JP4637686B2 - Color conversion apparatus, image forming apparatus, color conversion method, and computer program - Google Patents

Description

  The present invention relates to a color conversion apparatus that performs color correction processing by a table reference method, an image forming apparatus including the color conversion apparatus, a color conversion method for realizing the color conversion apparatus, and a computer program.

  Image processing apparatuses such as digital color copiers and digital color multifunction peripherals usually receive RGB signals of the first color system (scanner RGB in the case of copiers, sRGB in the case of printer apparatuses, etc.) and receive the signals. A desired output image is obtained by converting into a CMY signal or CMYK signal of the second color system. This conversion is currently performed by linear interpolation using a three-dimensional lookup table. Since this interpolation is linear interpolation, good color conversion can be performed when the relationship between the color space of the first color system and the color space of the second color system is linear. There is a problem that calculation error becomes large. Usually, since the relationship between the RGB signal of the first color system and the CMY signal or CMYK signal of the second color system is not linear, the calculation error increases.

  Therefore, a one-dimensional lookup table is usually placed before interpolation using a three-dimensional lookup table, and between the first color system RGB signal and the second color system CMY signal or CMYK signal. By correcting the non-linearity, an increase in calculation error is prevented (see, for example, Patent Document 1).

  Further, since the color material used differs between the character image or halftone dot image and the photographic image, the spectral characteristics of the character image or halftone dot image are different. For this reason, even if it looks to the same color for humans, the scanner's spectral sensitivity is different from that of the human eye, so the reading value of the scanner device may be different. Conversely, even if the reading value of the scanner device is the same, it may be viewed differently by humans. From such a problem, it is known that when the color material is different, each image can be reproduced well by changing the color correction table.

  Further, the optimum halftone processing differs between a character image or halftone dot image and a photographic image. For example, halftone processing that enhances the edge effect for text, processing that makes moire inconspicuous for halftone images, and halftone processing that emphasizes highlight and tone reproduction for photographic images It has become. As described above, when the halftone is changed in accordance with the characteristics of each image, if the color correction processing is not performed in accordance with that, the color difference becomes large, and it is difficult to perform high-precision color reproduction. In other words, in order to perform color conversion with high image quality, depending on the color material of the input image (printing or silver salt photography distinction) and its characteristics (for example, distinction between character images, halftone dot images, photographic images, etc.), It is required to change the color conversion process.

  In order to solve such a problem, there is known an image processing apparatus that performs color processing and halftone processing according to the characteristics of a photographic image and a character image included in a document image and reproduces the document image satisfactorily ( For example, see Patent Document 2). 51 and 52 are block diagrams showing the configuration of main parts of a conventional image processing apparatus. The image processing apparatus shown in FIG. 51 includes a color conversion unit 100 including a one-dimensional LUT calculation unit 101 and a three-dimensional LUT calculation unit 102. The one-dimensional LUT calculation unit 101 receives a digital RGB signal and a region identification signal indicating a distinction between a character / halftone dot region (referred to as a first region) and a photograph region (referred to as a second region). Based on the input region identification signal, the address reference unit 101a of the one-dimensional LUT calculation unit 101 switches the one-dimensional lookup table to be used to the one for the first region or the one for the second region, and performs the switched one-dimensional lookup. Based on the table, non-linearity correction is performed.

  The three-dimensional LUT calculation unit 102 receives the RGB signal output from the one-dimensional LUT calculation unit 101, calculates a desired weight by the weight calculation unit 102a, and uses the address calculation unit 102b based on the region identification signal. The color correction value is acquired by switching the three-dimensional lookup table to be used for the first region or the second region and accessing the switched three-dimensional lookup table. Then, based on the weight calculated by the weight calculation unit 102a and the color correction value acquired by the address calculation unit 102b, the interpolation calculation unit 102c performs an interpolation calculation to acquire a CMY signal that is an output image signal.

The image processing apparatus shown in FIG. 52 includes several types of color conversion units 110a to 110d that perform predetermined color correction, and the switching unit 111 switches the output according to the region identification signal, so that the color can be favorably selected according to the region. The corrected output image signal (CMY signal) is obtained.
Japanese Patent Laid-Open No. 2003-110865 Japanese Patent No. 2999611 JP-A-10-70669

  However, in the image processing apparatus shown in FIGS. 51 and 52, it is possible to improve the accuracy of color conversion, but the color conversion circuit of the color correction table (or matrix) has a circuit for a necessary image. Become. That is, the same number of color conversion circuits is required as the number of image types to be switched. For this reason, the switching process makes the circuit redundant and increases the circuit scale, resulting in an increase in cost.

  In order to deal with such problems, arithmetic processing is performed on the input of a specific color area, interpolation is performed by linear mapping in another area where no color exists in the input color space, and vertices per unit volume A color conversion device that performs color conversion by increasing the number has been proposed (see, for example, Patent Document 3).

  However, the means for determining the specific color region is complicated, and at the same time, the calculation means is necessary because the calculation is performed and the mapping is performed. In addition, depending on the shape of the color gamut of the input image data, the calculation method for mapping a specific color area to an area where no color exists may be complicated, resulting in an increase in circuit scale and an increase in cost. I will. In addition, since mapping is performed in an area without data, there is a problem that the size and number of data to be stored are limited, and the accuracy may be reduced.

  SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and a color conversion device capable of efficiently holding a table memory and performing color conversion with a simple circuit configuration without executing switching of a color correction table, An object of the present invention is to provide an image forming apparatus provided with a color conversion apparatus, a color conversion method for realizing the color conversion apparatus, and a computer program.

The color conversion device according to the present invention is a color conversion device that includes a plurality of pixels and performs color conversion on each pixel of an input image including one or more identifiable regions to generate an output image. A conversion means for converting the pixel value of each pixel of the input image so that the pixel value after conversion is uniquely determined according to the characteristics of the region and the range of the converted pixel value is continuous between the regions ; Stores color correction values at grid points obtained by equally dividing the color space determined by the range of pixel values, a color correction table in which the number of grid points for each color component is predetermined, and colors read from the color correction table And a calculation means for performing an interpolation calculation on the pixel using a correction value.

  In the present invention, conversion is performed such that the range of pixel values after conversion differs according to the characteristics of the area to which each pixel belongs, and a color correction table that defines a color space determined by the range of pixel values after conversion is defined. Therefore, for example, a plurality of types of one-dimensional lookup tables determined according to the characteristics of the region to which each pixel belongs and a range of pixel values after conversion by the one-dimensional lookup table By using one three-dimensional lookup table defined according to the above, it is possible to perform an appropriate color conversion process according to the area or document type to which each pixel of the input image belongs. Further, since the interval between the grid points is constant for all color components, when calculating the grid points in the vicinity of each pixel in the input image, it is sufficient to divide each color component by the same number so that all the colors are obtained by one arithmetic circuit. Component grid points are calculated.

  In the color conversion apparatus according to the present invention, the conversion means sets means for setting an offset to be added to the pixel value of each pixel of the input image according to the region, and means for adding the set offset to the pixel value. And a one-dimensional lookup table defining the relationship between the pixel value of each pixel to which the offset is added and the pixel value after conversion, and means for converting the pixel value based on the one-dimensional lookup table It is characterized by.

  In the present invention, the offset to be added to the pixel value of each pixel of the input image is set according to the region, and after the set offset is added, the pixel value is converted based on the one-dimensional lookup table. Therefore, even when a plurality of one-dimensional lookup tables are not prepared, data with different pixel value ranges according to the regions can be obtained.

  In the color conversion apparatus according to the present invention, the conversion means determines a pixel value of each pixel of the input image based on a one-dimensional lookup table that defines a relationship between pixel values before and after the conversion, and the one-dimensional lookup table. A conversion unit; a unit for setting an offset to be added to the pixel value converted by the unit according to the region; and a unit for adding the set offset to the pixel value.

  In the present invention, pixel value conversion using a one-dimensional lookup table is performed for each pixel of the input image, an offset to be added to the pixel value after conversion is set, and the set offset is set to the pixel value. Therefore, even if a plurality of one-dimensional lookup tables are not prepared, data with different pixel value ranges can be obtained depending on the region.

In the color conversion apparatus according to the present invention, the interval between the lattice points is 2 ⁿ (where n is an integer smaller than the number k of bits of each color component of the input image), and the number of lattice points Dn is 2 ^(m−1) +1. <Dn <2 ^{(m + 1)} +1 (where Dn ≠ 2 ^m +1 and m = k−n).

In the present invention, since the interval between the lattice points is 2 ⁿ , the lattice points near each pixel are calculated by dividing each color component by 2 ⁿ . The number of grid points Dn is an integer satisfying 2 ^(m−1) +1 <Dn <2 ^{(m + 1)} +1 (where Dn ≠ 2 ^m +1 and m = k−n). In response, the color component of the input image is converted. For example, the range of R, G, and B pixel values in the input image is 0 to 255, the interval between lattice points is 16 (n = 4), and the number of R, G, and B lattice points in the color correction table is 18. In the case of (the pixel value range increases from 0 to 272), the conversion of the color components of the input image is executed according to the number of grid points increased with the division number 17 (= 272 / (2 ⁴ )) as a reference. . The number of grid points can be set finely such as (2 ^m +1) ± 1, (2 ^m +1) ± 2, (2 ^m +1) ± 3, and the like. When the number of grid points is increased, the conversion accuracy is improved, but the memory capacity of the color correction table is increased. Further, when the lattice points are reduced, the conversion accuracy is lowered, but the memory capacity of the color correction table is reduced.

  The color conversion apparatus according to the present invention is characterized by comprising means for identifying the characteristics of the region to which each pixel of the input image belongs based on the pixel value of the pixel.

  In the present invention, since the feature of the area to which each pixel of the input image belongs is identified based on the pixel value, the area can be automatically determined.

  The color conversion apparatus according to the present invention is characterized in that the area included in the input image is any one of a halftone dot area, a character area, and a photograph area.

  In the present invention, since an area included in the input image is any one of a halftone dot area, a character area, and a photograph area, appropriate color conversion is performed according to these areas.

  The color conversion device according to the present invention defines a feature of a region to be identified by a range of pixel values, and determines whether or not a pixel value of each pixel of the input image is included in the defined range. Means are provided.

  In the present invention, hardware can be simplified because appropriate color correction processing can be performed according to the region to which the input image data belongs only by switching the one-dimensional lookup table.

  In the color conversion device according to the present invention, the characteristics of the region to be identified are defined by color distribution, and each pixel belongs to the defined color distribution and means for obtaining color information of each pixel of the input image Means for determining whether or not based on the acquired color information.

  In the present invention, since it is determined whether or not each pixel belongs to a color distribution defined in advance, a region included in the input image is identified by the color distribution. In addition, in order to perform color conversion according to the identified area, for example, appropriate color conversion according to the content of the input image is performed.

  The color conversion apparatus according to the present invention is characterized in that an area included in the input image is identified based on a type of document that is a source of the input image.

  In the present invention, since the area included in the input image is identified based on the type of document that is the source of the input image, appropriate color conversion is performed according to the type of document.

  An image forming apparatus according to the present invention includes the color conversion apparatus according to any one of the above-described inventions, and a unit that forms an image on a sheet based on an image obtained by color conversion by the color conversion apparatus. It is characterized by providing.

  Since the present invention includes means for forming an image on a sheet based on an image obtained by color conversion, the present invention can be applied to a digital copying machine, a printer device, a digital multi-function peripheral, and the like.

The color conversion method according to the present invention is a color conversion method in which each pixel of an input image including a plurality of pixels and including one or more identifiable areas is subjected to color conversion to generate an output image. A first step of converting the pixel value of each pixel of the input image so that the range of pixel values after conversion is uniquely determined according to the characteristics of the region and the range of pixel values after conversion is continuous between the regions ; Stores color correction values at grid points obtained by equally dividing the color space determined by the range of pixel values after conversion, and reads color correction values read from a color correction table in which the number of grid points for each color component is determined in advance. And a second step of performing an interpolation operation on the pixel.

  In the present invention, conversion is performed such that the range of pixel values after conversion differs according to the characteristics of the area to which each pixel belongs, and a color correction table that defines a color space determined by the range of pixel values after conversion is defined. Therefore, for example, a plurality of types of one-dimensional lookup tables determined according to the characteristics of the region to which each pixel belongs and a range of pixel values after conversion by the one-dimensional lookup table By using one three-dimensional lookup table defined according to the above, it is possible to perform an appropriate color conversion process according to the area or document type to which each pixel of the input image belongs.

  In the color conversion method according to the present invention, in the first step, an offset to be added to a pixel value of each pixel of the input image is set according to the region, and the set offset is added to the pixel value. The pixel value is converted based on a one-dimensional lookup table that defines the relationship between the pixel value of each pixel to which the offset is added and the pixel value after conversion.

  In the present invention, in the first step, an offset to be added to the pixel value of each pixel of the input image is set according to the area, and after adding the set offset, the pixel is determined based on the one-dimensional lookup table. Since value conversion is performed, even when a plurality of one-dimensional lookup tables are not prepared, data with different pixel value ranges can be obtained depending on the region.

  In the color conversion method according to the present invention, the first step converts the pixel value of each pixel of the input image based on a one-dimensional lookup table that defines the relationship between the pixel values before and after the conversion, and the converted pixel value The offset to be added is set in accordance with the region, and the set offset is added to the pixel value.

  In the present invention, in the first step, pixel values are converted using a one-dimensional lookup table for each pixel of the input image, and an offset to be added to the converted pixel value is set and set. Since the offset is added to the pixel value, even if a plurality of one-dimensional lookup tables are not prepared, data with different pixel value ranges depending on the area can be obtained.

A computer program according to the present invention causes a computer, a plurality of pixels, the computer program for generating an output image by performing color conversion on each pixel of the input image including an identifiable region or regions, the computer, A step of converting the pixel value of each pixel of the input image so that the range of pixel values after conversion is uniquely determined according to the characteristics of the region to which each pixel belongs , and the range of pixel values after conversion is continuous between the regions. And the computer stores color correction values at grid points obtained by equally dividing the color space determined by the converted pixel value range, and reads the number of grid points for each color component from a predetermined color correction table. A step of performing an interpolation operation on the pixel using the color correction value.

  In the present invention, the computer executes a conversion in which the range of pixel values after conversion differs depending on the characteristics of the region to which each pixel belongs, and the color specified for the color space determined by the range of pixel values after conversion Since the interpolation calculation is executed by the computer using the correction table, for example, conversion is performed using a plurality of types of one-dimensional lookup tables and one-dimensional lookup tables determined according to the characteristics of the region to which each pixel belongs. By using a single three-dimensional lookup table defined according to the range of pixel values later, appropriate color conversion processing according to the area or document type to which each pixel of the input image belongs can be performed.

  In the case of the present invention, conversion is performed such that the range of pixel values after conversion differs according to the characteristics of the region to which each pixel belongs, and a color correction table that defines a color space determined by the range of pixel values after conversion is used. Interpolation calculation is performed. Therefore, for example, a plurality of types of one-dimensional lookup tables determined according to the characteristics of the region to which each pixel belongs, and a single three-dimensional look defined according to the range of pixel values converted by the one-dimensional lookup table. By using the uptable, it is possible to execute an appropriate color conversion process according to the region to which each pixel of the input image belongs or the document type. Further, since the interval between the grid points is constant for all color components, when calculating the grid points in the vicinity of each pixel in the input image, it is sufficient to divide each color component by the same number so that all the colors are obtained by one arithmetic circuit. Component lattice points can be calculated. That is, when switching the one-dimensional lookup table according to the area, hardware can be simplified and cost can be increased as compared with a case where a grid point for each color component is calculated using a plurality of arithmetic circuits. Can be suppressed.

  In the case of the present invention, the offset to be added to the pixel value of each pixel of the input image is set according to the region, and after the set offset is added, the pixel value is converted based on the one-dimensional lookup table. I have to. Therefore, even when a plurality of one-dimensional lookup tables are not prepared, data with different pixel value ranges can be obtained according to the region.

  In the case of the present invention, pixel values are converted using a one-dimensional lookup table for each pixel of the input image, an offset to be added to the pixel value after conversion is set, and the set offset is set as the pixel value. I try to add. Therefore, even when a plurality of one-dimensional lookup tables are not prepared, data with different pixel value ranges can be obtained according to the region.

According to the present invention, since the interval between the lattice points is 2 ⁿ , the lattice points near each pixel can be calculated by dividing each color component by 2 ⁿ . In addition, the accuracy of color conversion can be improved by increasing the number of grid points within the limit of the memory capacity of the color correction table, and conversely, the number of grid points to be decreased according to the memory capacity limit of the color correction table is minimized. Therefore, a decrease in color conversion accuracy can be minimized.

  According to the present invention, since the feature of the region to which each pixel of the input image belongs is identified based on the pixel value, the region can be automatically determined.

  In the case of the present invention, since the area included in the input image is any one of a halftone dot area, a character area, and a photograph area, appropriate color conversion according to these areas can be performed.

  According to the present invention, the feature of the region to be identified is defined by the range of pixel values, and means for determining whether or not the pixel value of each pixel of the input image is included in the defined range. Therefore, the hardware can be simplified because appropriate color correction processing can be performed according to the region to which the input image data belongs only by switching the one-dimensional lookup table.

  In the case of the present invention, since it is determined whether each pixel belongs to a color distribution defined in advance, the area included in the input image can be identified according to the color distribution. Thus, by identifying the area according to the color distribution, for example, the sky or the sea when belonging to the blue area, the plant or leaf when belonging to the green area, and the person and the input image when belonging to the skin color area Content can be determined, and appropriate color conversion processing according to the content can be performed.

  According to the present invention, since the area included in the input image is identified based on the type of document that is the source of the input image, appropriate color conversion can be performed according to the type of document.

  In the case of the present invention, since a means for forming an image on a sheet based on an image obtained by color conversion is provided, it can be applied to a digital copying machine, a printer device, a digital multi-function peripheral, and the like.

Hereinafter, the present invention will be specifically described with reference to the drawings showing embodiments thereof.
Embodiment 1 FIG.
FIG. 1 is a block diagram illustrating the internal configuration of an image forming apparatus according to the present invention. The image forming apparatus according to the present invention includes a control unit 1, an image input unit 2, an image processing unit 3, an image output unit 7, a storage unit 8, and an operation unit 9. The control unit 1 is composed of a CPU that controls each part of the hardware and a RAM that temporarily holds data necessary for the control. The storage unit 8 is, for example, a nonvolatile semiconductor memory, and stores a control program for controlling each unit of hardware, various tables necessary for color conversion processing, and the like. The control unit 1 loads the control program and various tables from the storage unit 8 when the power is turned on and the like, and executes the loaded control program, thereby operating the image forming apparatus according to the present invention as a whole. The operation unit 9 includes various hardware keys for receiving operation instructions from the user.

  The image input unit 2 is a unit that optically reads an image of a document, and includes a light source that irradiates light to a document for reading, an image sensor such as a CCD (Charge Coupled Device), and the like. In the image input unit 2, a reflected light image from a document set at a predetermined reading position is formed on the image sensor, and RGB (R: Red, G: Green, B: Blue) analog electrical signals are output. . The analog electrical signal output from the image input unit 2 is input to the image processing unit 3.

  The image processing unit 3 converts the analog electrical signal output from the image input unit 2 into a digital signal, and then performs various processes according to the type of image to generate an output image signal. The generated image signal is output to the image output unit 7. In the present embodiment, CMYK signals (C: Cyan, M: Magenta, Y: Yellow, K: Black) are generated as output image signals. The internal configuration and operation of the image processing unit 3 will be described later.

  The image output unit 7 is a unit that forms an image on a sheet such as paper or an OHP film based on the image signal output from the image processing unit 3. Therefore, the image output unit 7 generates a latent image on the photosensitive drum by emitting a laser beam in accordance with the photosensitive drum, a charger for charging the photosensitive drum to a predetermined potential, and image data received from the outside. Laser writing device to be generated, developing device for supplying toner to the electrostatic latent image formed on the surface of the photosensitive drum to make it visible, transfer device for transferring the toner image formed on the surface of the photosensitive drum onto the paper Etc. (not shown), and an image desired by the user is formed on a sheet by electrophotography. In addition to image formation by an electrophotographic method using a laser writing apparatus, an image formation may be performed by an inkjet method, a thermal transfer method, a sublimation method, or the like.

  In this embodiment, the image input unit 2 is configured as an image reading unit and the image output unit 7 is configured as an image forming unit. However, the image input unit 2 receives image data from the outside, and transmits the image data to the outside. You may make it comprise as a means.

  FIG. 2 is a block diagram illustrating the configuration of the image processing unit 3. The image processing unit 3 includes an AD conversion unit 31, a shading correction unit 32, an input tone correction unit 33, a region separation processing unit 34, a color correction unit 35A, a black generation / under color removal unit 36, a spatial filter processing unit 37, and an output. A gradation correction unit 38 and a gradation reproduction processing unit 39 are provided.

  The AD conversion unit 31 converts the analog RGB signal input from the image input unit 2 into a digital RGB signal. The shading correction unit 32 performs processing for removing various distortions generated in the illumination system, the imaging system, and the imaging system of the image input unit 2 on the digital RGB signal output from the AD conversion unit 31. In the present embodiment, AD conversion and shading correction are performed inside the image processing unit 3, but a digital RGB signal obtained by performing AD conversion and shading correction may be received from the outside. Good.

  The input tone correction unit 33 converts the RGB reflectance signal into a density signal, and executes image quality adjustment processing such as background density correction and contrast. The input tone correction unit 33 may perform processing for adjusting color balance in addition to the processing described above.

  The region separation processing unit 34 separates each pixel in the input image into one of a character region, a halftone dot region, and a photographic region based on the RGB signal. Based on the separation result, the region separation processing unit 34 outputs a region identification signal indicating which region each pixel belongs to, a color correction unit 35A, a black generation / under color removal unit 36, a spatial filter processing unit 37, an output gradation While outputting to the correction | amendment part 38 and the gradation reproduction process part 39, the RGB signal output from the input gradation correction | amendment part 33 is output to the color correction part 35A of a back | latter stage as it is.

  Hereinafter, processing executed by the region separation processing unit 34 will be described. As a method for separating an input image into a character area, a halftone dot area, and a photographic area, for example, a method described in JP-A-2002-232232 can be used. This method is the sum of the maximum density difference that is the difference between the minimum density value and the maximum density value in an n × m block (for example, 15 × 15) including the target pixel and the absolute value of the density difference between adjacent pixels. A certain total density busyness is calculated and compared with a plurality of predetermined threshold values to separate the background area / printing paper (photo) area from the character edge area / halftone dot area.

In performing the region separation, the following feature amount is used.
(1) Since the density distribution of the background region is usually small in density change, both the maximum density difference and the total density busyness become very small.
(2) The density distribution of the photographic paper photographic area has a smooth density change, and the maximum density difference and the total density busyness are both small and slightly larger than the background area.
(3) Concentration distribution in the halftone dot area varies depending on the halftone dot, but the total density busyness varies by the number of halftone dots, so the ratio of the total density busyness to the maximum density difference Becomes larger. Therefore, when the total density busyness is larger than the product of the maximum density difference and the character / halftone dot determination threshold (one of the plurality of thresholds), it can be determined that the pixel is a halftone pixel.
(4) In the density distribution of the character area, the maximum density difference is large, and the total density busyness increases accordingly. However, since the density change is smaller than that of the halftone dot area, the total density busyness is smaller than that of the halftone dot area. . Therefore, when the total density busyness is smaller than the product of the maximum density difference and the character / halftone dot determination threshold, it is possible to determine that the pixel is a character edge pixel.

Region separation can be performed as follows using the above-described feature amount.
First, the calculated maximum density difference is compared with the maximum density difference threshold, and the calculated total density busyness is compared with the total density busyness threshold. When it is determined that the maximum density difference is smaller than the maximum density difference threshold and the total density busyness is smaller than the total density busyness threshold, it is determined that the target pixel is the background / printing paper region, Otherwise, it is determined to be a character / halftone dot region.

  Next, when it is determined that the area is the background / printing paper area, the calculated maximum density difference is compared with the background / printing paper determination threshold, and if the maximum density difference is smaller, the background area is determined. If the maximum density difference is larger, it is determined that the area is a photographic paper area.

  If it is determined that the region is the character / halftone dot region, the calculated total density busyness is compared with the value obtained by multiplying the maximum density difference by the character / halftone determination threshold value. Is small, it is determined to be a character edge region, and if the total density busyness is larger, it is determined to be a halftone dot region.

Hereinafter, the description will be returned to FIG. 2 to describe the remaining hardware configuration of the image processing unit 3.
The color correction unit 35A performs a process of removing color turbidity based on the spectral characteristics of CMY color materials including unnecessary absorption components in order to realize faithful color reproduction. The present invention relates particularly to the color correction unit 35A, and a region identification signal output from the region separation processing unit 34 is used to correct nonlinearity using a one-dimensional lookup table (hereinafter referred to as a one-dimensional LUT). Then, color conversion is performed based on a three-dimensional lookup table (hereinafter referred to as a three-dimensional LUT). Details of the color correction unit 35A will be described later.

  The black generation / undercolor removal unit 36 performs black generation processing for generating a black (K) signal from the CMY three-color signals after color correction, and subtracts the K signal obtained by black generation from the original CMY signal to create a new CMY. A signal generation process is performed, and a CMY three-color signal is converted into a CMYK four-color signal. As an example of black generation / undercolor removal processing, a method using skeleton black is known. In this method, the input characteristic of the skeleton curve is y = f (x), the input data is C, M, Y, the output data is C ′, M ′, Y ′, K ′, and the undercolor removal rate is If α (0 <α <1), the black generation / under color removal processing is K ′ = f {min (C, M, Y)}, C ′ = C−αK ′, M ′ = M−αK ′. , Y ′ = Y−αK ′.

  The spatial filter processing unit 37 performs spatial filter processing using a digital filter on the image data of the CMYK signal input from the black generation / undercolor removal unit 36 based on the region identification signal, thereby correcting the spatial frequency characteristics. Processing for preventing blurring of output images, deterioration of graininess, and the like is performed. For example, a region separated as a character by the region separation processing unit 34 performs a sharp enhancement process to enhance the reproducibility of a black character or a color character, and emphasizes a high frequency component. For the region separated as a halftone dot region by the region separation processing unit 34, low-pass filter processing for removing the input halftone dot component is performed.

  The output gradation correction unit 38 performs an output gradation correction process for converting a signal such as a density signal into a halftone dot area ratio corresponding to the characteristics of the output destination.

  The gradation reproduction processing unit 39 performs gradation reproduction processing that finally separates an image into pixels and performs processing so that each gradation can be reproduced. For example, for a region separated into photographs by the region separation processing unit 34, binarization or multi-value processing is performed on the screen with an emphasis on gradation reproducibility, and the region separation processing unit 34 separates the characters into characters. The binarization or multi-value processing is performed on a high-resolution screen suitable for high-frequency reproducibility.

  Next, the operation of the color correction unit 35A will be described. 3 is a schematic explanatory diagram for explaining the operation of the color correction unit 35A according to the first embodiment. FIG. 4 is a conceptual diagram of a one-dimensional LUT used in the color correction unit 35A. FIG. 5 is a conceptual diagram of the three-dimensional LUT. FIG. As described above, the RGB signal and the region identification signal output from the region separation processing unit 34 are input to the color correction unit 35A. The color correction unit 35A can identify whether the pixel to be corrected belongs to a halftone dot region, a character region, or a photo region based on the region identification signal output from the region separation processing unit 34. The one-dimensional LUT to be used is switched based on the above, and nonlinearity correction is performed for each color component (for each color component of R, G, B). In the correction using the one-dimensional LUT, conversion is performed so that a three-dimensional LUT (color correction table) mapped to a different area in the RGB color space can be accessed according to the identified area. In color correction using a three-dimensional LUT, color correction is performed using one three-dimensional LUT defined for the RGB color space.

  The one-dimensional LUT is set so that the range of the output signal varies depending on the region to which each pixel of the input image belongs. For example, when each pixel of the input image belongs to a photographic paper photograph area (referred to as a first area), the nonlinearity is corrected by converting the pixel value using the one-dimensional LUT shown in FIG. If each pixel of the input image belongs to a character area or a halftone dot area (referred to as a second area), nonlinearity can be reduced by converting the pixel value using the one-dimensional LUT shown in FIG. Make corrections. Note that the horizontal axis of the graph shown in FIG. 4 indicates the pixel value of the input signal, and the vertical axis indicates the pixel value of the output signal, and the curve drawn in each graph is a conversion curve between the input signal and the output signal. Is shown. These conversion curves are defined as a look-up table (one-dimensional LUT). When pixel values are converted according to these conversion curves, the pixels belonging to the first region are converted so that the pixel values belong to the range of 256 to 511, and the pixels belonging to the second region have a pixel value of 0. To be within the range of ~ 255.

  In this way, by using a one-dimensional LUT that outputs values in different ranges depending on the region to which the pixel belongs, mapping is performed on points in different regions on the color space. That is, when the one-dimensional LUT for the first region is used, as shown in FIG. 5, 256 ≦ R ≦ 511, 256 ≦ G ≦ 511, 256 ≦ B ≦ 511 (where R, G, B are When a one-dimensional LUT for the second region is used and mapped to a point in a region satisfying (which indicates a pixel value of each color component converted by the one-dimensional LUT), 0 ≦ R ≦ 255, 0 ≦ G ≦ 255, 0 It is mapped to a point in the region that satisfies ≦ B ≦ 255. Therefore, by preparing a color correction table corresponding to the points in each region (that is, a three-dimensional LUT in which the first region three-dimensional LUT and the second region three-dimensional LUT are described in one table). Therefore, it is not necessary to switch the three-dimensional LUT according to the area identification signal, and appropriate color correction processing can be performed for each area.

Next, color correction using a three-dimensional LUT will be described. FIG. 6 is a schematic diagram showing an input color space. In the present embodiment has an input color space to RGB color space, the origin P _K [0,0,0] black, the P _W [1,1,1] point of the diagonal and white, the P _K P _R A color space is formed with the R component, P _K P _G as the G component, and P _K P _B as the B component. In this RGB color space, an arbitrary color can be expressed as rP _R + gP _G + bP _B using parameters r, g, and b between 0 and 1. By making the input color space an RGB color space, the axis connecting P _K [0,0,0] and P _W [1,1,1] is on a diagonal line, and the K component is maintained by maintaining this gray axis. Can be maintained, and the image quality does not deteriorate.

  In the present embodiment, pixel values are converted with respect to points (pixels) in the input color space using the above-described one-dimensional LUT, and color conversion processing is performed using the three-dimensional LUT shown in FIG. As the three-dimensional LUT used in this color conversion process, a color correction table proposed by the present applicant in Japanese Patent Application No. 2004-160043 can be used. In this color correction table, the number of grid points in which color correction values are stored is not fixed to 17 or 33 as in the prior art. For example, the number of grid points of a certain color component is increased based on other color components, or It is determined for each color component, such as decreasing, and the color component of the input image data is converted in accordance with the increase or decrease in the number of grid points. Note that the interval between grid points is constant for all color components. For this reason, the three-dimensional LUT can be flexibly set according to the character region, halftone dot region, and photo region that are the region separation results, or according to the document type determination result described later. It is possible to efficiently use the memory capacity while suppressing deterioration in image quality.

  Color correction using a three-dimensional LUT is classified into several types according to the color space dividing method. Here, color correction using a tetrahedron will be described. In color correction using a tetrahedron, the input color space is simply divided into a plurality of unit cells each having eight lattice points along the three color axes, and then the unit cell is further divided by the tetrahedron. FIG. 7 is a schematic diagram showing the unit cell divided by a tetrahedron. In each of FIGS. 7A to 7F, a broken line represents a unit cell, and a solid line represents a divided tetrahedron. For example, the tetrahedron T1 shown in FIG. 7A is a tetrahedron that satisfies the condition that the RGB value of the input signal is R ≧ G ≧ B. The same applies to the other tetrahedrons T2 to T6.

FIG. 8 is an explanatory diagram for explaining an output signal calculation method, and FIG. 9 is a table summarizing parameters used when calculating an output signal. When calculating the output signal, after determining which tetrahedron (T1 to T6) belongs to the size of the color component of the input signal, color correction stored in the three-dimensional LUT in advance corresponding to the four vertices The product sum of the value and the weight calculated by the input signal is calculated and used as the output signal (interpolated value). For example, when the coordinates of each vertex P _K , P _R , P _G , P _B , P _C , P _M , P _Y , P _W of the unit cell are defined as shown in FIG. The interpolation result of point P (x, y, z) is

bK × P _K + bP × P _P + bS × P _S + bW × P _W

It can be expressed as Here, P _P is the coordinate of the primary color (RGB) selected according to the tetrahedron to which the point P belongs, and P _S is the secondary color (CMY) selected according to the tetrahedron to which the point P belongs. (See FIG. 9). Further, bK, bW, bP, and bS indicate weights of the points P _K , P _W , P _P , and P _S , respectively, and take the values shown in FIG. 9 according to the tetrahedron to which the point P belongs. Here, fR, fG, and fB shown in the chart of FIG. 9 represent the lengths of the vectors P _K P in the respective axial directions, and fR = (x−x ₀ ) / (x ₁ −x ₀ ), fG = (y−y ₀ ) / (y ₁ −y ₀ ) and fB = (z−z ₀ ) / (z ₁ −z ₀ ).

  In this embodiment, tetrahedral interpolation is used for color correction using a three-dimensional LUT. However, tetrahedral interpolation is the smallest solid that forms a three-dimensional space, and the number of accesses at one time is small. Since there are four, there is a merit that the calculation cost can be reduced. However, since the three-dimensional space is finely divided, there is a possibility that discontinuity occurs at the boundary. Therefore, it is desirable that the interpolation method to be adopted is not limited to tetrahedral interpolation but is determined flexibly in consideration of calculation cost, accuracy, device characteristics, and the like.

  In the present embodiment, such color correction is realized by hardware processing. FIG. 10 is a block diagram illustrating the internal configuration of the color correction unit 35A according to the first embodiment. The color correction unit 35A includes a one-dimensional LUT calculation unit 351 that performs nonlinear correction using a one-dimensional LUT and a three-dimensional LUT calculation unit 352 that performs color correction using a three-dimensional LUT. The one-dimensional LUT used in the one-dimensional LUT calculation unit 351 and the three-dimensional LUT used in the three-dimensional LUT calculation unit 352 are stored in the storage unit 8 in advance, and the control unit is turned on when the image forming apparatus is turned on. 1 is configured to be loaded into the RAM in the memory.

  The one-dimensional LUT calculation unit 351 receives the RGB signal output from the region separation processing unit 34 and the region identification signal. The address reference unit 351a of the one-dimensional LUT calculation unit 351 accesses the one-dimensional LUT for the first region or the second region loaded in the control unit 1 based on the input RGB signal and region identification signal. An RGB signal whose nonlinearity is corrected is acquired. That is, the one-dimensional LUT calculation unit 351 functions as conversion means in the color conversion apparatus of the present invention. The RGB signal whose nonlinearity is corrected by the one-dimensional LUT calculation unit 351 is output to the weighting calculation unit 352a and the address calculation unit 352b of the three-dimensional LUT calculation unit 352.

  The weight calculation unit 352a calculates a desired weight from the input signal and passes the value to the interpolation calculation unit 352c. On the other hand, the address calculation unit 352b calculates an address to be accessed from the input signal, and obtains the color correction value held in the three-dimensional LUT by accessing the RAM in the control unit 1. The color correction value acquired by the address calculation unit 352b is output to the interpolation calculation unit 352c. The interpolation calculation unit 352c performs a three-dimensional interpolation calculation based on the weighting output from the weighting calculation unit 352a and the color correction value output from the address calculation unit 352b, and generates an output signal (CMY signal). That is, the three-dimensional LUT calculation unit 352 functions as a calculation unit in the color conversion apparatus of the present invention.

  In the three-dimensional LUT calculation unit 352 of the present embodiment, the output signal is obtained by interpolation calculation, but output image data for all combinations of input image data is calculated in advance without performing interpolation calculation. The color correction value may be acquired by a direct conversion method using a table. In this case, the three-dimensional LUT calculation unit 352 does not need the weighting calculation unit 352a and the interpolation calculation unit 352c, and is configured only by the address calculation unit 352b.

Embodiment 2. FIG.
In the color correction unit 35A shown in the first embodiment, the one-dimensional LUT is switched according to the region identification signal, but a different offset amount is added to the input signal (RGB signal) instead of switching the one-dimensional LUT. In addition, a configuration in which nonlinearity correction is performed using one one-dimensional LUT may be employed. The operation of the color correction unit 35B used in this embodiment will be described with reference to FIG. 11, and the internal configuration will be described with reference to FIG. Since the configuration other than the color correction unit 35B is the same as that of the first embodiment, the description thereof will be omitted.

  FIG. 11 is a schematic explanatory diagram for explaining the operation of the color correction unit 35B according to the second embodiment, and FIG. 12 is a conceptual diagram of a one-dimensional LUT used in the color correction unit 35B. The RGB signal and the region identification signal output from the region separation processing unit 34 are input to the color correction unit 35B. The color correction unit 35B can identify whether the pixel to be corrected belongs to a halftone dot region, a character region, or a photo region based on the region identification signal, and should be added to the RGB signal based on the identification result Set the offset amount. Then, after the set offset amount is added to the RGB signal, pixel value conversion using a one-dimensional LUT is performed for each color component (for each color component of R, G, and B) in order to correct nonlinearity. . In the conversion of the pixel value using the one-dimensional LUT, conversion is performed so that a three-dimensional LUT (color correction table) mapped in different regions of the RGB color space can be accessed according to the added offset amount. In color correction using a three-dimensional LUT, color correction is performed using one three-dimensional LUT defined for the RGB color space.

  In this embodiment, the input / output ranges are made different by switching the offset amount according to the region. For example, when each pixel of the input image belongs to a photographic paper photograph area (first area), the pixel value range of the input signal can be converted to 256 to 511 by setting the offset amount to 256, By using the one-dimensional LUT shown in FIG. 12, the pixel value range of the output signal can be converted into 256 to 511. When each pixel of the input image belongs to a character area or a halftone dot area (first area), the pixel value range of the input signal can be set to 0 to 255 by setting the offset amount to 0. Similarly, by using the one-dimensional LUT shown in FIG. 12, the range of the pixel value of the output signal can be set to 0-255.

  In this way, by using a one-dimensional LUT that outputs values in different ranges depending on the region to which the pixel belongs, mapping is performed on points in different regions on the color space. That is, for pixels belonging to the first region, 256 ≦ R ≦ 511, 256 ≦ G ≦ 511, 256 ≦ B ≦ 511 (where R, G, and B are pixels of each color component of the pixel converted by the one-dimensional LUT. The pixels belonging to the second region are mapped to points in the region satisfying 0 ≦ R ≦ 255, 0 ≦ G ≦ 255, and 0 ≦ B ≦ 255. It will be. Therefore, by preparing a color correction table corresponding to the points in each region (that is, a three-dimensional LUT in which the first region three-dimensional LUT and the second region three-dimensional LUT are described in one table). Therefore, it is not necessary to switch the three-dimensional LUT according to the area identification signal, and appropriate color correction processing can be performed for each area.

  FIG. 13 is a block diagram illustrating an internal configuration of the color correction unit 35B according to the second embodiment. The color correction unit 35B includes an offset calculation unit 353 that sets an offset amount according to the region identification signal, a one-dimensional LUT calculation unit 354 that corrects nonlinearity using a one-dimensional LUT, and a color correction that uses a three-dimensional LUT. The three-dimensional LUT calculation unit 355 performs the above. The one-dimensional LUT used in the one-dimensional LUT calculation unit 354 and the three-dimensional LUT used in the three-dimensional LUT calculation unit 355 are stored in the storage unit 8 in advance, and the control unit is turned on when the image forming apparatus is turned on. 1 is configured to be loaded into the RAM in the memory.

  The offset calculation unit 353 receives the RGB signal and the region identification signal output from the region separation processing unit 34. The offset amount switching unit 353b of the offset calculation unit 353 switches the offset amount to be added according to the input region identification signal. For example, the offset amount is set to 256 for a pixel belonging to the first region, and the offset amount is set to 0 for a pixel belonging to the second region. The offset amount set by the offset amount switching unit 353b is output to the offset amount adding unit 353a and added to the input RGB signal.

  The RGB signal to which the offset amount is added is output to the one-dimensional LUT calculation unit 354. The address reference unit 354a of the one-dimensional LUT calculation unit 354 accesses the one-dimensional LUT loaded in the control unit 1 based on the value obtained by adding the offset amount to the RGB signal, and acquires the RGB signal corrected for nonlinearity. To do. The RGB signal whose nonlinearity is corrected by the one-dimensional LUT calculation unit 354 is output to the weighting calculation unit 355a and the address calculation unit 355b of the three-dimensional LUT calculation unit 355.

  The weight calculation unit 355a calculates a desired weight from the input signal and passes the value to the interpolation calculation unit 355c. On the other hand, the address calculation unit 355b calculates an address to be accessed from the input signal, and acquires the color correction value held in the three-dimensional LUT by accessing the RAM in the control unit 1. The color correction value acquired by the address calculation unit 355b is output to the interpolation calculation unit 355c. The interpolation calculation unit 355c performs a three-dimensional interpolation calculation based on the weighting output from the weighting calculation unit 355a and the color correction value output from the address calculation unit 355b, and generates an output signal (CMY signal).

  When performing color correction with the above-described configuration, it is only necessary to switch the offset amount, and it is appropriate according to the region to which each pixel of the input image belongs without switching between the one-dimensional LUT and the three-dimensional LUT by the region identification signal. It is possible to perform a color correction process.

Embodiment 3 FIG.
In the color correction unit 35B shown in the second embodiment, a different offset amount is added to the input signal (RGB signal) according to the region identification signal, and nonlinearity correction using a one-dimensional LUT and a three-dimensional LUT are used. However, after correcting nonlinearity using a one-dimensional LUT, an offset amount corresponding to a region identification signal is added and color correction using a three-dimensional LUT is performed. It is good. The operation of the color correction unit 35C used in the present embodiment will be described with reference to FIG. 14, and the internal configuration will be described with reference to FIG. Since the configuration other than the color correction unit 35C is the same as that of the first embodiment, the description thereof will be omitted.

  FIG. 14 is a schematic explanatory diagram for explaining the operation of the color correction unit 35C according to the third embodiment, and FIG. 15 is a conceptual diagram of a three-dimensional LUT used in the color correction unit 35C. The RGB signal and the region identification signal output from the region separation processing unit 34 are input to the color correction unit 35C. First, the color correction unit 35C corrects nonlinearity with respect to an input signal (RGB signal) using a one-dimensional LUT prepared in advance. Further, since the color correction unit 35C can identify whether the pixel to be corrected belongs to a halftone dot region, a character region, or a photographic region based on the region identification signal, the nonlinearity is determined based on the identification result. The offset amount to be added to the RGB signal corrected for is set for each color component. Then, after adding the set offset amount to the RGB signal, color correction is performed using a three-dimensional LUT. In the present embodiment, the offset amount is set so as to be mapped to different regions in the RGB color space according to the region to which each pixel belongs.

  In this embodiment, after correcting nonlinearity using a one-dimensional LUT, an offset amount corresponding to a region is added. For example, when each pixel of the input image belongs to a photographic paper photograph area (first area), the offset amount for the R signal is set to 256, the offset amount for the G signal and the B signal is set to 0, and each pixel of the input image is set. Is in the character area or halftone dot area (second area), the offset amount for each RGB signal is set to zero. As a result, as shown in FIG. 15, the pixels belonging to the first region are mapped to points in the region satisfying 256 ≦ R ≦ 511, 0 ≦ G ≦ 255, and 0 ≦ B ≦ 255. The pixels belonging to the two regions are mapped to points in the region satisfying 0 ≦ R ≦ 255, 0 ≦ G ≦ 255, and 0 ≦ B ≦ 255. Therefore, by preparing a color correction table corresponding to the points in each region (that is, a three-dimensional LUT in which the first region three-dimensional LUT and the second region three-dimensional LUT are described in one table). Therefore, it is not necessary to switch the three-dimensional LUT according to the area identification signal, and appropriate color correction processing can be performed for each area. In addition, unlike the above-described embodiment, since the color correction value can be held only on the diagonal line, the table memory that has been wasted can be arranged efficiently, and the table memory can be efficiently held. It becomes.

  FIG. 16 is a block diagram illustrating an internal configuration of the color correction unit 35C according to the third embodiment. The color correction unit 35C includes a one-dimensional LUT calculation unit 356 that corrects nonlinearity using a one-dimensional LUT, an offset calculation unit 357 that sets an offset amount according to a region identification signal, and color correction using a three-dimensional LUT. The three-dimensional LUT calculation unit 358 performs the above. The one-dimensional LUT used in the one-dimensional LUT calculation unit 356 and the three-dimensional LUT used in the three-dimensional LUT calculation unit 358 are stored in the storage unit 8 in advance, and the control unit is turned on when the power of the image forming apparatus is turned on. 1 is configured to be loaded into the RAM in the memory.

  The one-dimensional LUT calculation unit 356 receives the RGB signal output from the region separation processing unit 34. The address reference unit 356a of the one-dimensional LUT calculation unit 356 accesses the one-dimensional LUT loaded in the control unit 1 based on the input RGB signal, and acquires an RGB signal in which nonlinearity is corrected. The RGB signal whose nonlinearity is corrected by the one-dimensional LUT calculation unit 356 is output to the offset calculation unit 357.

  The offset calculation unit 357 receives the RGB signal output from the one-dimensional LUT calculation unit 356 and the region identification signal output from the region separation processing unit 34. The offset amount switching unit 357b of the offset calculation unit 357 switches the offset amount to be added according to the input region identification signal. For example, in the case of a pixel belonging to the first region, the offset amount for the R signal is set to 256, and the offset amounts for the G signal and the B signal are set to 0. In the case of a pixel belonging to the second area, the offset amount is set to 0 for each RGB signal. The offset amount set by the offset amount switching unit 357b is output to the offset amount adding unit 357a and added to the input RGB signal. The RGB signal to which the offset amount is added is output to the weighting calculation unit 358a and the address calculation unit 358b of the three-dimensional LUT calculation unit 358.

  The weight calculation unit 358a calculates a desired weight from the input signal and passes the value to the interpolation calculation unit 358c. On the other hand, the address calculation unit 358b calculates an address to be accessed from the input signal, accesses the three-dimensional LUT loaded in the control unit 1, and acquires a color correction value. The color correction value acquired by the address calculation unit 358b is output to the interpolation calculation unit 358c. The interpolation calculation unit 358c performs a three-dimensional interpolation calculation based on the weighting output from the weighting calculation unit 358a and the color correction value output from the address calculation unit 358b, and generates an output signal (CMY signal).

  When performing color correction with the above-described configuration, it is only necessary to switch the offset amount, and it is appropriate according to the region to which each pixel of the input image belongs without switching between the one-dimensional LUT and the three-dimensional LUT by the region identification signal. It is possible to perform a color correction process. In addition, the three-dimensional LUT can be efficiently held.

  FIG. 17 is a schematic explanatory view for explaining a modification of the present embodiment. In the embodiment described above, the offset amount is added before the three-dimensional LUT calculation unit 358. However, the first offset amount is added before the one-dimensional LUT calculation unit 356, and the three-dimensional LUT calculation unit 358 is added. The second offset amount may be added in the previous stage.

  For example, in the addition of the first offset amount, similarly to the second embodiment, the offset amount is set to 256 in the case of a pixel belonging to the first region, and the offset amount is set in the case of a pixel belonging to the second region. Set to 0. On the other hand, in the addition of the second offset amount, for pixels belonging to the first region, the offset amount for the R signal is set to 0, the offset amount for the G signal and the B signal is set to −256, and the pixels belonging to the second region are set. In this case, the offset amount for each RGB signal is set to zero. As a result, similar to that shown in FIG. 15, the pixels belonging to the first region are mapped to points in the region satisfying 256 ≦ R ≦ 511, 0 ≦ G ≦ 255, and 0 ≦ B ≦ 255. Thus, the pixels belonging to the second region are mapped to points in the region satisfying 0 ≦ R ≦ 255, 0 ≦ G ≦ 255, and 0 ≦ B ≦ 255. Therefore, by preparing a color correction table corresponding to the points in each region (that is, a three-dimensional LUT in which the first region three-dimensional LUT and the second region three-dimensional LUT are described in one table). Therefore, it is not necessary to switch the three-dimensional LUT according to the area identification signal, and appropriate color correction processing can be performed for each area. Further, the table memory can be arranged efficiently, and the table memory can be efficiently held.

Embodiment 4 FIG.
In the above-described embodiment, the mode in which appropriate color correction processing is performed in accordance with the region identification signal has been described. However, appropriate color correction processing is performed in accordance with the type of document that is the source of the input image, instead of the region identification signal. The structure to perform may be sufficient.

  FIG. 18 is a block diagram illustrating a configuration of the image processing unit 4 included in the image forming apparatus according to the fourth embodiment. Since the configuration other than the image processing unit 4 is exactly the same as that of the first embodiment, the description thereof will be omitted. The image processing unit 4 includes an AD conversion unit 41, a shading correction unit 42, a document type determination unit 43, an input tone correction unit 44, a region separation processing unit 45, a color correction unit 46, a black generation / under color removal unit 47, a space A filter processing unit 48, an output tone correction unit 49A, and a tone reproduction processing unit 49B are provided.

  In the image processing unit 4 of the present embodiment, as a process preceding the document type determination unit 43, the analog RGB signal is converted into a digital RGB signal by the AD conversion unit 41, and the image input unit 2 is converted by the shading correction unit 42. A process of removing various distortions generated in the illumination system, the imaging system, and the imaging system (see FIG. 1) and a process of adjusting the color balance are performed.

  The document type determination unit 43 converts the RGB signal (RGB reflectance signal), from which various distortions have been removed by the shading correction unit 42 and the color balance has been adjusted, into a density signal (for example, a CMY signal). A document type is determined, for example, whether the input document image is a character document, a printed photograph document, a photographic paper photograph document, or a character / printed photograph document combining them.

  As a method for determining the document type, for example, a method described in JP-A-2002-218232 can be used. In this method, pre-scanning is performed, and from the density histogram, the number of low-frequency density sections that are smaller than a predetermined threshold value, the first maximum-frequency density section, and the density sections adjacent to the first maximum-frequency density section. A density category of the second maximum power having the maximum power value is obtained, and the ratio of the first maximum power value to the total number of pixels and the ratio of (first maximum power value−second maximum power value) to the total number of pixels. Is calculated. By comparing these values with predetermined first threshold value, second threshold value, and third threshold value, the document is classified into one of a character, a photograph, and a character / photo.

  If the image is determined to be a photograph, the input image data is binarized, a mask composed of a plurality of pixels including the target pixel is set, and “0” to “1”, “1” in the main scanning direction and the sub scanning direction are set. ”To“ 0 ”(only the number of change points in the main scanning direction may be used for speeding up), and if it is equal to or greater than a predetermined threshold value, a printed photograph (in the case of a printed photograph, local The image signal variation in the area is large.) If it is less than the threshold value, it is determined as a photographic paper photograph.

  The document type may be determined by performing a pre-scan as described above, or by storing the image signal after shading correction in a storage medium such as a hard disk and reading the stored image signal. good.

  A document type determination signal as a document type determination result is output to the color correction unit 46. The color correction unit 46 adjusts image quality such as background density and contrast in the input tone correction unit 44, performs region separation processing in the region separation processing unit 45, and then performs color correction according to the document type determination signal. Process. In this color correction processing, for example, in the case of a photographic paper photographic original or a character photographic paper photographic original, the one-dimensional LUT for the first area shown in FIG. 4A and the first area for the first area shown in FIG. Color correction processing is performed using a three-dimensional LUT. In the case of a character document, a character-printed photo document, or a printed photo document, the one-dimensional LUT for the second area shown in FIG. 4B and the three-dimensional LUT for the second area shown in FIG. Color correction processing is performed using

  After the color correction processing is performed by the color correction unit 46, the black generation / under color removal unit 47, the spatial filter processing unit 48, the output gradation correction unit 49A, and the gradation reproduction processing unit 49B are the same as in the first embodiment. Perform the process.

  In the present embodiment, the document type determination unit 43 determines the document type. However, the document type may be determined based on a manual input by the user. In this case, information related to the type of document is received through the operation unit 9, and the control unit 1 determines the type of document based on the received information. The control unit 1 outputs a document type determination signal as a determination result to the color correction unit 46, and the color correction unit 46 performs an appropriate color correction process according to the document type determination signal.

  In FIG. 18, a document type determination signal, which is a document type determination result, is fed back only to the color correction unit 46. However, the input tone correction unit 44, the region separation processing unit 45, the spatial filter processing unit 48, and the floor A configuration may be adopted in which processing is performed in accordance with the type of document input to the tone reproduction processing unit 49B.

Embodiment 5 FIG.
In the above-described embodiments, the present invention is realized by the image forming apparatus configured by hardware. However, the present invention may be configured by software processing.

  FIG. 19 is a block diagram illustrating an internal configuration of an image processing apparatus in which a computer program according to the present invention is installed. In the figure, reference numeral 10 denotes an image processing apparatus according to the present embodiment, specifically a personal computer, a workstation, or the like. The image processing apparatus 10 includes a CPU 11. The CPU 11 loads various control programs from the ROM 13 connected to the bus 12 to the RAM 14 and executes them. The operation unit 15, auxiliary storage unit 16, storage unit 17, image input The hardware such as the IF 18 and the image output IF 19 is controlled.

  The operation unit 15 includes a keyboard, a mouse, and the like in order to select image data to be processed, input parameters necessary for image processing, an instruction to start image processing, and the like. The auxiliary storage unit 16 includes a reading device for reading the computer program from the recording medium 20 on which the computer program of the present invention is recorded. As the recording medium 20, an FD (Flexible Disk), a CD-ROM, or the like can be used. The storage unit 17 includes a hard disk device having a disk-shaped magnetic recording medium, and stores a computer program read by the auxiliary storage unit 16, image data input through the image input IF 18, and the like.

  The image input IF 18 is an interface for connecting the image input device 21. An interface such as SCSI or USB can be used as the image input IF 18, and examples of the image input device 21 connected to the image input IF 18 include a scanner device and a digital still camera. The image output IF 19 is an interface for connecting the image output device 22. An interface such as USB can be used as the image output IF 19, and examples of the image output device 22 connected to the image output IF 19 include a printer device and a digital multi-function peripheral.

  As the recording medium 20 for recording the computer program according to the present invention, in addition to the above-mentioned FD and CD-ROM, optical recording medium such as MO, MD, DVD, magnetic recording medium such as hard disk, IC card, memory card Further, it is also possible to use a semiconductor memory such as a card-type recording medium such as an optical card, a mask ROM, an EPROM (Erasable Programmable Read Only Memory), an EEPROM (Electrically Erasable Programmable Read Only Memory), a flash ROM, or the like.

  When the image processing apparatus 10 includes a modem or the like as a communication means for connecting to a server apparatus or the like via a communication network, the computer program of the present invention is stored in advance in the server apparatus, and the server The computer program may be downloaded from the apparatus and installed in the image processing apparatus 10.

  FIG. 20 is a flowchart for explaining the procedure of color correction processing executed by the image processing apparatus 10. The CPU 11 of the image processing apparatus 10 performs a region separation process on the image data read from the storage unit 17 and the image data input through the image input IF 18 (step S11). For the region separation process, a method similar to the method described in the first embodiment can be used, and information (region identification information) about a region to which each pixel belongs is generated. In addition, as a pre-process of this region separation process, a configuration in which processes such as shading correction and input tone correction are performed may be employed.

Next, the CPU 11 performs conversion of input image data using the one-dimensional LUT switched based on the area identification information (step S12). For example, when it can be determined that the target pixel belongs to the photographic paper photograph area (referred to as the first area), nonlinearity is obtained by converting the pixel value using a one-dimensional LUT as shown in FIG. When it can be determined that the image belongs to the character region or the halftone dot region (referred to as the second region), the pixel value is converted using a one-dimensional LUT as shown in FIG. Perform sex correction.
Note that the one-dimensional LUT used in this conversion may be provided as an internal parameter of the computer program of the present invention, or may be provided separately from the computer program.

Next, the CPU 11 calculates a weight using the converted data (step S13), obtains an address using the converted data, and reads a color correction value from the three-dimensional LUT (step S14). That is, by converting the pixel value using a one-dimensional LUT corresponding to the region, the conversion destination is mapped to a different region in the color space. For example, the three-dimensional as shown in FIG. By using the LUT, it is possible to perform appropriate color conversion according to the area.
The three-dimensional LUT used in the color conversion may be configured as an internal parameter of the computer program of the present invention, or may be provided separately from the computer program.

  Next, the CPU 11 performs an interpolation operation based on the weight calculated in step S13 and the correction value read in step S14, and generates output image data (step S15).

  As described above, in the present embodiment, the one-dimensional LUT to be used is switched based on the region identification information, and after the pixel value is converted using the switched one-dimensional LUT, the color conversion using the three-dimensional LUT is performed. Do. In other words, the image forming apparatus described in the first embodiment is realized by software processing.

Embodiment 6 FIG.
In the fifth embodiment, the computer program for realizing the image forming apparatus of the first embodiment has been described. However, the color correction unit 35B described in the second embodiment may be realized by a computer program. The internal configuration of the image processing apparatus 10 that installs the computer program according to the present embodiment is the same as that described in the fifth embodiment.

  FIG. 21 is a flowchart for explaining the procedure of color conversion processing executed by the image processing apparatus 10. The CPU 11 of the image processing apparatus 10 performs a region separation process on the image data read from the storage unit 17 and the image data input through the image input IF 18 (step S21). For the region separation process, a method similar to the method described in the first embodiment can be used, and information (region identification information) about a region to which each pixel belongs is generated. In addition, as a pre-process of this region separation process, a configuration in which processes such as shading correction and input tone correction are performed may be employed.

  Next, the CPU 11 sets an offset amount for each pixel based on the generated region identification information (step S22), and adds the set offset amount to the input image data. (Step S23). Then, using the one-dimensional LUT as shown in FIG. 12, conversion is performed on the data added with the offset amount (step S24). For example, when each pixel of the input image data belongs to the photographic paper photograph area (first area), the pixel value range of the input image data can be set to 256 to 511 by setting the offset amount to 256. By using the one-dimensional LUT as shown in FIG. 12, the pixel value range of the output image data can be set to 256 to 511. When each pixel of the input image data belongs to a character area or a halftone dot area (second area), the pixel value range of the input image data is set to 0 to 255 by setting the offset amount to 0. Similarly, by using a one-dimensional LUT as shown in FIG. 12, the range of pixel values of output image data can be set to 0-255.

  Next, the CPU 11 calculates a weight using the converted data (step S25), obtains an address using the converted data, and reads a color correction value from the three-dimensional LUT (step S26). That is, in the process of step S24, the conversion destination is mapped to a different area in the color space depending on the area. For example, an appropriate color corresponding to the area can be obtained by using a three-dimensional LUT as shown in FIG. Conversion can be performed.

  Next, the CPU 11 performs an interpolation operation based on the weight calculated in step S25 and the correction value read in step S26, and generates output image data (step S27).

  Thus, in this embodiment, it is only necessary to switch the offset amount, and an appropriate color can be selected according to the region to which each pixel of the input image belongs without switching between the one-dimensional LUT and the three-dimensional LUT according to the region identification information. Conversion processing can be performed.

Embodiment 7 FIG.
In the fifth embodiment, the computer program for realizing the image forming apparatus of the first embodiment has been described. However, the color correction unit 35C described in the third embodiment may be realized by a computer program. The internal configuration of the image processing apparatus 10 that installs the computer program according to the present embodiment is the same as that described in the fifth embodiment.

  FIG. 22 is a flowchart illustrating the procedure of the color conversion process executed by the image processing apparatus 10. The CPU 11 of the image processing apparatus 10 performs a region separation process on the image data read from the storage unit 17 and the image data input through the image input IF 18 (step S31). For the region separation process, a method similar to the method described in the first embodiment can be used, and information (region identification information) about a region to which each pixel belongs is generated. In addition, as a pre-process of this region separation process, a configuration in which processes such as shading correction and input tone correction are performed may be employed.

  Next, the CPU 11 performs conversion of the input image data using the one-dimensional LUT (step S32). One-dimensional LUT used in this conversion is one type, and switching based on area identification information is not performed.

  Next, the CPU 11 adds an offset amount corresponding to the area identification information to the converted input image data (step S33). For example, when the target pixel belongs to the first region based on the region identification information, the offset amount 256 is added to the R signal, and the offset amount 0 is added to the G signal and the B signal. Further, when the target pixel belongs to the second region based on the region identification information, an offset amount 0 is added to each of the RGB signals.

  Next, the CPU 11 calculates a weight using the data added with the offset amount (step S34), obtains an address using the data added with the offset amount, and reads the color correction value from the three-dimensional LUT (step S34). S35). That is, by adding an offset amount corresponding to the region in step S33, it is possible to map to a different region in the color space. For example, using a three-dimensional LUT as shown in FIG. It is possible to perform appropriate color conversion.

  Next, the CPU 11 performs an interpolation operation based on the weight calculated in step S34 and the correction value read in step S35, and generates output image data (step S36).

  Thus, in this embodiment, it is only necessary to switch the offset amount, and an appropriate color can be selected according to the region to which each pixel of the input image belongs without switching between the one-dimensional LUT and the three-dimensional LUT by the region identification signal. Correction processing can be performed. In addition, the three-dimensional LUT can be efficiently held.

Embodiment 8 FIG.
In the above-described embodiment, the mode in which appropriate color correction processing is performed according to the area identification signal or the document type determination signal has been described. However, instead of using these signals, color correction is performed according to the content of the input image. It is good. Here, the content of the input image is obtained by classifying image data as a meaningful group on the color space and the original image plane. For example, in the case of a blue region on the color space and those pixels grouped together on the original image plane to cover the entire image, it can be predicted as the sea or the sky. Further, if the skin color is gathered on the original image plane, it can be predicted that the image is a person image. Each of them can be seen favorably by performing an enhancement process with color correction.

  In this way, an impression of a specific content depends on a memory color empirically nurtured by an individual's preconception and expectation. This is based on the fact that, for example, a healthy and bright skin color is preferred, and a bright color is preferred for plant flowers and leaves. In particular, it has been proposed that skin color, plant green, sky blue, and the like give a visually good impression by performing selective emphasis processing that approaches a memory color. In this embodiment, by using the color distribution information, for example, when there are many blue areas in the color space of the image data, it is determined that the area is the sea or sky, and the saturation is emphasized only for the area. Color correction can be performed. In addition, when there are many skin color areas, it is determined that the image is a person image, and color correction with enhanced lightness can be performed only for human skin. In this way, by switching the color correction processing for each content of the input image, it is possible to perform preferable color reproduction.

  FIG. 23 is a block diagram illustrating a configuration of the image processing unit 5 provided in the image forming apparatus according to the eighth embodiment. The image processing unit 5 includes an AD conversion unit 51, a shading correction unit 52, a document type determination unit 53, an input tone correction unit 54, a color distribution information extraction unit 55, a region separation processing unit 56, a color correction unit 57, a black generation / An under color removal unit 58, a spatial filter processing unit 59, an output tone correction unit 60A, and a tone reproduction processing unit 60B are provided. Among these components, the components other than the color distribution information extraction unit 55 are exactly the same as the respective components of the image processing unit 4 shown in the fourth embodiment, for example.

  The color distribution information extraction unit 55 determines whether a specific region such as a blue region, a green region, or a skin color region is included in the input image based on the output signal (CMY signal) of the input tone correction unit 54. A color distribution information signal indicating which region each pixel belongs to is output. When not included in the specific area, the color distribution information signal is output as an area not included in the specific area. Then, the color distribution information signal is output to the color correction unit 57 and the CMY signal input from the input tone correction unit 54 is output to the subsequent region separation processing unit 56 as it is. In FIG. 23, the color distribution information extraction unit 55 is provided before the region separation processing unit 56, but may be provided after the region separation processing unit 56. The document type determination unit 53 is not always necessary. When the document type determination unit 53 is not provided, the RGB signal output from the shading correction unit 52 is input to the input tone correction unit 54, and the RGB signal whose tone is corrected by the input tone correction unit 54 is color. The information is input to the distribution information extraction unit 55.

Hereinafter, processing in the color distribution information extraction unit 55 will be described. FIG. 24 is a block diagram illustrating the internal configuration of the color distribution information extraction unit 55. The color distribution information extracting unit 55, and color measurement in advance of each color patch of the color chart document with calorimeter, for example, CIE1976L ^* a ^* b ^* signal (CIE: Commission International del'Eclairage, L *: lightness, a ^* , B ^* : chromaticity) and the like, and color space conversion characteristics are formulated from CMY signals output from the input tone correction unit 54 (RGB signals when the document type determination unit 53 is not installed). The signal conversion unit 551 of the color distribution information extraction unit 55 converts the input RGB signal to an L ^* a ^* b ^* color signal that does not depend on the characteristics of the device by using such a conversion formula. This conversion can be realized by a masking equation.

Then, the determination unit 554 is within a color space constituting a pattern portion to be extracted (in this case, a blue region, a green region, and a skin color region and represents the content of the input image), that is, the CIE 1976 L ^* a ^* b ^* color space. A region in the region is specified, and pixels included in the region are extracted based on the L ^* a ^* b ^* color signal. A specific extraction method is as follows. The average value (a ^* bar, b ^* bar), variance value (σa ^* , σb ^* ) of each luminance (L *) and corresponding color difference information (chromaticity) in a pattern portion to be extracted from a sample image or the like in advance Ask for. This is held as an LUT 553 corresponding to each luminance (L *). When the signal obtained by color-converting the output signal of the input tone correction unit 54 is L ^* , a ^* , b ^* , whether or not it belongs to the pattern part to be extracted is satisfied by applying the following formula at the same time. Depending on whether or not.

Here, n is a natural number. This expression represents a determination expression for determining whether the chromaticities a ^* and b ^* are within a predetermined range. The color distribution information extraction unit 55 obtains an average value (a ^* bar, b ^* bar) and variance value (σa ^* , σb ^* ) of color difference information with respect to the color-converted luminance signal L * by referring to the LUT553. If the average value and variance value of the color difference information are formulated in advance for each luminance, it is possible to directly obtain the average value and the calculation unit 552 of the color difference information corresponding to each luminance without having the LUT 553. . Then, the calculation unit 552 multiplies the variance value (σa ^* , σb ^* ) of the color difference information by a natural number n and adds / subtracts the average value (a ^* bar, b ^* bar) of the color difference information as shown in Equation 1. Is output to the determination unit 554. The determination unit 554 determines whether the color-converted color difference information (a ^* , b ^* ) is within the range of values obtained by the calculation unit 552 as in Equation 1. Further, the determination unit 554 also determines whether or not the total number of pixels extracted by Equation 1 is greater than a predetermined threshold with respect to the number of pixels of all images. For example, even if the pixel extracted by Equation 1 is in the blue region, if it is only a small amount (for example, about 1%) with respect to the number of pixels in the entire image, the blue region does not cover the entire image. It cannot be judged as sky or sea (rather, it is judged as sudden noise). On the contrary, if it is about 50% or more with respect to the number of pixels of the whole image, it is considered that the whole image is covered, and it can be judged that it is the sky or the sea. When the total number of pixels thus extracted is larger than a predetermined threshold, the determination unit determines that the extracted pixels belong to the pattern part to be extracted, and sets a value indicating the area of the pattern part to be extracted. Output as a color distribution information signal.

  The color distribution information extraction may be performed at the time of pre-scanning, or may be performed by storing the image signal after the shading correction processing is performed in a storage medium such as a hard disk and reading out the stored image signal.

Next, the operation of the color correction unit 57 (color conversion device) will be described. 25 is a schematic explanatory diagram for explaining the operation of the color correction unit 57 according to the eighth embodiment. FIG. 26 is a conceptual diagram of a one-dimensional LUT used in the color correction unit 57. FIG. 27 is a conceptual diagram of the three-dimensional LUT. FIG. As described above, the color correction unit 57 receives the CMY signal from the region separation processing unit 56 and the color distribution information signal output from the color distribution information extraction unit 55. In the present embodiment, first, the offset amount switched according to the color distribution information signal is added to the input CMY signal (first offset addition), and data using the one-dimensional LUT is added to the CMY signal to which the offset is added. Perform conversion. Then, an offset amount switched according to the color distribution information signal is added to the converted data (second offset addition), and color correction using the three-dimensional LUT is performed on the CMY to which the offset is added. In the three-dimensional LUT used in the present embodiment, all lattice point intervals (lattice width) are 2 ⁿ (n is an integer), and the number of lattice points is arbitrary. For example, when the grid width of the three-dimensional LUT is 16 (= 2 ⁴ ), if the output value range of the preceding one-dimensional LUT is 0 to 255, the number of grid points is 17 (256/16 + 1), If the output value range of the one-dimensional LUT in the previous stage is 0 to 335, the number of grid points is 22 (336/16 + 1 = 22).

  FIG. 28 is a block diagram showing an internal configuration of the color correction unit 57 according to the eighth embodiment. For example, if the output signal CMY of the input tone correction unit 54 is (255, 255, 30) and belongs to the blue region (referred to as the first region shown in FIG. 26), the offset calculation unit 571 The offset amount switching unit 571b switches the offset amount to (0, 0, 0) based on the color distribution information signal. When the offset amount adding unit 571a adds the offset amount to the CMY signal, the signal output from the offset calculating unit 571 is (255, 255, 30). The added signal is input to the one-dimensional LUT calculation unit 572, and the address reference unit 572a accesses the one-dimensional LUT based on the added signal value and acquires a value corresponding thereto. At this time, since the added signal is in the first area (range 0 to 255) shown in FIG. 26, the one-dimensional LUT for the first area is used. Now, let the signal acquired from the one-dimensional LUT be (255, 255, 20). Next, when the offset amount set by the offset amount switching unit 573b in the offset calculation unit 573 based on the color distribution information signal is (256, 0, 0), the offset amount adding unit 573a sets the offset amount to 1. Since the signal is added to the signal (255, 255, 20) acquired from the dimension LUT, the signal output from the offset calculation unit 573 is (511, 255, 20). The output signal of the offset calculation unit 573 is input to the three-dimensional LUT calculation unit 574. When signals (511, 255, 20) are input to the three-dimensional LUT calculation unit 574, the signals are for the first area shown in FIG. 27 (C is 256 to 511, M is 0 to 255, and Y is 0 to 0). Therefore, the color correction process is performed using the three-dimensional LUT for the first area. Note that the color correction processing method using the three-dimensional LUT can use the same method as the interpolation calculation method described in the first embodiment. This three-dimensional LUT for the first area holds data such that, for example, blue saturation is high.

  Color correction processing is performed in the same manner for green (second region), skin color (third region), and regions that do not belong to any one (fourth region). As in the case of the first area, the 3D LUT for the second area holds data that increases the saturation of the green color, and the 3D LUT for the third area has a higher skin color brightness. Data is stored.

  After the color correction processing is performed by the color correction unit 57, the black generation / under color removal unit 58, the spatial filter processing unit 59, the output gradation correction unit 60A, and the gradation reproduction processing unit 60B are the same as in the first embodiment. Perform the process.

  In FIG. 23, the color distribution information signal, which is the determination result of the color distribution information extraction, is fed back only to the color correction unit 57, but is input to the spatial filter processing unit 59 and the gradation reproduction processing unit 60B, and the input image It is good also as a structure which performs the process according to this content. Further, by combining with the region separation identification signal and the document type determination signal used in the above-described embodiment, color correction adapted to various documents and contents can be performed.

In this embodiment, the offset calculation units 571 and 573 are arranged before and after the one-dimensional LUT calculation unit 572. However, the offset calculation unit is arranged in front of the one-dimensional LUT calculation unit 572 as in the second embodiment. The offset calculation unit may be configured only after the one-dimensional LUT calculation unit 572 as in the third embodiment. The input / output relationship in the one-dimensional LUT can be as shown in FIG. 4 and FIG. 12 as well as FIG.

  When performing color correction with the above configuration, it is only necessary to switch the offset amount, and without switching the one-dimensional LUT and the three-dimensional LUT by the color distribution information signal, the color region to which each pixel of the input image belongs is assigned. Accordingly, appropriate color correction processing can be performed. In addition, the three-dimensional LUT can be efficiently held.

Embodiment 9 FIG.
In the eighth embodiment, the present invention is realized by hardware. However, the processing performed in the eighth embodiment may be realized by a computer program. The internal configuration of the image processing apparatus that installs the computer program according to the present embodiment is the same as that described in the fifth embodiment. That is, the image processing apparatus 10 in which the computer program according to the present embodiment is installed includes a CPU 11. The CPU 11 loads the computer program on the RAM 14 and executes the computer program, which is read from the storage unit 17. The image data or the image data input through the image input IF 18 is configured to perform color correction processing.

  FIG. 29 is a flowchart for explaining the color correction processing procedure executed by the image processing apparatus 10. The CPU 11 in the image processing apparatus 10 performs color distribution information extraction processing on the image data read from the storage unit 17 and the image data input through the image input IF 18 (step S41). The color distribution information extraction process can use a method similar to the method described in the eighth embodiment, and generates information (color distribution information) regarding the region to which each pixel belongs. In addition, as a pre-process of this color distribution information extraction process, a configuration for performing processes such as shading correction and input tone correction may be employed. Further, it may be combined with region separation processing.

  Next, the CPU 11 sets an offset amount for each pixel based on the generated color distribution information (step S42), and adds the set offset amount to the input image data. (Step S43). Then, using the one-dimensional LUT as shown in FIG. 26, data conversion is performed on the data added with the offset amount (step S44). For example, when each pixel of the input image data belongs to a blue region (referred to as a first region), the pixel value range of the input image data is set by setting the offset amount to 0 for each signal of CMY (RGB). The range of pixel values of the output image data can be set to 0 to 255 by using the one-dimensional LUT shown in FIG. In addition, when each pixel of the input image data belongs to the green region (referred to as the second region), the pixel value range of the input image data can be set to 256 to 511 by setting the offset amount to 256. By using the one-dimensional LUT shown in FIG. 26, the range of pixel values of output image data can be set to 0-255. Similarly, the offset amount may be set to 512 if it belongs to the skin color region (referred to as the third region), and to 768 if it belongs to the other region (referred to as the fourth region).

  Next, the CPU 11 adds an offset amount corresponding to the color distribution information to the converted data (step S45), and calculates a weight using the data obtained by adding the offset amount (step S46). Then, an address is obtained using the data added with the offset amount, and the color correction value is read from the three-dimensional LUT (step S47). That is, by adding an offset amount corresponding to the color distribution information in step S45, it can be distributed to different areas in the color space. For example, by using a three-dimensional LUT as shown in FIG. Appropriate color correction according to information can be performed.

Next, the CPU 11 performs an interpolation operation based on the weight calculated in step S46 and the correction value read in step S47, and generates output image data (step S48). Note that as the interpolation calculation method, a method similar to the method described in Embodiment 1 can be used.
In this embodiment, the offset amount is added before and after the data conversion processing by the one-dimensional LUT. However, as in the sixth embodiment, the offset amount addition is performed before the data conversion processing by the one-dimensional LUT. Even in a process that exists only in this case, as in the seventh embodiment, the offset amount may be added only after the data conversion process using the one-dimensional LUT. The input / output relationship in the one-dimensional LUT can be as shown in FIG. 4 and FIG. 12 as well as FIG.

  As described above, in this embodiment, it is only necessary to switch the offset amount, and an appropriate color is selected according to the region to which each pixel of the input image belongs without switching the one-dimensional LUT and the three-dimensional LUT according to the color distribution information. Correction processing can be performed. In addition, the three-dimensional LUT can be efficiently held.

Embodiment 10 FIG.
In the above-described embodiment, the mode in which appropriate color correction processing is performed according to the area identification signal, the document type determination signal, and the color distribution information signal has been described. Instead of using these signals, the input image data is stored in the color space. The correction may be performed in accordance with the position signal in the color space indicating whether or not the image belongs to a predetermined region.

  FIG. 30 is a block diagram illustrating a configuration of the image processing unit 6 provided in the image forming apparatus according to the tenth embodiment. The image processing unit 6 includes an AD conversion unit 61, a shading correction unit 62, an input gradation correction unit 63, a color space position signal generation unit 64, a region separation processing unit 65, a color correction unit 66A, a black generation / under color removal unit. 67, a spatial filter processing unit 68, an output tone correction unit 69A, and a tone reproduction processing unit 69B. Among these components, the components other than the color space position signal generation unit 64 are, for example, exactly the same as the respective components of the image processing unit 3 shown in the first embodiment.

  In the color space position signal generation unit 64, a signal (color space position signal indicating whether or not it belongs to a predetermined region in the RGB color space based on the output signal (RGB signal) of the input tone correction unit 63. ) And output to the subsequent color correction unit 66A. The color correction unit 66A switches the one-dimensional LUT in accordance with the position signal in the color space from the position signal generation unit 64 in the color space, and performs data conversion using the switched one-dimensional LUT. Then, color correction using a three-dimensional LUT is performed on the converted data.

  Hereinafter, processing in the color space position signal generation unit 64 will be described. FIG. 31 is a block diagram illustrating an internal configuration of the color space position signal generation unit 64. The color space position signal generation unit 64 includes a signal generation unit 641, and the RGB signal from the input tone correction unit 63 is input to the signal generation unit 641. Further, the color space position signal generation unit 64 outputs the RGB signal from the input tone correction unit 63 to the subsequent region separation processing unit 65 as it is.

  The signal generation unit 641 in the color space position signal generation unit 64 generates a color space position signal according to the determination table 641a and outputs the color space position signal to the color correction unit 66A. FIG. 32 is a conceptual diagram illustrating an example of the determination table 641a. In the determination table 641a, a specific area in the color space is defined by a range of pixel values. In the example shown in FIG. 32, the region indicated by the ranges of 240 ≦ R ≦ 255, 240 ≦ G ≦ 255, 240 ≦ B ≦ 255 is defined as the highlight region, and the other regions are defined as the base region. ing. That is, the signal generation unit 641 can determine whether the input RGB signal belongs to the highlight area or the base area by referring to the determination table 641a. “1” is output as the position signal in the color space, and “0” is output as the position signal in the color space when it belongs to the base region.

  In this embodiment, the region in the color space is defined in advance by the range of pixel values, and the signal generation unit 641 determines which region belongs to, but the user himself / herself identifies it. An area to be set may be arbitrarily set.

  Next, the operation of the color correction unit 66A (color conversion device) will be described. FIG. 33 is a schematic explanatory diagram for explaining the operation of the color correction unit 66A according to the tenth embodiment, FIG. 34 is a conceptual diagram of a one-dimensional LUT used in the color correction unit 66A, and FIG. 35 is a conceptual diagram of the three-dimensional LUT. FIG. As described above, the RGB signal from the region separation processing unit 65 and the color space position signal output from the color space position signal generation unit 64 are input to the color correction unit 66A. In the present embodiment, first, data conversion is performed using a one-dimensional LUT switched according to a color space position signal, and then color correction is performed on the converted data using a three-dimensional LUT.

  The one-dimensional LUT is set so that the range of the output signal varies depending on the region to which each pixel of the input image belongs. For example, when each pixel of the input image belongs to the highlight area, the pixel value is converted using the one-dimensional LUT shown in FIG. When each pixel of the input image belongs to the base region, the pixel value is converted using the one-dimensional LUT shown in FIG. Note that the horizontal axis of the graph shown in FIG. 34 indicates the pixel value of the input signal, and the vertical axis indicates the pixel value of the output signal, and the curve drawn in each graph is a conversion curve between the input signal and the output signal. Is shown. These conversion curves are defined as a look-up table (one-dimensional LUT). When the pixel values are converted according to these conversion curves, the pixels belonging to the highlight area are converted so that the pixel values are within the range of 256 to 319, and the pixels belonging to the base area are 0 to 0. It is converted to belong to the range of 255.

  In this way, by using a one-dimensional LUT that outputs values in different ranges depending on the region to which the pixel belongs, mapping is performed on points in different regions on the color space. That is, when a one-dimensional LUT for highlighting is used, as shown in FIG. 35, the highlight is mapped to a point in an area that satisfies 256 ≦ R ≦ 319, 256 ≦ G ≦ 319, and 256 ≦ B ≦ 319. When the one-dimensional LUT is used, it is mapped to a point in the region that satisfies 0 ≦ R ≦ 255, 0 ≦ G ≦ 255, and 0 ≦ B ≦ 255. Therefore, by preparing a color correction table (that is, a highlight three-dimensional LUT and a base three-dimensional LUT) corresponding to the points in each region, 3 corresponding to the position signal in the color space is prepared. There is no need to switch the dimension LUT, and appropriate color correction processing for each region is possible.

In the three-dimensional LUT used in the present embodiment, all the lattice point intervals (lattice width) are 2 ⁿ (n is an integer), and the number of lattice points is arbitrary. For example, the base three-dimensional LUT has a grid width of 16 (= 2 ⁴ ), and the number of grid points is 17 for each RGB color component. On the other hand, in the highlight region, 15 ranges per color component are expanded to 63 ranges per color component in the one-dimensional LUT. Therefore, it is possible to further increase the number of grid points per color component by setting the grid width to 16. That is, it is possible to perform color correction with high accuracy for the highlight area. Since the actual range of the highlight area is 15, the actual grid width is 4 (= 16 × 16/64). In other words, the three-dimensional LUT in the highlight area has an actual grid width of 4, and more detailed interpolation is executed.

  FIG. 36 is a block diagram illustrating an internal configuration of the color correction unit 66A according to the tenth embodiment. The color correction unit 66A includes a one-dimensional LUT calculation unit 661 that performs data conversion using a one-dimensional LUT and a three-dimensional LUT calculation unit 662 that performs color correction using a three-dimensional LUT.

  The one-dimensional LUT calculation unit 661 receives the color space position signal output from the color space position signal generation unit 64 and the RGB signal output from the region separation processing unit 65. The address reference unit 661a of the one-dimensional LUT calculation unit 661 accesses the highlight or base one-dimensional LUT based on the input RGB signal and color space position signal, and acquires the converted RGB signal. The RGB signals converted by the one-dimensional LUT calculation unit 661 are output to the weighting calculation unit 662a and the address calculation unit 662b of the three-dimensional LUT calculation unit 662.

  The weight calculation unit 662a calculates a desired weight from the input signal and passes the value to the interpolation calculation unit 662c. On the other hand, the address calculation unit 662b calculates an address to be accessed from the input signal, and acquires a color correction value by accessing the three-dimensional LUT based on the address. The color correction value acquired by the address calculation unit 662b is output to the interpolation calculation unit 662c. The interpolation calculation unit 662c performs a three-dimensional interpolation calculation based on the weighting output from the weighting calculation unit 662a and the color correction value output from the address calculation unit 662b, and generates an output signal (CMY signal).

  As described above, in this embodiment, since a one-dimensional LUT is used to map to a different point for each position in the color space, if a color correction value is held at a point corresponding to that position, the color Appropriate color correction processing can be performed in accordance with the position signal in space. At this time, it is not necessary to switch the three-dimensional LUT simply by switching the one-dimensional LUT according to the position signal in the color space. Further, it is not necessary to hold the memory start address and the like, and the conventional technique can be used, so that it is not necessary to perform complicated memory access calculation. Therefore, it is possible to provide a color conversion apparatus and an image forming apparatus that can perform appropriate processing with simple hardware.

Embodiment 11 FIG.
In the tenth embodiment, the present invention is realized by hardware. However, the processing performed in the tenth embodiment may be realized by a computer program. The internal configuration of the image processing apparatus that installs the computer program according to the present embodiment is the same as that described in the fifth embodiment. That is, the image processing apparatus 10 in which the computer program according to the present embodiment is installed includes a CPU 11. The CPU 11 loads the computer program on the RAM 14 and executes the computer program, which is read from the storage unit 17. The image data or the image data input through the image input IF 18 is configured to perform color correction processing.

  FIG. 37 is a flowchart for explaining the procedure of color correction processing executed by the image processing apparatus 10. The CPU 11 of the image processing apparatus 10 performs area determination processing on the image data read from the storage unit 17 and the image data input through the image input IF 18 (step S51). The area determination process can use a technique similar to the technique described in the tenth embodiment, and generates information (position information in the color space) indicating which area in the color space each pixel belongs to. In addition, as a pre-process of this area determination process, a configuration in which processes such as shading correction and input tone correction are performed may be used.

  Next, the CPU 11 performs conversion of the input image data using the one-dimensional LUT switched based on the area determination information (step S52). For example, when it can be determined that the target pixel belongs to the highlight area, the pixel value is converted using a one-dimensional LUT as shown in FIG. Further, when it can be determined that it does not belong to the highlight area, that is, when it can be determined that it belongs to the base area, pixel values are converted using a one-dimensional LUT as shown in FIG.

  Next, the CPU 11 calculates a weight using the converted data (step S53), obtains an address using the converted data, and reads a color correction value from the three-dimensional LUT (step S54). That is, by converting the pixel value using a one-dimensional LUT corresponding to the region, the conversion destination is mapped to a different region in the color space. For example, the three-dimensional as shown in FIG. By using the LUT, it is possible to perform appropriate color conversion according to the area. Then, the CPU 11 performs an interpolation operation based on the weight calculated in step S53 and the correction value read in step S54, and generates output image data (step S55).

  As described above, in this embodiment, the one-dimensional LUT to be used is switched based on the position information in the color space, the pixel value is converted using the switched one-dimensional LUT, and then the color using the three-dimensional LUT is used. Perform conversion. In other words, the image forming apparatus described in the first embodiment is realized by software processing.

Embodiment 12 FIG.
In the color correction unit 66A shown in the tenth embodiment, the one-dimensional LUT is switched in accordance with the position signal in the color space. Instead of switching the one-dimensional LUT, different offset amounts are used for the input signal (RGB signal). May be added, and nonlinearity correction may be performed using one one-dimensional LUT. The operation of the color correction unit 66B used in the present embodiment will be described with reference to FIG. 38, and the internal configuration will be described with reference to FIG. The configuration other than the color correction unit 66B is the same as that of the tenth embodiment.

  FIG. 38 is a schematic explanatory diagram for explaining the operation of the color correction unit 66B according to the twelfth embodiment, and FIG. 39 is a conceptual diagram of a one-dimensional LUT used in the color correction unit 66B. The color correction unit 66B receives the color space position signal output from the color space position signal generation unit 64 and the RGB signal output from the region separation processing unit 65. The color correction unit 66B can determine which region (for example, highlight region or base region) in the color space belongs based on the position signal in the color space, and the RGB signal based on the determination result. Set the offset amount to be added to. After the set offset amount is added to the RGB signal, pixel values are converted for each color component using a one-dimensional LUT in order to correct nonlinearity. In the conversion of the pixel value using the one-dimensional LUT, conversion is performed so that a three-dimensional LUT (color correction table) mapped in different regions of the RGB color space can be accessed according to the added offset amount. In color correction using a three-dimensional LUT, color correction is performed using one three-dimensional LUT defined for the RGB color space.

  In the present embodiment, the input / output ranges are made different by switching the offset amount according to the determined area. For example, when each pixel of the input image belongs to the highlight area, the pixel value range of the input signal can be converted to 256 to 271 by setting the offset amount to 256, and the one-dimensional LUT shown in FIG. Can be used to convert the pixel value range of the output signal to 256 to 319. Also, when each pixel of the input image belongs to the base region, the range of the pixel value of the input signal can be set to 0 to 255 by setting the offset amount to 0. Similarly, the one-dimensional LUT shown in FIG. By using it, the range of the pixel value of the output signal can be set to 0-255.

  In this way, by using a one-dimensional LUT that outputs values in different ranges depending on the region to which the pixel belongs, mapping is performed on points in different regions on the color space. That is, the pixel belonging to the highlight region is mapped to a point in the region satisfying 256 ≦ R ≦ 511, 256 ≦ G ≦ 511, 256 ≦ B ≦ 511, and the pixel belonging to the base region is 0 ≦ R ≦ 255, 0 ≦ G ≦ 255, and 0 ≦ B ≦ 255. Therefore, by preparing a color correction table (that is, a three-dimensional LUT in which a base three-dimensional LUT and a highlight three-dimensional LUT are described in one table) corresponding to the points in each region, a color space is prepared. Switching of the three-dimensional LUT according to the internal position signal is not necessary, and appropriate color correction processing can be performed for each region.

  FIG. 40 is a block diagram illustrating the internal configuration of the color correction unit 66B according to the twelfth embodiment. The color correction unit 66B uses an offset calculation unit 663 that sets an offset amount according to a position signal in the color space, a one-dimensional LUT calculation unit 664 that converts a range of pixel values using a one-dimensional LUT, and a three-dimensional LUT. The three-dimensional LUT calculation unit 665 performs color correction.

  The offset calculation unit 663 receives the RGB signal output from the region separation processing unit 65 and the color space position signal output from the color space position signal generation unit 64. The offset amount switching unit 663b of the offset calculation unit 663 switches the offset amount to be added according to the input color space position signal. For example, the offset amount is set to 256 in the case of a pixel belonging to the highlight region, and the offset amount is set to 0 in the case of a pixel belonging to the base region. The offset amount set by the offset amount switching unit 663b is output to the offset amount adding unit 663a and added to the input RGB signal.

  The RGB signal to which the offset amount is added is output to the one-dimensional LUT calculation unit 664. The address reference unit 664a of the one-dimensional LUT calculation unit 664 accesses the one-dimensional LUT based on the value obtained by adding the offset amount to the RGB signal, and acquires the RGB signal whose nonlinearity is corrected. The RGB signal whose nonlinearity is corrected by the one-dimensional LUT calculation unit 664 is output to the weighting calculation unit 665a and the address calculation unit 665b of the three-dimensional LUT calculation unit 665.

  The weight calculation unit 665a calculates a desired weight from the input signal and passes the value to the interpolation calculation unit 665c. On the other hand, the address calculation unit 665b calculates an address to be accessed from the input signal, and accesses the three-dimensional LUT based on the address to obtain a color correction value. The color correction value acquired by the address calculation unit 665b is output to the interpolation calculation unit 665c. The interpolation calculation unit 665c performs a three-dimensional interpolation calculation based on the weighting output from the weighting calculation unit 665a and the color correction value output from the address calculation unit 665b, and generates an output signal (CMY signal).

  When performing color correction with the above-described configuration, it is only necessary to switch the offset amount. Depending on the region to which each pixel of the input image belongs without switching between the one-dimensional LUT and the three-dimensional LUT by the color space position signal. Therefore, it is possible to perform appropriate color correction processing.

Embodiment 13 FIG.
In the twelfth embodiment, the present invention is realized by hardware. However, the processing performed in the twelfth embodiment may be realized by a computer program. The internal configuration of the image processing apparatus that installs the computer program according to the present embodiment is the same as that described in the fifth embodiment. That is, the image processing apparatus 10 in which the computer program according to the present embodiment is installed includes a CPU 11. The CPU 11 loads the computer program on the RAM 14 and executes the computer program, which is read from the storage unit 17. The image data or the image data input through the image input IF 18 is configured to perform color correction processing.

  FIG. 41 is a flowchart for explaining the procedure of color conversion processing executed by the image processing apparatus 10. The CPU 11 of the image processing apparatus 10 performs region determination processing on the image data read from the storage unit 17 and the image data input through the image input IF 18 (step S61). The area determination process can use a technique similar to the technique described in the tenth embodiment, and generates information (position information in the color space) indicating which area in the color space each pixel belongs to. In addition, as a pre-process of this area determination process, a configuration in which processes such as shading correction and input tone correction are performed may be used.

  Next, the CPU 11 sets an offset amount for each pixel based on the generated position information in the color space (step S62), and adds the set offset amount to the input image data. (Step S63). Then, conversion is performed on the data to which the offset amount is added using the one-dimensional LUT (step S64). For example, when each pixel of the input image data belongs to the highlight area, the pixel value range of the input image data can be set to 256 to 271 by setting the offset amount to 256, as shown in FIG. By using the one-dimensional LUT, the pixel value range of the output image data can be set to 256 to 319. When each pixel of the input image data belongs to the base area, the pixel value range of the input image data can be set to 0 to 255 by setting the offset amount to 0, as shown in FIG. By using the one-dimensional LUT, the pixel value range of the output image data can be set to 0-255.

  Next, the CPU 11 calculates a weight using the converted data (step S65), obtains an address using the converted data, and reads a color correction value from the three-dimensional LUT (step S66). That is, in the process of step S64, the conversion destination is mapped to a different area in the color space depending on the area. For example, an appropriate color corresponding to the area can be obtained by using a three-dimensional LUT as shown in FIG. Conversion can be performed. Then, the CPU 11 performs an interpolation operation based on the weight calculated in step S65 and the correction value read in step S66, and generates output image data (step S67).

  As described above, in this embodiment, it is only necessary to switch the offset amount, and it is appropriate according to the region to which each pixel of the input image belongs without switching between the one-dimensional LUT and the three-dimensional LUT according to the position information in the color space. It is possible to perform a simple color conversion process.

Embodiment 14 FIG.
In the color correction unit 66B shown in the twelfth embodiment, a different offset amount is added to the input signal (RGB signal) according to the position signal in the color space, the nonlinearity correction using the one-dimensional LUT, and the three-dimensional LUT. However, after correcting nonlinearity using a one-dimensional LUT, an offset amount corresponding to a position signal in the color space is added, and a color using a three-dimensional LUT is used. It is good also as composition which performs amendment. The operation of the color correction unit 66C used in this embodiment will be described with reference to FIG. 42, and the internal configuration will be described with reference to FIG. The configuration other than the color correction unit 66C is the same as that of the twelfth embodiment.

  FIG. 42 is a schematic explanatory diagram for explaining the operation of the color correction unit 66C according to the fourteenth embodiment, and FIG. 43 is a conceptual diagram of a three-dimensional LUT used in the color correction unit 66C. The color correction unit 66C receives the color space position signal output from the color space position signal generation unit 64 and the RGB signal output from the region separation processing unit 65. First, the color correction unit 66C corrects nonlinearity with respect to an input signal (RGB signal) using a one-dimensional LUT prepared in advance. In addition, since the color correction unit 66C can determine to which region (for example, a highlight region or a base region) in the color space the target pixel belongs based on the position signal in the color space. An offset amount to be added to the RGB signal whose nonlinearity is corrected based on the determination result is set for each color component. Then, after adding the set offset amount to the RGB signal, color correction is performed using a three-dimensional LUT. In the present embodiment, the offset amount is set so as to be mapped to different regions in the RGB color space according to the region to which each pixel belongs.

  In this embodiment, after correcting nonlinearity using a one-dimensional LUT, an offset amount corresponding to a region is added. For example, when each pixel of the input image belongs to the highlight region, the offset amount for the R signal is set to 256, the offset amount for the G signal and the B signal is set to 0, and when each pixel of the input image belongs to the base region, RGB The offset amount for each signal is set to zero. As a result, as shown in FIG. 43, the pixels belonging to the highlight region are mapped to points in the region satisfying 256 ≦ R ≦ 319, 0 ≦ G ≦ 255, and 0 ≦ B ≦ 255. The pixels belonging to the region are mapped to points in the region that satisfy 0 ≦ R ≦ 255, 0 ≦ G ≦ 255, and 0 ≦ B ≦ 255. Therefore, by preparing a color correction table corresponding to the points in each area (that is, a three-dimensional LUT in which a highlight three-dimensional LUT and a base three-dimensional LUT are described in one table), It is not necessary to switch the three-dimensional LUT according to the position signal in the color space, and appropriate color correction processing can be performed for each area. In addition, unlike the above-described embodiment, since the color correction value can be held only on the diagonal line, the table memory that has been wasted can be arranged efficiently, and the table memory can be efficiently held. It becomes.

  FIG. 44 is a block diagram illustrating the internal configuration of the color correction unit 66C according to the fourteenth embodiment. The color correction unit 66C includes a one-dimensional LUT calculation unit 666 that corrects nonlinearity using a one-dimensional LUT, an offset calculation unit 667 that sets an offset amount according to a region identification signal, and color correction using a three-dimensional LUT. The three-dimensional LUT calculation unit 668 performs the above.

  The RGB signal output from the region separation processing unit 65 is input to the one-dimensional LUT calculation unit 666. The address reference unit 666a of the one-dimensional LUT calculation unit 666 accesses the one-dimensional LUT based on the input RGB signal and acquires an RGB signal in which nonlinearity is corrected. The RGB signal whose nonlinearity is corrected by the one-dimensional LUT calculation unit 666 is output to the offset calculation unit 667.

  The offset calculation unit 667 receives the RGB signal output from the one-dimensional LUT calculation unit 666 and the color space position signal output from the color space position signal generation unit 64. The offset amount switching unit 667b of the offset calculation unit 667 switches the offset amount to be added according to the input color space signal. For example, in the case of a pixel belonging to the highlight area, the offset amount for the R signal is set to 256, and the offset amounts for the G signal and the B signal are set to 0. In the case of a pixel belonging to the base region, the offset amount is set to 0 for each RGB signal. The offset amount set by the offset amount switching unit 667b is output to the offset amount adding unit 667a and added to the input RGB signal. The RGB signal to which the offset amount is added is output to the weighting calculation unit 668a and the address calculation unit 668b of the three-dimensional LUT calculation unit 668.

  The weight calculation unit 668a calculates a desired weight from the input signal, and passes the value to the interpolation calculation unit 668c. On the other hand, the address calculation unit 668b calculates an address to be accessed from the input signal, and accesses the three-dimensional LUT based on the address to acquire a color correction value. The color correction value acquired by the address calculation unit 668b is output to the interpolation calculation unit 668c. The interpolation calculation unit 668c performs a three-dimensional interpolation calculation based on the weight output from the weight calculation unit 668a and the color correction value output from the address calculation unit 668b, and generates an output signal (CMY signal).

  When performing color correction with the above-described configuration, it is only necessary to switch the offset amount, and it is appropriate according to the region to which each pixel of the input image belongs without switching between the one-dimensional LUT and the three-dimensional LUT by the region identification signal. It is possible to perform a color correction process. In addition, the three-dimensional LUT can be efficiently held.

Embodiment 15 FIG.
In the fourteenth embodiment, the present invention is realized by hardware. However, the processing performed in the fourteenth embodiment may be realized by a computer program. The internal configuration of the image processing apparatus that installs the computer program according to the present embodiment is the same as that described in the fifth embodiment. That is, the image processing apparatus 10 in which the computer program according to the present embodiment is installed includes a CPU 11. The CPU 11 loads the computer program on the RAM 14 and executes the computer program, which is read from the storage unit 17. The image data or the image data input through the image input IF 18 is configured to perform color correction processing.

  FIG. 45 is a flowchart for explaining the procedure of color conversion processing executed by the image processing apparatus 10. The CPU 11 of the image processing apparatus 10 performs a region determination process on the image data read from the storage unit 17 and the image data input through the image input IF 18 (step S71). The area determination process can use a technique similar to the technique described in the tenth embodiment, and generates information (position information in the color space) indicating which area in the color space each pixel belongs to. In addition, as a pre-process of this area determination process, a configuration in which processes such as shading correction and input tone correction are performed may be used.

  Next, the CPU 11 performs conversion of input image data using a one-dimensional LUT prepared in advance (step S72). Further, the CPU 11 sets an offset amount for each signal based on the position information in the color space (step S73), and adds the set offset amount for each signal (step S74). For example, when each pixel of the input image belongs to the highlight region, the offset amount for the R signal is set to 256, the offset amount for the G signal and the B signal is set to 0, and the data converted using the one-dimensional LUT is used. Add these offsets. When each pixel of the input image belongs to the base region, the offset amount for each signal is set to zero.

  Next, the CPU 11 calculates the weight using the data added with the offset amount (step S75), obtains an address using the data added with the offset amount, and reads the color correction value from the three-dimensional LUT (step S75). S76). That is, by adding an offset amount corresponding to a region in step S73, it is possible to map to a different region in the color space. For example, using a three-dimensional LUT as shown in FIG. It is possible to perform appropriate color conversion. Then, the CPU 11 performs an interpolation operation based on the weight calculated in step S75 and the correction value read in step S76, and generates output image data (step S77).

  Thus, in this embodiment, it is only necessary to switch the offset amount, and an appropriate color can be selected according to the region to which each pixel of the input image belongs without switching between the one-dimensional LUT and the three-dimensional LUT by the region identification signal. Correction processing can be performed. In addition, the three-dimensional LUT can be efficiently held.

Embodiment 16 FIG.
In the embodiment described above, it is configured to determine whether each pixel of the input image data belongs to the highlight area or the base area. However, the determination area is not necessarily limited to two, and three or more determination areas are not necessarily included. It is good also as a structure which provides a determination area | region. In the twelfth embodiment, a different offset amount is added to the input signal (RGB signal) according to the position signal in the color space, and then the nonlinearity correction using the one-dimensional LUT and the color correction using the three-dimensional LUT. In the fourteenth embodiment, after correcting nonlinearity using a one-dimensional LUT, an offset amount corresponding to the position signal in the color space is added, and then a three-dimensional LUT is used. However, a configuration may be adopted in which an offset amount corresponding to the position signal in the color space is added before and after the nonlinearity correction by the one-dimensional LUT.

  FIG. 46 is a conceptual diagram illustrating another example of the determination table 641a used by the signal generation unit 641 of the position signal generation unit 64 in the color space. In the example shown in FIG. 46, the region indicated by the range of 0 ≦ R ≦ 47, 0 ≦ G ≦ 47, 0 ≦ B ≦ 47 is a dark region, 0 ≦ R ≦ 63, 0 ≦ G ≦ 63, 80 ≦ B. The region indicated by the range of ≦ 143 is defined as the blue region, and the region indicated by the ranges of 240 ≦ R ≦ 255, 240 ≦ G ≦ 255, and 240 ≦ B ≦ 255 is defined as the highlight region. Further, an area that does not belong to any of these areas is defined as a base area. That is, the signal generation unit 641 can determine whether the input RGB signal belongs to the dark region, the blue region, the highlight region, or the base region by referring to the determination table 641a. Different position signals in the color space can be output in accordance with.

  FIG. 47 is a schematic explanatory diagram illustrating color correction processing according to the sixteenth embodiment. In this embodiment, first, the offset amount switched according to the position signal in the color space is added to the RGB signal (first offset addition), and data conversion using the one-dimensional LUT is performed on the RGB signal to which the offset is added. Do. Then, an offset amount switched according to the position signal in the color space is added to the converted data (second offset addition), and color correction using the three-dimensional LUT is performed on the RGB signal to which the offset has been added.

  In this embodiment, first, the input / output ranges are made different by switching the offset amount according to the determined area and performing data conversion using a one-dimensional LUT. FIG. 48 is a conceptual diagram showing an example of a one-dimensional LUT used in the present embodiment, and FIG. 49 is a table summarizing the relationship between offsets and mapping destinations defined for each area. The order of storage in the one-dimensional LUT is assumed to be a base area, a dark area, a blue area, and a highlight area. Since the one-dimensional LUT for the dark area is stored immediately after the one-dimensional LUT for the base area and the one-dimensional LUT for the base area is up to 255, the offset amount of the dark area is 256. That is, the minimum RGB values in the dark area are mapped to 256, respectively. On the other hand, the maximum value can be expressed by (base point grid point distance) × (number of grids of each color component−1) + (minimum value of each component−1). Although the maximum value of the input signal in the dark region was 47, the number of grid points is 9, so that 16 × (9-1) + 256-1 = 383.

  Next, an example of the blue area will be described. Since the blue area is stored next to the dark area, the blue area is stored from 384 which is the next value of 383 which is the maximum mapping value of the dark area. Since the minimum input values of the R component and G component in the blue area are 0, 384 is the offset amount. However, since the minimum value of the B component in the blue region is 80, 80 is mapped to 385, so the offset amount is 305. Regarding the maximum value, the grid point distance to be interpolated is 8, the number of grids of each color component is 11, the offset amount is 385 for the R component and G component, and 305 for the B component. Is 16 × (11-1) + 383-1 = 543, and the maximum value of the B component is 16 × (11-1) + 383-1 = 543.

  The one-dimensional LUT and the offset amount are determined by such a calculation method, and the pixel value of the input data is converted. Further, by performing color correction using the three-dimensional LUT shown in the conceptual diagram of FIG. 50, appropriate color correction can be performed according to each region.

  In the above, the method for performing an appropriate color correction process according to the position signal in the color space has been described. However, the appropriate color correction process is performed in conjunction with not only the position signal in the color space but also the area identification signal. Also good. For example, only when the area identification signal is a photographic area (continuous tone area such as a photographic paper photograph) and the position signal in the color space is a highlight, the lattice point interval is reduced by using any of the methods described above. Thus, processing such as interpolation processing can be performed.

  Further, an appropriate color correction process may be performed based on the position signal in the color space and the document type determination signal. For example, when the document type is determined to be a photographic paper photographic document or a character photographic paper photographic document, and the color space position signal is a dark region, a blue region, or a highlight region, 1 shown in FIGS. Processing is performed using a three-dimensional LUT and a three-dimensional LUT. Alternatively, a color correction table for dark areas, blue areas, and highlight areas is provided in areas different from those for photographic paper photographic originals and character photographic paper photographic originals for printed photographic originals and character printed photographic originals to perform processing. Anyway.

1 is a block diagram illustrating an internal configuration of an image forming apparatus according to the present invention. It is a block diagram explaining the structure of an image process part. 6 is a schematic explanatory diagram for explaining the operation of the color correction unit according to Embodiment 1. FIG. It is a conceptual diagram of the one-dimensional LUT used in a color correction part. 3 is a conceptual diagram of a three-dimensional LUT used in a color correction unit. FIG. It is a schematic diagram which shows an input color space. It is a schematic diagram which shows a mode that the unit cell was divided | segmented by the tetrahedron. It is explanatory drawing explaining the calculation method of an output signal. It is the graph which put together the parameter used when calculating an output signal. 2 is a block diagram illustrating an internal configuration of a color correction unit according to Embodiment 1. FIG. FIG. 10 is a schematic explanatory diagram for explaining an operation of a color correction unit according to Embodiment 2. It is a conceptual diagram of the one-dimensional LUT used in a color correction part. 6 is a block diagram illustrating an internal configuration of a color correction unit according to Embodiment 2. FIG. FIG. 10 is a schematic explanatory diagram for explaining an operation of a color correction unit according to Embodiment 3. 3 is a conceptual diagram of a three-dimensional LUT used in a color correction unit. FIG. 10 is a block diagram illustrating an internal configuration of a color correction unit according to Embodiment 3. FIG. It is typical explanatory drawing which shows the modification of this Embodiment. FIG. 10 is a block diagram illustrating a configuration of an image processing unit included in an image forming apparatus according to a fourth embodiment. It is a block diagram explaining the internal structure of the image processing apparatus in which the computer program based on this invention was installed. 6 is a flowchart illustrating a procedure of color correction processing executed by the image processing apparatus. 6 is a flowchart illustrating a procedure of color correction processing executed by the image processing apparatus. 6 is a flowchart illustrating a procedure of color correction processing executed by the image processing apparatus. FIG. 10 is a block diagram illustrating a configuration of an image processing unit included in an image forming apparatus according to an eighth embodiment. It is a block diagram explaining the internal structure of a color distribution information extraction part. FIG. 20 is a schematic explanatory diagram for explaining the operation of a color correction unit according to an eighth embodiment. It is a conceptual diagram of the one-dimensional LUT used in a color correction part. 3 is a conceptual diagram of a three-dimensional LUT used in a color correction unit. FIG. FIG. 20 is a block diagram illustrating an internal configuration of a color correction unit according to an eighth embodiment. 6 is a flowchart illustrating a processing procedure of color correction processing executed by the image processing apparatus. FIG. 16 is a block diagram illustrating a configuration of an image processing unit included in an image forming apparatus according to a tenth embodiment. It is a block diagram explaining the internal structure of the position signal generation part in color space. It is a conceptual diagram explaining an example of the determination table. FIG. 20 is a schematic explanatory diagram for explaining the operation of a color correction unit according to the tenth embodiment. It is a conceptual diagram of the one-dimensional LUT used in a color correction part. 3 is a conceptual diagram of a three-dimensional LUT used in a color correction unit. FIG. FIG. 20 is a block diagram illustrating an internal configuration of a color correction unit according to Embodiment 10. 6 is a flowchart illustrating a procedure of color correction processing executed by the image processing apparatus. FIG. 20 is a schematic explanatory diagram for explaining the operation of a color correction unit according to Embodiment 12. It is a conceptual diagram of the one-dimensional LUT used in a color correction part. FIG. 20 is a block diagram illustrating an internal configuration of a color correction unit according to Embodiment 12. 6 is a flowchart illustrating a color conversion process performed by the image processing apparatus. FIG. 20 is a schematic explanatory diagram for explaining the operation of a color correction unit according to a fourteenth embodiment. 3 is a conceptual diagram of a three-dimensional LUT used in a color correction unit. FIG. FIG. 25 is a block diagram illustrating an internal configuration of a color correction unit according to a fourteenth embodiment. 6 is a flowchart illustrating a color conversion process performed by the image processing apparatus. It is a conceptual diagram which shows the other example of the determination table which the signal generation part of the position signal generation part in color space uses. FIG. 20 is a schematic explanatory diagram for explaining color correction processing according to a sixteenth embodiment. It is a conceptual diagram which shows an example of the one-dimensional LUT used by this Embodiment. It is the table | surface which put together the relationship between the offset prescribed | regulated about each area, and a mapping destination. It is a conceptual diagram of a three-dimensional LUT. It is a block diagram which shows the principal part structure of the conventional image processing apparatus. It is a block diagram which shows the principal part structure of the conventional image processing apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Control part 2 Image input part 3 Image processing part 7 Image output part 8 Storage part 9 Operation part 35A, 35B, 35C Color correction part 351,354,356 One-dimensional LUT calculation part 352,355,358 Three-dimensional LUT calculation part 353 , 357 Offset calculator

Claims (14)

In a color conversion apparatus that includes a plurality of pixels and generates an output image by performing color conversion on each pixel of an input image including one or more identifiable regions,
Conversion means for converting the pixel value of each pixel of the input image so that the pixel value after conversion is uniquely determined according to the characteristics of the region to which each pixel belongs and the range of pixel values after conversion is continuous between the regions ; Storing color correction values at grid points obtained by equally dividing a color space determined by the range of pixel values after conversion, and a color correction table in which the number of grid points for each color component is predetermined, and the color correction table A color conversion apparatus comprising: a calculation unit that performs an interpolation calculation on the pixel using the color correction value read from the pixel.   The converting means includes means for setting an offset to be added to the pixel value of each pixel of the input image according to the region, means for adding the set offset to the pixel value, and each pixel to which the offset is added. 2. The apparatus according to claim 1, further comprising: a one-dimensional lookup table defining a relationship between the pixel value of the pixel and the pixel value after conversion; and means for converting the pixel value based on the one-dimensional lookup table. Color conversion device.   The conversion means includes a one-dimensional lookup table that defines a relationship between pixel values before and after conversion, means for converting the pixel value of each pixel of the input image based on the one-dimensional lookup table, and conversion by the means The color conversion apparatus according to claim 1, further comprising: means for setting an offset to be added to the pixel value according to the region; and means for adding the set offset to the pixel value. The interval between the lattice points is 2 ⁿ (where n is an integer smaller than the number k of bits of each color component of the input image), and the number of lattice points Dn is 2 ^(m−1) +1 <Dn <2 ^{(m + 1).} 4. The color conversion apparatus according to claim 1, wherein the color conversion apparatus is an integer satisfying +1 (where Dn ≠ 2 ^m +1 and m = k−n).   5. The color conversion apparatus according to claim 1, further comprising a unit that identifies a feature of a region to which each pixel of the input image belongs based on a pixel value of the pixel.   6. The color conversion apparatus according to claim 5, wherein the area included in the input image is any one of a halftone dot area, a character area, and a photograph area.   A feature of an area to be identified is defined by a range of pixel values, and means for determining whether or not a pixel value of each pixel of the input image is included in the defined range is provided. Item 6. The color conversion device according to Item 5.   The characteristics of the area to be identified are defined by color distribution, and means for acquiring color information of each pixel of the input image, and the acquired color information indicating whether each pixel belongs to the specified color distribution The color conversion apparatus according to claim 5, further comprising: a determination unit based on the determination unit.   5. The color conversion according to claim 1, wherein an area included in the input image is identified based on a type of a document that is a source of the input image. 6. apparatus.   A color conversion device according to any one of claims 1 to 9, and means for forming an image on a sheet based on an image obtained by color conversion by the color conversion device. Image forming apparatus. In a color conversion method for generating an output image by performing color conversion on each pixel of an input image that includes a plurality of pixels and includes one or more identifiable regions,
The pixel value of each pixel of the input image is converted so that the range of pixel values after conversion is uniquely determined according to the characteristics of the region to which each pixel belongs , and the range of pixel values after conversion is continuous between the regions . Stores color correction values at grid points obtained by equally dividing the color space determined by one step and the converted pixel value range, and the number of grid points for each color component is read from a predetermined color correction table. And a second step of performing an interpolation operation on the pixel using a color correction value.   In the first step, an offset to be added to the pixel value of each pixel of the input image is set according to the region, the set offset is added to the pixel value, and the pixel value of each pixel obtained by adding the offset The color conversion method according to claim 11, wherein the pixel value is converted based on a one-dimensional lookup table that defines a relationship between the pixel value and the converted pixel value.   The first step converts a pixel value of each pixel of the input image based on a one-dimensional lookup table that defines a relationship between pixel values before and after conversion, and adds an offset to be added to the converted pixel value. The color conversion method according to claim 11, wherein the color conversion method is set according to a region, and the set offset is added to the pixel value. In a computer program that causes a computer to generate an output image by performing color conversion on each pixel of an input image that includes a plurality of pixels and includes one or more identifiable regions,
The computer determines the range of pixel values after conversion according to the characteristics of the region to which each pixel belongs , and sets the pixel value of each pixel of the input image so that the range of converted pixel values is continuous between the regions. The color correction value at grid points obtained by equally dividing the color space determined by the range of pixel values after conversion is stored in the computer, and the number of grid points for each color component is predetermined. And a step of executing an interpolation operation on the pixel using the color correction value read from the table.

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