JPH0614315B2 - Color correction method - Google Patents

Description

The present invention relates to a color correction method, and more particularly to a recording device such as a color printer based on a color separation signal obtained from an input device such as a color television tuner or a color scanner. The present invention relates to a color correction method for performing color reproduction faithful to image information when recording with.

[Prior art and its problems]

In the field of color recording such as color printing and color copying machines,
A color original is color-separated, processed, and reproduced.The color signal is often affected by the spectrum of the illumination light source, the characteristics of the dichroic mirror, and the color characteristics of the photoelectric conversion element. There are many factors that affect color reproducibility, such as the color characteristics of color photographic paper or color ink. Therefore, it is extremely difficult to faithfully reproduce the color of the original document as it is.

Therefore, in order to faithfully reproduce the color of the original, highlight level adjustment, gamma adjustment, masking adjustment, under color removal (U
Various color corrections such as (CR) adjustment and gradation adjustment are performed.

Among them, the masking process is for adding or subtracting in proportion to the density of each signal, controlling the color tone during color reproduction while maintaining color balance, and improving saturation, and is used for recording color images. Since the color materials such as cyan, magenta, and yellow inks, toners, and ink donor films have unnecessary absorption due to their spectral characteristics, saturation,
It has an effect of correcting various characteristics relating to color, such as when the hue is out of alignment and a preferable image cannot be obtained.

Usually, red (R), green (G), to facilitate processing,
The blue (B) signal, the yellow (Y) signal, the magenta (M) signal, and the cyan (C) signal are the E. N. Often expressed as D (equivalent neutral concentration). In this case, the color separation signal when an achromatic color is read is R = G = B (hereinafter, all equivalent neutral density), and conversely, C = M = Y (hereinafter, equivalent neutral density) signal. Is sent to the output device (recording unit), an achromatic color is reproduced.

However, R of the color separation signal corresponding to a pixel,
This masking process is performed on an achromatic component in which G and B have a relationship of R = G = B, that is, When a is performed, since a _ij is conventionally determined from unnecessary absorption densities of solid printing of C, M, and Y, the output signals C, M, and Y of cyan, magenta, and yellow corresponding to this pixel are determined.
Was independently determined without maintaining the relationship of C = M = Y, and it was difficult to reproduce an achromatic color.

Therefore, a method has been proposed in which a circuit is separately provided for an achromatic signal, that is, a signal of R = G = B so that masking processing is not performed, but the circuit becomes complicated and
There is a problem that a color difference due to a difference in processing between a color close to an achromatic color and an achromatic color is noticeable, and an unnatural image is likely to be formed.

The present invention has been made in view of the above circumstances, facilitates reproduction of an achromatic color without complicating the circuit configuration, and maintains a smooth connection between a color close to an achromatic color and an achromatic color without separation. An object of the present invention is to provide a good masking correction method capable of

[Means for solving problems]

Therefore, in the present invention, cyan (C), magenta (M), and yellow (Y) color recording signals are generated based on the equivalent neutral densities of the red (R), green (G), and blue (B) color separation signals. When correcting the discoloration due to the error of the image recording device when outputting, L _1s : first column number of primary group (1) L _1t : last column number of primary group (3) L _2s : first column number of secondary group (4) L _2t : last column of secondary group Number (9) L _3s : First column number of the third group (10) L _3t : Last column number of the third group L _ns : First column number of the nth group L _nt : Last column number of the nth group ( m) When the terms obtained by multiplication from the three transformations of R, G, and B are grouped into the terms of the primary group, the terms of the secondary group, ... The color correction matrix a _{ij that} satisfies the constraint condition that
It is characterized by using.

In the second aspect of the present invention, the colors of cyan (C), magenta (M), and yellow (Y) are calculated based on the equivalent neutral densities of the red (R), green (G), and blue (B) color separation signals. When correcting the discoloration due to the error of the image recording device when outputting the recording signal, a _i1 + a _i2 + a _i3 = 1 (i = 1,2,3) and a _i4 + a
It is characterized in that a color correction matrix a _ij that satisfies a constraint condition such that _i5 ... a _i9 = 0 (i = 1, 2, 3) is used.

[Action]

FIG. 1 shows a basic configuration of an image forming apparatus for implementing the method of the present invention, which is separated into three colors by using a dichroic mirror or the like and read by an image reading unit 1 which is read by using an image sensor or the like. The color separation signals D _R , D _G , and D _B of the three colors R, G, and B thus obtained are converted into equivalent neutral densities in the input correction device 2 and read as color separation signals R, G, and B, and masking processing is performed. After the color correction processing is performed in the device 3, the output value conversion device converts the image into a form suitable for the image recording unit 5, and the image output device 4 reproduces the image.

For example, When a masking processing device that satisfies a _i1 + a _i2 + a _i3 = 1 and a _i4 + a _i5 ...... a _i9 = 0 is used, R, G, B
There When a respective equivalent neutral density _{E, C = (a 11 +} a 12 + a 13) E + (a 14 + a 15 + a 16 ... a 19) = E 2 = E M = (a 21 + a 22 + a 23) _{E + (a 24 + a 25} + a 26 ... a 29) = E 2 = E Y = (a 31 + a 32 + a 33) E + (a 34 + a 35 + a 36 ... a 39) = E 2 = E therefore C = M = Y, and the achromatic color is reproduced correctly.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 2 is a diagram showing the configuration of the halftone image forming apparatus according to the embodiment of the present invention.

This apparatus includes a power calculation circuit 6 for calculating the exponentiation of the signals R, G, and B that have been color-separated by a red filter, a green filter, and a blue filter, respectively.
A cyan signal operation circuit for calculating cyan, magenta, and yellow recording signals from a multiplication circuit 7 for calculating each product and a correction matrix determined based on the exponentiation and product of these color separation signals and color correction conditions. 8, a magenta signal arithmetic circuit 9, and a yellow signal arithmetic circuit 10. These cyan signal arithmetic circuit 8, magenta signal arithmetic circuit 9, and yellow signal arithmetic circuit 10 are cyan signal adjusting circuit 11 and magenta signal adjusting circuit, respectively. 12 and the yellow signal adjusting circuit 13, and the correction matrix is a _i1 + a _i2 + a _i3 by these respective color signal adjusting circuits.
= 1, a _i4 + a _i5 ...... a _i9 = 0 is adjusted so as to always satisfy the constraint condition.

For example, the correction matrix adjusted by each color signal adjustment circuit In and, a ₁₁ ... a ₁₉ a cyan signal calculation circuit, a a ₂₁ ... a ₂₉ is the yellow signal operation circuit magenta signal operation circuit
₃₁ ... a ₃₉ are input as coefficients.

Here, when each color separation signal is an ideal gray input, for example, (R, G, B) = (0.7,0.7,0,7), the output signal (C, M, Y) of each color = (0.7,0.
7, 0.7) was obtained, and completely achromatic color reproduction was realized.

In addition, for skin colors that the general person is extremely sensitive to discoloration, the input signals (R, G, B) = (0.24, 0.
46, 0.52) output is (C, M, Y) =
(0.16, 0.39, 0.51), and the desired output (C ₀ , M ₀ , Y ₀ ) as the skin color = (0.16,
It is a value very close to 0.41, 0.47).

Furthermore, when we tried to reproduce 8 kinds of achromatic colors with different densities,
An average color difference of 2.1 (CIELab) was achieved, and almost ideal achromatic color reproduction was confirmed.

Furthermore, when color patches of 45 colors were individually reproduced, an average color difference of 5.2 was obtained, and good color reproducibility could be realized over all colors.

Although the color correction is performed in this way, in the reproduction of an actual document, a _i1 + a _i2 + a _i3 = 1 and a _i4
By adjusting the color correction coefficient (matrix) according to the state of the original while maintaining the constraint condition of a _i9 = 0, it is possible to perform better color reproduction without destroying the gray balance.

In the embodiment, the case where the color correction matrix has a 3 × 9 matrix configuration considering up to the second-order item has been described. However, considering up to the higher-order items, the present invention is also applicable to the case of the 3 × n matrix configuration. The method is effective. That is, when outputting the color recording signals of cyan (C), magenta (M), and yellow (Y) based on the equivalent neutral density of the color separation signals of red (R), green (G), and blue (B), the image is output. When correcting the discoloration due to the error of the recording device in advance, L _1s : first column number of primary group (1) L _1t : last column number of primary group (3) L _2s : first column number of secondary group (4) L _2t : last column of secondary group Number (9) L _3s : First column number of the third group (10) L _3t : Last column number of the third group L _ns : First column number of the nth group L _nt : Last column number of the nth group ( m) When the terms obtained by multiplication from the three transformations of R, G, and B are grouped into the terms of the primary group, the terms of the secondary group, ... The color correction matrix a _{ij that} satisfies the constraint condition that
The color correction method is characterized in that

In addition, the items to be considered may be appropriately selected as necessary.

〔effect〕

As described above, according to the present invention, it is possible to ensure color reproducibility for achromatic color image information and to smoothly reproduce colors from achromatic colors to high-saturation colors without color separation.

Further, the masking circuit may have the same circuit configuration as the conventional one, and extremely effective color correction can be realized.

[Brief description of drawings]

FIG. 1 is a block diagram showing the basic configuration of the image forming apparatus of the present invention, and FIG. 2 is a block diagram of the halftone image forming apparatus of the embodiment of the present invention. 1 ... Image reading unit, 2 ... Input correction device, 3 ... Masking processing device, 4 ... Image output device, 5 ... Image recording unit, 6 ...
Power calculation circuit, 7 ... Multiplication circuit, 8 ... Cyan signal calculation circuit, 9 ... Magenta signal calculation circuit, 10 ... Yellow signal calculation circuit, 11 ... Cyan signal adjustment circuit, 12 ... Magenta signal adjustment circuit, 13 ... Yellow signal adjustment circuit .

Claims (2)

[Claims] 1. Color recording signals of cyan (C), magenta (M), and yellow (Y) based on equivalent neutral densities of red (R), green (G), and blue (B) color separation signals. When correcting the discoloration due to the error of the image recording device when outputting, L _1s : first column number of primary group (1) L _1t : last column number of primary group (3) L _2s : first column number of secondary group (4) L _2t : last column of secondary group Number (9) L _3s : First column number of the third group (10) L _3t : Last column number of the third group L _ns : First column number of the nth group L _nt : Last column number of the nth group ( m) When the terms obtained by multiplication from the three transformations of R, G, and B are grouped into the terms of the primary group, the terms of the secondary group, ... The color correction matrix a _{ij that} satisfies the constraint condition that
A color correction method characterized in that the color correction method is used. 2. Cyan (C), magenta (M), and yellow (Y) color recording signals based on the equivalent neutral densities of the red (R), green (G), and blue (B) color separation signals. When correcting the discoloration due to the error of the image recording device when outputting, a _i1 + a _i2 + a _i3 = 1 (i = 1,2,3) and a _i4 + a
A color correction method characterized in that a color correction matrix a _ij that satisfies a constraint condition such that _i5 ... a _i9 = 0 (i = 1, 2, 3) is used.