Patent Publication Number: US-8970920-B2

Title: Image processing apparatus and image processing method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image processing technique of recognizing the characteristics of input image data by, for example, obtaining the frequency distribution of the input image data and, more particularly, to an image processing apparatus and image processing method for the technique. 
     2. Description of the Related Art 
     Conventionally, there is available a technique based on frequency distribution (histogram) generation as a method for recognizing the characteristics of image data captured by an imaging apparatus such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor). This method is characterized in analyzing the number of pixels existing in the input image data captured by an imaging apparatus for each tone of pixel values in the main scanning direction and sub-scanning direction of the image data. This method uses values for discriminating the characteristics of image data and values dependent on a system which analyzes image data as elements (the tones of pixel values) constituting a frequency distribution or the intervals between pixel positions where sampling is performed or both of them. Examples of analysis on images using frequency distributions include under color determination for read image data, color count determination for a document (color document/monochrome document), determination for the presence/absence of a content such as characters and thin lines, and blank page determination. 
     An analysis based on a frequency distribution is used to adopt image correction parameters suitable for a specific object included in an image or to identify the boundary between an object and a background. Japanese Patent Laid-Open No. 10-210288 discloses a technique of segmenting the image captured by an imaging apparatus into a plurality of small images, generating a histogram of the level distribution of pixel signals constituting each small image, and specifying a small image including the boundary between an object and a background. 
     In such a frequency distribution analysis for each pixel value, the size of the image data of an analysis target need to be determined in advance to decide the pixel position where sampling is performed and perform uniform segmentation for small regions. 
     When reading a document using an ADF (Automatic Document Feeder), the size of image data is generally determined for the first time when the feeder completely reads one page of the document. It is therefore essential to store image data corresponding to one page. This requires a resource such as a memory or the like. In addition, a frequency distribution analysis is performed for the first time after the storage of image data corresponding to one page, and subsequent image processing is performed based on the analysis result. For this reason, a document reading process, a frequency distribution analysis process, and a subsequent image process constitute a critical path for processing corresponding to one page. This is a factor that determines a total processing time. 
     Attaching a sensor for document size detection to an ADF can detect a document size before reading operation. This, however, leads to a cost increase corresponding to the sensor. In addition, if document pages having different sizes are stacked in a mixed state, some sensor may detect a uniform size different from actual document sizes. 
     SUMMARY OF THE INVENTION 
     The present invention provides an image processing apparatus and image processing method which properly execute an analysis on a frequency distribution even if no document size is informed in advance. 
     The present invention has the following arrangement. That is, an image processing apparatus comprising: a reading unit configured to read a document from an end portion of the document for each line in a predetermined direction and outputs image data; a detection unit configured to detect a size of the document in the predetermined direction; an obtainment unit configured to obtain a frequency distribution of pixel values by using output image data, for each reference region having a predetermined width in the predetermined direction, while reading the document by using the reading unit; a totalization unit configured to segment a frequency distribution obtained by the obtainment unit for each of the reference regions into a predetermined number of groups, and totalizes and stores the frequency distribution for each of the groups; a control unit configured to control totalization by the totalization unit, upon detection of a size of the document by the detection unit, so as to equalize the numbers of the reference regions sorted to the respective groups by using the size of the document, a width of the reference region, and the number of the groups; and an analysis unit configured to analyze the input image data by using a frequency distribution stored for each group which is totalized by control by the control unit. 
     According to the above arrangement, it is possible to perform a proper frequency distribution analysis even on image data whose image size is not determined. This makes it possible to concurrently perform image data reading and a frequency distribution analysis, thereby contributing to an increase in processing speed. In addition, if the only purpose is to perform a frequency distribution analysis, there is no need to store read image data. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are block diagrams showing the overall arrangement of an apparatus; 
         FIG. 2  is a block diagram showing the arrangement of a region control unit  109 ; 
         FIG. 3  is a view showing region segmentation of a read document page; 
         FIG. 4  is a view showing an example of a detection target for frequency distribution generation; 
         FIG. 5  is a block diagram showing the arrangement of a frequency distribution generation unit  110  in the first embodiment; 
         FIG. 6  is a view showing the arrangement of an ADF  105 ; 
         FIG. 7  is a chart showing an example of a synchronization signal; 
         FIGS. 8A and 8B  are views showing a frequency distribution generation target region (other than a region width 2N) in the first embodiment; 
         FIGS. 9A and 9B  are views showing a frequency distribution generation target region (the region width 2N) in the first embodiment; 
         FIGS. 10A and 10B  are graphs showing the arrangements of first and second region frequency distributions; 
         FIG. 11  is a flowchart showing an overall control procedure in the apparatus; 
         FIG. 12  is a flowchart showing a control procedure for a CPU  101  in the first embodiment; 
         FIG. 13  is a flowchart showing a control procedure for a region control unit  109  in the first embodiment; 
         FIG. 14  is a flowchart showing a control procedure for a frequency distribution generation unit  110  in the first embodiment; 
         FIG. 15  is a block diagram showing the arrangement of a frequency distribution generation unit  110  in the second embodiment; 
         FIGS. 16A and 16B  are views showing a frequency distribution generation target region (other than a region width 2N) in the second embodiment; 
         FIGS. 17A and 17B  are views showing a frequency distribution generation target region (the region width 2N) in the second embodiment; 
         FIG. 18  is a flowchart showing a control procedure for the frequency distribution generation unit  110  in the second embodiment; 
         FIG. 19  is a block diagram showing a control procedure for a region control unit  109  in the third embodiment; 
         FIGS. 20A ,  20 B, and  20 C are views each showing a frequency distribution generation target region in the third embodiment; 
         FIG. 21  is a flowchart showing a control procedure for a CPU  101  in the third embodiment; and 
         FIG. 22  is flowchart showing a control procedure for the region control unit  109  in the third embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     [First Embodiment] 
     &lt;Arrangement of Image Processing Apparatus&gt; 
     The embodiments of the present invention will be described below with reference to the accompanying drawings.  FIGS. 1A and 1B  are block diagrams showing the arrangement of an image processing apparatus ( FIG. 1B ) according to an embodiment and the arrangement of a frequency distribution generation unit ( FIG. 1A ) arranged in the apparatus. Referring to  FIG. 1B , a controller unit  100  is a controller which is connected to a scanner  114  as an image input unit and a printer  115  as an image output unit. The controller  100  is also connected to a LAN  117  or public line  118  to input and output image data and device information. An ADF (Automatic Document Feeder)  105  is attached to the scanner  114 . A CPU  101  functions as a controller which controls the overall digital multifunction peripheral as an image processing apparatus. A RAM (Random Access Memory)  103  temporarily stores control data or is used as a work memory under the control of the CPU  101 . A ROM (Read Only Memory)  102  stores programs executed by the CPU  101 . An HDD (Hard Disk Drive)  122  stores system software, image data, and the like. An operation unit I/F  119  is an interface unit for an operation unit  116  and outputs image data to be displayed on the operation unit  116  to it. The operation unit I/F  119  also functions to transfer the information input by the user from the operation unit  116  to the CPU  101 . A network I/F  120  is connected to the LAN  117  to input and output information. A modem  121  is connected to the public line  118  to perform modulation/demodulation processing for data transmission/reception. The above components are arranged on a system but  124 . An image bus I/F  125  is a bus bridge which connects the system but  124  to an image bus I/F  123  which transfers image data at high speed, and converts a data structure. The image bus I/F  123  is formed from a fast bus such as a PCI bus or IEEE1394 bus. A device I/F  111  connects the scanner  114  and the printer  115 , which are image input/output devices, to the controller  100  to perform synchronous system/asynchronous system conversion of image data. The device I/F  111  includes a reading I/F control unit  108  which converts the signal output from the reading device into digital data and a region control unit  109  which, for example, decides the size of image data. A frequency distribution generation unit  110  generates the frequency distribution, that is, the histogram, of the luminances of the pixels of the image data input by the scanner  114 . An image processing unit  113  performs correction, processing, and editing for the image data input by the scanner  114  based on, for example, a histogram, and performs processing suitable for subsequent print output or image transmission. The image processing unit  113  performs correction, resolution conversion processing, and the like in accordance with the printer  115 . 
       FIG. 1A  shows a detailed arrangement associated with frequency distribution generation. An illustration of some of the buses, interfaces, and the like is omitted in  FIG. 1A . A register  104  is set by the CPU  101  and holds the operation mode of the frequency distribution generation unit or an instruction concerning control. The ADF  105  feeds document pages stacked on the mount table to the document glass plate one by one. The ADF  105  includes, at a predetermined detection position, a trailing end detection sensor  107  which detects the trailing end of a document page at the time of reading operation. A reading device  106  such as a CCD or CMOS reads the document page conveyed on the document glass plate at a reading position spaced apart from the detection position by a predetermined distance, and transmits the resultant image data to the frequency distribution generation unit  110 . The RAM  103 , the HDD  122 , or the like stores the data, as needed. Although  FIG. 1A  shows the reading device  106  as if it were part of the ADF  105 , the reading device  106  is a device belonging to the scanner  114 . The trailing end detection sensor  107  is spaced apart from the mount position of a document page by a predetermined distance in a predetermined conveying direction, and hence can also detect the leading end of the document as well as the trailing end. Since the conveying speed is determined, it is possible to decide the document size in the conveying direction from the detection results on the leading and trailing ends. That is, the trailing end detection sensor can be regarded as an end portion detection sensor. 
       FIG. 6  shows the schematic arrangement of the ADF  105 . The same reference numerals as in  FIG. 6  denote the same components in  FIGS. 1A and 1B . A document glass plate  601  is a glass plate on which a document is mounted and which is provided on the image processing apparatus. A document  602  is a document bundle constituted by document pages having different sizes set on the installation portion of the ADF  105 . The page located at the bottom of the document  602  is conveyed onto the document glass plate  601  by rollers and the like (not shown). A document page  603  is a document page image data on which is being read and which is conveyed by document convey units such as rollers in the ADF  105  (not shown) in the direction indicated by reference numeral  606  in  FIG. 6 . The reading device  106  reads the document page  603  at a reading position  604 . The trailing end detection sensor  107  detects the trailing end position of the document page  603  and notifies the CPU  101  of the detection result. That is, the CPU  101  can detect the trailing end position of the image on the document page  603  during reading operation when the trailing end of the document page  603  is conveyed to a position passing through the trailing end detection sensor  107 . At this time point, the portion of the image printed on the document page  603  which has not been read corresponds to the range from the position of the trailing end detection sensor  107  to the reading position  604 . Since the range from the position of the trailing end detection sensor  107  and the reading position  604  is uniquely obtained based on the mechanical arrangement of the image processing apparatus, the number of lines of the document image data defining this range is determined in accordance with the distance from the position of the trailing end detection sensor  107  to the reading position  604  and the conveying speed of the document page. That is, it is possible to decide the length of a document page (that is, the length in the sub-scanning direction) when the trailing end detection sensor  107  detects the trailing end of the document page  603 . Note that the number of lines sometimes changes in accordance with the conveying speed of a document page. For example, when reading a document page at a resolution of 600 dpi and at a resolution of 300 dpi in the conveying direction, since the ratio between the conveying speeds is two to one, the number of lines in the interval from the position of the trailing end detection sensor  107  to the reading position  604  of the reading device  106  changes. For this reason, the CPU  101  controls frequency distribution generation processing upon managing the number of lines in accordance with the reading resolution of a document image in the sub-scanning direction by the ADF  105 . When feeding a document bundle having a constant width like a document including both A4 and A3 pages, it is possible to specify the widthwise direction of the document by detecting the position of a regulation plate which is set in accordance with the width of a document page and prevents a shift of the document page in the widthwise direction. Obviously, this apparatus may include a mechanism for detecting the width of each document page itself by using a mechanical or optical sensor. Refer back to the arrangement of the frequency distribution generation unit shown in  FIGS. 1A and 1B . The reading I/F control unit  108  receives the analog image data read by the reading device  106  and outputs the data to the subsequent-stage processing unit. That is, the reading I/F control unit  108  has a function of converting the analog image data output from the reading device  106  into digital image data and outputting the converted digital image data to the subsequent-stage processing unit. Note that the data output to the subsequent-stage processing unit includes both digital image data and a synchronization signal which allows control of read image data.  FIG. 7  shows an example of this synchronization signal.  FIG. 7  indicates that the apparatus outputs a page synchronization signal (negative logic) as the reading start timing of a document image and a line synchronization signal (negative logic) as a reading line start timing, together with image data (the hatched portions are ineffective image data). 
     The region control unit  109  controls a region for frequency distribution generation based on the image data and synchronization signal output from the reading I/F control unit  108 . When reading the document page  603  by using the ADF  105 , the detection of the leading and trailing ends of the document page depends on the conveying arrangement for the document page  603  and the light source arrangement of the reading device  106 . For this reason, the apparatus sometimes cannot accurately detect an end portion and erroneously detects the shadows of a document.  FIG. 3  shows this situation. Referring to  FIG. 3 , an overall line count  301  indicates the number of pixels of a document image in the sub-scanning direction, and a effective line count  302  indicates the number of pixels of a portion in the sub-scanning direction which is free from the influence of the shadows formed at the leading and trailing ends of a document page by the light source at the time of reading operation. In addition, a shadow portion  303  is the leading end shadow portion of the document page  603  and a shadow portion  304  is a trailing end shadow portion at the time of reading operation. The effective line count  302  is the line count obtained by subtracting the line counts of the shadow portions  303  and  304  from the overall line count  301 . As described above, the regions of the image data read by the ADF  105  which correspond to the leading and trailing end portions of the document page sometimes include shadow data. Using values other than the image data printed on a document page will interfere with an accurate analysis after frequency distribution generation. It is therefore necessary to perform control so as not to set these shadow portions as frequency distribution generation targets. 
     The region control unit  109  performs control so as not to output any image data of such shadow portions for subsequent-stage processing.  FIG. 2  shows the detailed arrangement of the region control unit  109 . The same reference numerals as in  FIG. 2  denote the same components in  FIGS. 1A and 1B . Referring to  FIG. 2 , image data  201  is the image data output from the reading I/F control unit  108 , and a synchronization signal  202  is the synchronization signal (line synchronization signal) in the sub-scanning direction which is output together with the image data. A register  1041  holds the leading end effective position of document image data. A register  1042  holds the trailing end effective position of the document image data. A register  1043  holds a reference region width for the acquisition of a frequency distribution. A reference region is a band obtained by segmenting image data into congruent shapes parallel to the main scanning direction. The length of the band in the sub-scanning direction is a reference region width. Note that the leading end effective position  1041 , the trailing end effective position  1042 , and the reference region width  1043  which are held in the registers are predetermined widths set by the CPU  101 . A first line counter  1091  is a line counter which is arranged in the region control unit  109  and counts the number of lines of image data input based on the synchronization signal  202 . A first comparison unit  1092  compares the count value of the first line counter  1091  with the leading end effective position  1041 , and the count value of the first line counter  1091  with the trailing end effective position  1042 , thereby determining a region free from the influence of shadows, that is, a effective region obtained by subtracting the shadow portions  303  and  304  in  FIG. 3  from one-page image data corresponding to the overall scanned document page  603 . Therefore, the number of lines of the shadow portion  303  is set at the leading end effective position  1041 . For example, this value can be empirically decided in advance. In addition, the value obtained by subtracting the number of lines of the shadow portion  304  from the overall line count  301  is set at the trailing end effective position  1042 . The overall line count  301  can be determined at the time of the detection of the trailing end of the document page by the trailing end detection sensor  107 , and the number of lines of the shadow portion  304  is decided in advance. It is therefore possible to set the value of the trailing end effective position  1042  at the time of the trailing end of the document page by the trailing end detection sensor  107 . 
     The region control unit  109  performs region control for frequency distribution generation based on the set value of the reference region width  1043 . The set value of the reference region width  1043  is reflected in a width  305  in  FIG. 3  and can be decided in advance. The region control unit  109  switches regions for frequency distribution generation, with the reference region width  1043  being a unit. Assume that the region control unit  109  performs control to generate (totalize) the frequency distributions of two regions by sampling the same number of pixels in the respective regions at the same intervals. In this case, the region control unit  109  alternately switches the first and second regions for each reference region width and totalizes the frequency distributions of pixel values in the respective regions. For example, the region control unit  109  controls totalization to set the frequency distribution of a region  306  as the frequency distribution of a first region, that is, a first frequency distribution, and controls totalization to set the frequency distribution of a next region  307  as the frequency distribution of a second region, that is, a second frequency distribution. In this example, the region control unit  109  performs control to set the frequency distribution of a subsequent region  308  as a first frequency distribution to generate the frequency distributions of the two regions which alternately appear in the sub-scanning direction. Note that the number of regions for frequency distribution generation is not limited to two in the sub-scanning direction and may be three or more. That is, it is possible to segment an image into reference regions (bands) parallel to the main scanning direction, classify them into N types of totalization target regions, and totalize a frequency distribution for each totalization target region, that is, a frequency distribution for each pixel value. In this case, the numbers of pixels sampled for the totalization of frequency distributions are the same in all the reference regions (at least almost the same). Pixels to be sampled may be all pixels, selected at predetermined intervals in a raster order, or selected at predetermined intervals in the main scanning and sub-scanning directions. When, for example, generating frequency distributions in three types of reference regions, the region control unit  109  performs region control to set the frequency distribution of the region  308  as a third frequency distribution, and performs control to set the frequency distribution of the next region as a first frequency distribution. 
     The set value of the reference region width  1043  is set in accordance with a detection target in the frequency distribution generation unit. For example, when analyzing the presence/absence of character data in document image data, the region control unit  109  decides the set value of the reference region width  1043  in accordance with the font size of target character data and a reading resolution.  FIG. 4  shows an example of this setting. Referring to  FIG. 4 , a character  401  indicates a character in 10.5 pt, and a pixel count  402  indicates the number of pixels of the character  401  in the sub-scanning direction. If the reading device  106  has a reading resolution of 600 dpi, the pixel count  402  in the sub-scanning direction which forms the character  401  in 10.5 pt shown in  FIG. 4  is approximately 88. The reference region width  1043  in this case, therefore, is set to 64 which is a power of 2 in the neighborhood of 88 pixels. According to the above description, the reference region width is set to a power of 2 for the simplification of the processing arrangement in consideration of the hardware arrangement and control. However, the set value of the reference region width  1043  in this embodiment is not limited to this value. That is, it is possible to use sub-scanning pixel values constituting a detection target without any change or the multiplication or division result with a predetermined coefficient. 
     &lt;Region Classification Control&gt; 
     The region control unit  109  performs control based on the reference region width  1043  by using a line counter  1093 , a comparison unit  1094 , and an output control unit  1095  in  FIG. 2 . Although not shown, when detecting the leading end portion of a document page, the region control unit receives a reset signal to start line counting. Assume that the detection signal based on a document leading end portion is input with a delay or the like, as needed, to synchronize with the start of input of the line synchronization signal  202 . The line counter  1091  counts the number of pulses of the line synchronization signal  202  input after resetting and inputs the resultant value to the comparison unit  1092 . If the line counter value matches the leading end effective position  1041 , the comparison unit  1092  inputs a signal representing the match (reset signal) to the line counter  1093 . The line counter  1093  is reset by the signal representing the match from the comparison unit  1092  and the signal representing the match from the comparison unit  1094  on the subsequent stage to resume counting. That is, the line counter  1093  starts counting upon receiving a determination result on the leading end of the effective region obtained by eliminating the influence of the shadow portion  303  by the comparison unit  1092 . That is, the line counter  1093  performs counting operation for only an interval corresponding to the effective region. The comparison unit  1094  compares the count value of the line counter  1093  with the set value of the reference region width  1043 . If the count value of the line counter  1093  matches the reference region width  1043 , the comparison unit  1094  outputs a control signal for switching regions as frequency distribution totalization targets to the output control unit  1095 . At the same time, the comparison unit  1094  outputs a reset (load) signal for causing the line counter  1093  to start counting from the initial state. Upon receiving the reset (load) signal from the comparison unit  1094 , the line counter  1093  returns the count value to the initial value, and performs counting upon receiving the line synchronization signal  202  again. The output control unit  1095  outputs a signal for controlling first frequency distribution totalization target regions to the processing unit on the subsequent stage until it receives the frequency distribution switching signal output from the comparison unit  1094 . That is, the output control unit  1095  outputs, as output image data  204 , a region identification signal  203  indicating a first frequency distribution totalization target region and the image data  201  input from the reading I/F control unit  108 . In this case, the image data output from the output control unit  1095  belongs to the region indicated by the region identification signal. That is, since the two signals need to be synchronous, the output control unit  1095  gives a proper delay to the image data. Upon receiving a frequency distribution switching signal from the comparison unit  1094 , the output control unit  1095  switches the region identification signal  203  to a signal indicating a second frequency distribution totalization target region and outputs the signal. That is, the output control unit  1095  performs this control to accumulate the frequency distributions of the region  306  in  FIG. 3  as the first frequency distribution, and the frequency distributions of the region  307  as the second frequency distribution. The output control unit  1095  performs control to accumulate the frequency distributions of the subsequent region  308  as the first frequency distribution. In this manner, the output control unit  1095  totalizes frequency distributions while alternately switching the first and second frequency distribution totalization target regions. Note that the detailed operation control of the region control unit  109  will be described later by using other drawings. 
     &lt;Frequency Distribution Generation Unit&gt; 
     The frequency distribution generation unit  110  totalizes and generates frequency distributions for two types of regions segmented in the sub-scanning direction. The frequency distribution generation unit  110  generates a frequency distribution by using the region identification signal  203  and output image data  204  output from the region control unit  109 . Although this embodiment will exemplify frequency distribution generation for two types of regions segmented in the sub-scanning direction, the embodiment is not limited to this, as described with reference to the region control unit  109 . That is, if the regions indicated by the region identification signal  203  are three types of regions segmented in the sub-scanning direction, the frequency distribution generation unit  110  is also configured to generate frequency distributions of the three regions accordingly.  FIG. 5  shows the internal arrangement of the frequency distribution generation unit  110 . Referring to  FIG. 5 , a data identification unit  1101  refers to the value of the image data  204  and outputs the value to the subsequent stage. Assume that the image data  204  is constituted by 8 bits per pixel, that is, has 256 tones, and a frequency distribution is to be quantized and generated into 32 tones (5 bits). In this case, the data identification unit  1101  outputs the upper 5-bit value of each pixel value, from which the lower 3-bit value is removed, to the subsequent stage. It is possible to determine the degree of quantization in advance and make it programmable. A data sorting unit  1102  reflects the quantized image data output from the data identification unit  1101  in frequency distributions on the subsequent stage in accordance with the region identification signal  203 . For example, if the region identification signal  203  indicates a first region, the data sorting unit  1102  reflects the frequencies of the pixel values represented by quantized image data in a first region frequency distribution  1103 . If the region identification signal  203  indicates a second region, the data sorting unit  1102  reflects the frequencies of the pixel values represented by the quantized image data in a second region frequency distribution  1104 . The first region frequency distribution  1103  and the second region frequency distribution  1104  store the frequency distributions of the respective regions. 
     &lt;Data Identification Unit&gt; 
     The concrete operation of the data sorting unit  1102  will be described next. The region  306  in  FIG. 3  is a first region in the effective region. Therefore, the data sorting unit  1102  sorts the frequencies of values represented by image data belonging to the region  306 , and reflects them in the first region frequency distribution  1103 . More specifically, the data sorting unit  1102  adds 1 to the frequency of the density, of the frequency distribution stored in the first region frequency distribution  1103 , which is represented by image data. The region  307  is a second region of the effective region. Therefore, the data sorting unit  1102  sorts the frequencies of the values represented by the image data belonging to the region  307 , and reflects the resultant data in the second region frequency distribution  1104 . More specifically, the data sorting unit  1102  adds 1 to the frequency of the density, of the frequency distribution stored in the second region frequency distribution  1104 , which is represented by image data. The subsequent region  307  is a third region of the effective region. In this embodiment, to generate frequency distributions of two regions in the sub-scanning direction, the data sorting unit  1102  reflects the value indicated by the image data belonging to the third region in the first region frequency distribution  1103 . 
     Region control by the region control unit  109  and the operation of the frequency distribution generation unit  110  will be described in more detail.  FIG. 8A  shows the concept of a case in which the number of lines of a effective region is not 2N times (N is an integer equal to or more than 1) the set value of the reference region width  1043 . That is, the number of lines in the effective region is not an odd-number multiple of the reference region width in this case.  FIG. 8A  also shows a state in which the distance from the position of the trailing end detection sensor  107  to the reading position of the reading device  106  at this time exceeds two times the set value of the reference region width  1043  when converted into the number of lines. Assume that in  FIG. 8A , each white rectangular region indicates a region the data acquired from which is reflected in the first region frequency distribution  1103 , and each black rectangular region indicates a region the data acquired from which is reflected in the second region frequency distribution  1104 . Assume that the region control unit  109  starts region switching from a white rectangular region. That is, the following description is based on the assumption that the region control unit  109  subsequently alternately switches black and white rectangular regions in the order named. 
     Referring to  FIG. 8A , an arrow  801  indicates the line read by the reading device  106  at the timing when the trailing end detection sensor  107  has detected the trailing end of a document page. In this embodiment, the amount of image data belonging to the interval from the position of the trailing end detection sensor  107  to the reading position  604  is equal to or more than two times the set value of the reference region width  1043 , and hence the apparatus reads image data corresponding to a portion denoted by reference numeral  802 . 
     At the timing when the trailing end detection sensor  107  has detected the trailing end of a read document page, the read line is the one at the position  801 , and a frequency distribution generation target is a white rectangular region. That is, the frequency distribution of the image is the timing reflected in the first region frequency distribution  1103 . In order to equalize the number of pixels sampled in the two types of regions, namely the first and second regions, it is necessary to generate the second region frequency distribution  1104  by using the image data belonging to the next black rectangular region, that is, the second region. The image processing apparatus uses a frequency distribution to determine the attribute type of a document page. For example, the apparatus sometimes determines whether read image data belongs to a color document page or a monochrome document page. A frequency distribution represents the frequency of each value of pixel data constituting a read document page. A monochrome pixel can be regarded as including R, G, and B components whose values are almost equal to each other. It is therefore possible to determine whether a read document page is a color or monochrome document page by analyzing whether the frequency distributions of the color components constituting each pixel of the read image data have the same tendency or different tendencies. Assume that read pixel data is constituted by red (R), green (G), and blue (B) components. In this case, if the frequency distributions of the R, G, and B components exhibit the same tendency in terms of peak value or frequency, that is, if the histograms of the respective color components match each other, it is possible to determine that the read document page is a monochrome document page. In this case, however, the overall frequency distribution of one read document page includes too large frequencies to perform accurate determination. For this reason, the apparatus segments the image data of the read document page into a plurality of regions and generates a frequency distribution for each segmented region. The apparatus then analyzes each generated frequency distribution to improve the determination accuracy. Note that in this case, segmenting the image data into regions having the same size will equalize the total numbers of frequencies of the respective frequency distributions, thereby further improving the determination accuracy. 
     In addition, a frequency distribution is sometimes used to analyze the presence/absence of a content in read image data. That is, a frequency distribution is used to determine whether a read document page is a blank page or a page containing a content such as characters and thin lines. If the variance value obtained from this frequency distribution is smaller than a threshold, the apparatus determines that the read document page is a blank page. 
     In this case as well, it is preferable to segment a read document page into a plurality of regions and generate a frequency distribution for each segmented region instead of generating a frequency distribution for one overall read document page. In addition, equalizing the total numbers of frequencies of the frequency distributions of the respective segmented regions can improve the accuracy of information statistically obtained for an analysis on each frequency distribution. 
       FIG. 8A  shows a case in which the remaining number of lines at the time of the detection of the trailing end of a document page by the trailing end detection sensor  107  exceeds two times the reference region width  1043 . In this case, therefore, for example, it is possible to generate a frequency distribution of the black rectangular region next to the white rectangular region to which the line  801  belongs. The region control unit  109  therefore performs control to generate a second region frequency distribution by using the image data belonging to the next black rectangular region along with the progression of document reading operation. In addition, the frequency distribution generation unit  110  generates the second region frequency distribution  1104  in accordance with region control by the region control unit  109 . Upon generating a frequency distribution for a second region after the detection of the trailing end of a document page, the region control unit  109  handles subsequently input image data as a region including the influence of the shadow of the trailing end, and does not control the frequency distribution generation unit  110  to generate a frequency distribution. The distance between the trailing end detection sensor  107  and the reading device  106  is fixed and can be converted into the number of lines in accordance with the resolution in the sub-scanning direction. In addition, the reference region width  1043  is provided before document reading operation. It is therefore possible to determine whether the remaining number of lines at the time of the detection of the trailing end of a document page by the trailing end detection sensor  107  exceeds two times the reference region width  1043  at the time of the detection of the trailing end of the document page by the trailing end detection sensor  107 . 
     If, however, the amount of image data read after the detection of the trailing end of the read image data by the trailing end detection sensor  107  greatly exceeds two time the reference region width  1043 , for example, becomes four times the reference region width  1043 , the region control unit  109  performs different control. That is, upon generating a frequency distribution of the subsequent black rectangular region, if there is a data amount including a pair of next white and black rectangular regions, the region control unit  109  performs control to generate frequency distributions based on equal sample counts in the two regions in the sub-scanning direction. 
       FIG. 8B  shows the regions of the document image data  603  reflected in the first region frequency distribution  1103  and the second region frequency distribution  1104  in such control operation. Referring to  FIG. 8B , the position denoted by reference numeral  803  is the boundary between regions at which the sample count of the first region frequency distribution  1103  is equal to that of the second region frequency distribution  1104 . That is, the data acquired from the gray hatched region after the position  803  is not reflected in the first region frequency distribution  1103  and the second region frequency distribution  1104 . Note that the above trailing end control for read image data by the region control unit  109  is based on the set value of the trailing end effective position  1042  set by the CPU  101  or the set value of the number of pixels read from document image data (not shown) in the sub-scanning direction. That is, this control is based on the assumption that the CPU  101  recognizes the timing of the detection of a trailing end by the trailing end detection sensor  107  and sets the number of pixels read up to the trailing end effective position or trailing end of the read image data based on information at the time of reading operation such as a reading resolution. Control by the CPU  101  and control by the region control unit  109  will be described later with reference with other drawings. 
     The following is a case in which the number of pixels of read image data in the sub-scanning direction from which the region of the shadow of the leading end included in the read image data is 2N times (N is an integer equal to or more than 1) the set value of the reference region width  1043 .  FIG. 9A  is a view showing a concept of this case.  FIG. 9A  shows a state in which the remaining region of the read image data at the timing of the trailing end of the document image by the detection of the trailing end detection sensor  107  exceeds two times the set value of the reference region width  1043 . As described with reference to  FIGS. 8A and 8B , the remaining region of the read image data indicates the amount of image data belonging to the interval from the trailing end detection sensor  107  to the reading position  604 . Assume also that in  FIG. 9A , each white rectangular region indicates a region the data acquired from which is reflected in the first region frequency distribution  1103 , and each black rectangular region indicates a region the data acquired from which is reflected in the second region frequency distribution  1104 . Assume that the apparatus starts region switching from a white rectangular region as in the case described with reference to  FIGS. 8A and 8B . That is, the following description is based on the assumption that the apparatus subsequently alternately switches black and white rectangular regions in the order named. 
     Referring to  FIG. 9A , reference numeral  901  denotes the timing when the trailing end detection sensor  107  has detected the trailing end of the read image data. In this embodiment, since the amount of image data belonging to the interval from the trailing end detection sensor  107  to the reading position  604  is equal to or more than two times the set value of the reference region width  1043 , the apparatus subsequently reads image data corresponding to the portion denoted by reference numeral  902 . 
     At the timing (the position denoted by reference numeral  901 ) when the trailing end detection sensor  107  has detected the trailing end of the read document page, a black rectangular region generates a frequency distribution. That is, this is the timing when the generated frequency distribution is reflected as the frequency distribution of the read image data in the second region frequency distribution  1104 . In this embodiment, the amount of image data belonging to the interval from the trailing end detection sensor  107  to the reading position  604  is equal to or more than two times the set value of the reference region width  1043 . In addition, the embodiment is based on the assumption that the number of pixels in the sub-scanning direction from which the influence of the shadow of the leading end portion of the read image data is removed is 2N times the reference region width  1043 . Therefore, even after the timing when the trailing end detection sensor  107  has detected the trailing end of a read document page, the apparatus inputs read image data at least two times the reference region width  1043 . After the generation of the current second region frequency distribution, the sample count of the frequency distribution  1103  acquired from the first region becomes the same as that of the frequency distribution  1104  acquired from the second region. Even if subsequently input white and black rectangular regions are respectively reflected in the first region frequency distribution  1103  and the second region frequency distribution, it is possible to equalize the sample counts. In this case, however, the lower portion denoted by reference numeral  902  corresponds to the image trailing end of the document image data  603 . As has been described, this trailing end portion includes a shadow portion in read image data. Therefore, this portion should not be reflected in the first region frequency distribution  1103  and the second region frequency distribution  1104 . For this reason, the region control unit  109  performs control to reflect, in the generation of each frequency distribution, data acquired in the interval from the time point (reference numeral  901 ) as a reference when the trailing end detection sensor  107  has detected the document trailing end to the earliest timing when the sample count of the frequency distribution  1103  acquired from the first region becomes equal to the sample count of the frequency distribution  1104  acquired from the second region.  FIG. 9B  shows the regions of the document image data  603  which are respectively reflected in the first region frequency distribution  1103  and the second region frequency distribution  1104  when such control is performed. Referring to  FIG. 9B , the position denoted by reference numeral  903  is between regions at which the sample count of the first region frequency distribution  1103  is equal to that of the second region frequency distribution  1104  without any influence of the shadow of the trailing end of the document image data. That is, the data acquired from the gray hatched regions after the position  903  are not reflected in the first region frequency distribution  1103  and the second region frequency distribution  1104 . Note that the above trailing end control for read image data by the region control unit  109  is based on the set value of the trailing end effective position  1042  set by the CPU  101  or the set value of the number of pixels read in the sub-scanning direction of document image data (not shown) as in the case described with reference to  FIGS. 8A and 8B . That is, this control is based on the assumption that the CPU  101  recognizes the timing of the detection of a trailing end by the trailing end detection sensor  107  and sets the trailing end effective position of read image data or the set value of the number of read pixels in the sub-scanning direction of document image data based on information at the time of reading operation such as a reading resolution. 
       FIGS. 8A ,  8 B,  9 A, and  9 B show a case in which the predetermined number of groups of reference regions is set to two. The following is a more general description of this case. That is, the apparatus further divides the remaining number of lines of the effective region at the time of the detection of the trailing end of the document page by the trailing end detection sensor  107  by the number of lines corresponding to the reference region width. This converts the number of lines into the number of reference regions (which is represented by J). In this case, the number of groups of reference regions as frequency distribution totalization targets is represented by K (K=2 in the above case), and the region in which totalization is currently performed is the ith region (i=0 to K−1). Note however that the first reference region of the effective region is the 0th region. The apparatus then obtains (J+i)/K (the remainder will be rounded off). When reference regions are classified into K groups respectively assigned with numbers from 0 to K−1, the value of (J+i)/K represents the number of sets of reference regions, with K regions being one set, included in the unread portion of the effective region at the time of the detection of the trailing end of the document page. Note that the number of sets of reference regions includes a set being read. In this case, the apparatus cyclically and sequentially reads sets of reference regions, which are respectively assigned with the numbers 0 to K−1, and hence can easily determine how a given reference region in the course of reading ranks in terms of the assigned numbers. Therefore, upon detecting the trailing end of a document page, the apparatus therefore totalizes frequency distributions for each group until it reads (J+i)/K reference regions of the last group (that is, the (K−1)th group), and sets the subsequent regions other than totalization targets. In other words, the apparatus sets, as frequency distribution totalization targets, the remaining K×((J+i)/K)−i (operation “/” represents an integer quotient) reference regions excluding the currently read reference region, and sets the subsequent regions other than targets. This is because the remaining regions do not include one set of reference region, that is, M reference regions, and setting them as totalization targets will lead to variation in the sample counts of the respective reference regions. In this case, J is decided from the distance from the trailing end detection sensor  107  to the reading device  106  and the reference region width, and the number of groups, that is, the number K of reference regions constituting one set, is decided in advance. Note that (J+i)/K=0 indicates that when the trailing end of a document page is detected, the unread portion does not include one set of reference regions, that is, K reference regions. Since it is difficult to undo a total value by a predetermined step after totalization, it is necessary to decide a reference region width and the reference region count K corresponding to one set in advance so as to satisfy (J+i)/K&gt;0. For this purpose, J and K are decided in advance so as satisfy (J+i)/K&gt;0 (since “/” represents an integer quotient, (J+i)/K≧1, that is, J≧K) even in a case in which the value of i is 0, which is the minimum value. Since the value of J is decided from the distance from the trailing end detection sensor  107  to the reading device  106  and a reference region width, a reference region width and the number K of reference regions constituting one set are decided to satisfy J≧K. 
     The purpose of this operation is to decide the size of a document page in the conveying direction and the size of the effective region in the conveying direction at the time of the detection of the trailing end detection sensor by the trailing end of the document page. The purpose of the above procedure is to sample the effective region for each group so as to equalize the numbers of reference regions of the respective groups and totalize the frequency distributions of pixel values. 
     As described above, even if a document size is not determined at the start of reading operation by region control by the region control unit  109  based on the reference region width  1043  and frequency distribution generation by the frequency distribution generation unit  110 , it is possible to generate frequency distributions with an equal number of frequencies for each reference region width  1043 .  FIGS. 10A and 10B  show the arrangement concept of the first region frequency distribution  1103  and second region frequency distribution  1104  in this case.  FIG. 10A  is a graph showing the state of the first region frequency distribution  1103 .  FIG. 10B  is a graph showing the state of the second region frequency distribution  1104 . Although frequency distributions in  FIGS. 10A and 10B  differ in shape, the sample counts of the respective regions are equal. That is, it is possible to generate frequency distributions based on an equal sample count without any influence of shadows. 
     &lt;Frequency Distribution Generation Procedure&gt; 
     Control by the CPU  101 , the region control unit  109 , and the frequency distribution generation unit  110  will be described by using other drawings.  FIG. 11  shows the overall operation procedure in the frequency distribution generation unit. In the operation procedure in the frequency distribution generation unit in this embodiment, the region control unit  109  performs region control on the image data read by the reading device  106  so as to equalize the sample counts in the respective reference regions in the sub-scanning direction (step S 101 ). The frequency distribution generation unit  110  generates frequency distributions in accordance with the control signal  203  and image data  204  output from the region control unit  109  (step S 102 ). 
       FIG. 12  is a flowchart showing a control procedure in the CPU  101  which controls the overall frequency distribution generation unit in this embodiment. At the startup of the apparatus in the embodiment, the CPU  101  performs setting necessary for region control by the frequency distribution generation unit, that is, the leading end effective position  1041 , the trailing end effective position  1042 , and the reference region width  1043  (step S 201 ). Note that since a document size is not determined at the start of document image data reading when the ADF  105  performs reading operation, the CPU  101  sets the trailing end effective position  1042  to the maximum set value which can be set in step S 201 . The CPU  101  then sets the number of pixels read in the sub-scanning direction of the document image data (step S 202 ). Note that like setting of the trailing end effective position  1042  in step S 201 , the CPU  101  sets the number of pixels read in the sub-scanning direction to the maximum set value which can be set in step S 202  because the document size is not determined at the start of reading operation. 
     Upon completing the above setting, the CPU  101  issues a reading operation start instruction (step S 203 ). The CPU  101  does not change the set values set in steps S 201  and S 202  until the trailing end detection sensor  107  detects a trailing end during reading operation for the document image data  603  (NO in step S 204 ). Upon receiving a trailing end detection signal concerning the document image data  603  from the trailing end detection sensor  107  (YES in step S 204 ), the CPU  101  calculates the remaining number of pixels in the sub-scanning direction in accordance with the mechanical arrangement of the image processing apparatus and parameters at the time of operation such as the reading resolution of the document image data  603  (step S 205 ). More specifically, the CPU  101  determines the number of pixels read from the region from the position of the trailing end detection sensor  107  to the reading position  604  in the sub-scanning direction, as a document image size, based on parameters such as a resolution at the time of document reading operation. 
     Upon completing the calculation of the remaining number of pixels read in the sub-scanning direction in step S 205 , the CPU  101  sets the trailing end effective position  1042  of the document image data and the number of pixels read in the sub-scanning direction upon determination of the document size (step S 206 ). That is, the CPU  101  sets a value for the setting target, for which the maximum value which can be set is set in step S 201 , in accordance with the determination of the document size. In addition, the CPU  101  notifies the region control unit  109  of the completion of setting to indicate the completion of setting of the number of pixels read in the sub-scanning direction upon determination of the document size (step S 207 ). The CPU  101  then waits for a reading completion notification concerning the document image data  603 . Upon receiving the reading completion notification, the CPU  101  terminates the control procedure (step S 208 ). 
     An operation procedure in the region control unit  109  under the control of the CPU  101  shown in  FIG. 12  will be described next.  FIG. 13  is a flowchart showing a control procedure in the region control unit  109 . When the region control unit  109  issues a reading operation start instruction for document image data under the control of the CPU  101  in step S 203 , the reading device  106  starts reading the document image data  603 . The reading I/F control unit  108  converts the image data read by the reading device  106  into digital data and outputs it to the region control unit  109 . At this time, the reading I/F control unit  108  outputs the digitally converted image data, together with the synchronization signal  202 . The region control unit  109  does not start region control until receiving the synchronization signal  202  from the reading I/F control unit  108  (NO in step S 301 ). Upon receiving the synchronization signal  202  from the reading I/F control unit  108  (YES in step S 301 ), the line counter  1091  counts the number of input lines (step S 302 ). If the count value obtained in step S 302  has not reached the set value of the leading end effective position  1041  set by the CPU  101  in step S 201 , the line counter  1091  waits for an input of read image data again (NO in step S 303 ). The control performed by the region control unit  109  in step S 303  can eliminate the influence of the shadow included in the leading end read image data of the document image data. If the input line count value obtained in step S 302  has reached the set value of the leading end effective position  1041 , the region control unit  109  shifts to the next control (YES in step S 303 ). 
     That is, the output control unit  1095  outputs the one-line read image data input following the synchronization signal  202  as the image data  204  to the frequency distribution generation unit  110  (step S 304 ). The output control unit  1095  also outputs the region identification signal  203 , together with the image data  204 , to the frequency distribution generation unit  110 . The region control unit  109  repeatedly executes the processing from step S 301  to step S 304  until it receives the notification of the completion of setting of a document reading size issued by the CPU  101  in step S 207  (NO in step S 305 ). Upon receiving the notification of the completion of setting of a document reading size from the CPU  101  (YES in step S 305 ), the region control unit  109  calculates the difference between the value counted in step S 302  and the set value of the document reading size (step S 306 ). The difference calculated in step S 306  indicates the number of pixels of the read image data of the subsequently input document image data  603  in the sub-scanning direction. That is, the CPU  101  obtains the number of pixels in the sub-scanning direction (that is, the number of lines) from the position of the trailing end detection sensor  107  to the reading position  604 . Since this value is obtained in accordance with the distance between the trailing end detection sensor and the reading device, which are structurally fixed, and the pixel density in the sub-scanning direction, the CPU  101  obtains the value by using the pixel density in the sub-scanning direction as a parameter in practice. A pixel density is generally selected from choices such as 200 dpi, 300 dpi, 400 dpi, 600 dpi, and 1,200 dpi. The region control unit  109  calculates the remaining number of read regions by using the calculation result obtained in step S 306  and the set value of the reference region width  1043  set by the CPU  101  in step S 201  (step S 307 ). In this case, the region control unit  109  performs subsequent control based on cases in which the number of pixels in the sub-scanning direction from which the number of pixels corresponding to the leading end shadow portion of the document image data  603  is deleted is not 2N times the reference region width  1043  and is 2N times the reference region width  1043 . Note that control to be performed when the number of pixels is not 2N times the reference region width  1043  or is 2N times the reference region width  1043  has already been described with reference to  FIGS. 8A and 8B  or  FIGS. 9A and 9B , a description of the control will be omitted. If the number of groups of reference regions is K, the CPU  101  decides a reference region as a frequency distribution totalization target according to the general rule described after the description of  FIGS. 8A and 8B  or  FIGS. 9A and 9B . 
     The region control unit  109  stands by until the next synchronization signal  202  is input (NO in step S 308 ). Upon receiving the synchronization signal  202  (YES in step S 308 ), the region control unit  109  counts input lines (step S 309 ). If the count value obtained in step S 309  indicates a region to be output to the frequency distribution generation unit  110  on the subsequent stage, the region control unit  109  outputs the region identification signal  203  and the image data  204  to the frequency distribution generation unit  110  (YES in step S 310 ). That is, if regions should be reflected in the first region frequency distribution  1103  and the second region frequency distribution  1104 , the region control unit  109  outputs the data to the frequency distribution generation unit  110  (step S 311 ). The region control unit  109  switches regions in accordance with the region identification signal  203  in response to a signal indicating that the count value has reached the reference region width output from the comparison unit  1094  as a trigger. The region control unit  109  may sequentially switch reference regions from the first region to the next (i←i+1 (where i≦K))th region if K reference regions constitute one set. If the input read image data falls outside a frequency distribution totalization target region, the region control unit  109  outputs no data to the frequency distribution generation unit  110  (NO in step S 310 ). That is, the region control unit  109  performs control to output no data to the frequency distribution generation unit  110  if the count value belongs to a region in which the sample count of the first region frequency distribution  1103  is not equal to that of the second region frequency distribution  1104 . 
     That is, the region control unit  109  performs control so as not to use, for frequency distribution generation, any read image data input after a region determined not as a target region. More specifically, upon receiving the synchronization signal  202  input after a region determined not as a target region is input (step S 312 ), the region control unit  109  counts the number of input lines by using the line counter  1091  (step S 313 ). The region control unit  109  continuously performs the processing from step S 312  to step S 313  until the count value as the number of input lines in step S 313  has reached the set value of the number of pixels in the sub-scanning direction of document image data which is set by the CPU  101  (NO in step S 314 ). When the input line count value obtained in step S 313  reaches the number of pixels read in the sub-scanning direction of the document image data (YES in step S 314 ), the region control unit  109  outputs an end notification indicating the end of reading of the document image data  603  to the CPU  101 . 
     An operation procedure in the frequency distribution generation unit  110  under the control of the region control unit  109  shown in  FIG. 13  will be described next.  FIG. 14  is a flowchart showing a control procedure in the frequency distribution generation unit  110 . The frequency distribution generation unit  110  performs no control until it receives the region identification signal  203  and the image data  204  from the region control unit  109  (NO in step S 401 ). When the region control unit  109  outputs the region identification signal  203  and the image data  204  (YES in step S 401 ), the data sorting unit  1102  switches frequency distributions, in which the value of the input image data  204  is to be reflected, in accordance with the value represented by the region identification signal  203  (step S 402 ). That is, if the region identification signal  203  from the region control unit  109  indicates reflection in the first region frequency distribution (YES in step S 402 ), the data sorting unit  1102  performs control to reflect the value of the image data  204  in the first region frequency distribution  1103  (step S 403 ). If the region identification signal  203  indicates reflection in the second region frequency distribution (NO in step S 402 ), the data sorting unit  1102  performs control to reflect the value of the image data  204  in the second region frequency distribution  1104  (step S 404 ). Since the region control unit  109  performs control to equalize the sample counts of the first and second region frequency distributions  1103  and  1104 , the data sorting unit  1102  performs control to switch frequency distribution generation targets in accordance with the region identification signal  203  output from the region control unit  109 . The frequency distribution generation unit  110  continuously performs the processing from step S 401  to step S 403  or from step S 401  to step S 404  (step S 405 ) until the region control unit  109  notifies the end of document reading (step S 313 ). 
     Referring to  FIG. 14 , reference regions are classified into two types of regions, namely the first and second regions, that is, two groups of regions, and two groups constitute one set. Even if one set includes three or more groups, frequency distributions are totalized for each region determined in step S 402  as in the case shown in  FIG. 14 . 
     Although the first embodiment has been described on the assumption that the region control unit  109  is directly connected to the frequency distribution generation unit  110 , this embodiment is not limited to this. For example, the apparatus may be configured to include a module for executing image processing for the image data  204  on the subsequent stage of the region control unit  109  and input an image processing result to the frequency distribution generation unit  110 . In this case, the region identification signal  203  output from the region control unit  109  may be input to the frequency distribution generation unit  110  via the image processing module. 
     As described above, in the first embodiment, when a document size is not determined at the start of reading operation and the remaining read image data at the time of the detection of the trailing end of the document page is equal to or more than 2N times the reference region width  1043 , it is possible to equalize the frequency distribution sample counts of a plurality of regions in the sub-scanning direction. In addition, this embodiment has exemplified the frequency distribution generation arrangement which sets the set value of the reference region width  1043  in frequency distribution generation in accordance with a detection target in frequency distribution generation, thereby improving the accuracy of subsequent frequency distribution analysis. 
     [Second Embodiment] 
     The first embodiment has exemplified control on the region control unit  109  and the frequency distribution generation unit  110  when the read image data region after the detection of the trailing end of a document image by the trailing end detection sensor  107  exceeds two times (K times in general; the same will apply hereinafter) the set value of the reference region width  1043 . In the arrangement of the image processing apparatus, however, the number of pixels (the number of lines) in the sub-scanning direction from the installation position of the trailing end detection sensor  107  to the reading position  604  is sometimes less than two times the reference region width  1043 . According to the above description, the first embodiment is configured to set a reference region width and the number K of reference regions constituting one set so as to avoid such a case. However, the value of the reference region width  1043  is set in accordance with a detection target in frequency distribution generation, and hence it is required to cope with this setting. In addition, even if the arrangement of the image processing apparatus remains the same, when the value of the reference region width  1043  is set to a value larger than that described in the first embodiment, a read image data region after the detection of the trailing end of a document page can be a value roughly corresponding to one reference region width  1043 . The second embodiment will exemplify a case in which a read image data region after the detection of the trailing end of the document page by the trailing end detection sensor  107  is very near/less than the set value of the reference region width  1043 . 
     The internal arrangement of a region control unit  109  in this embodiment is the same as that in the first embodiment. The internal arrangement of a frequency distribution generation unit  110  in the second embodiment differs from that in the first embodiment.  FIG. 15  shows the arrangement of the frequency distribution generation unit  110  in a case in which the second embodiment is implemented. The same reference numerals as in  FIG. 15  denote the same components of the frequency distribution generation unit  110  in the first embodiment. 
     The constituent elements of the frequency distribution generation unit  110  in this embodiment will be described in detail. Referring to  FIG. 15 , a first region preceding-stage frequency distribution  1501  is configured to reflect the value of image data  204  in a frequency distribution when the value of a region identification signal  203  output from a data sorting unit  1102  indicates frequency distribution generation for a first region. A second region preceding-stage frequency distribution  1502  is configured to reflect the value of the image data  204  in a frequency distribution when the value of the region identification signal  203  output from the data sorting unit  1102  indicates frequency distribution generation for a second region. The first region preceding-stage frequency distribution  1501  is connected to a first region frequency distribution  1103 . The second region preceding-stage frequency distribution  1502  is connected to a second region frequency distribution  1104 . A frequency distribution reflection signal  1503  is a signal output from the data sorting unit  1102  at the timing when the value represented by the region identification signal  203  input from the region control unit  109  changes by two regions. More specifically, this signal is a control signal which changes at the timing when a value indicating a first region switches to a value indicating a second region and then switches to a value indicating a first region. That is, the signal is an output signal which becomes significant information at the timing when the value switches to the value indicating a first region in the second and subsequent steps. In more general, when the apparatus obtains frequency statistics for each reference region with K reference regions constituting one set, the data sorting unit  1102  outputs the frequency distribution reflection signal  1503  at the timing when the region identification signal  203  switches from the Kth reference region to the first reference region. The first region frequency distribution  1103  and the second region frequency distribution  1104  respectively allow the values of the preceding-stage frequency distributions connected to the preceding stages to be reflected in them at the output timing of the frequency distribution reflection signal  1503 . That is, when the frequency distribution reflection signal  1503  is output, the first region frequency distribution  1103  cumulatively adds the value of the first region preceding-stage frequency distribution  1501  to the value of the frequency distribution held so far. As a result, the total of past frequency distributions is reflected in the first region frequency distribution  1103 . Likewise, when the frequency distribution reflection signal  1503  is output, the second region frequency distribution  1104  cumulatively adds the value of the second region preceding-stage frequency distribution  1502  to the value of the frequency distribution held so far. As a result, the total of past frequency distributions is reflected in the second region frequency distribution  1104 . That is, each preceding stage frequency distribution is provided to temporarily store frequency distributions corresponding to one set of reference regions. When the totalization of frequency distributions corresponding to one set of reference regions is complete, the resultant data is collectively reflected in the frequency distribution. 
     The reason why the frequency distribution generation unit  110  has this arrangement will be described by using other drawings. Described first is a case in which the number of pixels of read image data in the sub-scanning direction from which the number of pixels corresponding to the leading end shadow portion included in the read image data is removed is not 2N times (N is an integer equal to or more than 1) the set value of a reference region width  1043 .  FIGS. 16A and 16B  show a state in which a read image data region after the detection of the trailing end of the document image by a trailing end detection sensor  107  is less than the set value of the reference region width  1043 . As described in the first embodiment, the remaining region of the read image data indicates the amount of image data belonging to the interval from the position of the trailing end detection sensor  107  to a reading position  604 . Assume also that in  FIGS. 16A and 16B , each white rectangular region indicates a region the data from which is reflected in the first region frequency distribution  1103 , and each black rectangular region indicates a region the data acquired from which is reflected in the second region frequency distribution  1104 . Assume that the apparatus starts region switching from a white rectangular region. That is, the following description is based on the assumption that the apparatus subsequently alternately switches black and white rectangular regions in the order named. 
     Referring to  FIG. 16A , reference numeral  1601  denotes the timing when the trailing end detection sensor  107  has detected the trailing end of the read image data. In this description, since the amount of image data belonging to the interval from the position of the trailing end detection sensor  107  to the reading position  604  is less than the set value of the reference region width  1043 , the image data region denoted by reference numeral  1602  is less than a value corresponding to one region of the set value of the reference region width  1043 . 
     At the timing (the position denoted by reference numeral  1601 ) when the trailing end detection sensor  107  has detected the trailing end of the read document page, the data acquired from the white rectangular region generates a frequency distribution. That is, this is the timing when the generated frequency distribution is reflected as the frequency distribution of the read image data in the first region preceding-stage frequency distribution  1501 . In order to generate frequency distributions with the same sample count in two regions in the sub-scanning direction, it is necessary to generate the second region preceding-stage frequency distribution  1502  by using image data belonging to the next black rectangular region. Referring to  FIG. 16A , however, since the amount of read image data after the detection of the trailing end of the read image data by the trailing end detection sensor  107  is less than the reference region width  1043 , there is insufficient image data to generate the frequency distribution of the next black rectangular region. If, therefore, the first region preceding-stage frequency distribution  1501  generated at the timing (denoted by reference numeral  1601 ) of the detection of the trailing end of the document page by the trailing end detection sensor  107  is reflected in the first region frequency distribution  1103 , the sample count of the first region frequency distribution becomes different from that of the second region frequency distribution. In order to prevent this, therefore, the internal arrangement of the frequency distribution generation unit  110  includes the first region preceding-stage frequency distribution  1501  and the second region preceding-stage frequency distribution  1502 . That is, in the case shown in  FIG. 16A , the apparatus generates frequency distributions for the first region preceding-stage frequency distribution  1501  and the second region preceding-stage frequency distribution  1502  after the timing (reference numeral  1601 ) of the detection of the trailing end of the document page by the trailing end detection sensor  107 . However, since the width of the last black rectangular region is less than the reference region width  1043 , there is no timing of switching to the next white rectangular region. That is, the apparatus does not output the control signal  1503 . As a consequence, the first region frequency distribution  1103  and the second region frequency distribution  1104  at the time of the completion of reading of document image data  603  become those from which the last white rectangular region and the last black rectangular region are removed.  FIG. 16B  shows the arrangement of input image data and the arrangement of image data reflected in the first region frequency distribution  1103  and the second region frequency distribution  1104 . Referring to  FIG. 16B , reference numeral  1603  denotes the boundary position of reflection in the first region frequency distribution  1103  and the second region frequency distribution  1104 . 
     Described next is a case in which the number of pixels of read image data in the sub-scanning direction from which the number of pixels corresponding to the leading end shadow portion included in the read image data is removed is 2N times (N is an integer equal to or more than 1) the set value of a reference region width  1043 . 
       FIGS. 17A and 17B  show a state in which a read image data region after the detection of the trailing end of a document image by the trailing end detection sensor  107  is less than the set value of the reference region width  1043 . Referring to  FIGS. 17A and 17B , as in the case shown in  FIGS. 16A and 16B , each white rectangular region indicates a region reflected in the first region frequency distribution  1103 , and each black rectangular region indicates a region reflected in the second region frequency distribution  1104 . Assume that the apparatus starts region switching from a white rectangular region. 
     Referring to  FIG. 17A , reference numeral  1701  denotes the timing when the trailing end detection sensor  107  has detected the trailing end of the read image data. In this embodiment, the amount of image data belonging to the interval from the position of the trailing end detection sensor  107  to the reading position  604  is less than the set value of the reference region width  1043 . Therefore, the image data region denoted by reference numeral  1702  is less than a value corresponding to one region of the set value of the reference region width  1043 . 
     At the timing (the position denoted by reference numeral  1701 ) when the trailing end detection sensor  107  has detected the trailing end of the read document page, the data acquired from the black rectangular region generates a frequency distribution. That is, this is the timing when the generated frequency distribution is reflected as the frequency distribution of the read image data in the second region preceding-stage frequency distribution  1502 . Referring to  FIG. 17A , the amount of read image data after the detection of the trailing end of the read image data by the trailing end detection sensor  107  is less than the reference region width  1043 . 
     The arrangement shown in  FIGS. 17A and 17B  differs from that shown in  FIGS. 16A and 16B . 
     That is, reflecting all input image data in frequency distribution generation can make the sample count of the frequency distribution generated by the data acquired in a white rectangular region equal to the sample count of the frequency distribution generated by the data acquired in a black rectangular region. The image data of the last black rectangular region in  FIG. 17A  includes the influence of the shadow of the trailing end of the document page. When, however, performing an analysis by using a generated frequency distribution, shadow image data makes it impossible to perform accurate determination. 
     In order to prevent this, the internal arrangement of the frequency distribution generation unit  110  includes the first region preceding-stage frequency distribution  1501  and the second region preceding-stage frequency distribution  1502 . That is, in the case shown in  FIG. 17A , after the timing (reference numeral  1701 ) of the detection of the trailing end of the document page by the trailing end detection sensor  107 , the apparatus generates frequency distributions for the first region preceding-stage frequency distribution  1501  and the second region preceding-stage frequency distribution  1502 . After the input of the image data of the last black rectangular region, there is no image data of a white rectangular region. That is, the apparatus does not output the control signal  1503 . This removes the frequency distributions of the last white and black rectangular regions from the first region frequency distribution  1103  and the second region frequency distribution  1104  at the time of the completion of reading of the document image data  603 .  FIG. 17B  shows the arrangement of the input image data and the arrangement of image data reflected in the first region frequency distribution  1103  and the second region frequency distribution  1104 . Referring to  FIG. 17B , reference numeral  1703  denotes the boundary position of reflection in the first region frequency distribution  1103  and the second region frequency distribution  1104 . 
     A control procedure in the frequency distribution generation unit  110  in the second embodiment will be described next. Since a control procedure in the CPU  101  and a control procedure in the region control unit  109  in the second embodiment are the same as those in the first embodiment, a description of the procedures will be omitted. 
       FIG. 18  is a flowchart showing a control procedure in the frequency distribution generation unit  110  in the second embodiment. The frequency distribution generation unit  110  does not perform control until it receives the region identification signal  203  and the image data  204  from the region control unit  109  (NO in step S 501 ). When the region control unit  109  outputs the region identification signal  203  and the image data  204  (YES in step S 501 ), the frequency distribution generation unit  110  causes the data sorting unit  1102  to recognize that the input image data is the image data for the first region and switch frequency distributions in which the value of the input image data  204  is to be reflected, in accordance with the value represented by the region identification signal  203 , without outputting the control signal  1503  (step S 506 ). If the region identification signal  203  from the region control unit  109  indicates reflection in the frequency distribution of the first region (YES in step S 506 ), the data sorting unit  1102  reflects the value of the image data  204  in the first region preceding-stage frequency distribution  1501  (step S 507 ). If the region identification signal  203  indicates reflection in the frequency distribution of the second region (NO in step S 506 ), the data sorting unit  1102  reflects the value of the image data  204  in the second region preceding-stage frequency distribution  1502  (step S 508 ). The frequency distribution generation unit  110  checks whether it has received a document reading end notification from the region control unit  109 , and waits for the next input data (step S 501 ) if it has received no document reading end notification (NO in step S 509 ). Upon receiving data from the region control unit  109 , the data sorting unit  1102  outputs the control signal  1503  if the input image data is switched to image data of the first region for the second or subsequent time. When the data sorting unit  1102  outputs the control signal  1503 , the first region frequency distribution  1103  reflects the frequency distribution held in the first region preceding-stage frequency distribution  1501  connected to the preceding stage in the value which has been held in the first region frequency distribution  1103 . At the same timing, the second region frequency distribution  1104  reflects the frequency distribution, held in the second region preceding-stage frequency distribution  1502  connected to the preceding stage, in the value which has been held in the second region frequency distribution  1104  (step S 503 ). In response to reflection in the first region frequency distribution  1103 , the first region preceding-stage frequency distribution  1501  initializes the held frequency distribution to prepare for frequency distribution generation indicated by the next image data  204  (step S 504 ). In response to reflection in the second region frequency distribution  1104 , the second region preceding-stage frequency distribution  1502  initializes the held frequency distribution to prepare for frequency distribution generation indicated by the next image data  204  (step S 505 ). Upon completion of reflection in the first region frequency distribution  1103  and the second region frequency distribution  1104 , the data sorting unit  1102  performs the processing from step S 506  to step S 508  described above based on the region identification signal  203  and image data  204  input from the region control unit  109 . The frequency distribution generation unit  110  repeatedly executes the processing from step S 501  to step S 508  until it receives a document reading end notification from the region control unit  109  (NO in step S 509 ), and terminates the processing in accordance with the notification (YES in step S 509 ). 
     As described above, in this embodiment, upon completion of the totalization of frequency distributions of one set of reference regions, the apparatus reflects the resultant data in the totalization of frequency distributions of the overall image. For this reason, any data concerning a set of reference regions in which totalization has not been complete is not reflected in the frequency distribution of the overall image. 
     As described above, according to the second embodiment, even if a read image data region after the detection of the trailing end of a document page is very near/less than the reference region width  1043 , it is possible to generate frequency distributions whose sample counts in the sub-scanning direction are equal. 
     [Third Embodiment] 
     The first and second embodiments have exemplified the case in which the influence of the shadow of the trailing end of a document image is eliminated and the frequency distribution sample counts of a plurality of regions in the sub-scanning direction are equalized in accordance with the relationship between a read image data region after the detection of the trailing end of the document image and the set value of the reference region width  1043 . Assume, however, that the set value of the reference region width  1043  is large. In this case, if the apparatus performs control so as not to acquire the frequency distribution of a read image data region less than the reference region width  1043  to equalize the frequency distribution sample counts, a larger region is not reflected in frequency distributions relative to the document image data. The larger a region which is not reflected in frequency distributions, the higher the possibility of interfering with an analysis using frequency distributions. The third embodiment is configured to solve this problem. 
     The internal arrangement of a frequency distribution generation unit  110  in this embodiment is the same as that in the first embodiment. The internal arrangement of a region control unit  109 , a control procedure in the unit, and a control procedure in a CPU  101  in the third embodiment differ from those in the first embodiment.  FIG. 19  shows the arrangement of the region control unit  109  in a case in which the third embodiment is implemented. The same reference numerals as in  FIG. 19  denote the same components of the region control unit  109  in the first embodiment. 
     The respective constituent elements which are arranged in the region control unit  109  to implement this embodiment will be described. Reference numerals  1044  and  1045  denote new registers which are arranged in a register  104  and required to implement the third embodiment. Reference numeral  1044  denotes a trailing end region width to be applied to a document trailing end portion; and  1045 , a region width switching instruction unit for issuing a region width switching instruction to be applied to the document trailing end portion. Note that the trailing end region width  1044  and the region width switching instruction unit  1045  allow the CPU  101  to make settings. Reference numeral  1096  denotes a region counter which counts at the timing when the count value of a line counter  1093  reaches the set value of a reference region width  1043  or the set value of the trailing end region width  1044 ; and  1097 , a region width switching unit which performs selective output control on the set value of the reference region width  1043  and the set value of the trailing end region width  1044  in accordance with the count value of the region counter  1096  and the value of the region width switching instruction unit  1045 . 
     The necessity of the third embodiment will be described by using other drawings.  FIGS. 20A ,  20 B, and  20 C show the relationship between the region segmented state of document image data  603  and the reference region width  1043  after the detection of the trailing end of the document image by a trailing end detection sensor  107 . Referring to  FIGS. 20A ,  20 B, and  20 C, as in the above description, each white rectangular region indicates a read image data region reflected in a first region frequency distribution  1103 , and each black rectangular region indicates a read image data region reflected in a second region frequency distribution  1104 . Assume that the apparatus starts region switching from a white rectangular region. 
     Referring to  FIG. 20A , reference numeral  2001  denotes the timing when the trailing end detection sensor  107  has detected the trailing end of a document page; and  2002 , the amount of read image data input after the detection of the trailing end of the document page. Note that  FIG. 20A  shows a case in which the amount  2002  of read image data input after the detection of the trailing end of the document page is equal to or more than two regions of the set value of the reference region width  1043 , and the number of pixels in the sub-scanning direction from which the influence of the shadow of the leading end of the document page is removed is not 2N times the reference region width  1043 . Reference numeral  2003  denotes the number of pixels in the sub-scanning direction which conforms with the set value of the reference region width  1043 . The first embodiment is configured to perform control to equalize the numbers of white and black rectangular regions reflected in frequency distributions to equalize the sample counts of frequency distributions of a plurality of regions in the sub-scanning direction and acquire no frequency distribution of a region including the shadow of the trailing end of the document page.  FIG. 20B  shows the result. Referring to  FIG. 20B , reference numeral  2004  denotes the boundary between regions reflected in the first region frequency distribution  1103  and the second region frequency distribution  1104 . As shown in  FIG. 20B , the image data (the gray hatched region) input after the boundary  2004  is equal to or more than one region of the set value of the reference region width  1043 . As the set value of the reference region width  1043  increases, the amount of image data which is not reflected in a frequency distribution increases. This may lead to the inability to perform an accurate analysis. 
     Even in the state shown in  FIG. 20A , the third embodiment is configured to equalize the frequency distribution sample counts of a plurality of regions in the sub-scanning direction and increase the amount of image data to be reflected in frequency distributions. Note that the implementation of the third embodiment requires control by the CPU  101  and control by region control unit  109 , and hence the corresponding procedures will be described in detail by using other drawings. 
       FIG. 21  is a flowchart showing a control procedure in the CPU  101  in the third embodiment. At the startup of the apparatus in the embodiment, the CPU  101  performs setting necessary for region control by the frequency distribution generation unit, that is, a leading end effective position  1041 , a trailing end effective position  1042 , the reference region width  1043 , and the trailing end region width  1044  (step S 601 ). Note that since a document size is not determined at the start of document image data reading when an ADF  105  performs reading operation, the CPU  101  sets the trailing end effective position  1042  to the maximum set value which can be set in step S 601 . The CPU  101  then sets the number of pixels read in the sub-scanning direction of the document image data (step S 602 ). Note that like setting of the trailing end effective position  1042  in step S 601 , the CPU  101  sets the number of pixels read in the sub-scanning direction to the maximum set value which can be set in step S 602  because the document size is not determined at the start of reading operation. 
     Upon completing the above setting, the CPU  101  issues a reading operation start instruction (step S 603 ). The CPU  101  does not change the set values set in steps S 601  and S 602  until the trailing end detection sensor  107  detects a trailing end during reading operation for the document image data  603  (NO in step S 604 ). Upon receiving a trailing end detection signal concerning the document image data  603  from the trailing end detection sensor  107  (YES in step S 604 ), the CPU  101  calculates the remaining number of pixels in the sub-scanning direction in accordance with the mechanical arrangement of the image processing apparatus and parameters at the time of operation such as the reading resolution of the document image data  603  (step S 605 ). More specifically, the CPU  101  determines the number of pixels read from the region from the position of the trailing end detection sensor  107  to a reading position  604  in the sub-scanning direction, as a document image size, based on parameters such as a resolution at the time of document reading operation. 
     Upon completing the calculation of the remaining number of pixels read in the sub-scanning direction in step S 605 , the CPU  101  sets the trailing end effective position  1042  of the document image data and the number of pixels read in the sub-scanning direction upon determination of the document size (step S 606 ). That is, the CPU  101  sets a value for the setting target, for which the maximum value which can be set is set in step S 601 , in accordance with the determined document size. Since the trailing end detection sensor  107  has detected the trailing end of the document page, the CPU  101  performs setting to subsequently switch frequency distribution generation region units. That is, the CPU  101  performs setting to switch the set value of the reference region width  1043 , which has been used so far, to the trailing end region width  1044 . The CPU  101  performs this setting by performing setting for the region width switching instruction unit  1045 . Assume that the region width switching instruction unit  1045  before the detection of the trailing end by the trailing end detection sensor  107  is 0. In this case, when the trailing end detection sensor  107  detects a trailing end, the CPU  101  switches the region width switching instruction unit  1045  to 1 (step S 607 ). Although this description is based on the assumption that the reference region width  1043  differs in arrangement from the trailing end region width  1044 , the present invention is not limited to this. The apparatus may have an arrangement configured to include one region width setting holding arrangement and allow the CPU  101  to overwrite the setting as long as the arrangement is guaranteed to be free from any trouble in accurate frequency distribution generation by a control procedure in the CPU  101  or region width switching control. Upon completing region width switching setting for frequency distribution generation to be applied after the detection of the trailing end of a document page, the CPU  101  notifies the completion of setting to indicate the completion of setting of the number of pixels read in the sub-scanning direction upon determination of a document size (step S 608 ). The CPU  101  waits for the notification of the completion of reading of document image data, and terminates the control procedure at the time of receiving the notification (step S 609 ). 
     An operation procedure in the region control unit  109  under the control of the CPU  101  shown in  FIG. 21  will be described next.  FIG. 22  is flowchart showing a control procedure in the region control unit  109 . When the region control unit  109  issues a reading operation start instruction for document image data under the control of the CPU  101  in step S 603 , the reading device  106  starts reading the document image data  603 . The reading I/F control unit  108  converts the image data read by the reading device  106  into digital data and outputs it to the region control unit  109 . At this time, the reading I/F control unit  108  outputs the digitally converted image data, together with the synchronization signal  202 . The region control unit  109  does not start region control until receiving the synchronization signal  202  from the reading I/F control unit  108  (NO in step S 701 ). Upon receiving the synchronization signal  202  from the reading I/F control unit  108  (YES in step S 701 ), a line counter  1091  counts the number of input lines (step S 702 ). If the count value obtained in step S 702  has not reached the set value of the leading end effective position  1041  set by the CPU  101  in step S 601 , the region control unit  109  waits for the input of the next synchronization signal  202  (NO in step S 703 ). The control performed by the region control unit  109  in step S 703  can eliminate the influence of the shadow included in the leading end read image data of the document image data. If the input line count value obtained in step S 702  has reached the set value of the leading end effective position  1041 , the region control unit  109  shifts to the next control (YES in step S 703 ). That is, the region control unit  109  shifts the process to control for making read image data after the input of the synchronization signal  202  be image data to be reflected in frequency distribution generation. The line counter  1093  receives an output signal from the comparison unit  1092  and counts the number of lines every time the synchronization signal  202  is input. The comparison unit  1094  compares the value counted by the line counter  1093 . Since the reference region width  1043  corresponds to the setting of a plurality of pixels in the sub-scanning direction, the count value of the line counter  1093  does not match the set value of the reference region width  1043  at the start of reading operation. Therefore, the apparatus does not perform any specific control by the comparison unit  1094 . On the other hand, after the input of the synchronization signal  202 , an output control unit  1095  outputs the region identification signal  203  and image data  204 , input to the region control unit  109 , to the frequency distribution generation unit  110  (step S 704 ). 
     The comparison unit  1094  compares the count value of the line counter  1093 , which counts every time the synchronization signal  202  is input, with the value of the reference region width  1043 . With this comparison control, the apparatus performs determination control to determine whether image data after the input of the synchronization signal  202  belongs to a first region or second region. More specifically, the apparatus performs control based on the check made by the comparison unit  1094  whether the count value of the line counter  1093  matches the set value of the reference region width  1043  (step S 705 ). If the count value of the line counter  1093  does not match the set value of the reference region width  1043  (NO in step S 705 ), the region control unit  109  waits for the input of the next synchronization signal  202  (NO in step S 706 ). When the next synchronization signal  202  is input (step S 706 ), the line counter  1093  counts the number of input lines (step S 707 ). The output control unit  1095  then outputs read image data after the input of the synchronization signal  202  to the frequency distribution generation unit  110 , together with a region identification signal  203 . The apparatus continuously performs the processing from step S 706  to step S 708  until the count value of the line counter  1093  matches the reference region width  1043  (NO in step S 705 ). 
     If the comparison unit  1094  determines that the count value of the line counter  1093  matches the set value of the reference region width  1043  (YES in step S 705 ), the comparison unit  1094  outputs a signal indicating the match to the line counter  1093 , the region counter  1096 , and the region width switching unit  1097 . Upon receiving the signal, the line counter  1093  initializes the count value to handle subsequently input image data as data belonging to a region different from the immediately preceding input region (step S 709 ). The region counter  1096  receives the signal and counts (step S 710 ). According to this description, since the frequency distributions generated by the frequency distribution generation unit  110  concern two regions in the sub-scanning direction, the region counter  1096  is a 1-bit counter. That is, the region counter  1096  indicates 0 while the line counter  1093  and the comparison unit  1094  handle the read image data of a first region as target data, and indicates 1 while they handle the read image data of a second region as target data. 
     When a value from the region counter  1096  shows a state that the second region frequency distribution  1104  is controlled and the comparison unit  1094  outputs a match signal, the sample count of the first region frequency distribution  1103  is equal to that of the second region frequency distribution  1104 . Therefore, the region width switching unit  1097  checks whether the value indicated by the region counter  1096  shows a state that the second region frequency distribution  1104  is controlled, at the timing when the comparison unit  1094  outputs the match signal. If the value indicated by the region counter  1096  shows the state that the first region frequency distribution  1103  is controlled at the timing when the comparison unit  1094  outputs the match signal (NO in step S 711 ), the apparatus performs the processing from step S 706  to step S 708 . 
     Upon determining that the value indicated by the region counter  1096  controls the second region frequency distribution  1104  at the timing when the comparison unit  1094  outputs a match signal, the region width switching unit  1097  outputs a count value clear signal to the region counter  1096 . Upon receiving the clear signal from the region width switching unit  1097 , the region counter  1096  clears the count value (step S 712 ). The region width switching unit  1097  then checks whether the CPU  101  has performed region width switching setting in step S 607  (step S 713 ). More specifically, the region width switching unit  1097  checks whether the CPU  101  has performed setting for the region width switching instruction unit  1045 . If the CPU  101  has not performed region width switching setting (NO in step S 713 ), the apparatus continuously performs the processing from step S 706  to step S 708  and the processing from step S 705  to step S 712  again. 
     Upon determining in step S 713  that the CPU  101  has performed region width switching setting, that is, the CPU  101  has performed setting for the region width switching instruction unit  1045 , the region width switching unit  1097  shifts to the next control (YES in step S 713 ). That the CPU  101  has performed setting for the region width switching instruction unit  1045  means that the trailing end detection sensor  107  has detected the trailing end of the document page. Therefore, the region width switching unit  1097  performs region control to switch to the second region width, that is, the trailing end region width  1044  (step S 714 ). 
     The region width switching unit  1097  calculates the number of pixels of the remaining read image data in the sub-scanning direction by using the count value of the line counter  1091 , the set value of the leading end effective position  1041 , and the set value of the trailing end effective position  1042  (step S 715 ). The region width switching unit  1097  then calculates the remaining number of regions, which allow to equalize the frequency distribution sample counts of the first region frequency distribution  1103  and second region frequency distribution  1104 , based on the calculated number of pixels in the sub-scanning direction and the set value of the trailing end region width  1044  (step S 716 ). That is, the region width switching unit  1097  obtains the number of regions which allow to equalize the frequency distribution sample counts of the first region frequency distribution  1103  and second region frequency distribution  1104  by using the second region width set to the trailing end region width  1044 . 
     When the region width switching unit  1097  obtains the remaining number of regions in step S 716 , the region control unit  109  waits for the input of the next synchronization signal  202  (step S 717 ). When the region control unit  109  receives the synchronization signal  202 , the line counter  1091  and the line counter  1093  count (step S 718 ). The output control unit  1095  then outputs the read image data input after the input of the synchronization signal  202  to the frequency distribution generation unit  110  (step S 719 ). 
     The comparison unit  1094  compares the second region width, that is, the set value of the trailing end region width  1044 , with the count value of the line counter  1093  to perform region control (step S 720 ). If the count value of the line counter  1093  does not match the trailing end region width  1044  (NO in step S 720 ), the apparatus continuously performs the processing from step S 717  to step S 719 . Upon determining in step S 720  that the set value of the trailing end region width  1044  matches the count value of the line counter  1093 , the comparison unit  1094  outputs a match signal. Upon receiving the match signal, the line counter  1093  initializes the count value (step S 721 ). 
     If the count value of the line counter  1091  is equal to or less than the number of pixels in the sub-scanning direction which corresponds to the remaining number of regions in step S 716 , the region control unit  109  determines that the corresponding data is effective region data, and continuously performs the processing from step S 717  to step S 721  (NO in step S 722 ). If the count value of the line counter  1091  exceeds the number of pixels in the sub-scanning direction which corresponds to the remaining number of regions in step S 716  (YES in step S 722 ), the region control unit  109  determines that the subsequently input read image data is ineffective region data. That is, the region control unit  109  determines, even with the set value of the trailing end region width  1044 , that the subsequently input image data is not image data to be reflected in frequency distribution generation. Subsequently, the region control unit  109  counts the synchronization signal  202  (steps S 723  and S 724 ), and continuously performs the processing in steps S 723  and S 724  until the count value of the line counter  1091  becomes equal to the number of pixels read in the sub-scanning direction which is set in step S 606  executed by the CPU  101  (NO in step S 725 ). If the region control unit  109  determines that the count value of the line counter  1091  matches the number of pixels read in the sub-scanning direction which is set in step S 606  executed by the CPU  101 , the region control unit  109  determines that the document reading operation is complete (YES in step S 725 ). The region control unit  109  then outputs a document reading end notification to the CPU  101  (step S 726 ). 
       FIG. 20C  shows a change in the frequency distribution of the read image data shown in  FIG. 20A  due to the control procedure shown in  FIG. 21  which is executed by the CPU  101  and the control procedure shown in  FIG. 22  which is executed by the region control unit  109 . Referring to  FIG. 20C , reference numeral  2005  denotes the number of pixels in the sub-scanning direction based on the set value of the trailing end region width  1044 ; and  2006 , the boundary between image data reflected in the frequency distribution by the frequency distribution generation unit  110 . In the first embodiment, the read image data input after the position denoted by reference numeral  2004  include many image data (data indicated by the gray hatching) which are not reflected in frequency distributions. In the third embodiment shown in  FIG. 20C , the read image data input after the position denoted by reference numeral  2006  include fewer gray hatched portions. This is the effect obtained by switching the reference region width  1043  as a unit for the generation of frequency distributions to the trailing end region width  1044  for the document trailing end upon detection of the document trailing end by the trailing end detection sensor  107 . That is, the apparatus performs control to minimize read image data which is not reflected in frequency distribution generation by decreasing the set value of the trailing end region width  1044  relative to the set value of the reference region width  1043 . Even in this case, it is possible to equalize the frequency distribution sample counts of the first region frequency distribution  1103  and second region frequency distribution by equally controlling the numbers of white and black rectangular regions complying with the trailing end region width  1044 . 
     Although the above description is based on the assumption that the reference region width  1043  differs from the trailing end region width  1044  in terms of arrangement, the present invention is not limited to this. It is important in the third embodiment to decrease the region width as a reference for frequency distribution generation, after the detection of the trailing end by the trailing end detection sensor  107 , relative to the region width used before the detection of the trailing end. Therefore, the apparatus may perform control to decrease the value of the reference region width  1043  used so far by a predetermined factor after the detection of the trailing end by the trailing end detection sensor  107 . 
     As has been described above, according to the third embodiment, it is possible to further reduce read image data which is not reflected in frequency distributions by switching a reference region width for frequency distribution generation after the detection of the document trailing end in a case in which the frequency distribution sample counts of a plurality of regions in the sub-scanning direction are equalized. 
     [Other Embodiments] 
     Aspects of the present invention can also be realized by a computer of a system or apparatus (or devices such as a CPU or MPU) that reads out and executes a program recorded on a memory device to perform the functions of the above-described embodiment(s), and by a method, the steps of which are performed by a computer of a system or apparatus by, for example, reading out and executing a program recorded on a memory device to perform the functions of the above-described embodiment(s). For this purpose, the program is provided to the computer for example via a network or from a recording medium of various types serving as the memory device (for example, computer-readable medium). 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2012-276117, filed Dec. 18, 2012, which is hereby incorporated by reference herein in its entirety.