KR20200098433A - Method and apparatus for encoding/decoding image and recording medium for storing bitstream - Google Patents

Abstract

The present specification discloses an image decoding method. The image decoding method comprises the following steps of: deriving a cross component linear model (CCLM) parameter for a current color difference block; determining a down-sampling filter for the current color difference block; applying the down-sampling filter to a current luminance block corresponding to the current color difference block; and generating a predicted pixel value of the current color difference block by using the CCLM parameter and a pixel value of the luminance block to which the down-sampling filter is applied, wherein the step of determining the down-sampling filter can be determined based on the position of the current luminance block. According to the present invention, compression efficiency can be improved.

Description

Video encoding/decoding method, apparatus, and recording medium storing the bitstream {METHOD AND APPARATUS FOR ENCODING/DECODING IMAGE AND RECORDING MEDIUM FOR STORING BITSTREAM}

The present invention relates to a video encoding/decoding method, an apparatus, and a recording medium storing a bitstream. Specifically, the present invention relates to a method and apparatus for predicting an intra-screen color difference signal selectively applying a filter.

Recently, demand for high-resolution and high-quality images such as high definition (HD) images and ultra high definition (UHD) images is increasing in various application fields. The higher the resolution and quality of the video data, the higher the amount of data is compared to the existing video data. Therefore, if the video data is transmitted using a medium such as a wired/wireless broadband line or stored using an existing storage medium, the transmission cost and The storage cost increases. In order to solve these problems that occur as image data becomes high-resolution and high-quality, a high-efficiency image compression technology for images having higher resolution and quality is required.

Inter-screen prediction technology that predicts pixel values included in the current picture from pictures before and/or after the current picture using image compression technology, and intra prediction that predicts pixel values included in the current picture using pixel information in the current picture There are various technologies such as technology, transformation and quantization technology to compress the energy of the residual signal, and entropy coding technology that allocates short codes to symbols with high frequency of appearance and long codes to symbols with low frequency of appearance. Video data can be effectively compressed and transmitted or stored using video compression technology.

Cross component between the luminance channel and the chrominance channel even when color space conversion is performed from RGB to YCbCr color space in the case of color images by applying various technologies such as predictive coding and transform coding using transformation to high-resolution images such as 4K or 8K. This still exists.

To remove this redundancy, a linear model is generated through the correlation between a given color difference channel block and a luminance reference block spatially corresponding thereto, and using this model to perform intra prediction of the color difference channel is CCLM (Cross Component Linear Model) prediction technology.

When the CCLM prediction coding technique, which is an intra-screen prediction method for a color difference channel using a relational expression between a color difference pixel and a luminance pixel spatially corresponding thereto, is applied, the pixel values of the luminance block spatially corresponding to the color difference block to be coded are downsampled ( downsampling).

The present invention provides a method and apparatus for selecting a downsampling filter used for downsampling according to the CCLM-Above-Left mode, CCLM-Left mode, and CCLM-Above mode selected for CCLM prediction. It is for sake

In addition, it is to provide a method and apparatus for selecting a filter in consideration of the location, size, and shape of the block to be sampled downward.

An image decoding method according to an embodiment of the present invention includes: deriving a Cross Component Linear Model (CCLM) parameter for a current color difference block, determining a down-sampling filter for the current color difference block, and Applying the down-sampling filter to a corresponding current luminance block and generating a predicted pixel value of the current chrominance block by using the CCLM parameter and pixel values of the luminance block to which the down-sampling filter is applied, the The step of determining the down-sampling filter may be determined based on the position of the current luminance block.

According to the present invention, a video encoding/decoding method and apparatus with improved compression efficiency can be provided.

The present invention is a method and apparatus for down-sampling a luminance pixel by selectively applying a down-sampling filter, different from the conventional method, when applying a CCLM prediction coding technique that predicts a color difference block in a screen through correlation between channels. Particularly, when the conventional technology is applied to an image in which the degree of change of pixel values is spatially concentrated, problems of compression ratio and image quality deteriorate seriously.The present invention relates to a CCLM mode, block shape and length, block aspect ratio, By selectively applying various down-sampling filters according to the position of the luminance block, it is possible to improve encoding efficiency or improve encoding quality.

1 is a block diagram showing a configuration according to an embodiment of an encoding apparatus to which the present invention is applied.
2 is a block diagram showing a configuration according to an embodiment of a decoding apparatus to which the present invention is applied.
3 is a diagram schematically illustrating a split structure of an image when encoding and decoding an image.
4 is a diagram for describing an embodiment of an intra prediction process.
5 is a diagram for describing an embodiment of an inter prediction process.
6 is a diagram for describing a process of transformation and quantization.
7 is a diagram for describing reference samples usable for intra prediction.
8 is a diagram for explaining an embodiment of predicting a color difference block from a reconstructed luminance block.
9 is a diagram illustrating a 6-tap down-sampling filter according to an embodiment of the present invention.
10 is a diagram illustrating various embodiments of a down-sampling filter according to an embodiment of the present invention.
11 is a diagram for explaining an embodiment in which a down-sampling filter is determined according to a CCLM mode.
12 is a diagram for describing a filter according to an embodiment of the present invention.
13 is a diagram illustrating relative downward-sampling positions of luminance and color difference pixels according to an embodiment of the present invention.
14 is a diagram illustrating a position of a luminance block to be sampled downward in an upper block according to an embodiment of the present invention.
15 is a diagram for explaining an embodiment in which a down-sampling filter is determined according to a position of a luminance block in FIG. 14.
16 is a diagram for describing an embodiment in which a down-sampling filter is determined according to a shape of a block.
17 is a diagram for describing an image decoding method according to an embodiment of the present invention.

In the present invention, various modifications may be made and various embodiments may be provided, and specific embodiments will be illustrated in the drawings and described in detail in the detailed description. However, this is not intended to limit the present invention to a specific embodiment, it is to be understood to include all changes, equivalents, and substitutes included in the spirit and scope of the present invention. Like reference numerals in the drawings refer to the same or similar functions over several aspects. The shapes and sizes of elements in the drawings may be exaggerated for clearer explanation. For a detailed description of exemplary embodiments described below, reference is made to the accompanying drawings, which illustrate specific embodiments as examples. These embodiments are described in detail sufficient to enable a person skilled in the art to practice the embodiments. It should be understood that the various embodiments are different from each other but need not be mutually exclusive. For example, specific shapes, structures, and characteristics described herein may be implemented in other embodiments without departing from the spirit and scope of the present invention in relation to one embodiment. In addition, it is to be understood that the location or arrangement of individual components within each disclosed embodiment may be changed without departing from the spirit and scope of the embodiment. Accordingly, the detailed description to be described below is not intended to be taken in a limiting sense, and the scope of exemplary embodiments, if properly described, is limited only by the appended claims, along with all scope equivalents to those claimed by the claims.

In the present invention, terms such as first and second may be used to describe various components, but the components should not be limited by the terms. These terms are used only for the purpose of distinguishing one component from another component. For example, without departing from the scope of the present invention, a first element may be referred to as a second element, and similarly, a second element may be referred to as a first element. The term and/or includes a combination of a plurality of related listed items or any of a plurality of related listed items.

When a component of the present invention is referred to as being “connected” or “connected” to another component, it may be directly connected or connected to the other component, but other components exist in the middle. It should be understood that it may be possible. On the other hand, when a component is referred to as being "directly connected" or "directly connected" to another component, it should be understood that there is no other component in the middle.

Components shown in the embodiments of the present invention are independently shown to represent different characteristic functions, and does not mean that each component is formed of separate hardware or a single software component. That is, each constituent part is listed and included as a respective constituent part for convenience of explanation, and at least two of the constituent parts are combined to form one constituent part, or one constituent part is divided into a plurality of constituent parts to perform a function. Integrated embodiments and separate embodiments of the components are also included in the scope of the present invention unless departing from the essence of the present invention.

The terms used in the present invention are only used to describe specific embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In the present invention, terms such as "comprises" or "have" are intended to designate the presence of features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, but one or more other features. It is to be understood that the presence or addition of elements or numbers, steps, actions, components, parts, or combinations thereof, does not preclude in advance. That is, in the present invention, the description of “including” a specific configuration does not exclude configurations other than the corresponding configuration, and means that additional configurations may be included in the scope of the implementation of the present invention or the technical idea of the present invention.

Some of the components of the present invention are not essential components that perform essential functions in the present invention, but may be optional components only for improving performance. The present invention can be implemented by including only the components essential to implement the essence of the present invention excluding components used for performance improvement, and a structure including only essential components excluding optional components used for performance improvement Also included in the scope of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In describing the embodiments of the present specification, if it is determined that a detailed description of a related known configuration or function may obscure the subject matter of the present specification, the detailed description thereof will be omitted, and the same reference numerals are used for the same elements in the drawings. Used and redundant descriptions for the same components are omitted.

Hereinafter, an image may mean one picture constituting a video, and may represent a video itself. For example, “encoding and/or decoding of an image” may mean “encoding and/or decoding of a video” and “encoding and/or decoding of one of the images constituting a video” May be.

Hereinafter, the terms "movie" and "video" may be used with the same meaning, and may be used interchangeably.

Hereinafter, the target image may be an encoding target image that is an encoding target and/or a decoding target image that is a decoding target. In addition, the target image may be an input image input through an encoding device or an input image input through a decoding device. Here, the target image may have the same meaning as the current image.

Hereinafter, the terms "image", "picture", "frame" and "screen" may be used with the same meaning, and may be used interchangeably.

Hereinafter, the target block may be an encoding target block that is an object of encoding and/or a decoding object block that is an object of decoding. Also, the target block may be a current block that is a target of current encoding and/or decoding. For example, the terms "target block" and "current block" may have the same meaning, and may be used interchangeably.

Hereinafter, the terms "block" and "unit" may be used with the same meaning, and may be used interchangeably. Or “block” may represent a specific unit.

Hereinafter, the terms "region" and "segment" may be used interchangeably.

Hereinafter, the specific signal may be a signal indicating a specific block. For example, the original signal may be a signal representing a target block. The prediction signal may be a signal representing a prediction block. The residual signal may be a signal indicating a residual block.

In embodiments, each of the specified information, data, flag, index and element, attribute, and the like may have a value. A value "0" of information, data, flags, indexes, elements, attributes, etc. may represent a logical false or a first predefined value. That is to say, the value "0", false, logical false, and the first predefined value may be replaced with each other and used. A value "1" of information, data, flags, indexes, elements, attributes, etc. may represent a logical true or a second predefined value. That is to say, the value "1", true, logical true and the second predefined value may be used interchangeably.

When a variable such as i or j is used to indicate a row, column, or index, the value of i may be an integer greater than or equal to 0, or may be an integer greater than or equal to 1. That is to say, in embodiments, rows, columns, and indexes may be counted from 0, and may be counted from 1.

Glossary of terms

Encoder: refers to a device that performs encoding. That is, it may mean an encoding device.

Decoder: refers to a device that performs decoding. That is, it may mean a decoding device.

Block: MxN array of samples. Here, M and N may mean positive integer values, and a block may often mean a two-dimensional array of samples. A block can mean a unit. The current block may mean an encoding object block, which is an object of encoding during encoding, and a decoding object block, which is an object of decoding when decoding. Also, the current block may be at least one of a coding block, a prediction block, a residual block, and a transform block.

Sample: A basic unit that composes a block. It may be expressed as a value from 0 to 2 ^Bd -1 according to the bit depth (B _d ). In the present invention, a sample may be used in the same sense as a pixel or a pixel. That is, samples, pixels, and pixels may have the same meaning.

Unit: It may mean a unit of image encoding and decoding. In encoding and decoding of an image, a unit may be a region obtained by dividing one image. Further, a unit may mean a divided unit when one image is divided into subdivided units and encoded or decoded. That is, one image may be divided into a plurality of units. In encoding and decoding of an image, a predefined process may be performed for each unit. One unit may be further divided into sub-units having a smaller size than the unit. Depending on the function, the units are Block, Macroblock, Coding Tree Unit, Coding Tree Block, Coding Unit, Coding Block, and Prediction. It may mean a unit (Prediction Unit), a prediction block (Prediction Block), a residual unit (Residual Unit), a residual block (Residual Block), a transform unit (Transform Unit), a transform block (Transform Block), and the like. In addition, a unit may mean including a luminance component block, a chrominance component block corresponding thereto, and a syntax element for each block in order to distinguish it from a block. The unit may have various sizes and shapes, and in particular, the shape of the unit may include not only a square, but also a geometric figure that can be expressed in two dimensions, such as a rectangle, a trapezoid, a triangle, and a pentagon. In addition, the unit information may include at least one of a type of a unit indicating a coding unit, a prediction unit, a residual unit, a transform unit, and the like, a size of a unit, a depth of a unit, and an order of encoding and decoding units.

Coding Tree Unit: It is composed of two color difference component (Cb, Cr) coded tree blocks related to one luminance component (Y) coded tree block. In addition, it may mean including the blocks and a syntax element for each block. Each coding tree unit uses one or more partitioning methods, such as a quad tree, a binary tree, and a ternary tree, to construct subunits such as coding units, prediction units, and transform units. Can be divided. Like division of an input image, it may be used as a term to refer to a sample block that becomes a processing unit in an image decoding/encoding process. Here, the quad tree may mean a quadrilateral tree.

When the size of the coded block falls within a predetermined range, it may be divided only into a quad tree. Here, the predetermined range may be defined as at least one of a maximum size and a minimum size of a coding block that can be divided only by a quadtree. Information indicating the maximum/minimum size of a coding block in which quadtree-type division is allowed can be signaled through a bitstream, and the information is in at least one unit of a sequence, a picture parameter, a tile group, or a slice (segment). Can be signaled. Alternatively, the maximum/minimum size of the coding block may be a fixed size pre-set in the encoder/decoder. For example, when the size of the coding block corresponds to 256x256 to 64x64, it may be split only into a quadtree. Alternatively, when the size of the coding block is larger than the size of the maximum transform block, it may be divided only into a quadtree. In this case, the divided block may be at least one of a coding block or a transform block. In this case, the information indicating splitting of the coding block (eg, split_flag) may be a flag indicating whether to split the quadtree. When the size of the coded block falls within a predetermined range, it may be divided into a binary tree or a three-division tree. In this case, the above description of the quad tree can be applied equally to a binary tree or a three-division tree.

Coding Tree Block: It can be used as a term for referring to any one of a Y-coded tree block, a Cb-coded tree block, and a Cr-coded tree block.

Neighbor block: May mean a block adjacent to the current block. A block adjacent to the current block may refer to a block facing the current block or a block located within a predetermined distance from the current block. The neighboring block may mean a block adjacent to the vertex of the current block. Here, the block adjacent to the vertex of the current block may be a block vertically adjacent to a neighboring block horizontally adjacent to the current block or a block horizontally adjacent to a neighboring block vertically adjacent to the current block. The neighboring block may mean a restored neighboring block.

Reconstructed Neighbor Block: This may mean a neighboring block that has already been encoded or decoded in a spatial/temporal manner around the current block. In this case, the restored neighboring block may mean a restored neighboring unit. The reconstructed spatial neighboring block may be a block in the current picture and already reconstructed through encoding and/or decoding. The reconstructed temporal neighboring block may be a reconstructed block or a neighboring block at a position corresponding to the current block of the current picture in the reference image.

Unit Depth: It may mean the degree to which a unit is divided. The root node in the tree structure may correspond to the first undivided unit. The highest node may be referred to as a root node. Also, the highest node may have a minimum depth value. In this case, the uppermost node may have a depth of level 0. A node having a depth of level 1 may represent a unit generated as the first unit is divided once. A node with a depth of level 2 may represent a unit created as the first unit is divided twice. A node having a depth of level n may represent a unit generated when the first unit is divided n times. The leaf node may be the lowest node, and may be a node that cannot be further divided. The depth of the leaf node may be at the maximum level. For example, a predefined value of the maximum level may be 3. It can be said that the root node has the shallowest depth, and the leaf node has the deepest depth. In addition, when a unit is expressed in a tree structure, the level at which the unit exists may mean the unit depth.

Bitstream: May mean a sequence of bits including coded image information.

Parameter Set: Corresponds to header information among structures in the bitstream. At least one of a video parameter set, a sequence parameter set, a picture parameter set, and an adaptation parameter set may be included in the parameter set. Also, the parameter set may include tile group, slice header, and tile header information. In addition, the tile group may mean a group including several tiles, and may have the same meaning as a slice.

The adaptation parameter set may refer to a parameter set that can be shared by referring to different pictures, subpictures, slices, tile groups, tiles, or bricks. In addition, information in the adaptation parameter set may be used in subpictures, slices, tile groups, tiles, or bricks within a picture by referring to different adaptation parameter sets.

In addition, the adaptation parameter set may refer to different adaptation parameter sets by using identifiers of different adaptation parameter sets in subpictures, slices, tile groups, tiles, or bricks within a picture.

In addition, the adaptation parameter set may refer to different adaptation parameter sets by using identifiers of different adaptation parameter sets in a slice, a tile group, a tile, or a brick within a subpicture.

In addition, the adaptation parameter sets may refer to different adaptation parameter sets by using identifiers of different adaptation parameter sets in tiles or bricks within a slice.

In addition, the adaptation parameter sets may refer to different adaptation parameter sets by using identifiers of different adaptation parameter sets in bricks within the tile.

By including information on an adaptation parameter set identifier in the parameter set or header of the subpicture, an adaptation parameter set corresponding to the adaptation parameter set identifier may be used in the subpicture.

By including information on the adaptation parameter set identifier in the parameter set or header of the tile, an adaptation parameter set corresponding to the adaptation parameter set identifier may be used in the tile.

By including information on the adaptation parameter set identifier in the header of the brick, an adaptation parameter set corresponding to the adaptation parameter set identifier may be used in the brick.

The picture may be divided into one or more tile rows and one or more tile columns.

The subpicture may be divided into one or more tile rows and one or more tile columns within the picture. The subpicture is an area having a rectangular/square shape within a picture, and may include one or more CTUs. In addition, at least one tile/brick/slice may be included in one subpicture.

The tile is an area having a rectangular/square shape within a picture, and may include one or more CTUs. Also, a tile can be divided into one or more bricks.

The brick may mean one or more CTU rows in the tile. A tile can be divided into one or more bricks, and each brick can have at least one or more CTU rows. Tiles that are not divided into two or more can also mean bricks.

The slice may include one or more tiles in a picture, and may include one or more bricks in the tile.

Parsing: It may mean determining a value of a syntax element by entropy decoding a bitstream, or it may mean entropy decoding itself.

Symbol: It may mean at least one of a syntax element of an encoding/decoding target unit, a coding parameter, and a value of a transform coefficient. Also, the symbol may mean an object of entropy encoding or a result of entropy decoding.

Prediction Mode: This may be information indicating a mode encoded/decoded by intra prediction or a mode encoded/decoded by inter prediction.

Prediction Unit: It may mean a basic unit when prediction is performed, such as inter prediction, intra prediction, inter prediction, inter-screen compensation, and motion compensation. One prediction unit may be divided into a plurality of partitions having a smaller size or a plurality of sub prediction units. The plurality of partitions may also be basic units in performing prediction or compensation. A partition generated by division of a prediction unit may also be a prediction unit.

Prediction Unit Partition: This may mean a form in which a prediction unit is divided.

Reference Picture List: This may mean a list including one or more reference pictures used for inter prediction or motion compensation. The types of the reference image list may include LC (List Combined), L0 (List 0), L1 (List 1), L2 (List 2), L3 (List 3), and more than one reference image for inter prediction. Lists can be used.

Inter prediction indicator: may mean an inter prediction direction (unidirectional prediction, bidirectional prediction, etc.) of the current block. Alternatively, it may mean the number of reference pictures used when generating a prediction block of the current block. Alternatively, it may mean the number of prediction blocks used when inter prediction or motion compensation is performed on the current block.

Prediction list utilization flag: Indicates whether a prediction block is generated using at least one reference image in a specific reference image list. An inter prediction indicator can be derived using the prediction list utilization flag, and conversely, the prediction list utilization flag can be derived using an inter prediction indicator. For example, when the prediction list utilization flag indicates a first value of 0, it may indicate that a prediction block is not generated using a reference image in the reference image list, and when a second value of 1 is indicated, the reference It may indicate that a prediction block can be generated using an image list.

Reference Picture Index: This may mean an index indicating a specific reference picture in the reference picture list.

Reference Picture: This may mean an image referenced by a specific block for inter-screen prediction or motion compensation. Alternatively, the reference image may be an image including a reference block referenced by the current block for inter prediction or motion compensation. Hereinafter, the terms "reference picture" and "reference image" may be used with the same meaning, and may be used interchangeably.

Motion Vector: It may be a 2D vector used for inter-screen prediction or motion compensation. The motion vector may mean an offset between an encoding/decoding object block and a reference block. For example, (mvX, mvY) may represent a motion vector. mvX may represent a horizontal component, and mvY may represent a vertical component.

Search Range: The search range may be a two-dimensional area in which a motion vector is searched during inter prediction. For example, the size of the search area may be MxN. M and N may each be a positive integer.

Motion Vector Candidate: When predicting a motion vector, it may mean a block to be a prediction candidate or a motion vector of the block. Also, the motion vector candidate may be included in the motion vector candidate list.

Motion Vector Candidate List: This may mean a list constructed by using one or more motion vector candidates.

Motion Vector Candidate Index: May mean an indicator indicating a motion vector candidate in the motion vector candidate list. It may be an index of a motion vector predictor.

Motion Information: At least one of a motion vector, a reference image index, an inter prediction indicator, as well as a prediction list utilization flag, reference image list information, reference image, motion vector candidate, motion vector candidate index, merge candidate, merge index, etc. It may mean information including one.

Merge Candidate List: This may mean a list formed by using one or more merge candidates.

Merge Candidate: May mean a spatial merge candidate, a temporal merge candidate, a combined merge candidate, a combined bi-prediction merge candidate, a zero merge candidate, and the like. The merge candidate may include motion information such as an inter prediction indicator, a reference image index for each list, a motion vector, a prediction list utilization flag, and an inter prediction indicator.

Merge Index: May mean an indicator indicating a merge candidate in the merge candidate list. Also, the merge index may indicate a block from which a merge candidate is derived from among blocks reconstructed spatially/temporally adjacent to the current block. Also, the merge index may indicate at least one of motion information of the merge candidate.

Transform Unit: It may mean a basic unit when encoding/decoding a residual signal such as transform, inverse transform, quantization, inverse quantization, and transform coefficient encoding/decoding. One transform unit may be divided into a plurality of sub-transform units having a smaller size. Here, the transform/inverse transform may include at least one of a first-order transform/inverse transform and a second-order transform/inverse transform.

Scaling: This may mean a process of multiplying a quantized level by a factor. Transform coefficients can be generated as a result of scaling for the quantized level. Scaling can also be called dequantization.

Quantization Parameter: In quantization, it may mean a value used when generating a quantized level using a transform coefficient. Alternatively, it may mean a value used when generating a transform coefficient by scaling a quantized level in inverse quantization. The quantization parameter may be a value mapped to a quantization step size.

Residual quantization parameter (Delta Quantization Parameter): This may mean a difference value between the predicted quantization parameter and the quantization parameter of the encoding/decoding target unit.

Scan: This can mean a method of arranging the order of coefficients within a unit, block, or matrix. For example, sorting a two-dimensional array into a one-dimensional array is called a scan. Alternatively, arranging a one-dimensional array into a two-dimensional array may also be referred to as a scan or an inverse scan.

Transform Coefficient: This may mean a coefficient value generated after transformation is performed by an encoder. Alternatively, it may mean a coefficient value generated after performing at least one of entropy decoding and inverse quantization in the decoder. A quantized level obtained by applying quantization to a transform coefficient or a residual signal or a quantized transform coefficient level may also be included in the meaning of the transform coefficient.

Quantized Level: This may mean a value generated by quantizing a transform coefficient or a residual signal in an encoder. Alternatively, it may mean a value that is the target of inverse quantization before the decoder performs inverse quantization. Similarly, a quantized transform coefficient level resulting from transform and quantization may also be included in the meaning of the quantized level.

Non-zero transform coefficient (Non-zero Transform Coefficient): This may mean a transform coefficient whose size is not 0, or a transform coefficient level whose size is not 0 or a quantized level.

Quantization Matrix: This may mean a matrix used in a quantization or inverse quantization process in order to improve subjective or objective quality of an image. The quantization matrix may also be called a scaling list.

Quantization Matrix Coefficient: May mean each element in a quantization matrix. The quantization matrix coefficient may also be referred to as a matrix coefficient.

Default matrix: This may mean a predetermined quantization matrix defined in advance in an encoder and a decoder.

Non-default Matrix: This may mean a quantization matrix that is not predefined by an encoder and a decoder and is signaled by a user.

Statistical value: The statistical value for at least one of the variables, encoding parameters, constants, etc. that has specific operable values is the average value, weighted average value, weighted sum, minimum value, maximum value, mode, median value, interpolation It may be at least one or more of the values.

1 is a block diagram showing a configuration according to an embodiment of an encoding apparatus to which the present invention is applied.

The encoding device 100 may be an encoder, a video encoding device, or an image encoding device. A video may include one or more images. The encoding apparatus 100 may sequentially encode one or more images.

1, the encoding apparatus 100 includes a motion prediction unit 111, a motion compensation unit 112, an intra prediction unit 120, a switch 115, a subtractor 125, a transform unit 130, and a quantization unit. A unit 140, an entropy encoder 150, an inverse quantization unit 160, an inverse transform unit 170, an adder 175, a filter unit 180, and a reference picture buffer 190 may be included.

The encoding apparatus 100 may encode an input image in an intra mode and/or an inter mode. Also, the encoding apparatus 100 may generate a bitstream including information encoded by encoding an input image, and may output the generated bitstream. The generated bitstream may be stored in a computer-readable recording medium or streamed through a wired/wireless transmission medium. When the intra mode is used as the prediction mode, the switch 115 may be switched to intra, and when the inter mode is used as the prediction mode, the switch 115 may be switched to inter. Here, the intra mode may mean an intra prediction mode, and the inter mode may mean an inter prediction mode. The encoding apparatus 100 may generate a prediction block for an input block of an input image. Also, after the prediction block is generated, the encoding apparatus 100 may encode the residual block by using a residual between the input block and the prediction block. The input image may be referred to as a current image that is a current encoding target. The input block may be referred to as a current block or a current block to be encoded.

When the prediction mode is an intra mode, the intra prediction unit 120 may use a sample of a block already encoded/decoded around the current block as a reference sample. The intra prediction unit 120 may perform spatial prediction for the current block using the reference sample, and may generate prediction samples for the input block through spatial prediction. Here, intra prediction may mean intra prediction.

When the prediction mode is the inter mode, the motion prediction unit 111 may search for an area that best matches the input block from the reference image in the motion prediction process, and may derive a motion vector using the searched area. . In this case, a search area may be used as the area. The reference image may be stored in the reference picture buffer 190. Here, when encoding/decoding of the reference image is processed, it may be stored in the reference picture buffer 190.

The motion compensation unit 112 may generate a prediction block for the current block by performing motion compensation using a motion vector. Here, inter prediction may mean inter prediction or motion compensation.

When the motion vector value does not have an integer value, the motion prediction unit 111 and the motion compensation unit 112 may generate a prediction block by applying an interpolation filter to a partial region of a reference image. . In order to perform inter prediction or motion compensation, the motion prediction and motion compensation method of the prediction unit included in the corresponding coding unit based on the coding unit is a skip mode, merge mode, and improved motion vector prediction ( It is possible to determine whether the method is an Advanced Motion Vector Prediction (AMVP) mode or a current picture reference mode, and perform inter prediction or motion compensation according to each mode.

The subtractor 125 may generate a residual block by using a difference between the input block and the prediction block. The residual block may be referred to as a residual signal. The residual signal may mean a difference between the original signal and the predicted signal. Alternatively, the residual signal may be a signal generated by transforming, quantizing, or transforming and quantizing a difference between the original signal and the predicted signal. The residual block may be a residual signal in units of blocks.

The transform unit 130 may transform the residual block to generate a transform coefficient, and may output the generated transform coefficient. Here, the transform coefficient may be a coefficient value generated by performing transform on the residual block. When the transform skip mode is applied, the transform unit 130 may omit the transform of the residual block.

A quantized level may be generated by applying quantization to a transform coefficient or a residual signal. Hereinafter, in embodiments, the quantized level may also be referred to as a transform coefficient.

The quantization unit 140 may generate a quantized level by quantizing a transform coefficient or a residual signal according to a quantization parameter, and may output the generated quantized level. In this case, the quantization unit 140 may quantize the transform coefficient using a quantization matrix.

The entropy encoding unit 150 may generate a bitstream by performing entropy encoding according to a probability distribution on values calculated by the quantization unit 140 or values of a coding parameter calculated during an encoding process. Yes, and can output a bitstream. The entropy encoder 150 may perform entropy encoding on information about a sample of an image and information for decoding an image. For example, information for decoding an image may include a syntax element or the like.

When entropy coding is applied, a small number of bits are allocated to a symbol having a high probability of occurrence, and a large number of bits are allocated to a symbol having a low probability of occurrence, and the symbol is expressed. The size of the column can be reduced. The entropy encoding unit 150 may use an encoding method such as exponential Golomb, Context-Adaptive Variable Length Coding (CAVLC), and Context-Adaptive Binary Arithmetic Coding (CABAC) for entropy encoding. For example, the entropy encoding unit 150 may perform entropy encoding using a variable length encoding (VLC) table. In addition, the entropy encoding unit 150 derives the binarization method of the target symbol and the probability model of the target symbol/bin, and then the derived binarization method, probability model, and context model. Arithmetic coding can also be performed using.

The entropy encoder 150 may change a two-dimensional block form coefficient into a one-dimensional vector form through a transform coefficient scanning method in order to encode a transform coefficient level (quantized level).

The coding parameter may include information (flags, indexes, etc.) encoded by the encoder and signaled by the decoder, such as syntax elements, as well as information derived during the encoding process or the decoding process, and the image is to be encoded or decoded. It can mean the information you need at the time. For example, unit/block size, unit/block depth, unit/block division information, unit/block type, unit/block division structure, quadtree type division, binary tree type division, binary tree type division Direction (horizontal or vertical direction), binary tree-type division type (symmetric division or asymmetric division), 3-division tree type division, 3-division tree type division direction (horizontal or vertical direction), 3-division tree type Division type (symmetric division or asymmetric division), whether to divide complex tree type, division direction of complex tree type (horizontal or vertical direction), division type of complex tree type (symmetric division or asymmetric division), complex Segmented tree in the form of a tree (binary tree or three-segmented tree), prediction mode (in-screen prediction or inter-screen prediction), intra-screen luminance prediction mode/direction, intra-screen color difference prediction mode/direction, intra-screen split information, inter-screen Split information, coded block split flag, prediction block split flag, transform block split flag, reference sample filtering method, reference sample filter tab, reference sample filter coefficient, prediction block filtering method, prediction block filter tab, prediction block filter coefficient, prediction block Boundary filtering method, prediction block boundary filter tap, prediction block boundary filter coefficient, intra prediction mode, inter prediction mode, motion information, motion vector, motion vector difference, reference image index, inter prediction direction, inter prediction indicator, Prediction list utilization flag, reference picture list, reference picture, motion vector prediction index, motion vector prediction candidate, motion vector candidate list, whether to use merge mode, merge index, merge candidate, merge candidate list, skip mode, Interpolation filter type, interpolation filter tab, interpolation filter coefficient, motion vector size, motion vector expression accuracy, transform type, transform size, information on whether to use first-order transform, information on whether to use second-order transform, first-order transform index, second-order transform index , Residual signal presence information, coded block pattern, coded block pattern Lag (Coded Block Flag), quantization parameter, residual quantization parameter, quantization matrix, whether to apply in-screen loop filter, in-screen loop filter coefficient, in-screen loop filter tab, in-screen loop filter shape/shape, and deblocking filter applied Whether to apply, deblocking filter coefficient, deblocking filter tap, deblocking filter strength, deblocking filter shape/shape, whether to apply adaptive sample offset, adaptive sample offset value, adaptive sample offset category, adaptive sample offset type, adaptation Whether to apply an adaptive loop filter, adaptive loop filter coefficient, adaptive loop filter tap, adaptive loop filter shape/shape, binarization/inverse binarization method, context model determination method, context model update method, regular mode execution, bypass mode Execution status, context bin, bypass bin, significant coefficient flag, last significant coefficient flag, coefficient group unit encoding flag, last significant coefficient position, flag for whether the coefficient value is greater than 1, flag for whether the coefficient value is greater than 2, Flag for whether the coefficient value is greater than 3, remaining coefficient value information, sign information, reconstructed luminance sample, reconstructed chrominance sample, residual luminance sample, residual chrominance sample, luminance transform coefficient, chrominance transform coefficient, luminance quantized Level, chrominance quantized level, transform coefficient level scanning method, size of a motion vector search area on the side of the decoder, shape of a motion vector search area on the side of the decoder, the number of search motion vectors on the side of the decoder, CTU size information, minimum block size information, Maximum block size information, maximum block depth information, minimum block depth information, image display/output order, slice identification information, slice type, slice division information, tile group identification information, tile group type, tile group division information, tile identification information, At least one of tile type, tile segmentation information, picture type, input sample bit depth, reconstructed sample bit depth, residual sample bit depth, transform coefficient bit depth, quantized level bit depth, information on luminance signals, and information on color difference signals of A value or a combined form may be included in the encoding parameter.

Here, signaling a flag or index may mean that the encoder entropy encodes the flag or index and includes the corresponding flag or index in the bitstream. It may mean entropy decoding.

When the encoding apparatus 100 performs encoding through inter prediction, the encoded current image may be used as a reference image for another image to be processed later. Accordingly, the encoding apparatus 100 may reconstruct or decode the encoded current image again, and store the reconstructed or decoded image as a reference image in the reference picture buffer 190.

The quantized level may be dequantized by the inverse quantization unit 160. It may be inverse transformed by the inverse transform unit 170. The inverse quantized and/or inverse transformed coefficient may be summed with the prediction block through the adder 175, and a reconstructed block may be generated by adding the inverse quantized and/or inverse transformed coefficient and the prediction block. Here, the inverse quantized and/or inverse transformed coefficient means a coefficient in which at least one of inverse quantization and inverse transform is performed, and may mean a reconstructed residual block.

The restoration block may pass through the filter unit 180. The filter unit 180 converts at least one such as a deblocking filter, a sample adaptive offset (SAO), and an adaptive loop filter (ALF) to a reconstructed sample, a reconstructed block, or a reconstructed image. Can be applied. The filter unit 180 may also be referred to as an in-loop filter.

The deblocking filter may remove block distortion occurring at the boundary between blocks. In order to determine whether to perform the deblocking filter, it may be determined whether to apply the deblocking filter to the current block based on samples included in several columns or rows included in the block. When applying a deblocking filter to a block, different filters can be applied according to the required deblocking filtering strength.

An appropriate offset value may be added to a sample value to compensate for an encoding error using a sample adaptive offset. The sample adaptive offset may correct an offset from the original image in units of samples for the deblocking image. After dividing the samples included in the image into a certain number of areas, a method of determining an area to perform offset and applying an offset to the corresponding area, or a method of applying an offset in consideration of edge information of each sample may be used.

The adaptive loop filter may perform filtering based on a value obtained by comparing the reconstructed image and the original image. After dividing the samples included in the image into predetermined groups, a filter to be applied to the corresponding group may be determined, and filtering may be performed differentially for each group. Information related to whether to apply the adaptive loop filter may be signaled for each coding unit (CU), and the shape and filter coefficients of the adaptive loop filter to be applied may vary according to each block.

The reconstructed block or reconstructed image that has passed through the filter unit 180 may be stored in the reference picture buffer 190. The reconstructed block that has passed through the filter unit 180 may be a part of the reference image. In other words, the reference image may be a reconstructed image composed of reconstructed blocks that have passed through the filter unit 180. The stored reference image can then be used for inter-screen prediction or motion compensation.

2 is a block diagram showing a configuration according to an embodiment of a decoding apparatus to which the present invention is applied.

The decoding device 200 may be a decoder, a video decoding device, or an image decoding device.

Referring to FIG. 2, the decoding apparatus 200 includes an entropy decoding unit 210, an inverse quantization unit 220, an inverse transform unit 230, an intra prediction unit 240, a motion compensation unit 250, and an adder 255. , A filter unit 260 and a reference picture buffer 270 may be included.

The decoding apparatus 200 may receive a bitstream output from the encoding apparatus 100. The decoding apparatus 200 may receive a bitstream stored in a computer-readable recording medium or a bitstream streamed through a wired/wireless transmission medium. The decoding apparatus 200 may perform decoding on a bitstream in an intra mode or an inter mode. Also, the decoding apparatus 200 may generate a reconstructed image or a decoded image through decoding, and may output a reconstructed image or a decoded image.

When the prediction mode used for decoding is an intra mode, the switch may be switched to intra. When the prediction mode used for decoding is the inter mode, the switch may be switched to inter.

The decoding apparatus 200 may obtain a reconstructed residual block by decoding the input bitstream, and may generate a prediction block. When the reconstructed residual block and the prediction block are obtained, the decoding apparatus 200 may generate a reconstructed block to be decoded by adding the reconstructed residual block and the prediction block. The block to be decoded may be referred to as a current block.

The entropy decoding unit 210 may generate symbols by performing entropy decoding according to a probability distribution for a bitstream. The generated symbols may include quantized level symbols. Here, the entropy decoding method may be a reverse process of the entropy encoding method described above.

The entropy decoder 210 may change a one-dimensional vector form coefficient into a two-dimensional block form through a transform coefficient scanning method in order to decode a transform coefficient level (quantized level).

The quantized level may be inverse quantized by the inverse quantization unit 220 and may be inversely transformed by the inverse transform unit 230. The quantized level is a result of performing inverse quantization and/or inverse transformation, and may be generated as a reconstructed residual block. In this case, the inverse quantization unit 220 may apply a quantization matrix to the quantized level.

When the intra mode is used, the intra prediction unit 240 may generate a prediction block by performing spatial prediction using a sample value of an already decoded block adjacent to the decoding target block on the current block.

When the inter mode is used, the motion compensation unit 250 may generate a prediction block by performing motion compensation on the current block using a motion vector and a reference image stored in the reference picture buffer 270. When the motion vector does not have an integer value, the motion compensation unit 250 may generate a prediction block by applying an interpolation filter to a partial region of a reference image. In order to perform motion compensation, it is possible to determine whether the motion compensation method of the prediction unit included in the coding unit is a skip mode, a merge mode, an AMVP mode, or a current picture reference mode based on the coding unit, and each mode According to the motion compensation can be performed.

The adder 255 may generate a reconstructed block by adding the reconstructed residual block and the prediction block. The filter unit 260 may apply at least one of a deblocking filter, a sample adaptive offset, and an adaptive loop filter to the reconstructed block or reconstructed image. The filter unit 260 may output a reconstructed image. The reconstructed block or reconstructed image may be stored in the reference picture buffer 270 and used for inter prediction. The reconstructed block that has passed through the filter unit 260 may be a part of the reference image. In other words, the reference image may be a reconstructed image composed of reconstructed blocks that have passed through the filter unit 260. The stored reference image can then be used for inter-screen prediction or motion compensation.

3 is a diagram schematically illustrating a split structure of an image when encoding and decoding an image. 3 schematically shows an embodiment in which one unit is divided into a plurality of sub-units.

In order to efficiently divide an image, a coding unit (CU) may be used in encoding and decoding. An encoding unit may be used as a basic unit of image encoding/decoding. In addition, when encoding/decoding an image, an encoding unit may be used as a unit into which an intra prediction mode and an inter prediction mode are classified. The coding unit may be a basic unit used for a process of prediction, transform, quantization, inverse transform, inverse quantization, or encoding/decoding of transform coefficients.

Referring to FIG. 3, an image 300 is sequentially segmented in units of a largest coding unit (LCU), and a segmentation structure is determined in units of an LCU. Here, the LCU may be used in the same meaning as a coding tree unit (CTU). The division of a unit may mean division of a block corresponding to a unit. The block division information may include information on the depth of the unit. The depth information may indicate the number and/or degree of division of the unit. One unit may be hierarchically divided into a plurality of sub-units with depth information based on a tree structure. In other words, a unit and a sub-unit generated by the division of the unit may correspond to a node and a child node of the node, respectively. Each divided sub-unit may have depth information. The depth information may be information indicating the size of the CU, and may be stored for each CU. Since the unit depth indicates the number and/or degree of division of the unit, the division information of the sub-unit may include information on the size of the sub-unit.

The split structure may refer to a distribution of a coding unit (CU) within the CTU 310. This distribution may be determined according to whether or not to divide one CU into a plurality (a positive integer of 2 or more including 2, 4, 8, 16, etc.). The horizontal and vertical dimensions of the CU generated by the division are either half the horizontal size and half the vertical size of the CU before division, or a size smaller than the horizontal size and the vertical size of the CU before division, depending on the number of divisions. Can have. The CU can be recursively divided into a plurality of CUs. By recursive partitioning, at least one of a horizontal size and a vertical size of a divided CU may be reduced compared to at least one of a horizontal size and a vertical size of the CU before division. The partitioning of the CU can be recursively performed up to a predefined depth or a predefined size. For example, the depth of the CTU may be 0, and the depth of the Smallest Coding Unit (SCU) may be a predefined maximum depth. Here, the CTU may be a coding unit having the largest coding unit size as described above, and the SCU may be a coding unit having the smallest coding unit size. The division starts from the CTU 310, and the depth of the CU increases by one whenever the horizontal size and/or the vertical size of the CU is reduced by the division. For example, for each depth, a CU that is not divided may have a size of 2Nx2N. In addition, in the case of a divided CU, a CU having a size of 2Nx2N may be divided into four CUs having a size of NxN. The size of N can be halved for each increase in depth by 1.

Also, information on whether the CU is divided may be expressed through partition information of the CU. The division information may be 1-bit information. All CUs except the SCU may include partition information. For example, if the value of the split information is a first value, the CU may not be split, and if the value of the split information is a second value, the CU can be split.

Referring to FIG. 3, a CTU having a depth of 0 may be a 64x64 block. 0 can be the minimum depth. An SCU of depth 3 may be an 8x8 block. 3 can be the maximum depth. CUs of 32x32 blocks and 16x16 blocks may be represented by depth 1 and depth 2, respectively.

For example, when one coding unit is split into four coding units, the horizontal and vertical sizes of the four split coding units may each have a size of half compared to the horizontal and vertical sizes of the coding units before being split. have. For example, when a 32x32 coding unit is divided into four coding units, each of the divided four coding units may have a size of 16x16. When one coding unit is divided into four coding units, it can be said that the coding unit is divided into a quad-tree (quad-tree partition).

For example, when one coding unit is split into two coding units, the horizontal or vertical size of the two split coding units may have a size of half compared to the horizontal or vertical size of the coding unit before being split. . For example, when a 32x32 coding unit is vertically split into two coding units, each of the two split coding units may have a size of 16x32. For example, when an 8x32 coding unit is horizontally split into two coding units, each of the two split coding units may have a size of 8x16. When one coding unit is divided into two coding units, it can be said that the coding unit is divided into a binary-tree (binary-tree partition).

For example, when one coding unit is split into three coding units, it may be split into three coding units by dividing the horizontal or vertical size of the coding unit before splitting in a ratio of 1:2:1. For example, when a coding unit having a size of 16x32 is horizontally split into three coding units, the three split coding units may have sizes of 16x8, 16x16, and 16x8, respectively, from the top. For example, when a 32x32 coding unit is vertically split into three coding units, the split three coding units may have sizes of 8x32, 16x32, and 8x32 from the left, respectively. When one coding unit is divided into three coding units, it can be said that the coding unit is divided into a ternary-tree (ternary-tree partition).

The CTU 320 of FIG. 3 is an example of a CTU to which quadtree division, binary tree division, and three-division tree division are all applied.

As described above, in order to divide the CTU, at least one of quadtree division, binary tree division, and three-division tree division may be applied. Each division can be applied based on a predetermined priority. For example, quadtree division may be preferentially applied to the CTU. An encoding unit that can no longer be divided into a quadtree may correspond to a leaf node of a quadtree. The coding unit corresponding to the leaf node of the quadtree may be a root node of a binary tree and/or a three-division tree. That is, the coding unit corresponding to the leaf node of the quadtree may be divided into a binary tree, divided into a three-divided tree, or may not be further divided. At this time, the coding unit corresponding to the leaf node of the quadtree is divided into a binary tree or generated by dividing a three-divided tree so that quadtree division is not performed again, so that block division and/or signaling of division information is performed. It can be done effectively.

The division of the coding unit corresponding to each node of the quadtree may be signaled using quad division information. Quad splitting information having a first value (eg, '1') may indicate that the corresponding coding unit is quadtree split. Quad segmentation information having a second value (eg, '0') may indicate that the corresponding coding unit is not quadtree segmented. The quad division information may be a flag having a predetermined length (eg, 1 bit).

Priority may not exist between the binary tree division and the three-division tree division. That is, the coding unit corresponding to the leaf node of the quadtree may be divided into a binary tree or divided into a three-division tree. In addition, the coding unit generated by the binary tree division or the three-division tree division may be again divided into the binary tree or the three-division tree, or may not be further divided.

Partitioning when there is no priority between binary tree partitioning and three-partition tree partitioning can be referred to as a multi-type tree partition. That is, the coding unit corresponding to the leaf node of the quad tree may be the root node of the multi-type tree. The division of the coding unit corresponding to each node of the complex type tree may be signaled using at least one of information about whether to divide the complex type tree, information about a division direction, and information about a division tree. In order to divide a coding unit corresponding to each node of the composite tree, information about whether to be divided, information about a division direction, and information about a division tree may be sequentially signaled.

The information on whether to split the composite type tree having the first value (eg, '1') may indicate that the corresponding coding unit is split the composite type tree. The information on whether to split the composite type tree having the second value (eg, '0') may indicate that the corresponding coding unit is not split the composite type tree.

When the coding unit corresponding to each node of the composite tree is split into the composite tree, the coding unit may further include split direction information. The division direction information may indicate the division direction of the complex type tree division. Split direction information having a first value (eg, '1') may indicate that the corresponding encoding unit is split in the vertical direction. The division direction information having the second value (eg, '0') may indicate that the corresponding encoding unit is divided in the horizontal direction.

When the coding unit corresponding to each node of the composite tree is split into the composite tree, the coding unit may further include split tree information. The split tree information can indicate a tree used for splitting a composite tree. Split tree information having a first value (eg, '1') may indicate that the corresponding coding unit is divided into a binary tree. Split tree information having a second value (eg, '0') may indicate that the corresponding coding unit is split into a three-divided tree.

The information on whether to be divided, information on the division tree, and information on the division direction may be flags each having a predetermined length (eg, 1 bit).

At least one of quad split information, information on whether to split the composite tree, split direction information, and split tree information may be entropy encoded/decoded. For entropy encoding/decoding of the information, information on a neighboring encoding unit adjacent to the current encoding unit may be used. For example, the split form (whether or not, the split tree and/or the split direction) of the left coding unit and/or the upper coding unit is likely to be similar to the split form of the current coding unit. Accordingly, it is possible to derive context information for entropy encoding/decoding of information of the current encoding unit based on information of the neighboring encoding unit. In this case, the information on the neighboring coding unit may include at least one of quad split information of the corresponding coding unit, information on whether to split a complex type tree, information on a split direction, and information on a split tree.

As another embodiment, among the binary tree division and the three-division tree division, the binary tree division may be performed preferentially. That is, the binary tree division is applied first, and the coding unit corresponding to the leaf node of the binary tree may be set as the root node of the three-division tree. In this case, quadtree splitting and binary tree splitting may not be performed for the coding unit corresponding to the node of the three-division tree.

A coding unit that is no longer split by quadtree splitting, binary tree splitting, and/or three-divided tree splitting may be a unit of coding, prediction, and/or transformation. That is, the coding unit may no longer be split for prediction and/or transformation. Therefore, a split structure for splitting the coding unit into a prediction unit and/or a transform unit, split information, etc. may not exist in the bitstream.

However, when the size of the coding unit serving as the division unit is larger than the size of the largest transform block, the corresponding coding unit may be recursively split until the size of the coding unit becomes equal to or smaller than the size of the largest transform block. For example, when the size of the coding unit is 64x64 and the size of the maximum transform block is 32x32, the coding unit may be divided into four 32x32 blocks for transformation. For example, when the size of the coding unit is 32x64 and the size of the maximum transform block is 32x32, the coding unit may be divided into two 32x32 blocks for transformation. In this case, whether or not to split the coding unit for transformation is not separately signaled, and may be determined by comparing the width or height of the coding unit and the width or height of the largest transform block. For example, when the width of the coding unit is larger than the width of the largest transform block, the coding unit may be vertically divided into two. In addition, when the length of the coding unit is greater than the length of the maximum transform block, the coding unit may be horizontally divided into two.

Information about the maximum and/or minimum size of the coding unit, and information about the maximum and/or minimum size of the transform block may be signaled or determined at a higher level of the coding unit. The higher level may be, for example, a sequence level, a picture level, a tile level, a tile group level, a slice level, and the like. For example, the minimum size of the coding unit may be determined to be 4x4. For example, the maximum size of the transform block may be determined to be 64x64. For example, the minimum size of the transform block may be determined to be 4x4.

Information on the minimum size (minimum quadtree size) of the coding unit corresponding to the leaf node of the quadtree and/or the maximum depth from the root node to the leaf node of the complex tree (maximum depth of the complex tree) is encoded. It can be signaled or determined at a higher level of the unit. The higher level may be, for example, a sequence level, a picture level, a slice level, a tile group level, and a tile level. The information on the minimum quadtree size and/or the maximum depth of the composite tree may be signaled or determined for each of an intra-screen slice and an inter-screen slice.

Difference information about the size of the CTU and the maximum size of the transform block may be signaled or determined at a higher level of the coding unit. The higher level may be, for example, a sequence level, a picture level, a slice level, a tile group level, and a tile level. Information on the maximum size (the maximum size of the binary tree) of the coding unit corresponding to each node of the binary tree may be determined based on the size of the coding tree unit and the difference information. The maximum size of the coding unit corresponding to each node of the three-division tree (the maximum size of the three-division tree) may have a different value according to the type of the slice. For example, in the case of an intra-screen slice, the maximum size of a three-division tree may be 32x32. Further, for example, in the case of an inter-screen slice, the maximum size of a three-division tree may be 128x128. For example, the minimum size of the coding unit corresponding to each node of the binary tree (the minimum size of the binary tree) and/or the minimum size of the coding unit corresponding to each node of the three-division tree (the minimum size of the three-division tree) is the minimum size of the coding block. Can be set to size.

As another example, the maximum size of the binary tree and/or the maximum size of the three-division tree may be signaled or determined at the slice level. Also, the minimum size of the binary tree and/or the minimum size of the three-divided tree may be signaled or determined at the slice level.

Based on the size and depth information of various blocks described above, quad split information, information on whether to split a composite tree, split tree information, and/or split direction information may or may not exist in the bitstream.

For example, if the size of the coding unit is not larger than the quadtree minimum size, the coding unit does not include quad split information, and the quad split information may be inferred as a second value.

For example, if the size (horizontal and vertical) of the coding unit corresponding to the node of the composite tree is larger than the maximum size of the binary tree (horizontal and vertical) and/or the maximum size of the three-segment tree (horizontal and vertical), the coding unit The binary tree may not be divided and/or the three divided tree may not be divided. Accordingly, information on whether to split the composite tree is not signaled and can be inferred as the second value.

Alternatively, the size (horizontal and vertical) of the coding unit corresponding to the node of the complex tree is the same as the minimum size of the binary tree (horizontal and vertical), or the size of the coding unit (horizontal and vertical) is the minimum size of the three-segment tree (horizontal). And vertical), the coding unit may not be divided into a binary tree and/or a three-divided tree. Accordingly, information on whether to split the composite tree is not signaled and can be inferred as the second value. This is because when the coding unit is divided into a binary tree and/or a three-division tree, a coding unit smaller than the minimum size of the binary tree and/or the minimum size of the three-division tree is generated.

Alternatively, the binary tree division or the three-division tree division may be limited based on the size of the virtual pipeline data unit (hereinafter, the size of the pipeline buffer). For example, when an encoding unit is divided into sub-coding units that are not suitable for the pipeline buffer size by binary tree division or 3-division tree division, the corresponding binary tree division or 3-division tree division may be limited. The pipeline buffer size may be the size of the maximum transform block (eg, 64X64). For example, when the pipeline buffer size is 64X64, the partition below may be limited.

-Three-division tree division for NxM (N and/or M is 128) coding units

-Splitting a binary tree in the horizontal direction for 128xN (N <= 64) coding units

-Split vertical binary tree for Nx128 (N <= 64) coding units

Alternatively, when the depth in the complex type tree of the coding unit corresponding to the node of the complex type tree is equal to the maximum depth of the complex type tree, the coding unit may not be divided into a binary tree and/or a three-divided tree. Accordingly, information on whether to split the composite tree is not signaled and can be inferred as the second value.

Alternatively, only when at least one of vertical binary tree division, horizontal binary tree division, vertical 3-division tree division, and horizontal 3-division tree division is possible for the coding unit corresponding to the node of the complex tree, the complex type It is possible to signal whether the tree is divided. Otherwise, the coding unit may not be divided into a binary tree and/or a three-divided tree. Accordingly, information on whether to split the composite tree is not signaled and can be inferred as the second value.

Alternatively, only when both vertical binary tree division and horizontal binary tree division are possible for the coding unit corresponding to the node of the complex tree, or when both vertical and horizontal 3-division tree divisions are possible, the above Split direction information can be signaled. Otherwise, the division direction information is not signaled and may be deduced as a value indicating a direction in which division is possible.

Alternatively, only when both the vertical direction binary tree division and the vertical direction 3 division tree division are possible for the coding unit corresponding to the node of the complex tree, or both the horizontal direction binary tree division and the horizontal direction 3 division tree division are possible, the above Split tree information can be signaled. Otherwise, the split tree information is not signaled and may be inferred as a value indicating a splittable tree.

4 is a diagram for describing an embodiment of an intra prediction process.

Arrows from the center to the outside of FIG. 4 may indicate prediction directions of intra prediction modes.

Intra-picture encoding and/or decoding may be performed using reference samples of neighboring blocks of the current block. The neighboring block may be a restored neighboring block. For example, intra-picture encoding and/or decoding may be performed using values of reference samples or encoding parameters included in reconstructed neighboring blocks.

The prediction block may mean a block generated as a result of performing intra prediction. The prediction block may correspond to at least one of CU, PU, and TU. The unit of the prediction block may be the size of at least one of CU, PU, and TU. The prediction block may be a square-shaped block having a size of 2x2, 4x4, 16x16, 32x32, or 64x64, or a rectangular block having a size of 2x8, 4x8, 2x16, 4x16, and 8x16.

Intra prediction may be performed according to an intra prediction mode for the current block. The number of intra prediction modes that the current block can have may be a predefined fixed value, and may be differently determined according to the property of the prediction block. For example, the properties of the prediction block may include the size of the prediction block and the shape of the prediction block.

The number of intra prediction modes may be fixed to N regardless of the block size. Alternatively, for example, the number of intra prediction modes may be 3, 5, 9, 17, 34, 35, 36, 65, or 67. Alternatively, the number of intra prediction modes may be different according to the size of the block and/or the type of color component. For example, the number of intra prediction modes may differ depending on whether a color component is a luma signal or a chroma signal. For example, as the size of the block increases, the number of intra prediction modes may increase. Alternatively, the number of intra prediction modes of the luminance component block may be greater than the number of intra prediction modes of the color difference component block.

The intra prediction mode may be a non-directional mode or a directional mode. The non-directional mode may be a DC mode or a planar mode, and the angular mode may be a prediction mode having a specific direction or angle. The intra prediction mode may be expressed as at least one of a mode number, a mode value, a mode number, a mode angle, and a mode direction. The number of intra prediction modes may be one or more M including the non-directional and directional modes. Whether samples included in neighboring blocks reconstructed for intra prediction of the current block are available as reference samples of the current block The step of checking may be performed. If there is a sample that cannot be used as a reference sample of the current block, a sample value of a sample that cannot be used as a reference sample by using a value obtained by copying and/or interpolating at least one sample value among samples included in the reconstructed neighboring block After replacing with, it can be used as a reference sample of the current block.

7 is a diagram for describing reference samples usable for intra prediction.

As shown in FIG. 7, for intra prediction of a current block, at least one of reference sample lines 0 to 3 may be used. In FIG. 7, samples of segment A and segment F may be padded with nearest samples of segment B and segment E, respectively, instead of being taken from a reconstructed neighboring block. Index information indicating a reference sample line to be used for intra prediction of the current block may be signaled. When the upper boundary of the current block is the boundary of the CTU, only the reference sample line 0 may be available. Therefore, in this case, the index information may not be signaled. When a reference sample line other than the reference sample line 0 is used, filtering on a prediction block to be described later may not be performed.

During intra prediction, a filter may be applied to at least one of a reference sample or a prediction sample based on at least one of an intra prediction mode and a size of a current block.

In the planner mode, when generating the prediction block of the current block, the weighted sum of the upper and left reference samples of the current sample and the upper right and lower left reference samples of the current block is used according to the position of the prediction target sample in the prediction block. A sample value of a sample to be predicted can be generated. In addition, in the case of the DC mode, when generating a prediction block of the current block, an average value of upper and left reference samples of the current block may be used. In addition, in the case of the directional mode, a prediction block may be generated using reference samples at the top, left, top right, and/or bottom left of the current block. Real-level interpolation can also be performed to generate predicted sample values.

In the case of intra-screen prediction between color components, a prediction block for the current block of the second color component may be generated based on the corresponding reconstructed block of the first color component. For example, the first color component may be a luminance component, and the second color component may be a color difference component. For intra prediction between color components, a parameter of a linear model between the first color component and the second color component may be derived based on a template. The template may include upper and/or left peripheral samples of the current block and upper and/or left peripheral samples of the reconstructed block of the first color component corresponding thereto. For example, the parameter of the linear model is a sample value of a first color component having a maximum value among samples in the template, a sample value of a second color component corresponding thereto, and a sample value of a first color component having a minimum value among samples in the template. And the sample value of the second color component corresponding thereto. When the parameters of the linear model are derived, a prediction block for the current block may be generated by applying the corresponding reconstructed block to the linear model. Depending on the image format, sub-sampling may be performed on neighboring samples of the reconstructed block of the first color component and the corresponding reconstructed block. For example, when one sample of the second color component corresponds to four samples of the first color component, one corresponding sample may be calculated by sub-sampling the four samples of the first color component. In this case, parameter derivation of the linear model and intra-screen prediction between color components may be performed based on sub-sampled corresponding samples. Whether or not intra prediction between color components is performed and/or a range of a template may be signaled as an intra prediction mode.

The current block may be divided into two or four sub-blocks in a horizontal or vertical direction. The divided sub-blocks may be sequentially restored. That is, the sub-prediction block may be generated by performing intra prediction on the sub-block. In addition, inverse quantization and/or inverse transformation may be performed on the sub-block to generate a sub residual block. A reconstructed sub block may be generated by adding the sub prediction block to the sub residual block. The reconstructed sub-block may be used as a reference sample for intra prediction of a subsequent sub-block. The sub-block may be a block including a predetermined number (eg, 16) or more. Thus, for example, when the current block is an 8x4 block or a 4x8 block, the current block may be divided into two sub-blocks. In addition, when the current block is a 4x4 block, the current block cannot be divided into sub-blocks. When the current block has a size other than that, the current block may be divided into four sub-blocks. Information on whether the sub-block-based intra prediction is performed and/or a division direction (horizontal or vertical) may be signaled. The subblock-based intra prediction may be limited to be performed only when the reference sample line 0 is used. When the sub-block-based intra prediction is performed, filtering on a prediction block to be described later may not be performed.

A final prediction block may be generated by performing filtering on the prediction block predicted in the screen. The filtering may be performed by applying a predetermined weight to a sample to be filtered, a left reference sample, an upper reference sample, and/or an upper left reference sample. The weight and/or reference sample (range, location, etc.) used for the filtering may be determined based on at least one of a block size, an intra prediction mode, and a location of the filtering target sample in the prediction block. The filtering may be performed only in a predetermined intra prediction mode (eg, DC, planar, vertical, horizontal, diagonal, and/or adjacent diagonal mode). The adjacent diagonal mode may be a mode obtained by adding or subtracting k to the diagonal mode. For example, k may be a positive integer of 8 or less.

The intra prediction mode of the current block may be predicted from the intra prediction mode of a block existing around the current block and entropy encoding/decoding may be performed. When the intra prediction mode of the current block and the neighboring block are the same, information indicating that the intra prediction mode of the current block and the neighboring block is the same may be signaled using predetermined flag information. In addition, among the intra prediction modes of a plurality of neighboring blocks, indicator information for an intra prediction mode identical to the intra prediction mode of the current block may be signaled. When the intra prediction modes of the current block and the neighboring block are different from each other, entropy encoding/decoding may be performed based on the intra prediction mode of the neighboring block to entropy encoding/decoding information on the intra prediction mode of the current block.

5 is a diagram for describing an embodiment of an inter prediction process.

The square shown in FIG. 5 may represent an image. In addition, arrows in FIG. 5 may indicate a prediction direction. Each image may be classified into an I picture (Intra Picture), a P picture (Predictive Picture), and a B picture (Bi-predictive Picture) according to the encoding type.

The I picture may be encoded/decoded through intra prediction without inter prediction. The P picture may be encoded/decoded through inter prediction using only a reference image existing in one direction (eg, forward or reverse). The B picture may be encoded/decoded through inter prediction using reference images existing in the bidirectional direction (eg, forward and backward). Further, in the case of a B picture, it may be encoded/decoded through inter prediction using reference pictures existing in bidirectional directions or inter prediction using reference pictures existing in one of the forward and reverse directions. Here, the two directions may be forward and reverse. Here, when inter prediction is used, the encoder may perform inter prediction or motion compensation, and the decoder may perform motion compensation corresponding thereto.

Hereinafter, inter prediction according to an embodiment will be described in detail.

Inter prediction or motion compensation may be performed using a reference image and motion information.

Motion information about the current block may be derived during inter prediction by each of the encoding apparatus 100 and the decoding apparatus 200. The motion information may be derived using motion information of a reconstructed neighboring block, motion information of a collocated block, and/or a block adjacent to the collocated block. The collocated block may be a block corresponding to a spatial position of the current block in a collocated picture (col picture) that has already been restored. Here, the collocated picture may be one picture from among at least one reference picture included in the reference picture list.

The method of deriving motion information may differ according to the prediction mode of the current block. For example, as a prediction mode applied for inter prediction, AMVP mode, merge mode, skip mode, merge mode with motion vector difference, sub-block merge mode, triangulation mode, inter intra combined prediction mode, afine inter There may be modes, etc. Here, the merge mode may be referred to as a motion merge mode.

For example, when AMVP is applied as a prediction mode, at least one of a motion vector of a reconstructed neighboring block, a motion vector of a collocated block, a motion vector of a block adjacent to the collocated block, and a (0, 0) motion vector is a motion vector. It is determined as a candidate, and a motion vector candidate list can be generated. A motion vector candidate can be derived using the generated motion vector candidate list. Motion information of the current block may be determined based on the derived motion vector candidate. Here, the motion vector of the collocated block or the motion vector of the block adjacent to the collocated block may be referred to as a temporal motion vector candidate, and the motion vector of the reconstructed neighboring block may be referred to as a spatial motion vector candidate. ) Can be referred to.

The encoding apparatus 100 may calculate a motion vector difference (MVD) between a motion vector of a current block and a motion vector candidate, and entropy-encode the MVD. Also, the encoding apparatus 100 may generate a bitstream by entropy encoding the motion vector candidate index. The motion vector candidate index may indicate an optimal motion vector candidate selected from motion vector candidates included in the motion vector candidate list. The decoding apparatus 200 may entropy-decode the motion vector candidate index from the bitstream, and select a motion vector candidate of the decoding target block from among the motion vector candidates included in the motion vector candidate list by using the entropy-decoded motion vector candidate index. . In addition, the decoding apparatus 200 may derive a motion vector of a decoding target block through the sum of the entropy-decoded MVD and the motion vector candidate.

Meanwhile, the encoding apparatus 100 may entropy-encode the calculated resolution information of the MVD. The decoding apparatus 200 may adjust the resolution of the entropy-decoded MVD using the MVD resolution information.

Meanwhile, the encoding apparatus 100 may calculate a motion vector difference (MVD) between a motion vector of a current block and a motion vector candidate based on the affine model, and may entropy-encode the MVD. The decoding apparatus 200 may derive an affine control motion vector of the decoding target block through the sum of the entropy-decoded MVD and the affine control motion vector candidate to derive the motion vector in sub-block units.

The bitstream may include a reference picture index indicating a reference picture. The reference image index may be entropy-encoded and signaled from the encoding apparatus 100 to the decoding apparatus 200 through a bitstream. The decoding apparatus 200 may generate a prediction block for a decoding object block based on the derived motion vector and reference image index information.

Another example of a method of deriving motion information is a merge mode. The merge mode may mean merging of motions for a plurality of blocks. The merge mode may mean a mode in which motion information of a current block is derived from motion information of a neighboring block. When the merge mode is applied, a merge candidate list may be generated using motion information of a reconstructed neighboring block and/or motion information of a collocated block. The motion information may include at least one of 1) a motion vector, 2) a reference image index, and 3) an inter prediction indicator. The prediction indicator may be unidirectional (L0 prediction, L1 prediction) or bidirectional.

The merge candidate list may represent a list in which motion information is stored. The motion information stored in the merge candidate list includes motion information of neighboring blocks adjacent to the current block (spatial merge candidate) and motion information of a block collocated to the current block in a reference image (temporal merge candidate). temporal merge candidate)), new motion information generated by a combination of motion information already in the merge candidate list, motion information of a block encoded/decoded before the current block (history-based merge candidate) And at least one of a zero merge candidate.

The encoding apparatus 100 may entropy-encode at least one of a merge flag and a merge index to generate a bitstream and then signal to the decoding apparatus 200. The merge flag may be information indicating whether to perform a merge mode for each block, and the merge index may be information about which block of neighboring blocks adjacent to the current block is to be merged. For example, neighboring blocks of the current block may include at least one of a left neighboring block, an upper neighboring block, and a temporal neighboring block of the current block.

Meanwhile, the encoding apparatus 100 may entropy-encode correction information for correcting a motion vector among motion information of the merge candidate and may signal the correction information to the decoding apparatus 200. The decoding apparatus 200 may correct the motion vector of the merge candidate selected by the merge index based on the correction information. Here, the correction information may include at least one of information on whether or not to be corrected, information on a correction direction, and information on a correction size. As described above, a prediction mode for correcting a motion vector of a merge candidate based on signaled correction information may be referred to as a merge mode having a motion vector difference.

The skip mode may be a mode in which motion information of a neighboring block is applied as it is to a current block. When the skip mode is used, the encoding apparatus 100 may entropy-encode information on which motion information of a block is to be used as motion information of the current block, and may signal the decoding apparatus 200 through a bitstream. In this case, the encoding apparatus 100 may not signal to the decoding apparatus 200 a syntax element relating to at least one of motion vector difference information, an encoding block flag, and a transform coefficient level (quantized level).

The subblock merge mode may refer to a mode in which motion information is derived in units of subblocks of a coding block (CU). When the sub-block merge mode is applied, motion information (sub-block based temporal merge candidate) and/or an affine control point of the sub-block collocated with the current sub-block in the reference image A subblock merge candidate list may be generated using an affiliate control point motion vector merge candidate.

In a triangle partition mode, each motion information is derived by dividing the current block in a diagonal direction, and each prediction sample is derived using each of the derived motion information, and each of the derived prediction samples is derived. It may mean a mode in which a predicted sample of the current block is derived by weighting.

The inter-intra combined prediction mode may mean a mode in which a prediction sample of a current block is derived by weighting a prediction sample generated by inter prediction and a prediction sample generated by intra prediction.

The decoding apparatus 200 may self-correct the derived motion information. The decoding apparatus 200 may search for a predefined area based on a reference block indicated by the derived motion information, and may derive the motion information having the minimum SAD as the corrected motion information.

The decoding apparatus 200 may compensate for a prediction sample derived through inter prediction using an optical flow.

6 is a diagram for describing a process of transformation and quantization.

As shown in FIG. 6, a quantized level may be generated by performing a transform and/or quantization process on the residual signal. The residual signal may be generated as a difference between an original block and a prediction block (an intra prediction block or an inter prediction block). Here, the prediction block may be a block generated by intra prediction or inter prediction. Here, the transformation may include at least one of a first order transformation and a second order transformation. When a first-order transform is performed on a residual signal, a transform coefficient may be generated, and a second-order transform coefficient may be generated by performing a second-order transform on the transform coefficient.

The primary transform may be performed using at least one of a plurality of pre-defined transform methods. For example, a plurality of pre-defined transformation methods may include a Discrete Cosine Transform (DST), a Discrete Sine Transform (DST), or a Karhunen-Loeve Transform (KLT) based transformation. Secondary transform may be performed on transform coefficients generated after the first transform is performed. The transformation method applied during the first transformation and/or the second transformation may be determined according to at least one of encoding parameters of the current block and/or the neighboring block. Alternatively, conversion information indicating a conversion method may be signaled. The DCT-based conversion may include, for example, DCT2, DCT-8, and the like. DST-based conversion may include, for example, DST-7.

A quantized level may be generated by performing quantization on a result of performing a first-order transformation and/or a second-order transformation or a residual signal. The quantized level may be scanned according to at least one of an upper-right diagonal scan, a vertical scan, and a horizontal scan based on at least one of an intra prediction mode or a block size/shape. For example, by scanning the coefficients of a block using up-right diagonal scanning, it can be changed to a one-dimensional vector form. Depending on the size of the transform block and/or the intra prediction mode, a vertical scan that scans a two-dimensional block shape coefficient in a column direction instead of a diagonal scan in the upper right corner, or a horizontal scan that scans a two-dimensional block shape coefficient in a row direction may be used. . The scanned quantized level may be entropy-coded and included in the bitstream.

The decoder may entropy-decode the bitstream to generate a quantized level. The quantized levels may be inverse scanned and arranged in a two-dimensional block shape. At this time, at least one of an upper right diagonal scan, a vertical scan, and a horizontal scan may be performed as a reverse scanning method.

Inverse quantization can be performed on the quantized level, second-order inverse transformation can be performed depending on whether or not the second-order inverse transformation is performed, and the result of performing the second-order inverse transformation is restored by performing a first-order inverse transformation depending on whether or not the first-order inverse transformation is performed. A residual signal can be generated.

Before in-loop filtering, inverse mapping of a dynamic range may be performed on a luminance component restored through intra prediction or inter prediction. The dynamic range can be divided into 16 equal pieces, and a mapping function for each piece can be signaled. The mapping function may be signaled at a slice level or a tile group level. An inverse mapping function for performing the inverse mapping may be derived based on the mapping function. In-loop filtering, storage of reference pictures, and motion compensation are performed in the demapped region, and the prediction block generated through inter prediction is converted to the mapped region by mapping using the mapping function, and then a reconstructed block is generated. Can be used for However, since intra prediction is performed in a mapped region, a prediction block generated by intra prediction can be used to generate a reconstructed block without mapping/demapping.

When the current block is a residual block of a color difference component, the residual block may be converted to an inversely mapped area by performing scaling on the color difference component of the mapped area. Whether the scaling is available may be signaled at a slice level or a tile group level. The scaling can be applied only when the mapping for the luma component is available and the division of the luminance component and the division of the chrominance component follow the same tree structure. The scaling may be performed based on an average of sample values of a luminance prediction block corresponding to the color difference block. In this case, when the current block uses inter prediction, the luminance prediction block may mean a mapped luminance prediction block. A value required for the scaling can be derived by referring to a lookup table using an index of a piece to which the average of the sample values of the luminance prediction block belongs. Finally, by scaling the residual block using the derived value, the residual block may be converted into an inversely mapped region. Subsequent reconstruction of a color difference component block, intra prediction, inter prediction, in-loop filtering, and storage of a reference picture may be performed in the demapped region.

Information indicating whether the mapping/inverse mapping of the luminance component and the color difference component is available may be signaled through a sequence parameter set.

The prediction block of the current block may be generated based on a block vector representing a displacement between the current block and a reference block in the current picture. In this way, a prediction mode for generating a prediction block with reference to a current picture may be referred to as an intra block copy (IBC) mode. The IBC mode can be applied to an MxN (M<=64, N<=64) coding unit. The IBC mode may include a skip mode, a merge mode, an AMVP mode, and the like. In the case of the skip mode or the merge mode, a merge candidate list is configured, and a merge index is signaled, so that one merge candidate may be specified. The specified merge candidate block vector may be used as a block vector of the current block. The merge candidate list may include at least one or more such as a spatial candidate, a history-based candidate, a candidate based on an average of two candidates, or a zero merge candidate. In the case of the AMVP mode, a differential block vector may be signaled. In addition, the prediction block vector may be derived from a left neighboring block and an upper neighboring block of the current block. An index on which neighboring block to use may be signaled. The prediction block of the IBC mode is included in the current CTU or the left CTU, and may be limited to a block in a previously reconstructed region. For example, the value of the block vector may be limited so that the predicted block of the current block is located within three 64x64 block regions prior to the 64x64 block to which the current block belongs in an encoding/decoding order. By limiting the value of the block vector in this way, it is possible to reduce memory consumption and device complexity according to the implementation of the IBC mode.

Hereinafter, a specific embodiment according to the present invention will be described with reference to FIGS. 8 to 17.

In the case of a color image, it is known that redundancy (or correlation or correlation) remains between the luminance signal (Y channel) and the color difference signal (Cb and Cr channels) even when color space conversion is performed from RGB to YCbCr color space.

This redundancy between the luminance and color difference signals is also referred to as cross component redundancy. When encoding/decoding an image, a cross component prediction technique that further improves encoding performance by additionally removing a correlation existing between a luminance signal and a color difference signal may be used. This can be referred to as CCLM (Cross Component Linear Model) in the same meaning. This may mean a technique of predicting a color difference signal from an encoded/decoded luminance signal and then encoding/decoding the color difference signal using the predicted value.

At this time, in order to predict the color difference signal from the encoded/decoded luminance signal, the relationship between the two signals is linearly modeled as shown in Equation 1, and the Cross Component Linear Model (CCLM) is used. A predicted value for a color difference pixel value can be calculated using the coded/decoded luminance signal value and the CCLM of Equation 1.

Here, (i, j) represents the spatial position of the predicted color difference pixel in the color difference block, pred _c (i, j) represents the predicted color difference pixel value at the (i, j) position in the color difference block, and rec _L ( i, j) may mean a luminance pixel value corresponding to a color difference pixel at a position (i, j) in the reconstructed luminance block spatially corresponding to the color difference block.

8 is a diagram for explaining an embodiment of predicting a color difference block from a reconstructed luminance block.

8 shows a luminance block (Luma) and a color difference block (Chroma) in a 4:2:0 color subsampling format. Here, the color difference block may be an NH x NV size block, and the luminance block may be a 2NH x 2NV size block.

In the case of a 4:2:2 color subsampling format, the color difference block may be a block having a size of NH x 2NV. In addition, in the present invention, the size of the block may be a square (ie, NH=NV) having the same horizontal and vertical size, or a rectangular shape having different NH and NV values.

The parameters α and β of Equation 1 are the reconstructed color difference signal values of the upper and left sides adjacent to the current color difference block, and the upper and left (Above) and left sides adjacent to the luminance block corresponding to the current color difference block. Left) can be obtained through a method of minimizing a regression error using a linear correlation between the restored luminance signal values.

Rec _L (i, j) of Equation 1, that is, the restored luminance pixel may be obtained by downsampling the luminance pixel value according to the spatial resolution of the color difference signal. This is because the spatial resolution of the color difference signal is lower than that of the luminance signal due to 4:2:2 or 4:2:0 color subsampling.

In FIG. 8, square boxes indicate spatial positions of reconstructed pixel values, and a position of a filled circle in a luminance block may indicate a corresponding spatial position of a pixel value obtained by downward sampling of the restored luminance pixel values. That is, it is shown that each position in the luminance block with a filled circle is a spatial position corresponding to each position in the color difference block with a filled circle. In order to obtain a corresponding luminance channel pixel value required for CCLM prediction of a color difference signal, luminance pixels must be sampled downward, and the downward sampling filter used at this time may be a 6-tap filter shown in FIG. 9.

The 6-tap filter of FIG. 9 is a filter designed in consideration of a case where the color difference pixels are located in the same position as the luminance pixels in the horizontal direction and the luminance pixels in the vertical direction (this case is generally referred to as a Type 0 position).

The filled circles of the luminance block shown in FIG. 8 indicate the locations of pixel values obtained by downward sampling the restored luminance pixels at the upper and left sides of the luminance block. Here, the filled circles of the luminance block are at the same position as the original luminance pixels in the horizontal direction, but it can be seen that they are the Type 0 positions located in the middle between two lines of luminance pixels in the vertical direction.

<Equation between pixels obtained by down-sampling the restored color difference pixels located at the top and left and the restored luminance pixels located at the top and left indicated by the circles filled in FIG. 8 Parameters α and β values can be obtained through the regression analysis according to 1>. The above method can be referred to as CCLM-Above-Left mode or simply CCLM mode in the sense that parameters α and β values of the CCLM model are obtained using only pixels located at the top and left.

In addition, in some cases, the regression according to <Equation 1> between the pixels obtained by down-sampling the restored color difference pixels located at the upper part (Above) and the restored luminance pixels located at the upper part (Above) indicated by the circles filled in FIG. It is also possible to obtain the parameters α and β values through regression analysis. The above method can be referred to as the CCLM-Above mode in the sense that the parameters α and β of the CCLM model are obtained using only pixels located at the top.

In addition, in some cases, the regression according to <Equation 1> between pixels obtained by down-sampling the restored color difference pixels located on the left and the restored luminance pixels located on the left indicated by the circles filled in FIG. It is also possible to obtain the parameters α and β values through regression analysis. The above method can be referred to as the CCLM-Left mode in the sense that the parameters α and β of the CCLM model are obtained using only pixels located on the left.

For the color difference block, CCLM prediction may be performed by selecting one of the modes of CCLM-Above-Left, CCLM-Left, and CCLM-Above in terms of encoding efficiency.

In addition, the encoder can signal which of these three modes is used to the decoder.

The values of the parameters α and β may be determined by one of the CCLM-Above-Left mode, the CCLM-Left mode, or the CCLM-Above mode. A predicted value for each pixel value of the current color difference block may be obtained by applying the determined parameters α and β values and the down-sampled luminance pixel values to Equation 1.

When performing CCLM prediction, regardless of whether the CCLM-Above-Left mode, the CCLM-Left mode, or the CCLM-Above mode is used, the 6-tap filter down-sampling filter of FIG. 9 can always be used.

However, it may be more advantageous in terms of coding efficiency to use a down-sampling filter adaptively according to the characteristics of the current color difference block. That is, any one of a plurality of down-sampling filters may be selected and used for CCLM prediction.

According to an embodiment of the present invention, the encoder may signal information indicating a filter used when performing CCLM prediction to a decoder.

Meanwhile, when signaling which filter is used separately, it may be disadvantageous in terms of coding efficiency. Accordingly, a down-sampling filter used for CCLM prediction may be implicitly determined based on at least one of the CCLM mode of the current color difference block, the position of the block, the shape of the block, and the size of the block. In this case, since signaling of information on the down-sampling filter is not required, coding efficiency can be improved.

10 is a diagram illustrating various embodiments of a down-sampling filter according to the present invention.

Referring to FIG. 10, at least one of a 3x3 filter, a 3x2 filter, a 3x1 filter, a 1x3 filter, a 2x3 filter, a 2x2 filter, a 2x1 filter, a 1x2 filter, a cross filter, and an X-shaped filter may be used as a downsampling filter.

The shape of the filter shown in FIG. 10 represents the types of possible filters.

Meanwhile, since the shape of the filter determines the number of pixels to which the down-sampling filtering is applied and the location of the pixels, it can be an important factor in determining the amount of computation, memory amount, and buffer amount required for image encoding and decoding. In general, in order to reduce the complexity and cost of the system, it is preferable that the number of pixels involved in down-sampling filtering is small, and the position of the pixels is designed so that the required amount of the line buffer is small.

11 is a diagram for explaining an embodiment in which a down-sampling filter is determined according to a CCLM mode.

It may be implemented to use a predetermined down-sampling filter based on the CCLM mode of the current color difference block. If this method is used, there is an advantage that it is not necessary to separately transmit the selection information of the down-sampling filter or to have the selection information in the bitstream. That is, there is an advantage in that coding efficiency can be increased by selecting and using a predetermined filter without additional down-sampling filter selection information by using only CCLM mode selection information.

On the other hand, when different down-sampling filters to be used for the CCLM-Left mode and CCLM-Above mode are previously set, one down-sampling filter may be rotated 90 degrees to configure another down-sampling filter. The configurations of Embodiment 1-5 and Embodiment 1-6 of FIG. 11 may correspond to this. In this case, one filter has a shape in which another filter is rotated 90 degrees, but the filter coefficient values of each filter may be set differently as shown in FIG. 12.

13 is a diagram illustrating relative downward sampling positions of luminance and color difference pixels.

Specifically, FIG. 13 shows vertical and horizontal positions of luminance and color difference pixels according to a 4:2:0 subsampling format, a 4:2:2 subsampling format, and a 4:4:4 subsampling format. It may be implemented to use a predetermined down-sampling filter based on the relative down-sampling position of the luminance and color difference pixels. That is, the down-sampling filter used for CCLM prediction may be determined according to the subsampling format.

14 is a diagram showing a position of a luminance block to be sampled downward in an upper block. Here, the upper block may be one of a coding tree unit (CTU) and a coding unit (CU).

It may be implemented to use a predetermined down-sampling filter according to the position of the luminance block spatially corresponding to the current color difference block.

The image unit to be encoded/decoded may be a coding tree unit (CTU). Since encoding is performed for each CTU, it is impossible to use a pixel value of a CTU other than the current CTU for encoding/decoding, or it may take a lot of time because memory access is complicated. Therefore, when the luminance block to be used for CCLM prediction is close to the CTU boundary and the pixels required for down-sampling filtering exist outside the current CTU, a pixel belonging to another CTU is not required by using a smaller filter type. There is an advantage that the system can be simplified if it is configured so that the number of pixels is minimized or the number of pixels required is minimized.

When the luminance block to be used for CCLM prediction is close to the CTU boundary, such as C1, C2, C3, C4, C7 of FIG. 14, and the pixel required for down-sampling filtering exists outside the current CTU, the current CTU is Pixel values in the external area may be used. In this case, it is preferable in that the system can be simplified to avoid the need for pixels belonging to different CTUs by using smaller types of possible filters. Therefore, according to the pixels required for filtering, the down-sampling filter can be selectively applied by dividing into the following four cases.

One. When the pixels required for filtering at the top of the block and the left of the block do not exceed the current CTU: C5, C6, C8, and C9 of FIG. 14

2. When the pixel required for filtering at the top of the block exceeds the current CTU: C2, C3 of FIG. 14

3. When the pixel required for filtering on the left side of the block exceeds the current CTU: C4 and C7 of FIG. 14

4. When all the pixels required for filtering at the top and the left of the block exceed the current CTU: C1 of FIG. 14

15 is a diagram for explaining an embodiment in which a down-sampling filter is determined according to a position of a luminance block in FIG. 14.

When the down-sampling filter is determined according to the position of the luminance block as shown in FIG. 15, there is an advantage in that selection information of the down-sampling filter is not separately transmitted or the selection information is not provided in the bitstream. That is, there is an advantage in that encoding efficiency can be increased by selecting and using a predetermined filter without additional down-sampling filter selection information.

For example, when the location of the luminance block is located at the CTU boundary, the down-sampling filter used for CCLM prediction may be determined as a 1x3 filter or a 3x1 filter. On the other hand, when the location of the luminance block is not located at the CTU boundary, it may be determined as a 5-point cross-shaped filter, a 3x2 filter, or a 2x3 filter. That is, when the location of the luminance block is located at the CTU boundary, the down-sampling filter used for CCLM prediction may be determined as a filter using one sample line.

16 is a diagram for describing an embodiment in which a down-sampling filter is determined according to a shape of a block.

The shape of the block can be distinguished using the aspect ratio. The block aspect ratio represents the ratio of the horizontal length to the vertical length of a block, and can be expressed as a single number by calculating the horizontal length/vertical length. Also, the shape of the block can be expressed as a square, rectangle, or triangle.

The possible sizes of blocks are 2x2, 2x4, 2x8, 2x16, 4x2, 4x4, 4x8, 4x16, 4x32, 4x64, 8x2, 8x4, 8x8, 8x16, 8x32, 8x64,16x2, 16x4, 16x8, 16x16, 16x32,16x64,32x4 , 32x8, 32x16, 32x32, 32x64, 64x4, 64x8, 64x16, 64x32, 64x64.

If this is expressed as an aspect ratio representing the horizontal length/vertical length, it can be expressed as 1, 2, 4, 8, 16, 1/2, 1/4, 1/8, 1/16. Therefore, it can be divided into 6 cases according to the shape of the block as follows.

One. Blocks with an aspect ratio of 1 (i.e. square blocks with the same length and width)

2. Blocks with an aspect ratio greater than 1 (i.e., a block with a long horizontal rectangle)

3. Blocks with an aspect ratio less than 1 (i.e., a block with a long vertical rectangle)

4. Blocks in the shape of a right triangle with an aspect ratio of 1 (that is, blocks in the shape of a right isosceles triangle with the same length and width)

5. Blocks in the shape of a right triangle with an aspect ratio greater than 1 (i.e. blocks in the shape of a right triangle with a long horizontal length)

6. Blocks in the shape of a right triangle with an aspect ratio of less than 1 (i.e. blocks in the shape of a right triangle with a long vertical length)

16 shows an embodiment of a down-sampling filter determined according to six block shapes. If this method is used, there is an advantage that it is not necessary to separately transmit the selection information of the down-sampling filter or to have the selection information in the bitstream. That is, by adaptively applying a down-sampling filter according to the shape of a block, there is an advantage of increasing coding efficiency.

Meanwhile, according to an embodiment of the present invention, the down-sampling filter may be determined according to the size of the block.

Downsampling can be effectively performed by applying a minimum filter for a relatively small block and a larger filter for a relatively large block.

The size of the block can also be variously composed of 2,4,8,16,32,64 horizontally and 2,4,8,16,32,64 vertically.

For example, when the size of a block is larger than a predefined size, at least one of a 3x3 filter, a 3x2 filter, a 2x3 filter, a 2x2 filter, a cross filter, and an X filter may be selected as a downsampling filter for CCLM prediction.

On the other hand, when the size of the block is less than or equal to the predefined size, at least one of a 3x1 filter, a 1x3 filter, a 2x1 filter, and a 1x2 filter may be selected as a downsampling filter for CCLM prediction.

If this method is used, there is an advantage that it is not necessary to separately transmit the selection information of the down-sampling filter or to have the selection information in the bitstream. That is, there is an advantage in that coding efficiency can be improved by adaptively applying a down-sampling filter according to a block size.

In addition, according to an embodiment of the present invention, a down-sampling filter may be determined based on features of one or more blocks. That is, a method of implementing the above embodiments in a complex manner can be considered.

17 is a flowchart illustrating a video decoding method according to an embodiment of the present invention.

Referring to FIG. 17, the video decoder may derive a cross component linear model (CCLM) parameter for a current color difference block (S1701). Specifically, the CCLM parameter may be determined based on the CCLM mode. Here, the CCLM mode may include a CCLM-Above-Left mode, a CCLM-Above mode, and a CCLM-Left mode as described above.

Meanwhile, the CCLM parameters may be parameters α and β of Equation 1 above.

Then, the image decoder may determine a downlink sampling filter for the current color difference block (S1702).

As in an embodiment of the present invention, the down-sampling filter may be determined based on the position of the current luminance block. Specifically, the down-sampling filter may be determined according to whether the location of the current luminance block is at the CTU boundary.

Also, the down-sampling filter may be determined based on at least one of the shape and size of the current luminance block.

In addition, the down-sampling filter may be determined based on the CCLM mode.

In addition, the image decoder may apply the down-sampling filter determined in step S1702 to the current luminance block corresponding to the current color difference block (S1703).

In addition, the image decoder may generate the predicted pixel value of the current color difference block by using the CCLM parameter and the pixel value of the luminance block to which the down-sampling filter is applied (S1704). Specifically, the CCLM parameter and the down-sampling filter are applied to Equation 1, and the predicted pixel value of the current color difference block may be generated by applying the pixel value of the luminance block.

Meanwhile, in order for the image encoder to derive the same prediction result as the image decoder, the image decoding method of FIG. 17 may also be performed as an image encoding method.

The bitstream generated by the image encoding method of the present invention may be temporarily stored in a computer-readable non-transitory recording medium, and may be a bitstream encoded by the image encoding method described above.

Specifically, as a non-transitory computer-readable recording medium storing a bitstream generated by an image encoding method, the image encoding method includes: deriving a Cross Component Linear Model (CCLM) parameter for a current color difference block, and the current color difference Determining a down-sampling filter for a block, applying the down-sampling filter to a current luminance block corresponding to the current color difference block, and using the CCLM parameter and pixel values of the luminance block to which the down-sampling filter is applied. Generating a predicted pixel value of a current color difference block, and determining the down-sampling filter may be determined based on a position of the current luminance block.

The present invention is a method and apparatus for down-sampling luminance pixels by selectively applying a down-sampling filter, unlike a conventional method, when applying a CCLM prediction technique that predicts a color difference block in a screen through correlation between channels.

Particularly, when the conventional technology is applied to an image in which the degree of change in pixel values is spatially concentrated, problems of compression rate and image quality deteriorate seriously. The present invention relates to the CCLM mode, block size, block shape, and luminance block position. Accordingly, by selectively applying various down-sampling filters, it is possible to improve encoding efficiency or improve encoding quality.

The above embodiments may be performed in the same manner in an encoder and a decoder.

An image may be encoded/decoded using at least one or a combination of at least one of the above embodiments.

The order of applying the embodiment may be different between the encoder and the decoder, and the order of applying the embodiment may be the same between the encoder and the decoder.

The above embodiments may be performed for each of the luminance and color difference signals, and the above embodiments may be similarly performed for the luminance and color difference signals.

The shape of the block to which the above embodiments of the present invention are applied may have a square shape or a non-square shape.

The embodiments of the present invention may be applied according to the size of at least one of a coding block, a prediction block, a transform block, a block, a current block, a coding unit, a prediction unit, a transform unit, a unit, and a current unit. The size here may be defined as a minimum size and/or a maximum size in order to apply the above embodiments, or may be defined as a fixed size to which the above embodiments are applied. In addition, in the above embodiments, the first embodiment may be applied to the first size, and the second embodiment may be applied to the second size. That is, the always-on embodiments can be applied in combination according to the size. Further, the above embodiments of the present invention may be applied only when the size is greater than or equal to the minimum size and less than or equal to the maximum size. That is, the above embodiments may be applied only when the block size is included within a certain range.

For example, the above embodiments can be applied only when the size of the current block is 8x8 or more. For example, the above embodiments can be applied only when the size of the current block is 4x4. For example, the above embodiments can be applied only when the size of the current block is 16x16 or less. For example, the above embodiments can be applied only when the size of the current block is 16x16 or more and 64x64 or less.

The above embodiments of the present invention can be applied according to a temporal layer. A separate identifier is signaled to identify a temporal layer to which the above embodiments are applicable, and the above embodiments may be applied to a temporal layer specified by the corresponding identifier. The identifier here may be defined as the lowest layer and/or the highest layer to which the embodiment is applicable, or may be defined as indicating a specific layer to which the embodiment is applied. In addition, a fixed temporal layer to which the above embodiment is applied may be defined.

For example, the above embodiments can be applied only when the temporal layer of the current image is the lowest layer. For example, the above embodiments can be applied only when the temporal layer identifier of the current image is 1 or more. For example, the above embodiments can be applied only when the temporal layer of the current image is the highest layer.

A slice type or a tile group type to which the above embodiments of the present invention are applied is defined, and the above embodiments of the present invention may be applied according to the corresponding slice type or tile group type.

In the above-described embodiments, the methods are described on the basis of a flow chart as a series of steps or units, but the present invention is not limited to the order of steps, and certain steps may occur in a different order or concurrently with other steps as described above. I can. In addition, those of ordinary skill in the art understand that the steps shown in the flowchart are not exclusive, other steps are included, or one or more steps in the flowchart may be deleted without affecting the scope of the present invention. You can understand.

The above-described embodiments include examples of various aspects. Although not all possible combinations for representing the various aspects can be described, those of ordinary skill in the art will recognize that other combinations are possible. Accordingly, the present invention will be said to include all other replacements, modifications and changes falling within the scope of the following claims.

The embodiments according to the present invention described above may be implemented in the form of program instructions that can be executed through various computer components and recorded in a computer-readable recording medium. The computer-readable recording medium may include program instructions, data files, data structures, etc. alone or in combination. The program instructions recorded in the computer-readable recording medium may be specially designed and configured for the present invention, or may be known and usable to those skilled in the computer software field. Examples of computer-readable recording media include magnetic media such as hard disks, floppy disks, and magnetic tapes, optical recording media such as CD-ROMs and DVDs, and magnetic-optical media such as floptical disks. media), and a hardware device specially configured to store and execute program instructions such as ROM, RAM, flash memory, and the like. Examples of the program instructions include not only machine language codes such as those produced by a compiler, but also high-level language codes that can be executed by a computer using an interpreter or the like. The hardware device may be configured to operate as one or more software modules to perform processing according to the present invention, and vice versa.

In the above, the present invention has been described by specific matters such as specific elements and limited embodiments and drawings, but this is provided only to help a more general understanding of the present invention, and the present invention is not limited to the above embodiments. , Anyone with ordinary knowledge in the technical field to which the present invention pertains can make various modifications and variations from these descriptions.

Accordingly, the spirit of the present invention is limited to the above-described embodiments and should not be defined, and all modifications that are equally or equivalent to the claims as well as the claims to be described later fall within the scope of the spirit of the present invention. I would say.

Claims (1)

Deriving a cross component linear model (CCLM) parameter for a current color difference block;
Determining a down-sampling filter for the current color difference block;
Applying the down-sampling filter to a current luminance block corresponding to the current color difference block; And
Generating a predicted pixel value of the current color difference block using the CCLM parameter and the pixel value of the luminance block to which the down-sampling filter is applied,
The step of determining the down-sampling filter is an image decoding method, characterized in that it is determined based on a position of the current luminance block.

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