JP2000022949A - Screening method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the quality of an output image by increasing the number of lines which can be represented when hexagonal cells which are not regularly hexagonal on a square lattice is applied to a printer having a relatively small number of gray scales. SOLUTION: Hexagonal cells 1-1, 1-2 and 1-3 of 24 (=(5×5)-(1×1)) pixels having two sides (oblique side) after a couple of opposite corners of a 5×5 square cell are omitted are formed and those three hexagonal cells 1-1, 1-2 and 1-3 are combined to form one half-tone cell 1. The number of gradations that this half-tone cell can represent is 24×3=72 and the number gray scales which can be represented when this is applied to a printer of 600 dpi is 120.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a screening method for forming a hexagonal cell on a square lattice to suppress the occurrence of horizontal and vertical noise patterns.

[0002]

2. Description of the Related Art Generally, an output device such as a printer having a medium-to-low resolution uses halftones based on a distributed rule dither instead of a collective (centralized) dither or a distributed random dither (error diffusion). The reason for this is that when the collective dither is used, blurring and rosette due to the denseness of the ink are conspicuous, and when the distributed random dither is used, the memory and calculation cost increase. For this reason, the technique of the distributed rule dither becomes important especially in the color ink jet printer.

In the distributed rule dither screening, a square halftone cell is usually used. However, due to the nature of the dither pattern, horizontal and vertical noise patterns that are easily recognized by human eyes at a certain density are generated. As a method for preventing the generation of the horizontal and vertical noise patterns, the reference "Robert Uli
"Chney. Digital Halftoning. The MIT Press." introduces a regular hexagonal dither pattern, and it was thought that this dither could realize distributed rule-based dither screening in which the above pattern was inconspicuous.

However, since the hexagonal dither in the above-mentioned document is a regular hexagonal halftone cell realized on a 60-degree grid, an output device for printing dots on the 60-degree grid is required. , Cannot be applied to output devices that dot a square grid.

[0005] Since it is difficult to generate a screen in which square hexagonal cells are laid on a square lattice, the inventor of the present application has reduced the pair of opposite corners of the square cell in the earlier application to two sides (oblique sides). By expressing hexagonal cells (not regular hexagons), it is proposed to suppress the occurrence of horizontal and vertical noise patterns seen in screening using square halftone cells.

Here, in this method, in order to prevent the pattern from being biased, it is necessary to arrange the cells so that the distance between the centers of the adjacent cells is as equal as possible. should not. FIG. 4 shows a case where hexagonal cells are formed on a 5 × 5 square lattice as an example. In this case, corners of two opposing 5 × 5 pixels shown in FIG. 4A are cut off. , 24 (= 5 × 5−5) which is not a regular hexagon as shown in FIG.
A hexagonal cell of 1 × 1) pixels is formed. Then, as shown in FIG. 4C, the hexagonal cells are arranged on the square lattice without any gap.

FIGS. 5 to 7 show a method of generating a hexagonal cell for expressing a gradation of about 256 gradations.
First, the cell needs a central pixel,
First, as shown in FIG. 5A, hexagonal basic cells are formed on a 3 × 3 square lattice. In addition, eight gradations (= 3 × 3-1 × 1) can be expressed by the hexagonal cells.
Next, as shown in FIG. 5 (b), expansion is performed by forming a larger hexagonal cell with the center point of each cell in contact with the side of the basic cell as the vertex.

In this case, it is preferable that the vertices of the acute angle portion are included in the hexagon, and the vertices at both ends of the oblique side are not included (the acute angle portion of the contacting cell). In addition, 24 gradations (= 5 × 5-1 × 1) can be expressed by the hexagonal cell shown in FIG. By repeating the same expansion for the expanded cell, 72 gray levels (= 9 × 9−
3 × 3) and 216 gradations (= 15 × 15) as shown in FIG.
-3 x 3) to form a hexagonal size cell that can be expressed,
Next, a threshold is set for each pixel as shown in FIG.

As another conventional example, as disclosed in Japanese Patent Application Laid-Open No. Hei 8-265568, for example, a digital raster device that sequentially compares input image data with each element of a threshold value array and binarizes or multivalues the data is used. , A smooth gradation characteristic of 150 gradations or more is achieved by using a dither matrix composed of eight clusters capable of expressing 15 gradations or more.
A method has been proposed which is realized by repeating a small pattern of about five gradations to achieve both high resolution and low cost.

As another conventional example, as disclosed in Japanese Patent Application Laid-Open No. 7-264403, for example, a threshold matrix is divided into a plurality of sub-matrices having the same size, and a plurality of thresholds are randomly assigned to each sub-matrix. A method has been proposed in which a threshold matrix having a high spatial frequency and capable of smooth gradation expression can be created by arranging the threshold matrix.

[0011]

By the way, as described above, in the conventional method of expressing a hexagonal cell which is not a regular hexagon on a square lattice, the hexagonal cell is formed in such a shape that the distance between the centers of the cells is as equal as possible. Once determined, the number of gradations that can be represented by the cell is 8 in FIG.
5 (b), then 72 cells in the next expanded cell, and 216 in FIGS. 6 and 7.

However, in order to use a medium or low-gradation printer, a cell that can normally express 72 or more gradations in terms of the number of gradations to be expressed is required.
When a cell of 72 gradations is used for a 0 dpi printer, the number of lines is about 60, and there is a problem that an output image becomes rough.

The present invention has been made in view of the above-mentioned problems of the prior art, and increases the number of lines that can be expressed when a hexagonal cell that is not a regular hexagon on a square lattice is applied to a printer with a relatively low number of gradations. It is an object of the present invention to provide a screening method capable of improving the image quality of a subject.

[0014]

In order to achieve the above object, a first means is to cut off a pair of corners of a square cell on a square lattice to form a hexagonal cell which is not a regular hexagon, and forms this hexagonal cell. One halftone cell is formed by combining a plurality of cells, and a halftone is expressed by setting a threshold value in each lattice of the halftone cell.

A second means is that, in the first means, the halftone cells are arranged on a square grid without gaps, and the distance between the threshold setting start grids of adjacent halftones is as equal as possible. It is characterized by.

The third means is characterized in that the threshold values set in each of the plurality of hexagonal cells constituting the halftone cell in the first and second means are not similar.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an explanatory view showing a halftone cell in one embodiment of the screening method according to the present invention, FIG. 2 is an explanatory view showing a state in which the halftone cells of FIG. 1 are arranged on a square lattice, and FIG. FIG. 4 is an explanatory diagram showing threshold values set in a halftone cell.

In FIG. 1, first, a pair of opposing corners of a 5 × 5 square cell is dropped to form two sides (oblique sides).
Hexagonal cell 1-1 of 4 (= 5 × 5-1 × 1) pixels,
1-2, 1-3, and these three hexagonal cells 1-
One halftone cell 1 is formed by combining 1, 1-2, and 1-3. Therefore, the number of gradations that can be expressed by the halftone cell 1 is 24 × 3 = 72. Also,
When this is applied to a 600 dpi printer, the number of lines that can be expressed is as follows. Since the size of one dot is about “5 × 5”, the number of lines = resolution / length of one side of cell 600/5 = 120.

As shown in FIG. 2, the plurality of halftone cells 1 can be arranged on the screen grid without any gaps, and at this time, the distance between the threshold value setting start cells of adjacent halftones is possible. Since the fill start pixel indicated by a black square in each halftone cell 1 is not orthogonal to the horizontal and vertical directions, it is possible to prevent horizontal and vertical noise patterns from being generated.

FIG. 3 shows an example in which a threshold value is set at each pixel position of the halftone cell 1. As can be seen from FIG.
By setting the thresholds so that the order in which pixels 1, 1-2, and 1-3 are filled with the thresholds is not similar,
It is possible to prevent the occurrence of a periodic noise pattern that is easily noticeable.

[0021]

As described above, according to the first aspect of the present invention, a set of opposite corners of a square cell on a square lattice is cut off to form a hexagonal cell which is not a regular hexagon.
Since one halftone cell is formed by combining a plurality of these hexagonal cells, the number of lines that can be expressed increases when a hexagonal cell that is not a regular hexagon on a square lattice is applied to a printer with a relatively low gradation number. As a result, the image quality of the output image can be improved.

According to the second aspect of the present invention, since the halftone cells are arranged on the square lattice without gaps, screening can be easily performed, and the threshold value between adjacent halftone threshold setting start lattices can be set. Since the distances are as equal as possible, horizontal and vertical noise patterns can be prevented from occurring.

According to the third aspect of the present invention, since the threshold values set for each of the plurality of hexagonal cells constituting the halftone cell are not similar, it is possible to prevent the generation of a noticeable periodic noise pattern. Can be.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a halftone cell in one embodiment of a screening method according to the present invention.

FIG. 2 is an explanatory diagram showing a state in which the halftone cells of FIG. 1 are arranged on a square lattice.

FIG. 3 is an explanatory diagram showing a threshold value set in the halftone cell of FIG. 1;

FIG. 4 is an explanatory diagram showing a conventional hexagonal cell.

FIG. 5 is an explanatory diagram showing a method of expanding the hexagonal cell of FIG.

FIG. 6 is an explanatory diagram showing a hexagonal cell obtained by expanding the hexagonal cell of FIG. 4;

FIG. 7 is an explanatory diagram showing threshold values set in the hexagonal cell of FIG. 6;

[Description of Signs] 1 Halftone cell 1-1, 1-2, 1-3 Hexagonal cell

Claims (3)

[Claims] 1. Opposite ones of square cells on a square lattice
A set of corners is cut off to form a hexagonal cell that is not a regular hexagon, a plurality of these hexagonal cells are combined to form one halftone cell, and a halftone is set by setting a threshold value for each grid of the halftone cell. A screening method characterized by expressing. 2. The halftone cell according to claim 1, wherein the halftone cells are arranged on a square lattice without gaps, and the distance between the threshold setting start lattices of adjacent halftones is as equal as possible. Screening method. 3. The screening method according to claim 1, wherein threshold values set in each of the plurality of hexagonal cells constituting the halftone cell are not similar.