Patent Publication Number: US-9420167-B2

Title: Imaging apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Priority is claimed on Japanese Patent Application No. 2010-142656 filed on Jun. 23, 2010, the contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Field of the Invention 
     The invention relates to an imaging apparatus. 
     2. Description of Related Art 
     In the related art, a technique of retrieving a region in an image acquired continuously over time, which has characteristic information similar to characteristic information of a subject selected first to thereby track the position of the subject, is known. For example, Japanese Patent Application Publication No. 2007-272732A discloses a technique which adds a positional correction input from a user to the tracking position of a subject while performing the tracking of the subject. 
     According to the technique, when a plurality of regions having the characteristic Information similar to the characteristic information of the subject selected first is included in the retrieval target image acquired later, an imaging apparatus may extract a plurality of similar regions from the image as tracking positions. Thus, it may be difficult to determine one tracking position, and there is a problem in that a plurality of tracking positions may be displayed alternately. 
     A situation when a concrete problem occurs will be described with reference to  FIGS. 6A and 6B . A case where a train having a uniformly colored front is coming towards a viewer will be described as an example. A photographer selects a focus point on which the photographer wants to focus in advance from a plurality of rectangular regions (called focus points) displayed on a finder screen as shown in  FIG. 7  and stores the information thereof in a RAM of an imaging apparatus. Then, the imaging apparatus can continuously focus on a moving subject by focusing on a subject imaged at the selected focus point. 
     The photographer adjusts the position of a predetermined focus point on a screen while looking at the finder so as to match a position below the front window of the train which is the location of a subject being tracked first. When the two positions match each other, the photographer half-presses a shutter button. While the shutter button is being half-pressed, the imaging apparatus automatically changes the focus point and continuously performs focusing following the movement of the subject. 
     The imaging apparatus stores image characteristic information (for example, color information) as a template within an initial focus detection position  82   a  depicted by a bold frame in  FIG. 6A . The color within the bold frame is divided into 3-by-3 blocks, and the averages of the R, G, and B values within the respective blocks are used as the color information. That is, the image characteristic information within the initial focus detection position  82   a  is expressed as 27 kinds of data (R11, G11, B11) to (R33, G33, B33), for example. The imaging apparatus calculates and tracks a portion similar to the stored template information in the next tracking image. 
     SUMMARY 
     However, when the tracking position is calculated just by extracting a region most similar to the template information through template matching calculation, the region similar to the template information increases in size since the area of the front surface of the train having the same color increases as the train comes closer to the photographer. In this case, since the most similar position changes depending on subtle differences such as the way the light strikes or the like, there is a problem in that the tracking position  82   b  circles around within the position where the front surface of the train has a similar color. Thus, the user may feel that the tracking device is not operating stably. 
     A purpose of aspects of the present invention is to provide an imaging apparatus capable of calculating a tracking position stably. 
     According to an aspect of the invention, an imaging apparatus is provided, the imaging apparatus including an optical system; an imaging unit; an extractor that extracts a part of a first image as a reference image, the first image being received via the imaging unit; a first detector that obtains a first similar image from within the first image, the first similar image having a characteristic similar to a characteristic of the reference image; a calculator that obtains a positional relationship between the first similar image and the reference image; a second detector that obtains a second similar image from within a second image, the second image being received via the imaging unit later than the first image, the second similar image having a characteristic similar to the characteristic of the reference image; and a position detector that detects a position of a corresponding part of the second similar image in accordance with the positional relationship. 
     According to aspects of the present invention, it is possible to calculate the tracking position stably. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a configuration diagram showing a major part of a digital single-lens reflex camera according to an embodiment of the invention. 
         FIG. 2  is a block configuration diagram of a control unit. 
         FIGS. 3A and 3B  are diagrams illustrating a case in which one tracking position is determined when a train is coming toward a viewer according to an embodiment of the invention. 
         FIG. 4  is a flowchart illustrating the flow of a subject tracking process. 
         FIG. 5  is a diagram illustrating a method of calculating a tracking position when two similar regions are extracted. 
         FIGS. 6A and 6B  are diagrams illustrating a case in which one tracking position is not determined when a train is coming toward a viewer in the imaging apparatus of the related art. 
         FIG. 7  is a diagram illustrating focus points displayed on a finder screen. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the invention will be described with reference to the drawings.  FIG. 1  is a configuration diagram of a major part of a digital single-lens reflex camera according to an embodiment of the invention. In  FIG. 1 , a replacement lens  8  is attached to a camera body  10 . The replacement lens  8  is detachably attached to the camera body  10 . 
     The camera body  10  includes a quick return mirror  11 , a focusing glass  12 , a pentagonal prism  13 , a display  14 , an ocular lens  15 , a photometer lens  16 , a sub-image sensor (imaging unit)  17 , an imaging device  18 , a shutter  19 , a sub-mirror  20 , a control unit  21 , a focus detector  22 , a shutter controller  23 , a RAM  24 , and a ROM  25 . 
     The replacement lens  8  includes a photographic lens  1 , a diaphragm  2 , a lens driver  3 , a range detector  4 , and a diaphragm controller  5 . 
     The range detector  4  detects a signal indicating a movement distance of a focus adjustment lens included in the photographic lens  1  using a detection signal detected by a detection device (not shown). When the focus adjustment lens moves to a focus position, a sharp image of a major subject is formed on the imaging device  18  and the sub-image sensor (imaging unit)  17  which are disposed on the camera body  10 . 
     A subject beam entering the camera body  10  after passing through the photographic lens  1  and the diaphragm  2  is reflected upward by the quick return mirror  11  at the illustrated position and imaged on the focusing glass  12  before a release switch is full-pressed. The subject beam is then incident on the pentagonal prism  13 . The incident subject beam is refracted by the pentagonal prism  13  and imaged again on the ocular lens  15 . Moreover, the subject beam is split by the pentagonal prism  13 , and the split subject beam is incident on the photometer lens  16 . The beam incident on the photometer lens  16  is incident on the sub-image sensor (imaging unit)  17 , and the subject image is formed on the sensor. 
     The display unit  14  is also called a finder screen and has a configuration in which a PN liquid crystal layer is stacked on the focusing glass. The display unit  14  displays a focus point. 
     The photometer lens  16  includes an image sensor (for example, a CCD image sensor) which has a plurality of photoelectric conversion elements corresponding to pixels. The sub-image sensor (imaging unit) 17 images the subject image formed on the sensor and outputs a photoelectric conversion signal corresponding to the brightness of the subject image to the control unit  21 . 
     Moreover, part of the subject beam passes through the quick return mirror  11  and is guided downward to the focus detector  22  by the sub-mirror  20 . The focus detector  22  has an image sensor (for example, a CCD image sensor) for detecting the amount of defocus by a phase-difference detection method. The defocus amount detection image sensor images the subject image formed on the image sensor and outputs an analog photoelectric conversion signal corresponding to the brightness of the subject image to the control unit  21 . 
     The quick return mirror  11  rotates to be outside the optical path when the release switch is full-pressed. Then, the subject beam entering the camera body is incident on the photographic imaging device  18  through the shutter  19 , and the subject image is formed on the imaging surface thereof. 
     The imaging device  18  is configured by a photographic image sensor (for example, a CCD image sensor) which has a plurality of photoelectric conversion elements corresponding to pixels. The imaging device  18  images the subject image formed on the imaging surface and outputs a photoelectric conversion signal corresponding to the brightness of the subject image to the control unit  21 . 
     Next, the flow of the processes by the control unit  21  will be described with reference to the block diagram of the control unit  21  in  FIG. 2 . The control unit  21  includes an AF-CCD controller  31 , a focus adjustment controller  32 , a focus adjuster  33 , an A/D converter  34 , an exposure calculator  35 , an extractor  36 , a first detector  37 , a calculator  38 , a second detector  39 , a position detector  40 , an A/D converter  41 , and an image processor  42 . 
     The AF-CCD controller  31  reads the photoelectric conversion signal (stored charge) from the focus detector  22 . Moreover, the AF-CCD controller  31  performs A/D conversion on the read photoelectric conversion signal and supplies the A/D converted signal to the focus adjustment controller  32 . 
     The focus adjustment controller  32  performs AF calculation on the tracking position supplied from the position detector  40  based on the analog photoelectric conversion signal input from the focus detector  22  to the calculated amount of defocus described later and outputs the calculated amount of defocus to the focus adjustor  33 . 
     Specifically, the focus adjustment controller  32  calculates the amount of relative positional (positional relationship) shift (hereinafter referred to as amount of defocus) of two images which are formed at different positions on the sensor and which are formed by a pair of defocus amount detection beams incident through different areas of the photographic lens  1 . The pair of subject images are close to each other in a front-focus state in which the photographic lens  1  forms the sharp image of the subject in front of an intended focal plane. The pair of subject images are separated from each other in a back-focus state in which the photographic lens  1  forms the sharp image of the subject to the back of the intended focal plane. No relative positional shift occurs in the pair of subject images in an in-focus state in which the sharp image of the subject is formed on the intended focal plane. 
     The focus adjustor  33  calculates a lens driving amount based on the focus state (amount of defocus) detected by the focus adjustment controller  32  with respect to the tracking position calculated by the position detector  40  and outputs the calculated lens driving amount to the lens driver  3 . Upon receiving the lens driving amount from the focus adjustor  33 , the lens driver  3  moves the position of the photographic lens  1  in accordance with the lens driving amount. In this way, the focusing of the lens can be adjusted. 
     The A/D converter  34  converts the analog photoelectric conversion signal input from the sub-image sensor (imaging unit)  17  into a digital signal and outputs the photoelectric conversion information to an exposure calculator  35  and an extractor  36 . 
     The exposure calculator  35  performs a predetermined exposure calculation based on the input photoelectric conversion information to determine the aperture value of the diaphragm  2  and the shutter speed of the shutter  19 . Then, the exposure calculator  35  outputs the aperture value to the diaphragm controller  5  so as to change the aperture value of the diaphragm  2 . Moreover, the exposure calculator  35  outputs the shutter speed to the shutter controller  23  so as to open and close the shutter  19  at the shutter speed. 
     The extractor  36  reads a preset focus point from the RAM  24  and determines an initial focus detection position  52   a  shown in  FIG. 3A  based on the photoelectric conversion information of the sub-image sensor (imaging unit) input from the A/D converter  34 . The extractor  36  stores the image information of the initial focus detection position  52   a  as image information for template matching. 
     In the present embodiment, color information is used as image information. Specifically, the extractor  36  divides the color within the bold frame of the focus detection position  52   a  shown in  FIG. 3A  into 3-by-3 blocks and stores the average of the R, G, and B values within the respective blocks in the RAM  24 . 
     In addition, the luminance information of the image and information on designs such as edges may be used as the image information. 
     The first detector  37  performs a computation corresponding to template matching with respect to the entire image of which the template is acquired and calculates the similarity to the template at various positions with respect to all regions of the image while shifting the bold frame by a distance of one block. The first detector  37  calculates the similarity S to the template by summing the absolute values of the differences between the (Rij, Gij, Bij) values (i and j are integers from 1 to 3) at the respective positions and the (Rt, Gt, Bt) values, and the similarity S is expressed by Expression (1) below. 
     
       
         
           
             
               
                 
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     The first detector  37  calculates a region similar to the template in the image of which the template is acquired. In the following description, the distribution of similarities S at respective positions will be referred to as a similarity map. The first detector  37  sorts the values of the similarity map in ascending order. In this case, since the absolute sum of the differences is calculated in the same image as the image of which the template is acquired, the smallest value of the similarity S is 0 at the position where the template is acquired. 
     The first detector  37  creates a binarized similarity map in which the value in each block of the similarity map is binarized. That is, 1 is assigned to a region in which the value of the similarity map is smaller than a predetermined threshold and 0 is assigned to the other regions. Moreover, the first detector  37  calculates a binarized similar region (first similar region) which is occupied by blocks having the value of 1 in the image, and calculates the horizontal and vertical widths of the similar region. 
     In addition, the binarized similar region may be not only a region larger than the region of the template, but also a region which has the highest similarity in respect to the template within a same size as the region of the template among the regions in which the value of the similarity map is smaller than the predetermined threshold. 
     The calculator  38  calculates the relative position (positional relationship) of the template acquisition position within the binarized similar region and stores the relative position (positional relationship) in the RAM  24 . This position will be referred to as an initial relative position. 
     For example, the calculator  38  calculates the relative position of the template acquisition position in a similar region  53   a  in  FIG. 3A  in terms of its horizontal and vertical proportional positions from left and top of the similar region  53   a  and stores the calculated values in the RAM  24 . In  FIG. 3A , since the relative position (positional relationship) of the template acquisition position is 25% from left and 50% from top, the calculator  38  stores the information of the initial relative position (positional relationship), which is 25% from left and 50% from top, in the RAM  24 . 
     In addition, before calculating the relative position (positional relationship), the calculator  38  may perform the known image processing such as expansion and reduction on the binarized similarity map to remove noise regions and then calculate the relative position (positional relationship). 
     The second detector  39  reads initial image information (in the present embodiment, it uses the averages of the R, G, and B values in the respective blocks at the initial focus detection position) from the RAM  24  and calculates a similarity map similarly to the process performed by the first detector  37 . Then, the second detector  39  calculates a binarized similarity map from the calculated similarity map and determines a similar region (second similar region). 
     The second detector  39  performs template matching on the tracking image information which is acquired repeatedly. After that, the second detector  39  calculates the horizontal and vertical widths of the similar region and outputs the calculated values to the position detector  40 . 
     In addition, the similar region may be not only a region larger than the region of the template, but also a region which has the highest similarity in respect to the template within a same size as the region of the template among the regions in which the value of the similarity map is smaller than the predetermined threshold. 
     The position detector  40  reads the initial relative position (positional relationship) from the RAM  24  and applies the initial relative position (positional relationship) to the horizontal and vertical widths of the second similar region calculated by the second detector  39  to thereby determine the tracking position which is a stored absolute position. 
     Specifically, for example, the initial relative position (positional relationship) which is 25% from left and 50% from top with respect to the horizontal and vertical width of the similar region  53   b  in  FIG. 3B  is displayed as a rectangular frame on the display unit  14  as the final tracking position. 
     The position detector  40  outputs the tracking position to the focus adjustment controller  32 . 
     The A/D converter  41  converts the analog image signal input from the imaging device  18  into a digital signal and outputs the digital image signal to the image processor  42 . 
     The image processor  42  performs image processing and stores image data in the RAM  24 . 
     Respective control programs of the control unit  21  are stored in the ROM  25 . 
     Next, the flow of a subject tracking process will be described with reference to the flowchart of  FIG. 4 . First, when a user half-presses a release button (shutter button), a sub-image sensor (imaging unit)  17  images a subject, and the focus detector  22  receives the subject beam (step S 601 ). Subsequently, the extractor  36  calculates the initial focus detection position from the predetermined focus point (step S 602 ). Subsequently, the extractor  36  stores the image information of the initial focus detection position in the RAM  24  (step S 603 ), and the first detector  37  and the calculator  38  calculates the initial similar region and the initial relative position of the initial similar region, and stores the calculated information in the RAM  24  (step S 604 ). 
     Subsequently, the sub-image sensor (imaging unit)  17  images a subject again and the focus detector  22  receives the subject beam (step S 605 ). Subsequently, the second detector  39  (similar region calculator) reads the image information of the initial focus position from the RAM  24  as a template, performs the computation corresponding to template matching on the image information obtained presently to calculate a similar region, and calculates the horizontal and vertical widths of the similar region (step S 606 ). Subsequently, the position detector  40  applies the initial relative position (positional relationship) read from the RAM  24  to the calculated horizontal and vertical widths, calculates a tracking position, displays the initial relative position on the display unit  14 , and outputs the tracking position to the focus adjustment controller  32  (step S 607 ). The focus adjustment controller  32  calculates a focus detection value (amount of defocus) with respect to the calculated tracking position and outputs the focus detection value to the focus adjustor  33  (step S 608 ). 
     In this way, the user can keep track of an intended position of the train while preventing the tracking result from running unstably within the similar region which can occur when the computation according to the invention is not applied. 
     The focus adjustor  33  calculates a lens driving amount based on the calculated focus detection value, drives the lenses included in the photographic lens  1 , and adjusts the distance between the lenses to thereby adjust the focusing (step S 609 ). The processes of steps S 605  to S 609  are repeated (step S 610 : NO) until the half-press is released, and the process ends when the half-press is released (step S 610 : YES), 
     Through the above process, even when a number of tracking position candidates having the same characteristics (color, luminance, or design) as the first target subject are present in the tracking screen, the imaging apparatus can keep track of the first focusing position as intended by the user while preventing the tracking position from being frequently changed between a plurality of candidates. 
     In addition, two or more binarized similar regions may be present on an image. In this case, the second detector  39  unifies the two or more binarized similar regions and the position detector  40  calculates the tracking position from the initial relative position (positional relationship) with respect to the horizontal and vertical widths of the unified similar region. 
     Specifically, for example, the second detector  39  sets a similar region  74  so as to include two similar regions  72  and  73  shown in  FIG. 5  and outputs the similar region  74  to the position detector  40 . Upon receiving the unified similar region  74  from the second detector  39 , the position detector  40  applies the initial relative position (positional relationship) (40% from left and 50% from top) to the horizontal and vertical widths of the unified similar region  74  to calculate a tracking position  71  which is an absolute position. 
     The second detector  39  calculates the actual size of the initial similar region (first similar region) from an imaging magnification and calculates the actual size of a similar region (second similar region) which is calculated subsequently from an image magnification used subsequently. When the actual size of the second similar region is not within a predetermined range of magnification of the first similar region, the extraction of the second similar region may be regarded as an error. By doing so, even when an object having information similar to the image information of the initial focus detection position appears suddenly, it is possible to exclude an image region including the object from the tracking position based on the size of the similar region. 
     Moreover, the position detector  40  may update the template with the image information at the tracking position whenever the tracking position is calculated. 
     Furthermore, when the second similar region calculated by the second detector  39  includes a plurality of regions, the focus adjustment controller  32  may calculate the distances of each region and exclude the regions of which the distance is outside a predetermined range from the second similar region. In this way, regions which are erroneously extracted as the similar region can be excluded from the similar region based on the distance to the subject. 
     The focus adjustor  33  may stop adjusting the focusing of the optical system when the focus state (defocus amount) detected repeatedly by the focus adjustment controller  32  lacks continuity. Specifically, the focus adjustor  33  controls the lens driver  3  so as to temporarily halt the movement of lenses. In this way, even when an object having information similar to the image information of the initial focus detection position appears suddenly, it is possible to exclude the object from the tracking position based on the distance to the object. 
     In the present embodiment, although the tracking position is displayed on the finder screen (the display unit  14 ) in the case of the digital single-lens reflex camera, the tracking position may be displayed on the back liquid-crystal display of a camera in the case of a compact digital camera. 
     While the embodiment of the invention has been described with reference to the drawings, the specific configuration is not limited to the embodiment, and various modifications can be made without departing from the spirit of the invention.