A survey of classical methods and new trends in pansharpening of multispectral images <s> MS <s> Abstract To take advantage of the high spectral resolution of Landsat TM images and the high spatial resolution of SPOT panchromatic images (SPOT PAN), we present a wavelet transform method to merge the two data types. In a pyramidal fashion, each TM reflective band or SPOT PAN image was decomposed into an orthogonal wavelet representation at a given coarser resolution, which consisted of a low frequency approximation image and a set of high frequency, spatially-oriented detail images. Band-by-band, the merged images were derived by performing an inverse wavelet transform using the approximation image from each TM band and detail images from SPOT PAN. The spectral and spatial features of the merged results of the wavelet methods were compared quantitatively with those of intensity-hue-saturation (IHS), principal component analysis (PCA), and the Brovey transform. It was found that multisensor data merging is a trade-off between the spectral information from a low spatial-high spectral resolution sensor an... <s> BIB001 </s> A survey of classical methods and new trends in pansharpening of multispectral images <s> MS <s> This paper presents a novel multi-sensor image fusion algorithm, which extends pan-sharpening of multispectral (MS) data through intensity modulation to the integration of MS and SAR imagery. The method relies on SAR texture, extracted by ratioing the despeckled SAR image to its lowpass approximation. SAR texture is used to modulate the generalized intensity (GI) of the MS image, which is given by a linear transform extending Intensity-Hue-Saturation (IHS) transform to an arbitrary number of bands. Before modulation, the GI is enhanced by injection of highpass details extracted from the available Pan image by means of the “a-trous” wavelet decomposition. The texturemodulated pan-sharpened GI replaces the GI calculated from the resampled original MS data; then the inverse transform is applied to obtain the fusion product. Experimental results are presented on Landsat-7/ETM+ and ERS-2 images of an urban area. The results demonstrate accurate spectral preservation on vegetated regions, bare soil, and also on textured areas (buildings and road network) where SAR texture information enhances the fusion product, which can be usefully applied for both visual analysis and classification purposes. <s> BIB002
Algorithm 2 Relative spectral contribution pansharpening The Brovey transform (BT), named after its author, is a simple method to merge data from different sensors based on the chromaticity transform  , with the limitation that only three bands are involved BIB001  . A pansharpened image using the BT method is shown in Figure 2(c) . The Brovey transform provides excellent contrast in the image domain but greatly distorts the spectral characteristics  . The Brovey sharpened image is not suitable for pixel-based classification as the pixel values are changed drastically  . A variation of the BT method subtracts the intensity of the MS image from the PAN image before applying Equation 3  . Although the first BT method injects more spatial details, the second one preserves better the spectral details. The concept of intensity modulation (IM) was originally proposed by Wong et al.  in 1980 for integrating Landsat MSS and Seasat SAR images. Later, this method was used by Cliche et al.  for enhancing the spatial resolution of three-band SPOT MS (XS) images. As a method in the relative spectral contribution family, we can derive IM from Equation 3, by replacing the sum of all MS bands, by the intensity component of the IHS transformation  . Note that the use of the IHS transformation limits to three the number of bands utilized by this method. The intensity modulation may cause color distortion if the spectral range of the intensity replacement (or modulation) image is different from the spectral range covered by the three bands used in the color composition BIB002 . In the literature, different versions based on the IM concept have been used   BIB002 . The relations between RSC and CS families have been deeply studied in   where these families are considered as a particular case of the GIHS and GIF protocols, respectively. The authors also found that RSC methods are closely CS, with the difference, as already commented, that the contribution of the PAN varies locally.