Patent Publication Number: US-2022217375-A1

Title: Scalable Nesting SEI Messages For OLSs

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a continuation of International Application No. PCT/US2020/050395, filed Sep. 11, 2020 by Ye-Kui Wang, and titled “Scalable Nesting SEI Messages For OLSs,” which claims the benefit of U.S. Provisional Patent Application No. 62/905,143 filed Sep. 24, 2019 by Ye-Kui Wang, and titled “Scalable Nesting of SEI Messages for Output Layer Sets,” which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure is generally related to video coding, and is specifically related to scalable nesting supplemental enhancement information (SEI) messages used to support encoding layers into output layer sets (OLSs) in 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 
     In an embodiment, the disclosure includes a method implemented by a decoder, the method comprising: receiving, by a receiver of the decoder, a bitstream comprising one or more layers and a scalable nesting supplemental enhancement information (SEI) message, wherein the scalable nesting SEI message includes one or more scalable-nested SEI messages and a scalable nesting output layer set (OLS) flag, and wherein the scalable nesting OLS flag is set to specify whether the scalable-nested SEI messages apply to specific OLSs or specific layers; and decoding, by a processor of the decoder, a coded picture from the one or more layers, based on the scalable-nested SEI messages, to produce a decoded picture. 
     Some video coding systems employ SEI messages. An SEI message contains information that is 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 parameters used to check a bitstream for conformance with standards. A hypothetical reference decoder (HRD) can read the SEI messages to determine how to check the bitstream for standards conformance. Such systems may employ separate types of SEI messages for data relating to layers and data relating to OLSs that contain layers. This may result in a system that is complicated and redundant. The present example includes a scalable nesting SEI message configured to contain parameters related to either layers or OLSs. For example, the scalable nesting SEI message may contain a scalable nesting OLS flag, which can be set to indicate whether the scalable nesting SEI message contains parameters related to layers or parameters related to OLSs. The scalable nesting SEI message may also contain one or more scalable-nested SEI messages that relate to the layers or the OLSs. When the scalable nesting SEI message relates to OLSs, the scalable nesting SEI message also includes flags indicating a number of OLSs associated with the scalable nesting SEI message and indicating OLS indices to correlate the OLSs to the scalable-nested SEI messages. When the scalable nesting SEI message relates to layers, the scalable nesting SEI message also includes flags indicating a number of layers associated with the scalable nesting SEI message and indicating layer identifiers (IDs) to correlate the layers to the scalable-nested SEI messages. In this way, the number of SEI message types may be reduced, which decreases complexity and decreases a total number of message types. This in turn reduces the length of message ID data used to identify each type of message. As a result, coding efficiency is increased, which reduces processor, memory, and/or network signaling resource usage at both the encoder and the decoder. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the scalable nesting OLS flag is set to one when specifying that the scalable-nested SEI messages apply to specific OLSs, and wherein the scalable nesting OLS flag is set to zero when specifying that the scalable-nested SEI messages apply to specific layers. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the scalable nesting OLS flag is set to one when the scalable nesting SEI message contains an SEI message that has a payload type of buffering period, picture timing, or decoding unit information. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the scalable nesting SEI message includes a scalable nesting number of OLSs minus one (num_olss_minus1) syntax element when the scalable nesting OLS flag is set to one, and wherein the scalable nesting num_olss_minus1 syntax element specifies a number of OLSs to which the scalable-nested SEI messages apply, and wherein a value of the scalable nesting num_olss_minus1 syntax element is in a range of zero to a total number of OLSs (TotalNumOlss)−1, inclusive. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the scalable nesting SEI message includes a scalable nesting OLS delta minus one (ols_idx_delta_minus1[i]) syntax element used to derive a nesting OLS index (NestingOlsIdx[i]) that specifies an OLS index of an i-th OLS to which the scalable-nested SEI messages apply when the scalable nesting OLS flag is equal to one, wherein a value of the scalable nesting ols_idx_delta_minus1[i] syntax element is in a range of zero to the TotalNumOlss−2, inclusive. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, further comprising deriving NestingOlsIdx[i] as follows: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 if( i = = 0 ) 
               
               
                   
                  NestingOlsIdx[ i ] = scalable nesting ols_idx_delta_minus1[ i ] 
               
               
                   
                 else 
               
               
                   
                  NestingOlsIdx[ i ] = NestingOlsIdx[ i − 1 ] + scalable nesting 
               
               
                   
                 ols_idx_delta_minus1[ i ] + 1. 
               
               
                   
                   
               
            
           
         
       
     
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the scalable nesting SEI message includes a scalable nesting number of layers minus one (num_layers_minus1) syntax element when the scalable nesting OLS flag is set to zero, and wherein the scalable nesting num_layers_minus1 syntax element specifies a number of layers to which the scalable-nested SEI messages apply. 
     In an embodiment, the disclosure includes a method implemented by an encoder, the method comprising: encoding, by a processor of the encoder, a bitstream comprising one or more layers; encoding into the bitstream, by the processor, a scalable nesting SEI message, wherein the scalable nesting SEI message includes one or more scalable-nested SEI messages and a scalable nesting OLS flag, and wherein the scalable nesting OLS flag is set to specify whether the scalable-nested SEI messages apply to specific OLSs or specific layers; performing, by the processor, a set of bitstream conformance tests based on the scalable nesting SEI message; and storing, by a memory coupled to the processor, the bitstream for communication toward a decoder. 
     Some video coding systems employ SEI messages. An SEI message contains information that is 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 parameters used to check a bitstream for conformance with standards. A HRD can read the SEI messages to determine how to check the bitstream for standards conformance. Such systems may employ separate types of SEI messages for data relating to layers and data relating to OLSs that contain layers. This may result in a system that is complicated and redundant. The present example includes a scalable nesting SEI message configured to contain parameters related to either layers or OLSs. For example, the scalable nesting SEI message may contain a scalable nesting OLS flag, which can be set to indicate whether the scalable nesting SEI message contains parameters related to layers or parameters related to OLSs. The scalable nesting SEI message may also contain one or more scalable-nested SEI messages that relate to the layers or the OLSs. When the scalable nesting SEI message relates to OLSs, the scalable nesting SEI message also includes flags indicating a number of OLSs associated with the scalable nesting SEI message and indicating OLS indices to correlate the OLSs to the scalable-nested SEI messages. When the scalable nesting SEI message relates to layers, the scalable nesting SEI message also includes flags indicating a number of layers associated with the scalable nesting SEI message and indicating layer IDs to correlate the layers to the scalable-nested SEI messages. In this way, the number of SEI message types may be reduced, which decreases complexity and decreases a total number of message types. This in turn reduces the length of message ID data used to identify each type of message. As a result, coding efficiency is increased, which reduces processor, memory, and/or network signaling resource usage at both the encoder and the decoder. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the scalable nesting OLS flag is set to one when specifying that the scalable-nested SEI messages apply to specific OLSs, and wherein the scalable nesting OLS flag is set to zero when specifying that the scalable-nested SEI messages apply to specific layers. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the scalable nesting OLS flag is set to one when the scalable nesting SEI message contains an SEI message that has a payload type of buffering period, picture timing, or decoding unit information. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the scalable nesting SEI message includes a scalable nesting num_olss_minus1 syntax element when the scalable nesting OLS flag is set to one, and wherein the scalable nesting num_olss_minus1 syntax element specifies a number of OLSs to which the scalable-nested SEI messages apply, and wherein a value of the scalable nesting num_olss_minus1 syntax element is in a range of zero to a TotalNumOlss−1, inclusive. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the scalable nesting SEI message includes a scalable nesting ols_idx_delta_minus1[i] syntax element used to derive a NestingOlsIdx[i] that specifies an OLS index of an i-th OLS to which the scalable-nested SEI messages apply when the scalable nesting OLS flag is equal to one, wherein a value of the scalable nesting ols_idx_delta_minus1[i] syntax element is in a range of zero to the TotalNumOlss−2, inclusive. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, further comprising deriving NestingOlsIdx[i] as follows: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 if( i = = 0 ) 
               
               
                   
                  NestingOlsIdx[ i ] = scalable nesting ols_idx_delta_minus1[ i ] 
               
               
                   
                 else 
               
               
                   
                  NestingOlsIdx[ i ] = NestingOlsIdx[ i − 1 ] + scalable nesting 
               
               
                   
                 ols_idx_delta_minus1[ i ] + 1. 
               
               
                   
                   
               
            
           
         
       
     
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the scalable nesting SEI message includes a scalable nesting num_layers_minus1 syntax element when the scalable nesting OLS flag is set to zero, and wherein the scalable nesting num_layers_minus1 syntax element specifies a number of layers to which the scalable-nested SEI messages apply. 
     In an embodiment, the disclosure includes a video coding device comprising: a processor, a receiver coupled to the processor, a memory coupled to the processor, and a transmitter coupled to the processor, wherein the processor, receiver, memory, and transmitter are configured to perform the method of any of the preceding aspects. 
     In an embodiment, the disclosure includes a non-transitory computer readable medium comprising a computer program product for use by a video coding device, the computer program product comprising computer executable instructions stored on the non-transitory computer readable medium such that when executed by a processor cause the video coding device to perform the method of any of the preceding aspects. 
     In an embodiment, the disclosure includes a decoder comprising: a receiving means for receiving a bitstream comprising one or more layers and a scalable nesting SEI message, wherein the scalable nesting SEI message includes one or more scalable-nested SEI messages and a scalable nesting OLS flag, and wherein the scalable nesting OLS flag is set to specify whether the scalable-nested SEI messages apply to specific OLSs or specific layers; a decoding means for decoding a coded picture from the one or more layers, based on the scalable-nested SEI messages, to produce a decoded picture; and a forwarding means for forwarding the decoded picture for display as part of a decoded video sequence. 
     Some video coding systems employ SEI messages. An SEI message contains information that is 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 parameters used to check a bitstream for conformance with standards. A HRD can read the SEI messages to determine how to check the bitstream for standards conformance. Such systems may employ separate types of SEI messages for data relating to layers and data relating to OLSs that contain layers. This may result in a system that is complicated and redundant. The present example includes a scalable nesting SEI message configured to contain parameters related to either layers or OLSs. For example, the scalable nesting SEI message may contain a scalable nesting OLS flag, which can be set to indicate whether the scalable nesting SEI message contains parameters related to layers or parameters related to OLSs. The scalable nesting SEI message may also contain one or more scalable-nested SEI messages that relate to the layers or the OLSs. When the scalable nesting SEI message relates to OLSs, the scalable nesting SEI message also includes flags indicating a number of OLSs associated with the scalable nesting SEI message and indicating OLS indices to correlate the OLSs to the scalable-nested SEI messages. When the scalable nesting SEI message relates to layers, the scalable nesting SEI message also includes flags indicating a number of layers associated with the scalable nesting SEI message and indicating layer IDs to correlate the layers to the scalable-nested SEI messages. In this way, the number of SEI message types may be reduced, which decreases complexity and decreases a total number of message types. This in turn reduces the length of message ID data used to identify each type of message. As a result, coding efficiency is increased, which reduces processor, memory, and/or network signaling resource usage at both the encoder and the decoder. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the decoder is further configured to perform the method of any of the preceding aspects. 
     In an embodiment, the disclosure includes an encoder comprising: an encoding means for: encoding a bitstream comprising one or more layers; and encoding into the bitstream a scalable nesting SEI message, wherein the scalable nesting SEI message includes one or more scalable-nested SEI messages and a scalable nesting OLS flag, and wherein the scalable nesting OLS flag is set to specify whether the scalable-nested SEI messages apply to specific OLSs or specific layers; a HRD means for performing a set of bitstream conformance tests based on the scalable nesting SEI message; and a storing means for storing the bitstream for communication toward a decoder. 
     Some video coding systems employ SEI messages. An SEI message contains information that is 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 parameters used to check a bitstream for conformance with standards. A HRD can read the SEI messages to determine how to check the bitstream for standards conformance. Such systems may employ separate types of SEI messages for data relating to layers and data relating to OLSs that contain layers. This may result in a system that is complicated and redundant. The present example includes a scalable nesting SEI message configured to contain parameters related to either layers or OLSs. For example, the scalable nesting SEI message may contain a scalable nesting OLS flag, which can be set to indicate whether the scalable nesting SEI message contains parameters related to layers or parameters related to OLSs. The scalable nesting SEI message may also contain one or more scalable-nested SEI messages that relate to the layers or the OLSs. When the scalable nesting SEI message relates to OLSs, the scalable nesting SEI message also includes flags indicating a number of OLSs associated with the scalable nesting SEI message and indicating OLS indices to correlate the OLSs to the scalable-nested SEI messages. When the scalable nesting SEI message relates to layers, the scalable nesting SEI message also includes flags indicating a number of layers associated with the scalable nesting SEI message and indicating layer IDs to correlate the layers to the scalable-nested SEI messages. In this way, the number of SEI message types may be reduced, which decreases complexity and decreases a total number of message types. This in turn reduces the length of message ID data used to identify each type of message. As a result, coding efficiency is increased, which reduces processor, memory, and/or network signaling resource usage at both the encoder and the decoder. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the encoder is further configured to perform the method of any of the preceding aspects. 
     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 bitstream. 
         FIG. 8  is a schematic diagram of an example video coding device. 
         FIG. 9  is a flowchart of an example method of encoding a video sequence into a bitstream including scalable nesting SEI messages. 
         FIG. 10  is a flowchart of an example method of decoding a video sequence from a bitstream including scalable nesting SEI messages. 
         FIG. 11  is a schematic diagram of an example system for coding a video sequence using a bitstream including scalable nesting SEI messages. 
     
    
    
     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 coded picture is a coded representation of a picture comprising video coding layer (VCL) network abstraction layer (NAL) units with a particular value of NAL unit header layer identifier (nuh_layer_id) within an access unit (AU) and containing all coding tree units (CTUs) of the picture. A decoded picture is a picture produced by applying a decoding process to a coded picture. A 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 interspersed as desired with emulation prevention bytes. 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. A layer is a set of VCL NAL units that share a specified characteristic (e.g., a common resolution, frame rate, image size, etc.) as indicated by layer identifier (ID) and associated non-VCL NAL units. A NAL unit header layer identifier (nuh_layer_id) is a syntax element that specifies an identifier of a layer that includes a NAL unit. A video parameter set (VPS) is a data unit that contains parameters related to an entire video. A coded video sequence is a set of one or more coded pictures. A decoded video sequence is a set of one or more decoded pictures. 
     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 hypothetical reference decoder (HRD) is a decoder model operating on an encoder that 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. HRD parameters may be included in supplemental enhancement information (SEI) messages. A 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 scalable nesting SEI message is a message that contains a plurality of SEI messages that correspond to one or more OLSs or one or more layers. A buffering period (BP) SEI message is a SEI message that contains HRD parameters for initializing an HRD to manage a coded picture buffer (CPB). A picture timing (PT) SEI message is a SEI message that contains HRD parameters for managing delivery information for access units (AUs) at the CPB and/or a decoded picture buffer (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 scalable nesting SEI message includes a set of scalable-nested SEI messages. A scalable-nested SEI message is a SEI message that is nested inside a scalable nesting SEI message. A flag is a variable or single-bit syntax element that can take one of the two possible values: 0 and 1. A scalable nesting OLS flag is a flag that specifies whether scalable-nested SEI messages apply to specific OLSs or specific layers. A scalable nesting number of OLSs minus one (num_olss_minus1) is a syntax element that specifies the number of OLSs to which the scalable-nested SEI messages apply. A total number of OLSs minus one (TotalNumOlss−1) is a syntax element that specifies a total number of OLSs specified in a VPS. A scalable nesting OLS delta minus one (ols_idx_delta_minus1[i]) is a syntax element that contains data sufficient to derive a nesting OLS index. A nesting OLS index (NestingOlsIdx) is a syntax element that specifies the OLS index of the OLS to which the scalable-nested SEI messages apply. A scalable nesting number of layers minus one (num_layers_minus1) is a syntax element that specifies the number of layers to which the scalable-nested SEI messages apply. A scalable nesting layer id (layer_id[i]) is a syntax element that specifies the nuh_layer_id value of an i-th layer to which the scalable-nested SEI messages apply. 
     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), 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. 
     Some video coding systems employ supplemental enhancement information (SEI) messages. An SEI message contains information that is 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 parameters used to check a bitstream for conformance with standards. A hypothetical reference decoder (HRD) can read the SEI messages to determine how to check the bitstream for standards conformance. Such systems may employ separate types of SEI messages for data relating to layers and data relating to output layer sets (OLSs) that contain layers. This may result in a system that is complicated and redundant. 
     Disclosed herein is a scalable nesting SEI message configured to contain parameters related to either layers or OLSs. For example, the scalable nesting SEI message may contain a scalable nesting OLS flag, which can be set to indicate whether the scalable nesting SEI message contains parameters related to layers or parameters related to OLSs. The scalable nesting SEI message may also contain one or more scalable-nested SEI messages that relate to the layers or the OLSs. As used herein, one or more indicates any positive number of a corresponding item, which includes one or a plurality of such an item. When the scalable nesting SEI message relates to OLSs, the scalable nesting SEI message also includes flags indicating a number of OLSs associated with the scalable nesting SEI message and indicating OLS indices to correlate the OLSs to the scalable-nested SEI messages. When the scalable nesting SEI message relates to layers, the scalable nesting SEI message also includes flags indicating a number of layers associated with the scalable nesting SEI message and indicating layer identifiers (IDs) to correlate the layers to the scalable-nested SEI messages. In this way, the number of SEI message types may be reduced, which decreases complexity and decreases a total number of message types. This in turn reduces the length of message ID data used to identify each type of message. As a result, coding efficiency is increased, which reduces processor, memory, and/or network signaling resource usage at both the encoder and the decoder. 
       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 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 Access Unit (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 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 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. 
     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 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 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 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 to ensure that the resulting sub-bitstream can be correctly decoded under the conditions that are expected for the sub-bitstream. Accordingly, the HRD parameters in the bitstream  551  can indicate the CPB delivery schedules as well as include sufficient data to allow the HRD  500  to determine the CPB delivery schedules and correlate the CPB delivery schedules 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 show, 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 and can be included in the same AU. A temporal ID 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. 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. In some cases, an OLS  625  contains only an output layer referred to as a simulcast 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, 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 . Scalable nesting SEI messages can be employed to indicate the parameters needed to check the layers  631 - 632  and the OLSs  625  for standard conformance. 
       FIG. 7  is a schematic diagram illustrating an example bitstream  700 . For example, the bitstream  700  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  700  may include a multi-layer video sequence  600 . In addition, the bitstream  700  may include various parameters to control the operation of a HRD, such as HRD  500 . Based on such parameters, the HRD can check the bitstream  700  for conformance with standards prior to transmission toward a decoder for decoding. 
     The bitstream  700  includes a VPS  711 , one or more SPSs  713 , a plurality of picture parameter sets (PPSs)  715 , a plurality of slice headers  717 , image data  720 , and SEI messages  719 . A VPS  711  contains data related to the entire bitstream  700 . For example, the VPS  711  may contain data related OLSs, layers, and/or sublayers used in the bitstream  700 . An SPS  713  contains sequence data common to all pictures in a coded video sequence contained in the bitstream  700 . For example, each layer may contain one or more coded video sequences, and each coded video sequence may reference a SPS  713  for corresponding parameters. The parameters in a SPS  713  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  713 , a single SPS  713  can contain data for multiple sequences in some examples. The PPS  715  contains parameters that apply to an entire picture. Hence, each picture in the video sequence may refer to a PPS  715 . It should be noted that, while each picture refers to a PPS  715 , a single PPS  715  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  715  may contain data for such similar pictures. The PPS  715  can indicate coding tools available for slices in corresponding pictures, quantization parameters, offsets, etc. 
     The slice header  717  contains parameters that are specific to each slice in a picture. Hence, there may be one slice header  717  per slice in the video sequence. The slice header  717  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  700  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  717  may be used interchangeably in some contexts. For example, certain parameters may be moved between the slice header  717  and a picture header depending on whether such parameters are common to all slices in a picture. 
     The image data  720  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  720  may include OLSs  721 , layers  723 , pictures  725 , and/or slices  727 . An OLS  721 , is a set of layers  723  for which one or more layers are specified as output layer(s). An OLS  721  may be substantially similar to OLS  625 . A layer  723  is a set of VCL NAL units that share a specified characteristic (e.g., a common resolution, frame rate, image size, etc.) as indicated by a layer ID, such as a nuh_layer_id, and associated non-VCL NAL units. For example, a layer  723  may include a set of pictures  725  that share the same nuh_layer_id. A layer  723  may be substantially similar to layers  631  and/or  632 . A picture  725  is an array of luma samples and/or an array of chroma samples that create a frame or a field thereof. For example, a picture  725  is a coded image that may be output for display or used to support coding of other picture(s)  725  for output. A picture  725  contains one or more slices  727 . A slice  727  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  725  that are exclusively contained in a single NAL unit. The slices  727  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  700  can be coded as a sequence of NAL units. A NAL unit is a container for video data and/or supporting syntax. A NAL unit can be a VCL NAL unit or a non-VCL NAL unit. A VCL NAL unit is a NAL unit coded to contain video data, such as image data  720  and an associated slice header  717 . 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. For example, a non-VCL NAL unit can contain a VPS  711 , a SPS  713 , a PPS  715 , a SEI message  719 , or other supporting syntax. 
     A SEI message  719  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. For example, the SEI messages  719  may contain data to support HRD processes or other supporting data that is not directly relevant to decoding the bitstream  700  at a decoder. The SEI message  719  may be scalable nesting SEI messages. A scalable nesting SEI message is a message that contains a plurality of scalable-nested SEI messages that correspond to one or more OLSs  721  or one or more layers  723 . Accordingly, a scalable nesting SEI message is a SEI message  719  that contains a set of scalable-nested SEI messages of the same type. SEI messages  719  may include a BP SEI message that contains HRD parameters for initializing an HRD to manage a CPB. SEI messages  719  may also include a PT SEI message that contains HRD parameters for managing delivery information for AUs at the CPB and/or the DPB. SEI messages  719  may also include a DUI SEI message that contains HRD parameters for managing delivery information for DUs at the CPB and/or the DPB. 
     The bitstream  700  includes various flags to signal the configuration of the SEI messages  719 . For example, a SEI message  719  may include a scalable nesting (SN) OLS flag  731 , a scalable nesting number of OLSs minus one (num_olss_minus1)  733 , a scalable nesting OLS delta minus one (ols_idx_delta_minus1[i])  735 , a scalable nesting number of layers minus one (num_layers_minus1)  737 , and/or a scalable nesting layer ID (layer_id[i])  739  when the SEI message  719  is a scalable nesting SEI message. 
     The scalable nesting OLS flag  731  is a syntax element that specifies whether scalable-nested SEI messages in a scalable nesting SEI message apply to specific OLSs  721  or specific layers  723 . For example, the scalable nesting OLS flag  731  can be set to one when the scalable-nested SEI messages apply to specific OLSs  721  (and not layers). Further, the scalable nesting OLS flag  731  can be set to zero when the scalable-nested SEI messages apply to specific layers  723  (and not OLSs). Accordingly, a HRD can read a scalable nesting OLS flag  731  in a SEI message  719  and determine whether all scalable-nested SEI messages contained therein describe the OLSs  721  or the layers  723 . 
     The scalable nesting num_olss_minus1  733  is used when the SEI message  719  relates to OLSs  721  as indicated by the scalable nesting OLS flag  731 . The scalable nesting num_olss_minus1  733  is a syntax element that specifies the number of OLSs  721  to which the scalable-nested SEI messages in a scalable nesting SEI message apply. The scalable nesting num_olss_minus1  733  employs minus one format and hence includes one less than the actual value. For example, if a scalable nesting SEI message includes scalable-nested SEI messages that relate to five OLSs  721 , then the scalable nesting num_olss_minus1  733  is set to a value of four. 
     The scalable nesting ols_idx_delta_minus1[i]  735  is used when the SEI message  719  relates to OLSs  721  as indicated by the scalable nesting OLS flag  731 . The scalable nesting ols_idx_delta_minus1[i]  735  is a syntax element that contains data sufficient to derive a nesting OLS index. Specifically, the scalable nesting ols_idx_delta_minus1[i]  735  contains an OLS index for each scalable-nested SEI message in a scalable nesting SEI message. As such, the scalable nesting ols_idx_delta_minus1[i]  735  can be used to correlate the scalable-nested SEI messages to the OLSs  721 . In a specific example, the ols_idx_delta_minus1[i]  735  is used to determine a nesting OLS index (NestingOlsIdx) for each scalable-nested SEI message. The NestingOlsIdx is a syntax element that specifies the OLS index of the OLS  721  to which a corresponding scalable-nested SEI message applies. In an example, the NestingOlsIdx[i] is derived as follows: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 if( i = = 0 ) 
               
               
                   
                  NestingOlsIdx[ i ] = scalable nesting ols_idx_delta_minus1[ i ] 
               
               
                   
                 else 
               
               
                   
                  NestingOlsIdx[ i ] = NestingOlsIdx[ i − 1 ] + scalable nesting 
               
               
                   
                 ols_idx_delta_minus1[ i ] + 1. 
               
               
                   
                   
               
            
           
         
       
     
     The scalable nesting num_layers_minus1  737  is used when the SEI message  719  relates to layers  723  as indicated by the scalable nesting OLS flag  731 . The scalable nesting num_layers_minus1  737  is a syntax element that specifies the number of layers  723  to which the scalable-nested SEI messages in a scalable nesting SEI message apply. The scalable nesting num_layers_minus1  737  employs minus one format and hence includes one less than the actual value. For example, if a scalable nesting SEI message includes scalable-nested SEI messages that relate to five layers  723 , then the scalable nesting num_layers_minus1  737  is set to a value of four. 
     The layer_id[i]  739  is used when the SEI message  719  relates to layers  723  as indicated by the scalable nesting OLS flag  731 . The layer_id[i]  739  is a syntax element that specifies the nuh_layer_id value of an i-th layer to which the scalable-nested SEI messages apply. As such, the layer_id[i]  739  can be used to correlate each of the scalable-nested SEI messages to the corresponding layers  723 . 
     Accordingly, the flags described in bitstream  700  allow a HRD and/or a decoder to quickly determine the configuration of the SEI messages  719 . The HRD/decoder can employ the scalable nesting OLS flag  731  to determine if a set of scalable-nested messages relate to OLSs  721  or layers  723 . The HRD/decoder can then determine the number of corresponding OLSs  721  using scalable nesting num_olss_minus1  733  and an index of each corresponding OLS  721  using scalable nesting ols_idx_delta_minus1[i]  735  to determine how to apply the scalable-nested messages when the scalable-nested messages relate to OLSs  721 . Further, the HRD/decoder can then determine the number of corresponding layers  723  using scalable nesting num_layers_minus1  737  and an index of each corresponding layers  723  using layer_id[i]  739  to determine how to apply the scalable-nested messages when the scalable-nested messages relate to layers  723 . This approach reduces the number of SEI message  719  types. This decreases complexity and decreases a total number of message types. This in turn reduces the length of message ID data used to identify each type of message. As a result, coding efficiency is increased, which reduces processor, memory, and/or network signaling resource usage at both the encoder and the decoder. 
     The preceding information is now described in more detail herein below. Layered video coding is also referred to as scalable video coding or video coding with scalability. Scalability in video coding may be supported by using multi-layer coding techniques. A multi-layer bitstream comprises a base layer (BL) and one or more enhancement layers (ELs). Example of scalabilities includes spatial scalability, quality/signal to noise ratio (SNR) scalability, multi-view scalability, frame rate scalability, etc. When a multi-layer coding technique is used, a picture or a part thereof may be coded without using a reference picture (intra-prediction), may be coded by referencing reference pictures that are in the same layer (inter-prediction), and/or may be coded by referencing reference pictures that are in other layer(s) (inter-layer prediction). A reference picture used for inter-layer prediction of the current picture is referred to as an inter-layer reference picture (ILRP).  FIG. 6  illustrates an example of multi-layer coding for spatial scalability in which pictures in different layers have different resolutions. 
     Some video coding families provide support for scalability in separated profile(s) from the profile(s) for single-layer coding. Scalable video coding (SVC) is a scalable extension of the advanced video coding (AVC) that provides support for spatial, temporal, and quality scalabilities. For SVC, a flag is signaled in each macroblock (MB) in EL pictures to indicate whether the EL MB is predicted using the collocated block from a lower layer. The prediction from the collocated block may include texture, motion vectors, and/or coding modes. Implementations of SVC may not directly reuse unmodified AVC implementations in their design. The SVC EL macroblock syntax and decoding process differs from the AVC syntax and decoding process. 
     Scalable HEVC (SHVC) is an extension of HEVC that provides support for spatial and quality scalabilities. Multiview HEVC (MV-HEVC) is an extension of HEVC that provides support for multi-view scalability. 3D HEVC (3D-HEVC) is an extension of HEVC that provides support for 3D video coding that is more advanced and more efficient than MV-HEVC. Temporal scalability may be included as an integral part of a single-layer HEVC codec. In the multi-layer extension of HEVC, decoded pictures used for inter-layer prediction come only from the same AU and are treated as long-term reference pictures (LTRPs). Such pictures are assigned reference indices in the reference picture list(s) along with other temporal reference pictures in the current layer. Inter-layer prediction (ILP) is achieved at the prediction unit (PU) level by setting the value of the reference index to refer to the inter-layer reference picture(s) in the reference picture list(s). Spatial scalability resamples a reference picture or part thereof when an ILRP has a different spatial resolution than the current picture being encoded or decoded. Reference picture resampling can be realized at either picture level or coding block level. 
     VVC may also support layered video coding. A VVC bitstream can include multiple layers. The layers can be all independent from each other. For example, each layer can be coded without using inter-layer prediction. In this case, the layers are also referred to as simulcast layers. In some cases, some of the layers are coded using ILP. A flag in the VPS can indicate whether the layers are simulcast layers or whether some layers use ILP. When some layers use ILP, the layer dependency relationship among layers is also signaled in the VPS. Unlike SHVC and MV-HEVC, VVC may not specify OLSs. An OLS includes a specified set of layers, where one or more layers in the set of layers are specified to be output layers. An output layer is a layer of an OLS that is output. In some implementations of VVC, only one layer may be selected for decoding and output when the layers are simulcast layers. In some implementations of VVC, the entire bitstream including all layers is specified to be decoded when any layer uses ILP. Further, certain layers among the layers are specified to be output layers. The output layers may be indicated to be only the highest layer, all the layers, or the highest layer plus a set of indicated lower layers. 
     The preceding aspects contain certain problems. HEVC, including scalable extensions SHVC and MV-HEVC, may employ scalable nesting SEI message for associating SEI messages with bitstream subsets corresponding to various operation points or with specific layers or sub-layers. HEVC may also employ bitstream partition nesting for associating SEI messages with a bitstream partition in an OLS. A bitstream partition includes one or more layers of a multi-layer bitstream. Each bitstream partition nesting SEI message may be contained within a scalable nesting SEI message. This two-level nesting scheme for SEI messages for OLSs is complicated. 
     In general, this disclosure describes approaches for scalable nesting of SEI messages for output layer sets in multi-layer video bitstreams. The descriptions of the techniques are based on VVC. However, the techniques also apply to layered video coding based on other video codec specifications. 
     One or more of the above-mentioned problems may be solved as follows. Specifically, this disclosure includes methods for simple and efficient scalable nesting of SEI messages for OLSs in multi-layer video bitstreams. Instead of using a two-level nesting scheme, just one nesting SEI message is defined to directly include nesting SEI messages that apply to one or more layers in an OLS. 
     An example implementation of the preceding mechanisms is as follows. An example scalable nesting SEI message syntax is as follows. 
     
       
         
           
               
               
             
               
                   
               
               
                 scalable_nesting( payloadSize ) { 
                 Descriptor 
               
               
                   
               
             
            
               
                  nesting_ols_flag 
                 u(1) 
               
               
                  if( nesting_ols_flag ) { 
                   
               
               
                   nesting_num_olss_minus1 
                 ue(v) 
               
               
                   for( i = 0; i &lt;= nesting_num_olss_minus1; i++ ) { 
                   
               
               
                   nesting_ols_idx_delta_minus1[ i ] 
                 ue(v) 
               
               
                   if( NumLayersInOls[ NestingOlsIdx[ i ] ] &gt; 1) { 
                   
               
               
                    nesting_num_ols_layers_minus1[ i ] 
                 ue(v) 
               
               
                    for( j = 0; j &lt;= nesting_num_ols_layers_minusl[ i ]; 
                   
               
               
                    j++ ) 
                   
               
               
                     nesting_ols_layer_idx_delta_minusl[ i ][ j ] 
                 ue(v)  
               
               
                    } 
                   
               
               
                   } 
                   
               
               
                  } else { 
                   
               
               
                   nesting_all_layers_flag 
                 u(1) 
               
               
                   if( !nesting_all_layers_flag ) { 
                   
               
               
                    nesting_num_layers_minus1 
                 ue(v) 
               
               
                    for( i = 1; i &lt;= nesting_num_layers_minus1; i++ ) 
                   
               
               
                     nesting_layer_id[ i ] 
                 u(6) 
               
               
                   } 
                   
               
               
                  } 
                   
               
               
                  nesting_num_seis_minus1 
                 ue(v) 
               
               
                  while( !byte_aligned( ) ) 
                   
               
               
                   nesting_zero_bit /* equal to 0 */ 
                 u(1) 
               
               
                  for( i = 0; i &lt;= nesting_num_seis_minus1 ; i++ ) 
                   
               
               
                   sei_message( ) 
                   
               
               
                 } 
               
               
                   
               
            
           
         
       
     
     In an alternative example, a flag may be added when nesting_ols_flag is equal to one. This flag may be set equal to one to indicate that the scalable-nested SEI messages apply to all OLSs and are applicable to all layers in each OLS. When this flag is set equal to one, all syntax elements after this flag until nesting_num_seis_minus1 are not signaled. In another alternative example, a flag may be employed and set equal to one to indicate that the scalable-nested SEI messages apply to all OLSs. When this flag is equal to one, the syntax element nesting_num_olss_minus1 and the list of syntax elements nesting_ols_idx_delta_minus1[i] are not signaled. In another alternative example, the nesting OLS index values signaled by the syntax elements nesting_ols_idx_delta_minus1[i] are directly coded instead of being delta coded. In another alternative example, a flag may be employed and set equal to one to indicate that the scalable-nested SEI messages apply to all layers of the OLS. When this flag is equal to one, the lists of syntax elements nesting_num_ols_layers_minus1[i] and nesting_ols_layer_idx_delta_minus1[i][j] are not signaled. In another alternative example, the nesting OLS layer index values signaled by the syntax elements nesting_ols_layer_idx_delta_minus1[i][j] are directly coded instead of being delta coded. 
     An example scalable nesting SEI message semantics is as follows. 
     A scalable nesting SEI message provides a mechanism to associate SEI messages with specific layers in the context of specific OLSs or with specific layers not in the context of an OLS. A scalable nesting SEI message contains one or more SEI messages. The SEI messages contained in the scalable nesting SEI message are also referred to as the scalable-nested SEI messages. Bitstream conformance may require that the following restrictions apply when SEI messages are contained in a scalable nesting SEI message. 
     An SEI message that has payloadType equal to one hundred thirty two (decoded picture hash) or one hundred thirty three (scalable nesting) should not be contained in a scalable nesting SEI message. When a scalable nesting SEI message contains a buffering period, picture timing, or decoding unit information SEI message, the scalable nesting SEI message should not contain any other SEI message with payloadType not equal to zero (buffering period), one (picture timing), or one hundred thirty (decoding unit information). 
     Bitstream conformance may also require that the following restrictions apply on the value of the nal_unit_type of the SEI NAL unit containing a scalable nesting SEI message. When a scalable nesting SEI message contains an SEI message that has payloadType equal to zero (buffering period), one (picture timing), one hundred thirty (decoding unit information), one hundred forty five (dependent RAP indication), or one hundred sixty eight (frame-field information), the SEI NAL unit containing the scalable nesting SEI message should have a nal_unit_type set equal to PREFIX_SEI_NUT. When a scalable nesting SEI message contains an SEI message that has payloadType equal to one hundred thirty two (decoded picture hash), the SEI NAL unit containing the scalable nesting SEI message should have a nal_unit_type set equal to SUFFIX_SEI_NUT. 
     A nesting_ols_flag may be set equal to one to specify that the scalable-nested SEI messages apply to specific layers in the context of specific OLSs. The nesting_ols_flag may be set equal to zero to specify that that the scalable-nested SEI messages generally apply (e.g., not in the context of an OLS) to specific layers. 
     Bitstream conformance may require that the following restrictions are applied to the value of nesting_ols_flag. When the scalable nesting SEI message contains an SEI message that has payloadType equal to zero (buffering period), one (picture timing), or one hundred thirty (decoding unit information), the value of nesting_ols_flag should be equal to one. When the scalable nesting SEI message contains an SEI message that has payloadType equal to a value in VclAssociatedSeiList, the value of nesting_ols_flag should be equal to zero. 
     A nesting_num_olss_minus1 plus one specifies the number of OLSs to which the scalable-nested SEI messages apply. The value of nesting_num_olss_minus1 should be in the range of zero to TotalNumOlss−1, inclusive. The nesting_ols_idx_delta_minus1[i] is used to derive the variable NestingOlsIdx[i] that specifies the OLS index of the i-th OLS to which the scalable-nested SEI messages apply when nesting_ols_flag is equal to one. The value of nesting_ols_idx_delta_minus1[i] should be in the range of zero to TotalNumOlss−2, inclusive. The variable NestingOlsIdx[i] may be derived as follows: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 if( i = = 0 ) 
               
               
                   
                  NestingOlsIdx[ i ] = nesting_ols_idx_delta_minus1[ i ] 
               
               
                   
                 else 
               
               
                   
                  NestingOlsIdx[ i ] = NestingOlsIdx[ i − 1 ] + 
               
               
                   
                  nesting_ols_idx_delta_minus1[ i ] + 1 
               
               
                   
                   
               
            
           
         
       
     
     The nesting_num_ols_layers_minus1[i] plus one specifies the number of layers to which the scalable-nested SEI messages apply in the context of the NestingOlsIdx[i]-th OLS. The value of nesting_num_ols_layers_minus1[i] should be in the range of zero to NumLayersInOls[NestingOlsIdx[i]]−1, inclusive. 
     The nesting_ols_layer_idx_delta_minus1[i][j] is used to derive the variable NestingOlsLayerIdx[i][j] that specifies the OLS layer index of the j-th layer to which the scalable-nested SEI messages apply in the context of the NestingOlsIdx[i]-th OLS when nesting_ols_flag is equal to one. The value of nesting_ols_layer_idx_delta_minus1[i] should be in the range of zero to NumLayersInOls[nestingOlsIdx[i]]−two, inclusive. 
     The variable NestingOlsLayerIdx[i][j] may be derived as follows: 
     
       
         
           
               
             
               
                   
               
             
            
               
                 if( j = = 0 ) 
               
               
                  NestingOlsLayerIdx[ i ][ j ] = nesting_ols_layer_idx_delta_minus1[ 
               
               
                  i ][ j ] 
               
               
                 else 
               
               
                  NestingOlsLayerIdx[ i ][ j ] = NestingOlsLayerIdx[ i ][ j − 1 ] + 
               
               
                   nesting_ols_layer_idx_delta_minus1[ i ][ j ] + 1 
               
               
                   
               
            
           
         
       
     
     The lowest value among all values of LayerIdInOls[NestingOlsIdx[i]][NestingOlsLayerIdx[i][0]] for i in the range of zero to nesting_num_olss_minus1, inclusive, should be equal to nuh_layer_id of the current SEI NAL unit (e.g., the SEI NAL unit containing the scalable nesting SEI message). The nesting_all_layers_flag may be set equal to one to specify that the scalable-nested SEI messages generally apply to all layers that have nuh_layer_id greater than or equal to the nuh_layer_id of the current SEI NAL unit. The nesting_all_layers_flag may be set equal to zero to specify that the scalable-nested SEI messages may or may not generally apply to all layers that have nuh_layer_id greater than or equal to the nuh_layer_id of the current SEI NAL unit. 
     The nesting_num_layers_minus1 plus one specifies the number of layers to which the scalable-nested SEI messages generally apply. The value of nesting_num_layers_minus1 should be in the range of zero to vps_max_layers_minus1−GeneralLayerIdx[nuh_layer_id], inclusive, where nuh_layer_id is the nuh_layer_id of the current SEI NAL unit. The nesting_layer_id[i] specifies the nuh_layer_id value of the i-th layer to which the scalable-nested SEI messages generally apply when nesting_all_layers_flag is equal to zero. The value of nesting_layer_id[i] should be greater than nuh_layer_id, where nuh_layer_id is the nuh_layer_id of the current SEI NAL unit. 
     When the nesting_ols_flag is equal to one, the variable NestingNumLayers, specifying the number of layer to which the scalable-nested SEI messages generally apply, and the list NestingLayerId[i] for i in the range of zero to NestingNumLayers−1, inclusive, specifying the list of nuh_layer_id value of the layers to which the scalable-nested SEI messages generally apply, are derived as follows, where nuh_layer_id is the nuh_layer_id of the current SEI NAL unit: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 if( nesting_all_layers_flag ) { 
               
               
                   
                  NestingNumLayers = 
               
               
                   
                 vps_max_layers_minus1 + 1 − GeneralLayerIdx[ nuh_layer_id ] 
               
               
                   
                  for( i = 0; i &lt; NestingNumLayers; i ++) 
               
               
                   
                   NestingLayerId[ i ] = vps_layer_id[ GeneralLayerIdx[ 
               
               
                   
                   nuh_layer_id ] + i ] (D-2) 
               
               
                   
                 } else { 
               
               
                   
                  NestingNumLayers = nesting_num_layers_minus1 + 1 
               
               
                   
                  for( i = 0; i &lt; NestingNumLayers; i ++) 
               
               
                   
                   NestingLayerId[ i ] = ( i = = 0 ) ? nuh_layer_id : 
               
               
                   
                   nesting_layer_id[ i ] 
               
               
                   
                 } 
               
               
                   
                   
               
            
           
         
       
     
     The nesting_num_seis_minus1 plus one specifies the number of scalable-nested SEI messages. The value of nesting_num_seis_minus1 should be in the range of zero to sixty three, inclusive. The nesting_zero_bit should be set equal to zero. 
       FIG. 8  is a schematic diagram of an example video coding device  800 . The video coding device  800  is suitable for implementing the disclosed examples/embodiments as described herein. The video coding device  800  comprises downstream ports  820 , upstream ports  850 , and/or transceiver units (Tx/Rx)  810 , including transmitters and/or receivers for communicating data upstream and/or downstream over a network. The video coding device  800  also includes a processor  830  including a logic unit and/or central processing unit (CPU) to process the data and a memory  832  for storing the data. The video coding device  800  may also comprise electrical, optical-to-electrical (OE) components, electrical-to-optical (EO) components, and/or wireless communication components coupled to the upstream ports  850  and/or downstream ports  820  for communication of data via electrical, optical, or wireless communication networks. The video coding device  800  may also include input and/or output (I/O) devices  860  for communicating data to and from a user. The I/O devices  860  may include output devices such as a display for displaying video data, speakers for outputting audio data, etc. The I/O devices  860  may also include input devices, such as a keyboard, mouse, trackball, etc., and/or corresponding interfaces for interacting with such output devices. 
     The processor  830  is implemented by hardware and software. The processor  830  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  830  is in communication with the downstream ports  820 , Tx/Rx  810 , upstream ports  850 , and memory  832 . The processor  830  comprises a coding module  814 . The coding module  814  implements the disclosed embodiments described herein, such as methods  100 ,  900 , and  1000 , which may employ a multi-layer video sequence  600  and/or a bitstream  700 . The coding module  814  may also implement any other method/mechanism described herein. Further, the coding module  814  may implement a codec system  200 , an encoder  300 , a decoder  400 , and/or a HRD  500 . For example, the coding module  814  may be employed to implement a HRD. Further, the coding module  814  may be employed to encode scalable nesting SEI messages with corresponding flags to support clear and concise signaling of scalable-nested SEI messages in the scalable nesting SEI messages. Accordingly, the coding module  814  may be configured to perform mechanisms to address one or more of the problems discussed above. Hence, coding module  814  causes the video coding device  800  to provide additional functionality and/or coding efficiency when coding video data. As such, the coding module  814  improves the functionality of the video coding device  800  as well as addresses problems that are specific to the video coding arts. Further, the coding module  814  effects a transformation of the video coding device  800  to a different state. Alternatively, the coding module  814  can be implemented as instructions stored in the memory  832  and executed by the processor  830  (e.g., as a computer program product stored on a non-transitory medium). 
     The memory  832  comprises one or more memory types such as disks, tape drives, solid-state drives, read only memory (ROM), random access memory (RAM), flash memory, ternary content-addressable memory (TCAM), static random-access memory (SRAM), etc. The memory  832  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. 
       FIG. 9  is a flowchart of an example method  900  of encoding a video sequence into a bitstream, such as bitstream  700 , including scalable nesting SEI messages. Method  900  may be employed by an encoder, such as a codec system  200 , an encoder  300 , and/or a video coding device  800  when performing method  100 . Further, the method  900  may operate on a HRD  500  and hence may perform conformance tests on a multi-layer video sequence  600 . 
     Method  900  may begin when an encoder receives a video sequence and determines to encode that video sequence into a multi-layer bitstream, for example based on user input. At step  901 , the encoder encodes the video sequence into one or more layers and encodes the layers into a multi-layer bitstream. A layer may include a set of VCL NAL units with the same layer ID and associated non-VCL NAL units. For example, a layer may include a set of VCL NAL units that contain video data of encoded pictures as well as any parameter sets used to code such pictures. The layers may be included in OLSs. For example, an OLS may contain an output layer and any supporting layers that may be used to decode the output layer according to inter-layer prediction. As such, an OLS may contain sufficient data to decode a representation of the video sequence, for example at a corresponding image size, SNR, frame rate, etc. As a video sequence may be coded into several representations, the video sequence can include several layers that are organized into several OLSs as desired. In this way, the encoder can select an OLS with corresponding layers for transmission to a decoder upon request. 
     At step  903 , the encoder encodes SEI messages into the bitstream. A SEI message is a syntax structure that contains data that is not used for decoding. For example, SEI messages may contain data to support conformance testing to ensure the bitstream conforms to standards. To support simplified signaling when used in conjunction with a multi-layer bitstream, the SEI messages are encoded as scalable nesting SEI messages. A scalable nesting SEI message includes one or more scalable-nested SEI messages. The scalable-nested SEI messages may each apply to one or more of the OLSs and/or one or more of the layers. In order to support simplified signaling, a scalable nesting SEI message includes a scalable nesting OLS flag. The scalable nesting OLS flag may be set to specify whether the scalable-nested SEI messages in the scalable nesting SEI message apply to specific OLSs or to specific layers. For example, the scalable nesting OLS flag can be set to one when specifying that the scalable-nested SEI messages apply to specific/corresponding OLSs (e.g., and not layers). As another example, the scalable nesting OLS flag can be set to zero when specifying that the scalable-nested SEI messages apply to specific/corresponding layers (e.g., and not OLSs). A scalable nesting SEI message may contain several types of scalable-nested SEI messages. As a specific example, the scalable-nested SEI messages may include buffering period SEI messages, picture timing SEI messages, and/or decoding unit information SEI messages. The scalable nesting OLS flag can be set to one to indicate the scalable-nested SEI messages apply to specific OLSs (e.g., and not layers) when the scalable nesting SEI message contains any SEI message that has a payload type of buffering period, picture timing, or decoding unit information. 
     The scalable nesting SEI messages may contain other data to indicate how corresponding scalable-nested SEI messages should be employed by the HRD at the encoder. For example, a scalable nesting SEI message may include a scalable nesting num_olss_minus1 syntax element that specifies a number of OLSs to which the corresponding scalable-nested SEI messages apply. The scalable nesting num_olss_minus1 syntax element may be employed when the scalable nesting OLS flag is set to one indicating the scalable-nested SEI messages apply to OLSs. A value of the scalable nesting num_olss_minus1 syntax element may be constrained to remain in a range of zero to a TotalNumOlss−1, inclusive. In a similar manner, a scalable nesting SEI message may include a scalable nesting num_layers_minus1 that specifies a number of layers to which the corresponding scalable-nested SEI messages apply when the scalable nesting OLS flag is set to zero indicating the scalable-nested SEI messages apply to layers. 
     A scalable nesting SEI message may also include a scalable nesting ols_idx_delta_minus1[i] syntax element, which is used to derive a nesting OLS index (NestingOlsIdx[i]) that specifies an OLS index of an i-th OLS to which the scalable-nested SEI messages apply when the scalable nesting OLS flag is equal to one indicating the scalable-nested SEI messages apply to OLSs. Specifically, the scalable nesting ols_idx_delta_minus1[i] syntax element can be employed to specify a corresponding OLS for each scalable-nested SEI message. As such, the scalable nesting num_olss_minus1 can be used to determine the number of OLSs that are referenced by a scalable nesting SEI message and the scalable nesting ols_idx_delta_minus1[i] can be used to correlate each scalable-nested SEI message to a corresponding OLS. A value of the scalable nesting ols_idx_delta_minus1[i] syntax element may be constrained to remain in a range of zero to the TotalNumOlss−2, inclusive. In a specific example, NestingOlsIdx[i] is derived as follows: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 if( i = = 0 ) 
               
               
                   
                  NestingOlsIdx[ i ] = scalable nesting ols_idx_delta_minus1[ i ] 
               
               
                   
                 else 
               
               
                   
                  NestingOlsIdx[ i ] = NestingOlsIdx[ i − 1 ] + scalable nesting 
               
               
                   
                 ols_idx_delta_minus1[ i ] + 1. 
               
               
                   
                   
               
            
           
         
       
     
     In a similar manner, the scalable nesting SEI message may include a scalable nesting layer_id[i] when the scalable nesting OLS flag is equal to zero indicating the scalable-nested SEI messages apply to layers. The scalable nesting layer_id[i] specifies the layer ID (e.g., nuh_layer_id) value of the i-th layer to which the scalable-nested SEI messages apply. 
     At step  905 , a HRD operating at the encoder can perform a set of bitstream conformance tests based on the scalable nesting SEI message. For example, the HRD can read the flags in a scalable nesting SEI message to determine how to interpret the scalable-nested SEI messages contained in the scalable nesting SEI message. The HRD can then read the scalable-nested SEI messages to determine how to check OLSs and/or layers for conformance to standards. The HRD can then perform conformance tests on the OLS and/or layers based on the scalable-nested SEI messages and/or corresponding flags in the scalable nesting SEI message. At step  907 , the encoder can store the bitstream for communication toward a decoder upon request. 
       FIG. 10  is a flowchart of an example method  1000  of decoding a video sequence from a bitstream, such as bitstream  700 , including scalable nesting SEI messages. Method  1000  may be employed by a decoder, such as a codec system  200 , a decoder  400 , and/or a video coding device  800  when performing method  100 . Further, method  1000  may be employed on a multi-layer video sequence  600  that has been checked for conformance by a HRD, such as HRD  500 . 
     Method  1000  may begin when a decoder begins receiving a bitstream of coded data representing a multi-layer video sequence, for example as a result of method  900 . At step  1001 , the decoder receives a bitstream comprising one or more layers. A layer may include a set of VCL NAL units with the same layer ID and associated non-VCL NAL units. For example, a layer may include a set of VCL NAL units that contain video data of encoded pictures as well as any parameter sets used to code such pictures. The layers may be included in an OLS. For example, an OLS may contain an output layer and any supporting layers that may be used to decode the output layer according to inter-layer prediction. As such, an OLS may contain sufficient data to decode a representation of the video sequence, for example at a corresponding image size, SNR, frame rate, etc. As a video sequence may be coded into several representations, the video sequence can include several layers that are organized into several OLSs as desired. In this way, the decoder can request and receive a specified OLS with corresponding layers as desired to decode and display a particular representation of the video sequence. 
     The bitstream also includes one or more scalable nesting SEI messages. A SEI message is a syntax structure that contains data that is not used for decoding. For example, SEI messages may contain data to support conformance testing to ensure the bitstream conforms to standards. To support simplified signaling when used in conjunction with a multi-layer bitstream, the SEI messages are coded in scalable nesting SEI messages. A scalable nesting SEI message includes one or more scalable-nested SEI messages. The scalable-nested SEI messages may each apply to one or more of the OLSs and/or one or more of the layers. In order to support simplified signaling, a scalable nesting SEI message includes a scalable nesting OLS flag. The scalable nesting OLS flag may be set to specify whether the scalable-nested SEI messages in the scalable nesting SEI message apply to specific OLSs or to specific layers. For example, the scalable nesting OLS flag can be set to one when specifying that the scalable-nested SEI messages apply to specific/corresponding OLSs (e.g., and not layers). As another example, the scalable nesting OLS flag can be set to zero when specifying that the scalable-nested SEI messages apply to specific/corresponding layers (e.g., and not OLSs). A scalable nesting SEI message may contain several types of scalable-nested SEI messages. As a specific example, the scalable-nested SEI messages may include buffering period SEI messages, picture timing SEI messages, and/or decoding unit information SEI messages. The scalable nesting OLS flag can be set to one to indicate the scalable-nested SEI messages apply to specific OLSs (e.g., and not layers) when the scalable nesting SEI message contains any SEI message that has a payload type of buffering period, picture timing, or decoding unit information. 
     The scalable nesting SEI messages may contain other data to indicate how corresponding scalable-nested SEI messages should be employed by a HRD at the encoder. For example, a scalable nesting SEI message may include a scalable nesting num_olss_minus1 syntax element that specifies a number of OLSs to which the corresponding scalable-nested SEI messages apply. The scalable nesting num_olss_minus1 syntax element may be employed when the scalable nesting OLS flag is set to one indicating the scalable-nested SEI messages apply to OLSs. A value of the scalable nesting num_olss_minus1 syntax element may be constrained to remain in a range of zero to a TotalNumOlss−1, inclusive. In a similar manner, a scalable nesting SEI message may include a scalable nesting num_layers_minus1 that specifies a number of layers to which the corresponding scalable-nested SEI messages apply when the scalable nesting OLS flag is set to zero indicating the scalable-nested SEI messages apply to layers. 
     A scalable nesting SEI message may also include a scalable nesting ols_idx_delta_minus1[i] syntax element, which is used to derive a nesting OLS index (NestingOlsIdx[i]) that specifies an OLS index of an i-th OLS to which the scalable-nested SEI messages apply when the scalable nesting OLS flag is equal to one indicating the scalable-nested SEI messages apply to OLSs. Specifically, the scalable nesting ols_idx_delta_minus1[i] syntax element can be employed to specify a corresponding OLS for each scalable-nested SEI message. As such, the scalable nesting num_olss_minus1 can be used to determine the number of OLSs that are referenced by a scalable nesting SEI message and the scalable nesting ols_idx_delta_minus1[i] can be used to correlate each scalable-nested SEI message to a corresponding OLS. A value of the scalable nesting ols_idx_delta_minus1[i] syntax element may be constrained to remain in a range of zero to the TotalNumOlss−2, inclusive. In a specific example, NestingOlsIdx[i] is derived as follows: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 if( i = = 0 ) 
               
               
                   
                  NestingOlsIdx[ i ] = scalable nesting ols_idx_delta_minus1[ i ] 
               
               
                   
                 else 
               
               
                   
                  NestingOlsIdx[ i ] = NestingOlsIdx[ i − 1 ] + scalable nesting 
               
               
                   
                 ols_idx_delta_minus1[ i ] + 1. 
               
               
                   
                   
               
            
           
         
       
     
     In a similar manner, the scalable nesting SEI message may include a scalable nesting layer_id[i] when the scalable nesting OLS flag is equal to zero indicating the scalable-nested SEI messages apply to layers. The scalable nesting layer_id[i] specifies the layer ID (e.g., nuh_layer_id) value of the i-th layer to which the scalable-nested SEI messages apply. 
     At step  1003 , the decoder can decode a coded picture from the one or more layers, based on the scalable-nested SEI messages, to produce a decoded picture. For example, the presence of the scalable nesting SEI message can indicate that the bitstream has been checked by a HRD at the encoder and hence conforms to standards. Accordingly, the presence of the scalable nesting SEI message indicates the bitstream can be decoded. At step  1005 , the decoder can forward the decoded picture for display as part of a decoded video sequence. 
       FIG. 11  is a schematic diagram of an example system  1100  for coding a video sequence using a bitstream including scalable nesting SEI messages. System  1100  may be implemented by an encoder and a decoder such as a codec system  200 , an encoder  300 , a decoder  400 , and/or a video coding device  800 . Further, the system  1100  may employ a HRD  500  to perform conformance tests on a multi-layer video sequence  600  and/or a bitstream  700 . In addition, system  1100  may be employed when implementing method  100 ,  900 , and/or  1000 . 
     The system  1100  includes a video encoder  1102 . The video encoder  1102  comprises an encoding module  1103  for encoding a bitstream comprising one or more layers. The encoding module  1103  is further for encoding into the bitstream a scalable nesting supplemental enhancement information (SEI) message, wherein the scalable nesting SEI message includes one or more scalable-nested SEI messages and a scalable nesting output layer set (OLS) flag, and wherein the scalable nesting OLS flag is set to specify whether the scalable-nested SEI messages apply to specific OLSs or specific layers. The video encoder  1102  further comprises a HRD module  1105  for performing a set of bitstream conformance tests based on the scalable nesting SEI message. The video encoder  1102  further comprises a storing module  1106  for storing the bitstream for communication toward a decoder. The video encoder  1102  further comprises a transmitting module  1107  for transmitting the bitstream toward a video decoder  1110 . The video encoder  1102  may be further configured to perform any of the steps of method  900 . 
     The system  1100  also includes a video decoder  1110 . The video decoder  1110  comprises a receiving module  1111  for receiving a bitstream comprising one or more layers and a scalable nesting supplemental enhancement information (SEI) message, wherein the scalable nesting SEI message includes one or more scalable-nested SEI messages and a scalable nesting output layer set (OLS) flag, and wherein the scalable nesting OLS flag is set to specify whether the scalable-nested SEI messages apply to specific OLSs or specific layers. The video decoder  1110  further comprises a decoding module  1113  for decoding a coded picture from the one or more layers, based on the scalable-nested SEI messages, to produce a decoded picture. The video decoder  1110  further comprises a forwarding module  1115  for forwarding the decoded picture for display as part of a decoded video sequence. The video decoder  1110  may be further configured to perform any of the steps of method  1000 . 
     A first component is directly coupled to a second component when there are no intervening components, except for a line, a trace, or another medium between the first component and the second component. The first component is indirectly coupled to the second component when there are intervening components other than a line, a trace, or another medium between the first component and the second component. The term “coupled” and its variants include both directly coupled and indirectly coupled. The use of the term “about” means a range including ±10% of the subsequent number unless otherwise stated. 
     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 may 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, components, techniques, or methods without departing from the scope of the present disclosure. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and may be made without departing from the spirit and scope disclosed herein.