A COMPARISON AND EVALUATION OF MOTION INDEXING ...

3.1.6.1 Inverted File Index
Inverted file index is a type of indexing used in text-based retrieval systems.
It is an index data structure storing a mapping from content to its locations in
a database file, or in a document, or in a set of documents. It is also known as
postings file or an inverted file. The database Γ is indexed with respect to F and
inverted indices are used. For each feature vector υ ∈ {0, 1} f , an inverted file index
or inverted file, L (υ), is created. This file consists of all indices m ∈ [0, M] of the
sequence w = (w0, w1, · · · , wM) with v = wm. Hence, L (υ) gives information about
the F -segments of Γ which exhibit the feature vector υ. Figure 3.2 shows the flow of
inverted list creation.
Inverted lists are sets represented as sorted, repetition-free sequences. The
number of inverted lists for the feature vector depends on the number of boolean
functions it is composed of. The vector υ ∈ {0, 1} f has f boolean functions and has
2 f inverted lists associated with it. An inverted file index of the database Γ consists
of these 2 f inverted lists L(υ) for vector υ and is denoted by I Γ F .
The inverted lists contain the position of the segments and the segment lengths of
the motion file. Each segment position appears in exactly one inverted list, the size
of the inverted file index is proportional to the number M of segments of Γ.
3.1.6.2 Algorithm for Feature-based Indexing
The indexing or matching of the similar motion clips depends on the definition
of similarity. There can be different notions for declaring two motion clips as similar.
Searching a similar motion can be as restrictive as finding motion clips which are
exactly the same or it can be little less restrictive as fuzzy search which only checks
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