Patent Publication Number: US-11659207-B2

Title: General constraints of syntax elements for video coding

Description:
This application claims the benefit of U.S. Provisional Patent Application 63/036,320, filed Jun. 8, 2020, the entire content of which is incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to video encoding and video decoding. 
     BACKGROUND 
     Digital video capabilities can be incorporated into a wide range of devices, including digital televisions, digital direct broadcast systems, wireless broadcast systems, personal digital assistants (PDAs), laptop or desktop computers, tablet computers, e-book readers, digital cameras, digital recording devices, digital media players, video gaming devices, video game consoles, cellular or satellite radio telephones, so-called “smart phones,” video teleconferencing devices, video streaming devices, and the like. Digital video devices implement video coding techniques, such as those described in the standards defined by MPEG-2, MPEG-4, ITU-T H.263, ITU-T H.264/MPEG-4, Part 10, Advanced Video Coding (AVC), ITU-T H.265/High Efficiency Video Coding (HEVC), and extensions of such standards. The video devices may transmit, receive, encode, decode, and/or store digital video information more efficiently by implementing such video coding techniques. 
     Video coding techniques include spatial (intra-picture) prediction and/or temporal (inter-picture) prediction to reduce or remove redundancy inherent in video sequences. For block-based video coding, a video slice (e.g., a video picture or a portion of a video picture) may be partitioned into video blocks, which may also be referred to as coding tree units (CTUs), coding units (CUs) and/or coding nodes. Video blocks in an intra-coded (I) slice of a picture are encoded using spatial prediction with respect to reference samples in neighboring blocks in the same picture. Video blocks in an inter-coded (P or B) slice of a picture may use spatial prediction with respect to reference samples in neighboring blocks in the same picture or temporal prediction with respect to reference samples in other reference pictures. Pictures may be referred to as frames, and reference pictures may be referred to as reference frames. 
     SUMMARY 
     In general, this disclosure describes techniques for signalling syntax elements to constrain other syntax elements. In accordance with one or more techniques of this disclosure, a video encoder may encode, and a video decoder may decode, general constraint informatuon (GCI) flags and/or modify the semantics of existing GCI flags to further constrain various other syntax elements. Some example syntax elements that may be constrained include, but are not limited to, a flag that specifies whether subpicture information is present in a bitstream and whether more than one subpicture may be present in the bitstream (e.g., sps_subpic_info_present_flag), a flag that specifies whether reference picture list (RPL) syntax elements are present in slice headers of instantaneous decoding refresh (IDR) pictures (e.g., sps_idr_rpl_present_flag), a flag that specifies whether a video parameter set (VPS) includes hypothetical reference decoder parameters (e.g., vps_general_hrd_params_present_flag), a flag that specifies whether a sequence parameter set (SPS) includes hypothetical reference decoder parameters (e.g., sps_general_hrd_params_present_flag), and/or a flag that specifies whether a bitstream conveys pictures that represent fields (e.g., sps_field_seq_flag). 
     By encoding the aforementioned flags (e.g., a flag that specifies a constraint for a second syntax element that specifies whether subpicture information is present in the coded video bitstream and whether more than one subpicture is allowed to be present in the coded video bitstream), a video encoder may enable a video decoder to discard bitstreams with specific coding features not supported without the video decoder having to parse sequence parameter set (SPS) or picture parameter set (PPS). Additionally, by encoding the aforementioned flags, the video encoder may enable the receiver to identify the appropriate bitstream among multiple candidates (e.g., adaptive streaming). For situations in which the video encoder is aware of the receivers&#39; capability, the video encoder may selectively disable coding tools by setting the GCI at high level. The video encoder may also turn of specific coding tools when it is aware of the content, as some coding tools may only be useful for particular video content. In this way, the techniques of this disclosure may reduce power consumption, compression complexity, and decoding complexity. 
     As one example, a method of decoding video data includes decoding, from a coded video bitstream, a first syntax element that specifies a constraint for a second syntax element that specifies whether subpicture information is present in the coded video bitstream and whether more than one subpicture is allowed to be present in the coded video bitstream; and decoding, based on the constraint specified by the first syntax element, the second syntax element. 
     As another example, a method of encoding video data includes encoding, in a coded video bitstream, a first syntax element that specifies a constraint for a second syntax element that specifies whether subpicture information is present in the coded video bitstream and whether more than one subpicture is allowed to be present in the coded video bitstream; and encoding, based on the constraint specified by the first syntax element, the second syntax element 
     As another example, a device for decoding video data includes a memory configured to store at least a portion of a coded video bitstream; and one or more processors implemented in circuitry and configured to: decode, from the coded video bitstream, a first syntax element that specifies a constraint for a second syntax element that specifies whether subpicture information is present in the coded video bitstream and whether more than one subpicture is allowed to be present in the coded video bitstream; and decode, based on the constraint specified by the first syntax element, the second syntax element. 
     As another example, a computer-readable storage medium stores instructions that, when executed, cause one or more processors to: decode, from a coded video bitstream, a first syntax element that specifies a constraint for a second syntax element that specifies whether subpicture information is present in the coded video bitstream and whether more than one subpicture is allowed to be present in the coded video bitstream; and decode, based on the constraint specified by the first syntax element, the second syntax element. 
     The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description, drawings, and claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a block diagram illustrating an example video encoding and decoding system that may perform the techniques of this disclosure. 
         FIGS.  2 A and  2 B  are conceptual diagrams illustrating an example quadtree binary tree (QTBT) structure, and a corresponding coding tree unit (CTU). 
         FIG.  3    is a block diagram illustrating an example video encoder that may perform the techniques of this disclosure. 
         FIG.  4    is a block diagram illustrating an example video decoder that may perform the techniques of this disclosure. 
         FIG.  5    is a flowchart illustrating an example method for encoding a current block, in accordance with one or more techniques of this disclosure. 
         FIG.  6    is a flowchart illustrating an example method for decoding a current block, in accordance with one or more techniques of this disclosure. 
         FIG.  7    is a flowchart illustrating an example method for signaling syntax elements that constrain other syntax elements used to represent video data, in accordance with one or more techniques of this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Versatile video coding (VVC) has been developed by a Joint Video Experts Team (JVET) of ITU-T and ISO/IEC to achieve substantial compression capability beyond HEVC for a broadened range of applications. VVC is expected to be finalized in July 2020, resulting in an aligned text that will be published by both ITU-T and ISO/IEC. 
     A current draft of the VVC standard (Ye-Kui Wang. “AHG2: Editorial input of a text integration for the May 2020 HLS AHG meeting outcome,” Joint Video Experts Team (JVET) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 19 th  Meeting: by teleconference, 22 Jun.-1 Jul. 2020, JVET-S0152v5 (hereinafter “VVC Draft 10”)) specifies normative bitstream and picture formats, high level syntax (HLS) and semantics, and the parsing and decoding process. VVC also specifies profiles/tiers/levels (PTL) restrictions, byte stream format, hypothetical reference decoder and supplemental enhancement information (SEI) in the annex. 
     VVC inherits a number of high-level features from HEVC, such as network abstraction layer (NAL) unit and parameter set concepts, tile and wave-front parallel processing, layered coding and the use of SEI messages for supplemental data signalling. More new high-level features are introduced in VVC, including rectangular slice and subpicture concept, picture resolution adaptation, mixed NAL unit types, picture header (PH), gradual decoding refresh (GDR) picture, virtual boundary, and the reference picture list (RPL) for reference picture management. 
     Numerous signaling conditions or semantic constraints are specified in VVC on the parameter set syntax elements, and the variables derived from the syntax elements are used in the decoding process. For instance, VVC specifies the general constraint information (GCI) to constrain specific coding tools, NAL unit types and functionalities. In particular, VVC Draft 10 includes the following GCI syntax elements and semantics: 
     7.3.3.2 General Constraint Information Syntax 
                                 general_constraint_info( ) {   Descriptor                   general_non_packed_constraint_flag   u(1)        general_frame_only_constraint_flag   u(1)        general_non_projected_constraint_flag   u(1)        general_one_picture_only_constraint_flag   u(1)        intra_only_constraint_flag   u(1)        max_bitdepth_minus8_constraint_idc   u(4)        max_chroma_format_constraint_idc   u(2)        single_layer_constraint_flag   u(1)        all_layers_independent_constraint_flag   u(1)        no_ref_pic_resampling_constraint_flag   u(1)        no_res_change_in_clvs_constraint_flag   u(1)        one_tile_per_pic_constraint_flag   u(1)        pic_header_in_slice_header_constraint_flag   u(1)        one_slice_per_pic_constraint_flag   u(1)        one_subpic_per_pic_constraint_flag   u(1)        no_qtbtt_dual_tree_intra_constraint_flag   u(1)        no_partition_constraints_override_constraint_flag   u(1)        no_sao_constraint_flag   u(1)        no_alf_constraint_flag   u(1)        no_ccalf_constraint_flag   u(1)        no_joint_cbcr_constraint_flag   u(1)        no_mrl_constraint_flag   u(1)        no_isp_constraint_flag   u(1)        no_mip_constraint_flag   u(1)        no_ref_wraparound_constraint_flag   u(1)        no_temporal_mvp_constraint_flag   u(1)        no_sbtmvp_constraint_flag   u(1)        no_amvr_constraint_flag   u(1)        no_bdof_constraint_flag   u(1)        no_dmvr_constraint_flag   u(1)        no_cclm_constraint_flag   u(1)        no_mts_constraint_flag   u(1)        no_sbt_constraint_flag   u(1)        no_lfnst_constraint_flag   u(1)        no_affine_motion_constraint_flag   u(1)        no_mmvd_constraint_flag   u(1)        no_smvd_constraint_flag   u(1)        no_prof_constraint_flag   u(1)        no_bcw_constraint_flag   u(1)        no_ibc_constraint_flag   u(1)        no_ciip_constraint_flag   u(1)        no_gpm_constraint_flag   u(1)        no_ladf_constraint_flag   u(1)        no_transform_skip_constraint_flag   u(1)        no_bdpcm_constraint_flag   u(1)        no_weighted_prediction_constraint_flag   u(1)        no_palette_constraint_flag   u(1)        no_act_constraint_flag   u(1)        no_lmcs_constraint_flag   u(1)        no_cu_qp_delta_constraint_flag   u(1)        no_chroma_qp_offset_constraint_flag   u(1)        no_dep_quant_constraint_flag   u(1)        no_sign_data_hiding_constraint_flag   u(1)        no_mixed_nalu_types_in_pic_constraint_flag   u(1)        no_trail_constraint_flag   u(1)        no_stsa_constraint_flag   u(1)        no_rasl_constraint_flag   u(1)        no_radl_constraint_flag   u(1)        no_idr_constraint_flag   u(1)        no_cra_constraint_flag   u(1)        no_gdr_constraint_flag   u(1)        no_aps_constraint_flag   u(1)        while( !byte_aligned( ) )             gci_alignment_zero_bit   f(1)        gci_num_reserved_bytes   u(8)        for( i = 0; i &lt; gci_num_reserved_bytes; i++ )             gci_reserved_byte[ i ]   u(8)       }                    
7.4.4.2 General Constraint Information Semantics
 
     general_non_packed_constraint_flag equal to 1 specifies that there shall not be any frame packing arrangement SEI messages present in the bitstream of the OlsInScope. general_non_packed_constraint_flag equal to 0 does not impose such a constraint.
         NOTE 1—Decoders may ignore the value of general non packed constraint flag, as there are no decoding process requirements associated with the presence or interpretation of frame packing arrangement SEI messages.       

     general_frame_only_constraint_flag equal to 1 specifies that OlsInScope conveys pictures that represent frames. general_frame_only_constraint_flag equal to 0 specifies that OlsInScope conveys pictures that may or may not represent frames.
         NOTE 2—Decoders may ignore the value of general_frame_only_constraint_flag, as there are no decoding process requirements associated with it.       

     general_non_projected_constraint_flag equal to 1 specifies that there shall not be any equirectangular projection SEI messages or generalized cubemap projection SEI messages present in the bitstream of the OlsInScope. general_non_projected_constraint_flag equal to 0 does not impose such a constraint.
         NOTE 3—Decoders may ignore the value of general_non_projected_constraint_flag, as there are no decoding process requirements associated with the presence or interpretation of equirectangular projection SEI messages and generalized cubemap projection SEI messages.       

     general_one_picture_only_constraint_flag equal to 1 specifies that there is only one coded picture in the bitstream. general_one_picture_only_constraint_flag equal to 0 does not impose such a constraint. 
     intra_only_constraint_flag equal to 1 specifies that sh_slice_type shall be equal to I. intra_only_constraint_flag equal to 0 does not impose such a constraint. 
     max_bitdepth_minus8_constraint_idc less than 8 specifies that sps_bitdepth_minus8 shall be in the range of 0 to max_bitdepth_minus8_constraint_idc, inclusive. max_bitdepth_minus8_constraint_idc equal to or greater than 8 does not impose a constraint. 
     max_chroma_format_constraint_idc less than 2 specifies that sps_chroma_format_idc shall be in the range of 0 to max_chroma_format_constraint_idc, inclusive. max_chroma_format_constraint_idc equal to 2 does not impose a constraint. 
     single_layer_constraint_flag equal to 1 specifies that the value of nuh_layer_id shall be the same for all VCL NAL units in OlsInScope. single_layer_constraint_flag equal to 0 does not impose such a constraint. 
     all_layers_independent_constraint_flag equal to 1 specifies that vps_all_independent_layers_flag shall be equal to 1. all_layers_independent_constraint_flag equal to 0 does not impose such a constraint. 
     no_ref_pic_resampling_constraint_flag equal to 1 specifies that sps_ref_pic_resampling_enabled_flag shall be equal to 0. no_ref_pic_resampling_constraint_flag equal to 0 does not impose such a constraint. 
     no_res_change_in_clvs_constraint_flag equal to 1 specifies that sps_res_change_in_clvs_allowed_flag shall be equal to 0. no_res_change_in_clvs_constraint_flag equal to 0 does not impose such a constraint. 
     one_tile_per_pic_constraint_flag equal to 1 specifies that each picture shall contain only one tile, i.e., the value of NumTilesInPic for each picture shall be equal to 1. one_tile_per_pic_constraint_flag equal to 0 does not impose such a constraint. 
     pic_header_in_slice_header_constraint_flag equal to 1 specifies that each picture shall contain only one slice and the value of sh_picture_header_in_slice_header_flag in each slice shall be equal to 1. pic_header_in_slice_header_constraint_flag equal to 0 does not impose such a constraint. 
     one_slice_per_pic_constraint_flag equal to 1 specifies that each picture shall contain only one slice, i.e., if pps_rect_slice_flag is equal to 1, the value of num_slices_in_pic_minus1 shall be equal to 0, otherwise, the value of num_tiles_in_slice_minus1 present in each slice header shall be equal to NumTilesInPic−1. one_slice_per_pic constraint flag equal to 0 does not impose such a constraint. 
     one_subpic_per_pic_constraint_flag equal to 1 specifies that each picture shall contain only one subpicture, i.e., the value of sps_num_subpics_minus1 for each picture shall be equal to 0. one_subpic_per_pic_constraint_flag equal to 0 does not impose such a constraint. 
     no_qtbtt_dual_tree_intra_constraint_flag equal to 1 specifies that sps_qtbtt_dual_tree_intra_flag shall be equal to 0. no_qtbtt_dual_tree_intra_constraint_flag equal to 0 does not impose such a constraint. 
     no_partition_constraints_override_constraint_flag equal to 1 specifies that sps_partition_constraints_override_enabled_flag shall be equal to 0. no_partition_constraints_override_constraint_flag equal to 0 does not impose such a constraint. 
     no_sao_constraint_flag equal to 1 specifies that sps_sao_enabled_flag shall be equal to 0. no_sao_constraint_flag equal to 0 does not impose such a constraint. 
     no_alf_constraint_flag equal to 1 specifies that sps_alf_enabled_flag shall be equal to 0. no_alf_constraint_flag equal to 0 does not impose such a constraint. 
     no_ccalf_constraint_flag equal to 1 specifies that sps_ccalf_enabled_flag shall be equal to 0. no_ccalf_equal to 0 does not impose such a constraint. 
     no_joint_cbcr_constraint_flag equal to 1 specifies that sps_joint_cbcr_enabled_flag shall be equal to 0. no_joint_cbcr_constraint_flag equal to 0 does not impose such a constraint. 
     no_mrl_constraint_flag equal to 1 specifies that sps_mrl_enabled_flag shall be equal to 0. no_mrl_constraint_flag equal to 0 does not impose such a constraint. 
     no_isp_constraint_flag equal to 1 specifies that sps_isp_enabled_flag shall be equal to 0. no_isp_constraint_flag equal to 0 does not impose such a constraint. 
     no_mip_constraint_flag equal to 1 specifies that sps_mip_enabled_flag shall be equal to 0. no_mip_constraint_flag equal to 0 does not impose such a constraint. 
     no_ref_wraparound_constraint_flag equal to 1 specifies that sps_ref_wraparound_enabled_flag shall be equal to 0. no_ref_wraparound_constraint_flag equal to 0 does not impose such a constraint. 
     no_temporal_mvp_constraint_flag equal to 1 specifies that sps_temporal_mvp_enabled_flag shall be equal to 0. no_temporal_mvp_constraint_flag equal to 0 does not impose such a constraint. 
     no_sbtmvp_constraint_flag equal to 1 specifies that sps_sbtmvp_enabled_flag shall be equal to 0. no_sbtmvp_constraint_flag equal to 0 does not impose such a constraint. 
     no_amvr_constraint_flag equal to 1 specifies that sps_amvr_enabled_flag shall be equal to 0. no_amvr_constraint_flag equal to 0 does not impose such a constraint. 
     no_bdof_constraint_flag equal to 1 specifies that sps_bdof_enabled_flag shall be equal to 0. no_bdof_constraint_flag equal to 0 does not impose such a constraint. 
     no_dmvr_constraint_flag equal to 1 specifies that sps_dmvr_enabled_flag shall be equal to 0. no_dmvr_constraint_flag equal to 0 does not impose such a constraint. 
     no_cclm_constraint_flag equal to 1 specifies that sps_cclm_enabled_flag shall be equal to 0. no_cclm_constraint_flag equal to 0 does not impose such a constraint. 
     no_mts_constraint_flag equal to 1 specifies that sps_mts_enabled_flag shall be equal to 0. no_mts_constraint_flag equal to 0 does not impose such a constraint. 
     no_sbt_constraint_flag equal to 1 specifies that sps_sbt_enabled_flag shall be equal to 0. no_sbt_constraint_flag equal to 0 does not impose such a constraint. 
     no_lfinst_constraint_flag equal to 1 specifies that sps_lfnst_enabled_flag shall be equal to 0. no_lfnst_constraint_flag equal to 0 does not impose such a constraint. 
     no_affine_motion_constraint_flag equal to 1 specifies that sps_affine_enabled_flag shall be equal to 0. no_affine_motion_constraint_flag equal to 0 does not impose such a constraint. 
     no_mmvd_constraint_flag equal to 1 specifies that sps_mmvd_enabled_flag shall be equal to 0. no_mmvd_constraint_flag equal to 0 does not impose such a constraint. 
     no_smvd_constraint_flag equal to 1 specifies that sps_smvd_enabled_flag shall be equal to 0. no_smvd_constraint_flag equal to 0 does not impose such a constraint. 
     no_prof_constraint_flag equal to 1 specifies that sps_affine_prof_enabled_flag shall be equal to 0. no_prof_constraint_flag equal to 0 does not impose such a constraint. 
     no_bcw_constraint_flag equal to 1 specifies that sps_bcw_enabled_flag shall be equal to 0. no_bcw_constraint_flag equal to 0 does not impose such a constraint. 
     no_ibc_constraint_flag equal to 1 specifies that sps_ibc_enabled_flag shall be equal to 0. no_ibc_constraint_flag equal to 0 does not impose such a constraint. 
     no_ciip_constraint_flag equal to 1 specifies that sps_ciip_enabled_flag shall be equal to 0. no_cipp_constraint_flag equal to 0 does not impose such a constraint. 
     no_gpm_constraint_flag equal to 1 specifies that sps_gpm_enabled_flag shall be equal to 0. no_gpm_constraint_flag equal to 0 does not impose such a constraint. 
     no_ladf_constraint_flag equal to 1 specifies that sps_ladf_enabled_flag shall be equal to 0. no_ladf_constraint_flag equal to 0 does not impose such a constraint. 
     no_transform_skip_constraint_flag equal to 1 specifies that sps_transform_skip_enabled_flag shall be equal to 0. no_transform_skip_constraint_flag equal to 0 does not impose such a constraint. 
     no_bdpcm_constraint_flag equal to 1 specifies that sps_bdpcm_enabled_flag shall be equal to 0. no_bdpcm_constraint_flag equal to 0 does not impose such a constraint. 
     no_weighted_prediction_constraint_flag equal to 1 specifies that sps_weighted_pred_flag and sps_weighted_bipred_flag shall both be equal to 0. no_weighted_prediction_constraint_flag equal to 0 does not impose such a constraint. 
     no_palette_constraint_flag equal to 1 specifies that sps_palette_enabled_flag shall be equal to 0. no_palette_constraint_flag equal to 0 does not impose such a constraint. 
     no_act_constraint_flag equal to 1 specifies that sps_act_enabled_flag shall be equal to 0. no_act_constraint_flag equal to 0 does not impose such a constraint. 
     no_lmcs_constraint_flag equal to 1 specifies that sps_lmcs_enabled_flag shall be equal to 0. no_lmcs_constraint_flag equal to 0 does not impose such a constraint. 
     no_cu_qp_delta_constraint_flag equal to 1 specifies that pps_cu_qp_delta_enabled_flag shall be equal to 0. no_cu_qp_delta_constraint_flag equal to 0 does not impose such a constraint. 
     no_chroma_qp_offset_constraint_flag equal to 1 specifies that pps_cu_chroma_qp_offset_list_enabled_flag shall be equal to 0. no_chroma_qp_offset_constraint_flag equal to 0 does not impose such a constraint. 
     no_dep_quant_constraint_flag equal to 1 specifies that sps_dep_quant_enabled_flag shall be equal to 0. no dep quant constraint flag equal to 0 does not impose such a constraint. 
     no_sign_data_hiding_constraint_flag equal to 1 specifies that sps_sign_data_hiding_enabled_flag shall be equal to 0. no_sign_data_hiding_constraint_flag equal to 0 does not impose such a constraint. 
     no_mixed_nalu_types_in_pic_constraint_flag equal to 1 specifies that it is a requirement of bitstream conformance that pps_mixed_nalu_types_in_pic_flag shall be equal to 0. no_mixed_nalu_types_in_pic_constraint_flag equal to 0 does not impose such a constraint. 
     no_trail_constraint_flag equal to 1 specifies that there shall be no NAL unit with nuh_unit_type equal to TRAIL NUT present in OlsInScope. no trail constraint flag equal to 0 does not impose such a constraint. 
     no_stsa_constraint_flag equal to 1 specifies that there shall be no NAL unit with nuh_unit_type equal to STSA_NUT present in OlsInScope. no_stsa_constraint_flag equal to 0 does not impose such a constraint. 
     no_rasl_constraint_flag equal to 1 specifies that there shall be no NAL unit with nuh_unit_type equal to RASL_NUT present in OlsInScope. no_rasl_constraint_flag equal to 0 does not impose such a constraint. 
     no_radl_constraint_flag equal to 1 specifies that there shall be no NAL unit with nuh_unit_type equal to RADL_NUT present in OlsInScope. no_radl_constraint_flag equal to 0 does not impose such a constraint. 
     no_idr_constraint_flag equal to 1 specifies that there shall be no NAL unit with nuh_unit_type equal to IDR_W_RADL or IDR_N_LP present in OlsInScope. no_idr_constraint_flag equal to 0 does not impose such a constraint. 
     no_cra_constraint_flag equal to 1 specifies that there shall be no NAL unit with nuh_unit_type equal to CRA_NUT present in OlsInScope. no_cra_constraint_flag equal to 0 does not impose such a constraint. 
     no_gdr_constraint_flag equal to 1 specifies that sps_gdr_enabled_flag shall be equal to 0. no_gdr_constraint_flag equal to 0 does not impose such a constraint. 
     no_aps_constraint_flag equal to 1 specifies that there shall be no NAL unit with nuh_unit_type equal to PREFIX_APS_NUT or SUFFIX_APS_NUT present in OlsInScope, and sps_lmcs_enabled_flag and sps_scaling_list_enabled_flag shall both be equal to 0. no aps constraint flag equal to 0 does not impose such a constraint. 
     gci.alignment_zero_bits shall be equal to 0. 
     gci_num_reserved_bytes specifies the number of the reserved constraint bytes. The value of gci_num_reserved_bytes shall be 0. Other values of gci_num_reserved_bytes are reserved for future use by ITU-T ISO/IEC and shall not be present in bitstreams conforming to this version of this Specification. 
     gci_reserved_byte[i] may have any value. Its presence and value do not affect decoder conformance to profiles specified in this version of this Specification. Decoders conforming to this version of this Specification shall ignore the values of all the gci_reserved_byte[i] syntax elements. 
     However, there are a few high level syntax elements that are not constrained yet and the semantics of existing constraint flags may not be coupled with the corresponding (e.g., unconsidered) syntax elements. 
     In accordance with one or more techniques of this disclosure, video encoder  200  may encode, and video decoder  300  may decode, GCI flags and/or modify the semantics of existing GCI flags to further constrain various other syntax elements. Some example syntax elements that may be constrained include, but are not limited to, a flag that specifies whether subpicture information is present in a bitstream and whether more than one subpicture may be present in the bitstream (e.g., sps_subpic_info_present_flag), a flag that specifies whether reference picture list (RPL) syntax elements are present in slice headers of instantaneous decoding refresh (IDR) pictures (e.g., sps_idr_rpl_present_flag), a flag that specifies whether a video parameter set (VPS) includes hypothetical reference decoder parameters (e.g., vps_general_hrd_params_present_flag), a flag that specifies whether a sequence parameter set (SPS) includes hypothetical reference decoder parameters (e.g., sps_general_hrd_params_present_flag), and/or a flag that specifies whether a bitstream conveys pictures that represent fields (e.g., sps_field_seq_flag). Detailed examples for each of the aforementioned proposals follow. The proposals include both new general constraint flags and modifications to existing general constraint flags. 
     In some examples, a video coder may signal (e.g., video encoder  200  may encode and video decoder  300  may decode) a general constraint flag (e.g., no_subpic_info_constraint_flag) to constrain sps_subpic_info_present_flag. When such constraint flag is equal to 1, the value of sps_subpic_info_present_flag shall be equal to 0. When such a constraint flag is equal to 0, no constraint is imposed. When no_subpic_info_constraint_flag is 1, pic_header_in_slice_header_constraint_flag shall be 0. 
     In some examples, a video coder may use a value of a general constraint syntax element (e.g., max_subpics_constraint_idc as described in U.S. Provisional Patent Application No. 63/024,331 filed on May 13, 2020) to constrain sps_subpic_info_present_flag. In accordance with one or more techniques of this disclosure, the semantics of max_subpics_constraint_idc may be modified as follows:
         max_subpics_constraint_idc specifies the number of subpictures within a picture. The value of sps_num_subpics_minus1 shall be in the range of 0 to max_subpics_idc−1, inclusive. When the value of max_subpics_constraint_idc is equal to 0, the value of sps_subpic_info_present_flag shall be equal to 0. When one_subpic_per_pic_constraint_flag is equal to 1, max_subpics_constraint_idc shall be equal to 1.       

     In some examples, a video coder may signal a general constraint flag (e.g., no_idr_rpl_constraint_flag) to constrain sps_idr_rpl_present_flag. When such constraint flag is equal to 1, the value of sps_idr_rpl_present_flag shall be equal to 0. 
     In some examples, a video coder may signal a general constraint flag (e.g., no_hrd_constraint_flag), constrain vps_general_hrd_params_present_flag and/or sps_general_hrd_params_present_flag. When such constraint flag is equal to 1, the value of vps_general_hrd_params_present_flag and sps_general_hrd_params_present_flag shall be equal to 0. vps_general_hrd_params_present_flag and sps_general_hrd_params_present_flag having the value of 0 may imply that no hypothetical reference decoder (HRD) parameters are provided for the decoder. 
     In some examples, a video coder may use modified semantics of a general constraint flag (e.g., general_frame_only_constraint_flag). For instance, the semantics of general_frame_only_constraint_flag may be modified such that general_frame_only_constraint_flag equal to 1 specifies that sps_field_seq_flag shall be equal to 0, and general_frame_only_constraint_flag equal to 0 does not impose such a constraint (e.g., general_frame_only_constraint_flag equal to 0 does not require that sps_field_seq_flag shall be equal to 0). 
     Sanchez et al. “AHG9: On OLS and sublayers,” Joint Video Experts Team (JVET) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 19 th  Meeting: by teleconference, 22 Jun.-1 Jul. 2020, JVET-S0100 (hereinafter “JVET-S0100”) proposes to constrain the temporal ID (tid) parameter among profile_tier_level (PTL), decoded picture buffer (DPB) and HRD parameter structure sharing the same index. However, in VVC Draft 10, PTL is specified for each output layer set (OLS) in VPS, while DPB and HRD are specified for each multiple layer OLS in VPS. Specifically, JVET-S0100 proposed to constrain the vps_dpb_max_temporal_id[i], vps_hrd_max_tid[i] and vps_ptl_max_temporal_id[i] as follows.
         The value of vps_ols_dpb_params_idx[vps_ols_dpb_params_idx[i]] and vps_hrd_max_tid[vps_ols_hrd_idx[i]] shall be greater than or equal to vps_ptl_max_temporal_id[vps_ols_ptl_idx[i]].   vps_ols_ptl_idx[i] is the index, to the list of profile_tier_level( ) syntax structures in the VPS, of the profile_tier_level( ) syntax structure that applies to the i-th OLS.   vps_ols_dpb_params_idx[i] is the index, to the list of dpb_parameters( ) syntax structures in the VPS, of the dpb_parameters( ) syntax structure that applies to the i-th multi-layer OLS.   vps_ols_hrd_idx[i] is the index, to the list of ols_hrd_parameters( ) syntax structures in the VPS, of the ols_hrd_parameters( ) syntax structure that applies to the i-th multi-layer OLS.       

     However, in case single layer OLS is specified in the VPS, constraining the tid among PTL, DPB and HRD with the same index may be problematic. For instance, in case there is single layer OLS in VPS, the size of OLS is different from the size of multi-layer OLS and, as a result, vps_dpb_max_temporal_id[i], vps_hrd_max_tid[i] and vps_ptl_max_temporal_id[i] may associate with different OLS and it may be problematic and undesirable to constrain the temporal id across different OLSs. 
     In accordance with one or more techniques of this disclosure, the constraints described in JVET-S0100 may be modified as follows (with additions shown between &lt;INSERT&gt; . . . &lt;/INSERT&gt; tags and removals shown between &lt;DELETE&gt; . . . &lt;/DELETE&gt; tags): 
     The value of vps_ols_dpb_params_idx[vps_ols_dpb_params_idx[i]] and vps_hrd_max_tid[vps_ols_hrd_idx[i]] shall be greater than or equal to vps_ptl_max_temporal_id[vps_ols_ptl_idx[&lt;DELETE&gt;i&lt;/DELETE&gt;&lt;INSERT&gt;j&lt;/INSERT&gt;]], &lt;INSERT&gt; where vps_ols_ptl_idx[j] specifies the index, to the list of profile_tier_level( ) syntax structures in the VPS, of the profile_tier_level( ) syntax structure that applies to the i-th multi-layer OLS&lt;/INSERT&gt;. 
     In another example, a list DpbHrdIdxToPtlIdx[i] for i ranging from 0 to NumMultiLayerOlss, inclusive, specifying the conversion from an index of multi-layer OLS to the corresponding index of total OLS is derived as follows: 
     
       
         
           
               
             
               
                   
               
             
            
               
                 NumLayersInOls[ 0 ] = 1 
               
               
                 LayerIdInOls[ 0 ][ 0 ] = vps_layer_id[ 0 ] 
               
               
                 NumMultiLayerOlss = 0 
               
               
                 for( i = 1; i &lt; TotalNumOlss; i++ ) { 
               
               
                  if( vps_each_layer_is_an_ols_flag ) { 
               
               
                   NumLayersInOls[ i ] = 1 
               
               
                   LayerIdInOls[ i ][ 0 ] = vps_layer_id[ i ] 
               
               
                  } else if( vps_ols_mode_idc = = 0 ∥ vps_ols_mode_idc = = 1 ) { 
               
               
                   NumLayersInOls[ i ] = i + 1 
               
               
                   for( j = 0; j &lt; NumLayersInOls[ i ]; j++ ) 
               
               
                    LayerIdInOls[ i ][ j ] = vps_layer_id[ j ] 
               
               
                  } else if( vps_ols_mode_idc = = 2 ) { 
               
               
                   for( k = 0, j = 0; k &lt;= vps_max_layers_minus1; k++ ) 
               
               
                    if( layerIncludedInOlsFlag[ i ][ k ] ) 
               
               
                     LayerIdInOls[ i ][ j++ ] = vps_layer_id[ k ] 
               
               
                   NumLayersInOls[ i ] = j 
               
               
                  } 
               
               
                  if( NumLayersInOls[ i ] &gt; 1 ) { 
               
               
                   MultiLayerOlsIdx[ i ] = NumMultiLayerOlss 
               
               
                   &lt;INSERT&gt;DpbHrdIdxToPtlIdx[ NumMultiLayerOlss] = i &lt;/INSERT&gt; 
               
               
                   NumMultiLayerOlss++ 
               
               
                  } 
               
               
                 } 
               
               
                   
               
            
           
         
       
     
     The constraint on the temporal ID may be modified as follows:
         The value of vps_ols_dpb_params_idx[vps_ols_dpb_params_idx[i]] and vps_hrd_max_tid[vps_ols_hrd_idx[i]] shall be greater than or equal to vps_ptl_max_temporal_id[vps_ols_ptl_idx[&lt;INSERT&gt;DpbHrdIdxToPtlIdx[i]]&lt;/INSERT&gt;], where i is in a range from 0 to NumMultiLayerOlss−1, inclusive.       

     A gradual decoding refresh (GDR) picture can be detected by ph_gdr_or_irappic_flag and ph_gdrpic_flag at picture header (PH) without parsing sps_gdr_enabled_flag. Since there is no specific sequence picture set (SPS) flag to control instantaneous decoder refresh (IDR), clean random access (CRA) or any other picture type, it is asserted that it is not necessary to specify sps_gdr_enabled_flag. 
     In accordance with one or more techniques of this disclosure, a video coder may refrain from signaling a sps_gdr_enabled_flag syntax element. In other words, video encoder  200  may not encode, and video decoder  300  may not decode, from a sequence picture set (SPS), a syntax element that specifies whether GDR is enabled. In particular, this disclosure recognizes that sps_gdr_enabled_flag may not be necessary to be signalled to control GDR pictures. The semantics of the no_gdr_constraint_flag may be modified as follows: 
     no_gdr_constraint_flag equal to 1 specifies that &lt;DELETE&gt; sps_gdr_enabled_flag shall be equal to 0&lt;/DELETE&gt;&lt;INSERT&gt; there shall be no NAL unit with nuh_unit_type equal to GDR_NUT present in OlsInScope&lt;/INSERT&gt;. no_gdr_constraint_flag equal to 0 does not impose such a constraint. 
     As such, the no_gdr_constraint_flag syntax element may not control the presence of the sps_gdr_enabled_flag. 
       FIG.  1    is a block diagram illustrating an example video encoding and decoding system  100  that may perform the techniques of this disclosure. The techniques of this disclosure are generally directed to coding (encoding and/or decoding) video data. In general, video data includes any data for processing a video. Thus, video data may include raw, unencoded video, encoded video, decoded (e.g., reconstructed) video, and video metadata, such as signaling data. 
     As shown in  FIG.  1   , system  100  includes a source device  102  that provides encoded video data to be decoded and displayed by a destination device  116 , in this example. In particular, source device  102  provides the video data to destination device  116  via a computer-readable medium  110 . Source device  102  and destination device  116  may comprise any of a wide range of devices, including desktop computers, notebook (i.e., laptop) computers, mobile devices, tablet computers, set-top boxes, telephone handsets such as smartphones, televisions, cameras, display devices, digital media players, video gaming consoles, video streaming device, broadcast receiver devices, or the like. In some cases, source device  102  and destination device  116  may be equipped for wireless communication, and thus may be referred to as wireless communication devices. 
     In the example of  FIG.  1   , source device  102  includes video source  104 , memory  106 , video encoder  200 , and output interface  108 . Destination device  116  includes input interface  122 , video decoder  300 , memory  120 , and display device  118 . In accordance with this disclosure, video encoder  200  of source device  102  and video decoder  300  of destination device  116  may be configured to apply the techniques for constraining high level syntax elements. Thus, source device  102  represents an example of a video encoding device, while destination device  116  represents an example of a video decoding device. In other examples, a source device and a destination device may include other components or arrangements. For example, source device  102  may receive video data from an external video source, such as an external camera. Likewise, destination device  116  may interface with an external display device, rather than include an integrated display device. 
     System  100  as shown in  FIG.  1    is merely one example. In general, any digital video encoding and/or decoding device may perform techniques for constraining high level syntax elements. Source device  102  and destination device  116  are merely examples of such coding devices in which source device  102  generates coded video data for transmission to destination device  116 . This disclosure refers to a “coding” device as a device that performs coding (encoding and/or decoding) of data. Thus, video encoder  200  and video decoder  300  represent examples of coding devices, in particular, a video encoder and a video decoder, respectively. In some examples, source device  102  and destination device  116  may operate in a substantially symmetrical manner such that each of source device  102  and destination device  116  includes video encoding and decoding components. Hence, system  100  may support one-way or two-way video transmission between source device  102  and destination device  116 , e.g., for video streaming, video playback, video broadcasting, or video telephony. 
     In general, video source  104  represents a source of video data (i.e., raw, unencoded video data) and provides a sequential series of pictures (also referred to as “frames”) of the video data to video encoder  200 , which encodes data for the pictures. Video source  104  of source device  102  may include a video capture device, such as a video camera, a video archive containing previously captured raw video, and/or a video feed interface to receive video from a video content provider. As a further alternative, video source  104  may generate computer graphics-based data as the source video, or a combination of live video, archived video, and computer-generated video. In each case, video encoder  200  encodes the captured, pre-captured, or computer-generated video data. Video encoder  200  may rearrange the pictures from the received order (sometimes referred to as “display order”) into a coding order for coding. Video encoder  200  may generate a bitstream including encoded video data. Source device  102  may then output the encoded video data via output interface  108  onto computer-readable medium  110  for reception and/or retrieval by, e.g., input interface  122  of destination device  116 . 
     Memory  106  of source device  102  and memory  120  of destination device  116  represent general purpose memories. In some examples, memories  106 ,  120  may store raw video data, e.g., raw video from video source  104  and raw, decoded video data from video decoder  300 . Additionally or alternatively, memories  106 ,  120  may store software instructions executable by, e.g., video encoder  200  and video decoder  300 , respectively. Although memory  106  and memory  120  are shown separately from video encoder  200  and video decoder  300  in this example, it should be understood that video encoder  200  and video decoder  300  may also include internal memories for functionally similar or equivalent purposes. Furthermore, memories  106 ,  120  may store encoded video data, e.g., output from video encoder  200  and input to video decoder  300 . In some examples, portions of memories  106 ,  120  may be allocated as one or more video buffers, e.g., to store raw, decoded, and/or encoded video data. 
     Computer-readable medium  110  may represent any type of medium or device capable of transporting the encoded video data from source device  102  to destination device  116 . In one example, computer-readable medium  110  represents a communication medium to enable source device  102  to transmit encoded video data directly to destination device  116  in real-time, e.g., via a radio frequency network or computer-based network. Output interface  108  may demodulate a transmission signal including the encoded video data, and input interface  122  may demodulate the received transmission signal, according to a communication standard, such as a wireless communication protocol. The communication medium may comprise any wireless or wired communication medium, such as a radio frequency (RF) spectrum or one or more physical transmission lines. The communication medium may form part of a packet-based network, such as a local area network, a wide-area network, or a global network such as the Internet. The communication medium may include routers, switches, base stations, or any other equipment that may be useful to facilitate communication from source device  102  to destination device  116 . 
     In some examples, source device  102  may output encoded data from output interface  108  to storage device  112 . Similarly, destination device  116  may access encoded data from storage device  112  via input interface  122 . Storage device  112  may include any of a variety of distributed or locally accessed data storage media such as a hard drive, Blu-ray discs, DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or any other suitable digital storage media for storing encoded video data. 
     In some examples, source device  102  may output encoded video data to file server  114  or another intermediate storage device that may store the encoded video data generated by source device  102 . Destination device  116  may access stored video data from file server  114  via streaming or download. File server  114  may be any type of server device capable of storing encoded video data and transmitting that encoded video data to the destination device  116 . File server  114  may represent a web server (e.g., for a website), a File Transfer Protocol (FTP) server, a content delivery network device, or a network attached storage (NAS) device. Destination device  116  may access encoded video data from file server  114  through any standard data connection, including an Internet connection. This may include a wireless channel (e.g., a Wi-Fi connection), a wired connection (e.g., digital subscriber line (DSL), cable modem, etc.), or a combination of both that is suitable for accessing encoded video data stored on file server  114 . File server  114  and input interface  122  may be configured to operate according to a streaming transmission protocol, a download transmission protocol, or a combination thereof. 
     Output interface  108  and input interface  122  may represent wireless transmitters/receivers, modems, wired networking components (e.g., Ethernet cards), wireless communication components that operate according to any of a variety of IEEE 802.11 standards, or other physical components. In examples where output interface  108  and input interface  122  comprise wireless components, output interface  108  and input interface  122  may be configured to transfer data, such as encoded video data, according to a cellular communication standard, such as 4G, 4G-LTE (Long-Term Evolution), LTE Advanced, 5G, or the like. In some examples where output interface  108  comprises a wireless transmitter, output interface  108  and input interface  122  may be configured to transfer data, such as encoded video data, according to other wireless standards, such as an IEEE 802.11 specification, an IEEE 802.15 specification (e.g., ZigBee™), a Bluetooth™ standard, or the like. In some examples, source device  102  and/or destination device  116  may include respective system-on-a-chip (SoC) devices. For example, source device  102  may include an SoC device to perform the functionality attributed to video encoder  200  and/or output interface  108 , and destination device  116  may include an SoC device to perform the functionality attributed to video decoder  300  and/or input interface  122 . 
     The techniques of this disclosure may be applied to video coding in support of any of a variety of multimedia applications, such as over-the-air television broadcasts, cable television transmissions, satellite television transmissions, Internet streaming video transmissions, such as dynamic adaptive streaming over HTTP (DASH), digital video that is encoded onto a data storage medium, decoding of digital video stored on a data storage medium, or other applications. 
     Input interface  122  of destination device  116  receives an encoded video bitstream from computer-readable medium  110  (e.g., a communication medium, storage device  112 , file server  114 , or the like). The encoded video bitstream may include signaling information defined by video encoder  200 , which is also used by video decoder  300 , such as syntax elements having values that describe characteristics and/or processing of video blocks or other coded units (e.g., slices, pictures, groups of pictures, sequences, or the like). Display device  118  displays decoded pictures of the decoded video data to a user. Display device  118  may represent any of a variety of display devices such as a liquid crystal display (LCD), a plasma display, an organic light emitting diode (OLED) display, or another type of display device. 
     Although not shown in  FIG.  1   , in some examples, video encoder  200  and video decoder  300  may each be integrated with an audio encoder and/or audio decoder, and may include appropriate MUX-DEMUX units, or other hardware and/or software, to handle multiplexed streams including both audio and video in a common data stream. If applicable, MUX-DEMUX units may conform to the ITU H.223 multiplexer protocol, or other protocols such as the user datagram protocol (UDP). 
     Video encoder  200  and video decoder  300  each may be implemented as any of a variety of suitable encoder and/or decoder circuitry, such as one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), discrete logic, software, hardware, firmware or any combinations thereof. When the techniques are implemented partially in software, a device may store instructions for the software in a suitable, non-transitory computer-readable medium and execute the instructions in hardware using one or more processors to perform the techniques of this disclosure. Each of video encoder  200  and video decoder  300  may be included in one or more encoders or decoders, either of which may be integrated as part of a combined encoder/decoder (CODEC) in a respective device. A device including video encoder  200  and/or video decoder  300  may comprise an integrated circuit, a microprocessor, and/or a wireless communication device, such as a cellular telephone. 
     Video encoder  200  and video decoder  300  may operate according to a video coding standard, such as ITU-T H.265, also referred to as High Efficiency Video Coding (HEVC) or extensions thereto, such as the multi-view and/or scalable video coding extensions. Alternatively, video encoder  200  and video decoder  300  may operate according to other proprietary or industry standards, such as ITU-T H.266, also referred to as Versatile Video Coding (VVC). A recent draft of the VVC standard is described in Bross, et al. “Versatile Video Coding (Draft 9),” Joint Video Experts Team (JVET) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 18th Meeting: by teleconference, 15-24 Apr. 2020, JVET-R2001-v10 (hereinafter “VVC Draft 9”). The techniques of this disclosure, however, are not limited to any particular coding standard. 
     In general, video encoder  200  and video decoder  300  may perform block-based coding of pictures. The term “block” generally refers to a structure including data to be processed (e.g., encoded, decoded, or otherwise used in the encoding and/or decoding process). For example, a block may include a two-dimensional matrix of samples of luminance and/or chrominance data. In general, video encoder  200  and video decoder  300  may code video data represented in a YUV (e.g., Y, Cb, Cr) format. That is, rather than coding red, green, and blue (RGB) data for samples of a picture, video encoder  200  and video decoder  300  may code luminance and chrominance components, where the chrominance components may include both red hue and blue hue chrominance components. In some examples, video encoder  200  converts received RGB formatted data to a YUV representation prior to encoding, and video decoder  300  converts the YUV representation to the RGB format. Alternatively, pre- and post-processing units (not shown) may perform these conversions. 
     This disclosure may generally refer to coding (e.g., encoding and decoding) of pictures to include the process of encoding or decoding data of the picture. Similarly, this disclosure may refer to coding of blocks of a picture to include the process of encoding or decoding data for the blocks, e.g., prediction and/or residual coding. An encoded video bitstream generally includes a series of values for syntax elements representative of coding decisions (e.g., coding modes) and partitioning of pictures into blocks. Thus, references to coding a picture or a block should generally be understood as coding values for syntax elements forming the picture or block. 
     HEVC defines various blocks, including coding units (CUs), prediction units (PUs), and transform units (TUs). According to HEVC, a video coder (such as video encoder  200 ) partitions a coding tree unit (CTU) into CUs according to a quadtree structure. That is, the video coder partitions CTUs and CUs into four equal, non-overlapping squares, and each node of the quadtree has either zero or four child nodes. Nodes without child nodes may be referred to as “leaf nodes,” and CUs of such leaf nodes may include one or more PUs and/or one or more TUs. The video coder may further partition PUs and TUs. For example, in HEVC, a residual quadtree (RQT) represents partitioning of TUs. In HEVC, PUs represent inter-prediction data, while TUs represent residual data. CUs that are intra-predicted include intra-prediction information, such as an intra-mode indication. 
     As another example, video encoder  200  and video decoder  300  may be configured to operate according to VVC. According to VVC, a video coder (such as video encoder  200 ) partitions a picture into a plurality of coding tree units (CTUs). Video encoder  200  may partition a CTU according to a tree structure, such as a quadtree-binary tree (QTBT) structure or Multi-Type Tree (MTT) structure. The QTBT structure removes the concepts of multiple partition types, such as the separation between CUs, PUs, and TUs of HEVC. A QTBT structure includes two levels: a first level partitioned according to quadtree partitioning, and a second level partitioned according to binary tree partitioning. A root node of the QTBT structure corresponds to a CTU. Leaf nodes of the binary trees correspond to coding units (CUs). 
     In an MTT partitioning structure, blocks may be partitioned using a quadtree (QT) partition, a binary tree (BT) partition, and one or more types of triple tree (TT) (also called ternary tree (TT)) partitions. A triple or ternary tree partition is a partition where a block is split into three sub-blocks. In some examples, a triple or ternary tree partition divides a block into three sub-blocks without dividing the original block through the center. The partitioning types in MTT (e.g., QT, BT, and TT), may be symmetrical or asymmetrical. 
     In some examples, video encoder  200  and video decoder  300  may use a single QTBT or MTT structure to represent each of the luminance and chrominance components, while in other examples, video encoder  200  and video decoder  300  may use two or more QTBT or MTT structures, such as one QTBT/MTT structure for the luminance component and another QTBT/MTT structure for both chrominance components (or two QTBT/MTT structures for respective chrominance components). 
     Video encoder  200  and video decoder  300  may be configured to use quadtree partitioning per HEVC, QTBT partitioning, MTT partitioning, or other partitioning structures. For purposes of explanation, the description of the techniques of this disclosure is presented with respect to QTBT partitioning. However, it should be understood that the techniques of this disclosure may also be applied to video coders configured to use quadtree partitioning, or other types of partitioning as well. 
     In some examples, a CTU includes a coding tree block (CTB) of luma samples, two corresponding CTBs of chroma samples of a picture that has three sample arrays, or a CTB of samples of a monochrome picture or a picture that is coded using three separate color planes and syntax structures used to code the samples. A CTB may be an N×N block of samples for some value of N such that the division of a component into CTBs is a partitioning. A component is an array or single sample from one of the three arrays (luma and two chroma) that compose a picture in 4:2:0, 4:2:2, or 4:4:4 color format or the array or a single sample of the array that compose a picture in monochrome format. In some examples, a coding block is an M×N block of samples for some values of M and N such that a division of a CTB into coding blocks is a partitioning. 
     The blocks (e.g., CTUs or CUs) may be grouped in various ways in a picture. As one example, a brick may refer to a rectangular region of CTU rows within a particular tile in a picture. A tile may be a rectangular region of CTUs within a particular tile column and a particular tile row in a picture. A tile column refers to a rectangular region of CTUs having a height equal to the height of the picture and a width specified by syntax elements (e.g., such as in a picture parameter set). A tile row refers to a rectangular region of CTUs having a height specified by syntax elements (e.g., such as in a picture parameter set) and a width equal to the width of the picture. 
     In some examples, a tile may be partitioned into multiple bricks, each of which may include one or more CTU rows within the tile. A tile that is not partitioned into multiple bricks may also be referred to as a brick. However, a brick that is a true subset of a tile may not be referred to as a tile. 
     The bricks in a picture may also be arranged in a slice. A slice may be an integer number of bricks of a picture that may be exclusively contained in a single network abstraction layer (NAL) unit. In some examples, a slice includes either a number of complete tiles or only a consecutive sequence of complete bricks of one tile. 
     This disclosure may use “N×N” and “N by N” interchangeably to refer to the sample dimensions of a block (such as a CU or other video block) in terms of vertical and horizontal dimensions, e.g., 16×16 samples or 16 by 16 samples. In general, a 16×16 CU will have 16 samples in a vertical direction (y=16) and 16 samples in a horizontal direction (x=16). Likewise, an N×N CU generally has N samples in a vertical direction and N samples in a horizontal direction, where N represents a nonnegative integer value. The samples in a CU may be arranged in rows and columns. Moreover, CUs need not necessarily have the same number of samples in the horizontal direction as in the vertical direction. For example, CUs may comprise N×M samples, where M is not necessarily equal to N. 
     Video encoder  200  encodes video data for CUs representing prediction and/or residual information, and other information. The prediction information indicates how the CU is to be predicted in order to form a prediction block for the CU. The residual information generally represents sample-by-sample differences between samples of the CU prior to encoding and the prediction block. 
     To predict a CU, video encoder  200  may generally form a prediction block for the CU through inter-prediction or intra-prediction. Inter-prediction generally refers to predicting the CU from data of a previously coded picture, whereas intra-prediction generally refers to predicting the CU from previously coded data of the same picture. To perform inter-prediction, video encoder  200  may generate the prediction block using one or more motion vectors. Video encoder  200  may generally perform a motion search to identify a reference block that closely matches the CU, e.g., in terms of differences between the CU and the reference block. Video encoder  200  may calculate a difference metric using a sum of absolute difference (SAD), sum of squared differences (SSD), mean absolute difference (MAD), mean squared differences (MSD), or other such difference calculations to determine whether a reference block closely matches the current CU. In some examples, video encoder  200  may predict the current CU using uni-directional prediction or bi-directional prediction. 
     Some examples of VVC also provide an affine motion compensation mode, which may be considered an inter-prediction mode. In affine motion compensation mode, video encoder  200  may determine two or more motion vectors that represent non-translational motion, such as zoom in or out, rotation, perspective motion, or other irregular motion types. 
     To perform intra-prediction, video encoder  200  may select an intra-prediction mode to generate the prediction block. Some examples of VVC provide sixty-seven intra-prediction modes, including various directional modes, as well as planar mode and DC mode. In general, video encoder  200  selects an intra-prediction mode that describes neighboring samples to a current block (e.g., a block of a CU) from which to predict samples of the current block. Such samples may generally be above, above and to the left, or to the left of the current block in the same picture as the current block, assuming video encoder  200  codes CTUs and CUs in raster scan order (left to right, top to bottom). 
     Video encoder  200  encodes data representing the prediction mode for a current block. For example, for inter-prediction modes, video encoder  200  may encode data representing which of the various available inter-prediction modes is used, as well as motion information for the corresponding mode. For uni-directional or bi-directional inter-prediction, for example, video encoder  200  may encode motion vectors using advanced motion vector prediction (AMVP) or merge mode. Video encoder  200  may use similar modes to encode motion vectors for affine motion compensation mode. 
     Following prediction, such as intra-prediction or inter-prediction of a block, video encoder  200  may calculate residual data for the block. The residual data, such as a residual block, represents sample by sample differences between the block and a prediction block for the block, formed using the corresponding prediction mode. Video encoder  200  may apply one or more transforms to the residual block, to produce transformed data in a transform domain instead of the sample domain. For example, video encoder  200  may apply a discrete cosine transform (DCT), an integer transform, a wavelet transform, or a conceptually similar transform to residual video data. Additionally, video encoder  200  may apply a secondary transform following the first transform, such as a mode-dependent non-separable secondary transform (MDNSST), a signal dependent transform, a Karhunen-Loeve transform (KLT), or the like. Video encoder  200  produces transform coefficients following application of the one or more transforms. 
     As noted above, following any transforms to produce transform coefficients, video encoder  200  may perform quantization of the transform coefficients. Quantization generally refers to a process in which transform coefficients are quantized to possibly reduce the amount of data used to represent the transform coefficients, providing further compression. By performing the quantization process, video encoder  200  may reduce the bit depth associated with some or all of the transform coefficients. For example, video encoder  200  may round an n-bit value down to an m-bit value during quantization, where n is greater than m. In some examples, to perform quantization, video encoder  200  may perform a bitwise right-shift of the value to be quantized. 
     Following quantization, video encoder  200  may scan the transform coefficients, producing a one-dimensional vector from the two-dimensional matrix including the quantized transform coefficients. The scan may be designed to place higher energy (and therefore lower frequency) transform coefficients at the front of the vector and to place lower energy (and therefore higher frequency) transform coefficients at the back of the vector. In some examples, video encoder  200  may utilize a predefined scan order to scan the quantized transform coefficients to produce a serialized vector, and then entropy encode the quantized transform coefficients of the vector. In other examples, video encoder  200  may perform an adaptive scan. After scanning the quantized transform coefficients to form the one-dimensional vector, video encoder  200  may entropy encode the one-dimensional vector, e.g., according to context-adaptive binary arithmetic coding (CABAC). Video encoder  200  may also entropy encode values for syntax elements describing metadata associated with the encoded video data for use by video decoder  300  in decoding the video data. 
     To perform CABAC, video encoder  200  may assign a context within a context model to a symbol to be transmitted. The context may relate to, for example, whether neighboring values of the symbol are zero-valued or not. The probability determination may be based on a context assigned to the symbol. 
     Video encoder  200  may further generate syntax data, such as block-based syntax data, picture-based syntax data, and sequence-based syntax data, to video decoder  300 , e.g., in a picture header, a block header, a slice header, or other syntax data, such as a sequence parameter set (SPS), picture parameter set (PPS), or video parameter set (VPS). Video decoder  300  may likewise decode such syntax data to determine how to decode corresponding video data. 
     In this manner, video encoder  200  may generate a bitstream including encoded video data, e.g., syntax elements describing partitioning of a picture into blocks (e.g., CUs) and prediction and/or residual information for the blocks. Ultimately, video decoder  300  may receive the bitstream and decode the encoded video data. 
     In general, video decoder  300  performs a reciprocal process to that performed by video encoder  200  to decode the encoded video data of the bitstream. For example, video decoder  300  may decode values for syntax elements of the bitstream using CABAC in a manner substantially similar to, albeit reciprocal to, the CABAC encoding process of video encoder  200 . The syntax elements may define partitioning information for partitioning of a picture into CTUs, and partitioning of each CTU according to a corresponding partition structure, such as a QTBT structure, to define CUs of the CTU. The syntax elements may further define prediction and residual information for blocks (e.g., CUs) of video data. 
     The residual information may be represented by, for example, quantized transform coefficients. Video decoder  300  may inverse quantize and inverse transform the quantized transform coefficients of a block to reproduce a residual block for the block. Video decoder  300  uses a signaled prediction mode (intra- or inter-prediction) and related prediction information (e.g., motion information for inter-prediction) to form a prediction block for the block. Video decoder  300  may then combine the prediction block and the residual block (on a sample-by-sample basis) to reproduce the original block. Video decoder  300  may perform additional processing, such as performing a deblocking process to reduce visual artifacts along boundaries of the block. 
     In accordance with the techniques of this disclosure, video encoder  200  may encode, and video decoder  300  may decode, one or more general constraint information (GCI) flags. As one example, video decoder  300  may decode, from a coded video bitstream, a first syntax element that specifies a constraint for a second syntax element that specifies whether subpicture information is present in the coded video bitstream and whether more than one subpicture may be present in the coded video bitstream; and decode, based on the constraint specified by the first syntax element, the second syntax element. In some examples, the first syntax element may be a no_subpic_info_constraint_flag syntax element. In some examples, the first syntax element may be a max_subpics_constraint_idc syntax element. In some examples, the second syntax element may be a sps_subpic_info_present_flag syntax element. In some examples, decoding the second syntax element based on the constraint may include determining that a value of the second syntax element is 0 where a value of the first syntax element is 0. 
     As another example, video decoder  300  may decode, from a coded video bitstream, a first syntax element that specifies a constraint for a second syntax element that specifies whether reference picture list (RPL) syntax elements are present in slice headers of instantaneous decoding refresh (IDR) pictures; and decode, based on the constraint specified by the first syntax element, the second syntax element. In some examples, the first syntax element may be a no_idr_rpl constraint flag syntax element. In some examples, the second syntax element may be a sps_idr_rpl_present_flag syntax element. In some examples, decoding the second syntax element based on the constraint may include determining that a value of the second syntax element is 0 where a value of the first syntax element is 1. 
     As another example, video decoder  300  may decode, from a coded video bitstream, a first syntax element that specifies a constraint for a second syntax element that specifies whether a video parameter set (VPS) of the coded video bitstream includes hypothetical reference decoder parameters; and decode, based on the constraint specified by the first syntax element, the second syntax element. In some examples, the first syntax element may be a no_hrd_constraint_flag syntax element. In some examples, the second syntax element may be a vps_general_hrd_params_present_flag syntax element. In some examples, decoding the second syntax element based on the constraint may include determining that a value of the second syntax element is 0 where a value of the first syntax element is 1. In some examples, video decoder  300  may further decode, based on the constraint specified by the first syntax element, a third syntax element that specifies whether a sequence parameter set (SPS) of the coded video bitstream includes hypothetical reference decoder parameters. In some examples, the third syntax element may include a sps_general_hrd_params_present_flag syntax element. In some examples, decoding the third syntax element based on the constraint may include determining that a value of the third syntax element is 0 where a value of the first syntax element is 1. 
     As another example, video decoder  300  may decode, from a coded video bitstream, a first syntax element that specifies a constraint for a second syntax element that specifies whether the coded video bitstream conveys pictures that represent fields; and decode, based on the constraint specified by the first syntax element, the second syntax element. In some examples, the first syntax element may include a general_frame_only_constraint_flag syntax element. In some examples, the second syntax element may include a sps_field_seq_flag syntax element. In some examples, to decode the second syntax element based on the constraint, video decoder  300  may determine that a value of the second syntax element is 0 where a value of the first syntax element is 1. 
     Video encoder  200  may encode the syntax elements in the above-described examples. In any of the examples, the coded video bitstream may be a VVC compliant bitstream. As such, in some examples, the techniques of this disclosure may apply to VVC video coding. 
     This disclosure may generally refer to “signaling” certain information, such as syntax elements. The term “signaling” may generally refer to the communication of values for syntax elements and/or other data used to decode encoded video data. That is, video encoder  200  may signal values for syntax elements in the bitstream. In general, signaling refers to generating a value in the bitstream. As noted above, source device  102  may transport the bitstream to destination device  116  substantially in real time, or not in real time, such as might occur when storing syntax elements to storage device  112  for later retrieval by destination device  116 . 
       FIGS.  2 A and  2 B  are conceptual diagrams illustrating an example quadtree binary tree (QTBT) structure  130 , and a corresponding coding tree unit (CTU)  132 . The solid lines represent quadtree splitting, and dotted lines indicate binary tree splitting. In each split (i.e., non-leaf) node of the binary tree, one flag is signaled to indicate which splitting type (i.e., horizontal or vertical) is used, where 0 indicates horizontal splitting and 1 indicates vertical splitting in this example. For the quadtree splitting, there is no need to indicate the splitting type, because quadtree nodes split a block horizontally and vertically into 4 sub-blocks with equal size. Accordingly, video encoder  200  may encode, and video decoder  300  may decode, syntax elements (such as splitting information) for a region tree level of QTBT structure  130  (i.e., the solid lines) and syntax elements (such as splitting information) for a prediction tree level of QTBT structure  130  (i.e., the dashed lines). Video encoder  200  may encode, and video decoder  300  may decode, video data, such as prediction and transform data, for CUs represented by terminal leaf nodes of QTBT structure  130 . 
     In general, CTU  132  of  FIG.  2 B  may be associated with parameters defining sizes of blocks corresponding to nodes of QTBT structure  130  at the first and second levels. These parameters may include a CTU size (representing a size of CTU  132  in samples), a minimum quadtree size (MinQTSize, representing a minimum allowed quadtree leaf node size), a maximum binary tree size (MaxBTSize, representing a maximum allowed binary tree root node size), a maximum binary tree depth (MaxBTDepth, representing a maximum allowed binary tree depth), and a minimum binary tree size (MinBTSize, representing the minimum allowed binary tree leaf node size). 
     The root node of a QTBT structure corresponding to a CTU may have four child nodes at the first level of the QTBT structure, each of which may be partitioned according to quadtree partitioning. That is, nodes of the first level are either leaf nodes (having no child nodes) or have four child nodes. The example of QTBT structure  130  represents such nodes as including the parent node and child nodes having solid lines for branches. If nodes of the first level are not larger than the maximum allowed binary tree root node size (MaxBTSize), then the nodes can be further partitioned by respective binary trees. The binary tree splitting of one node can be iterated until the nodes resulting from the split reach the minimum allowed binary tree leaf node size (MinBTSize) or the maximum allowed binary tree depth (MaxBTDepth). The example of QTBT structure  130  represents such nodes as having dashed lines for branches. The binary tree leaf node is referred to as a coding unit (CU), which is used for prediction (e.g., intra-picture or inter-picture prediction) and transform, without any further partitioning. As discussed above, CUs may also be referred to as “video blocks” or “blocks.” 
     In one example of the QTBT partitioning structure, the CTU size is set as 128×128 (luma samples and two corresponding 64×64 chroma samples), the MinQTSize is set as 16×16, the MaxBTSize is set as 64×64, the MinBTSize (for both width and height) is set as 4, and the MaxBTDepth is set as 4. The quadtree partitioning is applied to the CTU first to generate quad-tree leaf nodes. The quadtree leaf nodes may have a size from 16×16 (i.e., the MinQTSize) to 128×128 (i.e., the CTU size). If the quadtree leaf node is 128×128, the leaf quadtree node will not be further split by the binary tree, because the size exceeds the MaxBTSize (i.e., 64×64, in this example). Otherwise, the quadtree leaf node will be further partitioned by the binary tree. Therefore, the quadtree leaf node is also the root node for the binary tree and has the binary tree depth as 0. When the binary tree depth reaches MaxBTDepth (4, in this example), no further splitting is permitted. When the binary tree node has a width equal to MinBTSize (4, in this example), it implies that no further vertical splitting is permitted. Similarly, a binary tree node having a height equal to MinBTSize implies that no further horizontal splitting is permitted for that binary tree node. As noted above, leaf nodes of the binary tree are referred to as CUs, and are further processed according to prediction and transform without further partitioning. 
       FIG.  3    is a block diagram illustrating an example video encoder  200  that may perform the techniques of this disclosure.  FIG.  3    is provided for purposes of explanation and should not be considered limiting of the techniques as broadly exemplified and described in this disclosure. For purposes of explanation, this disclosure describes video encoder  200  according to the techniques of VVC (ITU-T H.266, under development), and HEVC (ITU-T H.265). However, the techniques of this disclosure may be performed by video encoding devices that are configured to other video coding standards. 
     In the example of  FIG.  3   , video encoder  200  includes video data memory  230 , mode selection unit  202 , residual generation unit  204 , transform processing unit  206 , quantization unit  208 , inverse quantization unit  210 , inverse transform processing unit  212 , reconstruction unit  214 , filter unit  216 , decoded picture buffer (DPB)  218 , and entropy encoding unit  220 . Any or all of video data memory  230 , mode selection unit  202 , residual generation unit  204 , transform processing unit  206 , quantization unit  208 , inverse quantization unit  210 , inverse transform processing unit  212 , reconstruction unit  214 , filter unit  216 , DPB  218 , and entropy encoding unit  220  may be implemented in one or more processors or in processing circuitry. For instance, the units of video encoder  200  may be implemented as one or more circuits or logic elements as part of hardware circuitry, or as part of a processor, ASIC, of FPGA. Moreover, video encoder  200  may include additional or alternative processors or processing circuitry to perform these and other functions. 
     Video data memory  230  may store video data to be encoded by the components of video encoder  200 . Video encoder  200  may receive the video data stored in video data memory  230  from, for example, video source  104  ( FIG.  1   ). DPB  218  may act as a reference picture memory that stores reference video data for use in prediction of subsequent video data by video encoder  200 . Video data memory  230  and DPB  218  may be formed by any of a variety of memory devices, such as dynamic random access memory (DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM), resistive RAM (RRAM), or other types of memory devices. Video data memory  230  and DPB  218  may be provided by the same memory device or separate memory devices. In various examples, video data memory  230  may be on-chip with other components of video encoder  200 , as illustrated, or off-chip relative to those components. 
     In this disclosure, reference to video data memory  230  should not be interpreted as being limited to memory internal to video encoder  200 , unless specifically described as such, or memory external to video encoder  200 , unless specifically described as such. Rather, reference to video data memory  230  should be understood as reference memory that stores video data that video encoder  200  receives for encoding (e.g., video data for a current block that is to be encoded). Memory  106  of  FIG.  1    may also provide temporary storage of outputs from the various units of video encoder  200 . 
     The various units of  FIG.  3    are illustrated to assist with understanding the operations performed by video encoder  200 . The units may be implemented as fixed-function circuits, programmable circuits, or a combination thereof. Fixed-function circuits refer to circuits that provide particular functionality, and are preset on the operations that can be performed. Programmable circuits refer to circuits that can be programmed to perform various tasks, and provide flexible functionality in the operations that can be performed. For instance, programmable circuits may execute software or firmware that cause the programmable circuits to operate in the manner defined by instructions of the software or firmware. Fixed-function circuits may execute software instructions (e.g., to receive parameters or output parameters), but the types of operations that the fixed-function circuits perform are generally immutable. In some examples, one or more of the units may be distinct circuit blocks (fixed-function or programmable), and in some examples, one or more of the units may be integrated circuits. 
     Video encoder  200  may include arithmetic logic units (ALUs), elementary function units (EFUs), digital circuits, analog circuits, and/or programmable cores, formed from programmable circuits. In examples where the operations of video encoder  200  are performed using software executed by the programmable circuits, memory  106  ( FIG.  1   ) may store the instructions (e.g., object code) of the software that video encoder  200  receives and executes, or another memory within video encoder  200  (not shown) may store such instructions. 
     Video data memory  230  is configured to store received video data. Video encoder  200  may retrieve a picture of the video data from video data memory  230  and provide the video data to residual generation unit  204  and mode selection unit  202 . Video data in video data memory  230  may be raw video data that is to be encoded. 
     Mode selection unit  202  includes a motion estimation unit  222 , a motion compensation unit  224 , and an intra-prediction unit  226 . Mode selection unit  202  may include additional functional units to perform video prediction in accordance with other prediction modes. As examples, mode selection unit  202  may include a palette unit, an intra-block copy unit (which may be part of motion estimation unit  222  and/or motion compensation unit  224 ), an affine unit, a linear model (LM) unit, or the like. 
     Mode selection unit  202  generally coordinates multiple encoding passes to test combinations of encoding parameters and resulting rate-distortion values for such combinations. The encoding parameters may include partitioning of CTUs into CUs, prediction modes for the CUs, transform types for residual data of the CUs, quantization parameters for residual data of the CUs, and so on. Mode selection unit  202  may ultimately select the combination of encoding parameters having rate-distortion values that are better than the other tested combinations. 
     Video encoder  200  may partition a picture retrieved from video data memory  230  into a series of CTUs, and encapsulate one or more CTUs within a slice. Mode selection unit  202  may partition a CTU of the picture in accordance with a tree structure, such as the QTBT structure or the quad-tree structure of HEVC described above. As described above, video encoder  200  may form one or more CUs from partitioning a CTU according to the tree structure. Such a CU may also be referred to generally as a “video block” or “block.” 
     In general, mode selection unit  202  also controls the components thereof (e.g., motion estimation unit  222 , motion compensation unit  224 , and intra-prediction unit  226 ) to generate a prediction block for a current block (e.g., a current CU, or in HEVC, the overlapping portion of a PU and a TU). For inter-prediction of a current block, motion estimation unit  222  may perform a motion search to identify one or more closely matching reference blocks in one or more reference pictures (e.g., one or more previously coded pictures stored in DPB  218 ). In particular, motion estimation unit  222  may calculate a value representative of how similar a potential reference block is to the current block, e.g., according to sum of absolute difference (SAD), sum of squared differences (SSD), mean absolute difference (MAD), mean squared differences (MSD), or the like. Motion estimation unit  222  may generally perform these calculations using sample-by-sample differences between the current block and the reference block being considered. Motion estimation unit  222  may identify a reference block having a lowest value resulting from these calculations, indicating a reference block that most closely matches the current block. 
     Motion estimation unit  222  may form one or more motion vectors (MVs) that defines the positions of the reference blocks in the reference pictures relative to the position of the current block in a current picture. Motion estimation unit  222  may then provide the motion vectors to motion compensation unit  224 . For example, for uni-directional inter-prediction, motion estimation unit  222  may provide a single motion vector, whereas for bi-directional inter-prediction, motion estimation unit  222  may provide two motion vectors. Motion compensation unit  224  may then generate a prediction block using the motion vectors. For example, motion compensation unit  224  may retrieve data of the reference block using the motion vector. As another example, if the motion vector has fractional sample precision, motion compensation unit  224  may interpolate values for the prediction block according to one or more interpolation filters. Moreover, for bi-directional inter-prediction, motion compensation unit  224  may retrieve data for two reference blocks identified by respective motion vectors and combine the retrieved data, e.g., through sample-by-sample averaging or weighted averaging. 
     As another example, for intra-prediction, or intra-prediction coding, intra-prediction unit  226  may generate the prediction block from samples neighboring the current block. For example, for directional modes, intra-prediction unit  226  may generally mathematically combine values of neighboring samples and populate these calculated values in the defined direction across the current block to produce the prediction block. As another example, for DC mode, intra-prediction unit  226  may calculate an average of the neighboring samples to the current block and generate the prediction block to include this resulting average for each sample of the prediction block. 
     Mode selection unit  202  provides the prediction block to residual generation unit  204 . Residual generation unit  204  receives a raw, unencoded version of the current block from video data memory  230  and the prediction block from mode selection unit  202 . Residual generation unit  204  calculates sample-by-sample differences between the current block and the prediction block. The resulting sample-by-sample differences define a residual block for the current block. In some examples, residual generation unit  204  may also determine differences between sample values in the residual block to generate a residual block using residual differential pulse code modulation (RDPCM). In some examples, residual generation unit  204  may be formed using one or more subtractor circuits that perform binary subtraction. 
     In examples where mode selection unit  202  partitions CUs into PUs, each PU may be associated with a luma prediction unit and corresponding chroma prediction units. Video encoder  200  and video decoder  300  may support PUs having various sizes. As indicated above, the size of a CU may refer to the size of the luma coding block of the CU and the size of a PU may refer to the size of a luma prediction unit of the PU. Assuming that the size of a particular CU is 2N×2N, video encoder  200  may support PU sizes of 2N×2N or N×N for intra prediction, and symmetric PU sizes of 2N×2N, 2N×N, N×2N, N×N, or similar for inter prediction. Video encoder  200  and video decoder  300  may also support asymmetric partitioning for PU sizes of 2N×nU, 2N×nD, nL×2N, and nR×2N for inter prediction. 
     In examples where mode selection unit  202  does not further partition a CU into PUs, each CU may be associated with a luma coding block and corresponding chroma coding blocks. As above, the size of a CU may refer to the size of the luma coding block of the CU. The video encoder  200  and video decoder  300  may support CU sizes of 2N×2N, 2N×N, or N×2N. 
     For other video coding techniques such as an intra-block copy mode coding, an affine-mode coding, and linear model (LM) mode coding, as a few examples, mode selection unit  202 , via respective units associated with the coding techniques, generates a prediction block for the current block being encoded. In some examples, such as palette mode coding, mode selection unit  202  may not generate a prediction block, and instead generate syntax elements that indicate the manner in which to reconstruct the block based on a selected palette. In such modes, mode selection unit  202  may provide these syntax elements to entropy encoding unit  220  to be encoded. 
     As described above, residual generation unit  204  receives the video data for the current block and the corresponding prediction block. Residual generation unit  204  then generates a residual block for the current block. To generate the residual block, residual generation unit  204  calculates sample-by-sample differences between the prediction block and the current block. 
     Transform processing unit  206  applies one or more transforms to the residual block to generate a block of transform coefficients (referred to herein as a “transform coefficient block”). Transform processing unit  206  may apply various transforms to a residual block to form the transform coefficient block. For example, transform processing unit  206  may apply a discrete cosine transform (DCT), a directional transform, a Karhunen-Loeve transform (KLT), or a conceptually similar transform to a residual block. In some examples, transform processing unit  206  may perform multiple transforms to a residual block, e.g., a primary transform and a secondary transform, such as a rotational transform. In some examples, transform processing unit  206  does not apply transforms to a residual block. 
     Quantization unit  208  may quantize the transform coefficients in a transform coefficient block, to produce a quantized transform coefficient block. Quantization unit  208  may quantize transform coefficients of a transform coefficient block according to a quantization parameter (QP) value associated with the current block. Video encoder  200  (e.g., via mode selection unit  202 ) may adjust the degree of quantization applied to the transform coefficient blocks associated with the current block by adjusting the QP value associated with the CU. Quantization may introduce loss of information, and thus, quantized transform coefficients may have lower precision than the original transform coefficients produced by transform processing unit  206 . 
     Inverse quantization unit  210  and inverse transform processing unit  212  may apply inverse quantization and inverse transforms to a quantized transform coefficient block, respectively, to reconstruct a residual block from the transform coefficient block. Reconstruction unit  214  may produce a reconstructed block corresponding to the current block (albeit potentially with some degree of distortion) based on the reconstructed residual block and a prediction block generated by mode selection unit  202 . For example, reconstruction unit  214  may add samples of the reconstructed residual block to corresponding samples from the prediction block generated by mode selection unit  202  to produce the reconstructed block. 
     Filter unit  216  may perform one or more filter operations on reconstructed blocks. For example, filter unit  216  may perform deblocking operations to reduce blockiness artifacts along edges of CUs. Operations of filter unit  216  may be skipped, in some examples. 
     Video encoder  200  stores reconstructed blocks in DPB  218 . For instance, in examples where operations of filter unit  216  are not needed, reconstruction unit  214  may store reconstructed blocks to DPB  218 . In examples where operations of filter unit  216  are needed, filter unit  216  may store the filtered reconstructed blocks to DPB  218 . Motion estimation unit  222  and motion compensation unit  224  may retrieve a reference picture from DPB  218 , formed from the reconstructed (and potentially filtered) blocks, to inter-predict blocks of subsequently encoded pictures. In addition, intra-prediction unit  226  may use reconstructed blocks in DPB  218  of a current picture to intra-predict other blocks in the current picture. 
     In general, entropy encoding unit  220  may entropy encode syntax elements received from other functional components of video encoder  200 . For example, entropy encoding unit  220  may entropy encode quantized transform coefficient blocks from quantization unit  208 . As another example, entropy encoding unit  220  may entropy encode prediction syntax elements (e.g., motion information for inter-prediction or intra-mode information for intra-prediction) from mode selection unit  202 . Entropy encoding unit  220  may perform one or more entropy encoding operations on the syntax elements, which are another example of video data, to generate entropy-encoded data. For example, entropy encoding unit  220  may perform a context-adaptive variable length coding (CAVLC) operation, a CABAC operation, a variable-to-variable (V2V) length coding operation, a syntax-based context-adaptive binary arithmetic coding (SBAC) operation, a Probability Interval Partitioning Entropy (PIPE) coding operation, an Exponential-Golomb encoding operation, or another type of entropy encoding operation on the data. In some examples, entropy encoding unit  220  may operate in bypass mode where syntax elements are not entropy encoded. 
     Video encoder  200  may output a bitstream that includes the entropy encoded syntax elements needed to reconstruct blocks of a slice or picture. In particular, entropy encoding unit  220  may output the bitstream. 
     The operations described above are described with respect to a block. Such description should be understood as being operations for a luma coding block and/or chroma coding blocks. As described above, in some examples, the luma coding block and chroma coding blocks are luma and chroma components of a CU. In some examples, the luma coding block and the chroma coding blocks are luma and chroma components of a PU. 
     In some examples, operations performed with respect to a luma coding block need not be repeated for the chroma coding blocks. As one example, operations to identify a motion vector (MV) and reference picture for a luma coding block need not be repeated for identifying an MV and reference picture for the chroma blocks. Rather, the MV for the luma coding block may be scaled to determine the MV for the chroma blocks, and the reference picture may be the same. As another example, the intra-prediction process may be the same for the luma coding block and the chroma coding blocks. 
     Video encoder  200  represents an example of a device configured to encode video data including a memory configured to store video data, and one or more processing units implemented in circuitry and configured to encode a first syntax element that specifies a constraint for a second syntax element; and encode the second syntax element with a value based on the constraint. 
       FIG.  4    is a block diagram illustrating an example video decoder  300  that may perform the techniques of this disclosure.  FIG.  4    is provided for purposes of explanation and is not limiting on the techniques as broadly exemplified and described in this disclosure. For purposes of explanation, this disclosure describes video decoder  300  according to the techniques of VVC (ITU-T H.266, under development), and HEVC (ITU-T H.265). However, the techniques of this disclosure may be performed by video coding devices that are configured to other video coding standards. 
     In the example of  FIG.  4   , video decoder  300  includes coded picture buffer (CPB) memory  320 , entropy decoding unit  302 , prediction processing unit  304 , inverse quantization unit  306 , inverse transform processing unit  308 , reconstruction unit  310 , filter unit  312 , and decoded picture buffer (DPB)  314 . Any or all of CPB memory  320 , entropy decoding unit  302 , prediction processing unit  304 , inverse quantization unit  306 , inverse transform processing unit  308 , reconstruction unit  310 , filter unit  312 , and DPB  314  may be implemented in one or more processors or in processing circuitry. For instance, the units of video decoder  300  may be implemented as one or more circuits or logic elements as part of hardware circuitry, or as part of a processor, ASIC, of FPGA. Moreover, video decoder  300  may include additional or alternative processors or processing circuitry to perform these and other functions. 
     Prediction processing unit  304  includes motion compensation unit  316  and intra-prediction unit  318 . Prediction processing unit  304  may include additional units to perform prediction in accordance with other prediction modes. As examples, prediction processing unit  304  may include a palette unit, an intra-block copy unit (which may form part of motion compensation unit  316 ), an affine unit, a linear model (LM) unit, or the like. In other examples, video decoder  300  may include more, fewer, or different functional components. 
     CPB memory  320  may store video data, such as an encoded video bitstream, to be decoded by the components of video decoder  300 . The video data stored in CPB memory  320  may be obtained, for example, from computer-readable medium  110  ( FIG.  1   ). CPB memory  320  may include a CPB that stores encoded video data (e.g., syntax elements) from an encoded video bitstream. Also, CPB memory  320  may store video data other than syntax elements of a coded picture, such as temporary data representing outputs from the various units of video decoder  300 . DPB  314  generally stores decoded pictures, which video decoder  300  may output and/or use as reference video data when decoding subsequent data or pictures of the encoded video bitstream. CPB memory  320  and DPB  314  may be formed by any of a variety of memory devices, such as DRAM, including SDRAM, MRAM, RRAM, or other types of memory devices. CPB memory  320  and DPB  314  may be provided by the same memory device or separate memory devices. In various examples, CPB memory  320  may be on-chip with other components of video decoder  300 , or off-chip relative to those components. 
     Additionally or alternatively, in some examples, video decoder  300  may retrieve coded video data from memory  120  ( FIG.  1   ). That is, memory  120  may store data as discussed above with CPB memory  320 . Likewise, memory  120  may store instructions to be executed by video decoder  300 , when some or all of the functionality of video decoder  300  is implemented in software to be executed by processing circuitry of video decoder  300 . 
     The various units shown in  FIG.  4    are illustrated to assist with understanding the operations performed by video decoder  300 . The units may be implemented as fixed-function circuits, programmable circuits, or a combination thereof. Similar to  FIG.  3   , fixed-function circuits refer to circuits that provide particular functionality, and are preset on the operations that can be performed. Programmable circuits refer to circuits that can be programmed to perform various tasks, and provide flexible functionality in the operations that can be performed. For instance, programmable circuits may execute software or firmware that cause the programmable circuits to operate in the manner defined by instructions of the software or firmware. Fixed-function circuits may execute software instructions (e.g., to receive parameters or output parameters), but the types of operations that the fixed-function circuits perform are generally immutable. In some examples, one or more of the units may be distinct circuit blocks (fixed-function or programmable), and in some examples, one or more of the units may be integrated circuits. 
     Video decoder  300  may include ALUs, EFUs, digital circuits, analog circuits, and/or programmable cores formed from programmable circuits. In examples where the operations of video decoder  300  are performed by software executing on the programmable circuits, on-chip or off-chip memory may store instructions (e.g., object code) of the software that video decoder  300  receives and executes. 
     Entropy decoding unit  302  may receive encoded video data from the CPB and entropy decode the video data to reproduce syntax elements. Prediction processing unit  304 , inverse quantization unit  306 , inverse transform processing unit  308 , reconstruction unit  310 , and filter unit  312  may generate decoded video data based on the syntax elements extracted from the bitstream. 
     In general, video decoder  300  reconstructs a picture on a block-by-block basis. Video decoder  300  may perform a reconstruction operation on each block individually (where the block currently being reconstructed, i.e., decoded, may be referred to as a “current block”). 
     Entropy decoding unit  302  may entropy decode syntax elements defining quantized transform coefficients of a quantized transform coefficient block, as well as transform information, such as a quantization parameter (QP) and/or transform mode indication(s). Inverse quantization unit  306  may use the QP associated with the quantized transform coefficient block to determine a degree of quantization and, likewise, a degree of inverse quantization for inverse quantization unit  306  to apply. Inverse quantization unit  306  may, for example, perform a bitwise left-shift operation to inverse quantize the quantized transform coefficients. Inverse quantization unit  306  may thereby form a transform coefficient block including transform coefficients. 
     After inverse quantization unit  306  forms the transform coefficient block, inverse transform processing unit  308  may apply one or more inverse transforms to the transform coefficient block to generate a residual block associated with the current block. For example, inverse transform processing unit  308  may apply an inverse DCT, an inverse integer transform, an inverse Karhunen-Loeve transform (KLT), an inverse rotational transform, an inverse directional transform, or another inverse transform to the transform coefficient block. 
     Furthermore, prediction processing unit  304  generates a prediction block according to prediction information syntax elements that were entropy decoded by entropy decoding unit  302 . For example, if the prediction information syntax elements indicate that the current block is inter-predicted, motion compensation unit  316  may generate the prediction block. In this case, the prediction information syntax elements may indicate a reference picture in DPB  314  from which to retrieve a reference block, as well as a motion vector identifying a location of the reference block in the reference picture relative to the location of the current block in the current picture. Motion compensation unit  316  may generally perform the inter-prediction process in a manner that is substantially similar to that described with respect to motion compensation unit  224  ( FIG.  3   ). 
     As another example, if the prediction information syntax elements indicate that the current block is intra-predicted, intra-prediction unit  318  may generate the prediction block according to an intra-prediction mode indicated by the prediction information syntax elements. Again, intra-prediction unit  318  may generally perform the intra-prediction process in a manner that is substantially similar to that described with respect to intra-prediction unit  226  ( FIG.  3   ). Intra-prediction unit  318  may retrieve data of neighboring samples to the current block from DPB  314 . 
     Reconstruction unit  310  may reconstruct the current block using the prediction block and the residual block. For example, reconstruction unit  310  may add samples of the residual block to corresponding samples of the prediction block to reconstruct the current block. 
     Filter unit  312  may perform one or more filter operations on reconstructed blocks. For example, filter unit  312  may perform deblocking operations to reduce blockiness artifacts along edges of the reconstructed blocks. Operations of filter unit  312  are not necessarily performed in all examples. 
     Video decoder  300  may store the reconstructed blocks in DPB  314 . For instance, in examples where operations of filter unit  312  are not performed, reconstruction unit  310  may store reconstructed blocks to DPB  314 . In examples where operations of filter unit  312  are performed, filter unit  312  may store the filtered reconstructed blocks to DPB  314 . As discussed above, DPB  314  may provide reference information, such as samples of a current picture for intra-prediction and previously decoded pictures for subsequent motion compensation, to prediction processing unit  304 . Moreover, video decoder  300  may output decoded pictures (e.g., decoded video) from DPB  314  for subsequent presentation on a display device, such as display device  118  of  FIG.  1   . 
     In this manner, video decoder  300  represents an example of a video decoding device including a memory configured to store video data, and one or more processing units implemented in circuitry and configured to decode a first syntax element that specifies a constraint for a second syntax element; and decode the second syntax element based on the constraint. 
       FIG.  5    is a flowchart illustrating an example method for encoding a current block. The current block may comprise a current CU. Although described with respect to video encoder  200  ( FIGS.  1  and  3   ), it should be understood that other devices may be configured to perform a method similar to that of  FIG.  5   . 
     In this example, video encoder  200  initially predicts the current block ( 350 ). For example, video encoder  200  may form a prediction block for the current block. Video encoder  200  may then calculate a residual block for the current block ( 352 ). To calculate the residual block, video encoder  200  may calculate a difference between the original, unencoded block and the prediction block for the current block. Video encoder  200  may then transform the residual block and quantize transform coefficients of the residual block ( 354 ). Next, video encoder  200  may scan the quantized transform coefficients of the residual block ( 356 ). During the scan, or following the scan, video encoder  200  may entropy encode the transform coefficients ( 358 ). For example, video encoder  200  may encode the transform coefficients using CAVLC or CABAC. Video encoder  200  may then output the entropy encoded data of the block ( 360 ). In addition to the transform coefficients and in accordance with one or more techniques of this disclosure, video encoder  200  may encode one or more general constraint syntax elements. 
       FIG.  6    is a flowchart illustrating an example method for decoding a current block of video data. The current block may comprise a current CU. Although described with respect to video decoder  300  ( FIGS.  1  and  4   ), it should be understood that other devices may be configured to perform a method similar to that of  FIG.  6   . 
     Video decoder  300  may receive entropy encoded data for the current block, such as entropy encoded prediction information and entropy encoded data for transform coefficients of a residual block corresponding to the current block ( 370 ). Video decoder  300  may entropy decode the entropy encoded data to determine prediction information for the current block and to reproduce transform coefficients of the residual block ( 372 ). In addition to the transform coefficients and in accordance with one or more techniques of this disclosure, video decoder  300  may receive one or more general constraint syntax elements. Video decoder  300  may predict the current block ( 374 ), e.g., using an intra- or inter-prediction mode as indicated by the prediction information for the current block, to calculate a prediction block for the current block. Video decoder  300  may then inverse scan the reproduced transform coefficients ( 376 ), to create a block of quantized transform coefficients. Video decoder  300  may then inverse quantize the transform coefficients and apply an inverse transform to the transform coefficients to produce a residual block ( 378 ). Video decoder  300  may ultimately decode the current block by combining the prediction block and the residual block ( 380 ). 
       FIG.  7    is a flowchart illustrating an example method for signaling syntax elements that constrain other syntax elements used to represent video data, in accordance with one or more techniques of this disclosure. Although described with respect to video decoder  300  ( FIGS.  1  and  4   ), it should be understood that other devices may be configured to perform a method similar to that of  FIG.  7   . For instance, video encoder  200  ( FIGS.  1  and  3   ) may perform a method similar to that of  FIG.  7   . 
     Video decoder  300  may decode a first syntax element that specifies a constraint for a second syntax element, the second syntax element specifying whether subpicture information is present and whether more than one subpicture may be present ( 702 ). For instance, entropy decoding unit  302  may decode the first syntax element from a coded video bitstream (stored at least in part at CPB memory  320 ) and provide a value of the first syntax element to prediction processing unit  304 . In some examples, the first syntax element may be a no_subpic_info_constraint_flag syntax element. In some examples, the second syntax element may be a sps_subpic_info_present_flag syntax element. 
     Video decoder  300  may decode, based on the constraint specified by the first syntax element, the second syntax element ( 704 ). For instance, entropy decoding unit  302  may decode the second syntax element from the coded video bitstream (stored at least in part at CPB memory  320 ) and provide a value of the second syntax element to prediction processing unit  304 . The value of the second syntax element is constrained based on the value of the first syntax element. For instance, when a value of the first syntax element is 1, entropy decoding unit  302  may determine that the value of the second syntax element is 0 (e.g., the value of the second syntax element may be constrained to be 0 where the value of the first syntax element is 1). 
     The semantics of the first syntax element may be as follows: no_subpic_info_constraint_flag equal to 1 specifies that sps_subpic_info_present_flag for all pictures in OlsInScope shall be equal to 0. no_subpic_info_constraint_flag equal to 0 does not impose such a constraint. 
     The semantics of the second syntax element may be as follows: sps_subpic_info_present_flag equal to 1 specifies that subpicture information is present for the CLVS and there might be one or more than one subpicture in each picture of the CLVS. sps_subpic_info_present_flag equal to 0 specifies that subpicture information is not present for the CLVS and there is only one subpicture in each picture of the CLVS. 
     Additionally or alternatively to decoding the first and second syntax elements, video decoder  300  may decode a third syntax element that specifies a constraint for a fourth syntax element, the fourth syntax element specifying whether reference picture list (RPL) syntax elements are present in slice headers of instantaneous decoding refresh (IDR) pictures ( 706 ). For instance, entropy decoding unit  302  may decode the third syntax element from the coded video bitstream (stored at least in part at CPB memory  320 ) and provide a value of the third syntax element to prediction processing unit  304 . In some examples, the third syntax element may be a no_idr_rpl_constraint_flag syntax element. In some examples, the fourth syntax element may be a sps_idr_rpl_present_flag syntax element. 
     Video decoder  300  may decode, based on the constraint specified by the third syntax element, the fourth syntax element ( 708 ). For instance, entropy decoding unit  302  may decode the fourth syntax element from the coded video bitstream (stored at least in part at CPB memory  320 ) and provide a value of the fourth syntax element to prediction processing unit  304 . The value of the fourth syntax element is constrained based on the value of the third syntax element. For instance, when a value of the third syntax element is 1, entropy decoding unit  302  may determine that the value of the fourth syntax element is 0 (e.g., the value of the fourth syntax element may be constrained to be 0 where the value of the third syntax element is 1). 
     The semantics of the third syntax element may be as follows: no_idr_rpl_constraint_flag equal to 1 specifies that sps_idr_rpl_present_flag for all pictures in OlsInScope shall be equal to 0. no_idr_rpl_constraint_flag equal to 0 does not impose such a constraint. 
     The semantics of the fourth syntax element may be as follows: sps_idr_rpl_present_flag equal to 1 specifies that RPL syntax elements could be present in slice headers of slices with nal_unit_type equal to IDR_N_LP or IDR_W_RADL. sps_idr_rpl_present_flag equal to 0 specifies that RPL syntax elements are not present in slice headers of slices with nal_unit_type equal to IDR_N_LP or IDR_W_RADL. 
     By encoding the aforementioned flags, a video encoder (e.g., video encoder  200 ) may enable a video decoder (e.g., video decoder  300 ) to discard bitstreams with specific coding features not supported without the video decoder having to parse sequence parameter set (SPS) or picture parameter set (PPS). For instance, where the constraint for the second syntax element that specifies that subpicture information is not present in the coded video bitstream, the video decoder may avoid having to parse SPS and/or PPS syntax elements to determine whether or not the subpicture information is present in the coded video bitstream. Additionally, by encoding the aforementioned flags, the video encoder may enable the receiver to identify the appropriate bitstream among multiple candidates (e.g., adaptive streaming). For situations in which the video encoder is aware of the receivers&#39; capability, the video encoder may selectively disable coding tools by setting the GCI at high level. The video encoder may also turn of specific coding tools when it is aware of the content, as some coding tools may only be useful for particular video content. In this way, the techniques of this disclosure may reduce power consumption, compression complexity, and decoding complexity. 
     The following numbered clauses may illustrate one or more aspects of the disclosure: 
     Clause 1A. A method of coding video data, the method comprising: coding, from a coded video bitstream, a first syntax element that specifies a constraint for a second syntax element that specifies whether subpicture information is present in the coded video bitstream and whether more than one subpicture may be present in the coded video bitstream; and coding, based on the constraint specified by the first syntax element, the second syntax element. 
     Clause 2A. The method of clause 1A, wherein the first syntax element comprises a no_subpic_info_constraint_flag syntax element. 
     Clause 3A. The method of clause 1A or clause 2A, wherein the second syntax element comprises a sps_subpic_info_present_flag syntax element. 
     Clause 4A. The method of clause 1A or clause 3A, wherein the first syntax element comprises a max_subpics_constraint_idc syntax element. 
     Clause 5A. The method of any of clauses 1A-4A, wherein coding the second syntax element based on the constraint comprises determining that a value of the second syntax element is 0 when a value of the first syntax element is 0. 
     Clause 6A. The method of any of clauses 1A-5A, wherein the coded video bitstream comprises a versatile video coding (VVC) compliant bitstream. 
     Clause 7A. The method of any of clauses 1A-6A, wherein coding comprises decoding. 
     Clause 8A. The method of any of clauses 1A-7A, wherein coding comprises encoding. 
     Clause 1B. A method of coding video data, the method comprising: coding, from a coded video bitstream, a first syntax element that specifies a constraint for a second syntax element that specifies whether reference picture list (RPL) syntax elements are present in slice headers of instantaneous decoding refresh (IDR) pictures; and coding, based on the constraint specified by the first syntax element, the second syntax element. 
     Clause 2B. The method of clause 1B, wherein the first syntax element comprises a no_idr_rpl_constraint_flag syntax element. 
     Clause 3B. The method of clause 1B or clause 2B, wherein the second syntax element comprises a sps_idr_rpl_present_flag syntax element. 
     Clause 4B. The method of any of clauses 1B-3B, wherein coding the second syntax element based on the constraint comprises determining that a value of the second syntax element is 0 when a value of the first syntax element is 1. 
     Clause 5B. The method of any of clauses 1B-4B, wherein the coded video bitstream comprises a versatile video coding (VVC) compliant bitstream. 
     Clause 6B. The method of any of clauses 1B-5B, wherein coding comprises decoding. 
     Clause 7B. The method of any of clauses 1B-6B, wherein coding comprises encoding. 
     Clause 1C. A method of coding video data, the method comprising: coding, from a coded video bitstream, a first syntax element that specifies a constraint for a second syntax element that specifies whether a video parameter set (VPS) of the coded video bitstream includes hypothetical reference decoder parameters; and coding, based on the constraint specified by the first syntax element, the second syntax element. 
     Clause 2C. The method of clause 1C, wherein the first syntax element comprises a no_hrd_constraint_flag syntax element. 
     Clause 3C. The method of clause 1C or clause 2C, wherein the second syntax element comprises a vps_general_hrd_params_present_flag syntax element. 
     Clause 4C. The method of any of clauses 1C-3C, wherein coding the second syntax element based on the constraint comprises determining that a value of the second syntax element is 0 where a value of the first syntax element is 1. 
     Clause 5C. The method of any of clauses 1C-4C, further comprising: coding, based on the constraint specified by the first syntax element, a third syntax element that specifies whether a sequence parameter set (SPS) of the coded video bitstream includes hypothetical reference decoder parameters. 
     Clause 6C. The method of clause 5C, wherein the third syntax element comprises a sps_general_hrd_params_present_flag syntax element. 
     Clause 7C. The method of clause 5C or clause 6C, wherein coding the third syntax element based on the constraint comprises determining that a value of the third syntax element is 0 when a value of the first syntax element is 1. 
     Clause 8C. The method of any of clauses 1C-7C, wherein the coded video bitstream comprises a versatile video coding (VVC) compliant bitstream. 
     Clause 9C. The method of any of clauses 1C-8C, wherein coding comprises decoding. 
     Clause 10C. The method of any of clauses 1C-9C, wherein coding comprises encoding. 
     Clause 1D. A method for coding video data, the method comprising: coding, from a coded video bitstream, a first syntax element that specifies a constraint for a second syntax element that specifies whether the coded video bitstream conveys pictures that represent fields; and coding, based on the constraint specified by the first syntax element, the second syntax element. 
     Clause 2D. The method of clause 1D, wherein the first syntax element comprises a general_frame_only_constraint_flag syntax element. 
     Clause 3D. The method of clause 1D or clause 2D, wherein the second syntax element comprises a sps_field_seq_flag syntax element. 
     Clause 4D. The method of any of clauses 1D-3D, wherein coding the second syntax element based on the constraint comprises determining that a value of the second syntax element is 0 when a value of the first syntax element is 1. 
     Clause 5D. The method of any of clauses 1D-4D, wherein the coded video bitstream comprises a versatile video coding (VVC) compliant bitstream. 
     Clause 6D. The method of any of clauses 1D-5D, wherein coding comprises decoding. 
     Clause 7D. The method of any of clauses 1D-6D, wherein coding comprises encoding. 
     Clause 1E. A method for coding video data, the method comprising: refraining from coding, from a sequence parameter set (SPS) of a coded video bitstream, a first syntax element that specifies whether gradual decoding refresh (GDR) is enabled. 
     Clause 2E. The method of clause 1E, wherein the first syntax element comprises a sps_gdr_enabled_flag syntax element. 
     Clause 3E. The method of any clauses 1E or 2E, further comprising: coding, from the coded video bitstream, a second syntax element that specifies whether there shall be no network abstraction layer (NAL) unit with nuh_unit_type equal to GDR NUT present in one or more output layer sets (OLSs) specified by a video parameter set (VPS) (OlsInScope). 
     Clause 4E. The method of clause 3E, wherein the second syntax element comprises a no_gdr_constraint_flag syntax element. 
     Clause 5E. The method of any of clauses 1E-4E, wherein the coded video bitstream comprises a versatile video coding (VVC) compliant bitstream. 
     Clause 6E. The method of any of clauses 1E-5E, wherein coding comprises decoding. 
     Clause 7E. The method of any of clauses 1E-6E, wherein coding comprises encoding. 
     Clause 1F. A device for coding video data, the device comprising one or more means for performing the method of any of clauses 1A-7E. 
     Clause 2F. The device of clause 1F, wherein the one or more means comprise one or more processors implemented in circuitry. 
     Clause 3F. The device of any of clauses 1F and 2F, further comprising a memory to store the video data. 
     Clause 4F. The device of any of clauses 1F-3F, further comprising a display configured to display decoded video data. 
     Clause 5F. The device of any of clauses 1F-4F, wherein the device comprises one or more of a camera, a computer, a mobile device, a broadcast receiver device, or a set-top box. 
     Clause 6F. The device of any of clauses 1F-5F, wherein the device comprises a video decoder. 
     Clause 7F. The device of any of clauses 1F-6F, wherein the device comprises a video encoder. 
     Clause 8F. A computer-readable storage medium having stored thereon instructions that, when executed, cause one or more processors to perform the method of any of clauses 1A-7E. 
     It is to be recognized that depending on the example, certain acts or events of any of the techniques described herein can be performed in a different sequence, may be added, merged, or left out altogether (e.g., not all described acts or events are necessary for the practice of the techniques). Moreover, in certain examples, acts or events may be performed concurrently, e.g., through multi-threaded processing, interrupt processing, or multiple processors, rather than sequentially. 
     In one or more examples, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media, or communication media including any medium that facilitates transfer of a computer program from one place to another, e.g., according to a communication protocol. In this manner, computer-readable media generally may correspond to (1) tangible computer-readable storage media which is non-transitory or (2) a communication medium such as a signal or carrier wave. Data storage media may be any available media that can be accessed by one or more computers or one or more processors to retrieve instructions, code and/or data structures for implementation of the techniques described in this disclosure. A computer program product may include a computer-readable medium. 
     By way of example, and not limitation, such computer-readable storage media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage, or other magnetic storage devices, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer. Also, any connection is properly termed a computer-readable medium. For example, if instructions are transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. It should be understood, however, that computer-readable storage media and data storage media do not include connections, carrier waves, signals, or other transitory media, but are instead directed to non-transitory, tangible storage media. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc, where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. 
     Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the terms “processor” and “processing circuitry,” as used herein may refer to any of the foregoing structures or any other structure suitable for implementation of the techniques described herein. In addition, in some aspects, the functionality described herein may be provided within dedicated hardware and/or software modules configured for encoding and decoding, or incorporated in a combined codec. Also, the techniques could be fully implemented in one or more circuits or logic elements. 
     The techniques of this disclosure may be implemented in a wide variety of devices or apparatuses, including a wireless handset, an integrated circuit (IC) or a set of ICs (e.g., a chip set). Various components, modules, or units are described in this disclosure to emphasize functional aspects of devices configured to perform the disclosed techniques, but do not necessarily require realization by different hardware units. Rather, as described above, various units may be combined in a codec hardware unit or provided by a collection of interoperative hardware units, including one or more processors as described above, in conjunction with suitable software and/or firmware. 
     Various examples have been described. These and other examples are within the scope of the following claims.