1. A method comprising: obtaining an image; estimating a spectral image of the obtained image; calculating one or both of a detection component of the image and a residual component of the image, wherein the detection component of the image is calculated based on the spectral image, a spectral-power distribution for a specific illuminant, and spectral sensitivities of a detector, and wherein the residual component of the image is calculated based on the spectral image, the spectral power distribution for the specific illuminant, and the spectral sensitivities of the detector or is calculated based on the spectral image and the calculated detection component; and generating an image signature based on one or both of the detection component and detection-component basis functions, and the residual component and residual-component basis functions.