1. A method of comparing a first histogram h 1 to a second histogram h 2 , comprising determining a diffusion distance that is a function of h 1 , h 2 and a diffusion filter wherein the diffusion distance is K ⁡ ( h 1 , h 2 ) = ∑ l = 0 L ⁢ k ⁡ (  d l ⁡ ( x )  ) , wherein ⁢ ⁢ d 0 ⁡ ( x ) = h 1 ⁡ ( x ) - h 2 ⁡ ( x ) ⁢ ⁢ and ⁢ ⁢ d l ⁡ ( x ) = [ d l - 1 ⁡ ( x ) * ϕ ⁡ ( x , σ ) ] ⁢ ↓ 2 and wherein L is a number of pyramid layers, σ is the constant standard deviation for φ and ↓ 2 denotes half size downsampling.