Question
When using Modify-Move Vertex(Edge), the model becomes distorted.
Answer
CIM has rules for maintaining the gemometry. If the modeling is done without considering these rules, the resulting shape may be distorted.
1. Principle of the cross-sectional shape change operation of Edit Vertex(Edge)
Move Vertex(Edge) provides a model editing function that is similar to the STRETCH function of 2D CAD.
Move Vertex(Edge) is a shape editing function with high degrees of freedom, but sometimes it produces a shape that differs from the user's expected result.
Unlike 2D, 3D models must follow rules to maintain their shape.
Let's take a look at the Move Edge example below.
(Although there is a difference in the Target attribute in this function, the operating principles of Vertex and Edge are the same.)
| Fig. 1 Example of Move Edge-1 | Fig. 2 Example of Move Edge-2 |
The two figures are Preview screens of the Body's one side Edge of Box modified 0.4m in the -Z / -Y direction, respectively.
While Fig. 1 creates the intended shape of the user,
Fig. 2 creates a distorted shape that is difficult for the user to predict.
The reason for this shape distortion in 3D models is due to the operation of the X/Y/Z axis to maintain the shape of the plane that makes up the object.
It can be easily understood once you know the shape maintenance rules of 3D models.
Let's look at the premise conditions of Solid shape in Move Edge as follows.
ㆍAn Edge in a 3D model is composed of two Points, and a Vertex has at least three Edges
and three Surfaces touching it.
ㆍThe number of surfaces and edges that make up the initial shape cannot be changed.
Based on these two assumptions, let's consider the process of moving an edge in the model shown in Figure 3.
Figure 3. Target Edge and Vertex forming Face ①~③
1. When the Target Edge moves through Edit Edge, the Vertex that makes up the Edge moves with it.
As the edge moves in the -Y direction, the shape of the face adjacent to the vertex also changes.
2. The movement path of the Vertex is as follows:
Face ① ②: exists within Infinite Face*
Face ③: goes out of Infinite Face (cannot maintain the shape in one plane)
*Infinite Face - a plane area obtained by infinitely extending the original face
3. Face ③, where four points that make up a rectangle do not lie on the same plane, must perform one of three operations.
A. Edit Face ③ to a curved surface
B. Edit Face ③ into two separate triangular faces by changing it to a rectangle
C. Slightly adjust adjacent points to maintain the rectangular plane
In this process, CIM creates the shape through the C operation to maintain the flat condition of the initial surface.
Therefore, to ensure that the Move Vertex (Edge) function performs the appropriate operation, it is necessary to consider whether the condition that 'the face adjacent to each point that will move maintains a flat shape' is maintained.
2. Comparison of Shape Changes by Edit Vertex (Edge)
ㆍCross-sectional shape change by Move Vertex
| Fig. 4 Setting Move Vertex | Fig. 5 Executing Move Vertex |
ㆍChanges in cross-sectional shape by Move Edge
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| Fig. 6 Setting Move Edge |
Fig. 7 Executing Move Edge |
Changes in shape by Move Edge do not apply to Member elements.