JP4957693B2 - Image reading apparatus and image reading method - Google Patents

Description

  The present invention relates to an image reading apparatus and an image reading method.

2. Description of the Related Art Conventionally, in an image reading apparatus that reads an original while it is conveyed by an ADF (Auto Document Feeder), an apparatus that stores shading correction data according to the paper thickness is known in order to prevent image quality deterioration due to the deflection (lift) of the original (For example, refer to Patent Document 1). Since how the float occurs depends on the paper thickness, the conventional image reading apparatus stores shading correction data for each paper thickness, and the image quality deteriorates by correcting the shading correction data according to the paper thickness of the original to be read. Is preventing.
Japanese Patent Laid-Open No. 6-311359

  The conventional image reading apparatus is based on the premise that if the paper thickness is constant, the way the floating occurs is the same. However, the float fluctuates randomly during the conveyance of the document, and even if the paper thickness is the same, the manner in which the float occurs does not necessarily match completely. For this reason, according to the conventional image reading apparatus, there is a problem that correction cannot be appropriately performed according to the degree of floating that randomly varies during conveyance.

  The present invention has been completed based on the above-described circumstances, and an object thereof is to provide an image reading apparatus and an image reading method that appropriately correct the influence of the floating of a document.

A first invention is an image reading apparatus, and includes a line sensor that reads a document and outputs image data, and one of the lines of the image data using one of the lines of the image data as a reference line. Each line group includes correction means for correcting the pixel density of each pixel in the line group in accordance with a ratio between the maximum pixel density in the line group and the maximum pixel density in the reference line.
In many cases, the maximum pixel density in each line group represents the density of the background color of the document, in other words, the density of the ground color. Therefore, for each line group, if the pixel density of each pixel is corrected in accordance with the ratio of the background color density (maximum pixel density) in the line group to the background color density (maximum pixel density) in the reference line. The influence of each pixel in each line due to the floating can be made to be the same as the influence of each pixel in the reference line due to the floating. That is, the effect of floating is constant for any line, and variations in pixel density between lines when the same color is read can be reduced.
Since the maximum pixel density of each pixel has a difference in density depending on the degree of floating, correction based on the maximum pixel density is performed according to the degree of floating that varies randomly. become. That is, according to this correction, it can correct appropriately according to the float. Therefore, according to the present invention, it is possible to appropriately correct the influence of the floating of the document.

A second invention is the image reading apparatus according to the first invention, wherein the correcting means displays a line corresponding to the leading edge of the document or the vicinity thereof, or a line corresponding to the trailing edge of the document or the vicinity thereof. Use as a reference line.
According to the present invention, the pixel density of each pixel can be corrected to the pixel density read in a state where there is no floating.

A third invention is the image reading apparatus according to the first or second invention, wherein the maximum pixel density is an average value of the upper predetermined number of pixel densities.
According to the present invention, since the average value of the upper predetermined number of pixel densities is used as the maximum pixel density, the influence of electrical noise and the like can be reduced as compared with the case where the largest pixel density is used as the maximum pixel density. Can be corrected accurately.

A fourth invention is the image reading apparatus according to any one of the first to third inventions, wherein one line of the image data is set as one line group.
According to the present invention, one line is set as one line group. In other words, since correction is performed for each line, correction can be performed with higher accuracy.

A fifth invention is the image reading apparatus according to any one of the first to fourth inventions, wherein for each line group, whether or not the correction is performed for the line group is determined as a maximum pixel density in the line group. And determining means based on the difference between the maximum pixel density in the other line group and the correcting means, and the correcting means performs the correction only when it is determined to be corrected by the determining means.
In many cases, for example, a document on which document data is printed, a ground color exists in all lines of the document. However, for example, a document having ruled lines extending in the main scanning direction may not have a ground color. If the above correction is performed even for a line having no ground color, the maximum pixel density is corrected to the ground color density even though the maximum pixel density does not represent the ground color density. It will be corrected.
By the way, for example, when the difference in the maximum pixel density between the two line groups is large, there is a high possibility that the maximum pixel density in at least one of the line groups is not the background color density. In this case, inappropriate correction can be reduced if correction is not performed for at least one line group.
According to the present invention, whether or not to perform the correction is determined based on the difference from the maximum pixel density in the other line group, so that inappropriate correction can be reduced.

A sixth aspect of the invention is the image reading apparatus according to the fifth aspect of the invention, wherein the determination means includes, for each line group, the maximum pixel density in the line group and the line group in which the correction has been performed immediately before. Whether or not the difference from the maximum pixel density is equal to or less than a threshold value, or whether or not the difference from the maximum pixel density in the reference line is equal to or less than the threshold value when there is no line group corrected immediately before If it is equal to or less than the threshold, it is determined that the correction is made.
If you go back in order from the line group that has been corrected immediately before, you will eventually reach the reference line. In other words, if the difference between the maximum pixel density in the line group corrected immediately before and the maximum pixel density in the reference line is small, the maximum pixel density in the reference line represents the density of the ground color. It can be said that the maximum pixel density in the line group corrected immediately before represents the density of the ground color. Therefore, if the difference from the maximum pixel density in the line group corrected immediately before is small, it can be said that the maximum pixel density in the line group to be determined is the background color density.
According to the present invention, since it is determined that correction is performed if the difference from the maximum pixel density in the line group that has been corrected immediately before or in the reference line is equal to or less than the threshold value, inappropriate correction can be more reliably reduced.

A seventh invention is the image reading apparatus according to the sixth invention, wherein the determination means sequentially determines from the line group closer to the line corresponding to the leading edge of the document.
If the distance between the line group to be determined and the other line group is short, the two line groups are close to each other. When the degree of floating is close, if both the maximum pixel density in the line group to be determined and the maximum pixel density in the other line group represent the density of the ground color, the two maximum pixel densities The difference is smaller.
According to the present invention, since the determination is made in order from the line group closest to the line corresponding to the leading edge of the document, the distance between the two line groups to be compared can be reduced. That is, the difference in maximum pixel density is reduced. Thereby, a threshold value can be made small and inappropriate correction can be reduced more.

An eighth invention is the image reading apparatus according to the seventh invention, wherein the correction means corrects the pixel density of each pixel in the line group determined not to be corrected by the determination means immediately before. Correct with the ratio used in.
Since the line group determined not to perform the correction only has no ground color and is affected by the floating itself, it is desirable to perform some correction. As described above, when the distance between the two line groups is short, the two line groups are close to each other. If the degree of floating is close, even if the other line group is corrected with the ratio used for correcting one line group, the effect of floating can be reduced to some extent with high accuracy.
According to the present invention, since the determination is made in order from the line group closest to the line corresponding to the leading edge of the document, the distance between the line group to be determined and the line group corrected immediately before becomes shorter. Therefore, by correcting with the ratio used in the previous correction, it is possible to reduce the influence of floating even for the line group determined not to perform the correction.

A ninth invention is the image reading apparatus according to any one of the fifth to eighth inventions, wherein each of the line groups is composed of lines within a certain width in the sub-scanning direction, and the determination means is the line group. The determination is made based on the difference between the maximum pixel density in any one of the lines and the maximum pixel density in any one line of the other group of lines.
When a plurality of lines constituting one line group are within a certain width in the sub-scanning direction, the distance between the lines is close, so that the influence of floating on each line is also close. Therefore, even if the determination is made using the maximum pixel density in any one line, it can be correctly determined in many cases. When the determination is made using the maximum pixel density in any one line, the pixel density of all the pixels included in the line group to be determined is acquired and the maximum pixel density is specified in a shorter time. Processing can be performed. Therefore, according to this invention, it can correct more efficiently.

A tenth aspect of the invention is the image reading apparatus according to any one of the first to ninth aspects of the invention, wherein the number of lines of the line group when correcting a portion where the change of the floating of the document is large is changed by the change of the floating. Line number changing means for making the number smaller than the number of lines when correcting a small portion is provided.
Since the difference in the maximum pixel density between the lines in the image data is large in the portion where the change in the floating of the document is large, the correction can be appropriately performed by reducing the number of lines. On the other hand, since the difference in the maximum pixel density between the lines is small in the portion where the change in floating is small, the correction can be made more efficiently by correcting by increasing the number of lines.
According to the present invention, the number of lines when correcting a line corresponding to a portion with a large change in the float of the original is made smaller than the number of lines when correcting a line corresponding to a portion with a small change in the float. It is possible to appropriately and efficiently correct the influence of the above.

An eleventh invention is the image reading apparatus according to the tenth invention, wherein the line number changing means changes the number of lines based on the line group that has already been corrected.
The magnitude of the change in the float of the portion to be corrected can be estimated to some extent from the already corrected line group. Therefore, by changing the number of lines based on a group of lines that have already been corrected, the number of lines can be changed according to the magnitude of the change in floating.

A twelfth invention is the image reading apparatus according to the tenth invention, wherein the line number changing means changes the number of lines based on the paper type of the document.
The magnitude of the change in floating of the portion to be corrected can be estimated to some extent from the paper type of the original. Therefore, by changing the number of lines based on the paper type of the document, the number of lines can be changed according to the magnitude of the change in floating.

A thirteenth aspect of the invention is the image reading apparatus according to any one of the sixth to eighth aspects of the invention, wherein the threshold value when correcting a portion where the change in floating of the original is large is corrected, and the portion where the change in floating is small is corrected. A threshold value changing means for making the threshold value larger than the threshold value when
In the portion where the change in floating is large, the difference in the maximum pixel density between the two lines becomes large. For this reason, if the threshold value is small, the difference becomes larger than the threshold value, and there is a possibility that the correction is not performed despite the ground color density in the line group to be determined. Therefore, it is desirable to increase the threshold value in a portion where the change in floating is large.
On the other hand, it is desirable to reduce the threshold value in order to reduce inappropriate correction in a portion where the change in floating is small.
According to the present invention, errors such as correcting a line group that should not be corrected or not correcting a line group that should be corrected can be reduced.

A fourteenth aspect of the invention is an image reading method using an image reading apparatus including a line sensor that reads a document and outputs image data, wherein any line of the image data is used as a reference line, Obtaining a maximum pixel density, and, for each line group comprising one or more lines of the image data, the pixel density of each pixel in the line group is determined as the maximum pixel density in the line group and the reference line Correcting according to the ratio to the maximum pixel density.
According to the present invention, it is possible to appropriately correct the influence of the floating of the document.

  According to the present invention, it is possible to appropriately correct the influence of the floating of the document.

<Embodiment 1>
A first embodiment of the present invention will be described with reference to FIGS.
(1) Configuration of Image Reading Device FIG. 1 is a schematic diagram of an image scanner 1 (an example of an image reading device). The image scanner 1 is a flatbed image scanner that reads a document conveyed by an ADF 14 (automatic document feeder) and a document placed on the first platen glass 11 by a CIS (Contact Image Sensor) method.

The housing 10 is generally formed in a box shape, and a first platen glass 11 and a second platen glass 12 made of a colorless and transparent glass plate are provided side by side on the top.
The document cover 13 is connected to the housing 10 so as to be rotatable between a closed posture that covers the first platen glass 11 and an open posture that opens the first platen glass 11. The document cover 13 includes an ADF (automatic document feeder) 14, a document tray 15, a discharge tray 16, and the like.

  The ADF 14 transports documents placed on the document tray 15 one by one by a pair of transport rollers 17, transports the documents to a position facing the second platen glass 12, and then uses a pair of paper discharge rollers 18. It is discharged onto the discharge tray 16.

  The document reading unit 19 is housed in the housing 10, and includes an image sensor 20 (an example of a line sensor), a light source 21 configured by RGB light emitting diodes, and the like, and reflected light reflected by the document is transmitted to the image sensor 20. A rod lens array 22 that forms an image on each light receiving element, a carriage 23 on which these are mounted, a stepping motor (not shown) that reciprocates the carriage 23 in the sub-scanning direction (A direction in the figure), and the like.

  The image sensor 20 includes a plurality of light receiving elements arranged in a line in the main scanning direction (the vertical direction on the paper), an output unit (not shown) that outputs pixel signals corresponding to the charges accumulated in each light receiving element. . The image sensor 20 is an image sensor, and receives light reflected by the document by each light receiving element, and outputs an electric signal (pixel signal) corresponding to the electric charge accumulated in each light receiving element. The image sensor 20 may be a COMS type image sensor, a CCD type image sensor, or an image sensor instead of these.

  When reading an original placed on the first platen glass 11, the image sensor 20 is driven by a stepping motor and moves in the sub-scanning direction (the direction of arrow A in FIG. 1). 1 is read line by line in the main scanning direction (perpendicular to the plane of FIG. 1). When reading a document conveyed by the ADF 14, the image sensor 20 stops under the second platen glass 12, and reads the document conveyed on the second platen glass 12 line by line in the main scanning direction.

  The image sensor control unit 24 is configured as an ASIC, and is connected to the document reading unit 19 via a flexible flat cable 25. The image sensor control unit 24 includes an A / D conversion unit (not shown) and an image processing unit (not shown). The A / D converter converts the analog pixel signal output from the image sensor 20 into a digital pixel signal (pixel density). The image processing unit performs various corrections such as shading correction and gamma correction on the pixel density output from the A / D conversion unit, and outputs the result to the control unit 26.

  The control unit 26 (an example of a correction unit, a determination unit, a line number changing unit, and a threshold changing unit) includes a CPU, a ROM, a RAM, an NVRAM, and the like (not shown). When the controller 26 is instructed to read a document from an external device such as a so-called personal computer via the interface unit 27, or is instructed to read the document from an operation panel (not shown) provided in the housing 10, Each unit of the image scanner 1 is controlled to read a document, and image data is generated based on the pixel density output from the image sensor control unit 24. The control unit 26 converts the generated image data into a predetermined data format such as JPEG format and outputs the converted data to the interface unit 27.

  The interface unit 27 includes a USB interface, a network interface, and the like.

(2) Document Lifting Document lifting that occurs when a document is read using the ADF 14 will be described.
FIG. 2A shows a state in which the leading edge of the document M is conveyed to the reading position on the second platen glass 12. In this state, the leading end side of the original M is not held by the paper discharge roller 18 and is in a free state. When the front end is in a free state, the original M attempts to return to a state without deflection due to its own repulsive force, whereby the front end and the vicinity thereof are in close contact with the second platen glass 12. Therefore, the leading edge of the document M and the vicinity thereof are read without floating.

  FIG. 2B shows a state where the conveyance of the document M has progressed and the leading end side is nipped by the paper discharge roller 18 while the trailing end side of the document M is nipped by the conveyance roller 17. In this state, if the speed at which the paper discharge roller 18 feeds the document M toward the discharge tray 16 is faster than the speed at which the transport roller 17 feeds the document M, the document M is pulled in the discharge direction. Is higher than the position of the upper surface of the second platen glass 12, that is, the document M is lifted from the second platen glass 12.

  FIG. 2C shows a state in which the conveyance of the original M further proceeds and the rear end side of the original M is released from the conveyance roller 17. In this state, the rear end side of the original M is in a free state, and the rear end of the original M and the vicinity thereof are in close contact with the second platen glass 12. Therefore, the rear end of the document M and its vicinity are read without being lifted.

  The graph shown in the upper part of FIG. 3 is a graph showing the distance between the document and the image sensor 20 when reading the document conveyed by the ADF 14 in time series. As described above, the leading edge of the document and the vicinity thereof, and the trailing edge of the document and the vicinity thereof are in close contact with the second platen glass 12. Therefore, the distance between the leading edge of the document and the vicinity thereof and the image sensor 20, the trailing edge of the document, The distance between the vicinity and the image sensor 20 is equal to the distance from the upper surface of the second platen glass 12 to the image sensor 20.

  On the other hand, in the intermediate state, specifically, the state shown in FIG. 2B, the intermediate portion of the document is lifted from the second platen glass 12, so that the distance between the document and the image sensor 20 is increased. This distance varies randomly according to the rotation speeds of the transport roller 17 and the discharge roller 18.

The graph shown in the lower part of FIG. 3 is a graph showing the pixel density output when the float changes as shown in the upper part of the graph. More specifically, it is a graph showing the pixel density output from one light receiving element when a document whose entire surface is uniformly filled with one color is read.
Since the leading edge of the document and the vicinity thereof, and the trailing edge of the document and the vicinity thereof are in close contact with the second platen glass 12, the pixel density read without being lifted is output.
On the other hand, since the middle portion of the document is read in a floating state, the distance between the light source 21 and the image sensor 20 is increased, thereby reducing the amount of light received by the image sensor 20, and the pixel density is low. Value.

(3) Correction of influence due to floating In this embodiment, a case will be described as an example in which a plurality of pixels arranged in a line in the main scanning direction are set as one line in image data, and the influence due to floating is corrected for each line. That is, a case where one line corresponds to one line group will be described as an example.
4A is a schematic diagram illustrating an example of a document, and FIG. 4B is a schematic diagram illustrating an image represented by image data output by reading the document illustrated in FIG. 4A. The document in the illustrated example is obtained by printing document data with black characters on a printing paper having a white background. In the case of such a document, a ground color (white) exists in any main scanning region 31 of the document. A broken line 32 shown in FIG. 4B indicates a line corresponding to the main scanning region 31.

(3-1) Outline of Correction Processing Although the degree of floating influence differs between different lines, the degree of floating influence is almost the same at any position in the main scanning direction within the same line. That is, the degree to which the pixel density decreases due to floating is substantially the same for any pixel in the same line. Therefore, in order to correct the influence of floating for a certain line, it is only necessary to specify the degree to which the pixel density has decreased for that line and correct the pixel density of each pixel in that line accordingly.

Specifically, for example, a ratio between a pixel density obtained by reading a certain color in the line and a pixel density read in a state where the color does not float regardless of the line is obtained, and each pixel of the line is obtained. The pixel density may be corrected according to the ratio.
In order to obtain this ratio, for one color read in the line, the pixel density read in a state where the color does not float is necessary. Therefore, in this embodiment, the background color of the document, in other words, the background color of the document is used as the certain color. The reason for using the ground color will be described below.

  (Reason 1) Since the ground color is likely to exist in any main scanning area of the document as described above, if the ground color is read in a state in which there is no floating in at least one line, its pixel density Can be used to correct all other lines.

  (Reason 2) In general, the background color is often a light color, in other words, a color with a high pixel density, and the density of the background color is the highest among all the pixel densities output by reading a document. Tend to be. In other words, the density of the ground color can be specified by the maximum pixel density.

  (Reason 3) Since the leading edge of the document and the vicinity thereof, and the trailing edge of the document and the vicinity thereof are in close contact with the second platen glass 12, the line corresponding to the leading edge of the document or the vicinity thereof, or the trailing edge of the document or the vicinity thereof There is a high possibility that the maximum pixel density of the corresponding line is the density of the ground color read without floating.

  From the above, the line corresponding to the leading edge of the document or the vicinity thereof, or the line corresponding to the trailing edge of the document or the vicinity thereof is set as a reference line, and each pixel in the line is expressed for each line as shown in the following Expression 1. If the pixel density is corrected in accordance with the ratio of the maximum pixel density in the line and the maximum pixel density in the reference line, the influence of floating can be appropriately corrected for the line.

  Here, i is a pixel address in the main scanning direction, j is a column number (line number) in the sub-scanning direction, x (i, j) is a pixel density before correction at the j-th column i address, and A (j) is j The maximum pixel density of the column, K is the maximum pixel density of the reference line, and y (i, j) is the corrected pixel density of the j-th column i address. K / A (j) corresponds to the “ratio” described above.

(3-2) Judgment of whether or not to correct In many cases of originals on which document data is printed, there are ground colors in all main scanning areas of the originals. For example, as shown in FIG. In a document with extended ruled lines, there is also a main scanning area without a background color. If the above-described correction is performed for the line corresponding to the main scanning region having no background color, the maximum pixel density does not represent the background color density, but the background color density is corrected. An appropriate correction will be made.

  Therefore, in this embodiment, for each line, whether or not to correct the line is determined based on the difference between the maximum pixel density in the line and the maximum pixel density in other lines. For example, when the difference in maximum pixel density between two lines is large, it is highly possible that at least one of the lines does not have a background color density. Therefore, it can be seen that when the difference in maximum pixel density is large, at least one line should not be corrected.

  However, it is not possible to determine which line should be corrected only by the difference in the maximum pixel density. In addition, even when the maximum pixel density in one line does not represent the density of the ground color and the maximum pixel density in the other line does not represent the density of the ground color, the maximum pixel between the two lines The difference in density may be small. That is, even if the difference in maximum pixel density is small, there is a case where no line should be corrected.

Therefore, in the present embodiment, if the difference from the maximum pixel density of the line corrected immediately before is equal to or less than the threshold value, it is determined that correction is performed. Since there is no line corrected immediately before the first correction line, it is determined that correction is performed if the difference from the maximum pixel density of the reference line is equal to or less than a threshold value.
In this way, as shown by the arrow 33 in FIG. 4B, if the line corrected immediately before is traced back in order, it finally reaches the reference line. In FIG. 4B, the retroactive interval is widened for easy understanding. Since the difference between the maximum pixel density in the line corrected immediately before and the maximum pixel density in the line corrected immediately before is equal to or smaller than the threshold value, that is, the difference is small, the difference from the maximum pixel density in the reference line is traced back. Will also be small. Therefore, if the maximum pixel density in the reference line represents the density of the ground color, the maximum pixel density of the line corrected immediately before also represents the density of the ground color. That is, if the difference from the maximum pixel density of the line corrected immediately before is small, it can be said that the maximum pixel density in the line is the density of the ground color.

  Accordingly, if it is determined that correction is performed if the difference between the maximum pixel density in the line corrected immediately before or the maximum pixel density in the reference line is equal to or less than the threshold value, inappropriate correction can be more reliably reduced.

(3-3) Determination Order In this embodiment, a line (front end line) corresponding to the front end of the document is used as a reference line, and the determination is made in order from the line closer to the front end line as shown in FIG.
When the distance between the two lines is short, the two lines are close to each other. For example, since the distance in the sub-scanning direction between the main scanning region (region extending in the direction perpendicular to the paper surface) 35 shown in FIG. 3 and the main scanning region 36 is short, the degree of floating of the main scanning region 35 and the degree of floating of the main scanning region 36 are shown. Is close. On the other hand, for example, the distance between the main scanning region 36 and the main scanning region 37 is long, so the degree of floating is not close.

  When the degree of floating is close, if the maximum pixel density of one line and the maximum pixel density of the other line both represent the density of the ground color, the difference between the maximum pixel densities of these two lines (FIG. 3). The distance D) shown in FIG. When the difference in maximum pixel density is small, it is desirable to reduce the threshold value. When the threshold value is large, for example, when there is no background color density in the line to be determined, the threshold value is set even though there is some difference from the maximum pixel density of the line corrected immediately before (the line with the background color). This is because the difference is less than the threshold value due to the large value, thereby improper correction.

  If the determination is made in order from the line closest to the line corresponding to the leading edge of the document, the distance between the two lines to be compared can be reduced. That is, the difference in maximum pixel density is reduced. Thereby, a threshold value can be made small and inappropriate correction can be reduced more.

(3-4) Setting of Threshold Value In this embodiment, the threshold value when correcting a portion with a large change in the float of the document is set larger than the threshold value when correcting a portion with a small change in the float.
For example, it is assumed that there are two adjacent lines, both of which have a background color density. In this case, the difference in maximum pixel density between the two lines is relatively large when the change in floating is large, and the difference is relatively small when the change in floating is small.

For this reason, in the portion where the change in the float is large, the difference in the maximum pixel density becomes larger than the threshold due to the large change in the float even though there is a background color density in the determination target line if the threshold is small. Therefore, there is a possibility that the correction is not performed even though it should be corrected. Therefore, it is desirable to increase the threshold value in a portion where the change in floating is large.
On the other hand, in a portion where the change in floating is small, it is desirable to reduce the threshold value in order to reduce inappropriate correction.

  Whether or not the change in the floating of the document is large can be estimated based on the already corrected line. Specifically, for example, “the difference between the maximum pixel density of the line corrected immediately before and the maximum pixel density of the line corrected immediately before”, “the maximum pixel density of the line corrected immediately before, and the correction just before that. It can be estimated from the difference from the “difference from the maximum pixel density of the line”. If there is no change in the degree of floating regardless of whether the float is large or small, the difference should be 0. Therefore, if the difference is large, the change in float around the judgment target line is large.

(3-5) Correction for a line determined not to be corrected A line determined not to be corrected, in other words, a line having no ground color, has no ground color and is affected by floating itself. Therefore, some correction is desirable. Therefore, in the present embodiment, the line determined not to be corrected is corrected using the ratio used in the previous correction.

As mentioned above, if the distance between the two lines is short, the degree of floating will also be close, so even if the other line is corrected with the ratio used for correcting one line, the effect of floating will be affected to some extent with high accuracy. Can be reduced.
If the determination is made in order from the line closest to the line corresponding to the leading edge of the document, the distance between the line determined not to be corrected and the line corrected immediately before becomes short. Therefore, by correcting with the ratio used in the previous correction, the effect of floating can be reduced to some extent with respect to a line where no ground color exists.

(4) Flow of Correction Processing In the following description, an example will be described in which correction is performed in order from the line closest to the reference line, with the first output line from the image sensor control unit 24, that is, the tip line as the reference line. .

FIG. 6 is a flowchart showing the flow of processing for the reference line. This process is executed when the leading edge line (reference line) is output from the image sensor control unit 24.
Since it is unlikely that the reference line is affected by floating, correction is not performed, and only processing for obtaining the maximum pixel density and storing it in the RAM is performed.

In S101, the control unit 26 acquires the maximum pixel density in the reference line. Specifically, the control unit 26 sets the average value of the pixel densities of the upper 16 pixels in the reference line as the maximum pixel density. When the average value of the pixel density of the upper predetermined number (16 in this case) is used as the maximum pixel density, the influence of electrical noise or the like can be reduced as compared with the case where the largest pixel density is used as the maximum pixel density. Can be corrected accurately.
Note that the “predetermined upper number” is not limited to the first highest pixel density, the second highest pixel density, and the third, but may be, for example, the second, third, fourth, It may be fourth, fifth.
In S102, the control unit 26 stores the calculated average value in the RAM as the maximum pixel density of the reference line.

FIG. 7 is a schematic diagram showing a flow of processing for lines output after the leading end line (reference line). This process is executed every time a line after the reference line is output from the image sensor control unit 24.
In S201, the control unit 26 sets a threshold according to the magnitude of the change in floating. Specifically, for example, the control unit 26 determines that "the difference between the maximum pixel density of the line corrected immediately before and the maximum pixel density of the line corrected immediately before" and "the maximum pixel density of the line corrected immediately before that" Furthermore, the difference from the “difference from the maximum pixel density of the corrected line immediately before” is compared with a predetermined value. If the difference is greater than or equal to the predetermined value, the change in floating is large. A large threshold is set.
Note that the threshold value set when the change in floating is large does not have to be a constant value, and the threshold value may be set according to the magnitude of the difference.

  In S202, the control unit 26 acquires the maximum pixel density of the output line (hereinafter referred to as “correction target line”). Here, as in the case of the reference line, the average pixel density of the upper 16 pixels is set as the maximum pixel density. It should be noted that the “predetermined upper number” may be different between the reference line and the correction target line, and the pixel numbers acquired as the upper predetermined number may be different.

  In S203, the control unit 26 determines whether or not a difference between the maximum pixel density of the correction target line obtained in S202 and the maximum pixel density of the line corrected immediately before is equal to or less than a threshold value. In the case of a line to be corrected first, that is, a line adjacent to the reference line, there is no line corrected immediately before, so it is determined whether or not the difference from the maximum pixel density of the reference line is equal to or less than a threshold value.

The control unit 26 proceeds to S204 when the difference is equal to or smaller than the threshold value, and proceeds to S206 when larger than the threshold value.
In S204, the control unit 26 calculates the above-described “ratio” by dividing the maximum pixel density of the reference line by the maximum pixel density of the correction target line.

In S205, the control unit 26 corrects the pixel density of each pixel in the correction target line according to the ratio calculated in S204.
In S206, the control unit 26 corrects the pixel density of each pixel in the correction target line according to the ratio used in the previous correction.

  As a result of the above processing, the maximum pixel density in each line becomes constant at the maximum pixel density in the reference line as shown in FIG. 8, in other words, the density of the ground color read in a state without floating.

  FIG. 9 is a graph showing the pixel density before and after correction of each pixel for a certain line. More specifically, FIG. 9 is a graph showing the pixel density before and after correction of each pixel for a certain line 38 shown in FIG. 8 as seen from the horizontal axis direction of the graph shown in FIG. In this embodiment, since the same ratio is used uniformly for pixels in the same line, the graph shape of the pixel density after correction is the same as that of the pixel density before correction, and the maximum pixel density matches the maximum pixel density of the reference line. The correction is made while maintaining the graph shape up to the height.

(5) Effects of Embodiment According to the image scanner 1 according to the first embodiment of the present invention described above, the pixel density of each pixel in the line for each line is set to the maximum pixel density in the line and the maximum in the reference line. Since the correction is performed in accordance with the ratio with the pixel density, it is possible to appropriately correct the influence due to the floating of the document.

  Further, according to the image scanner 1, since the line corresponding to the front end of the document (front end line) is used as the reference line, the pixel density of each pixel can be corrected to the pixel density read without floating.

  Further, according to the image scanner 1, since the average value of the upper predetermined number of pixel densities is used as the maximum pixel density, the influence of electrical noise and the like is reduced as compared with the case where the largest pixel density is used as the maximum pixel density. Can be corrected more accurately.

  Furthermore, according to the image scanner 1, since one line of image data is made into one line group, in other words, since it corrects for every line, it can correct more accurately.

  Furthermore, according to the image scanner 1, it is determined, for each line, whether to perform the above-described correction for the line based on the difference between the maximum pixel density in the line and the maximum pixel density in the other lines. Improper correction can be reduced.

  Further, according to the image scanner 1, for each line, whether the difference between the maximum pixel density in the line and the maximum pixel density in the line corrected immediately before is equal to or less than a threshold value, or there is a line corrected immediately before. If not, since it is determined whether or not the difference from the maximum pixel density in the reference line is equal to or less than the threshold value, inappropriate correction can be more reliably reduced.

  Further, according to the image scanner 1, since the determination is made in order from the line group closest to the line corresponding to the leading edge of the document, the threshold value can be reduced and inappropriate correction can be further reduced.

  Further, according to the image scanner 1, since the pixel density of each pixel in the line determined not to be corrected is corrected by the ratio used in the previous correction, the line determined not to be corrected is also applied to the line. The effect of floating can be reduced.

  Further, according to the image scanner 1, since the threshold value for correcting a portion where the change in floating of the document is large is set larger than the threshold value for correcting a portion where the change in floating is small, a line that should not be corrected is corrected. Or errors such as not correcting the line to be corrected can be reduced.

<Embodiment 2>
Next, Embodiment 2 of the present invention will be described.
In the second embodiment, one line group includes a plurality of lines such as 10 to 20, for example.
If there are too many lines that make up one line group, the effect of floating may not be corrected properly. Conversely, if the number is too small, the time required for correction will increase, so the number of lines included in one line group will be It is desirable to set it appropriately through experiments.

  In the second embodiment, since one line group includes a plurality of lines, one ratio is obtained for the plurality of lines. Specifically, the ratio is obtained by using the maximum pixel density in the plurality of lines as the maximum pixel density of the line group, and the pixel density of each pixel in the plurality of lines is uniformly corrected with the obtained ratio. If correction is performed using one ratio, the processing amount for obtaining the ratio can be reduced as compared with the case where the ratio is obtained for each line, so that the time required for reading can be shortened.

  The plurality of lines constituting one line group are preferably continuous lines. If the lines are continuous, the effect of the floating of each line is close, so even if correction is performed using one ratio, correction can be made with high accuracy.

In the second embodiment, the number of lines in the line group when correcting a portion with a large change in the float of the document is made smaller than the number of lines when correcting a portion with a small change in the float.
For example, as described above, when the change in floating is large, the difference in the maximum pixel density between the two lines becomes large. Therefore, there is a difference in the degree of reduction in pixel density due to floating between one line and the other line. If these are included in the same line group, correction is performed at the same ratio, so that the accuracy of correction is reduced.
Accordingly, it is possible to appropriately correct the portion where the change in the floating of the document is large by correcting the number of lines to be small. On the other hand, since the difference in the maximum pixel density between the lines is small in the portion where the change in floating is small, the correction can be made more efficiently by correcting by increasing the number of lines.
The second embodiment is substantially the same as the first embodiment in other points.

<Other embodiments>
The present invention is not limited to the embodiments described with reference to the above description and drawings. For example, the following embodiments are also included in the technical scope of the present invention.

(1) In the above embodiment, the line corresponding to the leading edge of the document (leading line) has been described as an example of the reference line, but it may be a line corresponding to the vicinity of the leading edge of the document or the trailing edge of the document or the vicinity thereof. . Since these lines are highly likely to have been read with the document in close contact with the second platen glass 12, when these lines are used as reference lines, the pixel density of each pixel is read without floating. The pixel density can be corrected.
Further, a line other than the line corresponding to the leading edge of the document or the vicinity thereof, or the trailing edge of the document or the vicinity thereof may be used as the reference line. Since such a line may be read with a floating state, the pixel density of each pixel cannot always be corrected to the pixel density when it is read without a floating state. By matching the color density to the ground color density of the reference line, the ground color unevenness can be reduced.

  (2) In the second embodiment, it has been described that it is desirable that a plurality of lines constituting one line group are continuous. For example, if the plurality of lines are within a certain width in the sub-scanning direction, It does not have to be continuous. This is because the effect of floating of each line is close if it is within a certain width in the sub-scanning direction.

(3) In the second embodiment, an example has been described in which the ratio is obtained using the maximum pixel density in a plurality of lines as the maximum pixel density of the line group. However, the maximum pixel density in any one line of the line group is determined as the line. The ratio may be obtained as the maximum pixel density of the group.
For example, when a plurality of lines constituting one line group are within a certain width in the sub-scanning direction, the distance between the lines is close, so that the effect of floating on each line is close. Therefore, even if the ratio is obtained using the maximum pixel density in any one line, it can be accurately corrected in many cases.
If the ratio is obtained by using the maximum pixel density in any one line, the ratio can be obtained in a short time, so that correction can be made more efficiently.

(4) In the above embodiment, whether or not the change in the float of the document is large is determined by “the difference between the maximum pixel density of the line corrected immediately before and the maximum pixel density of the line corrected immediately before” and “ The determination is based on the difference between the maximum pixel density of the corrected line and the maximum pixel density of the line corrected just before that, but it may be determined according to the paper type.
Since how the document floats varies depending on the paper type depending on the thickness and material of the paper, if the paper type is known, the change in the float increases in which part of the document, and the change in the float decreases in which part Can be guessed to some extent. Therefore, by collecting data on how the float occurs for each paper type through experiments, etc., and changing the number of lines based on the data according to the paper type of the original, the magnitude of the change in float The number of lines can be changed according to The paper type may be specified by, for example, making an operator specify.

  (5) In the above embodiment, the case where the original is conveyed by the ADF 14 and read is described as an example. However, as shown in FIG. 10, the book 39 may be placed on the first platen glass 11 and read. When the book is opened for reading, both end portions are in close contact with the first platen glass 11 and the center portion is lifted from the first platen glass 11. Therefore, by setting the lines corresponding to both ends as the reference lines, the pixel density of each pixel can be corrected to the pixel density when read without floating.

  (6) Although the control unit 26 has been described as an example of the correction unit, the determination unit, the line number changing unit, and the threshold value changing unit in the above embodiment, these units may be realized by the image sensor control unit 24. The image sensor control unit 24 and the control unit 26 may share these means. Further, a dedicated ASIC for realizing these means may be provided.

  (7) In the above embodiment, the image scanner 1 is described as an example of the image reading apparatus. However, the present invention may be applied to a so-called multi-function machine having a scanner function, a printer function, a copy function, and a facsimile function.

1 is a schematic diagram of an image reading apparatus according to an embodiment of the present invention. FIG. 4 is a schematic diagram for explaining the floating of a document. FIG. 4 is a schematic diagram for explaining the floating of a document. FIG. 4 is a schematic diagram for explaining the floating of a document. The graph which shows the distance of a manuscript and an image sensor, and the graph which shows pixel density. FIG. 3 is a schematic diagram illustrating an example of a document. The schematic diagram which shows an example of image data. FIG. 3 is a schematic diagram illustrating an example of a document. The flowchart which concerns on one Embodiment of this invention. The flowchart which concerns on one Embodiment of this invention. The graph which shows the pixel density after correction | amendment which concerns on one Embodiment of this invention. The graph which shows the pixel density after correction | amendment which concerns on one Embodiment of this invention. FIG. 3 is a schematic diagram illustrating an example of a document.

Explanation of symbols

1. Image scanner (image reading device)
10 ... Case 20 ... Image sensor (line sensor)
26... Control unit (correction means, determination means, line number change means, threshold value change means)

Claims (12)

A line sensor that reads a document and outputs image data;
Any line of the image data is used as a reference line, and for each line group composed of one or more lines of the image data, the pixel density of each pixel in the line group is set to the maximum pixel density in the line group and the pixel group. Correction means for correcting according to the ratio of the maximum pixel density in the reference line;
Line number changing means for making the number of lines of the line group when correcting a portion where the change in floating of the original is large smaller than the number of lines when correcting a portion where the change in floating is small;
An image reading apparatus comprising: The image reading apparatus according to claim 1,
The line number changing unit is an image reading device that changes the number of lines based on the line group that has already been corrected. The image reading apparatus according to claim 1,
The line number changing unit is an image reading apparatus that changes the number of lines based on a paper type of a document. An image reading apparatus according to any one of claims 1 to 3,
The image reading apparatus, wherein the maximum pixel density is an average value of a predetermined upper number of pixel densities in the line group. An image reading apparatus according to any one of claims 1 to 4,
The image reading apparatus, wherein the correction unit uses, as the reference line, a line corresponding to the leading edge of the document or the vicinity thereof, or a line corresponding to the trailing edge of the document or the vicinity thereof. An image reading apparatus according to any one of claims 1 to 5,
For each line group, a determination unit that determines whether to perform the correction for the line group based on a difference between a maximum pixel density in the line group and a maximum pixel density in the other line group,
The image reading apparatus, wherein the correction unit performs the correction only when it is determined to be corrected by the determination unit. The image reading apparatus according to claim 6,
For each line group, the determination means determines whether or not the difference between the maximum pixel density in the line group and the maximum pixel density in the line group corrected immediately before is equal to or less than a threshold value. When the corrected line group does not exist, it is determined whether or not a difference from the maximum pixel density in the reference line is equal to or less than the threshold value. apparatus. The image reading apparatus according to claim 7,
The image reading apparatus, wherein the determination unit sequentially determines the line group closer to the line corresponding to the leading edge of the document. The image reading apparatus according to claim 8,
The image reading apparatus, wherein the correction unit corrects the pixel density of each pixel in the line group determined not to be corrected by the determination unit by a ratio used in the previous correction. An image reading apparatus according to any one of claims 6 to 9,
Each said line group consists of lines within a certain width in the sub-scanning direction,
The determination unit is an image reading apparatus that makes a determination based on a difference between a maximum pixel density in any one line of the line group and a maximum pixel density in any one line of the other line group. An image reading apparatus according to any one of claims 7 to 9,
An image reading apparatus comprising: a threshold value changing unit configured to make the threshold value when correcting a portion where the change in floating of the document is large larger than the threshold value when correcting a portion where the change in floating is small. An image reading method using an image reading apparatus including a line sensor that reads a document and outputs image data,
Using any line of the image data as a reference line, obtaining a maximum pixel density in the reference line;
For each line group composed of one or more lines of the image data, the pixel density of each pixel in the line group is determined according to the ratio of the maximum pixel density in the line group and the maximum pixel density in the reference line. The stage of correction,
Making the number of lines of the line group when correcting a portion with a large change in floating of the original smaller than the number of lines when correcting a portion with a small change in floating;
An image reading method including:

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