JPH0213346B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention uses a long line image sensor.
The present invention relates to an image input device such as an OCR (optical character reader), and particularly relates to a white level follower circuit of an A/D conversion section of the image input device.

Generally, in image input devices such as OCR,
Light is applied to an image, such as a form, and the reflected light is converted into an electrical signal and then converted into a digital value.
This digital value is converted into a black and white binary signal using a certain threshold value and is stored in the video memory.

A/ which converts the above electrical signal into digital value
A white level follower circuit generates a white level reference potential for the D converter. This reference potential depends on the amount of light reflected from the background portion (white background) of the form, that is, the white level, and this white level changes due to fluctuations in the light source, unevenness in light irradiation, reflectance of the paper, and the like.

In order to change the white level reference potential each time the white level changes, a conventional white level follower circuit includes a register, a D/A converter that converts the value of the register into a first potential, and a first A voltage divider circuit that divides the potential of and generates a second potential, and compares the input signal potential with the first and second potentials, and increases the value of the register when the input signal potential is greater than the first potential. a down signal that decreases the value of the register when the input signal potential is between the first potential and the second potential, and a down signal that maintains the value of the register when the input signal potential is less than the second potential. an image input device for optically reading an image of the first potential, comprising: a comparison means for sending out a clamp signal, and a conversion means for converting the value of the register according to the up signal, the down signal, and the clamp signal; This was used as the reference potential of the A/D converter. As a result, a stable reference potential can be obtained during character detection (see:
54-90063).

However, in the conventional type described above, since the white level is tracked independently for each sampling bit (pixel) in the main scanning direction, the diagonal boundary part (gray level) of a part of the character is measured pixel by pixel. In some cases, a down signal is sent out at a point where a clamp signal should be sent out assuming that the level is white, that is, a black level. As a result, the white level reference potential follows and falls.
Even if the next pixel is at the black level, a further down signal may be sent. As a result, an image is obtained in which part of the characters are omitted, which poses a problem that is inconvenient for character recognition.

An object of the present invention is to focus on a plurality of consecutive pixels in the main scanning direction, and to convert the down signal into a clamp signal when a down signal is followed by a clamp signal. It prevents the pixel from tracking below the level reference potential, eliminates the influence on the next black level pixel, and therefore reduces the omission of part of the character, thus reducing the inconvenience in character recognition. The purpose is to eliminate the above-mentioned problems in the conventional type.

Hereinafter, the present invention will be explained with reference to the drawings in comparison with a conventional example.

FIG. 1 is a block circuit diagram of a general image input device. In FIG. 1, the form 1 is transported in the direction of arrow
is projected onto the one-dimensional image sensor 3 via.
The image sensor 3 scans this one-dimensional image and generates an output signal according to the amount of light reflection. This output signal is amplified by the video amplifier 4, and then converted by the A/D converter 5 into a digital value indicating the amount of light reflected relative to the background (white background) of the form 1. The converted digital value is sent to a preprocessing circuit 6 that has the effect of increasing the sharpness of the image by a predetermined amount.
The signal is converted into a black and white binary signal in which characteristic elements of the drawing line used for recognition are recovered and stored sequentially in the image memory 7. Thereafter, in the character cutting circuit 8, data is cut out character by character and sent to a character checking section (not shown).

According to the present invention, in the A/D converter 5 of FIG.
This invention relates to a white level follower circuit that generates a reference potential for an A/D converter. This A/D converter 5
In this process, as a step before the image analog signal is converted into black and white binary signals "0" and "1", the image analog signal is converted into a digital value with an appropriate white level as a reference. In this case, the white level as a reference potential changes depending on the nature of the form, location, light source, etc., and therefore needs to be determined each time.

FIG. 2 is a block circuit diagram of an A/D conversion section of a conventional image input device. In Figure 2, A/D
The converter 5 includes an A/D converter 51 and a white level follower circuit that provides a reference potential to the A/D converter 51. More specifically, the white level follower circuit includes comparison circuits 52 and 53, and an exchange table.
ROM 54, register 55 that holds the output of ROM 54, and D/A that converts the value of register 55 from D/A.
It consists of an A converter 56 and a voltage dividing circuit 57 that divides the output voltage of the D/A converter 56. in this case,
The output voltage of the D/A converter 56 is the same as that of the A/D converter 51.
, and is therefore the current white level reference potential. Further, the output voltage of the voltage dividing circuit 57 indicates the lowest level of the white level. Therefore, if the input signal potential from the video amplifier 4 is equal to or higher than the output voltage of the voltage dividing circuit 57, the white level reference potential is the input signal potential. On the other hand, if the input signal potential is lower than the output voltage of the voltage dividing circuit 57, the white level reference potential does not follow and holds the current level.

That is, if the input signal potential is equal to or higher than the reference potential, both the comparison circuits 52 and 53 send out a "1" signal, and as a result, the ROM 54 sends out a value obtained by multiplying the contents of the register 55 by a fixed rate, for example, 1.1. The contents of register 55 increase. In this case, the combination of the output signals of comparison circuits 52 and 53 becomes an up signal for register 55. Also, when the input signal potential is between the reference potential and the divided potential,
The comparison circuits 52 and 53 respectively receive a “0” signal,
A "1" signal is sent, and as a result, the ROM 54 sends out a value obtained by multiplying the contents of the register 55 by a fixed rate, for example, 0.9, and the contents of the register 55 decrease. In this case, the combined signal of the output signals of comparison circuits 52 and 53 becomes a down signal for register 55.
Further, if the input signal potential is equal to or less than the divided potential, both comparison circuits 52 and 53 send out a "0" signal, and as a result, the ROM 54 sends out the contents of the register 55 as is. Therefore, the contents of register 55 are retained. In this case, the comparison circuits 52, 5
The combination of the three output signals becomes a clamp signal for the register 55.

However, in order to be able to read information for many pixels at once, the register 55 has been changed to a shift register to correspond to a long line image sensor having multiple elements, instead of one reading element (one dot). In contrast, in the circuit on the device of the long line image sensor, in which pixel information for multiple elements is held in this shift register according to each reading element of the long line image sensor, the white level follower circuit operates on a pixel by pixel basis. Therefore, the white level reference potential may sometimes follow even gray level pixels (down signal transmission), and as a result, the white level reference potential decreases,
Furthermore, since the white level reference potential has also decreased in the black level pixel during the next scan, it may follow further (continuous down signal transmission).
In other words, the black level is treated as the white level, which is unfavorable for character recognition.

Figure 3A to Figure 3C, Figure 4A and Figure 4B
FIG. 2 is a diagram for explaining the principle of the present invention. Third
In Figure A, when the shaded area is scanned in the scanning direction in the order of pixels X _o-2 , X _o-1 , X _o , ..., X _{o +5} in the nth sub-scan, the pixel X _o becomes gray level. .
As shown in FIG. 3B, if this gray level potential is higher than the output voltage of the voltage dividing circuit 57 in FIG.
A down signal "D" is sent out by the comparator circuit, and the white level reference potential is lowered. As a result, the down signal "D" may also be generated for the black level at the (n+1)th pixel _Yo in the next scan.
As a result, pixels X _o and Y _o are considered to be at the white level. In this case, the pixels in the shaded area
The parts X _o and Y _o are deleted. In other words,
In this case, when the pixel of the character "8" shown in FIG. 4A is read, a character with both ends cut off is written in the image memory and recognized, as shown in FIG. 4B. Unfavorable for character recognition.

In the present invention, as shown in FIG. _3C , in consecutive pixels, for example,
occurs and a clamp signal "C" is generated in the pixel X _o+1 next to it, the down signal "D" at the pixel X _o is converted into the clamp signal "C".

FIG. 5 is a block circuit diagram of an A/D converter for an image input device as an embodiment of the present invention. Fifth
In the figure, the same reference numerals are given to the same elements as those in FIG. 2. That is,
In order to convert the above-mentioned down signal "D" into a clamp signal "C", shift registers 61 and 62 monitor the comparison results of the comparison circuits 52 and 53 for five pixels.
Add. In other words, shift registers 61, 62
holds the information of 5 pixels, and the ROM6 described below
It is used to supply the retained state information (signals) of the five pixels to the ROM 63 for signal conversion in Step 3. Furthermore, a combination of 10 values (signals) for 5 pixels held in these shift registers 61 and 62 is converted into 2 signals.
ROM63 is added. This is because two or more clamps are considered to be required to recognize a character line segment. As shown in FIG. 3C, let "C" and "D" next to "C" be "C". The outputs of the comparison circuits 52 and 53 are input to shift registers 61 and 62, and then shifted by the shift registers 61 and 62. As a result, the shift registers 61 and 62 have comparison circuits for the five pixels read by the successive elements. The comparison result values at 52 and 53 are stored, and these comparison result values are input to the ROM 63. In this case, the ROM 63 has a ²¹⁰ address x 2 bit configuration, and data is written in advance, and data corresponding to the comparison result values for the aforementioned five pixels stored in the shift registers 61 and 62, that is, ,
Data such as "C" and "D" next to "C" as described above are output as "C". Therefore, the combination of the comparison result values stored in the two shift registers 61 and 62 becomes an up signal, a down signal, or a clamp signal for the ROM 54 as means for converting the contents of the register 55.

In addition, registers 64, 65, and 66 are stored in the ROM 54.
The reason why they are provided between the input of the register 55 and the output of the register 55 is to compensate for the delay time of the up signal, down signal, or clamp signal due to the presence of the elements 61, 62, and 63.

As explained above, according to the present invention, it is possible to reduce reading errors of the next black level pixel due to a decrease in the white level reference potential due to a gray level pixel, and therefore, it is possible to reduce the omission of part of a character. That is, it is possible to eliminate inconveniences in character recognition, and is useful for solving the problems in the conventional type described above.

[Brief explanation of drawings]

Fig. 1 is a block circuit diagram of a general image input device, Fig. 2 is a block circuit diagram of an A/D conversion section of a conventional image input device, Figs. 3A to 3C, Figs. 4A and 4. FIG. 4B is a diagram for explaining the principle of the present invention, and FIG. 5 is a block circuit diagram of an A/D conversion section of an image input device as an embodiment of the present invention. 1... Form, 3... Image sensor, 4... Video amplifier, 5... A/D converter, 51... A/D converter,
52, 53... Comparison circuit, 54... ROM (conversion means), 55... Shift register according to the number of bits of the image sensor, 56... A/D converter, 57... Voltage dividing circuit, 61, 62... Shift register, 63 …
ROM (conversion means), 64, 65, 66... registers.

Claims (1)

[Claims] 1. A shift register, a D/A converter that converts the value of the shift register into a first potential, and a voltage dividing circuit that divides the first potential to generate a second potential. an up signal that compares the input signal potential with the first and second potentials and increases the value of the shift register when the input signal potential is greater than the first potential; a down signal that decreases the value of the shift register when the potential is between a first potential and the second potential, and maintains the value of the shift register when the input signal potential is less than the second potential; and means for converting the value of the shift register according to the UP signal, the DOWN signal, and the clamp signal, and writing the value into the shift register. In the white level follower circuit, in which the reference potential of an A/D converter for an image input device that optically reads an image is set as a reference potential, a white level follower circuit is provided with , further comprising means for converting the down signal into the clamp signal when the output signal of the comparing means obtained by scanning in the main scanning direction becomes the clamp signal following the down signal. white level follower circuit. 2. The white level follower circuit according to claim 1, further comprising a delay compensation register group provided between the output of the shift register and the input of the conversion means for converting the value of the shift register.