JP2996523B2 - Image processing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
Input value image data, and
The present invention relates to an image processing method for converting into binary data by a difference diffusion method .

[0002]

2. Description of the Related Art Conventionally, a laser beam printer (LB) is used.
P) and printers of the ink jet system often use a binary recording system of "whether to output or not" the printing dot. For this reason, in a copying apparatus to which this is applied, in order to copy an image having a halftone density, such as a photograph or a halftone original, the read halftone image data is simulated by the image processing circuit. Has been performed to reproduce.

As one of such pseudo halftone processing methods, there is a so-called "dither method" which is widely used at present. The dither method has an advantage that the above-described pseudo halftone can be performed at low cost because the hardware configuration is simple. However, this method also has the following disadvantages.

When the original is a halftone image such as a print, a periodic stripe pattern (moire) occurs in the copied image, and the image is deteriorated. If the original contains line drawings, characters, etc., the line reproducibility is poor and the image is degraded. In order to cope with the drawback, the smoothing process (spatial filtering process) is performed on the read halftone image data.

[0005] The disadvantage of [1] is addressed by edge enhancement. When any of these methods is used, it is difficult to obtain an image with good reproducibility for all kinds of images such as photographs, halftone images, line drawings, and characters. Also,
Due to such processing, the circuit scale is large and complicated, and the original advantage of the dither method is lost.

[0006] From such a background, there is a method called "error diffusion method" as a pseudo halftone processing method that has recently attracted attention. The error diffusion method is a method characterized in that a density error at the time of binarization is diffused to peripheral pixels so that the density can be preserved. W Floyd and L.St
einberg “An Adaptive Algorithm for Spatial Gray S
cale "There is SID 75 Digest.

[0007]

The error diffusion method has been widely used because it has better image reproducibility than the dither method and the like, but has the following disadvantages. In some cases, dots are connected in a halftone portion, and a band is formed, which adversely affects the visual sense.

Since the printing of dots is delayed in the highlight image area, the image is distorted in a direction in which the error is diffused, that is, a so-called “brush” occurs, which is a net for image reproduction. As for the method, various improvement methods have been proposed, such as devising a distribution matrix of error data and a method of correlating with an adjacent pixel.

[0009] Regarding the above, there has been proposed a method of observing the printing dots around the pixel of interest, for example, to make the retouching development inconspicuous. As for (2), the same occurs in the dark image area as well as in the highlight image area (clearing the area without dots). However, since the print dots usually have a large dot diameter so as to intersect adjacent pixels, Normally, there is no problem because the part that is not hit is crushed.

[0010] The countermeasure against the drawback is to consider processing of one color, that is, a single color, and does not solve the following problem in the case of color image processing in which three or four colors are superimposed. Was. In other words, with regard to the above-mentioned clearing, when multiple colors are both highlighted image parts,
Since there is no correlation between the dots of each color, depending on the image, if the dots of each color are concentrated or dispersed, image unevenness may occur, which may have an undesirable effect on the image quality.

FIG. 9 shows an example of two-color printing by a conventional method. The diagonally right and left circles represent dots of different colors. In FIG. 9, dots are arranged on the same line because of the image data of the same density level for both colors, and there are portions where the two colors are overlapped and printed like dots indicated by *. Therefore, when the image is viewed from a distance, the streak is conspicuous, deteriorating the image quality.

[0012]

SUMMARY OF THE INVENTION The present invention relates to the above-mentioned prior art.
It eliminates the disadvantages of surgery, and is a multi-valued image data of the first color.
A threshold used for binarizing the data and a multi-valued image of the second color.
Different thresholds for binarizing image data
Is the dot after each binarization in the highlight part
Can be suppressed, and the print density can be determined and
To set the threshold change cycle,
To provide an image processing device that can improve reproducibility
And The following configuration is provided as one means for achieving such an object . That is, the first and second multi-valued image data of a plurality of colors are
And input each multi-valued image data by the error diffusion method.
In an image processing apparatus for converting into binary data,
A first threshold value that periodically varies the multivalued image data of the color
Of binarizing the binary data of the first color by comparing
1 and a multi-level image data of the second color are periodically
And the ratio of the first threshold value to the second threshold value different from the first threshold value
A second process of binarizing binary data of a second color by comparison
Binarization circuit and binarization from first and second binarization circuits
The print density is determined based on the already-existing binary data.
Setting the first and second threshold values based on the determination result
Setting means, wherein the setting means determines that the printing density is high.
If it is set, set a short threshold value of the fluctuation cycle and
If it is determined to be low, set a threshold with a long fluctuation cycle.
And features.

[0013]

In the above arrangement, the multivalued image data of the first color
Threshold used for binarizing data and a multi-valued image of a second color
Different thresholds used for binarizing data
Is the dot after each binarization in the highlight part
Can be suppressed, and the print density can be determined and
To set the threshold change cycle,
Reproducibility can be improved .

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment according to the present invention will be described below in detail with reference to the drawings.

[0015]

FIG. 1 is a block diagram showing the structure of a first embodiment according to the present invention. In FIG. 1, reference numerals 1 to 4 indicate cyan, magenta, yellow, and black (C, M, Y, K).
Is an error diffusion processing unit that executes a binarization process on the pixel of interest while performing error diffusion on the multi-valued color signal of the above by the error diffusion method. The multi-valued color signals of cyan, magenta, yellow, and black (C, M, Y, K) are input to the error diffusion processing units 1 to 4, respectively, and threshold values TC,
By providing TM, TY, and TK from outside, binarized color signals CB, MB, YB, and KB are generated and output, respectively.

Since all of the error diffusion processing sections 1 to 4 have the same configuration, only the detailed configuration of the error diffusion processing section 1 for performing the binarization processing of the cyan signal is shown, and the other is omitted. The error diffusion processing unit includes an error adder 10, an error calculator 11, and a comparator 12. Error adder 10
Is a circuit block for adding an error density component (having positive and negative values) generated at the time of binarization with a predetermined matrix and a predetermined distribution ratio to an input multivalued image to be binarized. is there. For example, when an error diffusion matrix of 5 pixels × 3 lines is provided, a data holding circuit, an addition circuit, and a data delay circuit corresponding to the matrix size are provided inside.

After the error density component is added by the error adder 10, the image data is binarized by the comparator 12.
The comparator 12 outputs a logical value "1" (dot printing) if the input data is larger than the threshold value, for example, and the input data is smaller than the threshold value, or If the values are equal, a logical value "0" (dot not printed) is output (signal CB).

The error calculator 11 calculates error density data based on the values "0" and "1" of the output signal CB from the comparator 12, and distributes the data to each pixel of the error diffusion matrix at a predetermined distribution ratio. , And a circuit provided to the error adding unit 10. The threshold value generating circuits 5 to 8 are circuits that generate threshold values a, b, c, and d as shown in FIG. The waveforms of a, b, c, and d shown in FIG.
For example, the threshold values may be replaced by TC, TM, TY, and TK. By changing the threshold value for each color in this manner, a binary image as shown in FIG. 3 can be obtained.

The phase of this waveform may be shifted for each binary line. As described above, in the first embodiment, instead of checking the correlation between the printing dots for each color, the open loop control is performed so that the printing positions of the colors are not easily overlapped. FIG. 3 is an example of image reproduction in the present embodiment. In the first embodiment shown in FIG. 3, printing positions of two colors are shifted by a distance 1 from FIG. 9 which is a printing example of the conventional method. That is, the threshold value generating circuits 5 to 8
By changing the threshold value for binarization for each color,
FIG. 2 shows whether the threshold value at a specific time for each color is different from the others, and whether the multivalued data is easy to output as binary data “1” or difficult to output at the same time. Conversion unconditionally. As a result, FIG.
An image as shown in FIG. That is, when the threshold value of the binarization is high, the dot is hardly hit. On the contrary, when the threshold value is low, the dot is easily hit, and this is made different for each color. is there.

As described above, according to this embodiment,
The threshold value generation circuits 5 to 8 can change the threshold value for binarization for each color. By shifting the original print position in this way, the image quality of the highlight part can be particularly improved.

[0021]

[Second Embodiment] The above description is based on the assumption that the threshold value generation circuits 5 to 8 change the threshold value for binarization for each color, and shift the printing positions of the two colors by a distance l, thereby making the highlight portion. The example in which the image quality is improved has been described. However, the present invention is not limited to the above-described example. Even in a highlight image, if the density is particularly low, it is desirable that the interval between dots is wider.

A second embodiment according to the present invention will be described below, in which the intervals at which dots are drawn are increased when the density of a highlight image is low. FIG. 4 is a detailed circuit block diagram of a second embodiment according to the present invention. The same reference numerals are given to the same components as those of the first embodiment shown in FIG. 1, and the detailed description will be omitted. That is, also in the second embodiment, the error diffusion processing units 1 to 4 have the same configuration as in FIG.

In FIG. 4, an OR gate 13 is a gate circuit for calculating the logical sum of the input printing states of the four colors. In the threshold generation circuit 14 of the second embodiment, the dots in a predetermined area are viewed, and the C, M, Y,
Threshold value T with reference to each multi-valued image data of K
C ', TM', TY ', TK' are changed. That is, in the threshold value generation circuit 14, for example, the OR gate 13
If it is determined from the result that the print density is high, a waveform as shown in FIG. 2 similar to that of the first embodiment is output. If it is determined that the print density is low,
Is selected to output the waveform as shown in (1). For example,
In the case where the overlapping portion of the dots as shown in FIG. 3 is equal to or more than a predetermined value, it may be determined that the printing density is high.

The change of the threshold value is not limited to the above two examples. Further, waveforms having several types of cycles may be prepared and may be changed stepwise according to image information. By controlling in this way,
A better binary image can be obtained. FIG. 7 shows a binarization control flow chart in the second embodiment described above. Also,
In some cases, it is difficult to determine whether the image data is the same as highlight image data of the same color or an image including a line drawing or an edge component by using only binary information. In such a case, each of C, M, Y, and K is used. 2
The multi-valued image data before the binarization is also referred to, and the timing at which the waveform is changed by the multi-valued data or the multi-valued data and the binarized data is controlled so that the image area is changed. May be controlled so that dots are naturally formed.

[0025]

Third Embodiment In the above description, an example in which a good binary image is obtained by controlling the threshold value has been described. However, the present invention is not limited to the above example, and the same effect can be achieved without controlling the threshold value. A third embodiment according to the present invention which achieves the same operation and effect without controlling the threshold value will be described below.

FIG. 8 is a control block diagram of a third embodiment according to the present invention. In FIG. 8, error diffusion processing units 21 to 2
4 has substantially the same circuit configuration as the error diffusion processing units 1 to 4 in FIG. 1, but the threshold value of the comparator 12 is a fixed value Th, and the output of the comparator 12 is
5 and is different from the first embodiment in that a highlight determination unit 16 is added.

The highlight judging section 16 is a circuit for judging whether or not a pixel binarized by the comparator 12 from the input multi-value data (cyan signal) is a part of a highlight image. Yes, the output value is sent to the highlight determination processing unit 17 as CH. The AND gate 15 is a circuit for forcibly setting the binary signal CB to the value “0” by the print inhibition signal CK sent from the highlight determination processing unit 17.

The highlight determination processing unit 17 includes binary image data CB, sent from the error diffusion processing units 21 to 24,
MB, YB, KB and highlight determination signals CH, MH,
YH and KH are input, and based on these data, the print inhibit signals CK and M to the error diffusion processing units 21 to 24 so that the print result becomes a uniform print pattern as shown in FIG.
K, YK, and KK are generated.

The control algorithm of the highlight judgment processing unit 17 is described in, for example, Japanese Patent Application No. 1-130945.
No., Japanese Patent Application No. 1-130946, Japanese Patent Application No. 2-60771
The uniform processing with a single color described in the item can be referred to. That is, a control method in which dots are arranged uniformly by examining the dot printing state and the density of image data around the target pixel can be applied. The above-described control algorithm is applied to the case of multi-color printing, and it is only necessary to look at the dot correlation of each color and perform control so that the dots of each color do not overlap.

Even when the control of this embodiment is used, the same operation and effect as those of the first and second embodiments can be achieved, and dots can be formed naturally even if the image area changes. In addition,
The present invention may be applied to a system including a plurality of devices, or may be applied to an apparatus including a single device. Needless to say, the present invention can also be applied to a case where the above is achieved by supplying a program to a system or an apparatus.

[0031]

As described above, according to the present invention,
Threshold used when binarizing multi-valued image data of the first color
And used when binarizing multi-valued image data of the second color
By making the threshold different, each
The dot overlap after binarization can be suppressed, and the printing density can be further improved.
To determine the degree and set the threshold fluctuation cycle accordingly.
Thus, the image reproducibility of the highlight part can be improved.

[Brief description of the drawings]

FIG. 1 is a circuit block diagram of a first embodiment according to the present invention.

FIG. 2 is a diagram showing a waveform indicating a change in a threshold value in the first embodiment.

FIG. 3 is a diagram showing an output example of a highlight image in the first embodiment.

FIG. 4 is a circuit block diagram of a second embodiment according to the present invention.

FIG. 5 is a diagram illustrating one of the waveforms indicating a change in the threshold value in the second embodiment.

FIG. 6 is a diagram illustrating an output example of a highlight image in a second embodiment.

FIG. 7 is a flowchart showing output control of a highlight image in a second embodiment.

FIG. 8 is a circuit block diagram of a third embodiment according to the present invention.

FIG. 9 is a diagram showing an example of output of a conventional highlight image.

[Explanation of symbols]

 1 to 4, 21 to 24 Error diffusion processing unit 5 to 8, 14 Threshold value generation circuit 10 Error addition unit 11 Error calculation unit 12 Comparator 16 Highlight determination unit 17 Highlight determination processing unit.

Claims (1)

(57) [Claims] 1. Multi-valued image data of a first and a second plurality of colors are
And input each multi-valued image data by the error diffusion method.
An image processing method for converting multi-valued image data of a first color into a binary data,
Binarization processing is performed on the first color binary data by comparison with the value.
A first binarizing circuit, and a first threshold for periodically changing the multi-valued image data of the second color
The value is compared with a second threshold value having a different value.
A second binarization circuit for performing binarization processing on the binary data, and binarized binary data from the first and second binarization circuits;
Print density based on the data, and based on the result of the determination.
Setting means for setting the first and second threshold values.
If the setting unit determines that the print density is high,
When a threshold with a short period is set and it is determined that the print density is low
Is a method for setting a threshold value having a long fluctuation period .