JPH06181519A - Print image linear density conversion method - Google Patents

Abstract

(57) [Abstract] [Object] The present invention relates to a linear density conversion method for a printed image, and realizes a high-quality linear density conversion method. [Structure] A table of halftone dot data for each gradation is created corresponding to the combination of the line density of the image data and the number of screen lines defined for each printing device, and the created halftone dot data table is created. Based on the line density of the original image data and the screen frequency, select the halftone dot data to be compared, compare the selected halftone dot data and the original image data, and the halftone image data part, It is divided into line image data parts, and for the halftone image data part, the gradation value of the matched halftone dot data is obtained as pseudo multi-valued data, or the output assumed line density of the original halftone image data and the screen The period information of the halftone dot pattern is obtained from the number of lines, and the number of black pixels in the area having half of the halftone dot period as one side is counted, and the count value is converted into 256 gradations to simulate. Multivalued data is obtained, and the above obtained pseudo Based on the value data, the linear density of the output assumed, searching for the halftone dot data table by the number of screen lines, obtain dot data after the conversion.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for converting linear density of printed images. In particular, the present invention relates to a linear density conversion method for forming a print image on a plate making that needs to be created in the previous step of printing.

In recent years, optical and electronics technologies have been widely used for plate making of newspapers and advertisements. Original images of newspapers and advertisements include line drawings such as characters, tint blocks, and cuts that do not require gradation reproduction, and photographs that require gradation reproduction.

Therefore, "area gradation", which is a technique of gradation reproduction, is generally performed as an expression of a photograph in plate making such as newspapers and advertisements, and recently, a dither method and a density pattern method using an electronic technique are applied. "Electronic shading" by etc.
It is the mainstream to apply.

By the "electronic halftone" technique, a halftone image is a target for which gradation reproduction such as a photograph is required. By applying the above-mentioned electronic halftone to the original image, the original image can be converted into electrical image data composed of electronic halftone dots. This image data is generally called halftone data.

The halftone data is stored on a magnetic tape or the like, read out when necessary, and used to create a print image composed of a line image or a halftone dot image. That is, the halftone dot image is formed by the halftone dots created by the halftone data. The density of these halftone dots in the halftone dot image (hereinafter referred to as the halftone dot density) takes into consideration the resolution of the printing device, the material of the printed matter, the image quality expected of the printed matter, etc., and the expected line drawing or halftone image is obtained in the printed matter. Be converted to This conversion is generally called "linear density conversion".

With the recent development of the printing industry, various printing devices have come to be able to obtain a wide variety of printing images, and linear density conversion according to these printing devices and printing images is performed. In particular, now that high quality printing is emphasized, it is desired to perform linear density conversion so as not to deteriorate the quality of the original image as much as possible.

Conventionally, the linear density conversion has been performed by a technique of simply enlarging or reducing a print image (hereinafter referred to as simple enlargement or simple reduction), as will be described later. The deterioration of the quality of the printed image that occurs, especially the moire that occurs in the halftone image, has been a problem. Therefore,
There is a demand for a linear density conversion method that causes less deterioration of the printed image, such as moire of a halftone dot image that occurs when converting the linear density of the printed image.

[0008]

12 to 14 are views for explaining a conventional linear density conversion method, and FIG. 12 is a block diagram showing a conventional linear density conversion process. 13 shows a specific example of the conventional linear density conversion method,
FIG. 14 shows a problem of the conventional linear density conversion method.

The linear density conversion is performed, for example, on the original image data read from a magnetic tape (not shown).
This is performed along the Y axis and X axis of the dimension address. In FIG. 12, the original image data stored in the first image storage unit 11 is read by the first read / write control unit 21, converted in the Y-axis direction at a predetermined conversion rate, and then converted into the second image storage unit. 1
Stored in 2. The data stored in the second image storage unit 12 is read out by the second read / write control unit 22 and X
It is converted in the axial direction at a predetermined conversion rate and stored in the third image storage unit 13.

Thus, the data stored in the third image storage section 13 is image data converted in the Y-axis direction and the X-axis direction at a predetermined conversion rate, and this image data is read to make a plate. .

The first and second read / write control units 2 when the conversion rates in the X and Y axis directions are respectively set to 1.5 times.
The aspect of the read / write control by 1 and 22 is shown in FIGS.
Explain by (c). 13 (a), (b), and (c) show the first,
The storage patterns of the image data stored in the second and third image storage units 11, 12, and 13 are shown arranged in two-dimensional addresses.

FIG. 13A shows a part of the storage pattern of the original image data stored in the first image storage section 11.
That is, FIG. 13A shows digital data of halftone dot pixels arranged by address. When the halftone dot pixel has digital data, it is indicated by a black circle (“1”), and when there is no digital data, a white circle (“0”). ).

Assuming that the conversion rates in the X-axis and Y-axis directions are both 1.5 times, first, the first read / write control unit 21 functions to store the data in the first image storage unit 11 shown in FIG.
The data in (a) has the same addresses as those shown in FIG. 13 (a) on the X-axis, and has 1.5 times the addresses shown in FIG. 13 (a) on the Y-axis in the second image storage unit 12. Fig. 13
It is stored as shown in (b), and then the second read / write control unit 2
The data (black circles) in FIG. 13B stored in the second image storage unit 12 due to the action of 2 is shown in FIG.
The address shown in (b) is the same as that shown in FIG.
The address is stored in the third image storage unit 13 as shown in FIG. 13C at an address 1.5 times the address shown in FIG.
By sequentially reading the data stored in the image storage unit 13, the image data subjected to the linear density conversion of 1.5 times can be sent to the printing unit in the next stage.

[0014]

[Problems to be Solved by the Invention] FIGS. 11 (a), 11 (b) and 11 (c)
Is an example of simple expansion when the conversion rate is 1.5 times,
In the case of the line drawing as shown in FIG. 11, this simple enlargement does not cause a problem such as distortion or deformation as another pattern. However, the situation is different for halftone images.

An example of halftone dot data is shown in FIG. In FIG. 14A, white circles and black circles indicate digital signals arranged at the addresses of the halftone dot data. A white circle indicates that the signal does not exist, that is, "0", and a black circle indicates that the signal exists, that is, "1". It is called a halftone dot.

Generally, the density of the pixels forming the halftone dots is determined by considering the image quality of the original image and the image quality of the print image to be created. Generally, the density of the above-mentioned halftone dots per inch is formed. It is indicated by the number of existing pixels (dots) and is expressed by dots / inch (dpi).

Assuming that the density in FIG. 14A is 454 dpi and the halftone image is linearly density-converted to 800 dpi, the number of pixels is 800/45 for both the X and Y axes.
4 = 1.76 times.

Therefore, the halftone image after the linear density conversion is shown in FIG.
As shown in (b). In FIG. 14B, it should be noted that the signal pattern of one halftone dot formed by the black circles in FIG. 14B is at four corners as compared with the signal pattern in FIG. 14A before conversion. It lacks (pixels) and looks like a cross. Originally, it needs to be converted as shown in FIG.

In general, the grayscale of the original image often changes gently on the screen. Therefore, since the halftone dot image also exists while changing gently up, down, left, and right on the paper surface, it has a circular shape as shown in FIG. 14B. When a signal pattern having no directivity is continuously printed, it appears as moire on a printed image.

As shown in FIGS. 13 (a), 13 (b) and 13 (c),
When the printed image appears as a line drawing, moire does not occur because the result of electronic halftoning is a set of digital signals "1" or "0".

As described above, FIGS. 13 (a), (b), (c) and FIGS. 14 (a),
(b) is an example of a linear density conversion in which a line drawing and a halftone dot image are simply enlarged. However, even when the line density conversion is a simple reduction, the moire problem does not appear in the line drawing, and the halftone dot image Appear in.

In view of the above-mentioned conventional drawbacks, the present invention selects data to be a halftone dot image from image data forming a line image and a halftone dot image, and selects each halftone dot data for the selected halftone dot image data. It is an object of the present invention to provide a linear density conversion method for a print image, which performs linear density conversion so that the signal pattern has no directionality and is as circular as possible.

[0023]

FIG. 1 is a diagram for explaining the principle of the present invention. The above problems can be solved by the linear density conversion method configured as follows.

(1) Corresponding to a combination of a line density indicating the number of pixels per unit length of image data and a screen frequency indicating the number of halftone dots per unit length of the image data, which is determined for each printing device. Then, create a halftone dot data table consisting of halftone dot data for each gradation, and by combining the above-mentioned line density and screen frequency of the original image data, the halftone dot data of the halftone dot data table created above And select the halftone dot data to be compared when extracting the halftone dot image data part in the original image data, and select the selected halftone dot data and the original image data in units of a predetermined area. By comparison, it is divided into a halftone dot image data portion that matches and a line image data portion that does not match. For the halftone dot image data portion, the gradation value of each halftone dot data that matches is divided into pseudo multi-valued data. Obtained as data D, above Based on the simulated multi-valued data D, the table of the halftone dot data is searched by the line density and the screen frequency which are supposed to be output, and the converted halftone dot data is obtained. Enlargement, simple reduction, linear density conversion is performed to obtain converted line drawing data, and the obtained halftone dot data and line drawing data are combined to obtain converted image data. .

(2) Corresponding to a combination of a line density indicating the number of pixels per unit length of image data and a screen frequency indicating the number of halftone dots per unit length of the image data, which is determined for each printing device. Then, create a halftone dot data table consisting of halftone dot data for each gradation, and by combining the above-mentioned line density and screen frequency of the original image data, the halftone dot data of the halftone dot data table created above And select the halftone dot data to be compared when extracting the halftone dot image data part in the original image data, and select the selected halftone dot data and the original image data in units of a predetermined area. By comparison, it is divided into a matched halftone dot image data part and a non-matched halftone dot image data part, and the halftone dot image data part is divided into halftone dots according to the line density and screen frequency of the original halftone dot image data. Halftone dots in the pattern Period, the number of black pixels in an area having half of the halftone dot period as one side is counted, the count value is converted into 256 gradations, and pseudo multi-valued data D is obtained. Based on the obtained pseudo multi-valued data D, the table of the halftone dot data is searched by the above-mentioned assumed line density and screen frequency to obtain the converted halftone dot data, and the line image data portion is simply It is configured such that enlargement and simple reduction are performed, line density conversion is performed to obtain converted line drawing data, and the obtained dot data and line drawing data are combined to obtain converted image data.

[0026]

In the present invention, first, the linear density (n dpi) indicating the number of pixels per unit length (for example, inches) of image data determined for each printing device, and the per unit length of the image data (for example, , Inch) halftone dot data (L line / i) combination corresponding to the number of screen lines formed (binarization in units of the area with halftone dot cycle m as one side) Create a data pattern table.

As shown in FIG. 3, this halftone dot data table is a linear density (n dp) indicating the number of pixels per unit length (inch) of the image data defined for each printing device.
i) and the number of screen lines (L lines / i) formed by halftone dots per unit length of the image data (L lines / i), corresponding to each combination, the halftone dot data for each gradation is tabulated. It is a thing.

Therefore, if there are J types of linear densities (n dpi) and K types of screen rulings (L lines / i) in the printing device, there will be J × K number of dots in the halftone dot data table. ,
Halftone dot data having different halftone dot periods m are generated.

Generally, assuming that the linear density is n dpi, the halftone dot period is m pixels, the screen ruling is L lines / i, and the angle of the halftone dot pattern is θ, m = n / L sin θ. In Fig. 3, line density: 454 dpi, screen ruling: 65
If the line / i and the angle θ of the halftone dot pattern are 45 degrees, the halftone dot period is
Approximately m = 10, linear density after linear density conversion: 800dpi,
If the screen frequency is 65 lines / i and the angle θ of the halftone dot pattern is 45 degrees, the halftone dot period is about m = 20, so the halftone dot data corresponding to each is shown in FIG. As described above, a binary data pattern in which a region having different halftone dot periods m as one side is filled with “0” (shown by ◯) pixels and “1” (shown by ●) pixels.

When the gradation is "0", the entire area is "0", and when the gradation is "255", the area is "1" and the gradation is, for example, " In the case of 100 ", for example, the data pattern forms a halftone dot in which" 1 "is gathered in the central portion in the area.

An original image to be subjected to linear density conversion according to the present invention,
The relationship between the converted image and the converted image will be described with reference to FIG. The original image data is, for example,
With the above linear density n = 454 dpi, the above screen frequency L = 65
If the line / i and the angle of the halftone dot pattern are θ = 45 degrees (see FIG. 9), then the halftone dot period is approximately m = 10 pixels,
The image data is as shown on the left side of FIG.

In this original image, as shown in the figure, halftone dot data of a predetermined gradation and line image data are mixed in a unit of an area having a halftone dot period of m = 10 pixels. It is a thing.

The halftone dot data in the original image data has the linear density n = 454 dpi and the screen frequency L
In the halftone dot data table created in advance corresponding to the combination of = 65 lines / i, the halftone dot data corresponding to each gradation is formed.

For this original image data, a linear density of 454 d
pi, screen frequency 65 lines / i to linear density 800 dpi, screen frequency 65 lines / i
The halftone dot data table created in advance corresponding to the line density and screen frequency of each printing device is searched, and the line density of 800 dpi and the screen frequency of 65 lines / i are combined. The line density conversion according to the present invention is performed by selecting the provided halftone dot data corresponding to each gradation and storing it in the corresponding region of the address region for each halftone dot period m. {Refer to the converted image data shown on the right side of FIG. 2} Hereinafter, the above-described linear density conversion operation and operation will be described based on the flowchart of FIG.

Based on the halftone dot data table created as described above, it is provided corresponding to the combination of the above-mentioned line density and screen frequency based on the line density and screen frequency of the original image data. The set of halftone dot data is searched, and the halftone dot data to be compared, which is necessary for extracting the halftone dot image data part from the original image, is selected in order from the gradation “0”, for example. The extracted halftone data is compared with the image data extracted by searching the area divided by the halftone dot period m of the original image data, and the matched halftone image data portion and any halftone It is divided into a line image data portion that does not match the dot data.

Regarding the halftone dot image data portion of the divided image data, the gradation value of each matched halftone dot data is obtained as the pseudo multi-valued data D, or the halftone dot image data Partial linear density: n dpi,
Assuming that the halftone dot period is m pixels, the number of screen lines is L, and the angle of the halftone dot pattern is θ, the output expected line density of the original halftone dot image data and the number of screen lines are considered in view of the fact that m = n / L sin θ. To obtain the halftone dot period m of the halftone dot pattern, count the number of black pixels in the area having 1/2 of the halftone dot period m as one side, and calculate the count value (pseudo multi-valued data Da). Two
Pseudo multi-valued data D is obtained by converting to 56 gradations.

Based on the obtained pseudo multivalued data D, the halftone dot data table after conversion is obtained by searching the table of the halftone dot data according to the assumed line density and screen frequency of the output.

Then, the line image data portion is subjected to line density conversion by simply enlarging and contracting, which is a conventional line density converting method, to obtain line image data after conversion. {Refer to FIG. 5} In this way, the obtained halftone data and line drawing data are combined to obtain converted image data.

That is, in the image data linear density conversion method according to the present invention, a halftone dot data portion whose image quality cannot be guaranteed is simply extracted by simple enlargement or simple reduction of the entire image, and the extracted halftone dot data portion is extracted. By using the halftone dot data that takes into consideration the line density to be output and the screen frequency, it is possible to realize high-performance line density conversion with high performance and faithfully reproducing the density of the original image. .

[0040]

Embodiments of the present invention will be described in detail below with reference to the drawings. The above-mentioned FIGS. 1, 2 and 3 are diagrams for explaining the principle of the present invention, FIGS. 4 to 8 are diagrams showing an embodiment of the present invention, and FIGS. 9 to 11 are diagrams of the present invention. It is a figure showing other examples.

In the present invention, a line density indicating the number of pixels per unit length (inch) of image data and a screen indicating the number of halftone dots per unit length (inch) of the image data are determined for each printing device. Corresponding to the combination of the number of lines,
Create a table of halftone dot data for each gradation and extract the halftone dot image data part from the original image data based on the created halftone dot data table based on the above line density and screen ruling of the original image data. For comparison, the halftone dot data to be compared is selected in order from, for example, the gradation “0”, and the selected halftone dot data and the original image data are compared to determine a matched halftone dot image data portion. , The line image data portion that does not match is obtained, and for the halftone image data portion, the gradation value of each matched halftone dot data is obtained as the pseudo multi-valued data D, or the halftone image For the data part, the halftone dot period m of the halftone dot pattern is obtained from the line density and screen ruling of the original halftone dot image data, and the black pixel in the area whose half side is 1/2 of the halftone dot period m Count the number of The pseudo multi-valued data D is obtained by converting into 6 gradations, and the table of the halftone dot data is searched based on the obtained pseudo multi-valued data D according to the expected line density and screen frequency. , The converted halftone dot data is obtained, and the line image data portion is subjected to line density conversion by simply enlarging and reducing, obtaining the converted line drawing data, and obtaining the obtained halftone dot data and line drawing data. To obtain the converted image data by combining
It is a necessary means for carrying out the present invention. The same reference numerals indicate the same objects throughout the drawings.

Hereinafter, referring to FIGS. 1 to 3, FIGS.
The linear density conversion method of the present invention will be described with reference to FIGS. 8 and 9 to 11. 1) In the present invention, first, the halftone dot data table shown in FIGS. 3 and 4 (a) is generated. As described above, this halftone dot data table has a line density (pixels per unit length (for example, inch), which is indicated by dpi (dots / inch)) determined for each printing device, and a screen line. Corresponding to each combination of the number (the number of lines formed by halftone dots per unit length (for example, inch)), each gradation (0 to 25)
This is a table of halftone dot data corresponding to 5 gradations. As described above, assuming that there are 10 types of line density and 10 types of screen frequency, 10 × 10 = 100 types of halftone dot data Exists in the halftone dot data table.

Given the linear density (454 dpi) and screen ruling (65 lines / i) of a printing device, the image output from the printing device is as shown in FIGS. 3 and 4 (a). Halftone dot data corresponding to each gradation can be obtained corresponding to the combination of the above-mentioned linear density (454 dpi) and screen frequency (65 lines / i).

2) Based on the linear density and screen ruling of the original image data output from the printing device, the halftone dot data to be compared when the halftone dot image data portion is extracted from the original image data is, for example, gradation. Select in order from "0".

FIG. 4 (b) shows an example of selecting the above halftone dot data, where the halftone dot image data of the original image has a line density of 454 dpi and a screen frequency of 65 lines / i. . By searching the halftone dot data table corresponding to the combination of the above line density and screen frequency using the above line density and screen frequency, the target dot conversion corresponding to each gradation (0 to 255) You can ask for data.

3) The original image data is searched, for example, in units of an area having a halftone dot period (obtained from the above-mentioned line density and screen ruling) as one unit, and converted into the target halftone dot selected in 2) above. Compare with the data, the part that matches any of the target halftone data is the halftone data,
The original image data is divided using the portion that does not match any of the target halftone dot data as the line drawing data. {See Fig. 5}
In the above division processing, in the case of halftone dot data, the gradation value (for example, gradation “12” in the present embodiment) of each matched halftone dot data is obtained, and this gradation value is obtained. As shown in FIG. 5, the obtained value “12” is set as the pseudo multi-valued data D in the halftone dot data storage area on the main memory, and the halftone dot period m of the original image is set as a unit. The pseudo multi-valued data D obtained above is stored together with the head address (simply called an address).

As for the line drawing data, as shown in FIG.
The value data is continuously stored in units of line data (binary data) in that area.

4) As for the above-mentioned halftone dot data, as shown in FIG. 6, the halftone dot is converted into the above-mentioned halftone dots by the expected output line density (800 dpi) and the screen frequency of the halftone dot image (65 lines / i). Search the data table (search in the order of the arrows) and
3) The halftone dot data corresponding to the value (for example, "12" in this embodiment) of the pseudo multi-valued data D obtained in 3) is searched,
The retrieved halftone dot data (binary data) is expanded on the memory as the converted halftone dot data together with the address of the original image.

5) As for the line drawing data, as shown in FIG. 7, the line density conversion is performed by the simple enlargement and the simple reduction, which are the same as the conventional line density conversion method, and the address of the original image is obtained as the converted line drawing data. At the same time, it is expanded in a predetermined area on the main memory.

In this embodiment, the linear density is 454 dpi to 800.
Since it is a conversion to dpi, it becomes 800/454 ≈ 1.8, and the expanded data (see Fig. 11 above) obtained by converting the original image data to 1.8 times the vertical and horizontal directions is converted to the specified area on the main memory. Store as back line drawing data.

6) The data length of the converted image data obtained in 4) and 5) above is calculated, and the converted data for the address of each unit area (area having halftone dot period m as one side) in the original image is calculated. The address in the image data is obtained, and the halftone dot data portion and the line drawing data portion are combined based on the obtained address and expanded as the converted image data in a predetermined converted image data area on the main memory. To do.

In the case of this embodiment, the original image data is 800 × 60.
When it is set to 0 pixel, when converting from 454 dpi to 800 dpi, it becomes 1.8 times as described above, so the image data after conversion is (800 × 1.8) × (600 × 1.8) pixel, that is,
Since it is 1440 × 1080 pixels, the address in the converted image data can be obtained by calculating the address of the original image by original image address × (800 × 600) / (1440 × 1080).

On the basis of the address thus obtained, the halftone dot data and the line drawing data are combined,
By expanding in a predetermined area on the main memory, it is possible to obtain image data that has undergone a predetermined linear density conversion. {Fig. 8
Reference} Next, FIGS. 9 to 11 show another embodiment in the case of performing the linear density conversion on the divided halftone dot data.
Described in.

1) Also in this case, a halftone dot data table corresponding to each gradation corresponding to the combination of the line density and the screen frequency determined for each printing device {FIG. 3, FIG.
Generating "see FIG. 4 (a)" is the same as in the case of the embodiment described with reference to FIGS.

2) As described above, the line density: n dpi, the halftone dot period: m pixels, the screen ruling: L, the angle of the halftone dot pattern:
If θ is set, in consideration of the fact that m = n / L sin θ is obtained, the halftone dot period m of the halftone dot pattern of the original image is obtained from the expected output line density of the original halftone dot image data and the screen ruling.

In this embodiment, the original image data is the linear density.
If the screen frequency is 454 dpi and the screen frequency is 65, and the angle of the screen frequency is 45 degrees (see Fig. 9), the halftone dot period is about 10 pixels from the above calculation formula.

3) Here, the original image data is divided into 5 × 5 pixel areas each having 1/2 of the calculated halftone dot period as one side,
When counting the number of black pixels in the divided area,
As shown in FIG. 7, since the number of black pixels (indicated by a dot pattern) in each area is 4 pixels, 6 pixels, ..., In this embodiment, the obtained black pixels are obtained. Is the pseudo multi-valued data Da.

4) The value of the pseudo multi-valued data Da obtained in 3) is converted into 255 gradations by the calculation formula shown in FIG. That is, the pseudo multi-valued data Da is 5 × 5 (= 25)
Since it is the number of black pixels in the pixel area, it is necessary to convert it to the number of black pixels in the area of 255 pixels in order to convert it to 255 gradations. It is necessary to multiply the pseudo multi-valued data Da (N) by 255/5 × 5 as shown in FIG.

In this way, the converted pseudo multi-valued data D2 of each area is obtained as "40" and "61", respectively. 5) Using the value of the pseudo multi-valued data D obtained in 4), the halftone dot data of the line density and screen frequency to be converted are obtained by searching the above halftone dot data table, and the predetermined area on the main memory is obtained. Expand to.

In this embodiment, the linear density after conversion is 800 dpi,
When the screen frequency is 65, the halftone dot data corresponding to the gradation 61 can be obtained. Therefore, the obtained halftone dot data is converted to the image data after conversion described in FIG. It is possible to obtain the converted halftone dot data which has been subjected to the linear density conversion by expanding it to the address of.

With respect to the line drawing data obtained by dividing the original image, the line drawing data after conversion can be obtained by performing simple expansion and contraction in the same method as described with reference to FIGS.

Hereinafter, by combining the line drawing data and the halftone dot data by the same method as described with reference to FIGS. 4 to 8, the image data after the conversion of the line density can be obtained. As described above, the linear density conversion method according to the present invention creates a table of halftone dot data for each gradation corresponding to the combination of the linear density of image data and the screen frequency, which is determined for each printing device. , Based on the created halftone dot data table, select the halftone dot data to be compared for extracting the halftone dot image data part in the original image data according to the line density of the original image data and the screen frequency. , The selected halftone dot data and the original image data are compared, and divided into a halftone dot image data portion and a line image data portion,
For the halftone dot image data portion, the gradation value of each matched halftone dot data is obtained as the pseudo multi-valued data D,
Alternatively, the halftone dot period m of the halftone dot pattern is obtained from the line density of the original halftone dot image data and the number of screen lines, and the number of black pixels in the area having one half of the halftone dot period m as one side is calculated. The pseudo multi-valued data D is obtained by counting and converting the count value into 256 gradations. Based on the obtained pseudo multi-valued data D, the halftone dot data of the halftone dot data is obtained according to the expected line density and screen frequency. The table is searched to obtain the converted halftone dot data, and the line image data portion is subjected to linear density conversion by simply enlarging and reducing to obtain the converted line image data and the obtained halftone dot data. Is characterized in that the converted image data is obtained by synthesizing the image data with the line drawing data.

[0063]

As described above in detail, according to the linear density conversion method for a printed image of the present invention, the halftone dot data portion whose image quality cannot be guaranteed is extracted by simple enlargement and simple reduction of the entire image. Then, regarding the extracted halftone dot data part,
By using halftoned data that takes into consideration the desired line density and screen frequency, it is possible to realize high-quality line density conversion that is highly efficient and can faithfully reproduce the density of the original image. Is obtained.

[Brief description of drawings]

FIG. 1 is an explanatory diagram (1) of the principle of the present invention.

FIG. 2 is an explanatory diagram of the principle of the present invention (No. 2)

FIG. 3 is an explanatory diagram of the principle of the present invention (No. 3)

FIG. 4 is a diagram showing an embodiment of the present invention (No. 1).

FIG. 5 is a diagram showing an embodiment of the present invention (part 2).

FIG. 6 is a diagram showing an embodiment of the present invention (part 3).

FIG. 7 is a diagram showing an embodiment of the present invention (Part 4).

FIG. 8 is a diagram showing an embodiment of the present invention (No. 5).

FIG. 9 is a diagram showing another embodiment of the present invention (No. 1).

FIG. 10 is a diagram showing another embodiment of the present invention (No. 2).

FIG. 11 is a diagram showing another embodiment of the present invention (part 3).

FIG. 12 is a diagram (part 1) explaining a conventional linear density conversion method.

FIG. 13 is a diagram (part 2) for explaining a conventional linear density conversion method.

FIG. 14 is a diagram (part 3) explaining a conventional linear density conversion method.

[Explanation of Codes] Halftone data table Target halftone data Line drawing data, Line drawing data part Halftone dot data, Halftone data part Matched target halftone data Post-conversion halftone data Post-conversion linework data Post-conversion image data

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location H04N 1/40 104 9068-5C

Claims (2)

[Claims] 1. Corresponding to a combination of a line density indicating the number of pixels per unit length of image data and a screen frequency indicating the number of halftone dots per unit length of the image data, which is determined for each printing device. , A halftone dot data table () consisting of halftone dot data for each gradation is created, and the halftone dot data table () created above is combined by combining the above-mentioned line density and screen frequency of the original image data. Search the dot data and select the dot data () to be compared when extracting the dot image data part () in the original image data, and select the selected dot data () and the original image. The data is compared in units of a predetermined area, and it is divided into a halftone dot image data part () that matches and a line image data part () that does not match, and the halftone dot image data part () is matched. The gradation value of each halftone dot data () Pseudo multi-valued data D is obtained, and based on the obtained pseudo multi-valued data D, the halftone dot data table () is searched according to the expected line density and screen frequency, and the converted halftone data is obtained. The point data () is obtained, and the line image data portion () is subjected to line density conversion by performing simple expansion and contraction to obtain line image data () after conversion. A method for converting linear density of a print image, which comprises synthesizing with line drawing data () to obtain converted image data (). 2. Corresponding to a combination of a line density indicating the number of pixels per unit length of image data and a screen frequency indicating the number of halftone dots per unit length of the image data, which is determined for each printing device. , A halftone dot data table () consisting of halftone dot data for each gradation is created, and the halftone dot data table () created above is combined by combining the above-mentioned line density and screen frequency of the original image data. Search the dot data and select the dot data () to be compared when extracting the dot image data part () in the original image data, and select the selected dot data () and the original image. The data is compared with a predetermined area as a unit, and it is divided into a halftone dot image data portion () that matches and a line image data portion () that does not match. Halftone image data line density and screen line The halftone dot period of the halftone dot pattern is obtained by the number, and the number of black pixels in the area having one half of the halftone dot period as one side is counted, and the count value is set to 25.
Pseudo multi-valued data D is obtained by converting to 6 gradations, and based on the obtained pseudo multi-valued data D, a table of the halftone dot data according to the expected line density and screen ruling ().
To obtain the converted halftone dot data (), and the line image data part () is simply enlarged or reduced to perform the linear density conversion to obtain the converted line image data (). A method for converting a line density of a print image, which comprises combining the obtained dot data () and line drawing data () to obtain converted image data ().