JP5906871B2 - Image processing apparatus, image forming apparatus, and program - Google Patents

Description

  The present invention relates to an image processing apparatus, an image forming apparatus, and a program.

  When the number of gradations per halftone dot is small, the gradation difference is easily recognized and so-called tone jump is likely to occur. In Patent Document 1, in an image data processing apparatus that converts continuous tone data into binary image data, random data is added to lower bits of the original threshold data for the purpose of preventing tone jumps that occur on a halftone image. It describes that continuous tone data is compared using the added threshold data.

JP 7-336536 A

  An object of the present invention is to increase the number of gradations that can be reproduced by changing the number of threshold values constituting the threshold data without changing the number of threshold values.

The image processing apparatus according to claim 1 of the present invention specifies threshold data that describes a plurality of threshold values defined in a matrix of 8 × 8 or 16 × 16 in order to convert a gradation image into a halftone image. Storage means for storing a plurality of adjacent threshold values at different positions, and comparison means for comparing gradation values of the gradation image with the threshold values stored in the storage means. And comparing means for switching the threshold data used for the comparison in accordance with the gradation value , wherein the threshold data includes a plurality of the specific positions, and the plurality of threshold data are the specific positions at the specific positions. It has a configuration in which the threshold magnitude relationship is different .

An image processing apparatus according to a second aspect of the present invention is the configuration according to the first aspect, wherein the threshold value at the specific position is equal to or lower than the gradation value when compared with a specific gradation value. Have the same number of values.
An image processing apparatus according to a third aspect of the present invention has the configuration according to the first or second aspect , wherein the comparison means repeatedly uses the plurality of threshold data in a predetermined order.
An image processing apparatus according to a fourth aspect of the present invention is the image processing apparatus according to the third aspect , wherein the plurality of threshold data includes first threshold data and second threshold data, The first threshold data is used when the gradation value is an even number, and the second threshold data is used when the gradation value is an odd number.

According to a fifth aspect of the present invention, there is provided an image forming apparatus for forming a halftone dot image on a recording medium according to the comparison result of the image processing apparatus according to any one of the first to fifth aspects and the comparison unit. And a forming unit.

According to a sixth aspect of the present invention, there is provided a program for storing threshold data describing a plurality of threshold values defined in a matrix of 8 × 8 or 16 × 16 for converting a gradation image into a halftone image. A means for reading out the threshold data from a storage means for storing a plurality of the threshold values in different positions at a specific position, and comparing a gradation value of the gradation image with the plurality of threshold values stored in the storage means A program for causing the threshold data used for the comparison to function as a means for switching according to the gradation value , wherein the threshold data includes a plurality of the specific positions, and the plurality of threshold data Are different in the magnitude relation of the threshold at the specific position .

According to the configuration of the first, sixth, and seventh aspects, compared to the case of using only a single threshold data, the number of gradations that can be reproduced by the number without changing the number of threshold values constituting the threshold data. Can be increased.
According to the structure of Claim 2, compared with the case where the magnitude relationship of a threshold value does not differ, it is possible to make the shape of a halftone image differ in a specific position.
According to the configuration of claim 3, when compared with a specific gradation value, a halftone image with the same number of pixels as compared to a case where the values which are equal to or less than the gradation value are not the same number. It is possible to vary the shape of the.
According to the configuration described in claim 4, it is possible to reduce the number of threshold data to be stored as compared with the case where threshold data is not used repeatedly.
According to the configuration described in claim 5, it is further possible to minimize the number of threshold data to be stored.

Block diagram showing configuration of image forming apparatus The figure which shows an example of a structure of an image formation part roughly Block diagram showing the configuration of the image processing unit The figure which illustrates two types of threshold data Flow chart showing comparison operation of image processing unit Diagram showing gradation change (growth pattern of halftone dots) Diagram showing gradation change (growth pattern of halftone dots) The figure which shows the comparative example of threshold value data Diagram for explaining the relationship between the number of on-pixels and the potential integral value The figure for demonstrating the relationship between arrangement | positioning of an ON pixel, and an electric potential integral value

[Embodiment]
FIG. 1 is a block diagram illustrating a configuration of an image forming apparatus 100 according to an embodiment of the present invention. Here, it is assumed that the image forming apparatus 100 is an electrophotographic printer. The configuration of the image forming apparatus 100 is roughly divided into an image processing unit 110 and an image forming unit 120. The image processing unit 110 is a unit that converts image data into a format suitable for image formation by the image forming unit 120. The image forming unit 120 is a unit that forms an image corresponding to the image data supplied from the image processing unit 110 on a recording medium such as paper. The image processing unit 110 and the image forming unit 120 correspond to an example of the image processing apparatus and the image forming unit of the present invention, respectively.

  FIG. 2 is a diagram schematically illustrating an example of the configuration of the image forming unit 120. The image forming unit 120 includes a photosensitive drum 121, a charging roll 122, an exposure device 123, a developing device 124, a transfer roll 125, and a fixing device 126. Note that an arrow A1 indicated by a broken line in the drawing indicates a sheet conveyance path.

  The photosensitive drum 121 is a cylindrical member having a photoconductive layer, and rotates in a direction indicated by an arrow A2 in the drawing. The charging roll 122 is a member that charges the photosensitive drum 121 to a predetermined potential. The exposure device 123 irradiates the photosensitive drum 121 with laser light corresponding to the image data, and forms an electrostatic latent image on the photosensitive drum 121. In the present embodiment, the exposure apparatus 123 includes a VCSEL (Vertical Cavity Surface Emitting Laser) that simultaneously irradiates a plurality of laser beams, thereby forming an electrostatic latent image corresponding to a halftone dot. The developing device 124 applies toner to a portion of the photosensitive drum 121 corresponding to the electrostatic latent image to develop the electrostatic latent image. The developing device 124 attaches toner to the photosensitive drum 121 by a potential difference (bias voltage) generated between the developing device 124 and the photosensitive drum 121. The transfer roll 125 is in contact with the photosensitive drum 121 to form a nip region, and the toner is transferred to a sheet passing through the nip region by a potential difference (transfer voltage). The fixing device 126 heats and pressurizes the paper on which the toner is transferred, and fixes the toner on the paper.

  Note that the image forming apparatus 100 may include a plurality of part or all of the configuration shown in FIG. That is, the image forming apparatus 100 may be a monochrome printer or a color printer. The image forming apparatus 100 in the case of a color printer may include an intermediate transfer body such as an intermediate transfer belt, but is not limited thereto, and may be configured as a so-called rotary developing system.

FIG. 3 is a block diagram illustrating a configuration of the image processing unit 110. The image processing unit 110 includes an acquisition unit 111, a storage unit 112, a comparison unit 113, and an output unit 114.
Note that the image processing unit 110 may be configured by an ASIC (Application Specific Integrated Circuit) dedicated to image processing or the like, but a CPU (Central Processing Unit) uses a memory and executes a program (that is, software) (By processing).

  The acquisition unit 111 is means for acquiring image data. The image data acquired by the acquisition unit 111 is data representing an image in which each pixel is expressed with a predetermined number of gradations (hereinafter referred to as “gradation image”). Such image data is hereinafter referred to as “tone image data”.

  The storage unit 112 is means for storing threshold data. Here, the threshold data refers to data describing a plurality of threshold values defined in a matrix for displaying a gradation image with a halftone dot in a pseudo gradation display. That is, the threshold data is a set of a plurality of threshold values. The storage unit 112 stores a plurality of the threshold data. In this embodiment, as will be described later, two types of threshold data are stored in the storage unit 112.

  The comparison unit 113 is means for comparing the gradation image data with the threshold data. The comparison unit 113 compares the gradation value included in the gradation image data with the threshold value included in the threshold data, and outputs a value corresponding to the comparison result. For example, the comparison unit 113 outputs “1” when the gradation value is equal to or less than the threshold value, and outputs “0” when the gradation value is smaller than the threshold value. The comparison unit 113 compares the gradation values included in the gradation image data in units of one or a plurality of pixels.

  Further, the comparison unit 113 switches threshold data used for comparison of gradation images according to a predetermined rule. For example, the comparison unit 113 switches the threshold data used for comparison according to the gradation value that is the comparison target. That is, the comparison unit 113 changes the threshold data used for comparison according to the gradation value.

  The output unit 114 is a unit that outputs a comparison result from the comparison unit 113 and supplies the comparison result to the image forming unit 120. The output unit 114 outputs the comparison result from the comparison unit 113 as two-gradation image data. Hereinafter, the image data of two gradations is referred to as “halftone image data”, and the image displayed in pseudo gradation represented by the halftone image data is referred to as “halftone image”. In the halftone image data of the present embodiment, the value of the portion where the halftone dot is formed by irradiating the laser beam is “1”, and the portion where the halftone dot is not formed without irradiating the laser beam (the background and the halftone dot) It is assumed that the data is binary data with a value of “0”.

  The configuration of the image forming apparatus 100 is as described above. With this configuration, the image forming apparatus 100 acquires image data from an external device such as a personal computer, and forms an image (halftone dot image) corresponding to the acquired image data on a sheet. The image forming apparatus 100 may acquire the gradation image data from the external apparatus, but acquires the image data in another format from the external apparatus, and converts the image data in the other format into the gradation image data. Processing may be executed.

  The following specific example shows a case where 600 dpi (dots per inch) gradation image data is converted into 1200 dpi halftone image data. Here, it is assumed that one halftone dot is composed of 8 × 8 pixels, that is, 64 pixels. Therefore, the number of lines in this case is 1200/8 lpi (lines per inch), that is, 150 lpi.

  In the following description, it is assumed that the number of gradations of gradation image data is 68 gradations for convenience of explanation. The reason for using such a number of gradations is to explain that gradation expression of 68 gradations is realized by a halftone dot composed of 64 pixels. Therefore, when actually implementing the present invention, the number of gradations may be a general value such as “64” or “256”. In this case, the number of pixels constituting one halftone dot may be smaller than 8 × 8 pixels (in the case of 64 gradations) or 16 × 16 pixels (in the case of 256 gradations).

  FIG. 4 is a diagram illustrating two types of threshold data T1 and T2 used in the present embodiment. Each of the threshold data T1 and T2 is a set of thresholds for 64 pixels in which the minimum value is “1” and the maximum value is “68”. Here, the thresholds constituting the threshold data T1 and T2 are expressed as Th (i, j). Here, i and j are indexes for indicating the position of each pixel in the row direction and the column direction, respectively. For example, Th (1,1) means the threshold value of the first row and first column, and the value is “65”. Similarly, Th (1,2) means the threshold value of the first row and the second column, and its value is “61”.

  The threshold data T1 and T2 have different threshold arrangements at specific positions. In FIG. 4, when the threshold data T1 and T2 are compared, the positions of Th (4,6) and Th (4,7) aligned in the row direction (first position) and Th (2,4) aligned in the column direction. ) And Th (3,4) positions (second position), Th (5,2) and Th (5,3) positions aligned in the row direction (third position), Th lines aligned in the column direction The arrangement of threshold values is different at four positions (4, 5) and Th (7, 5) (fourth position), and the same threshold is arranged at other positions.

  The threshold values in the first position to the fourth position have different magnitude relationships in the adjacent directions. More specifically, the threshold values of the first position to the fourth position have a relationship in which the magnitudes are reversed in adjacent directions. For example, at the first position (fifth row), the threshold data T1 has a larger threshold value in the second column, while the threshold data T2 has a larger threshold value in the third column.

  FIG. 5 is a flowchart showing the comparison operation of the image processing unit 110 when the threshold data T1 and T2 are used. In this example, the image processing unit 110 first refers to the gradation value (n) at a specific position in the gradation image data, and determines whether or not it is an even number (step S1). The image processing unit 110 changes threshold data used for comparison according to the determination result of the gradation value. Specifically, when the gradation value is an even number (S1: YES), the image processing unit 110 refers to the threshold data T1 (step S2), whereas when the gradation value is an odd number ( S1: NO), the threshold value data T2 is referred to, and the referred threshold value data is compared with the gradation value (step S3).

  The image processing unit 110 compares the gradation image data and the threshold data (step S4), and outputs a value corresponding to the comparison result. If the gradation value at a specific position of the gradation image data is equal to or less than the threshold value of the position corresponding to the position (S1: YES), the image processing unit 110 outputs a value “1” that turns on the pixel ( Step S5) If the gradation value is larger than the threshold value, a value of “0” that does not light the pixel is output (Step S6).

  Then, the image processing unit 110 determines whether or not such comparison has been performed for the entire gradation image data (step S7). When there is an unprocessed gradation value (S7: NO), the image processing unit 110 executes the processing after step S1 for the unprocessed gradation value, and when there is no unprocessed gradation value. (S7: YES), the process is terminated.

  FIG. 6 is a diagram showing a gradation change in this embodiment, that is, a halftone dot growth pattern. Here, a case where an area of 4 × 4 pixels having the same gradation value is compared with the threshold data T1 and T2 is shown. FIG. 6 shows halftone dots when the gradation values of the gradation image data are “15”, “16”, and “17”. Of the threshold values of the threshold data, those with hatching are those that are equal to or lower than the gradation value of the gradation image data.

  When the gradation value is “15”, that is, when it is an odd number, the image processing unit 110 uses the threshold data T2 for comparison. In the threshold data T2, since Th (4,7) is “15”, the pixel corresponding to this position (pixel after binarization) is set to “1”. The threshold data T2 also sets “1” for other pixels whose threshold is “15” or less. Hereinafter, a pixel having a binarized gradation value of “1” is referred to as an “on pixel”, and a pixel having a binarized gradation value of “0” is referred to as an “off pixel”. An on-pixel is a pixel corresponding to a position where laser light is irradiated. At this time, the pixel corresponding to Th (4,6) is an off pixel.

  Next, when the gradation value is “16”, that is, when the gradation value is an even number, the image processing unit 110 switches the threshold data used for comparison from the threshold data T2 to the threshold data T1. The threshold data T1 is different from the threshold data T1 in that Th (4,6) is “16” and Th (4,7) is “17”. As a result, the pixels in the fourth row and the sixth column are turned on, and the pixels in the fourth row and the seventh column are turned off.

  Here, comparing the case where the gradation value is “15” and the case where the gradation value is “16”, the number of ON pixels is 13 in any case, which is the same number. However, the position of the on pixel is different in the third position. In the third position, the position of the ON pixel is the fourth row and the seventh column when the gradation value is “15”, while the fourth row and the sixth column when the gradation value is “16”. It is.

  Next, when the gradation value is “17”, that is, when it is an odd number again, the image processing unit 110 uses the threshold data T2 for comparison. That is, the image processing unit 110 switches the threshold data used for comparison from the threshold data T1 to the threshold data T2. At this time, the number of ON pixels is 14, which is one more than when the gradation value is “15” or “16”. Further, the position of the ON pixel at this time corresponds to a position obtained by performing a logical sum when the gradation value is “15” and “16”.

  FIG. 7 is a diagram showing the gradation change in the present embodiment in comparison with the case where a plurality of threshold data is not used. Further, FIG. 8 is threshold data shown as a comparative example and is a diagram showing 64-gradation threshold data T3 in which the threshold increases by one. For example, when the number of ON pixels increases from 8 to 11, the gradation change when the threshold data T3 is used is as follows: state 1 → state 2 → state 3 → state 4. The change of the gradation value at this time is 8 → 9 → 10 → 11.

  On the other hand, the gradation change when the threshold data T1 and T2 are used together as in the present embodiment is as follows: state 1 → state 2a → state 2 → state 3 → state 4a → state 4. The change of the gradation value at this time is 8 → 9 → 10 → 11 → 12 → 13. That is, in this case, the comparison is made with the threshold data T1 in the states 1, 2, 4a, and the comparison is made with the threshold data T2 in the states 2a, 3, 4.

  When the state 2a and the state 2 are compared, the number of ON pixels is the same, but the position of the ON pixels is different. More specifically, in the case of the state 2, compared to the case of the state 2, the positions of the ON pixels are more dense, and the position of the ON pixels as a whole is closer to the center of the halftone dot. It can be said that. The same applies to the case where the state 4a and the state 4 are compared.

  FIG. 9 is a diagram showing an integral value (hereinafter referred to as “potential integral value”) of the surface potential of the photosensitive drum 121 generated by one halftone dot, which contributes to development. The potential integral value is an integral value obtained by subtracting a voltage corresponding to the bias voltage from the surface potential of the photosensitive drum 121. In general, the image density (the color density of the image) is proportional to this potential integral value. Therefore, the larger the potential integration value, the higher the image density and the higher the dot area ratio.

  Among the potential integration values shown in FIG. 9, the gray scales indicated by hatching are the gradations added by the configuration of the present embodiment. The other potential integration values are gradations generated in both the configuration of the present embodiment and the threshold data T3 of the comparative example. That is, according to the configuration of the present embodiment, different gradations (that is, image densities) can be generated by the same number of on pixels. Therefore, the gradations added in this way moderate the gradation change due to the stepwise growth (enlargement) of the halftone dots and suppress the tone jump.

FIG. 10 is a diagram for explaining the relationship between the on-pixel arrangement and the potential integral value. Here, the surface potential by the laser beam is indicated by a solid line, and the sum is indicated by a broken line. As shown in FIG. 10, the intensity distribution of the laser light applied to the photosensitive drum 121 is almost Gaussian. In general, assuming that the size of each pixel is about 1200 dpi, the diameter of the laser beam at the position of 1 / e ² of the peak intensity is about 50 μm to 60 μm, and is set larger than the pixel size. . Therefore, when irradiating laser light for two pixels, when these pixels are adjacent to each other, the laser light irradiated to one pixel is also partially irradiated to the other pixel. As a result, the intensity of the laser light applied to these pixels is greater (that is, stronger) than when only one laser light is applied.

When the intensity of the laser beam is superimposed in this way, a difference also occurs in the potential integration value. As shown in FIG. 10, in the case of irradiating laser light for two pixels, a portion where the surface potential exceeds the bias voltage (V _bias ) between when these pixels are adjacent to each other and when they are not adjacent to each other. This is because the total sum (that is, the area) of the potential integral values can be different. In other words, in the case of irradiating a plurality of pixels with laser light, when these pixels are adjacent to each other (in other words, when the pixels are not adjacent to each other (that is, when an off pixel is included in between), (development) The potential integral value (which contributes to) increases, and as a result, the image density increases.

[Modification]
Embodiment of this invention is not limited to embodiment mentioned above, For example, the form shown below may be sufficient. Moreover, these forms may be combined with each other by using them together or by replacing a part thereof as necessary.

(1) In the present invention, the relationship in which pixels and thresholds are “adjacent” does not indicate only when they are continuous in a row direction or a column direction. For example, in the threshold data T1 and T2 shown in FIG. 4, the thresholds adjacent to Th (2,2) are Th (1,1), Th (1,2), Th (1,3), Th (2, 1), Th (2,3), Th (3,1), Th (3,2), Th (3,3).

(2) The above-described threshold data T1 and T2 are only examples of threshold data for realizing the present invention. Therefore, the specific arrangement of the threshold values in the threshold data may be changed according to the required image quality. In addition, the “specific position” in the present invention, that is, the position where the threshold value arrangement is different between one threshold value data and another threshold data may be at any position in the threshold data. It may be the position of various gradation values for which jump suppression is required. In the case where the magnitude relations of the threshold values are different, the adjacent threshold values are not necessarily two, and may be three or more. Furthermore, the comparison in the present invention may be an off pixel when the gradation value is equal to or greater than the threshold value, and an on pixel when the gradation value is less than the threshold value.

(3) In the present invention, the threshold data used for comparison of gradation values may be any number as long as there are two or more types. For example, if three types of threshold data are used, pay attention to the remainder when the gradation value is divided by 3, and the first threshold data when the remainder is “1” and the second when the remainder is “2”. The threshold data may be referred to and the third threshold data may be referred to in the case of “0”. The threshold data is preferably repeatedly used in a predetermined order, but different values may be set for each of the maximum value and the minimum value of the gradation values.

(4) The image processing apparatus according to the present invention may be configured as a part of another apparatus like the image processing unit 110, or may be configured as a single apparatus. Further, as described above, the image processing apparatus according to the present invention may be provided as a program, or may be provided in the form of a recording medium on which the program is recorded. Furthermore, the program according to the present invention may be acquired from an external device via a network or other communication means and downloaded to a computer.

DESCRIPTION OF SYMBOLS 100 ... Image forming apparatus, 110 ... Image processing part, 111 ... Acquisition part, 112 ... Memory | storage part, 113 ... Comparison part, 114 ... Output part, 120 ... Image forming part

Claims (6)

In order to convert a gradation image into a halftone dot image, threshold data describing a plurality of threshold values defined in a matrix of 8 × 8 or 16 × 16 is obtained by arranging two or more threshold values adjacent to each other at a specific position. Storage means for storing a plurality of differently,
Comparing means for comparing gradation values of the gradation image with the plurality of threshold values stored in the storage means, and comprising comparison means for switching the threshold value data used for the comparison according to the gradation values. ,
There are a plurality of the specific positions in the threshold data,
The plurality of threshold data is an image processing apparatus in which the threshold value is different in magnitude at the specific position . The image processing apparatus according to claim 1, wherein the threshold value at the specific position has the same number of values that are equal to or lower than the gradation value when compared with a specific gradation value. The image processing apparatus according to claim 1 , wherein the comparison unit repeatedly uses the plurality of threshold data in a predetermined order. The plurality of threshold data includes first threshold data and second threshold data,
The image processing according to claim 3 , wherein the comparison unit uses the first threshold data when the gradation value is an even number, and uses the second threshold data when the gradation value is an odd number. apparatus. An image processing apparatus according to any one of claims 1 to 4 ,
An image forming apparatus comprising: an image forming unit that forms, on a recording medium, a halftone image corresponding to a comparison result by the comparing unit. Computer
A plurality of threshold data describing a plurality of threshold values defined in an 8 × 8 or 16 × 16 matrix for converting a gradation image into a halftone image so that the arrangement of the threshold values at a specific position is different. Means for reading out the threshold data from storage means for storing;
Means for comparing the gradation value of the gradation image with the plurality of threshold values stored in the storage means, and for functioning as means for switching the threshold data used for the comparison according to the gradation value A program ,
There are a plurality of the specific positions in the threshold data,
The plurality of threshold data is a program in which the threshold value is different in magnitude at the specific position .

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