JPH01195777A - Color picture processing system - Google Patents

Abstract

PURPOSE:To obtain a color tone with high picture quality and accuracy while using a few color materials by applying signal conversion so that the color of a color picture outputted by a picture output device is coincident with or closest to a color of an original picture and executing the elimination of the background color and expressing all colors by three color materials including black color among 4 color materials. CONSTITUTION:The picture information of an original is separated into a red (R) color separation image, a green (G) color separation image and a blue (B) color separation image by a scanner 1 and outputted while being converted into a digital color signal. The digital color signal is converted into a chrominance signal of an output color of the picture output device by a signal conversion table 2. The color tone reproduction processing information in the signal conversion table 2 is stored in a color tone reproduction processing ROM so that the same color tone as much as possible as that of the original is kept and a color closest to the color of the original is selected out and outputted if the color of the original is unable to be outputted by a printer 3. The color correction means applies the elimination of background color till one of four colors is eliminated. Thus, a print output with accurate color tone is obtained with less amount of toner.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a color image processing system, and more particularly, to a color image processing system that can accurately and stably reproduce image information such as color tone. .

(Background of the invention) Generally, color images include red (R) and green (G).

Separated into three colors of blue (B) and read, color material (toner)
Usually, an image is formed by converting it into four colors corresponding to the colors yellow (Y), magenta (M), cyan (C), and black (K). That is, the image data read as RGB positive number fIi (for example, Pa tone level in dither method) is Y, which corresponds to the adhesion area ratio of each color toner,
It is common to convert into positive values for each M and C.

Conventionally, as such a method, masking processing that takes into account the characteristics of toner has been considered effective. This is R, G, B
The conversion from Y to M to C is performed by linear or quadratic or higher order matrix operations.

In addition, in laser printers, etc., in order to change the degree of toner transferred to transfer paper, the amount of toner adhering to the photosensitive drum can be changed in several stages by changing the pulse width of the writing laser. There are ways to express gradation. Furthermore, in color printers, there is a method in which toners of different colors (y, M, C, K) are layered on a drum and transferred to transfer paper at once.

By combining the above methods, golden hard copies can be obtained quickly.

(Problem to be Solved by the Invention) However, when a plurality of color toners are layered on a photosensitive drum, the developability of the color toner that is developed first and the toner that is superimposed and developed on top of it are different. It will be quite different. For this reason, the relationship between the writing laser and the amount of transfer, and therefore the relationship between conversion from R, G, and B to Y, M, and C, becomes complicated, and it may be difficult to approximate it using polynomials or the like.

Furthermore, it is theoretically possible to express all colors using three toner colors including K instead of four toner colors YMCK. Change this to 100% UCR (undercolor removal)
That's what it means. According to this method, toner can be erased! ! It is possible to reduce the amount. However, since this method is difficult to handle, the reality is that it is not used much other than for certain purposes, such as improving the reproducibility of edge portions.

The present invention has been made in view of the above-mentioned problems, and its purpose is to provide a color image processing system that can obtain print output with accurate tones using a small amount of toner and without deteriorating image quality. It is about realization.

(Means for Solving the Problems) The present invention to solve the above problems includes an image reading means for reading a document image for each of a plurality of color components and generating digitized color image data for each color; and an image forming means for forming a color image on a recording medium by sequentially superimposing images made of color materials of multiple colors including black based on the color image forming means formed by the image forming means. and signal conversion means for performing color reproduction processing so that the color of the original image matches or is closest to the color of the original image, and undercolor removal processing that reduces the density of color materials other than black and increases the density of black color materials, This signal conversion means is configured to perform undercolor removal processing so that the density of at least one coloring material other than black becomes zero level.

(Function) The color reproduction means is a color image 1 formed by the color image forming means! A color is selected so that the color of A matches or is closest to the color of the original image. The color correction means performs undercolor removal until one of the four colors is used up.

(Example) Examples of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention. In the figure, 1 is a scanner that constitutes an image reading means that separates the image information of a document into the three primary colors of RGB and outputs it as a digital signal, and 2 is a scanner that converts the RGB signals into Y (yellow), 1M (magenta), and C. (cyan), K (black) and selects the color closest to the original that can be output by the image output device and outputs it as image data. 3 is a signal conversion table that obtains a hard copy of the original from the image data. This is a printer that constitutes an image forming apparatus for.

The operation will be explained below. The image information of the original is scanned by scanner 1.
A color-separated image of red (R), a color-separated image of green (G),
The signal is separated into a blue (B) color separation image, converted into a digital color signal of a predetermined number of bits (for example, 8 bits), and output. This digital color signal is converted using the signal conversion table 2.
It is converted into a color signal of the output color (here, four colors of Y, M, and C) of the image output device. This signal conversion table 2 is
It has a color tone reproduction processing ROM (not shown) that stores image processing information. Among the color tone reproduction processing information, one unit is a total of 8 bits including 6-bit color information and 2-bit color code, and there are 44M (for 4 colors of YMCK). This color tone reproduction processing information is used to maintain the same color tone as the original as much as possible, and to select and output the color closest to the original color if the color of the original cannot be output by the printer 3 (hereinafter referred to as color tone reproduction). ), stored in the color tone reproduction processing ROM. In other words, using the concentration signal as an address,
By accessing the color tone reproduction processing ROM, 8-bit color tone reproduction processing information can be obtained. The obtained color tone reproduction processing information is multivalued and output as image data.

This image data is sent to the printer 3 and output as a dot pattern.

The color tone reproduction in the signal conversion table 2 will be explained in detail below.

In order to reproduce a hard copy with the same color tone as the original, it is necessary to know the characteristics of the scanner and printer in advance. If a signal conversion table is connected between the scanner and the printer, and this signal conversion table complements the characteristics of both, the printer can output a hard copy with a color tone that is extremely similar to the color of the original. At this time, the signal conversion table was created using computer color matching (C
CM) and stored in the color tone reproduction processing ROM.

■Scanner characteristics measurement First, we must examine the characteristics of the scanner.

Therefore, a plurality of colored papers are carried out from among color samples prepared in advance (for example, Munsell color samples) and measured by a color shading meter to obtain values of the colored papers in the CI EXYZ coordinate system. next,
By placing colored paper on the document table of a scanner and scanning it, RGB brightness signals from the colored paper scanner are obtained.

Since there is a linear relationship between the two types of colored paper values obtained in this way, the following equation holds true.

Here, the parameters a to i are obtained from the two types of values by approximation using the least squares method. That is, by determining the parameters a to i, the RGB signals from the scanner can be converted to XYZ, and the characteristics of the scanner can be investigated.

Depending on the characteristics of the scanner, the RGB-XYZ conversion accuracy can be improved by increasing the nine parameters a to 1 to 30 and understanding the scanner characteristics as a secondary conversion matrix.

■ Measuring Printer Characteristics All you have to do is print out the dither pattern on the printer, measure it, and find the relationship between the dither pattern signal (YMCK) and the color. However, it is necessary to output and color measure all the dither patterns. takes too much time and effort. For example, if YMCK is to be displayed with 64 gradations each, the number of patterns is 6.
It will be about 44. Therefore, patterns are printed out at appropriate intervals and the colors are measured. Data may be interpolated using linear interpolation or the like between the two. Further, by smoothing the data after interpolation, it is also possible to suppress data variations.

The type of dither pattern is dot distributed type.

There is a centralized type or a medium between them, and you can use any of them.However, if there is a problem with the stability of the printer, it is preferable to use the centralized type.

If the signal conversion table supplements the characteristics of the scanner and printer determined in (1) and (2) above, the printer can output a hard copy with a color tone close to that of the original. Data for color tone reproduction at this time is stored in the color tone reproduction processing ROM.

In order to ensure the stability of overlapping toners during image formation with a printer, it is desirable that the number of types of overlapping toners is small. Therefore, in the present invention, 100% tJ
Execute cR (undercolor removal).

However, in UCR, since the black component obtained by the combination of Y, M, and C is replaced with K, it is not possible to reproduce the deep color that is created by superimposing other colors on top of the maximum density of black.

Therefore, the following method is used.

First, the reproduced color gamut of the printer is divided into three parts according to YMC. FIG. 2 is an explanatory diagram showing the reproduced color gamut of the printer divided into three parts in this way. In this figure, the reproduced color gamut is represented by a circle for convenience of explanation. Also, Y, M, on the circumference
The value from the center of the 0 circle where C is placed and K is placed at the center of the circle to MMC on the circumference is the dither level. Here, (1) is displayed using only the MCK coloring material and no Y coloring material is used. Similarly, (2) is displayed using only the YMK coloring material and no C coloring material is used. Similarly, (3) is Y
It is displayed using only the CK coloring material, and no M coloring material is used at all.

According to this method, the three color materials Y, M, and C are not used simultaneously, so that substantially 100% UCR is executed.

Moreover, in each area (1), (2>, and (3)), even if K is at the maximum density (maximum dither level or solid), the other two types of color materials must be further overlapped to the maximum density. This makes it possible to expand the reproduction color gamut, especially in dark areas.

In addition, since at least one of Y, M, and C is not used in any case, the number of color materials superimposed on the photosensitive drum is reduced, which saves color materials and improves stability during superimposition. You can aim for sex.

Next, an example of creating a conversion table will be described.

The relationship between the pattern in the printer and the reproduced colors is input to the computer as data. This will be referred to as reproduction color data here. The structure of the reproduced color data is
The input is Y, M, C, dither level (in this example, 0~
64), the output is reproduced color data as shown below.

In this way, values converted into CIELAB space are used.

Also, the reproduction color data is one of the four (Y, M, C, K) other than K (any one of Y, M, C)
Must be composed only of things such that is always 0 (for example, C = 15.M = O, Y = 20. = 8)
. This is necessary in order to perform the 100% UCR and three-part division of expression color as described above.

Then, set the scanner signals RGB (for example, R
=3. G=50. B=40>.

Thereafter, the scanner signal is converted into a CIEXYZ value by the RGB→XYZ conversion described above.

Furthermore, it is developed into a density space or a uniform color space. In this example, CIELAB space is adopted.

Matching is performed between the above-mentioned reproduced color data and the color signal of the scanner. That is, the structure is such that the input is the RGB signal of the scanner and the output is the YMCK dither level (0 to 64). The discrimination amount for matching is CIELAB
All you have to do is compare the values in space. That is, the color difference ΔEa
The combination that minimizes b is the matching condition for equal colors.

The above operations are performed for all scanner signals to create matched data.

There may be discontinuous parts in the matched data. This is likely to occur mainly when the color of the pattern produced by the printer is unstable or when the matching method is poor. In this case, it is possible to reduce discontinuities by smoothing the data.

The data matched in this manner is stored in the ROM and placed in the signal conversion table 2. As a result,
A hard copy with a color tone as close as possible to the original image read by the scanner 1 can be obtained.

As described above, by printing out the signal after color reproduction and undercolor removal using an image output device, it is possible to maintain the same color tone as the original with a small amount of color material without deteriorating the image quality of the color image. .

In this embodiment, the printer uses four color materials (toners) of Y, M, and C.
The present invention is not limited to this, and it is also possible to use a coloring material of a different color, or it is also possible to use a different number of colors.

(Effects of the Invention) As described in detail above, in the present invention, signal conversion is performed so that the color of the color image output by the image output device and the color of the original image match or are closest to each other. By performing undercolor removal so that all colors are expressed using three color materials including black, it is possible to obtain print output with high image quality and accurate tones using fewer color materials. It is possible to realize a color image processing system that can.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention, and FIG. 2 is an explanatory diagram of color materials and expression colors. 1...Scanner 2...Signal conversion table 3...Printer

Claims (1)

[Claims] An image reading means that reads a document image for each of a plurality of color components and generates digitized color image data for each color, and sequentially superimposes images made of color materials of a plurality of colors including black based on the color image data. In a color image processing system having an image forming means for forming a color image on a recording medium by using a method, color reproduction is performed such that the color of the color image formed by the image forming means and the color of the original image match or are closest to each other. The signal converting means performs undercolor removal processing to reduce the density of colorants other than black and increase the density of colorants other than black. A color image processing system characterized in that the system is configured to perform undercolor removal processing so that the density becomes zero level.