JP2021197709A - Image processing apparatus, image processing method, and program - Google Patents

Abstract

To provide a technique to prevent a deterioration in image quality at an end of a printed image.SOLUTION: In an MFP, an image processing unit (halftone processing unit) has: an image acquisition unit 301 that acquires raster image data indicating an image; an edge extraction unit 302 that extracts an edge of an object in the image; an angle determination unit 305 that determines the direction of the edge; a correction processing unit 1601 that performs sharpening processing for an edge in the raster image data, the edge having been determined to be directed in a predetermined direction; and an image output unit 206 that outputs, to a printing unit, print image data generated based on data obtained as a result of the processing performed by the correction processing unit 1601.SELECTED DRAWING: Figure 16

Description

The present disclosure relates to a technique for suppressing deterioration of image quality of edges in image data.

In printing by an image forming apparatus, a density change may occur in which the density of the edge portion of the image printed on the recording medium becomes darker or lighter. "Sweeping" and "scraping" are known as the phenomenon of concentration change. In an image forming apparatus using an electrophotographic method such as a laser beam printer, "sweep" is generated by excessive adhesion of toner (developer) to the upper end or the lower end of an image. On the other hand, "scraping" occurs when the toner at the lower end is scraped.

Patent Document 1 describes a method of suppressing a density change at an edge of an image by correcting an edge region using a plurality of density sample.

Japanese Unexamined Patent Publication No. 2009-171439

However, even if the correction is performed by the method according to Patent Document 1, the occurrence of "sweep" and "scraping" may cause deterioration of image quality such as deterioration of sharpness at the edge of the image. be.

The image processing apparatus of the present disclosure includes acquisition means for acquiring raster image data indicating an image, extraction means for extracting edges of objects in the image, determination means for determining the orientation of the edges, and raster image data. , A correction means for sharpening an edge determined to be oriented in a predetermined direction, and a print image data generated based on the data obtained as a result of the processing by the correction means are output to the printing means. It is characterized by having an output means.

According to the technique of the present disclosure, deterioration of image quality at the edge of a printed image can be suppressed.

A block diagram showing the hardware configuration of the MFP. The block diagram which shows the functional structure of an image processing part. The block diagram which shows the functional structure of the halftone processing part. A flowchart for explaining the entire processing of the halftone processing. A flowchart for explaining the edge extraction process. The schematic diagram which shows an example of the table for determining an edge judgment value. The schematic diagram which shows an example of the pattern which shows the angle of an edge. Schematic diagram for explaining the angle of the edge. A flowchart for explaining the angle determination process. The schematic diagram which shows an example of the table for conversion from a density value to a correction value. A flowchart for explaining the image composition process. The schematic diagram which shows the correction factor determination table. Schematic diagram showing the correspondence between the edge angle and the correction factor determination table. Schematic diagram of the chart for determining the correction value. The figure for demonstrating the blurring which occurs in an edge. The block diagram which shows the functional structure of the halftone processing part. A flowchart for explaining the entire processing of the halftone processing. A flowchart for explaining the correction process of raster image data. The schematic diagram which shows an example of the image data for explaining a halftone process. A flowchart for explaining the sharpening process. Schematic diagram showing the correspondence between the edge angle and the correction parameter. The figure which shows an example of the image which performed the sharpening process. The schematic diagram which shows the relationship of the brightness profile.

Hereinafter, embodiments for carrying out the technique of the present disclosure will be described with reference to the drawings. It should be noted that the following embodiments do not limit the invention according to the claims, and that all combinations of features described in the following embodiments are indispensable for the means for solving the technique of the present disclosure. Not exclusively.

<First Embodiment>
In the present embodiment, the MFP (MultiFaction Peripheral) 100, which has a print function for printing an image on a recording medium using an electrophotographic process and has a scan function, a transmission function, and the like, will be described as an example of the image forming apparatus. Shall be done. However, the present embodiment is not limited to this, and the present embodiment can be applied to an image forming apparatus that performs another printing method. The MFP 100 also functions as an image processing device.

In the following embodiment, the color corresponding to each color space held by the image data is represented by an alphabetic character such as R, G, B or L, a, b. That is, R indicates a red component in the RGB color space. Similarly, the color material to be recorded on the recording medium is represented by the alphabetic characters C (cyan), M (magenta), Y (yellow), and K (black) for each color. Image data is two-dimensional data of a plurality of planes having planes for each color. For example, the image data in the RGB color space indicates layer structure data of three two-dimensional planes for each of R, G, and B.

[Hardware configuration of image forming device]
FIG. 1 is a block diagram showing a hardware configuration of the MFP 100 according to the present embodiment. The MFP 100 includes a CPU 101, a ROM 102, a RAM 103, a large-capacity storage unit 104, a display unit 105, an operation unit 106, an engine I / F 107, a network interface (I / F) 108, and a scanner I / F 109. Each of these parts is connected to each other via the system bus 110. Further, the MFP 100 further includes a printing unit 111 and a scanner unit 112. The printing unit 111 and the scanner unit 112 are connected to the system bus 110 via the engine I / F 107 and the scanner I / F 109, respectively.

The CPU 101 controls the operation of the entire MFP 100. The CPU 101 executes various processes described later by reading the program stored in the ROM 102 into the RAM 103 and executing the program. The ROM 102 is a read-only memory, and stores a system startup program, a program for controlling the printing unit 111, character data, character code information, and the like. The RAM 103 is a volatile random access memory, which is used as a work area of the CPU 101 and a temporary storage area of various data. For example, the RAM 103 is used as a storage area for storing font data additionally registered by downloading, an image file received from an external device, and the like. The large-capacity storage unit 104 is, for example, an HDD or SSD, and various data are spooled and used as a storage of programs, information files, image data, and the like, or as a work area.

The display unit 105 is composed of, for example, a liquid crystal display (LCD) and is used for displaying the setting state of the MFP 100, the status of processing being executed, the error status, and the like. The operation unit 106 is composed of an input device such as a hard key and a touch panel provided on the display unit 105, and receives an input (instruction) by a user operation. The operation unit 106 is used to change the setting of the MFP 100, reset the setting, and the like, and is used to execute the color adjustment processing mode of the MFP 100 when executing the color adjustment processing.

The engine I / F 107 functions as an interface for controlling the printing unit 111 in response to an instruction from the CPU 101 when printing is executed. Engine control commands and the like are transmitted and received between the CPU 101 and the printing unit 111 via the engine I / F 107. The network I / F 108 functions as an interface for connecting the MFP 100 to the network 113. The network 113 may be, for example, a LAN or a telephone line network (PSTN).

Based on the image data (printed image data) received from the system bus 110 side, the printing unit 111 uses a developer (toner) of a plurality of colors (here, four colors of CMYK) to produce a multicolor image on paper or the like. Formed on a recording medium.

The scanner I / F 109 functions as an interface for controlling the scanner unit 112 in response to an instruction from the CPU 101 when the scanner unit 112 reads a document. Scanner unit control commands and the like are transmitted and received between the CPU 101 and the scanner unit 112 via the scanner I / F 109. The scanner unit 112 reads the image of the original and generates the scanned image data under the control of the CPU 101, and transmits the image data to the RAM 103 or the large-capacity storage unit 104 via the scanner I / F 109.

[Functional configuration of image processing unit]
FIG. 2 is a block diagram showing an internal configuration of an image processing unit, which is a functional unit responsible for image processing in the MFP 100. Each process in the image processing unit 200 described below is realized by the CPU 101 expanding the control program held in the ROM 102 to the RAM 103 and executing the process. Alternatively, some or all of the functions of the image processing unit 200 may be realized by hardware such as an ASIC, FPGA, or an electronic circuit. Alternatively, the image processing unit 200 may be configured as part of the function of the device independent of the MFP 100, and the MFP 100 may acquire the data obtained as a result of processing by the image processing unit of the device.

The image processing unit 200 according to the present embodiment includes an image input unit 201, a control command generation unit 202, a color conversion processing unit 203, a Raster Image Processor (hereinafter RIP) unit 204, a halftone processing unit 205, and an image output unit 206. ..

The image input unit 201 acquires data of an image to be printed. The input image data is, for example, image data input from the host PC 115 via the network 113 and the network I / F 108. Alternatively, the image input unit 201 may acquire the image data stored in the large-capacity storage unit 104. The input image data will be described as being three-layer data in which each RGB signal corresponding to the sRGB color space is expressed by 8-bit 256 gradations. In addition, sRGB in this embodiment refers to the standard of RGB color space defined by IEC (International Electrotechnical Commission). The image data input to the image input unit 201 is sent to the control command generation unit 202.

The control command generation unit 202 controls the color conversion processing unit 203 and generates control commands for the RIP unit 204 based on the input image data. The generated control command is sent to the RIP unit 204.

The color conversion processing unit 203 further performs color conversion processing for converting RGB values of input image data into color values (pixel values) in the CMYK color space by using a color conversion table stored in advance. The color conversion table is stored in the RAM 103 or the large-capacity storage unit 104. For example, the color conversion processing unit 203 converts the color value of the input image data from the RGB color space independent of the printing unit 111 to the color value of the devRGB color space. Further, the color conversion processing unit 203 converts the devRGB color space ^{into the color value in the L *} a ^* b ^{* color space, and converts the color from the L *} a ^* b ^* color space into the color value in the CMYK color space. By performing the color conversion process in this way, the color value of the input image data is converted into the color value of the CMYK color space. The L ^* a ^* b ^* color space refers to a three-dimensional visual uniform color space defined by the CIE (International Commission on Illumination) that does not depend on the printing unit 111 in consideration of human visual characteristics. By passing through a color space that does not depend on the printing unit 111, it is possible to reproduce colors that humans recognize as the same color. The method of color conversion processing from the RGB color space to the CMYK color space, which does not depend on the printing unit 111, is not limited to the above method. For example, the color conversion processing unit 203 may convert the color value of the RGB color space into the color value of the CMYK color space by using the color conversion table in which the three color conversion tables are combined.

The RIP unit 204 generates raster image data in the CMYK color space using the control commands generated by the control command generation unit 202. In the present embodiment, the raster image data in the CMYK color space is four-plane two-dimensional data indicating the recording amount for each color of the coloring material to be recorded on the recording medium. In this embodiment, it is assumed that the data is 8-bit data for each color.

The halftone processing unit 205 performs halftone processing on the raster image generated by the RIP unit 204. Even if the input image data is data expressed in 256 gradations for each color, the number of gradations that the printing unit can handle is a low gradation number less than 256 gradations such as 2, 4, and 16 gradations. In many cases. The halftone processing unit 205 performs halftone processing so that stable halftone expression can be output even with a number of gradations smaller than 256 gradations. Through these processes, print image data, which is data that can be expressed by the print unit 111, is generated. Hereinafter, the number of gradations that can be expressed by the printing unit 111 of the present embodiment will be described as 16.

When the image output unit 206 receives the print image data from the halftone processing unit 205, the image output unit 206 transmits the print image data to the print unit 111 via the engine I / F 107. The CPU 101 instructs the printing unit 111 to form an image based on the printed image data. The printing unit 111 prints a color image according to the input image data on the recording medium by executing each process of exposure, development, transfer, and fixing.

[Functional configuration of halftone processing unit]
FIG. 3 is a functional block diagram showing a functional configuration of the halftone processing unit 205. The halftone processing unit 205 includes an image acquisition unit 301, an edge extraction unit 302, a quantization processing unit 303, a correction amount determination unit 304, an angle determination unit 305, and an image composition unit 306.

The image acquisition unit 301 acquires raster image data expressed in the CMYK color space from the RIP unit 204. When the image acquisition unit 301 acquires raster image data in the CMYK color space, it outputs the raster image data to each processing unit of the edge extraction unit 302, the quantization processing unit 303, and the correction amount determination unit 304.

The edge extraction unit 302 acquires raster image data from the image acquisition unit 301. The edge extraction unit 302 extracts the end portion (edge) of each object in the image based on the raster image data, and holds a value indicating whether the pixel is a pixel constituting the edge for each pixel. Generate edge image data. The edge image data is output to the angle determination unit 305. The details of the edge extraction process will be described later.

The angle determination unit 305 acquires the edge image data, and uses the pixels constituting the edge and the reference pixels which are pixels around the edges in the edge image data, and the angle of the edge composed of the pixels (edge orientation). ) Is determined. The method of determining the edge angle will be described later.

The quantization processing unit 303 generates screen data (halftone image data) by performing quantization by a systematic dither method based on raster image data. The halftone image data is 4-bit (16 gradations) image data that can be printed by the printing unit 111. The quantization process is performed for each CMYK, and halftone image data for each CMYK is generated. Since the systematic dither method is a well-known technique, the description thereof will be omitted. In this embodiment, it is described as being quantized by the systematic dither method, but various other methods such as a concentration pattern method and an error diffusion method can be applied.

The correction amount determination unit 304 generates edge correction data for correcting the rattling of the edge.
The generated edge correction data is sent to the image compositing unit 306. The edge correction data is data for correcting the halftone image data so that the edges of the image formed by the printing unit 111 are bordered by dots. The detailed processing of the correction amount determination unit 304 will be described later.

The image composition unit 306 acquires edge angle data, halftone image data, and edge correction data. The image composition unit 306 corrects the edge correction data according to the angle of the edge based on the edge angle data. Further, the corrected edge correction data and the halftone image data are combined to generate print image data to be output by the print unit 111. The detailed processing of the image composition unit 306 will be described later.

[About rattling]
The “rattling” described in the present embodiment is a stepped jaggedness generated by area gradation processing of a raster image having a halftone density in the quantization processing unit 303. In addition to the number of lines on the screen used in the systematic dither method and the resolution that can be expressed by the printing unit 111, the reproducibility of dots at the edges according to the transport direction causes a difference in rattling.

As a method of correcting rattling, it is conceivable to generate edge correction data in which dots are formed so as to border the edges, and combine the edge correction data and screen data to generate print image data. Be done. However, for example, when the edge facing the downstream side in the transport direction of the recording medium on which the image is printed has low dot reproducibility by the printing unit, the rattling cannot be completely corrected by the method of uniformly correcting the edge. There is.

Therefore, in the present embodiment, a method of improving the accuracy of rattling correction and suppressing deterioration of the printed image by correcting the edge correction data generated by the correction amount determination unit 304 according to the direction of the edge will be described. do.

[Processing flow for halftone processing]
FIG. 4 is a flowchart showing the overall processing flow in the halftone processing unit 205. The entire processing flow of the halftone processing according to the present embodiment will be described with reference to FIG. The series of processes shown in the flowchart of FIG. 4 is performed by the CPU 101 of the MFP 100 expanding the program code stored in the ROM 102 into the RAM 103 and executing the program code. Further, some or all the functions of the steps in FIG. 4 may be realized by hardware such as an ASIC or an electronic circuit. The symbol "S" in the description of each process means that the step is a step in the flowchart, and the same applies to the subsequent flowcharts.

In S401, the image acquisition unit 301 acquires the raster image data of the CMYK color space output from the RIP unit 204. The image acquisition unit 301 outputs the acquired raster image data to the edge extraction unit 302, the quantization processing unit 303, and the correction amount determination unit 304.

In S402, an edge extraction process for generating edge image data from raster image data is performed.

FIG. 5 is a flowchart for explaining the details of the edge extraction process. The details of the processing of S402 will be described with reference to FIG. In the edge extraction process, edge image data having a 1-bit value indicating whether or not each pixel is a pixel constituting an edge is created. The edge extraction process is performed for each color material data based on the raster image data of the CMYK color space, and is generated in the edge image data for each color of CMYK. In the following flowchart, the process of generating the edge image data of the color material from the raster image of any of the color materials of CMYK will be described.

In S501, the edge extraction unit 302 selects a pixel of interest from the pixels of the raster image data.

In S502, the edge extraction unit 302 is the largest signal among the signal values which are the pixel values of the reference region indicated by a total of 9 pixels (3 × 3 pixels) having a width of 3 pixels and a height of 3 pixels centered on the pixel of interest. Determine the value as the maximum value [MAX]. The reference area in the edge extraction process of the present embodiment will be described as an area of 3 × 3 pixels centered on the pixel of interest, but the reference area is not limited to 3 × 3 pixels and has 5 × 5 pixels or more. It may be an area. The pixels that make up the reference area are called reference pixels.

In S503, the edge extraction unit 302 determines the smallest signal value among the signal values in the reference region as the minimum value [MIN].

In S504, the edge extraction unit 302 subtracts the minimum value [MIN] determined in S503 from the maximum value [MAX] determined in S502 to calculate the contrast value [CONT]. As a result, the step amount of the signal value in the reference area is calculated.

In S505, the edge extraction unit 302 determines the edge determination value [Sub] using a one-dimensional look-up table (hereinafter referred to as LUT) in which the minimum value [MIN] determined in S503 is input.

FIG. 6 is an example of a one-dimensional LUT used in this step. The edge determination value [Sub] is a threshold value for determining whether the pixel of interest is a pixel constituting the edge of the object.

In S506, the edge extraction unit 302 compares the contrast value [CONT] determined in S504 with the edge determination value [Sub] determined in S505, and determines whether the contrast value [CONT] is larger. As a result of the determination, when the contrast value [CONT] is larger than the edge determination value [Sub] (YES in S506), the process proceeds to S507.

Since the contrast value [CONT] is larger than the threshold value and the contrast value [CONT] is sufficiently large in the region where the signal value changes sharply, it is considered that the pixel of interest is a pixel constituting the edge. Be done. Therefore, in S507, the edge extraction unit 302 sets the edge determination signal of the pixel of interest to "1 (ON)" in order to indicate that the pixel of interest selected in S501 is the target of edge correction.

On the other hand, when the contrast value [CONT] is not larger than the edge determination value [Sub] (S506 is NO), the process proceeds to S508. If the contrast value [CONT] is not larger than the edge determination value [Sub], it is considered that the edge correction process is not necessary. Therefore, in S508, the edge extraction unit 302 sets the edge determination signal of the pixel of interest to “0 ( OFF) ”.

In S509, the edge extraction unit 302 determines whether or not the edge extraction process has been performed on all the pixels of the raster image data. If the processing is not completed for all the pixels, the process returns to S501, the pixel for which the edge extraction processing has not been processed is selected as the pixel of interest, and the processing of S502 to S508 is repeated. When the edge extraction process is completed for all the pixels, edge image data in which the value of the edge determination signal is held for each pixel is generated from the raster image data in the CMYK color space. The generated edge image data is output to the angle determination unit 305. Then, the edge extraction process ends.

Returning to FIG. 4, the description of the entire processing flow of the halftone processing will be continued. In S403, the angle determination unit 305 performs an edge angle determination process for generating edge angle data indicating the edge angle. In the edge angle determination process, pattern matching is performed on the edge image data by performing a convolution operation with a pattern of 5 × 5 pixels and calculating the degree of matching with the pattern. A reference area is defined as 5 × 5 pixels in the edge image data referred to when calculating the degree of coincidence with the pattern. Further, the pixel located in the center of the reference region is defined as the pixel of interest, and the pixels other than the pixel of interest are defined as peripheral pixels.

FIG. 7 shows an example of a pattern used in the edge angle determination process. The pattern has a size of 5 × 5 pixels, and the pixel values are three values of 0, 1, and 2. 0 represents a background pixel, 1 represents an object pixel, and 2 represents either a background or an object pixel. In this embodiment, eight types of patterns 700 to 707 are used. Independent ID values are set for each pattern. As shown in FIG. 7, the pattern ID values 0 to 7 are described as corresponding to the patterns 700 to 707, respectively.

FIG. 8 is a schematic diagram for explaining the angle of the edge. As shown in FIG. 8, in the present embodiment, the axis orthogonal to the transport direction, which is the direction in which the recording medium such as paper when the printing unit 111 prints an image is conveyed, is 0 °, and the axis is clockwise from 0 °. The angle of rotation is defined as the angle of the edge.

Each pattern represents an edge having a different angle, and in the edge angle determination process, the angle of the edge (direction of the edge) indicated by the pattern having a high degree of matching is determined as the angle of the edge of the pixel of interest (direction of the edge). For example, patterns 700 to 702 are patterns representing an edge (upper end portion) whose edge faces the upstream side in the transport direction of the recording medium. Of these, the pattern 700 is a pattern in which the edge angle represents 0 °. Further, for example, the patterns 705 to 707 are patterns of edges (lower end portions) whose edges face the downstream side in the transport direction of the recording medium, which is the opposite side to the upstream side. That is, it is a pattern representing an edge facing in the same direction as the transport direction. Of these, for example, the pattern 706 is a pattern having an edge angle of 180 °. Further, the patterns 703 and 704 are patterns representing edges facing in a direction orthogonal to the transport direction of the recording medium.

FIG. 9 is a flowchart for explaining the details of the edge angle determination process. The details of the processing of S403 will be described with reference to FIG. The edge angle determination process is performed for each color material data based on the raster image data of the CMYK color space, and the edge angle data for each color of CMYK is generated. In the following flowchart, a process of generating edge image data of a color from a raster image of any color of CMYK will be described.

In S901, the angle determination unit 305 acquires patterns 700 to 707 used for pattern matching.

In S902, the angle determination unit 305 selects a pixel of interest from the pixels of the edge image data.

In S903, the angle determination unit 305 determines whether the edge determination signal of the pixel of interest is 1 indicating ON.

When the edge determination signal of the pixel of interest is 1 (YES in S903), the pixel of interest is a pixel constituting the edge. Therefore, in S904 to S907, a process for determining the angle of the edge is performed. First, in S904, the angle determination unit 305 selects one pattern of interest from the patterns 700 to 707 to perform pattern matching processing.

In S905, the angle determination unit 305 performs pattern matching processing for calculating the degree of matching between the reference region composed of the pixel of interest and the peripheral pixels and the pattern of interest selected in S904. In pattern matching, the value of 25 pixels in the reference region including the pixel of interest in the edge image data is compared with the value of 25 pixels of the pattern of interest. Then, it is determined whether or not the values of the pixels in the pattern of interest and the corresponding pixels in the reference area match. At this time, when the value of the pixel of the attention pattern is 2, it is determined that the value of the corresponding pixel in the reference area matches any value. In this way, in this step, the degree of coincidence between the reference area of the pixel of interest and the pattern of interest is calculated.

In S906, the angle determination unit 305 determines whether or not the calculation of the degree of coincidence with all the patterns 700 to 707 acquired in S901 is completed. If it is not completed, the process returns to S904, the pattern for which the matching degree has not been calculated is selected as the pattern of interest, and the process of S905 is repeated. When the pattern matching process with all the patterns is completed, the process proceeds to S907.

In S907, the angle determination unit 305 selects the pattern having the highest degree of matching from the patterns 700 to 707. Then, the ID value of the pattern having the highest degree of matching is set to a value indicating the edge angle of the pixel of interest, and the edge angle data is generated. For example, when it is determined that the reference region of the pixel of interest has a high degree of coincidence with the pattern 706, the pixel of interest is a pixel constituting an edge having an angle of 180 ° on the downstream side in the transport direction. Is set.

On the other hand, when the edge determination signal of the pixel of interest is 0 (NO in S903), the pixel of interest is not an edge pixel but a pixel not subject to edge correction. Therefore, in S908, the angle determination unit 305 sets “-1” indicating that the pixel of interest is a non-processed pixel that is not subject to edge correction.

In S909, the angle determination unit 305 determines whether or not the edge angle determination process has been performed on all the pixels. If the processing is not completed for all the pixels, the process returns to S902, the unprocessed pixel is selected as the pixel of interest, and the processing of S902 to S908 is repeated. When the processing is completed for all the pixels, the edge angle data is generated. The edge angle data in the present embodiment is generated as two-dimensional data in which any value of 0 to 7, which is the ID value of the pattern, and -1, which indicates non-edge, is held for each pixel. The generated edge angle data is output to the image composition unit 306. Then, the edge extraction process ends.

In addition, although it was explained that edge angle data is generated based on the result of calculating the degree of coincidence with the pattern for each pixel, correction processing is performed so that the angle difference with the periphery is reduced after determining the angle for each pixel. It is preferable to do so. Further, an example of calculating the angle is shown on the premise that the upper end of the image data is always adjusted to be the upper end in the transport direction. However, the effect of this embodiment is not limited to the above example, and more preferably, the angle is determined in consideration of the paper orientation specified by the user and the imposition on the inside of the page.

Returning to FIG. 4, the description of the entire processing flow of the halftone processing will be continued. In S404, the quantization processing unit 303 generates halftone image data from the raster image data in the CMYK color space by multi-value dither processing. The quantization processing unit 303 compares the dither matrix stored in advance with the raster image data, and generates halftone image data. That is, when the number of gradations that can be expressed by the printing unit 111 is 16 gradations as in the present embodiment, for example, a 4-bit halftone image that can express 16 gradations is generated. In this case, four screen data having the same number of bits are independently held for each color material. The generated halftone image data is sent to the image composition unit 306.

In S405, the correction amount determination unit 304 generates edge correction data from the raster image data in the CMYK color space. The correction amount determination unit 304 selects the pixel of interest from the raster image and determines the correction value of the pixel of interest. By performing this process on all pixels, edge correction data holding the correction value for each pixel is generated. Since the correction value corrects the rattling of the edge, it is used to correct the pixel value of the halftone image data of the pixel determined to be the edge in the image composition processing described later.

FIG. 10 is a graph showing the one-dimensional LUT1000 used for determining the correction value. As shown in FIG. 10, the correction value of the pixel of interest is determined by inputting a signal value for each CMYK indicating the density value of the pixel of interest in the raster image. The LUT 1000 is designed so that the correction value is determined by a value of 0 to 15 according to the number of gradations that can be expressed by the printing unit 111. That is, the edge correction data is the same 4-bit image data as the halftone image data. The LUT 1000 is held independently for each coloring material. Then, the generation of the edge correction data is performed independently for each color material as in the edge extraction process and the quantization process, and the edge correction data for each color material is generated. The generated edge correction data is output to the image composition unit 306.

In S406, an image composition process for generating print image data output to the print unit 111 is performed based on the edge angle data, edge correction data, and halftone image data.

[Image composition processing]
FIG. 11 is a flowchart for explaining the details of the image composition process. The details of the processing of S406 will be described with reference to FIG. The image composition process is performed for each color material, and print image data for each CMYK color is generated. In the following flowchart, a process of generating print image data of any of the CMYK color materials will be described.

In S1101, the image composition unit 306 selects the pixel of interest. Then, in S1102 to S1106, a process of determining a signal value representing a gradation value of 0 to 15 of the pixel of interest is performed. The determined signal value is used as a signal value (pixel value) of the pixel of interest in the printed image data, and the printed image data is generated.

In S1102, the image synthesizing unit 306 determines whether the pixel of interest is a pixel constituting the edge based on the edge angle data. When the value of the pixel corresponding to the pixel of interest in the edge angle data is other than "-1", the image composition unit 306 determines that the pixel of interest is a pixel constituting the edge. On the other hand, when the value in the pixel corresponding to the pixel of interest in the edge angle data is "-1", it is determined that the pixel of interest is not a pixel constituting the edge.

When the pixel of interest is not an edge (NO in S1102), it is not necessary to correct the rattling. Therefore, the process proceeds to S1106, and the image synthesizing unit 306 sets the signal value of the halftone image data generated as a result of the quantization process to the signal value of the pixel of interest as it is.

When the pixel of interest is an edge (YES in S1102), the signal value of the pixel of interest is determined in consideration of the correction of rattling. Therefore, first, the process proceeds to S1103, and the image synthesizing unit 306 corrects the correction value of the pixel of interest in the edge correction data according to the angle of the edge formed by the pixel of interest.

In the present embodiment, the correction factor is determined according to the minimum value [MIN] of the pixel of interest determined in S503 of the edge extraction process, and the determined correction factor is multiplied by the correction value determined in the correction data generation process of S405. By doing so, the corrected correction value is calculated. The corrected value after this correction is also referred to as a corrected value. The conversion from the minimum value [MIN] to the correction factor is based on a one-dimensional LUT.

FIG. 12 is a schematic diagram showing a graph of a one-dimensional LUT used for inputting a minimum value [MIN] to determine a correction factor. As shown in FIG. 12, the present embodiment includes a plurality of one-dimensional LUTs (hereinafter, referred to as “correction factor determination table”) for determining the correction factor. ID values are set in each of the plurality of correction factor determination tables.

In the present embodiment, it is assumed that 0 (LUT_ID = 0) is set as the ID value in the correction factor determination table of FIG. 12 (a). Further, 1 (LUT_ID = 1) is set as the ID value in the correction factor determination table of FIG. 12 (b), and 2 (LUT_ID = 2) is set in the correction factor determination table of FIG. 12 (c). And. In the present embodiment, a plurality of correction factor determination tables are selected and used properly according to the angle of the edge formed by the pixel of interest. In consideration of the reproducibility of dots according to the transport direction of the recording medium, the correction factor determination tables suitable for the edge angles are designed, and are stored in advance before the halftone processing is performed.

The correction factor determination table is designed so that the larger the ID value, the larger the correction factor is determined. The correction factor determination table with LUT_ID = 1 is designed so that the correction factor is determined to be larger than the correction factor determination table with LUT_ID = 0. Further, the correction factor determination table with LUT_ID = 2 is designed so that the correction factor is determined to be larger than the correction factor determination table with LUT_ID = 1.

The corrected correction value (correction value) is determined by multiplying the correction value by the correction factor. When the signal value of the halftone image data is corrected, the correction value determined in this step is replaced with the signal value of the halftone image data. When the correction factor determination table having a large ID value is selected, the correction factor is largely determined, and as a result, the signal value can be corrected so that the dots formed by the printing unit 111 are expressed darkly.

FIG. 13 is a diagram showing a table that holds the ID value of the correction factor determination table corresponding to the angle of the edge. The image composition unit 306 uses the table of FIG. 13 to select a correction factor determination table according to the angle of the edge of the pixel of interest.

In the present embodiment, as in the case of the influence of the scraping phenomenon, the edge facing the downstream side in the transport direction of the recording medium causes more rattling than the edge facing the other side. explain. As shown in FIG. 13, the ID values 1 or 2 of the correction factor determination table correspond to the patterns 705, 706, and 707 (pattern ID values 5, 6, and 7) indicating that the edges are facing the downstream side. It is attached. In this way, when the rattling of the lower end portion is large, the correction factor is determined so that the correction factor of the edge of the lower end portion is larger than that of the upper end portion. Therefore, at the lower end portion, the correction value is determined so that dots darker than the edges other than the lower end portion are printed. As described above, in the present embodiment, even if the size of the rattling differs depending on the orientation of the edge, the rattling can be corrected according to the orientation of the edge.

When the dot density of the edge becomes high due to the correction, even if the edge faces the downstream side in the transport direction, the edge rattles at the edge diagonal to the transport direction such as patterns 705 and 707. May stand out. This is because, as shown in the patterns 705 and 707, the diagonal edges themselves are not straight lines. Therefore, in the present embodiment, the correction factor determination table selected by the angle of the edge is changed even for the edge facing the downstream side in the transport direction. As shown in FIG. 13, when the edge is slanted (when the pattern ID is 5 or 7), the correction factor is smaller than that of the edge which is almost parallel to the transport direction (when the pattern ID is 6). A correction factor determination table is associated with the determination. As described above, in the present embodiment, it is possible to correct the rattling suitable for the direction of the edge.

In S1104, the image synthesizing unit 306 compares the correction value (correction value after correction) determined in S1103 with the signal value of the pixel of interest in the halftone image data, and determines whether the correction value is larger.

When the correction value is larger (YES in S1104), the image synthesizing unit 306 replaces the signal value in the pixel of interest of the printed image data with the correction value obtained as a result of S1103 in S1105. In this way, the print image data is generated by replacing the edge signal value in the halftone image data with the correction value. Therefore, the halftone image data is corrected so that dots bordering the edges are added, and rattling is suppressed.

On the other hand, when the signal value of the halftone image data is larger (S1104 is NO), the process proceeds to S1106. Then, the image synthesizing unit 306 sets the signal value of the halftone image data generated as a result of the quantization process as it is as the signal value of the pixel of interest in the printed image data.

When the pixel of interest in the halftone image data is a pixel constituting an edge, the signal value of the halftone image data should be uniformly replaced with a correction value regardless of the signal value of the halftone image data. Corrections may be made.

In the present embodiment, when the signal value of the halftone image data is larger than the correction value, the signal value of the halftone image data is not replaced with the correction value as in the process of S1105. In this method, rattling is corrected by utilizing the arrangement of dots in the halftone image data generated as a result of the quantization process. Therefore, it is possible to correct the rattling while suppressing the appearance of the printed image from being spoiled.

In S1107, the image synthesizing unit 306 determines whether or not the image synthesizing process has been performed on all the pixels. If the processing is not completed for all the pixels, the process returns to S1101, the unprocessed pixel is selected as the pixel of interest, and the processing of S1102 to S1106 is repeated. When the processing is completed for all the pixels, print image data is generated.

As a comparative example, considering an example in which the correction value is determined by one correction factor determination table in S1103, the signal value of the edge in the printed image data is determined without considering the reproducibility of dots due to the difference in the direction of the edge. It ends up. Therefore, in the method of the comparative example, the correction amount may be insufficient on the lower end side in the transport direction where the density of the printed dots decreases due to a cause typified by the scraping phenomenon.

On the other hand, according to the present embodiment, the correction value can be determined according to the direction of the edge. Therefore, even if the reproducibility of the edge dots differs depending on the transport direction, it is possible to correct the rattling suitable for the edge orientation. Therefore, according to the present embodiment, deterioration of image quality at the edge of the printed image can be suppressed.

[Association between correction factor determination table and edge orientation]
In the above description, when the rattling of the lower end of the object occurs larger than that of the other edges, the dots for correcting the rattling of the lower end are printed darker than the other end. Explained how to correct the correction value. However, depending on the rotation direction of the exposure drum constituting the printing unit 111 and the developing sleeve, rattling of edges other than the lower end portion such as the upper end portion of the object may be conspicuous. Therefore, the edge to which the correction value is corrected so that the dot is expressed darkly is not limited to the lower end portion. The correction factor determination table according to the direction of the edge may be selected so that the quality of the edge is confirmed in advance and the rattling is uniformly corrected for the edges in all directions. For example, if the upper end portion is more prone to rattling, the table of FIG. 13 may be designed so that the LUT_ID is selected as 1 or 2 when the pattern ID is 0 to 2.

[Number of gradations that can be expressed by the print unit]
In the above description, since the number of gradations that can be expressed by the printing unit 111 is three or more gradations, an example of correcting a correction value of three or more values according to the direction of the edge has been described. In addition, the method of this embodiment can be applied even if the number of gradations that can be expressed by the printing unit 111 is two gradations. For example, when the number of gradations that can be expressed by the printing unit 111 is binary, the number of dots to be added other than the halftone image data is defined by the correction value. Then, a method of correcting the correction value so that the number of dots to be added other than the halftone image data is increased or decreased according to the direction of the edge so that the density is expressed deeply in the direction of the edge where the rattling becomes large. But it may be. With this method as well, it is possible to correct the rattling according to the direction of the edge.

[Correction using contrast value]
The correction value of the edge correction data may be corrected in consideration of, for example, the contrast value [CONT]. For example, when it is desired to correct the correction value so that the correction value is increased, the correction value may be increased by controlling the contrast value [CONT] to be large. Alternatively, at the edge where the reproducibility of the dots is low, the rattling may be corrected by controlling the signal value so that the contrast value [CONT] becomes larger than a predetermined value.

[Calibration of the correspondence between the edge orientation and the correction factor determination table]
In the above description, a method of correcting rattling suitable for the edge orientation has been described by selecting a correction factor determination table according to the edge orientation. The reproducibility of dots with halftone density at the edges also varies depending on the type of printing unit or recording medium. Therefore, it may have a function of correcting the correction value in consideration of the type of the printing unit or the recording medium.

FIG. 14 is an example of a chart for confirming the state of edges facing in different directions. For example, the correction value is corrected using any one of the three correction factor determination tables shown in FIGS. 12 (a) to 12 (c), and the chart of FIG. 14 is printed. This is done for all three correction factor determination tables shown in FIGS. 12 (a) to 12 (c). The user visually confirms each of the three printed matter on which the chart of FIG. 14 is printed, identifies the correction factor determination table in which the rattling can be corrected, according to the direction of the edge, and shows it in FIG. The correspondence between the edge orientation and the correction factor determination table may be modified. By performing the above processing, it is possible to perform processing so that the correction value is corrected according to the type of the printing unit 111 and the recording medium. In addition, instead of the method in which the user visually confirms the printed chart of FIG. 14, the rattling is quantified from the image data obtained by reading the printed matter on which the chart is printed by the scanner unit 112, and is appropriate. It may be a method of correcting to a correct correction value.

<Second embodiment>
In the first embodiment, a method for correcting "rattling", which is a deterioration in the quality of the halftone density at the edge, has been described. In this embodiment, a method of suppressing "blurring", which is a deterioration of the quality of the halftone density at the edge, and improving sharpness will be described. This embodiment will be described focusing on the difference from the first embodiment. The parts not specified in particular have the same configuration and processing as those in the first embodiment.

FIG. 15 is a diagram for explaining the blurring of the halftone density at the edge of the image. FIG. 15A is a diagram showing an image printed with ideal edges. In FIG. 15A, the upper end portion, which is the upstream edge in the transport direction, and the lower end portion, which is the downstream edge in the transport direction, are sharp and no blurring is observed.

FIG. 15B is a diagram showing a printed image in which the edges are blurred. The lower end portion B in FIG. 15B is blurred as compared with the region A other than the lower end portion, and the sharpness is lowered. Due to the characteristics of the printing unit 111, "scraping" in which toner is scraped off occurs at the lower end portion, or "sweep" caused by applying toner to a portion that should be the background color occurs at the lower end portion. In that case, blurring may occur at the lower end.

In the present embodiment, a method of suppressing blurring that occurs at an edge having such a specific orientation will be described. Although the method of suppressing the blurring of the lower end portion is described in the present embodiment, the present embodiment can be applied to the blurring of the edge other than the lower end portion. For example, the method of the present embodiment can be applied to the blur generated at the upper end portion.

[Structure of halftone processing unit]
FIG. 16 is a block diagram showing a functional configuration of the halftone processing unit 205 of the present embodiment. The same components as those in the first embodiment are designated by the same reference numerals and the description thereof will be omitted. The halftone processing unit 205 of the present embodiment includes an image acquisition unit 301, an edge extraction unit 302, an angle determination unit 305, a correction processing unit 1601, and a quantization processing unit 303.

The correction processing unit 1601 acquires the raster image data acquired by the image acquisition unit 301 and the edge angle data generated by the angle determination unit 305. The correction processing unit 1601 performs correction processing for suppressing the occurrence of edge blur on the raster image data based on the edge angle data. The detailed processing of the correction processing unit 1601 will be described later.

[Processing flow for halftone processing]
FIG. 17 is a flowchart showing a processing flow in the halftone processing unit 205. The entire processing flow of the halftone processing according to the present embodiment will be described with reference to FIG.

In S1701, the image acquisition unit 301 acquires the raster image data of the CMYK color space output from the RIP unit 204. The image acquisition unit 301 outputs the acquired raster image data to the edge extraction unit 302 and the correction processing unit 1601.

In S1702, an edge extraction process for generating edge image data from raster image data is performed. Since the details of the processing are the same as those of S402, the description thereof will be omitted.

In S1703, the angle determination unit 305 performs an edge angle determination process for generating edge angle data indicating the angle (direction of the edge) of each edge. Since the details of the processing are the same as those of S403, the description thereof will be omitted.

In S1704, the correction processing unit 1601 performs correction processing of the raster image data.

FIG. 18 is a flowchart for explaining the details of the correction process of the raster image data. The details of the processing of S1704 will be described with reference to FIG. The correction processing of the raster image data is performed for each color material data of the raster image data in the CMYK color space. In the following flowchart, processing for a raster image of any color of CMYK will be described.

In S1801, the correction processing unit 1601 performs smoothing processing on the raster image data. The correction processing unit 1601 cuts out the raster image data into a window of a predetermined size (for example, a size of 3 horizontal pixels and 7 vertical pixels) composed of a plurality of pixels, and smoothes the pixels in the window of the predetermined size. Perform the conversion. Smoothing is performed, for example, by obtaining the average value of the signal values in the window and replacing the average value with the signal value of the pixel of interest. The entire raster image is smoothed by performing a process of selecting a pixel of interest from the pixels in the raster image and performing smoothing until all the pixels are selected as the pixel of interest.

The method of determining the signal value of the pixel of interest in the smoothing process is not limited to the method of using the average value of the signal values in the window. Alternatively, for example, it may be determined by using a smoothing filter in which a weight is placed at the center of the line by using a filtering process.

FIG. 19 is a diagram for explaining a correction process of raster image data. FIG. 19A is a diagram showing an example of a raster image before the smoothing process is performed. FIG. 19B is a diagram showing a raster image obtained as a result of performing the smoothing process of this step on the raster image of FIG. 19A. As a result of the smoothing process, in FIG. 19B, the edges of the object are blurred and the density of the halftone is low.

In S1802, the correction processing unit 1601 calculates the difference between the signal value of the pixel of the raster image after the smoothing process and the signal value of the corresponding pixel of the raster image before the smoothing process to generate a difference image. FIG. 19C is a diagram showing an example of a difference image obtained as a result of this step. As shown in FIG. 19 (c), the processing of this step extracts the edges of the raster image before the processing shown in FIG. 19 (a). The difference image information is added to the raster image data based on the angle data and the correction parameters described later.

In S1803, the correction processing unit 1601 performs sharpening processing on the edges in a predetermined direction based on the difference image and the edge angle data.

FIG. 20 is a flowchart for explaining the details of the sharpening process. The details of the processing of S1803 will be described with reference to FIG.

In S2001, the correction processing unit 1601 selects a pixel of interest from the pixels of the raster image data. In S2002, the correction processing unit 1601 acquires a value representing the edge angle of the pixel of interest from the edge angle data. FIG. 19D is a diagram showing edge angle data corresponding to pixels of raster image data. As described in the first embodiment, in the edge angle data, any one of 0 to 7, which is an ID value of patterns 700 to 707, and -1, which indicates a non-edge, is held for each pixel. It is dimensional data.

In S2003, the correction processing unit 1601 acquires correction parameters according to the angle of the edge of the pixel of interest.

FIG. 21 is a diagram for explaining a table used for acquiring correction parameters. The correction parameter is a value between 0 and 1 including 0 and 1, and is used for calculating the correction value of the pixel of interest described later. The angle of the edge is the angle of the edge composed of the pixels of interest, and the ID value of the pattern of FIG. 7 is held as a value indicating the angle of the edge. Therefore, the correction processing unit 1601 can acquire the correction parameter of the pixel of interest from the value of the edge angle data of the pixel of interest using the table of FIG. 21.

The association between the edge angle and the correction parameter is predetermined based on the characteristics of the printing unit 111. Then, the table of FIG. 21 generated based on the correspondence between the edge angle and the correction parameter is stored in the MFP 100. When the pixel of interest is not a pixel constituting the edge, that is, when the value of the pixel of interest in the edge angle data is -1, the correction parameter is determined to be 0, for example.

In S2004, the correction processing unit 1601 multiplies the pixel value of the difference image at the same position as the pixel of interest by the correction parameter of the pixel of interest acquired in S2003 to obtain a correction value for correcting the signal value of the raster image. calculate.

In S2005, the correction processing unit 1601 adds the correction value of the attention pixel calculated in S2004 to the signal value of the attention pixel of the raster image data, and the value obtained as a result of the addition is the signal value of the attention pixel of the raster image data. Replace with. When the value obtained as a result of adding the correction values exceeds the maximum value of the signal value of the raster image data, the signal value of the pixel of interest is replaced with the maximum value. That is, when the signal value of the raster image is 8 bits, if the result of adding the correction value to the signal value of the pixel of interest in the raster image data is, for example, 300, it is clipped to 255 and the signal of the pixel of interest in the raster image data. Replace the value with 255.

As shown in FIG. 21, the correction parameter values include 0, which is a parameter whose correction value is 0, 1 which is used for general unsharp mask processing, and 0, which is a value larger than 0 and less than 1. 7 etc. can be considered. Although the sharpening process in the correction processing unit 1601 will be described as a process using an unsharp mask, a bilateral filter may be used.

As shown in FIG. 21, the correction parameter of the pixel of interest is configured by the pixel of interest and is determined according to the direction (angle) of the edge. For example, when the edge composed of the pixel of interest is determined to face the upstream side in the transport direction (when the ID value of the pattern is 0 to 2), 0 is associated as the correction parameter of the pixel of interest. Has been done. Further, when the direction of the edge composed of the pixel of interest is determined to be the direction orthogonal to the transport direction (when the pattern ID is 3 or 4), 0 is associated as the correction parameter of the pixel of interest. ing. That is, in the present embodiment, when the ID value of the edge angle of the pixel of interest is 0 to 4 or -1, the correction parameter is determined to be 0 so that the edge is not sharpened.

When the edge composed of the pixel of interest is determined to face the downstream side in the transport direction (when the pattern ID is 5 to 7), a value other than 0 is associated with the correction parameter of the pixel of interest. Has been done. That is, the edge determined to face the downstream side in the transport direction is sharpened.

FIG. 19 (e) is a diagram showing an example of a raster image that has undergone correction processing obtained as a result of this step. By the processing of this step, the signal value is corrected so that the lower end portion of the raster image before the correction processing shown in FIG. 19A becomes darker. Therefore, even if the halftone image data is generated by the quantization process using the raster image data and the printing unit 111 prints based on the printed image data based on the halftone image data, as shown in FIG. 15B. The occurrence of blurring at the lower end is suppressed. As described above, in the present embodiment, by changing the correction parameter according to the direction of the edge, it is possible to perform the sharpening process on the edge in a predetermined direction. Therefore, even when the edge in a predetermined direction is likely to be blurred, the edge blur can be suppressed.

Further, in the present embodiment, among the edges facing the downstream side in the transport direction, when the direction of the edges as shown in the patterns 705 and 707 in FIG. 7 is slanted with respect to the transport direction, the correction parameter is set. They are associated so that they are greater than 0 and less than 1. For example, in the table of FIG. 21, when the value indicating the angle (direction) of the edge composed of the pixel of interest is 5 or 7, 0.7 is associated as the correction parameter of the pixel of interest. That is, when the direction of the edge is not parallel to the transport direction but at an angle, the correction parameter is set to a smaller value than when the direction of the edge as shown in the pattern 706 is in the same direction as the transport direction. The table of FIG. 21 is designed. The reason for this will be explained.

FIG. 22 is a diagram showing raster image data, and is a diagram for explaining an edge facing the downstream side in the transport direction in which the signal value is corrected. 22 (a) is the same as FIG. 19 (e), and the corrected edge orientation is the same as the paper transport direction. FIG. 22B is a diagram showing a raster image after correction at an edge that is oblique with respect to the transport direction.

Here, consider a case where the sharpening process is performed so that the signal value of the downstream edge of FIG. 22B becomes the same as the corrected signal value of the downstream edge of FIG. 22A. In this case, the halftone of the edge becomes high density due to the correction, so that the edge rattling may be conspicuous at the diagonal edge as shown in FIG. 22 (b). This is because the diagonal edge is not a straight line due to the structure of the raster image data.

Therefore, in the present embodiment, the correction parameter is changed depending on the angle of the edge even if the edge is facing the downstream side in the transport direction. As described above, when the edge formed by the pixel of interest is an edge oblique to the transport direction, sharpening processing is performed using a correction parameter having a smaller value than the edge whose direction is close to parallel to the transport direction. Is done. Therefore, among the edges facing the downstream side, the edges that are oblique in the transport direction can be corrected to suppress blurring while suppressing rattling. In this way, by changing the correction parameter according to the direction of the edge, it is possible to carry out the sharpening process of the halftone edge suitable for the direction of the edge.

In S2006, the correction processing unit 1601 determines whether or not processing has been performed on all the pixels. If the processing is not completed for all the pixels, the process returns to S2001, the unprocessed pixel is selected as the pixel of interest, and the processing of S2002 to S2005 is repeated. When the processing is completed for all the pixels, raster image data in which the signal value is corrected so that the edge blur is corrected is generated. The modified raster image data is output to the quantization processing unit 303. Then, the sharpening process is completed.

Returning to FIG. 17, the description of the entire processing flow of the halftone processing will be continued. Halftone image data is generated from the raster image data of the modified CMYK color space by the quantization processing unit 303 in S1705. Since the process of generating halftone image data is the same as that of S404, the description thereof will be omitted. Printed image data is generated based on the halftone image data, and the printed image data is output to the image output unit 206.

FIG. 23 is a diagram for explaining the effect of the correction process of the present embodiment on the edge. The vertical axis of FIGS. 23 (a) to 23 (c) represents the transport direction, and the horizontal axis represents the luminance. FIG. 23 (a) is a diagram expressing the luminance profile of the raster image of FIG. 19 (a) in the transport direction, and ideally, the density of the printed image also changes as shown in FIG. 23 (a). preferable. However, if blurring occurs at the lower end as a result of printing the raster image of FIG. 19 (a), the density of the printed image changes as shown in FIG. 23 (b). The blur portion A in FIG. 23 (b) is a blur caused by a change in density caused by “scraping”. The bokeh portion B is a bokeh generated by "sweep" caused by applying toner to a portion that should be originally a background color.

23 (c) shows the density change of the printed image based on the raster image obtained as a result of performing the correction processing in the present embodiment on the raster image of FIG. 19 (a) in the same manner as in FIG. 23 (a). It is a figure shown in. At the location C in FIG. 23 (c), which was the blurred portion A in FIG. 23 (b), a sharpening process is performed to correct the raster image data so that the density increases. Further, the concentration decreases at the location D. Therefore, the density change at the lower end of the image printed by the correction process of the present embodiment approaches FIG. 23 (a). Therefore, according to the present embodiment, blurring at the lower end is suppressed. It is possible to suppress deterioration of image quality at the edges of the printed image.

In addition, the method of the first embodiment and the method of this embodiment may be applied in combination. For example, after the halftone image data is generated in S1705, the correction data generation process of S405 and the image composition process of S405 described in the first embodiment are performed to correct the edge of the halftone image data. You may.

<Other embodiments>
A program that realizes one or more functions of the above-described embodiment is supplied to a system or device via a network or a storage medium. It can also be realized by the process of reading and executing a program by one or more processors in the computer of the system or apparatus. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

301 Image acquisition unit 302 Edge extraction unit 1601 Correction processing unit 206 Image output unit 200 Image processing unit 100 MFP

Claims (12)

An acquisition method for acquiring raster image data indicating an image,
An extraction means for extracting the edges of an object in the image,
A determination means for determining the orientation of the edge and
A correction means for sharpening an edge determined to be oriented in a predetermined direction in the raster image data, and a correction means.
An output means for outputting the printed image data generated based on the data obtained as a result of the processing by the correction means to the printing means, and an output means.
An image processing apparatus characterized by having. The predetermined direction is characterized in that the image is oriented toward either the downstream side or the upstream side in the transport direction of the recording medium when the image is printed on the recording medium by the printing means. The image processing apparatus according to claim 1. The correction means
Of the edges determined to be facing one side, if the direction of the edge is the first direction forming a predetermined angle with respect to the one side, the direction of the edge is the first direction. The image processing apparatus according to claim 2, wherein the image processing apparatus is sharpened so that the density is lower than that in the case of the second direction, which is closer to the direction parallel to the transport direction. .. The sharpening treatment for the edges is performed by an unsharp mask.
In the process of adding the difference image to the raster image data, when the sharpening process is performed on the edge determined to be facing the first direction, the pixel value of the difference image is less than 1 and larger than 0. The image processing apparatus according to claim 3, wherein the values obtained as a result of multiplying the values are added together. The correction means
The second aspect of the present invention is characterized in that the sharpening process is not performed on the edge determined to be facing the opposite side to the one side and the determined edge facing the direction orthogonal to the transport direction. The image processing apparatus according to any one of 4. The image processing apparatus according to any one of claims 2 to 5, wherein one side is the downstream side. The extraction means is
In the raster image data showing the image, it is determined whether the pixel of interest is a pixel constituting an edge based on the difference between the maximum value and the minimum value of the pixel values of the pixel of interest and the peripheral pixels of the pixel of interest. The image processing apparatus according to any one of claims 1 to 6, wherein the edge is extracted by the image processing apparatus. The determination means is
The aspect according to any one of claims 1 to 7, wherein the orientation of the edge is determined based on the degree of coincidence between the extracted edge and each pattern showing the edge pointing in a plurality of directions. The image processing device described. Further having a generation means for generating halftone image data by processing the data obtained as a result of the sharpening process by the correction means.
The image processing apparatus according to any one of claims 1 to 8, wherein the output means outputs the printed image data generated based on the halftone image data to the printing means. Further, it has a second correction means for correcting the value of the pixel of the halftone image data corresponding to the pixel constituting the edge according to the direction in which the edge is facing.
The image processing apparatus according to claim 9, wherein the printed image data is halftone image data obtained as a result of correction by the second correction means. The acquisition step to acquire raster image data showing an image,
An extraction step that extracts the edges of objects in the image,
The decision step to determine the orientation of the edge and
A correction step for sharpening the edge in a predetermined direction in the raster image data, and a correction step.
An output step for outputting print image data generated based on the data obtained as a result of the processing by the correction step to the printing means, and an output step.
An image processing method characterized by having. A program for making a computer function as each means of the image processing apparatus according to any one of claims 1 to 10.

Images