JP2000295444A - Device and method for reading image and storage medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a proper digital density value with respect to dot images which form intermediate density. SOLUTION: An A/D converting part 102 converts an analog signal outputted by a line sensor 101 to a digital signal, on the basis of a reference voltage. A data processing part 103 performs shading correction. A data processing part 104 performs gamma-correction processing and binarizing processing. On the basis of digital signals for one line shading-corrected by the data processing part 103, a counter 105 counts the number of pixels, which have an image signal level higher than the reference voltage of the A/D converting part 102. When the counted number of pixels is more than a prescribed upper limit value or less than a prescribed lower limit value, a discrimination part 106 changes the reference voltage of the A/D converting part 102.

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, an image reading method, and a storage medium, and more particularly, to an image reading apparatus having a function of adjusting the density of a document image, and an image reading method applied to the image reading apparatus. And a storage medium storing a program for executing the image reading method.

[0002]

2. Description of the Related Art Conventionally, as a method of binarizing an image signal, a method is known in which an image analog signal input from a sensor or the like is A / D converted and then binarized at a predetermined threshold. In this method, when an image signal is obtained by reading a document with a scanner or the like, binarization may not be properly performed if the background of the document is gray or if the document paper is colored.

As a countermeasure in such a case, density adjustment has been conventionally performed. As a conventional density adjustment method, an analog signal from a sensor or the like is converted into an analog signal by an A / D converter.
The image density is adjusted at the time of D conversion. Specifically, a reference voltage is created for each pixel based on an analog signal from a sensor or the like, and the A / D converter has the same level as the reference voltage. A digital value representing white is given to the analog signal, and a digital value (multi-value) close to black is given every time the level of the analog signal becomes lower than the reference voltage by a predetermined level. By varying the reference voltage, density adjustment can be performed.

[0004]

However, the above-mentioned conventional density adjustment method has the following problems.

That is, in recent years, documents used in offices and the like are digital copiers and laser beam printers (L
Many are digital images due to the spread of BP). A digital image is composed of fine points, and is a halftone image in order to form an intermediate density. Further, in order to prevent toner from scattering, no image is formed on the end face of the document.

When the conventional density adjustment method is used for the halftone image forming the intermediate density, as described above, the reference voltage of the A / D converter is set for each pixel based on the analog signal from the sensor or the like. Is set. Therefore, the reference voltage of the A / D converter is shifted to the white level side when a dot of the halftone image is read, and the reference voltage is shifted to the white level side when a portion other than the dot of the halftone image is read. . Therefore, when a halftone image forming an intermediate density is read, the reference voltage of the A / D converter changes for each part constituting the halftone image, and as a result, the The original halftone image cannot be reproduced.

As described above, many originals used in offices and the like have no image on the end face. In the conventional density adjustment method, since the density adjustment is also performed on the end face as described above, there is a problem that the reference voltage when reading the end face does not become an appropriate value.

Also, when there is small dust or dirt on the original, the reference voltage is changed by the dust or dirt, causing a deviation from the reference voltage representing the original background of the original. is there.

The present invention has been made in view of such a problem, and an image reading apparatus, an image reading method, and an image reading method capable of obtaining an appropriate density digital value for a halftone image forming an intermediate density. And providing the storage medium is the first
The purpose of.

It is a second object of the present invention to provide an image reading apparatus, an image reading method, and a storage medium capable of obtaining an appropriate density digital value for a white background such as a document end face.

It is a third object of the present invention to provide an image reading apparatus, an image reading method, and a storage medium capable of obtaining an appropriate density digital value for an original having small dust and dirt.

[0012]

According to the first aspect of the present invention, there is provided an image reading apparatus having a function of adjusting the density of a document image by reading an optical signal incident from the document. A / D conversion means for converting an analog signal output from the line sensor into a digital signal based on a reference voltage, and first data processing means for performing shading correction on the digital signal converted by the A / D conversion means. A second data processing unit for performing a gamma correction process and a binarization process on the digital signal that has been subjected to shading correction by the first data processing unit, and a first signal that has been subjected to shading correction by the first data processing unit. Based on the digital signal for the line, the image signal level is A / A
Counting means for counting the number of pixels that are equal to or higher than the reference voltage of the D conversion means, and wherein the number of pixels obtained by the counting means counting over one line of the digital signal is equal to or more than a predetermined upper limit value or equal to or less than a predetermined lower limit value If
Reference voltage changing means for changing a reference voltage of the A / D conversion means.

According to the third aspect of the present invention, in an image reading apparatus having a function of adjusting the density of a document image, an analog signal output from a line sensor that reads an optical signal incident from the document is converted based on a reference voltage. A / D conversion means for converting into a digital signal, first data processing means for performing shading correction on the digital signal converted by the A / D conversion means, and digital data which has been subjected to shading correction by the first data processing means. A second data processing means for performing gamma correction processing and binarization processing on the signal; and a digital signal for one line, which is included in the first data processing means and has been subjected to the shading correction, based on the digital signal for one line. Extracting means for extracting an image signal relating only to the image portion excluding the end face of the document, and one line of the image signal extracted by the extracting means, Counting means for counting the number of pixels whose image level is equal to or higher than the reference voltage of the A / D conversion means, and wherein the number of pixels obtained by counting over one line of the image signal is equal to or higher than a predetermined upper limit value Alternatively, a reference voltage changing means for changing a reference voltage of the A / D conversion means when the reference voltage is equal to or less than a predetermined lower limit value is provided.

According to the fifth aspect of the present invention, in an image reading apparatus having a function of adjusting the density of a document image, an analog signal output from a line sensor that reads an optical signal incident from a document is converted based on a reference voltage. A / D conversion means for converting into a digital signal, first data processing means for performing shading correction on the digital signal converted by the A / D conversion means, and digital data which has been subjected to shading correction by the first data processing means. A second data processing means for performing gamma correction processing and binarization processing on the signal, and an image signal level based on the digital signal for one line which has been subjected to shading correction by the first data processing means. When a predetermined number of pixels that are equal to or higher than the reference voltage of the A / D conversion means are continuously detected, a count for counting the number of the pixels. And when the number of pixels obtained by the counting means counting over one line of the digital signal is equal to or more than a predetermined upper limit value or equal to or less than a predetermined lower limit value, the reference voltage of the A / D conversion means is changed. Reference voltage changing means for changing the reference voltage.

According to a seventh aspect of the present invention, in the image reading method applied to the image reading apparatus having the function of adjusting the density of the original image, the line sensor that reads the optical signal incident from the original is output. A / D for converting an analog signal to be converted into a digital signal based on a reference voltage
A first data processing step of performing shading correction on the digital signal converted in the A / D conversion step, and a gamma correction processing on the digital signal that has been subjected to shading correction in the first data processing step. Based on a second data processing step of performing a binarization process and a digital signal of one line subjected to shading correction in the first data processing step, the number of pixels whose image signal level is equal to or higher than the reference voltage is determined. A counting step of counting, and a reference voltage changing step of changing the reference voltage when the number of pixels obtained in the counting step is equal to or more than a predetermined upper limit or equal to or less than a predetermined lower limit. .

According to the ninth aspect of the present invention, in the image reading method applied to the image reading apparatus having the function of adjusting the density of the original image, the analog signal output from the line sensor which reads the optical signal incident from the original is provided. An A / D conversion step of converting a signal into a digital signal based on a reference voltage, a first data processing step of performing shading correction on the digital signal converted in the A / D conversion step, and the first data processing A second data processing step of performing gamma correction processing and binarization processing on the digital signal that has been subjected to shading correction in the step, and a digital signal for one line that has been subjected to shading correction in the first data processing step. An extracting step of extracting an image signal relating only to an image portion excluding an end surface portion of the original; A counting step of counting the number of pixels whose image level is equal to or higher than the reference voltage based on the image signal of one line extracted by the flip-flop; and determining whether the number of pixels obtained in the counting step is equal to or higher than a predetermined upper limit value or a predetermined value. A reference voltage changing step of changing the reference voltage when the value is equal to or less than the lower limit of the reference voltage.

According to the eleventh aspect of the present invention, in the image reading method applied to the image reading apparatus having the function of adjusting the density of the original image, the analog signal output from the line sensor which reads the optical signal incident from the original is provided. An A / D conversion step of converting a signal into a digital signal based on a reference voltage, a first data processing step of performing shading correction on the digital signal converted in the A / D conversion step, and the first data processing A second data processing step of performing gamma correction processing and binarization processing on the digital signal that has been subjected to shading correction in the step, and a digital signal for one line that has been subjected to shading correction in the first data processing step. When a predetermined number of pixels whose image signal levels are equal to or higher than the reference voltage are detected consecutively, A counting step of counting the number of pixels, and the reference step is performed when the number of pixels obtained by counting over one line of the digital signal is equal to or more than a predetermined upper limit or equal to or less than a predetermined lower limit. And a reference voltage changing step of changing a voltage.

Further, according to the invention of claim 13,
An image reading method applied to an image reading apparatus having a density adjustment function of a document image is stored as a program. In a computer-readable storage medium, the image reading method reads an optical signal incident from a document. An A / D conversion step of converting an analog signal output by the line sensor into a digital signal based on a reference voltage; a first data processing step of performing shading correction on the digital signal converted in the A / D conversion step; Gamma correction processing is performed on the digital signal subjected to shading correction in the first data processing step.
A second data processing step for performing a binarization process;
A counting step of counting the number of pixels whose image signal level is equal to or higher than the reference voltage based on the digital signals for the lines, and the number of pixels obtained in the counting step is equal to or higher than a predetermined upper limit or equal to or lower than a predetermined lower limit. A reference voltage changing step of changing the reference voltage in some cases.

According to a fifteenth aspect of the present invention, there is provided a computer-readable storage medium storing, as a program, an image reading method applied to an image reading apparatus having a density adjusting function of a document image. An A / D conversion step of converting an analog signal output by a line sensor that reads an optical signal incident from a document into a digital signal based on a reference voltage;
A first data processing step of performing shading correction on the digital signal converted in the A / D conversion step;
A second data processing step of performing gamma correction processing and binarization processing on the digital signal that has been subjected to shading correction in the first data processing step, and one line that has been subjected to shading correction in the first data processing step Extracting an image signal relating to only the image portion excluding the end face portion of the document based on the digital signal for one minute, based on the image signal for one line extracted by the extracting step,
A counting step of counting the number of pixels whose image level is equal to or higher than the reference voltage; and changing the reference voltage when the number of pixels obtained in the counting step is equal to or higher than a predetermined upper limit or equal to or lower than a predetermined lower limit. And a reference voltage changing step.

According to a seventeenth aspect of the present invention, there is provided a computer-readable storage medium storing, as a program, an image reading method applied to an image reading apparatus having a function of adjusting the density of a document image. An A / D conversion step of converting an analog signal output by a line sensor that reads an optical signal incident from a document into a digital signal based on a reference voltage;
A first data processing step of performing shading correction on the digital signal converted in the A / D conversion step;
A second data processing step of performing gamma correction processing and binarization processing on the digital signal that has been subjected to shading correction in the first data processing step, and one line that has been subjected to shading correction in the first data processing step A counting step of counting the number of pixels, when a predetermined number of pixels whose image signal levels are equal to or higher than the reference voltage are detected consecutively based on the digital signal for minutes,
A reference voltage changing step of changing the reference voltage when the number of pixels obtained by counting over one line of the digital signal is equal to or more than a predetermined upper limit or equal to or less than a predetermined lower limit. It is characterized by the following.

[0021]

Embodiments of the present invention will be described below with reference to the drawings.

(First Embodiment) FIG. 1 is a block diagram showing a configuration of a data processing portion of an image reading apparatus according to a first embodiment of the present invention.

Although not shown in the figure, the image reading apparatus is composed of, for example, a document table, an illumination light source, a line sensor, an optical device, a driving device, etc., and the illumination light source emits light to a document placed on the document table. And the reflected light is sent to the line sensor via the optical system device, and the line sensor outputs an image signal. The line sensor has a configuration in which a number of image sensors are arranged in a line, and the driving device moves the line sensor relative to a document in a direction perpendicular to the direction in which the image sensors are arranged.

In FIG. 1, reference numeral 101 denotes this line sensor. An image signal for one line detected by the line sensor 101 is sent to the A / D converter 102. The A / D converter 102 changes the reference voltage according to the level of the analog signal sent from the D / A converter 107 as described later, sets the 0 level using the reference voltage, and then sets the image signal. Is converted to a multi-valued digital signal.
This digital signal is sent to the data processing unit 103. The data processing unit 103 performs shading correction and sends the result to the data processing unit 104 and the counter 105. In the data processing 104, image processing such as gamma correction is performed, and further binarized and output.

The counter 105 receiving the image data subjected to the shading correction by the data processing unit 103 counts the number of pixels whose signal level is equal to or higher than the reference voltage based on the image data for one line. The count value is sent to the determination unit 106, and the determination unit 106 compares the sent count value with a preset upper limit value and lower limit value. When the count value is larger than the upper limit value or when the count value is smaller than the lower limit value, the digital signal indicating the difference is converted into a digital signal by the D / A converter 107.
Send to The D / A converter 107 creates an analog signal indicating the difference based on the transmitted digital signal, and sends the analog signal to the A / D converter 102. The A / D converter 102 changes the reference voltage according to the level of the transmitted analog signal.

The image data to be counted by the counter 105 is the image data after the shading correction. This is performed by using the image data after the shading correction, so that each image sensor ( This is because it is possible to eliminate the influence of the variation in sensitivity of the chip) and the unevenness in the light amount of the illumination light source.

The image data to be counted by the counter 105 is the image data before the gamma correction. However, since the image data is changed by the gamma correction, the image data before the gamma correction is not affected. This is to use the data.

FIG. 2 is a diagram showing the relationship between the image data input to the A / D converter 102 and the reference voltage of the A / D converter 102 changed by the D / A converter 107.

When the image data for one line exceeds the reference voltage set in the A / D converter 102, the counter 105 corresponds to the shaded area as shown by the shaded area in FIG. Count the number of pixels. If the count value is larger than a preset upper limit value, the reference voltage is increased according to the difference as shown in FIG. Further, as shown in FIG. 2C, when the image data for one line is smaller than the reference voltage set in the A / D conversion unit 102, the counter 105
Is 0, which is naturally smaller than the preset lower limit, so that the reference voltage is reduced according to the difference as shown in FIG. 2 (D).

As described above, the reference voltage of the A / D converter 102 is changed in a range in which the count value counted by the counter 105 based on one line of image data is divided by the upper limit value and the lower limit value. , And the A / D converter 102 uses the same reference voltage for one line of image data read by the line sensor 101.

Therefore, even when a halftone image forming an intermediate density is read, the reference voltage of the A / D converter 102 does not change for each part constituting the halftone image. An accurate density digital value of each of the black and white portions constituting the halftone dot image can be obtained, and the original halftone dot image can be accurately reproduced.

If the upper limit and the lower limit are set so that the range divided by the upper limit and the lower limit with which the count values counted by the counter 105 are compared is large, the reference density with respect to the average density of the entire original image is set. Density adjustment for setting the voltage is performed, and even when there is an extremely different portion of the density in a part of the image, appropriate density adjustment according to the entire original image can be performed.

The upper limit and the lower limit with which the count values counted by the counter 105 are compared may not be set to fixed values but may be changed. This makes it possible to adjust the density according to the original image to be read, even for an image having a large density change or an image having a small density change. This will be described with reference to FIG.

FIG. 3 shows image data input to the A / D converter 102 when the density change of the background portion of the document is large, and a reference of the A / D converter 102 changed by the D / A converter 107. FIG. 3 is a diagram illustrating a relationship with a voltage.

It is preferable that the reference voltage when the A / D converter 102 converts the original image data when the density change of the background portion of the original is large into a digital value is set at the position shown in FIG. In such a case, if the reference voltage of the A / D conversion unit 102 changes following the change of the background of the document, the density greatly changes between the surfaces of the document. Therefore, the upper limit value for comparing the count value of the counter 105 is changed to a large value.
The density of the document is stabilized while the reference voltage of the conversion unit 102 is set at the position shown in FIG.

(Second Embodiment) FIG. 4 is a block diagram showing a configuration of a data processing portion of an image reading apparatus according to a second embodiment of the present invention. The configuration of the second embodiment is as follows.
Since the configuration is basically the same as that of the first embodiment, the same components are denoted by the same reference numerals and description thereof will be omitted.

In the second embodiment, the data processing unit 103
However, as in the first embodiment, the image data for one line after the shading correction is sent to the data processing unit 104, and the edge of the document is removed from the one-line image data after the shading correction. Only the extracted image portion is extracted and sent to the count 105. The image reading device includes a unit that detects a position and a width of the document, and the data processing unit 103
Extracts only the image portion excluding the end face of the document from the image data after shading correction based on the detected result. In the count 105, counting is performed in the same manner as in the first embodiment based on image data relating to an image portion from which the end face of the document has been removed, and the A / A data is passed through the determination unit 106 and the D / A conversion unit 107. The reference voltage of the D conversion unit 102 is set.

FIG. 5 shows the A / D in the second embodiment.
FIG. 3 is a diagram illustrating a relationship between image data input to a conversion unit and a reference voltage of the A / D conversion unit changed by a D / A conversion unit.

Usually, in a manuscript created in an office or the like,
As shown in FIG. 5A, an image such as a document is not printed on the end face of the document.

The image data sent to the counter 105 is
The image data is limited to the image data in the range W1 shown in FIG.
Therefore, as shown in FIG. 5C, the reference voltage can be set at an optimum position for the image data image in the range W1 regardless of the end face of the document.

(Third Embodiment) The configuration of the third embodiment is basically the same as the configuration of the first embodiment. Therefore, the configuration of the first embodiment will be described.

In the third embodiment, as in the first embodiment, the counter 105 receives one line of image data after shading correction from the data processing unit 103 and converts the received one line of image data. On the basis of this, it is determined whether or not the signal level of the pixel is equal to or more than a predetermined set value. When detected, the number of those pixels is counted.

If there is small dust or dirt on the original, they are not counted by the counter 105. Therefore, the reference voltage of the A / D converter 102 is
The optimum reference voltage is set without being affected by the small dust or dirt.

Although each of the above embodiments has been described by taking as an example the case where it is constituted by hardware, the present invention can also be applied to software.

When the present invention is realized by software, a storage medium storing program codes of software for realizing the functions of the above-described embodiments is replaced with a storage medium.
It goes without saying that the present invention can also be achieved by supplying the program or the program to the system or the apparatus and causing the computer (or CPU or MPU) of the system or the apparatus to read and execute the program code stored in the storage medium.

In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiments, and the storage medium storing the program code constitutes the present invention.

As a storage medium for supplying the program code, for example, a floppy disk, hard disk, optical disk, magneto-optical disk, CD-ROM, CD
-R, a magnetic tape, a nonvolatile memory card, a ROM, or the like can be used.

When the computer executes the readout program code, not only the functions of the above-described embodiments are realized, but also an OS or the like running on the computer based on the instruction of the program code. It is needless to say that the present invention includes a case where a part or all of the actual processing is performed and the function of each embodiment described above is realized by the processing.

Further, after the program code read from the storage medium is written into a memory provided in a function expansion board inserted into the computer or a function expansion unit connected to the computer, based on the instructions of the program code, It is needless to say that the present invention includes a case where the CPU or the like provided in the function expansion board or the function expansion unit performs part or all of the actual processing, and the processing realizes the functions of the above-described embodiments. .

[0050]

As described in detail above, claims 1 and 7 have been described.
According to the thirteenth aspect of the present invention, the number of pixels whose image signal level is equal to or higher than the reference voltage in the image data for one line sensor is counted, and the counted value is used as the reference value of the A / D converter. Used to make a decision to change the voltage.

As a result, the A / D converter sets the 0 level to the image signal for one line sensor using the same reference voltage, and therefore, the halftone image forming the intermediate density can be displayed. There is no change in the reference voltage during reading for one line, and an accurate density digital value of a halftone image is obtained. As a result, an accurate halftone image can be reproduced.

According to the invention described in claim 2, claim 4, claim 6, claim 8, claim 10, claim 12, claim 14, claim 16, or claim 18, a line sensor is provided. The above-described counting is performed over the image data for one line, and when the obtained count value is equal to or more than a predetermined upper limit value or equal to or less than a lower limit value, the reference voltage of the A / D conversion unit is changed. The predetermined upper and lower limits are set to values that can be set arbitrarily.

Thus, even for an image having a large density change,
In addition, it is possible to adjust the density according to the original image to be read even with an image having a small density change.

Further, claim 3, claim 9, or claim 1
According to the fifth aspect of the invention, in a document having no image on the end face of the document, the image data to be counted is only the image portion excluding the end face of the document.

As a result, it is possible to prohibit the reference voltage of the A / D converter from being changed by the image data of the end face portion where no image is formed, and it is possible to obtain an appropriate density digital value.

According to the fifth, eleventh, or seventeenth aspect of the present invention, pixels having an image signal level equal to or higher than the reference voltage in the image data for one line sensor are continuously determined. When the number is detected, the above counting is performed.

As a result, an appropriate density digital value can be obtained without treating small dust or dirt on the document as an image.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a data processing portion of an image reading apparatus according to a first embodiment of the present invention.

FIG. 2 illustrates image data input to an A / D conversion unit and D / D conversion data.
FIG. 5 is a diagram illustrating a relationship between an A / D converter and a reference voltage changed by the A converter.

FIG. 3 shows A / A when the density change of the background portion of the original is large.
FIG. 4 is a diagram illustrating a relationship between image data input to a D / A converter and a reference voltage of the A / D converter changed by the D / A converter.

FIG. 4 is a block diagram illustrating a configuration of a data processing portion of an image reading apparatus according to a second embodiment of the present invention.

FIG. 5 is a diagram illustrating a relationship between image data input to an A / D converter and a reference voltage of the A / D converter changed by the D / A converter in the second embodiment.

[Explanation of symbols]

 101 line sensor 102 A / D conversion unit (A / D conversion unit) 103 data processing unit (first data processing unit) 104 data processing unit (second data processing unit) 105 counter (counting unit) 106 determination unit ( Reference voltage changing means) 107 D / A converter (reference voltage changing means)

Claims (18)

[Claims] An image reading apparatus having a function of adjusting the density of a document image, wherein an analog signal output from a line sensor that reads an optical signal incident from the document is converted into a digital signal based on a reference voltage. Conversion means; first data processing means for performing shading correction on the digital signal converted by the A / D conversion means; and gamma correction processing for the digital signal subjected to shading correction by the first data processing means. A second data processing unit for performing a binarization process; and a reference signal of the A / D conversion unit for an image signal level based on the digital signal of one line which has been subjected to shading correction by the first data processing unit. A counting means for counting the number of pixels, the counting means counting over one line of digital signal If the number of pixels obtained by is less than a predetermined upper limit value or a predetermined lower limit value, the image reading apparatus characterized by having a reference voltage changing means for changing the reference voltage of the A / D converter. 2. The image reading apparatus according to claim 1, wherein the predetermined upper limit value and the predetermined lower limit value are changeable values. 3. An image reading apparatus provided with a function of adjusting the density of a document image, wherein an analog signal output from a line sensor that reads an optical signal incident from the document is converted into a digital signal based on a reference voltage. Conversion means; first data processing means for performing shading correction on the digital signal converted by the A / D conversion means; and gamma correction processing for the digital signal subjected to shading correction by the first data processing means. A second data processing unit that performs a binarization process; and an image unit that is included in the first data processing unit and that excludes an end surface of the document based on the digital signal for one line that has been subjected to the shading correction. Extracting means for extracting an image signal relating only to the image signal, and based on the image signal for one line extracted by the extracting means,
Counting means for counting the number of pixels whose image level is equal to or higher than the reference voltage of the A / D conversion means; and wherein the number of pixels obtained by the counting means counting over one line of the image signal is equal to or higher than a predetermined upper limit. An image reading apparatus comprising: a reference voltage changing unit that changes a reference voltage of the A / D conversion unit when the reference voltage is equal to or less than a predetermined lower limit. 4. The image reading apparatus according to claim 3, wherein the predetermined upper limit value and the predetermined lower limit value are changeable values. 5. An image reading apparatus having a function of adjusting the density of a document image, wherein the A / D converter converts an analog signal output from a line sensor reading an optical signal incident from the document into a digital signal based on a reference voltage. Conversion means; first data processing means for performing shading correction on the digital signal converted by the A / D conversion means; and gamma correction processing for the digital signal subjected to shading correction by the first data processing means. A second data processing unit for performing a binarization process; and a reference signal of the A / D conversion unit for an image signal level based on the digital signal of one line which has been subjected to shading correction by the first data processing unit. Counting means for counting the number of pixels when a predetermined number of the above pixels are continuously detected, and the counting means Reference voltage changing means for changing the reference voltage of the A / D conversion means when the number of pixels obtained by counting over one line of the digital signal is not less than a predetermined upper limit value or not more than a predetermined lower limit value; An image reading apparatus comprising: 6. The image reading apparatus according to claim 5, wherein the predetermined upper limit value and the predetermined lower limit value are changeable values. 7. An image reading method applied to an image reading apparatus having a function of adjusting the density of a document image, wherein an analog signal output from a line sensor that reads an optical signal incident from the document is digitalized based on a reference voltage. An A / D conversion step of converting the digital signal into a signal, a first data processing step of performing shading correction on the digital signal converted in the A / D conversion step, and a digital signal subjected to shading correction in the first data processing step A second data processing step of performing a gamma correction process and a binarization process on the image signal level, and based on the digital signal of one line subjected to shading correction in the first data processing step, the image signal level is set to the reference value. A counting step of counting the number of pixels that are equal to or higher than the voltage; Image reading method was the number of pixels and wherein if less than a predetermined upper limit value or more or a predetermined lower limit value, it has a reference voltage change step of changing the reference voltage. 8. The image reading method according to claim 7, wherein said predetermined upper limit value and said predetermined lower limit value are changeable values. 9. An image reading method applied to an image reading apparatus having a function of adjusting the density of a document image, wherein an analog signal output from a line sensor reading an optical signal incident from the document is digitally converted based on a reference voltage. An A / D conversion step of converting the digital signal into a signal, a first data processing step of performing shading correction on the digital signal converted in the A / D conversion step, and a digital signal subjected to shading correction in the first data processing step A second data processing step of performing a gamma correction process and a binarization process on the image data; and an edge portion of the document based on the digital signal of one line that has been subjected to shading correction in the first data processing step. An extraction step of extracting an image signal relating only to an image portion to be removed, and one line extracted by the extraction step A counting step of counting the number of pixels whose image level is equal to or higher than the reference voltage, based on the image signal of the above, when the number of pixels obtained in the counting step is equal to or higher than a predetermined upper limit or equal to or lower than a predetermined lower limit. A reference voltage changing step of changing the reference voltage. 10. The image reading method according to claim 9, wherein said predetermined upper limit value and said predetermined lower limit value are changeable values. 11. An image reading method applied to an image reading apparatus having a function of adjusting the density of a document image, wherein an analog signal output from a line sensor that reads an optical signal incident from the document is digitally converted based on a reference voltage. An A / D conversion step of converting the digital signal into a signal, a first data processing step of performing shading correction on the digital signal converted in the A / D conversion step, and a digital signal subjected to shading correction in the first data processing step A second data processing step of performing a gamma correction process and a binarization process on the image signal level, and based on the digital signal of one line subjected to shading correction in the first data processing step, the image signal level is set to the reference value. When a predetermined number of pixels at or above the voltage are detected consecutively, the number of pixels is counted. A counting step; and a reference voltage change for changing the reference voltage when the number of pixels obtained by counting over one line of the digital signal in the counting step is equal to or more than a predetermined upper limit or equal to or less than a predetermined lower limit. And an image reading method. 12. The image reading method according to claim 11, wherein said predetermined upper limit value and said predetermined lower limit value are changeable values. 13. A computer-readable storage medium storing, as a program, an image reading method applied to an image reading apparatus having a density adjustment function of a document image, wherein the image reading method is incident from a document. An A / D conversion step of converting an analog signal output by the line sensor that has read the optical signal into a digital signal based on a reference voltage; and a first step of performing shading correction on the digital signal converted in the A / D conversion step. A data processing step; a second data processing step of performing gamma correction processing and binarization processing on the digital signal that has been subjected to shading correction in the first data processing step; and shading in the first data processing step. Based on the corrected digital signal for one line, the image signal level is A counting step of counting the number of pixels that are equal to or higher than a reference voltage; and a reference voltage changing step of changing the reference voltage when the number of pixels obtained in the counting step is equal to or more than a predetermined upper limit or equal to or less than a predetermined lower limit. And a storage medium comprising: 14. The storage medium according to claim 13, wherein said predetermined upper limit value and said predetermined lower limit value are changeable values. 15. A computer-readable storage medium storing, as a program, an image reading method applied to an image reading apparatus having a density adjusting function of a document image, wherein the image reading method is incident from a document. An A / D conversion step of converting an analog signal output by the line sensor that has read the optical signal into a digital signal based on a reference voltage; and a first step of performing shading correction on the digital signal converted in the A / D conversion step. A data processing step; a second data processing step of performing gamma correction processing and binarization processing on the digital signal that has been subjected to shading correction in the first data processing step; and shading in the first data processing step. Based on the corrected digital signal for one line, the end face of the original is An extracting step of extracting an image signal relating to only the image portion; a counting step of counting the number of pixels whose image level is equal to or higher than the reference voltage based on the image signal of one line extracted by the extracting step; A reference voltage changing step of changing the reference voltage when the number of pixels obtained in the counting step is equal to or more than a predetermined upper limit value or equal to or less than a predetermined lower limit value. 16. The storage medium according to claim 15, wherein said predetermined upper limit value and said predetermined lower limit value are changeable values. 17. A computer-readable storage medium storing, as a program, an image reading method applied to an image reading apparatus having a density adjustment function of a document image, wherein the image reading method is incident from a document. An A / D conversion step of converting an analog signal output by the line sensor that has read the optical signal into a digital signal based on a reference voltage; and a first step of performing shading correction on the digital signal converted in the A / D conversion step. A data processing step; a second data processing step of performing gamma correction processing and binarization processing on the digital signal that has been subjected to shading correction in the first data processing step; and shading in the first data processing step. Based on the corrected digital signal for one line, the image signal level is A counting step of counting the number of pixels when a predetermined number of pixels having a voltage equal to or higher than the reference voltage are continuously detected; and a step of counting the number of pixels obtained by counting the digital signals for one line. A reference voltage changing step of changing the reference voltage when the value is not less than a predetermined upper limit value or not more than a predetermined lower limit value. 18. The storage medium according to claim 17, wherein said predetermined upper limit value and said predetermined lower limit value are changeable values.