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• #8173 headhunter addikt headhunter #8171

  Új Válasz 2011-02-12 16:57:08 #8173

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  addikt

  válasz headhunter #8171 üzenetére

  "3f. What's GMC and what is it good for?

  GMC stands for Global Motion Compensation and that pretty much tells the story of what it does. If used, it will look at the whole frame and see if there is an amount of motion that all the parts of the frame have in common. It will then take this amount of motion and put it in a single value. The parts of the frame are the macroblocks, and the amount of motion is called a 'motion vector' which has both a direction and a value (as a sort of two-dimensional X,Y value) .
  All the macroblocks normally have their own motion vectors, but with GMC the one motion vector that they all have in common (that's why it's called 'Global') will be compensated and put into a single motion vector. Some macroblocks' movement will be completely compensated for by the GMC vector, getting completely nullified by the compensation process. These macroblocks' motion vector will then be removed, as it is the same and is only extra information. The possible benefit is that you can remove many or all the motion vectors of the macroblocks (or even the blocks themselves if there is no altered texture information) in a frame by a single value, thereby making it much smaller.
  Note however that this is for one-warppoint GMC. With Multiple warppoints the proces is much more complex, but the principle is the same.

  3g. Warppoints, hmm...what's a warppoint?

  A warppoint is a motion vector that defines a displacement of one *edge* of the video. Take a piece of paper and move it by its edges and you'll see what I mean.
  -The first warppoint defines displacement of top-left edge. If it's the only warppoint, the rest of the picture just has the same vector and the whole picture moves. Think panning.
  -A second warppoint defines displacement of top-right edge (not *precisely* true but close enough without getting too technical). Together with the first warppoint, this is enough to define panning *and* zoom. Note that it could be used to define panning and rotation instead, but isn't.*
  -A third warppoint means displacement of down-left edge and three warpoints are enough to define panning, zoom and rotation.
  -A fourth warppoint would create perspective-like movement.
  Note that XviD's GMC uses 3 warppoints, whereas DivX's GMC uses only one. Warppoints are stored in the frame's header, and only if they are used.
  "

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