Patent Publication Number: US-2022217388-A1

Title: Decoded Picture Buffer Operation For Resolution Changes

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a continuation of International Application No. PCT/US2020/051314, filed Sep. 17, 2020 by Ye-Kui Wang, and titled “Decoded Picture Buffer Operation For Resolution Changes,” which claims the benefit of U.S. Provisional Patent Application No. 62/905,236 filed Sep. 24, 2019 by Ye-Kui Wang, and titled “Video Coding Improvements,” which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure is generally related to video coding, and is specifically related to improvements in signaling parameters to support coding of multi-layer bitstreams. 
     BACKGROUND 
     The amount of video data needed to depict even a relatively short video can be substantial, which may result in difficulties when the data is to be streamed or otherwise communicated across a communications network with limited bandwidth capacity. Thus, video data is generally compressed before being communicated across modern day telecommunications networks. The size of a video could also be an issue when the video is stored on a storage device because memory resources may be limited. Video compression devices often use software and/or hardware at the source to code the video data prior to transmission or storage, thereby decreasing the quantity of data needed to represent digital video images. The compressed data is then received at the destination by a video decompression device that decodes the video data. With limited network resources and ever increasing demands of higher video quality, improved compression and decompression techniques that improve compression ratio with little to no sacrifice in image quality are desirable. 
     SUMMARY 
     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 a plurality of pictures; setting, by the processor, a value of a no output of prior pictures flag (NoOutputOfPriorPicsFlag) when a value of maximum picture width in luma samples (PicWidthMaxInSamplesY) for a current access unit (AU) is different from a value of PicWidthMaxInSamplesY for a preceding AU in decoding order; emptying a decoded picture buffer (DPB) without output of contained pictures based on the value of the NoOutputOfPriorPicsFlag; decoding, by the processor, a current picture; and storing, by the DPB, the current picture. 
     Video coding systems may code a video sequence into multiple layers. Such layers can be extracted from an encoded bitstream and transmitted to a decoder to allow the decoder to display the video sequence using different characteristics as desired. For example, pictures can be transmitted at a higher resolution/larger size when network conditions are optimal and transmitted at a lower resolution/lower size when network conditions deteriorate. One problem with this implementation is that a DPB at the decoder can overflow during layer changes, which can cause a poor user experience. A NoOutputOfPriorPicsFlag can be used to address this issue. The NoOutputOfPriorPicsFlag can be set to cause the DPB to be emptied to make room for new pictures. This may occur even if the preceding pictures in the DPB are designated for output. In some video coding systems, the NoOutputOfPriorPicsFlag is set based on a picture size change that is detected due to a change in the width or height between successive pictures. Adaptive resolution change (ARC) is a mechanism for assisting with resolution changes. ARC may dynamically change the resolution of the pictures in the DPB to match the resolution of subsequent pictures in order to allow resolution changes to occur at pictures other than IRAP pictures. The usage of ARC may prevent the NoOutputOfPriorPicsFlag from operating properly. Specifically, ARC may change the spatial resolution of a preceding picture be the same as a current picture, which prevents the check for picture size changes from operating correctly. As such, ARC may result in DPB overflows by effectively disabling usage of the NoOutputOfPriorPicsFlag. 
     The present aspect provides a mechanism for enabling a NoOutputOfPriorPicsFlag when ARC is employed. For example, the decoder can check for a change in maximum picture height and/or maximum picture width instead of checking for picture height and width. The maximum picture height and maximum picture width are not altered by ARC. Accordingly, the NoOutputOfPriorPicsFlag can be properly set to prevent DPB overflows due to spatial resolution changes even when ARC is used to harmonize resolutions during layer changes. Hence, various errors may be avoided. A similar process can also be used by a hypothetical reference decoder (HRD) operated at an encoder when checking an encoded bitstream for standards conformance. As a result, the functionality of the encoder and the decoder is increased. Further, coding efficiency may be 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, further comprising setting, by the processor, the value of the NoOutputOfPriorPicsFlag when a value of maximum picture height in luma samples (PicHeightMaxInSamplesY) for the current AU is different from a value of PicHeightMaxInSamplesY for the preceding AU in decoding order. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the DPB is emptied before decoding the current picture. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the DPB is emptied after parsing a slice header of a first slice of the current picture. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, further comprising decrementing a DPB fullness variable for each picture removed when emptying the DPB. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, further comprising constructing, by the processor, a reference picture list and performing reference picture list marking for the current AU prior to setting the NoOutputOfPriorPicsFlag. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the DPB is emptied when a preceding picture is marked as unused for reference. 
     In an embodiment, the disclosure includes a method implemented by an encoder, the method comprising: encoding, by a processor of the encoder, a plurality of pictures into a bitstream; checking, by a HRD operating on the processor, the bitstream for conformance by: setting, by the processor, a value of a NoOutputOfPriorPicsFlag when a value of PicWidthMaxInSamplesY for a current AU is different from a value of PicWidthMaxInSamplesY for a preceding AU in decoding order; emptying a DPB without selecting contained pictures for output based on the value of the NoOutputOfPriorPicsFlag; decoding, by the processor, a current picture; storing, by the DPB, the current picture; and storing, by a memory coupled to the processor, the bitstream for communication toward a decoder. 
     Video coding systems may code a video sequence into multiple layers. Such layers can be extracted from an encoded bitstream and transmitted to a decoder to allow the decoder to display the video sequence using different characteristics as desired. For example, pictures can be transmitted at a higher resolution/larger size when network conditions are optimal and transmitted at a lower resolution/lower size when network conditions deteriorate. One problem with this implementation is that a DPB at the decoder can overflow during layer changes, which can cause a poor user experience. A NoOutputOfPriorPicsFlag can be used to address this issue. The NoOutputOfPriorPicsFlag can be set to cause the DPB to be emptied to make room for new pictures. This may occur even if the preceding pictures in the DPB are designated for output. In some video coding systems, the NoOutputOfPriorPicsFlag is set based on a picture size change that is detected due to a change in the width or height between successive pictures. ARC is a mechanism for assisting with resolution changes. ARC may dynamically change the resolution of the pictures in the DPB to match the resolution of subsequent pictures in order to allow resolution changes to occur at pictures other than TRAP pictures. The usage of ARC may prevent the NoOutputOfPriorPicsFlag from operating properly. Specifically, ARC may change the spatial resolution of a preceding picture be the same as a current picture, which prevents the check for picture size changes from operating correctly. As such, ARC may result in DPB overflows by effectively disabling usage of the NoOutputOfPriorPicsFlag. 
     The present aspect provides a mechanism for enabling a NoOutputOfPriorPicsFlag when ARC is employed. For example, the decoder can check for a change in maximum picture height and/or maximum picture width instead of checking for picture height and width. The maximum picture height and maximum picture width are not altered by ARC. Accordingly, the NoOutputOfPriorPicsFlag can be properly set to prevent DPB overflows due to spatial resolution changes even when ARC is used to harmonize resolutions during layer changes. Hence, various errors may be avoided. A similar process can also be used by a hypothetical reference decoder (HRD) operated at an encoder when checking an encoded bitstream for standards conformance. As a result, the functionality of the encoder and the decoder is increased. Further, coding efficiency may be 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, further comprising setting, by the HRD operating on the processor, the value of the NoOutputOfPriorPicsFlag when a value of PicHeightMaxInSamplesY for the current AU is different from a value of PicHeightMaxInSamplesY for the preceding AU in decoding order. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the DPB is emptied before decoding the current picture. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the DPB is emptied after parsing a slice header of a first slice of the current picture. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, further comprising decrementing, by the HRD operating on the processor, a DPB fullness variable for each picture removed when emptying the DPB. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, further comprising constructing, by the HRD operating on the processor, a reference picture list and performing reference picture list marking. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the DPB is emptied when a preceding picture is marked as unused for reference. 
     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 a plurality of pictures; a setting means for setting a value of a NoOutputOfPriorPicsFlag when a value of PicWidthMaxInSamplesY for a current AU is different from a value of PicWidthMaxInSamplesY for a preceding AU in decoding order; an emptying means for emptying a DPB without output of contained pictures based on the value of the NoOutputOfPriorPicsFlag; a decoding means for decoding a current picture; a storing means for storing the current picture in a DPB; and an outputting means for outputting the current picture from the DPB for display as part of a decoded video sequence. 
     Video coding systems may code a video sequence into multiple layers. Such layers can be extracted from an encoded bitstream and transmitted to a decoder to allow the decoder to display the video sequence using different characteristics as desired. For example, pictures can be transmitted at a higher resolution/larger size when network conditions are optimal and transmitted at a lower resolution/lower size when network conditions deteriorate. One problem with this implementation is that a DPB at the decoder can overflow during layer changes, which can cause a poor user experience. A NoOutputOfPriorPicsFlag can be used to address this issue. The NoOutputOfPriorPicsFlag can be set to cause the DPB to be emptied to make room for new pictures. This may occur even if the preceding pictures in the DPB are designated for output. In some video coding systems, the NoOutputOfPriorPicsFlag is set based on a picture size change that is detected due to a change in the width or height between successive pictures. ARC is a mechanism for assisting with resolution changes. ARC may dynamically change the resolution of the pictures in the DPB to match the resolution of subsequent pictures in order to allow resolution changes to occur at pictures other than TRAP pictures. The usage of ARC may prevent the NoOutputOfPriorPicsFlag from operating properly. Specifically, ARC may change the spatial resolution of a preceding picture be the same as a current picture, which prevents the check for picture size changes from operating correctly. As such, ARC may result in DPB overflows by effectively disabling usage of the NoOutputOfPriorPicsFlag. 
     The present aspect provides a mechanism for enabling a NoOutputOfPriorPicsFlag when ARC is employed. For example, the decoder can check for a change in maximum picture height and/or maximum picture width instead of checking for picture height and width. The maximum picture height and maximum picture width are not altered by ARC. Accordingly, the NoOutputOfPriorPicsFlag can be properly set to prevent DPB overflows due to spatial resolution changes even when ARC is used to harmonize resolutions during layer changes. Hence, various errors may be avoided. A similar process can also be used by a hypothetical reference decoder (HRD) operated at an encoder when checking an encoded bitstream for standards conformance. As a result, the functionality of the encoder and the decoder is increased. Further, coding efficiency may be 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 plurality of pictures into a bitstream; a HRD means for checking the bitstream for conformance by: setting a value of a NoOutputOfPriorPicsFlag when a value of PicWidthMaxInSamplesY for a current AU is different from a value of PicWidthMaxInSamplesY for a preceding AU in decoding order; emptying a DPB without selecting contained pictures for output based on the value of the NoOutputOfPriorPicsFlag; decoding a current picture; and storing the current picture in the DPB; and a storing means for storing the bitstream for communication toward a decoder. 
     Video coding systems may code a video sequence into multiple layers. Such layers can be extracted from an encoded bitstream and transmitted to a decoder to allow the decoder to display the video sequence using different characteristics as desired. For example, pictures can be transmitted at a higher resolution/larger size when network conditions are optimal and transmitted at a lower resolution/lower size when network conditions deteriorate. One problem with this implementation is that a DPB at the decoder can overflow during layer changes, which can cause a poor user experience. A NoOutputOfPriorPicsFlag can be used to address this issue. The NoOutputOfPriorPicsFlag can be set to cause the DPB to be emptied to make room for new pictures. This may occur even if the preceding pictures in the DPB are designated for output. In some video coding systems, the NoOutputOfPriorPicsFlag is set based on a picture size change that is detected due to a change in the width or height between successive pictures. ARC is a mechanism for assisting with resolution changes. ARC may dynamically change the resolution of the pictures in the DPB to match the resolution of subsequent pictures in order to allow resolution changes to occur at pictures other than TRAP pictures. The usage of ARC may prevent the NoOutputOfPriorPicsFlag from operating properly. Specifically, ARC may change the spatial resolution of a preceding picture be the same as a current picture, which prevents the check for picture size changes from operating correctly. As such, ARC may result in DPB overflows by effectively disabling usage of the NoOutputOfPriorPicsFlag. 
     The present aspect provides a mechanism for enabling a NoOutputOfPriorPicsFlag when ARC is employed. For example, the decoder can check for a change in maximum picture height and/or maximum picture width instead of checking for picture height and width. The maximum picture height and maximum picture width are not altered by ARC. Accordingly, the NoOutputOfPriorPicsFlag can be properly set to prevent DPB overflows due to spatial resolution changes even when ARC is used to harmonize resolutions during layer changes. Hence, various errors may be avoided. A similar process can also be used by a hypothetical reference decoder (HRD) operated at an encoder when checking an encoded bitstream for standards conformance. As a result, the functionality of the encoder and the decoder is increased. Further, coding efficiency may be 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. 
     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 a plurality of pictures; emptying a DPB without output of contained pictures when a value of PicWidthMaxInSamplesY for a current AU is different from a value of PicWidthMaxInSamplesY for a preceding AU in decoding order; decoding, by the processor, a current picture; and storing, by the DPB, the current picture. 
     Video coding systems may code a video sequence into multiple layers. Such layers can be extracted from an encoded bitstream and transmitted to a decoder to allow the decoder to display the video sequence using different characteristics as desired. For example, pictures can be transmitted at a higher resolution/larger size when network conditions are optimal and transmitted at a lower resolution/lower size when network conditions deteriorate. One problem with this implementation is that a DPB at the decoder can overflow during layer changes, which can cause a poor user experience. A NoOutputOfPriorPicsFlag can be used to address this issue. The NoOutputOfPriorPicsFlag can be set to cause the DPB to be emptied to make room for new pictures. This may occur even if the preceding pictures in the DPB are designated for output. In some video coding systems, the NoOutputOfPriorPicsFlag is set based on a picture size change that is detected due to a change in the width or height between successive pictures. ARC is a mechanism for assisting with resolution changes. ARC may dynamically change the resolution of the pictures in the DPB to match the resolution of subsequent pictures in order to allow resolution changes to occur at pictures other than IRAP pictures. The usage of ARC may prevent the NoOutputOfPriorPicsFlag from operating properly. Specifically, ARC may change the spatial resolution of a preceding picture be the same as a current picture, which prevents the check for picture size changes from operating correctly. As such, ARC may result in DPB overflows by effectively disabling usage of the NoOutputOfPriorPicsFlag. 
     The present aspect provides a mechanism for enabling a NoOutputOfPriorPicsFlag when ARC is employed. For example, the decoder can check for a change in maximum picture height and/or maximum picture width instead of checking for picture height and width. The maximum picture height and maximum picture width are not altered by ARC. Accordingly, the NoOutputOfPriorPicsFlag can be properly set to prevent DPB overflows due to spatial resolution changes even when ARC is used to harmonize resolutions during layer changes. Hence, various errors may be avoided. A similar process can also be used by a hypothetical reference decoder (HRD) operated at an encoder when checking an encoded bitstream for standards conformance. As a result, the functionality of the encoder and the decoder is increased. Further, coding efficiency may be 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, further comprising emptying the DPB without output of contained pictures when a value of PicHeightMaxInSamplesY for the current AU is different from a value of PicHeightMaxInSamplesY for the preceding AU in decoding order. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the DPB is emptied before decoding the current picture. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the DPB is emptied after parsing a slice header of a first slice of the current picture. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, further comprising decrementing a DPB fullness variable for each picture removed when emptying the DPB. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, further comprising constructing, by the processor, a reference picture list and performing reference picture list marking for the current AU prior to emptying the DPB. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the DPB is emptied when a preceding picture is marked as unused for reference. 
     In an embodiment, the disclosure includes a method implemented by an encoder, the method comprising: encoding, by a processor of the encoder, a plurality of pictures into a bitstream; checking, by a HRD operating on the processor, the bitstream for conformance by emptying a DPB without output of contained pictures when a value of PicWidthMaxInSamplesY for a current AU is different from a value of PicWidthMaxInSamplesY for a preceding AU in decoding order; decoding, by the processor, a current picture; storing, by the DPB, the current picture; and storing, by a memory coupled to the processor, the bitstream for communication toward a decoder. 
     Video coding systems may code a video sequence into multiple layers. Such layers can be extracted from an encoded bitstream and transmitted to a decoder to allow the decoder to display the video sequence using different characteristics as desired. For example, pictures can be transmitted at a higher resolution/larger size when network conditions are optimal and transmitted at a lower resolution/lower size when network conditions deteriorate. One problem with this implementation is that a DPB at the decoder can overflow during layer changes, which can cause a poor user experience. A NoOutputOfPriorPicsFlag can be used to address this issue. The NoOutputOfPriorPicsFlag can be set to cause the DPB to be emptied to make room for new pictures. This may occur even if the preceding pictures in the DPB are designated for output. In some video coding systems, the NoOutputOfPriorPicsFlag is set based on a picture size change that is detected due to a change in the width or height between successive pictures. ARC is a mechanism for assisting with resolution changes. ARC may dynamically change the resolution of the pictures in the DPB to match the resolution of subsequent pictures in order to allow resolution changes to occur at pictures other than TRAP pictures. The usage of ARC may prevent the NoOutputOfPriorPicsFlag from operating properly. Specifically, ARC may change the spatial resolution of a preceding picture be the same as a current picture, which prevents the check for picture size changes from operating correctly. As such, ARC may result in DPB overflows by effectively disabling usage of the NoOutputOfPriorPicsFlag. 
     The present aspect provides a mechanism for enabling a NoOutputOfPriorPicsFlag when ARC is employed. For example, the decoder can check for a change in maximum picture height and/or maximum picture width instead of checking for picture height and width. The maximum picture height and maximum picture width are not altered by ARC. Accordingly, the NoOutputOfPriorPicsFlag can be properly set to prevent DPB overflows due to spatial resolution changes even when ARC is used to harmonize resolutions during layer changes. Hence, various errors may be avoided. A similar process can also be used by a hypothetical reference decoder (HRD) operated at an encoder when checking an encoded bitstream for standards conformance. As a result, the functionality of the encoder and the decoder is increased. Further, coding efficiency may be 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, further comprising emptying the DPB without output of contained pictures when a value of PicHeightMaxInSamplesY for the current AU is different from a value of PicHeightMaxInSamplesY for the preceding AU in decoding order. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the DPB is emptied before decoding the current picture. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the DPB is emptied after parsing a slice header of a first slice of the current picture. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, further comprising decrementing a DPB fullness variable for each picture removed when emptying the DPB. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, further comprising constructing, by the processor, a reference picture list and performing reference picture list marking for the current AU prior to emptying the DPB. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides, wherein the DPB is emptied when a preceding picture is marked as unused for reference. 
     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. 
         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 to support usage of a no output of prior pictures flag (NoOutputOfPriorPicsFlag) in conjunction with adaptive resolution change (ARC). 
         FIG. 10  is a flowchart of an example method of decoding a video sequence from a bitstream while employing a NoOutputOfPriorPicsFlag in conjunction with ARC. 
         FIG. 11  is a schematic diagram of an example system for coding a video sequence into a bitstream while employing a NoOutputOfPriorPicsFlag in conjunction with ARC. 
     
    
    
     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 luma sample is a brightness value of a point/pixel in a picture. A picture width is a horizontal distance between left boundary of a picture and a right boundary of a picture as measured in luma samples. A picture height is a vertical distance between top boundary of a picture and a bottom boundary of a picture as measured in luma samples. A slice is 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 that are exclusively contained in a single network abstraction layer (NAL) unit. 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) 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. Parsing is a process of analyzing a string of data to obtain logical data components. A decoded picture is a picture produced by applying a decoding process to a coded picture. 
     An AU is a set of coded pictures that are included in different layers and are associated with the same time for output from a decoded picture buffer (DPB). A current AU is an AU that corresponds to a particular output time in a decoding process and/or a HRD conformance checking process. A preceding AU is an AU that occurs prior to (e.g., immediately preceding) a current AU in decoding order. A decoding order is an order in which syntax elements are processed by a decoding process. 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 ID and associated non-VCL NAL units. 
     A DPB is a buffer holding decoded pictures for reference, output reordering, or output delay. A DPB fullness variable is a data element that indicates a number of pictures stored in a DPB. Decrementing is a reduction in value. A reference picture list is a list of reference pictures used for inter-prediction. Reference picture list marking is a process of marking a decoded picture in a DPB with a reference status, including unused for reference, used for short-term reference, and used for long-term reference, based on a constructed reference picture list for a current slice. Unused for reference is a reference status indicating a decoded picture is not used as a reference picture for any further pictures to be decoded. A no output of prior pictures flag (NoOutputOfPriorPicsFlag) is a derived variable that specifies when the DPB should be emptied without outputting pictures contained in the DPB. A maximum picture width in luma samples (PicWidthMaxInSamplesY) is a derived variable that indicates a maximum width, in units of luma samples, of each decoded picture in a corresponding sequence and/or layer. A maximum picture height in luma samples (PicHeightMaxInSamplesY) is a derived variable that indicates a maximum height, in units of luma samples, of each decoded picture in a corresponding sequence and/or layer. 
     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). A picture parameter set (PPS) is a syntax structure containing syntax elements that apply to entire coded pictures as determined by a syntax element found in each picture header. A picture header is a syntax structure containing syntax elements that apply to all slices of a coded picture. A slice header is a part of a coded slice containing data elements pertaining to all tiles or CTU rows within a tile represented in the slice. 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. 
     The following acronyms are used herein, Access Unit (AU), Coding Tree Block (CTB), Coding Tree Unit (CTU), Coding Unit (CU), Coded Layer Video Sequence (CLVS), Coded Layer Video Sequence Start (CLVSS), Coded Video Sequence (CVS), Coded Video Sequence Start (CVSS), Joint Video Experts Team (JVET), Hypothetical Reference Decoder HRD, Motion Constrained Tile Set (MCTS), Maximum Transfer Unit (MTU), Network Abstraction Layer (NAL), Output Layer Set (OLS), Picture Order Count (POC), Random Access Point (RAP), Raw Byte Sequence Payload (RBSP), Sequence Parameter Set (SPS), Video Parameter Set (VPS), Versatile Video Coding (VVC). 
     Many video compression techniques can be employed to reduce the size of video files with minimal loss of data. For example, video compression techniques can include performing spatial (e.g., intra-picture) prediction and/or temporal (e.g., inter-picture) prediction to reduce or remove data redundancy in video sequences. For block-based video coding, a video slice (e.g., a video picture or a portion of a video picture) may be partitioned into video blocks, which may also be referred to as treeblocks, coding tree blocks (CTBs), coding tree units (CTUs), coding units (CUs), and/or coding nodes. Video blocks in an intra-coded (I) slice of a picture are coded using spatial prediction with respect to reference samples in neighboring blocks in the same picture. Video blocks in an inter-coded unidirectional prediction (P) or bidirectional prediction (B) slice of a picture may be coded by employing spatial prediction with respect to reference samples in neighboring blocks in the same picture or temporal prediction with respect to reference samples in other reference pictures. Pictures may be referred to as frames and/or images, and reference pictures may be referred to as reference frames and/or reference images. Spatial or temporal prediction results in a predictive block representing an image block. Residual data represents pixel differences between the original image block and the predictive block. Accordingly, an inter-coded block is encoded according to a motion vector that points to a block of reference samples forming the predictive block and the residual data indicating the difference between the coded block and the predictive block. An intra-coded block is encoded according to an intra-coding mode and the residual data. For further compression, the residual data may be transformed from the pixel domain to a transform domain. These result in residual transform coefficients, which may be quantized. The quantized transform coefficients may initially be arranged in a two-dimensional array. The quantized transform coefficients may be scanned in order to produce a one-dimensional vector of transform coefficients. Entropy coding may be applied to achieve even more compression. Such video compression techniques are discussed in greater detail below. 
     To ensure an encoded video can be accurately decoded, video is encoded and decoded according to corresponding video coding standards. Video coding standards include International Telecommunication Union (ITU) Standardization Sector (ITU-T) H.261, International Organization for Standardization/International Electrotechnical Commission (ISO/IEC) Motion Picture Experts Group (MPEG)-1 Part 2, ITU-T H.262 or ISO/IEC MPEG-2 Part 2, ITU-T H.263, ISO/IEC MPEG-4 Part 2, Advanced Video Coding (AVC), also known as ITU-T H.264 or ISO/IEC MPEG-4 Part 10, and High Efficiency Video Coding (HEVC), also known as ITU-T H.265 or MPEG-H Part 2. AVC includes extensions such as Scalable Video Coding (SVC), Multiview Video Coding (MVC) and Multiview Video Coding plus Depth (MVC+D), and three dimensional (3D) AVC (3D-AVC). HEVC includes extensions such as Scalable HEVC (SHVC), Multiview HEVC (MV-HEVC), and 3D HEVC (3D-HEVC). The joint video experts team (JVET) of ITU-T and ISO/IEC has begun developing a video coding standard referred to as Versatile Video Coding (VVC). VVC is included in a Working Draft (WD), which includes JVET-O2001-v14. 
     Video coding systems may code a video sequence into multiple layers. Such layers can be extracted from an encoded bitstream and transmitted to a decoder to allow the decoder to display the video sequence using different characteristics as desired. For example, pictures can be transmitted at a higher resolution/larger size when network conditions are optimal and transmitted at a lower resolution/lower size when network conditions deteriorate. One problem with this implementation is that a decoded picture buffer (DPB) at the decoder can overflow during layer changes, which can cause a poor user experience. A no output of prior pictures flag (NoOutputOfPriorPicsFlag) can be used to address this issue. The NoOutputOfPriorPicsFlag can be set to cause the DPB to be emptied to make room for new pictures. This may occur even if the preceding pictures in the DPB are designated for output. In some video coding systems, the NoOutputOfPriorPicsFlag is set based on a picture size change that is detected due to a change in the width or height of the pictures. Adaptive resolution change (ARC) is a mechanism for assisting with resolution changes. ARC may dynamically change the resolution of the pictures in the DPB to match subsequent pictures in order to allow resolution changes to occur at pictures other than intra random access point (IRAP) pictures. The usage of ARC may prevent the NoOutputOfPriorPicsFlag from operating properly. Specifically, ARC may change the spatial resolution of a preceding picture be the same as a current picture, which prevents the check for picture size changes from operating correctly. As such, ARC may result in DPB overflows by effectively disabling usage of the NoOutputOfPriorPicsFlag. 
     Disclosed herein is a mechanism for enabling a NoOutputOfPriorPicsFlag when ARC is employed. For example, the decoder can check for a change in maximum picture height and maximum picture width instead of checking for picture height and width. The maximum picture height and maximum picture width are not altered by ARC. Accordingly, the NoOutputOfPriorPicsFlag can be properly set to prevent DPB overflows due to spatial resolution changes even when ARC is used to harmonize resolutions during layer changes. Hence, various errors may be avoided. A similar process can also be used by a hypothetical reference decoder (HRD) operated at an encoder when checking an encoded bitstream for standards conformance. As a result, the functionality of the encoder and the decoder is increased. Further, coding efficiency may be 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 containing prediction data and/or a transform unit (TU) containing transformed residual data for the CU. The motion estimation component  221  generates motion vectors, prediction units, 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 prediction unit of a video block in an inter-coded slice by comparing the position of the prediction unit 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 prediction unit 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 . 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 that share the same layer ID and associated non-VCL NAL units. A VCL NAL unit is a NAL unit coded to contain video data, such as a coded slice of a picture. A non-VCL NAL unit is a NAL unit that contains non-video data such as syntax and/or parameters that support decoding the video data, performance of conformance checking, or other operations. 
     In the example shown, layer N+1  632  is associated with a larger image size than layer N  631 . Accordingly, pictures  611 ,  612 ,  613 , and  614  in layer N+1  632  have a larger picture size (e.g., larger height and width and hence more samples) than pictures  615 ,  616 ,  617 , and  618  in layer N  631  in this example. However, such pictures can be separated between layer N+1  632  and layer N  631  by other characteristics. While only two layers, layer N+1  632  and layer N  631 , are shown, a set of pictures can be separated into any number of layers based on associated characteristics. Layer N+1  632  and layer N  631  may also be denoted by a layer ID. A layer ID is an item of data that is associated with a picture and denotes the picture is part of an indicated layer. Accordingly, each picture  611 - 618  may be associated with a corresponding layer ID to indicate which layer N+1  632  or layer N  631  includes the corresponding picture. For example, a layer ID may include a NAL unit header layer identifier (nuh_layer_id), which is a syntax element that specifies an identifier of a layer that includes a NAL unit (e.g., that include slices and/or parameters of the pictures in a layer). A layer associated with a lower quality/smaller image size/smaller 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/larger image size/larger 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 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. 
     The pictures  611 - 618  may also be included in AUs. An AU is a set of coded pictures that are included in different layers and are associated with the same output time during decoding. For clarity of discussion of concepts described herein, AUs include a current AU  628  and a preceding AU  627 . A current AU  628  is an AU that corresponds to a particular output time in a decoding process and/or a HRD conformance checking process. A preceding AU  627  is an AU that occurs prior to a current AU  628  in decoding order. For example, the preceding AU  627  maybe the AU that immediately precedes the current AU  628  in decoding order. 
     Coded pictures in the same AU are scheduled for output from a DPB at a decoder at the same time. For example, pictures  613  and  617  are in the same current AU  628 . Pictures  612  and  616  are in a preceding AU  627  that is different from the current AU  628  including pictures  613  and  617 . Pictures  612  and/or  616  may be decoded prior to pictures  613  and/or  617  because the preceding AU  627  precedes the current AU  628  in decoding order. Also, pictures  613  and  617  in the current AU  627  may be displayed in the alternative. For example, picture  617  may be displayed when a small picture size is desired and picture  613  may be displayed when a large picture size is desired. When the large picture size is desired, picture  613  is output and picture  617  is used only for interlayer prediction  621 . In this case, picture  617  is discarded without being output once interlayer prediction  621  is complete. In addition, in the event of a switch from layer N  631  to layer N+1  632 , picture  616  of the preceding AU  627  may be output while picture  613  from the current AU  628  may be output. This sort of switch may occur, for example, when a user increases the size of the video output (e.g., increases the size of a video window or switches to a larger screen) and/or when network conditions improve. Likewise, in the event of a switch from layer N+1  632  to layer N  631 , picture  612  of the preceding AU  627  may be output while picture  617  from the current AU  628  may be output. This sort of switch may occur, for example, when a user reduces the size of the video output (e.g., reduces the size of a video window or switches to a smaller screen) and/or when network conditions improve. 
     An AU can be further divided into one or more picture units (PUs)  625 . A PU  625  is a subset of an AU that contains a single coded picture. A PU  625  can be formally defined as a set of NAL units that are associated with each other according to a specified classification rule, are consecutive in decoding order, and contain exactly one coded picture. It should be noted that a PU  625  can be referred to as a decoding unit (DU) when discussed in terms of a HRD and/or associated conformance tests. 
       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 adaptation parameter sets (APSs)  716 , a plurality of picture headers  718 , 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. 
     An APS  716  is syntax structure containing syntax elements/parameters that apply to one or more slices  727  in one or more pictures  725 . Such correlations can be determined based on syntax elements found in slice headers  717  associated with the slices  727 . For example, an APS  716  may apply to at least one, but less than all, slices  727  in a first picture  721 , to at least one, but less than all, slices  727  in a second picture  725 , etc. An APS  716  can be separated into multiple types based on the parameters contained in the APS  716 . Such types may include adaptive loop filter (ALF) APS, luma mapping with chroma scaling (LMCS) APS, and scaling list (Scaling) APS. An ALF is an adaptive block based filter that includes a transfer function controlled by variable parameters and employs feedback from a feedback loop to refine the transfer function. Further, the ALF is employed to correct coding artifacts (e.g., errors) that occur as a result of block based coding, such as blurring and ringing artifacts. As such, ALF parameters included in an ALF APS may include parameters selected by the encoder to cause an ALF to remove block based coding artifacts during decoding at the decoder. LMCS is a process that is applied as part of the decoding process that maps luma samples to particular values and in some cases also applies a scaling operation to the values of chroma samples. The LMCS tool may reshapes luma components based on mappings to corresponding chroma components in order to reduce rate distortion. As such, a LMCS APS includes parameters selected by the encoder to cause a LMCS tool to reshape luma components. A scaling list APS contains coding tool parameters associated with quantization matrices used by specified filters. As such, an APS  716  may contain parameters used to apply various filters to coded slices  727  during conformance testing at a HRD and/or upon decoding at a decoder. 
     A picture header  718  is a syntax structure containing syntax elements that apply to all slices  727  of a coded picture  725 . For example, a picture header  718  may contain picture order count information, reference picture data, data relating in intra-random access point (IRAP) pictures, data related to filter application for a picture  725 , etc. A PU may contain exactly one picture header  718  and exactly one picture  725 . As such, the bitstream  700  may include exactly one picture header  718  per picture  725 . A slice header  717  contains parameters that are specific to each slice  727  in a picture  725 . Hence, there may be one slice header  717  per slice  727  in the video sequence. The slice header  717  may contain slice type information, filtering information, prediction weights, tile entry points, deblocking parameters, etc. In some instances, syntax elements may be the same for all slices  727  in a picture  725 . In order to reduce redundancy, the picture header  718  and slice header  717  may share certain types of information. For example, certain parameters (e.g., filtering parameters) may be included in the picture header  718  when they apply to an entire picture  725  or included in a slice header  717  when they apply to a group of slices  727  that are a subset of the entire picture  725 . 
     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 layers  723 , pictures  725 , and/or slices  727 . 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 nuh_layer_id is a syntax element that specifies an identifier of a layer  723  that includes at least one NAL unit. For example, the lowest quality layer  723 , known as a base layer, may include the lowest value of nuh_layer_id with increasing values of nuh_layer_id for layers  723  of higher quality. Hence, a lower layer is a layer  723  with a smaller value of nuh_layer_id and a higher layer is a layer  723  with a larger value of nuh_layer_id. 
     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 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. 
     Various parameters are signaled in the bitstream  700  to support decoding the pictures  725 , slices  727 , and/or other partitions thereof to reconstruct a decoded video sequence. For example, the SPS  713  may contain a SPS picture width maximum in luma samples (sps_pic_width_max_in_luma_samples)  731  and a SPS picture height maximum in luma samples (sps_pic_height_max_in_luma_samples)  732 . The sps_pic_width_max_in_luma samples  731  is a signaled data element that specifies the maximum width, in units of luma samples, of each decoded picture  725  referring to the SPS  713 . The sps_pic_width_max_in_luma samples  732  is a signaled data element that specifies the maximum height, in units of luma samples, of each decoded picture  725  referring to the SPS  713 . Accordingly, the sps_pic_width_max_in_luma_samples  731  and the sps_pic_width_max_in_luma samples  732  specify the maximum width and height, respectively, of each picture  725  in a coded video sequence related to the SPS  713 . 
     As noted above, an encoder can transmit various layers  723  of pictures  725  to a decoder depending on conditions at the decoder. Further, the transmitted layers  723  may change when conditions at the decoder change. For example, pictures  725  can be transmitted at a higher resolution/larger size when network conditions are optimal and transmitted at a lower resolution/lower size when network conditions deteriorate. One problem with this implementation is that a DPB at the decoder can overflow during layer  723  changes, which can cause a poor user experience. For example, the DPB may be full of pictures  725  at a lower resolution prior to a layer  723  change to a higher resolution. As such, the DPB may not have enough space to store the higher resolution pictures  725  received from a higher layer  723  after the switch. This may cause the DPB to overflow, which results in an error and prevents proper decoding and display of the higher resolution pictures  725 . 
     A no output of prior pictures flag (NoOutputOfPriorPicsFlag) can be used to address this issue. A NoOutputOfPriorPicsFlag is variable that is derived at the decoder and/or derived at a HRD during conformance testing. The NoOutputOfPriorPicsFlag specifies when the DPB should be emptied without outputting pictures  725  contained in the DPB. Accordingly, the NoOutputOfPriorPicsFlag can be set to cause the DPB to be emptied to make room for new pictures  725 . This may occur even if the preceding pictures  725  in the DPB are designated for output. In some video coding systems, the NoOutputOfPriorPicsFlag is set based on a picture  725  size change that is detected due to a change in the picture width and/or picture height between a current picture  725  and a preceding picture  725 . 
     ARC is a mechanism for assisting with resolution changes. Generally, a decoder swaps between sequences and/or layers  723  at TRAP pictures. This is because an IRAP picture can be decoded without reference to other pictures  725 , and can therefore always be decoded. Non-IRAP pictures are coded with reference to other pictures  725 , and hence a decoder may not have sufficient information to decode a non-IRAP picture as the first picture  725  in a sequence and/or layer  723 . ARC may assist with such transitions. ARC may dynamically change the resolution of the pictures  725  in the DPB to match the resolution subsequent pictures  725 . Accordingly, a reference picture  725  in the DPB can be changed from a first resolution to a second resolution so that a non-IRAP picture at the second resolution can be decoded based on the reference picture  725  without requiring an intervening IRAP picture. The problem with ARC is that ARC may prevent the NoOutputOfPriorPicsFlag from operating properly. Specifically, ARC may change the spatial resolution of a preceding picture  725  be the same as a current picture  725 , which prevents the check for picture size changes from operating correctly. As such, ARC may result in DPB overflows by effectively disabling usage of the NoOutputOfPriorPicsFlag. 
     Bitstream  700  is configured to allow the NoOutputOfPriorPicsFlag to be correctly enabled when ARC is employed. For example, the decoder can check for a change in maximum picture height and maximum picture width instead of checking for picture height and width. The maximum picture height and maximum picture width are not altered by ARC. Accordingly, the NoOutputOfPriorPicsFlag can be properly set to prevent DPB overflows due to spatial resolution changes even when ARC is used to during layer  723  changes. 
     In a specific example, the decoder can employ a PicWidthMaxInSamplesY variable and a PicHeightMaxInSamplesY variable when setting the NoOutputOfPriorPicsFlag. PicWidthMaxInSamplesY is a derived variable that indicates a maximum width, in units of luma samples, of each decoded picture  725  in a corresponding sequence and/or layer  723 . PicHeightMaxInSamplesY is a derived variable that indicates a maximum height, in units of luma samples, of each decoded picture  725  in a corresponding sequence and/or layer  723 . The PicWidthMaxInSamplesY can be derived based on sps_pic_width_max_in_luma samples  731  in the SPS  713 . Further, the PicHeightMaxInSamplesY can be derived based on sps_pic_height_max_in_luma_samples  732  in the SPS  713 . 
     The NoOutputOfPriorPicsFlag can then be set based on PicHeightMaxInSamplesY and/or PicWidthMaxInSamplesY. For example, the NoOutputOfPriorPicsFlag can be set when a value of PicWidthMaxInSamplesY for an AU (including a current picture  725 ) is different from a value of PicWidthMaxInSamplesY for a preceding AU (including a preceding picture  725 ) in decoding order. In another example, the NoOutputOfPriorPicsFlag can be set when a value of PicHeightMaxInSamplesY for an AU (including a current picture  725 ) is different from a value of PicHeightMaxInSamplesY for a preceding AU (including a preceding picture  725 ) in decoding order. 
     Once the NoOutputOfPriorPicsFlag is set, the DPB can be emptied as needed to prevent DPB overflows. For example, the DPB can be emptied after parsing a slice header  717  of a first slice  727  of the current picture  725  and before decoding the current picture  725 . This empties the preceding picture  725  from the preceding AU from the DPB before the current picture  725  is decoded, and hence before storage space in the DPB for the current picture  725  is needed. 
     In another example, a reference picture list can be constructed for the slice  727  and a reference picture list marking process can be performed prior to setting the NoOutputOfPriorPicsFlag. The reference picture list construction process can determine which pictures  725  are reference pictures for the current picture  725 . The reference picture list marking process can then indicate whether the pictures  725  in the DPB are unused for reference, used for short-term reference, and/or used for long-term reference based on the reference picture list construction process. In a specific example, the NoOutputOfPriorPicsFlag is set and the DPB is emptied when each of preceding pictures  725  in the DPB are marked as unused for reference. In this way, the DPB is not emptied if the DPB contains reference pictures. Further, the DPB may employ a DPB fullness variable, which is a data element that indicates a number of pictures stored in a DPB. The DPB fullness variable can be decremented by one for each picture removed when emptying the DPB. 
     By employing the examples described herein the NoOutputOfPriorPicsFlag can be set to empty the DPB as needed to prevent DPB overflows. Hence, the embodiments included herein may be employed to avoid various decoding errors related to switching between layers  723 . A similar process can also be used by a HRD operated at an encoder when checking an encoded bitstream for standards conformance. This process can prevent the HRD from having DPB overflows when checking layers  723  for conformance, and hence reduces the occurrence of conformance test failures during encoding. As a result, the functionality of the encoder and the decoder is increased. Further, coding efficiency may be 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 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. For example, the nuh_layer_id values for SPS, PPS, and APS NAL units may not be properly constrained. Further, the TemporalId value for SEI NAL units may not be properly constrained. In addition, setting of NoOutputOfPriorPicsFlag may not be properly specified when reference picture resampling is enabled and pictures within a CLVS have different spatial resolutions. Also, in some video coding systems suffix SEI messages cannot be contained in a scalable nesting SEI message. As another example, buffering period, picture timing, and decoding unit information SEI messages may include parsing dependencies on VPS and/or SPS. 
     In general, this disclosure describes video coding improvement approaches. 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 abovementioned problems may be solved as follows. The nuh_layer_id values for SPS, PPS, and APS NAL units are properly constrained herein. The TemporalId value for SEI NAL units is properly constrained herein. Setting of the NoOutputOfPriorPicsFlag is properly specified when reference picture resampling is enabled and pictures within a CLVS have different spatial resolutions. Suffix SEI messages are allowed to be contained in a scalable nesting SEI message. Parsing dependencies of BP, PT, and DUI SEI messages on VPS or SPS may be removed by repeating the syntax element decoding_unit_hrd_params_present_flag in the BP SEI message syntax, the syntax elements decoding_unit_hrd_params_present_flag and decoding_unit_cpb_params_in_pic_timing_sei_flag in the PT SEI message syntax, and the syntax element decoding_unit_cpb_params_in_pic_timing_sei_flag in the DUI SEI message. 
     An example implementation of the preceding mechanisms is as follows. An example general NAL unit semantics is as follows. 
     A nuh_temporal_id_plus1 minus 1 specifies a temporal identifier for the NAL unit. The value of nuh_temporal_id_plus1 should not be equal to zero. The variable TemporalId may be derived as follows: 
       TemporalId=nuh_temporal_id_plus1−1
 
     When nal_unit_type is in the range of IDR_W_RADL to RSV_IRAP_13, inclusive, TemporalId should be equal to zero. When nal_unit_type is equal to STSA_NUT, TemporalId should not be equal to zero. 
     The value of TemporalId should be the same for all VCL NAL units of an access unit. The value of TemporalId of a coded picture, a layer access unit, or an access unit may be the value of the TemporalId of the VCL NAL units of the coded picture, the layer access unit, or the access unit. The value of TemporalId of a sub-layer representation may be the greatest value of TemporalId of all VCL NAL units in the sub-layer representation. 
     The value of TemporalId for non-VCL NAL units is constrained as follows. If nal_unit_type is equal to DPS_NUT, VPS_NUT, or SPS_NUT, TemporalId is equal to zero and the TemporalId of the access unit containing the NAL unit should be equal to zero. Otherwise if nal_unit_type is equal to EOS_NUT or EOB_NUT, TemporalId should be equal to zero. Otherwise, if nal_unit_type is equal to AUD_NUT, FD_NUT, PREFIX_SEI_NUT, or SUFFIX_SEI_NUT, TemporalId should be equal to the TemporalId of the access unit containing the NAL unit. Otherwise, when nal_unit_type is equal to PPS_NUT or APS_NUT, TemporalId should be greater than or equal to the TemporalId of the access unit containing the NAL unit. When the NAL unit is a non-VCL NAL unit, the value of TemporalId should be equal to the minimum value of the TemporalId values of all access units to which the non-VCL NAL unit applies. When nal_unit_type is equal to PPS_NUT or APS_NUT, TemporalId may be greater than or equal to the TemporalId of the containing access unit. This is because all PPSs and APSs may be included in the beginning of a bitstream. Further, the first coded picture has TemporalId equal to zero. 
     An example sequence parameter set RBSP semantics is as follows. An SPS RBSP should be available to the decoding process prior to being referenced. The SPS may be included in at least one access unit with TemporalId equal to zero or provided through external mechanism. The SPS NAL unit containing the SPS may be constrained to have a nuh_layer_id equal to the lowest nuh_layer_id value of PPS NAL units that refer to the SPS. 
     An example picture parameter set RBSP semantics is as follows. A PPS RBSP should be available to the decoding process prior to being referenced. The PPS should be included in at least one access unit with TemporalId less than or equal to the TemporalId of the PPS NAL unit or provided through external mechanism. The PPS NAL unit containing the PPS RBSP should have a nuh_layer_id equal to the lowest nuh_layer_id value of the coded slice NAL units that refer to the PPS. 
     An example adaptation parameter set semantics is as follows. Each APS RBSP should be available to the decoding process prior to being referenced. The APS should also be included in at least one access unit with TemporalId less than or equal to the TemporalId of the coded slice NAL unit that refers the APS or provided through an external mechanism. An APS NAL unit is allowed to be shared by pictures/slices of multiple layers. The nuh_layer_id of an APS NAL unit should be equal to the lowest nuh_layer_id value of the coded slice NAL units that refer to the APS NAL unit. Alternatively, an APS NAL unit may not be shared by pictures/slices of multiple layers. The nuh_layer_id of an APS NAL unit should be equal to the nuh_layer_id of slices referring to the APS. 
     In an example, removal of pictures from the DPB before decoding of the current picture is discussed as follows. The removal of pictures from the DPB before decoding of the current picture (but after parsing the slice header of the first slice of the current picture) may occur at the CPB removal time of the first decoding unit of access unit n (containing the current picture). This proceeds as follows. The decoding process for reference picture list construction is invoked and the decoding process for reference picture marking is invoked. 
     When the current picture is a coded layer video sequence start (CLVSS) picture that is not picture zero, the following ordered steps are applied. The variable NoOutputOfPriorPicsFlag is derived for the decoder under test as follows. If the value of pic_width_max_in_luma_samples, pic_height_max_in_luma_samples, chroma_format_idc, separate_colour_plane_flag, bit_depth_luma_minus8, bit_depth_chroma_minus8 or sps_max_dec_pic_buffering_minus1[Htid] derived from the SPS is different from the value of pic_width_in_luma_samples, pic_height_in_luma_samples, chroma_format_idc, separate_colour_plane_flag, bit_depth_luma_minus8, bit_depth_chroma_minus8 or sps_max_dec_pic_buffering_minus1[Htid], respectively, derived from the SPS referred to by the preceding picture, NoOutputOfPriorPicsFlag may be set to one by the decoder under test, regardless of the value of no_output_of_prior_pics_flag. It should be noted that, although setting NoOutputOfPriorPicsFlag equal to no_output_of_prior_pics_flag may be preferred under these conditions, the decoder under test is allowed to set NoOutputOfPriorPicsFlag to one in this case. Otherwise, NoOutputOfPriorPicsFlag may be set equal to no_output_of_prior_pics_flag. 
     The value of NoOutputOfPriorPicsFlag derived for the decoder under test is applied for the HRD, such that when the value of NoOutputOfPriorPicsFlag is equal to 1, all picture storage buffers in the DPB are emptied without output of the pictures they contain, and the DPB fullness is set equal to zero. When both of the following conditions are true for any pictures k in the DPB, all such pictures k in the DPB are removed from the DPB. Picture k is marked as unused for reference, and picture k has PictureOutputFlag equal to zero or a corresponding DPB output time is less than or equal to the CPB removal time of the first decoding unit (denoted as decoding unit m) of the current picture n. This may occur when DpbOutputTime[k] is less than or equal to DuCpbRemovalTime[m]. For each picture that is removed from the DPB, the DPB fullness is decremented by one. 
     In an example, output and removal of pictures from the DPB is discussed as follows. The output and removal of pictures from the DPB before the decoding of the current picture (but after parsing the slice header of the first slice of the current picture) may occur when the first decoding unit of the access unit containing the current picture is removed from the CPB and proceeds as follows. The decoding process for reference picture list construction and decoding process for reference picture marking are invoked. 
     If the current picture is a CLVSS picture that is not picture zero, the following ordered steps are applied. The variable NoOutputOfPriorPicsFlag can be derived for the decoder under test as follows. If the value of pic_width_max_in_luma_samples, pic_height_max_in_luma_samples, chroma_format_idc, separate_colour_plane_flag, bit_depth_luma_minus8, bit_depth_chroma_minus8 or sps_max_dec_pic_buffering_minus1[Htid] derived from the SPS is different from the value of pic_width_in_luma_samples, pic_height_in_luma_samples, chroma_format_idc, separate_colour_plane_flag, bit_depth_luma_minus8, bit_depth_chroma_minus8 or sps_max_dec_pic_buffering_minus1[Htid], respectively, derived from the SPS referred to by the preceding picture, NoOutputOfPriorPicsFlag may be set to one by the decoder under test, regardless of the value of no_output_of_prior_pics_flag. It should be noted that although setting NoOutputOfPriorPicsFlag equal to no_output_of_prior_pics_flag is preferred under these conditions, the decoder under test can set NoOutputOfPriorPicsFlag to one in this case. Otherwise, NoOutputOfPriorPicsFlag can be set equal to no_output_of_prior_pics_flag. 
     The value of NoOutputOfPriorPicsFlag derived for the decoder under test can be applied for the HRD as follows. If NoOutputOfPriorPicsFlag is equal to one, all picture storage buffers in the DPB are emptied without output of the pictures they contain and the DPB fullness is set equal to zero. Otherwise (NoOutputOfPriorPicsFlag is equal to zero), all picture storage buffers containing a picture that is marked as not needed for output and unused for reference are emptied (without output) and all non-empty picture storage buffers in the DPB are emptied by repeatedly invoking a bumping process and the DPB fullness is set equal to zero. 
     Otherwise (the current picture is not a CLVSS picture), all picture storage buffers containing a picture which are marked as not needed for output and unused for reference are emptied (without output). For each picture storage buffer that is emptied, the DPB fullness is decremented by one. When one or more of the following conditions are true, the bumping process is invoked repeatedly while further decrementing the DPB fullness by one for each additional picture storage buffer that is emptied until none of the following conditions are true. A condition is that the number of pictures in the DPB that are marked as needed for output is greater than sps_max_num_reorder_pics[Htid]. Another condition is that a sps_max_latency_increase_plus1[Htid] is not equal to zero and there is at least one picture in the DPB that is marked as needed for output for which the associated variable PicLatencyCount is greater than or equal to SpsMaxLatencyPictures[Htid]. Another condition is that the number of pictures in the DPB is greater than or equal to SubDpbSize[Htid]. 
     An example general SEI message syntax is as follows. 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 sei_payload( payloadType, payloadSize ) {  
                 Descriptor 
               
               
                  if( nal_unit_type = = PREFIX_SEI_NUT ) 
                   
               
               
                   if( payloadType = = 0 ) 
                   
               
               
                    buffering_period( payloadSize ) 
                   
               
               
                   else if( payloadType = = 1 ) 
                   
               
               
                    pic_timing( payloadSize ) 
                   
               
               
                   else if( payloadType = = 3 ) 
                   
               
               
                    filler_payload( payloadSize ) 
                   
               
               
                   else if( payloadType = = 130 ) 
                   
               
               
                    decoding_unit_info( payloadSize ) 
                   
               
               
                   else if( payloadType = = 133 ) 
                   
               
               
                    scalable_nesting( payloadSize ) 
                   
               
               
                   else if( payloadType = = 145 ) 
                   
               
               
                    dependent_rap_indication( payloadSize ) 
                   
               
               
                     // Specified in ITU-T H.SEI | ISO/IEC 23002-7. 
                   
               
               
                   else if( payloadType = = 168 ) 
                   
               
               
                    frame_field_info( payloadSize ) 
                   
               
               
                   else 
                   
               
               
                    reserved_sei_message( payloadSize ) 
                   
               
               
                  else /* nal_unit_type = = SUFFIX_SEI_NUT */ 
                   
               
               
                   if( payloadType = = 3 ) 
                   
               
               
                    filler_payload( payloadSize ) 
                   
               
               
                   if( payloadType = = 132 ) 
                   
               
               
                    decoded_picture_hash( payloadSize ) 
                   
               
               
                     // Specified in ITU-T H.SEI | ISO/IEC 23002-7. 
                   
               
               
                   else if( payloadType = = 133 ) 
                   
               
               
                    scalable_nesting( payloadSize ) 
                   
               
               
                   else 
                   
               
               
                    reserved_sei_message( payloadSize ) 
                   
               
               
                  if( more_data in_payload( ) ) { 
                   
               
               
                   if( payload_extension_present( )) 
                   
               
               
                   reserved_payload_extension_data  
                 u(v) 
               
               
                   payload_bit_equal_to_one /* equal to 1 */  
                 f(1) 
               
               
                   while( !byte_aligned( )) 
                   
               
               
                    payload_bit_equal_to_zero /* equal to 0 */  
                 f(1) 
               
               
                  } 
                   
               
               
                 } 
               
               
                   
               
            
           
         
       
     
     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_minus1[ i ]; j++ ) 
                   
               
               
                      nesting_ols_layer_idx_delta minus1[ 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( ) 
                   
               
               
                 } 
               
               
                   
               
            
           
         
       
     
     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 signal and/or read various parameters as described herein. Further, the coding module may be employed to encode and/or decode a video sequence based on such parameters. As such, the signaling changes described herein may increase the efficiency and/or avoid errors in the coding module  814 . 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 , to support usage of a NoOutputOfPriorPicsFlag in conjunction with ARC. 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 a plurality of pictures into a bitstream. For example, the pictures may be included in AUs. Further, the pictures may be included in layers. The encoder can encode one or more layers including the pictures 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. 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. One or more of the layers may be output layers. Layers that are not an output layer are encoded to support reconstructing the output layer(s), but such supporting layers are not intended for output at a decoder. In this way, the encoder can encode various combinations of layers for transmission to a decoder upon request. The layers can be transmitted as desired to allow the decoder to obtain different representations of the video sequence depending on network conditions, hardware capabilities, and/or user settings. When the bitstream is encoded, the encoder can employ a HRD to check the bitstream for conformance. 
     At step  903 , the HRD can decode a preceding picture in a preceding AU and store the preceding picture in a DPB. For example, the preceding picture may be part of a first layer. The HRD can then check for problems that occur during a switch to a second layer. 
     At step  905 , the HRD can prepare to decode a current picture in the second layer of a current AU. The HRD can parse a slice header of a first slice of the current picture. The HRD can also construct a reference picture list for the first slice based on the reference picture list. The HRD can also perform a reference picture list marking process on the pictures in the DPB (e.g., including the preceding picture) to mark each picture as unused for reference, used for short-term reference, and/or used for long-term reference. 
     At step  907 , the HRD can set a NoOutputOfPriorPicsFlag to a corresponding value (e.g., to one) when a value of PicWidthMaxInSamplesY for the current picture in a current AU is different from a value of PicWidthMaxInSamplesY for the preceding picture in a preceding AU in decoding order. The PicWidthMaxInSamplesY can be derived based on a sps_pic_width_max_in_luma_samples obtained from a SPS in the bitstream. Further, the HRD can set the NoOutputOfPriorPicsFlag when a value of PicHeightMaxInSamplesY for the current picture in the current AU is different from a value of PicHeightMaxInSamplesY for the preceding picture in the preceding AU in decoding order. The PicHeightMaxInSamplesY can be derived based on a sps_pic_height_max_in_luma_samples obtained from a SPS in the bitstream. The NoOutputOfPriorPicsFlag can be set to one to specify that all picture storage buffers in the DPB should emptied without output of the pictures they contain, and that a DPB fullness variable should be set equal to zero. 
     At step  909 , the HRD can empty the DPB without selecting the contained pictures for output based on a value of the NoOutputOfPriorPicsFlag (e.g., when the NoOutputOfPriorPicsFlag is set equal to one). The HRD can also decrement a DPB fullness variable (e.g., reduce by one) for each picture removed when emptying the DPB. It should be noted that the NoOutputOfPriorPicsFlag may only be set at step  907 , and hence the DPB may only be emptied at step  909 , when all of the pictures in the DPB including the preceding picture are marked as unused for reference according to the reference picture list marking process of step  905 . 
     The HRD can then decode the current picture in the current AU and store the current picture in the DPB a step  911 . Accordingly, the DPB is emptied at step  909  before the current picture in the current AU is decoded at step  911 . Further, the DPB is emptied at step  909  after parsing the slice header of the first slice of the current picture at step  905 . 
     In some instances, the NoOutputOfPriorPicsFlag may prevent a DPB overflow when switching between a first layer containing the preceding picture and a second layer containing the current picture. Accordingly, the HRD conformance test may be successful when checking the current picture for conformance. Presuming no other errors are encountered, the HRD conformance test is completed. Presuming the conformance test is successful, the bitstream is stored in long term memory for communication toward a decoder at step  913 , for example via a content server. Layers of the bitstream can then be transmitted toward the 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 , while employing a NoOutputOfPriorPicsFlag in conjunction with ARC. 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  and/or in response to a request by the decoder. At step  1001 , the decoder receives a bitstream a bitstream comprising a plurality of pictures. For example, the pictures may be included in AUs. Further, the pictures may be included in layers. A layer may include a set of VCL NAL units with the same layer ID and associated non-VCL NAL units. For example, the set of VCL NAL units are part of a layer when the set of VCL NAL units all have the same layer ID. A layer may include a set of VCL NAL units that contain video data of pictures as well as any parameter sets used to code such pictures in associated non-VCL NAL units. One or more of the layers may be output layers. Layers that are not an output layer are encoded to support reconstructing the output layer(s), but such supporting layers are not intended for output. In this way, the decoder can obtain different representations of the video sequence depending on network conditions, hardware capabilities, and/or user settings. 
     At step  1003 , the decoder can decode a preceding picture in a preceding AU and store the preceding picture in a DPB. For example, the preceding picture may be part of a first layer. The decoder can determine to switch to a second layer, for example due to user input and/or due to a change in operating conditions. For example, the decoder may request to switch to the second layer. 
     At step  1005 , the decoder can prepare to decode a current picture in the second layer of a current AU. The decoder can parse a slice header of a first slice of the current picture. The decoder can also construct a reference picture list for the first slice based on the reference picture list. The decoder can also perform a reference picture list marking process on the pictures in the DPB (e.g., including the preceding picture) to mark each picture as unused for reference, used for short-term reference, and/or used for long-term reference. 
     At step  1007 , the decoder can set a NoOutputOfPriorPicsFlag to a corresponding value (e.g., to one) when a value of PicWidthMaxInSamplesY for the current picture in a current AU is different from a value of PicWidthMaxInSamplesY for the preceding picture in a preceding AU in decoding order. The PicWidthMaxInSamplesY can be derived based on a sps_pic_width_max_in_luma_samples obtained from a SPS in the bitstream. Further, the decoder can set the NoOutputOfPriorPicsFlag when a value of PicHeightMaxInSamplesY for the current picture in the current AU is different from a value of PicHeightMaxInSamplesY for the preceding picture in the preceding AU in decoding order. The PicHeightMaxInSamplesY can be derived based on a sps_pic_height_max_in_luma_samples obtained from a SPS in the bitstream. The NoOutputOfPriorPicsFlag can be set to one to specify that all picture storage buffers in the DPB should emptied without output of the pictures they contain, and that a DPB fullness variable should be set equal to zero. 
     At step  1009 , the decoder can empty the DPB without selecting the contained pictures for output based on a value of the NoOutputOfPriorPicsFlag (e.g., when the NoOutputOfPriorPicsFlag is set equal to one). The decoder can also decrement a DPB fullness variable (e.g., reduce by one) for each picture removed when emptying the DPB. It should be noted that the NoOutputOfPriorPicsFlag may only be set at step  1007 , and hence the DPB may only be emptied at step  1009 , when all of the pictures in the DPB including the preceding picture are marked as unused for reference according to the reference picture list marking process of step  1005 . 
     The decoder can then decode the current picture in the current AU and store the current picture in the DPB a step  1011 . Accordingly, the DPB is emptied at step  1009  before the current picture in the current AU is decoded at step  1011 . Further, the DPB is emptied at step  1009  after parsing the slice header of the first slice of the current picture at step  1005 . In some instances, the NoOutputOfPriorPicsFlag may prevent a DPB overflow when switching between a first layer containing the preceding picture and a second layer containing the current picture. Accordingly, the usage of NoOutputOfPriorPicsFlag may increase the functionality of the decoder. 
     At step  1013 , the decoder can output the current picture from the DPB 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 into a bitstream while employing a NoOutputOfPriorPicsFlag in conjunction with ARC. 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 plurality of pictures into a bitstream. The video encoder  1102  further comprises a HRD module  1105  for checking the bitstream for conformance by: setting a NoOutputOfPriorPicsFlag when a value of PicWidthMaxInSamplesY for a current AU is different from a value of PicWidthMaxInSamplesY for a preceding AU in decoding order; emptying a DPB without selecting contained pictures for output based on a value of the NoOutputOfPriorPicsFlag; decoding a current picture; and storing the current picture in the DPB. 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 a plurality of pictures. The video decoder  1110  further comprises a setting module  1113  for setting a NoOutputOfPriorPicsFlag when a value of PicWidthMaxInSamplesY for a current AU is different from a value of PicWidthMaxInSamplesY for a preceding AU in decoding order. The video decoder  1110  further comprises an emptying module  1115  for emptying a DPB without output of contained pictures based on a value of the NoOutputOfPriorPicsFlag. The video decoder  1110  further comprises a decoding module  1117  for decoding a current picture. The video decoder  1110  further comprises a storing module  1119  for storing the current picture in a DPB. The video decoder  1110  further comprises a outputting module  1121  for outputting the current picture from the DPB 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.