JPS61189774A - Formation system of multi-gradation picture - Google Patents

Abstract

PURPOSE:To improve the reproducibility as well as the smooth display of gradation and to prevent the color variance, by preparing a concentrated threshold value matrix containing (k) units (k>=2) of cores for formation a screen angle and reducing the degree of appearance down to (k) units or less for dots having the density unequal to the highest level. CONSTITUTION:The original picture data PIX expressed in 6 bits is supplied for each picture element as the address information on a table converter 60 which produces the picture data on recording dots. At the same time, the position information (x) and (y) of two bits each on the X and Y directions are supplied. The converter 60 decides the density of the output dots to the input picture elements by a multi-value dither process and with use of a multi-value threshold matrix of (4 rows X 4 columns). This density information is sent to a recording system 62 in the form of the 2-bit data. In the multi-value dither process each picture data is compared with a piece of threshold data corresponding to each threshold value matrix. Then the highest one of density levels that has the picture data value higher than the threshold value is selected as the density level of the output dot.

Description

Detailed Description of the Invention [Technical Field] The present invention relates to an image forming (print 1 display) system that can control the dot density directly or indirectly by changing the dot diameter to three or more levels. , relates to a method for forming a multi-tone image.

[Prior art]

The dither method, density pattern method, and submatrix method are known as methods for pseudo-expressing multi-gradation images using binary dots. The dither method compares each pixel data of an original image with a corresponding element (threshold data) of a threshold matrix to determine whether the output dot is white or black. The density pattern method is a method in which each image data of an original image is compared with all elements of a threshold matrix, and black and white of an output dot corresponding to each element is determined. The submatrix method is the previous 2
This is an intermediate method, in which each image data of the original image is compared with all the elements of the partial matrix of the threshold value matrix, and the black and white of the output dot corresponding to each element is determined. In any case, the principle of determining whether the output dots are black or white by comparing the threshold value data of the threshold value matrix with the original image data is the same, and can be considered as a unified multi-value dither method in a broad sense.

In an image forming system that can control the density of dots at multiple levels, even when forming a multi-tone image, by using a multi-value threshold matrix corresponding to each density level,
Similar methods (multi-value dither method, multi-value density pattern method, multi-value submatrix method) can be applied.

As a conventional technique that can be considered to be included in the scope of the multilevel dither method, there is known, for example, the "Image Display Method" disclosed in Japanese Patent Application Laid-Open No. 147728/1983.

However, such conventional multilevel dithering methods have a problem in that dots at low density levels appear frequently and it is difficult to obtain stable image quality with good reproducibility.

Furthermore, when a threshold matrix having a screen angle such as a halftone dot pattern is used and the dot density level is expressed by the dot diameter, there is a problem that the clarity of one image is impaired. Furthermore, when peta-printing with low-density dots, the screen angle disappears, so if you try to express multi-color, multi-gradation by overlapping dots of three or more colors, color misalignment will occur due to misalignment of the plates for each color. There was a problem in that it was easy to occur.

Note that if the appearance of low-density dots is simply reduced in order to stabilize the image quality, the resolution will deteriorate.

〔the purpose〕

An object of the present invention is to stabilize the image quality while maintaining sufficient resolution in an image forming system that can control the density level of dots directly or indirectly by changing the dot diameter to three or more levels. An object of the present invention is to provide an improved multi-gradation image forming method that can improve reproducibility, achieve smooth gradation expression, and prevent color shift when applied to multi-color, multi-gradation images.

〔composition〕

The present invention is, in principle, a multi-tone image forming method using a multi-value dither method in a broad sense (including a multi-value density pattern method and a multi-value submatrix method), and its main feature lies in the configuration of a threshold value matrix. That is, in the present invention, the screen angle is formed by using a threshold matrix as a concentrated type having k≧2 nuclei, and the number of dots whose density is not at the highest level appears within the threshold matrix is set to be less than or equal to k. This improves reproducibility (it is generally difficult to control low-density dots with good reproducibility), forms halftone dots with smooth gradation, and improves the density level between dots within the threshold matrix. To express each halftone dot in a well-balanced manner while keeping the resolution within an allowable range by preventing the difference from exceeding 1, and to prevent the screen angle from disappearing.

Furthermore, when the present invention is applied to the formation of a multicolor and multitone image,
To prevent color shift, dark value matrices with different screen angles are used to determine the dot density level of each color.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic block diagram of a multi-tone image forming apparatus according to an embodiment of the present invention. In this figure, 62 is a recording system that can control the density level of dots including white directly or indirectly by changing the dot diameter at four levels (white is level 1). .

Reference numeral 60 denotes a table converter that generates image data of recorded dots, to which original image data PIX expressed in 6 bits is serially input pixel by pixel as address information, and position information in the X and Y directions ( (2 bits each) x+y is input. Position information
' It is created by dividing the frequency by 4.66. This table converter 60 determines the density of an output dot for an input pixel by a multi-value dither method using a multi-value threshold matrix of 4 rows x 4 columns, and the density information is sent to the recording system 62 as 2-bit data. sent to.

Conceptually, the threshold matrix is, as shown in FIG. 2, a threshold matrix 52. It consists of a threshold matrix 54 whose elements are threshold data of density level 3, and a threshold 71-lix 56 whose elements are threshold data of density level 4.

Here, assume that data of an original image 50 as shown in FIG. 2 is input (each grid of the original image 50 means one pixel, and the number is the value of the pixel data). In the multi-level dither method, basically each image data is compared with one corresponding threshold value data (element specified by x, y) of each darkness value matrix, and if the image data value is equal to or greater than the threshold value, Among the density levels, the highest density level is selected as the density level of the output dot. Therefore, an output image 58 is obtained for the original image 50. Each grid in the output image 58 represents a dot, and the number is a density level.

In this embodiment, instead of directly executing such a comparison operation, the results of the comparison operation are registered in advance as a table inside the table converter 60, and the corresponding comparison operation is performed based on the input pixel data and position information X+Y. I am trying to read the results from the table.

The threshold matrix used in this example can express 49 gradations, is a dot-concentrated type with a kernel number k of 2, and the appearance degree of low-density dots is a maximum of 2 in a 4×4 matrix. It is individual. Also, the screen angle θ
is 45°.

Since such a threshold matrix is used, according to this embodiment, a multi-gradation image with stable image quality, smooth gradation, and satisfactory resolution can be recorded.

FIG. 3 is a block diagram of a multi-color, multi-gradation image forming apparatus according to another embodiment of the present invention. This device records a multi-color, multi-gradation image by overlapping yellow, magenta and cyan plates in a field-sequential manner.

70 is the yellow dot density level (level 1 to 4, level 1
is a table converter to determine white).

This is the same as the table converter 60 in FIG.

That is, this table converter 70 uses the threshold value matrix for the screen angle of 45° shown in FIG.
Performs dither processing on the image data and outputs yellow dot density data (2 bits). This threshold matrix is 4
×4, the pixel clock Hclk of the original image data and the line synchronization signal Vclk are each counted 4
The output of the decimal count 76.78 is input to the table converter 70 as X, Y position information X, V of the input pixel.

Reference numeral 72 denotes a table converter that determines magenta dot density levels (levels 1 to 4) by a multilevel dither method. Conceptually, the multivalued threshold matrix used here consists of threshold matrices 92, 94, and 96 whose elements are threshold data of each density level shown in FIG. This dark value matrix is a dot-concentrated type consisting of four cores,
The screen angle θ is selected so that tan-' θ=1/2, the number of low-density dots appearing is 4 or less in the matrix, and 61 gradations can be expressed.

When this threshold matrix is arranged as shown in FIG. 5, it becomes a 10×10 repeating pattern as shown by the hatched area. Therefore, the 4-bit output data of the decimal counter 80.82 that counts the pixel clock Hclk and the line synchronization signal Vclk, respectively, is input to the table converter 72 as X, Y position information x, y.

Reference numeral 74 denotes a table converter that determines the cyan dot density level (levels 1 to 4) by a multivalued dither method. Conceptually, the multivalued threshold matrix used here is
It consists of a threshold matrix 102, 104, and 106 whose elements are threshold data of each density level shown in the figure. This threshold matrix is a dot-concentrated type consisting of four nuclei,
The screen angle θ is chosen so that tan-10=2.

The number of low-density dots appearing is 4 or less in the matrix, and 61 gradations can be expressed.

The threshold matrix of the table converter 74 also has a repeating pattern of 10XIO, similar to the threshold matrix of the table converter 72. Therefore, the outputs of the decimal counters 80 and 82 are X.

It is given to the table converter 74 as Y position information Xr1.

The multiplexer 84 selects the output data of any one of the table converters 70, 72, 74 specified by the 2-bit color information CLR, and supplies it to the recording system 86. This recording system 86 can control the dot density of each color of yellow, magenta, and cyan into four levels (level 1 is white) directly or by changing the dot diameter. Therefore, one color information CLR
By controlling the , a multi-color, multi-gradation image can be obtained by recording plates of desired colors and overlapping each plate.

Generally, when expressing multi-color and multi-gradation using a plate of three or more colors, if the threshold matrix for each color is made the same, color shift will occur due to misalignment of dots.

However, if the screen angles of the threshold matrices for each color are made different as in this embodiment, such color shift can be prevented. Further, the threshold matrix used in this embodiment has the same characteristics as in the previous embodiment. Therefore, according to this embodiment, it is possible to prevent color shift due to dot position shift, improve reproducibility (stabilize image quality), achieve smooth gradation expression, and achieve sufficient resolution.

Although each of the embodiments described above utilizes a multi-value dither method, it is clear that the present invention can be similarly implemented using a multi-value density pattern method or a multi-value submatrix method.

〔effect〕

As explained above, according to the present invention, it is possible to achieve the effects of stabilizing image quality and achieving smooth gradation expression while maintaining sufficient resolution.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing one embodiment of the present invention, Fig. 2 is an explanatory diagram of a multi-value threshold matrix, 8 images, and an output image used in the same embodiment, and Fig. 3 is another embodiment of the present invention. FIG. 4 and FIG. 5 are explanatory diagrams of a multi-value threshold matrix for magenta. FIG. 6 is an explanatory diagram of a multi-value threshold matrix for cyan. 60.70.72.74...Table converter. 62.86... Recording system. 64.66.76.78... Quaternary counter, 80.8
2... Decimal counter, 84... Multiplexer. Figure 1 (Shi~Tsushi 3)
Figure 6

Claims (2)

[Claims] (1) In an image forming system that controls the density of dots to 3 levels or more directly or indirectly by dot diameter, the density level of dots is determined by comparing the original image data with the threshold data of the threshold matrix. This is a multi-tone image forming method that forms a multi-tone image consisting of a combination of dots.The threshold matrix is a concentrated type having k (k≧2) nuclei, and inside it, A multi-gradation image forming method characterized in that the number of dots whose density is not at the highest level is k or less, and the density level difference between the dots is 1 or less. (2) By using threshold matrices with different screen angles to determine the dot density level of each color,
2. The multi-gradation image forming method according to claim 1, wherein a multi-color, multi-gradation image consisting of a combination of three or more colors is formed.