Patent Publication Number: US-2022217376-A1

Title: Picture Timing And Decoding Unit Information For Temporal Scalability

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a continuation of International Application No. PCT/US2020/051826 filed on Sep. 21, 2020, by Futurewei Technologies, Inc., and titled “Picture Timing And Decoding Unit Information For Temporal Scalability,” which claims the benefit of U.S. Provisional Patent Application No. 62/905,147 filed Sep. 24, 2019 by Futurewei Technologies, Inc., and titled “Picture Timing and Decoding Unit Information for Temporal Scalability,” each of which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure is generally related to video coding, and is specifically related to hypothetical reference decoder (HRD) parameter changes to support efficient encoding and/or conformance testing of multi-layer bitstreams. 
     BACKGROUND 
     The amount of video data needed to depict even a relatively short video can be substantial, which may result in difficulties when the data is to be streamed or otherwise communicated across a communications network with limited bandwidth capacity. Thus, video data is generally compressed before being communicated across modern day telecommunications networks. The size of a video could also be an issue when the video is stored on a storage device because memory resources may be limited. Video compression devices often use software and/or hardware at the source to code the video data prior to transmission or storage, thereby decreasing the quantity of data needed to represent digital video images. The compressed data is then received at the destination by a video decompression device that decodes the video data. With limited network resources and ever increasing demands of higher video quality, improved compression and decompression techniques that improve compression ratio with little to no sacrifice in image quality are desirable. 
     SUMMARY 
     A first aspect relates to a method implemented by a video decoder, comprising receiving, by the video decoder, a bitstream comprising a coded picture and a supplemental enhancement information (SEI) message, wherein the SEI message includes coded picture buffer (CPB) parameters corresponding to decoding unit (DU)-based hypothetical reference decoder (HRD) operations on sublayers; and decoding, by the video decoder, the coded picture from the bitstream to obtain a decoded picture. 
     The method provides techniques that ensure picture-level coded picture buffer (CPB) parameters corresponding to decoding unit (DU)-based HRD operations on sublayers are included in a supplemental enhancement information (SEI) message. Because the picture-level CPB parameters are included in the SEI message, the HRD can use the DU-based HRD operations to test the sublayers in the bitstream for bitstream conformance, which ensures that the sublayers are properly coded and/or can be properly decoded. Thus, the coder/decoder (a.k.a., “codec”) in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the CPB parameters specify a duration between CPB removal times of two decoding units. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the SEI message is a picture timing (PT) SEI message. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the CPB parameters comprise a common CPB removal delay increment and a CPB removal delay increment for an access unit (AU) associated with the PT SEI message. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the SEI message is a PT SEI message that specifies a number of decoding units in the AU associated with the PT SEI message. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the SEI message is a PT SEI message that specifies a number of network abstraction layer (NAL) units in a decoding unit (DU) of the AU associated with the PT SEI message. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the SEI message is a decoding unit information (DUI) SEI message that provides a temporal identifier (ID) of an SEI NAL unit containing the DUI SEI message. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the temporal ID specifies a highest sublayer for which CPB removal delay information is contained in the DUI SEI message. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides displaying the decoded picture on a display of an electronic device. 
     A second aspect relates to a method implemented by a video encoder, comprising generating, by the video encoder, a bitstream comprising a coded picture and a supplemental enhancement information (SEI) message, wherein the SEI message includes coded picture buffer (CPB) parameters corresponding to decoding unit (DU)-based hypothetical reference decoder (HRD) operations on sublayers; performing, by the video encoder, the DU-based HRD operations on the sublayers using the CPB parameters to determine whether the bitstream is conforming; and storing, by the video encoder, the bitstream for communication toward a video decoder when the bitstream is conforming based on performance of the DU-based HRD operations. 
     The method provides techniques that ensure picture-level coded picture buffer (CPB) parameters corresponding to decoding unit (DU)-based HRD operations on sublayers are included in a supplemental enhancement information (SEI) message. Because the picture-level CPB parameters are included in the SEI message, the HRD can use the DU-based HRD operations to test the sublayers in the bitstream for bitstream conformance, which ensures that the sublayers are properly coded and/or can be properly decoded. Thus, the coder/decoder (a.k.a., “codec”) in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the CPB parameters specify a duration between CPB removal times of two decoding units, and wherein the bitstream is conforming when the duration between the CPB removal times is not exceeded. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the SEI message is a picture timing (PT) SEI message. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the CPB parameters comprise a common CPB removal delay and a CPB removal delay for an access unit (AU) associated with the PT SEI message. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the PT SEI message specifies a number of decoding units in the AU associated with the PT SEI message and a number of network abstraction layer (NAL) units in a decoding unit (DU) of the AU associated with the PT SEI message. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the SEI message is a decoding unit information (DUI) SEI message that provides a temporal identifier (ID) of an SEI NAL unit containing the DUI SEI message. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the DUI SEI message specifies the temporal ID of a highest sublayer for which CPB removal delay information is contained in the DUI SEI message. 
     A third aspect relates to a decoding device, comprising a receiver configured to receive a bitstream comprising a coded picture and a supplemental enhancement information (SEI) message, wherein the SEI message includes coded picture buffer (CPB) parameters corresponding to decoding unit (DU)-based hypothetical reference decoder (HRD) operations on sublayers; a memory coupled to the receiver, the memory storing instructions; and a processor coupled to the memory, the processor configured to execute the instructions to cause the decoding device to decode the coded picture from the bitstream to obtain a decoded picture. 
     The decoding device provides techniques that ensure picture-level coded picture buffer (CPB) parameters corresponding to decoding unit (DU)-based HRD operations on sublayers are included in a supplemental enhancement information (SEI) message. Because the picture-level CPB parameters are included in the SEI message, the HRD can use the DU-based HRD operations to test the sublayers in the bitstream for bitstream conformance, which ensures that the sublayers are properly coded and/or can be properly decoded. Thus, the coder/decoder (a.k.a., “codec”) in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the CPB parameters specify a duration between CPB removal times of two decoding units. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the SEI message is a PT SEI message that specifies a number of decoding units in an access unit (AU) associated with the PT SEI message, and wherein the CPB parameters comprise a common CPB removal delay and a CPB removal delay for the AU associated with the PT SEI message. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the SEI message is a decoding unit information (DUI) SEI message that provides a temporal identifier (ID) of an SEI NAL unit containing the DUI SEI message. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the temporal ID specifies a highest sublayer for which CPB removal delay information is contained in the DUI SEI message. 
     A fourth aspect relates to an encoding device, comprising a memory containing instructions; a processor coupled to the memory, the processor configured to implement the instructions to cause the encoding device to: generate a bitstream comprising a coded picture and a supplemental enhancement information (SEI) message, wherein the SEI message includes coded picture buffer (CPB) parameters corresponding to decoding unit (DU)-based HRD operations on sublayers; perform the DU-based HRD operations on the sublayers using the CPB parameters to determine whether the bitstream is conforming; and a transmitter coupled to the processor, the transmitter configured to transmit the video bitstream toward a video decoder when the bitstream is conforming based on performance of the DU-based HRD operations. 
     The encoding device provides techniques that ensure picture-level coded picture buffer (CPB) parameters corresponding to decoding unit (DU)-based HRD operations on sublayers are included in a supplemental enhancement information (SEI) message. Because the picture-level CPB parameters are included in the SEI message, the HRD can use the DU-based HRD operations to test the sublayers in the bitstream for bitstream conformance, which ensures that the sublayers are properly coded and/or can be properly decoded. Thus, the coder/decoder (a.k.a., “codec”) in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the CPB parameters specify a duration between CPB removal times of two decoding units, and wherein the bitstream is conforming when the duration between the CPB removal times is not exceeded. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the SEI message is a picture timing (PT) SEI message. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the CPB parameters comprise a common CPB removal delay and a CPB removal delay for an access unit (AU) associated with the PT SEI message. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the PT SEI message specifies a number of decoding units in the AU associated with the PT SEI message and a number of network abstraction layer (NAL) units in a decoding unit (DU) of the AU associated with the PT SEI message. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the SEI message is a decoding unit information (DUI) SEI message that provides a temporal identifier (ID) of an SEI network abstraction layer (NAL) unit containing the DUI SEI message. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the DUI SEI message specifies a temporal identifier (ID) of a highest sublayer for which CPB removal delay information is contained in the DUI SEI message. 
     A fifth aspect relates to a coding apparatus. The coding apparatus includes a receiver configured to receive a picture to encode or to receive a bitstream to decode; a transmitter coupled to the receiver, the transmitter configured to transmit the bitstream to a decoder or to transmit a decoded image to a display; a memory coupled to at least one of the receiver or the transmitter, the memory configured to store instructions; and a processor coupled to the memory, the processor configured to execute the instructions stored in the memory to perform any of the methods disclosed herein. 
     The coding apparatus provides techniques that ensure picture-level coded picture buffer (CPB) parameters corresponding to decoding unit (DU)-based HRD operations on sublayers are included in a supplemental enhancement information (SEI) message. Because the picture-level CPB parameters are included in the SEI message, the HRD can use the DU-based HRD operations to test the sublayers in the bitstream for bitstream conformance, which ensures that the sublayers are properly coded and/or can be properly decoded. Thus, the coder/decoder (a.k.a., “codec”) in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides a display configured to display a decoded picture. 
     A sixth aspect relates to a system. The system includes an encoder; and a decoder in communication with the encoder, wherein the encoder or the decoder includes the decoding device, the encoding device, or the coding apparatus disclosed herein. 
     The system provides techniques that ensure picture-level coded picture buffer (CPB) parameters corresponding to decoding unit (DU)-based HRD operations on sublayers are included in a supplemental enhancement information (SEI) message. Because the picture-level CPB parameters are included in the SEI message, the HRD can use the DU-based HRD operations to test the sublayers in the bitstream for bitstream conformance, which ensures that the sublayers are properly coded and/or can be properly decoded. Thus, the coder/decoder (a.k.a., “codec”) in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed. 
     A seventh aspect relates to a means for coding. The means for coding includes receiving means configured to receive a picture to encode or to receive a bitstream to decode; transmission means coupled to the receiving means, the transmission means configured to transmit the bitstream to a decoding means or to transmit a decoded image to a display means; storage means coupled to at least one of the receiving means or the transmission means, the storage means configured to store instructions; and processing means coupled to the storage means, the processing means configured to execute the instructions stored in the storage means to perform any of the methods disclosed herein. 
     The means for coding provides techniques that ensure picture-level coded picture buffer (CPB) parameters corresponding to decoding unit (DU)-based HRD operations on sublayers are included in a supplemental enhancement information (SEI) message. Because the picture-level CPB parameters are included in the SEI message, the HRD can use the DU-based HRD operations to test the sublayers in the bitstream for bitstream conformance, which ensures that the sublayers are properly coded and/or can be properly decoded. Thus, the coder/decoder (a.k.a., “codec”) in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed. 
     For the purpose of clarity, any one of the foregoing embodiments may be combined with any one or more of the other foregoing embodiments to create a new embodiment within the scope of the present disclosure. 
     These and other features will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts. 
         FIG. 1  is a flowchart of an example method of coding a video signal. 
         FIG. 2  is a schematic diagram of an example coding and decoding (codec) system for video coding. 
         FIG. 3  is a schematic diagram illustrating an example video encoder. 
         FIG. 4  is a schematic diagram illustrating an example video decoder. 
         FIG. 5  is a schematic diagram illustrating an example hypothetical reference decoder (HRD). 
         FIG. 6  is a schematic diagram illustrating an example multi-layer video sequence configured for inter-layer prediction. 
         FIG. 7  is a schematic diagram illustrating an example multi-layer video sequence configured for temporal scalability. 
         FIG. 8  is a schematic diagram illustrating an example bitstream. 
         FIG. 9  is an embodiment of a method of decoding a coded video bitstream. 
         FIG. 10  is an embodiment of a method of encoding a video bitstream. 
         FIG. 11  is a schematic diagram of a video coding device. 
         FIG. 12  is a schematic diagram of an embodiment of a means for coding. 
     
    
    
     DETAILED DESCRIPTION 
     It should be understood at the outset that although an illustrative implementation of one or more embodiments are provided below, the disclosed systems and/or methods may be implemented using any number of techniques, whether currently known or in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalents. 
     The following terms are defined as follows unless used in a contrary context herein. Specifically, the following definitions are intended to provide additional clarity to the present disclosure. However, terms may be described differently in different contexts. Accordingly, the following definitions should be considered as a supplement and should not be considered to limit any other definitions of descriptions provided for such terms herein. 
     A bitstream is a sequence of bits including video data that is compressed for transmission between an encoder and a decoder. An encoder is a device that is configured to employ encoding processes to compress video data into a bitstream. A decoder is a device that is configured to employ decoding processes to reconstruct video data from a bitstream for display. A picture is an array of luma samples and/or an array of chroma samples that create a frame or a field thereof. A picture that is being encoded or decoded can be referred to as a current picture for clarity of discussion. A network abstraction layer (NAL) unit is a syntax structure containing data in the form of a Raw Byte Sequence Payload (RBSP), an indication of the type of data, and emulation prevention bytes, which are interspersed as desired. A video coding layer (VCL) NAL unit is a NAL unit coded to contain video data, such as a coded slice of a picture. A non-VCL NAL unit is a NAL unit that contains non-video data such as syntax and/or parameters that support decoding the video data, performance of conformance checking, or other operations. An access unit (AU) is a set of NAL units that are associated with each other according to a specified classification rule and pertain to one particular output time. A decoding unit (DU) is an AU or a sub-set of an AU and associated non-VCL NAL units. A layer is a set of VCL NAL units that share a specified characteristic (e.g., a common resolution, frame rate, image size, etc.) and associated non-VCL NAL units. A decoding order is an order in which syntax elements are processed by a decoding process. A video parameter set (VPS) is a data unit that contains parameters related to an entire video. 
     A temporal scalable bitstream is a bitstream coded in multiple layers providing varying temporal resolution/frame rate (e.g., each layer is coded to support a different frame rate). A sublayer is a temporal scalable layer of a temporal scalable bitstream including VCL NAL units with a particular temporal identifier value and associated non-VCL NAL units. For example, a temporal sublayer is a layer that contains video data associated with a specified frame rate. A sublayer representation is a subset of the bitstream containing NAL units of a particular sublayer and the lower sublayers. Hence, one or more temporal sublayers may be combined to achieve a sublayer representation that can be decoded to result in a video sequence with a specified frame rate. An output layer set (OLS) is a set of layers for which one or more layers are specified as output layer(s). An output layer is a layer that is designated for output (e.g., to a display). An OLS index is an index that uniquely identifies a corresponding OLS. A zeroth (0-th) OLS is an OLS that contains only a lowest layer (layer with a lowest layer identifier) and hence contains only an output layer. A temporal identifier (ID) is a data element that indicates data corresponds to temporal location in a video sequence. A sub-bitstream extraction process is a process that removes NAL units from a bitstream that do not belong to a target set as determined by a target OLS index and a target highest temporal ID. The sub-bitstream extraction process results in an output sub-bitstream containing NAL units from the bitstream that are part of the target set. 
     An HRD is a decoder model operating on an encoder. The HRD checks the variability of bitstreams produced by an encoding process to verify conformance with specified constraints. A bitstream conformance test is a test to determine whether an encoded bitstream complies with a standard, such as Versatile Video Coding (VVC). HRD parameters are syntax elements that initialize and/or define operational conditions of an HRD. Sequence-level HRD parameters are HRD parameters that apply to an entire coded video sequence, while picture-level HRD parameters are HRD parameters that apply to pictures in a coded video sequence. A maximum HRD temporal ID (Htid) specifies the Temporal ID of the highest sublayer representation for which the HRD parameters are contained in an i-th set of OLS HRD parameters. An operation point (OP) is a temporal subset of an OLS that is identified by an OLS index and a highest temporal ID. A coded picture buffer (CPB) is a first-in first-out buffer in a HRD that contains coded pictures in decoding order for use during bitstream conformance verification. A decoded picture buffer (DPB) is a buffer for holding decoded pictures for reference, output reordering, and/or output delay. 
     A supplemental enhancement information (SEI) message is a syntax structure with specified semantics that conveys information that is not needed by the decoding process in order to determine the values of the samples in decoded pictures. A buffering period (BP) SEI message is a SEI message that contains HRD parameters for initializing an HRD to manage a CPB. A picture timing (PT) SEI message is a SEI message that contains HRD parameters for managing delivery information for AUs at the CPB and/or the DPB. A decoding unit information (DUI) SEI message is a SEI message that contains HRD parameters for managing delivery information for DUs at the CPB and/or the DPB. 
     A CPB removal delay is a period of time that a corresponding current AU can remain in the CPB prior to removal and output to a DPB. An initial CPB removal delay is a default CPB removal delay for each picture, AU, and/or DU in a bitstream, OLS, and/or layer. A CPB removal offset is a location in the CPB used to determine boundaries of a corresponding AU in the CPB. An initial CPB removal offset is a default CPB removal offset associated with each picture, AU, and/or DU in a bitstream, OLS, and/or layer. A decoded picture buffer (DPB) output delay information is a period of time that a corresponding AU can remain in the DPB prior to output. A CPB removal delay information is information related to removal of a corresponding DU from the CPB. A delivery schedule specifies timing for delivery of video data to and/or from a memory location, such as a CPB and/or a DPB. 
     A maximum number of temporal sublayers is a maximum number of sublayers for which the initial CPB removal delay and the initial CPB removal offset are indicated in the BP SEI message. A common CPB removal delay increment specifies the duration, in units of clock sub-ticks, between the nominal CPB removal times of any two consecutive DUs in decoding order in the AU associated with the picture timing SEI message. The common CPB removal delay increment is also used to calculate an earliest possible time of arrival of decoding unit data into the CPB for a hypothetical stream scheduler (HSS). 
     A number of decoding units specifies the number of DUs in the AU the picture timing SEI message is associated with. A number of NAL units specifies the number of NAL units in the i-th DU of the AU the PT SEI message is associated with. A common CPB removal delay flag specifies whether the syntax elements common CPB removal delay increment are present in the PT SEI message. 
     A CPB removal delay increment specifies the duration, in units of clock sub-ticks, between the nominal CPB removal times of the (i+1)-th DU and the i-th DU, in decoding order, in the AU associated with the PT SEI message. 
     A VPS maximum sublayers minus one (vps_max_sublayers_minus1) syntax element is a syntax element that specifies the maximum number of temporal sublayers that may be present in a layer specified by the VPS. 
     The following acronyms are used herein, Access Unit (AU), Coding Tree Block (CTB), Coding Tree Unit (CTU), Coding Unit (CU), Coded Layer Video Sequence (CLVS), Coded Layer Video Sequence Start (CLVSS), Coded Video Sequence (CVS), Coded Video Sequence Start (CVSS), Joint Video Experts Team (JVET), Hypothetical Reference Decoder (HRD), Motion Constrained Tile Set (MCTS), Maximum Transfer Unit (MTU), Network Abstraction Layer (NAL), Output Layer Set (OLS), Picture Order Count (POC), Random Access Point (RAP), Raw Byte Sequence Payload (RBSP), Sequence Parameter Set (SPS), Video Parameter Set (VPS), Versatile Video Coding (VVC). 
     Many video compression techniques can be employed to reduce the size of video files with minimal loss of data. For example, video compression techniques can include performing spatial (e.g., intra-picture) prediction and/or temporal (e.g., inter-picture) prediction to reduce or remove data redundancy 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 treeblocks, coding tree blocks (CTBs), coding tree units (CTUs), coding units (CUs), and/or coding nodes. Video blocks in an intra-coded (I) slice of a picture are coded using spatial prediction with respect to reference samples in neighboring blocks in the same picture. Video blocks in an inter-coded unidirectional prediction (P) or bidirectional prediction (B) slice of a picture may be coded by employing 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/or images, and reference pictures may be referred to as reference frames and/or reference images. Spatial or temporal prediction results in a predictive block representing an image block. Residual data represents pixel differences between the original image block and the predictive block. Accordingly, an inter-coded block is encoded according to a motion vector that points to a block of reference samples forming the predictive block and the residual data indicating the difference between the coded block and the predictive block. An intra-coded block is encoded according to an intra-coding mode and the residual data. For further compression, the residual data may be transformed from the pixel domain to a transform domain. These result in residual transform coefficients, which may be quantized. The quantized transform coefficients may initially be arranged in a two-dimensional array. The quantized transform coefficients may be scanned in order to produce a one-dimensional vector of transform coefficients. Entropy coding may be applied to achieve even more compression. Such video compression techniques are discussed in greater detail below. 
     To ensure an encoded video can be accurately decoded, video is encoded and decoded according to corresponding video coding standards. Video coding standards include International Telecommunication Union (ITU) Standardization Sector (ITU-T) H.261, International Organization for Standardization/International Electrotechnical Commission (ISO/IEC) Motion Picture Experts Group (MPEG)-1 Part 2, ITU-T H.262 or ISO/IEC MPEG-2 Part 2, ITU-T H.263, ISO/IEC MPEG-4 Part 2, Advanced Video Coding (AVC), also known as ITU-T H.264 or ISO/IEC MPEG-4 Part 10, and High Efficiency Video Coding (HEVC), also known as ITU-T H.265 or MPEG-H Part 2. AVC includes extensions such as Scalable Video Coding (SVC), Multiview Video Coding (MVC) and Multiview Video Coding plus Depth (MVC+D), and three dimensional (3D) AVC (3D-AVC). HEVC includes extensions such as Scalable HEVC (SHVC), Multiview HEVC (MV-HEVC), and 3D HEVC (3D-HEVC). The joint video experts team (JVET) of ITU-T and ISO/IEC has begun developing a video coding standard referred to as Versatile Video Coding (VVC). VVC is included in a Working Draft (WD), which includes JVET-02001-v14. 
     The latest VVC draft provides specifics for picture timing (PT) SEI messages, decoding unit information (DUI) SEI messages, an AU-based HRD operation (e.g., an HRD operation that applies to the entire AU), and a decoding unit (DU)-based HRD operation (e.g., an HRD operation that applies to one decoding unit, or picture, in the AU). 
     Picture-level coded picture buffer (CPB) parameters needed for the AU-based HRD operations for both layers and sublayers are signaled in the PT SEI messages. Picture-level CPB parameters needed for DU-based HRD operations for layers are signaled in either the PT SEI message or the DUI SEI message. However, picture-level CPB parameters needed for the DU-based HRD operations for sublayers are missing from the PT SEI message and the DUI SEI message. 
     Disclosed herein are techniques that ensure picture-level CPB parameters corresponding to DU-based HRD operations on sublayers are included in an SEI message. Because the picture-level CPB parameters are included in the SEI message, the HRD can use the DU-based HRD operations to test the sublayers in the bitstream for bitstream conformance, which ensures that the sublayers are properly coded and/or can be properly decoded. Thus, the coder/decoder (a.k.a., “codec”) in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed. 
       FIG. 1  is a flowchart of an example operating method  100  of coding a video signal. Specifically, a video signal is encoded at an encoder. The encoding process compresses the video signal by employing various mechanisms to reduce the video file size. A smaller file size allows the compressed video file to be transmitted toward a user, while reducing associated bandwidth overhead. The decoder then decodes the compressed video file to reconstruct the original video signal for display to an end user. The decoding process generally mirrors the encoding process to allow the decoder to consistently reconstruct the video signal. 
     At step  101 , the video signal is input into the encoder. For example, the video signal may be an uncompressed video file stored in memory. As another example, the video file may be captured by a video capture device, such as a video camera, and encoded to support live streaming of the video. The video file may include both an audio component and a video component. The video component contains a series of image frames that, when viewed in a sequence, gives the visual impression of motion. The frames contain pixels that are expressed in terms of light, referred to herein as luma components (or luma samples), and color, which is referred to as chroma components (or color samples). In some examples, the frames may also contain depth values to support three dimensional viewing. 
     At step  103 , the video is partitioned into blocks. Partitioning includes subdividing the pixels in each frame into square and/or rectangular blocks for compression. For example, in High Efficiency Video Coding (HEVC) (also known as H.265 and MPEG-H Part 2) the frame can first be divided into coding tree units (CTUs), which are blocks of a predefined size (e.g., sixty-four pixels by sixty-four pixels). The CTUs contain both luma and chroma samples. Coding trees may be employed to divide the CTUs into blocks and then recursively subdivide the blocks until configurations are achieved that support further encoding. For example, luma components of a frame may be subdivided until the individual blocks contain relatively homogenous lighting values. Further, chroma components of a frame may be subdivided until the individual blocks contain relatively homogenous color values. Accordingly, partitioning mechanisms vary depending on the content of the video frames. 
     At step  105 , various compression mechanisms are employed to compress the image blocks partitioned at step  103 . For example, inter-prediction and/or intra-prediction may be employed. Inter-prediction is designed to take advantage of the fact that objects in a common scene tend to appear in successive frames. Accordingly, a block depicting an object in a reference frame need not be repeatedly described in adjacent frames. Specifically, an object, such as a table, may remain in a constant position over multiple frames. Hence the table is described once and adjacent frames can refer back to the reference frame. Pattern matching mechanisms may be employed to match objects over multiple frames. Further, moving objects may be represented across multiple frames, for example due to object movement or camera movement. As a particular example, a video may show an automobile that moves across the screen over multiple frames. Motion vectors can be employed to describe such movement. A motion vector is a two-dimensional vector that provides an offset from the coordinates of an object in a frame to the coordinates of the object in a reference frame. As such, inter-prediction can encode an image block in a current frame as a set of motion vectors indicating an offset from a corresponding block in a reference frame. 
     Intra-prediction encodes blocks in a common frame. Intra-prediction takes advantage of the fact that luma and chroma components tend to cluster in a frame. For example, a patch of green in a portion of a tree tends to be positioned adjacent to similar patches of green. Intra-prediction employs multiple directional prediction modes (e.g., thirty-three in HEVC), a planar mode, and a direct current (DC) mode. The directional modes indicate that a current block is similar/the same as samples of a neighbor block in a corresponding direction. Planar mode indicates that a series of blocks along a row/column (e.g., a plane) can be interpolated based on neighbor blocks at the edges of the row. Planar mode, in effect, indicates a smooth transition of light/color across a row/column by employing a relatively constant slope in changing values. DC mode is employed for boundary smoothing and indicates that a block is similar/the same as an average value associated with samples of all the neighbor blocks associated with the angular directions of the directional prediction modes. Accordingly, intra-prediction blocks can represent image blocks as various relational prediction mode values instead of the actual values. Further, inter-prediction blocks can represent image blocks as motion vector values instead of the actual values. In either case, the prediction blocks may not exactly represent the image blocks in some cases. Any differences are stored in residual blocks. Transforms may be applied to the residual blocks to further compress the file. 
     At step  107 , various filtering techniques may be applied. In HEVC, the filters are applied according to an in-loop filtering scheme. The block based prediction discussed above may result in the creation of blocky images at the decoder. Further, the block based prediction scheme may encode a block and then reconstruct the encoded block for later use as a reference block. The in-loop filtering scheme iteratively applies noise suppression filters, de-blocking filters, adaptive loop filters, and sample adaptive offset (SAO) filters to the blocks/frames. These filters mitigate such blocking artifacts so that the encoded file can be accurately reconstructed. Further, these filters mitigate artifacts in the reconstructed reference blocks so that artifacts are less likely to create additional artifacts in subsequent blocks that are encoded based on the reconstructed reference blocks. 
     Once the video signal has been partitioned, compressed, and filtered, the resulting data is encoded in a bitstream at step  109 . The bitstream includes the data discussed above as well as any signaling data desired to support proper video signal reconstruction at the decoder. For example, such data may include partition data, prediction data, residual blocks, and various flags providing coding instructions to the decoder. The bitstream may be stored in memory for transmission toward a decoder upon request. The bitstream may also be broadcast and/or multicast toward a plurality of decoders. The creation of the bitstream is an iterative process. Accordingly, steps  101 ,  103 ,  105 ,  107 , and  109  may occur continuously and/or simultaneously over many frames and blocks. The order shown in  FIG. 1  is presented for clarity and ease of discussion, and is not intended to limit the video coding process to a particular order. 
     The decoder receives the bitstream and begins the decoding process at step  111 . Specifically, the decoder employs an entropy decoding scheme to convert the bitstream into corresponding syntax and video data. The decoder employs the syntax data from the bitstream to determine the partitions for the frames at step  111 . The partitioning should match the results of block partitioning at step  103 . Entropy encoding/decoding as employed in step  111  is now described. The encoder makes many choices during the compression process, such as selecting block partitioning schemes from several possible choices based on the spatial positioning of values in the input image(s). Signaling the exact choices may employ a large number of bins. As used herein, a bin is a binary value that is treated as a variable (e.g., a bit value that may vary depending on context). Entropy coding allows the encoder to discard any options that are clearly not viable for a particular case, leaving a set of allowable options. Each allowable option is then assigned a code word. The length of the code words is based on the number of allowable options (e.g., one bin for two options, two bins for three to four options, etc.) The encoder then encodes the code word for the selected option. This scheme reduces the size of the code words as the code words are as big as desired to uniquely indicate a selection from a small sub-set of allowable options as opposed to uniquely indicating the selection from a potentially large set of all possible options. The decoder then decodes the selection by determining the set of allowable options in a similar manner to the encoder. By determining the set of allowable options, the decoder can read the code word and determine the selection made by the encoder. 
     At step  113 , the decoder performs block decoding. Specifically, the decoder employs reverse transforms to generate residual blocks. Then the decoder employs the residual blocks and corresponding prediction blocks to reconstruct the image blocks according to the partitioning. The prediction blocks may include both intra-prediction blocks and inter-prediction blocks as generated at the encoder at step  105 . The reconstructed image blocks are then positioned into frames of a reconstructed video signal according to the partitioning data determined at step  111 . Syntax for step  113  may also be signaled in the bitstream via entropy coding as discussed above. 
     At step  115 , filtering is performed on the frames of the reconstructed video signal in a manner similar to step  107  at the encoder. For example, noise suppression filters, de-blocking filters, adaptive loop filters, and SAO filters may be applied to the frames to remove blocking artifacts. Once the frames are filtered, the video signal can be output to a display at step  117  for viewing by an end user. 
       FIG. 2  is a schematic diagram of an example coding and decoding (codec) system  200  for video coding. Specifically, codec system  200  provides functionality to support the implementation of operating method  100 . Codec system  200  is generalized to depict components employed in both an encoder and a decoder. Codec system  200  receives and partitions a video signal as discussed with respect to steps  101  and  103  in operating method  100 , which results in a partitioned video signal  201 . Codec system  200  then compresses the partitioned video signal  201  into a coded bitstream when acting as an encoder as discussed with respect to steps  105 ,  107 , and  109  in method  100 . When acting as a decoder, codec system  200  generates an output video signal from the bitstream as discussed with respect to steps  111 ,  113 ,  115 , and  117  in operating method  100 . The codec system  200  includes a general coder control component  211 , a transform scaling and quantization component  213 , an intra-picture estimation component  215 , an intra-picture prediction component  217 , a motion compensation component  219 , a motion estimation component  221 , a scaling and inverse transform component  229 , a filter control analysis component  227 , an in-loop filters component  225 , a decoded picture buffer component  223 , and a header formatting and context adaptive binary arithmetic coding (CABAC) component  231 . Such components are coupled as shown. In  FIG. 2 , black lines indicate movement of data to be encoded/decoded while dashed lines indicate movement of control data that controls the operation of other components. The components of codec system  200  may all be present in the encoder. The decoder may include a subset of the components of codec system  200 . For example, the decoder may include the intra-picture prediction component  217 , the motion compensation component  219 , the scaling and inverse transform component  229 , the in-loop filters component  225 , and the decoded picture buffer component  223 . These components are now described. 
     The partitioned video signal  201  is a captured video sequence that has been partitioned into blocks of pixels by a coding tree. A coding tree employs various split modes to subdivide a block of pixels into smaller blocks of pixels. These blocks can then be further subdivided into smaller blocks. The blocks may be referred to as nodes on the coding tree. Larger parent nodes are split into smaller child nodes. The number of times a node is subdivided is referred to as the depth of the node/coding tree. The divided blocks can be included in coding units (CUs) in some cases. For example, a CU can be a sub-portion of a CTU that contains a luma block, red difference chroma (Cr) block(s), and a blue difference chroma (Cb) block(s) along with corresponding syntax instructions for the CU. The split modes may include a binary tree (BT), triple tree (TT), and a quad tree (QT) employed to partition a node into two, three, or four child nodes, respectively, of varying shapes depending on the split modes employed. The partitioned video signal  201  is forwarded to the general coder control component  211 , the transform scaling and quantization component  213 , the intra-picture estimation component  215 , the filter control analysis component  227 , and the motion estimation component  221  for compression. 
     The general coder control component  211  is configured to make decisions related to coding of the images of the video sequence into the bitstream according to application constraints. For example, the general coder control component  211  manages optimization of bitrate/bitstream size versus reconstruction quality. Such decisions may be made based on storage space/bandwidth availability and image resolution requests. The general coder control component  211  also manages buffer utilization in light of transmission speed to mitigate buffer underrun and overrun issues. To manage these issues, the general coder control component  211  manages partitioning, prediction, and filtering by the other components. For example, the general coder control component  211  may dynamically increase compression complexity to increase resolution and increase bandwidth usage or decrease compression complexity to decrease resolution and bandwidth usage. Hence, the general coder control component  211  controls the other components of codec system  200  to balance video signal reconstruction quality with bit rate concerns. The general coder control component  211  creates control data, which controls the operation of the other components. The control data is also forwarded to the header formatting and CABAC component  231  to be encoded in the bitstream to signal parameters for decoding at the decoder. 
     The partitioned video signal  201  is also sent to the motion estimation component  221  and the motion compensation component  219  for inter-prediction. A frame or slice of the partitioned video signal  201  may be divided into multiple video blocks. Motion estimation component  221  and the motion compensation component  219  perform inter-predictive coding of the received video block relative to one or more blocks in one or more reference frames to provide temporal prediction. Codec system  200  may perform multiple coding passes, e.g., to select an appropriate coding mode for each block of video data. 
     Motion estimation component  221  and motion compensation component  219  may be highly integrated, but are illustrated separately for conceptual purposes. Motion estimation, performed by motion estimation component  221 , is the process of generating motion vectors, which estimate motion for video blocks. A motion vector, for example, may indicate the displacement of a coded object relative to a predictive block. A predictive block is a block that is found to closely match the block to be coded, in terms of pixel difference. A predictive block may also be referred to as a reference block. Such pixel difference may be determined by sum of absolute difference (SAD), sum of square difference (SSD), or other difference metrics. HEVC employs several coded objects including a CTU, coding tree blocks (CTBs), and CUs. For example, a CTU can be divided into CTBs, which can then be divided into CBs for inclusion in CUs. A CU can be encoded as a prediction unit (PU) containing prediction data and/or a transform unit (TU) containing transformed residual data for the CU. The motion estimation component  221  generates motion vectors, PUs, and TUs by using a rate-distortion analysis as part of a rate distortion optimization process. For example, the motion estimation component  221  may determine multiple reference blocks, multiple motion vectors, etc. for a current block/frame, and may select the reference blocks, motion vectors, etc. having the best rate-distortion characteristics. The best rate-distortion characteristics balance both quality of video reconstruction (e.g., amount of data loss by compression) with coding efficiency (e.g., size of the final encoding). 
     In some examples, codec system  200  may calculate values for sub-integer pixel positions of reference pictures stored in decoded picture buffer component  223 . For example, video codec system  200  may interpolate values of one-quarter pixel positions, one-eighth pixel positions, or other fractional pixel positions of the reference picture. Therefore, motion estimation component  221  may perform a motion search relative to the full pixel positions and fractional pixel positions and output a motion vector with fractional pixel precision. The motion estimation component  221  calculates a motion vector for a PU of a video block in an inter-coded slice by comparing the position of the PU to the position of a predictive block of a reference picture. Motion estimation component  221  outputs the calculated motion vector as motion data to header formatting and CABAC component  231  for encoding and motion to the motion compensation component  219 . 
     Motion compensation, performed by motion compensation component  219 , may involve fetching or generating the predictive block based on the motion vector determined by motion estimation component  221 . Again, motion estimation component  221  and motion compensation component  219  may be functionally integrated, in some examples. Upon receiving the motion vector for the PU of the current video block, motion compensation component  219  may locate the predictive block to which the motion vector points. A residual video block is then formed by subtracting pixel values of the predictive block from the pixel values of the current video block being coded, forming pixel difference values. In general, motion estimation component  221  performs motion estimation relative to luma components, and motion compensation component  219  uses motion vectors calculated based on the luma components for both chroma components and luma components. The predictive block and residual block are forwarded to transform scaling and quantization component  213 . 
     The partitioned video signal  201  is also sent to intra-picture estimation component  215  and intra-picture prediction component  217 . As with motion estimation component  221  and motion compensation component  219 , intra-picture estimation component  215  and intra-picture prediction component  217  may be highly integrated, but are illustrated separately for conceptual purposes. The intra-picture estimation component  215  and intra-picture prediction component  217  intra-predict a current block relative to blocks in a current frame, as an alternative to the inter-prediction performed by motion estimation component  221  and motion compensation component  219  between frames, as described above. In particular, the intra-picture estimation component  215  determines an intra-prediction mode to use to encode a current block. In some examples, intra-picture estimation component  215  selects an appropriate intra-prediction mode to encode a current block from multiple tested intra-prediction modes. The selected intra-prediction modes are then forwarded to the header formatting and CABAC component  231  for encoding. 
     For example, the intra-picture estimation component  215  calculates rate-distortion values using a rate-distortion analysis for the various tested intra-prediction modes, and selects the intra-prediction mode having the best rate-distortion characteristics among the tested modes. Rate-distortion analysis generally determines an amount of distortion (or error) between an encoded block and an original unencoded block that was encoded to produce the encoded block, as well as a bitrate (e.g., a number of bits) used to produce the encoded block. The intra-picture estimation component  215  calculates ratios from the distortions and rates for the various encoded blocks to determine which intra-prediction mode exhibits the best rate-distortion value for the block. In addition, intra-picture estimation component  215  may be configured to code depth blocks of a depth map using a depth modeling mode (DMM) based on rate-distortion optimization (RDO). 
     The intra-picture prediction component  217  may generate a residual block from the predictive block based on the selected intra-prediction modes determined by intra-picture estimation component  215  when implemented on an encoder or read the residual block from the bitstream when implemented on a decoder. The residual block includes the difference in values between the predictive block and the original block, represented as a matrix. The residual block is then forwarded to the transform scaling and quantization component  213 . The intra-picture estimation component  215  and the intra-picture prediction component  217  may operate on both luma and chroma components. 
     The transform scaling and quantization component  213  is configured to further compress the residual block. The transform scaling and quantization component  213  applies a transform, such as a discrete cosine transform (DCT), a discrete sine transform (DST), or a conceptually similar transform, to the residual block, producing a video block comprising residual transform coefficient values. Wavelet transforms, integer transforms, sub-band transforms or other types of transforms could also be used. The transform may convert the residual information from a pixel value domain to a transform domain, such as a frequency domain. The transform scaling and quantization component  213  is also configured to scale the transformed residual information, for example based on frequency. Such scaling involves applying a scale factor to the residual information so that different frequency information is quantized at different granularities, which may affect final visual quality of the reconstructed video. The transform scaling and quantization component  213  is also configured to quantize the transform coefficients to further reduce bit rate. The quantization process may reduce the bit depth associated with some or all of the coefficients. The degree of quantization may be modified by adjusting a quantization parameter. In some examples, the transform scaling and quantization component  213  may then perform a scan of the matrix including the quantized transform coefficients. The quantized transform coefficients are forwarded to the header formatting and CABAC component  231  to be encoded in the bitstream. 
     The scaling and inverse transform component  229  applies a reverse operation of the transform scaling and quantization component  213  to support motion estimation. The scaling and inverse transform component  229  applies inverse scaling, transformation, and/or quantization to reconstruct the residual block in the pixel domain, e.g., for later use as a reference block which may become a predictive block for another current block. The motion estimation component  221  and/or motion compensation component  219  may calculate a reference block by adding the residual block back to a corresponding predictive block for use in motion estimation of a later block/frame. Filters are applied to the reconstructed reference blocks to mitigate artifacts created during scaling, quantization, and transform. Such artifacts could otherwise cause inaccurate prediction (and create additional artifacts) when subsequent blocks are predicted. 
     The filter control analysis component  227  and the in-loop filters component  225  apply the filters to the residual blocks and/or to reconstructed image blocks. For example, the transformed residual block from the scaling and inverse transform component  229  may be combined with a corresponding prediction block from intra-picture prediction component  217  and/or motion compensation component  219  to reconstruct the original image block. The filters may then be applied to the reconstructed image block. In some examples, the filters may instead be applied to the residual blocks. As with other components in  FIG. 2 , the filter control analysis component  227  and the in-loop filters component  225  are highly integrated and may be implemented together, but are depicted separately for conceptual purposes. Filters applied to the reconstructed reference blocks are applied to particular spatial regions and include multiple parameters to adjust how such filters are applied. The filter control analysis component  227  analyzes the reconstructed reference blocks to determine where such filters should be applied and sets corresponding parameters. Such data is forwarded to the header formatting and CABAC component  231  as filter control data for encoding. The in-loop filters component  225  applies such filters based on the filter control data. The filters may include a deblocking filter, a noise suppression filter, a SAO filter, and an adaptive loop filter. Such filters may be applied in the spatial/pixel domain (e.g., on a reconstructed pixel block) or in the frequency domain, depending on the example. 
     When operating as an encoder, the filtered reconstructed image block, residual block, and/or prediction block are stored in the decoded picture buffer component  223  for later use in motion estimation as discussed above. When operating as a decoder, the decoded picture buffer component  223  stores and forwards the reconstructed and filtered blocks toward a display as part of an output video signal. The decoded picture buffer component  223  may be any memory device capable of storing prediction blocks, residual blocks, and/or reconstructed image blocks. 
     The header formatting and CABAC component  231  receives the data from the various components of codec system  200  and encodes such data into a coded bitstream for transmission toward a decoder. Specifically, the header formatting and CABAC component  231  generates various headers to encode control data, such as general control data and filter control data. Further, prediction data, including intra-prediction and motion data, as well as residual data in the form of quantized transform coefficient data are all encoded in the bitstream. The final bitstream includes all information desired by the decoder to reconstruct the original partitioned video signal  201 . Such information may also include intra-prediction mode index tables (also referred to as codeword mapping tables), definitions of encoding contexts for various blocks, indications of most probable intra-prediction modes, an indication of partition information, etc. Such data may be encoded by employing entropy coding. For example, the information may be encoded by employing context adaptive variable length coding (CAVLC), CABAC, syntax-based context-adaptive binary arithmetic coding (SBAC), probability interval partitioning entropy (PIPE) coding, or another entropy coding technique. Following the entropy coding, the coded bitstream may be transmitted to another device (e.g., a video decoder) or archived for later transmission or retrieval. 
       FIG. 3  is a block diagram illustrating an example video encoder  300 . Video encoder  300  may be employed to implement the encoding functions of codec system  200  and/or implement steps  101 ,  103 ,  105 ,  107 , and/or  109  of operating method  100 . Encoder  300  partitions an input video signal, resulting in a partitioned video signal  301 , which is substantially similar to the partitioned video signal  201 . The partitioned video signal  301  is then compressed and encoded into a bitstream by components of encoder  300 . 
     Specifically, the partitioned video signal  301  is forwarded to an intra-picture prediction component  317  for intra-prediction. The intra-picture prediction component  317  may be substantially similar to intra-picture estimation component  215  and intra-picture prediction component  217 . The partitioned video signal  301  is also forwarded to a motion compensation component  321  for inter-prediction based on reference blocks in a decoded picture buffer component  323 . The motion compensation component  321  may be substantially similar to motion estimation component  221  and motion compensation component  219 . The prediction blocks and residual blocks from the intra-picture prediction component  317  and the motion compensation component  321  are forwarded to a transform and quantization component  313  for transform and quantization of the residual blocks. The transform and quantization component  313  may be substantially similar to the transform scaling and quantization component  213 . The transformed and quantized residual blocks and the corresponding prediction blocks (along with associated control data) are forwarded to an entropy coding component  331  for coding into a bitstream. The entropy coding component  331  may be substantially similar to the header formatting and CABAC component  231 . 
     The transformed and quantized residual blocks and/or the corresponding prediction blocks are also forwarded from the transform and quantization component  313  to an inverse transform and quantization component  329  for reconstruction into reference blocks for use by the motion compensation component  321 . The inverse transform and quantization component  329  may be substantially similar to the scaling and inverse transform component  229 . In-loop filters in an in-loop filters component  325  are also applied to the residual blocks and/or reconstructed reference blocks, depending on the example. The in-loop filters component  325  may be substantially similar to the filter control analysis component  227  and the in-loop filters component  225 . The in-loop filters component  325  may include multiple filters as discussed with respect to in-loop filters component  225 . The filtered blocks are then stored in a decoded picture buffer component  323  for use as reference blocks by the motion compensation component  321 . The decoded picture buffer component  323  may be substantially similar to the decoded picture buffer component  223 . 
       FIG. 4  is a block diagram illustrating an example video decoder  400 . Video decoder  400  may be employed to implement the decoding functions of codec system  200  and/or implement steps  111 ,  113 ,  115 , and/or  117  of operating method  100 . Decoder  400  receives a bitstream, for example from an encoder  300 , and generates a reconstructed output video signal based on the bitstream for display to an end user. 
     The bitstream is received by an entropy decoding component  433 . The entropy decoding component  433  is configured to implement an entropy decoding scheme, such as CAVLC, CABAC, SBAC, PIPE coding, or other entropy coding techniques. For example, the entropy decoding component  433  may employ header information to provide a context to interpret additional data encoded as codewords in the bitstream. The decoded information includes any desired information to decode the video signal, such as general control data, filter control data, partition information, motion data, prediction data, and quantized transform coefficients from residual blocks. The quantized transform coefficients are forwarded to an inverse transform and quantization component  429  for reconstruction into residual blocks. The inverse transform and quantization component  429  may be similar to inverse transform and quantization component  329 . 
     The reconstructed residual blocks and/or prediction blocks are forwarded to intra-picture prediction component  417  for reconstruction into image blocks based on intra-prediction operations. The intra-picture prediction component  417  may be similar to an intra-picture estimation component  215  and an intra-picture prediction component  217 . Specifically, the intra-picture prediction component  417  employs prediction modes to locate a reference block in the frame and applies a residual block to the result to reconstruct intra-predicted image blocks. The reconstructed intra-predicted image blocks and/or the residual blocks and corresponding inter-prediction data are forwarded to a decoded picture buffer component  423  via an in-loop filters component  425 , which may be substantially similar to decoded picture buffer component  223  and in-loop filters component  225 , respectively. The in-loop filters component  425  filters the reconstructed image blocks, residual blocks and/or prediction blocks, and such information is stored in the decoded picture buffer component  423 . Reconstructed image blocks from decoded picture buffer component  423  are forwarded to a motion compensation component  421  for inter-prediction. The motion compensation component  421  may be substantially similar to motion estimation component  221  and/or motion compensation component  219 . Specifically, the motion compensation component  421  employs motion vectors from a reference block to generate a prediction block and applies a residual block to the result to reconstruct an image block. The resulting reconstructed blocks may also be forwarded via the in-loop filters component  425  to the decoded picture buffer component  423 . The decoded picture buffer component  423  continues to store additional reconstructed image blocks, which can be reconstructed into frames via the partition information. Such frames may also be placed in a sequence. The sequence is output toward a display as a reconstructed output video signal. 
       FIG. 5  is a schematic diagram illustrating an example HRD  500 . A HRD  500  may be employed in an encoder, such as codec system  200  and/or encoder  300 . The HRD  500  may check the bitstream created at step  109  of method  100  before the bitstream is forwarded to a decoder, such as decoder  400 . In some examples, the bitstream may be continuously forwarded through the HRD  500  as the bitstream is encoded. In the event that a portion of the bitstream fails to conform to associated constraints, the HRD  500  can indicate such failure to an encoder to cause the encoder to re-encode the corresponding section of the bitstream with different mechanisms. 
     The HRD  500  includes a hypothetical stream scheduler (HSS)  541 . A HSS  541  is a component configured to perform a hypothetical delivery mechanism. The hypothetical delivery mechanism is used for checking the conformance of a bitstream or a decoder with regards to the timing and data flow of a bitstream  551  input into the HRD  500 . For example, the HSS  541  may receive a bitstream  551  output from an encoder and manage the conformance testing process on the bitstream  551 . In a particular example, the HSS  541  can control the rate that coded pictures move through the HRD  500  and verify that the bitstream  551  does not contain non-conforming data. 
     The HSS  541  may forward the bitstream  551  to a CPB  543  at a predefined rate. The HRD  500  may manage data in decoding units (DU)  553 . A DU  553  is an AU or a sub-set of an AU and associated non-video coding layer (VCL) network abstraction layer (NAL) units. Specifically, an AU contains one or more pictures associated with an output time. For example, an AU may contain a single picture in a single layer bitstream, and may contain a picture for each layer in a multi-layer bitstream. Each picture of an AU may be divided into slices that are each included in a corresponding VCL NAL unit. Hence, a DU  553  may contain one or more pictures, one or more slices of a picture, or combinations thereof. Also, parameters used to decode the AU, pictures, and/or slices can be included in non-VCL NAL units. As such, the DU  553  contains non-VCL NAL units that contain data needed to support decoding the VCL NAL units in the DU  553 . The CPB  543  is a first-in first-out buffer in the HRD  500 . The CPB  543  contains DUs  553  including video data in decoding order. The CPB  543  stores the video data for use during bitstream conformance verification. 
     The CPB  543  forwards the DUs  553  to a decoding process component  545 . The decoding process component  545  is a component that conforms to the VVC standard. For example, the decoding process component  545  may emulate a decoder  400  employed by an end user. The decoding process component  545  decodes the DUs  553  at a rate that can be achieved by an example end user decoder. If the decoding process component  545  cannot decode the DUs  553  fast enough to prevent an overflow of the CPB  543 , then the bitstream  551  does not conform to the standard and should be re-encoded. 
     The decoding process component  545  decodes the DUs  553 , which creates decoded DUs  555 . A decoded DU  555  contains a decoded picture. The decoded DUs  555  are forwarded to a DPB  547 . The DPB  547  may be substantially similar to a decoded picture buffer component  223 ,  323 , and/or  423 . To support inter-prediction, pictures that are marked for use as reference pictures  556  that are obtained from the decoded DUs  555  are returned to the decoding process component  545  to support further decoding. The DPB  547  outputs the decoded video sequence as a series of pictures  557 . The pictures  557  are reconstructed pictures that generally mirror pictures encoded into the bitstream  551  by the encoder. 
     The pictures  557  are forwarded to an output cropping component  549 . The output cropping component  549  is configured to apply a conformance cropping window to the pictures  557 . This results in output cropped pictures  559 . An output cropped picture  559  is a completely reconstructed picture. Accordingly, the output cropped picture  559  mimics what an end user would see upon decoding the bitstream  551 . As such, the encoder can review the output cropped pictures  559  to ensure the encoding is satisfactory. 
     The HRD  500  is initialized based on HRD parameters in the bitstream  551 . For example, the HRD  500  may read HRD parameters from a VPS, a SPS, and/or SEI messages. The HRD  500  may then perform conformance testing operations on the bitstream  551  based on the information in such HRD parameters. As a specific example, the HRD  500  may determine one or more CPB delivery schedules  561  from the HRD parameters. A delivery schedule specifies timing for delivery of video data to and/or from a memory location, such as a CPB and/or a DPB. Hence, a CPB delivery schedule  561  specifies timing for delivery of AUs, DUs  553 , and/or pictures, to/from the CPB  543 . It should be noted that the HRD  500  may employ DPB delivery schedules for the DPB  547  that are similar to the CPB delivery schedules  561 . 
     Video may be coded into different layers and/or OLSs for use by decoders with varying levels of hardware capabilities as well for varying network conditions. The CPB delivery schedules  561  are selected to reflect these issues. Accordingly, higher layer sub-bitstreams are designated for optimal hardware and network conditions and hence higher layers may receive one or more CPB delivery schedules  561  that employ a large amount of memory in the CPB  543  and short delays for transfers of the DUs  553  toward the DPB  547 . Likewise, lower layer sub-bitstreams are designated for limited decoder hardware capabilities and/or poor network conditions. Hence, lower layers may receive one or more CPB delivery schedules  561  that employ a small amount of memory in the CPB  543  and longer delays for transfers of the DUs  553  toward the DPB  547 . The OLSs, layers, sublayers, or combinations thereof can then be tested according to the corresponding delivery schedule  561  to ensure that the resulting sub-bitstream can be correctly decoded under the conditions that are expected for the sub-bitstream. The CPB delivery schedules  561  are each associated with a schedule index (ScIdx)  563 . A ScIdx  563  is an index that identifies a delivery schedule. Accordingly, the HRD parameters in the bitstream  551  can indicate the CPB delivery schedules  561  by ScIdx  563  as well as include sufficient data to allow the HRD  500  to determine the CPB delivery schedules  561  and correlate the CPB delivery schedules  561  to the corresponding OLSs, layers, and/or sublayers. 
       FIG. 6  is a schematic diagram illustrating an example multi-layer video sequence  600  configured for inter-layer prediction  621 . The multi-layer video sequence  600  may be encoded by an encoder, such as codec system  200  and/or encoder  300  and decoded by a decoder, such as codec system  200  and/or decoder  400 , for example according to method  100 . Further, the multi-layer video sequence  600  can be checked for standard conformance by a HRD, such as HRD  500 . The multi-layer video sequence  600  is included to depict an example application for layers in a coded video sequence. A multi-layer video sequence  600  is any video sequence that employs a plurality of layers, such as layer N  631  and layer N+1  632 . 
     In an example, the multi-layer video sequence  600  may employ inter-layer prediction  621 . Inter-layer prediction  621  is applied between pictures  611 ,  612 ,  613 , and  614  and pictures  615 ,  616 ,  617 , and  618  in different layers. In the example shown, pictures  611 ,  612 ,  613 , and  614  are part of layer N+1  632  and pictures  615 ,  616 ,  617 , and  618  are part of layer N  631 . A layer, such as layer N  631  and/or layer N+1  632 , is a group of pictures that are all associated with a similar value of a characteristic, such as a similar size, quality, resolution, signal to noise ratio, capability, etc. A layer may be defined formally as a set of VCL NAL units and associated non-VCL NAL units. A VCL NAL unit is a NAL unit coded to contain video data, such as a coded slice of a picture. A non-VCL NAL unit is a NAL unit that contains non-video data such as syntax and/or parameters that support decoding the video data, performance of conformance checking, or other operations. 
     In the example shown, layer N+1  632  is associated with a larger image size than layer N  631 . Accordingly, pictures  611 ,  612 ,  613 , and  614  in layer N+1  632  have a larger picture size (e.g., larger height and width and hence more samples) than pictures  615 ,  616 ,  617 , and  618  in layer N  631  in this example. However, such pictures can be separated between layer N+1  632  and layer N  631  by other characteristics. While only two layers, layer N+1  632  and layer N  631 , are shown, a set of pictures can be separated into any number of layers based on associated characteristics. Layer N+1  632  and layer N  631  may also be denoted by a layer ID. A layer ID is an item of data that is associated with a picture and denotes the picture is part of an indicated layer. Accordingly, each picture  611 - 618  may be associated with a corresponding layer ID to indicate which layer N+1  632  or layer N  631  includes the corresponding picture. For example, a layer ID may include a NAL unit header layer identifier (nuh layer id), which is a syntax element that specifies an identifier of a layer that includes a NAL unit (e.g., that include slices and/or parameters of the pictures in a layer). A layer associated with a lower quality/bitstream size, such as layer N  631 , is generally assigned a lower layer ID and is referred to as a lower layer. Further, a layer associated with a higher quality/bitstream size, such as layer N+1  632 , is generally assigned a higher layer ID and is referred to as a higher layer. 
     Pictures  611 - 618  in different layers  631 - 632  are configured to be displayed in the alternative. As such, pictures in different layers  631 - 632  can share a temporal ID  622  as long as the pictures are included in the same AU. A temporal ID  622  is a data element that indicates data corresponds to temporal location in a video sequence. An AU is a set of NAL units that are associated with each other according to a specified classification rule and pertain to one particular output time. For example, an AU may include one or more pictures in different layers, such as picture  611  and picture  615  when such pictures are associated with the same temporal ID  622 . As a specific example, a decoder may decode and display picture  615  at a current display time if a smaller picture is desired or the decoder may decode and display picture  611  at the current display time if a larger picture is desired. As such, pictures  611 - 614  at higher layer N+1  632  contain substantially the same image data as corresponding pictures  615 - 618  at lower layer N  631  (notwithstanding the difference in picture size). Specifically, picture  611  contains substantially the same image data as picture  615 , picture  612  contains substantially the same image data as picture  616 , etc. 
     Pictures  611 - 618  can be coded by reference to other pictures  611 - 618  in the same layer N  631  or N+1  632 . Coding a picture in reference to another picture in the same layer results in inter-prediction  623 . Inter-prediction  623  is depicted by solid line arrows. For example, picture  613  may be coded by employing inter-prediction  623  using one or two of pictures  611 ,  612 , and/or  614  in layer N+1  632  as a reference, where one picture is referenced for unidirectional inter-prediction and/or two pictures are referenced for bidirectional inter-prediction. Further, picture  617  may be coded by employing inter-prediction  623  using one or two of pictures  615 ,  616 , and/or  618  in layer N  631  as a reference, where one picture is referenced for unidirectional inter-prediction and/or two pictures are referenced for bidirectional inter-prediction. When a picture is used as a reference for another picture in the same layer when performing inter-prediction  623 , the picture may be referred to as a reference picture. For example, picture  612  may be a reference picture used to code picture  613  according to inter-prediction  623 . Inter-prediction  623  can also be referred to as intra-layer prediction in a multi-layer context. As such, inter-prediction  623  is a mechanism of coding samples of a current picture by reference to indicated samples in a reference picture that is different from the current picture where the reference picture and the current picture are in the same layer. 
     Pictures  611 - 618  can also be coded by reference to other pictures  611 - 618  in different layers. This process is known as inter-layer prediction  621 , and is depicted by dashed arrows. Inter-layer prediction  621  is a mechanism of coding samples of a current picture by reference to indicated samples in a reference picture where the current picture and the reference picture are in different layers and hence have different layer IDs. For example, a picture in a lower layer N  631  can be used as a reference picture to code a corresponding picture at a higher layer N+1  632 . As a specific example, picture  611  can be coded by reference to picture  615  according to inter-layer prediction  621 . In such a case, the picture  615  is used as an inter-layer reference picture. An inter-layer reference picture is a reference picture used for inter-layer prediction  621 . In most cases, inter-layer prediction  621  is constrained such that a current picture, such as picture  611 , can only use inter-layer reference picture(s) that are included in the same AU and that are at a lower layer, such as picture  615 . When multiple layers (e.g., more than two) are available, inter-layer prediction  621  can encode/decode a current picture based on multiple inter-layer reference picture(s) at lower levels than the current picture. 
     A video encoder can employ a multi-layer video sequence  600  to encode pictures  611 - 618  via many different combinations and/or permutations of inter-prediction  623  and inter-layer prediction  621 . For example, picture  615  may be coded according to intra-prediction. Pictures  616 - 618  can then be coded according to inter-prediction  623  by using picture  615  as a reference picture. Further, picture  611  may be coded according to inter-layer prediction  621  by using picture  615  as an inter-layer reference picture. Pictures  612 - 614  can then be coded according to inter-prediction  623  by using picture  611  as a reference picture. As such, a reference picture can serve as both a single layer reference picture and an inter-layer reference picture for different coding mechanisms. By coding higher layer N+1  632  pictures based on lower layer N  631  pictures, the higher layer N+1  632  can avoid employing intra-prediction, which has much lower coding efficiency than inter-prediction  623  and inter-layer prediction  621 . As such, the poor coding efficiency of intra-prediction can be limited to the smallest/lowest quality pictures, and hence limited to coding the smallest amount of video data. The pictures used as reference pictures and/or inter-layer reference pictures can be indicated in entries of reference picture list(s) contained in a reference picture list structure. 
     In order to perform such operations, layers such as layer N  631  and layer N+1  632  may be included in an OLS  625 . An OLS  625  is a set of layers for which one or more layers are specified as an output layer. An output layer is a layer that is designated for output (e.g., to a display). For example, layer N  631  may be included solely to support inter-layer prediction  621  and may never be output. In such a case, layer N+1  632  is decoded based on layer N  631  and is output. In such a case, the OLS  625  includes layer N+1  632  as the output layer. When an OLS  625  contains only an output layer, the OLS  625  is referred to as a 0-th OLS. A 0-th OLS is an OLS that contains only a lowest layer (layer with a lowest layer identifier) and hence contains only an output layer. In other cases, an OLS  625  may contain many layers in different combinations. For example, an output layer in an OLS  625  can be coded according to inter-layer prediction  621  based on a one, two, or many lower layers. Further, an OLS  625  may contain more than one output layer. Hence, an OLS  625  may contain one or more output layers and any supporting layers needed to reconstruct the output layers. A multi-layer video sequence  600  can be coded by employing many different OLSs  625  that each employ different combinations of the layers. The OLSs  625  are each associated with an OLS index  629 , which is an index that uniquely identifies a corresponding OLS  625 . 
     Checking a multi-layer video sequence  600  for standards conformance at a HRD  500  can become complicated depending on the number of layers  631 - 632  and OLSs  625 . A HRD  500  may segregate the multi-layer video sequence  600  into a sequence of operation points  627  for testing. An operation point  627  is a temporal subset of an OLS  625  that is identified by an OLS index  629  and a highest temporal ID  622 . As a specific example, a first operation point  627  could include all pictures in a first OLS  625  from temporal ID zero to temporal ID two hundred, a second operation point  627  could include all pictures in the first OLS  625  from temporal ID two hundred and one to temporal ID four hundred, etc. The operation point  627  selected for testing at a specified instant is referred to as an OP under test (targetOp). Hence, a targetOp is an operation point  627  that is selected for conformance testing at a HRD  500 . 
       FIG. 7  is a schematic diagram illustrating an example multi-layer video sequence  700  configured for temporal scalability. The multi-layer video sequence  700  may be encoded by an encoder, such as codec system  200  and/or encoder  300  and decoded by a decoder, such as codec system  200  and/or decoder  400 , for example according to method  100 . Further, the multi-layer video sequence  700  can be checked for standard conformance by a HRD, such as HRD  500 . The multi-layer video sequence  700  is included to depict another example application for layers in a coded video sequence. For example, the multi-layer video sequence  700  may be employed as a separate embodiment or may be combined with the techniques described with respect to the multi-layer video sequence  600 . 
     The multi-layer video sequence  700  includes sublayers  710 ,  720 , and  730 . A sublayer is a temporal scalable layer of a temporal scalable bitstream that includes VCL NAL units (e.g., pictures) with a particular temporal identifier value as well as associated non-VCL NAL units (e.g., supporting parameters). The sublayer  710  may be referred to as a base layer and sublayers  720  and  730  may be referred to as enhancement layers. As shown, the sublayer  710  includes pictures  711  at a first frame rate, such as thirty frames per second. The sublayer  710  is a base layer because the sublayer  710  includes the base/lowest frame rate. The sublayer  720  contains pictures  721  that are temporally offset from the pictures  711  of sublayer  710 . The result is that sublayer  710  and sublayer  720  can be combined, which results in a frame rate that is collectively higher than the frame rate of the sublayer  710  alone. For example, sublayer  710  and  720  may have a combined frame rate of sixty frames per second. Accordingly, the sublayer  720  enhances the frame rate of the sublayer  710 . Further, sublayer  730  contains pictures  731  that are also temporally offset from the pictures  721  and  711  of sublayers  720  and  710 . As such, the sublayer  730  can be combined with sublayers  720  and  710  to further enhance the sublayer  710 . For example, the sublayers  710 ,  720 , and  730  may have a combined frame rate of ninety frames per second. 
     A sublayer representation  740  can be dynamically created by combining sublayers  710 ,  720 , and/or  730 . A sublayer representation  740  is a subset of a bitstream containing NAL units of a particular sublayer and the lower sublayers. In the example shown, the sublayer representation  740  contains pictures  741 , which are the combined pictures  711 ,  721 , and  731  of sublayers  710 ,  720 , and  730 . Accordingly, the multi-layer video sequence  700  can be temporally scaled to a desired frame rate by selecting a sublayer representation  740  that includes a desired set of sublayers  710 ,  720 , and/or  730 . A sublayer representation  740  may be created by employing an OLS that includes sublayer  710 ,  720 , and/or  730  as layers. In such a case, the sublayer representation  740  is selected as an output layer. As such, temporal scalability is one of several mechanisms that can be accomplished using multi-layer mechanisms. 
       FIG. 8  is a schematic diagram illustrating an example bitstream  800 . For example, the bitstream  800  can be generated by a codec system  200  and/or an encoder  300  for decoding by a codec system  200  and/or a decoder  400  according to method  100 . Further, the bitstream  800  may include a multi-layer video sequence  600  and/or  700 . In addition, the bitstream  800  may include various parameters to control the operation of an HRD, such as HRD  500 . Based on such parameters, the HRD can check the bitstream  800  for conformance with standards prior to transmission toward a decoder for decoding. 
     The bitstream  800  includes a VPS  811 , one or more SPSs  813 , a plurality of picture parameter sets (PPSs)  815 , a plurality of slice headers  817 , image data  820 , a BP SEI message  819 , a PT SEI message  818 , and a DUI SEI message  816 . A VPS  811  contains data related to the entire bitstream  800 . For example, the VPS  811  may contain data related OLSs, layers, and/or sublayers used in the bitstream  800 . An SPS  813  contains sequence data common to all pictures in a coded video sequence contained in the bitstream  800 . For example, each layer may contain one or more coded video sequences, and each coded video sequence may reference a SPS  813  for corresponding parameters. The parameters in a SPS  813  can include picture sizing, bit depth, coding tool parameters, bit rate restrictions, etc. It should be noted that, while each sequence refers to a SPS  813 , a single SPS  813  can contain data for multiple sequences in some examples. The PPS  815  contains parameters that apply to an entire picture. Hence, each picture in the video sequence may refer to a PPS  815 . It should be noted that, while each picture refers to a PPS  815 , a single PPS  815  can contain data for multiple pictures in some examples. For example, multiple similar pictures may be coded according to similar parameters. In such a case, a single PPS  815  may contain data for such similar pictures. The PPS  815  can indicate coding tools available for slices in corresponding pictures, quantization parameters, offsets, etc. 
     The slice header  817  contains parameters that are specific to each slice in a picture. Hence, there may be one slice header  817  per slice in the video sequence. The slice header  817  may contain slice type information, POCs, reference picture lists, prediction weights, tile entry points, deblocking parameters, etc. It should be noted that in some examples, a bitstream  800  may also include a picture header, which is a syntax structure that contains parameters that apply to all slices in a single picture. For this reason, a picture header and a slice header  817  may be used interchangeably in some contexts. For example, certain parameters may be moved between the slice header  817  and a picture header depending on whether such parameters are common to all slices in a picture. 
     The image data  820  contains video data encoded according to inter-prediction and/or intra-prediction as well as corresponding transformed and quantized residual data. For example, the image data  820  may include AUs  821 , DUs  822 , and/or pictures  823 . An AU  821  is a set of NAL units that are associated with each other according to a specified classification rule and pertain to one particular output time. A DU  822  is an AU or a sub-set of an AU and associated non-VCL NAL units. A picture  823  is an array of luma samples and/or an array of chroma samples that create a frame or a field thereof. In plain language, an AU  821  contains various video data that may be displayed at a specified instant in a video sequence as well as supporting syntax data. Hence, an AU  821  may contain a single picture  823  in a single layer bitstream or multiple pictures from multiple layers that are all associated with the same instant in a multi-layer bitstream. Meanwhile, a picture  823  is a coded image that may be output for display or used to support coding of other picture(s)  823  for output. A DU  822  may contain one or more pictures  823  and any supporting syntax data needed for decoding. For example, a DU  822  and an AU  821  may be used interchangeably in simple bitstreams (e.g., when an AU contains a single picture). However, in more complex multi-layer bitstreams (e.g., the bitstream containing the multi-layer video sequence  600 ), a DU  822  may only contain a portion of the video data from an AU  821 . For example, an AU  821  may contain pictures  823  at several layers (e.g., layers  631 ,  632 ) and/or sublayers (e.g., sublayers  710 ,  720 ,  730 ) where some of the pictures  823  are associated with different OLSs. In such a case, a DU  822  may only contain picture(s)  823  from a specified OLS and/or a specified layer/sublayer. 
     A picture  823  contains one or more slices  825 . A slice  825  may be defined as an integer number of complete tiles or an integer number of consecutive complete coding tree unit (CTU) rows (e.g., within a tile) of a picture  823  that are exclusively contained in a single NAL unit  829 . The slices  825  are further divided into CTUs and/or coding tree blocks (CTBs). A CTU is a group of samples of a predefined size that can be partitioned by a coding tree. A CTB is a subset of a CTU and contains luma components or chroma components of the CTU. The CTUs/CTBs are further divided into coding blocks based on coding trees. The coding blocks can then be encoded/decoded according to prediction mechanisms. 
     A bitstream  800  is a sequence of NAL units  829 . A NAL unit  829  is a container for video data and/or supporting syntax. A NAL unit  829  can be a VCL NAL unit or a non-VCL NAL unit. A VCL NAL unit is a NAL unit  829  coded to contain video data, such as a coded slice  825  and an associated slice header  817 . A non-VCL NAL unit is a NAL unit  829  that contains non-video data such as syntax and/or parameters that support decoding the video data, performance of conformance checking, or other operations. For example, a non-VCL NAL unit can contain a VPS  811 , a SPS  813 , a PPS  815 , a BP SEI message  819 , a PT SEI message  818 , a DUI SEI message  816 , or other supporting syntax. 
     The bitstream  800  can include one or more SEI messages that support conformance testing by an HRD, such as HRD  500 . An SEI message is a syntax structure with specified semantics that conveys information not needed by the decoding process in order to determine the values of the samples in decoded pictures. For example, the SEI messages may contain data to support HRD processes or other supporting data that is not directly relevant to decoding the bitstream  800  at a decoder. For example, bitstream  800  may include a BP SEI message  819 , a PT SEI message  818 , and a DUI SEI message  816 . 
     A BP SEI message  819  is a SEI message that contains HRD parameters  870  for initializing a HRD to manage a CPB. For example, the BP SEI message  819  may contain data describing the CPB delivery schedules, such as CPB delivery schedule  561 , that may be employed when performing conformance tests on the bitstream  800 . A delivery schedule may be described by a pair of values describing the timing of the delivery schedule (e.g., how often to remove data) and describing the amount of data to be transferred (e.g., how much data to remove at each occurrence). The BP SEI message  819  indicates the AU or DU that should be the starting point of the conformance check (e.g., an AU  821  or a DU  822 ) and a data pair indicating the default schedule to use for each data unit. In a specific example, the BP SEI message  819  may include an initial CPB removal delay  837  and an initial CPB removal offset  839 . An initial CPB removal delay  837  is a default CPB removal delay for each picture, AU, and/or DU in a bitstream, OLS, and/or layer. An initial CPB removal offset  839  is a default CPB removal offset associated with each picture, AU, and/or DU in a bitstream, OLS, and/or layer. By employing the initial CPB removal delay  837  and the initial CPB removal offset  839  pair, a HRD can determine a CPB delivery schedule to use when removing data units (AUs or DUs) from the CPB during conformance testing. 
     In an embodiment, the BP SEI message  819  includes a maximum number of temporal sublayers  841  for which the initial CPB removal delay  837  and the initial CPB removal offset  839  are indicated in the BP SEI message  819 . This maximum number of temporal sublayers  841  is designated bp_max_sublayers_minus1. The value of bp_max_sublayers_minus1 shall be in the range of 0 to a maximum number of sublayers  843  specified in the VPS  811 , which is designated vps_max_sublayers_minus1, inclusive. vps_max_sublayers_minus1 plus 1 specifies the maximum number of temporal sublayers that may be present in a layer specified by the VPS  811 . The value of vps_max_sublayers_minus1 shall be in the range of 0 to 6, inclusive. 
     A PT SEI message  818  is a SEI message that contains HRD parameters  880  (a.k.a., picture level CPB parameters) for managing delivery information for AUs at the CPB and/or the DPB. For example, a PT SEI message  818  may contain additional parameters for use in performing a HRD conformance test on a corresponding AU. In a specific example, the PT SEI message  818  may contain a CPB removal delay  835  and a DPB output delay  833 . A CPB removal delay  835  is period of time that a corresponding current AU can remain in the CPB prior to removal and output to a DPB. For example, the CPB removal delay  835  may be used to calculate the number of clock ticks between the removal of the current AU and a preceding AU in decoding order where the preceding AU is associated with a BP SEI message  819 . Accordingly, the CPB removal delay  835  indicates that a removal delay for a current AU is different than the default removal delay described by the initial CPB removal delay  837  in the BP SEI message  819 . Further, the CPB removal delay  835  contains a value of the difference of the removal delay for a current AU from the default value. A DPB output delay  833  is information describing a period of time that a corresponding AU can remain in the DPB prior to output. Specifically, the DPB output delay  833  may be employed to determine an output time of a picture from the DPB, and hence the amount of time the picture/AU can remain in the DPB after removal from the CPB. The output time at the HRD corresponds with an expected output of a picture for display at a decoder. 
     In an embodiment, the PT SEI message  818  includes a common CPB removal delay increment  845 , which is designated pt_du_common_cpb_removal_delay_increment_minus1. The common CPB removal delay increment  845  plus 1 specifies the duration, in units of clock sub-ticks, between the nominal CPB removal times of any two consecutive DUs (e.g., DUs  822 ) in decoding order in the AU (e.g., AU  821 ) associated with the picture timing SEI message  818  when Htid i is equal to i, where Htid identifies the highest temporal sublayer to be decoded. This value is also used to calculate an earliest possible time of arrival of decoding unit data into the CPB for a hypothetical stream scheduler (HSS). The length of this syntax element is du_cpb_removal_delay_increment_length_minus1+1 bits. 
     In an embodiment, the PT SEI message  818  includes a number of decoding units  847 , which is designated pt_num_decoding_units_minus1. The number of decoding units  847  plus 1 specifies the number of DUs (e.g., DUs  822 ) in the AU (e.g., AU  821 ) the picture timing SEI message  818  is associated with. The value of num_decoding_units_minus1 shall be in the range of 0 to PicSizeInCtbsY−1, inclusive. In an embodiment, the PicSizeInCtbsY syntax element represents a size of a picture measured in CTBs (e.g., a width of the picture measured in CTBs x a height of the picture measured in CTBs). 
     In an embodiment, the PT SEI message  818  includes a number of NAL units  849  in the i-th DU of the AU the PT SEI message  818  is associated with. The number of NAL units  849  is designated as pt_num_nalus_in_du_minus1[i]. The value of pt_num_nalus_in_du_minus1[i] shall be in the range of 0 to PicSizeInCtbsY−1, inclusive. 
     In an embodiment, the PT SEI message  818  includes a common CPB removal delay flag  851 , which is designated as pt_du_common_cpb_removal_delay_flag. The common CPB removal delay flag  851  equal to 1 specifies that the syntax elements common CPB removal delay increment  845 , which are designated as pt_du_common_cpb_removal_delay_increment_minus1[i]), are present in the PT SEI message  818 . The DU common CPB removal delay flag  851  equal to 0 specifies that the syntax elements common CPB removal delay increment  845  are not present. When not present in the PT SEI message  818 , the common CPB removal delay flag  851  is inferred to be equal to 0. 
     In an embodiment, the first DU of the AU is the first pt_num_nalus_in_du_minus1[0]+1 consecutive NAL units in decoding order in the AU. The i-th (with i greater than 0) DU of the AU is the pt_num_nalus_in_du_minus1[i]+1 consecutive NAL units immediately following the last NAL unit in the previous DU of the AU, in decoding order. In an embodiment, there is at least one VCL NAL unit in each DU, and all non-VCL NAL units associated with a VCL NAL unit are included in the same DU as the VCL NAL unit. 
     In an embodiment, the PT SEI message  818  includes a CPB removal delay increment  853 , which is designated pt_du_cpb_removal_delay_increment_minus1. The CPB removal delay increment  853  plus 1 specifies the duration, in units of clock sub-ticks, between the nominal CPB removal times of the (i+1)-th DU and the i-th DU, in decoding order, in the AU associated with the PT SEI message  818  when Htid is equal to j. This value is also used to calculate an earliest possible time of arrival of DU data into the CPB for the HSS. The length of this syntax element is bp_du_cpb_removal_delay_increment_length_minus1+1 bits. 
     In an embodiment, the PT SEI message  818  includes the maximum number of sublayers  843  instead of, or in addition to, the VPS  811 . 
     A DUI SEI message  816  is a SEI message that contains HRD parameters  890  (a.k.a., picture level CPB parameters) for managing delivery information for DUs at the CPB and/or the DPB. For example, the DUI SEI message  816  may contain additional parameters for use in performing a HRD conformance test on a corresponding DU. As noted above, an AU may contain one or more DUs. Hence, information for checking a DU may be different than information for checking an AU. As a specific example, the DUI SEI message  816  may contain CPB removal delay information  831 . A CPB removal delay information  831  is information related to removal of a corresponding DU from the CPB. For example, the CPB removal delay information  831  may be used to calculate the number of clock ticks between the removal of the current DU and a preceding DU in decoding order. 
     In an embodiment, the DUI SEI message  816  includes the maximum number of temporal sublayers  841  for which the initial CPB removal delay  837  and the initial CPB removal offset  839  are indicated in the BP SEI message  819 . This maximum number of temporal sublayers  841  is designated bp_max_sublayers_minus1. The value of bp_max_sublayers_minus1 shall be in the range of 0 to a maximum number of sublayers  843  specified in the VPS  811 , which is designated vps_max_sublayers_minus1, inclusive. vps_max_sublayers_minus1 plus 1 specifies the maximum number of temporal sublayers that may be present in a layer specified by the VPS  811 . The value of vps_max_sublayers_minus1 shall be in the range of 0 to 6, inclusive. 
     In an embodiment, the DUI SEI message  816  includes the maximum number of sublayers  843  instead of, or in addition to, the VPS  811 . 
     As can be appreciated by the preceding description, the BP SEI message  819 , the PT SEI message  818 , and the DUI SEI message  816  contain a significant amount of information. In an embodiment, the HRD parameters  880  and/or  890  (a.k.a., picture-level CPB parameters) in the PT SEI message  818  and/or the DUI SEI message  816  are used to perform DU-based HRD operations on sublayers to test for bitstream conformance. 
     By way of example, an HRD determines, for each layer, whether the duration, as specified in the PT SEI message  818  and/or the DUI SEI message  816 , between the nominal CPB removal times of any two consecutive decoding units in decoding order in the access unit associated with the picture timing SEI message is exceeded. When the duration is exceeded, the bitstream does not conform and a new bitstream with revised CPB parameters is generated and tested by the encoder. That process may repeat until the duration is not exceeded, which means that the bitstream conforms to the standard (e.g., the VVC standard). 
     The HRD may also determine, for each layer, whether the duration, as specified in the PT SEI message  818  and/or the DUI SEI message  816 , between the CPB removal times of the (i+1)-th decoding unit and the i-th decoding unit, in decoding order, in the access unit associated with the picture timing SEI message is exceeded. When the duration is exceeded, the bitstream does not conform and a new bitstream with revised CPB parameters is generated and tested by the encoder. That process may repeat until the duration is not exceeded, which means that the bitstream conforms to the standard (e.g., the VVC standard). 
     Once a conforming bitstream is obtained, that bitstream may be stored and communicated toward the decoder. In an embodiment, the BP SEI message  819 , the PT SEI message  818 , and the DUI SEI message  816  remain included in the bitstream even though the decoder may not use this information in decoding any of the pictures included in the bitstream. 
     An example implementation of the HRD using the DU-based HRD operations to test the sublayers in the bitstream for bitstream conformance is provided in the following syntax and semantics. 
     A picture timing SEI message syntax is as follows. 
     
       
         
           
               
               
             
               
                   
                   
               
               
                   
                 Descriptor 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 pic_timing( payloadSize ) { 
                   
               
               
                  pt_max_sub_layers_minus1 
                 u(3) 
               
               
                  cpb_removal_delay_minus1[ pt_max_sub_layers_minus1 ] 
                 u(v) 
               
               
                  for( i = TemporalId; i &lt; pt_max_sub_layers_minus1; i++ ) { 
               
               
                   sub_layer_delays_present_flag[ i ] 
                 u(1) 
               
               
                   if( sub_layer_delays_present_flag[ i ] ) { 
               
               
                    cpb_removal_delay_delta_enabled_flag[ i ] 
                 u(1) 
               
               
                    if( cpb_removal_delay_delta_enabled_flag[ i ] ) 
               
               
                     cpb_removal_delay_delta_idx[ i ] 
                 u(v) 
               
               
                    else 
               
               
                     cpb_removal_delay_minus1[ i ] 
                 u(v) 
               
               
                   } 
               
               
                  } 
               
               
                  dpb_output_delay 
                 u(v) 
               
               
                  if( decoding_unit_hrd_params_present_flag ) 
               
               
                   pic_dpb_output_du_delay 
                 u(v) 
               
               
                  if( decoding_unit_hrd_params_present_flag &amp;&amp; 
               
               
                    decoding_unit_cpb_params_in_pic_timing_sei_flag ) { 
               
               
                   num_decoding_units_minus1 
                 ue(v) 
               
               
                   du_common_cpb_removal_delay_flag 
                 u(1) 
               
               
                   if( du_common_cpb_removal_delay_flag ) 
               
               
                    for( i = TemporalId; i &lt; pt_max_sub_layers_minus1; i++ ) 
               
               
                     du_common_cpb_removal_delay_increment_minus1[ i ] 
                 u(v) 
               
               
                   for( i = 0; i &lt;= num_decoding_units_minus1; i++ ) { 
               
               
                    num_nalus_in_du_minus1[ i ] 
                 ue(v) 
               
               
                    if( !du_common_cpb_removal_delay_flag &amp;&amp; i &lt; 
               
               
                 num_decoding_units_minus1 ) 
               
               
                     for( j = TemporalId; j &lt; pt_max_sub_layers_minus1; j++ ) 
               
               
                      du_cpb_removal_delay_increment_minus1[ i ][ j ] 
                 u(v) 
               
               
                   } 
               
               
                  } 
               
               
                 } 
               
               
                   
               
            
           
         
       
     
     An example of picture timing SEI message semantics is as follows. 
     The picture timing SEI message provides CPB removal delay and DPB output delay information for the access unit associated with the SEI message. 
     num_decoding_units_minus1 plus 1 specifies the number of decoding units in the access unit the picture timing SEI message is associated with. The value of num_decoding_units_minus1 shall be in the range of 0 to PicSizeInCtbsY−1, inclusive. 
     du_common_cpb_removal_delay_flag equal to 1 specifies that the syntax elements du_common_cpb_removal_delay_increment_minus1 [i] are present. du_common_cpb_removal_delay_flag equal to 0 specifies that the syntax elements du_common_cpb_removal_delay_increment_minus1[i] are not present. 
     du_common_cpb_removal_delay_increment_minus1[i] plus 1 specifies the duration, in units of clock sub-ticks (see clause C.1), between the nominal CPB removal times of any two consecutive decoding units in decoding order in the access unit associated with the picture timing SEI message when Htid i equal to i. This value is also used to calculate an earliest possible time of arrival of decoding unit data into the CPB for the HSS, as specified in Annex C of the VVC standard. The length of this syntax element is du_cpb_removal_delay_increment_length_minus1+1 bits. 
     num_nalus_in_du_minus1[i] plus 1 specifies the number of NAL units in the i-th decoding unit of the access unit the picture timing SEI message is associated with. The value of num_nalus_in_du_minus1[i] shall be in the range of 0 to PicSizeInCtbsY−1, inclusive. 
     The first decoding unit of the access unit consists of the first num_nalus_in_du_minus1[0]+1 consecutive NAL units in decoding order in the access unit. The i-th (with i greater than 0) decoding unit of the access unit consists of the num_nalus_in_du_minus1[i]+1 consecutive NAL units immediately following the last NAL unit in the previous decoding unit of the access unit, in decoding order. There shall be at least one VCL NAL unit in each decoding unit. All non-VCL NAL units associated with a VCL NAL unit shall be included in the same decoding unit as the VCL NAL unit. 
     du_cpb_removal_delay_increment_minus1[i][j] plus 1 specifies the duration, in units of clock sub-ticks, between the nominal CPB removal times of the (i+1)-th decoding unit and the i-th decoding unit, in decoding order, in the access unit associated with the picture timing SEI message when Htid i equal to j. This value is also used to calculate an earliest possible time of arrival of decoding unit data into the CPB for the HSS, as specified in Annex C of the VVC standard. The length of this syntax element is du_cpb_removal_delay_increment_length_minus1+1 bits. 
     An example decoding unit information SEI message syntax is as follows. 
     
       
         
           
               
               
             
               
                   
                   
               
               
                   
                 Descriptor 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 decoding_unit_info( payloadSize ) { 
                   
               
               
                   
                  decoding_unit_idx 
                 ue(v) 
               
               
                   
                  dui_max_sub_layers_minus1 
                 u(3) 
               
               
                   
                  if( !decoding_unit_cpb_params_in_pic_timing_sei_flag ) 
               
               
                   
                   for( i = TemporalId; i &lt; dui_max_sub_layers_minus1; i++ ) 
               
               
                   
                    du_spt_cpb_removal_delay_increment[ i ] 
                 u(v) 
               
               
                   
                  dpb_output_du_delay_present_flag 
                 u(1) 
               
               
                   
                  if( dpb_output_du_delay_present_flag ) 
               
               
                   
                   pic_spt_dpb_output_du_delay 
                 u(v) 
               
               
                   
                 } 
               
               
                   
                   
               
            
           
         
       
     
     An example of picture timing SEI message semantics is as follows. 
     The decoding unit information SEI message provides CPB removal delay information for the decoding unit associated with the SEI message. 
     The following applies for the decoding unit information SEI message syntax and semantics.
         The syntax elements decoding_unit_hrd_params_present_flag, decoding_unit_cpb_params_in_pic_timing_sei_flag and dpb_output_delay_du_length_minus1, and the variable CpbDpbDelaysPresentFlag are found in or derived from syntax elements in the general_hrd_parameters( ) syntax structure that is applicable to at least one of the operation points to which the decoding unit information SEI message applies.   The bitstream (or a part thereof) refers to the bitstream subset (or a part thereof) associated with any of the operation points to which the decoding unit information SEI message applies.       

     The presence of decoding unit information SEI messages for an operation point is specified as follows.
         If CpbDpbDelaysPresentFlag is equal to 1, decoding_unit_hrd_params_present_flag is equal to 1 and decoding_unit_cpb_params_in_pic_timing_sei_flag is equal to 0, one or more decoding unit information SEI messages applicable to the operation point shall be associated with each decoding unit in the CVS.   Otherwise, if CpbDpbDelaysPresentFlag is equal to 1, decoding_unit_hrd_params_present_flag is equal to 1 and decoding_unit_cpb_params_in_pic_timing_sei_flag is equal to 1, one or more decoding unit information SEI messages applicable to the operation point may or may not be associated with each decoding unit in the CVS.   Otherwise (CpbDpbDelaysPresentFlag is equal to 0 or decoding_unit_hrd_params_present_flag is equal to 0), in the CVS there shall be no decoding unit that is associated with a decoding unit information SEI message applicable to the operation point.       

     The set of NAL units associated with a decoding unit information SEI message consists, in decoding order, of the SEI NAL unit containing the decoding unit information SEI message and all subsequent NAL units in the access unit up to but not including any subsequent SEI NAL unit containing a decoding unit information SEI message with a different value of decoding_unit_idx. Each decoding unit shall include at least one VCL NAL unit. All non-VCL NAL units associated with a VCL NAL unit shall be included in the decoding unit containing the VCL NAL unit. 
     The TemporalId in the decoding unit information SEI message syntax is the TemporalId of the SEI NAL unit containing the decoding unit information SEI message. 
     decoding_unit_idx specifies the index, starting from 0, to the list of decoding units in the current access unit, of the decoding unit associated with the decoding unit information SEI message. The value of decoding_unit_idx shall be in the range of 0 to PicSizeInCtbsY−1, inclusive. 
     A decoding unit identified by a particular value of duIdx includes and only includes all NAL units associated with all decoding unit information SEI messages that have decoding_unit_idx equal to duIdx. Such a decoding unit is also referred to as associated with the decoding unit information SEI messages having decoding_unit_idx equal to duIdx. 
     For any two decoding units duA and duB in one access unit with decoding_unit_idx equal to duIdxA and duIdxB, respectively, where duIdxA is less than duIdxB, duA shall precede duB in decoding order. 
     A NAL unit of one decoding unit shall not be present, in decoding order, between any two NAL units of another decoding unit. 
     dui_max_sub_layers_minus1 plus 1 specifies the the TemporalId of the highest sub-layer representation for which the CPB removal delay information is contained in the decoding unit information SEI message. The value of dui_max_sub_layers_minus1 shall be in the range of 0 to vps_max_sub_layers_minus1, inclusive. 
     du_spt_cpb_removal_delay_increment[i] specifies the duration, in units of clock sub-ticks, between the nominal CPB times of the last decoding unit in decoding order in the current access unit and the decoding unit associated with the decoding unit information SEI message when Htid i equal to i. This value is also used to calculate an earliest possible time of arrival of decoding unit data into the CPB for the HSS, as specified in Annex C. The length of this syntax element is du_cpb_removal_delay_increment_length_minus1+1. When the decoding unit associated with the decoding unit information SEI message is the last decoding unit in the current access unit, the value of du spt_cpb_removal_delay_increment[i] shall be equal to 0. 
     dpb_output_du_delay_present_flag equal to 1 specifies the presence of the pic_spt_dpb_output_du_delay syntax element in the decoding unit information SEI message. dpb_output_du_delay_present_flag equal to 0 specifies the absence of the pic_spt_dpb_output_du_delay syntax element in the decoding unit information SEI message. 
     pic_spt_dpb_output_du_delay is used to compute the DPB output time of the picture when DecodingUnitHrdFlag is equal to 1. It specifies how many sub clock ticks to wait after removal of the last decoding unit in an access unit from the CPB before the decoded picture is output from the DPB. When not present, the value of pic_spt_dpb_output_du_delay is inferred to be equal to pic_dpb_output_du_delay. The length of the syntax element pic_spt_dpb_output_du_delay is given in bits by dpb_output_delay_du_length_minus1+1. 
     It is a requirement of bitstream conformance that all decoding unit information SEI messages that are associated with the same access unit, apply to the same operation point, and have dpb_output_du_delay_present_flag equal to 1 shall have the same value of pic_spt_dpb_output_du_delay. 
     The output time derived from the pic_spt_dpb_output_du_delay of any picture that is output from an output timing conforming decoder shall precede the output time derived from the pic_spt_dpb_output_du_delay of all pictures in any subsequent CVS in decoding order. 
     The picture output order established by the values of this syntax element shall be the same order as established by the values of PicOrderCntVal. 
     For pictures that are not output by the “bumping” process because they precede, in decoding order, a CLVSS picture that has no_output_of_prior_pics_flag equal to 1 or inferred to be equal to 1, the output times derived from pic_spt_dpb_output_du_delay shall be increasing with increasing value of PicOrderCntVal relative to all pictures within the same CVS. 
     For any two pictures in the CVS, the difference between the output times of the two pictures when DecodingUnitHrdFlag is equal to 1 shall be identical to the same difference when DecodingUnitHrdFlag is equal to 0. 
       FIG. 9  is an embodiment of a method  900  of decoding implemented by a video decoder (e.g., video decoder  400 ). The method  900  may be performed after a bitstream has been directly or indirectly received from a video encoder (e.g., video encoder  300 ). The method  900  improves the decoding process by ensuring picture-level coded picture buffer (CPB) parameters used to perform DU-based HRD operations on sublayers are included in a supplemental enhancement information (SEI) message. Because the picture-level CPB parameters are included in the SEI message, the HRD can use the DU-based HRD operations to test the sublayers in the bitstream for bitstream conformance, which ensures that the sublayers are properly coded and/or can be properly decoded. Thus, the coder/decoder (a.k.a., “codec”) in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed. 
     In block  902 , the video decoder receives a bitstream comprising a coded picture and an SEI message. The SEI message includes CPB parameters (e.g., the HRD parameters  880  and/or  890  referred to as picture-level CPB parameters) used to perform DU-based HRD operations on sublayers (e.g., sublayers  710 ,  720 ,  730 ). The DU-based HRD operations, which correspond to a DU such as DU  822 , are different from AU-based HRD operations, which correspond to an AU such as AU  821 . As noted above, the DU-based HRD operations are implemented by an HRD (e.g., HRD  500 ) for the purpose of testing for a bitstream such as bitstream  800 , which includes a multi-layer video sequence  600  and/or a multi-layer video sequence  700 , for bitstream conformance. 
     In an embodiment, the CPB parameters specify a duration between CPB removal times of two decoding units. In an embodiment, the SEI message is a picture timing (PT) SEI message. In an embodiment, the CPB parameters comprise a common CPB removal delay and a CPB removal delay for AU associated with the PT SEI message. 
     In an embodiment, the SEI message is a PT SEI message that specifies a number of decoding units in the AU associated with the PT SEI message. In an embodiment, the SEI message is a PT SEI message that specifies a number of NAL units in a DU of the AU associated with the PT SEI message. As used herein, the highest sublayer is the enhancement layer (e.g., sublayer  730 ) furthest away from the base layer (e.g., sublayer  710 ). 
     In an embodiment, the SEI message is a decoding unit information (DUI) SEI message that provides a temporal ID of an SEI NAL unit containing the DUI SEI message. In an embodiment, the temporal ID specifies a highest sublayer for which CPB removal delay information is contained in the DUI SEI message. 
     In block  904 , the video decoder decodes the coded picture from the bitstream to obtain a decoded picture. Thereafter, the decoded picture may be used to generate or produce an image or video sequence for display to a user on the display or screen of an electronic device (e.g., a smart phone, tablet, laptop, personal computer, etc.). In an embodiment, the picture-level CPB parameters contained in the PT SEI message  818  and/or the DUI SEI message  816  are not used in decoding the coded picture. 
       FIG. 10  is an embodiment of a method  1000  of encoding a video bitstream implemented by a video encoder (e.g., video encoder  300 ). The method  1000  may be performed when a picture (e.g., from a video) is to be encoded into a video bitstream and then transmitted toward a video decoder (e.g., video decoder  400 ). The method  1000  improves the encoding process by ensuring picture-level coded picture buffer (CPB) parameters used to perform DU-based HRD operations on sublayers are included in a supplemental enhancement information (SEI) message. Because the picture-level CPB parameters are included in the SEI message, the HRD can use the DU-based HRD operations to test the sublayers in the bitstream for bitstream conformance, which ensures that the sublayers are properly coded and/or can be properly decoded. Thus, the coder/decoder (a.k.a., “codec”) in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed. 
     In block  1002 , the video encoder generates a bitstream comprising a coded picture and an SEI message. The SEI message includes CPB parameters (e.g., the HRD parameters  880  and/or  890  referred to as picture-level CPB parameters) used to perform DU-based HRD operations on sublayers (e.g., sublayers  710 ,  720 ,  730 ). The DU-based HRD operations, which correspond to a DU such as DU  822 , are different from AU-based HRD operations, which correspond to an AU such as AU  821 . As noted above, the DU-based HRD operations are implemented by an HRD (e.g., HRD  500 ) for the purpose of testing for a bitstream such as bitstream  800 , which includes a multi-layer video sequence  600  and/or a multi-layer video sequence  700 , for bitstream conformance. 
     In an embodiment, the CPB parameters specify a duration between CPB removal times of two decoding units. In an embodiment, the SEI message is a picture timing (PT) SEI message. In an embodiment, the CPB parameters comprise a common CPB removal delay and a CPB removal delay for an access unit (AU) associated with the PT SEI message. In an embodiment, the PT SEI message specifies a number of decoding units in the AU associated with the PT SEI message and a number of network abstraction layer (NAL) units in a decoding unit (DU) of the AU associated with the PT SEI message. 
     In an embodiment, the SEI message is a decoding unit information (DUI) SEI message that provides a temporal identifier (ID) of an SEI NAL unit containing the DUI SEI message. In an embodiment, the DUI SEI message specifies a temporal identifier (ID) of a highest sublayer for which CPB removal delay information is contained in the DUI SEI message. 
     In block  1004 , the video encoder performs the DU-based HRD operations on the sublayers using the CPB parameters to determine whether the bitstream is conforming. 
     In an embodiment, the bitstream is conforming when the duration between the CPB removal times is not exceeded. 
     In block  1006 , the video encoder stores the bitstream for communication toward a video decoder when the bitstream is conforming based on performance of the DU-based HRD operations. The bitstream may be stored in memory until the bitstream is transmitted toward the video decoder. Once received by the video decoder, the encoded bitstream may be decoded (e.g., as described above) to generate or produce an image or video sequence for display to a user on the display or screen of an electronic device (e.g., a smart phone, tablet, laptop, personal computer, etc.). 
       FIG. 11  is a schematic diagram of a video coding device  1100  (e.g., a video encoder  300  or a video decoder  400 ) according to an embodiment of the disclosure. The video coding device  1100  is suitable for implementing the disclosed embodiments as described herein. The video coding device  1100  comprises ingress ports  1110  and receiver units (Rx)  1120  for receiving data; a processor, logic unit, or central processing unit (CPU)  1130  to process the data; transmitter units (Tx)  1140  and egress ports  1150  for transmitting the data; and a memory  1160  for storing the data. The video coding device  1100  may also comprise optical-to-electrical (OE) components and electrical-to-optical (EO) components coupled to the ingress ports  1110 , the receiver units  1120 , the transmitter units  1140 , and the egress ports  1150  for egress or ingress of optical or electrical signals. 
     The processor  1130  is implemented by hardware and software. The processor  1130  may be implemented as one or more CPU chips, cores (e.g., as a multi-core processor), field-programmable gate arrays (FPGAs), application specific integrated circuits (ASICs), and digital signal processors (DSPs). The processor  1130  is in communication with the ingress ports  1110 , receiver units  1120 , transmitter units  1140 , egress ports  1150 , and memory  1160 . The processor  1130  comprises a coding module  1170 . The coding module  1170  implements the disclosed embodiments described above. For instance, the coding module  1170  implements, processes, prepares, or provides the various codec functions. The inclusion of the coding module  1170  therefore provides a substantial improvement to the functionality of the video coding device  1100  and effects a transformation of the video coding device  1100  to a different state. Alternatively, the coding module  1170  is implemented as instructions stored in the memory  1160  and executed by the processor  1130 . 
     The video coding device  1100  may also include input and/or output (I/O) devices  1180  for communicating data to and from a user. The I/O devices  1180  may include output devices such as a display for displaying video data, speakers for outputting audio data, etc. The I/O devices  1180  may also include input devices, such as a keyboard, mouse, trackball, etc., and/or corresponding interfaces for interacting with such output devices. 
     The memory  1160  comprises one or more disks, tape drives, and solid-state drives and may be used as an over-flow data storage device, to store programs when such programs are selected for execution, and to store instructions and data that are read during program execution. The memory  1160  may be volatile and/or non-volatile and may be read-only memory (ROM), random access memory (RAM), ternary content-addressable memory (TCAM), and/or static random-access memory (SRAM). 
       FIG. 12  is a schematic diagram of an embodiment of a means for coding  1200 . In an embodiment, the means for coding  1200  is implemented in a video coding device  1202  (e.g., a video encoder  300  or a video decoder  400 ). The video coding device  1202  includes receiving means  1201 . The receiving means  1201  is configured to receive a picture to encode or to receive a bitstream to decode. The video coding device  1202  includes transmission means  1207  coupled to the receiving means  1201 . The transmission means  1207  is configured to transmit the bitstream to a decoder or to transmit a decoded image to a display means (e.g., one of the I/O devices  1180 ). 
     The video coding device  1202  includes a storage means  1203 . The storage means  1203  is coupled to at least one of the receiving means  1201  or the transmission means  1207 . The storage means  1203  is configured to store instructions. The video coding device  1202  also includes processing means  1205 . The processing means  1205  is coupled to the storage means  1203 . The processing means  1205  is configured to execute the instructions stored in the storage means  1203  to perform the methods disclosed herein. 
     It should also be understood that the steps of the exemplary methods set forth herein are not necessarily required to be performed in the order described, and the order of the steps of such methods should be understood to be merely exemplary. Likewise, additional steps may be included in such methods, and certain steps may be omitted or combined, in methods consistent with various embodiments of the present disclosure. 
     While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods might be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented. 
     In addition, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled or directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein.