JPH10257323A - Image reader - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize an appropriate density correction corresponding to base density, in spite of original conditions and operating characteristics. SOLUTION: This reader is provided with a means 104 for detecting peak luminance Bp with a maximum frequency, based on luminance frequency distribution data DN generated from the output of reading means, a storage means 106 for correspondingly storing the peak luminance, base luminance and scanning position, and a means 101 for calculating the base luminance corresponding to the detected peak luminance Bp and the scanning position, based on information T1 stored in the storage means.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reader for converting image information of various documents into electric signals.

[0002]

2. Description of the Related Art Many image readers used as image input means for digital copiers and computer systems have a density correction function for preventing so-called "underground fog". That is, the background density of the document is detected, an appropriate threshold is set, and a portion of the read image that is lighter than the threshold is regarded as the background (background), and its data value is replaced with a white background data value (the brightest value). . This allows
For example, even if the background color is other than white (for example, pale yellow) as in a discolored newspaper, it is possible to reproduce a high-contrast image similar to the case where the background color is white. In some cases, the data value of a portion darker than the threshold value is corrected according to the background density.

A mode method is known as a method for setting the above-mentioned threshold value in a character image. This is based on the premise that the density histogram (frequency distribution diagram) of the character image has a bimodal shape with a large frequency between the character density and the background density. It is a method. Further, a method of approximating a histogram with a normal distribution function and setting a threshold value by an average / variance calculation is also used.

As the threshold value, the value obtained by accumulating the frequencies of the background density range corresponding to the background portion in the histogram from the low density side is 70% to 80% of the total frequency of the background density range.
Density values that give values of the order of magnitude are suitable. If all pixels in the background density range are reproduced white using the limit value on the high density side of the background density range as a threshold, fine characters may be thinned and white portions of a halftone image may be whitened out. On the other hand, when 70% to 80% of all pixels in the background density range are reproduced in white, the pixel positions are usually dispersed, and the remaining 20% to 30% of non-white pixels are reproduced. The presence of is inconspicuous.

[0005]

Conventionally, since the shape of the histogram becomes complicated due to unevenness of the background and the mixture of characters and halftone images, it is difficult to appropriately set the threshold value. there were. For example, in reading an image printed on glossy paper, a valley of the histogram may not be detected due to reflection on the document surface.

[0006] In addition, operating characteristics inherent to the apparatus, such as uneven illumination and variations in imaging sensitivity between pixels, also cause the histogram of the read density to be complicated. In particular, an image reading apparatus (book scanner) of the upward setting type suitable for reading a book has a more complicated histogram due to the operation characteristics as compared with a general line scanning type image reader used for reading a sheet document. Was found to be significant. The following can be cited as the reasons. In a book scanner, reading is performed with the entire document surface illuminated, and the height of the document surface is not constant. Since the illumination is wide and the distance between the document surface and the imaging device is large, the amount of incident light on the imaging device is small and the influence of noise is large. Since the space between the document surface and the imaging device is open, it is affected by ambient light.

SUMMARY OF THE INVENTION It is an object of the present invention to realize appropriate density correction according to the background density regardless of the document conditions and the operation characteristics.

[0008]

[Means for Solving the Problems] Surface to be read (original surface)
The frequency distribution of the luminance depends on image information, characteristics of the imaging system, disturbance, and the like. However, in an image such as a character image, which is mostly a background portion, as long as the background density is the same, there is a difference in the distribution shape between a plurality of images, but the luminance at which the frequency is maximum (this is the peak value). Luminance) is the same. By utilizing this fact, a threshold value for density correction for preventing fogging of the background is appropriately set.

That is, a sample image (chart) showing a representative example of the background density is read in advance to obtain a frequency distribution of luminance, and a luminance having a maximum frequency in a luminance range corresponding to the background is detected as a peak luminance. Then, a luminance threshold value for correction is calculated based on the frequency distribution, and stored as sample data in association with the detected peak luminance. Similar processing is performed for a plurality of sample images, and a plurality of sets of sample data are stored. Thereafter, even when an unspecified image is read, the frequency distribution of the luminance is obtained to detect the peak luminance. Then, basically, the density of the read image is corrected using the threshold value of the sample data having the same peak luminance. If there is no sample data having the same peak luminance, a threshold value suitable for the read image is calculated from the correlation between the peak luminance specified by the sample data and the threshold value. The relational expression may be stored as sample data.

When the density of the entire image is uniform, in principle, the frequency distribution of the luminance in each of the arbitrarily divided areas of the image is equal to each other (if the size of the area is the same, the frequency is also equal). . However, in practice, there is a slight difference in the appearance of the power distribution due to unevenness of illumination, distortion of the lens system, and variation in imaging sensitivity. Therefore, by storing sample data for each area and calculating a threshold value for each area using the sample data, fine density correction can be realized.

According to a first aspect of the present invention, there is provided an apparatus having reading means for subdividing an optical image of a surface to be read into pixels and converting it into electric signals, and performing data processing of the read image in accordance with the background density of the surface to be read. An image reading apparatus for detecting a peak luminance having a maximum frequency based on luminance frequency distribution data generated from an output of the reading means, and associating the peak luminance, the background luminance, and the scanning position with each other. There is provided a storage unit for storing, and a unit for calculating a base luminance corresponding to the detected peak luminance and the scanning position based on information stored in the storage unit. In this specification, the luminance means the degree of lightness and darkness of the surface to be read, and is substantially synonymous with "density".

[0012]

FIG. 1 is a perspective view showing the appearance of a book scanner 1 to which the present invention is applied, and FIG.
FIG.

The book scanner 1 is an image reading apparatus suitable for reading book originals (spread originals) BD such as books and magazines, and includes a main body housing 10 for accommodating a power supply circuit and the like, a dark original platen for supporting the originals. 20, an imaging unit 30 for converting a document image into an electric signal, and a lamp unit 40 for illuminating the document. The document table 20 is arranged on the front side of the main body housing 10. The imaging unit 30 is arranged above the document table 20 and is supported in a cantilever manner by a support column 12 extending upward from the upper surface of the main body housing 10. The lamp unit 40 is fixed to a position on the lower surface side of the imaging unit 30 on the support 12. A space 80 between the document table 20 and the imaging unit 30 is open to a free space outside the apparatus, and has a sufficient size for setting a book document. The distance between the platen 20 and the lower surface of the imaging unit 30 is 30 cm
As described above, since the pages can be turned on the document table 20, the user can efficiently perform the reading operation of a large number of pages.

An operation panel OP is provided on the upper end of the front surface of the main body housing 10, and a projection plate 18 for detecting the height of the document surface (spread surface) S1 is fixed to the lower end. On the operation panel OP, together with a liquid crystal display, buttons for designating reading modes and conditions (reading size, magnification, number of output sheets, density, etc.) are arranged. The front surface of the projection plate 18 is a glossy surface and is a 45 ° inclined surface with respect to the upper surface of the document table 20. An image of the end surface S3 of the book document BD in a direction orthogonal to the left-right direction is reflected on the projection plate 18, and the projection plate 18 in that state is photographed together with the document image.

Operation panel O in main body housing 10
A main switch 51 is provided on a side surface on the right side toward P. A start key 5 for the user to instruct the start of reading is provided on both sides of the platen 20 in the left-right direction.
2, 53 are provided one by one.

In FIG. 1, the image pickup unit 30 has a CC
A line sensor 31 composed of a D array, an imaging lens 32,
And a mirror 33. The original image is projected on the light receiving surface of the line sensor 31 by the optical system OS including the mirror 33 and the imaging lens 32. Line sensor 31
Is mounted on a movable body (scanner) of a sub-scanning mechanism (not shown), and translates along the left-right direction (sub-scanning direction) M2 while keeping the arrangement direction of the CCD elements in the vertical direction. By this parallel movement, a two-dimensional document image is captured. That is, in the book scanner 1, a two-dimensional imaging surface is formed by the movement of the line sensor 31. When an area sensor is used in place of the line sensor 31, its light receiving surface becomes an imaging surface. The main scanning direction of the document image is a front-back direction on the document table 20,
The vertical direction is on the imaging surface.

When using the book scanner 1, the user places the book document BD upward on the document table 20 with the book document BD opened. At this time, the boundary between the left and right pages is aligned with a mark indicating the center of the document table 20 in the left and right direction, and the book document BD is pressed against the lower edge of the projection plate 18 to perform positioning in the front and rear direction. The boundary between the projection plate 18 and the document table 20 is a reference line for setting the document. The center of the reference line is the reference position Ps (see FIG. 2). In addition, the user presses the edge portion of the book document BD by hand as necessary to maintain the facing state. When the start keys 52 and 53 are turned on, the book scanner 1 is turned on.
Performs preliminary scanning and main scanning in order. In the preliminary scanning, the document size, the document surface S1
The original conditions such as the background luminance of the original are measured, and the operation setting of the main scanning is performed based on the measurement result. In the main scanning, the read images are sequentially output to an external device. Examples of the external device include a printer, a display, an image memory, and an image editing device (computer system). For example, when a printer is connected, the read image is printed out in real time.

FIG. 3 is a block diagram of a control system of the book scanner 1. The book scanner 1 is controlled by a CPU 101 having a microcomputer. CPU1
Reference numeral 01 denotes an operation of the imaging control circuit 130 that controls the photoelectric conversion of the line sensor 31, the sub-scanning mechanism 131 that moves the line sensor 31, the lamp control circuit 140 that controls the illumination, and the display device of the operation panel OP. Give instructions. The CPU 101 also controls the signal processing system 100 that performs quantization of the output of the line sensor 31 and image correction. A main switch 51 and start keys 52 and 53 are connected to the CPU 101.

FIG. 4 is a block diagram of a main part of the signal processing system 100, FIG. 5 is a diagram showing an example of a luminance frequency distribution, and FIG. 6 is a diagram showing input / output characteristics of the density correction section 103C. The signal processing system 100 includes a CPU 101, an AD converter 102, an image processing circuit 103, a luminance detection unit 104, a RAM 105, and a nonvolatile memory 106.

In the preliminary scanning, the photoelectric conversion signal output from the line sensor 31 is applied to the AD converter 10.
2, for example, 8-bit image data (luminance data)
D10 is input to the luminance detecting unit 104 in the order of pixel arrangement. The luminance detecting unit 104 includes a comparator and a counter, and generates luminance frequency distribution data DN of a read image by counting the imaging data D10 for one line for each value class. Here, in the case of a book mainly composed of characters as a typical book original BD, the luminance frequency distribution has local maximum values in the character range and the background range as shown in FIG. When the photograph and the character are mixed, the frequency in the halftone range increases, but most of the frequency corresponds to the background. The CPU 101 takes in the luminance frequency distribution data DN and detects the data value of the class having the highest frequency in the background range as the peak luminance BP. Then, by referring to the table T1 in the memory 106, the base setting value BS is calculated and stored in the RAM 105 in a manner described later. This series of operations is repeated for each line.

In the main scanning, the imaging data D
10 are input to the image processing circuit 103 in the order of pixel arrangement. The image processing circuit 103 performs processes such as MTF correction for improving image quality, density correction, correction of image distortion, and masking for erasing an image outside the effective reading area. Image data D20 that has passed through the image processing circuit 103 is output to the external device.

The density correction is a data processing for correcting the image data, which has been subjected to the luminance / density conversion according to the equation (1), as shown in FIG. 6, and is performed by the density correction unit 103C. D = log ₁₀ (1 / R) (1) D: Density value R: Luminance value By passing through the density correction unit 103C, the base setting value BS
The photographing data value of the lower luminance pixel is replaced with a white background data value. The density correction unit 103 is provided with a base setting value BS in line units from the CPU 101 in accordance with the progress of sub-scanning.

As described above, in addition to the operation reading mode in which the preliminary scanning and the main scanning are performed and a read image is output to the outside (that is, the reading mode at the time of use), the book scanner 1 has a function of density correction. An auxiliary reading mode specific to the present invention is provided. The auxiliary reading mode is specified by a predetermined operator in an adjustment process before shipping the book scanner 1 from the factory, a periodic inspection after the start of use, and the like. Hereinafter, the operation in the auxiliary reading mode will be described.

FIG. 7 shows a luminance histogram of a sample image,
FIG. 8 is a diagram illustrating a data configuration of the table T1, and FIG. 9 is a diagram illustrating an example of a correlation between the peak luminance Bp and the background setting value Bs.

After designating the auxiliary reading mode, the operator reads a plurality of sample images (charts).
The chart is a uniform density image whose density is a value within the range of the background density of a general book (a reflection density of about 0.1 to 0.4). The number of charts is selected according to the accuracy required for density correction, and about 2 to 8 is a practical value. Here 3
It is assumed that two charts are used.

The operator places the first chart on the document table 20.
And press the start keys 52 and 53. Similarly to the above-described preliminary scanning, the imaging data D10 obtained by reading the chart is supplied from the AD converter 102 to the luminance detection unit 104.
Transferred to The luminance detection unit 104 repeats the operation of counting the imaging data D10 for one line for each value class, and generates luminance frequency distribution data DN for each line of the chart. The frequency distribution of each line is approximately a normal distribution, but the distribution curve is slightly distorted due to the sensitivity difference between the pixels of the line sensor 31 and the like. The distribution between the lines is slightly different due to uneven illuminance and lens distortion.

The CPU 101 fetches the luminance frequency distribution data DN of each line, detects the data value of the class having the highest frequency for each line as the peak luminance Bp, and calculates the total of the frequencies counted from the low luminance side with respect to the total frequency. The luminance that is a value of a fixed ratio (for example, 20%) is calculated as the base setting value (lower limit setting sample value) Bs of the chart. Then, the detected peak luminance Bp, the calculated background setting value Bs, and the line number (scanning position) are stored in the nonvolatile memory 106 in association with each other. This series of operations is repeated for each line.

When the reading of the first sheet is completed, the operator sequentially moves the charts of the second sheet and the third sheet in the same manner as the first sheet.
And instruct each reading. 2nd, 3rd
As for the sheet chart, the luminance detection unit 10
4 generates luminance frequency distribution data DN,
According to 101, the peak luminance Bp and the background setting value Bs are stored in the memory 106. By the above operation, the memory 1
The table 06 is provided with a table T1 in which one line number Y is associated with three peak luminances Bp and three base setting values Bs as shown in FIG. FIG. 7 shows a luminance frequency distribution of each chart at a certain line position.

In this embodiment, since the number of samples per line is 3, in the above-mentioned operation reading mode,
There are few cases where a peak luminance Bp that matches the peak luminance BP of the luminance frequency distribution when an actual document is read exists in the table T1. If there is no matching peak luminance Bp, the CPU 101 obtains a background setting value BS that matches the actual original peak luminance Bp by interpolation using the data in the table T1. To speed up the interpolation operation,
The arithmetic expression may be stored in the memory 106.

FIG. 10 is a flowchart showing the general operation of the auxiliary reading mode. The illumination lamp is turned on in response to the start instruction, and the movement of the scanner is started (# 11 to # 11).
13). During the scanning, the luminance frequency distribution data DN is created for each line, the peak luminance Bp is detected, and the background setting value (sample value) Bs is calculated (# 1).
4, # 15). When the scanning is completed, the illumination lamp is turned off once and the next start instruction is waited (# 15 to # 15).
# 17). When reading of the predetermined number of charts is completed, a relational expression between the peak luminance Bp and the background setting value Bs at each line position is obtained, and the table T1 is completed (# 18).

FIG. 11 is a flowchart showing the general operation in the operation reading mode. The illumination lamp is turned on in response to the start instruction, and the preliminary scanning is started (# 21)
~ # 23). During the preliminary scanning, the luminance frequency distribution data DN is created for each line, and the peak luminance BP is detected (# 24, # 25). When the preliminary scanning is completed, the above-described interpolation calculation is performed for each line with reference to the table T1 to calculate a base setting value BS that matches the peak luminance BP (# 26). Subsequently, the main scanning is started, and during the main scanning, density correction is performed for each line using the base set value BS as a threshold (# 27, # 28). When the main scanning is completed, the illumination lamp is turned off and the apparatus stands by (# 29, # 30).

As is clear from the above description, in the present embodiment, the CPU 101 includes means for detecting peak luminance, means for calculating the lower limit set sample value, means for creating a data table, and the lower limit set value data table. It functions as a means to calculate based on.

The operation in the auxiliary reading mode may be controlled by an external device. If the characteristics are different for each device, the table T is set for each device as in the above-described embodiment.
1 is desirable. However, when there is no difference between the individual devices (when illumination unevenness or the like also appears in another device), the table T1 created in one device may be diverted to another device.

[0034]

According to the first aspect of the present invention, it is possible to realize appropriate density correction in accordance with the background density irrespective of document conditions and operation characteristics.

[Brief description of the drawings]

FIG. 1 is a perspective view showing the appearance of a book scanner to which the present invention is applied.

FIG. 2 is a schematic diagram of reading by a book scanner.

FIG. 3 is a block diagram of a control system of the book scanner.

FIG. 4 is a block diagram of a main part of a signal processing system.

FIG. 5 is a diagram illustrating an example of a luminance frequency distribution.

FIG. 6 is a diagram illustrating input / output characteristics of a density correction unit.

FIG. 7 is a luminance histogram of a sample image.

FIG. 8 is a diagram showing a data configuration of a table.

FIG. 9 is a diagram illustrating an example of a correlation between a peak luminance and a background setting value.

FIG. 10 is a flowchart of a schematic operation in an auxiliary reading mode.

FIG. 11 is a flowchart of a schematic operation in an operation reading mode.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Book scanner (image reading apparatus) 31 Line sensor (reading means) 101 CPU 106 Memory (storage means) DN luminance frequency distribution data BP peak luminance S1 Document surface (reading target surface) T1 table (stored information)

Claims (1)

[Claims] 1. An image reading apparatus, comprising: reading means for subdividing an optical image of a surface to be read into pixels and converting it into electric signals, and performing data processing of the read image in accordance with the background density of the surface to be read. Means for detecting peak luminance having the highest frequency based on luminance frequency distribution data generated from the output of the reading means; storage means for storing peak luminance, background luminance, and scanning position in association with each other; Means for calculating background brightness corresponding to the obtained peak brightness and scanning position based on information stored in the storage means.