Image encoding / decoding method and apparatus for signaling HRD parameters, and computer-readable recording medium storing a bitstream - Patents.com

Abstract

To provide an image encoding / decoding method and apparatus for signaling HRD parameters, and a method of transmitting a bitstream.SOLUTION: An image decoding method may comprise obtaining first information specifying the number of one or more HRD parameter syntax structures in a 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 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 is applied to a current OLS based on the second information, and processing the current OLS based on the selected HRD parameter syntax structure.SELECTED DRAWING: Figure 16

Description

[Technical field]

[0001] The present disclosure relates to an image encoding / decoding method and apparatus, and more particularly to an image encoding / decoding method and apparatus that signal HRD (Hypothetical reference decoder) related parameters, as well as a computer-readable recording medium storing a bitstream generated by the image encoding method / apparatus of the present disclosure. [Background technology]

[0002] Recently, the demand for high-resolution, high-quality images, for example, HD (High Definition) images and UHD (Ultra High Definition) images, is increasing in various fields. As the resolution and quality of image data increases, the amount of information or bits transmitted increases relatively compared to conventional image data. The increase in the amount of information or bits transmitted leads to an increase in transmission costs and storage costs.

[0003] This requires a highly efficient image compression technique for effectively transmitting, storing, and reproducing high-resolution, high-quality image information. Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present disclosure is to provide an image encoding / decoding method and apparatus with improved encoding / decoding efficiency.

[0005] Another object of the present disclosure is to provide an image encoding / decoding method and apparatus that improves the efficiency of encoding / decoding by efficiently signaling HRD parameters.

[0006] Another object of the present disclosure is to provide a method for transmitting a bitstream generated by the image encoding method or apparatus according to the present disclosure.

[0007] Another object of the present disclosure is to provide a recording medium storing a bitstream generated by the image encoding method or apparatus according to the present disclosure.

[0008] Another object of the present disclosure is to provide a recording medium storing a bitstream that is received by the image decoding device according to the present disclosure, decoded, and used to restore an image.

[0009] The technical problems to be solved by the present disclosure are not limited to the above-mentioned technical problems, and other technical problems not described above will be clearly understood by a person having ordinary skill in the art to which the present disclosure pertains from the following description. [Means for solving the problem]

[0010] An image decoding method performed by an image decoding device according to one embodiment of the present disclosure may include a step of acquiring first information indicating the number of one or more hypothetical reference decoder (HRD) parameter syntax structures in a video parameter set (VPS), acquiring the one or more HRD parameter syntax structures from the VPS based on the first information, acquiring second information regarding 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 an HRD parameter syntax structure to be applied to a current OLS based on the second information, and processing the current OLS based on the selected HRD parameter syntax structure.

[0011] 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.

[0012] 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 of the one or more multi-layer OLSs.

[0013] In the image decoding method of the present disclosure, the second information can be obtained from the VPS based on the number of the one or more HRD parameter syntax structures in the VPS being greater than 1 and the number of the one or more HRD parameter syntax structures in the VPS not being equal to the number of the one or more multi-layer OLSs.

[0014] In the image decoding method of the present disclosure, based on the number of the one or more HRD parameter syntax structures in the VPS being 1, the second information is not obtained from the VPS and the second information can be inferred to be a value of 0.

[0015] In the image decoding method of the present disclosure, based on the number of the one or more HRD parameter syntax structures in the VPS being greater than 1 and the number of the one or more HRD parameter syntax structures in the VPS being equal to the number of the one or more multi-layer OLSs, the second information is not obtained from the VPS, and the second information for the i-th multi-layer OLS can be inferred to be the value of i.

[0016] In the image decoding method of the present disclosure, based on the fact that the current OLS includes only a single layer, the HRD parameter syntax structure applied to the current OLS can be obtained from a Sequence Parameter Set (SPS).

[0017] An image decoding device according to another aspect of the present disclosure includes a memory and at least one processor, wherein the at least one processor acquires first information indicating the number of one or more hypothetical reference decoder (HRD) parameter syntax structures in a video parameter set (VPS), acquires the one or more HRD parameter syntax structures from the VPS based on the first information, acquires second information regarding mapping between one or more multi-layer output layer sets (OLS) and the one or more HRD parameter syntax structures from the VPS based on the first information, selects an HRD parameter syntax structure to be applied to a current OLS based on the second information, and processes the current OLS based on the selected HRD parameter syntax structure.

[0018] An image encoding method performed by an image encoding device according to another aspect of the present disclosure may include the steps of: encoding first information indicating 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 into the VPS based on the first information; encoding second information regarding mapping between one or more multi-layer output layer sets (OLSs) and the one or more HRD parameter syntax structures into the VPS based on the first information; and processing the current OLS based on the HRD parameter syntax structure applied to the current OLS.

[0019] In the image coding 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.

[0020] In the image coding method of the present disclosure, each of the one or more HRD parameter syntax structures in the VPS can be mapped to at least one multi-layer OLS of the one or more multi-layer OLSs.

[0021] In the image encoding method of the present disclosure, the second information can be encoded in the VPS based on the number of the one or more HRD parameter syntax structures in the VPS being greater than 1 and the number of the one or more HRD parameter syntax structures in the VPS not being equal to the number of the one or more multi-layer OLSs.

[0022] In the image encoding method of the present disclosure, based on the number of the one or more HRD parameter syntax structures in the VPS being 1, the second information is not encoded in the VPS and the second information can be inferred to be a value of 0.

[0023] In the image encoding method of the present disclosure, based on the number of the one or more HRD parameter syntax structures in the VPS being greater than 1 and the number of the one or more HRD parameter syntax structures in the VPS being equal to the number of the one or more multi-layer OLSs, the second information is not encoded in the VPS, and the second information for the i-th multi-layer OLS can be inferred to be the value of i.

[0024] In the image coding method of the present disclosure, based on the fact that the current OLS includes only a single layer, the HRD parameter syntax structure applied to the current OLS can be coded into a Sequence Parameter Set (SPS).

[0025] A transmission method according to another aspect of the present disclosure can transmit a bitstream generated by the image encoding device or image encoding method of the present disclosure.

[0026] A computer-readable recording medium according to another aspect of the present disclosure can store a bitstream generated by the image encoding method or image encoding device of the present disclosure.

[0027] The features described above in the brief summary of the present disclosure are merely exemplary embodiments of the detailed description of the present disclosure that follows and are not intended to limit the scope of the present disclosure. Effect of the Invention

[0028] According to the present disclosure, an image encoding / decoding method and apparatus with improved encoding / decoding efficiency can be provided.

[0029] Furthermore, the present disclosure can provide an image encoding / decoding method and apparatus capable of improving the efficiency of encoding / decoding by efficiently signaling HRD parameters.

[0030] According to the present disclosure, there can also be provided a method for transmitting a bitstream generated by the image encoding method or apparatus according to the present disclosure.

[0031] According to the present disclosure, a recording medium storing a bitstream generated by the image encoding method or apparatus according to the present disclosure can be provided.

[0032] In addition, the present disclosure may provide a recording medium storing a bitstream that is received by the image decoding device according to the present disclosure, decoded, and used to restore an image.

[0033] The effects obtained by the present disclosure are not limited to the effects described above, and other effects not described above will be clearly understood by those having ordinary skill in the art to which the present disclosure pertains from the following description. [Brief description of the drawings]

[0034] [Figure 1] FIG. 1 is a schematic diagram illustrating a video coding system to which an embodiment of the present disclosure can be applied. [Diagram 2] 1 is a diagram illustrating an image encoding device to which an embodiment of the present disclosure can be applied; [Diagram 3] 1 is a diagram illustrating an image decoding device to which an embodiment of the present disclosure can be applied; [Figure 4] 1 shows an example of a general picture decoding procedure to which the embodiments of the present disclosure can be applied. [Diagram 5] 1 shows an example of a general picture encoding procedure to which the embodiments of the present disclosure can be applied. [Figure 6] FIG. 2 shows an example of a hierarchical structure for coded images / video. [Figure 7] FIG. 2 is an exemplary diagram illustrating a syntax structure of a VPS according to one embodiment of the present disclosure. [Figure 8] A diagram showing a syntax structure of an SPS for signaling HRD parameters according to one embodiment of the present disclosure. [Figure 9] A diagram showing a general_hrd_parameter() syntax structure according to one embodiment of the present disclosure. [Figure 10] A diagram illustrating an ols_hrd_parameters() syntax structure according to one embodiment of the present disclosure. [Figure 11] A diagram showing a sublayer_hrd_parameters() syntax structure according to one embodiment of the present disclosure. [Figure 12] FIG. 1 is a diagram for explaining an example of an image encoding method to which an embodiment of the present disclosure can be applied. [Figure 13] FIG. 1 is a diagram for explaining an example of an image decoding method to which an embodiment of the present disclosure can be applied. [Figure 14] FIG. 11 is a diagram for explaining another example of an image decoding method to which an embodiment of the present disclosure can be applied. [Figure 15] FIG. 13 is a diagram illustrating a process of encoding HRD parameters based on num_ols_hrd_params_minus1 according to another embodiment of the present disclosure. [Figure 16]FIG. 13 is a diagram for explaining a process of decoding HRD parameters based on num_ols_hrd_params_minus1 according to another embodiment of the present disclosure. [Figure 17] FIG. 1 illustrates a content streaming system to which an embodiment of the present disclosure can be applied. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] The present disclosure will be described in detail below with reference to the accompanying drawings so that those skilled in the art can easily implement the present disclosure. However, the present disclosure may be embodied in various different forms and is not limited to the embodiments described herein.

[0036] In describing the embodiments of the present disclosure, if it is determined that a specific description of a known configuration or function may make the gist of the present disclosure unclear, the detailed description thereof will be omitted. In addition, in the drawings, parts that are not related to the description of the present disclosure are omitted, and similar parts are denoted by similar reference numerals.

[0037] In the present disclosure, when a certain component is "connected," "coupled," or "connected" to another component, this includes not only a direct connection relationship, but also an indirect connection relationship in which another component exists between them. Furthermore, when a certain component is described as "including" or "having" another component, this does not mean that the other component is excluded, but that the other component can be further included, unless otherwise specified.

[0038] In this disclosure, terms such as "first" and "second" are used only for the purpose of distinguishing one component from another component, and do not limit the order or importance of the components unless otherwise specified. Therefore, within the scope of this disclosure, a first component in one embodiment may be called a second component in another embodiment, and similarly, a second component in one embodiment may be called a first component in another embodiment.

[0039] In this disclosure, components that are distinguished from one another are used to clearly describe the characteristics of each component, and do not necessarily mean that the components are separate. In other words, multiple components may be integrated and configured as a single hardware or software unit, or one component may be distributed and configured as multiple hardware or software units. Thus, even if not otherwise stated, such integrated or distributed embodiments are also included in the scope of the present disclosure.

[0040] In the present disclosure, the components described in the various embodiments are not necessarily essential components, and some may be optional components. Therefore, an embodiment consisting of a subset of the components described in one embodiment is also included in the scope of the present disclosure. In addition, an embodiment including other components in addition to the components described in the various embodiments is also included in the scope of the present disclosure.

[0041] The present disclosure relates to image encoding and decoding, and terms used in this disclosure may have ordinary meanings in the technical field to which the present disclosure belongs, unless they are newly defined in this disclosure.

[0042] In this disclosure, a "picture" generally means a unit indicating any one image in a particular time period, 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. Also, a slice / tile may include one or more coding tree units (CTUs).

[0043] In this disclosure, a "pixel" or a "pel" may refer to the smallest unit constituting one picture (or image). A term corresponding to a pixel may be a "sample." A sample may generally indicate a pixel or a pixel value, may indicate only a pixel / pixel value of a luma component, or may indicate only a pixel / pixel value of a chroma component.

[0044] In this disclosure, a "unit" may refer to a basic unit of image processing. A unit may include at least one of a specific region of a picture and information related to the region. A unit may be used interchangeably with terms such as "sample array," "block," or "area," depending on the case. In a general case, an M×N block may include a set (or array) of samples or transform coefficients consisting of M columns and N rows.

[0045] In the present disclosure, a "current block" may refer to any one of a "current coding block," a "current coding unit," a "block to be coded," a "block to be decoded," or a "block to be processed." If prediction is performed, a "current block" may refer to a "current predicted block" or a "block to be predicted." If transformation (inverse transformation) / quantization (inverse quantization) is performed, a "current block" may refer to a "current transformed block" or a "block to be transformed." If filtering is performed, a "current block" may refer to a "block to be filtered."

[0046] Furthermore, in this disclosure, unless there is an explicit reference to a chroma block, a "current block" may refer to a block including both a luma component block and a chroma component block, or a "luma block of the current block." The luma component block of the current block may be expressed by explicitly including the reference to a luma component block, such as a "luma block" or a "current luma block." Furthermore, the chroma component block of the current block may be expressed by explicitly including the reference to a chroma component block, such as a "chroma block" or a "current chroma block."

[0047] In the present disclosure, "A or B" can mean "A only," "B only," or "both A and B." In other words, in the present disclosure, "A or B" can be interpreted as "A and / or B." For example, in the present disclosure, "A, B or C" can mean "A only," "B only," "C only," or "any combination of A, B and C."

[0048] As used in this disclosure, " / " and ", (comma)" can mean "and / or." For example, "A / B" can mean "A and / or B." Thus, "A / B" can mean "A only," "B only," or "both A and B." For example, "A, B, C" can mean "A, B, or C."

[0049] In the present disclosure, "at least one of A and B" can mean "A only," "B only," or "both A and B." In addition, in the present disclosure, the expressions "at least one of A or B" and "at least one of A and / or B" can be interpreted as being the same as "at least one of A and B."

[0050] In addition, in this disclosure, "at least one of A, B and C" can mean "A only," "B only," "C only," or "any combination of A, B and C." Furthermore, "at least one of A, B or C" and "at least one of A, B and / or C" can mean "at least one of A, B and C."

[0051] Furthermore, parentheses used in the present disclosure may mean "for example." Specifically, when "prediction (intra prediction)" is displayed, "intra prediction" may be proposed as an example of "prediction." In other words, "prediction" in the present disclosure is not limited to "intra prediction," and "intra prediction" may be proposed as an example of "prediction." Furthermore, even when "prediction (i.e., intra prediction)" is displayed, "intra prediction" may be proposed as an example of "prediction."

[0052] In the present disclosure, technical features described separately in one drawing may be realized separately or simultaneously.

[0053] Video Coding System Overview

[0054] FIG. 1 illustrates a video coding system according to this disclosure.

[0055] A video coding system according to an embodiment may include an encoding device 10 and a decoding device 20. The encoding device 10 may transmit encoded video and / or image information or data to the decoding device 20 in a file or streaming format via a digital storage medium or a network.

[0056] The encoding device 10 according to an embodiment may include a video source generating unit 11, an encoding unit 12, and a transmitting unit 13. The decoding device 20 according to an embodiment may include a receiving unit 21, a decoding unit 22, and a rendering unit 23. The encoding unit 12 may be referred to as a video / image encoding unit, and the decoding unit 22 may be referred to as a video / image decoding unit. The transmitting unit 13 may be included in the encoding unit 12. The receiving unit 21 may be included in the decoding unit 22. The rendering unit 23 may include a display unit, which may be configured as a separate device or an external component.

[0057] The video source generating unit 11 can obtain videos / images through a process of capturing, synthesizing, or generating videos / images. The video source generating unit 11 can include a video / image capture device and / or a video / image generation device. The video / image capture device can include, for example, one or more cameras, a video / image archive containing previously captured videos / images, etc. The video / image generation device can include, for example, a computer, a tablet, a smartphone, etc., and can (electronically) generate videos / images. For example, a virtual video / image can be generated through a computer, etc., in which case the video / image capture process can be replaced by a process in which related data is generated.

[0058] The encoder 12 may encode the input video / image. The encoder 12 may perform a series of steps such as prediction, transformation, quantization, etc. for compression and encoding efficiency. The encoder 12 may output the encoded data (encoded video / image information) in a bitstream format.

[0059] The transmitting unit 13 may transmit the encoded video / image information or data output in a bitstream format to the receiving unit 21 of the decoding device 20 in a file or streaming format via a digital storage medium or a network. The digital storage medium may include various storage media such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, etc. The transmitting unit 13 may include elements for generating a media file through a predetermined file format and may include elements for transmitting via a broadcasting / communication network. The receiving unit 21 may extract / receive the bitstream from the storage medium or network and transmit it to the decoding unit 22.

[0060] The decoding unit 22 can decode the video / image by performing a series of steps such as inverse quantization, inverse transformation, and prediction corresponding to the operations of the encoding unit 12.

[0061] The rendering unit 23 can render the decoded video / images. The rendered video / images can be displayed via a display unit.

[0062] Overview of the image encoding device

[0063] FIG. 2 is a diagram illustrating an image encoding device to which an embodiment of the present disclosure can be applied.

[0064] As shown in Fig. 2, the image coding device 100 may include an image division unit 110, a subtraction unit 115, a transformation unit 120, a quantization unit 130, an inverse quantization unit 140, an inverse transformation unit 150, an addition unit 155, a filtering unit 160, a memory 170, an inter prediction unit 180, an intra prediction unit 185, and an entropy coding unit 190. The inter prediction unit 180 and the intra prediction unit 185 may be collectively referred to as a "prediction unit." The transformation unit 120, the quantization unit 130, the inverse quantization unit 140, and the inverse transformation unit 150 may be included in a residual processing unit. The residual processing unit may further include a subtraction unit 115.

[0065] All or at least some of the components constituting the image encoding device 100 may be realized by a single hardware component (e.g., an encoder or a processor) depending on the embodiment. Also, the memory 170 may include a decoded picture buffer (DPB) and may be realized by a digital storage medium.

[0066] The image division unit 110 may divide an input image (or picture, frame) input to the image encoding device 100 into one or more processing units. As an example, the processing units may be called coding units (CUs). The coding units may be obtained by recursively dividing a coding tree unit (CTU) or a largest coding unit (LCU) according to a QT / BT / TT (Quad-tree / Binary-tree / Ternal-tree) structure. For example, one coding unit may be divided into a plurality of coding units of a deeper depth based on a quad-tree structure, a binary-tree structure, and / or a ternary-tree structure. For dividing the coding units, a quad-tree structure may be applied first, and a binary-tree structure and / or a ternary-tree structure may be applied later. A coding procedure according to the present disclosure may be performed based on a final coding unit that is not further divided. The maximum coding unit may be used as the final coding unit, and a lower depth coding unit obtained by dividing the maximum coding unit may be used as the final coding unit. Here, the coding procedure may include procedures such as prediction, transformation, and / or restoration, which will be described later. As another example, a 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 divided or partitioned from the final coding unit, respectively. The prediction unit may be a unit of sample prediction, and the transform unit may be a unit for deriving transform coefficients and / or a unit for deriving a residual signal from the transform coefficients.

[0067] The prediction unit (inter prediction unit 180 or intra prediction unit 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 prediction unit may determine whether intra prediction or inter prediction is applied in units of a current block or a CU. The prediction unit may generate various information related to prediction of the current block and transmit it to the entropy encoding unit 190. The information related to prediction may be encoded by the entropy encoding unit 190 and output in a bitstream format.

[0068] The intra prediction unit 185 may predict the current block by referring to samples in the current picture. The referenced samples may be located in the neighborhood of the current block or may be located away from the current block according to an intra prediction mode and / or an intra prediction technique. The intra prediction mode 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 depending on the fineness of the prediction direction. However, this is merely an example, and more or less directional prediction modes may be used depending on the setting. The intra prediction unit 185 may also determine a prediction mode to be applied to the current block using prediction modes applied to neighboring blocks.

[0069] The inter prediction unit 180 may derive a predicted block for a current block based on a reference block (reference sample array) identified 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 a block, a sub-block, or a sample based on the correlation of motion information between a neighboring block and a 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 from each other. The temporal neighboring block may be called a collocated reference block, a collocated CU (colCU), etc. The reference picture including the temporal neighboring block may be called a collocated picture (colPic). For example, the inter prediction unit 180 may generate information indicating which candidate is used to derive a motion vector and / or a reference picture index of the current block by forming a motion information candidate list based on neighboring blocks. Inter prediction may be performed based on various prediction modes, and for example, in the case of a skip mode and a merge mode, the inter prediction unit 180 may use motion information of neighboring blocks as motion information of the current block. In the case of the skip mode, unlike the merge mode, a residual signal may not be transmitted.In the case of a motion vector prediction (MVP) mode, the motion vector of the current block can be signaled by using the motion vector of a neighboring block as a motion vector predictor and encoding a motion vector difference and an indicator for the motion vector predictor. The motion vector difference can mean the difference between the motion vector of the current block and the motion vector predictor.

[0070] The prediction unit may generate a prediction signal based on various prediction methods and / or prediction techniques, which will be described later. For example, the prediction unit may apply intra prediction or inter prediction for prediction of the current block, and may simultaneously apply intra prediction and inter prediction. A prediction method that simultaneously applies intra prediction and inter prediction for prediction of the current block may be called combined inter and intra prediction (CIIP). The prediction unit may also perform intra block copy (IBC) for prediction of the current block. Intra block copy can be used for content image / video coding such as games, for example, as in screen content coding (SCC). IBC is a method of predicting a current block using an already restored reference block in a current picture that is located a predetermined distance away from the current block. When IBC is applied, the position of the reference block in the current picture may be coded as a vector (block vector) corresponding to the predetermined distance. IBC is basically performed in the current picture, but may be performed similarly to inter prediction in that a reference block is derived in the current picture. That is, the IBC may use at least one of the inter prediction techniques described in this disclosure.

[0071] The prediction signal generated by the prediction unit may be used to generate a restored signal or a residual signal. The subtraction unit 115 may subtract the prediction signal (predicted block, prediction sample array) output from the prediction unit from the input image signal (original block, original sample array) to generate a residual signal (residual block, residual sample array). The generated residual signal may be transmitted to the conversion unit 120.

[0072] The transform unit 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-Loeve transform (KLT), a graph-based transform (GBT), or a conditionally non-linear transform (CNT). Here, the GBT refers to a transform obtained from a graph when the relationship information between pixels is represented by a graph. The CNT refers to a transform obtained based on a predicted signal generated using all previously reconstructed pixels. The transform process may be applied to pixel blocks having the same square size, or may be applied to non-square, variable-sized blocks.

[0073] The quantization unit 130 may quantize the transform coefficients and transmit the quantized transform coefficients to the entropy coding unit 190. The entropy coding unit 190 may code the quantized signal (information on the quantized transform coefficients) and output the coded signal in a bitstream format. The information on the quantized transform coefficients may be referred to as residual information. The quantization unit 130 may rearrange the quantized transform coefficients in a block format into a one-dimensional vector format based on a coefficient scan order, and may generate information on the quantized transform coefficients based on the quantized transform coefficients in the one-dimensional vector format.

[0074] The entropy coding unit 190 may perform various coding methods, such as exponential Golomb, context-adaptive variable length coding (CAVLC), and context-adaptive binary arithmetic coding (CABAC). The entropy coding unit 190 may also code information required for video / image restoration (e.g., values ​​of syntax elements, etc.) together or separately in addition to the quantized transform coefficients. The coded information (e.g., coded video / image information) may be transmitted or stored in a network abstraction layer (NAL) unit unit in a bitstream format. 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). The video / image information may further include general constraint information. The signaling information, transmitted information and / or syntax elements referred to in this disclosure may be encoded through the above-mentioned encoding procedures and included in the bitstream.

[0075] The bitstream may be transmitted via a network or may be stored in a digital storage medium. Here, the network may include a broadcasting network and / or a communication network, and the digital storage medium may include various storage media such as a USB, SD, CD, DVD, Blu-ray, HDD, SSD, etc. A transmitting unit (not shown) for transmitting the signal output from the entropy encoding unit 190 and / or a storing unit (not shown) for storing the signal may be provided as an internal / external element of the image encoding device 100, or the transmitting unit may be provided as a component of the entropy encoding unit 190.

[0076] The quantized transform coefficients output from the quantization unit 130 can be used to generate a residual signal. For example, the quantized transform coefficients are subjected to inverse quantization and inverse transformation via the inverse quantization unit 140 and the inverse transformation unit 150, so that a residual signal (residual block or residual sample) can be restored.

[0077] The adder 155 may generate a reconstructed signal (reconstructed picture, reconstructed block, reconstructed sample array) by adding the reconstructed residual signal to a prediction signal output from the inter prediction unit 180 or the intra prediction unit 185. When there is no residual for the current block to be processed, such as when a skip mode is applied, a predicted block may be used as a reconstructed block. The adder 155 may be referred to as a reconstruction unit or a reconstructed block generation unit. The generated reconstructed signal may be used for intra prediction of the next current block to be processed in the current picture, and may also be used for inter prediction of the next picture after filtering as described below.

[0078] The filtering unit 160 may apply filtering to the reconstructed signal to improve subjective / objective image quality. For example, the filtering unit 160 may apply various filtering methods to the reconstructed picture to generate a modified reconstructed picture, and may store the modified reconstructed picture in the memory 170, specifically, in the DPB of the memory 170. The various filtering methods may include, for example, deblocking filtering, sample adaptive offset, an adaptive loop filter, a bilateral filter, and the like. The filtering unit 160 may generate various information related to filtering, as will be described later in the description of each filtering method, and transmit the information to the entropy coding unit 190. The information related to filtering may be coded by the entropy coding unit 190 and output in a bitstream format.

[0079] The modified reconstructed picture transmitted to the memory 170 may be used as a reference picture in the inter prediction unit 180. When inter prediction is applied through this, the image encoding device 100 may avoid a prediction mismatch between the image encoding device 100 and the image decoding device, and may also improve encoding efficiency.

[0080] The DPB in the memory 170 may store modified reconstructed pictures to be used as reference pictures in the inter prediction unit 180. The memory 170 may store motion information of blocks from which motion information in the current picture is derived (or coded) and / or motion information of already reconstructed intra-picture blocks. The stored motion information may be transmitted to the inter prediction unit 180 to be used as motion information of spatial surrounding blocks or motion information of temporal surrounding blocks. The memory 170 may store reconstructed samples of reconstructed blocks in the current picture and transmit them to the intra prediction unit 185.

[0081] Overview of the image decoding device

[0082] FIG. 3 is a diagram illustrating an image decoding device to which an embodiment of the present disclosure can be applied.

[0083] 3, the image decoding device 200 may include an entropy decoding unit 210, an inverse quantization unit 220, an inverse transform unit 230, an adder 235, a filtering unit 240, a memory 250, an inter prediction unit 260, and an intra prediction unit 265. The inter prediction unit 260 and the intra prediction unit 265 may be collectively referred to as a "prediction unit." The inverse quantization unit 220 and the inverse transform unit 230 may be included in a residual processing unit.

[0084] All or at least some of the components constituting the image decoding device 200 may be realized by one hardware component (e.g., a decoder or a processor) depending on the embodiment. Also, the memory 170 may include a DPB and may be realized by a digital storage medium.

[0085] The image decoding device 200, which receives a bitstream including video / image information, can reconstruct an image by executing a process corresponding to the process performed by the image encoding device 100 of Fig. 2. For example, the image decoding device 200 can perform decoding using a processing unit applied in the image encoding device. Thus, the processing unit for decoding can be, for example, a coding unit. The coding unit can be obtained by dividing a coding tree unit or a maximum coding unit. Then, the reconstructed image signal decoded and output by the image decoding device 200 can be reproduced by a reproduction device (not shown).

[0086] 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 via the entropy decoding unit 210. For example, the entropy decoding unit 210 may derive information (e.g., video / image information) required for image restoration (or picture restoration) by parsing the 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 image decoding apparatus may further use information on the parameter set and / or the general constraint information to decode an image. The signaling information, received information, and / or syntax elements referred to in the present disclosure may be obtained from the bitstream by being decoded via the decoding procedure. For example, the entropy decoding unit 210 may decode information in a bitstream based on a coding method such as exponential Golomb coding, CAVLC, or CABAC, and output values ​​of syntax elements required for image restoration and quantized values ​​of transform coefficients related to the residual. More specifically, the CABAC entropy decoding method may receive bins corresponding to each syntax element from the bitstream, determine a context model using syntax element information to be decoded and decoded information of neighboring blocks and a block to be decoded, or information of a symbol / bin decoded in a previous step, predict the occurrence probability of the bin based on the determined context model, and perform arithmetic decoding of the bin to generate a symbol corresponding to the value of each syntax element. In this case, the CABAC entropy decoding method may update the context model using information of the decoded symbol / bin for the context model of the next symbol / bin after determining the context model.Among the information decoded by the entropy decoding unit 210, information related to prediction is provided to a prediction unit (inter prediction unit 260 and intra prediction unit 265), and residual values ​​entropy-decoded by the entropy decoding unit 210, i.e., quantized transform coefficients and related parameter information, may be input to the inverse quantization unit 220. Also, among the information decoded by the entropy decoding unit 210, information related to filtering may be provided to the filtering unit 240. Meanwhile, a receiving unit (not shown) for receiving a signal output from the image encoding device may be further provided as an internal / external element of the image decoding device 200, or the receiving unit may be provided as a component of the entropy decoding unit 210.

[0087] Meanwhile, the image decoding device according to the present disclosure may be called a video / image / picture decoding device. The image decoding device may include an information decoder (video / image / picture information decoder) and / or a sample decoder (video / image / picture sample decoder). The information decoder may include an entropy decoding unit 210, and the sample decoder may include at least one of an inverse quantization unit 220, an inverse transform unit 230, an adder 235, a filtering unit 240, a memory 250, an inter prediction unit 260, and an intra prediction unit 265.

[0088] The inverse quantization unit 220 may inverse quantize the quantized transform coefficients to output transform coefficients. The inverse quantization unit 220 may rearrange the quantized transform coefficients in a two-dimensional block format. In this case, the rearrangement may be performed based on a coefficient scan order performed in the image encoding device. The inverse quantization unit 220 may perform inverse quantization on the quantized transform coefficients using a quantization parameter (e.g., quantization step size information) to obtain transform coefficients.

[0089] The inverse transform unit 230 can inversely transform the transform coefficients to obtain a residual signal (residual block, residual sample array).

[0090] The prediction unit may perform prediction on a current block and generate a predicted block including a prediction sample for the current block. The prediction unit may determine whether intra prediction or inter prediction is applied to the current block based on the prediction information output from the entropy decoding unit 210, and may determine a specific intra / inter prediction mode (prediction technique).

[0091] The prediction unit can generate a prediction signal based on various prediction methods (techniques) described below, as has been described in the explanation of the prediction unit of the image encoding device 100.

[0092] The intra prediction unit 265 may predict the current block by referring to samples in the current picture. The description of the intra prediction unit 185 may be similarly applied to the intra prediction unit 265.

[0093] The inter prediction unit 260 may derive a predicted block for the current block based on a reference block (reference sample array) identified 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, sub-blocks, or samples based on correlation of motion information between neighboring blocks 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 blocks may include spatial neighboring blocks present in the current picture and temporal neighboring blocks present in the reference picture. For example, the inter prediction unit 260 may configure a motion information candidate list based on the neighboring blocks, and derive a motion vector and / or a reference picture index for the current block based on the received candidate selection information. Inter prediction may be performed based on various prediction modes (techniques), and the prediction information may include information indicating a mode (technique) of inter prediction for the current block.

[0094] The adder 235 may generate a reconstructed signal (reconstructed picture, reconstructed block, reconstructed sample array) by adding the acquired residual signal to a prediction signal (predicted block, predicted sample array) output from a prediction unit (including the inter prediction unit 260 and / or the intra prediction unit 265). When there is no residual for the current block, such as when a skip mode is applied, the predicted block can be used as the reconstructed block. The description of the adder 155 may also be applied to the adder 235. The adder 235 may also be referred to as a reconstruction unit or a reconstructed block generator. The generated reconstructed signal may be used for intra prediction of the next current block in the current picture, and may also be used for inter prediction of the next picture via filtering, as described below.

[0095] The filtering unit 240 may apply filtering to the reconstructed signal to improve subjective / objective image quality. For example, the filtering unit 240 may apply various filtering methods to the reconstructed picture to generate a modified reconstructed picture, and may store the modified reconstructed picture in the memory 250, specifically, in the DPB of the memory 250. The various filtering methods may include, for example, deblocking filtering, sample adaptive offset, an adaptive loop filter, a bilateral filter, etc.

[0096] The (modified) reconstructed picture stored in the DPB of the memory 250 may be used as a reference picture in the inter prediction unit 260. The memory 250 may store motion information of a block from which motion information in the current picture is derived (or decoded) and / or motion information of a block in an already reconstructed picture. The stored motion information may be transmitted to the inter prediction unit 260 to be used as motion information of a spatial surrounding block or motion information of a temporal surrounding block. The memory 250 may store a reconstructed sample of a reconstructed block in the current picture and transmit it to the intra prediction unit 265.

[0097] In this specification, the embodiments described for the filtering unit 160, inter prediction unit 180 and intra prediction unit 185 of the image encoding device 100 can also be applied in a similar or corresponding manner to the filtering unit 240, inter prediction unit 260 and intra prediction unit 265 of the image decoding device 200, respectively.

[0098] General image / video coding procedures

[0099] In image / video coding, pictures constituting an image / video can be coded / decoded according to a series of decoding orders. A picture order corresponding to an output order of decoded pictures can be set to be different from the decoding order. Based on this, not only forward prediction but also backward prediction can be performed during inter prediction.

[0100] FIG. 4 shows an example of a general picture decoding procedure to which the embodiments of the present disclosure can be applied.

[0101] Each procedure shown in Fig. 4 may be performed by the image encoding device of Fig. 3. For example, step S410 may be performed by the entropy decoding unit 210, step S420 may be performed by the prediction unit including the intra prediction unit 265 and the inter prediction unit 260, step S430 may be performed by the residual processing unit including the inverse quantization unit 220 and the inverse transform unit 230, step S440 may be performed by the addition unit 235, and step S450 may be performed by the filtering unit 240. Step S410 may include the information decoding procedure described in this disclosure, step S420 may include the inter / intra prediction procedure described in this disclosure, step S430 may include the residual processing procedure described in this disclosure, step S440 may include the block / picture reconstruction procedure described in this disclosure, and step S450 may include the in-loop filtering procedure described in this disclosure.

[0102] Referring to FIG. 4, the picture decoding procedure may include, as shown in the description of FIG. 3, an image / video information acquisition procedure (S410) from a bitstream (by decoding), a picture reconstruction procedure (S420 to S440), and an in-loop filtering procedure (S450) for the reconstructed picture. The picture reconstruction procedure may be performed based on a prediction sample and a residual sample obtained through the inter / intra prediction (S420) and residual processing (S430, inverse quantization and inverse transform for quantized transform coefficients) processes described in the present disclosure. A modified reconstructed picture may be generated through an in-loop filtering procedure for the reconstructed picture generated by the picture reconstruction procedure, and the modified reconstructed picture may be output as a decoded picture or may be stored in a decoded picture buffer or memory 250 of the decoding device and used as a reference picture in an inter prediction procedure when decoding a subsequent picture. In some cases, the in-loop filtering procedure may be omitted, in which case the reconstructed picture may be output as a decoded picture, or may be stored in a decoded picture buffer or memory 250 of the decoding device and used as a reference picture in an inter-prediction procedure when decoding a subsequent picture. The in-loop filtering procedure (S450) 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, and some or all of them 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 the bi-lateral filter procedure may be applied sequentially, or all of them may be applied sequentially. For example, the SAO procedure may be performed after the deblocking filtering procedure is applied to the reconstructed picture.Or, for example, the ALF procedure can be performed after a deblocking filtering procedure is applied to the reconstructed picture, which can be done in the encoding device as well.

[0103] FIG. 5 shows an example of a schematic picture encoding procedure to which the embodiments of the present disclosure can be applied.

[0104] Each procedure shown in Fig. 5 may be performed by the image encoding device of Fig. 2. For example, step S510 may be performed in a prediction unit including an intra prediction unit 185 or an inter prediction unit 180, step S520 may be performed in a residual processing unit including a transform unit 120 and / or a quantization unit 130, and step S530 may be performed in an entropy encoding unit 190. Step S510 may include an inter / intra prediction procedure described in this disclosure, step S520 may include a residual processing procedure described in this disclosure, and step S530 may include an information encoding procedure described in this disclosure.

[0105] Referring to FIG. 5, the picture encoding procedure may include not only a procedure of encoding information for picture reconstruction (e.g., prediction information, residual information, partitioning information, etc.) and outputting it in a bitstream format as shown in the description of FIG. 2, but also a procedure of generating a reconstructed picture for a current picture and a procedure of applying in-loop filtering to the reconstructed picture (optional). The encoding apparatus may derive a (modified) residual sample from the quantized transform coefficient through the inverse quantization unit 140 and the inverse transform unit 150, and may generate a reconstructed picture based on the prediction sample output from step S510 and the (modified) residual sample. The reconstructed picture generated in this manner may be the same as the reconstructed picture generated in the above-mentioned decoding apparatus. A modified reconstructed picture may be generated through an in-loop filtering procedure on the reconstructed picture, which may be stored in the decoded picture buffer or memory 170, and may be used as a reference picture in an inter prediction procedure when encoding a subsequent picture, as in the case of 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 (parameters) can be coded in the entropy coding unit 190 and output in bitstream format, and the decoding device can perform the in-loop filtering procedure in the same manner as the coding device based on the filtering related information.

[0106] Through such an in-loop filtering procedure, noises generated during image / video coding, such as blocking artifacts and ringing artifacts, can be reduced, and subjective / objective visual quality can be improved. In addition, by performing the in-loop filtering procedure in both the encoding device and the decoding device, the encoding device and the decoding device can derive the same prediction result, thereby improving the reliability of picture coding and reducing the amount of data to be transmitted for picture coding.

[0107] As described above, a picture reconstruction procedure may be performed not only in a decoding apparatus but also in an encoding apparatus. A reconstruction block may be generated based on intra prediction / inter prediction for each block, and a reconstruction picture including the reconstruction block may be generated. If 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 only on intra prediction. Meanwhile, if a 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. Color components of a picture may include luma components and chroma components, and unless expressly limited in this disclosure, the methods and embodiments proposed in this disclosure may be applied to luma components and chroma components.

[0108] Example of coding hierarchy and structure

[0109] Video / images coded according to this disclosure may be processed, for example, according to the coding hierarchy and structure described below.

[0110] FIG. 6 shows an example of a hierarchical structure for coded images / video.

[0111] The coded image / video can be divided into a VCL (video coding layer), which handles the image / video decoding process and itself, a subsystem that transmits and stores the coded information, and a NAL (network abstraction layer), which exists between the VCL and the subsystem and is responsible for network adaptation functions.

[0112] In VCL, it is possible to generate VCL data including compressed image data (slice data), or to generate parameter sets including information such as a Picture Parameter Set (PPS), a Sequence Parameter Set (SPS), and a Video Parameter Set (VPS), or a Supplemental Enhancement Information (SEI) message that is additionally required for image decoding processing.

[0113] In NAL, NAL units can be generated by adding header information (NAL unit header) to RBSP (Raw Byte Sequence Payload) generated in VCL. In this case, RBSP refers to slice data, parameter set, SEI message, etc. generated in VCL. NAL unit header can include NAL unit type information identified by RBSP data included in the corresponding NAL unit.

[0114] As shown in Fig. 6, NAL units can be classified into VCL NAL units and non-VCL NAL units according to the type of RBSP generated in the VCL. The VCL NAL unit can refer to a NAL unit that includes information (slice data) on an image, and the non-VCL NAL unit can refer to a NAL unit that includes information (parameter set or SEI message) required for decoding an image.

[0115] The above-mentioned VCL NAL unit and non-VCL NAL unit can be transmitted over a network with header information according to the data standard of the lower system. For example, the NAL unit can be transformed into a data format of a predetermined standard such as H.266 / VVC file format, RTP (Real-time Transport Protocol), TS (Transport Stream), etc., and transmitted over various networks.

[0116] As described above, the NAL unit type of an NAL unit can be identified according to an RBSP data structure included in the NAL unit, and information on the NAL unit type can be stored and signaled in a NAL unit header. For example, NAL units can be largely classified into VCL NAL unit types and Non-VCL NAL unit types depending on whether the NAL unit includes information on an image (slice data). The VCL NAL unit types can be classified according to the nature and type of a picture included in the VCL NAL unit, and the Non-VCL NAL unit types can be classified according to the type of parameter set.

[0117] Below, examples of NAL unit types identified by the types of parameter sets / information contained in the Non-VCL NAL unit types are listed.

[0118] -DCI (Decoding capability information) NAL unit type (NUT): Type for NAL units containing DCI

[0119] -VPS (Video Parameter Set) NUT: Type for NAL units containing VPS

[0120] -SPS (Sequence Parameter Set) NUT: Type for NAL units containing SPS

[0121] -PPS (Picture Parameter Set) NUT: Type for NAL units containing PPS

[0122] -APS (Adaptation Parameter Set) NUT: Type for NAL units including APS

[0123] -PH(Picture header) NUT: Type for NUL units containing picture headers

[0124] The above-mentioned NAL unit type has syntax information for the NAL unit type, and the syntax information can be stored in a NAL unit header and signaled. For example, the syntax information is nal_unit_type, and the NAL unit type can be specified using the value of nal_unit_type.

[0125] Meanwhile, 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 for 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 pictures. The slice header (slice header syntax) may include information / parameters commonly applicable to the slices. 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 include information / parameters commonly applicable to multi-layers. The DCI may include information / parameters related to decoding capability.

[0126] In the present disclosure, the 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, and the slice header syntax. In addition, in the present disclosure, the low level syntax (LLS) may include, for example, a slice data syntax, a CTU syntax, a coding unit syntax, a transform unit syntax, and the like.

[0127] Meanwhile, in the present disclosure, the image / video information encoded from the encoding device to the decoding device and signaled in a bitstream format may include not only partitioning-related information within a picture, intra / inter prediction information, residual information, in-loop filtering information, etc., but also the slice header information, the picture header information, the APS information, the PPS information, the SPS information, the VPS information, and / or the DCI information. In addition, the image / video information may further include general constraint information and / or NAL unit header information.

[0128] High level syntax signaling and semantics

[0129] As mentioned above, the 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.

[0130] Video Parameter Set signaling

[0131] A video parameter set (VPS) is a parameter set used for transmitting 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, information on a relationship between an OLS and a DPB, etc. The VPS may not be essential for decoding a bitstream.

[0132] Before it can be referenced, the VPS raw byte sequence payload (RBSP) must be available to the decoding process, either by being included in at least one Access Unit (AU) with TemporalID equal to 0, or by being provided via external means.

[0133] All VPS NAL units with a particular value of vps_video_parameter_set_id in a coded video sequence (CVS) MUST have the same content.

[0134] FIG. 7 is a diagram illustrating an exemplary syntax structure of a VPS according to one embodiment of the present disclosure.

[0135] The syntax structure of the VPS shown in FIG. 7 includes only syntax elements relevant to the present disclosure, and various other syntax elements not shown in FIG. 7 may be included in the VPS.

[0136] In the example shown in Figure 7, vps_video_parameter_set_id provides an identifier for the VPS. Other syntax elements can reference the VPS using vps_video_parameter_set_id. The value of vps_video_parameter_set_id must be greater than 0.

[0137] The value of vps_max_layers_minus1 plus 1 may indicate the maximum number of layers allowable within each CVS that references the VPS.

[0138] The value of vps_max_sublayers_minus1 plus 1 may indicate the maximum number of temporal sublayers that may exist in a hierarchy in each CVS that references the VPS. vps_max_sublayers_minus1 may have a value of 0 to 6.

[0139] vps_all_layers_same_num_sublayers_flag may be signaled if vps_max_layers_minus1 is greater than 0 and vps_max_sublayers_minus1 is greater than 0. A first value (e.g., 1) of vps_all_layers_same_num_sublayers_flag may indicate that the number of temporal sublayers is the same for all layers in each CVS that references the VPS. A second value (e.g., 0) of vps_all_layers_same_num_sublayers_flag may indicate that layers in each CVS that references the VPS may not have the same number of temporal sublayers. When vps_all_layers_same_num_sublayers_flag is not present, its value may be inferred to be the first value (e.g., 1).

[0140] vps_all_independent_layers_flag may be signaled if vps_max_layers_minus1 is greater than 0. A first value (e.g., 1) of vps_all_independent_layers_flag may indicate that all layers in the CVS are coded independently without using inter-layer prediction. A second value (e.g., 0) of vps_all_independent_layers_flag may indicate that one or more layers in the CVS may use inter-layer prediction. If vps_all_independent_layers_flag is not present, its value may be inferred to be the first value (e.g., 1).

[0141] Each_layer_is_an_ols_flag may be signaled when vps_max_layers_minus1 is greater than 0. Also, each_layer_is_an_ols_flag may be signaled when vps_all_independent_layers_flag is a first value. A first value (e.g., 1) of each_layer_is_an_ols_flag may indicate whether each OLS includes only one stratum. Also, a first value (e.g., 1) of each_layer_is_an_ols_flag may indicate that each stratum in a CVS that references a VPS is itself an OLS (i.e., the one stratum included in the OLS is the only output layer). Also, a second value (e.g., 0) of each_layer_is_an_ols_flag may indicate that at least one OLS can include more than one stratum. If vps_max_layers_minus1 is 0, then the value of each_layer_is_an_ols_flag can be inferred to be 1. Otherwise, if vps_all_independent_layers_flag is 0, then the value of each_layer_is_an_ols_flag can be inferred to be 0.

[0142] If each_layer_is_an_ols_flag is a second value (e.g., 0) and vps_all_independent_layers_flag is a second value (e.g., 0), ols_mode_idc may be signaled.

[0143] An ols_mode_idc having a first value (e.g., 0) may indicate that the total number of OLSs identified by the VPS is equal to vps_max_layers_minus1+1. In this case, the i-th OLS may include layers with layer indexes from 0 to i. Also, for each OLS, only the layer with the highest layer index (top layer) within the OLS may be output.

[0144] An ols_mode_idc having a second value (e.g., 1) may indicate that the total number of OLSs identified by the VPS is equal to vps_max_layers_minus1+1. In this case, the i-th OLS may include layers with layer indexes from 0 to i. Furthermore, for each OLS, all layers within the OLS may be output.

[0145] An ols_mode_idc with a third value (e.g., 2) can indicate that the total number of OLSs specified by the VPS is explicitly signaled. In addition, for each OLS, the output layer is explicitly signaled. The other layer that is not the output layer can be a direct reference layer or an indirect reference layer of the OLS output layer.

[0146] If vps_all_independent_layers_flag is 1 and each_layer_is_an_ols_flag is 0, the value of ols_mode_idc can be inferred to be a third value (e.g., 2).

[0147] If ols_mode_idc is 2, num_output_layer_sets_minus1 and ols_output_layer_flag[i][j] can be explicitly signaled.

[0148] The value of num_output_layer_sets_minus1 plus 1 can indicate the total number of OLSs specified by the VPS.

[0149] When ols_mode_idc is 2, ols_output_layer_flag[i][j] may indicate whether the jth stratum of the ith OLS is an output layer. A first value (e.g., 1) of ols_output_layer_flag[i][j] may indicate that the stratum having the same stratum identifier (nuh_layer_id) as vps_layer_id[j] is the output layer of the ith OLS. A second value (e.g., 0) of ols_output_layer_flag[i][j] may indicate that the stratum having the same stratum identifier (nuh_layer_id) as vps_layer_id[j] is not the output layer of the ith OLS.

[0150] The HRD parameters signaled by the VPS are described below.

[0151] If each_layer_is_an_ols_flag is a second value (e.g., 0), then vps_general_hrd_params_present_flag may be signaled. A first value (e.g., 1) of vps_general_hrd_params_present_flag may indicate that HRD parameters different from the general_hrd_parameters() syntax structure are present in the VPS. A second value (e.g., 0) of vps_general_hrd_params_present_flag may indicate that HRD parameters different from the general_hrd_parameters() syntax structure are not present in the VPS. If vps_general_hrd_params_present_flag is not present, then its value may be inferred to be the second value (e.g., 0).

[0152] If the i-th OLS contains one stratum (NumLayersInOls[i] is equal to 1), the general_hrd_parameters() syntax structures that apply to the i-th OLS may be present in a Sequence Parameter Set (SPS) referenced by a stratum in the i-th OLS.

[0153] vps_sublayer_cpb_params_present_flag may be signaled if vps_max_sublayers_minus1 is greater than 0. A first value (e.g., 1) of vps_sublayer_cpb_params_present_flag may indicate that the i-th ols_hrd_parameters() syntax structure in the VPS includes HRD parameters for a sublayer whose temporal layer identifier (TemporalId) is 0 to hrd_max_tid[i]. A second value (e.g., 0) of vps_sublayer_cpb_params_present_flag may indicate that the i-th ols_hrd_parameters() syntax structure in the VPS includes HRD parameters for a sublayer whose temporal layer identifier (TemporalId) is only hrd_max_tid[i]. If vps_max_sublayers_minus1 is 0, then vps_sublayer_cpb_params_present_flag can be inferred to a second value (e.g., 0).

[0154] When vps_sublayer_cpb_params_present_flag is a second value (e.g., 0), it can be inferred that the HRD parameters for sublayers whose temporal layer identifier (TemporalId) is 0 to hrd_max_tid[i]-1 are identical to the HRD parameters for sublayers whose temporal layer identifier (TemporalId) is hrd_max_tid[i].

[0155] The value of num_ols_hrd_params_minus1 plus 1 may indicate the number of ols_hrd_parameters() syntax structures in the VPS. num_ols_hrd_params_minus1 may have a value from 0 to TotalNumOlss-1. TotalNumOlss may indicate the total number of OLSs identified by the VPS. In this disclosure, HRD parameters may refer to ols_hrd_parameters(). Therefore, the number of HRD parameter syntax structures may refer to the number of ols_hrd_parameters() syntax structures.

[0156] hrd_max_tid[i] may be signaled if vps_max_sublayers_minus1 is greater than 0 and vps_all_layers_same_num_sublayers_flag is a second value (e.g., 0). hrd_max_tid[i] may indicate the temporal hierarchical identifier (TemporalId) of the top-most sublayer whose associated HRD parameters are included in the i-th ols_hrd_parameters() syntax structure.

[0157] hrd_max_tid[i] can have a value from 0 to vps_max_sublayers_minus1. If vps_max_sublayers_minus1 is 0, the value of hrd_max_tid[i] can be inferred to be 0. If vps_max_sublayers_minus1 is greater than 0 and vps_all_layers_same_num_sublayers_flag is 1, the value of hrd_max_tid[i] can be inferred to be equal to vps_max_sublayers_minus1.

[0158] As shown in Figure 7, a variable firstSubLayer indicating a temporal layer identifier (TemporalId) of the first sublayer can be induced to 0 or hrd_max_tid[i] based on vps_sublayer_cpb_params_present_flag. Specifically, if vps_sublayer_cpb_params_present_flag is 1, firstSubLayer can be induced to 0, otherwise, firstSubLayer can be induced to hrd_max_tid[i]. Based on the induced firstSubLayer and hrd_max_tid[i], an ols_hrd_parameters() syntax structure can be signaled.

[0159] If the value of num_ols_hrd_params_minus1 plus 1 is not equal to TotalNumOlss 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 if the number of layers (NumLayersInOls[i]) included in the i-th OLS is greater than 1. ols_hrd_idx[i] is an index to a list of ols_hrd_parameters() in the VPS and may be an index of ols_hrd_parameters() applied to the i-th OLS. ols_hrd_idx[i] may have a value from 0 to num_ols_hrd_params_minus1. If the number of strata contained in the i-th OLS (NumLayersInOls[i]) is 1, then the ols_hrd_parameters() syntax structure that applies to the i-th OLS can be present in an SPS referenced by a hierarchy in the i-th OLS.

[0160] In the present disclosure, ols_hrd_idx[i] is the index of ols_hrd_parameters() applied to the i-th OLS or the i-th multilayer OLS, and can be referred to as mapping information (information about mapping) between the (multilayer) OLS and the HRD parameter syntax structure (ols_hrd_parameters()).

[0161] If num_ols_hrd_param_minus1 plus 1 is equal to TotalNumOlss, the value of ols_hrd_idx[i] can be inferred to be equal to i. Otherwise, if NumLayersInOls[i] is greater than 1 and num_ols_hrd_params_minus1 is 0, the value of ols_hrd_idx[i] can be inferred to be 0.

[0162] HRD signalling in VPS and SPS

[0163] The signaling of HRD parameters according to the present disclosure will be described in more detail below. HRD parameters can be signaled for each output layer set (OLS). A hypothetical reference decoder (HRD) is a hypothetical decoder model that specifies limitations on the variability of a conforming NAL unit stream or a conforming byte stream that can be generated during the encoding process.

[0164] The HRD parameters may be signaled in the VPS as described with reference to Fig. 7. Alternatively, the HRD parameters may be signaled in the SPS.

[0165] FIG. 8 is a diagram illustrating a syntax structure of an SPS for signaling HRD parameters according to one embodiment of the present disclosure.

[0166] In the example shown in FIG. 8, a first value (e.g., 1) of sps_ptl_dpb_hrd_params_present_flag may indicate that a profile_tier_level() syntax structure and a dpb_parameters() syntax structure are present in the SPS. The profile_tier_level() may be a syntax structure for transmitting parameters for a profile layer level, and the dpb_parameters() may be a syntax structure for transmitting DPB (decoded picture buffer) parameters. In addition, a first value (e.g., 1) of sps_ptl_dpb_hrd_params_present_flag may indicate that a general_hrd_parameters() syntax structure and an ols_hrd_parameters() syntax structure may be present in the SPS. A second value (e.g., 0) of sps_ptl_dpb_hrd_params_present_flag may indicate that the four syntax structures are not present in the SPS. The value of sps_ptl_dpb_hrd_params_present_flag may be the same as the value of vps_independent_layer_flag[GeneralLayerIdx[nuh_layer_id]], i.e., 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]].

[0167] In the above, vps_independent_layer_flag[i] may be a syntax element that is included in the VPS and transmitted. A first value (e.g., 1) of vps_independent_layer_flag[i] may indicate that the layer with index i is an independent layer that does not use inter-layer prediction. A second value (e.g., 0) of vps_independent_layer_flag[i] may indicate that the layer with index i can use inter-layer prediction. If vps_independent_layer_flag[i] does not exist, its value may be inferred to be the first value (e.g., 1).

[0168] If sps_ptl_dpb_hrd_params_present_flag is 1, sps_general_hrd_params_present_flag can be signaled.

[0169] A first value (e.g., 1) of sps_general_hrd_params_present_flag may indicate that the SPS includes the general_hrd_parameters() and ols_hrd_parameters() syntax structures. A second value (e.g., 0) of sps_general_hrd_params_present_flag may indicate that the SPS does not include the general_hrd_parameters() or ols_hrd_parameters() syntax structures.

[0170] As shown in FIG 8, sps_sublayer_cpb_params_present_flag may be signaled if sps_max_sublayers_minus1 is greater than 0. In this case, a value of sps_max_sublayers_minus1 plus 1 may indicate the maximum number of temporal sublayers that may exist in each coded layer video sequence (CLVS) that references the SPS. A first value (e.g., 1) of sps_sublayer_cpb_params_present_flag may indicate that the ols_hrd_parameters() syntax structure in the SPS includes HRD parameters for sublayers whose temporal layer identifiers (TemporalId) are 0 to sps_max_sublayers_minus1. A second value (e.g., 0) of sps_sublayer_cpb_params_present_flag may indicate that the ols_hrd_parameters() syntax structure in the SPS includes HRD parameters for sublayers whose temporal layer identifier (TemporalId) is only sps_max_sublayers_minus1. If sps_max_sublayers_minus1 is 0, the value of sps_sublayer_cpb_params_present_flag may be inferred to be the second value (e.g., 0).

[0171] When sps_sublayer_cpb_params_present_flag is a second value (e.g., 0), the HRD parameters for sublayers whose temporal layer identifier (TemporalId) is 0 to sps_max_sublayers_minus1-1 can be inferred to be equal to the HRD parameters for sublayers whose temporal layer identifier (TemporalId) is sps_max_sublay_minus1.

[0172] FIG. 9 is a diagram illustrating a general_hrd_parameters() syntax structure according to one embodiment of the present disclosure.

[0173] As shown in Figure 9, the general_hrd_parameters() syntax structure can include some of the sequence level HRD parameters used for HRD operations. As a requirement for bitstream consistency, the contents of general_hrd_parameters() present in the VPS or SPS in the bitstream must be the same.

[0174] When the general_hrd_parameters() syntax structure is included in a VPS, the general_hrd_parameters() syntax structure can be applied 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 can be applied to an OLS including only the lowest layer among the layers that refer to the SPS. In this case, the lowest layer may be an independent layer.

[0175] As shown in Fig. 9, the general_hrd_parameters() syntax structure may include syntax elements such as num_units_in_tick, time_scale, and general_nal_hrd_params_present_flag as HRD parameters. The HRD parameters shown in Fig. 9 may have the same meaning as conventional HRD parameters. Therefore, a detailed description of the HRD parameters that are less relevant to the present disclosure will be omitted.

[0176] FIG. 10 is a diagram illustrating an ols_hrd_parameters() syntax structure according to one embodiment of the present disclosure.

[0177] When the ols_hrd_parameters() syntax structure is included in a VPS, OLSs to which the ols_hrd_parameters() syntax structure is applied 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 be applied to an OLS including only the lowest layer among layers that refer to the SPS. In this case, the lowest layer may be an independent layer.

[0178] As shown in Fig. 10, the ols_hrd_parameters() syntax structure may include syntax elements such as fixed_pic_rate_general_flag, fixed_pic_rate_within_cvs_flag, and elemental_duration_in_tc_minus1 as HRD parameters. The HRD parameters shown in Fig. 10 may have the same meaning as conventional HRD parameters. Therefore, a detailed description of the HRD parameters that are less relevant to the present disclosure will be omitted.

[0179] FIG. 11 is a diagram illustrating a sublayer_hrd_parameters() syntax structure according to one embodiment of the present disclosure.

[0180] The sublayer_hrd_parameters() syntax structure can be signaled included in the ols_hrd_parameters() syntax structure of FIG.

[0181] As shown in Fig. 11, the sublayer_hrd_parameters() syntax structure may include syntax elements such as bit_rate_value_minus1, cpb_size_value_minus1, and cpb_size_du_value_minus1 as HRD parameters. The HRD parameters shown in Fig. 11 may have the same meaning as conventional HRD parameters. Therefore, a detailed description of the HRD parameters that are less relevant to the present disclosure will be omitted.

[0182] For reference, the output time may refer to the time at which a picture restored from the DPB is output. The output time may be specified by the HRD according to an output timing DPB operation.

[0183] Two sets of HRD parameters are available: NAL HRD parameters and VCL HRD parameters. The HRD parameters can be signaled via 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 can be signaled in the VPS or in the SPS.

[0184] For example, DPB management can be performed based on the HRD parameters, e.g., removal of pictures from the DPB before decoding of the current picture and / or output of (decoded) pictures can be performed based on the HRD parameters.

[0185] According to the signaling method of the HRD parameters described with reference to FIGS. 7 to 11, at least the following problems may occur.

[0186] As mentioned above, num_ols_hrd_params_minus1 can be limited to have values ​​from 0 to TotalNumOlss-1. However, there is no restriction that each HRD parameter signaled in the VPS should be associated with at least one OLS. Therefore, there is a problem that the VPS can include unused HRD parameters, which reduces the efficiency of signaling.

[0187] As mentioned above, num_ols_hrd_params_minus1 can be restricted to have a value between 0 and TotalNumOlss-1. However, since there can be more than one OLS that contains only one layer, the restriction allows unused HRD parameter structures to be signaled in the VPS. An OLS that contains only one layer is not associated with an HRD parameter structure signaled in the VPS.

[0188] Signaling of HRD parameters in relation to OLS may include other drawbacks not mentioned in this disclosure, including the problems mentioned above.

[0189] To solve at least one of the problems, an embodiment of the present disclosure may include at least one of the following features, which may be applied individually or in combination with other features.

[0190] Configuration 1: Each HRD parameter structure signaled in the VPS may be restricted to be associated with at least one OLS.

[0191] Configuration 2: The number of HRD parameter structures signaled in the VPS (i.e., num_ols_hrd_params_minus1) must not be greater than the number of OLSs that include more than one hierarchy. That is, the number of HRD parameter structures signaled in the VPS may be limited to not be greater than the total number of OLSs minus the number of OLSs that include only one hierarchy.

[0192] FIG. 12 is a diagram for explaining an example of an image coding method to which an embodiment of the present disclosure can be applied.

[0193] The image encoding apparatus may derive HRD parameters (S1210) and encode image / video information (S1220). At this time, the image / video information may include information related to the induced HRD parameters.

[0194] Although not shown in FIG. 12, the image coding apparatus may perform DPB management based on the HRD parameters derived in step S1210.

[0195] FIG. 13 is a diagram for explaining an example of an image decoding method to which an embodiment of the present disclosure can be applied.

[0196] The image decoding apparatus may obtain image / video information from a bitstream (S1310). At this time, the image / video information may include information related to the HRD parameters.

[0197] The image decoding apparatus can decode the picture based on the obtained HRD parameters (S1320).

[0198] FIG. 14 is a diagram for explaining another example of an image decoding method to which the embodiment of the present disclosure can be applied.

[0199] The image decoding apparatus may obtain image / video information from a bitstream (S1410). At this time, the image / video information may include information related to the HRD parameters.

[0200] The image decoding apparatus can perform DPB management based on the acquired HRD parameters (S1420).

[0201] The image decoding apparatus may decode a picture based on the DPB (S1430). For example, a block / slice in a current picture may be decoded based on inter prediction using an already reconstructed picture in the DPB as a reference picture.

[0202] In the examples described with reference to Figures 12 to 14, the information related to the HRD parameters may include at least one of the information / syntax elements described in relation to at least one of the embodiments of the present disclosure. Also, as described above, DPB management may be performed based on the HRD parameters. For example, removal of a picture from the DPB before decoding of the current picture and / or output of a (decoded) picture may be performed based on the HRD parameters.

[0203] According to one embodiment of the present disclosure for solving at least some of the above-mentioned problems, each HRD parameter structure signaled in a VPS can be restricted to be associated with at least one OLS.

[0204] As described above, the ols_hrd_parameters() applied to the i-th OLS can be identified by ols_hrd_idx[i]. According to this embodiment, each of all ols_hrd_parameters() signaled in the VPS can be restricted to be applied to at least one OLS. That is, each ols_hrd_parameters() in the VPS can be identified by at least one ols_hrd_idx[i].

[0205] According to this embodiment, each of the ols_hrd_parameters() in the VPS is used at least once. In other words, unused ols_hrd_parameters() are not signaled. Therefore, according to this embodiment, there is an advantage that the signaling of ols_hrd_parameters() can be performed efficiently.

[0206] According to another embodiment of the present disclosure to address at least some of the above-mentioned problems, the number of HRD parameter structures signaled in the VPS (i.e., num_ols_hrd_params_minus1) may be limited to be no greater than the number of OLSs including more than one hierarchy.

[0207] According to the example described with reference to FIG. 7, the value of num_ols_hrd_params_minus1 plus 1 indicates the number of ols_hrd_parameters() syntax structures in the VPS, and num_ols_hrd_params_minus1 can have a value from 0 to TotalNumOlss-1.

[0208] However, as mentioned above, an OLS including only one layer may exist, and an OLS including only one layer is not associated with an HRD parameter structure signaled in the VPS. The range of values ​​of num_ols_hrd_params_minus1 in the example of FIG. 7 may cause inaccurate signaling. Therefore, accurate signaling can be performed if the range of the number of ols_hrd_parameters() syntax structures is specified by the number of OLSs including multiple layers (multi-layer OLSs) (NumMultiLayerOlss) instead of the total number of OLSs (TotalNumOlss).

[0209] According to the present disclosure, the value of num_ols_hrd_params_minus1 plus 1 indicates the number of ols_hrd_parameters() syntax structures in the VPS, and num_ols_hrd_params_minus1 may have a value from 0 to NumMultiLayerOlss-1. In this case, the number of OLSs (multi-layer OLSs) including multiple layers (NumMultiLayerOlss) may be equal to the total number of OLSs (TotalNumOlss) minus the number of OLSs including only one layer (NumSingleLayerOlss).

[0210] As described above, each HRD parameter structure signaled in the VPS may be restricted to be associated (mapped) with at least one OLS. Also, the number of HRD parameter structures signaled in the VPS (i.e., num_ols_hrd_params_minus1) may be restricted to not be greater than the number of OLSs including multiple layers (multi-layer OLS). The above two embodiments may be combined to form another embodiment as follows.

[0211] FIG. 15 is a diagram illustrating a process of encoding HRD parameters based on num_ols_hrd_params_minus1 according to another embodiment of the present disclosure.

[0212] The image coding apparatus can code num_ols_hrd_params_minus1 into a VPS (S1510). The value obtained by adding 1 to num_ols_hrd_params_minus1 indicates the number of ols_hrd_parameters() syntax structures in the VPS, and num_ols_hrd_params_minus1 can have a value from 0 to NumMultiLayerOlss-1.

[0213] The image coding device can code num_ols_hrd_params_minus1+1 ols_hrd_parameters() syntax structures into a VPS (S1520).

[0214] The image encoding device can determine the following condition 1 (S1530).

[0215] Condition 1: (num_ols_hrd_params_minus1+1!=NumMultiLayerOlss&&num_ols_hrd_params_minus1>0)?

[0216] The condition 1 is a condition for encoding ols_hrd_idx into a bitstream, and if the condition 1 is satisfied (S1530-Yes), the image encoding apparatus can encode ols_hrd_idx into a VPS (S1540). According to an embodiment described with reference to FIG. 15, ols_hrd_idx[i] is an index to a list of ols_hrd_parameters() in the VPS, and may be an index of ols_hrd_parameters() applied to an OLS including the i-th multiple layers (multi-layer OLS). That is, ols_hrd_idx[i] in the VPS is signaled for multi-layer OLSs, and may have a value from 0 to num_ols_hrd_params_minus1.

[0217] If the condition 1 is not satisfied (S1530-No), the image coding apparatus does not code ols_hrd_idx to a VPS, and the value may be inferred (S1550). Specifically, if num_ols_hrd_params_minus1 is 0, the value of ols_hrd_idx[i] may be inferred to be 0. Otherwise, if the value of 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.

[0218] In addition, an ols_hrd_parameters() syntax structure that applies to an OLS that contains only a single stratum may not be coded into a VPS, but may be coded into an SPS referenced by that stratum within the OLS.

[0219] Furthermore, each ols_hrd_parameters() in the VPS can be referenced by at least one ols_hrd_idx[i] (i has a value from 0 to NumMultiLayerOlss-1).

[0220] The image coding apparatus may code the picture based on the HRD parameters (S1560). In this case, the HRD parameters may be ols_hrd_parameters() in the VPS referenced by ols_hrd_idx[i] obtained in step S1540 or inferred in step S1550. Alternatively, in the case of an OLS including only a single layer, the HRD parameters may be ols_hrd_parameters() in the SPS referenced by the single layer.

[0221] FIG. 16 is a diagram for explaining a process of decoding HRD parameters based on num_ols_hrd_params_minus1 according to another embodiment of the present disclosure.

[0222] The image decoding apparatus can acquire num_ols_hrd_params_minus1 from the VPS (S1610). The value of num_ols_hrd_params_minus1 plus 1 indicates the number of ols_hrd_parameters() syntax structures in the VPS, and num_ols_hrd_params_minus1 can have a value from 0 to NumMultiLayerOlss-1.

[0223] The image decoding apparatus can acquire num_ols_hrd_params_minus1+1 ols_hrd_parameters() syntax structures from the VPS (S1620).

[0224] The image decoding apparatus can determine the following condition 2 (S1630).

[0225] Condition 2:(num_ols_hrd_params_minus1+1!=NumMultiLayerOlss&&num_ols_hrd_params_minus1>0)?

[0226] The condition 2 is a condition for acquiring ols_hrd_idx from a bitstream, and if the condition 2 is satisfied (S1630-Yes), the image decoding apparatus can acquire ols_hrd_idx from the VPS (S1640). According to an embodiment described with reference to Fig. 16, ols_hrd_idx[i] is an index to a list of ols_hrd_parameters() in the VPS, and may be an index of ols_hrd_parameters() applied to the i-th OLS including multiple layers (multi-layer OLS).

[0227] That is, ols_hrd_idx[i] in the VPS is signaled to the multi-layer OLS and can have values ​​from 0 to num_ols_hrd_params_minus1.

[0228] If the condition 2 is not satisfied (S1630-No), the image decoding apparatus does not acquire ols_hrd_idx from the VPS, and can infer its value (S1650). Specifically, if num_ols_hrd_params_minus1 is 0, the value of ols_hrd_idx[i] can be inferred to be 0. Otherwise, if the value of num_ols_hrd_param_minus1 plus 1 is equal to NumMultiLayerOlss, the value of ols_hrd_idx[i] can be inferred to be equal to i.

[0229] Additionally, the ols_hrd_parameters() syntax structure that applies to an OLS that contains only a single stratum may not be obtained from the VPS, but from the SPS referenced by that stratum in the OLS.

[0230] Furthermore, each ols_hrd_parameters() in the VPS can be referenced by at least one ols_hrd_idx[i] (i has a value from 0 to NumMultiLayerOlss-1).

[0231] The image decoding apparatus may decode the picture based on the HRD parameters (S1660). In this case, the HRD parameters may be ols_hrd_parameters() in the VPS referenced by ols_hrd_idx[i] obtained in step S1640 or inferred in step S1650. Alternatively, in the case of an OLS including only a single layer, the HRD parameters may be ols_hrd_parameters() in the SPS referenced by the single layer.

[0232] According to the embodiments described with reference to Figures 15 and 16, unnecessary signaling of the ols_hrd_parameters() syntax structure in an unreferenced VPS can be prevented, and the ols_hrd_parameters() syntax structure for an OLS including only a single layer is signaled only via the SPS, thereby achieving more accurate and efficient signaling of HRD parameters.

[0233] In the methods described with reference to Figures 15 and 16, some steps may be omitted, or the order of other steps may be changed. Also, steps not shown in Figures 15 and 16 may be added at any position.

[0234] Although the exemplary method of the present disclosure is expressed as a series of operations for clarity of explanation, this is not intended to limit the order in which the steps are performed, and each step may be performed simultaneously or in a different order, if necessary. To realize the method according to the present disclosure, the steps illustrated may include other steps, some steps may be omitted and the remaining steps may be omitted, or some steps may be omitted and additional steps may be included.

[0235] In the present disclosure, an image encoding device or an image decoding device that performs a predetermined operation (step) can perform the operation (step) to check the execution conditions or circumstances of the operation (step). For example, if it is described that a predetermined operation is performed when a predetermined condition is satisfied, the image encoding device or the image decoding device can perform the predetermined operation after performing an operation to check whether the predetermined condition is satisfied or not.

[0236] The various embodiments of the present disclosure are not intended to enumerate all possible combinations, but 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.

[0237] Additionally, various embodiments of the present disclosure may be implemented using hardware, firmware, software, or a combination thereof, etc. In the case of a hardware implementation, the implementation may be implemented using one or more 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.

[0238] In addition, the image decoding device and the image encoding device to which the embodiments of the present disclosure are applied may be included in a multimedia broadcast transmitting / receiving device, a mobile communication terminal, a home cinema video device, a digital cinema video device, a surveillance camera, a video conversation device, a real-time communication device such as a video communication, a mobile streaming device, a storage medium, a camcorder, a custom video (VoD) service providing device, an over-the-top video (OTT) device, an internet streaming service providing device, a three-dimensional (3D) video device, an image telephone video device, and a medical video device, and may be used to process a video signal or a data signal. For example, the over-the-top video (OTT) device may include a game console, a Blu-ray player, an internet-connected TV, a home theater system, a smartphone, a tablet PC, a digital video recorder (DVR), and the like.

[0239] FIG. 17 is a diagram illustrating a content streaming system to which an embodiment of the present disclosure can be applied.

[0240] As shown in FIG. 17, a content streaming system to which an embodiment of the present disclosure is applied may broadly include an encoding server, a streaming server, a Web server, a media storage, a user device, and a multimedia input device.

[0241] The encoding server compresses content input from a multimedia input device such as a smartphone, camera, camcorder, etc. into digital data to generate a bitstream and transmits the bitstream to the streaming server. As another example, if a multimedia input device such as a smartphone, camera, video camera, etc. directly generates a bitstream, the encoding server can be omitted.

[0242] The bitstream may be generated by an image encoding method and / or image encoding device to which an 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.

[0243] The streaming server transmits multimedia data to a user device based on a user request via a web server, and the web server can act as a medium to inform the user of available services. When a user requests a desired service from the web server, the web server transmits the request to the streaming server, and the streaming server can transmit the multimedia data to the user. At this time, the content streaming system can include a separate control server, and in this case, the control server can control commands / responses between devices in the content streaming system.

[0244] The streaming server may receive the content from a media storage and / or an encoding server. For example, when receiving the content from the encoding server, the content may be received in real time. In this case, the streaming server may store the bitstream for a certain period of time in order to provide a smooth streaming service.

[0245] Examples of the user devices include mobile phones, smart phones, laptop computers, digital broadcasting terminals, personal digital assistants (PDAs), portable multimedia players (PMPs), navigation systems, slate PCs, tablet PCs, ultrabooks, wearable devices such as smartwatches, smart glass, head mounted displays (HMDs), digital TVs, desktop computers, and digital signage.

[0246] Each server in the content streaming system can be operated as a distributed server, in which case data received from each server can be processed in a distributed manner.

[0247] The scope of the present disclosure includes software or machine-executable commands (e.g., operating systems, applications, firmware, programs, etc.) that cause operations according to the methods of the various embodiments to be performed on a device or computer, and non-transitory computer-readable medium on which such software or commands can be stored and executed on a device or computer. [Industrial Applicability]

[0248] The embodiments of the present disclosure can be used to encode / decode images.

Claims

1. An image decoding method performed by an image decoding device, the image decoding method comprising: obtaining first information indicating a 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 regarding a mapping between one or more multi-layer output layer sets (OLS) and the one or more HRD parameter syntax structures from the VPS based on the first information; selecting an HRD parameter syntax structure to be applied to the current OLS based on the second information; and processing the current OLS based on the selected HRD parameter syntax structure; The first information is obtained based on whether the VPS includes the one or more HRD parameter syntax structures; Each of the one or more HRD parameter syntax structures in the VPS is mapped to at least one multi-layer OLS of the one or more multi-layer OLSs.

2. The image decoding method of claim 1 , wherein a number of the one or more HRD parameter syntax structures in the VPS is not greater than a number of the one or more multi-layer OLSs.

3. 2. The image decoding method of claim 1, wherein the second information is obtained from the VPS based on the number of the one or more HRD parameter syntax structures in the VPS being greater than one and the number of the one or more HRD parameter syntax structures in the VPS not being equal to the number of the one or more multi-layer OLSs.

4. 4. The image decoding method of claim 3, wherein, based on a number of the one or more HRD parameter syntax structures in the VPS being 1, the second information is not obtained from the VPS and the second information is inferred to be equal to a value of 0.

5. 4. The image decoding method of claim 3, wherein, based on the number of the one or more HRD parameter syntax structures in the VPS being greater than 1 and the number of the one or more HRD parameter syntax structures in the VPS being equal to the number of the one or more multi-layer OLSs, the second information is not obtained from the VPS, and it is inferred that the second information of an i-th multi-layer OLS is equal to the value of i.

6. The image decoding method of claim 1 , wherein the HRD parameter syntax structure applied to the current OLS is obtained from a Sequence Parameter Set (SPS) on the basis that the current OLS includes only a single layer.

7. An image coding method performed by an image coding device, the image coding method comprising: encoding first information indicating a 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 regarding a mapping between one or more multi-layer output layer sets (OLS) in the VPS and the one or more HRD parameter syntax structures based on the first information; and processing the current OLS based on an HRD parameter syntax structure applied to the current OLS; the first information is encoded based on whether the VPS includes the one or more HRD parameter syntax structures; Each of the one or more HRD parameter syntax structures in the VPS is mapped to at least one multi-layer OLS of the one or more multi-layer OLSs.

8. The image coding method of claim 7 , wherein a number of the one or more HRD parameter syntax structures in the VPS is not greater than a number of the one or more multi-layer OLSs.

9. 8. The image encoding method of claim 7, wherein the second information is encoded within the VPS based on the number of the one or more HRD parameter syntax structures in the VPS being greater than one and the number of the one or more HRD parameter syntax structures in the VPS not being equal to the number of the one or more multi-layer OLSs.

10. 10. The image encoding method of claim 9, wherein the second information is not encoded in the VPS and the second information is inferred to be equal to a value of 0 based on the number of the one or more HRD parameter syntax structures in the VPS being 1.

11. 10. The image encoding method of claim 9, wherein, based on the number of the one or more HRD parameter syntax structures in the VPS being greater than one and the number of the one or more HRD parameter syntax structures in the VPS being equal to the number of multi-layer OLSs, the second information is not encoded in the VPS, and it is inferred that the second information of an i-th multi-layer OLS is equal to the value of i.

12. The image encoding method of claim 7 , wherein the HRD parameter syntax structure applied to the current OLS is encoded within a Sequence Parameter Set (SPS) based on the current OLS including only a single layer.

13. 1. A method for transmitting a bitstream, comprising: generating the bitstream; transmitting the bitstream; The bitstream comprises: encoding first information indicating a 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 regarding a mapping between one or more multi-layer output layer sets (OLS) in the VPS and the one or more HRD parameter syntax structures based on the first information; processing the current OLS based on an HRD parameter syntax structure applied to the current OLS; the first information is encoded based on whether the VPS includes the one or more HRD parameter syntax structures; The method of claim 1, wherein each of the one or more HRD parameter syntax structures in the VPS is mapped to at least one multi-layer OLS of the one or more multi-layer OLSs.

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