JP2001157072A - Image processing method, image processor and image- forming device provided with the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image processing method and an image processor capable of executing fast chromatic conversion processing, while suppressing the size of an LUT. SOLUTION: When an RGB image data is inputted to a chromatic conversion processing part 1, a cyan data separation part 2a separates G and B to data G' and B' being upper 4 bits and data G'' and B'' being lower 4 bits with R, being the complementary color of C as is. A cyan table access part 3a reads prescribed number of the table values of C prepared concerning only grid point in an input color space expressed by address data of coordinates (G', R', B') and a cyan two-dimensional interpolation arithmetic part 4a obtains output image data of C concerning points, except for the grid points by two-dimensional interpolation arithmetic by using there table values and G'' and B''. Concerning M and Y, RGB image data except for the complementary color is also separated to be upper bit data and power bit data to obtain output image data by two-dimensional interpolation arithmetic.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing method for converting image data by a table reference method used in an image forming apparatus such as a full-color copying machine, a color management system, and the like, and an image processing for executing the image processing method. A color correction process and a color space conversion process are suitably performed in a method and an apparatus and an image forming apparatus including the same.

[0002]

2. Description of the Related Art In a digital image forming apparatus such as a full-color copying machine, a color correction method for converting input image data from an image input device into color-corrected output image data, and converting input image data into uniform color space data. There have been many proposals for color conversion methods for this purpose. Examples of such methods include the new edition of the Color Science Handbook, pp. 1137-1149 (edited by the Japan Society of Color Science, published by The University of Tokyo Press), and the Journal of the Imaging Society of Japan, Vol. 37, No. 4, (1998),
Refer to the table on pages 555 to 559 (Look Up
Table: Lookup Table). In the following, a process of correcting a color signal that would otherwise be an inappropriate signal as an output, a process of converting a color signal to change the color itself, and a coordinate of the color signal even if the color itself does not change All processes for converting the system will be referred to as color conversion.

In the direct conversion method, which is one of the LUT methods, color conversion data is calculated in advance for all combinations of input image data, the result is stored in a color conversion table, and table values for the input image data are stored. And outputting as output image data. This direct conversion method accesses a color conversion table, has a simple circuit configuration, can perform relatively high-speed processing, and can be applied to any nonlinear characteristic.

In another three-dimensional interpolation method, which is another LUT method, a table value for a combination of some selected input image data is calculated in advance, stored in a color conversion table, and the table value is stored in the color conversion table. Is calculated by three-dimensional interpolation using the table values stored in the color conversion table. As an example, each image data of 8 bits and 3 colors constituting the input image data is defined as X, Y, Z coordinates, and each coordinate axis is 4 bits, that is,
FIG. 8 shows the input color space divided into 2 ⁴ = 16.

In this case, the input color space is 4096 (16 × 16 × 1).
6) It is divided into cubes, and the total number of grid points, or table values, is 4913 (17 × 17 × 17). The coordinates of each grid point p _i (i = 0, 1,..., 7) of the cube are represented by the upper 4 bits of each color image data, and the grid point p _i stores color conversion data corresponding to the input image data. ing. Grid point p _i
For an input image data to be a point other than the lower 4 bits of each color image data, the color conversion data obtained by interpolation using the table values of the eight grid points p _i in the vicinity.

[0006] Now, as shown in FIG. 8, each relative ratio grid width in each grid of any of the input image data p, X-axis grid point p ₀ in the standard, Y-axis, the Z-axis direction a,
Assuming that b and c (0 ≦ a, b, c ≦ 1), the interpolation value f (p) in the eight-point interpolation (cubic interpolation) is expressed by the following table, where f (p _i ) is the table value at the grid point p _i . It is determined by equation (1).

F (p) = (1-a) (1-b) (1-c) · f (p₀) + A (1-b) (1-c) ・ f (p₁) + Ab (1-c) ・ f (p_Two) + (1-a) b (1-c) ・ f (p_Three) + (1-a) (1-b) c ・ f (p_Four) + A (1-b) c ・ f (p_Five) + Abc ・ f (p₆) + (1-a) bc ・ f (p₇) (1) FIG. 9 shows an example of a conventional color conversion processing unit 51 that performs the above processing.
An example of the configuration is shown. The figure shows R, which constitutes the input image data.
From each of the G and B image data,
This is a configuration for obtaining image data of Ann (C). First, 8
R, G, and B image data are converted to cyan data separation unit 5
2, the cyan data separation unit 52 outputs
The upper 4 bits of data R ', G', B 'and the lower 4 bits.
Bit data R ", G", B ".
Each bit data is input to the cyan table access unit 53.
Each of the lower 4 bits is cyan 3D interpolation
It is input to the calculating unit 54. Cyan table access unit 53
Is an input color space having coordinates of R, G, and B image data.
Search for a cube containing the input image data in _i
Table values f (p_i) Is the cyan LUT
Read from

When these table values f (p _i ) are input to the cyan three-dimensional interpolation calculating unit 54, the cyan three-dimensional interpolation calculating unit 54 converts the lower 4 bits of data R ″, G ″, B ″ into a cube. The relative ratios a, b, and c corresponding to the grid widths are obtained, and the interpolation value f (p) is calculated by performing an interpolation operation according to Equation (1), thereby obtaining cyan output image data. The same configuration as in FIG. 9 is applied to the color conversion processing to magenta and yellow.

This three-dimensional interpolation method can obtain color conversion values for all combinations of input image data even when the number of input image data for which table values are to be calculated in advance is limited. Can be reduced in size.

[0010]

However, each of the above image processing methods has the following problems. In the direct conversion method, high conversion accuracy can be obtained even when the relationship between the input image data and the output image data has an arbitrary nonlinearity. However, table values for all the input image data are stored in the color conversion table. Need to be
There is a problem that the size of the color conversion table becomes very large. For example, input image data of 8 bits and 3 colors requires a table size of 48 Mbytes and input image data of 10 bits and 4 colors requires a table size of 4 Gbytes, which is problematic in terms of cost.

On the other hand, in the three-dimensional interpolation method, since the memory table is constituted by the table values of only some of the input image data among all the input image data, there is no problem regarding the table size. The three-dimensional interpolation calculation process requires a large amount of calculation, and a complicated circuit for this process is additionally required, so that there is a problem that the processing speed is reduced.

The present invention has been made in view of the above-mentioned conventional problems, and a first object of the present invention is to provide an image processing method and an image processing apparatus capable of performing high-speed color conversion processing while suppressing the size of an LUT. Another object of the present invention is to provide an image forming apparatus.

A second object of the present invention is to provide a color conversion process using a conventional three-dimensional interpolation method, including a color conversion process using the LUT of the present invention capable of achieving the first object, and An image processing method capable of obtaining an output image with higher image quality by individually setting a color conversion process or the like using a direct conversion method for each color of image data of each coordinate constituting output image data. An object of the present invention is to provide an image processing apparatus and an image forming apparatus.

[0014]

According to a first image processing method of the present invention, in order to achieve the first object, a first colorimetric system in which image data of each coordinate is represented by predetermined bits. An image processing method for individually obtaining image data of each coordinate of a second color system from input image data and performing a color conversion process as output image data, wherein the output to be obtained from the input image data is The image data of the specific two coordinates defined for each image data of each coordinate of the image data is subjected to data separation processing for separating into upper bit data and lower bit data, while image data of other coordinates of the input image data is performed. Is not subjected to the data separation processing, the upper bit data and the image data not subjected to the data separation processing are used as address data, and the input image represented by the address data For the data, a table value is read from a look-up table in which image data of each coordinate of the corresponding output image data is stored in advance, and for the input image data not represented by the address data, a table is read from the look-up table. By reading a predetermined number of values and performing a two-dimensional interpolation operation using the predetermined number of table values and the lower-order bit data, image data of each coordinate of the corresponding output image data is obtained.

According to the above invention, of the input image data of the first color system, the image data of the specific two coordinates is subjected to the process of separating into upper bit data and lower bit data. Also, the extracted upper bit data,
Image data of other coordinates that do not perform data separation processing is used as address data, and image data of each coordinate of corresponding output image data is prepared as a table value only for input image data represented by the address data. I do. For other input image data not represented by the address data, that is, input image data to which an address should be finely specified by lower-order bit data of the image data, a predetermined number of table values such as a table value in address data in the vicinity thereof Is read, and a two-dimensional interpolation operation is performed using these table values and the lower-order bit data to obtain image data at each coordinate of the output image data.

As described above, since the table value corresponding to the lower bit data is omitted, the size of the LUT (look-up table) is much smaller than that used in the direct conversion method, and the time of the interpolation calculation is three-dimensional. It is shorter than the interpolation method. As a result, high-speed color conversion processing from input image data having the first color system to output image data having the second color system can be performed while suppressing the size of the LUT.

Further, in order to achieve the first object, the first image processing apparatus of the present invention converts input image data of a first color system in which image data of each coordinate is represented by predetermined bits. In an image processing apparatus having a color conversion processing unit that individually obtains image data of each coordinate of the second color system and uses it as output image data, the color conversion processing unit may obtain the input image data. The image data of the specific two coordinates defined for each image data of each coordinate of the output image data is subjected to a data separation process of separating upper bit data and lower bit data, while image data of other coordinates is performed. Is a data separation unit that does not perform the data separation processing, and the upper bit data extracted by the data separation unit and the image data that is not subjected to the data separation processing are address data. For the input image data represented by the address data, read a table value from a look-up table in which image data of each coordinate of the corresponding output image data is stored in advance, and read the input image data not represented by the address data. A lookup table access unit that reads a predetermined number of table values from the lookup table; a predetermined number of table values read by the lookup table access unit; and lower bit data extracted by the data separation unit. And a two-dimensional interpolation operation unit that obtains image data of each coordinate of the output image data corresponding to the input image data not represented by the address data by performing a two-dimensional interpolation operation.

According to the above-mentioned invention, of the input image data of the first color system, image data of specific two coordinates is separated by the data separation unit into upper bit data and lower bit data. Do. Further, the extracted upper bit data and the image data of the other coordinates that are not subjected to the data separation processing are used as address data, and only the input image data represented by the address data is used for each coordinate of the corresponding output image data. Are prepared in advance as table values. The look-up table access unit, for other input image data not represented by the address data, that is, for input image data whose address is to be finely specified by the lower bit data of the image data, a table in the address data in the vicinity thereof A predetermined number of table values, such as values, are read out, and the two-dimensional interpolation operation unit performs two-dimensional interpolation operation using these table values and lower-order bit data to obtain image data of each coordinate of the output image data.

As described above, since the table value corresponding to the lower bit data is omitted, the size of the LUT (look-up table) is much smaller than that used in the direct conversion method, and the time of the interpolation operation is three-dimensional. It is shorter than the interpolation method. As a result, high-speed color conversion processing from input image data having the first color system to output image data having the second color system can be performed while suppressing the size of the LUT.

Incidentally, among the input image data of the first color system, the image data having a complementary color relationship with the image data of the predetermined coordinates in the output image data of the second color system is the second image data.
Has the greatest effect on the conversion accuracy to the color image output image data.

Therefore, in the first image processing apparatus of the present invention, the image data which has not been subjected to the data separation processing by the data separation unit is further converted into image data of coordinates to be obtained for output image data and complementary color data. The image data may be configured to be related image data.

According to this, the data separation processing is not performed on the image data having the complementary color relationship, thereby minimizing the conversion error due to the interpolation operation. Therefore, color conversion processing can be performed accurately and efficiently without causing color shift.

Further, in the first image processing apparatus of the present invention, when black image data is included in the output image data, the data separation processing is not performed by the data separation unit when obtaining black image data. The image data can be configured to be image data of coordinates having a high contribution rate to the brightness of the output image data.

According to this, when black image data is included in the second colorimetric output image data, the first colorimetric input image data of the first colorimetric input image data is used to obtain black image data. Data separation processing is not performed on the image data having a high contribution rate to the brightness of the output image data of the second color system. As a result, a conversion error due to the interpolation operation with respect to the gray component of the output image data is minimized. Therefore, color conversion processing can be performed accurately and efficiently without causing color shift.

Further, in the first image processing apparatus according to the present invention, when the input image data includes lightness data and chromaticity data, the data separation unit is used when obtaining image data of each coordinate of the output image data. Accordingly, the image data not subjected to the data separation processing can be configured to be the brightness data.

According to this, when the input image data of the first color system includes lightness data and chromaticity data, the data separation processing is not performed on the lightness data. This allows
An error due to the interpolation calculation is avoided as much as possible for the brightness component of the output image data. Therefore, color conversion processing can be performed accurately and efficiently without causing color shift.

Further, in the first image processing apparatus of the present invention, the color conversion processing unit is separately provided with a black image data generation unit for obtaining black image data by a three-dimensional interpolation operation using a look-up table method. It can also be configured.

When the two-dimensional interpolation operation is used, the color conversion accuracy is high and accurate, but a slight ripple error occurs. Colors near gray formed by black image data are sensitive to human eyes, and when ripples occur in white to black gradations and the like, they are more conspicuous than when ripples occur in other colors. Therefore, as described above, by separately providing a black image data generation unit that obtains black image data by three-dimensional interpolation using the look-up table method, it is possible to prevent the image quality from deteriorating due to the above-described ripple. Thus, the image quality can be further improved.

Further, in the first image processing apparatus of the present invention, the color conversion processing unit is separately provided with a black image data generation unit for obtaining black image data without performing an interpolation operation using a look-up table method. It is also possible to adopt a configuration, whereby it is possible to prevent the occurrence of ripples in a gray image represented by the above-described black image data, and to further improve the image quality.

In the second image processing method of the present invention, in order to achieve the second object, an input image of a first color system in which image data of each coordinate is represented by predetermined bits. An image processing method in which image data of each coordinate of the second color system is individually obtained from the data and color conversion processing as output image data is performed using a lookup table method.
A color conversion process suitable for the color of the image data is performed for each image data at each coordinate of the output image data.

As a color conversion process using a lookup table method, a direct conversion method and a three-dimensional interpolation method have been conventionally known. The present applicant has conducted various studies on each of the color conversion processes such as the direct conversion method, the three-dimensional interpolation method, and the two-dimensional interpolation method which is the configuration of the above-described first image processing method and image processing apparatus of the present invention. However, for each color conversion process, there is a difference in processing speed, ripple error, interpolation error, etc., depending on whether interpolation calculation is used, and the interpolation calculation used, and even if the same interpolation calculation method is used, It has been found that the image quality of the output image depends on the image, and more specifically, on the colors constituting the output image data.

Therefore, in the above-described second image processing method of the present invention, a color conversion process using a look-up table method is performed according to the color of image data of each coordinate of a second color system forming output image data. It is set individually.
This makes it possible to perform a color conversion process according to the color of each image data constituting the output image data by utilizing each feature of the color conversion process using the lookup table method. By applying the present invention to an image forming apparatus, it is possible to obtain an output image with improved image quality as a whole.

For example, if the image data of the second color system forming the output image data is composed of cyan, magenta, and yellow, for cyan and magenta,
By performing a three-dimensional interpolation operation with a small ripple error to enhance gradation, and performing a two-dimensional interpolation operation for yellow, color conversion accuracy can be increased and accurate colors can be obtained, so that the overall image quality can be improved.

If the image data of the second color system constituting the output image data is composed of cyan, magenta, and yellow and black, the two-dimensional interpolation is performed for cyan, magenta, and yellow. By performing the processing, the color conversion accuracy is higher than the three-dimensional interpolation processing,
Although accurate colors can be output, as described above, if two-dimensional interpolation processing is performed on black image data, image quality may be degraded due to ripple errors.

Therefore, in the above-described second image processing method of the present invention, when the image data is black, a color conversion process using a three-dimensional interpolation method or a table value read from a lookup table is used as it is. It is more preferable to adopt a configuration that uses a color conversion process that performs a color conversion process, thereby preventing generation of ripples in a gray image represented by black image data even in a configuration that generates up to black image data by the color conversion process. However, for other colors, the color conversion accuracy is increased to produce accurate colors, and the image quality can be improved as a whole.

An image processing apparatus for realizing the second image processing method of the present invention has the following configuration.

The second image processing apparatus according to the present invention comprises the second image processing apparatus.
In order to achieve the object of (1), the color conversion processing unit, for each coordinate of the output image data, the input image data of the first color system,
According to the type of the interpolation operation including the presence or absence of the interpolation operation, a combination of the image data of each coordinate in the input image data and the data separation unit for separating only the address data or the address data and the data for the interpolation is described above. A lookup table suitable for the type of interpolation calculation processing, in which image data at predetermined coordinates of output image data corresponding to the address data is stored as a table value as the address data, is separated by the data separation unit. A lookup table access unit for reading a table value corresponding to address data, and only a table value read from a lookup table by the lookup table access unit, or an interpolation unit separated from the table value by a data separation unit Interpolation operation processing unit that performs predetermined interpolation operation processing using data The interpolation calculation processing of each system including the data separation unit, the look-up table access unit, and the interpolation calculation processing unit provided for each coordinate of the output image data is performed on the image data of each coordinate of the output image data. It is characterized in that it is set according to the color.

According to the above arrangement, the data separation unit converts the input image data of the first color system into the address data and the interpolation data in accordance with the interpolation calculation processing suitable for the color of the image data to be obtained in the output image data. And separate it into data and data format. Here, if the interpolation calculation process corresponding to the color of the image data to be obtained is a process in which no interpolation calculation is performed, no interpolation data is required, and only the address data is extracted.

The address data indicates an address for reading a table value stored in a look-up table of a look-up table access unit provided at a subsequent stage of the data separating unit. The combination of the image data of each coordinate in the data is stored as the address data, and the image data of each coordinate of the output image data corresponding to the address data is stored as a table value. Such a look-up table is created for each interpolation calculation process.

The look-up table access unit reads a table value corresponding to the address data by using the provided look-up table, and outputs the read table value to the interpolation operation processing unit at the subsequent stage. The interpolation calculation processing unit performs a predetermined interpolation calculation process using the table values read by the lookup table access unit and the interpolation data separated by the data separation unit. If the interpolation calculation process that does not perform the interpolation calculation is set, the table value is output as it is.

This makes it possible to perform a color conversion process according to the color of each image data constituting the output image data by utilizing each feature of the color conversion process using the lookup table method. In an image forming apparatus provided with such an image processing apparatus, it is possible to obtain an output image with improved image quality as a whole. The image forming apparatus is realized by, for example, a digital color copying machine using an inkjet system or an electrophotographic system.

Further, in the above-mentioned second image processing apparatus of the present invention, in a system where the image data is black, it is more preferable that the color conversion processing using three-dimensional interpolation is performed. Accordingly, it is possible to prevent the occurrence of ripples in the gray image represented by the above-described black image data, and to further improve the image quality.

In the above-described second image processing apparatus of the present invention, in a system where the image data is black, a color conversion process using the table value read from the lookup table as it is is performed. Thus, it is possible to prevent the occurrence of ripples in the gray image represented by the above-described black image data, and to further improve the image quality.

[0044]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [Embodiment 1] An embodiment of an image processing method, an image processing apparatus, and an image forming apparatus according to the present invention will be described below with reference to FIGS. .

FIG. 1 shows a configuration of a color conversion processing unit 1 which is a characteristic part of the image processing apparatus according to the present embodiment.
The color conversion processing unit 1 performs RGB (R: red, G: green, B: blue)
Input image data of the first color system represented by coordinates
The image data is converted into output image data of a second color system represented by Y (C: cyan, M: magenta, Y: yellow) coordinates. The data separation unit 2 and the color conversion table access unit (look-up table) (Access unit) 3, two-dimensional interpolation operation unit 4, and black generation / under color removal unit 5. Further, the data separation unit 2 includes a cyan data separation unit 2a, a magenta data separation unit 2b, and a yellow data separation unit 2c, and the color conversion table access unit 3 includes a cyan table access unit 3a, a magenta table access unit 3b, and a yellow table access. The two-dimensional interpolation operation unit 4 includes a cyan two-dimensional interpolation operation unit 4a and a magenta 2
Dimensional interpolation calculation unit 4b and yellow two-dimensional interpolation calculation unit 4
c.

The cyan data separation unit 2a, cyan table access unit 3a, and cyan two-dimensional interpolation operation unit 4a
From the R, G, and B image data constituting the input image data, C image data of the output image data is obtained.
The magenta data separation unit 2b, the magenta table access unit 3b, and the magenta two-dimensional interpolation calculation unit 4b convert the M image data of the output image data from the R, G, and B image data constituting the input image data. Ask. Yellow data separation unit 2c, yellow table access unit 3
The c and yellow two-dimensional interpolation calculation unit 4c obtains Y image data of the output image data from each of the R, G, and B image data constituting the input image data.

The data separation unit 2 determines the image data C, M, and Y of the three types of coordinates to be obtained among the image data R, G, and B of the three types of coordinates each represented by 8 bits. The specific two types of image data are subjected to a data separation process of separating upper 4-bit data and lower 4-bit data, and the remaining image data is left as 8-bit data without performing the data separation process. Output. The upper 4-bit data separated and the 8-bit data not separated are input to the color conversion table access unit 3, and the separated lower 4-bit data is 2 bits.
It is input to the dimensional interpolation calculator 4. Note that each image data constituting the first color system input image data is not limited to 8 bits, but may be another bit number such as 10 bits, and even if the number of upper bits and the number of lower bits to be separated are arbitrary. The good thing is clear from the following description.

Here, of the input image data of the first color system, image data having a complementary color relationship with image data of coordinates to be obtained in output image data of the second color system. No separation processing is performed. For example, the cyan data separation unit 2a outputs the R image data, which is the complementary color of C, as it is, without any processing, as it is in 8 bits, and outputs the upper 4 bits of the G image data and the B image data. G'.B 'and lower 4-bit data G ".B" are output separately. Similarly, no data separation processing is performed for G for M and for B for Y.

The color conversion table access unit 3 determines whether the signal input from the data separation unit 2, that is, the upper 4 bits of data extracted by the data separation unit 2, is not separated.
Using the bit image data as address data, a color conversion table (lookup table) for color system conversion stored therein is accessed to read a table value and read out.
It is input to the dimensional interpolation calculator 4.

The color conversion table is created corresponding to the input color space as shown in FIG. In the figure, the X-axis, Y-axis, and Z-axis coordinates correspond to any of the R, G, and B image data, respectively, and the coordinates (x, (y, z) corresponds to one input image data. The input color space is 2 in the X-axis and Z-axis directions.
⁴ = 16, and the X coordinate and Z coordinate of each grid point correspond to the upper 4 bits of the two types of image data. The input color space is divided into 2 ⁸ = 256 in the Y-axis direction, and the Y coordinate of the grid point corresponds to image data represented by 8 bits. And the coordinates (x, y, z) of these grid points
Corresponds to the address data, and the color conversion table stores a table value for each address data.

For example, the color conversion table of the cyan table access unit 3a is (x, y, z) = (G ', R,
B ') is created corresponding to the input color space with the coordinates of the grid points, and the image data of C corresponding to each grid point on a one-to-one basis is stored as a table value. When the coordinates (G ', R, B') of a certain grid point are designated, the cyan table access unit 3a reads a table value corresponding to this grid point from the C color conversion table. Here, R is a complementary color of C and has the greatest influence on the conversion accuracy of C into output image data. Therefore, for R, 8-bit (256 steps) grid points are prepared, and interpolation to be described later is performed. We try to avoid errors due to calculations as much as possible. As a result, color conversion processing can be performed accurately and efficiently without causing color shift.

The method of creating the color conversion table is performed, for example, in the following procedure. 1) The color image forming unit outputs each color patch of CMY. 2) Each output color patch is read from the color image input device to obtain RGB image data. 3) The output CMY values are associated with the read RGB values, and the input / output characteristics of the image forming apparatus, that is, the coefficients between the CMY values and the RGB values are defined by a neural network or a masking operation coefficient determination method. 4) Color conversion calculation is performed using the coefficients obtained by the above method.

In the input color space, grid points are provided only for every 16 bits in the input image data in the X-axis direction and the Z-axis direction. The table value corresponding to the address to be specified by the lower 4-bit data is not stored.
Thus, the color conversion table according to the present embodiment
Since the table value corresponding to the lower bit data is omitted for the type of image data, L used in the direct conversion method is used.
It is much smaller in size than UT.

To obtain output image data corresponding to a point other than a grid point, the two-dimensional interpolation operation unit 4 determines four grid points p _i (i = 0, i = 0,
, 3) are extracted and a two-dimensional interpolation operation is performed using a table value g (p _i ) stored corresponding to each grid point p _i of the unit square. The position coordinates of a point in the unit square of the arbitrary input image data p are determined by G ″, B ″ for lower 4 bits of data extracted by the data separation unit 2, for example, C. Given the position coordinates, the relative ratio to the grid width in each grid is determined.

As shown in FIG. 2, this relative ratio is calculated by
_A , b (0 ≦
a, b ≦ 1), an interpolated value g (p) for any input image data p by four-point interpolation is obtained by the following equation (2), where g (p _i ) is the table value at the grid point p _i . Can be

G (p) = (1-a) (1-b) · g (p ₀ ) + a (1-b) · g (p ₁ ) + ab · g (p ₂ ) + (1-a) b G (p ₃ ) (2) The amount of calculation of equation (2) by four-point interpolation is only about one third of the amount of calculation of equation (1) by eight-point interpolation. In comparison, the speed of the color conversion processing can be increased.

The black generation / under color removal section 5 performs black generation processing / under color removal processing on the CMY color conversion values (output image data) output from the two-dimensional interpolation calculation section 4. As a method of performing black generation / undercolor removal, a method of performing black generation using skeleton black as shown in FIG. 7 is general. The input / output 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 UCR (Under Color Remova).
l) If the rate is γ (0 <γ <1), the black generation / undercolor removal processing is represented by the following equation (3).

K ′ = f ｛min (C, M, Y)｝ C ′ = C−γK ′ (3) M ′ = M−γK ′ Y ′ = Y−γK ′ Next, from the RGB input image data, K in addition to CMY
FIG. 3 shows the configuration of the color conversion processing unit 11 when generating (black) output image data. The color conversion processing unit 11 further includes a black data separation unit 2d, a black table access unit 3d, and a black two-dimensional interpolation calculation unit 4d in the color conversion processing unit 1 of FIG. 2, and omits the black generation / under color removal unit 5. This is the configuration. In addition, the result of black generation is stored in the K color conversion table of the color conversion table access unit 3, and the result of under color removal is stored in each of the CMY color conversion tables in advance.

The black data separating section 2d receives the input RGB data.
Among the image data, the data separation process is not performed on image data having a high contribution ratio to the gray component (or brightness component) of the output image data. The respective contribution rates of RGB to the gray component (or lightness component) are generally G ≧ R ≧ B, and the accuracy of the gray component (or lightness component) included in the color to be finally generated is G
Image data is the most influential. Therefore, the G image data is not subjected to data separation processing to avoid errors due to the interpolation operation as much as possible, and the R and B image data is separated into upper 4-bit data and lower 4-bit data. As a result, color conversion processing can be performed accurately and efficiently without causing color shift.

Next, the input image data of the first color system is C
IE (Commission Internationaled l'Eclairage:
FIG. 4 shows the configuration of the color conversion processing unit 21 in the case of L ^* a ^* b ^* data (L ^* is brightness data, a ^* and b ^* are chromaticity data). The color conversion processing unit 21 includes a color conversion table access unit 3, a two-dimensional interpolation operation unit 4, a black generation / under color removal unit 5 of the color conversion processing unit 1, a color coordinate conversion unit 6,
This is a configuration in which a color conversion / data processing unit 7 and a data separation unit 8 are added.

The color coordinate converter 6 converts the input RGB image data into CIEL ^* a ^* b, which is uniform color space data.
^* Convert to data. The color coordinate conversion is performed in the following procedure. 1) Measure each color patch of a color chart original with a colorimeter to obtain L ^* a ^* b ^* values. 2) The color chart original used for color measurement is read from a color image input device to obtain RGB values. 3) In order to associate the measured L ^* a ^* b ^* value with the read RGB value, a coefficient between the L ^* a ^* b ^* value and the RGB value is obtained by a neural network or a masking operation coefficient determination method. 4) Perform color coordinate conversion using the coefficients obtained by the above method.
At this time, the circuit configuration may be a masking operation using a matrix, may be an LUT method using a three-dimensional interpolation operation, or may be a two-dimensional interpolation operation as in the configuration of the present embodiment. The LUT method using the LUT may be used.

The color conversion / data processing section 7 performs color conversion processing by image editing on the uniform color space data L ^* a ^* b ^* obtained by the color coordinate conversion section 6 and brightness / lightness conversion by color reproduction area interpolation.
Data processing such as saturation compression is performed, and each is output as 8-bit image data L ^{* (1)} · a ^{* (1)} · b ^{* (1)} .
The data separation unit 8 does not perform the data separation process on the brightness data L ^{* (1)} that easily affects the accuracy of the color conversion process among the image data input from the color conversion / data processing unit 7, a ^{* (1)} · b ^{* (1)} is the upper bit data a
^{* (2)} · b ^{* (2)} and lower bit data a ^{* (3)} · b ^{* (3)} . As described above, for the brightness data L ^{* (1)} , the error due to the interpolation calculation is avoided as much as possible without performing the data separation processing, so that the color conversion processing is performed accurately and efficiently without causing color shift. be able to.

The 8-bit lightness data L ^{* (1)} and the upper 4 bits of data a ^{* (2)} · b ^{* (2)} are stored in the cyan table access section 3a, the magenta table access section 3b of the color conversion table access section 3, and And yellow table access unit 3
c is input in common to each of
^{* (3)} · b ^{* (3)} is commonly input to each of the cyan two-dimensional interpolation calculator 4a, the magenta two-dimensional interpolation calculator 4b, and the yellow two-dimensional interpolation calculator 4c of the two-dimensional interpolation calculator 4. . Note that the color conversion processing unit 21 generates /
Although the under color removal process is performed, a configuration including a K color conversion table as in the case of FIG. 3 may be employed.

As is apparent from the above, in the color conversion processing units 1, 11, and 21 according to the present embodiment, the combination of the first color system and the second color system is arbitrary.

Next, a series of color conversion processes from the input image data having the first color system to the output image data having the second color system by the color conversion processing sections 1, 11, 21 are shown in FIG. Shown in the flowchart. First, in S1, the data separation unit 2
8, among the input image data of the first color system,
No data separation process is performed for the given image data,
Other image data is separated into upper bit data and lower bit data. Next, at S2, the color conversion table access unit 3 uses the color conversion table as a table value from the color conversion table, using the image data not subjected to the data separation processing and the upper bit data of the other image data subjected to the data separation processing as address data. read out. Then, in S3, the two-dimensional interpolation calculation unit 4 performs a two-dimensional interpolation calculation using the table values read from the color conversion table and the lower-order bit data of the image data subjected to the data separation processing.
Finally, the value obtained by the two-dimensional interpolation calculation is output as output image data (color conversion value) of the second color system.

Here, Table 1 summarizes the characteristics of the two-dimensional interpolation method according to the present embodiment and those of the conventional direct conversion method and three-dimensional interpolation method.

[0067]

[Table 1]

As can be seen from the above table, the two-dimensional interpolation method according to this embodiment can simultaneously reduce the size of the circuit, increase the conversion speed, and reduce the cost.

Next, the color conversion processing unit 1 (or 11.2)
FIG. 6 shows the configuration of the image processing apparatus 31 using 1). The image processing device 31 includes an A / D conversion unit 32, a shading correction unit 33, an input gradation correction unit 34, a color conversion processing unit 1 (or 11/21), a spatial filter processing unit 35, an output gradation correction unit 36, And a tone reproduction processing section 37.
Further, a color image input device 15 is provided at a stage preceding the image processing device 31, and a color image forming unit 16 is provided at a stage subsequent to the image processing device 31. The image processing device 31 that performs the above-described two-dimensional interpolation calculation includes:
Since the color conversion processing unit 1 (or 11/21) performs high-speed color conversion processing using a small-sized color conversion table, it is particularly effective for a copying machine or a multifunction peripheral in which high-speed image output and low cost are desired. It is something.

The color image input device 15 has, for example, a scanner, and converts a reflected light image from an original into RGB analog signals as a CCD (Charge Coupled Device).
And input it to the image processing device 31. In the image processing device 31, first, an A / D (analog / digital) converter converts an input RGB signal into digital image data. Next, the shading correction unit 33 performs a process of removing various distortions generated in the illumination system, the imaging system, and the imaging system of the image input device from the image data. After that, the input tone correction unit 34 performs a process of converting the RGB reflectance signals into signals that are easy to handle by the image processing system, such as density signals, while adjusting the color balance.

Next, the color conversion processing unit 1 (or 11.2)
1) converts the RGB image data into CMY image data by a two-dimensional interpolation operation, and converts the CMY image data from CMY image data into spectral characteristics of CMY color materials including unnecessary absorption components in order to realize faithful color reproduction. A process for removing color turbidity based on the image is performed. Further, black generation processing for generating K (black) image data from the CMY three-color image data after the color conversion and under color removal processing are performed to generate CMYK four-color image data.

Next, the spatial filter processing section 35 performs a spatial filter process using a digital filter on the obtained image data, and corrects the spatial frequency characteristics so as to prevent blurring and graininess deterioration of the output image. To process. Then, the output tone correction unit 36 performs an output tone correction process of converting a signal such as a density signal into a dot area ratio which is a characteristic value of the color image forming unit 16, and finally, a tone reproduction processing unit 37. Performs a tone reproduction process (halftone generation) that divides an image into pixels and reproduces each tone.

The color image forming section 16 outputs image data on a recording medium (for example, paper). For example, a color image forming apparatus using an electrophotographic system or an ink jet system can be used. It is not limited.

As described above, according to the image processing method and the image processing apparatus of the present embodiment, the second color expression is obtained from the input image data of the first color system while suppressing the size of the LUT. It is possible to perform high-speed color conversion processing on output image data comprising a system.

[Embodiment 2] Another embodiment of an image processing method, an image processing apparatus, and an image forming apparatus according to the present invention will be described below with reference to FIGS. For convenience of explanation, components having the same functions as those shown in the drawings of Embodiment 1 are given the same reference numerals, and descriptions thereof are omitted.

In the present embodiment, the LU including the color conversion processing using the two-dimensional interpolation operation described in detail in the first embodiment is used.
An image processing method and an image processing method capable of obtaining an output image with higher image quality by individually setting the color conversion processing using the T method for each color of the image data of each coordinate constituting the output image data The present invention relates to an apparatus and an image forming apparatus.

FIG. 10 is a block diagram showing a configuration of a digital color copying machine according to an embodiment including an image processing device 39 to which the configuration of the image processing method and the image processing device of the present invention is applied.

As shown in FIG. 10, the image processing device 39
Are an A / D conversion unit 32, a shading correction unit 33, an input gradation correction unit 34, an area separation processing unit 40, a color conversion processing unit 41, a black generation / under color removal unit 5, a spatial filter processing unit 3.
5, an output tone correction section 36, and a tone reproduction processing section 37. The color image input device 15 and the color image forming section 16 are connected to the output tone correction section 36 and the color image forming section 16 to constitute a digital color copying machine as a whole. are doing.

The RGB analog signals read by the color image input device 15 are transmitted to the A /
D conversion unit 32, shading correction unit 33, input gradation correction unit 34, area separation processing unit 40, color conversion processing unit 41, black generation / under color removal unit 5, spatial filter processing unit 35, output gradation correction unit 36 , And the tone reproduction processing section 37 in this order.
It is output to the color image forming unit 16 as a CMYK digital color signal.

Although the description partially overlaps with FIG. 6 used in the first embodiment, the members shown in FIG.
The / D (analog / digital) conversion unit 32 converts an RGB analog signal into a digital signal, and the shading correction unit 33 converts a digital RGB signal sent from the A / D conversion unit 32 into a digital signal. The color image input device 15 performs a process of removing various distortions generated in the illumination system, the imaging system, and the imaging system.

The input tone correction section 34 outputs the RGB signals (R
In addition to adjusting the color balance of the (GB reflectance signal), the image signal is converted into a signal, such as a density signal, which can be easily handled by an image processing system employed in the image processing apparatus 39.

The segmentation processing section 40 converts the RGB signals into
Each pixel in the input image is separated into a character area, a halftone dot area, and a photograph area. The area separation processing unit 40
Based on the separation result, an area identification signal indicating which area the pixel belongs to is output to the black generation / under color removal unit 5, the spatial filter processing unit 35, and the tone reproduction processing unit 37.

The color conversion processing section 41 converts the input image data of the first color system represented by RGB coordinates into CMY (C: cyan, M: magenta, Y: yellow) coordinates.
Is converted into output image data of the color system. In order to realize faithful color reproduction, processing for removing color turbidity based on the spectral characteristics of CMY color materials including unnecessary absorption components is performed. The details of the color conversion processing unit 41 will be described later.

The black generation / under color removal unit 5 outputs the CM after color correction.
The CMY is a process for generating a new CMY signal by subtracting the K signal obtained by the black generation from the original CMY signal from black generation for generating a black (K) signal from three Y color signals.
Are converted into CMYK four-color signals. As an example of the black generation processing, there is a method of performing black generation using skeleton black (general method). However, since this method is as described in the first embodiment, further description is omitted for convenience.

The spatial filter processing unit 35 performs a spatial filter process on the image data of the CMYK signal input from the black generation / under color removal unit 5 using a digital filter based on the region identification signal, and removes the spatial frequency characteristics. The output gradation correction unit 36 and the gradation reproduction processing unit 37 perform the CMYK correction in the same manner as the spatial filter processing unit 35.
A predetermined process is performed on the image data of the signal based on the region identification signal.

For example, in the area separated into characters by the area separation processing section 40, the sharpening processing by the spatial filter processing section 35 has a large amount of high-frequency enhancement in order to particularly enhance the reproducibility of black characters or color characters. Is done. At the same time, the tone reproduction processing section 37 selects binarization or multi-value processing on a high-resolution screen suitable for reproduction of high-frequency components.

Further, with respect to the area separated into halftone dots by the area separation processing section 40, the spatial filter processing section 35 performs low-pass filter processing for removing an input halftone dot component. The output tone correction unit 36 performs an output tone correction process of converting a signal such as a density signal into a dot area ratio which is a characteristic value of the color image forming unit 16,
In the tone reproduction processing section 37, a tone reproduction process (halftone generation) for finally separating an image into pixels and performing processing so that each tone can be reproduced is performed. With respect to the region separated into photographs by the region separation processing unit 40, binarization or multi-value processing is performed on a screen that emphasizes tone reproducibility.

The image data subjected to each of the above-described processes is
The data is temporarily stored in the storage unit, read out at a predetermined timing, and input to the color image forming unit 16.

Hereinafter, the color conversion correction section 41 will be described in detail. The present invention is also applicable to a system in which an image input device such as a scanner, an image processing device, an image forming device such as a color printer, and an image display device such as a display are configured via a network.

FIG. 11 shows a configuration of the color conversion processing section 41 in the image processing device 39. As shown in FIG. 11, the color conversion processing unit 41 includes a data separation unit 62, a color conversion table access unit 63, and an interpolation calculation processing unit 64. The data separation unit 62, the color conversion table access unit 63, and the interpolation operation processing unit 64 are provided for each of CMYK image data of CMYK constituting output image data. The color conversion table access unit 63 includes a cyan data separation unit 62a, a magenta data separation unit 62b, and a yellow data separation unit 62c. The color conversion table access unit 63 performs interpolation from the cyan table access unit 63a, the magenta table access unit 63b, and the yellow table access unit 63c. The arithmetic processing unit 64 includes a cyan three-dimensional interpolation arithmetic processing unit 64a, a magenta three-dimensional interpolation arithmetic processing unit 64b, and a yellow two-dimensional interpolation arithmetic processing unit 64
c.

The cyan data separation section 62a, cyan table access section 63a, and cyan three-dimensional interpolation calculation processing section 64a convert the C image data of the output image data from the RGB image data constituting the input image data. A magenta data separation unit 62b, a magenta table access unit 63b, and a magenta three-dimensional interpolation calculation processing unit 64b determine a M image data from each of the RGB image data, a yellow data separation unit 62c,
The yellow table access unit 63c and the yellow two-dimensional interpolation calculation processing unit 64c are systems for obtaining Y image data from RGB image data.

Of these three systems, the system that obtains C and M image data is configured to perform three-dimensional interpolation processing to obtain output image data. Output image data is obtained by performing a dimensional interpolation process.

Here, using Table 2, two-dimensional interpolation processing,
Each feature of the three-dimensional interpolation processing and the processing without interpolation calculation without performing the interpolation calculation will be described. Table 2 shows the results of comparing the conversion speed and gradation between these three types of interpolation calculation processing.

[0094]

[Table 2]

As shown in Table 2, when the table sizes are the same, the three-dimensional interpolation processing has a characteristic that although the conversion speed is slow, the ripple error is small and the gradation is excellent. Ripple is light and dark stripes of density.
If the ripple error is large, ripples such as ripples appear in the image. On the other hand, the two-dimensional interpolation processing using the two-dimensional interpolation method has a small and accurate interpolation error, has a higher conversion speed than the three-dimensional interpolation processing, and the gradation is not as good as the three-dimensional interpolation processing. It has the characteristic point that it is good. In addition, the processing without interpolation calculation is used in the third embodiment described later. However, since interpolation calculation is not performed, if the table size is set to the same as that for performing interpolation, gradation cannot be expected. The conversion speed is extremely fast and high speed.

In consideration of such features of the respective interpolation calculation processes, the interpolation calculation process of the system for obtaining each image data constituting the output image data is set as described above. That is, for a system that obtains C and M image data, gradation is improved by performing a three-dimensional interpolation process with a small ripple error. On the other hand, since two-dimensional interpolation processing is used for yellow, color conversion accuracy is higher than in three-dimensional interpolation processing, and an accurate color can be obtained. In the case of the two-dimensional interpolation processing, there is a possibility that ripples may occur, but there is no problem because yellow is not visually noticeable.

Returning to FIG. 11, the data separation unit 62 converts the RGB signals, which are input image data represented by predetermined bits, into higher-order bits serving as address data of a color conversion table provided in the color conversion table access unit 63. The data separation process is performed to separate the data into lower-order bit data that is used as interpolation data used for the interpolation calculation. The separation mode differs depending on the interpolation calculation process.

Table 3 shows an example of data separation in the data separation section 62 when each of the RGB signals is 8 bits. Note that Table 3 shows the number of bits of higher-order bit data serving as address data.

[0099]

[Table 3]

As described in the first embodiment, the yellow data separation section 62c of the system that performs the two-dimensional interpolation process, among the RGB image data, the color having a complementary color relationship with Y, that is, the B data Is output as it is with 8 bits without separating the data, and the R and G data are compared with the upper 4-bit data R ′ · G ′ and the lower 4 bits.
It is separated into bit data R ″ and G ″.

The cyan data separation unit 62a and the magenta data separation unit 62b of the system for performing the three-dimensional interpolation process
Are the upper 5 bits of data R ′
G'B 'and lower 3 bits R "G"
・ B ”.

The upper bit data separated by the data separation section 62 is output to the corresponding color conversion table access section 63, and the lower bit data is output to the corresponding interpolation operation processing section 64.

Here, each of the RGB image data constituting the input image data is 8 bits. However, the present invention is not limited to this, and other bit numbers such as 10 bits may be used. May be arbitrary as long as the above specific conditions are satisfied.

The color conversion table access section 63 has a color conversion table (look-up table) suitable for the type of interpolation calculation processing.
a and the magenta table access unit 63b have a color conversion table for the three-dimensional interpolation process, and the yellow table access unit 63c has a color conversion table for the two-dimensional interpolation process.

In each color conversion table, predetermined coordinates (a certain color in CMY) of output image data corresponding to a combination of coordinates (each color of RGB) represented by upper N bits of data of the input image data are stored. Table values are calculated and stored in advance. The table value of the address corresponding to the upper bit data of the input image data is read from the color conversion table, and is input to the corresponding interpolation operation processing unit 64. Note that a color conversion table for three-dimensional interpolation processing is conventionally known, and a color conversion table for two-dimensional interpolation processing is also described in the first embodiment. The above description is omitted.

The cyan three-dimensional interpolation calculation processing part 64a and the magenta three-dimensional interpolation calculation processing part 64b in the interpolation calculation processing part 64 use the upper bit data of (5,5,5) for RGB input image data. And the upper 15-bit data is divided into lower 3 bits × 3 bits × 3
Three-dimensional interpolation is performed using bits to obtain C and M image data. In the yellow two-dimensional interpolation calculation processing unit 64c, the RGB
For the input image data of
4, 8), and the upper 16-bit data is two-dimensionally interpolated by lower 4 bits × 4 bits to obtain Y data.
Is obtained.

Finally, the processing procedure of the color conversion processing section 41 will be described with reference to the flowchart of FIG. In the color conversion process, first, the data separation unit 62 converts the image data of the first color system, that is, here, the image data of RGB into address data in a form conforming to the interpolation calculation method of each system. It is separated into upper bit data and lower bit data serving as interpolation data (S11).

Next, the color conversion table access unit 63
Reads the table value from the color conversion table using the upper bid data as address data (S12).

The interpolation operation processing unit 64 uses the table values read from the color conversion table and the lower-order bit data to perform three-dimensional interpolation operations in the C and M systems and two-dimensional interpolation operations in the Y system. Interpolation processing is performed (S13), and the value obtained by the interpolation processing is output as CMY image data of the second color system (S14).

As described above, in the digital color copying machine according to the present embodiment, in the system for obtaining the C and M image data constituting the output image data, the three-dimensional interpolation process with a small ripple error is performed, and the Y image is output. The system for obtaining data is configured to perform accurate two-dimensional interpolation processing with high conversion accuracy.

Therefore, the gradation is improved in cyan and magenta, and the color conversion accuracy is improved for yellow because of the use of the two-dimensional interpolation, compared to the three-dimensional interpolation, so that accurate colors can be obtained. As a result, the output image obtained by the digital color copying machine of the present embodiment is C
Compared to the case where all the MYs are subjected to the three-dimensional interpolation processing, the accuracy and gradation are improved, and the image quality of the output image is improved as a whole.

[Embodiment 3] Another embodiment of an image processing method, an image processing apparatus, and an image forming apparatus according to the present invention will be described below with reference to FIGS. Note that, for convenience of explanation, components having the same functions as those shown in the drawings of the above-described embodiments are given the same reference numerals, and descriptions thereof will be omitted.

A digital color copying machine according to another embodiment of the present invention is similar to the digital color copying machine shown in FIG.
The color conversion processing unit 41 and the black generation / under color removal unit 5 in the image processing device 39 shown in FIG. 0 are integrated to perform color conversion processing including black generation. Instead of the generation / under color removal unit 5, the color conversion /
A black generator 44 is provided.

As shown in FIG. 13, the color conversion / black generation section 44 provided in the image processing device 39 of the digital color copying machine according to the present embodiment includes a data separation section 65, a color conversion table access section 66, It comprises an interpolation operation processing section 67.

The data separation section 65 includes a cyan data separation section 65a, a magenta data separation section 65b, a yellow data separation section 65c, and a black data separation section 65d. The color conversion table access section 66 includes a cyan table access section 66a. , Magenta table access unit 6
6b, the yellow table access unit 66c, and the black table access unit 66d.
Is composed of a cyan two-dimensional interpolation calculation processing unit 67a, a magenta two-dimensional interpolation calculation processing unit 67b, a yellow two-dimensional interpolation calculation processing unit 67c, and a black three-dimensional interpolation calculation processing unit 67d.

The cyan data separation unit 65a, cyan table access unit 66a, and cyan two-dimensional interpolation operation processing unit 67a convert the C image data of the output image data from the RGB image data constituting the input image data. A magenta data separation unit 65b, a magenta table access unit 66b, and a magenta two-dimensional interpolation calculation processing unit 67b determine a M image data from each of the RGB image data, a yellow data separation unit 65c,
A yellow table access unit 66c and a yellow two-dimensional interpolation calculation processing unit 67c are provided with a system for obtaining Y image data from RGB image data, and a black data separation unit 65.
d, a black table access unit 66d, and a black three-dimensional interpolation calculation processing unit 67d are systems for obtaining K image data from each of the RGB image data.

Among these, the system for obtaining C, M, and Y image data is configured to obtain output image data by performing two-dimensional interpolation processing, and the system for obtaining K image data (black image data generation). ), Output image data is obtained by performing three-dimensional interpolation processing.

Table 4 shows an example of data separation in the data separation section 65 when each of the RGB signals is 8 bits. Note that Table 4 shows the number of bits of higher-order bit data serving as address data.

[0119]

[Table 4]

The data separation unit 65 includes a CMY cyan data separation unit 65a, a magenta data separation unit 65b,
And the yellow data separation unit 65c performs separation suitable for two-dimensional interpolation processing. That is, the cyan data separation unit 6
In 5a, as described in Embodiment 1, RG
Among the image data of B, the color having a complementary color relationship with C, that is, the data of R is output as it is with 8 bits without separating the data, and the upper 4 bits are compared with the data of G and B.
It is separated into G'.B 'which is bit data and G ".B" which is data of lower 4 bits. Similarly, the magenta data separation unit 65b outputs the G data as it is with 8 bits without separating the data, and replaces the B and R data with B ′ · R ′, which are the upper 4 bits of data.
And B ".R" which is data of lower 4 bits. In addition, the yellow data separation unit 65c outputs the B data as it is with 8 bits without separating the data, and outputs R ′ and G ′, which are the upper 4 bits of data, for the R and G data. , The lower 4-bit data R ″
・ G ”is separated.

In the black data separation section 65d for performing the three-dimensional interpolation processing, each of the RGB image data is R'G'B 'which is the upper 5 bits of data and R "G which is the lower 3 bits of data. ".B".

In the color conversion table access section 66, the 3
Black image data is stored as table values in the color conversion table for dimensional interpolation, and undercolor removal is performed in the color conversion table for two-dimensional interpolation provided in each of the CMY table access units 66a to 66c. The values after application (the values obtained by subtracting the black image data) are stored.

Each of CMY in interpolation calculation processing section 67
In the dimensional interpolation calculation processing units 67a to 67c, high-order bit data is added to (8, 4,
4; in the case of cyan), and the upper 16 bits of data are two-dimensionally interpolated with lower 4 bits × 4 bits. On the other hand, the black three-dimensional interpolation calculation processing unit 67d sets the upper bit data to 15 bits of (5, 5, 5) with respect to the RGB input image data, and converts the upper 15 bits of data into lower 3 bits × Three-dimensional interpolation is performed with 3 bits × 3 bits.

As described above, by performing two-dimensional interpolation processing on a system for obtaining CMY image data, the color conversion accuracy is higher than that of three-dimensional interpolation processing, and accurate colors can be obtained. . On the other hand, for K, the ripple becomes less noticeable by performing three-dimensional interpolation processing with a small ripple error. The color near gray formed by the image data of K is sensitive to human eyes, and if ripples occur in gradation from white to black, the image becomes conspicuous and the image quality deteriorates. By using the three-dimensional interpolation processing, there is an effect of suppressing gray, in particular, in which ripples are noticeable.

As a result, in the digital color copying machine of the present embodiment, it is possible to improve the quality of the output image as a whole, and to obtain an output image with high image quality.
Further, with such a configuration, it is not necessary to separately provide the black generation / under color removal unit 5, so that the circuit configuration of the image processing device 39 can be simplified.

By the way, as shown in Table 4 above,
For a system for obtaining K image data, a process without interpolation calculation can be set in addition to the three-dimensional interpolation process. K
However, the configuration in which the process without interpolation calculation is performed has an advantage that the image quality is slightly reduced but the processing speed is faster than the configuration in which the three-dimensional interpolation process is performed on K.

FIG. 14 shows a system for obtaining K image data.
The configuration of the color conversion / black generation unit 44 'in which no interpolation calculation processing is set is shown.

Normally, in the case of processing without interpolation calculation, RGB
Is separated so that the number of higher-order bits is larger than that of the other color data. However, in the case of K, the color data has a complementary color relationship. Since there is no color, a gray component (or lightness component) is applied to each input RGB image data.
The number of upper bits to be extracted is increased for image data having a high contribution rate to the image data.

In general, the contribution rate to the gray component (or lightness component) in each of the RGB image data is represented by G ≧
R ≧ B. Therefore, the data separation unit 65 '
In the black data separation unit 65d ′, it is preferable that the number of upper bits to be extracted is larger than that of R and B for the G component. Further, when the interpolation calculation is not performed, it is necessary to extract a larger number of high-order bits than in the case where the interpolation calculation is performed, in order to increase the accuracy of the output image data. From these, as shown in Table 4, here, G
It is assumed that 7 bits are extracted for the component and 6 bits are extracted for each of R and B.

In the color conversion table of the black table access section 66d 'in the color conversion table access section 66', the combination of the number of high-order bits of the process without interpolation calculation is used as address data, and the black image data is used as a table value. Is stored.

In the interpolation operation processing section 67 ′, the upper bit data is set to 19 bits of (6, 7, 6) with respect to the RGB input image data, and the upper 19 bit data is used as is for K. Output as image data.

In each of the color conversion / black generation units 44 and 44 'shown in FIGS. 13 and 14, three-dimensional interpolation is used for C and M as in the example shown in the second embodiment. Two-dimensional interpolation processing may be used for Y. Further, with respect to a system for obtaining K image data, a direct conversion method of the LUT method may be used, and the LUT may have a table value of K image data for all combinations of input image data. In this case, the table size becomes large, but the gradation from white to black, which is conspicuous, is very accurately performed, so that further improvement in image quality can be expected.

[Embodiment 4] Another embodiment of an image processing method, an image processing apparatus, and an image forming apparatus according to the present invention will be described below with reference to FIG. Note that, for convenience of explanation, components having the same functions as those shown in the drawings of the above-described embodiments are given the same reference numerals, and descriptions thereof will be omitted.

A digital color copying machine according to another embodiment of the present invention is similar to the digital color copying machine shown in FIG.
The color conversion processing unit 41 and the black generation / under color removal unit 5 in the image processing device 39 shown in FIG. 0 are integrated to perform color conversion processing including black generation. Instead of the generation / under color removal unit 5, the color conversion /
A black generator 46 is provided.

The color conversion / black generation section 46 shown in FIG.
A system for obtaining each image data of MY is described in the first embodiment with reference to FIG.
And a configuration for performing a three-dimensional interpolation process in the color conversion / black generation unit 44 shown in FIG. 13 in the third embodiment in a system for obtaining K image data. Are combined.

The color conversion / black generation section 46 comprises a color coordinate conversion section 6, a color conversion / data processing section 7, a data separation section 72, a color conversion table access section 73, and an interpolation calculation processing section 74. Among them, the data separation unit 72 has a CMY data separation unit 72a equal to the data separation unit 8 in FIG. 4 and a black data separation unit 72b. The color conversion table access unit 73 is equivalent to the cyan, magenta, and yellow table access units 3a to 3c in FIG.
a to 73c, and a black table access unit 73d. The interpolation operation processing unit 74 is equivalent to the cyan, magenta, and yellow two-dimensional interpolation operation units 4a to 4c in FIG. The three-dimensional interpolation calculation processing units 74a to 74c and the black three-dimensional interpolation calculation processing unit 74 equivalent to the black three-dimensional interpolation calculation processing unit 67d in FIG.
d.

In the configuration of FIG. 15, two-dimensional interpolation processing is set for each system for obtaining CMY image data, and three-dimensional interpolation processing is set for the system for obtaining K image data. Also in this case, a three-dimensional interpolation process is set for each system for obtaining C and M image data, a two-dimensional interpolation process is set for a system for obtaining Y image data, and a system for obtaining K image data. With regard to, processing without interpolation calculation or direct conversion processing can also be set. In the case where the processing without interpolation calculation is used for the system for obtaining the K image data, extraction is performed so that the number of high-order bits of the lightness component data is increased.

Table 5 shows an example of data separation in the data separation section 72 when each of the RGB signals is 8 bits. Note that Table 5 shows the number of bits of higher-order bit data serving as address data.

[0139]

[Table 5]

Also in this case, in the system for obtaining the image data of K, by using the three-dimensional interpolation processing with little ripple, the processing without interpolation calculation, or the direct conversion method, the image near the gray where the ripple is conspicuous can be obtained with high image quality. Can be reproduced.

[0141]

As described above, according to the first image processing method of the present invention, the image of the specific two coordinates determined for each coordinate of the output image data to be obtained from the input image data is obtained. Data is subjected to a data separation process for separating upper bit data and lower bit data, while image data of other coordinates of the input image data is not subjected to the data separation process, and the upper bit data and the data separation are not performed. Unprocessed image data and address data, and for the input image data represented by the address data, a table value from a lookup table in which the image data of each coordinate of the corresponding output image data is stored in advance. For the input image data which is read out and not represented by the address data, the data is read from the lookup table. Predetermined number of read Le values, by performing two-dimensional interpolation operation by using the above-mentioned predetermined number of table value and the lower bit data, a configuration for obtaining the image data of each coordinate of the corresponding said output image data.

Therefore, a table value is prepared only for the input image data represented by the address data. For the input image data whose address is to be specified in detail by the lower bit data of the image data, a table of the address data is prepared. Image data of each coordinate of the output image data is obtained by performing a two-dimensional interpolation operation using the value and the lower bit data.

As described above, since the table value corresponding to the lower bit data is omitted, the size of the LUT (look-up table) is much smaller than that used in the direct conversion method, and the time of the interpolation operation is three-dimensional. It is shorter than the interpolation method. As a result, there is an effect that high-speed color conversion processing can be performed from input image data of the first color system to output image data of the second color system while suppressing the size of the LUT.

In the first image processing apparatus of the present invention, as described above, the color conversion processing section performs
The image data of the specific two coordinates defined for each image data of each coordinate of the output image data to be obtained is subjected to data separation processing for separating upper bit data and lower bit data, while image data of other coordinates is performed. With respect to the data separation unit that does not perform the data separation processing, the upper bit data extracted by the data separation unit and the image data that is not subjected to the data separation processing are used as address data, and the data represented by the address data For the input image data, read the table value from the lookup table in which the image data of each coordinate of the corresponding output image data is stored in advance, and for the input image data not represented by the address data, read from the lookup table. Look-up table access unit that reads a predetermined number of table values By performing a two-dimensional interpolation operation from a predetermined number of table values read by the lookup table access unit and lower-order bit data extracted by the data separation unit, the input image data not represented by the address data is obtained. And a two-dimensional interpolation calculation unit for obtaining image data of each coordinate of the output image data corresponding to the above.

Therefore, a table value is prepared only for the input image data represented by the address data. For the input image data to which the address should be specified in detail by the lower bit data of the image data, a table of the address data is prepared. Image data of each coordinate of the output image data is obtained by performing a two-dimensional interpolation operation using the value and the lower bit data.

As described above, since the table value corresponding to the lower bit data is omitted, the size of the LUT (look-up table) is much smaller than that used in the direct conversion method, and the time of the interpolation operation is three-dimensional. It is shorter than the interpolation method. As a result, there is an effect that high-speed color conversion processing can be performed from input image data of the first color system to output image data of the second color system while suppressing the size of the LUT.

Further, in the first image processing apparatus of the present invention, the image data which is not subjected to the data separation processing by the data separation unit has a relationship between the image data of the coordinates to be obtained for the output image data and the complementary color. It may be configured to be certain image data.

According to this, the data separation processing is not performed on the image data having the relationship of the complementary color which has the greatest influence on the conversion accuracy, thereby minimizing the conversion error due to the interpolation operation. Therefore, there is an effect that color conversion processing can be performed accurately and efficiently without causing color shift.

Further, in the first image processing apparatus according to the present invention, when black image data is included in the output image data, the data separation processing is not performed by the data separation unit when obtaining black image data. The image data may be configured to be image data of coordinates having a high contribution rate to the brightness of the output image data.

According to this, of the input image data of the first color system, the data separation processing is not performed on the image data having a high contribution to the brightness of the output image data of the second color system. As a result, a conversion error due to the interpolation operation for the gray component of the output image data is minimized. Therefore, there is an effect that color conversion processing can be performed accurately and efficiently without causing color shift.

Further, in the first image processing apparatus according to the present invention, when the input image data includes lightness data and chromaticity data, the data separation unit is used when obtaining image data of each coordinate of the output image data. Accordingly, the image data not subjected to the data separation processing may be the brightness data.

According to this, the data separation processing is not performed on the brightness data, so that an error due to the interpolation operation on the brightness component of the output image data is avoided as much as possible. Therefore, there is an effect that color conversion processing can be performed accurately and efficiently without causing color shift.

Further, in the first image processing apparatus of the present invention, the color conversion processing unit is separately provided with a black image data generation unit for obtaining black image data by three-dimensional interpolation using a look-up table method. It is also possible to adopt a configuration that has been used.

When the two-dimensional interpolation operation is used, the color conversion accuracy is high and accurate, but a slight ripple error occurs, and the color near gray formed by black image data is sensitive to human eyes, and the When a ripple occurs in a black gradation or the like, it is more conspicuous than in a case where a ripple occurs in other colors, but as described above, a black image data for obtaining black image data by a three-dimensional interpolation operation using a lookup table method is used. By providing a configuration including the image data generating unit separately, it is possible to prevent the image quality from being degraded due to the above-described ripple, and to further improve the image quality.

Further, in the first image processing apparatus of the present invention, the color conversion processing unit is separately provided with a black image data generation unit for obtaining black image data without performing an interpolation operation using a lookup table method. This also has the effect of preventing the occurrence of ripples in the gray image represented by the above-described black image data and further improving the image quality. According to the second image processing method of the present invention, as described above, the image data of each coordinate of the second color system is converted from the input image data of the first color system in which the image data of each coordinate is represented by predetermined bits. In an image processing method in which data is individually obtained and color conversion processing to be output image data is performed using a look-up table method, an image data is generated for each image data of each coordinate of the output image data. It is configured to perform color conversion processing suitable for the color.

Each of the color conversion processes using the look-up table method has a difference in processing speed, ripple error, interpolation error, etc., depending on whether or not interpolation calculation is used and the interpolation calculation to be used. Even if the interpolation method is used,
The image quality depends on the colors constituting the output image data. According to this, the color conversion processing using the look-up table method is performed based on the colors of the image data at each coordinate of the second color system constituting the output image data. Are set individually.

Therefore, the color conversion processing according to the color of each image data constituting the output image data can be performed by utilizing the respective features of the color conversion processing using the look-up table method. By applying the method to the image forming apparatus, it is possible to obtain an output image with improved image quality as a whole.

In the above-described second image processing method of the present invention, when the image data is black, a color conversion process using a three-dimensional interpolation method or a table value read from a lookup table is used as it is. It is more preferable to employ a configuration using color conversion processing. With this configuration, in a configuration in which even the black image data is generated by the color conversion processing, it is possible to improve the color conversion accuracy and output accurate colors, thereby improving the image quality as a whole. This has the effect.

In the second image processing apparatus of the present invention, as described above, the color conversion processing unit converts the input image data of the first color system into the presence or absence of an interpolation operation for each coordinate of the output image data. According to the type of the interpolation calculation processing including, a data separation unit that separates only the address data or the form of the address data and the interpolation data, and the combination of the image data of each coordinate in the input image data as the address data,
A lookup table in which image data at predetermined coordinates of the output image data corresponding to the address data is stored as a table value, and which has a lookup table suitable for the type of interpolation processing, and which corresponds to the address data separated by the data separation unit. A lookup table access unit that reads out a table value to be read, and only the table value read from the lookup table by the lookup table access unit, or the table value and the interpolation data separated by the data separation unit. An interpolation operation processing unit that performs a predetermined interpolation operation process, the data separation unit, the look-up table access unit, provided for each coordinate of the output image data;
The interpolation calculation processing of each system including the interpolation calculation processing unit is set in accordance with the color of the image data at each coordinate of the output image data.

As a result, the color conversion processing according to the color of each image data constituting the output image data can be performed by utilizing the respective features of the color conversion processing using the look-up table method. The image forming apparatus provided with such an image processing apparatus has an effect that it is possible to obtain an output image with improved image quality as a whole.

In the above-described second image processing apparatus of the present invention, in a system where the image data is black, it is more preferable that color conversion processing using three-dimensional interpolation is performed. Accordingly, it is possible to prevent the occurrence of ripples in the gray image represented by the above-described black image data, and to further improve the image quality.

In the above-described second image processing apparatus of the present invention, in a system where the image data is black, the color conversion processing using the table values read from the lookup table as they are is performed. This is more preferable, whereby the effect of preventing the occurrence of ripples in the gray image represented by the above-described black image data and further improving the image quality can be obtained.

[Brief description of the drawings]

FIG. 1 illustrates an image processing apparatus according to an embodiment of the present invention.
FIG. 3 is a block diagram illustrating a configuration example of a color conversion processing unit.

FIG. 2 is an explanatory diagram illustrating a two-dimensional interpolation calculation method performed by a color conversion processing unit in FIG. 1;

FIG. 3 illustrates an image processing apparatus according to an embodiment of the present invention.
It is a block diagram showing other examples of composition of a color conversion processing part.

FIG. 4 illustrates an image processing apparatus according to an embodiment of the present invention.
FIG. 21 is a block diagram illustrating yet another configuration example of the color conversion processing unit.

FIG. 5 is a flowchart illustrating an image processing method according to an embodiment of the present invention.

FIG. 6 is a block diagram illustrating a configuration of an image processing apparatus according to an embodiment of the present invention.

FIG. 7 is a graph showing an example of a skeleton curve used when generating black.

FIG. 8 is an explanatory diagram illustrating a three-dimensional interpolation calculation method performed by a conventional color conversion processing unit.

FIG. 9 is a block diagram illustrating a configuration of a color conversion processing unit of a conventional image processing apparatus.

FIG. 10 is a block diagram illustrating a configuration of an image processing apparatus provided in a digital color copying machine according to another embodiment of the present invention.

11 is a block diagram showing a configuration of a color conversion processing unit in the image processing device shown in FIG.

FIG. 12 is a flowchart illustrating a color conversion processing procedure in a color conversion processing unit illustrated in FIG. 11;

FIG. 13 is a block diagram illustrating a configuration of a color conversion processing unit in an image processing apparatus provided in a digital color copying machine according to another embodiment of the present invention.

FIG. 14 is a block diagram illustrating a configuration of a color conversion processing unit in an image processing apparatus provided in a digital color copying machine according to another embodiment of the present invention.

FIG. 15 is a block diagram illustrating a configuration of a color conversion processing unit in an image processing apparatus provided in a digital color copying machine according to another embodiment of the present invention.

[Explanation of symbols]

Reference Signs List 1 color conversion processing unit 2 data separation unit 3 color conversion table access unit (lookup table access unit) 4 2D interpolation calculation unit 6 color coordinate conversion unit 8 data separation unit 11 color conversion processing unit 21 color conversion processing unit 31 image processing Device 39 Image processing device 41 Color conversion processing unit 44 Color conversion / black generation unit (color conversion processing unit) 44 'Color conversion / black generation unit (color conversion processing unit) 46 Color conversion / black generation unit (color conversion processing unit) 62 data separation unit 63 color conversion table access unit (lookup table access unit) 64 interpolation calculation processing unit 65 data separation unit 65 'data separation unit 66 color conversion table access unit (lookup table access unit) 66' color conversion table access Section (lookup table access section) 67 interpolation calculation processing section 67 ′ interpolation calculation processing section 72 data separation section 73 Conversion table access unit (lookup table access unit) 74 interpolation processing section

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Masanori Minami 22-22, Nagaike-cho, Abeno-ku, Osaka City, Osaka F-term (reference) 5B057 CA01 CA08 CA12 CA16 CB01 CB08 CB12 CB16 CC01 CE17 CE18 CH07 5C077 LL17 LL18 MP08 NP05 PP31 PP32 PP33 PP36 PP37 PP38 PQ12 PQ23 RR19 5C079 HB01 HB03 HB06 HB08 HB12 LA28 LB02 MA04 MA11 NA02 NA03 NA10 NA11 NA13 PA02

Claims (14)

[Claims] An image data of each coordinate of a second color system is individually obtained from input image data of a first color system in which image data of each coordinate is represented by predetermined bits. In the image processing method of performing color conversion processing, among the input image data, image data of two specific coordinates defined for each image data of each coordinate of the output image data to be obtained is determined by upper bit data and lower bit data. While performing data separation processing for separating the input image data into bit data, the image data of the other coordinates of the input image data is not subjected to the data separation processing, and the upper bit data and the image data not subjected to the data separation processing are separated. Regarding the input image data represented by the address data as the address data, the image data of each coordinate of the corresponding output image data is stored in advance. For the input image data not represented by the address data, a predetermined number of table values are read from the lookup table, and a predetermined number of table values are read from the lookup table, and the predetermined number of table values and the lower bit data are used. An image processing method characterized by obtaining image data of each coordinate of the corresponding output image data by performing a two-dimensional interpolation operation. 2. The image data of each coordinate of the second color system is individually obtained from the input image data of the first color system in which the image data of each coordinate is represented by predetermined bits. In the image processing apparatus having a color conversion processing section, the color conversion processing section includes an image of specific two coordinates defined for each image data of each coordinate of the output image data to be obtained from the input image data. The data is subjected to a data separation process for separating upper bit data and lower bit data, while the image data of other coordinates is not subjected to the data separation process. The upper bit data and the image data not subjected to the data separation processing are used as address data, and the input image data represented by the address data has a corresponding A table value is read from a lookup table in which image data of each coordinate of the output image data is stored in advance, and for the input image data not represented by the address data, a predetermined number of table values are read from the lookup table. By performing a two-dimensional interpolation operation on a table access unit and a predetermined number of table values read by the lookup table access unit and lower-order bit data extracted by the data separation unit, the data is not represented by the address data. An image processing apparatus comprising: a two-dimensional interpolation calculating unit that obtains image data of each coordinate of the output image data corresponding to the input image data. 3. The image data which is not subjected to the data separation processing by the data separation unit is image data having a complementary color relationship with image data of coordinates to be obtained for output image data. 3. The image processing device according to 2. 4. When the output image data includes black image data, the image data that is not subjected to the data separation processing by the data separation unit when obtaining the black image data has a contribution to the brightness of the output image data. The image processing apparatus according to claim 2, wherein the image data is coordinate data having a high rate. 5. When the input image data includes brightness data and chromaticity data, the image data not subjected to the data separation processing by the data separation unit when obtaining image data of each coordinate of the output image data is the same as the image data. The image processing apparatus according to claim 2, wherein the image processing apparatus is lightness data. 6. The color conversion processing unit according to claim 2, further comprising a black image data generation unit for obtaining black image data by a three-dimensional interpolation operation using a look-up table method. 6. The image processing device according to 3 or 5. 7. The color conversion processing unit further comprises a black image data generation unit for obtaining black image data without performing an interpolation operation using a look-up table method. 6. The image processing device according to 3 or 5. 8. The image data of each coordinate of the second color system is individually obtained from the input image data of the first color system in which the image data of each coordinate is represented by a predetermined bit. An image processing method for performing color conversion processing using a look-up table method, wherein color conversion processing suitable for the color of the image data is performed for each image data at each coordinate of the output image data. . 9. A color conversion process using a three-dimensional interpolation operation when the image data is black.
The image processing method according to 1. 10. The image processing method according to claim 8, wherein when the image data is black, a color conversion process is performed using the table value read from the lookup table as it is. 11. An image data of each coordinate of a second color system is individually obtained from input image data of a first color system in which image data of each coordinate is represented by predetermined bits, and output image data and In the image processing apparatus having a color conversion processing unit, the color conversion processing unit performs, for each coordinate of the output image data, the input image data of the first color system based on an interpolation operation process including the presence or absence of an interpolation operation. Depending on the type, only the address data,
Alternatively, a data separation unit that separates the data into a form of address data and interpolation data; and a combination of image data of each coordinate in the input image data as the address data, and image data of predetermined coordinates of output image data corresponding to the address data. A lookup table access unit that has a lookup table suitable for the type of interpolation calculation processing stored as a table value, and reads a table value corresponding to the address data separated by the data separation unit; An interpolation operation processing unit that performs a predetermined interpolation operation using only the table value read from the look-up table by the table access unit or using the table value and the interpolation data separated by the data separation unit These data are provided for each coordinate of the output image data. Image processing, wherein the interpolation calculation processing of each system including the data separation unit, the lookup table access unit, and the interpolation calculation processing unit is set according to the color of the image data at each coordinate of the output image data. apparatus. 12. The image processing apparatus according to claim 11, wherein a color conversion process using a three-dimensional interpolation operation is performed in a system where the image data is black. 13. The system according to claim 1, wherein in a system in which the image data is black, a color conversion process using the table value read from the lookup table as it is is performed.
2. The image processing device according to 1. 14. An image forming apparatus comprising the image processing apparatus according to any one of claims 2 to 7, and 11 to 13.