JP3945457B2 - Image reading method - Google Patents

Description

[0001]
[Technical field continued]
The present invention relates to an image reading method in an image input apparatus such as a scanner or a facsimile. Specifically, the image on a reading medium is optically read and converted into an analog electric signal, and the analog signal is further converted into a digital signal. The present invention relates to a reading method of an image input device that outputs the image.
[0002]
[Prior art]
Conventionally, there is an image input device shown in FIG. 12 as an image reading circuit (patent application number: Sho-63-296557). According to this image reading apparatus, a peak holding circuit for holding a peak value of a reading value based on one line reading of an input document is provided, and this peak value is used as a reference value for generating a binarization threshold value.
[0003]
[Problems to be solved by the invention]
However, in the conventional technology, when the fluctuation of the reading value caused by the sensitivity variation of the sensor or the variation of the light source is large, the white peak value of one line is held, and even if this value is used as the reference value, the variation of the input level is large. Therefore, there is a problem that the threshold value cannot be generated stably and the binarization processing of the image is not performed correctly.
[0004]
SUMMARY OF THE INVENTION An object of the present invention is to provide an image input apparatus that can perform stable binarization and simplify user operations.
[0005]
[Means for Solving the Problems]
The present invention is an image reading method in a scanner that manually moves an apparatus to read an entire medium, and includes a step of inputting optically read data for an image on a reading medium, and a binarization process of input data. Prior to performing shading correction for correcting density unevenness, and after shading correction of the input data, the input data of the first line of the image including characters on the reading medium is temporarily stored in the input data holding unit. A step of generating, a step of generating a density histogram distribution from the input data of the first line of the image including characters temporarily stored in the input data holding unit, and a binarization threshold value from the density histogram distribution And an input obtained from the reading medium based on the binarization threshold at the time of reading the medium. And performing a binarization processing of data, an image reading method characterized by having a.
[0006]
In a scanner that reads the entire medium by moving the device manually, the device is always positioned at the reading head position when reading.
[0007]
In the present invention, by creating a threshold value from the head data at the start of reading, the time to shift from threshold value creation to actual reading is reduced as much as possible, and the operator has to change the position when shifting from threshold value creation to reading. It is something to omit.
[0008]
As another feature, a correction unit that performs shading correction of input data is provided, and the corrected input data is held in the input data holding unit.
[0009]
The threshold generation unit obtains the white peak value (WP) of the white portion of the paper and the black peak value (BP) of the black portion of the character from the density histogram distribution generated by the density histogram distribution generation portion, and sets the threshold value to α × (WP The threshold value is generated under the condition of −BP) + BP (0 <α <1).
[0010]
The density of the input data is distributed between the density of the background of the paper and the density of the character portion (ink). In general, since the character area includes a space between characters, the density of the background of the paper is distributed at a constant rate. If the optical resolution of the scanner is high, the distribution of character lines also includes the ink density level at a constant rate. Therefore, the background level and the black level of the character portion can be detected from the density histogram distribution.
[0011]
Even for sensor input data that varies in sensitivity due to the effects of sensors, lighting, etc., the white peak value (white peak: WP) of the input level of the background (white) of the paper and characters from the density histogram distribution of the input data The black peak value (black peak value: BP) of the input level is held as a reference value. By generating a threshold value from this reference value, the determined threshold value does not exceed the white and black levels, and correct binarization can always be performed.
[0012]
As a method of correcting static sensitivity variations such as sensors and illumination, there is also a shading correction process in which correction is performed at a black and white input level read in advance. By performing the shading correction process, there is an effect of expanding the dynamic range of the input over the entire sensor area. First, the input data after the shading correction process is held. A reference value for generating a threshold is obtained from this input data, and a threshold is generated. In this method, the dynamic range is expanded, and a pseudo halftone input using dither (defined by combining a plurality of binarized data in a certain area) can always be processed with a correct threshold value.
[0013]
The threshold generated by the threshold generation unit is
Threshold = α × (white peak value−black peak value) + black peak value (0 <α <1)
As a result, the optimum threshold value that does not exceed the white and black levels can be generated. In particular, α = 0.5 corresponds to the case where the maximum margin is set for the white and black levels.
[0014]
When the paper to be read is shorter than the sensor length of the image reading unit (for example, when reading a business card with an A6 size scanner), the density histogram distribution obtains white peak values and black peak values including those other than the target paper. End up. By generating a threshold value using a white peak value and a black peak value from the density histogram distribution of input data in a limited place (for example, business card size) on one line specified in advance, other than the density range to be read The binarization threshold can be generated even when the background of the image is input at the same time.
[0015]
[Embodiment]
FIG. 1 is a diagram showing the configuration of an image reading apparatus according to an embodiment of the present invention.
[0016]
The image reading apparatus according to the present embodiment optically reads an image on a medium on which an image to be read is recorded, converts the image into an electric signal, and binarizes an image signal output from the image reading unit. An image signal processing unit (binarization unit) to perform, an input data holding unit for temporarily holding input data from the image reading unit at the time of line reading, and a density histogram distribution generating unit for generating a density histogram distribution from the input data And a white peak value of the background value of the density histogram distribution, and a threshold value generation unit that generates a binarization threshold value from the black peak value of the character to the image signal processing unit.
[0017]
The image reading unit includes a sensor unit and an A / D conversion unit. In the present embodiment, a line sensor composed of a CCD or the like is used as the sensor unit. Since the sensor type is not directly related to the present invention, any other type of sensor can be used. The sensor unit outputs an analog electric signal having a value corresponding to the density of the image read from the medium.
[0018]
The A / D conversion unit converts the analog electric signal output from the sensor unit into a digital signal. Assuming that an 8-bit digital signal is handled, 255 is output from the A / D converter when the read result is white, and 0 is output when the read result is black.
[0019]
FIG. 2 is a drawing showing the flow of binarization processing according to this embodiment. As shown in FIG. 2, in the image reading apparatus of the present embodiment, threshold value generation processing and binarization processing of the read image are executed.
[0020]
First, in order to generate a threshold value, one line of an image of a background (white part) including characters on a medium is read, data is input, and the data is stored in a line data holding unit. Also, the input data is passed to the density histogram distribution generation unit. The density histogram distribution generation unit generates a density histogram distribution from the density value for each pixel of the input data. In the threshold value generation unit, based on the density histogram generated by the density histogram distribution generation unit, a white peak value indicating the background white and a black peak indicating the black character are obtained, and a threshold value is generated from the black and white peak value, The image is sent to the image processing unit (binarization unit). When the reading target area of one line is all black or all white, the white peak value and the black peak value cannot be obtained only by reading one line. In this case, a threshold value is generated using a white maximum value (255) or a black minimum value (0) prepared in advance, and the subsequent processing is performed.
[0021]
Next, in the image reading process, the image processing unit performs a process of binarizing the digital image signal from the image reading unit using the threshold value of the threshold value generation unit. The threshold value generated by the threshold value generation unit is generated from input data obtained by reading the background (white portion) and character portion (black portion) of the medium. Therefore, the threshold value is optimal for the original and is correctly binarized.
[0022]
FIG. 3 shows an overview of a pen-type scanner device and an internal configuration diagram thereof, which is an example in which the image reading device shown in FIG. 1 is incorporated. The pen-type scanner device according to the present embodiment employs a form in which an image reading unit is placed in close contact with the original to read the original. The pen-type scanner is moved on the read original in this way to scan the original, and the obtained input data is binarized based on the determined threshold value to output binarized image data. Yes.
[0023]
FIG. 4 illustrates control and data flow at the time of image reading in the pen type scanner shown in FIG. In this example, an example of reading data for four lines is shown. In FIG. 4, the encoder pulse corresponds to a movement amount signal output from the movement amount detector. Further, the line sensor activation signal is a signal generated every driving cycle of the line sensor, and signal transfer from the line sensor is started based on this signal. In FIG. 3, it corresponds to a drive signal. The line sensor image signal is an image signal output from the line sensor based on the line sensor activation signal. A signal that has been subjected to image processing is an image signal that has undergone binarization processing.
[0024]
Although details will be described later, the data transfer interruption to the apparatus is performed from the image reading apparatus to the information processing apparatus when image data is transferred from the image reading apparatus to an external information processing apparatus. Hereinafter, control of reading operation and data flow will be described with reference to FIGS.
[0025]
The image reading unit includes a line sensor, a light source (LED or cold cathode tube), and a lens, and performs image reading in a contact system with a reading medium. During reading, the light source is turned on, and the line sensor is started to read with an activation signal, whereby the amount of reflected light at each position on the sensor is converted into an electrical signal and output as an analog signal. The movement amount detection unit converts the movement amount on the reading medium into a pulse signal by the encoder and outputs the pulse signal. Thereby, the relative movement amount of the image reading apparatus with respect to the medium is detected. The image signal processing unit digitizes the image signal from the image reading unit by an A / D conversion circuit, and performs binarized image processing with a threshold value determined from the density histogram distribution. The control unit performs overall reading control and data transfer to the information processing apparatus. The activation signal to the line sensor is always generated during the reading period of the medium, and the line data (image data) after image processing is stored in a buffer inside the image reading apparatus.
[0026]
When the control unit detects a movement pulse (encoder pulse) from the movement amount detection unit, the control unit issues an interrupt to the information processing apparatus and performs data transfer of the image-processed line data stored in the buffer. Only one movement pulse is output in one driving cycle of the line sensor during normal operation (line sensor image signal 1 in FIG. 4). When the binarization process is performed, a data transfer interrupt is performed from the image reading apparatus to the information processing apparatus, and the image data is transferred to the information processing apparatus.
[0027]
However, if the movement pulse is detected twice or more within one driving cycle of the line sensor, the control unit determines that the movement of the scanner has exceeded the reading performance of the scanner, and only the data for one movement pulse is obtained. Forward. In the example of FIG. 4, this corresponds to the line sensor image signal 2. Here, two encoder pulses appear between the line sensor activation signals. On the other hand, if the movement pulse is not detected within one driving cycle of the line sensor, it means that the scanner is not moving. In FIG. 4, it corresponds to the part of the line sensor image signal 3. Here, no encoder pulse appears between the line sensor activation signals. In this case, the image-processed line data stored in the buffer is discarded, and the read data (line sensor image signal 4) of the next line is held.
[0028]
The I / F unit is an electrical interface to the information processing apparatus connected to the image reading apparatus, and performs interrupts and data access to the information processing apparatus. The binarized data transferred to the information processing apparatus is displayed on the display unit of the information processing apparatus. Therefore, the operator can check the image data being read from the display unit.
[0029]
FIG. 5 is a block diagram showing an image reading apparatus according to the second embodiment. The image reading apparatus shown in FIG. 5 optically reads an image on a reading medium and converts it into an electrical signal, and an image signal that binarizes the image signal output from the image reading unit after shading correction. A processing unit (binarization unit), an input data holding unit that holds input data after shading correction at the time of line reading, a density histogram generation unit that generates a density histogram distribution from data in the input data holding unit, and a density histogram A threshold generation unit that generates a binarization threshold value from the white peak value of the background value and the black peak value of the character to the image signal processing unit;
[0030]
The image reading unit includes a sensor unit and an A / D conversion unit, and digitizes and outputs an image signal to be read (for example, in the case of 8 bits, white is 255 and black is 0). The output of the image reading unit is converted into an output from which influences such as surface unevenness are removed by the shading correction unit. The background value read in advance is stored as a white shading correction value, and the input image signal that is input is always converted and output according to the following equation (1).
[0031]
Input image signal: x (i), corrected signal: y (i)
White shading correction value: ww (i)
y (i) = 255x (i) / ww (i). . . (1)
The image signal converted by the above equation is binarized as in the first embodiment. For this purpose, input data for which one line of shading correction has been completed is held in the input data holding unit. On the other hand, a threshold value is generated from input data as in the first embodiment. Here, white shading correction processing is shown, but black shading correction may be performed in the same manner.
[0032]
FIG. 6 is a diagram for explaining the image reading operation according to the third embodiment of the present invention, and is a graph showing the relationship between the input waveform of the background, the density histogram distribution, and the threshold value to be determined. The graph on the left side of the drawing shows the input value of the read signal for one line including the background of the read original and the black portion corresponding to the character. The horizontal axis represents pixels in the lateral direction of the medium, and the vertical axis represents density values. Further, the graph on the right side of the drawing shows the density histogram distribution of the pixels. The horizontal axis in the figure indicates the density value, and the vertical axis indicates the frequency of occurrence of the density value.
[0033]
Here, even if the sensitivity of the sensor varies, a white peak value (WP) indicating the background and a black peak value (BP) indicating the character appear in the density histogram distribution. Therefore, if the threshold value is generated inside the white peak value and the black peak value, the read image can be binarized. That is, if a threshold value is generated at the position shown in the figure, the signal near the black peak value is output as black, and the signal near the white peak value is output as white. The threshold value is calculated by the following formula.
[0034]
Threshold = α × (W P−BP) + BP (1 <α <1)
Α is a coefficient for determining the threshold, and an arbitrary value can be applied. The value of α may be appropriately selected according to the density of the original to be read, the desired output image quality, and the like.
[0035]
Thus, by using the white peak value (WP) and the black peak value (BP) to generate the threshold value, correct binarization can be performed.
[0036]
FIG. 7 shows an image reading apparatus according to the fourth embodiment. If the paper width to be read is shorter than the reading area of the image reading unit, even if the line input data value is obtained, the non-target portion (outside the document) is also read together. Therefore, data irrelevant to the medium is input, and the density histogram distribution of the paper cannot be obtained correctly. For this reason, in the present embodiment, the effective area designation unit uses as input data values only for data in a limited place on a line designated in advance, and generates a density histogram distribution. The effective area may be specified, for example, by specifying the width of the medium to be read. The image reading apparatus recognizes a portion to be treated as a valid signal from signals read based on the designation of the effective region, generates a density histogram only for that portion, and generates a threshold value (FIG. 7 effective region). As a result, an optimum binarization threshold can be generated even for a sheet shorter than the line length of the image reading unit.
[0037]
FIG. 8 illustrates an image reading apparatus according to a fifth embodiment of the present invention. 9 to 11 are flowcharts showing the procedure of the reading process in the image reading apparatus shown in FIG.
[0038]
The above drawing shows the relationship between holding the line input data for determining the threshold and the actual reading process. There are four line inputs for determining the threshold.
1) Separate processing type Separately from actual image reading, one line of paper is input and a threshold is determined in advance from the density histogram distribution, and subsequent reading is binarized using the determined threshold.
2) One leading line for starting reading before reading type is input, a threshold value is determined from the density histogram distribution obtained as a result of reading, and binarization processing is performed with the threshold value determined at the beginning at the time of reading.
3) One line is input as the end line of post-reading type reading, and the threshold is determined from the density histogram distribution. Subsequent reading of the document is binarized using a threshold determined at the end of the previous reading.
4) Simultaneously with sequential one-line image input, a threshold value is determined for each line from the density histogram distribution, and binarization processing is always performed with the latest threshold value using the determined threshold value.
[0039]
By adopting such a configuration, a configuration corresponding to the hardware configuration and the priority of the processing time can be selected.
[0040]
[Effect of the present invention]
As described above, according to the present invention, by creating a threshold value from the data of the first line at the start of reading in the present invention, the time required to shift from threshold value creation to actual reading is reduced as much as possible. It is possible to save the labor of changing the position when the operator shifts from threshold value creation to reading, and to realize an accurate reading process according to the medium, at high speed and without burdening the operator.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating an image reading apparatus according to an embodiment of the present invention. FIG. 2 is a diagram illustrating an image reading processing procedure according to an embodiment. FIG. 3 is a specific configuration example of an image reading apparatus. FIG. 5 is a timing chart of reading. FIG. 5 is a diagram showing the relationship between the read image signal and the density histogram. FIG. 7 is an effective area narrower than the reading range of the image reading apparatus. FIG. 8 illustrates an example of image reading according to another embodiment. FIG. 9 illustrates a binarization processing procedure according to another embodiment. FIG. 10 illustrates another binarization processing procedure. FIG. 11 shows still another binarization processing procedure. FIG. 12 shows a conventional binarization processing.

Claims (1)

A method of reading an image in a scanner that manually moves the apparatus and reads the entire medium,
Inputting the Lud over data reading optically the image on the scanned medium,
Prior to the binarization processing of the input data, performing a shading correction for correcting density unevenness;
Temporarily holding the input data of the first line of the image including characters on the reading medium in the input data holding unit after the shading correction of the input data;
Generating a concentration histogram distribution temporarily stored characters in the input data holding unit from the input data of the head of a line of including images,
Generating a binarization threshold from the density histogram distribution;
Performing binarization processing of input data obtained from the reading medium based on the binarization threshold when reading the medium;
An image reading method comprising:

Images