JPH078007B2 - Color image processing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a color image processing method.

[Prior Art] As a conventional color image forming method, four color inks of yellow, magenta, cyan, and black are used, and a full color image is obtained by a subtractive color mixing method. Further, the undercolor removal method (UCR) used in the above-mentioned conventional method considers the minimum value of yellow, magenta, and cyan to be black, reduces the black amount from each color ink at a specific ratio, and removes that portion. It is a method of replacing with black.

This makes it possible to reproduce black with a higher density than black created by overlapping yellow, magenta, and cyan.
In addition, the consumption of yellow, magenta, and cyan ink can be reduced.

However, a method of replacing all black with black ink (100% U
When CR) is adopted, 100% UCR has not been realized because the entire full-color image becomes darker than necessary and the image quality becomes rough. Therefore, yellow, magenta, and cyan inks slightly remain in the black portion.

[Problems to be Solved by the Invention] In the above method, even in the black portion, yellow,
Since a small amount of magenta and cyan ink remains, the yellow, magenta, and cyan inks are scattered in the black portion, and the color inks are scattered in the black character portion, which lowers the quality of the black character.

To solve this problem, a character area may be detected and 100% UCR of the detected character may be performed, as disclosed in Japanese Patent Laid-Open No. 61-13262. However, in this conventional example, the halftone dot image has periodical points where the waveform of the reading signal of the document crosses the reference value, so that the halftone dot image is erroneously determined to be a character region and a color photograph is obtained. A halftone dot image such as the above may be printed in black.

It is an object of the present invention to provide an image processing method capable of reliably preventing the color ink from scattering in a black character portion and not printing a color image other than a character such as a halftone dot image in black. To do.

[Means for Solving the Problem] According to the present invention, an area in which, among pixels of a color image signal, pixels having a predetermined feature amount or more indicating an edge is continuously present is determined to be a character area. However, black monochromatic processing is performed on the color image signal of the area determined to be the character area and further determined to be the black area.

[Operation] According to the present invention, each pixel of a color image signal is determined to be a character region, and a region in which pixels having a predetermined amount of feature amount indicating an edge are continuously present is determined to be a character region. In addition, since the black monochromatic processing is performed on the color image signal in the area determined to be the black area, it is possible to reliably prevent the color ink from scattering in the black character portion, and moreover, if the non-character image such as the halftone image is used. Never print color images in black.

[Embodiment] FIG. 1 is a block diagram showing an embodiment of the present invention.

The input sensor 10 has a photoelectric conversion element such as a CCD camera,
The original is read and three color separation signals of red (R), green (G) and blue (B) are output. The A / D converter 11 converts each of the above signals into an 8-bit digital signal, which enables gradation expression of 256 levels for each color. The log converter 12 performs density conversion of digital signals of each color expressed in 256 gradations, and signals C ₁ , M ₁ , and Y ₁ representing the amounts of three color inks of cyan, magenta, and yellow.
Is output.

The masking circuit 13 performs color correction processing on the signals C ₁ , M ₁ and Y ₁ , and this color correction is a correction performed to remove the turbid component of the color separation filter of the sensor 10 and the turbid component of the ink. . The masking is performed according to the following formula.

Y ₂ ＝ K ₁₁ Y ₁ ＋ K ₁₂ M ₁ ＋ K ₁₃ C ₁ M ₂ ＝ K ₂₁ Y ₁ ＋ K ₂₂ M ₁ ＋ K ₂₃ C ₁ C ₂ ＝ K ₃₁ Y ₁ ＋ K ₃₂ M ₁ ＋ K ₃₃ C ₁ Where, above K _{11 to} K ₃₃ are experimentally determined parameters.

The UCR circuit 14 outputs the output signals C ₂ , M ₂ and Y ₂ of the masking circuit 13.
With respect to, the under color removal processing and the calculation of black ink are performed. This processing is performed according to the following formula.

_{Y 3 = Y 2 -α (K} 2 -β) M 3 = M 2 -α (K 2 -β) C 3 = C 2 -α (K 2 -β) , _{where, K 2 = min (Y 2} , M ₂ and C ₂ ), K ₂ is the output of the black detection circuit 17 described later, and α and β are constants. When α = 1 and β = 0, it is called 100% UCR and
0% UCR is a case where black created by three colors of yellow, magenta, and cyan is all replaced with black ink.

The dither circuit 15 is a circuit that binarizes the output signals C ₃ , M ₃ , Y ₃ , and K ₃ of the UCR 14, and the signals C ₄ , M ₄ , Y ₄ , and K ₄ output by the dither circuit 15 are 1 bit each is sent to the color printer 16. That is, a color image is formed by turning on / off each ink dot.

The black detection circuit 17 outputs the output signals C ₂ , M ₂ of the masking circuit 13,
The minimum value of Y ₂ is detected and this is output as a black signal K ₂ . Further, a black detection circuit 17, when the black amount is above a certain threshold, in which the output K ₁ is "1".

Further, the character detection circuit 18 detects the continuity of the image based on the M ₁ output of the log conversion circuit 12. That is,
If M ₁ has continuity, the output R ₁ becomes “1”. AND circuit 1
9 is the AND of the outputs K ₂ and R ₁ . Therefore,
If it is black and continuity about it is detected,
The output U1 becomes "1", and the UCR amount is switched by inputting this signal to the UCR circuit 14.

The above embodiment is a color image processing method for processing a color image signal obtained by photoelectrically converting a color original, and the black detection circuit 17 determines a black area of the color original based on the color image signal. This is an example of realizing the detection step, and the character detection circuit 18 is a pixel in which each pixel of the color image signal has a feature amount indicating that it is an edge or more, which is a predetermined value or more. Is an example of realizing a character detection step of determining that a region that continuously exists is a character region, the UCR circuit 14, the color of the region that is determined to be a character region, and further determined to be a black region This is an example of realizing a black monochromatic processing step of performing monochromatic black processing on an image signal.

FIG. 2 is a block diagram showing an example of the black detection circuit 17.

Comparator 21a, 21b, 21c, the color signal C _2, M _2, compared one by 2 among Y _2, in which sends the comparison result to the judgment unit 22. The determiner 22 selects the minimum value of the three signals and sends the result to the selector 23, which outputs the minimum value of the signals C ₂ , M ₂ and Y ₂ . The signal of this minimum value is K ₂ . The comparator 24 compares the minimum value signal K ₂ with an M constant threshold value,
When it is equal to or more than the threshold, the output K _{1 is set} to “1”.

FIG. 3 is a block diagram showing an example of the character detection circuit 18.

The averaging circuit 30 calculates an average value of the input M ₁ within a specific region, and the comparator 31 uses this average value as the original input signal M ₁
Is output when M ₁ is large, and “0” is output when M ₁ is small.
Binarization processing is performed.

The density difference detection circuit 32 determines the magnitude of the absolute value of the difference between the output M ₁ and the average value. When the absolute value is larger than a specific threshold value, “1” is output, and when the absolute value is smaller, "0" is output to. The correction circuit 33 allows the output of the comparator 31 to pass only when the output of the density difference detector 32 is "1", and always holds "0" when the output of the density difference detector 32 is "0". It is a gate circuit. As a result, binarization is selectively performed only when the density difference is large. This data is stored in the line buffer 34.

FIG. 5 is a block diagram showing a second embodiment of the character detection circuit in the above embodiment.

In this embodiment, the continuity detecting circuit 35a detects the continuity of binary "1" pixels in four directions of vertical, horizontal and diagonal directions. A circuit example of this continuity detecting circuit is shown in FIG.

In FIG. 4, the pixel arrays 40, 41, 42, 43 are respectively
The pixel positions in the horizontal direction, the vertical direction, and the diagonal two directions are shown. The binary data of each pixel are AND circuits 44a, 44b, 44
When the logical operation is performed by c and 44d and all the input 5 pixels are "1", each AND circuit outputs "1". Then, these outputs are logically operated by the OR circuit 45. Therefore, when the pixels of "1" are continuous in any one of the vertical, horizontal, and diagonal directions, the OR circuit 45 outputs "1". This output "1" is a signal showing continuity, and the signal showing this continuity is designated as R ₁ and is sent to the UCR circuit 14.

In FIG. 5, the maximum value detector 50 and the minimum value detector 51 are
Based on the signal M ₁ , the maximum value and the minimum value in the specific area (for example, 3 × 3 pixels) are detected. The averaging circuit 52 is a circuit that averages the maximum value and the minimum value obtained as described above, and divides the value obtained by adding the maximum value and the minimum value by two. The comparator 54 compares the average value and the signal M ₁ and binarizes them, and when the signal M ₁ is larger,
"1" is output, and when it is smaller, "0" is output.

The subtractor 53 performs an operation of subtracting the minimum value from the maximum value. This output is compared with a specific threshold value in the comparator 55, and when the output of the subtractor 53 is large, "1" is output. When it is small, "0" is output. Correction circuit 33a
Is the same as the correction circuit 33 of FIG. 3, receiving the outputs of the comparators 54 and 55. The line buffer 34a is similar to the line buffer 34 shown in FIG.

FIG. 6 is a block diagram for explaining selection of the UCR circuit 14 in the above embodiment.

The UCR circuits 60a, 60b, 60c perform halftone UCR processing, and the UCR circuits 61a, 61b, 61c perform UCR processing of character portions.

FIG. 7 is a diagram for explaining the halftone UCR processing and the UCR processing of the character portion.

7 (1), upon reading the black document shows a state in which the color signal data _{Y 2, M 2, C 2} , K 2 is entered at the same level. Fig. 2 (2) shows the UCR of the halftone part in the above case.
It is a figure showing the result, the amount of color signal data Y ₂ , M ₂ , and C ₂ is reduced to about half, and Y ₃ , M ₃ , and C ₃ are replaced, and black data K _{3 is used} instead.
Is generated by the black generation circuit 62. This is called 50% UCR.

On the other hand, in the character part, as shown in FIG.
The amount of the color signal data Y ₃ , M ₃ , and C ₃ is set to almost 0, and instead, the black data K ₃ is generated in the black generation circuit 63 and replaced with the color ink. This is called 100% UCR.

Select circuits 64a, 64b, 64 according to the select signal U1
Select the above data by switching c and 64d,
Halftone portion, the ink amount C ₃ adapted to the character portion, M _3, Y _3, and outputs a K _3.

[Effect of the Invention] According to the present invention, it is possible to reliably prevent the color ink from scattering in the black character portion, and, moreover, to colorize the color image in the area of the fine dots formed by the halftone dots such as the halftone dot image in black. This has the effect of not printing.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention. FIG. 2 is a block diagram showing an example of the black detection circuit in the above embodiment. FIG. 3 is a block diagram showing a first embodiment of the character detector in the above embodiment. FIG. 4 is a diagram showing an example of the continuity detecting circuit in the above embodiment. FIG. 5 is a block diagram showing a second embodiment of the character detection circuit in the above embodiment. FIG. 6 is a diagram for explaining selection of the UCR circuit in the above embodiment. FIG. 7 is a diagram for explaining the UCR processing in the halftone portion and the character portion in the above embodiment. 17 ... Black detection circuit, 18 ... Character detection circuit, 14 ... UCR circuit, 3
5, 35a ... Continuity detection circuit.

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Claims (1)

[Claims] 1. A color image processing method for processing a color image signal obtained by photoelectrically converting a color original, wherein the color image signal is an edge based on the color image signal. A character detection step of determining an area where pixels having a predetermined feature amount or more continuously exist as a character area; a black area detection step of determining a black area of the color original document based on the color image signal; A monochromatic black processing step of performing monochromatic black processing on the color image signal of the area determined to be the character area and further determined to be the black area;