JP2001218053A - Halftone screening method and information recording medium with computer program performing the method recorded thereon - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the generation of a conspicuous dot pattern in a highlight part. SOLUTION: In the case of performing digital halftone screening that uses a hexagon cell 1 obtained by cutting off a pair of facing corners of a square cell on a square lattice and defining the as oblique sides as a halftone cell, one area 2 that does not come in contact with dots in an adjacent hexagon cell 1 is set in the hexagon cell 1, and the threshold for binarization of each dot position in the hexagon cell 1 is set so that one dot plotting start position 3 in the individual area 2 can be random in each hexagon cell 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a halftone screening method using a hexagonal cell which cuts off a pair of opposite corners of a square cell on a square lattice and sets the corner as a hypotenuse, and implements the halftone screening method. The present invention relates to an information recording medium on which a program to be recorded is recorded.

[0002]

2. Description of the Related Art Conventionally, in order to compensate for the disadvantages of the collective dither and the error diffusion method, several methods have been proposed in which both are used in combination depending on the density. For example, Japanese Patent Application Laid-Open No. H10-56568 proposes a method in which a dither matrix method is used for low and high density areas and an error diffusion method is used for medium density parts in order to perform good halftoning in all density areas. Have been.

[0003]

By the way, in halftone screening using hexagonal cells in which a pair of opposite corners of a square cell on a square lattice is cut off and the hypotenuse is set as the hypotenuse, the center of each cell is horizontally and vertically. Since they do not exist at the same position in the directions, it is possible to suppress the occurrence of noise patterns in the horizontal and vertical directions.

FIG. 22 shows, as an example, a 5 × 5 pixel square matrix in which the upper right pixel is cut down to １ ／ and the lower left pixel is cut down to と し て and these are set as hypotenuses.
A hexagonal cell 1 having 5-1 = 24 pixels is shown. However, in the conventional digital halftone screening using the hexagonal cells, if the first point (dot drawing start point) for filling each cell is the center of the cell, it has no randomness. There is a problem that the dot pattern is easily visible.

Also, in the error diffusion method, generally, the calculation of diffusing the error between the density and the threshold value for all pixels to a plurality of neighboring pixels is performed “total number of pixels + total number of pixels × number of neighboring pixels” times. There is also a problem that the amount of calculation is large.

In the case of using a dot concentration type dither matrix, if the diameter of each dot is too large in the highlight portion, there is a problem that the dots become too sparse and thin and thin lines cannot be expressed. As a method of solving this problem, a method of dividing the cell into smaller cells is conceivable. However, in this method, in the medium density portion, adjacent dots are more likely to be fused by the dot gain, and therefore, the number of expressed gradations is reduced. There is a problem of lowering.

The present invention has been made in view of such a conventional problem, and has as its object to provide a halftone screening method and a halftone screening method capable of suppressing the occurrence of a dot pattern that is easily visible in a highlight portion. An object of the present invention is to provide an information recording medium on which a program for executing the method is recorded.

Another object of the present invention is to provide a halftone screening method which requires less calculation cost than the error diffusion method, and an information recording medium on which a program for executing the method is recorded.

[0009]

SUMMARY OF THE INVENTION In order to achieve the above-mentioned object, the present invention provides a digital device which uses a hexagonal cell as a halftone cell, which cuts off a pair of opposite corners of a square cell on a square lattice and uses this as a hypotenuse. In the halftone screening method, one area that does not come into contact with a dot in an adjacent hexagonal cell is set in the hexagonal cell, and one dot drawing start position in this individual area is randomly set for each hexagonal cell. The threshold value for binarization of each dot position in the hexagonal cell is set so that

In this case, based on the remainder obtained by dividing the random number by the number of pixels in the area, the dot drawing start position for each hexagonal cell is determined to be random. Further, the position to be filled as the second dot and thereafter is a position adjacent to the dot drawing start position, and is random for each hexagonal cell. Further, after filling the inside of the area, the remaining dot positions in the hexagonal cell are grown by a dot aggregation type halftone dither having the same shape.

According to another aspect of the present invention, there is provided a halftone screening method using a hexagonal cell as a halftone cell, in which a pair of corners of a square cell on a square lattice are cut off and formed as hypotenuses. A plurality of areas in the hexagonal cell that do not contact the dots in adjacent hexagonal cells, and do not contact other points in the same cell; Are set so as not to reverse, and a threshold value for binarization of each dot position in the hexagonal cell is set so that each dot drawing start position in the plurality of regions is random for each of the hexagonal cells. Is set.

In this case, based on the remainder obtained by dividing the random number by the number of pixels in the area, it is determined that the dot drawing start position for each hexagonal cell and each area is random. Further, the position to be filled as the second dot and thereafter is a position adjacent to the dot drawing start position, and is random for each hexagonal cell. Further, after the plurality of regions are filled, the remaining dot positions in the hexagonal cell are grown by using a collective halftone dither having the same shape.

In addition, the dots of each color in the hexagonal cell are arranged so as not to overlap as much as possible. Also, a dot of a certain color is arranged at the center of the hexagonal cell, and a dot of another color is arranged at the vertex of the hexagonal cell. Further, a dot of one color of black, magenta, and cyan is placed at the center of the hexagonal cell, dots of another color are placed at the apex of the hexagonal cell, and yellow is placed at another position.

The hexagonal cells are divided into sub-matrices. Also, a sub-matrix is used for auxiliary dots. Further, all the sub-matrices in the hexagonal cell need not have the same shape, and may be configured to include those having an irregular shape.

Further, a rectangular halftone dither tile having the same aspect ratio as the resolution ratio of the image output device having different resolutions in the main scanning direction and the sub-scanning direction is created, and the plurality of hexagonal cells are replaced with the halftone dither tile. Place within.
Further, the hexagonal cells are deformed vertically or horizontally according to the aspect ratio of the halftone dither tile. The halftone dither tile is one halftone dither cell. In the highlight portion, the same threshold value is set for a plurality of pixels in the halftone dither cell.

The halftone screening method can be executed by a computer program, and can be easily implemented by recording the computer program on an information recording medium and downloading it to a personal computer or the like. .

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS <First Embodiment> An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is an explanatory view showing a hexagonal cell used in an embodiment of the halftone screening method according to the present invention and a filled area of a highlight portion, and FIG. 2 is a diagram showing the start of dot filling in the hexagonal cell of FIG. Explanatory diagram showing points,
FIG. 3 is an explanatory diagram showing second and third filling points in the hexagonal cell of FIG. 1, and FIG. 3 is an explanatory diagram showing a filling region of a high density portion in the hexagonal cell of FIG.

Here, FIG. 22 described above shows an example of 5
For the square matrix of × 5 pixels, the upper right one pixel is 1 /
2 shows a hexagonal cell 1 of 5 × 5-1 = 24 pixels in which one pixel at the lower left is cut off to １ ／ and these are hypotenuses. In the conventional digital halftone screening using the hexagonal cells, the first cell to fill each cell is used.
When the point is set at the center of the cell, the dot pattern in the highlighted portion is easily noticeable because it has no randomness.

On the other hand, in the present invention, in order to make the dot drawing start point of each hexagonal cell 1 in the highlight portion different at random, first, as shown in FIG. A non-contact area 2 (3 × 3 pixels “0” to “8”) is taken. And the dot drawing start point 3
Is determined, the dot drawing start point 3 is randomly moved for each cell 1 in the 3 × 3 dot area 2 as shown in FIG. That is, the binarization threshold value of each dot position of the hexagonal cell 1 is set so that the dot drawing start position 3 in the 3 × 3 dot area 2 of each cell 1 is randomly different for each cell 1. .

In this case, since the movement is in the area 2 where the points of the adjacent cells 1 are not in contact with each other, a large deviation in density due to dot fusion or the like does not occur in the highlight portion. In addition, since one dot is always painted in each cell 1 as shown in FIG. 2, a phenomenon such as a start delay that occurs when the error diffusion method is used does not occur. Also, as shown in FIG. 1, the random position for each cell 1 is 3 random numbers.
The determination is made based on the remainder obtained by dividing the number of pixels in the area 2 of × 3 dots = 9.

After the dot of the first point is placed on all the cells 1, the dot arrangement after the second point is a position adjacent to the dot placed before the dot as shown in FIG. It is determined to be random for each one, and the above 3 × 3
This is repeated until all the pixels in the dot range 2 are filled. During this time, contact between the filled pixels does not occur between the adjacent cells 1.

After filling the area 2 of the above 3 × 3 dots, that is, for medium and high densities, as shown in FIG. 4, dots are grown in the same shape as in the case of a normal cluster type cell halftone. In addition, it is possible to suppress variations in density in the middle and high density portions of each cell 1.

<Second Embodiment> Next, a second embodiment will be described with reference to FIGS.

As described above, when the centralized dither matrix is used, if the diameter of each dot is too large in the highlight portion, the dots become too sparse and thin and thin lines cannot be represented. As a method of solving this problem, a method of dividing the cell into smaller cells is conceivable. However, in this method, in the medium density portion, adjacent dots are more likely to be fused by the dot gain, and therefore, the number of expressed gradations is reduced. There is a problem of lowering.

Therefore, in the second embodiment, the reproduction of fine lines in a highlight portion is improved, and the drawing of the halftone cells is started as a centralized dither matrix capable of preventing a decrease in the number of expressed gradations. A method of growing a plurality of dots in a low-density portion can be considered. In this case, it is necessary to take measures to suppress the generation of a noise pattern in the low density portion.

FIG. 5 shows a 9 × 9 as an example of the hexagonal cell.
For the square matrix of pixels, the upper left 3 × 3 pixel is 1 /
2 shows a hexagonal cell 1 of 9 × 9−3 × 3 = 72 pixels, in which the lower right 3 × 3 pixels are cut off by と し て and these are hypotenuses. However, in the conventional digital halftone screening using the hexagonal cell 1, the starting point for filling each cell 1 is set to the center point 3-1 of the upper right 5 × 5 pixel and the lower left 5 × 5 pixel in the hexagonal cell 1. The center point 3-2 has no randomness, so that the dot pattern in the highlight portion is easily noticeable.

Therefore, in the second embodiment, in order to make the two starting points 3-1 and 3-2 of each cell 1 different at random, first, as shown in FIG. Similarly to the above, the second embodiment does not make contact with a point in an adjacent cell, and in the second embodiment, does not make contact with another point in the same cell, and does not cause the other point in the same cell to be vertically and horizontally reversed. Two × 3 dot areas (2-1 and 2-2 in the figure) are taken. Then, when the two dot start points 3-1 and 3-2 are determined, as shown in FIG. 7, each of the start dots 3--3 in two areas 2-1 and 2-2 of 3 × 3 dots respectively. 1,3
−2 is randomly moved for each cell 1. That is, the dot drawing start positions 3-1 and 3-2 in the two areas 2-1 and 2-2 of each cell 1 are randomly different for each cell 1 so that the dot drawing start positions 3-1 and 3-2 of the cell 1 Set the threshold for value conversion.

In this case, since the movement is performed in a region where the points of adjacent cells do not touch each other, a large deviation in density due to dot fusion or the like does not occur in a highlight portion. In addition, since the relationship with other points in the same cell does not reverse, there is no occurrence of dot alignment abnormality. further,
Since two dots are always painted in each cell 1 as shown in FIG. 7, a phenomenon such as a start delay that occurs when the error diffusion method is used does not occur. As shown in FIG. 6, a method of determining a position to be randomly moved for each cell 1 includes a method of generating a random number and determining the random number by a remainder obtained by dividing the number by the number of pixels in the areas 2-1 and 2-2. , Other methods of imparting randomness can be applied.

The first dot 3-
The dot arrangement after the second point after placing 1, 3-2 is a position adjacent to the dot placed before the dot as shown in FIG. 8 and is random for each cell 1,
Also, as shown in FIG. 9, this is repeated until all the pixels in the above ranges 2-1 and 2-2 are filled. During this time, there is no contact of the filled pixels between adjacent cells.

After the areas 2-1 and 2-2 are painted, that is, for medium and high densities, the dots are painted in the same shape so that the dots in the same cell 1 are fused. After the dots are merged, this is treated as one dot. The reason for this is that if microdots are handled as they are,
This is because as the density becomes higher, the fusion of dots occurs earlier due to the influence of the dot gain, and the number of expressed gradations decreases.

<Third Embodiment> Next, FIGS.
The third embodiment will be described with reference to FIG.

For example, as shown in FIG. 10, a set of opposite corners of a square cell on a square lattice is cut off to form a halftone cell with a hexagonal cell which is not a regular hexagon, thereby forming a full-color image of Y, M, C and black. When expressing
When halftone cells having the same shape for each color are screened from the same origin (dot drawing start position), the center position of the dot on the screen for each color becomes the same. Therefore,
Since the dots, ie, ink, overlap, the color reproducibility deteriorates. In particular, there is a problem that the other color in which the black dots overlap is a dark image. FIG. 10 shows a 3 × cell in a hexagonal cell.
This indicates that black (indicated by a vertical line) of 2 × 2 pixels overlaps other colors (indicated by oblique lines) of the three pixels.

Japanese Patent Application Laid-Open No. 8-237496 proposes a method in which a black dot screen has a shift of half the screen frequency on a square lattice in order to effectively separate black dots from color dots. However, this method cannot be applied to the hexagonal cell 1.

Therefore, in the third embodiment, when halftone cells are formed by hexagonal cells to express a color halftone, interference of each color is reduced and color reproducibility is improved. In addition, a screening method is realized in which overlapping of dots of different colors is unlikely to occur even in a high density region. Further, the halftone cell having a simple configuration reduces interference of yellow, magenta, cyan and black, thereby improving color reproducibility.

In FIG. 11, first, as an example, 9 × 9
For the square matrix of pixels, the upper left 3 × 3 pixel and the lower right 3 × 3 pixel are both cut off in half to form a basic hexagonal cell of 72 (= 9 × 9−3 × 3) pixels that is not a regular hexagon. Form. Then, each vertex of the basic hexagonal cell is centered, and the basic hexagonal cell is divided so as to be smaller than the basic hexagonal cell. Is cut off in half to form a hexagonal cell of 24 (= 5 × 5-1) pixels which is not a regular hexagon. The hexagonal cell has three colors (shown by solid black, horizontal lines, and vertical lines in FIG. 11).
And by arranging the dots of each color at the center of the hexagon, the colors can be arranged as far apart from each other as possible.

However, in this case, only dots for three colors can be arranged. Therefore, in order to express full color, three colors other than yellow, which is not visually noticeable, are arranged by the above-described method. As shown in the figure, dots of four colors can be arranged by arranging them at other appropriate places (indicated by halftone dots).

In FIGS. 10 to 12, the dot drawing start position for each hexagonal cell of the same color is shown at the same position. However, this is merely to make the drawing easier to see. As in the first embodiment, one area that does not come into contact with a dot in an adjacent hexagonal cell is set in the hexagonal cell, and one dot drawing start position in this individual area is randomly set for each hexagonal cell. Thus, a binarization threshold value for each dot position in the hexagonal cell is set.

As in the second embodiment, a plurality of areas in the hexagonal cell that do not contact the dots in adjacent hexagonal cells, do not contact other points in the same cell, and Set a plurality of areas where the top, bottom, left and right positions do not reverse with other points in the same cell, and set each dot drawing start position in the plurality of areas to be random for each hexagon cell. A threshold for binarization of the dot position is set.

<Fourth Embodiment> Next, FIGS.
The fourth embodiment will be described with reference to FIG.

For example, as shown in FIG. 13, a set of opposite corners of a square cell on a square lattice is cut off to form a hexagonal cell that is not a regular hexagon, and a plurality of hexagonal cells (four in FIG. 13) are formed. In the method of forming one halftone cell by combining the halftone cells, since attention is paid to the dots used for drawing the peripheral portion of the rectangular area parallel to the x-axis direction, the arrangement of the dots is acute, so the distance between the dots is large. Becomes larger. For this reason, “missing” occurs in a portion where the interval between the dots is wide, and the dots in the vicinity of the dots on the line are not symmetric with respect to the center between the dots. There is a problem of being noticeable.

Therefore, in the fourth embodiment, the undulation of the image is suppressed when one halftone cell is formed by combining a plurality of hexagonal cells. FIG. 14 is an explanatory diagram showing a sub-matrix, and FIG. 15 is an explanatory diagram showing auxiliary points in the sub-matrix of FIG.

Here, as described above, when attention is paid to the dots used for drawing the peripheral portion of the rectangular area parallel to the x-axis direction, since the dot arrangement is acute, the distance between the dots is large. Become. For this reason, “missing” occurs in a portion where the interval between the dots is wide, and the dots near the dots on the line are not symmetric with respect to the center between the dots.
There is a problem that if the dot is grown as it is, it becomes "undulation" and becomes noticeable.

Therefore, in order to prevent this, the arrangement of dots is set to an appropriate angle, or the dot interval (cell itself) is used.
Can be reduced. Therefore, in the fourth embodiment, as shown in FIG. 15, each hexagonal cell in the composite hexagonal cell, which is one halftone cell, is divided to form a sub-matrix, and as shown in FIG. An auxiliary point (auxiliary dot) is provided so as to be small.

More specifically, first, as shown in FIG. 14, a square matrix of 6.times.6
Two pixels and the lower right 2 × 2 pixel are cut off in half to form one hexagonal cell of 32 (= 6 × 6−2 × 2) pixels which is not a regular hexagon, and four hexagonal cells are combined. To form a composite hexagonal cell. Then, in each hexagonal cell, one pixel at the upper left and one pixel at the lower right are cut off in half with respect to a square matrix of 3 × 3 pixels to form a hexagon of 8 (= 3 × 3-1) pixels that are not regular hexagons. Form a square sub-matrix. When the center cell of the hexagonal sub-matrix is set as an auxiliary point as shown in FIG. 15, the dot interval becomes small, and therefore, “missing” which causes undulations
Since the portion can be filled, the undulation of the image can be suppressed.

As a first method of expressing halftones by this method, first, cells in a sub-matrix where no auxiliary dots are provided are filled and grown, and this sub-matrix is filled with all cells. , The cells in the sub-matrix provided with the auxiliary points are painted out. As a second method, as shown in FIG. 16, if the auxiliary dots are treated and grown in the same manner as the original dots, the number of lines can be increased without reducing the number of gradations. However, in this case, since the distance between the dots is small, the coupling by the dot gain is likely to occur. Therefore, it is better to determine which method is to be used depending on the shape and size of the sub-matrix.

Here, the sub-matrices in a hexagonal cell need not all be the same shape, and need not be hexagonal. FIG. 17 shows, as an example, a composite hexagonal cell in which four hexagonal cells are combined, and the oblique side of the upper left is set to 8
A pentagonal sub-matrix of pixels, the lower right hypotenuse is a 2-pixel triangular sub-matrix, and the rest are 3 × 4, 4 ×
This shows a case where a rectangular sub-matrix of 3, 3 × 3 pixels is used.

In FIGS. 13 to 17, the dot drawing start position for each hexagonal cell is shown at the same position, but this is merely for the purpose of making the drawing easier to see. Similarly, in the hexagonal cell, set one area that does not contact the dots in the adjacent hexagonal cell,
A threshold for binarization of each dot position in a hexagonal cell is set so that one dot drawing start position in each of the regions is random for each hexagonal cell.

Similarly to the second embodiment, a plurality of areas in the hexagonal cell that do not contact the dots in the adjacent hexagonal cells, do not contact other points in the same cell, and Set a plurality of areas where the top, bottom, left and right positions do not reverse with other points in the same cell, and set each dot drawing start position in the plurality of areas to be random for each hexagon cell. A threshold for binarization of the dot position is set.

<Fifth Embodiment> Next, a fifth embodiment will be described with reference to FIGS.

A tile used in halftone screening for an image output device (printer, display) having different resolutions in the main scanning direction and the sub-scanning direction needs to have a shape corresponding to the resolution ratio of the image output device. Therefore, in the fifth embodiment, when the resolution ratio between the main scanning direction and the sub-scanning direction of the image output device is, for example, 2: 1, FIG.
8, the same 2: 1 halftone dither tile 1 (main scanning direction = 32 pixels × sub scanning direction = 16 in the figure)
Pixel). The area for each pixel is 2
In order to express the halftone by the digitized data, a threshold value for binarization in comparison with the multi-valued image data is set.

Further, in the fifth embodiment, in order to suppress the noise patterns in the horizontal and vertical directions, FIG.
As shown in FIG. 9, in the 2: 1 halftone dither tile 11, a plurality of hexagonal cells 1 made of a square and not a regular hexagon are arranged. Here, as for the size of the dither tile 11, the size matched to the byte boundary is more efficient in terms of the storage speed in the memory and the calling speed from the memory. Therefore, the size of the dither tile 11 is appropriate for the byte boundary, but the dither cell of the hexagonal cell 1 made of a square may not fit well in the tile size limited to 2: 1 as described above. .

Therefore, a hexagonal cell dither cell is created from a rectangle obtained by slightly deforming a square, and is created while adjusting it in consideration of an asymmetric resolution ratio. FIG. 20A shows, as an example, a 5 × 5 pixel square 13 in which 5 × 5 pixels are cut off by cutting off ２ of the upper left two pixels and the lower right two pixels.
A hexagonal cell 14 of −2 × 2 = 21 pixels is created, and then this hexagonal cell 14 is horizontally expanded by a factor of 2 to fit a tile size limited to 2: 1 as described above.
This shows a case where a hexagonal cell 15 of 0 × 5−4 × 2 = 42 pixels is created.

FIG. 20B shows another example of 4 ×
A hexagonal cell 17 of 4 × 5-2 × 2 = 16 pixels having two oblique sides by cutting off the upper left two pixels and the lower right two pixels of the 5-pixel rectangle 16 is created, and then the hexagonal cell 17 is formed.
Is horizontally extended twice to fit the tile size limited to 2: 1 as described above, and 8 × 5−4 × 2 = 32
This shows a case where a hexagonal cell 18 of pixels is created. Note that one hexagonal cell 15 shown in FIG.
Gray scale can be expressed, and one hexagonal cell 18 shown in FIG. 20B can express 32 gray scales.

As shown in FIG. 19, the plurality of hexagonal cells created in this way are combined so as to fit in a tile size limited to 2: 1.
Create Therefore, by treating the dither tile as one unit (one super cell), the rectangular dither tile 11 shown in FIG.
The gradation can be expressed.

Here, when a pixel is painted from the low density portion as a tile = cell one pixel at a time, a noise pattern in the horizontal direction and the vertical direction is generated. Therefore, a plurality of pixels (for example, FIG. By setting the same threshold for the indicated pixel (1)), it is possible to suppress the horizontal and vertical noise patterns.

Although the dot drawing start position for each hexagonal cell is not shown in FIGS. 18 to 21, it is actually similar to the first embodiment in the hexagonal cell adjacent to the hexagonal cell. Set one area that does not touch the dot of
A threshold for binarization of each dot position in a hexagonal cell is set so that one dot drawing start position in each of the regions is random for each hexagonal cell.

Similarly to the second embodiment, a plurality of areas in the hexagonal cell that do not contact the dots in the adjacent hexagonal cell, do not contact other points in the same cell, and Set a plurality of areas where the top, bottom, left and right positions do not reverse with other points in the same cell, and set each dot drawing start position in the plurality of areas to be random for each hexagon cell. A threshold for binarization of the dot position is set.

[0059]

As described above, according to the first aspect of the present invention, one area that does not come into contact with a dot in an adjacent hexagonal cell is set in a hexagonal cell, and one area in each individual area is set. The threshold value for binarization of each dot position in the hexagonal cell is set so that one dot drawing start position is random for each hexagonal cell. Generation can be suppressed.

According to the second aspect of the present invention, the dot drawing start position for each hexagonal cell is determined to be random based on the remainder obtained by dividing the random number by the number of pixels in the area. As compared with the method that requires complicated calculation such as described above, it is possible to suppress an increase in calculation cost and to make the dot arrangement random.

According to the third aspect of the present invention, since the position to be filled as the second dot or later is a position adjacent to the dot drawing start position and is random for each hexagonal cell, It is possible to suppress the occurrence of a dot pattern that is easily visible.

According to the fourth aspect of the present invention, after the area is painted, the remaining dot positions in the hexagonal cell are grown by a dot-assembled halftone dither having the same shape, so that the density in the middle and high density portions is increased. Can be suppressed.

According to the fifth aspect of the present invention, a plurality of areas in the hexagonal cell that do not contact the dots in the adjacent hexagonal cells, and do not contact other points in the same cell, and Set a plurality of areas where the other points in the cell and the top, bottom, left and right positions do not reverse, so that the dot drawing start position in the plurality of areas is random for each hexagon cell, Since the threshold for binarization at each dot position is set, it is possible to suppress the occurrence of a dot pattern that is easily noticeable in a highlight portion.

According to the sixth aspect of the invention, the dot drawing start position for each hexagonal cell and each region is determined to be random based on the remainder obtained by dividing the random number by the number of pixels in the region. As compared with a method requiring a complicated calculation such as an error diffusion method, it is possible to suppress an increase in calculation cost and to provide a random arrangement of dots.

According to the seventh aspect of the present invention, since the position to be filled as the second dot and thereafter is a position adjacent to the dot drawing start position and is random for each hexagonal cell, It is possible to suppress the occurrence of a dot pattern that is easily visible.

According to the eighth aspect of the present invention, after the area is painted, the remaining dot positions in the hexagonal cell are grown by the dot aggregation type halftone dither having the same shape. Can be suppressed.

According to the ninth aspect of the present invention, the dots of each color in the hexagonal cell are arranged so as not to overlap as much as possible. In addition to the above-described effects, the color reproduction is reduced by reducing the interference of each color. Performance can be improved.

According to the tenth aspect, a dot of a certain color is arranged at the center of a hexagonal cell, and a dot of another color is arranged at a vertex of a hexagonal cell. It is possible to prevent overlapping of dots of different colors from occurring even in a high density region.

According to the eleventh aspect of the present invention, dots of a certain color among black, magenta, and cyan are arranged at the center of a hexagonal cell, and dots of another color are arranged at vertices of a hexagonal cell. Since yellow is arranged at another position, in addition to the above-mentioned effects, interference of yellow, magenta, cyan, and black can be reduced by a halftone cell having a simple configuration, and color reproducibility can be improved.

According to the twelfth aspect of the present invention, one halftone cell is formed by combining a plurality of hexagonal cells, and the hexagonal cells are divided into sub-matrices. In addition, the swell of the image can be suppressed.

According to the thirteenth aspect of the present invention, since the sub-matrix is used for auxiliary dots, undulation of an image can be suppressed in addition to the above effects.

According to the fourteenth aspect of the present invention, all the sub-matrices in the hexagonal cell do not need to have the same shape, so that in addition to the above effects, the dot shape can be varied.

According to the fifteenth aspect of the present invention, a rectangular halftone dither tile having the same aspect ratio as the resolution ratio between the main scanning direction and the sub-scanning direction of the image output device is created, and a regular hexagon formed on a square lattice Since a plurality of hexagonal cells that are not arranged in the halftone dither tile, in addition to the above effects, when performing halftone screening for an image output device having different resolutions in the main scanning direction and the sub scanning direction, horizontal Noise patterns in the direction perpendicular to the direction can be prevented.

According to the sixteenth aspect of the present invention, since the hexagonal cells are deformed in the vertical direction or the horizontal direction according to the aspect ratio of the halftone dither tile, the processing speed is improved in addition to the above effects. And the memory capacity can be reduced.

According to the seventeenth aspect of the present invention, since the halftone dither tile is one halftone dither cell, the number of gradations that can be expressed can be increased.

According to the eighteenth aspect of the present invention, the same threshold value is set for a plurality of pixels in the halftone dither cell in the highlight portion. Noise patterns can be prevented.

According to the nineteenth aspect, the present invention can be realized not only by hardware but also by a computer program.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a hexagonal cell used in a halftone screening method according to a first embodiment and a filled area of a highlight portion.

FIG. 2 is an explanatory diagram showing a dot filling start point in the hexagonal cell of FIG. 2;

FIG. 3 is an explanatory diagram showing second and third filling points in the hexagonal cell of FIG. 2;

FIG. 4 is an explanatory diagram showing a filled region of a high density portion in the hexagonal cell of FIG. 2;

FIG. 5 is an explanatory diagram showing another general hexagonal cell and a dot drawing start position.

FIG. 6 is an explanatory diagram showing a hexagonal cell used in the halftone screening method of the second embodiment and a filled area of a highlight portion.

FIG. 7 is an explanatory diagram showing a dot drawing start position in the hexagonal cell of FIG. 6;

FIG. 8 is an explanatory diagram showing second and third filling points in the hexagonal cell of FIG. 6;

FIG. 9 is an explanatory diagram showing a filled area of a high density portion in the hexagonal cell of FIG. 6;

FIG. 10 is an explanatory diagram showing a conventional arrangement position of black, yellow, magenta, and cyan.

FIG. 11 is an explanatory diagram illustrating a halftone screening method according to a third embodiment.

FIG. 12 is an explanatory diagram showing hexagonal cells used in a halftone screening method according to a third embodiment and arrangement positions of black, yellow, magenta, and cyan.

FIG. 13 is an explanatory diagram showing a general matrix.

FIG. 14 is an explanatory diagram showing a sub-matrix used in the halftone screening method of the fourth embodiment.

FIG. 15 is an explanatory diagram showing auxiliary points in the sub-matrix of FIG.

FIG. 16 is an explanatory diagram showing a gradation expression method according to a modified example of the sub-matrices of FIGS. 14 and 15;

FIG. 17 is an explanatory diagram showing another shape of the sub-matrix in FIGS. 14 and 15;

FIG. 18 is an explanatory diagram illustrating a configuration of a halftone dither tile used in a halftone screening method according to a fifth embodiment.

FIG. 19 is an explanatory diagram showing a state where hexagonal cells are arranged in the halftone dither tile of FIG. 10;

FIG. 20 is an explanatory diagram showing a method for creating the hexagonal cell in FIG. 19;

FIG. 21 is an explanatory diagram showing a threshold setting process for a highlight portion.

FIG. 22 is an explanatory diagram showing a general hexagonal cell and a dot drawing start position.

[Explanation of symbols]

1 Hexagonal cell 2,2-1,2-2 Area 3,3-1,3-2 Dot drawing start position (drawing start dot)

 ──────────────────────────────────────────────────続 き Continuing on the front page F term (reference) 2C262 AA24 AB05 BB03 BB06 EA08 GA21 5B057 CA01 CA08 CA12 CA16 CB01 CB07 CB12 CB16 CE13 CE16 CH01 CH11 DA08 DB02 DB06 DB09 DC22 5C077 MP08 NN09 PP33 PP38 PP52 PP68 RR02 H03 5 LC04 LC07 LC13 NA02 NA03 NA10 NA11 NA25

Claims (19)

[Claims] 1. Opposite ones of square cells on a square lattice
In a halftone screening method using a hexagonal cell as a halftone cell, which cuts off a set of corners and sets the hypotenuse as a hypotenuse, sets one area that does not contact a dot in an adjacent hexagonal cell in the hexagonal cell, A half-value threshold value for each dot position in said hexagonal cell is set so that one dot drawing start position in each of said regions is random for each said hexagonal cell. Tone screening method. 2. The halftone according to claim 1, wherein a dot drawing start position for each of the hexagonal cells is determined to be random based on a remainder obtained by dividing a random number by the number of pixels in the area. Screening method. 3. A position to be filled as a second dot and thereafter is a position adjacent to the dot drawing start position,
3. The halftone screening method according to claim 1, wherein each of the hexagonal cells is random. 4. The method according to claim 1, wherein after the area is filled, the remaining dot positions in the hexagonal cell are grown by a dot aggregation type halftone dither having the same shape. The halftone screening method according to any one of the first to third aspects. 5. Opposite ones of square cells on a square lattice
In a halftone screening method using a hexagonal cell having a hypotenuse as a halftone cell by cutting off a corner of a set, a plurality of areas not in contact with dots in adjacent hexagonal cells in the hexagonal cell, Set a plurality of areas that do not touch other points in the same cell and that the positions of the other points in the same cell do not reverse up, down, left, and right, and each dot drawing start position in the plurality of areas is the hexagon. A halftone screening method, wherein a binarization threshold value for each dot position in the hexagonal cell is set so as to be random for each cell. 6. The method according to claim 5, wherein a dot drawing start position for each hexagonal cell and each area is determined based on a remainder obtained by dividing a random number by the number of pixels in the area. The described halftone screening method. 7. A position to be filled as a second dot or later is a position adjacent to the dot drawing start position,
7. The halftone screening method according to claim 5, wherein each of the hexagonal cells is random. 8. After filling the plurality of regions,
The halftone screening method according to any one of claims 5 to 7, wherein the remaining dot positions in the hexagonal cell are grown by a collective halftone dither having the same shape. 9. The halftone screening method according to claim 1, wherein dots of each color in the hexagonal cell are arranged so as not to overlap as much as possible. 10. The halftone screening method according to claim 9, wherein a dot of a certain color is arranged at the center of the hexagonal cell, and a dot of another color is arranged at a vertex of the hexagonal cell. 11. A dot of one color of black, magenta and cyan is placed at the center of a hexagonal cell, a dot of another color is placed at the vertex of a hexagonal cell, and yellow is placed at another position. The halftone screening method according to claim 9 or 10, wherein: 12. The halftone screening method according to claim 1, wherein the inside of the hexagonal cell is divided into a sub-matrix. 13. The halftone screening method according to claim 12, wherein said sub-matrix is used for auxiliary dots. 14. The halftone screening method according to claim 12, wherein the sub-matrices in the hexagonal cells include those having different shapes. 15. A rectangular halftone dither tile having the same aspect ratio as the resolution ratio of the image output device having different resolutions in the main scanning direction and the sub-scanning direction, and the plurality of hexagonal cells are replaced with the halftone dither tile. The halftone screening method according to any one of claims 1 to 14, wherein the halftone screening method is arranged in a screen. 16. The halftone screening method according to claim 15, wherein the hexagonal cell is deformed in a vertical or horizontal direction according to an aspect ratio of the halftone dither tile. 17. The halftone screening method according to claim 15, wherein the halftone dither tile is one halftone dither cell. 18. The halftone screening method according to claim 17, wherein the same threshold value is set for a plurality of pixels in the halftone dither cell in the highlight portion. 19. An information recording medium on which a computer program for executing the halftone screening method according to claim 1 is recorded.