JPH029657A - Reproduction of medium contrast image - Google Patents

Abstract

PURPOSE:To reproduce a medium contrast image excellent in both of gradation reproducibility and resolving power by selecting the first dither matrix when the mean density of the original picture image is a predetermined value or less while selecting the second dither matrix when the mean density exceeds the predetermined density. CONSTITUTION:A CPU 40A calculates the mean density of an original picture matrix to calculate which gradation said matrix corresponds to, on reference to the conversion table of an ROM 40B. The mean density of the original picture of (A) is 58 and that of the original picture of (B) is 119 and both densities respectively correspond to 4 and 8 in the number of gradations. The number of gradations are compared with the reference number of gradations of 5 and, in the case of the reference number of gradations or less, a Bayer type either matrix is selected and, in the case of above the reference number of gradations, a Fattening type dither matrix is selected. Each pixel unit density of the original picture matrix is compared with the threshold value of the selected dither matrix to determine the recording and non-recording of each dot. An order is issued to a semiconductor laser emitting element 8 to form a recording matrix.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a halftone image reproduction method for reproducing an original picture by displaying the halftones of the original picture in pseudo gradations using the l11m dither method.

[Prior Art] Conventionally, in this type of halftone image reproduction method, a dither matrix with a predetermined threshold value arrangement pattern is used to compare the threshold value with the pixel unit density of the original image, and the process is performed according to the comparison result. By determining recording/non-recording (on/off) for each dot and forming a recording matrix, the halftones of the original image are displayed in pseudo gradation.

One of the threshold placement patterns is a dot concentration type in which the smallest threshold is placed in the center, and as shown in FIG. ) can be concentrated in the center (
The figure shows one of them, the Fatteng type.) In addition, there is a dot dispersion type in which the dots are arranged so that adjacent threshold values are greatly different, and as shown in Fig. 6 (B), by using this type, recording dots can be dispersed in the recording matrix (the figure shows that type). One Bay
er type).

[Problems to be Solved by the Invention] However, in an image reproducing apparatus that implements the halftone image reproducing method described above, for example, in a laser printer, the spot irradiation of about 11M of laser light becomes a recorded dot, and in a dot printer, etc. The stamped points made by the bottle become recording dots, and both recording dots become approximately circular black dots. Therefore,
In order to fill a pixel with a black dot, the size of the recording dot must be at least as large as the circle circumscribing the rectangular pixel. Therefore, the recording driver [•] is a large black dot that protrudes from the pixel. Therefore, when the number of recorded dots increases, adjacent non-recorded dots (off dots)
The total area of the recording dots increases as they protrude into the area, and the number of recording dots no longer corresponds to the density actually reproduced by the recording matrix, impairing the linearity of gradation reproduction.

Therefore, when the above-mentioned dot dispersion type (for example, Bayer type) dither matrix is used, the recording dots are dispersively arranged in the recording matrix, so the density of the image reproduced by the recording matrix is low. Although it is possible to reproduce an image with high resolution in the area, as the number of recorded dots increases, the area of protrusion to the non-recorded dot area increases. ) The gradation reproducibility is poor in areas with high agricultural production.

On the other hand, dot concentration type (for example, Fattening type)
When using a dither matrix, the recording dots will be concentrated in the recording matrix, so
The recording dots are adjacent and the protruding parts overlap each other,
The area of the protruding portion is not relatively large. Therefore,
As shown in the solid line graph in FIG. 7, the gradation reproducibility is excellent. However, because the recording dots are concentrated, the spatial frequency of the recording dots is low. In other words, the dot concentrated type has a problem of poor resolution compared to the dot distributed type.

FIG. 7 shows the results of an experimental study on the relationship between the number of recorded dots and the reproduced density, and shows the results of measuring the reflectance of the recording matrix each time the number of recorded dots increases by one.

As mentioned above, conventional halftone image reproduction methods use either a dot-dispersed type or a dot-concentrated type of dither matrix, and each has advantages and disadvantages in terms of tone reproduction and resolution.

The present invention has been made with the object of providing a halftone image reproduction method that is excellent in both tone reproduction and resolution.

[Means for Solving the Problem] As illustrated in FIG. 1, the gist of the present invention is to compare the threshold 1 @ constituting the dither matrix with the pixel unit density of the original image, and to calculate the result of the comparison. In the halftone image reproduction method, the average density of the original image is determined (Pl), and if the average density is less than a predetermined density (P2-'i'ES
), if the above average density exceeds the predetermined density (P2-NO), the arrangement pattern of the above threshold value is dot concentrated. The halftone image reproduction method is characterized by selecting a second dither matrix that is a type (P4).

[Operation] According to the above-described method of the present invention, when the average density of the original image is less than a predetermined density, the first dither matrix is selected, so that when the recording matrix is formed, the ON dots are dispersedly arranged. . On the other hand, if the average density of the original image exceeds a predetermined density, the second dither matrix is selected, so that when the recording matrix is formed, the ON dots are arranged in a concentrated manner. Therefore, when the density is below a predetermined density, there is a suitable amount of space between the on dots, making it possible to reproduce images with high resolution.
When the density exceeds a predetermined value, all the ON dots become one, resulting in excellent gradation reproducibility and no loss of resolution.

[Example code] An embodiment of the present invention will be described based on the drawings.

As shown in FIG. 2, a halftone image reproducing apparatus (hereinafter referred to as a reproducing apparatus) 1 in which the halftone image reproducing method of the present invention is carried out uses a laser beam to generate a photoreceptor using a laser beam, based on manually inputted image density information. This is a type of printer that forms an electrostatic latent image on a drum and develops it onto recording paper to form an image. Its configuration is generally the same as that of a well-known laser printer. That is, a charger 4, a transfer device 6, a cleaner (not shown), etc. are provided on the circumferential surface of the photoreceptor drum 2, and these devices 4 and 6,
An electrostatic latent image drawn on the photoreceptor drum 2 by laser light is transferred and developed onto a recording paper 20, which will be described later.

Laser light (indicated by a dashed line in the figure) is emitted from the semiconductor laser emitting element 8 toward the polygon mirror 10 . The polygon mirror 10 is a regular octagonal polygon mirror (a regular hexagonal polygon mirror may also be used) that is rotated at a stable speed, so that the laser beam is scanned at a constant speed in the horizontal direction as shown in the figure. become. The laser beam scanned in the horizontal direction enters the synchronization mirror 14 and the rectangular reflection mirror 16 provided opposite the side wall of the photosensitive drum 2 through the f-θ lens 12, and the synchronization mirror 1
The laser beam reflected by 4 irradiates the optical sensor 18, and
The laser beam reflected by the reflection mirror 16 is directed to the photoreceptor drum 2.
will be irradiated.

As described above, when the drum 2 rotates in the direction of the arrow in the figure, the drawn portion of the photosensitive drum 2 that has been exposed to the laser beam is transferred to the recording paper 20 fed by the well-known paper feeding rollers 22, 24, etc. closely followed. As a result, the electrostatic latent image on the photosensitive drum 2 is transferred and developed onto the recording paper 20, and an image is reproduced.

Each of the above-mentioned devices is centrally managed by an electronic control device 40 so that they operate in synchronization. This electronic control device 40 includes a well-known CPU4OA and a ROM40B.
, RAM40C, human output boat 40C, R
The OM 40B stores a dither matrix with a Bayer-type threshold arrangement shown in ■ in FIG. 3(A) and a dither matrix with a Fattening-type threshold arrangement shown in ■ in FIG. 3(B). It is connected to an image reading section 50 that reads the original image as an 81 light image. Furthermore, Ba
The dither matrix with the Yer type threshold value arrangement corresponds to the first dither matrix, and the dither matrix with the Fattening type threshold value arrangement corresponds to the second dither matrix.

When the density read in 256 levels for each pixel of the original image is manually input as multi-value density information from the image reading unit 50, the electronic control device 40 uses the density information and the detection result of the optical sensor 18 to calculate the density. The electrostatic latent image is transferred and developed by controlling the laser emission of the semiconductor laser emitting element 8 and the rotation of the photoreceptor drum 2 and polygon mirror 10.

In the electronic control device 40 described above, when density information is manually input from the image reading section 50, the CPU 40A executes image reproduction processing. The image reproduction process will be explained below based on the flowchart shown in FIG.

The CPU 4OA calculates the average density of the 4×4 original image matrix and stores it in the ROM 40 to determine which gradation it corresponds to.
Obtain by referring to the conversion table stored in B (S
l). For example, the average density of the original picture shown in ■ in Figure 3 (A) is 58, and the average density of the original picture shown in ■ in (R) is 11.
Therefore, according to the conversion table shown in FIG. 3(C), the number of gradations corresponds to 4 and 8, respectively. Next, compare the number of gradations with a predetermined standard number of gradations, for example, 5 (S2), and if it is less than the standard number of gradations (if affirmative in S2), a Bayer-type dither matrix is used. (S3). On the other hand, when the obtained number of gradations exceeds the reference number of gradations (when a negative judgment is made in S2), Fat
Select a ten ing type dither matrix (S
11). Subsequently, each pixel unit density of the 4×4 original image matrix is compared with the corresponding threshold value of the selected dither matrix to determine whether or not each dot should be recorded (S5).

Then, a command is given to the semiconductor laser emitting element 8, etc., to print a recording dot (as shown in the figure) in a square (recording pixel) having the same area as the read pixel, as shown in (■) in Figure 3 (A) and (■) in (B). (indicated by a broken line circle and hatching) to form a 4×4 recording matrix (S6) 6 The reference number of gradations mentioned above is determined in advance for the reason shown below. That is, as is clear from the above-mentioned FIG.
In a recording matrix using a Yer-type dither matrix, when the number of gradations is 5 or less, the increase in the number of gradations is almost directly proportional to the reflection density, and the resolution of the recording mudrisk is high. However, when the number of gradations exceeds 5, the agricultural products (reflection) suddenly rise to 81! ] When the number is 8 or more, the maximum value is almost reached, and the gradation reproducibility is poor.

On the other hand, in a recording matrix using a fattening dither matrix, the number of gradations and the reflection density are almost directly proportional. However, as mentioned above, the resolution is poorer than that of the 8ayer type.

In short, if the number of gradations is 5 as a standard, then Bayer
When the limit is exceeded, a fattening type is used. Note that reproduction apparatuses with different recorded dot sizes and dot pitches have different gradation reproduction characteristics, so the reference gradation is determined in accordance with the characteristics.

When the reference number of gradations is set to 5 and the above-described image reproduction process is executed, the recording matrix shown in FIG. 5(A) is formed. Figure 5 (B
). From the figure, it is clearly proportional.

As described in detail above, in the halftone image reproduction method of this embodiment, the average density of the original image is determined as the number of gradations, and the number of gradations is equal to or less than the predetermined standard number of gradations).
An er-type dither matrix is used, and a fattening-type dither matrix is used in the density region where the number of gradations exceeds the standard number of gradations. The relationship between the number of gradations and the reproduction density is linear, and images with high resolution are reproduced.

In this embodiment, the Bayer type is used as a dot-concentrated dither matrix, and the F is used as a dot-distributed dither matrix.
Although the attening type is used, various other threshold arrangement patterns may be used without departing from the gist of the present invention. For example, a halftone dot type, an area separation halftone dot type, a spiral type, etc. may be used as the dot concentration type dither matrix.

[Effects of the Invention] As explained above, in the halftone image reproduction method of the present invention, when the average density of the original image is less than a predetermined density, the first dither matrix of the dot dispersion type is selected, and the average density is If the density exceeds a predetermined value, the second dither matrix having a medium dot size is selected, so that an image with excellent halftone reproduction and resolution can be reproduced.

[Brief explanation of the drawing]

FIG. 1 is a basic configuration diagram illustrating the halftone image reproduction method of the present invention, FIG. 2 is a schematic configuration diagram of a halftone image reproduction apparatus in which the present invention is implemented, and FIG. 3(A) is a Bayer type FIG. 3 (B) is an explanatory diagram showing the density information and recording matrix when using a dither matrix, and FIG.
) is an explanatory diagram showing a conversion table between density information and number of gradations,
FIG. 4 is a flowchart showing the image reproduction process executed by the electronic control unit, FIG. 5(A) is an explanatory diagram of the recording matrix formed by executing the image reproduction process, and FIG. 5(B)
is a graph showing the relationship between the density reproduced by the same recording matrix and the number of gradations, FIG. 6(A) is an explanatory diagram of the recording matrix when a fattening type dither matrix is used, and FIG. 6(B) is a Bayer An explanatory diagram of the recording matrix when using a type dither matrix, Fig. 7 is Baye
7 is a graph showing the relationship between the density reproduced by a recording matrix and the number of gradations when an r-type and a fattening-type dither matrix are used, respectively. 1... Halftone image reproduction device 40... Electronic control device 4 OA CP 0 40 B-ROM
2O... Image reading section

Claims (1)

[Claims] In a halftone image reproduction method that compares a threshold value constituting a dither matrix with the pixel unit density of an original image, and determines on/off for each dot according to the comparison result to reproduce the original image. Calculate the average density, and if the average density is below a predetermined density, select a first dither matrix in which the arrangement pattern of the threshold values is a dot dispersion type as the dither matrix, and if the average density exceeds the predetermined density, In this case, a second dither matrix in which the arrangement pattern of the threshold values is a dot-concentrated type is selected as the dither matrix.