JP2002140183A - Image processing apparatus and method, printer apparatus having the image processing apparatus, and storage medium storing computer readable program - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide an image compression technique capable of suppressing a character stain on an edge portion and achieving a high compression ratio without deteriorating image quality in an image in which texts and photographs are mixed. At the time of compression, a specific pixel on a compression target image that satisfies a predetermined condition has a gray level of 1 / m.
(M> 1), the gray level of pixels other than the specific pixel is converted to 1 / n times (n> m), and a predetermined compression method is used using the gray level of each pixel after the conversion. Perform image compression. At the time of decompression, the compressed image is decompressed by a decompression method corresponding to the predetermined compression method, and for each pixel of the decompressed image, the density level is multiplied by p times (p
> M).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an image compression technique. In particular, the present invention relates to an image compression technique used for data handled by a printer, a copier, a scanner, a facsimile, a display, and the like, and suitable for an image in which text and photographs are mixed.

[0002]

2. Description of the Related Art Generally, image data has a large capacity.
When transmitting image data, compress on the transmitting side,
In many cases, the receiving side expands the data. For example, in a printer system including a host device and a printer device, the host device compresses an image to be printed to generate compressed data, and transmits the compressed data to the printer device. The printer receives the compressed data,
The print target image is obtained by decompressing this.

As an algorithm for compressing such an image, for example, there is an algorithm based on the JPEG system. JP
The image compression method based on the EG method is based on a method of performing irreversible encoding based on DCT (discrete cosine transform). Specifically, the input image is divided into blocks of 8 × 8 pixels, and the DCT operation is performed for each block. Then, a DC component (DC component) is obtained for the obtained DCT coefficient.
And the AC component (AC component) are independently quantized, and the DC component coefficient is encoded by a difference value using the DC component coefficient of the immediately preceding block as a predicted value, and the Huffman encoding is performed for the AC component coefficient. .

[0004]

When an image or the like in which text (characters) and a photograph are mixed is transmitted and received, an image compression technique suitable for both the photograph and the text 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.

[0007] However, in a text image having a sharp outline, information loss due to cut / quantization of high-frequency components occurs in the process of compression / expansion. Is degraded.

[0008] One pattern of image quality degradation will be described by taking as an example a case where each of the RGB gradation values takes a value of -128 to 127. In the input image, (R, G, B) = (127, 127,
127) in the decoded image due to the information loss due to the cut / quantization of the high-frequency component, (R, G,
B) = Pure white pixels such as (127, 125, 126). Such a symptom originally causes spot-like stains (character stains) in portions that should be white without printing anything, which is a major factor in deteriorating image quality.

In the JPEG system, by setting an image quality parameter (degree of quantization) to a high image quality side, it is possible to prevent image quality deterioration such as character smear to some extent.
However, in this case, there is a problem that the compression ratio is greatly reduced. In particular, the compression ratio of an image including a photograph or the like may be lower than that of a lossless compression method (run-length or the like).

It is an object of the present invention to provide an image compression technique capable of suppressing a character stain at an edge portion and achieving a high compression ratio without deteriorating image quality in an image in which texts and photographs are mixed. With the goal.

[0011]

An image processing method according to the present invention comprises the steps of: scanning a compression target image and detecting a pixel satisfying a predetermined condition as a specific pixel; a conversion step of converting the pixels other than the specific pixel into m-times (m> 1), and converting the gray level to 1 / n times (n>m); And a compression step of performing image compression using a predetermined compression method.

Further, the image processing method of the present invention is an image processing method for receiving a compressed image and decompressing the compressed image, wherein the compressed image has a gray level of a specific pixel satisfying a predetermined condition on the image to be compressed. Is converted to 1 / m times (m> 1), the gray level of pixels other than the specific pixel is converted to 1 / n times (n> m), and a predetermined level is determined using the gray level of each pixel after the conversion. A compressed image obtained by performing image compression according to the compression method, wherein the image processing method is performed by a decompression step of decompressing the compressed image by a decompression method corresponding to the predetermined compression method, and decompressed by a decompression step. A conversion step of converting the gray level to p times (p> m) for each pixel of the image. In this case, it is desirable that p = n.

[0013] In the image processing method of the present invention, the predetermined condition is that a pixel gives a maximum gray level in the image to be compressed and a pixel gives a minimum gray level in the image to be compressed. It is desirable to include at least one of the conditions.

Preferably, the gray level is a gray value in an RGB color space or a Y (luminance) value in a YUV color space, and the predetermined compression method is JPEG.
It is desirable to use a compression method.

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

The image processing apparatus according to the present invention includes a detecting means for scanning a compression target image to detect a pixel satisfying a predetermined condition as a specific pixel, and increasing the shading level of the specific pixel by 1 / m (m> 1). ), And for a pixel other than the specific pixel, a conversion means for converting the gray level to 1 / n times (n> m), and a predetermined compression method using the gray level of each pixel converted by the conversion means. And compression means for performing image compression by

Further, the image processing apparatus of the present invention is an image processing apparatus for receiving a compressed image and decompressing the compressed image, wherein the compressed image has a gray level of a specific pixel satisfying a predetermined condition on the image to be compressed. Is converted to 1 / m times (m> 1), the gray level of pixels other than the specific pixel is converted to 1 / n times (n> m), and a predetermined level is determined using the gray level of each pixel after the conversion. A compressed image obtained by performing image compression by the compression method, wherein the image processing device is expanded by the expansion means for expanding the compressed image by an expansion method corresponding to the predetermined compression method, and expanded by the expansion means. Conversion means for converting the gray level to p times (p> m) for each pixel of the image. In this case, it is desirable that p = n.

In the image processing apparatus of the present invention, the predetermined condition is that a pixel gives a maximum gray level in the image to be compressed and a pixel gives a minimum gray level in the image to be compressed. It is desirable to include at least one of the conditions.

Preferably, the gray level is a gray value in an RGB color space or a Y (luminance) value in a YUV color space, and the predetermined compression method is JPEG.
It is desirable to use a compression method.

A printer device according to the present invention includes the image processing device according to the present invention.

The printer system according to the present invention detects a pixel satisfying a predetermined condition by scanning an image to be compressed as a specific pixel, and the density level of the specific pixel is 1 / m times (m> 1). Conversion means for converting the pixels other than the specific pixel into a gray level of 1 / n (n> m), and a predetermined compression method using the gray level of each pixel converted by the conversion means. A host device including compression means for performing image compression, and a printer device according to the present invention are provided.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) A first embodiment of the present invention will be described with reference to FIG. First of the present invention
The image processing system 1 according to the first embodiment includes a detecting unit 1
0, the first including the first conversion unit 11 and the image compression unit 12
, And a second image processing apparatus 3 including an image decompression unit 13 and a second conversion unit 14.

In the present embodiment, the description will be made on the assumption that the input image (image to be compressed) is a 256-gradation gray-scale image having a value of -128 to 127. However, 0-2
The same can be applied to a grayscale image of 256 tones taking a value of 55 or a grayscale image of a grayscale other than 256 grayscales (for example, 64 grayscales, 512 grayscales, etc.). The input image may be an RGB color image. In this case, the present invention may be applied to each plane such as RGB. Further, when the input image is represented by a color system such as YUV, the present invention can be applied to a Y plane indicating a luminance value.

The detecting means 10 scans the input image and detects a pixel satisfying a predetermined condition as a specific pixel in which character contamination is likely to occur. It is assumed that the input image is acquired by a scanner, a digital camera, or the like, and is stored in the first image processing device 2.

The character stain is particularly likely to occur at a location where a spatial frequency of a high-frequency component is provided, such as a location where a pixel having a minimum value of -128 and a pixel having a maximum value of 127 are adjacent to each other. Edges are dull due to loss of information on high-frequency components in the process of compression / decompression, and pixels taking 127 (or -128) in the input image take values smaller than 127 (or values larger than -128) after decompression. This is because it may be decoded as a pixel.

Therefore, in the present embodiment, the conditions for detecting a specific pixel in which character stains are likely to occur are that the input image has the maximum gray value (127) and the input image has the minimum gray value (127). −128). Whether to use both conditions or only one of the conditions may be determined according to the design.

The first converter 11 converts the input image based on the detection result of the detector 10 according to the following procedure.
Generate an intermediate image.

For the specific pixel detected by the detecting means 10, the gray scale value is converted to 1 / m times (m> 1). Further, for pixels other than the specific pixel, the grayscale value is converted to 1 / n times (n> m). FIG. 2 shows an example where m = 2 and n = 3. FIG. 2A shows an input image, and FIG. 2B shows an intermediate image. Pixels surrounded by a thick frame indicate specific pixels.

The image compression means 12 applies a predetermined compression method to the intermediate image generated by the
Compression is performed by the PEG method to generate compressed data (JPEG file). Here, it is assumed that the predetermined compression method is an irreversible compression method. This is because in the case of the reversible compression method, information is not lost, and the image quality does not deteriorate in the first place.

The image compression procedure according to the basic JPEG method will be described below. There are several variations in the JPEG method, 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 the frequency distribution is obtained by DCT for each of the divided partial images. FIG. 3A shows an example of a 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. 3C shows an example after quantization.

Here, the value of the DC component (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.

When the input image (intermediate image) is an RGB color image, the image is converted to a YUV color system,
It is desirable to execute EG compression processing. RGB → YU
The V conversion can be calculated based on equation (1). However, another value may be used for the coefficient in equation (1) according to the standard to be adopted. Y = 0.299 x R + 0.587 x G + 0.114 x B V = 0.500 x R-0.419 x G-0.081 x BU = -0.169 x R-0.331 x G + 0.500 × B (1) The image decompression means 13 receives the compressed data of the intermediate image compressed by the first image processing apparatus 1 as input, and performs image decompression by a decompression method corresponding to the compression method used in the image compression means 12. Do. For example, when the JPEG system is used in the image compression means 12, decoding of Huffman code and decoding of run-length compression are performed on the AC component for each of the compressed data of the partial images divided at the time of compression, and the decoding result is obtained. The frequency distribution is calculated by performing inverse quantization on the component in which the DC component of the partial image is inserted, and inverse DCT is performed on the frequency distribution.
Decode the intermediate image.

The second conversion means 14 converts the intermediate image decoded by the image decompression means 13 according to the following procedure to generate an output image.

First, a tentative output image is generated by multiplying the gray value for each pixel of the decoded intermediate image by p times (p> m). Next, when there is a pixel whose gray level exceeds 127 (or smaller than -128) in the temporary output image,
The gray value of that pixel is 127 (or -128)
And an output image is generated.

When the decoded intermediate image is represented by the YUV color system, the above procedure may be executed after conversion to the RGB color system. In this case, the gray value of the provisional output image can be calculated based on equation (2). Different values may be used for the coefficients in the equation according to the standard used. When the gradation value at which the center becomes 0 is not taken as in -128 to 127, RGB → YUV conversion, YU
In each equation of the V → RGB conversion, an offset value may be further added. R = (1.000 × Y + 0 × U + 1.4020 × V) × p G = (1.000 × Y−0.34414 × U−0.71414 × V) × p B = (1.000 × Y + 1.77200) × U + 0 × V) × p (2) (Suppression of character stain) When an output image is generated using the second image processing device with respect to the data compressed by the first image processing device 2, The reason why no stain occurs will be described below.

As described above, the cause of the character stain is that the pixel having the maximum grayscale value (or the minimum grayscale value) in the input image due to the cut of the information loss and the high-frequency component due to the quantization becomes the maximum grayscale after expansion. That is, the pixel is decoded as a pixel having a value−decoding error α (or a minimum gray level + decoding error α).

According to the present invention, in a color space, one pixel is assigned to a maximum pixel value (or minimum pixel value) at the time of compression.
/ M times, and p times (p>
m). That is, focusing only on the processing in the color space, the expansion ratio is larger than the compression ratio. Under such a configuration, even if a decoding error occurs in the process of compression / expansion at a specific pixel of the intermediate image, the specific pixel of the temporary output image must be The value exceeds the maximum gray value (or a value smaller than the minimum gray value), that is, a value exceeding the dynamic range in the output image. Pixels exceeding the maximum gray value (or smaller than the minimum gray value) in the temporary output image are corrected to the maximum gray value (or the minimum gray value) in the output image. Or the minimum gray value).

That is, a pixel having a maximum gray value (or a minimum gray value) in an input image is decoded as a pixel having a maximum gray value (or a minimum gray value) also in an output image. Is decoded as black, and no character stain occurs. α <maximum gray value (or absolute value of minimum gray value) × (pm) / pm (3)
Here, α is a maximum decoding error α ^* determined based on a quantization parameter or the like at the time of compression, and m and p are determined based on the maximum decoding error α ^* so as to satisfy Expression (3). Generation of dirt can be completely suppressed.

In the JPEG system, when various parameters are set within a practical range, the maximum decoding error α ^* is usually 2 or less. Therefore, α = 2, maximum gray level = 127
FIG. 4 shows a practical combination of m and p that satisfies Expression (3).

(Improvement of compression ratio) In an apparatus such as a personal computer,
In many cases, one pixel is represented by 8-bit information for each color of RGB. That is, the information amount of one pixel is 2
^{It is 8} . On the other hand, in a printer apparatus, one pixel is often represented by information of about 2 bits for each color of CMYK (or 6 colors including light cyan and light magenta). That is, the information amount of one pixel is 2 ⁴ (or 2 ⁶ ). This is for the printer, the image data represented in RGB 8 bits per color means that the invention will include more information than necessary amount, by thinning out the 2 ^{4-2 2} about information from (and compressed in a color space Even after giving it to the printer device,
It means that color quality on printing is not significantly affected.

Therefore, according to the present invention, before the compression processing of the JPEG method or the like in the image compression means 12, the first conversion means 1
In 1, the dynamic range in the color space is narrowed by multiplying the density value of the input image by 1 / m or 1 / n to compress the amount of information. With this configuration, the compression ratio of the present invention is (compression ratio in the conversion unit 11) × (compression ratio in the image compression unit 12), which is smaller than the conventional method in which image compression such as JPEG is directly applied to an input image. Thus, a high compression ratio can be achieved. Incidentally, the degree of in theory does not affect the color quality be thinned out 2 ^{4-2 2} about information, considering that the image is represented by a set of dots in the dither or the like, the degree of thinning (compression ) is preferably set to ² 1 to 2 ^2.

Here, in the present invention, the degree of compression in the color space is made different between the specific pixel and the non-specific pixel, and the compression ratio of the non-specific pixel (1 / n) <the compression ratio of the specific pixel (1/1). m). This is because, in the provisional output image, at least a specific pixel is configured to take a value exceeding the maximum grayscale value, and a non-specific pixel is configured to be within the dynamic range as much as possible. In the tentative output image, whether or not the gray value of the non-specific pixel falls within the dynamic range is n
And p. If n ≧ p, it can be guaranteed that the values fall within the dynamic range.
On the other hand, when n <p, contrast in the output image can be enhanced, but a pixel having a large gray value in the input image may exceed the maximum gray value in the output image. The relationship between n and p may be determined according to the design. However, when it is desired to obtain the same contrast between the input image and the output image, it is desirable that n = p.

An image having a large number of specific pixels, for example, DTP
In the case of a (Desk Top Publishing) image, the compression rate is reduced by 1 / m times for a specific pixel as compared with a case where 1 / n times for all pixels.

(Second Embodiment) Next, a second embodiment of the present invention will be described with reference to FIG. A printer system 4 according to a second embodiment of the present invention includes a host device 5 including a detection unit 10, a first conversion unit 11, an image compression unit 12, and a transmission unit 15, a reception unit 16, an image decoding unit 14, It is configured to include the printer device 6 including the second conversion unit 15 and the printing unit 17.

The host device 5 and the printer device 6 are physically provided with a CPU, a memory, and the like. Each of the above-described units is functionally realized by the CPU executing an application program stored in the memory. You.

The configurations and operations of the detecting means 10, the first converting means 11, the image compressing means 12, the image expanding means 13 and the second converting means 15 are the same as in the first embodiment.

The transmitting means 15 generates print data based on the data compressed by the image compressing means 12 in accordance with a predetermined printer control language. Then, via a network or the like, for example, IEEE1284. Print data and commands are output to the printer device 5 in the D4 format.

The receiving means 16 receives print data and commands from the host device 5 and stores them. Then, compressed data is extracted from the print data and output to the image decompression means 13.

The printing means 17 comprises a printer control means, a print engine including a print head, a paper feed mechanism (carriage, carriage drive motor, etc.) and the like.

When receiving a command or the like for requesting a printing operation from the host device 5 via the receiving unit 16, the printer control unit outputs a paper feed command to the print engine. The print engine receives the feed instruction and controls a paper feed mechanism and the like to prepare a state necessary for starting printing. Then, a data transfer request is output to the printer control means. In response to the data transfer request, the printer control unit responds to the synchronization signal sent from the print engine and converts the second conversion unit 1
The output image generated in step 4 is transferred to the print engine, and printing is executed while controlling the print engine.

(Third Embodiment) Next, a third embodiment of the present invention will be described. The third embodiment includes a recording medium on which an information processing program is recorded. This recording medium may be a CD-ROM, a magnetic disk, a semiconductor memory, or another recording medium, and includes a case where the recording medium is distributed via a network. In addition, the present invention includes a case where the card is distributed as a card or an option board for a host device or a printer device.

The information processing program is read from the recording medium into the information processing device and controls the operation of the information processing device.
The information processing apparatus scans the compression target image and detects a pixel satisfying a predetermined condition as a specific pixel under the control of an information processing program. The information processing apparatus increases the gray level of the specific pixel by 1 / m times (m> 1), and for the pixels other than the specific pixel, the gray level is multiplied by 1 / n (n>
m) and a compression step of performing image compression by a predetermined compression method using the gray level of each pixel after the conversion in the conversion step.

The information processing apparatus receives a predetermined compressed image under the control of the information processing program, and executes processing for expanding the compressed image. In the predetermined compressed image, the gray level is converted to 1 / m times (m> 1) for a specific pixel satisfying a predetermined condition on the image to be compressed, and the gray level is reduced to 1 / m for pixels other than the specific pixel. This is a compressed image obtained by performing n-fold conversion (n> m) and performing image compression by a predetermined compression method using the gray level of each pixel after the conversion. The decompression process includes: a decompression step of decompressing the compressed image by a decompression method corresponding to the predetermined compression method; and for each pixel of the image decompressed by the decompression step,
A conversion step of converting the gray level to p times (p> m).

That is, the information processing apparatus executes the same processing as the processing by the image processing apparatus 2 and / or the image processing apparatus 3 in FIG. 1 under the control of the information processing program.

(Others) The present invention is not limited to the above embodiment, but can be applied in various modifications. For example, in the second embodiment, a case in which the present invention is applied to a printer system has been described. It is possible to obtain the effects of the invention. For example, it is conceivable to apply the present invention to a system including a copying machine, a scanner, a facsimile, and the like.

When the present invention is applied to an RGB color image, the values of m, n, and p may be set independently for each of RGB. For example, since the G plane contains many luminance components, it can be said that a decoding error in the G plane has a large effect on character contamination. Therefore, in order to suppress the decoding error in the G plane more reliably than in the other planes, ( _pg / _mg )> ( _pr / _mr ), ( _pb / _mb )
It is conceivable to set such that

When the dynamic range of the grayscale value of the input image is 0 to MAX, the minimum value of each pixel of the input image is set to a negative value such as X → X-MAX / 2. The correction may be performed so that the detection unit 10 detects the specific pixel. In this case, the second conversion unit 14 is configured to correct each pixel of the output image such as Y → Y + MAX / 2 so that the dynamic range of the grayscale value of the output image is 0 to MAX.

[0059]

According to the structure of the present invention, since the information is compressed in the color space and then compressed by the JPEG method or the like, a high compression ratio can be achieved.

Further, since the expansion rate in the color space is larger than the compression rate in the color space for the pixel that may cause the character stain, the character at the edge portion is It is possible to suppress the occurrence of dirt and prevent the image quality from lowering.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a functional configuration diagram of an image processing system according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating an example of an input image and an intermediate image.

FIG. 3 is a schematic diagram for explaining an image compression method according to the JPEG method.

FIG. 4 is a diagram for explaining an example of a combination of m and p.

FIG. 5 is a block diagram illustrating a functional configuration diagram of a printer system according to a second embodiment of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 image processing system 2, 3 image processing device 4 host device 5 printer device 10 detection means 11 first conversion means 12 image compression means 13 image decompression means 14 second conversion means 15 output means 16 reception means 17 printing means

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H03M 7/40 H04N 1/00 E 5C062 H04N 1/00 1/41 B 5C077 1/409 B41J 3/00 A 5C078 1/41 H04N 1/40 101D 5J064 F-term (reference) 2C087 AA03 AA09 BA03 BA06 BA07 BA12 BD40 2C187 AD03 2C262 AA24 AA26 AA27 AB13 AC02 AC07 BB03 BB15 BC03 BC10 BC19 CA08 DA17 EA04 A01 A01 A01 A01 A0B5B5 CA16 CB12 CB18 CC01 CE11 CG02 CH18 DC22 5C062 AA05 AB22 AB40 AC04 AC25 5C077 LL19 MP01 PP54 PQ08 RR06 RR21 TT02 TT06 5C078 AA04 AA09 BA44 BA57 DA01 DA02 DA17 DA18 DB05 5J064 AA02 BA08 BA09 BA13 BA12

Claims (15)

[Claims] 1. A detecting means for scanning a compression target image to detect a pixel satisfying a predetermined condition as a specific pixel, and for increasing / decreasing a gray level of the specific pixel by 1 / m (m>
(1) converting means for converting pixels other than the specific pixel into 1 / n times (n> m) the gray level for pixels other than the specific pixel; and performing predetermined compression using the gray level of each pixel after conversion by the converting unit. An image processing apparatus comprising: compression means for performing image compression by a method. 2. An image processing apparatus for receiving a compressed image and decompressing the compressed image, wherein the compressed image has a shading level of 1 / m times (m>) for a specific pixel satisfying a predetermined condition on an image to be compressed. 1), the gray level of pixels other than the specific pixel is converted to 1 / n times (n> m), and image compression is performed by a predetermined compression method using the gray level of each pixel after the conversion. A compressed image obtained by the above, wherein the image processing apparatus expands the compressed image by an expansion method corresponding to the predetermined compression method, and for each pixel of the image expanded by the expansion means,
A conversion means for converting the gray level into p-fold (p> m). 3. The apparatus according to claim 2, wherein p = n.
The image processing apparatus according to any one of the preceding claims. 4. The predetermined condition is a pixel that gives a maximum gray level in the image to be compressed and a pixel that gives a minimum gray level in the image to be compressed. The image processing device according to claim 1, wherein the image processing device includes the image processing device. 5. The image processing apparatus according to claim 1, wherein the gray level is a gray value in an RGB color space or a Y (luminance) value in a YUV color space. . 6. The image compression apparatus according to claim 1, wherein the predetermined compression method is a JPEG compression method. 7. A printer comprising the image processing apparatus according to claim 1. Description: 8. A printer system, comprising: a host device having the image processing device according to claim 1; and a printer device having the image processing device according to claim 2 or 3. 9. A detecting step of scanning a compression target image to detect a pixel satisfying a predetermined condition as a specific pixel, and increasing a shading level of the specific pixel by 1 / m (m> m).
1), and converting a pixel other than the specific pixel into a gray level of 1 / n times (n>m); and a predetermined compression using the gray level of each pixel after the conversion in the converting step. And a compression step of performing image compression by a method. 10. An image processing method for receiving a compressed image and decompressing the compressed image, wherein the compressed image has a gradation level of 1 / m times (m>) for a specific pixel satisfying a predetermined condition on an image to be compressed. 1), the gray level of pixels other than the specific pixel is converted to 1 / n times (n> m), and image compression is performed by a predetermined compression method using the gray level of each pixel after the conversion. The image processing method comprises: a decompression step of decompressing the compressed image by a decompression method corresponding to the predetermined compression method; and a shading level for each pixel of the image decompressed by the decompression step. And a conversion step of converting p into p times (p> m). 11. The image processing method according to claim 10, wherein p = n. 12. The predetermined condition is a pixel that gives a maximum gray level in the image to be compressed and a pixel that gives a minimum gray level in the image to be compressed. The image processing method according to any one of claims 9 to 11, wherein the image processing method includes: 13. The image processing method according to claim 9, wherein the gray level is a gray value in an RGB color space or a Y (luminance) value in a YUV color space. . 14. The image compression apparatus according to claim 9, wherein the predetermined compression method is a JPEG compression method. 15. A computer-readable recording medium storing a program for causing a computer to execute the image processing method according to claim 9. Description: