High level syntax control of loop filter

1. A video decoding method, comprising:

receiving a bitstream;

decoding the bitstream by using control flags for luma mapping with chroma scaling (LMCS) 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;

decoding a first control flag indicating whether the LMCS is enabled at a first level; and

in response to the first control flag indicating the LMCS is enabled at the first level, signaling a second control flag indicating whether LMCS is enabled at a second level, wherein:

the LMCS is enabled at the second level when a value of the second control flag equals to 1;

the LMCS is disabled at the second level when the value of the second control flag equals to 0; and

the second level is a lower level than the first level.

2. The video decoding method of claim 1 , wherein the first control flag is a control flag for the LMCS at the sequence level, and the second control flag is a control flag for the LMCS at the picture level.

3. The video decoding method of claim 2 , further comprising:

in response to the second control flag indicating the LMCS is enabled at the second level, signaling a third control flag indicating whether chroma scaling (CS) is enabled at the second level, wherein:

the CS is enabled at the second level when a value of the third control flag equals to 1; and

the CS is disabled at the second level when the value of the third control flag equals to 0.

4. The video decoding method of claim 1 , further comprising:

in response to the second flag indicating the LMCS is enabled at the second level, signaling a fourth control flag indicating whether the LMCS is applied at a third level, wherein:

luma mapping (LM) is applied and the CS can be applied at the third level when a value of the fourth control flag equals to 1;

the LMCS is not applied at the third level when the value of the fourth control flag equals to 0; and

the third level is a lower level than the second level.

5. The video decoding method of claim 4 , wherein the third level is the slice level.

6. A video encoder, comprising:

a memory storing a set of instructions; and

at least one processor configured to execute the set of instructions to:

receive a video sequence;

encode the video sequence by using control flags for luma mapping with chroma scaling (LMCS) 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;

signal a first control flag indicating whether the LMCS is enabled at a first level; and

in response to the first control flag indicating the LMCS is enabled at the first level, signal a second control flag indicating whether LMCS is enabled at a second level, wherein:

the LMCS is enabled at the second level when a value of the second control flag equals to 1;

the LMCS is disabled at the second level when the value of the second control flag equals to 0; and

the second level is a lower level than the first level.

7. The video encoder of claim 6 , wherein the first control flag is a control flag for the LMCS at the sequence level, and the second control flag is a control flag for the LMCS at the picture level.

8. The video encoder of claim 7 , wherein the at least one processor is further configured to:

in response to the second control flag indicating the LMCS is enabled at the second level, signal a third control flag indicating whether chroma scaling (CS) is enabled at the second level, wherein:

the CS is enabled at the second level when a value of the third control flag equals to 1; and

the CS is disabled at the second level when the value of the third control flag equals to 0.

9. The video encoder of claim 6 , wherein the at least one processor is further configured to:

in response to the second flag indicating the LMCS is enabled at the second level, signal a fourth control flag indicating whether the LMCS is applied at a third level, wherein:

luma mapping (LM) is applied and the CS can be applied at the third level when a value of the fourth control flag equals to 1;

the LMCS is not applied at the third level when the value of the fourth control flag equals to 0; and

the third level is a lower level than the second level.

10. The video encoder of claim 9 , wherein the third level is the slice level.

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

receiving a video sequence;

encoding the video sequence by using control flags for luma mapping with chroma scaling (LMCS) 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 LMCS is enabled at a first level; and

in response to the first control flag indicating the LMCS is enabled at the first level, signaling a second control flag indicating whether LMCS is enabled at a second level, wherein:

the LMCS is enabled at the second level when a value of the second control flag equals to 1;

the LMCS is disabled at the second level when the value of the second control flag equals to 0; and

the second level is a lower level than the first level.

12. The non-transitory computer readable medium of claim 11 , wherein the first control flag is a control flag for the LMCS at the sequence level, and the second control flag is a control flag for the LMCS at the picture level.

13. The non-transitory computer readable medium of claim 12 , wherein the method further comprising:

in response to the second control flag indicating the LMCS is enabled at the second level, signaling a third control flag indicating whether chroma scaling (CS) is enabled at the second level, wherein:

the CS is enabled at the second level when a value of the third control flag equals to 1; and

the CS is disabled at the second level when the value of the third control flag equals to 0.

14. The non-transitory computer readable medium of claim 11 , wherein the method further comprising:

in response to the second flag indicating the LMCS is enabled at the second level, signaling a fourth control flag indicating whether the LMCS is applied at a third level, wherein:

luma mapping (LM) is applied and the CS can be applied at the third level when a value of the fourth control flag equals to 1;

the LMCS is not applied at the third level when the value of the fourth control flag equals to 0; and

the third level is a lower level than the second level.

15. The non-transitory computer readable medium of claim 14 , wherein the third level is the slice level.