JPH03208468A - Picture binarizing system - Google Patents

Abstract

PURPOSE:To attain binarization with a proper threshold level by correcting a shading waveform obtained through the read of a white reference plate based on the peak value and an original peak value, correcting a picture signal according to the corrected shading waveform and applying binarization to the result. CONSTITUTION:A picture signal Vin is read sequentially from a picture input circuit 1 and D/A conversion is implemented at a specified sampling period by an A/D converter 6. In this case, a shading waveform VS1 is given to a reference voltage terminal of the A/D converter 6. The shading waveform VS1 is obtained by applying A/D conversion to a shading waveform VS read from a shading memory 7 based on a reference voltage being an original peak value VP and is the result of amplitude correction to the shading waveform VS by VP/VSP. Thus, the A/D-converted picture signal Vin is subject to shading correction and is a signal with a maximum amplitude where the dynamic range whose peak value reaches a full range is secured.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an image binarization method in image input means such as an image scanner and a figure reading device, and particularly relates to an image binarization method involving shading correction processing. .

[Prior Art] Conventionally, when converting an image signal into a binary image signal, for example, the peak value of the original level is divided by resistance to determine a threshold level, or the gain of an FET, etc. is controlled based on the peak value of the original level. The level of the image signal is adjusted to a predetermined threshold level.

However, these image binarization methods cannot eliminate the influence of shading caused by variations in the sensitivity of the image sensor, the output of the light source, and the like.

On the other hand, shading correction of image signals based on signals read from a white reference plate has been conventionally performed, but when converting image signals into binary values, the level of the white reference plate and the level of the original are different. Therefore, it is difficult to determine the optimal threshold level.

Therefore, for example, the previous pixel level and the previous threshold) Lt
A method in which the current threshold level is dynamically changed by a counter etc. by referring to the P level, etc. -aX/Wi
Also known are a method in which the threshold is determined by n calculation or a table, and a method in which a shading correction function is added by adjusting the discharge time constant of a peak follower circuit that follows the peak value of the document level.

[Problem to be solved by the invention] However, among these methods, the first two are dedicated LS
The disadvantage is that the circuit becomes complicated and expensive, such as the need for I and peripheral timing circuits, and in the latter case, it is difficult to select a time constant, and it is difficult to accurately follow a plurality of peak values.

The present invention has been made in view of such problems, and an object of the present invention is to provide an image binarization method that can perform appropriate binarization processing in consideration of shading waveforms and has a simple circuit configuration. .

[Means for Solving the Problems] The image binarization method according to the present invention reads a white reference plate and holds the shading waveform and its peak value, and then reads the original to obtain an image signal and converts the image signal into holding the peak value, correcting the shading waveform based on the peak value of the shading waveform and the peak value of the image signal, correcting the image signal with this corrected shading waveform, and then correcting the corrected shading waveform. It is characterized in that the image signal is binarized using a predetermined threshold value.

[Function] According to the present invention, the shading waveform obtained by reading the white reference plate is corrected based on its peak value and the original peak value before reading the original, so that the shading waveform is adjusted to the original white level. can be adapted. By correcting the image signal using this modified shading waveform, shading can be removed and the amplitude of the image signal can always be kept at a constant value. Therefore, it becomes easy to set the threshold value.

In addition, in the present invention, gain adjustment for the shading waveform and image signal is the main focus of the processing, and the threshold
Since no calculation is required to determine the field level, it can be implemented with a simple circuit.

By determining the original peak value based on multiple sample data, even if the correct original peak value cannot be obtained due to the image pattern of the original, the correct original peak value can be estimated using other sample data. Therefore, the reliability of binarization can be improved.

In addition, when the corrected image signal is binarized, by quantizing it with two threshold values, the levels of fine black lines in a white image, thin white lines in a black image, pale black, darkened white, etc. of,
At least one of the two threshold values can be detected. Then, by binarizing the current pixel based on the quantization pattern that is the detection result, the quantization pattern of the previous pixel, and the binarization result, it is possible to accurately binarize even the thin lines mentioned above. can.

[Embodiments] Hereinafter, embodiments of the present invention will be described based on the accompanying drawings.

FIG. 1 is a block diagram showing the configuration of an image processing apparatus for implementing an image binarization method according to an embodiment of the present invention.

The image input circuit 1 is composed of a line image sensor, a synchronization signal generation circuit, etc., and is controlled by a microcomputer 2, reads the original 3 and the white reference plate 4, and outputs an image signal vI@. This image signal v1 is
The signal is input to an A/D converter 6 via an amplifier 5. A
The /D converter 6 A/D converts the shading waveform Vs when reading the white reference plate 4, and A/D converts the image signal Vlfi when reading the original 3. The output of this A/D converter 8 is supplied to a shading memory 7.

The shading memory 7 holds the shading waveform Vs, and is capable of exchanging data with the microcomputer 2 via the data bus 1o. Moreover, the output of this shading memory 7 is D/A
Changeover switch SwI via converter 11 and amplifier 12
is supplied to the B terminal of The A terminal of the changeover switch SWI receives the peak value v of the shading waveform Vs, which will be described later.
SP is now given. Changeover switch SW
I is the output of the amplifier 12 and the shading peak value VS
p and is supplied to the reference voltage terminal of the A/D converter 6.

On the other hand, the output of the amplifier 5 is also supplied to the peak follower circuit 13. The peak follower circuit 13 holds the output peak value of the amplifier 5 after being reset by the reset signal RES. The output of the peak follower circuit 13 is supplied to the B terminal of the changeover switch SW2. The output of the amplifier 5 is directly applied to the A terminal of the changeover switch sw2. The changeover switch SW2 selects the output of the peak follower circuit 13 and the output of the amplifier 5, and converts the output to the A/
It is supplied to the D converter 14. The reference voltage terminal of the A/D converter 14 has a shading peak value V8P and a fixed voltage V8P.
. and are selectively provided by being switched by a changeover switch sw3. The output of the A/D converter 14 is supplied to the microcomputer 2 via the data bus 1o.

Further, the output of the A/D converter 14 and the data on the data bus 10 can be taken into the D/A converter 15. The D/A converter 15 has a fixed voltage V. The input data is D/A converted using the reference voltage, and the output is sent to the amplifier 16.
The voltage is applied to the reference voltage terminal of the D/A converter 11 via.

Furthermore, the data of the shading peak value vsp given from the microcomputer 2 via the data bus 10 is given to the D/A converter 17 which uses the fixed voltage Vc as a reference voltage, and after being D/A converted there, The signal is supplied to each A terminal of the changeover switches S W r and S W3 via the amplifier 18 . Note that the changeover switch SW1. sw2
．． sw3 is switched and controlled by switching signals CI+C2 and C3 from the microcomputer 2, respectively.

Further, the output of the A/D converter 6 is sent to two comparators 21.2
It is also supplied to one terminal of each of the two terminals.

A threshold level Tu+TL is applied from the microcomputer 2 to the other terminals of the comparators 21 and 22, respectively. The outputs of the comparators 21 and 22 are input to the pattern determination circuit 23. The pattern determination circuit 23 determines and outputs a binarized output based on the output of the comparator 21.22, the output of the comparator 21.22 for the previous pixel held internally, and the binarization result.

Next, the operation of the image processing apparatus according to this embodiment configured as described above will be explained.

Image processing is performed in the following order: capture of a shading waveform by the white reference plate 4, initialization before conveyance of paper, and binarization of image data.

FIG. 2 is a flowchart showing the shading waveform import process.

First, the microcomputer 2 selects the changeover switch sw2.
Connect sw3 to terminal B, respectively (step S,). Next, the microcomputer 2 triggers the image input circuit 1 to read the white reference plate 4 (step 51).
2). As a result, the white reference plate 4 is controlled by the image input circuit 1.
is read, and the peak value VSP of the shading waveform is held in the peak follower circuit 3. This peak value v
sp is A/D converted by the A/D converter 14 using the fixed voltage V0 as a reference voltage, and then read by the microcomputer 2 (step S13) and set in the D/A converter 17. (Step s, 4).Subsequently,
Changeover switch SW1 is connected to terminal A (step 51).
5) The peak value VsP is given as the reference voltage of the A/D converter 6.

The microcomputer 2 again triggers the image input circuit 1 to read the shading waveform, reads the white reference plate 4 (step S to), and then the A/D converter 6
A/D conversion is performed. At this time, since the peak value V8P of the shading waveform is given as the reference voltage of the A/D converter 6, the A/D conversion output has a peak value in the full range [e.g. In the case of bits, the shading waveform data has the maximum amplitude and has a dynamic range of (FF)h].

This data is stored in the shading memory 7 and also copied to another memory (not shown) (step 517). Further, since the shading memory 7 can be read/written from the microcomputer 2, the data may be corrected if necessary.

Subsequently, the initialization process shown in FIG. 3 before conveying the paper is performed.

First of all, the microcomputer 2 is the selector switch SW.
1 to terminal B, and selector switch S W 3 to terminal A (step S2). Next, the D/A converter 15
Set the initial data of the original peak value Vp in (
step 822). That is, since it is unknown what value the document peak value Vp will be in the first line, a value expected in advance is used as the initial data. Next, the peak follower circuit 13 is reset, and the selector switch SW 51 is set to terminal A (step 52).
3).

Next, binarization processing of the image data is started.

A flowchart of this process is shown in FIG.

First, conveyance of paper is started by a start trigger from the microconvenitor 2 (step 831).
. At the same time, a threshold level TLI=TL is set in each of the comparators 21 and 22 (step 532).

Subsequently, the image input circuit 1 outputs the image signal V2. ．． are sequentially read, and A/D conversion is performed in the A/D converter 6 at a prescribed sampling period (step 533). At this time, the shading waveform v8m is applied to the A/D conversion converter reference voltage terminal.

This shading waveform VSI is obtained by A/D converting the shading waveform vs read out from the shading memory 7 using the document peak value VP (initial data in the first line) as a reference voltage. As shown by VB2 in FIG. 5, the shading waveform Vs is modified in amplitude by V p /V tap. Therefore, the shading waveform v,1
The image signal vl, which has been A/D converted using m as the reference voltage, has its shading corrected and the peak value is in the full range [for example, in the case of 8 bits (FF)h], as shown by Vlvt8 in FIG. This results in a signal of maximum amplitude with a guaranteed dynamic range. This eliminates the influence of shading as well as the level difference between the white reference plate and the document, making it easier to set the threshold level for binarization.

This signal is then quantized in comparators 21, 22 by two threshold shield levels Tu, Tt, as shown in FIG. In the pattern determination circuit 23, when the quantized output from the comparators 21 and 22 is "00", the output is "O".
When it is “11”, it outputs “1”, and when it is “01”, it outputs “1”.
Refers to the quantization pattern of the previous pixel, and when it is “00”, it is “1”.
When it is "11," when it is "0," and when it is "01," the binarized data of the previous pixel is output as binarized data, respectively.

As a result, as shown in Figure 5, when the image signal level is easily affected by the brightness of the surroundings, such as pale black and dark white, or thin black lines on a white background and thin white lines on a black background, However, accurate binarization can be performed (step 833).

When the binarization for one line is completed (step 534),
Set the changeover switch SW2 to terminal B, and set the peak value Vp of the document level held in the peak follower circuit 13.
Vsp is sampled as a reference voltage (step S
36). Next, reset the peak follower circuit 13 and set the selector switch SW2 to terminal A (
Step 538).

Then, the document peak value Vp to be used when reading the next line is determined and set in the D/A converter 15 (step 537). This is repeated until the entire document is read (step 538).

In this way, according to the circuit of this embodiment, the A/D converter 6
, 14 and the D/A converter 11, the gain is adjusted for the shading waveform and the image signal, and the amplitude of the image signal V, □ is normalized. This method has the advantage that the circuit configuration is simpler than the conventional method of dynamically varying the shield level, and shading can be reliably removed.

Note that in the above processing, it is assumed that there is a white pixel at the position where the document peak value Vp is obtained, but if there is a black pixel at that position, the correct document peak value Vp cannot be obtained. Therefore, it is preferable to determine the document peak value Vp by referring to several lines of data, rather than determining it based on only one line of data.

Furthermore, in order to determine the document peak value Vp, a plurality of data of one line of image data may be sampled, and the document peak value Vp may be determined by referring to these sampled data.

This process is shown in FIG.

That is, after applying the start trigger to start paper conveyance (step 541), when the horizontal synchronization signal is not in the ON state (step S4□), at the same time as the A/D conversion and binarization of the image signal Vlt+, The image signal Vlf,k (k=1 to m) is sampled to a memory (not shown) via the changeover switch SW2 and the A/D converter 14 at regular time intervals (step 543). When the sampling of one line is completed (step 544), the selector switch SW2 terminal B is set to sample the document peak value Vp held in the peak follower circuit 13 using vsp as the reference voltage (step 546). Then, while resetting the peak follower circuit 13, the changeover switch SW2
is set to terminal A (step 548). Subsequently, the document peak value Vp is determined as follows, and the D/A converter 15
Set to . This continues until the paper is finished (step 548).

FIG. 7 is a diagram showing an example of a method for determining the document peak value Vp.

First, the distribution of the ratio between the sample data Vlnk and the shading data Vsk at the corresponding position is examined (step SI5.). Then, as shown in FIG. 8, it is checked whether vP/vsp is included in the group g having the largest value of /VElk in Vl (step S5□), and if it is not included, the group g is The product of the representative value a and VsP is set as the document peak value Vp (step 553). With this process, even if the peak follower circuit 13 cannot obtain the correct document peak value Vp, the process can be performed without any problem.

[Effects of the Invention] As described above, according to the present invention, the shading waveform obtained by reading the white reference plate is corrected based on its peak value and the original peak value, and the shading waveform obtained by reading the white reference plate is corrected based on the corrected shading waveform. Since binarization is performed after correcting the image signal, the discrepancy between the white reference plate level and the document white level is corrected, shading is removed, and binarization is performed using an appropriate threshold. can be done.

Further, according to the present invention, since processing is performed mainly on gain adjustment for shading waveforms and image signals, there is an advantage that the circuit configuration is simple.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an image processing device according to an embodiment of the present invention, FIG. 2 is a flowchart of shading waveform capture processing in the same device, FIG. 3 is a flowchart of initialization processing in the same device, and FIG. 2 of the image data in the same device
A flowchart of the value conversion process, FIG. 5 is a waveform diagram showing the operation of the device, FIGS. 6 and 7 are flowcharts showing an example of the method for determining the document peak value, and FIG. 8 is a waveform diagram showing the operation of the device. It is a graph diagram showing distribution of data.

Claims (3)

[Claims] (1) After reading the white reference plate and holding the shading waveform and its peak value, reading the original to obtain an image signal and holding the peak value of the image signal, and converting the shading waveform to the peak value of the shading waveform. and a peak value of the image signal, and after correcting the image signal with the corrected shading waveform, the corrected image signal is binarized using a predetermined threshold value. Binarization method. (2) The image binarization method according to claim 1, wherein the peak value of the image signal is determined by referring to a plurality of sample data of the image signal. (3) The corrected image signal is quantized using two threshold values, and the quantization pattern, the quantization pattern of the previous pixel, and the quantization pattern of the previous pixel are
2. The image binarization method according to claim 1, wherein the current pixel is binarized based on the digitization result.