JPH02192363A - Method for converting image data of dot image - Google Patents

Abstract

PURPOSE:To prevent moire occurring even when a dot image is reproduced by converting the image data of an original image that is the dot image to new image data by applying position interpolation on the mean value of the image data at every picture element block. CONSTITUTION:The image data D0 read by an input scanner 1 is preserved in the memory 23 of an image processing device 2. The device 2 divides and sets the inside of the original image into plural picture element blocks consisting of picture elements of 3X3. Next, part of the data D0 preserved in the memory 23 i.e., the image data of partial image is read out, and is stored in a buffer memory 12. Following that, an arithmetic circuit 22 calculates the mean value of each picture element block based on the image data of the partial image. The new data of peripheral picture element of the picture element block can be found by applying weighted mean by multiplying the mean value of the picture element block and that of a neighboring picture element block by coefficients (m) and (1-m), respectively.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to an image data conversion method used in an image processing device such as a plate-making scanner, and in particular to an image data conversion method used in an image processing device such as a prepress scanner. The present invention relates to an image data conversion method for converting data into continuous multi-tone image data, mainly for preventing the occurrence of moiré.

(Prior Art) Generally, a photographic image is used as an original image to be read by a plate-making scanner. However, there are cases where there is no photographic image to be used as the original image, and it is unavoidable to use a halftone image from a halftone film or halftone printed matter as the original image.

However, it is generally known that when a halftone image is used as an original image to be read by a plate-making scanner and printing plates and printed matter are created according to normal processing procedures, moire is likely to occur. This moiré is caused by the halftone dot pattern that makes up the original image and the halftone dot pattern used when processing image data with a plate-making scanner (usually called a "screen").
This is due to interference between

Conventional methods to prevent the occurrence of moiré are as follows:
For example, there is one disclosed in Japanese Unexamined Patent Publication No. 58-14136.

(Problem to be Solved by the Invention) However, in this conventional method, the original image, which is a halftone image, and the scanning t
This is a method of obtaining average image data in the direction across the scanning line by moving the i position relative to the scanning line and generating irregular lateral movement in the direction across the scanning line. Therefore, this method is essentially equivalent to blurring the original image, and there is a problem in that image data that can visually faithfully reproduce the original image cannot be obtained.

(Purpose of the Invention) The present invention is intended to solve the above-mentioned problems in the conventional technology, and provides new image data that can visually faithfully represent the original image and prevent the occurrence of moiré. The purpose of the present invention is to provide an image data conversion method for reducing the number of images.

(Means for Solving the Problems) In order to solve the above problems, the present invention provides a method of converting image data obtained by reading an original image composed of halftone dots pixel by pixel into multi-tone image data. In (a)
Set multiple pixel blocks consisting of multiple pixels, and (b
) For each of the plurality of image blocks, determine the average value of the image data of each pixel, and (c) For each of the plurality of pixel blocks, calculate the new image data value for the central pixel of the pixel. (d) setting the new image data value at each pixel other than the center of the pixel block from the center of the pixel block toward the center of the adjacent pixel block; Set interpolation so that it changes continuously. Note that the number of pixels at the center is not limited to one, and may include a plurality of pixels.

(Function) Image data obtained by reading an original image composed of halftone dots has a density distribution state according to the halftone dot pattern of the image, that is, high density pixels and low density pixels are mixed closely together. It represents the state of being.

In this invention, each pixel block includes a plurality of pixels, and the average value of image data within the pixel block is set as the new image data value at the center of the block. Then, based on the new image data values at the center of each adjacent pixel block, new image data values for pixels other than the center are obtained.

For this reason, the halftone dot pattern of the original image has disappeared.
Moiré does not occur even when an image is reproduced using a predetermined screen based on new image data.

Furthermore, since new image data is obtained based on the image data obtained by reading the halftone dots of the original image without blurring the original image, image data that is faithful to the original image can be obtained.

Roughly speaking, for pixels other than the center, interpolation is set so that the image data changes continuously between the centers of adjacent pixel blocks, so the gradation changes expressed by halftone dots in the original image are , it can be converted into new image data that is expressed as smooth gradation changes, and image data that visually faithfully reproduces the original image can be obtained.

(Embodiment) A. Schematic structure and operation of apparatus FIG. 1 is a block diagram showing an example of an image data processing system IPS that converts image data by applying an embodiment of the present invention. In FIG. 1, the image data processing system IPS includes an input scanner 11, an image processing device 2, and an output scanner 3. This image Ill! The device 2 has a buffer memory 21 . It includes an arithmetic circuit 22 and a memory 23. In this embodiment, the image processing device 2 is configured as a cutout mask device for creating a cutout mask, and is connected with a graphic display 4, a mouse 5, etc. for use in creating the cutout mask.

FIG. 2 is a flowchart showing the operation of the embodiment.

First, in step S1, an original image, which is a halftone dot image, is read by an input scanner.

FIG. 3 is a conceptual diagram showing an enlarged portion of the original image O.

This original image O is a halftone dot film, and the image is expressed by halftone dots H to H7 of various sizes. Note that in this original image 0, only the area within halftone dots H-H7 is black, and the other areas are transparent. Further, the original image O is virtually divided by the reading pixels P when reading with the input scanner.

When the original image O is read in step S1, image data D expressing the brightness (1 m degree) of each pixel P in multiple gradations is generated for each pixel P. is obtained. For example, image data D. When expressed as an 8-bit binary number (0 to 255 in decimal), the value (in decimal) of the image data D of pixel P8 in FIG.

The value of the image data Dbo of O is set to O, and the value of the pixel P
. The value of the image data Dc0 is set to an intermediate value between 0 and 255.

In step S2, image data read by the input scanner (hereinafter referred to as 1 initial image data) Do
is stored in the memory 23 of the image processing device 2.

Steps 83 to S9 are steps of arithmetic processing within the image processing device 2.

In step S3, the interior of the original image 0 is divided into a plurality of pixel blocks PB each including (3×3) pixels. FIG. 4 is a conceptual diagram showing a pixel block PB. Pixel block PB includes pixels P at the center. and the surrounding 8
It is composed of four pixels P1 to P8. In addition, in the pixel block PB, the halftone dot area ratio of the halftone dots of the original image 0 is 100%.
The size is approximately the same as that of the halftone dot shape (hereinafter referred to as 1100% halftone dot shape). In other words, the size of the pixel P is determined so that the pixel block PB is approximately the same size as the 100% halftone dot shape. The reason for this will be explained later.

In step S4, the image data D stored in the memory 23. , that is, image data D of a partial image.

- not shown) is read out and stored in the buffer memory 21.

In step S5, the arithmetic circuit 22 calculates the image data D of the partial image. , the average value of each pixel block PB is calculated as follows: e, -(e□ +e1+...+e8)/9...(
1) Here, e: Average value of pixel block P8 ei (i=0 to 8): Pixel P in pixel block PB
In the image data step S6, the pixel P at the center of each pixel block PB. The new data "vO" is set to the value of the average value e obtained by equation (1).

That is, vO...(2) e=ev FIG. 5 shows the pixel block PB in the original image 0 shown in FIG.
~PB, is a conceptual diagram showing the distribution state of image data. In FIG. 5, for pixel blocks other than pixel block PBo, only new data (ie, the average value of the pixel block) a, ˜i of the central pixel is shown.

Next, in step S7, each pixel block PBa to PB,
Based on the average value a, ~i, of each pixel block PBa
-PB, new data of peripheral pixels within is calculated. Taking pixel block PB as an example, new data ev1 to ev8 of each peripheral pixel in it are interpolated as follows: e, 1-mev+ (1-m)a, . . . 3a
ev2-mev+ (1-m)bV-3be,=
meV+ (1-m)CV ・3CeV4=me
V ten (1-m) fV...3dev5-mev+ (1
-m) i, =13eev6=meV+
(1-m)h, ・(34ev7=meV+ (
1-m) Q, -3(]ev8=me, + (1
-m) dv -3h Here, m: a constant of 0.5≦m≦1.0, that is, new data evl to ev8 of peripheral pixels of pixel block PB8 are PB of that pixel block. The average of 1i1! It is obtained by multiplying ev and the average values av to iv of adjacent pixel blocks by weighting coefficients m and (1-m), respectively, and performing a weighted average. Therefore, the new data eV1 to ev8 of the surrounding pixels take a value intermediate between the average value e of the relevant pixel block and the average value av-i of the adjacent pixel block. Also, since the weighting coefficient m is set in the range of 0.5 to 1.0, the new data ev1 to ev of the surrounding pixels
8 is the average value a of adjacent pixel blocks, ~1. The value is closer to the average value ev of its own pixel block PBo.

By interpolating and setting new data for each peripheral pixel in this way, new image data evo"""v8 representing a state in which the density changes smoothly on the image plane is obtained.

Note that the weighting coefficient m may be changed according to the difference between the average value e and the average value a, ~i, of adjacent pixel blocks, for example, as follows: When 0≦Iev−aV1≦10, m-o, s...(4a) 10<l
e, -a, l≦30 17),! : Ki, m-o, s
...(4b)250<le -a
vl (When D, ■ m= 1.0...(4
C) Here, the average values e and a as image data
It is assumed that v is expressed in 8 bits, and therefore, the average values e and a take values of O to 255 in decimal notation.

The above equations (4a) to (4C) mean that the larger the difference between the average values of adjacent pixel blocks, the larger the value of the weighting coefficient m. By changing the value of the weighting coefficient m in this way, when the difference between the average values of adjacent pixel blocks is small, image data can be obtained that expresses the two-dimensional change in density for each pixel gradually. It will be done. On the other hand, when the difference between the average values of adjacent pixel blocks is large, image data that expresses m degrees as changing rapidly at the boundary between pixel blocks is obtained. In particular, when the boundaries of adjacent pixel blocks represent the outline of a figure, large changes in density in the outline are expressed as image data, making the outline clear and faithfully reproducing the original image. It has the advantage that image data can be obtained.

It should be noted that equations (4a) to (4c) apply to pixel blocks PB, P
Although the expression is shown for B8, a similar formula is generally set for any two pixel blocks. Also,
The values at both ends of the inequality signs in equations (4a) to (4C) and the value of the weighting coefficient m are determined empirically through the results of actual image processing.

Please note that the new data requested as described above (hereinafter referred to as “
This is called "corrected data.")evO"ev& may be different from the average value eV of data eo"""'a before being converted to corrected data. Correction data evo~”v
Although the average value of 8 does not necessarily have to be equal to the average value eV before conversion, the modified data e, is adjusted so that these average values are equal to each other. A predetermined operation may be performed, such as adding equal values to each of ~e and 8.

As described above, the new image data (hereinafter referred to as “corrected image data”) DI that has been screened as corrected data by the arithmetic circuit 22 is stored in the buffer memory 21. This modified image data DI is then transferred from the buffer memory 21 to the memory 23 and stored in step S8.

In step S9, it is determined whether or not the processing has been completed for the entire image of the original image O. If the processing has not been completed, the process returns to step S4, and processing for the next partial image is performed at step 84.
The processes from ~S8 are executed.

In this way, the image data D of the original image O, which is a mesh image. is converted into modified image data D11 that represents an image whose density changes smoothly between pixels. Then, the corrected image data D is transferred from the image processing device 2 to the output scanner 3.
There, it undergoes image processing such as halftone conversion and is recorded as a halftone film. Correction ii! ii image data D does not have a halftone dot pattern as shown in FIG. Moiré caused by interference with other objects does not occur.

In this embodiment, the image processing device 2 is configured as a cutout mask device. Therefore, the image expressed by the modified image data D (hereinafter referred to as the "modified image") is displayed on the graphic display 4,
A cutout mask can also be created by the operator specifying a desired contour in the corrected image using the mouse 5 or the like. At this time, since the outline of the image in the corrected image is not blurred and is clearly represented, there is also the advantage that an accurate cutout mask can be obtained.

B. Modifications The present invention is not limited to the embodiments described above, and the following modifications are also possible.

■ The original image is not limited to a halftone dot film, but may be a multicolor halftone printed matter. In this case, a color scanner is used as an input scanner for reading the original image, and the read image data is separated into color components (color-separated image data) such as Y, M, C, and so on. After the color-separated image data is obtained, steps $2 to S9 in FIG. 2 are executed for at least one of the color-separated image data, and the same effects as in the above embodiment can be obtained.

(2) In the above embodiment, a pixel block PB composed of (3×3) pixels was used, but in general, a pixel block composed of (MXN) pixels can be used (here, M and N are integers).

ffleA diagram is a pixel block PB4 of (4×4) pixels.
FIG. Further, FIG. 6B is a conceptual diagram showing image data when original image 0 is divided into pixel blocks PB4. In these figures, four pixels P at the center of one pixel block. ~P3 new data evo-ev3
is assumed to be equal to the average value e of the pixel block.

Also, new data of peripheral pixels P4 to P, 5 are (3a) to (
The interpolation setting is performed using the following equation, which is similar to equation 3h).

e, 4”mev+(1-m) a, ...(
5a) e, 5=8,6=rT1ev+ (1m)bv
−(5b)ev7=me,+(1−m)Cv
・(5c) V14 v15=meV+(1-m)
dv...(5d) e = e Here, m: constant ai of 0.5≦m≦1.0: average value v of adjacent pixel blocks In other words, new data e −e of surrounding pixels is v
4 v15 Weighting coefficients m and (1-
m> is calculated by multiplying and averaging.

It goes without saying that the weighting coefficient m can be changed as in equations (4a) to (4c) above.

FIG. 7A is a conceptual diagram showing a pixel block PB5 of (5×5) pixels. Moreover, FIG. 78 is a conceptual diagram showing image data when the original image O is divided into pixel blocks PB5. In these figures, a pixel P at the center of one pixel block. The new data evo is assumed to be equal to the average value e of the pixel block.

Also, the positions sandwiched between the center pixels of adjacent pixel blocks (that is, the upper, lower, left, and right positions shown in FIG. 7C).

New data (
In the example of the central pixel block in FIG. 7B, the data 8v1゛v3°v5゛v7゛v9゛0v11゜eeee, e, e-e) are interpolated v13 v15 v17 v24 as follows.

e, 1=m1 ev+ (1ml)av...(
6a) e, 7=m2 ev+(1-2nd) a
−(6b) e v3”” ml e v + (1−
ml ) b y ... (6C) ev18 = m
,, ev+ (1-2nd) bv-(6d) where ml: constant rT1 of 0.5≦m1≦1.0
2: Constant of 0.5≦m2≦1.0 Also, m
1≦m2, that is, according to formulas (6a) to (ed),
New data for pixels near the center of a pixel block is set to have a value close to the average value of the pixel block.

Equations (6a) to (6d) contain new data e, 19, 17.
Although only ev3゜e, 18 is shown, the data e
V5°eV9' ev13 is expressed by a formula similar to the above-mentioned data evl. Similarly, data e, ev19
v21.e is data e and data eV7. Vll
'v23 V17 eV15 is data e and data eV2oV22゜v3
・ e e is expressed by the same formula as data e V24
VlB will be applied.

On the other hand, new data of pixels that are not located between the center pixels of adjacent pixel blocks (in the example of the center pixel block in Fig. 7B, the circled data ev2, v4, v)
6・”V8・0v10・0v12・evle
e4 and e) are given for example by: e,2=m3ev17+(1-m)b
...(7) v23 Here, 3rd: constant bv2 of 0.5≦m3≦1.0
3: New data at pixel position ffP23 of pixel block PB5b Two data e, bV? on the right side of equation (7)? 7
V23 is calculated based on equations (6a) to (6d). however,
The data bV23 is specifically calculated using the following formula, which is similar to formula (6b): b = m b + (1-m) d ・=
(8) v232v 2v Therefore, new data ev12. After finding bv23, (
Using equation 1), data ev2 can be newly calculated.

Note that from equations (6b), (7), and (8), the value of new data ev2 can also be expressed as follows: e, 2 = m2 3rd ev + (1st 2nd) 3rd aV
+m < 1-3rd) bV + (1-2nd) < 1-3rd) dv (9) That is, the new data ev2 is its own i! ! ii the average value eV of the pixel block and the average value av of the adjacent pixel block.

The weighting coefficient m 2 m 3゜v (1-m) is added to b and d, respectively. It is obtained by multiplying the second (1st - 3rd) and (1 2nd) < 1 3rd) and averaging.

In this way, a pixel block can generally be composed of (MXN) pixels, but the size of one pixel block is approximately the same size as the 100% halftone dot shape of the original image. is maintained. That is, when a pixel block is composed of (MXN) pixels, if the values of M and N are increased, the length of one side of one pixel is decreased in inverse proportion to this. This means that the size of one pixel block is 100%
This is because if it becomes too large compared to the halftone dot shape, there is a problem in that the image data is over-corrected so that a portion that should originally have a steep density change has a gentle density change. Furthermore, if the size of one pixel block becomes too small compared to the 100% halftone dot shape, an image close to the halftone dot pattern of the halftone image will be expressed by the corrected image data, which will prevent the occurrence of moiré. This is because the effect becomes lower.

(2) The new data of the pixel in the center is the arithmetic average of each data of the pixel block as shown in equation (1), but other average values based on the image data of the pixel block may be used. For example, the following weighted average may be used.

Here, l is a predetermined constant of 1.0 or more.

Also, data evk (k=o
-MXN-1) may generally be interpolated according to a predetermined function f that uses representative data of adjacent pixel blocks as variables.

evk”” f (a, b, ”・,
i ) ”・(11) v V
vThe above equations (3a) to (3h) and (5a)
) ~(5d) formula, (6a) ~(6d) formula, (7)
Equations (9) and the like are specific examples of the above equation (11). Note that, as can be seen from these equations, the new data of pixels existing between the center pixels of adjacent pixel blocks is interpolated so that its value changes monotonically in the middle of the center pixels.

(Effects of the Invention) As explained above, according to the present invention, image data of an original image, which is a halftone image, is converted into new image data by positionally interpolating the average value of image data for each pixel block. Therefore, this new image data does not have the halftone dot pattern of the original image, and even if the halftone dot image is reproduced based on this new image data, it is effective in preventing the occurrence of moiré.

Furthermore, since new image data for each pixel is obtained based on the image data obtained by reading the halftone dots of the original image without blurring the original image, it is possible to obtain image data that is faithful to the original image.

Furthermore, since the image data is interpolated so that it changes continuously between the centers of adjacent pixel blocks, the gradation changes that were expressed as halftone dots in the original image can be expressed as smooth gradation changes. image data can be obtained,
This has the effect that image data that visually faithfully reproduces the original image can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of an image processing system as a device to which an embodiment of the present invention is applied, FIG. 2 is a flowchart showing the procedure of the embodiment, FIG. 3 is a conceptual diagram showing an original image, and FIG. Figures 6A and 7A are diagrams showing pixel blocks, Figures 5, 6B and 7B are conceptual diagrams showing the distribution of image data of the original image, and Figure 7C connects adjacent center pixels. It is an explanatory view of directions. IPS...Image processing system, 2...Image processing device, 0...Original picture, PB...
Pixel block, P, P-P8... pixels, a to i
...Average value, ■v "vl~"v8...New data

Claims (1)

[Claims] (1) A method of converting image data obtained by reading an original image composed of halftone dots pixel by pixel into multi-tone image data, which method includes: (a) setting a plurality of pixel blocks each consisting of a plurality of pixels; (b) For each of the plurality of pixel blocks, calculate the average value of the image data of each pixel, and (c) For each of the plurality of pixel blocks, calculate the new image data value of the central pixel. , set equal to the average value of the pixel block, and (d) change the value of new image data at each pixel other than the center of the pixel block from the center of the pixel block to the center of the adjacent pixel block. 1. A method for converting image data of a halftone dot image, characterized by setting interpolation so as to continuously change toward the .