1. A method of processing video data, comprising: receiving from a motion capturing circuit a first frame and a second frame; dividing the first frame into a plurality of pixel blocks having a first size; calculating motion vectors of each of the first-size pixel blocks between the first frame and the second frame; grouping proximal motion vectors to identify first groups of motion vectors, wherein each identified group among the first groups of motion vectors corresponds to one or more objects or to a portion of one or more objects in a scene; calculating an affine transform Ti(n) for each identified group among the first groups of motion vectors, wherein Ti(n) is the affine transform of the ith group, where n denotes its time sequence position; calculating a number of selected points within an extent of the affine transform Ti(n) of each group among the first groups of motion vectors; and identifying a first-group principal transform candidate by selecting among the affine transforms Ti(n) of first groups of motion vectors using scoring functions, wherein the scoring functions include at least one of transform-point-number scores and transform-extent scores, wherein the first-group principal transform candidate is a first candidate for selection as a principal transform P(n) of the scene.