JPH0814843B2 - Color conversion method and color conversion device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an application for inputting a color image signal or a color video signal and performing arbitrary color conversion in real time, for example, a color hard copy device, a color display device, a color television camera device, The present invention relates to a color recognition device, a video editing device, and the like.

[0002]

2. Description of the Related Art Conventionally, in image processing of a monochrome image, the information contained in one pixel of the image is one-dimensional information called brightness (density), and the brightness conversion is so-called gamma curve conversion.
LUT (look-up table) for various nonlinear curves
If you write it in, it was possible to convert colors in real time.
Even if the image to be handled is a color image, it is treated as three monochrome images of R (red) plane, G (green) plane, and B (blue) plane for the purpose of performing color conversion in real time, and each is independent. It was often converted by the LUT. However, in this type of processing, the color conversion that can be handled is essentially one-dimensional processing, and R '= hR (R), G' = hG (G), B '= hB (B). Only color conversion is possible.

In color image processing, the information contained in one pixel is three-dimensional information (R, G, B), and color conversion in the original sense means three-dimensional conversion R '= fR. (R, G, B) G '= fG (R, G, B)
B '= fB (R, G, B).

For example, as a technique which has become important in color image processing in recent years, in HLS conversion for converting a color represented by (R, G, B) into a hue H, a lightness L, and a saturation S,
Like H = H (R, G, B), one output is a function of three inputs and belongs to the above three-dimensional conversion. However, if it is attempted to convert these with a general-purpose table, assuming that one color is an 8-bit signal, 16 (Mbyte) is required for conversion per color.
The memory capacity of e) is also required. Therefore, conventionally, there is required hardware that can perform three-dimensional color conversion universally and in real time for arbitrary color conversion. On the other hand, for the main purpose of color correction for color hard copy and color scanner, the input color space is divided into a plurality of color spaces, and a plurality of color correction information located at the vertices are selected.
There is an example of a color signal interpolation method in which weighting processing is performed and interpolation output is performed (Japanese Patent Publication No. 58-16180). In this example, a unit cube, which is the basic solid in the three-dimensional color signal space, is set for interpolation processing, this unit cube is divided into multiple tetrahedra, and interpolation calculation is simplified from the output signals at each vertex of the tetrahedron. The idea to change is disclosed.

By using this as a general-purpose color conversion device, it is possible to perform non-linear free color conversion at high speed while maintaining the gradation of an image, such as changing the color of a specific color in a color space. .

[0006]

However, in the prior art, the input color space in which there is a regular three-dimensional grid forming the unit interpolation section as in the example of the above patent, the three-color separated reflectance from the color scanner is used. , A transmittance, or a device-dependent space such as three-color separation density, and their three-dimensional coordinate axes are divided at equal intervals.
Therefore, the important characteristic that "human visual characteristics require many gradations in the lightness direction but not so many gradations in the chromaticity direction" is not effectively utilized. There is a problem. Further, in the prior art, since a tetrahedron obtained by dividing a unit cube in the input color space into a plurality of units is assumed as a unit interpolation section, when the input color changes on a straight line parallel to the lightness direction, a plurality of different input colors are used. While passing through the tetrahedron, each tetrahedron will be linearly interpolated independently. Therefore, when the color conversion is a non-linear conversion, there is a problem in that the interpolation output value for an input having a gray scale in the gray direction, which is the most important in human visual characteristics, is not linearly interpolated smoothly and becomes an unnatural polygonal line. It was

[0007]

In order to achieve the above object, the present invention provides an interpolation means by converting a three-color separated input signal into lightness / chromaticity and then performing an interpolation operation in this lightness / chromaticity space. The present invention provides a color conversion method and a color conversion device that divides a three-dimensional space into triangular prism unit interpolation sections and linearly interpolates the inside.

[0008]

In the color conversion method and the color conversion apparatus of the present invention, the color space of R, G, B, etc. formed by the output signals of the color scanner as described above is called a device depend, and is unique to each color device. belongs to. These are lacking in generality as a color system, and it is hard to say that each axis accurately reflects human visual characteristics. Therefore, in recent years, there is a movement to unify the color expression among many color devices (color display, various color hard copies). This is called device-independent color, and it is expected that a separation space of lightness and chromaticity such as CIE-Lab or YIQ will be used among various color spaces. Also, when adjusting the color of an image, it is convenient that the color is separated into lightness and chromaticity because only the color can be changed while maintaining the gradation. From these conditions, a combination of lightness and two kinds of chromaticity (Y, I, Q or Y, RY, BY) is most suitable for the input space of a general-purpose color conversion device using interpolation. . Therefore, as a solution to the first problem, even if the input signal is three signals of red, green, and blue of a color scanner etc., it is converted to the brightness / chromaticity separation type before interpolation is performed. Set the interpolation section. At this time, the unit interpolation section in the lightness direction can be made fine in order to emphasize the lightness gradation.
Furthermore, in order to improve the interpolation accuracy when inputting with gray-scale gradation, it is necessary to set the unit interpolation section so that there is no possibility that a polygonal line will occur in the linear interpolation straight line. Can be solved by setting a triangular prism type single interpolation section having a bottom surface parallel to the lightness direction and linearly interpolating the inside.

[0009]

EXAMPLE An example of the color conversion method of the present invention will be described below.

In this embodiment, (R, G,
B) Convert the color represented in the color space into the color space represented by the lightness and the color difference. There are various definitions of brightness and color difference,
Here, Y, (RY), and (BY) are adopted. This is because the conversion formula is simple, and other color spaces such as YIQ may be used. The conversion formula from RGB to Y uses the following formula.

[0011]

[Equation 1]

Next, the three-dimensional color space represented by Y, (RY), and (BY) is divided into axes as shown in FIG. 3 so that the color space is roughly divided into unit rectangular parallelepiped regions. To divide. If the number of divisions of each axis is fine in the lightness Y direction and coarse in the (RY) and (BY) chromaticity directions, it is effective for human visual characteristics and the limited memory capacity is efficiently used. Although it can be used, FIG. 3 illustrates the case where the number of divisions of each axis is the same for simplicity. Each unit rectangular parallelepiped has a rectangular shape in the (RY) (BY) chromaticity plane, but is further divided into two triangular prisms along the diagonal of this rectangle. Figure 4 shows this unit rectangular parallelepiped ABCD-E.
The FGH and the triangular prism obtained by dividing the FGH are shown. FIG. 5 shows one of the triangular prisms ABC-EFG divided into two. When a color point is input in this triangular prism, the corresponding output value can be interpolated from the output value at each vertex of the triangular prism. Now, in FIG. 5, the vertices A, B, and
The output values at C, E, F and G are (A), (B), (C),
When (E), (F), and (G) are set, the input color point P
The output value (P) corresponding to is the component of the vector AP from the point A to the input point P on the Y, (RY), and (BY) axes of ΔY, Δ (RY). ) And Δ (BY) can be used to interpolate in three steps as follows. In the first stage, P
Draw a straight line parallel to the Y-axis from triangle ABC and triangle E
The intersections with FG are P ₁ and P ₂ , respectively. And the triangle AB
In C, the output value at P ₁ to (P _1),

[0013]

[Equation 2]

Interpolation is performed as follows. This has the following graphical meaning. FIG. 6 is a view of the triangular prism of FIG. 5 observed from the Y-axis direction. From this direction, the two bottom faces of the triangular prism completely overlap, and P and P ₁ and P ₂ , A and E, B and F, C.
And G overlap. Therefore, it may be considered that this triangle is the triangle ABC. Δ (RY), which is a component of the vector AP in FIG. 5 to the input first difference vector AB (601), and the input second difference vector BC (6
02) which is a component to the output first difference value {(B)-(A)} corresponding to each input difference vector and the output second difference value {(C ) −
(B)} is weighted with Δ (R−Y) and Δ (B−Y) to obtain the first and second output increments and added to the output value (A) at A. . In the present embodiment, the linear interpolation operation always uses the difference, but even if (A), (B), and (C) are weighted and added without using the difference, the multiplication only increases once. , It is clear that it gives the same result. In the second stage, the output value (P ₂₎ a first difference value output the same weighting in the P ₂ considers 6 a triangle EFG {(F) - (E )}, outputs the second difference value {(G ) −
(F)},

[0015]

(Equation 3)

It is calculated as In the third stage, (P ₁ ) and (P ₂ ) are linearly interpolated as follows on the line segment P ₁ -P _{2 in} FIG.

[0017]

[Equation 4]

Substituting equations 2 and 3 into equation 4 and rearranging,

[0019]

(Equation 5)

[0020]

As described above, the input point P is the triangular prism ABC.
-The output value at P is determined when in EFG. Input color point is the other triangular prism that divides the rectangular parallelepiped ACD-EG
When it exists in H, it becomes as shown in FIG. 7 and FIG. As before, from P to triangle ACD and triangle EGH
A straight line parallel to the Y-axis is drawn to obtain intersections P ₁ and P _2, and Y, (RY), and (BY) of the vector AP in the axial direction ΔY, Δ (RY), Δ (B−Y) is obtained, and the output value at P ₁ in the first step is

[0022]

(Equation 6)

In the second stage, the output value at P ₂ is

[0024]

(Equation 7)

In the third step, the output interpolation value at P is

[0026]

(Equation 8)

It is calculated as follows. (Equation 8) to (Equation 6),
Substituting (Equation 7) and rearranging,

[0028]

[Equation 9]

It becomes as follows. The determination of whether the input color point P is included in the above-mentioned two types of triangular prism ABC-EFG or triangular prism ACD-EGH is made by determining the component Δ (R-
Y) and Δ (B−Y) are compared, and when Δ (R−Y) ≧ Δ (B−Y), triangular prism AB
When C-EFG △ (RY) <△ (BY), triangular prism AC
It becomes like D-EGH (Equation 10).

As described above, the color conversion method of this embodiment is
The RGB input value is a three-dimensional space Y (RY) (B-
Y), and when the output value is obtained from this Y (RY) (BY) input space, the input space is divided into rectangular parallelepipeds, and the rectangular parallelepiped is further divided into triangular prisms.
The interpolation calculation is performed using the output value at each vertex position and the difference value between the output values.

Next, an embodiment of the color conversion device of the present invention will be described with reference to FIGS.

FIG. 2 is an overall block connection diagram of the color conversion apparatus in one embodiment of the present invention. In FIG. 2, only the part where the output signal R ′ is generated from the input signal (R, G, B) is shown, so that (R, G, B) to (R ′,
G ′, B ′) is generated, the three-dimensional interpolation unit 1 of FIG.
You need three 01s. The brightness generation unit 102 generates a Y signal from the input signal. The lightness generation unit 102 executes (Equation 1) and can be configured by a multiplier and an adder, or a lookup table and an adder. The difference between the Y signal and the R and B signals is calculated in the subtractor 103 to generate color difference signals (RY) and (BY). Then, the Y signal, the (RY) signal, and the (BY) signal are input to the three-dimensional interpolation unit.

In a practical application, RGB signals may be used instead of Y signals.
The G signal that has the largest contribution to the Y signal
It is also possible to use it instead of a signal. In that case, the G, R, and B signals are not subjected to the lightness / chromaticity signal conversion, and
Three-dimensional interpolation unit 101 as (R-G) and (B-G) signals
Is input to

FIG. 1 shows details of the three-dimensional interpolation unit 101 of FIG.
It is a block diagram shown . In the three-dimensional interpolation unit 101, Y (R-
Y) (BY) The input space is divided into a rectangular parallelepiped and further divided into triangular prisms for interpolation. The division into the rectangular parallelepiped is performed by the Y signal, the (RY) signal, and the (BY) in FIG. ) It is performed by dividing a digital signal representing a signal into a high-order signal and a low-order signal. As an example, Y, (RY),
Assume that the (BY) signal is an 8-bit signal, the respective upper signals are 3, 3, and 3 bits from the most significant bit of the signal, and the lower signals are the remaining 5, 5, and 5 bits. In this case, Y, (R-
The three-dimensional space represented by Y) and (B-Y) is 2 in the Y-axis direction.
³ = 8 interpolation intervals, to be divided into (R-Y) axially 2 ³ = 8 interpolation interval, (B-Y) axially 2 ³ = 8 interpolation segment, the three-dimensional space The total is (8) x (8) x
(8) = Divided into 512 rectangular parallelepipeds, and the lengths of the sides of each rectangular parallelepiped are 3 in the Y, (RY), and (BY) directions, respectively.
2, 32, 32. In FIG. 1, these Y, (R-
Y) and (B-Y) upper signals are (201),
(202) and (203), and lower signals are shown (204), (205), and (206), respectively. That is, in FIG. 3 and FIG. 4, the upper signal is Y, (R-
Y) and (B-Y) are represented by numerical values from 0 to 7, 0 to 7, and 0 to 7, respectively, in the axial directions, and the lower-order signal is the input color of the input color with point A as a reference point in each unit rectangular parallelepiped. The position is 0 to 31 in each of the Y, (RY), and (BY) axis directions,
Expressed with numerical values from 0 to 31, 0 to 31. Therefore, the lower order signals are ΔY, Δ (RY), and Δ (B−) described above.
Y) will be expressed. This lower signal Δ (R-
(205) and (206) of Y) signal and Δ (B-Y) signal
Is input to the triangular prism determination unit 207 and outputs a 1-bit triangular prism determination signal (208). This follows (Equation 10)
By the size judgment, it is judged by outputting 1 or 0 which one of the two triangular prisms obtained by dividing the rectangular parallelepiped contains the input color point. Now, it is assumed that the input color is included in the triangular prism ABC-EFG as the determination result. At the same time △
(RY) is the first multiplier 219 and the third multiplier 22.
6 and the Δ (B−Y) signal is sent to the second multiplier 220 and
And a fourth multiplier 227. Upper signal set (20
1), (202), (203) and the triangular prism determination signal (20
8) are collectively used as a memory address signal (209) of 10 bits in total (3 + 3 + 3 + 1 =). The memory address signals are the first to sixth color conversion table memories 210 to 215.
Is input to Each color conversion table memory 210-215
In, information of terms necessary for interpolation when one triangular prism is specified is stored in advance, and the stored value is output in parallel with the memory address input. When the triangular prism ABC-EFG is specified as described above, the first, second and third
The color conversion table memories 210, 211, and 212 are respectively the output value (A), the output first difference value {(B) − (A)}, and the output second difference value { (C) − in (Equation 5).
(B) } data is output. These data are shown by (216), (217), and (218) in FIG. According to (Equation 5), the data (217) and (205) are
The data is multiplied by the first multiplier 219, the data (218) and (206) are multiplied by the second multiplier 220, and the two multiplication results are added by the first adder 221. The data (216) is added to the second adder 222 to generate a result obtained by adding the first term to the third term of (Equation 5). The fourth, fifth and sixth color conversion table memories 213, 214 and 215 respectively output the third difference value {(E)-(A)} and the fourth output difference value {(F) in (Equation 5).
-(E)-(B) + (A)}, output fifth difference value {(G)
-(F)-(C) + (B)} is output. These data are (223), (224), (225) in FIG.
Indicated by. According to the expression in [] of the fourth term of (Expression 5),
The data (224) and (205) are multiplied by the third multiplier 226 and the data (225) and (206) are multiplied by the fourth multiplier.
It is multiplied by vessel 227, the two multiplication results to the third addition of
Are added by the vessel 228, the result is the data (223) first
4 is added by the adder 229 . The result is (20
4) The lower signal ΔY of the Y signal and the fifth multiplier 2
Is multiplied by 30, to complete calculations paragraph of (Equation 5), the
The addition result of the first to third terms is further added by the adder 231 of 5 and the final result (P) of (Equation 5) is output (232). When the input color is included in the triangular prism ACD-EGH as a result of the triangular prism determination unit 208, the table memory is accessed in a state where only 1 bit of the data (208) in the memory address signal (209) is different. At that time, the output values of the table memories 210 to 215 become the values shown in the respective terms of (Equation 9), and the interpolation is performed by operating so as to calculate (Equation 9). It should be noted that the number of bits of each signal and the bit allocation of the higher-order signal and the lower-order signal shown here are only one embodiment, and may be other values.

The difference between the straight lines to be interpolated between the conventional tetrahedral interpolation and the interpolation using the triangular prism having the bottom surface parallel to the chromaticity direction described in the present embodiment will be described below. FIG. 9 shows a state in which an input space represented by R, G, and B is divided into cubes, and the cubes are divided into six tetrahedra. Three input colors are represented in the first cube abcd-efgh and the second cube bijc-fklg as the loci of gradation changes in the lightness direction. Since the lightness direction is the diagonal direction of the cube, these Is the line segment a-g, which is the diagonal direction of the cube, and the line segment m-n connecting the midpoint m of b-1 and a-b and the midpoint n of g-1. Figure 10
Shows the distorted hexahedron formed by the input cube of FIG. 9 in the output space (R′G′B ′). The hexahedron corresponding to the first cube in the input space is a'b'c'd'-e'f'g'h '.
And the hexahedron corresponding to the second cube is b'i'j'c '
-F'k'l'g '. Input color locus a-
Output interpolation straight lines corresponding to g and bl are diagonal lines a'-g 'and b'-l' of the output cube. However, as the input color locus, m'-n 'corresponding to m-n which should exist between ag and bl is not a straight line, and m'-o'-n'
It becomes a straight line bent at the point o'as shown in. This is because the line segments ag and b-1 of the input trajectory in FIG. 9 are the diagonals of the cube, and the cube is divided into six tetrahedra that share this diagonal, so these two straight lines are Although it is completely included in one of the tetrahedrons that is a unit interpolation solid, the line segment m
This is because -n passes through different tetrahedral regions such that the first cube is included in the tetrahedron abfg and the second cube is included in the tetrahedron bfgl. In this case, since linear interpolation is performed independently in each tetrahedron, unnatural bending occurs although the continuity is maintained at the boundaries between the tetrahedra. In this way, in the interpolation using the tetrahedron, when the color is moved parallel to the lightness direction, the nonlinearity of the color conversion is,
Originally, in the output space, the shape of the interpolated straight line, which should be the straight line m′-n ′, collapses, which adversely affects the gradation of the output image.
On the other hand, FIG. 11 shows an interpolation method using the triangular prism of the present embodiment, in which the triangular prism is divided in the Y (RY) (BY) space and two triangular prism areas ABC-DEF and DEF-GHI are formed.
Are formed. At this time, as input color loci, parallel straight lines ADG and CFI changing in the lightness Y direction and a straight line JKL existing in the middle are set. FIG. 12 shows A'B'C'-D'E'F 'and D'E'F'-G'H' corresponding to two triangular prisms in the input space in the output space (R'G'B ').
A state in which I ′ exists in a distorted state due to the non-linear conversion is shown. The three straight lines of the input color are the polygonal lines A'D 'in this space.
G'and C'F'I ', and J'K'L'. They all pass through adjacent triangular prisms, and none of them pass through different triangular prisms. Therefore, the polygonal line after interpolation has no unnaturalness. For example, in this case, among the three trajectories, the polygonal line J'K'L 'at the intermediate position in the input space is the polygonal line A'D'G' passing through the grid point and the polygonal line passing through the grid point as well. C'F '
It is a broken line in the middle of I '.

As described above, in this embodiment, it is possible to perform better linear interpolation than the conventional method on the locus of the color changing in the lightness direction.

[0037]

As described above, the present invention has an advantage that any color conversion whose input and output is three-dimensional can be processed in real time by an interpolation method having no discontinuity.

Further, in the conventional three-dimensional color signal interpolation method, which is a linear interpolation method using tetrahedron division, each three-dimensional axis cannot have a special meaning, and the memory cannot be effectively used. However, according to the present invention, by using a solid body called a triangular prism, it is possible to give the meaning of lightness in the principal axis direction of the triangular prism and chromaticity plane to the bottom surface of the triangular prism, which effectively separates the memory into lightness and chromaticity. There is an advantage that can be utilized. In addition, there is an advantage that, when a color change in the lightness direction is sensitive to humans, the unnatural bending of the interpolation line at the boundary of the unit interpolation section unlike the conventional tetrahedral interpolation does not occur. Further, since the input space when performing the interpolation is the lightness / chromaticity space, it is convenient when the color conversion method of the present invention is applied to the field of color adjustment, and the effect in the color image processing field is very high. large.

[Brief description of drawings]

FIG. 1 is a detailed block connection diagram of a three-dimensional interpolation unit of a color conversion device according to an embodiment of the present invention.

FIG. 2 is an overall block connection diagram of the color conversion device in the embodiment.

FIG. 3 is a conceptual diagram in which a three-dimensional space created by Y (RY) (BY) showing the concept of the color conversion method in one embodiment of the present invention is divided into a plurality of rectangular parallelepipeds.

FIG. 4 is a conceptual diagram in which the same rectangular parallelepiped is divided into two triangular prisms.

FIG. 5 is a rectangular parallelepiped triangular prism ABC-EF in FIG.
Conceptual diagram of G

FIG. 6 is a diagram of the triangular prism ABC-EFG in FIG. 5 observed from the Y-axis direction.

7 is a rectangular parallelepiped triangular prism ACD-EG in FIG.
Conceptual diagram of H

8 is a diagram of the triangular prism ACD-EGH in FIG. 7 observed from the Y-axis direction.

FIG. 9 is a conceptual diagram showing a locus of input colors in a conventional input space.

FIG. 10 is a conceptual diagram showing an output interpolation straight line in a conventional output space.

FIG. 11 is a conceptual diagram showing a locus of an input color in an input space of a color conversion method according to an embodiment of the present invention.

FIG. 12 is a conceptual diagram showing an output interpolation line in an output space of a color conversion method according to an embodiment of the present invention.

[Explanation of symbols]

101 three-dimensional interpolation unit 102 lightness generation unit 103 subtractor 210 first color conversion table memory 211 second color conversion table memory 212 third color conversion table memory 213 fourth color conversion table memory 214 fifth color conversion Table memory 215 Sixth color conversion table memory 219 First multiplier 220 Second multiplier 221 First adder 222 Second adder 226 Third multiplier 227 Fourth multiplier 228 Third Adder 229 fourth adder 230 fifth multiplier 231 fifth adder

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication location G09G 5/06 9377-5H H04N 9/74 Z (72) Inventor Toshiharu Kurosawa Tama Ward, Kawasaki City, Kanagawa Prefecture 3-10-1 Higashisanda Matsushita Giken Co., Ltd. (72) Teruo foot Inventor 3-10-1 Higashimita 3-chome, Tama-ku, Kawasaki-shi, Kanagawa Matsushita Giken

Claims (3)

[Claims] 1. An input color signal is converted into a lightness / chromaticity separation signal, each axis of a three-dimensional input color space formed by lightness / chromaticity is divided, and first divided into a plurality of rectangular parallelepipeds, and then, Each rectangular parallelepiped is divided into two triangular prisms having a bottom surface in a plane parallel to the chromaticity plane, and it is determined by selecting which one of the two triangular prisms the input color is included in and selected in advance. The color conversion table accumulating the output colors after conversion on the coarse grid points in the lightness / chromaticity separation signal space is read, and the output values at the six grid points corresponding to the vertices of the selected triangular prism. In each triangular prism, draw a straight line parallel to the lightness axis from the input color to the two triangular bases, find the two intersections of this straight line and the base, and within the two triangular bases, Output values at three intersections are output at the three vertices of the triangle read from the color conversion table. A color conversion method characterized by obtaining the output color signal with respect to the input color signal by linearly interpolating the output values at the two intersections in the lightness direction. 2. A three-dimensional input color sky created by lightness and chromaticity.
Each axis in between is divided into a plurality of rectangular parallelepipeds,
Two triangular prisms with a base in the plane parallel to the plane of chromaticity
The input color signal expressed by the lightness / chromaticity separation signal is input, and the triangular column including the input color signal is determined by determining the magnitude of the lower bit signal of the two input chromaticity signals. A triangular prism determining unit, a first color conversion table that stores output values at a first reference vertex that is the vertex of the first bottom surface of the triangular prism, and the first reference vertex of the first bottom surface A second color conversion table that stores an output first difference value that is a difference between the output value at the first vertex that is one of the two vertices other than the above and the output value at the first reference vertex. , An output second difference value that is the difference between the output value at the second vertex that is the other of the two vertices other than the first reference vertex on the first bottom surface and the output value at the first vertex And the output value at the first reference vertex of the triangular prism. A fourth color conversion table that stores an output third difference value that is a difference from the output value at the second reference vertex that is the vertex of the second bottom surface, and the second color conversion table of the second bottom surface. Difference between the output value at the third vertex which is one of the two vertices other than the reference vertex and the output value at the second reference vertex
A fifth color conversion table that stores an output fourth difference value that is a difference between the minute value and the output first difference value, and two vertexes of the second bottom surface other than the second reference vertex. A sixth difference value, which is the difference between the output value at the other fourth vertex and the output value at the third apex, and the difference between the output second difference value and the sixth output value. A color conversion table, a first multiplier and a second multiplier for multiplying each of the output first difference value and the output second difference value by a lower signal of the two chromaticity signals of the input color signal, A first adder for adding the outputs of the first multiplier and the second multiplier, and an output of the first adder to an output value at the first reference vertex to obtain a first adder. A second adder for obtaining an interpolation result, and the two of the input color signals for the output fourth difference value and the output fifth difference value, respectively. A third adder for adding the third multiplier and the fourth multiplier for multiplying the lower signal of the chrominance signal, respectively, an output of said third multiplier and fourth multiplier,
A fourth adder that obtains a second interpolation result by adding the output of the third adder to the output third difference value, a lower signal of the lightness signal of the input color signal, and the second interpolation result. And a fifth multiplier for multiplying the output of the fifth multiplier by the first multiplier
And a fifth adder for adding the interpolation result to the color conversion device. 3. A lightness generation unit for generating a lightness signal from color signals represented by three primary colors or tristimulus values, and an arithmetic unit for generating a chromaticity signal, and the generated lightness / chromaticity separation signal is input. The color conversion device according to claim 2, wherein the color conversion device is a signal.