JP2000354170A - Image compressor, its method and printer with the same image compressor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To guarantee reversibility of a specified pixel in the case of decoding and to prevent image degradation of a contour part by storing position, density value information as a compressed result for the specified pixel to which high space frequency is given on an inputted image. SOLUTION: An inputted 256 gradation image is scanned, a specified image to satisfy a specified condition is detected and its position and density value are stored by a specified pixel detecting means 10. Namely, when characters are overwritten on a dark image, the brightest part is generally detected as the specified pixel, the reversibility of image quality to constitute the characters is guaranteed and the image degradation of the characters is prevented. The density value is replaced with a specified density value by receiving the position and density value information of the inputted image and the specified pixel, an intermediate image by which high compression rate is achieved without image degradation is generated by an intermediate image generating means 11, the intermediate image is compressed by receiving the position and density value information of the intermediate image and the specified pixel, its result is calculated, defined as a compressed result of the intermediate image, the result, the position of the specified pixel and the density value are outputted as the compressed results by an image compressing means 12.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image compression technique.
In particular, an image that is used for data handled by printers, copiers, scanners, facsimile machines, displays, etc., and that can achieve high compression ratios without deteriorating image quality in images that contain content such as photographs and text Related to compression technology.

2. Description of the Related Art In a device such as a printer, it is required to hold a large number of input images. Therefore, in general, an image is compressed in the device. As an algorithm for performing such image compression, for example, there is an algorithm based on the JPEG method.

An image compression method according to the JPEG method is DC
It is based on a method of performing irreversible encoding based on T (discrete cosine transform). Specifically, the input image is 8 ×
It is divided into 8-pixel blocks, and the DCT is
Perform the operation. Then, a DC component (DC component) and an AC component (AC component) are independently quantized with respect to the obtained DCT coefficient, and the difference between the DC component coefficient and the DC component coefficient of the immediately preceding block is used as a predicted value. , And Huffman coding is performed on the coefficient of the AC component.

In a printer or the like, since it is necessary to input and output a wide variety of images from a photograph to a text, an image compression technique suitable for both a photographic image and a text image is required. .

However, in the conventional image compression techniques, those suitable for compressing photographic images and the like are not suitable for compressing text images and the like, while those suitable for compressing text images and the like are not suitable for compressing photographic images and the like. There was a problem that it was not suitable for compression.

For example, the JPEG method can support many color spaces and can achieve a high compression ratio particularly when there is little change in image density. It is a suitable compression method.

However, the compression ratio is deteriorated for an image having a sharp outline, and high-frequency components are
This is not suitable for compressing a text image or the like because there is a problem in that the picture quality around the contour (edge portion) in the decoded image is degraded due to dropout in the expansion process.

Accordingly, when the JPEG method is applied to a case where characters or graphics are overwritten in a photographic image, a sufficient compression ratio cannot be achieved as compared with a case where no characters or the like are present. The problem that outlines of characters and the like are blurred occurs.

On the other hand, outline information, which is a high-frequency component, is important for characters and the like, and it is desirable that such a portion be compressed by a reversible method in which the high-frequency component is not lost.

Accordingly, the present invention ensures that reversibility is assured for text and the like in which high-frequency components have important information, thereby preventing image quality deterioration and achieving a high compression ratio. It is an object to provide a simple image compression technique.

According to the present invention, a pixel which gives a high-frequency spatial frequency on an input image is detected as a specific pixel, and the specific pixel is replaced with a pixel which gives a low-frequency spatial frequency in the input image. The image compression is performed, and the result of the compression and the information on the position and the density value of the specific pixel are used as the compression result of the input image.

Further, according to the present invention, a pixel satisfying a predetermined condition is detected as a specific pixel by scanning an input image, and information on the position and density value of the specific pixel is stored in a storage means. After the density value of the specific pixel is replaced with a predetermined density value, image compression is performed by the JPEG method, and the compression result by the JPEG method and the stored information on the position and density value of the specific pixel are output as the compression result of the input image. It is characterized by doing.

The image compression method of the present invention can be implemented by a computer, and a computer program for the method is installed or loaded on the computer through various media such as a CD-ROM, a magnetic disk, a semiconductor memory, and a communication network. Can be.

The present invention also provides a specific pixel detecting means for scanning an input image, detecting a pixel satisfying a predetermined condition as a specific pixel, and storing information on the position and density value of the specific pixel. An intermediate image generating means for generating an intermediate image by replacing the density value of the specific pixel with a predetermined density value, and performing image compression on the intermediate image according to the JPEG method; Image compression means for outputting information on the position and density value of the specific pixel as an input image compression result.

It is preferable that the specific pixel detecting means detects a pixel which gives a maximum density value in the input image, a pixel which gives a minimum density value in the input image, or both, as at least a specific pixel.

It is preferable that the specific pixel detecting means detects a pixel having a density value or an edge intensity within a predetermined range as at least a specific pixel, and the predetermined range is calculated based on an input image. preferable.

The intermediate image generating means may calculate the density value of the specific pixel as an average density value of an input image, an average density value of pixels surrounding the specific pixel, or a pixel other than a pixel detected as the specific pixel. It is preferable to replace with any one of the above average density values.

Further, according to the present invention, the intermediate image compression result is subjected to image expansion by the JPEG method, and the density value of a pixel at the position of the specific pixel in the expanded image is replaced with the density value of the specific pixel. Therefore, it is preferable to provide an image decoding means for performing image decoding.

[Effect] Normally, when characters or figures are present in an image,
In order to be distinguishable from the background portion, the luminance value of a character or the like is set to be high when the background is dark, and to be low when the background is bright. That is, since the luminance value changes rapidly in the outline portion of a character or the like, the pixel of the outline portion gives a high-frequency spatial frequency component among the spatial frequency components of the entire image.

Therefore, according to the present invention having the above configuration, for a pixel (specific pixel) that gives a high-frequency spatial frequency on an input image, information on the position and density value is stored as a compression result, so that the specific It is possible to guarantee the reversibility of the pixel at the time of decoding and prevent the image quality of the outline portion from deteriorating.

In the JPEG system, a high compression ratio can be achieved when the change in image density is small,
By replacing the luminance value of a specific pixel that provides a high-frequency component with a luminance value that provides a low-frequency component, an image suitable for the JPEG method can be converted.

Therefore, according to the present invention having the above configuration, image compression is performed after replacing the luminance value of a specific pixel with a luminance value giving a low-frequency spatial frequency. be able to.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described with reference to FIG. The image compression apparatus 1 according to the first embodiment of the present invention includes a specific pixel detection unit 10, an intermediate image generation unit 11, and an image compression unit 12.

Here, the description is made on the assumption that the input image is a grayscale image of 256 gradations. However, the input image may be a color image. In this case, the present invention may be applied to each plane such as RGB. .

The specific pixel detecting means 10 scans the input image, detects specific pixels satisfying predetermined conditions, and stores information on the position and density value of the specific pixels.

Here, the predetermined condition may be various depending on what portion in the image is desired to guarantee reversibility at the time of decoding.

For example, if it is desired to guarantee reversibility for the brightest part, the predetermined condition may be to give the maximum density value in the image.

Generally, when a character or the like is overwritten on a dark image, the character or the like is written in a lighter color (a part such as a character has a higher density value than other parts in the image). (See FIG. 2). Therefore, by detecting the brightest part as a specific pixel, reversibility can be assured for pixels constituting characters and the like,
It is possible to prevent image quality deterioration of characters and the like.

On the other hand, depending on the conditions at the time of reading an image by a scanner or the like, even if a character or the like is overwritten by the same person at the same time, the density value of pixels constituting the character or the like does not become constant. May be distributed in a certain range. FIG. 4 shows an example of an input image in such a case.
In FIG. 4, the numerical value on the input image represents the density value of each pixel.

In the example shown in FIG. 4, the density values of the pixels constituting the character 400 are distributed in the range of 248 to 255. In such a case, assuming that the maximum density value in the image is given as the predetermined condition, only a part of pixels constituting characters or the like can be detected as specific pixels.

Therefore, in such a case, the pixels constituting characters or the like are detected as specific pixels by setting the density value within a predetermined range as the predetermined condition.
In the case of the example of FIG. 4, if the threshold value is set to 248 and a pixel having a density value equal to or higher than the threshold value is detected as a specific pixel, all the pixels constituting the character 400 can be detected. it can.

Here, the threshold value for defining the predetermined range may be given from the outside or may be stored in advance. Also, the threshold value can be automatically determined based on the input image, and in that case, various conventional techniques can be used. For example, the density value is small (or large) using the density histogram of the input image.
Calculate the density value at which the cumulative distribution from the two becomes P percent, and use the P tile method as a threshold, the mode method to find the valley of the histogram and set the threshold, and the discriminant analysis to maximize the class separation , A difference histogram method using a histogram in which the frequencies in the density histogram are weighted by the sum of the differences in the neighborhood, and a density histogram for only the pixels having a large absolute value of the second derivative. Various threshold value selection methods such as a method of creating and using a mode method or a discrimination threshold value selection method can be used. These prior arts are described in detail in "Image Processing Handbook, Shokodo Co., Ltd., First Edition, 5th Edition, 1992".

For the detected specific pixel, for example, (X coordinate, Y coordinate, density value) is stored as its position and density value information. In the example of FIG. 4, the information on the position and the density value of the specific pixel is (1, 1, 255) (1, 2, 255).
(1,3,255) (2,2,248) (3,2,24
8) (4, 1, 250) (4, 2, 250) (4, 3,
250).

On the other hand, if it is desired to guarantee reversibility for the darkest part, the predetermined condition may be to give the minimum density value in the image.

Generally, when a character or the like is overwritten on a bright image, the character or the like is written in a dark color (see FIG. 3). Therefore, by detecting the darkest part as a specific pixel, reversibility can be assured for pixels constituting characters and the like, and image quality degradation of characters and the like can be prevented.

In the case where the density values of pixels constituting characters and the like are distributed below a certain density value, the predetermined condition is that the density value is below a predetermined threshold value. This makes it possible to detect a pixel constituting a character or the like as a specific pixel. The threshold value in such a case can be determined in the same manner as in the case described above.

Here, when it is desired to guarantee reversibility for both the brightest part and the darkest part, the predetermined condition may be to give the maximum density value or the minimum density value in the image. By determining the conditions in this way, it is possible to detect a character or the like as a specific pixel even if it is unknown whether the character or the like is overwritten with a bright color or a dark color in the image. Become.

Further, when it is desired to guarantee reversibility of an edge portion in an image, the predetermined condition is that the edge strength calculated for each pixel is within a certain range (for example, a predetermined threshold value or more). Good.

In general, it is expected that the density of a character or the like is sufficiently different from that of a background image. It is considered to be composed. Therefore, by detecting an edge portion as a specific pixel, reversibility can be guaranteed for pixels in a contour portion such as a character, and blurring of a contour portion of a character or the like can be prevented.

Here, as a method of calculating the edge strength, a conventional edge detection technique can be used. For example, So
The vertical differential and horizontal spatial differential values are obtained by the bel operator, and the root of the sum of the squares can be used as the edge strength. As a method of determining the threshold value, the above-described various conventional techniques can be applied by replacing the density value with the edge intensity.

The intermediate image generating means 11 receives the input image and the information on the position and the density value of the specific pixel, and replaces the density value of the specific pixel in the input image with a predetermined density value. Generate an image. Here, the image suitable for the JPEG method means an image that can achieve a high compression ratio without deteriorating the image quality.

In the JPEG method, a high compression ratio can be achieved when the high-frequency component is small in the target image. Therefore, the density value of a specific pixel is replaced so that the high-frequency component is reduced in the input image. To an intermediate image suitable for. This means that a specific pixel is smoothed, and various smoothing techniques can be used in such processing.

For example, the average value of the density value is calculated for pixels in a certain range, and the specific pixel is smoothed by replacing the density value of the specific pixel with the average value. As the average value, for example, an average density value of all pixels of the input image,
An average density value of pixels other than the specific pixel among all the pixels of the input image, an average density value of surrounding pixels within a certain range of the specific pixel to be replaced, and the like can be used. Also, based on the concept of the median filter,
A median may be used instead of the average.

FIG. 5 shows an intermediate image generated by replacing the density value of a specific pixel with the average density value of pixels other than the specific pixel in all the pixels of the input image in the example of FIG.

The image compressing means 12 receives the intermediate image and the information on the position and density value of the specific pixel, and compresses the intermediate image according to the JPEG method.

The image compression procedure according to the basic JPEG system will be described below. There are several variations in the JPEG system, and in the present invention, these variations can be selected according to the design. it can.

First, the intermediate image is divided into blocks of 8 × 8 pixels, and a frequency distribution is obtained by DCT for each of the divided partial images. FIG. 6A shows an example of the partial image.
(B) shows an example of a frequency distribution obtained by DCT.
Next, quantization is performed by dividing each element of the frequency distribution by each element of a predetermined quantization matrix. FIG. 6C shows an example after quantization.

Here, the value of the DC component (the upper left element) is directly used as the compression result. AC components (elements other than the upper left) are zigzag-scanned and arranged in a row, run-length compressed, Huffman-encoded, and the encoded sequence is used as a compression result. Note that run-length compression and Huffman coding are well-known techniques in the field of image compression, and are described, for example, in “Introduction to Illustrated Digital Image Processing”, Sanno University Press, 1996.

The image compression means 12 obtains the compression result for each partial image as described above, and stores the compression results for all the partial images as intermediate image compression results. Then, the intermediate image compression result and the information on the position and density value of the specific pixel are output as an input image compression result.

Here, the image compressing means 12 may output the information of the position and density value of the specific pixel after performing compression by the following method, for example.

First, label the input image.
N-value. Here, N is 1 for the number of types of density values of a specific pixel.
Is the value added. More specifically, N-value conversion is performed by assigning a label to each density value for a specific pixel and assigning one common label to pixels other than the specific pixel. FIG. 7A shows the result of labeling the example of FIG. In this example, it is quaternized (N = 4).

A lossless compression algorithm such as run-length compression is applied to the N-valued image. FIG.
When basic run-length compression is attempted in the scanning direction on the quaternary image of (a), the compression result is (0, 9) (1,
3) (0, 6) (2, 1) (0, 7) (2, 1) (0,
6) (3, 3) (0, 28). Where (a, b)
Represents (label value, continuous length).

The compression result obtained as described above and the correspondence information between the label and the density value as shown in FIG.
It is output as information on the position and density value of the specific pixel.

In the above embodiment, the input image is replaced with the density value of the specific pixel before the JPEG
It is configured to perform image compression by the method
A configuration in which the present invention is applied to each partial image (for example, a block of 8 × 8 pixels) generated in the process of image compression by the G method is also conceivable. In this case, each of the partial images is an input image in the present invention, a specific pixel is detected for each partial image, and information on the position and density value of the specific pixel is also output for each partial image.

Next, a second embodiment of the present invention will be described with reference to FIG. A printer device 2 according to a second embodiment of the present invention includes an image input unit 20, an image compression unit 2
1, a compressed image storage means 22, an image decoding means 23, and a printing means 24.

The image input means 20 receives digital image data obtained by a scanner, a digital camera or the like, and outputs the digital image data to the image compression means 21 as an input image. Here, in addition to directly accepting digital image data,
Receives data in a form that includes control information of the printer device, etc., generates digital image data based on it,
You may make it output as an input image.

The image compression means 21 receives an input image from the image input means 20, performs image compression by the image compression method of the present invention, and stores the input image compression result in the compressed image storage means 2.
Output to 2. The image compression unit 21 can be realized by the image compression device disclosed in the first embodiment of the present invention. That is, the input image compression result output from the image compression unit 21 includes the intermediate image compression result and information on the position and density value of the specific pixel.

The compressed image storage means 22 stores the image compression device 2
1 to receive and store the input image compression result. Such a compressed image storage means 22 can be realized by, for example, a semiconductor memory or a hard disk device.

The image decoding means 23 reads the input image compression result from the compressed image storage means 22 in response to a request from the printing means 24, executes a decoding process, and outputs the obtained decoded image to the printing means 24.

The image decoding means 23 performs image decoding by the JPEG method on the intermediate image compression result included in the input image compression result. Specifically, for each compression result of the partial image divided at the time of compression by the JPEG method, an AC
Huffman code decoding and run-length compression decoding are performed on the components, and a frequency distribution is calculated by performing inverse quantization on a result obtained by inserting the DC component of the partial image into the decoding result, and inverse DCT is performed on the frequency distribution. Thus, the intermediate image is decoded.

Then, a pixel corresponding to the position of the specific pixel in the decoded intermediate image is obtained based on the information on the position and the density value of the specific pixel included in the input image compression result, and the density value of the pixel is calculated. , The decoded image is obtained by substituting the density value of the corresponding specific pixel.

Here, when the information on the position of the specific pixel and the density value is in a compressed state, the density value is replaced after expansion according to the algorithm used for compression.

The printing unit 24 issues a decoding start request to the image decoding unit 23 according to the progress of the printing process, receives the decoded image from the image decoding unit 23, performs halftoning on the decoded image, and performs halftone data processing. Is generated, and printing is performed based on the halftone data. Conventional techniques such as an error diffusion method, a dither method, and a screen method can be used as a halftoning method.

Next, a third embodiment of the present invention will be described. The third embodiment includes a recording medium on which an image compression program is recorded. This recording medium is CD-RO
M, a magnetic disk, a semiconductor memory, and other recording media, including those distributed through a network.

The image compression program is read from the recording medium into the data processing device, and controls the operation of the data processing device. Under the control of the image compression program, the data processing device scans the input image, detects a pixel satisfying a predetermined condition as a specific pixel, stores information on the position and density value of the specific pixel in the storage unit, After replacing the density value of the specific pixel with a predetermined density value, image compression is performed by the JPEG method, and the compression result by the JPEG method and the stored information on the position and density value of the specific pixel are used as the compression result of the input image. Output.

That is, the data processing device executes the same processing as the processing by the specific pixel detecting means 10, the intermediate image generating means 11, and the image compressing means 12 in FIG. 1 under the control of the image compression program.

The present invention is not limited to the above embodiments, but can be applied in various modifications.

According to the present invention, the position and the density value of a specific pixel satisfying a predetermined condition are stored.
It is possible to guarantee reversibility at the time of decoding and prevent image quality deterioration. In particular, by making the pixels constituting characters, figures, and the like specific pixels, it is possible to prevent the outline of the characters, figures, and the like from blurring.

Also, since the image compression is performed by the JPEG system after replacing the density value of a specific pixel with a predetermined density value, the image compression is performed on an intermediate image suitable for the JPEG system, and a high compression ratio is obtained. Can be achieved. In particular, a high compression rate can be achieved by replacing the density value of a specific pixel so that the high-frequency component is reduced on the intermediate image.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a first exemplary embodiment of the present invention.

FIG. 2 is a conceptual diagram illustrating an example of an input image according to the first embodiment of the present invention.

FIG. 3 is a conceptual diagram illustrating an example of an input image according to the first embodiment of the present invention.

FIG. 4 is a diagram illustrating an example of an input image according to the first embodiment of the present invention.

FIG. 5 is a diagram illustrating an example of an intermediate image according to the first embodiment of the present invention.

FIG. 6 is a schematic diagram for explaining an image compression method according to the JPEG method.

FIG. 7 is a schematic diagram for explaining a method of storing position and density value information of a specific pixel.

FIG. 8 is a block diagram illustrating a configuration of a second exemplary embodiment of the present invention.

DESCRIPTION OF SYMBOLS 1 Image compression device 2 Printer device 10 Specific image detection means 11 Intermediate image generation means 12 Image compression means 20 Image input means 21 Image compression means 22 Compressed image storage means 23 Image decoding means 24 Printing means 400 Characters

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5C059 KK00 LA00 MA00 MA23 MC15 ME02 ME05 PP01 PP04 PP15 PP20 SS15 SS20 SS28 UA02 UA05 UA38 5C078 BA21 BA57 CA02 CA21 DA01 DB12 9A001 EE04 HH27 KK42

Claims (16)

[Claims] An image compression method comprising: detecting a pixel that gives a high-frequency spatial frequency on an input image as a specific pixel; replacing the specific pixel with a pixel that gives a low-frequency spatial frequency in the input image; And information of the position and density value of the specific pixel as a compression result of the input image. 2. A method according to claim 1, wherein a pixel satisfying a predetermined condition is detected as a specific pixel by scanning the input image, and information on the position and density value of the specific pixel is stored in a storage unit. After the value is replaced with a predetermined density value, image compression is performed by the JPEG method.
An image compression apparatus for outputting a compression result by a PEG method and information of the stored position and density value of a specific pixel as a compression result of an input image. 3. A specific pixel detecting means for scanning an input image to detect a pixel satisfying a predetermined condition as a specific pixel, and storing information on a position and a density value of the specific pixel; An intermediate image generating means for generating an intermediate image by replacing the density value with a predetermined density value; performing image compression on the intermediate image by the JPEG method; An image compression apparatus comprising: an image compression unit that outputs information on a position and a density value as an input image compression result. 4. The apparatus according to claim 2, wherein said predetermined condition is a condition that a pixel gives a maximum density value in the input image or a pixel gives a minimum density value in the input image. Or the image compression device according to any one of 3. 5. The apparatus according to claim 2, wherein the predetermined condition is a condition that a pixel gives a maximum density value or a minimum density value in an input image. Image compression device. 6. The image according to claim 2, wherein the predetermined condition is a condition that the density value of the input image is a pixel within a predetermined range. Compression device. 7. The image according to claim 2, wherein the predetermined condition is a condition that the edge intensity of the input image is within a predetermined range. Compression device. 8. The image compression apparatus according to claim 6, wherein said predetermined range is calculated based on an input image. 9. The image compression apparatus according to claim 2, wherein said predetermined density value is an average density value of an input image. 10. The method according to claim 2, wherein the predetermined density value is a value determined for each specific pixel, and is an average density value of pixels surrounding each specific pixel. An image compression device according to claim 1. 11. The image compression method according to claim 2, wherein the predetermined density value is an average density value of pixels other than the pixel detected as the specific pixel. apparatus. 12. The image compression method according to claim 1, further comprising:
An image decoding means for performing image expansion by EG method and performing image decoding by replacing a density value of a pixel at the position of the specific pixel with a density value of the specific pixel in the expanded image. Items 3 to 1
The image compression device according to claim 1. 13. A printer comprising the image compression device according to claim 1. Description: 14. A pixel which gives a high-frequency spatial frequency on an input image is detected as a specific pixel, and the specific pixel is replaced with a pixel which gives a low-frequency spatial frequency in the input image, and then image compression is performed. And information on the position and density value of the specific pixel as a compression result of the input image. 15. A pixel which satisfies a predetermined condition is detected as a specific pixel by scanning an input image, information on the position and density value of the specific pixel is stored in a storage means, and the density of the specific pixel in the input image is stored. Image compression is performed by the JPEG method after replacing the value with a predetermined density value, and the result of the compression by the JPEG method and the stored information on the position and density value of the specific pixel are output as the compression result of the input image. Image compression method. 16. A computer-readable recording medium storing a program for causing a computer to execute the image compression method according to claim 14.