The disclosure concerns hyperspectral and multispectral image. The disclosure concerns the computation of a descriptor for such an image. The descriptor can be used in a range of image post-processing applications, such as classification, recognition, tracking and localization. Aspects of the invention include a computer implemented method and a computer system. To computer the descriptor for a set of wavelength indexed bands, a band representation of the radiance measurements for that band based on a probability distribution function is computed 300. Then a wavelength-dependent cross-correlation between the band representations is computed 302. Finally, a descriptor based on the computed cross-correlation between the band representations is computed 304. It is an advantage that the descriptor captures a discriminative and descriptive representation of scene captured in the hyperspectral or multispectral image.