JPH0588589B2 - - Google Patents

Description

[Detailed description of the invention]

<Technical Field> The present invention relates to a color image processing method, and more particularly to a color image processing method when the color reproduction range of an output system is different from the color reproduction range of an input system. <Prior Art> A description will be given below using an inkjet video printer as an example. Generally, in this type of device, the color reproduction range of the input system and the color reproduction range of the output system are different. For example, as shown in the x,y chromaticity diagram (CIE chromaticity diagram) of FIG. 2, the color reproduction range R _TV of a television is wider than the color reproduction range R _IJ of an inkjet printer. Note that R total is the entire color range. Conventionally, there was no solution to this problem, so when trying to reproduce colors that cannot be reproduced by an inkjet printer, such as those shown in Figure 1 A and B, they are equivalently reproduced as A' and B'. As a result, the saturation difference and gradation difference that originally existed between A and B are lost. Such a phenomenon appears particularly in highly pure colors, such as the color of a red rose or the color of a deep blue sea, and significantly deteriorates the quality of reproduced images. <Objective> In view of the above-mentioned problems, the present invention aims to provide a color image processing method capable of reproducing a high-quality image even when the color reproduction ranges of the input system and the output system are different. <Example> FIG. 1 shows the concept of the present invention. Color space is originally three-dimensional, but to simplify the explanation, it will be explained using two-dimensional chromaticity. Now, let W be the white point and consider reproducing colors P and Q. Colors W, P, and Q exist on a straight line.
That is, the hues of P and Q are the same. The straight line WP is
Let T be the point where it intersects with the limit line R _TV indicating the color reproduction range of the television, and S be the point where it intersects with the limit line R _IJ indicating the color reproduction range of the inkjet printer. At this time, it is possible to reproduce the color of Q as it is, but it is impossible to reproduce the color of P as it is. Therefore, Q is on the same straight line ′=×
For Q′ that satisfies WS/, P is′=×/
By converting each to P' that satisfies WT and performing color reproduction, the hue of P and Q will not change, and the difference in saturation between P and Q can be reproduced in a natural manner. Become. The same idea can be applied even if the color reproduction range of the inkjet printer is partially out of focus, as shown in FIG. If you want to reproduce P, as before, you can convert it to P' that satisfies '=×/ and then perform color reproduction. In fact, it can be done in the same way in the case of three dimensions. Let the coordinates of the white point be W→=(X _W , Y _W , Z _W ). _{Consider the} case of reproducing a certain color _P →= _{( P} . If the points where W→P intersect with V _TV and V _IJ are T→ and S→, then T→ and S→ are uniquely determined. At this time, the color P'→ to be reproduced is given by P'→=W→+|WS|/|WT|·WP/|WP|. By doing this, even if the color reproduction range between input and output is different, the hue is constant, the brightness and saturation are compressed to a natural form, and a high-quality image can be obtained. FIG. 5 describes an embodiment of a signal processing circuit that implements the present invention. The color compression unit 51 converts the video signals R, G, 8.
Data R′, G′, which receives 6-bit digital input and undergoes color compression as described below.
It is converted to B′. Here, we use a table conversion method using 2 ¹⁸ -byte ROM, which is
It can be configured with three 256Kbit ROMs. The density conversion section 52 converts these R', G', and B' into densities C, M, and Y of cyan, magenta, and yellow, respectively, by a table conversion method. For example, R′ is C′=
Convert as −logR′. The masking unit 53 performs masking in order to correct the asymmetric color components of each color ink and achieve C, M, and Y densities on the paper surface. For example, C′=a ₁₁ c+a ₁₂ m+a ₁₃ y,
All you have to do is correct M′ and Y′ using the linear equations of C, M, and Y.
This is implemented using table transformations. Next, the density values C', M', and Y' to be output are converted into respective head drive voltages by a head drive section 54, and the inkjet heads are driven accordingly. Next, the ROM data stored in advance in the color compression section 51 will be explained. Let the coordinates of the color solid be an orthogonal system of tristimulus values X, Y, and Z. First, the color solid V _TV that can be expressed by television is given as follows. According to NTSC standard

It is [ ]. Here, 0≦R≦1, 0≦G≦1, 0≦
If we include the condition B≦1, this represents a rectangular parallelepiped in the XYZ system. In this case, the six plane equations f ₁ (X, Y, Z) = 0,... f ₆ (X, Y, Z) = 0
The territory S _TV is determined. In addition, the color reproduction area of the inkjet V _IJ area
S _IJ is generally multiplied by f(X, Y, Z) = 0, or
For example, it can be approximated as follows. Considering it as a solid as shown in Figure 6, where the radius changes as the brightness goes in the Y direction, we can assume that aX ² +bZ ² = f(Y) ² (2). Now, according to the NTSC standard for input R, G, and B.

Convert to [ ]. The white point is W → = (W _X , W _Y , W _Z ) and the straight line PW →
The equation of

[ka] Multiply. When formula (4) is substituted into formula (1) and solved, two solutions T ₁ → and T ₂ → are obtained, and of these, the one in the direction of WP→ is defined as T→. Furthermore, by substituting equation (4) into equation (2) and solving it, two solutions S ₁ → and S ₂ → are obtained. Among these, the one in the direction of WP→ is assumed to be S→. These are uniquely determined.
here,

[ka] as

Find X′, Y′, and Z′ that satisfy [Formula]. This is the color-compressed point P'. Calculate R′, G′, and B′ again using equation (6) of the NTSC standard, and use this data as

[C] In this way, R', G, determined from R, G, and B,
The value of B' may be stored in the ROM 51 whose input is the address. Here, we use the tristimulus values XYZ as the coordinates of the color solid.
I thought of this, but you may also use Lab ^* series, Luv〓, etc. In this way, the color reproduction range of the input system can be compressed to the color reproduction range of the output system without changing the hue, so even when forming color images with different color reproduction ranges of the input and output systems, good color images can be obtained. It will be done. In this embodiment, a television signal is used as an input system signal, but a direct input signal from another solid-state image sensor or the like may also be used.
Furthermore, although the inkjet printer has been selected as the output system in the explanation, other color printers such as electrophotographic printers, thermal printers, etc. can also be used. <Effects> As described above, according to the present invention, even if the color reproduction ranges of the input system and the output system are different, it is possible to compression map the saturation or brightness in a natural manner without changing the hue. This makes it possible to obtain high-quality color images.

[Brief explanation of the drawing]

Figure 1 is a chromaticity diagram showing the concept of the present invention, Figure 2 is a chromaticity diagram explaining a conventional example, Figure 3 is a chromaticity diagram explaining another example of the present invention, and Figure 4 is a three-dimensional diagram. 5 is a circuit diagram showing an example of a signal processing circuit that implements the present invention, and FIG. 6 is a diagram showing the color reproduction range by an inkjet printer in a three-dimensional color space. be. In the figure, R _{TV and} V _TV indicate the color reproduction range of a color television, and R _IJ and _VIJ indicate the color reproduction range of an inkjet printer.

Claims (1)

[Claims] 1. When the color reproduction range of the output system differs from that of the input system, the hue is kept constant around the white point on the chromaticity diagram, and points outside the color reproduction range of the output system are compressed and mapped to points within the range. A color image processing method characterized by: