TABLE II.

The objective functions for all the comparison methods.

Method	Objective function
RR	$\mathrm{m}\mathrm{i}{\mathrm{n}}_{\mathbf{W}}{\Vert {\mathbf{W}}^T{\mathbf{X}}_2-{\mathbf{X}}_1\Vert }_F^2+{\lambda }_1\Vert \mathbf{W}{\Vert }_F^2$
GFS	$\mathrm{m}\mathrm{i}{\mathrm{n}}_{\mathbf{W}}{\Vert {\mathbf{W}}^T{\mathbf{X}}_2-{\mathbf{X}}_1\Vert }_F^2+{\lambda }_1\Vert \mathbf{W}{\Vert }_{G_{2,1}}+{\lambda }_2\Vert \mathbf{W}{\Vert }_{2,1}$
L21	$\mathrm{m}\mathrm{i}{\mathrm{n}}_{\mathbf{W}}{\Vert {\mathbf{W}}^T{\mathbf{X}}_2-{\mathbf{X}}_1\Vert }_F^2+{\lambda }_1\Vert \mathbf{W}{\Vert }_{2,1}$
SLR	${\min }_{\mathbf{A},\mathbf{B},\mathbf{b}}{\Vert {\mathbf{X}}_1^T-{\mathbf{X}}_2^T\mathbf{B}{\mathbf{A}}^T-\mathbf{eb}\Vert }_F^2+{\lambda }_1\Vert \mathbf{A}{\Vert }_{2,1}+{\lambda }_2\Vert \mathbf{B}{\Vert }_{2,1}$
RFS	minW ||WTX2–X1||2,1 + λ1 ||W||2,1