JPH0547016B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a method for correcting color image information, and in particular to obtaining a copy of an original image by printing or the like from color image information given by a color image signal based on RGB three primary color data. Regarding the color correction method in case.

[Background of the invention]

Conventionally, so-called printing plate-making scanners have been widely used for printing color images, but in recent years, as various data processing techniques using computers have been proposed and put into practical use, such scanners have become increasingly popular. By applying various data processing techniques to this image data after converting it into electrical image data, it has become possible to obtain even more effective printed images. There are color correction techniques.

Incidentally, the methods conventionally used for such color correction have been mainly based on the Neugepauer equation or the photographic masking method.

However, these conventional color correction methods require a large number of parameters to be considered, require a high level of skill for color correction work, and are difficult to manage colors that match the three attributes of color perception: hue, saturation, and brightness. It had the disadvantage of being.

Therefore, in order to eliminate these drawbacks of the conventional technology, color image information is separated into three components: hue, cloudiness, and gradation. A color correction method that can be easily and accurately controlled has been filed as an invention in Japanese Patent Application No. 156,514/1985, and this previously filed method will be described below.

In this previously applied method, each of the three primary color data of color image information is rearranged for each pixel to form three data arranged in order of size, and based on these three data, the hue and cloudiness are determined for each pixel.
In addition, information representing each gradation is obtained, so as shown in Figure 2, the color image information is preprocessed.

Separation conversion processing of three attribute data based on preprocessed signals.

Synthesis processing of 3 attribute data.

Post-processing consisting of sumi version calculation and UCR.

Through these four processes, color-corrected data for the original image data is obtained.

(Preprocessing) The contents of this process are as follows.

(1-1) Color information (r, g, b) is input for each pixel of color image information read from the original image using a scanner or the like and stored in advance in memory.

(1-2) Normalize the magnitude of color information (r, g, b) between 0 (low brightness) and 1 (high brightness),
Next, this is converted into a complement to obtain complementary color information (c, m,
y). That is, 0≦r, g, b≦1 c△ = 1-r, m△ = 1-g, y△ = 1-b.

(1-3) Sorting the complementary color information (c, m, y) and arranging them in descending order.The largest one is data a 1 , the second one is data a ₂ , and the third one is data a ₁ .
Define a ₃ .

Here, (a ₁ , a ₂ , a ₃ ) is referred to as preprocessed data.

(Separation conversion process) This process converts the preprocessed data (a ₁ , a ₂ , a ₃ )
It consists of the following three processes that proceed in parallel based on the following.

(a) Separation and conversion processing of hue information (c ₁ , m ₁ , y ₁ ).

(b) Separation and conversion processing of cloudy information (c ₁ ′, m ₁ ′, y ₁ ′).

(c) Separation and conversion processing of gradation information g.

(Separation and conversion processing of hue information (a)) (a-1) The factors that determine the hue are determined as follows.

(a ₁ − _{a 3} ) and (a ₂ − _{a 3} ) However, in the case of (a ₂ ≠ a ₃ ).

(a ₁ − a ₃ ) However, in the case of (a ₂ = a ₃ ).

This is due to the following reason. That is, (a ₂ ≠ a ₃ )
This is because the data a ₃ when (a ₂ = a ₃ ) and the data a ₂ and a ₃ when (a 2 = a 3 ) each function only as cloudy components and do not serve as factors that change the hue.

And the hue in the above case is (a ₂ −
a ₃ )/(a ₁ − _{a 3} ), as shown in Figure 3, the data is a ₁ among the points that internally divide the real number line on the diagram in which the hues are arranged in order of wavelength. The point immediately before any of (c, m, y),
In the above case, data a is ₁ (c,
m, y) is the point where one of them is located.

For example, data (c, m, y) is c=0.3,
Assuming that m=0.1 and y=0, in this case,
a ₁ = c = 0.3, a ₂ = m = 0.1, a ₃ = y = 0,
Therefore, since (a ₂ ≠ a ₃ ), (a ₂ − a ₃ )/(a ₁ −
a ₃ ) = 1/3 and a ₁ = c (cyan), the third
The point * in the figure is the hue of the above data.

(A-2) Next, as shown in Figure 4, the horizontal axis shows the hue in order of wavelength, and the vertical axis shows the amount of ink (called dot % or sepa density) at the highest gradation for each hue. By preparing the obtained hue table and converting it by referring to the above hue position, hue information corresponding to the amount of ink at the highest gradation required for each plate (separation plate) of C, M, and Y can be obtained. (c ₁ , m ₁ , y ₁ ) can be obtained.

(Separation and conversion processing of cloudy information (b)) (b-1) The amount of cloudiness is expressed by the amount of the opposite color (complementary color) contained in a certain color,
Therefore, the opposite colors for the original data (c, m, y) are (1-c, 1-m, 1-y). Sorting this and arranging it in descending order (a ₃ ,
a ₂ , a ₁ ), and therefore, a ₃ /a ₁ is the ratio of cloudiness to be added to the hue information (c ₁ , m ₁ , y ₁ ).

(B-2) Therefore, by using the data (c ₁ , m ₁ , y ₁ ) obtained in the process of (A-2) above and the data a ₁ , a ₃ , (c ₁ ′, m ₁ ′, y ₁ ′) = a ₃ / a ₁ (1-c ₁ , 1-m ₁ , 1-y ₁ ) ...(5) and obtain the cloudy information (c ₁ ′, m ₁ ′, y ₁ ′) do it like this.

(Separation and conversion processing of gradation information (c)) (c-1) The gradation can be expressed by the value of the data _a1 itself. This is because in preprocessing, color information (r, g, b) changes from minimum value = 0 to maximum value =
This is because it is normalized to 1, and therefore, the maximum gradation of the hue is always when a ₁ =1.
In other words, the gradation is given by the ratio of the data a ₁ at that time to the maximum value of this data a ₁ , but the data a ₁
Since the maximum value of is 1, the gradation is (a ₁ /1)=a ₁ .

(C-2) Next, prepare a gradation conversion table as shown in FIG. 5, and refer to this table using data _a1 to obtain gradation information g with desired characteristics.

(Synthesizing Process) This process is to obtain dot % or sepa concentration data c ₂ , m ₂ , y ₂ necessary for each separation plate other than the black plate, and the calculation process is as follows.

c ₂ = (c ₁ + c ₁ ′) × g m ₂ = (m ₁ + m ₁ ′) × g y ₂ = (y ₁ + y ₁ ′) × g ... (6) (Post-processing) Here, the sumi version Create data _k2 that represents the amount of ink required for the process, and perform processing when performing UCR (undercolor removal).

(4-1) In the sumi-ban calculation, data k ₂ is first defined and calculated as follows.

k ₂ △ = min (c ₂ , m ₂ , y ₂ ) where min represents a function that gives the minimum value.

(4-2) Prepare the sumi version table shown in Figure 6, and use it to calculate the final data to be used from data k _2.
By converting to _k3 , it is possible to arbitrarily realize either full black or skeleton black by selecting characteristics a and b.

(4-3) When applying UCR, the final data (c ₃ , m ₃ , y ₃ ) should be obtained by the following calculations.

c ₃ = c ₂ −α・k ₃ m ₃ = m ₂ −α・k ₃ y ₃ = y ₂ −α・k ₃ ...(7) However, α: UCR rate 0α≦1 Based on the data obtained in this way, cyan and magenta,
By exposing each of the yellow separation plates and the blackout plate, it is possible to obtain a printed matter with the desired color development.

According to this method, the color correction work is
For hue, the task is to select the characteristics of the hue table shown in Figure 4, and for cloudiness, the task is to adjust the magnitude of a ₃ /a ₁ in equation (5) above for each color. Then, for the gradation, the task is to select the characteristics of the gradation table shown in Figure 5, which are mutually independent tasks and correspond to the three perceptual attributes of color. , can be performed extremely easily and accurately.

By the way, among the color image information supplied as a printing document, there is often information that does not have a good gray balance.

Also, gray data originally has complementary color information c, m,
This is realized when y is given in equal amounts, and therefore, if equal amounts of c, m, and y are used as printing inks, correct gray color containing no hue component should be printed. However, due to the characteristics of currently commonly used printing inks, correct gray cannot be reproduced even if equal amounts of c, m, and y are used, and only a slightly reddish gray can be expressed.

On the other hand, in the conventional method described above, the calculation of the sepa density data c ₂ , m ₂ , y ₂ necessary for each color separation plate is performed using the same gradation information g as shown in equation (6) above. It is now being carried out by

Therefore, the conventional method described above has the disadvantage that it is not possible to correct the loss of gray balance in the original image or the loss of gray balance due to the characteristics of the printing ink.

[Purpose of the invention]

An object of the present invention is to provide a color correction method that eliminates the drawbacks of the above-mentioned conventional methods and allows printed matter to always be obtained with correct gray balance, regardless of the characteristics of the original image or the printing ink used. be.

[Summary of the invention]

To achieve this object, the present invention provides a color correction method in which color image information is separated into three components of hue, muddiness, and gradation and processed independently. It is characterized by the following points.

[Embodiments of the invention]

Hereinafter, the color correction method according to the present invention will be explained in detail with reference to illustrated embodiments.

FIG. 1 shows an embodiment of the present invention, in which 1
0, 11, 12 are gradation conversion processing, others are 2nd
This is the same as the conventional example shown in the figure.

The gradation conversion processes 10 to 12 each have independent gradation conversion tables as shown in _FIG . _c ,
The calculation process using g _n and g _y is as follows.

c ₂ = (c ₁ + c ₁ ′) × g _c m ₂ = (m ₁ + m ₁ ′) × g _n y ₂ = (y ₁ + y ₁ ′) × g _y ……(6)′ All processing is the same as in the case of FIG.

Therefore, according to this embodiment, the characteristics of the gradation conversion table to be referred to in each of the gradation conversion processes 10 to 12 are determined according to the gray balance state of the image based on the original color image report and the characteristics of the printing ink used. By selecting , you can always obtain printed matter with correct gray balance.

Note that the characteristics of the gradation conversion table used at this time may be determined by actually performing trial printing, or if the necessary data, such as the characteristics of the ink, are quantitatively provided, they may be determined based on that. It may also be determined by calculation. The example shown in FIG. 7 is set by referring to a table used in a printing plate scanner, etc. For information c (cyan), a large number of changes are made for data a ₁ , and information m ( Magenta) and y (yellow) have almost the same characteristics.

By the way, in the above embodiment, the color information (r, g, b) from the original image is read out from the memory and processed, but it may be processed in real time while being read with a scanner.

Furthermore, if the color information from the original image data is directly given as complementary color information (c, m, y), it goes without saying that inversion processing in preprocessing is unnecessary.

In the above explanation, the present invention has been explained in relation to printing plate making, but the present invention is not limited to printing plate making.
It goes without saying that it can be applied to color correction in various color hard copy production devices such as inkjet type and electrostatic type printers and copiers, and can be expected to have the same effects as the above-mentioned embodiments. Nor.

〔Effect of the invention〕

As explained above, according to the present invention, in the conventional method of separating color image information into the three components of hue, turbidity, and gradation for processing, gradation conversion is further performed based on characteristics that differ for each color. Since these processes are performed independently of each other, the effect of this conventional method is that color correction processing can be performed according to the perceptual classification of colors (hue, saturation, brightness), and there is no concept of parameters. , color matching can be done intuitively and easily.

Hue and brightness can be controlled completely independently, making corrections easy and accurate.

Color communication between multiple workers such as designers and color separation operators becomes smoother.

It can be implemented as either a hardware system or a software system, and has a wide range of applications.

Even when it comes to a hardware system, it can be implemented using inexpensive and highly accurate digital circuits, without requiring the precise and expensive analog circuits used in the past.

Completely independent hue control is possible for each arbitrarily set hue.

In addition to the above effects, it is possible to easily provide a color correction method that provides the excellent effects listed below.

No matter what kind of original image is given, a printed image that always maintains correct gray balance can be obtained.

Gray balance can always be maintained correctly regardless of the characteristics of printing ink.

Even if the characteristics of printing ink change, it can be easily accommodated and there is no need to compromise on gray balance.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing the operation of an embodiment of the color correction method according to the present invention, FIG. 2 is an explanatory diagram showing the operation of the conventional color correction method, FIG. 3 is an explanatory diagram of hue determination processing, and FIG. The figure is an explanatory diagram showing an example of a hue table,
FIG. 5 is an explanatory diagram showing an example of a gradation conversion table;
FIG. 6 is an explanatory diagram showing an example of a blackout table, and FIG. 7 is an explanatory diagram showing an example of a gradation conversion table. ...Pre-processing, ...separation conversion processing, ...composition processing, ...post-processing, 10 to 12...gradation conversion processing.

Claims (1)

[Claims] 1 The three primary color data of each pixel of color image information are sorted and processed, and each pixel is arranged in order of size.
In the color correction method, the three primary color data of each pixel is divided into hue information, cloudiness information, and gradation information and processed based on these three data. 3
This table consists of a table search using one of the following data, and this table has three
A color correction method characterized by being composed of tables of types.