Patent Publication Number: US-2023141607-A1

Title: Image encoding/decoding method and device for signaling hrd parameters, and computer-readable recording medium in which bitstream is stored

Description:
TECHNICAL FIELD 
     The present disclosure relates to an image encoding/decoding method and apparatus and, more particularly, to an image encoding/decoding method and apparatus for signaling a hypothetical reference decoder (HRD) related parameter and a computer-readable recording medium storing a bitstream generated by the image encoding method/apparatus of the present disclosure. 
     BACKGROUND ART 
     Recently, demand for high-resolution and high-quality images such as high definition (HD) images and ultra high definition (UHD) images is increasing in various fields. As resolution and quality of image data are improved, the amount of transmitted information or bits relatively increases as compared to existing image data. An increase in the amount of transmitted information or bits causes an increase in transmission cost and storage cost. 
     Accordingly, there is a need for high-efficient image compression technology for effectively transmitting, storing and reproducing information on high-resolution and high-quality images. 
     DISCLOSURE 
     Technical Problem 
     An object of the present disclosure is to provide an image encoding/decoding method and apparatus with improved encoding/decoding efficiency. 
     Another object of the present disclosure is to provide an image encoding/decoding method and apparatus for improving encoding/decoding efficiency by efficiently signaling a HRD parameter. 
     Another object of the present disclosure is to provide a method of transmitting a bitstream generated by an image encoding method or apparatus according to the present disclosure. 
     Another object of the present disclosure is to provide a recording medium storing a bitstream generated by an image encoding method or apparatus according to the present disclosure. 
     Another object of the present disclosure is to provide a recording medium storing a bitstream received, decoded and used to reconstruct an image by an image decoding apparatus according to the present disclosure. 
     The technical problems solved by the present disclosure are not limited to the above technical problems and other technical problems which are not described herein will become apparent to those skilled in the art from the following description. 
     Technical Solution 
     An image decoding method performed by an image decoding apparatus according to an aspect of the present disclosure may comprise obtaining first information specifying the number of one or more hypothetical reference decoder (HRD) parameter syntax structures in a video parameter set (VPS), obtaining the one or more HRD parameter syntax structures from the VPS based on the first information, obtaining second information on mapping between one or more multi-layer output layer sets (OLSs) and the one or more HRD parameter syntax structures from the VPS based on the first information, selecting a HRD parameter syntax structure that applies to a current OLS based on the second information, and processing the current OLS based on the selected HRD parameter syntax structure. 
     In the image decoding method of the present disclosure, the number of the one or more HRD parameter syntax structures in the VPS may not be greater than the number of the one or more multi-layer OLSs. 
     In the image decoding method of the present disclosure, each of the one or more HRD parameter syntax structures in the VPS may be mapped to at least one multi-layer OLS among the one or more multi-layer OLSs. 
     In the image decoding method of the present disclosure, based on that the number of the one or more HRD parameter syntax structures in the VPS is greater than 1 and the number of the one or more HRD parameter syntax structures in the VPS is not equal to the number of the one or more multi-layer OLSs, the second information may be obtained from the VPS. 
     In the image decoding method of the present disclosure, based on that the number of the one or more HRD parameter syntax structures in the VPS is 1, the second information may not be obtained from the VPS, and the second information may be inferred to be equal to a value of 0. 
     In the image decoding method of the present disclosure, based on that the number of the one or more HRD parameter syntax structures in the VPS is greater than 1 and the number of the one or more HRD parameter syntax structures in the VPS is equal to the number of the one or more multi-layer OLSs, the second information may not be obtained from the VPS, and the second information of an i-th multi-layer OLS may be inferred to be equal to a value of i. 
     In the image decoding method of the present disclosure, based on that the current OLS contains only a single layer, the HRD parameter syntax structure that applies to the current OLS may be obtained from a sequence parameter set (SPS). 
     An image decoding apparatus according to another aspect of the present disclosure may comprise a memory and at least one processor. The at least one processor may be configured to obtain first information specifying the number of one or more hypothetical reference decoder (HRD) parameter syntax structures in a video parameter set (VPS), to obtain the one or more HRD parameter syntax structures from the VPS based on the first information, to obtain second information on mapping between one or more multi-layer output layer sets (OLSs) and the one or more HRD parameter syntax structures from the VPS based on the first information, to select a HRD parameter syntax structure that applies to a current OLS based on the second information, and to process the current OLS based on the selected HRD parameter syntax structure. 
     An image encoding method performed by an image encoding apparatus according to another aspect of the present disclosure may comprise encoding first information specifying the number of one or more hypothetical reference decoder (HRD) parameter syntax structures in a video parameter set (VPS), encoding the one or more HRD parameter syntax structures in the VPS based on the first information, encoding second information on mapping between one or more multi-layer output layer sets (OLSs) and the one or more HRD parameter syntax structures in the VPS based on the first information, and, based on a HRD parameter syntax structure that applies to a current OLS, processing the current OLS. 
     In the image encoding method of the present disclosure, the number of the one or more HRD parameter syntax structures in the VPS may not be greater than the number of the one or more multi-layer OLSs. 
     In the image encoding method of the present disclosure, each of the one or more HRD parameter syntax structures in the VPS may be mapped to at least one multi-layer OLS among the one or more multi-layer OLSs. 
     In the image encoding method of the present disclosure, based on that the number of the one or more HRD parameter syntax structures in the VPS is greater than 1 and the number of the one or more HRD parameter syntax structures in the VPS is not equal to the number of the one or more multi-layer OLSs, the second information may be encoded into the VPS. 
     In the image encoding method of the present disclosure, based on that the number of the one or more HRD parameter syntax structures in the VPS is 1, the second information may not be encoded into the VPS, and the second information may be inferred to be equal to a value of 0. 
     In the image encoding method of the present disclosure, based on that the number of the one or more HRD parameter syntax structures in the VPS is greater than 1 and the number of the one or more HRD parameter syntax structures in the VPS is equal to the number of the one or more multi-layer OLSs, the second information may not be encoded into the VPS, and the second information of an i-th multi-layer OLS may be inferred to be equal to a value of i. 
     In the image encoding method of the present disclosure, based on that the current OLS contains only a single layer, the HRD parameter syntax structure that applies to the current OLS may be encoded into a sequence parameter set (SPS). 
     A transmission method according to another aspect of the present disclosure may transmit the bitstream generated by the image encoding apparatus or the image encoding method of the present disclosure. 
     A computer-readable recording medium according to another aspect of the present disclosure may store the bitstream generated by the image encoding apparatus or the image encoding method of the present disclosure. 
     The features briefly summarized above with respect to the present disclosure are merely exemplary aspects of the detailed description below of the present disclosure, and do not limit the scope of the present disclosure. 
     Advantageous Effects 
     According to the present disclosure, it is possible to provide an image encoding/decoding method and apparatus with improved encoding/decoding efficiency. 
     Also, according to the present disclosure, it is possible to provide an image encoding/decoding method and apparatus for improving encoding/decoding efficiency by efficiently signaling HRD parameters. 
     Also, according to the present disclosure, it is possible to provide a method of transmitting a bitstream generated by an image encoding method or apparatus according to the present disclosure. 
     Also, according to the present disclosure, it is possible to provide a recording medium storing a bitstream generated by an image encoding method or apparatus according to the present disclosure. 
     Also, according to the present disclosure, it is possible to provide a recording medium storing a bitstream received, decoded and used to reconstruct an image by an image decoding apparatus according to the present disclosure. 
     It will be appreciated by persons skilled in the art that that the effects that can be achieved through the present disclosure are not limited to what has been particularly described hereinabove and other advantages of the present disclosure will be more clearly understood from the detailed description. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG.  1    is a view schematically showing a video coding system, to which an embodiment of the present disclosure is applicable. 
         FIG.  2    is a view schematically showing an image encoding apparatus, to which an embodiment of the present disclosure is applicable. 
         FIG.  3    is a view schematically showing an image decoding apparatus, to which an embodiment of the present disclosure is applicable. 
         FIG.  4    is a view showing an example of a schematic picture decoding procedure, to which embodiment(s) of the present disclosure is applicable. 
         FIG.  5    is a view showing an example of a schematic picture encoding procedure, to which embodiment(s) of the present disclosure is applicable. 
         FIG.  6    is a view showing an example of a layer structure for a coded image/video. 
         FIG.  7    is a view illustrating a syntax structure of a VPS according to an embodiment of the present disclosure. 
         FIG.  8    is a view illustrating the syntax structure of an SPS for signaling HRD parameters according to an embodiment of the present disclosure. 
         FIG.  9    is a view illustrating a general_hrd_parameters( ) syntax structure according to an embodiment of the present disclosure. 
         FIG.  10    is a view illustrating an ols_hrd_parameters( ) syntax structure according to an embodiment of the present disclosure. 
         FIG.  11    is a view illustrating a sublayer_hrd_parameters( ) syntax structure according to an embodiment of the present disclosure. 
         FIG.  12    is a view illustrating an example of an image encoding method, to which an embodiment of the present disclosure is applicable. 
         FIG.  13    is a view illustrating an example of an image decoding method, to which an embodiment of the present disclosure is applicable. 
         FIG.  14    is a view illustrating another example of an image decoding method, to which an embodiment of the present disclosure is applicable. 
         FIG.  15    is a view illustrating a process of encoding HRD parameters based on num_ols_hrd_params_minus1 according to another embodiment of the present disclosure. 
         FIG.  16    is a view illustrating a process of decoding HRD parameters based on num_ols_hrd_params_minus1 according to another embodiment of the present disclosure. 
         FIG.  17    is a view showing a content streaming system, to which an embodiment of the present disclosure is applicable. 
     
    
    
     MODE FOR INVENTION 
     Hereinafter, the embodiments of the present disclosure will be described in detail with reference to the accompanying drawings so as to be easily implemented by those skilled in the art. However, the present disclosure may be implemented in various different forms, and is not limited to the embodiments described herein. 
     In describing the present disclosure, if it is determined that the detailed description of a related known function or construction renders the scope of the present disclosure unnecessarily ambiguous, the detailed description thereof will be omitted. In the drawings, parts not related to the description of the present disclosure are omitted, and similar reference numerals are attached to similar parts. 
     In the present disclosure, when a component is “connected”, “coupled” or “linked” to another component, it may include not only a direct connection relationship but also an indirect connection relationship in which an intervening component is present. In addition, when a component “includes” or “has” other components, it means that other components may be further included, rather than excluding other components unless otherwise stated. 
     In the present disclosure, the terms first, second, etc. may be used only for the purpose of distinguishing one component from other components, and do not limit the order or importance of the components unless otherwise stated. Accordingly, within the scope of the present disclosure, a first component in one embodiment may be referred to as a second component in another embodiment, and similarly, a second component in one embodiment may be referred to as a first component in another embodiment. 
     In the present disclosure, components that are distinguished from each other are intended to clearly describe each feature, and do not mean that the components are necessarily separated. That is, a plurality of components may be integrated and implemented in one hardware or software unit, or one component may be distributed and implemented in a plurality of hardware or software units. Therefore, even if not stated otherwise, such embodiments in which the components are integrated or the component is distributed are also included in the scope of the present disclosure. 
     In the present disclosure, the components described in various embodiments do not necessarily mean essential components, and some components may be optional components. Accordingly, an embodiment consisting of a subset of components described in an embodiment is also included in the scope of the present disclosure. In addition, embodiments including other components in addition to components described in the various embodiments are included in the scope of the present disclosure. 
     The present disclosure relates to encoding and decoding of an image, and terms used in the present disclosure may have a general meaning commonly used in the technical field, to which the present disclosure belongs, unless newly defined in the present disclosure. 
     In the present disclosure, a “picture” generally refers to a unit representing one image in a specific time period, and a slice/tile is a coding unit constituting a part of a picture, and one picture may be composed of one or more slices/tiles. In addition, a slice/tile may include one or more coding tree units (CTUs). 
     In the present disclosure, a “pixel” or a “pel” may mean a smallest unit constituting one picture (or image). In addition, “sample” may be used as a term corresponding to a pixel. A sample may generally represent a pixel or a value of a pixel, and may represent only a pixel/pixel value of a luma component or only a pixel/pixel value of a chroma component. 
     In the present disclosure, a “unit” may represent a basic unit of image processing. The unit may include at least one of a specific region of the picture and information related to the region. The unit may be used interchangeably with terms such as “sample array”, “block” or “area” in some cases. In a general case, an M×N block may include samples (or sample arrays) or a set (or array) of transform coefficients of M columns and N rows. 
     In the present disclosure, “current block” may mean one of “current coding block”, “current coding unit”, “coding target block”, “decoding target block” or “processing target block”. When prediction is performed, “current block” may mean “current prediction block” or “prediction target block”. When transform (inverse transform)/quantization (dequantization) is performed, “current block” may mean “current transform block” or “transform target block”. When filtering is performed, “current block” may mean “filtering target block”. 
     In addition, in the present disclosure, a “current block” may mean a block including both a luma component block and a chroma component block or “a luma block of a current block” unless explicitly stated as a chroma block. The “luma block of the current block” may be expressed by including an explicit description of a luma component block, such as “luma block” or “current luma block”. The “chroma block of the current block” may be expressed by including an explicit description of a chroma component block, such as “chroma block” or “current chroma block”. 
     In the present disclosure, “A or B” may mean “only A”, “only B” or “both A and B”. In other words, in the present disclosure, “A or B” may be interpreted as “A and/or B”. For example, in the present disclosure, “A, B or C” may mean “only A, “only B”, “only C” or “any combination of A, B and C”. 
     A slash (/) or comma used in the present disclosure may mean “and/or”. For example, “A/B” may mean “A and/or B”. Therefore, “A/B” may mean “only A”, “only B” or “both A and B”. For example, “A, B, C” may mean “A, B or C”. 
     In the present disclosure, “at least one of A and B” may mean “only A”, “only B” or “both A and B”. In addition, in the disclosure, “at least one of A or B” or “at least one of A and/or B” may be interpreted as being the same as “at least one of A and B”. 
     In addition, in the present disclosure, “at least one of A, B and C” may mean “only A”, “only B”, “only C” or “any combination of A, B and C”. In addition, in the disclosure, “at least one of A, B or C” or “at least one of A, B and/or C” may be interpreted as being the same as “at least one of A, B and C”. 
     In addition, parentheses used in the present disclosure may mean “for example”. Specifically, when “prediction (intra prediction” is described, “intra prediction” may be proposed as an example of “prediction”. In other words, “prediction” of the present disclosure is not limited to “intra prediction” and “intra prediction” may be proposed as an example of “prediction”. In addition, even when “prediction (that is, intra prediction)” is described, “intra prediction” may be proposed as an example of “prediction”. 
     In the present disclosure, technical features individually described in one drawing may be implemented individually or simultaneously. 
     Overview of Video Coding System 
       FIG.  1    is a view showing a video coding system according to the present disclosure. 
     The video coding system according to an embodiment may include a encoding apparatus  10  and a decoding apparatus  20 . The encoding apparatus  10  may deliver encoded video and/or image information or data to the decoding apparatus  20  in the form of a file or streaming via a digital storage medium or network. 
     The encoding apparatus  10  according to an embodiment may include a video source generator  11 , an encoding unit  12  and a transmitter  13 . The decoding apparatus  20  according to an embodiment may include a receiver  21 , a decoding unit  22  and a renderer  23 . The encoding unit  12  may be called a video/image encoding unit, and the decoding unit  22  may be called a video/image decoding unit. The transmitter  13  may be included in the encoding unit  12 . The receiver  21  may be included in the decoding unit  22 . The renderer  23  may include a display and the display may be configured as a separate device or an external component. 
     The video source generator  11  may acquire a video/image through a process of capturing, synthesizing or generating the video/image. The video source generator  11  may include a video/image capture device and/or a video/image generating device. The video/image capture device may include, for example, one or more cameras, video/image archives including previously captured video/images, and the like. The video/image generating device may include, for example, computers, tablets and smartphones, and may (electronically) generate video/images. For example, a virtual video/image may be generated through a computer or the like. In this case, the video/image capturing process may be replaced by a process of generating related data. 
     The encoding unit  12  may encode an input video/image. The encoding unit  12  may perform a series of procedures such as prediction, transform, and quantization for compression and coding efficiency. The encoding unit  12  may output encoded data (encoded video/image information) in the form of a bitstream. 
     The transmitter  13  may transmit the encoded video/image information or data output in the form of a bitstream to the receiver  21  of the decoding apparatus  20  through a digital storage medium or a network in the form of a file or streaming. The digital storage medium may include various storage mediums such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, and the like. The transmitter  13  may include an element for generating a media file through a predetermined file format and may include an element for transmission through a broadcast/communication network. The receiver  21  may extract/receive the bitstream from the storage medium or network and transmit the bitstream to the decoding unit  22 . 
     The decoding unit  22  may decode the video/image by performing a series of procedures such as dequantization, inverse transform, and prediction corresponding to the operation of the encoding unit  12 . 
     The renderer  23  may render the decoded video/image. The rendered video/image may be displayed through the display. 
     Overview of Image Encoding Apparatus 
       FIG.  2    is a view schematically showing an image encoding apparatus, to which an embodiment of the present disclosure is applicable. 
     As shown in  FIG.  2   , the image encoding apparatus  100  may include an image partitioner  110 , a subtractor  115 , a transformer  120 , a quantizer  130 , a dequantizer  140 , an inverse transformer  150 , an adder  155 , a filter  160 , a memory  170 , an inter predictor  180 , an intra predictor  185  and an entropy encoder  190 . The inter predictor  180  and the intra predictor  185  may be collectively referred to as a “predictor”. The transformer  120 , the quantizer  130 , the dequantizer  140  and the inverse transformer  150  may be included in a residual processor. The residual processor may further include the subtractor  115 . 
     All or at least some of the plurality of components configuring the image encoding apparatus  100  may be configured by one hardware component (e.g., an encoder or a processor) in some embodiments. In addition, the memory  170  may include a decoded picture buffer (DPB) and may be configured by a digital storage medium. 
     The image partitioner  110  may partition an input image (or a picture or a frame) input to the image encoding apparatus  100  into one or more processing units. For example, the processing unit may be called a coding unit (CU). The coding unit may be acquired by recursively partitioning a coding tree unit (CTU) or a largest coding unit (LCU) according to a quad-tree binary-tree ternary-tree (QT/BT/TT) structure. For example, one coding unit may be partitioned into a plurality of coding units of a deeper depth based on a quad tree structure, a binary tree structure, and/or a ternary structure. For partitioning of the coding unit, a quad tree structure may be applied first and the binary tree structure and/or ternary structure may be applied later. The coding procedure according to the present disclosure may be performed based on the final coding unit that is no longer partitioned. The largest coding unit may be used as the final coding unit or the coding unit of deeper depth acquired by partitioning the largest coding unit may be used as the final coding unit. Here, the coding procedure may include a procedure of prediction, transform, and reconstruction, which will be described later. As another example, the processing unit of the coding procedure may be a prediction unit (PU) or a transform unit (TU). The prediction unit and the transform unit may be split or partitioned from the final coding unit. The prediction unit may be a unit of sample prediction, and the transform unit may be a unit for deriving a transform coefficient and/or a unit for deriving a residual signal from the transform coefficient. 
     The predictor (the inter predictor  180  or the intra predictor  185 ) may perform prediction on a block to be processed (current block) and generate a predicted block including prediction samples for the current block. The predictor may determine whether intra prediction or inter prediction is applied on a current block or CU basis. The predictor may generate various information related to prediction of the current block and transmit the generated information to the entropy encoder  190 . The information on the prediction may be encoded in the entropy encoder  190  and output in the form of a bitstream. 
     The intra predictor  185  may predict the current block by referring to the samples in the current picture. The referred samples may be located in the neighborhood of the current block or may be located apart according to the intra prediction mode and/or the intra prediction technique. The intra prediction modes may include a plurality of non-directional modes and a plurality of directional modes. The non-directional mode may include, for example, a DC mode and a planar mode. The directional mode may include, for example, 33 directional prediction modes or 65 directional prediction modes according to the degree of detail of the prediction direction. However, this is merely an example, more or less directional prediction modes may be used depending on a setting. The intra predictor  185  may determine the prediction mode applied to the current block by using a prediction mode applied to a neighboring block. 
     The inter predictor  180  may derive a predicted block for the current block based on a reference block (reference sample array) specified by a motion vector on a reference picture. In this case, in order to reduce the amount of motion information transmitted in the inter prediction mode, the motion information may be predicted in units of blocks, subblocks, or samples based on correlation of motion information between the neighboring block and the current block. The motion information may include a motion vector and a reference picture index. The motion information may further include inter prediction direction (L0 prediction, L1 prediction, Bi prediction, etc.) information. In the case of inter prediction, the neighboring block may include a spatial neighboring block present in the current picture and a temporal neighboring block present in the reference picture. The reference picture including the reference block and the reference picture including the temporal neighboring block may be the same or different. The temporal neighboring block may be called a collocated reference block, a co-located CU (colCU), and the like. The reference picture including the temporal neighboring block may be called a collocated picture (colPic). For example, the inter predictor  180  may configure a motion information candidate list based on neighboring blocks and generate information indicating which candidate is used to derive a motion vector and/or a reference picture index of the current block. Inter prediction may be performed based on various prediction modes. For example, in the case of a skip mode and a merge mode, the inter predictor  180  may use motion information of the neighboring block as motion information of the current block. In the case of the skip mode, unlike the merge mode, the residual signal may not be transmitted. In the case of the motion vector prediction (MVP) mode, the motion vector of the neighboring block may be used as a motion vector predictor, and the motion vector of the current block may be signaled by encoding a motion vector difference and an indicator for a motion vector predictor. The motion vector difference may mean a difference between the motion vector of the current block and the motion vector predictor. 
     The predictor may generate a prediction signal based on various prediction methods and prediction techniques described below. For example, the predictor may not only apply intra prediction or inter prediction but also simultaneously apply both intra prediction and inter prediction, in order to predict the current block. A prediction method of simultaneously applying both intra prediction and inter prediction for prediction of the current block may be called combined inter and intra prediction (CIIP). In addition, the predictor may perform intra block copy (IBC) for prediction of the current block. Intra block copy may be used for content image/video coding of a game or the like, for example, screen content coding (SCC). IBC is a method of predicting a current picture using a previously reconstructed reference block in the current picture at a location apart from the current block by a predetermined distance. When IBC is applied, the location of the reference block in the current picture may be encoded as a vector (block vector) corresponding to the predetermined distance. IBC basically performs prediction in the current picture, but may be performed similarly to inter prediction in that a reference block is derived within the current picture. That is, IBC may use at least one of the inter prediction techniques described in the present disclosure. 
     The prediction signal generated by the predictor may be used to generate a reconstructed signal or to generate a residual signal. The subtractor  115  may generate a residual signal (residual block or residual sample array) by subtracting the prediction signal (predicted block or prediction sample array) output from the predictor from the input image signal (original block or original sample array). The generated residual signal may be transmitted to the transformer  120 . 
     The transformer  120  may generate transform coefficients by applying a transform technique to the residual signal. For example, the transform technique may include at least one of a discrete cosine transform (DCT), a discrete sine transform (DST), a karhunen-loève transform (KLT), a graph-based transform (GBT), or a conditionally non-linear transform (CNT). Here, the GBT means transform obtained from a graph when relationship information between pixels is represented by the graph. The CNT refers to transform acquired based on a prediction signal generated using all previously reconstructed pixels. In addition, the transform process may be applied to square pixel blocks having the same size or may be applied to blocks having a variable size rather than square. 
     The quantizer  130  may quantize the transform coefficients and transmit them to the entropy encoder  190 . The entropy encoder  190  may encode the quantized signal (information on the quantized transform coefficients) and output a bitstream. The information on the quantized transform coefficients may be referred to as residual information. The quantizer  130  may rearrange quantized transform coefficients in a block type into a one-dimensional vector form based on a coefficient scanning order and generate information on the quantized transform coefficients based on the quantized transform coefficients in the one-dimensional vector form. 
     The entropy encoder  190  may perform various encoding methods such as, for example, exponential Golomb, context-adaptive variable length coding (CAVLC), context-adaptive binary arithmetic coding (CABAC), and the like. The entropy encoder  190  may encode information necessary for video/image reconstruction other than quantized transform coefficients (e.g., values of syntax elements, etc.) together or separately. Encoded information (e.g., encoded video/image information) may be transmitted or stored in units of network abstraction layers (NALs) in the form of a bitstream. The video/image information may further include information on various parameter sets such as an adaptation parameter set (APS), a picture parameter set (PPS), a sequence parameter set (SPS), or a video parameter set (VPS). In addition, the video/image information may further include general constraint information. The signaled information, transmitted information and/or syntax elements described in the present disclosure may be encoded through the above-described encoding procedure and included in the bitstream. 
     The bitstream may be transmitted over a network or may be stored in a digital storage medium. The network may include a broadcasting network and/or a communication network, and the digital storage medium may include various storage media such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, and the like. A transmitter (not shown) transmitting a signal output from the entropy encoder  190  and/or a storage unit (not shown) storing the signal may be included as internal/external element of the image encoding apparatus  100 . Alternatively, the transmitter may be provided as the component of the entropy encoder  190 . 
     The quantized transform coefficients output from the quantizer  130  may be used to generate a residual signal. For example, the residual signal (residual block or residual samples) may be reconstructed by applying dequantization and inverse transform to the quantized transform coefficients through the dequantizer  140  and the inverse transformer  150 . 
     The adder  155  adds the reconstructed residual signal to the prediction signal output from the inter predictor  180  or the intra predictor  185  to generate a reconstructed signal (reconstructed picture, reconstructed block, reconstructed sample array). If there is no residual for the block to be processed, such as a case where the skip mode is applied, the predicted block may be used as the reconstructed block. The adder  155  may be called a reconstructor or a reconstructed block generator. The generated reconstructed signal may be used for intra prediction of a next block to be processed in the current picture and may be used for inter prediction of a next picture through filtering as described below. 
     The filter  160  may improve subjective/objective image quality by applying filtering to the reconstructed signal. For example, the filter  160  may generate a modified reconstructed picture by applying various filtering methods to the reconstructed picture and store the modified reconstructed picture in the memory  170 , specifically, a DPB of the memory  170 . The various filtering methods may include, for example, deblocking filtering, a sample adaptive offset, an adaptive loop filter, a bilateral filter, and the like. The filter  160  may generate various information related to filtering and transmit the generated information to the entropy encoder  190  as described later in the description of each filtering method. The information related to filtering may be encoded by the entropy encoder  190  and output in the form of a bitstream. 
     The modified reconstructed picture transmitted to the memory  170  may be used as the reference picture in the inter predictor  180 . When inter prediction is applied through the image encoding apparatus  100 , prediction mismatch between the image encoding apparatus  100  and the image decoding apparatus may be avoided and encoding efficiency may be improved. 
     The DPB of the memory  170  may store the modified reconstructed picture for use as a reference picture in the inter predictor  180 . The memory  170  may store the motion information of the block from which the motion information in the current picture is derived (or encoded) and/or the motion information of the blocks in the picture that have already been reconstructed. The stored motion information may be transmitted to the inter predictor  180  and used as the motion information of the spatial neighboring block or the motion information of the temporal neighboring block. The memory  170  may store reconstructed samples of reconstructed blocks in the current picture and may transfer the reconstructed samples to the intra predictor  185 . 
     Overview of Image Decoding Apparatus 
       FIG.  3    is a view schematically showing an image decoding apparatus, to which an embodiment of the present disclosure is applicable. 
     As shown in  FIG.  3   , the image decoding apparatus  200  may include an entropy decoder  210 , a dequantizer  220 , an inverse transformer  230 , an adder  235 , a filter  240 , a memory  250 , an inter predictor  260  and an intra predictor  265 . The inter predictor  260  and the intra predictor  265  may be collectively referred to as a “predictor”. The dequantizer  220  and the inverse transformer  230  may be included in a residual processor. 
     All or at least some of a plurality of components configuring the image decoding apparatus  200  may be configured by a hardware component (e.g., a decoder or a processor) according to an embodiment. In addition, the memory  250  may include a decoded picture buffer (DPB) or may be configured by a digital storage medium. 
     The image decoding apparatus  200 , which has received a bitstream including video/image information, may reconstruct an image by performing a process corresponding to a process performed by the image encoding apparatus  100  of  FIG.  2   . For example, the image decoding apparatus  200  may perform decoding using a processing unit applied in the image encoding apparatus. Thus, the processing unit of decoding may be a coding unit, for example. The coding unit may be acquired by partitioning a coding tree unit or a largest coding unit. The reconstructed image signal decoded and output through the image decoding apparatus  200  may be reproduced through a reproducing apparatus (not shown). 
     The image decoding apparatus  200  may receive a signal output from the image encoding apparatus of  FIG.  2    in the form of a bitstream. The received signal may be decoded through the entropy decoder  210 . For example, the entropy decoder  210  may parse the bitstream to derive information (e.g., video/image information) necessary for image reconstruction (or picture reconstruction). The video/image information may further include information on various parameter sets such as an adaptation parameter set (APS), a picture parameter set (PPS), a sequence parameter set (SPS), or a video parameter set (VPS). In addition, the video/image information may further include general constraint information. The image decoding apparatus may further decode picture based on the information on the parameter set and/or the general constraint information. Signaled/received information and/or syntax elements described in the present disclosure may be decoded through the decoding procedure and obtained from the bitstream. For example, the entropy decoder  210  decodes the information in the bitstream based on a coding method such as exponential Golomb coding, CAVLC, or CABAC, and output values of syntax elements required for image reconstruction and quantized values of transform coefficients for residual. More specifically, the CABAC entropy decoding method may receive a bin corresponding to each syntax element in the bitstream, determine a context model using a decoding target syntax element information, decoding information of a neighboring block and a decoding target block or information of a symbol/bin decoded in a previous stage, and perform arithmetic decoding on the bin by predicting a probability of occurrence of a bin according to the determined context model, and generate a symbol corresponding to the value of each syntax element. In this case, the CABAC entropy decoding method may update the context model by using the information of the decoded symbol/bin for a context model of a next symbol/bin after determining the context model. The information related to the prediction among the information decoded by the entropy decoder  210  may be provided to the predictor (the inter predictor  260  and the intra predictor  265 ), and the residual value on which the entropy decoding was performed in the entropy decoder  210 , that is, the quantized transform coefficients and related parameter information, may be input to the dequantizer  220 . In addition, information on filtering among information decoded by the entropy decoder  210  may be provided to the filter  240 . Meanwhile, a receiver (not shown) for receiving a signal output from the image encoding apparatus may be further configured as an internal/external element of the image decoding apparatus  200 , or the receiver may be a component of the entropy decoder  210 . 
     Meanwhile, the image decoding apparatus according to the present disclosure may be referred to as a video/image/picture decoding apparatus. The image decoding apparatus may be classified into an information decoder (video/image/picture information decoder) and a sample decoder (video/image/picture sample decoder). The information decoder may include the entropy decoder  210 . The sample decoder may include at least one of the dequantizer  220 , the inverse transformer  230 , the adder  235 , the filter  240 , the memory  250 , the inter predictor  160  or the intra predictor  265 . 
     The dequantizer  220  may dequantize the quantized transform coefficients and output the transform coefficients. The dequantizer  220  may rearrange the quantized transform coefficients in the form of a two-dimensional block. In this case, the rearrangement may be performed based on the coefficient scanning order performed in the image encoding apparatus. The dequantizer  220  may perform dequantization on the quantized transform coefficients by using a quantization parameter (e.g., quantization step size information) and obtain transform coefficients. 
     The inverse transformer  230  may inversely transform the transform coefficients to obtain a residual signal (residual block, residual sample array). 
     The predictor may perform prediction on the current block and generate a predicted block including prediction samples for the current block. The predictor may determine whether intra prediction or inter prediction is applied to the current block based on the information on the prediction output from the entropy decoder  210  and may determine a specific intra/inter prediction mode (prediction technique). 
     It is the same as described in the predictor of the image encoding apparatus  100  that the predictor may generate the prediction signal based on various prediction methods (techniques) which will be described later. 
     The intra predictor  265  may predict the current block by referring to the samples in the current picture. The description of the intra predictor  185  is equally applied to the intra predictor  265 . 
     The inter predictor  260  may derive a predicted block for the current block based on a reference block (reference sample array) specified by a motion vector on a reference picture. 
     In this case, in order to reduce the amount of motion information transmitted in the inter prediction mode, motion information may be predicted in units of blocks, subblocks, or samples based on correlation of motion information between the neighboring block and the current block. The motion information may include a motion vector and a reference picture index. The motion information may further include inter prediction direction (L0 prediction, L1 prediction, Bi prediction, etc.) information. In the case of inter prediction, the neighboring block may include a spatial neighboring block present in the current picture and a temporal neighboring block present in the reference picture. For example, the inter predictor  260  may configure a motion information candidate list based on neighboring blocks and derive a motion vector of the current block and/or a reference picture index based on the received candidate selection information. Inter prediction may be performed based on various prediction modes, and the information on the prediction may include information indicating a mode of inter prediction for the current block. 
     The adder  235  may generate a reconstructed signal (reconstructed picture, reconstructed block, reconstructed sample array) by adding the obtained residual signal to the prediction signal (predicted block, predicted sample array) output from the predictor (including the inter predictor  260  and/or the intra predictor  265 ). If there is no residual for the block to be processed, such as when the skip mode is applied, the predicted block may be used as the reconstructed block. The description of the adder  155  is equally applicable to the adder  235 . The adder  235  may be called a reconstructor or a reconstructed block generator. The generated reconstructed signal may be used for intra prediction of a next block to be processed in the current picture and may be used for inter prediction of a next picture through filtering as described below. 
     The filter  240  may improve subjective/objective image quality by applying filtering to the reconstructed signal. For example, the filter  240  may generate a modified reconstructed picture by applying various filtering methods to the reconstructed picture and store the modified reconstructed picture in the memory  250 , specifically, a DPB of the memory  250 . The various filtering methods may include, for example, deblocking filtering, a sample adaptive offset, an adaptive loop filter, a bilateral filter, and the like. 
     The (modified) reconstructed picture stored in the DPB of the memory  250  may be used as a reference picture in the inter predictor  260 . The memory  250  may store the motion information of the block from which the motion information in the current picture is derived (or decoded) and/or the motion information of the blocks in the picture that have already been reconstructed. The stored motion information may be transmitted to the inter predictor  260  so as to be utilized as the motion information of the spatial neighboring block or the motion information of the temporal neighboring block. The memory  250  may store reconstructed samples of reconstructed blocks in the current picture and transfer the reconstructed samples to the intra predictor  265 . 
     In the present disclosure, the embodiments described in the filter  160 , the inter predictor  180 , and the intra predictor  185  of the image encoding apparatus  100  may be equally or correspondingly applied to the filter  240 , the inter predictor  260 , and the intra predictor  265  of the image decoding apparatus  200 . 
     General Image/Video Coding Procedure 
     In image/video coding, a picture configuring an image/video may be encoded/decoded according to a decoding order. A picture order corresponding to an output order of the decoded picture may be set differently from the decoding order, and, based on this, not only forward prediction but also backward prediction may be performed during inter prediction. 
       FIG.  4    shows an example of a schematic picture decoding procedure, to which embodiment(s) of the present disclosure is applicable. 
     Each procedure shown in  FIG.  4    may be performed by the image decoding apparatus of  FIG.  3   . For example, step S 410  may be performed by the entropy decoder  210 , step S 420  may be performed by a predictor including the predictors  265  and  260 , step S 430  may be performed by a residual processor  220  and  230 , step S 440  may be performed by the adder  235 , and step S 450  may be performed by the filter  240 . Step S 410  may include the information decoding procedure described in the present disclosure, step S 420  may include the inter/intra prediction procedure described in the present disclosure, step S 430  may include a residual processing procedure described in the present disclosure, step S 440  may include the block/picture reconstruction procedure described in the present disclosure, and step S 450  may include the in-loop filtering procedure described in the present disclosure. 
     Referring to  FIG.  4   , the picture decoding procedure may schematically include a procedure (S 410 ) for obtaining image/video information (through decoding) from a bitstream, a picture reconstruction procedure (S 420  to S 440 ) and an in-loop filtering procedure (S 450 ) for a reconstructed picture. The picture reconstruction procedure may be performed based on prediction samples and residual samples obtained through inter/intra prediction (S 420 ) and residual processing (S 430 ) (dequantization and inverse transform of the quantized transform coefficient) described in the present disclosure. A modified reconstructed picture may be generated through the in-loop filtering procedure for the reconstructed picture generated through the picture reconstruction procedure. In this case, the modified reconstructed picture may be output as a decoded picture, stored in a decoded picture buffer (DPB) of a memory  250  and used as a reference picture in the inter prediction procedure when decoding the picture later. The in-loop filtering procedure (S 450 ) may be omitted. In this case, the reconstructed picture may be output as a decoded picture, stored in a DPB of a memory  250 , and used as a reference picture in the inter prediction procedure when decoding the picture later. The in-loop filtering procedure (S 450 ) may include a deblocking filtering procedure, a sample adaptive offset (SAO) procedure, an adaptive loop filter (ALF) procedure and/or a bi-lateral filter procedure, as described above, some or all of which may be omitted. In addition, one or some of the deblocking filtering procedure, the sample adaptive offset (SAO) procedure, the adaptive loop filter (ALF) procedure and/or the bi-lateral filter procedure may be sequentially applied or all of them may be sequentially applied. For example, after the deblocking filtering procedure is applied to the reconstructed picture, the SAO procedure may be performed. Alternatively, after the deblocking filtering procedure is applied to the reconstructed picture, the ALF procedure may be performed. This may be similarly performed even in the encoding apparatus. 
       FIG.  5    shows an example of a schematic picture encoding procedure, to which embodiment(s) of the present disclosure is applicable. 
     Each procedure shown in  FIG.  5    may be performed by the image encoding apparatus of  FIG.  2   . For example, step S 510  may be performed by the predictors  185  and  180 , step S 520  may be performed by a residual processor  115 ,  120  and  130 , and step  3530  may be performed in the entropy encoder  190 . Step S 510  may include the inter/intra prediction procedure described in the present disclosure, step  3520  may include the residual processing procedure described in the present disclosure, and step S 530  may include the information encoding procedure described in the present disclosure. 
     Referring to  FIG.  5   , the picture encoding procedure may schematically include not only a procedure for encoding and outputting information for picture reconstruction (e.g., prediction information, residual information, partitioning information, etc.) in the form of a bitstream but also a procedure for generating a reconstructed picture for a current picture and a procedure (optional) for applying in-loop filtering to a reconstructed picture, as described with respect to  FIG.  2   . The encoding apparatus may derive (modified) residual samples from a quantized transform coefficient through the dequantizer  140  and the inverse transformer  150 , and generate the reconstructed picture based on the prediction samples which are output of step S 510  and the (modified) residual samples. The reconstructed picture generated in this way may be equal to the reconstructed picture generated in the decoding apparatus. The modified reconstructed picture may be generated through the in-loop filtering procedure for the reconstructed picture. In this case, the modified reconstructed picture may be stored in the decoded picture buffer or a memory  170 , and may be used as a reference picture in the inter prediction procedure when encoding the picture later, similarly to the decoding apparatus. As described above, in some cases, some or all of the in-loop filtering procedure may be omitted. When the in-loop filtering procedure is performed, (in-loop) filtering related information (parameter) may be encoded in the entropy encoder  190  and output in the form of a bitstream, and the decoding apparatus may perform the in-loop filtering procedure using the same method as the encoding apparatus based on the filtering related information. 
     Through such an in-loop filtering procedure, noise occurring during image/video coding, such as blocking artifact and ringing artifact, may be reduced and subjective/objective visual quality may be improved. In addition, by performing the in-loop filtering procedure in both the encoding apparatus and the decoding apparatus, the encoding apparatus and the decoding apparatus may derive the same prediction result, picture coding reliability may be increased and the amount of data to be transmitted for picture coding may be reduced. 
     As described above, the picture reconstruction procedure may be performed not only in the image decoding apparatus but also in the image encoding apparatus. A reconstructed block may be generated based on intra prediction/inter prediction in units of blocks, and a reconstructed picture including reconstructed blocks may be generated. When a current picture/slice/tile group is an I picture/slice/tile group, blocks included in the current picture/slice/tile group may be reconstructed based on only intra prediction. On the other hand, when the current picture/slice/tile group is a P or B picture/slice/tile group, blocks included in the current picture/slice/tile group may be reconstructed based on intra prediction or inter prediction. In this case, inter prediction may be applied to some blocks in the current picture/slice/tile group and intra prediction may be applied to the remaining blocks. The color component of the picture may include a luma component and a chroma component and the methods and embodiments of the present disclosure are applicable to both the luma component and the chroma component unless explicitly limited in the present disclosure. 
     Example of Coding Layer Structure 
     Coded video/image according to the present disclosure may be, for example, processed according to the coding layer and structure described below. 
       FIG.  6    is a view showing a layer structure for a coded image. 
     The coded image is classified into a video coding layer (VCL) for an image decoding process and handling itself, a lower system for transmitting and storing encoded information, and a network abstraction layer (NAL) present between the VCL and the lower system and responsible for a network adaptation function. 
     In the VCL, VCL data including compressed image data (slice data) may be generated or a supplemental enhancement information (SEI) message additionally required for a decoding process of an image or a parameter set including information such as a picture parameter set (PPS), a sequence parameter set (SPS) or a video parameter set (VPS) may be generated. 
     In the NAL, header information (NAL unit header) may be added to a raw byte sequence payload (RBSP) generated in the VCL to generate a NAL unit. In this case, the RBSP refers to slice data, a parameter set, an SEI message generated in the VCL. The NAL unit header may include NAL unit type information specified according to RBSP data included in a corresponding NAL unit. 
     As shown in  FIG.  6   , the NAL unit may be classified into a VCL NAL unit and a non-VCL NAL unit according to the RBSP generated in the VCL. The VCL NAL unit may mean a NAL unit including information on an image (slice data), and the Non-VCL NAL unit may mean a NAL unit including information (parameter set or SEI message) required to decode an image. 
     The VCL NAL unit and the Non-VCL NAL unit may be attached with header information and transmitted through a network according to the data standard of the lower system. For example, the NAL unit may be modified into a data format of a predetermined standard, such as H.266/VVC file format, RTP (Real-time Transport Protocol) or TS (Transport Stream), and transmitted through various networks. 
     As described above, in the NAL unit, a NAL unit type may be specified according to the RBSP data structure included in the corresponding NAL unit, and information on the NAL unit type may be stored in a NAL unit header and signaled. For example, this may be largely classified into a VCL NAL unit type and a non-VCL NAL unit type depending on whether the NAL unit includes information on an image (slice data). The VCL NAL unit type may be classified according to the property and type of the picture included in the VCL NAL unit, and the Non-VCL NAL unit type may be classified according to the type of a parameter set. 
     An example of the NAL unit type specified according to the type of the parameter set/information included in the Non-VCL NAL unit type will be listed below.
         DCI (Decoding capability information) NAL unit type (NUT): Type for NAL unit including DCI   VPS(Video Parameter Set) NUT: Type for NAL unit including VPS   SPS(Sequence Parameter Set) NUT: Type for NAL unit including SPS   PPS(Picture Parameter Set) NUT: Type for NAL unit including PPS   APS (Adaptation Parameter Set) NUT: Type for NAL unit including APS   PH (Picture header) NUT: Type for NAL unit including PH       

     The above-described NAL unit types may have syntax information for a NAL unit type, and the syntax information may be stored in a NAL unit header and signaled. For example, the syntax information may be nal_unit_type, and the NAL unit types may be specified using nal_unit_type values. 
     Meanwhile, as described above, one picture may include a plurality of slices, and one slice may include a slice header and slice data. In this case, one picture header may be further added to a plurality of slices (slice header and slice data set) in one picture. The picture header (picture header syntax) may include information/parameters commonly applicable to the picture. The slice header (slice header syntax) may include information/parameters commonly applicable to the slice. The APS (APS syntax) or PPS (PPS syntax) may include information/parameters commonly applicable to one or more slices or pictures. The SPS (SPS syntax) may include information/parameters commonly applicable to one or more sequences. The VPS (VPS syntax) may information/parameters commonly applicable to multiple layers. The DCI (DCI syntax) may include information/parameters related to decoding capability. 
     In the present disclosure, a high level syntax (HLS) may include at least one of the APS syntax, the PPS syntax, the SPS syntax, the VPS syntax, the DCI syntax, the picture header syntax or the slice header syntax. In addition, in the present disclosure, a low level syntax (LLS) may include, for example, a slice data syntax, a CTU syntax, a coding unit syntax, a transform unit syntax, etc. 
     Meanwhile, in the present disclosure, image/video information encoded in the encoding apparatus and signaled to the decoding apparatus in the form of a bitstream may include not only in-picture partitioning related information, intra/inter prediction information, residual information, in-loop filtering information but also information on the slice header, information on the picture header, information on the APS, information on the PPS, information on the SPS, information on the VPS and/or information on the DCI. In addition, the image/video information may further include general constraint information and/or information on a NAL unit header. 
     High Level Syntax Signalling and Semantics 
     As described above, image/video information according to the present disclosure may include a high level syntax (HLS). An image encoding method and/or an image decoding method may be performed based on the image/video information. 
     Video Parameter Set Signalling 
     A video parameter set (VPS) is a parameter set which is used for the carriage of layer information. The layer information may include, for example, information on an output layer set (OLS), information on a profile tier level, information on a relationship between an OLS and a hypothetical reference decoder and information on a relationship between an OLS and a decoded picture buffer (DPB). The VPS may not be essential for decoding of a bitstream. 
     A VPS raw byte sequence payload (RBSP) shall be available to a decoding process prior to it being referenced, included in at least one access unit (AU) with TemporalId equal to 0 or provided through external means. 
     All VPS NAL units with a particular value of vps_video_parameter_set_id in a coded video sequence (CVS) shall have the same content. 
       FIG.  7    is a view illustrating a syntax structure of a VPS according to an embodiment of the present disclosure. 
     The syntax structure of the VPS shown in  FIG.  7    includes only syntax elements related to the present disclosure, and various other syntax elements not shown in  FIG.  7    may be included in the VPS. 
     In the example shown in  FIG.  7   , vps_video_parameter_set_id provides an identifier for the VPS. Other syntax elements may refer to the VPS using vps_video_parameter_set_id. The value of vps_video_parameter_set_id shall be greater than 0. 
     vps_max_layers_minus1 plus 1 may specify the maximum allowed number of layers in each CVS referring to the VPS. 
     vps_max_sublayers_minus1 plus 1 may specify the maximum number of temporal sublayers that may be present in a layer in each CVS referring to the VPS. vps_max_sublayers_minus1 may be in the range of 0 to 6, inclusive. 
     vps_all_layers_same_num_sublayers_flag may be signaled when vps_max_layers_minus1 is greater than 0 and vps_max_sublayers_minus1 is greater than 0. vps_all_layers_same_num_sublayers_flag equal to a first value (e.g., 1) may specify that the number of temporal sublayers is the same for all the layers in each CVS referring to the VPS. vps_all_layers_same_num_sublayers_flag equal to a second value (e.g., 0) may specify that the layers in each CVS referring to the VPS may not have the same number of temporal sublayers. When vps_all_layers_same_num_sublayers_flag is not present, the value thereof may be inferred to be equal to a first value (e.g., 1). 
     vps_all_independent_layers_flag may be signaled when vps_max_layers_minus1 is greater than 0. vps_all_independent_layers_flag equal to a first value (e.g., 1) may specify that all layers in the CVS are independently coded without using inter-layer prediction. vps_all_independent_layers_flag equal to a second value (e.g., 0) may specify that one or more of the layers in the CVS may use inter-layer prediction. When vps_all_independent_layers_flag is not present, the value thereof may be inferred to be equal to a first value (e.g., 1). 
     each_layer_is_an_ols_flag may be signaled when vps_max_layers_minus1 is greater than 0. In addition, each_layer_is_an_ols_flag may be signaled when vps_all_independent_layers_flag is equal to a first value. each_layer_is_an_ols_flag equal to a first value (e.g., 1) may specify that each OLS contains only one layer. In addition, each_layer_is_an_ols_flag equal to a first value (e.g., 1) may specify that each layer itself in a CVS referring to the VPS is an OLS (that is, one layer contained in the OLS is the only output layer). In addition, each_layer_is_an_ols_flag equal to a second value (e.g., 0) may specify that at least one OLS may contain more than one layer. If vps_max_layers_minus1 is equal to 0, the value of each_layer_is_an_ols_flag may be 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 may be inferred to be equal to 0. 
     When each_layer_is_an_ols_flag is equal to a second value (e.g., 0) and vps_all_independent_layers_flag is equal to a second value (e.g., 0), ols_mode_idc may be signaled. 
     ols_mode_idc equal to a first value (e.g., 0) may specify that the total number of OLSs specified by the VPS is equal to vps_max_layers_minus1+1. In this case, an i-th OLS may contain the layers with layer indices from 0 to i, inclusive. In addition, for each OLS, only a layer having a highest layer index (highest layer) in the OLS may be output. 
     ols_mode_idc equal to a second value (e.g., 1) may specify that the total number of OLSs specified by the VPS is equal to vps_max_layers_minus1+1. In this case, an i-th OLS may contain layers with layer indices from 0 to i, inclusive. In addition, for each OLS, all layers in the OLS may be output. 
     ols_mode_idc equal to a third value (e.g., 2) may specify that the total number of OLSs specified by the VPS is explicitly signaled. In addition, for each OLS, the output layers are explicitly signaled. Other layers which are not output layers are the layers that are direct or indirect reference layers of the output layers of the OLS. 
     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 may be inferred to be equal to a third value (e.g., 2). 
     When ols_mode_idc is 2, num_output_layer_sets_minus1 and ols_output_layer_flag[i][j] may be explicitly signaled. 
     num_output_layer_sets_minus1 plus 1 may specify the total number of OLSs specified by the VPS. 
     When ols_mode_idc is 2, ols_output_layer_flag[i][j] may specify whether a j-th layer of an i-th OLS is an output layer. ols_output_layer_flag[i] [j] equal to a first value (e.g., 1) may specify that a layer with a layer identifier nuh_layer_id equal to vps_layer_id[j] is an output layer of an i-th OLS. ols_output_layer_flag[i][j] equal to a second value (e.g., 0) may specify that a layer with a layer identifier nuh_layer_i equal to vps_layer_id [j] is not an output layer of an i-th OLS. 
     Hereinafter, HRD parameters signaled in the VPS will be described. 
     When each_layer_is_an_ols_flag is equal to a second value (e.g., 0), vps_general_hrd_params_present_flag may be signaled. vps_general_hrd_params_present_flag equal to a first value (e.g., 1) may specify that the general_hrd_parameters( ) syntax structure and other HRD parameters are present in the VPS. vps_general_hrd_params_present_flag equal to a second value (e.g., 0) may specify that the general_hrd_parameters( ) syntax structure and other HRD parameters are not present in the VPS. When vps_general_hrd_params_present_flag is not present, the value thereof may be inferred to be equal to a second value (e.g., 0). 
     When an i-th OLS contains one layer (NumLayersInOls[i] is equal to 1), the general_hrd_parameters( ) syntax structure that applies to the i-th OLS may be present in a sequence parameter set (SPS) referred to by the layer in the i-th OLS. 
     vps_sublayer_cpb_params_present_flag may be signaled when vps_max_sublayers_minus1 is greater than 0. vps_sublayer_cpb_params_present flag equal to a first value (e.g., 1) may specify that the i-th ols_hrd_parameters( ) syntax structure in the VPS contains HRD parameters for the sublayers with a temporal layer identifier TemporalId in the range of 0 to hrd_max_tid[i], inclusive. vps_sublayer_cpb_params_present_flag equal to a second value (e.g., 0) may specify that the i-th ols_hrd_parameters( ) syntax structure in the VPS contains HRD parameters for the sublayer with a temporal layer identifier TemporalId equal to hrd_max_tid[i] only. When vps_max_sublayers_minus1 is equal to 0, vps_sublayer_cpb_params_present_flag may be inferred to be equal to a second value (e.g., 0). 
     When vps_sublayer_cpb_params_present_flag is equal to a second value (e.g., 0), the HRD parameters for the sublayers with TemporalId in the range of 0 to hrd_max_tid[i]−1, inclusive, are inferred to be the same as that for the sublayer with a temporal layer identifier TemporalId equal to hrd_max_tid[i]. 
     num_ols_hrd_params_minus1 plus 1 may specify the number of ols_hrd_parameters( ) syntax structures in the VPS. num_ols_hrd_params_minus1 may be in the range of 0 to TotalNumOlss−1. TotalNumOlss may specify the total number of OLSs specified by the VPS. In the present disclosure, the HRD parameter may mean ols_hrd_parameters ( ). Accordingly, the number of HRD parameter syntax structures may mean the number of ols_hrd_parameters( ) syntax structures. 
     hrd_max_tid[i] may be signaled when vps_max_sublayers_minus1 is greater than 0 and vps_all_layers_same_num_sublayers_flag is equal to a second value (e.g., 0). hrd_max_tid[i] may specify the temporal layer identifier TemporalId of the highest sublayer for which the related HRD parameters are contained in the i-th ols_hrd_parameters( ) syntax structure. 
     hrd_max_tid[i] may be in the range of 0 to vps_max_sublayers_minus1, inclusive. When vps_max_sublayers_minus1 is equal to 0, the value of hrd_max_tid[i] may be inferred to be equal to 0. When vps_max_sublayers_minus1 is greater than 0 and vps_all_layers_same_num_sublayers_flag is equal to 1, the value of hrd_max_tid[i] may be inferred to be equal to vps_max_sublayers_minus1. 
     As shown in  FIG.  7   , a variable firstSubLayer specifying the temporal layer identifier TemporalId of a first sublayer may be derived to be 0 or hrd_max_tid[i] based on vps_sublayer_cpb_params_present_flag. Specifically, when vps_sublayer_cpb_params_present_flag is equal to 1, firstSubLayer may be derived to be 0, and, otherwise, firstSubLayer may be derived to be hrd_max_tid[i]. Based on the derived firstSubLayer and hrd_max_tid[i], the ols_hrd_parameters( ) syntax structure may be signaled. 
     When num_ols_hrd_params_minus1 plus 1 and TotalNumOlss are not equal and num_ols_hrd_params_minus1 is greater than 0, ols_hrd_idx[i] may be signaled. In this case, ols_hrd_idx[i] may be signaled for the i-th OLS, when the number (NumLayersInOls[i]) of layers contained in the i-th OLS is greater than 1. ols_hrd_idx[i] specifies the index, to the list of ols_hrd_parameters( ) syntax structures in the VPS, of the ols_hrd_parameters( ) syntax structure that applies to the i-th OLS. The value of ols_hrd_idx[[i] may be in the range of 0 to num_ols_hrd_params_minus1, inclusive. When the number (NumLayersInOls[i]) of layers contained in the i-th OLS is equal to 1, the ols_hrd_parameters( ) syntax structure that applies to the i-th OLS may be present in an SPS referred to by the layer in the i-th OLS. 
     In the present disclosure, ols_hrd_idx[i] is the index of ols_hrd_parameters( ) that applies to an i-th OLS or an i-th multi-layer OLS and may be referred to as mapping information (information on mapping) between (multi-layer) OLSs and HRD parameter syntax structures (ols_hrd_parameters( )). 
     When num_ols_hrd_param_minus1 plus 1 is equal to TotalNumOlss, the value of ols_hrd_idx[i] may be inferred to be equal to i. Otherwise, when NumLayersInOls[i] is greater than 1 and num_ols_hrd_params_minus1 is equal to 0, the value of ols_hrd_idx[i] may be inferred to be equal to 0. 
     HRD Signalling in VPS and SPS 
     Hereinafter, signaling of HRD parameters according to the present disclosure will be described in greater detail. The HRD parameters may be signaled for each output layer set (OLS). A hypothetical reference decoder (HRD) is a hypothetical decoder model that specifies constraints on the variability of conforming NAL unit streams or conforming byte streams that an encoding process may produce. 
     The HRD parameters may be included and signaled in a VPS as described with reference to  FIG.  7    or may be included and signaled in an SPS. 
       FIG.  8    is a view illustrating the syntax structure of an SPS for signaling HRD parameters according to an embodiment of the present disclosure. 
     In the example shown in  FIG.  8   , sps_ptl_dpb_hrd_params_present_flag equal to a first value (e.g., 1) may specify that a profile_tier_level( ) syntax structure and a dpb_parameters( ) syntax structure are present in the SPS. profile_tier_level( ) may be a syntax structure for transmitting parameters for a profile tier level, and dpb_parameters( ) may be a syntax structure for transmitting decoded picture buffer (DPB) parameters. In addition, sps_ptl_dpb_hrd_params_present_flag equal to a first value (e.g., 1) may specify that a general_hrd_parameters( ) syntax structure and an ols_hrd_parameters( ) syntax structure may be present in an SPS. sps_ptl_dpb_hrd_params_present_flag equal to a second value (e.g., 0) may specify that the above-described four syntax structures are not present in the SPS. The value of sps_ptl_dpb_hrd_params_present_flag may be equal to the value of vps_independent_layer_flag[GeneralLayerIdx[nuh_layer_id] ]. That is, the value of sps_ptl_dpb_hrd_params_present_flag may be encoded as the value of vps_independent_layer_flag[GeneralLayerIdx[nuh_layer_id] ]. 
     In the above, vps_independent_layer_flag[i] may be a syntax element included and transmitted in the VPS. vps_independent_layer_flag[i] equal to a first value (e.g., 1) may specify that a layer with an index i is an independent layer which does not use inter-layer prediction. vps_independent_layer_flag[i] equal to a second value (e.g., 0) may specify that a layer with an index i may use inter-layer prediction. When vps_independent_layer_flag[i] is not present, the value thereof is inferred to be equal to a first value (e.g., 1). 
     When sps_ptl_dpb_hrd_params_present_flag is equal to 1, sps_general_hrd_params_present_flag may be signaled. 
     sps_general_hrd_params_present_flag equal to a first value (e.g., 1) may specify that the SPS includes a general_hrd_parameters( ) syntax structure and an ols_hrd_parameters( ) syntax structure. sps_general_hrd_params_present_flag equal to a second value (e.g., 0) may specify that the SPS does not include a general_hrd_parameters( ) syntax structure or an ols_hrd_parameters( ) syntax structure. 
     As shown in  FIG.  8   , when sps_max_sublayers_minus1 is greater than 0, sps_sublayer_cpb_params_present_flag may be signaled. In this case, sps_max_sublayers_minus1 plus 1 may specify the maximum number of temporal sublayers which may be present in each coded layer video sequence (CLVS) referring to the SPS. sps_sublayer_cpb_params_present_flag equal to a first value (e.g., 1) may specify that the ols_hrd_parameters( ) syntax structure in the SPS includes HRD parameters for sublayers with the temporal layer identifer TemporalId in the range of 0 to sps_max_sublayers_minus1, inclusive. sps_sublayer_cpb_params_present_flag equal to a second value (e.g., 0) may specify that the ols_hrd_parameters( ) syntax structure in the SPS includes HRD parameters for the sublayer with the temporal layer identifier TemporalId equal to sps_max_sublayers_minus1 only. When sps_max_sublayers_minus1 is equal to 0, the value of sps_sublayer_cpb_params_present_flag is inferred to be equal to a second value (e.g., 0). 
     When sps_sublayer_cpb_params_present_flag is equal to a second value (e.g., 0), the HRD parameters for the sublayers with the temporal layer identifier TemporalId in the range of 0 to sps_max_sublayers_minus1−1, inclusive, are inferred to be the same as that for the sublayer with the temperal layer identifier TemporalId equal to sps_max_sublayers_minus1. 
       FIG.  9    is a view illustrating a general_hrd_parameters( ) syntax structure according to an embodiment of the present disclosure. 
     As shown in  FIG.  9   , the general_hrd_parameters( ) syntax structure may include some of the sequence-level HRD parameters used in the HRD operations. It is a requirement of bitstream conformance that the content of the general_hrd_parameters( ) present in any VPSs or SPSs in the bitstream shall be identical. 
     When the general_hrd_parameters( ) syntax structure is included in a VPS, the general_hrd_parameters( ) syntax structure may apply to all OLSs specified by the VPS. When the general_hrd_parameters( ) syntax structure is included in an SPS, the general_hrd_parameters( ) syntax structure may apply to the OLS that contains only the lowest layer among the layers that refer to the SPS. In this case, the lowest layer is an independent layer. 
     As shown in  FIG.  9   , the general_hrd_parameters( ) syntax structure is a HRD parameter and may include syntax elements such as num_units_in_tick, time_scale, and general_nal_hrd_params_present_flag. The HPRD parameters shown in  FIG.  9    may have the same meanings as the conventional HRD parameters. Accordingly, a detailed description of HRD parameters that are less relevant to the present disclosure will be omitted. 
       FIG.  10    is a view illustrating an ols_hrd_parameters( ) syntax structure according to an embodiment of the present disclosure. 
     When the ols_hrd_parameters( ) syntax structure is included in a VPS, OLSs, to which the ols_hrd_parameters( ) syntax structure applies, may be specified by the VPS. When the ols_hrd_parameters( ) syntax structure is included in an SPS, the ols_hrd_parameters( ) syntax structure may apply to an OLS that contains the lowest layer among the layers that refer to the SPS. In this case, the lowest layer is an independent layer. 
     As shown in  FIG.  10   , the ols_hrd_parameters( ) syntax structure is a HRD parameter and may include syntax elements such as fixed_pic_rate_general_flag, fixed_pic_rate_within_cvs_flag, and elemental_duration_in_tc_minus1. The HRD parameters shown in  FIG.  10    may have the same meanings as the conventional HRD parameters. Accordingly, a detailed description of HRD parameters that are less relevant to the present disclosure will be omitted. 
       FIG.  11    is a view illustrating a sublayer_hrd_parameters( ) syntax structure according to an embodiment of the present disclosure. 
     The sublayer_hrd_parameters( ) syntax structure may be included and signaled in the ols_hrd_parameters( ) syntax structure of  FIG.  10   . 
     As shown in  FIG.  11   , the sublayer_hrd_parameters( ) syntax structure is a HRD parameter and may include syntax elements such as bit_rate_value_minus1, cpb_size_value_minus1, and cpb_size_du_value_minus1. The HRD parameters shown in  FIG.  11    may have the same meanings as the conventional HRD parameters. Accordingly, a detailed description of HRD parameters that are less relevant to the present disclosure will be omitted. 
     For reference, an output time may be a time when a reconstructed picture is to be output from a DPB. The output time may be specified by the HRD according to the output timing DPB operation. 
     Two sets of HRD parameters such as NAL HRD parameter and VCL HRD parameter may be used. The HRD parameters may be signaled through the general_hrd_parameters( ) syntax structure and the ols_hrd_parameters( ) syntax structure. The general_hrd_parameters( ) syntax structure and the ols_hrd_parameters( ) syntax structure may be included and signaled in the VPS or may be included and signaled in the SPS. 
     For example, DPB management may be performed based on the HRD parameters. As an example, removal of picture(s) from the DPB before decoding of the current picture and/or (decoded) picture output may be performed based on the HRD parameters. 
     The signaling method of the HRD parameters described with reference to  FIGS.  7  to  11    have at least the following problems. 
     As described above, num_ols_hrd_params_minus1 may be constrained to be in the range of 0 to TotalNumOlss−1, inclusive. However, it is not constrained that each HRD parameter that is signalled in the VPS must be associated with at least one OLS. Accordingly, since the VPS may include unused HRD parameters, signaling efficiency may deteriorate. 
     As described above, num_ols_hrd_params_minus1 may be constrained to be in the range of 0 to TotalNumOlss−1, inclusive. However, the constraint allows unused HRD structure to be included and signalled in the VPS, since there may be one or more OLSs which contain only one layer. Such OLSs which contain only one layer are not associated with HRD parameter structure signalled in the VPS. 
     Signaling of the HRD parameters associated with the OLS includes the above-described problems and include disadvantages which are not described in the present disclosure. 
     The embodiments according to the present disclosure for solving at least one of the problems may include at least one of the following configurations. The following configurations are applicable individually or in combinations. 
     Configuration 1: It may be constrained that each HRD parameter structure that is signalled in the VPS is associated with at least one OLS. 
     Configuration 2: The number of HRD parameter structures signalled in the VPS (i.e., num_ols_hrd_params_minus1) shall not be greater than the number of OLSs that contains more than one layer. That is, the number of HRD parameter structures signalled in the VPS shall not be greater than the total number of OLSs minus the number of OLS that contains only one layer. 
       FIG.  12    is a view illustrating an example of an image encoding method, to which an embodiment of the present disclosure is applicable. 
     The image encoding apparatus may derive HRD parameters (S 1210 ) and encode image/video information (S 1220 ). In this case, the image/video information may include information related to the derived HRD parameters. 
     Although not shown in  FIG.  12   , the image encoding apparatus may perform DPB management based on the HRD parameters derived in step S 1210 . 
       FIG.  13    is a view illustrating an example of an image decoding method, to which an embodiment of the present disclosure is applicable. 
     The image decoding apparatus may obtain image/video information (S 1310 ). In this case, the image/video information may include information related to the HRD parameters. 
     The image decoding apparatus may decode a picture based on the obtained HRD parameters (S 1320 ). 
       FIG.  14    is a view illustrating another example of an image decoding method, to which an embodiment of the present disclosure is applicable. 
     The image decoding apparatus may obtain image/video information from a bitstream ( 51410 ). In this case, the image/video information may include information related to the HRD parameters. 
     The image decoding apparatus may perform DPB management based on the obtained HRD parameters (S 1420 ). 
     The image decoding apparatus may decode a picture based on the DPB (S 1430 ). For example, blocks/slices in a current picture may be decoded based on inter prediction using a picture already reconstructed in the DPB as a reference picture. 
     In the example described with reference to  FIGS.  12  to  14   , the information related to the HRD parameters may include at least one of information/syntax elements described in connection with at least one of the embodiments of the present disclosure. In addition, as described above, DPB management may be performed based on the HRD parameters. For example, removal of picture(s) from the DPB before decoding of the current picture and/or (decoded) picture output may be performed based on the HRD parameters. 
     According to an embodiment of the present disclosure for solving at least some of the above-described problems, each HRD parameter structure signaled in the VPS may be constrained to be associated with at least one OLS. 
     As described above, ols_hrd_parameters( ) that applies to an i-th OLS may be specified by ols_hrd_idx[i]. According to the present embodiment, each of all ols_hrd_parameters( ) signaled in the VPS may be constrained to apply to at least one OLS. That is, each ols_hrd_parameters( ) in the VPS may be specified by at least one ols_hrd_idx[i]. 
     According to the present embodiment, each of ols_hrd_parameters( ) in the VPS is used at least once. That is, unused ols_hrd_parameters( ) is not signaled. Therefore, according to the present embodiment, signaling of ols_hrd_parameters( ) may be efficiently performed. 
     According to another embodiment of the present disclosure for solving at least some of the above-described problems, the number (i.e., num_ols_hrd_params_minus1) of HRD parameter structures signaled in the VPS may be constrained not to be greater than the number of OLSs containing more than one layer. 
     According to the example described with reference to  FIG.  7   , num_ols_hrd_params_minus1 plus 1 may specify the number of ols_hrd_parameters( ) syntax structures in the VPS, and num_ols_hrd_params_minus1 may be in the range of 0 to TotalNumOlss−1, inclusive. 
     However, as described above, there may be an OLS containing only one layer, and the OLS containing only one layer is not associated with a HRD parameter structure signaled in the VPS. The range of the value of num_ols_hrd_params_minus1 in the example of  FIG.  7    may cause inaccurate signaling. Accordingly, by specifying the range of the number of ols_hrd_parameters( ) syntax structures by the number (NumMultiLayerOlss) of OLSs containing multiple layers (multi-layer OLSs) instead of the total number (TotalNumOlss) of OLSs, accurate signaling may be performed. 
     According to the present embodiment, num_ols_hrd_params_minus1 plus 1 may specify the number of ols_hrd_parameters( ) syntax structures in the VPS, and num_ols_hrd_params_minus1 may be in the range of 0 to NumMultiLayerOlss−1, inclusive. In this case, the number (NumMultiLayerOlss) of OLSs containing multiple layers (multi-layer OLSs) may be equal to the total numbers (TotalNumOlss) of OLSs minus the number (NumSingleLayerOlss) of OLSs containing only one layer. 
     As described above, each HRD parameter signaled in the VPS may be constrained to be associated with (mapped to) at least one OLS. In addition, the number of HRD parameters signaled in the VPS (i.e., num_ols_hrd_params_minus1) may be constrained not to be greater than the number of OLSs containing multiple layers (multi-layer OLSs). The two embodiments may be combined to construct another embodiment as follows. 
       FIG.  15    is a view illustrating a process of encoding HRD parameters based on num_ols_hrd_params_minus1 according to another embodiment of the present disclosure. 
     The image encoding apparatus may encode num_ols_hrd_params_minus1 into a VPS (S 1510 ). num_ols_hrd_params_minus1 plus 1 may specify the number of ols_hrd_parameters( ) syntax structures in the VPS, and num_ols_hrd_params_minus1 may be in the range of 0 to NumMultiLayerOlss−1, inclusive. 
     The image encoding apparatus may encode num_ols_hrd_params_minus1+one ols_hrd_parameters( ) syntax structure into the VPS (S 1520 ). 
     The image encoding apparatus may determine the following condition 1 (S 1530 ). 
     Condition 1: (num_ols_hrd_params_minus1+1 !=NumMultiLayerOlss &amp;&amp; num_ols_hrd_params_minus1&gt;0)? 
     Condition 1 is a condition for encoding ols_hrd_idx into a bitstream. When Condition 1 is satisfied (S 1530 —Yes), the image encoding apparatus may encode ols_hrd_idx into the VPS (S 1540 ). According to the embodiment described with reference to  FIG.  15   , ols_hrd_idx[i] is an index for a list of ols_hrd_parameters( ) in the VPS, and may be an index of ols_hrd_parameters( ) that applies to an OLS containing multiple i-th layers (multi-layer OLS). That is, ols_hrd_idx[i] in the VPS may be signaled for multi-layer OLSs, and may be in the range of 0 to num_ols_hrd_params_minus1, inclusive. 
     When condition 1 is not satisfied (S 1530 —No), the image encoding apparatus may infer ols_hrd_idx without encoding ols_hrd_idx into the VPS (S 1550 ). Specifically, when num_ols_hrd_params_minus1 is equal to 0, the value of ols_hrd_idx[i] may be inferred to be equal to 0. Otherwise, when num_ols_hrd_param_minus1 plus 1 is equal to NumMultiLayerOlss, the value of ols_hrd_idx[i] may be inferred to be equal to i. 
     In addition, the ols_hrd_parameters( ) syntax structure that applies to an OLS containing only a single layer may not be encoded into the VPS, and may be encoded into an SPS referred to by the layer in the OLS. 
     In addition, each ols_hrd_parameters( ) in the VPS may be referred to by at least one ols_hrd_idx[i] (i being in the range 0 to NumMultiLayerOlss−1, inclusive). 
     The image encoding apparatus may encode a picture based on the HRD parameters (S 1560 ). In this case, the HRD parameters may be ols_hrd_parameters( ) in the VPS referred to by ols_hrd_idx[i] obtained in step S 1540  or inferred in step S 1550 . Alternatively, in the case of an OLS containing only a single layer, the HRD parameters may be ols_hrd_parameters( ) in the SPS referred to by the single layer. 
       FIG.  16    is a view illustrating a process of decoding HRD parameters based on num_ols_hrd_params_minus1 according to another embodiment of the present disclosure. 
     The image decoding apparatus may obtain num_ols_hrd_params_minus1 from a VPS (S 1610 ). num_ols_hrd_params_minus1 plus 1 may specify the number of ols_hrd_parameters( ) syntax structures in the VPS, and num_ols_hrd_params_minus1 may be in the range of 0 to NumMultiLayerOlss−1, inclusive. 
     The image decoding apparatus may obtain num_ols_hrd_params_minus1+one ols_hrd_parameters( ) syntax structure from the VPS (S 1620 ). 
     The image decoding apparatus may determine the following condition 2 (S 1630 ). 
     Condition 2: (num_ols_hrd_params_minus1+1 !=NumMultiLayerOlss &amp;&amp; num_ols_hrd_params_minus1&gt;0)? 
     Condition 2 is a condition for obtaining ols_hrd_idx from a bitstream, and, when condition 2 is satisfied (S 1630 —Yes), the image decoding apparatus may obtain ols_hrd_idx from the VPS (S 1640 ). According to the embodiment described with reference to  FIG.  16   , ols_hrd_idx[i] is an index for a list of ols_hrd_parameters( ) in the VPS, and may be an index of ols_hrd_parameters( ) that applies to an OLS containing multiple i-th layers (multi-layer OLS). 
     That is, ols_hrd_idx[i] in the VPS may be signaled for the multi-layer OLSs, and may be in the range of 0 to num_ols_hrd_params_minus1, inclusive. 
     When condition 2 is not satisfied (S 1630 —No), the image decoding apparatus may infer ols_hrd_idx without obtaining ols_hrd_idx from the VPS (S 1650 ). Specifically, when num_ols_hrd_params_minus1 is equal to 0, the value of ols_hrd_idx[i] may be inferred to be equal to 0. Otherwise, when num_ols_hrd_param_minus1 plus 1 is equal to NumMultiLayerOlss, the value of ols_hrd_idx[i] may be inferred to be equal to i. 
     In addition, the ols_hrd_parameters( ) syntax structure that applies to an OLS containing only a single layer may not be obtained from the VPS and may be obtained from an SPS referred to by the layer in the OLS. 
     In addition, each ols_hrd_parameters( ) in the VPS may be referred to by at least one ols_hrd_idx[i](i being in the range of 0 to NumMultiLayerOlss−1, inclusive). 
     The image decoding apparatus may decode a picture based on the HRD parameters (S 1660 ). In this case, the HRD parameters may be ols_hrd_parameters( ) in the VPS referred to by ols_hrd_idx[i] obtained in step S 1640  or inferred in step S 1650 . Alternatively, in the case of an OLS containing only a single layer, the HRD parameters may be ols_hrd_parameters( ) in an SPS referred by the single layer. 
     According to the embodiments described with reference to  FIGS.  15  to  16   , it is possible to prevent unnecessary signaling of ols_hrd_parameters( ) syntax structure in the VPS which is not referred to, and to more accurately and efficiently perform signaling of the HRD parameters, by signaling the ols_hrd_parameters( ) syntax structure for the OLS containing the single layer only through the SPS. 
     In the method described with reference to  FIGS.  15    and  16 , some steps may be omitted or the order thereof may be changed. In addition, a step(s) which is(are) not shown in  FIGS.  17  and  18    may be added at any location. 
     While the exemplary methods of the present disclosure described above are represented as a series of operations for clarity of description, it is not intended to limit the order in which the steps are performed, and the steps may be performed simultaneously or in different order as necessary. In order to implement the method according to the present disclosure, the described steps may further include other steps, may include remaining steps except for some of the steps, or may include other additional steps except for some steps. 
     In the present disclosure, the image encoding apparatus or the image decoding apparatus that performs a predetermined operation (step) may perform an operation (step) of confirming an execution condition or situation of the corresponding operation (step). For example, if it is described that predetermined operation is performed when a predetermined condition is satisfied, the image encoding apparatus or the image decoding apparatus may perform the predetermined operation after determining whether the predetermined condition is satisfied. 
     The various embodiments of the present disclosure are not a list of all possible combinations and are intended to describe representative aspects of the present disclosure, and the matters described in the various embodiments may be applied independently or in combination of two or more. 
     Various embodiments of the present disclosure may be implemented in hardware, firmware, software, or a combination thereof. In the case of implementing the present disclosure by hardware, the present disclosure can be implemented with application specific integrated circuits (ASICs), Digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), general processors, controllers, microcontrollers, microprocessors, etc. 
     In addition, the image decoding apparatus and the image encoding apparatus, to which the embodiments of the present disclosure are applied, may be included in a multimedia broadcasting transmission and reception device, a mobile communication terminal, a home cinema video device, a digital cinema video device, a surveillance camera, a video chat device, a real time communication device such as video communication, a mobile streaming device, a storage medium, a camcorder, a video on demand (VoD) service providing device, an OTT video (over the top video) device, an Internet streaming service providing device, a three-dimensional (3D) video device, a video telephony video device, a medical video device, and the like, and may be used to process video signals or data signals. For example, the OTT video devices may include a game console, a blu-ray player, an Internet access TV, a home theater system, a smartphone, a tablet PC, a digital video recorder (DVR), or the like. 
       FIG.  17    is a view showing a content streaming system, to which an embodiment of the present disclosure is applicable. 
     As shown in  FIG.  17   , the content streaming system, to which the embodiment of the present disclosure is applied, may largely include an encoding server, a streaming server, a web server, a media storage, a user device, and a multimedia input device. 
     The encoding server compresses content input from multimedia input devices such as a smartphone, a camera, a camcorder, etc. into digital data to generate a bitstream and transmits the bitstream to the streaming server. As another example, when the multimedia input devices such as smartphones, cameras, camcorders, etc. directly generate a bitstream, the encoding server may be omitted. 
     The bitstream may be generated by an image encoding method or an image encoding apparatus, to which the embodiment of the present disclosure is applied, and the streaming server may temporarily store the bitstream in the process of transmitting or receiving the bitstream. 
     The streaming server transmits the multimedia data to the user device based on a user&#39;s request through the web server, and the web server serves as a medium for informing the user of a service. When the user requests a desired service from the web server, the web server may deliver it to a streaming server, and the streaming server may transmit multimedia data to the user. In this case, the content streaming system may include a separate control server. In this case, the control server serves to control a command/response between devices in the content streaming system. 
     The streaming server may receive content from a media storage and/or an encoding server. For example, when the content is received from the encoding server, the content may be received in real time. In this case, in order to provide a smooth streaming service, the streaming server may store the bitstream for a predetermined time. 
     Examples of the user device may include a mobile phone, a smartphone, a laptop computer, a digital broadcasting terminal, a personal digital assistant (PDA), a portable multimedia player (PMP), navigation, a slate PC, tablet PCs, ultrabooks, wearable devices (e.g., smartwatches, smart glasses, head mounted displays), digital TVs, desktops computer, digital signage, and the like. 
     Each server in the content streaming system may be operated as a distributed server, in which case data received from each server may be distributed. 
     The scope of the disclosure includes software or machine-executable commands (e.g., an operating system, an application, firmware, a program, etc.) for enabling operations according to the methods of various embodiments to be executed on an apparatus or a computer, a non-transitory computer-readable medium having such software or commands stored thereon and executable on the apparatus or the computer. 
     INDUSTRIAL APPLICABILITY 
     The embodiments of the present disclosure may be used to encode or decode an image.