JPS58218272A - Halftone detector - Google Patents

Abstract

PURPOSE:To prevent a moire from occurring at the time of halftone picture image scanning in a halftone picture image detector for a scanning paper used for a facsimile terminal equipment, by detecting whether or not the variable point interval of a white and black two-valued picture signal exists within the range of a specified cycle. CONSTITUTION:A picture signal (a) is inputted in a block 1, and a variable point interval signal (b) of positive pulse is outputted at the rise of the picture signal (a). A block 2 outputs a positive reset pulse (c) to a counter 3 at the fall of the signal (b). The counter 3 counts a sampling clock (d) after being reset and outputs a count value N to comparators 4 and 5. The comparator 4 compares the count value N with a set value and outputs a discrimination result (e) of 1, if the count value N is small. The comparator 5 compares the count value N with the set value and outputs a discrimination result (f) of 1, if the N is large. If the signals (b), (e) and (f) become 1 at the same time in an AND gate 6, a halftone detection signal (g) of 1 is outputted.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a halftone dot detection device used in a facsimile machine to detect a halftone dot image included in a scanned paper surface.

Generally, it is known that when a paper surface consisting of non-halftone images such as characters and line drawings and halftone images of photographs is scanned at a certain value of sub-scanning line density, moiré occurs in the halftone image area. In order to prevent this moire, it is necessary to scan the sub-scanning line density at a value that makes moire less noticeable in the area including the halftone image. In order to automatically switch the sub-scanning line density at a halftone dot area during scanning, it is essential to detect the halftone dot area by determining the characteristics of the image.

Conventionally, a method has been proposed in which the sub-scanning line density needs to be changed by marking a mark line or the like on the edge of the paper in advance and reading the mark.

Furthermore, in recent years, in order to avoid the trouble of making such marks, a system has been developed and put into practical use that automatically switches the image by determining the characteristics of the image on the paper by analyzing the image signal.

As a method of determining the characteristics of this image, there is a method of measuring the density of points where the image signal changes from white to black or from black to white. This method uses images with thin text.

Is it made up of detailed designs, etc., or has large letters?
It can be determined whether the image consists of a simple pattern or the like. However, since the original purpose of this method is to determine the complexity of an image, it cannot necessarily be used as a method for detecting halftone images.

In general, halftone images are characterized by a high density of changing points because they consist of thin white dots and black dots, but unlike non-halftone areas, a distinctive feature of halftone images is that the white dots and black dots have a regular periodicity. Since they are lined up in parallel, the interval between one black dot and the next black dot is the length of the halftone dot period.

The present inventors focused on the distinctive features of such a halftone image, and determined the difference between a black pixel in an image signal that changes from a white pixel to a black pixel, and a black pixel that changes from a white pixel to a black pixel. The present invention was developed based on the knowledge that if the interval is close to the halftone dot period, the image signal can be determined to be a signal of the halftone dot area.

That is, the present invention provides a halftone image detection device fcl, lit; which distinguishes between halftone images and non-halftone images on a scanning paper surface and prevents the occurrence of moiré during halftone image scanning.
IC,! ``2'' = this -゛016゜ or less - of the present invention
will be explained using drawings.

If sampling is performed between one halftone dot using 20 reading clocks, the halftone dot period is defined as 20 pixels. The following explanation will be given using an example in which the halftone dot period is 20 pixels.

FIG. 1 is a block diagram showing an embodiment of the halftone dot detection device of the present invention. FIG. 2 is a timing chart of signals flowing between the blocks in FIG. 1. The numbers assigned to the signals are the same in FIG. 1 and FIG. 2.

The image signal a is input to a block 1 which outputs a change point interval signal. This block 1 outputs a positive pulse of the change point interval signal S at the rising edge of the image signal a.

The block 2 outputs a positive reset pulse C to the counter 3 at the falling edge of the change point interval signal S. After being reset, the counter 3 detects the falling edge of the sampling clock d and outputs the count value N to the comparators 4 and 6. :, ;Comparator 4 outputs the count value N and the previous (.

Also °1 setting 7. , T value T+β (B is natural Iβ<T−
), and if the ζ count value N is small, the determination result ef: is output as °1°'. Further, the comparator 5 calculates the count value N and a previously set value T-a<atrz natural a
a. ), and if the currant value N is large, then
A determination result f of "°1" is output.

AND gate 6 is a change point interval signal, but when it is "1", comparator 4
, 6 simultaneously occur that the judgment results e and f are 1", a signal of 1", that is, a halftone dot detection signal q is output. That is, a change point interval signal indicating that the image signal a has changed from white to black is outputted as a reset pulse cf to set the counter 3 to zero. The counter 3 counts the number of sampling clocks from the next sampling clock set to 0 until it becomes 0 again, that is, the number of pixels N. This value N is constantly input to comparators 4 and 5 and T+β2N>T-
It is determined whether the condition α is satisfied. The AND gate 6 determines whether or not the above conditions are satisfied when the next transition from white to black occurs. The solid line timing chart in FIG. 2 shows an example where the condition is satisfied and a halftone detection signal is output, and the broken line shows an example where N is larger than T+β and the condition is not satisfied.

By the way, the reason why the width α+β is set for the halftone dot period T to determine the changing point interval N is that when scanning a part where the density of the halftone dot is changing, the changing point interval N is not necessarily equal to the halftone period T.
This is because it does not have the same value.

FIG. 3^) shows a halftone dot portion A with the same density and its image signal port, and FIG. 36) shows a halftone dot portion A whose density becomes darker depending on the scanning method and its image signal port. If the density is the same, the change point interval N will be the halftone dot period T (20 pixels), but where the density slopes, the third
As in the example in Figure (B), the change point interval N is 18 pixels.

On the other hand, where the density is close to each other, the interval between changing points is 20 pixels or more. The maximum width of change for both α and β may be up to T/2, or it may be about T/10 for both α and β since the density of normal photographs changes relatively gradually.

As described above, the present invention provides a means for measuring the interval between changing points of a black and white binary image signal, and a means for measuring whether the interval between changing points measured by the measuring means is within a predetermined range based on the halftone period. Because it has a means for determining, it is possible to capture the characteristics peculiar to halftone images, effectively separate non-halftone areas from halftone areas, and detect halftone areas in photographs, etc. with high accuracy. Based on this, it is possible to suppress the occurrence of moiré in the halftone dot area during scanning.

[Brief explanation of the drawing]

FIG. 1 shows halftone image maintenance in one embodiment of the present invention. FIG. 2 is a block diagram showing the hanging device; FIG. 2 is a timing chart showing the operation of the main parts of the device; FIG. It is a figure showing an interval. 1... Changing point interval signal output block, 2...
...Reset pulse output block, 3...Counter, 4.5...Comparator, 6...And gate. Name of agent: Patent attorney Toshio Nineo and one other person (Akebono)

Claims (1)

[Claims] a change point interval measuring means for measuring an interval between a change point at which a black and white binary image signal obtained by scanning a paper surface changes from white to black and a change point at which it changes from white to black again; 1. A halftone dot detection device comprising: a determining means for determining whether the interval measured by the dot interval measuring means is within a predetermined range based on the period of the halftone dots on the paper surface.