JP2592824B2 - Image signal processing device - 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, digital copiers have been configured with a laser beam printer using electrophotographic technology for the image recording and output unit, and a scanner that performs photoelectric conversion reading of main scanning with a line sensor such as a CCD as the image reading unit. Have been. 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, a Laplacian filter performs secondary differentiation in the main scanning direction and the sub-scanning direction,
It is known that a pixel of interest is corrected based on the result of the secondary differentiation.

 FIG. 9 shows an example of the edge enhancement processing circuit.

The digital image signal 801 inputted for each line is converted into the line memories 820 and 82 of the delay buffer memory 802 for three lines.
The buffer memory 802 outputs three lines of the image signal 803 of the current line, the image signal 804 of one line before, and the image signal 805 of two lines before. These image signals are delayed by the latch 806 in pixel units.

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 in the main scanning direction by (−1).
The multiplied result is added by an adder 811 to obtain a secondary differential signal 312 in the main scanning direction for the pixel of interest.

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 become clear that the output image has various adverse effects due to the scaling method and the edge enhancement processing described above.

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 transferred by a separate charge transfer unit 602, 603 for each of the even-numbered pixel and the odd-numbered pixel.

Therefore, variations in the sensitivity of each light receiving cell, variations in the DC offset amount due to differences in the transfer unit, and non-linear amplification due to the minute signal of the amplifier are the causes of the variations in the digital image signal image by pixel. Has become.

DC drift removal to correct this variation,
Various correction means such as shading correction have been proposed, but all use the property that the output of the CCD line sensor is linear with respect to the light amount, and the nonlinearity of the light receiving element with respect to a minute light amount and the nonlinearity of the amplifier are used. Correction cannot be made if there is a property.

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. 11A due to the above-described CCD configuration. This variation in the main scanning direction is corrected by the edge enhancement circuit shown in FIG.
It is emphasized as shown in FIG.

This variation is further emphasized as shown in FIG. 11 (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 scattered, such as portion C-2, dark pixels continue, and as a copy output, a sharp black line is conspicuous.

On the other hand, in the case of enlargement, the image information subjected to edge emphasis processing is watered in the main scanning direction as shown in FIG. 11 (d), so that the density variation is emphasized and output by the increased 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. 12 (a), 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. A CCD line sensor having an opening length a in the sub-scanning direction moves and scans the original by b in the sub-scanning direction to read one pixel of the original, and as shown in FIG. × (a
The area of + b) is read as one pixel.

Here, assuming that the reading movement amount b in the sub-scanning direction is the scan length at the time of variable-magnification reading, an image read in the original area S1 on a certain scanning line as shown in FIG.
The image recorded by the pixel P1 and read in the original area S2 by the same pixel in the next scanning line is printed on the P2 by the printer.
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 rate of blur per pixel of the recorded image is Becomes

Next, as shown in FIG. 12D, 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.

Thus, conventionally, as the magnification is increased by using the edge enhancement in the sub-scanning direction having a fixed strength,
There is a disadvantage that the amount of blur contained in the image increases.

〔Purpose〕

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above-described drawbacks of the conventional configuration, and has as its object to reduce unevenness in main scanning due to zooming and to eliminate an increase in blur in sub-scanning due to an increase in magnification.

〔Example〕

 Hereinafter, the present invention will be described using preferred embodiments.

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. Document 102 placed on platen glass 101 and held down by document cover 100
A CCD line sensor 103 in which a plurality of light receiving elements are arranged on one line is used to read the image information of
The illumination light from 4 is reflected on the surface of the original 102, and the mirrors 105 and 10
An image is formed on the CCD line sensor 103 by the lens 18 via 6,107.

Optical unit 113 including light source 104 and mirror 105 and mirror
The optical unit 114 composed of 106 and 107 moves at a relative speed of 2: 1. The optical unit moves from left to right at a constant speed while performing PLL control by the DC servo motor 109. This moving speed is variable according to the magnification on the outward path, and is 180 mm / sec at the same magnification and 22.5 mm at 800% magnification.
/ sec, 360mm / sec at 50% reduction. The main scanning direction (hereinafter referred to as Y direction) orthogonal to the sub-scanning direction (hereinafter referred to as X direction) in which the optical unit moves is the CCD line sensor 1.
The optical unit is moved from the home position at the left end to a predetermined position to the right while reading at a resolution of 400 dots / inch according to 03, and then moved back 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.

In synchronization with the synchronization signal 205, the reference clock generator 206
The image signals for one line read from the CCD line sensor 103 by the two-phase clocks 207 and 208 from the A / D converter 210 are amplified by the amplifier 209, and then are amplified by the A / D converter 210.
After being converted into an 8-bit digital image signal 212 for each pixel synchronized with 1, it is input to an edge enhancement 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 in this manner is supplied to the line memories 227 and 2 of the double buffer memory 214.
The data is written in the line 28 one line at a time, and is 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.

First clock control circuit 2 for W-address counter 215
17 is, for example, a cascade of two TTLs of model number 74167 used in cascade, and is input according to a magnification signal MM from a rotary encoder SW for setting a main scanning magnification described later.
Controls the number of passing clocks in 100 clocks.

As the second clock control circuit for the R-address counter 216, for example, a circuit in which two TTs of model number 7497 are connected in cascade is used. Controls the number of passing clocks inside.

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. A read clock 222 formed by outputting the image signal written in the memory 214 as it is without thinning out the pixel clock 211 by the second clock control circuit 218.
Is read out with the R-address 223 generated by the R-address counter 216 in accordance with the following. The image signal 224 read from the memory 214 in this manner is obtained by changing the write image signal 219 in the main scanning direction.
The result is 50% magnification.

As described above, the reduction M (%) is determined by the following equation based on the clock passing number P in 100 clocks set in the first clock control circuit 217.

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 read, the second clock control circuit 218 causes the pixel clock 211 to pass through two clocks at a rate of one clock, thereby forming a read clock 222. In accordance with the read clock 222, the image signal 224 read by the R-address 223 generated by the R-address counter 216 has one pixel cycle twice that of the write image signal 219, and is 200% in the main scanning direction.
It will be a scaled version.

As described above, the enlargement ratio M (%) is determined by the following equation based on the number Q of passing clocks in 4096 clocks set in the second clock control circuit 218.

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 beam whose light emission amount is controlled, and an electrostatic latent image corresponding to an image signal is formed on the drum 203.
The latent image is formed line by line, and this latent image is output as a toner developed image whose density is modulated for each pixel by an electrophotographic process (not shown).

A DC servo motor 230 generates a driving force for reciprocating the optical unit, and an encoder 231 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. Rotary encoder SW for setting magnification in sub-scanning direction by clock control circuit 234
The number of clocks passing through the clock signal is controlled in accordance with the magnification signal SM from.

The clock signal 238 from the third clock control circuit 234 is
The signal is input to the L control circuit 233, and the PLL control circuit 233 controls the driver 23 so that the clock signal 238 and the clock signal 239 match.
A drive signal is output to 2 to control the rotation of the motor 230 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 by a delay buffer 302 having line memories 810 to 812 for three lines via a filter circuit 809 described later.

That is, the image signal 303 of the current line, the image signal 304 of the previous line, and the image signal 30 of the previous line are provided by the delay buffer memory 302 for three lines addressed by the output of the address counter 301 for identifying the pixels for one line.
Image signals for three lines of 5 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.
The result obtained by multiplying 8,309 by (−1) is added by an adder 311 to obtain a secondary differential signal 312 in the main scanning direction for the target pixel.

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 main scanning edge enhancement signal multiplication coefficient corresponding thereto. 80
This is a ROM that outputs 7. 802 is a sub-scanning edge enhancement signal 317
804 is a ROM which receives as an address the output of the rotary encoder SW 806 that sets the sub-scanning magnification in percent, and outputs a corresponding sub-scanning edge enhancement signal multiplication coefficient 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.

In FIG. 6, the horizontal axis is the main scanning magnification set by the rotary encoder SW805, and the vertical axis is the output multiplication coefficient 807.
Shows the value of

As can be seen from this figure, the multiplication coefficient at 100% magnification is 1
As the white streak and black streak increase as the magnification decreases, the multiplication coefficient is reduced, and the multiplication coefficient is set to 0.5 at a magnification of 50%.
And 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.

However, at a magnification of 600% or more, even if edge enhancement is not performed as described above, density unevenness included in the digital image signal 212 significantly deteriorates the quality of an output image.

When the main scanning magnification is 600% or more, it is necessary to positively remove density unevenness included in the digital image signal 212.

For this reason, in this embodiment, two types of smoothing filters having different intensities are prepared and used depending on the main scanning magnification.

In other words, 600% magnification at which the main scanning edge cannot be emphasized
We use weak filters up to 800% and strong filters for magnifications over 800%.

The filter processing is not performed for the magnification of 600% or less.

This filter processing is executed by the filter circuit 809.
As a weak filter, the target pixel is doubled, and the pixel before and after in the main scanning direction is doubled and smoothed by doubling the preceding and succeeding A filter. As a strong filter, a total of four pixels including the target pixel, two pixels before and one pixel after it are used. This is a B filter that performs a smoothing process.

The filter circuit 809 is configured as shown in FIG. 8th
In the figure, reference numerals 90 to 904 denote flip-flops for latching the input digital image signal 212 for each pixel in accordance with a pixel clock 211, and 905 denotes a multiplier for doubling the target pixel signal. The target pixel signal doubled by the multiplier 905 and the pixel signals before and after the target pixel are input to the adder 906, and the value obtained by adding them is multiplied by 乗 算 in the multiplier 907 and supplied to the selector 121.

 The output of the multiplier 907 is the output of the filter A.

Also, the target pixel signal, two pixels before the target pixel and one after the target pixel
The pixel signal of the pixel is input to an adder 909, and a value obtained by adding them is multiplied by に て by a multiplier and supplied to a selector 121.

 The output of the multiplier 909 is the output of the filter B.

The use of the two filters A and B is selected by the selector 121 in accordance with the signal 812, and whether or not to perform the filter processing is selected by the selector 122 by the signal 811.

The use / non-use of the filter and the selection of the filter A / B are set by the filter control circuit 810 according to the main scanning magnification.

The filter condition setting circuit 810 receives the main scanning magnification signal from the main scanning magnification setting SW 805, and sets the signal 811 to a low level when the main scanning magnification is less than 60% without using a filter, and sets the signal 811 to a high level when the filtering is not more than 600%. At a magnification of 600% or more, the signal 812 is set to a low level so that the A filter is used at a magnification of less than 800%, and at a rate of 800% or more, the signal 812 is set to a high level so that the B filter is used.

As described above, using a filter with a large enlargement ratio in which the pixel unevenness in the main scanning is strongly emphasized in area makes it possible to obtain a uniform output image with less unevenness even with a large enlargement ratio.

On the other hand, a ROM 8 for outputting a sub-scanning edge enhancement signal multiplication coefficient
04 is configured as shown in FIG. 7 corresponding 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, the coefficient from 100% to 800% is determined by setting the multiplication coefficient at 100% magnification to 1 and connecting the coefficient 2 at 800% magnification 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. Also, 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 sub-scanning speed increases and blur included in a pixel decreases.

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 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 stored in the double buffer memory 214 described above.
And a scaling process in the main scanning direction according to the scaling factor 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 according to 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 noted. It is also effective to increase or decrease the physical size of the Laplacian filter according to the magnification ratio, not only the lines before and after the line, but also the lines before and after the line.

In the main scanning filter processing, at a boundary magnification at which edge enhancement processing is not performed, for example, at a magnification of about 600%, a smoother output image can be obtained by using both the filter processing and edge enhancement processing.

Further, it is more effective to prepare not only two types of filters, weak / strong, but also different types of filters, and to use several types according to the magnification.

If the pixel unevenness in the main scanning direction is a regular one, such as unevenness of even and odd pixels of a CCD, use a filter that removes the unevenness at a fairly low magnification to improve the sharpness of the image by edge enhancement. Becomes more effective. For example, in the case of unevenness for each pixel as shown in FIG. 11 (a), the unevenness completely disappears when the A filter (1, 2, 1) of the filter circuit 809 is always used.

The strong filters include not only filters for pixels in the main scanning direction but also filters including sub-scanning directions.
Dimensional filters are also used.

(Effect)

As described above, in the present invention, edge blurring and image processing which are problematic at low magnifications and image unevenness which is problematic at high magnifications can be optimally removed by selectively using edge enhancement and filtering depending on the magnification. Thus, there is an effect that a uniform output image is obtained without being affected by the magnification.

[Brief description of the drawings]

1 is a sectional view of an image reading apparatus, FIG. 2 is a block diagram of an image processing circuit, FIG. 3 is a timing chart of a main scanning reduction process, FIG. 4 is a timing chart of a main scanning enlargement process, and FIG. FIG. 6 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, and FIG. 8 is a configuration of a filter. FIG. 9 is a diagram showing a conventional example of an edge enhancement circuit. FIG. 10 is a configuration diagram of an image reading line sensor. FIG. 11 is a diagram showing main scanning pixel unevenness due to edge enhancement and scaling. Is a diagram showing scanning blur in the sub-scanning direction, 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 Is a SW.

Claims (2)

(57) [Claims] A reading means for electrically reading a document image by photoelectric conversion means; a first processing means for performing edge enhancement processing on an image signal from the reading means; Second processing means for performing smoothing processing by using the first processing means, and determining whether to use or not use the second processing means independently of each other according to the magnification of the image. Image signal processing device. 2. An image signal processing apparatus according to claim 1, wherein the strength of edge enhancement processing and the strength of smoothing processing are independently set according to the magnification of the image.