“…It is known to us that just one evaluation index could not well demonstrate the quality of fused images in quantitative assessment. Thus, for the sake of making a comprehensive evaluation for the fusion images, six popular fusion evaluation metrics are introduced in this section, namely visual information fidelity for fusion (VIFF) [ 29 , 30 , 31 , 32 , 33 ], Q S [ 34 ], average gradient (AG) [ 20 , 35 , 36 ], correlation coefficient (CC) [ 20 , 37 , 38 ], spatial frequency (SF) [ 20 , 39 , 40 , 41 ], and Q W [ 34 , 42 ]. In terms of all the six metrics, the higher the value data of the evaluation index, the better the fusion performance will be.…”