Constraints on reference picture lists

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:

• perform a conversion between a video comprising one or more pictures comprising one or more subpictures and a bitstream of the video,
• wherein the bitstream conforms to a format rule that specifies that a subpicture cannot be a random access decodable leading subpicture or a random access skipped leading subpicture when the subpicture is not a leading subpicture of an intra random access point subpicture, and
• wherein the leading subpicture precedes the intra random access point subpicture in output order,
• wherein the intra random access point subpicture is a subpicture for which all video coding layer (VCL) network abstraction layer (NAL) units have a same value of NAL unit type in a range of IDR_W_RADL to CRA_NUT, inclusive,
• wherein the random access decodable leading subpicture is a subpicture for which each VCL NAL unit has NAL unit type equal to RADL_NUT,
• wherein the random access skipped leading subpicture is a subpicture for which each video coding layer (VCL) network abstraction layer (NAL) has NAL unit type equal to RADL_NUT,
• wherein the format rule specifies an order by which pictures are indicated in the bitstream,
• wherein when a current subpicture is a clean random access subpicture, the format rule disallows an entry in a reference picture list of a current slice from including a first picture that precedes, according to a first order, a second picture that precedes, according to a second order, a current picture,
• wherein the second picture comprises an intra random access point subpicture having a same layer identifier of a network abstraction unit (NAL) unit and a same subpicture index as the current subpicture, and
• wherein the clean random access subpicture is an intra random access point subpicture for which each VCL NAL unit has nal_unit_type equal to CRA_NUT.

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

• perform a conversion between a video comprising one or more pictures comprising one or more subpictures and a bitstream of the video,
• wherein the bitstream conforms to a format rule that specifies that a subpicture cannot be a random access decodable leading subpicture or a random access skipped leading subpicture when the subpicture is not a leading subpicture of an intra random access point subpicture, and
• wherein the leading subpicture precedes the intra random access point subpicture in output order,
• wherein the intra random access point subpicture is a subpicture for which all video coding layer (VCL) network abstraction layer (NAL) units have a same value of NAL unit type in a range of IDR_W_RADL to CRA_NUT, inclusive,
• wherein the random access decodable leading subpicture is a subpicture for which each VCL NAL unit has NAL unit type equal to RADL_NUT,
• wherein the random access skipped leading subpicture is a subpicture for which each video coding layer (VCL) network abstraction layer (NAL) has NAL unit type equal to RADL_NUT,
• wherein the format rule specifies an order by which pictures are indicated in the bitstream,
• wherein when a current subpicture is a clean random access subpicture, the format rule disallows an entry in a reference picture list of a current slice from including a first picture that precedes, according to a first order, a second picture that precedes, according to a second order, a current picture,
• wherein the second picture comprises an intra random access point subpicture having a same layer identifier of a network abstraction unit (NAL) unit and a same subpicture index as the current subpicture, and
• wherein the clean random access subpicture is an intra random access point subpicture for which each VCL NAL unit has nal_unit_type equal to CRA_NUT.

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:

• generating the bitstream of the video comprising one or more pictures comprising one or more subpictures,
• wherein the bitstream conforms to a format rule that specifies that a subpicture cannot be a random access decodable leading subpicture or a random access skipped leading subpicture when the subpicture is not a leading subpicture of an intra random access point subpicture, and
• wherein the leading subpicture precedes the intra random access point subpicture in output order,
• wherein the intra random access point subpicture is a subpicture for which all video coding layer (VCL) network abstraction layer (NAL) units have a same value of NAL unit type in a range of IDR_W_RADL to CRA_NUT, inclusive,
• wherein the random access decodable leading subpicture is a subpicture for which each VCL NAL unit has NAL unit type equal to RADL_NUT,
• wherein the random access skipped leading subpicture is a subpicture for which each video coding layer (VCL) network abstraction layer (NAL) has NAL unit type equal to RADL_NUT,
• wherein the format rule specifies an order by which pictures are indicated in the bitstream,
• wherein when a current subpicture is a clean random access subpicture, the format rule disallows an entry in a reference picture list of a current slice from including a first picture that precedes, according to a first order, a second picture that precedes, according to a second order, a current picture,
• wherein the second picture comprises an intra random access point subpicture having a same layer identifier of a network abstraction unit (NAL) unit and a same subpicture index as the current subpicture, and
• wherein the clean random access subpicture is an intra random access point subpicture for which each VCL NAL unit has nal_unit_type equal to CRA_NUT.