JPH0550910B2 - - Google Patents

Description

Detailed Description of the Invention (Technical Field of the Invention) The present invention is directed to converting a color image signal obtained by reading an original image with an imaging device such as a television camera into a dye density for obtaining a color hard copy, etc. The present invention relates to a color image signal conversion method.

(Technical background of the invention and its problems) Conventionally, when obtaining a color hard copy using a color scanner or inkjet using color image signals R ₀ , G ₀ , B ₀ obtained by reading an original image, for example, the following procedure is used. The color image signal was converted into dye density for color hard copy using the following formula. That is, D _C D _M D _Y = a ₁₁ a ₂₁ a ₃₁ a ₁₂ a ₂₂ a ₃₂ a ₁₃ a ₂₃ a ₃₃ R ₀ G ₀ B ₀ …Using the matrix calculation formula (1), for color hard copy The dye concentrations D _C , _DM , and _DY were obtained.
The matrix coefficient a _ij in equation (1) is the dye concentration D _C ,
Multiple regression analysis was used to determine that D _M and D _Y faithfully represent the original colors.

However, in the matrix correction calculation using equation (1), the color difference ΔE between the reproduced color image of the color hard copy and the original image is large;
It was not possible to faithfully reproduce color images. This is because the description of the color reproduction system is insufficient in the calculation using the determinant of equation (1).

(Object of the invention) This invention was made in view of the above-mentioned circumstances, and an object of the invention is to sufficiently reduce the color difference between a reproduced color image and an original image, and to achieve faithful color reproduction. An object of the present invention is to provide a method for converting color image signals.

(Summary of the Invention) The color image signal conversion method of the present invention relates to a color image signal conversion method for converting a color image signal obtained by reading an original image into a dye density for obtaining a color hard copy. Color image signals R ₀ , G ₀ , B ₀ according to formula (2) below
The dye density D _C to obtain a color hard copy,
It is used to convert to D _M and D _Y.

D _C D _M D _Y =a ₁₁ a ₂₁ a ₃₁ a ₁₂ a ₂₂ a ₃₂ a ₁₃ a ₂₃ a ₃₃ ×−logS _{11 S 21} S ₃₁ S ₁₂ S 22 S ₃₂ S ₁₃ S ₂₃ S ₃₃ R ₀ G ₀ B ₀ ...(2) However, the matrix coefficients a _ij and S _ij are determined based on the spectral reflection density characteristics of the dye used to obtain the color hard copy and the spectral sensitivity characteristics of the original image reading system. The settings are made so as to minimize the color difference between the image and the image.

(Embodiment of the Invention) FIG. 1 shows an example of an apparatus for realizing the method of the invention, in which color image signals R ₀ , G ₀ , B ₀ obtained by reading an original image are input to a matrix calculation circuit 1. At the same time, the coefficients S ₁₁ to _{S 33} set in the coefficient setter 4 are input, and the calculations in { } in the logarithm of equation (2) are performed, and the results are R ₁ , G ₁ , _B1 is sent to the logarithmic conversion circuit 2. In the logarithmic conversion circuit 2, R ₂ =log R ₁ , G ₂ =log G ₁ ,
A logarithmic conversion of B ₂ =log B ₁ is performed, and the converted concentration signals R ₂ , G ₂ , and B ₂ are sent to the next matrix calculation circuit 3. Coefficients a ₁₁ to a ₃₃ from the coefficient setter 5 are input to the matrix calculation circuit 3.
By performing the calculation shown in equation (2) above, the dye concentrations D _C , _DM , and _DY are obtained.

The matrix coefficients a _ij and S _ij are calculated based on the spectral reflection characteristics of the dye used to obtain the color hard copy and the spectral sensitivity characteristics of the reading system for the original image. It is set to the minimum.

FIG. 2 shows an example of the spectral reflection density characteristics of a dye for obtaining a color hard copy, and FIG. 3 shows an example of the spectral sensitivity characteristics of an original image reading system. Specific examples of matrix coefficients a _ij and S _ij determined from these to minimize the color difference between the original image and the reproduced color hard copy image will be shown.

a ₁₁ a ₂₁ a ₃₁ a ₁₂ a ₂₂ a ₃₂ a ₁₃ a ₂₃ a ₃₃ =1.66424 −0.34025 0.3209 −0.76798 3.11038 −1.39352 0.11179 −1.78656 2.35608 …(3) S ₁₁ S ₂₁ S ₃₁ S ₁₂ S ₂₂ S ₃₂ S ₁₃ S ₂₃ S ₃₃ =0.00938 −0.00027 −0.00031 0.00041 0.01210 0.00243 0.00026 −0.00182 0.00788 (4) The matrix coefficients a _ij and S _ij determined in this way are set in advance in the coefficient setters 5 and 4, respectively.

(Effects of the Invention) As explained above, according to the color image signal conversion method of the present invention, the input color image signal
After correcting R ₀ , G ₀ , and B ₀ by matrix coefficient S _ij ,
Since the density signal is obtained by logarithmic transformation and this density signal is corrected again using the matrix coefficient a _ij , a faithful color reproduction image can be obtained.

[Brief explanation of the drawing]

Figure 1 is a block configuration diagram showing an example of an apparatus for realizing the method of this invention, and Figures 2 and 3 show the spectral reflection density distribution of the reproduction system dye and the spectral sensitivity characteristics of the reading system for obtaining color hard copies. FIG. 1, 3... Matrix calculation circuit, 2... Logarithmic conversion circuit, 4, 5... Coefficient setting device.

Claims (1)

[Claims] 1. In a color image signal conversion method for converting a color image signal obtained by reading an original image into a dye density for obtaining a color hard copy, the color image signal R ₀ , G ₀ , B ₀ is the dye concentration D _C to obtain a color hard copy,
A method of converting a color image signal, characterized by converting it into D _M and D _Y. D _C D _M D _Y =a ₁₁ a ₂₁ a ₃₁ a ₁₂ a ₂₂ a ₃₂ a ₁₃ a ₂₃ a ₃₃ ×−logS _{11 S 21} S ₃₁ S ₁₂ S 22 S ₃₂ S ₁₃ S ₂₃ S ₃₃ R ₀ G ₀ B ₀ However, the matrix coefficients a _ij and S _ij are based on the spectral reflection density characteristics of the dye used to obtain the color hard copy and the spectral sensitivity characteristics of the original image reading system. It is set so that the color difference is minimized.