Modeling 1: Polygons - An Introduction to Computer Graphics for Artists - page 71

6.3 Modifi cation

6.3.1 Introduction

In this section, a number of polygon modifi cation tools will be explored. Each tool
modifi es an existing object by adding to it, moving its elements, or deleting parts
of it.

6.3.2 Adding Vertices

6.3.2.1 Subdivision

This is a tool that draws new edges on every selected face or every face if the entire
object is selected. It can draw them from existing vertices towards the center of each
face (diamond pattern), or from the center of each edge to the center of each face
(square pattern) (Fig. 6.3 ). The fi rst variant results in one new vertex in the center of
each face, the second creates a minimum of four new vertices (1 for the center, one
for each edge, with a minimum of three edges).

6.3.2.2 Extrusion

Extrusion detaches the selected face or faces from its neighboring faces, then con-
nects the two groups with new faces. This results in one new face for every edge in
the original extruded face. After these faces have been added, the original face is
transformed away from its original position. Because it is now connected to the rest
of the original object with new faces, the transformed face has sides that would not
have been present otherwise.

Edge Extrusion

Edge extrusion creates a new edge, joins it to the original edge with new edges (cre-
ating a face in the process), and then offsets the new edge a specifi ed distance from
the original edge (Fig. 6.3 ). A variant of this tool is a two-dimensional version. With
it, the edge itself is offset, and new edges created as the sides of a new face to join
the original edge and the new one. This does not create as many new vertices as the
other method, and results in non-manifold faces (see Sect. 9.3.11 ) .

Vertex Extrusion

Vertex extrusion subdivides a face around a selected vertex (Fig. 6.3 ). The tool suf-
fers from the same problem as edge extrusion, if the extrusion offset is too close or