Methods and apparatuses of coding pictures partitioned into subpictures in video coding systems

1. A video processing method for coding pictures in a video encoding or decoding system, comprising:

receiving input video data associated with a current picture, wherein the current picture is composed of a plurality of Coding Tree Units (CTUs) for encoding or decoding;

determining a number of subpictures for the current picture;

partitioning the current picture into one or more subpictures according to the number of subpictures, wherein each subpicture contains a plurality of complete CTUs and boundaries of each subpicture are aligned with grids of the current picture in units of CTUs, wherein the number of subpictures in the current picture is limited by an allowed maximum number of slices, wherein the number of subpictures in the current picture is limited to be not greater than the allowed maximum number of slices under a specified profile and level constraint, wherein the allowed maximum number of slices is a maximum number of slices each Access Unit (AU) allowed to be partitioned into, wherein the AU is a set of Prediction Units (PUs) that belong to different layers and contains coded pictures associated with a same time for output from a Decoded Picture Buffer (DPB); and

encoding the one or more subpictures in the current picture to generate a video bitstream or decoding the one or more subpictures in the current picture to generate decoded video, wherein the allowed maximum number of slices is derived by a syntax element parsed from the video bitstream or a syntax element indicating the allowed maximum number of slices is signaled in the video bitstream.

2. The method of claim 1 , wherein the number of subpictures in the current picture is indicated by a syntax element sps_num_subpics_minus1 signaled in or parsed from a Sequence Parameter Set (SPS).

3. The method of claim 2 , wherein each picture in a Coded Layered Video Sequence (CLVS) referred to the SPS is determined to be partitioned into a plurality of subpictures when the syntax element sps_num_subpics_minus1 is greater than 0.

4. The method of claim 1 , wherein a subpicture layout for the current picture is specified based on a grid of the current picture in units of CTUs.

5. The method of claim 1 , wherein the number of subpictures in the current picture is limited by a minimum of a number of CTUs in the current picture and the allowed maximum number of slices.

6. The method of claim 1 , wherein the allowed maximum number of slices indicates a maximum number of slices each picture allowed to be partitioned into.

7. The method of claim 1 , wherein the current picture is partitioned into slices each containing a number of complete CTUs, wherein each subpicture in the current picture contains one or more slices that collectively cover a rectangular region of the current picture.

8. The method of claim 1 , further comprising determining a syntax element indicating whether subpicture ID mapping information is present in a Picture Parameter Set (PPS) referred by the current picture, and inferring rectangular slices are used for partitioning the current picture when the syntax element indicates subpicture ID mapping information is present in the PPS.

9. The method of claim 8 , wherein a presence of a syntax element indicating whether the current picture is partitioned in rectangular slices or raster scan slices is conditioned on the syntax element indicating whether subpicture ID mapping information is present in the PPS.

10. The method of claim 1 , further comprising determining one or more reference pictures for inter coding the current picture, wherein each reference picture has a same CTU size as the current picture when the current picture is partitioned into a plurality of subpictures and the reference picture is not an Inter Layer Reference Picture (ILRP) containing one subpicture.

11. The method of claim 10 , wherein each reference picture for inter coding the current picture is a reference picture in a same layer as the current picture or an ILRP in a different layer as the current picture.

12. The method of claim 11 , wherein a Sequence Parameter Set (SPS) referred to by the current picture and a SPS referred to by each reference picture have a same value of sps_log 2_ctu_size_minus5 for inter-layer coding, wherein sps_log 2_ctu_size_minus5 indicates a CTU size.

13. An apparatus of video processing method in a video encoding or decoding system, the apparatus comprising one or more electronic circuits configured for:

receiving input video data associated with a current picture, wherein the current picture is composed of a plurality of Coding Tree Units (CTUs) for encoding or decoding;

determining a number of subpictures for the current picture;

partitioning the current picture into one or more subpictures according to the number of subpictures, wherein each subpicture contains a plurality of complete CTUs and boundaries of each subpicture are aligned with grids of the current picture in units of CTUs, wherein the number of subpictures in the current picture is limited by an allowed maximum number of slices, wherein the number of subpictures in the current picture is limited to be not greater than the allowed maximum number of slices under a specified profile and level constraint, wherein the allowed maximum number of slices is a maximum number of slices each Access Unit (AU) allowed to be partitioned into, wherein the AU is a set of Prediction Units (PUs) that belong to different layers and contains coded pictures associated with a same time for output from a Decoded Picture Buffer (DPB); and

encoding the one or more subpictures in the current picture to generate a video bitstream or decoding the one or more subpictures in the current picture to generate decoded video, wherein the allowed maximum number of slices is derived by a syntax element parsed from the video bitstream or a syntax element indicating the allowed maximum number of slices is signaled in the video bitstream.

14. A non-transitory computer readable medium storing program instruction causing a processing circuit of an apparatus to perform a video processing method for pictures partitioned into subpictures, and the method comprising:

receiving input video data associated with a current picture, wherein the current picture is composed of a plurality of Coding Tree Units (CTUs) for encoding or decoding;

determining a number of subpictures for the current picture;

partitioning the current picture into one or more subpictures according to the number of subpictures, wherein each subpicture contains a plurality of complete CTUs and boundaries of each subpicture are aligned with grids of the current picture in units of CTUs, wherein the number of subpictures in the current picture is limited by an allowed maximum number of slices, wherein the number of subpictures in the current picture is limited to be not greater than the allowed maximum number of slices under a specified profile and level constraint, wherein the allowed maximum number of slices is a maximum number of slices each Access Unit (AU) allowed to be partitioned into, wherein the AU is a set of Prediction Units (PUs) that belong to different layers and contains coded pictures associated with a same time for output from a Decoded Picture Buffer (DPB); and

encoding the one or more subpictures in the current picture to generate a video bitstream or decoding the one or more subpictures in the current picture to generate decoded video, wherein the allowed maximum number of slices is derived by a syntax element parsed from the video bitstream or a syntax element indicating the allowed maximum number of slices is signaled in the video bitstream.