Patent Publication Number: US-7593037-B2

Title: Imaging device and method for capturing image

Description:
RELATED APPLICATION 
     The present disclosure relates to the subject matter contained in Japanese Patent Application No. 2005-252268 filed on Aug. 31, 2005, which is incorporated herein by reference in its entirety. 
     FIELD 
     The present invention relates to an imaging device capable of taking both a motion picture and a high-quality still image. 
     BACKGROUND 
     An imaging device capable of capturing a motion picture of an existing television size and a high-quality still image has recently been proposed. 
     However, as described in JP-A-2005-057378, a related-art imaging device has two systems, each of which consists of capturing unit and buffering unit used for concurrently capturing a motion picture and a still picture. However, this method encounters a problem of an increase in a circuitry scale and a cost hike. 
     In order to solve this problem, JP-A-2005-197910 describes an imaging device that, during capture of a motion picture, records a frame number assigned to the motion picture—which is now in the course of being captured—when a shutter button is pressed to photograph a still image. High resolution operation is performed by means of: taking a frame designated by the frame number as a core frame to be imparted with high resolution; and taking frames straddling the core frame as reference frames used for the high resolution operation. The imaging device enables photographing of a high-resolution still image of a desired moment without involvement of a substantial increase in the amount of processing performed during capture of a motion picture and in circuitry scale. 
     However, according to the above described method, the reference frames, which straddle the core frame, are not optimal, which in turn poses a limitation in operation for imparting high resolution to the still image. 
     As mentioned above, the related art involves a necessity for two systems, each of which consists of capturing unit and buffering unit used for concurrently capturing a motion picture and a still picture, and encounters a problem of a failure to reduce a circuitry scale. Moreover, a frame number assigned to a motion picture—which has been captured at the moment of the shutter button being pressed to capture a still image—is recorded. Subsequently, high resolution operation is performed by means of: taking the frame designated by the frame number as a core frame to be imparted with high resolution; and taking frames straddling the core frame as reference frames used for the high resolution operation. According to this method, restraints are imposed on the reference frames straddling the core frame, and limitations are encountered in imparting high resolution to the core frame. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, there is provided an imaging device including: a capturing unit that captures a motion picture at one of a plurality of frame rates; a buffer that temporary stores the motion picture captured by the capturing unit; a signal generation unit that generates a signal used for specifying a core frame that is to be subjected to a resolution enhancement processing, while the capturing unit captures the motion picture at a first frame rate; a capturing control unit that controls the capturing unit to capture the motion picture, after the core frame specified by the signal, at a second frame rate that is higher than the first frame rate; a reference frame determination unit that determines a frame stored in the buffer at the second frame rate as a reference frame; and an image processing unit that performs the resolution enhancement processing by converting the core frame into a high-resolution frame by reference to the reference frame determined by the reference frame determination unit. 
     According to a second aspect of the invention, there is provided a method for capturing image, including: capturing a motion picture at one of a plurality of frame rates; temporary storing the captured motion picture; generating a signal used for specifying a core frame that is to be subjected to a resolution enhancement processing, while capturing the motion picture at a first frame rate; controlling the capturing of the motion picture to capture frames, after the core frame specified by the signal, at a second frame rate that is higher than the first frame rate; determining a frame temporary stored at the second frame rate as a reference frame; and performing the resolution enhancement processing by converting the core frame into a high-resolution frame by reference to the reference frame. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a block diagram showing a configuration of an imaging device according to a first embodiment; 
         FIG. 2  is a flowchart showing operation of the imaging device according to the first embodiment; 
         FIG. 3  is a block diagram showing a configuration of an imaging device according to a second embodiment; 
         FIG. 4  is a flowchart showing operation of the imaging device according to the second embodiment; 
         FIG. 5  is a block diagram showing a configuration of an imaging device according to a third embodiment; 
         FIG. 6  is a flowchart showing operation of the imaging device according to the third embodiment; 
         FIG. 7  is a block diagram showing a configuration of an imaging device according to a fourth embodiment; 
         FIG. 8  is a flowchart showing operation of the imaging device according to the fourth embodiment; 
         FIG. 9  is a view showing that a motion picture is taken for a given period of time after capture of a core frame at a frame rate which is higher than that employed in a time other than the predetermined time; 
         FIG. 10  is a view showing that a motion picture is taken for a given period of time after capture of a core frame at a frame rate and spatial resolution which are higher than those employed in a time other than the predetermined time; 
         FIG. 11  is a view showing that a motion picture is taken for a given period of time before and after capture of a core frame at a frame rate which is higher than that employed in a time other than the predetermined time; 
         FIG. 12  is a view showing that a motion picture is taken for a given period of time before and after capture of a core frame at a frame rate and spatial resolution which are higher than those employed in a time other than the predetermined time; 
         FIG. 13  is a view showing that a picture is taken for a given period of time before and after capture of a core frame at a frame rate and spatial resolution which are higher than those employed in a time other than the predetermined time, and that respective frames are captured with higher resolution; and 
         FIG. 14  is a view showing that the shutter button remains depressed, and a point in time when a first signal is generated and another point in time when a second signal is generated. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the accompanying drawings, a description will be given in detail of embodiments of the invention. 
     First Embodiment 
       FIG. 1  is a block diagram showing an imaging device according to a first embodiment. 
     The imaging device of the first embodiment includes a capturing unit  101  that captures a motion picture at a plurality of frame rates; a storage unit  102  that stores the captured motion picture; a signal generation unit  103  that generates a signal used for specifying a core frame, which is to be imparted with high resolution, in the motion picture; a capturing control unit  104  that switches the frame rate for capturing the capturing unit  101  in accordance with the signal input from the signal generation unit  103 ; a reference frame determination unit  105  that determines reference frames used for imparting the core frame with high resolution; and a image processing unit  106  that converts the core frame into a high-resolution image through use of the reference frames. 
     The capturing unit  101  captures a motion picture by means of an imaging device such as a CCD (Charge-Coupled Device) element. For instance, the capturing unit captures, at a frame rate of 30 fps in time series, a still image of VGA size (640×480 pixels) into which the NTSC size of an existing television set is digitized. 
     The storage unit  102  stores, in chronological order, the images captured by the capturing unit  101  for capturing a motion picture. The motion picture is stored in a storage medium such as SD memory; an HDD (Hard Disk Drive); a DVD; a tape medium; or the like. The motion picture is usually stored in a data format into which a motion picture is compressed, such as MPEG-2 or MPEG-4. 
     The signal generation unit  103  generates a signal used for specifying a core frame, which is to be imparted with high resolution, in the motion picture. For instance, during the capture of a motion picture, the user generates, as a signal used for specifying a core frame, a frame number assigned to a motion picture captured at the moment of the shutter button  103   a  being pressed for acquiring a still image. 
     The reference frame determination unit  105  determines reference frames, which are used in operation for imparting high resolution to the core frame in the motion picture, from the time-series images stored in the storage unit  102  by means of the signal generated by the signal generation unit  103 . 
     The image processing unit  106  converts the core frame, for which the signal generation unit  103  has generated a signal used for specification, into a high-resolution image through use of the reference frames determined by the reference frame determination unit  105 . 
     Operation of the imaging device according to the first embodiment will now be described by reference to  FIGS. 1 and 2 .  FIG. 2  is a flowchart showing operation of the imaging device according to the first embodiment. 
     The capturing unit  101  starts capturing a motion picture at a first frame rate (step S 201 ). 
     The storage unit  102  stores the captured motion picture (step S 202 ). 
     Next, the signal generating unit  103  determines whether or not the shutter button  103   a  is pressed (step S 203 ). When the shutter button  103   a  is pressed (YES in step S 203 ), the signal generation unit  103  records a frame number assigned to a motion picture, which is captured at the moment of the shutter button  103   a  being pressed to acquire a still image, as a signal used for specifying a core frame which is to be imparted with high resolution in a motion picture (step S 204 ). The capturing unit  101  returns to the first frame rate from when capturing is commenced at a second frame rate—higher than the first frame rate—until a required number of frames is achieved. Processing proceeds to the next processing (step S 205 ). When the shutter button  103   a  is not pressed (NO in step S 203 ), a signal is not generated, and processing proceeds to the next process (step S 205 ). 
     The capturing unit  101  determines whether or not taking of a motion picture is terminated (step S 205 ). When taking of a motion picture is not terminated (NO in step S 205 ), the capturing unit  101  captures the next image (step S 201 ). The storage unit  102  records the captured image (step S 202 ). For instance, a determination as to whether or not taking of a motion picture is terminated may be made on the basis of whether or not the user has issued an instruction for terminating capturing action or whether or not a given period has elapsed since initiation of taking of the motion picture. 
     When taking of a motion picture has been completed (YES in step S 205 ), the reference frame determination unit  105  determines from the storage unit  102  reference frames used for imparting the core frame with high resolution, on the basis of the signal generated by the signal generation unit  103  for specifying the core frame; namely, a frame number assigned to the motion picture captured at the moment of the shutter button  103   a  being pressed to capture a still image (step S 206 ). As to the reference frames, the frames—which have been recorded after the core frame and stored at the second frame rate higher than the first frame rate—are selected as reference frames. Alternatively, one frame taken before the core frame and two frames subsequent to the core frame may be selected; or a larger number of reference frames may be selected as the scaling factor used for imparting high resolution to the core frame is increased. For instance, when the scaling factor used for rendering the resolution of the core frame high is two in both the vertical and horizontal directions, four frames, including the core frame, may be selected. When the scaling factor used for rendering the resolution of the core frame high is three in both the vertical and horizontal directions, nine frames, including the core frame, may be selected. 
       FIG. 9  shows an example where the first frame rate is 1/30 second and the second frame rate is 1/60 second. After the thus-designated core frame, a motion picture is shot at a frame rate, which is higher than the preceding frame rate, within a given period of time. Thus, when the resolution of the core frame is made higher, the scaling factor can be increased. 
     The image processing unit  106  converts the core frame, for which the signal generation unit  103  has generated a signal, into a high-resolution image by use of the reference frames determined by the reference frame determination unit  105  (step S 207 ), and image processing is completed. A method for enhancing resolution is to detect motions between the core frame and the reference frames; and to perform high-resolution operation by reference to the reference frames that have undergone motion compensation on the basis of the motions. A high resolution operation technique based on MAP (Maximization of a posterior probability sequence estimation) method, which is classified into Reconstruction super-resolution processing, will be described hereinbelow. The MAP method is to determine a high-resolution image (hereinafter called an “estimated high-resolution image”) which maximizes a posterior probability while a core frame and reference frames (hereinafter called “observed low-resolution images”) among captured and stored motion picture frames are taken as conditions. The posterior probability is formulated as a cost function, and an optimization problem is resolved by use of the steepest-descent method, the conjugate gradient method, genetic algorithms, or the like, to thus minimize the cost function and estimate a high-resolution image. The cost function for the minimization problem can be computed according to Equation (1). 
     
       
         
           
             
               
                 
                   
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     In the Equation (1), “x” denotes a vector representation of an estimated high-resolution image; “y k ” denotes a vector representation of the k-th observed low-resolution image; “W k ” denotes a matrix representing conversion of an estimated high-resolution image into the k-th observed image (information about motions between a core frame and reference frames, a point spread function of the capturing unit  101 , down-sampling, or the like); “K” denotes the number of observed images; “C” denotes a matrix (generally embodied as a high-pass filter) representing anterior information; “α” denotes a parameter representing the strength of restraint; and “∥ ∥” represents L2 norm. An estimated high-resolution image “x” is computed by means of solving the problem of minimizing the formulated cost function. The method for rendering the resolution of an image high is not limited to the MAP method. All commonly-available methods can be adopted; for instance, the non-uniform interpolation method, the POCS (Projection Onto Convex Sets) method, the back projection method, and the like. 
     As mentioned above, according to the imaging device of the first embodiment, a frame number assigned to a motion picture, which is captured at the moment of a shutter button  103   a  being pressed for capturing a still image, is recorded during capture of a motion picture. Subsequently, the frame designated by the frame number is taken as a core frame to be imparted with high resolution, and frames which have been captured at a higher frame rate are taken as reference frames used for high resolution operation. Thus, high resolution operation is performed. As a result, a high-resolution still image of desired moment can be captured without involvement of a substantial increase in the amount of processing performed during capturing of a motion picture and an increase in circuitry scale. 
     Second Embodiment 
     An imaging device according to a second embodiment will now be described. This imaging device observes a larger volume of information used for rendering the resolution of the core frame high by use of excessive processing capability of the imaging device, to thus render the resolution of the core frame higher. 
       FIG. 3  is a block diagram showing the imaging device according to the second embodiment. 
     As shown in  FIG. 3 , this imaging device differs from its counterpart of the first embodiment in terms of the function of a capturing unit  301  and that of a signal generation unit  303  having been changed. In connection with other configurations and functions,  FIG. 3  is identical with  FIG. 1 , which is a block diagram showing the configuration of the first embodiment, and hence the same reference numerals are assigned to corresponding elements in  FIG. 3 , and their explanations are omitted here. 
     The capturing unit  301  has a first capturing mode and a second capturing mode, and captures a motion picture while switching between the modes. 
     The signal generation unit  303  generates a signal used for specifying a core frame, which is a frame to be subjected to high-resolution operation in the motion picture, and a signal used for effecting switching between the capturing modes of the capturing unit  301 . 
     Operation for effecting switching between the modes of the imaging device of the second embodiment having the above configuration will now be described by reference to  FIGS. 3 and 4 .  FIG. 4  is a flowchart showing operation of the imaging device of the second embodiment. 
     The capturing unit  301  starts capturing a motion picture in the first capturing mode (step S 401 ). The first capturing mode is a mode for taking in time sequence a still image of VGA size (640□480 pixels), to which the NTSC size of the existing television is digitized, at a frame rate of 30 fps; namely, a common mode for capturing a motion picture. 
     Next, the storage unit  102  stores the captured motion picture as in the case of step S 202  (step S 402 ). 
     The signal generation unit  303  determines whether or not the shutter button  103   a  is pressed (step S 403 ). When the shutter button  103   a  is pressed (Yes in step S 403 ), the signal generation unit  303  switches the capturing mode of the capturing unit  301  to the second capturing mode (step S 404 ); records, as a signal used for specifying a core frame whose resolution is to be enhanced in a motion picture, the frame number assigned to the motion picture captured at the moment of the shutter button  103   a  being pressed to capture a still image (step S 405 ); and proceeds to the next process (step S 406 ). When the shutter button  103   a  is not pressed (No in step S 403 ), no signal is generated, and processing proceeds to the next process (step S 406 ). The second capturing mode is a mode for taking an image in the core frame and/or the reference frames at spatial resolution which is higher than the VGA size (640×480 pixels), which is a resolution for another frame. As a result of the capturing mode being switched to the second capturing mode, a larger volume of information used for rendering the resolution of the core frame higher can be observed by use of excessive processing capability of the imaging device. 
       FIG. 10  shows an example first frame rate of 1/30 seconds and an example second frame rate of 1/60 seconds. The core frame and the reference frames are arranged to take motion pictures of spatial resolution which is higher than that used for other frames. During a predetermined period of time after the thus-designated core frame, a motion picture of higher spatial resolution is taken at a frame rate which is higher than that used before the core frame. Thus, a scaling factor can be increased when the resolution of the core frame is enhanced. 
     Motion picture capturing completion determination processing pertaining to step S 406 , reference frame determination processing pertaining to step S 407 , and core frame high resolution operation processing pertaining to step S 408  are analogous to processing pertaining to steps S 205  to S 207 , which pertain to the imaging device of the first embodiment, and their explanations are omitted. 
     As mentioned above, according to the imaging device of the second embodiment, during a given period of the core frame a motion picture is taken at a higher frame rate and at higher spatial resolution than those employed at times other than the period of the core frame, by use of excessive processing capability of the imaging device. As a result, the resolution of the core frame can be further enhanced. 
     Third Embodiment 
     An imaging device having a capturing unit, such as a video camera or the like, is usually equipped with a hand shake compensation function. Initially, the hand shake correction function detects the amounts of hand shakes by means of a method called an image detection method and a method called an angular velocity detection method. Next, hand shakes are corrected on the basis of the amounts of hand shakes by means of an electronic hand shake correction method for compensating for movements in an image by means of image processing or an optical hand shake correction method for taking an image by means of shifting an optical system. However, in relation to the camera equipped with such a hand shake correction function, when high-resolution operation processing is performed after capture of a motion picture, the amounts of hand shakes are detected, and the thus-detected amounts of hand shakes are stored. If a motion picture is stored without involvement of operation for compensating movements of the image by means of the electronic hand shake correction method or the optical hand shake correction method, a larger volume of information which can be utilized for enhancing resolution can be obtained. 
     The imaging device of the third embodiment stores the amounts of hand shakes detected by the video detection method or the angular velocity detection method; and stores a motion picture without performing motion compensation based on the electronic hand shake correction method or the optical hand shake correction method, to thus enable enhance the resolution of the core frame with higher accuracy. 
       FIG. 5  is a block diagram showing an imaging device according to a third embodiment. 
     As shown in  FIG. 5 , the imaging device differs from its counterpart of the first embodiment in that a hand shake detection unit  506  and a hand shake amount storage unit  507  are added to the imaging device of the first embodiment; and in that the function of a reference frame determination unit  504  and that of a image processing unit  505  are changed from those of their counterparts. In connection with other configurations and functions,  FIG. 5  is identical with  FIG. 1 , which is a block diagram showing the configuration of the first embodiment, and hence the same reference numerals are assigned to corresponding elements in  FIG. 5 , and their explanations are omitted here. 
     The hand shake detection unit  506  detects the extent to which the imaging device has become displaced from a reference position due to the hand shakes. 
     The hand shake amount storage unit  507  stores the amounts of hand shakes detected by the hand shake detection unit  506 . 
     The reference frame determination unit  504  determines, from time-series images stored in the storage unit  102 , reference frames used for increasing the resolution of the core frame whose resolution is to be enhanced in the motion picture, by use of the signal generated by the signal generation unit  103  and the amounts of hand shakes stored in the hand shake amount storage unit  507 . 
     The image processing unit  505  converts the core frame, for which the signal generation unit  103  has generated a designation signal, into a high-resolution image, through use of the reference frames determined by the reference frame determination unit  105  and the amounts of hand shakes stored in the hand shake amount storage unit  507 . 
     The high-resolution operation using the amounts of hand shakes will now be described by reference to  FIGS. 5 and 6 .  FIG. 6  is a flowchart showing operation of the imaging device of the third embodiment. 
     First, the capturing unit  101  starts capturing a motion picture at the first frame rate (step S 601 ). 
     The hand shake detection unit  506  detects the amount of displacement from the reference position for the image captured by the capturing unit  101  as amounts of hand shakes (step S 602 ). In relation to the amounts of hand shakes, the amount of displacement, where correlation values between the current frame and the reference frame are maximized, is computed as the quantity of hand shakes according to the video detection method. Alternatively, according to the angular velocity detection method, movements of the imaging device attributable to hand shakes are detected by two angular velocity sensors for vertical and horizontal directions, which are called gyroscopes. 
     The hand shake amount storage unit  507  stores the amounts of hand shakes detected by the hand shake detection unit  506  (step S 603 ). 
     Storage processing pertaining to step S 604 , shutter button  103   a  determination processing pertaining to step S 605 , core frame signal generation processing pertaining to step S 606 , and motion picture capturing completion determination processing pertaining to step S 607  are analogous to processing of the imaging device of the first embodiment pertaining to steps S 202  to S 205 , and hence their explanations are omitted. 
     On the basis of the signal which has originated from the signal generation unit  103  and is used for specifying the core frame and the amounts of hand shakes stored in the hand shake amount storage unit  507 , the reference frame determination unit  504  determines from the storage unit  102  the reference frames used for enhancing the resolution of the core frame (step S 608 ). 
     In general, the amounts of hand shakes between the core frame and the reference frame preferably correspond to one-half a pixel; namely, assume a phase difference of one-half. By means of reference frame determination processing, a frame whose phase difference is close to one-half is selected as a reference frame. However, if a time lag between the core frame and the reference frame becomes greater, the frame assuming a phase difference of one-half will become inappropriate as a reference frame to be used for enhancing resolution, depending on the motion of the subject. Therefore, it is also preferable to select, as a reference frame, a frame whose phase difference is close to one-half within a given period of the core frame. 
     The image processing unit  505  then converts the core frame into a high-resolution image by use of the reference frame determined by the reference frame determination unit  504  and the amounts of hand shakes stored in the hand shake amount storage unit  507  (step S 609 ), and completes image processing. During high-resolution operation processing, the accuracy of the high-resolution image is greatly dependent on the accuracy of a matrix W k , which represents conversion of the estimated high-resolution image into the k-th observed image, as indicated by Equation (1). The matrix W k  is estimated by use of the amounts of hand shakes, which are detected by the hand shake detection unit  506  and recorded in the hand shake amount storage unit  507 , whereby the estimation accuracy of the matrix W k  is increased, and the image quality of the high-resolution image is enhanced, as well. 
     The hand shake detection unit  506  does not detect hand shakes, and the hand shake amount storage unit  507  does not retain the amounts of hand shakes. In short, when high-resolution operation processing is performed after capture of a motion picture in the camera equipped with the hand shake correction function, an image—where a phase difference required for resolution enhancement has arisen—can be observed by means of merely deactivating the hand shake correction function and storing the image having undergone a phase difference in the storage unit  102 , and higher image quality is achieved. 
     As mentioned above, according to the imaging device of the third embodiment, there are stored the amounts of hand shakes which have been detected according to the video detection method or the angular velocity detection method, and the motion picture is stored without involvement of motion compensation based on the electronic hand shake correction method or the optical hand shake correction method. Thus, the resolution of the core frame can be further enhanced. 
     Fourth Embodiment 
     An imaging device according to a fourth embodiment captures information, which is effective for performing resolution enhancing operation prior to the core frame, as a result of the signal generation unit generating a signal when pressing of the shutter button has been started and when the shutter button has been fully pressed (pushed). Subsequently, in the fourth embodiment, a motion picture is described as a time-series image in order to describe operation for enhancing the resolution of the core frame during capture of a motion image and operation for enhancing the resolution of a plurality of still images with use of a still camera. 
       FIG. 7  is a block diagram showing an imaging device according to a fourth embodiment. 
     As shown in  FIG. 7 , the imaging device differs from its counterpart of the first embodiment in that the function of a capturing unit  701  and that of a signal generation unit  703  are changed from those of their counterparts. In connection with other configurations and functions,  FIG. 7  is identical with  FIG. 1 , which is a block diagram showing the configuration of the first embodiment, and hence the same reference numerals are assigned to corresponding elements in  FIG. 7 , and their explanations are omitted here. 
     The capturing unit  701  has a first capturing mode and a second capturing mode, and captures a picture, in time sequence, while effecting switching between the modes. 
     The signal generation unit  703  generates a first signal which is a signal for effecting switching between the capturing modes of the capturing unit  701  and a second signal used for specifying a core frame whose resolution is to be enhanced in the time-series images. 
     Operation of the thus-configured imaging device induced by the first and second signals will now be described by reference to  FIGS. 7 and 8 .  FIG. 8  is a flowchart showing operation of the imaging device according to the fourth embodiment. 
     The capturing unit  701  starts capturing time-series images in the first capturing mode (step S 801 ). In the embodiment, it is assumed that the first capturing mode includes the following three modes. 
     Capturing mode A1, Capturing mode B1: Capturing modes for taking in time sequence a still image of VGA size (640×480 pixels) into which the NTSC size of the existing television is digitized, at a frame rate of 30 fps; and is a mode generally used for capturing a motion picture. 
     Capturing mode C1: Capturing mode where time-series images are not taken. 
     Next, as in step S 402 , the storage unit  102  stores the captured time-series images (step S 802 ). 
     Next, the signal generation unit  703  determines whether or not pressing of the shutter button  703   a  has been started (step S 803 ). At the moment at which pressing of the shutter button  703   a  is started (Yes in step S 803 ), the signal generation unit  703  generates a first signal used for switching the capturing mode of the capturing unit  701 , and switches the capturing mode of the capturing unit  701  to the second capturing mode (step S 804 ). Processing then proceeds to the next process (step S 807 ). In the embodiment, it is assumed that the second capturing mode is a mode for taking a still image of VGA size (640×480 pixels) at the following frame rate. 
     Capturing mode A2: Captures image at a frame rate which is higher than a frame rate of 30 fps employed prior to capture of the still image 
     Capturing mode B2: Captures image at spatial resolution which is higher than the VGA size (640×480 pixels) that is resolution for another frame. 
     Capturing mode C2: Captures time-series images. 
     When the present time is not the moment at which pressing of the shutter button  703   a  has been started (No in step S 803 ); namely, when the user has not yet started pressing the shutter button  703   a  or has already started pressing the shutter button  703   a , processing proceeds to the next process (step S 805 ). A larger volume of information used for enhancing the resolution of a core frame can be observed even in connection with the frames acquired prior to the core frame to be specified later (for which the shutter button  703   a  is fully pressed), by means of switching the capturing mode between the capturing mode A2 that is the second capturing mode and the capturing mode B2, as well as by use of excessive processing capability of the imaging device obtained during capture of a motion picture. By means of switching the capturing mode to the capturing mode C2 that is the second capturing mode, capture of a still image can be started before the shutter button  703   a  is pressed to capture a still image when a still image (a core frame) is taken by a still camera. 
       FIG. 14  shows that the shutter button  703   a  is pressed, as well as showing a point in time when a first signal is generated and another point in time when a second signal is generated. 
     As shown in  FIG. 14 , when force is exerted on the shutter button  703   a , pressing of the shutter button  703   a  is started, and the shutter button  703   a  returns to its original position after the force has been eliminated. When pressing of the shutter button  703   a  is commenced, the first signal is generated. When the shutter button  703   a  has been fully pushed, the second signal is generated. The second signal becomes a signal used for designating the core frame. By means of generation of the first signal, storage of reference frames is commenced at a frame rate which is higher than that achieved earlier. 
     The signal generation unit  703  then determines whether or not the shutter button  703   a  has fully been pushed (step S 805 ). When the shutter button  703   a  has been fully pushed (Yes in step S 805 ), the signal generation unit  703  records a frame number assigned to a time-series image, which has been captured at the moment of the shutter button  703   a  having been fully pressed, as a signal used for specifying a core frame whose resolution is to be enhanced in the time-series image, to thus generate a signal used for again switching the capturing mode of the capturing unit  701  to the first capturing mode after lapse of a given period of time since capture of the core frame (step S 806 ). Processing proceeds to the next process (step S 807 ). When the shutter button  703   a  has not been fully pushed (No in step S 805 ), processing proceeds to the next process (step S 807 ). 
     Image capturing completion determination processing pertaining to step S 807 , reference frame determination processing pertaining to step S 808 , and core frame high-resolution operation processing pertaining to S 809  are analogous to processing of the imaging device of the first embodiment in steps S 406  to S 408 . Hence, their explanations are omitted. 
       FIG. 11  shows an embodiment where the first frame rate is 1/30 second and the second frame rate is 1/60 second. In this embodiment, the core frame and reference frames straddling the core frame are captured at the second frame rate of 1/60 second. Thus, a motion picture is captured at a frame rate which is higher than that employed for a time other than the given period of time before and after the core frame. As a result, the scaling factor can be increased to a greater extent when the resolution of the core frame is enhanced further. 
     As mentioned above, according to the imaging device of the fourth embodiment, the signal generation unit generates a signal when pressing of the shutter button  703   a  is started and when the shutter button  703   a  is fully pressed. By means of acquiring information which has been available prior to the core frame and which is effective for enhancing resolution, the resolution of the core frame can be enhanced further. 
     In the fourth embodiment, as shown in  FIG. 12 , the core frame and the reference frames can be used for capturing a motion picture of higher spatial resolution, in addition to the above-mentioned image, at a frame rate which is higher than that acquired in the time other than the given period of time before and after the core frame. As shown in  FIG. 13 , for a given period of time before and after the frames, only the core frame can also be captured at a frame rate higher than that achieved in the time other than the given time and with higher spatial resolution. 
     As described above, according to the embodiments, there is recorded a frame number assigned to the motion picture captured at the moment of a shutter button being pressed to capture a still image while capturing motion picture. Subsequently, a core frame used for rendering the resolution of the frame designated by the frame number high and subsequent frames or frames straddling the core frame are recorded at a frame rate higher than that used during ordinary capturing operation. These frames are captured as reference frames used for high-resolution operation, and high-resolution operation is performed. As a result, a high-resolution still image of a desired moment can be taken without involvement of a substantial increase in the amount of processing to be performed during capture of a motion picture and an increase in circuitry scale.