ITU-T T.800标准：JPEG2000核心编码系统详解

JPEG2000

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"ITU-T T.800标准是国际电信联盟(ITU)关于JPEG 2000图像编码系统的详细规范。这个标准定义了核心编码系统，用于高效地压缩和解压缩静止图像数据，旨在提供高质量的图像传输和存储解决方案。JPEG 2000是JPEG（Joint Photographic Experts Group）标准的升级版，它采用了更先进的压缩技术，包括分块编码、小波变换以及多分辨率表示等特性。" JPEG 2000的核心编码系统由以下几个关键技术组成： 1. 小波变换(Wavelet Transform)：与传统的基于离散余弦变换(DCT)的JPEG相比，JPEG 2000采用小波变换来处理图像数据。小波变换允许同时在空间和频率域上进行分析，提供了更精细的图像分解。 2. 多分辨率表示(Multi-resolution Pyramid)：JPEG 2000支持自底向上的多分辨率表示，图像数据可以被分解成多个分辨率级别的细节，这使得图像的渐进显示和区域感兴趣区域的优先解码成为可能。 3. 分块编码(Block Coding)：图像被分割成多个小块进行独立编码，这样可以更好地适应不同区域的复杂性，并且允许并行处理，提高了编码效率。 4. 有损和无损编码(Lossy and Lossless Coding)：JPEG 2000支持两种编码方式，可以根据需求选择保留原始数据完整性或牺牲部分质量来实现更高的压缩率。 5. 自适应编码(Adaptive Coding)：编码过程会根据图像内容和信道条件动态调整编码策略，确保最佳的压缩效果和传输效率。 6. 区域编码(Region-of-Interest Coding)：允许用户优先解码图像中的特定区域，这对于医学影像、遥感等领域特别有用。 7. 嵌入式位流(Embedded Bitstream)：JPEG 2000的位流是嵌入式的，这意味着可以在不完全解码整个图像的情况下获取图像的低分辨率版本，这对于网络传输和实时应用非常有利。 8. 颜色管理(Color Management)：JPEG 2000标准考虑了颜色空间的转换和管理，确保在不同的设备和环境中图像色彩的一致性。 9. 元数据(Metadata)：支持添加和保留元数据，如EXIF信息，这有助于图像的管理和检索。 ITU-T T.800标准详细阐述了JPEG 2000编码系统的设计和实现，涵盖了从图像分析、编码算法到解码过程的各个方面，旨在提供一种高效、灵活且高质量的静止图像压缩标准。此标准适用于各种应用场景，如数字图像存储、网络传输、远程监控、医疗影像处理等。

ISO/IEC 15444-1:2016 (E)

8 Rec. ITU-T T.800 (11/2015)

QCC Quantization component marker

QCD Quantization default marker

RGN Region-of-interest marker

SIZ Image and tile size marker

SOC Start of codestream marker

SOP Start of packet marker

SOD Start of data marker

SOT Start of tile-part marker

TLM Tile-part lengths marker

5 General description

This Recommendation | International Standard describes an image compression system that allows great flexibility, not

only for the compression of images, but also for the access into the codestream. The codestream provides a number of

mechanisms for locating and extracting portions of the compressed image data for the purpose of retransmission, storage,

display or editing. This access allows storage and retrieval of compressed image data appropriate for a given application,

without decoding.

The division of both the original image data and the compressed image data in a number of ways leads to the ability to

extract image data from the compressed image data to form a reconstructed image with a lower resolution or lower

precision, or regions of the original image. This allows the matching of a codestream to the transmission channel, storage

device or display device, regardless of the size, number of components and sample precision of the original image. The

codestream can be manipulated without decoding to achieve a more efficient arrangement for a given application.

Thus, the sophisticated features of this Recommendation | International Standard allow a single codestream to be used

efficiently by a number of applications. The largest image source devices can provide a codestream that is easily processed

for the smallest image display device, for example.

In general, this Recommendation | International Standard deals with three domains: spatial (samples), transformed

(coefficients) and compressed image data. Some entities (e.g., tile-component) have meaning in all three domains. Other

entities (e.g., code-block or packet) have meaning in only one domain (e.g., transformed or compressed image data,

respectively). The splitting of an entity into other entities in the same domain (e.g., component to tile-components) is

described separately for each of the domains.

5.1 Purpose

There are four main elements described in this Recommendation | International Standard:

– Encoder: An embodiment of an encoding process. An encoder takes as input digital source image data and

parameter specifications, and by means of a set of procedures generates as output a codestream.

– Decoder: An embodiment of a decoding process. A decoder takes as input compressed image data and

parameter specifications, and by means of a specified set of procedures generates as output digital

reconstructed image data.

– Codestream syntax: A representation of the compressed image data that includes all parameter

specifications required by the decoding process.

– Optional file format: The optional file format is for exchange between application environments. The

codestream can be used by other file formats or stand-alone without this file format.

5.2 Codestream

The codestream is a linear stream of bits from the first bit to the last bit. For convenience, it can be divided into (8-bit)

bytes, starting with the first bit of the codestream, with the "earlier" bit in a byte viewed as the most significant bit of the

byte when given e.g., a hexadecimal representation. This byte stream may be divided into groups of consecutive bytes.

The hexadecimal value representation is sometimes implicitly assumed in the text when describing bytes or a group of

bytes that do not have a "natural" numeric value representation.

ISO/IEC 15444-1:2016 (E)

Rec. ITU-T T.800 (11/2015) 9

5.3 Coding principles

The main procedures for this Recommendation | International Standard are shown in Figure 5-1. This shows the decoding

order only. The compressed image data is already conceptually assigned to portions of the image data. Procedures are

presented in the annexes in the order of the decoding process. The coding process is summarized below.

NOTE 1 – Annexes A to I are considered normative to this Recommendation | International Standard. However, certain denoted

subclauses and notes and all examples are informative.

Many images have multiple components. This Recommendation | International Standard has a multiple component

transformation to decorrelate three components. This is the only function in this Recommendation | International Standard

that relates components to each other (see Annex G).

The image components may be divided into tiles. These tile-components are rectangular arrays that relate to the same

portion of each of the components that make up the image. Thus, tiling of the image actually creates tile-components that

can be extracted or decoded independently of each other. This tile independence provides one of the methods for

extracting a region of the image (see Annex B).

The tile-components are decomposed into different decomposition levels using a wavelet transformation. These

decomposition levels contain a number of sub-bands populated with coefficients that describe the horizontal and vertical

spatial frequency characteristics of the original tile-components. The coefficients provide frequency information about a

local area, rather than across the entire image like the Fourier transformation. That is, a small number of coefficients

completely describe a single sample. A decomposition level is related to the next decomposition level by a spatial factor

of two. That is, each successive decomposition level of the sub-bands has approximately half the horizontal and half the

vertical resolution of the previous one. Images of lower resolution than the original are generated by decoding a selected

subset of these sub-bands (see Annex F).

Although there are as many coefficients as there are samples, the information content tends to be concentrated in just a

few coefficients. Through quantization, the information content of a large number of small-magnitude coefficients is

further reduced (Annex E). Additional processing by the entropy coder reduces the number of bits required to represent

these quantized coefficients, sometimes significantly compared to the original image (see Annexes C, D and B).

Figure 5-1 – Specification block diagram

The individual sub-bands of a tile-component are further divided into code-blocks. These rectangular arrays of

coefficients can be extracted independently. The individual bit-planes of the coefficients in a code-block are coded with

three coding passes. Each of these coding passes collects contextual information about the bit-plane compressed image

data (see Annex D). An arithmetic coder uses this contextual information, and its internal state, to decode a compressed

bit stream (see Annex C.) Different termination mechanisms allow different levels of independent extraction of this coding

pass compressed image data.

The bit stream compressed image data created from these coding passes is grouped in layers. Layers are arbitrary

groupings of coding passes from code-blocks (see Annex B).

NOTE 2 – Although there is great flexibility in layering, the premise is that each successive layer contributes to a higher quality

image.

Sub-band coefficients at each resolution level are partitioned into rectangular areas called precincts (see Annex B).

Packets are a fundamental unit of the compressed codestream. A packet contains compressed image data from one layer

of a precinct of one resolution level of one tile-component. Packets provide another method for extracting a spatial region

T.800_F5-1

Codestream syntax (Annex A)

ROI

(Annex H)

File format (optional, Annex I)

Data ordering

(Annex B)

Arithmetic

coding

(Annex C)

Coefficient

bit modelling

(Annex D)

Quantization

(Annex E)

Transformation

(Annex F)

DC, comp.

transformation

(Annex G)

Codestream

Reconstructed and rendered image

ISO/IEC 15444-1:2016 (E)

10 Rec. ITU-T T.800 (11/2015)

independently from the codestream. These packets are interleaved in the codestream using a few different methods

(see Annex B).

A mechanism is provided that allows the compressed image data corresponding to regions of interest in the original tile-

components to be coded and placed earlier in the bit stream (see Annex H).

Several mechanisms are provided to allow the detection and concealment of bit errors that might occur over a noisy

transmission channel (see D.5 and J.7).

The codestream relating to a tile organized in packets, are arranged in one or more tile-parts. A tile-part header, comprised

of a series of markers and marker segments, contains information about the various mechanisms and coding styles that

are needed to locate, extract, decode and reconstruct every tile-component. At the beginning of the entire codestream is a

main header, comprised of markers and marker segments that offers similar information, as well as information about the

original image (see Annex A).

The codestream is optionally wrapped in a file format that allows applications to interpret the meaning of, and other

information about the image. The file format may contain data besides the codestream (see Annex I).

In review, procedures that divide the original image are the following:

– The components of the image are divided into rectangular tiles. The tile-component is the basic unit of the

original or reconstructed image.

– Performing the wavelet transformation on a tile-component creates decomposition levels.

– These decomposition levels are made up of sub-bands of coefficients that describe the frequency

characteristics of local areas (rather than across the entire tile-component) of the tile-component.

– The sub-bands of coefficients are quantized and collected into rectangular arrays of code-blocks.

– Each bit-plane of the coefficients in a code-block is entropy coded in three types of coding passes.

– Some of the coefficients can be coded first to provide a region of interest.

At this point the image data is fully converted to compressed image data. The procedures that reassemble these bit stream

units into the codestream are the following:

– The compressed image data from the coding passes are collected in layers.

– Packets are composed compressed image data from one precinct of a single layer of a single resolution

level of a single tile-component. The packets are the basic unit of the compressed image data.

– All the packets from a tile are interleaved in one of several orders and placed in one or more tile-parts.

– The tile-parts have a descriptive tile-part header and can be interleaved in some orders.

– The codestream has a main header at the beginning that describes the original image and the various

decomposition and coding styles.

– The optional file format describes the meaning of the image and its components in the context of the

application.

6 Encoder requirements

An encoding process converts source image data to compressed image data. Annexes A, B, C, D, E, F, G and H describe

the encoding process. All encoding processes are specified informatively.

An encoder is an embodiment of the encoding process. In order to conform to this Recommendation | International

Standard, an encoder shall convert source image data to compressed image data that conform to the codestream syntax

specified in Annex A.

7 Decoder requirements

A decoding process converts compressed image data to reconstructed image data. Annexes A to H describe and specify

the decoding process. All decoding processes are normative.

A decoder is an embodiment of the decoding process. In order to conform to this Recommendation | International

Standard, a decoder shall convert all, or specific parts of, any compressed image data that conform to the codestream

syntax specified in Annex A to a reconstructed image.

There is no normative or required implementation for the encoder or decoder. In some cases, the descriptions use particular

implementation techniques for illustrative purposes only.

ISO/IEC 15444-1:2016 (E)

12 Rec. ITU-T T.800 (11/2015)

Annex A

Codestream syntax

(This annex forms an integral part of this Recommendation | International Standard.)

In this annex and all of its subclauses, the flow charts and tables are normative only in the sense that they are defining an

output that alternative implementations shall duplicate.

This annex specifies the marker and marker segment syntax and semantics defined by this Recommendation | International

Standard. These markers and marker segments provide codestream information for this Recommendation | International

Standard. Furthermore, this annex provides a marker and marker segment syntax that is designed to be used in future

specifications that include this Recommendation | International Standard as a normative reference.

This Recommendation | International Standard does not include a definition of compliance or conformance. The parameter

values of the syntax described in this annex are not intended to portray the capabilities required to be compliant.

A.1 Markers, marker segments, and headers

This Recommendation | International Standard uses markers and marker segments to delimit and signal the characteristics

of the source image and codestream. This set of markers and marker segments is the minimal information needed to

achieve the features of this Recommendation | International Standard and is not a file format. A minimal file format is

offered in Annex I.

Main and tile-part headers are collections of markers and marker segments. The main header is found at the beginning of

the codestream. The tile-part headers are found at the beginning of each tile-part (see below). Some markers and marker

segments are restricted to only one of the two types of headers while others can be found in either.

Every marker is two bytes long. The first byte consists of a single 0xFF byte. The second byte denotes the specific marker

and can have any value in the range 0x01 to 0xFE. Many of these markers are already used in Rec. ITU-T T.81 |

ISO/IEC 10918-1 and Rec. ITU-T T.84 | ISO/IEC 10918-3 and shall be regarded as reserved unless specifically used.

A marker segment includes a marker and associated parameters, called marker segment parameters. In every marker

segment the first two bytes after the marker shall be an unsigned value that denotes the length in bytes of the marker

segment parameters (including the two bytes of this length parameter but not the two bytes of the marker itself). When a

marker segment that is not specified in this Recommendation | International Standard appears in a codestream, the decoder

shall use the length parameter to discard the marker segment.

A.1.1 Types of markers and marker segments

Six types of markers and marker segments are used: delimiting, fixed information, functional, in-bit stream, pointer and

informational. Delimiting markers and marker segments are used to frame the main and tile-part headers and the

bit-stream data. Fixed information marker segments give required information about the image. The location of these

marker segments, like delimiting marker and marker segments, is specified. Functional marker segments are used to

describe the coding functions used. In-bit-stream markers and marker segments are used for error resilience. Pointer

marker segments provide specific offsets in the bit stream. Informational marker segments provide ancillary information.

A.1.2 Syntax similarity with Rec. ITU-T T.81 | ISO/IEC 10918-1

The marker and marker segment syntax uses the same construction as defined in Rec. ITU-T T.81 | ISO/IEC 10918-1.

The marker range 0xFF30 to 0xFF3F is reserved by this Recommendation | International Standard for markers without

marker segment parameters. Table A.1 shows in which specification these markers and marker segments are defined.

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