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Old 16th December 2016, 08:43   #20  |  Link
LigH
German doom9/Gleitz SuMo
 
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Join Date: Oct 2001
Location: Germany, rural Altmark
Posts: 6,753
That's the wrong question, IMHO. All three are able to find the optimal motion vector – even if it is rather distant from a previous estimation (due to recursive search steps to continue into the direction of the best match so far). They will merely differ by the required time to find the optimum, but when it's found, the quality will be the same in all cases.

Assumed there are no misleading constellations which provoke the recursive search to fail when led into a wrong direction, in which case the remaining difference to be quantized would be of a bigger magnitude, or an intra coded block would be required, which would take more space and reduce the efficiency.

Star has a smaller scope right away, it will terminate faster for rather regular motion, which may differ a bit more in mostly vertical or horizontal direction. The scope of UMH is wider, there are already 49 samples always in a first step (16 directions * 3 radii + the origin of the estimation), which will make it probably one of the slowest of these three on the average, yet may be more suitable for the case of a rather shaky camera or otherwise randomly changing motion. And SEA is effectively exhaustive over the whole motion range square, its speed would be constantly high, equal to the dual logarithm of the range for 8..9 samples in each refinement step; to beat UMH on average speed, it has to calculate at most 6 steps which would cover a square of +/- 32 pixels around the estimation, if I'm not completely wrong...

For encoding a TV broadcast of a sports event, I would probably trust the star-shaped motion search because motion will most often be quite regular (well estimated), which will save some time in comparison to UMH (probably good for action body camera footage) and SEA (probably best for those who insist in "the best" regardless of costs).
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Last edited by LigH; 16th December 2016 at 09:09.
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