JP2007166562A - Color conversion apparatus and method, color conversion program, and storage medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image processor and image processing method in which acceleration of operation is attained by decreasing throughput required for unwanted color conversion processing. <P>SOLUTION: Color conversion sections 2-1 to 2-M perform color conversion processing with color conversion techniques of different throughput. For example, a three-dimensional lookup table is used and a color conversion technique is applied of which throughput is less than a conventional tetrahedral interpolation scheme such as outputting a value of a most nearby lattice point, performing interpolation from two lattice points in an L* direction or performing interpolation from four lattice points of an a*-b* plane. Which one of these color conversion sections 2-1 to 2-M is to perform color conversion processing is selected in a selection section 1. The color conversion section can be selected in accordance with a pixel value or color component of an image to be processed. For example, a color conversion section of little throughput is selected in a high-speed processing mode, thereby attaining acceleration of color conversion processing. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image processing technique for performing color conversion processing on an image.

  Color conversion processing for images is used in various situations. For example, when an image read by an image input device in a copying machine is formed by an image output device, color conversion processing is required due to the difference in the color space of the image input device and the image output device. As one of conventional color conversion processing techniques, for example, there is a method using a multidimensional lookup table with interpolation called DLUT (Direct Look-up Table).

FIG. 14 is an explanatory diagram of an example of a color conversion method using DLUT. Here, the three-dimensional L ^* a ^* b ^* color space is divided by a predetermined number of axes of L ^* , a ^* , and b ^* . The space is divided into cubes or rectangular parallelepipeds, but the vertices are called lattice points, and the value after color conversion is held for each lattice point. For example, when converting to the CMYK color space, the CMYK values may be stored in association with the grid points.

The divided cube or rectangular parallelepiped is further divided into six tetrahedrons for interpolation calculation as shown in FIG. Which tetrahedron belongs is determined from the input L ^* , a ^* , and b ^* values, and an interpolation operation is performed using CMYK values corresponding to the four vertices (grid points) of the tetrahedron to which it belongs. In this way, any input L ^* a ^* b ^* value can be converted into a CMYK value. Such a color conversion method is described in Patent Document 1, for example.

  In this color conversion method using the DLUT, since the value of the lattice point can be easily changed, nonlinear color conversion can be easily realized, so that color conversion can be performed with high accuracy. There is also an advantage that colors outside the color reproduction range can be handled in a unified manner. Further, by using a tetrahedron as shown in FIG. 14, the amount of processing is reduced compared to the case where the interpolation processing is performed using 8 points which are vertices of a cube or a rectangular parallelepiped.

  However, even in such a method using a tetrahedron, a process for determining which tetrahedron a pixel value to be converted belongs to, and the vertex coordinates of the tetrahedron are used to access the three-dimensional lookup table to obtain four points. Each of the processes of obtaining the converted values and the process of performing the interpolation calculation from these four values requires a large amount of processing and requires a large amount of processing time. In particular, when this color conversion processing is realized by software, a large amount of processing time is required due to a very large memory access and calculation amount. In addition, since the DLUT is multi-dimensional, the addresses to be accessed are not continuous, and therefore, memory access takes a lot of time.

  The increase in the processing amount in such color conversion processing has become more serious with the recent increase in image resolution, and there has been a demand for speeding up the color conversion processing.

  On the other hand, as an apparatus having a plurality of color conversion means, there is a color image forming apparatus described in Patent Document 2, for example. In Patent Document 2, a plurality of DLUTs having different numbers of grid points are provided and switched according to the processing mode. However, the color conversion method itself is the same when any DLUT is used. Therefore, even if the DLUT is switched, the processing amount does not change and the color conversion process cannot be accelerated.

JP-A-53-123201 Japanese Patent Laid-Open No. 10-200772

  The present invention has been made in view of the above-described circumstances, and an object thereof is to provide an image processing apparatus and an image processing method that reduce the amount of processing required for unnecessary color conversion processing and increase the speed. It is. It is another object of the present invention to provide an image processing program for causing a computer to execute such a function of the image processing apparatus or an image processing method, and a storage medium storing the image processing program.

  The present invention relates to a color conversion apparatus and a color conversion method for performing color conversion processing on an image, and includes a plurality of color conversion units having different processing amounts, and appropriately selects the color conversion units and selects the selected color. The color conversion processing is performed by the conversion means. The selection of the color conversion means can be performed according to the attribute of the image, the image processing mode, the pixel value or color component of the image to be processed. Alternatively, the color region may be selected. For example, a color region in which the variation amount of the output value with respect to the variation of the input pixel value exceeds a certain value, or the square sum of the color difference components of the input pixel value is a constant value. For a color region exceeding 1, a color conversion means with high accuracy can be selected even if the amount of processing is large, and a decrease in color conversion accuracy can be suppressed.

  As a plurality of color conversion means, an N-dimensional lookup table is used to obtain the values of the four points that are the vertices of the tetrahedron including the pixel values in the same manner as in the past, and the pixels are interpolated from the output values of the four points In addition to a method for obtaining a corresponding output value, a method in which interpolation calculation is not performed using a value at one point in the vicinity of the pixel value as an output value, or interpolating from two values in the vicinity of the pixel value for one component of the pixel value, etc. The amount of processing, such as a method of obtaining an output value corresponding to a pixel by fixing a component of the pixel, a method of obtaining an output value corresponding to a pixel by interpolating from values of a plurality of points in the vicinity of the pixel value, in which one component of the pixel value is fixed It is sufficient to provide several color conversion means having different colors.

  For example, in a method in which the value of one point in the vicinity of the pixel value is used as the output value, the output value can be obtained simply by acquiring the value of one point in the vicinity of the pixel value by referring to the N-dimensional lookup table using the upper predetermined number of bits of the pixel value. And an output value as a color conversion result can be obtained with a very small processing amount. Further, even in the method of performing interpolation for one component of the pixel value, only two N-dimensional lookup tables and two points of interpolation processing are required, and an output value can be obtained with a small processing amount. Further, even in a method of interpolating from a plurality of values by fixing one component of the pixel value, for example, if it is three-dimensional, it becomes four-point interpolation, and only four N-dimensional lookup tables and four-point interpolation processing are required. In this case, it is not necessary to determine which tetrahedron is included in comparison with the four-point interpolation of the tetrahedron, and the interpolation process in this case is easier than the interpolation process from the vertex of the tetrahedron. An output value can be obtained with a smaller amount of processing.

  When color conversion processing is performed by such a color conversion method with a small amount of processing, the color conversion accuracy differs depending on each color conversion method, and there is a concern that the color conversion accuracy may be reduced due to a reduction in processing amount. In such a case, for a color conversion method with a small amount of processing as described above, an N-dimensional lookup table having a larger number of grid points than an N-dimensional lookup table used by a color conversion means having a large amount of processing is used. By doing so, it is possible to suppress a decrease in color conversion accuracy.

  In addition, the N-dimensional lookup table used in each color conversion unit is held by each color conversion unit, and may be arranged in several or may be configured to share one N-dimensional lookup table as a whole. .

  According to the present invention, a plurality of color conversion units having different processing amounts are provided, and the color conversion unit is selected and used according to the image attribute, image processing mode, pixel value, color component, color region, etc. of the image to be processed. . As a result, color conversion can be performed with the minimum necessary accuracy for the image or pixel to be processed, so the amount of color conversion processing performed with unnecessary accuracy can be reduced, and color conversion processing for the entire image can be performed. It is possible to increase the speed.

  FIG. 1 is a block diagram showing a first embodiment of the present invention. In the figure, 1 is a selection unit, and 2-1 to N are color conversion units. The color conversion units 2-1 to 2 -M perform color conversion processing on input image data by color conversion methods having different processing amounts. An example of various color conversion methods will be described later.

  The selection unit 1 selects one of the plurality of color conversion units 2-1 to M and passes the input image data. The color conversion unit can be selected for each image or for one or more pixels. In this example, an image attribute and a processing mode are input to the selection unit 1. When the selection unit 1 selects a color conversion unit, the image attribute and the processing mode can be referred to. Of course, a configuration in which only image attributes or only processing modes are referred to may be used.

  Examples of the image attribute include a background, a character, and a design. Further, the background can be further divided into a color background and a white background, the character can be further divided into a color character and a black character, and the picture can be divided into a photograph and a CG. As processing modes, there are a high-speed mode, a high-quality mode, and an intermediate standard mode. Other processing modes include a photo original mode, a character original mode, and a character / photo original mode. It is not necessary to prepare all these attributes and processing modes, and they can be appropriately selected and used. Of course, other attributes and processing modes may exist, and these attributes and processing modes may be used for selection of the color conversion unit.

An example of the color conversion method used in the color conversion units 2-1 to M will be described. Here, it is assumed that pixel values in the L ^* a ^* b ^* color space are input as input image data, and a case where color conversion processing is performed using a three-dimensional lookup table will be described. Of course, the color space of the input image data and the color space of the image data after color conversion are arbitrary. FIG. 2 is an explanatory diagram of a specific example of address generation when referring to a three-dimensional lookup table. Here, it is assumed that each component of the pixel value in the L ^* a ^* b ^* color space is 8 bits, and the upper 4 bits thereof are used as an address for referring to the three-dimensional lookup table. For example, in the L ^* component, the upper 4 bits are Lh, the lower 4 bits are Ll, and Lh is used as an address to the three-dimensional lookup table. Similarly, in the a ^* component, the upper 4 bits are ah and the lower 4 bits are al, ah is used as an address to the three-dimensional lookup table, and in the b ^* component, the upper 4 bits are bh and the lower 4 bits are bl. bh is used as an address to the 3D lookup table. Accordingly, the grid points of the three-dimensional lookup table to be used are 17 points (16 divisions) for each component.

  FIG. 3 is an explanatory diagram of an example of the first color conversion method. In the figure, black circles indicate lattice points in the three-dimensional lookup table, and stars indicate pixel values in the color space. The same applies to the following explanation of the color conversion method. In this first color conversion method, the value of the grid point closest to the pixel value is used as the output value. In the example shown in FIG. 3, since the pixel value indicated by the star is closest to the lattice point S (Lh + 1, ah, bh), the value of this lattice point S may be used as the output value. As a specific example, the most significant bit of the lower 4 bits of each color component may be referred to as to which grid point is closer. For example, if the most significant bit of L1 is 0, Lh + 1, if L1, Lh + 1, if the most significant bit of al is 0, ah + 1, if a most significant bit of bl is 0, bh, Then, it is sufficient to refer to the three-dimensional lookup table as bh + 1.

  In this first color conversion method, the three-dimensional lookup table needs to be accessed only once and no interpolation processing is performed, so that the amount of processing is very small and the color conversion processing can be performed at high speed. As a further simplified method, Lh, ah, and bh can be used as they are to access a three-dimensional lookup table, obtain a converted value, and use it as an output value. This method can further improve the processing speed.

FIG. 4 is an explanatory diagram of an example of the second color conversion method. This second color conversion method is a method of obtaining an output value corresponding to a pixel by interpolating from two values near the pixel value for one component of the pixel value. Here, L ^* is interpolated using two different points. In the example shown in FIG. 4, since the pixel value indicated by the star is closest to the lattice point S (Lh + 1, ah, bh), the value of this lattice point S and the lattice point P (Lh obtained by subtracting L ^* from −1 ⁾ are used. , Ah, bh). In fact, as in the case of the first color conversion method, any of ah or ah + 1 for a ^*, also b ^* to determine either bh or bh + 1 for, for L ^* of Lh and Lh + 1 Both are used to generate an address and access the 3D lookup table. Then, an output value may be calculated by interpolation from the obtained two converted values.

  In the second color conversion method, access to the three-dimensional lookup table is only required twice, and the interpolation processing is interpolation between two points, so that it can be performed by simple calculation. Therefore, although not as much as the first color conversion method, the amount of processing is very small compared to the method using the conventional tetrahedron, and the color conversion processing can be performed at high speed.

FIG. 5 is an explanatory diagram of an example of the third color conversion method. In this third color conversion method, one point value closest to the pixel value and one component at the one point are fixed and the other components are output by interpolation from the other three point values near the pixel value. It is a method of calculating a value. Here, one component to be fixed is L ^* . In the example shown in FIG. 5, since the pixel value indicated by an asterisk is closest to the lattice point S (Lh + 1, ah, bh), the value of this lattice point S and Lh + 1 are fixed and a ^* , b ^* for grid points of combinations respectively +1 T (Lh + 1, ah + 1, bh), the grid point U (Lh + 1, ah, bh + 1), using the value of the grid point V (Lh + 1, ah + 1, bh + 1), a * -b * plane The output value can be obtained by performing the interpolation at.

  In the third color conversion method, access to the three-dimensional lookup table is four times, which is the same number of times as the method using the conventional tetrahedron, but it is necessary to determine which tetrahedron is included. In addition, the interpolation processing on the plane is easier than the interpolation of the tetrahedron. Therefore, the amount of processing can be reduced as compared with the conventional method using a tetrahedron, and color conversion processing can be performed at high speed.

  In addition to the first to third color conversion methods as described above, in order to perform color conversion with high accuracy, the conventional tetrahedral interpolation and cubic interpolation that performs interpolation from eight points described with reference to FIG. 14 are used. It is also preferable to provide a color conversion unit that performs the above. Furthermore, as an example of special color conversion, a color conversion unit that outputs an output value indicating white or black can be provided. Of course, in addition to this, a color conversion unit that performs color conversion by various color conversion methods having different processing amounts can be provided. For example, a method of performing color conversion by matrix calculation without using a three-dimensional lookup table may be adopted.

  The color conversion process can be performed by appropriately using a plurality of color conversion units 2-1 to M having different processing amounts. In the various color conversion methods as described above, a color conversion method with a small amount of processing tends to have a low color conversion accuracy, and a color conversion method with a large amount of processing tends to provide a high color conversion accuracy. In addition, for example, in the second color conversion method, the hue and saturation are low in color conversion accuracy, but the brightness gradation is high in accuracy, and conversely, in the third color conversion method, the color in terms of hue and saturation is high. It has characteristics such as high conversion accuracy but low brightness accuracy. The selection unit 1 determines which color conversion unit to use in consideration of such processing amount, color conversion accuracy, and characteristics of each color conversion method.

  FIG. 6 is an explanatory diagram of a first selection example of the color conversion method in the selection unit. Here, three modes of high image quality, standard, and high speed can be selected by the user as processing modes, and a case where five attribute values of white background, color background, text, CG, and photograph can be taken as image attributes is shown. Yes. Note that “white output” indicates conversion to an output value corresponding to white regardless of the input pixel value, “re-neighboring” is the first color conversion method shown in FIG. 3, and “L interpolation” is The second color conversion method shown in FIG. 4, “ab interpolation” indicates the third color conversion method shown in FIG. 5, and “Lab interpolation” indicates the conventional color conversion method for performing tetrahedral interpolation.

For example, when the processing mode is “high image quality”, a color conversion method that performs tetrahedral interpolation is used for any image attribute regardless of the processing time. As a result, color conversion can be performed with high accuracy, and a high-quality image can be output. When the processing mode is "High speed", select a color conversion method with as little processing amount as possible, and always output white for pixels with an image attribute of white background, and color reproduction for color background and characters. Therefore, the first color conversion method using the value of the nearest grid point as the output value is used. Furthermore, since color change is more important than gradation for CG, the third color conversion method that performs interpolation processing on the a ^* -b ^* plane is used, and gradation is emphasized for photographs. A second color conversion method that performs L ^* interpolation processing may be used. When the processing mode is “standard”, the color conversion method is selected so that the processing is performed at a certain high speed and the color conversion is performed with a certain degree of accuracy. Here, white is output for the white background, and the first color conversion method is used for the color background. A second color conversion method is used for characters and CG, and a color conversion method such as tetrahedral interpolation that can perform color conversion with high accuracy is used for photographs.

  In this way, by selecting the color conversion method according to the processing mode and the image attribute and performing color conversion, the processing amount can be reduced according to each processing mode. Processing time can be shortened. Of course, in the “high image quality” mode, highly accurate color conversion is possible, and color conversion processing can be performed with the color conversion accuracy and processing speed requested by the user.

  FIG. 7 is an explanatory diagram of a second selection example of the color conversion method in the selection unit. In this example, in addition to “high speed”, “standard”, and “high quality” as processing modes, there are document modes of “photo”, “text”, and “text / photo”. Then, it indicates which color conversion method is selected for each character attribute pixel according to the combination. For example, even in the “High Speed” mode, if the “Photo” document mode is selected, color conversion accuracy is emphasized for the image portion of the photo attribute. In the “Text” document mode, the image attribute is color converted for the character portion. The accuracy is improved, and if it is “character / photo”, the color conversion accuracy is improved to some extent in both the character portion and the photo portion of the image attribute. Even when the processing mode is “standard”, the color conversion accuracy is improved when the image attribute is “color background” and CG in the “photo” document mode, and when the image attribute is CG in the “text / photo” document mode. ing. When the processing mode is “High Quality”, high-accuracy color conversion is basically performed for all image attributes. In this example, however, the “Text” document mode has a white background to make the characters stand out. In this case, white is always output. In this way, the color conversion method can be selected in consideration of the document mode.

  The selection of the color conversion method based on the processing mode and the image attribute shown in FIGS. 6 and 7 is an example, and the color conversion method can be selected without being limited to this example. Needless to say, the processing mode and image attributes are not limited to the examples shown in FIGS. For example, for a black character, it is possible to select a color conversion method that always outputs an output value formed of a single black color.

  FIG. 8 is a block diagram showing a first modification of the first embodiment of the present invention. In the figure, 3 is an N-dimensional lookup table, 21 is an address generation unit, and 22 is an interpolation unit. In the first modification, an example in which the N-dimensional lookup table 3 is shared by a plurality of color conversion units 2 is shown. Each color conversion unit 2 uses an address generation unit 21 that generates an address for accessing the N-dimensional lookup table 3 in the corresponding color conversion method, and a value read from the N-dimensional lookup table 3. And an interpolation unit 22 for performing an interpolation process. Since the N-dimensional lookup table 3 has a large amount of data, it can be used by a plurality of color conversion units 2 to reduce the required memory amount.

  Of course, as in the first color conversion method described above, the color conversion unit 2 that does not require interpolation processing may be configured without providing the interpolation unit 22. Further, the color conversion unit 2 that always outputs white or black does not need to refer to the N-dimensional lookup table 3, and therefore it is not necessary to provide both the address generation unit 21 and the interpolation unit 22.

  FIG. 9 is an explanatory diagram of a second modification of the first embodiment of the present invention. In the first to third color conversion methods shown in FIGS. 3 to 5 described above, the amount of processing is small, but the three-dimensional interpolation processing is not performed, so that the color conversion accuracy is lowered. One method of preventing this decrease in color conversion accuracy to some extent without increasing the amount of processing is to increase the number of grid points in the N-dimensional lookup table. FIG. 9A shows a cube or a rectangular parallelepiped formed by lattice points of the N-dimensional lookup table accessed by the upper 4 bits as shown in FIG. 2, and the black circles are lattice points at this time. is there. Here, when an N-dimensional lookup table is created by halving the lattice point interval of each axis, lattice points indicated by white circles are added. By using the grid points indicated by the black and white circles, the color conversion accuracy can be improved.

  For example, in the color conversion method for selecting the grid point closest to the pixel value shown in FIG. 3, the error between the pixel value and the selected grid point is reduced by the increase of the grid point. Therefore, color conversion accuracy can be improved. At this time, since the reference of the N-dimensional lookup table is changed only once, the processing amount is the same. Similarly, for other color conversion methods, the color conversion accuracy can be improved without changing the processing amount. However, the amount of memory for holding the N-dimensional lookup table increases as the number of grid points increases.

  When referring to the N-dimensional lookup table in which the lattice point interval is halved and the lattice points are increased in this way, for example, as shown in FIG. 9B, for accessing the N-dimensional lookup table. When generating an address, the upper 5 bits from each component of the pixel value may be used as Lh, ah, bh.

  As shown in FIG. 8, when the N-dimensional lookup table is shared, some color conversion units share an N-dimensional lookup table with a large number of grid points, and some color conversion units have a small number of grid points. Use such as sharing an N-dimensional lookup table is also possible.

  FIG. 10 is a block diagram showing a second embodiment of the present invention. In the figure, the same parts as those in FIG. Reference numeral 4 denotes a color component detection unit. In this example, when the selection unit 1 selects the color conversion units 2-1 to 2 -M, an example is shown in which the selection is performed according to the pixel value of the input image.

  The color component detection unit 4 detects what color the pixel value of the input image to be processed is. More specifically, it can be determined whether the color is in a specific color region. For example, whether or not the color region has an output value fluctuation amount that exceeds a certain value with respect to the input pixel value variation, or a color region in which the square sum of the color difference components of the input pixel value exceeds a certain value. Can be determined. These colors affect the image quality when the color conversion accuracy decreases. Conversely, in high-lightness color regions, especially yellow color regions and low-lightness color regions, even if the output value changes greatly, it is a region where the color change does not seem so much. In these regions, Since the color conversion accuracy may be somewhat worse, the processing speed can be increased. The color component detection unit 4 detects whether or not the color is in such a region from the pixel value. Of course, the color region to be detected is not limited to the above example, and may be set in advance.

  The selection unit 1 selects a color conversion unit that actually performs color conversion processing from among the plurality of color conversion units 2-1 to M in accordance with the detection result of the color component detection unit 4. As described above, the input pixel has a color region in which the variation amount of the output value with respect to the variation of the input pixel value exceeds a certain value, or the square sum of the color difference components of the input pixel value exceeds the certain value. For a color region where a color conversion error greatly affects the reproduced color, such as a color in the color region, a color conversion unit that performs color conversion processing by a color conversion method with high color conversion accuracy is selected. As a result, the color conversion accuracy is not greatly reduced. In addition, for a color region that does not have much influence even if a color conversion method with low color conversion accuracy is selected, such as a color region with high lightness or low lightness, a color conversion unit with a small processing amount is selected. Thereby, the processing time can be shortened. Note that the configuration may be such that the processing of the color component detection unit 4 is performed in the selection unit 1.

FIG. 11 is a flowchart illustrating an example of the operation of the selection unit 1 according to the second embodiment of the present invention. Here, as an example, an operation in the case of selecting a color conversion unit with a small amount of processing in a high brightness color region is shown. First, in S11, it is determined whether or not the L ^* component of the pixel value is greater than a predetermined threshold value, and it is determined whether or not the pixel is a high brightness color. If the L ^* component of the pixel value is equal to or less than the threshold value and is not a high-lightness color, in S12, a color conversion unit that performs color conversion using a highly accurate color conversion method such as tetrahedral interpolation conventionally performed is selected. To do.

If the L ^* component of the pixel value is larger than the threshold value and has a high brightness color, in this example, it is further determined in S13 whether or not the pixel color is yellow. As described above, yellow is a color that is particularly difficult to perceive a color change among high-lightness colors. Therefore, it is possible to select a color conversion method with a smaller amount of processing, particularly for yellow among high-lightness colors. If it is determined in S13 that the color of the pixel is yellow, in S15, for example, color conversion is performed using a color conversion method that approximates the value of the nearest grid point described in FIG. Select fewer color converters. If the color has a high lightness other than yellow, in S14, for example, the color conversion method for performing the L ^* interpolation process described in FIG. 4 or the a ^* -b ^* plane described in FIG. Select a color conversion unit that maintains a certain level of color conversion accuracy, such as the color conversion method to be performed, and has a small amount of processing.

  Then, pixel values are passed to the selected color conversion unit, and color conversion processing is performed to obtain an output value. Thus, by selecting the color conversion unit according to the pixel value of the input image, the processing amount of the color conversion process can be reduced, and the processing time can be shortened.

  Here, an example of the operation of the selection unit 1 in the case where the processing amount is reduced for the high brightness color area and the color conversion process is speeded up is described. However, the color is similarly applied to other color areas, for example, the low brightness color area. Conversion unit selection processing can be performed. For example, only when the input pixel value indicates white or black, the output value of white or black can be output without performing complicated color conversion processing.

Also in the second embodiment, the color conversion unit can be selected in consideration of the processing mode and the image attribute as in the first embodiment. FIG. 12 is an explanatory diagram of a third selection example of the color conversion method in the selection unit. In this example, as in the case described with reference to FIG. 11, an example is shown in which the input pixels are classified according to whether they are high brightness color and further yellow, and the color conversion unit is selected using the processing mode. Yes. When the processing mode is “high image quality”, a color conversion unit corresponding to a color conversion method capable of performing color conversion with high accuracy for high brightness colors including yellow is selected. When the processing mode is “high speed”, the selection process described with reference to FIG. 11 is used as it is. Further, when the processing mode is “standard”, the color conversion method for performing the L ^* interpolation processing described with reference to FIG. 4 and the a ^* -b ^* plane described with reference to FIG. A color conversion unit corresponding to a color conversion method to be processed is selected, and a color conversion unit corresponding to a color conversion method capable of performing color conversion with high accuracy is selected. In this way, the color conversion unit can be selected according to the value of the pixel to be processed input together with the processing mode.

  Further, the color conversion unit can be selected in consideration of the document mode as shown in FIG. 7 and also in consideration of the image attribute as described in FIGS.

  For example, in the method for selecting a color conversion unit shown in FIGS. 6 and 7, when the processing mode is “high speed” or “standard”, a color conversion unit with a small processing amount may be selected depending on the image attribute. However, in the above-described color region in which the variation amount of the output value with respect to the variation of the input pixel value exceeds a certain value or the color region in which the square sum of the color difference components of the input pixel value exceeds the certain value, the processing amount It is conceivable that the color conversion accuracy is lowered and the image quality is affected by selecting a color conversion unit with a small amount of color. Therefore, in these areas, even when a color conversion unit with a small amount of processing is selected in the processing mode, a color conversion unit corresponding to a color conversion method capable of performing color conversion with high accuracy is selected. can do. Note that the constant value used as the criterion for determining the amount of variation at this time may be determined according to the color conversion accuracy, and may be set according to the grid point interval (size) of the N-dimensional lookup table, for example. For example, as described with reference to FIG. 9, when the grid point interval is narrow, the color conversion accuracy is improved. Therefore, it is possible to select a color conversion unit with a small processing amount as much as possible by increasing a certain value.

In the above description, L ^* a ^* b ^* is shown as an example of the color space of the input image data. However, the present invention is not limited to this. For example, any color space such as YCbCr color space or RGB color space may be used. It can be similarly configured in a color space. Of course, the color space after color conversion is also arbitrary.

  FIG. 13 is an explanatory diagram of an example of a computer program and a storage medium storing the computer program when the function of the color conversion apparatus or the color conversion method of the present invention is realized by the computer program. In the figure, 31 is a program, 32 is a computer, 41 is a magneto-optical disk, 42 is an optical disk, 43 is a magnetic disk, 44 is a memory, 51 is a magneto-optical disk apparatus, 52 is an optical disk apparatus, and 53 is a magnetic disk apparatus.

  A part or all of the functions of the selection unit 1, the color conversion units 2-1 to M, and the color component detection unit 4 described in the above embodiments can be realized by a program 31 that can be executed by a computer. is there. In that case, the program 31 and data used by the program (including data of the N-dimensional lookup table) can be stored in a computer-readable storage medium. A storage medium is a signal format that causes a state of change in energy such as magnetism, light, electricity, etc. according to the description of a program to a reader provided in the hardware resources of a computer. Thus, the description content of the program can be transmitted to the reading device. For example, there are a magneto-optical disk 41, an optical disk 42 (including a CD and a DVD), a magnetic disk 43, a memory 44 (including an IC card and a memory card), and the like. Of course, these storage media are not limited to portable types.

  By storing the program 31 in these storage media and mounting these storage media in, for example, the magneto-optical disk device 51, the optical disk device 52, the magnetic disk device 53, or a memory slot (not shown) of the computer 32, the computer 31 The program 31 can be read to execute the function of the color conversion apparatus or the color conversion method of the present invention. Alternatively, a storage medium may be attached to the computer 32 in advance, and the program 31 may be transferred to the computer 32 via a network, for example, and the program 31 may be stored and executed on the storage medium.

  Of course, some functions may be configured by hardware, or all may be configured by hardware. Alternatively, it can be configured as a program including the configuration of the color conversion apparatus of the present invention together with other configurations. For example, it can be configured as one program together with a control program in a copying machine or an image forming apparatus. Of course, in the case of application to other purposes, integration with a program for that purpose is also possible.

It is a block diagram which shows the 1st Embodiment of this invention. It is explanatory drawing of the specific example of an address generation at the time of referring a three-dimensional lookup table. It is explanatory drawing of an example of the 1st color conversion system. It is explanatory drawing of an example of the 2nd color conversion system. It is explanatory drawing of an example of the 3rd color conversion system. It is explanatory drawing of the 1st selection example of the color conversion system in a selection part. It is explanatory drawing of the 2nd selection example of the color conversion system in a selection part. It is a block diagram which shows the 1st modification in the 1st Embodiment of this invention. It is explanatory drawing of the 2nd modification in the 1st Embodiment of this invention. It is a block diagram which shows the 2nd Embodiment of this invention. It is a flowchart which shows an example of operation | movement of the selection part in the 2nd Embodiment of this invention. It is explanatory drawing of the 3rd selection example of the color conversion system in a selection part. It is explanatory drawing of an example of the storage medium which stored the computer program in the case of implement | achieving the function of the color conversion apparatus or color conversion method of this invention with a computer program, and the computer program. It is explanatory drawing of an example of the color conversion method by DLUT.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Selection part, 2-1-N ... Color conversion part, 3 ... N-dimensional lookup table, 4 ... Color component detection part, 21 ... Address generation part, 22 ... Interpolation part, 31 ... Program, 32 ... Computer, 41 ... magneto-optical disk, 42 ... optical disk, 43 ... magnetic disk, 44 ... memory, 51 ... magneto-optical disk apparatus, 52 ... optical disk apparatus, 53 ... magnetic disk apparatus.

Claims (31)

  In a color conversion apparatus that performs color conversion processing on an image, a plurality of color conversion units having different processing amounts, and a color conversion unit that selects one of the color conversion units according to the attribute of the image and selects the image A color conversion apparatus comprising a selection unit for passing.   In a color conversion apparatus that performs color conversion processing on an image, a plurality of color conversion units having different processing amounts and one of the color conversion units according to an image processing mode are selected and the image is transferred to the selected color conversion unit A color conversion apparatus comprising a selection unit.   In a color conversion device that performs color conversion processing on an image, a plurality of color conversion units having different processing amounts and a color obtained by selecting the color conversion unit according to the pixel value of the image to be processed and selecting the image A color conversion apparatus comprising selection means for passing to the conversion means.   In a color conversion apparatus that performs color conversion processing on an image, a plurality of color conversion units having different processing amounts, a detection unit that detects color components of an image to be processed, and the color conversion unit according to a detection result by the detection unit A color conversion apparatus comprising: a selection unit that selects any of the above and passes the image to the selected color conversion unit.   The color conversion apparatus according to claim 1, wherein the selection unit further uses an image processing mode when the color conversion unit is selected.   6. The color conversion apparatus according to claim 1, wherein the plurality of color conversion units have different color conversion accuracy.   The color conversion apparatus according to claim 1, wherein the attribute can take any one of at least two values of a background, a pattern, and a character.   The color conversion apparatus according to claim 7, wherein either the color background or the white background can be taken as the attribute of the background.   One of the color conversion means outputs pixel data representing white, and the selection means selects a color conversion means for outputting pixel data representing white when the attribute is a white background. The color conversion device according to claim 8.   The color conversion apparatus according to claim 7, wherein either the photograph or the CG can be taken as the attribute of the pattern.   The color conversion apparatus according to claim 7, wherein the character attribute can be either a black character or a color character.   One of the color conversion means outputs pixel data representing black, and the selection means selects a color conversion means for outputting pixel data representing black when the attribute is a black character. The color conversion apparatus according to claim 11.   The color conversion apparatus according to any one of claims 1 to 12, wherein at least one of the color conversion units performs color conversion using an N-dimensional lookup table.   14. The color conversion apparatus according to claim 13, wherein at least one of the color conversion means uses a value near one pixel value as an output value using an N-dimensional lookup table.   The color conversion apparatus according to claim 14, wherein a value of one point in the vicinity of the pixel value is obtained by referring to the N-dimensional lookup table based on a predetermined number of upper bits of the pixel value.   At least one of the color conversion means obtains an output value corresponding to a pixel by interpolating from two values near the pixel value for one component of the pixel value using an N-dimensional lookup table. The color conversion device according to any one of claims 13 to 15.   A value of one point in the vicinity of the pixel value obtained by referring to the N-dimensional lookup table and a pixel value for one component are referred to the N-dimensional lookup table as +1 or −1 in the upper predetermined number of bits. The color conversion apparatus according to claim 16, wherein interpolation is performed from the value of one point obtained in this way.   At least one of the color conversion means uses an N-dimensional lookup table to fix one point value near the pixel value and a plurality of points near the pixel value for the other component by fixing one component of the one point. The color conversion device according to any one of claims 13 to 17, wherein an output value corresponding to a pixel is obtained by interpolating from the value of.   Using the N-dimensional lookup table, the value of one point in the vicinity of the pixel value and one component of the one point are fixed, and the other components are each set to +1 or −1 for the upper predetermined number of bits, respectively. The color conversion apparatus according to claim 18, wherein interpolation is performed from values of a plurality of points obtained by referring to the table.   At least one of the color conversion means uses an N-dimensional lookup table to obtain four values that are the vertices of a tetrahedron including pixel values, and corresponds to the pixel by interpolation from the output values of the four points 20. The color conversion apparatus according to claim 13, wherein an output value to be obtained is obtained.   21. The color conversion apparatus according to claim 13, wherein the N-dimensional lookup table is provided in common for a plurality of the color conversion units.   20. The color conversion unit uses an N-dimensional lookup table having a larger number of grid points than an N-dimensional lookup table used by a color conversion unit having a large amount of processing. The color conversion device described in 1.   The selection means selects a color conversion means with high accuracy even if the amount of processing is large, for a color region in which a variation amount of an output value with respect to a variation of an input pixel value exceeds a certain value. The color conversion device according to any one of claims 1 to 21.   The selection unit selects a color conversion unit with high accuracy even if the amount of processing is large for a color region in which the variation amount of the output value with respect to the variation of the input pixel value exceeds a certain value. The color conversion apparatus according to any one of claims 13 to 21, wherein a value is set according to a size of the N-dimensional lookup table.   The selection means selects a color conversion means with high accuracy even if the amount of processing is large for a color region in which the sum of squares of color difference components of input pixel values exceeds a certain value. The color conversion device according to any one of claims 21 to 21.   In a color conversion method for performing color conversion processing on an image, the color conversion method includes a plurality of color conversion units having different processing amounts, and the selection unit selects one of the color conversion units according to the attribute of the image, and the selected color conversion A color conversion method characterized by performing color conversion processing of the image by means.   In a color conversion method for performing color conversion processing on an image, the color conversion method includes a plurality of color conversion units having different processing amounts, the selection unit selects one of the color conversion units according to an image processing mode, and the selected color conversion unit A color conversion method characterized by performing color conversion processing of the image by the above.   In a color conversion method for performing color conversion processing on an image, the image processing apparatus includes a plurality of color conversion units having different processing amounts, and the selection unit selects one of the color conversion units according to the pixel value of the image to be processed. A color conversion method comprising: performing color conversion processing of the image by the color conversion means.   A color conversion method for performing a color conversion process on an image has a plurality of color conversion units with different processing amounts, detects a color component of an image to be processed by a detection unit, and determines any of the color conversion units according to a detection result. A color conversion method comprising: selecting a color by a selection unit, and performing color conversion processing of the image by the selected color conversion unit.   A color conversion program for performing color conversion processing on an input image in a computer, the function of the color conversion device according to any one of claims 1 to 25 or any one of claims 26 to 29. A color conversion program that causes a computer to execute the color conversion method according to claim 1.   The color conversion according to any one of claims 1 to 25, in a computer-readable storage medium storing a color conversion program for causing a computer to execute a color conversion process for performing a color conversion process on an input image. 30. A computer-readable storage medium storing a function of the apparatus or a color conversion program for causing a computer to execute the color conversion method according to any one of claims 26 to 29.

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