JP2004072245A - Image processor and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To apply black thinning processing to black characters and black lines when an original is a positive original, and apply white thinning processing to white characters and white lines when an original is a negative original. <P>SOLUTION: An original background gray level detection circuit 1 decides whether received image data indicate a positive original or a negative original, an edge detection circuit 2 detects whether or not surrounding pixels around a target pixel of the received image data depict an edge area, and a thinning processing circuit 3 applies black thinning processing to a target pixel decided to be an edge area when the image data are decided to be a positive original and applies white thinning processing to the target pixel decided to be the edge area when the image data are decided to be a negative original. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image processing apparatus that performs thinning processing of characters and lines on an image in an image forming apparatus such as a copying machine, a scanner, and a facsimile that performs digital image processing, and an image forming apparatus including the image processing apparatus.
[0002]
[Prior art]
In an image processing apparatus such as a monochrome digital copying machine or a scanner, when a copy original is copied (copy copy), characters and lines of the copy image may be thinner and thicker than the original. In such a case, an edge portion of a character or a line is detected by image processing and a thinning process is performed, and the character or the line is thinned to reproduce the original more faithfully. A conventional example of this type is disclosed, for example, in Japanese Patent Application Laid-Open No. 8-65520.
[0003]
[Problems to be solved by the invention]
However, since it is a process for thinning the characters and black lines of a positive original (white background and black characters and the like), it is usually the case when the original is dark and the characters and lines are white such as a negative original. There is a problem that thinning is not performed on white characters and white line portions, and conversely, thickening processing is performed in which white lines are thickened. Furthermore, in the case of a circuit configuration that performs black-and-white reversal processing of image data before performing fine line detection and thinning processing when normal positive original pain is reversed, the black characters and black lines of the original image are reversed. As a result, there is a problem that white characters and white lines are formed, and the white area is thickened.
[0004]
In addition, if the matrix size for performing thinning detection is small, the edge portion of the halftone dot portion may be erroneously recognized as the edge portion of the thin line portion, and the image of the halftone dot portion becomes unsightly and causes deterioration in image quality.
[0005]
Therefore, an object of the present invention is to provide an image processing apparatus and an image forming apparatus capable of performing an optimal thinning process on a positive document or a negative image and outputting a high-quality image.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, an image processing apparatus according to a first means includes: a document determining means for determining whether input image data is a positive document or a negative document; Edge detection means for detecting whether or not a peripheral pixel centered on the pixel is an edge area; and a determination result of the document determination means for a target pixel determined to be an edge area by the edge detection means. And thinning processing means for performing white thinning processing when it is determined that the document is a negative original. With this configuration, if the original is a positive original, black thinning processing is performed on black characters and black lines, and if the original is a negative original, white thinning processing is performed on white characters and white lines. Therefore, it is possible to perform optimal thinning processing for each document.
[0007]
The second means is that, in the first means, the edge detecting means further detects an edge amount, and the thinning processing means further changes the degree of thinning according to the edge amount detected by the edge detecting means. It is characterized by the following. With this configuration, in addition to performing the edge determination, the edge amount can also be detected and the level of the degree of thinning can be adjusted based on the magnitude of the edge amount, so that the optimal thinning processing can be performed on the edges of the image. It becomes possible.
[0008]
The third means is characterized in that, in the first or second means, when the edge detection means determines that the area is not an edge area, the pixel data of interest is output as it is. With this configuration, the thinning processing is performed when it is determined to be an edge, and the thinning processing is not performed when it is determined to be not an edge (halftone dot). Thinning can not be performed on the portion, and it is possible to prevent image quality deterioration.
[0009]
The fourth means comprises an image processing apparatus according to the first to third means, and an image forming means for forming an image on recording paper based on image data processed by the image processing apparatus. It is characterized by having done. With this configuration, it is possible to perform black thinning processing on black characters and black lines in the case of a positive original, and perform white thinning processing on white characters and white thin lines in the case of a negative original. This makes it possible to output an image obtained by performing an optimal thinning process on the image.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of an image processing apparatus according to the present invention, FIG. 2 is a block diagram showing a peripheral pixel edge detection circuit of FIG. 1 in detail, and FIG. 3 is a block diagram showing a matrix generation unit of FIG. 4, FIG. 4 is an explanatory diagram showing the coefficients of the filter processing unit of FIG. 2 in detail, FIG. 5 is an explanatory diagram showing the processing of the thinning processing circuit of FIG. 1, and FIG. 6 is an explanatory diagram showing the processing of FIG. 7 is an explanatory diagram collectively showing the processing of the thinning processing circuit of FIG. 1, and FIG. 8 is an explanatory diagram showing an example of an input image.
[0011]
In FIG. 1, input image data is applied to a document background density detecting circuit 1 for detecting the background of the document, an edge detecting circuit 2 for detecting an edge around a pixel of interest and calculating an edge amount, and a thinning processing circuit 3. The outline of the operation of this embodiment is as follows. First, the background density detection circuit 1 detects the background density level of the input image data to detect whether the document is a positive document or a negative document. Next, the surrounding pixel edge detection circuit 2 determines an edge around the target pixel, and further calculates the magnitude of the edge amount.
[0012]
When the original is a positive original, the thinning processing circuit 3 performs black thinning processing, that is, processing for dividing the thinning target pixel data by a certain value to white data based on the detection result of the original background density detection circuit 1. On the other hand, if the original is a negative original, white thinning processing, that is, processing of adding a certain value to thinning target pixel data to make black data is performed. The data handled here is multi-valued 8-bit data, white is 0, and black is 255.
[0013]
Further, when the thinning processing circuit 3 determines that the edge determination result by the surrounding pixel edge detection circuit 2 is an edge, the edge amount from the surrounding pixel edge detection circuit 2 indicates that the edge amount is, for example, a positive document and the edge amount is When it is large, the thinning level is increased by dropping to 25% of the original data with respect to the thinning target pixel data. When it is a positive original and the edge amount is small, the original data is reduced with respect to the thinning target pixel data. And the line thinning level is reduced. Further, when it is determined that the edge determination result is not an edge, the thinning processing circuit 3 does not perform the thinning processing.
[0014]
Next, the operation of each unit will be described. First, the document background density detection circuit 1 will be described. As a method of detecting the background density of a document, there is a method in which a press-scan operation is performed to check the background density in advance. In this method, the density of each pixel of a document is detected, the number of pixels having that density is counted for each density, a histogram is created based on the result, and a density representative of the background is obtained. The density is compared with a certain threshold value. If the density is smaller than a certain threshold value, it is determined that the background density is a white (light) positive document. If the density is more than a certain threshold value, the background density is a black (dark) negative document. Judge.
[0015]
This method is premised on pre-scanning. However, if the calculation is performed by detecting the background density in units of scan lines or pixel blocks, the determination result of the background density can be updated at any time. In addition, various methods other than those described above are conceivable as the original background density detection method, and the present invention is not limited to these methods.
[0016]
Next, the surrounding pixel edge detection circuit 2 will be described. As shown in FIG. 2, the edge detection circuit 2 includes a matrix generation unit 201, a filter processing unit 202, and an edge detection unit 203. As shown in FIG. 3, the matrix generation unit 203 forms a matrix of 5 × 5 pixels by using four run memories 210 to 213 and five flip-flops 220 for each line, that is, a total of 25 flip-flops 220.
[0017]
The filter processing unit 202 performs four types of first-order differential filter processing on each of the 5 × 5 pixel data from the matrix generation unit 201 as shown in FIGS. The edge detection unit 203 converts the four data processed by the filter processing unit 202 into absolute values, and obtains the maximum value. Then, the edge amount is quaternized by three preset threshold values = thO, thl, and th2 to obtain a 2-bit edge signal. When the edge signal is 0, the edge is not an edge. When the edge signal is 1, the edge amount is small. When the edge signal is 2, the edge amount is medium. When the edge signal is 3, the edge amount is large. Here, the edge amount is set to 2 bits, but may be further divided into multiple bits. Various methods other than the above-described methods are conceivable as the edge detection method, and the present invention is not limited to these methods.
[0018]
Next, the thinning processing circuit 3 will be described. In the thinning processing, detection is performed by the following determination method. As shown in FIG. 5, a 3 × 3 matrix is formed for the target pixel x. The formation of the matrix may be performed in the same manner as in the case of the edge detection circuit 203 described above, and a description thereof will be omitted. In the main scanning direction, thinning processing is performed when the relationship between the left and right pixel data Dr and Dl of the target pixel x and the threshold values ThA and ThB is established as follows.
[0019]
(Main scanning direction)
Dr ≧ ThA and Dl ≦ ThB (1)
Or, Dr ≦ ThB and Dl ≧ ThA (2)
Equation (1) is illustrated in FIG.
[0020]
Similarly, in the sub-scanning direction, the thinning process is performed when the relationship between the pixel data Du and Dd above and below the target pixel X and the threshold values ThA and ThB is established as follows.
[0021]
(Sub scanning direction)
Du ≧ ThA and Dd ≦ ThB (3)
Or Du ≦ ThB and Dd ≧ ThA (4)
For the target pixel for which any of the conditional expressions (1) to (4) is satisfied, the thinning level is determined based on the output from the original background density detection circuit 1 and the edge amount from the edge detection circuit 2. When the output from the document background density detection circuit 1 is a positive document and the edge amount from the edge detection circuit 2 is large (3 for a 2-bit signal), the thinning level of black characters and black lines is increased. Then, the target pixel is divided by 4 to make the original data 25%.
[0022]
When the edge amount is medium (2 for a 2-bit signal), the target pixel is divided by 2 to make the original data 50%. When the edge amount is small (1 for a 2-bit signal), the target pixel is divided by 4/3 to reduce the thinning level, and the original data is set to 75%. When it is determined that the edge is not an edge (0 in a two-bit signal), the pixel data of interest is output as it is in order to prevent thinning of black characters and black lines.
[0023]
Further, when the output from the document background density detection circuit 1 is a negative document, and when the edge amount from the edge detection circuit 2 is large, 128 pixels are added to the target pixel in order to increase the thinning level of white characters and white lines. Is added. When the edge amount is medium, 64 is added to the target pixel. When the edge amount is small, 32 is added to the year pixel. If it is determined that the pixel is not an edge, the pixel data of interest is output as it is in order to prevent white characters and white lines from being thinned (data handled here is multi-valued 8-bit data. 0 and black are 255). The above contents are summarized in FIG.
[0024]
Next, a specific example will be described. First, calculation is performed for data of 5 × 5 pixels as shown in FIG. This data is a collection of data of almost vertical edges. The edge amount is calculated by the edge detection circuit 203. The result of taking the absolute value of each of the four filter processes is:
After vertical edge detection filter processing = 1980
After horizontal edge detection filter processing = 75
After oblique edge 1 detection filter processing = 1370
After oblique edge 2 detection filter processing = 1230
It becomes.
[0025]
The maximum value among these is 1980 after the vertical edge detection filter processing. In order to convert the data into 8-bit data (0 to 255), the result is divided by 10 to obtain 198. Here, assuming that the thresholds for quaternizing the edge amount are ThO = 64, Thl = 128, and Th2 = 192, the edge amount is 3, and the edge amount is large.
[0026]
Next, a process of determining whether or not to perform the thinning process is performed. Assuming that the thresholds are 200 for ThA and 50 for ThB, the conditional expression (1) holds for the data in FIG. Therefore, the thinning processing is performed on the target pixel value = 130. When the background density of the document is a positive document, the pixel value of interest (= 130) × 0.25 = 32.5 = 32 (rounded down to the decimal point). If the result of the original background density detection is a negative original, the target pixel (= 130) + 128 = 258 = 255 (255 or more is fixed to 255).
[0027]
Next, calculation is performed for data of 5 × 5 pixels as shown in FIG. This data is 2 × 2 dot data. The edge amount is calculated by the edge detection circuit 203. The result of taking the absolute value of each of the four filter processes is:
After vertical edge detection filter processing = 180
After horizontal edge detection filter processing = 180
After oblique edge 1 detection filter processing = 360
After oblique edge 2 detection filter processing = 0
It becomes. The maximum value among these is 360 after the oblique edge 1 detection filter processing. In addition, it is 36 when divided by 10 to convert it into 8-bit data (0 to 255). Here, assuming that the thresholds for quaternizing the edge amount are Th0 = 64, Thl = 128, and Th2 = 192, the edge amount is 0, and it is determined that the edge is not an edge.
[0028]
Next, it is determined whether thinning processing is to be performed or not. Assuming that the threshold value is 200 for ThA and 50 for ThB, the data in FIG. 8B satisfies the conditional expressions (1) and (4), but since the edge detection result is determined not to be an edge, the document Regardless of whether the background density is a positive document or a negative document, the target pixel 220 is output as it is.
[0029]
The image data processed by the image processing apparatus in this way is output to an image forming means (printer engine), where an image is formed on a recording sheet by the printer engine and printed out. As the printer engine, for example, one that forms an image by an electrophotographic process is used. It is to be noted that since this is a known technology of a printer engine for forming an image by an electrophotographic process, a detailed description is omitted here.
[0030]
【The invention's effect】
As described above, according to the present invention, black thinning processing is performed on black characters and black lines in the case of a positive original, and white thinning processing is performed on white characters and white lines in the case of a negative original. Since the thinning processing is performed, the optimum thinning processing is performed for both a positive document and a negative image, and an image processing apparatus and an image forming apparatus capable of outputting high-quality images can be provided.
[Brief description of the drawings]
FIG. 1 is a block diagram illustrating an embodiment of an image processing apparatus according to the present invention.
FIG. 2 is a block diagram illustrating a peripheral pixel edge detection circuit of FIG. 1 in detail;
FIG. 3 is a block diagram illustrating a matrix generating unit of FIG. 2 in detail;
FIG. 4 is an explanatory diagram showing in detail coefficients of a filter processing unit in FIG. 2;
FIG. 5 is an explanatory diagram showing the processing of the thinning processing circuit of FIG. 1;
FIG. 6 is an explanatory diagram showing the processing of FIG. 5 in detail;
FIG. 7 is an explanatory diagram collectively showing processing of the thinning processing circuit of FIG. 1;
FIG. 8 is an explanatory diagram showing an example of an input image.
[Explanation of symbols]
1 Document background density detection circuit 2 Edge detection circuit 3 Thinning processing circuit

Claims (4)

Document determining means for determining whether the input image data is a positive document or a negative document,
Edge detection means for detecting whether or not peripheral pixels around the pixel of interest of the input image data are edge regions;
When the determination result of the document determination unit is determined to be a positive document, black thinning processing is performed on the target pixel determined to be an edge region by the edge detection unit, and the pixel is determined to be a negative document. An image processing apparatus comprising: a thinning processing unit that performs white thinning processing in some cases. 2. The apparatus according to claim 1, wherein the edge detecting means further detects an edge amount, and the thinning processing means further changes a degree of thinning according to the edge amount detected by the edge detecting means. Image processing device. 3. The image processing apparatus according to claim 1, wherein the pixel data of interest is output as it is when the edge detection unit determines that the pixel is not an edge area. An image processing apparatus according to any one of claims 1 to 3,
Image forming means for forming an image on recording paper based on image data processed by the image processing apparatus;
An image forming apparatus comprising:

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