JP2016066982A - Image reading device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image reading device that can reduce a DC power source noise component superimposed on a read-out signal due to a periodical ripple voltage in a power source for a reading device.SOLUTION: A controller executes average gradation value calculation processing of reading a reference board and calculating an average value of gradation values of all pixels of one line corrected by a correction part to calculate a whole average gradation value, group average gradation value calculation processing of dividing the gradation values of all the pixels of one line into plural gradation value groups every fixed noise pixel number corresponding to a fixed noise period, and averaging the gradation values of all pixels of the same rank in the respective gradation value groups to calculate a group average gradation value, thinning-out candidate pixel settling processing of successively settling thinning-out candidate pixels for the gradation value group from a pixel having a larger difference between the whole average gradation value and the group average gradation value, and line thinning-out pixel setting processing of settling line thinning-out pixels on the basis of the thinning-out candidate pixels for all the pixels of one line, and setting the line thinning-out pixel to a set value.SELECTED DRAWING: Figure 7

Description

  The present invention relates to an image reading apparatus.

In the image reading apparatus, there is a problem that noise is generated in the read signal due to fluctuations in the power input to the reading device of the image reading apparatus. For example, a DC power supply noise component due to a ripple voltage in the power supply of the reading device is superimposed on the read signal.
The image reading apparatus described in Patent Document 1 extracts a noise component to be superimposed on a read signal, and subtracts the extracted noise component from the read signal to remove the noise component.

JP 2005-348340 A

  However, if the reading device outputs the electric signal as a reading signal at the same time as the photoelectric conversion element in the reading device outputs the electric signal, it can be switched to a different light source color until all the reading signals are output. The reading speed becomes slow. Therefore, in recent years, the reading device has built-in analog memories as many as the number of photoelectric conversion elements, and after reading one line, temporarily stores an electrical signal in the analog memory, so that the previous one line is stored. Immediately after reading, the color can be switched to a different light source for reading the next one line.

  The reading device configured as described above outputs a signal obtained by reading the previous one line as a read signal while reading the next one line. While the noise component for one line is the noise component for the next line, it is not the noise component for the common line. Therefore, even if the technique for removing the noise component described in Patent Document 1 is used for a reading device used in recent years without considering the timing for removing the noise component, there is a problem that the noise component cannot be removed. It was.

  An object of the present application is to reduce a DC power supply noise component superimposed on a read signal due to a periodic ripple voltage in a power supply of a reading device.

  In order to achieve the above object, according to an aspect of the present invention, a reading unit that reads a document for each line at a predetermined reading line cycle and generates analog image data of each pixel in the main scanning direction, and the reading unit A reference plate disposed at a position opposite to the power supply, a power supply unit that converts the first DC voltage to a second DC voltage to be supplied to the reading unit by switching control with a switching frequency, and a sampling period for the analog image data A conversion unit that samples and converts the digital image data into digital image data, a correction unit that corrects the digital image data to a gradation value, and a resolution of the digital image data by thinning out pixels in the main scanning direction according to a set value A resolution conversion unit for converting from the first resolution to the second resolution, and a control unit, wherein the control unit reads the reference plate. Average gradation value calculation processing for calculating the total average gradation value by averaging the gradation values of all pixels of one line corrected by the correction unit, the period corresponding to the switching frequency, and the sampling of the conversion unit Dividing the gradation values of all pixels of the one line into a plurality of gradation value groups for each constant noise pixel number corresponding to a constant noise period that is a common multiple of the period, and the same among the gradation value groups A group average gradation value calculation process for calculating a group average gradation value by averaging the gradation values of all the pixels in the rank, and a pixel having a large difference between the total average gradation value and the group average gradation value Thinning candidate pixel determination processing for determining thinning candidate pixels in the gradation value group in order, and line thinning pixels are determined based on the thinning candidate pixels for all pixels in the one line. Run a line thinning pixel setting process for setting the line thinning pixels in the set value, and the line scanning cycle is determined to an integral multiple of the constant noise cycle.

  According to a specific aspect of the present invention, the control unit calculates an integer corresponding to the first resolution from a ratio of an integer corresponding to the first resolution and an integer corresponding to the second resolution. Calculating the number of pixels, performing a decimation calculation process for calculating a value obtained by subtracting an integer corresponding to the second resolution from the number of pixels of the decimation group as a decimation pixel number decimation from the decimation group; The group is a group for sequentially grouping consecutive pixels from the head to the tail of all the pixels of the one line, and determining the line thinning pixels for each group, and the thinning candidate pixels are the gradation values The group includes a first thinned pixel and a second thinned pixel, and the first thinned pixel is a pixel related to the first resolution and the second resolution. And the second thinned pixel is a pixel not related to the first resolution and the second resolution, and the line thinned pixel setting process includes the first thinned pixel in the process of determining the line thinned pixel in each thinned group. The line-thinned pixels corresponding to the number of thinned-out pixels are determined in preference to the thinned-out pixels over the second thinned-out pixels.

  In a specific aspect according to claim 3, the control unit calculates a thinning rate by dividing the second resolution by the first resolution, the constant noise pixel number, and the thinning-out rate. A pixel determination value calculation process for calculating a pixel determination value by multiplying the rate and a first decimation pixel determination process in which the integer of the integer part of the pixel determination value is set to the first decimation pixel number are executed.

  5. The specific aspect according to claim 4, wherein the first decimation pixel determination process includes the first decimation pixel as a first decimation pixel from a pixel having a large difference between the total average gradation value and the group average gradation value. Set the number.

  In the specific aspect according to claim 5, when the first thinned pixel of the number of thinned pixels is determined in the thinned group, the control unit determines that the first thinned pixel is not determined as the first thinned pixel. On the other hand, forbidden pixel setting processing is performed for setting as a forbidden pixel for prohibiting the setting of the first thinned pixel.

  In a specific aspect according to claim 6, the control unit determines that the second thinned pixel is determined when the pixel determination value is not an integer, or when the first thinned pixel number is not determined for the first thinned pixel. The second decimation pixel determination process for setting the second decimation pixel setting flag for confirming to ON is executed.

  In the specific aspect according to claim 7, the second thinned pixel determination process includes the first thinned pixel and the second thinned pixel in each thinned group when the second thinned pixel setting flag is on. The second thinned pixel is determined so that the total number of pixels becomes the thinned pixel number.

  According to an eighth aspect of the present invention, a document is read line by line at a predetermined reading line cycle, and an analog image data of each pixel in the main scanning direction is generated, and arranged at a position facing the reading unit. A reference plate, a power supply unit that converts the first DC voltage to a second DC voltage to be supplied to the reading unit by performing switching control with a switching frequency, and the analog image data is sampled at a sampling period into digital image data. A conversion unit for converting, a correction unit for shading correction of the digital image data to a gradation value, and a resolution of the digital image data are converted from the first resolution to the second resolution by thinning out pixels in the main scanning direction. The resolution conversion unit reads the reference plate and averages the gradation values of all pixels of one line corrected by the correction unit. An average gradation value calculation unit for calculating the total average gradation value, and a plurality of levels for each number of fixed noise pixels corresponding to a fixed noise period that is a common multiple of a period corresponding to the switching frequency and a sampling period of the conversion unit. Divide the gradation values of all the pixels in one line into a tone value group, and calculate the group average gradation value by averaging the gradation values of all the pixels in the same order in each gradation value group A group average gradation value calculation unit, and a thinning candidate pixel determination unit that determines a thinning candidate pixel for the gradation value group in order from a pixel having a large difference between the total average gradation value and the group average gradation value A line thinning pixel determination unit that determines all the pixels of the one line as line thinning pixels based on the thinning candidate pixels, and the resolution conversion unit includes the line thinning pixels Thinning, the line scanning cycle is determined to an integral multiple of the constant noise cycle.

  In the first aspect of the invention, the average gradation value calculation processing is performed by calculating an average value of gradation values of all the pixels of one line corrected by reading the reference plate and corrected by the correction unit. Is a process for calculating. The group average tone value calculation process divides the tone value of all pixels in one line into a plurality of tone value groups for each fixed noise pixel number corresponding to a fixed noise period, A group average gradation value is calculated by averaging the gradation values of all the pixels. In the thinning candidate pixel determination process, thinning candidate pixels are determined for the gradation value group in order from the pixel having the largest difference between the total average gradation value and the group average gradation value. In the line thinning pixel setting process, for all pixels in one line, line thinning pixels are determined based on the thinning candidate pixels, and the line thinning pixels are set to set values. Therefore, since the thinning candidate pixels are determined every fixed noise period and the thinning pixels are determined based on the thinning candidate pixels, the noise component can be reduced.

  In a specific aspect according to claim 2, for a thinning group for determining thinned pixels of all pixels in one line, the thinning calculation processing is performed by calculating a ratio between an integer corresponding to the first resolution and an integer corresponding to the second resolution. Of these, an integer corresponding to the first resolution is calculated as the number of pixels in the thinning group, and a value obtained by subtracting an integer corresponding to the second resolution from the number of pixels in the thinning group is calculated as a thinning number to be thinned out from the thinning group. The thinning candidate pixels are composed of a first thinning pixel and a second thinning pixel with respect to the gradation value group. In the line thinning pixel setting process, the line thinning pixel is determined in preference to the second thinning pixel not associated with the first thinning pixel related to the first resolution and the second resolution. Therefore, since the first thinned pixel related to the resolution is determined for each thinned group in preference to the second thinned pixel not related to the resolution, the line thinned pixel is determined, so that the conversion from the first resolution to the second resolution is performed with high accuracy. be able to.

  According to a specific aspect of the present invention, the thinning rate calculation process calculates the thinning rate by dividing the first resolution and the second resolution. The pixel determination value calculation unit calculates a pixel determination value by multiplying a certain number of noise pixels and a thinning rate. In the first decimation pixel determination process, the integer of the integer part of the pixel determination value is set as the first decimation pixel number. Therefore, by setting the number of first thinned pixels as an integer in the integer part of the pixel determination value, it is possible to calculate a thinned number that can always be thinned in a fixed noise pixel group, and to always thin out pixels that are affected by noise. Noise components can be reduced.

  According to a specific aspect of the present invention, the first thinned pixel determination process sets the first thinned pixel from pixels having a large difference between the total average gradation value and the group average gradation value. Therefore, pixels with a large amount of noise can be removed.

  According to a specific aspect of the present invention, when the first thinned-out pixel of the number of thinned-out pixels is determined in the thinned-out group, the prohibited pixel setting process is performed for the pixels that are not determined as the first thinned-out pixels. It is set as a setting prohibition pixel that prohibits the determination of the thinned pixel. Therefore, since only the first thinned pixels equal to or smaller than the number of thinned pixels are determined in the thinned pixel group, the first thinned pixels can always be thinned.

  According to a specific aspect of the present invention, when the pixel determination value is not an integer or when the first number of thinned pixels is not determined, the second thinned pixel determination process sets the second thinned pixel. Turn on the flag. Therefore, when it is not necessary to determine the second thinned pixel, the thinning process can be executed at high speed by turning off the second thinned pixel setting flag.

  According to a specific aspect of the present invention, in the second thinning pixel determination process, when the second thinning flag is ON, the total number of pixels of the first thinning pixel and the second thinning pixel is thinned in each thinning group. The second thinned pixels are determined so that the number of pixels is reached. Accordingly, in each thinning group, the thinned-out pixels can be determined by adding up the first thinned-out pixels and the second thinned-out pixels, so that the conversion from the first resolution to the second resolution can be performed with high accuracy. .

1 is a front view showing an internal configuration of an image reading apparatus 1 according to an embodiment of the present invention. 2 is an enlarged view illustrating a configuration of a reading unit 24 of the image reading apparatus 1. 2 is a block diagram illustrating a power supply configuration of the image reading apparatus 1. FIG. 2 is a block diagram illustrating an electrical configuration of the image reading apparatus 1. FIG. It is a flowchart which shows a maintenance main process. It is a flowchart which shows a reading main process. It is a flowchart which shows a thinning setting process. It is a flowchart which shows a 1st thinning pixel determination process. It is a flowchart which shows a 2nd thinning pixel determination process. It is a flowchart which shows 1 line thinning pixel setting processing. It is drawing explaining a 1st specific example. It is drawing explaining the 2nd specific example.

[Embodiment]
Hereinafter, an image reading apparatus 1 according to an embodiment of the present invention will be described with reference to the drawings. In FIG. 1, the vertical direction and the front-back direction are indicated by arrows.

<Mechanical Configuration of Image Reading Apparatus 1>
In FIG. 1, the image reading apparatus 1 includes a paper feed tray 2, a main body 3, and a paper discharge tray 4. The operation unit 5 and the display unit 6 are disposed on the upper surface of the main body unit 3. The operation unit 5 includes a power switch and various setting buttons, and receives operation commands and the like from the user. For example, the operation unit 5 includes a selection button for selecting one of a three-color mode and a mono mode and an operation button for setting a resolution. The display unit 6 includes an LCD and displays the status of the image reading apparatus 1.

  A conveyance path 20 is formed inside the main body 3. The document GS placed on the paper feed tray 2 is transported in the transport direction FD along the transport path 20 and is discharged to the paper discharge tray 4. A paper feed roller 21, a separation pad 22, a pair of upstream transport rollers 23, a reading unit 24, a platen glass 25, and a pair of downstream transport rollers 26 are arranged along the transport path 20.

  The paper feed roller 21 cooperates with the separation pad 22 to feed a plurality of originals GS placed on the paper feed tray 2 one by one. The upstream side conveyance roller 23 and the downstream side conveyance roller 26 are driven by a conveyance motor MT (see FIG. 4). The platen glass 25 is light transmissive and is disposed along the transport path 20 below the transport path 20. The transport rollers 23 and 26 transport the document GS so that the document GS fed from the paper feed roller 21 passes over the platen glass 25.

  In the present embodiment, the document GS is placed on the paper feed tray 2 so that the reading surface of the document GS faces the placement surface of the paper feed tray 2. The reading unit 24 is disposed below the conveyance path 20 and reads an image on the reading surface of the document GS passing through the platen glass 25. The document sensor 27 is disposed in the paper feed tray 2 and is turned on when the document GS is placed on the paper feed tray 2 and turned off when the document GS is not placed on the paper feed tray 2. Is done.

(Detailed configuration of the reading unit 24)
A detailed configuration of the reading unit 24 will be described with reference to FIG. In FIG. 2, the reading unit 24 includes a light source 30, a light receiving unit 31, and an optical member 32. The light source 30 includes light emitting diodes of three colors of red, green, and blue. When the light emitted from the light source 30 is reflected by the reading surface of the document GS or the like, the optical member 32 guides the reflected light to the light receiving unit 31. In the present embodiment, when the color mode is selected, the three-color light emitting diodes are sequentially turned on to read an image of the original GS for one line. When the mono mode is selected, an image of the original GS for one line is read by turning on a specific one of the three colors, for example, a green light emitting diode.

  The gray reference plate 34 is disposed at a position facing the reading unit 24 via the conveyance path 20. The gray reference plate 34 has a lower reflectance than white, which is the background color of the document GS. When the document GS is not present on the conveyance path 20, the light emitted from the light source 30 is reflected by the gray reference plate 34, and the reflected light is received by the light receiving unit 31 through the optical member 32. The optical member 32 includes a rod lens extending in the main scanning direction MD.

  The light receiving unit 31 has a large number of sensor IC chips arranged linearly in the main scanning direction MD, and each sensor IC chip includes a large number of photoelectric conversion elements 33 arranged in the main scanning direction MD, which are not shown. Built-in shift register and amplifier. Here, each photoelectric conversion element 33 is an element that outputs one pixel.

<Electrical Configuration of Image Reading Apparatus 1>
The electrical configuration of the image reading apparatus 1 will be described with reference to FIGS. In FIG. 3, the image reading apparatus 1 includes an AC / DC conversion unit 48, a DC / DC conversion unit 49, a drive unit 47, a reading unit 24, and a control unit 50 as a part of constituent elements. Among these components, the AC / DC converter 48 and the DC / DC converter 49 are converters that convert voltages, and supply power to the drive unit 47, the reading unit 24, and the control unit 50 at different voltages. doing. The control unit 50 includes a CPU 40, a ROM 41, a RAM 42, a device control unit 44, and an image processing unit 46, and controls the image reading apparatus 1. Details will be described later.

  The AC / DC converter 48 converts the AC voltage 100V into a DC voltage 24V and outputs the DC voltage to the DC / DC converter 49. That is, the AC / DC converter 48 converts the AC voltage 100V into the DC voltage 24V using a known circuit configuration such as a diode bridge and an integration circuit.

  The DC / DC converter 49 generates the necessary DC voltage by performing PWM control on the DC voltage 24V input from the AC / DC converter 48 according to the PWM setting value, and then integrating using an integration circuit. Yes. For example, the DC voltage 3.3V required by the reading unit 24 is generated by PWM control of the DC voltage 24V so that the conduction ratio is 13.75% (3.3V / 24V = 13.75%). Is done. The DC voltage 1.8V required by the controller 50 and the DC voltage 24V required by the drive circuit 47 are similarly generated by PWM control.

  In FIG. 4, the image reading apparatus 1 includes a CPU 40, a ROM 41, a RAM 42, a flash PROM 43, a device control unit 44, an analog front end (hereinafter referred to as AFE) 45, an image processing unit 46, and a drive circuit 47. Prepare as a part. These components are connected to the operation unit 5, the display unit 6, and the document sensor 27 via the bus 48.

  The ROM 41 stores programs for executing various operations of the image reading apparatus 1 such as a maintenance main process, a reading main process, and a subroutine process in each main process, which will be described later. The CPU 40 controls each unit according to the program read from the ROM 41. The flash PROM 43 is a readable / writable nonvolatile memory, and stores various data generated by the control process of the CPU 40, such as black data BK and white data WH acquired by the maintenance main process. The RAM 42 temporarily stores the calculation result generated by the control process of the CPU 40.

  The device control unit 44 is connected to the reading unit 24 and transmits to the reading unit 24 a signal for controlling turning on / off of the light source 30 and a signal for controlling a current value flowing through the light source 30 based on a command from the CPU 40. To do. In addition, the device control unit 44 receives the clock signal CLK and the serial-in signal SI in order to sequentially operate a large number of photoelectric conversion elements 33 of each sensor IC chip of the light receiving unit 31 based on a command from the CPU 40. Transmit to the light receiving unit 31. When receiving the lighting control signal from the device control unit 44, the reading unit 24 turns on the light source 30 and transmits an analog signal corresponding to the amount of light received by the light receiving unit 32 to the AFE 45.

  The AFE 45 is connected to the reading unit 24 and converts an analog signal transmitted from the reading unit 24 into a digital signal based on a command from the CPU 40. The AFE 45 has a predetermined input range and resolution. For example, if the resolution is 10 bits, the gradation is from “0” to “1023”. In this case, the AFE 45 converts the analog signal transmitted from the reading unit 24 into 10-bit (0 to 1023) digital image data as a digital signal. The digital image data converted by the AFE 45 is transmitted to the image processing unit 46.

  The image processing unit 46 is composed of an ASIC that is a dedicated IC for image processing, and performs various types of image processing on the digital image data. The image processing includes various correction processing such as shading correction, resolution conversion processing, and the like. The image processing unit 46 performs shading correction on the digital image data using the black data BK and the white data WH to correct the gradation value. Further, the image processing unit 46 performs resolution conversion processing by thinning out and outputting the gradation values. Specifically, the image processing unit 46 thins out a plurality of thinned pixels set for all the pixels from the first pixel to the last pixel, and outputs a gradation value. The tone values after the thinning are stored in the RAM 42 via the bus 48.

  The drive circuit 47 is connected to the transport motor MT and drives the transport motor MT based on a drive command transmitted from the CPU 40. The drive circuit 47 rotates the transport motor MT according to the rotation amount and the rotation direction commanded by the drive command. When the transport motor MT rotates by a predetermined amount, the transport rollers 23 and 26 rotate by a predetermined angle, and the document GS is transported by a predetermined distance on the transport path 20.

<Operation of Embodiment>
Next, the operation of the image reading apparatus 1 will be described with reference to the drawings. The image reading apparatus 1 mainly executes a maintenance main process executed before reading the document GS and a read main process for reading the document GS. The processing of steps M1 to M10 during the maintenance main processing, the processing of steps MA1 to MA13 during the reading main processing, and the processing of the steps of each subroutine are processing executed by the CPU 40.

(Maintenance main processing)
The maintenance main process shown in FIG. 5 is performed when an operator such as a service person operates the operation unit 5 of the image reading apparatus 1 before the image reading apparatus 1 is shipped from the factory or when a service person performs maintenance inspection after the shipment. Start by operating according to a special operating method.

  First, when an operator places a special document GS serving as a white reference on the paper feed tray 2, the document sensor 27 detects the document GS. It is determined whether there is a document GS according to the detection signal from the document sensor 27 (M1). When document GS is present (M1: YES), the process proceeds to step M2. When there is no document GS (M1: NO), the process proceeds to step M10, an error message notifying that there is no document GS is displayed on the display unit 6 (M10), and the maintenance main process ends.

  The CPU 40 sets reading setting values for the device control unit 44 and the AFE 45 (M2). Specifically, the CPU 40 acquires a clock signal CLK corresponding to each resolution from the flash PROM 43 and sets it in the device control unit 44. Further, the CPU 40 acquires the offset adjustment value, gain adjustment value, and sampling value of the AFE 45 from the flash PROM 43 and sets them in the AFE 45. The set value of the clock signal CLK is a set value for transferring the signal of one pixel of the shift register included in the light receiving unit 31. Therefore, the set value of the clock signal CLK is a value corresponding to the output speed per pixel of the analog signal output from the light receiving unit 31 (hereinafter referred to as pixel speed). The sampling value is a value indicating timing for converting an analog signal into a digital signal, and is set for each pixel. In other words, the sampling period is the same as the pixel period. The offset adjustment value is a value for shifting the level of the analog signal input to the AFE 45, and the gain adjustment value is a value for adjusting the gain of the analog signal input to the AFE 45.

  The CPU 40 calculates a constant noise period (M3). Specifically, the CPU 40 acquires the PWM setting value from the flash PROM 43 and calculates the switching cycle SWT of the DC / DC converter 49. The CPU 40 calculates the pixel period IPT from the pixel speed. The CPU 40 calculates the least common multiple of the switching period SWT and the pixel period IPT as the constant noise period FNT. Further, a value obtained by dividing the constant noise period FNT by the pixel period IPT is stored in the RAM 42 as the constant noise pixel number FNC.

  The CPU 40 sets a line cycle (M4). Specifically, the CPU 40 calculates the set value of the serial-in signal SI corresponding to each resolution so as to be an integral multiple of the constant noise period FNT, and sets it as a line period in the device control unit 44.

  The CPU 40 adjusts the light amount of the light source 30 (M5). The CPU 40 irradiates light from the light source 30 toward the white reference document GS, and adjusts the light amount ST so that the analog signal when the reflected light is read becomes the maximum value of the AFE 45. Here, the light quantity ST is determined by the lighting time and current value in one line of the light source 30.

  The CPU 40 acquires black data BK (M6). The CPU 40 turns off the light source 30 and reads the white reference document GS. Then, the read digital image data for one line is acquired as black data BK and stored in the flash PROM 43.

  The CPU 40 acquires white data WH (M7). The CPU 40 turns on the light source 30 with the light quantity ST and reads the white reference document GS. Then, the CPU 40 acquires the read digital image data for one line as white data WH and stores it in the flash PROM 43.

  The CPU 40 determines whether or not there is a document GS according to the detection signal from the document sensor 27 (M8). If CPU 40 determines that there is no document GS (M8: YES), CPU 40 advances the process to the maximum value detection process (M9) of ash data. If CPU 40 determines that there is document GS (M8: NO), CPU 40 advances the process to error display process (M10). The CPU 40 causes the display unit 6 to display an error message notifying that the document GS has been erroneously placed (M10), and the maintenance main process ends.

  The CPU 40 detects the ash data maximum value GRmax (M9). The CPU 40 turns on the light source 30 with the light amount ST and reads the gray reference plate 34. Then, the CPU 40 detects the maximum value as the gray data maximum value GRmax among the read digital image data for one line. The CPU 40 stores the ash data maximum value GRmax in the flash PROM 43. When the gray data maximum value GRmax is stored, the maintenance main process ends.

(Reading main process)
The reading main process shown in FIG. 6 is started when the user places a reading document GS on the paper feed tray 2 and operates a reading start button of the operation unit 5.

  When the user operates the reading start button, the CPU 40 sets a reading setting value (MA1). In accordance with the resolution RES set by the user operating the operation unit 5, the CPU 40 sets reading setting values for the device control unit 44, the AFE 45, and the image processing unit 46. Similar to the reading setting process (M2) and the thinning-related setting (M4) in the maintenance main process, the CPU 40 sets the clock signal CLK and the serial-in signal SI to the device control unit 44 and adjusts the offset to the AFE 45. Set the value, gain adjustment value, and sampling value. Further, the CPU 40 performs shading correction on the image processing unit 46 and performs setting to save the gradation value in the RAM 42.

  The CPU 40 adjusts the light quantity ST of the light source 30 (MA2). The CPU 40 irradiates light from the light source 30 toward the gray reference plate 34 and adjusts the light quantity ST so that the maximum value of the digital image data when the reflected light is read becomes the gray data maximum value GRmax.

  The CPU 40 determines whether or not the set resolution RES set by the user is the resolution for executing the thinning process (MA3). Specifically, when the set resolution RES does not match the output resolution RED output from the reading unit 24 (MA3: Yes), the CPU 40 proceeds to a thinning setting process (MA4) for setting thinned pixels, and sets the setting resolution. When RES matches the output resolution RED (MA3: No), the CPU 40 executes a reading process (MA5).

  The CPU 40 sets pixels to be thinned out for one line in the image processing unit 46 (MA4). Although details will be described later, the thinning setting process (MA4) acquires gradation values for one line, and determines the positions of the first thinned pixels and the second thinned pixels in the fixed noise group described later from the acquired gradation values. To do. Further, the thinning setting process (MA4) determines thinned pixels for one line from the determined first thinned pixels and second thinned pixels, and sets the determined thinned pixels in the image processing unit 46.

  The CPU 40 reads the document GS (MA5). Specifically, the CPU 40 reads the document GS based on the reading setting value set in the reading setting process (MA1), and outputs it to the analog signal image processing unit 46. The CPU 40 causes the image processing unit 46 to perform shading correction on the output analog signal to generate a gradation value. The CPU 40 thins out the gradation values generated according to the thinning pixel setting for one line set in the image processing unit 46. The CPU 40 stores the thinned gradation value in the RAM 42. When no thinning pixels are set in the image processing unit 46, the gradation values are stored in the RAM 42 without being thinned out.

(Thinning setting process MA4)
When the thinning setting process (MA4) is started, the CPU 40 acquires the gradation value DT for one line (TA1). The CPU 40 irradiates the gray reference plate 34 with the light amount ST, reads the reflected light, acquires the gradation value DT for one line, and stores it in the RAM 42.

  The CPU 40 calculates the total average gradation value DTave (TA2). Specifically, the CPU 40 calculates the average value of all the gradation values DT for one line as the total average gradation value DTave and stores it in the RAM 42.

  The CPU 40 calculates each rank pixel average gradation value DTgr (TA3). Specifically, the CPU 40 calculates the average value of the gradation values for each fixed noise pixel number FNC from the gradation value of the first pixel among the gradation values DT for one line as the first rank pixel average gradation value. . The CPU 40 calculates the average value of the gradation values for each fixed noise pixel number FNC from the gradation value of the second pixel among the gradation values DT for one line as the second-order pixel average gradation value. Similarly, the CPU 40 calculates each rank pixel average gradation value DTgr for a certain number of noise pixels FNC. A group that is grouped for each fixed number of noise pixels FNC is called a fixed noise group. In the fixed noise group, a pixel on the first pixel side is a first rank pixel, a next pixel is a second rank pixel, and each rank pixel is Define. For example, when the gradation value of the gray reference plate is 600, an example of the first rank pixel average gradation value is 630, and an example of the second rank pixel average gradation value is 575.

  The CPU 40 calculates each average difference value DFave (TA4). Specifically, the CPU 40 calculates a difference value between each rank pixel average gradation value DTgr and the total average gradation value DTave as each average difference value DFave. The difference value calculation method is a method of calculating the absolute value of the subtracted value. Each average difference value DFave is calculated by a number corresponding to the fixed noise pixel number FNC for one line, and stored in association with each rank pixel.

  The CPU 40 calculates the thinning interval TOI, the thinning number TON, and the thinning rate TOR (TA5). Specifically, the CPU 40 calculates an integer ratio between the set resolution RES set by the user and the output resolution RED output from the reading unit 24. The CPU 40 calculates an integer corresponding to the output resolution RED out of the integer ratio as the thinning interval TOI, and calculates a value obtained by subtracting an integer corresponding to the set resolution RES from the thinning interval TOI as the thinning number TON. . Further, the CPU 40 calculates a value obtained by dividing the thinning number TON by the thinning interval TOI as the thinning rate TOR. Here, the thinning interval TOI corresponds to the number of pixels in a thinning group described later.

  The CPU 40 determines the first thinned pixel (TA6). The CPU 40 determines the first thinned pixel for the constant noise group. Although details will be described later, the number of pixels of the fixed noise group and the number of pixels of the thinning group may be different. Therefore, if a thinned pixel is set for each fixed noise group instead of a thinned group, it is necessary to set a first thinned pixel that is thinned preferentially and a second thinned pixel that is thinned when there is no first thinned pixel. is there. In this process (TA6), the first thinned pixels to be thinned out with priority are determined.

  The CPU 40 determines the second thinning pixel (TA7). The CPU 40 determines the second thinning pixel for the constant noise group. Although details will be described later, in this processing, in the thinning group, when the number of pixels of the first thinning pixel is less than the thinning number TON, the second thinning pixel to be thinned is determined.

  The CPU 40 determines the thinned pixels for one line and sets the determined thinned pixels in the image processing unit 46 (TA8). Although details will be described later, the CPU 40 sets one or both of the first thinned pixels and the second thinned pixels in the image processing unit 46 as thinned pixels for each thinning group in order from the top pixel.

(First thinning pixel determination process TA6)
When the first thinned pixel determination process (TA6) is started, the CPU 40 calculates a pixel determination value JV (TB1). Specifically, the CPU 40 calculates the pixel determination value JV by multiplying the constant noise pixel number FNC and the thinning rate TOR.

  The CPU 40 calculates the first thinning-out number TON1 (TB2). Specifically, the CPU 40 calculates the integer part of the pixel determination value JV as the first thinning number TON1.

  The CPU 40 determines whether it is necessary to set the second thinning pixel (TB3). The second thinned pixel is a pixel that is set when further thinning is necessary when the first thinned pixel is thinned in the thinning group. Therefore, when the pixel determination value JV is an integer, the image processing unit 46 can convert the setting resolution RES by only thinning out the first thinned pixels. However, if the pixel determination value JV is not an integer, the image processing unit 46 cannot convert to the set resolution RES unless other pixels are thinned out in addition to the first thinned pixels. In this process (TB3), when the pixel determination value JV is an integer (TB3: No), the process proceeds to the maximum pixel position calculation process (TB5), and when the pixel determination value JV is not an integer (TB3: Yes). Then, the process proceeds to the process of turning on the second thinning pixel setting flag (TB4). In the process of turning on the second thinned pixel setting flag (TB4), the CPU 40 sets the second thinned pixel setting flag for performing the process of setting the second thinned pixel to ON.

  The CPU 40 calculates the maximum rank pixel that is the maximum value of each average difference value DFave (TB5). The CPU 40 calculates a pixel whose average difference value DFave is a maximum value among pixels that are not first-decimation pixel setting prohibition pixels, which will be described later in the fixed noise pixel group, and in which the first decimation pixel is not determined, and sets the maximum value Determine the associated maximum rank pixel. Here, the first decimation pixel setting prohibition pixel is a pixel that prohibits the setting of the first decimation pixel, and is determined in TB8.

  The CPU 40 determines the highest-order pixel as the first thinned pixel (TB6). The CPU 40 determines the highest rank pixel as the first thinned pixel in the constant noise pixel group.

  The CPU 40 determines whether or not the determined number of first thinned pixels matches the thinned number TON in all thinned groups including the highest-order pixel (TB7). In each thinning group, the number of pixels that can be thinned is up to the thinning number TON. Therefore, the CPU 40 determines whether or not the determined number of the first thinned pixels matches the thinned number TON in all the thinned groups including the highest-order pixel that is the latest first thinned pixel. : Yes) proceeds to the process (TB8) for making the pixels included in all the matched thinning groups the first thinned pixel setting prohibition pixel, and if they do not match (TB7: No), the first thinned number TON1 The process proceeds to a process (TB9) for determining whether the minute has been confirmed.

  The CPU 40 sets all pixels not determined as the first thinned-out pixels as the first thinned-out pixel setting prohibited pixels for the thinned-out group in which the determined number of the first thinned-out pixels matches the thinned-out number TON (TB8).

  The CPU 40 determines whether or not the number of first thinned pixels determined in the constant noise pixel group matches the first thinned pixel number TON1 (TB9). When the number of first thinned pixels determined in the constant noise pixel group matches the first thinned pixel number TON1 (TB9: Yes), the CPU 40 ends the first thinned pixel determination process (TA6) and does not match. In the case (TB9: No), the process proceeds to the process (TB10) for determining whether or not to continue the first thinned pixel determination process.

  The CPU 40 determines whether or not to continue the first thinning pixel determination process (TB10). Specifically, the CPU 40 determines whether or not there is a pixel that is not the first thinned pixel setting prohibition pixel and is not determined as the first thinned pixel in the constant noise pixel group. If there is a pixel that is not the first decimation pixel setting prohibition pixel and is not determined as the first decimation pixel in the constant noise pixel group (TB10: Yes), the process proceeds to the maximum rank pixel determination process (TB5) again, and the fixed noise When there is only a pixel determined as the first thinned pixel setting prohibited pixel or the first thinned pixel in the pixel group (TB10: No), the process proceeds to a process of turning on the second thinned pixel setting flag (TB11). In the processing of TB11, the CPU 40 sets a second thinning pixel setting flag for performing processing for setting the second thinning pixel to ON. When the process of TB11 ends, the first thinned pixel determination process (TA6) ends.

(Second thinning pixel determination process TA7)
When the second thinned pixel determination process (TA7) is started, the CPU 40 determines whether or not the second thinned pixel setting flag is ON (TC1). The second decimation pixel setting flag is ON when the first decimation pixel determination process (TA6) cannot be converted into the set resolution RES only by decimation of the first decimation pixel.

  The CPU 40 selects the next thinning group (TC2). Specifically, when the thinning group is not currently selected, the CPU 40 selects, as a thinning group, pixels corresponding to the number of pixels of the thinning group from the first pixel of the 1-line gradation value DT acquired in the TA1 process. When the group is selected, pixels corresponding to the number of pixels in the thinning group are selected as the thinning group from the next pixel of the currently selected thinning group.

  The CPU 40 determines whether or not the first thinning pixel is determined as the thinning number TON for the selected thinning group (TC3). When the first thinning pixel is determined as the thinning number TON (TC3: Yes), the process proceeds to processing (TC6) for determining whether the selected thinning group is the final thinning group, and the first thinning pixel is the thinning number. If the TON has not been determined (TC3: No), the process proceeds to the process (TC4) for calculating the second thinning-out number TON2.

  The CPU 40 calculates the second thinning number TON2 (TC4). Specifically, the CPU 40 calculates the second thinning number TON2 by subtracting the first thinning pixel number set in the selected thinning group from the thinning number TON.

  The CPU 40 determines the second thinned pixel (TC5). Specifically, the CPU 40 calculates a rank pixel in which each average difference value DFave has a maximum value, except for a pixel determined as the first thinning pixel in the selected thinning group, and sets the rank pixel as the first pixel. Determined as 2 thinned pixels.

  The CPU 40 determines whether or not the selected thinning group is the final thinning group (TC6). When the selected thinning group is not the final thinning group (TC6: No), the process proceeds to the process of selecting the thinning group (TC2). When the selected thinning group is the final thinning group (TC6: Yes) Then, the second thinning pixel determination process (TA7) is terminated.

(1 line thinning pixel setting processing TA8)
When the one-line thinning pixel setting process (TA8) is started, the CPU 40 selects the next thinning group (TD1). Specifically, when the thinning group is not currently selected, the CPU 40 selects, as a thinning group, pixels corresponding to the number of pixels of the thinning group from the first pixel of the 1-line gradation value DT acquired in the TA1 process. When the thinning group is selected, pixels corresponding to the number of pixels of the thinning group are selected as the thinning group from the next pixel of the currently selected thinning group.

  The CPU 40 determines whether or not there is a first thinning pixel in the currently selected thinning group (TD2). When there is no first thinned pixel (TD2: No), the process proceeds to the process of setting the second thinned pixel as a thinned pixel (TD5), and when there is a first thinned pixel (TD2: Yes), the first thinned pixel is selected. It progresses to the process (TD3) set as a thinning pixel.

  The CPU 40 determines and sets the first thinned pixel as the thinned pixel (TD3). Specifically, the CPU 40 determines the first thinned pixel as a thinned pixel and sets it in the image processing unit 46.

  The CPU 40 determines whether or not the image processing unit 46 has been set for the thinning number TON for the pixels of the currently selected thinning group (TD4). When the setting of the thinning number TON is executed for the pixels of the currently selected thinning group (TD4: Yes), a process for determining whether or not the currently selected thinning group is the final thinning group (TD6). If the setting of the thinning number TON is not executed for the pixels of the currently selected thinning group (TD4: No), the process proceeds to the process of setting the second thinning pixel as a thinning pixel (TD5).

  The CPU 40 determines and sets the second thinned pixel as the thinned pixel (TD5). Specifically, the CPU 40 determines the second thinned pixel as a thinned pixel and sets it in the image processing unit 46.

  The CPU 40 determines whether or not the selected thinning group is the final thinning group (TD6). When the selected thinning group is not the final thinning group (TD6: No), the process proceeds to the process of selecting the thinning group (TD1). When the selected thinning group is the final thinning group (TC6: Yes) Then, the second thinning pixel determination process (TA7) is terminated.

(Specific case)
(First example)
Next, various cases realized by the processes of FIGS. 6 to 10 will be described with reference to FIGS. 11 and 12. A common part of the first specific example and the second specific example will be described. 11 and 12 show an analog signal output from the reading unit 24 from the first pixel to the sixth pixel and a direct current supplied to the reading unit 24 of the DC / DC conversion unit 49 when the photoelectric conversion element 33 operates. It is the figure which showed the voltage fluctuation of voltage 3.3V. Since the fifth pixel from the top pixel is the first rank pixel and the sixth pixel from the top is the second rank pixel, only the numbers are shown in parentheses. When voltage fluctuation occurs in the DC voltage 3.3 V supplied to the reading unit 24, the voltage fluctuation is superimposed on the analog signal output from the reading unit 24. In the waveform chart shown in the figure, the vertical axis indicates the output of each signal, and the horizontal axis indicates time.

  Specific example 1 will be described with reference to FIG. In the specific example 1, the switching cycle SWT of the DC / DC converter 49 is 1 μs, and the pixel cycle IPT is 250 ns. In this specific example, the constant noise period calculation process (M3) of the maintenance main process calculates the least common multiple of 1 μs of the switching period SWT and 250 ns of the pixel period IPT, and calculates 1 μs of the constant noise period FNT.

  Next, a thinning setting process (MA4) in the reading main process sets thinning pixels for one line in the image processing unit 46. Specifically, it will be described in order. In the total average gradation value DTave calculation process (TA2), the average value of all the gradation values DT for one line is calculated, and the total average gradation value DTave illustrated by the broken line is calculated.

  Next, each rank pixel average gradation value DTgr calculation process (TA3) calculates rank pixel average gradation values DTgr from the first rank pixel to the fourth rank pixel. Each average difference value DFave calculation process (TA4) calculates an average difference value DFave illustrated by arrows from the first rank pixel to the fourth rank pixel. In this specific example, the average difference value DFave of the first rank pixel is the largest, the average difference value DFave of the fourth rank pixel is the second largest, and the average difference value DFave of the second rank pixel is the third largest. .

  In the first specific example, the output resolution RED output by the reading unit 24 is 300 DPI, and the setting resolution RES set by the user is 200 DPI. In this case, the TA5 process in the thinning setting process (MA4) calculates an integer ratio 3: 2 of 300 DPI of the output resolution RED and 200 DPI of the setting resolution RES, and sets 3 which is an integer corresponding to the output resolution RED as a thinning interval. Calculated as TOI (in other words, the number of pixels of the number of thinning groups). Further, this process calculates 1 as the thinning-out number TON by subtracting 2 as an integer corresponding to the set resolution RES from 3 as the thinning-out interval TOI. Further, in this process, 0.33, which is a value obtained by dividing 1 as the thinning number TON from 3 as the thinning interval TOI, is calculated as the thinning rate TOR.

  The pixel determination value JV calculation process (TB1) multiplies the fixed noise pixel number FNC by 4 and the thinning rate TOR 0.33 to calculate 1.32 as the pixel determination value JV. In the first thinned pixel number TON1 calculation process (TB2), the integer part 1 of the pixel determination value JV is calculated as the first thinned pixel number TON1. In the process of TB3, since the pixel determination value JV is not an integer (TB3: Yes), it is determined that the second thinned pixel needs to be set, and the second thinned pixel setting flag is turned ON (TB4).

  Next, in the maximum rank pixel determination process (TB5), the first rank pixel is determined as the maximum rank pixel. In the first thinned pixel determination process (TB6), the first rank pixel that is the highest rank pixel is confirmed as the first thinned pixel.

  Next, the processing of TB7 includes a first rank pixel, a second rank pixel, a group of third rank pixels, a third rank pixel, and a fourth rank pixel that are all thinning groups including the first rank pixel that is the maximum rank pixel. In the rank pixel, first rank pixel group, and fourth rank pixel, first rank pixel, and second rank pixel group, the first decimation pixel setting number is 1, and the decimation number TON is also 1. , TB7 is determined as Yes, and the process proceeds to TB8. In the process of TB8, the pixels included in these groups are set as the first thinned pixel setting prohibited pixels. That is, the second rank pixel, the third rank pixel, and the fourth rank pixel are set as the first thinned pixel setting prohibition pixels.

  Next, since the second thinning pixel setting flag is ON (TC1: Yes), the second thinning pixel is determined. When the thinning group selection process (TC2) selects a thinning group including the first rank pixel that is the first thinning pixel, the processing of TC3 is determined for one pixel that is the thinning pixel number TON. The process does not proceed to TC5 for determining two thinned pixels, and the process returns to the thinning group selection process (TC2) or the second thinned pixel determination process (TA7) ends.

  When the thinning group selection process (TC2) selects a group of second-ranked pixels, third-ranked pixels, and fourth-ranked pixels that are groups that do not include the first-ranked pixels that are first-thinned pixels, Since one pixel of the thinning-out number TON has not been determined (TC3: No), the process proceeds to the second thinning-out pixel determination process (TC5), and the process of TC5 is the fourth rank having the largest average difference value DFave in the group. The pixel is determined as the second thinned pixel.

  In the one-line thinning pixel setting process (TA8), thinning pixels are determined by the first rank pixels that are the first thinning pixels and the second rank pixels that are the second thinning pixels, and are determined by the image processing unit 46. Set. Specifically, in the thinning group selection process (TD1), the third pixel from the top pixel is set as a thinning group. In this case, since the first rank pixel that is the first decimation pixel is included (TD2: Yes), the first rank pixel is determined as the decimation pixel, and the first pixel is set as the decimation pixel in the image processing unit 46 (TD3 ). The process of TD4 is set to one pixel which is the thinning-out number TON (TD4: Yes), so it is determined as Yes, and the process proceeds to the process of determining whether it is the final thinning-out group (TD6). Next, since the third pixel thinning group from the currently selected first pixel is not the final thinning group (TD6: No), the flow proceeds to the thinning group selection processing (TD1).

  Next, in the thinning group selection process (TD1), the fourth to sixth pixels, which are the next thinning group, are set as the thinning group. In this case, since the first rank pixel that is the first thinned pixel is included (TD2: Yes), the first rank pixel is determined as the thinned pixel, and the fifth pixel is the thinned pixel as in the previous case. (TD3). Similar processing is repeatedly executed up to the final thinning group, thinning pixels for one line are determined, and the determined pixels are set in the image processing unit 46.

(Second specific example)
A specific example 2 will be described with reference to FIG. In the second specific example, the DC / DC converter 49 has a switching cycle SWT of 333 ns and a pixel cycle IPT of 250 ns. In this specific example, the constant noise period calculation process (M3) of the maintenance main process calculates the least common multiple of 333 ns of the switching period SWT and 250 ns of the pixel period IPT, and calculates 1 μs of the constant noise period FNT and the constant noise pixel number FNC. 4 is calculated.

  Next, a thinning setting process (MA4) in the reading main process sets thinning pixels for one line in the image processing unit 46. Specifically, it will be described in order. In the total average gradation value DTave calculation process (TA2), the average value of all the gradation values DT for one line is calculated, and the total average gradation value DTave illustrated by the broken line is calculated.

  Next, each rank pixel average gradation value DTgr calculation process (TA3) calculates rank pixel average gradation values DTgr from the first rank pixel to the fourth rank pixel. Each average difference value DFave calculation process (TA4) calculates an average difference value DFave illustrated by arrows from the first rank pixel to the fourth rank pixel. In this specific example, the average difference value DFave of the second rank pixel is the largest, the average difference value DFave of the third rank pixel is the second largest, and the average difference value DFave of the first rank pixel is the third largest. .

  In this specific example 2, the output resolution RED output by the reading unit 24 is 1200 DPI, and the setting resolution RES set by the user is 600 DPI. In this case, the TA5 process in the thinning setting process (MA4) calculates an integer ratio 2: 1 between 1200 DPI of the output resolution RED and 600 DPI of the setting resolution RES, and subtracts 2 which is an integer corresponding to the output resolution RED. Calculated as TOI (in other words, the number of pixels of the number of thinning groups). Further, this process calculates 1 as a thinning number TON by subtracting 1 as an integer corresponding to the set resolution RES from 2 as the thinning interval TOI. Further, in the present process, 0.5, which is a value obtained by dividing 1 as the thinning number TON by 2 as the thinning interval TOI, is calculated as the thinning rate TOR.

  The pixel determination value JV calculation process (TB1) multiplies the fixed noise pixel number FNC by 4 and the thinning rate TOR by 0.5 to calculate 2 as the pixel determination value JV. In the first thinned pixel number TON1 calculation process (TB2), the integer part 2 of the pixel determination value JV is calculated as the first thinned pixel number TON1. In the process of TB3, since the pixel determination value JV is an integer (TB3: No), it is determined that it is not necessary to set the second thinned pixel, and the process proceeds to the process of turning on the second thinned pixel setting flag (TB4). First, the process proceeds to the maximum rank pixel determination process (TB5).

  Next, in the maximum rank pixel determination process (TB5), the second rank pixel is determined as the maximum rank pixel. Next, in the first thinned pixel determination process (TB6), the second rank pixel that is the highest rank pixel is confirmed as the first thinned pixel.

  Next, the processing of TB7 is performed in the first rank pixel group and the second rank pixel group which are all thinning groups including the second rank pixel which is the maximum rank pixel, and in the second rank pixel group and the third rank pixel group. Since the first decimation pixel setting number is 1 and the decimation number TON is 1, it is determined Yes in the TB7 process, and the process proceeds to the TB8 process. In the process of TB8, the pixels included in these groups are set as the first thinned pixel setting prohibited pixels. That is, the first rank pixel and the third rank pixel are the first thinned pixel setting prohibition pixels.

  Next, in the process of TB9, since the determined number of first thinned pixels is 1 and the first number of thinned pixels TON1 is 2 (TB9: No), the process proceeds to TB10. Since the pixel that is not the one-thinned pixel setting prohibition pixel and is not determined as the first thinned-out pixel is the fourth rank pixel (TB10: Yes), the process proceeds to the maximum rank pixel determination process (TB5) again.

  Next, the maximum rank pixel determination process (TB5) calculates a fourth rank pixel that is not the first decimation pixel setting prohibition pixel and has the maximum average difference value DFave among the pixels that are not definite as the first decimation pixel. Then, the fourth rank pixel is determined as the first thinned pixel.

  In the second specific example, since the second thinning pixel setting flag is not ON (TC1: No), the second thinning pixel is not set.

  In the one-line thinning pixel setting process (TA8), thinning pixels are determined by the second and fourth rank pixels, which are the first thinning pixels, and the determined thinning pixels are set in the image processing unit 46. Specifically, in the thinning group selection process (TD1), the second pixel from the top pixel is set as a thinning group. In this case, since the second rank pixel that is the first thinned pixel is included (TD2: Yes), the second rank pixel is determined as the thinned pixel, and the second pixel is set as the thinned pixel in the image processing unit 46 ( TD3). The process of TD4 is set to one pixel which is the thinning-out number TON (TD4: Yes), so it is determined as Yes, and the process proceeds to the process of determining whether it is the final thinning-out group (TD6). Next, since the thinning group for the second pixel from the currently selected first pixel is not the final thinning group (TD6: No), the process proceeds to the thinning group selection process (TD1).

  Next, in the thinning group selection process (TD1), the third to fourth pixels, which are the next thinning group, are set as the thinning group. In this case, since the fourth rank pixel that is the first decimation pixel is included (TD2: Yes), the fourth rank pixel is determined as the decimation pixel, and the fourth pixel is the decimation pixel. (TD3). Similar processing is repeatedly executed up to the final thinning group, thinning pixels for one line are determined, and the determined pixels are set in the image processing unit 46.

<Effect of embodiment>
In the present embodiment, in each rank pixel average gradation value DTgr calculation process (TA3) of the thinning setting process (MA4), a fixed noise pixel group with respect to the gradation value DT when the gray reference plate having a uniform density is read. Each rank pixel average gradation value DTgr is calculated by averaging the gradation values for each rank pixel. Therefore, each rank pixel average gradation value DTgr represents the influence of the noise component on the gradation value for each rank pixel. Further, since the difference value between the total average gradation value DTave obtained by averaging the gradation values of one line and each rank pixel average gradation value DTgr is calculated as each average difference value DFave, the gradation value of the noise component is changed. This makes it possible to identify pixels that have a large influence. Since the first decimation pixel determination process (TA6) and the second decimation pixel determination process (TA7) determine the first decimation pixel and the second decimation pixel for a certain noise pixel group, the gradation of the noise component It is possible to specify only pixels that have a large influence on the value. In the one-line thinning pixel setting process (TA8), the thinning pixels are determined with priority given to the first thinning pixels for each thinning group, and the thinning pixels are set for one line. Therefore, it has the effect of improving resolution conversion accuracy and the effect of removing pixels that have a large influence on the tone value of noise components. it can.

[Correspondence between embodiment and invention]
The image reading device 1, the reading unit 24, and the gray reference plate 34 are examples of the image reading device, the reading unit, and the reference plate of the present invention. The DC / DC conversion unit 49 is an example of the power supply unit of the present invention, and the image processing unit 46 is an example of the conversion unit, the correction unit, and the resolution conversion unit of the present invention. The CPU 40 is an example of the control unit of the present invention. The total average gradation value DTave calculation process (TA2) is an example of the average gradation value calculation process of the present invention. Each rank pixel average gradation value DTgr calculation process (TA3) is an example of the group average gradation value calculation process of the present invention. The first thinning pixel determination process (TA6) and the second thinning pixel determination process (TA7) are examples of the thinning candidate pixel determination process of the present invention. One line thinning pixel setting process (TA8) is an example of the line thinning pixel setting process of the present invention.

[Modification]
The present invention is not limited to this embodiment, and various modifications can be made without departing from the spirit of the present invention. An example of the modification will be described below.

(1) The image reading apparatus 1 according to the present embodiment may be applied to a multi-function peripheral including a printer unit. Although the power supply of the DC voltage 3.3V has been described as the power supply of the reading unit 24, another DC voltage may be used, or a power supply common to other circuits may be used.

(2) In this embodiment, the constant noise period FNT and the constant noise pixel number FNC are calculated in the constant noise period calculation process (M3), and become an integral multiple of the constant noise period FNT in the line period setting process (M4). The line cycle is set as described above, but the switching cycle SWT and the pixel cycle IPT of the DC / DC converter 49 are determined as fixed values in the product design stage, and the fixed noise pixel number FNC and the line cycle are set in advance in the apparatus main body. You may remember it.

(3) Although the thinning setting process (MA4) is realized by software executed by the CPU in this embodiment, this thinning setting process may be realized by a hardware function.

(4) In the present embodiment, the description has been given using the gray reference plate 34. However, even if a non-gray color, for example, a white reference plate or a black reference plate is used, the influence of the ripple voltage similarly occurs. The same effect is produced.

(5) In the present embodiment, in the one-line thinning pixel setting process (TA8), the thinning pixel determination process and the setting process are realized in the same process. It may be realized by one process.

DESCRIPTION OF SYMBOLS 1 ... Image reading apparatus, 5 ... Operation part, 24 ... Reading part, 30 ... Light source, 31 ... Light receiving part, 33 ... Photoelectric conversion element, 40 ... CPU, 43 ... Flash PROM, 45 ... AFE, 46 ... Image processing part

Claims (8)

A reading unit that reads an original for each line at a predetermined reading line cycle and generates analog image data of each pixel in the main scanning direction;
A reference plate disposed at a position facing the reading unit;
A power supply unit that converts the first DC voltage to the second DC voltage supplied to the reading unit by switching control at a switching frequency;
A converter that samples the analog image data at a sampling period and converts the analog image data into digital image data;
A correction unit for correcting the shading of the digital image data into gradation values;
A resolution conversion unit that converts the resolution of the digital image data from the first resolution to the second resolution by thinning out pixels in the main scanning direction according to a set value;
A control unit,
The controller is
An average gradation value calculation process for calculating the total average gradation value by reading the reference plate and averaging the gradation values of all pixels in one line corrected by the correction unit;
The gradation values of all pixels of the one line are assigned to a plurality of gradation value groups for each number of fixed noise pixels corresponding to a fixed noise period that is a common multiple of a period corresponding to the switching frequency and a sampling period of the conversion unit. A group average gradation value calculation process for calculating a group average gradation value by dividing and averaging the gradation values of all the pixels of the same rank in each gradation value group;
A thinning candidate pixel determination process for determining thinning candidate pixels in the gradation value group in order from a pixel having a large difference between the total average gradation value and the group average gradation value;
A line thinning pixel setting process for determining a line thinning pixel based on the thinning candidate pixels and setting the line thinning pixel to the set value for all pixels of the one line;
The image reading apparatus, wherein the reading line cycle is determined to be an integral multiple of the constant noise cycle. The controller is
Of the ratio of the integer corresponding to the first resolution and the integer corresponding to the second resolution, an integer corresponding to the first resolution is calculated as the number of pixels of the thinning group, and the number of pixels of the thinning group is calculated based on the number of pixels. Performing a thinning calculation process for calculating a value obtained by subtracting an integer corresponding to two resolutions as the number of thinned pixels to be thinned from the thinning group;
The thinning group is a group for sequentially grouping consecutive pixels from the beginning to the tail of all the pixels of the one line, and determining the line thinning pixels for each group,
The thinning candidate pixel is composed of a first thinning pixel and a second thinning pixel in the gradation value group, and the first thinning pixel is a pixel related to the first resolution and the second resolution, The second thinned pixel is a pixel not related to the first resolution and the second resolution,
In the line thinning pixel setting process, in the process of determining the line thinning pixel in each thinning group, the first thinning pixel is given priority over the second thinning pixel, and the number of thinning pixels is determined. The image reading apparatus according to claim 1. The controller is
A decimation rate calculation process for calculating a decimation rate by dividing the second resolution by the first resolution;
A pixel determination value calculation process for calculating a pixel determination value by multiplying the constant noise pixel number and the thinning rate;
The image reading apparatus according to claim 1, wherein a first thinned pixel determination process is performed in which an integer in the integer part of the pixel determination value is a first thinned pixel number. The first thinned-out pixel determination process sets the first thinned-out pixel number as a first thinned-out pixel from a pixel having a large difference between the total average gradation value and the group average gradation value. The image reading apparatus according to 3. The controller is
In the thinning-out group, when the first thinned-out pixel of the number of thinned-out pixels is fixed, as a determination prohibition pixel that prohibits the determination of the first thinned-out pixel with respect to a pixel that is not fixed as the first thinned-out pixel 5. The image reading apparatus according to claim 2, wherein a prohibited pixel setting process to be set is executed. When the pixel determination value is not an integer, or when the first thinned pixel number is not determined for the first thinned pixel number, the control unit turns on a second thinned pixel setting flag for determining the second thinned pixel. 5. The image reading apparatus according to claim 3, wherein a second thinning pixel determination process is set to “5”. In the second decimation pixel determination process, when the second decimation pixel setting flag is on, the total number of pixels of the first decimation pixel and the second decimation pixel is the number of decimation pixels in each decimation group. The image reading apparatus according to claim 6, wherein the second thinned pixels are determined so that A reading unit that reads an original for each line at a predetermined reading line cycle and generates analog image data of each pixel in the main scanning direction;
A reference plate disposed at a position facing the reading unit;
A power supply unit that converts the first DC voltage to the second DC voltage supplied to the reading unit by switching control at a switching frequency;
A converter that samples the analog image data at a sampling period and converts the analog image data into digital image data;
A correction unit for correcting the shading of the digital image data into gradation values;
A resolution converter that converts the resolution of the digital image data from a first resolution to a second resolution by thinning out pixels in the main scanning direction;
An average gradation value calculating unit that calculates the total average gradation value by reading the reference plate and averaging the gradation values of all pixels of one line corrected by the correction unit;
The gradation values of all pixels of the one line are assigned to a plurality of gradation value groups for each number of fixed noise pixels corresponding to a fixed noise period that is a common multiple of a period corresponding to the switching frequency and a sampling period of the conversion unit. A group average gradation value calculation unit that calculates a group average gradation value by dividing and averaging the gradation values of all the pixels of the same rank in each of the gradation value groups;
A thinning candidate pixel determining unit that determines a thinning candidate pixel for the gradation value group in order from a pixel having a large difference between the total average gradation value and the group average gradation value;
A line thinning pixel determining unit that determines all pixels of the one line as line thinning pixels based on the thinning candidate pixels;
The resolution conversion unit thins the line thinning pixels,
The image reading apparatus, wherein the reading line cycle is determined to be an integral multiple of the constant noise cycle.

Images