1. 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.

2.  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

3. (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.

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

5. 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.

6. (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.

7. 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.

8.  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.