JPH0752951B2 - Image data compression processing method and apparatus - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image data compression processing apparatus and method.

Recording of semiconductor memory, magnetic recording media, optical disks, etc., because the amount of digital image data obtained from a camera equipped with a solid-state electronic image pickup device such as CCD, a camera tube, an optical scanning reader (OCR), etc. is extremely large. When recording on a medium, data compression is required to reduce the amount of data. A typical method of image data compression is W.CHEN
and W.PRATT “Scene Adaptive Coder” IEEE Trans.on Co
mm.Vol.CoM-32, No. 3, March 1984, pp225-232. It divides the original image data into multiple blocks, orthogonally transforms the original image data for each block, obtains a transform coefficient, normalizes by dividing this transform coefficient by an appropriate normalization coefficient, and rounds the fraction. Then, the data is quantized, and the quantized value is encoded to obtain compressed data.

An example of applying this data compression method to video data transmission is described in USP 4,394,774. The digital video compression system described in this document transmits video data through an appropriate medium while compressing the video data at a video rate and expands the data on the receiving side. It discloses a method of adjusting the normalization coefficient so that the compressed data length of each block falls within a certain range. However, in this method, one screen is divided into a plurality of blocks and the normalization coefficient is adjusted for each block based on the compressed data length of each block. There is a problem that the image quality is deteriorated in a portion having a fine pattern or the like by performing a normalization process with a larger normalization coefficient than that of

On the other hand, there is a demand to always keep the compressed data length for one screen almost constant. For example, when a still image taken by a digital still camera is stored in a digital memory, it is possible to compress the image data and store it in the memory in order to use the memory capacity as effectively as possible. It is preferable that the number of frames that can be stored in one memory chip or one memory card is fixedly determined in advance. For that purpose, it is necessary to fix the compressed data length for one screen. In many fields as well as in digital still cameras, there are such demands, and the recording medium is not limited to the semiconductor memory but may be the optical disk or the like depending on the application field.

SUMMARY OF THE INVENTION According to the present invention, the compressed data length for one screen can be fixed almost constant, and even if there is a partial difference in fineness of the picture, the image quality is not partially deteriorated, and further compression processing is performed. An object of the present invention is to provide an image data compression processing device and method capable of shortening the time.

The present invention divides one screen into blocks each consisting of a plurality of pixels, performs orthogonal transformation on the original image data of the pixels contained in each block, and obtains the orthogonal transformation coefficient for one screen. In the image data compression processing method for normalizing the obtained orthogonal transform coefficient using the normalization coefficient and assigning a predetermined code to each of the normalized values to obtain encoded data, prior to the compression processing of the original image data, , A block group is formed by a plurality of adjacent blocks, representative blocks representative of those blocks are extracted from each block group, and the orthogonal transformation, normalization, and encoding are performed on each of the extracted representative blocks. The total amount of encoded data obtained by this is counted over all the representative blocks, and the count value is within the specified range. The normalization, encoding, and data counting process are repeated while changing the normalization factor so that the optimum normalization factor is determined. In this way, the optimum normalization factor is determined, and then the original image data compression process is performed. In this compression process, the orthogonal transformation coefficient is normalized by using the determined optimum normalization coefficient, and the normalized value is encoded to obtain the final compressed image data. To do.

Further, the present invention divides one screen into blocks each composed of a plurality of pixels, performs orthogonal transformation on the original image data of the pixels included in the block for each block, and obtains the orthogonal transformation coefficient for one screen, In an image data compression device that obtains encoded data by normalizing the obtained orthogonal transform coefficient using a normalization coefficient and assigning a predetermined code to each of the normalized values, prior to compression processing of the original image data, A block group forming means for forming a block group from a plurality of adjacent blocks, and a representative block extracting means for extracting a representative block representative of the blocks from each block group formed by the block group forming means. , The above orthogonal transformation,
Normalization and encoding are performed for each of the representative blocks extracted by the representative block extracting means, and the data amount of encoded data obtained by this is counted by all the representative blocks. Normalization coefficient determining means for determining the optimum normalization coefficient by repeating normalization, encoding and data counting processing while changing the normalization coefficient so that the count value falls within a predetermined range, and the normalization coefficient determining means. After determining the optimum normalization coefficient determined by, the process moves to the compression processing of the original image data, and in this compression processing, the orthogonal transformation coefficient is normalized using the optimum normalization coefficient determined and is normalized. It is characterized by comprising an encoding means for obtaining final compressed image data by encoding a value.

The extraction of the representative block may be performed by selecting the representative block from the plurality of blocks, or the representative block is created by obtaining the average value of the corresponding pixel data of the plurality of blocks for each pixel data of the representative block. You may perform by doing.

In the second and subsequent normalization processing of the representative block, those orthogonal transform coefficients may be normalized by a new normalization coefficient, or in the second and subsequent normalization processing, the previously normalized values may be used. A new normalized value may be obtained by normalizing.

According to the present invention, a representative block is extracted from the image data of one screen prior to the compression processing of the original image data, the compression processing is performed on this representative block, and the normalization coefficient is set so that the compressed data length becomes almost constant. Has been decided. Then, the image data of the entire one screen is compressed by using the determined optimum normalization coefficient to obtain the final compressed image data. Therefore, it can be expected that the compressed data length for one screen becomes almost constant. Since the compression processing is performed only for the representative block in the determination of the normalization coefficient, the amount of calculation is smaller than that in the case of performing the compression processing of the entire screen, and the processing time required for the determination of the normalization coefficient can be shortened. The processing time of the entire data compression processing can also be shortened. Further, when the normalization coefficient is determined, processing such as orthogonal transformation, normalization, and coding is performed for each divided block, and one determined normalization coefficient can be used for the entire screen. There is no problem that the normalization coefficient is partially different due to the difference in fineness and the image quality is partially deteriorated, and it is possible to maintain a relatively high image quality even for a fine pattern or the like.

The optimal normalization coefficient in the original image data is determined, and the orthogonal transformation coefficient obtained from the original image data is normalized and encoded using the determined optimal normalization coefficient. In other words, since the normalization coefficient for the image data of the one screen is determined by the data amount corresponding to the data amount of the image data of the one screen, an appropriate normalization coefficient is always obtained regardless of the change of the image. Regardless of the image change, the compressed data length for one screen can always be set to a substantially constant value regardless of the image.

Description of Embodiments Embodiments of the present invention will be described in detail with reference to FIGS. 1 to 4. FIG. 1 shows the image data compression processing in functional blocks, and FIG. 2 shows the flow of the processing in a diagram. FIG. 3 shows the relationship between the divided blocks and the representative block on the screen. FIG. 4 is a graph showing a normalization coefficient table.

Referring to FIG. 3, the original image data for one screen is composed of N × M pixels, and one pixel is represented by 8-bit data, for example. Such original image data is divided into a plurality of blocks B in the vertical and horizontal directions of the screen. One block is composed of 8 × 8 pixels, for example.

First, the data compression processing for one screen will be described.

The original image data is converted by the conversion unit 11 for each block.
DCT (Discrete Cosine Transformation) transformation (a type of orthogonal transformation) is performed, and the DCT coefficient F (u, v) is obtained (step 21). The DCT coefficient F (u, v) is stored in the coefficient memory 12 for each block (step 22). This DCT
The processing is performed on all original image data for one screen.

After the optimum normalization coefficient Dd, which will be described in detail later, is determined, the normalization unit 13 performs normalization processing for one screen (step 28). In this process, the DCT coefficient F (u, v) is read from the memory 12 for each block, and this coefficient F (u, v) is read.
v) is divided by the normalization factor Dd. The fractional result of this division is quantized by being rounded off. Generally, threshold value processing is performed simultaneously with or before this normalization processing. The threshold value process is a process of subtracting a certain value T from the DCT coefficient F (u, v). When F (u, v) ≦ T, the subtraction result is set to 0. F (0,0), that is, the lowest order (lowest frequency) component of each block (this is referred to as a DC component, which is data representing the average luminance of each block) is not subjected to threshold processing. Further, the DC component is also normalized by a normalization coefficient different from other components (AC component) in the normalization process. The optimal normalization coefficient Dd adjusted by the feedback processing described later is basically applied to the AC component. This normalization process is performed for each block and for the entire screen.

The normalized data is then encoded by the encoding unit 14 (step 29). Encoding includes Huffman encoding, run-length encoding, and the like. Usually, at the same time as or before DCT processing, normalization or encoding, two-dimensional array image data is zigzag scanned and converted into one-dimensional array data. The encoded data for one screen is output as compressed data and written in, for example, a memory (step 30).

The process of determining the optimum normalization coefficient Dd is performed as follows. In this processing, the data of the representative block is used. As shown in FIG. 3, a plurality of blocks B are put together to form one group. In this example, each block group consists of 4 × 4 blocks B. One representative block Br is selected from each block group.

After the DCT calculation process for the entire screen, the DCT coefficients stored in the memory 12 and belonging to the selected representative block Br are read out and normalized by the normalization unit 13 (step 23). In the first normalization process, an initial value D ₀ of a predetermined normalization coefficient (denoted by a symbol D) is used. That is, the normalization unit 13 is given the initial value D ₀ through the switching unit 17.

The normalized data of the representative block Br is next encoded block 14
Is encoded in (step 24). Next, the encoded data is given to the counter 15 and its representative block Br
Is calculated, that is, the data length of all representative blocks is counted (step 25). The normalization process and the encoding process are performed for each representative block, and the encoded data of each representative block is sequentially given to the counter 15. When normalization and encoding of the representative block in one screen are completed, the count value of the counter 15 represents the encoded data length of the representative block in one screen. The normalization coefficient D is set so that this count value substantially matches a predetermined value (this is called a target value). Therefore, it is checked whether the difference between the coefficient value of the counter 15 and the target value is smaller than a certain allowable amount E (step 26). If the above difference exceeds the allowable value E, a conversion table is used to obtain a new normalization coefficient D.
16 is referenced (step 27).

The conversion table 16 has a counter 15 as shown in FIG.
A normalization coefficient D that is considered to be appropriate and is calculated empirically for the count value of is set in advance. A new normalization coefficient D is set by referring to the conversion table 16, and is given to the normalization unit 13 via the switching unit 17. Then, this new normalization coefficient D is used to similarly normalize and code all representative blocks Br, and the data length of the coded data is counted by the counter 15 (steps 23, 2).
4,25). Then, the above process is repeated according to the determination result of step 26.

When the difference between the count value of the counter 15 and the target value is within the allowable value E, the last-used normalization coefficient D is considered to be the optimum amount. Normalization and coding processing is performed on the entire screen (steps 28 and 29).

In the flowchart of FIG. 2, first, the DCT coefficient is obtained for the entire screen, and then the process of determining the optimum normalization coefficient Dd is performed. First, the DCT coefficient is obtained for the representative block Br, the normalization coefficient is determined, and after the optimum normalization coefficient Dd is determined, the DCT operation is performed for the entire screen.
Further, normalization and encoding may be performed.

In the above embodiment, one block is selected from the 4 × 4 = 16 blocks and this block is set as the representative block.
A representative block composed of a set of pixels having average value data of corresponding pixels in a plurality of blocks (for example, 4 × 4 = 16 blocks) may be created.

As shown in FIG. 5, data L _ij (x _{i + k × 8} , y _{j + l × 8} ) (k
= 0 to 3, l = 0 to 3) The average value _ij shown in the following equation is set as one pixel data of the representative block Br.

A set of such data _ij collected over i = 0 to 7, j = 0 to 7 is one representative block. Then, such a representative block is created for each of 4 × 4 = 16 block groups, and the normalization coefficient Dd is determined using the data of such a plurality of representative blocks.

Here, the data L _ij and _ij may be luminance data before orthogonal transformation or orthogonal transformation coefficients after orthogonal transformation.

In the above embodiment, since the DCT coefficient stored in the memory 12 is normalized by the new normalization constant in each normalization process, the amount of data to be normalized is always constant.

Next, an embodiment in which the amount of data to be normalized is reduced for each normalization process and the processing time can be shortened will be described with reference to FIGS. 6 and 7. In these figures, the same functional blocks and the same processes as those shown in FIGS. 1 and 2 are designated by the same reference numerals.

The one belonging to the selected representative block Br is the normalization unit 13A.
Given to. The normalization unit 13A is provided with a normalization coefficient initial value D ₀ of a sufficiently low value so that the compressed data length of any image data does not exceed the target value.
The value of the representative block Br normalized by the normalization unit 13A using the initial value D ₀ is stored in the memory 12A (steps 23A, 33). When the normalized values in each block are arranged in a one-dimensional array by zigzag scanning, the normalized values arranged in the rear part are often 0 in many cases. When continuous 0s exist over a certain range until the end, an EOB (End of Block) code is added to the last non-zero value (step 33), and 0 data after that is discarded. Instead of actually adding the EOB, the position (order) to which the EOB should be added may be stored.

The normalized value of the representative block Br is then sequentially encoded, and the code amount is counted by the counter 15 (steps 24 and 25). Counter 15 is a representative block Br for one screen
The encoded data length of is counted. Since the initial value D ₀ of the normalization coefficient is selected to be a small value as described above, the count value of the counter 15 obtained for the image data of the representative block Br for one screen generally exceeds the target value (step 26
A), the normalization coefficient D is selected by the normalization coefficient setting unit 16A to be one step larger, and is supplied to the normalization unit 13A via the switching unit 17 (step 34).

The data of the normalized representative block Br stored in the memory 12A is normalized again using the new normalization coefficient D, and the result is stored in the memory 12A (step 2
3A, 33). Since this normalization processing may be performed up to the position of the EOB code, the normalization processing amount is smaller than that in the case of normalizing all representative block data, and the processing time can be shortened. The previous normalization coefficient is D ₁ and the current normalization coefficient is D ₂
Then, the representative block B stored in the memory 12A
Since the normalized data of r has already been normalized by the coefficient D ₁ , it will be normalized by dividing by the coefficient D ₂ / D ₁ this time. It goes without saying that if the range in which 0s continue is widened by this new normalization, the EOB addition position is changed to the position next to the last non-zero data of the new normalized data. In this way, the data amount of the representative block to be normalized is reduced.

After that, the encoding and the counting process of the encoded data are performed in the same manner, and the obtained count values of all the representative blocks for one screen are compared with the target value (step 26A). If there are more count values, a larger value is selected as the normalization coefficient, and the above process is repeated (step
34, 23A, 33, 24, 25).

When the count value falls below the target value, the entire screen is normalized with the obtained optimum normalization coefficient, coded and output as compressed data, and then written in the memory (steps 28, 2).
9,30).

It goes without saying that in the functional block diagrams shown in FIGS. 1 and 6, any part may be realized by a hardware circuit or software processing using a binary computer.

[Brief description of drawings]

FIG. 1 is a functional block diagram showing an embodiment of the present invention, and FIG.
The flow chart shows the processing procedure of the block diagram shown in FIG.
A chart, FIG. 3 is a diagram showing a relationship between divided blocks on a screen and a representative block, FIG. 4 is a graph showing a conversion table, and FIG. 5 is a diagram showing an example of creating a representative block. 6 and 7 show another embodiment. FIG. 6 is a functional block diagram and FIG. 7 is a flow chart. 11 …… DCT section (orthogonal transformation section), 12,12A …… memory, 13,13A …… normalization section, 14 …… encoding section, 15 …… counter, 16 …… conversion table, 16A …… normalization Coefficient setting section.

Claims (5)

[Claims] 1. A screen is divided into blocks each made up of a plurality of pixels, and the original image data of the pixels contained in each block is subjected to orthogonal transformation to obtain orthogonal transformation coefficients for one screen. In the image data compression processing method, in which the obtained orthogonal transform coefficient is normalized using the normalization coefficient and a predetermined code is assigned to each of the normalized values to obtain encoded data, the compression processing of the original image data is performed. Then, a block group is formed by a plurality of adjacent blocks, representative blocks representative of these blocks are extracted from each block group, and the above orthogonal transformation, normalization, and encoding are performed on the extracted representative blocks. For each representative block, the data amount of the encoded data obtained by each is counted, and the count value is within the specified range. While changing the normalization coefficient so that it fits in, the optimum normalization coefficient is determined by repeating the normalization, encoding, and data counting processing. After determining the optimum normalization coefficient in this way, compression of the original image data is performed. Moving to the processing, in this compression processing, the above-mentioned orthogonal transform coefficient is normalized using the determined optimum normalization coefficient, and the normalized value is encoded to obtain the final compressed image data. Compression processing method. 2. The image data compression processing method according to claim 1, wherein the representative block is selected from a plurality of blocks. 3. The image data compression processing method according to claim 1, wherein the representative block is created by calculating an average value of corresponding pixel data of a plurality of blocks for each pixel data of the representative block. 4. The image data compression processing method according to claim 1, wherein a new normalized value is obtained by normalizing the previous normalized value in the second and subsequent normalization processing. 5. A screen is divided into blocks each consisting of a plurality of pixels, and the original image data of the pixels included in the block is subjected to orthogonal transformation for each block to obtain orthogonal transformation coefficients for one screen. In the image data compression device that obtains encoded data by normalizing the obtained orthogonal transform coefficient using the normalization coefficient and assigning a predetermined code to each of the normalized values, prior to the compression processing of the original image data, , Blocks that form a block group from multiple adjacent blocks
Group forming means, representative block extracting means for extracting a representative block representing each block from each block group formed by the block group forming means, the orthogonal transformation, normalization and encoding, the representative block Counting means for performing each of the representative blocks extracted by the extracting means and counting the data amount of the coded data obtained thereby over all the representative blocks, so that the count value counted by the counting means falls within a predetermined range. The normalization coefficient determining means for determining the optimum normalization coefficient by repeating the normalization, encoding and data counting processing while changing the normalization coefficient, and the optimum normalization coefficient determined by the normalization coefficient determining means After determining, move to the compression processing of the original image data, And compressing the orthogonal transformation coefficient by using the determined optimum normalization coefficient, and encoding the normalized value to obtain final compressed image data. Processing equipment.