JP2007081793A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technology for suitably detecting the line number boundary of the screen line number on an original to check the growth of a moire in a low line number region, and to restrain the image quality deterioration in a high line number region. <P>SOLUTION: The image forming apparatus detects the screen line number of pixels of image data of a read original, counts the number of pixels having the same screen line number to specify the screen line numbers that its value exceeds a threshold th1, corrects all the values of the specified screen line numbers to a mean value thereof, compares the corrected screen line numbers with a threshold th2 to classify into high and low line number regions, and applies different screen processes and filter processes to the high and low line number regions. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a technique for forming an image with good image quality in an image forming apparatus.

  2. Description of the Related Art Conventionally, image forming apparatuses that express halftones with screens such as halftone dots and lines have been widely used. In such an image forming apparatus, when copying (copying), there is a problem that moire occurs and the image quality is impaired. This moire occurs when the screen structure (halftone dot structure) of the original scanned and read interferes with the screen structure used when the image forming apparatus forms an image.

  A technique for smoothing the screen structure of a document with a digital filter is known as means for avoiding the above-described moire. However, when the number of screen lines of the document is low (for example, about 100 lines), strong smoothing processing is necessary to avoid the occurrence of moire, but strong smoothing is performed on the screen structure of the document. When the processing is performed, there is a problem that the sharpness of the image is extremely lowered, and the image is extremely out of focus as a whole, and the image quality is extremely deteriorated.

In order to avoid such a problem, the image processing apparatus described in Patent Document 1 detects an area with a specific number of lines (for example, 100 lines) in which moire is likely to occur. The smoothing process is switched so that a weak smoothing process is performed for the other regions. By doing so, it is possible to suppress degradation of image quality due to moire and to suppress degradation of a halftone image in a line number region other than the specific line number.
JP 2001-119575 A

  By the way, a technique for detecting a screen line number by reading a printed document is known. However, a certain amount of error often occurs in the detection of the screen line number, and an erroneous screen line number is detected. There is. The technique described in Patent Document 1 has a problem that the area of the specific halftone dot cannot be detected properly when the screen line number cannot be detected accurately.

  Further, the erroneous detection (detection error) of the number of screen lines described above often occurs near the boundary between a specific halftone dot area and other areas. Therefore, there is a problem that the boundary between the specific halftone dot area and the other area cannot be properly detected particularly when a document in which the specific halftone dot area and other areas are mixed is read.

  The present invention has been made in view of the above-described background, preferably detects a line number boundary of the screen line number of an original, detects a boundary of an area having a different screen line number, and detects an area with a low line number. It is an object of the present invention to provide a technique for suppressing the occurrence of moiré in the image forming area and suppressing image quality deterioration in a high-line-number area.

  In order to solve the above-described problems, the present invention provides an image reading unit that reads a document and generates image data representing the document, and a minute region detection unit that detects a minute region of the image data generated by the image reading unit. And, for each minute region of the image data, a line number estimating unit that estimates the number of screen lines in the region, and the number of minute regions in which the screen line number estimated by the line number estimating unit is the same value. The high frequency line number specifying means for specifying the number of screen lines whose number is equal to or more than a predetermined threshold, and the screen line number value specified by the high frequency line number specifying means continuously appear in a certain range. In this case, a line number correcting unit that determines one value belonging to the range as the correction screen line number, and replaces the screen line number of the minute area corresponding to the range with the correction screen line number. , Image processing means for applying image processing corresponding to the number of correction screen lines for each of the micro areas, and output means for outputting image data in which image processing is applied to each micro area by the image processing means. An image forming apparatus is provided.

  In a preferred aspect of the present invention, the line number correcting means may calculate an average value of all screen line numbers in the range to obtain the corrected screen line number.

Further, the image processing means may perform screen processing according to the corrected screen line number determined by the line number correcting means.
Alternatively, the image processing unit may perform a filtering process according to the corrected screen line number determined by the line number correcting unit.

Further, in a preferred aspect of the present invention, the image processing means comprises a classifying unit that classifies each of the micro regions into a low line number region where the correction screen line number is equal to or less than a predetermined threshold and other regions. The image processing according to the classification result by the classification unit may be applied to each of the minute regions.
In this aspect, the image processing means may apply FM screen processing in the low line number region.
The image processing means may apply a smoothing process in the low line number region.

Further, in a preferred aspect of the present invention, the line number estimation means specifies a halftone dot center pixel of a screen halftone of the image data generated by the image reading means, and the specified halftone dot center pixel and its re-neighborhood. The screen line number may be estimated by measuring the distance from the halftone dot center pixel.
In a further preferred aspect of the present invention, the minute area may be an area of a unit pixel of image data.

According to the present invention, the screen line number is estimated from the image data of the read original, and the estimated screen line number value is corrected according to the appearance frequency, so that the screen line number detection error can be reduced. . In particular, it is possible to suitably detect a boundary between regions having different screen line numbers.
Further, according to the present invention, the image data area of the document is divided into a low-line area and a high-line area, and image processing corresponding to each screen line number is performed in each area. As a result, it is possible to avoid moiré in an area where the number of screen lines is low and to prevent image quality deterioration in an area where the number of screen lines is high.

Embodiments of the present invention will be described below.
<A: Number of screen lines of the document>
Prior to describing the configuration of the embodiment of the present invention, first, a document scanned by the image forming apparatus according to the present embodiment will be described below with reference to FIGS. 1 to 3.
FIG. 1 is a diagram illustrating an example of a document on which an image is formed with the same number of screen lines or a document having no screen structure. FIG. 1A illustrates an example of a document on which only characters are printed. Generally, when printing only characters, printing is performed without using a screen, so that a character image having no screen structure is formed on the paper.

  FIG. 1B is a diagram illustrating an example of a document on which an image is formed using an image forming apparatus that expresses a halftone with a screen such as a halftone dot or a line. In this case, the entire surface of the document is formed with the same number of screen lines. An image is generally formed with a screen line number of 75 to 200 lines.

FIG. 1C is a diagram illustrating an example of a document on which an image is formed by exposure on photographic paper. The image formed by exposing the photographic paper is an image having no screen structure.
In the following description, for convenience of description, an image formed with a halftone expressed on a screen is referred to as a “screen image”, and an image that does not have a screen structure formed by exposure to photographic paper or the like. This will be described as “non-screen image”. An image formed by printing characters will be referred to as a “character image”.

  Next, FIG. 2 is a diagram illustrating an example of a document on which characters and a screen image I1 are mixedly formed. In this case, the area where the characters are printed does not have a screen structure, and the image area is formed with a certain number of screen lines. There is also an image forming apparatus that forms an image by performing screen processing only in the image area.

  Next, FIG. 3A is a diagram showing an example of a document in which characters and a plurality of screen images I1 and I2 (for example, 100 lines and 200 lines, etc.) having different numbers of screen lines are mixed. FIG. 3B is a diagram showing an example of a document in which characters and a non-screen image I3 are mixed. FIG. 3C is a document in which a screen image I1 and a non-screen image I3 are mixed. It is a figure which shows an example. For example, when the user creates a single original by pasting a plurality of images, an original as shown in FIG. 3 is used.

  Many of the originals read by the image forming apparatus are roughly classified into any of the originals shown in FIGS. In the case of the original shown in FIG. 1B, the number of screen lines is constant in all areas of the original. The document shown in FIG. 2 has two types of areas, an area having no screen structure and an area other than the area. In the case of the original shown in FIG. 3A, the screen line number has three types of areas, for example, an area of 100 lines, an area of 200 lines, and other areas. In the case of the original shown in FIG. 3A, a plurality of screen lines are mixed, but the number of screen lines is constant in each image area.

When a histogram is generated by statistically calculating the screen line number of the original for each pixel, for example, when the screen line number of the screen image I1 is 200 lines and the screen line number of the screen image I2 is 100 lines, FIG. In the histogram shown in Fig. 4, a peak appears on the 100th line. In the case of the document shown in FIG. 3A, peaks appear at the 100th and 200th line portions. In this way, a peak appears at a certain number of screen lines and does not have a continuous distribution.
From the above, it can be said that “the screen line number distribution of the document has a peak, and the screen line number does not frequently change in one document”.
The present invention utilizes this tendency, and corrects erroneous detection of the number of screen lines using this tendency.

<B: Configuration of Image Forming Apparatus>
Next, the configuration of the image forming apparatus according to the embodiment of the present invention will be described.
FIG. 4 is a block diagram showing a schematic configuration of the image forming apparatus 1 according to the embodiment of the present invention. The image forming apparatus 1 is an apparatus having a copying function. In the figure, reference numeral 10 denotes a control unit having an arithmetic device such as a CPU (Central Processing Unit). A storage unit 20 includes a storage device such as a RAM (Random Access Memory), a ROM (Read Only Memory), or a hard disk, and stores various programs for operating each unit of the image forming apparatus 1. The control unit 10 reads out and executes the program stored in the storage unit 20 to control each unit of the image forming apparatus 1 via the bus BUS.

  Reference numeral 30 denotes an image reading unit that optically reads a document and generates image data representing an image of the document. The image reading unit 30 includes an optical system member (not shown) configured by a CCD (Charge Coupled Device) or the like, and reads an image of a document placed on a platen glass (not shown) by the optical system member. Then, RGB format image data representing the read image in pixel units is generated. 40 includes, for example, an ASIC (Application Specific Integrated Circuit) and an LSI (Large Scale Integration), and executes predetermined image processing to be described later on the image data generated by the image reading unit 30 to obtain image data of each color of YMCK. An image processing unit to be generated. Reference numeral 50 denotes an image forming unit that forms a toner image of each color of YMCK based on the image data of each color of YMCK generated in the image processing unit 40 and transfers the image to a sheet to form an image. An operation unit 60 includes a touch panel type liquid crystal display, various buttons, and the like, and receives various instructions regarding image formation from the user.

Here, the details of the image processing executed by the image processing unit 40 will be described below with reference to FIG. FIG. 5 is a block diagram schematically illustrating a functional configuration of the image processing unit 40 of the image forming apparatus 1. In the figure, reference numeral 41 denotes an input color conversion processing unit that converts RGB format image data generated by the image reading unit 30 into L ^* a ^* b ^* format image data. The input color conversion processing unit 41 includes a storage unit that stores image data, and stores the converted image data in the storage unit.

  A pre-processing unit 420 detects the screen line number of the image data, corrects the screen line number, and the like. The image forming apparatus 1 performs this preprocessing on the image data stored in the storage unit of the input color conversion processing unit 41, and then performs filter processing and screen processing, which will be described later, according to the processing result of the preprocessing. Execute.

  The line number detection processing unit 42 detects the screen line number of each pixel of the image data generated by the input color conversion processing unit 41. This processing is performed as follows, for example. First, each pixel of the image data is binarized, and the binarized image data is subjected to processing such as pattern matching to identify a pixel that is the center of the halftone dot of the screen halftone of the read document. Next, measure the distance to the nearest halftone dot center pixel for each pixel that is at the specified halftone dot center, and calculate the number of screen lines from the measured distance using a table, threshold value, etc. To do. Specifically, for example, as shown in FIG. 6, the distance is calculated by extracting the nearest halftone dot center pixel in the pixel P in an n × n pixel area centered on the pixel P that is the halftone dot center. The number of screen lines is calculated from the distance between the extracted halftone dot center pixel and the pixel P. The closer the distance, the higher the number of lines, and the farther the distance, the lower the number of lines. By performing this process for each pixel of the image data, the number of screen lines for each pixel is detected.

  In this line number detection process, the screen line number may be erroneously detected. For example, when colored paper, recycled paper, or the like is used as a document, or in a character area, the screen line number may be erroneously detected. In particular, many detection errors occur near the boundary between the screen image area and the other areas. For example, when a document as shown in FIG. 3A is read, the vicinity of the boundary between the area of the screen image I1 and its adjacent area, or the vicinity of the boundary between the area of the screen image I2 and its adjacent area. , The number of screen lines may be erroneously detected. Even in the screen image area, the screen line number is not accurately detected for all pixels, and the value of the screen line number causing a certain degree of error from the actual screen line number is detected. The line number correction process described later is a process for reducing such a detection error by correcting the detected screen line number.

  Next, the line number extraction processing unit 43 shown in FIG. 5 measures the number of pixels having the same screen line number, and specifies the number of screen lines whose number is equal to or greater than the threshold value. Specifically, the line number extraction processing unit 43 first generates a histogram by measuring the number of pixels having the same screen line number specified by the line number detection process. FIG. 7A is a diagram illustrating an example of a generated histogram. When the original illustrated in FIG. 3A is read, a histogram as shown in FIG. 7A is generated. The reason why the peaks of this histogram have a continuous width (n1 to n2, n3 to n4) is that, as described above, the number of screen lines may be erroneously detected in the area of the screen image or the boundary thereof. It is.

  Subsequently, the line number extraction processing unit 43 specifies the number of screen lines in which the number of pixels having the same screen line number (frequency of appearance of the screen lines) is equal to or greater than the threshold th1. In the example shown in FIG. 4A, the number of screen lines n1 to n2 and the number of screen lines n3 to n4 are specified.

  Next, the line number correction processing unit 44 shown in FIG. 5 corrects the screen line number of each pixel of the image data. As described above, the distribution of the screen line number of the document has a peak, and the screen line number tends not to change frequently in one document. Therefore, in this embodiment, when the screen line number value specified by the line number extraction process described above appears continuously in a certain range, the line number correction processing unit 44 is extracted by the line number extraction process. An average value of the screen line number is calculated, and the extracted value of the screen line number is corrected to this average value.

  Specifically, for example, when the histogram shown in FIG. 7A is generated by the line number extraction process, the specified screen line number ranges are n1 to n2 and n3 to n4. It is a range. The line number correction processing unit 44 corrects the screen line number values of n1 to n2 to the average value, and also looks up a table for correcting the screen line number values in the range of n3 to n4 to the average value. And correct the number of screen lines using the created table. FIG. 7B is a diagram illustrating an example of the contents of the look-up table. The horizontal axis represents the value of the screen line number before correction, and the vertical axis represents the value of the screen line number after correction. As illustrated, the number of screen lines in the range of n1 to n2 is corrected to the average value n12, and the number of screen lines in the range of n3 to n4 is corrected to the average value n34.

  For example, as shown in FIG. 3, when a document having two images with different screen line numbers is read in which the screen line number of the screen image I1 is 200 lines and the screen line number of the screen image I2 is 100 lines. The value of the number of screen lines to be detected from the document is either 100 or 200. However, as shown in FIG. 7A, the screen line number value is detected with a slight error. By correcting the number of screen lines detected with such an error to a constant value by the above-described line number correction process, the number of lines in the same line number area in the document can be corrected to a constant value. Thereby, detection errors can be reduced.

Next, the output line number determination unit 45 shown in FIG. 5 determines each pixel whose screen line number has been corrected by the above-described line number correction process as a low line number region having a screen line number equal to or less than a threshold value and a high line number. This is a process of classifying into areas.
FIG. 8 is a diagram illustrating processing performed by the output line number determination unit 45. In the figure, the output line number determination unit 45 determines whether or not the screen line number is equal to or greater than the threshold th2 for each pixel whose screen line number has been corrected by the line number correction process (step S1). When the threshold is greater than or equal to the threshold th2 (step S1; YES), a high line number tag (identification information) indicating the high line number is generated (step S2). On the other hand, if it is less than the threshold th2, a low line number tag (identification information) indicating a low line number is generated (step S3). And the filter process part 47 is performed with respect to the image data to which the tag produced | generated by step S2 or step S3 was provided for every pixel.

In this output line number determination process, the screen line number for each pixel is corrected by the line number correction process described above so that the high line number region and the low line number region can be appropriately classified. It has become. Specifically, for example, when the threshold th2 is a value between n3 and n4 as shown in FIG. 7B, the number of screen lines is set to n3 by performing the above-described line number correction processing. All regions between n4 are classified as low-line regions. However, when the above-described line number correction processing is not performed, the pixel area having the screen line numbers n3 to n4 is divided into a high line number area and a low line number area. That is, an area having pixels with screen line numbers n3 to n4 is processed as an area with different line numbers in spite of being an area with the same line number in a document. As described above, by performing the above-described line number correction processing, even when the threshold value th2 is a value between n3 and n4, all the pixels with the screen line number n3 to n4 have a high line number or a low line number. Since it is specified as either one, it is possible to suitably execute a filter process and a screen process described later without dividing the area into two.
In addition, by performing such a line number correction process, it becomes possible to more accurately detect the boundaries of areas with different screen line numbers, and in particular, it is possible to more accurately detect edges of image areas. Become.

  Next, the character processing unit 46 shown in FIG. 5 is a process of separating the image data into a character area and an image area. The character processing unit 46 separates the image data into a character region and an image region, and generates a tag indicating whether the image region or the character region for each pixel.

  The filter processing unit 47 converts the image data stored in the storage unit of the input color conversion processing unit 41 into a tag generated by the output line number determining unit 45 and a tag generated by the character processing unit 46. Accordingly, filter processing having a different smoothing degree is applied to each region. Specifically, a strong filter process is applied to an image area having a low screen line number (for example, 100 line number). Conversely, a weak filter process is applied to an image area with a high screen line number or a non-screen image area. Further, the emphasis process is performed on the character area without performing the smoothing process.

  Thus, moire can be avoided by performing the smoothing process on the low-line-number region. Moreover, the problem that the sharpness of the image is deteriorated can be avoided by performing a weak smoothing process on a high line number region.

The output color conversion processing unit 48 converts image data in the L ^* a ^* b ^* format into image data in the YMCK format.
The screen conversion processing unit 49 performs screen processing on the image data converted by the output conversion processing using an AM screen or FM screen. This screen processing also applies screen processing according to the tags generated in the output line number determination processing and the character processing processing, as in the above-described filter processing. Specifically, for example, screen processing is performed with an FM screen in an image region with a low number of lines, and conversely, screen processing is performed with an AM screen in an image region with a high number of lines. By performing FM screen processing in a low line number region, it is possible to reduce the occurrence of moire in the low line number region.

<C: Operation>
Next, the operation of this embodiment will be described. First, the image reading unit 30 of the image forming apparatus 1 reads a placed document and generates RGB format image data representing an image of the document. The input color conversion processing unit 41 in the image processing unit 40 converts the RGB format image data supplied from the image reading unit 30 into L ^* a ^* b ^* format image data.

  Subsequently, the preprocessing unit 420 performs a line number detection process, a line number extraction process, a line number correction process, and an output line number determination process on the converted image data, and performs a high line for each pixel of the image data. A tag indicating whether the region is a few region or a low line number region is generated, and a character processing process is performed on the image data to generate a tag indicating whether the region is a character region or an image region.

When each process described above is completed, the image processing unit 40 continues to store the image data stored in the storage unit of the input color conversion processing unit 41 according to the tag generated by the output line number determination process and the character processing process. A different filtering process is applied to each area. Then, an output color conversion process is performed to convert the filtered image data in the L ^* a ^* b ^* format into image data in the YMCK format.

  Next, the image processing unit 40 performs screen processing corresponding to the tag on the color-converted YMCK image data, and supplies the image processing unit 50 with the screen processing. The image forming unit 50 creates toner images of each color of YMCK according to the supplied image data, and transfers the image to a sheet to form an image.

  As described above, in the present embodiment, for each pixel of the image data of the read document, the screen line number of the pixel is estimated, and the estimated screen line number value is corrected according to the appearance frequency. The detection error of the screen line number can be reduced. In particular, it is possible to suitably detect a boundary between regions having different screen line numbers.

  In addition, since the image data area of the document is divided into a low-line area and a high-line area, and filter processing and screen processing corresponding to each screen line number are applied to each area. In addition, it is possible to avoid moiré in an area where the number of screen lines is low and to prevent image quality deterioration in a high line number area or a non-screen image area.

<D: Modification>
As mentioned above, although embodiment of this invention was described, this invention is not limited to embodiment mentioned above, It can implement with another various form. An example is shown below.
(1) In the above-described embodiment, the weak filter process is performed in the high line number region and the strong filter process is performed in the low line number region. However, the present invention is not limited to this. For example, the screen line number is low. The smoothing process may be performed for the line number region, and the smoothing process may not be performed for the high line number region. Further, edge enhancement processing may be performed in a high line number region.

(2) In the above-described embodiment, AM screen processing and FM screen processing are applied according to the number of screen lines. However, for example, a screen may not be applied in a low line number region. In the case of an image forming apparatus having the capability of reproducing a screen halftone dot (or tens of thousands of lines) with a low screen line number as it is and forming an image, the screen is not used in the low line number region. In addition, the occurrence of moire can be reduced by performing the image forming apparatus.

(3) In the above-described embodiment, the threshold value th2 is used to classify the image data into a low-line-number area and a high-line-number area. May be classified into a plurality of areas of 3 or more, and processing according to the number of screen lines in the area may be performed.

(4) In the above-described embodiment, for each pixel of the image data, the screen line number of the pixel is estimated. However, the screen line number is not estimated for each pixel, and a predetermined minute area is determined. The number of screen lines may be estimated every time. In this case, the number of screen lines is determined for each minute area, and image processing is performed according to the number of screen lines. Further, as an example of how to determine the number of screen lines in each minute area, the number of screen lines may be once determined for each pixel in the same manner as in the above-described embodiment, and averaged in the minute area. Further, the number of screen lines of a specific pixel (for example, the center pixel) in the minute area may be represented as the number of screen lines of the minute area.

It is a figure which shows an example of the original document concerning this invention. It is a figure which shows an example of the original document concerning this invention. It is a figure which shows an example of the original document concerning this invention. 1 is a block diagram illustrating a schematic configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a block diagram schematically illustrating a functional configuration of an image processing unit according to the embodiment. It is a figure for demonstrating the line number detection process of the embodiment. It is a figure for demonstrating the line number extraction process of the embodiment. It is a figure for demonstrating the output line number determination process of the embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 10 ... Control part, 20 ... Memory | storage part, 30 ... Image reading part, 40 ... Image processing part, 50 ... Image forming part, 60 ... Operation part.

Claims (9)

Image reading means for reading a document and generating image data representing the document;
A minute area detecting means for detecting a minute area of the image data generated by the image reading means;
For each minute area of the image data, a line number estimation means for estimating the number of screen lines in the area;
A high frequency line number specifying unit for measuring the number of the micro regions where the number of screen lines estimated by the line number estimating unit is the same, and specifying the number of screen lines whose number is equal to or greater than a predetermined threshold;
When the value of the screen line number specified by the high-frequency line number specifying unit continuously appears in a certain range, one value belonging to the range is determined as the corrected screen line number, and a minute region corresponding to the range A line number correcting means that replaces the screen line number with the corrected screen line number;
Image processing means for applying image processing according to the number of correction screen lines for each of the micro regions;
An image forming apparatus comprising: output means for outputting image data obtained by applying image processing to each minute region by the image processing means. The image forming apparatus according to claim 1, wherein the line number correcting unit calculates an average value of all screen line numbers in the range to obtain the corrected screen line number. The image forming apparatus according to claim 1, wherein the image processing unit includes a screen process corresponding to the corrected screen line number determined by the line number correcting unit. The image forming apparatus according to claim 1, wherein the image processing unit performs a filtering process according to the corrected screen line number determined by the line number correcting unit. Classifying means for classifying each of the micro regions into a low line number region where the correction screen line number is equal to or less than a predetermined threshold and other regions,
The image forming apparatus according to claim 1, wherein the image processing unit applies image processing according to a classification result by the classification unit to each of the minute regions. The image forming apparatus according to claim 5, wherein the image processing unit applies FM screen processing in the low line number region. The image forming apparatus according to claim 5, wherein the image processing unit applies a smoothing process in the low line number region. The line number estimation unit specifies a halftone dot center pixel of a screen halftone of the image data generated by the image reading unit, and measures a distance between the specified halftone dot center pixel and a halftone dot center pixel near the halftone dot center pixel. The number of screen lines is then estimated. The image forming apparatus according to claim 1, wherein the number of screen lines is estimated. The image forming apparatus according to claim 1, wherein the minute region is a region of a unit pixel of image data.

Images