JP2003116000A - Dither pattern generating method and apparatus, computer program, and computer-readable storage medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow dots of print objects to be easily produced even when a multilevel image with a comparatively light density is received while maintaining gradation reproducibility. SOLUTION: When threshold values for generating a 8×8 dither matrix are 0, 4, 8, etc., positions of the threshold value 28 being the eighth order from the minimum threshold value 0 are exclusive to each other by avoiding rows and columns to which they belong from being overlapped with each other. As a result, the minimum values of the threshold values of the rows and columns are 0, 4, 28, and even when a multilevel image with a comparatively light density is received, dots can be formed on any row or column with high possibility.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dither pattern creating method and apparatus, a computer program, and a computer-readable storage medium.

[0002]

2. Description of the Related Art The halftone method is a method of area-modulating a multi-valued image and expressing it as a binary image. Among them, the dither method has a merit that it can be processed at high speed because it is a process of simply comparing a dither matrix in which an input value and a threshold value are lined up, and is also a widely used method.

[0003]

However, since the dither method truncates input data that does not reach the threshold value by simple comparison, for example, thin line data of thin and thin lines (for example, 1 dot line) may be completely rounded. It may disappear. This will be described with reference to FIG.

FIG. 1 shows an example of a so-called dot concentration type dither pattern. The size of the dithering target is 8x8,
The input value is 8 bits (256 gradations from 0 to 255).
The number of gradations is 8 for input values that can take the range of 0-255.
Since only x8 = 64 can be obtained, an input value per 4 gradations is taken every 4 levels. Only pixels whose pixel values in the pattern are smaller than the input value are printed.

Now, using the dither matrix shown in the drawing, when a vertical line of 1 pixel width at level "64" overlaps the leftmost column in FIG. 1 (the hatched part in the drawing), each of the input lines The pixel value is less than any threshold in the leftmost column of the dither matrix. Therefore, all are binarized as "0", and as a result, the dots to be printed disappear.

The present invention has been made in view of the above problems, and it is possible to easily generate dots to be printed even if a multi-valued image having a relatively low density is input while maintaining gradation reproducibility. An object of the present invention is to provide a dither pattern creating method and apparatus, a computer program, and a computer-readable storage medium.

[0007]

In order to solve this problem, for example, the dither pattern forming method of the present invention comprises the following steps. That is, it is a dither matrix creation method in the dither method in which multi-valued image data in which one pixel is represented by a plurality of bits is quantized into a smaller number of bits, and each grid of the dither matrix has a first gradation. When sequentially allocating from the threshold value, as the grid to which the i-th threshold value is allocated, the row and column to which the threshold value allocated before the i-1th is set as the allocation prohibition area, and selected from the grids other than the allocation prohibition area. The i-th threshold value is assigned to the selected cell.

[0008]

DETAILED DESCRIPTION OF THE INVENTION Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

First, an outline of the procedure in the embodiment is shown in FIG.
It will be described with reference to FIGS. Here, the size of the dither matrix is 8 × 8, and the input image has 256 gradations in which one pixel has 8 bits (values from 0 to 255 can be taken).
And

Therefore, the dither matrix is 8 × 8 = 6.
Since four thresholds are needed, the threshold is 0,
It is a value every 4 levels such as 4, 8, 12 ...

First, as shown in FIG. 2, a level (threshold) of the first gradation "0" is assigned to the upper left pixel of the dither matrix.

Next, referring to FIG. 3, the second gradation "4"
Excludes rows and columns (areas indicated by diagonal lines in the figure) including "0" of the first gradation, selects from the remaining cells (pixel positions), and assigns to that position.

Further, referring to FIG. 4, "8" of the third gradation is a row and a column containing "0" of the first gradation and a row containing "4" of the second gradation. The column (hatched portion in FIG. 4) is excluded, selected from the remaining squares, and assigned to that position.

Further, referring to FIG. 5, "12" at the fourth gradation is a row and column including pixels before the second gradation and a row including "8" at the third gradation. And row (hatched area in Figure 5)
Select from the remaining squares that exclude, and assign to that position.

As described above, when the position to which the threshold value of the i-th gradation is assigned is sequentially determined, pixels are selected from the rows and columns excluding the pixels of the i-1th gradation and before. Then, as shown in FIG. 6, when the threshold “28” for the eighth gradation is determined, only one pixel remains in the selectable place.

With respect to the gradation after this, the pixel position is appropriately determined up to the threshold value "255" of the 64th gradation without being bound by the above rule, and for example, as shown in FIG.
The dither matrix is completed as shown in. For example,
The process of selecting and assigning one with a random number from the cells to which the threshold is not assigned is repeated until the final threshold "255".

The minimum values of the threshold values in each row and each column of the dither matrix of FIG. 7 are obtained as shown in FIG. The maximum threshold value in the minimum value group is “28”. Therefore, when the dither matrix of FIG. 7 is used, as long as the density of the input multivalued image data to be binarized is at least the value “29”, at least one pixel exceeding the threshold value is secured. In other words, it is guaranteed that there is at least one printed dot, and it is possible to make it difficult for a line image having a low density to disappear.

As described above, the idea of the present invention is that, of the threshold values assigned to the cells of the dither matrix, the smaller value (0, 4, 8, ..., 28) is the row to which the assigned cells belong. Its main feature is that the columns adopt mutually exclusive ones.

Therefore, as shown in FIG. 9, in a square dither matrix, a gray scale corresponding to the number of pixels on one side of the dither matrix, that is, a pixel size of 8 in this case.
Since it is a x8 dither matrix, "2" of the eighth gradation
Up to 8 ″, if the pixels are arranged on the diagonal pixels of the dither matrix, the same effect as the example described above can be obtained. The threshold values for the ninth gradation and higher are appropriately arranged and determined. Is similar to the above example.

Next, an example of an apparatus for creating the above dither matrix is shown in FIG. 10, and its operation processing procedure will be described with reference to the flowchart of FIG.

FIG. 10 is a block diagram of a general information processing apparatus (for example, personal computer).

In the figure, 1 is a CPU which controls the entire apparatus, 2 is a ROM which stores a BIOS and a boot program, and 3 is a RAM which is used as a work area of the CPU 1. 4 is OS, various applications,
It is a hard disk device (HDD) that stores the dither matrix creation processing program in the embodiment. 5
Is a display device having a built-in VRAM and a controller for controlling its drawing. Reference numeral 6 is a keyboard (KB), a pointing device (PD) such as a mouse, and 7 is an interface for connecting the printer 8. The printer 8 may be of any type, but, for example, a type that ejects ink liquid is adopted.

In the above configuration, when the power of the apparatus is turned on, the CPU 1 loads the OS in the HDD 4 according to the boot program of the ROM 2 into the RAM 3, and accordingly edits using various applications. However, in the embodiment, a dither matrix creation program will be described.

FIG. 10 is a flow chart showing the operation procedure of the dither matrix creation program. Hereinafter, description will be made in order with reference to FIG.

First, in step S1, the size of the dither matrix to be created is determined. Since the dither matrix does not have to be square, M × N as shown
Can be expressed as Therefore, M and N are first determined. As for the determination method, for example, a message inquiring the user is displayed, and M and N are determined from the answer.

Next, in step S2, an increment value ΔT for assigning a threshold value is calculated. In the case of an 8-bit multi-valued image, 256 possible values from 0 to 255 can be taken, and therefore the value is calculated by dividing it by the size of the dither matrix (the number of threshold values = M × N). It should be noted that the calculated ΔT is not an integer, but is calculated to a decimal point with an appropriate number of digits.

Next, in step S3, a variable T for storing the threshold to be assigned is initialized to 0, and a variable i used as a counter is also initialized to 0.

In step S4, the value n of the dither matrix size M.times.N input earlier and the smaller value n of N are determined. In the case of the size of 8 × 8 described above, n = 8, but this means that when assigning a threshold value from its minimum value, it is the eighth, and as shown in FIG. 6, there is only one. This is because there is no pixel position to be assigned.

In step S5, the row and column to which the position of the grid to which the threshold value has already been assigned belong is set as an allocation prohibited area, and one of the remaining grids is selected. In the initial state, all M × N are selected. When the grid to which the threshold is assigned is determined, the process proceeds to step S6, and the integer part of the threshold T (0 in the initial state) is assigned to the position, that is, the fractional part is truncated and the threshold is assigned.

In the next step S7, the threshold value T is increased by ΔT and the variable i is increased by "1".

In step S8, the value of the variable i is compared with n. If there is a relation of i <n, the process returns to step S5 and the next threshold value is assigned.

Thus, when the variable i becomes equal to n, the process proceeds to step S9, and the remaining thresholds are sequentially assigned to unassigned grid positions. For example, it suffices to rearrange the unallocated cells in a random number, and thereafter increase the threshold value by ΔT and allocate the cells. When this allocation is completed, the dither matrix created in step S10 is stored in the HDD 4.

Although the procedure for creating the dither matrix has been described in the above example, for example, when a print instruction is given on an application, the printer driver is activated. In the printer driver, the above-described procedure described in this embodiment is used. The processing may be started. That is, the processing procedure of FIG. 10 may be incorporated as one processing of the printer driver.

<Second Embodiment> In the above embodiment, when the size of the dither matrix is, for example, 8 × 8, and the thresholds “0” to “28”, the rows and columns to which they belong are mutually exclusive. However, the present invention is not limited to this.

For example, as shown in FIG. 6, when the threshold value "28" is assigned, only one square is considered, but when the assignment is completed, the assignment prohibited area may be reset and rubbed. Of course, with respect to the squares that already have the assigned threshold value, the value is not changed.

Therefore, when the threshold value "32" is assigned,
Since the allocation prohibition area before that is reset, as shown in FIG. 12, it is permitted to select from anywhere except the shaded area. Therefore, eight threshold values after "32" can be similarly assigned. And 8
When the allocation of the individual threshold values is completed, the allocation prohibited area may be reset again and the same processing may be repeated.

In order to realize such processing, it suffices to perform processing according to the procedure of FIG.

In the figure, step S is different from FIG.
9 is replaced with steps S11 and S12. In other words, in step S11, the allocation-prohibited area is reset while leaving the cells to which the threshold has already been allocated.
In step S12, it is determined whether or not allocation of all thresholds has been completed, and if not, the processes in step S5 and subsequent steps are repeated.

As a result of the above, according to the second embodiment,
For example, in an 8x8 dither matrix, "28"
When multi-valued data that exceeds P times P is input, it is guaranteed that the number of “1” s when binarized will be P.

Since the above-described embodiment can be realized as a computer program as described above, the present invention also includes a computer program in its category. Further, since the computer program usually means setting a storage medium for storing the computer program in the device and copying or installing the program in a memory (hard disk etc.) in the device, the present invention provides such a computer-readable storage medium. It is clear that this is also a category.

In the embodiment, the dither matrix is described as an example of binarization. However, the dither matrix may be adapted to ternarization in which the quantized values are 0, 1, and 2. If it is applied to ternarization, first, the quantized data is set to 2, or one dither matrix that is binarized to a value other than 2 (either 0 or 1) is used, and it is determined that it is not 2. It suffices to prepare a dither matrix that binarizes the generated pixels into either 0 or 1. The same applies to quantization with four levels or more.

[0042]

As described above, according to the present invention, it is possible to easily generate dots to be printed even if a multi-valued image having a relatively low density is input while maintaining gradation reproducibility. become.

[Brief description of drawings]

FIG. 1 is a diagram showing a structure of a conventional dither matrix and a state of binarization.

FIG. 2 is a diagram showing an example of a dither matrix in a state in which a first threshold value is set in the embodiment.

FIG. 3 is a diagram showing an example of a dither matrix having an allocation prohibited area when setting a second threshold value in the embodiment.

FIG. 4 is a diagram showing an example of a dither matrix having an allocation prohibited area when setting a third threshold value in the embodiment.

FIG. 5 is a diagram showing an example of a dither matrix having an allocation prohibited area when setting a fourth threshold value in the embodiment.

FIG. 6 is a diagram showing an example of a dither matrix having an allocation prohibited area when setting an eighth threshold value in the embodiment.

FIG. 7 is a diagram showing a structure of a dither matrix in the embodiment.

FIG. 8 is a diagram showing extracted minimum values of rows and columns of the dither matrix of FIG.

FIG. 9 is a block configuration diagram of an apparatus according to an embodiment.

FIG. 10 is a flowchart showing an operation processing procedure in the embodiment.

FIG. 11 is a flowchart showing an operation processing procedure in the second embodiment.

FIG. 12 is a diagram showing an example of a dither matrix when setting a ninth threshold value according to the second embodiment.

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Claims (5)

[Claims] 1. A dither matrix creation method in a dither method for quantizing multi-valued image data in which one pixel is represented by a plurality of bits into a smaller number of bits, wherein the first floor is formed in each cell of the dither matrix. When sequentially allocating from the threshold value of the key, as the grid to which the i-th threshold value is allocated, the row and the column to which the threshold value allocated before the i-1th is set as the allocation-prohibited area, and the cells other than the allocation-prohibited area are A method for creating a dither matrix, which is characterized by selecting from among and assigning an i-th threshold value to the selected cell. 2. The dither matrix creating method according to claim 1, further comprising the step of resetting the allocation prohibited area when a grid to which a target threshold is allocated is not found. 3. A dither matrix creation device in a dither method for quantizing multi-valued image data in which one pixel is represented by a plurality of bits into a smaller number of bits, wherein a first floor is provided in each cell of the dither matrix. When sequentially allocating from the threshold value of the key, as the grid to which the i-th threshold value is allocated, the row and the column to which the threshold value allocated before the i-1th is set as the allocation-prohibited area, and the cells other than the allocation-prohibited area are An apparatus for creating a dither matrix, comprising: selecting means for selecting from among them and assigning means for assigning an i-th threshold value to a selected square. 4. A computer program that functions as a dither matrix creating device in a dither method for quantizing multi-valued image data in which one pixel is represented by a plurality of bits into a smaller number of bits, and each cell of the dither matrix. At the time of sequentially allocating from the threshold value of the first gradation, the row and column to which the threshold value assigned before the i−1th threshold value belongs is set as an allocation prohibition area as a cell to which the i-th threshold value is allocated. A computer program comprising: a program code of a step of selecting from other squares; and a program code of a step of assigning an i-th threshold value to the selected square. 5. A computer-readable storage medium further storing the computer program according to claim 4.