Method and device for encoding/decoding image on basis of mixed NAL unit type, and recording medium for storing bitstream

1 . An image decoding method performed by an image decoding apparatus, the image decoding method comprising:

obtaining, from a bitstream, network abstraction layer (NAL) unit type information of at least one NAL unit including coded image data of a current picture;

determining a NAL unit type of each of a plurality of slices in the current picture based on the obtained NAL unit type information; and

decoding the plurality of slices based on the determined NAL unit type,

wherein the current picture includes a first subpicture and a second subpicture, the first subpicture includes one or more slices having a first NAL unit type and the second subpicture includes one or more slices having a second NAL unit type,

wherein, based on subpicture bitstreams for the first subpicture and the second subpicture having the same coding structure and parameters of the same value, the bitstream has a form in which the subpicture bitstreams are merged,

wherein the current picture includes the first subpicture and the second subpicture based on the first NAL unit type and the second NAL unit type being different from each other,

wherein all slices included in the first subpicture have the same NAL unit type and all slices included in the second subpicture have the same NAL unit type,

wherein the current picture is constrained not to include a NAL unit type equal to GDR_NUT based on the first NAL unit type and the second NAL unit type being different from each other, and

wherein a coded layer video sequence start (CLVSS) picture unit includes an intra random access point (IRAP) picture unit or a gradual decoding refresh (GDR) picture unit, which precedes a recovery point picture in a decoding order and follows the recovery point picture in an output order.

2 . The image decoding method of claim 1 , wherein the parameters include a plurality of decoded picture buffer (DPB) parameters, and wherein the DPB parameters have the same value for the bitstream and the subpicture bitstreams.

3 . The image decoding method of claim 1 ,

wherein the second NAL unit type is limited to be TRAIL_NUT, based on the first NAL unit type being equal to either CRA_NUT, IDR_N_LP or IDR_W_RADL.

4 . The image decoding method of claim 1 ,

wherein, the second NAL unit type is limited to be either TRAIL_NUT or STSA_NUT, based on the first NAL unit type being equal to either CRA_NUT, IDR_N_LP or IDR_W_RADL.

5 . The image decoding method of claim 1 ,

wherein an end-of-sequence (EOS) NAL unit preceding the current picture in a decoding order is limited not to presence in the bitstream, based on the first NAL unit type and the second NAL unit type being different from each other.

6 . The image decoding method of claim 1 ,

wherein a second picture unit belonging to a first layer after a first picture unit which belongs to the first layer and includes an end-of-sequence (EOS) NAL unit is limited to have a single NAL unit type.

7 . The image decoding method of claim 1 ,

wherein a picture unit following an end-of-sequence (EOS) NAL unit in a decoding order is treated as the CLVSS picture unit only when the picture unit has a single NAL unit type.

8 . The image decoding method of claim 1 ,

wherein whether the first NAL unit type and the second NAL unit type are different from each other is determined based on a mixed NAL unit type flag obtained from a picture parameter set (PPS).

9 . An image encoding method performed by an image encoding apparatus, the image encoding method comprising:

partitioning a current picture into two or more subpictures;

determining a network abstraction layer (NAL) unit type of each of the subpictures; and

encoding the subpictures based on the determined NAL unit type,

wherein the current picture includes a first subpicture and a second subpicture, the first subpicture includes one or more slices having a first NAL unit type and the second subpicture includes one or more slices having a second NAL unit type,

wherein, based on subpicture bitstreams for the first subpicture and the second subpicture having the same coding structure and parameters of the same value, the subpicture bitstreams are merged into a single bitstream,

wherein the current picture includes the first subpicture and the second subpicture based on the first NAL unit type and the second NAL unit type being different from each other,

wherein all slices included in the first subpicture have the same NAL unit type and all slices included in the second subpicture have the same NAL unit type,

wherein the current picture is constrained not to include a NAL unit type equal to GDR_NUT based on the first NAL unit type and the second NAL unit type being different from each other, and

wherein a coded layer video sequence start (CLVSS) picture unit includes an intra random access point (IRAP) picture unit or a gradual decoding refresh (GDR) picture unit, which precedes a recovery point picture in a decoding order and follows the recovery point picture in an output order.

10 . The image encoding method of claim 9 ,

wherein, based on that at least one of the plurality of subpictures has the same NAL unit type as CRA_NUT, IDR_N_LP or IDR_W_RADL, the remaining of the plurality of subpictures are limited to have the same NAL unit type as TRAIL_NUT.

11 . A non-transitory computer-readable recording medium storing a bitstream generated by the image encoding method of claim 9 .

12 . A method of transmitting a bitstream generated by an image encoding method, the image encoding method comprising:

partitioning a current picture into two or more subpictures;

determining a network abstraction layer (NAL) unit type of each of the subpictures; and

encoding the subpictures based on the determined NAL unit type,

wherein the current picture includes a first subpicture and a second subpicture, the first subpicture includes one or more slices having a first NAL unit type and the second subpicture includes one or more slices having a second NAL unit type,

wherein, based on subpicture bitstreams for the first subpicture and the second subpicture having the same coding structure and parameters of the same value, the subpicture bitstreams are merged into a single bitstream,

wherein the current picture includes the first subpicture and the second subpicture based on the first NAL unit type and the second NAL unit type being different from each other,

wherein all slices included in the first subpicture have the same NAL unit type and all slices included in the second subpicture have the same NAL unit type,

wherein the current picture is constrained not to include a NAL unit type equal to GDR_NUT based on the first NAL unit type and the second NAL unit type being different from each other, and

wherein a coded layer video sequence start (CLVSS) picture unit includes an intra random access point (IRAP) picture unit or a gradual decoding refresh (GDR) picture unit, which precedes a recovery point picture in a decoding order and follows the recovery point picture in an output order.