1. A method for decoding a current block of a picture, the method comprising: deriving one or more valid candidates of a first type for a motion vector of the current block, each of the one or more valid candidates of the first type being derived from a motion vector of a first block; deriving a valid candidate of a second type for the motion vector of the current block when a total number of the one or more valid candidates of the first type is less than a threshold value, the valid candidate of the second type being different from the one or more valid candidates of the first type, and the valid candidate of the second type being derived from a motion vector of a second block different from the first block; selecting the motion vector of the current block for decoding the current block, from among the one or more valid candidates of the first type and the valid candidate of the second type; predicting the current block using the selected motion vector of the current block; and generating an image using the predicted current block.