Loop filter utilized in encoder and associated signal processing method

A loop filter utilized in an encoder, comprising:

• a constrained directional enhancement filter, arranged to process multiple frames, wherein for a first frame in the multiple frames, the constrained directional enhancement filter determines a best filter strength of each block in the first frame in a first filter strength list, and determines a second filter strength list according to content of the first frame;
• and for a second frame in the multiple frames, the constrained directional enhancement filter determines a best filter strength of each block in the second frame in the second filter strength list, and determines a third filter strength according to content of the second frame;
• and
• a decision circuit, coupled to the constrained directional enhancement filter, and arranged to record which index in the first filter strength list is the best filter strength corresponding to each block in the first frame, and provide the first filter strength list and the index corresponding to each block to an encoding circuit of the encoder as an output of the encoder;
• wherein the constrained directional enhancement filter utilizes multiple preset filter strengths to perform a filter operation upon each block in the first frame to generate multiple filter results, and determines the second filter strength list according to the multiple filter results of each block in the first frame.