1. A method comprising, by a computing device: at time t 2 , determining a delta set of first objects representing a difference between a first set of first objects at time t 1 and a second set of first objects at time t 2 , wherein each first object is a first image, wherein the first set of first objects is represented as a first k-dimensional tree, and wherein the second set of first objects is represented as a second k-dimensional tree; for each first object, generating n elements, x 1. .. x n , representing the content of the first object; comparing the delta set of first objects with a set of second objects represented as a balanced k-dimensional tree, wherein each second object is a second image; for each second object, generating n elements, y 1. .. y n , representing the content of the second object, wherein when comparing the content of a second object with the content of a first object, the content of the second object substantially matches the content of the first object if ∑ i = 1 n ⁢ ( x i - y i ) 2 is less than a pre-defined threshold; and identifying each second object whose content substantially matches content of at least one first object from the delta set of first objects.