1st EARSeL Workshop of the SIG Urban Remote Sensing Humboldt-Universität zu Berlin, 2-3 March 2006 1 A CRITICAL REVIEW OF FUSION METHODS FOR TRUE COLOUR DISPLAY OF VERY HIGH RESOLUTION IMAGES OF URBAN AREAS Luciano Alparone 1 , Bruno Aiazzi 2 , Stefano Baronti 2 , Andrea Garzelli 3 and Filippo Nencini 3 1. University of Florence, Dept. Electr. & Telecoms., Via S. Marta, 3, I-50139 Florence, ITALY; Tel.: +39 055 4796563; Fax: +39 055 494569; e-mail: alparone@lci.det.unifi.it 2. Institute of Applied Physics “Nello Carrara”, National Research Council, Florence, ITALY; e-mail: {b.aiazzi s.baronti}@ifac.cnr.it 3. University of Siena, Dept. of Information Engineering, Via Roma, 56, I-53100 Siena, ITALY; e-mail: {garzelli nencini}@dii.unisi.it ABSTRACT This paper critically reviews state-of-the-art and advanced methods for multi-spectral (MS) and panchromatic (Pan) image fusion based on either intensity-hue-saturation (IHS) transformation, or redundant multi-resolution analysis (MRA). In either cases, lower-resolution MS bands are sharp- ened by injecting details taken from the higher-resolution Pan image. Crucial point is modelling the relationships between detail coefficients of a generic MS band and the Pan image at the same resolution. Once calculated at the coarser resolution, where both types of data are available, such a model shall be extended to the finer resolution to weight the Pan details to be injected. Two in- jection models embedded in a generalised Laplacian pyramid (GLP) decomposition will be com- pared on a test set of very high resolution QuickBird MS+Pan data. INTRODUCTION Remote-sensing image fusion techniques aim at integrating the information conveyed by data ac- quired with different spatial and spectral resolution from satellite and aerial platforms. The most straightforward goal is photo-analysis, but also automated tasks such as features extraction and segmentation/classification have been found to benefit from fusion. A variety of image fusion tech- niques is devoted to merge multi-spectral (MS) and panchromatic (Pan) images, which exhibit complementary characteristics of spatial and spectral resolutions. Pan-sharpened MS is a fusion product in which the MS bands are sharpened by the higher-resolution Pan image. When exactly three MS bands are concerned, the most straightforward fusion method is to resort to an Intensity-Hue-Saturation (IHS) transformation. This procedure is equivalent to inject, i.e., add, the difference between the sharp Pan and the smooth intensity into the re-sampled MS bands (i). Since the histogram-matched Pan and the intensity component I do not generally have the same radiometry, i.e. local mean, when the fusion product is displayed in colour composition, large spec- tral distortion, i.e. colour changes, may be noticed. This occurs because the spectral response of I, as synthesised by means of the MS bands, may be far different from that of Pan. Thus, also radi- ance offsets, slowly space-varying, and not only spatial details, are locally injected. When more than three spectral bands are available, IHS fusion may be applied to three consecutive spectral components at a time, or better the IHS transformation may be replaced with principal component analysis (PCA). The latter does not avoid spectral distortion, even if it may be less noticeable. Generally speaking, if the spectral responses are not perfectly overlapped with the Pan bandwidth, as it happens with Ikonos and QuickBird, IHS- and PCA-based methods yield poor results in terms of spectral fidelity. To definitely overcome this inconvenience, methods based on injecting spatial details only, taken from the Pan image without resorting to IHS transformation, have been introduced and have dem- onstrated superior performances. Multi-resolution analysis (MRA) provides effective tools, like wavelets and Laplacian pyramids, to help carry out data fusion tasks (ii). However, in the case of