JPS6356069A - Color picture recording method - Google Patents

Abstract

PURPOSE:To weaken the visual intensity of a rosette and to improve the quality of a picture by making each impression of yellow magenta and cyan dot- concentration type and making a Chinese ink impression a dot-dispersion type in a threshold matrix. CONSTITUTION:A picture processor 2 converts each inputted picture data of R, G and B into yellow(Y), magenta(M) and cyan (C) signals. Threshold generation parts 10-Y, 10-M and 10-C in a screen generation part 8 respectively stores the threshold pattern in the dot-concentration type and the threshold generation part 10-K for the Chinese ink(K) impression stores the threshold pattern in the dot-dispersion type. Thus the space frequency in black dot obtained with the threshold pattern of the K impression is made to shift to a high pass in order to be inconspicuous, so that the low frequency of rosette which occurs due to superposing four impressions such as Y, M, C and K can be accurately removed without losing gradation.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a color image recording method using a pseudo area gradation method used in laser printers, inkjet printers, thermal printers, etc. This is intended to prevent deterioration in image quality.

[Conventional technology]

In color halftone image recording, in order to avoid moire when overlapping the four colors of cyan (C), magenta (M), yellow (Y), and black (K), a method is usually used to change the screen angle for each color. It is taken.

One method is image recording using the so-called dither method, which compares each threshold of an nxn threshold matrix with the brightness level of an input image signal and records the halftone of an image according to the comparison result. There is a device in which the threshold value array of the threshold value matrix is changed depending on the screen angle.

Conventionally, in this method, emphasis has been placed on smoothness of gradation characteristics, and a dot-concentrated threshold matrix as shown in FIG. 4 is used for each color. That is, in FIG. 4, (a), (b), (c), and (d) show threshold matrices for yellow, magenta, cyan, and black, respectively, but in all of these threshold matrices, the threshold values vary from the center of the matrix to the periphery. It was arranged so that it spread out.

[Problem that the invention seeks to solve]

However, using such a threshold matrix,
When recording is performed by superimposing the four colors Y, M, and C, there is a problem in that low-frequency rosettes R as shown in FIG. 5 occur, resulting in a significant deterioration of image quality. This rosette R is Y.

This is caused by interference between the small-diameter high-frequency rosettes generated in the M and C versions and the black plate with a screen angle of 45°, and is particularly noticeable when the resolution of the printer is low.

This invention was made in view of these circumstances,
This is intended to prevent image quality deterioration due to rosettes caused by overlapping screens.

[Means and effects for solving the problem] Therefore, in the present invention, the threshold matrices for yellow, magenta, and cyan are of the same dot-concentrated type as in the past, and a dot-dispersed type of threshold matrix is used only for black. .

In other words, it is said that the dot concentrated type has excellent gradation display characteristics, and the dot dispersed type has excellent resolution.In this invention, by making only the black plate the dot dispersed type, the gradation characteristics are also taken into account. At the same time, the spatial frequency of the dots created by the black dot pattern is moved to a higher frequency range so that they are less noticeable to the eye, thereby visually weakening the strength of the rosette.

〔Example〕

FIG. 1 shows the overall configuration of a digital color copying apparatus to which the present invention is applied.

This digital color copying apparatus is composed of an image input device 1, an image processing device 2, and an image output device 3.

The image input device 1 is composed of an image sensor, an A/D converter, etc., and separates the original image using red (R), green (G), and blue-violet (B) filters, and converts the color-separated optical signals into images. input to the sensor, and convert the photoelectric conversion output of the image sensor into A/
The D converter performs A/D conversion and outputs R, G, and B multivalue brightness data.

Each of the R, G, and B read data is input to the color processing section 4 of the image processing device 2. The color processing section 4 includes a masking circuit 5 and a UCR (Under Color Rem).
The masking circuit 5 performs a color correction operation on each of the input R, G, and B data to change each R, G, and B image data to yellow (
Y), magenta (M), and cyan (C) signals. UCR6 performs black plate calculation and undercolor removal,
Y was human-powered.

From M and C image signals, yellow (Y) and magenta (M)
, cyan (C), and black (K).

These Y, M, and C image data are temporarily stored in the storage section 7 at the next stage. Each memory 7-Y, 7- of the storage unit 7
Each of M, 7-C and 7- is a page memory, and can store one page of each image data of Y, M, C, respectively.

The multivalued brightness data of Y, M, and C temporarily stored in the memories 7-Y, 7-M, 7-C, and 7-K are sent to each comparator 9-Y, 9-M in the screen generator 8. ,9-C,
9-K. Each comparator 9-Y, 9-M,
The other input terminal of each of 9-C and 9- is connected to a threshold value generator 1.
Threshold data from 0-Y, 10-M, 10-C, and 10-K are input, respectively. These threshold value generation units 10-Y, 10-
M, 10-C, and 10- are used to generate a dither pattern for halftone reproduction, the details of which will be described later. Each comparator 9-Y, 9-M, 9
-C, 9-, memory 7-Y, 7-M, 7-C, 7
-Y input from K.

Each multi-value brightness data of M, C, is sent to a threshold generation unit 10-Y,
10-M, 10-C, and 10-K are sequentially compared with each threshold value input, and each comparison result consisting of two values, large and small, is sent to each image output module 3-Y, 3-M, and 3- of the image output device 3. C,
3-Output to K. The image output device 3 is, for example, a laser beam printer, and includes comparators 9-Y, 9-M, 9
By controlling the on/off of each laser beam to each module 3-Y, 3-M, 3-C, 3- according to the comparative magnitude signals output from -C and 9-K, respectively, the intermediate value for each color is Outputs a tone image.

In such a configuration, the threshold value generating section 10-Y.

10-M, 10-C, and 10- each include, for example,
It is composed of a ROM that stores a threshold value matrix, an address counter that outputs an address for the ROM, etc. In this case, threshold value generation units 10-Y, 10-M, and 1
0-C stores the same dot-concentrated threshold patterns as the conventional ones shown in FIGS. 4(a), 4(b) and 4(c), respectively, but the black threshold generation section 10-K In this case, a dot-distributed threshold pattern as shown in FIG. 2 is stored. That is, the threshold value generation unit 10-Y has a
) The screen angle O° dot concentration type threshold pattern from "1" to "36" as shown in FIG.
40'' and a screen angle of 18'' are stored, and the threshold generating section 10-C stores the dot concentration type threshold pattern as shown in FIG.
) are stored in the screen angle 72'' dot concentration threshold patterns from "1" to "40" as shown in FIG. A dot-dispersed threshold pattern with a screen angle of 45° up to 45 degrees is stored. It is said that the dot-concentrated threshold arrangement has excellent gradation expression, and the dot-dispersed threshold arrangement has excellent resolution. In this example, Y, M,
By changing the threshold arrangement of the K version to a dot dispersion type as shown in Figure 2 without changing the threshold arrangement of the 0 version, the spatial frequency of the black dots obtained by the threshold pattern of the K version is moved to a higher frequency range. Try to make it less noticeable and do not spoil the gradation (, Y, M.

Interference with the 3rd edition of C is to be suppressed.

FIG. 3 shows the overlapping state of four Y, M, and C plates according to this embodiment, and it can be seen that the strength of the rosette is clearly weaker than in FIG. 5.

In this way, according to this embodiment, it is possible to appropriately remove low frequency rosettes caused by the overlapping of Y, M, C, K, and four versions having different screen angles. In addition to this, this embodiment has the advantage of alleviating the additive law failure phenomenon that occurs due to overlapping of dots during undercolor removal in UCR6.

〔Effect of the invention〕

As explained above, according to the present invention, the rosettes generated by overlapping the four versions of cyan, magenta, yellow, and black are moved to the high frequency side where they do not visually cause problems, so that the visual strength of the rosettes is improved. can be weakened and image quality can be improved.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an example of the overall configuration of a color copying apparatus to which an embodiment of the present invention is applied, FIG. 2 is a diagram showing an example of a black plate threshold matrix used in the embodiment, and FIG. is a diagram showing an example of a four-color superimposed image according to the same embodiment,
FIG. 4 is a diagram illustrating a threshold matrix for each color used in the conventional device, and FIG. 5 is a diagram illustrating an example of a four-color superimposed image by the conventional device. 1... Image input device, 2... Image processing device, 3...
- Image output device, 4... Color processing unit, 5... Masking circuit, 6... UCR, 7... Storage unit, 8...
・Screen generator, 9...Comparator, 10...
・Threshold value generation part, R... rosette. ￥ Katagu (Souffle〉Angle 45°) Fig. 3 (Q) Y"II 7":':l-:fhO'l
(b) M =, HIZl, +-241B
°) (C) C lira (scree) @721 (
d) Krupu (also 7 ri 12 parrot 45″i Fig. 4○ 5th old

Claims (2)

[Claims] (1) A halftone image is created by comparing each image data of yellow, magenta, cyan, and black with a threshold matrix for each color in which a plurality of different threshold values are arranged, and superimposing the four colors according to these comparison outputs. In the color image recording method according to the present invention, the threshold matrix is a dot-concentrated type for each of yellow, magenta and cyan plates, and a dot-dispersed type for a black plate. (2) The color image recording method according to claim (1), wherein each of the yellow, magenta, cyan, and black threshold matrices is provided with a different screen angle.