JP2607661B2 - Image signal processing device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image signal processing apparatus for converting a continuous tone image into a halftone image, and more particularly, to an image signal obtained from the continuous tone image. And a threshold signal, and when the difference between these signals is within a predetermined range, an image signal processing apparatus configured to generate an image signal related to a halftone image based on a signal set stochastically.

[Background of the Invention] In recent years, in the field of printing and plate making, image scanning reading and recording apparatuses for electrically processing image information of a document to create a film original for the purpose of streamlining work processes, improving image quality, and the like have been widely used. It is used for

This image scanning reading and recording apparatus basically includes an image input unit, an image processing unit, and an image output unit. That is, in the image input unit, the document conveyed to the predetermined image reading position is scanned by laser light or the like, and the image information is converted into an electric signal corresponding to the intensity of the reflected light such as the laser light.
Next, the photoelectrically converted image information is subjected to arithmetic processing such as gradation correction and contour emphasis in accordance with plate making conditions in an image processing unit, and is then converted again into an optical signal such as a laser beam, and the recording medium is formed of a photosensitive material. It is recorded and played back. The record carrier is developed by a predetermined developing device, and is used as a film original for printing or the like.

By the way, when the original to be printed and copied is a continuous tone image such as a photograph or a painting, it is necessary to decompose the original image in order to express the shading of the image. That is, the continuous tone image is converted into a halftone image composed of a group of halftone dots having a size corresponding to the density of the image. Here, as a method of halftone decomposition, there is a method of irradiating an optical signal related to the continuous tone image onto a record carrier through a contact screen in which peripheral blurred points are arranged in a mesh shape. On the other hand, the image scanning reading and recording apparatus employs a method of electrically generating a halftone screen signal corresponding to a contact screen and forming a halftone image based on the halftone screen signal.

Here, the mesh screen signal is composed of a threshold signal having a number of gradations according to the resolution, and the threshold signal is compared with an image signal relating to a continuous gradation image to obtain a 2D signal.
A digitized halftone image signal is obtained. In this case, if the number of gradations of the threshold signal is small, there is a problem that a tone jump occurs in the obtained halftone dot image.

That is, FIG. 1 shows a relationship between a threshold signal and an output density of an image which is halftone-decomposed based on the threshold signal. Ideally, as shown by a dashed line, a linear gradation is shown. Although it is desirable to have a, the output density of the network resolved image becomes like a ₁ to a ₆ by influence of the diameter of the laser beam relative to the threshold signal in the image scanning reading and recording apparatus in reality. In this case, while the threshold signal goes from 2 to 3,
Alternatively, the output density of the halftone-decomposed image fluctuates greatly between 5 and 6. A so-called tone jump occurs.

Therefore, in order to suppress the occurrence of such a tone jump, it is conceivable to increase the number of gradations of the threshold signal and set the network resolution high. However, in this case, the circuit configuration becomes complicated, the device becomes expensive, and there is a disadvantage that a long time is required for signal processing. In another method, a halftone image is obtained by comparing an image signal with a threshold signal to which noise generated at random without increasing the number of tones of the threshold signal. In this case, the tone jump is relatively difficult to visually recognize. However, it has not been possible to completely eliminate fluctuations in output density due to low resolution.

[Object of the Invention] The present invention has been made to overcome the above-mentioned disadvantages, and compares an image signal obtained from a continuous tone image with a threshold signal, and a difference between these signals is within a predetermined range. By generating an image signal related to a halftone image using a signal set stochastically, it is possible to obtain a halftone image that is smoother than a continuous tone image at high speed and at low cost without increasing resolution. It is an object of the present invention to provide an image signal processing device that can perform the processing.

[Means for Achieving the Object] In order to achieve the above object, the present invention compares an image signal relating to a continuous tone image with a threshold signal, and determines that the image signal is larger than the threshold signal by a predetermined amount. 1 and outputs a second comparison signal when the image signal is smaller than the threshold signal by a predetermined amount, and outputs a third comparison signal when the difference between the image signal and the threshold signal is within a predetermined range. A comparing means for outputting, a random number generating means for generating a random number signal, a probability signal output means for outputting a probability signal set in advance according to the image signal, a predetermined probability based on the random number signal and the probability signal A selection signal output means for outputting a selection signal, and a halftone image signal based on the first or second comparison signal when the first or second comparison signal is output from the comparison means. The third comparison signal is output Signal selection means for outputting a halftone image signal based on the selection signal from the selection signal output means when the signal is input.

[Embodiment] Next, a preferred embodiment of an image signal processing apparatus according to the present invention will be described in detail with reference to the accompanying drawings.

In FIG. 2, reference numeral 10 indicates an image scanning reading and recording apparatus to which the image signal processing apparatus according to the present embodiment is applied. In the image scanning reading and recording apparatus 10, the continuous tone image carried on the document S is converted into an electric signal and then output on the film F as a halftone tone image.

The original S is configured to be conveyed in the sub-scanning direction by a conveying unit (not shown) in the direction of arrow A. The main scanning is performed in the B direction. The continuous tone image photoelectrically converted by the CCD 14 is converted into an image signal P as a digital signal by the A / D converter 18 based on the main scanning clock signal X from the clock generator 16, and then the image according to the present embodiment. It is supplied to an image processing unit 20 which is a signal processing device. The image processing unit 20 performs image processing such as halftone decomposition processing on the image signal P, and outputs the binarized halftone image signal R to the image recording unit 22. The image recording unit 22 converts the halftone image signal R into an optical signal such as a laser beam and guides it on the film F to record a halftone image.

FIG. 3 shows the configuration of the halftone decomposition processing circuit in the image processing section 20 of FIG. In this case, the image processing unit 20
Is a line memory 24, a threshold data table 26, a data comparator 28 as comparison means, a data selector 30 as signal selection means, a random number generation circuit 32 as random number generation means, a random number probability control table 34 as selection signal output means, Counter 36, 38
And an auxiliary bit data table 40 as a probability signal output unit.

The line memory 24 holds the image signal P relating to the continuous tone image for each main scanning line. On the other hand, the threshold data table 26 stores a threshold signal Q for converting a continuous tone image into a halftone image.
Hold. The data comparator 28 receives the image signal P and the threshold signal Q
When the image signal P is larger than the threshold signal Q, when the image signal P is smaller than the threshold signal Q, and when the image signal P is equal to the threshold signal Q, the comparison signals S ₁ and S are respectively obtained. and outputs the ₂ and S ₃ to the data selector 30. The data selector 30 outputs the comparison signals S ₁ and S ₂ from the data comparator 28.
Or selection signal from S ₃ and the random number probability control table 34 Z
And outputs a halftone dot image signal R which is a binary signal based on.

In this case, the random number probability control table 34
And a 2-bit random number signal r ₁ , r _2, and a 2-bit auxiliary bit signal i ₁ , i ₂ which is probability code data from the auxiliary bit data table 40, and outputs a selection signal Z. The settings are made as shown in FIG. The random number generation circuit 32 is described in, for example, Transactions of the Society of Instrument and Control Engineers Vol.2.
3, No. 8 (August 1987).

The auxiliary bit data table 40 holds a probability signal for setting the probability that the selection signal Z is in a predetermined state, for example, the state of 1. The image signal is based on the tone jump of the output density shown in FIG. The auxiliary bit signals i ₁ and i ₂ for P are preset as shown in FIG. The counters 36 and 38 output a timing signal x for extracting the auxiliary bit signals i ₁ and i ₂ from the auxiliary bit data table 40 in accordance with the main scanning clock signal X and the sub scanning clock signal Y.
Generate y.

The image scanning reading and recording apparatus to which the image signal processing apparatus according to the present embodiment is applied is basically configured as described above. Next, the operation and effects thereof will be described.

First, the CCD 14 scans the document S conveyed in the sub-scanning direction in the direction of arrow A.
The main image is converted into an electric signal by performing main scanning in the direction of arrow B through the light condensing optical system 12 on the continuous tone image carried by the image forming apparatus. Next, the continuous tone image converted into the electric signal is converted into a digital signal by the A / D converter 18 based on the main scanning clock signal X from the clock generator 16, and an image signal P is obtained. This image signal P is temporarily stored in the line memory 24 of the image processing unit 20 for each main scanning line, and then compared with the threshold signal Q stored in the threshold data table 26 by the data comparator 28 (see FIG. 3). ).

In this case, the data comparator 28 outputs the image signal P and the threshold signal Q
And compares the comparison signals S ₁ , S ₂ and S ₃ with the data selector according to the case of P> Q, P <Q and P = Q, respectively.
Output to 30. When the data selector 30 is output comparison signals S ₁ from the data comparator 28, and outputs the dot image signal R is ON signal (= 1) to the image recording section 22. Also outputs dot image signal R is OFF signal (= 0) in the image recording unit 22 when the output is a comparison signal S _2. Furthermore, if the output is a comparison signal S _3, the ON signal based on the selection signal Z from the random number probability control table 34 (= 1) or off signal (= 0) to the image recording section 22 as the halftone dot image signal R Output. The image recording unit 22 controls an optical signal such as a laser beam in accordance with the on / off signals, and records a halftone image on the film F.

Here, a random number probability control table for outputting the selection signal Z
34 is set as shown in FIG. That is, the random number probability control table 34 is a combination of the 2-bit random number signals r ₁ and r ₂ supplied from the random number generation circuit 32 and the 2-bit auxiliary bit signals i ₁ and i ₂ supplied from the auxiliary bit table 40. On the other hand, the selection signal Z is set. In this case, the probability that the selection signal Z becomes 1 is 0%, 25%, 50% and 75% for the four combinations of the auxiliary bit signals i ₁ and i ₂ , respectively.
Set so that On the other hand, the auxiliary bit data table 40 is set as shown in FIG. 5 based on the relationship between the threshold signal and the output density shown in FIG. For example, when the image signal P is 3, the probability that the halftone image signal R will be an ON signal (= 1) is set to 50% for the threshold signal Q of 3 and the output density is a _{2 in} FIG. and set to be an intermediate between a _3. Therefore,
For the image signal P of _No. 3, i ₁ = 1 and i ₂ = 0.
Further, i ₁ = i ₂ = 1 is set so that the halftone dot image signal R becomes an ON signal (= 1) with a probability of 75% for the image signal P of 6.

Therefore, the random number probability control table 34 and the auxiliary bit data table set as shown in FIGS.
The selection signal Z is obtained using 40. For example, when the image signal P is 3, from the auxiliary bit data table 40, i ₁ =
1, i ₂ = 0 auxiliary bit signals i ₁ , i ₂
It is supplied to the random number probability control table 34 based on the timing signals x and y from 38. The random number probability control table 34 outputs to the data selector 30 a selection signal Z which becomes 1 with a probability of 75% based on these auxiliary bit signals i ₁ and i ₂ . on the other hand,
The data selector 30 outputs the comparison signals S ₁ and S ₂ from the data comparator 28.
Are supplied to the image recording unit 22 based on the comparison signals S ₁ and S ₂ . Then, when receiving the comparison signal S ₃ from the data comparator 28 outputs a halftone dot image signal R based on the selection signal Z from the random number probability control table 34 to the image recording unit 22.

FIG. 6c shows a halftone image signal R obtained by comparing the image signal P of 3 (FIG. 6a) with the threshold signal Q of 0 to 8 (FIG. 6b). . In this case, the halftone dot image signal R, which is an ON signal (= 1) corresponding to the threshold signal Q of 3, is generated with a probability of 50%. Therefore, the recording unit 22
Thus, the input density of the halftone image recorded on the film F is the halftone density (a ₂ + a ₃ ) / 2 between a ₂ and a _{3 in} FIG. As a result, a smooth image with less noticeable tone jump can be obtained.

In the above description, the 2-bit random number signals r ₁ and r ₂ and the 2-bit auxiliary bit signal are used to obtain the selection signal Z.
Although i ₁ and i ₂ are used, if the number of bits of these signals is increased, the probability that the selection signal Z becomes 1 can be set more finely, whereby a smoother image can be displayed on the film F. It is possible to record.

[Effects of the Invention] As described above, according to the present invention, an image signal obtained from a continuous tone image is compared with a threshold signal, and when a difference between these signals is within a predetermined range, generation is performed with a predetermined probability. The image signal related to the halftone image is obtained based on the selected selection signal. In this case, since the variation of the halftone image can be suppressed by the generation probability of the selection signal, a smooth halftone image can be obtained without increasing the number of tones at the time of halftone decomposition. . Moreover, since it is not necessary to increase the resolution in order to obtain a smooth image, there is an advantage that the image signal can be processed at low cost and at high speed.

As described above, the present invention has been described with reference to the preferred embodiments. However, the present invention is not limited to these embodiments, and various improvements and design changes can be made without departing from the gist of the present invention. Of course.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining the relationship between a threshold signal and an output density according to the prior art and the fact that a new output density is generated by the image signal processing apparatus according to the present invention. FIG. 2 is an image signal processing apparatus according to the present invention. 3 is a schematic block diagram of an image signal processing apparatus according to the present invention, FIG. 4 is an explanatory diagram of a random number probability control table shown in FIG. 3, and FIG. 6 is an explanatory diagram of the auxiliary bit data table shown in FIG. 3, and FIGS. 6a to 6c are explanatory diagrams of an image signal, a threshold signal, and a halftone image signal in the image signal processing device according to the present invention. 10 image scanning reading and recording device, 14 CCD 20 image processing unit 22, image recording unit 26 threshold data table 28 data comparator 30 data selector 32 random number generation Circuit 34: Random number probability control table 40: Auxiliary bit data table F: Film, S: Original

Claims (2)

(57) [Claims] An image signal relating to a continuous tone image is compared with a threshold signal, and a first comparison signal is output when the image signal is larger than the threshold signal by a predetermined amount. A comparison unit that outputs a second comparison signal when the difference between the image signal and the threshold signal is within a predetermined range, and outputs a third comparison signal when the difference between the image signal and the threshold signal is within a predetermined range; Probability signal output means for outputting a preset probability signal in accordance with the image signal, selection signal output means for outputting a selection signal at a predetermined probability based on the random number signal and the probability signal, and the comparison means 1st or 2nd
When a comparison signal is output, a halftone image signal based on the first or second comparison signal is output, and when a third comparison signal is output by the comparison means, based on a selection signal from the selection signal output means. An image signal processing apparatus comprising: a signal selection unit that outputs a halftone image signal. 2. An image signal processing apparatus according to claim 1, wherein said probability signal output means comprises a data table set as probability code data for the image signal.