High level syntax control of loop filter

A video decoder, comprising:

• a memory storing a set of instructions;
• and
• at least one processor configured to execute the set of instructions to cause the decoder to perform:
• receive a bitstream;
• decode the bitstream by using control flags for adaptive loop filter (ALF) at a sequence level, a picture level, or a slice level, wherein the sequence level, the picture level, and the slice level are levels ranking from high to low;
• decode a first control flag indicating whether the ALF is enabled at a first level;
• and
• in response to the first control flag indicating the ALF at the first level is enabled, decode a second control flag indicating whether ALF is enabled at a second level, wherein:
• the ALF is enabled at the second level when a value of the second control flag equals to 1;
• the ALF is disabled at the second level when the value of the second control flag equals to 0;
• the second level is a lower level than the first level;
• and
• the first control flag is a control flag for ALF at the sequence level, and the second control flag is a control flag for ALF at one of the picture level or the slice level depending on a third control flag signaled at a Picture Parameter Set (PPS).

A non-transitory computer readable medium storing a bitstream that is processed according to a method comprising:

• receiving a video sequence corresponding to the bitstream;
• encoding the video sequence by using control flags for adaptive loop filter (ALF) at a sequence level, a picture level, or a slice level, wherein the sequence level, the picture level, and the slice level are levels ranking from high to low;
• signaling a first control flag indicating whether the ALF is enabled at a first level;
• and
• in response to the first control flag indicating the ALF at the first level is enabled, signaling a second control flag indicating whether ALF is enabled at a second level, wherein:
• the ALF is enabled at the second level when a value of the second control flag equals to 1;
• the ALF is disabled at the second level when the value of the second control flag equals to 0;
• the second level is a lower level than the first level;
• and
• wherein the first control flag is a control flag for ALF at the sequence level, and the second control flag is a control flag for ALF at one of the picture level or the slice level depending on a third control flag signaled at a Picture Parameter Set (PPS).