JPH0822063B2 - Image signal encoding device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image signal coding apparatus and method for shortening the transmission time of an image signal or reducing the storage capacity.

(Prior Art) As a data compression method for a multi-valued image (for example, 1 pixel 8 bits, 256 levels), there are an information storage type coding system and an information non-storage type coding system. Information-preserving encoding does not include quantization in the encoding process, and it is possible to reproduce the exact same image as the original image by encoding / compositing processing, but a high compression rate is not obtained. I can't. On the other hand, the information non-conservation type encoding includes some quantization processing in the encoding process, and the reproduced image includes quantization noise and deterioration of image quality due to the encoding / decoding processing, but high compression is required. The rate is obtained.

The performance of information non-conservative coding is generally quantized distortion (S / N
Ratio) and the data compression rate (information amount), but as one method to realize a good S / N ratio vs. information amount, the transform coefficient after orthogonal transformation is quantized and the variable length code is used. There is a method to make it.

In general, the power of the transform coefficient is concentrated in a part of the transform coefficients, so in this method, a large amount of information is assigned to the transform coefficient of large power, and a small amount of information is assigned to the transform coefficient of low power. , Enables a significant amount of information compression.

Further, the distribution of the normal image signal differs greatly depending on the image, but the distribution of this conversion coefficient often follows a certain model regardless of the image. Therefore, by using the variable-length code designed based on this model, it is possible to realize the compression of the information amount independent of the image.

Further, there is a progressive coding system as a coding system for multi-valued images. The progressive coding method is a method of displaying a rough image by using rough information of the entire image as a first step and displaying a finer image by sequentially using fine information. .

In this progressive coding method, if the amount of rough information of the entire image used in the first stage is reduced, even if the transmission speed of information in communication is slow, the first step is to convert a rough image into high speed. Can be displayed on. In this first stage, the entire image is displayed although it is a rough image, so the receiver can know the contents of the entire image more quickly than in the case where a normal image is sequentially decoded as a fine image. be able to. Therefore, even if the transmission speed is slow, the burden on the user can be greatly reduced.

Further, as the detailed information is received, the entire image becomes finer and finer, so that the image can be discriminated before all the information is received. Therefore, when it is desired to retrieve only a necessary image from a large number of images, the transmission of information can be stopped at the time when the determination can be made, and the retrieval efficiency can be greatly improved.

Such a progressive coding system can be easily realized by applying a coding system using orthogonal transform. That is, instead of transmitting all the orthogonal transform coefficients, only the orthogonal transform coefficients whose power is concentrated are encoded and transmitted for the entire image. Then, only the transmitted orthogonal transform coefficient is inversely transformed to display the composite image.

In this case, since only some of the orthogonal transform coefficients are transmitted, the amount of information is much smaller than when all the orthogonal transform coefficients are transmitted. Therefore, even if the information transmission speed is low, the information can be transmitted at high speed. Further, since the information of the entire image is transmitted, the entire image can be displayed although it is a rough image.

Then, by transmitting the remaining orthogonal transform coefficients sequentially, a finer decoded image can be obtained.

Further, in contrast to such a progressive encoding method, a method of sequentially encoding and decoding an image on a row-by-row basis is called a sequential encoding method. To realize this method, all orthogonal transform coefficients may be encoded from the beginning.

(Problem to be Solved by the Invention) When progressive coding is realized by using a method of quantizing such an orthogonal transform coefficient to perform variable length coding,
First, orthogonal transformation and quantization processing of the entire image are performed in advance, and the quantized orthogonal transformation coefficient is stored. Then, as the first step of progressive, only some orthogonal transform coefficients in which power is concentrated are encoded and transmitted for the entire image, and the remaining orthogonal transform coefficients are sequentially encoded.

However, when the progressive coding is realized by such a method, there is a problem that an enormous storage capacity is required to store the orthogonal transform coefficient when the image size is large.

Therefore, when the orthogonal transform coefficient is stored in the memory, a very large number of memories are required, and there is a problem that the encoding device becomes expensive and large.

Instead of storing the orthogonal transform coefficient in this way, the orthogonal transform and the quantization process can be redone for each progressive step. However, even in such a case, since the input image signal must be stored, the same problem occurs.

Further, since the processing time of orthogonal transformation is generally long when the image size is large, there is a problem that the entire processing time becomes enormous by repeating the same processing a number of times.

The present invention, instead of storing the quantized transform coefficient, stores the code for each stage of progressive encoding for each stage, so that the progressive code can be stored without using a large storage capacity. It is an object of the present invention to provide an image signal encoding method and an apparatus therefor that can realize the encoding.

(Means for Solving the Problem) An image signal coding apparatus according to the present invention includes a block reading unit that reads an image signal in a block unit including a plurality of pixels, and a plurality of transform coefficients by performing orthogonal transform in the block unit. An orthogonal transformation unit to be obtained, a quantization unit that quantizes the plurality of transform coefficients and outputs a plurality of quantization indexes, and transmits each stage to divide the plurality of quantization indexes into a plurality of stages for transmission. A position storage unit that stores the position of the quantization index to be processed in the block, an encoding unit that performs variable length encoding of the quantization index based on the position, and outputs a code corresponding to each stage, and the code A code storage unit for storing each stage, and a code for transmitting the code stored in the code storage unit for each stage in order from the first stage. Signal transmission unit.

Further, another image signal encoding device of the present invention includes a block reading unit that reads an image signal in a block unit including a plurality of pixels, and an orthogonal transform unit that performs orthogonal transform in the block unit to obtain a plurality of transform coefficients. , A quantizer that quantizes the plurality of transform coefficients and outputs a plurality of quantization indexes, and a quantization index that should be transmitted in each stage in order to divide the plurality of quantization indexes into a plurality of stages for transmission. A position storage unit that stores the position in the block, a coding unit that performs variable length coding on the quantization index based on the position, and outputs a code corresponding to each stage, and a coding unit that outputs the code corresponding to each stage. A code storage unit that stores a code for each stage, and first transmits the code of the first stage output from the encoding unit, and then transmits the code. Composed of the code transmission unit to be transmitted to each stage parts stored in the code in order from the second stage.

(Operation) An image signal encoding apparatus of the present invention will be described.

First, the block reading unit reads an image signal in units of blocks including a plurality of pixels. A square block composed of n × n pixels is often used as this block.

Next, the orthogonal transform unit performs orthogonal transform on this block unit to obtain a plurality of transform coefficients. For this orthogonal transformation,
An arbitrary orthogonal transform such as a two-dimensional discrete cosine transform or Hadamard transform can be used. If a square block consisting of n × n pixels is used, the number of transform coefficients is also n × n per block.

Then, the quantizer performs quantization by dividing each transform coefficient by a given quantization step, and obtains a quantization index corresponding to each transform coefficient. However, here, all transform coefficients are quantized in the same quantization step, but different quantization steps may be used depending on the position of each transform coefficient in the block.

Here, in order to divide and transmit the image information to a plurality of stages, the position in the block of the quantization index to be transmitted in each stage is stored in the position storage unit in advance. At this time, the number of quantization indexes to be transmitted at each stage is determined in consideration of image quality and transmission rate, and the positions of the quantization indexes to be transmitted are determined and stored accordingly. However, this position, that is, the number of quantization indexes transmitted in each stage, can be made variable for each image based on the statistical amount of the image.

Based on this position, the coding unit performs variable length coding on the quantization index corresponding to each stage, and outputs the code corresponding to each stage. That is, the position of the quantization index to be transmitted in the first stage is read from the position storage unit, this is subjected to variable length coding and output as the code of the first stage. Similarly, the code of each stage is output.

By the way, as a method of variable-length coding the quantization indexes, a method of individually variable-length coding all the quantization indexes in the block is generally used. However, there are various methods such as a method in which the length is grouped and the length is variable length coded.

An example of these methods is, for example, Reference 1: Mutsumi Ota, Toshio Koga, "Comparison of Variable Length Coding in Motion Compensated Interframe Hybrid Coding Systems," 1986, IEICE Communications Division National Conference Collection, Volumes 1, 1-206 and Reference 2: Yukitoshi Tsuboi, Sadaji Okamoto, "Comparative Study of Various Entropy Coding Methods for Color Still Image Coding", Image Coding Symposium, 2nd Symposium Material, It is described on pages 71-72.

Then, the code storage unit stores the code separately for each stage.

After performing such a process for all the blocks in the image, the code transmission unit sequentially transmits the codes stored in the code storage unit from the first stage. That is, first, all the codes stored as the codes of the first stage are transmitted. Next, all the codes stored as the codes of the second stage are transmitted. Then, after transmitting all the codes of all the stages, the transmission of the codes is completed.

Further, instead of first storing the code of each stage in the code storage unit for each stage and then transmitting the code as described above, only the code of the first stage is not stored in the code storage unit, but is transmitted from the encoding unit. It can also be transmitted from the code transmission unit at the same time as being output. Then, the codes of the second and subsequent stages are similarly stored in the code storage unit. By doing so, code transmission can be started earlier, and the time required for the entire encoding process can be shortened.

On the decoding side, first, the image is decoded based on the quantization index decoded from the code transmitted in the first stage, and a rough image is displayed. Then, by obtaining the information of the second and third stages and decoding the image together with the information transmitted in the previous stage, the fine images are sequentially decoded and displayed.

By thus dividing and transmitting the quantization index to each stage, progressive encoding can be realized.

By the way, what is stored in the code storage unit is the code after the variable length coding. Therefore, the code amount is significantly reduced as compared with the original image signal. Therefore, this code storage unit can be configured with a storage capacity much smaller than that in the case where the quantization index is stored and the progressive coding is performed. That is, a compact and low-priced encoding device can be easily realized.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 (a) is a block diagram showing an embodiment of an image signal encoding apparatus of the first invention. In the following description, the two-dimensional discrete cosine transform is used as the orthogonal transform, but it is also possible to use an orthogonal transform such as Hadamard transform.

As shown in FIG. 1A, the block reading unit 1 reads an image signal in units of blocks for DCT conversion. For example, an 8-bit image signal per pixel is 8 pixels vertically,
A total of 64 pixels of 8 pixels horizontally are read as one block. Then, the DCT transform unit 2 performs a two-dimensional discrete cosine transform of the read image signal 101 for one block, and calculates 8 × 8 transform coefficients 102.

Upon receiving the transform coefficients 102 calculated in this way, the quantizing unit 3 quantizes the transform coefficients 102 by dividing the transform coefficients 102 by a given quantization step, and outputs a quantization index 103 corresponding to each transform coefficient 102. . However, here, all transform coefficients are quantized in the same quantization step, but different quantization steps may be used depending on the position of each transform coefficient 102 in the block.

Here, in order to divide and transmit a plurality of quantization indexes 103 of a block to a plurality of stages, the position storage unit 8 stores in advance the positions 108 in the block of the quantization indexes 103 to be transmitted in each stage. Keep it.

FIGS. 2A and 2B are explanatory diagrams of the positions of the quantization indexes to be transmitted in each stage. As shown in FIG. 2A, a number indicating the position is assigned to each quantization index 103 in the block. Then, as shown in FIG. 2B, the number of the quantization index 103 to be transmitted in each stage is stored in the position storage unit 8 as the position 108. In the example of FIG. 2B, one quantization index 103 is transmitted in the first stage and three quantization indexes 103 are transmitted in the second stage.

Then, the encoding unit 4 first reads the position 108 of the quantization index 103 to be transmitted in the first stage from the position storage unit 8, and reads the quantization index to be transmitted in the first stage.
The variable length encoding of 103 is performed, and the code 111 corresponding to the first stage is output. The variable length coding method of the quantization index 103 is shown in Documents 1 and 2.

Next, the encoding unit 4 reads the position 108 of the quantization index 103 to be transmitted in the second stage from the position storage unit 8, and the quantization index 103 to be transmitted in the second stage.
The variable length encoding of the
Output 2

Similarly, the encoding unit 4 performs the encoding process such as the code 113 corresponding to the third stage and the code 114 corresponding to the fourth stage.

The code storage unit 6 stores the codes 111, 112, 113, 114 and the like output from the coding unit 4 for each stage.

After performing such a storage process for all the blocks in the image, the code transmission unit 7 first reads all the codes 111 of the first stage from the code storage unit 6 as code 106 and transmits them as code 107. . Next, the code transmission unit 7 reads all the codes 112 of the second stage as code 106 from the code storage unit 6 and transmits them as code 107. Hereinafter, the same processing is repeated for each stage, and when all the codes 106 of all the stages are read from the code storage unit 6 and transmitted as the code 107, the transmission processing of the code 107 is ended.

FIG. 1 (b) is a block diagram showing an embodiment of an image signal encoding apparatus of the second invention. As shown in FIG. 1 (b), the code 111 in the first stage is output from the coding unit 4 and at the same time transmitted from the code transmission unit 7 as code 107. As described above, by transmitting only the code 111 in the first stage without storing it in the code storage unit 6, it is possible to speed up the start of transmission of the code 107 and reduce the overall encoding processing time.

In the coding apparatus described above, the code stored in the code storage unit 6 is the code after the variable length coding. Therefore, the code amount is much smaller than the initial image signal 101 and the quantization index 103. Therefore, it is possible to realize progressive coding with a much smaller storage capacity than in the case where the image signal 101 and the quantization index 103 are stored and the progressive coding is realized. That is, the code storage unit 6 can be configured with a very small number of memories, and a compact and low-priced encoding device can be realized.

By changing the position 108 stored in the position storage unit 8, it is possible to freely set the amount of information to be transmitted in each stage when progressively transmitting image information and the manner in which the image becomes finer in stages. . Therefore, even when various transmission speeds and images are targeted, it is possible to realize a progressive coding method corresponding to them. At that time, the position 108 can be made variable for each image according to the statistical amount of the image.

Further, by specifying the total quantization index 103 in the block as the position 108 of the code to be transmitted in the first stage, it is possible to execute sequential image transmission using this encoding device.

In the above description, the block size is 8 × 8, but another size or shape may be used.

Further, in the above description, although not specifically defined as an image signal, a multi-valued black-and-white image, RGB color component images, luminance / color difference signals such as Y, (RY), (BY), etc. Are all included in this image signal. Similarly, the present invention can be applied to a frame difference signal in a moving image such as a television signal, and a sufficient effect can be obtained. This inter-frame difference signal is described in Reference 3: “TV Band Widows Compression Transmission Bi-Motion Compensated Inter-frame Coding (Television Bandwidth C
ompression transmission by Motion-compensated Inte
rframe Coding) ”IEEE Communication Magazin
e) magazine, November 1982, pages 24-30.

(Effects of the Invention) As described above, by using the image signal coding apparatus of the present invention, the codes of each stage of progressive coding are stored, so that the progressive coding can be performed without using a large storage capacity. Coding can be realized, and a compact and low-priced coding device can be realized.

[Brief description of drawings]

FIG. 1 (a) is a block diagram showing an embodiment of an image signal encoding apparatus of the first invention, and FIG. 1 (b) is an embodiment of an image signal encoding apparatus of the second invention. The block diagrams shown in FIGS. 2A and 2B are explanatory diagrams of the positions of the quantization indexes to be transmitted in each stage. In the figure, 1 ... block reading unit, 2 ... DCT conversion unit, 3 ... quantization unit, 4 ... encoding unit, 6 ... code storage unit, 7 ... code transmission unit,
8 ... Position storage unit.

Claims (2)

[Claims] 1. A block reading unit for reading an image signal in a block unit composed of a plurality of pixels, an orthogonal transform unit for performing a orthogonal transform in the block unit to obtain a plurality of transform coefficients, and quantizing the plurality of transform coefficients. And a position for storing the position in the block of the quantization index to be transmitted in each stage in order to divide and transmit the plurality of quantization indexes into a plurality of stages. A storage unit, a coding unit that performs variable-length coding on the quantization index based on the position and outputs a code corresponding to each stage, a code storage unit that stores the code for each stage, and the code storage unit And an image signal encoding device configured to transmit the above-described code stored in step 1 in order from the first stage to each stage. 2. A block reading unit for reading an image signal in a block unit composed of a plurality of pixels, an orthogonal transform unit for performing a orthogonal transform in the block unit to obtain a plurality of transform coefficients, and quantizing the plurality of transform coefficients. And a position for storing the position in the block of the quantization index to be transmitted in each stage in order to divide and transmit the plurality of quantization indexes into a plurality of stages. A storage unit, a coding unit that performs variable-length coding on the quantization index based on the position and outputs a code corresponding to each stage, and a code that stores the code of each stage after the second stage for each stage. The storage unit, first transmitting the code of the first stage output from the encoding unit, and then transmitting the code stored in the code storage unit to the second stream. Encoding apparatus of the image signal composed of over di a code transmitter for transmitting in sequence for each stage.