Encoder, decoder, encoding method, and decoding method

A decoding method comprising:

• determining whether or not current images are progressive;
• in response to determining that the current images are progressive, decoding one syntax element indicating a chroma location type from a bitstream, the chroma location type indicating locations of chroma samples relative to luma samples for a frame included in the current images;
• and
• in response to determining that the current images are not progressive, decoding two syntax elements from the bitstream, each of the two syntax elements indicating the chroma location type for a different one of fields of two types included in the current images.

A non-transitory computer readable medium storing a bitstream and computer-executable instructions,

• wherein the bitstream includes (i) one syntax element which indicates a chroma location type indicating locations of chroma samples relative to luma samples for a frame included in current images or (ii) two syntax elements each of which indicates the chroma location type for a different one of fields of two types included in the current images,
• wherein the bitstream includes a parameter according to which the computer-executable instructions cause a decoder to perform:
• determining whether or not the current images are progressive;
• in response to determining that the current images are progressive, decoding the one syntax element from the bitstream;
• and
• in response to determining that the current images are not progressive, decoding the two syntax elements from the bitstream.