Encoder and decoder

A decoder which decodes a picture from a bitstream, the picture including subpictures, the picture including slices, each of the slices being included in one of the subpictures, the decoder comprising:

• circuitry;
• and
• memory coupled to the circuitry,
• wherein, in operation, the circuitry:
• for each of the slices included in the picture, decodes a subpicture level slice index of the slice from a slice header of the slice;
• and
• decodes each of the slices from a bitstream,
• a picture level slice index and a subpicture level slice index are assigned to each of the slices, the picture level slice index being assigned at a picture level, the subpicture level slice index being assigned at a subpicture level, and
• the picture level slice index assigned to a current slice to be processed that is included in a current subpicture to be processed is calculated by adding (i) the subpicture level slice index of the current slice and (ii) a total number of slices included in one or more subpictures that have been encoded prior to the current subpicture out of the subpictures.