Patent Publication Number: US-8125544-B2

Title: Image processing apparatus for extracting quadrangle area in image

Description:
CROSS-REFERENCE TO THE RELATED APPLICATION(S) 
     The present application is based upon and claims priority from prior Japanese Patent Application No. 2008-224709, filed on Sep. 2, 2008, and from prior Japanese Application No. 2009-072298, filed on March 24, 2009, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an image processing apparatus and a computer program for extracting a quadrangle area including a subject contour from an image. 
     BACKGROUND 
     There is known an image capturing devices having a function, which is called a contour quadrangle extracting function, for extracting a quadrangle area including a subject contour from an image to enable image processing such as coordinate conversion on a subject image that is included in a captured image. Such image capturing devices extract the quadrangle area by detecting plural straight lines constituting a subject contour from edge images that include edge pixels and represent the subject contour using Hough transform and then selecting straight lines that form the quadrangle area from the detected plural straight lines. An example of such image capturing devices is disclosed in JP-A-2005-267457. 
     In the conventional image capturing devices, the number of edge pixels located on each of detected straight lines in edge images is calculated and straight lines to form a quadrangle area are selected according to the largeness of the calculated numbers of edge pixels. However, according to this configuration, an inadequate quadrangle area may be extracted in the case where the size of a quadrangle area to be extracted is unknown or plural subject images exist in one image. 
     Furthermore, in the conventional image capturing devices, in the case where a captured image includes plural subject images, plural quadrangle area candidates (hereinafter abbreviated as “rectangle candidates”) are displayed on a display screen in descending order of the evaluation value such as the size of the quadrangle area and the user selects a rectangle candidate to be used for image processing from the plural rectangle candidates being displayed on the display screen. However, in the above configuration of the conventional image capturing devices, the user cannot select a rectangle candidate smoothly because of phenomena that a displayed rectangle candidate is switched between different subject images and that another rectangle candidate is displayed for a subject image for which a rectangle candidate has already been selected. 
     SUMMARY 
     One aspect of the invention provides an image processing apparatus including: a line segment detecting unit configured to detect vertical line segments and horizontal line segments in an image; a facing-lines candidate generating unit configured to generate vertical facing-lines candidates and horizontal facing-lines candidates from the vertical line segments and the horizontal line segments, the vertical facing-lines candidates and horizontal facing-lines candidates being candidates for pairs of facing lines configuring quadrangle areas in the image; a rectangle candidate generating unit configured to generate a plurality of pairs of one of the vertical facing-lines candidates and one of the horizontal facing-lines candidates, and to generate the quadrangle areas as rectangle candidates, the quadrangle areas having intersecting points of each pair of the vertical facing-lines candidates and the horizontal facing-lines candidates as four corner points; and a calculating unit configured to calculate likelihood of each of the rectangle candidates based on a relationship between line segments constituting the vertical facing-lines candidates and the horizontal facing-lines candidates and the rectangle candidates. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A general configuration that implements the various features of the present invention will be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the invention and not to limit the scope of the invention. 
         FIGS. 1A and 1B  are perspective views showing a configuration of a digital camera according to a first embodiment of the invention, wherein  FIG. 1A  is a perspective view mainly showing components provided in the front face, and wherein  FIG. 1B  is a perspective view mainly showing components provided in the back face. 
         FIG. 2  is a block diagram showing a configuration of a control system of the digital camera shown in  FIGS. 1A and 1B . 
         FIG. 3  is a flowchart of a contour quadrangle extraction process according to the first embodiment. 
         FIGS. 4A-4C  show example image data and example edge images extracted by edge image extraction processing in step S 5  of the process shown in  FIG. 3 . 
         FIG. 5  shows the configuration of a Sobel filter which is used at step S 5  of the process shown in  FIG. 3 . 
         FIGS. 6A and 6B  show example edge images as obtained by thinning and binarization of step S 6  of the process shown in  FIG. 3 . 
         FIGS. 7A and 7B  show line example pieces of segment information obtained by labeling processing of step S 7  of the process shown in  FIG. 3 . 
         FIG. 8  is a schematic diagram illustrating line segment division processing of step S 8  of the process shown in  FIG. 3 . 
         FIG. 9  is a schematic diagram illustrating line segment connection processing of step S 9  of the process shown in  FIG. 3 . 
         FIGS. 10A and 10B  show an example horizontal facing-lines candidate and an example vertical facing-lines candidate, respectively, obtained by pairing processing of step S 10  of the process shown in  FIG. 3 . 
         FIG. 11  shows an example rectangle candidate obtained at step S 11  of the process shown in  FIG. 3 . 
         FIG. 12  shows that a perimeter length of the example rectangle candidate shown in  FIG. 11  is calculated at step S 12  of the process shown in  FIG. 3 . 
         FIG. 13  illustrates scoring processing of step S 12  of the process shown in  FIG. 3 . 
         FIGS. 14A-14D  show example rectangle candidates that are arranged imaginarily in descending order of scores calculated at step S 12  of the process shown in  FIG. 3  and displayed one at a time. 
         FIG. 15  shows how a transition is made between rectangle candidates, which are arranged imaginarily in descending order of the score, and the destination rectangle candidate is displayed every time the user makes a operation. 
         FIG. 16  shows a manner of display of rectangle candidates in a case that plural subject images are included in a captured image. 
         FIG. 17  illustrates a flow of a process that subject images are subjected to image processing based on displayed rectangle candidates. 
         FIG. 18  is a flowchart of a grouping process according to a second embodiment of the invention. 
         FIGS. 19A and 19B  show an example rectangle candidate for description of step S 21  of the process shown in  FIG. 18 . 
         FIG. 20A  shows plural example rectangle candidates and  FIG. 20B  shows calculated sets of coordinates of corner points, coordinates of the center of gravity, and perimeter length of each rectangle candidate. 
         FIG. 21  is a flowchart of a rectangle candidate selection process according to the second embodiment. 
         FIG. 22  shows plural example rectangle candidates. 
         FIG. 23  shows a result of grouping of the plural rectangle candidates shown in  FIG. 22  by the coordinates of the center of gravity. 
         FIG. 24  shows a result of grouping of the plural rectangle candidates shown in  FIG. 22  by the x coordinate of the center of gravity and the size. 
         FIG. 25  shows a result of grouping of the plural rectangle candidates shown in  FIG. 22  by the y coordinate of the center of gravity and the size. 
         FIG. 26  shows example state transitions of the selection process. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments according to the present invention will be described in detail with reference to the accompanying drawings. The scope of the claimed invention should not be limited to the examples illustrated in the drawings and those described below. 
     First Embodiment 
     A configuration of a digital camera according to a first embodiment of the invention will be hereinafter described in detail. 
     First, the entire configuration of the digital camera  1  according to the first embodiment of the invention will be described with reference to  FIGS. 1A and 1B . 
     As shown in  FIG. 1A , the digital camera  1  according to the first embodiment of the invention is equipped with an imaging lens  3 , a timer indicator  4 , a finder window  5 , a strobe emission unit  6 , and a microphone  7  in the front face of a generally rectangular, flat-box-shaped body (hereinafter abbreviated as a body)  2 . The top face of the body  2  is provided with a power switch  8  and a shutter button  9  at right-hand positions (as viewed from the user). The imaging lens  3 , with which a zoom function of steplessly varying the focal length and an AF (auto focus) function are realized, is retracted in the body  2  in a power-off state and in a playing mode. The power switch  8  is a switch that is operated to turn on or off the power and the shutter button  9  is a button that is operated to command shooting timing in a recording mode. 
     As shown in  FIG. 1B , a recording mode (R) key  10 , a playing mode (P) key  11 , an electronic view finder (EVF)  12 , a speaker  13 , a macro key  14 , a strobe key  15 , a menu key  16 , a ring key  17 , a set key  18 , and a display unit  19  are provided n the back face of the body  2 . When the recording mode key  10  is operated in a power-off state, the power is turned on automatically and a transition is made to a still image recording mode. On the other hand, when the recording mode key  10  is operated repeatedly in a power-on state, the still image recording mode and a moving image recording mode are set cyclically. In the first embodiment, the still image recording mode includes a single shot mode in which an ordinary shooting operation is performed at a given exposure time and a multi-shot mode in which a subject is shot consecutively at an exposure time that is shorter than in the single shot mode and a single image is generated by combining plural image frames. 
     When the playing mode key  11  is operated in a power-off state, the power is turned on automatically and a transition is made to the playing mode. The EVF  12 , which is an eyepiece-type finder using a liquid crystal display screen, displays a live view image on the liquid crystal display screen in the recording mode and reproduction-displays a selected image, for example, in the playing mode. The macro key  14  is operated to switch between ordinary shooting and macro shooting in the still image recording mode. The strobe key  15  is operated to switch the emission mode of the strobe emission unit  6 . The menu key  16  is operated to perform a selection such as selecting one of various menu items. The ring key  17  is a monolithically formed with upward, downward, rightward, and leftward item selection keys. The set key  18  which is located at the center of the ring key  17  is operated to set a configuration on a currently selected item. 
     The display unit  19 , which is a color liquid crystal display panel with a backlight, monitor-displays a live view image in the recording mode and reproduction-displays a selected image, for example, in the playing mode. A display device other than a liquid crystal display panel may be used as a display unit that replaces the display unit  19 . Although not shown in  FIG. 1A  or  1 B, the bottom face of the digital camera  1  is provided with, for example, a memory card slot in or from which a memory card as a storage medium is inserted or removed, a USB (universal serial bus) connector which is a serial interface connector for connection to an external personal computer or the like. 
     Next, the configurations of an imaging system and a control system of the digital camera  1  according to the first embodiment will be described with reference to  FIGS. 2 and 3 . 
     In the digital camera  1  according to the first embodiment, a CCD  33  is an imaging device disposed behind a lens optical system  32  on its imaging optical axis. The lens optical system  32  constitutes the imaging lens  3  and its focusing position and stop position are moved when it is driven by a motor (M)  31 . In the recording mode, the CCD  33  is scan-driven by a timing generator (TG)  34  and a vertical driver  35  and produces, every predetermined period, a photoelectric conversion output of one frame that corresponds to an optical image formed. The photoelectric conversion output, more specifically, analog signals of respective primary color components (R, G, and B), is gain-adjusted properly, sampled and held by a sample hold circuit (S/H)  36 , and converted by an A/D converter  37  into digital data, which are subjected to color processing including pixel interpolation and γ correction by a color processing circuit  38 . As a result, a digital luminance signal Y and color-difference signals Cb and Cr are generated, which are output to a DMA (direct memory access) controller  39 . 
     The DMA controller  39  writes the luminance signal Y and the color-difference signals Cb and Cr to a buffer that is provided in the DMA controller  39  using a composite sync signal, a memory write enable signal, and a clock signal which are also output from the color processing circuit  38 , and then DMA-transfers those signals to a DRAM  41  (used as a buffer memory) via a DRAM interface (I/F)  40 . A controller  42 , which is composed of a CPU, a ROM in which computer programs to be run by the CPU are stored in a fixed manner, a RAM which is used as a work memory, and other devices, controls operations of the entire digital camera  1 . 
     After completion of the DMA transfer of the luminance signal Y and the color-difference signals Cb and Cr to the DRAM  41 , the controller  42  reads the luminance signal Y and the color-difference signals Cb and Cr from the DRAM  41  via the DRAM I/F  40  and writes those signals to a VRAM  44  via a VRAM controller  43 . A digital video encoder  45  reads the luminance signal Y and the color-difference signals Cb and Cr from the VRAM  44  via the VRAM controller  43  on a regular basis, generates a video signal based on those signals, and outputs the generated video signal to the EVF  12  and the display unit  19 . Each of the EVF  12  and the display unit  19  performs display based on the video signal supplied from the digital video encoder  45  and thereby displays, in real time, an image corresponding to current image information being taken in from the VRAM controller  43 . 
     If the shutter button  9  is operated with timing of intended still image shooting in a state that what is called a live view image is displayed (i.e., as described above a current image is displayed as a monitor image in real time on each of the EVF  12  and the display unit  19 ), a trigger signal is generated. In response to the trigger signal, the controller  42  suspends the DMA transfer, to the DRAM  41 , of a luminance signal Y and color-difference signals Cb and Cr of one frame being taken in from the CCD  33 , acquires a new luminance signal Y and color-difference signals Cb and Cr of one frame by newly driving the CCD  33  with an aperture and a shutter speed that are suitable for proper exposure conditions, and transfers those signals to the DRAM  41 . Then, the controller  42  stops the use of this route and causes a transition to a recording (storing) state. 
     In the recording (storing) state, the controller  42  reads the luminance signal Y and the color-difference signals Cb and Cr individually from the DRAM  41  via the DRAM interface  40 , and writes those signals to an image processor  47 . The image processor  47  compresses the data by ADCT (adaptive discrete cosine transform), Huffman coding which is an entropy coding method, or the like. The resulting code data is read from the image processor  47  and written to one of a memory card  48  which is inserted in a detachable manner as a recording medium of the digital camera  1  and a built-in memory (not shown) which is built in the digital camera  1  in a fixed manner. Upon completion of the compression of the luminance signal Y and the color-difference signals Cb and Cr and writing of all the compressed data to the memory card  48  or the built-in memory, the controller  42  again activates the route from the CCD  33  to the DRAM  41 . 
     A user interface  49 , an audio processor  50 , and a strobe driver  51  are connected to the controller  42 . The user interface  49  is composed of the above-described power switch  8 , shutter button  9 , recording mode key  10 , a playing mode key  11 , macro key  14 , strobe key  15 , menu key  16 , ring key  17 , set key  18 , etc. A signal produced by operating each of those keys is sent directly to the controller  42 . Equipped with a sound source circuit such as a PCM sound source, during recording of a sound the audio processor  50  digitizes an audio signal that is supplied from the microphone  7 , generates an audio data file by compressing the data according to a given data file format, for example, according to the MPS (MPEG-1 audio layer 3) standard, and sends the compressed data to the memory card  48  or the built-in memory. On the other hand, during reproduction of a sound, the audio processor  50  expands an audio data file sent from the memory card  48  or the built-in memory, converting the data into an analog signal, and outputs an amplified sound by driving the speaker (SP)  13 . When performing still image shooting, the strobe driver  51  charges up a strobe large-capacitance capacitor (not shown) and drives the strobe emission unit  6  to flash under the control of the controller  42 . 
     The above-configured digital camera  1  extracts a quadrangle area including a subject contour by performing a contour quadrangle extraction process (described below). How the digital camera  1  operates in performing the contour quadrangle extraction process will be described below with reference to a flowchart shown in  FIG. 3 . 
     Image capturing (shooting) is performed after the user selects such a mode as “shooting of a name card or a document” or “shooting of a white board” from recording modes for respective scenes by operating the ring key  17  and the set key  18 . In these modes, a subject skewing correction is performed. An image taken in one of these modes is taken in by the image processor  47 , and the contour quadrangle extraction process shown in  FIG. 3  is started from step S 1  with timing that it has become executable. The operation to be described below of the digital camera  1  is implemented in such a manner that the CPU of the controller  42  loads a computer program stored in the ROM into the RAM and runs the computer program to control the image processing performed by the image processor  47 . 
     Here, it is assumed that the following image processing including various processing units is performed by the image processor  47  under the control of the controller  42 . Which means that each of the processing units included in the image processing is implemented as a software configuration. However, any one or more of the processing units included in the image processing may be implemented as a hardware configuration, such as by an ASIC. In addition, all of those processing units may not be performed by a single component. Any one or more of the processing units included in the image processing may be performed by a plurality of cooperatively operating components. 
     At step S 1 , the image processor  47  performs distortion correction processing on the input captured image, whereby the captured image which is distorted due to the lens characteristics of the lens optical system  32  is corrected. Step S 1  is thus completed and the contour quadrangle extraction process proceeds to step S 2 . 
     At step S 2 , the image processor  47  reduces the size of the captured image (image size) as subjected to the distortion correction to a given size. More specifically, the image processor  47  calculates a size of the captured image as subjected to the distortion correction and reduces the horizontal and vertical lengths of the captured image based on the calculated size so that the captured image size becomes 320 (horizontal)×240 (vertical) pixels. Step S 2  is thus completed and the contour quadrangle extraction process proceeds to step S 3 . 
     At step S 3 , the image processor  47  converts the representation form of the color information of the captured image from the bit map form into the YUV form (Y: luminance signal, U: difference between the luminance signal and the blue component, V: difference between the luminance signal and the red component) Step S 3  is thus completed and the contour quadrangle extraction process proceeds to step S 4 . 
     At step S 4 , the image processor  47  eliminates noise components from the image data of the captured image by applying a median filter to the image data. The median filter used in the embodiment is such that pixel values of a local area of 3×3 pixels are arranged in ascending order and the pixel value of a center pixel of the arrangement is employed as a pixel value of the center pixel of the area. Step S 4  is thus completed and the contour quadrangle extraction process proceeds to step S 5 . 
     At step  5 , as shown in  FIGS. 4A ,  4 B, and  4 C, the image processor  47  extracts edge images by extracting vertical (X direction) edges and horizontal (Y direction) edges from the noise-components-eliminated image data. In the embodiment, the image processor  47  extracts each of a vertical edge image and a horizontal edge image using a Sobel filter which detects a contour by calculating a spatial first-order derivative (see  FIG. 5 ). Step S 5  is thus completed and the contour quadrangle extraction process proceeds to step S 6 . 
     At step S 6 , as shown in  FIGS. 6A and 6B , the image processor  47  performs thinning and binarization on each of the vertical edge image and the horizontal edge image that were extracted at step S 5 . More specifically, the image processor  47  detects, from the edge pixels included in the vertical edge image, pixels whose x coordinates satisfy a condition (pixel value at x−1)&lt;(pixel value at x)≧(pixel value at x+1). Likewise, the image processor  47  detects, from the edge pixels included in the horizontal edge image, pixels whose y coordinates satisfy a condition (pixel value at y−1)&lt;(pixel value at y)≧(pixel value at y+1). Then, the image processor  47  sets the pixel values of the extracted pixels at the coordinates x or y among the pixels constituting the vertical edge image or the horizontal edge image to “255” and those of the other pixels to “0.” Step S 6  is thus completed and the contour quadrangle extraction process proceeds to step S 7 . 
     At step S 7 , the image processor  47  performs labeling processing on each of the vertical edge image and the horizontal edge image, whereby pieces of vertical line segment information and pieces of horizontal line segment information to constitute a subject contour (see  FIGS. 7A and 7B ) are generated. In the embodiment, for the horizontal edge image, the image processor  47  tries to detect an edge pixel included in the edge image by performing a scan in the X direction from x=0 while referring to pixels that are adjacent in the Y direction. When an edge pixel is detected, the image processor  47  determines whether the detected edge pixel has the pixel value “255” and is connected to other pixels. In a case where the detected edge pixel has the pixel value of “255” but is not connected to any other pixel, the image processor  47  starts tracing a line segment including the detected edge pixel in the X direction. More specifically, the image processor  47  performs tracing on three points (x+1, y−1), (x+1, y), and (x+1, y+1) that are located on the right of the tracing start position (x, y). 
     If one of the following conditions is satisfied, the image processor  47  assigns a unique number to the line segment (i.e., performs labeling) and finishes its tracing. Where the tracing should be continued, the image processor  47  sets the x coordinate of the last-detected edge pixel as a start position of the next tracing. 
     Condition 1: Among the three points, at least one point is already labeled. 
     Condition 2: Among the three points, at least two points are pixels that belong to the edge image. 
     Condition 3: During the tracing, a pixel belonging to the edge image is not detected for two of the three points. 
     On the other hand, for the vertical edge image, the image processor  47  detects an edge pixel included in the edge image by performing a scan in the X direction from y=0 and performs the same processing as the processing performed for the horizontal edge image. Then, for each line segment labeled by the tracing, the image processor  47  calculates, as line segment information, start point coordinates, end point coordinates, a slope (calculated from the start point coordinates and the end point coordinates), an average of errors, from the slope line, of the respective points constituting the line segment (i.e., displacements in the X direction (in the case of a vertical line segment or in the Y direction (in the case of a horizontal line segment), coordinates of a point having a maximum error, and the maximum error. Step S 7  is thus completed and the contour quadrangle extraction process proceeds to step S 8 . 
     At step S 8 , by referring to the pieces of line segment information generated at step S 7 , the image processor  47  determines whether there exists a line segment whose maximum value of errors from the slope line is larger than or equal to a given value. If such a line segment exists, as shown in  FIG. 8  the image processor  47  divides the line segment into two line segments at the maximum-error point (in the example shown in  FIG. 8 , point P). The dividing point may be included in the shorter one of the divisional line segments. The image processor  47  does not divide the line segment if its length is greater than or equal to a first threshold value or the length of a divisional line segment will be shorter than or equal to a second threshold value. The image processor  47  updates the pieces of line segment information if a line segment(s) is divided. Step S 8  is thus completed and the contour quadrangle extraction process proceeds to step S 9 . 
     At step S 9 , the image processor  47  extracts, as connection source line segments, a given number of line segments whose lengths are greater than or equal to a given value (selected in order from the longest one) by referring to the pieces of line segment information that were updated at step S 8  and, as shown in  FIG. 9 , connects each connection source line segment to a line segment (connection destination line segment) that satisfies the following three conditions. Then, the image processor  47  calculates, by the least squares method, start point coordinates and end point coordinates of a line segment formed by connecting each connection source line segment to a connection destination line segment. Step S 9  is thus completed and the contour quadrangle extraction process proceeds to step S 10 . 
     Condition 1: The distance between the connection destination line segment and the connection source line segment is smaller than a given value. 
     Condition 2: The connection source line segment is not completely included in the connection destination line segment. 
     Condition 3: When the connection source line segment is extended from its start point or end point to the connection destination line segment, errors of the extension of the connection source line segment from the positions of its start point and end point are smaller than a given value. 
     At step S 10 , as shown in  FIGS. 10A and 10B , the image processor  47  generates a candidate for a pair of facing edge lines; hereinafter simply referred to as “facing-lines candidate”) of each quadrangle from each of the set of vertical line segments and the set of horizontal line segments as subjected to the division processing of step S 8  and the connection processing of step S 9 . (In the example of  FIGS. 10A and 10B , a pair of line segments H 1  and H 2  constitutes a horizontal facing-lines candidate and a pair of line segments V 1  and V 2  constitutes a vertical facing-lines candidate.) More specifically, the image processor  47  generates, as facing-lines candidates, plural pairs of line segments in each of the vertical direction and the horizontal direction, the line segments of each pair being such that their distance is greater than or equal to a given value and the ratio between their lengths is within a given range (e.g., ⅓ to 3). Step S 10  is thus completed and the contour quadrangle extraction process proceeds to step S 11 . 
     At step S 11 , as shown in  FIG. 11 , the image processor  47  generates combinations from the vertical facing-lines candidates and the horizontal facing-lines candidates generated at step S 10 . Then, for each combination, the image processor  47  calculates four intersecting points of the facing-lines candidates using only the slope information of each line segment. An intersecting point may be set on an extension(s) of a line segment(s), that is, the line segments concerned need not intersect there actually. The image processor  47  then generates plural rectangle candidates S each having calculated four intersecting points as corner points (see  FIG. 12 ). Step S 11  is thus completed and the contour quadrangle extraction process proceeds to step S 12 . 
     At step S 12 , the image processor  47  calculates a perimeter length L 1  of each rectangle candidate S generated at step S 11 . The perimeter length L 1  can be calculated by adding up the distances between the four corner points of the rectangle candidate S. The image processor  47  also calculates, as shown in  FIG. 13 , a total length L 2  of parts of the vertical and horizontal line segments L, the parts being located on circumferential edges of each of the rectangle candidates S. Then, according to the following Expression (1), the image processor  47  calculates, as a score of each rectangle candidate S (likelihood of a quadrangle area), a ratio of the total length L 2  of the line segments L to the perimeter length L 1  of the rectangle candidate S (scoring processing). In Expression (1), the coefficient P means a penalty coefficient for reducing the score of the rectangle candidate S in the case where a line segment has a portion that extends past any of the four corner points of the rectangle candidate S (e.g., portions shown in regions R 1  and R 2  in  FIG. 13  which project from the perimeter of the rectangle candidate S). For example, the penalty coefficient is set at 1.0 if there is no such portion, 0.8 if one such portion exists, and 0.64 if two such portions exist. The values of the penalty coefficient are not limited to these values and various modifications are possible in this respect. For example, where a subject has a fixed shape and its aspect ratio is known, the penalty coefficient may be set smaller (smaller than 1.0) as the calculated aspect ratio deviates more from the known value. Where the perimeter length of a subject is known, the penalty coefficient may be set smaller (smaller than 1.0) as the calculated perimeter length deviates more from the known value. Step S 12  is thus completed and the contour quadrangle extraction process proceeds to step S 13 .
 
score=( L 2/ L 1)×100 ×P   (1)
 
     At step S 13 , for example, as shown in  FIGS. 14A-14D , the image processor  47  arranges rectangle candidates S 1 -S 4  imaginarily in descending order of scores (i.e., degrees of likelihood) calculated at step S 12  and displays them one at a time to be overlapped on the captured image. More specifically, as shown in  FIG. 15 , the image processor  47  displays rectangle candidates, which are arranged imaginarily in descending order of likelihood, on the display unit  19  one at a time (cyclically) every time the user operates the ring key  17 . Although in the example shown in  FIG. 15  the rectangle candidates which are arranged imaginarily in descending order of likelihood are displayed on the display unit  19  one at a time, they may be displayed together on the display unit  19  in different colors according to their scores. 
     A captured image may include plural subject images. In view of this, for example, it is possible to prepare an on/off-switchable plural correction mode and allow the user to decide whether to select from plural rectangle candidates. More specifically, where the plural correction mode is off, the image processor  47  displays rectangle candidates, which are arranged imaginarily in descending order of likelihood (cyclically), on the display unit  19  one at a time every time the user operates the ring key  17  (e.g., in a manner shown in see  FIG. 16 ), performs image processing such as coordinate conversion on the pixels in the area enclosed by a rectangle candidate selected by the user, and finishes the contour quadrangle extraction process. Where the plural correction mode is on, for example, the image processor  47  displays rectangle candidates, which are arranged imaginarily in descending order of likelihood (cyclically), on the display unit  19  one at a time every time the user operates the ring key  17  in a manner shown in  FIG. 17  (sections (a) and (b)), performs image processing such as coordinate conversion on the pixels in the area enclosed by a rectangle candidate selected by the user (section (c) shown in  FIG. 17 ), and renders selectable the rectangle candidates that were not selected by the user so that further image processing can be performed (sections (d), (e), and (f) shown in  FIG. 17 ). This procedure allows the user to perform image correction while sequentially selecting correct rectangle candidates for plural subjects. Step S 13  is thus completed and the contour quadrangle extraction process are finished. 
     As is apparent from the above description, in the contour quadrangle extraction process according to the first embodiment, the image processor  47  detects pieces of vertical line segment information and pieces of horizontal line segment information from a captured image and generates vertical facing-lines candidates and horizontal facing-lines candidates to constitute quadrangle areas from the detected pieces of vertical and horizontal line segment information. Then, the image processor  47  generates plural pairs of a vertical facing-lines candidate and a horizontal facing-lines candidate and generates, for each pair, a quadrangle area (rectangle candidate S) having, as corner points, intersecting points of the vertical facing-lines candidate and the horizontal facing-lines candidate. Then, the image processor  47  calculates, as a score of each rectangle candidate S, a ratio of a total length L 2  of the line segments L constituting the rectangle candidate S to a perimeter length  11  of the rectangle candidate S, and displays the rectangle candidates S one at a time according to the calculated scores to be overlapped on the captured image. As such, the contour quadrangle extraction process can present extracted rectangle candidates S to the user in such a manner that the likelihood of each rectangle candidate S is taken into consideration. Therefore, the digital camera  1  which executes the above-described contour quadrangle extraction process allows the user to select a rectangle candidate S smoothly. 
     Although in the above description the presentation by concurrent display using different colors and the presentation by sequential (cyclic) display are given as examples of the method for presenting plural rectangle candidates S generated by the contour quadrangle extraction process to the user, the method for presenting plural rectangle candidates S to the user is not limited to any particular method. And it is not always necessary to present all rectangle candidates S generated by the contour quadrangle extraction process to the user; for example, the number of rectangle candidates S to be presented to the user may be limited in such a manner that only a given number of rectangle candidates S selected in order from the highest score are presented. In this case, the number of rectangle candidates S presented to the user can be reduced and hence the user&#39;s manipulations of selecting a rectangle candidate S can be prevented from becoming complicated. 
     In the above description, plural rectangle candidates S generated by the contour quadrangle extraction process are presented to the user and the user is thereafter caused to select a rectangle candidate S to be subjected to later image processing. Alternatively, a rectangle candidate S to be subjected to later image processing may be selected automatically according to calculated scores. In this case, it is not necessary to request the user to make a selection operation, whereby a series of steps including the later image processing can be executed smoothly while user manipulations are simplified. 
     The image processing which is performed on the pixels in a selected rectangle candidate S after the contour quadrangle extraction process may be any of various kinds of image processing such as skew correction by coordinate conversion, image extraction, enlargement/reduction, contrast adjustment, and level correction or a combination thereof. 
     Second Embodiment 
     Next, a second embodiment of the invention will be described. In the above-described first embodiment, plural rectangle candidates S are generated at step S 11 , scored at step S 12 , and presented sequentially to the user according to their calculated scores at step S 13 . The second embodiment is different from the first embodiment in that steps S 12  and S 13  of the first embodiment are replaced by a step of grouping the plural rectangle candidates S (described below). 
     The first embodiment is suitable for a case that the number of subject images included in an image, that is, the number of areas in an image each of which is to be selected from rectangle candidates S as an area to be subjected to image processing is one or a few. On the other hand, the second embodiment is suitably applied to an image including a few or more similar subject images. Specific examples of such a case are a case of taking an overview image of a photo album and then extracting each photograph from the image taken and a case of taking a snapshot of a bulletin board having plural memos pinned down and then extracting each memo from the image taken. 
     In a digital camera  1  according to the second embodiment, after plural rectangle candidates S have been generated from one captured image by the part (steps S 1 -S 11 ) of the contour quadrangle extraction process according to the first embodiment, they are grouped according to their sets of coordinates of the centers of gravity (hereinafter may be referred to as “center positions”) and their sizes (sizes of quadrangle areas) by performing a grouping process described below. How the digital camera  1  operates in performing the grouping process will be described below with reference to a flowchart shown in  FIG. 18 . 
     The grouping process shown in  FIG. 18  is started from step S 21  with timing that step S 11  of the above-described contour quadrangle extraction process has completed. The operation to be described below of the digital camera  1  is realized in such a manner that the CPU of the controller  42  loads a computer program stored in the ROM into the RAM and runs it. 
     At step S 21 , the image processor  47  calculates coordinates of the center of gravity of each rectangle candidate S. More specifically, first, the image processor  47  calculates sets of coordinates (Ax, Ay), (Bx, By), (Cx, Cy), and (Dx, Dy) of the corner points of each rectangle candidate S (see  FIGS. 19A and 19B ) and calculates sets of coordinates of the centers of gravity G 1  and G 2  of triangles ABD and BDC constituting the rectangle candidate S according to the following Equations (2) and (3) (see  FIG. 19A ). Then, the image processor  47  calculates sets of coordinates of the centers of gravity G 3  and G 4  of triangles ABC and ACD constituting the rectangle candidate S according to the following Equations (4) and (5) (see  FIG. 19B ). Then, the image processor  47  calculates a straight line connecting the centers of gravity G 1  and G 2  and a straight line connecting the centers of gravity G 3  and G 4  and calculates an intersecting point K (Kx, Ky) of the two straight lines which is given by the following Equation (6) as coordinates of the center of gravity of the rectangle candidates S. More specifically, when five rectangle candidates S 1 -S 5  have been generated from one captured image by the contour quadrangle extraction process (see  FIG. 20A ), the image processor  47  calculates sets of coordinates of the corner points and sets of coordinates of the centers of gravity of each of the rectangle candidates S 1 -S 5  (see  FIG. 20B ). Step S 21  is thus completed and the grouping process proceeds to step S 22 . 
     
       
         
           
             
               
                 
                   
                     
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     At step S 22 , the image processor  47  determines, for the coordinates of the center of gravity of the rectangle candidate S concerned that were calculated at step S 21 , whether there exists a group of such sets of coordinates of centers of gravity that the sum of the absolute values of the differences of the x and y coordinates from those of the rectangle candidate S concerned is smaller than or equal to a given threshold value α. More specifically, for the coordinates (x 1 , y 1 ) of the rectangle candidate S 1 , the image processor  47  determines whether there exists a group of such sets of coordinates (x 2 , y 2 ) of centers of gravity each of which satisfies the inequality |x 1 −x 2 |+|y 1 −y 2 |≦α. If there exists a group of such sets of coordinates of centers of gravity that the sum of the absolute values of the differences of the x and y coordinates from those of the rectangle candidate concerned is smaller than or equal to the given threshold value α (i.e., a group of rectangle candidates whose centers of gravity are close to the center of gravity of the rectangle candidate concerned), at step S 23  the image processor  47  registers the set concerned of the coordinates of the center of gravity in the thus-found group and moves to step S 25 . On the other hand, if there is no group of such sets of coordinates of centers of gravity that the sum of the absolute values of the differences of the x and y coordinates from those of the rectangle candidate concerned is smaller than or equal to the given threshold value α, at step S 24  the image processor  47  generates a new group of sets of coordinates of centers of gravity and registers the set concerned of coordinates of the center of gravity in the generated new group and moves to step S 25 . The grouping methods may be any of various methods. One method is to assign identification information unique to a group to attribute information of each set of coordinates of a center of gravity. 
     At step S 25 , the image processor  47  determines whether step S 22  has been executed for all the sets of coordinates of the centers of gravity calculated at step S 21 . If determined that step S 22  has not been executed yet for all the sets of coordinates of the centers of gravity calculated at step S 21 , the image processor  47  returns to step S 22 . If determined that step S 22  has been executed for all the sets of coordinates of the centers of gravity calculated at step S 21 , the image processor  47  moves to step S 26 . 
     At step S 26 , the image processor  47  calculates a perimeter length L of each rectangle candidate S according to Equation 7. More specifically, when the five rectangle candidates S 1 -S 5  have been generated as shown in  FIG. 20A , the image processor  47  calculates a perimeter length L of each of the rectangle candidates S 1 -S 5 . Step S 26  is thus completed and the grouping process proceeds to step S 27 .
 
 L =√{square root over (( Bx−Ax ) 2 +( By−Ay ) 2 )}{square root over (( Bx−Ax ) 2 +( By−Ay ) 2 )}+√{square root over (( Dx−Bx ) 2 +( Dy−By ) 2 )}{square root over (( Dx−Bx ) 2 +( Dy−By ) 2 )}+√{square root over (( Cx−Dx ) 2 +( Cy−Dy ) 2 )}{square root over (( Cx−Dx ) 2 +( Cy−Dy ) 2 )}+√{square root over (( Ax−Cx ) 2 +( Ay−Cy ) 2 )}{square root over (( Ax−Cx ) 2 +( Ay−Cy ) 2 )}  (7)
 
     At step S 27 , the image processor  47  determines whether the group concerned of sets of coordinates of centers of gravity includes a rectangle candidate S whose perimeter length calculated at step S 26  exceeds a given threshold value β. If determined that the group concerned includes such a rectangle candidate S, at step S 28  the image processor  47  generates a new group of sets of coordinates of centers of gravity (i.e., a group of rectangle candidates that are close to in the coordinates of centers of gravity but different in size from the rectangle candidates of the original group), registers the coordinates of the center of gravity of the rectangle candidate S concerned in the generated new group, and moves to step S 29 . On the other hand, if determined that the group does not include such a rectangle candidate S, the image processor  47  proceeds the process to step S 29 . 
     At step S 29 , the image processor  47  determines whether step S 27  has been executed for all the groups of sets of coordinates of centers of gravity. If determined that step S 27  has not been executed yet for all the groups of sets of coordinates of centers of gravity, the image processor  47  returns to step S 27 . On the other hand, if determined that step S 27  has been executed for all the groups of sets of coordinates of centers of gravity, the image processor  47  finishes the grouping process. 
     As a result of the execution of the grouping process, all the rectangle candidates S included in the one captured image are grouped by the sets of coordinates of the center of gravity (i.e., the center position) and the size. 
     In the above description, a perimeter length L of each rectangle candidate S is calculated and used as information indicating the size of each rectangle candidate S in grouping the rectangle candidates S. Alternatively, the image processor  47  may group rectangle candidates S using, as information indicating the size of each rectangle candidate S, the average Z of the lengths of the four sides of each rectangle candidate S, the internal area of each rectangle candidate S, the average of the lengths of the diagonals of each rectangle candidate S, or the like, instead of the perimeter length L. 
     By performing the following rectangle candidate selection process after completion of the grouping process, the digital camera  1  allows the user to select a rectangle candidate S smoothly even in the case where plural rectangle candidates S exist in one captured image. How the digital camera  1  operates in performing this selection process will be described below with reference to a flowchart shown in  FIG. 21 . 
     The selection process shown in  FIG. 21  is started from step S 31  with timing that the above-described grouping process has completed. The selection process will be described below in a specific manner by using an example that a total of 17 rectangle candidates A, B 1 , B 2 , C 1 , C 2 , D 1 , D 2 , E 1 , E 2 , F 1 , F 2 , G 1 , G 2 , H 1 , H 2 , I 1 , and I 2  have been extracted as shown in  FIG. 22  and grouped by each of combinations from the coordinates (x, y) of the center of gravity and the size z (the average of the lengths of the four sides) as shown in  FIGS. 23-25 . The operation to be described below of the digital camera  1  is realized in such a manner that the CPU of the controller  42  loads a computer program stored in the ROM into the RAM and runs it. 
     At step S 31 , the controller  42  determines whether the ringing key  17  has been operated. The controller  42  proceeds the process to step S 32  with timing that the ringing key  17  has been operated. 
     At step S 32 , the controller  42  highlights a rectangle candidate (correction candidate) selected by the ring key  17  to render the selected rectangle candidate visually recognizable. More specifically, if the largest rectangle candidate A shown in  FIG. 22  has been selected by the ring key  17 , the controller  42  highlights the rectangle candidate A by changing the color of its frame from white to green. Step S 32  is thus completed and the selection process proceeds to step S 33 . 
     At step S 33 , the controller  42  determines whether the user has selected the rectangle candidate, which was selected by the user at step S 32  by pressing the set key  18 , as the correction candidate. If determined that the user has not pressed the set key  18 , the controller  42  returns to step S 31 . If determined that the user has pressed the set key  18 , the controller  42  proceeds the process to step S 34 . 
     In the example shown in  FIG. 22 , if the user makes a downward operation on the ring key  17  instead of operating the set key  18  in a state that the rectangle candidate A is selected, the controller  42  highlights the rectangle candidate B 1  which belongs to the group whose size is closest to the size of the rectangle candidate A (i.e., the group that is closest to the rectangle candidate A in the Z-axis direction) based on the grouping results shown in  FIGS. 24 and 25 . If the user makes a downward operation again on the ring key  17  in a state that the rectangle candidate B 1  is highlighted, the controller  42  highlights the rectangle candidate B 2  which belongs to the same group as the rectangle candidate B 1  in the XY plane. 
     On the other hand, if the user makes a rightward operation on the ring key  17  in a state that the rectangle candidate B 1  is highlighted, the controller  42  highlights the rectangle candidate C 1  which belongs to the same group as the rectangle candidate B 1  in the Z-axis direction and belongs to the group adjacent in clockwise direction on the XY plane (i.e., the group which is close in center position to the group of the rectangle candidate B 1 ). If the user makes a leftward operation on the ring key  17  in a state that the rectangle candidate B 1  is highlighted, the controller  42  highlights the rectangle candidate F 1  which belongs to the same group as the rectangle candidate B 1  in the Z-axis direction and belongs to the group adjacent in counterclockwise direction on the XY plane (i.e., the group which is close in center position to the group of the rectangle candidate B 1 ). 
     If the user makes a rightward operation on the ring key  17  in a state that the rectangle candidate C 1  is highlighted, the controller  42  highlights the rectangle candidate G 1  which belongs to the same group as the rectangle candidate C 1  in the Z-axis direction and belongs to the group adjacent in clockwise direction on the XY plane. In causing a transition from one group to another in the Z-axis direction, the controller  42  highlights a rectangle candidate in the destination group after causing a given offset. This measure is taken because when the user is searching for a rectangle candidate while changing the rectangle candidate size, it is meaningless to highlight a correction candidate that is approximately the same in size as and different in position from the current one. More specifically, after the rectangle candidate B 2 , the controller  42  highlights the rectangle candidate E 1  rather than one of the rectangle candidate C 1 , C 2 , G 1 , and G 2  which are smaller than the rectangle candidate B 2  only a little.  FIG. 26  shows example transitions as described above. 
     At step S 34 , the image processor  47  calculates a projective transformation matrix for a quadrangle area that is the rectangle candidate determined at step S 33 , and generates a quadrangle image by applying the calculated projective transformation matrix to the rectangle candidate and the pixels enclosed by the rectangle candidate. Step S 34  is thus completed and the selection process proceeds to step S 35 . 
     At step S 35 , the image processor  47  erases, from the display, the rectangle candidates corresponding to all the sets of coordinates of centers of gravity included in the group to which the coordinates of the center of gravity of the rectangle candidate determined at step S 33  belong. Step S 35  is thus completed and the selection process proceeds to step S 36 . 
     At step S 36 , the controller  42  determines whether the user has given an instruction to finish the selection process by operating the user interface  49 . If determined that the user has not given an instruction to finish the selection process, the controller  47  returns to step S 31 . On the other hand, if determined that the user has given an instruction to finish the selection process, the controller  42  finishes the selection process. 
     As is apparent from the above description, in the digital camera  1  according to the second embodiment, the image processor  47  groups plural rectangle candidates S by the coordinates of the center of gravity and the size. When a rectangle candidate S to be subjected to image processing is selected from the plural rectangle candidates S, the image processor  47  stops displaying the rectangle candidates S belonging to the same group as the selected rectangle candidate S according to results of the grouping process. This configuration allows the user to select a desired rectangle candidate smoothly even in the case where there exist plural rectangle candidates S that differ from each other only a little in center position or size. 
     Although each of the first and second embodiments is directed to the digital camera, the invention can also be applied to a digital video camera for taking a moving image, an image processing apparatus not having an imaging portion, or the like. That is, the contour quadrangle extraction process according to each of the above embodiments may be executed after an image taken by an external image capturing device is taken in by using a memory card, a USB cable, or the like. In each of the above embodiments, correction by coordinate conversion is performed after rectangle candidates are presented to the user in descending order of scores and one of them is selected by the user. An alternatively procedure is possible in which rectangle candidates are corrected by coordinate conversion in descending order of scores while correction results are presented to the user sequentially and the user selects a most preferable one. 
     It is to be understood that the present invention is not limited to the specific embodiments described above and that the invention can be embodied with the components modified without departing from the spirit and scope of the invention. The invention can be embodied in various forms according to appropriate combinations of the components disclosed in the embodiments described above. For example, some components may be deleted from all of the components shown in each embodiment. Further, components in different embodiments may be used appropriately in combination.