JPH0698183A - Method and apparatus for making a halftone screen for color printing - Google Patents

Abstract

(57) [Abstract] [Object] To provide a method of using a digital computer to create a set of halftone screens for color printing, without the need to rotate the screens relative to each other to avoid the moire effect. To do. [Constitution] A value representing the required chromaticity level of each primary color in an image to be printed by creating and storing a separate matrix in which converted light intensity level values are arranged in a predetermined pattern as a screen function matrix unique to each halftone screen. Create a picture matrix consisting of
Storing and dividing the area of each screen into a plurality of cells each formed as a binary matrix of transparent or opaque unit compartments, and for each of said cells in the corresponding screen to be created said series for each primary color Each of the screens is created by assigning one of the converted intensity level values of the, and the converted intensity level value of the particular primary color is compared with the respective value in the corresponding screen function matrix.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method and apparatus for producing color prints, and more particularly to a method and apparatus for producing halftone screens for color prints.

[0002]

2. Description of the Related Art In the conventional halftone color copying method,
3 original color prints or color slides
Scanning is performed by a photodetector that senses changes in luminosity at the primary color frequencies, namely red, blue and green, respectively. The average luminous intensity level of each primary color in each incremental area (pixel) of the scanned original is quantified and stored in digital form. The red, green and blue digital values can also be obtained from other sources (eg programmed computers). By processing this resulting data, the respective digital values are converted from the dye three primary colors cyan, magenta and yellow to the equivalent luminous intensity levels required to produce the same color. In addition to the three primary colors, a "black" component must also be added to the processed output data.

Four screens are created from these four sets of numbers using a suitable plotter. This screen, called a halftone screen, has a grid pattern,
Depending on the printing method, the grid takes the form of physical apertures or clear areas of photographic negatives. In the halftone method, the area ratio of each opening or transparent area to the surrounding opaque area is determined by the chromaticity required for a particular pixel in the reproduced image. A color print copy is then obtained by utilizing each screen to distribute each of the three primary colors as dots in a grid pattern. The color print created is a copy of the original image, but it is composed of several patterns that combine dots of four colors of different sizes, and the human eye integrates these dot patterns into various patterns of the original image. It is reproduced as color and detail.

Interference fringes or patterns called "moiré effect" are formed as a problem that occurs when superposing various types of gratings. The Moire effect always occurs when multiple sets of parallel lines are superposed so as to form an angle with each other. That is, at a position where a plurality of sets of parallel lines intersect, they are mutually strengthened so that a dark stripe appears in the image. As a method of making the moire effect as small as possible when producing a color print by the halftone method, the superposed gratings are at an accurate angle with each other, that is, 0 ° for yellow and +15 for cyan.
There is a method of positioning at °, magenta at -15 °, and black at + 45 °. This method and the grating angle control method are disclosed in US Pat. Nos. 4,456,924 and 4,499,489. However, in this method, if there is any deviation in the above-mentioned accurate angle, a remarkable moire effect appears. By aligning the screens at the same angle, the moire effect can be reduced as much as possible. However, if the geometrical conditions of the screen to be used are the same, if there is any deviation in the lateral shift, the amount of dot overlap will change, which changes the area ratio of the white area and the color area. This approach is not practical because it results in significant changes in color reproduction.

[0005]

SUMMARY OF THE INVENTION The present invention relates to an improved method of making halftone screens for color reproduction, in which the screens do not have to be rotated relative to each other to avoid moire effects and each halftone screen is sequentially linear. Make color copies by shifting in the directions. The shape and relative position of dots on different screens
The choice is made to minimize color shifts due to variations in alignment during the printing stage. Since the superposition pattern is relatively regular, the conversion from red, green and blue to cyan, magenta, yellow and black is performed more accurately than before. Traditionally, this transformation required complicated and geometrical analysis utilizing some assumptions and simplifications for simplification, which inevitably resulted in poor color quality. The present invention can reduce the number of calculation steps required for processing color data.

The above and other advantages of the invention are summarized as follows: a color original is scanned and the luminous intensity in a series of pixel areas is measured for each of the three primary color frequencies, or else color video data is obtained, for example, by computer graphics operations. Is achieved by seeking. A digital computer is then used to convert the intensity level of each pixel into a set of numbers representing the intensity level of each primary color to be printed (e.g. cyan, magenta, yellow and black). A screen function matrix corresponding to each of the four color screens to be created for color reproduction is created and stored in computer memory. Each screen function matrix consists of at least one set of numbers covering all possible intensity level numbers for the relevant color. However, the numerical value pattern or positional relationship in each matrix differs for each color in a predetermined manner.

For example, using a suitable xy plotter,
Each screen is created by dividing the screen area into a plurality of cells, each cell corresponding to one halftone period. Each cell is formed by a plotter as a binary pixel area matrix, where the plotter creates each pixel area in one of two states, eg, transparent or opaque. Since the dimensions of the screen function matrix are the same as the cell matrix, each pixel area in the cell will have a corresponding intensity level value in the screen function matrix. Each cell of the picture screen is located in one or more specific pixel areas of the image to be reproduced. The binary state of each pixel area in the cell is determined by comparing the corresponding intensity level value in the screen function matrix with the intensity level stored value obtained from the associated pixel area.

[0008]

The detailed contents of the present invention will be described below with reference to the accompanying drawings.

In FIG. 1, 10 is a scanner loaded with an original color image to be copied, which may be a color photograph, such as a print or slide. The scanner may be any known optical scanning device which raster scans an image by means of light receiving elements which detect the level of reflected or transmitted light of each primary color, namely red, green and blue. The detected light intensity level is 3
It is converted into one electric analog signal and supplied to an analog / digital (A / D) converter 12. In addition, the scanner 10 outputs two signals x and y that define the pixel position and scan line of the scanning element that moves with respect to the photograph. A /
Since the D converter 12 is synchronized with the pixel position signal x, it produces a digital output each time the scanning element advances in predetermined increments. Thus, the image is converted into a series of pixels by the scanner 10 and the A / D converter 12, and the average luminous intensity at each pixel of each of the three primary colors becomes a digital value set on a scale of preferably 0 to 1. Although the scanner is shown as an example, the present invention is not limited to a specific digital color image data creating method, and for example, graphic information created by a computer may be used as the color image data supply source.

The three digitized red, green and blue output signals R, G and B are input to the digital computer 14 along with the x and y position signals. The computer is A in FIG.
Data is stored in the memory in the form of a picture matrix shown in. The positions Xm, Yn in the stored picture matrix correspond to the scanned original image positions. The three intensity level values R, G and B for each pixel are stored at corresponding locations in the picture matrix.

Once the data for the original is stored in the picture matrix, the computer processes the data to convert it into a set of intensity values equivalent to the data for the printing primaries cyan, magenta and yellow. The mathematics for this transformation are well known and are described in, for example, A. C. Yule “Principles of Color Reprod
uction ”(wiley & Sons, 1967), Chapters 10 and 11
See chapter. At the same time, the fourth about black
It is preferable to also calculate the intensity value set. The obtained intensity value data is stored in a print matrix as shown in FIG. 2B, and four intensity level values C,
M, Y and K are the corresponding positions in the print matrix x,
stored in y.

The computer utilizes this print matrix data to supply control information to the plotter 16 which draws the four halftone screens 18 required to make a color print. The plotter is a known high resolution xy
It's a plotter. Depending on the type of screen, a laser beam, an ink jet, or the like may be used as a print element or stylus of the plotter, and contrast dots may be formed in a unit section of the screen according to a command regardless of the medium forming the screen. The stylus can be positioned in any unit zone within the drawing area by an xy coordinate digital input signal. The screen can take various forms, such as a photographic negative, depending on the printing method employed. A known printer 20 processes the created screen to create a color print.

The command signal for the stylus and the position control signal for the plotter are generated by the computer in the following manner. As is apparent from FIG. 3, each of the created screens 18 is a screen / half-screen.
The cells 22 are logically divided into cells 22 corresponding to the cell cycle. The cell size (CS) may vary depending on the sharpness of the printing method employed and may be relatively large, for example when printing on a newspaper, since the sharpness is relatively low. The cell size is also limited by the plotter stylus unit (element) size (ES), and the more dots of the plotter in each cell, the better the color range of the printed copy. For example, halftone copying is not possible if the cell size is the same as the pixel area size. It is preferable to set the cell size to at least 8 times the unit size so that each cell includes at least 64 unit partitions. The physical size ES of the plotter dot or unit block 24 is determined by the minimum spacing resolution of the particular plotter used. The cell size CS is an integer n times the plotter unit size ES. Therefore, each cell consists of a unit partition or an n × n binary matrix of plotter dots 24. In the illustrated embodiment, n = 8.

In the present invention, the plotter, under the control of a computer, places dots in each cell in a unique pattern in each of the four halftone screens required for duplicate printing. The pattern varies from cell to cell, depending on the halftone or color intensity level requirements of each cell. At the same time, the pattern must be constructed so that the overlap of the individual colors is minimized when the screens are overlaid. Therefore, the computer stores four screen function matrices 26. The dimensions of this function matrix are the cell matrix,
That is, it is equal to the dimensions of the n × n matrix. Therefore, each position in the screen function matrix corresponds to the position 24 of the plotter dot (unit block) in the cell. The computer stores at each position in the screen function matrix different values taken from a set of numbers representing every chromaticity level on a scale of 0 to 1. The number of increments dividing the intensity scale is equal to the number of positions in the matrix, ie n × n. 4A to 4D illustrate four screen function matrices 26, one for each of the four primary colors cyan, magenta, yellow, and black. In this figure, instead of the actual intensity level value, the number of each incremental position of the intensity scale is shown.

The computer that calculates and stores the screen function matrix controls the plotter to advance the stylus from one unit block to the next according to a predetermined sequence. At each position, the computer sends a binary control signal to the stylus to activate or deactivate the stylus. This binary control signal is the intensity stored in the screen function matrix for a particular stylus position (unit compartment) within the cell for the particular halftone period or chromaticity level value required for the cell obtained from the print matrix (FIG. 2B). Set by comparing with the level value. For example, the computer activates the stylus (or does not activate the stylus depending on the printing method) only if the required chromaticity level is higher than the value obtained from the screen function matrix. 4 (1) to (4) show cells plotted for each color at four chromaticity levels. Each cell contains at least one pixel of the original scan image stored in the computer and associated with each cell. It is convenient for understanding the present invention to assume a case where one pixel and one cell correspond to each other, but such a correspondence does not necessarily have to be present. The positional relationship between pixels and cells is controlled by a computer so that pixel values are assigned to each unit section of the plotter. Therefore, the positional relationship differs depending on the relative size of the print image to the original image size. For example, if the image size needs to be greatly stretched, the chromaticity of multiple cells or halftone periods can be controlled by utilizing a single pixel of the original image. The number of pixels formed by scanning is to some extent independent of the number of cells and is determined by the size of each pixel (SS) relative to the size of the original image scanned. Therefore, as is apparent from FIG. 3, the cell boundary does not always coincide with the pixel boundary, and one cell may include intensity values from two or more pixels in the adjacent pixel group. It is also possible to utilize the intensity value of a single pixel to control the plotter in more than one cell.

In FIG. 4, for example, a plotter unit section is 8 ×.
For a cell formed as a binary matrix containing 8 cells, a series of screen function matrices corresponding to the four primary colors cyan, magenta, yellow and black respectively are shown. As already mentioned, the size of the binary matrix is determined by the ratio of the required cell size to the size of the unit compartment created by the particular plotter. The intensity level values stored in each matrix are arranged in a predetermined position pattern that causes a unique color pattern to appear within the screen cell. 4 (1) to (4) show color patterns of respective colors at four chromaticity levels, that is, 12.5%, 37.5%, 50% and 81.8% chromaticity.

The relationship between the screen function matrices corresponding to the four colors can be expressed as follows. Assuming that V ₁ (i, j) is a basic matrix corresponding to cyan, i, j = 0,1, ... n, and n is an integer multiple of 4, (black) V ₄ (i , J) = V ₁ (i, (j + n /
2) mod n) (magenta) V ₂ (i, j) = V ₁ ((i + n / 4) m
od n, (j + n / 4) mod n) (yellow) V ₃ (i, j) = V ₂ (i, (j + n /
2) mod n) However, "mod n" represents modulo n. n is 4
Otherwise, the mathematical definition of analog conversion becomes complicated.

The point of the present invention is that not only the screen function matrix forms a different pattern for each color, but each cell corresponding to each color has two clearly separated color gamuts. Occupy different positions for each of the four colors. In the illustrated preferred embodiment, the numbered array in the screen function matrix of FIG. 4A for cyan has two color gamuts that are approximately centered in the two diagonal quadrants of the cell (FIG. 4 (1)-(4)). 4) for black, the numbered array in the screen function matrix of FIG. 4D shows two color gamuts that are approximately centered in the two quadrants along the other diagonal of the cell. In the numbered array in the screen function matrix for magenta and yellow, the two color gamuts appear approximately centered on the centers of the two adjacent borders of the cell, respectively, and at the center and one corner of the cell. When the screens are overlapped, the color gamut centers of all four primary colors are arranged in the pattern shown in FIG.

If the chromaticity is high, the numbered array in the screen function matrix widens the color gamut in each cell by adding more continuous unit partitions to each color gamut. In the case of magenta and yellow, since the color gamut is concentrated on the cell boundary, the color gamut extends to adjacent cells. FIG. 5 shows a mosaic of cells that appear with four screens superimposed. For cyan and yellow, the color gamut is evenly spaced along the first diagonal line A, and for magenta and black the second diagonal line B.
There are color areas at evenly spaced locations along. Arranged in this way, the colors obtained by the four screens have the greatest spacing and the least overlap.
Although it is visually preferable to arrange the color dots in a diagonal direction, the lines A and B may be set at different angles. Although the primary colors for printing are assigned to the screen matrix in the above description, this assignment can be changed arbitrarily.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

2A is a diagram showing a format of data created by a scanner from a color photographic original to be copied, and FIG. 2B is a diagram showing a format of data converted for printing.

FIG. 3 is a diagram showing a screen cell formed by a plotter.

FIG. 4 shows four screen function matrices and their four.
It is a figure which shows the screen cell pattern of each primary color in various different color intensities.

FIG. 5 is a diagram showing superimposed color positions.

[Explanation of symbols]

 10 Scanner 12 Analog / Digital Converter 14 Digital Computer 16 Screen Plotter 18 Halftone Screen 20 Printer 22 Cell 24 Unit Partition 26 Screen Function Matrix

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location G06F 15/68 310 9191-5L 320 A 9191-5L

Claims (20)

[Claims] 1. A method of making a color copy using a plurality of halftone screens printing each of at least three primary colors, comprising: (a) scanning a color original image to be copied in a predetermined pattern; and (b) The light intensity corresponding to each of the three primary color frequencies in a series of pixel regions of the color original image is sensed, and (c) the light intensity level corresponding to each primary color frequency for each pixel region scanned represents the required chromaticity level of each primary color. A screen color matrix consisting of a set of intensity level values corresponding to the chromaticity of a particular color from zero chromaticity to maximum chromaticity in increments (d) Created and stored so that they are arranged in the position pattern of (e) Each halftone screen area is transparent or opaque. Each number in the set of numbers that divides the cell into a plurality of cells formed as a binary matrix of elementary regions and forms the screen color matrix corresponding to this halftone screen for each pixel region in the halftone screen to be created. To create each halftone screen from the screen color matrix, and (f) set the pixel area binary value of each halftone screen depending on whether it is greater or less than the value stored in the screen function matrix. And (g) a step of making a color copy from the halftone screen by positioning the halftone screen so that the primary colors partially overlap and overlap at the same screen angle. . 2. The method according to claim 1, wherein the same two values obtained based on a predetermined intensity level value are used.
A method of setting the position of a numerical value in each screen function matrix such that a binary pixel area is included in a group of consecutive pixel areas that are symmetrical to each other when combined with adjacent cells of the created screen. 3. A method according to claim 2, characterized in that the intervals between the pixel area groups in each of the created screens are substantially equal. 4. A method according to claim 3, characterized in that the spacing between the pixel areas associated with the screen obtained for each primary color with the screens superposed is approximately equal. . 5. A method according to claim 1, characterized in that the converting step comprises the step of converting the intensity levels at the primary color frequencies into the four primary colors to be printed. 6. The method according to claim 5, wherein said creating and storing step includes the step of creating and storing four screen function matrices, the two lowest numerical values being the first screen function matrix. At approximately the center and one corner, at the second screen function matrix at the center of each of two adjacent edges, and at the third screen function matrix at the center of each of the two diagonal quadrants. In the 4-screen function matrix, the two quadrants in the diagonal direction are positioned at the center of each of the remaining two quadrants. 7. The method according to claim 6, wherein the step of creating and storing each of the four matrices comprises successively increasing intensity level values surrounding two lowest intensity values. A method comprising placing at a location. 8. A method of producing a halftone color separated image on a display surface, the first set of numerical values corresponding to a first color representing intensity levels of the first color in a plurality of linear pixel areas of a master image to be produced. To generate a plurality of spots of the first color on the display surface, the number and position of which are related to the pixel area and the size of which is related to the intensity level of the first color in each pixel area of the master image. Creating a second set of numerical values corresponding to a second color that represents the intensity level of the second color in the pixel area, the number and position of which are associated with the pixel area, and the size of which is the intensity level of the second color in each pixel area of the master image The plurality of spots of the second color associated with the first color spot are linearly shifted, and depending on the intensity level, the spots of the first color may be overlapped. And one or two corresponding to the one or more additional colors representing the intensity level of the one or more additional colors in the pixel area of the master image. Create the above set of numerical values, and set the number of spots and the number of positions are related to the pixel area, and the size is related to the intensity level of the additional color in each pixel area of the master image. A half comprising the steps of linearly shifting from each other and from the spots of the first and second colors so that they appear on the display surface, depending on the intensity level, overlapping the first and second colors. Tone color separation image creation method. 9. The method according to claim 8, wherein the display surface is a piece of paper. 10. The method according to claim 8, wherein the master image is graphic data created by a computer. 11. A method according to claim 8 wherein the master image is a color photograph. 12. The method of claim 8 wherein the spot of the first color is horizontally shifted from the spot of the second color and one spot of the additional color is of the first and second colors. A method characterized by shifting vertically from the spot. 13. The method of claim 8 wherein the spot of the first color is vertically shifted from the spot of the second color and one spot of the additional color is of the first and second colors. A method characterized by shifting horizontally from the spot. 14. A method according to claim 8 wherein each spot has two centers and is divided into two parts below a given intensity level and above a given intensity level into a single part. A method characterized by being integrated into one part. 15. The method according to claim 14, wherein the second color spot is horizontally shifted from the first color spot by 50% of the pixel interval, and the second color spot is vertically shifted from the first color spot. A method characterized by not shifting to. 16. The method according to claim 15, wherein the one or more additional sets are the third set and the fourth set, and the one or more additional colors are the third. Color and a fourth color. 17. A method according to claim 16, wherein the spot of the third color is horizontally shifted from the spot of the second color by 25% of the pixel spacing and the spots of the first and second colors. A method characterized by shifting from the spot in the vertical direction by 25% of the pixel spacing. 18. A method according to claim 16, wherein the spot of the fourth color is horizontally shifted from the spot of the first color by 25% of the pixel spacing,
A method of shifting from a color spot in the vertical direction by 25% of the pixel spacing. 19. A method according to claim 8, characterized in that the spots in each pixel area of the color overlap each other at high intensity levels. 20. The method according to claim 8, characterized in that the spots in each pixel of the color overlap each other at an intensity level of 50% or more.