JPH11261819A - Image processor and image output device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the generation of a pseudo contour in an image processor adopting a multivalued error propagation method. SOLUTION: The density value i(x, y) of a picture element is inputted, and which zone the inputted density value is included is judged in a zone judgement part 7. A threshold for deciding the gradation based on the judged result is decided in a threshold decision part 8. A boundary value for zone judgement is modulated corresponding to the (x) coordinate of the picture element. The threshold is modulated corresponding to the (y) coordinate of the picture element. Thus, the generation of the pseudo contour is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus and an image output apparatus, and more particularly to an image processing apparatus and an image output apparatus for performing a multi-value pseudo gradation processing capable of expressing a plurality of gradations.

[0002]

2. Description of the Related Art In recent years, printers that output pictorial images with high image quality have been developed. In particular, most of the conventional ink jet printers express the gradation by a dot on / off, so-called binary value.
With the development of various technologies (shading ink, dot overprinting, dot diameter modulation), a multi-value gradation ink jet printer capable of expressing a plurality of gradation levels has appeared, and it has become possible to output high-quality images.

[0003] Also, similarly, a thermal transfer printer, an electrophotographic printer, and the like, capable of expressing a multi-valued image are known.

In a binary printer, a technique has been used in which a plurality of dots are used to express a gray scale in terms of area in order to simulate the gray scale. These methods are called an area gradation method or a pseudo gradation method, and as typical methods, an organized dither method and an error diffusion method are known.

[0005] In a printer which has a resolution of about 360 dpi, which is currently the mainstream, an error diffusion method is often used because excellent results in image quality can be obtained.

[0006] Even in the above-described printer capable of expressing multi-valued gradations, if it is not enough to express a sufficient number of gradations in terms of image quality using only the inherent gradation number, the pseudo gradation method is used. Is expressed in combination with.

Further, even in a printer capable of expressing a large number of gradations in principle, the number of gradations is limited and used in combination with the pseudo gradation method to improve the stability of the system. What expresses is known.

As a pseudo gradation method used in combination with these multi-value printers, a method that is an extension of the conventional binary pseudo gradation method is used. For example, as an extension of the error diffusion method to multiple values, Kato, Arai, Yasuda: Sho53
A multi-level error diffusion method has been disclosed in the IEICE Communications Division 504 (Prior Art 1).

In the multi-level error diffusion method according to the prior art 1, usually, most dots are printed at random. However, dots are regularly arranged only when the input image data is in the vicinity of a value close to the multi-valued output value. However, there is a drawback that it stands out unnaturally. This unnatural area occurs in a band shape around an area where pixels having the same gradation level are distributed, and appears as a so-called pseudo contour. This false contour significantly degrades the image quality.

FIG. 15 is a diagram showing an image which is an output result of image processing by the conventional multi-level error diffusion method, and FIG. 16 is an enlarged view of a portion surrounded by a square in FIG.

Referring to the figure, according to the conventional multi-valued error diffusion method, a pseudo contour occurs due to a stepwise change in the gradation of a portion where a gentle gradation change is required. .

Also, in the systematic dither method, when a dither matrix having high dot dispersibility is used, a similar phenomenon occurs due to a difference in a generated texture pattern.

For this reason, several methods have been proposed so far as a method for suppressing the occurrence of a pseudo contour.

Ochi: Proceedings 94-01 of the Institute of Image Electronics Engineers of Japan
-05 (Prior Art 2) reports a technique for suppressing the occurrence of false contours by hierarchizing error diffusion processing.

S. Sugiura and T. Makita: J. Ima
g. Sci. Tech., 39, 495 (1995) and JP-A-9-98.
No. 290 (Prior Art 3) discloses a technique of suppressing a false contour by adding noise to an area of the input image data where the pseudo contour occurs.

Furthermore, Japanese Patent Publication No. 6-101788 (prior art 4) proposes a method of modulating a threshold value of error diffusion by a dither matrix.

[0017]

However, in the method of the prior art 2, the processing is hierarchized and the processing is performed a plurality of times.
When the number of gradations increases to eight and sixteen values, there is a disadvantage that the time required for processing becomes very long.

In the technique of the prior art 3, since a means for adding noise is required, a dither matrix is used as a noise source. The dither matrix is
Since it is configured as a two-dimensional table, there is a problem that the configuration of the apparatus becomes more complicated and the cost associated therewith increases. In addition, since the area to which noise is added increases in accordance with the number of gray levels, there is a disadvantage that as the number of gray levels increases, hardware or processing steps required for calculations such as area determination increase.

Further, in the technique of the prior art 4, the dither matrix is used as in the technique of the prior art 3, so that the configuration of the apparatus becomes complicated. In addition, as the number of gradations increases, the number of required dither matrices increases, so that there is a problem that the cost is further increased.

[0020]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and an image processing apparatus capable of preventing occurrence of a false contour and performing high-speed processing. Its purpose is to provide at low cost.

In order to achieve the above object, according to one aspect of the present invention, an image processing apparatus includes an input unit for inputting a density value of a pixel, and a plurality of zones in which the input density value is determined in accordance with the density. Determining means for determining which of the zones is included using the boundary value of the zone, first changing means for changing the boundary value of the zone according to the address of the pixel, and based on the determined zone, Processing means for performing pseudo gradation conversion processing of the pixel density value.

Preferably, a threshold value is used for the pseudo gradation conversion processing, and the image processing apparatus further includes a second changing means for changing the threshold value according to the address of the pixel.

[0023] More preferably, the address of the pixel includes a vertical coordinate and a horizontal coordinate, and the first changing means stores the coordinate in one of the vertical coordinate and the horizontal coordinate. The boundary value of the zone is changed accordingly, and the second changing means changes the threshold value according to one of the coordinates in the vertical direction and the coordinates in the horizontal direction.

[0024] More preferably, the processing means performs a pseudo gradation process using an organized dither method.

More preferably, the processing means performs a pseudo-gradation processing using an error diffusion method.

According to another aspect of the present invention, an image output device includes any one of the above-described image processing devices.

[0027] More preferably, the image output device is an ink jet printer.

More preferably, the ink jet printer is a multi-valued ink jet printer that modulates a dot diameter.

According to the present invention, it is possible to provide an inexpensive image processing apparatus capable of suppressing the occurrence of false contours and performing high-speed processing.

[0030]

FIG. 1 shows a first embodiment of the present invention.
FIG. 1 is a block diagram illustrating a configuration of an inkjet printer 100 including one image processing apparatus.

The ink jet printer 100 has a CPU 101 for controlling the entire apparatus, a pseudo gradation processing section 110 for executing the image processing of the present invention, a RAM 102 for storing image data and the like, and a program for operating the apparatus. , A data receiving unit 104 for receiving image data from an external device, a head ejection drive unit 105 for controlling the ejection of ink from the inkjet head, and a head movement drive unit 106 for controlling the movement of the inkjet head. And a paper feed drive unit 107 for controlling paper feed, a recovery motor drive unit 108 for driving a motor for performing maintenance of the ink jet head, and various sensors 109.

A CPU 101 for controlling the entire apparatus executes a program stored in a ROM 103 by using a RAM 102 as necessary. This program includes a part for recording an image on a recording sheet (a recording medium, such as paper) and a part for restoring the nozzle surface of the inkjet head to a good state.

Image data is input from an external device via a data receiving unit 104 by a program for recording an image. The head ejection drive unit 105, the head movement drive unit 106, the paper feed drive unit 107, and various sensors 110 are controlled, and an image is recorded on a recording sheet.

A recovery system motor drive unit 108 and various sensors 109 are controlled when necessary by a program for recovering the nozzle surface of the ink jet head to a good state, and the nozzle surface is cleaned.

Under the control of the CPU 101, the head ejection drive unit 105 drives the piezoelectric element of the ink jet head, and the head movement drive unit 106 drives a drive motor that moves the carriage holding the ink jet head in the horizontal direction. The paper feed drive unit 107 drives a paper feed roller. Further, based on the control of the CPU 101, the recovery motor drive unit 108 drives a motor and the like necessary to recover the nozzle surface of the inkjet head to a good state.

The ink jet printer 100 is
By adjusting the voltage applied to the piezoelectric element for ejecting the ink, dots having a size of eight gradations from 0 to 7 (however, 0 is a state without dots) can be printed on the recording sheet.

FIG. 2 is a flowchart for explaining image processing performed by the ink jet printer 100 shown in FIG.

Referring to the figure, image data is input from an external device via data receiving unit 104 in step S1. In step S2, the gradation of the image included in the input image data is converted so as to be printed with visually favorable characteristics. In step S3, color conversion (or color correction) of the converted image data is performed.

In step S4, a process of separating the gray component in the input image data and replacing it with a black signal (black generation + UCR) is performed. In step S5, a pseudo tone conversion process of converting image data into a tone suitable for a tone that can be output by the inkjet printer 100 is performed. This processing is executed in the pseudo gradation processing unit 110.

In step S6, the image data subjected to the pseudo gradation is output to a recording sheet. FIG. 3 is a diagram for explaining the processing performed in the pseudo gradation conversion processing (S5) of FIG.

Referring to FIG. 3, the image data to be processed in the pseudo tone conversion has an x coordinate (horizontal coordinate).
And y-coordinate (vertical coordinate). One pixel corresponds to each of the coordinates (x, y). The density value i (x, y) of each pixel of the image data (input image) to be processed takes any integer value from 0 to 255. The pseudo gradation conversion converts the image data into eight gradations. That is, the density value o (x, y) of one pixel of the output image data takes one of the values 0, 36, 72, 109, 146, 182, 219 or 255.

Next, an outline of pseudo gradation conversion of input image data will be described. FIG. 4 is a diagram for explaining pseudo gradation.

To the density value i (x, y) of one pixel of the input image data, an error value diffused from a peripheral pixel already processed is added. The density value to which the diffused error has been added is first determined to which zone the density belongs based on the zone boundary value. Here, the zone boundary values are 0, 36, 72, 109, 146, and 18 as their reference values.
2, 219 and 255 have been defined. For example, if the density value i (x, y) of the pixel to which the diffused error is added is 0 or more and less than 36, it is determined that the zone to which the density value belongs is Z1. Similarly, the density value of the pixel to which the diffused error has been added is Z2 to Z2 due to the zone boundary value.
It is determined which zone of Z7 belongs. In each of the zones Z1 to Z7, a gradation determination threshold value in the zone is defined. Here, the tone determination threshold value is a threshold value for determining a density value output in each zone. As a reference value of this threshold value, 18, 54, 90, 12 in each of the zones Z1 to Z7.
7, 164, 200 and 237 are set.

For example, in the zone Z1, if the density value of the pixel to which the diffused error is added is 18 or more, the gradation of the pixel is set to "1". That is, the density value o (x, y) of the pixel of the output image data is set to 36. On the other hand, if the density value of the pixel to which the diffused error is added is less than 18, the gradation is set to “0”, and the density value o (x, y) of the output pixel is set to 0. Similarly, in each zone, by using the gradation determination threshold,
The density value of the pixel of the output image data is determined.

In FIG. 4, reference values are described for each of the zone boundary value and the gradation determination threshold value. However, in the present embodiment, these reference values are set in accordance with the input pixel address. A number is added to the value. As a result, the boundary value and the threshold value are modulated, and the occurrence of a false contour in the output image data is prevented.

FIG. 5 is a block diagram showing a configuration of an image processing device included in pseudo tone processing section 110 of FIG. In this block, the pseudo gradation process in step S5 in FIG. 2 is executed.

Referring to the figure, the image processing apparatus performs processing such that the density value i (x, y) of one pixel of the input image data and the corresponding diffusion error e (x,
y), a comparator 2 that compares the output of the adder 1 with the threshold value Th, and an output of the comparator 2 and an input pixel density value i (x, y). Pixel density value o output based on the included zone number Zn
An output value determination unit 6 for determining (x, y), and an output value o
The value of -o (x, y) with the sign of (x, y) inverted and i
Adder 3 for adding the value of (x, y) + e (x, y)
A diffusion filter 4 for diffusing the output of the adder 3 to pixels around the pixel to be processed; an error memory 5 for storing the diffused error; A zone determination unit 7 for determining a zone by inputting a density value i (x, y) and its x coordinate, an output of the zone determination unit 7 and a y coordinate of a pixel to be processed are input. And a threshold value determination unit 8 for determining a gradation determination threshold value.

The zone judging section 7 adds ± α to each of the reference values of the zone boundary values (see FIG. 4) depending on whether the input x-coordinate is an odd number or an even number. Specifically, when x is an even number, α is added to each of the reference values of the zone boundary values shown in FIG. On the other hand, when x is an odd number, each of the reference values of the zone boundary value is-
α is added. Thus, the zone boundary value is modulated by the value of x. By comparing the modulated zone boundary value with the density value i (x, y) of the input pixel, the zone to which the density value i (x, y) belongs is Z1 to Z1.
It is determined which of Z7. The determined zone number Zn is output to each of the output value determination unit 6 and the threshold value determination unit 8.

In the threshold value determining section 8, first, a reference value (see FIG. 4) of the gradation determination threshold value corresponding to the input zone number Zn is determined. Then, a value of ± β is added to the reference value according to the input y coordinate. Specifically, when y is an even number, a fixed value β is added to the reference value of the gradation determination threshold value, and when y is an odd number, β is subtracted. Thereby, the gradation determination threshold is modulated. The modulated gradation determination threshold value is input to the comparator 2 as the threshold value Th.

FIG. 6 is a diagram showing a case where the same image as the image shown in FIG. 16 is output by the image processing in this embodiment.

Referring to the figure, in this embodiment, the zone boundary value and the gradation determination threshold value are modulated according to the address (x, y). It is possible to prevent the occurrence of a problematic false contour, and to prevent the image quality of the image from being reduced. Further, in the image device according to the present embodiment, it is possible to prevent the occurrence of a false contour without lowering the calculation speed and without complicating the configuration of the device. The advantage of these high computation speeds and the simplicity of the configuration is that
It becomes more conspicuous as the number of multi-valued gradation levels increases. Therefore, the present invention can be particularly effectively applied to an ink jet printer having a large number of gradation levels which can be relatively expressed among multi-value output devices. Further, among the ink jet printers, particularly in a printer that performs gradation control by dot diameter modulation as in the present embodiment, the present invention is particularly advantageous because the number of gradation levels that can be expressed is large.

In this embodiment, the image output device is an ink jet printer. However, the output device is not limited to this, and may be used in an image output device such as a CRT, a liquid crystal display, a thermal transfer printer, and an electrophotographic printer. The present invention can also be applied to the present invention.

FIG. 7 is a block diagram showing a configuration of an image processing apparatus according to another embodiment of the present invention. This image processing apparatus can be applied to an image output apparatus such as an ink jet printer, similarly to the image processing apparatus of FIG.

In this embodiment, the comparator 10 compares the density value i (x, y) of the pixel of the input image data with the threshold value Th. The output of the comparator 10 is input to the output value determination unit 11. The output value determination unit 11 determines the output value o (x, y) based on the zone Zn to which the density value i (x, y) belongs.

The zone to which i (x, y) belongs is determined by the zone determination unit 12. This determination is the same as that shown in FIG. 4, and thus description thereof will not be repeated. That is, in the determination of the zone, the reference value of the zone boundary value shown in FIG.
Is used.

In the dither matrix selection section 13, a dither matrix corresponding to the determined zone is selected. Specifically, the dither matrices shown in FIGS. 8 to 14 are selected for each of the zones Z1 to Z7. The dither matrix selection unit 13 further outputs a threshold value Tj corresponding to the address (x, y) from the selected dither matrix.

The modulation output determining section 14 calculates the y of i (x, y).
When the coordinates are even, a constant value δ is output, and when the coordinates are odd, −δ is output. The output value from the modulation output determining unit 14 is added to the value of Tj by the adder 15 and the threshold value T
h. The threshold value Th is input to the comparator 10.

In this embodiment, the systematic dither method is adopted for image processing. However, the reference value for determining the zone is modulated by the x coordinate of the pixel, and the threshold value of the dither matrix is changed. Modulated by the y coordinate of the pixel. As a result, it is possible to prevent the occurrence of a false contour similarly to the image processing circuit shown in FIG.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of an inkjet printer according to one embodiment of the present invention.

FIG. 2 is a flowchart illustrating a process performed by the inkjet printer of FIG. 1;

FIG. 3 is a diagram for explaining image data processed in pseudo gradation (S5) of FIG. 2;

FIG. 4 is a diagram for explaining the principle of pseudo-gradation.

FIG. 5 is a block diagram illustrating a configuration of an image processing circuit included in the pseudo gradation processing unit 110 of FIG.

FIG. 6 is a diagram for explaining an effect of the present invention.

FIG. 7 is a block diagram illustrating a configuration of an image processing apparatus according to another embodiment of the present invention.

FIG. 8 is a diagram showing a dither matrix in a zone Z1.

FIG. 9 is a diagram showing a dither matrix in a zone Z2.

FIG. 10 is a diagram showing a dither matrix in a zone Z3.

FIG. 11 is a diagram showing a dither matrix in a zone Z4.

FIG. 12 is a diagram showing a dither matrix in a zone Z5.

FIG. 13 is a diagram showing a dither matrix in a zone Z6.

FIG. 14 is a diagram showing a dither matrix in a zone Z7.

FIG. 15 is a diagram for explaining a problem of the related art.

FIG. 16 is an enlarged view of a portion surrounded by a square in FIG.

[Explanation of symbols]

 2 Comparator 4 Diffusion filter 5 Error memory 6 Output value determination unit 7 Zone determination unit 8 Threshold value determination unit 100 Inkjet printer

Claims (8)

[Claims] An input unit for inputting a density value of a pixel; and a zone boundary value of a zone determined as to which input density value is included in a plurality of zones determined according to the density. Determining means for changing the boundary value of the zone according to the address of the pixel; and performing pseudo tone conversion processing of the density value of the pixel based on the determined zone. An image processing apparatus comprising: 2. The method according to claim 1, further comprising a second changing unit that changes a value of the threshold value according to an address of the pixel, using a threshold value in the pseudo gradation conversion process. The image processing apparatus according to any one of the preceding claims. 3. The address of the pixel includes a coordinate in a vertical direction and a coordinate in a horizontal direction, and the first changing unit determines one of the coordinate in the vertical direction and the coordinate in the horizontal direction. The boundary value of the zone is changed according to the above, The second changing means changes the threshold value according to any one of the coordinates in the vertical direction and the coordinates in the horizontal direction. Item 3. The image processing device according to item 2. 4. The image processing apparatus according to claim 1, wherein said processing means performs a pseudo gradation process using a systematic dither method. 5. The image processing apparatus according to claim 1, wherein said processing means performs a pseudo gradation process using an error diffusion method. 6. An image output device including the image processing device according to claim 1. 7. The image output device according to claim 6, wherein the image output device is an ink jet printer. 8. The image output apparatus according to claim 7, wherein the inkjet printer is a multi-value inkjet printer that modulates a dot diameter.