JPS6215666A - Converting system for form description - Google Patents

Abstract

PURPOSE:To minimize the checking times for crosses of faces and increase the processing speed by adopting a local combination (face loop) of face candidates which are arranged around an apex to form a loop. CONSTITUTION:The face loops detected at each apex are rearranged at a face loop generating part 3 so that the face candidates sharing the apexes form a loop. The number of face candidates forming the loop is equal to the number of ridges sharing the apexes. The crosses are checked between the face candidates (excluding adjacent ones) forming the produced face loop. Then only the combination of candidate faces including no cross as a face loop. The face loops around each apex are combined at a shell producing part 4 to decide the shell 1 forming face and its direction. The face loops are selected with no conflict produced against the validity/invalidity and the surface/rear sides of the decided faces. In such a way, an answer is obtained in the form of a shell containing all faces turned outside.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a method for converting a description of a three-dimensional shape expressed in a simplified format into a more detailed description, for example in computer-aided design in the mechanical field. be.

(Prior art) A representation format that describes a three-dimensional shape using only vertices and edges (
A boundary representation format (reference: Computing Surveys Com
putting 5urveys), Vol, 12°N
o, 4. pp. 437-464.1980, hereinafter referred to as solid representation), is divided into a stage of finding surface candidates and a stage of combining these candidates to generate a physically possible shell.

What is obtained in the first stage are those that satisfy the requirements for surfaces (hereinafter referred to as surface candidates), but not all of them become actual surfaces. One of the criteria for determining whether a surface candidate is valid or invalid is the intersection of two surface candidates. By checking the presence or absence of intersections, surfaces that cannot exist are removed, and edges and vertices are added as necessary to divide the surface candidates into smaller surface candidates.

By performing the above preprocessing, a set of several closed surface candidates (hereinafter referred to as virtual blocks) is formed in which the surface candidates do not intersect with each other and do not overlap with each other. Next, the state of the entire shape is determined by determining whether the contents of each virtual block are full or empty (Reference: IBM Journal of Research and Development (IBM J, Res.
, Develop, ) Vol. 24. No, 5. pp
.

582-597.1980).

(Problems to be solved by the invention) Conventional methods require cross-checking for all combinations of obtained surface candidates, or add auxiliary edges and vertices, resulting in an increase in unnecessary surfaces and edges. However, there are problems in that the amount of calculation increases and the processing speed is slow.

An object of the present invention is to provide a processing method that is faster than conventional methods in generating an interpretable solid representation from a wireframe representation of a three-dimensional polyhedral shape.

(Means for Solving the Problem) The gist of the present invention is to use surfaces arranged to form a loop around one vertex as an intermediate representation when generating a shell by combining surface candidates. The point is that a surface loop, which is a local combination of candidates, is adopted.

The method of the present invention includes an edge loop generation unit that detects a set of edges that are on the same plane and form a loop, and a surface candidate generation unit that detects surface candidates by combining several edge loops. A surface loop generation section that arranges surface corrections to form a loop around each vertex, and a shell generation section that detects possible shells by combining surface loops.

(Operation) Surface candidates are generated by detecting loops formed by edges on the same plane. This process creates several surface candidates around the vertex, but these usually include surfaces other than the actual surface, and the local state around the vertex is not always accurate. It cannot be determined uniquely.

Next, a surface loop is generated as a combination of valid surfaces around the vertex. At this time, intersection checks are performed on each surface that makes up the surface loop at the same time, but the number of times this is checked is fewer than when checking all combinations of surfaces, so processing speed is improved compared to conventional methods. do.

By selecting one of the generated surface loops, the local surface state around its vertex is determined. If multiple surface loops exist for one vertex, selecting one of them so that it does not conflict with the surface loops of other vertices that have already been selected will enable or disable all surface candidates around that vertex. direction is determined. By doing this for all vertices, a combination of surfaces representing a shell is obtained.

Furthermore, even when there are multiple solutions that can be interpreted from a single wireframe representation, each solution is given by a different combination of surface loops.

(Example) FIG. 1 is a diagram clearly showing the overall processing flow of the present invention. Conversion is performed by sequentially creating edge loops, surface candidates formed by edge loops, surface loops around each vertex, and information on the entire shell from information about vertices and edges.

In order to detect all edge loops, the edge loop generation unit 1 selects any two edges that share a vertex and finds the smallest loop on a plane that includes them.

The surface correction is composed of one outer circumferential loop and zero or more inner circumferential loops included therein. Detection of surface candidates is performed in the surface candidate generation unit 2 through the steps of grouping edge loops on the same plane, checking inclusion relationships between edge loops, and extracting surface information from the inclusion relationship tree.

The surface loop detected for each vertex in the surface loop generation unit 3 is obtained by rearranging the surface candidates that share the vertex to form a loop, and the number of both candidates that make up the loop is the same as the number of candidates that share the vertex. Equal to the number of edges.

In the three-dimensional shape shown in FIG. 2(a), the surface loop shown in FIG. 2(b) is obtained from both three candidates around the vertex A.

In the three-dimensional shape shown in Figure 3, in addition to the four actual surfaces ABC, ACD, BCF, and CDF around vertex C, two apparent upper surfaces ACFE and BCDE are detected. Ru. Since the number of edges that share the vertex C is four, the surface loop selects four out of the six candidates to create the surface loop ABC-BCF-CDF--ACD-ABC. On the other hand, the plane loop ABC-BCDE -CDF - -ACFE -ABC also satisfies the condition, but both candidates BCD
E and ACFE intersect, which is incorrect. In order to eliminate such combinations, check the intersection of both candidates (excluding those that are adjacent to each other) that make up the generated surface loop (check whether there is a common part outside the vertex and edge) and register only combinations that do not include intersections as surface loops.

A single vertex may have multiple surface loops. For example, in the three-dimensional shape shown in Fig. 4, six candidates F1 to F6 are considered around vertex A, and by combining these, Fl -F2- F6- F4- FIFl -F5-
F6- F3- FIF5- F2- F3- F4
- Three types of surface loops of F5 are obtained.

The shell generator 4 combines the surface loops around each vertex to determine the surfaces forming the shell and their orientations. By selecting one surface loop, it is determined whether all candidates around that vertex are valid or invalid, and the relative front-back relationship between valid surfaces is also determined. For each vertex, a surface loop is selected so as not to contradict the validity/invalidity and front and back sides of the surface determined so far. The above processing is performed in two stages: assuming a surface loop, checking the front-back relationship of each surface constituting the shell, and determining the front and back sides.

When selecting a surface loop, the target surface loop is enabled/
Since it is necessary to include at least one of both candidates that have been determined to be invalid, the next surface loop is selected preferentially starting from the vertices that share an edge with the vertex for which the surface loop has already been selected.

Next, the final front and back sides of each surface forming the shell are determined based on the locally defined surface orientation within each selected surface loop. At the same time, consistency with other surfaces whose front and back sides have already been determined is checked. On this occasion,
Regarding the order of surface loops to be examined, priority is given to vertices that share a surface with vertices that have already been searched. This results in a solution as a shell with all faces facing outward.

(Effects of the invention) As described above, by employing a local intermediate representation called a surface loop, the number of surface intersection checks, which require calculation time, is minimized, resulting in faster processing. .

[Brief explanation of drawings]

FIG. 1 is a diagram showing the overall processing flow of this method. Figure 2(a) is a diagram showing an example of a three-dimensional shape, and Figure 2(a) is a diagram showing an example of a three-dimensional shape.
b) is a diagram showing a surface loop found around vertex A in the shape of FIG. 2(a). FIGS. 3 and 4 are diagrams showing examples of different three-dimensional shapes, respectively. E 1 Exit 2 ((1) (b) 3rd Exit 8 4th Exit

Claims (1)

[Claims] Shape description from a representation format that describes a three-dimensional polyhedral shape only by vertices and edges to a boundary representation format that describes an interpretable three-dimensional shape using a hierarchical data structure of shells, faces, edge loops, edges, and vertices. This conversion method includes an edge loop generator that detects a set of edges forming a loop on the same plane, and a surface candidate generator that detects surface candidates by combining several edge loops. a surface loop generation section that arranges surface candidates to form a loop around each vertex, and a shell generation section that detects possible shells by combining surface loops. Conversion method.