Method of determining motion vectors for a bi-predictive 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, reference index information from 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 reference picture referred to by the reference index information, the reference picture being for the co-located image block, the type being one of long term and short term, the type characterizing a temporal distance of the reference picture for the co-located image block with respect to the bi-predictive image block, the determining step determining the motion vectors according to a first set of expressions if the reference index information refers to a reference picture of the short term type and the determining step determining the motion vectors according to a second set of expressions if the reference index information refers to a reference picture of the long term type, the second set of expressions being different than the first set of expressions; and

decoding, by the moving picture coding system, the bi-predictive image block based on the determined motion vectors.

2. The method of claim 1 , wherein the second set of expressions sets a first motion vector of the bi-predictive image block equal to a motion vector of the co-located image block.

3. The method of claim 2 , wherein the first motion vector of the bi-predictive image block is a forward motion vector.

4. The method of claim 2 , wherein the determining step is performed in a direct mode of operation.

5. The method of claim 2 , wherein the first set of expressions determines the first and second motion vectors of the bi-predictive image block by scaling the motion vector of the co-located block.

6. The method of claim 1 , wherein the second set of expressions sets a motion vector of the bi-predictive image block equal to zero.

7. The method of claim 6 , wherein the motion vector of the bi-predictive image block is a backward motion vector.

8. The method of claim 6 , wherein the determining step is performed in a direct mode of operation.

9. The method of claim 6 , wherein the first set of expressions determines first and second motion vectors of the bi-predictive image block by scaling the motion vector of the co-located block.

10. The method of claim 1 , wherein the second set of expressions sets a first motion vector of the bi-predictive image block equal to a motion vector of the co-located image block and sets a second motion vector of the bi-predictive image block equal to zero.

11. The method of claim 10 , wherein the first motion vector of the bi-predictive image block is a forward motion vector and the second motion vector of the bi-predictive image block is a backward motion vector.

12. The method of claim 10 , wherein the determining step is performed in a direct mode of operation.

13. The method of claim 10 , wherein the first set of expressions determines the first and second motion vectors of the bi-predictive image block by scaling the motion vector of the co-located block.