JPH05176173A - Picture data compression method - Google Patents

Abstract

PURPOSE:To facilitate selection of a compression parameter by which a decoded picture is obtained with stable picture quality by obtaining a picture quality index obtained through the comparison between original picture data and expanded picture data and displaying the index together with a processing object picture. CONSTITUTION:A compression circuit 302 of a picture data compressor 3 selects one set of compression parameter registered in a parameter management table 303 and compresses original picture data Do to generate compressed picture data Dc. An expansion circuit 304 expands the data Dc to generate expanded picture data Dr. An arithmetic operation circuit 307 compares the data Do, Dr, a discrimination circuit 308 obtains a picture quality index representing the picture quality of the decoded picture to generate picture data Dir and the data are displayed on a monitor 312 together with picture data Drr of a reduced picture. Thus, the operator has only to discriminate whether or not the picture quality rank to be displayed reaches a prescribed level and the compression parameter by which a decoded picture is obtained with stable picture quality is easily selected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for compressing image data for printing and plate making, and more particularly to a method for compressing multivalued image data by lossy coding.

[0002]

2. Description of the Related Art Since image data for printing plate making has a huge amount of data, a huge memory capacity is required to store the image data as it is, and a large amount of time is required for data transfer. Therefore, in general, an image data compression method is used in which the image data is encoded and the image data is compressed to reduce the data amount.

Techniques such as so-called vector quantization and orthogonal transformation are used as a method for encoding multi-valued image data. Discrete cosine transform (hereinafter referred to as "DCT transform") and Hadamard transform are known as the orthogonal transform. These encoding methods can compress the image data at a high compression rate, but on the other hand, the image data obtained by restoring the compressed image data does not completely match the image data before compression, This is a so-called lossy encoding method. Since the amount of multi-valued image data for commercial printing is enormous, it is one of the items in which the need for compression by the lossy encoding method is particularly high.

By the way, in the image for commercial printing, high image quality (for example, smoothness of skin, sharpness of edges, etc.) is required. However, when the image data compressed by the irreversible encoding method is restored and the image is reproduced, the deterioration of the image quality may be visible to the naked eye depending on the image, and the image may not be used as an image for commercial printing. Generally, the higher the compression rate, the greater the deterioration of the image quality of the restored image.

As a method of increasing the compression rate while suppressing the deterioration of the image quality of the restored image, the method described in Japanese Patent Laid-Open No. 62-57367 is known. In this conventional method, a plurality of compression parameters for realizing a plurality of compression ratios are prepared, and an error image between a restored image corresponding to each compression ratio and an original image is displayed. Then, one error image is selected from a plurality of error images by visual judgment, and the original image is compressed using a compression parameter corresponding to the error image.

[0006]

In the above-mentioned conventional method, one error image is selected from a plurality of error images by visual judgment. However, which error image is selected, sufficient image quality can be obtained. The decision as to whether to be made depends on the experience and skill of the operator. Therefore, for inexperienced operators,
It is difficult to choose a compression parameter that gives satisfactory image quality. In addition, even an experienced operator cannot always select an error image corresponding to the same image quality, so that the quality of the restored image may vary. As described above, conventionally, there is a problem that it is difficult to select a compression parameter so that a restored image with stable quality can be obtained.

The present invention has been made to solve the above problems in the prior art, and an object thereof is to provide an image data compression method capable of obtaining a restored image of stable quality.

[0008]

In order to solve the above-mentioned problems, the invention described in claim 1 is a method for compressing multi-valued image data by lossy encoding, which comprises (a) a plurality of sets of compression. Preparing parameters, and (b) selecting at least one set of the plurality of sets of compression parameters,
(C) generating compressed image data by irreversibly encoding and compressing the original image data using the selected compression parameter, and (d) using the selected compression parameter to compress the compressed image data. And (e) an image quality indicating the image quality of the restored image represented by the expanded image data by comparing the original image data with the expanded image data. A step of obtaining an index, and (f)
Compressing the image quality of the restored image to a predetermined quality level by performing (g) the steps (b) to (f) at least once, displaying the image quality index together with the image to be processed. Selecting a parameter.

Further, in the image data compression method according to the second aspect, in the step (e), the S / N ratio of the compressed image data is calculated, and the image quality rank determined in advance is based on the S / N ratio. According to the definition, determining the image quality rank of the restored image as an image quality index, and step (f)
Includes displaying the image quality rank together with the image to be processed.

In the image data compression method according to the third aspect, in the step (e), the image quality index is determined for each pixel block of a predetermined size in the image to be processed, and the image quality index is predetermined. Identifying pixel blocks whose image quality index does not reach the threshold value by comparing with a threshold value, step (f) revealing pixel blocks whose image quality index does not reach the threshold value. Further, the step (g) includes a step of selecting, for each pixel block, a compression parameter at which the image quality of the restored image reaches the threshold value.

[0011]

According to the method of claim 1, the image quality index obtained by comparing the original image data and the decompressed image data is obtained and displayed together with the image to be processed. Therefore, the image quality index is selected by looking at the image quality index. The image quality achieved by the compression parameters can be seen quantitatively. And
Since the compression parameter that reaches the predetermined quality level is selected by comparing the displayed image quality index with the predetermined quality level, it is possible to select the compression parameter that can obtain a restored image of stable quality.

According to the method of claim 2, since the image quality rank according to the S / N ratio is displayed, the compression parameter can be easily selected so that the image quality rank of the restored image reaches a predetermined level.

According to the method of claim 3, the image quality index is compared with a predetermined threshold value for each pixel block, a pixel block whose image quality index does not reach the threshold value is specified, and the pixel block is also specified. Since the compression parameter is selected so that the image quality index reaches the threshold value, the compression parameter can be easily selected so that the entire image reaches the predetermined image quality level.

[0014]

EXAMPLES A. First Embodiment FIG. 1 is a block diagram showing the configuration of an image processing system including an image data compression device to which an embodiment of the present invention is applied. This image processing system uses the image data Do of the original image.
A reading scanner 1 for reading (hereinafter referred to as “original image data”), a first switch 2, an image data compression device 3 for compressing the original image data Do, a second switch 4,
The interface 5 and the image processing device 6 are provided.
In the state of FIG. 1, the original image data Do is given to the image data compression device 3 and compressed. The generated compressed image data Dc is given to the image processing device 6 via the switch 4 and the interface 5. When the switches 2 and 4 are switched, the original image data Do is directly given to the image processing device 6 via the interface 5.

FIG. 2 is a block diagram showing the internal structure of the image data compression device 3. The original image data Do supplied from the reading scanner 1 is given to the compression circuit 302 via the first buffer 301. The compression method (Hadamard transform, DCT transform, etc.) and the compression parameter (quantization table, coding code, number of coding coefficients to be stored, etc.) for determining the compression rate are controlled by the control unit 3 according to the operator's designation.
It is registered in the parameter management table 303 by 30. At this time, a plurality of sets of compression parameters having different compression rates are registered. The compression circuit 302 selects a set of compression parameters registered in the parameter management table 303 and compresses the original image data Do according to the compression parameters to generate compressed image data Dc. The compressed image data Dc is output from the image data compression device 3 and given to the image processing device 6 via the switch 4 and the interface 5 (FIG. 1).

The image data compression device 3 further includes a decompression circuit 304. The decompression circuit 304 decompresses (decompresses) the compressed image data Dc to generate decompressed image data Dr. The decompressed image data Dr is given to the arithmetic circuit 307 via the second buffer 305. The decompressed image data Dr is also given to the reduction circuit 320 via the switches 321 and 322, where the image is reduced (for example, thinned) and the reduced image data Dr representing the reduced image is obtained.
r is generated. The reduced image data Drr is displayed on the switch 3
23 to the frame buffer 310.

In synchronization with the fact that the decompressed image data Dr is given from the decompression circuit 304 to the arithmetic circuit 307 via the second buffer 305, the original image data Do output from the first buffer 301 is also the third image data Do. It is given to the arithmetic circuit 307 via the buffer 306.

The arithmetic circuit 307 has a function of calculating an error between the original image data Do and the expanded image data Dr. In this embodiment, the S / N ratio is used as an error index. The S / N ratio is defined by the following equation. S / N = 20.log [Dmax / (MSE) ^ 0.5] (1) Here, the operator "log" represents common logarithm, and the operator "^" represents exponentiation. Dmax is the maximum value that the image data can have, and for example, for 8-bit image data, Dmax = 255. MSE is the mean square error and is defined by the following equation. MSE = ΣΣ [{Do (i, j) -Dr (i, j)} ^ 2] / (M × N) (2) ”where M is the vertical direction of the image to be processed. The number of pixels, N is the number of pixels in the horizontal direction. Further, i and j are pixel coordinates in the vertical direction and the horizontal direction, respectively, and the operator ΣΣ accumulates the values in parentheses from 1 to M for i, and the values in parentheses from 1 to N for j as well. It shows an operation for accumulating.

When the image data is composed of image data components corresponding to four color inks of yellow (Y), magenta (M), cyan (C), and black (K), the arithmetic circuit 307 is The S / N ratio of each color component is calculated, and the average value Asn (dB) of the S / N ratios of the four colors is calculated.

The average value Asn of the S / N ratio is calculated by the arithmetic circuit 307.
To the discrimination circuit 308. The discrimination circuit 308
The image quality rank is determined according to the following conditions. Image quality rank 1:50 <Asn (dB) Image quality rank 2:45 <Asn ≦ 50 (dB) Image quality rank 3:40 <Asn ≦ 45 (dB) Image quality rank 4:35 <Asn ≦ 40 (dB) Image quality rank 5: 30 <Asn ≦ 35 (dB) Image quality rank 6: Asn ≦ 30 (dB) The discriminant circuit 3 shows the relationship between the image quality rank and the average value Asn.
It is previously registered in the memory 309 connected to 08.

The discriminating circuit 308 further generates image data Dir for displaying the image quality rank thus determined, and writes it in the frame buffer 310. As described above, the image data Drr of the reduced image is written in the frame buffer 310. The frame image data representing the reduced image and the image quality rank is stored in the frame buffer 31.
A monitor 312 is provided from 0 through the D / A converter 311.

FIG. 3 is a conceptual diagram showing a reduced image RI and a table of image quality ranks displayed on the monitor 312. In the example of FIG. 3, it is displayed that the image quality rank is 2.
The image quality rank of the restored image can be determined in advance depending on the type and use of the image. Therefore, the operator can easily determine from the display of the image quality rank whether the restored image has reached the desired image quality level. Since the image quality rank depends on the compression parameter, if the image quality rank does not reach the desired level, the compression parameters for obtaining the restored image with better image quality are stored in the parameter management table 303. The compression parameters may be selected to achieve the desired image quality rank. When selecting the compression parameter, the monitor 312 displays a selection branch for selecting a plurality of sets of compression parameters registered in the parameter management table 303, and the operator selects one of them.

FIG. 4 is a flow chart showing a processing procedure until a desired image quality rank is achieved. Step S1
After compressing the original image data in step S2, the reduced image RI as shown in FIG. 3 is displayed in step S2. The reduced image RI is an image obtained by reducing the restored image represented by the decompressed image data. In step S2, the image quality rank is also displayed at the same time. In step S3, the operator monitors 3
Observing No. 12 and looking at the table of image quality ranks, it is determined whether the image quality has reached a desired level. If the image quality has not reached the desired level, the compression parameter is changed in step S4, and steps S1 to S are performed again.
Execute 3.

As described above, in the first embodiment, the operator only has to determine whether or not the displayed image quality rank has reached a predetermined level, so that a compression parameter for obtaining a restored image of stable quality is obtained. There is an advantage that can be selected.

In the image processing system of FIG. 1, the image data is read in the following procedure. First, the reading scanner 1 performs a preliminary scan (prescan) to obtain image data of a reduced image. Preliminary scanning is the process of reading image data while thinning out,
It is performed before the main scan for reading image data without thinning. This reduced image data is the image data compression device 3
A compression parameter is selected that provides sufficient image quality as described above. Then, the image data is read in the main scan, and the image data compression device 3 compresses the image data using the same compression parameters as in the preliminary scan. Since the read time of the preliminary scan is shorter than that of the main scan, the compression parameter can be determined in a shorter time by determining the compression parameter in the preliminary scan.

The compressed image data is transmitted from the image data compression device 3 to an external device (image processing device 6 or other device).
When outputting to, the compression parameters are transferred together with the compressed image data.

B. Second Example In the first example, as shown in the equation (1), the image quality of the entire image was evaluated by obtaining the S / N ratio of the entire image.
In the second embodiment, the image is divided into blocks of m × n pixels, the S / N ratio is obtained for each block, and the image quality is evaluated for each block. When the image data is compressed by DCT conversion, the image data is compressed for each pixel block (for example, a block of 8 × 8 pixels). Further, in order to increase the compression rate of the entire image, it is possible to change the number of transform coefficients (also one of the compression parameters) stored as compressed image data for each pixel block. In this case, since the error level between the decompressed image data and the original image data differs depending on the pixel block, it is preferable to obtain the S / N ratio for each pixel block.

In the second embodiment, the arithmetic circuit 307 obtains the S / N ratio of each image data component of YMCK for each pixel block, and the average value Asn of the S / N ratios of these four components for each pixel block. calculate. Memory 309
Stores a threshold value (for example, 30 dB) designated in advance by the operator. The discrimination circuit 308
The threshold value stored in the memory 309 and the average value Asn of the S / N ratio are compared for each pixel block, and the average value As
A pixel block in which n is smaller than the threshold value is detected as an image quality deterioration block. The determination circuit 308 further creates image data for painting the image quality deterioration block,
It is supplied to the monitor 312 via the frame buffer 310 and the DA converter 311.

FIG. 5 shows a monitor 31 in the second embodiment.
It is a conceptual diagram which shows the example of the image displayed on 2. The entire reduced image RIa is displayed lightly, and the image quality deterioration block (indicated by diagonal lines in the drawing) is clearly shown in a color such as red or a reversed color that is easily visible. Also, the boundaries of the pixel blocks are displayed. In the example of FIG. 5, since the image quality of the character portion of the book is greatly deteriorated, the pixel block including many character portions of the book is the image quality deterioration block.

If the image quality deterioration block is not a very important part of the entire image and it is not necessary to improve the image quality, the operation is terminated. On the other hand, when it is necessary to improve the image quality of the image quality deterioration block, the operator selects the image quality deterioration block with a mouse or the like, and
The compression parameter applied to the pixel block is changed to a compression parameter with which a restored image with higher image quality can be obtained (that is, a compression parameter with a low compression rate). Then, the image data of the pixel block is compressed again. This procedure is almost the same as that shown in FIG.

As in the second embodiment, if the compression parameter of the image quality deteriorated pixel block is changed to one having a lower compression rate, the compression rate of the image data of the entire image is not excessively reduced. There is an advantage that the quality of the entire image can be easily improved by improving the image quality of the portion where the image quality is easily deteriorated (for example, the portion including fine characters).

The present invention is not limited to the above-described embodiment, but can be carried out in various modes without departing from the scope of the invention, and the following modifications can be made.

(1) In the first embodiment, the calculated image quality rank is displayed on the monitor 312. However, the control section 330 further determines the image quality rank obtained by the discrimination circuit 308 and a preset image quality rank. You may make it compare with the required level of. At this time, when the image quality rank does not reach the required level, the control unit 330 sequentially selects a plurality of sets of compression parameters registered in the parameter management table 303, and automatically obtains the compression parameter at which the image quality rank reaches the required level. It is also possible to do so.

(2) In the first and second embodiments, YM
Although the image quality was evaluated by obtaining the average value Asn of the respective S / N ratios of the four color image components of CK, the image quality was evaluated using the S / N ratio of at least one image component of YMCK. May be. Further, the present invention is not limited to image data composed of YMCK image components, but can be applied to compression of image data composed of BGR components and uniform color space components. It is also applicable to image data containing only one color image component. In general, the present invention can be applied to the compression of multi-valued image data.

(3) Although the S / N ratio, the average value Asn, or the image quality rank is used as the image quality evaluation index in the above embodiment, any other image quality evaluation index may be used.

(4) In the above embodiment, the image data is compressed by encoding the image data by the DCT transform, but the present invention is generally applicable to the case of compressing multi-valued image data by the lossy encoding. For example, the present invention can also be applied to compression by orthogonal transform coding such as Fourier transform or Hadamard transform or vector quantization coding.

(5) In the first embodiment, the image quality rank of the entire image is displayed as shown in FIG. 3, but as shown in FIG. 6, the image quality of the area designated by the operator is displayed. May be. At this time, the operator may specify the positions of the diagonal end points P1 and P2 of the rectangular area for which the image quality rank is to be calculated on the monitor 312 by using a mouse or the like.

[0038]

As described above, according to the method described in claim 1, the image quality index obtained by comparing the original image data and the decompressed image data is obtained and displayed together with the image to be processed. Therefore, by looking at the image quality index, the image quality achieved by the selected compression parameter can be seen quantitatively. Then, by comparing the displayed image quality index with a predetermined quality level,
Since a compression parameter that reaches a predetermined quality level is selected, there is an effect that a compression parameter that can obtain a restored image of stable quality can be selected.

In the method of claim 2, since the image quality rank according to the S / N ratio is displayed, it is possible to easily select the compression parameter so that the image quality rank of the restored image reaches a predetermined level. effective.

According to the method of claim 3, the image quality index is compared with a predetermined threshold value for each pixel block, a pixel block whose image quality index does not reach the threshold value is specified, and the pixel block is also specified. Since the compression parameter is selected so that the image quality index reaches the threshold value, it is possible to easily select the compression parameter so that the entire image reaches a predetermined image quality level.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of an image processing system including an image data compression device to which an embodiment of the present invention is applied.

FIG. 2 is a block diagram showing an internal configuration of an image data compression device.

FIG. 3 is a conceptual diagram showing a reduced image displayed on a monitor and a table of image quality ranks.

FIG. 4 is a flowchart showing a processing procedure until a desired image quality rank is achieved.

FIG. 5 is a conceptual diagram showing an example of an image displayed on a monitor in the second embodiment.

FIG. 6 is a conceptual diagram showing a modification of the first embodiment.

[Explanation of symbols]

 1 Scanning Scanner 2 Switch 3 Image Data Compressor 4 Switch 5 Interface 6 Image Processor 301 Buffer 302 Compression Circuit 303 Parameter Management Table 304 Decompression Circuit 305 Buffer 306 Buffer 307 Arithmetic Circuit 308 Discrimination Circuit 309 Memory 310 Frame Buffer 311 D / A Converter 312 monitor 320 reduction circuit 321 switch 322 switch 323 switch 330 control unit Dc compressed image data Dir image data of image quality rank table Do original image data Dr expanded image data Drr reduced image data RI reduced image

Claims (3)

[Claims] 1. A method for compressing multivalued image data by lossy encoding, comprising: (a) preparing a plurality of sets of compression parameters; and (b) at least one set of the plurality of sets of compression parameters. And (c) generating compressed image data by lossy encoding and compressing the original image data using the selected compression parameter, and (d) using the selected compression parameter. Decompressing the compressed image data to generate decompressed image data, and (e) comparing the original image data with the decompressed image data to obtain an image of a decompressed image represented by the decompressed image data. A step of obtaining an image quality index indicating quality; (f) a step of displaying the image quality index together with an image to be processed; (g) the step (b) Step (f) is at least 1
The image data compression method comprises the step of: selecting a compression parameter by which the compression parameter is reached such that the image quality of the restored image reaches a predetermined quality level. 2. The image data compression method according to claim 1, wherein the step (e) calculates the S / N ratio of the compressed image data and sets a predetermined image quality rank based on the S / N ratio. The image data compression method includes the step of determining the image quality rank of the restored image as an image quality index according to the definition of, and the step (f) displaying the image quality rank together with the image to be processed. 3. The image data compression method according to claim 1, wherein the step (e) obtains an image quality index for each pixel block of a predetermined size in the image to be processed and Identifying a block of pixels whose image quality index does not reach said threshold by comparison with a predetermined threshold, step (f) comprising pixels whose image quality index does not reach said threshold An image data compression method including a step of clearly indicating a block, and step (g) including a step of selecting, for each pixel block, a compression parameter at which an image quality of a restored image reaches the threshold value.