by an optional 1728-byte table (usually not present,

except

in special applications) followed by a sequence of stripe data

entities (SDE). Each SDE encodes the content of one single stripe

information

blocks (called floating marker segments) can also be present.

They are

image or contain additional application specific information. A

looks like this:

| |

+------------------------------------------------+

| |

| optional 1728-byte table |

| |

+------------------------------------------------+

| |

| optional floating marker segments |

| |

+------------------------------------------------+

| |

+------------------------------------------------+

| |

| stripe data entity |

| |

+------------------------------------------------+

...

+------------------------------------------------+

| |

| stripe data entity |

| |

+------------------------------------------------+

possible to store various resolution layers in several BIEs. The

BIE

header contains the number of the first and the last resolution

layer stored in this BIE. Progressive coding is deactivated by

storing the image in one single resolution layer.

It is possible to use the raw BIE data stream as specified by the

JBIG

standard directly as the format of a file used for storing

images.

This is what the pbmtojbg and jbgtopbm conversion tools do that

are

provided in this package as demonstration applications. However,

as

the BIE format has been designed for a large number of very

algorithm. Many features expected from a good file format are

missing

in the BIE data stream:

- no "magic code" in the first few bytes to allow identification

of the file format on a typeless file system and to allow

automatic distinction from other compression algorithms

- no standardized way to encode additional information such as a

textual description, information about the meaning of various

bit

planes, the physical size and resolution of the document, etc.

- a checksum to ensure image integrity

- many things more

document archiving and exchange. A standard format for this

would typically combine a BIE representing the image data with an

On the other hand, in database applications for instance, a BIE

might

be stored directly in a variable length field. Auxiliary

information

would then be stored in other fields of the same record, to

simply

search operations.

2 Compressing an image

contains

eight pixels, where the most significant bit represents the

leftmost

of these. Each line of a bitmap has to be stored in an integral

number

of bytes. If the image width is not an integral multiple of

eight,

then the final byte has to be padded with zero bits.

For example the 23x5 pixels large single plane image:

.XXXXX..XXX...X...XXX..

.....X..X..X..X..X.....

.....X..XXX...X..X.XXX.

is represented by the 15 bytes

00000100 11100010 01011100

00111000 11100010 00111000

directly by the Xlib library of the X Window System.

As JBIG can also handle images with multiple bit planes, the

JBIG-KIT

library functions accept and return always arrays of pointers to

bitmaps with one pointer per plane.

For single-plane images, the standard recommends that a 0 pixel

represents the background and a 1 pixel represents the foreground

color of an image, in other words, 0 is white and 1 is black for

For greyscale images, by using a Gray code instead of a simple

binary

weighted representation of the pixel intensity, some increase in

coding efficiency can be reached.

A Gray code is also a binary representation of integer numbers,

but it

has the property that the representations of the integer numbers

i and