Table 2. Definitions for the First and Second Order Statistical Features Used in This Study.

Mean (1st order stat.)	$\frac{1}{N}{\displaystyle {\sum }_{i=1}^N{I}_k}$	where N is the number of pixels in a given region and Ik the int. value of a pixel
Co-occurrence probability (2nd order stat.)	$C_{ij}=\frac{P_{i,j}}{{\displaystyle {\sum }_{i,j=1}^G{P}_{i,j}}}$	where Pi,j represents the number of occurrences of gray levels i and j for a certain (d, θ) pair in a given region
Contrast	${\displaystyle \sum C_{ij}{(i-j)}^2}$	${\displaystyle \sum ={\displaystyle {\sum }_{i=1}^G{\displaystyle {\sum }_{j=1}^G}}}$
Cluster shade	${\displaystyle \sum C_{ij}{(i+j-{\mu }_i-{\mu }_j)}^3}$	${\mu }_i={\displaystyle \sum i{C}_{ij}}$
		${\mu }_j={\displaystyle \sum j}{C}_{ij}$
Energy	${\displaystyle \sum C_{ij}{}^2}$
Entropy*	$-{\displaystyle \sum C_{ij}\log (C_{ij}+eps)}$
Homogeneity	${\displaystyle \sum \frac{C_{ij}}{1+\left|i-j\right|}}$
Information measures of correlation*	${(1-\exp [-2(h_{ij2}-h_{ij})])}^{1/2}$	hij = entropy
		$h_{ij2}=-{\displaystyle \sum p_i{p}_j\log (p_i{p}_j+eps)}$
		$p_i={\displaystyle {\sum }_i{C}_{ij}}$and $p_j={\displaystyle {\sum }_j{C}_{ij}}$
Inverse difference moment	${\displaystyle \sum \frac{C_{ij}}{1+{(i-j)}^2}}$

*eps is an arbitrarily small constant chosen to be 2⁻⁵² (floating point accuracy).