JP2011120114A - Image forming device, and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technology for effectively acquiring a screen within a short time period, which is similar to a screen desired by a user, and in which moire between engines and moire in an original document are not conspicuous. <P>SOLUTION: A screen setting unit 102 acquires number of lines information indicating a reciprocal of a distance between dots and screen angle information indicating an angle of a dot position. A screen cycle-calculating unit 103 acquires a frequency space vector corresponding to a vector indicating the dot position specified by the number of lines information and the screen angle information. A screen adjusting unit 105 adjusts the number of lines information and/or the screen angle information in order to raise: an interference frequency of a periodic vector indicated by periodic vector information expressing by a frequency space a pitch of printing unevenness caused by an image forming device; and the frequency space vector. A binarization processing unit 106 converts a printing object image into a binary image using the number of lines information and the screen angle information to be each acquired after the adjusting process. The binary image is printed and outputted by a printer engine 2. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming technique of a binary image using halftone dots.

  Most of images output by an image forming apparatus such as an electrophotographic printer in recent years are expressed by binary values of dot on / off. Such an image expresses gradation by changing the printing area. Generally, a halftone screen or a halftone dot is used as a method for determining on / off of dots. In a digitized image forming apparatus, a halftone screen is constituted by a threshold matrix, and binarization is performed by comparing a threshold matrix continuously arranged on a two-dimensional plane with a multi-value input image. For this reason, the output binary image has periodicity defined by the threshold matrix.

  In color printing using a plurality of colors, an inevitable problem is the misalignment of halftone dots of each color. The misalignment of halftone dots is inevitable even when offset printing is used. In particular, in an electrophotographic printer, for example, misregistration of halftone dots occurs due to uneven rotation of the photosensitive drum. The displacement of the halftone dots causes a change in the density of the output image and a color shift. Therefore, it is known to change the screen angle of the halftone dot for each color in order to prevent color misregistration.

  However, when the screen angle of the halftone dot is changed in order to prevent color misregistration, the screen period for each color causes interference, and an interference draft called moire occurs between colors. Hereinafter, moire generated between screens of colors is referred to as intercolor moire. When the inter-color moire generated in an image has a low frequency that is easily noticeable by humans, the image quality is significantly degraded. For this reason, there is a method of generating a combination of four-color screens such that the generated inter-color moire has a high frequency (Patent Documents 1 and 2).

  On the other hand, the image forming apparatus main body also has a unique period due to uneven rotation of the photosensitive drum as described above. For this reason, the inherent period of the image forming apparatus (hereinafter referred to as the engine period) interferes with the screen period, and moire occurs. Hereinafter, moire generated due to interference between the engine cycle and the screen cycle is referred to as inter-engine moire.

  Further, when a document image having a halftone period is input as an input image, the halftone period of the document and the screen period interfere with each other, and moire occurs. Hereinafter, the moire generated by the interference between the halftone dot cycle of the document and the cycle of the screen is referred to as a document moire. Furthermore, there is also a moire that occurs due to interference between the halftone dot period of the original image and the engine period.

JP 2003-125216 A JP 2004-193759 A

  However, in an image output by an image forming apparatus in which a user can set an arbitrary screen such as a proofer application, the engine cycle and the screen cycle interfere with each other, and an inter-engine moire may occur. It is very difficult for the user to set a screen that avoids the moire between engines because there are many combinations of screens. Further, when a document image is input, document moire may occur. Similarly to the inter-color moire, it is very difficult for the user to set a screen to be avoided by the user.

  As one method for solving these problems, for example, in Patent Document 1, all conceivable screens are generated, and the interference frequency is calculated to select a screen having a high frequency moire. However, since the entire search is performed in this method, the calculation time is enormous, and it is difficult to obtain a screen close to the screen desired by the user in a short time.

  The present invention has been made in view of the above problems, and provides a technique for efficiently acquiring in a short time a screen that is close to the screen desired by the user and in which moire between engines and document moire are not noticeable. With the goal.

  In order to achieve the object of the present invention, for example, an image forming apparatus of the present invention comprises the following arrangement. That is, an image forming apparatus, means for obtaining line number information indicating the reciprocal of a distance between halftone dots and screen angle information indicating an angle of a halftone dot position, the line number information and the screen angle information Means for obtaining a vector in the frequency space as a frequency space vector corresponding to a vector indicating a halftone dot position defined by the above, and means for obtaining period vector information expressing the period of uneven printing by the image forming apparatus in the frequency space And adjusting means for adjusting the line number information and / or the screen angle information in order to increase the interference frequency between the periodic vector indicated by the periodic vector information and the frequency space vector, and after the adjustment processing by the adjusting means Using the obtained line number information and the screen angle information, the print target image is converted into a binary image that is a set of halftone dots, and the converted binary Characterized in that it comprises a means for printing out the image.

  According to the configuration of the present invention, it is possible to efficiently obtain a screen close to a screen desired by a user and having less engine-moire and document moire conspicuous in a short time.

FIG. 3 is a block diagram illustrating an example of a main functional configuration of the image forming apparatus. FIG. 3 is a diagram showing a more detailed configuration of the printer engine 2. 10 is a flowchart of a series of processes for printing out a binary image. The flowchart which shows the detail of the process in step S306. The figure which shows a part of halftone dot group which comprises the screen processed image. The figure which shows the frequency space vectors F1 and F2 in a frequency space. The figure which shows an example of the change range. The figure which shows frequency space vector F1, F2, F3, F1 ', F2'. The figure which shows frequency space vector F1, F2, F3, F1 ', F2'. The figure which shows frequency space vector F1, F2, F3, F1 ', F2'. 10 is a flowchart of a series of processes for printing out a binary image.

  Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. The embodiment described below shows an example when the present invention is specifically implemented, and is one of the specific examples of the configurations described in the claims.

[First Embodiment]
FIG. 1 is a block diagram illustrating an example of a main functional configuration in the image forming apparatus according to the present embodiment. That is, FIG. 1 shows only the part touched in the following description, and does not show the overall configuration of the image forming apparatus according to the present embodiment. However, FIG. 1 shows the operation unit 3, the printer controller 1, and the printer engine 2.

  The printer controller 1 includes a control unit such as a CPU and a memory unit such as a RAM and a ROM, and controls the operation of each unit constituting the image forming apparatus. Further, the printer controller 1 transmits various information including a control command and print target image data to the printer engine 2, and conversely receives necessary information from the printer engine 2. That is, the printer controller 1 controls data communication related to printing processing with the printer engine 2.

  As shown in FIG. 1, the printer controller 1 has an input image data storage unit 101, a screen setting unit 102, a screen cycle calculation unit 103, an engine cycle storage unit 104, a screen adjustment unit 105, and a binarization processing unit 106. Each of these units may be configured by hardware, but the input image data storage unit 101 and the engine cycle storage unit 104 are configured by a memory unit such as a RAM or a ROM, and the other units are stored in the ROM. The computer program may be configured. In this case, the computer program is executed by a control unit such as the CPU included in the printer controller 1, thereby realizing the functions of these units.

  The input image data storage unit 101 is for temporarily storing data of an image to be printed input from the outside of the image forming apparatus (for example, a computer connected to the image forming apparatus).

  The screen setting unit 102 acquires screen information input by the user using the operation unit 3. In this screen information, in addition to the line number information indicating the reciprocal of the distance between the halftone dots and the screen angle information indicating the angle of the halftone dot position, the halftone dot shape information, the output resolution information, the expression gradation number, etc. Contains condition information related to screen design.

  In order to obtain the period of the image after screen processing, the screen period calculation unit 103 uses a vector in the frequency space corresponding to a vector indicating a halftone dot position defined by the line number information and the screen angle information as a frequency space vector. Ask.

  The engine cycle storage unit 104 stores cycle vector information that expresses the cycle of uneven printing by the image forming apparatus in a frequency space. This period vector information is obtained in advance.

  The screen adjustment unit 105 is configured to increase the interference frequency between the frequency space vector obtained by the screen cycle calculation unit 103 and the cycle vector indicated by the cycle vector information stored in the engine cycle storage unit 104, and Adjust screen angle information. As a result, the screen adjustment unit 105 adjusts the screen defined by the line number information and the screen angle information.

  The binarization processing unit 106 uses the line number information and the screen angle information adjusted by the screen adjustment unit 105 for the print target image data temporarily stored in the input image data storage unit 101. Binarization processing (screen processing) is performed. Thus, a binary image that is a set of halftone dots is generated as the print target image. Then, the binarization processing unit 106 sends the generated binary image to the printer engine 2.

  Next, the printer engine 2 will be described. The printer engine 2 is for printing out a binary image generated by the printer controller 1, and includes a printer engine control unit 201, devices 202, and sensors 203 as shown in FIG.

  Each device 202 is configured by a motor or the like used for driving an image carrier or a paper conveyance system, and each sensor 203 is configured by a temperature sensor, a humidity sensor, a density sensor, a speed sensor, a position sensor, or the like. The printer engine control unit 201 controls the operation of each device 202 according to the input from the printer controller 1 and the sensing result by each sensor 203.

  In the present embodiment, the operation unit 3 is described as an information input device that can be operated by the user. However, other types of information input devices are conceivable, for example, transmitted from other devices via a network. It may be used for inputting information.

  FIG. 2 is a diagram showing a more detailed configuration of the printer engine 2. A photosensitive drum 402 is used for charging, exposure, development, and transfer. Reference numeral 403 denotes a charger that charges the photosensitive drum 402. Reference numeral 404 denotes a laser scanner that drives a semiconductor laser in accordance with image data to be printed and scans the photosensitive drum 402 with a polygon mirror to record a latent image. Reference numeral 405 denotes a developing unit that develops toner on the latent image recorded on the photosensitive drum 402. A transfer unit 407 transfers the toner on the photosensitive drum 402 to a medium such as paper. A fixing device 408 fixes the toner image transferred to the medium by heat and pressure. Reference numeral 414 denotes a developing sleeve facing the photosensitive drum 402. Reference numeral 415 denotes a density sensor that measures the density of a patch developed on the photosensitive drum 402.

  In the present embodiment, a single image forming station (including a photosensitive drum 402, a charger 403, a developing device 405, etc.) is shown for the sake of simplicity. However, in the case of a color image forming apparatus, for example, image forming stations for cyan, magenta, yellow, and black colors may be sequentially arranged on the transfer unit 407 along the moving direction. In some cases, the developing units 405 of the respective colors are arranged around one photosensitive drum 402 along the periphery. Alternatively, the developing devices 405 for the respective colors may be arranged in a rotatable housing. As described above, in the case of a color image forming apparatus, a desired developing device 405 is opposed to the photosensitive drum 402 so as to develop a desired color.

  Next, the line number information and the screen angle information are adjusted, and using the adjusted information, the print target image is converted into a binary image that is a set of halftone dots, and the binary image is printed out. This process will be described with reference to FIG. 3 showing a flowchart of the process.

  First, in step S301, the input image data storage unit 101 acquires print target image data input from the outside of the image forming apparatus (for example, a computer connected to the image forming apparatus), and temporarily stores this data. To do.

  In step S <b> 302, the screen setting unit 102 acquires the above-described screen information input by the user using the operation unit 3. In the present embodiment, to simplify the description, it is assumed that the screen information is for defining a set of halftone dots arranged in a square lattice pattern. However, it will be clear from the following description that the essence of the following description can be applied to screen information defining other dot arrangement patterns.

  In step S302, the screen setting unit 102 creates a screen-processed image by quantizing a single-color image of a certain size that is stored in advance using the acquired screen information. In this embodiment, an irrational tangent method (hereinafter referred to as an irrational screen) is used as an example of several methods for creating a screen-processed image. Here, since a method of quantizing a monochromatic image using an irrational screen is well known, the technique will be briefly described here.

  An irrational screen is defined by a vector of its own coordinate system (u, v). The vector of the coordinate system (u, v) is projected and used on the coordinates (x, y) of the input pixel of the input image in accordance with the arbitrarily set screen line number and screen angle. Here, a quantization threshold is set for each vector, and the projected vector can be used as a threshold matrix when the input image is quantized.

  Therefore, in this embodiment, first, the coordinates (u, v) of the screen coordinate system of the output pixel corresponding to the coordinates (x, y) of the pixel of interest of the monochromatic image are obtained by affine transformation. Next, the output pixel density threshold for determining whether or not to output the output pixel to the screen processed image is calculated by using the coordinates (u, v) of the output pixel obtained by the affine transformation, and the determination is made. The output pixel density is quantized. By performing the above processing on all the pixels constituting the monochrome image, the monochrome image can be quantized using an irrational screen to create a screen-processed image. As a method for obtaining the threshold value, a method using a function specifying a halftone dot shape, that is, a method for growing a halftone dot, or a method of easily rotating a threshold matrix, enlarging / reducing, and interpolation calculation may be used.

  FIG. 5 is a diagram showing a part of a halftone dot group constituting the screen-processed image created in this way. In FIG. 5, the halftone dots are indicated by black circles. As described above, in the present embodiment, the screen-processed image is assumed to be a set of halftone dots arranged in a square lattice pattern. Therefore, as shown in FIG. 5, each halftone dot is arranged in a square lattice pattern. ing. That is, the distance between halftone dots is constant both vertically and horizontally. Further, the screen-processed image constituted by such a halftone dot group is arranged to be inclined with respect to the horizontal axis (X axis) of the space (real space) in which the screen-processed image is arranged.

  In FIG. 5, the line number information indicates the reciprocal of the distance between the target halftone dot 501 and a halftone dot adjacent thereto (for example, halftone dot 502). For example, when the output resolution is 1200 DPI and the distance between halftone dots is about 180 μm, the number of lines indicated by the line number information is about 141 LPI.

  The screen angle information indicates the inclination θ of the screen processed image with respect to the horizontal axis (X) of the space where the screen processed image is arranged. In FIG. 5, the inclination θ is an angle formed by the vertical axis of the screen-processed image (parallel to the axis passing through the target halftone dot 501 and the halftone dot 502 immediately above) and the horizontal axis (X) in the real space.

  In the following description, it is assumed that the screen processed image shown in FIG. 5 has been created in step S302. Returning to FIG. 3, next in step S303, the screen period calculation unit 103 sets the vector from the target halftone dot 501 to the halftone dot 502 immediately above it as R1 (x1, y1), and is adjacent to the left from the target halftone dot 501. A vector to the halftone dot 503 is obtained as R2 (x2, y2). Then, the screen cycle calculation unit 103 obtains a vector in the frequency space corresponding to the vector R1 (x1, y1) as a frequency space vector F1 (x1, y1). Similarly, the screen cycle calculation unit 103 obtains a vector in the frequency space corresponding to the vector R2 (x2, y2) as a frequency space vector F2 (x2, y2). Here, a method for obtaining the frequency space vectors F1 and F2 will be described.

  First, the area A of the region defined by the frequency space vectors R1 and R2 is obtained based on the following equation.

A = | x1 · y2−x2 · y1 |
Then, assuming that the frequency space vectors F1 and F2 are F1 = (u1, v1) and F2 = (u2, v2), u1, v1, u2, and v2 are obtained based on the following equations.

u1 = −y1 / A
v1 = x1 / A
u2 = −y2 / A
v2 = x2 / A
FIG. 6 is a diagram showing frequency space vectors F1 and F2 in the frequency space obtained from the vectors R1 and R2 shown in FIG. Returning to FIG. 3, next, in step S <b> 304, the screen adjustment unit 105 stores period vector information (period data) that is stored in the engine period storage unit 104 and represents the period of uneven printing by the image forming apparatus in a frequency space. ) As a frequency space vector F3. The period vector information may be expressed in frequency space as a period read from the paper surface that has been printed out from the image forming apparatus in advance, or the period of the photosensitive drum 402 measured using each sensor 203. May be expressed in frequency space. For example, a speed sensor may be used to detect the speed of the paper feed belt and to express paper feed pitch unevenness in a frequency space.

  In the present embodiment, there is one frequency space vector F3. However, since an actual image forming apparatus has a plurality of engine cycles, there may be a plurality of frequency space vectors F3. In that case, each frequency space vector F3 may be stored in advance in the engine cycle storage unit 104, and the subsequent processing may be performed for each frequency space vector F3.

  In step S306, the screen adjustment unit 105 adjusts the line number information and / or screen angle information acquired in step S302 to increase the interference frequency between the frequency space vectors F1 and F2 and the frequency space vector F3. Details of the processing in this step will be described later.

  In step S307, the binarization processing unit 106 uses the line number information and the screen angle information obtained after the adjustment process in step S306 to convert the print target image stored in the input image data storage unit 101 in step S301 into a binary image. Convert to

  Finally, in step S308, the binarization processing unit 106 sends the binary image created in step S307 to the printer engine 2. Accordingly, the printer engine control unit 201 included in the printer engine 2 controls each device such as a laser scanner, a photosensitive drum, and a paper conveyance system, and performs print output based on the binary image.

  Next, the process performed by the screen adjustment unit 105 in step S306 will be described in more detail. FIG. 4 is a flowchart showing details of the process in step S306.

  The frequency of the generated moire can be expressed by the difference between the frequency space vectors representing the two periods. However, the screen adjustment unit 105 calculates the frequency space vectors F1 'and F2' of the screen whose distance from the frequency space vector F3 of the engine cycle is long in the frequency space from the frequency space vectors F1 and F2, respectively. That is, when the distance between F3 and F1 'or the distance between F3 and F2' is short, the moire between the engines has a low frequency, so the moire is conspicuous in the output image, and the image quality is significantly lowered. Therefore, by determining F1 'and F2' such that the distance to F3 is long from F1 and F2, the moire between engines becomes a high frequency and the moire can be made inconspicuous. Note that the distance L1 (uL1, vL1) between F3 (u3, v3) and F1 ′ (u1, v1) and the distance L2 (uL2, vL2) between F3 (u3, v3) and F2 ′ (u2, v2) are And can be calculated based on the following equation.

uL1 = u3-u1
vL1 = v3-v1
uL2 = u3-u2
vL2 = v3-v2
First, in step S401, the screen adjustment unit 105 sets the frequency space vectors F1 and F2 to be used in the subsequent processing. In step S402, the screen adjustment unit 105 sets the frequency space vector F3 to be used in the subsequent processing.

  Next, in step S403, the screen adjustment unit 105 sets a change range (design range) of the frequency space vectors F1 and F2. This change range indicates the change range of the lengths and angles of the frequency space vectors F1 and F2, and as described above, the lengths of the frequency space vectors F1 and F2 so that the distance from the frequency space vector F3 is increased. The change range when the angle is changed is shown. In other words, such a change range is also a change range of the line number information and the screen angle information.

  For example, as shown in FIG. 7, the change range for the line number information may be 120 to 200 LPI, and the change range for the screen angle information may be 0 to 180 degrees. Such a change range may be determined in advance, or may be appropriately input by the user using the operation unit 3. FIG. 7 is a diagram illustrating an example of the change range.

  Next, in step S404, the currently set adjustment modes are the angle priority mode (first mode), the line number priority mode (second mode), and the line number / angle optimization mode (third mode). It is judged which one is. The mode setting may be set in advance, or may be input by the user via the operation unit 3 by prompting the user to set the mode in this step.

  Here, each selectable mode will be described. First, the angle priority mode will be described. The angle priority mode is a mode in which the screen angle information acquired by the screen setting unit 102 is fixed and only the line number information acquired by the screen setting unit 102 is adjusted.

  FIG. 8 is a diagram illustrating the frequency space vectors F1, F2, and F3 in the frequency space and the frequency space vectors F1 ′ and F2 ′ after adjustment for the frequency space vectors F1 and F2. In FIG. 8, the gray portion indicates the change range set in step S403.

  In the angle priority mode, the frequency space vectors F1, F2 are fixed so that the L1 (uL1, vL1) and L2 (uL2, vL2) are maximized within the change range. Each component of F2 is changed. As a result, in FIG. 8, the length of any of the frequency space vectors F1 and F2 is extended to the outer frame of the change range, and the result is the frequency space vectors F1 'and F2'. These frequency space vectors F1 'and F2' are frequency space vectors after adjustment for the frequency space vectors F1 and F2, respectively.

  Therefore, if it is determined in step S404 that the angle priority mode is selected, the process proceeds to step S405. In step S405, the screen adjustment unit 105 obtains the adjusted frequency space vectors F1 'and F2' by executing the processing in the angle priority mode described above.

  Next, the line number priority mode will be described. The line number priority mode is a mode in which the line number information acquired by the screen setting unit 102 is fixed and only the screen angle information acquired by the screen setting unit 102 is adjusted.

  FIG. 9 is a diagram illustrating the frequency space vectors F1, F2, and F3 in the frequency space and the frequency space vectors F1 ′ and F2 ′ after adjustment for the frequency space vectors F1 and F2. In FIG. 9, the gray portion indicates the change range set in step S403.

  In the line number priority mode, the frequency space vectors F1 and F2 remain fixed, and the frequency space vector is set so that L1 (uL1, vL1) and L2 (uL2, vL2) are maximized within the change range. Each component of F1 and F2 is changed. As a result, in FIG. 9, the frequency space vector F1, the frequency space vector F1, and the frequency space vector F3 have an angle difference of 45 degrees, and the frequency space vector F2 and the frequency space vector F3 have an angle difference of 45 degrees. F2 is rotating. The frequency space vector F1 after rotation is F1 ', and the frequency space vector F2 after rotation is F2'. These frequency space vectors F1 'and F2' are frequency space vectors after adjustment for the frequency space vectors F1 and F2, respectively.

  Therefore, if it is determined in step S404 that the line number priority mode is selected, the process proceeds to step S406. In step S406, the screen adjustment unit 105 obtains the adjusted frequency space vectors F1 'and F2' by executing the processing in the above-described line number priority mode.

  Next, the line number / angle optimization mode will be described. The line number / angle optimization mode is a mode in which both the line number information and the screen angle information acquired by the screen setting unit 102 are appropriately adjusted.

  FIG. 10 is a diagram showing the frequency space vectors F1, F2, and F3 in the frequency space and the frequency space vectors F1 ′ and F2 ′ after adjustment for the frequency space vectors F1 and F2. In FIG. 10, the gray portion indicates the change range set in step S403.

  In the line number / angle optimization mode, the components of the frequency space vectors F1 and F2 are changed so that L1 (uL1, vL1) and L2 (uL2, vL2) are maximized within the change range. . As a result, in FIG. 10, for each of the frequency space vectors F1 and F2, the angle difference from the frequency space vector F3 is 45 degrees, and the length is extended to the outer frame of the change range. Space vectors F1 ′ and F2 ′ are obtained. These frequency space vectors F1 'and F2' are frequency space vectors after adjustment for the frequency space vectors F1 and F2, respectively.

  Accordingly, if it is determined in step S404 that the line number / angle optimization mode is selected, the process proceeds to step S407. In step S407, the screen adjustment unit 105 obtains adjusted frequency space vectors F1 'and F2' by executing the processing in the above-described line number / angle optimization mode.

  In this way, by adjusting the screen by the distance from the frequency space vector of the engine cycle, the number of screen lines and the screen angle are uniquely determined, and thus can be obtained in a shorter time.

  Returning to FIG. 4, in step S408, the frequency space vectors F1 'and F2' obtained in any of steps S405, S406, and S407 are set as the final adjusted frequency space vectors. That is, in step S408, the adjusted line number information and screen angle information can be obtained. Actually, the processing from step S401 to step S408 is performed for all target halftone dots.

  In step S409, the screen adjustment unit 105 creates a halftone dot shape from all the adjusted line number information and screen angle information determined in step S408. The halftone dot shape may be a dot shape or a line shape. Also, it may be a halftone dot shape created by the user or may be defined by a spot function.

  As described above, according to the present embodiment, by adjusting the line number information and the screen angle information according to the setting mode, a screen that is close to the screen desired by the user and has less noticeable moire between engines can be efficiently obtained in a short time. Can be created.

  In the present embodiment, the screen condition set by the user is handled as being changed by the screen adjustment unit. However, the image may be output under the screen condition set by the user. In that case, the user may be notified of the evaluation value (frequency value) of moire.

[Second Embodiment]
In the first embodiment, the interference with the engine cycle is suppressed by generating a screen having the highest interference frequency with the engine cycle. In this embodiment, a screen generation method for preventing interference with a halftone period when the input image has a halftone period will be described.

  About a series of processes for adjusting line number information and screen angle information, converting the image to be printed into a binary image which is a set of halftone dots using these adjusted information, and printing out the binary image The process will be described with reference to FIG. In FIG. 11, the parts denoted by the same step numbers as those in FIG. 3 are the same as the processes described in the first embodiment. Therefore, in the following, different parts from FIG. 3, that is, steps S604 and S605 are performed. Processing will be described. The points other than the following description are the same as those in the first embodiment. In the present embodiment, the description will be made assuming that the print target image is a document image having a halftone dot period.

  In step S604, the screen adjustment unit 105 obtains the frequency component of the print target image acquired in step S301. For example, the frequency component is obtained by performing Fourier transform on the print target image.

  Next, in step S605, the screen adjustment unit 105 obtains a halftone frequency space vector F4 (mainly line number information and screen angle information) of the original image using the frequency component obtained in step S604. For example, the line number information and screen angle information of the document image are obtained by examining the peak of the frequency component obtained in step S604. Note that the processing performed to obtain the line number information and the screen angle information in this step is not limited to such a method.

  For example, a method may be used in which the image to be printed is shifted several pixels vertically and horizontally and an autocorrelation function between the shifted image and the original image is obtained. Also, when moving within the range of several pixels in the vertical and horizontal directions, if the autocorrelation coefficient is maximized when shifting 6 pixels and 2 pixels horizontally and 6 pixels horizontally and 6 pixels horizontally in a 1200 DPI image, the angle of the halftone dot About 71.6 degrees and about -18.4 degrees, and the number of lines can be calculated as about 190 LPI. Further, when a supercell is used for a halftone dot of an original image, the above method cannot be used because the number of vertical and horizontal pixels differs depending on the halftone cell. However, since a pattern consisting of several halftone cells is repeatedly used, autocorrelation is taken over a wide range, the angle is calculated from the point where autocorrelation is maximum, and the number of peak points that are several points up to that point. The number of lines can be calculated by obtaining.

  In addition, the number of lines and the angle may be obtained from pattern matching with halftone dot pattern images of several types of lines and angles. For example, generally used halftone dot angles are 15 degrees, 45 degrees, 75 degrees, 90 degrees, 30 degrees, 60 degrees, etc., and the number of lines includes 100 LPI, 120 LPI, 133 LPI, 150 LPI, 175 LPI, and the like. Therefore, if the pattern matching is performed with the halftone dot image of these angles and the number of lines and the most suitable angle and the number of lines are selected, the number of lines and the angle can be obtained.

  In step S306, the processing in step S306 described in the first embodiment is performed using the frequency space vectors F1 and F2 and the frequency space vector F4. In this step, only the frequency space vector F4 is used instead of the frequency space vector F3, and the others are the same as in the first embodiment.

  As described above, according to the present embodiment, it is possible to efficiently generate a screen that is close to the screen desired by the user and in which the document moire is not noticeable in a short time. In the present embodiment, the halftone dot period of the document image has been described as one, but a plurality of frequency space vectors may be set. In the present embodiment, the document moire has been described. In addition, interference with the engine cycle may be considered.

Claims (6)

An image forming apparatus,
Means for obtaining line number information indicating the reciprocal of the distance between halftone dots and screen angle information indicating the angle of the halftone dot positions;
Means for obtaining a vector in a frequency space as a frequency space vector corresponding to a vector indicating a halftone dot position defined by the line number information and the screen angle information;
Means for obtaining periodic vector information representing a period of uneven printing by the image forming apparatus in a frequency space;
Adjusting means for adjusting the line number information and / or the screen angle information in order to increase the interference frequency between the periodic vector indicated by the periodic vector information and the frequency space vector;
Using the line number information and the screen angle information obtained after the adjustment processing by the adjustment means, the print target image is converted into a binary image that is a set of halftone dots, and the converted binary image is printed out. And an image forming apparatus.   The adjusting means adjusts the line number information and / or the screen angle information so that a distance between the periodic vector and the frequency space vector becomes large within a predetermined range. The image forming apparatus according to claim 1. The adjusting means operates in accordance with one mode selected from the first mode, the second mode, and the third mode,
When the first mode is selected,
The screen angle information is fixed and the line number information is adjusted so that the distance between the periodic vector and the frequency space vector is increased within a predetermined range.
When the second mode is selected,
The line number information is fixed and the screen angle information is adjusted so that the distance between the periodic vector and the frequency space vector is increased within a predetermined range.
When the third mode is selected,
The line number information and the screen angle information are adjusted so that a distance between the periodic vector and the frequency space vector becomes large within a predetermined range. The image forming apparatus described in 1. An image forming apparatus,
Means for obtaining line number information indicating the reciprocal of the distance between halftone dots and screen angle information indicating the angle of the halftone dot positions;
Means for obtaining a vector in a frequency space as a frequency space vector corresponding to a vector indicating a halftone dot position defined by the line number information and the screen angle information;
Means for obtaining period vector information representing a halftone dot period in a print target image in a frequency space;
Adjusting means for adjusting the line number information and / or the screen angle information in order to increase the interference frequency between the periodic vector indicated by the periodic vector information and the frequency space vector;
Using the line number information and the screen angle information obtained after the adjustment processing by the adjustment unit, the print target image is converted into a binary image that is a set of halftone dots, and the converted binary image is printed out. And an image forming apparatus. An image forming method of an image forming apparatus,
Obtaining line number information indicating the reciprocal of the distance between halftone dots and screen angle information indicating the angle of the halftone dot positions;
Obtaining a vector in a frequency space as a frequency space vector corresponding to a vector indicating a halftone dot position defined by the line number information and the screen angle information;
Obtaining periodic vector information representing a period of uneven printing by the image forming apparatus in a frequency space;
An adjustment step of adjusting the line number information and / or the screen angle information in order to increase the interference frequency between the periodic vector indicated by the periodic vector information and the frequency space vector;
Using the line number information and the screen angle information obtained after the adjustment process in the adjustment step, the print target image is converted into a binary image that is a set of halftone dots, and the converted binary image is printed out. An image forming method comprising the steps of: An image forming method performed by an image forming apparatus,
Obtaining line number information indicating the reciprocal of the distance between halftone dots and screen angle information indicating the angle of the halftone dot positions;
Obtaining a vector in a frequency space as a frequency space vector corresponding to a vector indicating a halftone dot position defined by the line number information and the screen angle information;
Obtaining periodic vector information representing a halftone dot period in a print target image in a frequency space;
An adjustment step of adjusting the line number information and / or the screen angle information in order to increase the interference frequency between the periodic vector indicated by the periodic vector information and the frequency space vector;
Using the line number information and the screen angle information obtained after the adjustment process in the adjustment step, the print target image is converted into a binary image that is a set of halftone dots, and the converted binary image is printed. An image forming method comprising: an output step.

Images