JP2751256B2 - Color image output method and apparatus - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image output apparatus such as a color copying machine, and more particularly, to a color image output method and apparatus for accurately reproducing the color of a document.

[Conventional technology]

2. Description of the Related Art Conventionally, for example, in a color copying machine, the density of each color of a document is read by a color image input device, and red,
Green and blue document density signals are generated, and these color document density signals are converted into yellow, magenta, and cyan color material density signals by a color correction circuit. Then, in accordance with these color material density signals, a color image is formed by transferring ink of each color or the like to paper in a color image output device.

For example, when the document is yellow, the density of each color document read by the color image input device is 0 for red, 0 for green, and 1 for blue. However, the density is normalized from 0 to 1. Then, these document densities are converted into color material densities,
Yellow density is 1, magenta density is 0, cyan density is 0
Becomes However, this is the ideal case,
Actually, since the wavelength characteristics of the optical system and the conversion characteristics of the color correction circuit are limited, in addition to the required color, ie, yellow in the above example, unnecessary colors, ie, magenta and cyan, are reproduced in the above example. As a result, color reproducibility is reduced. In terms of color materials, a small amount of the magenta ink and the cyan ink are mixed in the yellow ink.

Therefore, in order to solve this problem, it has been proposed that the output is not performed when the color material density is equal to or less than a certain fixed value. Thereby, unnecessary colors are removed, and only necessary colors are reproduced. Therefore, color reproducibility is improved.

[Problems to be solved by the invention]

However, in the above-described method, when the color material density is equal to or less than a certain fixed value, the output is uniformly stopped, so that
There is a problem that a highlight portion of the document, that is, a light color portion is not reproduced.

Accordingly, it is an object of the present invention to prevent the mixing of unnecessary colors without deteriorating the reproducibility of the highlighted portion of the document by making the input / output characteristics for the required colors different from the input / output characteristics for the unnecessary colors.

[Means for solving the problem]

According to the color image output method of the present invention, in order to achieve the object, red, green and blue original density signals obtained by reading a color original are converted into at least yellow, magenta and cyan color material density signals and output. In the color image output method, a saturation and a hue are obtained based on the original density signal based on the signal, a necessary color and an unnecessary color for a color to be reproduced are determined from the hue, and a color material density is determined for the required color. The input / output characteristics in the signal path are substantially linear, and the input / output characteristics for unnecessary colors are such that the reduction amount increases as the density decreases from a constant density to a low density area. .

In the input / output characteristics of the unnecessary color in a low density region, it is preferable that a change in the reduction amount is large when the saturation is high, and a change in the reduction amount is small when the saturation is low.

The color image output apparatus according to the present invention is a color image output apparatus for converting a red, green, and blue original density signal of a color original into at least yellow, magenta, and cyan color material density signals and outputting the same. Means for calculating the saturation and hue based on the calculated hue, means for determining the required and unnecessary colors for the color to be reproduced from the determined hue, and input / output characteristics in the color material density signal path for the required color. A correction means is provided which is substantially linear, and which reduces the input / output characteristics of unnecessary colors from a constant density to a low density in a low density area, the amount of reduction increasing as the density decreases.

The input / output characteristics of the correction means in the low-density region with respect to the unnecessary color are characterized in that the change in the reduction amount is large when the saturation is high, and the change in the reduction amount is small when the saturation is low.

[Action]

In the present invention, the input / output characteristics for the color material density signal are such that the output is reduced in the low density region for unnecessary colors as shown in FIG. It has a linear shape as shown in the figure. That is, the reduction amount is increased in the low density region for the unnecessary color, but the reduction is not performed for the necessary color. As a result, color mixing of unnecessary colors can be prevented, and a highlight portion can be well reproduced for the necessary colors.

In addition, if the characteristics are switched between unnecessary colors and required colors continuously or stepwise according to the saturation, the output density does not suddenly change and the image does not become unnatural.

〔Example〕

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

FIG. 2 is a block diagram of a color image output apparatus according to the embodiment of the present invention.

For example, red, green,
The blue original density signals D _R , D _G , and D _B are supplied to a color correction circuit 1 and converted into cyan, magenta, and yellow color material density signals D _C , D _M , and D _Y by a well-known matrix operation or the like. You. The original density signals D _R , D _G , and D _B of each color are supplied to a hue detection circuit 2 and a saturation detection circuit 3, and a hue signal H and a saturation signal C are obtained. In the embodiment shown in FIG.
The hue signal H is a signal indicating where the hue of the document exists in the Munsell hue circle. These hue signals H
The chroma signal C and the color material density signals D _C , D _M , and D _Y are supplied to correction circuits 4, 5, and 6 for cyan, magenta, and yellow, respectively. The correction circuits 4, 5, 6 for these colors are, for example,
It consists of a look-up table format ROM as schematically shown in Table 1. In the table,, indicates the input / output characteristics of the color material density shown in FIG.

That is, focusing on one color, the conversion characteristic of the color material density, for example, the ink density, that is, the input / output characteristic in the color image output device has the characteristic as shown in FIG. Now, when the target color is a necessary color, the input / output characteristic of the color material density becomes linear as shown in the characteristic of FIG. 5, and when it is an unnecessary color, as shown in the characteristic of FIG. , when there is an input density becomes constant concentration D ₂ or less, the output density is drastically reduced, the input density is the output disappears D ₁ below.

Further, the input / output characteristics for the unnecessary colors are not fixed to the characteristics but are changed according to the saturation. That is, when the saturation is low, the characteristics are substantially linear over the entire density region, but when the saturation is high, when the input density falls below a certain density, the output density sharply decreases. Further, the change of this characteristic is not performed in a binary manner, but the degree of reduction also changes depending on the saturation, and approaches the input / output characteristic at high saturation (see the characteristic in FIG. 1), and the input / output characteristic at low saturation. It approaches the characteristic (see FIG. 1 characteristic).

The necessary color means the color of the color material necessary to reproduce the target color, and the unnecessary color means the color of the color material which is originally unnecessary but is actually mixed. There is a relationship shown in the table.

Now, assuming that the color of the document is bright yellow, the document density of each color read by the color image input device is high in red density and green density and low in blue density. Then, these document densities are converted into color material densities, so that the yellow density is high and the magenta and cyan densities are low.

At this time, the hue detection circuit 2 detects which hue region of the red, yellow, green, cyan, blue, and magenta colors of the original belongs to the Munsell hue circle. That is,
Here, it is detected that the color of the document belongs to the hue area of yellow, and it is found that the necessary color is yellow and the unnecessary colors are magenta and cyan.

Based on the output of the hue detection circuit 2, the input / output characteristics of the yellow correction circuit 6 are set to the input / output characteristics shown in FIG. 1, and the input / output characteristics of the magenta correction circuit 5 and the cyan correction circuit 4 are set to FIG. The input / output characteristics shown by. As a result, for yellow, which is a necessary color, the output changes linearly with respect to the input, whereas for unnecessary colors, that is, magenta and cyan, the output in the low-level region is reduced. , And no low-level component is output for the unnecessary color.

Therefore, unnecessary colors do not mix with the required colors, and the required colors can be accurately reproduced up to the highlight portion, that is, the light portion.

Further, in the present embodiment, as described above, a plurality of input / output characteristics are prepared between the input / output characteristics, and according to the saturation detected by the saturation detection circuit 3, The input / output characteristics are switched by switching the table. That is, when the saturation is low,
That is, when the color is close to gray, the input / output characteristic close to the input / output characteristic is used, and when the saturation is high, the input / output characteristic close to the input / output characteristic is used. As described above, by switching the input / output characteristics according to the saturation, the image does not become unnatural due to a sudden change in the color tone.

In other words, the present embodiment prevents the occurrence of color mixture by rapidly reducing unnecessary colors in a low-density region for a high-saturation image in which color mixture is conspicuous,
Gray is originally composed of three colors and mixed colors are hardly conspicuous, and is output linearly to improve reproducibility in a low density region.

In the above-described embodiment, since the input / output characteristics are switched by switching the table, the control is stepwise. However, by increasing the number of input / output characteristics, substantially continuous control is performed. be able to. Further, if the input / output characteristics are controlled using a variable analog non-linear circuit capable of changing the linear characteristic by an external signal, complete continuous control can be performed.

In the above-described embodiment, the hue detection circuit 2 and the saturation detection circuit 3 are provided independently for detecting the hue and the saturation. However, the circuit for detecting the hue and the saturation in the color image output device. If exists, that circuit can be used.

Hereinafter, such an embodiment will be described with reference to FIGS. 3 and 4. FIG.

FIG. 3 is an overall block diagram of a color copying machine to which the color image output method of the present invention is applied.

The output of the color image input device 11 for reading a color original is supplied to the RGB color separation circuit 12, the red, green, document density signal D _R of blue, D _G, is converted to D _B, further, HVC conversion circuit 13 As a result, it is converted into a hue signal H ₀ , a lightness signal V ₀ , and a saturation signal C ₀ . Note that RGB means red, green, blue, and HVC means
hue, value, chroma. These signals H ₀ , V ₀ , C ₀
Are supplied to the HVC adjustment circuit 14. An operation panel unit (not shown) is connected to the HVC adjustment circuit 14, and the hue, lightness, and saturation signals H ₀ , V ₀ , and C _{0 are} input according to instructions from the operation panel unit.
Is to be adjusted.

The adjusted hue, lightness, and saturation signals H ₁ , V ₁ , and C ₁ are again converted into red, green, and blue density signals D _R , D _G , and D _B by the HVC inverse conversion circuit 15.
After that, the color correction circuit 16 converts them into yellow, magenta, and cyan color material density signals D _Y , D _M , and D _C.

These color material density signals D _Y , D _M and D _C are END (equivalent
After being converted into a corresponding equivalent neutral density signal by a neutral density conversion circuit 17, the signal is supplied to a color correction circuit 18.
The hue signal H ₁ from the HVC adjustment circuit 14 is supplied to the color correction circuit 18.
And chroma signal C ₁ is supplied. Note that the color correction circuit 16
The color correction circuit corresponds to the color correction circuit 1 shown in FIG.
Reference numeral 18 corresponds to the cyan, magenta, and yellow correction circuits 4, 5, and 6 shown in FIG. That is, the color correction circuit 18
Performs the same conversion as the look-up table shown in Table 1 based on the detected hue and saturation. HVC
The hue and saturation may be detected based on the output of the conversion circuit 13.

In the black amount determining circuit 19, a black signal for inking is generated from the color-corrected equivalent neutral density signal, and in the under color removing circuit 20, the above-mentioned equivalent neutral density of yellow, magenta and cyan is obtained. Subtract the black signal from the signal, and vice versa
Colorant concentration signal D _Y the equivalent neutral density signals in END converter 21, D _M, reconverted to D _C. These color material density signals D _Y , D _M ,
Black signal from D _C and black ink quantity determination circuit 19, the gradation correction circuit 22
In, gradation correction is performed according to the output characteristics of the color image output device. Finally, a color copy is obtained by adhering yellow, magenta, cyan, and black coloring materials corresponding to the image of the document to the paper.

Next, adjustment of hue, lightness and saturation using the above-described HVC conversion circuit 13, HVC adjustment circuit 14, and HVC inverse conversion circuit 15 will be described with reference to FIG.

 First, the hue adjustment will be described.

In this embodiment, the original density signal D _R, D _G, Noting that can determine the position of the color wheel from the magnitude relation of D _B, the density signal D _R, D _G, approximates the hue angle H from D _B I do. Here, for the sake of simplicity, the signal name and the signal value or the hue angle are represented by the same reference numerals.

 Hereinafter, a procedure for approximating the hue angle H will be described.

 First, the approximation by the reflectance expression will be considered.

Red, green, and blue reflectance signals are denoted by R _R , R _G , and R _B, and are arranged in descending order. That is, max (R _R , R _G , R _B ), mid (R _R , R _G , R _B ), min
(R _R , R _G , R _B ) is obtained.

For example, when R _R > R _G > R _B , max (R _R , R _G , R _B ) = _R mid (R _R , R _G , R _B ) = R _G min (R _R , R _G , the R _B) = R _B.

In the following description, max (R _R , R _G , R _B ), mid (R _R , R
_G , R _B ), and min (R _R , R _G , R _B ) are simply represented by max, mid, and min, respectively.

min represents a white component. Therefore, the hue range is divided by the six hue axes R _Y , G _Y , G _Y , Which part of G _C , B _C , B _M , or R _M belongs to is determined. The angles of the hue axes are 0 degree, 60 degrees, 120 degrees, 180 degrees, 240 degrees, and 300 degrees, respectively.
Degrees.

Table 3 shows the relationship between the magnitude relationship of the reflectance and the hue region.

Here, the hue angle ratio _Hr is In the definitions, the hue angle ratio H _r varies in a range of 0 to 1,
It is 0 when it is on the red, green, and blue axes, and is 1 when it is on the yellow, cyan, and magenta axes.

Therefore, the reflectance signal R _R, R _G, the magnitude relation of R _B, hue can identify whether belonging to any of the six hue regions, further, the hue angle H by hue angle ratio H _r is, H = F ( H _r ).

Where the function F is It is a function that becomes

Next, the hue angle H thus defined is converted into a Munsell hue by a conversion table shown in Table 4. This is because the hue angle obtained by the above-described calculation does not always correspond exactly to the hue angle of the Munsell hue circle, and thus requires correction. The intermediate angle is obtained by interpolation.

Next, the approximation based on the reflectance expression is converted into the approximation based on the density.

When the density is D _i and the reflectance is R _i , D _i = log ₁₀ R _i . However, if the density D _i is regarded as an expression of the absorption rate A _i (= 1−R _i ), the expression (1) see, can define the hue angle ratio H _r by the density by the following equation.

This change in hue angle ratio H _r is the same as the case of approximation by the reflectance representation. In the following description, max
(D _R , D _G , D _B ), mid (D _R , D _G , D _B ), and min (D _R , D _G , D _B ) are simply represented by max, mid, and min, respectively.

The hue angle division by the hue region division and the function F can be performed in the same manner as the approximation by the reflectance expression.

Obtain the above hue angle H. Therefore, the three-color density signals D _R from RGB color separation circuit 12, D _G, is D _B, is supplied for color conversion provided HVC conversion circuit 13 ROM (not shown). The hue conversion ROM is a look-up table that receives the three-color density signals D _R , D _G , and D _B and outputs the hue obtained by the above calculation. Therefore, from the hue conversion ROM, 3
Color density signal D _R, D _G, the hue signal H ₀ corresponding to D _B obtained.

This hue signal H ₀ is supplied to a hue adjustment circuit (not shown) provided in the HVC adjustment circuit 14. The hue adjustment circuit independently performs the specified hue angle adjustment for each of the red, green, and blue regions, and includes a look-up table format ROM. The hue adjustment circuit includes a hue signal H ₀ , a red area adjustment signal S _R , a green area adjustment signal S _G , and a blue area adjustment signal.
As input S _B, to adjust the hue within the range of the hue shown in Table 5.

If the input hue angle H belongs to any of the three regions, the hue angle is adjusted in the relationship of H ₁ = H ₀ + 2 × f _H. However, H ₁ is the hue angle after adjustment, f _H is the adjustment factor. Adjustment factor f _H is changed by adjusting the operation panel unit.

 Next, the brightness adjustment will be described.

In the present embodiment, three-color density signals D _R, D _G, obtained by the following approximate expression luminous density D from D _B.

D = α ₁ × D _R + α ₂ × D _G + α ₃ × D _B = 0.5 × D _R + 0.45 × D _G + 0.05 × D _B Further, from this luminous density D, the lightness V is obtained by the following approximate expression.
Ask for.

However, β: 2.362 The above approximation calculation is performed by a brightness conversion ROM (not shown) provided in the HVC conversion circuit 13. Note that β
Is not limited to the above value, and the brightness V can be sufficiently approximated in the range of 2.30 ≦ β ≦ 2.45.

The brightness conversion ROM is a look-up table that receives the three-color density signals D _R , D _G , and D _B and outputs the brightness obtained by the calculation. Therefore, from the lightness conversion ROM, 3-color density signals D _R, D _G, the lightness signal V ₀ corresponding to the D _B obtained.

This brightness signal V ₀ is supplied to a brightness adjustment circuit (not shown) provided in the HVC adjustment circuit 14. Brightness adjustment circuit is constituted by a ROM look-up table format, when the brightness of the output to lightness V ₀ which input was V _1, The following calculation is performed.

Here, f _V in is the degree of adjustment that changes in accordance with an instruction from the operation panel unit, for example, by varying in a range of "2.5" - "5.5", and adjust the brightness V, copy density as a result Can be adjusted.

Finally, the saturation adjustment will be described. In this embodiment, the saturation C is obtained from the maximum value max, the minimum value min, and the lightness V of the density previously obtained by the following approximate expression.

C = γ × V × (max−min) Here, γ: 2.44 The above-described approximate calculation is performed by a saturation conversion ROM (not shown) provided in the HVC conversion circuit 13. Note that γ
Is not limited to the above value, and the saturation C can be sufficiently approximated in the range of 2.30 ≦ γ ≦ 2.60.

The saturation conversion ROM is a look-up table that receives the three-color density signals D _R , D _G , and D _B and outputs the saturation determined by the above calculation. Therefore, from the chroma conversion ROM, 3-color density signals D _R, D _G, chroma signal C ₀ corresponding to the D _B obtained.

The saturation signal C ₀ is supplied to a saturation adjustment circuit (not shown) provided in the HVC adjustment circuit 14. Chroma adjustment circuit is constituted by a ROM look-up table format, when the saturation of the output was set to C ₁ with respect to the saturation C ₀ input, The following calculation is performed.

Here, f _C is an adjustment coefficient instructed from the operation panel unit. For example, the saturation C can be adjusted by changing the adjustment coefficient in a range of “5” to “11”.

As described above, in the present embodiment, the density information is once converted into saturation, brightness, and hue information, and after adjusting each of these information, the density information is combined with other information as described later. Since it is converted into a signal, saturation, brightness and hue can be adjusted independently.

The hue, lightness, and saturation signals H ₁ , V ₁ , and C ₁ adjusted as described above are supplied to the HVC inverse conversion circuit 15, and again, the red, green, and blue density signals D _R , It is converted to D _G and D _B.

That is, first, the density D is obtained from the lightness V, the max-min is obtained from the lightness V and the saturation C, and the hue angle H is inversely converted by a look-up table to obtain the hue angle ratio H _r And hue area information. Further, the max-min and hue angle ratio H _r, determine the max-mid.

By the way, the relationship among the densities D, max, mid, and min is given in the form of Table 6 for each hue region.

Therefore, for each hue area, the density D, max-min, max-mid
Max, mid, min can be determined from the values of D _R ,
Correspondence to D _G and D _B is also required.

For example, that the hue region R _Y, and examples of conversion in _{D B> D G> D R} , Becomes

The min, mid, and max are assigned to the respective color densities based on the hue area information obtained previously.

In this way, the hue, lightness, and saturation signals H, V, C
Is again converted into red, green, and blue density signals D _R , D _G , and D _B ,
It is supplied to the color correction circuit 16.

Note that, in the above-described embodiment, for ease of understanding, the description has been made by dividing into blocks according to functions.
As shown in FIG. 4, from the HVC inverse conversion circuit 15 to the gradation correction circuit 22, the ROMs 30Y, 30M, 30 are used as lookup tables for the respective color materials of yellow, magenta, cyan, and black.
C, 30K.

That is, a table is formed in a form in which all the intermediate calculation processes are integrated, and the signals H ₀ , V ₀ , and C _{0 of} hue, lightness, and saturation are formed.
And the number of dots for each color material of yellow, magenta, cyan, and black in the color image output device 23
N _Y , N _M , N _C , and N _K are directly output. For this reason, in the portion after the HVC inverse conversion circuit 15, the density signal and the like do not occur on the circuit, but are incorporated in the table in the form of coefficients. In the present embodiment, the color material signal is not limited to a signal that changes in an analog manner directly corresponding to the density, but also includes the number of dots in the color image output device 23.

According to the embodiment shown in FIG. 3, the hue and saturation signals generated for adjusting the hue and saturation can be used, so that there is no need to provide a dedicated hue detection circuit and saturation detection circuit. There is.

〔The invention's effect〕

As described above, according to the present invention, the input / output characteristics for the unnecessary colors are such that the amount of reduction increases as the density decreases from a constant density to a low density in the low density region. On the other hand, the input / output characteristics in the color material density signal path are substantially linear. Thus, for example, when a yellow document is reproduced, unnecessary colors magenta and cyan can be removed while accurately reproducing yellow as a necessary color even when the input density is low. Therefore, unnecessary color mixing can be prevented without impairing the reproducibility of the highlighted portion of the document.

[Brief description of the drawings]

FIG. 1 is a graph showing a change in input / output characteristics of a color material density in the color image output device of the present embodiment, FIG. 2 is a block diagram of a main part of the color image output device, and FIG. FIG. 4 is a block diagram of a color copying machine to which the image output device is applied, and FIG. 4 shows a look-up table type ROM for directly outputting the number of dots for each color material in the color image output device from each signal of hue, lightness and saturation. It is a block diagram. 1: Color correction circuit, 2: Hue detection circuit 3: Saturation detection circuit, 4: Cyan correction circuit 5: Magenta correction circuit, 6: Yellow correction circuit 11: Color image input device 12: RGB color separation circuit, 13: HVC Conversion circuit 14: HVC adjustment circuit, 15: HVC inverse conversion circuit 16: Color correction circuit, 17: END conversion circuit 18: Color correction circuit, 19: Black amount determination circuit 20: Under color removal circuit, 21: Reverse END conversion circuit 22: gradation correction circuit, 23: color image output device 30Y: ROM for yellow conversion 30M: ROM for magenta conversion 30C: ROM for cyan conversion 30K: ROM for black conversion

Claims (4)

(57) [Claims] 1. A color image output method for converting and outputting at least red, green and blue original density signals of a color original into respective yellow, magenta and cyan color material density signals and outputting the same based on the original density signals And the hue are determined, and the necessary and unnecessary colors for the color to be reproduced are determined from the hue. For the required color, the input / output characteristics in the color material density signal path are made substantially linear. 3. A color image output method according to claim 1, wherein said input / output characteristic is a characteristic in which a reduction amount increases as the density decreases from a constant density to a low density region. 2. The input / output characteristics of the unnecessary color in a low density region are such that a change in the reduction amount is large when the saturation is high, and a change in the reduction amount is small when the saturation is low. The color image output method according to claim 1. 3. A color image output apparatus for converting a red, green, and blue original density signal of a color original into at least yellow, magenta, and cyan color material density signals and outputting the converted signal. And a means for determining a hue, a means for determining a required color and an unnecessary color for a color to be reproduced from the determined hue, and an input / output characteristic in a color material density signal path for the required color being substantially linear, Correcting means for reducing the input / output characteristics of the unnecessary colors from a constant density to a low density in a low density area as the density decreases. 4. The input / output characteristic of the correction means in a low-density region for the unnecessary color is such that a change in the reduction amount is large when the saturation is high, and a change in the reduction amount is small when the saturation is low. 4. The color image output device according to claim 3, wherein: