JPH08335260A - Color image processor - Google Patents

Abstract

PURPOSE: To provide a color image processor which can prevent color slurring and darkening in an edge area of an image. CONSTITUTION: This color image processor consists of an edge detecting means 203 which detects the edge position in the image, an edge correcting means 204 which substitutes density information on peripheral pixels for the detected edge, a base color removal rate detecting means 205 which detects a base color removal rate from the substituted density information, and a base color removal and black adding means 206 which subtracts the generation quantity of black from the substituted density information and base color removal rate and a black quantity from respective color quantities. Consequently, a color signal of adjacent non-edge pixels is substituted for respective color signals in a color transition state at the edge position, so the color transition state is eliminated and the blurring of the edge area can be suppressed in a print result. Further, a block addition quantity and a base color removal quantity are found from the base color removal rate found from the values to equalize the rate of replacement with the black quantity at peripheral pixels, thereby preventing trouble such as the darkening in the edge area, variation in color tone, etc.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image forming apparatus such as a color printer or a color copying machine for printing a color image with a plurality of color components.

[0002]

2. Description of the Related Art Conventionally, this type of apparatus expresses all colors by changing the dye amounts of cyan (C), magenta (M), and yellow (Y) when outputting an image on recording paper.
For example, a black image is represented by superimposing saturation amounts of cyan, magenta, and yellow. However, in this case, the print result is not completely black, but bluish black. If you try to express a black thin line with these three colors,
It is difficult for the printer to have the same printing positions of the three colors with respect to one pixel because of the mechanical structure of the printer.

Therefore, conventionally, by replacing the black component with a black dye, the black is expressed as a complete black, the color shift of the black edge portion is eliminated, and the dye amount for three colors is replaced with one color. Therefore, the amount of dye is saved. This process is generally called undercolor removal or black printing. Specifically, the minimum value in the C, M, and Y color signals expressing one pixel is selected, and the black (K) dye amount is obtained from this value (blackening). Then, the obtained amount of K is subtracted from each color signal of C, M, Y to obtain a new color signal of C, M, Y (undercolor removal). As a method for obtaining this black pigment amount, in the color image forming apparatus described in Japanese Patent Laid-Open No. 63-183460, it becomes a measure of color vividness from a plurality of color component signals expressing the color of each pixel. The saturation is calculated, and the lower the saturation, the higher the amount of black printing is controlled.

Further, in the color image processing apparatus described in Japanese Patent Laid-Open No. 61-1171, in order to reduce the color shift of the edge portion, the black printing amount and the undercolor are proportional to the change amount of the color signal of the edge portion. Control is performed to increase the removal amount.

[0005]

In the above prior art, black printing and undercolor removal are realized by calculating each color signal of one pixel (target pixel) to be processed. However, when the pixel of interest is located in the edge area, the color signal is in a transition state from one color to another color. Therefore, it does not represent accurate saturation (brightness) or brightness (brightness). Therefore, the amount of saturation required in the color image forming apparatus described in JP-A-63-183460 and JP-A-61-11
The black printing amount before edge enhancement, which is required in the color image processing apparatus described in Japanese Patent No. 71, is inaccurate. For example, even if the saturation of the peripheral pixels is high, if the C, M, and Y values in the density transition area of the edge have the same value, the saturation will be low, and as a result, the black printing amount However, there is a problem that the amount of undercolor removal increases, and the edges are expressed as black in either method.

On the other hand, when the C, M, and Y values in the edge area have large variations despite the low saturation image area, the saturation is erroneously recognized as high, and the black printing amount and the undercolor removal are performed. As the amount becomes smaller, color misregistration occurs in the edge area, and faithful black reproduction cannot be realized.

[0007]

According to the color image processing apparatus of the present invention, the amount of black-printing of a pixel of interest is determined based on the input / output pixel of interest and the color-separated density information of a plurality of pixels located around the pixel of interest. And a color signal processing means for obtaining the undercolor removal amount.

Further, the color image processing apparatus of the present invention determines whether or not the pixel of interest is located in the edge region of the image based on the color-separated grayscale information of the pixel of interest and a plurality of pixels located around the pixel of interest. Edge detection means for detecting the pixel value, edge correction means for resetting the grayscale information of the pixel of interest based on the grayscale information of peripheral pixels when the pixel of interest is located in the edge region of the image, and the edge correction means. Undercolor removal rate detecting means for detecting the undercolor removal rate of the grayscale information of the pixel of interest reset by means, and grayscale information of the pixel of interest reset by the edge correction means and the undercolor removal rate detecting means It is characterized in that it has a color signal processing means comprising an undercolor removal amount and an undercolor removal amount for obtaining an undercolor removal amount and an undercolor removal amount from the undercolor removal rate obtained from.

Further, the color image processing apparatus of the present invention is characterized in that the edge detecting means performs the operation based on a result of convolution with a coefficient set by the edge detecting filter for each color-separated grayscale information. .

Further, in the color image processing apparatus of the present invention, the edge correction unit may correct one of the adjacent pixels in the direction orthogonal to the direction in which the edges are continuous, which is closer to the grayscale information of the pixel of interest whose grayscale information is corrected. It is characterized by selecting as.

In the color image processing apparatus of the present invention, the undercolor removal rate detecting means includes maximum value detecting means for detecting a maximum value of a plurality of color separated color signals, and a plurality of color separated color signals. Minimum value detecting means for detecting the minimum value, average value calculating means for calculating the average value of the outputs of the maximum value detecting means and the minimum value detecting means, and the difference between the outputs of the maximum value detecting means and the minimum value detecting means. Difference calculating means for calculating the quantity, inverting means for subtracting the output of the difference calculating means from the saturation value of the density level of the color signal, and output of the inverting means for division by the output of the average value calculating means, and the division And a dividing means for outputting the result as an undercolor removal rate.

Further, in the color image processing apparatus of the present invention, the undercolor removal / blackening means is a minimum value detecting means for detecting a minimum value of a plurality of color signals of the color-separated pixels, and the minimum value detecting means. Multiplication means for multiplying the output of the means and the undercolor removal rate obtained for the pixel, and outputting the multiplication result as a black-printing amount; a plurality of color signals of the color-separated pixels and the multiplication means; And a difference calculation means for calculating the difference amount from the multiplication result of 1 as the undercolor removal amount.

Further, the color image processing apparatus of the present invention detects whether or not the pixel of interest is located in the edge area from the color-separated grayscale information of the pixel of interest and a plurality of pixels located around it. The edge detecting means, the adjacent pixel detecting means for detecting an adjacent pixel in the direction orthogonal to the direction in which the edges are continuous when the target pixel is determined to be located in the edge region by the edge detecting means, and the adjacent pixel detecting means. The present invention is characterized by having undercolor removal / blackening means for obtaining the undercolor removal amount and blackening amount of the target pixel from the grayscale information of the adjacent pixel detected by the pixel detection means and the target pixel.

Also, in the color image processing apparatus of the present invention, the undercolor removal / blackening means for obtaining the undercolor removal amount and blackening amount of the target pixel from the grayscale information of the adjacent pixel and the target pixel is set to the target pixel. From the undercolor removal rate detecting means for obtaining the undercolor removal rate from the pixel having the higher density among the adjacent pixels, and the undercolor removal rate obtained by the undercolor removal rate detecting means and the grayscale information of the target pixel. And a black-printing amount calculating means for obtaining a black-printing amount.

In the color image processing apparatus of the present invention, the undercolor removal / blackening means for obtaining the undercolor removal amount and blackening amount of the target pixel from the grayscale information of the adjacent pixel and the target pixel is the target pixel. It is characterized in that it has means for making the color-separated grayscale information of the pixel having a lower density among the pixels adjacent to the color-shifted pixel to be the color-separated grayscale information after the undercolor removal of the pixel of interest.

Further, in the color image processing apparatus of the present invention, the undercolor removal rate detecting means includes maximum value detecting means for detecting a maximum value of a plurality of color separated color signals, and a plurality of color separated color signals. Minimum value detecting means for detecting the minimum value of, the doubling arithmetic means for outputting a value twice the output of the minimum value detecting means, and the output of the maximum value detecting means from the output of the doubling arithmetic means. And a difference calculation means for calculating the difference of

[0017]

In the color image processing apparatus having the above structure, each color signal in the transition state at the edge position of the pixel of interest of the image is replaced by the color signal of the non-edge pixel located in the periphery thereof, and as a result, the transition of the color signal in the edge region is performed. The state disappears, and blurring of the edge portion can be suppressed in the print result.

Further, by obtaining the black printing amount and the undercolor removal amount from the undercolor removal ratio obtained from the value, the replacement ratio of the black signal to the peripheral pixels can be made the same, and the blackening of the edge portion of the image, Problems such as changes in color tone can be prevented.

[0019]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of a color laser printer which is an output device of a computer as an example of an image processing apparatus to which the present invention is applied. In the figure, reference numeral 101 denotes a personal computer (PC), which outputs color information of one pixel as blue (B), green (G), and red (R) color signals as image output. The page memory 102 is a memory that stores each color signal for one page of image data to be printed.

The color conversion unit 103 converts the image information represented by R, G, and B into color signals of C, M, and Y which are dyes for printing on recording paper. This can be realized, for example, by configuring a conversion table with a semiconductor memory, inputting R, G, B color signals at addresses, and outputting C, M, Y color signals after color conversion as data.

The edge detection undercolor removal unit 104 is means for performing black printing and undercolor removal, which is a feature of the present invention.
The color shift of the edge part is corrected, and the optimum black (K) signal corresponding to the image and new C ', M', Y'corresponding to it
Generate a signal.

The pulse width modulator 105 is a means for changing the size of the dot of each color printed on the recording paper in proportion to the value of each color signal. This can be realized, for example, by using a digital-analog converter and a semiconductor laser and changing the output of the semiconductor laser in proportion to the digital value of each input color component.

The printing unit 106 is means for adhering a dye corresponding to each color signal onto the recording paper. This can be realized by a printing mechanism of a laser printer, for example.

The color image information for one page generated on the PC 101 is reproduced from the printing unit 106 on the recording paper by sequentially executing the processing by each of the above means.

FIG. 2 shows the configuration of an embodiment of the edge detection undercolor removal unit 104 in FIG.

In the figure, the edge detection undercolor removal unit 10
Reference numeral 4 has line memories 201a and 201b, latch circuits 202a to 202i, an edge detection circuit 203, an edge correction circuit 204, an undercolor removal rate detection circuit 205, and an undercolor removal / blackening circuit 206. are doing.

The line memories 201a and 201b are means for storing image information on the main scanning lines one line before and two lines before the main scanning line currently being input. For example, a total of 24 bits of digital signals of 8 bits for each color of C, M, and Y per pixel are stored for the number of pixels in the main scanning direction. This is a random access memory (RAM) which is one of the semiconductor memories
Can be achieved with.

The latch circuits 202a to 202i are means for temporarily storing the image information on each main scanning line and sequentially shifting to the right by one pixel after the processing of one pixel is completed. This can be composed of a flip-flop which is a semiconductor element. The nine latch circuits 202a to 202i sequentially output the image information P00 to P22 of a total of 9 pixels including 3 pixels in the main scanning direction and 3 pixels in the sub scanning direction. Of these, the target of the image processing of the present invention is the image information P11 of the pixel located at the center of the image of 9 pixels.

The edge detection circuit 203 has a function of detecting whether or not the pixel P11 located at the center of the image is located in the edge area by using the image information of the outputs P00 to P22 of the latch circuits 202a to 202i.

Next, FIG. 3 shows a specific configuration of the edge detection circuit 203. In the figure, the edge detection circuit 203 includes edge determination matrices 302a to 302d, convolution operation circuits 303a to 303d, and absolute value circuits 304a to 304.
d, the maximum value circuit 305, and the edge reference value setting circuit 30
6 and comparison circuits 307a to 307e.

The pixel matrix 301 is a latch circuit 202.
The outputs P00 to P22 of a to 202i are shown. Each pixel is represented by a 24-bit digital signal of 8 bits for each of C, M, and Y.

Edge determination matrices 302a to 302d
Is a coefficient for determining whether or not the pixel of interest P11 is located at the edge portion. Here, 302a is the target pixel P1
1 is a coefficient for detecting a contour continuous in the horizontal direction (horizontal edge), that is, a region where color transition occurs in the vertical direction. Similarly, 302b is a coefficient for detecting an edge continuous in the vertical direction (vertical edge), 302c is a contour continuous in the lower right direction (right oblique edge), and 302d is a coefficient for detecting a continuous contour in the lower left direction (left oblique edge). The values of these coefficients are set to output large values when the pixel of interest P11 actually corresponds to the edge area when the convolution operation described below is performed.

The convolution operation circuits 303a to 303d are provided in the pixel matrix 301 and the edge determination matrices 302a to 30d.
It has a function of performing a calculation for measuring the degree of the edge of the target pixel P11 between 2d.

The specific configuration and operation of the convolutional operation circuit 303 will be described with reference to FIG. FIG. 4A is a block diagram showing a case where a convolution operation is executed between the image signals P00 to P22 of the pixel matrix 301 and the coefficients K00 to K22 of the edge determination matrix 302.

FIG. 4B shows a concrete configuration of the convolutional operation circuit 303. The convolution operation circuit 303 is
Multipliers 401a to 401i and these multipliers 401a
˜401i for adding the multiplication results.

The multipliers 401a to 401i are C for each pixel,
The result of multiplying the 8-bit value for each of M and Y by the coefficient of the edge determination matrix 302 corresponding to the pixel position is output. These multipliers 401a to 401i can be easily configured by combining logic circuits. The adder 402 has a function of adding the multiplication results of the nine multipliers 401a to 401i. This adder 402 can be easily constructed by combining logic circuits. The output of the adder 402 becomes the convolution operation result of the pixel matrix 301 and the edge determination matrix 302.

Next, the absolute value circuits 304a to 304d shown in FIG.
Has a function of obtaining the absolute value of the output of the operation result of the convolution operation circuits 303a to 303d. This absolute value circuit 304a
˜304d can be easily realized by a subtracter composed of a logic circuit.

The maximum value circuit 305 is the absolute value circuit 304a ...
The maximum value in the output of 304d is detected. This can be configured by, for example, a combination of a comparator that compares the magnitudes of two values and a selector that performs a binary selection according to the comparison result.
The comparator and the selector can be easily realized by a logic circuit.

The edge reference value setting circuit 306 has a function of setting a threshold value for determining an edge. This can be easily configured with flip-flops.

The comparator circuits 307a to 307e output 1 when the + side input data is greater than or equal to the-side input data. At other times, 0 is output.

Next, the operation of the edge detection circuit 203 will be described. First, the image signal P0 of the pixel matrix 301
0 to P22 and edge determination matrices 302a to 302d
The maximum value of the four absolute values of the operation outputs of the convolution operation circuits 303a to 303d is detected. Here, the edge corresponding to the coefficient indicating the maximum value is a candidate for the edge where the target pixel P11 is located. The types of edges detected with each coefficient are shown in FIG.
As shown in. Next, the output of the maximum value circuit 305 is compared with the outputs of the absolute value circuits 304a to 304d. This time,
For example, when the output of the absolute value circuit 304a is the maximum value, the output b3 of the comparison circuit 307b outputs 1 and the outputs b2 to b0 output 0.

Next, the maximum value circuit 305 is compared with the edge reference value set by the edge reference value setting circuit 306 by the comparison circuit 307a. Here, when the maximum value of the absolute value circuits 304a to 304d is smaller than the edge reference value, it is not considered as an edge. At this time, the output b4 of the comparison circuit 307a outputs 1. This process is a process for preventing erroneous determination of a minute change in density as an edge. The correspondence between the values of the above processing results b4 to b0 and the detected edges is as shown in FIG.

The edge correction circuit 204 of FIG. 2 has a function of replacing the image signal of the target pixel P11 with the image signals of the peripheral pixels based on the output of the edge detection circuit 203.

Next, FIG. 6 shows a specific configuration of the edge correction circuit 204. In the figure, the edge correction circuit 204 is
Selection circuits 601a to 601d and difference calculation circuit 602
a, 602b, absolute value circuits 603a, 603b, and a comparison circuit 604.

The selection circuits 601a to 601d have a function of selecting one of the signals input from the left side and outputting it to the right side according to the value of the signal input from the upper side.

The difference calculation circuits 602a and 602b have a function of obtaining the difference between two input signals. This can be easily realized by a subtractor composed of a logic circuit.

The absolute value circuits 603a and 603b have a function of obtaining the absolute value of the difference output of the difference calculation circuits 602a and 602b. This can be realized by the same logic circuit as the absolute values 304a to 304d in FIG. For example, when the difference output is output in 2's complement notation, when the most significant sign bit is 1, the numerical value is negative, so the value of the bit representing the numerical value is inverted and 1 is added to the least significant bit. It can be realized with a circuit.

The comparison circuit 604 is an absolute value circuit 603a, 6
03b Compares the magnitude of two inputs and outputs 1 if the + side input value is greater than or equal to the − side input value, 0 otherwise
Is output. The comparison circuit 604 can be easily realized by a combination of logic circuits.

Next, the operation of the edge correction circuit 204 will be described. First, the output b4 of the edge detection circuit 203 is 1
, The pixel of interest P11 is a non-edge at this time, as shown in FIG. Therefore, the selection circuit 601d outputs the image information P11 itself of the pixel of interest as the correction result PM11.

Next, when only the output b3 of the edge detection circuit 203 is 1, it is determined to be a horizontal edge at this time as shown in FIG. 5, so that the pixel of interest P11 is the pixel P01 above and below it.
It is located in the transition state of the color signal from P21 to P21. Therefore, the target pixel P11 is the pixel P01 and the pixel P21.
Which of the two is the closest value is detected, and the closer value is substituted for the value of the target pixel P11. At this time, each pixel has 8 pixels for each of C, M, and Y.
Although expressed by a bit color signal, the calculation is performed using the color component detected by the pixel of interest P11 as an edge. For example, if the value of C of the target pixel P11 is detected as the maximum value by the maximum value circuit 305 of the edge detection circuit 203 and is determined as a horizontal edge, the processing by the edge correction circuit 204 is performed by the image signal P00 of the pixel matrix 301. ~ C of P22
The calculation is performed using the density information of the (cyan) component.

Therefore, P01 is selected as the output Pa of the selection circuit 601a, and P21 is selected as the output Pb of the selection 601b.
Select Next, the absolute value of the difference between P11 and Pa is calculated by the difference calculation circuit 602a and the absolute value circuit 603a. Also P
The absolute value of the difference between 11 and Pb is calculated by the difference calculation circuit 602b and the absolute value circuit 603b. These two outputs are compared by the comparison circuit 604, and a value close to the pixel of interest P11 is selected from the outputs Pa and Pb of the selection circuits 601a and 601b, and is set as the edge correction value PM11. In this case, if P21 is selected as the correction value for P11, the C, M, and Y color signals of the target pixel P11 are all replaced with the C, M, and Y of the pixel P21.

Outputs b2, b1, of the edge detection circuit 203
Similar processing is performed when b0 is 1, respectively. Selection circuit 6
FIG. 7 shows a state in which the image information pixels Pa and Pb according to 01a and 601b are selected. FIG. 7 shows a 9-pixel region centered on the target pixel P11 and adjacent to the target pixel P11. The hatching in the figure indicates the transition region of the color signal, that is, the edge region. For example, FIG. 7A shows the target pixel P1.
1 belongs to the horizontal edge. In this case, the selection circuit 6
P01 is selected as the output Pa of 01a and P21 is selected as the output Pb of the selection circuit 601b. Similarly, FIG. 7B shows a case of a vertical edge, and FIG. 7C shows a case of a right-oblique edge.
(D) is a selection circuit 601a in the case of a left oblique edge,
The positions of the pixels selected as the outputs Pa and Pb of 601b are shown.

FIG. 10 shows another structure of the edge correction circuit 204. This is because when the target pixel P11 is located in the edge area, of the two pixels at both ends of the edge,
The pixel with the higher density is output as PM11, the pixel with the lower density is output as PL11, and if the target pixel P11 is located in the non-edge region, P1 is output to PM11 and PL11.
1 is an embodiment of a circuit that operates so as to output 1 as it is. In the figure, the edge correction circuit 204 has selection circuits 1001 a to 1001 d and a comparison circuit 1002.

The selection circuits 1001a to 1001d have a function of selecting one of the signals input from the left terminal according to the value of the signal input from the upper terminal and outputting it to the right terminal. There is. This selection circuit 1001a-10
01d can be easily realized by a combination of logic circuits.

The comparison circuit 1002 includes a selection circuit 1001a,
The magnitudes of the two inputs of 1001b are compared, and 1 is output if the input value of the + side input terminal is greater than or equal to the input value of the − side input terminal, and 0 is output otherwise. Comparison circuit 1002
Can be easily realized by a combination of logic circuits.

Next, the operation of the edge correction circuit 204 will be described. First, when b4 is 1, at this time, P11 is a non-edge as shown in FIG. In that case, P11 is output from the selection circuit 1001c to PM11. Similarly PL
P11 is output from the selection circuit 1001d to 11.

Next, when only b3 is 1, at this time, as shown in FIG. 5, since it is determined to be a horizontal edge, P11 is located in the transition state of the color signal from the pixels P01 to P21 above and below it. Will be there. Therefore, the larger one of P01 and P21 is selected and output as PM11. The smaller one is output as PL11. At this time, each pixel is C, M,
Although each Y is represented by a color signal of 8 bits, the calculation is performed by the color component detected by P11 as an edge. For example, P1
The C value of 1 is the maximum value circuit 30 in the edge detection circuit 203.
If it is detected as a horizontal edge and is determined as a horizontal edge, the processing in the edge correction circuit 204 is C of P00 to P22.
Calculation is performed using the concentration information of the components. Therefore, the selection circuit 100
The selection circuit 100 selects P01 as the output Pa of 1a.
P21 is selected as the output Pb of 1b. Next, the comparison circuit 1002 compares Pa and Pb, and when Pa is a value equal to or larger than Pb, the comparison circuit 1002 outputs 1. As a result, Pa is output from the selection circuit 1001c and Pb is output from the selection circuit 1001d. Further, in this case, since b4 is 0, the selection circuits 1001e to Pa change to the selection circuit 10
Pb is output from 01f. When b2, b1 and b0 are 1, the same processing is performed. Selection of pixels of Pa and Pb is as described in FIG.

Next, the lower color removal rate detection circuit 205 of FIG.
It has a function of detecting the ratio of undercolor removal / black printing from the color information of the edge correction result PM11 of the edge correction circuit 204.

Next, FIG. 8 shows a specific configuration of the undercolor removal rate detection circuit 205. In the figure, the undercolor removal rate detection circuit 2
Reference numeral 05 denotes a maximum value circuit 801, a minimum value circuit 802, a difference calculation circuit 803, an average value circuit 804, and a division circuit 8.
05 and an inverting circuit 806.

The maximum value circuit 801 detects the edge correction value PM11.
It has a function of detecting the maximum value from among the C, M, and Y signals. This can be easily realized by a comparator and a selector which are composed of logic circuits.

The minimum value circuit 802 calculates the edge correction value PM1.
It has a function of detecting the minimum value among the C, M, and Y signals of 1. This can be easily realized by a comparator and a selector which are composed of logic circuits.

The difference calculation circuit 803 is a maximum value circuit 801.
And a function of obtaining the difference amount between the output of the minimum value circuit 802. This can be easily realized by a subtractor composed of a logic circuit. The inversion circuit 806 has a function of subtracting the output of the difference calculation circuit 803 from the saturation value of the color signal. For example, when the color signal of each color component is represented by 8 bits, the saturation value is 255. This can be easily realized by a subtractor composed of a logic circuit.

The average value circuit 804 has a function of obtaining an average value of the outputs of the maximum value circuit 801 and the minimum value circuit 802. This can be easily realized by setting the carry signal output from the adder constituted by the logic circuit to the most significant bit and combining the upper 7 bits of the addition result into an 8-bit signal.

The division circuit 805 has a function of dividing the output of the inverting circuit 806 by the output of the average value circuit 804. For example, enter the values of dividend and divisor as addresses,
This can be realized by a look-up table whose data output is the division result. The look-up table can be easily realized by a random access memory which is one of semiconductor memories.

Next, the operation of the undercolor removal rate detection circuit 205 will be described. In this example, the lower color removal rate is increased as the color is achromatic.

Generally, an achromatic (gray) area is C, M, Y.
The respective color signals of have the same value. Therefore, the smaller the difference between the maximum value and the minimum value of the C, M, and Y color signals, the greater the achromaticity. Therefore, the output of the difference calculation circuit 803 is subtracted from the saturation value by the inversion circuit 804 so that the smaller the difference amount, the larger the inversion result.

However, even with the same difference value, the achromaticity is higher when the average color signal value of C, M and Y is low and when it is high. Therefore, the ratio of the output of the inverting circuit 806 to the average color signal obtained by the average value circuit 804 is obtained by the dividing circuit 805, and is output to obtain the undercolor removal rate.

The undercolor removal / blackening circuit 206 shown in FIG. 2 generates a K (black) signal from the C, M, Y signals of the edge-corrected target pixel PM11 and the output of the undercolor removal rate detection circuit 205. , And has a function of subtracting the K signal from the C, M, Y signals to generate new C ′, M ′, Y ′ signals.

FIG. 11 shows another configuration of the undercolor removal rate detection circuit 205. In the figure, the undercolor removal rate detection circuit 205 includes a minimum value circuit 1101 and a maximum value circuit 110.
2, a doubling circuit 1103, a difference value circuit 1104,
And a bit shift circuit 1105.

The minimum value circuit 1101 has an edge correction value PM1.
It has a function of detecting the minimum value from the C, M, and Y signals of 1. This can be easily realized by a comparator and a selector that are composed of logic circuits.

The maximum value circuit 1102 has an edge correction value PM1.
It has a function of detecting the maximum value from the C, M, and Y signals of 1. This can be easily realized by a comparator and a selector that are composed of logic circuits.

The doubling circuit 1103 has a function of doubling the input signal value and outputting it. This can be easily realized by a shift circuit that shifts one bit in the direction of the most significant bit of the signal value that is simply expressed by a digital value.

The difference value circuit 1104 is a doubling circuit 1103.
From the output of the maximum value circuit 1102. 0 if the difference value is 0 or less
Is output. This can be easily realized by a subtraction circuit composed of a logic circuit.

The bit shift circuit 1105 is the difference value circuit 1
It has a function of multiplying the digital signal value of 104 by 1/256. This function can be easily realized by a circuit that shifts 8 bits in the direction of the least significant bit of a signal value represented by a digital value.

Next, the operation of the lower color removal rate detection circuit 205 shown in FIG. 11 will be described. This embodiment shows a case where the undercolor removal rate is increased as the brightness of the pixel of interest is smaller and the brightness is smaller. First, the function of obtaining the amount of saturation, which is a measure of vividness, will be described. Generally, in an achromatic (gray) area, the C, M, and Y color signals have the same value. Therefore, the smaller the difference between the maximum value and the minimum value of each of the C, M, and Y color signals, the smaller the amount of saturation. However, even with the same difference value, when the minimum value of each of the C, M, and Y color signals is low and high, the saturation amount is smaller when the minimum value is higher. Therefore, the saturation amount, which is a scale for expressing vividness, is represented by V and is represented by the formula (1).

[0076]

## EQU1 ## V = (MAX (C, M, Y) -MIN (C, M, Y)) / MIN (C, M, Y) (1) MAX (C, M, Y) in equation (1) Is a color component of one pixel C, M,
It represents the maximum value of Y. Similarly, MIN (C, M, Y) represents the minimum value of the color components C, M, and Y of one pixel. In Expression (1), the smaller the value of V, the closer to achromatic color.

Next, the function of obtaining the lightness amount, which is a measure of brightness, will be described. Generally, the brightness of colors is C, M,
The smaller the minimum value among the Y color signals, the brighter the color. Therefore, the brightness of the color is set to L and is represented by the equation (2).

[0078]

## EQU2 ## L = MIN (C, M, Y) / 256 (2) The reason for dividing by 256 in equation (2) is to normalize the value of L from 1 to 0. Further, in Expression (2), the smaller the value of L, the brighter the pixel.

Next, the undercolor removal rate is set to α and is expressed by the equation (3).

[0080]

## EQU3 ## α = (1-V) * L = (2 * MIN (C, M, Y) -MAX (C, M, Y)) / 256 (3) Equation (3) shows that the smaller the amount of saturation, , Or the smaller the lightness,
This indicates that the undercolor removal rate is large. The calculation of the equation (3) is performed in FIG.
102, a doubling circuit 1103, and a bit shift circuit 1
By carrying out at 104, the undercolor removal rate α is obtained.

Next, FIG. 9 shows a specific configuration of the undercolor removal / blackening circuit 206. In the figure, the undercolor removal / blackening printing circuit 206 has a minimum value circuit 901, a multiplier 902, and difference calculation circuits 903a to 903c.

The minimum value circuit 901 calculates the edge correction value PM1.
It has a function of detecting the minimum value among the C, M, and Y signals of 1.

The multiplier 902 has a function of multiplying the output of the minimum value circuit 901 by the undercolor removal rate. Multiplier 90
2 can be easily realized by a combination of adders. Alternatively, a data output corresponding to the multiplier and the multiplicand as an address may be configured as a lookup table as a multiplication result. The look-up table can be easily realized by a random access memory which is one of semiconductor memories.

The difference calculation circuits 903a to 903c have a function of subtracting the output of the multiplier 902 from the C, M, and Y signals of the edge correction value PM11, and can be easily realized by a subtracter composed of a logic circuit.

Next, the operation of the undercolor removal / blackening circuit 206 will be described. Generally, the minimum value of the C, M, and Y signals corresponds to the density of the achromatic region. Then, in the case of an achromatic region, the minimum value is multiplied by the undercolor removal rate 1 to obtain the black amount.
For example, in a completely black area, the C, M and Y signals are all 25
Since it has a value of 5, the minimum value is also 255. The undercolor removal rate obtained by the undercolor removal rate detection circuit 205 is 1.
Therefore, the multiplication result of the minimum value 255 and the undercolor removal rate 1 is 255.
And the black printing amount is 255. On the contrary, in the area which is not the achromatic area (the area with high saturation), the undercolor removal rate has a value close to 0, and the minimum value is also a small value.
The black printing amount, which is the output of 2, is close to 0, and the undercolor removal amount is also close to 0. Therefore, the C, M, and Y signals after the undercolor removal hardly change from those before the undercolor removal.

FIG. 12 shows another structure of the undercolor removal / blackening circuit 206. In this embodiment, the undercolor removal rate α obtained from the edge-corrected pixel PM11 of the target pixel is used to obtain C ′, C ′ after removal of the undercolor from C, M, and Y of the target pixel P11 itself.
The case where M ', Y', and K are calculated is shown. In the figure, the minimum value circuit 1201 indicates C, M, and Y of the target pixel P11.
It has the function of detecting the minimum value from the signal.

The multiplier 1202 has a function of multiplying the output of the minimum value circuit 1201 by the undercolor removal rate. The multiplier can be easily realized by a combination of adders. Alternatively, the multiplier and the multiplicand may be used as addresses, and the corresponding data output may be configured by a lookup table that is a multiplication result. The look-up table can be easily realized by a random access memory which is one of semiconductor memories.

The difference calculation circuit 1203 detects the target pixel P11.
It has a function of subtracting the output of the multiplier 1202 from the C, M, and Y signals, and can be easily realized by a subtracter configured by a logic circuit.

The selection circuit 1204 has a function of selecting one of the signals input from the left input terminal according to the value of the signal input from the upper input terminal and outputting it to the output terminal on the right side. ing. The selection circuit 1204 can be easily realized by a combination of logic circuits.

Next, the undercolor removal / blackening circuit 206 shown in FIG.
The operation of will be described. In the figure, when it is first determined that the target pixel P11 is located in the non-edge region, b4 = 0 at this time. In this case, the target pixel P1
K is calculated by the multiplier 1202 from the minimum value of the C, M, and Y signals of 1 and the undercolor removal rate α calculated in FIG. 8 or 11. Further, K is subtracted from the C, M, and Y signals of the target pixel P11 by the difference calculation circuit 1203 to obtain C ′, M ′, and Y ′ signals after undercolor removal. In this case, since the PM 11 used to obtain the undercolor removal rate is the target pixel P11 itself from the operation of FIG. 10, only the target pixel is determined when it is determined that the target pixel P11 is located in the non-edge region. Undercolor removal / blackening processing is performed from the value.

Next, when it is determined that the pixel of interest P11 is located in the edge area, b4 = 1, and the adjacent pixel PM of high density detected by the edge correction circuit 204 of FIG.
11 is used to detect the lower color removal rate detection circuit 2 of FIG.
From the undercolor removal rate α obtained in step 05 by the multiplier 1202.
Ask for. As for the C ′, M ′, and Y ′ signals after the undercolor removal, the minimum value of the C, M, and Y signals of the adjacent pixel PL11 having the low density detected by the edge correction circuit 204 of FIG. .

As described above, when the pixel of interest is located in the edge area, the amount of black is increased by obtaining the undercolor removal rate from the adjacent high density pixel and generating K, thereby increasing the amount of black and blurring the edge portion. Is prevented and the effect of edge enhancement is obtained. In that case, if the K value is simply subtracted from the C, M, and Y color signals of the pixel of interest, the values of the C ′, M ′, and Y ′ signals after the undercolor removal are close to zero. By the way, in color laser printers and inkjet printers, C, M, and
Generally, the printing positions of Y and K signals do not match,
If the value of the C ', M', and Y'signals after undercolor removal is 0, K
White spots occur in the direction in which the C, M, and Y signals are displaced from the signal print position. Therefore, C of adjacent low-density pixels,
White spots can be prevented by using the M and Y signals as the values after the undercolor removal of the pixel of interest.

Next, the edge detection undercolor removal unit 1 in FIG.
The operation of 04 will be described. In FIG. 2, the target pixel P
The edge detection circuit 203 detects from the C, M, and Y color signals of 11 and its surrounding pixels whether or not the pixel of interest P11 corresponds to an edge region, and if so, the edge correction circuit 204 detects the edge. Among the color signals of the pixel in the direction orthogonal to the direction, the color signal value close to the target pixel P11 is set to the target pixel P11.
Of the target pixel PM11 whose edge has been corrected by the undercolor removal rate detection circuit 205, and the undercolor removal rate is obtained from the color signal of the target pixel PM11, and the K signal is generated by the undercolor removal / blackening circuit 206. And C, M, Y are reset. By performing the above-described processing one pixel at a time, it is possible to perform optimal undercolor removal and blackening for both the non-edge area and the edge area, and it is possible to prevent problems such as darkening and color tone change in the edge area.

[0094]

According to the present invention, since each color signal in the color transition state at the edge position of the image is replaced with the color signal of the adjacent non-edge pixel, the color transition state is eliminated and the blur of the edge area is suppressed in the print result. it can.

By obtaining the black printing amount and the undercolor removal amount from the color signal value of the pixel of interest whose edge has been corrected and the undercolor removal ratio obtained from the value, the rate of replacing the peripheral pixels with the black amount is the same. It is possible to prevent problems such as darkening of the edge region and change of color tone.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a color laser printer as an example of a color image processing apparatus to which the present invention is applied.

FIG. 2 is a block diagram showing a configuration of an embodiment of an edge detection undercolor removal unit in FIG.

3 is a block diagram showing a specific configuration of an edge detection circuit in FIG.

FIG. 4 is a block diagram showing a specific configuration of a convolution operation circuit in FIG.

5 is an explanatory diagram showing an example of an edge determination output by the edge detection circuit shown in FIG.

FIG. 6 is a block diagram showing a specific configuration of the edge correction circuit in FIG.

7 is an explanatory diagram showing a state in which a target pixel is replaced with a peripheral pixel by the edge correction circuit shown in FIG.

8 is a block diagram showing a specific configuration of an undercolor removal rate detection circuit in FIG.

9 is a block diagram showing a specific configuration of an undercolor removal / blackening circuit in FIG.

10 is a block diagram showing the configuration of another embodiment of the edge correction circuit in FIG.

11 is a block diagram showing a configuration of another embodiment of the undercolor removal rate detection circuit in FIG.

FIG. 12 is a block diagram showing the configuration of another embodiment of the undercolor removal / blackening circuit in FIG.

[Explanation of symbols]

 101 personal computer 102 page memory 103 color conversion unit 104 edge detection undercolor removal unit 105 pulse width modulation unit 106 printing units 201a and 201b line memories 202a to 202i latch 203 edge detection circuit 204 edge correction circuit 205 undercolor removal rate detection circuit 206 Undercolor removal / blackening circuit 301 Pixel matrix 302, 302a to 302d Edge detection matrix 303a to 303d Convolution operation circuit 304a to 304d Absolute value circuit 305 Maximum value circuit 306 Edge reference value setting circuit 307a to 307e Comparison circuit 401a to 401i Multiplier 402 Adder 601a to 601d Selection circuit 602a Difference calculation circuit 602b Difference calculation circuit 603a Absolute value circuit 603b Absolute value circuit 604 Comparison circuit 801 Maximum value circuit 802 Small value circuit 803 Difference calculation circuit 804 Average value circuit 805 Division circuit 806 Inversion circuit 901 Minimum value circuit 902 Multiplier 903a Difference calculation circuit 903b Difference calculation circuit 903c Difference calculation circuit 1001a Selection circuit 1001b Selection circuit 1001c Selection circuit 1001d Selection circuit 1002 Comparison Circuit 1101 Minimum value circuit 1102 Maximum value circuit 1103 Doubler circuit 1104 Difference value circuit 1105 Bit shift circuit 1201 Minimum value circuit 1202 Multiplier 1203 Difference value circuit 1204 Selection circuit

Claims (10)

[Claims] 1. A color signal processing means for obtaining a black printing amount and an undercolor removal amount of a pixel of interest based on inputted and shaded pixel information of the pixel of interest and color-separated density information of a plurality of pixels located around the pixel of interest. A color image processing device characterized by the above. 2. Edge detection means for detecting whether or not the pixel of interest is located in an edge region of an image based on color-separated grayscale information of the pixel of interest and a plurality of pixels located in the periphery thereof. When the target pixel is located in the edge area of the image, an edge correction unit that resets the grayscale information of the target pixel based on the grayscale information of the peripheral pixels, and the attention target reset by the edge correction unit. Undercolor removal rate detection means for detecting the undercolor removal rate of the pixel grayscale information, and the undercolor removal rate obtained from the grayscale information of the pixel of interest reset by the edge correction means and the undercolor removal rate detection means. Undercolor removal to obtain the black printing amount and the undercolor removal amount from
A color image processing device comprising a color signal processing means including black-and-black printing means. 3. The color image according to claim 2, wherein the edge detection unit performs the color separation based on a result of a convolution operation with a coefficient set by an edge detection filter for each grayscale information. Processing equipment. 4. The edge correction means selects, as a correction value, one of adjacent pixels in a direction orthogonal to a direction in which edges are continuous, which is closer to the grayscale information of a target pixel whose grayscale information is to be corrected. The color image processing apparatus according to claim 2. 5. The under color removal rate detecting means includes maximum value detecting means for detecting a maximum value of the color separated color signals, and minimum value for detecting a minimum value of the color separated color signals. A detection means; an average value calculation means for calculating an average value of outputs of the maximum value detection means and the minimum value detection means; and a difference calculation means for calculating a difference amount of outputs of the maximum value detection means and the minimum value detection means. An inverting means for subtracting the output of the difference computing means from the saturation value of the density level of the color signal; an output of the inverting means divided by an output of the average value computing means; and a result of the division being output as a lower color removal rate. 5. The color image processing apparatus according to claim 2, further comprising: 6. The undercolor removal / blackening means is a minimum value detection means for detecting the minimum value of a plurality of color signals of color-separated pixels, and an output of the minimum value detection means and the pixel are obtained. A multiplication means for multiplying the obtained undercolor removal rate and outputting the multiplication result as a black-printing amount; and a difference amount between the color signals of the color-separated pixels and the multiplication result of the multiplication means. The color image processing apparatus according to claim 2, further comprising a difference calculation unit that calculates a color removal amount. 7. Edge detection means for detecting whether or not the pixel of interest is located in an edge region from color-separated grayscale information of the pixel of interest and a plurality of pixels located around the pixel of interest, and the edge detection. When it is determined by the means that the pixel of interest is located in the edge region, the adjacent pixel detecting means for detecting the adjacent pixel in the direction orthogonal to the edge continuous direction, and the adjacent pixel detected by the adjacent pixel detecting means A color image processing apparatus comprising: a pixel and an undercolor removal / blackening unit for obtaining the undercolor removal amount and blackening amount of the pixel of interest from the grayscale information of the pixel of interest. 8. Undercolor removal for obtaining the undercolor removal amount and black-printing amount of the target pixel from the grayscale information of the adjacent pixel and the target pixel
The black-printing means includes an undercolor removal rate detecting means for obtaining an undercolor removal rate from a pixel having a higher density among the pixels adjacent to the pixel of interest, and an undercolor removal rate obtained by the undercolor removal rate detecting means. 8. The color image processing apparatus according to claim 7, further comprising black-printing amount calculation means for obtaining a black-printing amount from the ratio and the shade information of the pixel of interest. 9. Undercolor removal for obtaining the undercolor removal amount and black-printing amount of the target pixel from the grayscale information of the adjacent pixel and the target pixel
The black-printing means has means for making the color-separated light and shade information of the pixel having the lower density among the pixels adjacent to the target pixel the color-separated light and shade information after the undercolor removal of the target pixel. The color image processing device according to claim 7. 10. The undercolor removal rate detecting means includes maximum value detecting means for detecting a maximum value of the color separated color signals, and minimum value for detecting a minimum value of the color separated color signals. Detection means, doubling operation means for outputting a value twice the output of the minimum value detecting means, and difference operation means for taking the difference between the output of the doubling operation means and the output of the maximum value detecting means. A color image processing device comprising: