TWI450216B - Computer system and method for extracting boundary elements - Google Patents

Description

Boundary element extraction method and computer system thereof

The invention relates to a boundary element extraction method and a computer system thereof, in particular to a method for extracting boundary elements from a messy point cloud and a computer system thereof.

In the development and manufacturing process of 3D products, error analysis has become an important part of product quality verification. The traditional approach is to use a point cloud acquisition device to acquire a point cloud of the workpiece (ie, a collection of points consisting of multiple three-dimensional discrete points), and then log the point cloud data to the computer, and execute the corresponding software to process the point cloud data to extract points. The boundary element corresponding to the cloud, that is, the boundary point and boundary feature of the workpiece. Then, the actual workpiece and the theoretical design image obtained by measuring the workpiece are aligned by an iterative fitting method to determine whether there is an error between the actual workpiece and the image in the theoretical design image. However, the alignment of this method is not efficient, and the special positions are not fully aligned.

In addition, with the development of imaging technology, the reference of technologies such as face recognition, feature matching, and finite element analysis, the requirements of the extraction precision and extraction speed of boundary elements are getting higher and higher in the industry, while the boundary elements in the traditional technology are used. The extraction method still does not meet these requirements.

In view of the above, it is necessary to provide a boundary element extraction method and a computer system thereof, which can automatically extract boundary elements of a chaotic point cloud, including extracting boundary points and creating boundary features, and improving extraction precision and extraction speed of boundary elements.

A boundary element extraction method, comprising: obtaining a point cloud data generated by the measurement workpiece and a maximum side length of a triangle constituting a point cloud triangle meshing surface; and the acquired point cloud is formed into a triangle according to the maximum side length Meshing the surface; extracting the boundary points from the triangular meshed surface; grouping the boundary points according to the vector variation of the boundary points; fitting each set of boundary points into a straight line and a circle respectively; determining the linear tolerance And the round tolerance judgment determines the boundary features suitable for each set of boundary points; and outputs the boundary features.

A computer system for extracting boundary elements, the computer system comprising: an acquisition unit, configured to acquire a point cloud data generated by the measurement workpiece and a maximum side length of a triangle constituting a point cloud triangle meshed surface; a triangular meshing unit, And a boundary point extraction unit configured to extract a boundary point from the triangular meshed surface; and a unit for forming the boundary point according to the maximum side length; The vector variation case groups the boundary points, and each set of boundary points is respectively fitted into a straight line or a circle, and after determining the linear tolerance judgment and the round tolerance judgment, determining boundary features suitable for each set of boundary points, and outputting the Border features.

Compared with the prior art, the boundary element extraction method and the computer system thereof can not only automatically extract boundary elements of a single layer of disordered point clouds, but also extract boundary elements of multiple layers of disordered point clouds, thereby improving the accuracy and speed of boundary element extraction. .

Referring to Figure 1, there is shown an operational environment diagram of a preferred embodiment of a computer system for extracting boundary elements of the present invention. The operating environment map includes an image measuring machine The table 1 and the computer 2 connected to the image measuring machine 1 are provided.

The image measuring machine 1 is used for measuring the workpiece 3 and acquiring point cloud data of the workpiece 3. The point cloud data includes the three-dimensional coordinates of the point, the identification of the point, and the total number of point clouds. In this embodiment, since the density of each part of the workpiece 3 is different and the structure is different, the point cloud acquired by the image measuring machine 1 is a messy point cloud.

A boundary element extraction system 20 is stored in the computer 2, and as shown in FIG. 2, the boundary element extraction system 20 includes an acquisition unit 21, a triangular meshing unit 23, a boundary point extraction unit 25, and a creation unit 27 for use from a messy point. A boundary element is extracted from the cloud, the boundary element including boundary points and boundary features. Therefore, extracting the boundary element in the embodiment includes extracting the boundary point and creating the boundary feature. Specifically, the boundary element extraction system 20 establishes a triangular meshed surface for the acquired point cloud (hereinafter referred to as a “triangular mesh surface”). ), extracting boundary points from the triangular mesh surface, and creating boundary features of the point cloud according to the extracted boundary points.

Referring to Figure 3, there is shown a flow chart of a preferred embodiment of the boundary element extraction method of the present invention.

In step S1, the acquiring unit 21 acquires the point cloud data generated by the image measuring machine 1 for measuring the workpiece 3 and obtains the maximum side length of the triangle of the triangular mesh surface input by the user. The obtaining includes the acquiring unit 21 receiving the point cloud data of the measuring workpiece 3 on the image measuring machine 1 on the line, and acquiring the point cloud obtained by the image measuring machine 1 previously measuring the workpiece 3 by opening the data file. data. The maximum side length is a user-defined side length that is approximately equal to the average of the distance between adjacent points in the point cloud.

In step S3, the triangular meshing unit 23 establishes the acquired point cloud into a triangular mesh surface according to the maximum side length.

In step S5, the boundary point extracting unit 25 extracts a boundary point from the above-described triangular mesh surface.

In step S7, the creating unit 27 creates a boundary feature of the point cloud according to the extracted boundary point, and outputs the created boundary feature.

Referring to FIG. 4, a specific operation flowchart of establishing a triangular mesh surface in step S3 in FIG. 3 is shown.

Step S300, the triangle meshing unit 23 calculates the bounding box of the point cloud according to the three-dimensional coordinates of the point in the point cloud data, groups the bounding box, and fills in the identifier of each point in the point cloud data into corresponding In the grouping. The side length of each unit small cube after the grouping can be calculated from the maximum side length and the total number of point clouds.

Step S302: Obtain any point in the point cloud that has not formed a triangle TO with other points, as the first vertex D1 of the triangle TO, and obtain a point closest to the first vertex D1 as the second vertex D2 of the triangle TO.

Step S304, connecting the first vertex D1 and the second vertex D2 to obtain one side B0, and adding the side B0 to the side array.

Step S306, finding a third vertex D3 constituting the triangle TO according to the maximum side length, constructing the triangle T0 and adding the other two sides of the triangle T0 to the edge array, that is, the other two except the edge B0 Side. Wherein, the inner angle of the triangle T0 cannot be too small, such as 1 degree and 2 degrees. When searching for the third vertex D3, priority should be given to the points formed by the angle greater than 25 degrees, and then the angle formed is less than 25 degrees. Angle can not be too small point. In the preferred embodiment, the angle of the composition cannot be less than 3 degrees.

Step S308, performing circumscribed ball determination and obtuse angle judgment on the triangle T0 to find a point D' which can form a triangle with the side B0 except the third vertex D3. The circumscribed ball judgment refers to fitting a triangle T0 into a ball by using a mathematical rule such as a least square method, the triangle T0 is connected to the ball through its three vertices, and no point exists in the ball, and the triangular meshing unit 23 The point D' is found on the ball by the obtuse angle principle, and the point D' and the side B0 form a triangle T', and the angle between the triangle T' and the triangle T0 must be an obtuse angle.

In step S310, the point D' and the side B0 form a triangle T', and the triangular meshing unit 23 adds the other two sides of the triangle T, that is, the other two sides except the side B0, to the side line.

In step S312, the triangular meshing unit 23 loops back the edges in the edge array, and repeats steps S308 to S312 until all the edges are circled, and the completed triangle constitutes a triangular mesh surface of the point cloud.

In step S308, in the case of obtuse angle determination, the preferred embodiment preferentially considers a triangle that forms an angle of 120 degrees with the triangle T0. However, the triangle meshing unit 23 may find a plurality of points D' at the same time by using the method of circumscribing and obtuse angle judgment, but it can only determine that one point D' is the best point, and the side B0 forms a triangle T'. For example, if the triangular meshing unit 23 finds the point D1' and the point D2' simultaneously by the method of the circumscribing ball judgment and the obtuse angle judgment, the edge B0 and the point D1' may form a triangle T1', the edge B0 and the point D2' may form a triangle. T2`, the triangular meshing unit 23 needs to determine which of the diagonals of the edge B0 in the triangles T1' and T2' is larger, if the diagonal of the edge B0 in the triangle T1' is larger than the diagonal in the triangle T2', Final point D1` is the best point to be found; conversely, if the diagonal of edge B0 in triangle T2' is greater than the diagonal in triangle T1', then the final point D2' is the best point to be found.

Referring to FIG. 5, a specific operation flowchart of extracting boundary points in step S5 in FIG. 3 is shown.

In step S500, the boundary point extracting unit 25 acquires a triangle around the current judgment point and a vertex of each triangle from the triangular mesh plane.

Step S502, calculating the number of times each vertex is occupied by the triangle around the current judgment point, as shown in FIG. 6 and FIG. 7, point 1, point 2, point 3, point 4, and point 5 are respectively around the current judgment point 0. The vertices of the triangle. In FIG. 6, points 1, 2, 3, 4, and 5 are occupied by the triangles around the current judgment point 0, respectively; in FIG. 7, points 2, 3, and 4 are currently The number of times the triangle around point 0 is occupied is 2, and the number of times 1 and 5 are occupied is 1.

Step S504, it is sequentially determined whether the number of times the vertex is occupied by the triangle around the current judgment point 0 is greater than 1.

If the result of the determination in step S504 is YES, that is, the number of times the vertices are occupied is greater than 1, step S506 determines that the current determination point is an internal point, and then directly proceeds to step S510. As shown in FIG. 6, since all the vertices are occupied by the triangle around the current judgment point 0 by more than 1, the current judgment point 0 shown in FIG. 6 is determined as the inner point.

If the result of the determination in step S504 is NO, that is, the number of times at least one vertex in the vertex is occupied is equal to 1, step S508 determines that the current judgment point is a boundary point, as shown in FIG. 7, since point 1 and point 5 are Occupied The number of times is 1, respectively, and therefore, the current judgment point 0 shown in Fig. 7 is determined as the boundary point.

In step S510, the boundary point extracting unit 25 determines whether or not the points in the triangular mesh plane have been subjected to the boundary point determination. If so, the flow is directly ended; if there is still no boundary point determination, the process returns to step S500, and the point is taken as the current determination point, and steps S500 to S510 are executed again until all the points are judged by the boundary point.

Referring to FIG. 8, a specific operation flowchart of creating a boundary feature in step S7 in FIG. 3 is shown.

In step S700, the creating unit 27 respectively uses the boundary point extracted in FIG. 5 as the current point, and expands outwardly with the current point as a center layer, and calculates an angle between the vectors of the expanded boundary point on the triangular mesh surface. relationship.

In step S702, the creating unit 27 groups all the boundary points according to the angle change condition, and fits each set of boundary points to a straight line. Specifically, the creating unit 27 places the boundary points whose angle changes differ little in one group in step S702, and thereby obtains a plurality of sets of boundary points, and then fits each set of boundary points into a straight line. For example, if the angle between the boundary point a and the vector of the boundary point b on the triangular mesh surface is 60 degrees, and the angle between the boundary point a and the vector of the boundary point c on the triangular mesh surface is 1 degree, then It indicates that the angle between the vectors of the boundary points a and b varies greatly, and the angle change between the boundary point a and the vector of the boundary point c is small, so the boundary point a and the boundary point b are divided into different groups. , and the boundary point a and the boundary point c are divided into the same group.

Step S704, determining a point for fitting the straight line with the fitted Whether the difference between the lines is less than a preset line tolerance.

If the result of the determination in step S704 is YES, the process directly proceeds to step S714; if the result of the determination in step S704 is negative, it indicates that the fitted straight line is not suitable for the set of boundary points, and in step S706, the creating unit 27 uses the least squares method. The set of boundary points is fitted to a circle.

Step S708, determining whether the difference between the point for fitting the circle and the fitted circle is less than a preset circular tolerance.

If the result of the determination in step S708 is no, go directly to step S714; if the result of the determination in step S708 is YES, and at least two circles are fitted in step S706, the creating unit 27 in step S710 is based on the radius of the circle, The relationship between the circle vector and the center line and the vector to determine whether a cylinder can be constructed between the fitted circles. In this embodiment, the condition that the creating unit 27 constructs the cylinder is that the radius of the circle fitted above is the same, the angle between the vectors of the circle is 0 degree or 180 degrees, and the angle between the center line and the vector is Also 0 or 180 degrees.

If the result of the determination in step S710 is NO, the process proceeds directly to step S714; if the result of the determination in step S710 is YES, then in step S712, the creation unit 27 constructs a circle satisfying the above condition into a cylinder.

In step S714, the creating unit 27 outputs boundary features suitable for each set of boundary points including straight lines, circles, and cylinders. For example, if all the boundary points extracted by the boundary point extraction unit 25 are as shown in FIG. 9(a), the boundary features created by the creation unit 27 are as shown in FIG. 9(b).

In this embodiment, the linear tolerance and the round tolerance can be calculated in advance by the user, which is approximately equal to the average of the distance between adjacent points in the point cloud. In addition, The density of point clouds in different regions may be different. In the region where the density of the point cloud is large, the average value may be multiplied by a coefficient to obtain the linear tolerance and the round tolerance of the region. Wherein, the coefficient may be 0.1, 0.2 or 0.3.

In addition, in this embodiment, the creating unit 27 may also first fit each set of boundary points into a circle. If the difference between the point used to fit the circle and the fitted circle is not less than the circle tolerance, it indicates that the fitted circle is not suitable for the set of boundary points, and the creating unit 27 uses the least squares method to The group boundary points are fitted to a straight line, and then the line tolerance is judged. When the creation unit 27 fits at least two circles, it is judged whether or not a cylinder can be constructed between the fitted circles.

The present invention has been described above in terms of preferred embodiments, and is not intended to limit the invention. Any person skilled in the art will be able to make changes and refinements without departing from the spirit and scope of the invention, and the scope of the invention is defined by the scope of the appended claims.

Image measuring machine ‧‧1

Computer ‧‧2

Workpiece ‧‧3

Border element extraction system ‧‧20

Access unit ‧‧21

Triangular grid unit ‧‧23

Boundary point extraction unit ‧‧‧25

Creation unit ‧‧27

Get the point cloud data obtained by measuring the workpiece ‧‧‧S1

Create the acquired point cloud into a triangular meshed surface ‧‧‧S3

Extracting boundary points from the triangular meshed surface ‧‧‧S5

Create boundary features of point clouds based on extracted boundary points ‧‧‧S7

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagram showing the operational environment of a preferred embodiment of a computer system for extracting boundary elements of the present invention.

Figure 2 is a functional unit diagram of the boundary element extraction system of the present invention.

3 is a flow chart showing the operation of a preferred embodiment of the boundary element extraction method of the present invention.

FIG. 4 is a flow chart showing the specific operation of establishing a triangular meshed surface in step S3 of FIG. 3.

FIG. 5 is a flow chart showing the specific operation of extracting the boundary point in step S5 of FIG. 3.

6 and 7 show that the present invention obtains a triangle around the current judgment point and A schematic representation of the vertices of each triangle.

FIG. 8 is a specific operational flowchart of creating a boundary feature in step S7 of FIG. 3.

Figure 9 is a schematic illustration of the creation of boundary features in accordance with the present invention.

Get the point cloud data obtained by measuring the workpiece ‧‧‧S1

Create the acquired point cloud into a triangular meshed surface ‧‧‧S3

Extracting boundary points from the triangular meshed surface ‧‧‧S5

Create boundary features of point clouds based on extracted boundary points ‧‧‧S7

Claims (10)

A boundary element extraction method, wherein the boundary element comprises a boundary point and a boundary feature, the method comprises: (a) acquiring a point cloud data generated by the measurement workpiece and a maximum side length of a triangle constituting the point cloud triangle meshed surface (b) the acquired point cloud is formed into a triangular meshed surface according to the maximum side length; (c) extracting the boundary point from the triangular meshed surface: (d) the boundary is changed according to the vector variation of the boundary point Points are grouped; (e) each set of boundary points is fitted to a straight line and a circle respectively; (f) a boundary feature suitable for each set of boundary points is determined after straight line tolerance judgment and round tolerance judgment; and (g) output Describe the boundary features. The boundary element extraction method according to claim 1, wherein the step (b) comprises the following steps: (b1) calculating a bounding box of the point cloud according to a three-dimensional coordinate of a point in the point cloud data, The bounding box is grouped, and the identifier of each point in the point cloud data is filled into the corresponding group; (b2) any point in the point cloud that has not formed a triangle with other points is obtained as the first vertex of the triangle, and Taking the point closest to the first vertex as the second vertex of the triangle; (b3) connecting the first vertex and the second vertex to obtain an edge B0, and adding the side B0 to the edge array; (b4) according to the The largest side length finds the third top that makes up the triangle Pointing, constructing the triangle and adding the other two sides of the triangle to the edge array; (b5) performing circumscribed ball judgment and obtuse angle judgment on the triangle to find a triangle which can form a triangle with the edge B0 except the third vertex (b6) the found point and the side B0 form another triangle, adding the other two sides of the other triangle except the side B0 to the side 伫 column; and (b7) looping the side 伫 column The middle edge and repeat steps (b5) through (b6) until all the edges complete the task of building the triangle. The boundary element extraction method according to claim 1, wherein the step (c) comprises the following steps: (c1) acquiring a triangle around the current judgment point in the triangular meshed surface and a vertex of each triangle; C2) calculating the number of times each vertex is occupied by a triangle around the current judgment point; (c3) if the number of times the vertex is occupied is greater than 1, determining that the current judgment point is an inner point; (c4) if the vertex is If the number of times the vertex is occupied is equal to 1, it is determined that the current judgment point is a boundary point; (c5) determining whether the points in the triangular mesh plane are all judged by the boundary point; and (c6) if the step (c5) is judged If the result is YES, the process is directly terminated. If the result of the step (c5) is that the boundary point is not determined, the process returns to the step (c1), and the point is used as the current judgment point, and the step (c1) to the step is re-executed. (c5) until all points have been judged by the boundary point. The boundary element extraction method of claim 1, wherein the boundary features comprise a straight line, a circle, and a cylinder. The boundary element extraction method of claim 4, wherein between the step (f) and the step (g), the following step is included: if at least two fits are determined in the step (f) If a circle exists, it is determined whether a cylinder can be constructed between the fitted circles according to the radius of the circle, the vector of the circle, and the relationship between the line and the vector; and if the result of the determination is yes, according to the Fit the circle to build the cylinder. A computer system for extracting boundary elements, wherein the boundary element comprises a boundary point and a boundary feature, the computer system comprises: an acquisition unit, configured to acquire a point cloud data generated by the measurement workpiece, and a triangle mesh surface of the point cloud a maximum side length of the triangle; a triangular meshing unit configured to establish the acquired point cloud as a triangular meshed surface according to the maximum side length; and a boundary point extracting unit for using the triangular meshed surface from the triangle Extracting boundary points; and creating a unit for grouping the boundary points according to the vector variation of the boundary points, respectively fitting each set of boundary points into a straight line or a circle, and determining the straight line tolerance judgment and the round tolerance judgment The boundary features of each set of boundary points are adapted and the boundary features are output. The computer system of claim 6, wherein the point cloud data comprises a three-dimensional coordinate of a point and an identification of a point. The computer system of claim 6, wherein the boundary point extraction unit repeats when extracting a boundary point from the triangular meshed surface Perform the following steps until the points on the triangular meshed surface are evaluated by the boundary point: obtain the triangle around the current judgment point in the triangular meshed surface and the vertices of each triangle; mark each vertex to be judged by the current point The number of times the surrounding triangle is occupied; if the number of times the vertex is occupied is greater than 1, it is determined that the current judgment point is an inner point; and if the number of times the vertex is occupied in the vertex is equal to 1, the current judgment is determined The point is the boundary point. The computer system of claim 6, wherein the boundary feature comprises a straight line, a circle or a cylinder. The computer system according to claim 9, wherein the creating unit is further configured to: after determining the linear tolerance judgment and the round tolerance judgment, if it is determined that at least two fitted circles exist, according to the circle The radius, the vector of the circle, and the relationship between the line and the vector to determine whether the cylinder can be constructed between the fitted circles. If the result of the judgment is yes, the cylinder is constructed according to the fitted circle, and the output is The constructed cylinder.