JP2002290731A - Image processing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an error diffusion method that allows easily acquiring high quality image data. SOLUTION: In the image processing method for binarizing pixels of image data represented by multi-valued gradation values or converting them to a predetermined plurality of values with a predetermined threshold as a criterion, when value pluralizing, each pixel of multi-valued image data is scanned in reversed right and left direction for each line, a target pixel is binarized, and an error between the binarized value and an original value is calculated. The error of the pixel to be binarized is distributed to a pixel toward the front of scanning direction and a pixel toward the rear and on the skew of the scanning direction in a predetermined ratio of the targeted pixels. The distribution ratio of the error is one second for the front pixel of the error, while one second for the rear and skew of the error.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for outputting the gradation value of each pixel of multi-valued image data as a predetermined plural value such as a binary value, and diffusing an error of the pixel to be multi-valued to another pixel. The present invention relates to an image processing method described above.

[0002]

2. Description of the Related Art Conventionally, image data having many gradations is
A dither method and an error diffusion method are known as methods for converting the value. The error diffusion method is basically a method of binarizing multi-gradation pixel data. For example, an image representing one pixel in 256 levels is represented by 1
This is a method of converting a pixel into an image that is expressed in two stages (in terms of an ink jet printer, whether or not to hit a dot).

When the pixel value is 0 to 127, the value is basically "0". When the pixel value is 128 to 255, the value is basically "255". This “255” is generally expressed as “1”. Thus, simply 1,0
If only the conversion is performed, an error occurs between the original numerical value at the time of the conversion and the converted value. These errors are respectively distributed or distributed to pixels around the pixel to be binarized, and the error of the target pixel is diffused to surrounding pixels. In the conventional error diffusion method, when the scanning direction is moved in one direction to the right,
3/10 of the error to the next (right) pixel, 7/10
To the pixel directly below, or 7/16 of the error
Is distributed to the next (right) pixel, 5/16 to the pixel immediately below, 3/16 to the lower left pixel, and 1/16 to the lower right pixel.

Japanese Patent Application Laid-Open Nos. Hei 3-151762, Hei 4-37256 and Hei 11-88685 disclose a method of alternately or randomly changing a scanning direction. Also, JP-A-63-155952
As disclosed in Japanese Patent Application Laid-Open Publication No. H10-157, a method of selecting and using a plurality of distribution coefficient sets together with pixel processing without making the distribution ratio of error by binarization of a target pixel to peripheral pixels constant,
Many error diffusion systems have been proposed, such as the one described in JP-A-8-228284, which takes a more complicated method.

[0005]

If the error diffusion method is simple, the occurrence of stripes in the image will increase. Conversely, if the processing is complicated, it will be eliminated, but it will take time to process the image. There is a problem that it takes place. Also,
The conventional ordinary error diffusion method has a problem that an unnatural tendency occurs in the diagonally lower right direction of the output image. An object of the present invention is to solve the above problems.

[0006]

In order to achieve the above object, the present invention binarizes a pixel of image data expressed by a multi-valued gradation value into a predetermined value or a predetermined value based on a predetermined threshold value. An image processing method for calculating an error between a converted value of a target pixel for multi-value conversion and an original value, distributing this error to other pixels, and diffusing the error of the target pixel, comprising: The scanning of each pixel of the data is performed by inverting the direction of each line for each line, and the error of the target pixel to be multi-valued is determined at a predetermined ratio by a pixel in front of the target pixel in the scanning direction and a target pixel. Are distributed to pixels obliquely below the rear side. Further, the present invention is characterized in that the distribution ratio of the error is one half of the error with respect to the front pixel, and is one half of the error with respect to the pixel obliquely below the rear side. It is.

[0007]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In FIG.
(6) is a computer C functioning as an image processing device.
An error diffusion processing unit including a PU (Central Processing Unit) and an image processing program stored in a computer;
(7) is a multi-valued image memory for storing image data capable of expressing three or more gradation values for each pixel, and (8) is image data binarized by the error diffusion processing unit (6). Is an error buffer memory for temporarily storing the error of the binarized pixel.

[0008] The error diffusion processing unit (6) reads the gradation value of the pixel of interest read from the multi-valued image memory (7) in order from the top for each line of the multi-valued image data (10). The scanning is performed from the left end to the right end of the line, and when the scanning position reaches the right end of the image data (10), the scanning direction is reversed this time as shown in FIG. Scan leftward toward. The error diffusion processing unit (6) repeats this scanning until the end of the image data (10) is reached.

In FIG. 1A, (12) shows the scanning direction of the first line, and (14) shows the scanning direction of the second line. According to the present invention, the pixels to which the error is assigned, that is, the pixels to be distributed are different between the case where the pixels are scanned rightward and the case where the pixels are scanned leftward. First, a case where pixels are scanned in the right direction will be described. FIG. 2 shows a part of the multi-valued image data, showing a target pixel and its surrounding pixels. (X) is the pixel of interest.

As shown in FIG. 3, the pixel (1) on the left of the target pixel (X) has already been binarized, and the gradation value (A) has already been determined. The error of the pixel of interest (X) is not distributed to the pixel (4) immediately below the pixel of interest (X) and the pixel (5) on the right side thereof. Therefore, the gradation values (D) and (E) of the pixels (4) and (5) do not change. The error diffusion processing unit (6) reads the pixel of interest from the multi-valued image memory (7).
The gradation value of (X) is read, and the error value given to the target pixel (X) is read from the error buffer memory (9). Then, the two values are added and compared with the threshold value. If the two values are larger, the gradation value is “1”, and if not, the gradation value is “0”.

The binarized gradation values are recorded in the binary image memory (8). The error from the original gradation value caused by this binarization is one half that as shown in FIG. 1 (B) and FIG.
Is distributed to the front (right) pixel (2), and the remaining half is distributed to the rear (left) lower pixel (3). The error value distributed to each pixel is
Record in the error buffer memory (9). Distributing the error value to each pixel specifically means changing the gradation value of each pixel as follows. For example, if the gradation value is 0-2
As shown in FIG. 4, in the multi-valued image having 256 stages of 55, the target pixel (X) has an error-distributed gradation value of 90, the pixel (2) has a gradation value of 98, and the pixel (3) has a gradation value of 98. The gradation value is 92,
The gradation value of the pixel (4) is 100, and the gradation value of the pixel (5) is 1
If the threshold value is 128 and the threshold value is 128, the pixel of interest (X)
Becomes “0” after the binarization conversion.

The difference "+90" between the converted value "0" and the original value 90 is distributed to the pixels (2) and (3) by half. As a result, the value of the pixel (2) is 98+ [90 × (1
/ 2)] = 143, and the value of pixel (3) is 92+ [90 ×
(1/2)] = 137. The values of the pixels (4) and (5) remain as they are. Similarly, when the scanning direction is performed from left to right from right to left, FIGS.
As shown in the figure, one half of the error of the target pixel (X) is in the front (left) pixel (1), and the remaining half is in the pixel (5) obliquely below the target pixel (X). ).

For example, as shown in FIG. 6, the error-distributed gradation value of the target pixel (X) is 150, the value of the pixel (1) is 98,
Assuming that the value of (3) is 92, the value of pixel (4) is 100, and the value of pixel (5) is 136, the target pixel (X) becomes “255” after the binarization conversion.
The difference “−105” between the converted value “255” and the original value 150 is calculated as 2
Pixels are distributed to pixels (1) and (5) in units of one minute. As a result, pixel (1)
Is 98 + [− 105 × (１ ／)] = 46, the pixel
The value of (5) is 136 + [− 105 − (− 105 × (1)]] = 83 The values of the pixels (3) and (4) remain as they are.

Pixels having 256 gradation levels from 0 to 255 are 2
After the value conversion, the gradation value becomes 0 or 255. If the conversion result of each pixel is 0, set "0"; if it is 255, set "1".
Record in (7). In the above-described embodiment, since the distribution processing of 誤差 of the error is processing in which only the value is shifted horizontally in the computer, the processing is relatively easy and the processing is relatively quick in the computer. In the present embodiment, the error diffusion processing when the target pixel is binarized has been described. However, the present invention is not particularly limited to this binarization, and can be applied to quaternary and other cases. .

[0015]

As described above, according to the present invention, by performing scanning alternately in both directions, the right and left errors of the target pixel are offset to a certain extent, and the portion behind the target pixel is the obliquely lower pixel which is the closest pixel to it. By assigning an error to the pixel of the target pixel, the processing can be performed in substantially the same manner as when an error is assigned to the left and right pixels of the target pixel, and high-quality image data can be obtained by a simple method. Also, the distribution of the error is 2
The processing speed can be improved by setting the ratio to one-half.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of the present invention.

FIG. 2 is an explanatory diagram of the present invention.

FIG. 3 is an explanatory diagram of the present invention.

FIG. 4 is an explanatory diagram of the present invention.

FIG. 5 is an explanatory diagram of the present invention.

FIG. 6 is an explanatory diagram of the present invention.

FIG. 7 is a functional block diagram of the present invention.

[Explanation of symbols]

 1 pixel 2 pixel 3 pixel 4 pixel 5 pixel 6 error diffusion processing unit 7 multi-level image memory 8 binary image memory 9 error buffer memory 10 multi-level image data 12 scanning direction 14 scanning direction

Claims (2)

[Claims] When a pixel of image data represented by a multi-level gradation value is binarized or multi-valued with a predetermined threshold as a reference, a converted value of a multi-valued target pixel and an original value This is an image processing method that calculates an error with the value of the target pixel, distributes this error to other pixels, and diffuses the error of the target pixel. And scanning, the error of the target pixel of the multi-valued is distributed at a predetermined ratio to a pixel in front of the target pixel in the scanning direction and a pixel obliquely below and behind the target pixel. Characteristic image processing method. 2. The method according to claim 1, wherein a distribution ratio of the error is one half of the error with respect to a front pixel, and is one half of the error with respect to a diagonally lower pixel on the rear side. The image processing method according to claim 1.