1. A computer implemented method for decomposing hyperspectral or multispectral image data, the image data comprising for each pixel location a sampled image spectrum that is a mixture of plural reflectance spectra, the method comprising: accessing a data source for plural linearly independent basis vectors, characterizing plural hyperplanes, each hyperplane representing an estimate of one of the plural reflectance spectra; and determining for each pixel location, a contribution of the plural basis vectors of each hyperplane to the image spectrum of that pixel location.