Wireframe Models

Modeling of objects & pictures mean description of the objects & pictures to the computer so as to produce a visual display that simulates the real thing.

One way to do is to use set of primitives or geometric form that are simple enough to be easily implemented on the computer & flexible to represent model.

Simple geometric form are;

(i) Points and Lines: We specify a line by giving its coordinates in 3-D such as endpoints of a line P1(x1,y1,z1) & P2….

(ii) Polylines: is a chain of connected line segments. It is specified by giving the vertices P0,P1….Pn defining the line segment.

(iii) Polygons: Closed polylines

(iv) Polyhedron : is a closed polygonal net (i.e one which encloses a definite volume) in which each polygonal is planar.The polygons are called the faces of the polyhedron.

Wireframe Model : consists of edges, vertices & polygons.Here vertices are connected by edges & polygons are sequences of vertices or edges. The edges may be curved or straight line segment. Wireframe Model is called a polygonal net or polygonal mesh.

Sample rendering of a wireframe cube, icosahedron, and approximate sphere

Representing a polygonal Net Model:

(i) Explicit Vertex list V= {p0,p1,p2….pn).

The points pi(xi,yi,zi) are the vertices of the

polygon net, stored in the order in which they

would be encountered by traveling around the

model. Although this form of representation is

useful for single polygons, it is quite

inefficient for a complete polygonal net, in

that shared vertices are repeated several times.

(ii) Polygon listing In this form of representation, each vertex is stored exactly once in a vertex list V = (p0,…..pn), & each polygon is defined by pointing or indexing into this vertex list. Again, shared edges are drawn several times in displaying the model.

(iii) Explicit edge listing In this form of representation, we keep a vertex list in which each vertex is stored exactly once and an edge listing in which each edge is stored exactly once.

Each edge in the edge list points to the two vertices in the vertex list which define that edge. A polygon is now represented as a list of pointers or indices into edge list. Additional information, such as those polygon sharing a given edge, can also be stored in the edge list. Explicit edge listing can be used to represent the more general wireframe model.

Advantages :(i) Easy to create.(ii) Provides a convenient geometric

definition for many engineering applications.

Disadvantages :(i) Inability to produce blended and rounded surfaces.(ii) Inability to differentiate b/w the inside &

outside of objects.(iii) In 3-D, backside of the model is visible.