Polygon Table
objects in a scene are generally described by a set of polygon patches. In fact, graphics packages often provide functions that describe surface shapes in polygonal meshes. A description of each objectincludes geometry information and other surface parameters (such as color, transparency, and light reflection properties) that specify polygon patches. as you enter information for each polygon, the data is put into tables awaiting subsequent processing, display, and object management of the scene. These polygon data tables are organized into two groups: a geometric data table and a property data table. A geometric data table contains vertex coordinates and parameters that identify the spatial direction of polygon patches. The property information for an object contains the degree of transparency of the specified object and the reflection and texture characteristics of its surface.
the geometry of the object in the scene is simply organized into three tables: the vertex table, the edge table, and the Polygon table (the Polygon table, for short). The coordinates of each vertex of the object are stored in the vertex table. The edge table contains a pointer to the vertex table to determine the end point of each polygon's edge. The patch table contains a pointer to the edge table to determine the edges of each polygon. Figure 3.50 shows the related table of two adjacent polygon patches on the object surface. In addition, objects and their constituent polygons can be assigned to object and polygon labels, which makes it easier to refer to them.
3.50 The practice of placing geometry data in three tables provides the convenience of referencing individual components (vertex, edge, and polygon slices for each object). Using edge table data that identifies polygon boundaries can also be used to display objects efficiently.
Another arrangement is to use only two tables:vertex table and polygon sheet. But this is not easy enough, and some edges will be drawn two times in the online block diagram.
Another possibility is to use only a single patch table, which causes the duplication of coordinate information, because explicit coordinate values are used in each patch. The edge and polygon relationships must also be rebuilt from the vertex list of the patch table.
you can increase the speed of information extraction by adding additional information to the data table in Figure 3.50. For example, expanding the edge table to include a pointer to a patch table so that the common edges belonging to two polygons can be quickly identified (see Figure 3.51). This is particularly useful for drawing processes that require shading from one polygon to the next. Similarly, a vertex table can extend pointers to corresponding edges to quickly extract information.
Other geometric information that is often stored in a data table includes the slope and polygon edges of each edge, polygon patches, and the coordinate range of each object in the scene. when entering vertices, we can calculate the slope of the edges and scan the coordinate values to confirm the smallest and largest x, Y, z values of individual segments and polygons. Because a geometric data table can contain the vertices and edges of a large number of complex objects in a scene, it is important to check their consistency and integrity. In particular, in interactive applications, it is possible to create some input errors that distort the display of objects in the process of describing vertices, edges, and polygons.
The more information that is contained in a data table, the easier it is to check for errors. Therefore, it is easier to check for errors when using three data tables (vertices, edges, and polygons) because this scenario provides the most information. The tests that can be done by the graphics package are:
- every a vertex has at least two edges as its endpoint
- Each edge is at least part of a polygon
- each polygon is closed.
- Each polygon has at least one shared edge
- If the edge table contains a pointer to a polygon, then the polygon pointer each side of the reference has a reverse pointer pointing back to the polygon.
Computer Graphics (ii) OUTPUT element _10_ polygon fill Area _4_ polygon table
