Syntax design method and apparatus for performing coding by using syntax

An encoding apparatus for an image encoding, the encoding apparatus comprising:

• a memory;
• and
• at least one processor connected to the memory, the at least one processor configured to:
• derive affine enabled flag information and sub-block temporal motion vector prediction enabled flag information;
• derive a prediction sample for a current block by applying inter prediction to the current block;
• derive specific flag information related to whether a sub-block based specific merge mode is applied to the current block;
• determine whether to signal a specific merge index for the sub-block based specific merge mode based on the specific flag information, the affine enabled flag information and the sub-block temporal motion vector prediction enabled flag information;
• and
• encode image information including at least one of the affine enabled flag information, the sub-block temporal motion vector prediction enabled flag information, the specific flag information, or the specific merge index,
• wherein the specific flag information is derived based on:
• determining whether to signal the specific flag information based on at least one of the affine enabled flag information or the sub-block temporal motion vector prediction enabled flag information, and
• deriving the specific flag information based on the result of the determining whether to signal the specific flag information,
• wherein based on a case that a value of the specific flag information is equal to 1, a value of the affine enabled flag information is equal to 0 and a value of the sub-block temporal motion vector prediction enabled flag information is equal to 1, it is determined that the specific merge index is not signaled, and
• wherein the sub-block temporal motion vector prediction enabled flag information is configured in a sequence parameter set.

An apparatus for transmitting data for an image, the apparatus comprising:

• at least one processor configured to obtain a bitstream for the image, wherein the bitstream is generated based on deriving affine enabled flag information and sub-block temporal motion vector prediction enabled flag information, deriving a prediction sample for a current block by applying inter prediction to the current block, deriving specific flag information related to whether a sub-block based specific merge mode is applied to the current block, determining whether to signal a specific merge index for the sub-block based specific merge mode based on the specific flag information, the affine enabled flag information and the sub-block temporal motion vector prediction enabled flag information, and generating the bitstream by encoding image information including at least one of the affine enabled flag information, the sub-block temporal motion vector prediction enabled flag information, the specific flag information, or the specific merge index;
• and
• a transmitter configured to transmit the data comprising the bitstream,
• wherein the deriving the specific flag information comprises:
• determining whether to signal the specific flag information based on at least one of the affine enabled flag information or the sub-block temporal motion vector prediction enabled flag information, and
• deriving the specific flag information based on the result of the determining whether to signal the specific flag information,
• wherein based on a case that a value of the specific flag information is equal to 1, a value of the affine enabled flag information is equal to 0 and a value of the sub-block temporal motion vector prediction enabled flag information is equal to 1, it is determined that the specific merge index is not signaled, and
• wherein the sub-block temporal motion vector prediction enabled flag information is configured in a sequence parameter set.