JPH0468965A - Picture reader - Google Patents

Abstract

PURPOSE:To prevent production of moire by providing a low pass filter and an arithmetic section deciding its cut-off frequency based on the resolution of an input device, resolution of a current device, size of a dither matrix and a picture output magnification to the reader. CONSTITUTION:An arithmetic section 2 calculates a cut-off frequency based on a resolution Id of an input device, a resolution Od of an output device, a size Ds of a dither matrix and a picture output magnification Gn and gives the result to a low pass filter 1. Thus, when a picture is outputted to a display or recording device which reduces or magnified a picture having a strong periodicity such as a dot picture and outputs it as a pseudo half tone, moire caused by reflection of a dot frequency of a dot picture due to the reduced picture and moire caused by reflection of a dot frequency of a dot picture due to dither modulation are suppressed and a dot picture with high picture quality is always generated.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an image reading device that suppresses moiré that occurs when displaying pseudo halftones by reducing or enlarging a halftone image to an arbitrary magnification. It is.

[Prior Art] FIG. 5 is a block diagram showing a conventional halftone image reduction method disclosed in, for example, Japanese Patent Application Laid-Open No. 60-142670. A rate setting device (22) is a reduction target rectangle setting device that sets a reduction target rectangle, and (23) is connected to the output side of the reduction rate setting device (21) and reduction target rectangle setting device (22), and is connected to the processing unit. A dither section determination logic circuit that calculates a dither section which is a block; (24) is an image memory to which the address and size of the dither section are given from this dither section decision logic circuit (23); (25) is a dither section decision logic circuit; A reduced pixel point calculation circuit (26) receives the size and reduction rate of the dither partitioned area from the partitioned area determination logic circuit (23) and creates position information of the reduced pixel point (26).
a weighting coefficient calculating circuit which receives position information from the dithering area determination logic circuit (25) and also receives the size and reduction rate of the dithering area from the dithering area determination logic circuit (23), and calculates a weighting coefficient;
(27) is a weighting coefficient memory that stores the weighting coefficient from this weighting coefficient calculation, and (28) is a reduced pixel point calculation circuit (25
), a reduced pixel point coordinate memory that stores position information from (
29) are a large number of horizontal line shift registers in the dither section connected in parallel to each other on the output side of the image memory (24), and (30) are connected in parallel to each other on the output side of these horizontal line shift registers in the dither section (29). (31) is a matrix adder (comparator) that adds the outputs of all the vertical dot reduction pixel level calculation logic circuits (30), (32) is connected to this matrix addition. black and white determination circuit (33) connected to the output side of the pixel point coordinate memory (28) and the reduced pixel point coordinate memory (28);
is a dither matrix generation circuit connected to the input side of the matrix adder (31), and (34) is a black and white judgment circuit (32).
) connected to the output side of the reduced dither image memory, (50
) is a high-speed clock generator that generates a high-speed clock signal, (51) is a counter that is connected to the output side of this high-speed clock generator (50) and generates a clock signal, (52)
) is connected to the output side of this counter (51), the dither section determination logic circuit (23) and the dither matrix generation circuit (33).
) is connected between the input side of the counter and generates the black signal, and (53) is the dither matrix generating circuit (33).
) is connected to the input side of the clock generator.

The conventional halftone image reduction method is configured as described above, and the dither section determination logic circuit (23) is generated by the high speed clock generator (50); Synchronized with the clock signals sequentially given by (51) and (52), the reduction rate given by the reduction rate setter (21) and the processing target rectangle of the original image given by the reduction target rectangle setter (22). According to the information, the NXN dots of the pixel points on the original image corresponding to the 4×4 dot block of the reduced output pixel points are divided into blocks, and a dither segmentation area, which is a processing unit block, is calculated. The dithering area determination logic circuit (23) provides the image memory (24) with the address and dithering area size of this dithering area in the image memory (24), and also provides the dithering area size and reduction. The ratio is output to the reduced pixel point calculation circuit (25) and the weighting coefficient calculation circuit (26).

The tarok signal of the counter (52), which provides the timing for determining the dither section, is further input to a dither matrix generation circuit (33), where a two-dimensional threshold pattern corresponding to the dither section is created.

The reduced pixel point calculation circuit (25) calculates reduced pixel points R(1,1) to R(
4, 4), and the weighting coefficient calculation circuit (26
) and output to the reduced pixel point coordinate memory (28).

The weighting coefficient calculation circuit (26) calculates pixel points S(i, j) to S(i The weighting coefficients (j+n) are calculated and output to the weighting coefficient memory (27).

From the image memory (24), pixel information on the original image in the dither section (40) is stored for each horizontal line (every j to j+n lines).
and a horizontal line shift register (29) within each dither section.
It is stored for -10 days. This pixel information on the original image is synchronized with the clock signal of the counter (51) and is stored in the array S(:, j
〜jten), S(i+1.j〜j+n) ・ ・ ・
The signals are input from the horizontal line shift register (29) in each dither section to the fir dot reduction pixel level calculation logic circuit (30) in units of S (i+m, j to j+n). The reduced pixel level calculation logic circuit (30) calculates the pixel level of the reduced pixel points stored in the reduced pixel point coordinate memory (28) and the weighting coefficient memory (27) from the sequentially input pixel information on the original image. By referring to the necessary reduced pixel point coordinates and weighting coefficient information, each column of reduced pixel points (R(1,1~
4), R(2,1-4), R(3,1-4), R
Continuous pixel levels for each column (4, 1 to 4) are calculated and created, and in synchronization with the clock signal from the clock generator (53), the pixel levels for each column are sequentially added to the matrix adder (3).
1).

Further, the clock signal from the clock generator (53) is also input to the dither matrix generation circuit (33), and the threshold string of the two-dimensional threshold pattern created when generating the dither section (40) described above is applied to the clock signal. output in sync with.

The matrix adder (31) compares the successive pixel levels of the reduced pixel points of each column that are input synchronously with each threshold value of the threshold value column, and outputs the comparison result signal of each column to determine black and white. The black and white level of each column is determined and binarized in the circuit (32), and the reduced pixel point address is determined by referring to the reduced pixel point coordinate memory (28), and the reduced dither image memory (
34).

Similarly, the inside of the rectangle to be reduced is divided into dither sections (40
) and outputs the reduced pixels to the reduced dither image memory (34).

[Problems to be Solved by the Invention] In the conventional halftone image reduction method, when reducing an image with strong periodicity such as a halftone image, the halftone frequency of the halftone image due to the reduction of the 0 image is reduced. There is a problem in that moiré occurs in the output image and the image quality deteriorates due to two causes: (1) reversal of the halftone dot frequency of the halftone image due to dither modulation;

This invention was made to solve the above-mentioned problems, and it reduces or enlarges an image with strong periodicity, such as a halftone image, and transmits the image to a display or recording device that outputs it in pseudo halftones. When outputting, the moiré caused by the reversal of the halftone frequency of the halftone image due to the reduction of the 0 image and the moire caused by the reversal of the halftone dot frequency of the halftone image due to O dither modulation are suppressed, and the The object of the present invention is to obtain an image reading device that generates high-quality halftone images.

[Means for Solving the Problems] An image reading device according to the present invention is provided with a low-pass filter that limits the band of a halftone image, and a calculation section that determines the cutoff frequency of this filter.

[Function] The arithmetic unit in the present invention calculates the resolution Id of the input device.
, output device resolution ○d, dither matrix size Ds, and image output magnification Gn to sampling period Sn
and the dither matrix period Dn, and these periods Sn
, Dn and the minimum halftone dot period A min of the halftone dot image, further determine the period Ma of moire that occurs in the case of ○, and the period Mb of the moire that occurs in the case of O, and calculate these periods Na and Mb by 2.
The cutoff frequency to be set at ^1ain is calculated and given to the low-pass filter. In addition, the minimum halftone dot period A
Ring in is "R3: Halftone Expression 3-1 = Halftone Formation" by Jun Ishii (Journal of the Institute of Image Electronics Engineers, Vol. 10, No. 5, No. 38
From Fig. 4 quoted from Fig-3 on page 1, Am1n = 25.4/ 133-〇 in the case of monochrome printing,
1.91 (lam), and for color printing
m1n=25.4/200=0.127<ms).

[Example] An embodiment of the present invention will be described below with reference to the drawings.

In FIG. 1 showing a block diagram of an embodiment of the present invention, (1) is a low-pass filter that limits the passband of a halftone image signal input from an input device (not shown);
(2) is a calculation unit that calculates the cutoff frequency Fc according to the resolution Id of the input device, the resolution Od of the output device (not shown), the output magnification of the image, such as the reduction ratio Gn, and the size Ds of the dither matrix.

FIG. 2 is a circuit diagram showing a specific example of the configuration of the low-pass filter (1) shown in FIG. 1. In the figure (201a) ~
(201b) is a switch, (202) is a switch (20
A resistor group (203) is connected between switches (201c) and (201d), and is connected between switches (201c) and (201d).
A capacitor group (204) is an operational amplifier connected to switches (201b) and (201d), which are composed of capacitors 01 to co having different values.

Next, the operation of the embodiment shown in FIG. 1 will be explained.

When a halftone image is reduced to an arbitrary size and output, interference fringes called moiré often appear in the output image, resulting in a deterioration in image quality. In particular, when the output device displays images in pseudo-halftones, moiré occurs significantly.

There are two reasons why this moire occurs:

That is, the halftone synchronization An of the input halftone image and the sampling period of this halftone image 5n=110d−Gn
and the halftone dot period An of the halftone dot image inputted as 0 and the period Dn of the dither matrix = 110d-D
This is a case that occurs between s.

The moiré periods Ma and Mb of (1) and (2) are given by the following equations 0 and 0, respectively.

Ma=1/II/^n-1/Sn l
■Mb=1/+1/^n-1/Dn1 ■
Theoretically, according to the sampling theorem, O moiré and O moire occur when the sampling period Sn and dither matrix period Dn become large and the relationships of the following equations 0 and 0 are established, respectively.

1 ■ Sn =, An Dn = Yoshi An ■ Moire occurs when the moiré periods Ma and Mb become equal to or greater than An, according to the 0 formula where the 0 formula is substituted into the 0 formula, and the 0 formula where the 0 formula is substituted into the 0 formula.

Ma=1/II/80-2/^n1=An
■Mb=1/+1/^n-2/^nI=An
■Therefore, for any sampling period Sn and any dither matrix period Dn, the moiré periods Ma, M
Moiré can be suppressed by making b equal to or less than the minimum period Am1n of the halftone image.

A win> 1/11/^n-1/Sn1
■A akin> 1/II/^n-1/Dn
l ■Therefore, by cutting the high frequency components of the halftone image using a low-pass filter (1), the
The occurrence of moiré can be suppressed by satisfying the relationships of the formula and the formula 0.

However, if the cutoff frequency Fc of the low-pass filter (1) is set by considering only the effect of suppressing moire, there is a risk that information about the image itself of the halftone dot image will be lost, and as a result, it will not lead to high image quality.

The halftone dot image is expressed using its halftone period An. Therefore, from the sampling theorem, it can be estimated that the shortest period Z win of the image itself is about half the halftone period An.

Therefore, in the present invention, in order to prevent the moiré of ■ and the moiré of 0 from returning to the frequency band FZ of the halftone image itself (FZ x Zsin = 2.Am1n), in the calculation unit (2), From the resolution Id of the input device, the resolution Od of the output device, the size DS of the dither matrix, and the image output magnification Gn, calculate the cutoff frequency Fc and set the moiré periods Ma and Mb to 2.
Suppress to A sin. In addition, in order to avoid losing the information of the halftone image itself, the maximum cutoff frequency F
s is the maximum halftone frequency F a wax (-1/^win)
Set to 1/2 of

By controlling the cutoff frequency Fc of the low-pass filter (1) in this way, a high-quality reduced halftone image that suppresses moiré and does not lose the sharpness of the input halftone image can be obtained using any pseudo-halftone device. be able to.

Hereinafter, the calculation method of the calculation unit (2) will be explained with reference to the flowchart of FIG.

In step (Sl), from FIG. 4, the minimum halftone dot period Am1
Determining n 6 For example, from FIG. 4, since the halftone dot pitch of a monochromatic printed matter is at most 133 screen lines/inch, it can be seen from equation 0 that the minimum halftone dot period A win is 0.191 mm.

Am1n=25.4/133=0.191mm ■In step (S2), the maximum cutoff frequency Fs1 is calculated from the minimum halftone dot period A min found in step (Sl).
/(2·Am1n) is obtained as described above.

In step (S3), the input device resolution 1d, the output device resolution Od, the dither matrix size D
s, obtain the image output magnification Gn.

In step (S4), the sampling period 5n=110d-Gn is obtained from Od and Gn obtained in step (S3), and the dither matrix period Dn=
Find 110d-Ds.

In step (S5), A obtained in step (Sl)
From m i n and Sn and Dn obtained in step (S4), the moire period Ma1 /
l l/^win-1/Snl and Mb=1/11/
^win1/Dn l is derived. The moiré periods Ma and Mb derived in this way are then the minimum halftone period A+l.
It is determined whether or not it is twice or more l1n.

If Ma≧2·All1n and Mb≧2−Am1n, the process proceeds from step (S6) to step (S7), where it is further determined whether Ma is larger than Mb.

If Ma>Mb, proceed to step (S8) and set Ma to 2.
・Image halftone period A1 that can be suppressed to below A win is 2 ・A11n=1/(1/^1-17S
n), proceed to step (S9) to obtain the cutoff frequency F1=1/A, 1. Next, check whether the cutoff frequency F1 obtained in this way is greater than the maximum cutoff frequency Fs. It is determined in step (SIO>) whether or not the case is true. If no, Fc is set to Fl, but if yes, Fc is set to Fs. In step (S7), Ma>M
If it is found that Ma is not b, that is, Ma<Mb, the process proceeds to step (Sll) and the image halftone period A2 that can suppress Mb to 2.Am1n or less is set to 2.A.
After finding m1n=1/(1/^2-1/Sn), proceed to step (S12) and calculate the cutoff frequency F2.
= 1/A2 is calculated. Next, it is determined in step (S13) whether the cutoff frequency F2 obtained in this way is greater than the maximum cutoff frequency Fs. If no, Fc is set to F2, but if YES Then change Fc to Fs.

Also, Ma≧2・As1n and M b < 2・A+
If un, step (S14) to step (S1
After proceeding to step 5) and obtaining the image dot period A1 that can suppress Ma to 2·Aw+in or less in the same manner as in step (S8), proceeding to step (S16) and setting the cutoff frequency F1 in step ( If it is found in step (S17) that the cutoff frequency F1 obtained in this way is not larger than the maximum cutoff frequency Fs, Fc is set to Fl, but if it is larger, Fc is set to Fl. Change Fc to Fs.

Also, M a < 2 ・Am1n and Mb≧2 ・A
In the case of men, step (S18) to step (S
After proceeding to step 19) and obtaining the image dot period A2 that can suppress Mb to 2·A win or less in the same manner as in step (Sit), proceeding to step (S20) and step-up the cutoff frequency F2. If it is found in step (S21) that the cutoff frequency F2 obtained in this way is not larger than the maximum cutoff frequency Fs, Fc is set to F2.
If it is larger, change Fc to Fs.

After I&, M a < 2 ・Am1n and Mb < 2-
In the case of logic in, there is no need to suppress the moiré periods Ma and Mb, so the cutoff frequency Fc of the low-pass filter (1) may be infinite.

In the above embodiment, the input halftone dot image was explained only for monochrome printed matter, but if the minimum halftone dot period A + ain in the case of color printing is determined using FIG. effective.

Further, when the pitch of the input halftone dot image is more limited, it is more effective to newly set the minimum halftone dot period A win based on the limited halftone dot image pitch.

[Effects of the Invention] As described above, the present invention includes a low-pass filter and an arithmetic unit that determines its cutoff frequency from the resolution of the input device, the resolution of the output device, the size of the dither matrix, and the image output magnification. As a result, moiré caused by changing the image output magnification or dither matrix is suppressed, and a halftone image can be more clearly reproduced on any pseudo-halftone output device at any magnification.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a circuit diagram showing a specific example of the low-pass filter in FIG. 1, and FIG. 3 is a calculation method of the calculation section in FIG. 1. FIG. 4 is a diagram showing the periodic distribution of halftone dots used in printed matter, FIG. 5 is a block diagram showing the prior art, and FIG. 6 is a diagram showing a dither section. In the figure, (1) is a low-pass filter, and (2) is a calculation section. In addition, in the figures, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Claims] (1) In an image reading device that reads an image with an input device, performs pseudo gradation processing such as dithering processing, and then outputs the reading result to an output device, the passband of the image input from the input device is limited. and an arithmetic unit that determines a cutoff frequency of the low-pass filter, and the cutoff frequency is determined from the resolution of the input device, the resolution of the output device, the size of the dither matrix, and the output magnification of the image. An image reading device characterized by: