representative frame in GoF (Group of frames) of a video is formed by taking spatial and temporal gradients sequentially for image frames and by selecting the pixel of the largest spatial-temporal gradient (STG) for all co-located pixels in the GoF. As a result, the boundary of the moving object in the video is highlighted by the STG operation. Therefore, an optimal bounding box for a moving object can be determined by choosing the maximum spatial density of the STG for various sizes of the bounding box. The bounding box includes the boundary trace of the MOOI (moving object of interest) in the GoF and is used to differentiate the MOOI from the non-MOOI. That is, the pixels outside the bounding box are the non-MOOI and they are the main target for the size reduction of the video frames for a pre-processing of video compression.
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