Object description - Feature Extraction and Image Processing - page 247

7

Object description

7.1 Overview

Objects are represented as a collection of pixels in an image. Thus, for purposes of recognition
we need to describe the properties of groups of pixels. The description is often just a set of
numbers - the object's descriptors . From these, we can compare and recognise objects by
simply matching the descriptors of objects in an image against the descriptors of known
objects. However, in order to be useful for recognition, descriptors should have four important
properties. First, they should define a complete set . That is, two objects must have the same
descriptors if and only if they have the same shape. Secondly, they should be congruent. As
such, we should be able to recognise similar objects when they have similar descriptors.
Thirdly, it is convenient that they have invariant properties. For example, rotation invariant
descriptors will be useful for recognising objects whatever their orientation . Other important
invariance properties naturally include scale and position and also invariance to affine and
perspective changes. These last two properties are very important when recognising objects
observed from different viewpoints. In addition to these three properties, the descriptors
should be a compact set. Namely, a descriptor should represent the essence of an object in
an efficient way. That is, it should only contain information about what makes an object
unique, or different from the other objects. The quantity of information used to describe
this characterisation should be less than the information necessary to have a complete
description of the object itself. Unfortunately, there is no set of complete and compact
descriptors to characterise general objects. Thus, the best recognition performance is obtained
by carefully selected properties. As such, the process of recognition is strongly related to
each particular application with a particular type of object.
In this chapter, we present the characterisation of objects by two forms of descriptors.
These descriptors are summarised in Table 7.1 . Region and shape descriptors characterise

Table 7.1 Overview of Chapter 7

Chain codes

Shape boundary Cumulative angular function

Fourier descriptors

Elliptic descriptors

Object description Area

Perimeter

Basic

Compactness

Region Dispersion

First order

Moments

Centralised
Zernike