Information-theoretic image fusion assessment without reference

In this work we investigate the use of Shannon's information theory for the goal of devising quality scores of image fusion results, that do not require reference originals. In particular, the mutual information between re-sampled original and fused MS bands is used to measure the spectral quality, while the mutual information between the Pan image and the fused bands yields a measure of spatial quality. The rationale is that the mutual information calculated either between any couple of bands, or between each band and the Pan image, should be unchanged after fusion, i.e. when the spectral information is translated from the coarse scale of the MS data to the fine scale of the Pan image. Experimental results carried out on Ikonos data demonstrate that the results provided by the proposed information-theoretic method are in trend with analysis performed on spatially degraded data by means of average SAM, Walds's ERGAS and the novel Q4 index based on quaternion theory and recently proposed by the authors. However, the novel method requires no reference and is therefore usable in all practical cases.