Patent Publication Number: US-2022217329-A1

Title: Disallowing Unused Layers in Multi-Layer Video Bitstreams

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a continuation of International Application No. PCT/US2020/050904 filed on Sep. 15, 2020, by Futurewei Technologies, Inc., and titled “Disallowing Unused Layers In Multi-Layer Video Bitstreams,” which claims the benefit of U.S. Provisional Patent Application No. 62/905,136 filed Sep. 24, 2019, by Futurewei Technologies, Inc., and titled “Disallowing Unnecessary And Unused Layers In Multi-Layer Video Bitstreams,” each of which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     In general, this disclosure describes techniques for multi-layer video bitstreams in video coding. More specifically, this disclosure ensures that unnecessary and/or unused layers are disallowed within a multi-layer bitstream in video coding. 
     BACKGROUND 
     The amount of video data needed to depict even a relatively short video can be substantial, which may result in difficulties when the data is to be streamed or otherwise communicated across a communications network with limited bandwidth capacity. Thus, video data is generally compressed before being communicated across modern day telecommunications networks. The size of a video could also be an issue when the video is stored on a storage device because memory resources may be limited. Video compression devices often use software and/or hardware at the source to code the video data prior to transmission or storage, thereby decreasing the quantity of data needed to represent digital video images. The compressed data is then received at the destination by a video decompression device that decodes the video data. With limited network resources and ever increasing demands of higher video quality, improved compression and decompression techniques that improve compression ratio with little to no sacrifice in image quality are desirable. 
     SUMMARY 
     A first aspect relates to a method of decoding implemented by a video decoder, comprising receiving, by the video decoder, a video bitstream including a video parameter set (VPS) and a plurality of layers, wherein each layer is included in at least one output layer set (OLS) specified by the VPS; and decoding, by the video decoder, a picture from one of the plurality of layers. 
     The method provides techniques that disallow unused layers in multi-layer video bitstreams. That is, any layer not used in at least one output layer set (OLS) is constrained from being included in multi-layer bitstreams. This avoids having extraneous information in the coding process and increases coding efficiency. Thus, the coder/decoder (a.k.a., “codec”) in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the picture is included in an output layer of the at least one OLS, and wherein each layer is specified in the OLS. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides selecting an output layer from the at least one OLS for the decoding. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides selecting the picture for the decoding from the selected output layer. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that each layer in the plurality of layers comprises a set of video coding layer (VCL) network abstraction layer (NAL) units that all have a particular value of a layer identifier (ID) and associated non-VCL NAL units. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the at least one OLS comprises two output layers, and wherein one of the two output layers references another of the two output layers. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the at least one OLS contains more than one output layer. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that each of the plurality of layers with a particular value of layer ID specified in the VPS, one of the layers in the at least one OLS shall also have the particular value of layer ID. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides displaying the decoded picture on a display of an electronic device. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that receiving, by the video decoder, a second video bitstream including a second video parameter set (VPS) and a second plurality of layers, wherein at least one layer is not included in at least one output layer set (OLS) specified by the second VPS; and in response to the receiving, taking some other corrective measures to ensure that a conforming bitstream corresponding to the second video bitstream is received prior to decoding a picture from one of the second plurality of layers. 
     A second aspect relates to a method of encoding a video bitstream implemented by a video encoder. The method includes a method of encoding implemented by a video encoder, the method comprising generating, by the video encoder, a plurality of layers and a video parameter set (VPS) specifying one or more output layer sets (OLSs), wherein the video encoder is constrained such that each layer from the plurality of layers is included in at least one of the OLSs specified by the VPS; encoding, by the video encoder, the plurality of layers and the VPS into a video bitstream; and storing, by the video encoder, the video bitstream for communication toward a video decoder. 
     The method provides techniques that disallow unused layers in multi-layer video bitstreams. That is, any layer not used in at least one output layer set (OLS) is constrained from being included in multi-layer bitstreams. This avoids having extraneous information in the coding process and increases coding efficiency. Thus, the coder/decoder (a.k.a., “codec”) in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that each of the one or more OLSs contains one or more output layers, wherein each of the output layers comprises one or more pictures, and wherein there is no layer excluded from all of the one or more OLSs. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that each layer in the plurality of layers comprises a set of video coding layer (VCL) network abstraction layer (NAL) units that all have a particular value of a layer identifier (ID) and associated non-VCL NAL units. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that one of the OLSs comprises two output layers, and wherein one of the two output layers references another of the two output layers. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that for each of the plurality of layers with a particular value of layer ID specified in the VPS, one of the layers in the one or more OLSs shall also have the particular value of layer ID. 
     A third aspect relates to a decoding device. The decoding device includes a decoding device, comprising a receiver configured to receive a video bitstream including a video parameter set (VPS) and a plurality of layers, wherein each layer is included in at least one output layer set (OLS) specified by the VPS; a memory coupled to the receiver, the memory storing instructions; and a processor coupled to the memory, the processor configured to execute the instructions to cause the decoding device to decode a picture from one of the plurality of layers to obtain a decoded picture. 
     The decoding device provides techniques that disallow unused layers in multi-layer video bitstreams. That is, any layer not used in at least one output layer set (OLS) is constrained from being included in multi-layer bitstreams. This avoids having extraneous information in the coding process and increases coding efficiency. Thus, the coder/decoder (a.k.a., “codec”) in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the picture is included in an output layer of the at least one OLS, and wherein each layer is specified in the OLS. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the processor is further configured to select an output layer from the at least one OLS prior to the picture being decoded. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the processor is further configured to select the picture from the output layer after the output layer has been selected. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that each layer in the plurality of layers comprises a set of video coding layer (VCL) network abstraction layer (NAL) units that all have a particular value of a layer identifier (ID) and associated non-VCL NAL units. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the at least one OLS comprises two output layers, and wherein one of the two output layers references another of the two output layers. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the at least one OLS contains one or more output layers. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that the processor is configured to execute the instructions to further cause the decoding device to receive a second video bitstream including a second video parameter set (VPS) and a second plurality of layers, wherein at least one layer is not included in at least one output layer set (OLS) specified by the second VPS; and in response to receipt of the second video bitstream, taking some other corrective measures to ensure that a conforming bitstream corresponding to the second video bitstream is received prior to decoding a picture from one of the second plurality of layers. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that a display configured to display the decoded picture. 
     A fourth aspect relates to a decoding device. The encoding device includes an encoding device, comprising a memory containing instructions; a processor coupled to the memory, the processor configured to implement the instructions to cause the encoding device to generate a plurality of layers and a video parameter set (VPS) specifying one or more output layer sets (OLSs), wherein the video encoder is constrained such that each layer from the plurality of layers is included in at least one of the OLSs specified by the VPS; encode the plurality of layers and the VPS into a video bitstream; and a transmitter coupled to the processor, the transmitter configured to transmit the video bitstream toward a video decoder. 
     The decoding device provides techniques that disallow unused layers in multi-layer video bitstreams. That is, any layer not used in at least one output layer set (OLS) is constrained from being included in multi-layer bitstreams. This avoids having extraneous information in the coding process and increases coding efficiency. Thus, the coder/decoder (a.k.a., “codec”) in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that each of the one or more OLSs contains one or more output layers, and wherein each of the output layers comprises one or more pictures. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that each layer in the plurality of layers comprises a set of video coding layer (VCL) network abstraction layer (NAL) units that all have a particular value of a layer identifier (ID) and associated non-VCL NAL units. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that one of the OLSs comprises two output layers, and wherein one of the two output layers references another of the two output layers. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides that each of the plurality of layers with a particular value of layer ID specified in the VPS, one of the layers in the one or more OLSs shall also have the particular value of layer ID. 
     A fifth aspect relates to a coding apparatus. The coding apparatus includes a coding apparatus, comprising a receiver configured to receive a picture to encode or to receive a bitstream to decode; a transmitter coupled to the receiver, the transmitter configured to transmit the bitstream to a decoder or to transmit a decoded image to a display; a memory coupled to at least one of the receiver or the transmitter, the memory configured to store instructions; and a processor coupled to the memory, the processor configured to execute the instructions stored in the memory to perform any of the methods disclosed herein. 
     The coding apparatus provides techniques that disallow unused layers in multi-layer video bitstreams. That is, any layer not used in at least one output layer set (OLS) is constrained from being included in multi-layer bitstreams. This avoids having extraneous information in the coding process and increases coding efficiency. Thus, the coder/decoder (a.k.a., “codec”) in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed. 
     Optionally, in any of the preceding aspects, another implementation of the aspect provides a display configured to display a decoded picture. 
     A sixth aspect relates to a system. The system includes an encoder; and a decoder in communication with the encoder, wherein the encoder or the decoder includes the decoding device, the encoding device, or the coding apparatus disclosed herein. 
     The system provides techniques that disallow unused layers in multi-layer video bitstreams. That is, any layer not used in at least one output layer set (OLS) is constrained from being included in multi-layer bitstreams. This avoids having extraneous information in the coding process and increases coding efficiency. Thus, the coder/decoder (a.k.a., “codec”) in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed. 
     A seventh aspect relates to a means for coding. The means for coding includes receiving means configured to receive a picture to encode or to receive a bitstream to decode; transmission means coupled to the receiving means, the transmission means configured to transmit the bitstream to a decoding means or to transmit a decoded image to a display means; storage means coupled to at least one of the receiving means or the transmission means, the storage means configured to store instructions; and processing means coupled to the storage means, the processing means configured to execute the instructions stored in the storage means to perform any of the methods disclosed herein. 
     The means for coding provides techniques that disallow unused layers in multi-layer video bitstreams. That is, any layer not used in at least one output layer set (OLS) is constrained from being included in multi-layer bitstreams. This avoids having extraneous information in the coding process and increases coding efficiency. Thus, the coder/decoder (a.k.a., “codec”) in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed. 
     For the purpose of clarity, any one of the foregoing embodiments may be combined with any one or more of the other foregoing embodiments to create a new embodiment within the scope of the present disclosure. 
     These and other features will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts. 
         FIG. 1  is a flowchart of an example method of coding a video signal. 
         FIG. 2  is a schematic diagram of an example coding and decoding (codec) system for video coding. 
         FIG. 3  is a schematic diagram illustrating an example video encoder. 
         FIG. 4  is a schematic diagram illustrating an example video decoder. 
         FIG. 5  illustrates an example of multi-layer coding for spatial scalability. 
         FIG. 6  illustrates an example of multi-layer coding using output layer sets (OLSs). 
         FIG. 7  illustrates an embodiment of a video bitstream. 
         FIG. 8  is an embodiment of a method of decoding a coded video bitstream. 
         FIG. 9  is an embodiment of a method of encoding a coded video bitstream. 
         FIG. 10  is a schematic diagram of a video coding device. 
         FIG. 11  is a schematic diagram of an embodiment of a means for coding. 
     
    
    
     DETAILED DESCRIPTION 
     It should be understood at the outset that although an illustrative implementation of one or more embodiments are provided below, the disclosed systems and/or methods may be implemented using any number of techniques, whether currently known or in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalents. 
     The following terms are defined as follows unless used in a contrary context herein. Specifically, the following definitions are intended to provide additional clarity to the present disclosure. However, terms may be described differently in different contexts. Accordingly, the following definitions should be considered as a supplement and should not be considered to limit any other definitions of descriptions provided for such terms herein. 
     A bitstream is a sequence of bits including video data that is compressed for transmission between an encoder and a decoder. An encoder is a device that is configured to employ encoding processes to compress video data into a bitstream. A decoder is a device that is configured to employ decoding processes to reconstruct video data from a bitstream for display. A picture is an array of luma samples and/or an array of chroma samples that create a frame or a field thereof. A picture that is being encoded or decoded can be referred to as a current picture for clarity of discussion. A reference picture is a picture that contains reference samples that can be used when coding other pictures by reference according to inter-prediction and/or inter-layer prediction. A reference picture list is a list of reference pictures used for inter-prediction and/or inter-layer prediction. Some video coding systems utilize two reference picture lists, which can be denoted as reference picture list one and reference picture list zero. A reference picture list structure is an addressable syntax structure that contains multiple reference picture lists. Inter-prediction 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. A reference picture list structure entry is an addressable location in a reference picture list structure that indicates a reference picture associated with a reference picture list. A slice header is a part of a coded slice containing data elements pertaining to all video data within a tile represented in the slice. A picture parameter set (PPS) is a parameter set that contains data related to an entire picture. More specifically, the PPS is a syntax structure containing syntax elements that apply to zero or more entire coded pictures as determined by a syntax element found in each picture header. A sequence parameter set (SPS) is a parameter set that contains data related to a sequence of pictures. An access unit (AU) is a set of one or more coded pictures associated with the same display time (e.g., the same picture order count) for output from a decoded picture buffer (DPB) (e.g., for display to a user). An access unit delimiter (AUD) is an indicator or data structure used to indicate the start of an AU or the boundary between AUs. A decoded video sequence is a sequence of pictures that have been reconstructed by a decoder in preparation for display to a user. 
     A network abstraction layer (NAL) unit is a syntax structure containing data in the form of a Raw Byte Sequence Payload (RBSP), an indication of the type of data, and interspersed as desired with emulation prevention bytes. A video coding layer (VCL) NAL unit is a NAL unit coded to contain video data, such as a coded slice of a picture. A non-VCL NAL unit is a NAL unit that contains non-video data such as syntax and/or parameters that support decoding the video data, performance of conformance checking, or other operations. A layer is a set of VCL NAL units that share a specified characteristic (e.g., a common resolution, frame rate, image size, etc.) and associated non-VCL NAL units. The VCL NAL units of a layer may share a particular value of a NAL unit header layer identifier (nuh_layer_id). A coded picture is a coded representation of a picture comprising VCL NAL units with a particular value of a NAL unit header layer identifier (nuh_layer_id) within an access unit (AU) and containing all coding tree units (CTUs) of the picture. A decoded picture is a picture produced by applying a decoding process to a coded picture. 
     An output layer set (OLS) is a set of layers for which one or more layers are specified as output layer(s). An output layer is a layer that is designated for output (e.g., to a display). A zeroth (0-th) OLS is an OLS that contains only a lowest layer (layer with a lowest layer identifier) and hence contains only an output layer. A video parameter set (VPS) is a data unit that contains parameters related to an entire video. Inter-layer prediction is a mechanism of coding a current picture in a current layer by reference to a reference picture in a reference layer, where the current picture and the reference picture are included in the same AU and the reference layer includes a lower nuh_layer_id than the current layer. 
     The following acronyms are used herein, Coding Tree Block (CTB), Coding Tree Unit (CTU), Coding Unit (CU), Coded Video Sequence (CVS), Joint Video Experts Team (JVET), Motion-Constrained Tile Set (MCTS), Maximum Transfer Unit (MTU), Network Abstraction Layer (NAL), Picture Order Count (POC), Picture Parameter Set (PPS), Raw Byte Sequence Payload (RBSP), Sequence Parameter Set (SPS), Versatile Video Coding (VVC), and Working Draft (WD). 
       FIG. 1  is a flowchart of an example operating method  100  of coding a video signal. Specifically, a video signal is encoded at an encoder. The encoding process compresses the video signal by employing various mechanisms to reduce the video file size. A smaller file size allows the compressed video file to be transmitted toward a user, while reducing associated bandwidth overhead. The decoder then decodes the compressed video file to reconstruct the original video signal for display to an end user. The decoding process generally mirrors the encoding process to allow the decoder to consistently reconstruct the video signal. 
     At step  101 , the video signal is input into the encoder. For example, the video signal may be an uncompressed video file stored in memory. As another example, the video file may be captured by a video capture device, such as a video camera, and encoded to support live streaming of the video. The video file may include both an audio component and a video component. The video component contains a series of image frames that, when viewed in a sequence, gives the visual impression of motion. The frames contain pixels that are expressed in terms of light, referred to herein as luma components (or luma samples), and color, which is referred to as chroma components (or color samples). In some examples, the frames may also contain depth values to support three dimensional viewing. 
     At step  103 , the video is partitioned into blocks. Partitioning includes subdividing the pixels in each frame into square and/or rectangular blocks for compression. For example, in High Efficiency Video Coding (HEVC) (also known as H.265 and MPEG-H Part 2) the frame can first be divided into coding tree units (CTUs), which are blocks of a predefined size (e.g., sixty-four pixels by sixty-four pixels). The CTUs contain both luma and chroma samples. Coding trees may be employed to divide the CTUs into blocks and then recursively subdivide the blocks until configurations are achieved that support further encoding. For example, luma components of a frame may be subdivided until the individual blocks contain relatively homogenous lighting values. Further, chroma components of a frame may be subdivided until the individual blocks contain relatively homogenous color values. Accordingly, partitioning mechanisms vary depending on the content of the video frames. 
     At step  105 , various compression mechanisms are employed to compress the image blocks partitioned at step  103 . For example, inter-prediction and/or intra-prediction may be employed. Inter-prediction is designed to take advantage of the fact that objects in a common scene tend to appear in successive frames. Accordingly, a block depicting an object in a reference frame need not be repeatedly described in adjacent frames. Specifically, an object, such as a table, may remain in a constant position over multiple frames. Hence the table is described once and adjacent frames can refer back to the reference frame. Pattern matching mechanisms may be employed to match objects over multiple frames. Further, moving objects may be represented across multiple frames, for example due to object movement or camera movement. As a particular example, a video may show an automobile that moves across the screen over multiple frames. Motion vectors can be employed to describe such movement. A motion vector is a two-dimensional vector that provides an offset from the coordinates of an object in a frame to the coordinates of the object in a reference frame. As such, inter-prediction can encode an image block in a current frame as a set of motion vectors indicating an offset from a corresponding block in a reference frame. 
     Intra-prediction encodes blocks in a common frame. Intra-prediction takes advantage of the fact that luma and chroma components tend to cluster in a frame. For example, a patch of green in a portion of a tree tends to be positioned adjacent to similar patches of green. Intra-prediction employs multiple directional prediction modes (e.g., thirty-three in HEVC), a planar mode, and a direct current (DC) mode. The directional modes indicate that a current block is similar/the same as samples of a neighbor block in a corresponding direction. Planar mode indicates that a series of blocks along a row/column (e.g., a plane) can be interpolated based on neighbor blocks at the edges of the row. Planar mode, in effect, indicates a smooth transition of light/color across a row/column by employing a relatively constant slope in changing values. DC mode is employed for boundary smoothing and indicates that a block is similar/the same as an average value associated with samples of all the neighbor blocks associated with the angular directions of the directional prediction modes. Accordingly, intra-prediction blocks can represent image blocks as various relational prediction mode values instead of the actual values. Further, inter-prediction blocks can represent image blocks as motion vector values instead of the actual values. In either case, the prediction blocks may not exactly represent the image blocks in some cases. Any differences are stored in residual blocks. Transforms may be applied to the residual blocks to further compress the file. 
     At step  107 , various filtering techniques may be applied. In HEVC, the filters are applied according to an in-loop filtering scheme. The block based prediction discussed above may result in the creation of blocky images at the decoder. Further, the block based prediction scheme may encode a block and then reconstruct the encoded block for later use as a reference block. The in-loop filtering scheme iteratively applies noise suppression filters, de-blocking filters, adaptive loop filters, and sample adaptive offset (SAO) filters to the blocks/frames. These filters mitigate such blocking artifacts so that the encoded file can be accurately reconstructed. Further, these filters mitigate artifacts in the reconstructed reference blocks so that artifacts are less likely to create additional artifacts in subsequent blocks that are encoded based on the reconstructed reference blocks. 
     Once the video signal has been partitioned, compressed, and filtered, the resulting data is encoded in a bitstream at step  109 . The bitstream includes the data discussed above as well as any signaling data desired to support proper video signal reconstruction at the decoder. For example, such data may include partition data, prediction data, residual blocks, and various flags providing coding instructions to the decoder. The bitstream may be stored in memory for transmission toward a decoder upon request. The bitstream may also be broadcast and/or multicast toward a plurality of decoders. The creation of the bitstream is an iterative process. Accordingly, steps  101 ,  103 ,  105 ,  107 , and  109  may occur continuously and/or simultaneously over many frames and blocks. The order shown in  FIG. 1  is presented for clarity and ease of discussion, and is not intended to limit the video coding process to a particular order. 
     The decoder receives the bitstream and begins the decoding process at step  111 . Specifically, the decoder employs an entropy decoding scheme to convert the bitstream into corresponding syntax and video data. The decoder employs the syntax data from the bitstream to determine the partitions for the frames at step  111 . The partitioning should match the results of block partitioning at step  103 . Entropy encoding/decoding as employed in step  111  is now described. The encoder makes many choices during the compression process, such as selecting block partitioning schemes from several possible choices based on the spatial positioning of values in the input image(s). Signaling the exact choices may employ a large number of bins. As used herein, a bin is a binary value that is treated as a variable (e.g., a bit value that may vary depending on context). Entropy coding allows the encoder to discard any options that are clearly not viable for a particular case, leaving a set of allowable options. Each allowable option is then assigned a code word. The length of the code words is based on the number of allowable options (e.g., one bin for two options, two bins for three to four options, etc.) The encoder then encodes the code word for the selected option. This scheme reduces the size of the code words as the code words are as big as desired to uniquely indicate a selection from a small sub-set of allowable options as opposed to uniquely indicating the selection from a potentially large set of all possible options. The decoder then decodes the selection by determining the set of allowable options in a similar manner to the encoder. By determining the set of allowable options, the decoder can read the code word and determine the selection made by the encoder. 
     At step  113 , the decoder performs block decoding. Specifically, the decoder employs reverse transforms to generate residual blocks. Then the decoder employs the residual blocks and corresponding prediction blocks to reconstruct the image blocks according to the partitioning. The prediction blocks may include both intra-prediction blocks and inter-prediction blocks as generated at the encoder at step  105 . The reconstructed image blocks are then positioned into frames of a reconstructed video signal according to the partitioning data determined at step  111 . Syntax for step  113  may also be signaled in the bitstream via entropy coding as discussed above. 
     At step  115 , filtering is performed on the frames of the reconstructed video signal in a manner similar to step  107  at the encoder. For example, noise suppression filters, de-blocking filters, adaptive loop filters, and SAO filters may be applied to the frames to remove blocking artifacts. Once the frames are filtered, the video signal can be output to a display at step  117  for viewing by an end user. 
       FIG. 2  is a schematic diagram of an example coding and decoding (codec) system  200  for video coding. Specifically, codec system  200  provides functionality to support the implementation of operating method  100 . Codec system  200  is generalized to depict components employed in both an encoder and a decoder. Codec system  200  receives and partitions a video signal as discussed with respect to steps  101  and  103  in operating method  100 , which results in a partitioned video signal  201 . Codec system  200  then compresses the partitioned video signal  201  into a coded bitstream when acting as an encoder as discussed with respect to steps  105 ,  107 , and  109  in method  100 . When acting as a decoder, codec system  200  generates an output video signal from the bitstream as discussed with respect to steps  111 ,  113 ,  115 , and  117  in operating method  100 . The codec system  200  includes a general coder control component  211 , a transform scaling and quantization component  213 , an intra-picture estimation component  215 , an intra-picture prediction component  217 , a motion compensation component  219 , a motion estimation component  221 , a scaling and inverse transform component  229 , a filter control analysis component  227 , an in-loop filters component  225 , a decoded picture buffer component  223 , and a header formatting and context adaptive binary arithmetic coding (CABAC) component  231 . Such components are coupled as shown. In  FIG. 2 , black lines indicate movement of data to be encoded/decoded while dashed lines indicate movement of control data that controls the operation of other components. The components of codec system  200  may all be present in the encoder. The decoder may include a subset of the components of codec system  200 . For example, the decoder may include the intra-picture prediction component  217 , the motion compensation component  219 , the scaling and inverse transform component  229 , the in-loop filters component  225 , and the decoded picture buffer component  223 . These components are now described. 
     The partitioned video signal  201  is a captured video sequence that has been partitioned into blocks of pixels by a coding tree. A coding tree employs various split modes to subdivide a block of pixels into smaller blocks of pixels. These blocks can then be further subdivided into smaller blocks. The blocks may be referred to as nodes on the coding tree. Larger parent nodes are split into smaller child nodes. The number of times a node is subdivided is referred to as the depth of the node/coding tree. The divided blocks can be included in coding units (CUs) in some cases. For example, a CU can be a sub-portion of a CTU that contains a luma block, red difference chroma (Cr) block(s), and a blue difference chroma (Cb) block(s) along with corresponding syntax instructions for the CU. The split modes may include a binary tree (BT), triple tree (TT), and a quad tree (QT) employed to partition a node into two, three, or four child nodes, respectively, of varying shapes depending on the split modes employed. The partitioned video signal  201  is forwarded to the general coder control component  211 , the transform scaling and quantization component  213 , the intra-picture estimation component  215 , the filter control analysis component  227 , and the motion estimation component  221  for compression. 
     The general coder control component  211  is configured to make decisions related to coding of the images of the video sequence into the bitstream according to application constraints. For example, the general coder control component  211  manages optimization of bitrate/bitstream size versus reconstruction quality. Such decisions may be made based on storage space/bandwidth availability and image resolution requests. The general coder control component  211  also manages buffer utilization in light of transmission speed to mitigate buffer underrun and overrun issues. To manage these issues, the general coder control component  211  manages partitioning, prediction, and filtering by the other components. For example, the general coder control component  211  may dynamically increase compression complexity to increase resolution and increase bandwidth usage or decrease compression complexity to decrease resolution and bandwidth usage. Hence, the general coder control component  211  controls the other components of codec system  200  to balance video signal reconstruction quality with bit rate concerns. The general coder control component  211  creates control data, which controls the operation of the other components. The control data is also forwarded to the header formatting and CABAC component  231  to be encoded in the bitstream to signal parameters for decoding at the decoder. 
     The partitioned video signal  201  is also sent to the motion estimation component  221  and the motion compensation component  219  for inter-prediction. A frame or slice of the partitioned video signal  201  may be divided into multiple video blocks. Motion estimation component  221  and the motion compensation component  219  perform inter-predictive coding of the received video block relative to one or more blocks in one or more reference frames to provide temporal prediction. Codec system  200  may perform multiple coding passes, e.g., to select an appropriate coding mode for each block of video data. 
     Motion estimation component  221  and motion compensation component  219  may be highly integrated, but are illustrated separately for conceptual purposes. Motion estimation, performed by motion estimation component  221 , is the process of generating motion vectors, which estimate motion for video blocks. A motion vector, for example, may indicate the displacement of a coded object relative to a predictive block. A predictive block is a block that is found to closely match the block to be coded, in terms of pixel difference. A predictive block may also be referred to as a reference block. Such pixel difference may be determined by sum of absolute difference (SAD), sum of square difference (SSD), or other difference metrics. HEVC employs several coded objects including a CTU, coding tree blocks (CTBs), and CUs. For example, a CTU can be divided into CTBs, which can then be divided into CBs for inclusion in CUs. A CU can be encoded as a prediction unit (PU) containing prediction data and/or a transform unit (TU) containing transformed residual data for the CU. The motion estimation component  221  generates motion vectors, PUs, and TUs by using a rate-distortion analysis as part of a rate distortion optimization process. For example, the motion estimation component  221  may determine multiple reference blocks, multiple motion vectors, etc. for a current block/frame, and may select the reference blocks, motion vectors, etc. having the best rate-distortion characteristics. The best rate-distortion characteristics balance both quality of video reconstruction (e.g., amount of data loss by compression) with coding efficiency (e.g., size of the final encoding). 
     In some examples, codec system  200  may calculate values for sub-integer pixel positions of reference pictures stored in decoded picture buffer component  223 . For example, video codec system  200  may interpolate values of one-quarter pixel positions, one-eighth pixel positions, or other fractional pixel positions of the reference picture. Therefore, motion estimation component  221  may perform a motion search relative to the full pixel positions and fractional pixel positions and output a motion vector with fractional pixel precision. The motion estimation component  221  calculates a motion vector for a PU of a video block in an inter-coded slice by comparing the position of the PU to the position of a predictive block of a reference picture. Motion estimation component  221  outputs the calculated motion vector as motion data to header formatting and CABAC component  231  for encoding and motion to the motion compensation component  219 . 
     Motion compensation, performed by motion compensation component  219 , may involve fetching or generating the predictive block based on the motion vector determined by motion estimation component  221 . Again, motion estimation component  221  and motion compensation component  219  may be functionally integrated, in some examples. Upon receiving the motion vector for the PU of the current video block, motion compensation component  219  may locate the predictive block to which the motion vector points. A residual video block is then formed by subtracting pixel values of the predictive block from the pixel values of the current video block being coded, forming pixel difference values. In general, motion estimation component  221  performs motion estimation relative to luma components, and motion compensation component  219  uses motion vectors calculated based on the luma components for both chroma components and luma components. The predictive block and residual block are forwarded to transform scaling and quantization component  213 . 
     The partitioned video signal  201  is also sent to intra-picture estimation component  215  and intra-picture prediction component  217 . As with motion estimation component  221  and motion compensation component  219 , intra-picture estimation component  215  and intra-picture prediction component  217  may be highly integrated, but are illustrated separately for conceptual purposes. The intra-picture estimation component  215  and intra-picture prediction component  217  intra-predict a current block relative to blocks in a current frame, as an alternative to the inter-prediction performed by motion estimation component  221  and motion compensation component  219  between frames, as described above. In particular, the intra-picture estimation component  215  determines an intra-prediction mode to use to encode a current block. In some examples, intra-picture estimation component  215  selects an appropriate intra-prediction mode to encode a current block from multiple tested intra-prediction modes. The selected intra-prediction modes are then forwarded to the header formatting and CABAC component  231  for encoding. 
     For example, the intra-picture estimation component  215  calculates rate-distortion values using a rate-distortion analysis for the various tested intra-prediction modes, and selects the intra-prediction mode having the best rate-distortion characteristics among the tested modes. Rate-distortion analysis generally determines an amount of distortion (or error) between an encoded block and an original unencoded block that was encoded to produce the encoded block, as well as a bitrate (e.g., a number of bits) used to produce the encoded block. The intra-picture estimation component  215  calculates ratios from the distortions and rates for the various encoded blocks to determine which intra-prediction mode exhibits the best rate-distortion value for the block. In addition, intra-picture estimation component  215  may be configured to code depth blocks of a depth map using a depth modeling mode (DMM) based on rate-distortion optimization (RDO). 
     The intra-picture prediction component  217  may generate a residual block from the predictive block based on the selected intra-prediction modes determined by intra-picture estimation component  215  when implemented on an encoder or read the residual block from the bitstream when implemented on a decoder. The residual block includes the difference in values between the predictive block and the original block, represented as a matrix. The residual block is then forwarded to the transform scaling and quantization component  213 . The intra-picture estimation component  215  and the intra-picture prediction component  217  may operate on both luma and chroma components. 
     The transform scaling and quantization component  213  is configured to further compress the residual block. The transform scaling and quantization component  213  applies a transform, such as a discrete cosine transform (DCT), a discrete sine transform (DST), or a conceptually similar transform, to the residual block, producing a video block comprising residual transform coefficient values. Wavelet transforms, integer transforms, sub-band transforms or other types of transforms could also be used. The transform may convert the residual information from a pixel value domain to a transform domain, such as a frequency domain. The transform scaling and quantization component  213  is also configured to scale the transformed residual information, for example based on frequency. Such scaling involves applying a scale factor to the residual information so that different frequency information is quantized at different granularities, which may affect final visual quality of the reconstructed video. The transform scaling and quantization component  213  is also configured to quantize the transform coefficients to further reduce bit rate. The quantization process may reduce the bit depth associated with some or all of the coefficients. The degree of quantization may be modified by adjusting a quantization parameter. In some examples, the transform scaling and quantization component  213  may then perform a scan of the matrix including the quantized transform coefficients. The quantized transform coefficients are forwarded to the header formatting and CABAC component  231  to be encoded in the bitstream. 
     The scaling and inverse transform component  229  applies a reverse operation of the transform scaling and quantization component  213  to support motion estimation. The scaling and inverse transform component  229  applies inverse scaling, transformation, and/or quantization to reconstruct the residual block in the pixel domain, e.g., for later use as a reference block which may become a predictive block for another current block. The motion estimation component  221  and/or motion compensation component  219  may calculate a reference block by adding the residual block back to a corresponding predictive block for use in motion estimation of a later block/frame. Filters are applied to the reconstructed reference blocks to mitigate artifacts created during scaling, quantization, and transform. Such artifacts could otherwise cause inaccurate prediction (and create additional artifacts) when subsequent blocks are predicted. 
     The filter control analysis component  227  and the in-loop filters component  225  apply the filters to the residual blocks and/or to reconstructed image blocks. For example, the transformed residual block from the scaling and inverse transform component  229  may be combined with a corresponding prediction block from intra-picture prediction component  217  and/or motion compensation component  219  to reconstruct the original image block. The filters may then be applied to the reconstructed image block. In some examples, the filters may instead be applied to the residual blocks. As with other components in  FIG. 2 , the filter control analysis component  227  and the in-loop filters component  225  are highly integrated and may be implemented together, but are depicted separately for conceptual purposes. Filters applied to the reconstructed reference blocks are applied to particular spatial regions and include multiple parameters to adjust how such filters are applied. The filter control analysis component  227  analyzes the reconstructed reference blocks to determine where such filters should be applied and sets corresponding parameters. Such data is forwarded to the header formatting and CABAC component  231  as filter control data for encoding. The in-loop filters component  225  applies such filters based on the filter control data. The filters may include a deblocking filter, a noise suppression filter, a SAO filter, and an adaptive loop filter. Such filters may be applied in the spatial/pixel domain (e.g., on a reconstructed pixel block) or in the frequency domain, depending on the example. 
     When operating as an encoder, the filtered reconstructed image block, residual block, and/or prediction block are stored in the decoded picture buffer component  223  for later use in motion estimation as discussed above. When operating as a decoder, the decoded picture buffer component  223  stores and forwards the reconstructed and filtered blocks toward a display as part of an output video signal. The decoded picture buffer component  223  may be any memory device capable of storing prediction blocks, residual blocks, and/or reconstructed image blocks. 
     The header formatting and CABAC component  231  receives the data from the various components of codec system  200  and encodes such data into a coded bitstream for transmission toward a decoder. Specifically, the header formatting and CABAC component  231  generates various headers to encode control data, such as general control data and filter control data. Further, prediction data, including intra-prediction and motion data, as well as residual data in the form of quantized transform coefficient data are all encoded in the bitstream. The final bitstream includes all information desired by the decoder to reconstruct the original partitioned video signal  201 . Such information may also include intra-prediction mode index tables (also referred to as codeword mapping tables), definitions of encoding contexts for various blocks, indications of most probable intra-prediction modes, an indication of partition information, etc. Such data may be encoded by employing entropy coding. For example, the information may be encoded by employing context adaptive variable length coding (CAVLC), CABAC, syntax-based context-adaptive binary arithmetic coding (SBAC), probability interval partitioning entropy (PIPE) coding, or another entropy coding technique. Following the entropy coding, the coded bitstream may be transmitted to another device (e.g., a video decoder) or archived for later transmission or retrieval. 
       FIG. 3  is a block diagram illustrating an example video encoder  300 . Video encoder  300  may be employed to implement the encoding functions of codec system  200  and/or implement steps  101 ,  103 ,  105 ,  107 , and/or  109  of operating method  100 . Encoder  300  partitions an input video signal, resulting in a partitioned video signal  301 , which is substantially similar to the partitioned video signal  201 . The partitioned video signal  301  is then compressed and encoded into a bitstream by components of encoder  300 . 
     Specifically, the partitioned video signal  301  is forwarded to an intra-picture prediction component  317  for intra-prediction. The intra-picture prediction component  317  may be substantially similar to intra-picture estimation component  215  and intra-picture prediction component  217 . The partitioned video signal  301  is also forwarded to a motion compensation component  321  for inter-prediction based on reference blocks in a decoded picture buffer component  323 . The motion compensation component  321  may be substantially similar to motion estimation component  221  and motion compensation component  219 . The prediction blocks and residual blocks from the intra-picture prediction component  317  and the motion compensation component  321  are forwarded to a transform and quantization component  313  for transform and quantization of the residual blocks. The transform and quantization component  313  may be substantially similar to the transform scaling and quantization component  213 . The transformed and quantized residual blocks and the corresponding prediction blocks (along with associated control data) are forwarded to an entropy coding component  331  for coding into a bitstream. The entropy coding component  331  may be substantially similar to the header formatting and CABAC component  231 . 
     The transformed and quantized residual blocks and/or the corresponding prediction blocks are also forwarded from the transform and quantization component  313  to an inverse transform and quantization component  329  for reconstruction into reference blocks for use by the motion compensation component  321 . The inverse transform and quantization component  329  may be substantially similar to the scaling and inverse transform component  229 . In-loop filters in an in-loop filters component  325  are also applied to the residual blocks and/or reconstructed reference blocks, depending on the example. The in-loop filters component  325  may be substantially similar to the filter control analysis component  227  and the in-loop filters component  225 . The in-loop filters component  325  may include multiple filters as discussed with respect to in-loop filters component  225 . The filtered blocks are then stored in a decoded picture buffer component  323  for use as reference blocks by the motion compensation component  321 . The decoded picture buffer component  323  may be substantially similar to the decoded picture buffer component  223 . 
       FIG. 4  is a block diagram illustrating an example video decoder  400 . Video decoder  400  may be employed to implement the decoding functions of codec system  200  and/or implement steps  111 ,  113 ,  115 , and/or  117  of operating method  100 . Decoder  400  receives a bitstream, for example from an encoder  300 , and generates a reconstructed output video signal based on the bitstream for display to an end user. 
     The bitstream is received by an entropy decoding component  433 . The entropy decoding component  433  is configured to implement an entropy decoding scheme, such as CAVLC, CABAC, SBAC, PIPE coding, or other entropy coding techniques. For example, the entropy decoding component  433  may employ header information to provide a context to interpret additional data encoded as codewords in the bitstream. The decoded information includes any desired information to decode the video signal, such as general control data, filter control data, partition information, motion data, prediction data, and quantized transform coefficients from residual blocks. The quantized transform coefficients are forwarded to an inverse transform and quantization component  429  for reconstruction into residual blocks. The inverse transform and quantization component  429  may be similar to inverse transform and quantization component  329 . 
     The reconstructed residual blocks and/or prediction blocks are forwarded to intra-picture prediction component  417  for reconstruction into image blocks based on intra-prediction operations. The intra-picture prediction component  417  may be similar to intra-picture estimation component  215  and to 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. 
     Keeping the above in mind, video compression techniques perform spatial (intra-picture) prediction and/or temporal (inter-picture) prediction to reduce or remove redundancy inherent in video sequences. For block-based video coding, a video slice (i.e., 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 encoded using spatial prediction with respect to reference samples in neighboring blocks in the same picture. Video blocks in an inter-coded (P or B) slice of a picture may use spatial prediction with respect to reference samples in neighboring blocks in the same picture or temporal prediction with respect to reference samples in other reference pictures. Pictures may be referred to as frames, and reference pictures may be referred to as reference frames. The POC is a variable associated with each picture that uniquely identifies the associated picture among all pictures in the coded layer video sequence (CLVS), indicates when the associated picture is to be output from the DPB, and indicates the position of the associated picture in output order relative to the output order positions of the other pictures in the same CLVS that are to be output from the DPB. A flag is a variable or single-bit syntax element that can take one of the two possible values: 0 and 1. 
     Spatial or temporal prediction results in a predictive block for a block to be coded. Residual data represents pixel differences between the original block to be coded and the predictive block. 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, resulting in residual transform coefficients, which then may be quantized. The quantized transform coefficients, initially arranged in a two-dimensional array, may be scanned in order to produce a one-dimensional vector of transform coefficients, and entropy coding may be applied to achieve even more compression. 
     Image and video compression has experienced rapid growth, leading to various coding standards. Such video coding standards include ITU-T H.261, International Organization for Standardization/International Electrotechnical Commission (ISO/IEC) 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 3D AVC (3D-AVC). HEVC includes extensions such as Scalable HEVC (SHVC), Multiview HEVC (MV-HEVC), and 3D HEVC (3D-HEVC). 
     There is also a new video coding standard, named Versatile Video Coding (VVC), being developed by the joint video experts team (WET) of ITU-T and ISO/IEC. While the VVC standard has several working drafts, one Working Draft (WD) of VVC in particular, namely B. Bross, J. Chen, and S. Liu, “Versatile Video Coding (Draft 5),” JVET-N1001-v3, 13th JVET Meeting, Mar. 27, 2019 (VVC Draft 5) is referenced herein. 
     Scalability in video coding usually is supported by using multi-layer coding techniques. A multi-layer bitstream comprises a base layer (BL) and one or more enhancement layers (ELs). An example of scalabilities includes spatial scalability, quality/signal-to-noise (SNR) scalability, multi-view scalability, etc. When a multi-layer coding technique is used, a picture or a part thereof may be coded (1) without using a reference picture, i.e., using intra prediction; (2) by referencing to reference pictures that are in the same layer, i.e., using inter prediction; or (3) by referencing to reference pictures that are in other layer(s), i.e., using 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. 5  is a schematic diagram illustrating an example of layer based prediction  500 , for example as performed to determine MVs at block compression step  105 , block decoding step  113 , motion estimation component  221 , motion compensation component  219 , motion compensation component  321 , and/or motion compensation component  421 . Layer based prediction  500  is compatible with unidirectional inter-prediction and/or bidirectional inter-prediction, but is also performed between pictures in different layers. 
     Layer based prediction  500  is applied between pictures  511 ,  512 ,  513 , and  514  and pictures  515 ,  516 ,  517 , and  518  in different layers. In the example shown, pictures  511 ,  512 ,  513 , and  514  are part of layer N+1  532  and pictures  515 ,  516 ,  517 , and  518  are part of layer N  531 . A layer, such as layer N  531  and/or layer N+1  532 , 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. In the example shown, layer N+1  532  is associated with a larger image size than layer N  531 . Accordingly, pictures  511 ,  512 ,  513 , and  514  in layer N+1  532  have a larger picture size (e.g., larger height and width and hence more samples) than pictures  515 ,  516 ,  517 , and  518  in layer N  531  in this example. However, such pictures can be separated between layer N+1  532  and layer N  531  by other characteristics. While only two layers, layer N+1  532  and layer N  531 , are shown, a set of pictures can be separated into any number of layers based on associated characteristics. Layer N+1  532  and layer N  531  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  511 - 518  may be associated with a corresponding layer ID to indicate which layer N+1  532  or layer N  531  includes the corresponding picture. 
     Pictures  511 - 518  in different layers  531 - 532  are configured to be displayed in the alternative. As such, pictures  511 - 518  in different layers  531 - 532  can share the same temporal identifier (ID) and can be included in the same AU. As used herein, an AU is a set of one or more coded pictures associated with the same display time for output from a DPB. For example, a decoder may decode and display picture  515  at a current display time if a smaller picture is desired or the decoder may decode and display picture  511  at the current display time if a larger picture is desired. As such, pictures  511 - 514  at higher layer N+1  532  contain substantially the same image data as corresponding pictures  515 - 518  at lower layer N  531  (notwithstanding the difference in picture size). Specifically, picture  511  contains substantially the same image data as picture  515 , picture  512  contains substantially the same image data as picture  516 , etc. 
     Pictures  511 - 518  can be coded by reference to other pictures  511 - 518  in the same layer N  531  or N+1  532 . Coding a picture in reference to another picture in the same layer results in inter-prediction  523 , which is compatible unidirectional inter-prediction and/or bidirectional inter-prediction. Inter-prediction  523  is depicted by solid line arrows. For example, picture  513  may be coded by employing inter-prediction  523  using one or two of pictures  511 ,  512 , and/or  514  in layer N+1  532  as a reference, where one picture is referenced for unidirectional inter-prediction and/or two pictures are reference for bidirectional inter-prediction. Further, picture  517  may be coded by employing inter-prediction  523  using one or two of pictures  515 ,  516 , and/or  518  in layer N  531  as a reference, where one picture is referenced for unidirectional inter-prediction and/or two pictures are reference for bidirectional inter-prediction. When a picture is used as a reference for another picture in the same layer when performing inter-prediction  523 , the picture may be referred to as a reference picture. For example, picture  512  may be a reference picture used to code picture  513  according to inter-prediction  523 . Inter-prediction  523  can also be referred to as intra-layer prediction in a multi-layer context. As such, inter-prediction  523  is a mechanism of coding samples of a current picture by reference to indicated samples in a reference picture that are different from the current picture where the reference picture and the current picture are in the same layer. 
     Pictures  511 - 518  can also be coded by reference to other pictures  511 - 518  in different layers. This process is known as inter-layer prediction  521 , and is depicted by dashed arrows. Inter-layer prediction  521  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  531  can be used as a reference picture to code a corresponding picture at a higher layer N+1  532 . As a specific example, picture  511  can be coded by reference to picture  515  according to inter-layer prediction  521 . In such a case, the picture  515  is used as an inter-layer reference picture. An inter-layer reference picture is a reference picture used for inter-layer prediction  521 . In most cases, inter-layer prediction  521  is constrained such that a current picture, such as picture  511 , 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  515 . When multiple layers (e.g., more than two) are available, inter-layer prediction  521  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 layer based prediction  500  to encode pictures  511 - 518  via many different combinations and/or permutations of inter-prediction  523  and inter-layer prediction  521 . For example, picture  515  may be coded according to intra-prediction. Pictures  516 - 518  can then be coded according to inter-prediction  523  by using picture  515  as a reference picture. Further, picture  511  may be coded according to inter-layer prediction  521  by using picture  515  as an inter-layer reference picture. Pictures  512 - 514  can then be coded according to inter-prediction  523  by using picture  511  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  532  pictures based on lower layer N  531  pictures, the higher layer N+1  532  can avoid employing intra-prediction, which has much lower coding efficiency than inter-prediction  523  and inter-layer prediction  521 . 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. 
     Each AU  506  in  FIG. 5  may contain several pictures. For example, one AU  506  may contain pictures  511  and  515 . Another AU  506  may contain pictures  512  and  516 . Indeed, each AU  506  is a set of one or more coded pictures associated with the same display time (e.g., the same temporal ID) for output from a decoded picture buffer (DPB) (e.g., for display to a user). Each AUD  508  is an indicator or data structure used to indicate the start of an AU (e.g., AU  508 ) or the boundary between AUs. 
     Previous H.26x video coding families have provided support for scalability in separate profile(s) from the profile(s) for single-layer coding. Scalable video coding (SVC) is the scalable extension of the AVC/H.264 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 cannot directly reuse unmodified H.264/AVC implementations in their design. The SVC EL macroblock syntax and decoding process differs from H.264/AVC syntax and decoding process. 
     Scalable HEVC (SHVC) is the extension of the HEVC/H.265 standard that provides support for spatial and quality scalabilities, multiview HEVC (MV-HEVC) is the extension of the HEVC/H.265 that provides support for multi-view scalability, and 3D HEVC (3D-HEVC) is the extension of the HEVC/H.264 that provides supports for three dimensional (3D) video coding that is more advanced and more efficient than MV-HEVC. Note that the temporal scalability is included as an integral part of the single-layer HEVC codec. The design of the multi-layer extension of HEVC employs the idea where the decoded pictures used for inter-layer prediction come only from the same access unit (AU) and are treated as long-term reference pictures (LTRPs), and are assigned reference indices in the reference picture list(s) along with other temporal reference pictures in the current layer. Inter-layer prediction (ILP) is achieved at the prediction unit (PU) level by setting the value of the reference index to refer to the inter-layer reference picture(s) in the reference picture list(s). 
     Notably, both reference picture resampling and spatial scalability features call for resampling of a reference picture or part thereof. Reference picture resampling (RPR) can be realized at either the picture level or coding block level. However, when RPR is referred to as a coding feature, it is a feature for single-layer coding. Even so, it is possible or even preferable from a codec design point of view to use the same resampling filter for both the RPR feature of single-layer coding and the spatial scalability feature for multi-layer coding. 
       FIG. 6  illustrates an example of layer based prediction  600  utilizing output layer sets (OLSs), for example as performed to determine MVs at block compression step  105 , block decoding step  113 , motion estimation component  221 , motion compensation component  219 , motion compensation component  321 , and/or motion compensation component  421 . Layer based prediction  500  is compatible with unidirectional inter-prediction and/or bidirectional inter-prediction, but is also performed between pictures in different layers. The layer based prediction of  FIG. 6  is similar to that of  FIG. 5 . Therefore, for the sake of brevity, a full description of layer based prediction is not repeated. 
     Some of the layers in the coded video sequence (CVS)  690  of  FIG. 6  are included in an OLS. An OLS is a set of layers for which one or more layers are specified as the output layers. An output layer is a layer of an OLS that is output.  FIG. 6  depicts three different OLSs, namely OLS 1, OLS 2, and OLS 3. As shown, OLS 1 includes Layer N  631  and Layer N+1  632 . OLS 2 includes Layer N  631 , Layer N+1  632 , Layer N+2  633 , and Layer N+1  634 . OLS 3 includes Layer N  631 , Layer N+1  632 , and Layer N+2  633 . Despite three OLSs being shown, a different number of OLSs may be used in practical applications. 
     Each of the different OLSs may contain any number of layers. The different OLSs are generated in an effort to accommodate the coding capabilities of a variety of different devices having varying coding capabilities. For example, OLS 1, which contains only two layers, may be generated to accomodate a mobile phone with relatively limited coding capabilities. On the other hand, OLS 2, which contains four layers, may be generated to accommodate a big screen television, which is able to decode higher layers than the mobile phone. OLS 3, which contains three layers, may be generated to accommodate a personal computer, laptop computer, or a tablet computer, which may be able to decode higher layers than the mobile phone but cannot decode the highest layers like the big screen television. 
     The layers in  FIG. 6  can be all independent from each other. That is, each layer can be coded without using inter-layer prediction (ILP). In this case, the layers are referred to as simulcast layers. One or more of the layers in  FIG. 6  may also be coded using ILP. Whether the layers are simulcast layers or whether some of the layers are coded using ILP is signaled by a flag in a video parameter set (VPS), which will be more fully discussed below. When some layers use ILP, the layer dependency relationship among layers is also signaled in the VPS. 
     In an embodiment, when the layers are simulcast layers, only one layer is selected for decoding and output. In an embodiment, when some layers use ILP, all of the layers (e.g., the entire bitstream) are specified to be decoded, and certain layers among the layers are specified to be output layers. The output layer or layers may be, for example, 1) only the highest layer, 2) all the layers, or 3) the highest layer plus a set of indicated lower layers. For example, when the highest layer plus a set of indicated lower layers are designated for output by a flag in the VPS, layer N+3  634  (which is the highest layer) and layers N  631  and N+1  632  (which are lower layers) from OLS 2 are output. 
     Still referring to  FIG. 6 , some layers are not included in any of the OLSs. For example, Layer N+4  635  is not contained within any of OLS 1, OLS 2, and OLS 3. Such layers are referred to as unused layers. Unfortunately, SHVC and MV-HEVC allow such unused layers to be included in multi-layer video bitstreams, which unnecessarily burdens coding resources and decreases coding efficiency. 
     Disclosed herein are techniques that disallow unused layers in multi-layer video bitstreams. That is, any layer not used in at least one output layer set (OLS) is constrained from being included in multi-layer bitstreams. This avoids having extraneous information in the coding process and increases coding efficiency. Thus, the coder/decoder (a.k.a., “codec”) in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed. 
       FIG. 7  illustrates an embodiment of a video bitstream  700 . As used herein the video bitstream  700  may also be referred to as a coded video bitstream, a bitstream, or variations thereof. As shown in  FIG. 7 , the bitstream  700  comprises at least one picture unit (PU)  701 . While three of the PUs  701  are shown in  FIG. 7 , a different number of PUs  701  may be present in the bitstream  700  in practical applications. Each PU  701  is 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 (e.g., picture  714 ). 
     In an embodiment, each PU  701  contains one or more of the following: decoding capability information (DCI)  702 , a video parameter set (VPS)  704 , a sequence parameter set (SPS)  706 , a picture parameter set (PPS)  708 , a picture header (PH)  712 , and a picture  714 . Each of the DCI  702 , the VPS  704 , the SPS  706 , and the PPS  708  may be generically referred to as a parameter set. In an embodiment, other parameter sets not shown in  FIG. 7  may also be included in the bitstream  700  such as, for example, an adaption parameter set (APS), which is a syntax structure containing syntax elements that apply to zero or more slices as determined by zero or more syntax elements found in slice headers. 
     The DCI  702 , which may also be referred to a decoding parameter set (DPS) or decoder parameter set, is a syntax structure containing syntax elements that apply to the entire bitstream. The DCI  702  includes parameters that stay constant for the lifetime of the video bitstream (e.g., bitstream  700 ), which can translate to the lifetime of a session. The DCI  702  can include profile, level, and sub-profile information to determine a maximum complexity interop point that is guaranteed to be never exceeded, even if splicing of video sequences occurs within a session. It further optionally includes constraint flags, which indicate that the video bitstream will be constraint of the use of certain features as indicated by the values of those flags. With this, a bitstream can be labelled as not using certain tools, which allows among other things for resource allocation in a decoder implementation. Like all parameter sets, the DCI  702  is present when first referenced, and referenced by the very first picture in a video sequence, implying that it has to be sent among the first NAL units in the bitstream. While multiple DCIs  702  can be in the bitstream, the value of the syntax elements therein cannot be inconsistent when being referenced. 
     The VPS  704  includes decoding dependency or information for reference picture set construction of enhancement layers. The VPS  704  provides an overall perspective or view of a scalable sequence, including what types of operation points are provided, the profile, tier, and level of the operation points, and some other high-level properties of the bitstream that can be used as the basis for session negotiation and content selection, etc. 
     In an embodiment, when it is indicated that some of the layers use ILP, the VPS  704  indicates that a total number of OLSs specified by the VPS is equal to the number of layers, indicates that the i-th OLS includes the layers with layer indices from 0 to i, inclusive, and indicates that for each OLS only the highest layer in the OLS is output. 
     In an embodiment, the VPS  704  includes syntax and semantics corresponding to the OLSs in the CLVS and/or video bitstream. The following syntax and semantics corresponding to the VPS  704  may be employed to implement the embodiments disclosed herein. 
     The syntax for the VPS  704  may be as follows. 
     
       
         
           
               
               
             
               
                   
               
               
                   
                 Des- 
               
               
                   
                 criptor 
               
               
                   
               
             
            
               
                 video_parameter_set_rbsp( ) { 
                   
               
               
                  vps_video_parameter_set_id  
                 u(4)  
               
               
                  vps_max_layers_minus1  
                 u(6)  
               
               
                  vps_max_sub_layers_minus1  
                 u(3)  
               
               
                  if( vps_max_layers_minus1 &gt; 0 )  
                   
               
               
                   vps_all_independent_layers_flag  
                 u(1) 
               
               
                  for( i =0; i &lt;= vps_max_layers_minus1; i++ ) { 
                   
               
               
                   vps_layer_id[ i ]  
                 u(6) 
               
               
                   if( i &gt; 0 &amp;&amp; !vps_all_independent_layers_flag ) { 
                   
               
               
                    vps_independent_layer_flag[ i ] 
                 u(1)  
               
               
                    if( !vps_independent_layer_flag[ i ] )  
                   
               
               
                     for( j = 0; j &lt; i; j++ )  
                   
               
               
                      vps_direct_dependency_flag[ i ][ j ]  
                 u(1) 
               
               
                   } 
                   
               
               
                  }  
                   
               
               
                  if( vps_max_layers_minus1 &gt; 0 ) { 
                   
               
               
                   if( vps_all_independent_layers_flag )  
                   
               
               
                    each_layer_is_an_ols_flag 
                 u(1) 
               
               
                   if( !each_layer_is_an_ols_flag ) { 
                   
               
               
                    if( !vps_all_independent_layers_flag )  
                   
               
               
                     ols_mode_idc 
                 u(2) 
               
               
                    if( ols_mode_idc = = 2) { 
                   
               
               
                     num_output_layer_sets_minus1  
                 u(8) 
               
               
                     for( i = 1; i &lt; num_output_layer_sets_minus1 + 1; i ++ ) { 
                   
               
               
                      for( j = 0; j &lt;= vps_max_layers_minus1; j++ )  
                   
               
               
                       layer_included_flag[ i ][ j ]  
                 u(1) 
               
               
                      if( !vps_all_independent_layers_flag )  
                   
               
               
                       for( j = 0; j &lt; NumLayersInOls[ i ] − 1; j++ )  
                   
               
               
                        vps_output_layer_flag[ i ][ j ] 
                 u(1)  
               
               
                     } 
                   
               
               
                    } 
                   
               
               
                   } 
                   
               
               
                  } 
                   
               
               
                  vps_constraint_info_present_flag  
                 u(1) 
               
               
                  vps_reserved_zero_7bits  
                 u(7) 
               
               
                  if( vps_constraint_info_present_flag )  
                   
               
               
                   general_constraint_info( )  
                   
               
               
                  general_hrd_params_present_flag  
                 u(1) 
               
               
                  if( general_hrd_params_present flag) { 
                   
               
               
                   num_units_in_tick  
                 u(32)  
               
               
                   time_scale  
                 u(32) 
               
               
                   general_hrd_parameters( )  
                   
               
               
                  } 
                   
               
               
                  vps_extension_flag  
                 u(1) 
               
               
                  if( vps_extension_flag )  
                   
               
               
                   while( more_rbsp_data( ) )  
                   
               
               
                    vps_extension_data_flag  
                 u(1) 
               
               
                  rbsp trailing bits( )  
                   
               
               
                 }  
               
               
                   
               
            
           
         
       
     
     The semantics for the VPS  704  may be as follows. In an embodiment, the VPS  704  includes one or more of the flags and parameters noted below. 
     A VPS raw byte sequence payload (RBSP) shall be available to the decoding process prior to it being referenced, included in at least one access unit with TemporalId equal to 0 or provided through external means, and the VPS NAL unit containing the VPS RBSP shall have nuh_layer_id equal to vps_layer_id[0]. 
     All VPS NAL units with a particular value of vps_video_parameter_set_id in a CVS shall have the same content. vps_video_parameter_set_id provides an identifier for the VPS for reference by other syntax elements. vps_max_layers_minus1 plus 1 specifies the maximum allowed number of layers in each CVS referring to the VPS. vps_max_sub_layers_minus1 plus 1 specifies the maximum number of temporal sub-layers that may be present in each CVS referring to the VPS. The value of vps_max_sub_layers_minus1 shall be in the range of 0 to 6, inclusive. 
     vps_all_independent_layers_flag equal to 1 specifies that all layers in the CVS are independently coded without using inter-layer prediction. vps_all_independent_layers_flag equal to 0 specifies that one or more of the layers in the CVS may use inter-layer prediction. When not present, the value of vps_all_independent_layers_flag is inferred to be equal to 1. When vps_all_independent_layers_flag is equal to 1, the value of vps_independent_layer_flag[i] is inferred to be equal to 1. When vps_all_independent_layers_flag is equal to 0, the value of vps_independent_layer_flag[0] is inferred to be equal to 1. 
     vps_layer_id[i] specifies the nuh_layer_id value of the i-th layer. For any two non-negative integer values of m and n, when m is less than n, the value of vps_layer_id[m] shall be less than vps_layer_id[n]. vps_independent_layer_flag[i] equal to 1 specifies that the layer with index i does not use inter-layer prediction. vps_independent_layer_flag[i] equal to 0 specifies that the layer with index i may use inter-layer prediction and vps_layer_dependency_flag[i] is present in VPS. When not present, the value of vps_independent_layer_flag[i] is inferred to be equal to 1. 
     vps_direct_dependency_flag[i][j] equal to 0 specifies that the layer with index j is not a direct reference layer for the layer with index i. vps_direct_dependency_flag [i][j] equal to 1 specifies that the layer with index j is a direct reference layer for the layer with index i. When vps_direct_dependency_flag[i][j] is not present for i and j in the range of 0 to vps_max_layers_minus1, inclusive, it is inferred to be equal to 0. 
     The variable DirectDependentLayerIdx[i][j], specifying the j-th direct dependent layer of the i-th layer, is derived as follows. 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                   
                 for( i = 1; i &lt; vps_max_layers_minus1; i++ ) 
               
               
                   
                  if( !vps_independent_layer_flag[ i ] ) 
               
               
                   
                   for( j = i, k = 0; j &gt;= 0; j− − ) 
               
               
                   
                    if( vps_direct_dependency_flag[ i ][ j ] ) 
               
               
                   
                     DirectDependentLayerIdx[ i ][ k++ ] = j 
               
               
                   
               
            
           
         
       
     
     The variable GeneralLayerIdx[i], specifying the layer index of the layer with nuh_layer_id equal to vps_layer_id[i], is derived as follows. 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                   
                 for( i = 0; i &lt;= vps_max_layers_minus1; i++ ) 
               
               
                   
                  GeneralLayerIdx[ vps_layer_id[ i ] ] = i 
               
               
                   
               
            
           
         
       
     
     each_layer_is_an_ols_flag equal to 1 specifies that each output layer set contains only one layer and each layer itself in the bitstream is an outpu layer set with the single included layer being the only output layer. each_layer_is_an_ols_flag equal to 0 that an outpu layer set may contain more than one layer. If vps_max_layers_minus1 is equal to 0, the value of each_layer_is_an_ols_flag is inferred to be equal to 1. Otherwise, when vps_all_independent_layers_flag is equal to 0, the value of each_layer_is_an_ols_flag is inferred to be equal to 0. 
     ols_mode_idc equal to 0 specifies that the total number of OLSs specified by the VPS is equal to vps_max_layers_minus1+1, the i-th OLS includes the layers with layer indices from 0 to i, inclusive, and for each OLS only the highest layer in the OLS is output. ols_mode_idc equal to 1 specifies that the total number of OLSs specified by the VPS is equal to vps_max_layers_minus1+1, the i-th OLS includes the layers with layer indices from 0 to i, inclusive, and for each OLS all layers in the OLS are output. ols_mode_idc equal to 2 specifies that the total number of OLSs specified by the VPS is explicitly signalled and for each OLS the highest layer and an explicitly signalled set of lower layers in the OLS are output. The value of ols_mode_idc shall be in the range of 0 to 2, inclusive. The value 3 of ols_mode_idc is reserved for future use by ITU-T|ISO/IEC. When vps_all_independent_layers_flag is equal to 1 and each_layer_is_an_ols_flag is equal to 0, the value of ols_mode_idc is inferred to be equal to 2. 
     num_output_layer_sets_minus1 plus 1 specifies the total number of OLSs specified by the VPS when ols_mode_idc is equal to 2. 
     The variable TotalNumOlss, specifying the total number of OLSs specified by the VPS, is derived as follows: 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                   
                 if( vps_max_layers_minus1 = = 0 ) 
               
               
                   
                  TotalNumOlss = 1 
               
               
                   
                 else if( each_layer_is_an_ols_flag | | ols_mode_idc = = 0 | |  
               
               
                   
                 ols_mode_idc = = 1 )  
               
               
                   
                  TotalNumOlss = vps_max_layers_minus1 + 1 
               
               
                   
                 else if( ols_mode_idc = = 2 ) 
               
               
                   
                  TotalNumOlss = num_output_layer_sets_minus1 + 1 
               
               
                   
               
            
           
         
       
     
     layer_included_flag[i][j] specifies whether the j-th layer (i.e., the layer with nuh_layer_id equal to vps_layer_id[j]) is included in the i-th OLS when ols_mode_idc is equal to 2. layer_included_flag[i][j] equal to 1 specifies that the j-th layer is included in the i-th OLS. layer_included_flag[i][j] equal to 0 specifies the j-th layer is not included in the i-th OLS. 
     The variable NumLayersInOls[i], specifying the number of layers in the i-th OLS, and the variable LayerIdInOls[i][j], specifying the nuh_layer_id value of the j-th layer in the i-th OLS, are derived as follows. 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                   
                 NumLayersInOls[ 0 ] = 1 
               
               
                   
                 LayerIdInOls[ 0 ][ 0 ] = vps_layer_id[ 0 ] 
               
               
                   
                 for( i = 1, i &lt; TotalNumOlss; i++ ) { 
               
               
                   
                 if( each_layer_is_an_ols_flag ) { 
               
               
                   
                  NumLayersInOls[ i ] = 1 
               
               
                   
                  LayerIdInOls[ i ][ 0 ] = vps_layer_id[ i ] 
               
               
                   
                 } else if( ols_mode_idc = = 0 | | ols_mode_idc = = 1 ) { 
               
               
                   
                  NumLayersInOls[ i ] = i + 1 
               
               
                   
                  for( j = 0; j &lt; NumLayersInOls[ i ]; j++ ) 
               
               
                   
                   LayerIdInOls[ i ][ j ] = vps_layer_id[ j ]  
               
               
                   
                 } else if( ols_mode_idc = = 2 ) { 
               
               
                   
                  for( k = 0, j = 0; k &lt;= vps_max_layers_minus1; k++ ) 
               
               
                   
                   if( layer_included_flag[ i ][ k ] ) 
               
               
                   
                    LayerIdInOls[ i ][ j++ ] = vps_layer_id[ k ]  
               
               
                   
                  NumLayersInOls[ i ] = j 
               
               
                   
                 } 
               
               
                   
               
            
           
         
       
     
     The variable OlsLayeIdx[i][j], specifying the OLS layer index of the layer with nuh_layer_id equal to LayerIdInOls[i][j], is derived as follows. 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                   
                 for( i = 0, i &lt; TotalNumOlss; i++ ) 
               
               
                   
                  for j = 0; j &lt; NumLayersInOls[ i ]; j++ ) 
               
               
                   
                   OlsLayeIdx[ i ][LayerIdInOls[ i ][ j ] ] = j 
               
               
                   
               
            
           
         
       
     
     The lowest layer in each OLS shall be an independent layer. In other words, for each i in the range of 0 to TotalNumOlss−1, inclusive, the value of vps_independent_layer_flag[GeneralLayerIdx[LayerIdInOls[i][0]]] shall be equal to 1. 
     Each layer shall be included in at least one OLS specified by the VPS. That is, for each layer with a particular value of nuh_layer_id nuhLayerId equal to one of vps_layer_id[k] for k in the range of 0 to vps_max_layers_minus1, inclusive, there shall be at least one pair of values of i and j, where i is in the range of 0 to TotalNumOlss−1, inclusive, and j is in the range of NumLayersInOls[i]−1, inclusive, such that the value of LayerIdInOls[i][j] is equal to nuhLayerId. 
     Any layer in an OLS shall be an output layer of the OLS or a (direct or indirect) reference layer of an output layer of the OLS. 
     vps_output_layer_flag[i][j] specifies whether the j-th layer in the i-th OLS is output when ols_mode_idc is equal to 2. vps_output_layer_flag[i] equal to 1 specifies that the j-th layer in the i-th OLS is output. vps_output_layer_flag[i] equal to 0 specifies that the j-th layer in the i-th OLS is not output. When vps_all_independent_layers_flag is equal to 1 and each_layer_is_an_ols_flag is equal to 0, the value of vps_output_layer_flag[i] is inferred to be equal to 1. 
     The variable OutputLayerFlag[i][j], for which the value 1 specifies that the j-th layer in the i-th OLS is output and the value 0 specifies that the j-th layer in the i-th OLS is not output, is derived as follows. 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                   
                 for( i = 0, i &lt; TotalNumOlss; i++ ) { 
               
               
                   
                  OutputLayerFlag[ i ][ NumLayersInOls[ i ] − 1 ] = 1 
               
               
                   
                  for( j = 0; j &lt; NumLayersInOls[ i ] − 1; j++ ) 
               
               
                   
                   if( ols_mode_idc[ i ] = = 0 ) 
               
               
                   
                    OutputLayerFlag[ i ][ j ] = 0 
               
               
                   
                   else if( ols_mode_idc[ i ] = = 1 ) 
               
               
                   
                    OutputLayerFlag[ i ][ j ] = 1 
               
               
                   
                   else if( ols_mode_idc[ i ] = = 2 ) 
               
               
                   
                    OutputLayerFlag[ i ][ j ] = vps_output_layer_flag[ i ][ j ] 
               
               
                   
                 } 
               
               
                   
               
            
           
         
       
     
     NOTE—The 0-th OLS contains only the lowest layer (i.e., the layer with nuh_layer_id equal to vps_layer_id[0]) and for the 0-th OLS the only included layer is output. 
     vps_constraint_info_present_flag equal to 1 specifies that the general constraint info( ) syntax structure is present in the VPS. vps_constraint_info_present_flag equal to 0 specifies that the general constraint info( ) syntax structure is not present in the VPS. 
     vps_reserved_zero_7 bits shall be equal to 0 in bitstreams conforming to this version of this VVC draft. Other values for vps_reserved_zero_7 bits are reserved for future use by ITU-T|ISO/FEC. In an embodiment, decoders shall ignore the value of vps_reserved_zero_7 bits. 
     general_hrd_params_present flag equal to 1 specifies that the syntax elements num_units_in_tick and time_scale and the syntax structure general_hrd_parameters( ) are present in the SPS RB SP syntax structure. general_hrd_params_present flag equal to 0 specifies that the syntax elements num_units_in_tick and time_scale and the syntax structure general_hrd_parameters( ) are not present in the SPS RB SP syntax structure. 
     num_units_in_tick is the number of time units of a clock operating at the frequency time_scale Hz that corresponds to one increment (called a clock tick) of a clock tick counter. num_units_in_tick shall be greater than 0. A clock tick, in units of seconds, is equal to the quotient of num_units_in_tick divided by time_scale. For example, when the picture rate of a video signal is 25 Hz, time_scale may be equal to 27,000,000 and num_units_in_tick may be equal to 1,080,000, and consequently a clock tick may be equal to 0.04 seconds. 
     time_scale is the number of time units that pass in one second. For example, a time coordinate system that measures time using a 27 MHz clock has a time_scale of 27,000,000. The value of time_scale shall be greater than 0. 
     vps_extension_flag equal to 0 specifies that no vps_extension_data_flag syntax elements are present in the VPS RBSP syntax structure. vps_extension_flag equal to 1 specifies that there are vps_extension_data_flag syntax elements present in the VPS RBSP syntax structure. 
     vps_extension_data_flag may have any value. Its presence and value do not affect decoder conformance to profiles specified in this version of this Specification. Decoders conforming to this version of this Specification shall ignore all vps_extension_data_flag syntax elements. 
     The SPS  706  contains data that is common to all the pictures in a sequence of pictures (SOP). The SPS  706  is a syntax structure containing syntax elements that apply to zero or more entire CLVSs as determined by the content of a syntax element found in the PPS referred to by a syntax element found in each picture header. In contrast, the PPS  708  contains data that is common to the entire picture. The PPS  708  is a syntax structure containing syntax elements that apply to zero or more entire coded pictures as determined by a syntax element found in each picture header (e.g., PH  712 ). 
     The DCI  702 , the VPS  704 , the SPS  706 , and the PPS  708  are contained in different types of Network Abstraction Layer (NAL) units. A NAL unit is a syntax structure containing an indication of the type of data to follow (e.g., coded video data). NAL units are classified into video coding layer (VCL) and non-VCL NAL units. The VCL NAL units contain the data that represents the values of the samples in the video pictures, and the non-VCL NAL units contain any associated additional information such as parameter sets (important data that can apply to a number of VCL NAL units) and supplemental enhancement information (timing information and other supplemental data that may enhance usability of the decoded video signal but are not necessary for decoding the values of the samples in the video pictures). 
     In an embodiment, the DCI  702  is contained in a non-VCL NAL unit designated as a DCI NAL unit or a DPS NAL unit. That is, the DCI NAL unit has a DCI NAL unit type (NUT) and the DPS NAL unit has a DPS NUT. In an embodiment, the VPS  704  is contained in a non-VCL NAL unit designated as a VPS NAL unit. Therefore, the VPS NAL unit has a VPS NUT. In an embodiment, the SPS  706  is a non-VCL NAL unit designated as a SPS NAL unit. Therefore, the SPS NAL unit has an SPS NUT. In an embodiment, the PPS  708  is contained in a non-VCL NAL unit designated as a PPS NAL unit. Therefore, the PPS NAL unit has a PPS NUT. 
     The PH  712  is a syntax structure containing syntax elements that apply to all slices (e.g., slices  718 ) of a coded picture (e.g., picture  714 ). In an embodiment, the PH  712  is in a new type of non-VCL NAL unit designated a PH NAL unit. Therefore, the PH NAL unit has a PH NUT (e.g., PH NUT). In an embodiment, there is one and only one PH  712  included in each PU  701 . That is, the PU  701  contains a single or lone PH  712 . In an embodiment, exactly one PH NAL unit is present for each picture  701  in the bitstream  700 . 
     In an embodiment, the PH NAL unit associated with the PH  712  has a temporal ID and a layer ID. The temporal ID identifier indicates the position of the PH NAL unit, in time, relative to the other PH NAL units in the bitstream (e.g., bitstream  701 ). The layer ID indicates the layer (e.g., layer  531  or layer  532 ) that contains the PH NAL unit. In an embodiment, the temporal ID is similar to, but different from, the POC. The POC uniquely identifies each picture in order. In a single layer bitstream, temporal ID and POC would be the same. In a multi-layer bitstream (e.g., see  FIG. 5 ), pictures in the same AU would have different POCs, but the same temporal ID. 
     In an embodiment, the PH NAL unit precedes the VCL NAL unit containing the first slice  718  of the associated picture  714 . This establishes the association between the PH  712  and the slices  718  of the picture  714  associated with the PH  712  without the need to have a picture header ID signaled in the PH  712  and referred to from the slice header  720 . Consequently, it can be inferred that all VCL NAL units between two PHs  712  belong to the same picture  714  and that the picture  714  is associated with the first PH  712  between the two PHs  712 . In an embodiment, the first VCL NAL unit that follows a PH  712  contains the first slice  718  of the picture  714  associated with the PH  712 . 
     In an embodiment, the PH NAL unit follows picture level parameter sets (e.g., the PPS) or higher level parameter sets such as the DCI (a.k.a., the DPS), the VPS, the SPS, the PPS, etc. having both a temporal ID and a layer ID less than the temporal ID and layer ID of the PH NAL unit, respectively. Consequently, those parameter sets are not repeated within a picture or an access unit. Because of this ordering, the PH  712  can be resolved immediately. That is, parameter sets that contain parameters relevant to an entire picture are positioned in the bitstream before the PH NAL unit. Anything that contains parameters for part of a picture is positioned after the PH NAL unit. 
     In one alternative, the PH NAL unit follows picture level parameter sets and prefix supplemental enhancement information (SEI) messages, or higher level parameter sets such as the DCI (a.k.a., the DPS), the VPS, the SPS, the PPS, the APS, the SEI message, etc. 
     The picture  714  is an array of luma samples in monochrome format or an array of luma samples and two corresponding arrays of chroma samples in 4:2:0, 4:2:2, and 4:4:4 colour format. In an embodiment, there is one and only one picture  714  included in each PU  701 . As such, there is only one PH  712  and only one picture  714  corresponding to that PH  712  in each PU  701 . That is, the PU  701  contains a single or lone picture  714 . 
     The picture  714  may be either a frame or a field. However, in one CVS  716 , either all pictures  714  are frames or all pictures  714  are fields. The CVS  716  is a coded video sequence for every coded layer video sequence (CLVS) in the video bitstream  700 . Notably, the CVS  716  and the CLVS are the same when the video bitstream  700  includes a single layer. The CVS  716  and the CLVS are only different when the video bitstream  700  includes multiple layers (e.g., as shown in  FIGS. 5 and 6 ). 
     Each picture  714  contains one or more slices  718 . A slice  718  is an integer number of complete tiles or an integer number of consecutive complete CTU rows within a tile of a picture (e.g., picture  714 ). Each slice  718  is exclusively contained in a single NAL unit (e.g., a VCL NAL unit). A tile (not shown) is a rectangular region of CTUs within a particular tile column and a particular tile row in a picture (e.g., picture  714 ). A CTU (not shown) is a CTB of luma samples, two corresponding CTBs of chroma samples of a picture that has three sample arrays, or a CTB of samples of a monochrome picture or a picture that is coded using three separate color planes and syntax structures used to code the samples. A CTB (not shown) is an N×N block of samples for some value of N such that the division of a component into CTBs is a partitioning. A block (not shown) is an M×N (M-column by N-row) array of samples (e.g., pixels), or an M×N array of transform coefficients. 
     In an embodiment, each slice  718  contains a slice header  720 . A slice header  720  is the part of the coded slice  718  containing the data elements pertaining to all tiles or CTU rows within a tile represented in the slice  718 . That is, the slice header  720  contains information about the slice  718  such as, for example, the slice type, which of the reference pictures will be used, and so on. 
     The pictures  714  and their slices  718  comprise data associated with the images or video being encoded or decoded. Thus, the pictures  714  and their slices  718  may be simply referred to as the payload or data being carried in the bitstream  700 . 
     Those skilled in the art will appreciate that the bitstream  700  may contain other parameters and information in practical applications. 
       FIG. 8  is an embodiment of a method  800  of decoding implemented by a video decoder (e.g., video decoder  400 ). The method  800  may be performed after a bitstream has been directly or indirectly received from a video encoder (e.g., video encoder  300 ). The method  800  improves the decoding process by disallowing unneeded layers in multi-layer video bitstreams. That is, any layer not used in at least one output layer set (OLS) is constrained from being included in multi-layer bitstreams. This avoids having extraneous information in the coding process and increases coding efficiency. Thus, the coder/decoder (a.k.a., “codec”) in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed. 
     In block  802 , the video decoder receives a video bitstream including a VPS (e.g., VPS  704 ) and a plurality of layers (e.g., layer N  631 , layer N+1  632 , etc.). In an embodiment, each layer is included in at least one OLS (e.g., OLS 1, OLS 2, etc.) specified by the VPS. That is, for each layer with a particular value of nuh_layer_id nuhLayerId equal to one of vps_layer_id[k] for k in the range of 0 to vps_max_layers_minus1, inclusive, there shall be at least one pair of values of i and j, where i is in the range of 0 to TotalNumOlss−1, inclusive, and j is in the range of NumLayersInOls[i]−1, inclusive, such that the value of LayerIdInOls[i][j] is equal to nuhLayerId. 
     In an embodiment, the video decoder expects each layer to be included in at least one OLS (e.g., OLS 1, OLS 2, etc.) specified by the VPS as described above based on VVC or some other standard. If, however, the decoder determines that this condition is not true, the decoder may detect an error, signal an error, request that a revised bitstream (or a portion thereof) be resent, or take some other corrective measures to ensure that a conforming bitstream is received. 
     The nuh_layer_id is signaled in the bitstream and used to determine the nuhLayerId. Each of the nuh_layer_id and nuhLayerId comprise an identifier (ID) used to uniquely identify a layer. The vps_layer_id[k] is used to specify the layer IDs in the VPS  704 . The vps_max_layers_minus1 is used to specify a maximum number of layers in an OLS. The TotalNumOlss is used to specify the total number of OLSs included in the CLVS and/or bitstream. Finally, NumLayersInOls specifies the number of layers in an OLS. 
     In an embodiment, each and every layer available to the encoder is specified in the VPS received by the decoder. That is, the VPS received by the decoder specifies every layer, regardless of whether every layer is included in the OLS received by the decoder. In an embodiment, each and every OLSs generated by the encoder is specified in the VPS received by the decoder. That is, the VPS received by the decoder specifies every OLS, regardless of whether that OLS is received by the decoder. 
     In an embodiment, each layer in the plurality of layers comprises a set of video coding layer (VCL) network abstraction layer (NAL) units that all have a particular value of a layer identifier (ID) and associated non-VCL NAL units. In an embodiment, the at least one OLS comprises two output layers, and wherein one of the two output layers references another of the two output layers. In an embodiment, the at least one OLS contains one or more output layers. In an embodiment, for each of the plurality of layers with a particular value of layer ID specified in the VPS, one of the layers in the at least one OLS shall also have the particular value of layer ID. 
     In block  804 , the video decoder decodes a picture (e.g., picture  615 ) from one of the plurality of layers. In an embodiment, the picture is included in an output layer of the at least one OLS. 
     In an embodiment, the method  800  further comprises selecting an output layer from the at least one OLS prior to the decoding. In an embodiment, the method  800  further includes selecting the picture from the output layer after the output layer has been selected. 
     Once the picture has been decoded, the picture may be used to generate or produce an image or video sequence for display to a user on the display or screen of an electronic device (e.g., a smart phone, tablet, laptop, personal computer, etc.). 
       FIG. 9  is an embodiment of a method  900  of encoding a video bitstream implemented by a video encoder (e.g., video encoder  300 ). The method  900  may be performed when a picture (e.g., from a video) is to be encoded into a video bitstream and then transmitted toward a video decoder (e.g., video decoder  400 ). The method  900  improves the encoding process by disallowing unneeded layers in multi-layer video bitstreams. That is, any layer not used in at least one output layer set (OLS) is constrained from being included in multi-layer bitstreams. This avoids having extraneous information in the coding process and increases coding efficiency. Thus, the coder/decoder (a.k.a., “codec”) in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed. 
     In block  902 , the video encoder generates a plurality of layers (e.g., layer N  631 , layer N+1  632 , etc.) and a VPS (e.g., VPS  704 ) specifying one or more OLSs (e.g., OLS 1, OLS 2, etc.). In an embodiment, each layer from the plurality of layers is included in at least one of the OLSs specified by the VPS. That is, for each layer with a particular value of nuh_layer_id nuhLayerId equal to one of vps_layer_id[k] for k in the range of 0 to vps_max_layers_minus1, inclusive, there shall be at least one pair of values of i and j, where i is in the range of 0 to TotalNumOlss−1, inclusive, and j is in the range of NumLayersInOls[i]−1, inclusive, such that the value of LayerIdInOls[i][j] is equal to nuhLayerId. In an embodiment, the video encoder is constrained so that each layer from the plurality of layers is included in at least one of the OLSs specified by the VPS. That is, the video encoder is required to have each layer from the plurality of layers included in at least one of the OLSs specified by the VPS. Such a constraint or requirement ensures that the bitstream conforms with, for example, VVC or some other standard, modified as indicated herein. However, the encoder may also be capable of operating in other modes where it is not so constrained, such as when operating under a different standard or a different version of the same standard. 
     The nuh_layer_id is signaled in the bitstream and used to determine the nuhLayerId. Each of the nuh_layer_id and nuhLayerId comprise an identifier (ID) used to uniquely identify a layer. The vps_layer_id[k] is used to specify the layer IDs in the VPS  704 . The vps_max_layers_minus1 is used to specify a maximum number of layers in an OLS. The TotalNumOlss is used to specify the total number of OLSs included in the CLVS and/or bitstream. Finally, NumLayersInOls specifies the number of layers in an OLS. 
     In an embodiment, each of the one or more OLSs contains one or more output layers, and wherein each of the output layers comprises one or more pictures. In an embodiment, each layer in the plurality of layers comprises a set of video coding layer (VCL) network abstraction layer (NAL) units that all have a particular value of a layer identifier (ID) and associated non-VCL NAL units. 
     In an embodiment, one of the OLSs comprises two output layers, and wherein one of the two output layers references another of the two output layers. In an embodiment, for each of the plurality of layers with a particular value of layer ID specified in the VPS, one of the layers in the one or more OLSs shall also have the particular value of layer ID. 
     In an embodiment, a hypothetical reference decoder (HRD) disposed in the encoder will check all of the layers and the OLSs. When the HRD finds a layer that is not in any OLS as described herein, then the HRD will return a conformance test error. That is, the HRD conformance test ensures that there are no unused layers. Hence, the encoder encodes according to the no unused layer requirement, but the HRD enforces this requirement. 
     In block  904 , the video encoder encodes the plurality of layers and the VPS into a video bitstream. In block  906 , the video encoder stores the video bitstream for communication toward a video decoder. The video bitstream may be stored in memory until the video bitstream is transmitted toward the video decoder. Once received by the video decoder, the encoded video bitstream may be decoded (e.g., as described above) to generate or produce an image or video sequence for display to a user on the display or screen of an electronic device (e.g., a smart phone, tablet, laptop, personal computer, etc.). 
       FIG. 10  is a schematic diagram of a video coding device  1000  (e.g., a video encoder  300  or a video decoder  400 ) according to an embodiment of the disclosure. The video coding device  1000  is suitable for implementing the disclosed embodiments as described herein. The video coding device  1000  comprises ingress ports  1010  and receiver units (Rx)  1020  for receiving data; a processor, logic unit, or central processing unit (CPU)  1030  to process the data; transmitter units (Tx)  1040  and egress ports  1050  for transmitting the data; and a memory  1060  for storing the data. The video coding device  1000  may also comprise optical-to-electrical (OE) components and electrical-to-optical (EO) components coupled to the ingress ports  1010 , the receiver units  1020 , the transmitter units  1040 , and the egress ports  1050  for egress or ingress of optical or electrical signals. 
     The processor  1030  is implemented by hardware and software. The processor  1030  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  1030  is in communication with the ingress ports  1010 , receiver units  1020 , transmitter units  1040 , egress ports  1050 , and memory  1060 . The processor  1030  comprises a coding module  1070 . The coding module  1070  implements the disclosed embodiments described above. For instance, the coding module  1070  implements, processes, prepares, or provides the various codec functions. The inclusion of the coding module  1070  therefore provides a substantial improvement to the functionality of the video coding device  1000  and effects a transformation of the video coding device  1000  to a different state. Alternatively, the coding module  1070  is implemented as instructions stored in the memory  1060  and executed by the processor  1030 . 
     The video coding device  1000  may also include input and/or output (I/O) devices  1080  for communicating data to and from a user. The I/O devices  1080  may include output devices such as a display for displaying video data, speakers for outputting audio data, etc. The I/O devices  1080  may also include input devices, such as a keyboard, mouse, trackball, etc., and/or corresponding interfaces for interacting with such output devices. 
     The memory  1060  comprises one or more disks, tape drives, and solid-state drives and may be used as an over-flow data storage device, to store programs when such programs are selected for execution, and to store instructions and data that are read during program execution. The memory  1060  may be volatile and/or non-volatile and may be read-only memory (ROM), random access memory (RAM), ternary content-addressable memory (TCAM), and/or static random-access memory (SRAM). 
       FIG. 11  is a schematic diagram of an embodiment of a means for coding  1100 . In an embodiment, the means for coding  1100  is implemented in a video coding device  1102  (e.g., a video encoder  300  or a video decoder  400 ). The video coding device  1102  includes receiving means  1101 . The receiving means  1101  is configured to receive a picture to encode or to receive a bitstream to decode. The video coding device  1102  includes transmission means  1107  coupled to the receiving means  1101 . The transmission means  1107  is configured to transmit the bitstream to a decoder or to transmit a decoded image to a display means (e.g., one of the I/O devices  1080 ). 
     The video coding device  1102  includes a storage means  1103 . The storage means  1103  is coupled to at least one of the receiving means  1101  or the transmission means  1107 . The storage means  1103  is configured to store instructions. The video coding device  1102  also includes processing means  1105 . The processing means  1105  is coupled to the storage means  1103 . The processing means  1105  is configured to execute the instructions stored in the storage means  1103  to perform the methods disclosed herein. 
     It should also be understood that the steps of the exemplary methods set forth herein are not necessarily required to be performed in the order described, and the order of the steps of such methods should be understood to be merely exemplary. Likewise, additional steps may be included in such methods, and certain steps may be omitted or combined, in methods consistent with various embodiments of the present disclosure. 
     While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods might be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented. 
     In addition, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled or directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein.