17. The method of claim 1 , comprising enforcing a non-negative constraint on the projection matrix.
19. The method of claim 17 , comprising determining min U , A ⁢ 1 2 ⁢  X - UA  F 2 + λ ⁢ ∑ d = 1 D ⁢ ∑ j = 1 J ⁢ a dj subject ⁢ ⁢ to ⁢ ⁢ U T ⁢ U = I , ⁢ A ≥ 0.