Patent Publication Number: US-10309770-B2

Title: Three-dimensional sensor system and three-dimensional data acquisition method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the national phase entry of International Application PCT/CN2017/086258, filed on May 27, 2017, which is based upon and claims priority to Chinese Patent Application No. CN 201610824489.3, filed on Sep. 14, 2016, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to the field of three-dimensional scanning, and more particularly to a three-dimensional sensor system and a three-dimensional data acquisition method. 
     BACKGROUND 
     Currently, the optical three-dimensional scanning technology is gradually used in a variety of fields such as industry, archaeology, medical treatment, and teaching etc. The triangulation method is the most popular method due to a wide range of applications, high accuracy, and a cost-effective performance. The existing products that use the triangulation principle includes laser distance-measuring sensors, three-dimensional contour sensors, three-coordinate instrument laser probe, handheld laser three-dimensional scanner, handheld projection three-dimensional scanner, fixed projection three-dimensional scanner, etc. 
     A common method to implement triangulation is to project a single linear pattern by laser or pattern projector. The principle of this method is relatively simple. Given the premise that the location of light planes emitted by the pattern projector and the location of the camera are known, the photosensitive element of camera captures a projection linear pattern on the image. The points on the projection linear pattern are connected to the optical center of the camera. The intersection points of the connection line and the light planes projected by the pattern projector are the desired three-dimensional points on the surface of the scanned object. The triangulation method has the advantages of high scanning accuracy, fast scanning speed, uncomplicated hardware, and high cost performance. Thus, the triangulation method is widely used in the occasions of close-distance contactless scanning. However, generally, the scanner or three-dimensional sensor that uses the triangulation method only project one linear pattern. If a plurality of linear patterns are simultaneously projected, there will be a mismatch of the image, so the effective three-dimensional data cannot be obtained. Therefore, the point-determining speed of the traditional scanner or three-dimensional sensor that uses the triangulation method is greatly restricted, and is not satisfactory for the occasions that require high scanning speed. 
     SUMMARY OF THE INVENTION 
     Regarding the drawbacks of the prior art, the present invention aims to provide a three-dimensional sensor system that uses a multiple-linear-pattern projection manner, and has a high point-determining efficiency and fast scanning speed. 
     In order to realize the above objectives, the technical solution of the present invention is as follows. 
     A three-dimensional sensor system includes at least one pattern projector, at least two cameras, a two-dimensional image feature extractor, a three-dimensional point-cloud generator, and a three-dimensional point-cloud verifier. 
     The pattern projector is configured to project at least two linear patterns simultaneously. 
     The at least two cameras are configured to capture a two-dimensional image of a scanned object simultaneously. 
     The two-dimensional image feature extractor is configured to extract a two-dimensional line set of the at least two linear patterns on a surface of a scanned object on a two-dimensional image. 
     The three-dimensional point-cloud generator is configured to generate a candidate three-dimensional point set based on the two-dimensional line set. 
     The three-dimensional point-cloud verifier is configured to select an authentic three-dimensional point set that correctly matches with a projection contour line on a surface of the object from the candidate three-dimensional point set. 
     As a further improvement of the present invention, the two-dimensional image feature extractor is configured to extract a two-dimensional line set of the at least two linear patterns on a surface of a scanned object on a two-dimensional image. Specifically, the two-dimensional image feature extractor performs a distortion correction on the two-dimensional image according to an internal parameter of a camera corresponding to the two-dimensional image. A connected region of a line contour in a corrected image is extracted according to a pixel gray-level difference. The two-dimensional line set of a highlight center with a sub-pixel level is calculated according to a gray-level centroid of the connected region. 
     As a further improvement of the present invention, the three-dimensional point-cloud generator is configured to generate a candidate three-dimensional point set based on the two-dimensional line set. Specifically, the three-dimensional point-cloud generator extracts two-dimensional point data from the two-dimensional line sets of at least two synchronous two-dimensional images respectively. The candidate three-dimensional point set is calculated according to the principles of triangulation and epipolar constraints based on a correlation between the spatial location of the cameras. 
     The three-dimensional point-cloud verifier is configured to select an authentic three-dimensional point set that correctly matches with a projection contour line on a surface of the object from the candidate three-dimensional point set. Specifically, the three-dimensional point-cloud verifier determines whether a point belongs to the authentic three-dimensional point set based on whether the point in the candidate three-dimensional point set is located in a certain three-dimensional light plane projected by the pattern projector and performs a selection to obtain the authentic three-dimensional point set. 
     As a further improvement of the present invention, the three-dimensional point-cloud verifier determines whether a point belongs to the authentic three-dimensional point set based on whether the point in the candidate three-dimensional point set is located in a certain three-dimensional light plane projected by the pattern projector and performs a selection to obtain the authentic three-dimensional point set. Specifically, the candidate three-dimensional point set includes a plurality of sub-sets. On the basis of distances between the sub-sets and the three-dimensional light plane, a sub-set having a minimum distance value is selected by the three-dimensional point-cloud verifier as the authentic three-dimensional point set. 
     As a further improvement of the present invention, at least three cameras are provided. 
     The three-dimensional point-cloud generator is configured to generate the candidate three-dimensional point set based on the two-dimensional line set. Specifically, the three-dimensional point-cloud generator extracts the two-dimensional point data from the two-dimensional line sets of two synchronous two-dimensional images respectively. The candidate three-dimensional point set is calculated according to the principles of triangulation and epipolar constraints based on the correlation between the spatial location of the two cameras corresponding to the two synchronous two-dimensional images. 
     The three-dimensional point-cloud verifier is configured to select the authentic three-dimensional point set that correctly matches with a projection contour line on the surface of the object from the candidate three-dimensional point set. Specifically, the three-dimensional point-cloud verifier uses the two-dimensional image shot by a third camera or more cameras to perform a data verification on the candidate three-dimensional point set and to select the authentic three-dimensional point set. 
     As a further improvement of the present invention, the three-dimensional point-cloud verifier uses the two-dimensional image shot by a third camera or more cameras to perform a data verification for the candidate three-dimensional point set and to select the authentic three-dimensional point set. Specifically, the candidate three-dimensional point set includes a plurality of sub-sets. A connection line between the subset and a photocenter of the third camera has an intersection point set with respect to the two-dimensional image shot by the third camera. On the basis of distances between the intersection point set and the two-dimensional line on the two-dimensional image shot by the third camera, a sub-set corresponding to a minimum distance value is selected by the three-dimensional point-cloud verifier as the authentic three-dimensional point set. 
     As a further improvement of the present invention, the three-dimensional point-cloud generator is configured to generate a candidate three-dimensional point set based on the two-dimensional line set. Specifically, the three-dimensional point-cloud generator extracts two-dimensional point data from the two-dimensional line set of an image shot by any camera. The candidate three-dimensional point set is obtained according to the principles of triangulation based on a correlation between the spatial location of a plurality of spatial light planes projected by the pattern projector and the camera. 
     The three-dimensional point-cloud verifier is configured to select an authentic three-dimensional point set that correctly matches with a projection contour line on the surface of the object from the candidate three-dimensional point set. Specifically, the three-dimensional point-cloud verifier verifies the candidate three-dimensional point set with an image shot by at least one other camera and performs a selection to obtain the authentic three-dimensional point set. 
     As a further improvement of the present invention, the three-dimensional point-cloud verifier verifies the candidate three-dimensional point set with an image shot by at least one other camera and performs a selection to obtain the authentic three-dimensional point set. Specifically, the candidate three-dimensional point set includes a plurality of sub-sets. A connection line between the subset and a photocenter of the at least one other camera has an intersection point set with respect to an image shot by the at least one other camera. On the basis of distances between the intersection point set and the two-dimensional line on the image shot by the at least one other camera, a sub-set corresponding to a minimum distance value is selected by the three-dimensional point-cloud verifier and the sub-set corresponding to the minimum distance value is the authentic three-dimensional point set. 
     As a further improvement of the present invention, the linear pattern is projected by one pattern projector or by a plurality of pattern projectors synchronously. The linear pattern is a straight line or a curved line. 
     As a further improvement of the present invention, the pattern projector includes a linear laser and a DOE beam splitting element. The linear laser produces a plurality of laser line segments through the DOE beam splitting element. 
     As a further improvement of the present invention, the pattern projector includes a projector. The projector projects the at least two linear patterns directly. 
     As a further improvement of the present invention, the three-dimensional sensor system includes a synchronizing trigger. The synchronizing trigger is used to trigger the camera and the pattern projector to operate synchronously. 
     A three-dimensional data acquisition method includes the following steps: 
     projecting, by a pattern projector, at least two linear patterns; 
     capturing, synchronously by at least two cameras, a two-dimensional image; 
     extracting a two-dimensional line set of at least two linear patterns on a surface of a scanned object on a two-dimensional image; 
     generating an candidate three-dimensional point set based on the two-dimensional line set; 
     selecting an authentic three-dimensional point set that correctly matches with a projection contour line on the surface of the object from the candidate three-dimensional point set. 
     As a further improvement of the present invention, extracting a two-dimensional line set on a two-dimensional image of at least two linear patterns on a surface of a scanned object specifically includes performing a distortion correction for the two-dimensional image according to an internal parameter of the camera corresponding to the two-dimensional image, extracting a connected region of a line contour in a corrected image according to a pixel gray-level difference, and calculating the two-dimensional line set of a highlight center with a sub-pixel level according to a gray-level centroid of the connected region. 
     As a further improvement of the present invention, generating a candidate three-dimensional point set based on the two-dimensional line set specifically includes extracting two-dimensional point data from the two-dimensional line sets of at least two synchronous two-dimensional images respectively, and obtaining the candidate three-dimensional point set by a calculation according to the principles of triangulation and epipolar constraints based on a correlation between the spatial location of the cameras. 
     Selecting an authentic three-dimensional point set that correctly matches with a projection contour line on the surface of the object from the candidate three-dimensional point set specifically includes determining whether a point belongs to the authentic three-dimensional point set based on whether the point in the candidate three-dimensional point set is located in a certain three-dimensional light plane projected by the pattern projector and performing a selection to obtain the authentic three-dimensional point set. 
     As a further improvement of the present invention, whether a point belongs to the authentic three-dimensional point set is determined based on whether the point in the candidate three-dimensional point set is located in a certain three-dimensional light plane projected by the pattern projector and a selection is performed to obtain the authentic three-dimensional point set. Specifically, the candidate three-dimensional point set includes a plurality of sub-sets, and on the basis of distances between the sub-sets and the three-dimensional light plane, a sub-set with a minimum distance value is selected, and the sub-set with a minimum distance value is the authentic three-dimensional point set. 
     As a further improvement of the present invention, at least three cameras are provided. 
     Generating an candidate three-dimensional point set based on the two-dimensional line set specifically includes extracting two-dimensional point data from the two-dimensional line sets of two synchronous two-dimensional images respectively, and obtaining the candidate three-dimensional point set by a calculation according to the principles of triangulation and epipolar constraints based on a correlation between the spatial location of the two cameras corresponding to the two synchronous two-dimensional images. 
     Selecting an authentic three-dimensional point set that correctly matches with a projection contour line on a surface of the object from the candidate three-dimensional point set specifically includes using a two-dimensional image shot by a third camera or more cameras to perform a data verification for the candidate three-dimensional point set and selecting the authentic three-dimensional point set. 
     As a further improvement of the present invention, the two-dimensional image shot by a third camera or more cameras is used to perform a data verification for the candidate three-dimensional point set and the authentic three-dimensional point set is selected. Specifically, the candidate three-dimensional point set includes a plurality of sub-sets. A connection line between the subset and a photocenter of the third camera has an intersection point set with respect to the two-dimensional image shot by the third camera. On the basis of distances between the intersection point set and the two-dimensional line on the two-dimensional image shot by the third camera, a sub-set corresponding to a minimum distance value is selected as the authentic three-dimensional point set. 
     As a further improvement of the present invention, generating a candidate three-dimensional point set based on the two-dimensional line set specifically includes extracting the two-dimensional point data from the two-dimensional line set of an image shot by any camera, and obtaining the candidate three-dimensional point set according to the principles of triangulation based on a correlation between the spatial location of a plurality of spatial light planes projected by the pattern projector and the camera. 
     Selecting an authentic three-dimensional point set that correctly matches with a projection contour line on a surface of the object from the candidate three-dimensional point set specifically includes verifying the candidate three-dimensional point set with an image shot by at least one other camera and selecting the authentic three-dimensional point set. 
     As a further improvement of the present invention, the candidate three-dimensional point set is verified with an images shot by at least one other camera and the authentic three-dimensional point set is selected. Specifically, the candidate three-dimensional point set includes a plurality of sub-sets. A connection line of the subset and a photocenter of the at least one other camera has an intersection point set with respect to the image shot by the at least one other camera. On the basis of distances between the intersection point set and the two-dimensional line on the image shot by the at least one other camera, a sub-set corresponding to a minimum distance value is selected as the authentic three-dimensional point set. 
     Compared with the prior art, the advantages of the present invention areas follows. 
     Since the three-dimensional sensor system uses a projection manner in which a plurality of linear patterns are projected, the three-dimensional sensor system can recognize the plurality of linear patterns that are synchronously projected and calculate the three-dimensional point-cloud data on the surface of the object. Thus, the efficiency of point-determining is several times higher than the traditional single-line scanning. The scanning speed is significantly improved. 
     The foregoing illustrations are merely an overall description of the technical solution of the present invention. In order to make the technical solution of the present invention clearly understood, to implement the present invention in accordance with the description of the specification, and to make the above or other objectives, features, advantages of the present invention apparent and understandable, the preferred embodiments are provided along with the drawings and described in detail hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a three-dimensional sensor system according to an embodiment of the present invention; 
         FIG. 2  is a schematic diagram of pattern projection and image capturing of the three-dimensional sensor system according to an embodiment of the present invention; 
         FIG. 3  is a schematic diagram of a method for obtaining an authentic three-dimensional point set according to the first embodiment of the present invention; 
         FIG. 4  is a schematic diagram of a method for obtaining an authentic three-dimensional point set according to the second embodiment of the present invention; 
         FIG. 5  is a schematic diagram of a method for obtaining an authentic three-dimensional point set according to the third embodiment of the present invention. 
     
    
    
     Description of reference numerals is as follows.  100 . object;  210 . pattern projector;  220 . first camera;  221 . first image;  230 . second camera;  231 . second image;  240 . third camera;  241 . third image;  310 . pattern projector;  320 . first camera;  321 . first image;  330 . second camera;  331 . second image;  410 . first camera;  411 . first image;  420 . second camera;  421 . second image;  430 . third camera;  431 . third image;  510 . pattern projector;  520 . first camera;  521 . first image;  530 . second camera;  531 . second image. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described in detail hereinafter with reference to the drawings and specific embodiments. 
       FIG. 1  is a schematic diagram of a three-dimensional sensor system according to an embodiment of the present invention. The three-dimensional sensor includes a first camera, a second camera, a third camera, a synchronizing trigger, a pattern projector, a two-dimensional image extractor, and a three-dimensional point-cloud generator. It should be noted that the number of cameras is at least two, and the number of pattern projector is at least one. The number of camera and pattern projector is not limited thereto. The main operation procedure of the three-dimensional sensor system is as follows. 
     Step 1, at least two linear patterns are projected by the pattern projector. 
     Preferably, the linear pattern may be projected by one pattern projector, or synchronously projected by a plurality of pattern projectors. The linear pattern is a straight line or a curved line. The pattern projector includes a linear laser and a DOE beam splitting element. A plurality of laser line segments are produced by the linear laser through the DOE beam splitting element. 
     Preferably, the pattern projector may also be a projector, and the projector directly projects the at least two linear patterns. 
     Step 2, at least two cameras synchronously capture a two-dimensional image of a scanned object. 
     Preferably, step 1 and step 2 may be performed synchronously. Specifically, the exposure of the first camera and the second camera are triggered by the synchronizing trigger while the pattern projector is triggered by the synchronization trigger. The two frames of image captured by the two cameras are respectively output to the two-dimensional image extractor for feature extraction. 
     Step 3, the two-dimensional image feature extractor extracts a two-dimensional line set of at least two linear patterns on the surface of the scanned object on a two-dimensional image. 
     Preferably, after the two-dimensional image extractor performs a distortion correction on the image according to an internal parameter of a camera corresponding to the two-dimensional image, a connected region of a line contour with high gray-level in a corrected image is extracted according to a pixel gray-level difference. Next, the two-dimensional line set of a highlight center with a sub-pixel level is calculated according to a gray-level centroid of the connected region. Moreover, the obtained two-dimensional line set is output to the three-dimensional point cloud generator. 
     Step 4, the three-dimensional point-cloud generator generates a candidate three-dimensional point set based on the two-dimensional line set. 
     Step 5, the three-dimensional point-cloud verifier selects an authentic three-dimensional point set that correctly matches the projection counter line on the surface of the object from the candidate three-dimensional point set. 
     Specifically, step 4 and step 5 can be implemented by three methods presented as follows. 
     The first method is that the three-dimensional point-cloud generator extracts two-dimensional point data from the two-dimensional line sets of at least two synchronous two-dimensional images respectively. The candidate three-dimensional point set is calculated according to the principles of triangulation and epipolar constraints based on a correlation between the spatial location of the cameras. The three-dimensional point-cloud verifier determines whether a point belongs to the authentic three-dimensional point set based on whether the point in the candidate three-dimensional point set is located in a certain three-dimensional light plane projected by the pattern projector. A selection is performed to obtain the authentic three-dimensional point set. 
     Preferably, the candidate three-dimensional point set includes a plurality of sub-sets. On the basis of distances between the sub-sets and the three-dimensional light plane, the three-dimensional point-cloud verifier selects a sub-set having a minimum distance value as the authentic three-dimensional point set. 
     The second method is that the three-dimensional point-cloud generator extracts the two-dimensional point data from the two-dimensional line sets of two synchronous two-dimensional images respectively. The candidate three-dimensional point set is calculated according to the principles of triangulation and epipolar constraints based on a correlation between the spatial location of the two cameras corresponding to the two synchronous two-dimensional images. The three-dimensional point-cloud verifier uses the two-dimensional images shot by a third camera or other cameras to perform a data verification on the candidate three-dimensional point set. A selection is performed to obtain the authentic three-dimensional point set. 
     Preferably, the candidate three-dimensional point set includes a plurality of sub-sets. A connection line between the subset and a photocenter of the third camera has an intersection point set with respect to the two-dimensional image shot by the third camera. On the basis of distances between the intersection point set and the two-dimensional line on the two-dimensional image shot by the third camera, a sub-set corresponding to a minimum distance value is selected by the three-dimensional point-cloud verifier as the authentic three-dimensional point set. 
     The third method is that the three-dimensional point-cloud generator extracts two-dimensional point data from the two-dimensional line set of an image shot by any camera. The candidate three-dimensional point set is obtained according to the principles of triangulation based on a correlation between the spatial location of a plurality of spatial light planes projected by the pattern projector and the camera. The three-dimensional point-cloud verifier verifies the candidate three-dimensional point set with an image shot by at least one other camera and performs a selection to obtain the authentic three-dimensional point set. 
     Preferably, the candidate three-dimensional point set includes a plurality of sub-sets. A connection line between the subset and a photocenter of the at least one other camera has an intersection point set with respect to an image shot by the at least one other camera. On the basis of distances between the intersection point set and the two-dimensional line on the image shot by at least another camera, a sub-set corresponding to a minimum distance value is selected by the three-dimensional point-cloud verifier as the authentic three-dimensional point set. 
     In order to make the above three methods more understandable, examples are provided hereinafter. Firstly, referring to  FIG. 2 , which is a schematic diagram of the pattern projection and image capturing of the three-dimensional sensor system according to an embodiment of the present invention. In  FIG. 2 , an example is taken wherein there are three cameras with known spatial location correlation and a pattern projector which can synchronously project three linear patterns. Specifically, a three-dimensional sensor system includes pattern projector  210 , first camera  220 , first image  221 , second camera  230 , second image  231 , third camera  240 , and third image  241 . The specific steps to generate a two-dimensional line set are as follows. 
     S 1 , pattern projector  210  projects three light planes PL 1 , PL 2 , and PL 3 . Three three-dimensional spatial lines SR 1 , SR 2  and SR 3  are formed on the surface of object  100  by the light planes. First camera  220  and second camera  230  synchronously capture a two-dimensional image. First camera  220  and second camera  230  captures first image  221  and second image  231  respectively. First image  221  and second image  231  each includes a plurality of linear patterns which constitute a part of linear patterns on the surface of object  100 . The linear patterns are presented on the two-dimensional images in the form of two-dimensional lines including SA 1 , SA 2 , SA 3  and SB 1 , SB 2 , SB 3 . 
     S 2 , all two-dimensional lines SA 1 , SA 2 , SA 3  and SB 1 , SB 2 , SB 3  are extracted from first image  221  and second image  231  respectively. Given that the internal parameters of the camera are known, firstly, an image distortion correction is performed according to the internal parameters of the corresponding camera. The connected region of the line counter with high gray-level of the image is extracted according to the gray-level difference of pixels obtained by column traversal statistic. The two-dimensional line of a highlight center with a sub-pixel level is calculated according to a gray-level centroid of the connected region to obtain a two-dimensional line set. 
     Three methods of generating a candidate three-dimensional point set based on the two-dimensional line set and selecting an authentic three-dimensional point set from the candidate three-dimensional point set are described through three specific embodiments. Each of the three embodiments is based on the two-dimensional line set obtained by the three-dimensional sensor system shown in  FIG. 2 . Each of the three embodiments correspond to one of the three methods of obtaining the authentic three-dimensional point set mentioned above respectively. 
       FIG. 3  is a schematic diagram of a method for obtaining an authentic three-dimensional point set according to the first embodiment of the present invention. The schematic diagram contains pattern projector  310 , first camera  320 , first image  321 , second camera  330 , and second image  331 . Pattern projector  310  projects three light planes PL 1 , PL 2 , and PL 3 . First camera  320  and second camera  330  synchronously capture the two-dimensional image. First camera  320  and second camera  330  capture first image  321  and second image  331  respectively. O 1  and O 2  are the photocenters of first camera  320  and second camera  330  respectively. The internal and external parameters of the cameras are given. 
     The method includes the following steps. 
     3.1, first image  321  and second image  331  include a plurality of linear patterns which constitute a part of linear patterns on the surface of object  100 . The linear patterns are presented in the form of two-dimensional lines on the two-dimensional image, such as two-dimensional line SA 1  in first image  321 , wherein Pa i  is a two-dimensional point on the two-dimensional line SA 1 . The internal parameter M A  of first camera  320  and the internal parameter M B  of the second camera  330  are given, and the external parameter RT of first camera  320  with respect to second camera  330  is given. An Essential Matrix E=RS can be calculated according to the external parameter RT of first camera  320  with respect to second camera  330 , wherein R is the rotation matrix and T is the translation vector, 
             S   =       [         0         -     T   z             T   y               T   z         0         -     T   x                 -     T   y             T   x         0         ]     .           
A Fundamental Matrix F=(M A   −1 ) T E(M B   −1 ) can be obtained by internal parameters M A  and M B  of the two cameras. According to the principle of epipolar constraints, (Xa i ,Ya i ,1) T F(x,y,1)=0 is satisfied, wherein (Xa i ,Ya i ) is the coordinate of point Pa i , (x,y) is a two-dimensional coordinate of a point located on an epipolar line. After the epipolar line is obtained (N 1 , N 2  are the epipolar points), an intersection point set {Pb 1i , Pb 2i , Pb 3i } of the epipolar line intersecting with each of the two-dimensional lines on second image  331  can be obtained.
 
     3.2, the three-dimensional points of two-dimensional point Pa i  on first image  320  are calculated according to each point in the intersection point set {Pb 1i , Pb 2i , Pb 3i } by the triangular principle. Since the triangular principle is a common sense, the details of the triangular principle will not be described herein. These three-dimensional points is the set of all possible three-dimensional points corresponding to the two-dimensional point Pa′, i.e. the candidate three-dimensional point set M i ={P 1i , P 2i , P 3i }. The method to calculate the three-dimensional point coordinate is described below. First image  321  captured by first camera  320  is imaged on a photosensitive element plane PF 1 . Second image  331  captured by second camera  330  is imaged on a photosensitive element plane PF 2 . L 1  is a spatial connection line between point Pa i  on the two-dimensional line SA 1  and photocenter O 1  of first camera  320 . L 2  is a spatial connection line between point Pb 1i  on the two-dimensional line SB 1  on second image  331  and photocenter O 2  of second camera  330 . L 1  and L 2  intersects at a spatial point P 1i  which is one of the candidate three-dimensional points corresponding to the calculated point Pa i . If the spatial straight lines do not intersect with each other, a mid-point of an intersection line segment on a common perpendicular of the two spatial lines is used as a candidate three-dimensional point. Similarly, the three candidate three-dimensional points P 1i , P 2i , and P 3i  of point Pa i  can be calculated. It is apparent that only one point in the three candidate three-dimensional points of point Pa i  is authentic. 
     3.3, steps 3.1 and 3.2 are repeated to calculate the corresponding candidate three-dimensional points of other points on two-dimensional line SA 1  on first image  321  by same method. 
     3.4. Since many points in the candidate three-dimensional point set do not belong the authentic three-dimensional point set, a selection and a verification is necessary. Under the condition that the three-dimensional position calibration of the plurality of three-dimensional light plane projected by the pattern projector and the camera are known, the three-dimensional light planes and all the candidate three-dimensional point sets obtained from first image  321  and second image  331  are converted to the same coordinate system. {{P 1i |1≤i≤n}, {P 2i |1≤i≤n}, {P 3i |1≤i≤n}} are all candidate three-dimensional point sets corresponding to the two-dimensional point set {Pa i |1≤i≤n} on the two-dimensional line SA 1  on first image  321 , wherein each two-dimensional line on second image  331  corresponds to one subset, for example, SB 1  corresponds to {P 1i |1≤i≤n}. The location relationships between each subset in the candidate three-dimensional point set and the three light planes are recorded respectively. A sum of distances between each point in the sub-set and the three light planes is used as a criterion for the selection: 
               Wk   =       ∑     i   =   1     n     ⁢           ⁢     (     D   ⁡     (       PL   k     ,     P   ji       )       )         ,         
wherein D(PL k , P ji ) is the distance from a candidate three-dimensional point P ji  corresponding to two-dimensional line SB j  to a certain light plane PL k . After the minimum value W m =min (W k |k=1,2,3) is selected, P ji ∈PL m  is subsequently determined. That is, the three-dimensional point set {Pm i |1≤i≤n} on light plane PL m  is the authentic three-dimensional point set corresponding to two-dimensional line SA 1  on first image  321 . In other words, the authentic three-dimensional counter line projected to the surface of object  100  by light plane PL m  is imaged as two-dimensional line SA 1  on first camera  320 , and as two-dimensional line SB j  on second camera  330 .
 
       FIG. 4  is a schematic diagram of a method for obtaining an authentic three-dimensional point set according to the second embodiment of the present invention. The schematic diagram contains first camera  410 , first image  411 , second camera  420 , second image  421 , third camera  430 , third image  431 , and a pattern projector (not shown in the drawing). 
     The two-dimensional image captured by first camera  410  is first image  411 . The two-dimensional image captured by second camera  420  is second image  421 . The two-dimensional image captured by third camera  430  is third image  431 . First image  411  captured by first camera  410  is imaged on a photosensitive element plane PF 1 . Second image  421  captured by second camera  420  is imaged on a photosensitive element plane PF 2 . Third image  431  captured by third camera  430  is imaged on a photosensitive element plane PF 3 . O 1 , O 2 , and O 3  are the photocenters of first camera  410 , second camera  420 , and third camera  430  respectively. The internal and external parameters of the cameras are known. 
     Under the conditions that three or more cameras are provided and the internal and external parameters of the camera are known, the data verification can be performed by the third or other cameras. The system is not required to calibrate the spatial location correlation between the camera and the plurality of light planes projected by the pattern projector. Rather, the three-dimensional points in the obtained candidate three-dimensional point set are verified by the third camera to output the authentic three-dimensional point set. 
     The specific method is as follows. 
     4.1, The two-dimensional line set on third image  431  is obtained as {SC 1 , SC 2 , SC 3 } according to the method described in step S 2 . The three candidate three-dimensional points P 1i , P 2i , and P 3i  corresponding to a point Pa i  on the two-dimensional line SA 1  on first image  411  are calculated according to the method described in step  3 . 2 . The three candidate three-dimensional points P 1i , P 2i  and P 3i  are connected to a photocenter O 3  of third camera  430  respectively. The connection lines intersect with a focal plane PF 3  of the third camera (i.e. third image  431 ) at three points Pc 1i , Pc 2i  and Pc 3i . It is apparent that only one of the three points is the image of the authentic three-dimensional point. Similarly, {{P 1i |1≤i≤n}, {P 2i |1≤i≤n}, {P 3i |1≤i≤n}} are all the candidate three-dimensional point sets corresponding to the two-dimensional point set {Pa i |1≤i≤n} on the two-dimensional line SA 1  on first image  311 , wherein each subset corresponds to a two-dimensional line on second image  321 . For example, the two-dimensional line SB 1  corresponds to the subset {P 1i |1≤i≤n}. 
     4.2, Each point in the candidate three-dimensional point set {{P 1i |1≤i≤n}, {P 2i |1≤i≤n}, {P 3i |1≤i≤n}} is connected to the photocenter O 3  of third camera  430 . The intersection point sets of the connection lines and third image  431  on a photosensitive element PF 3  is {{Pc 1i |1≤i≤n}, {Pc 2i |1≤i≤n}, {Pc 3i |1≤i≤n}}. The location relationships between each subset in the intersection point set and the three two-dimensional lines {SC 1 , SC 2 , SC 3 } on third image  431  are recorded respectively. A sum of distances between each point in the recorded sub-set and a certain two-dimensional line SC k  is used as a criterion for the selection: 
             Wk   =       ∑     i   =   1     n     ⁢           ⁢       (     D   ⁡     (       SC   k     ,     Pc   ji       )       )     .             
An epipolar line (N 3  is the epipolar point) is calculated according to epipolar constraint principle based on the point set {Pa i |1≤i≤n} on the two-dimensional line SA 1  on first image  411 . The candidate three-dimensional points corresponding to the intersection point set of the epipolar line intersecting with two-dimensional line SB j  is the intersection point set {Pc ji |1≤i≤n} on third image  431  intersecting with the connection lines between P ji  and the photocenter O 3  of third camera  430 . The distances from point set {Pc ji |1≤i≤n} to two-dimensional line SC k  is D(SC k , Pc ji ). The minimum value W m =min(W k |k=1,2,3) is obtained through a selection. That is, two-dimensional SC m  is the image of the authentic three-dimensional point set {P mi |1≤i≤n} corresponding to two-dimensional line SA 1  imaged on third image  431 . In other words, two-dimensional line SA 1  on first image  411 , two-dimensional line SB j  on second image  421 , and two-dimensional line SC m  of third image  431  are the projected image of the same authentic three-dimensional point set.
 
       FIG. 5  is a schematic diagram of a method for obtaining an authentic three-dimensional point set according to the third embodiment of the present invention. The schematic diagram contains pattern projector  510 , first camera  520 , first image  521 , second camera  530 , and second image  531 . Pattern projector  510  projects three light planes PL 1 , PL 2  and PL 3 . The two-dimensional image captured by first camera  520  is first image  521 . The two-dimensional image captured by second camera  530  is second image  531 . First image  521  captured by first camera  520  is imaged on a photosensitive element plane PF 1 . Second image  531  captured by second camera  530  is imaged on a photosensitive element plane PF 2 . O 1  and O 2  are the photocenters of first camera  520  and second camera  530  respectively. The internal and external parameters of the camera are known. 
     The specific method is as follows. 
     5.1, The two-dimensional point set {Pa i |1≤i≤n} on the two-dimensional line SA 1  on first image  521  is obtained according to step S 2 . Based on the triangulation principle, the extension line of the connection lines between the two-dimensional set and the photocenter O 1  of first camera  520  intersects with the three light planes PL 1 , PL 2  and PL 3  at {{P 1i |1≤i≤n}, {P 2i |1≤i≤n}, {P 3i |1≤i≤n}} which are the candidate three-dimensional point sets. 
     5.2 The Each point in the candidate three-dimensional point set is connected to the photocenter O 2  of second camera  530  respectively, and the connection lines intersect with photosensitive element plane PF 2  of second camera  530  at point {{Pb 1i |1≤i≤n}, {Pb 2i |1≤i≤n}, {Pb 3i |1≤i≤n}}. The location relationships between each subset in the intersection point set and the three two-dimensional lines SB 1 , SB 2 , SB 3  on second image  531  are recorded. A sum of distances between each point in the sub-set and a certain two-dimensional line SB k  is used as a criterion for the selection: 
             Wk   =       ∑     i   =   1     n     ⁢           ⁢       (     D   ⁡     (       SB   k     ,     Pb   ji       )       )     .             
A candidate three-dimensional point set of the point set {Pa i |1≤i≤n} of a two-dimensional line SA 1  on first image  521  associated with light plane PL j  is {P ji |1≤i≤n}. The distance from the point set {Pb ji |1≤i≤n} of second image  531  associated with the light plane PL j  to a two-dimensional line SB k  is D(SB k ,Pb ji ). A minimum value W m =min(W k |k=1,2,3) is obtained by a selection. That is, SB m  is determined as the line corresponding to the light plane PL j  imaged on second camera  530 . In other words, the three-dimensional point set {P ji |1≤i≤n} on the light plane PL j  is an authentic three-dimensional point set corresponding to two-dimensional line SA 1  on first image  521 . Also, the authentic three-dimensional point set of the light plane PLj projected on the surface of object  100  is imaged as the two-dimensional line SA 1  on first camera  520 , and as two-dimensional line SB m  on second camera  530 .
 
     Since the three-dimensional sensor system employs the projection of a plurality of linear patterns, the efficiency of point-determining is several times as much of the traditional single line scanning. The scanning speed is significantly improved. 
     A three-dimensional data acquisition method according to the embodiment of the present invention includes the following steps. 
     Step  101 , at least two linear patterns are projected by a pattern projector. 
     Preferably, the at least two linear patterns can be projected by one pattern projector or synchronously projected by a plurality of pattern projectors. The linear pattern is a straight line or a curve line. The pattern projector includes a linear laser and a DOE beam splitting element. The linear laser produces a plurality of laser line segments through the DOE beam splitting element. 
     Preferably, the pattern projector can be a projector. The projector directly projects at least two linear patterns. 
     Step  102 , at least two cameras synchronously capture the two-dimensional images. 
     Step  103 , a two-dimensional line set of a linear projection on a surface of a scanned object on a two-dimensional image is extracted. 
     Preferably, the step of extracting the two-dimensional line set on a two-dimensional image of a linear projection on a surface of a scanned object specifically includes the following steps. A distortion correction is performed on the two-dimensional image according to an internal parameter of the camera corresponding to the two-dimensional image. A connected region of a line contour in a corrected image is extracted according to a pixel gray-level difference. The two-dimensional line set of a highlight center with a sub-pixel level is calculated according to a gray-level centroid of the connected region. 
     Step  104 , an candidate three-dimensional point set is generated based on the two-dimensional line set. 
     Step  105 , an authentic three-dimensional point set that correctly matches with a projection contour line on the surface of the object is selected from the candidate three-dimensional point set. 
     Specifically, step  104  and  105  can be realized through the three methods described hereinafter. 
     The first method is that the two-dimensional point data are extracted from the two-dimensional line sets of at least two synchronous two-dimensional images respectively. The candidate three-dimensional point set is calculated according to the principles of triangulation and epipolar constraints based on a correlation between the spatial location of the cameras. Whether a point belongs to the authentic three-dimensional point set is determined based on whether the point in the candidate three-dimensional point set is located in a certain three-dimensional light plane projected by the pattern projector. A selection is performed to obtain the authentic three-dimensional point set. 
     Preferably, the candidate three-dimensional point set includes a plurality of sub-sets. On the basis of distances between the sub-sets and the three-dimensional light plane, a sub-set having a minimum distance value is selected as the authentic three-dimensional point set. 
     The second method is that the two-dimensional point data are extracted from the two-dimensional line sets of two synchronous two-dimensional images respectively. The candidate three-dimensional point set is calculated according to the principles of triangulation and epipolar constraints based on a spatial location correlation between the two cameras corresponding to the two synchronous two-dimensional images. The two-dimensional images shot by a third camera or more cameras are used to perform a data verification on the candidate three-dimensional point set. The selection is performed to obtain the authentic three-dimensional point set. 
     Preferably, the candidate three-dimensional point set includes a plurality of sub-sets. A connection line between the subset and a photocenter of the third camera has an intersection point set with respect to the two-dimensional image shot by the third camera. On the basis of distances between the intersection point set and the two-dimensional line on the two-dimensional image shot by the third camera, a sub-set corresponding to a minimum distance value is selected as the authentic three-dimensional point set. 
     The third method is that the two-dimensional point data is extracted from the two-dimensional line set of an image shot by any camera. The candidate three-dimensional point set is obtained according to the principles of triangulation based on a correlation between the spatial location of a plurality of spatial light planes projected by the pattern projector and the camera. The candidate three-dimensional point set is verified with an image shot by at least another camera. A selection is performed to obtain the authentic three-dimensional point set. 
     Preferably, the candidate three-dimensional point set includes a plurality of sub-sets. A connection line between the subset and a photocenter of the at least one other camera has an intersection point set with respect to an image shot by the at least one other camera. On the basis of distances between the intersection point set and the two-dimensional line on the image shot by the at least one other camera, a sub-set corresponding to a minimum distance value is selected as the authentic three-dimensional point set. 
     It should be noted that the three-dimensional data acquisition method of the present embodiment and the three-dimensional sensor system in the foregoing embodiments are two aspects based on the same inventive concept. The implementation process of the method has been described in detail hereinbefore, so that a person of ordinary skill in the art can clearly understand the structure and the implementation process of the system described in the present embodiment according to the above description. To make the specification more compact, the details will not be repeated herein. 
     Various variations and modifications may be derived by those skilled in the art according to the above-mentioned technical solutions and concepts, and such variations and modifications should fall within the protective scope of the appended claims of the present invention.