Abstract:
An imaging device is provided that includes an image blur evaluator, an imaging processor, an edge contour extraction processor, an image divider, and a distance information detector. The image blur evaluator evaluates the amount of blurring in an image. The imaging processor captures secondary images of the same object at different lens positions. The edge contour extraction processor extracts edge contour components of the secondary images and creates an edge-contour extracted image for each of the secondary images. The image divider divides each edge-contour extracted image into a plurality of relatively large blocks when blurring is evaluated to be relatively substantial, and into relatively small blocks when the blurring is evaluated to be moderate. The distance information detector detects distance information of the objects captured in each block based on the contrast.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an imaging device that electronically captures an image through a lens system. 
     2. Description of the Related Art 
     In photography, a bokeh or blurring technique using an effect of the depth of field is often used to blur a background and/or foreground view that does not include the main subject, which is in focus. However, the depth of field deepens as the focal length is reduced; therefore the bokeh effect in a compact digital camera with a relatively small image sensor is ineffective because such a camera requires a short focal length. 
     For the above-mentioned problem, there is provided a method that determines in-focus areas and out-of-focus areas of a captured image to produce an artificial bokeh effect by applying a blurring process to the images within the out-of-focus areas. In this method, a captured image is divided into a plurality of areas and the in-focus areas and the out-of-focus areas are determined by evaluating the spatial frequencies of each area. Namely, an area with high spatial frequencies is determined to be an in-focus area and an area with low spatial frequencies is determined to be an out-of-focus area. This determination is based on the fact that the in-focus areas generally have high contrast (refer to U.S. Pat. No. 7,454,134). 
     SUMMARY OF THE INVENTION 
     However, the spatial frequencies of an image also depend upon the texture of a subject, and therefore high spatial frequencies in an image do not always mean that the image is in focus. For example, an image of a forest, a tiled roof and the like have high frequencies by nature. On the other hand, when regarding a subject such as human skin, the spatial frequencies can be lower than those of out-of-focus areas even though the image is in focus. Therefore, in order to obtain an appropriate artificial bokeh effect, the blurring process may preferably be performed on the basis of distances in the optical direction from the camera lens to objects in each of the areas that constitute the image. 
     Therefore, one aspect of the present invention is to provide an imaging device and method that is able to accurately distinguish distances in the optical direction between objects captured in each of the areas constituting an image. 
     According to the present invention, an imaging device is provided that includes an image blur evaluator, an imaging processor, an edge contour extraction processor, an image divider, and a distance information detector. The image blur evaluator evaluates the amount of blurring due to a camera shake. The imaging processor captures a plurality of secondary images of the same object at different lens positions by driving a photographing lens. The edge contour extraction processor extracts edge contour components of the secondary images and creates an edge contour extracted image for each of the secondary images. The image divider divides each edge contour extracted image into a plurality of relatively large blocks when the amount of the blurring is evaluated as relatively strong, and into relatively small blocks when the amount of the blurring is evaluated as moderate, where each block includes at least one pixel. The distance information detector detects distance information of the objects captured in each block based on the contrast in each of the blocks. 
     In another aspect of the invention, a method for detecting distance information is provided. The method involves evaluating the amount of blurring due to a camera shake; capturing a plurality of secondary images of the same object at different lens positions by driving a photographing lens; extracting edge contour components of the secondary images and creating an edge contour extracted image for each of the secondary images; dividing each edge contour extracted image into a plurality of relatively large blocks when the amount of the blurring is evaluated as relatively strong and into relatively small blocks when the amount of the blurring is evaluated as moderate, with each block including at least one pixel; and detecting distance information of the objects captured in each block based on the contrast in each of the blocks. 
     In another aspect of the invention, a computer readable medium having instructions for controlling a computer to execute the above method is provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects and advantages of the present invention will be better understood from the following description with references to the accompanying drawings in which: 
         FIG. 1  is a block diagram showing the structure of a digital camera of a first embodiment; 
         FIG. 2  is a flowchart of interrupt processing repeatedly carried out in a bokeh control mode; 
         FIGS. 3A and 3B  are the flowcharts of a distance map preparation process; 
         FIG. 4  schematically illustrates how a contrast map is created; 
         FIG. 5  schematically illustrates how contrast values are scanned from contrast maps; 
         FIG. 6  schematically illustrates the relationship between a contrast map number and a contrast value; 
         FIG. 7  is an example of a conceptual schematic of a distance map; 
         FIG. 8  is a flowchart of a mask-preparation process of the first embodiment; 
         FIG. 9  is an example of the mask when the contrast map number “8” in  FIG. 7  is designated as the main subject; 
         FIG. 10  is an example of the mask when the contrast map number “13” in  FIG. 7  is designated as the main subject; 
         FIG. 11  is an example when the contrast map number “20” in  FIG. 7  is designated as the main subject; 
         FIG. 12  schematically illustrates a conceptual image of the distance map DM with a partially enlarged drawing; 
         FIG. 13  illustrates an example of the contrast value distribution for a block in which the false contour emerges; 
         FIG. 14  represents a situation when the area of 4×4 blocks of the enlarged figure in  FIG. 12  is divided into 2×2 blocks; 
         FIG. 15  illustrates an example of a contrast value distribution where a false peak is eliminated by extending the size of the blocks; 
         FIG. 16  is a flowchart of interrupt processing carried out in a bokeh control mode of an alternative embodiment; 
         FIG. 17  schematically illustrates an example of a histogram describing the frequencies of the blocks assigned to each distance; 
         FIG. 18  indicates the variation in the blur intensity of the bokeh control process in the near-distance mode; 
         FIG. 19  indicates the variation in the blur intensity of the bokeh control process in the far-distance mode; 
         FIG. 20  schematically illustrates the relationship between the histogram and the bokeh control process in the near-distance mode; 
         FIG. 21  illustrates an example of an image captured in the near-distance mode; 
         FIG. 22  schematically illustrates the relationship between the histogram and the bokeh control process in the far-distance mode; 
         FIG. 23  illustrates an example of an image captured in the far-distance mode; and 
         FIG. 24  is a flowchart of the mask preparation process in the second embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is described below with references to the embodiments shown in the drawings. 
       FIG. 1  is a block diagram schematically illustrating the general structure of a digital camera to which a first embodiment of the present invention is applied. Although the digital camera also applies to a camera unit implemented in various miniature electronic devices including a compact camera, a cellular phone and the like, the first embodiment will be explained for a compact camera, as an example. 
     In the digital camera  10 , an object image produced through a photographing lens  11  is captured by an image sensor  12 . The image sensor  12  is driven by an image sensor driver  13  that is controlled according to instructions from a controller  14 . An image captured by the image sensor  12  is fed to the controller  14  and subjected to various types of digital image processing, and then temporarily stored in an image memory  15 . The image stored in the image memory  15  may further be recorded in a recording medium  16  when desired by a user. An image captured by the image sensor  12  or stored in either the image memory  15  or the recording medium  16  may be displayed on a monitor  17 , e.g. an LCD or the like. 
     The photographing lens  11  is driven by a lens driver  18 , and an iris  19  is driven by an iris driver  20 . Contrast detection autofocus is employed so that the controller  14  controls the lens driver  18  with reference to the contrast of an image captured by the image sensor  12  and controls the iris driver  20  in accordance with a photometric value. 
     The digital camera  10  also has a shake reduction unit (an image stabilization system)  22 . Although, a sensor-shift type image stabilization system is employed in the present embodiment, a lens-based type or other types may also be used. As it is well known in the art, the shake reduction unit  22  has sensors to detect the magnitude of shake so that the image sensor  12  can be moved to counterbalance a camera shake to reduce or prevent image blurring on the imaging surface. The operation of the shake reduction unit  22  is controlled by the controller  14 . An occurrence of false contour, which will be detailed later, may be evaluated from the size of the initial M×N blocks and the scale of the operation performed by the shake reduction unit  22 , such as the magnitude of camera shake and the amount of the image sensor&#39;s  12  shift. Then the evaluated value, which may be referred to as shake-reduction-driven performance, is transmitted to the controller  14 . Note that the image stabilization is controlled independently of the image sensor drive and it is significantly faster than the frame rate. 
     Further, the digital camera  10  is provided with a switch group  21  that includes a release switch, photometric switch, dial switch for selecting either functions or modes, four-way controller switches, and so on, where each of the switches is connected to the controller  14 . 
     With reference to  FIG. 1  and the flowchart of FIG.  2 , a bokeh or blur control process of the first embodiment will be explained. The flowchart of  FIG. 2  represents interrupt processing repeatedly carried out by the controller  14  when a bokeh control mode is selected. Note that the bokeh control mode is selected by a user operating a predetermined switch. 
     In Step S 100 , whether the release button (not shown) is half depressed and the photometric switch S 1  is in the ON state is determined. When it is determined that the photometric switch S 1  is not in the ON state, in other words when the release button has not been half depressed, interrupt processing is immediately terminated and reexecuted after other processes are carried out. However, when it is determined that the photometric switch S 1  is in the ON position in Step S 100 , the lens driver  18  drives the photographing lens  11  and moves the in-focus distance from the closest distance to the furthest distance (e.g., infinity) by a predetermined number of steps to set the lens in a plurality of positions. At each position, one image is captured so that Np (Np: number of positions) through-the-lens images (secondary images) are stored in the image memory  15 . Note that the process in Step S 102  is carried out in short time so that Np images captured during this process can be regarded as shots of the same object taken from the same position, in the same direction and at the same angle of view, but focusing on different distances. 
     In Step S 104 , the photographing lens  11  is moved to the in-focus position by the lens driver  18  using the well-known contrast detection autofocus. In Step S 106 , whether or not the photometric switch is in the ON state, namely whether or not the half-depressed position of the release button (not shown) has been maintained, is determined. If it is not maintained, interrupt processing is terminated and repeated after the other processes are carried out. 
     When the ON state of the photometric switch S 1  is maintained, whether or not the release switch S 2  is in the ON state is determined in Step S 108 . If the release switch S 2  has not been turned on, the process returns to Step  106  and the same determinations are repeated. On the other hand, when the release switch S 2  is in the ON state, the image capturing operation for capturing a recording image (a primary image), which is the image to be recorded, is carried out by the image sensor  12  in Step S 110 . Further, in Step S 112 , a developing process is applied to the RAW data of the recording image. Note that the recording image, which is captured in Step S 110 , is also regarded as an image of the same object shot from the same position in the same direction and at the same angle of view as the Np images captured in Step S 102 . 
     In Step S 114 , a distance map preparation process that uses the through-the-lens images (secondary images) stored in the image memory  15  in Step S 102  is carried out. The distance map is a map of data that assigns distance information for the objects in each area (block) constituting the recording image (or the through-the-lens image). The details of the map preparation process will be discussed later. 
     In Step S 116 , a mask preparation process (detailed later) that prepares a mask for applying the blur control process to all areas other than the main subject area of the recording image (primary image) is carried out with reference to the distance map prepared in Step S 114 . In Step S 118 , the bokeh control process is applied to the recording image that has already been masked so that a new composite recording image (a new composite primary image) is produced. The new composite recording image is recorded in the recording medium  16  in Step S 120 , which ends this interrupt processing in the bokeh control mode. 
     Note that the recording image (primary image) may be recorded without performing the bokeh control processing and may be configured to be recorded with the corresponding distance map in the recording medium  16  so that the bokeh control processing or other image processing may be carried out later with reference to the distance map. In such a case, the recording image (primary image) data and the distance map data may be stored in separate files or kept together in the same file. When they are stored in separate files, the correspondence between separate files may use the file names or information recorded in header files. 
       FIGS. 3A and 3B  are flowcharts of the distance map preparation process, which takes place in Step S 114  of  FIG. 2 . Further,  FIGS. 5-8  represent processes that are carried out in  FIGS. 3 and 4 . With reference to  FIGS. 3-8 , the distance map preparation process of the first embodiment will be explained. 
     In the distance map preparation process, the shake-reduction-driven performance information from the shake reduction unit  22  is received in Step S 201  and then outline or edge contour components are extracted or enhanced in Step S 201  by applying an edge contour extraction process, which may include a high-pass filter and a differential filter, to each of the through-the-lens images (secondary images) stored in the image memory  15  (the images stored in Step S 102  of  FIG. 2 ). Here,  FIG. 4(   a ) represents an example of a through-the-lens image in which the outline (edge contour) components have not been extracted.  FIG. 4(   b ) represents an example of the through-the-lens image after applying the edge contour extracting process to the image illustrated in  FIG. 4(   a ). 
     In Step S 202 , whether or not the value of the shake-reduction-driven performance is below a predetermined threshold value is determined. Namely, when the camera shake is sufficiently small and it can be regarded that false contour will not occur, the image ( FIG. 4(   b )) from which the outline (edge contour) components have been extracted is divided into M×N blocks (areas) in Step S 203 , as illustrated in  FIG. 4(   c ) as an example. Here, N denotes the number of the blocks in the vertical direction and thus is smaller than the number of pixels in the vertical direction of the through-the-lens image (secondary image). Further, M denotes the number of the blocks in the horizontal direction and thus is smaller than the number of pixels in the horizontal direction of the through-the-lens image. Note that in the first embodiment a singular pixel may also constitute a block. 
     On the other hand, when the camera shake is large and it can be regarded that false contouring will be substantial, the number of blocks is reduced to (M/2)*(N/2) in Step S 204  and the images from which the outline components are extracted are divided into (M/2)*(N/2) blocks. Namely, when it is evaluated that the camera shake is large, i.e., the amount of false contour components is large, the number of blocks is reduced to one fourth of the number of the blocks that is selected when it is evaluated that the camera shake is small, in other words the size of one block is quadrupled. 
     In Step S 205 , the sum of the outline (edge contour) components is calculated for each of the blocks, which were divided in either of Step S 203  or Step S 204 . Namely, the sum of the pixel values within each block is calculated as a contrast value (a value used to evaluate the degree of contrast) for the image ( FIG. 4(   b )) from which the outline components has been extracted. In Step S 206 , the contrast values calculated for each block are stored in a buffer memory (not shown) of the size corresponding to M*N or (M/2)*(N/2) number of blocks, as a contrast map (a map for evaluating the degree of contrast) for each of the through-the-lens images (secondary images).  FIG. 4(   d ) schematically illustrates an example of the contrast map corresponding to the blocks (M*N) shown in  FIG. 4(   c ). In  FIG. 4(   d ), a brighter block represents a higher contrast value. 
     In Step S 208 , whether the above-mentioned process (S 200 -S 206 ) has been applied to all of the Np through-the-lens images (secondary images), which are stored in the image memory  15 , is determined. When it is determined that the above-mentioned process has not been conducted for all of the through-the lens images, the process returns to Step S 201  and the process of Steps S 201 -S 206  is carried out for the through-the-lens images that have not been processed. 
     On the other hand, when it is determined in Step S 208  that the process of Steps S 200 -S 206  has been conducted for all of the Np through-the-lens images stored in the image memory  15 , the process proceeds to Step S 210  and Np contrast maps for Np through-the-lens images are stored in the buffer memory, as illustrated in  FIG. 4(   e ). 
     As schematically illustrated in  FIG. 5 , the distance map is obtained by referring to every Np contrast map for each of the blocks. In Step S 210 , an array for M*N blocks or (M/2)*(N/2) blocks is reserved in a memory to preserve the distance map. In Step S 212 , the initial target block among the M*N blocks or (M/2)*(N/2) blocks, which will be subjected to the distance detection process, is set. 
     For example, when the number of blocks is M*N and when the positions of M*N blocks in each of the Np contrast maps are identified by coordinates (i, j), where “i” denotes the i-th row and “j” denotes the j-th column with respect to the upper left position of the map in  FIG. 5  as the origin, the distance detection process starts from the block with coordinates (i, j)=(1, 1). Note that in this procedure, the target block is primarily moved from left to right in the same row, then to the next row and so on until it reaches the last block. 
     In Step S 214 , the contrast value for each corresponding target block (i, j) among all of the Np contrast maps is scanned and a contrast map with the maximum contrast values for each particular (i, j) target block is identified so that the identification numbers of the contrast map can be obtained. For example, when there are 23 contrast maps (Np=23) and the contrast value of the target block (i, j) varies with respect to the contrast maps as indicated in  FIG. 6 , the maximum contrast value for target block (i, j) is detected on the contrast map with identification number “20”. 
     In Step S 218 , the identification number of the contrast map that is detected in Step S 216  is stored in an element of the array reserved in Step S 210  that corresponds to the same (i, j) target block of the distance map. In Step S 220 , whether or not the distance information detection process has been performed for all of the blocks is determined. Namely, whether or not an identification number designating whichever contrast map has the maximum contrast value for a particular (i, j) target block has been detected for all of the blocks and preserved in each corresponding array element is determined. When the above detection and preservation for all of the blocks has not been completed, the target is moved to the next block in Step S 222  and the process from Step S 214  is repeated. Further, when it is determined that the detection and preservation for all of the blocks has been completed, the distance map preparation process ends. 
     Upon completion of the distance map preparation process, as illustrated in  FIG. 7 , the identification number of whichever contrast map has the highest contrast value for a particular block is preserved in the array element of each corresponding block in the distance map. Namely, the number assigned to each block represents the in-focus lens position for that particular block area, and it may also correspond to the distance in the optical direction from the lens to the object captured in that particular block of the image. In the example of a distance map shown in  FIG. 7 , the distance to the subject increases as the contrast map number increases, and with reference to  FIG. 4(   a ), the blocks with the contrast map number “8” correspond to a person, i.e., the main subject. Further, the blocks with the contrast map number “20” correspond to a distant view, such as the far-off background, while the blocks with the contrast map number “13” correspond to objects that exist between the main subject and the background, for example trees just behind the person who is the main subject. 
     With reference to the flowchart of  FIG. 8  and FIGS.  9 - 11 , the mask preparation process of the first embodiment, which is carried out in Step S 116  of  FIG. 2 , will be explained. 
     When the mask preparation process is started, the main subject is selected in Step S 300 . The main subject may be designated by a focus point used in the AF operation during image capturing, or it may be designated from the image by a user using a suitable input device. It may also be designated by a position detected with face detection technology. For example, any block may be determined to be part of the main subject if the difference (or distance) between its contrast map number and the contrast map number (or position) of the block containing the pre-determined main subject is less than or equal to a certain range (or distance). 
     In Step S 302 , a mask to exclude the acknowledged main subject blocks from the bokeh control process is generated and the image captured in Step S 110  of  FIG. 2  is subjected to the masking process. Note that the corresponding blocks in a through-the-lens image (secondary image) and recording image (primary image) are instantly recognized from the number of pixels in each of the images. 
       FIG. 9  illustrates an example of the mask when the blocks with contrast map identification number “8” (a person) in  FIG. 7  are designated as the main subject. Further,  FIG. 10  illustrates an example of the mask when the contrast map identification number “13”, which corresponds to an intermediate distance, is designated as the main subject, and  FIG. 11  is an example when the contrast map identification number “20”, which corresponds to a distant view, is designated as the main subject. 
     Note that in the first embodiment alpha compositing may be used in the masking process so that brighter blocks in  FIGS. 9-11  are subjected to weaker blurring and darker blocks are subjected to stronger blurring. 
     With reference to  FIGS. 12-15 , an explanation is provided for extending the size of one block when blurring due to camera shake is evaluated as substantial. Namely, the reason for reducing the number of blocks, for example from M*N to (M/2)*(N/2), when the evaluated value of the shake-reduction-driven performance is greater than the predetermined value, is explained. 
       FIG. 12  schematically illustrates a conceptual image of the distance map DM with M×N blocks. In  FIG. 12 , a block filled in with light hatching represents a closer object and a block filled in with dark hatching represents a farther object. Further, in  FIG. 12 , 4×4 blocks at the upper left portion of the distance map DM are enlarged. 
     For example, in the enlarged view of  FIG. 12 , it is assumed that an image of a circular subject S 1  within 2×2 blocks at the center is moved from the initial position S 1  to positions S 2 , S 3  and S 4  in three succeeding frames (or fields) of through-the-lens images, which are stored in memory in Step S 102  of  FIG. 2 . Namely,  FIG. 12  represents a situation when blurring occurs. In this situation, the outline edge of the image S 1  in the first frame is in the central four blocks (2, 2), (2, 3), (3, 2) and (3, 3), so that contrast values of these blocks in the contrast map corresponding to the first frame are relatively high. 
     On the other hand, in the second frame, the outline edge of the image S 2  also emerges in four peripheral blocks (2, 1), (3, 1), (4, 1) and (4, 2) in addition to three central blocks (2, 2), (2, 3) and (3, 3), due to the displacement of the image. Therefore, contrast values of these blocks in the contrast map of the second frame are also relatively high. Similarly, as for the contrast map representing the third frame, contrast values of four peripheral blocks (2, 4), (3, 4), (4, 2) and (4, 3) in addition to four central blocks (2, 2), (2, 3), (3, 2) and (3, 3) are relatively high. And as for the contrast map representing the forth frame, contrast values of five peripheral blocks (1, 2), (1, 3), (1, 4), (2, 4) and (3, 4) in addition to three central blocks (2, 2), (3, 2) and (3, 3) are relatively high. 
     As described above, when the outline edge of the subject is displaced repeatedly with respect to the peripheral blocks by camera shake induced during the through-the-lens image storage operation of Step S 102 , the contrast values of the peripheral blocks that should have relatively low values in the contrast map (or frame) if there were no blurring are increased and thus a false contour emerges. For example, in the example of  FIG. 12 , the false contour may emerge in the peripheral 20 blocks (1, 1)-(1, 4), (2, 1), (2, 4), (3, 1), (3, 4), (4, 1)-(4, 4), such that their contrast values may be higher than the actual values. 
       FIG. 13  illustrates an example of contrast value distribution for a block in which the false contour emerges. The horizontal axis represents the distance (or the contrast map number) to an object and the vertical axis represents the contrast value. For example, when a distance L 1 , in the graph of  FIG. 13 , is the actual distance to the object captured in this block, the contrast value of the block generally has a maximum peak at the distance L 1 . However, if a portion of the outline edge of an object that should have been included in another block is included in this block when a frame image is captured with the lens position (which may be represented by the contrast map number) set where a distance L 2  is in focus, the contrast values of the block in the preceding frame and the following frame are increased and a peak may appear at the distance L 2 . 
     In some situations, the peak value at the distance L 2  may possibly become greater than the correct peak value at the distance L 1 . In such situations, the contrast map number representing the distance with the maximum contrast value, which is detected in Step S 216  of  FIG. 3B , may be set to the value representing the distance L 2  instead of the correct value representing the distance L 1 . Namely, the distance information of such a block in the distance map would be inaccurate. Incidentally, as for the distance map DM of  FIG. 12 , it represents a situation when a false contour appears in the peripheral 20 blocks (1, 1)-(1, 4), (2, 1), (2, 4), (3, 1), (3, 4), and (4, 1)-(4, 4), which surround the central four blocks in the enlarged figure. Namely, the distance information of these 20 blocks, which actually correspond to a far background view, include an error and thus are erroneously determined to be at a close range view. 
     Therefore, a mask produced by the mask-preparation process of  FIG. 8  with the distance map including the above-mentioned erroneous distance information would be inaccurate and could cause in an unnatural bokeh control process. Accordingly, in the present embodiment, the size of each divided block area is enlarged when the blurring or the amount of false contour is evaluated to be relatively large, thereby limiting the motion of the outline edge to within the same blocks so that the above-mentioned errors do not occur. 
       FIG. 14  represents a situation when an area of 4×4 blocks of the enlarged figure in  FIG. 12  is divided into four blocks, i.e., 2×2 blocks. In this situation, the outline edge of the circular images S 1 -S 4  is kept within the blocks (1, 1), (1, 2), (2, 1) and (2, 2), and appears in all of the blocks in all frames. Therefore, if the motion of the outline edge is within the same degree as the motion in  FIG. 12 , the contour components calculated in each of these four blocks will be accurate, thus false contour will not appear in any of the peripheral blocks surrounding these four blocks. Thereby, the false peak that has emerged at the distance L 2  in  FIG. 13  will disappear as shown in  FIG. 15  and only the correct peak at the distance L 1  remains. Note that the actual shape of the distribution of the contrast value in  FIG. 15  will be slightly different from that illustrated in  FIG. 14  due to the disappearance of the false peak. 
     As described above, when the false contour components due to blurring are evaluated to be substantial and thus the distance information may include an error, the size of the divided blocks is enlarged so that the effects of the outline edges in each block, which are induced by the blurring, may be reduced and in turn the accuracy of the distance information for each block may be improved. 
     As described above, according to the first embodiment an appropriate bokeh may be applied to areas other than the main subject, even in a situation where a bokeh effect cannot be sufficiently obtained using the depth of field. Further, in the present embodiment the contrast is evaluated using an edge contour extraction process to facilitate and optimize the in-focus evaluation. Further, by dividing an image into a plurality of blocks (except for blocks comprising a single pixel), the processing speed is improved. Further, in the present embodiment, an error induced in the distance detection for each of the blocks is eliminated or reduced by controlling the size of the blocks in accordance with the magnitude of blurring. 
     Further, in the present embodiment, since a plurality of through-the-lens images (secondary images) is used in preparing the distance map, the bokeh control processing can be performed during the normal image capturing sequence. Thereby, a user may not be aware of or bothered by the bokeh control processing, compared to when it is carried out exclusively by capturing a plurality of images to obtain the distance map, resulting in less time being consumed in the process. 
     Incidentally, although the present embodiment is explained with the compact camera as an example, a flowchart that may be applied to an alternative embodiment, including a camera unit provided in a cellular phone, is shown in  FIG. 16 . Here,  FIG. 16  corresponds to the flowchart of  FIG. 2  in the first embodiment. The substantial difference between this alternative embodiment and the first embodiment is that this camera unit does not have a photometric switch S 1 . Namely, the photometric operation, autofocus operation and image capturing operation are started only when the release button is fully depressed, in other words when the release switch is turned on. Therefore, the flowchart of the alternative embodiment does not have steps corresponding to Steps S 106  and S 108  of  FIG. 2 , and Step S 100  is replaced by Step S 400  that determines whether or not the release switch S 2  is in the ON state. Note that Steps S 402  and S 404  correspond to Steps S 102  and S 104 , and Steps S 406 -S 416  correspond to Steps S 110 - 120 , respectively. 
     As described above, the same effect or advantage of the first embodiment can be obtained for a camera unit mounted on a cellular phone and the like, in which the photometric switch is turned on only when the release button is fully depressed. 
     With reference to  FIGS. 17-24 , a digital camera of a second embodiment will be explained. The features of the digital camera in the second embodiment are the same as those in the first embodiment except for the masking method (Step S 116  in  FIG. 5 ). In the second embodiment, the masking method is changed according to the distance from the lens to the main subject in the optical direction. Therefore, in the following descriptions only features dissimilar to the first embodiment will be explained, which corresponds to Step S 116  of  FIG. 2 . 
     The depth of field deepens as the distance to a photographing object increases, and the resolving power in a one-step drive motion (the minimum displacement) of an AF operation is reduced as the distance increases. Therefore, in the second embodiment, the bokeh control process is changed in accordance with the distance from the lens to the main subject in the optical direction. In the present embodiment, a bokeh control process that emulates subtle differences caused by depth is selected when the main subject is relatively nearby (a near-distance mode). On the other hand, a bokeh process that blurs only the background of the main subject in order to enhance the contrast between the main subject and the background is selected when the main subject is relatively far away (a far-distance mode). 
       FIG. 17  schematically illustrates an example of a histogram in which the horizontal axis represents the distance to the object and the vertical axis represents the frequency of the blocks assigned to each distance. In the present embodiment, either the above-mentioned near-distance mode or far-distance mode is selected according to whether or not the main subject is closer than a predetermined distance “D o ”. Namely, when the distance to the main subject is “D o ” or less, the near-distance mode is selected, otherwise the far distance mode is employed. 
     Note that the depth of field also depends on the f-number, thus the distance “D o ” may be changed according to the f-number. For example, the depth of field becomes relatively shallower as the f-number is reduced, so that the distance “D o ” may be set to a greater value as the f-number is reduced and to a lesser value as the f-number is increased. 
       FIG. 18  indicates variation in the blur intensity of the bokeh control process in the near-distance mode. Namely, the curves in  FIG. 18  define the mask density in the near-distance mode. In the near-distance mode, a block corresponding to the distance of the main subject is not blurred or subjected to the minimum blurring. As for blocks corresponding to the foreground view or background view, the intensity of blurring is gradually enhanced as the distance of the blocks increases with respect to the distance “D o ” of the main subject. For example, the blurring intensity is gradually increased up to the maximum (100%) until it reaches the distance “D f ” in the foreground view or “D b ” in the background view. 
       FIG. 19  indicates variation in the blur intensity of the bokeh control process in the far-distance mode. Namely, curves in  FIG. 19  define the mask density in the far-distance mode. In the far-distance mode, as discussed above, block areas in the background view are only blurred to enhance the contrast between the main subject and the background view, so that the blurring is accomplished in a step function manner with the distance “D o ” of the main subject as the threshold. Namely, the blocks corresponding to distances closer than the main subject (distance “D o ”) are not blurred while all of the blocks corresponding to distances farther than the main subject (distance “D o ”) are subjected to the maximum blurring (100%). 
     In the present embodiment, the mask density curves defined in each of the modes may be controllable by operating the four-way controller switches or the electronic dial switch in the switch group  21  of  FIG. 1 . For example, in the near-distance mode the depth of field may be controllable. Namely, the position of “D f ” and “D b ” in  FIG. 14  may be varied by a one-step unit in the AF operation. Further, in the far-distance mode the intensity of blurring may be variable. 
       FIG. 20  schematically illustrates an example of the relationship between the histogram (distance/frequency (the number of blocks) based) and the bokeh control process in the near-distance mode. In  FIG. 20 , the broken lines represent the variation in the resolving power of the depth depending on the distance. Further,  FIG. 21  schematically illustrates an example of an image captured in the near-distance mode. Assuming a person S 0  is the main subject, persons S 1 -S 3 , who are in front and behind the person S 0 , are blurred at various degrees depending on their depth-wise distance from the person S 0 . Namely, in the present embodiment, the degree of blurring increases for objects as their distance from the main subject increases, so that a detailed difference in the depth is enhanced and the solidity is increased compared to the original image. 
     On the other hand,  FIG. 22  schematically illustrates an example of the relationship between the histogram (distance/frequency (the number of blocks) based) and the bokeh control process in the far-distance mode. Similar to  FIG. 20 , the broken lines represent the variation of the resolving power of the depth depending on the distance. Further,  FIG. 23  schematically illustrates an example of an image captured in the far-distance mode. In  FIG. 23 , the blurring process is not carried out on the blocks that are determined to have distances that are equal or less than the distance to the main subject (the person S 0 ), while uniform strong blurring is applied to the blocks that are determined to have distances greater than the distance of person S 0 . Thereby, an image is obtained in which only the main subject, the person S 0 , is held clear while the rest of the view is blurred. 
       FIG. 24  is a flowchart of the mask preparation process in the second embodiment, which corresponds to Step S 116  of  FIG. 2 . With reference to this flowchart, the mask preparation process in the second embodiment will be explained. 
     In Step S 500 , the main subject is designated similar to Step S 300  of  FIG. 8  in the first embodiment. In Step S 502 , the contrast map number (which corresponds to the distance) of blocks including the main subject is obtained. In Step S 504 , whether or not the main subject is in the near distance is determined from the distance information (the contrast map number) of the main subject. For example, whether or not the contrast map number or the distance, which corresponds to the main subject, is less than or equal to a predetermined value is determined. 
     When the main subject is determined to exist in the near distance at Step S 504 , the mask density curve for the near-distance mode, which is shown in  FIG. 18 , is selected in Step S 506 , and in Step S 508  the depth-wise distance (difference) of the target block from the subject image is calculated. 
     On the other hand, when it is determined that the main subject is not in the near distance, thus in the far distance, the mask density curve for the far-distance mode, which is shown in  FIG. 19 , is selected in Step S 16 , and in Step S 508  the distance (a difference value) of the target block from the subject image is calculated. 
     In Step S 510 , the difference value of the current target block calculated in Step S 508  and the mask density curve selected in Step S 506  or S 516  are referenced so that the mask density value that is applied to the current target block can be determined. Further, in Step S 512 , the designated mask density is stored in an element of the array corresponding to the current block, which is used to create the mask. 
     In Step S 514 , whether or not the above process is carried out for all of the M*N blocks, i.e., whether or not the mask density is designated for all of the M*N blocks, is determined. When it is determined that the process has been carried out for all of the M*N blocks, this mask preparation process terminates. Otherwise, in Step S 518 , the target is changed to the next block and the process from Step S 508  is repeated. Note that the transition of the target block is carried out in the same manner as scanning the target block in  FIG. 3B  and  FIG. 5 . 
     As described above, according to the second embodiment, the bokeh or blur effect is controlled depending on the distance to the main subject in the optical direction, so that an improved bokeh effect can be realized. 
     Incidentally, although the through-the-lens images are used to obtain the distance information in the present embodiment, the secondary images for obtaining the distance information are not limited to the through-the-lens images. Sequential images that are captured at a plurality of different focus positions during a short period of time can be also used to obtain the distance information. Further, in the present embodiment, the contrast map number is used as an example to represent the distance information. However, any variable representing the distance information is available, and further, it does not have to be proportional to the actual distance unless it has a one-to-one relationship with the distance. Further, the blurring process is also not limited to this embodiment. 
     The present invention may have significant advantages in a compact camera but it is not limited thereto; the inventive embodiment may also applied to a general digital single reflex camera and the like. 
     Further, although in the present embodiment edge contour extraction is applied to improve the precision of evaluating the contrast within each area (block), the contrast may be evaluated without using edge contour extraction if an appropriate method for evaluating the focusing conditions in each area (block) is available for the position of the lens when the image is captured. 
     The mask preparing process, image composition process and image storing process (Steps S 116 -S 120  and Steps S 412 -S 416 ) of the present embodiment may also be available for a device other than a digital camera, such as a personal computer and the like. In such situation, these processes are carried out with reference to the above-discussed distance map (distance information), which may be generated in the digital camera, or with reference to the distance information of the object that is obtained by an alternative method. For example, when a personal computer is used, these processes may be provided as a software program stored in a recording medium. Namely, the image data may be obtained from a recording medium or external communication and subjected to the image processing, such as the bokeh control process, with reference to distance information that is obtained from a recording medium or external communication. The resulting image may be recorded in a recording medium or transmitted to an external device. 
     In the present embodiment, the distance information of the object is applied to the blurring process; however, the distance information may be applied to an image process other than the blurring process. For example, it may be applied to an image process including a process for an artistic watercolor effect, a process for an illustration effect and a process for overlaying characters (including figures, symbols, letters and designs) on the image. As for the process for the artistic watercolor effect, the colors and tints may be controlled on the basis of the distance to the main subject, and/or a method for controlling the colors and tints may be changed according to the above distance. When overlaying the characters, the size of overlay areas or the size of characters may be controlled on the basis of the distance to the main subject, and/or a method for controlling the above sizes may be changed according to the above distance. 
     Further, the processes from the preparation of the distance map to the image processing for applying the distance information may be employed as a control program for an imaging device provided with a lens drive mechanism and an image sensor. 
     Although the imaging device (a digital camera) of the present embodiments is provided with an image stabilization system, the image stabilization system is not essential. For example, a sensor to detect camera shake may be provided only for evaluating blurring or evaluating the amount of false contour components that would affect the contrast map. Further, although in the present embodiment two different sizes are proposed for the blocks, the number of different sizes may also be three or more. 
     Although the embodiment of the present invention has been described herein with reference to the accompanying drawings, obviously many modifications and changes may be made by those skilled in this art without departing from the scope of the invention. 
     The present disclosure relates to subject matter contained in Japanese Patent Application No. 2011-178021 (filed on Aug. 16, 2011), which is expressly incorporated herein, by reference, in its entirety.