JP2003298860A - Screen arrangement, method and device for image processing, program, and recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image evaluation method or the like for automatically evaluating color shift. <P>SOLUTION: With respect to each element color, a plurality of spectrum data are generated in a spectrum generation part 104 and a spectrum operation part 106 on the basis of data caused by a density of input data, screen angles, and a pitch. The plurality if spectrum data generated for each element color are composed by a convolution execution part 108. This operation is performed also after phases of spectrums are shifted by a phase shift extent setting part 124 so as to move relative positions between respective screens by one pixel in the direction of the x axis or the y axis. The amplitude of the composed spectrum data before phase shift and that after phase shift are compared with each other to evaluate color slippage of an image by an image evaluation part 116. A screen having the same amplitude before and after phase shift is the most suitable that minimizes color shift. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a screen arrangement, an image processing method, an apparatus, a program, and a recording medium for color electrophotography for reproducing an image by using toners of a plurality of colors such as a color printer and a color copy.

[0002]

2. Description of the Related Art Color image output devices such as color printers and color copiers reproduce color images using cyan, magenta, yellow and black inks. In particular, a page printer that forms a latent image on a photosensitive drum by using a laser printer among color printers, develops the latent image with charged toner, and transfers the developed toner to a transfer sheet is irradiated with a laser beam. Regions can be variously changed within dots, and color image data with higher resolution and higher gradation can be reproduced even if the number of dots per unit area is small. In a color laser printer, as a binarization method for gradation reproduction of a grayscale image,
The dither method is widely used. According to this dither method, with respect to gradation data of each color which is an input signal, a conversion table called a dither matrix or a threshold matrix having correspondence between gradation data and image reproduction information is referred to The presence / absence (1,0) of the dot display is determined. Then, a halftone dot is formed depending on the presence or absence of display of a plurality of adjacent dots, and the halftone of the grayscale image is reproduced according to the size of the halftone dot.

[0003]

However, as an unavoidable problem in color laser printers, there is a positional shift of halftone dots of each color. Such positional displacement cannot be avoided even if an industrial printing apparatus is used, but in the case of a color image output apparatus, it mainly occurs in the sub-scanning direction due to uneven rotation (jitter) of the photosensitive drum, for example. Such a positional shift causes a change in brightness or, in the worst case, a color shift that deviates from a desired color.

Therefore, the present invention has been made in order to solve the above problems, and an object thereof is to provide a screen layout, an image processing method, an apparatus, a program, and a recording medium in which the amount of color misregistration is suppressed. To do.

[0005]

In view of the above problems, the invention according to claim 1 is characterized in that a repetitive pattern generated by stacking two or more screens arbitrarily selected from a plurality of screens, Even if the relative position of the screen is changed by a predetermined amount, it is configured to be substantially the same.

In view of the above problems, the invention of claim 2 is the invention of claim 1, wherein the predetermined amount is one pixel in the main scanning direction or the sub scanning direction.

In view of the above problems, the invention of claim 3 is the invention of claim 1 or 2, wherein the halftone dots grow in the direction of adjacent halftone dots.

In view of the above problems, the invention of claim 4 is the invention of claim 3, wherein the directions in which halftone dots grow are configured to intersect between screens.

In view of the above problems, the invention described in claim 5 is the invention according to claim 3, wherein the halftone dots grow in a dot shape in a portion where the brightness is higher than a predetermined value. To be done.

In view of the above problems, the invention of claim 6 is the invention of claim 1, wherein the predetermined amount is an integral multiple of 0.5 pixels in the main scanning direction. It

In view of the above problems, the invention according to claim 7 is the invention according to claim 1, wherein the halftone dots are arranged in a non-square manner.

In view of the above problems, the invention of claim 8 is the invention of claim 7, wherein the halftone dots are arranged in a parallelogram shape.

In view of the above problems, the invention according to claim 9 is the invention according to claim 1, wherein the tangent of the angle of the straight line connecting the adjacent halftone dots to the horizontal axis is a rational number. It

In view of the above problems, the invention described in claim 10 is the invention described in claim 1, wherein the screen is configured to relate to cyan and magenta.

In view of the above-mentioned problems, the invention according to claim 11 is the invention according to claim 1, wherein the screen relates to any two or more of cyan, magenta, yellow and black. Is configured as follows.

In view of the above problems, the invention of claim 12 is configured to perform halftone processing by utilizing the invention of any one of claims 1 to 11.

In view of the above problems, the invention set forth in claim 13 is configured so as to include halftone processing means for carrying out halftone processing using the invention set forth in any one of claims 1 to 11. It

In view of the above problems, a fourteenth aspect of the present invention is a program for causing a computer to execute a halftone process using the invention according to any one of the first to eleventh aspects.

In view of the above problems, the invention according to a fifteenth aspect is a computer-readable recording medium in which the program according to the fourteenth aspect is recorded.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment FIG. 1 is a block diagram of an image output system according to a preferred embodiment of the present invention. In this example, the host computer 5
At 0, image data 56 composed of RGB gradation data (8 bits for each, a total of 24 bits) is generated and provided to an image output device 60 such as a page printer. The image output device 60 such as a page printer reproduces a color image based on the supplied image data 56. The image output device 60 is a controller 62 that performs image processing and supplies laser driving data 69 to the engine 70.
And an engine 70 for reproducing an image according to the drive data 69.

The host computer 50 is an application program 5 such as a word processor and a graphic tool.
2, character data, graphic data, bitmap data, etc. are generated. The respective data generated by these application programs 52 are rasterized by the driver 54 for the image output device installed in the host computer 50, and the image data 56 consisting of gradation data of each RGB color for each pixel or dot. Is converted to.

The image output device 60 also includes a microprocessor (not shown), and the color conversion unit 64, according to the microprocessor and the installed control program.
A controller 62 including a halftone processing unit 66, a pulse width modulation unit 68, etc. is configured. Also, the engine 7
The laser driver 72 in 0 drives the laser diode 74 for image drawing based on the driving data 69.
The engine 70 includes a photosensitive drum, a transfer belt and the like, and a drive unit thereof, but they are omitted in FIG.

The color conversion unit 64 in the controller 62 converts the supplied RGB gradation data 56 for each dot into gradation data 10 of C (cyan) M (magenta) Y (yellow) K (black). To do. CMYK gradation data 10
Is 8-bit gradation data for each color, and the maximum is 256.
Has gradation. Note that the color conversion unit 64 uses RGB for each dot.
The gradation data 56 is converted into the gradation data 10 for each dot of each plane of CMYK. Therefore, the halftone processing unit 66 is supplied with the gradation data 10 (see the input data 10 in FIG. 2) corresponding to the dots for each plane of each color.

The halftone processing unit 66 refers to the conversion table having the correspondence between the gradation data created in advance and the image reproduction information for the gradation data 10 for each dot,
Image reproduction data 30 for each dot is generated. The halftone processing unit 66 uses the dither method described below with reference to FIGS. 2 to 5 to generate the image reproduction data 30 representing halftone. For example, by using the conversion table including the threshold matrix 20 shown in FIG. 2, binary image reproduction data 30 can be generated for each dot.

FIG. 2 is a diagram for explaining a binarization method for tone reproduction of a grayscale image by the dither method. Image data is generated by an image generation program such as a host computer and supplied as input data to an image processing unit for image output. The input image data is, for example, gradation data 10 for each color dot. In FIG. 2, gradation data is written in each dot. According to the dither method, the conversion table 2 called a threshold matrix in which the threshold value of each dot is changed according to a predetermined rule is applied to the gradation data.
0 is used. More specifically, the gradation data of each dot is compared with the threshold value of the threshold matrix, and if the gradation data is equal to or more than the threshold value of the corresponding dot, the image reproduction data 30 with drawing is generated, and if it is smaller, the image reproduction data 30 is drawn. The image reproduction data 30 of none is generated. That is, the multi-gradation data 10 for each dot is converted into the binary image reproduction data 30 for each dot.

The threshold matrix shown in FIG. 2 is a conversion table having correspondence between gradation data and image reproduction information indicating the presence / absence of drawing. That is, the threshold value of each dot in the conversion table can be referred to as image reproduction information indicating whether or not the gradation data should be drawn. The threshold value matrix 20 shown in FIG. 2 is a 4 × 4 matrix, which is a so-called dot concentration type in which the central portion is low and the peripheral portion is high.

FIG. 3 is a diagram for explaining a threshold matrix of the dither method. FIG. 3 shows an image reproduced for each gradation when the dot concentration type threshold matrix 20 of FIG. 2 is used. The images that can be reproduced when the gradation increases in order from the upper left to the right and from the lower left to the right are shown. That is, it is understood that the halftone dots grow from the center of the 4 × 4 halftone dot cell. It is known that the dot concentration type threshold matrix is superior to the dot dispersion type when the dot density becomes high to some extent.

FIG. 4 is a diagram showing changes in the shape of halftone dots formed by using the dither method. By using the conversion table composed of the dot concentration type threshold matrix shown in FIGS. 2 and 3, the dots in the halftone dot cells formed by m × m become gradually thicker according to the gradation degree (brightness). It is understood that it will grow. The gradation of the grayscale image is expressed by this two-dimensional density.

As shown in the example of the halftone dot in the middle of FIG. 4 (halftone dot in the middle portion), the growing direction of the halftone dot is set to the vertical direction and the horizontal direction in this example. The arrangement of the threshold matrix or the arrangement of thresholds in the matrix determines the growth direction of this halftone dot. Therefore, in the example of FIG. 4, the screen angle is 0 ° or 90 °.

FIG. 5 is a diagram showing a method of forming a screen angle. As shown in FIG. 5, halftone dots having a predetermined screen angle can be formed by appropriately arranging halftone dot cells 40 composed of m × m dots so as to be arranged.
In the example of FIG. 5, the m × m halftone dot cell is 4 on the right (N in the drawing).
When they are moved, they are placed one above the other, so they are placed at a: b = 4: 1. Therefore, the screen angle θ is θ = tan ⁻¹ (b / a) = tan ⁻¹ (1 /
4).

Here, the size N of the square threshold matrix 42 corresponding to one halftone dot period is N = LCM (a, b) × (b / a + a / b). However, LCM means the least common multiple.

The above-mentioned arrangement of the halftone dot cells 40 can be arbitrarily set by shifting the correspondence between the dots of the input data 10 and the threshold matrix 20. Alternatively, the screen angle can also be set by preparing in advance different threshold matrices for the square threshold matrix size and comparing the threshold matrix for the square threshold matrix 42 size with the input data.

As described with reference to FIG. 5, the screen angle of each color can be appropriately set by setting the arrangement of the threshold matrix 20 which is the conversion table. When the binary image reproduction data 30 is generated, the pulse modulation unit 68 generates drive data 69 with and without a laser drive pulse for each dot. The image evaluation apparatus 100 is configured by executing an image evaluation processing program by the host computer 50. This image evaluation processing program is normally recorded in a recording medium such as a floppy (registered trademark) disk or a CD-ROM in a form readable by the host computer 50 and distributed. The program is a media reading device (C
It is read by a D-ROM drive, a floppy disk drive, etc.) and installed in the hard disk. Then, the CPU is configured to read out a desired program from the hard disk as appropriate and execute a desired process.

FIG. 6 is a block diagram of the image evaluation apparatus according to the first embodiment of the present invention.

The image evaluation apparatus 100 includes an input unit 102 for inputting a screen basic condition; a spectrum generation unit 1 for generating a predetermined spectrum based on the input screen condition and the set phase shift amount.
04; a spectrum calculation unit 106 that performs a predetermined calculation on the frequency components of the generated basic spectrum; a convolution execution unit 108 that performs a convolution calculation on the frequency spectrum of each screen; and a calculation of a convolution calculation. An image evaluation unit 116 that receives the resulting spectrum data and evaluates the degree of color misregistration by comparing the amplitude of the spectrum data with and without the phase shift; An optimal combination determining unit 118 that determines a combination of screens with a small number of screens; a screen setting unit 122 that sets a screen to be evaluated
And; Phase shift amount setting unit 12 for setting the amount of phase shift
4 and ;.

Next, the operation of each part of the image evaluation apparatus 100 will be described.

First, the input screen 102 inputs basic screen conditions (screen halftone dot spacing, halftone dot size (diameter), pitch, angle) for the number of screens.
In addition, in this embodiment, the halftone dot shape is described as a circle. Therefore, the dot size can be specified by the diameter. However, the halftone dot shape is arbitrary, and halftone dots having other shapes may be used (eg, rectangle, etc.). The pitch and angle of the screen will be described with reference to FIG. Halftone dot 10
2a is a halftone dot of the screen of one color (for example, cyan) of CMYK. The halftone dots 102a of the screen are arranged in a parallelogram shape, θ is an angle (screen angle), and P1 and P2, which are intervals between opposing sides of the parallelogram, correspond to a pitch. Thus, it is preferable to arrange the halftone dots in a non-square shape. Note that tan, which is the tangent of the screen angle θ,
It is preferable that θ be a rational number in order to reduce the threshold matrix.

The screen setting unit 122 includes the input unit 102.
Set the screen to input to. The screen relates to any two or more of cyan, magenta, yellow and black. The screens are described here as being for cyan and magenta. The screen angle (θ: see FIG. 7) and pitch (P1 and P2: see FIG. 7) can be set (preferably a plurality of types) for each screen.

The phase shift amount setting section 124 sets the phase shift amount of the spectrum. The setting of the phase shift amount is performed on the spectrum generation unit 104. The phase shift amount is, for example, the phase of a spectrum corresponding to moving one pixel of the cyan and magenta screens by 1 pixel (pixel) in the x direction (main scanning direction) or the y direction (sub scanning direction). . By shifting the phase, the relative position of the cyan screen with respect to the magenta screen is moved by 1 pixel (pixel) in the x direction or the y direction.

It should be noted that the phase shift amount is, for example, the phase of the spectrum corresponding to the movement of one of the cyan and magenta screens by an integral multiple of 0.5 pixel (pixel) in the x direction (main scanning direction). May be By shifting the phase, the relative position of the cyan screen with respect to the magenta screen moves in the x direction (main scanning direction) by an integral multiple of 0.5 pixel (pixel).

FIG. 8 shows the processing of the screen setting unit 122 and the phase shift amount setting unit 124. First, the screen setting unit 122 sets a screen for the input unit 102 (step 262). The setting contents are the screen color (eg, cyan and magenta), the screen angle of each element color screen, the pitch, and the like. After this, spectrum generation (spectrum generation unit 104), spectrum calculation (spectrum calculation unit 106), convolution (convolution execution unit 108), and measurement of the amplitude of the convolution result (image evaluation unit 116) are performed. The details will be described later. Then, the phase shift amount setting unit 124 sets the phase shift amount in the spectrum generation unit 104 (step 264). Even after this, spectrum generation (spectrum generation unit 104), spectrum calculation (spectrum calculation unit 106), convolution (convolution execution unit 10)
8), measurement of amplitude as a result of convolution (image evaluation unit 11
6) is performed, and details will be described later. Then, after all the screen settings have been set (step 266, Y
es), and the process ends. Note that setting all the screen settings means setting all the combinations of screen angles and pitches for cyan and magenta screens, for example. By doing so, it is possible to measure the amplitude of the convolution result when the phase shift is not performed and when it is performed.

Next, the processing in the spectrum generator 104 will be described. When the screen condition is input to the conversion formula prepared for each halftone dot shape, the spectrum generation unit 104 generates spectra of appropriate orders in the vertical and horizontal directions for the number of screens.

FIG. 9 shows the processing in the spectrum generator 104. The spectrum generation unit 104 stores a spectrum generation formula calculated in advance. This spectrum generation formula corresponds to the case where the halftone dot shape of the screen is a circle. First, the screen halftone dot spacing and halftone dot size input from the input unit 102 are substituted into the spectrum generation equation (step 202). Then, the phase shift amount is set (step 203). Further, a spectrum is generated (step 204). And step 2 above
After 02 to 204 have been executed for all screens (step 206, Yes), the spectrum generation unit 104
The process in is ended.

Next, a specific process in the spectrum generation unit 104 will be described. FIG. 10 shows a halftone dot pattern with uniform gradation (FIG. 10a) and its spectrum (FIG. 10).
b) is shown. As an example, the frequency spectrum P (μ, ν) of a halftone dot manufactured using an orthogonal screen is shown in FIG.
The density distribution of halftone dots with uniform gradation density as shown in a is p
(X, y), where T is the periodic pitch of halftone dots, using the formula of Fourier integration,

[0045]

[Equation 1] It is expressed as. The frequency spectrum P (μ, ν) is
It is the spectrum generated in step 204.

The spectrum generator 104 is adapted to the case where the halftone dot shape is a circle, and finds Am, n in this case. Figure 1
As shown in 1, when the halftone dot shape is a circle with a diameter of 2R,

[0047]

[Equation 2] Becomes

The spectrum when the phase is shifted in step 203 is

[0049]

[Equation 3] Becomes

For example, 1 pi in the x direction under the condition of resolution 600 dpi
When the screen is moved by xel, the amplitude is 5.
0, spatial frequency 100lpi in x direction, spatial frequency 150lp in y direction
The spectrum with the condition i is

[0051]

[Equation 4] Becomes

That is, in this specific example, step 20
In step 2, the halftone dot interval and halftone dot size of the screen input from the input unit 102 are substituted into the spectrum generation formula, and further, in step 203, the phase shift amount is set. In step 204, the spectral frequency spectrum P (μ, ν) is generated. Then, after the above steps 202 to 204 are executed for all screens (step 206, Yes), the process in the spectrum generation unit 104 is ended. FIG. 16 shows an example of the output spectrum generated by the spectrum generation unit 104.

Next, the processing in the spectrum calculation unit 106 will be described. The spectrum calculation unit 106 operates the frequency components of the basic spectrum generated by the spectrum generation unit 104, and corresponds to slide / rotation / parallel movement (origin movement) of the screen in order to make the screen into a parallelogram shape. Repeat the calculation for the number of screens.

FIG. 12 shows processing in the spectrum calculation unit 106. The spectrum calculation unit 106 performs screen slide processing (step 210). That is, an operation for arranging dots in a parallelogram grid is performed. FIG. 13 shows a diagram for explaining the screen slide process. As shown in FIG. 13, in the xy axis plane,
The vertical side of the screen in which halftone dots are arranged in a square lattice is transformed into a side inclined obliquely to the right (for example, y: x = 2:
The operation is 1) of the lateral displacement x on the frequency axis.
This corresponds to the operation of subtracting the displacement of / 2, that is, x / 2 from the frequency component in the y direction of the spectrum. When transforming into a parallelogram having an arbitrary slope, it is assumed that the displacement rate is a and the transformation of x ′ = x + ay is performed on the xy axis coordinate axes. Therefore, the transformation of y ′ = y−ax is performed on the frequency axis. Just go.

Next, the spectrum calculation unit 106 performs screen rotation processing (step 212). That is, the frequency of each spectrum is rotated around the origin using the following formula fx ′ = fxcos θ−fycos θ fy ′ = fxsin θ + fysin θ, and conversion corresponding to the rotation operation on the xy axes is performed. Where fx: frequency in the x direction before rotation (line per inch) fy: frequency in the y direction before rotation (line per inch) fx ': frequency in the x direction after rotation (line per inch) fy': rotation Later frequency in the y direction (line per inch) θ: rotation angle (screen angle).

Further, the spectrum calculation unit 106 performs screen parallel movement processing (step 214). That is, in order to adjust the origin of the screen, each spectrum is given a phase component of an amount determined by the frequency component of each spectrum, halftone dot spacing, and origin position. The given phase component phase is represented by phase = (px × fx / lx) + (py × fy / ly). Where fx: frequency in the x direction of the spectrum fy: frequency in the y direction of the spectrum lx: number of lines in the x direction (line per inch) ly: number of lines in the y direction (line per inch) px: halftone dot spacing is 2π The amount of movement of the origin position in the x direction (rad) py (rad) py: The amount of movement of the origin position in the y direction (rad) where the halftone dot spacing is 2π (rad).

After the processing in steps 210 to 216 has been executed for all screens (step 216, Yes), the processing in the spectrum calculation unit 106 is terminated. Among the calculation results of the spectrum calculation unit 106, the spectrum of the screen for C is
The spectrum of the screen for A _C and M is A _M , the spectrum of the screen for _Y is A _Y , and the spectrum of the screen for _K is A _K. In addition, when written as A _{Cm, n} , m and n indicate the dimension of the spectrum.

The processing in the spectrum generating section 104 and the spectrum calculating section 106 described above independently processes the information for each screen, but the convolution executing section 108 described below processes the information for each screen. Are combined into one piece of information.

Next, the processing in the convolution execution unit 108 will be described. The convolution execution unit 108 performs a convolution operation corresponding to the operation of multiplying each screen as an image of xy coordinates on the frequency spectrum of each screen.

FIG. 14 shows a diagram for explaining the convolution operation. Since only the calculation equivalent to the superposition of two sheets can be performed at the same time, when the superposition of four sheets is performed, three convolutional operations shown in the convolutional operation 1, the convolutional operation 2 and the convolutional operation 3 are required. Is. That is, in the convolution calculation 1, a calculation corresponding to the superposition of the screen 1 and the screen 2 is performed, and in the convolution calculation 2, a calculation corresponding to the superposition of the screen 3 and the screen 4 is performed. Then, in the convolution calculation 3, a calculation corresponding to the superposition of the screens 1 to 4 is performed. The convolution calculation result is supplied to the evaluation value calculation unit 116. The convolution operation has two screens (eg A _C and A _M ), 3 screens (eg A _C , A _M and A _K ) or 4 screens (eg A _C , A _M , A _Y)
And _AK ). The result of the convolution operation is written with the convoluted screen as a subscript. For example, A _CM . A _CM is A _C and A _M
Is the result of folding in. In addition, when written as A _{CMm, n} , m and n indicate dimensions.

FIG. 15 is a diagram for explaining the concept of the convolution operation. As shown in FIG. 15, the screen 1
Assuming the spectrum of screen 2 and the spectrum of screen 2, the spectrum after convolution is the origin of one spectrum (spectrum 1 in this example) superimposed on the other spectrum (spectrum 2 in this example). Then, the intensity of the spectrum corresponds to the product of each spectrum.

Next, the processing in the image evaluation section 116 will be described. The image evaluation unit 116 receives the spectrum data, which is the result of the convolution operation, from the convolution execution unit 108. The spectrum data may or may not be phase-shifted. Therefore, the amplitude of the spectrum data when the phase shift is performed is compared with the amplitude of the spectrum data when the phase shift is not performed. When both are the same, an image with less color shift can be obtained.

Since the rate of deviation of the amplitude is a scalar quantity, it can be used for relative comparison of screens and absolute evaluation of screens. That is, in the case of comparing the color misregistration when the screen angle and the pitch of the screen are changed, the presence or absence of the color misregistration can be ranked by comparing the ratios of the deviations of the amplitudes. Also, based on the ratio of the amplitude deviation in the screen angle and pitch of a certain screen,
It is also possible to judge the conspicuous degree of the color misregistration based on the comparison between the reference and the ratio of the amplitude deviation in the screen angle and the pitch of another screen.

The optimum combination determination unit 118 is the image evaluation unit 1.
In 16, the screen combination in which the amplitude of the spectrum data when the phase shift is performed and the amplitude of the spectrum data when the phase shift is not performed are equal to each other is a combination of the screens with less color shift. To decide. The fact that the amplitude of the spectrum data when the phase shift is performed is equal to the amplitude of the spectrum data when the phase shift is not performed means the following. Ie a screen (eg cyan)
And another screen (eg, magenta) are repeated, the repetitive pattern is generated by another screen (eg, cyan) or another screen (eg, magenta).
It means that it does not change (exactly the same) by moving the relative position with respect to 1 pixel (pixel) in the x direction or the y direction. If such a combination of screens is used for printing or the like, color misregistration can be reduced.

Next, the overall operation of the image evaluation apparatus 100 will be described.

The overall operation of the image evaluation apparatus 100 is shown in FIG.
Shown in. First, the screen setting unit 122 sets a screen to be input to the input unit 102 (step 40).
2). The setting contents are the screen color, screen angle, and pitch. The set screen is sent to the spectrum generation unit 104 via the input unit 102 to generate a spectrum (step 404). A predetermined calculation is performed on the generated spectrum by the spectrum calculation unit 106 (step 406). The calculation result is convolved by the convolution executing unit 108 (step 408), and the image evaluation unit 116 measures the amplitude of the convolved spectrum (step 410). Since the phase has not been shifted yet (step 412, No),
The phase shift amount setting unit 124 sets the phase of the spectrum to 1
The pixel is moved in the x-axis or y-axis direction (step 414). Then, the amplitude of the spectrum convolved from the spectrum generation is measured (step 404-
Step 410) is performed. After this, since the phase has already been shifted (step 412, Yes), it is determined whether or not all screens have been set (step 41).
6) If not finished (step 416, No), the process returns to the screen setting (step 402). If the processing has been completed (step 416, Yes), the optimum combination determination unit 118 determines that the convolved spectrum has the same amplitude regardless of the presence or absence of the phase shift (step 418).

The following effects can be obtained by using the screen with the small color shift determined in this way at the time of printing. First, the same overlapping pattern appears every time the relative position of the screen shifts by the unit resolution (1 pixel). Therefore, no matter how large the misregistration occurs, the color misregistration can be suppressed by the maximum color change amount that occurs due to the misregistration within one pixel. That is, even if a plate displacement of unit resolution (1 pixel) or more occurs, the color displacement can be suppressed to a constant amount.

In addition, since the screen having such a small color shift is arranged with the halftone dots of the screen so as to cancel the area shift of the secondary color caused by the shift within one pixel, the maximum color change amount can be suppressed to a small value.

The halftone dots may grow in the direction of the adjacent halftone dots. This is a so-called line screen. At this time, if the growth directions of the screens of different colors intersect each other, the area variation of the secondary color of the screen can be suppressed, and the color stability can be improved in the entire density range. However, in the highlight portion of the line screen, since the toner adheres to the paper surface as an extremely thin line, it is easily affected by noise, jitter, and the like, and the toner density of a single color may become unstable. Therefore, a method of maintaining the stability of toner density in a single color is adopted by growing dots in a dot shape only in the highlight portion. That is, a screen that grows in a dot shape for the highlight portion and a line shape for the high density portion is often used. At this time, in the conventional screen, if the halftone dots were grown in a dot shape, the area variation of the secondary color due to plate misalignment would be large and the color stability might be impaired. However, in the screen in which the position of the center of gravity of the halftone dots is taken into consideration, the halftone dots are arranged so as to cancel the area variation, so that the amount of color change can be suppressed to a small level. Therefore,
It is possible to obtain a screen having excellent color stability against misregistration in the entire density range and excellent density stability in a single color.

Second Embodiment FIG. 18 is a block diagram of an image evaluation apparatus 100 according to the second embodiment of the present invention. This image evaluation device 100 differs from the image evaluation device 100 described above in that it directly multiplies pixel values and does not use a spectrum.

The image evaluation apparatus 100 has an input unit 102 for inputting a screen basic condition; a pixel value multiplication unit 124 for multiplying the pixel value of each screen; An image evaluator 126 that evaluates the degree of color misregistration by comparing the result of the multiplication with the result of multiplication without pixel shift; and an optimum combination determination unit 128 that determines a combination of screens with less color misregistration. And; a screen setting unit 122 for setting a screen to be evaluated; and a pixel shift unit 123 for setting a pixel shift amount.

Next, the operation of each part of the image evaluation apparatus 100 will be described.

Input unit 102 and screen setting unit 12
2 is the same as in the first embodiment. Pixel shift unit 1
23 sets the shift amount of the pixel. The pixel shift amount is set for the input unit 102. Pixel shift means, for example, moving one of the cyan and magenta screens by 1 pixel (pixel) in the x direction or the y direction. As a result, the relative position of the cyan screen with respect to the magenta screen is 1 pixel in the x direction (main scanning direction) or the y direction (sub scanning direction).
(Pixel) will move.

Note that the phase shift amount is, for example, the phase of the spectrum corresponding to moving one of the cyan and magenta screens by an integral multiple of 0.5 pixel (pixel) in the x direction (main scanning direction). May be By shifting the phase, the relative position of the cyan screen with respect to the magenta screen moves in the x direction (main scanning direction) by an integral multiple of 0.5 pixel (pixel).

The pixel value multiplication unit 124 multiplies the pixel value of each screen. The pixel value is 1 for each pixel if there is a halftone dot, and 0 if there is no halftone dot. Therefore, the pixel value of the corresponding pixel is multiplied between the screens. The multiplication result is the number of pixels of each screen, and the value is 0 or 1.
Becomes

Next, the processing in the image evaluation unit 126 will be described. The image evaluation unit 126 uses the pixel value multiplication unit 12
Receive the multiplication result of 4. The result of this multiplication may be one in which the pixel is shifted or one in which the pixel is not shifted. Therefore, the number of those having a value of 1 in the multiplication result when the pixel is shifted and the number of those having a value of 1 in the multiplication result when the pixel is not shifted are counted and compared. When both are the same, an image with less color shift can be obtained.

Since the number of products that take a predetermined value (eg, 1) in the multiplication result is a scalar quantity, it can be used for relative comparison of screens and absolute evaluation of screens. That is, when comparing the color shift when changing the screen angle and pitch of the screen,
By comparing the deviation ratios of the number of products that take a predetermined value in the multiplication result, it is possible to rank the presence or absence of color misregistration. Further, based on the ratio of the deviation of the number of those having a predetermined value in the multiplication result at the screen angle and pitch of a certain screen, the deviation of the number of those having the predetermined value in the multiplication result at the screen angle and pitch of another screen. It is also possible to judge the degree of noticeable color misregistration based on the comparison with the ratio.

The optimum combination determination unit 128 is the image evaluation unit 1.
26, a combination of screens in which the number of those having a predetermined value in the multiplication result when the pixel is shifted is equal to the number of those having the predetermined value in the multiplication result when the pixel is not shifted To be a combination of screens with less color shift. The fact that the number of products that have a predetermined value in the multiplication result when the pixel is shifted is equal to the number of products that have a predetermined value in the multiplication result when the pixel is not shifted means the following. To do. That is, one screen (eg cyan) and another screen (eg cyan)
The repetitive pattern generated by overlapping with (magenta) is the relative position of one screen (eg, cyan) to another screen (eg, magenta) in the x direction or y.
It means that it does not change (moves exactly the same) by moving 1 pixel in the direction. If such a combination of screens is used for printing or the like, color misregistration can be reduced.

Next, the overall operation of the image evaluation apparatus 100 will be described.

The overall operation of the image evaluation apparatus 100 is shown in FIG.
Shown in. First, the screen setting unit 122 sets a screen to be input to the input unit 102 (step 40).
2). The setting contents are the screen color, screen angle, and pitch. The set screen is sent to the pixel value multiplication unit 124 via the input unit 102, the pixel value is determined,
The pixel values of pixels corresponding to each other are multiplied (step 4).
22). Then, the image evaluation unit 126 counts the number of multiplication results that take a predetermined value (eg, 1) (step 424). Since the pixels have not been shifted yet (step 426, No), the pixel shift unit 1
23, one of the cyan and magenta screens is moved by 1 pixel in the x direction or the y direction (step 428). Then, the multiplication of the pixel values and the count of those which take a predetermined value among the multiplication results (step 4
22, step 424). After this, since the pixels have already been shifted (step 426, Yes), it is determined whether or not all screens have been set (step 4).
30), if not finished (step 430, No),
Return to the screen setting (step 402). If completed (step 430, Yes), the optimum combination determination unit 118 determines the same number of multiplication results that have a predetermined value regardless of the presence or absence of pixel shift as a screen with less color shift (step 430). 432).

If a screen with a small color shift determined in this way is used at the time of printing or the like, the same effect as in the first embodiment can be obtained.

The halftone dots may grow in the direction of the adjacent halftone dots. This is a so-called line screen. At this time, if the growth directions of the screens of different colors intersect each other, the area variation of the secondary color of the screen can be suppressed, and the color stability can be improved in the entire density range. However, in the highlight portion of the line screen, since the toner adheres to the paper surface as an extremely thin line, it is easily affected by noise, jitter, and the like, and the toner density of a single color may become unstable. Therefore, a method of maintaining the stability of toner density in a single color is adopted by growing dots in a dot shape only in the highlight portion. That is, a screen that grows in a dot shape for the highlight portion and a line shape for the high density portion is often used. At this time, in the conventional screen, if the halftone dots were grown in a dot shape, the area variation of the secondary color due to plate misalignment would be large and the color stability might be impaired. However, in the screen in which the position of the center of gravity of the halftone dots is taken into consideration, the halftone dots are arranged so as to cancel the area variation, so that the amount of color change can be suppressed to a small level. Therefore,
It is possible to obtain a screen having excellent color stability against misregistration in the entire density range and excellent density stability in a single color.

Third Embodiment FIG. 20 is a block diagram of an image output system according to a third embodiment of the present invention. This system configuration example is a modification of the system configuration example shown in FIG. In the system of FIG. 20, the driver 80 installed in the host computer 50 includes a rasterize function 54, a color conversion function 64, a halftone processing function 66, and an image evaluation function 100.
And an image setting function 300. Each of these functions 5
4, 64, 66 and 100 have the same functions as those having the same reference numbers shown in FIG. Then, the image reproduction data (pulse width data) 30 for each color generated by the halftone processing function is supplied to the pulse width modulator 68 of the controller 62 in the image output device 60 such as a page printer,
It is converted into desired drive data 69 and given to the engine 70.

In the system example of FIG. 20, the driver 80 installed on the host computer side performs color conversion processing, halftone processing, and image evaluation processing. In the example of FIG. 1, the color conversion process and the halftone process are performed by the controller in the image output device.
In the example of FIG. 20, it is performed on the host computer 50 side.
When the price reduction of the image output device 60 is required, it is required to lower the price by lowering the capacity of the controller 62. In this case, it is effective to partially implement some of the functions performed by the controller of FIG. 1 by the driver program installed in the host computer. When halftone processing is realized by the driver 80, a recording medium in which a program for causing the computer to execute the above halftone processing procedure is stored is built in the host computer 50.

Fourth Embodiment FIG. 21 is a block diagram of an image output system according to a fourth embodiment of the present invention. This system configuration example is a modification of the system configuration example shown in FIG. The difference from the system configuration example shown in FIG. 1 is that the image evaluation apparatus 100 is separate from the host computer 50.

In this case, before the host computer 50 gives the image data 56 to the image output device 60, the image evaluation device 100 decides in advance a combination of screens with less color shift and gives it to the halftone processing unit 66. .
The halftone processing unit 66 processes the gradation data 10 by using the given combination of screens having less color shift.

For example, the host computer 50 is a personal computer and the image output device 60 is a laser printer. Further, the halftone processing unit 66 reads the ROM (Read
OnlyMemory). Before shipment of the image output device (laser printer) 60, the image evaluation device 100 records a combination of screens with less color shift in this ROM. The halftone processing unit 66 reads out a combination of screens with less color shift from this ROM, and outputs the gradation data 10
To process.

The halftone processing unit 66 may be configured by the image output device 60 executing the halftone processing program. This halftone processing program is normally used by the image output device 6
0-readable form of floppy disk, CD-RO
It is recorded on a recording medium such as M and distributed. The program is read by a media reading device (CD-ROM drive, floppy disk drive, etc.) and installed in the hard disk. Then, the CPU is configured to read out a desired program from the hard disk as appropriate and execute a desired process. Alternatively, the image output device 60 may have a recording medium such as a ROM, and the halftone processing program may be recorded in this ROM.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an image output system.

FIG. 2 is a diagram illustrating a binarization method for tone reproduction of a grayscale image by a dither method.

FIG. 3 is a diagram illustrating a threshold matrix of a dither method.

FIG. 4 is a diagram showing a shape change of a halftone dot formed by using a dither method.

FIG. 5 is a diagram showing a method of forming a screen angle.

FIG. 6 shows a block diagram of an image evaluation device according to a first embodiment of the present invention.

FIG. 7 is a diagram showing a pitch and an angle of a screen.

FIG. 8 is a flowchart for explaining the processing of the screen setting unit 122 and the phase shift amount setting unit 124.

FIG. 9 is a flowchart for explaining the processing in the spectrum generation unit 104.

FIG. 10 is a diagram showing a halftone dot pattern with uniform gradation (FIG. 10a) and its spectrum (FIG. 10b).

FIG. 11 is a diagram for explaining the size of a halftone dot shape when the halftone dot shape is circular.

FIG. 12 is a flowchart for explaining a process in spectrum calculation section 106.

FIG. 13 is a diagram for explaining screen slide processing.

FIG. 14 is a diagram for explaining a convolution operation.

FIG. 15 is a diagram for explaining the concept of a convolution operation.

FIG. 16 is a diagram showing an example of an output spectrum generated by the spectrum generation unit 104.

FIG. 17 is a flowchart showing a process of the image evaluation device according to the first embodiment of the present invention.

FIG. 18 shows a block diagram of an image evaluation device according to a second embodiment of the present invention.

FIG. 19 is a flowchart showing a process of the image evaluation device according to the second embodiment of the present invention.

FIG. 20 is a configuration diagram of an image output system according to a third embodiment of the present invention.

FIG. 21 is a configuration diagram of an image output system according to a fourth embodiment of the present invention.

[Explanation of symbols]

10 Input data, gradation data 20 Conversion table, threshold matrix 30 image reproduction data 50 host computer 60 controller 66 Halftone processing unit 70 engine

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 2C262 AB11 BB03 FA01 FA12 FA13                       GA04                 5C077 LL02 MP02 MP08 NN07 NN08                       PP33 PP39 PP68 PQ12 TT02                 5C079 HB03 LC04 LC13 LC14 MA04                       MA11 NA02 PA03

Claims (15)

[Claims] 1. A repeating pattern generated by stacking two or more screens arbitrarily selected from a plurality of screens is substantially the same even if the relative positions of the screens are changed by a predetermined amount. Screen layout. 2. The screen arrangement according to claim 1, wherein the predetermined amount is one pixel in the main scanning direction or the sub scanning direction. 3. The screen arrangement according to claim 1, wherein the halftone dots grow in the direction of adjacent halftone dots. 4. The screen arrangement according to claim 3, wherein the directions in which the halftone dots grow intersect with each other between the screens. 5. The screen arrangement according to claim 3, wherein the halftone dots grow in a dot shape in a portion where the brightness is higher than a predetermined value. 6. The screen arrangement according to claim 1, wherein the predetermined amount is an integral multiple of 0.5 pixel in the main scanning direction. 7. The screen arrangement according to claim 1, wherein halftone dots are arranged in a non-square shape. 8. The screen arrangement according to claim 7, wherein the halftone dots are arranged in a parallelogram shape. 9. The screen arrangement according to claim 1, wherein a tangent of an angle of a straight line connecting adjacent halftone dots with respect to a horizontal axis is a rational number. 10. The screen arrangement according to claim 1, wherein the screens are for cyan and magenta. 11. The screen arrangement according to claim 1, wherein the screen relates to any two or more of cyan, magenta, yellow and black. 12. An image processing method for carrying out halftone processing using the screen arrangement according to claim 1. Description: 13. An image processing apparatus comprising a halftone processing means for performing a halftone process using the screen arrangement according to claim 1. Description: 14. A program for causing a computer to execute halftone processing using the screen arrangement according to claim 1. Description: 15. A computer-readable recording medium in which the program according to claim 14 is recorded.