JPH08242383A - Color converter - Google Patents

Abstract

PURPOSE: To remarkably improve the availability of a three-dimensional look-up table though it is formed in simple configuration. CONSTITUTION: The three-dimensional look-up table 2 is provided with a table 2c for C signal, a table 2m for M signal, a table 2y for Y signal and a table 2k for K signal, and generates a C(cyan) signal, a M(magenta) signal, a Y(yellow) signal and a K(black) signal based on L*, (a*) and (b*) signals, respectively. The most significant bit of the L* signal and that of the (a*) signal are supplied to the input terminal of an exclusive OR circuit 1, respectively, and the output signal S1 of the exclusive OR circuit 1 is supplied to each table as the most significant bits of the L* signal and the a* signal again.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color conversion device using a look-up table, and more particularly to a color conversion device capable of efficiently using a look-up table.

[0002]

2. Description of the Related Art In a digital copying machine, an image input / output device, a digital image processing device, etc., color signals are often converted (color conversion) because the types of color signals handled are different between input and output. , L ^* a ^* b ^* signals are often converted into one to four-dimensional color signals (for example, CMYK signals). Further, in color conversion, a lot of devices of a system that refers to a look-up table have been developed.

By the way, in this type of conventional device,
When performing color conversion of coordinate points in the L ^* a ^* b ^* color space using a three-dimensional lookup table, Japanese Patent Application No. 6-14426
As described in No. 8, the input value of each L ^* a ^* b ^* axis or its representative value was directly used as an index of the three-dimensional lookup table.

On the other hand, when a three-dimensional look-up table is constructed by a digital device, the space is generally formed into a cubic or rectangular parallelepiped shape so as not to complicate the table structure. However, in the color space, the color reproduction range of each device does not almost have the shape of a rectangular parallelepiped. Therefore, the ratio of the color reproduction range of the printing device in the entire space of the three-dimensional lookup table (hereinafter,
In reality, the utilization rate is as low as about 1/6. Therefore, in order to increase this utilization rate, Japanese Patent Application No. 6-1
As shown in Japanese Patent No. 44268, it has been proposed to use lattice points outside the color reproduction range for color conversion of a specific color.

[0005]

However, even in this case, only some points that can be used for color conversion of a specific color are set, which is far from a drastic improvement in utilization rate. It was

The present invention has been made in view of the above circumstances, and an object thereof is to provide a color conversion device capable of improving the utilization rate of the color space in the lookup table with a very simple structure. I am trying.

[0007]

In order to solve the above-mentioned problems, the invention according to claim 1 provides an L ^* signal, an a ^* signal and a b signal indicating coordinates in the L ^* a ^* b ^* color space. ^In a color conversion device that refers to a three-dimensional lookup table by using the ^* signal as an address and outputs a color material amount signal of one-dimensional to four-dimensional subtractive color mixing by using the L ^* signal, the a ^* signal, and the b ^* signal. The exclusive OR means for obtaining the exclusive OR of the most significant bits of the two signals and outputting the result is provided, and the three-dimensional lookup table has the color reproduction mapped by the exclusive OR means. A color conversion device having a rectangular parallelepiped space including a region.

Further, according to the invention described in claim 2, L ^* a ^* b
^* L ^* signal, a ^* signal and b ^* that indicate coordinates in color space
In the color conversion device that outputs a color material amount signal of one-dimensional to four-dimensional subtractive color mixing by referring to a three-dimensional lookup table using the signal as an address, the L ^* signal, a ^*
An exclusive OR means for obtaining the exclusive OR of the most significant bits of the two signals of the signal and the b ^* signal and outputting the result, and an L ^* signal excluding the most significant bit of the two signals, A representative value / interpolation value calculating means for obtaining a representative value and an interpolation value of the a ^* signal and the b ^* signal, a color material amount signal output from the three-dimensional lookup table, and a representative material / interpolation value calculating means. Interpolating means for performing an interpolating process based on an interpolating value, and the three-dimensional lookup table has a rectangular parallelepiped space including a color reproduction area mapped by the exclusive OR means,
The output signal of the exclusive OR means and the representative value output from the representative value / interpolation value calculating means are used as addresses.

According to a third aspect of the invention, the L
It is characterized in that the ranges of ^* signal, a ^* signal and b ^* signal are converted and used.

[0010]

The color reproduction area is mapped to another area by the exclusive OR means. In this case, the color reproduction area after mapping generally approaches a rectangular parallelepiped shape. On the other hand, the three-dimensional lookup table has a rectangular parallelepiped space that includes the mapped color reproduction area. Therefore, the utilization rate of the three-dimensional lookup table is significantly improved (claims 1 to 1).
3).

According to the second aspect of the invention, since the grid point data corresponding to the representative value calculated by the representative value / interpolation value calculating means may be stored in the three-dimensional lookup table, the storage capacity Can be reduced.

Since the ranges of the L ^* signal, the a ^* signal, and the b ^* signal are converted, it is possible to store them in the color space of the three-dimensional lookup table so as to include the color reproduction range of the printing device without excess or deficiency. it can.

[0013]

【Example】

A: First Embodiment Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention. Reference numeral 1 shown in this figure is an exclusive OR circuit that takes the exclusive OR of the L ^* signal of the input L ^* a ^* b ^* signal and the most significant bit of the a ^* signal, and its output signal S1
Is supplied as the highest address of the L ^* and a ^* inputs of the three-dimensional lookup table 2.

In this case, the three-dimensional lookup table 2
As shown in FIG. 2, has a C signal table 2c, an M signal table 2m, a Y signal table 2y, and a K signal table 2k, each of which is based on the L ^* a ^* b ^* signals.
The (Cyan) signal, the M (magenta) signal, the Y (yellow) signal, and the K (black) signal are generated. Then, the most significant bit of the L ^* signal and the most significant bit of the a ^* signal are respectively supplied to the respective input terminals of the exclusive OR circuit 1, and the output signal S1 of this exclusive OR circuit 1 is again changed to L level.
The most significant bits of the ^* signal and the a ^* signal are supplied to each table described above.

FIG. 3 is a diagram showing the color reproduction range of the printing apparatus used in this embodiment. The color reproduction range in the L ^* a ^* b ^* space is L ^{* at} the position of b ^* = 0 ^. It shows a state cut along the a ^* plane. As shown in this figure, the color reproduction range of the printing apparatus is substantially within the rhombus RH shown. Diagonal of the rhombus RH is, L ^* in the axial direction has a length of from L ^* = 0 to L ^* = 100, a
^* The direction and has a length from a ^* = -80 to a ^* = 80.

Here, the areas separated by diagonal lines in the rhombus RH are defined as (1a), (2a), (3a), and (4a), and the areas outside the square are (1b), (2b), and (3).
b) (4b).

Now, by applying the color reproduction range shown in FIG. 3 to a rectangular parallelepiped look-up table, as in the conventional apparatus, L
^If each value of ^* , a ^* , and b ^* is used as an address as it is, the areas (1b) to (4b) not included in the diamond RH
Is a useless area that does not contribute to color conversion. Further, FIG. 3 shows the state of the L ^* a ^* plane at the position of b ^* = 0, but the color reproduction range is smaller in the L ^* a ^* plane when the value of b ^* is changed than in the state shown in FIG. Become. Therefore, the utilization rate of the lookup table is extremely low in the entire L ^* a ^* b ^* space, which is about 1/6 as described above.

Now, in this embodiment, the L ^* signal and a ^*
The exclusive OR circuit 1 calculates the exclusive OR of the most significant bit of the signal. In this case, the output signal S1 of the exclusive OR circuit 1 and the above-mentioned areas have the following relationship. Here, the most significant bit of the L ^* signal is x (however, when x is “1”, L ^* is large, and when x is “0”, L ^* is small), and the most significant bit of a ^* is y (however, when y is “1”, a ^* is large and y is “0”)
Then, a ^* becomes small). Area (1a): x = 0, y = 0 Color reproduction range S1 = 0 Area (1b): x = 0, y = 0 Outside color reproduction range S1 = 0 Area (2a): x = 0, y = 1 color Reproduction range S1 = 1 area (2b): x = 0, y = 1 Outside color reproduction range S1 = 1 area (3a): x = 1, y = 0 Color reproduction range S1 = 1 area (3b): x = 1 , Y = 0 outside the color reproduction range S1 = 1 region (4a): x = 1, y = 1 color reproduction range S1 = 0 region (4b): x = 1, y = 1 outside the color reproduction range S1 = 0

In this embodiment, of the areas (1b), (2b), (3b) and (4b) which do not contribute to color conversion,
Using the areas (1b) and (2b), the information DA-4a of the area (4a) is recorded in the area (1b) and the information DA-3b of the area (3b) is recorded in the area (2b). Then, using the signal S1, a process of moving access to the region (4a) to the region (1b) and moving access to the region (3b) to the region (2b) is performed.

That is, the three-dimensional lookup table 2
In FIG. 4, as shown in FIG. 4, color reproduction range information DA-4a and DA-3b are assigned to areas that have been conventionally wasted. When the area 4a is accessed, the output signal S of the exclusive OR circuit 1 is output.
Since 1 becomes "0", the value of the most significant bit of the L ^* signal and the a ^* signal is inverted from "1" to "0", and the access to the area 4a shown in FIG. Then, the area 1b is accessed.

Similarly, when the area 3a is accessed, the output signal S1 of the exclusive OR circuit 1 is output.
However, since the most significant bit of the L ^* signal does not change, the most significant bit of a ^* is inverted from "0" to "1", so that the area 3a shown in FIG. For the access, the area is moved to the area 2b by moving in parallel. As described above, when viewed from the outside, the inside of the rhombus RH shown in FIG. 3 is accessed, but inside the region, the area shown in FIG. 4 is accessed.

Therefore, in this embodiment, FIG.
When viewed on the L ^* a ^* plane of b ^* = 0 shown in FIG. 3, most of the space of the lookup table can be used, compared with the conventional usage rate of about 1/2 (see FIG. 3). In addition, the utilization factor can be significantly improved also in another L ^* a ^* plane in which the value of b ^* is changed. Therefore, if the capacity is about the same as the conventional one, it is possible to increase the accuracy by increasing the number of grid points in the lookup table, and if the accuracy is about the same as the conventional one, the capacity of the lookup table is increased. It can be significantly reduced.

(Modification) (1) Although the above-described embodiment is an embodiment in which no interpolation is performed, it may be configured to perform interpolation as shown in FIG.
In FIG. 5, 10 is a representative value / interpolation value calculation means,
A representative value and an interpolation value are obtained from the L ^* a ^* b ^* signal (excluding the most significant bit of L ^* a ^* ). In this case, generally, the representative value is the higher-order bits and the interpolation value is the lower-order bits. Here, the obtained representative value is supplied to the three-dimensional lookup table 2 as an address signal. Also, 1
Reference numeral 1 denotes an interpolation means, which performs interpolation based on the interpolation value output from the representative value / interpolation value calculation means 10 and the output signal of the three-dimensional lookup table 2.

(2) In the above embodiment, L ^* a ^*
Although the exclusive OR of the most significant bits of the L ^* signal and the a ^* signal of the b ^* signal is used as the address of the lookup table, instead of this, the most significant bit of the L ^* signal and the b ^* signal is excluded. OR may be used, and in some cases, exclusive OR of the most significant bits of the a ^* signal and the b ^* signal may be used. However, in general, as shown in this embodiment,
It is more efficient to use the exclusive OR of the most significant bits of the L ^* signal and the a ^* signal.

(3) The shape of the color reproduction range differs depending on the printing device, and therefore, for example, the color reproduction area may extend outside the rhombus RH shown in FIG. 3 (see FIG. 6). In such a case, the range may be appropriately converted so that the color reproduction area fits within the diamond RH.

(4) The operation of the exclusive OR circuit 1 in the embodiment may be processed by software.

(5) In the embodiment, L ^* a ^* b
^{* The} signal is converted into a CMYK signal, but the output signal is not limited to such a four-dimensional signal, but is three-dimensional (for example, RGB of three-color signal), two-dimensional (for example, two-color signal) or one-dimensional ( For example, a black and white signal) may be used.

[0028]

As described above, according to the present invention, the utilization factor of the color space in the look-up table can be greatly improved with a simple structure.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an exemplary embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a three-dimensional lookup table 2.

FIG. 3 is a diagram showing a color reproduction range of a general printing apparatus on an L ^* b ^* plane.

FIG. 4 is a diagram showing a color reproduction range in an L ^* b ^* space of the three-dimensional lookup table 2 in the example.

FIG. 5 is a block diagram showing a modified example of the same embodiment.

FIG. 6 is a diagram showing a color reproduction range of a general printing apparatus on an L ^* b ^* plane.

[Explanation of symbols]

 1 Exclusive OR Circuit 2 3D Look Up Table 10 Representative Value / Interpolation Value Calculation Means 11 Interpolation Means

Claims (3)

[Claims] 1. An L ^* designating coordinates in the L ^* a ^* b ^* color space ^.
A three-dimensional lookup table is referred to by using signals, a ^* signals, and b ^* signals as addresses, and one-dimensional to four
In a color conversion device that outputs a color material amount signal of dimensional subtractive color mixing, an exclusive OR of the most significant bits of two signals of the L ^* signal, a ^* signal and b ^* signal is obtained, and the result is obtained. A color conversion device having an exclusive OR unit for outputting, and the three-dimensional look-up table having a rectangular parallelepiped space including a color reproduction area mapped by the exclusive OR unit. 2. An L ^* indicating coordinates in the L ^* a ^* b ^* color space ^.
In the color conversion device, the three-dimensional lookup table is referred to by using the representative values of the signal, the a ^* signal, and the b ^* signal as an address, and the one-dimensional to four-dimensional subtractive color material amount signal is output ^. The exclusive OR means for obtaining the exclusive OR of the most significant bits of the two signals, the a ^* signal and the b ^* signal, and outputting the result, and excluding the most significant bits of the two signals Representative value / interpolation value calculation means for obtaining a representative value and an interpolation value of the L ^* signal, a ^* signal and b ^* signal, a color material amount signal output from the three-dimensional lookup table, and the representative value / interpolation value calculation Interpolation means for performing interpolation processing based on the interpolation value output from the means, and the three-dimensional lookup table is a rectangular shape including a color reproduction area mapped by the exclusive OR means. It has Jo space, the exclusive color conversion apparatus, which comprises using OR means output signal and said representative value / representative value output from the interpolation value calculating means as an address. 3. The color conversion device according to claim 1, wherein the range of the L ^* signal, the a ^* signal, and the b ^* signal is converted and used.