*4.3. Evaluation Index*

In the reduced-resolution experiment, the HR MS image is usually used as the reference image for evaluation, therefore, commonly used objective evaluation indicators, including spectral angle mapper (SAM) [54], the erreur relative global adimensionnelle de synthese (ERGAS) [55], the correlation coefficient(CC) [56] and the Q2n index [57], is used to evaluate all the approaches at reduced resolution. The optimal values of CC and Q2n are 1, and the best values of for ERGAS and SAM are 0.

Since there is no reference image in the full-resolution experiment, we adopt the popular non-reference quality evaluation index, i.e., the quality with no reference (QNR) [58] to evaluate the pansharpening performance. QNR consists of a spectral distortion index, *Dλ*

and a spatial distortion index, *Ds*, which can be computed as: *QNR* = (1 − *<sup>D</sup>λ*) *α* (1 − *DS*) *β* . In the following experiments, *α* and *β* are both set as to 1. The ideal value is 1 for QNR, and 0 for *Dλ* and *DS*
