Notes by Raul Torres for NMF Seminar

Outline • • • • •

Non-negative Matrix Factorization: An overview Kernel Non-negative Matrix Factorization Sub-pattern based KNMF Experiments Conclusions

Non-negative Matrix Factorization: An overview • The key ingredient of NMF is the non-negativity constraints imposed on matrix factors.

•Columns of W are called NMF bases •Columns of H are its combining coefficients •The dimensions of W and H are n × r and r ×m , respectively The rank r of the factorization is usually chosen such that (n+m)r < nm, and hence the dimensionality reduction is achieved.

NMF: An overview Multiplicative update rules:

Kernel Non-negative Matrix Factorization Given m objects O1, O2, …, Om,

v1

v2 1

1 2

3 4 5 6 7

8 . . . n

v3 2

3.

.

.

vm m

•Attribute values represented as an n x m matrix: V=[v1, v2, …, vm] •Each column represents one of the m objects •Define the nonlinear map from original input space V to a higher or infinite dimensional feature space F

KNMF •As in NMF, KNMF finds two non-negative factors:

• is the bases in feature space • H is its combining coefficients: each column is the dimension-reduced representation of each object • is unknown, so it is impractical to directly factorize it • So:

•Kernel: is a function in the input space and at the same time is the inner product in the feature space through the kernel-induced nonlinear mapping

KNMF

KNMF

KNMF

• • Y

is the learned bases of is the learned bases of K

Sub-pattern based KNMF(SpKNMF) •Assume n is divisible by p •Reassemble the original matrix V into n/p by mp matrix U

v1

u1

U

u2

v3

u3

u4

u5

u6

u7

u8

u9

u10

u11

=

v2 n=21

m=4

p=3

v4 n/p=7

m*p=12

u12

SpKNMF

•H={hj} with dimension of r by mp, where r is the number of reduced dimensions •Then reassemble the matrix H into rp by m matrix R as R1 h1

H

h2

R3 h3

h4

h5

h6

h7

h8

h9

h10

h11

=

r*p= 15 r=5

R2 m=4

p=3

R4 m*p=12

h12

SpKNMF

Experiments • Configurations: ▫ Gaussian kernel with standard variance ▫ Nearest neighborhood classifier (1-NN) ▫ UCI: 10 independent runs, avereged 50% training – 50% testing (random) Tested in different dimensions

Experiments

Ionosphere

Bupa

Glass

PID

Experiments FERET Face Database

•400 gray-level frontal view face images from 200 persons are used •2 images per person •There are 71 females and 129 males

Accuracy

NMF

KNMF

SpKNMF

69.23%

80.37%

84.44%

Conclusions • KNMF can: ▫ Extract more useful features hidden in the original data using some kernel-induced nonlinear mapping ▫ Deal with relational data where only the relationships between objects are known ▫ Process data with negative values by using some specific kernel functions (e.g. Gaussian)

• KNMF is more general than NMF • SpKNMF improves the performance of KNMF performing KNMF on sub-patterns of the original data. • Issues ▫ Selection of kernels and parameters ▫ Choosing the appropriate size for the reassembled matrix