JPH1051656A - Under color removing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate the increase in rough surface feeling and the deterioration in the saturation of a low density part by removing an under color lest each signal of yellow, magenta, cyanogen and black after removing the under color changes to a signal level recognized as a noise not as grey level information of a printer. SOLUTION: The medians and the minimums of Yellow(Y), magenta(M) and cyanogen(C) signals are extracted from the signals to obtain the difference X. An under color removing quantity calculation circuit 4 compares a signal level A having the gradation expression of the printer recognized not as grey information but as a noise with a difference X to calculate an under color removing quantity K. At the time of X>A, an under color removing quantity is made the minimum value. On the other hand, at the time of X<=A, the under color removing quantity is made a minimum value-A.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color printer for converting a gray component represented by a mixed color of three colors of Y, M and C from a signal of yellow (Y), magenta (M) and cyan (C) into black (K). ) An undercolor removal device (International Patent Classification H04N 1/58) for replacing with a signal.

[0002]

2. Description of the Related Art Conventionally, in color printing, Y, M,
Color printing is performed by using four types of signals using K in addition to three colors of C. It is assumed that the overlapping portion of Y, M, and C is theoretically K, and by replacing the portion with K, gray balance is easily obtained and the lack of density in the high density region is compensated.

[0003] For example, as shown in FIG.
A method using a full black method in which 100% of the overlap of the three colors M and C is replaced with K, and a method using a method disclosed in Japanese Patent Application Laid-Open No. 59-161981.
As shown in Japanese Patent Application Laid-Open Publication No. H06-214, the inking is performed with the value obtained by multiplying the minimum value of Y, M, and C by a coefficient as K, Y is (Y−K), M is (M−K), C has an undercolor removal as (CK).

[0004]

However, in the conventional undercolor removing apparatus, since the printing characteristics of the printer are not taken into account, there is a disadvantage that the image quality is deteriorated due to a decrease in saturation in a low density portion and an increase in roughness. was there.

[0005] Since a printer can normally express a tone only by two values, ie, dot printing and no dot printing, it is common practice to express grayscale information by using a dither method. A typical dither method is Bayer's systematic dither method. In this method, grayscale pixels are represented by a set of dots using the integration function of human vision. The spatial frequency of the pixel pattern represented by a set of dots is set to a high frequency so as to be hardly detected by human eyes as noise.

However, in the low-density area, since the number of dots for expressing the gray-scale pixels is small, the visual integration operation is difficult to work, and is detected as noise rather than the gray-scale information, resulting in a rough feeling. Such a phenomenon also occurs when an error diffusion method is used for gradation expression.

In such a conventional undercolor removing apparatus which does not consider the gradation expression characteristics of the printer, when the undercolor removal amount is large, the signal levels of the signals Y, M, and C after the undercolor removal processing are reduced. It changes to a signal level recognized as noise, not as shading information of the printer. For this reason, it is visually perceived as noise rather than color shading information, causing an increase in roughness and a decrease in saturation, making it difficult to reproduce colors.

[0008]

In order to solve the above-mentioned problems, an undercolor removing apparatus according to the present invention comprises a Y, M, and Y after undercolor removal processing.
The undercolor removal amount is determined so that each signal of C does not change to a signal level that is recognized as noise rather than density information.

According to the present invention, the undercolor removal processing can be performed without lowering the saturation in the low-density portion or increasing the roughness.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An undercolor removal apparatus according to a first aspect of the present invention is an apparatus for performing undercolor removal processing for replacing gray components with black signals from yellow, magenta, and cyan signals. The undercolor removal amount is determined so that each of the processed yellow, magenta, and cyan signals does not change to a signal level recognized as noise, not as shading information. Has the effect of preventing a decrease in the saturation of the low density region.

The undercolor removing apparatus according to claim 2 of the present invention uses a central value Mid (Y, M, C) and a minimum value Min (Y, M, C) from yellow, magenta, and cyan signals. And the difference X = Mid (Y, M, C) −Min
(Y, M, C) is compared with the signal level A at which the tone expression of the printer is recognized as noise, not as grayscale information. When X> A, the undercolor removal amount = Min (Y, M , C) However, when Min (Y, M, C) <A, the undercolor removal amount =
0 When X ≦ A, undercolor removal amount = Min (Y, M, C) −A However, when Min (Y, M, C) <2A, undercolor removal amount is determined as undercolor removal amount = 0. It is possible to prevent the Y, M, and C signals from being lower than a signal level A that is recognized as noise rather than grayscale information in the gradation expression of the printer, thereby generating a feeling of roughness and low density. This has the effect of suppressing a decrease in the saturation of the region.

(Embodiment 1) The first aspect of the present invention will be described below.
An embodiment of the invention described in claim 2 will be described with reference to FIGS.

FIG. 1 is a block diagram showing the basic configuration of the present invention. The magnitudes of the respective signals of input image data Y, M, and C are compared, and the median value Mid (Y, M, C) is calculated. The minimum value Min (Y, M, C) is obtained by the minimum value detection circuit 2 by the median value detection circuit 1. Next, the difference X between the median value and the minimum value is calculated by the subtractor 3, and the value is input to the under color removal amount calculation circuit 4. The undercolor removal amount output from the undercolor removal amount calculation circuit 4 becomes the K signal as it is. Subtractors 5, 6, 7
Subtracts the undercolor removal amount from the input data Y, M, and C,
Data Y ', M', and C 'after the undercolor removal are obtained.

The undercolor removal amount calculation circuit 4 is preliminarily input with a value A which is visually recognized as noise rather than density information based on the gradation expression characteristics of the printer. In the method of determining the value of A, each signal level is input to the printer in order from a high density signal to a low density signal, and the output result of the printing apparatus for each signal is observed. Then, the signal level at which the noise starts to be visually recognized as noise rather than the grayscale information can be determined as A. Alternatively, a signal level at which a set of drawing points cannot be measured as a density value using a density measuring device can be used as A.

The undercolor removal amount calculation circuit 4 compares the difference X between the median value and the minimum value of the input signal with the value of A to determine the signal level of each color when the undercolor removal process is performed. A determination is made as to whether the signal level changes to a signal level recognized as noise, not as information.

The difference X is compared with the value A. If the difference X is larger than the value A, the minimum value of the input signal is used as the undercolor removal amount, and the minimum value is subtracted from the median value and the maximum value to obtain the undercolor. Obtain data after removal. Since the median value and the maximum value after the undercolor removal are higher than the signal level recognized as noise, it can be printed not as noise but as shading information.

For example, as shown in FIG. 2, when the difference X between the median value M and the minimum value C of the input data is larger than A, the undercolor removal amount is C. Each of the data Y 'and M' after the undercolor removal has a value larger than the signal level A recognized as noise than the density information, and each signal after the undercolor removal can be printed as density information instead of noise. Recognize.

As described above, when X> A, the undercolor removal amount = Min (Y, M, C), but even when X> A, Min (Y, M, C) When <A, the minimum value is at a level that is originally recognized as noise. Therefore, when the minimum value is replaced with K, the noise is visually emphasized, so that the undercolor removal amount = 0.

Conversely, if the difference X between the median value and the minimum value is smaller than the value A, and the undercolor removal amount is the minimum value of the input signal, at least the median value after the undercolor removal is reduced to a level recognized as noise. Will change. Therefore, assuming that the undercolor removal amount is a value obtained by subtracting A from the minimum value, the maximum value and the median value are always A
, And can be printed as density information.

As shown in FIG. 3, the difference X between the median and the minimum is
Is smaller than A, the undercolor removal amount is set to Min (Y, M,
C), that is, C, the data Y ′,
Of M ′ and C ′, the median value M ′ is a value smaller than the signal level recognized as noise than the density information. Therefore, as shown in FIG. 4, by subtracting the undercolor removal amount by A, the data Y ′, M ′,
Both C ′ and K ′ can be made higher than the signal level A that causes noise.

As described above, when X ≦ A, the undercolor removal amount = Min (Y, M, C) −A. However, in this case, if Min (Y, M, C) <2A, the converted K
May be smaller than A or the minimum value may be a negative value, so that the undercolor removal amount = 0.

[0022]

As described above, according to the present invention, each of the yellow, magenta, cyan, and black signals after the undercolor removal processing does not change to the signal level recognized as noise, not as shading information of the printer. By determining the undercolor removal amount in this way, the undercolor removal increases the roughness,
Under color can be removed without lowering the saturation of the low density portion.

[Brief description of the drawings]

FIG. 1 is a block diagram of an undercolor removing apparatus according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of undercolor removal processing of the apparatus.

FIG. 3 is an explanatory diagram of an undercolor removal process of the apparatus.

FIG. 4 is an explanatory diagram of an undercolor removal process of the apparatus.

FIG. 5 is an explanatory diagram of an undercolor removal process of a conventional undercolor removal device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Median value detection circuit 2 Minimum value detection circuit 3 Subtractor 4 Under color removal amount calculation circuit 5, 6, 7 Subtractor

Claims (2)

[Claims] 1. From the signals of yellow, magenta and cyan,
In an apparatus that performs an undercolor removal process that replaces a gray component with a black signal, the undercolor removal process is performed so that the yellow, magenta, and cyan signals do not change to signal levels that are recognized as noise, but not as shading information. An undercolor removing device for determining an amount of color removal. 2. From the signals of yellow, magenta and cyan,
Median value Mid (Y, M, C) and minimum value Min (Y, M, C)
C), and the difference X = Mid (Y, M, C) -Mi
n (Y, M, C) is compared with the signal level A at which the tone representation of the printer is recognized as noise, not as grayscale information. When X> A, the undercolor removal amount = Min (Y, M, C) However, when Min (Y, M, C) <A, the undercolor removal amount =
0 When X ≦ A, undercolor removal amount = Min (Y, M, C) −A However, when Min (Y, M, C) <2A, undercolor removal amount is determined as undercolor removal amount = 0. An undercolor removing device, comprising: