1. A computer-implemented method comprising: generating a coarse resolution image from a finer resolution image; generating a coarse coverage mask for the coarse resolution image that identifies valid and non-valid data in the coarse resolution image, wherein data is valid if it corresponds to image data, and data is invalid if it does not correspond to image data; and determining whether data in the finer resolution image is valid or non-valid data based on whether corresponding data identified by the coarse coverage mask is valid or non-valid data; and wherein the method is performed by one or more processors.