A Method Based on Local Variance for Quality Assessment of Multiresolution Image Fusion

Several methods exist to combine a panchromatic image of high spatial resolution with lower resolution multispectral imagery. Of particular interest are those methods designed to simulate real multispectral images having the spatial resolution of a panchromatic image. To help justify an algorithm over another one, quantitative evaluation of the quality of a fused image is necessary. In most cases, the evaluation is performed with the original highand low-spatial resolution images degraded to a coarser resolution by pixel-block averaging. The multispectral image of the highest resolution serves as a reference image (real image). Most approaches proposed for quality assessment are based on statistical measures computed over the whole image; typical measures are the correlation coefficient and the root-mean-square deviation. However, these measures make no reference to the spatial domain. In this paper, we suggest measures based on local variance computed over a three-by-three pixel window as complementary measures to evaluate the quality of the fused images. The rationale is that an ideal fused image must replicate the variance of the reference image when estimated locally. To help discriminate between local variance induced by real details as opposed to artefacts, the variance is partitioned into two terms. Each term takes into consideration the expected direction of the added details over the multispectral image oversampled by pixel replication. The method is illustrated with different fusion models applied to an Ikonos image.