JP7167848B2 - Image processing apparatus, image forming apparatus, and image area determination method. - Google Patents

Description

The present invention relates to an image processing apparatus, an image forming apparatus, and an image area discrimination method.

In an image forming apparatus, a technique is known in which a predetermined screen is applied to form a halftone image with a density designated by a ratio of a colorant-applied area and a non-applied area. As screen processing, a halftone screen represented by dots and a line screen represented by lines extending in a predetermined direction are often used. On the other hand, when forming an image with sharp contours such as characters, the image quality is improved by performing processing for emphasizing the edges (contours).

When an image subjected to such screen processing is read by a reading device and the read image is further formed (copied, etc.), moire may occur due to the difference between the screen resolution and the reading resolution of the reading device. It is known that artificial unevenness such as Therefore, in the read image data, a smoothing process is performed on the portion other than the portion where the contour is emphasized. Since edge enhancement processing and smoothing processing are diametrically opposed processes, the image quality deteriorates unless the processing regions are accurately specified and separated. On the other hand, a technique for separating a character portion and a background portion with high accuracy (Patent Document 1) and a technique for appropriately reading a line screen are disclosed (Patent Document 2).

JP 2013-42415 A JP 2017-147485 A

However, it is currently difficult to identify and extract a character area within an area to which line screen application processing has been performed. Therefore, there is a problem that it is necessary to reliably identify and process an area to which the halftone screen application process and the area to which the line screen application process are applied, which are subjected to the same smoothing process.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an image processing apparatus, an image forming apparatus, and an image area determination method that can more appropriately specify a line screen application area.

In order to achieve the above object, the invention according to claim 1,
a distribution acquisition unit that determines a specified area in input data of an image in which each pixel value is represented by multiple gradations, and acquires a frequency distribution of the plurality of pixel values in the specified area;
a boundary setting unit that sets a boundary value for binarizing the pixel value based on the frequency distribution acquired by the distribution acquisition unit;
a binarization unit that generates binary image data by binarizing the plurality of pixel values in the designated area using the boundary value;
a first identifying unit that identifies a striped structure corresponding to line screen processing based on the binary image data;
with
The image processing apparatus is characterized in that the boundary setting unit sets the two boundary values based on a pixel value having a minimum frequency in the frequency distribution.

Further, the invention according to claim 2 is the image processing apparatus according to claim 1,
The first identification unit determines that the designated area is a line screen application area when the striped structure is identified from the binary image data obtained from at least one of the two boundary values. It is characterized by specifying.

Further, the invention according to claim 3 is the image processing apparatus according to claim 2,
The first identification unit identifies the striped structure based on an alternate appearance pattern of binary values in the binary image data.

Further, the invention according to claim 4 is the image processing apparatus according to any one of claims 1 to 3,
If there are no two pixel values with a minimum frequency in the frequency distribution, the boundary setting unit sets a value shifted up and down by a predetermined width from a predetermined representative value in the frequency distribution as the two boundaries. It is characterized by being set as a value.

Further, the invention according to claim 5 is the image processing apparatus according to claim 4,
The predetermined representative value is the average of the average value and the median value of each pixel value in the specified area,
The predetermined width is a value obtained by multiplying the difference between the maximum and minimum pixel values in the specified area by a coefficient indicating a predetermined ratio.

Further, the invention according to claim 6 is the image processing apparatus according to claim 5,
A storage unit for storing the coefficient is provided.

Further, the invention according to claim 7 is the image processing device according to claim 5 or 6,
The coefficient is a value obtained based on the boundary values when the two boundary values are obtained.

Further, the invention according to claim 8 is the image processing apparatus according to any one of claims 1 to 7,
the input data includes component data of a multi-tone image in which pixel values are determined for each of a plurality of color components;
The first identifying unit is characterized in that, when the striped structure of the designated area is identified by any of the component data, the designated area is identified as a line screen application area.

Further, the invention according to claim 9 is the image processing apparatus according to any one of claims 1 to 8,
a second specifying unit that specifies a character display portion in the specified area;
a first processing unit that performs a smoothing process on the designated area in which the striped structure is identified in the input data;
a second processing unit that performs an emphasis process for emphasizing an outline of a character in the specified area in which a character display portion is specified in the input data;
characterized by comprising

Further, the invention according to claim 10,
The image processing device according to any one of claims 1 to 9,
a forming operation unit that performs an image forming operation based on the input data;
An image forming apparatus comprising:

Further, the invention according to claim 11,
A distribution obtaining step of determining a designated area in the input data of an image in which each pixel value is expressed in multiple gradations and obtaining a frequency distribution of the plurality of pixel values in the designated area;
A boundary setting step of setting a boundary value for binarizing the pixel value based on the frequency distribution obtained in the distribution obtaining step;
a binarization step of generating binary image data by binarizing the plurality of pixel values in the designated area using the boundary value;
a first identifying step of identifying a striped structure corresponding to line screen processing based on the binary image data;
including
The image region determination method is characterized in that in the boundary setting step, two boundary values are set based on a pixel value having a minimum frequency in the frequency distribution.

ADVANTAGE OF THE INVENTION According to this invention, there exists an effect that the applicable area of a line screen can be specified more appropriately.

1 is a block diagram showing the configuration of an image forming apparatus according to an embodiment; FIG. It is a schematic diagram explaining the screen used for screen processing. FIG. 10 is a table showing a correspondence relationship between a determination result of an area and a process for the area; FIG. It is a figure explaining specification of the application area of a line screen. FIG. 10 is a diagram showing an example of a reading result of an image on which line screen application processing has been performed; FIG. 10 is a diagram showing an example of a read image of an area to which halftone screen application processing has been performed; FIG. 10 is a diagram showing another example of a read image of an area to which halftone screen application processing has been performed; FIG. 10 is a diagram showing an example of a binary image of an area to which halftone screen application processing has been performed; 5 is a flowchart showing procedures of boundary value setting processing and line screen determination processing;

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 the configuration of an image forming apparatus 1 of this embodiment. 1A is a block diagram showing the overall configuration of the image forming apparatus 1, and FIG. 1B is a block diagram showing the functional configuration of the image processing section 12. As shown in FIG.

As shown in FIG. 1A, the image forming apparatus 1 includes a communication section 11, an image processing section 12, a forming operation section 21, and the like.
The communication unit 11 transmits and receives data to and from an external device according to a predetermined communication standard (TCP/IP, etc.). The communication unit 11 has, for example, a network card or the like, and transmits and receives data via a LAN (Local Area Network). In addition, data transmitted and received by the communication unit 11 may include data via a dedicated line connecting to an external device, such as data using a USB cable. Communication without cables, such as communication, may be included. Here, the communication unit 11 receives image data read by a reading device (scanner) and sends the image data to the image processing unit 12 .

The image processing unit 12 is included in the image processing apparatus of this embodiment, and performs image processing for image formation based on input image data. The image processing unit 12 includes a control unit 121, a memory 122 (storage unit), and the like. The input image data is temporarily stored in the memory 122 and processed by the control section 121 as described later. The memory 122 may also store setting data, control programs, and the like.

The control unit 121 is a hardware processor including a hardware circuit that performs various types of image processing, a CPU (Central Processing Unit) that performs overall control of the overall operation of the image forming apparatus 1, and the like.

The forming operation unit 21 is an engine that performs an image forming operation such as applying and fixing coloring material according to image data to be formed. A specific image forming method of the forming operation unit 21 is not particularly limited, and may be, for example, an electrophotographic method or an inkjet method.

As shown in FIG. 1B, the image processing unit 12 includes a frequency distribution acquisition unit 151 (distribution acquisition unit, distribution acquisition step), a boundary value setting unit 152 (boundary setting unit, boundary setting step), and binarization. Processing unit 153 (binarization unit, binarization step), line screen discrimination unit 154 (first specification unit, first specification step), character discrimination unit 155 (second specification unit), halftone screen discrimination It has a unit 156, a formation data processing unit 157, and the like. The functional operation of each part will be detailed later. Each of these units may be, for example, a hardware circuit configured exclusively by an ASIC (Application Specific Integrated Circuit).

Next, image input data (input image data) will be described. Image data read by an external reading device (scanner) has a large number of pixels arranged according to the resolution of the reading device, and each pixel has a predetermined multi-gradation (three or more gradations, here, for example, 256 levels). When pixel values (gradation values, color images, etc.) are obtained with a plurality of color components such as RGB, the gradation value is determined for each color component, and multi-gradation component data is obtained for each. It is bitmap (pixmap) data expressed in On the other hand, the image to be read contains halftones expressed by processing (screen processing) that applies a screen according to the recording resolution and density gradation of the image forming apparatus that formed the image. good.

FIG. 2 is a schematic diagram illustrating a screen used for screen processing. FIG. 2(a) is an enlarged view of an image portion processed by halftone screen processing, and gradation expression is performed according to the distribution density and/or size of fine dots (points). FIG. 2(b) is an enlarged view of an image portion processed by line screen processing, and gradation expression is performed according to the density and/or width of lines extending in a predetermined direction. That is, in the area of the image subjected to the line screening process, a fine striped structure in which a large number of lines are arranged in parallel at equal intervals is generated. For example, in multi-tone images such as patterns, backgrounds, drawings, and photographic images, these screens are selected according to the situation to express the halftones. Display of characters, figures such as circles and polygons, signs such as table frames, lines, and arrows (collectively referred to as characters, etc.). An image is formed by solidly filling the inside of the outline such as .

Next, the detection operation of each area in the read data of the image subjected to each screen processing will be described.
Bitmap data obtained by digitally capturing an image that has undergone screen processing and reading it using an imaging unit of a reading device may cause artificial unevenness in the read image, especially when the resolution of the screen and the reading resolution are different. , Granularity is conspicuous. Therefore, image processing (smoothing processing) is performed to smooth each pixel value (gradation value) by applying smoothing to the read data. On the other hand, if smoothing is applied to characters, graphics, etc., the contours become blurred and unclear, so image processing (contour enhancement processing) is performed to detect and emphasize these contours. In order to carry out these image processes, character areas (which may include the contours of figures) are determined on the read image data, and image areas (screen processing area, screen application area) subjected to each screen process are identified. Judgment processing is performed. These treatments may be performed independently. These discrimination processes are performed by setting a minute discrimination region (designated area) according to the size that can be visually recognized by humans (here, for example, 37×37 pixels. Depending on the image resolution and/or the line density of the line screen, etc. 10 times the line interval, etc.), while moving the specified area within the read image.

Here, when a character (including graphics and signs) area is included in the area subjected to line screen processing (application area of line screen), a parallel linear structure is generated in any case. Currently, it may be difficult to extract a character region from the application area of the line screen.

FIG. 3 is a chart showing an example of the correspondence relationship between the area determination result and the process for the area.
A screen processing area determined to have undergone screen processing (halftone screen processing or line screen processing) is subjected to smoothing processing regardless of the result of character discrimination. In a character area that is determined not to be a screen processing area and is determined to be a character area, the contour of a character or the like is specified and contour enhancement processing is performed on the contour. If it is determined that the area is neither of these areas, neither the smoothing process nor the contour enhancement process is performed.

Screen type discrimination is performed differently for halftone screens and line screens. As shown in FIG. 4, in an applied image of a line screen with an inter-line spacing dp (which is also the total width of a line and its adjacent background color portion) (collectively referred to as a line pair), When pixel values are obtained in order along the direction perpendicular to each line (the direction of the arrow), binary values (for example, “0” and “1”) are generated at predetermined intervals (that is, at intervals within a predetermined width). dp) appear alternately (binary alternate appearance pattern). By counting the number of such replacements in each direction (for example, 16 directions at intervals of 22.5 degrees) with a certain point as a reference and taking the distribution thereof, a striped structure (repetition of line pairs) is specified. , it is determined whether or not the area has undergone line screen processing. However, as described above, in the read image, read image data that is the same as the gradation value of each pixel in the original image is not necessarily obtained according to the read resolution or the like.

FIG. 5 is a diagram showing an example of the result of reading an image that has undergone line screen processing. FIG. 5A shows an enlarged reading result of a reference pattern image having vertical black lines against a background color portion (here, white) at 153 lpi (lines per inch) at 256 gradations. there is As shown in this figure, it can be seen that intermediate tone pixels are arranged between the low tone line and the high tone line. These pixel values are almost the same in the vertical direction (direction along the black line). FIG. 5B shows the frequency distribution (histogram) of the gradation values of each pixel of this read image. When an image that has been subjected to such line screen processing is read, many intermediate gradation pixels appear between high gradation pixels and low gradation pixels. In the frequency distribution of the read image of the image subjected to the line screen processing, there are portions (frequency minimum values) where the frequency is minimal in the gradation values between the intermediate gradation and the high gradation and the low gradation. It has been found by the inventor to have the feature of appearing (two in total).

In the read image data having such a frequency distribution, a boundary value (threshold) is simply a representative value such as the average value or the median value of all pixel values or pixel values in a sample range, which is intermediate between high gradation and low gradation. When each pixel is binarized, a large amount of halftone data is irregularly separated into a high gradation side and a low gradation side, and the fine pattern, that is, each line, disappears due to the line screening process. When determining whether or not the image (area) has undergone line screen processing, these two frequency minimum values are specified based on the frequency distribution, and the two frequency minimum values are binarized. By binarizing each as the boundary value of , a binary image in which the striped structure corresponding to the line screen processing is maintained is obtained. By using this binary image to detect the striped structure (repetitive structure of line pairs, which are sets of lines of high gradation value and lines of low gradation value) as described above, the line screen processing area is It becomes possible to determine whether or not Note that the density of the binary image in this case may be higher or lower than the density gradation of the original image, but this binary image is only used to determine whether or not line screen processing has been performed. No problems arise because the

That is, in the image area determination method of the present embodiment, the frequency distribution of pixel values is acquired for each designated area of multi-tone image data (processing of the frequency distribution acquisition unit 151 shown in FIG. 1B), and the frequency A boundary value for binarizing the pixel values in the specified area is determined based on the distribution (processing of the boundary value setting unit 152 shown in FIG. 1B). Each pixel value in the designated area is binarized by these boundary values to generate binary image data (processing of the binarization processing unit shown in FIG. 1(b)), and the generated binary image data is By specifying the striped structure caused by (according to) the line screen processing, it is determined whether or not the specified area has been subjected to the line screen processing (the line screen determination unit 154 shown in FIG. 1B). processing). The frequency distribution acquisition unit 151 may divide between the minimum value “0” and the maximum value (for example, “256”) of the pixel value by an appropriate interval (for example, “20”) that can identify the minimum value. .

Separately from the above processing, processing by a character discrimination unit 155 that discriminates the display portion of characters (including signs and graphics), and a halftone screen discrimination unit 156 that discriminates a region to which a halftone screen is applied. Processing is performed on the pixel data in the designated area or in the set area appropriate for each process in the read image data.

Based on these determination results, the smoothing processing unit 157a (first processing unit) of the formation data processing unit 157 performs smoothing processing on the corresponding regions, and the contour processing unit 157b (second processing unit) performs smoothing processing. performs edge enhancement processing. The formation data processing unit 157 may also perform various processing necessary for image formation, such as halftone processing and/or color tone correction processing.

In the case where an area that has not been subjected to line screen processing is a designated area, the line screen determination unit 154 normally determines that the frequency distribution of pixel values acquired by the frequency distribution acquisition unit 151 has two minimum frequency values. )do not do. In this case, the two boundary values may be values that are vertically shifted by a predetermined width with respect to a predetermined central value (representative value). Here, the median value may be the average (A+M)/2 of the average value A and the median value M of a plurality of pixel values in the designated area. The predetermined width can be a value K·W obtained by multiplying a coefficient K (K is an appropriate value less than 1) indicating a predetermined ratio with respect to the total width W indicated by the difference between the maximum value and the minimum value of the frequency distribution. . That is, the boundary value is (A+M)/2±K·W. The coefficient K is obtained in advance and stored in the memory 122 . The coefficient K is specified based on the gradation value corresponding to the two minimum frequency values specified from the frequency distribution as described above and the representative value in the frequency distribution. This value may be calculated in advance and held as a fixed value, or updated by weighted averaging with the conventional value each time two boundary values (frequency minimum values) are specified by the boundary value setting unit 152. may be

On the other hand, in the area to which the halftone screen is applied, detection of the dot is determined by, for example, whether or not there is a low tone value area similar to the dot in each direction around the dot in the same manner as in the conventional art. obtain. However, as described above, in the read image data, a plurality of dots arranged in a predetermined direction may appear to be connected and read depending on the arrangement density and/or the arrangement direction of each dot in halftone screen processing. .

FIG. 6 is a diagram showing an example of a read image of an area subjected to halftone screen processing. FIG. 6(a) is an enlarged view of an area including a plurality of dots of the read image at the same magnification as FIG. 5(a), and FIG. 6(b) shows the frequency distribution of the gradation value of each pixel. It is a diagram.
In the diagram shown in FIG. 6A, high (bright) gradation areas spread around a dot portion with a low gradation value. When the frequency distribution of gradation values is obtained in such a halftone screen application area, as shown in FIG. It has been found by the inventors of the present invention. That is, in the halftone screen application area, there is a tendency for gradation values to vary around dots.

FIG. 7 is a diagram showing another example of a read image of an area subjected to halftone screen processing.
Depending on the reading situation, even in areas that have undergone halftone screen processing, there appears to be a tendency for multiple dots to appear to be connected in a predetermined direction, here from the upper left to the lower right (the direction of the arrow). Sometimes. Even in this case, as in the case shown in FIG. 6B, two minimum frequency values do not appear in the frequency distribution of the gradation values.

FIG. 8 is a diagram obtained by binarizing the area to which the halftone screen application process shown in FIG. 7 has been performed. White squares and black squares indicate binary values. FIG. 8(a) is a diagram showing an example of a binary image in the case of binarization using a representative value (e.g., average value) as in the conventional art. FIG. 8B is a diagram showing an example of a binary image obtained by binarizing with reference to the boundary value on the higher gradation side of the two boundary values calculated for the area subjected to the halftone screen processing described above. is.

As shown in FIG. 8(a), if this diagram is binarized in the conventional manner, pixels with high gradation values (white portions) and pixels with low gradation values (black portions) appear to appear in the direction along the arrow. ) extend linearly, and high gradation value portions and low gradation value portions appear alternately in the direction perpendicular to the arrow. Therefore, such an area may be erroneously determined to be a line screen application area in the conventional determination process. As described above, it is difficult to identify and extract a character area within the line screen application area, and the character area is also subjected to smoothing processing when it is erroneously determined to be the line screen application area. As a result, if it is erroneously determined to be the application area of the line screen, it becomes difficult to appropriately perform edge enhancement processing on the character area. A desire to discriminate arises.

As shown in FIG. 8(b), binarization based on the boundary value on the higher gradation side of the two boundary values calculated for the area subjected to the above halftone screen processing results in an apparent striped structure. disappears. Even when the boundary value on the low gradation side is used as a reference, the striped structure similarly disappears. Therefore, it is possible to suppress misidentification of a region subjected to halftone screen processing as a line screen application area.

FIG. 9 is a flow chart showing the procedure of the boundary value setting process and the line screen determination process executed by the image processing section 12. As shown in FIG.
The boundary value setting process shown in FIG. 9A is started when the frequency distribution of the designated area is obtained by the frequency distribution obtaining unit 151 and data of the frequency distribution is input to the boundary value setting unit 152 . As described above, the specified area is sequentially specified while changing the position in the input image data, so the process may be repeatedly executed according to the specification.

When the frequency distribution is acquired (step S201), the boundary value setting unit 152 determines whether or not there are two minimum values of frequency (frequency) in the frequency distribution (step S202). If it is determined that there are two minimum values ("YES" in step S202), the boundary value setting unit 152 determines pixel values corresponding to the two minimum values as two boundary values (step S203). ). Then, the boundary value setting process ends.

If it is determined that there are no two minimum values ("NO" in step S202), the boundary value setting unit 152 approximately calculates the median value M and the average value A based on the frequency distribution (step S213). Note that the median value M and the average value A may be calculated from the original input image data without using the frequency distribution.

The boundary value setting unit 152 calculates the full width W, which is the difference between the maximum and minimum pixel values (step S214). The maximum and minimum pixel values may be approximately obtained based on the frequency distribution, or may be specified from the original input image data. Two boundary values D=(M+A)/2±K·W for binarization are calculated and set from the determined median value M, average value A, full width W, and predetermined coefficient K (step S215). Then, the boundary value setting process ends.

The line screen determination process shown in FIG. 9B includes two types of input image data (specified area data) is input to the line screen determination unit 154. FIG.

The line screen determination unit 154 acquires the two input binarized data (step S301). As described above, in each binarized data, by determining whether or not an array in which gradation lines appear alternately exists a predetermined number of times or more along a line segment extending in a predetermined direction in the binarized data, A binary array (referred to as a line screen pattern) corresponding to the striped structure by line screen processing is detected (step S302).

The line screen determination unit 154 determines whether or not it is determined that a line screen pattern has been detected in any of the binarized data (step S303). If it is determined that a line screen pattern has been detected in at least one of them ("YES" in step S303), the line screen determination unit 154 determines that the specified area is the line screen application area (step S304). Then, the line screen discrimination process ends.

If it is determined that the line screen pattern is not detected in any of the binarized data ("NO" in step S303), the line screen determination unit 154 determines that the designated area is not the line screen application area. (step S314). Then, the line screen discrimination process ends.

In addition, when each component data of a plurality of color components is held as multi-tone image data (data of designated area), for two or more of the designated areas of each component data, If the line screen determination unit 154 identifies a striped structure in any of the color components, it may be determined to be a line screen applicable area (that is, logical OR).

As described above, the image processing unit 12 of the present embodiment defines a designated area in the input data of an image in which each pixel value is represented by multiple gradations (for example, 256 gradations), and a plurality of pixels in the designated area. A frequency distribution acquisition unit 151 that acquires the frequency distribution of pixel values; a boundary value setting unit 152 that sets a boundary value for binarizing pixel values based on the frequency distribution acquired by the frequency distribution acquisition unit 151; A binarization processing unit 153 that generates binary image data by binarizing a plurality of pixel values in an area using boundary values, and a striped structure that corresponds to line screen processing is specified based on the binary image data. and a line screen determination unit 154 . The boundary value setting unit 152 sets two boundary values based on a pixel value (minimum frequency value) having a minimum frequency in the frequency distribution.
In this way, based on the characteristics of the frequency distribution in the line screen application image discovered by the present inventor, the binarization determination is performed using the gradation value corresponding to the minimum value of the frequency in the frequency distribution as the boundary value. Therefore, the intermediate gradation values in the read image are not divided into two sides of the binary, resulting in an incomplete binary image, and one of the binary values is reflected more appropriately. As a result, in the image to which screen processing has been applied, a binary image that more faithfully reflects the screen pattern of the screen-applied portion can be obtained. Therefore, the striped structure (line pairs) in the area to which the line screen is applied is more reliably detected, while the apparent striped structure due to the reading of the area to which the halftone screen is applied is more reliably eliminated. becomes possible. Therefore, it is possible to more reliably specify the area to which the line screen is applied, thereby suppressing omission of specifying the character area outside the line screen application area.

Further, the line screen determination unit 154 determines that the designated area is the line screen application area when a striped structure is identified from the binary image data obtained from at least one of the two boundary values. . This makes it possible to more appropriately eliminate detection omissions in the application area of the line screen.

Also, the line screen determination unit 154 identifies a striped structure (repetition of line pairs) based on the alternate appearance pattern of binary values in the binary image data. In the case of line screens, binary values appear periodically and alternately in the direction perpendicular to the direction in which the lines are extended. can. Also, as described above, by appropriately setting the boundary value, it is possible to more reliably exclude an apparent striped structure even with such a simple process.

In addition, when there are not two gradation values (minimum frequency values) at which the frequency distribution is minimum, the boundary value setting unit 152 shifts a predetermined representative value in the frequency distribution up and down by a predetermined width. Set the values as two boundary values. In other words, instead of using the representative value directly as the boundary value, a value assumed to be appropriate for identifying the applicable area of the line screen is set as the boundary value, thereby eliminating the apparent striped structure and It is possible to more reliably determine that it is not the applicable area of the screen.

Further, the predetermined representative value is the average of the average value A and the median value M of each pixel value in the designated area, and the predetermined width is the predetermined width W which is the difference between the maximum value and the minimum value of the pixel values in the designated area. is a value multiplied by a coefficient K that indicates the ratio of As a result, it is possible to reduce the influence of the background color and the color of the coloring material to be applied, determine a more appropriate value as the boundary value, and more reliably determine that the area is not the line screen application area.

The image processing unit 12 also includes a memory 122 that stores the coefficient K. FIG. By storing an appropriate coefficient K in advance, it is possible to stably obtain an appropriate boundary value without calculating or estimating from data for a small designated area each time.

Also, the coefficient K is a value obtained based on two boundary values obtained when two boundary values are obtained. That is, the coefficient K is determined so that two boundary values are determined based on the boundary values actually specified in the line screen application area, so that the apparent striped structure can be more stably removed from the line screen. It can be determined that it is not the application area of

The input image data includes component data of a multi-gradation image in which pixel values are determined for each of a plurality of color components such as RGB. When the striped structure is identified, the designated area is identified as the line screen application area. Depending on the image, not all color components form component images with sufficient densities. can be identified as a line screen application area as long as a striped structure is identified in .

Further, a character discrimination unit 155 for specifying a character display portion in the specified area, a smoothing processing unit 157a for performing smoothing processing on the specified area in which the striped structure is specified in the input image data, and an input image and a contour processing unit 157b that performs an emphasis process for emphasizing the contour of a character in a specified area in which a character display portion is specified in the data.
In other words, opposite processing such as smoothing processing and contour enhancement processing are performed on the screen application area and the character display portion. , it is possible to more appropriately define the region where smoothing and edge enhancement are performed.

Further, the image forming apparatus 1 of this embodiment includes the image processing section 12 described above and a forming operation section 21 that performs an image forming operation based on input image data. In this image forming apparatus 1, the screen application area in the screen processing application image read by a reading device such as a scanner is more appropriately discriminated and smoothed, and on the other hand, the outline of the display portion such as characters is more reliably emphasized. Therefore, it is possible to output a high-quality image by copying and outputting a scanned image.

Further, in the image area determination method of the present embodiment, a specified area is determined in input data (input image data) of an image in which each pixel value is represented by multiple gradations, and the frequency distribution of a plurality of pixel values in the specified area is determined. , a boundary setting step of setting a boundary value for binarizing the pixel values based on the frequency distribution obtained in the distribution obtaining step, a plurality of pixel values in the specified area are binarized by the boundary value a binarization step of generating converted binary image data; and a first identification step of identifying striped structures corresponding to the line screen processing based on the binary image data. In the boundary setting step, two boundary values are set based on the pixel value with the minimum frequency in the frequency distribution.
In this way, based on the characteristics of the frequency distribution in the line screen application image discovered by the present inventor, the binarization determination is performed using the gradation value corresponding to the minimum value of the frequency in the frequency distribution as the boundary value. Therefore, the intermediate gradation values in the read image are not divided into two sides of the binary, resulting in an incomplete binary image, and one of the binary values is reflected more appropriately. As a result, in the image to which screen processing has been applied, a binary image that more faithfully reflects the screen pattern of the screen-applied portion can be obtained. Therefore, the striped structure (line pairs) in the area to which the line screen is applied is more reliably detected, while the apparent striped structure due to the reading of the area to which the halftone screen is applied is more reliably eliminated. becomes possible. Therefore, the area to which the line screen is applied can be identified more reliably, and this image area determination method can suppress omission of character area identification outside the line screen application area.

It should be noted that the present invention is not limited to the above embodiments, and various modifications are possible.
For example, the binarization processing of the designated area and the line screen discrimination processing based on the two binarization boundary values may be performed in parallel, or the binarization processing and line screen discrimination processing based on one of the boundary values may be performed. The other binarization processing and line screen determination processing using the boundary value may be performed only when the processing is performed and the area is not determined to be a line screen applicable area.

Further, in the above embodiment, if a striped structure is detected in one of two binary images based on two boundary values, it is determined to be a line screen applicable area. Alternatively, the other result may be determined by adding appropriate additional conditions.

Moreover, the method for detecting the striped structure is not limited to that shown in the above embodiment. For example, each line may be identified and then determined based on the regularity of its direction and spacing.

Further, in the above embodiment, the average value of the average value A and the median value M of the gradation values is used as the representative value of the specified area, but the present invention is not limited to this. The average value A or median value M may be used as is, or other values that specify the vicinity of the center of all pixel values may be used.

In the above embodiment, the representative value is shifted by the same width vertically, but if the distribution differs between the upper and lower sides, different predetermined widths may be set for the upper and lower sides. Also, the method of determining the predetermined width is not limited to the above-described method, and for example, the variance, skewness, and kurtosis of the distribution may be taken into consideration.

Further, in the above-described embodiment, an RGB color image has been described as an example, but the present invention is not limited to this. It may be a monochrome grayscale image, or image data represented by another color system. When the index is represented by another color system, the above processing may be performed for the index showing a difference in gradation depending on the screen according to the color system.

Further, in the above embodiment, only the line screen and the halftone screen have been described, but it is also possible to distinguish the screen of the image using a screen other than the halftone screen.

In the above embodiment, two minimum frequency values are specified or one or less frequency minimum values are specified. In that case, two may be selected in descending order of frequency, or two may be selected based on the magnitude of the difference from the most recent maximum value. Alternatively, the size of the designated area and/or the frequency width of the frequency distribution may be changed.

Further, although the image processing unit 12 included in the image forming apparatus 1 has been described in the above embodiment, the image processing unit 12 may be a computer and may be a device separate from the image forming apparatus 1 . In this case, the processed data of the image processing section 12 may be sent to the image forming apparatus 1 . Further, instead of image processing for image formation, image processing for display on a display or the like may be performed.

Further, in the above embodiment, the input image data is obtained from the outside, but the image forming apparatus 1 may incorporate a reading device.

Further, in the above embodiments, each process is described as being performed by a dedicated logic circuit such as an ASIC, but it may be performed by a CPU (hardware processor) in the form of software based on a program.
In addition, specific details such as the configuration, processing content, and processing procedure shown in the above embodiments can be changed as appropriate without departing from the scope of the present invention.

1 image forming apparatus 11 communication unit 12 image processing unit 121 control unit 122 memory 21 forming operation unit 151 frequency distribution acquisition unit 152 boundary value setting unit 153 binarization processing unit 154 line screen determination unit 155 character determination unit 156 halftone screen determination Unit 157 Formation data processing unit 157a Smoothing processing unit 157b Contour processing unit D Boundary value K Coefficient

Claims (11)

a distribution acquisition unit that determines a specified area in input data of an image in which each pixel value is represented by multiple gradations, and acquires a frequency distribution of the plurality of pixel values in the specified area;
a boundary setting unit that sets a boundary value for binarizing the pixel value based on the frequency distribution acquired by the distribution acquisition unit;
a binarization unit that generates binary image data by binarizing the plurality of pixel values in the designated area using the boundary value;
a first identifying unit that identifies a striped structure corresponding to line screen processing based on the binary image data;
with
The image processing apparatus, wherein the boundary setting unit sets the two boundary values based on a pixel value having a minimum frequency in the frequency distribution. The first identification unit determines that the designated area is a line screen application area when the striped structure is identified from the binary image data obtained from at least one of the two boundary values. 2. The image processing device according to claim 1, wherein the image processing device is specified. 3. The image processing apparatus according to claim 2, wherein the first identification unit identifies the striped structure based on an alternate appearance pattern of binary values in the binary image data. If there are no two pixel values with a minimum frequency in the frequency distribution, the boundary setting unit sets a value shifted up and down by a predetermined width from a predetermined representative value in the frequency distribution as the two boundaries. 4. The image processing apparatus according to any one of claims 1 to 3, which is set as a value. The predetermined representative value is the average of the average value and the median value of each pixel value in the specified area,
5. The image processing apparatus according to claim 4, wherein the predetermined width is a value obtained by multiplying the difference between the maximum and minimum pixel values in the designated area by a coefficient indicating a predetermined ratio. 6. The image processing apparatus according to claim 5, further comprising a storage unit for storing said coefficients. 7. The image processing apparatus according to claim 5, wherein said coefficient is a value obtained based on said boundary values when said two boundary values are obtained. the input data includes component data of a multi-tone image in which pixel values are determined for each of a plurality of color components;
3. The first identification unit identifies the designated area as a line screen application area when the striped structure of the designated area is identified by any of the component data. 8. The image processing device according to any one of 1 to 7. a second specifying unit that specifies a character display portion in the specified area;
a first processing unit that performs a smoothing process on the designated area in which the striped structure is identified in the input data;
a second processing unit that performs an emphasis process for emphasizing an outline of a character in the specified area in which a character display portion is specified in the input data;
The image processing apparatus according to any one of claims 1 to 8, comprising: The image processing device according to any one of claims 1 to 9,
a forming operation unit that performs an image forming operation based on the input data;
An image forming apparatus comprising: A distribution obtaining step of determining a designated area in the input data of an image in which each pixel value is expressed in multiple gradations and obtaining a frequency distribution of the plurality of pixel values in the designated area;
A boundary setting step of setting a boundary value for binarizing the pixel value based on the frequency distribution obtained in the distribution obtaining step;
a binarization step of generating binary image data by binarizing the plurality of pixel values in the designated area using the boundary value;
a first identifying step of identifying a striped structure corresponding to line screen processing based on the binary image data;
including
The image region determination method, wherein, in the boundary setting step, two boundary values are set based on a pixel value having a minimum frequency in the frequency distribution.

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