JP4926128B2 - Image processing apparatus, image reading apparatus, image forming apparatus, computer program, recording medium, and image processing method - Google Patents

Description

  The present invention relates to an image processing apparatus that compresses image data, an image reading apparatus, an image forming apparatus, a computer program that realizes the image processing apparatus, a recording medium that records the computer program, and an image processing method.

  Image data input from a scanner, digital camera, or the like is compressed by an irreversible compression method such as JPEG (Joint Photographic Experts Group) to reduce the amount of data, and processing such as transfer or storage is performed. In the JPEG compression method, the data amount and image quality of image data change by changing the quantization coefficient (numerical value indicating the fineness of quantization) used when performing quantization. For example, when the value of the quantization coefficient is reduced, the image quality of the image data is improved and the data amount is increased. Further, when the value of the quantization coefficient is increased, the image quality of the image data is lowered and the data amount is reduced. That is, when performing compression processing of image data, it is necessary to determine an appropriate quantization coefficient based on the compression rate and image quality of the image data set by the user.

Patent Document 1 discloses an image processing apparatus in which a plurality of quantization coefficients are allocated for each operation performed by the user so that the optimum quantization coefficient for the operation performed by the user is selected. Yes. According to Patent Document 1, it is possible to compress image data by automatically setting a quantization coefficient suitable for an operation performed by a user without being conscious of the user.
JP 2001-24897 A

  However, in Patent Document 1, the resolution of image data is not taken into consideration, and a high-frequency component of image data may be removed depending on a set compression parameter (quantization coefficient). When the high-frequency component is removed, information such as fine patterns and small characters in the image is lost, which causes excessive image quality degradation. FIG. 9 is a schematic diagram for explaining the relationship between resolution and frequency components. (A) is a document, (b) is image data read at a resolution of 600 dpi, and (c) is read at a resolution of 300 dpi. Image data. Note that (b) and (c) represent the same pixel size. The document is a plurality of line patterns parallel to the sub-scanning direction. When this original is read by a scanner or the like, the line pattern period (pixel interval) is narrower and the frequency component is higher when the original is read at 300 dpi than at 600 dpi. That is, as the resolution is lowered, the frequency characteristic of the image data shifts to the high frequency side. Therefore, when compressing image data read at a low resolution, if the compression parameter (quantization coefficient) that removes excessively high-frequency components is used, the image information itself is lost, causing extreme image quality degradation. become. For example, in the case of (c), deterioration occurs such that the line pattern is blurred and disappears. Similarly, even in a fine pattern such as a photograph, details of characters, small characters, etc., the lower the resolution to be read, the more information is lost during compression, causing excessive image quality deterioration.

  The present invention has been made in view of such circumstances, and an object of the present invention is to set an appropriate quantization coefficient in accordance with the resolution of the image so that the image data quality is not deteriorated. Is to provide an image processing apparatus, an image reading apparatus, an image forming apparatus, a computer program for realizing the image processing apparatus, a recording medium on which the computer program is recorded, and an image processing method. .

An image processing apparatus according to the present invention performs predetermined frequency conversion on each pixel constituting image data to convert a pixel value into a frequency coefficient, and quantizes and quantizes the converted frequency coefficient based on a quantization coefficient. resulting quantized data to encode the said image processing apparatus for compressing image data, respectively the resolution of the image data, obtaining means for obtaining the people two directions respectively substantially orthogonal to each other, said acquisition means acquires s The setting means for setting the quantization coefficient in accordance with the lower one of the resolutions of the image data, and the compression means for compressing the image data using the quantization coefficient set by the setting means.

  The image processing apparatus according to the present invention further includes a quantization coefficient storage unit that stores a plurality of different quantization coefficients in association with the resolution, and the setting unit is configured to perform the quantization based on the resolution acquired by the acquisition unit. The quantization coefficient extracted from the quantization coefficient storage means is set.

  The image processing apparatus according to the present invention is characterized in that the setting means sets a quantization coefficient using a predetermined arithmetic expression.

  The image processing apparatus according to the present invention is characterized in that the setting means sets the quantization coefficient to large or small corresponding to high or low resolution.

  The image processing apparatus according to the present invention is characterized in that the setting means sets a quantization coefficient to 1 when the resolution acquired by the acquisition means is lower than a predetermined value.

  An image reading apparatus according to the present invention includes a reading unit that scans a document and generates read image data, and the image processing apparatus of the present invention.

  The image reading apparatus according to the present invention is characterized in that the reading unit is configured to change a scanning speed.

  An image forming apparatus according to the present invention includes an image processing apparatus according to the present invention, an image data storage unit that stores image data compressed by the compression unit, and an expansion unit that expands image data stored in the image data storage unit. And an output means for outputting the decompressed image data.

  An image forming apparatus according to the present invention includes a reading unit that scans a document and generates read image data, and the reading unit is configured to be capable of changing a scanning speed.

A computer program according to the present invention causes a computer to perform predetermined frequency conversion on each pixel constituting image data to convert a pixel value into a frequency coefficient, quantize the converted frequency coefficient based on a quantization coefficient, In a computer program for compressing the image data by encoding quantized data obtained by quantization, an acquisition unit that acquires the resolution of the image data in each of two directions substantially orthogonal to each other ; The setting means for setting the quantization coefficient in accordance with the lower resolution of the resolutions, and the compression means for compressing the image data using the quantization coefficient set by the setting means. It is characterized by.

  The recording medium according to the present invention is recorded with the computer program according to the present invention.

The image processing method according to the present invention performs predetermined frequency conversion on each pixel constituting image data to convert a pixel value into a frequency coefficient, and quantizes and quantizes the converted frequency coefficient based on the quantization coefficient. In the image processing method for compressing the image data by encoding the obtained quantized data, the step of acquiring the resolution of the image data in each of two directions substantially orthogonal to each other , and among each of the acquired resolutions, The method includes a step of setting a quantization coefficient in accordance with a lower resolution, and a step of compressing the image data using the set quantization coefficient.

According to the present invention, when compressing image data, the compression is performed using a quantization coefficient corresponding to the resolution of the image data. Therefore, it is possible to provide an image processing apparatus capable of compressing image data of any resolution while maintaining an optimal balance between maintaining good image quality and reducing the amount of data. In particular, it is possible to achieve both the maintenance of image quality such as the legibility of characters and the visibility of fine patterns in image data and the reduction of the data amount (that is, the maintenance of the compression rate).
In the present invention, the resolution of each of the image data in two directions substantially orthogonal to each other is acquired, and the quantization coefficient is set based on the lower resolution. For this reason, even if the resolutions in both directions are different, the image data can be compressed with the optimum settings for the lower resolution, and image quality degradation such as missing information such as small characters and fine patterns can be accurately obtained. It is possible to provide an image processing apparatus that can be prevented.

  According to the present invention, a plurality of different quantization coefficients are stored in association with the resolution, and the corresponding quantization coefficient is extracted and set based on the resolution of the image data. Therefore, it is possible to hold in advance the quantization coefficient adjusted to obtain the optimum image quality for image data of various resolutions, and compressing the image data using the same makes it always good. An image processing apparatus capable of obtaining image quality can be provided.

  According to the present invention, the quantization coefficient is set using an arithmetic expression. Therefore, it is not necessary to prepare and hold a plurality of quantization coefficients in advance, and an image processing apparatus capable of automatically calculating and setting an optimal quantization coefficient according to the resolution of image data is provided. be able to. This also allows the quantization coefficient to be changed smoothly in accordance with the resolution of the image data, so that an undesirable phenomenon such as a sharp change in image quality due to a large change in the quantization coefficient due to a slight difference in resolution. Can be prevented.

  Further, according to the present invention, the quantization coefficient is set to be smaller (or larger) as the resolution becomes lower (or higher). Therefore, for image data with low resolution, compression is performed to maintain the image quality of the image data by performing fine quantization, and for image data with high resolution, the image is processed by coarse quantization. It is possible to provide an image processing apparatus capable of performing compression that significantly reduces the amount of data. As a result, it is possible to achieve both the maintenance of image quality such as the legibility of characters and the visibility of fine patterns in image data and the reduction of the data amount (that is, the maintenance of the compression rate).

  According to the present invention, when the resolution is lower than the predetermined value, the quantization coefficient is set to 1. Therefore, it is possible to provide an image processing apparatus that can prevent loss of information such as small characters and fine patterns to the maximum extent and maintain good image quality for image data with extremely low resolution. .

  Further, according to the present invention, it is possible to set an appropriate quantization coefficient corresponding to the resolution of the image data for the image data read by scanning the document, and to compress the image data using the set quantization coefficient. Therefore, it is possible to provide an image reading apparatus that can obtain image data with good image quality and a small amount of data.

  According to the present invention, the scanning speed of the original is variable. For this reason, even when the resolution of image data to be read (particularly the resolution in the sub-scanning direction) changes according to the scanning speed of the document, the image data can always be compressed using an appropriate quantization coefficient. A reader can be provided.

  Further, according to the present invention, an appropriate quantization coefficient is set according to the resolution of the image data, the image data is compressed and stored using it, and the stored image data is expanded and output. Therefore, image data of any resolution can be stored with a reduced amount of data while maintaining good image quality, and a good output image with little deterioration in image quality due to compression can be obtained. A possible image forming apparatus can be provided. In addition, this makes it possible to store image data compressed with an appropriate image quality in place of image data with a large amount of data, thereby reducing the capacity required for storing image data and further reducing the amount of image data. Transmission time required for writing and reading can be shortened.

  According to the present invention, an appropriate quantization coefficient is always set for image data read by scanning a document, and the image data is compressed and stored using the quantization coefficient. Is decompressed and output. For this reason, image data scanned at various resolutions by scanning a document can be stored with a reduced amount of data while maintaining good image quality at all times. Therefore, it is possible to provide an image forming apparatus capable of obtaining a good output image with a small amount of image data.

  According to the invention, the image processing apparatus can be realized by a computer.

  Further, according to the present invention, a general-purpose computer such as a personal computer is read through a recording medium such as a CD-ROM (Compact Disc-Read Only Memory) or downloaded from a network, and the image data is read. Appropriate compression according to the resolution can be performed. Similarly, for digital copiers and multi-function peripherals that perform software processing with a DSP (Digital Signal Processor) or the like, the program is read into a flash memory or a rewritable recording medium, and the image data according to the resolution. Appropriate compression processing can be performed.

  Hereinafter, the present invention will be described in detail with reference to the drawings illustrating preferred embodiments.

(Embodiment 1)
FIG. 1 is a block diagram illustrating a configuration of a compression device that is an image processing device according to the present invention and an image reading device including the compression device. The image reading apparatus according to the present embodiment includes an image input apparatus 1, a compression apparatus 2, and an interface 3. The compression apparatus 2 according to the present embodiment is disposed between the image input apparatus 1 and the interface 3, Connected to each. Note that the operation of the image reading apparatus is controlled by a controller (not shown) such as a CPU in accordance with a user instruction.

  The image input apparatus 1 is a scanner (or a digital camera) including a CCD line sensor (reading means) 1a in which reading elements are arranged in one direction (hereinafter referred to as a main scanning direction), and is set in the image input apparatus 1. Read image data from the original. For example, the image input apparatus 1 irradiates light on a document set on the image input apparatus 1 from a light source (not shown), and relatively moves in a direction orthogonal to the main scanning direction (hereinafter referred to as a sub-scanning direction). The CCD line sensor 1a receives reflected light reflected from the document, and converts the reflected light into an electrical signal (hereinafter referred to as RGB analog signal) that is color-separated into red (R), green (G), and blue (B). To do. The image input device 1 outputs an electrical signal to the compression device 2.

  In this case, the resolution in the main scanning direction is determined by the number of light receiving elements of the CCD line sensor 1a. For example, if 600 light receiving elements are arranged per inch, the resolution in the main scanning direction is 600 dpi. On the other hand, the resolution in the sub-scanning direction is determined by the moving speed of the CCD line sensor 1a. For example, if the timing is adjusted to read at intervals of 0.0423 mm for each line while moving at a constant reference speed, the resolution in the sub-scanning direction is 600 dpi. On the other hand, if it is moved at twice the reference speed, the reading interval for each line will be twice as long (0.0846 mm), so the resolution in the sub-scanning direction will be 300 dpi. Conversely, if it is moved at a speed that is ½ times the reference speed, the reading interval for each line will be ½ times the length (0.0212 mm), so the resolution in the sub-scanning direction is 1200 dpi. Become. As described above, in a general scanner, when the moving speed of the CCD line sensor is increased, the resolution in the sub-scanning direction is lowered (the reading interval is coarse), and when it is delayed, the resolution is increased (the reading interval is fine). Note that the moving speed is controlled by the controller, and the controller changes the moving speed according to the resolution instructed by the user. For example, the user places an original on an original setting table (not shown) of the image input apparatus 1 and operates an operation button (not shown) to instruct an operation of reading the original, and to determine what resolution (or reference) It may be a variable magnification with respect to the resolution). Then, the controller determines the moving speed of the CCD line sensor according to the resolution instructed by the user, and operates the image input apparatus 1 so as to read the document at the determined moving speed.

  The interface 3 is, for example, an I / O port to which a personal computer (hereinafter referred to as a personal computer) (not shown) is connected. The interface 3 transfers the image data subjected to the JPEG compression process in the compression device 2 to the connected personal computer. The personal computer has an output device such as a display or a printer, and the image data JPEG compressed by the compression device 2 is output from the output device.

  The compression device 2 includes an A / D conversion unit 20, a shading correction unit 21, an input processing unit 22, a JPEG compression unit (compression unit) 23, a resolution acquisition unit (acquisition unit) 24, a parameter setting unit (setting unit) 25, and the like. Is provided. The compression device 2 includes a flash memory and an EPROM (Erasable Programmable ROM) storage unit (quantization coefficient storage unit) 27, and the storage unit 27 includes a plurality of quantization tables (hereinafter referred to as quantization tables). 26 is referred to as table DB).

  The A / D converter 20 converts the RGB analog signal input from the image input device 1 into a digital signal (hereinafter referred to as RGB signal). The shading correction unit 21 removes distortion from the RGB signal converted by the A / D conversion unit 20. The distortion included in the RGB signal is, for example, illumination unevenness caused by light irradiating a document when reading image data. The input processing unit 22 performs gamma correction on each of the RGB signals from which distortion has been removed by the shading correction unit 21. Gamma correction is a process for correcting the gradation of image data so that the output image has a desired gradation.

  The resolution acquisition unit 24 acquires resolution information in the main scanning direction and the sub-scanning direction when the document is read from the controller (not shown), and outputs the acquired resolution to the parameter setting unit 25.

  Here, image data read by a scanner having a CCD line sensor 1a is taken up as image data to be input to the compression device 2, but as another example, image data read by a digital camera may be input. As another example, image data downloaded via a network may be input. When capturing image data from a digital camera, the information regarding the number of effective pixels of the camera (1024 × 768 pixels, 800 × 600 pixels, etc.) is used and the number of effective pixels is regarded as the resolution. For example, the effective pixel number may be directly replaced with the resolution unit, such as 1024 × 768 dpi or 800 × 600 dpi, or may be replaced with the resolution unit by performing a predetermined calculation. Also, when downloading downloaded image data, resolution information written in the header of the image data is used. For example, in the case of image data in the TIFF (Tagged Image File Format) format, the resolution in the main scanning direction and the sub-scanning direction is written in the header, so that information on the resolution can be used as it is. Further, the image data in the TIFF format may be converted into the bitmap format and then input to the compression device 2. In these examples as well, the operation of the entire apparatus is controlled by a controller (not shown), and the resolution acquisition unit 24 acquires resolution information from the controller.

  The parameter setting unit 25 extracts an optimal quantization table from the quantization table DB 26 according to the resolution acquired by the resolution acquisition unit 24. Note that the parameter setting unit 25 extracts a quantization table corresponding to the resolution in the sub-scanning direction or the coarser (lower) resolution of the resolutions in the main scanning direction and the sub-scanning direction using a setting table described later. To do. As described above, the resolution in the sub-scanning direction can be changed by changing the scanning speed of the CCD line sensor 1a. When the resolution in the main scanning direction is constant, the resolution in the sub-scanning direction is used as a reference. By extracting the quantization table, it is possible to set an appropriate quantization table according to the change of the input resolution. In addition, if the quantization table is extracted based on the lower resolution among the resolutions in the main scanning direction and the sub-scanning direction, image quality deterioration such as missing information such as small characters and fine patterns in the photo, It is possible to set a quantization table to prevent more accurately. As a result, it is possible to achieve both maintaining good image quality and improving the compression rate. The parameter setting unit 25 sets the quantization table extracted from the quantization table DB 26 in the JPEG compression unit 23.

  The JPEG compression unit 23 encodes the RGB signal using the quantization table set by the parameter setting unit 25.

  FIG. 2 is a block diagram illustrating a configuration of the JPEG compression unit 23. The JPEG compression unit 23 includes a color space conversion unit 231, a sampling unit 232, a DCT conversion unit 233, a quantization unit 234, a Huffman encoding unit 235, a header information generation unit 236, and the like.

  The color space conversion unit 231 converts the RGB signal into a YCbCr signal (Y: luminance signal, Cb: blue hue signal, Cr: red hue signal). The sampling unit 232 thins out pixels according to a predetermined sampling ratio (for example, 4 (Y): 4 (Cb): 4 (Cr)) with respect to the hue signal among the YCbCr signals converted by the color space conversion unit 231. Process. Further, each YCbCr signal is divided into blocks composed of 8 rows and 8 columns of pixels of 8 elements in the horizontal direction and 8 elements in the vertical direction.

  The DCT conversion unit 233 performs DCT conversion for each block divided by the sampling unit 232, and converts each block into a frequency coefficient that is a coefficient of a frequency component. In the integer value of 8 rows and 8 columns, the data in the first row and the first column indicate the DC component (direct current component) as the reference data, and the other data indicates the AC component (alternating current component). The quantization unit 234 performs a process of quantizing each element of the 8-by-8 frequency coefficient obtained by the DCT conversion unit 233 using the quantization table set by the parameter setting unit 25. As will be described in detail later, the quantization table has two types of tables for luminance signals and hue signals, and each table has a total of 64 integer values of 8 rows and 8 columns (hereinafter referred to as quantization coefficients). ). The quantization unit 234 performs quantization by dividing each element of the frequency coefficient of 8 rows and 8 columns after DCT by the corresponding element of the quantization table.

  The Huffman encoder 235 rearranges the YCbCr signals (quantized data) quantized by the quantizer 234 in a line, performs encoding processing based on a predetermined Huffman code table, and generates code data. The values in the Huffman code table are not particularly limited, and generally used values are used. The header information generation unit 236 adds header information generated in units of image data (one page of document) to the head of the code data encoded by the Huffman encoding unit 235. As a result, a JPEG code in a format compliant with the image data standard is output. In the header information, information such as the width and height of the image data, the quantization table used at the time of compression, the Huffman encoding table, and the sampling ratio are classified and described by a predetermined marker symbol. The JPEG code output from the header information generation unit 236 is transferred to a personal computer or the like connected to the interface 3.

  3 and 4 are diagrams schematically showing the quantization table.

  The quantization table has two types of tables for luminance signal and hue signal, and each table is represented by a total of 64 quantization coefficients of 8 rows and 8 columns. In the quantization table, the upper left of 8 rows and 8 columns (first row and first column) is the quantization coefficient for the frequency coefficient of the DC component, and the quantization coefficient for the frequency coefficient of the high frequency component in the horizontal and vertical directions toward the lower right. Have. A plurality of quantization tables constitute a quantization table DB 26, and a plurality of types are stored in the storage unit 27 in advance. Then, a quantization table corresponding to the resolution is extracted from the quantization table DB 26. In the present embodiment, the quantization table DB 26 stores a quantization table from set A to set E having different quantization coefficients corresponding to each frequency coefficient, and a quantization table in which all the quantization coefficients are “1”. Has been. The quantization tables from set A to set E are adjusted in advance so as to obtain optimum image quality and compression rate when image data having a predetermined resolution is compressed. As described above, the quantization coefficient adjusted so as to obtain the optimum image quality for image data with various resolutions is held in advance, and the image data is compressed using the quantization coefficient so that the image quality is always good. Can be obtained.

  FIG. 5 is a diagram schematically illustrating a setting table used when setting the quantization table.

  In the setting table illustrated in FIG. 5, the resolution R [dpi] is associated with the quantization table. For example, when the resolution is 400 [dpi], the quantization table of set B is set. When the resolution is smaller than 300 [dpi], a quantization table having quantization coefficients of “1” is set. In the setting table of FIG. 5, a quantization table having a small quantization coefficient is set as the resolution becomes coarse (lower). Therefore, for image data with low resolution, compression is performed to maintain the image quality of the image data by performing fine quantization, and for image data with high resolution, the image is processed by coarse quantization. Compression that significantly reduces the amount of data can be performed. As a result, it is possible to achieve both the maintenance of image quality such as the legibility of characters and the visibility of fine patterns in image data and the reduction of the data amount (that is, the maintenance of the compression rate). When the resolution is smaller than a predetermined value (300 [dpi]), a quantization table having a quantization coefficient of “1” is set. Therefore, even when compressing image data with extremely low resolution, it is possible to prevent loss of high-frequency component information to the maximum, and good reproducibility of small characters and fine patterns is maintained sufficiently Image quality can be obtained. For example, if a document contains small characters, a quantization table that does not provide clear image data when read at a low resolution, and that greatly eliminates high-frequency component information from such image data. When compression is performed using (that is, a quantization table having a large quantization coefficient), the readability of characters in the output image is extremely lost. The visibility of the pattern is extremely lost not only in the text but also in the case where the document includes a photograph with a fine pattern. For this reason, when the resolution is low, by setting a quantization table with a small quantization coefficient, it is possible to suppress the loss of high-frequency component information, and to improve image quality such as character readability and pattern visibility in the output image. Can be maintained. When the resolution is extremely low, by setting a quantization table with the least amount of missing information (that is, a quantization table with all the quantization coefficients of 1), the readability of the characters in the output image and the visibility of the pattern are set. Image quality deterioration such as loss of image quality can be minimized. Note that the resolution for setting the quantization table with a quantization coefficient of 1 and the resolution for setting each quantization table are determined by, for example, determining the limit (whether or not the character can be read) that can maintain the legibility of fine characters. can do. On the other hand, when reading at a high resolution, clear image data can be obtained. Therefore, even if compression is performed so that high-frequency component information is lost, character readability and pattern visibility are hardly lost. Therefore, the compression rate can be improved by setting a quantization table having a large quantization coefficient.

  In the present embodiment, a preset quantization table is stored in the storage unit 27 such as a flash memory and an EPROM. However, the quantization table is set using the acquired resolution and a predetermined arithmetic expression. You may do it. The quantization table can be calculated by an arithmetic expression of quantization table (i, j) = default quantization table (i, j) × (R ′ × α + β). (I, j) represents the position of (row, column) in the quantization table. R ′ is R / 100 (where 100 dpi ≦ resolution R ≦ 1200 dpi). The default quantization table is a predetermined quantization table serving as a reference. The default quantization table, coefficients α (for example, “9/110”) and β (for example, “1/55”) are set to desired values while evaluating image quality and compression rate using images of various resolutions, for example. Set. As a standard of image quality, it is only necessary to look at the legibility of characters of various sizes, or the reproducibility of patterns and details in photographs. In this case, since the optimum quantization coefficient can be automatically calculated and set according to the resolution, it is not necessary to prepare a plurality of quantization tables in advance and hold them in the storage unit 27. 27 storage capacity can be greatly reduced. In addition, since the quantization coefficient can be changed smoothly according to the resolution of the image data, it is possible to prevent an undesired phenomenon in which the quantization coefficient is largely switched due to a slight difference in resolution and the image quality is rapidly changed. Can do.

  Next, image data compression processing performed by the compression apparatus 2 configured as described above will be described. FIG. 6 is a flowchart showing the operation of the compression device 2.

  First, the compression device 2 acquires the resolution in the main scanning direction and the sub-scanning direction of the image data to be processed (S1). The resolution information is acquired from a controller that controls the operation of the entire apparatus. Next, the compression device 2 determines whether or not the resolution in the sub-scanning direction is lower than the resolution in the main scanning direction (S2).

  When the resolution in the sub-scanning direction is lower (S2: YES), the compression apparatus 2 sets the resolution in the sub-scanning direction as the resolution R of the image data (S3), and moves the process to S5. On the other hand, when the resolution in the main scanning direction is higher or the resolutions are equal (S2: NO), the compression device 2 sets the resolution in the main scanning direction as the resolution R of the image data (S4), and moves the process to S5. Since the compression apparatus 2 sets the quantization table based on the setting table shown in FIG. 5, the compression apparatus 2 determines whether or not the resolution R is less than 300 [dpi] (S5). When the resolution is less than 300 [dpi] (S5: YES), the compression device 2 extracts a quantization table having all quantization coefficients “1” from the quantization table DB 26 (S6). Thereafter, the compression device 2 moves the process to S8.

  When the resolution is not less than 300 [dpi] (S5: NO), the compression device 2 extracts a quantization table corresponding to the resolution from the quantization table DB 26 based on the setting table of FIG. 5 (S7). For example, when the resolution is 400 [dpi], the compression device 2 extracts the quantization table of set B from the quantization table DB 26. Thereafter, the compression device 2 moves the process to S8. Note that the quantization table when the resolution is less than 300 [dpi] is stored in advance in the quantization table DB 26. However, if the quantization table is not prepared and the resolution is less than 300 [dpi], quantization is performed. A program for setting the coefficient to 1 may be used.

  In S8, the compression apparatus 2 performs JPEG compression processing on the image data using the extracted quantization table. JPEG compression processing conforms to general standards, and in the quantization processing performed in the JPEG compression processing, by performing quantization using the extracted quantization table, optimal compression according to the resolution of the image data Processing can be performed.

  Note that the image input device 1 that acquires image data and the compression device 2 that compresses image data are configured separately, but may be a single device. For example, the compression device 2 may include a CCD line sensor.

(Embodiment 2)
Next, an image forming apparatus including the compression device 2 that compresses the image data described in the first embodiment will be described.

  FIG. 7 is a block diagram illustrating a configuration of the image forming apparatus. The image forming apparatus according to the present embodiment reproduces image data satisfactorily after the compression device 2, the hard disk (image data storage means) 4 that stores the JPEG code output from the compression device 2, and the JPEG code is decoded. And a decompression device 5 for performing the above processing. The hard disk 4 stores the image data compressed by the compression device 2. Hereinafter, functions of the decompressing device 5 will be described. Note that the function described below may be included in the compression device 2. Since the compression apparatus 2 is the same as that of Embodiment 1, description is abbreviate | omitted.

  The expansion device 5 includes a JPEG expansion unit 51, a color correction unit 52, a black generation / undercolor removal unit 53, a spatial filter unit 54, a halftone generation unit 55, a region separation unit 56, and the like.

  The JPEG decompression unit 51 decodes the JPEG code and decompresses it to RGB signal image data. Specifically, header information is extracted from the JPEG code, and values such as a Huffman encoding table, a quantization table, and a sampling ratio are extracted. Using the extracted Huffman coding table, the code data included in the JPEG code is decoded into data of the frequency coefficient of the YCbCr signal. At that time, the data is arranged in units of blocks of 8 rows and 8 columns, and the inverse quantization process is performed by multiplying each element of each block by the extracted quantization table. Thereafter, the frequency coefficient data of the YCbCr signal is returned to the image data by performing inverse DCT transform for each block. Further, after interpolating the thinned pixels according to the extracted sampling ratio, a process of converting the color space from the YCbCr signal to the RGB signal is performed, and finally the image data of the RGB signal is output.

  The color correction unit 52 converts the RGB signal into a density signal of CMY (C: cyan, M: magenta, Y: yellow) which is a complementary color of the RGB signal, and faithful color reproduction is performed in the output device (output means) 6. Color correction processing is performed on the CMY density signal so that it can be performed.

  The black generation / under color removal unit 53 performs black generation processing for generating a black (K) color signal based on the CMY color signals constituting the density signal input from the color correction unit 52. Further, the black generation / under color removal unit 53 performs under color removal processing on the CMY color signals. The under color removal process is a process for obtaining a new CMY color signal by subtracting the black color signal generated by the black generation process from the CMY color signal. As a result of these processes, the CMY density signal is converted into image data composed of CMYK color signals.

  The spatial filter unit 54 performs a spatial filter process using a digital filter on the CMYK image data obtained by the black generation / undercolor removal unit 53. By appropriately sharpening or smoothing the image data, it is possible to prevent the image output from the output device 6 from blurring or causing deterioration in graininess.

  The region separation unit 56 determines what kind of region each pixel of the image data belongs to, for example, black character / color character / halftone dot, etc., for the RGB signal input from the JPEG decompression unit 51. It is determined whether it is. The region separation unit 56 outputs the separation result to the black generation / undercolor removal unit 53, the spatial filter unit 54, and the halftone generation unit 55. In each unit that has received the separation result, an appropriate process is switched according to the region.

  The halftone generation unit 55 performs gradation correction processing and halftone generation processing on the CMYK image data. The halftone generation process is a process for dividing an image into a plurality of pixels so that gradation can be reproduced, and a binary or multi-value dither method / error diffusion method or the like can be used. Further, the halftone generation unit 55 may perform processing for converting the density value of the image data into a halftone dot area ratio that is a characteristic value of the output device 6. The halftone generation unit 55 outputs the processed CMYK image data to the output device 6 connected to the decompression device 5. Then, the output device 6 forms a final output image based on the CMYK signal. The output device 6 is a device that reproduces an image such as an electrophotographic printer or an inkjet printer, and may be an image display device such as a liquid crystal display.

  According to this configuration, since the JPEG compression parameters are appropriately switched according to the resolution of the image data, it is possible to realize a good image quality with little image quality degradation due to JPEG compression for the image finally output from the output device 6. . Further, since image data of any resolution can be compressed and stored without extremely degrading the image quality, it is not necessary to store the image data itself, and the data capacity of the hard disk 4 is reduced. In addition, it is possible to shorten the data transmission time to the outside, and to shorten the data transmission time inside the apparatus such as data writing to and reading from the hard disk 4.

  As described above, the compression device 2 according to the above-described embodiment extracts and sets the quantization table from the quantization table DB 26 based on the resolution of the image data. Thereby, an appropriate quantization coefficient according to the resolution of the image data can be set, and the image data can be compressed and the data amount can be reduced without degrading the image quality of the image data.

  As another application example of the present invention, the present image processing method (compression processing method) is recorded on a computer-readable recording medium in which program code (execution format program, intermediate code program, source program) to be executed by a computer is recorded. It can also be set as the structure recorded. According to this, it is possible to provide a portable recording medium on which a program code for performing this image processing is recorded.

  FIG. 8 is a schematic view showing another application example of the present invention. The personal computer 11 executes the program code from the recording medium, compresses the image data read by the scanner 10, and stores it. Further, the personal computer 11 decodes the stored image data and outputs it from the printer 12. As described above, even if the computer 11 can read the recording medium on which the program code is recorded, it is possible to perform the main image processing regardless of the place, even if the place is not a fixed place such as a multifunction machine.

  As the recording medium, a memory (not shown) such as a ROM itself may be a program medium because processing is performed by a microcomputer, or an external storage device (not shown) may be used. As a program medium, a program medium that can be read by inserting a recording medium therein may be used. In any case, the stored program code may be configured to be accessed and executed by a microprocessor, or in any case, the program code is read and the read program code is a microcomputer. The program code may be downloaded to a program storage area (not shown) and executed. It is assumed that this download program is stored in the main device in advance.

  The program medium is a recording medium configured to be separable from the main body, and includes a tape system such as a magnetic tape or a cassette tape, a magnetic disk such as a floppy (registered trademark) disk or a hard disk, a CD-ROM / MO (Magneto-Optical). Disc) / MD / DVD optical discs, IC (Integrated Circuit) cards (including memory cards) / optical cards, etc., or mask ROM, EPROM, EEPROM (Electrically Erasable Programmable ROM), flash ROM It may be a medium that carries a fixed program including a semiconductor memory.

  In addition, a medium that can connect a communication network including the Internet and that carries the program code in a fluid manner so as to download the program code from the communication network may be used. When the program code is downloaded from the communication network in this way, the program for downloading may be stored in the main device in advance or installed from another recording medium. The present invention can also be realized in the form of a computer data signal embedded in a carrier wave in which the program code is embodied by electronic transmission.

  The above-described image processing method is executed by reading the recording medium by a digital color image forming apparatus or a program reading apparatus provided in a computer system. In particular, in the latter case, the present image processing method can be used according to the user's preference. The computer system includes an image input device such as a flatbed scanner, a film scanner, or a digital camera, a computer that performs various processes such as the image processing method by loading a predetermined program, and a CRT that displays the processing results of the computer. (Cathode Ray Tube) An image display device such as a display or a liquid crystal display, and a printer that outputs a processing result of a computer to a paper medium or the like. Further, a modem or the like as a communication means for connecting to a server or the like via a network is provided.

1 is a block diagram illustrating a configuration of a compression device that is an image processing device according to the present invention and an image reading device including the compression device. FIG. It is a block diagram which shows the structure of a JPEG compression part. It is a figure which shows a quantization table typically. It is a figure which shows a quantization table typically. It is a figure which shows typically the setting table used when setting a quantization table. It is a flowchart which shows operation | movement of a compression apparatus. 1 is a block diagram illustrating a configuration of an image forming apparatus. It is the schematic which shows another example of application of this invention. It is a schematic diagram for demonstrating the relationship between a resolution and a frequency component, (a) is a manuscript, (b) is the image data read with the resolution of 600 dpi, (c) is the image data read with the resolution of 300 dpi It is.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image input device 1a CCD line sensor 2 Compression apparatus 3 Interface 20 A / D conversion part 21 Shading correction part 22 Input processing part 23 JPEG compression part 24 Resolution acquisition part 25 Parameter setting part 26 Quantization table DB

Claims (12)

Predetermined frequency conversion is performed on each pixel constituting the image data to convert the pixel value into a frequency coefficient, the converted frequency coefficient is quantized based on the quantization coefficient, and the quantized data obtained by quantization is encoded. In the image processing apparatus for compressing the image data,
Acquisition means for acquiring the resolution of the image data in each of two directions substantially orthogonal to each other ;
Setting means for setting a quantization coefficient in accordance with the lower resolution of the respective resolutions acquired by the acquisition means;
An image processing apparatus comprising: compression means for compressing the image data using the quantization coefficient set by the setting means. A quantization coefficient storage means for storing a plurality of different quantization coefficients in association with the resolution;
The setting means includes
Based on the resolution acquired by the acquisition unit, an image processing apparatus according to claim 1, characterized in that it is to be set the quantization coefficient extracted from the quantized coefficient storing means. The setting means includes
The image processing apparatus according to claim 1, wherein the quantization coefficient is set using a predetermined arithmetic expression. The setting means includes
The image processing apparatus according to any one of claims 1 to 3 , wherein the quantization coefficient is set to be large or small corresponding to high or low resolution. The setting means includes
5. The image processing apparatus according to claim 1, wherein when the resolution acquired by the acquisition unit is lower than a predetermined value, the quantization coefficient is set to 1. 5 . Reading means for scanning a document and generating read image data;
Image reading apparatus characterized by comprising an image processing apparatus according to claim 1, any one of 5. The reading means includes
The image reading apparatus according to claim 6 , wherein the scanning speed can be changed. An image processing apparatus according to any one of claims 1 to 5 ,
Image data storage means for storing the image data compressed by the compression means;
Decompression means for decompressing image data stored in the image data storage means;
An image forming apparatus comprising: an output unit that outputs image data expanded by the expansion unit. Scanning means that scans a document and generates scanned image data;
The reading means includes
The image forming apparatus according to claim 8 , wherein the scanning speed is changeable. Quantized data obtained by causing the computer to perform a predetermined frequency conversion on each pixel constituting the image data to convert the pixel value into a frequency coefficient, quantize the converted frequency coefficient based on the quantization coefficient, and quantize In a computer program for compressing the image data by encoding
The computer,
Acquisition means for acquiring the resolution of the image data in each of two directions substantially orthogonal to each other ;
Setting means for setting the quantization coefficient according to the lower resolution of the acquired resolutions , and
A computer program that functions as a compression unit that compresses the image data using a quantization coefficient set by the setting unit. A computer-readable recording medium on which the computer program according to claim 10 is recorded. Predetermined frequency conversion is performed on each pixel constituting the image data to convert the pixel value into a frequency coefficient, the converted frequency coefficient is quantized based on the quantization coefficient, and the quantized data obtained by quantization is encoded. In the image processing method for compressing the image data,
Obtaining the resolution of the image data in each of two directions substantially orthogonal to each other ;
A step of setting a quantization coefficient according to a lower resolution among the acquired resolutions ;
And a step of compressing the image data using a set quantization coefficient.

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