1. A method performed by a computing system for identifying objects within a medium, the method comprising: accessing a sequence of image frames of the medium, each image frame corresponding to an along-medium location, each image frame having voxels representing various depths and cross-medium locations at that along-medium location, each voxel having an intensity; generating a depth-compensated intensity for each voxel that is a normalization of the intensities across depths to account for variations resulting from differences in depth; and analyzing the depth-compensated intensities to identify cross-medium locations of image frames with object voxels that represent part of an object.