JPH0468823B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an apparatus for binarizing an analog video signal obtained by scanning an image.

In an imaging system such as a facsimile, even if a video signal such as waveform A shown in the time chart in Fig. 1 is recorded by electrical-to-optical conversion and then optical-to-electrical conversion is performed again, the resulting video signal will be waveform B. It's like a sluggish name. When the analog video signal B, which has been distorted in this way, is threshold-processed using the threshold value C and converted into a binary value, the thin line parts become blurred ("black" turns into "white"), and the narrow gaps between the lines are crushed ( A phenomenon occurs in which “white” becomes “black”).

In order to prevent such image distortion and blurring, conventionally, an operation is performed to emphasize high frequency components of an analog video signal prior to threshold processing.
By this operation, an analog video signal such as waveform B is compensated as shown in waveform D, so that blurring of thin lines and collapse of narrow line gaps can be improved. However, this operation cannot be expected to be effective for thin lines or gaps parallel to the image scanning direction.
Another disadvantage is that it easily picks up noise.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an analog video signal binarization device that eliminates the above-mentioned drawbacks.

However, the main feature of the present invention is that by observing the analog video signal to be binarized, it is possible to obtain two-dimensional structural information such as ridge points and valley points ( (This will be described later) is extracted, and a threshold value used in threshold processing of the analog video signal is determined so that these ridge points and valley points are reflected in the binary video signal as faithfully as possible.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

First, ridge points and valley points are basically extracted as follows. An image expressed by an analog video signal to be binarized is observed through a 3×3 pixel window as shown in FIG. 2a. The pixel of interest is _x0 , and the density (level of the analog video signal) is compared and determined for the pixel arrangement in four directions shown in FIG. 2b with this pixel _x0 as the center. Then, if the density of the pixel of interest x ₀ is maximum in adjacent pixel columns in two or more directions, the pixel of interest x ₀ is extracted as a ridge point. Conversely, if the density of the pixel of interest x ₀ exhibits a minimum value in adjacent pixel rows in two or more directions, the pixel of interest x ₀ is extracted as a valley point. For example, if the pixel of interest x ₀ has a higher density level than any of the adjacent pixels x ₃ or x ₇ , and higher density level than any of the adjacent pixels x ₁ or x ₅ , then the pixel of interest x ₀ is placed on a ridge. Extract as a point.

With this method, it is difficult to extract ridge points and valley points for thick lines that do not contain high spatial frequency components or wide line gaps. However, in general, it is blurred,
Collapse occurs in thin lines and narrow line gaps,
It is sufficient to be able to extract ridge points and valley points for portions containing such high spatial frequency components.

The ridge points and valley points extracted by observing the analog video signal as described above are structural information that should be preserved as "picture" and "ground", respectively, even after threshold processing. In other words, in Figure 1, the ridge point is
If they are not reflected in the binary video signal as levels of 1α and 0〓, blurring and distortion will occur.

Therefore, in the present invention, the threshold value for threshold processing is determined so that the ridge points and valley points are reflected in the binary video signal as faithfully as possible. More specifically, by counting ridge points and valley points that are not correctly reflected in the binary video signal as errors, and optimally setting the threshold based on the counted value, the binary video signal is created with less blurring and distortion. get.

FIG. 3 shows experimental results regarding the relationship between the error occurrence rate and the threshold value at ridge points and valley points. Curve 31 is the percentage of the number of ridge point errors relative to the total number of ridge points;
Curve 32 is the percentage of the number of trough errors relative to the total number of troughs. Curve 33 is the percentage of the simple sum of the ridge point error and the trough point error. Therefore, basically, if the threshold is set at the minimum point of the curve 33,
A good binary video signal can be obtained. However, if you want a reproduced image that is a little blurry or crushed, you can add appropriate weights to the number of ridge point errors and the number of valley point errors, calculate the sum, and set a threshold that minimizes this weighted sum. . According to experiments, it seems that a better image can generally be reproduced by setting the threshold so that the weight of the valley point error is minimized and making the image a little blurry.

FIG. 4 shows an analog video signal 2 according to the present invention.
FIG. 2 is a block diagram showing an example of a value converting device.

Analog video signals output from an image sensor of a facsimile imaging system are temporarily stored in a buffer memory 1. The buffer memory 1 is constituted by an analog storage element such as a BBD (Bucket Brigade Device).

The analog video signal from the buffer memory 1 is sequentially input pixel by pixel to the binarization circuit 2, where it is subjected to threshold processing using a threshold value given by the arithmetic control circuit 3.
It is output as a binary video signal.

On the other hand, the ridge point extraction circuit 4 and the valley point extraction circuit 5
observes the analog video signal (image) in the buffer memory 1 through the 3×3 pixel window shown in FIG. 2, and extracts ridge points and valley points, respectively. The ridge point error detection circuit 6 detects the ridge point if the binary video signal output from the binarization circuit 2 is at "0" (white) level at the time when the ridge point extraction signal is output from the ridge point extraction circuit 4. Outputs point error signal. The valley point error detection circuit 7 outputs a valley point error signal if the binary video signal is at the "1" (black) level at the time when the valley point extraction signal is output from the valley point extraction circuit 5. The above-mentioned ridge point error signal and valley point error signal are counted by counters 8 and 9, respectively.

The arithmetic control circuit 3 takes in the count values of the counters 8 and 9 for each scanning line and resets the counters 8 and 9. The arithmetic control circuit 3 calculates the simple sum (or weighted sum) of the counted values of the counters 8 and 9. And this simple sum (or weighted sum)
The threshold value of the binarization circuit 2 is controlled by referring to the simple sum (or weighted sum) calculated and held for each of the previous several scanning lines. That is,
From the change in the simple sum (or weighted sum) of the number of errors at the ridge points and valley points of several scanning lines, curve 33 in Fig. 3 is obtained.
The threshold of the binarization circuit 2 is optimized so that it approaches the threshold of the valley (or its vicinity) of .

Note that when the density level of the pixel of interest is clearly at the "ground" level, the ridge point extraction circuit 4 suppresses the output of the ridge point extraction signal even if the pixel of interest is at the maximum level in two or more directions. It is also easy to configure as follows. This is more preferable because noise is less likely to appear on the "ground" portion of the image where there are no characters or graphics.

FIG. 5 shows a block diagram of another example of the analog video signal binarization device according to the present invention, and will be described.

Analog video signals output from the image sensor are sequentially input to the delay circuit 10. This delay circuit 10 holds an analog video signal for about one scanning line, and is constructed of, for example, a BBD.

Now, as shown in Figure 6, the pixel on scanning line i
Assuming that an analog video signal of Q _i,j is input to the delay circuit 10, this pixel Q _i,j and its four adjacent pixels Q _i,j-1 , Q _{i-1,j-1 ,} Q _{i-1 ,j} , Q _i-1,j+1 analog video signals are input from the delay circuit 10 to the comparison circuit 11. The comparison circuit 11 compares the level (density) of the pixel Q _i,j with each of four adjacent pixels, and inputs the result in 8-bit parallel to the shift register 12 as a total of 8-bit digital information. This shift register has a number of digits (8 bits/digit) approximately equal to the number of pixels for one scan line.

At the time when the information for the pixel Q _i,j is input to the shift register 12, the analog video signal of the pixel Q _{i-1,j-1 is transferred} from the delay circuit 10 to the n binarization circuits 16 ₁
~16 _o are input simultaneously. Binarization circuit 16 ₁ ~
16 _o performs threshold value processing on the input analog video signal using different threshold values, and outputs it as a binary video signal. At this point, the shift register 12 has accumulated all the information on the level comparison results between the pixel Q _i-1,j-1 undergoing binarization processing and its eight adjacent pixels, so the ridge can be determined from this information. The point/valley point determination circuit 13 determines whether the pixel Q _i-1,j-1 is a ridge point or a valley point,
The determination result is sent to the error detection circuits 14 ₁ to 14 _o .

Note that the specific 5-digit contents of the shift register 12 for determining ridge points and valley points are referred to;
The number of bits of information actually referenced is 16 bits. In addition, the output of the ridge point/trough point determination circuit 13 is 2
It's bit.

Error detection circuits 14 ₁ to 14 _o are 2 corresponding to when the ridge point/trough point determination circuit 13 displays a ridge point.
When the outputs of the value converting circuits 16 ₁ to 16 _o are at the "0" (white) level, a ridge point error signal is output. Further, when the ridge point/trough point determination circuit 13 is displaying a trough point, if the output of the corresponding binarization circuit 16 ₁ to 16 _o is at "1" (black) level, a trough point error signal is output. Output. The counter 15 separately counts the ridge point error signal and the trough point error signal for each of the error detection circuits 14 ₁ to 14 _o .

The arithmetic control circuit 18 reads the counted value of the counter 15 for each scanning line, and based on the counted value and the counted value for several scanning lines read and held before, the binarization circuit 16 ₁ - 16 _o , one binarization circuit with the minimum ridge point error and valley point error is selected, and a signal specifying it is given to the selector circuit 17. The selector circuit 17 is
The output of the binarization circuit specified by this specification signal is selected and finally output as a binary video signal. Note that the designation signal output from the arithmetic control circuit 18 is updated every scanning line. Also, counter 1
5 is reset by the arithmetic control circuit 18 every scanning line.

Here, the threshold values of the binarization circuits 16 ₁ to 16 _o do not necessarily have to be fixed, and the threshold values of each of the binarization circuits 16 ₁ to 16 _o may be controlled according to the average level of the analog video signal, etc. It's okay. Also, in the ridge point/trough point determination circuit 13, similarly to the ridge point extraction circuit 4 in the previous example, the "ground" part is not determined to be a ridge point even if it is maximal in two or more directions. It can also be configured.

As detailed above, in the present invention, ridge points and valley points of an image are extracted by observing an analog video signal, and a threshold value is set so that the ridge points and trough points are reflected in the binary video signal as faithfully as possible. Since the processing threshold is optimally determined, it is possible to obtain a binary video signal in which undesirable phenomena such as blurring of thin lines and collapse of narrow line gaps are significantly reduced. That is, the valley points are particularly effective when the image is dirty or when the lines are narrowly spaced. Ridge points are effective when the image is blurred or the lines are thin. When inputting line figures in this way, it is necessary to treat black and white equally, and as a result, images of various quality can be input with good quality. In particular, the present invention extracts ridge points and valley points for each pixel on an image by comparing the density in multiple directions (four directions in the above example). It is also possible to reliably extract ridge points and valley points, and therefore it is extremely effective when applied to the binarization of analog video signals of such line shapes.

[Brief explanation of drawings]

Figure 1 is a time chart to explain the problems associated with binarization of analog video signals, Figure 2 is a diagram to explain the method of extracting ridge points and valley points, and Figure 3 is a diagram to explain the method for extracting ridge points and valley points. FIG. 4 is a diagram showing a graph of the relationship between point errors, valley point errors, and threshold values. FIG. 4 is a block diagram of an example of an analog video signal binarization device according to the present invention. FIG. FIG. 6 is a diagram showing a block diagram of another example of the video signal binarization device, and is a diagram for explaining the operation of the device shown in FIG. 5. 1...Buffer memory, 2,16 ₁ ~16 _o ...
Binarization circuit, 3, 18... Arithmetic control circuit, 4...
Ridge point extraction circuit, 5... Valley point extraction circuit, 6, 7,
14 ₁ ~ 14 _o ...Error detection circuit, 8, 9, 15
... Counter, 10 ... Delay circuit, 11 ... Comparison circuit, 12 ... Shift register, 13 ... Ridge point/trough point determination circuit, 17 ... Selector circuit.

Claims (1)

[Scope of Claims] 1. Binarization means for comparing an analog video signal obtained by scanning a document image with a predetermined threshold value and converting it into a binary video signal representing the black and white level of the document; and the analog video signal. ridge point/valley point extraction means for observing the signal in a window of at least 3 x 3 pixels and extracting two-dimensional ridge points and valley points of the density level of the original image; Ridge point/valley point error detection that detects ridge points that are not binarized to the black level as ridge point errors, and trough points that are not binarized to the white level as valley point errors, when compared with points and valley points. means for counting the number of ridge point/trough point errors; and said binarization for every few scanning lines so that the number of errors is minimized according to the counted value of said ridge point/trough point errors. An analog video signal 2 characterized by comprising: a threshold value control means for optimizing the threshold value of the means;
Value device. 2. A plurality of binarization means for comparing analog video signals obtained by scanning an original image with different threshold values and converting the analog video signals into binary video signals representing black and white levels of the original; ridge point/valley point extraction means for observing with a 3×3 pixel window and extracting two-dimensional ridge points and valley points of the density level of the original image; and binary video signals from the plurality of binarization means. Compared with the extracted ridge points and valley points, respectively,
ridge point/valley point error detection means for determining a ridge point that is not binarized to a black level as a ridge point error and a valley point that is not binarized to a white level as a valley point error; counting means for counting the number of said ridge point/trough point errors with respect to the binary video signal of said means; An analog video signal 2 characterized in that it has: a control means for selecting one binarization means that minimizes the number of errors among the converting means;
Value device.