Coded representation of history-based motion vector prediction tables

An apparatus for processing video data comprising a processor and a non-transitory memory with instructions thereon, wherein the instructions upon execution by the processor, cause the processor to:

• determine, for a conversion between a current video block of a video and a bitstream of the video, a first prediction mode is applied to the current video block,
• maintain, for the first prediction mode, a first history-based motion vector predictor (HMVP) table, and
• perform the conversion based on the first prediction mode,
• wherein in the first prediction mode, a vector predictor of the current video block is determined based on the first HMVP table, and prediction samples of the current video block is obtained based on the vector predictor,
• wherein the first HMVP table comprises one or more vector predictor candidates derived from previously processed video blocks,
• wherein a number of vector predictor candidates in the first HMVP table is included in the bitstream, and
• wherein when the number of vector predictor candidates in the first HMVP table is smaller than N, the first prediction mode is disallowed for the current video block, wherein N is a predefined integer number.

A non-transitory computer-readable storage medium storing instructions that cause a processor to:

• determine, for a conversion between a current video block of a video and a bitstream of the video, a first prediction mode is applied to the current video block,
• maintain, for the first prediction mode, a first history-based motion vector predictor (HMVP) table, and
• perform the conversion based on the first prediction mode,
• wherein in the first prediction mode, a vector predictor of the current video block is determined based on the first HMVP table, and prediction samples of the current video block is obtained based on the vector predictor,
• wherein the first HMVP table comprises one or more vector predictor candidates derived from previously processed video blocks,
• wherein a number of vector predictor candidates in the first HMVP table is included in the bitstream, and
• wherein when the number of vector predictor candidates in the first HMVP table is smaller than N, the first prediction mode is disallowed for the current video block, wherein N is a predefined integer number.

A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by a video processing apparatus, wherein the method comprises:

• determining a first prediction mode is applied to a current video block of the video,
• maintaining, for the first prediction mode, a first history-based motion vector predictor (HMVP) table, and
• generating the bitstream based on the first prediction mode,
• wherein in the first prediction mode, a vector predictor of the current video block is determined based on the first HMVP table, and prediction samples of the current video block is obtained based on the vector predictor,
• wherein the first HMVP table comprises one or more vector predictor candidates derived from previously processed video blocks,
• wherein a number of vector predictor candidates in the first HMVP table is included in the bitstream, and
• wherein when the number of vector predictor candidates in the first HMVP table is smaller than N, the first prediction mode is disallowed for the current video block, wherein N is a predefined integer number.