JPH08274991A - Generating method for gradation increment data in dot image and image processing method using the method - Google Patents

Abstract

PURPOSE: To generate a dot image with high quality by generating threshold level data when multi-value image data are converted into binary data based on gradation increment data and correcting an error in the binarization. CONSTITUTION: An output device 20 generates a test pattern on a film F or a printed matter P based on test data from a test pattern data file 24. Then a density measurement device 26 measures each test pattern density to obtain gradation increment data with a density difference when a noted picture element is set/cleared with respect to same black/white dot state of a peripheral picture element as a correction value. A density D1/D0 when the peripheral picture element is a black dot/or a white dot, is measured by the measurement device 26, a threshold level matrix generator 22 obtains the difference between the density sets D1 and D0 as a network % difference A for each test pattern and generates a dot screen based on the obtained correction amount. Thus, a microscopic gradation increment owing to the effect of surrounding picture elements is corrected and the dot image on the film F and the printed matter P is formed as a gradation image with high accuracy.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of creating gradation increment data in a halftone dot image for preparing effective gradation increment data when converting multi-valued image data into binary image data, and a method therefor. The present invention relates to an image processing method.

[0002]

2. Description of the Related Art As represented by a laser printer,
Converting multi-valued image data into binary image data using predetermined threshold data, and based on the obtained binary image data,
An output device that forms a halftone dot image having a gradation on an output medium by controlling on / off of a laser beam or the like is widely used.

In this case, as a method of expressing gradation, an amplitude modulation method (also referred to as "AM method") in which the number of dots (pixels) per unit area is modulated according to multi-valued image data,
The density of dots can be roughly classified into a frequency modulation method (also referred to as “FM method”) that modulates dot density according to multi-valued image data. In addition, in the frequency modulation method, when converting multi-valued image data into binary image data using predetermined threshold value data, an error that occurs between the multi-valued image data and the binary image data can be avoided. There are an error diffusion method of generating binary image data having a desired density by diffusing into pixels, a threshold method of converting multivalued image data into binary image data using a threshold set as a dither matrix, and the like. is there.

By the way, in an output device such as a laser printer, the laser beam for forming the halftone image has a circular or elliptical beam spot shape, and its intensity distribution is a normal distribution shape. Therefore, it is difficult to represent an accurate gradation because there is bleeding in the recording portion with respect to the peripheral pixels. Further, since the output medium itself also has a dot gain such as bleeding in a blackened portion or ink bleeding, there is a problem in that an error occurs in gradation.

Therefore, for example, when the multi-valued image data is converted into binary image data by using the error diffusion method, the dot generation pattern (binarization pattern) already determined in the peripheral pixels of the pixel concerned. According to the above, the error due to binarization is corrected using the correction value corresponding to the dot generation pattern stored in the pattern table memory,
There is a method in which an error diffusion process is performed using the corrected error as a new error (see Japanese Patent Laid-Open No. 5-3540).

In this case, a correction value can be set accurately for an ideal dot generation pattern that can be patterned, and thus a good image can be reproduced, but the beam spot shape However, it cannot be applied to all output devices and output media having output characteristics dependent on the above and various dot gain characteristics.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in order to solve the above-mentioned problems, and it is possible to create highly accurate gradation increment data according to the characteristics of an output device or an output medium. Provided is a gradation correction data creation method for a halftone dot image, which enables the creation of a high quality halftone dot image by correcting the influence of dot gain etc. by using this gradation increment data, and an image processing method using the same. The purpose is to do.

[0008]

In order to achieve the above object, the present invention provides an on / off state of a target pixel on a desired output medium when forming a halftone image on the desired output medium. After forming a plurality of test patterns in which the surrounding pixels are turned on / off, the density or halftone dot area ratio of each test pattern is measured, and when the surrounding pixels are in the same on / off state The density difference between the on-state and off-state of the pixel of interest or the difference in halftone dot area ratio,
It is characterized in that gradation increment data is created from the density difference or the dot area ratio difference.

According to the present invention, binary image data is generated by comparing multi-valued image data with threshold data,
In an image processing method for forming a halftone image on a desired output medium based on value image data, in the desired output medium, surrounding pixels are turned on / off with respect to an on / off state of a pixel of interest. After forming a plurality of test patterns, the density or halftone dot area ratio of each test pattern is measured, and the density difference between the ON state and the OFF state of the target pixel when the surrounding pixels are in the same ON / OFF state. Alternatively, it is characterized in that the difference in halftone dot area ratio is obtained, the gradation increment data is created from the density difference or the halftone dot area ratio difference, and then the binary image data is adjusted based on the gradation increment data. .

[0010]

In the present invention, the test pattern of the gradation image is formed by using the actually used output device or output medium,
The density or halftone dot area ratio of the test pattern is measured, and effective gradation increment data is obtained from the difference from the ideal density or ideal halftone dot area ratio. Then, by converting the multi-valued image data into binary image data using the gradation increment data, it is possible to create a highly accurate gradation image in consideration of the gradation increment due to the surrounding pixels.

[0011]

1 shows a system to which the method according to the invention is applied. In this system, first, a procedure for creating the printed matter P will be described.

The multi-valued image signal I supplied from the external device to the raster image processor (RIP) 10 is selected in the comparator 12 by the selecting means 14 in accordance with the output conditions such as the screen frequency and the output medium, and the threshold matrix storage section. It is compared with the threshold signal TH output from 16 and converted into a binary image signal B. The threshold signal TH is
The threshold value matrix creating device 22 creates the threshold value matrix creating device 22 as described later.
May be integral with the RIP 10. Then, said 2
The value image signal B is supplied to the output device 20, and, for example, a negative type halftone dot image is formed on the film F by turning on / off the laser beam. The film F on which the halftone image is formed corresponding to each color of YMCK is made into a positive type film original plate through the returning process, and then a plate is prepared from the film, and the plate is set on the printing machine. , A desired printed matter P on a predetermined printing paper which is a recording medium
Is created.

Next, when the film F or the printed matter P is created as described above, the beam spot shape of the laser beam output from the output device 20, the dot gain of the output medium such as the film F, the printed matter P, etc. A method of creating effective gradation increment data for correcting a microscopic gradation increment that occurs depending on the state of surrounding pixels will be described.

First, as shown in FIG.
Use RIP10 test pattern data file
Based on the test data T supplied from 24, shown in FIG.
3x3 on the film F or the printed matter P.
Create a test pattern composed of pixels. Soshi
The density of each of the test patterns is measured using the density measuring device 26.
Measurement, and as shown in FIG.₁, Q₂,
Q₃, Q_FourThe same on (black dot) / off (white dot) state
In contrast, the density when the target pixel Q is off and the density when it is on
Based on the difference in density from the
(Q₁, Q₂, Q ₃, Q_Four) As. Note that FIG.
, The target pixel Q and the peripheral pixel Q₁, Q₂,
Q₃, Q_FourIs 0 if it is off, 1 if it is on
Then, [Q₁Q₂Q ₃Q_Four-Q] = [0000-1]
It shall be expressed as a sea urchin.

[0015] Therefore, is measured using a density measuring instrument 26 concentrations D1 and _{[Q 1 Q 2 Q 3 Q} 4 -0] concentration D0 of _{[Q 1 Q 2 Q 3 Q} 4 -1]. Next, in the threshold matrix creating device 22, the difference between the densities D1 and D0 is obtained as the halftone dot difference Δ (%) between the test patterns. That is,
The base density of the film F or the printed material P before the image recording is Db, and the maximum density when the whole is blackened is Dma.
If x, the halftone dot difference Δ (%) is

[0016]

[Equation 1]

Can be obtained as In this case, since the ideal halftone dot difference R without the dot gain in the test pattern shown in FIG. 2 is R = 100/9 (%) (2), the correction amount Cor (Q ₁ , Q ₂ , Q ₃ , Q ₄ )
Can be obtained as Cor (Q ₁ , Q ₂ , Q ₃ , Q ₄ ) = Δ / R−1 (3).

Next, a method of creating an FM type threshold matrix (mesh screen) using the correction amount Cor (Q ₁ , Q ₂ , Q ₃ , Q ₄ ) obtained as described above will be described. . In the following description, the thresholds of 1 to 25 are set, but it goes without saying that the present invention can be extended to the case of setting thresholds of 26 or more.

Therefore, a method of creating a threshold matrix will be described with reference to the flow charts shown in FIGS.
In this embodiment, the visual frequency characteristic (MT
F) is Fourier-transformed to obtain the point spread function shown in FIG. 7, and the threshold is set so as to minimize the variation in the density distribution of the image obtained by converting the density distribution of the original image using the point spread function. By setting a matrix, the graininess can be made visually inconspicuous, and by arranging the threshold values in consideration of another threshold value matrix set adjacent to the threshold value matrix, a rule by repeating the threshold value matrix is set. It is possible to eliminate the appearance of dynamic patterns.

[0020] First, as shown in FIG. 8, threshold doors with array T _5, the threshold phased array T ₅ threshold phased array T ₁ of the periphery of the same configuration as the through T ₄ is the desired threshold matrix , T _{6 to} T ₉ are defined. In addition, each of the threshold arrays T _k (k =
1 to 9) 25 threshold values t (P _kl ) (k = 1 to 1)
9, l = 1 to 25, t = 1 to 25) are defined for each pixel point P _kl (step S200). In addition, each pixel point P _kl
Let x _kl and y _kl be the x and y coordinates of x.

After all threshold values t (P _kl ) of each threshold array T _k are set to 0 (step S202), two pixel points P _k1 and P _{k2 which} are appropriately separated from each threshold array T _k. Is selected, and the threshold value t (P
_k1 ) = 1 and t ( _Pk2 ) = 2 are set (step S20).
4, see FIG. 8).

Next, the pixel point P _kl for setting the threshold value t (P _kl ) from 3 to 25 is obtained.

First, z = 2 is set (step S20).
6), the correction amount Cor (Q ₁ , Q ₂ , Q obtained by the equation (3) for the weighting filter f _kl (P _kz ) shown in FIG.
₃ , Q ₄ ) and corrected weighting filter f
_kl (P _kz ) is calculated (step S207). The weighting filter f _kl (P _kz ) represents the distribution characteristic of the point spread function shown in FIG. 7 as a two-dimensional density distribution composed of a 5 × 5 matrix.

That is, the correction amount Cor (Q₁,
Q₂, Q₃, Q_Four) Is a threshold value t set as an initial value
(P_k1) = 1, threshold t (P_k2) = 2
Filter f _kl(P_kz). In this case, the threshold t
(P_k1), Threshold t (P_k2), As shown in FIG.
Since the peripheral pixels are 0, the threshold value t (P_k1), Threshold t
(P _k2) Only the pixel corresponding to the film F or printed matter
Dot gain correction amount C for blackening on P
or (Q₁, Q₂, Q₃, Q_Four) Is the test pattern of FIG.
Obtained from [0000-0] and [0000-1]
Correction amount Cor (0, 0, 0, 0, 1). There
Then, the correction amount Cor (0, 0, 0, 0, 1) is shown in FIG.
Weighting filter f shown_kl(P_kz)
Weighting considering the gradation increment due to dot gain, etc.
Filter f_kl(P_kz) Is obtained.

Next, the corrected weighting filter f
_kl (P _kz ) is applied to the threshold array T _k of FIG. 8 to obtain the weighted threshold array T _k having the concentration distribution D _kl shown in FIG. 10 (step S2).
08). That is, the weighting filter f shown in FIG.
_kl (P _kz ) is the threshold value t (P of the threshold array T _k
kl ) is applied around the pixel point P _kl for which it is set, and by adding them, the density distribution D _kl shown in FIG.
Can be obtained. In this case, it is possible to reduce the visually noticeable graininess by sequentially setting the threshold value t (P _kl ) so that the density distribution D _kl becomes uniform.

Therefore, z = 3, the minimum value D of the density distribution D _kl
min = 99999, the random number RND = 0, i = 1 and then (step S210, S214, S216, S218) , if the threshold t (P _5i) is 0 (step S222), density distribution D _5i of the pixel point P _5i And the minimum value D _min are compared (steps S224 and S230). In this case, when i = 1, the minimum value D _min is 99999 (step S214), so D _min = D _5i is set (step S226), and x _5z = x _5i and y _5z = y _5i are set (step S226). Step S228). Then, i is updated (step S21
8) Then, the density distribution D _5i is again compared with the minimum value D _min (steps S224 and S230).

Here, as long as D _5i <D _min (step S224), the coordinates x _5z and y _5z for setting the threshold value t (P _5i ) = 3 (z = 3) are sequentially updated. That is, by this process, already threshold t (P _kl) concentration than any pixel point P _kl are set distribution D _5i is lower pixel points P _kl
It is possible to obtain the coordinates x _5z and y _5z of.

On the other hand, in step S230, D_5i=
D_minIf it is determined that the
To the threshold t (P_kl) Set pixel point P_klConcentration of
Cloth D _5iPixel points P where_5iThere are two or more
(For example, as shown in FIG. 10, the threshold value t (P_5i) = 3
Concentration distribution P for which is set_5iPixel points P of_5iIs 3
There are two points, A and 3B. ). In this case, 0 or 1
A random number NEWWRND that takes a value of deviation is obtained (step S
232), in step S214 with this random number NEWRND
The set random number RND is compared (step S23
4), if NEWRND> RND, then RND = NEWR
ND (step S236), coordinates obtained later
x_5i, Y_5iIs a threshold t (P_5i) = 3 (z = 3) is set
Coordinate x_5z, Y_5z(Step S238). Also, N
If EWRND ≤ RND, the coordinates obtained first
x_5i, Y_5iIs a threshold t (P_5i) = 3 (z = 3) is set
Coordinate x_5z, Y_5zAnd As a result, 3A shown in FIG.
One of 3B is the threshold value t (P_5i) = 3 (z = 3)
Coordinate x to set_5z, Y_5zRandomly selected as
Will be.

In this way, the density distribution D shown in FIG.
_The pixel point P _5Z with the smallest density can be extracted from _kl . Therefore, the threshold value t (P _kz ) is set for the pixel point P _{kz of} each threshold array T _k including the pixel point P _5Z.
= 3 (z = 3) is set (steps S220 and S24).
0).

Similarly, the processing from step S207 is repeated, and each pixel point P _kl for which the threshold value t (P _kl ) is set.
By correcting the weighting filter f _kl (P _kz ) in consideration of the dot gain and setting the threshold value t (P _kl ) = 4 to 25 so that all the pixel points P in the threshold array T _k are corrected.
_A threshold t (P _kl ) is set for _kl (step S2
12).

From the threshold array T _k set as described above, only the central threshold array T ₅ is extracted and set as a threshold matrix in the threshold matrix storage unit 16 shown in FIG. The threshold matrix is created according to the output device 20 used, the film F as the output medium, and the printed matter P, and is set in the threshold matrix storage unit 16.

Therefore, in the RIP 10, the selecting means 14 selects a desired threshold value matrix from the threshold value matrix storage section 16 and supplies the threshold value signal TH (corresponding to the threshold value t (P _5l )) to the comparator 12. The binary image signal B is generated by comparing with the binary image signal I. And the above 2
The value image signal is output as a halftone image on the film F by the output device 20, or the printed matter P is created from the film F.

In this case, the threshold matrix obtained as described above takes into consideration the characteristics of the output device due to the beam spot shape of the laser beam, the dot gain of the output medium, etc. Since the tonal increment is corrected, the halftone image on the film F or the printed matter P is formed as an extremely accurate gradation image. Further, since the threshold value is arranged so as to reduce the graininess of the visually noticeable image, a good halftone image can be obtained. Further, since the threshold values are arranged in consideration of other threshold value matrixes arranged in the surroundings, a regular pattern due to the repetition of the threshold value matrix does not appear on the halftone dot image, which is more preferable. Images can be obtained.

In the above-described embodiment, the gradation correction data is obtained by measuring the density of the test pattern, but the gradation correction data is obtained by measuring the halftone dot area ratio of the test pattern. You may

Further, in the above-mentioned embodiment, when the threshold matrix is created, the microscopic gradation increment due to the influence of the surrounding pixels is corrected, but the multi-valued image signal is compared with the predetermined threshold signal. In the error diffusion method for creating a binary image signal by considering the error due to the binarization of the obtained binary image signal for the binarization of the next multi-valued image signal,
The error may be corrected with the gradation increment data based on the on / off pattern of the surrounding pixels which has already been determined. As a result, it is possible to form an image with highly accurate gradation even in the binarization processing by the error diffusion method.

[0036]

As described above, according to the present invention, a test pattern is created using a desired output device or a desired output medium, so that the output characteristics and the dot gain of the output device depending on the beam spot shape of recording and the like can be obtained. Corresponding highly accurate gradation increment data can be obtained. Therefore, by using the gradation increment data to create threshold data when converting multi-valued image data into binary image data, or by correcting the binarization error in the error diffusion processing, the surrounding pixels are It is possible to create an extremely high quality halftone image in consideration.

[Brief description of drawings]

FIG. 1 is a configuration block diagram of a system to which a method of the present invention is applied.

FIG. 2 is an explanatory diagram of a test pattern for obtaining gradation increment data.

FIG. 3 is an explanatory diagram of a target pixel of a test pattern and its surrounding pixels.

FIG. 4 is a processing flowchart for creating a threshold matrix.

FIG. 5 is a processing flowchart for creating a threshold matrix.

FIG. 6 is an explanatory diagram of visual characteristics.

FIG. 7 is an explanatory diagram of a point spread function.

FIG. 8 is an explanatory diagram of an initial value of a threshold array and threshold values set therein.

FIG. 9 is an explanatory diagram of a weighting filter.

FIG. 10 is an explanatory diagram of the density distribution when a weighting filter is applied to the threshold array.

FIG. 11 is an explanatory diagram in the case where a threshold value of 3 is set for the threshold array.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 ... RIP 12 ... Comparator 14 ... Selection means 16 ... Threshold matrix storage unit 20 ... Output device 22 ... Threshold matrix creation device 24 ... Test pattern data file 26 ... Density measuring device

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] May 17, 1995

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0014

[Correction method] Change

[Correction content]

First, as shown in FIG.
Use RIP10 test pattern data file
Based on the test data T supplied from 24, shown in FIG.
3x3 on the film F or the printed matter P.
Create a test pattern composed of pixels. Soshi
The density of each of the test patterns is measured using the density measuring device 26.
Measurement, and as shown in FIG.₁, Q₂,
Q₃, Q_FourThe same on (black dot) / off (white dot) state
In contrast, the density when the target pixel Q is off and the density when it is on
Based on the difference in density from the
(Q₁, Q₂, Q ₃, Q_Four) As.However, the figure
Pixels Q and Q among 9 pixels of 3₁, Q₂, Q ₃4 strokes excluding
The on / off state of the element is constant.In addition, in FIG.
The target pixel Q and the peripheral pixels Q₁, Q₂, Q₃, Q_Four
Is 0 when it is off, 1 when it is on, and [Q₁Q
₂Q₃Q_Four-Q] = what is expressed as [0000-1]
And

[Procedure Amendment 2]

[Document name to be amended] Statement

[Name of item to be corrected] 0023

[Correction method] Change

[Correction content]

First, z = 2 is set (step S20).
6), the weighting filter f shown in FIG._kl(P_kz) To
hand,Correction amount Cor (Q₁, Q₂,
Q₃, Q _Four)Based on 1 + Cor (Q ₁ , Q ₂ , Q ₃ ,
Q ₄₎ And the weighting filter f corrected by
_kl(P_kz) Is obtained (step S207). In addition, the above
Weighting filter f_kl(P_kz) Is shown in FIG.like,
Corresponds to visual responseThe distribution characteristic of the point spread function of 5 × 5
As a two-dimensional density distribution consisting of a matrixWhen applied
More advantageous.

[Procedure 3]

[Document name to be amended] Statement

[Name of item to be corrected] 0024

[Correction method] Change

[Correction content]

That is, the correction amount Cor (Q₁,
Q₂, Q₃, Q_Four) Is a threshold value t set as an initial value
(P_k1) = 1, threshold t (P_k2) = 2
Filter f _kl(P_kz). In this case, the threshold t
(P_k1), Threshold t (P_k2), As shown in FIG.
Since the peripheral pixels are 0, the threshold value t (P_k1), Threshold t
(P _k2) Only the pixel corresponding to the film F or printed matter
Dot gain correction amount C for blackening on P
or (Q₁, Q₂, Q₃, Q_Four) Is the test pattern of FIG.
Obtained from [0000-0] and [0000-1]
Correction amount Cor (0, 0, 0, 0,0). There
so,1+Cor (0, 0, 0, 0,0) Is the weight shown in FIG.
Finding filter f_kl(P_kz)
Weighted fill that considers the gradation increment due to gain
F_kl(P_kz) Is obtained.

[Procedure amendment 4]

[Document name to be amended] Statement

[Name of item to be corrected] 0025

[Correction method] Change

[Correction content]

Next, the corrected weighting filter f
_kl (P _kz ) is _{stored in} the threshold array T _k of FIG.
1 is cumulatively added around the position of the threshold value determined in FIG.
The density distribution D _kl visually perceived as 0 is obtained (step S208). That is, the weighting filter f _kl (P _kz ) shown in FIG. 9 is caused to act around the pixel point P _{kl for} which the threshold value t (P _kl ) of the threshold array T _k is set, and these are added, The concentration distribution D _kl shown in FIG. 10 can be obtained. In this case, it is possible to reduce the visually noticeable graininess by sequentially setting the threshold value t (P _kl ) so that the density distribution D _kl becomes uniform.

[Procedure Amendment 5]

[Document name to be corrected] Drawing

[Name of item to be corrected] Fig. 4

[Correction method] Change

[Correction content]

[Figure 4]

Claims (4)

[Claims] 1. When forming a halftone image on a desired output medium, a plurality of test patterns in which the surrounding pixels are turned on / off with respect to the on / off state of a target pixel on the desired output medium. After forming the test pattern, the density or halftone dot area ratio of each test pattern is measured, and the density difference or halftone dot area between the ON state and the OFF state of the target pixel when the surrounding pixels are in the same ON / OFF state. A method of creating gradation increment data in a halftone image, wherein a gradation difference is obtained and gradation gradation data is prepared from the density difference or the dot area ratio difference. 2. An image processing method for generating binary image data by comparing multi-valued image data with threshold value data and forming a halftone image on a desired output medium based on the binary image data, After forming a plurality of test patterns in which the surrounding pixels are turned on / off with respect to the on / off state of the target pixel on the desired output medium, the density or halftone dot area ratio of each test pattern is measured. , A density difference between the ON state and the OFF state of the pixel of interest or a halftone dot area ratio difference when the surrounding pixels are in the same ON / OFF state, and a gradation increment is obtained from the density difference or the halftone dot area ratio difference. An image processing method characterized in that after the data is created, the binary image data is adjusted based on the gradation increment data. 3. The image processing method according to claim 2, wherein the arrangement of the threshold data forming the threshold matrix is set based on the gradation increment data. 4. The image processing method according to claim 2, wherein the gradation increment data is correction data in the binary image data generated by error diffusion processing.