JP3014200B2 - Image data encoding apparatus and encoding method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a code for image data for compressing data by dividing a multi-valued image into blocks consisting of a plurality of pixels, and performing two-dimensional orthogonal transformation on the pixel data in the blocks and coding. The present invention relates to an encoding device and an encoding method.

[0002] Since image data has a large amount of information, it is necessary to efficiently encode and compress the data in order to store it in a storage device or to transfer it to another device at high speed. As a method of efficiently compressing image data, for example, there is an adaptive discrete cosine transform coding method.

An adaptive discrete cosine transform coding scheme (Ada
ptive Discrete Cosine Tra
nsform) divides an image into blocks of 8 × 8 pixels, and converts the pixel level of each block into a two-dimensional discrete cosine transform (Discrete Cosine Tra).
nsform (hereinafter abbreviated as DCT), transforms them into coefficients of a spatial frequency distribution, quantizes them with a threshold adapted to vision, and encodes the obtained quantized coefficients using a statistically obtained Huffman table.

[0004]

2. Description of the Related Art FIG. 9 shows a conventional encoding apparatus. FIG. 1 shows an apparatus configuration of a conventional two-dimensional discrete cosine transform method.

In FIG. 1, reference numeral 50 denotes a DCT transform unit, which performs two-dimensional discrete cosine transform on multi-valued image data,
It outputs CT coefficients. Reference numeral 51 denotes a quantization unit which compares the DCT coefficient with a quantization threshold and performs quantization. Reference numeral 52 denotes a quantization threshold table for storing the quantization threshold. Numeral 53 denotes a variable length coding unit for performing variable length coding on the quantized coefficients with reference to a Huffman code table (Huffman table). Reference numeral 54 denotes a Huffman code table (Huffman table).

The operation of the configuration shown in FIG. 9 will be described with reference to FIG. FIG. 10 is an explanatory diagram of a DCT coefficient encoding method. Figure (a) shows an example of the quantization coefficient.

FIG. 1 shows a coefficient obtained by quantizing a DCT coefficient obtained by DCT transforming image data of 8 × 8 pixels per block by comparing it with a quantization threshold. In the figure, 60
Represents a DC component in the spatial frequency of one block of image data, and represents a horizontal frequency component (AC component) toward the right and a vertical frequency component (AC component) below.

FIG. 2B shows the scanning order of the quantization coefficients.
The figure shows the order of scanning one block of pixels to encode the quantized coefficients. The data to be encoded starts from the pixel of the DC component, scans zigzag as shown in the figure, and sequentially encodes the quantization coefficients of the higher-order horizontal and vertical AC components.

FIG. 1C shows an example of data to be encoded. The figure shows encoding target data obtained as a result of zigzag scanning of the quantization coefficient in (a). As shown, starting from the DC component “5”, the AC components are “−2”, “−3”, “1”,.
The encoding is sequentially performed toward the higher-order AC components in the order of. The arrangement of “000” is run-length data, and since the ninth data (−1) and subsequent 0s are arranged up to the last pixel of the scanning of one block, the end code “EOB” of one block is used.
To delimit one block of data.

Referring to the Huffman table, the data to be encoded shown in the figure is sequentially encoded to obtain compressed data. FIG. 11 is a diagram showing an arrangement of quantization coefficients and blocks of one screen.

In FIG. 1, reference numeral 70 denotes an example of a quantization coefficient, which is an example in which all AC components are 0 (hereinafter, a block in which all AC components are 0 is referred to as a DC block). 71
Is an example of the arrangement of blocks on one screen, showing a case where there are five blocks in the horizontal direction and a plurality of blocks in the vertical direction. It is assumed that blocks 1 to 4 and blocks 7 to 10 are DC blocks, and blocks 5 and 6 have non-zero quantization coefficients in the AC component (hereinafter, non-zero quantization in the AC component). A block having coefficients is called an AC block.)

In block 1, reference numeral 72 denotes the DC component of block 1. In block 5, 73 is a two-dimensional AC component of block 5 in the horizontal direction, and 74 is A of block 5.
Represents the highest harmonic component of the C component.

FIG. 12 shows a conventional encoding method. The figure shows a conventional encoding method when encoding the quantized coefficients in the example of the block arrangement of one screen in FIG.

As shown in the figure, encoding is performed sequentially from the data of block 1 to the data of the last block. Then, each block starts from the DC component, and A
The C component is sequentially encoded. In each block, up to the last data (the last non-zero data in the case of zigzag scanning of each block, in which 0s are arranged up to the last pixel thereafter) are to be encoded, and the subsequent blocks Are not coded up to the last pixel (0 is lined up),
By adding "OB", the block is delimited.
Then, the continuous data of 0 in the data portion to be encoded is run-lengthized. If the last pixel of the block is not 0, the data of all pixels is set as the encoding target, and E
No OB code is added.

In the case of the figure, block 1 has a DC component of 5 and A
Since all C components are 0, 5 is arranged at the beginning, and then EO
B is added to the data to be encoded as one block. Similarly, in blocks 2 to 4, only the DC component is non-zero and the AC components are all zero.
EOB is added following the DC component, and is used as encoding target data of each block.

Block 5 includes a DC component "5" followed by A
Since there is the C component "2", the AC component 2 is arranged following the head 5, and the AC components are sequentially encoded by zigzag scanning. Since 0 follows the last non-zero data "1", the last non-zero data "1" is used as the encoding target data, and EOB is added as a block delimiter. The block 6 has a DC component and then an AC component 4, and the last quantization coefficient of the block is "1". Therefore, the AC component is sequentially encoded from "2" and up to the last "1". Since the block 6 can know the delimiter of the block even without the block end code EOB, the head data of the next block following the data of the block 6 is added as the encoding target data without adding the EOB.

Similarly, in blocks 7 to 10, only the DC component is non-zero and the AC components are all zero, so that EOB is sequentially encoded as the data to be encoded following the DC component.

[0018]

In a conventional encoding method, for example, in a portion where there is no image change such as a background portion of image data of animation, only a DC component is non-zero.
, Blocks having all zero AC components continue. And
Following the encoding of the DC component, the procedure of encoding "EOB" appears repeatedly, and each time "EOB" must be variable-length encoded, which is inefficient.

An object of the present invention is to provide an encoding device and an encoding method for image data which can be efficiently encoded when a block in which all AC components are 0 continues.

[0020]

According to the present invention, an evaluation index is determined based on the number of blocks in which all AC components are 0 or a continuous number thereof among blocks of one screen, and the number of blocks or the evaluation index is compared with a threshold value. If the threshold value is larger than the threshold value, the coding procedure is divided into a procedure for continuously coding only the DC component of each block for one screen and a procedure for continuously coding only the AC component of each block. (Hereinafter, DC component and A
A method of dividing and encoding the C component is referred to as division encoding.)

Prior to the basic configuration of the present invention shown in FIG.
Will describe the encoding method of the present invention. FIG.
An example (71) of the block arrangement of one screen of FIG. 1 shows a division encoding method in the encoding method of the present invention.

In the figure, reference numeral 10 denotes a DC component, which indicates that only the DC component of each block is taken out as data to be encoded in block order and is sequentially encoded. 11
Is an AC component, and indicates that only the AC component of each block is continuously scanned in a zigzag manner for each block in block order and sequentially encoded. When the AC component is used as the encoding target data, a block in which all the AC components are 0 is E
An OB code is attached, and a continuous EOB is run-lengthized.

FIG. 1 shows the basic configuration of the present invention. In the figure, reference numeral 1 denotes a quantization coefficient creation unit which performs DCT conversion on image data for each block, quantizes the image data by comparing it with a quantization threshold, and creates a quantization coefficient for each block. 2
Is an encoding means for encoding the quantized coefficient created by the quantized coefficient creating section. Reference numeral 3 denotes a DC block detection unit that detects a DC block. 4
Is an encoding procedure determination unit that calculates an evaluation index based on the number of DC blocks or the number of consecutive DC blocks for one screen and compares it with a threshold value to perform division encoding or block encoding as in the past. For each time, it is determined whether the DC component and the AC component are continuously encoded. Reference numeral 5 denotes a variable-length coding unit that performs variable-length coding on the quantization coefficient with reference to the Huffman code table in accordance with the coding procedure determined by the coding procedure determination unit 4.

[0024]

The operation of the basic configuration of the present invention will be described with reference to FIG. FIG. 3 shows a processing flow of the basic configuration of the present invention.

The description will be made according to the numbers shown. Please refer to FIG. (1) The quantization coefficient creation unit 1 quantizes a DCT coefficient created by performing DCT on image data for each block with reference to a quantization threshold, and creates a quantization coefficient for each block.

(2) The DC block detector 3 receives the quantized coefficient and determines whether the AC component has non-zero. (3) The coding procedure determination unit 4 determines whether there are many blocks in which the non-zero coefficient is composed of only DC (calculates an evaluation index based on the number of DC blocks or the number of continuous DC blocks, and compares it with a threshold. Do). If the number of blocks or the evaluation index is larger than the threshold value, go to (4); if smaller, go to (8)
Proceed to.

(4), (5) The variable-length coding unit 5 receives the quantized coefficient and performs variable-length coding on the DC component until coding of the quantized coefficients of all blocks of the image is completed. (6), (7) The variable length coding unit 5 receives the quantization coefficient and sequentially codes only the AC component for all blocks.

(8), (9) The variable-length coding unit 5 continuously performs variable-length coding of the DC component and the AC component for each block for all blocks (hereinafter, referred to as non-division coding).

[0029]

FIG. 4 shows an embodiment of the apparatus configuration of the present invention. In the figure, 15 is a DCT transform unit, 16 is a quantizer, 17
Is a quantization threshold table, which stores the quantization threshold. Reference numeral 18 denotes an output direction selection unit which inputs a quantization coefficient from the quantization unit 16 and switches the output direction to the DC block detection unit 19 or the variable length coding unit 21. 19 is a DC block detection unit,
It is to determine whether there is non-zero in the AC component and to detect the DC block. Then, the DC block detection unit 1
9 means for comparing the AC component of the quantization coefficient with zero,
Counting means for counting the number of determined blocks and whether the determined block is a DC block or
An output unit for outputting an identification signal indicating whether the block is a C block is provided (not shown). Reference numeral 20 denotes an encoding procedure determination unit that calculates an evaluation index based on the number of DC blocks detected by the DC block detection unit 19 or the number of continuous blocks, compares the evaluation index with a threshold value, and divides and encodes DC components and AC components. Or non-divided coding. Reference numeral 21 denotes a variable-length coding unit that receives a quantization coefficient and performs variable-length coding on the quantization coefficient according to the coding method determined by the coding procedure determination unit 20. Reference numeral 22 denotes a Huffman code table, and reference numeral 23 denotes a timing control unit which controls the start and end of the operation of each unit.

FIG. 5 shows a processing flow of the timing control section in the device configuration of the present invention. The operation of the timing control unit will be described according to the processing flow in the figure (see FIG. 4).

(1) The timing control unit 23 instructs the output direction selection unit 18 to input the quantification coefficient output from the quantization unit 16 to the DC block side. (2) The timing control unit 23 performs DCT for each block.
The change unit 15 is instructed to perform DCT conversion, and the quantization unit 16 is instructed to perform quantization.

(3) It is determined whether or not the quantization coefficients of the block for one screen have been obtained. If the quantization coefficient for one screen has been found, proceed to (4). If the quantization coefficient for one screen has not been found, DCT transformation and quantization of the next block are instructed (for the next block, see (2) Process).

(4) The timing control unit 23 sets the output direction selection unit 18 (DMPX) so that the output of the quantization unit 16 is input to the variable length coding unit 21. (5) The variable length coding unit 21 performs the number of times specified by the coding procedure determination unit 20 (two times for divided coding and one time for non-divided coding). The DCT transform and quantization unit 16 for one screen are instructed to quantize the DCT transform, and the variable length encoding unit 21 is instructed to perform variable length encoding.

As a result, in the case of division coding, DCT
The transformation unit 15 performs DCT transformation for one screen twice, and the quantization unit 16 compares the DCT coefficient of each block output from the DCT transformation unit 15 with a threshold value of the quantization threshold value table 17 to obtain a quantization coefficient. Create Then, the output direction selection unit 18 (DMPX) inputs the quantized coefficients created by the quantization unit 16 to the variable length encoding unit 21. Variable length coding unit 21
Performs variable length coding of only the DC component of each block for the first input quantized coefficient of one screen. Further, with respect to the quantization coefficient for one screen in the next round, only the AC component of each block is subjected to variable length coding.

In the case of non-division encoding, the DCT transform unit 1
In step 5, the image data of one screen is subjected to DCT conversion only once, and is input to the quantization unit 16. The quantization unit 16 uses the DCT
A quantization coefficient for one screen is created based on the DCT coefficient sent from the conversion unit 15. Further, the quantization coefficients for one screen are transferred to the variable length coding unit 21. Then, the variable length coding unit 21 non-divides and codes the quantization coefficients for one screen for each block.

FIG. 6 is a processing flow of the DC block detecting section of the present invention. In the figure, 21 'is a comparison means, 22'
Is a counting means, and 23 'is an output means. The processing flow will be described according to the illustrated numbers.

(1) According to the instruction of the timing control unit,
The quantization coefficient of the AC component is read. (2) The comparing means 21 'compares the value of the quantization coefficient with 0 and determines whether the AC component is 0. If 0
Go to (3), if not 0, go to (5).

(3) The counting means 22 'counts the determined number of pixels. When the count reaches 64, one block has been completed, and the process proceeds to (4). If the count is less than 64, the process returns to (2) and the next pixel is determined.

(4) The output means 23 'is a block of A
Since all C components are 0, the DC block detection signal D
C is output as "1". Then, a signal indicating the end of the processing for one block is output to the timing control unit.

(5) The output means 23 'outputs the block
Since the block is a block (a block having a non-zero AC component), the DC block detection signal DC is output as “0”. Then, a signal indicating the end of the processing for one block is output to the timing control unit.

FIG. 7 is a diagram showing an embodiment of the coding procedure determining section of the present invention. (a) is an embodiment of the encoding procedure determination unit (1)
FIG. In the figure, reference numeral 31 denotes a DC block number counting unit which counts DC block detection signals (1 for DC blocks and 0 for AC blocks) detected by the DC block detection unit. Reference numeral 32 denotes a threshold value holding unit, which holds a threshold value for determining whether to use divided coding or non-divided coding. A comparison unit 33 compares the number of DC blocks with a threshold. Reference numeral 34 denotes a coding division number determination unit which performs DC component coding and AC coding when performing division coding.
Outputs the number of encoding divisions 2 for instructing two divided encodings for encoding the components, and in the case of non-division encoding, instructs the DC component and the AC component to be encoded once continuously. It outputs the number of encoding divisions of one.

FIG. 4B shows an embodiment (2) of the coding procedure determining unit.
It is. In the figure, reference numeral 40 denotes a DC block continuous number counting unit which counts the number of continuous DC blocks. Reference numeral 41 denotes an address generation unit, which is a weight coefficient holding unit 4
An address for designating a weighting factor of 2 is generated. Reference numeral 42 denotes a weight coefficient holding unit that holds a weight coefficient applied to the number of DC blocks according to the number of continuous DC blocks. Reference numeral 43 denotes a data DC degree calculation unit that calculates an evaluation index (DC degree) for divided coding by applying a weighting factor to the number of continuous DC blocks for each continuous DC block. Reference numeral 44 denotes a threshold storage unit, and reference numeral 45 denotes a comparison unit that compares the evaluation index calculated by the DC degree calculation unit 43 with the threshold. 46 is a coding division number determination unit.

In the configuration shown in the figure, the DC block continuation number counting section 40 counts the DC block detection signal to count the continuation number of DC blocks. Then, the address generation unit 4 specifies the address of the weight coefficient holding unit 42 storing the weight count determined based on the continuous number.
Instruct 1 to generate an address. The address generation unit 41 specifies an address specified to the weight coefficient holding unit 42. The weight coefficient holding unit 42 outputs the weight coefficient of the designated address to the DC degree calculation unit 43.

The DC degree calculating section 43 multiplies the continuous number counted by the DC block continuous number counting section 40 by a weighting coefficient. Then, for one screen, the result of weighting the continuous number is cumulatively added to calculate an evaluation index (DC degree). Then, the comparing unit 45 compares the threshold value held by the threshold value holding unit 44 with D
The evaluation indices calculated by the C degree calculating section 43 are compared, and if the evaluation index is larger than the threshold value, the coding division number determining section 46 outputs the coding division number 2 to instruct the division coding, and the evaluation index is equal to the threshold value. If it is smaller, the coding division number determination unit 46 outputs the coding division number 1 and instructs non-division coding.

The evaluation index is calculated by multiplying a predetermined integer N by a negative coefficient when the number of consecutive DC blocks is shorter than N. Multiply by 0 or a positive coefficient. When the number of continuous DC blocks is smaller than N, the negative coefficient is constant regardless of the length of the continuous number, or the absolute value of the negative coefficient is increased as the continuous number is shorter. Alternatively, when the number of continuous DC blocks is larger than N, the positive coefficient may be constant regardless of the length of the number of continuous blocks, or the absolute value of the positive coefficient may be increased as the number of continuous blocks becomes longer. .

FIG. 8 is a flowchart showing the processing flow of the variable length coding unit according to the present invention. In the figure, reference numeral 47 denotes an encoding method instructing means in the variable length encoding unit, which judges whether the number of encoding divisions is two or one. 48 is an encoding means, which in accordance with the instruction of the encoding method instructing means 47,
This is to perform division coding or non-division coding.

The processing flow will be described according to the numbers shown. (1) The process is started according to the instruction of the timing control unit, and the encoding method instruction unit 47 determines whether there is an instruction of division encoding from the encoding procedure determination unit. If the number of coding divisions is two, it is an instruction of division coding, and the process proceeds to (2).
If the coding division number is 1, it is non-division coding, so
Proceed to (4).

(2) Since the instruction of the encoding method instruction means is divisional encoding, the encoding means 48 selects only the DC component of each block from the quantized coefficients input from the quantization unit and changes the value. Perform long encoding.

(3) Only the AC component of each block is selected from the quantized coefficients input from the quantizing unit and subjected to variable length coding. When the encoding for one screen is completed, a signal indicating this is output to the timing control unit, and the processing is terminated.

(4) Since the encoding instruction of the encoding method instructing means 47 is non-division encoding, the encoding means 48 sequentially and continuously outputs the quantized coefficients input from the quantizing section to a variable length code. Become When the encoding for one screen is completed, a signal indicating this is output to the timing control unit, and the processing is terminated.

[0051]

According to the present invention, two-dimensional orthogonally transformed quantized coefficients, such as a background portion of a character image or an animated image, have a continuous block composed of only AC components and only DC components. Image data that frequently appears in the image data can be efficiently encoded.

[Brief description of the drawings]

FIG. 1 is a diagram showing a basic configuration of the present invention.

FIG. 2 is an explanatory diagram of means for solving the problem of the present invention.

FIG. 3 is a diagram showing a processing flow of a basic configuration of the present invention.

FIG. 4 is a diagram showing an embodiment of the device configuration of the present invention.

FIG. 5 is a diagram showing a processing flow of a timing control unit of the present invention.

FIG. 6 is a diagram showing a processing flow of a DC block detection unit of the present invention.

FIG. 7 is a diagram illustrating an embodiment of an encoding procedure determination unit according to the present invention.

FIG. 8 is a diagram showing a processing flow of a variable length encoding unit of the present invention.

FIG. 9 is a diagram showing a conventional encoding device (two-dimensional discrete cosine transform method).

FIG. 10 is an explanatory diagram of a conventional quantization coefficient encoding method.

FIG. 11 is a diagram showing an arrangement of quantization coefficients and blocks of one screen.

FIG. 12 is a diagram illustrating a conventional encoding method.

[Explanation of symbols]

 1: Quantized coefficient creation unit 2: Encoding means 3: DC block detection unit 4: Encoding procedure determination unit 5: Variable length encoding unit

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) H04N ⁷ /24-7/68 H04N 1/41 G06T 9/00 H04N 1/41

Claims (6)

(57) [Claims] 1. Multi-valued image data is input, divided into blocks each including a plurality of pixels, and two-dimensional orthogonal transformation of pixel gradation values is performed for each of the divided blocks, and the obtained coefficients are quantized. A quantized coefficient creating unit (1) and a DC block detecting unit (3) that receives the quantized coefficient and detects a DC block in which all of the AC component quantized coefficients are zero
And calculating an evaluation index based on the number of DC blocks or the number of continuous DC blocks in one screen, and performing variable length coding of only DC components and variable length coding of only AC components of all blocks in one screen. An encoding procedure determination unit (4) for determining whether to perform divisional encoding in which divisional encoding is performed or non-divisional encoding in which DC and AC components are continuously variable-length encoded for each block; A variable-length coding unit (5) for performing variable-length coding of the quantization coefficient according to the division method determined by the coding procedure determination unit (4). 2. Multi-valued image data is input, divided into blocks each including a plurality of pixels, and two-dimensional orthogonal transformation of pixel gradation values is performed for each of the divided blocks, and the obtained coefficients are quantized. In a method of encoding image data comprising a quantization coefficient creation unit (1) and encoding means (2) for encoding the quantization coefficient, the encoding means (2) inputs the quantization coefficient, Detecting a DC block in which the AC component of the quantization coefficient is all zero, detecting the number of the DC blocks in one screen, obtaining an evaluation index based on the number of the DC blocks or the continuous number of the blocks, Alternatively, the evaluation index is compared with a threshold value. If the evaluation index is larger than the threshold value, coding is performed by dividing into coding of only DC components and coding of only AC components for all blocks of one screen, and the number of blocks or the evaluation index is larger than the threshold value If it is smaller, Method of encoding image data, characterized by continuously encoded DC and AC components for each. 3. The evaluation index according to claim 1, wherein the weight is zero or a large positive weight applied to the continuous number when the continuous number of blocks having all zero AC components is larger than a predetermined integer N. An image data encoding apparatus characterized in that when the number of consecutive blocks is smaller than N, the number of consecutive blocks is determined by acting on a negative or small number of consecutive weights. 4. The method according to claim 3, wherein the weight is reduced as the number of continuations of the block is shorter, or the absolute value is increased in the case of a negative weight, and the weight is increased as the number of continuations is longer. Image data encoding device. 5. The evaluation index according to claim 2, wherein, when the number of consecutive blocks of all zero AC components is larger than a predetermined integer N, the weight is applied to the consecutive number as zero or a large positive weight. Wherein the number of consecutive blocks is smaller than N, the negative or small weight is applied to the number of consecutive blocks to obtain the image data. 6. The method according to claim 5, wherein the weight is reduced as the number of consecutive blocks is shorter, or the absolute value is increased in the case of a negative weight, and the weight is increased as the number of consecutive blocks is longer. Image data encoding method.