JPH05236265A - Color correction calculation method and apparatus - Google Patents

Abstract

(57) [Abstract] [Purpose] A color correction calculation method and device capable of performing high-speed color correction processing with a memory device having a small capacity and a relatively simple circuit configuration, which can be applied to a system having complicated nonlinear characteristics. I will provide a. [Structure] The input color separation signal is first subjected to a masking calculation to calculate a corrected color signal with coarse accuracy, while the upper bit group of the input color separation signal is referred to the conversion table 12 to obtain a residual error correction value and the residual error correction is performed. A highly accurate corrected color signal is obtained by calculating the value and the primary corrected color signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color correction calculation method and apparatus for improving color reproducibility in a digital color image processing apparatus such as a copying machine or a printer.

[0002]

2. Description of the Related Art In recent years, with the development of digital image processing technology, a method of reading a color image with an image reading device and driving an image recording device using the digital image signal to reproduce the color image is becoming popular. .. FIG. 3 shows a simplified configuration example of a device for performing such processing.

In an apparatus having such a structure, an image reading apparatus usually outputs color system signals of three primary colors of R (red), G (green), and B (blue), while an image recording apparatus does. It is common to receive and control colorimetric signals of the three primary colors of C (cyan), M (magenta), and Y (yellow). Therefore, a mechanism for converting the color signal color system is required between the image reading device and the image recording device.

Further, in an image reading device, an accurate color separation signal cannot be obtained because the spectral characteristic of the color separation filter is not ideal, and in an image recording device, the ink used for recording is different. There is an unnecessary spectral absorption component called sub-absorption in each of the spectral absorption characteristics of each of the three primary colors, and when the inks of each color are mixed, the additive and proportional laws do not hold between the components of the three primary colors, resulting in color turbidity. Due to such reasons, not only the conversion process of the color system but also the process corresponding to the color correction calculation is required.

A masking method is well known as a conventional color correction processing method used for this purpose. The color correction processing by the masking method will be briefly described. The matrix calculation is performed on each color separation signal to correct the signals.

As an example, FIG. 4 shows a block diagram when a color correction arithmetic unit is constructed by the primary masking method. In FIG. 4, three color separation signals R, G, B obtained from the image reading device are input to the matrix calculator 45.
The matrix calculator 45 includes three multipliers 41, 42, 43.
And an adder 44. For the three multipliers 41, 42, and 43, if the correction signal output to be obtained is Y, a11 to a13, if M, a21 to a23, and C, respectively.
If so, a31 to a33 are set as color correction coefficients. The matrix calculation of the color separation signals R, G, B and the color correction coefficients a11 to a33 is expressed by the following equation.

C = a11.R + a12.G + a13.BM M = a21.R + a22.G + a23.BY = a31.R + a32.G + a33.B The color correction coefficients a11 to a33 are A value that minimizes the color difference between an original image such as a document or a subject and an output image formed by the recording apparatus is calculated by the least square method or the like.

In the masking calculation shown in the above example, a linear expression is used. However, in order to obtain a color correction process with higher accuracy, a color obtained by a masking method using a quadratic or cubic expression is used. A correction arithmetic unit may also be used.

Further, in the process of converting a discrete value signal into a corresponding discrete value, a method of using a conversion table can be easily recalled. This holds all corresponding correction values for all input values in a table.

[0010]

However, in the conventional method as described above, in the primary masking method, correction is performed over the entire range of the color separation signal by using only nine fixed color correction coefficients a11 to a33. Therefore, there is a limit in reducing the color difference after correction. That is, as described above, in an actual system, there are influences such as the presence of unnecessary spectral absorption in the ink used for image formation and the fact that the additive and proportional laws of color are not satisfied when the inks are superposed. In many cases, the image recording apparatus has a non-linear characteristic, and even if the color correction processing based on the primary masking method is performed, there is a sufficient gap between the read original image and the formed image recorded by the color correction processing. No color reproducibility was obtained.

Therefore, for the purpose of correcting even the non-linear characteristic of the image recording apparatus, there is also known a color correction operation apparatus by a masking method using a second-order or third-order operation expression to which a non-linear term is added. It is put to practical use. For example, in the second-order masking method, in addition to the first-order terms R, G, and B in the above equation, R · R, G · G, B · B and R · G, G · B,
The six quadratic terms B and R are introduced into the arithmetic expression, and the correction arithmetic operation is performed by setting the coefficient for each quadratic term.

However, when such a high-order masking method is used, when the color correction arithmetic unit is actually realized by a digital circuit, the number of arithmetic units, especially the number of multipliers is larger than that of the circuit using the primary masking method. Therefore, many circuit elements are required, and many circuit elements must be used, which is an economical problem. This also means that the circuit scale becomes large and complicated, and as a result it becomes difficult to operate the device at high speed, and applications that require rapid color correction processing of large-scale image data. Will not be suitable for.

On the other hand, if the method of using a simple conversion table utilizing the fact that the signal to be handled is a discrete value is applied to the color correction calculation processing, in order to increase the accuracy of the color signal to be handled, the conversion table is constructed. There is a drawback that the memory for doing so becomes extremely huge. For example, a memory of about 192 megabits is required to input a color separation signal of 8 bits for each of R, G, and B to obtain a correction signal of 12 bits for one color, which is not very realistic. Further, using such a large-scale conversion table has a problem in that all values in the table must be rewritten when it is desired to adjust the color tone or change the hue.

The present invention has been made in order to solve the above-mentioned problems and has a high precision and a high speed even when applied to a system having a relatively simple circuit configuration and complicated non-linear characteristics. It is an object of the present invention to provide a color correction calculation method and apparatus capable of performing various color correction processing.

[0015]

To achieve this object, the color correction calculation method of the present invention obtains a first conversion value by performing a masking calculation on each color separation signal Wi, and separately calculates each color separation. The upper bit group WUi is separated from the signal Wi, all the WUi are referred to and a table is drawn to obtain a second conversion value, and the sum or difference between the second conversion value and the first conversion value is calculated. A corrected color signal is obtained by calculation. Alternatively, a masking operation is performed on each color separation signal Wi to obtain a first conversion value, the upper bit group WUi is separated from each color separation signal Wi, and each WUi is referred to using n tables. To obtain the second conversion value of the n-term, further refer to all the second conversion values of the n-term to obtain a third conversion value, and calculate the third conversion value and the third conversion value. A corrected color signal is obtained by calculating the sum or difference with the conversion value of 1.

Then, the color correction arithmetic unit of the present invention comprises a plurality of color separation signals Wi (i = 1 ,.
n) and a masking calculation means for outputting a first conversion value; and a conversion table for inputting each bit string composed of upper Ui bits of each color separation signal Wi and outputting a corresponding second conversion value. , And a calculation means for calculating the sum or difference between the first conversion value and the second conversion value. Alternatively, a plurality of color separation signals Wi represented by discrete values (i =
1 ,. ． , N) to output a first converted value and n pieces to input a bit string composed of upper Ui bits of each color separation signal Wi and output a corresponding second converted value. And a second conversion table for inputting all the second conversion values and outputting a corresponding third conversion value.
Conversion table, and a calculation unit that calculates the sum or difference of the first conversion value and the third conversion value.

[0017]

The present invention having the above-described structure operates based on the theoretical background as described below.

First of all, the color correction arithmetic unit considered for the explanation here is assumed to input a color separation signal vector X and output a corrected color separation signal vector Q which matches the characteristics of some processing system. .. Generally, the number of dimensions of the color separation signal vector X and the correction color separation signal vector Q is arbitrary, but here, for simplification of description, it is considered that each is three-dimensional. If R, G, B and C, M, Y are used as a color signal expression system that is often used, X = (R, G, B) Q = (C, M, Y) where R is the color of the red component Separation signal G: Green component color separation signal B: Blue component color separation signal C: Cyan component correction color separation signal M: Magenta component correction color separation signal Y: Yellow component correction color separation signal At this time, it is considered that the color correction computing device has realized a function for performing signal conversion as shown in the following equation.

C = Fc (R, G, B) M = Fm (R, G, B) Y = Fy (R, G, B) where Fc: conversion function for cyan signal Fm: conversion function for magenta signal Fy: Conversion function for yellow signal In the above equation, the C, M, and Y signals have different function characteristics, but the format is the same. Therefore, in the following, a conversion function for one color signal will be realized. If we think about it, signals of other colors can be treated in exactly the same way. Therefore, hereinafter, a method of realizing a signal conversion function of the form Z = F (R, G, B) will be considered.
Since the color signal of interest here is a discrete value, the conversion function in the above equation is also a discrete value function.

For this purpose, first, the data bit lengths of the color separation signals R, G and B, which are the input values of the above equation, are set to mr and
The data bits of each color separation signal are divided as mg and mb as shown below.

R → Ru + Rv G → Gu + Gv B → Bu + Bv where Ru is the upper ru bits of R and Rv is the lower rv bit of R Gv takes the lower gv bits of G Gv takes the lower bv bits of B Bv takes the lower bv bits of B where ru + rv = mr gu + gv = mg bu + b v = mb ru <m r gu <mg bu <mb The transformation of the signal conversion function shown above is tried using this division of the color signal data bits. That is, Z = F (R, G, B) = F (Ru + Rv, Gu + Gv, Bu + Bv) = F1 (R, G, B) + F2 (Ru, Gu, Bu) ≈FM (R, G) , B) + FR (Ru, Gu, Bu).

Considering its meaning in the above equation, FM is a function that controls conversion by masking processing and performs signal correction with coarse accuracy, and FR is a correction value including an error generated by FM. It is a function that generates a residual correction value that cancels the error.

The present invention is constructed based on the above-described modified formula. That is, a masking arithmetic unit for inputting the input color separation signal vector X to perform masking correction processing and a high-order bit of each color component R, G, B of the same input color separation signal vector X are taken as ru, gu and bu bits. It has a conversion table that inputs the input signal and extracts the residual correction value.

Finally, a desired correction is made by having a calculator for calculating the sum or difference of the residual correction value obtained by this conversion table and the coarse correction value output by the masking calculation device. Get the color signal.

Here, the function F for generating this residual correction value
Since R does not generally have a monotonic characteristic, it is realized by a conversion table. However, since the FR conversion table refers only to the upper bits of the original color separation signal, a conversion table of such a large size is not required. Instead, the residual correction values generated by this conversion table are generated only with the roughness of the quantized space spanned by the upper bits of the separated color separation signal, so that some errors occur microscopically.

However, in an actual color correction processing system, the residual correction value does not change abruptly with respect to the change of the input color separation signal, so that a sufficient correction signal can be obtained in many cases. For this reason, it is preferable that the number of signal bits separated from the original color separation signal for drawing the residual correction value generation table be increased or decreased according to the non-linearity of the system. That is, if the residual correction value changes rapidly with respect to the change of the input color separation signal, many bits are used,
If it changes only smoothly, refer to a small number of bits.

If the system is highly nonlinear,
In order to improve the correction accuracy without making the size of the residual correction value generation table too large, it is said that the residual correction value generation table is drawn after passing through the conversion table provided separately for each color separately from the original color separation signal. Possible configurations. If this is expressed by the formula shown above, Z = F (R, G, B) = F (Ru + Rv, Gu + Gv, Bu + Bv) = F1 (R, G, B) + F2 (Ru , Gu, Bu) ≈ FM (R, G, B) + FR (Fr (Ru), Fg (Gu), Fb (Bu)). Fr, Fg, and Fb in the above equation are functions introduced to alleviate the non-linearity of each color component signal. These functions are also realized by the conversion table.

[0028]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of the color correction arithmetic unit of this embodiment. The color correction arithmetic unit of this embodiment inputs the three color separation signals 14r, 14g, 14b of R, G, B provided from an image reading apparatus (not shown) to C, M, Y for the image recording apparatus (not shown). One of the corrected color signals 18 is output.

The color correction arithmetic unit is a masking arithmetic unit 1 for inputting a color separation signal and calculating a corrected color signal with coarse accuracy.
1 and a conversion table 1 for generating the residual correction value by inputting the higher-order bit group of each color separation signal R, G, B component to be input
2 and an adder 13 for adding the corrected color signal output from the masking calculation device 11 and the residual correction value output from the conversion table 12.

Next, the operation will be described.

The R, G, and B color separation signals 14r, 14g, and 14b in FIG. 1 are digital values represented by 8 bits. The correction color signal 18 of any one of C, M, and Y is a digital value represented by 12 bits.

First, 3 given from the image reading device or the like
The color separation signals 14r, 14g, 14b of the colors are guided to the masking calculation device 11 and the primary correction color signal 16 is generated by the masking calculation. Although various configurations are conceivable for this masking arithmetic unit, here, the one based on the primary masking method of the configuration shown in FIG. 4 is considered.

The operation of the primary masking arithmetic unit will be briefly described once more. The input R, G, B color separation signals are guided to the three multipliers 41, 42, 43, respectively, and set for each color. Color correction coefficient value (correction signal output to be calculated is Y
A11 to a13 if M, a21 to a23 if M, a31 if C
To a33) are multiplied. Then, the resulting values thus obtained are respectively guided to the adder 44 and added to each other to obtain a corrected color signal. In this embodiment, since the bit length of the R, G, B color separation signals that are input is 8 bits, the multipliers 41, 42, 4 are used.
As for 3, input two 8-bit data each and input 12
A device that outputs a bit operation result, and an adder 44 that inputs three 12-bit data and outputs a 12-bit operation result may be used.

Here, the color correction coefficient values a11 to a33 of the masking calculation are regressed by measuring the relationship between the signal driving the image recording device and the signal output by the image reading device in the assumed image processing system. It is obtained using an analysis method, etc., but details thereof are omitted.

Since this masking arithmetic unit 11 performs only linear arithmetic processing, the obtained primary correction color signal 16 is obtained.
Is only coarse precision. That is, an accurate correction signal cannot be obtained when there is a non-linear component of the signal. The conversion table 12 generates a residual correction value 17 which is a correction value for correcting this error. In FIG. 1, three color separation signals 14r provided from the image reading device,
For 14g and 14b, 12-bit wide signals 15r and 15g, which are generated by extracting only the upper 4 bits of the respective bits forming the data from each color separation signal,
15b is given to the conversion table 12. That is, in the present embodiment, ru = gu = bu = 4 as the above-mentioned bit division. The conversion table 12 is realized by a readable and writable semiconductor memory (hereinafter referred to as RAM) having 12 address lines. Then, C which is adapted to a desired correction system through an access control mechanism (not shown) in advance,
A residual correction value for either the M or Y correction color signal is set.

The residual correction value set here is also obtained from the relationship between the drive signal of the image recording device and the output signal of the image reading device, as described above. In other words, the difference between the correct correction color signal expected for the input color separation signal and the correction color signal output by the masking calculation device 11 used in this embodiment at that time may be obtained and set. .. However,
Since the conversion table 12 refers only to the upper bits of the input color separation signal, the residual correction value can be generated only at coarse intervals with respect to the signal range. Therefore, the residual correction value that minimizes the error within the signal range is calculated at each input color separation signal level, and the conversion table 12
Set to.

The required degree of quantization of the residual correction value (depending on the number of higher-order bits to be referred) changes depending on the degree of non-linearity of the color separation signal to be handled. That is, if a signal having a relatively straightforward characteristic is to be handled, only a few high-order data bits need to be referenced, and the size of the conversion table 12 can be reduced. On the contrary, although the size of the conversion table 12 is increased by referring to many data bits, it is possible to cope with a system having more complicated characteristics. In this embodiment, the residual correction value 17 output from the conversion table 12 has a 12-bit width. Therefore, the total amount of RAM required to configure this is
It is 2 ^ 12 * 8 = 32768 (bits).

Next, 1 generated in each of the above blocks
The next corrected color signal 16 and the residual difference correction value 17 are guided to the adder 13, respectively. This adder 13 outputs the primary correction color signal 1
6 and the residual correction value 17 are added together, and as a result, a desired corrected color signal 18 is obtained at the output of the adder 13. It can be said that this calculation corrects the error of the primary correction color signal 16 of coarse accuracy calculated by the masking calculation device 11 with the residual correction value 17.

The value of the correction color signal 18 output from the adder 13 may cause overflow or underflow depending on the value of the input color separation signal. Therefore, although not shown in FIG. 1 because it is not an essential part of this embodiment, the adder 13 is not used in actual use.
It is advisable to provide a clipping circuit for performing overflow processing or underflow processing after the.

In the color correction calculation device according to the embodiment shown here, only the signal for one color is obtained as the correction color signal 18, but the matrix coefficient values set in the masking calculation device 11 and the values stored in the conversion table 12 are set. It will be easily understood that by replacing the above, it can function as a color correction calculation device for any of C, M, and Y correction color signals with exactly the same configuration. For this reason, even if the application requires high-speed real-time processing, if the assumed image recording apparatus is a frame sequential recording method for each of C, M, and Y colors, the image recording apparatus is for one color. Since it is sufficient to exchange the values stored in each table by using the setup time until the recording of the next color is started after the recording of all the images, the color correction arithmetic unit having the configuration as shown in the present embodiment. Requires only one set of implementations. However, if the assumed recording method of the image recording apparatus is line-sequential for each color or requires a recording signal for each color for each pixel, and the overhead of the time required to replace the values stored in each table poses a problem, a similar configuration is used. It is necessary to prepare the color correction calculation device for each color.

As is clear from the above description, the circuit elements required in the circuit configuration of the color correction arithmetic unit shown in this embodiment are RAM 32 kilobits and 8 bit × 8 bit multipliers. Three 3-input 12-bit adders are 1
There is one 2-input 12-bit adder, and the number of circuit elements required in the circuit configuration of the color correction arithmetic unit by the primary masking method is three multipliers and one 3-input adder. Even if there is, there is no big difference. However, this color correction calculation device has an advantage that a color correction calculation process with high accuracy can be realized even when applied to a system having a complicated nonlinear characteristic. Furthermore, since the circuit configuration is relatively simple, it can be sufficiently applied to applications requiring high-speed real-time processing.

In this embodiment, R is used as the data bit referred to in the conversion table 12 in the input color separation signal,
The same number of data bits are taken and referred to for each of the G and B color separation signals 14r, 14g, and 14b, but this is merely for the purpose of making the description easy to understand, and what is inevitable. is not. Rather, from the standpoint of further reducing the capacity of the RAM required for the configuration of the color correction arithmetic unit of the present invention, a smaller number of bits is used for a color separation signal that is visually insensitive and contributes little to color reproducibility. By doing so, it becomes possible to configure the circuit with a RAM having a smaller capacity without substantially degrading the accuracy.

When the non-linearity of the input color separation signal is extremely large, the number of data bits which must be extracted from each color separation signal and referred to in the conversion table 12 becomes large,
As a result, the size of the conversion table 12 may increase. In order to avoid this, a configuration may be considered in which a correction table for correcting non-linearity independent for each color separation signal is inserted before the conversion table 12. FIG. 2 shows a block diagram of the color correction arithmetic unit having such a configuration. The operation is similar to that of the color correction arithmetic unit shown in FIG. However, in this configuration, the R component signal correction table 21, the G component signal correction table 22 and the B component signal correction table 23 are provided, and the non-linearity correction is performed on each color separation signal of RGB. There is. In order to realize these correction tables, for example, if non-linearity correction from 7 bits to 5 bits is performed, the correction table is realized by a RAM or ROM each having 7 address lines.

In either of the configurations shown in FIGS. 1 and 2, the adder 13 is not limited to the adder in the narrow sense, and it may be convenient to use the subtracter depending on the nature of the signal to be handled. Conceivable.

The present invention is not limited to the embodiments described in detail above, and modifications can be made without departing from the spirit of the invention.

[0047]

As described above, according to the color correction calculation method and apparatus of the present invention, it can be applied to a system having a complicated nonlinear characteristic with a memory device having a small capacity and a relatively simple circuit configuration. Since it is possible to perform high-speed color correction processing with high accuracy, it is possible to realize an inexpensive color image processing apparatus with good color reproducibility.

[Brief description of drawings]

FIG. 1 is a block diagram of a color correction calculation device according to an embodiment.

FIG. 2 is a block diagram of a color correction calculation device according to another embodiment.

FIG. 3 is a configuration diagram showing an example of a color image processing apparatus.

FIG. 4 is a block diagram of a color correction calculation device using a primary masking method, which is an example of a conventional color correction calculation device.

[Explanation of symbols]

 11 Masking Arithmetic Unit 12 Conversion Table 13 Adder 14r R Component Color Separation Signal 14g G Component Color Separation Signal 1br B Component Color Separation Signal 15r R Component Color Separation Signal Upper Bit Group 15g G Component Color Separation Signal Upper Bit Group 15b B Component Color Separated signal upper bit group 16 Primary correction color signal 17 Residual correction value 18 Correction color signal 21 R component signal correction table 22 G component signal correction table 23 B component signal correction table 41 Multiplier (R component Signal) 42 multiplier (for G component signal) 43 multiplier (for B component signal) 44 adder 45 matrix calculator

Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location H04N 1/46 9068-5C

Claims (4)

[Claims] 1. A plurality of color separation signals W represented by discrete values.
i (i = 1, ..., N) is converted to obtain a corrected color signal, wherein a first conversion value is obtained by performing a masking operation on each color separation signal Wi, and separately The upper bit group WUi is separated from the signal Wi, a second conversion value is obtained by referring to all WUi, and a table is drawn. The sum or difference between the second conversion value and the first conversion value is calculated. A color correction calculation method characterized in that a correction color signal is obtained by calculation. 2. A plurality of color separation signals W represented by discrete values.
i (i = 1, ..., N) is converted to obtain a corrected color signal, wherein a first conversion value is obtained by performing a masking operation on each color separation signal Wi, and separately The upper bit group WUi is separated from the signal Wi, the second conversion value of the n term is obtained by referring to each WUi using the n tables, and all the second conversion values of the n term are referred to. And a table is drawn to obtain a third conversion value, and a correction color signal is obtained by calculating the sum or difference between the third conversion value and the first conversion value. Calculation method. 3. A plurality of color separation signals W represented by discrete values.
i (i = 1, ..., N) is input to output a first conversion value, and a bit string composed of upper Ui bits of each color separation signal Wi is input and corresponding second And a calculation table for calculating a sum or a difference between the first conversion value and the second conversion value. 4. A plurality of color separation signals W represented by discrete values.
i (i = 1, ..., N) is input to output a first conversion value, and a bit string composed of upper Ui bits of each color separation signal Wi is input and corresponding second N first conversion tables for outputting the conversion values of, the second conversion table for inputting all the second conversion values and outputting the corresponding third conversion values, and the first conversion values And a calculation unit that calculates a sum or a difference with the third conversion value.