Motion vector reconstruction order swap

A method for reconstructing motion vectors used for bi-prediction in video decoding of a block of a current picture in a video decoder, the method comprising:

• determining at least one candidate motion vector predictor for each of two different motion vector predictor lists, each candidate motion vector predictor associated with a reference picture;
• selecting a reconstruction order for two different motion vector predictor lists based on the candidate motion vector predictors or based on a syntax element retrieved from a bitstream representing an encoded version of the current picture, wherein the reconstruction order identifies one of the two different motion vector predictor lists as a first motion vector predictor list and the other of the two different motion vector lists as a second motion vector predictor list;
• generating a first reconstructed motion vector for the first motion vector predictor list using a candidate motion vector predictor of the first motion vector predictor list and a first motion vector delta retrieved from the bitstream;
• replacing one of the at least one candidate motion vector predictor in the second motion vector predictor list with the first reconstructed motion vector or a scaled version of the first reconstructed motion vector;
• and
• generating a second reconstructed motion vector for the second motion vector predictor list using a candidate motion vector predictor of the second motion vector predictor list and a second motion vector delta retrieved from the bitstream.

An apparatus for predicting motion vectors used for bi-prediction in video encoding of a block of a current picture in a video encoder, wherein

• the apparatus is configured to determine at least one candidate motion vector predictor for each of two different motion vector predictor lists, each candidate motion vector predictor associated with a reference picture;
• the apparatus is configured to select a reconstruction order for the motion vector predictor lists based on the candidate motion vector predictors, wherein the reconstruction order identifies one of the two different motion vector predictor lists as a first motion vector predictor list and the other of the two different motion vector lists as a second motion vector predictor list;
• the apparatus is configured to generate a first motion vector delta as a difference between a first actual motion vector for the block and a candidate motion vector predictor from the first motion vector predictor list;
• the apparatus is configured to replace one of the at least one candidate motion vector predictor in the second motion vector predictor list with the first actual motion vector or a scaled version of the first actual motion vector;
• and
• the apparatus is configured to generate a second motion vector delta as a difference between a second actual motion vector for the block and a candidate motion vector predictor from the second motion vector predictor list.

An apparatus for reconstructing motion vectors used for bi-prediction in video decoding of a block of a current picture in a video decoder, wherein

• the apparatus is configured to determine at least one candidate motion vector predictor for each of two different motion vector predictor lists, each candidate motion vector predictor associated with a reference picture;
• the apparatus is configured to select a reconstruction order for two different motion vector predictor lists based on the candidate motion vector predictors or based on a syntax element retrieved from a bitstream representing an encoded version of the current picture, wherein the reconstruction order identifies one of the two different motion vector predictor lists as a first motion vector predictor list and the other of the two different motion vector lists as a second motion vector predictor list;
• the apparatus is configured to generate a first reconstructed motion vector for the first motion vector predictor list using a candidate motion vector predictor of the first motion vector predictor list and a first motion vector delta retrieved from the bitstream;
• the apparatus is configured to replace one of the at least one candidate motion vector predictor in the second motion vector predictor list with the first reconstructed motion vector or a scaled version of the first reconstructed motion vector;
• and
• the apparatus is configured to generate a second reconstructed motion vector for the second motion vector predictor list using a candidate motion vector predictor of the second motion vector predictor list and a second motion vector delta retrieved from the bitstream.