JPH0879547A - Preparation of printing picture - Google Patents

Abstract

PURPOSE: To allocate the appropriate kind of screening by discriminating the application of FM screening or normal screening corresponding to the brightness of color pictures and selecting a screen angle corresponding to a hue. CONSTITUTION: A thinning/increasing processing part 70 thins out the picture data of the pattern of a CMYK display color system read by a scanner 42. A color system conversion part 74 converts the thinned-out picture data to an RGB color system and then, converts them to an HSL color system. An area division processing part 76 performs an area division processing corresponding to the brightness of the pattern and the hue. Thus, a CPU 30 reproduces the area of low lightness by using the FM screening and reproduces the area of high lightness by using the normal screening. Also, the area of the high lightness for which the hue is a warm color system is reproduced while exchanging the screen angle of a C version and an M version.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a print image for a plurality of color plates to reproduce a color image. The "print image" in this specification means
The image includes various images such as an image for a printing plate recorded on a recording medium such as a photosensitive film and a color image reproduced by directly applying inks of a plurality of colors on a printing paper according to image data.

[0002]

2. Description of the Related Art In color offset printing, an original image such as a photographic image is divided into four color separation images corresponding to four color inks of cyan (C), magenta (M), yellow (Y) and black (K). A printed matter is created by disassembling each of the inks and overprinting color-separated images of each ink.

The image on the printing plate corresponding to each ink (hereinafter referred to as "print image") is represented by the size of minute dots (dots) on which the ink is placed. The halftone dots are arranged in a grid pattern at regular intervals, and the image density is expressed by the area ratio of the halftone dots per unit area. The percentage of the area of halftone dots per unit area is called halftone dot percentage.

The interval between halftone dots is defined by the so-called screen ruling, and the array direction of halftone dots is defined by the screen angle. The screen ruling is a value indicating the number of halftone dots formed per inch, and the larger the screen ruling, the higher the resolution of an image that can be reproduced. Conventionally, the number of screen lines commonly used in high-quality color printing of photographic images has been about 150 to 200 lines / inch.

Further, due to recent advances in printing technology, high-resolution halftone dots (hereinafter referred to as "high-definition halftone dots") having a screen ruling of about 300 lines / inch or more can be used. . The higher the screen ruling, the smaller each halftone dot, and the original image can be reproduced with higher resolution.

The above-mentioned halftone dots have a constant arrangement, and the size of each halftone dot changes according to the density of the original image. In other words, the reproduction of an image by halftone dots is a method of expressing the density of an image by amplitude modulation, and is referred to as “normal screening” in this specification. On the other hand, in recent years,
A method called "FM screening" (or FM dot) that expresses the density of an image by frequency modulation has been put into practical use. In FM screening, the size of dots on which ink is placed is fixed, and the appearance frequency of dots changes according to the density of the image.

It should be noted that, unlike the conventional halftone dots, the dots used in the FM screening are not periodic because the dot arrangement is not periodic. Therefore,
In this specification, conventional halftone dots having a periodic array and dots that reproduce an image by frequency modulation are collectively referred to as “print dots” or simply “dots”.

[0008]

The above-mentioned FM screening can reproduce the details (details) of the original image more faithfully than the conventional screening using the halftone dots, and the dot arrangement is periodic. Therefore, there is an advantage that so-called rosette moire or other moire does not occur or is unlikely to occur. However, when FM screening is applied to an image area in a pattern having a relatively low density and a small color change or a low density flat screen (a line drawing of a uniform color), the reproduced color image has graininess (roughness). ) Is conspicuous.

The present invention has been made in order to solve the above-mentioned problems in the prior art, and an object thereof is to generate a print image that makes the best use of the advantages of FM screening and ordinary screening.

[0010]

In order to solve the above problems, the method according to claim 1 of the present invention comprises:
A method for producing a print image for a plurality of color plates including at least C plate and M plate as a color separation image for reproducing a color image, comprising: (A) lightness / saturation / hue representing the color image. A step of preparing image data of a color system; (B) a first region having the lightness lower than a predetermined threshold value based on the image data; and a hue having the lightness higher than the predetermined threshold value and the hue The second is relatively greenish
And a third region in which the lightness is higher than the predetermined threshold and the hue is a relatively warm color system, and (C) the first region is FM screened. And the second region is reproduced by using a normal screening, the third region is formed by using the normal screening, and the second region is a screen angle of C plate and M plate. And a step of creating a print image of the color image by reproducing while exchanging.

It is distinguished whether the FM screening or the normal screening is applied depending on the lightness of the color image, and the screen angles of the C plate and the M plate are selected according to the hue. Appropriate screening type can be assigned.

Further, in the method described in claim 2, the step (B) includes a step of forming a fourth region having a predetermined width between the second and third regions, and the step (B). C) is
Reproducing the fourth region using the FM screening for at least C and M plates.

By providing such a fourth region, it is possible to make it inconspicuous that the screen angles of the C plate and the M plate are exchanged at the boundary between the second and third regions.

In the method according to claim 3, the step (A) includes a step of preparing image data of a CMYK color system representing the color image, and the step (B) includes each color of each area. For the plate, including a step of indicating which of the FM screening and the normal screening is to be applied, and preparing screen angle data indicating a screen angle in the case of applying the normal screening, the step (C) includes: The screen pattern data is read from the screen pattern memory according to the screening type and the screen angle designated by the screen angle data, and the print image of each color plate is recorded by comparing the screen pattern data with the CMYK image data. Including a step of generating a dot signal for

In this way, the print image of each plate can be generated according to the screen angle data.

In the method according to claim 4, the step (A) includes a step of smoothing image data of the CMYK color system representing the color image with a predetermined spatial filter, and the smoothed image data. Is converted into image data of a lightness / saturation / hue color specification system, and each pixel in the color image is calculated based on the lightness and hue of the image data of the lightness / saturation / hue color specification system. Dividing the color image into first to third regions by classifying into the first to third regions, and performing pinhole erasing on the divided first to third regions. , The second and third
Forming a fourth region of a predetermined width between the regions of
And the step (C) includes a step of reproducing the fourth region using the FM screening for at least C plate and M plate.

By smoothing the image data of the CMYK color system, it is possible to prevent the area division from becoming too fine even when the density change in the image is relatively rapid. In addition, it is possible to prevent the generation of an excessively small area by erasing the pinhole. Further, by providing the fourth region, it is possible to make it inconspicuous that the screen angles of the C plate and the M plate are exchanged at the boundary between the second and third regions.

According to a fifth aspect of the present invention, at least a C plate and an M image are used as a color separation image for reproducing a color image.
A method for creating a print image for a plurality of color plates including a plate, the method comprising: (A) preparing image data of a CMYK color system representing the color image; and (B) based on the image data, A first region in which the density of the color image is lower than a predetermined threshold value, and a second region in which the density of the color image is higher than the predetermined threshold value and the density of the M plate is higher than the density of the C plate, Dividing the color image into a third region in which the density of the color image is higher than the predetermined threshold value and the density of the C plate is higher than that of the M plate, and (C)
The first region was reproduced using FM screening,
The second region is reproduced using normal screening, the third region is reproduced using the normal screening, and the second region is reproduced by exchanging the screen angles of the C plate and the M plate. Accordingly, a step of creating a print image of the color image is provided.

Whether the FM screening or the normal screening is applied is discriminated according to the density of the color image, and the C screening is carried out according to the relationship between the density of the C plate and the density of the M plate.
Since the screen angles of the plate and the M plate are selected, an appropriate screening type can be assigned according to the density of the color image.

The method according to claim 6 is a method for creating a print image for each color plate of CMYK as a color separation image for reproducing a color image, wherein (A) a CMYK color specification representing the color image. Preparing the image data of the system, and (B) comparing the density of each color plate of the image data with a predetermined threshold value for each color plate to obtain a first region in which the density is higher than the predetermined threshold value; Dividing the color image for each plate into a second region in which the density is lower than the predetermined threshold value; and (C) reproducing the first region using FM screening, By recreating the region using normal screening,
Creating a print image of each plate of the color image, respectively.

Since the type of screening is determined according to the comparison between the density of the color image and a predetermined threshold value for each color plate, it is possible to assign an appropriate type of screening according to the density for each color plate.

[0022]

【Example】

A. First Embodiment: FIG. 1 is a block diagram showing the configuration of a plate making system for producing a print image by applying one embodiment of the present invention. This plate making system includes a CPU 30, a ROM 32 and an RA as a storage unit (main memory).
M34 and mouse 36 as a pointing device
A display control circuit 38, a color monitor 40 as a display device, and an input scanner 42 as an image input means.
And three image memories 44, 46 and 48, two run length memories 50 and 52, a halftone dot generating device 60 for generating a beam on / off signal representing a print image, and a print image recording means. And a recorder 62.

The CPU 30 has a thinning / filling processing unit 70.
The functions of the smoothing processing unit 72, the color system conversion unit 74, the region division processing unit 76, the pinhole erasing unit 78, and the intermediate region forming unit 80 are executed. Each of these parts is in ROM3
The application program stored in 2 to the CPU 3
It is realized by executing 0. The functions of these units will be described later.

The first and second image memories 44 and 46 are 32 in number.
It is a memory of bits / pixel and stores image data of CMYK color system. The third image memory 48 is 24 bits /
This is a pixel memory and stores image data of RGB color system or HSL color system (lightness / saturation / hue color system). Each component of these image data is represented by 8 bits / pixel.

The two run length memories 50 and 52 are
It is a memory that stores a screen angle control file (screen angle data) described later, and has the same memory size. These two run length memories 50 and 52 are used for the source / destination processing by the CPU 30.

FIG. 2 is a flow chart showing the overall procedure in the first embodiment. In step S1, the image data of the pattern is read by the input scanner 42 and stored in the image memory 44. In step S2, the read design is displayed on the color monitor 40.

In step S3, step S4 described later is performed.
The variables used for the area division processing in step 1 are set by the interactive processing. The variables used in the area dividing process of the first embodiment are as follows, and the method of using each variable will be described later.

Thinning rate M: Image data is thinned to 1 / M in the thinning process. Number of smoothing processes Sn: Number of smoothing processes. Brightness threshold Lv (%): a threshold for determining whether to apply FM screening or normal screening to each pixel in the pattern. Hue center value Hv (%) and hue width Hd (%): values used for determining whether or not the screen angles of the C plate and the M plate are exchanged when the normal screening is applied. Pinhole pixel count Pn: The number of pixels in the area recognized as a pinhole in pinhole processing. Frame pixel number Fn: width of the intermediate region (fourth region).

In step S4, the area division processing is performed by the CPU.
Performed by 30. FIG. 3 is a flowchart showing a detailed procedure of the area division processing. In step S11, the thinning / filling processing unit 70 causes the first image memory 44 to operate.
The size (number of pixels) of the image data is reduced by thinning out the image data of the CMYK color system stored in. The value of the thinning rate M is set in step S3 of FIG. For example, when M = 2 is set,
A 2 × 2 pixel block is thinned out to one pixel. When M = 1 is set, the thinning process is not performed. The thinned image data is stored in the second image memory 46. The original image data stored in the first image memory 44 is stored as it is because it is necessary for the output process of the print image. The thinning-out process in step S11 is performed in order to perform the process after step S12 at high speed when the size of the design is large. Therefore, when the size of the picture is relatively small, it is not necessary to perform the thinning-out process. In this case, the thinning-out rate M is set to 1.

In step S12, the smoothing processing section 72 performs smoothing processing (smoothing processing) on the thinned image data. FIG. 4 is an explanatory diagram showing a spatial filter used for smoothing processing. This spatial filter is 3x
It is a window of three pixels, and the average value of nine pixel values in the window is replaced with the pixel value of the central pixel. By applying this spatial filter to the whole area of the pattern,
The pixel value of each pixel is smoothed. When the pixel value of the pattern changes rapidly, the distribution of pixel values becomes smoother by performing the smoothing process several times. The number of smoothing processes Sn is specified in step S3 of FIG. When Sn = 0, no smoothing process is performed.
When performing the smoothing process, the second image memory 46,
A line memory area for three scanning lines secured in the RAM 34 is used.

In step S13, the color system conversion unit 74
Of the smoothed CMYK color system image data into RGB
The image data is converted into color system image data, and further converted into HSL color system image data according to the following mathematical expression 2.

[0032]

[Equation 1]

In equation 1, the operators "max" and "m"
“In” indicates an operation that takes the maximum value and the minimum value of the values in parentheses. The hue value H, the saturation value S, and the lightness value L of each pixel are obtained by the mathematical expression 1. FIG. 5 is an explanatory diagram showing the HSL space. The HSL space is a cylindrical coordinate system (zrθ coordinate system), where the lightness L is the vertical coordinate z, the saturation S is the diameter r,
The hue H corresponds to the angle θ.

FIG. 6 is an explanatory diagram showing a hue circle. In this embodiment, the hue H of red is defined as 0%, and the value of the hue H is defined to change along the directions of yellow, green, and blue and purple. In addition, as the hue H,
It is defined to take a value in the range of 0 ° to ± 180 °. Formula 1 is in the unit system in this case. On the hue circle, colors close to skin color (yellow-red) are called warm colors, and colors close to blue are called cold colors.

The saturation S indicates vividness and takes a value from 0% (achromatic color) to 100%. The lightness L indicates brightness, and takes a value from 0% (darkest) to 100% (lightest).

In the conversion from the RGB color system to the HSL color system, instead of using the above equation 1, Japanese Patent Laid-Open No. 5-30
It is also possible to carry out according to another method such as the method described in Japanese Patent Publication No. 0375.

In step S14 of FIG. 3, the area division processing unit 76 divides the area of the pattern and creates run length data of the screen angle file showing the divided areas. This region division is a process of determining the type of screening to be applied to each pixel according to the lightness value L and the hue value H of each pixel, and as a result, the region number corresponding to the type of screening is assigned to each pixel. Assigned.

FIG. 7 is an explanatory diagram showing the region number RN assigned to each pixel as a result of region division and the type of screening indicated by each region number. Although the area numbers RN from 0 to 16 are shown in FIG. 7, only the four area numbers RN (= 0, 3, 15, 16) underlined are used in the first embodiment. Is.

For the pixel having the region number RN of 0, the normal screening using the screen angle in the standard mode is applied. Standard mode screen angle is CMYK
The screen angles θC, θM, θY, and θK of the respective color plates are 15 °, 45 °, 0 °, and 75 °, respectively. FM screening is applied to the pixel with the region number RN of 15. There is no screen angle in FM screening. For a pixel with a region number RN of 16, C
Normal screening using screen angles in M change mode is applied. Here, the CM change mode means that the screen angles of the C plate and the M plate in the standard mode are exchanged. Therefore, in the CM change mode,
Screen angles θ C, θ M, θ Y of CMYK color plates,
θK becomes 45 °, 15 °, 0 °, and 75 °, respectively.
The area having the area number RN of 3 will be described later.

The region number RN of each pixel is determined in accordance with the following determination condition based on the lightness threshold value Lv, the hue center value Hv, and the hue width Hd designated in step S4 of FIG.

Judgment condition C1: The area number RN is determined by the relationship between the lightness threshold value Lv (%) and the lightness value L. When L ≦ Lv: RN = 15 (FM screening) When Lv <L: RN = 0 or 16 (normal screening)

Pixels having a lightness value L lower than the threshold value Lv (that is, pixels having a relatively high density) are unlikely to show granularity in image quality due to FM screening.
Apply M screening. On the other hand, a pixel whose brightness is higher than the threshold Lv (that is, a pixel whose density is relatively low) is F
Since it is unlikely that the granularity of the image quality will appear due to the M screening, the normal screening is applied. In addition,
When the lightness threshold Lv is set to 0%, FM screening is applied to the entire design, and when it is set to 100%, normal screening is applied to the entire design. When the normal screening is applied, the screen angle mode is further determined according to the following determination condition C2.

Judgment condition C2: When it is judged that the normal screening is applied according to the judgment condition C1, the standard mode is selected according to the relationship between the hue value H of each pixel, the hue center value Hv, and the hue width Hd. Either (RN = 0) or CM change mode (RN = 16) is selected. When Hv−Hd <H <Hv + Hd: RN = 16 (CM
Change mode: θC = 45 °, θM = 15 °) When H ≦ Hv−Hd or Hv + Hd ≦ H: RN = 0
(Standard mode: θC = 15 °, θM = 45 °)

Here, the hue center value Hv is a value indicating the central hue of a greenish color, and for example, a value of HV = 40% is preferable. The hue width Hd is preferably about 10%. The determination condition C2 means that the CM change mode is applied to a relatively green color and the standard mode is applied to a relatively warm color.
The reason for this is as follows. In the normal screening, the screen angles of the CMYK color plates are set to different values in order to prevent the occurrence of moire. In particular, the 15 ° moiré involving the Y plate, that is, the moiré caused by the two plates, that is, the Y plate and the plate arranged at a screen angle of 15 ° with respect to the Y plate, have an angular interval of only 15 °. However, it may be noticeable in some cases. this is,
In particular, when a person or vegetation is present in the image, it occurs between the Y plate and the M plate in the skin color part of the person, and between the Y plate and the C plate in the green part of the plant. In order to prevent this moire, it is preferable to set the difference in screen angle between the M plate and the C plate having the higher density and the Y plate to 45 °. In this embodiment, since the screen angle of the Y plate is set to 0 °, the C plate screen angle θC is set to 45 ° by setting the screen angle θC of the C plate to 45 ° for a region of a color having a large amount of the C component and the Y component. The difference in screen angle between the plate and Y plate is 45
Can be °. By the way, a relatively green color is a color having a large amount of C and Y components. Therefore, for a relatively greenish color, the CM change mode is applied to set the screen angle θC of the C plate to 45 ° and the screen angle θM of the M plate to 15 °. On the other hand, a relatively warm color is M
Since there are many components and Y component, the standard mode is applied to the relatively warm colors and the screen angle θ C of the C plate is set to 15
°, set the screen angle θ M of the M plate to 45 °.

When the hue width Hd is set to 0%, the standard mode is always selected when normal screening is applied. On the other hand, when the hue width Hd is set to 51%, the CM change mode is always selected when normal screening is applied.

It is also possible to select the standard mode and the CM change mode by using the next judgment condition C2a instead of the above judgment condition C2.

Criteria C2a: When it is determined that the normal screening is applied according to the criteria C1,
Either the standard mode (RN = 0) or the CM change mode (RN = 16) is selected according to the relationship between the C component value D (C) and the M component value D (M) of each pixel. When D (M) ≦ D (C): RN = 16 (CM change mode: θC = 45 °, θM = 15 °) When D (C) <D (M): RN = 0 (standard mode: θ
C = 15 °, θM = 45 °)

In the above judgment condition C2a, the value of the M component is C
If it is larger than the component value, the standard mode is applied,
When the value of the C component is larger than the value of the M component, it means that the CM change mode is applied. If the judgment condition C2a is used, the standard mode and the CM change mode can be determined from the image data of the CMYK color system. Also,
The color plate with a screen angle of 45 ° has the advantages that the presence of screen meshes is less noticeable than other color plates and the edges of the image can be reproduced sharply. By setting the larger screen angle to 45 °, a sharp image can be reproduced.

On the other hand, when the above-mentioned determination condition C2 is used, it is possible to prevent moire due to the C plate and the Y plate for a relatively greenish color having a large amount of C component and Y component,
For relatively warm colors having a large amount of M and Y components, there is an advantage that moire due to the M and Y plates can be prevented.

When the type of screening applied to each pixel is determined as described above, a screen angle control file indicating the type of screening is generated in the form of run length data. FIG. 8 is an explanatory diagram showing the structure of the screen angle control file. The screen angle control file is composed of a header section, a palette section and a run length data section. A file name, file size, etc. are registered in the header section. The above-mentioned table shown in FIG. 7 is registered in the pallet portion. In the palette portion, the halftone code of each color plate is represented by 3 bits as shown in the lower part of FIG. 7, and the halftone code for one area number is 12 bits (for 4 colors). .

FIG. 9 is an explanatory diagram showing the area divisions represented by the screen angle control file. In this example, the entire rectangular pattern includes an area with an area number RN = 0 (standard mode) and an area with RN = 16 (CM change mode).
It is divided into three regions of RN = 15 (FM screening) region. The run length data shown in FIG. 8B is data along the scanning line L1 in FIG.

When the processing in step S14 of FIG. 3 is completed in this way, the pinhole erasing section 78 executes the pinhole erasing processing in step S15. What is recognized as a pinhole is an area having a pixel number equal to or less than the pinhole pixel number Pn set in step S3 of FIG. When the pinhole erasing process is executed, a pinhole-shaped small area is erased, so that it is possible to prevent a very different kind of screening from being applied to a very small area.

The pinhole erasing process itself is well known, and for example, the method described in Japanese Patent Laid-Open No. 5-284246 by the present applicant can be applied.

In step S16 of FIG. 3, the intermediate area forming section 80 divides the normal screening standard mode area (area number RN = 0 area) and the CM change mode area (area number RN = 16 area). An intermediate area (frame area) is formed between them. The area number RN is set in this intermediate area.
= 3 is assigned. As can be seen from FIG. 7, in the intermediate region, FM screening is applied to the C plate and the M plate, and Y screening is performed.
Screening is usually applied to plates and K plates. Since the screen angles of the C plate and the M plate are exchanged in the standard mode and the CM change mode, if these areas are reproduced as they are, discontinuity of the image may be noticeable at the boundary. Therefore, if an intermediate region is formed between the standard mode region and the CM change mode region, and FM screening is applied to the C plate and the M plate in this intermediate region, the change in the screen angle is noticeable. It can be lost.

In FIG. 10, in step S16, the area with the area number RN = 0 (standard mode) and the area number RN = 16.
Between the areas of (CM change mode), the area number RN =
It is explanatory drawing which shows a mode that the intermediate region of 3 is formed. As shown in FIG. 7, in the intermediate area (area where RN = 3), the FM screen is applied to the C plate and the M plate, and the normal screening is applied to the Y plate and the K plate. Note that such an intermediate region can be formed by thickening both sides of the boundary line between the region of region number RN = 0 and the region of RN = 16 by a predetermined width. The frame pixel number Fn that defines the width of the intermediate region is set in step S3 of FIG.

The processing itself for generating a new area on the boundary between areas by the thickening processing is well known. For example, the method described in Japanese Patent Laid-Open No. 3-311972 by the present applicant can be applied. it can.

Thus, when the entire picture is divided into four types of areas as shown in FIG. 10B, step S in FIG.
17, the thinning / padding processing unit 70 sets the thinning rate M.
According to the above, the padding process of the screen angle control file is executed. The "filling process" here is a reverse process of the thinning process. For example, when the thinning process for replacing 2 × 2 pixels with 1 pixel is executed in step S11 described above, 1 pixel is converted into 2 × 2 pixels by the padding process.
Replace with pixels. If the thinning process cannot be performed in step S11, the padding process in step S17 is also not executed.

The padding process itself is well known, and for example, the method described in Japanese Patent Application Laid-Open No. 3-101995 by the present applicant can be applied.

When the pattern area dividing process in step S4 of FIG. 2 is completed as described above, the next step S4
5, the state of area division is displayed on the color monitor 40 according to the screen angle control file. On this occasion,
The area represented by each area number RN is displayed in a transparent color different from each other so as to be superimposed on the design. For example, F
The area to which M screening is applied is displayed with a yellow color filter applied, and the area to which the standard mode of normal screening is applied (area of relatively warm color type) is red and the CM change mode is applied. The areas (areas of relatively green color) are displayed with the blue color filter applied. The user can determine whether or not the area division is appropriately performed by observing the area division image superimposed on the design.

If the area division is not appropriate, the process proceeds from step S6 to step S7, and the area division state is corrected. For this correction, for example, various image correction processing such as brush processing, pinhole erasing processing, and thickening / thinning processing can be used. The corrected area division state is displayed again in step S5, and steps S5 to S7 are repeated until a satisfactory area division is obtained.

When the area division is completed in this way and the final screen angle control file is created, step S8
At, a print image of each CMYK plate is output. In this output process, the halftone dot generator 60 generates a beam on / off signal from the image data of the CMYK color system, and the recorder 62 records the print image on the photosensitive film or the printing plate in response to this signal.

FIG. 11 is a block diagram showing the internal structure of the halftone dot generator 60. The halftone dot generator 60 includes a clock generation circuit 102, an image buffer 104, a plate selector 106, and a screen angle control file buffer 108.
A bitmap expansion unit 110 and a version selector 112
And five dot generation circuits 121 to 125 and a screening selector 130.

The clock generation circuit 102 includes a recorder 62.
It is a frequency dividing circuit that divides the reference clock CLK given from to generate a high speed clock CLKH and a low speed clock CLKL. FIG. 12 shows three types of clocks CLK and CL
It is explanatory drawing which shows the relationship of KH and CLKL. The high-speed clock CLKH is a clock signal indicating the timing of 1 dot PXd which is a recording unit in the recorder 62. The low-speed clock CLKL is a clock signal indicating the timing of 1 pixel PXe in the image data. The high-speed clock CLKH has five dot generation circuits 1
21 to 125, and specifies the timing of generation of beam on / off signals in these dot generation circuits. The low-speed clock CLKL is given to the image buffer 104 and the bit map expansion unit 110, and the read timing of the image data stored in the image buffer 104 and the halftone data Dan stored in the memory inside the bit map expansion unit 110. Defines the timing for reading the bitmap data of.

The image buffer 104 is a buffer for storing the CMYK image data Dim transferred from the image memory 44. The plate selector 106 selects any one color of CMYK according to the plate designation signal Ssep given from the CPU 30, and outputs 8-bit image data of that color. The plate designation signal Ssep is one of CMYK.
This is a 2-bit signal indicating a color. The image data output from the plate selector 106 includes five dot generation circuits 121.
Commonly input to ~ 125.

Screen angle control file buffer 10
A buffer 8 stores the screen angle control file Fan transferred from the run length memory 50. The bit map expansion unit 110 bit map expands the run length data part of the screen angle control file Fan and stores it in the internal memory. The plate selector 112 selects any one of CMYK colors according to the plate designation signal Ssep and outputs a 3-bit halftone angle code Dan for that color.

The five dot generation circuits 121 to 125 are
The given image data Dim is compared with predetermined screen pattern data (threshold data), and a beam on / off signal Sdot is generated according to the comparison result.

FIG. 13 is a block diagram showing the internal structure of one dot generation circuit. The dot generation circuit includes a main scanning clock generator (Y clock generator) 140, a sub scanning clock generator (X clock generator) 142, a main scanning address counter (Y address counter) 144, and a sub scanning address counter ( X address counter) 146
And a screen pattern memory (SPM) 148 and a comparator 150 as a comparison means. The SPM 148 is a memory that stores a threshold pattern for converting the image data Dim into the beam on / off signal Sdot. Note that different threshold patterns are stored in the five dot generation circuits 121 to 125 of FIG. 11, respectively.

The Y clock generator 140 and the X clock generator 142 respectively generate the main scanning clock YCLK and the sub scanning clock XCLK from the high speed clock CLKH. The Y address counter 144 has a main scanning clock YC.
SP by ring-counting the number of LK pulses
An address YADD in the main scanning direction of M148 is generated.
Further, the X address counter 146 controls the sub-scanning clock X
S by counting the number of CLK pulses
An address XADD of the PM 148 in the sub-scanning direction is generated.

The main scanning address YADD and the sub-scanning address XADD are given to the SPM 148, and in response to this, S
The threshold value DTH is output from the PM 148. Comparator 150
Compares the image data Dim with the threshold value DTH and switches the level of the beam on / off signal Sdot according to the comparison result. The beam on / off signal is a signal that defines on / off of the exposure beam at each dot when recording a print image in the recorder 62.

FIG. 14 is an explanatory diagram showing how the dot shape of a square dot, which is a type of halftone dot used for normal screening, changes according to the density. Also, FIG.
FIG. 5 is an explanatory diagram showing how the dot shape of FM dots used for FM screening changes depending on the density. Note that FIG. 14 shows a region corresponding to one halftone dot, and in this example, one side has a width of 23 spots. Here, the “spot” is a recording unit in the recorder 62. FIG. 15 shows how the FM dots in the same 23 × 23 spot region as in FIG. 14 change according to the density. In the square dot shown in FIG. 14, the area of one black dot grows as the density increases, whereas in the FM dot shown in FIG. 15, the frequency of occurrence of black dots (ie The frequency of appearance) is getting higher. One pixel of the pattern has a size of 10 × 10 spots, for example.

The FM dots have the advantage that no rosette moiré occurs because the dots are small in size and are randomly arranged. In addition, object moire caused by the interference between the pattern of the picture and the periodic array of dots is unlikely to occur. Further, since each dot is small, there is an advantage that the reproducibility of image details is good.

The first four dot generation circuits 121 to 124 shown in FIG. 11 have a square dot screen angle of 0.
The screen pattern data (threshold pattern data) of °, 15 °, 45 °, and 75 ° are stored in the SPM 148 (see
3), each of which is stored in FIG. 3B, and the beam on / off signal is generated by comparing these screen pattern data with the image data Dim. Further, the fifth dot generation circuit 125 stores the FM dot screen pattern data in the SPM 148.

Screening selector 130 of FIG.
Selects one of the five beam on / off signals respectively generated by the five dot generation circuits 121 to 125 according to the 3-bit halftone angle code Dan. The selected beam on / off signal is supplied to the recorder 62, and a print image is recorded according to the beam on / off signal.

In the above-described first embodiment, since FM screening is applied to a region having a relatively low lightness and normal screening is applied to a region having a relatively high lightness, a region having a high lightness (density It is possible to prevent the granularity of the image quality that occurs when the FM screening is applied to the low area). Further, in the low lightness region, the detail of the pattern can be better reproduced by the FM screening.

Further, when the normal screening is applied, since the standard mode and the CM change mode are assigned according to the hue, it is possible to effectively prevent the occurrence of moire. Further, since the intermediate area to which the FM screening is applied is provided between the standard mode area and the CM change mode area, the boundary between the two mode areas can be made inconspicuous.

B. Second Embodiment: The second embodiment is the first
The area division processing is different from that of the above embodiment. Therefore,
The configuration of the apparatus shown in FIGS. 1 and 11 and the overall procedure shown in FIG. 2 are the same as in the first embodiment. However, the variables set in step S3 of FIG. 2 of the second embodiment are as follows.

Thinning rate M: Image data is thinned to 1 / M in the thinning process. (Same as in the first embodiment) Number of smoothing processes Sn: Number of smoothing processes. (Same as in the first embodiment) Density threshold values Cv, Mv, Yv, Kv: threshold values used in determining whether FM screening or normal screening is applied to each color plate. Pinhole pixel count Pn: The number of pixels in the area recognized as a pinhole in pinhole processing. (Same as in the first embodiment) Frame pixel number Fn: Width of intermediate region. (Same as the first embodiment)

FIG. 16 is a flow chart showing the detailed procedure of the area division processing in the second embodiment, which corresponds to FIG. 3 in the first embodiment. Steps S11 (thinning-out process), S12 (smoothing process), S in FIG.
15 (Pinhole erasing process), S17 (Filling process)
Is the same as that of FIG. Further, in the procedure of FIG. 16, step S13 of FIG. 3 (conversion to the HSL color system),
There is no S16 (creation of intermediate area).

In the area division in step S14a of FIG. 16, the type of screening applied to each color plate is determined according to the following judgment conditions.

Judgment condition C3: Density D (C), D of each color plate
(M), D (Y), D (K) and density thresholds Cv, Mv, Y
The region number RN is determined by the relationship with v and Kv. C version; if Cv ≦ D (C): FM screening If D (C) <Cv: normal screening (θc = 1
5 °) M plate; if Mv ≦ D (M): FM screening If D (M) <Mv: normal screening (θM = 4
5 °) Y plate; if Yv ≦ D (Y): FM screening If D (Y) <Yv: normal screening (θ Y = 0
°) K version; if Kv ≦ D (K): FM screening If D (K) <Kv: normal screening (θK = 7
5 °)

By determining the type of screening of each pixel in the pattern by using the above judgment condition C3,
A region number RN (FIG. 7) of 0 to 15 is assigned to each pixel. As a result, for each color plate, FM screening can be applied to a relatively high density area, and normal screening can be applied to a relatively low density area. In step S14a, a screen angle control file (FIG. 8) indicating the area number RN of each pixel is generated.

Next, in step S15 of FIG. 16, the pinholes in the screen angle control file are deleted, and in step S17, padding processing is performed,
A screen angle control file indicating the screen type for all the pixels in the pattern is obtained.

According to the second embodiment described above, it is possible to select appropriate screening for each color plate according to the density of each color plate. Further, since it is not necessary to convert the image data of the CMYK color system to the image data of the HSL color system, there is also an advantage that the entire process can be performed at a higher speed than in the first embodiment.

Since the structure of the apparatus and the meaning of the area numbers shown in FIG. 7 are common to the first and second embodiments, any one of the area divisions of the first and second embodiments can be designated by the user. It is also possible to select either.

The present invention is not limited to the above-described embodiments, but can be implemented in various modes without departing from the scope of the invention, and the following modifications are possible.

(1) The halftone dots normally used for screening are not limited to square dots, and various halftone dots such as sand, bricks, and lines can be used.

(2) In addition to the FM screening, it is also possible to utilize the conventional screening using high-definition halftone dots having a high screen ruling. The high definition halftone dot is a halftone dot having a screen ruling of about 300 lines / inch. On the other hand, halftone dots having a screen ruling of about 200 lines / inch or less are called "low resolution halftone dots". In high-definition halftone dots, since the size of each dot is small, rosette moire is less likely to occur, and the reproducibility of image details is good.

As the FM dots (frequency modulation dots), not only the type in which the appearance frequency of each dot changes according to the density, but also the type in which both the appearance frequency of dots and the dot size change according to the density FM dots may be adopted. In this specification, an FM dot refers to a dot whose appearance frequency changes at least in accordance with the density. In order to say high resolution, the resolution of the FM dot is preferably about 600 dpi or higher, and particularly preferably about 800 dpi or higher. Some recorders have a high resolution of about 4000 dpi, but when recording FM dots using such a high resolution recorder, the size of each dot of the FM dots is 2 × 2 of the recorder. It may be set equal to spots or 3 × 3 spots. By so doing, it is possible to improve the reproducibility during printing without the size of each dot becoming excessively small.

[0089]

As described above, according to the invention described in claim 1, since it is possible to assign an appropriate type of screening according to the brightness and hue of a color image, the advantages of FM screening and ordinary screening can be obtained. It is possible to generate a print image that makes the most of the advantage of.

According to the invention described in claim 2, by providing the fourth region, it is not noticeable that the screen angles of the C plate and the M plate are exchanged at the boundary between the second and third regions. can do.

According to the third aspect of the invention, the print image of each plate can be generated according to the screen angle data.

According to the invention described in claim 4, CMY
By smoothing the image data of the K color system, it is possible to prevent the area division from becoming too fine even when the density change in the image is relatively rapid. In addition, it is possible to prevent the generation of an excessively small area by erasing the pinhole. Further, by providing the fourth region, C at the boundary between the second and third regions.
It is possible to make it inconspicuous that the screen angles of the plate and the M plate are exchanged.

According to the invention described in claim 5, it is distinguished whether the FM screening or the normal screening is applied according to the density of the color image, and the relationship between the density of the C plate and the density of the M plate. Since the screen angles of the C plate and the M plate are selected according to the above, it is possible to assign an appropriate screening type according to the density of the color image.

According to the invention described in claim 6, since the type of screening is determined in accordance with the comparison between the density of the color image and a predetermined threshold value for each color plate, an appropriate screening according to the density is performed for each color plate. The type of can be assigned.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a plate making system that creates a print image by applying an embodiment of the present invention.

FIG. 2 is a flowchart showing the overall procedure of the embodiment.

FIG. 3 is a flowchart showing a detailed procedure of area division processing according to the first embodiment.

FIG. 4 is an explanatory diagram showing a spatial filter used for smoothing processing.

FIG. 5 is an explanatory diagram showing an HSL space.

FIG. 6 is an explanatory diagram showing a hue circle.

FIG. 7 is an explanatory diagram showing contents indicated by area numbers.

FIG. 8 is an explanatory diagram showing the structure of a screen angle control file.

FIG. 9 is an explanatory diagram showing area divisions represented by a screen angle control file.

FIG. 10 is an explanatory diagram showing a state in which an intermediate region is formed.

FIG. 11 is a block diagram showing an internal configuration of a halftone dot generator 60.

FIG. 12 shows three types of clocks CLK, CLKH, CLK
Explanatory drawing which shows the relationship of L.

FIG. 13 is a block diagram showing an internal configuration of a dot generation circuit.

FIG. 14 is an explanatory diagram showing changes in dot shape of square dots used for normal screening.

FIG. 15 is an explanatory diagram showing changes in the dot shape of FM dots used for FM screening.

FIG. 16 is a flowchart showing a detailed procedure of area division processing according to the second embodiment.

[Explanation of symbols]

 30 ... CPU 32 ... ROM 34 ... RAM 36 ... Mouse 38 ... Display control circuit 40 ... Color monitor 42 ... Input scanner 44, 46, 48 ... Image memory 50, 52 ... Run length memory 60 ... Halftone dot generating device 62 ... Recorder 70 ... thinning / filling processing unit 72 ... smoothing processing unit 74 ... color system conversion unit 76 ... region division processing unit 78 ... pinhole erasing unit 80 ... intermediate region forming unit 102 ... clock generation circuit 104 ... image buffer 106 ... plate selector 108 ... Screen angle control file buffer 110 ... Bit map expansion unit 112 ... Plate selector 121-125 ... Dot generation circuit 130 ... Screening selector 140 ... Y clock generator 142 ... X clock generator 144 ... Y address counter 146 ... X address Counter 148 ... SPM 150 Comparator

Claims (6)

[Claims] 1. A method for producing a print image for a plurality of color plates including at least a C plate and an M plate as a color separation image for reproducing a color image, comprising: (A) lightness representing the color image. A step of preparing image data of a color saturation / saturation / hue color system, and (B) a first region in which the lightness is lower than a predetermined threshold based on the image data, and the lightness is lower than the predetermined threshold. The color image is divided into a second region in which the hue is relatively high and the hue is relatively green, and a third region in which the lightness is higher than the predetermined threshold and the hue is relatively warm. And (C) reproducing the first region using FM screening and reproducing the second region using normal screening,
Creating a print image of the color image by using the normal screening for the third area and reproducing while exchanging screen angles of the C plate and the M plate with the second area; A method for creating a printed image, comprising: 2. The method of creating a print image according to claim 1, wherein the step (B) includes a step of forming a fourth region having a predetermined width between the second and third regions. And a step (C) including a step of reproducing the fourth region using at least the C plate and the M plate by using the FM screening. 3. The method for creating a print image according to claim 2, wherein the step (A) includes a step of preparing image data of CMYK color system representing the color image, and the step (B). Includes, for each color plate of each area, indicating which of the FM screening and the normal screening is to be applied, and preparing screen angle data indicating a screen angle when the normal screening is applied, The step (C) reads the screen pattern data from the screen pattern memory according to the screening type and the screen angle specified by the screen angle data, and compares the screen pattern data with the CMYK image data. Generates dot signals to record the print image of each color plate. A method for creating a printed image, including the steps of: 4. The method for creating a print image according to claim 1, wherein the step (A) smoothes image data of the CMYK color system representing the color image with a predetermined spatial filter, And a step of converting the smoothed image data into image data of a lightness / saturation / hue color specification system, wherein the step (B) includes the lightness of the image data of the lightness / saturation / hue color specification system. Dividing the color image into first to third regions by classifying each pixel in the color image into the first to third regions based on the hue, and the first to third divided regions. A step of performing pinhole erasing on the third region and a step of forming a fourth region having a predetermined width between the second and third regions, wherein the step (C) comprises The fourth area is at least for the C plate and the M plate. Comprising the step of reproduced using M screening method creating a print image. 5. A method for producing a print image for a plurality of color plates including at least a C plate and an M plate as a color separation image for reproducing a color image, wherein (A) CMYK representing the color image. A step of preparing image data of a color system, (B) a first region in which the density of the color image is higher than a predetermined threshold based on the image data, and a density of the color image is the predetermined threshold A second region which is lower than the density of the M plate and higher than the density of the C plate, and a third region in which the density of the color image is higher than the predetermined threshold value and the density of the C plate is higher than the density of the M plate. (C) the first region is reproduced using FM screening, the second region is reproduced using normal screening, and the third region is reproduced. Is the normal scree And a step of creating a print image of the color image by reproducing the C plate and the M plate while exchanging the screen angles of the C plate and the M plate with the second region. How to create an image. 6. A method of creating a print image for each color plate of CMYK as a color separation image for reproducing a color image, the method comprising: (A) representing the color image.
A step of preparing image data of a color system, and (B) a first region in which the density is higher than the predetermined threshold value by comparing the density of each color plate of the image data with a predetermined threshold value for each color plate. And (C) dividing the color image for each plate into a second region in which the density is lower than the predetermined threshold, and (C) reproducing the first region using FM screening. And a step of creating a print image of each plate of the color image by reproducing the area 2 using normal screening.