Analysis of ArcGIS polygon (multipatch) (1)

From a geometric point of view, a complex 3D shape can be expressed by a combination of enough small triangles. For example, a rectangle can form two triangles, for example:

The cylindrical shape can be arranged by multiple triangles:

However, the polygon in ArcGIS is not organized by the simple triangle method, but by the following three infrastructures:

1)Triangle strip, translated by the author:Triangle

2)Triangle fan

3) rings

Next we will explain the three structures one by one. The first isTriangle strip. A triangle can be arranged in order to form a polygon. ArcGIS records that this polygon does not record all triangles and each node of the triangle, but only records the typeTriangle
Strip, and record by order, 4, 5... nNode order. For example, is the record much more concise?

A triangle fan, similar to a triangle, can be arranged in sequence to form a polygon, except that the node sequence recorded by ArcGIS is different. ArcGIS first records that the polygon below is composed of triangular fans, followed ,....., the following "Pyramid" shape can be described for the 0, N sequential record nodes. (Note: The image below is not a plane chart, but a three-dimensional shape of a pyramid. The 0 points are the pyramid tip, and the 1, 2, and 4 nodes are the Bottom of the Pyramid)

The difficult section below is the ring structure. A ring is a relatively broad definition. I briefly describe it as a polygon composed of more than three nodes (not including three). The nodes may not be in a common plane. For example, a quadrilateral, such as the following three irregular shapes, where the second of all nodes is in a common plane, the first and third are not, but a circle is "one stroke can be drawn ", in the middle of the process, you cannot "lift the pen ".

Figure 1

Figure 2

Figure 3

after three basic components, triangle slice, and ring, you can create a polygon according to the plane. A polygon consists of multiple planes. However, for some constraints, logically include in a plane? ArcGIS has specific provisions. We will introduce this part in the next article.