Storing geometric and attribute information

A 3D object is commonly represented by a set of polygon surfaces (boundary representation). The data are stored in polygon tables, which are of two types.

1. Geometric data tables: coordinates and shape of the polygons (vertex table, edge table, surface/polygon table).
2. Attribute tables: surface properties such as colour, transparency, reflectivity, texture, and so on.

Geometric tables

• Vertex table: coordinates (x, y, z) of each vertex.
• Edge table: each edge points to its two end vertices.
• Surface (polygon) table: each polygon points to the edges that bound it.

Example: an object made of two adjacent triangles S1 = (V1, V2, V3) and S2 = (V3, V2, V4) that share the edge V2V3:

Vertex table        Edge table          Surface table
V1: x1 y1 z1        E1: V1 V2           S1: E1 E2 E3
V2: x2 y2 z2        E2: V2 V3           S2: E2 E4 E5
V3: x3 y3 z3        E3: V3 V1
V4: x4 y4 z4        E4: V2 V4
                    E5: V4 V3

Additional pointers, such as the surfaces that share each edge (for example, E2: S1, S2), make error checking and traversal fast.

Attribute data

Attributes are kept in a separate table or as fields attached to each surface, for example S1: colour = red, reflectivity = 0.5, transparency = 0, and so on. The plane equation coefficients (A, B, C, D) are also stored for each surface, for use in visibility tests and shading.

Advantages: shared vertices are stored only once, the structure is compact, and edges and surfaces can be processed quickly during transformation and rendering.