JPH10336656A - Image encoding device and method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect block noise made to appear by orthogonal transformation encoding by using information obtained in the process of the transformation encoding. SOLUTION: A group number detection part 511 detects the case that the difference of DC components with a block immediately before becomes '+1' or '-1', that is the case that a group number outputted by a grouping part 504 becomes 1, and outputs the detection signal of one bit to an AND 513. A run length detection part 512 detects the case that a run length outputted by a run length counter 508 is 63 and outputs the detection signal of one bit for indicating detection to the AND 513. The AND 513 outputs the AND of the detection signals of the group number detection part 511 and the run length detection part 512 to an output terminal 601 as block noise detection information.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image encoding apparatus and an image encoding method, and more particularly to an apparatus and method for detecting block noise generated in an encoding process before decoding.

[0002]

2. Description of the Related Art An orthogonal transform coding method is widely used as a method for coding a continuous tone image with high efficiency. In particular, a discrete cosine transform (Discrett) is used for the orthogonal transform.
An algorithm using e Cosine Transform (hereinafter, referred to as DCT) is based on JPEG (Joint Photograph), which is an international standard for still image coding.
hic ExpertGroup).

FIG. 16 is a block diagram for explaining the configuration of a basic JPEG algorithm called a JPEG baseline system.

In FIG. 1, reference numeral 100 denotes an input terminal to which an image is input; 200, a block dividing unit which divides the input image into pixel blocks each of which is a rectangular area of 8 × 8 pixels;
0 is a DCT transform unit that performs DCT on a pixel block basis and outputs a transform coefficient, and 400 is a quantizer that linearly quantizes the transform coefficient using a quantization step width set for each frequency and outputs a quantized coefficient. The encoding unit 500 is a variable-length encoding unit that performs variable-length encoding of a quantization coefficient in a predetermined procedure and outputs code data, and 600 is an output terminal that outputs code data.

FIG. 17 is a block diagram illustrating the configuration of the variable length coding unit 500. In FIG. 17, reference numeral 101 denotes an input terminal to which a quantization coefficient is input; 501, the quantization coefficients are rearranged in the order shown in FIG. 10 (zigzag scan);
A scan conversion unit that individually outputs a DC (direct current) component and an AC (alternating current) component; 502, a block delay unit that delays the DC component for the processing time of one pixel block; 503, a block delay unit that is one block before the DC component A subtractor 504 that subtracts the DC component and outputs a difference between the DC components. The subtractor 504 divides the difference between the input DC components into groups based on absolute values as shown in FIG. A grouping unit 505 for outputting an additional bit indicating whether it is a numerical value
Is a one-dimensional Huffman coding unit that uses a one-dimensional Huffman code table as shown in FIG. 8A to perform variable-length coding on the DC component group number SSSS and outputs a DC Huffman code.

On the other hand, reference numeral 507 denotes a zero determining unit for determining whether the input AC component is zero or non-zero and switching the output destination of the AC component, and 508 calculates a continuous length of the zero coefficient. A run-length counter 509 that outputs a run-length NNNN determines the magnitude of the non-zero AC coefficient in a group according to the table of FIG. 7 similarly to the case of the DC component, and determines whether the group number SSSS and the AC coefficient are within the group. A grouping unit 510 that outputs an additional bit indicating whether it is a numerical value. The two-dimensional grouping unit 510 encodes a combination of the run length NNNN and the group number SSSS using the two-dimensional Huffman code table of FIG. 8B and outputs an AC Huffman code. A Huffman coding unit 506 multiplexes the DC Huffman code, the AC Huffman code, and the additional bits of the DC component and the AC component to generate code data. Multiplexing unit which, 600 is an output terminal of the code data is output.

The operation of the JPEG baseline system will be described below with reference to FIG. In FIG. 16, the image input from the input terminal 100 is
00 is divided into pixel blocks, which are rectangular areas of 8 × 8 pixels. FIG. 5A shows an example of a pixel block. The pixel block is converted by the DCT converter 300 into a DC
T is applied to obtain a transform coefficient as an 8 × 8 matrix. The transform coefficient is information corresponding to the power of the two-dimensional frequency component included in the pixel block.

FIG. 4 is a diagram showing a correspondence between each element of a transform coefficient and a frequency. The element at the upper left in the matrix is called a DC component and corresponds to the average value in the pixel block. The other components are called AC components, with the right one corresponding to higher frequencies in the horizontal direction and the lower one corresponding to higher frequencies in the vertical direction. FIG. 5B is a transform matrix obtained by performing DCT on the pixel block of FIG.

Subsequently, in the quantization section 400, the transform coefficients are linearly quantized with quantization characteristics determined for each element.

FIG. 13 is a diagram for explaining linear quantization, and shows that an input is quantized by a quantization width Q and discrete outputs arranged at intervals Q are obtained. The quantization characteristics in JPEG are given, for example, as a matrix shown in FIG. 5C, and the values are quantized by dividing and transforming the transform coefficient at the corresponding position by these values. FIG.
This is a quantized coefficient obtained by quantizing the transform coefficient of FIG. 5B with the quantization matrix of FIG. 5C.

[0011] In the variable length coding section 500, the quantized coefficients shown in FIG. 5D are variable length coded, and code data is output to an output terminal 600.

The operation of the variable length coding unit 500 will be described below with reference to FIG.

The scan conversion unit 501 converts the input AC coefficients of the quantized coefficients into one-dimensional data in the order shown in FIG. The DC component is output without any processing.

The block delay unit 502 stores a DC component during a period in which one pixel block is encoded.

The subtractor 503 calculates the difference between the DC component of the current pixel block and the previous DC component stored in the block delay unit 502.

In the grouping unit 504, as shown in FIG.
The difference of the C component is divided into groups according to the magnitude of the absolute value, and an additional bit indicating the numerical value of the group number and the difference in the group is output.

The one-dimensional Huffman encoding unit 505 has the structure shown in FIG.
The DC Huffman code corresponding to the group number is output according to the one-dimensional Huffman code table of FIG. The DC Huffman code is input to the multiplexing unit 506 together with the additional bits.

On the other hand, the output destination of the AC component output from scan conversion section 501 is switched by zero determination section 507 depending on whether it is zero or non-zero.

For the AC component determined to be zero, the run length of the zero AC component is counted in the run length counter 508 until a non-zero AC component appears, and the run length is output.

The grouping unit 509 determines a group of non-zero AC components as shown in FIG.
Additional bits indicating the group number and the numerical value of the AC component in the group are output.

As shown in FIG. 9, the two-dimensional Huffman encoding section 510 encodes a two-dimensional Huffman code table composed of a combination of a run length and a group number.
Each element of the table is assigned an AC Huffman code as shown in FIG. An AC Huffman code corresponding to the combination of the run length and the group number is output.
6 is output.

FIG. 11A shows a DC component Huffman code and additional bits of the quantized coefficient shown in FIG. 4D. Similarly, FIG. 11B shows an AC component Huffman code and additional bits, and FIG. 11C shows code data obtained from the multiplexing unit 506.

With the above procedure, the encoding of the image by the JPEG baseline system is completed.

In general, the transform coding method is irreversible coding in which the decoded image is not the same as the original image.
This is because information is lost in the process of quantization, and it is known that noise such as block noise is generated as the compression ratio is increased.

FIG. 12 is a diagram for explaining block noise. The occurrence of block noise can be explained as follows. In the transform coding method, since coding is performed for each pixel block, if all AC components are quantized to zero by coarse quantization, a block having only an average value is decoded. This is because the block boundaries become conspicuous because they are connected with the average value of the surrounding pixel blocks with a step.

The block noise is not so noticeable in a region where the amplitude change of the pixel is sharp, but in a region where the gradation change is gradual, the gradation changes on a rectangular step, which may cause a large deterioration of the image quality. Are known.

Various studies have been made on such block noise improvement techniques.

As an improvement method on the encoding side, for example, the method disclosed in Japanese Patent Application Laid-Open No. 4-200833 is provided with means for detecting a block having only a DC component as block noise.
If detected, a 1-bit flag is added, and this block is subjected to a smoothing process at the time of decoding.

In Japanese Unexamined Patent Publication No. Hei 5-328329, upon encoding, the quantization characteristic is switched in accordance with the flatness of the pixel block and the AC component.

In Japanese Patent Application Laid-Open No. 7-50758, when reading a single-color original, block distortion generated due to density fluctuation due to a reading error is detected, and a level difference from peripheral blocks is smoothed. Further, when the absolute values of the AC coefficients are all equal to or smaller than a predetermined value, the coding is performed with all the AC coefficients set to zero.

As an improvement method on the decoding side, Japanese Patent Laid-Open No.
In Japanese Patent No. 3997, the characteristics of a noise removal filter are determined by the distribution of transform coefficients for each block during decoding. When blocks of only the DC component continue,
The noise is removed only when the difference between the DC components of the blocks is equal to or smaller than the quantization step.

[0032]

SUMMARY OF THE INVENTION In the prior art,
Techniques using block noise detection flags and variable quantization can be realized only between specific terminals equipped with these functions, and image transmission using the JPEG baseline algorithm, such as unspecified color facsimile, etc. There is a problem that it cannot be applied to an application that assumes a large number of receiving terminals.

In the conventional technique using an image correction process at the time of transmission and an adaptive filter after decoding,
The purpose is to improve the image quality of the decoded image, and an image faithful to the original image is not always reproduced.

For this reason, depending on the original and the application, the image encoded on the transmitting side is temporarily decoded and the operator checks the image quality. If block noise occurs, the image is re-encoded with higher image quality parameters. Although the transmission is considered to be the most reliable method, it requires human labor.

It is considered that these problems originate from the fact that the encoding algorithm has no function of detecting the occurrence of block noise.

Accordingly, it is an object of the present invention to provide an image encoding apparatus and method having a function of detecting block noise in order to realize encoding without occurrence of block distortion.

In the prior art, the detection of block noise includes the following steps: (1) Detecting a pixel block in which only the DC component of the transform coefficient in the pixel block is quantized and only the average value is decoded. (2) The average value of the pixel block is adjacent to the average value of the peripheral pixel blocks with a step. In many cases, either or both of the two conditions are used. However, there is a problem that a new circuit for detecting these conditions is required and the amount of calculation increases.

The second object of the present invention is to detect block noise using information obtained in the process of transform coding represented by the JPEG baseline system.

[0039]

According to a first aspect of the present invention, there is provided an image encoding apparatus for dividing an image into N × M (N and M are positive integer) pixels. A dividing unit that obtains a pixel block that is a rectangular area, a transforming unit that performs orthogonal transform on the pixel block to obtain a transform coefficient, and a quantization that obtains a quantized coefficient by quantizing the transform coefficient with a predetermined quantization characteristic. Means for performing variable-length coding on a DC (direct current) component of the quantization coefficient by a difference between the DC component of the immediately preceding pixel block and an AC (alternating current) component of the quantization coefficient,
A two-dimensional variable-length coding unit that performs variable-length coding by a combination of a continuous length of an AC coefficient that becomes zero and a magnitude of a non-zero AC coefficient that follows the two-dimensional AC coefficient; First detection means for detecting a value and outputting a first detection signal, and second detection means for detecting a specific value of a continuous length of the AC coefficient and outputting a second detection signal. Determining means for outputting a logical product of the first detection signal and the second detection signal as a detection result for each block,
And performing re-encoding with the quantization characteristic changed based on the detection result for each block.

With this configuration, block noise can be detected based on information generated in the previous encoding process.

According to a second aspect of the present invention, there is provided the image encoding apparatus according to the first aspect, wherein the second detection signal in the immediately preceding pixel block is stored. Determining means for outputting a logical product of the first detection signal, the second detection signal, and the content stored in the storage means as a detection result for each block; Based on the result, execution of re-encoding with the quantization characteristic changed is determined.

According to a third aspect of the present invention, in the image encoding apparatus according to the first or second aspect, the specific value of the difference is +1 or -1. It is characterized by.

According to a fourth aspect of the present invention, there is provided the image coding apparatus according to the first or second aspect, wherein the specific length of the continuous length of the zero AC coefficient is specified. The value is (N × M−1).

According to a fifth aspect of the present invention, in the image coding apparatus of the first or second aspect, the determination means is an AND element.

According to a sixth aspect of the present invention, in the image coding apparatus according to the first or second aspect, the orthogonal transform is a discrete cosine transform. .

According to a seventh aspect of the present invention, there is provided the image encoding apparatus according to the first or second aspect, wherein the pixel block is composed of 8 × 8 pixels. Features.

According to an eighth aspect of the present invention, in the image encoding apparatus according to the first or second aspect, the second detecting means is configured so that the continuous length of the AC coefficient is equal to the length of the continuous AC coefficient. Instead of detecting a specific value, a specific code output from the two-dimensional variable length encoding means is detected and a second detection signal is output.

According to a ninth aspect of the present invention, there is provided an image encoding apparatus according to the first or second aspect, further comprising a counting means for counting the detection result for each block. When the image encoding operation is completed,
By comparing the count result of the counting means with the total number of pixels of the image, or the total number of pixel blocks included in the image, when the count result is equal to or greater than a predetermined ratio, the quantization characteristic is changed. It is characterized by performing re-encoding.

According to a tenth aspect of the present invention, there is provided the image encoding apparatus according to the first or second aspect, wherein the DC component of the predetermined quantization characteristic in the quantizing means is provided. When the quantization threshold value is smaller than a predetermined value, re-encoding in which the quantization characteristic is changed is not performed.

An image encoding apparatus according to claim 11 of the present invention is the image encoding apparatus according to claim 1 or 2, wherein the predetermined value is an output for outputting a decoded image. It is characterized in that it is determined according to the tone reproduction characteristics of the means.

In the image coding method according to the twelfth aspect of the present invention, the image is divided into N × M (N and M are positive integers).
Obtain a pixel block that is a rectangular area of pixels, perform orthogonal transformation on the pixel block to obtain a transform coefficient, quantize the transform coefficient with a predetermined quantization characteristic to obtain a quantized coefficient, and obtain a DC of the quantized coefficient. The (DC) component is variable-length coded according to the difference between the DC component of the immediately preceding pixel block and the AC (alternating current) component of the quantization coefficient is converted to the continuous length of the zero AC coefficient and the non- The variable length coding is performed by a combination of zero AC coefficient magnitudes. In particular, a specific value of the difference is detected, a first flag is set, and a specific length of a continuous length of the AC coefficient is determined. Detecting a value, setting a second flag, obtaining a detection result for each block from the first flag and the second flag, and changing the quantization characteristic based on the detection result for each block. Perform re-encoding The features.

Also in this configuration, block noise can be detected based on information generated in the previous encoding process.

According to a thirteenth aspect of the present invention, in the image encoding method of the twelfth aspect, the value of the second flag in the immediately preceding pixel block is set to a third flag. Obtaining a detection result for each block from the first flag, the second flag, and the third flag, and performing re-encoding with the quantization characteristic changed based on the detection result for each block. It is characterized by.

[0054]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.

The image encoding apparatus and method having the function of detecting block noise according to the present invention detects block noise based on information obtained during the encoding process. According to the process information,
(1) Detection of a pixel block with only an average value, and (2) Detection of a step with a peripheral block.

Hereinafter, each detection method will be described.

A pixel block having only an average value has all AC
It is determined whether the component is zero. In the variable length coding unit 500 of the related art shown in FIG. 17, the run length of the zero AC component is counted by the run length counter 508, so that the run length output from the run length counter 508 is 63. It can be seen that all AC components are zero.

On the other hand, the step from the peripheral pixel block is DC
It can be determined from the group number of the component difference.

In an image region having a gradual gradation change in which block noise is a problem, DC
The DC component obtained by applying T and quantizing is quantized to the same level or an adjacent level. When quantization is performed to adjacent levels, a difference between levels, that is, a quantization step width becomes a quantization error, and a step occurs at a boundary between decoded pixel blocks.

If the edge overlaps the boundary of the original pixel block, the DC components of the pixel block sandwiching the edge are not always quantized to adjacent levels.

For this reason, DC between successive pixel blocks
The case where the difference between the components is +1 or -1 can be regarded as a condition for generating block noise. This, according to FIG.
It is nothing but detecting that the group number becomes 1.

In the variable length coding unit 500 according to the prior art shown in FIG. 17, when the group number output from the grouping unit 504 is 1, when the current pixel block is decoded, the pixel block on the left side is decoded. It can be considered that there is a possibility that a step may occur at the boundary with.

As described above, in the present invention, block noise occurring in an image area having a gradual gradation change in a document is detected by using these two block noise detection methods together.

Hereinafter, embodiments of the present invention will be described. [First Embodiment] FIG. 1 shows a first embodiment of the present invention as a whole. In FIG. 1, the portions corresponding to those in FIG. Omitted. In FIG. 1, block noise is detected by a variable length coding unit 500, and the detection result is
Feedback is made to 0 to adapt the quantization characteristics.

Next, detection of block noise by the variable length coding unit 500 will be described. FIG. 2 shows the variable length coding unit 500 of this embodiment. In this figure, the parts corresponding to those in FIG.

In the figure, reference numeral 511 denotes a grouping unit 504
A group number detection unit that detects a specific group number from the group number SSSS output by and outputs a detection signal;
Reference numeral 512 denotes a run-length detecting unit that detects a specific run length from the run-length NNNN output from the run-length counter 508 and outputs a detection signal. An AND 601 that outputs a logical product of the signals as a block noise detection signal is an output terminal from which the block noise detection signal is output.

If the difference between the DC component and the immediately preceding block is +1 or −1,
That is, the case where the group number output by the grouping unit 504 is 1 is detected, and the 1-bit detection signal is AND5.
13 is output.

The run length detection unit 512 determines that the run length output from the run length counter 508 is 63
Is detected, and a 1-bit detection signal indicating the detection is set to AN
Output to D513.

AND 513 is a group number detector 51
The logical product of 1 and the detection signal of the run length detection unit 512 is output to the output terminal 601 as block noise detection information.

In the first embodiment of the present invention, when block noise is detected from the output terminal 601, re-encoding is performed by changing the quantization characteristics of the quantization unit 400. Regarding the change of the quantization characteristic, if the coarse quantization characteristic is set in advance, the fine quantization characteristic should be used, or if the fine quantization characteristic is set in advance, the coarse quantization We will change it to become a characteristic.

The decision to execute re-encoding may be made by detecting block noise only once, or by counting a block noise detection signal by a counting means (not shown).
The counting result may be compared with the total number of pixels of the image or the total number of pixel blocks, and it may be determined that the block noise has occurred at a predetermined ratio or more.

With the above configuration and operation, it is possible to detect the block noise generated during the encoding of the image by the image encoding device of the first embodiment of the present invention.

[Second Embodiment] Next, a second embodiment of the present invention will be described. In the first embodiment described above, whether or not the immediately preceding pixel block is a block having only the average value is not considered. For this reason,
Block noise has also been detected in a boundary between an edge region where block noise is relatively inconspicuous and a gentle gradation, for example, a case where a flat pixel block follows a pixel block including an edge.

On the other hand, when the detection range is limited to a gradual gradation region, a condition that the immediately preceding pixel block is a pixel block having only an average value may be added.

That is, only a case where a pixel block having only the average value is continuous and a step corresponding to the quantization width of the DC component occurs at the boundary is detected as block noise.

Hereinafter, the configuration of the second embodiment of the present invention will be described with reference to FIG. The same components as those of the prior art shown in FIG. 11 or the first embodiment of the present invention shown in FIG.

In the figure, reference numeral 514 denotes a storage unit for storing detection information indicating that a specific run length has been detected for the immediately preceding pixel block. Reference numeral 515 denotes a group number detection unit 511 and a run length detection unit 512. An AND that outputs a logical product of the output detection signal and the detection information stored in the storage unit 514 as a block noise detection signal.

The detection signal output from the run-length detecting section 512 is output to an AND 515 and the storage section 5
At 14, a period required for encoding one pixel block is delayed. This detection information is input to the AND 515 together with the detection signals output by the group number detection unit 511 and the run length detection unit 512 in the next block, and the logical product of them is output as a block noise detection signal.

With the above configuration and operation, it is possible to detect block noise generated during image coding by the image coding apparatus according to the second embodiment of the present invention.

According to the JPEG specification, when the run length is 63, the two-dimensional Huffman encoding
An OB (End of Block: block end) code is to be output. Therefore, in the above embodiment, the two-dimensional Huffman encoding unit 510 may be configured to detect the case where only the EOB code is output for one pixel block, and determine that all AC components become zero.

Further, in the above embodiment, similar to JPEG, a pixel block of 8 × 8 pixels is assumed,
Although 63 is detected as the run length when the coefficient is zero, the present invention is not limited to this value. For example, N × M
When using pixel blocks of (N and M are positive integers),
The DC component is removed from the number of transform coefficients (N × M) (N × M
The run length of -1) may be detected.

Further, with respect to the pixel block located at the leftmost end of the image, the immediately preceding block is the block located at the rightmost position of the block line immediately above, so that it is excluded from the block noise detection processing.

Further, since the quantization width of the DC component is relatively small and there is a case where block noise does not cause a visual problem in some cases, when the quantization width of the DC component is smaller than a predetermined value, Stop the block noise detection function,
Alternatively, the block noise detection signal may be invalidated. The predetermined value is determined according to the reproduction characteristics of an output device that outputs a decoded image, such as a display or a printer.

Next, the first encoding method of the present invention will be described. FIG. 14 is a flowchart showing the procedure of the encoding method. [S1] An 8 × 8 pixel block is cut out from the input image. [S2] DCT is applied to the pixel block to obtain a transform coefficient. [S3] The transform coefficients are quantized to obtain quantized coefficients. [S4] The quantization coefficients are made one-dimensional in zigzag scan order. [S5] If the quantization coefficient is a DC component, step S
6. In the case of the AC component, the process branches to step S11. [S6] The difference from the DC component of the immediately preceding block is calculated. [S7] The differences are grouped according to FIG. 6 to obtain a group number and additional bits. [S8] If the group number is 1, the flow branches to step S9. [S9] The flag 1 indicating the detection of the group number 1 is set. [S10] The group number is Huffman-coded to obtain a DC Huffman code. [S11] It is determined whether the AC component is zero. If the AC component is zero, the process branches to step S12. If the AC component is not zero, the process branches to step S14. [S12] The run length of the zero AC component is counted. [S13] If the run length is 63, execute step S1.
Branch to 5. [S14] The AC components are grouped according to FIG. 7 to obtain a group number and additional bits. [S15] The flag 2 indicating the detection of the run length 63 is set. [S16] Change the combination of run length and group number to 2
Find the dimensional Huffman code. [S17] The DC Huffman code and the AC Huffman code are multiplexed with both additional bits, and code data is output. [S18] If the flag 1 and the flag 2 are set, the flow branches to step S19. [S19] The block noise detection flag is set.

Next, the second encoding method of the present invention will be described. FIG. 15 is a flowchart showing the procedure of the encoding method. Steps S1 to S17 are the first
Are the same as those in the encoding method described in FIG. Hereinafter, step S18 and subsequent steps will be described. [S18] If the flag 1, the flag 2, and the flag 3 are set, the flow branches to step S19. [S19] The block noise detection flag is set. [S20] If the flag 2 is set, the flow branches to step S21. [S21] The flag 3 is set.

[0086]

As described above, according to the present invention, the block noise is detected by detecting the difference between the DC components and the A which becomes zero.
A group number detecting unit 510, a run detecting unit 511, and an AND 513 are added to a conventional image coding apparatus because the information is based on information obtained in the course of a conventional coding operation such as a run length of a C component. Can be realized. Therefore, it is suitable for mounting on an LSI having a limited circuit scale or software whose performance is likely to be reduced due to the addition of noise detection processing.

In JPEG, various methods have been devised for controlling the code amount to a constant value because the code amount varies depending on the contents of the document. For example, the quantization characteristic is changed by multiplying the quantization matrix by a constant value a (a real number of a> 0), and the target code amount a is converged by a Newton-Raphson method or the like based on the relationship between the generated code amount and a. There is a method that is determined by In this case, since the image quality is not taken into account, an image which has the target code amount but contains much block noise may be decoded. In such a case, by applying the present invention, it is possible to control the code amount while avoiding block noise.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an overall configuration of a first embodiment of the present invention.

FIG. 2 is a configuration diagram of a main part of the first embodiment of the present invention.

FIG. 3 is a configuration diagram of a second embodiment of the present invention.

FIG. 4 is a diagram illustrating conversion coefficients.

FIG. 5 is a diagram illustrating an example of encoding.

FIG. 6 is a diagram illustrating grouping of DC components.

FIG. 7 is a diagram illustrating grouping of AC components.

FIG. 8 is a diagram illustrating a Huffman code table.

FIG. 9 is a diagram illustrating a two-dimensional Huffman code table of AC components.

FIG. 10 is a view for explaining one-dimensionalization by zigzag scanning.

FIG. 11 is a diagram illustrating an example of variable-length coding.

FIG. 12 is an explanatory diagram of block noise.

FIG. 13 is an explanatory diagram of linear quantization.

FIG. 14 is a flowchart illustrating a procedure of a first encoding method according to the present invention.

FIG. 15 is a flowchart illustrating a procedure of a second encoding method according to the present invention.

FIG. 16 is a schematic configuration diagram of a conventional technique.

FIG. 17 is a configuration diagram of a variable-length encoding circuit according to the related art.

[Explanation of symbols]

 100, 101 input terminal 200 block division unit 300 DCT conversion unit 400 quantization unit 500 variable length coding unit 600 output terminal 601 block noise detection signal output terminal 501 scan conversion unit 502 block delay unit 503 subtractor 504 grouping unit 505 1 Dimensional Huffman coding unit 506 Multiplexing unit 507 Zero judgment unit 508 Run length counter 509 Grouping unit 510 Two-dimensional Huffman coding unit 511 Group number detecting unit 512 Run detecting unit 513 AND 514 Storage unit 515 AND

Claims (13)

[Claims] 1. A dividing means for dividing an image to obtain a pixel block which is a rectangular area of N × M pixels (N and M are positive integers), and a transforming means for performing an orthogonal transformation on the pixel block to obtain a transform coefficient. And a quantizing means for quantizing the transform coefficient with a predetermined quantization characteristic to obtain a quantized coefficient, wherein a DC (direct current) component of the quantized coefficient is varied by a difference between the DC component of the immediately preceding pixel block. Length-encoded and A of the quantized coefficient
The continuous length of the AC coefficient which makes the C (alternating current) component zero;
A two-dimensional variable-length encoding unit that performs variable-length encoding as a combination of the magnitudes of nonzero AC coefficients following this, comprising the steps of: A first detecting means for outputting a detection signal; and a second detecting means for detecting a specific value of a continuous length of the AC coefficient.
A second detection unit that outputs a detection signal of the first detection signal and a determination unit that outputs a logical product of the first detection signal and the second detection signal as a detection result for each block. An image encoding device, wherein re-encoding is performed with the quantization characteristic changed based on a detection result. 2. The image coding apparatus according to claim 1, wherein said storage means stores said second detection signal in the immediately preceding pixel block, and wherein said first detection signal and said second detection signal are stored. A determination unit that outputs a logical product of the contents stored in the storage unit as a detection result for each block, and determines execution of re-encoding in which the quantization characteristic is changed based on the detection result for each block. An image encoding device characterized by: 3. The specific value of the difference is +1 or −
The image encoding device according to claim 1, wherein the number is 1. 4. The image encoding apparatus according to claim 1, wherein the specific value of the continuous length of the zero AC coefficient is (N × M−1). 5. The image coding apparatus according to claim 1, wherein said determination means is an AND element. 6. The image encoding apparatus according to claim 1, wherein the orthogonal transform is a discrete cosine transform. 7. The image coding apparatus according to claim 1, wherein said pixel block is composed of 8 × 8 pixels. 8. The method according to claim 1, wherein the second detecting means includes the step of:
2. The method according to claim 1, wherein instead of detecting a specific value of the continuous length of the coefficient, a specific code output from the two-dimensional variable length encoding unit is detected and a second detection signal is output. Or the image encoding device according to 2. 9. A counting means for counting the detection result for each block, wherein when the coding operation of the image is completed, the counting result of the counting means and the total number of pixels of the image or the number of pixels included in the image are included. 3. The image according to claim 1, wherein the re-encoding is performed by changing the quantization characteristic when the count result is equal to or more than a predetermined ratio by comparing the total number of pixel blocks to be processed. Encoding device. 10. When the quantization threshold of the DC component is smaller than a predetermined value in the predetermined quantization characteristic of the quantization means, re-encoding in which the quantization characteristic is changed is not executed. The image encoding device according to claim 1 or 2, wherein 11. The image encoding apparatus according to claim 1, wherein the predetermined value is determined according to a tone reproduction characteristic of an output unit that outputs a decoded image. 12. An image is divided to obtain a pixel block which is a rectangular area of N × M (N and M are positive integers) pixels, and the pixel block is subjected to an orthogonal transform to obtain a transform coefficient. Quantizing with a predetermined quantization characteristic to obtain a quantization coefficient,
The DC (direct current) component of the quantization coefficient is subjected to variable-length coding by a difference between the DC component of the immediately preceding pixel block and the AC (alternating current) component of the quantization coefficient is converted to a continuous length of an AC coefficient of zero. And a variable length coding method using a combination of the following non-zero AC coefficient magnitudes: detecting a specific value of the difference, setting a first flag, The second flag is set by detecting a specific value of the length to be set, and the first flag and the second flag are set.
An image encoding method comprising: obtaining a detection result for each block from the flags of (a) to (c), and performing re-encoding with the quantization characteristic changed based on the detection result for each block. 13. The image encoding method according to claim 12, wherein the value of the second flag in the immediately preceding pixel block is set to a third flag, and wherein the first flag and the second flag are set. A detection result for each block from the flag and the third flag, and performing re-encoding with the quantization characteristic changed based on the detection result for each block.