Image processing device that performs resolution conversion, its control method, and program

Abstract

To solve the problem, in which, although there is a technology that performs intermediate tone processing at 600 dpi, etc., and converts edges to a higher resolution while obtaining a smoothing effect by pattern matching, pattern matching involves holding a huge number of conversion patterns and comparing a certain range of pixel areas with a pattern, and the reduction in the circuit scale was not sufficient.SOLUTION: The image processing device analyzes the intensity and direction of vertical and horizontal edges and converts them to a higher resolution on the basis of the analysis results.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a technique for converting resolution of image data after halftoning. [Background technology]

[0002] Recently, image forming devices have been developed with engines ranging from 600 dpi and 1200 dpi to 2400 dpi and 4800 dpi. This has enabled high-resolution rendering and image processing, making it possible to print high-quality output. However, image processing of high-resolution image data requires an increase in the number of pixels and circuit size. One example of this is halftoning (also known as screen processing). To speed up processing, screen processing essentially extracts one main-scanning line from the image and processes it in a high-speed line buffer. This process is then repeated for each sub-scanning pixel. Compared to 600 x 600 dpi image data, 2400 x 2400 dpi image data has four times the number of pixels in the main scanning direction and four times the number in the sub-scanning direction. Therefore, when performing halftone processing at 2400×2400 dpi within the halftone processing time for 600×600 dpi, a processing speed 16 times faster than that for 600×600 dpi is required, resulting in a larger circuit scale.

[0003] For this reason, several techniques have been proposed for improving halftone processing of high-resolution images in image forming devices (for example, Patent Document 1). In Patent Document 1, 600 dpi image data is first subjected to screen processing, and then converted to higher-resolution image data by pattern matching. In this way, image data that has been screen-processed at a resolution lower than the engine resolution is adjusted to the engine resolution. Specifically, pattern matching is used to detect the edge portions of the image, and the edge portions are converted into pixel groups that provide a smoothing effect, which are then used as pixels for the high-resolution image data. This makes it possible to generate high-resolution halftone images that provide a smoothing effect to the edge portions while keeping the circuit scale small. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2017-208739 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the above-mentioned method does not sufficiently reduce the circuit scale. This is because, although the screen processing itself is performed in 600 dpi units, the patterns used for pattern matching must be stored in memory, and a comparison process with the stored patterns is then performed. In Patent Document 1, 104 patterns must be stored, and each stored pattern requires a comparison process of 9 x 9 pixels.

[0006] However, if the number of patterns is reduced and the window size is made smaller in an attempt to further reduce the circuit scale, the smoothing effect of pattern matching becomes insufficient. [Means for solving the problem]

[0007] No. edge detection means for calculating the edge direction and strength from image data of a single resolution; The aforementioned Edge direction and The aforementioned Based on the strength, a selection means for selecting a conversion pattern from a plurality of conversion patterns for converting image data of the first resolution into binary image data of a second resolution higher than the first resolution; a generation means for generating an inverted pattern by inverting the conversion pattern selected by the selection means based on the direction of the edge; and a generation means for generating an inverted pattern based on the inverted pattern generated by the generation means. the image data at the first resolution of of the second resolution Converting the binary image data do conversion and a means for detecting a difference between the image and the image. [Effects of the Invention]

[0008] According to the present invention, By reducing the number of conversion patterns to be retained, It is possible to perform resolution conversion that can be expected to improve image quality while keeping circuit scale small. 。 [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a diagram showing a configuration of an image forming apparatus according to an embodiment of the present invention. [Figure 2] FIG. 2 is a diagram showing the configuration of an image processing unit for printing. [Figure 3] FIG. 2 is a diagram showing the configuration of a resolution conversion unit. [Figure 4] 10 is a flowchart of a process for calculating edge direction and intensity. [Figure 5] FIG. 10 is a diagram showing an example of filter coefficients used to calculate edge direction and strength. [Figure 6] FIG. 10 is a diagram showing an example of a two-dimensional matrix for calculating id from edge intensity. [Figure 7] FIG. 10 is a diagram showing an example of a one-dimensional lookup table for modulating a 4-bit signal value into a 5-bit signal value. [Figure 8] 10A and 10B are diagrams showing examples of conversion patterns used for resolution conversion. [Figure 9] 10A and 10B are diagrams showing a series of steps performed by a resolution conversion unit up to the point where a pixel of interest is subjected to resolution conversion; [Figure 10] 10A and 10B are diagrams showing examples of conversion patterns used for resolution conversion in the second embodiment. [Figure 11] 10 is a flowchart of a process executed by the conversion pattern acquisition unit 307 in the second embodiment. [Figure 12] FIG. 10 is a flowchart showing a sequence of steps for obtaining a conversion pattern according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. Note that the configurations shown in the following embodiments are merely examples, and the present invention is not limited to the configurations shown in the drawings. [Example]

[0011] In this embodiment, an electrophotographic digital multifunction peripheral (hereinafter referred to as MFP) having multiple functions such as copying, printing, faxing, etc. will be described as an example of the image forming apparatus. However, this is not limited to this, and the present embodiment can also be applied to devices using other processes, such as inkjet printing.

[0012] 1 is a schematic block diagram showing the configuration of an MFP 100 according to this embodiment. The MFP 100 is made up of a scanner unit 101, a controller 102, a printer unit 103, and an operation unit 104.

[0013] The scanner unit 101 optically reads an image of an original document and acquires it as image data.

[0014] The controller 102 includes a CPU, a ROM, and a RAM, and performs predetermined image processing on image data read by the scanner unit 101. The image data that has undergone image processing is stored in the RAM in the controller 102.

[0015] The printer unit 103 forms an image on recording paper using electrophotography based on the image data that has been subjected to image processing in accordance with the specified print setting conditions. In this embodiment, the printer unit 103 prints at an engine resolution of 2400 x 2400 dpi and 1 bit. However, the resolution may also be 1200 dpi, 4800 dpi, etc.

[0016] The operation unit 104 is a user interface for the user to perform various operations, and the user sets various printing conditions for image data to be printed via the operation unit 104.

[0017] MFP 100 is connected to a server 107 that manages image data via a network 105, a personal computer (PC) 106 that instructs MFP 100 to execute printing, and the like. When controller 102 receives an instruction to execute printing from server 107 or PC 106, controller 102 rasterizes the image data sent from server 107 or PC 106, converts it into image data (bitmap data) compatible with printer unit 103, and stores it in RAM.

[0018] Next, we will explain the image processing for printing that is executed within the controller 102. Figure 2 is a block diagram showing the internal configuration of the image processing unit as a functional unit responsible for image processing. The image processing unit 200 according to this embodiment is made up of a color correction unit 201, a gamma correction unit 202, a screen processing unit 203, and a resolution conversion unit 204.

[0019] The color correction unit 201 performs color correction processing on image data (bitmap data) acquired from RAM. Specifically, it converts the data into image data in the CMYK color space, which expresses density using four colors (image signals) of CMYK, using a color conversion LUT or matrix calculation. The converted image data has an 8-bit (0 to 255) value for each pixel in each color.

[0020] The gamma correction unit 202 performs a correction process (gamma correction process) on the input CMYK image data using a one-dimensional lookup table (LUT) so that the image has the desired density characteristics when transferred to recording paper.

[0021] The screen processing unit 203 performs screen processing on the input image data to generate screen data (or halftone image data, hereafter referred to as an HT image). The bit format of the HT image is assumed to be the general 1-bit to 4-bit format. The generated HT image is then sent to the resolution conversion unit 204.

[0022] The resolution conversion unit 204 performs resolution conversion processing, which will be described later, on the HT image received from the screen processing unit 203. In this embodiment, a case will be described in which a 600×600 dpi 4-bit image is converted into a 2400×2400 dpi 1-bit image.

[0023] <Resolution conversion processing> Next, the resolution conversion process in the resolution conversion unit 204 will be described in detail with reference to Fig. 3. Fig. 3 is a block diagram showing the internal configuration of the resolution conversion unit 204. The resolution conversion unit 204 is made up of an edge detection unit 301, a pattern ID calculation unit 304, a modulation unit 305, and a pattern conversion unit 306. The edge detection unit 301 is made up of a SobelV calculation unit 302 and a SobelH calculation unit 303. The pattern conversion unit 306 is made up of a conversion pattern acquisition unit 307 and a binary pattern calculation unit 308.

[0024] <Edge detection unit 301> The edge detection unit 301 (SobelV calculation unit 302, SobelH calculation unit 303) calculates edge strength and direction in the vertical and horizontal directions, and outputs the edge strength to the pattern ID calculation unit 304 and the direction to the conversion pattern acquisition unit 307.

[0025] In the edge detection unit 301, the SobelV calculation unit 302 calculates the vertical edge strength sobel_v and the vertical edge direction minus_sign_v. This will be described in detail with reference to Fig. 4. Fig. 4 is a flowchart of the processing performed by the SobelV calculation unit 302, and all of the processing described with reference to Fig. 4 is performed by the SobelV calculation unit 302.

[0026] In step S101, a convolution operation is performed on the HT image generated by the screen processing unit 203 using filter coefficients over a width of 3 pixels and a height of 3 pixels centered on a pixel of interest within the reference area, to calculate sobel_v. In this embodiment, a Sobel filter capable of detecting vertical edges as shown in FIG. 5(a) is used for explanation, but any known filter coefficient capable of edge detection will do. Here, since the HT image in this embodiment is 4 bits (values ​​0 to 15), the minimum value of sobel_v in the convolution operation with the filter coefficients shown in FIG. 5 is -60 and the maximum value is 60.

[0027] In step S 102 , if sobel_v is positive, minus_sign_v is set to 0, and if sobel_v is negative, minus_sign_v is set to 1, and the result is output to the pattern conversion unit 306 .

[0028] In step S103, sobel_v calculated in step S101 is normalized. Specifically, sobel_v is converted to an absolute value and shifted to the right by 3 bits. Since sobel_v after conversion to an absolute value has a value from 0 to 60, shifting it to the right by 3 bits normalizes sobel_v to a range from 0 to 7. By normalizing in this way, the size of the matrix storing ids used in pattern ID calculation unit 304, which will be described later, can be made 8×8. Next, the normalized sobel_v is output to pattern ID calculation unit 304. Note that the shift amount may be adjusted depending on the matrix size of pattern ID calculation unit 304, which will be described later.

[0029] The processing content of the SobelH calculation unit 303 is different from the processing content of the SobelV calculation unit 302 in that the filter coefficients used in the convolution calculation are changed to a Sobel filter that can detect horizontal edges as shown in FIG. 5(b). Therefore, details thereof will be omitted.

[0030] As described above, the edge strength and direction are determined using a filter, and pattern matching is not used.

[0031] <Pattern ID calculation unit 304> The pattern ID calculation unit 304 uses a two-dimensional matrix to calculate a pattern number (id) to be used in the conversion pattern acquisition unit 307 (described later) from sobel_v and sobel_h received from the edge detection unit 301. The pattern ID calculation unit will be described in detail with reference to Fig. 6. Fig. 6 is an image diagram of the two-dimensional matrix used by the pattern ID calculation unit 304. Reference numeral 601 denotes the area of ​​id=0, 602 the area of ​​id=1, 603 the area of ​​id=2, and 604 the area of ​​id=3.

[0032] The pattern ID calculation unit 304 acquires an ID number using the received sobel_v as a row index and sobel_h as a column index. For example, if sobel_v=1 and sobel_h=2, then id=0.

[0033] In this embodiment, the number of patterns is set to four, so the id takes a value from 0 to 3, but the range of the id value and the placement location may be adjusted depending on the number of patterns to be held.

[0034] <Modulation section 305> The modulation unit 305 modulates the 4-bit signal value (also called pixel value or gradation value) of the pixel of interest into a 5-bit signal value using a one-dimensional LUT, and outputs the 5-bit signal value to the conversion pattern acquisition unit 307. FIG. 7 is a diagram showing an example of the LUT used by the modulation unit 305. The left column shows the input signal value, and the right column shows the output signal value. For example, if the input signal value is 15, the maximum 4-bit value, the output signal value will be 16.

[0035] <Pattern conversion unit 306> The pattern conversion unit 306 (conversion pattern acquisition unit 307, binary pattern calculation unit 308) is a processing unit that acquires a conversion pattern and actually converts 1x1 pixels of 600x600 dpi into 4x4 pixels of 2400x2400 dpi. Here, within the pattern conversion unit 306, the conversion pattern acquisition unit 307 acquires the conversion pattern, and the binary pattern calculation unit 308 calculates the pixels after conversion and converts the pixels.

[0036] First, the acquisition of a conversion pattern will be described. The conversion pattern acquisition unit 307 acquires a conversion pattern using the received three pieces of information: id, minus_sign_v, and minus_sign_h.

[0037] The conversion patterns in this embodiment are shown in Figure 8. 801 is the conversion pattern when id=0, 802 when id=1, 803 when id=2, and 804 when id=3. Here, the numerical values ​​in the patterns are compared with the signal value of the pixel of interest, and indicate the order in which pixels with smaller numerical values ​​in the pattern become black pixels (dots = ON). In addition, in 801, the numerical values ​​increase from the center, and it can be seen that as the signal value of the pixel of interest increases, the number of black pixels after conversion increases from the center. Hereinafter, 801 will be referred to as the central growth pattern, 802 as the horizontal growth pattern, 803 as the vertical growth pattern, and 804 as the diagonal growth pattern.

[0038] Furthermore, 801 to 804 each hold four patterns. This is because, even for the same ID, the growth direction is reversed depending on the received minus_sign_v (vertical edge direction) and minus_sign_h (horizontal edge direction). For example, when minus_sign_v=1 (vertical edge direction is reversed), the conversion pattern in the upper left of area 801 is reversed left to right. Similarly, when minus_sign_h=1, it is reversed up and down, and when both minus_sign_v and h are 1, it is reversed up and down and left to right. Next, an example of acquiring a specific conversion pattern will be described with reference to FIG. 9. FIG. 9 is a diagram showing an example from acquiring a conversion pattern to converting pixels.

[0039] 9(a) shows a 3×3 pixel area, with the center pixel being the pixel of interest, in a 600×600 dpi HT image input to the edge detection unit 301. The numerical values ​​in FIG. 9(a) indicate signal values ​​(4 bits).

[0040] When the pixel in Figure 9(a) is processed according to the flow in Figure 4 described above, the following results: sobel_v=5, minus_sign_v=1, sobel_h=5, and minus_sign_h=1. Next, the pattern ID calculation unit 304 obtains id=3, and the conversion pattern acquisition unit 307 obtains the conversion pattern for the area with id=3, that is, area 804. Here, since both minus_sign_v and h are 1, the conversion pattern in the lower right corner of area 804 is obtained. The pattern shown in Figure 9(b) indicates the obtained conversion pattern. This concludes the specific explanation of how to obtain a conversion pattern.

[0041] Next, an example of pixel conversion using a conversion pattern will be described with reference to Fig. 9. Fig. 9(c) is the pixel obtained by modulating the pixel of interest in Fig. 9(a) by the modulation unit 305. Fig. 9(d) is the result of pattern conversion of Fig. 9(c).

[0042] The binary pattern calculation unit 308 compares the pixels in Figure 9(c) received from the modulation unit 305 with the numerical values ​​in the conversion pattern in Figure 9(b), and sets the parts where the numerical value in the conversion pattern is smaller as black pixels, and the rest as white pixels. Figure 9(d) shows the result. It can be seen that the parts in Figure 9(b) where the signal value is smaller than 8 are set as black pixels.

[0043] Furthermore, if there is a diagonal edge in the HT image as shown in Fig. 9(a), by using the diagonal growth conversion pattern shown in Fig. 9(b) (i.e., by replacing it with a conversion pattern), it is possible to convert it into an HT image with a smoothing effect that is diagonally shifted to the lower left as shown in Fig. 9(d). In other words, by using a conversion pattern according to the edge direction for the edge pixels in the HT image, it is possible to obtain an HT image with a smoothing effect, and by using a central growth pattern for pixels other than the edge, it is possible to appropriately convert the resolution of the HT image.

[0044] In addition, the present invention has a simple configuration in which processing is performed using a 3x3 edge detection filter and the calculated edge direction and intensity values ​​are used to obtain a conversion pattern, which makes it possible to further reduce the circuit scale. Note that, although the present embodiment has been described as a case in which 600x600 dpi is converted to 2400x2400 dpi, it goes without saying that other resolutions can be used before and after conversion. [Example]

[0045] The second embodiment of the present invention will be described below. In the first embodiment, four conversion patterns are stored for each growth pattern, and the pattern to be acquired is switched based on id (which growth pattern), minus_sign_v, and h (the direction of the vertical and horizontal edges) to perform the resolution conversion process.

[0046] However, it is also possible to store only one conversion pattern for each growth pattern, invert the acquired conversion pattern in the direction of the vertical and horizontal edges, and then perform the resolution conversion process. This makes it possible to further reduce the total number of conversion patterns from 16 to 4.

[0047] In the second embodiment, an example will be described in which the resolution conversion process is performed by performing an inverse calculation of the growth pattern described above. Note that only the differences from the first embodiment will be described in detail below.

[0048] The second embodiment differs from the first embodiment in the conversion patterns to be stored and the conversion pattern acquisition unit 307 .

[0049] First, the conversion patterns to be stored will be described with reference to Fig. 10. Fig. 10 shows the conversion patterns to be stored in the second embodiment. 1001 is the area with id=0 (central growth), 1002 is the area with id=1 (horizontal growth), 1003 is the area with id=2 (vertical growth), and 1004 is the area with id=3 (diagonal growth). Unlike the first embodiment, there are a total of four conversion patterns to be stored. In the second embodiment, these are used by performing inverse calculations.

[0050] Next, the processing of the conversion pattern acquisition unit 307 in the second embodiment will be described with reference to Fig. 11. Fig. 11 is a flowchart of the processing executed by the conversion pattern acquisition unit 307.

[0051] First, in step S201, a conversion pattern is obtained from the id.

[0052] Next, in step S202, if minus_sign_v<1, the process proceeds to step S204, and if not, the process proceeds to step S203.

[0053] Subsequently, if the process proceeds to step S203, the conversion pattern acquired in step S201 is flipped left and right.

[0054] Next, in step S204, if minus_sign_h<1, the process ends, and if not, the process proceeds to step S205.

[0055] In step S205, the conversion pattern is flipped upside down.

[0056] Here, a specific example of obtaining a conversion pattern when id=3 and minus_signv and h are both 1 will be described with reference to Fig. 12. Fig. 12 shows the processing results of the flow in Fig. 11.

[0057] First, if id=3, the conversion pattern shown in FIG. 12(a) is acquired.

[0058] Next, since minus_sign_v=1, the conversion pattern in FIG. 12(a) is flipped left and right to become the conversion pattern in FIG. 12(b).

[0059] Next, since minus_sign_h=1, the conversion pattern in Fig. 12(b) is inverted upside down to become the conversion pattern in Fig. 12(c). The above is the processing content of the conversion pattern acquisition unit 307 in the second embodiment.

[0060] Thereafter, the binary pattern calculation unit 308 receives the conversion pattern from the conversion pattern acquisition unit 307 and performs resolution conversion processing, similar to the first embodiment.

[0061] As described above, even if only one conversion pattern is stored for each growth direction, the same resolution conversion process as in the first embodiment can be performed by performing inverse calculation of the conversion pattern depending on the edge direction.

[0062] (Other Examples) The present invention can also be realized by supplying a program that realizes one or more functions of the above-described embodiments to a system or device via a network or a storage medium, and having one or more processors in the computer of the system or device read and execute the program.The present invention can also be realized by a circuit (e.g., ASIC) that realizes one or more functions.

Claims

1. An edge detection means for calculating edge direction and strength from image data of a first resolution; a selection means for selecting, from a plurality of conversion patterns, a conversion pattern for converting the image data with the first resolution into binary image data with a second resolution higher than the first resolution, based on the direction and intensity of the edge calculated by the edge detection means; a generating means for generating an inverted pattern by inverting the conversion pattern selected by the selecting means based on the direction of the edge; a conversion unit that converts the image data of the first resolution into the binary image data of the second resolution based on the inverted pattern generated by the generation unit.

2. 2. The image processing apparatus according to claim 1, wherein said edge detection means calculates said direction and said strength of said edge using a Sobel filter.

3. 3. The image processing device according to claim 2, wherein the edge detection means calculates the direction and the intensity of the edge using a Sobel filter without performing processing using pattern matching on the image data of the first resolution.

4. An image processing device described in any one of claims 1 to 3, characterized in that the first resolution is 600 dpi and the second resolution is 2400 dpi.

5. An image processing device as described in any one of claims 1 to 4, characterized in that it further has a reading means for reading an image of a document.

6. An image processing device as described in any one of claims 1 to 5, characterized in that it further has a printing means for printing an image on a sheet.

7. An edge detection step of calculating edge direction and strength from image data of a first resolution; a selection step of selecting, from a plurality of conversion patterns, a conversion pattern for converting the image data with the first resolution into binary image data with a second resolution higher than the first resolution, based on the direction and the strength of the edge calculated in the edge detection step; a generating step of generating an inverted pattern by inverting the conversion pattern selected in the selecting step in accordance with the direction of the edge; a conversion step of converting the image data of the first resolution into the binary image data of the second resolution based on the inverted pattern generated in the generation step.

8. A program for causing a computer to execute the control method according to claim 7.

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