Processing hyperspectral or multispectral image data

The disclosure concerns processing hyperspectral or multispectral images. Image data comprises a sampled image spectrum represented by first values for each pixel location representative of an intensity associated with a wavelength index. A processor determines for each pixel location second values based on a measure of similarity between pixel locations with respect to the first values such that two pixel locations that 5 are similar with respect to the first values are also similar with respect to the second values. The processor then stores for each pixel location the determined one or more second values associated with that pixel location on a data store. This way, the image data is made suitable for applications, such as clustering or displaying, while pixels that are similar in the input image are also similar in the output data. This means that a 10 structure between the pixels in the input image is preserved.