Can motion vectors be used to measure object velocity ?

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Can motion vectors be used to measure object velocity ?

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Motion vector information cannot be reliably used as a means of determining object velocity unless the encoder model specifically set out to do so. First, encoder models that optimize picture quality form vectors that typically minimize prediction error and, consequentially, the vectors often do not represent true object translation. Standards converters that re-sample one frame rate to another (as in NTSC to PAL) use different methods (field coding, edge detection, et al) that are not concerned with optimizing SNR vs bitrate. Secondly, motion vectors are not transmitted for all macroblocks anyway.

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A. Motion vector information cannot be reliably used as a means of determining object velocity unless the encoder model specifically set out to do so. First, encoder models that optimize picture quality generate vectors that typically minimize prediction error and, consequently, the vectors often do not represent true object translation. Standards converters that resample one frame rate to another (as in NTSC to PAL) use different methods (motion vector field estimation, edge detection, et al) that are not concerned with optimizing ratios such as SNR vs bitrate. Secondly, motion vectors are not transmitted for all macroblocks anyway.

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