JPH0546750A - Method and device for converting color - Google Patents

Abstract

PURPOSE:To improve an interpolating gradation characteristic in a color lightness direction comforming with a visual sense characteristic by matching a three-dimensional interpolating method for executing the interpolating operation of a color conversion value of a color image signal. CONSTITUTION:Respective input chrominance signals are converted into lightness/chromaticity, a three-dimensional space formed by the lightness and chromaticity is divided into plural rectangular parallelopipeds and then roughly divided into plural triangle prisms having bottoms parallel with a chromaticity plane. Which triangle prism out of plural ones includes an input color is decided by a triangle prism deciding part 207 and output values corresponding to input points constituting respective points of the triangle prism, difference values among respective output colors and differences among respective difference values are stored in plural color convertion table memories 210 to 215, the stored values of respective memories 210 to 215 are weighted by multipliers 219, 220, 226, 227, 230 based upon the power signals 205, 206 of the input chrominance signal, and finally the weighted data are added by adders 220, 221, 228, 229, 231 to execute linear interpolation using the output values of six vertexes constituting the triangle prism.

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, information contained in one pixel of an image is one-dimensional information called lightness (density), and lightness conversion is so-called gamma curve conversion.
LUT (look-up table) for various non-linear curves
It was possible to convert the color in real time by writing in.
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 kind 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 for arbitrary color conversion and in real time. 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 a 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 is the three-color separated reflectance from the color scanner. , 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 called. Further, in the conventional technology, 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. Each tetrahedron is linearly interpolated independently while passing through the tetrahedron. For this reason, 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 for 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 for dividing a three-dimensional space into triangular prism unit interpolation sections and linearly interpolating 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 device depend, and is unique to each color device. belongs to. These are lacking in generality as a color system, and it cannot be said 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) may be optimal 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 brightness / chromaticity separation type before interpolation Set the interpolation section. At that 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 interval 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 with 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 to roughly divide the color space into unit rectangular parallelepiped regions. To divide. If the number of divisions of each axis is fine in the lightness Y direction and rough in the chromaticity directions of (RY) and (BY), it is effective for human visual characteristics and the limited memory capacity is efficiently used. Although it can be used, FIG. 3 illustrates a 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.
An FGH and a 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, G are (A), (B), (C),
When (E), (F), and (G), 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 (B−Y) 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, 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 of the output first difference value {(B)-(A)} and the output second difference value {(C) corresponding to each input difference vector in the output space. ) −
(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 if (A), (B), and (C) are weighted and added without using the difference, the number of multiplications is increased only 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]

## EQU1 ## In the third stage, (P ₁ ) and (P ₂ ) are linearly interpolated on the line segment P ₁ -P ₂ in FIG. 5 as follows.

[0017]

[Equation 4]

Substituting equations 2 and 3 into equation 4, 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 another rectangular prism that divides a 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 intersection points P ₁ and P _2, and Y, (RY), and (BY) of the vector AP in the respective axial direction components ΔY, Δ (RY), Δ (B−Y) is calculated, 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. In (Equation 8), (Equation 6),
Substituting (Equation 7) and rearranging,

[0028]

[Equation 9]

It becomes like this. The determination of which of the two types of triangular prism ABC-EFG and triangular prism ACD-EGH the input color point P is included in is determined by 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. Since only the part where the output signal R'is generated from the input signals (R, G, B) is shown in FIG. 2, (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 subtracter 103 calculates the difference between the Y signal and the R and B signals 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 the Y,
It is input to the three-dimensional interpolation unit 101 as representing the (RY) and (BY) signals.

FIG. 1 is a block diagram showing the details of the three-dimensional interpolation unit 101 shown in FIG. 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 an upper signal and a lower signal. As an example, Y, (RY),
Assume that the (BY) signal is an 8-bit signal, the upper signal of each of them is 3, 3, 3 bits from the most significant bit of the signal, and the lower signal is the remaining 5, 5, 5 bits. In this case, Y, (R-
The three-dimensional space represented by Y) and (BY) 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 higher-order signal indicates that each unit rectangular parallelepiped including the input color is the position coordinate of the point A closest to the origin Y, (R-
Y) and (B-Y) are expressed by numerical values from 0 to 7, 0 to 7, and 0 to 7 in the respective axial directions, and the lower-order signal is the input color of each unit rectangular parallelepiped with point A as a reference point. The position is Y, (R-Y), 0-31 in each axial direction (B-Y),
It is expressed by 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 the Y) signal and the 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 determination, it is determined 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 △
The (RY) and Δ (BY) signals are multiplied by the multipliers 219 and 220.
And multipliers 226 and 227, respectively. A total of (3 + 3 + 3 + 1 =) 10 of the higher-order signal groups (201), (202), (203) and the triangular prism determination signal (208).
A bit memory address signal (209) is used. The memory address signal is input from the color conversion table memory 210 to the storage color conversion table memory 215. Each color conversion table memory 210 to 215 stores in advance information of terms necessary for interpolation when one triangular prism is designated, and outputs the stored value in parallel with the memory address input. When the triangular prism ABC-EFG is designated as described above, the color conversion table memories 210, 211,
212 is the output value (A), the output first difference value {(B)-(A)}, and the output second difference value {(C) in (Equation 5), respectively.
-(A)} data is output. These data are shown in Figure 1.
Are indicated by (216), (217), and (218). According to (Equation 5), the data (217) and (205) are multiplied by the multiplier 219 to obtain the data (218) and (20).
6) is multiplied by the multiplier 220, these two multiplication results are added by the adder 221, and the result is the data (21
6) is added by the adder 222 to generate a result obtained by adding the first term to the third term of (Equation 5). The color conversion table memories 213, 214, and 215 respectively store the output third difference value {(E)-(A)} and the output fourth difference value {(F)-(E)-(B) + (in (Equation 5). A)}, and output fifth difference value {(G)-(F)-(C) + (B)}. These data are (223), (224),
It is shown by (225). According to the expression in [] of the fourth term of (Expression 5), the data (224) and (205) are multiplied by the multiplier 22.
6 and the data (225) and (206) are multiplied by the multiplier 227, and the two multiplication results are added by the adder 228.
And the result is added to the data (223) and the adder 22.
It is added at 9. The result is Y shown in (204).
The lower signal ΔY of the signal is multiplied by the multiplier 230, the fourth term of (Equation 5) is calculated, and the addition result of the first to third terms is further added by the adder 231. The final result (P) of 5) is output (232). Triangular prism determination unit 2
When the input color is included in the triangular prism ACD-EGH in the result of 08, the table memory is accessed with only one bit of the data (208) in the memory address signal (209) being 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 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 numerical values.

The difference between the straight lines to be interpolated in 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. 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 a line segment a-g, which is a diagonal direction of the cube, and a 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 b-l is not a straight line, and m'-o'-n'
It becomes a straight line bent at the point o '. 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. Thus, in interpolation using a tetrahedron, when moving colors in parallel to the lightness direction, the nonlinearity of color conversion is
Originally, in the output space, the shape of the interpolation 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 ').
The state where 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 together, and none of them pass through different triangular prisms. Therefore, the polygonal line after interpolation has no unnaturalness. For example, in this case, the polygonal line J'K'L 'at the intermediate position in the input space among the three trajectories 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 arbitrary color conversion in which the input and output are three-dimensional can be processed in real time by an interpolation method having no discontinuity.

Also, 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. Moreover, there is an advantage in 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 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 a 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.

FIG. 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 output value (A) storage color conversion table memory 211 output first difference value storage color conversion table memory 212 output second difference value storage color conversion table memory 213 output Third difference value storage color conversion table memory 214 Output Fourth difference value storage color conversion table memory 215 Output Fifth difference value storage color conversion table memory 219 Multiplier 220 Multiplier 221 Adder 222 Adder 226 Multiplier 227 Multiplier 228 Adder 229 Adder 230 Multiplier 231 Adder

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Reference number within the agency FI Technical display location H04N 9/74 Z 8626-5C (72) Inventor Toshiharu Kurosawa 3 Higashimita, Tama-ku, Kawasaki City, Kanagawa Prefecture No. 10-1 Matsushita Giken Co., Ltd. (72) Inventor Teruo Foot 3-10-1 Higashisanda, Tama-ku, Kawasaki City, Kanagawa Prefecture Matsushita Giken Co., Ltd.

Claims (4)

[Claims] 1. When obtaining an output color signal for an input color signal using a color conversion table memory that stores a difference value between the output color signal and the output color signal for the input color signal, Roughly divide the color space into multiple triangular prism areas, determine which triangular prism area in the input color space the input color is in, and store the color conversion table memory values for the input color points that make up the triangular prism. A color conversion method characterized by interpolating an output color signal with respect to an input color signal using. 2. When calculating an output color for an input color using a color conversion table memory which converts an input color signal into a brightness / chromaticity separation signal and uses the brightness / chromaticity separation signal as an input, the brightness / chromaticity is obtained. Each axis of the three-dimensional space created by is divided into a plurality of rectangular parallelepipeds, then each rectangular parallelepiped is divided into two triangular prisms with bases in the plane parallel to the chromaticity plane, and the input color It is determined which of the triangular prisms is included in each triangular prism, and the straight line parallel to the lightness axis is drawn from the input color to the two triangular bases in each triangular prism, and the two intersections of this straight line and the base are obtained. , The output values at the two intersections in the bottom surface of the two triangles are obtained by linear interpolation of the output values at the vertices of the triangle, and they are linearly interpolated in the lightness direction to obtain the output signal for the input color. The color conversion method according to claim 1, wherein the color conversion method is a color conversion method. 3. A triangular prism determination unit that divides a three-dimensional space created by an input color signal into a plurality of rectangular parallelepipeds and determines which one of the triangular prisms the input color is included in when the rectangular parallelepiped is divided into two triangular prisms. The difference between the output value corresponding to one vertex position of the triangular prism region and the output value at the different vertex positions of the triangular prism, the output first difference value, the output second difference value, the output third difference value, and the difference value. A color conversion table memory that stores the output fourth difference value and the output fifth difference value, a multiplier that performs an operation from the output from the color conversion table memory, and an adder. Color conversion device. 4. The color conversion apparatus according to claim 3, further comprising a lightness generation unit that generates a lightness signal from an input color signal and a calculator that generates a chromaticity signal.