JPS59193459A - Dot generation method - Google Patents

Abstract

PURPOSE:To produce an image output of high resolution at high speed with a simple structure by generating principal scanning direction and subscanning direction screen signals in synchronism to light beam scanning, and comparing them with image signals to generate dotted signals. CONSTITUTION:Subscanning direction screen signals (a) generated in synchronism to light beam scanning and image signals (b) are added to form signals (c). These signals (c) are compared with principal scanning direction screen signals (d) generated in synchronism to the light beam scanning, and signals (d) exceeding the signals (c) as a threshold value are produced as the high level of dotted signals. As a result, an image output high in resolution and equivalent to the case of numerically arranging contact screens 2-dimensionally can be produced at high speed with a simple structure requiring no large capacity memory.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a dot generation method, that is, a method for forming a halftone dot image in response to an image signal.

In the plate-making process of printing, a contact screen is closely stacked on top of the lithographic sensitive material, and a continuous tone image is projected onto the contact screen to create a halftone screen image. In this contact screen, as shown in FIG. 1(a), patterns are lined up over the entire surface of the screen, with the transmission optical density being low at the center and increasing toward the periphery.

In FIG. 1(a), only one unit of the A-turn is extracted and shown, and the change in transmission optical density is shown by equal transmission optical density lines. If the transmitted optical density is plotted on the vertical axis and the position coordinates are plotted on the horizontal axis, it can be expressed as shown in FIG. 1 (1). The five curves on the graph are ■ in Figure 1 (a)
The graph shows the change in the concentration of the transparent water on the straight line from ■.

Like? When a contact screen with specularity and ξ-turn is closely stacked on a squirrel-shaped photosensitive material and a uniform source is projected from above, the central part of the contact screen where the transmitted optical density is low, that is, where the source easily passes through, is As many elements pass through the light, an image is formed from the photosensitive material directly below that part.

If the projection source is weak, only the center of the screen unit shown in Figure 1(a) will pass through, forming a small halftone dot on the photosensitive material, and if the projection source is strong, the screen shown in Figure 1(a) will pass through. Since the amount of light that passes through the high-concentration portion of the jet level also increases, large halftone dots are formed on the photosensitive material. Similarly, if a continuous-tone image is projected onto a photosensitive material overlaid with a contact screen, large halftone dots will be formed in the bright areas of the continuous-tone image, and small halftone dots will be formed in the dark areas. A negative halftone dot image corresponding to the toned image is formed.

Now, let us consider the case where a halftone image is formed by scanning exposure. A halftone image can also be formed in the same way as described above even if a contact screen is brought into close contact with the lithographic photosensitive material and a continuous tone image is projected by scanning exposure to form "halftone dots" with a plurality of scanning lines. However, with this method, the contact screen must be attached and removed each time exposure is performed, resulting in poor workability. , the cost of consumables is high, halftone dots cannot be formed correctly if a scratched or dirty contact screen is used, there is a large amount of light loss in the contact screen, and it is necessary to use a powerful light source. Highly sensitive photosensitive materials are required, exposure speeds need to be slowed down, and because the original beam is modulated in an analog manner, the light intensity of the light source itself does not change over time and can maintain a stable intensity. Otherwise, the change in light source intensity is equivalent to the change due to the signal, resulting in many disadvantages such as uneven halftone images.Therefore, in recent years, a scanning light beam of 0N has been developed without using a contact screen. −
A method of directly exposing a photosensitive material in the form of halftone dots by turning it off has been devised and used. This method will be referred to herein as a dot generation method, and a device using this method will be referred to as a dot generator.

In the dot generation method, one halftone dot is formed using multiple scanning lines, but unlike the method using a contact screen, the light is turned on only when the original scanning beam approaches the position of the halftone dot to be formed. The light goes out as soon as it passes the point. Exposure is carried out in the form of halftone dots by repeatedly shifting the exposure position little by little, that is, while performing sub-scanning, so as to form the shape of halftone dots.

In practice, a laser or the like is used as a light source, which is kept on at all times, and an acousto-optic modulator (AOM) or the like is interposed in the optical path, and the element is turned ON-OFF using this modulator. Such a dot generation system overcomes the various drawbacks described above associated with the use of contact screens.

Color scanners used in commercial printing and the like are examples of devices that expose using the dot generation method. In this method, a photosensitive material is attached to the drum surface, the drum is rotated, and an original beam is irradiated onto the photosensitive material from an optical head that moves along an axis parallel to the rotational axis of the drum to perform scanning exposure. Let's do it.

On the drum there is an exposure section where photosensitive material is mounted and exposed;
A reading section for optically reading a color original containing an image to be exposed is provided. A color original is illuminated with an original, and the reflected or transmitted original is filtered through an optical filter to detect red, green,
It is divided into three blue components, the light intensity of each component is detected, and based on this signal, the built-in calculator calculates the intensity of the four color components used in printing: yellow, red, indigo, and black. , the size and shape of the halftone dot are calculated from this signal, and based on the results, the scanning FI beam is turned off to expose the dot in the shape of the halftone dot.

Devices that read monochromatic reflective originals and create halftone images have also been developed for applications such as newspaper printing.

The Fttogene sheeting method that has been used so far involves dividing the screen unit of the contact screen shown in Figure 1(a) into, for example, 8 x 8 grids, and transmitting optical density data of the contact screen for each grid. This data is stored in a memory device as it is in advance, and the corresponding AP address is calculated every time a scan is performed, this memory device is accessed, the transmission optical density data of the contact screen is read out, and the data is used as a threshold value. The procedure is to determine whether to turn the source beam on or off. The data stored in the memory device is equivalent to a two-dimensional numerical array of an optical contact screen.

The conventional dot generating force method has the following problems. In other words, each time the scanning light beam approaches the halftone dot area to be formed, it is necessary to access the memory element and read out the contact screen data, and the time required for memory access becomes rate-limiting, making high-speed scanning exposure impossible. There is a point. Furthermore, from a circuit perspective, since a peripheral circuit is required to access the memory, there is a problem that the design is difficult and the manufacturing cost is high. Furthermore, if you try to increase the number of scanning lines to improve the dot shape or smooth the gradation, the total access time will increase because the number of accesses will increase due to the large number of contact screen optical density data stored in the memory element. , a large memory capacity is required, and the peripheral circuitry for memory access also becomes large (-5 problems).Furthermore, the contact screen is divided into board patterns, and contact screen data is determined for each board board. Therefore, the minimum unit of ON-OFF of the original beam corresponds to one square of the base grid, resulting in poor resolution.
Therefore, there is a problem that the sharpness of the output image is low. Conversely, in order to obtain an output image with good resolution and sharpness, it is necessary to reduce the size of one grid square of the substrate, which requires a large memory and complicated peripheral circuits, and furthermore requires access As the time becomes longer, it is not possible to speed up the output time, and it becomes necessary to expose multiple original beams, resulting in an increase in the cost of the apparatus. As a result, devices using such a dot generation method are either large and expensive, such as those used in commercial printing or newspaper printing, or are limited to low-quality and small-screen applications.

The present invention has been made in consideration of the above-mentioned points, and it is an object of the present invention to provide a dot generate force method that can output high-resolution images at high speed and can be implemented using a simple and inexpensive device. purpose.

To achieve this objective, in the present invention, when recording a -C halftone image using the original beam scanning page, a main scanning direction screen signal and a sub scanning direction screen signal corresponding to transmission optical density information of a contact screen are used as a basis. The object of the present invention is to provide a Pttogenerate method in which a halftone signal is generated by generating an image signal synchronized with beam scanning and comparing an image signal synchronized with scanning with both of the screen signals.

The present invention will be described below by way of example with reference to the accompanying drawings.

In order to make it easier to understand the concept of the present invention, the explanation will be made using the explanatory diagrams of the contact screen shown in FIGS. 1(a) and 1(b). Figure 1 (a) on top of the photosensitive material.
Place the contact screen shown in Figure 1 (a) in close contact.
), the original beam of each scanning line reaches the g-light material through the density/ξ turn of the contact screen as shown in Figure 1(b). become. It is the portion where the amount of light exceeds a certain threshold that is recorded as a halftone dot. When exposure is performed without using a contact screen, a signal corresponding to the curve shown in FIG. 1(b) may be generated for each scan. Figure 1 (b
) can be approximately created by moving upward in parallel based on the curve (■). Therefore, it can be considered that the equivalent screen signal in the case of exposure without using a contact screen is as shown in FIG. 1(C). In FIG. 1(C), p' indicates the time required to main scan the length of the screen pitch p. Furthermore, the signal in Figure 1 (C) is Prefectural Figure 1 f.
It can be considered that the main scanning direction screen signal of d) and the sub-scanning direction screen signal of FIG. 1(e) are added. In FIG. 1(e), p+ indicates the time required for sub-scanning the length of the screen pitch p, and t8 indicates the time required for one main scan. For example, if 2000 screen pitches are scanned during one main scan and the blanking time is tb, then t,=2000p';□1+tb.

Next, we will explain one method of halftone dotting an image signal using such a main scanning direction screen signal and a sub scanning direction screen signal.
An example is shown in FIG. FIG. 2(a) shows a screen signal in the sub-scanning direction. FIG. 2(b) shows an image signal. This example uses a gradation image signal that gradually becomes brighter from shadow to light. The image signal is shown as 15 cm for the peta part and +5 square for the brightest part (the part without halftone dots). FIG. 2(d) shows the screen signal in the main scanning direction. In this example, one main scan includes four screen pitches. First, the sub-running five-direction screen signal of FIG. 2(a) and the image signal of FIG. 2(b) are added to form the signal of FIG. 2(C). Compare the signal in Figure 2 (C) with the main scanning direction screen signal in Figure 2 (d), and when the main scanning direction screen signal exceeds the signal level in Figure 2 (C), the halftone signal is determined. The level of The halftone signal is shown in FIG. FIG. 4 shows the exposure state when the exposure beam is turned on when the halftone signal is high level 4.

In the example shown in Figure 2, the halftone signal is set to high level when the result of subtracting the sum of the sub-scanning direction screen signal and the image signal from the main-scanning direction screen signal is a positive value. There are other methods of matching the scanning direction screen signal, the sub-scanning direction screen signal, and the image signal to create the halftone signal. In other words, if the main scanning direction screen signal, sub-scanning direction screen signal, and image signal in Fig. 2 are used as they are, the main scanning direction screen signal and the inverted image signal are added, the sub-scanning direction screen signal is subtracted, and the result is If the value is positive, even if the halftone signal is set to high level, the halftone signal shown in Fig. 3 can be obtained.
If the result of adding the main scanning direction screen signal and the inverted sub-scanning direction screen signal and subtracting the image signal is a positive value, even if the halftone signal is set to high level, the halftone signal shown in Fig. 3 will not be generated. Even if the main scanning direction screen signal, the inverted sub-scanning direction screen signal, and the inverted image signal are added, and the halftone signal is set to high level when the result takes a positive value, the result shown in Fig. 3 is obtained. I get a signal. Each of these methods is a problem in how to take the codes of the main scanning direction screen signal, the sub scanning direction screen signal, and the image signal.

The main scanning direction screen signal and the sub-scanning direction screen signal do not have to have the shapes described in the example above, and the main scanning direction screen signal is a periodic signal whose period is the time required to main scan a length corresponding to the screen pitch. Any signal may be used, and in addition to the triangular waveform described above, a sawtooth waveform, a sine wave waveform, and any other waveform may be used for the purpose of creating a special halftone dot shape.

The screen signal in the sub-scanning direction may be any periodic signal as long as it is a constant value during one main scan and has a period of time required for sub-scanning a length corresponding to the screen pitch as shown in Fig. 2. Like the waveform shown in (a), a waveform that has a downward staircase wave and an upward staircase wave in one period, a waveform in which each level of the downward staircase wave and each level of the upward staircase wave are all different, and a downward or upward staircase wave. Any waveform can be used, such as one consisting of only

The amplitudes of the main scanning direction screen signal and the sub scanning direction screen signal can be arbitrarily selected. For example, the second
If the amplitudes are chosen to be the same as shown in Figures (a) and (d), the contact screen equivalent to these screen signals is shown in Figure 1 (
It will be like a). The amplitudes in the main and sub-scanning directions can be chosen to be unequal so as to be equivalent to a special contact screen.

Furthermore, halftone dots equivalent to a special contact screen can be created by variously selecting the DC offset components of the main scanning direction screen signal and the sub scanning direction screen signal.

By appropriately selecting the amplitude of the image signal and the DC offset component, the contrast of the recorded halftone image and the overall density level can be adjusted.

The main scanning direction screen signal and the sub-scanning direction screen signal differ depending on the fixed force type, but they can be easily generated without using a memory element or the like.

For example, when the main scanning is performed by the rotation of the drum and the sub-scanning is performed by the lateral movement of the optical head, as in the case of color scanners used in the Soye printing press, etc., a rotary encoder or the like is attached to the drum's rotation axis, and the zero position signal and the angle of the drum are If the signal is detected as a rectangular wave, a screen signal in the sub-scanning direction can be generated using the zero position signal as a trigger, and if one period of the angle signal is selected to be equal to p'Ill, the angle signal can be integrated to generate a triangular wave type screen signal. It is good to generate a sine wave type screen signal in the main scanning direction through the low/ξ filter. Instead of the angle signal of the rotary encoder, a signal synchronized with the zero position 41 word may be generated by a phase locked loop (PLL) circuit.

To synchronize the image signal with the movement, use the above-mentioned zero position signal,
A known method may be followed using an angle signal or the like.

As an example other than color scanners, when the photosensitive drum is rotated like in a laser beam printer, and the scanning source beam is scanned over the drum surface parallel to the drum's rotation axis using a rotating polygon mirror, the main scan is performed by rotating the drum. This is done by a polygon mirror, and the secondary movement is done by rotating a drum.

In this case, the main scanning start point is detected by a photodetector as a zero position signal, and a signal corresponding to the angle signal in the above example is sent to the PLL.
It is sufficient to create a signal synchronized with the zero position signal using a circuit.

As understood from the above description, the dot generation method of the present invention improves various problems faced by conventional dot generation methods, as detailed below. In other words, the dot generating method of the present invention does not require accessing a memory device and reading out contact screen data each time a set-cutting light beam approaches a halftone dot area to be formed, unlike the conventional method. This is because the contact screen data is always generated separately in the main scanning direction and the sub-scanning direction. Therefore, the dot generation method of the present invention has the advantage that it can be applied to high-speed constant distortion exposure, where the conventional dot generation method could not be used because the memory access time was the limiting factor.

Furthermore, when considered from a circuit perspective, the dot generation method of the present invention has the advantage that it does not require the use of memory, so there is no need for peripheral circuits to control the memory, making the design easy and manufacturing costs low. . In other words, when the conventional dot generation method tried to increase the number of scanning lines to improve the dot shape and smooth the gradation, the number of optical density data of the contact screen increased in the memory element, which required a large capacity memory. In contrast, the dot generation method of the present invention requires only an increase in the number of levels of the screen signal in the sub-scanning direction, resulting in a simple circuit, ease of design, and It has the advantage of improving halftone dot quality without sacrificing advantages such as high-speed scanning capability.

Furthermore, in the conventional bit generation method, the ON-OFF unit of the original beam in the main scanning direction depends on the density of memory data in the main scanning direction, resulting in poor resolution in the main scanning direction (and poor image sharpness). However, in the dot generation method of the present invention, the length of ON7 to OFF of the original beam in the main scanning direction is Even when the fixed line density is low, the resolution in the main scanning direction is maintained at a high level.Increasing the fixed line density only increases the number of levels of the sub-scanning direction screen signal, which increases circuit complexity. The advantage is that there is nothing to do.

From the above advantages, the dot generation method of the present invention has the great feature that it can generate pits at high speed with a small and inexpensive circuit, and can realize a dot generator that outputs a high-resolution halftone image. .

[Brief explanation of drawings]

Figures 1 (a) and (b) show the transmitted optical density and ξ turn at one halftone dot of the contact screen used for halftone dot formation;
Figure 1 (C), (
d) is a straight line of the concentration change curve in Figure 1(b);
and a diagram illustrating an example of a screen signal in the main scanning direction in the present invention, in which it is continued in the direction of a beggar's set meal,
FIG. 1(e) is a diagram showing an example of the sub-scanning direction screen signal used in combination with the above-mentioned main-scanning direction screen signal, and FIG. Scanning direction screen signal, image signal. a waveform diagram showing a weight signal of a sub-scanning direction screen signal and an image signal, and a main scanning direction screen signal, respectively;
Figure 3 is a waveform diagram showing a halftone signal formed by matching the image signal with the main and sub-scanning direction screen signals, and Figure 4 shows an example of a dot image formed using the halftone signal in Figure 3. FIG. p... Screen pitch, D... Transmission optical density, ■
...Heavy use. 62 Figure (b) also 3 (8) +0FF) 1 quantitative date 1st day 20th 30th 4th 50th 60B 7th 8th day Cell 4 Figure opening sub-scanning direction □ 371-

Claims (1)

[Claims] 1. In a method of forming a halftone image by original beam scanning, values are periodically set in accordance with transmitted optical density information when a halftone dot forming contact screen is scanned in each of the main and sub-scanning directions. A screen signal in the main scanning direction and a screen signal in the sub-scanning direction are generated in synchronization with the scanning of the original beam, and the image signal synchronized with the scanning of the original beam is compared with both of the screen signals to perform halftone dot formation. A dot generation method characterized in that a signal is formed and a dot image is formed based on the halftone signal. 2. The method according to claim 1, wherein the main scanning direction screen signal is applied to one of the contact screens.
It is a signal whose value changes in one cycle of the time required to main scan the length of the screen pitch, and the sub-scanning direction screen signal maintains a constant value over one main scanning period while changing the length of the one screen pitch. A pit generation method in which the image signal is a signal whose value changes periodically based on the time required for sub-scanning, and the image signal is an analog signal.