US Patent 7844116 Method for identifying images after cropping

Images within an image repository related to a selected image may be located despite cropping, rotating, and/or the application of a variety of other distortions to either the selected image and/or images in the image repository. Described exemplary approaches may use image digests produced using a multi-step process. In a first step, a discrete Fourier transform may be applied to produce an M×N frequency-based image matrix from an M×N image matrix containing elements that represent spatial image pixel values. In subsequent steps, matrix approximations may be applied to reduce the M×N frequency-based image matrix and to produce an image digest that reflects the content of the image. Further, an approach is described for generating a search threshold value that may be used to determine whether images digests associated with images in a repository are sufficiently similar to a search digest to be returned in a search result.