JPS61120574A - Color image forming method - Google Patents

Abstract

PURPOSE:To reproduce a color image of an original picture with fidelity by applying primary color conversion correcting the relation between a density value and a three primary color separation system. CONSTITUTION:A prescribed matrix conversion is applied by a primary color converting part 20 provided newly by using a parameter where the spectral characteristic of color filters Rlambda', Glambda', lambdaB' in the three color separating section 5 and the spectral characteristic of the color filters Rlambda', Glambda' and Blambda' of a densitometer are represented by a linear combination of spectral three stimulus values xlambda, ylambda and Zlambda and then a complementary color conversion is applied. Then an output DC subject to separate complementary color conversion is measured by a densitometer having a spectral characteristic R'lambda.

Description

[Detailed description of the invention] Technical fields> The present invention relates to a color image forming method.

Hereinafter, explanation will be made regarding a video printer, but as will be described later, the present invention also applies to a color copy 'A' machine, a color scanner,
Photoelectrically scans the original image or copy for color printing, separates it into three colors, obtains R, G, and B signals, performs color processing such as masking, and at least adjusts the density of cyan, magenta, and yellow on the paper. It can be applied to anything that can produce a color image by controlling it.

<Prior Art> A conceptual diagram of a conventional color reproduction method in this type of apparatus is shown in FIG. 1
2B separates 3 colors, photoelectric converters 14R, 14G, 14B
to obtain three primary color signals (R, G, B).

Next, this (R, G, B) signal is converted into (1) by the complementary color conversion section 6.
A computation in the form of the formula is performed to convert each color into C degrees (DC, Dm, Dy) of complementary colors (cyan, magenta, yellow).

(Ro, Go, Bo are R, G, which gives the highest brightness white,
Next, when reproducing the determined density values of each color on paper, the density output converter 7 uses masking processing to obtain the desired values (Dc, Dm, Dy ) is controlled by controlling the dot ratio of each plate. Vc, V
m and Vy are signals indicating the amount of cyan, magenta, and yellow ink. If necessary, black may be used here, but even in that case, the desired (Dc, Dm, D
In order to obtain y), the amount of cyan, magenta, yellow, and black ink must be controlled.

The problem with this conventional method is that the three primary colors R, G, and B are separated into three colors, and the density of their complementary colors (]) c, Dm, Dy)
The problem is that there is no chromatic relationship between the colors, and no measures have been taken to compensate for the discrepancies in color reproduction that will occur.

Depending on the spectral characteristics of the color filter used for the three-color separation, the R, G, and B signals separated into the three colors may become completely different depending on the filter used to perform the same signal manually.

Also, its complementary color k K D C+ D m , D y
If the spectral filter used in the densitometer used for measurement is not specified, even if the same thing is measured, the values will be completely different depending on the densitometer used.

This means that the spectral characteristics of the color separation filter of the three-color separation system match the spectral characteristics of the color separation filter used for concentration measurement, but if they do not match, then some correction is not necessary. This means that it cannot be reproduced.

Traditionally, this type of problem has been at the root of color reproduction.

An object of the present invention is to solve the above-mentioned problems and to provide a color image forming method capable of faithfully reproducing a color image of an original image.

<Example> A generalized block diagram of the configuration of the present invention is shown in FIG.

In FIG. 2, parts having the same functions as those in FIG. 1 are given the same reference numerals. Color filter R in the three-color separation section 5
The spectral characteristics of λ, Gλ, Bλ and the color filters in the densitometer (λ, G゛λ, B゛λ) are approximated as follows by a linear combination of spectral tristimulus values, 7λ, and iλ. Ru.

This approximation formula can be easily determined by measuring the spectral characteristics of the actual filter and applying the least squares method thereto.

Furthermore, if the density value is determined by the "optical density measurement method for colored light" proposed separately by the applicant, the parameters used at that time may be used as they are for the coefficients of (b-j). At this time, the newly provided primary color conversion unit 20 performs the following matrix conversion.

After that, complementary color conversion may be performed according to equation (1).

The reason for doing so is as follows.

The outputs R, G, and B of the three-color separation system have the characteristics of the spectral filter.

Therefore, the output R'G'B' obtained by converting this into the primary colors is as follows.・Furthermore, the outputs Dc, Dm, and Dy obtained by performing complementary color conversion are as follows. (Xo, Yo, Zo are tristimulus values of 0 (for example, standard white board)) Z=K l fρxzλPλdλ (9) For example, Dc is 1
From this equation, which is multiplied as follows, we can see that Dc must be the concentration measured with a densitometer that has λ as its spectral characteristic (however,
1! In other words, if there is no newly installed primary color converter, the density value used for processing and the density value actually measured will be completely different. In principle, it is impossible to obtain accurate color reproduction.

FIG. 3 is a block diagram of color processing when the present invention is applied to a color video printer connected to an NTSC television system. In FIG. 0, 21 is an NTSC decoder, 22
23 is a primary color converter, 23 is a complementary color converter, and 24 is a density output converter.

In this case, the density value used is measured using the method of calculating from xYZ shown in the above-mentioned "Optical density measurement method for one color light".

That is, (x, y, z) of the sample is measured using a colorimeter, and the density value is calculated using equation (8). In this case, matrix B can be set arbitrarily.

shall be.

On the other hand, NTSC color television signals were originally
This is the result of three-color separation using a color TV camera, etc.
This is specified in the NTSC standard. Therefore, using the same parameters, the characteristics of the three-color separation filter of an ideal color camera are as follows.Therefore, in the system shown in FIG. 3, the conversion matrix T of the primary color conversion section 22 may be as follows.

Also, as another implementation method, conversely.

In addition, in FIG. 3, another method (PAL) is used.

SECAM) has a similar configuration and can be handled by simply changing the coefficients of the primary color conversion matrix.

In addition, even in the case of color scanner input, the characteristics of the scanner's three-color separation filter are approximated as a linear combination of spectral tristimulus values using the least squares method, and the weight parameters at this time are used to calculate the primary colors. All you have to do is construct a transformation matrix.

Figure 4 is a block diagram of the signal processing of a color printer that can input two inputs of different primary colors.
Reference numerals A and 22B indicate different primary color converting units provided in accordance with, for example, an NTSC system input and an input from a color reading device, and 25 indicates an input changeover switch. By switching the switch 25 according to the input system, optimum color reproduction according to the input system can be achieved.

Effect> As described above, according to the present invention, by performing primary color conversion that corrects the relationship between the density value and the three-color separation system, it is possible to perform accurate color reproduction even when using the density value. becomes.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram for implementing the conventional color reproduction method;
3 is a circuit diagram of one embodiment of the present invention, FIG. 3 is a circuit diagram of another embodiment, and FIG. 4 is a circuit diagram of still another embodiment. In the figure, 5 is a three-color separation section, 6 is a complementary color conversion section, 7 is a density-output conversion section, and 20 is a primary color conversion section.

Claims (2)

[Claims] (1) When forming a color image using three-color separated RGB signals, the spectral characteristics of the filter for three-color separation are determined by parameters expressed as a linear combination of spectral tristimulus values. A color image forming method characterized by performing primary color conversion. (2) The color image forming method according to item 1, wherein the primary color conversion to be used is switched depending on the input in response to a plurality of inputs having different primary color systems.