JPH11225272A - Image reader - Google Patents

Abstract

PROBLEM TO BE SOLVED: To fast perform shading correction by a circuit that has a small scale by correcting the intensity of a signal of a read image through subtraction by a specific subtracter and multiplication by a specific multiplier in each pixel. SOLUTION: A pseudo-division circuit 21 calculates a division result K/(IW-IB) and operates when a standard image is read. When an image that becomes an object of shading correction is read, output signals of a line sensor are sequentially given to a substrater 24 from an AD converter. The minimum intensity IB stored in a memory 22 is sequentially given to the substrater 24 and the substrater 24 subtracts the IB form signal intensity IO before correction. The output is given to a multiplier 25, also, the K/(IW-IB) stored in a memory 23 is sequentially given to the multiplier 25 and the multiplier 25 multiplies a subtraction result given from the subtracter 24 by it, sequentially outputs the results and gives them to a gamma correction circuit 28. Here, the IW is the maximum intensity of a standard image signal, the IB is the minimum intensity and the K is a constant.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reading apparatus, and more particularly, to an image reading apparatus that performs shading correction.

[0002]

2. Description of the Related Art In an image reading apparatus such as a facsimile or an image scanner, a document image is irradiated with light, and the reflected light is detected by a sensor having a large number of pixels composed of photoelectric conversion elements. The image is read by converting to. However, the light applied to the document image is not constant and not always uniform. In addition, the sensitivities of the photoelectric conversion elements, which are pixels, vary. For this reason, there is no absolute reference for the intensity of the signal output by the sensor, and the intensity of the signal for each pixel does not exactly correspond to the true density of the document.

[0003] Therefore, the signal output from the sensor is corrected to remove the non-uniformity of irradiation light and the sensitivity difference of the photoelectric conversion element, and to keep the signals of all pixels within a certain intensity range. This correction is called shading correction.

[0004] Shading correction is usually performed based on the intensity of a signal when a standard image having a dark portion on which an image of a fixed density is formed and a bright portion on which no image is formed is read as a reference, according to equation (1). This is performed for each pixel. IC = (IO−IB) × K / (IW−IB) (1)

Here, IC is the intensity of the signal after correction, I0 is the intensity of the signal before correction, and K is a constant. IB is the minimum intensity of the signal of the standard image, and IW is the maximum intensity of the signal of the standard image. They correspond to the dark and light portions of the standard image, and are called the black level and the white level, respectively. The constant K is an arbitrary value. For example, if the signal strength is represented by 8 bits, it is 2
55.

The image reading apparatus uses a difference (I) between the maximum intensity and the minimum intensity of the standard image for each pixel of the sensor.
W-IB), and an arithmetic unit for performing subtraction, multiplication and division appearing on the right side of the equation (1). When an image is read, subtraction, multiplication and division are performed by each arithmetic unit on the signal intensity IO of each pixel output from the sensor, and the corrected signal intensity IC
Get.

In general, a divider has a significantly larger circuit scale than an arithmetic unit that performs addition, subtraction, and multiplication. For this reason, some image reading apparatuses include a circuit that performs an operation equivalent to division by repeating subtraction and bit shift. FIG. 3 shows the flow of processing when shading correction is performed on the signal strength represented by 8 bits by such an arithmetic circuit.

First, initialization is performed. That is, the intensity IO of the signal to be corrected is given (step # 5), and the intensity IO
, The minimum intensity IB of the stored standard image signal is subtracted and the result is stored in the register A (# 10), and the difference between the maximum intensity and the minimum intensity of the stored standard image signal (IW-I)
B) is read and put into the register B (# 15). The register C for indicating the corrected signal intensity IC is cleared (# 20), and 8 is set to a counter J indicating the number of repetitions of the subsequent processing (# 25).

Following the initial setting, the values of the registers A and B are compared (# 30). If the value of the register A is equal to or greater than the value of the register B, the value of the register B is subtracted from the register A (# 35), and the j-th (j)
⁽¹⁾ is set to 1 in the bit of the counter J), and 2 ^(j-1) is added to the value of the register C (# 40). #
If the value of the register A is smaller than the value of the register B in the judgment of 30, the processes of # 35 and # 40 are not performed.

Next, the contents of the register B are shifted by one bit to the lower side (# 45), and 1 is subtracted from the counter J (##
50), it is determined whether or not the value of the counter J is 0 (# 55). If the value of the counter J is not 0, the process returns to # 30 and the above process is repeated. If it is 0, the process is terminated.

[0011]

A circuit that performs shading correction in accordance with the processing shown in FIG. 3 has a smaller configuration than a circuit that performs the shading correction, so that it is easy to design and manufacture, and to reduce the cost. Is preferred.
However, this circuit has a low processing speed because it is necessary to repeat subtraction and bit shift. For this reason, the time required for the shading correction becomes longer, which hinders the speed of the entire image reading process.

In particular, when using a sensor composed of a very large number of pixels to read an image at high resolution,
The output cycle of the sensor must be long enough to allow shading correction to follow, and it is difficult to achieve both high resolution and high speed processing of an image. Further, since the number of repetition processes is equal to the number of bits for representing the signal strength, when the signal strength is represented by a larger number of bits in order to increase the gradation of the image, a longer time is required for shading correction. Thus, the processing speed is further reduced.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to provide an image reading apparatus capable of performing high-speed shading correction with a small-scale circuit.

[0014]

According to the present invention, there is provided an image reading sensor comprising a plurality of pixels.
The intensity difference between the maximum intensity and the minimum intensity of the signal of the read standard image is stored, and the intensity of the signal of the read image is corrected for each pixel of the sensor according to the stored intensity difference. In the image reading apparatus in which the signal of the pixel is within a certain intensity range, a first intensity storing the minimum intensity of the signal of the standard image corresponding to each pixel of the sensor is stored.
And a second memory for storing a value obtained by dividing an upper limit value of a certain intensity range by an intensity difference between a maximum intensity and a minimum intensity of a signal of the standard image, and a first memory for storing a first value from a signal intensity of a given image. And a multiplier for multiplying the subtraction result of the subtractor by the value stored in the second memory, and the subtractor subtracts the intensity of the signal of the read image by the subtractor. Correction is performed for each pixel by subtraction and multiplication by a multiplier.

This image reading apparatus performs shading correction on a signal output from a sensor, but does not perform division at the time of correction. That is, when reading the standard image, for each pixel of the sensor, determine the intensity difference between the maximum intensity and the minimum intensity of the signal of the standard image,
The upper limit of a certain intensity range, that is, a constant is divided by this intensity difference, and the result is stored.

Thus, at the time of correction, only the subtraction of subtracting the minimum intensity of the standard image signal from the intensity of the image signal and the multiplication of multiplying the subtraction result by the stored division result can be performed. The correction process can be performed at high speed. Since the process of reading the standard image does not need to be performed at a high speed, the division may be performed using the aforementioned small-scale circuit that repeats the subtraction and the bit shift.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is applied to a hand-held scanner which is a hand-held image scanner will be described below with reference to the drawings. FIG. 1 shows a handy scanner 1 of the present embodiment (hereinafter, simply referred to as a scanner).
The schematic configuration of is shown. The handy scanner 1 includes a lighting unit 1
1, line sensor 12, analog signal processing unit 13, A
D converter 14, timing generator 15, digital signal processing unit 16, image compression unit 17, and transmission unit 1
8, which reads a document image, compresses the image data, and transmits the image data to another image processing apparatus 2.

The illuminator 11 irradiates light to the original. The line sensor 12 is composed of a charge-coupled device (CCD) and has a number of pixels arranged in a straight line. Line sensor 1
Reference numeral 2 receives the light from the illumination unit 11 reflected by the document, and outputs an analog signal indicating the amount of received light for each pixel.
When the user holds the scanner 1 and moves the document over the document, the density of the document image is detected line by line by the line sensor 12, and the signal is output for each line. Thereby, a two-dimensional image is read.

The analog signal processing unit 13 performs double correlation sampling of the output signal of the line sensor 12 and performs automatic gain control. The AD converter 14 converts an analog signal input from the analog signal processing unit 13 into an 8-bit digital signal and outputs the digital signal to the digital signal processing unit 16.
The timing generator 15 supplies a timing signal to the line sensor 12, the analog signal processing unit 13, and the AD converter 14, and instructs a timing of each signal output, sampling, and signal conversion.

The digital signal processing section 16 performs various processes on the signal supplied from the AD converter 14 to generate image data representing a read image. Image compression unit 1
7 converts the image data generated by the digital signal processing unit 16 into JPEG (Joint Photographic image coding).
Experts Group) compression. The transmission unit 18 transmits the compressed image data to another image processing device 2 via the cable 2a.

The image processing apparatus 2 directly uses image data provided from the scanner 1 or transfers the image data to another apparatus, and is, for example, a personal computer. The image processing device 2 converts the compressed image data into JP
It has a function of decompressing according to the EG method, and can reproduce an image read by the scanner 1.

The digital signal processing section 16 includes a shading correction circuit 20 for shading correction of the output signal of the line sensor 12 converted into a digital signal, a gamma correction circuit 28 for gamma correction of the correction result, and further emphasizes the outline of the image. And an edge emphasizing / smoothing circuit 29 for smoothing.

The shading correction circuit 20 performs shading correction on the signal for each pixel of the line sensor 12 given from the AD converter 14 according to the equation (2). IC = (IO−IB) × KO (2)

Where KO is K / (IW-IB),
K is a constant 255. As described above, IC is the intensity of the signal after correction, I0 is the intensity of the signal before correction, IB is the minimum intensity of the signal of the standard image (black level), and IW is the maximum intensity of the signal of the standard image (white level). It is.

Equation (2) is equal to equation (1), so that the correction result by the shading correction circuit 20 is the same as the correction result of the conventional device. However, K / (IW
-IB) is performed when the standard image is read, and the result is stored as KO. Further, the minimum intensity IB is also stored at the time of reading the standard image, and at the time of correction, only the subtraction and the multiplication appearing on the right side of Expression (2) are performed.

FIG. 2 shows the configuration of the shading correction circuit 20.
Shown in The shading correction circuit 20 includes the pseudo division circuit 2
It comprises one and two memories 22, 23, a subtractor 24 and a multiplier 25. The memories 22 and 23 are for storing the minimum intensity IB and the division result KO, respectively.
Storage areas.

The pseudo division circuit 21 is for obtaining the division result KO, and operates when reading a standard image, but does not operate when correcting a general image. Although not shown, the pseudo division circuit 21 includes a subtractor, a register, a comparator, a counter, and the like, and performs substantially the same processing as the processing shown in FIG. 3 by these. The only difference is the processing of steps # 5 to # 15.

When a standard image having a dark area where an image of a fixed density is formed and a bright area where no image is formed is read,
The quasi-division circuit 21 calculates the minimum intensity IB (black level) and the maximum intensity IW of the signal output from the line sensor 12 for each pixel.
(White level) is given from the AD converter 14, the subtraction is performed, and the subtraction result (IW-IB) is stored in the register B (corresponding to step # 15 in FIG. 3). The value 1 is stored in the register A (corresponding to # 10).

Following these processes, # 20 to # 55
The processing up to is performed. As a result, K / (I
W-IB). The pseudo division circuit 21 performs the above-described processing for each signal for each pixel, and writes the contents of the register C to the memory 23 in order each time the processing ends.

The minimum intensity IB of the signal for each pixel when the standard image is read is obtained from the AD converter 14 in the memory 2
2 and are written in order. In this way, all the parameters necessary for the shading correction are stored in the memories 22 and 23.

When a normal image to be subjected to shading correction is read, the output signal of the line sensor 12 is sequentially supplied from the AD converter 14 to the subtractor 24.
In the meantime, the minimum intensity IB stored in the memory 22 is sequentially given to the subtractor 24, and the subtractor 24 subtracts the intensity IB from the intensity I0 and sequentially outputs a subtraction result (IO-IB).

The output of the subtractor 24 is provided to a multiplier 25. The multiplier 25 stores K /
(IW−IB) are successively applied, and the multiplier 25 multiplies these by the subtraction result supplied from the subtractor 24 and sequentially outputs the multiplication result. The output of the multiplier 25 is a gamma correction circuit 28
Given to.

Since the processing performed by the shading correction circuit 20 at the time of correction is only subtraction and multiplication, the time required for correction is extremely short. Also, the shading correction circuit 20
Compared with the shading correction circuit of the conventional image reading apparatus, the configuration is such that a memory 23, a subtractor 24 and a multiplier 25 are added, and the circuit scale is only slightly increased. That is, the handy scanner 1
It is possible to perform shading correction at a very high speed with a circuit of the same small scale as the conventional one.

The process of reading the standard image and generating the parameters to be stored in the memory 23 takes time because the pseudo division circuit 21 performs repetitive processing. However, the reading of the standard image is a process that does not need to be performed at high speed, and is a process that is performed during the manufacture or maintenance work of the scanner 1 and is not normally performed. No problem arises.

Moreover, when reading the standard image, the maximum intensity IW of the signal for each pixel of the line sensor 12 is stored in the memory 23 in order, and after the reading is completed,
K / (I) by the pseudo division circuit 21 using the contents of 2 and 23.
W-IB) is calculated, and the calculation result can be written to the memory 23. This makes it possible to read the standard image very quickly. In addition,
As the memories 22 and 23, rewritable and nonvolatile memories, for example, EEPROMs are used.

In this embodiment, an example of a handy scanner has been described. However, the shading correction circuit 20 can be applied to an image reading apparatus of a type in which a sensor is fixed and an image is moved. Examples of applications include facsimile machines and image copiers.

An area sensor may be used as the image reading sensor. The area sensor reads an image two-dimensionally as a surface, and is more efficient than a line sensor that reads an image one-dimensionally as a line, and generally has a larger number of pixels. Since the above-mentioned shading correction circuit has a very high processing speed, if it is used in combination with a sensor having a large number of pixels, such as an area sensor, its characteristics can be further exploited.

Although the signal strength is represented by 8 bits and 256 gradations can be represented here, the signal strength may be represented by a different number of bits. For example, if the signal strength is expressed by 10 bits, 102
It becomes possible to express four gradations. Even in this case, since the repetition processing of the number of bits equal to the number of bits is not performed in the shading correction, there is almost no effect on the time required for the correction, and the correction can be performed at substantially the same speed as in the case of 8 bits.

Note that the image compression unit 17 includes the image processing device 2
It is provided for the efficiency of data transfer to and the efficient use of memory for storing image data.
If there is no time problem in the data transfer and there is no problem in the storage capacity, it may be omitted.

[0040]

According to the image reading apparatus of the present invention, shading correction can be performed at high speed without using a large-scale divider. Even when a sensor having an extremely large number of pixels is used, since the shading correction can follow the output cycle of the sensor, it is easy to simultaneously read an image with high resolution and quickly read an image. Further, since the time required for the correction does not depend on the number of bits representing the signal strength, the gradation of the image can be set arbitrarily, and a high-quality image can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schematic configuration of a handy scanner according to an embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a shading correction circuit of the handy scanner.

FIG. 3 is a diagram showing a flow of a shading correction process of a conventional image reading apparatus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Handy scanner 2 Image processing apparatus 11 Illumination part 12 Line sensor 13 Analog signal processing part 14 AD converter 15 Timing generator 16 Digital signal processing part 17 Image compression part 18 Transmission part 20 Shading correction circuit 21 Pseudo division circuit 22 Memory 23 Memory 24 Subtraction Unit 25 multiplier 28 gamma correction circuit 29 edge emphasis / smoothing circuit

Claims (1)

[Claims] An image reading sensor comprising a plurality of pixels, wherein a difference between a maximum intensity and a minimum intensity of a signal of a read standard image is stored for each pixel of the sensor, and the read image is stored. In the image reading device, the intensity of the signal is corrected for each pixel of the sensor according to the stored intensity difference, and the signals of all the pixels of the image are within a certain intensity range. Corresponding to, a first memory that stores the minimum intensity of the signal of the standard image, and a value obtained by dividing an upper limit value of the predetermined intensity range by an intensity difference between the maximum intensity and the minimum intensity of the signal of the standard image. A subtractor for subtracting the intensity stored in the first memory from the intensity of a signal of a given image; and storing the subtraction result of the subtractor in the second memory. And and a multiplier for multiplying the values are, the image reading apparatus and correcting each pixel the intensity of the image signals read by the multiplication by the multiplier and the subtraction by the subtractor.