Method and apparatus of filtering image in image coding system

A filtering method performed by a decoder, the method comprising:

• deriving a motion vector of a current block based on a neighboring block to the current block;
• deriving a reference block on a reference picture based on the motion vector;
• deriving first neighboring samples to the current block and second neighboring samples to the reference block for derivation of information about a filter for the current block;
• deriving the filter information for the current block based on the first neighboring samples and the second neighboring samples;
• and
• filtering the current block based on the filter information.

A filtering method performed by an encoder, the method comprising:

• deriving a specific neighboring block to a current block for derivation of information about a filter for the current block;
• deriving a reference block in a reference picture corresponding to the specific neighboring block;
• deriving the information about the filter for the current block based on the specific neighboring block and the reference block;
• filtering the current block based on the information about the filter;
• and
• encoding and outputting information about the filtering performed on the current block.