From: Localizing global descriptors for content-based image retrieval
tf
df
Normalization
n(natural): tf t,d
n(no): 1
n(none): 1
l(log): 1+log(tf t,d )
t(idf): log(N/df t )
c (cos):\( {1}/{\sqrt {{w_{1}^{2}}+\dots + {w_{M}^{2}}}}\)