JPH06284402A - High definition picture coding method - Google Patents

Abstract

PURPOSE:To attain high definition picture coding with less distortion in a high frequency component. CONSTITUTION:A DCT coefficient obtained by a DCT device 2 is compared with plural comparison DCT coefficients STH1-STHm at comparators C1-Cm. When the DCT coefficient is larger than any of the DCT coefficients STH1-STHm, a relevant comparator is set and any relevant table among high frequency quantization tables Q1-Qm, a quantization device 4 makes quantization based thereon to prevent deterioration in a high frequency component.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image coding method,
In particular, the present invention relates to a high-quality image encoding method that requires high image quality.

[0002]

2. Description of the Related Art As shown in FIG.
The blocking circuit 21 converts the data into n × n data, and the discrete cosine transformer 22 performs discrete cosine transform (DCT) on the data.
In the conventional image encoding method for compressing / encoding image data by quantizing the result by the quantizer 23 (25 is a quantization table) and performing variable length encoding by the variable length encoder 24, , The coefficient after DCT is quantized by using a single quantization table in the same image (in some cases, different quantization tables are used for the luminance component and the color difference component).

[0003]

However, when encoding is performed using a single quantization table, the normally used quantization table has the character of reducing high frequency components due to human visual characteristics. As a result, the encoded and decoded image is conspicuously deteriorated at a rapid image change point (a character, an edge of a building, etc.) in which a large amount of high-frequency components occur.

On the other hand, in order to make these deteriorations inconspicuous,
If a quantization table having a property of storing more high-frequency components is set, the amount of code generated will increase, and effective image coding may not be performed.

The present invention has been made in consideration of the above points, and provides a high-quality image coding method capable of coping with a sudden image change point. It is what

[0006]

In order to solve the above problems, the present invention compresses image data by subjecting image data to discrete cosine transform, quantizing the result of the transform, and performing variable length coding. In the image encoding method for encoding, one of a plurality of quantization tables is selected according to the coefficient after the discrete cosine transform in the quantization.

[0007]

According to the present invention, the DCT coefficient after DCT is compared with the reference DCT coefficient, and as a result of the comparison, the DCT coefficient determined to have many high frequency components is quantized in the high frequency quantization table. By selecting and using, it is possible to perform encoding in which deterioration is not noticeable.

Further, a plurality of sets of comparators, comparison DCT coefficients, and high frequency quantization tables are provided, and different DCT coefficients and high frequency quantization tables are provided with different patterns, so that DCT coefficients to be output are output. By finely switching the high frequency quantization table in accordance with the characteristics of, the compression rate by encoding can be increased as compared with the case of using a single comparator.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is applied to an image coding apparatus will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of this embodiment.

In FIG. 1, the input image data is n
The pixel is divided into n × n (normally 8 × 8) pixel blocks by the × n block former 1. The following work is performed in this block unit. Next, a discrete cosine transform (DCT) is performed on this block by a discrete cosine transformer 2, and a DC
The T coefficient is determined. This is the DC on the upper left side of n × n.
The component is a (direct current) component, and the other components are AC (alternating current) components, and have higher-frequency image information as it goes leftward (x direction) or downward (y direction). The DCT coefficient of an image such as a character has a large coefficient in the high frequency range. Therefore, the DCT coefficients are calculated by m comparators C _{1 to} C _m , and m comparison DCT coefficients S _{TH1 to} S _TH.
If there is a DCT coefficient higher than a certain _value as compared with _THm , the comparator switches the quantization tables Q _{0 to} Q _m connected to the quantizer 4 via the switching unit 3.

For example, the normal quantizer 4 quantizes by the conventional quantization table Q ₀ . Now, an image such as a character is input, and its DCT coefficient is the comparison DCT coefficient S.
_Suppose that it is larger than S _TH1 of _{TH1 to} S _THm . Then, the output of the comparator C ₁ is turned on, whereby the switching unit 3 selects the high frequency quantization table Q ₁ and supplies it to the quantizer 4. Similarly, when C ₂ and C ₃ are ON, Q ₂ and Q ₃ are used, respectively. Where S
_{TH1 to} S _THm are quantization tables that leave higher frequencies as they get closer to m. Further, the switching unit 3 is designed to use a quantizer closer to m when a plurality of comparators are turned on. Also,
In order to transmit the number of the quantization table used to the decoding side,
The number of the table is transmitted to the variable length coding unit 5, and the code corresponding to the number is generated in the code.

[0013]

As described above, according to the present invention, the DCT
For images that contain a lot of high frequency components in the coefficients, by using a quantization table that emphasizes high frequency components, it is possible to reduce the cut of high frequency components when performing quantization. As a result, deterioration of the decoded image can be reduced and high-quality image transmission / storage can be performed.

[Brief description of drawings]

FIG. 1 is a block diagram of a high-quality image encoding device to which the present invention has been applied.

FIG. 2 is a block diagram of a conventional device.

[Explanation of symbols]

 1 n × n blocker 2 discrete cosine transformer 3 switching unit 4 quantizer 5 variable length encoder

Claims (4)

[Claims] 1. An image coding method for performing compression / encoding of image data by subjecting image data to discrete cosine transform, quantizing the result of the transform, and performing variable length coding, wherein the discrete cosine is used for the quantization. A high-definition image coding method, characterized in that one of a plurality of quantization tables is selected in accordance with a coefficient after conversion. 2. The high quality image coding method according to claim 1, wherein the plurality of quantization tables are selected by using a plurality of comparison discrete cosine transform coefficients. 3. The high quality image code according to claim 2, wherein a high quality image is realized for an arbitrary image by rewriting the contents of the comparison discrete cosine transform coefficient and the quantization table. Method. 4. The high-quality image coding method according to claim 1, wherein the number of the selected quantization table is included in the coded data.