Sub-picture motion vectors in video coding

A method implemented in an encoder, the method comprising:

• partitioning, by a one or more processors of the encoder, a current picture into a sub-picture, and the sub-picture into a current block;
• obtaining, by the one or more processors, candidate motion vectors for the current block of the sub-picture;
• deriving, by the one or more processors, a candidate list of candidate motion vectors for the current block according to temporal luma motion vector prediction, wherein the temporal luma motion vector prediction is performed according to:
• when yCb>>CtbLog2SizeY is equal to yColBr>>CtbLog2SizeY, yColBr is less than or equal to botBoundaryPos and xColBr is less than or equal to rightBoundaryPos, the following applies:
• deriving variables mvLXCol and availableFlagLXCol;
• otherwise, excluding collocated motion vectors of a collocated block of the current block from the candidate list by setting both components of the mvLXCol equal to zero and setting the availableFlagLXCol equal to zero;
• wherein
• xColBr = xCb + cb Width;
• yColBr = yCb + cbHeight;
• rightBoundaryPos = subpic_treated_as_pic_flag[ SubPicIdx ] ?
• SubPicRightBoundaryPos :
• pic_width_in_luma_samples − 1;
• and
• botBoundaryPos = subpic_treated_as_pic_flag[ SubPicIdx ] ?
• SubPicBotBoundaryPos :
• pic_height_in_luma_samples − 1,
• where xColBr and yColBR specify a location of the collocated block, xCb and yCb specify a top left sample of the current block relative to a top left sample of the current picture, cbWidth is a width of the current block, cbHeight is a height of the current block, SubPicRightBoundaryPos is a position of a right boundary of the sub-picture, SubPicBotBoundaryPos is a position of a bottom boundary of the sub-picture, pic_width_in_luma_samples is a width of the current picture measured in luma samples, pic_height_in_luma_samples is a height of the current picture measured in luma samples, botBoundaryPos is a computed position of the bottom boundary of the sub-picture, rightBoundaryPos is a computed position of the right boundary of the sub-picture, SubPicIdx is an index of the sub-picture, subpic_treated_as_pic_flag[SubPicIdx] is a flag indicating whether the sub-picture is treated as a picture during decoding excluding in-loop filtering operations, and CtbLog2SizeY indicates a size of a coding tree block;
• selecting, by the one or more processors, a current motion vector for the current block from the candidate list of candidate motion vectors;
• encoding, by the one or more processors, the current block into a bitstream based on the current motion vector.

A decoder comprising:

• a receiver configured to receive a bitstream comprising a coded sub-picture of a current picture;
• and
• one or more processors configured to:
• obtain candidate motion vectors for a current block of the coded sub-picture;
• derive a candidate list of candidate motion vectors for the current block according to temporal luma motion vector prediction, wherein the temporal luma motion vector prediction is performed according to:
• when yCb>>CtbLog2SizeY is equal to yColBr>>CtbLog2SizeY, yColBr is less than or equal to botBoundaryPos and xColBr is less than or equal to rightBoundaryPos, the following applies:
• deriving variables mvLXCol and availableFlagLXCol;
• otherwise, excluding collocated motion vectors of a collocated block of the current block from the candidate list by setting both components of the mvLXCol equal to zero and setting the availableFlagLXCol equal to zero;
• wherein
• xColBr = xCb + cbWidth;
• yColBr = yCb + cbHeight;
• rightBoundaryPos = subpic_treated_as_pic_flag[ SubPicIdx ] ?
• SubPicRightBoundaryPos :
• pic_width_in_luma_samples − 1;
• and
• botBoundaryPos = subpic_treated_as_pic_flag[ SubPicIdx ] ?
• SubPicBotBoundaryPos :
• pic_height_in_luma_samples − 1,
• where xColBr and yColBr specify a location of the collocated block, xCb and yCb specify a top left sample of the current block relative to a top left sample of the current picture, cbWidth is a width of the current block, cbHeight is a height of the current block, SubPicRightBoundaryPos is a position of a right boundary of the coded sub-picture, SubPicBotBoundaryPos is a position of a bottom boundary of the coded sub-picture, pic_width_in_luma_samples is a width of the current picture measured in luma samples, pic_height_in_luma_samples is a height of the current picture measured in luma samples, botBoundaryPos is a computed position of the bottom boundary of the coded sub-picture, rightBoundaryPos is a computed position of the right boundary of the coded sub-picture, SubPicIdx is an index of the coded sub-picture, subpic_treated_as_pic_flag[SubPicIdx] is a flag indicating whether the coded sub-picture is treated as a picture during decoding excluding in-loop filtering operations, and CtbLog2SizeY indicates a size of a coding tree block;
• determine a current motion vector for the current block from the candidate list of candidate motion vectors;
• and
• decode the current block based on the current motion vector.

An encoder comprising:

• one or more processors configured to:
• partition a current picture into a sub-picture, and the sub-picture into a current block;
• obtain candidate motion vectors for the current block of the sub-picture;
• derive a candidate list of candidate motion vectors for the current block according to temporal luma motion vector prediction, wherein the temporal luma motion vector prediction is performed according to:
• when yCb>>CtbLog2SizeY is equal to yColBr>>CtbLog2SizeY, yColBr is less than or equal to botBoundaryPos and xColBr is less than or equal to rightBoundaryPos, the following applies:
• deriving variables mvLXCol and availableFlagLXCol;
• otherwise, excluding collocated motion vectors of a collocated block of the current block from the candidate list by setting both components of the mvLXCol equal to zero and setting the availableFlagLXCol equal to zero;
• wherein
• xColBr = xCb + cb Width;
• yColBr = yCb + cbHeight;
• rightBoundaryPos = subpic_treated_as_pic_flag[ SubPicIdx ] ?
• SubPicRightBoundaryPos :
• pic_width_in_luma_
• samples − 1;
• and
• botBoundaryPos = subpic_treated_as_pic_flag[ SubPicIdx ] ?
• SubPicBotBoundaryPos :
• pic_height_in_luma_samples − 1,
• where xColBr and yColBR specify a location of the collocated block, xCb and yCb specify a top left sample of the current block relative to a top left sample of the current picture, cbWidth is a width of the current block, cbHeight is a height of the current block, SubPicRightBoundaryPos is a position of a right boundary of the sub-picture, SubPicBotBoundaryPos is a position of a bottom boundary of the sub-picture, pic_width_in_luma_samples is a width of the current picture measured in luma samples, pic_height_in_luma_samples is a height of the current picture measured in luma samples, botBoundaryPos is a computed position of the bottom boundary of the sub-picture, rightBoundaryPos is a computed position of the right boundary of the sub-picture, SubPicIdx is an index of the sub-picture, subpic_treated_as_pic_flag[SubPicIdx] is a flag indicating whether the sub-picture is treated as a picture during decoding excluding in-loop filtering operations, and CtbLog2SizeY indicates a size of a coding tree block;
• select a current motion vector for the current block from the candidate list of candidate motion vectors;
• and
• encode the current block into a bitstream based on the current motion vector.