Variable length coding method and variable length decoding method

Abstract

According to the present invention, an image coding apparatus (103) that encodes quantized coefficients corresponding to an image signal as target data to be processed is provided with a run-length encoding unit (RLE2) that assigns variable length codes to the quantized coefficients using code tables. The run-length encoding unit (RLE2) forms a second code table by optimizing a first code table to the target data to be processed, and selects one of the first and the second code tables as a code table that is to be employed for the assignment of the variable length codes in accordance with a quantization parameter (QP) or a variable length coding selection signal (VlcSel), whereby redundancy of information included in the target data to be processed can be effectively eliminated, and the compression ratio for the image signal or the like can be further increased.

Description

TECHNICAL FIELD [0001] The present invention relates to a variable length coding method and a variable length decoding method and, more particularly, to a method for converting plural coefficients which are obtained by quantizing frequency components of image data (quantization coefficients) into coded data according to a variable length coding process, and a method for decoding the coded data according to a variable length decoding process to reconstitute plural coefficients. BACKGROUND ART [0002] In recent years, we have entered the age of multimedia in which audio, video, and other data are integrally handled, and conventional information media (i.e., means for transmitting information to persons), such as newspapers, magazines, televisions, radios, and telephones have been adopted as subjects of multimedia. Generally, “multimedia” means to represent not only characters but also diagrams, speeches and especially images simultaneously in relation with each other. In order to handl...

AI Technical Summary

Benefits of technology

[0170] According to the present invention, in the above-mentioned variable length coding method, according to the coding process for the coefficients, the transformation of run-level pairs into codes is carried out for each block that is composed of a predetermined number of coefficients, and in the code table processing step, the second code table is formed on the basis of a sum of the number of already-processed coefficients which have been subjected to the coding process in an object block that is an object of the coding process, and the number of uncoded non-zero coefficients which have not yet been subjected to the coding process in the object block. Therefore, a code table that does not include pairs of run value and level value which will never occur can be used as the second code table, thereby further increasing the variable length coding efficiency.

[0171] According to the present invention, there is provided a variable length coding apparatus that encodes coefficient data that is composed of plural coefficients including: a coding unit for subjecting the respective coefficients to a coding process of transforming the coefficient data into coded data which is composed of plural codes, by using plural code tables indicating correspondences between numerical information representing the values of the coefficients and codes, and this coding unit includes: a code table selection unit for selecting one of the code tables in accordance with at least one of information about processed coefficients which have been subjected to the coding process, and a parameter relating to generation of the coefficients; and a code assignment unit for assigning a code to an uncoded coefficient which has not yet been subjected to the coding process, using the selected code table. Therefore, redundant information included in the coefficient data which are to be subjected to the variable length coding process can be effectively eliminated by the selection of a code table according to characteristics of coefficients that constitute the coefficient data or states of the coding process for the coefficients, thereby greatly increasing the coding efficiency in the variable length coding process for image signals or the like.

[0172] According to the present invention, in the above-mentioned variable length coding apparatus, the coefficients are obtained by quantizing frequency components of image data in accordance with a quantization step corresponding to the image data, and the code table selection unit selects the code table that is to be used by the code assignment unit, according to a value of the quantization step. Therefore, a code table that is adapted to the value of the quantization step and maximizes the coding efficiency can be always used.

[0173] According to the present invention, in the above-mentioned variable length coding apparatus, the code table selection unit selects the code table that is to be used by the code assignment unit, in accordance with the information about the processed coefficients which have been subjected to the coding process. Therefore, the coding process for quantized coefficients can be carried out always using a code table is adapted to the number of not-yet-processed coefficients and maximizes the coding efficiency.

Problems solved by technology

So, as for the information media described above, it is not practical to handle such massive data as it is in the digital format.

For example, picturephones have already been put to practical use by ISDN (Integrated Services Digital Network) having a transmission rate of 64 kbps˜1.5 Mbps, but it is impossible to transmit an output image of a television camera having a large quantity of data as it is by the ISDN.

In the variable length coding process by the coding unit, redundant information included in the quantized coefficient (data to be processed) cannot be removed sufficiently and, thus, the compression rate is susceptible to improvement.

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