JP4545766B2 - Image processing apparatus, image forming apparatus, image reading apparatus, image processing program, and recording medium - Google Patents

Description

  The present invention relates to an image processing apparatus including a region separation processing unit that performs region separation processing for separating input image data into a plurality of regions including at least a character region.

  With the advancement of digital image processing technology, color images can be reproduced with high image quality in image forming apparatuses such as electrophotographic processes and inkjet copying machines and printers. The machine has been commercialized.

  Document images copied using these image forming apparatuses include characters, halftone photographs, line drawings, photographs (continuous tone areas such as photographic paper photographs), or originals in which they are mixed. In order to obtain a reproducible image, it is necessary to perform image processing suitable for the configuration type of each document.

  In such a situation, in order to perform optimum processing according to the type of composition of the original, information on the mask centered on the pixel of interest is used to create a character area / dot area / other area (character area / network). There is an area separation process in which an original image is separated into an area that is not divided into point areas (for example, a photographic area, a background area, etc.). In general, image data read from an image input device such as a scanner is divided into blocks composed of a plurality of pixels, and is either a character area, halftone dot area, or other area for each pixel or block unit. Identified. Image processing is performed based on the identification result. In particular, for black character regions, it is important to improve the output image quality by emphasizing resolution and reproducing in black.

  For example, there is a method disclosed in Patent Document 1 for the region separation processing. In this method, attention is paid to properties such as the large area of the base, connectivity, and uniformity of density. Then, based on the density information of each pixel constituting the image, the background density area determination unit determines, as the background area, an area having a density equivalent to that of the background area or a background with a uniform color. Thereafter, based on the determination result of the background area and the non-background area, an area having a predetermined size or less in the non-background area is identified as a character area and an area having a predetermined size or more as a gradation image area. As a result, a character area surrounded by the base area can be detected.

In addition to the above region separation processing, Patent Document 2 provides background color edge detection means for detecting characters on the background color, detects the presence or absence of the background color for the pixel of interest, and has a background color. A method of performing resolution priority processing by performing processing similar to that of a character edge in the case of an edge region is proposed. In addition, it is described that the determination of the presence or absence of the background color is performed by threshold processing, and the determination accuracy is improved by changing the threshold. Further, a device for suppressing erroneous determination by controlling the amount of edge detection when it is determined that a background color exists is shown.
Japanese Patent Laid-Open No. 2-199588 (published on August 7, 1990) Patent No. 3725255 (issued on December 7, 2005)

  However, in the technique of Patent Document 1, when the background density is higher than a predetermined density, it is determined as a picture area, and an edge in the picture area is not erroneously detected as a character area. For this reason, it is impossible to detect characters on the background having a somewhat high density. If normal characters surrounded by a background with a certain high density cannot be detected, resolution priority processing that improves the reproducibility of the characters is not performed. Furthermore, in general, halftone processing using a plurality of pixels is performed in order to improve gradation, and there is a possibility that the output image quality of characters surrounded by the background is further deteriorated. Therefore, there is a problem that the sharpness of the characters in the background area having a relatively high density is inferior, and black characters are output as blurred characters.

  On the other hand, the technique of Patent Document 2 is a method of detecting characters on a background with a certain level of density, and performs character detection by controlling a threshold value for edge detection with respect to the background. The effect of the color background is not taken into consideration. There is a problem that the color determination accuracy for black characters surrounded by the color background is deteriorated due to the influence of the surrounding color background. This is because, even when a black character portion is read, the signal input as a black signal that is originally balanced is affected by the diffusely reflected external light of the surrounding color base, which is affected by being input with a slight tint. . If the achromatic color determination is performed using this colored signal, it may be erroneously determined as a color character (non-black character) different from the result to be determined originally. For example, if a yellow color base is present around a black character, even if the black character is read, the black character may be read with a yellowish tint containing a yellow component due to the influence of the surrounding yellow. If the black character area is mistakenly recognized as a color character, the amount of black generated will be reduced by the optimization processing for the color character, and at the same time, it may be reproduced as a character with enhanced bleeding due to the emphasis processing. is there. Also, in the method based on the control of the edge detection amount, the detection amount of characters on the normal white background and the detection amount of characters on the color background are different, and the detection amount of characters on the color background is reduced, and as a whole It may not improve the accuracy when viewed.

  As described above, the conventional detection method has problems that the character on the background having a certain density is not detected, or that the black image is erroneously determined as a color character and the output image quality is deteriorated.

  SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and the object thereof is to output a black image surrounded by a color background having a certain density, with a clear image quality with reduced blurring. It is to realize an image processing apparatus capable of performing the above.

  In order to solve the above problems, an image processing apparatus according to the present invention is an image processing apparatus including an area separation processing unit that performs an area separation process for separating input image data into a plurality of areas including at least a character area. A color background determination unit that determines whether or not a color background exists in the input image data; and a background that removes the color background from the input image data when the color background determination unit determines that the color background exists. A removal unit, and the region separation processing unit performs a region separation process on the input image data from which the color background is removed by the background removal unit.

  As described above, when a black character is read when a color background is used in the document, the original is read with some color influenced by the color components of the surrounding color background. Then, the chromatic / achromatic determination accuracy is deteriorated due to unnecessary color components. As a result, if the black character in the color background cannot be determined as a black character, an incorrect image quality optimization process is performed and it cannot be reproduced as a clear black single character.

  However, according to the above configuration, it is determined in advance whether or not a color background exists in the input image data. If a color background exists, a process for removing the color background is performed, and a region separation process is performed on the input image data from which the color background has been removed. As a result, even if there is a black character surrounded by the color background, the black character is not affected by the color background. As a result, the black character determination accuracy in the region separation processing unit is improved, and the black character reproduction performance can be improved. That is, it is possible to realize an image processing apparatus that can output even a black character surrounded by a color background having a certain density, with a clear image quality with reduced blurring.

  Further, in the image processing apparatus of the present invention, in addition to the above configuration, the background removal unit corrects input image data so that the color component of the color background becomes an achromatic color, and then corrects it to an achromatic color. It is preferable to remove the colored background.

  The conventional background removal processing is realized by calculating the density to be the background using a histogram or the like and then correcting the density of the area lighter than the background density to be the white density. However, with simple background removal processing, removal processing is applied only to the background portion, and the influence of the surrounding color background cannot be corrected. Therefore, the determination accuracy of the region separation process cannot be improved.

  However, according to the above configuration, after correcting the input image data so that the color component of the color background becomes an achromatic color, by removing the color background corrected to an achromatic color, It becomes possible to remove the influence of the located color background. As a result, it is possible to improve the determination accuracy of the achromatic color region determination process of the region separation processing unit.

  The image processing apparatus of the present invention is an image processing apparatus including an area separation processing unit that performs an area separation process for separating input image data into a plurality of areas including at least a character area, and the area separation processing unit includes: And a chromatic / achromatic determining unit that determines whether the character is chromatic or achromatic by comparing a maximum difference between a plurality of color components and a predetermined threshold value. A color background determination unit that determines whether or not a color background exists, and a control unit that increases the predetermined threshold when the color background determination unit determines that a color background exists. .

  Here, the maximum difference between the plurality of color components is large when the color component is chromatic. Therefore, a chromatic / achromatic determination unit determines that the color is chromatic when the maximum difference is larger than a predetermined threshold, and achromatic when the maximum difference is smaller.

  According to the above configuration, when the color background determination unit determines that a color background exists, the predetermined threshold that is a chromatic / achromatic determination criterion in the chromatic / achromatic determination unit is increased. Thus, when there is a color background, even if there is an effect of color blur on the black character, the control is performed so that the predetermined threshold value is increased. Can be determined. That is, even for a black character surrounded by a color base, it is possible to increase the accuracy of determining the region separation for the black character. As a result, black characters can be clearly expressed and can be expressed in a state without color blur.

  Further, since the color background is not removed, the color background can be output in the original color.

Furthermore, in the image processing apparatus of the present invention, the color ground determination unit uses a density histogram of each of a plurality of color components, and for each color component,
Condition 1: The lowest concentration peak position Condition 2: The ratio of the frequency at the peak position to the total frequency is greater than or equal to the first predetermined value Condition 3: The peak position satisfying all that the peak position is within the predetermined concentration range If there is a peak position that satisfies the above conditions 1 to 3 for all color components, and the maximum difference between these peak positions is greater than or equal to the second predetermined value, a color background exists. Judge that.

  Condition 1 is a condition set by paying attention to the fact that the base region often appears as a peak first when viewed from the low density side in the document. Condition 2 is a condition set by paying attention to the fact that the area of the base region often occupies a certain size in the document. Condition 3 is a condition that imposes the premise that the density of the background is somewhat bright. In other words, if the peak is darker than the desired background density, or if the peak is too bright, it is determined that it is not a background that will affect the region separation, so that it can be excluded. Yes.

  For this reason, the color background determination unit can determine that a color background exists only when there is a color background that may affect black characters. As a result, when there is a color background that has no possibility of affecting black characters, it is not necessary to operate the background removal unit or the control unit.

  An image forming apparatus according to the present invention includes the above-described image processing apparatus. As a result, black characters surrounded by colored characters can be output clearly and without blurring.

  An image reading apparatus according to the present invention includes an image reading unit that reads input image data from a document, and the image processing device that processes the image data read by the image reading unit as input image data. As a result, it is possible to generate image data that can output black characters surrounded by color characters clearly and without blurring.

  The image processing apparatus may be realized by a computer. In this case, a program for causing the image processing apparatus to be realized by the computer by causing the computer to operate as the respective units, and a computer-readable program that records the program. Recording media also fall within the scope of the present invention.

  An image processing apparatus according to the present invention is an image processing apparatus including an area separation processing unit that performs an area separation process for separating input image data into a plurality of areas including at least a character area. A color background determination unit that determines whether or not a color background exists, and a background removal unit that removes a color background from input image data when the color background determination unit determines that a color background exists. The separation processing unit performs a region separation process on the input image data from which the color background is removed by the background removal unit.

  The image processing apparatus of the present invention is an image processing apparatus including an area separation processing unit that performs an area separation process for separating input image data into a plurality of areas including at least a character area, and the area separation processing unit includes: And a chromatic / achromatic determining unit that determines whether the character is chromatic or achromatic by comparing a maximum difference between a plurality of color components and a predetermined threshold value. A color background determination unit that determines whether or not a color background exists, and a control unit that increases the predetermined threshold when the color background determination unit determines that a color background exists. .

  As a result, there is an effect that it is possible to realize an image processing apparatus that can output even a black character surrounded by a color background having a certain density, with a clear image quality that suppresses blurring.

[Embodiment 1]
Embodiment 1 of the present invention will be described below with reference to FIG. FIG. 1 is a block diagram showing a configuration of an image processing apparatus (color image processing apparatus) according to the present invention. In this embodiment, a digital color copying machine will be described as an example of the image forming apparatus.

  As shown in FIG. 1, the color image processing apparatus 1 is connected to an operation panel 12, a color image input apparatus 13, and a color image output apparatus 14, and constitutes a digital color copying machine as a whole.

  The operation panel 12 is a setting button for controlling the operation of the display unit such as a liquid crystal display and the entire digital color copying machine (for example, an image mode [character mode / character photo mode / photo mode etc.] indicating a document type to be copied). For example). Furthermore, it has a setting slide button for setting the density and background level.

  The color image input device (image reading means) 13 is composed of, for example, a scanner unit equipped with a CCD (Charge Coupled Device), and the reflected light image from the original is converted into RGB (R: red, G: green, B: blue). Are read by the CCD as analog signals and output to the color image processing apparatus 1 as RGB analog signals.

  The analog signal read by the color image input device 13 is subjected to A / D conversion processing and shading correction processing in the color image processing device 1, and then subjected to color ground determination processing and input tone correction processing. Separation processing is performed, followed by color correction processing, black generation and under color removal processing, spatial filter processing, output gradation correction processing, and gradation reproduction processing in this order, and C (cyan), M (magenta), and Y ( Yellow) and K (black) digital color signals are output to the color image output device 14.

  The color image output device 14 is a device that outputs a result of predetermined image processing performed by the image processing device 2.

  The color image processing apparatus 1 includes an A / D (analog / digital) conversion unit 2, a shading correction unit 3, a color background determination unit 4, an input tone correction unit 5, a region separation processing unit 6, a color correction unit 7, and a black generation. An under color removal unit 8, a spatial filter processing unit 9, an output tone correction unit 10, and a tone reproduction processing unit 11 are provided.

  The A / D converter 2 converts the RGB analog signal read by the color image input device 13 into a digital signal.

  The shading correction unit 3 performs a process of removing various distortions generated in the illumination system, the imaging system, and the imaging system of the color image input device 13 on the digital RGB signal sent from the A / D conversion unit 2. Is. The shading correction unit 3 converts RGB signals (RGB reflectance signals) into signals that are easy to handle in the image processing system employed in the color image processing apparatus 1, such as density signals, and performs processing for adjusting color balance. Do.

  The signal corrected by the shading correction unit 3 is input to the color background determination unit 4 and the input tone correction unit 5.

  The color background determination unit 4 determines whether or not the background portion of the document is a color background that affects the region separation process, and calculates a density conversion characteristic for the input tone correction unit 5 to remove the color background. And output to the input tone correction unit 5.

  The input tone correction unit 5 performs density correction processing on the input image signal input from the shading correction unit 3 based on the density conversion characteristics for removing the background calculated by the color background determination unit 4. Processing for removing the background is performed, and image quality adjustment processing such as contrast is further performed. Further, the input tone correction unit 5 can control the output density to be increased or decreased according to a user-specified density change instruction.

  The region separation processing unit 6 separates each pixel in the input image based on the RGB signal as a black character region, a color character region, a halftone dot region, and other regions if they do not belong to any region. .

  Further, the region separation processing unit 6 generates a region identification signal indicating which region the pixel belongs to based on the separation result, and generates a black generation and under color removal unit 8, a spatial filter processing unit 9, and a gradation reproduction processing unit 11. In addition, the RGB signal input from the input tone correction unit 5 is output to the subsequent stage as it is.

  The color correction unit 7 removes color turbidity based on spectral characteristics of CMY (C: cyan, M: magenta, Y: yellow) color materials including unnecessary absorption components in order to realize color reproduction faithfully. Is to do.

  The black generation and under color removal unit 8 performs black generation processing for generating a black (K) signal from the CMY three-color signals after color correction, while subtracting the K signal obtained by black generation from the original CMY signal to newly The undercolor removal process for generating a simple CMY signal is performed. In other words, the black generation and under color removal unit 8 changes the CMY three-color signal to the CMYK four-color signal.

  The spatial filter processing unit 9 performs spatial filter processing using a digital filter on the image data of the CMYK signal input from the black generation and under color removal unit 8 based on the region identification signal, thereby correcting the spatial frequency characteristics. Processing is performed to prevent blurring of the output image and deterioration of graininess.

  The output tone correction unit 10 performs output tone correction processing for converting a signal such as a density signal into a halftone dot area ratio that is a characteristic value of the color image output device 14.

  The gradation reproduction processing unit 11 performs gradation reproduction processing (processing for making it possible to reproduce the respective gradations by finally separating the image into pixels based on the region identification signal for the image data of the CMYK signal. Halftone generation).

(Configuration of region separation processing unit)
Next, the internal configuration of the region separation processing unit 6 will be described. The area separation processing unit 6 performs a process of separating a character area (divided into a black character area and a color character area), a halftone dot area, and other areas. FIG. 2 is a block diagram showing an internal configuration of the region separation processing unit 6. As shown in FIG. 2, the region separation processing unit 6 includes a character determination unit 20, an achromatic color determination unit 21, a halftone dot determination unit 22, an achromatic color character determination unit 23, and a final determination unit 24.

  As a method for separating input image data into characters, halftone dots, and other areas, for example, a method described in “Image Electronics Society of Japan Proceedings 90-06-04” can be used. A general method can be used as a method of further separating the separated character area into a black character area and a color character area.

  In the present embodiment, region separation processing is performed using a block of M × N (M and N are natural numbers) pixels centered on the pixel of interest, and a region identification signal indicating the region to which the pixel of interest belongs is generated.

  The region separation processing unit 6 obtains an average value (Dave) of the signal level for the pixels in the block, and binarizes each pixel in the block using the average value. Further, the maximum pixel signal level (Dmax) and the minimum pixel signal level (Dmin) are also obtained at the same time. Details of each block will be described below.

The character determination unit 20 identifies a character area. Specifically, in the character area, since the difference between the maximum pixel signal level and the minimum pixel signal level is large and the density is considered to be high, the character determination unit 20 satisfies the following expression (1). The character area is determined.
Dsub> PC or Dmin <PB (1)
PC and PB are predetermined threshold values.

The achromatic color determination unit 21 determines whether or not the target pixel is an achromatic region. Specifically, the achromatic color determination unit 21 calculates Daver (: average value of R plane signal) and Daveg (: average value of G plane signal) of the target pixel and neighboring pixels for each RGB plane. , Daveb (: average value of B plane signal) is used to calculate the maximum difference Ddiff between the average values according to the following equation:
Ddiff = MAX (Daver, Daveg, Daveb) -MIN (Daver, Daveg, Daveb)
MAX and MIN are functions that output the maximum value and the minimum value, respectively, of the input Daver, Daveg, and Daveb. The achromatic color determination unit 21 sets the target pixel as a chromatic pixel when the Ddiff value is equal to or greater than a predetermined threshold (THcolor), and sets the target pixel as an achromatic pixel when the Ddiff value is smaller than the threshold THcolor. judge.

  The achromatic character determination unit 23 combines the determination result of the character determination unit 20 and the determination result of the achromatic color determination unit 21 to determine whether the target pixel is an achromatic character region or an achromatic character (color character). Whether it is an area or a non-character area is determined.

  The halftone dot determination unit 22 determines whether or not the target pixel is a halftone dot region. In the halftone dot region, attention is paid to the fact that the fluctuation of the image signal in the small region is large and the density is higher than the background, and it is identified whether or not it is a halftone dot region.

  Specifically, the halftone dot determination unit 22 obtains the number of change points from 0 to 1 in the main scanning and sub-scanning directions for the binarized data, respectively, and the number of change points from 1 to 0. , KV, and when both exceed the threshold values TH and TV, the halftone dot region is determined. Further, in order to prevent erroneous determination with respect to the background, the previously obtained Dmax, Dmin, Dave are compared with threshold values B1, B2.

That is, when the following expression (2) is satisfied, the halftone dot determination unit 22 determines that the region is a halftone dot region.
Dmax-Dave> B1, Dave-Dmin> B2, KH> TH, and KV> TV (2).

  The final determination unit 24 uses the determination result of the achromatic character determination unit 23 and the determination result of the halftone dot determination unit 22 to make a final determination as shown in Table 1 below, and outputs the output from the region separation processing unit 6 (region separation). Signal).

  As described above, the region separation processing unit 6 can separate a black character region, a color character region, a halftone dot region, and other regions.

(About the overall image processing flow)
Next, the overall flow of the image processing method by the digital color copying machine having the above configuration will be described.

  First, the color image input device 13 reads document image data as RGB analog signals and outputs them to the color image processing device 1.

  In the color image processing apparatus 1, the input analog signal is converted into an RGB digital signal by the A / D conversion unit 2 and output to the shading correction unit 3. The shading correction unit 3 removes various distortions generated in the illumination system, imaging system, and imaging system of the color image input device 13 from the RGB digital signal and converts the RGB reflectance signal into a density signal. .

  Thereafter, the RGB signal is input to the color ground determination unit 4 and the input tone correction unit 5.

  The color background determination unit 4 determines whether or not the background portion of the document is a color background (determination process), and density conversion characteristics for removing the color background by the input tone correction unit 5 (details will be described later). ) Is calculated. Then, the density conversion characteristic calculated in the density conversion characteristic calculation process is output to the input tone correction unit 5. Details of the determination process and density conversion characteristic calculation process of the color background determination unit 4 will be described later.

  Then, the input tone correction unit 5 performs density correction processing using the density conversion characteristics calculated by the color background determination unit 4 on the input image signal input from the shading correction unit 3 and removes the background. After performing the removal process, and further adjusting the color balance and performing the image quality adjustment process such as contrast, the image is output to the region separation processing unit 6.

  Thereafter, the region separation processing unit 6 separates each pixel in the input image into any one of a black character region, a color character region, a halftone dot region, and other regions from the input RGB signal. The region separation processing unit 6 outputs a region identification signal indicating which region the pixel belongs to based on the separation result, as a color correction unit 7, a black generation and under color removal unit 8, a spatial filter processing unit 9, and a tone reproduction processing unit. 11 and the input RGB signal is output to the color correction unit 7 as it is.

  The color correction unit 7 performs a process of removing color turbidity on the input RGB signal based on the spectral characteristics of the CMY color material including unnecessary absorption components, and then converts the CMY signal after color correction into a black generation and under color removal unit. Output to 8.

  The black generation and under color removal unit 8 performs black generation processing for generating a K signal from the input CMY signal and under color removal processing for generating a new CMY signal by subtracting the K signal from the input CMY signal. . The CMY three-color signal is converted into a CMYK four-color signal and then output to the spatial filter processing unit 9.

  As an example of the black generation processing, there is a method (general method) for generating black by UCR (under color removal processing). In this method, the input / output characteristic of the black generation curve is y = f (x), the input data is C, M, Y, the output data is C ′, M ′, Y ′, K ′, UCR (Under When the color removal rate is α (0 <α <1), the black generation and under color removal processing is expressed by the following equations (3) to (6).

K ′ = f {min (C, M, Y)} (3)
C ′ = C−αK ′ (4)
M ′ = M−αK ′ (5)
Y ′ = Y−αK ′ (6)
After that, the spatial filter processing unit 9 performs spatial filter processing using a digital filter on the input image data of the CMYK signal based on the region identification signal, and corrects the spatial frequency characteristics to thereby blur or graininess of the output image. Process to prevent deterioration. Further, similar to the spatial filter processing unit 9, the gradation reproduction processing unit 11 performs predetermined processing on the image data of the CMYK signal based on the region identification signal.

  For example, in the region separated by the region separation processing unit 6 into characters or halftone characters, sharp enhancement processing is performed in the spatial filter by the spatial filter processing unit 9 in order to improve the reproducibility of black characters or color characters in particular. The amount of high frequency enhancement is increased. At the same time, the gradation reproduction processing unit 11 selects binarization or multi-value conversion on a high-resolution screen suitable for high-frequency reproduction.

  Further, with respect to the region separated into halftone dot regions by the region separation processing unit 6, the spatial filter processing unit 9 performs low-pass filter processing for removing the input halftone component. The output tone correction unit 10 performs output tone correction processing for converting a signal such as a density signal into a halftone dot area ratio that is a characteristic value of the color image output device 14, and then the tone reproduction processing unit 11. Then, gradation reproduction processing (halftone generation) is performed so that the image is finally separated into pixels and each gradation is reproduced.

  For the region separated into other regions by the region separation processing unit 6, a low-pass filter is applied in the spatial filter processing unit 9 to such an extent that sharpness for noise removal is not reduced.

(Details of processing in the color ground determination unit and input tone correction unit)
Next, an outline of processing in the color ground determination unit 4 and the input tone correction unit 5 which is characteristic processing of the present invention will be described. FIG. 3 is a flowchart showing the flow of these processes.

  First, the color background determination unit 4 performs a histogram generation process (S1). Specifically, the color ground determination unit 4 creates a density histogram (RGB input because of RGB input, the higher the density, the lower the value, the lower the density, and the higher the value for paper white). To do.

  However, if a density histogram is created with all density values from 0 to 255, a memory for holding the calculation amount and the count value of each density category (hereinafter referred to as bin) is required. Therefore, in the present embodiment, for example, the entire density area is equally divided into 32 density sections (bins) to create a density histogram. Accordingly, the density range bin (i) of the i-th bin is as shown in Equation 1 below.

Note that bin (i) is common to RGB in order to set the same bin density range for RGB.

  Then, a density histogram of each color of R, G, and B is created in the entire pixel range of the document (S2). The density histogram is created by a method according to the following equation (2). In the following example, the calculation method of the R plane is shown, but the calculation can be performed in the same way for the G and B planes.

  Here, R (x, y) indicates the pixel value (density value) of the R plane at the pixel (x, y). In the above equation 2, the value of R (x, y) is the bin bin (i). This indicates that 1 is added to the i-th count value when the density range is entered. As a result, by performing the counting process on the entire document, it is possible to calculate a density histogram of all the pixels and the RGB planes.

  FIG. 4 shows an example in which a density histogram is calculated for a document. In FIG. 4, the horizontal axis indicates the density value (displayed as a pixel value in the figure).

  Next, the color background determination unit 4 performs background determination processing for determining whether there is a background that may affect the region separation processing (S3). Specifically, the color background determination unit 4 uses the density histogram having 32 bins for each of the calculated RGB planes to determine that a predetermined background exists when the following three conditions are satisfied. To do.

  Condition 1 is a condition set by paying attention to the fact that the base region often appears as a peak first when viewed from the low density side in the document. Condition 2 is a condition set by paying attention to the fact that the area of the base region often occupies a certain area in the document. Here, THbg = 0.05 is set as an example as the threshold value. Condition 3 is a condition that imposes the premise that the density of the background is somewhat bright. In other words, if the peak is darker than the desired background density, or if the peak is too bright, it is determined that it is not a background that will affect the region separation, so that it can be excluded. Yes.

  When the above three conditions are satisfied, it is determined that the base area exists in the i-th bin. The above processing is performed for each of the RGB planes, and the bin position where the background area exists in each plane is calculated.

  Then, the color background determination unit 4 determines whether there is a bin that satisfies the conditions 1 to 3 in each of the RGB planes. That is, the color background determination unit 4 determines whether there is a background that affects the region separation process.

  If any one of the RGB planes does not satisfy the above conditions 1 to 3, the following process is canceled because the influence on the determination accuracy of the region separation is small, and the region using an input image that is not subjected to normal correction processing Perform separation processing. That is, the input tone correction unit 5 does not perform density correction processing.

In the following description, it is assumed that bins satisfying the above conditions 1 to 3 exist in the RGB plane. Here, a case will be described in which it is determined that the background exists in the i _R , i _G , and i _B th bins in each of the RGB planes.

  Next, in S4, the color background determination unit 4 performs color background determination processing for determining whether or not a color background exists.

  Specifically, the color background determination unit 4 uses the density histogram of the entire document created for each RGB signal to determine the frequency bias between the RGB planes in the vicinity of the background with a low density. It is determined whether or not to do so. That is, the color background determination unit determines the presence / absence of the background density deviation based on the maximum difference between the RGB positions of the bin where the background exists, which is calculated in the background determination process (S3). In the present embodiment, it is determined that there is a bias when the following conditional expression 4 is satisfied, and the target color background exists.

According to this conditional expression, when the difference between the maximum value and the minimum value of the bin positions (here, i _R , i _G , i _B ) determined as the background is equal to or greater than a predetermined value (Thdiff), the color background is determined. Is what you do. The difference between the maximum value and the minimum value is a parameter related to the density difference between RGB of the bin corresponding to the background, that is, the density of the background, so if the background has color, On the other hand, when the background has no color, the value on the left side decreases. Therefore, the color background can be determined when the value on the left side of the conditional expression is equal to or greater than a predetermined value (THdiff).

  When the conditional expression (4) is not satisfied, the input tone correction unit does not perform density correction processing, and performs region separation processing using an input image that is not subjected to normal correction processing. Below, the case where the conditional expression of Formula 4 is satisfied is demonstrated.

  Thereafter, the color background determination unit 4 performs density conversion characteristic calculation processing for calculating density conversion characteristics for removing the background while maintaining the balance of the color components of the color background, and the calculated density conversion characteristics are input to the input tone correction unit. 5 (S5).

  In the present embodiment, in order to remove the background while maintaining the balance of the color components, density conversion characteristics for performing gamma correction processing for matching the balance of the color components in the lower area are calculated. Specifically, in order to balance the background color, the high density side is fixed using the background bin position and the high density bin position calculated for each RGB plane in S3, By adjusting the density so that the low density side is aligned, the color balance of the ground is aligned.

Therefore, the color background determination unit 4 calculates the position of the bin on the high density side as the density conversion characteristic. Specifically, the color ground determination unit 4 uses the density histogram of each of the RGB planes created in S1 to add the frequency from the highest density, and set the bin position exceeding a predetermined threshold to the high density side. Bin position (j _R , j _G , j _B ).

That is, the bin position j _R on the high density side of the R plane is calculated using the following equation (5). The bin positions j _G and j _B on the high density side of the G and B planes are similarly calculated.

Equation 5, by sequentially adding the frequencies of the density histogram from the high concentration side, means setting the position of the bottle exceeding a threshold THhigh first as j _R.

Then, the color background determination unit 4 uses the calculated high density side bin position (j _R , j _G , j _B ) and the low density side bin position (i _R , i _G , i _B ) as density conversion characteristics. And output to the input tone correction unit 5.

Next, the input gradation correction unit 5 determines the input high density side bin position (j _R , j _G , j _B ) and low density side bin position (i _R , i _G , i _B ). Then, the R signal is corrected according to the following equation 6 and density correction processing for generating a new R signal R _new is performed (S6). The same correction is performed for the G and B signals, and new G and B signals G _new and B _new are generated.

  FIG. 5 shows a frequency distribution state when the density histogram is calculated again for the image corrected by the density correction process. FIG. 5 shows that the color components between RGB are balanced by the density correction process. That is, the color of the background region is gray (achromatic color). As described above, it is possible to adjust the entire pixel distribution while maintaining the density balance on the low density (highlight) side. Thereafter, the input tone correction unit 5 performs a background removal process for removing the color components in the lower area on the image data on which the density correction process has been performed.

  FIG. 6 is a correction table used in the input tone correction unit 5. The correction table represents both density correction processing and background removal processing. In FIG. 6, the horizontal axis indicates the value of the image data input to the input gradation correction unit, and the vertical axis indicates the value of the image data after the density correction process and the background removal process are performed. In FIG. 6, in order to balance the density of the RGB plane, the density correction of the B plane is performed, and then the under color is removed from each of the RGB planes. Note that the background removal area is determined in advance based on various image data.

  Then, the input tone correction unit 5 outputs the image data on which the density correction process and the undercolor removal process have been performed to the region separation processing unit 6. Then, the region separation processing unit 6 performs the region separation processing described above on the input image data.

  As described above, the image subjected to the density correction process and the under color removal process by the input tone correction unit 5 is set as a region separation target, thereby eliminating the influence of the color component of the background region and eliminating the region separation. A saturation determination process can be performed. As a result, it is possible to improve the determination accuracy of the color determination. In this way, the density correction process prevents erroneous determination due to color cast on black characters due to the color background. Therefore, the density conversion characteristic used for the density correction process can also be expressed as a color cast correction signal.

  If the detection accuracy of color judgment can be improved, the detection accuracy of characters in the color gamut can be improved, and image processing that increases the desired resolution is performed on the characters in the color gamut, resulting in high image quality. An output signal can be obtained.

In the present embodiment, the background component is removed from the entire image in the background removal process. As a result, the color of the image slightly changes in the area other than the background, but it is not so influential. In order to prevent this effect,
(1) The image data that has been subjected to density correction processing and background removal processing by the input tone correction section 5 is used as image data for region separation processing, and image data that has not been subjected to the density correction processing and background removal processing is color. The image processing target of the subsequent block including the correction unit 7 is a target.
(2) The background area / non-background area is determined, and density correction processing and background removal processing are performed only on the background area.
(3) In the correction table in the input tone correction unit 5, a strong correction is applied to the background density area, and weighting is performed so that the amount of change decreases as the density increases.
The following measures can be considered.

  In the above description, the color image processing apparatus 1 is newly provided with the color background determination unit 4, but the input tone correction unit 5 may perform processing executed by the color background determination unit 4.

[Embodiment 2]
In the first embodiment, when it is determined that there is a color background that affects the determination accuracy of the region separation, the color background is removed after balancing the color tone of the color background, and is used as an input image for the region separation process. Handled. However, depending on the mode selected by the user, the color background may not be removed.

  In the second embodiment, even when the color background cannot be removed, black characters surrounded by the color background can be clearly expressed and output as an image without blur.

  In this embodiment, the process for determining whether or not there is a color background that affects the region separation is performed in the same manner as in the first embodiment, and if it is determined that there is a color background that affects the region separation process, Instead of removing the background, the threshold value for color determination in the achromatic determination processing in the region separation processing is controlled.

  The color image processing apparatus 1 according to the present embodiment has the same configuration as that of the first embodiment shown in FIG. However, the color background determination unit 4 of the present embodiment performs the color background determination processing (S1 to S5 in FIG. 3) as in the first embodiment, but instead of the density conversion characteristic calculation processing, the threshold control processing described below is performed. Do.

As described above, the achromatic color determination unit 21 of the region separation processing unit 6 calculates the maximum difference Ddiff between the average values using Daver, Daveg, and Daveb obtained by averaging the pixel of interest and the neighboring pixels by the RGB planes. Calculate according to the following formula.
Ddiff = MAX (Daver, Daveg, Daveb) -MIN (Daver, Daveg, Daveb)
The achromatic color determination unit 21 sets the target pixel as a chromatic pixel when the Ddiff value is equal to or greater than a predetermined threshold (THcolor), and sets the target pixel as an achromatic pixel when the Ddiff value is smaller than the predetermined value. judge.

  The color background determination unit 4 of the present embodiment performs a threshold control process for controlling the threshold THcolor. Specifically, the threshold THcolor is controlled according to Equation 7 and output to the region separation processing unit 6 (see the broken line in FIG. 1).

  For example, the setting values are set to values val1 = 25 and val2 = 20. As a result, the threshold value val1 is selected when determining black characters in the color background, so even if the black character has a slight color due to the influence of the color background, it can be determined as a black character. become. On the other hand, when there is no color background, val2 is selected and the black character is determined with a stricter threshold value. Therefore, it is less likely that a color character with low saturation is erroneously determined as a black character.

  As described above, by controlling the threshold THcolor of the achromatic color determination unit 21 in the region separation process without removing the color background, the detection accuracy of characters on the color background and characters on the normal white background is improved. Is possible.

[Others]
In the first and second embodiments, the color image processing apparatus 1 includes the color correction unit 7, the black generation and under color removal unit 8, the spatial filter processing unit 9, the output tone correction unit 10, and the tone reproduction processing unit. . However, the configuration of the color image processing apparatus is not limited to this. FIG. 7 is a block diagram illustrating a configuration of an image reading apparatus (flatbed scanner) including a color image processing apparatus according to another embodiment.

  As shown in FIG. 7, the color image processing apparatus 1 a includes an A / D conversion unit 2, a shading correction unit 3, a color background determination unit 4, an input tone correction unit 5, and a region separation processing unit 6. Then, a color image input device 13 is connected to the color image processing device 1a to constitute an image reading device as a whole.

  An analog signal read by the color image input device 13 is converted into an A / D conversion unit 2, a shading correction unit 3, a color background determination unit 4, an input tone correction unit 5, and an area separation process in the color image processing device 1a. The signals are sent in the order of the unit 6 and output as RGB digital color signals to a personal computer or a printer.

  The processing contents of the A / D conversion unit 2, the shading correction unit 3, the color background determination unit 4, the input tone correction unit 5, and the region separation processing unit 6 are the same as those of the color image processing apparatus 1 described above.

  The image data subjected to the above-described processes is input to a computer or a printer, and color correction processing, spatial filter processing, gradation reproduction processing, and the like are performed. The above processing is controlled by a CPU (Central Processing Unit) (not shown).

  Further, the image input to the region separation processing according to the present invention is corrected by determining whether or not a color background exists, or the threshold THcolor of the achromatic color determination unit 21 of the region separation processing unit 6 is controlled. May be realized by software (application program). In this case, it is possible to provide a computer or scanner with a scanner driver that incorporates software that determines whether or not a color background exists and corrects the threshold THcolor.

  In addition, the present invention performs input tone correction processing and controls the threshold value of region separation processing on a computer-readable recording medium that records a program to be executed by a computer according to the result of color background determination processing. You can also record how to do it.

  As a result, it is possible to provide a portable recording medium on which a processing program capable of improving the detection accuracy of characters surrounded by the color background is recorded.

  The recording medium may be a non-illustrated memory, for example, a program medium such as a ROM because processing is performed by a microcomputer, and a program reading device as an external storage device (not illustrated) is provided, and the recording medium is stored therein. It may be a program medium that can be read by being inserted.

  In any case, the stored program may be configured to be accessed and executed by the microprocessor, and the program is read out, and the read program is stored in a program storage area (not shown) of the microcomputer. A method of downloading and executing the program may be used. In this case, it is assumed that the download program is stored in the main device in advance.

  Here, the program medium is a recording medium configured to be separable from the main body, and includes a tape system such as a magnetic tape and a cassette tape, a magnetic disk such as a floppy (registered trademark) disk and a hard disk, and a CD-ROM / MO /. Disk system for optical disks such as MD / DVD, card system such as IC card (including memory card) / optical card, mask ROM, EPROM (Erasable Programmable Read Only Memory), EEPROM (Electrically Erasable Programmable Read Only Memory), flash It may be a medium that carries a fixed program including a semiconductor memory such as a ROM.

  In this case, since the system configuration is capable of connecting a communication network including the Internet, the medium may be a medium that fluidly carries the program so as to download the program from the communication network. When the program is downloaded from the communication network in this way, the download program may be stored in the main device in advance or installed from another recording medium. The present invention can also be realized in the form of a computer data signal embedded in a carrier wave in which the program code is embodied by electronic transmission.

  The recording medium is read by a program reading device provided in a digital color image forming apparatus or a computer system, whereby the above-described image processing method is executed.

  The computer system includes an image input device such as a flatbed scanner, a film scanner, and a digital camera, a computer that performs various processes such as the image processing method by loading a predetermined program, and displays the processing results of the computer. An image display device such as a CRT display / liquid crystal display and a printer that outputs the processing results of the computer to paper. Furthermore, a network card, a modem, and the like are provided as communication means for connecting to a server or the like via a network.

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims, and embodiments obtained by appropriately combining technical means disclosed in different embodiments. Is also included in the technical scope of the present invention.

  The image processing apparatus of the present invention can be applied to an image forming apparatus such as a copying machine or a multifunction machine.

1 is a block diagram showing a configuration of a digital color copying machine according to the present invention. It is a block diagram which shows the internal structure of the area | region separation process part with which a color image processing apparatus is provided. It is a flowchart which shows the flow of a process in a color ground determination part and an input gradation correction | amendment part. It is a figure which shows an example of a density histogram. It is a figure which shows frequency distribution at the time of calculating a density histogram again with respect to the image correct | amended by the density correction process. It is a figure which shows the correction table used by the input gradation correction | amendment part. 1 is a block diagram illustrating a configuration of an image reading apparatus according to the present invention.

Explanation of symbols

1 Color image processing device (image processing device)
2 A / D conversion unit 3 Shading correction unit 4 Color background determination unit 5 Input gradation correction unit (background removal unit, control unit)
6 Region separation processing unit 7 Color correction unit 8 Black generation and under color removal unit 9 Spatial filter processing unit 10 Output tone correction unit 11 Tone reproduction processing unit 12 Operation panel 13 Color image input device (image reading means)
14 color image output device 20 character determination unit 21 achromatic color determination unit 22 halftone dot determination unit 23 achromatic color character determination unit (chromatic / achromatic determination unit)
24 Final judgment part

Claims (6)

An image processing apparatus including an area separation processing unit that performs an area separation process for separating input image data into a plurality of areas including at least a character area,
The region separation processing unit identifies a character region of the input image data, and compares the maximum difference between a plurality of color components in the character region with a predetermined threshold value, so that the character is chromatic or achromatic. It has a chromatic / achromatic determination unit that determines whether
A color background determination unit that determines whether or not a color background exists in the input image data;
An image processing apparatus comprising: a control unit that increases the predetermined threshold when the color background determination unit determines that a color background exists. The color ground determination unit uses a density histogram of each of a plurality of color components, and for each color component,
Condition 1: The lowest concentration peak position Condition 2: The ratio of the frequency at the peak position to the total frequency is greater than or equal to the first predetermined value Condition 3: The peak position satisfying all that the peak position is within the predetermined concentration range Determine whether there is
It is determined that a color background exists when there is a peak position satisfying the above conditions 1 to 3 for all color components and the maximum difference between these peak positions is equal to or greater than a second predetermined value. Item 8. The image processing apparatus according to Item 1 . An image forming apparatus comprising the image processing apparatus according to claim 1 or 2. Image reading means for reading input image data from a document;
Image reading apparatus characterized by comprising an image processing apparatus according to claim 1 or 2 performs processing image data by the image reading unit read as the input image data. An image processing program for operating the image processing apparatus according to claim 1 or 2 for causing a computer to function as each unit. A computer-readable recording medium on which the image processing program according to claim 5 is recorded.

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