A moving-object region is specified on an image captured by an imaging apparatus, and a plurality of calculation points are arranged on the specified region. A group of local motion vectors is determined for the calculation points, and a moving-object motion vector is obtained based on the determined local motion vectors. A group of calculation points whose local motion vectors are the same as or similar to the moving-object motion vector is continuously used to calculate the local motion vectors, and is updated by moving the position of the group of calculation points along the moving-object motion vector, and a calculation point whose local motion vector is different from the moving-object motion vector is rearranged by placing a new calculation point at a position near any of the continuously used calculation points. A movable mechanism including the imaging apparatus is controlled in accordance with the moving-object motion vector.
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