JP2017085456A - Image processing system, image processing method, and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce loosing of the shapes of character and line using an object of a graphics attribute or image attribute as background, while matching them with the density of the background.SOLUTION: An image processing system comprises: means that produces attribute information indicating an attribute of each pixel for input image data to be printed; color conversion processing means that converts a color space of the input image data to a color space corresponding to the color of a color material used for printing; and halftone processing means that performs a halftone processing corresponding to the attribute of each pixel on the basis of the attribute information. If the attribute of each pixel of image data after the color conversion is the attribute of a character or line, the halftone processing means produces dot image data such that at a dot composing a background area for an object of the character or line relating to this attribute, the size of the dot in a predetermined vicinity area of the object of the character or line is made smaller than the size of a dot outside the vicinity area.SELECTED DRAWING: Figure 8

Description

  The present invention relates to an image processing technique for suppressing, when printing characters or lines overlapping a background, the object shape of the characters or lines is distorted by a halftone dot in the background and the readability is reduced.

  Generally, when printing a document on a printer, the attributes (characters, graphics, photographs, etc.) of each pixel are determined from information described in the page description language (PDL), and an image corresponding to the attribute is displayed. Processing is in progress. For example, image processing is performed by generating attribute signals according to features such as Text attribute for pixels in the character / line area in the document to be printed, Graphics attribute for pixels in the graphic area, and Image attribute for pixels in the photograph area. It is carried out. In addition, an attribute signal such as a small-point character (SmallText attribute) or a thin line (ThinLine attribute) is generated for an area requiring reproduction with higher accuracy.

  At the time of image processing, for example, a screen having a relatively high resolution of 200 lines or the like is used for an image area in which the Text attribute is set, and an image quality that makes the outline of a character clear is realized. In addition, a screen having a relatively low resolution, such as 100 lines, is used for the image area in which the Graphics attribute and the Image attribute are set, and the image quality with less roughness is realized by suppressing the variation of halftone dots.

  However, in the above conventional method, halftone dots are connected in the Text attribute portion (near the boundary where the screen is switched) in contact with the Graphics attribute and the Image attribute, and as a result, the shape of the Text attribute object collapses, resulting in image quality degradation. It was.

  In order to deal with such a problem, a method has been proposed in which density control such as enhancement or suppression of contours is performed near the boundary where the screen is switched. (See Patent Document 1).

JP 2005-341249 A

  However, in the technique of the above-mentioned Patent Document 1, since the density is merely changed in the outline region of characters or the like, the halftone dot size in the vicinity of the boundary where the screen is switched does not change. As a result, line and character objects, particularly small-point character and thin line object shapes, are greatly collapsed, and readability may deteriorate.

  An image processing apparatus according to the present invention, means for generating attribute information indicating attributes for each pixel of input image data to be printed, and a color space of the input image data to a color space corresponding to a color material color used for printing Color conversion processing means for performing color conversion; and halftone processing means for performing halftone processing according to the attribute of each pixel based on the attribute information with respect to the image data after color conversion; When the attribute of each pixel in the color-converted image data is a character or line attribute, the character or line at a halftone dot constituting the background area of the character or line object related to the attribute Halftone dot image data in which the size of halftone dots in a predetermined neighboring area of the object is smaller than the size of halftone dots outside the neighboring area is generated.

  According to the present invention, it is possible to reduce the collapse of the shape of characters and lines with the graphics attribute and image attribute objects in the background while being familiar with the background density.

1 is a diagram illustrating an overall configuration of a printing system. 1 is a block diagram illustrating a configuration of an image forming apparatus. It is a figure which shows the internal structure of an image process part. FIG. 6 is a diagram illustrating an example of a main UI screen when performing print settings. FIG. 6 is a diagram illustrating an example of a UI screen for performing settings relating to print quality. It is a figure which shows the sub screen for the detailed setting of the "character / line improvement in halftone dot" function. It is a flowchart which shows the flow of the initialization process of the "character / line improvement in halftone dot" function. 3 is a flowchart illustrating a processing flow in an image processing unit according to the first exemplary embodiment. It is a figure which shows an example of a raster image and its attribute information. It is a figure which shows an example of the attribute information at the time of rewriting to a new attribute. 6 is a flowchart illustrating a flow of gradation correction processing according to the first exemplary embodiment. (A) is a graph showing output characteristics of the printer, and (b) is a graph showing differences in characteristics depending on object attributes. 6 is a flowchart illustrating a flow of halftone processing according to the first embodiment. It is a figure explaining the effect of this invention. 10 is a flowchart illustrating a processing flow in an image processing unit according to the second embodiment. It is a flowchart which shows the flow of a monochrome ratio determination process. It is a flowchart which shows the flow of the necessity determination process of a monochrome ratio determination.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. The configurations shown in the following embodiments are merely examples, and the present invention is not limited to the illustrated configurations.

[Example 1]
FIG. 1 is a diagram illustrating an overall configuration of a printing system according to the present embodiment. The printing system 100 includes an image forming apparatus 101, a host computer 102, a mobile terminal 103, and a server 104, which are connected to each other via a network 105.

  The image forming apparatus 101 is an apparatus having a printing function such as an MFP (Multi Function Peripheral). The image forming apparatus 101 performs print output in response to a print instruction from the host computer 102, mobile terminal 103, and server 104 via the network 105. The image data read using the attached scanner can be printed out (copy function) or transmitted to the server 104 or the like (SEND function).

  Then, the image forming apparatus 101 performs image processing so that the size of halftone dots constituting the background is reduced in the vicinity of characters and lines (particularly small-point characters and thin lines) at the time of print output. In the following, an aspect in which a printing process involving the image processing is realized with a closed configuration in the image forming apparatus 101 will be described. However, the present invention is not limited to such a closed configuration, and may be realized by distributed processing in cooperation with the host computer 102, the mobile terminal 103, the server 104, and the like.

  Next, the configuration of the image forming apparatus 101 will be described. FIG. 2 is a block diagram illustrating a configuration of the image forming apparatus 101. The image forming apparatus 101 includes a control unit 200, an operation unit 210, a scanner 220, and a printer unit 230. In addition to the operation unit 210, the control unit 200 is connected to a scanner 220 that is an image input device and a printer unit 230 that is an image output device.

  The control unit 200 includes an operation unit I / F 201, a CPU 202, a RAM 203, a device I / F 204, an HDD 205, a ROM 206, a network I / F 207, and an image processing unit 208, which are connected to each other via a system bus 209. Yes.

  The CPU 202 activates the OS by a boot program stored in the ROM 206. The CPU 202 executes application programs stored in the HDD 205 on this OS, thereby executing various processes. A RAM 203 is used as a work area for the CPU 202. The RAM 203 provides a work area and an image memory area for temporarily storing image data. The HDD 205 stores the application program, image data, and various setting values.

  The operation unit I / F 201 is an interface with the operation unit 210 having a touch panel, and outputs various screen data to be displayed on the operation unit 210. The operation unit I / F 201 sends information input by the user through the operation unit 210 to the CPU 202. The device I / F 204 is an interface with the scanner 220 and the printer unit 230. The image forming apparatus 101 exchanges various data such as image data to be printed with the host computer 102, the mobile terminal 103, and the server 104 via the network I / F 207 and the network 105.

  The image processing unit 208 performs image processing such as color conversion processing, rendering processing, gradation correction processing, and halftone processing on image data to be printed. At this time, processing for improving characters and lines in the halftone dots is also performed based on various print settings. The printer unit 230 forms an image using a color material on a recording medium based on the printing data processed by the image processing unit 208. Such an image processing unit 208 that functions under the control unit 200 may be configured as an image processing device independent of the image forming apparatus 101.

  FIG. 3 is a diagram illustrating an internal configuration of the image processing unit 208, which includes a rendering processing unit 301, a color conversion processing unit 302, a switching processing unit 303, a gradation correction processing unit 304, and a halftone processing unit 305. Yes.

  A rendering processing unit 301 performs processing for interpreting input image data described in a page description language (PDL) and converting it into raster image data. Further, the characteristics (attributes such as characters, graphics, and photographs) of each pixel are discriminated from the information described in the PDL, and information (attribute information) indicating attributes for each pixel such as Text, Graphics, and Image is generated. At that time, attribute information such as a SmallText attribute is generated for a character (small point character) whose character size is smaller than a predetermined size, and a ThinLine attribute is generated for a line (thin line) whose line width is narrower than a predetermined width.

  The color conversion processing unit 302 converts the color space (for example, RGB) of the input image data into a color space (for example, CMYK) corresponding to a plurality of color material colors used in the printer unit 230.

  The switching processing unit 303 performs a determination process (neighbor attribute determination process) as to whether or not the pixel having the SmallText attribute or the ThinLine attribute exists in the vicinity of the target pixel. Then, according to the determination result, switching control information for controlling switching of the screen content (for example, the number and type of screen lines) used in the halftone processing unit 305 is generated, or a new attribute is added. Perform processing.

  The tone correction processing unit 304 performs tone correction processing using an LUT or the like that describes the tone characteristics of the printer unit 230.

  A halftone processing unit 305 performs halftone processing according to the attribute of the object on the image data on which the gradation correction processing has been performed, and a halftone dot image for printing expressed by halftone dots (halftone dots). Convert to data. In this case, in the halftone processing of this embodiment, the halftone dots in the background area of the character or line object are set to the halftone dots outside the vicinity area in the vicinity area of the character or line object. Halftone dot image data smaller than the size of is generated. Details thereof will be described later.

(Print setting screen)
Next, a UI screen used when the user sets printing conditions will be described. FIG. 4 is a diagram illustrating an example of a main UI screen (print setting screen) when performing print settings. Such a UI screen is displayed on the operation unit 210 or the monitor of the host computer 102 or the mobile terminal 103.

  The print setting screen 400 shown in FIG. 4 includes a page setting tab, a finishing tab, a paper feed tab, and a print quality tab. The user selects a tab according to the purpose and performs various print settings. Now, on the print setting screen 400, the print quality tab 401 is selected. When a processing option button 402 in the print quality tab 401 is pressed, a UI screen for performing detailed settings related to print quality is displayed in, for example, a separate window. In the color mode setting field 403, it is possible to select whether to print in color, monochrome, or color / monochrome automatic switching according to the page contents and other settings.

  FIG. 5 is a diagram illustrating an example of a UI screen (processing option screen) for performing settings related to print quality. In the processing option screen 500 shown in FIG. 5, the item list display column 501 displays a list of setting items that can be specified as processing options and the current setting contents corresponding to the setting items. When the user selects an arbitrary item from the list displayed in the item list display field 501, the selected item is displayed in the selection item display field 502, and the setting content is changed in the set value input field 503. Will be able to. In the example of FIG. 5, “character / line improvement in halftone dot” which is a characteristic function of this embodiment is selected, and “use” is set as the setting content. When further details about the item being selected can be set, a detail button 504 is displayed, and when this is pressed, a UI screen for detailed setting is displayed. FIG. 6 shows a detailed setting sub-screen 600 that is displayed when the detail button 504 is pressed while “characters in dot / line improvement” is selected. In this sub-screen 600, when using the “character / line improvement in halftone dot” function, the search range centered on the target pixel and the threshold value for discriminating the pixel to which the SmallText attribute and ThinLine attribute are assigned are set. Can be set. In the sub-screen 600 shown in FIG. 6, the SmallText object setting field 601 is provided with two types of information input fields necessary for determining the character to which the SmallText attribute is assigned. Specifically, there is a “character size” input field 602 for the number of points of the character size handled as a small point character, and an “applicable width” input field 603 for designating a search range for pixels with the SmallText attribute. In addition, the ThinLine object setting field 604 is provided with two types of information input fields necessary for determining the thin line to which the ThinLine attribute is assigned. Specifically, there is a “line width” input field 605 that defines the fineness of a line to be handled as a thin line, and an “applied width” input field 606 that specifies a range for searching for pixels with a ThinLine attribute. In the example of FIG. 6, one value is specified in the two “applicable width” input fields 603 and 606 on the assumption that the vertical and horizontal widths are the same, but the vertical and horizontal widths are separately set. May be specified.

  In addition, by selecting “halftone” from the list displayed in the item list display field 501, a pattern defining the content of halftone processing can be set. In the example of FIG. 5, “Pattern 2” is set. Here, pattern 2 is assumed to be a default pattern (combination) in which a screen with a high line number (212 lines) is assigned to the Text attribute and a screen with a low line number (106 lines) is assigned to the Graphics attribute and the Image attribute. And in this pattern 2, screens with the same high number of lines (212 lines) as normal characters (Text attribute) are used for small text characters with SmallText attributes and thin lines with ThinLine attributes where reproduction of details is important. To do. The user can arbitrarily adjust the content of the halftone process to be applied by selecting “halftone” from the list displayed in the item list display field 501 and changing the setting content (pattern). That is, it is possible to change the combination of screen line numbers assigned for each attribute, apply a common screen line number to all attributes of an object, or assign an error diffusion process.

  Further, by selecting “gray compensation” from the list displayed in the item list display field 501, it is possible to specify an attribute of an object for which gray compensation is performed. The gray compensation is a function of printing by replacing the color image signal values of the RGB values that appear gray on the display with equal amounts of K single-color signal values.

(Initial setting processing of halftone dot character / line improvement function)
As described above, when the user sets the “character / line improvement in halftone dot” item to “use” on the print setting screen, the initial setting process for effectively using the function according to the contents of the current print setting Is made. FIG. 7 is a flowchart showing the flow of the initial setting process. This series of processing is realized by developing a program stored in the HDD 205 or the ROM 206 in the RAM 203 and executing it by the CPU 202.

  In step 701, the setting content (pattern) of “halftone setting” in the item list display field 501 of the processing option screen 500 described above is acquired.

  In step 702, information on the number of screen lines assigned to the Graphics attribute and the Image attribute is extracted from the acquired pattern information, and the extracted screen line number is compared with a predetermined threshold (Lpi_Th). The threshold value (Lpi_Th) is for determining whether or not there is a possibility that the shape collapse occurs in an object in which the reproducibility of details is important and the readability may be lowered. Here, the threshold value (Lpi_Th) is set to 150 lpi. This means that if the number of screen lines assigned to the Graphics or Image attribute is less than or equal to the specified number of lines, the shape of the Text attribute object (character or line) surrounded by the object (background) of that attribute can be corrupted. It is determined that there is a sex. As a result of the determination, if the number of screen lines assigned to the Graphics attribute or Image attribute is smaller than the threshold value Lpi_Th, the process proceeds to step 703. On the other hand, if the number of screen lines assigned to the Graphics attribute or Image attribute is larger than the threshold value Lpi_Th, the process proceeds to step 704.

  In step 703, the CPU 202 sets the image processing unit 208 to perform the neighborhood attribute determination process in the switching processing unit 303 during the printing process of the input image data. As a result, in the gradation correction processing unit 304 and the halftone processing unit 305, the processing content is switched based on the attribute information (and the switching control information) described above.

  In step 704, settings for performing various processes as specified by the user on the UI screen (the item list display field 501 of the processing option screen 500 described above) are performed on the image processing unit 208 without performing the neighborhood attribute determination processing. This is done by the CPU 202.

  The above is the contents of the initial setting process for enabling the “character / line improvement in halftone dot” function.

  Next, the operation of the image processing unit 208 when printing out using the printer unit 230 will be described. FIG. 8 is a flowchart showing the flow of processing in the image processing unit 208 according to the present embodiment. This series of processing is also realized by developing a program stored in the HDD 205 or the ROM 206 in the RAM 203 and executing it by the CPU 202.

  In step 801, image data described in PDL is acquired as input image data to be printed. The acquired input image data is sent to the rendering processing unit 301.

  In step 802, the rendering processing unit 301 interprets the PDL included in the input image data and converts it into raster image data. The raster image data is sent to the color conversion processing unit 302. Further, the rendering processing unit 301 analyzes the PDL to determine the characteristics of each pixel (attributes such as characters, graphics, and photographs), and generates attribute information indicating the attributes for each pixel. In this embodiment, attribute information such as a Text attribute for characters, a Graphics attribute for graphics, an Image attribute for photographs, a SmallText attribute for small-point characters, and a ThinLine attribute for thin lines is generated for each pixel. The generated attribute information for each pixel is sent to the switching processing unit 303. FIG. 9 is a diagram showing an example of a raster image (monochrome) and its attribute information. The number “4” is present in a gray rectangle whose upper and lower ends are bordered by black lines. The number “4” is assigned SmallText representing a small point character, the gray rectangle is Graphics, and the black lines at the top and bottom are assigned Thinline attributes.

  In step 803, the pixel of interest in the raster image data generated in step 802 is determined. For example, each pixel is determined as a pixel of interest in order from the pixel at the left corner in the raster image.

  In step 804, the switching processing unit 303 executes neighborhood attribute determination processing for the pixel of interest using the attribute information generated in step 802. Here, an example of generating a switching flag as the switching control information will be mainly described. First, a search is performed to determine whether a SmallText attribute or ThinLine attribute pixel exists in an M × N pixel area centered on the pixel of interest. The “M × N pixel range, which is the search range in this case, and the threshold value for discriminating the attributes of SmallText and ThinLine” are described in the “Details in halftone dot / line improvement” screen 600 shown in FIG. The set contents are applied. In this embodiment, since it is specified in mm (refer to the input fields 603 and 604 in the detailed setting screen 600), when used in this step, it is necessary to convert to pixel. That is, it is converted (rounded off) to a pixel unit according to the setting content of “resolution” in the “item list” display field 501 of the processing option screen 500 shown in FIG. For example, if 600 dpi is 0.2 mm, then 0.2 mm / (25.4 mm × 600 pix) ≈5 pix, and the presence or absence of a pixel having a SmallText or ThinLine attribute is searched within a 5 × 5 pix range centered on the target pixel. It will be. As a result of the search, if there is a SmallText attribute or ThinLine attribute within the range of M x N pixels centered on the pixel of interest, the value of the above switching flag is used to switch the number of screen lines used in halftone processing. Set to ON to indicate that On the other hand, if it does not exist, it is set to OFF indicating that switching is not performed. If the setting for executing the neighborhood attribute determination process is not made in the initial setting process described above, the value of the switching flag is fixed to OFF. The generated switching flag is sent to the color conversion processing unit 302 together with the attribute information.

  In step 805, the color conversion processing unit 302 executes color conversion processing for converting the color space (for example, RGB) of the pixel of interest into a color space (for example, CMYK) corresponding to the printer unit 230. In the color conversion process, conversion is performed while switching the presence / absence of gray compensation based on the content of the above-described “gray compensation setting” and attribute information. At this time, gray compensation may be set independently of the attribute type for the region where the switching flag is ON, and color conversion processing may be performed according to the setting. By setting the gray compensation setting different from that of the SmallText or ThinLine attribute portion for the area where the switching flag is ON, the boundary surface of the gray portion can be emphasized and the shape of the object can be reinforced. The pixel-of-interest data (CMYK data) after the color conversion process is sent to the gradation correction processing unit 304 together with the attribute information and the switching flag.

  In step 806, the tone correction processing unit 304 executes tone correction processing in consideration of the tone characteristics of the printer unit 230 for the pixel of interest that has been color-converted into the CMYK color space. Note that the gradation characteristics vary depending on the halftone processing method described later, and therefore, it is necessary to switch the content of the gradation correction processing in accordance with the halftone processing method. That is, the content of the gradation correction process is switched according to the content of “halftone setting” in the attribute information, the switching flag, and the item list display field 501. The details of the gradation correction process with this switching will be described later. The pixel-of-interest data after gradation correction is sent to the halftone processing unit 305 together with the attribute information and the switching flag.

  In step 807, the halftone processing unit 305 performs halftone processing on the pixel-of-interest data after the gradation correction, and is a halftone dot represented by a halftone dot, which is image data that can be used by the printer unit 230. Image data is generated. In the present embodiment, halftone processing is performed according to the attribute information, the switching flag, and the contents of “halftone setting” in the item list display field 501 so that a finer halftone dot is formed at a location where the switching flag is ON. The contents are switched. Details of the halftone process with this switching will be described later.

  In step 808, it is determined whether all the pixels in the raster image data generated in step 802 have been processed as the target pixel. If there is an unprocessed pixel, the process returns to step 803 to determine the next pixel of interest and continue the process. On the other hand, if the processing for all the pixels has been completed, this processing ends. The halftone dot image data generated in this way is sent to the printer unit 230 and used for printing output. When the input image data is composed of a plurality of pages, the above processing is repeated for each page.

  The above is the content of the processing in the image processing unit 208 according to the present embodiment. In step 804 described above, the switching flag is generated when a pixel having the SmallText attribute or the ThinLine attribute exists in the vicinity region of the target pixel. However, the present invention is not limited to this. Instead of generating the switching flag, the attribute (for example, Graphics attribute) given to the target pixel is changed to a new attribute (NearArea attribute in this embodiment) indicating that the pixel is in the vicinity of the small point character or the thin line. You may rewrite (refer FIG. 10). Alternatively, since the character portion is generally assigned a high line number, it may be rewritten to an existing Text (may be SmallText or ThinLine) attribute. When rewriting to an existing attribute, it is not necessary to switch between gradation correction processing and halftone processing using a switching flag described later. By rewriting the attribute of the neighborhood area to the Text attribute, the halftone dot size near the character can be reduced only by the switching operation of halftone processing based on the conventional attribute information.

(Tone correction processing)
Next, the gradation correction processing that involves switching of the processing contents based on the object attribute and the switching flag in the gradation correction processing unit 304 will be described. FIG. 11 is a flowchart showing the flow of gradation correction processing according to the present embodiment. In this flowchart, for the sake of simplification of explanation, it is assumed that the above-mentioned “pattern 2” is set as the content of “halftone setting”. In other words, in the case of Text attribute (including SmallText and ThinLine), gradation correction processing for high line number is applied, and in the case of Graphics attribute and Image attribute, for neighboring high line number or low line number depending on the switching flag The tone correction process is applied.

Here, each gradation correction for the number of high lines, the number of adjacent high lines, and the number of low lines will be described. For example, in monochrome image formation by an electrophotographic method, there are only two types of states: toner as a color material is attached to a recording medium (black) or not (white). Therefore, gradation expression is performed with a halftone dot area per unit area. Since each halftone dot is very small, each halftone dot cannot be identified by the human eye and appears to be gray. The brightness of the area is determined by the ratio of the area of the black halftone dot and the white background. In such an area gradation, even if the black-and-white ratio per unit area is the same, if the number of lines is different, the size of each lump of halftone dots changes. The number of lines is an index indicating the number of halftone dots per inch. For example, when there are 106 halftone dots per inch, the number of lines is 106 lines. When printing is actually performed by a printer, the toner placement changes depending on the size of the halftone dots. FIG. 12A is a graph showing the output characteristics of the printer with the output density value on the vertical axis and the input density value (dot ratio) on the horizontal axis. The solid line is 212 lines and the broken line is 106 lines. Respectively. As is clear from the graph of FIG. 12A, when the number of lines is high, the individual halftone dots are small, and therefore, toner is not easily applied in a low density portion (bright portion), and a high density portion (dark portion). Then, a small white background cannot be expressed and tends to be crushed. And when a line number is low, it turns out that the reverse tendency is shown. And the point which attaches importance in image quality for every attribute of an object also differs. For example, for text, readability is important, and for photographs and graphics, the gradation of bright parts is important. FIG. 12B is a graph showing differences in characteristics depending on attributes when the horizontal axis represents the input density value (dot ratio) and the vertical axis represents the target output density value. For example, in the case of characters, tone correction is performed using an LUT having a characteristic indicated by a solid line that protrudes upward so that the readability of the characters can be ensured even if the density is low. On the other hand, in the case of photographs and graphics, gradation correction is performed using an LUT having a characteristic indicated by a dashed line protruding downward so that a smooth gradation is obtained in a bright part. From the above viewpoint, it is necessary to perform gradation correction suitable for each of the high line number, the neighboring high line number, and the low line number. The characteristics of each gradation correction are as follows.
High line number: Since gradation correction is performed on pixels constituting characters and fine lines, the characteristics of 212 lines (characteristics indicated by solid lines in FIG. 12A) are corrected so as to be convex upward.
・ For the number of nearby high lines: This is a tone correction for the pixels that make up the area where characters and fine lines exist in the vicinity, and we want to adjust to the background, so the characteristics of the 212 lines so that it protrudes downward like the background ( The characteristic indicated by the solid line in FIG.
Low number of lines: Since this is gradation correction for pixels constituting a photograph or graphics, the characteristics of the 106 lines (characteristics indicated by broken lines in FIG. 12A) are corrected so as to protrude downward.

  Then, a table (LUT) for correcting the input density value (signal value) to the target output density value so as to realize the above features is prepared in advance, and the LUT corresponding to the object attribute is used. The input density value is corrected. This will be described in detail below.

  In step 1101, with reference to the attribute information of the pixel of interest, it is first determined whether or not any of the Text attribute, SmallText attribute, and ThinLine attribute is met. As a result of the determination, if any of the attributes is met, the process proceeds to step 1102. On the other hand, if the attribute of the pixel of interest is an attribute other than these (that is, either the Graphics attribute or the Image attribute), the process proceeds to step 1103.

  In step 1102, the above-described tone correction processing for the number of high lines is performed on the pixel of interest. At this time, gamma correction that enhances contrast so that characters and lines are clear may be additionally applied.

  In step 1103, the processing is switched with reference to the switching flag. Specifically, if the switching flag is ON, the process proceeds to step 1104, and if the switching flag is OFF, the process proceeds to step 1105.

  In step 1104, the above-described tone correction processing for the number of adjacent high lines is performed on the pixel of interest. At this time, a gamma correction according to the original attribute (Graphics / Image) may be additionally applied so as to adjust to the surrounding density.

  In step 1105, gradation correction processing for a low line number is performed on the pixel of interest. At this time, a gamma correction that makes the gradation more smooth may be additionally applied.

  The above is the content of the gradation correction processing according to the object attribute and the switching flag.

(Halftone processing)
Next, halftone processing that involves switching of processing contents based on object attributes and switching flags in the halftone processing unit 305 will be described. FIG. 13 is a flowchart illustrating the flow of halftone processing according to the present embodiment. In this flowchart, for the sake of simplification of description, it is assumed that monochrome printing is performed, and the above-described “pattern 2” is set as the content of “halftone setting”. In other words, in the case of the Text attribute (including SmallText and ThinLine), the screen for the high line number is applied, and in the case of the Graphics attribute and the Image attribute, the screen for the neighboring high line number or the low line number is used depending on the switching flag. Shall apply.

  In step 1301, with reference to the attribute information of the pixel of interest, it is first determined whether or not any of the Text attribute, SmallText attribute, and ThinLine attribute is met. As a result of the determination, if any of the attributes is met, the process proceeds to step 1302. On the other hand, if the attribute of the pixel of interest is an attribute other than these (that is, either the Graphics attribute or the Image attribute), the process proceeds to step 1303.

  In step 1302, halftone processing is performed on the pixel of interest in the CMYK data after gradation correction using a screen having a predetermined number of lines (for example, 212 lines) for the number of high lines, and halftone image data is converted into halftone image data. Generated. The generated halftone dot image data is sent to the printer unit 230.

  In step 1303, the processing is switched with reference to the switching flag. Specifically, if the switching flag is ON, the process proceeds to step 1304, and if the switching flag is OFF, the process proceeds to step 1305.

  In step 1304, the target pixel in the CMYK data after gradation correction is subjected to halftone processing using a screen having a predetermined number of lines (for example, 212 lines) for the number of neighboring high lines, and the above-described halftone dot is obtained. Image data is generated. As a result, the halftone dot size of a limited area (Graphics attribute or Image attribute pixel portion) near the pixels of the SmallText attribute or ThinLine attribute can be reduced. It should be noted that the number of lines for the number of neighboring high lines is determined to be equal to or more than the number of lines assigned to at least the background area (the graphics attribute or the image attribute and the switching flag is OFF). Here, for example, when the number of lines assigned to the background area is the same as the number of lines in the background area, if the screen type assigned to the background area is a line screen that represents the object as a line, the dot screen is represented to represent the object as a dot. Just be fine. This is because the line screen has a stronger dot connection than the dot screen, and has a stronger ability to break down the object shape of lowercase letters and fine lines. The generated halftone dot image data is sent to the printer unit 230.

  In step 1305, halftone processing using a screen having a predetermined number of lines (for example, 106 lines) for low line number is performed on the pixel of interest in the CMYK data after gradation correction, and the above-described halftone image is obtained. Data is generated. The generated halftone dot image data is sent to the printer unit 230.

  The above is the content of the halftone process according to the object attribute and the switching flag.

  FIG. 14 is a diagram for explaining the effect when the image processing according to the present embodiment is performed. In the table of FIG. 14, a row 1401 indicates a raster image as an input image, and is common to the case where the conventional image processing is performed and the case where the image processing of the present embodiment is performed. A row 1402 is an image showing an area for each attribute. When the image processing of this embodiment is performed, there is an area with a Neararea attribute (area shown in white in the figure) that is not in the conventional image processing. I can see that A row 1403 shows a halftone image after halftone processing. When the image processing of the present embodiment is performed, the size of the surrounding halftone dot that touches the object of the SmallText attribute or the ThinLine attribute is different from that in the background. It can be seen that it is smaller than the halftone dot. A row 1404 shows an actual print result, and it can be seen that, when the image processing of this embodiment is performed, the shape of the object is prevented from being deformed by small point characters and fine lines, and the readability is improved.

  As described above, according to the present embodiment, it is possible to reduce the size of surrounding dots for characters and lines (particularly small-point characters and thin lines) while being familiar with the background density. Thereby, the collapse of the object shape can be reduced and the readability of characters and the like can be prevented from being lowered.

[Example 2]
In the first embodiment, the mode in which the screen is switched using the switching flag or the like in the vicinity of the character or the line has been described. When the method of the first embodiment is applied as it is, there may be a problem that a streak or the like occurs at a screen switching portion in halftone processing. In order to suppress the adverse effects as much as possible, it can be said that it is desirable to apply the present invention only to a portion where the degree of deformation of the object and the degree of deterioration of readability are large. From experience, it has been found that the area of a small point character or thin line expressed in a single color is more likely to collapse and the readability is greatly reduced. This is because, when printing using multiple color materials, the outline of characters, etc. is blurred due to color misalignment during printing, but at the final printing stage, the characters, etc. This is because the shape is reinforced. There is also a reason that the boundary between the background and characters is conspicuous due to the difference in toner color, and as a result, the contour is emphasized. Therefore, the monochrome ratio of the switching location detected in the neighborhood attribute determination process is examined, and the screen is switched using the switching flag only when the monochrome ratio is low (when it is expected that the shape of the object is deformed or the readability is greatly reduced). A mode of performing the above will be described as a second embodiment.

  Note that description of points in common with the first embodiment will be omitted, and description will be made focusing on processing contents in the switching processing unit 303 which are different points.

  FIG. 15 is a flowchart illustrating the flow of processing in the image processing unit 208 according to the present embodiment.

  Steps 1501 to 1505 correspond to steps 801 to 805 in the flow of FIG. That is, PDL data to be printed is acquired (S1501), the PDL is interpreted by the rendering processing unit 301 and converted into raster image data, and attribute information indicating an attribute for each pixel is generated (S1502). . Next, a pixel of interest in the generated raster image is determined (S1503), and the neighborhood attribute determination process using the attribute information is executed in the switching processing unit 303, and a switching flag is generated (S1504). Subsequently, the color conversion processing unit 302 executes color conversion processing for converting the color space of the pixel of interest into a color space corresponding to the printer unit 230 (S1505). The pixel-of-interest data after the color conversion processing is sent to the switching processing unit 303 together with the attribute information and the switching flag.

In step 1506, the switching processing unit 303 performs a monochrome ratio determination process for the pixel of interest using the attribute information and the switching flag. FIG. 16 is a flowchart showing details of the monochrome ratio determination process. This will be described in detail below.
In step 1601, the process is divided with reference to the switching flag. If the switch flag is ON, the process proceeds to step 1602. On the other hand, if the switching flag is OFF, the process is exited.

In step 1602, the monochrome ratio of each color is derived by referring to the data of the pixel of interest (here, CMYK data) received from the color conversion processing unit 302. Determine whether or not. Specifically, first, the monochromatic ratio is obtained for each color of CMYK using the following formulas (1) to (4).
Crate = C / (C + M + Y + K) (1)
Mrate = M / (C + M + Y + K) (2)
Yrate = Y / (C + M + Y + K) (3)
Krate = K / (C + M + Y + K) (4)
Then, the single color ratio of each color of CMYK is compared with a preset threshold value Thre, and if any of the single color ratios of CMYK is equal to or greater than the threshold value Thre, the process exits to switch the screen. On the other hand, if the monochromatic ratio of all CMYK is lower than the threshold value Thre, the process proceeds to step 1603 in order to postpone the screen switching.

  In step 1603, the switching flag of the target pixel having a low single color ratio is rewritten from ON to OFF. If the attribute is rewritten instead of generating the switching flag in the neighborhood attribute determination process (S1504), the attribute may be returned to the attribute before rewriting.

  The above is the content of the monochrome ratio determination process. Returning to the flowchart of FIG.

  The gradation correction processing in step 1507 and the halftone processing in step 1508 correspond to steps 806 and 807 in the flow of FIG. That is, gradation correction processing and halftone processing according to the object attribute and the switching flag are executed.

  Here, the monochromatic ratio is obtained for each pixel, but the present invention is not limited to this. For example, it may be obtained in area units such as M × N pixels. Further, although the threshold value Thre is set to the same value for each color, since the conspicuousness of the collapse of the object shape may vary depending on the color, a different threshold value may be set for each color.

  As described above, according to the present embodiment, the screen switching control is executed only in the portion where the monochrome ratio is low, and the screen switching control is not performed for the portion where the monochrome ratio is high in consideration of the adverse effects. . This makes it possible to reduce the size of surrounding dots for characters and lines (particularly small-point characters and thin lines) while being familiar with the density of the background while taking into account the adverse effects.

[Example 3]
The determination of the monochromatic ratio described in the second embodiment is a process that refers to the image data after color conversion (CMYK data in the second embodiment), so the load is large and the process takes time. Therefore, a mode in which the monochrome ratio determination process is executed only when the effect of the second embodiment can be expected will be described as a third embodiment. In addition, description is abbreviate | omitted about a common point with Example 2, and suppose that it demonstrates centering on a difference point below.

  FIG. 17 is a flowchart illustrating the flow of a process for determining whether to execute a monochrome ratio determination process according to the present embodiment. The monochrome ratio determination execution necessity determination process is executed by the switching processing unit 303 in the above-described flow of FIG. 15, for example, immediately after the execution of the color conversion process (it may be before step 1506). .

  In step 1701, it is determined whether the image forming apparatus 101 that performs printing is a color machine or a monochrome machine. In the case of a monochrome machine, since all pages to be output are printed in a single color and there is no point in determining the single color ratio, the present process is exited without deciding to execute the single color ratio determination process (that is, to make it non-executable). . On the other hand, if it is a color machine, the process proceeds to step 1702.

In step 1702, it is determined whether the processing target page is a page printed with a single color (single color page) or a page printed with a plurality of color materials (multiple color page). This determination is made as follows, for example.
1) When “color printing” is set in the color mode setting field 403: In principle, it is determined that the page is a multi-color page, and the process proceeds to step 1703.
However, if one color (any of CMYK) is set in the “number of colors” of the item list display field 501, the process exits without deciding to execute the monochrome ratio determination process.
2) When “color / monochrome automatic switching” is set in the color mode setting field 403 • If it is determined that the page is a color page in the separately executed color / monochrome determination process, it is determined as a multi-color page, and step 1703 Proceed to
If it is determined that the page is a monochrome page in the color / monochrome determination process, the process exits without determining to execute the monochrome ratio determination process.
3) When “Monochrome printing” is set in the color mode setting field 403 • It is determined that the page is a single-color page, and the process exits without determining to execute the single-color ratio determination process.
In step 1703, execution of the monochrome ratio determination process is determined.

  The above is the content of the necessity determination process for executing the monochrome ratio determination. If execution of the monochrome ratio determination is determined in step 1703 as a result of the execution necessity determination process, control is performed so that the monochrome ratio determination process in step 1506 is executed in the flow of FIG. 15 described above. On the other hand, if this determination process is completed without going through step 1703, the monochrome ratio determination process in step 1506 is skipped in the flow of FIG. 15 described above, and the gradation correction process in step 1507 is executed. Be controlled.

  As described above, according to the present embodiment, since the monochrome ratio determination process is executed only when the effect of the second embodiment can be expected, the process can be performed efficiently.

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

Claims (20)

Means for generating attribute information indicating attributes for each pixel of the input image data to be printed;
Color conversion processing means for color-converting the color space of the input image data into a color space corresponding to a color material color used for printing;
Halftone processing means for performing halftone processing according to the attribute of each pixel based on the attribute information for the image data after color conversion;
With
The halftone processing means, when the attribute of each pixel in the image data after the color conversion is a character or line attribute, in a halftone dot constituting the background region of the character or line object according to the attribute, An image processing apparatus that generates halftone dot image data in which a halftone dot size in a predetermined neighborhood region of the character or line object is smaller than a halftone dot size outside the neighborhood region. When there is a character or line attribute pixel in the vicinity of the pixel of interest in the image data after the color conversion, switching control information for controlling switching of the contents of the screen used in the halftone process is displayed for the pixel of interest. Further comprising means for generating,
The halftone processing means generates the halftone image data by switching the screen content for the pixel of interest in which pixels having character or line attributes exist in the vicinity based on the switching control information. The image processing apparatus according to claim 1. When there is a character or line attribute pixel in the vicinity of the pixel of interest in the image data after color conversion, the attribute of the pixel of interest is another attribute indicating that there is a character or line attribute pixel in the vicinity It further comprises a rewriting means for rewriting to
The halftone processing means generates the halftone image data by applying a screen having a predetermined content for the other attribute to the target pixel rewritten to the other attribute. The image processing apparatus according to claim 1.   The image processing apparatus according to claim 3, wherein the screen having the predetermined content is a screen having a number of lines equal to or more than the number of lines assigned to a graphic or photograph attribute. The background area is composed of pixels of graphic or photo attributes,
The means for generating the switching control information includes a character or line in the vicinity of the pixel of interest in the color-converted image data when the number of screen lines assigned to the graphic and photo attributes is equal to or less than a predetermined number of lines. The image processing apparatus according to claim 2, wherein the switching control information is generated by performing a neighborhood attribute determination process for determining whether or not a pixel having a specific attribute exists. The background area is composed of pixels of graphic or photo attributes,
When the number of screen lines assigned to the graphic and photo attributes is equal to or less than a predetermined number of lines, the rewriting means has a character or line attribute pixel in the vicinity of the pixel of interest in the color-converted image data. The image processing apparatus according to claim 3, wherein neighborhood attribute determination processing is performed to determine whether or not an image exists and the attribute is rewritten.   The neighborhood attribute determination process uses the attribute information as to whether or not a pixel having a character or line attribute exists in a range of M × N pixels centering on the pixel of interest in the image data after the color conversion. The image processing apparatus according to claim 5, wherein the determination is performed. When the switching control information indicates that the content of the screen to be used is switched,
The halftone processing unit generates the halftone image data by switching a screen to be used to a screen having a higher number of lines, or switching a screen type from a line screen to a dot screen. 2. The image processing apparatus according to 2.   The image processing apparatus according to claim 8, wherein the screen having a higher number of lines is a screen having a number of lines equal to or greater than the number of lines assigned to a graphic or photo attribute.   When the attribute of the pixel of interest in the image data after color conversion is a graphic or photo attribute, and there is a character or line attribute pixel in the vicinity thereof, the attribute of the pixel of interest is converted to a character or line The image processing apparatus according to claim 1, further comprising a rewriting unit that rewrites the attribute. The color space corresponding to the color material color used for the printing is CMYK,
A single color ratio of each color of CMYK is derived for each pixel, and for any pixel of the derived CMYK, the switching control information of the pixel switches the contents of the screen to be used for a pixel whose single color ratio is smaller than a predetermined threshold. The image processing apparatus according to claim 2, further comprising means for changing to a switching control information indicating that the contents of a screen to be used are not switched. The color space corresponding to the color material color used for the printing is CMYK,
For each pixel, the monochromatic ratio of each color of CMYK is derived, and for any pixel of the derived CMYK where the monochromatic ratio is smaller than a predetermined threshold, the attribute of the pixel has been rewritten by the rewriting means, the original The image processing apparatus according to claim 3, further comprising means for returning the attribute.   13. The image processing apparatus according to claim 11, wherein the single color ratio is derived only for pages that are printed using color materials of a plurality of colors among pages constituting the input image data.   The image according to any one of claims 1 to 13, further comprising gradation correction means for performing gradation correction on the image data after the color conversion processing based on the attribute information. Processing equipment.   When the pixel of interest in the image data after the color conversion processing is a pixel with a character or line attribute, or when the pixel with a character or line attribute pixel is present in the vicinity, 15. The image processing apparatus according to claim 14, wherein when the pixel is a pixel having a photo attribute, gradation correction of different contents is performed in each case.   16. The image processing apparatus according to claim 1, wherein the character is a character whose character size is smaller than a predetermined size, and the line is a line whose line width is narrower than a predetermined width. .   The image processing apparatus according to claim 1, further comprising setting means for setting the predetermined neighborhood area. The input image data is PDL data,
The means for generating the attribute information generates the attribute information from information described in PDL in the PDL data,
The image data after the color conversion is raster image data obtained by interpreting the PDL data.
The image processing apparatus according to claim 1, wherein the image processing apparatus is an image processing apparatus. Generating attribute information indicating attributes for each pixel of the input image data to be printed;
Color-converting the color space of the input image data into a color space corresponding to a color material color used for printing;
Performing halftone processing according to the attribute of each pixel based on the attribute information for the image data after the color conversion;
Including
In the step of performing the halftone process, when the attribute of each pixel in the image data after color conversion is a character or line attribute, a halftone dot constituting a background area of the character or line object related to the attribute An image processing method, comprising: generating halftone dot image data in which a halftone dot size in a neighborhood area of the character or line object is smaller than a halftone dot size outside the neighborhood area.   A program for causing a computer to function as the image processing apparatus according to any one of claims 1 to 18.