Image analysis – Image compression or coding – Adaptive coding
Reexamination Certificate
1999-08-31
2002-10-29
Johnson, Timothy M. (Department: 2623)
Image analysis
Image compression or coding
Adaptive coding
C382S251000
Reexamination Certificate
active
06473531
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a system for decoding coded digital image information and subjecting the decoded digital image information to recording.
2. Description of the Related Art
(Remote Printing System)
Recently, a remote printing system has become popular in which an image output device is connected to a network and the output of an image is indicated from a terminal on the network. Various digital images are handled therein, including those inputted from a scanner, those generated by a computer, and those transmitted through a network. With higher definition of digital images and colorization, a demand now exists for an image processing technique capable of attaining both high speed and high image quality.
In such a system, an image is generally handled after coding for the purpose of reducing the image transmission time and storage capacity.
(Necessity of Precollation Technique)
In a remote printing system with an image output device of high speed and high resolution connected thereto, a precollation technique is needed for operating the image output device efficiently. According to this technique, for continuous output of a large amount of images, images are stored in a storage device and are read from the same device in conformity with the speed of the image output device after sorting if necessary. In this case, image coding is absolutely necessary for the following two reasons. (The image coding will be referred to below in the same sense as image compression.)
(1) Reducing the Storage Capacity
Colored image information of a high definition is very large in the amount of information. In a printing system which handles a large amount of information, it is necessary in point of cost to reduce the amount of information by image compression before storage.
(2) Shortening the Read Time of a Storage Device
For storing a large amount of digital images there usually is employed an HDD as a storage device. Since the HDD is low in its read/write processing speed, it is necessary that the amount of input and output data be reduced in advance, or else it will become impossible to follow up the processing speed of an image output device.
Among the images handled by the system are included those (hereinafter referred to as “coded images”) which are inputted in an already coded state for the purpose of reducing the transmission time for example. In many cases, coded images are different in code format from those for precollation or require image processing such as color conversion or resolution conversion, so they are usually once decoded and are then coded again, which coded images are stored.
(High Efficient Image Coding)
As image coding methods there are known a reversible method and an irreversible method. In the reversible method, an image (hereinafter referred to as “decoded image”) which has been subjected to coding and decoding coincides completely with the original image. On the other hand, in the irreversible method, a decoded image does not coincide completely with the original image and the image quality may be degraded at a certain compression rate. The foregoing printing system often adopts the irreversible method in which a high compression rate can be expected.
(Problem involved in Irreversible Coding)
Generally, in irreversible coding, the higher the compression rate is, the lower the quality of a decoded image is (“decoded image quality” hereinafter), assuming that coding conditions are the same. That is, the compression rate and the decoded image quality are in a relation of trade-off. This is because the irreversible coding realizes a high compression rate by discarding information which is considered relatively difficult to influence the vision in the image concerned. Thus, if the compression rate is low, the amount of information discarded is small and there is attained a good decoded image quality, but if the compression rate is high, the amount of information discarded increases, resulting in degradation of the decoded image quality.
In the foregoing printing system there is a strong demand for a high compression rate and a high decoded image quality. Therefore, a decoded image quality control method is needed which can guarantee a high image quality and achieve the maximum compression rate.
(Conventional Decoded Image Quality Controlling Method in the Irreversible Coding Technique)
In an image there exists a region easy to be compressed and a region difficult to be compressed. Decoded image quality control applied to the irreversible coding technique is realized by analyzing image characteristics and deciding a coding parameter suitable for each region. For example, as a coding method to which such a decoded image quality control is applied, there is known an “image coding control method” (see Japanese Published Unexamined Patent Application No. Hei 6-165149), the construction of which is shown in FIG.
26
.
In this example, as a coding method there is illustrated a combined method of DCT (Discrete Cosine Transform) and conversion coefficient quantization, in which the quantizing step size (quantization matrix) is changed over from one to another for image quality control.
In
FIG. 26
, the numeral
2601
denotes a blocking unit, numeral
2602
denotes a DCT unit,
2603
denotes quantizing unit,
2604
denotes a coding unit,
2605
denotes an image quality control unit,
2606
denotes a quantization selecting unit,
2607
denotes input image information,
2608
denotes blocked image information,
2609
denotes image quality control information,
2610
denotes quantization matrix information, and
2611
denotes coded image information.
The image quality control unit
2605
measures a statistic of the blocked image information and outputs the image quality information
2609
. In accordance with the image quality control information
2609
the quantization selecting unit
2606
decides a quantization matrix. More specifically, if it is determined in the image quality control unit
2605
that the blocked image information is an image involving a coding distortion called mosquito noise, a quantization matrix difficult to cause a coding distortion (small in the quantizing step size) is selected in the quantization selecting unit
2606
. As a result, it becomes possible to suppress the occurrence of a coding distortion.
As another example of a coding method with the decoded image quality control applied thereto there is “an image quality predicting apparatus and method as well as an image quality controlling apparatus and method” (Japanese Published Unexamined Patent Application No. Hei 10-075369), whose construction is shown in
FIGS. 27 and 28
. The coding method is a combined method of DCT and conversion coefficient quantization, in which there is used a technique of changing over from one quantization matrix to another for image quality control.
In
FIG. 27
, the numeral
2701
denotes a blocking unit, numeral
2702
denotes a DCT unit, numeral
2703
denotes a quantizing unit, numeral
2704
denotes a coding unit, numeral
2705
denotes an image quality control unit, numeral
2706
denotes a coding system characteristic output unit, numeral
2707
denotes an image output device characteristic output unit, numeral
2708
denotes a quantization selecting unit, numeral
2709
denotes input image information,
2710
denotes blocking image information,
2711
denotes coding system characteristic information,
2712
denotes image output device characteristic information,
2713
denotes image quality control information,
2714
denotes quantization matrix information, and
2715
denotes coding image information.
In
FIG. 28
, the numeral
2801
denotes an image quality degradation item-by-item coding device characteristic output unit, numeral
2802
denotes an image quality degradation item-by-item image analyzing unit, numeral
2803
denotes an image quality degradation item-by-item output device characteristic output unit, numeral
2804
denotes an image quality item-by-item determining unit, and numera
Fuji 'Xerox Co., Ltd.
Johnson Timothy M.
Morgan & Lewis & Bockius, LLP
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