JPH0127627B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for recording an original image having continuous gradation as a halftone image using a mesh used in printing, etc., and in particular, by electrically synthesizing mesh pattern signals. The present invention relates to a method for recording halftone images relatively easily.

In recent years, various methods have been proposed for converting an original image having continuous gradation into a halftone image and recording it.

The basic method is to use a color scanner to scan an original image with continuous gradation, modulate a recording light beam with the image signal obtained, and use this modulated recording light beam to cover the entire surface of the contact screen. There is a method in which a photosensitive material, such as a lithium film, is scanned and exposed to obtain a halftone image. However, the contact screen originally lacks durability, is easily damaged when it comes into close contact with a photosensitive material, and has the disadvantage that it takes a lot of effort to put it in close contact with the material.

In addition, instead of using the method of directly attaching the contact screen to the photosensitive material as described above, it is also possible to create an original image with continuous gradation, a screen with a mesh pattern copied from the contact screen, and a method for wrapping the photosensitive material around the outer periphery of each. Three drums are provided, the drums are rotated in synchronization with each other, the original image and the screen are photoelectrically scanned, and the drive circuit of the light source of the recording light beam is controlled by the electrical signal obtained as a result of the photosensitive material. A method of recording a halftone image on top is proposed in JP-A-53-85601. However, even in this case, since a plurality of drums are required, there is a drawback that the apparatus becomes larger.

using such contact screens or
In order to improve the shortcomings of the recording methods used, various methods have already been proposed for recording halftone images by electro-optical means without using a contact screen.

A typical method is to photoelectrically scan an original image with continuous gradation and read out the image signal obtained by photoelectrically scanning and mesh density distribution information stored in read-only memory (ROM) in synchronization with the photoelectrically scanning. The obtained signals are sequentially compared to obtain a halftone dot image pulse signal, and based on this pulse signal, the driving circuit of the light source of the recording light beam is controlled in the same manner as described above, thereby forming halftone dots on the photosensitive material. There is a way to record images. However, the above recording method also has the following drawbacks. Since all of the mesh density distribution information must be stored in a storage element such as ROM, a large capacity storage element is required.
As a result, a problem arises in economical efficiency, and furthermore, a modulation signal is calculated based on a large amount of information such as the number of mesh lines, angle, position information, and input image density, in accordance with the scanning position of the recording light beam. Since the halftone image signal is obtained by doing this, if the mesh angle is tilted, for example, 15 degrees or 30 degrees, complicated calculations are required, making it necessary to introduce a microcomputer, etc. However, since this digital calculation also takes time, it has the drawback of being limited in terms of recording speed.

The present invention has been made in view of the shortcomings of the above-mentioned conventional methods, and by electrically synthesizing mesh density distribution information, it is possible to relatively easily create halftones without requiring a large-capacity storage element. It provides a method for recording images.

Generally, when the concentration distribution of a contact screen is expressed in two-dimensional (x, y) coordinates, It can be expressed by the approximate expression:

In the above equation (1), K is a constant determined according to the output voltage level of the input image signal. p is the pitch of the mesh to be recorded and generally corresponds to the length of the diagonal of the mesh with an area ratio of 50%. θ is the angle of the mesh; here, when the mesh with an area ratio of 50% is oriented as 〓, θ = 0°, and when it is oriented as 〓, θ = 45°.

In the above equation (1), if we specify θ and p and perform calculations according to the position information (x, y) scanned by the light beam, we can find F(x, y), and then It is sufficient to obtain a mesh image signal by comparing it with the input image signal of , but the calculation of equation (1) cannot be performed very quickly on a small microcomputer. Therefore, in the conventional method, a method was used in which a value calculated in advance was stored in a storage element.

However, as is well known, equation (1) can be decomposed as shown below.

F(x,y)=Kcos[2π/p{−xsin(θ−π/4
)+ysib(θ+π/4)}] +Kcos[2π/p{xsin(θ+π/4)−ysin
(θ−π/4)}]+2K…(2) In this way, F(x, y) can be expressed as the sum of two sine wave patterns and a DC component, and therefore two sine wave patterns are generated. This can be synthesized using a container.

This will be explained below with reference to the drawings.

FIG. 1 is a block diagram for explaining an embodiment to _which the halftone image recording method according to the present invention is applied. tsinA+M/NτsinB
)} ...(3) Generates a function.

Also, in the second function generator 2, ₂ (t) = K・cos {2π/τ(tsinB+M/NτsinA
)} ...(4) Generates a function.

Here, A=θ-π/4, B=θ+π/4 θ gives the angle of the mesh to be recorded. τ is the desired mesh pitch p on the recording screen.
This corresponds to the time it takes to scan a distance corresponding to . The value of τ is determined by assuming the main scanning distance by the light beam as x,
(assuming scanning in the x-axis direction), and the scanning time is t
It is expressed as τ=p/x・t.

Further, N and M are each integer values, and N is an integer fraction of the pitch p of the mesh where the pitch q of the main scanning lines of the light beam (the interval between the main scanning lines in the Y-axis direction) is recorded. (i.e. N=p/
q), where M is a value representing the current number of scanning lines. The constant K is a coefficient for balancing the output voltage level of the input image signal. N and θ are specified before the start of recording, and M is given by counting in accordance with the main scan. Each function ₁ (t) and f ₂ (t) is generated in synchronization with main scanning. The clock signal generating circuit 10 determines the timing.

The adder 4 is a circuit that adds the DC components from the first and second function generators 1 and 2 and the DC signal generation circuit 3 that supplies the DC component. The signal F(t) added by adder 4 is finally A two-dimensional mesh pattern signal F is obtained. This mesh signal F and the input image signal T from the input image signal generation circuit 5 are input to the comparator 6, and the calculation of S=T-F is performed, and as a result, if S0, it is ON;
If S<0, signal processing of OFF is performed in the ON-OFF circuit 7, and the signal controls the optical modulator processor 8 to modulate the recording light beam.

The operation order of the above embodiment will be explained in more detail as follows.

The clock signal from the clock signal generating circuit 10 serves as a timing signal for starting main scanning by the light beam, and also serves as a trigger signal for the first and second function generators 1 and 2. Furthermore, this clock signal is input to a counter 11, which counts the number of times main scanning has been performed, and uses this value as M to instruct the first and second function generators 1 and 2 as phase values. The signals from the first and second function generators 1, 2 generate respective functions ₁ (t) and ₂ (t) at this phase value. Further, the frequency of this function is automatically set by initially specifying the mesh angle θ.

A specific example of a halftone dot image generated by the embodiment shown in FIG. 1 is shown in FIGS. 2A and 3A. FIG. 2A shows the case when θ=45°, while FIG. 3A shows the case when θ=30°.

The case where θ=45° will be described in detail below with reference to FIGS. 2A to 2D.

When θ=45°, the above formulas (3) and (4) are ₁ (t) = Kcos (2π/τ・M/Nτ) ...(3)′ ₂ (t)=Kcos (2π/τ・t) ...(4)', and ₁ (t) is t as shown in Figure 2B.
₂ (t) is a cosine wave with period τ, and its waveform is shown in FIG. 2C.
FIG. 2D shows the waveform of F(t) synthesized by adding the DC component 2K to the above (3)' and (4)'.

The above F(t) indicates the mesh pattern signal when θ=45°. ₁ (t), M=
0, that is, at the start of scanning, the DC signal becomes ₁ (t) = K, and when M = 1, that is, at the start of recording the second scanning line, the DC signal becomes K・cos (2π/τ・1/Nτ). It becomes a constant value, and thereafter becomes a DC signal whose level fluctuates as M=N. In contrast, ₂
Since (t) does not include the term M, it is a cosine wave (period τ) always emitted from a constant phase value (σ). Combining this and adding the DC component, we get θ
The mesh pattern signal F(t) when =45° has the same period, the same amplitude, and the same phase value, but the bias amount varies for each scanning line. When a halftone dot image is recorded by turning the light beam ON and OFF while comparing such a halftone pattern signal F(t) with an input image signal T having an area ratio of exactly 50%, the halftone dot image shown in FIG. 2A is obtained. A mesh as shown in is formed.

Next, the case where θ=30° will be explained with reference to FIGS. 3A to 3D. In this case as well, the aforementioned θ=
Needless to say, the principle is the same as in the 45° case.

In this case, the above equations (3) and (4) become as follows.

₁ (t) = Kcos {2π/τ(−0.259×t+0.966M/
N τ)} ...(3)″ ₂ (t) = Kcos {2π/τ(0.966t+0.259M/Nτ
)} ...(4)″ Function F obtained from the sum of (3)″ and (4)″ above and 2K
(t) has a more complicated shape than in the case of θ=45° described above.

From equations (3) and (4), function ₁ (t) has period τ/0.259, phase 0.966/0.259 M/Nτ, and function ₂ (t) has period τ/0.966, phase 0.259/0.966 M/Nτ. It is a cosine function.

In Figures 3B and 3C, M = 0.1~
10, that is, as the scanning lines are sequentially recorded, the phases of ₁ (t) and ₂ (t) shift by the amount expressed by the above equation. This can be easily accomplished using well-known techniques on electronic circuits. If θ and p, and therefore τ, are specified before starting recording, ₁ (t) and
₂ (t) and set the frequency corresponding to θ and p, and also set the basic phase value, for example,
Since it can be stored in ROM etc.
All you have to do is read out the value as needed and send a signal to each function generator. In this case, it goes without saying that a ROM as large in capacity as the ROM used in the conventional system is not required.

As described above, according to the present invention, mesh pattern signals of any angle and any pitch can be synthesized relatively easily, and by using a method of performing main scanning with a light beam and simultaneously recording with a recording light beam. ,
Extremely high quality halftone images can be recorded.

In the above embodiment, a method of synthesizing a mesh pattern signal using a sine wave was explained.
The present invention is not limited to this sine wave,
It is also possible to synthesize the mesh pattern signal using a continuous function such as a triangular wave or a trapezoidal wave.

[Brief explanation of drawings]

FIG. 1 is a block diagram for explaining an embodiment to which the halftone image recording method according to the present invention is applied, and FIG.
The figure shows a specific example of a halftone image when the angle θ = 45°.
Figure 2B shows the first function signal when the angle θ = 45°.
₁ (t), Figure 2C is a waveform diagram showing the second function signal ₂ (t) when the angle θ = 45°, and Figure 2D is the mesh pattern when the angle θ = 45°. The waveform diagram, FIG. 3A, showing the signal F(t) shows the angle θ=
A specific example of a halftone image when the angle is 30 degrees, Figure 3B is a waveform diagram showing the first function signal ₁ (t) when the angle θ = 30 degrees, and Figure 3C is a waveform diagram when the angle θ = 30 degrees. FIG. _3D is a waveform diagram showing the mesh pattern signal F(t) when the angle θ=30°. DESCRIPTION OF SYMBOLS 1...First function generator, 2...Second function generator, 3...DC signal generation circuit, 4...Adder, 5
...Input image signal generation circuit, 6...Comparator, 7...
...ON-OFF circuit, 8...recording light modulation processor, 9...main scanning signal, 10...clock signal generation circuit, 11...counter, 12...y value setting circuit.

Claims (1)

[Claims] 1. An input image signal T(t) obtained by photoelectrically scanning an original image having continuous gradations, and a mesh pattern signal F(t) output in synchronization with the photoelectrically scanning the original image.
In the method of recording a halftone image by sequentially comparing the first function signal ₁ (t) ₁ (t) = K cos
B)} and second function signal ₂ (t) ₂ (t) = Kcos {2π/τ(tsinB+M/N・τ・sin
A)} {However, A=θ−π/4, B=θ+π/4, τ=p/x・t, N=p/q, (θ: mesh angle of halftone image, x : above-mentioned photoelectric scanning one scanning distance, t : one scanning time of the photoelectric scanning, p: mesh pitch of the halftone image, q: pitch of the photoelectric scanning line, N is an integer), and M is the time point at which the photoelectric scanning starts. So M=
0, M=0 at the start of one photoelectric scan, and M=1, 2, 3, . . ., N each time one photoelectric scan ends and one line halftone image is recorded. The above first function signal ₁
(t) and the second function signal _{2 (} an integer that determines the phase value of The sum of the DC signal 2K (K is a constant determined by the output voltage level of the input image signal) from the signal generator is taken as the mesh pattern signal F(t), and the result is sequentially compared with the input image signal T(t). and recording a halftone image using the obtained signal.