JPH10327325A - Picture reader - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve a read picture in a case of including an extremely light part on an original surface by measuring luminance distribution in each of plural areas obtained by dividing the original surface and executing data processing with respect to the reading picture according to base luminance calculated from measuring data of luminance distribution in another area concerning an area including the highlight of excessive luminance. SOLUTION: It is usual that in the case of including such a tilting part where an illuminating light is positively reflected to enter a photographing system within the original surface, the excessively highlight Ah is included on an upper picture G1 and a peak luminance Bp at the histogram of a line Lb including highlight Ah makes maximum luminance. In this case, when the total of a frequency on a high luminance side is not smaller than a fixed value, the existence of the highlight Ah is judged from the set luminance of a value close to maximum luminance and the base luminance Bu of the line Lb is set by interpolation based on the base luminance of another line. The read picture of the upper picture G1 including the highlight Ah is improved by this.

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 suitable for reading a spread document such as a book or a magazine.

[0002]

2. Description of the Related Art An image reading apparatus called a book scanner has been commercialized. In this apparatus, a photographing system is arranged above a platen, and an open space having a height of several tens cm exists between the platen and the photographing system. When reading a bound manuscript such as a book or a magazine (this is referred to as a "book manuscript"), the user sets the book manuscript upward on a manuscript table with the book manuscript open. The book scanner turns on the illumination, scans the original surface (spread side), reads the image, and sequentially outputs images to an external device. If the external device is a printer, a copied image is formed in real time. When a book scanner is used, the pages can be turned on the platen, so that the efficiency of the operation of reading many pages can be improved.

Such a book scanner is provided with a density correction function for preventing a so-called "underground fog" as in other image reading apparatuses such as a digital copying machine and an image reader. That is, the book scanner measures the luminance distribution of the document to calculate the background luminance (threshold), regards a portion of the read image that is brighter than the background luminance as the background (background part), and uses the data value as a white background data value. Replace with Thus, even if the background color of the document is other than white (for example, pale yellow) due to discoloration or fading, it is possible to reproduce a high-contrast image similar to the case where the background color is white. In particular, in a book scanner, since the document surface of a book document is curved, the background luminance is calculated for each minute area that is divided in the spread direction of the document surface, and density correction taking into account uneven illumination due to the curvature is performed. I have.

When calculating the background luminance, a histogram (luminance frequency distribution) of the luminance (pixel value) is obtained. In a document mainly composed of binary images represented by characters, the histogram has a bimodal shape. The background luminance is calculated according to a predetermined rule based on the peak luminance on the high luminance side in the histogram. For example, when the luminance obtained by subtracting a certain value from the peak luminance is used as the background luminance, the luminance whose frequency is half of the peak luminance and lower than the peak luminance may be used as the background luminance.

[0005]

As described above, if the document surface has undulations, the facing angle between the document surface and the illuminating light source becomes non-uniform, so that shadows and highlights (light portions) are inevitably mixed. (Illumination unevenness) occurs.

Conventionally, when there is an inclined portion on the document surface such that the illumination light is specularly reflected and enters the photographing system,
There has been a problem that density correction including prevention of background fog becomes inappropriate. In other words, the amount of specular reflection hardly depends on the object color of the reflecting surface,
The inclined portion is photographed extremely bright regardless of whether it is a base portion or a printed portion. The so-called "light source reflection" occurs. A conventional book scanner performs data processing on a read image by applying background luminance based on a luminance histogram that is not faithful to the brightness of the original document itself. Documents that are likely to cause problems, for example,
Newspapers were spread out, folded phone books (timetables), and books with page corners turned back.

SUMMARY OF THE INVENTION An object of the present invention is to improve the image quality of reading when an extremely bright portion exists on the document surface.

[0008]

When a document surface is divided into a plurality of areas and data processing relating to image quality is performed for each area,
For a region where an excessively bright highlight (bright portion) presumed to be due to the regular reflection of the illumination light, the processing content is set based on the luminance distribution of the other region. That is, the output density characteristic is adjusted by applying the background luminance of another area or applying the background luminance calculated by interpolation from the background luminance of the other plural areas. The presence or absence of highlighting can be determined, for example, by the magnitude of the total number of pixels whose luminance is equal to or greater than a certain value. When the document surface is subdivided into small areas,
Since the image information of adjacent areas is similar, there is no practical problem even if the same processing is performed on one of the areas as with the other areas.

According to a first aspect of the present invention, there is provided an image reading apparatus of an upward setting type in which a photographing system for photographing a manuscript is arranged above a manuscript table. Means for measuring the luminance distribution, means for determining whether or not there is a highlight having excessive luminance in each of the regions based on the measurement data of the luminance distribution, and for the region where the highlight does not exist, Data processing is performed on the read image according to the background luminance calculated from the measured data of the luminance distribution, and for the area where the highlight is present, according to the background luminance calculated from the measured data of the luminance distribution of another area. Means for performing data processing on the read image.

[0010] In the apparatus according to the second aspect of the present invention, different data processing is performed on the area where the highlight exists, as compared with the area where the highlight does not exist.

[0011]

FIG. 1 is a perspective view showing the appearance of a book scanner 1, and FIG.

The book scanner 1 is an image reading apparatus suitable for reading a book original (spread original) BD.
It has a main body housing 10 for accommodating a power supply circuit and the like, a dark document table 20 for supporting a document, an imaging unit 30 for converting a document image into an electric signal, and a lamp unit 40 for illuminating the document. The platen 20 is the main body housing 1
0 is located on the front side. 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. 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 BD.
The distance between the platen 20 and the lower surface of the imaging unit 30 is 30c.
m or more.

The lamp unit 40 includes a straight tube type lamp 4.
1 and a reflector 42, and is attached to the rear end of the lower surface of the imaging unit 30 so as to illuminate the book document BD from obliquely above. The reflector 42 is designed to illuminate the maximum reading range on the upper surface of the document table 20 almost uniformly. The outer cover 43 of the lamp unit 40 prevents unnecessary irradiation to the user and the surroundings. Since the lamp unit 40 is disposed behind the document table 20, the lamp unit 40 does not hinder the setting operation including the page turning.

An operation panel OP is provided at 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 is fixed to the lower end. On the operation panel OP, together with the liquid crystal display, buttons for specifying a reading mode and reading 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 vertical end face S3 of the book document BD is projected on the projection plate 18, and the projection plate 18 in that state is photographed together with the document image. The upper end surface 19 of the projection plate 18 is used as a white plate for shading correction.

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 and 53 are provided one by one, and an armrest 25 is provided on the front side.

The image pickup unit 30 includes a line sensor 31 composed of a CCD array, an image forming lens 32, and a mirror 33.
have. An original image is projected on the light receiving surface of the line sensor 31 by an optical system including the mirror 33 and the imaging lens 32. The imaging lens 32 is provided movably in the front-rear direction, and is positioned by the AF mechanism 132. The line sensor 31 is attached to a movable body (scanner) of a sub-scanning mechanism (not shown), and moves in a horizontal 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. The main scanning direction of the document image is a front-back direction on the document table 20 and a vertical direction on the imaging surface. In the present embodiment, the imaging unit 3
0 and the above-mentioned projection plate 18 constitute an imaging means for the end face S3, and the end face S3 is imaged simultaneously with the original face S1 in the preliminary scanning. Note that an area sensor can be used instead of the line sensor 31.

When using the book scanner 1, the user places the book document BD upward on the document table 20 such that the facing direction of the book document BD is the left-right direction of the document table 20. At this time, the boundary (spread center) between the left and right pages of the spread surface, which is the document surface S1, is aligned with the mark indicating the center of the document table 20 in the left-right direction, and the book document BD is pressed onto the lower edge of the projection plate 18. To perform positioning in the front-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 holds the left and right ends (or one end) of the book document BD as needed to maintain the spread state. The start keys 52 and 53 can be operated while holding the book document BD. The document table 20 is supported by a spring mechanism so that it can move up and down independently of the left and right in accordance with the weight of the document.
Thus, regardless of the thickness of the book document BD, the height of the left and right pages becomes almost the same regardless of which page is opened,
The shooting distance becomes a value within a certain range.

The book scanner 1 performs preliminary scanning and main scanning on one document. The preliminary scanning is performed in order to obtain document information (height distribution of the document surface S1, document size, background luminance, and the like) necessary for setting the operation of the main scanning. That is, the book scanner 1 responds to the information obtained by the preliminary scanning,
The main scanning operation conditions such as a focus adjustment amount (lens movement amount), a magnification for distortion correction, a masking range, and a density conversion characteristic are set. The image output to the external device is performed during the main scanning. The reading mode of the book scanner 1 includes a mode for reading both left and right pages at once (sheet document mode) and a mode for reading left and right pages separately (book division mode). In either mode, preliminary scanning and main scanning are performed on each page.

FIG. 3 is a view for explaining a method of measuring the height distribution of the document surface. The broken line in the figure indicates the position of the line of interest. The read image G0 is a photographed image (upper surface image) G1 of the upper surface of the book document BD, and a photographed image G2 of the document table 20.
0, and a photographed image G18 of the projection plate 18. The image G181 of the photographed image G18 indicates the shape of the end surface S3 of the set book document BD.
Image 1 of photographed image G18 other than image (end face image) G181
Reference numeral 80 denotes a background image reflected on the projection plate 18. A subject closer to the imaging surface is photographed larger than a distant subject,
Both edges of the upper surface image G1 in the main scanning direction (vertical direction) are curved.

Since the platen 20 is dark, the photographed image G2
0 is a darker image than other images. Further, since the end surface S3 is also illuminated together with the document surface S1, the end surface image G181 is brighter than the background image G180. Therefore, it is possible to extract the top surface image G1 and the end surface image G181 by judging the magnitude of the luminance, and measure the document size and the height distribution of the document surface S1. Specifically, for each sampling position (line) at a predetermined interval, the luminance (pixel value) is sequentially determined from the top pixel in the main scanning direction.
Is checked, and the pixel position (pixel number) n1 on the head side of the range where the luminance exceeds the first threshold th1 and the luminance is set to the second threshold th2
Pixel positions n3 and n4 on the leading and trailing ends of the range exceeding (th2> th1) are detected. The number of pixels between the pixel position n1 and the known pixel position n2 corresponding to the front edge of the projection plate 18 corresponds to the height of the document surface S1 in the line of interest,
The number of pixels between pixel position n3 and pixel position n4 corresponds to the document size in the main scanning direction. The actual size is obtained by the calculation of dividing the number of pixels by the imaging resolution. The curved state of the document surface S1, that is, the height distribution is specified by a set of data indicating the height of the document surface S1 in each of all the sampled lines. Further, the respective positions of the first line and the last line where the pixel positions n3 and n4 are detected correspond to both end positions of the document in the sub-scanning direction.

FIG. 4 is a diagram for explaining the procedure of calculating the background luminance. The upper surface image G1 extracted by the luminance discrimination from the above-described read image G0 is subdivided in the opening direction, and a luminance histogram is obtained for each minute area. Here, a description will be given assuming that the minute region is one line (vertical pixel column).
However, a plurality of lines may be used as unit areas for density correction to reduce the load of control.

In a book mainly composed of characters, the histogram of the line La in which the lighting condition is appropriate has a bimodal shape having peaks in the character range and the background range as shown in FIG. Usually, most of the frequencies correspond to the background, and the frequency of the peak on the high luminance side is larger. The background luminance Bu is calculated based on the luminance (peak luminance) Bp corresponding to the peak on the high luminance side. In the illustrated example, a fixed amount Δ
The luminance obtained by subtracting b is the background luminance Bu (= Bp−Δb). The certain amount Δb is optimized so that the background fog is eliminated and the character is not thinned or blurred. Note that the calculation formula is not limited to the exemplification, and the background brightness Bu may be a brightness at which a cumulative value obtained by counting the frequencies on the low brightness side from the peak brightness Bp, a brightness at which the frequency is half the peak brightness Bp, or the like is used. . The background luminance Bu calculated in this way is
It is used as the upper limit (threshold) of the image data value range to be replaced with white background data in the density correction described later.

On the other hand, if there is an inclined portion in the document surface where the illumination light is specularly reflected and enters the photographing system, FIG.
As shown in (A), the top image G1 is an excessively bright highlight A
h. That is, since the amount of specular reflection hardly depends on the object color of the reflecting surface, even if the above-described inclined portion has characters, the entirety of the inclined portion is photographed brighter than the background. The histogram of the line Lb including the highlight Ah has a feature that the frequency near the maximum luminance is large as shown in FIG. At the time of photographing, brightness exceeding the upper limit of the light receiving dynamic range of the photoelectric conversion device (line sensor 31) is rounded to the upper limit. Therefore, even when the area of the highlight Ah is small, the pixel value is the maximum value (in the case of 8-bit data, the value of decimal 2
55), the peak luminance Bp in the histogram often becomes the maximum luminance. In this case, since the peak brightness Bp does not correspond to the background range, if the background brightness Bu is calculated based on the peak brightness Bp, an appropriate density correction result cannot be obtained. In particular, in the case of a document with a dark background such as a newspaper or a telephone directory, the difference between the true background luminance and the calculated background luminance is larger than in the case of a document using white paper. Notable. Therefore, the book scanner 1 determines, for each minute area (here, one line) that is a correction unit, the presence or absence of a highlight Ah that may cause an error in the calculation of the background luminance Bu. In practice, for example, if the sum of the frequencies on the higher luminance side than the set luminance of a value close to the maximum luminance is equal to or more than a certain value, it is determined that the highlight Ah exists. And highlight A
For the line where h exists, the background luminance Bu is set by interpolation based on the background luminance Bu calculated for the other lines.

FIG. 5 is a diagram showing a background luminance distribution. In general, the document surface S1 of the book document BD is curved in a smooth arc shape having a convex upper side, and the paper quality of the pages is uniform. Therefore, the distribution curve of the background luminance Bu in the spread direction also changes smoothly. Therefore, even if there is a line for which the background luminance Bu cannot be correctly calculated due to the highlight Ah due to the regular reflection of the illumination light, the background of the line including the highlight Ah is determined based on the appropriate background luminance Bu of the nearby line. The luminance Bu can be estimated with accuracy that does not hinder practical use. Specifically, linear interpolation is performed using the data of the two nearest lines not including the highlight Ah. This makes it possible to obtain a background luminance distribution over the entire length in the spread direction.

FIG. 6 is a functional block diagram of a main part of the book scanner 1. As shown in FIG. The book scanner 1 is controlled by a CPU 101 having a microcomputer. CPU
101 is a CC that supplies a clock to the line sensor 31.
D drive circuit 130, sub-scanning mechanism 131, AF mechanism 13
2. An operation instruction is given to a control target such as the lamp control circuit 140 and the image processing circuit 103 which are responsible for the lighting control of the lamp unit 40. The operation panel OP, the switch group 50, and various sensors are connected to the CPU 101.

The photoelectric conversion signal output from the line sensor 31 of the imaging unit 30 is converted by the AD converter 102 into, for example, 8-bit image data D1. In the preliminary scanning, the image data D1 is input to the height measuring unit 104 and the luminance detecting unit 105 in the order of pixel arrangement.
The height measuring unit 104 includes a comparator and a counter, and counts the number of pixels exceeding the threshold th1. That is, the height of the document surface S1 is measured. The count value by the height measuring unit 104 is taken into the CPU 101 as height data DH. The luminance detection unit 105 includes a line memory for creating a histogram, and a counter for counting pixels (high-luminance pixels) having brightness equal to or higher than a predetermined value.
p and luminance data DB indicating the presence or absence of highlight Ah
Output to PU101. The CPU 101 calculates the height data D
Control data for main scanning is prepared according to H and the luminance data DB. The RAM 106 is a work area for executing programs by the CPU 101, and is used for storing control data for data processing for each line.
In the main scanning, focusing is performed according to the progress in the sub-scanning direction, and image data D <b> 1 captured in a focused state is input to the image processing circuit 103. The image processing circuit 206 performs luminance correction, which is data processing according to the background luminance Bu, distortion correction for scaling an image in accordance with a height distribution, and mask processing for erasing unnecessary images. Image data that has undergone image processing is output to an external device in real time. External devices include copiers, printers,
There are a display, an image memory device, and an image editing device (computer system).

FIG. 7 is a diagram showing the contents of data processing relating to density reproduction. Background luminance B set as described above
u is converted to “background density” by applying a logarithmic conversion curve. This is because, in the case of a medium that does not emit light by itself, such as paper, the light and darkness perceived by a human is not the luminance R but the density D.
Because it is proportional to The conversion formula is D = -logR. In the case of a light emitting display medium such as a CRT,
This conversion is not required.

Next, a conversion curve (γ conversion curve) is set so that all data having a density value lower than the obtained background density are set to “0” and a certain proportional relationship is provided between the background density and the maximum density. I do. Then, the image data of all the pixels in each line is converted into the reproduction density based on the γ conversion curve. Thus, data lighter than the background density can be replaced with white (0), and data darker than the background density can be reproduced linearly. By performing the same operation on all the lines using the same device, the background fog can be eliminated and the contrast can be increased. Appropriate density control can be performed even on a line to which background luminance by interpolation is applied.

FIG. 8 is a flowchart showing the outline of the reading operation of the book scanner 1. In response to the start key 52 (or 53) being turned on, the lighting lamp 41 is turned on (# 1, # 2), and the preliminary scanning is executed (#
3). A predetermined operation setting based on the document information obtained by the preliminary scanning is performed (# 4), and the main scanning is executed (# 5). When the main scanning is completed, the lamp 41 is turned off and the apparatus stands by (# 6).

FIG. 9 is a flowchart of the prescanning processing subroutine. The scanner is moved at a constant speed (# 30), and the height is measured by determining the shape of the end face image G181 in the read image G0 (# 31). The upper surface image G1 of the read image G0 is transferred to the luminance detecting unit 105 (# 32) and stored in the line memory of the luminance detecting unit 105 (# 33). At this time, the address of the line memory is associated with each possible value of 0 to 255 of the image data D1 on a one-to-one basis, and each time the image data D1 of one pixel is transferred, the address corresponding to the data value is transferred. Record (frequency) is increased by one. That is, a luminance histogram is created (# 34). When the histogram for one line is completed, the counter of the luminance detecting unit 105 counts high-luminance pixels of a set value or more (# 35). For example, when the set value is 250, the frequencies of addresses 250 to 255 are read from the line memory and added. Then, the sum is compared with the threshold value (# 36).

If the sum does not exceed the threshold value, it is determined that the illumination state of the line of interest is proper, and the background luminance Bu is calculated based on the histogram (# 37). That is, the peak luminance Bp is detected, and a certain amount ΔB is subtracted from the peak luminance Bp. Specifically, the peak luminance Bp is detected from the maximum luminance (255) to the lower limit of the background luminance (for example, 1).
Until 00), the frequency is compared in order and the larger one is selected. On the other hand, if the above added value exceeds the threshold,
Since there is a possibility that the highlight Ah exists and the histogram does not correspond to the brightness of the image, the line number is stored without calculating the background luminance Bu (# 38). For this line, the background luminance Bu is set by the above-described interpolation in the operation setting routine.

The above processing is repeated according to the progress in the sub-scanning direction. When the processing for all the lines is completed, the process returns to the main routine (# 39). In order to reduce the influence of noise, the background luminance Bu may be calculated based on the average of data for a plurality of lines.

FIG. 10 is a perspective view showing a document in which highlight Ah is likely to occur. In FIG. 10A, a folded newspaper is set as a document BD2. It was folded in four before, so there are folds in the center on the left and right. Since the fold is curved to distortion,
Highlight Ah occurs at the front edge. In FIG. 10B, the opened book is set as the document BD3. The corner of the page has a habit, and a highlight Ah is generated at a front corner rising to the rear side where the lighting unit is arranged.

[0034]

According to the first or second aspect of the present invention,
It is possible to improve the image quality of reading when an extremely bright portion exists in the document surface.

[Brief description of the drawings]

FIG. 1 is a perspective view illustrating an appearance of a book scanner.

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

FIG. 3 is a diagram for explaining a method of measuring a height distribution of a document surface.

FIG. 4 is a diagram for explaining a point of calculation of background luminance.

FIG. 5 is a diagram showing a background luminance distribution.

FIG. 6 is a functional block diagram of a main part of the book scanner.

FIG. 7 is a diagram showing the contents of data processing relating to density reproduction.

FIG. 8 is a flowchart schematically showing a reading operation of the book scanner.

FIG. 9 is a flowchart of a preliminary scanning processing subroutine.

FIG. 10 is a perspective view showing a document in which highlight Ah easily occurs.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 book scanner (image reading device) 20 platen 30 image pickup unit (photographing system) 101 CPU 103 image processing circuit (means for performing data processing) 105 luminance detector Ah highlight BD book document (document) BD2, BD3 document Bu base Luminance G1 Top surface image (read image) La line (region where no highlight exists) Lb line (region where highlight exists) S1 Original surface

Claims (2)

[Claims] 1. An upward-setting-type image reading apparatus in which a photographing system for photographing a document is disposed above a document table, and means for measuring a luminance distribution for each of a plurality of areas defining a document surface. Means for judging whether or not there is a highlight having excessive luminance in each of the regions based on the measurement data of the luminance distribution; and measuring the luminance distribution of the region where the highlight does not exist. Data processing is performed on the read image in accordance with the background luminance calculated from the data, and for the area where the highlight is present, the read image is processed in accordance with the background luminance calculated from the measurement data of the luminance distribution of another area. An image reading apparatus, comprising: means for performing data processing. 2. An image reading apparatus of an upward setting type in which a photographing system for photographing a document is disposed above a document table, wherein a means for measuring a luminance distribution for each of a plurality of areas defining a document surface is provided. Means for judging whether or not there is a highlight having excessive luminance in each of the regions based on the measurement data of the luminance distribution; and measuring the luminance distribution of the region where the highlight does not exist. Means for performing data processing on the read image in accordance with the background luminance calculated from the data, and performing other data processing on the area where the highlight exists.