Patent Publication Number: US-2013235086-A1

Title: Electronic zoom device, electronic zoom method, and program

Description:
TECHNICAL FIELD 
     The present invention relates to an electronic zoom device, an electronic zoom method, and a program. 
     BACKGROUND ART 
     Capturing devices, such as digital cameras, are often equipped with a zoom function. Although the zoom function is typically an optical zoom which changes the size of a capturing region by adjusting the focal distance of an optical lens, a function called a digital zoom or an electronic zoom is often loaded in devices such as a digital camera in which digital processing is possible. In the electronic zoom, an image acquired by capturing is electronically processed, and a region in the image is extracted so that the region can be expanded and displayed or reduced and displayed. Besides a scaling processing to the center of a screen as achieved in the optical zoom, a scaling processing to an arbitrary position can be performed in the electronic zoom. 
     Such electronic zoom is used when an image in the capturing region is displayed for preview on a display screen at the time of capturing of a digital camera, when an image after capturing is reproduced with any display devices, or the like. The electronic zoom can be used in various display devices such as a photo frame which have a display region expanding function beside the capturing devices such as a digital camera. 
     As techniques related to the electronic zoom, for example, techniques disclosed in Patent Literature 1 and Patent Literature 2 are known. 
     Patent Literature 1 discloses an electronic zoom which makes it possible to automatically follow a target subject even if the subject moves in the photographable region in a capturing device such as a video camera. In particular, the features of images are compared between a plurality of feeding and a motion vector is detected, and electronic zoom processing and the movement of a display region are automatically performed so that the movement and the size of the subject may be followed. 
     It is disclosed in Patent Literature 2 that, when a subject which moves is monitored with a television camera, the motion of the subject is followed automatically and the subject is always expanded and displayed in the center of a screen. In particular, a photographable area is divided into a plurality of regions, the changes of features such as a luminance level or a color for each region are detected in a plurality of fields, and the center position of the zoom and the zoom magnifying factor are adjusted automatically according to the regions where the change is detected. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP-A-6-350897 
     Patent Literature 2: JP-A-11-75103 
     SUMMARY OF INVENTION 
     Technical Problem 
     In recent years, a technique of recognizing various objects contained in an image is developed quickly. Therefore, not only objects such as a person included in the image are recognized, but also it becomes possible to recognize a region of the person&#39;s face, to distinguish between a plurality of persons&#39; faces, and to further recognize small parts of the faces. 
     Therefore, it is also possible to combine the technique of object recognition and the electronic zoom. For example, when a still picture is displayed, it is possible to detect a region where a specific person exists in the still picture, and to control to expand and display (zoom-in) the region so that an expanded image is displayed in the center of a display screen. 
     However, in the related techniques disclosed in Patent Literatures 1 and 2, the object which should be traced is assumed to be only one object and a plurality of objects cannot be traced. Therefore, when a plurality of objects are simultaneously recognized in a still picture, a user has to perform some input operation to choose one of the objects in order to specify the object to be traced. Thus, the operability is insufficient. 
     The present invention is intended to provide an electronic zoom device, an electronic zoom method, and a program which can change the object to be zoomed with high operability even if a plurality of object/regions are recognized within an image to be processed. 
     Solution to Problem 
     An electronic zoom device according to the invention is configured by including: an image display unit which is configured to display an image on a display screen; an object detection unit which is configured to detect a plurality of objects in the image which is displayed on the display screen; a reference position detection unit which is configured to detect reference positions of the objects which are detected by the object detection unit; a ratio calculating unit which is configured to calculate respective ratio parameters that change according to sizes of the objects detected by the object detection unit relative to the display screen; a zoom center position setting unit which is configured to set, as a zoom center position, a reference position of a first object whose ratio parameter calculated by the ratio calculating unit meets a predetermined criterion; and a zoom processing unit which is configured to perform zoom processing of the image displayed on the display screen with a focus on the zoom center position, wherein the zoom center position setting unit changes the zoom center position from the reference position of the first object to a reference position of a second object whose ratio parameter meets the predetermined criterion when the ratio parameter of the first object does not meet the predetermined criterion any more. 
     According to this electronic zoom device, even if a plurality of objects are recognized in an image to be processed, it is possible to change the object for zoom processing with high operability. Therefore, when a user looks at the display screen, a natural zoom processing can be performed. 
     In the electronic zoom device according to the invention, the ratio calculating unit calculates a ratio of an image of an object detected by the object detection unit to the whole display screen as the ratio parameter, and when the zoom center position setting unit has set, as the zoom center position, the reference position of the first object whose image ratio calculated by the ratio calculating unit is less than a predetermined ratio and the image ratio of the first object becomes or more than the predetermined ratio, the zoom center position setting unit changes the zoom center position from the reference position of the first object to the reference position of the second object whose image ratio is less than the predetermined ratio. 
     According to this electronic zoom device, at the time of zoom processing, the object to be zoom-processed can be shifted to a suitable object, according to the ratio of the display size of each of the objects to the whole display screen. 
     In the electronic zoom device according to the invention, the second object is the maximum in image ratio among the objects whose image ratio is less than the predetermined ratio. 
     According to this electronic zoom device, at the time of zoom processing, the object to be zoom-processed can be shifted to a suitable object according to the sizes of the objects with the change of the display size of each of the objects. Therefore, a natural zoom processing can be performed, without a sudden change of the screen display. 
     In the electronic zoom device according to the invention, the reference position of the second object is closest to the center position of the display screen. 
     According to this electronic zoom device, at the time of zoom processing, the object to be zoom-processed can be shifted to an object with a short distance from the display screen central position with the change of the display size of each of the objects. Therefore, a natural zoom processing can be performed, without a sudden change of the screen display. 
     In the electronic zoom device according to the invention, the reference position of the second object is located in a region of the first object. 
     According to this electronic zoom device, at the time of zoom processing, the object to be zoom-processed can be shifted to an object with a short distance from the reference position of the first object with the change of the display size of each of the objects. Therefore, a natural zoom processing can be performed, without a sudden change of the screen display. 
     In the electronic zoom device according to the invention, when a state changes to an undetected state that no objects are detected by the object detection unit, the zoom center position setting unit forbids change of the zoom center position until the state reaches a widest angle state. 
     According to this electronic zoom device, even if a state that no objects are detected is reached, the zoom center position will not be changed suddenly and a natural zoom processing can be performed. 
     In the electronic zoom device according to the invention, when the zoom center position is changed by the zoom center position setting unit, the image display unit indicates that the zoom center position is changed. 
     According to this electronic zoom device, the object to be zoom-processed can be reliably grasped with the visible information displayed. 
     An electronic zoom method according to the invention is an electronic zoom method in an electronic zoom device which displays an image on a display screen, the electronic zoom method including the steps of: detecting a plurality of objects in the image displayed on the display screen; detecting reference positions of the detected objects; calculating respective ratio parameters that change according to sizes of the detected objects relative to the display screen; setting a reference position of a first object whose calculated ratio parameter meets a predetermined criterion as a zoom center position; performing zoom processing of the image displayed on the display screen with a focus on the zoom center position; and changing the zoom center position from the reference position of the first object to a reference position of a second object whose ratio parameter meets the predetermined criterion when the ratio parameter of the first object does not meet the predetermined criterion any more. 
     According to this electronic zoom method, even if a plurality of objects are recognized in an image to be processed, it is possible to change the object for zoom processing with high operability. Therefore, when a user looks at the display screen, a natural zoom processing can be performed. 
     A program according to the invention is a program for causing a computer to execute each step of the electronic zoom method. 
     According to this program, even if a plurality of objects are recognized in an image to be processed, it is possible to change the object for zoom processing with high operability. Therefore, when a user looks at the display screen, a natural zoom processing can be performed. 
     Advantageous Effects of Invention 
     According to the invention, even if a plurality of objects are recognized in an image to be processed, it is possible to change the object to be subject to zoom processing with high operability. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram which shows an example of main components of a digital camera according to an embodiment of the present invention. 
         FIG. 2  is a flow chart which shows an example of a zoom center position control of the digital camera according to the embodiment of the present invention. 
       In  FIG. 3 , (A) to (H) show display examples when the zoom center position control shown in  FIG. 2  is performed. 
         FIG. 4  is a flow chart which shows a variation  1  of the zoom center position control with the digital camera according to the embodiment of the present invention. 
       In  FIG. 5 , (A) to (D) show display examples when the zoom center position control shown in  FIG. 4  is performed. 
         FIG. 6  is a flow chart which shows a variation  2  of the zoom center position control with the digital camera according to the embodiment of the present invention. 
       In  FIG. 7 , (A) to (F) show display examples when the zoom center position control shown in  FIG. 6  is performed. 
         FIG. 8  is a flow chart which shows a variation  3  of the zoom center position control with the digital camera according to the embodiment of the present invention. 
       In  FIG. 9 , (A) to (C) show display examples when the zoom center position control shown in  FIG. 8  is performed. 
     
    
    
     MODES FOR CARRYING OUT INVENTION 
     Hereinafter, an electronic zoom device, an electronic zoom method, and a program according to an embodiment of the present invention are explained in detail with reference to the figures. 
     The electronic zoom device of this embodiment can be considered as a digital camera which captures an image, an image display device which reads, reproduces and displays image data obtained by capturing beforehand from recording media such as a memory card. Here, a digital camera is assumed as an example of the electronic zoom device. 
       FIG. 1  is a block diagram which shows an example of main components of the digital camera according to the embodiment of the present invention. 
     A digital camera  10  shown in  FIG. 1  includes an image capturing unit  11 , an image storage unit  12 , an object detection unit  13 , a reference position detection unit  14 , a ratio calculating unit  15 , a center position changing unit  16 , a zoom control unit  17 , a manipulation unit  18 , and an image display unit  19 . 
     The image capturing unit  11  is provided with an imaging device, such as a CCD imaging sensor, in order to capture a predetermined capturing area including any subjects. Images produced by capturing with the imaging device are outputted to the object detection unit  13  as image data of still pictures such as a photograph, or videos which include a set of a plurality of frames of images. 
     The image storage unit  12  includes a nonvolatile memory storage such as a memory card, and accumulates the image data produced by capturing with the image capturing unit  11  or images which are zoom-processed by the zoom control unit  17  as data of still pictures or videos. 
     The image display unit  19  is formed as a liquid crystal display device or the like. The image display unit  19  is used for the preview display of an image to be captured, and a reproduction display for a user to appreciate the captured image. For example, the image data outputted from the image capturing unit  11  are zoom-processed by the zoom control unit  17  and then inputted into the image display unit  19 . When the image is displayed in real time, the image to be captured can be displayed for preview. Further, when any image data, which are captured beforehand and stored in the image storage unit  12 , are read out from the image storage unit  12 , zoom-processed by the zoom control unit  17 , and inputted into the image display unit  19 , the captured still pictures or videos can be reproduced and displayed. 
     The zoom control unit  17  provides an electronic zoom function which extracts a part of an image to be processed, and performs zoom processing with digital image processing. In the zoom processing, at least one of an expanded display control (zoom-in) and a reduced display control (zoom-out) is performed. That is, the zoom control unit  17  extracts an area which is decided with a zoom magnifying factor from the image to be processed, performs at least one of the expansion and the reduction with a position specified in the region of the image (zoom center position) centered, and outputs. The zoom control unit  17  outputs the image after the zoom processing to the image display unit  19  or the image storage unit  12 . 
     The manipulation unit  18  includes a plurality of operation buttons for receiving input operations from a user. For example, a zoom-in button, a zoom-out button, a capturing button (refer to  FIG. 9 ), and the like are contained in the manipulation unit  18 . With an input operation, the manipulation unit  18  outputs a signal indicating a zoom-in, a zoom-out or the like to the zoom control unit  17 . The manipulation unit  18  may be a touch panel which is arranged on a display screen of the image display unit  19 . 
     The object detection unit  13  recognizes various objects contained in an image by analyzing the contents of the image data from the image capturing unit  11  or the image storage unit  12 , and outputs information on each of the objects to the reference position detection unit  14 . That is, a plurality of objects in the image displayed by the image display unit  19  are detected. In this embodiment, “a plurality of objects” also has the meaning of a plurality of regions of one object. The object detecting function can be realized by using various publicly known image recognition techniques. 
     For example, when persons are included in an image as subjects, the object detection unit  13  can recognize the region of the whole body of each person, the region of the face of each person and the like as an independent object, respectively. When a table in which various kinds of dishes are lined up is contained in an image as a subject, the object detection unit  13  can recognize a region including all the dishes as one object, and also can recognize only a specific dish as one object. 
     The object detection unit  13  outputs area information showing the region of each of the one or more detected objects to the ratio calculating unit  15 . The area information includes information showing the position or the size of a predetermined region such as a rectangle surrounding the detected object. 
     The reference position detection unit  14  detects the reference position of each of the objects detected by the object detection unit  13 . For example, a position near the center of an objective such as the center position of a region surrounding the detected object, or almost the center position of the detected object is detected as a reference position. 
     The ratio calculating unit  15  calculates a ratio parameter Px for each of the objects detected by the object detection unit  13 . The ratio parameter Px changes according to the size of the object detected by the object detection unit  13  relative to the display screen. The ratio of the display size (height or width pixels) of a region surrounding each of the objects displayed on the display screen to the size (height or width pixels) of the whole display screen of the image display unit  19  can be considered as an example of the ratio parameter Px. For example, when the size of the whole display screen is 1000 pixels and the size of an object is 850 pixels, the ratio parameter Px is set to 0.85. When a ratio is calculated, a zoom magnifying factor from the zoom control unit  17  is also considered. The ratio calculating unit  15  outputs information on the calculated ratio to the center position changing unit  16 . The ratio of the area of an object to the area of the whole display screen may be also considered as a ratio parameter. 
     Since the size of the whole display screen of the image display unit  19  is generally constant, a part of the calculation of the ratio calculating unit  15  may be omitted. For example, it is also possible to use the display size of a region surrounding each of the objects as it is as a ratio parameter Px. It is also possible to use the result of subtracting the display size of an object from the size of the whole display screen as a ratio parameter Px. 
     With reference to the ratio parameter Px from the ratio calculating unit  15  for each of the plurality of objects detected by the object detection unit  13 , the center position changing unit  16  determines candidates (candidates for zoom) of objects to be zoom-processed by the zoom control unit  17  (objects for zoom). The center position changing unit  16  determines an object for zoom from the candidates for zoom. The center position changing unit  16  outputs the reference position of the determined object for zoom to the zoom control unit  17  as a center position at the time of performing zoom processing (zoom center position). The object for zoom is changed with the movement of an object or the change of a zoom magnifying factor. 
     That is, the center position changing unit  16  sets the reference position of an object whose ratio parameter Px meets a predetermined criterion (for example, the ratio of the size of the object to the whole display screen is equal to or less than a predetermined ratio Pth), as a zoom center position. For example, when the predetermined ratio Pth is set to 0.9, since the ratio parameter Px, which is 0.85, is smaller than the predetermined ratio, the predetermined criterion is met. When the ratio parameter of an object which has the reference position set as the zoom center position does not meet the predetermined criterion any more, the center position changing unit  16  changes the zoom center position to the reference position of another object which meets the predetermined criterion. 
     Next, a zoom center position control with the digital camera  10  is described. 
       FIG. 2  is a flow chart which shows an example of the zoom center position control with the digital camera  10 . The control shown in  FIG. 2  is performed when an image outputted in real time from the image capturing unit  11  is displayed for preview by the image display unit  19 , when an image read out from the image storage unit  12  is displayed by the image display unit  19 , or the like. 
     In Step S 11 , the zoom control unit  17  initializes a zoom center position. In the initialization, the center position in the display screen of the image display unit  19  is set as the zoom center position. The zoom magnifying factor is assumed as, for example, 1. In the initialization state, the image outputted from the image capturing unit  11  or the image read out from the image storage unit  12  is displayed on the screen of the image display unit  19 , without being zoom-processed. 
     In Step S 12 , the object detection unit  13  identifies whether one or more objects are detected from the image displayed on the display screen of the image display unit  19 , which is the image inputted into the object detection unit  13  from the image capturing unit  11  or the image storage unit  12 . If an object is detected, the flow proceeds to the following step S 13 . 
     In Step S 13 , the object detection unit  13  refers to the information on the detected object. It is identified whether a plurality of objects are detected. When a plurality of objects are detected, in order to specify an object for zoom, the flow proceeds to Step S 14 . On the other hand, when only one object is detected, the object is determined as an object for zoom and the flow proceeds to Step S 16 . 
     In Step S 14 , the center position changing unit  16  identifies whether the condition for determining candidates for zoom is met for each of the plurality of objects detected by the object detection unit  13 . For example, it is identified whether the ratio parameter Px outputted from the ratio calculating unit  15  is equal to or less than a threshold value Pth. The object which meets the condition of “Px&lt;=Pth” is determined as a candidate for zoom. For example, when the threshold value Pth is 0.8, only those objects whose display sizes on the screen are 80% or less of the size of the whole display screen meet the condition of candidates for zoom, and those objects whose display sizes are more than 80% are excluded from the candidates for zoom. 
     In Step S 15 , the center position changing unit  16  determines an object for zoom from the determined candidates for zoom. When there is only one object in the candidates for zoom, the candidate is determined as the object for zoom. When there are a plurality of candidates for zoom, it is preferable that the object which has the largest ratio parameter Px among those candidates is determined as the object for zoom. Thereby, since the ratio that the object which has the greatest Px has occupied in the display screen is larger than other objects, the change of the display screen at the time of zoom processing can be smoother. When there are a plurality of candidates for zoom with an equivalent display size, the object whose reference position is closer to the center of the display screen in those candidates for zoom may be preferentially determined as the object for zoom. Thereby, since the change of the zoom center position is finished with the minimum amount, the change of the display screen at the time of zoom processing can be smoother. 
     In Step S 16 , the reference position detection unit  14  calculates the reference position of the object for zoom determined by the center position changing unit  16 . 
     In Step S 17 , the center position changing unit  16  determines the reference position calculated by the reference position detection unit  14  as a new zoom center position. The information on this zoom center position is given to the zoom control unit  17 . When the new zoom center position is input, the zoom control unit  17  performs zoom processing on the image with the zoom center position as a center according to the specified zoom magnifying factor. The zoom control unit  17  gives the processed image to the image display unit  19 . 
     In Step S 18 , the center position changing unit  16  identifies whether the candidates for zoom are changed. In particular, when a new object is added to the candidates for zoom with the change of the zoom magnifying factor or an object of the existing candidates for zoom does not meet the predetermined condition any more, the candidates for zoom are changed. When there is a change in the candidates for zoom, the flow returns to Step S 13 , and when there is no change, the Step S 18  is performed again. 
     According to the zoom center position control shown in  FIG. 2 , even if a plurality of objects are recognized in the image to be processed, it is possible to change the object for zoom with high operability. That is, even if there are a plurality of candidates for zoom, it is possible to automatically follow a specific object as an object for zoom, without receiving an instruction through the manipulation unit  18 . 
     Next, particular display examples of the image display unit  19  are described. 
       FIG. 3  shows display examples when the zoom center position control shown in  FIG. 2  is performed. 
     In  FIG. 3(A) , a state before the zoom center position control is started is shown, and the whole image outputted from the image capturing unit  11  has appeared on a display screen  31 . In  FIG. 3(A) , one person is included as a subject. In  FIG. 3(A) , the reference position of the whole image displayed by the image display unit  19  is assumed as a zoom center position O. 
     When the preview display of  FIG. 3(A)  is performed, the object detection unit  13  conducts image analysis, and as shown in  FIG. 3(B) , objects  33  and  34  are detected. In  FIG. 3(B) , the object  33  which is equivalent to the whole body of one person  32  is detected as one object, and the object  34  which is equivalent to the region of the person  32 &#39;s face is detected as one object. In the example of  FIG. 3(B) , rectangular frames are drawn by the image display unit  19  so that each of the detected objects may be surrounded. Something instead of a rectangular shape may be drawn, and in particular, such frame does not need to be displayed. 
     In the display of the image in  FIG. 3(B) , the object  33  showing the whole body of the person  32  and the object  34  showing the region of the face become candidates for zoom, respectively, and the object with a larger display size, that is, the object  33  showing the whole body of the person  32  is specified as an object for zoom. As shown in  FIG. 3(C) , the center neighborhood of the region of the object for zoom is detected as a reference position, and this position is specified as a new zoom center position O. 
     For example, if an instruction of zoom-in is given to the zoom control unit  17  by operating the zoom-in button of the manipulation unit  18 , the zoom processing is started. According to the instruction of zoom-in, the zoom magnifying factor becomes larger, and the region containing the object  33  which is the object for zoom is expanded and displayed on the display screen  31 , as shown in  FIGS. 3(D) and 3(E) . Since the reference position of the object  33  which is the object for zoom becomes the zoom center position O, the expanded object  33  is displayed in the center of the display screen  31 , as shown in  FIG. 3(E) . 
     On the other hand, in the state of  FIG. 3(E) , the object  33  which is the object for zoom is expanded to a size which is equivalent to the whole display screen  31 , and it is considered that the condition of Px&lt;=Pth is not met. In this case, the object for zoom does not meet the condition of the candidates for zoom, and an object which meets the condition in other candidates for zoom is chosen as an object for zoom. Here, the object  34  showing the region of the face which meets the condition of the candidates for zoom is chosen as the next object for zoom. 
     As shown in  FIG. 3(F) , the reference position of the object  34  showing the region of the face is detected, and this position is specified as a new zoom center position O. Thus, even if a user does not perform a special operation, the object for zoom is changed into the object  34  with change of the situation, and the zoom center position O is also changed. 
     If the operation of the zoom-in button is repeated, the zoom-in will be performed with the new zoom center position O as a center. Therefore, as shown in  FIGS. 3(G) and 3(H) , the part near the center of the object  34  which is the changed object for zoom moves to the center of the display screen  31 , and is expanded and displayed. 
     Therefore, when there are a plurality of objects that are recognized in the display screen  31 , even if a user does not perform a selecting operation on any one of the objects, an object suitable for display can be chosen automatically and the operability is improved. 
     When the zoom-out button is operated, the zoom magnifying factor becomes smaller, the display size of each of the objects which appear in the display screen  31  will be changed, and a new object may be detected in the display screen  31 . In that case, since the candidates for zoom are changed, the object for zoom or the zoom center position may be changed if necessary. For example, when the state of  FIG. 3(G)  is changed to the state of  FIG. 3(D)  by performing a zoom-out, in  FIG. 3(G) , the object  34  is detected as the object for zoom, and in  FIG. 3(D) , the object  33  is detected as the object for zoom. Thus, even if the ratio parameter continuously meets the predetermined criterion, the object for zoom may be changed to a more suitable object in accordance with change of the situation. Here, it is a decision criterion whether a ratio parameter is the maximum ratio parameter in the objects which is equal to or less than a predetermined ratio. 
     The following scenes can be also assumed. 
     If the person  32  which is a subject moves himself and approaches the image capturing unit  11  after the zoom processing in the state of  FIG. 3(D)  is finished, the person  32  will be displayed on the display screen  31  as shown in  FIG. 3(E) . In the state of  FIG. 3(E) , the object  33  which is equivalent to the whole body of the person  32  is chosen as the object for zoom. However, since the person  32  approaches the image capturing unit  11 , the person  32 &#39;s display size increases to a size which is equivalent to the size of the whole display screen  31 . If the ratio parameter Px of the object  33  which is equivalent to the whole body of the person  32  does not meet the condition of the candidates for zoom, another object except the object  33  will be determined as the object for zoom. Here, the object  34  showing the region of the face smaller than the whole body of the person  32  among the objects detected in the display screen  31  of  FIG. 3(E)  is newly chosen as the object for zoom. Therefore, as shown in  FIG. 3(F) , the reference position of the object  34  is specified as a new zoom center position O. 
     Thus, even if the detected object moves and does not meet the condition of the candidates for zoom, it is possible to follow another object as the object for zoom with high operability, without receiving an instruction through the manipulation unit  18 . 
     (Modification 1) 
     Next, a modification 1 of the zoom center position control with the digital camera  10  is described. 
       FIG. 4  is a flow chart which shows the modification 1 of the zoom center position control with the digital camera  10 . The same numbers are given and shown in  FIG. 4  to the same steps as the steps shown in  FIG. 2 . In the control shown in  FIG. 4 , Step S 14 B differs from Step  14  of  FIG. 2 . 
     In Step S 14 B, the center position changing unit  16  determines the candidates for zoom from the objects in the region of the already determined object for zoom (under selection). The reference position of the candidate for zoom is located in the region of the object for zoom whose reference is already determined. 
       FIG. 5  shows display examples at the time of performing the zoom center position control shown in  FIG. 4 . 
       FIG. 5(A)  is a state before the zoom center position control is started, and the whole image outputted from the image capturing unit  11  has appeared on a display screen  51 . In  FIG. 5(A) , two persons  52  and  53  are included as subjects. In the state of  FIG. 5(A) , an object  54  which is equivalent to the whole body of the right side person  52 , an object  55  which is equivalent to the region of the right side person  52 &#39;s face, and an object  56  which is equivalent to the region of the left side person  53 &#39;s face are detected by the object detection unit  13 , respectively. 
     When the preview display shown in  FIG. 5(A)  is performed, the object  54  which is equivalent to the whole body of the person  52 , the object  55  which is equivalent to the region of the person  52 &#39;s face and the object  56  which is equivalent to the region of the person  53 &#39;s face become candidates for zoom, respectively. In these candidates for zoom, for example, the object  54  which is equivalent to the whole body of the person  52  which is the object with the greatest display size is chosen as the object for zoom. 
     If a zoom-in operation is performed by the manipulation unit  18 , as shown in  FIG. 5(B) , the reference position of the object  54  which is equivalent to the whole body of the person  52  is specified as the zoom center position O, and the object  54  is expanded and displayed with the zoom center position O as a center. 
     As shown in  FIG. 5(A) , after the object  54  which is equivalent to the whole body of the person  52  is determined as the object for zoom, when a zoom-in operation is performed as shown in  FIG. 5(B) , the ratio parameter Px of the object  54  which is equivalent to the whole body of the person  52  does not meet the condition of the candidates for zoom any more. In  FIG. 5(A) , two objects, that is, the object  55  which is equivalent to the region of the person  52 &#39;s face and the object  56  which is equivalent to the region of the person  53 &#39;s face appear on the display screen  51  with a mutually equivalent display size. In this case, by performing the control shown in  FIG. 4 , only the object  52  serves as a candidate for zoom, and is chosen as a new object for zoom. This is because the object  55  which is equivalent to the region of the person  52 &#39;s face is inside the object  54  which is equivalent to the region of the whole body of the person  52  who is the already determined present object for zoom. Therefore, as shown in  FIG. 5(C) , the zoom processing is performed by making the reference position of the object  55  which is equivalent to the region of the person  52 &#39;s face to be the zoom center position O. 
     On the other hand, since the object  56  which is equivalent to the region of the person  53 &#39;s face is located outside the object  54  which is equivalent to the region of the whole body of the person  52 , the object  56  does not serve as a candidate for zoom. Therefore, when the control shown in  FIG. 4  is performed, the zoom processing is not performed by making the object  56  which is equivalent to the region of the person  53 &#39;s face to be the zoom center position O, as shown in  FIG. 5(D) . 
     Thus, in the zoom-in operation, the reference position of the object  55  which is equivalent to the region of the person  52 &#39;s face instead of the object  56  which is equivalent to the region of the person  53 &#39;s face is specified as a zoom center position next to the object  54  which is equivalent to the region of the whole body of the person  52 . Therefore, an unnatural display from the viewpoint of a user, such as a sudden change of the position of the displayed object, will not occur, and a more natural change of the followed object at the time of zoom processing can be seen. 
     (Modification 2) 
     Next, a modification 2 of the zoom center position control with the digital camera  10  is described. 
       FIG. 6  is a flow chart which shows the modification 2 of the zoom center position control with the digital camera  10 . The same numbers are given and shown in  FIG. 6  to the same steps as the steps shown in  FIG. 2 . The control shown in  FIG. 6  differs from that of  FIG. 2  in that Steps S 21  and S 22  are added. 
     In Step S 21 , after Step S 17 , the object detection unit  13  identifies whether no objects in the image displayed by the image display unit  19  are detected. If one or more objects are detected, the flow proceeds to Step S 18 , and if there is no detected object, the flow proceeds to Step S 22 . 
     In Step S 22 , the zoom control unit  17  identifies whether the image displayed by the image display unit  19  is an image which is captured by the image capturing unit  11  in a state of the widest angle (the widest angle state). If the image is captured in the widest angle state, the flow proceeds to Step S 11 , and if the image is not captured in the widest angle state, the flow proceeds to Step S 21 . 
     Therefore, the center position changing unit  16  forbids the change of the zoom center position until the zoom-out is performed to reach the widest angle state, even if the object which has been determined as the object for zoom becomes undetected by the object detection unit  13 . Therefore, it can be seen from the viewpoint of a user that each of the objects is zoom-processed more naturally. If the state reaches the widest angle state, since the processing is resumed from Step S 11 , the zoom center position O will be initialized. 
       FIG. 7  shows display examples at the time of performing the zoom center position control shown in  FIG. 6 . 
     Here, a preview display is performed on a display screen  71  as shown in  FIG. 7(A) , and it is assumed that a person  72  is zoomed in as an object for zoom as shown in  FIG. 7(B) . During the zoom-in of the person  72 , as shown in  FIG. 7(C) , it is assumed that an object  73  which is equivalent to the whole body of the person  72  who is the object for zoom until then becomes undetected by the object detection unit  13  for some reason. 
     In the state that the object for zoom is not detected, if the zoom center position O is temporarily returned to the initial state, that is, if the zoom center position O is suddenly returned to the reference position of the whole image captured by the image capturing unit  11 , and the zoom-in is continued, an expanded image will be displayed with the zoom center position O in the initial state as a center, as shown in  FIG. 7(D) . That is, even if the user of the digital camera  10  is observing the person  72  who is the object for zoom, since other objects are expanded and displayed suddenly, the image may look unnatural. 
     Thus, as shown in  FIG. 7(E) , the zoom center position O is not changed to the reference position of the whole image captured by the image capturing unit  11 , but is maintained to the reference position of the object for zoom which is not detected any more. In this case, if a zoom-in is performed, the zoom magnifying factor will be changed, and an expanded display is performed with the zoom center position O as a center, as shown in  FIG. 7(F) , but the zoom center position O is not changed until the widest angle state is reached. On the other hand, if a zoom-out is continued and the widest angle state is reached, the zoom center position O is changed and initialized as shown in  FIG. 7(G) . 
     Thus, according to the zoom center position control shown in  FIG. 6 , the image displayed on the display screen will not be suddenly changed, and a natural zoom processing is possible. 
     (Modification 3) 
     Next, a modification 3 of the zoom center position control with the digital camera  10  is described. 
       FIG. 8  is a flow chart which shows the modification 3 of the zoom center position control with the digital camera  10 . The same numbers are given and shown in  FIG. 8  to the same steps as the steps shown in  FIG. 2 . The control shown in  FIG. 8  is different from that in  FIG. 2  in that Step S 31  is added after Step S 17 . 
     In Step S 31 , according to instructions of the zoom control unit  17 , the image display unit  19  displays a frame surrounding the determined object for zoom, a zoom-in button, a zoom-out button, a move button, or the like. The shape of the frame may be, for example, a rectangular shape, and may be an elliptical shape or other shapes. The zoom-in button is a button for increasing the zoom magnifying factor by a predetermined magnification. The zoom-out button is a button for decreasing the zoom magnifying factor by a predetermined magnification. The change of the display of the frame surrounding an object for zoom is equivalent to indicating that the zoom center position is changed. 
       FIG. 9  shows display examples at the time of performing the zoom center position control shown in  FIG. 8 . 
     In the examples shown in  FIG. 9 , it is assumed that a part of the manipulation unit  18  is arranged to be put on the display screen  91  of the image display unit  19  as a touch panel. In this case, a user can operate buttons which appears on a display screen  91  to perform input operations. 
     In  FIG. 9(A) , a zoom-in button  95  and a zoom-out button  96  are displayed on the display screen  91 . In  FIG. 9(B) , a move button  97  besides the zoom-in button  95  and the zoom-out button  96  is displayed on the display screen  91 . In  FIG. 9(C) , a move button  98  besides the zoom-in button  95 , the zoom-out button  96  and the move button  97  is displayed on the display screen  91 . 
     As shown in  FIGS. 9(A) and 9(B) , it is assumed that a state that a person  92  does not exist in the display screen  91  is changed to a state that the person  92  exists in the display screen  91 . In this case, an object which is equivalent to the whole body of the person  92  in the display screen  91  is detected as one object, and an object which is equivalent to the region of the person  92 &#39;s face is also detected as one object. 
     Then, as shown in  FIG. 9(B) , frames  93  and  94 , such as a rectangle surrounding each of the objects, are displayed on the display screen  91  as visible information. The zoom control unit  17  changes the display styles (for example, design of an icon) of the zoom-in button  95  and the zoom button  96  so that a user may understand that each of the objects surrounded by the frames  93  and  94  can be chosen as an object for zoom for. The move button  97  is also displayed on the display screen  91 . The move button  97  has a function of giving an instruction of moving at a stretch to the initially determined object for zoom-in order to realize the function of the move button  97 , information on the reference position and the zoom magnifying factor of an object for zoom which is determined initially are saved in the image storage unit  12 . If the move button  97  is pushed, an object which is equivalent to the whole body of the person  92  is determined as the object for zoom, and the object is expanded and displayed in the center of the display screen  91 . 
     In  FIG. 9(C) , since the person  92 &#39;s display size becomes a size more than predetermined and does not meet the condition of the candidates for zoom, the object which is equivalent to the region of the person  92 &#39;s face is determined as the object for zoom-in this case, the move button  98  is displayed on the display screen  91 . The move button  98  has a function of giving an instruction of returning at a stretch to the display of the previously determined object for zoom (object which is equivalent to the whole body of the person  92  here). In order to realize the function of the move button  98 , information on the reference position and the zoom magnifying factor of the previously selected object for zoom are saved in the image storage unit  12 . If the move button  98  is pushed, the object which is equivalent to the whole body of the person  92  is determined as the object for zoom, and the display is changed to the display of the  FIG. 9(B)  in which the zoom processing is performed. 
     Here, although examples are shown in which the frames are displayed, a frame does not need to be displayed when a subject is known even if there is no frame. 
     Thus, according to the zoom center position control shown in  FIG. 8 , a user can recognize the object for zoom or the change of the object for zoom easily. 
     Although the present invention is described in detail with reference to the embodiments, it is apparent that various modifications and amendments may be made by those skilled in the art without departing from the spirit and scope of the invention. 
     The present application is based on Japanese Patent Application No. 2010-052163 filed on Mar. 9, 2010, the contents of which are incorporated herein by reference. 
     INDUSTRIAL APPLICABILITY 
     The present invention is useful in electronic zoom because even if a plurality of objects are recognized in an image to be processed, it is possible to change the object for zoom processing with high operability. 
     REFERENCE SIGNS LIST 
       10 : digital camera 
       11 : image capturing unit 
       12 : image storage unit 
       13 : object detection unit 
       14 : reference position detection unit 
       15 : ratio calculating unit 
       16 : center position changing unit 
       17 : zoom control unit 
       18 : manipulation unit 
       19 : image display unit