Bidirectional prediction method and video decoding apparatus

A video encoding apparatus for inter-predicting a current block using any one of a plurality of bi-prediction modes, the apparatus comprising at least one processor configured to:

• encode enabled information indicating whether a first mode of the plurality of bi-prediction modes is allowed;
• encode, at a block level for the current block, mode information indicating whether the first mode is applied to the current block;
• when the mode information indicates that the first mode is applied,
• encode, at the block level, first motion information including differential motion vector information and predicted motion vector information for a first motion vector, and second motion information not including at least a portion of predicted motion vector information and differential motion vector information for a second motion vector,
• wherein in the first mode at least a portion of the first motion information is used together with the second motion information for derivation of the second motion vector at a video decoding apparatus;
• and
• generate a residual block that is a difference between the current block and a predicted block thereof, wherein the predicted block is generated by using a reference block indicated by the first motion vector in a first reference picture and a reference block indicated by the second motion vector in a second reference picture,
• generate transform coefficients by transforming the residual block, and encode information on the transform coefficients,
• wherein the first and second reference pictures are determined at a high level that is upper than the block level, and thereby are used in common as reference pictures for not only the current block but also blocks, associated with the high level, to which the first mode is applied.

A non-transitory computer readable medium storing a bitstream encoded by an inter prediction method using any one of a plurality of bi-prediction modes, the bitstream decoded by a process comprising:

• decoding enabled information indicating whether a first mode of the plurality of bi-prediction modes is allowed;
• decoding, at a block level for a current block, mode information indicating whether the first mode is applied to the current block, when the enabled information indicates that the first mode is allowed;
• when the mode information indicates that the first mode is applied to the current block,
• decoding, at the block level, first motion information including differential motion vector information and predicted motion vector information for a first motion vector and second motion information not including at least a portion of predicted motion vector information and differential motion vector information for a second motion vector, and
• deriving the first motion vector based on the first motion information, and deriving the second motion vector based on at least a portion of the first motion information and based on the second motion information;
• and
• predicting the current block using a reference block indicated by the first motion vector in a first reference picture and a reference block indicated by the second motion vector in a second reference picture,
• generating a residual block and reconstructing the current block based on the predicted block and the residual block,
• wherein the first and second reference pictures are determined at a high level that is upper than the block level, and thereby are used in common as reference pictures for not only the current block but also blocks, associated with the high level, to which the first mode is applied.