Method of determining motion vectors for bi-predictive image block
In one embodiment, the method includes determining a motion vector of a current image block equal to a motion vector of an image block based on a reference picture index indicating a reference picture for the image block. For example, the motion vector of the current image block may be equal to the motion vector of the image block if the reference picture index indicates a long-term reference picture for the image block.
1. A method of determining and using motion vectors for a bi-predictive image block, the method performed by a moving picture coding system and comprising:
obtaining, by the moving picture coding system, a motion vector of a co-located image block for the bi-predictive image block;
determining, by the moving picture coding system, motion vectors of the bi-predictive image block based on a type of temporal distance information between a reference picture for the co-located image block and the bi-predictive block,
the type of the temporal distance information being one of a first type and a second type, the second type being longer than the first type,
the determining step determining the motion vectors from the motion vector of a co-located image block according to a first set of expressions if the type of the temporal distance information is the first type and
the determining step determining the motion vectors from the motion vector of a co-located image block according to a second set of expressions if the type of the temporal distance information is the second type,
the second set of expressions being different than the first set of expressions;
obtaining motion-compensated blocks based on the two motion vectors; and
decoding, by the moving picture coding system, the bi-predictive image block by averaging the motion-compensated blocks.
2. The method of claim 1 , wherein the first type is a short term type and the second type is a long term type.