JP2005031759A - Image processor and image processing method to which spatial filter is applied - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image processor and an image processing method for optimally setting a digital filter for correcting an input image for every sensor with the direction dependency of the frequency characteristics of a sensor in a scanner taken into consideration. <P>SOLUTION: This image processor 100 for changing the edge emphasis degree of an input image by a digital filter is configured to apply filters whose edge emphasis degrees are different in mutually different directions, and to change the edge emphasis degrees in the different directions. Especially, as processing to change the edge emphasis degrees, frequency response characteristics in the different directions of a scanner 200 are detected, and filter coefficients to change the emphasis degrees are calculated by using a linear interpolation arithmetic operation according to the characteristics. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to image processing in an image forming system, and more particularly to processing for correcting an influence due to a frequency response characteristic of a sensor of an image input unit.
[0002]
[Prior art]
Conventionally, when a document image is read by a scanner, the read data is corrected by applying a digital filter in consideration of the frequency response characteristic (MTF) of the sensor. In that case, it is common to use a fixed digital filter having the same characteristics in all directions based on the typical MTF of the sensor.
[0003]
Various types of sensors are employed for scanners used in copiers and the like for each model. Among these, for example, a contact image sensor, the frequency response characteristics vary greatly depending on the direction. In addition, the sharpness of the input image may differ between the main scanning direction and the sub-scanning direction depending on the usage form of the sensor.
[0004]
[Patent Document 1]
JP-A-11-177836 [Patent Document 2]
Japanese Patent Laid-Open No. 2001-245157
[Problems to be solved by the invention]
However, in such a case, if a digital filter having the same characteristics in all directions is used, the sharpness of the input image is processed depending on the direction. Generally speaking, the image processing function in the device is often premised on the same handling in all directions, so if the frequency response characteristics of the sensor differ greatly depending on the direction, the sharpness of the processed image However, there is a problem that other image processing may not be performed as intended.
[0006]
The present invention has been made in view of such problems, and the problem is that a digital filter for correcting input image data is considered in consideration of the direction dependency of the frequency response characteristics of the sensor in the image input apparatus. An object of the present invention is to provide an image processing apparatus and an image processing method which are set optimally for each sensor.
[0007]
[Means for Solving the Problems]
In order to solve the above-described problem, an image apparatus according to the present invention is an image processing apparatus that changes the edge enhancement degree of an image by a digital filter, and performs filter processing for applying filters having different edge enhancement degrees in different directions. And a changing means for changing the difference in the edge enhancement degree in different directions. The image processing apparatus according to the present invention further includes a detection unit that detects a frequency response characteristic of the sensor of the image input device, a storage unit that stores the frequency response characteristic detected by the detection unit in different directions, and a storage unit. Filter calculating means for determining a coefficient to be input to the changing means according to the stored frequency response characteristic, and calculating and storing a digital filter. Further, the changing means stores a set of filter coefficients for a plurality of pixels corresponding to a frequency response characteristic having a minimum difference in edge enhancement in different directions and a frequency response characteristic having the maximum difference in edge enhancement. Based on this, filter coefficients for a plurality of pixels corresponding to other edge enhancement differences are calculated by linear interpolation. The linear interpolation calculation is performed using a ratio based on the difference in edge enhancement of the filter having the largest difference in edge enhancement depending on the direction as a coefficient. Note that the total sum of the filter coefficients calculated by the linear interpolation calculation is represented by a power of 2. Among the calculated filter coefficients, the coefficient corresponding to the DC component absorbs an error that can be accumulated by the calculation of the coefficient corresponding to the component other than the DC component, and the sum of the filter coefficients is expressed by a power of 2. Is set as follows.
[0008]
An image processing method according to the present invention is an image processing method for changing an edge enhancement degree of an image using a digital filter, and includes a filter processing step for applying filters having different edge enhancement degrees in different directions, and edges in different directions. And a change step for changing the difference in the degree of emphasis. The image processing method according to the present invention further includes a detection step of detecting a frequency response characteristic of a sensor of the image input device, and a storage step of storing the frequency response characteristic in a different direction detected in the detection step in a memory. And a filter calculating step for determining a coefficient to be input in the changing step in accordance with the frequency response characteristic stored in the memory, and calculating and storing a digital filter. In the changing step, a set of filter coefficients for a plurality of pixels corresponding to the frequency response characteristic having the smallest difference in edge enhancement in different directions and the frequency response characteristic having the largest difference in edge enhancement is stored. Based on this, filter coefficients for a plurality of pixels corresponding to other differences in edge enhancement are calculated by linear interpolation. Note that the linear interpolation calculation is performed using a ratio based on the difference in edge enhancement of the filter having the largest difference in edge enhancement depending on the direction as a coefficient. Further, the total sum of the filter coefficients calculated by the linear interpolation calculation is represented by a power of 2. Further, among the calculated filter coefficients, the coefficient corresponding to the DC component absorbs an error that can be accumulated by the calculation of the coefficient corresponding to other than the DC component, and the sum of the filter coefficients is represented by a power of 2. Is set to
[0009]
DETAILED DESCRIPTION OF THE INVENTION
DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described in detail with reference to the accompanying drawings. First, an image processing apparatus system according to an embodiment of the present invention will be described for each component.
[0010]
<Hardware configuration>
FIG. 1 is a diagram showing a control system configuration of a digital multi-function peripheral suitable for applying the present embodiment.
[0011]
The controller unit 100 is connected to a scanner 200 that is an image input device and a printer 300 that is an image output device, while being connected to a LAN 500 and a telephone line 600. The controller unit 100 controls input / output of image information and device information. Actually, the CPU 103 has a function of controlling the entire digital multi-function peripheral. A RAM 107 is a system work memory in which data necessary for the operation of the CPU 103 is stored, and is also used as an image memory for temporarily storing image data. The ROM 108 is used as a boot ROM, and stores a boot program for the digital multifunction peripheral. An HDD 109 is a hard disk drive and stores system software, image data, and the like. The HDD 109 may store information such as an image output speed and an installation position related to a node connected to the network (LAN 500) for each address. An operation unit I / F (interface) 104 serves as an interface between the operation unit 400 and the system bus 101, and outputs image data to be displayed on a liquid crystal unit (not shown) of the operation unit 400 to the operation unit 400. The operation unit I / F 104 serves to transmit information input by the user from the operation unit 400 to the CPU 103. The system bus 101 and the LAN 500 are connected via a network I / F 105 to input / output information. Further, the system bus 101 and the telephone line 600 are connected via a modem 106. The modem 106 performs modulation / demodulation processing for data transmission / reception. Each device is arranged on the system bus 101 as described above.
[0012]
The image bus I / F 110 is for connecting the system bus 101 and the image bus 102 for transferring image data at high speed, and functions as a bus bridge for converting the data structure. The image bus 102 is configured by a high-speed bus such as a PCI bus or IEEE1394.
[0013]
The following devices are arranged on the image bus 102. A raster image processor (RIP) 111 expands a PDL code into a bitmap image. The device I / F unit 112 connects the scanner 100 and the printer 300, which are image input / output devices, to the controller 100, and performs synchronous / asynchronous conversion of image data. A scanner image processing unit 700 corrects, processes, and edits input image data. The printer image processing unit 800 performs correction, resolution conversion, and the like according to the printer on the print output image data. The image compression unit 115 performs compression / decompression processing of JPEG for multi-value image data and JBIG, MMR, and MH for binary image data.
[0014]
<Image input unit (scanner)>
FIG. 2 is a diagram illustrating an appearance of a scanner which is an image input unit. The scanner unit 200, which is an image input device, illuminates an image on paper as a document and scans a CCD line sensor (not shown) to convert it into an electrical signal as raster image data. The manuscript paper is set on the tray 202 of the manuscript feeder 201, and when the user gives a reading start instruction from the operation unit 400, the controller CPU 103 gives an instruction to the scanner 200, and the feeder 201 feeds the manuscript paper one by one to copy the manuscript image. Perform a read operation.
[0015]
<Image output unit (printer)>
FIG. 3 is an external view of a printer that is an image output unit. The printer unit 300, which is an image output device, is a part that converts raster image data into an image on paper. The method is an electrophotographic method using a photosensitive drum or a photosensitive belt, and ink is ejected from a micro nozzle array. There is an ink jet method for printing an image directly on paper, but any method may be used. Activation of the printing operation is started by an instruction from the controller CPU 103. The printer unit 300 has a plurality of paper feed stages so that different paper sizes or different paper orientations can be selected, and paper cassettes 302, 303, 304, and 305 corresponding to the paper feed stages are mounted. A paper discharge tray 306 receives paper that has been printed.
[0016]
<Scanner image processing unit>
FIG. 4 is a diagram illustrating a configuration of the scanner image processing unit 700. The image bus I / F controller 701 is connected to the image bus 102 and has a function of controlling the bus access sequence, and also has a function of generating control and timing of each device in the scanner image processing unit 700. The MTF correction filter processing unit 702 is a part that executes the processing method according to the present embodiment. In order to correct the frequency characteristics of the image by the sensor characteristics of the scanner and perform edge enhancement, the MTF correction filter processing unit 702 performs a convolution operation using a digital spatial filter. Do. That is, in the MTF correction filter 702, a filter coefficient that matches the characteristics of the scanner 200 is determined by a process described later, and edge enhancement processing is performed depending on the direction by this filter. The image area separation processing unit 703 determines an image area by detecting a character part from the input image, and generates an image area signal used for subsequent image processing. The table processing unit 704 is a block that performs table conversion in order to convert luminance data that has undergone direction-dependent edge enhancement processing by the MTF correction filter into density data. For example, conversion is performed so that luminance data (steps 0 to 255) is represented by black and white data (for example, steps 0 to 255). The filter processing unit 705 performs a convolution operation with a digital spatial filter according to a purpose such as edge enhancement on the entire image. The MTF correction filter has direction dependency, but the filter 705 does not have direction dependency. For example, the editing unit 706 recognizes a closed region surrounded by a pointing device such as a marker pen from input image data, and performs image processing such as shading, shading, and negative / positive inversion on the image data in the closed region. Process. Then, the processed image data is transferred to the image bus 102 via the image I / F bus controller 701 again. Note that the coefficient of the MTF correction filter 702 can be determined so that the process of the filter 705 and the process 702 of the MTF correction filter are performed at once, but the details are omitted.
[0017]
<Printer image processing unit>
FIG. 5 is a diagram illustrating a configuration of the printer image processing unit 800. The image bus I / F controller 801 is connected to the image bus 102 and has a function of controlling the bus access sequence and a function of generating control and timing of each device in the printer image processing unit 800. The background removal processing unit 802 removes the background color when image data or the like obtained by reading a document having a light background color is sent. A color conversion processing unit 803 performs color conversion in accordance with the output characteristics of the printer. A resolution conversion unit 804 performs resolution conversion for converting image data received from the LAN 500 or the telephone line 600 into the resolution of the printer 300. The smoothing processing unit 805 performs a process of smoothing jaggy (image roughness appearing at a black and white border such as an oblique line) of the image data after resolution conversion.
[0018]
<Filter coefficient setting process peculiar to this embodiment>
Next, a digital filter coefficient setting process unique to this embodiment will be described. The sensors employed in the image input apparatus may have different frequency response characteristic differences depending on directions for each type and for each individual. In order to cope with various image processing after image input, it is desirable to individually correct their frequency response characteristics.
[0019]
However, if only sensors are replaced within a device with the same mechanism, if there are individual differences among sensors in mass production, or if a device or application that handles images such as input devices, image editing functions, and output devices is used by using a network. At present, it is difficult to deal with only a fixed correction coefficient when the combination becomes complicated. For this reason, it is desirable to prepare a large number of filters for various frequency characteristic differences. However, storing a large amount of individual filter coefficients requires a large amount of memory capacity and is not practical. is there.
[0020]
Therefore, in the present embodiment, the above problem is solved by using a method of calculating and applying a filter suitable for a sensor having various frequency response characteristics, particularly differences in characteristics depending on directions, using linear interpolation calculation. It has been solved.
[0021]
6 and 7 are diagrams showing filter coefficients to be applied to the input digital image data. In this example, both employ 7 × 7 pixel filter coefficients. 8 is a diagram showing the frequency response of the filter coefficient of FIG. 6, and FIG. 9 is a diagram showing the frequency response of the filter coefficient of FIG.
[0022]
8 and 9, the horizontal axis represents the spatial frequency and the vertical axis represents the power. The unit of the spatial frequency on the horizontal axis is lp / mm (line pair per millimeter), which is a unit indicating how many line pairs are present per mm. Further, the power on the vertical axis indicates to what percentage each frequency component is emphasized. The power of 100% means that the input image is output as it is.
[0023]
The filters shown in FIGS. 6 and 8 should be applied when the difference in the frequency response between the X-axis direction and the Y-axis direction is the greatest, and the frequency response in the Y-axis direction of the sensor is in the X-axis direction. Since it is higher than the above, it is a filter having a characteristic that suppresses the enhancement amount in the Y-axis direction lower than that in the X-axis direction. The filters shown in FIGS. 7 and 9 are to be applied when the frequency responses in the X-axis direction and the Y-axis direction are equal, and are filters having characteristics having the same enhancement amount in both directions. That is, the frequency response characteristic has no difference in edge enhancement degree in both directions. Here, the embodiment without difference in edge enhancement is used as an embodiment, but it may not be at all, and the difference is sufficiently larger than the difference in enhancement shown in FIG. If it is small, it can be applied as the present invention. That is, the filter in FIG. 8 shows the largest difference in edge enhancement in both XY axes, and the filter in FIG. 9 shows the smallest difference in edge enhancement in both XY directions. The fact that there is no difference in edge enhancement is an ideal state when the difference is minimized.
[0024]
In the filter shown in FIG. 8, the maximum values of the frequency responses in the X-axis direction and the Y-axis direction are about 270% and 170% near 6 lp / mm, respectively. The filter shown in FIG. 9 is about 270% in both directions. A procedure for generating a filter having a maximum value in the X-axis direction of about 270% and a maximum value in the Y-axis direction of about 170% to 270% from these two filters will be described.
[0025]
The filter coefficient in FIG. 6 is represented as SF1 (M, N), the filter coefficient in FIG. 7 is represented as SF2 (M, N), the argument M is defined as the X direction, the argument N is defined as the Y direction, and the upper left is defined as 0 rows and 0 columns. To do. For example, the value of SF1 (2, 3) is 38.
[0026]
A first specific example using this filter calculation function will be described. There is a case where F (M, N) having a characteristic exactly halfway between SF1 (M, N) and SF2 (M, N) (here, expressed as 0.5) is obtained. It can be expressed by a formula.
[0027]
F (M, N) = SF1 (M, N) × 0.5 + SF2 (M, N) × 0.5
In this equation, the fractional part is rounded off, and when the coefficient for 7 × 7 pixels is calculated, the filter coefficient shown in FIG. 10 is obtained. However, since the value of the DC (direct current) component F (3, 3) absorbs rounding errors accumulated in other coefficient operations, the sum of all F (M, N) coefficients is a power of 2, in this example Then, the adjustment is made to be 128. This is because, when the present embodiment is realized by hardware, the power of 2 is more convenient when performing a shift operation.
[0028]
Here, the middle (0.5) represents an intermediate point of about 170% to 270%, which is the range of the maximum value in the Y-axis direction of the filter to be obtained. In this example, the maximum value in the Y-axis direction, This means that G described later is 220%. The ratio of the linear interpolation calculation for SF1 (M, N) and SF2 (M, N) depends on the frequency characteristic specific to the connected scanner 200, that is, the frequency response characteristic in the X direction and the frequency in the Y direction. It depends on the difference in response characteristics. The characteristics of the scanner 200 may be acquired when a certain image is read, or the scanner 200 itself has data relating to its own characteristics, and the controller 100 may read it when connected. Further, the linear interpolation calculation according to the present embodiment may be executed only at the time of connection, or may be executed periodically.
[0029]
Further, as a second specific example using this filter calculation function, the frequency response characteristic at the frequency at which the power of the edge enhancement filter is maximum is 40% in the X-axis direction (main scanning direction), and the Y-axis direction. There is a case in which the image data input by 60% of the sensor in the (sub-scanning direction) is subjected to processing for applying the same degree of edge enhancement in both directions, and the filter F (M, N) for this is expressed by the following equation: It is represented by
[0030]
F (M, N) = SF1 (M, N) × (270−G) + SF2 (M, N) × (G−170)
G is the maximum power value in the Y-axis direction of the filter to be obtained. In this case, G = 270 × 40/60 = 180. Even in this case, the coefficient value of the DC component is adjusted so that the total sum of all the obtained filter coefficients F (M, N) is a power of 2, 128 in this example.
[0031]
The filters obtained as a result are as shown in FIGS.
[0032]
Further, the linear interpolation calculation shown in the present embodiment is generalized and described as follows.
[0033]
F (M, N) = SF1 (M, N) × (MAX _y2− G) + SF2 (M, N) × (G−MAX _y1 ),
G = (MAX _x ) × (A _x / A _y )
Here, MAX _x is the maximum power value in the X-axis direction of the edge enhancement filter, MAX _y1 is the maximum power value in the Y-axis direction of the filter having the maximum direction difference among the basic filters, and G is the Y-axis direction of the filter to be obtained. of maximum power, a _x X-axis direction of the sensor frequency response characteristics at a certain frequency, a _y shows a Y-axis direction of the sensor frequency response characteristics at the same frequency.
[0034]
FIG. 14 is a flowchart showing the outline of the above processing. When a process for changing the difference in edge enhancement is instructed, the process proceeds to step S101. In step S101, frequency response characteristics in at least two directions (different directions) of the sensor in the scanner (image input apparatus) are detected. The The different directions are the X-axis direction and the Y-axis direction in the present embodiment. And it transfers to step S102 and the filter coefficient corresponding to the frequency response characteristic in each direction is acquired. Furthermore, the process proceeds to step S103, and the difference in edge enhancement is changed by linearly interpolating the filter coefficient acquired in step S102. This linear interpolation is performed using the above-described linear interpolation formula. If the difference in edge enhancement is changed, the process ends.
[0035]
In the processing according to the embodiment of the present invention, a storage medium in which a program code of software embodying each function is recorded is provided to a system or apparatus, and a computer (or CPU or MPU) of the system or apparatus stores the storage medium in the storage medium. It is also achieved by reading and executing the programmed program code. In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiments, and the storage medium storing the program code constitutes the present invention. As a storage medium for supplying such a program code, for example, a floppy (registered trademark) disk, hard disk, optical disk, magneto-optical disk, CD-ROM, CD-R, magnetic tape, nonvolatile memory card, ROM Etc. can be used.
[0036]
Further, by executing the program code read by the computer, not only the functions of the above-described embodiments are realized, but also an OS (operating system) running on the computer based on the instruction of the program code Includes a case where the function of the above-described embodiment is realized by performing part or all of the actual processing.
[0037]
Furthermore, after the program code read from the storage medium is written in the memory provided in the function expansion board inserted into the computer or the function expansion unit connected to the computer, the function is determined based on the instruction of the program code. This includes the case where the CPU of the expansion board or function expansion unit performs part or all of the actual processing, and the functions of the above-described embodiments are realized by the processing.
[0038]
【The invention's effect】
As described above, according to the present invention, there is an effect of reducing the direction dependency of the sharpness of an output image by applying digital filters having different characteristics depending on directions. Furthermore, according to the present invention, the digital filter to be applied can be calculated according to the difference in characteristics for each direction, so that there is an effect of saving the data capacity of the filter stored in the device.
[Brief description of the drawings]
FIG. 1 is a diagram showing a control system configuration of a digital multi-function peripheral suitable for applying the present embodiment.
FIG. 2 is a diagram illustrating an appearance of a scanner.
FIG. 3 is a diagram illustrating an appearance of a printer.
FIG. 4 is a diagram illustrating a configuration of a scanner image processing unit.
FIG. 5 is a diagram illustrating a configuration of a printer image processing unit.
FIG. 6 is a diagram showing a first specific example of digital filter coefficients used for linear interpolation in the present embodiment.
FIG. 7 is a diagram illustrating a second specific example of digital filter coefficients used for linear interpolation in the present embodiment.
FIG. 8 is a diagram showing frequency response characteristics of the digital filter shown in FIG. 6;
9 is a diagram showing frequency response characteristics of the digital filter shown in FIG.
FIG. 10 is a diagram illustrating a first specific example of a filter coefficient after changing the edge enhancement degree, calculated by linear interpolation according to the present embodiment.
11 is a diagram showing frequency response characteristics of the digital filter shown in FIG.
FIG. 12 is a diagram illustrating a second specific example of the filter coefficient after changing the edge enhancement degree, calculated by linear interpolation according to the present embodiment.
13 is a diagram showing frequency response characteristics of the digital filter shown in FIG.
FIG. 14 is a flowchart showing an outline of edge enhancement degree change processing according to the present embodiment;
[Explanation of symbols]
100 Controller unit 103 CPU
107 RAM
108 ROM
400 Operation Unit 700 Scanner Image Processing Unit 702 MTF Correction Filter Processing Unit

Claims (14)

An image processing apparatus for changing an edge enhancement degree of an image by a digital filter,
Filter processing means for applying filters having different edge enhancement levels in different directions;
An image processing apparatus comprising: changing means for changing a difference in the edge enhancement degree in the different directions. Furthermore, detection means for detecting the frequency response characteristics of the sensor of the image input device;
Storage means for storing the frequency response characteristics in the different directions detected by the detection means;
Filter calculating means for determining a coefficient to be input to the changing means according to the frequency response characteristic stored in the storage means, and calculating and storing a digital filter;
The image processing apparatus according to claim 1, further comprising: The changing unit stores a set of filter coefficients for a plurality of pixels corresponding to a frequency response characteristic having a minimum difference in edge enhancement in the different directions and a frequency response characteristic having the maximum difference in edge enhancement. The image processing apparatus according to claim 2, wherein, based on this, filter coefficients for a plurality of pixels corresponding to other differences in edge enhancement are calculated by linear interpolation calculation. The image processing apparatus according to claim 3, wherein the linear interpolation calculation is performed using, as a coefficient, a ratio based on a difference in edge enhancement degree of a filter having a maximum difference in edge enhancement degree depending on a direction. The image processing apparatus according to claim 4, wherein the total sum of the filter coefficients calculated by the linear interpolation calculation is represented by a power of two. Among the calculated filter coefficients, a coefficient corresponding to a DC component absorbs an error that can be accumulated by calculation of a coefficient corresponding to the DC component other than the DC component, and the sum of the filter coefficients is expressed by a power of 2. The image processing apparatus according to claim 5, wherein the image processing apparatus is set as follows. An image processing method for changing an edge enhancement degree of an image by a digital filter,
A filtering process for applying filters having different edge enhancement levels in different directions;
An image processing method comprising: a changing step for changing a difference between the edge enhancement levels in the different directions. Furthermore, a detection step of detecting a frequency response characteristic of the sensor of the image input device;
A storage step of storing in the memory the frequency response characteristics in the different directions detected in the detection step;
8. A filter calculating step for determining a coefficient to be input in the changing step according to the frequency response characteristic stored in the memory, and calculating and storing a digital filter. Image processing method. In the changing step, a set of filter coefficients for a plurality of pixels corresponding to the frequency response characteristic having the smallest difference in edge enhancement in the different directions and the frequency response characteristic having the largest difference in edge enhancement is stored. 9. The image processing method according to claim 8, wherein filter coefficients for a plurality of pixels corresponding to other edge enhancement differences are calculated based on this by linear interpolation calculation. The image processing method according to claim 9, wherein the linear interpolation calculation is performed using, as a coefficient, a ratio based on a difference in edge enhancement of a filter having a maximum difference in edge enhancement depending on a direction. The image processing method according to claim 10, wherein the sum of the filter coefficients calculated by the linear interpolation calculation is represented by a power of two. Among the calculated filter coefficients, a coefficient corresponding to a DC component absorbs an error that can be accumulated by calculation of a coefficient corresponding to the DC component other than the DC component, and the sum of the filter coefficients is expressed by a power of 2. The image processing method according to claim 11, wherein the image processing method is set as follows. A program that executes the image processing method according to any one of claims 7 to 12. A computer-readable storage medium storing the program according to claim 13.

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