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Master Thesis - Department of Computer Science

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matrices, which can be organized in a matrix form as:<br />

⎡<br />

⎢<br />

SM = ⎢<br />

⎣<br />

S 1,1<br />

w<br />

S 2,1<br />

b<br />

S 1,2<br />

b . . . . . . S 1,C<br />

b<br />

S 2,2<br />

w . . . . . . S 2,C<br />

b<br />

. . . . . . . . . . . . . . .<br />

S i,1<br />

b . . . Si,i w . . . S i,C<br />

b<br />

. . . . . . . . . . . . . . .<br />

S C,1<br />

b<br />

The diagonal elements <strong>of</strong> SM, S i,i<br />

w<br />

S C,2<br />

b . . . . . . SC,C w<br />

⎤<br />

⎥<br />

⎦<br />

i = 1, 2, ...., C, represent the within-class scat-<br />

ter matrices for each <strong>of</strong> the C classes. Similarly, the non-diagonal elements <strong>of</strong> SM,<br />

S i,j<br />

b<br />

where i, j = 1, 2, ..., C and i �= j, are the between-class scatter matrices for two<br />

different classes i and j. Sb and Sw for C classes, denoted by SbC and SwC, can be<br />

obtained by summing up all non-diagonal and diagonal entries <strong>of</strong> SM respectively. So<br />

SwC =<br />

SbC =<br />

C�<br />

i=1<br />

C� C�<br />

S<br />

i=1 i�=j<br />

j=1<br />

i,j<br />

b<br />

S i,i<br />

w , (4.49)<br />

(4.50)<br />

Once SbC and SwC are obtained, optimum linear features can be selected by maxi-<br />

mizing tr(S −1<br />

wCSbC).<br />

4.4.3 Algorithm for Decision Fusion<br />

In this section, we describe the overall algorithm for our proposed method <strong>of</strong> decision<br />

fusion <strong>of</strong> two classifiers constructed on range space and null space <strong>of</strong> within-class<br />

scatter. We attempt to combine the complementary information present in null space<br />

and range space by decision fusion. The steps <strong>of</strong> our algorithm is given below:<br />

1. Construction <strong>of</strong> two classifiers (DNull and DRange) based on null space<br />

and range space.<br />

i) Compute Sw from training set X = [x 1 1, x 1 2, ..., x 1 N, x 2 1, ..., x C N] using Eqn. 4.1.<br />

ii) Perform eigen-analysis <strong>of</strong> Sw and select first r eigenvectors to form the<br />

basis vector set Q for range space V .<br />

90

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