Abstract:
The disclosure relates to a linking-up photographing system and a control method of linked-up cameras thereof including the following steps. A first image is acquired via a first camera. A second image is acquired via a second camera and presents a field of view (FOV) which partially overlaps a FOV of the first image at least. A control look-up table is established according to the first image and the second image. A designating command specifying a region of interest (ROI) in the first image is received. A FOV of the second camera is adjusted according to the ROI and the control look-up table, and then the second camera photographs a view specified by the ROI, so as to obtain a third image. In this way, the second camera can link up and cooperate with the first camera easily.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No(s). 101138328 filed in Taiwan, R.O.C. on Oct. 17, 2013, and on Patent Application No(s). 102121251 filed in Taiwan, R.O.C. on Jun. 14, 2013, the entire contents of which are hereby incorporated by reference. 
     TECHNICAL FIELD 
     The disclosure relates to a linking-up photographing system and a control method for linked-up cameras thereof, more particularly to a linking-up photographing system and a control method for linked-up cameras thereof which are capable of fast linking up linked-up cameras which cooperate with each other. 
     BACKGROUND 
     Recently more and more surveillance systems are popularly being used for protecting people&#39;s wealth and security. A panoramic camera was invented so that a surveillance system could monitor environments without any dead angles. The panoramic camera can be embodied by a multi-lenses assembly, or by a single fisheye lens. The panoramic camera supports the wide-angle photographing without any dead angles. 
     However, the panoramic camera cannot support an optical zoom, so an observer can not zoom in on an object under observation in a panoramic image captured by the panoramic camera. Even though the panoramic camera supports a digital zoom to enlarge its panoramic image for observing an object under observation in the enlarged panoramic image, the panoramic camera can not fulfill the actual requirements yet. Because this digital zoom is accomplished by cropping an image down to a centered area with the same aspect ratio as the original, and usually also interpolating the result back up to the pixel dimensions of the original, the resolution of the enlarged panoramic image will greatly reduce. If there is an accident that happened far away from the panoramic camera, it will be difficult for an observer to clearly observe the object under observation in the obtained panoramic image. 
     As compared with the panoramic camera, a pan-tilt-zoom (PTZ) camera can support the optical zoom so that it can zoom in to change its field of view (FOV), so as to zoom in on a remote object under observation. The FOV of the PTZ camera is smaller than the FOV of the panoramic camera. Thus, the PTZ camera is applied to cooperate with the panoramic camera in order to capture the details of the same view. 
     The angles of view of cameras disposed in different locations may be not the same, so when the PTZ camera is driven according to what the panoramic camera focuses on in the same environment, the FOV of the panoramic camera and the FOV of the PTZ camera will have differences therebetween. Therefore, the panoramic camera and the PTZ camera in such a surveillance system have to be disposed as close to each other as possible. 
     SUMMARY 
     A control method for linked-up cameras according to an embodiment of the disclosure includes the following steps. A first image is acquired via a first camera, and a second image is acquired via a second camera. A field of view (FOV) of the first image partially overlaps a FOV of the second image at least. A control look-up table is established according to the first image and the second image. A designating command specifying a region of interest (ROI) in the first image is received. According to the ROI and the control look-up table, a FOV of the second camera is adjusted for the second camera to photograph a view specified by the ROI, to obtain a third image. 
     A linking-up photographing system according to an embodiment of the disclosure includes a first camera configured to acquire a first image, a second camera configured to acquire a second image, a calibration unit configured to establish a control look-up table according to the first image and the second image, and a control unit. A FOV of the second image partially overlaps a FOV of the first image at least. The control unit is configured to receive a designating command which specifies a ROI in the first image, and to adjust a FOV of the second camera according to the ROI and the control look-up table, so as to control the second camera to photograph a view specified by the ROI, to obtain a third image. 
     A control method of linked-up cameras according to an embodiment of the disclosure includes the following steps: acquiring a first image via a first camera; acquiring a second image via a second camera, wherein a FOV of the second image partially at least overlaps a FOV of the first image; setting M position points in the first image and in the second image, and the M position points of the first image corresponding to the M position points of the second image respectively, where M is a positive integer greater than or equal to 3; receiving a designating command which specifies a ROI in the first image; calculating a coordinate transformation parameter set according to at least three selected position points; calculating a mapping point of the second image corresponding to a center point of the ROI according to the coordinate transformation parameter set; and adjusting a FOV of the second camera according to the mapping point, and then controlling the second camera to photograph a view specified by the ROI, to obtain a third image. 
     A linking-up photographing system according to an embodiment of the disclosure includes a first camera configured to acquire a first image; a second camera configured to acquire a second image presenting a FOV which partially overlaps a FOV of the first image at least; and a control module configured to set M position points in the first image and in the second image, to receive a designating command which specifies a ROI in the first image, to calculate a coordinate transformation parameter set according to at least three selected position points, to calculate a mapping point of the second image corresponding to a center point of the ROI according to the coordinate transformation parameter set, and to adjust a FOV of the second camera according to the mapping point, so as to control the second camera to photograph a view specified by the ROI, to obtain a third image, where the M position points of the first image corresponding to the M position points of the second image respectively, and M is a positive integer greater than or equal to 3. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow along with the accompanying drawings which are for illustration only, thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a schematic diagram of a linking-up photographing system according to an embodiment of the disclosure; 
         FIG. 2A  is a schematic disposition diagram of a first camera and a second camera in  FIG. 1  according to an embodiment of the disclosure; 
         FIG. 2B  is a schematic disposition diagram of a first camera and a second camera in  FIG. 1  according to another embodiment of the disclosure; 
         FIG. 3  is a flowchart of a control method of linked-up cameras according to an embodiment of the disclosure; 
         FIG. 4A  is a schematic diagram of a first image according to an embodiment of the disclosure; 
         FIG. 4B  is a schematic diagram of a fourth image according to an embodiment of the disclosure; 
         FIG. 4C  is a schematic diagram of a second image according to an embodiment of the disclosure; 
         FIG. 5  is a flowchart of the step S 200  in  FIG. 3  according to an embodiment of the disclosure; 
         FIG. 6A  to  FIG. 6K  are schematic diagrams for showing the panoramic stitching of the fourth images according to an embodiment of the disclosure; 
         FIG. 7  is a flowchart of the step S 300  in  FIG. 3  according to an embodiment of the disclosure; 
         FIG. 8  is a schematic diagram of the selection of position points in the first image and the second image according to an embodiment of the disclosure; 
         FIG. 9  is a flowchart of the step S 340  in  FIG. 7  according to an embodiment of the disclosure; 
         FIG. 10  is a schematic diagram of a relation between a Cartesian coordinate system and a round coordinate system according to an embodiment of the disclosure; 
         FIG. 11  is a flowchart of the step S 500  in  FIG. 3  according to an embodiment of the disclosure; 
         FIG. 12  is a schematic diagram of an interpolation method according to an embodiment of the disclosure; 
         FIG. 13A  is a flowchart of the step S 510  in  FIG. 12  according to an embodiment of the disclosure; 
         FIG. 13B  is a flowchart of the step S 510  in  FIG. 12  according to an embodiment of the disclosure; 
         FIG. 14  is a schematic diagram of the setting of boundary mapping points according to an embodiment of the disclosure; 
         FIG. 15  is a schematic diagram of a second reference circle an embodiment of the disclosure; 
         FIG. 16  is a schematic diagram of a linking-up photographing system according to an embodiment of the disclosure; and 
         FIG. 17  is a flowchart of a control method of linked-up cameras according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing. 
       FIG. 1  illustrates a linking-up photographing system according to an embodiment of the disclosure. The linking-up photographing system includes a first camera  12 , a second camera  14  and a control module  11  which includes a calibration unit  16  and a control unit  18 . The first camera  12 , the second camera  14  and the calibration unit  16  respectively connect to the control unit  18 . The first camera  12  and the second camera  14  respectively capture images according to their FOVs, so the control unit  18  can acquire a first image via the first camera  12  and a second image via the second camera  14 . A FOV of the first image partially overlaps a FOV of the second image at least. 
     For example, the first camera  12  and the second camera  14  are lens sets with charge-coupled devices (CCDs), lens sets with complementary metal-oxide-semiconductors (CMOSs) or internet protocol (IP) cameras. Specifically, the first camera  12  can be, for example, a fisheye camera, a panoramic camera or a wide-angle camera. The disclosure takes a fisheye camera as an example as the first camera  12 , so the first image is a panoramic image, i.e. a fisheye image. Generally, a panoramic image can present a FOV of 360 degrees. For instance, the second camera  14  is a pan-tilt-zoom (PTZ) camera or a digital PTZ camera, and can perform an optical zoom or a digital zoom. 
     The location of the first camera  12  and the location of the second camera  14  are adjacent to each other in an exemplary embodiment, or have a preset distance therebetween in another exemplary embodiment. For instance, the location of the first camera  12  and the location of the second camera  14  are close to each other in the same room as shown in  FIG. 2A , or are disposed apart from each other in the same room as shown in  FIG. 2B . 
     The calibration unit  16  and the control unit  18  in this and some embodiments can be embodied by a personal computer, a network video recorder (NVRs), an embedded system, or other electronic devices with a computing function. In this and some embodiments, the calibration unit  16  and the control unit  18  can be disposed in the same computer or server and connect to the first camera  12  and the second camera  14  through a network. 
     The link-up control procedure performed by the linking-up photographing system is described as follows.  FIG. 3  illustrates a control method of linked-up cameras according to an embodiment of the disclosure. Through the control method of linked-up cameras, the linking-up photographing system controls the second camera to pan, tilt or zoom in or out according to the sensed result of the first camera and/or a designating command, for capturing a third image. In this and some embodiments, the designating command can be provided manually by observers or be provided automatically by performing a motion detection procedure or an object detection procedure, or be provided by an external computing device. 
     Firstly, a first image as shown in  FIG. 4A  is acquired via the first camera (step S 100 ), and a second image as shown in  FIG. 4C  is acquired via the second camera (step S 200 ). A FOV of the second image partially overlaps a FOV of the first image at least. Specifically, refer to  FIG. 5 , the calibration unit  16  or the control unit  18  in step S 200  performs step S 210  and S 220  to obtain the second image. Firstly, the calibration unit  16  controls the second camera  14  to, according to various FOVs, capture a plurality of fourth images which are planar images as shown in  FIG. 4B  (step S 210 ). Then, the calibration unit  16  stitches these fourth images to generate a panoramic image which is set as the second image as shown in  FIG. 4C  (step S 220 ). 
     The detail of panoramically stitching the fourth images is described as follows. In  FIG. 6A  to  FIG. 6E , many fourth images  30  are respectively captured according to various FOVs, and the FOVs of these fourth images  30  have scenes adjacent to each other. Thus, the contents of some of these fourth images  30  overlap each other. Because the first image  22  is a panoramic image, the fourth images  30  need to be curved to be corresponding fifth images  32  shown in  FIG. 6F  to  FIG. 6J . Specifically, the fourth image  30  in  FIG. 6A  becomes the fifth image  32  in  FIG. 6F , the fourth image  30  in  FIG. 6B  becomes the fifth image  32  in  FIG. 6G , the fourth image  30  in  FIG. 6C  becomes the fifth image  32  in  FIG. 6H , and the rest can be deduced by analogy. Then, the fifth images  32  in  FIG. 6F  to  FIG. 6J  are stitched to generate one section  34 , shown in  FIG. 6K , of a panoramic image according to features of the fifth images  32  during the panoramic stitching, and the features can be specific objects or scenes in the images. In the same way, the second camera  14  can photograph scenes corresponding to other parts of the FOV of the first image, to obtain and process more fourth images for obtaining more sections  34  of the panoramic image. Finally, all sections  34  are further stitched to generate the panoramic image which is set as the second image similar to the first image. 
     Since the fourth images  30  are curved to be the fifth images  32 , the horizontal lines in each fourth image  30  become arcs. For example, a reference axis  31  horizontally across the fourth image  30  in  FIG. 6D  is straight, and a reference axis  33  in the fifth image  32  is an arc. Thus, the reference axis  35  in the section  34  in  FIG. 6K  is an arc as the same as the reference axis  33 . 
     Refer to  FIG. 3 , after receiving the first image and the second image, the calibration unit  16  establishes a control look-up table according to the first image and the second image (step S 300 ). Herein, the calibration unit  16  can directly receive the first image or the fourth images, or receive one of them through the control unit  18 . If the control unit  18  outputs the second image, the calibration unit  16  can receive the second image from the control unit  18 . 
     The detail of the establishment of the control look-up table is described as follows. Refer to  FIG. 7  and  FIG. 8 , the above step S 300  and the setting of position points in a mapping transformation procedure are illustrated. In  FIG. 8 , the first image  40  has a first central point  41 , and the second image  50  has a second central point  52 . The first central point  41  corresponds to a first central mapping point  51  in the second image  50 , and the second central point  52  corresponds to a second central mapping point  42  in the first image  40 . The first central mapping point  51  can be thought as formed by projecting the first central point  41  on the second image  50 , and the second central mapping point  42  can be thought as formed by projecting the second central point  52  on the first image  40 . 
     The first camera  12  and the second camera  14  are disposed at different locations, whereby the FOV of the first image  40 , and the FOV of the second image  50  are partially the same. In the second image  50 , the location of the first central mapping point  51  differs from the location of the second central point  52 , and the distance between the first central mapping point  51  and the second central point  52  increases with the disposition distance of the first camera  12  and the second camera  14  increased. Similarly, in the first image  40 , the location of the second central mapping point  42  also differs from the location of the first central point  41 . 
     In  FIG. 7 , firstly the calibration unit  16  sets M position points  43  in the first image  40 , and M corresponding position points  53  in the second image  50 , where M is a positive integer greater than or equal to 3 (step S 310 ). For example, M is 50 or 100. The position point  43  and the position point  53  specify the same position in the first image  40  and the second image  50 . Because the first camera  12  and the second camera  14  are disposed at different locations, the position of an object in the FOV of the first image  40  and the position of this object in the FOV of the second image  50  are different. For example, a coordinate of a point in an upper right corner of a picture in the first image  40  is (160, 180), and a coordinate of the same point in the second image  50  is (270, 80). 
     The selection and setting of the position points can be performed manually or be performed by a feature point extraction and matching technology to find out M pairs of the position point  43  of the first image  40  and the position point  53  of the second image  50 . 
     The calibration unit  16  groups every three of the M position points  43  of the first image  40  together to generate N position point sets according to an order of all combinations, where N is a positive integer (step S 320 ). Specifically, N is a number of combinations C 3   M  representing that three of the M position points  43  are selected every time. Assume that M is 4. The calibration unit  16  will select the first one, second one and third one of the four position points  43  to be a first position point set, select the first one, second one and fourth one of the four position points  43  to be a second position point set, select the first one, third one and fourth one of the four position points  43  to be a third position point set, and select the second one, third one and fourth one of the four position points  43  to be a fourth position point set. Thus, there will totally be four position point sets (C 3   4 =4). 
     Subsequently, the calibration unit  16  calculates N coordinate transformation parameter sets corresponding to the N position point sets respectively (step S 330 ). In other words, the calibration unit  16  performs a mapping transformation procedure on each position point  43  of each of the N position point sets and on each corresponding position point  53  to calculate and obtain a coordinate transformation parameter set. Assume that the locations of the M position points  43  of the first image  40  and the locations of the M position points  53  of the second image  50  are known, the mapping transformation procedure can be performed by the following equation: 
     
       
         
           
             
               
                 
                   
                     
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     where (X i , Y i ) represents a coordinate of the position point  43  of the first image  40 , (X o , Y o ) represents a coordinate of the position point  53  of the second image  50 , (a1, a2, a3, b1, b2, b3) represents a coordinate transformation parameter set of a pair of the position points  43  and  53 . In this way, the N coordinate transformation parameter sets can be calculated and obtained according to the N position point sets. In the subsequent steps, the control unit  18  can map any point in the first image  40  onto a corresponding point in the second image  50  according to the obtained coordinate transformation parameter sets. In other words, according to the coordinate transformation parameter sets, the control unit  18  can utilize a location of one point in the first image  40  to calculate and obtain a location of a corresponding point in the second image  50 . 
     Subsequently, the calibration unit  16  sets a plurality of coordinate points  44  in the first image  40 . In an embodiment, all the pixels in the first image  40  can be set as the coordinate points  44  except the pixels at the position points  43 . In another embodiment, some pixels in the first image  40  are sampled and set as coordinate points  44  in order to reduce the computing quantity and time. For instance, one pixel in every 8×8 pixels or every 16×16 pixels is selected and set as one coordinate point  44  as shown in  FIG. 8 . 
     The calibration unit  16  performs the mapping transformation procedure on each of the coordinate points  44  in the first image  40  according to one of the N coordinate transformation parameter sets to obtain a capture position parameter corresponding to the coordinate point  44  for the second camera  14  (step S 340 ), and stores the obtained capture position parameter in the control look-up table (step S 350 ). In other words, the calibration unit  16  in step S 340  can sequentially select one coordinate point  44  to perform the mapping transformation procedure on the selected coordinate point  44 , and then can determine whether all the coordinate points  44  have been selected or not. If some coordinate points  44  have not been selected, the calibration unit  16  can successively select next one of these coordinate points  44  to perform the mapping transformation procedure until all the coordinate point  44  are selected. 
       FIG. 9  is a flowchart of the step S 340  in  FIG. 7  according to an embodiment of the disclosure. One of the N position point sets in the first image  40  is selected according to current one of the coordinate points  44  (step S 341 ). Specifically, the selected position point set has three position points  43  which are the closest to the current coordinate point  44  in the first image  40 . The calibration unit  16  sums up a distance between the current coordinate point  44  and each of the three position points  43 , and then selects the position point set corresponding to the minimum sum total. 
     Then, the mapping point of the second image  50  corresponding to the current coordinate point  44  is calculated according to the coordinate transformation parameter set corresponding to the selected position point set (step S 342 ), and the capture position parameter corresponding to the mapping point is obtained (step S 343 ). The coordinate transformation parameter set can be expressed as a transformation matrix as in equation (1), and the calibration unit  16  can multiply the coordinate of the current coordinate point  44  by the transformation matrix to obtain a coordinate of the corresponding mapping point. 
     For example, the coordinate of the coordinate point  44  and the coordinate of the mapping point can be expressed by a rectangular coordinate (x, y) in the Cartesian coordinate system or by a round coordinate (pan, tilt) in the Polar coordinate system. Assume that the disclosure takes the round coordinate manner as an exemplary embodiment, the capture position parameter includes a tilt coefficient and a pan coefficient for the second camera  14 , and the tilt coefficient and the pan coefficient respectively correspond to a pan angle and a tilt angle which belong to the mapping point. The rectangular coordinate and the round coordinate can be defined as shown in  FIG. 10 . The rectangular coordinate and the round coordinate can be transformed to each other by using trigonometric functions. For example, the rectangular coordinate (x, y) is transformed to the round coordinate (pan, tilt) by the following equations: 
     
       
         
           
             
               
                 
                   
                     pan 
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     In a nutshell, the calibration unit  16  in the step S 340  calculates the mapping points which respectively correspond to the coordinate points  44 , sets the round coordinate of the mapping points to be capture position parameters, and stores the capture position parameters in the control look-up table. Through the control look-up table, the control unit  18  can look up a mapping point, corresponding to each coordinate point  44 , in the second image  50  much faster. 
     In the control method, the steps S 100  to S 300  are for calibration, and the steps S 400  and S 500  are performed to control the second camera  14  according to the designating command and the control look-up table in real time. The control unit  18  repeats the steps S 400  and S 500  to respond every designating command. Refer to  FIG. 3 , after the control look-up table is established, the control unit  18  can unceasingly determine whether any designating command is provided by a user, and the designating command in this and some embodiments specifies the ROI in the first image. If a designating command is provided, the control unit  18  receives the designating command (step S 400 ). In an exemplary embodiment, the linking-up photographing system supports a graphical user interface (GUI) which allows users to intuitively input designating commands into it. The ROI in this and some embodiments can be circular, so that users can use a fixed circle figure to set the ROI. The GUI also allows users to set the location of the center of circle of the ROI and set the radius of the ROI. Moreover, the shape of the ROI can be a rectangle, a rectangle with round-corners, or an ellipse, but does not limit the disclosure. 
     Then, the FOV of the second camera  14  is adjusted according to the ROI and the control look-up table, and then the control unit  18  controls the second camera  14  to acquire a third image corresponding to the ROI (step S 500 ). The control unit  18  utilizes the tilt coefficient and the pan coefficient to control the second camera  14 , and then the second camera  14  turns to a defined angle according to the tilt coefficient and the pan coefficient to obtain the third image under the FOV corresponding to the two coefficients. 
       FIG. 11  illustrates the step S 500  according to an embodiment of the disclosure. Firstly, the control unit  18  looks up a capture position parameter corresponding to the ROI in the control look-up table according to a third central point of the ROI (step S 510 ), and then controls the second camera  14  to capture a view according to the capture position parameter, so as to obtain the third image (step S 520 ). If the ROI is circular, the third central point is set as a center of this circle. If the third central point is located at one coordinate point  44 , the control unit  18  can look up a capture position parameter, e.g. a tilt coefficient and a pan coefficient, corresponding to this coordinate point  44  in the control look-up table. If the third central point is not located at any coordinate point  44 , the control unit  18  can look up the coordinate point  44  adjacent to the third central point in the control look-up table, and then calculate the capture position parameter corresponding to the third central point, by using an interpolation method. 
       FIG. 12  illustrates the interpolation method according to an embodiment of the disclosure. Assume that the third central point  61  is located in between four adjacent coordinate points  44   a ,  44   b ,  44   c  and  44   d , a distance between two adjacent coordinate points  44 , e.g. the coordinate point  44   a  and the coordinate point  44   b , is one unit, a distance between the third central point  61  and the coordinate point  44   a  on the traverse is m, and a distance between the third central point  61  and the coordinate point  44   a  on the longitudinal axis is n, where m and n are positive integers less than 1. In this way, the capture position parameter of the third central point  61  can be obtained through the interpolation method by using the following equation:
 
 P=[A ×(1 −m )+ B×m ]×(1 −n )+[ C ×(1 −m )+ D×m]×n   (4);
 
     where P is the capture position parameter of the third central point  61 , e.g. a tilt coefficient and a pan coefficient, and A, B, C and D are capture position parameters of the coordinate points  44   a ,  44   b ,  44   c  and  44   d . P, A, B, C and D can be expressed as number pairs, e.g. (pan, tilt). 
     The control unit  18  looks up the tilt coefficients and pan coefficients of the coordinate points  44   a  to  44   d , and then calculates the corresponding tilt coefficient and pan coefficient of the third central point  61  by the interpolation method. In this way, the control unit  18  can control the second camera  14  to turn to a defined angle corresponding to the tilt coefficient and the pan coefficient, according to the location of the third central point, so as to obtain the third image under the FOV based on the defined angle. 
     In some embodiments, the second camera  14  can perform an optical zoom or a digital zoom according to the size of the ROI. In other words, a scaling parameter of the second camera  14  is changed according to the ROI. Thus, the capture position parameter further includes a scaling angle specifying the scaling parameter which can be a zoom ratio, a focus value or a view angle for capturing images. The zoom ratio is the relative ratio between the longest and the shortest zoom focal length of the camera lens module, and the focus value or the view angle can be absolute values and be designed and controlled according to different lens sets. 
     Refer to  FIG. 13A  and  FIG. 14 , the detail of the step S 510  and the boundary mapping points are illustrated. In this embodiment, the ROI  60  is circular and includes a third central point  61  and a plurality of boundary points  62 . The boundary points  62  surround the ROI  60  to form a boundary. The third central point  61  corresponds to a third central mapping point  71  of a second image  50 , and the boundary points  62  correspond to a plurality of boundary mapping points  72  of the second image  50  respectively. 
     The control unit  18  calculates the third central mapping point  71  of the second image  50  corresponding to the third central point  61 , and the boundary mapping points  72  of the second image  50  corresponding to the boundary points  62  respectively (step S 511 ). The coordinate of the third central mapping point  71  and the coordinate of the boundary mapping points  72  can be obtained fast by using the above mapping procedure and the above control look-up table or by using the above control look-up table and the above interpolation method. 
     Line  73  contains the second central point  52  and the third central mapping point  71 . The control unit  18  checks every acute angle formed by every mapping point  72  located at one side of line  73 , the second central point  52 , and the third central mapping point  71 , and selects the corresponding mapping point  72  to the maximum acute angle as  72   a . The control unit  18  checks every acute angle formed by every mapping point  72  located at the other side of line  73 , the second central point  52 , and the third central mapping point  71 , and selects the corresponding mapping point  72  to the maximum acute angle as  72   b . The control unit  18  selects a maximum one of angles formed by the boundary mapping point  72   a , the second central point  52  and the third central mapping point  71  or by the boundary mapping point  72   b , the second central point  52  and the third central mapping point  71 , and multiplies the selected maximum angle by 2 to obtain a scaling angle  56  (step S 512 ). 
     The scaling angle  56  is not directly formed by the boundary mapping point  72   a , the second central point  52  and the boundary mapping point  72   b  but by multiplying the maximum acute angle by 2. The graph formed by these boundary mapping points  72  which are looked up in the control look-up table according to the boundary points  62 , is asymmetrical as compared with line  73 . This causes that the FOV corresponding to a scaling angle which is directly formed by the boundary mapping point  72   a , the second central point  52  and the boundary mapping point  72   b , can not fully cover the boundary mapping points  72   a  and  72   b . Therefore, when the maximum angle is multiplied by 2 to obtain the scaling angle and when a central point of the third image is the third central mapping point  71 , i.e. the third central point  61 , the left side and right side of the third image will cover the boundary mapping point  72   a , i.e. the boundary point  62   a , and the boundary mapping point  72   b , i.e. the boundary point  62   b . The scaling angle  56  herein is rather calculated and obtained according to the ROI  60  in real time than looked up in the control look-up table. 
     Then, the control unit  18  looks up the capture position parameter corresponding to the ROI  60  in the control look-up table according to the third central mapping point  71  (step S 513 ). The capture position parameter includes a tilt coefficient and a pan coefficient. 
     The horizontal lines in the fourth images  60  are curved to be arcs, e.g. the reference axis  33  in  FIG. 6I . All the reference axes at the same horizontal attitude of the fourth images  60  which are captured according to the same tilt coefficient and various pan coefficients will be curved and form a circle in the second image  50 . For example, a second reference circle  55  is shown in  FIG. 14  and  FIG. 15 . 
     The greater the ROI  60  in  FIG. 14  is, the greater the distance between the boundary mapping points  72   a  and  72   b , and the scaling angle  56  in  FIG. 15  are. This means that the ROI  60  presents a wider FOV. Herein, the second camera  14  can reduce its scaling parameter to obtain a wider FOV, or can increase its scaling parameter to obtain a narrower FOV to obtain more detailed information about remote objects. Thus, the scaling angle  56  can represent a scaling parameter for capturing the third image. 
     Furthermore, refer to  FIG. 13B  and  FIG. 14 , the detail of the step S 510  and the boundary mapping points are illustrated according to another embodiment of the disclosure. In this embodiment, the ROI  60  is circular. 
     In step S 510 , the control unit  18  calculates a second central mapping point  42  of the first image  40  corresponding to the second central point  52 , and calculates a third central mapping point  71  of the second image  50  corresponding to the third central point  61  (step S 515 ). Similarly, the coordinate of the third central mapping point  71  and the coordinate of the second central mapping point  42  can be looked up in the control look-up table or further be obtained by using the interpolation method. 
     The control unit  18  can set the second central mapping point  42  to be a center of circle, and set a distance between the second central mapping point  42  and the third central point  61  to be a radius, so as to calculate and obtain a first reference circle  45 . The first reference circle  45  joins two of the boundary points  62  (step S 516 ), e.g. the boundary points  62   a  and  62   b  in  FIG. 14 . 
     Subsequently, the control unit  18  calculates and obtains the boundary mapping points  72   a  and  72   b  of the second image  50  corresponding to the boundary points  62   a  and  62   b  joining the first reference circle  45  (step S 517 ), and calculates the scaling angle  56  according to the boundary mapping points  72   a  and  72   b  and the second central point  52  (step S 518 ). The boundary mapping points  72   a  and  72   b  obtained in step S 517  may differ from the boundary mapping points  72   a  and  72   b  obtained in step S 512 . 
     After the scaling angle  56  is obtained, the control unit  18  looks up the capture position parameter corresponding to the ROI  60 , in the control look-up table according to the third central mapping point  71  (step S 519 ). The capture position parameter includes a tilt coefficient and a pan coefficient. When calculating the scaling angle  56 , the control unit  18  in this embodiment only utilizes the boundary points  62   a  and  62   b  joining the first reference circle  45 , and the corresponding boundary mapping points  72   a  and  72   b  and does not need to calculate other boundary points  62  or other boundary mapping points  72 . Thus, the disclosure may calculate and obtain the scaling angle  56  faster and meanwhile keep high accuracy. 
     In this way, according to the control look-up table established by the calibration unit  16  in advance and according to the ROI  60  defined by users, the control unit  18  can calculate and obtain the corresponding tilt coefficient and pan coefficient in a capture position parameter fast in real time, and then control the second camera  14  according to the capture position parameter, for obtaining the third image under the FOV corresponding to the ROI  60 . Specifically, the FOV of the third image is similar to the ROI  60  but the third image has a higher resolution and more details than the ROI  60  in the first image  40 . 
     On the other hand, the disclosure further provides another embodiment of the linking-up photographing system. Refer to  FIG. 16 , the linking-up photographing system includes a first camera  22 , a second camera  24  and a control module  26 . The first camera  22  and the second camera  24  connect to the control module  26 . The disposition, operation and type of the first camera  22  and of the second camera  24  are the same as the embodiment in  FIG. 1  and are not described hereinafter thereby. 
     The control module  26  can be embodied by a personal computer, a network video recorder (NVR), an embedded system or an electronic device having a computing function in this and some embodiments. After the first camera  22  and the second camera  24  connect to the control module  26 , the processing unit  131  performs a control method of linked-up cameras according to the first image and the second image which are equal to the first image and the second image obtained in  FIG. 1 . The differences between the linking-up photographing systems in  FIG. 1  and  FIG. 16  are that the control module  26  in  FIG. 16  includes a processing unit  131  which can perform a link-up control procedure and even perform a motion detection procedure or an object detection procedure, and that the control module  26  rather directly calculates a capture position parameter according to a designating signal, the first image and the second image in real time than looks up the capture position parameter in a control look-up table, for controlling the second camera  24  to operate according to the capture position parameter. 
     Refer to  FIG. 16  and  FIG. 17 , the control method in the link-up control procedure performed by the linking-up photographing system includes the following steps. Firstly, the processing unit  131  controls the first camera  22  to capture the first image shown in  FIG. 4A  according to its FOV, and controls the second camera  24  to capture many fourth images shown in  FIG. 4B  according to various FOVs (step S 610 ). The fourth images are planar and panoramically stitched to generate the second image shown in  FIG. 2C  (step S 620 ). 
     Subsequently, at least three position points are set in the first image and in the second image (step S 630 ), and the locations of the three position points in the first image correspond to the locations of the three position points in the second image respectively. Then, a desired object under observation, is set in the first image (step S 640 ), three position points, which are the closest to the object under observation in the first image, are selected from the at least three position points to calculate a coordinate transformation parameter through which the first image can map onto the second image (step S 650 ). 
     Finally, after a location, i.e. the capture position parameter, of the object under observation in the second image is calculated according to the coordinate transformation parameter (step S 660 ); the FOV of the second camera  24  is adjusted to photograph the object under observation (step S 670 ). 
     In this embodiment, the panoramic stitching can refer to the above description of  FIG. 6A  to  FIG. 6K , the setting of position points in the first image and in the second image can refer to the above description of  FIG. 8 , the setting of the object under observation can refer to the description of  FIG. 8 , the selection of the three position points can refer to the description of  FIG. 8  and  FIG. 9 , and the calculation of the location of the object under observation in the second image can refer to the description of equation (1),  FIG. 14  and  FIG. 15 , whereby they will not be illustrated hereinafter. 
     Additionally, the control module in the disclosure can further include a storage unit, a first signal input unit, a second signal input unit, a signal output unit, a communication unit and a display unit all of which are not shown in the figures and connect to the processing unit. The storage unit can store the first image, the second image, the fourth images, the mapping transformation procedure, and the link-up control procedure, and can even further store the motion detection procedure or the object detection procedure. The storage unit is a random access memory (RAM), a flash memory or a hard disk. The first signal input unit connects to the first camera and the second camera, for receiving the first image, the second image and the fourth images. The types and quantity of connection ports of the first signal input unit can be designed according to various application requirements. The second signal input unit receives the designating command for selecting the desired object under observation. As above, in this and some embodiments, the designating command can be provided manually by observers or be provided automatically by performing a motion detection procedure or an object detection procedure, or be provided by an external computing device. The signal output unit connects to the second camera and sends a driving signal to the second camera for adjusting the FOV of the second camera. The signal output unit and the first signal input unit can be embodied in the same component or in two separate components. Similarly, the signal output unit and the second signal input unit can be embodied in the same component or in two separate components. The communication unit connects the linking-up photographing system to a network. The display unit can display the first image and the fourth images in real time. The linking-up photographing system can further connect to a server or terminal display device through the network. 
     As set forth above, the disclosure uses a fisheye camera, a panoramic camera or a wide-angle camera to capture a panoramic image, i.e. the above first image, and uses a PTZ camera to capture another panoramic image, i.e. the above second image, whose a FOV overlaps that of the first image. Then, the disclosure establishes the above control look-up table according to the mapping relation between these two panoramic images, and then obtains a desired capture position parameter according to the designating command fast by the look-up manner. By using the capture position parameter, the disclosure can perform the above control method to fast control many linked-up cameras to operate. Therefore, the disclosure may save great computing quantity caused by calculating the capture position parameter in real time. Moreover, the disclosure also uses the look-up manner to directly obtain the tilt coefficient and pan coefficient corresponding to the ROI and uses the first reference circle to fast calculate a scaling angle corresponding to the ROI. Thus, the disclosure can fast control many cameras to operate in linking-up manner, and reduce the response time which users have to wait.