Patent Publication Number: US-2022226710-A1

Title: Mark for club head measurement and image processing device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2021-007703, filed on Jan. 21, 2021. The above applications are hereby expressly incorporated by reference, in their entireties, into the present application. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to a mark for club head measurement in use for measuring information about a state of a club head of a golf club and an image processing device that detects the mark for club head measurement from a photographic image thereof. 
     2. Description of the Related Art 
     Conventionally, a variety of devices that take photos of a golfer&#39;s swing and provide photographic information thereof have been proposed. The photographic information of the swing is provided to the golfer to encourage the golfer to improve golfing or encourage the golfer to select a golf club suitable for the golfer. 
     As an example of the device described above, Japanese Patent Application Laid-open No. 2018-61729 proposes photographing a golf club and a golf ball from above when a golfer swings the golf club, and projecting a full-size image of the photographed club head and golf ball on the floor. 
     SUMMARY OF THE INVENTION 
     Here, important factors of a golfer&#39;s swing include the motion of the face of a club head and the velocity of a club head during a swing. Recognizing the motion of the face of the club head or the velocity of the club head during a swing enables understanding of the characteristics of the golfer&#39;s swing and enables selection of a golf club suited to the characteristics. 
     However, when a club head in motion is simply photographed from above, for example, as in Japanese Patent Application Laid-open No. 2018-61729, it is difficult to detect the position or the face of the club head from the photographic image with high accuracy, and it is difficult to obtain information about a state of the club head, such as the velocity of the club head or the motion of the face, with high accuracy. 
     In view of the aforementioned problem, an object of the present invention is to provide a mark for club head measurement and an image processing device that can obtain information about a state of a club head with high accuracy. 
     A mark for club head measurement of the present invention is used to measure information about a state of a club head of a golf club and is provided on the club head. The mark includes a central portion formed with a pattern having a lightness lower than the periphery thereof and a peripheral portion formed with a pattern surrounding the periphery of the central portion and having a lightness higher than that of the central portion. 
     In the mark for club head measurement of the present invention, the central portion may be circular, and the peripheral portion may have a doughnut shape, and it is preferable for the peripheral portion to have a width smaller than a radius of the central portion. 
     In the mark for club head measurement of the present invention, it is preferable for a maximum brightness value of the central portion to be 32 or less, and a difference between the maximum brightness value and for a minimum brightness value of the central portion to be 15 or less. 
     In the mark for club head measurement of the present invention, two doughnut-shaped marks each including the central portion and the peripheral portion may be arranged side by side and formed integrally. 
     In the mark for club head measurement of the present invention, it is preferable for the mark to be a sticker on which glue is applied to enable the mark to be affixed to the club head. 
     A first image processing device of the present invention includes: a mark detector configured to acquire time-series photographic images in which a club head is photographed when a golf club having a club head with the mark for club head measurement is swung, and to detect the mark for club head measurement from each of the photographic images; and a head information generator configured to generate information about a state of the club head, based on the mark for club head measurement detected by the mark detector. 
     In the first image processing device of the present invention, when the number of the marks for club head measurement detected from a certain photographic image of the photographic images is larger than a preset number, the mark detector specifies the mark for club head measurement included in the certain photographic image, based on information on the mark for club head measurement in another photographic image in which the preset number of the marks for club head measurement are detected. 
     In the first image processing device of the present invention, the head information generator may generate a trajectory image representing a trajectory of the club head of the golf club and a group of a plurality of face images each representing a face of the club head during the swing, as the information about a state of the club head, and the image processing device may include a control unit configured to display the trajectory image and the group of face images. 
     A second image processing device of the present invention includes: an image generating unit configured to generate a trajectory image representing a trajectory of a club head of a golf club during a swing of the golf club and a group of a plurality of face images each representing a face of the club head during the swing; and a control unit configured to display the trajectory image and the group of face images. 
     In the second image processing device of the present invention, the image generating unit may generate an image representing the face by a straight line, as each of the face images. 
     According to the mark for club head measurement and the first image processing device of the present invention, the mark includes a central portion formed with a pattern having a lightness lower than the periphery and a peripheral portion formed with a pattern having a lightness higher than the central portion, so that information about a state of a club head can be obtained with high accuracy. 
     According to the second image processing device of the present invention, a trajectory image representing a trajectory of a club head of a golf club during a swing of the golf club and a group of a plurality of face images each representing a face of the club head during the swing are generated, and the trajectory image and the group of face images are displayed, so that the trajectory of the club head and the motion of the face can be understood more clearly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an overall configuration of a golf impact analysis system including an embodiment of an image processing device of the present invention. 
         FIG. 2  is a diagram illustrating an example of doughnut-shaped marks provided on a club head. 
         FIG. 3  is a diagram illustrating the details of the doughnut-shaped marks. 
         FIG. 4  is a flowchart illustrating an exemplary method of detecting the doughnut-shaped marks from a photographic image. 
         FIG. 5  is a flowchart illustrating an exemplary method of detecting the doughnut-shaped marks from a photographic image. 
         FIG. 6  is a diagram illustrating an example of the photographic image before a Laplacian filter process. 
         FIG. 7  is a diagram illustrating an example of the photographic image after the Laplacian filter process. 
         FIG. 8  is a diagram illustrating an example of a cut-out image after a gray-scale inversion process. 
         FIG. 9  is a diagram illustrating an example of a binarization process result when a binarization level (threshold) is varied in a range of ±6. 
         FIG. 10  is a diagram illustrating a projection display example of a group of face images and a trajectory image generated when a golf club is swung. 
         FIG. 11  is a diagram illustrating a projection display example of groups of face images and trajectory images TG generated by two swings of a golf club. 
         FIG. 12  is a diagram illustrating another example of the mark for club head measurement. 
         FIG. 13  is a diagram illustrating an example of a photographic image obtained by photographing five kinds of black sheets. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A golf impact analysis system including an embodiment of a mark for club head measurement and an image processing device of the present invention will be described in detail below with reference to the drawings. The golf impact analysis system of the present embodiment is characterized by a mark for club head measurement for measuring information about a state of a club head. First, the overall golf impact analysis system will be described.  FIG. 1  is an overall configuration diagram of a golf impact analysis system  1  of the present embodiment. 
     As illustrated in  FIG. 1 , the golf impact analysis system  1  of the present embodiment includes a photography device  10 , an image processing device  20 , and a projection device  30 . The photography device  10  and the image processing device  20  as well as the image processing device  20  and the projection device  30  are communicatively connected by wire or wirelessly so that a variety of signals can be exchanged. 
     The photography device  10  takes a photograph of a club head  41   a  of a golf club  41  from above when a golfer  40  swings the golf club  41 . The photography device  10  is installed above the golfer  40  who swings the golf club  41  and is installed immediately above a region in which the club head  41   a  passes through the vicinity of a floor so that the region can be photographed. Specifically, the photography device  10  may be installed immediately above a predetermined photography region R including the vicinity around a preset placement position P of a golf ball  42  on the floor, for example. The photography device  10  may be installed on a support member such as a stand or may be installed on a ceiling. 
     The photography device  10  includes an illumination unit  11  and a camera unit  12 . The illumination unit  11  of the present embodiment has an infrared light source and irradiates the photography region R with infrared light emitted from the infrared light source. The camera unit  12  has an image pickup element, such as a complementary metal-oxide-semiconductor (CMOS) image sensor or a charge-coupled device (CCD) image sensor, and an IR filter arranged in front of the image pickup element. The IR filter is an optical filter that absorbs visible light and transmits infrared light. 
     The photography device  10  emits infrared light from the illumination unit  11  and takes a photograph in the photography region R with the camera unit  12 , based on a control signal output from a control unit  22  of the image processing device  20  to be described later. Specifically, the photography device  10  emits infrared light from the illumination unit  11  at a predetermined frame rate and takes photographs of the club head  41   a  passing through the photography region R. Photographic images taken by the photography device  10  at a predetermined frame rate are output to the image processing device  20 . 
     The image processing device  20  is configured with, for example, a computer and includes a central processing unit (CPU), a semiconductor memory such as a read-only memory (ROM) and a random access memory (RAM), a storage such as a hard disk, and hardware such as a communication I/F. 
     The image processing device  20  includes an image generating unit  21 , a control unit  22 , a display unit  23 , and an input unit  24 . 
     A golf impact analysis program is installed in the semiconductor memory or the hard disk of the image processing device  20 . This program is executed by the CPU to enable the image generating unit  21  and the control unit  22  described above to function. In the present embodiment, all of the functions described above are implemented by the golf impact analysis program. However, the present invention is not limited thereto, and some or all of the functions may be configured with hardware such as an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or any other electric circuit. 
     The components of the image processing device  20  will be described in detail below. 
     The image generating unit  21  acquires a plurality of time-series photographic images of the club head  41   a  taken by the photography device  10  and generates a group of face images each representing the face of the club head  41   a , based on the acquired photographic images. The term “face” used throughout the present specification refers to a surface in the club head  41   a  that impacts a golf ball. The image generating unit  21  of the present embodiment corresponds to the mark detector and the head information generator of the present invention. 
     Specifically, as illustrated in  FIG. 2 , the top surface of the club head  41   a  of the golf club  41  swung by the golfer  40  has two doughnut-shaped (double circle-shaped) marks M 1  and M 2  serving as indicators of the face  41   b . The doughnut-shaped marks M 1  and M 2  correspond to the mark for club head measurement of the present invention. 
     The doughnut-shaped marks M 1  and M 2  are stickers on which glue is applied so that the doughnut-shaped marks M 1  and M 2  can be affixed to the club head  41   a . The doughnut-shaped marks M 1  and M 2  are affixed to the top surface of the club head  41   a  at a predetermined distance from each other along the face  41   b.    
     As illustrated in  FIG. 3 , the doughnut-shaped marks M 1  and M 2  each include a central portion C formed with a pattern having a lightness lower than the periphery and a peripheral portion Ph formed with a pattern surrounding the periphery of the central portion C having a lightness higher than the central portion C. 
     In the present embodiment, the doughnut-shaped marks M 1  and M 2  each have the central portion C formed with a circular pattern and the peripheral portion Ph formed in a doughnut shape. The central portion C is filled black, and the peripheral portion Ph is white. 
     The doughnut-shaped marks M 1  and M 2  of the present embodiment are each formed such that the width w of the peripheral portion Ph is smaller than the radius r of the central portion C. The width w of the peripheral portion Ph is preferably less than or equal to half the radius r of the central portion C, and more preferably less than or equal to one third the radius r. Specifically, for example, the width w of the peripheral portion Ph may be 1.5 mm, and the radius r of the central portion C may be 4.5 mm. 
     The reason why the width w of the peripheral portion Ph is smaller than the radius r of the central portion C in this manner is that if the width w of the peripheral portion Ph is larger, the image of the peripheral portion Ph in a photographic image taken by the photography device  10  will exhibit blown-out highlights, which may influence the accuracy of detection of the central portion C. 
     In the present embodiment, making the width w of the peripheral portion Ph smaller than the radius r of the central portion C can suppress the influence of the blown-out highlights described above and can improve the accuracy of detection of the central portion C. 
     As described above, the central portion C is formed to be black. When a photograph is taken by the photography device  10 , infrared light is reflected at the central portion C even if it is formed to be black, depending on the material thereof or the like, imparting an influence on the accuracy of detection of the central portion C. It is therefore preferable for the material of the central portion C to be selected such that the maximum brightness value of the photographic image of the central portion C is 32 or less and the difference between the maximum brightness value and the minimum brightness value is 15 or less. Each pixel of the photographic image is eight bits (256 gray levels), and the brightness value is a value from 0 to 255. The measurement conditions of the brightness value will be described later in examples and comparative examples. 
     Selecting the material of the central portion C such that the brightness value falls within the range described above can suppress reflection of infrared light at the central portion C and can improve the accuracy of detection of the central portion C. 
     The image generating unit  21  detects the doughnut-shaped marks M 1  and M 2  described above from each of the time-series photographic images. Then, the image generating unit  21  generates a face image that represents the face  41   b  by a straight line by connecting the center of the central portion C of the doughnut-shaped mark M 1  and the center of the central portion C of the doughnut-shaped mark M 2  with a straight line. 
     Here, a method of detecting the doughnut-shaped marks M 1  and M 2  from a photographic image will be described with reference to the flowchart illustrated in  FIG. 4  and  FIG. 5 . 
     First, an edge enhancement process is performed by performing a Laplacian filter process on the photographic image (S 10 ). For example, a 3×3 filter is used as a Laplacian filter.  FIG. 6  is a diagram illustrating an example of the photographic image before the Laplacian filter process, and  FIG. 7  is a diagram illustrating an example of the photographic image after the Laplacian filter process. As illustrated in  FIG. 6 , in addition to the doughnut-shaped marks M 1  and M 2 , a reflection image of the golf ball and a bright spot that is considered to be a reflection by the club head  41   a  are included in the photographic image, but can be caused to fade by performing the Laplacian filter process described above, so that erroneous detection of the doughnut-shaped marks M 1  and M 2  can be reduced. 
     A binarization process is then performed on the photographic image which was subjected to the Laplacian filter process (S 12 ). As the binarization process, for example, a binarization process by the discrimination analysis method (Otsu&#39;s method) is performed. 
     Subsequently, an opening process is performed on the photographic image which was subjected to the binarization process (S 14 ). This process removes small patterns or fine patterns other than the doughnut-shaped marks M 1  and M 2  and makes the doughnut-shaped marks M 1  and M 2  more distinctive. 
     Subsequently, a labeling process is performed on the photographic image which was subjected to the opening process (S 16 ). 
     If the number of labels generated by the labeling process is 100 or more (NO at S 18 ), it is determined that the doughnut-shaped marks M 1  and M 2  are not detected appropriately, and the process ends. 
     On the other hand, if the number of labels is less than 100 (YES at S 18 ), a region three times the area of each label is defined, and a photographic image included in the region (a part of the photographic image illustrated in  FIG. 6 ) is cut out (S 20 ). 
     Then, a 2×2 dilation filter process is performed on the images cut out at S 20  (hereinafter referred to as cut-out images) (S 22 ). The cut-out images include a white portion that is the peripheral portion Ph of the doughnut-shaped marks M 1 , M 2 , but the detected white portion may be discontinuous at some points. Performing the dilation filter process described above on the cut-out images can eliminate the discontinuation described above and improve the accuracy of detection of the peripheral portion Ph. 
     Then, after the dilation filter process is performed on the cut-out images described above, if the maximum brightness value of the cut-out image corresponding to a predetermined label is 30 or less (NO at S 22 ), the process proceeds to the cut-out image corresponding to the next label, and if the maximum brightness value exceeds 30 (YES at S 22 ), the cut-out image corresponding to the predetermined label is possibly the doughnut-shaped marks M 1 , M 2  and is left as a candidate for the doughnut-shaped marks M 1 , M 2  (S 28 ). The cut-out image having the maximum brightness value of 30 or less is not left as a candidate for the doughnut-shaped marks M 1 , M 2  and is not subjected to the following process. 
     Subsequently, a gray-scale inversion process is performed on the cut-out image left as a candidate for the doughnut-shaped marks M 1 , M 2  among the cut-out images cut out at S 20  (S 30 ).  FIG. 8  is a diagram illustrating an example of the cut-out image subjected to the gray-scale inversion process. 
     Then, a binarization process is performed on the cut-out image which was subjected to the gray-scale inversion process (S 32 ). As the binarization process, for example, a binarization process by the discrimination analysis method (Otsu&#39;s method) is performed. 
     Subsequently, the binarization process is performed again by changing the binarization level at two-pitch intervals in the range of ±6 around the binarization level (threshold) used in the binarization process at S 32  (S 34 ). This process generates seven binarized images as illustrated in  FIG. 9  for one cut-out image. Then, a white portion of the binarized image is masked and labeling is performed again (S 36 ). 
     Subsequently, the contour of the label closest to the center of each binarized image is extracted (S 38 ). Then, an oval is fitted to the contour extracted at S 38  (S 40 ). Then, the median of the areas of the ovals extracted for seven binarized images is determined, and the mean values of height and width are determined for the oval having the median, the ovals having up to the second largest value from the median value, and the ovals having up to the second smallest value from the median value, that is, five ovals excluding the oval having the largest area and the oval having the smallest area among the seven extracted ovals. The height and the width of an oval mean that the height is the length in the y direction and the width is the length in the x direction when the coordinates system of the photographic image is two-dimensional coordinates of x and y. 
     When the mean height and width of an oval is within a preset range of thresholds and the brightness value of a portion in the cut-out image corresponding to the oval is 100 or greater, the contour thereof is recognized as the doughnut-shaped marks M 1 , M 2  (S 42 ). 
     The above-described steps S 10  to S 42  are performed on each photographic image to detect the doughnut-shaped marks M 1  and M 2  included in each photographic image. 
     Here, if a mark having a uniform simple circle, dot, or line in the same color are attached to the club head  41   a  instead of the doughnut-shaped marks M 1  and M 2  as in the present embodiment, for example, the binarization determination process is adversely affected when position recognition of the mark on a black and white image is performed using mechanical image processing. Specifically, reflection light caused by near-infrared illumination emitted from immediately above and impinging and reflected on a portion other than the mark is indistinguishable from reflection light reflected by the mark, so that the position of the mark tends to be erroneously recognized. That is, it is difficult to obtain information about a state of the club head with high accuracy. 
     In order to prevent reflection from a portion other than the mark as described above, a black sticker may be affixed to the entire surface in the photography region of the club head  41   a  and a white circular sticker affixed on the black sticker may be detected by image processing. This method, however, is inconvenient because a relatively large black sticker needs to be affixed to the club head  41   a  and the glue on the black sticker may be left on a surface of the club head  41   a.    
     On the other hand, the doughnut-shaped marks M 1  and M 2  of the present embodiment each include the central portion C formed with a pattern having a lightness lower than the periphery and the peripheral portion Ph formed with a pattern having a lightness higher than the central portion C. More specifically, when uniform illumination is applied in photographing the club head  41   a , the characteristics of reflection of light from the club head  41   a  are such that reflection on a section other than the doughnut-shaped marks M 1  and M 2  has the highest lightness at the center of the reflection portion in the section. However, the doughnut-shaped marks M 1  and M 2  of the present embodiment can be clearly distinguished from the reflection portion because the lightness at the central portion C is low and the lightness of the peripheral portion Ph is high. The erroneous recognition described above therefore can be avoided, and information about a state of the club head can be obtained with high accuracy. 
     Furthermore, the doughnut-shaped marks M 1  and M 2  are convenient because they can be formed with stickers that are small relative to the size of the club head  41   a , and glue remaining on the club head  41   a  when a large black sticker is affixed thereto can be prevented. 
     In theory, the two doughnut-shaped marks M 1  and M 2  described above can be detected from one photographic image by performing the process of detecting the doughnut-shaped marks M 1  and M 2 . However, three or more patterns may be detected as the doughnut-shaped marks, for example, because of reflection on the club head  41   a . A process of identifying two doughnut-shaped marks M 1  and M 2  in such a case will be described below. 
     First, the image generating unit  21  specifies the photographic images immediately before and after the club head  41   a  impacts the golf ball  42 , and determines whether only two doughnut-shaped marks M 1  and M 2  are detected from theses photographic images. 
     Then, when only two doughnut-shaped marks M 1  and M 2  are detected from these photographic images, the image generating unit  21  calculates the heights and the widths of the detected doughnut-shaped marks M 1  and M 2  and the distance between the doughnut-shaped mark M 1  and the doughnut-shaped mark M 2 . 
     Then, for a photographic image in which three or more patterns are detected as the doughnut-shaped marks, two patterns closest to the heights and the widths of the doughnut-shaped marks M 1  and M 2  described above and the distance between the doughnut-shaped mark M 1  and the doughnut-shaped mark M 2  are specified among three patterns, and these two patterns are detected as the doughnut-shaped marks M 1  and M 2 . 
     When three or more patterns are detected in both of the photographic images immediately before and after the club head  41   a  impacts the golf ball  42 , photographic images advanced one frame forward and backward sequentially from the moment of the impact are specified, and it is determined whether only two doughnut-shaped marks M 1  and M 2  are detected. 
     Generation of a group of face images by the image generating unit  21  has been described above. In addition to a group of face images, the image generating unit  21  also generates a trajectory image that represents the trajectory of the club head  41   a  by a line. 
     Specifically, the image generating unit  21  generates a trajectory image by extracting an image portion of the club head  41   a  from each photographic image, calculating the center portion of each of the extracted image portions of the club head  41   a , and connecting the calculated center positions to fit a curve. 
     Returning now to  FIG. 1 , the control unit  22  includes a CPU and controls the entire golf impact analysis system  1 . Specifically, the control unit  22  communicates with the photography device  10  through the communication I/F and outputs a control signal to instruct the photography device  10  of a photography start timing and a frame rate of photography. 
     The photography start timing may be specified and input, for example, by a user using the input unit  24  that includes a mouse and a keyboard. Alternatively, the placement of the golf ball  42  at the golf ball placement position P in the photography region R may be detected, and photographing may be started in accordance with the detection signal. The placement of the golf ball  42  may be detected by using a contact sensor or an optical sensor, or by using a photographic image taken by the photography device  10 . In the method of using a photographic image, for example, preliminary photographing in the photography region is performed by the photography device  10  before a swing of the golf club  41  is started. The photographic image taken by preliminary photographing is input to the control unit  22 . 
     Then, the control unit  22  detects the placement of the golf ball  42  by detecting that an image of the golf ball  42  appears at a preset position in the photographic image. The control unit  22  starts main photographing in response to the placement of the golf ball  42  and starts photographing the club head  41   a.    
     The control unit  22  also outputs a group of face images and a trajectory image generated by the image generating unit  21  to the projection device  30  and allows the projection device  30  to project the group of face images and the trajectory image on the floor. 
       FIG. 10  is a diagram illustrating a projection display example of a group of face images FG and a trajectory image TG generated when the golf club  41  is swung. As illustrated in  FIG. 10 , for example, the projection device  30  projects a group of face images FG and a trajectory image TG on the floor. The element denoted by CG illustrated in  FIG. 10  is a three-dimensional object image representing the club head. 
     Unlike the photographic image taken by the photography device  10 , the three-dimensional object image CG is an image that schematically represents the club head as a three-dimensional image. This three-dimensional object image CG representing the club head is also generated by the image generating unit  21 . Then, the control unit  22  outputs the three-dimensional object image CG to the projection device  30 , and the three-dimensional object image CG is projected on the floor by the projection device  30 . 
       FIG. 10  depicts the three-dimensional object image CG of the club head at a predetermined position on the trajectory image TG. However, the projection device  30  may display an animation of the three-dimensional object image CG moving along the line of the trajectory image TG under control by the control unit  22 . 
       FIG. 10  illustrates an example in which a group of face images FG and a trajectory image TG in one swing is projected. However, groups of face images and trajectory images in two or more swings may be projected in an overlapped manner.  FIG. 11  illustrates an example in which a set of a group of face images FG 1  and a trajectory image TG 1  in a first swing and a set of a group of face images FG 2  and a trajectory image TG 2  in a second swing are projected. It is preferable for the set of the group of face images FG 1  and the trajectory image TG 1  in the first swing and the set of the group of face images FG 2  and the trajectory image TG 2  in the second swing to be projected in different colors. 
     The control unit  22  also displays the group of face images and the trajectory image generated by the image generating unit  21  on the display unit  23  of the image processing device  20 . 
     The display unit  23  includes, for example, a display device such as a liquid crystal display. The input unit  24  includes, for example, an input device such as a mouse and a keyboard. The image processing device  20  may be configured with a tablet terminal, and the display unit  23  and the input unit  24  may be configured with a touch panel. 
     The projection device  30  is configured with a projector and projects a group of face images and a trajectory image on the floor under the control of the control unit  22  of the image processing device  20  as described above. The projection device  30  is installed above a golfer who swings the golf club  41 , or the projection device  30  is installed below the golfer and adjacent to the photography device  10  using a short-focus/proximity lens. The projection device  30  has a projection distance and brightness that enable a group of face images and a trajectory image to be displayed on the floor with sufficient lightness and clearness. 
     The projection device  30  is installed such that a group of face images and a trajectory image can be projected in a predetermined projection region including the vicinity around the placement position P of the golf ball  42  on the floor. The projection device  30  may be installed, for example, together with the photography device  10  on a support member such as a stand or may be installed on a ceiling. 
     The projection device  30  also projects a three-dimensional object image of the club head on the floor as described above. 
     In the foregoing embodiment, the club head  41   a  provided with the doughnut-shaped marks M 1  and M 2  is photographed from immediately above by the photography device  10 . However, the present invention is not limited to such a configuration. For example, a stereo camera may be provided as the photography device  10 , and the club head  41   a  provided with the doughnut-shaped marks M 1  and M 2  may be photographed stereoscopically. 
     In this configuration, the positions of the doughnut-shaped marks M 1  and M 2  in a three-dimensional space can be detected, and the motion of the club head  41   a  can be analyzed in more detail. 
     In the foregoing embodiment, the doughnut-shaped mark M 1  and the doughnut-shaped mark M 2  are formed separately. However, the present invention is not limited to such a configuration. For example, as illustrated in  FIG. 12 , the two doughnut-shaped marks M 1  and M 2  may be formed integrally by being arranged side by side on both ends of a rectangular sticker having a length equivalent to the length of the face of the club head  41   a . In such a configuration, the rectangular sticker is affixed to the club head  41   a  such that the length direction of the rectangular sticker provided with the doughnut-shaped marks M 1  and M 2  is parallel to the face, whereby the direction of the face can be detected more precisely. 
     In the foregoing embodiment, doughnut-shaped marks are employed. However, the shape of the mark for club head measurement is not limited thereto and may be, for example, a double polygon such as a double quadrangle or triangle as long as the mark has a central portion and a peripheral portion as described above. 
     For example, when groups of face images and trajectory images of a plurality of golf clubs  41  are generated, the shape of the mark for club head measurement may be changed for each golf club  41 . In this manner, a plurality of golf clubs  41  can be identified automatically, and the group of face images and the trajectory image can be stored and controlled for each golf club  41 . 
     Example 
     Examples and comparative examples of the doughnut-shaped marks M 1  and M 2  of the foregoing embodiment are listed in Table 1 below. Note that the present invention is not limited to these examples. Table 1 shows the measurement results of the maximum brightness value, the minimum brightness value, and the difference between the maximum brightness value and the minimum brightness value of five kinds of black sheets BL 1  to BL 5  used as a material for the central portion of the doughnut-shaped mark, and the detection accuracy indicated by “high” and “low” obtained when each of the black sheets BL 1  to BL 5  was used as the central portion of the doughnut-shaped mark.  FIG. 13  illustrates a photographic image obtained by taking a photograph of five kinds of black sheets BL 1  to BL 5 . Table 2 shows the photography conditions and the illumination conditions of five kinds of black sheets BL 1  to BL 5 . 
     As illustrated in Table 1 and  FIG. 13 , as for Example 1 (BL 4 ) and Example 2 (BL 5 ), the maximum brightness value was 32 or less, and the difference between the maximum brightness value and the minimum brightness value was 15 or less. When Examples 1 and 2 were used as the central portion of the doughnut-shaped mark, high-accuracy detection was achieved. 
     As for Example 3 (BL 1 ), the difference between the maximum brightness value and the minimum brightness value was 15 or less but the maximum brightness value was 35, and, therefore, when Example 3 was used as the central portion of the doughnut-shaped mark, a portion having a high brightness value was detected as noise and the detection accuracy was low. 
     As for Example 4 (BL 2 ), the reflectivity was high, the maximum brightness value was 255, and the difference between the maximum brightness value and the minimum brightness value was 238 or less. When Example 4 was used as the central portion of the doughnut-shaped mark, a portion having a high brightness value and a portion with a large difference in brightness value were detected as noise, and the detection accuracy was lowest. 
     As for Example 5 (BL 3 ), the maximum brightness value exceeded 32, and the difference between the maximum brightness value and the minimum brightness value exceeded 15. When Example 5 was used as the central portion of the doughnut-shaped mark, a portion having a high brightness value and a portion with a large difference in brightness value were detected as noise, and the detection accuracy was low. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
               
               
                   
                 ple 1 
                 ple 2 
                 ple 3 
                 ple 4 
                 ple 5 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Black sheet 
                 BL4 
                 BL5 
                 BL1 
                 BL2 
                 BL3 
               
               
                 Maximum brightness 
                 32 
                 26 
                 35 
                 255 
                 54 
               
               
                 value 
               
               
                 Minimum brightness 
                 17 
                 13 
                 21 
                  17 
                 22 
               
               
                 value 
               
               
                 Difference between 
                 15 
                 13 
                 14 
                 238 
                 32 
               
               
                 maximum brightness 
               
               
                 value and minimum 
               
               
                 brightness value 
               
               
                 Detection accuracy 
                 High 
                 High 
                 Low 
                 Low 
                 Low 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                 TABLE 2 
               
               
                   
               
             
            
               
                 Photography conditions 
                   
               
               
                 Camera 
                 acA640-750 um made by Basler 
               
               
                 Gain 
                 12 
               
               
                 Black level 
                 10 
               
               
                 Gamma 
                 0.8 
               
               
                 Lens 
                 FL-CC1614A-VG made by RICOH 
               
               
                 Aperture 
                 1.4 
               
               
                 Exposure time 
                 100 ms 
               
               
                 Illumination conditions 
               
               
                 Light source 
                 L760-66-60-130 (two light sources) 
               
               
                   
                 made by Ushio Inc., powersource 6 A, 20 V 
               
               
                 Work distance 
                  3 m