JP2020046556A - Image processing device, image processing method and program - Google Patents

Abstract

To provide an image processing device capable of generating print image data for forming an image with suppressed density unevenness in the main scanning direction or enhancing the quality of printed matter on the basis of image data to be screen-processed.SOLUTION: The image processing device is an image processing device for generating print image data by performing screen processing on input image data. The image processing device has setting means that sets a dither matrix used for the screen processing or a dot pattern combined with the print image data according to the density unevenness in the main scanning direction.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an image processing device, an image processing method, and a program.

  In electrophotographic image forming apparatuses, image quality such as in-plane uniformity (hereinafter referred to as image quality) is a factor that impairs sheet quality due to equipment such as a laser scanner that forms an electrostatic latent image on a photosensitive drum. And density unevenness occurring in the main scanning direction.

  Patent Document 1 discloses a technique for adjusting the amount of exposure light emitted from a scanner unit based on a result of detection of a test pattern by a color sensor when a sheet on which a test pattern is formed is fed while being rotated by 90 degrees. Has been disclosed. Japanese Patent Application Laid-Open No. H11-163873 discloses a technique for correcting an image signal value so as to cancel density unevenness by creating an LUT (Look Up Table) for converting an input image signal value into a different image signal value for each main scanning position. Are also disclosed.

  On the other hand, in the image forming processing system, image processing is performed in advance on the input image data by using the host computer, and the obtained print image data is received by the image forming apparatus, and based on the received print image data, a recording medium such as paper is stored. There are systems for performing image formation. As an example of image processing executed by such a system, a multi-valued continuous tone (for example, 8 bits) using a dither matrix is represented by the density and size of dots per unit area, that is, the area tone Screen processing, which is a process of converting to The number of gradations per pixel of the print image data after the screen processing depends on the configuration of the image forming apparatus (generally, 1 bit, 2 bits, 4 bits, etc.).

JP 2006-58565 A

  However, in the case of the above-described system, since the LUT is a method of converting a signal value into another signal value as described above, it is difficult to correct an image represented by area gradation. For this reason, in the conventional technology (main scanning shading correction) for correcting density unevenness in the main scanning direction described in Patent Literature 1, it is difficult to correct image data to be subjected to screen processing.

  The present invention has been made in view of at least one of the above problems. An object of the present invention is to generate print image data for forming an image in which density unevenness in the main scanning direction is suppressed, based on image data on which screen processing is performed. Another object of the present invention is to improve the quality of printed matter.

  According to one embodiment of the present invention, there is provided an image processing apparatus configured to perform screen processing on input image data to generate print image data, the image processing apparatus including a dithering apparatus used for the screen processing. The image processing apparatus further includes a setting unit that sets a matrix or a dot pattern to be combined with the print image data in accordance with density unevenness in the main scanning direction.

  According to one aspect of the present invention, it is possible to generate print image data for forming an image in which density unevenness in the main scanning direction is suppressed, based on image data on which screen processing is performed. According to another aspect of the present invention, the quality of a printed matter can be improved.

FIG. 1 is a diagram illustrating a configuration example of an image forming system. FIG. 3 is a block diagram illustrating a configuration example of a host computer. FIG. 2 is a block diagram illustrating a configuration example of a controller unit of the image forming apparatus. It is a figure for explaining screen processing. It is a figure for explaining a copy-forgery-inhibited pattern image. FIG. 4 is a diagram for explaining density unevenness. FIG. 4 is a diagram for explaining a relationship between a dot pattern and a density tendency. FIG. 9 is a diagram for explaining a density step generated by switching of dot patterns. It is a figure showing the example of a dot pattern which has a submatrix. FIG. 7 is a diagram for explaining a relationship between a switching position and a density tendency in dot matrix sub-matrix units. It is a figure for explaining switching of a dot pattern using a submatrix. It is a figure showing the example of a screen profile. FIG. 4 is a diagram illustrating an example of a calibration image. It is a flowchart which shows the example of a screen profile creation processing procedure. FIG. 9 is a diagram illustrating an example of a GUI for creating a screen profile. It is a flowchart which shows the example of a dot pattern switching processing procedure. FIG. 4 is a diagram for explaining a relationship among image data, dither matrices, and screen image data. FIG. 4 is a diagram illustrating a relationship example between a threshold value of a dither matrix and screen image data. FIG. 3 is a diagram illustrating an example of a dither matrix. FIG. 9 is a diagram illustrating an example of a relationship between density unevenness that occurs in each gradation and a main scanning position in a screen process using a dither matrix. FIG. 4 is a diagram illustrating an example of a screen profile for correcting density unevenness.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the components described in this embodiment are merely examples, and are not intended to limit the scope of the present invention thereto. Further, all combinations of the components described in the embodiments are not necessarily essential to the means for solving the problems, and within the scope of the present invention described in the claims, Various modifications and changes are possible.

[Embodiment 1]
FIG. 1 is a block diagram illustrating a configuration example of an image processing system according to the present embodiment.

  In the image processing system, a host computer 101 and an image forming apparatus 102 are connected via a network 107 so as to be able to transmit and receive signals to and from each other. Note that the number of connections between the host computer 101 and the image forming apparatus 102 is not limited to one, but may be plural. In the present embodiment, the network 107 is applied as a connection method, but the present invention is not limited to this. For example, a serial transmission system such as USB, a parallel transmission system such as Centronics or SCSI, and the like are also applicable.

  The host computer (hereinafter, referred to as PC) 101 has a function of a personal computer. The PC 101 is configured to be able to transmit and receive files using the network 107 and to transmit print image data to the image forming apparatus 102 via a printer driver 201 which will be described in detail later. Further, the PC 101 is configured to generate copy-forgery-inhibited pattern data and add it to print image data.

  The image forming apparatus 102 includes a controller unit 103, a scanner unit 104, a printer unit 105, and a device operation unit 106.

  The controller unit 103 is a control unit including, for example, a processor and an integrated circuit, and controls the operation of the entire image forming apparatus 102. The controller unit 103 is connected to the PC 101 and an external image processing device (not shown) via a network 107. Thus, the controller unit 103 can input and output print image data and device information.

  The scanner unit 104 performs a document reading process for generating scanned image data obtained by scanning a document. The scanner unit 104 generates calibration image data by scanning the calibration image.

  The printer unit 105 performs a printing process on a recording medium or the like using the image data. Specifically, the printer unit 105 forms an electrostatic latent image according to the image data after the image processing, and develops the formed electrostatic latent image to form a monochromatic toner image. Next, the printer unit 105 forms a multicolor toner image by superimposing the single color toner images, transfers the formed multicolor toner image to a recording medium, and fixes the multicolor toner image on the recording medium.

  The device operation unit 106 includes an input unit such as a button or a touch panel, and a display unit such as a liquid crystal panel or an LED. The device operation unit 106 receives an operation instruction from a user for the image forming apparatus 102 and displays a screen.

  FIG. 2 is a block diagram illustrating a configuration example of the PC 101. The PC (image processing apparatus) 101 includes a printer driver 201, a driver display unit 202, and a driver operation unit 203. The printer driver 201 includes a printer driver I / F 204, an image processing unit 205, and a screen profile creation unit 206.

  The printer driver 201 converts the image data to be printed into a format of print image data that can be received by the image forming apparatus 102, and transmits the converted print image data to the image forming apparatus 102 via the network 107. In the present embodiment, the printer driver 201 transmits to the image forming apparatus 102 screen image data obtained by performing a screen process for expressing the gradation based on the density of the dots and the size of the dots themselves on the input image data. . Further, the printer driver 201 acquires device information of the image forming apparatus 102 via the network 107.

  The driver display unit 202 displays parameters of image processing performed by the printer driver 201 and device information of the image forming apparatus 102 received by the printer driver 201 via the network 107 on a display device (not shown) such as a liquid crystal monitor.

  The driver operation unit 203 is an operation screen that is displayed by the driver display unit 202 and receives an operation related to setting of parameters for image processing performed by the printer driver 201 and the like. The driver operation unit 203 displays, for example, a graphical user interface (GUI) described later in detail, which is an operation screen on which a setting operation can be performed by a user of the present system. Thereby, the user of the present system can instruct the image forming apparatus 102 to perform desired image processing.

  The printer driver I / F 204 transmits image processing parameter information displayed on the driver display unit 202, receives image processing parameters set by the driver operation unit 203, and transmits image processing parameters used by the image processing unit 205. Further, the printer driver I / F 204 transmits print image data to the image forming apparatus 102 and receives device information.

  The image processing unit 205 converts the input image data into a format of print image data that can be received by the image forming apparatus 102 in order to transmit the image data to the image forming apparatus 102. When performing format conversion to print image data, the image processing unit 205 performs screen processing based on the screen profile created by the screen profile creation unit 206.

  The screen profile creation unit 206 creates a screen profile used in screen processing, which is a process of creating print image data by the image processing unit 205. The screen profile is a profile indicating how to switch a screen (such as a dot pattern) used at a specific main scanning position so as to correct density unevenness of the image forming apparatus 102 in the main scanning direction. That is, the screen profile is data indicating the relationship between the main scanning position at which the screen is switched and the screen to be used after the switching. By switching the screen based on this screen profile, density unevenness in the main scanning direction is suppressed, as will be described later in detail.

  In the present embodiment, a PC is exemplified as an example of the image processing apparatus, but the present invention is not limited to this. For example, the present invention can be applied to a mobile terminal or a tablet terminal having a function of generating print image data.

  FIG. 3 is a block diagram illustrating a configuration example of the controller unit 103 of the image forming apparatus 102. The controller unit 103 includes a CPU 301, a RAM 302, a ROM 303, an HDD 304, a scanner unit I / F 305, a device operation unit I / F 306, a network I / F 307, an image forming unit 308, a printer I / F 309, and a calibration unit 310. They are communicably connected by a bus 311.

  A CPU (Central Processing Unit) 301 is a central processing unit that controls the entire system of the image forming apparatus 102. The CPU 301 executes an OS (Operating System) and various programs stored in the ROM 303 and the HDD 304 using the RAM 302 as a work memory. The CPU 301 comprehensively controls access to various connected devices based on a control program or the like stored in the ROM 303, and also comprehensively controls various processes such as image processing performed inside the controller unit 103. .

  A RAM (Random Access Memory) 302 is a system work memory for the operation of the CPU 301 and a memory for temporarily storing image data. The RAM 302 is composed of an SRAM that retains the stored contents even after the power is turned off, and a DRAM from which the stored contents are erased after the power is turned off. A ROM (Read Only Memory) 303 stores a boot program of the apparatus. An HDD (Hard Disk Drive) 304 stores various data such as various programs and image data.

  The scanner unit I / F 305 is an interface unit for connecting to the scanner unit 104. The scanner unit I / F 305 performs data communication with the scanner unit 104 and communicates image data read by the scanner unit 104 and control data used for scanner processing.

  The device operation unit I / F 306 is an interface unit for connecting to the device operation unit 106. The device operation unit I / F 306 outputs image data to be displayed on the device operation unit 106 to the device operation unit 106, and acquires information input from the device operation unit 106.

  The network I / F 307 is connected to the network 107 and transmits and receives image data and information.

  The image forming unit 308 receives the print image data transmitted by the printer driver 201 via the network 107, and converts the received print image data into a format that the printer unit 105 can draw on a recording medium.

  The printer unit I / F 309 is connected to the printer unit 105 and the image forming unit 308, performs data communication with the printer unit 105, and performs communication of image data and control data used in printer processing.

  The calibration unit 310 corrects various parameters used in image processing performed by the image forming unit 308, based on calibration image data generated by reading the calibration image by the scanner unit 104.

(Screen processing)
In the present embodiment, an area gray scale method is used to realize gray scale reproduction. The area gray scale is a method of expressing a gray scale by changing a ratio of a colored area per unit area, and an AM (amplitude modulation) screen and an FM (frequency modulation) screen are known as typical examples. Have been. The AM screen expresses a gradation by modulating the size of a colored region (so-called dot size). The FM screen arranges minute isolated dots of a fixed size in a pseudo-random manner, and expresses a tone by dot density. In the screen processing, a dither matrix corresponding to a threshold table for controlling the growth of the screen is used, and the pixel value of each pixel of the image data to be processed is binarized by the dither matrix. Image data obtained by the screen processing is called dot image data.

  FIGS. 4A and 4B show a screen processing method when the pixel value of all pixels is 64 in a 4 × 4 pixel area composed of 8 bits for each pixel, that is, a pixel value of 256 gradations. It is a figure for explaining. FIGS. 4A and 4B are diagrams illustrating different screen processing methods. FIG. 4A shows a method of expressing a gradation using a dot size per a predetermined unit area. FIG. 4B shows a method of expressing gradation using the number of dots per predetermined unit area. Here, binarization will be described for simplification of the description.

  The image data 401 represents a pixel area of 4 × 4 pixels with a total pixel value of 64. Since the maximum value of the pixel value is 255, the pixel value is approximately 1/4. Therefore, when the binarization process is performed, a process of drawing a quarter of the pixel of the unit area is performed. The screen image data 402 shown in FIG. 4A and the screen image data 403 shown in FIG. 4B both show the state after the screen processing in a unit area of 4 × 4 pixels. Each of the screen image data 402 and 403 has a unit area of 16 pixels, and is in a state where four pixels are drawn. The screen image data 402 is data obtained by a dot concentration type method that expresses gradation using the size of one dot. In the screen image data 402, the image is drawn such that one pixel has a size of four pixels per unit area. The screen image data 403 is data obtained by a dot-dispersion type method of expressing gradation by increasing the number of dots in the minimum unit. The screen image data 403 is drawn such that four dots of one pixel are present per unit area.

  Screen processing methods (dot-concentration type, dot-dispersion type) are generally used depending on the image to be printed and the attribute information (characters, photos, etc.) of the drawing object included in the image. It is.

  There is a tint block image as an example of properly using the screen processing. The copy-forgery-inhibited pattern image will be described with reference to FIG. 5A and 5B are diagrams for explaining a copy-forgery-inhibited pattern image. FIG. 5A shows an example of a dot pattern used in the background portion of the copy-forgery-inhibited pattern image, and FIG. FIG. 5 (c) shows an example of a dot pattern before and after copying. The copy-forgery-inhibited pattern image has the same density and includes a background portion 501 in which dots disappear after copying, and a latent image portion 502 in which dots remain after copying. In the present embodiment, a dot pattern of a tint block image having a unit area of 8 × 8 pixels is assumed.

  In the present embodiment, as shown in FIG. 5A, the dot pattern used for the background portion 501 is discrete as shown in FIG. Designed to be deployed. That is, the background portion 501 is drawn using the first dot pattern in which each dot is not adjacent in the unit area. In addition, the dot pattern used for the image corresponding to the latent image portion 502 is arranged in a concentrated manner as shown in FIG. 5B by using a dot concentration type screen processing method such as the screen image data 402. Designed to be. That is, the latent image portion 502 is drawn using a second dot pattern in which dots are concentrated at one place in a unit area.

  Further, screen processing is performed by inputting appropriate input image signals so that the background portion and the latent image portion have substantially the same density when printing (image formation) on a recording medium using the image forming apparatus 102. Thus, a binary image forming the latent image portion and the background portion is generated.

  As a result, a copy-forgery-inhibited pattern image in which the background portion and the latent image portion cannot be visually distinguished is formed on the recording medium output by printing. On the other hand, in the copy image obtained by copying the copy-forgery-inhibited pattern image using the copy function of the multifunction device, the latent image portion is drawn with dots of a certain size. Can be imaged. However, since the background portion is drawn with the minimum unit of dots, the dots cannot be resolved by the scanner unit or the like of the multifunction peripheral. As a result, only the latent image portion remains in the copy image and is drawn on the recording medium.

  However, when the printer unit 105 of the image forming apparatus 102 draws on a recording medium, main scanning is performed due to deterioration of a photosensitive drum or a laser scanner included in the printer unit 105 or a developing device that develops an electrostatic latent image. The density unevenness may occur in the direction. In particular, an image formed by dot dispersion screen processing such as a background portion of a copy-forgery-inhibited pattern image may be hardly rendered in an electrophotographic image forming process.

  FIG. 6 is a diagram illustrating an example of density unevenness that occurs in the main scanning direction in the background portion of the copy-forgery-inhibited pattern image. Specifically, FIG. 6 is a graph showing an example of density unevenness as a result of performing the dot dispersion screen processing shown in FIG. 5A at specific main scanning positions X0 to X4. It can be seen that at the main scanning position X2 near the center of the main scanning position, a density equal to the target density Dt of the background is output. However, it can be seen that the density gradually decreases with respect to the target density Dt in the background as it goes to one end from the main scanning position X2 to X1 and from X1 to X0. Also in the case of going from the main scanning position X2 to X3 and from X3 to X4 toward the other end, similarly to the case of going from the main scanning position X2 to X0, the density gradually decreases with respect to the target density Dt of the background portion. You can see that there is. For example, despite the dot pattern being adjusted so that the background portion and the latent image portion in the copy-forgery-inhibited pattern image match with the target density Dt, the copy-forgery-inhibited pattern image due to the density unevenness that occurred in the main scanning direction was used. Even if there is, there is a possibility that the latent image portion will appear.

  Therefore, in the present embodiment, the dot pattern is switched in accordance with the density unevenness in the main scanning direction. A method of switching the dot pattern will be described with reference to FIGS. 7, 8, 9, 10, and 11. FIG.

  FIG. 7 is a diagram illustrating an example of three types of dot patterns used in the present embodiment and an example of a density tendency obtained thereby. The dot pattern 701 shown in FIG. 7A is the same as the dot pattern shown in FIG. 5A, and indicates a default dot pattern serving as a reference. The default dot pattern 701 indicates a dot pattern obtained by the screen processing at the main scanning position X2.

  On the other hand, a dot pattern 702 shown in FIG. 7B represents a dot pattern obtained by the screen processing at the main scanning positions X1 and X3. Compared to the default dot pattern 701, the dot pattern 702 shows that the number of dots to be drawn is increased by two. By using the dot pattern 702 having a larger number of dots drawn at the main scanning positions X1 and X3 than the dot pattern used at the main scanning position X2, the main scanning positions X2, X1, and X3 shown in FIG. Density unevenness that occurs when drawing with a pattern is suppressed.

  A dot pattern 703 shown in FIG. 7C represents a dot pattern obtained by the screen processing at the main scanning positions X0 and X4. Compared to the default dot pattern 701, the dot pattern 703 shows that the number of dots to be drawn is increased by four. At the main scanning positions X0 and X4, by using the dot pattern 703 having a larger number of drawn dots than the dot pattern used at the main scanning position X2, the main scanning positions X2 and X0 and X4 shown in FIG. Density unevenness that occurs when drawing with a pattern is suppressed.

  FIG. 7D is a graph showing the density at each main scanning position of an image obtained using the dot patterns shown in FIGS. 7A, 7B, and 7C.

  The curve d0 indicates the density at each main scanning position when the dot pattern 701 is used, the curve d1 indicates the density when the dot pattern 702 is used, and the curve d2 indicates the density at each main scanning position when the dot pattern 703 is used.

  As shown in FIG. 7D, for example, by using the dot pattern 701 corresponding to the curve d0 at the main scanning position X2 and the dot pattern 702 corresponding to the curve d1 at the main scanning positions X1 and X3, the target density Dt is obtained. Can be drawn in accordance with. By using the dot pattern 703 corresponding to the curve d2 at the main scanning positions X0 and X4, it is possible to perform drawing in accordance with the target density Dt. That is, by switching the dot pattern according to each main scanning position, it is possible to perform drawing in accordance with the target density Dt.

  Even when there is density unevenness, if a dot pattern is suddenly switched and drawn, a density step may occur. That is, even if an attempt is made to suppress the occurrence of density unevenness by switching the dot pattern, a density step may occur due to the switching of the dot pattern.

  An example of dot pattern switching when a density step occurs will be described with reference to FIG.

  FIG. 8 is a diagram illustrating an example of dot pattern switching when a density step occurs. The main scanning position X2_3 represents an arbitrary main scanning position between the main scanning position X2 and the main scanning position X3. Here, at the main scanning position X2_3, the number of dots to be drawn is increased by two from the dot pattern 701 where the number of dots to be drawn by 8 × 8 pixels is 8, and the number of dots to be drawn by 8 × 8 pixels is increased. 10 shows a state in which the dot pattern 702 is switched to a dot pattern 702 in which the number of dots is 10.

  As shown in FIG. 8, when the dot pattern is suddenly switched at a certain point between the main scanning position X2 and the main scanning position X3, there is a possibility that the density step becomes conspicuous before and after the dot pattern switching.

  To cope with this, when a different dot pattern is used between the adjacent main scanning positions Xn and Xn + 1, the dot pattern is switched stepwise using a dot pattern including a plurality of sub-matrices.

  FIG. 9 is a diagram illustrating an example of a dot pattern including a plurality of sub-matrices obtained by the screen processing.

  FIG. 10 is a diagram showing the relationship between the switching position and the density of the dot pattern shown in FIG. 9 in sub-matrix units between the main scanning positions X2 and X3. Note that the curve showing the density tendency is extracted from FIG.

  FIG. 11 is a diagram showing an example of a dot pattern obtained by screen processing at a dot pattern switching position between the main scanning positions X2 and X3 shown in FIG.

  The dot pattern shown in FIG. 9 is a dot dispersion type dot pattern and is composed of 16 × 16 256 pixels. This dot pattern can be divided into sub-matrices 901, 902, 903, 904 of 8 × 8 pixels, which are unit areas. That is, the dot pattern includes four 8 × 8 pixel sub-matrices (first sub-matrix, second sub-matrix, third sub-matrix, and fourth sub-matrix) 901, 902, 903, and 904. Each of the first, second, third, and fourth sub-matrices 901, 902, 903, and 904 has eight dots to be drawn. By switching the dot patterns stepwise using these sub-matrices, it is possible to suppress the density step due to the switching of the dot patterns.

  With reference to FIGS. 10 and 11, a method of switching the dot pattern step by step in sub-matrix units will be described.

  As shown in FIG. 10, between the main scanning positions X2 and X3, the main scanning positions X2_3 (0), X2_3 (1), X2_3 (2), and X2_3 (3) at which stepwise dot pattern switching is performed. Is divided into Here, X2_3 (0) represents the same main scanning position as X2.

  In the range from the main scanning position X2 (X2_3 (0)) to X2_3 (1), as shown in FIG. 11A, the pattern is of a dot dispersion type, and the number of dots to be drawn by 8 × 8 pixels is small. 8, a dot pattern with four sub-matrices 1101 is used.

  In the range from the main scanning positions X2_3 (1) to X2_3 (2), as shown in FIG. 11B, one of the four sub-matrices 1101 has 10 dots drawn by 8 × 8 pixels. The dot pattern switched to a certain sub matrix 1102 is used.

  In the range from the main scanning positions X2_3 (2) to X2_3 (3), as shown in FIG. 11C, a dot pattern in which two of the four sub-matrices 1101 are switched to the sub-matrix 1102 is used.

  In the range from the main scanning positions X2_3 (3) to X3, as shown in FIG. 11D, a dot pattern in which three of the four sub-matrices 1101 are switched to the sub-matrix 1102 is used.

  In the range after the main scanning position X3, as shown in FIG. 11E, a dot pattern in which all four sub-matrices 1101 are switched to the sub-matrix 1102 is used.

  As described above, by switching the dot pattern of one of the plurality of sub-matrices, the dot pattern can be switched in a stepwise manner. That is, it is possible to switch the number of dots to be drawn at a division position corresponding to the number of sub-matrices forming one dot pattern in a certain section of the main scanning position in sub-matrix units. This makes it possible to switch the dot pattern step by step in sub-matrix units, thereby suppressing a sudden change in the dot pattern and suppressing a sudden change in the density. As a result, it is possible to switch the dot pattern from X2 to X3 without noticeable density steps.

  In the above, the case where the occurrence of density unevenness is suppressed by switching the five types of dot patterns has been described, but the present invention is not limited to this. It is also possible to adjust the number of types of dot patterns (the number of switching) according to the size of the density unevenness and the position in the main scanning direction. Although the case where the number of dots to be drawn is increased by two by switching the dot pattern has been described, the present invention is not limited to this. It is also possible to adjust the number of dots to be drawn by switching the dot pattern according to the size of the density unevenness and the position in the main scanning direction. Although the case where the stepwise dot pattern switching using the sub-matrix is applied to the range from the main scanning positions X2 to X3 has been described, the range from the main scanning positions X0 to X1, the range from the main scanning positions X1 to X2, the main scanning position It is also possible to apply to the range from the position X3 to the position X4.

(How to create a screen profile)
A method of creating a screen profile by the screen profile creation unit 206 will be described with reference to FIGS. The screen profile is a profile indicating which dot pattern is used at each main scanning position. That is, the screen profile is data in which a dot pattern to be used is set for each main scanning position.

  FIG. 12 is a diagram illustrating a configuration example of a screen profile. The figure shows what dot pattern P (Xn) is used for the main scanning position Xn which is a screen switching point. That is, the dot pattern P (Xn) is used in a range from the main scanning position Xn to a position not including the main scanning position Xn + 1 in the range from the main scanning position Xn + 1. What is indicated by the dot pattern P (Xn) may be, for example, the number of dots per unit area or a threshold arrangement of a dither matrix described later.

  The screen profile may be created by the calibration unit 310 of the image forming apparatus 102 based on the calibration image read by the scanner unit 104. The screen profile may be created by the user through the driver operation unit 203 by the screen profile creation unit 206, or may be created by the calibration unit 310 through the device operation unit 106.

  In the present embodiment, a method in which the calibration unit 310 creates a screen profile based on the calibration image read by the scanner unit 104 will be described with reference to FIGS.

  FIG. 13 is a schematic diagram of an example of a calibration image used by the calibration unit 310 to create a screen profile. This image is formed on a recording medium by the printer unit 105 and output. The output recording medium includes a first area 1301, a second area 1302, a third area 1303, a fourth area 1304, from the upstream side in the sub-scanning direction orthogonal to the main scanning direction to the downstream side. A patch is formed in each of the fifth regions 1305. The patches formed in the first area 1301, the second area 1302, the third area 1303, the fourth area 1304, and the fifth area 1305 are created with the same dot pattern for each area. Each of the areas 1301 to 1305 indicates a range over the entire main scanning direction. In each of the areas 1301 to 1305, the patch is formed at a position corresponding to the main scanning position Xn of the screen switching point shown in FIG. Is done.

  The numerical value described in each patch in each of the areas 1301 to 1305 indicates that the patch of “± 0” is a patch drawn with the default dot pattern. In addition, the plus or minus notation indicates that the patch is drawn by the number of dot patterns obtained by increasing or decreasing the number of dots per unit area compared to the default dot pattern. Specifically, in the first area 1301, patches drawn by a dot pattern in which the number of dots per unit area is increased by two compared to the default dot pattern are formed at locations corresponding to the respective main scanning positions. . In the second area 1302, a patch drawn with a dot pattern in which the number of dots per unit area is increased by one compared to the default dot pattern is formed at a position corresponding to each main scanning position. In the third area 1303, patches drawn with the default dot pattern are formed at locations corresponding to the respective main scanning positions. In the fourth area 1304, patches drawn with a dot pattern in which the number of dots per unit area is reduced by one compared to the default dot pattern are formed at locations corresponding to the respective main scanning positions. In the fifth area 1305, patches drawn with a dot pattern in which the number of dots per unit area is reduced by two compared to the default dot pattern are formed at locations corresponding to the respective main scanning positions. That is, a patch drawn in a predetermined dot pattern is formed corresponding to each main scanning position.

  By reading the calibration image shown in FIG. 13 by the scanner unit 104, it is possible to measure the density on the recording medium output when the main scanning position and the dot pattern are determined at each main scanning position. Become. The calibration unit 310 creates a screen profile based on the density information obtained as a result.

  FIG. 14 is a flowchart illustrating an example of a screen profile creation processing procedure. This processing is executed by the calibration unit 310. Pm described in the flowchart indicates the dot pattern when the numerical value m in the patch is described in the calibration image. Hereinafter, the symbol S means a step in the flowchart.

  In step S1401, the calibration unit 310 receives the calibration image data acquired by the scanner unit 104. Next, in S1402, the calibration unit 310 acquires the patch density D (Xn, Pm) when the dot pattern is Pm at the main scanning position Xn. For example, when the user measures the patch density when the dot pattern is Pm at the main scanning position Xn and the dot pattern is Pm, and inputs the patch density to a GUI described later in detail, the calibration unit 310 obtains the patch density D. become. In the present embodiment, when the process of S1402 is performed for the first time, the patch density D (X0, P (−2)) when the main scanning position is X0 and the number of dots per unit area is the smallest dot pattern Shall be measured. The processing target when the processing of S1402 is performed for the first time is not limited to the patch (X0, P (-2)). If the measurement target can be arbitrarily updated in S1406 and S1408 described later, any patch (Xn, Pm) can be set as the measurement target even when the process of S1402 is performed for the first time.

  Next, in S1403, the calibration unit 310 calculates a density difference (error) between the target density Dt and the patch density D (Xn, Pm). Then, it is determined whether or not the calculation result is smaller than the density difference (error) from the patch density D (Xn, P (Xn)) for the dot pattern P (Xn) currently set at the main scanning position Xn. I do. If it is not determined that the density difference (error) from the patch density D (Xn, Pm) for the dot pattern Pm (Xn) is smaller, the process skips S1404 and proceeds to S1405. On the other hand, if it is determined that the density difference (error) from the patch density D (Xn, Pm) for the dot pattern Pm (Xn) is smaller, the process advances to the next step S1404. In S1404, the calibration unit 310 updates the current dot pattern P (Xn) at the main scanning position Xn to the dot pattern Pm.

  Next, in step S1405, the calibration unit 310 determines whether or not the density of patches of all dot patterns has been measured at the main scanning position Xn. If it is determined that measurement has not been performed, the process advances to step S1406. In step S1406, the dot pattern to be measured at the main scanning position Xn is updated, and the process returns to step S1402. On the other hand, when it is determined that the patch densities of all the dot patterns have been measured at the main scanning position Xn, the process proceeds to S1407, and it is determined whether the dot patterns at all the main scanning positions Xn have been determined at S1407. If it is determined that the dot patterns at all the main scanning positions Xn have not been determined, the process proceeds to S1408, where the main scanning positions and dot patterns to be processed are updated at S1408, and the process returns to S1402.

  If it is determined that the dot patterns at all the main scanning positions have been determined, the process ends.

  The screen profile created in this way is sent to the printer driver 201 of the PC 101 via the network 107. The printer driver 201 that has received the screen profile stores the screen profile in the image processing unit 205. Thus, when the copy-forgery-inhibited pattern image is formed in the printer driver 201, it is possible to form a background image by suppressing the density unevenness in the main scanning direction by referring to the screen profile.

  The screen profile may be created by a method in which the user directly determines a dot pattern at each main scanning position. A method in which the user directly specifies a screen profile and the screen profile generation unit 206 generates a screen profile based on the screen profile will be described with reference to FIG.

  FIG. 15 is a diagram illustrating an example of a GUI provided by the driver operation unit 203.

  The user sets information of the default dot pattern using the GUI shown in FIG. The user inputs default dot pattern information into an input field 1501 for inputting default dot pattern information in the GUI shown in FIG. Information input to the input field 1501 is information on how many pixels are drawn per unit area. At this time, the user may directly input a numerical value or select a numerical value prepared in advance on the GUI side. FIG. 16 shows a case where 16 is input as a default dot pattern in the input field 1501.

  Next, upon detecting the selection of the calibration image output key 1502 by the user, the processor of the PC 101 instructs the image forming apparatus 102 to output a calibration image. The calibration image used for output may be generated by the printer driver 201 or may be generated by the image forming apparatus 102 based on an instruction. Then, the user looks at the output product and determines which patch has a latent image inconspicuous. Subsequently, when receiving the selection operation for the dot pattern input area 1503, the processor of the PC 101 selects an optimal density patch at each main scanning position based on the selection, and selects a screen to be used for each main scanning position. Generate a profile. The screen profile is used for subsequent printing. The selection at this time may be a form in which the user directly inputs a numerical value, or may be a form in which the user selects from a list of numerical values prepared in advance on the GUI side. Thus, the user can directly set a desired screen profile. In FIG. 16, in the dot pattern input area 1503, 0 is assigned to a location corresponding to the main operation position X2, +1 is assigned to a location corresponding to the main operation position X1, X3, and a location corresponding to the main operation position X0, X4. In the example shown in FIG.

(Dot pattern switching process using screen profile)
An example of a dot pattern switching process using a screen profile performed by the image processing unit 205 will be described with reference to FIG.

  FIG. 16 is a flowchart illustrating an example of a dot pattern switching processing procedure using a screen profile. It is assumed that the dot pattern used at this time has a size of I × J pixels and is composed of L sub-matrices.

  In step S1601, the image processing unit 205 acquires a main scanning position Xn that satisfies x ≧ Xn and x <Xn + 1 among main scanning positions Xn specified in the screen profile for the main scanning position x of interest.

  In S1602, the image processing unit 205 sets intermediate positions Xn_n + 1 (0) to Xn_n + 1 (L-1) corresponding to the number L of sub-matrices between the main scanning positions Xn to Xn + 1. At this time, Xn_n + 1 (0) represents the same main scanning position as Xn.

  In step S1603, the image processing unit 205 acquires an intermediate position 1 that satisfies x ≧ Xn_n + 1 (l) and x <Xn_n + 1 (l + 1) for the main scanning position x.

  In step S1604, regarding the dot pattern at the main scanning position x, the image processing unit 205 sets L-1 of the L sub-matrices to a dot pattern based on the screen profile Pn, and sets l to a dot pattern based on the screen profile Pn + 1. decide. At this time, the method of arranging the sub-matrices may be a predetermined one or a random one.

  In step S1605, the image processing unit 205 determines whether a dot pattern has been determined at all main scanning positions. If it is not determined that the dot pattern has been determined at all the main scanning positions, the process proceeds to S1606, and the main scanning position x of interest is updated to x + 1 in S1606. That is, the image processing unit 205 changes the processing target from the main scanning position to the next main scanning position. Then, the process returns to S1601. On the other hand, if it is determined that the dot pattern has been determined at all the main scanning positions, the flow ends.

  As described above, in the image forming system, the image processing unit 205 performs image processing on the input image data using a predetermined dot pattern to generate print image data. Then, the image forming apparatus 102 receives the print image data generated by the image processing unit 205, and outputs a print image to a recording medium based on the received print image data. However, the image processing unit 205 sets the dot pattern to be synthesized with the print image data according to the density unevenness in the main scanning direction. Thus, based on image data on which screen processing is performed, print image data for forming an image in which density unevenness in the main scanning direction is suppressed by switching a dot pattern used for drawing in accordance with density unevenness in the main scanning direction. Can be generated.

  In the present embodiment, the case where the present invention is applied to a dot pattern used for screen processing has been described, but the present invention is not limited to this. It is also possible to apply to a dither matrix instead of a dot pattern.

[Embodiment 2]
In the first embodiment, print image data is generated by performing image processing on image data using a dot pattern switched according to the main scanning position based on the screen profile.

  Depending on the configuration of the image forming system, the printer driver 201 may transmit print image data when printing all images. In this case, screen processing using not only a specific dot pattern but also a dither matrix is performed on the image, and print image data obtained by the screen processing is transmitted to the image forming apparatus 102. Therefore, it is necessary to consider not only the density unevenness at a specific tone density but also the density unevenness at all the tone densities.

  Therefore, in the present embodiment, a method of switching the dither matrix according to the density unevenness and creating a profile for switching the dither matrix will be described.

(Screen processing using dither matrix)
Screen processing (binarization processing) using a dither matrix performed by the image processing unit 205 will be described with reference to FIG. Screen processing refers to converting image data from continuous tone to area tone, that is, the area of a colored (toner-attached) area and the area of an uncolored (toner-unattached) area per unit area. This is a process of expressing with a gradation expressed by the ratio of. A dither matrix representing the growth of the screen is used for the screen processing, and the binarization of the pixel value of each pixel of the image data to be processed is performed by the dither matrix. The dither matrix corresponds to a threshold table representing the growth of the screen according to the increase in the pixel value (density) of the input image. FIG. 17 is a diagram illustrating an example of the screen processing.

  FIG. 17 shows input image data 1701 to be subjected to screen processing, a dither matrix 1702 used for screen processing, and screen image data 1703 obtained by screen processing.

  When the image data 1701 is input, the image processing unit 205 pays attention to the pixel value of each pixel of the image data 1701, and compares the pixel value with a threshold value of the dither matrix corresponding to the pixel of interest. The image processing unit 205 outputs 0 or the maximum gradation value as the pixel value of the target pixel according to the result of the comparison. Specifically, as a result of the comparison, when the pixel value of the target pixel of the image data 1701 exceeds the threshold value of the dither matrix 1702, the pixel value of the target pixel of the screen image data 1703 becomes the maximum gradation value. On the other hand, when the pixel value of the pixel of interest in the image data 1701 is equal to or smaller than the threshold value of the dither matrix 1702, the pixel value of the pixel of interest in the screen image data 1703 is 0. In the example shown in FIG. 17, the image data 1701 having 9 pixels of 3 × 3 and the pixel value of each pixel is 130 is used to binarize the pixel values of the input pixels to 0 and 255 with 9 pixels of 3 × 3. Screen processing is performed using the dither matrix 1702 to be processed. When the pixel value of each pixel exceeds the corresponding threshold value as a result of the screen processing, the pixel value of the corresponding pixel in the screen image data 1703 is 255. On the other hand, when the pixel value of each pixel is equal to or smaller than the corresponding threshold value, the pixel value of the corresponding pixel in the screen image data 1703 is 0.

  In the present embodiment, by changing each threshold included in the dither matrix, an image (screen image data) expressed according to screen image data obtained by the screen processing can be changed.

  An example of a change in an image (screen image) obtained by changing each threshold of the dither matrix will be described with reference to FIG.

  FIG. 18 is a diagram illustrating a change example of an image (screen image) obtained by changing each threshold of the dither matrix.

  FIG. 18A shows a dither matrix serving as a base, which converts an 8-bit image of 4 × 4 pixels into binary. Using the dither matrix shown in FIG. 18A, screen image data obtained by performing screen processing on image data in which the pixel values of all 4 × 4 16 pixels are 128 is shown in FIG. ). It can be seen from the screen image data shown in FIG. 18B that half of all 16 pixels, ie, 8 pixels, are drawn.

  In FIG. 18C, the order of pixels to be drawn is the same as that of the dither matrix shown in FIG. 18A, but the closer the threshold value is to 128, the more the threshold value of the dither matrix shown in FIG. 9 shows a dither matrix that has been subjected to a process of correcting a large value. Using the dither matrix shown in FIG. 18C, screen image data obtained by performing screen processing on image data in which the pixel values of all 16 pixels of 4 × 4 are 128 are shown in FIG. ). When the dot shape of the screen image data shown in FIG. 18D is compared with the dot shape of the screen image data shown in FIG. 18B, the shape is almost the same, but since the threshold value near 128 has been increased, one pixel You can see that it is not drawn. That is, a small dot is drawn for the pixel value of the image data to be screen-processed.

  In FIG. 18E, the order of pixels to be drawn is the same as the dither matrix shown in FIG. 18A, but the closer the threshold is to 128, the more the threshold of the dither matrix shown in FIG. 2 shows a dither matrix that has been subjected to a process of correcting the value of to a small value. Using the dither matrix shown in FIG. 18E, screen image data obtained by performing screen processing on image data in which the pixel values of all 4 × 4 16 pixels are 128 is shown in FIG. ). When the dot shape of the screen image data shown in FIG. 18 (f) is compared with the dot shape of the screen image data shown in FIG. 18 (b), the shape is almost the same, but since the threshold value near 128 has been reduced, one pixel has been reduced. It can be seen that drawing is increasing. That is, a large dot is drawn for the pixel value of the image data to be screen-processed. Thus, by adjusting the threshold value of the dither matrix, it is possible to adjust the dot growth.

  Using this property, a method of creating a screen profile when performing a screen process using a dither matrix will be described with reference to FIGS.

  FIG. 19 is a diagram illustrating an example of a dither matrix used in the present embodiment. FIG. 19A shows an example of a default dither matrix, and FIG. 19B shows an example of a dither matrix used in the present embodiment.

  The default dither matrix shown in FIG. 19A is a dither matrix of 8 × 8 pixels, and is a dither matrix for drawing four dots that grow greatly according to gradation.

  The dither matrix shown in FIG. 19B is a dither matrix created for correcting the main scanning positions X0 and X4 shown in FIG.

  FIG. 20 is a diagram showing, for each gradation value, density unevenness in the main scanning direction that occurs when screen processing is performed using a default dither matrix. Curve D1 shows the density unevenness when the tone value is 64, curve D2 shows the density unevenness when the tone value is 128, curve D3 shows the density unevenness when the tone value is 192, and curve D4. Represents density unevenness when the gradation value is 255. The main scanning positions X0, X1, X2, X3, X4 are the same as in FIG. That is, the main scanning position X2 indicates the vicinity of the center of the main scanning position, the main scanning positions X1 and X0 indicate specific positions from the center of the main scanning position toward one end, and the main scanning positions X3 and X4 indicate the main scanning positions. A specific position is shown from the center toward the other end.

  From the density unevenness of each tone value in the main scanning direction shown in FIG. 20, the printer unit 105 of the image forming apparatus 102 of the present embodiment has the largest density unevenness at the time of the curve D2 corresponding to the tone value 128. You can see that. Next, density unevenness occurs in the curve D1 corresponding to the tone value 64 and the curve D3 corresponding to the tone value 192, and the density unevenness substantially occurs in the curve D4 corresponding to the maximum tone value (solid image) 255. You can see that it is not.

  A screen profile for correcting density unevenness shown in FIG. 20 will be described with reference to FIG.

  FIG. 21 is a diagram showing an example of a screen profile for correcting the density unevenness shown in FIG. This example of a screen profile is a profile indicating how many additional pixels can be used to reproduce the target density at each gradation value and each main scanning position. That is, the screen profile shown in FIG. 21 is data indicating the increase / decrease of the number of dots to be drawn as compared with the default dither matrix, which is necessary for setting the target density at each main scanning position for each gradation value. .

  Here, a curve D0 not shown in FIG. 20 represents a case of paper white having a gradation value of 0. Therefore, in the curve D0, it is assumed that the drawing pixels are not added even at the main scanning positions X0, X1, X2, X3, and X4. Also, in the case of a dither matrix used when drawing characters or thin lines, if the number of pixels to be drawn decreases even though the gradation is the maximum, jaggies or breaks in the thin lines may be caused. Also, the increase / decrease of the drawing pixel in the curve D4 is not performed. For example, the curve D2 indicates that the target density can be reproduced by drawing two additional pixels at the main scanning positions X0 and X4 and one pixel at the main scanning positions X1 and X3. This screen profile is created by performing each of the gradations 64, 128, 192, 255 corresponding to the curves D1 to D4 except for the gradation 0 representing the paper white corresponding to the curve D0. Note that a method similar to the screen profile creation method (FIG. 14) used in the first embodiment is used to create the screen profile of the present embodiment.

  Then, the image processing unit 205 corrects the dither matrix used at each main scanning position based on the created screen profile. For example, the dither matrix used for X0 and X4 is one pixel when the gradation value is 64 or 192 corresponding to the curves D1 and D3, and is two pixels when the gradation value is 128 corresponding to the curve D2. Correction is required to increase the number of pixels to be drawn. The size of the dither matrix used in this embodiment is 64 pixels of 8 × 8 pixels, and the maximum gradation value is 255. Therefore, the gradation value from drawing one pixel to drawing the next pixel is used. Are in increments of four.

  Accordingly, the threshold value of the value close to the gradation value 128 corresponding to the curve D2 is 8 and the threshold value of the value close to the gradation value 64 corresponding to the curve D1 and the gradation value 192 corresponding to the curve D3 is reduced by 4. This enables drawing corresponding to the screen profile. Also, the correction value of the threshold value at D1_2 between the gradation values, for example, between the gradation value 64 corresponding to the curve D1 and the gradation value 128 corresponding to the curve D2, is, for example, the difference value between D1_2 and D1 and D1_2. And a weighted average of the correction values based on the difference between D2 and D2. The resulting dither matrix is shown in FIG.

  By using the dither matrix corrected based on the screen profile created by the above method while switching at each main scanning position, it is possible to form an image with suppressed density unevenness even in the case of the screen processing using the dither matrix. Further, similarly to the method of switching the screen stepwise using the sub-matrix performed in the first embodiment, it is possible to suppress the density step by switching the dither matrix used stepwise.

[Other Embodiments]
The present invention can be applied to a system including a plurality of devices (for example, a host computer, an interface device, a reader, a printer, etc.), but also to a device including one device (for example, a copying machine, a facsimile machine, etc.). You may. Another object of the present invention is to supply a recording medium storing program codes for realizing the functions of the above-described embodiments to a system or an apparatus, and a computer of the system or the apparatus reads out and executes the program code stored in the storage medium. It is also achieved by doing In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiment, and the program code itself and the storage medium storing the program code constitute the present invention.

  According to the present invention, an operating system (OS) running on a computer performs part or all of actual processing based on instructions of a program code, and the processing realizes the functions of the above-described embodiments. This is also the case. Further, the present invention is also applied to a case where the program code read from the storage medium is written in a memory provided in a function expansion card inserted into the computer or a function expansion unit connected to the computer. In that case, based on the instruction of the written program code, the CPU provided in the function expansion card or the function expansion unit performs part or all of the actual processing, and the processing realizes the functions of the above-described embodiments. .

  The present invention supplies a program for realizing one or more functions of the above-described embodiments to a system or an apparatus via a network or a storage medium, and one or more processors in a computer of the system or the apparatus read and execute the program. This processing can be realized. Further, it can also be realized by a circuit (for example, an ASIC) that realizes one or more functions.

101 Host Computer 102 Image Forming Apparatus 103 Controller 104 Scanner 105 Printer 106 Device Operation 107 Network

Claims (9)

An image processing apparatus that performs screen processing on input image data to generate print image data,
An image processing apparatus, comprising: setting means for setting a dither matrix used for the screen processing or a dot pattern to be synthesized with the print image data in accordance with density unevenness in a main scanning direction. The image processing apparatus according to claim 1, wherein the setting unit sets the dot pattern or the dither matrix according to a position in the main scanning direction. The image processing apparatus according to claim 1, wherein the setting unit sets the dot pattern or the dither matrix according to a position in the main scanning direction based on a patch created in advance. The setting means has a plurality of switching points for switching the dither matrix or the dot pattern in the main scanning direction, and switches the dither matrix or the dot pattern to be used according to each of the plurality of switching points. The image processing apparatus according to claim 1. The dither matrix or the dot pattern has a plurality of sub-matrices,
When different dither matrices or dot patterns are set for a first switching point and a second switching point, the setting means sets the dither as the distance from the first switching point approaches the second switching point. In a unit of a matrix or a sub-matrix included in the dot pattern, the dither matrix or the dot pattern set at the first switching point is switched stepwise to the dither matrix or the dot pattern set at the second switching point. The image processing device according to claim 4.   The image processing apparatus according to claim 1, wherein the setting unit sets the dot pattern or the dither matrix according to a gradation. The image represented by the image data is a tint block image having a background portion that disappears after copying and a latent image portion that remains after copying,
The background portion is drawn using a first dot pattern in which each dot is not adjacent in a unit area,
The latent image portion is drawn using a second dot pattern in which dots are concentrated at one place in a unit area,
7. The apparatus according to claim 1, wherein the setting unit sets the number of dots per unit area of the first dot pattern according to the density unevenness in the main scanning direction. 8. Image processing device. An image processing method for performing screen processing on input image data to generate print image data,
An image processing method, wherein a dither matrix used for the screen processing or a dot pattern synthesized with the print image data is set in accordance with density unevenness in a main scanning direction.   A program for causing a computer to function as the image processing device according to any one of claims 1 to 7.