JP2592825C - - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image signal processing apparatus that processes an image signal obtained by reading a document image using an image sensor such as a CCD. [Prior art] Conventionally, as a configuration of a digital copying machine, a known laser beam printer using electrophotographic technology is used for an image recording output unit, and a main scanning photoelectric conversion reading by a line sensor such as a CCD is used as an image reading unit. Scanner to do is used. Reading of the scanner in the sub-scanning direction is performed by mechanically moving the document relatively in a direction perpendicular to the reading direction of the photoelectric conversion element. When scaling the output image with this apparatus configuration, the main scanning laser scan in the axial direction of the photoreceptor of the laser beam printer, and both the vertical scanning and the sub-scanning drum rotation, change the scanning speed stably. Since it is extremely difficult, the zooming operation is performed exclusively on the scanner side. That is, the magnification in the sub-scanning direction is reduced by increasing the scanning speed of the original with respect to the rotation speed of the drum, and enlarged by decreasing the scanning speed. The magnification in the main scanning direction is reduced by thinning out the image signal for one line in the main scanning read by the line sensor for each predetermined pixel, and the image signal for one line is overlapped for each predetermined pixel. Magnification is performed by recording. On the other hand, a process called an edge enhancement process is performed to enhance the outline of the read image and obtain a sharp image. As an example of the edge enhancement processing, it is known that a Laplacian filter performs secondary differentiation in the main scanning direction and the sub-scanning direction, and corrects a pixel of interest based on a result of the secondary differentiation. FIG. 8 shows an example of the edge enhancement processing circuit. A digital image signal 801 input for each line is stored in a delay buffer memory 802 for three lines and each line memory 820, 821, 822.
3 of image signal 803 of current line, image signal 804 of one line before, and image signal 805 of two lines before
Image signals for the lines are output. These image signals are delayed by the latch 806 on a pixel-by-pixel basis. Here, the target pixel is a signal 807 obtained by delaying the image signal 804 of the previous line by two pixels, and the multiplier 810 doubles the target pixel 807 by two pixels 808 and 809 before and after the main scan direction.
Is multiplied by (−1) and added by an adder 811 to obtain a value of 2 in the main scanning direction with respect to the pixel of interest.
The next differential signal 312 is obtained. Further, the multiplier 815 multiplies the pixel signals 813 and 814 of the lines before and after the same main scanning positional relationship as the pixel of interest by (−1), and the result of doubling the pixel signal of interest 807 is added by the adder 816. A secondary differential signal 817 in the sub-scanning direction is obtained. By adding these secondary differential signals 812 and 817 to the pixel of interest by the adder 818, an edge-enhanced image signal 819 is obtained. It has been clarified that the above-described scaling method and edge enhancement processing have various adverse effects on the output image. First, even when a document having a uniform density is read, the digital image signal is not uniform. This is partly due to the internal configuration of the CCD line sensor as shown in FIG. That is, each pixel output of the light receiving cell 601 is supplied to a separate charge transfer unit 6 for each even-numbered pixel and odd-numbered pixel.
02, 603, each passing through a separate amplifier 604, 605,
It is output as a line image signal. Accordingly, variations in the sensitivity of each light receiving cell, variations in the DC offset amount due to the difference in the transfer unit, and non-linear amplification due to the small signal of the amplifier, etc., cause variations in the digital image signal from pixel to pixel. I have. Various correction means such as DC drift removal and shading correction have been proposed to correct this variation, but all use the property that the output of the CCD line sensor is linear with respect to the light amount, and If there is non-linearity of the light receiving element or non-linearity of the amplifier, correction cannot be completed. This correction error is included in a large amount of black information, which is a very small amount of light, and has a variation for each pixel as shown in FIG. 10A due to the above-described CCD configuration. This variation in the main scanning direction is emphasized by the edge emphasizing circuit shown in FIG. 8 as shown in FIG. This variation is further emphasized as shown in FIG. 10 (c) by the above-described image reduction processing by image blinking. That is, bright pixels continue in a portion where a high-density portion is scattered, such as the portion C-1, and the copy output is conspicuous as a sharp white line. Further, in a portion where a portion having a low density is spread as in the portion C-2, dark pixels continue, and as a copy output, the sharpness is conspicuous as a black line. On the other hand, in the case of enlargement, the image information subjected to edge emphasis processing is flooded in the main scanning direction as shown in FIG. 10 (d), so that the density variation is emphasized and output by an increase in the output area per pixel. Is done. As described above, in the related art, there is a disadvantage that the unevenness of the CCD emphasized by the edge emphasis is further emphasized by the main scanning magnification changing process. Similarly, in the sub-scanning, there is a problem due to a mismatch between magnification and edge enhancement. As shown in FIG. 11A, one pixel of the CCD line sensor has a certain opening length in both the main scanning direction and the sub-scanning direction. In the figures, both are indicated by the length a. When a CCD line sensor having an opening length of a in the sub-scanning direction moves and scans the original by b in the sub-scanning direction and reads one pixel of the original, as shown in FIG. The area of × (a + b) is read as one pixel. Here, assuming that the reading movement amount b in the sub-scanning direction is the scan length when reading at the same size,
As shown in FIG. 11 (c), an image read in the original area S1 on a certain scan line is recorded by a pixel P1 in the printer, and an image read in the original area S2 by the same pixel in the next scan line is written in the printer P2. In each of the pixels P1 and P2, a portion corresponding to the opening area of the CCD line sensor indicated by oblique lines is commonly included as a blur. Here, the blur per pixel of the recorded image is Becomes Next, as shown in FIG. 11D, the original reading movement amount in the sub-scanning direction per pixel is calculated as follows. Assuming that the recording magnification in the sub-scanning direction is 400%, similarly, the ratio of blur per pixel of a recorded image is It can be seen that as the magnification increases, the sub-scanning length decreases, and the defocusing in which the denominator of the expression for the ratio of blurring decreases increases. As described above, conventionally, there is a disadvantage that the amount of blur included in an image increases as the enlargement magnification increases by using the ridge enhancement in the sub-scanning direction having a fixed strength. [Purpose] The present invention has been made in view of the above-described drawbacks in the conventional configuration, and has as its object to reduce the unevenness of main scanning due to zooming and to eliminate the increase in blur in sub-scanning due to an increase in magnification. I do. EXAMPLES Hereinafter, the present invention will be described using preferred examples. FIG. 1 is a diagram showing an example of a document reading apparatus (hereinafter, referred to as a scanner) to which the present invention is applied. CCD in which a plurality of light receiving elements are arranged on one line in order to read image information of an original 102 placed on an original platen glass 101 while being held down by an original cover 100.
The line sensor 103 is used, and the illumination light from the light source 104 is reflected on the surface of the document 102 and is imaged on the CCD line sensor 103 by the lens 18 via the mirrors 105, 106, and 107. An optical unit 113 composed of a light source 104 and a mirror 105 and an optical unit 114 composed of mirrors 106 and 107 move at a relative speed of 2: 1. This optical unit is DC
The servo motor 109 moves from left to right at a constant speed while performing PLL control.
This moving speed is variable according to the magnification on the outward path, and is 180 mm / sec at the same magnification, 800%
It is 22.5 mm / sec at the time of enlargement and 360 mm / sec at the time of 50% reduction. The optical unit is read at a resolution of 400 dots / inch by the CCD line sensor 103 in a main scanning direction (hereinafter, referred to as a Y direction) orthogonal to a sub-scanning direction (hereinafter, referred to as an X direction) in which the optical unit moves. After moving from the position to the right to a predetermined position, the device is returned to the home position again to complete one scan. The home position is detected when the light shielding plate 111 crosses the home position sensor 110 composed of a photo interrupter. The standard density plate 112 is used for shading correction and light amount control of the light source 104. The position where the home position sensor 110 detects the light shielding plate 111 is the position where the standard density plate 112 can be read by the CCD line sensor 103. Becomes FIG. 2 is a block diagram of signal processing from image reading to recording. Reference numeral 201 denotes a laser light emitting unit, and the laser light emitted from the laser light is scanned in the axial direction on the surface of a drum 203 rotating at a constant speed by a polygon scanner 202 rotating at a high speed at a constant speed. At this time, the passage of the laser beam is detected by the photodiode 204 disposed close to the drum on the extension of the scanning line, and the main scanning synchronization signal 205 is generated. Synchronizing with the synchronization signal 205, the two-phase clocks 207,
An image signal for one line read from the CCD line sensor 103 by the
After being amplified by the A / D converter 210, it is converted into an 8-bit digital image signal 212 for each pixel synchronized with the pixel clock 211, and then input to the edge emphasizing circuit 213. The edge emphasizing circuit 213 is composed of a Laplacian filter using a three-line delay buffer, and performs a secondary differentiation independently in the main scanning direction and the sub-scanning direction to perform edge emphasis processing. The image signal edge-enhanced by the edge emphasizing circuit 213 is written to the line memories 227 and 228 of the double buffer memory 214 line by line, and read out again to perform the scaling process in the main scanning direction. A write address counter (hereinafter, W-address counter) 215 and a read address counter (hereinafter, R-address counter) 216 for performing the scaling process operate in synchronization with the main scanning synchronization signal 205, respectively. The scaling process is performed by changing the ratio of the operating speeds of the W-address counter and the R-address counter. To change the operation speed of the counter, first and second clock control circuits 217 and 218 are used. The first clock control circuit 217 for the W-address counter 215 is, for example, model number 74167.
A cascade of two TTLs is used, and the number of passing clocks out of 100 clocks to be input is controlled according to a magnification signal MM from a rotary encoder SW for setting a main scanning magnification described later. The second clock control circuit for the R-address counter 216 is, for example, model number 7497.
A cascade of two TTLs is used, and the number of passing clocks in the input 4096 clocks is controlled in accordance with the magnification signal MM in the same manner as the first clock control circuit 217. Hereinafter, the case of reduction and the case of enlargement will be described by taking magnifications of 50% and 200% as examples. In the case of reduction to 50%, as shown in FIG. 3, the first clock control circuit 217 thins out the pixel clock 211 by two clocks at a rate of one clock to form the write clock 220. The W-address 221 generated by the W-address counter 216 in accordance with the write clock 220 changes by one address for every two pixels of the edge-emphasized write image signal. Only the even-numbered pixels are written. The image signal written in the memory 214 is output as it is without thinning out the pixel clock 211 by the second clock control circuit 218, and the R-address counter 2 according to the read clock 222 formed.
Reading is performed with the R-address 223 generated in step 16. The image signal 224 read from the memory 214 in this manner is a signal obtained by scaling the write image signal 219 by 50% in the main scanning direction. As described above, the reduction rate M (%) is set in the first clock control circuit 217 by 100.
It is determined by the following equation according to the clock passing number P in the clock. That is, the same value as the reduction rate M (%) is set as the clock passage number P. On the other hand, in the case of enlargement to 200%, the first clock control circuit 217 does not thin out the pixel clock 211 but directly writes the W-address counter as shown in FIG. Supply 215. When the image signal written in the memory 214 is generated, the second clock control circuit 2
At 18, a read clock 222 is created by passing the pixel clock 211 through two clocks at a rate of one clock. According to the read clock 222, the image signal 224 read by the R-address 223 generated by the R-address counter 216 is 1
The pixel period becomes twice as long as the writing image signal 219, and the magnification is 200% in the main scanning direction. As described above, the enlargement ratio M (%) is set in the second clock control circuit 218.
The number of passing clocks Q during the clock is determined as follows. The image signal thus scaled in the main scanning direction is modulated into an analog signal by the D / A converter 225, then amplified by the amplifier 226, and the laser driver 201 controls the light emission amount of the laser corresponding to one pixel. I do. The amount of charge is controlled on the drum 203 by the laser light whose emission amount is controlled, and an electrostatic latent image corresponding to an image signal is formed line by line on the drum 203, and the latent image is formed by an electrophotographic process (not shown). The image is output as a toner developed image whose density is modulated for each pixel. Reference numeral 230 denotes a DC servo motor that generates a driving force for causing the optical unit to reciprocate, and reference numeral 231 denotes an encoder that generates a clock signal 239 synchronized with the rotation of the motor 230. Reference numeral 236 denotes a reference clock generating unit for generating a reference clock for controlling the rotation of the motor 230. The reference clock from the reference clock generating unit 236 is divided by a frequency dividing circuit 235 into a clock signal 237 of a predetermined frequency, and then a third clock is generated. The clock control circuit 234 controls the number of passing clocks of the clock signal according to the magnification signal SM from the rotary encoder SW for setting the magnification in the sub-scanning direction. The clock signal 238 from the third clock control circuit 234 is input to the PLL control circuit 233, and the PLL control circuit 233 outputs a drive signal to the driver 232 so that the clock signal 238 and the clock signal 239 match. The rotation of the motor 230 is controlled to move the optical unit forward at a speed corresponding to the magnification. FIG. 5 shows a detailed configuration of the edge enhancement circuit shown in FIG. The image signal input from the A / D converter (210 in FIG. 2) is delayed in a delay buffer 302 having line memories 810 to 812 for three lines. That is, a delay buffer memory 302 for three lines addressed by the output of an address counter 301 for identifying a pixel for one line provides an image signal 303 for the current line, an image signal 304 for the previous line, and an image signal 305 for the previous line. Are output. These image signals are delayed by the latch 306 in pixel units. Here, the pixel of interest is a pixel signal 307 obtained by delaying the image signal 304 of the previous line by two pixels.
In the multiplier 310, the result of multiplying the pixel of interest 307 by two and the pixel signals 308 and 309 before and after in the main scanning direction by (−1) is added by the adder 311 and the second derivative of the pixel of interest in the main scanning direction is obtained. A signal 312 is obtained. Further, the multiplier 315 multiplies the pixel signals 313 and 314 of the previous and subsequent lines having the same main scanning positional relationship as the target pixel by (−1), and adds the result of doubling the target pixel signal 307 by the adder 316. A secondary differential signal 317 in the sub-scanning direction is obtained. 801 is a multiplier for increasing / decreasing the main scanning edge enhancement signal 312, 803 is input as an address the output of the rotary encoder SW805 for setting the main scanning magnification in percentage, and a corresponding main scanning edge enhancement signal multiplication coefficient. ROM that outputs 807
It is. 802 is a multiplier for increasing or decreasing the sub-scanning edge enhancement signal 317, and 804 is a rotary encoder SW 806 for setting the sub-scanning magnification in percent.
Is input as an address and the corresponding sub-scanning edge enhancement signal multiplication coefficient
ROM that outputs 808. The coefficient ROM 803 is configured as shown in FIG. 6 in order to prevent white streaks and black streaks due to reduction in the main scanning direction and pixel density unevenness due to enlargement from being further strengthened in area. FIG. 6 shows the main scanning magnification set by the horizontal axis rotary encoder SW805.
The vertical axis indicates the value of the output multiplication coefficient 807. As can be seen from the figure, the multiplication coefficient at a magnification of 100% is set to 1, and the multiplication coefficient is reduced to 0.5 at a magnification of 50% as the white streak and the black streak increase as the magnification decreases. At magnifications below 50%, the multiplication factor is not reduced to prevent loss of information. Further, in the case of magnification of 100% or more, since the area unevenness of the density unevenness of the output becomes remarkable from about 200%, the multiplication coefficient is gradually reduced, and when the density becomes 600%, the density included in the digital image signal 212 is reduced. Even unevenness becomes noticeable with an increase in the area, so that the edge enhancement in the main scanning direction is not performed by setting the multiplication coefficient to 0. As described above, in this embodiment, in order to prevent the density unevenness of each pixel from being conspicuous due to the scaled image processing, the coefficient ROM 803 controls the edge enhancement amount of the main scanning in accordance with the main scanning magnification to adjust the unevenness amount. I have. On the other hand, the ROM 804 that outputs the sub-scanning edge enhancement signal multiplication coefficient is configured as shown in FIG. 7 in response to an increase in the amount of blur included in one pixel due to an increase in the magnification in the sub-scanning direction. That is, a coefficient from 100% to 800% is determined by connecting a multiplication coefficient at a magnification of 100% and a coefficient 2 at a magnification of 800% with a straight line. The reason why the multiplication coefficient is not increased at a magnification of 800% or more is to prevent the contour of the shaded edge portion of the output image from being enhanced and output due to excessive edge enhancement. The reason why the multiplication coefficient is not reduced at a magnification of 100% or less is to remove a certain amount of blur included in an optical system such as a lens even if the blur included in the pixel decreases due to an increase in the sub-scanning speed. In this way, the multipliers 801 and 802 add the secondary differential signal 312 in the main scanning direction and the secondary differential signal 317 in the sub-scanning direction independently increased and decreased according to the main scanning magnification and the sub-scanning magnification, respectively, to the adder 318. Is added to the pixel signal 307 of the pixel of interest to obtain an edge-enhanced image signal 219. This image signal 219 is input to the above-mentioned double buffer memory 214, and a scaling process in the main scanning direction according to the scaling ratio is executed. Since the image signal subjected to the scaling process is subjected to edge enhancement processing in consideration of the scaling factor, it is possible to minimize the occurrence of density unevenness and streaks in the image due to the scaling process. In this embodiment, the second differential signal is increased or decreased in accordance with the magnification by using the multiplier. However, the line for taking the second differential by increasing the number of delay lines in the line buffer memory 302 is set as the target line. It is also effective to increase or decrease the physical size of the Laplacian filter in accordance with the magnification, such as not only the front and rear lines but also the front front line and the rear rear line. As described above, in the present embodiment, the edge emphasis strength can be set independently for each of the main scanning direction and the sub-scanning direction. Best output is obtained. Further, even when only one of the correction amounts has to be suppressed in consideration of another image processing such as a scaling process, the control becomes simple because there is no influence on the other correction. In addition, by changing the edge enhancement amount by changing the magnification, it is possible to suppress the deterioration of the image caused by the magnification change such as the magnification streak in the main scanning direction, the density unevenness and the blur in the sub-scanning direction. can get. [Effects] As described above, according to the present invention, the intensity of edge enhancement in each of the main scanning direction and the sub-scanning direction is independently changed based on the reading magnification of the image in each direction. It is possible to suppress the occurrence of density unevenness and streaks due to the scaling process in each of the sub-scanning directions .

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of an image reading device, FIG. 2 is a block diagram of an image processing circuit, FIG. 3 is a timing chart of a main scanning reduction process, and FIG. 5 is a diagram showing a configuration example of an edge enhancement circuit; FIG. 6 is a diagram showing a main scanning edge enhancement multiplication coefficient table; FIG. 7 is a diagram showing a sub-scanning edge enhancement multiplication coefficient table; 8 is a diagram showing a conventional example of an edge enhancement circuit, FIG. 9 is a configuration diagram of an image reading line sensor, FIG. 10 is a diagram showing main scanning pixel unevenness due to edge enhancement and scaling, and FIG. 11 is a sub-scanning direction. 103 is a diagram showing a scanning blur, 103 is a CCD line sensor, 213 is an edge enhancement circuit, 214 is a double buffer memory,
301 is an address counter, 302 is a delay buffer memory, 803 and 804 are ROM, 805 and
806 is a rotary encoder SW.

Claims (1)

And Claims (1) reading reads the document image into electrical by a photoelectric conversion element means includes a processing means for performing edge enhancement processing on the image signal from said reading means, a main scanning direction, the sub
The reading magnification of each image in the scanning direction can be set independently, and the main scanning direction
, The strength of edge enhancement relative to the image reading magnification in each of the sub-scanning directions
The relationship is made different depending on the reading characteristics in the main scanning direction and the sub-scanning direction.
An image signal processing apparatus wherein the strength of edge enhancement in each of the sub-scanning directions is independently changed based on the image reading magnification in each direction .