Video coding in relation to subpictures

An encoder comprising:

• at least one processor configured to perform operations including:
• dividing a particular picture into a plurality of subpictures;
• encoding, via context-adaptive binary arithmetic coding (CABAC), the particular picture into a bitstream by inserting one or more zero words into the bitstream such that a bin-to-bit constraint is fulfilled corresponding to a subpicture of the plurality of subpictures;
• and
• providing an indication to treat the subpicture as a picture and to apply, to the subpicture, boundary extension for motion compensation.

A method of data decoding comprising:

• receiving a bitstream that includes a plurality of pictures, wherein a particular picture of the plurality of pictures is divided into a plurality of subpictures;
• interpreting, based on the bitstream, an indication to treat the subpicture as a picture and to apply boundary extension for motion compensation to the subpicture;
• and
• decoding, via context-adaptive binary arithmetic coding (CABAC), the particular picture from the bitstream, which includes parsing one or more zero words from the bitstream such that a bin-to-bit constraint is fulfilled corresponding to the subpicture.

A decoder comprising:

• at least one processor configured to perform operations including:
• receiving a bitstream that includes a plurality of pictures, wherein a particular picture of the plurality of pictures is divided into a plurality of subpictures;
• interpreting, based on the bitstream, an indication to treat the subpicture as a picture and to apply boundary extension for motion compensation to the subpicture;
• and
• decoding, via context-adaptive binary arithmetic coding (CABAC), the particular picture from the bitstream, which includes parsing one or more zero words from the bitstream such that a bin-to-bit constraint is fulfilled corresponding to the subpicture.