JP2006245674A - Image forming apparatus and gradation expressing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a gradation expressing method capable of being used in a printer utilizing an area gradation method, in which texture such as a periodic structure or granularity is inconspicuous and gradation is excellent. <P>SOLUTION: In the gradation expressing method with an AM screen in an area gradation system printer, dots having the same areas and different shapes are prepared as dots for the AM screen and are allocated to different gradation values. For example, using a dither method utilizing a dot concentrated threshold matrix, two or more threshold matrices having the same areas and different shapes are prepared to select a threshold matrix in accordance with gradation. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus for forming an area gradation image and a gradation expression method therefor.

AM screens used as halftones for electrophotography, ink jet printing, and printing express the color shading with the size of each halftone dot with a constant interval between each halftone dot. However, the AM screen has a higher number of lines (unit length). (Increase the number of halftone dots per unit = decrease the halftone dot interval), the halftone dot interval is reduced, so that an image with high resolution can be obtained. However, by increasing the number of lines, the dot interval, that is, the unit cell of the AM screen becomes smaller, and if the unit cell is small, the number of included pixels decreases and the number of gradations that can be expressed decreases, resulting in poor gradation expression. Become.
As a method for improving the gradation expression with a high number of lines, a supercell method is generally used. This is a group of adjacent cells (eg, 2 × 2 halftone dots, 3 × 3 halftone dots, etc.) grouped into one unit supercell, and the size of each halftone dot in the supercell This is a method for improving gradation expression by applying modulation.
For example, if the unit cell of a halftone dot is represented by a group of 5 × 5 = 25 pixels, it can only represent 25 gradations of the dot area of each halftone dot. On the other hand, when the supercell is composed of four 2 × 2 halftone dots, four halftones (collection of 5 × 5 × 4 = 100 pixels) represent four times 100 gradations. be able to.
However, since a dot of each halftone dot in the supercell is modulated, a large periodic pattern in supercell units is generated. As a result, the periodic structure is noticeable and the image quality is degraded. In addition, since the dots of each halftone dot in the supercell are modulated, a granular component is generated, which also causes deterioration in image quality.
Patent Document 1: Halftone dot pattern generation method: This patent relates to a halftone dot pattern generation method, and is an invention made to improve gradation with a low resolution printer. As a method, a block pattern in which a plurality of basic patterns are collected is generated, and the number of gradations is increased by increasing the number of constituent pixels per unit pattern.

JP-A-8-87105

  Therefore, the present invention can also be used in a printer that uses an area gradation method such as electrophotography or ink jet, and gradation expression that does not show texture such as periodic structure and graininess and has excellent gradation property. The purpose is to obtain a method.

The inventor uses the phenomenon that even if the area ratio is the same as digital data, the macro density is different (dot gain characteristics change) when printed out by electrophotography or the like due to the difference in dot geometry. The present inventors have found that tone expression can be improved without modulating the dot size. In other words, regarding the dots of the dot of the area gradation method AM screen, even if the dot area as the digital data is the same, when the dot shape is different, the printed out density is slightly different. The inventors have realized that the gradation can be improved not only by the dot area of the halftone dots but also by the difference in shape, thereby improving the gradation.
That is, the subject of this invention is achieved by the following means.

1) An image forming apparatus characterized in that dots having the same area and different shapes are prepared as halftone dot digital data of an AM screen and assigned to different gradation signal values.
2) Using a dither method using a dot concentration type threshold matrix, preparing two or more threshold matrices having the same dot area but different shapes, and selecting the threshold matrix corresponding to the gradation signal value The image forming apparatus according to claim 1.
3) The image forming apparatus according to claim 1 or 2, wherein the image is formed by electrophotography.
4) Gradation characterized by increasing the number of gradations in the image forming apparatus by preparing dots having the same area and different shapes as the halftone dot digital data of the AM screen and assigning them to different gradation signal values. expression methed.

  According to the present invention, even if the area ratio is the same, a phenomenon in which the macro density is different due to the difference in the geometrical shape of the dots is utilized, and the periodic structure and graininess are generated by assigning them to different gradation values. Therefore, the gradation expression can be improved.

According to the present invention, in the gradation expression method using an AM screen, halftone dots of an AM screen having the same area and different shapes are prepared and assigned to different gradation values.
That is, in the present invention, the area and shape of each dot of the AM screen representing a certain density are the same, but the area of each dot of the AM screen representing a different density while having a substantially the same density is the same as the previous area. The dots are the same but use different shapes. Due to the different geometric shapes, the dot gain characteristic during printing by the image forming apparatus changes, and the macro density of the printing by the image forming apparatus changes even if the area of the dots constituting the AM screen is the same. be able to.
Specifically, for example, dots having the same area and different shapes can be assigned to different gradation values as follows.
(1) When a continuous tone image is converted into an area tone image using an AM screen, the dither matrix is used to convert the continuous tone image using the dither method.
(2) At that time, the threshold matrix M1 is designed so that each dot having the same area ratio has the same area and shape.
(3) Next, another threshold matrix M2,... Is prepared so that each dot having the same area ratio has the same area and shape, but when the area is the same as the threshold matrix M1, the dot shape is To be different.
(4) D1 (a), D2 (a),... Are output macro densities by the image forming apparatus having the area ratio a of the threshold matrixes M1, M2,.
(5) The macro densities D1 (a), D2 (a),... Are rearranged in order of density.
(6) When the macro density to be output is determined, one threshold matrix and one area ratio are selected correspondingly. Threshold processing is performed using the selected threshold matrix to convert it into an area gradation image.

  In the above example, the case where the threshold matrixes are M1, M2,..., 3 or more is considered. However, when the threshold matrixes are two, that is, M1, M2, the middle from the dot of the normal threshold matrix M1. The threshold value matrix M2 can be easily designed so that the dots excluding these pixels correspond to one less pixel number.

  Hereinafter, for the sake of simplicity, it is assumed that a unit cell of a halftone dot of an AM screen is represented by a collection of 5 × 5 = 25 pixels.

  Consider a dot concentration threshold matrix. For example, when a dot having 8 pixels is placed on a halftone dot composed of 5 × 5 = 25 pixels, even if the number of pixels is 8 and the same area, as shown in FIG. Each threshold matrix M1, M2, M3 can be designed to be (8), M2 (8), M3 (8).

  Using each of these threshold matrixes M1, M2, and M3, the print density when the number of pixels is 8 and a certain wide area (see FIG. 2) is printed out by the image forming apparatus, that is, each macro density D1 (8 ), D2 (8), and D3 (8) do not actually have the same density, but, for example, the order is D1 (8) <D2 (8) <D3 (8).

Similarly, when a dot having 7 pixels, for example, is placed on a halftone dot made up of 5 × 5 = 25 pixels, each threshold value matrix has the same area of 7 pixels as shown in FIG. The threshold matrixes M1, M2, and M3 can be designed so that the dots in M1, M2, and M3 are dots M1 (7), M2 (7), and M3 (7) having different shapes.
Then, using each threshold matrix M1, M2, M3, each macro density D1 (7), D2 (7), D7 (7), D2 (7), when the number of pixels is set to 7 and a large area (see FIG. 4) is printed out by the image forming apparatus. D3 (7) does not actually have the same concentration, and for example, D1 (7) <D2 (7) <D3 (7).

  The reason why the dot gain characteristic changes depending on the dot shape is that, for example, in an image forming apparatus such as an electrophotographic printer, even if the dot area ratio is the same as digital data, Due to the difference in dot shape, the area ratio and potential of the electrostatic latent image after image exposure have changed, and the difference in the toner adhesion due to the difference in the shape of the latent image has resulted in macro density. It is possible that there is a difference in

In the above example, the halftone dot is composed of 5 × 5 = 25 pixels. However, in an actual apparatus, one halftone dot includes a larger number of pixels, for example, 8 × 8 = 64 pixels. A halftone dot composed of 10 × 10 = 100 pixels can be used.
When the number of pixels contained in one halftone dot increases in this way, when dots with a large number of pixels are placed, various shapes of dots having the same area ratio can be designed. A shape having a sufficient difference in macro density can be used.
Further, for example, if two or three dots having different shapes are prepared for each area ratio, and two or three threshold matrixes are designed based on them, the gradation expression is about two to three times. However, it is not always necessary to select each threshold matrix for each macro density, and it is also possible to appropriately select the threshold matrix to be selected according to the purpose, such as smooth gradation expression. it can.

By the way, even in the same image forming apparatus, it is conceivable that there is a difference in how the macro density is output depending on the operating conditions of the apparatus. It is possible that the macro densities D1 (7) and D2 (7) are reversed so that D1 (7)> D2 (7).
Further, in the above example, the halftone dot having the number of pixels of 5 × 5 = 25 is used. However, when the number of pixels included in the halftone dot is considerably increased, a dot having the pixel number c is placed in the threshold value matrix M3. And the macro density of D3 (c)> D1 (c + 1) may be reversed in the case where a dot having a pixel number c + 1 which is one more in the threshold value matrix M1 is placed.

  Therefore, the operation of arranging in order of macro density can be designed so that each macro density output each time the apparatus is started up is actually measured and arranged in order of measurement density.

Three threshold matrixes M1, M2, and M3 were designed by using a threshold matrix of a 10 × 10 matrix by the method shown in FIGS. Using a color electrophotographic output machine Docu-color 1256GA (manufactured by Fuji Xerox, 2400 dpi), the threshold matrixes M1, M2, and M3 are output at 100 × 100 halftone dots for each area ratio (dot pixels), and black toner Each macro density D1 (a), D2 (a), D3 (a) of the printed image developed in step 1 is measured with a densitometer X-rite, and is rearranged in the order of density. A set of rates was selected to give 256 gradations. Threshold processing is performed using a set of threshold matrix and area ratio selected in this way, and the image is converted into an area gradation image.
A grayscale 1-byte gradation [256 gradations] (4800 pixels × 4800 pixels) was converted into a 4800 pixel × 4800 pixel · K half-tone image using the combination of the threshold matrix and the area ratio.
When the grayscale image converted into the area gradation image was output by the above-described color electrophotographic output machine Docu-color 1256GA (Fuji Xerox, 2400 dpi), a smooth gradation could be obtained. In addition, the periodic pattern of dots was inconspicuous compared with the case where a conventional supercell halftone was used.

Example of differently shaped dots M1 (8), M2 (8), M3 (8) when the same number of pixels is 8 in each threshold matrix M1, M2, M3 An example of printing a wide area to some extent using each threshold value matrix M1, M2, M3 and 8 pixels. Examples of dots M1 (7), M2 (7), M3 (7) having different shapes when the number of pixels is 7 in each threshold value matrix M1, M2, M3 Example of printing a wide area to some extent using each threshold value matrix M1, M2, M3 with 7 pixels

Claims (4)

  An image forming apparatus characterized by preparing dots of the same area and different shapes as halftone dot digital data of an AM screen and assigning them to different gradation signal values.   A dither method using a dot concentration type threshold matrix is used, two or more threshold matrixes having the same dot area but different shapes are prepared, and the threshold matrix is selected corresponding to the gradation signal value. Item 2. The image forming apparatus according to Item 1.   The image forming apparatus according to claim 1, wherein the image is formed by electrophotography.   A gradation expression method characterized by increasing the number of gradations in an image forming apparatus by preparing dots having the same area and different shapes as digital data for halftone dots of an AM screen and assigning them to different gradation signal values .

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