function d = d_gen(m_a,m_b,S_a,S_b);

% function d = d_gen(m_a,m_b,S_a,S_b);
% 
% d = generalized d' (Thomas, 99, 03)
% m_x = mean vector for density x
% S_x = covariance matrix for density x

m_dif = m_b - m_a;
S_sum = S_a + S_b;
d = ( m_dif' * m_dif ) / sqrt( .5*( m_dif' * S_sum * m_dif ) );