Method of determining motion vectors for bi-predictive image block

1. A method of determining and using motion vectors for a current image block in a current picture, the method performed by a moving picture coding system that includes a buffer and comprising:

obtaining, by the moving picture coding system, a motion vector of a co-located image block in a co-located picture for the current image block;

determining, by the moving picture coding system, motion vectors of the current image block based on a type of a reference picture of the current image block and the motion vector of the co-located image block,

the type being one of a long-term reference picture and a short-term reference picture,

the type characterizing a temporal distance of the reference picture of the current image block with respect to the current image block,

the determining step determining the motion vectors of the current image block, including at least one motion vector, obtained by scaling the motion vector of the co-located image block,

if the type of a reference picture for the co-located image block is the short-term reference picture and

the determining step determining the motion vectors of the current image block, including at least one motion vector, which equal to the motion vector of the co-located image block,

if the type of the reference picture for the co-located image block is the long-term reference picture;

obtaining, by the moving picture coding system, a motion compensated block of the current image block by using the determined motion vectors of the current image block and the reference picture of the current image block; and

determining, by the moving picture coding system, the current image block based on the motion compensated block.

2. The method of claim 1 , wherein the at least one motion vector of the current image block equal to zero, if the type of the reference picture for the co-located image block is the long-term reference picture.

3. The method of claim 1 , wherein the long-term reference picture is coded before the short-term reference picture.

4. A moving picture coding system that includes a buffer to determine and use motion vectors for a current image block in a current picture, comprising:

the moving picture coding system to obtain a motion vector of a co-located image block in a co-located picture for the current image block, determine motion vectors of the current image block based on a type of a reference picture of the current image block being one of a long-term reference picture and a short-term reference picture and the motion vector of the co-located image block, obtain a motion compensated block of the current image block by using the determined motion vectors of the current image block and the reference picture of the current image block, and determine the current image block based on the motion compensated block,

the type characterizing a temporal distance of the reference picture of the current image block with respect to the current image block,

wherein the moving picture coding system to determine the motion vectors of the current image block, including at least one motion vector, obtained by scaling the motion vector of the co-located image block, if the type of a reference picture for the co-located image block is the short-term reference picture and

the moving picture coding system to determine the motion vectors of the current image block, including at least one motion vector, which equal to the motion vector of the co-located image block, if the type of the reference picture for the co-located image block is the long-term reference picture.

5. The moving picture coding system of claim 4 , wherein the at least one motion vector of the current image block equal to zero, if the type of the reference picture for the co-located image block is the long-term reference picture.

6. The moving picture coding system of claim 4 , wherein the long-term reference picture is coded before the short-term reference picture.