Image encoding / decoding method and device based on CCLM intra prediction, and recording medium for storing bitstreams

Abstract

An image encoding / decoding method and apparatus are provided. The image decoding method according to the present disclosure includes: deriving chroma reference samples located in the periphery of a chroma block and luma reference samples corresponding to the chroma reference samples based on an intra prediction mode of the chroma block being a CCLM (Cross-component linear model) mode; inducing CCLM parameters based on the chroma reference samples and the luma reference samples; and generating predicted samples of the chroma block based on the CCLM parameters, where the chroma reference samples can be obtained from two or more chroma reference lines adjacent to the chroma block.

Description

[Technical field]

[0001] The present disclosure relates to an image encoding / decoding method, an apparatus, and a recording medium for storing a bitstream, and more particularly, to an image encoding / decoding method and apparatus based on CCLM (Cross-component linear model) intra prediction, and a recording medium for storing a bitstream generated by the image encoding method / apparatus of the present disclosure. [Background technology]

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

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

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

[0005] Another object of the present disclosure is to provide an image encoding / decoding method and device that performs CCLM (Cross-component linear model) intra prediction.

[0006] Another object of the present disclosure is to provide an image encoding / decoding method and apparatus that performs CCLM intra prediction based on multiple reference lines.

[0007] Another object of the present disclosure is to provide an image encoding / decoding method and apparatus for performing improved LMS (Linear-Mean-Square)-based CCLM intra prediction.

[0008] Another object of the present disclosure is to provide an image encoding / decoding method and apparatus that performs improved linear model CCLM intra prediction using a predetermined number of pixels.

[0009] Another object of the present disclosure is to provide an image encoding / decoding method and apparatus that adaptively performs CCLM intra prediction based on multiple reference lines.

[0010] Another object of the present disclosure is to provide a non-transitory computer-readable recording medium for storing a bitstream generated by the image encoding method or apparatus according to the present disclosure.

[0011] Another object of the present disclosure is to provide a non-transitory computer-readable recording medium for storing a bitstream that is received by an image decoding device according to the present disclosure, decoded, and used to restore an image.

[0012] Another object of the present disclosure is to provide a method for transmitting a bitstream generated by the image encoding method or apparatus according to the present disclosure.

[0013] The technical problems to be solved by the present disclosure are not limited to the above-mentioned technical problems, and other technical problems not described above will be clearly understood by a person having ordinary skill in the art to which the present disclosure pertains from the following description. [Means for solving the problem]

[0014] According to one embodiment of the present disclosure, there is provided an image decoding method performed by an image decoding device, the image decoding method including: a step of deriving a chroma reference sample located in the periphery of a chroma block and a luma reference sample corresponding to the chroma reference sample based on the intra prediction mode of the chroma block being a CCLM (Cross-component linear model) mode; a step of deriving a CCLM parameter based on the chroma reference sample and the luma reference sample; and a step of generating a predicted sample of the chroma block based on the CCLM parameter, wherein the chroma reference sample may be obtained from two or more chroma reference lines adjacent to the chroma block.

[0015] According to one embodiment of the present disclosure, the step of deriving the CCLM parameters may be performed based on a Linear-Mean-Square (LMS) method using the chroma reference samples present in two or more of the chroma reference lines adjacent to the chroma block.

[0016] According to an embodiment of the present disclosure, the step of deriving the CCLM parameters may use a predetermined number of the chroma reference samples present in two or more of the chroma reference lines adjacent to the chroma block.

[0017] According to one embodiment of the present disclosure, based on the fact that the chroma block is a non-square block, a subsampling method for a first reference line adjacent to the chroma block and a subsampling method for a second reference line adjacent to the chroma block may be the same.

[0018] According to one embodiment of the present disclosure, based on the fact that the chroma block is a non-square block, a subsampling method for a first reference line adjacent to the chroma block may be different from a subsampling method for a second reference line adjacent to the chroma block.

[0019] According to an embodiment of the present disclosure, the number of chroma reference lines for deriving the chroma reference sample may be determined to be different depending on whether the top boundary of the chroma block coincides with the boundary of a CTU.

[0020] According to one embodiment of the present disclosure, the multiple upper luma reference lines include first to fourth upper reference lines, a first reference line is generated that is downsampled based on the first upper reference line and the second upper reference line, a second reference line is generated that is downsampled based on the third upper reference line and the fourth upper reference line, and the luma reference sample may be obtained based on the downsampled first reference line and the second reference line.

[0021] According to one embodiment of the present disclosure, the multiple left luma reference lines include the first to fifth left reference lines, a third reference line is generated based on the first left reference line, the second left reference line, and the third left reference line, a fourth reference line is generated based on the third left reference line, the fourth left reference line, and the fifth left reference line, and the luma reference sample may be obtained based on the downsampled third reference line and the fourth reference line.

[0022] According to one embodiment of the present disclosure, the number of chroma reference lines used to derive the CCLM parameters may be determined to be different based on a comparison between the size of the chroma block and a predefined threshold.

[0023] According to an embodiment of the present disclosure, the number of chroma reference lines used to derive the CCLM parameters may be determined to be different depending on the type of CCLM applied to the chroma block.

[0024] According to one embodiment of the present disclosure, the step of deriving the CCLM parameters may further include the steps of classifying the luma reference samples and the chroma reference samples into two or more groups based on a predetermined threshold, and deriving CCLM parameters for each of the classified groups.

[0025] According to one embodiment of the present disclosure, there is provided an image encoding method performed by an image encoding device, the encoding method including: a step of deriving a chroma reference sample located in the periphery of a chroma block and a luma reference sample corresponding to the chroma reference sample based on the intra prediction mode of the chroma block being a CCLM (Cross-component linear model) mode; a step of deriving a CCLM parameter based on the chroma reference sample and the luma reference sample; and a step of generating a predicted sample of the chroma block based on the CCLM parameter, wherein the chroma reference sample may be obtained from two or more chroma reference lines adjacent to the chroma block.

[0026] According to one embodiment of the present disclosure, in a computer-readable recording medium storing a bitstream generated by an image encoding method, the image encoding method includes a step of deriving a chroma reference sample located in the vicinity of the chroma block and a luma reference sample corresponding to the chroma reference sample based on the intra prediction mode of the chroma block being a CCLM (Cross-component linear model) mode, a step of deriving a CCLM parameter based on the chroma reference sample and the luma reference sample, and a step of generating a predicted sample of the chroma block based on the CCLM parameter, wherein the chroma reference sample may be obtained from two or more chroma reference lines adjacent to the chroma block.

[0027] According to one embodiment of the present disclosure, in a method for transmitting a bitstream generated by an image encoding method, the image encoding method includes a step of deriving a chroma reference sample located in the vicinity of the chroma block and a luma reference sample corresponding to the chroma reference sample based on the intra prediction mode of the chroma block being a CCLM (Cross-component linear model) mode, a step of deriving a CCLM parameter based on the chroma reference sample and the luma reference sample, and a step of generating a predicted sample of the chroma block based on the CCLM parameter, wherein the chroma reference sample may be obtained from two or more chroma reference lines adjacent to the chroma block. Effect of the Invention

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

[0029] Furthermore, according to the present disclosure, an image encoding / decoding method and device that performs CCLM (Cross-component linear model) intra prediction can be provided.

[0030] In addition, the present disclosure may provide an image encoding / decoding method and apparatus for performing CCLM intra prediction based on multiple reference lines.

[0031] In addition, the present disclosure may provide an image encoding / decoding method and apparatus for performing improved LMS (Linear-Mean_Square)-based CCLM intra prediction.

[0032] Furthermore, according to the present disclosure, an image encoding / decoding method and apparatus can be provided that performs improved linear model CCLM intra prediction using a predetermined number of pixels.

[0033] In addition, the present disclosure may provide an image encoding / decoding method and apparatus that adaptively performs CCLM intra prediction based on multiple reference lines.

[0034] According to the present disclosure, a non-transitory computer-readable recording medium can be provided that stores a bitstream generated by the image encoding method or apparatus according to the present disclosure.

[0035] According to the present disclosure, a non-transitory computer-readable recording medium can be provided that stores a bitstream that is received by an image decoding device according to the present disclosure, decoded, and used to restore an image.

[0036] According to the present disclosure, a method for transmitting a bitstream generated by an image encoding method or apparatus according to the present disclosure can be provided.

[0037] The effects obtained by the present disclosure are not limited to the effects described above, and other effects not described above will be clearly understood by those having ordinary skill in the art to which the present disclosure pertains from the following description. [Brief description of the drawings]

[0038] [Figure 1] FIG. 1 is a schematic diagram illustrating a video coding system to which an embodiment of the present disclosure can be applied. [Diagram 2] 1 is a diagram illustrating an image encoding device to which an embodiment of the present disclosure can be applied; [Diagram 3] 1 is a diagram illustrating an image decoding device to which an embodiment of the present disclosure can be applied; [Figure 4] 1 is a flowchart illustrating a video / image coding method based on intra-prediction. [Diagram 5] FIG. 13 is a diagram illustrating an example configuration of an intra prediction unit according to the present disclosure. [Figure 6] 1 is a flowchart illustrating a video / image decoding method based on intra prediction. [Figure 7]FIG. 13 is a diagram illustrating an example configuration of an intra prediction unit according to the present disclosure. [Figure 8] FIG. 2 is a diagram illustrating multiple reference lines for intra prediction according to one embodiment of the present disclosure. [Figure 9] FIG. 2 is a diagram showing the locations of adjacent samples used for CCLM (Cross-component linear model) parameter derivation according to one embodiment of the present disclosure. [Figure 10] FIG. 1 is a diagram showing an example in which adjacent samples are classified into two groups to derive parameters of a multi-model linear model (MMLM) according to an embodiment of the present disclosure. [Figure 11] 1 is a diagram for explaining a reference sample selection method for deriving CCLM parameters according to one embodiment of the present disclosure. [Figure 12] A diagram for explaining a reference sample selection method for deriving CCLM parameters in LM_A mode according to one embodiment of the present disclosure. [Figure 13] 1 illustrates an example of a method for selecting reference samples in a non-square block according to one embodiment of the present disclosure. [Figure 14a] 13 illustrates an example of a method for selecting reference samples in a non-square block according to another embodiment of the present disclosure. [Figure 14b] 4 illustrates an example of a method for selecting reference samples in a non-square block according to another embodiment of the present disclosure. [Figure 15] 1 is a flowchart illustrating a CCLM prediction method according to an embodiment of the present disclosure. [Figure 16] An example of how to apply CCLM when the top boundary of the current chroma block coincides with the boundary of the CTU is shown below. [Figure 17] 13 is a diagram illustrating an embodiment of the present disclosure in which it is determined whether to apply CLM based on multiple reference lines based on the size of a current chroma block. [Figure 18]11A to 11C are diagrams for explaining an embodiment of the present disclosure in which it is determined whether or not to apply CCLM based on the type of CCLM applied to a current chroma block. [Figure 19] FIG. 1 is a diagram illustrating an example of a content streaming system to which an embodiment of the present disclosure can be applied. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present disclosure will be described in detail below with reference to the accompanying drawings so that those skilled in the art can easily implement the present disclosure. However, the present disclosure may be embodied in various different forms and is not limited to the embodiments described herein.

[0040] In describing the embodiments of the present disclosure, if it is determined that a specific description of a known configuration or function may make the gist of the present disclosure unclear, the detailed description thereof will be omitted. In addition, in the drawings, parts that are not related to the description of the present disclosure are omitted, and similar parts are denoted by similar reference numerals.

[0041] In the present disclosure, when a certain component is "connected," "coupled," or "connected" to another component, this includes not only a direct connection relationship, but also an indirect connection relationship in which another component exists between them. Furthermore, when a certain component is described as "including" or "having" another component, this does not mean that the other component is excluded, but that the other component can be further included, unless otherwise specified.

[0042] In this disclosure, terms such as "first" and "second" are used only for the purpose of distinguishing one component from another component, and do not limit the order or importance of the components unless otherwise specified. Therefore, within the scope of this disclosure, a first component in one embodiment may be called a second component in another embodiment, and similarly, a second component in one embodiment may be called a first component in another embodiment.

[0043] In this disclosure, components that are distinguished from one another are used to clearly describe the characteristics of each component, and do not necessarily mean that the components are separate. In other words, multiple components may be integrated and configured as a single hardware or software unit, or one component may be distributed and configured as multiple hardware or software units. Thus, even if not otherwise stated, such integrated or distributed embodiments are also included in the scope of the present disclosure.

[0044] In the present disclosure, the components described in the various embodiments are not necessarily essential components, and some may be optional components. Therefore, an embodiment consisting of a subset of the components described in one embodiment is also included in the scope of the present disclosure. In addition, an embodiment including other components in addition to the components described in the various embodiments is also included in the scope of the present disclosure.

[0045] The present disclosure relates to image encoding and decoding, and terms used in this disclosure may have ordinary meanings in the technical field to which the present disclosure belongs, unless they are newly defined in this disclosure.

[0046] In this disclosure, "video" may refer to a collection of a series of images over time.

[0047] In this disclosure, a "picture" generally means a unit indicating any one image in a particular time period, a slice / tile is a coding unit constituting a part of a picture, and one picture may be composed of one or more slices / tiles. Also, a slice / tile may include one or more coding tree units (CTUs).

[0048] In this disclosure, a "pixel" or a "pel" may refer to the smallest unit constituting one picture (or image). A term corresponding to a pixel may be a "sample." A sample may generally indicate a pixel or a pixel value, may indicate only a pixel / pixel value of a luma component, or may indicate only a pixel / pixel value of a chroma component.

[0049] In this disclosure, a "unit" may refer to a basic unit of image processing. A unit may include at least one of a specific region of a picture and information related to the region. A unit may be used interchangeably with terms such as "sample array," "block," or "area," depending on the case. In a general case, an M×N block may include a set (or array) of samples or transform coefficients consisting of M columns and N rows.

[0050] In the present disclosure, a "current block" may refer to any one of a "current coding block," a "current coding unit," a "block to be coded," a "block to be decoded," or a "block to be processed." If prediction is performed, a "current block" may refer to a "current predicted block" or a "block to be predicted." If transformation (inverse transformation) / quantization (inverse quantization) is performed, a "current block" may refer to a "current transformed block" or a "block to be transformed." If filtering is performed, a "current block" may refer to a "block to be filtered."

[0051] Furthermore, in this disclosure, unless there is an explicit reference to a chroma block, a "current block" may refer to a block including both a luma component block and a chroma component block, or a "luma block of the current block." The luma component block of the current block may be expressed by explicitly including the reference to a luma component block, such as a "luma block" or a "current luma block." Furthermore, the chroma component block of the current block may be expressed by explicitly including the reference to a chroma component block, such as a "chroma block" or a "current chroma block."

[0052] In the present disclosure, " / " and "," can be interpreted as "and / or." For example, "A / B" and "A, B" can be interpreted as "A and / or B." Also, "A / B / C" and "A, B, C" can mean "at least one of A, B, and / or C."

[0053] In this disclosure, "or" can be interpreted as "and / or." For example, "A or B" can mean 1) only "A," 2) only "B," or 3) "A and B." Alternatively, in this disclosure, "or" can mean "additionally or alternatively."

[0054] In this disclosure, "at least one of A, B, and C" can mean "A only," "B only," "C only," or "any and all combinations of A, B, and C." Also, "at least one of A, B, or C" or "at least one of A, B, and / or C" can mean "at least one of A, B, and C."

[0055] Parentheses used in the present disclosure may mean "for example." For example, when "prediction (intra prediction)" is displayed, "intra prediction" may be proposed as an example of "prediction." In other words, "prediction" in the present disclosure is not limited to "intra prediction," and "intra prediction" may be proposed as an example of "prediction." Also, when "prediction (i.e., intra prediction)" is displayed, "intra prediction" may be proposed as an example of "prediction."

[0056] Video Coding System Overview

[0057] FIG. 1 is a schematic diagram of a video coding system to which embodiments of the present disclosure can be applied.

[0058] A video coding system according to an embodiment may include an encoding device 10 and a decoding device 20. The encoding device 10 may transmit encoded video and / or image information or data to the decoding device 20 in a file or streaming format via a digital storage medium or a network.

[0059] The encoding device 10 according to an embodiment may include a video source generating unit 11, an encoding unit 12, and a transmitting unit 13. The decoding device 20 according to an embodiment may include a receiving unit 21, a decoding unit 22, and a rendering unit 23. The encoding unit 12 may be referred to as a video / image encoding unit, and the decoding unit 22 may be referred to as a video / image decoding unit. The transmitting unit 13 may be included in the encoding unit 12. The receiving unit 21 may be included in the decoding unit 22. The rendering unit 23 may include a display unit, which may be configured as a separate device or an external component.

[0060] The video source generating unit 11 can obtain videos / images through a process of capturing, synthesizing, or generating videos / images. The video source generating unit 11 can include a video / image capture device and / or a video / image generation device. The video / image capture device can include, for example, one or more cameras, a video / image archive containing previously captured videos / images, etc. The video / image generation device can include, for example, a computer, a tablet, a smartphone, etc., and can (electronically) generate videos / images. For example, a virtual video / image can be generated through a computer, etc., in which case the video / image capture process can be replaced by a process in which related data is generated.

[0061] The encoder 12 may encode the input video / image. The encoder 12 may perform a series of steps such as prediction, transformation, quantization, etc. for compression and encoding efficiency. The encoder 12 may output the encoded data (encoded video / image information) in a bitstream format.

[0062] The transmitting unit 13 may obtain the encoded video / image information or data output in a bitstream format, and may transmit the same to the receiving unit 21 of the decoding device 20 or other external objects in a file or streaming format via a digital storage medium or a network. The digital storage medium may include various storage media such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, etc. The transmitting unit 13 may include elements for generating a media file via a predetermined file format, and may include elements for transmitting the same via a broadcasting / communication network. The transmitting unit 13 may be provided as a transmitting device separate from the encoding unit 120, and in this case, the transmitting device may include at least one processor for obtaining the encoded video / image information or data output in a bitstream format, and a transmitting unit for transmitting the same in a file or stream format. The receiving unit 21 may extract / receive the bitstream from the storage medium or network, and transmit the same to the decoding unit 22.

[0063] The decoding unit 22 can decode the video / image by performing a series of steps such as inverse quantization, inverse transformation, and prediction corresponding to the operations of the encoding unit 12.

[0064] The rendering unit 23 can render the decoded video / images. The rendered video / images can be displayed via a display unit.

[0065] Overview of the image encoding device

[0066] FIG. 2 is a diagram illustrating an image encoding device to which an embodiment of the present disclosure can be applied.

[0067] As shown in Fig. 2, the image coding device 100 may include an image division unit 110, a subtraction unit 115, a transformation unit 120, a quantization unit 130, an inverse quantization unit 140, an inverse transformation unit 150, an addition unit 155, a filtering unit 160, a memory 170, an inter prediction unit 180, an intra prediction unit 185, and an entropy coding unit 190. The inter prediction unit 180 and the intra prediction unit 185 may be collectively referred to as a "prediction unit." The transformation unit 120, the quantization unit 130, the inverse quantization unit 140, and the inverse transformation unit 150 may be included in a residual processing unit. The residual processing unit may further include a subtraction unit 115.

[0068] All or at least some of the components constituting the image encoding device 100 may be realized by a single hardware component (e.g., an encoder or a processor) depending on the embodiment. Also, the memory 170 may include a decoded picture buffer (DPB) and may be realized by a digital storage medium.

[0069] The image division unit 110 may divide an input image (or picture, frame) input to the image encoding device 100 into one or more processing units. As an example, the processing units may be called coding units (CUs). The coding units may be obtained by recursively dividing a coding tree unit (CTU) or a largest coding unit (LCU) according to a QT / BT / TT (Quad-tree / Binary-tree / Ternal-tree) structure. For example, one coding unit may be divided into a plurality of coding units of a deeper depth based on a quad-tree structure, a binary-tree structure, and / or a ternary-tree structure. For dividing the coding units, a quad-tree structure may be applied first, and a binary-tree structure and / or a ternary-tree structure may be applied later. A coding procedure according to the present disclosure may be performed based on a final coding unit that is not further divided. The maximum coding unit may be used as the final coding unit, and a lower depth coding unit obtained by dividing the maximum coding unit may be used as the final coding unit. Here, the coding procedure may include procedures such as prediction, transformation, and / or restoration, which will be described later. As another example, a processing unit of the coding procedure may be a prediction unit (PU) or a transform unit (TU). The prediction unit and the transform unit may be divided or partitioned from the final coding unit, respectively. The prediction unit may be a unit of sample prediction, and the transform unit may be a unit for deriving transform coefficients and / or a unit for deriving a residual signal from the transform coefficients.

[0070] The prediction unit (inter prediction unit 180 or intra prediction unit 185) may perform prediction on a block to be processed (current block) and generate a predicted block including prediction samples for the current block. The prediction unit may determine whether intra prediction or inter prediction is applied in units of a current block or a CU. The prediction unit may generate various information related to prediction of the current block and transmit it to the entropy encoding unit 190. The information related to prediction may be encoded by the entropy encoding unit 190 and output in a bitstream format.

[0071] The intra prediction unit 185 may predict the current block by referring to samples in the current picture. The referenced samples may be located in the neighborhood of the current block or may be located away from the current block according to an intra prediction mode and / or an intra prediction technique. The intra prediction mode may include a plurality of non-directional modes and a plurality of directional modes. The non-directional mode may include, for example, a DC mode and a planar mode. The directional mode may include, for example, 33 directional prediction modes or 65 directional prediction modes depending on the fineness of the prediction direction. However, this is merely an example, and more or less directional prediction modes may be used depending on the setting. The intra prediction unit 185 may also determine a prediction mode to be applied to the current block using prediction modes applied to neighboring blocks.

[0072] The inter prediction unit 180 may derive a predicted block for a current block based on a reference block (reference sample array) identified by a motion vector on a reference picture. In this case, in order to reduce the amount of motion information transmitted in the inter prediction mode, the motion information may be predicted in units of a block, a sub-block, or a sample based on the correlation of motion information between a neighboring block and a current block. The motion information may include a motion vector and a reference picture index. The motion information may further include inter prediction direction (L0 prediction, L1 prediction, Bi prediction, etc.) information. In the case of inter prediction, the neighboring block may include a spatial neighboring block present in the current picture and a temporal neighboring block present in the reference picture. The reference picture including the reference block and the reference picture including the temporal neighboring block may be the same or different from each other. The temporal neighboring block may be called a collocated reference block, a collocated CU (colCU), etc. The reference picture including the temporal neighboring block may be called a collocated picture (colPic). For example, the inter prediction unit 180 may generate information indicating which candidate is used to derive a motion vector and / or a reference picture index of the current block by forming a motion information candidate list based on neighboring blocks. Inter prediction may be performed based on various prediction modes, and for example, in the case of a skip mode and a merge mode, the inter prediction unit 180 may use motion information of neighboring blocks as motion information of the current block. In the case of the skip mode, unlike the merge mode, a residual signal may not be transmitted.In the case of a motion vector prediction (MVP) mode, the motion vector of the current block can be signaled by using the motion vector of a neighboring block as a motion vector predictor and encoding a motion vector difference and an indicator for the motion vector predictor. The motion vector difference can mean the difference between the motion vector of the current block and the motion vector predictor.

[0073] The prediction unit may generate a prediction signal based on various prediction methods and / or prediction techniques, which will be described later. For example, the prediction unit may apply intra prediction or inter prediction for prediction of the current block, and may simultaneously apply intra prediction and inter prediction. A prediction method that simultaneously applies intra prediction and inter prediction for prediction of the current block may be called combined inter and intra prediction (CIIP). The prediction unit may also perform intra block copy (IBC) for prediction of the current block. Intra block copy can be used for content image / video coding such as games, for example, as in screen content coding (SCC). IBC is a method of predicting a current block using an already restored reference block in a current picture that is located a predetermined distance away from the current block. When IBC is applied, the position of the reference block in the current picture may be coded as a vector (block vector) corresponding to the predetermined distance. IBC is basically performed in the current picture, but may be performed similarly to inter prediction in that a reference block is derived in the current picture. That is, the IBC may use at least one of the inter prediction techniques described in this disclosure.

[0074] The prediction signal generated by the prediction unit may be used to generate a restored signal or a residual signal. The subtraction unit 115 may subtract the prediction signal (predicted block, prediction sample array) output from the prediction unit from the input image signal (original block, original sample array) to generate a residual signal (residual block, residual sample array). The generated residual signal may be transmitted to the conversion unit 120.

[0075] The transform unit 120 may generate transform coefficients by applying a transform technique to the residual signal. For example, the transform technique may include at least one of a discrete cosine transform (DCT), a discrete sine transform (DST), a Karhunen-Loeve transform (KLT), a graph-based transform (GBT), or a conditionally non-linear transform (CNT). Here, the GBT refers to a transform obtained from a graph when the relationship information between pixels is expressed as a graph. The CNT refers to a transform obtained based on a predicted signal generated using all previously reconstructed pixels. The transform process may be applied to pixel blocks having the same square size, or may be applied to non-square, variable-sized blocks.

[0076] The quantization unit 130 may quantize the transform coefficients and transmit the quantized transform coefficients to the entropy coding unit 190. The entropy coding unit 190 may code the quantized signal (information on the quantized transform coefficients) and output the coded signal in a bitstream format. The information on the quantized transform coefficients may be referred to as residual information. The quantization unit 130 may rearrange the quantized transform coefficients in a block format into a one-dimensional vector format based on a coefficient scan order, and may generate information on the quantized transform coefficients based on the quantized transform coefficients in the one-dimensional vector format.

[0077] The entropy coding unit 190 may perform various coding methods, such as exponential Golomb, context-adaptive variable length coding (CAVLC), and context-adaptive binary arithmetic coding (CABAC). The entropy coding unit 190 may also code information required for video / image restoration (e.g., values ​​of syntax elements, etc.) together or separately in addition to the quantized transform coefficients. The coded information (e.g., coded video / image information) may be transmitted or stored in a network abstraction layer (NAL) unit unit in a bitstream format. The video / image information may further include information on various parameter sets, such as an adaptation parameter set (APS), a picture parameter set (PPS), a sequence parameter set (SPS), or a video parameter set (VPS). The video / image information may further include general constraint information. The signaling information, transmitted information and / or syntax elements referred to in this disclosure may be encoded through the above-mentioned encoding procedures and included in the bitstream.

[0078] The bitstream may be transmitted via a network or may be stored in a digital storage medium. Here, the network may include a broadcasting network and / or a communication network, and the digital storage medium may include various storage media such as a USB, SD, CD, DVD, Blu-ray, HDD, SSD, etc. A transmitting unit (not shown) for transmitting the signal output from the entropy encoding unit 190 and / or a storing unit (not shown) for storing the signal may be provided as an internal / external element of the image encoding device 100, or the transmitting unit may be provided as a component of the entropy encoding unit 190.

[0079] The quantized transform coefficients output from the quantization unit 130 can be used to generate a residual signal. For example, the quantized transform coefficients are subjected to inverse quantization and inverse transformation via the inverse quantization unit 140 and the inverse transformation unit 150, so that a residual signal (residual block or residual sample) can be restored.

[0080] The adder 155 may generate a reconstructed signal (reconstructed picture, reconstructed block, reconstructed sample array) by adding the reconstructed residual signal to a prediction signal output from the inter prediction unit 180 or the intra prediction unit 185. When there is no residual for the current block to be processed, such as when a skip mode is applied, a predicted block may be used as a reconstructed block. The adder 155 may be referred to as a reconstruction unit or a reconstructed block generation unit. The generated reconstructed signal may be used for intra prediction of the next current block to be processed in the current picture, and may also be used for inter prediction of the next picture after filtering as described below.

[0081] Meanwhile, luma mapping with chroma scaling (LMCS) may be applied during picture encoding and / or reconstruction.

[0082] The filtering unit 160 may apply filtering to the reconstructed signal to improve subjective / objective image quality. For example, the filtering unit 160 may apply various filtering methods to the reconstructed picture to generate a modified reconstructed picture, and may store the modified reconstructed picture in the memory 170, specifically, in the DPB of the memory 170. The various filtering methods may include, for example, deblocking filtering, sample adaptive offset, an adaptive loop filter, a bilateral filter, and the like. The filtering unit 160 may generate various information related to filtering, as will be described later in the description of each filtering method, and transmit the information to the entropy coding unit 190. The information related to filtering may be coded by the entropy coding unit 190 and output in a bitstream format.

[0083] The modified reconstructed picture transmitted to the memory 170 may be used as a reference picture in the inter prediction unit 180. When inter prediction is applied through this, the image encoding device 100 may avoid a prediction mismatch between the image encoding device 100 and the image decoding device, and may also improve encoding efficiency.

[0084] The DPB in the memory 170 may store modified reconstructed pictures to be used as reference pictures in the inter prediction unit 180. The memory 170 may store motion information of blocks from which motion information in the current picture is derived (or coded) and / or motion information of already reconstructed intra-picture blocks. The stored motion information may be transmitted to the inter prediction unit 180 to be used as motion information of spatial surrounding blocks or motion information of temporal surrounding blocks. The memory 170 may store reconstructed samples of reconstructed blocks in the current picture and transmit them to the intra prediction unit 185.

[0085] Overview of the image decoding device

[0086] FIG. 3 is a diagram illustrating an image decoding device to which an embodiment of the present disclosure can be applied.

[0087] 3, the image decoding device 200 may include an entropy decoding unit 210, an inverse quantization unit 220, an inverse transform unit 230, an adder 235, a filtering unit 240, a memory 250, an inter prediction unit 260, and an intra prediction unit 265. The inter prediction unit 260 and the intra prediction unit 265 may be collectively referred to as a "prediction unit." The inverse quantization unit 220 and the inverse transform unit 230 may be included in a residual processing unit.

[0088] All or at least some of the components constituting the image decoding device 200 may be realized by one hardware component (e.g., a decoder or a processor) depending on the embodiment. Also, the memory 170 may include a DPB and may be realized by a digital storage medium.

[0089] The image decoding device 200, which receives a bitstream including video / image information, can reconstruct an image by executing a process corresponding to the process performed by the image encoding device 100 of Fig. 2. For example, the image decoding device 200 can perform decoding using a processing unit applied in the image encoding device. Thus, the processing unit for decoding can be, for example, a coding unit. The coding unit can be obtained by dividing a coding tree unit or a maximum coding unit. Then, the reconstructed image signal decoded and output by the image decoding device 200 can be reproduced by a reproduction device (not shown).

[0090] The image decoding apparatus 200 may receive a signal output from the image encoding apparatus of FIG. 2 in a bitstream format. The received signal may be decoded via the entropy decoding unit 210. For example, the entropy decoding unit 210 may derive information (e.g., video / image information) required for image restoration (or picture restoration) by parsing the bitstream. The video / image information may further include information on various parameter sets such as an adaptation parameter set (APS), a picture parameter set (PPS), a sequence parameter set (SPS), or a video parameter set (VPS). In addition, the video / image information may further include general constraint information. The image decoding apparatus may further use information on the parameter set and / or the general constraint information to decode an image. The signaling information, received information, and / or syntax elements referred to in the present disclosure may be obtained from the bitstream by being decoded via the decoding procedure. For example, the entropy decoding unit 210 may decode information in a bitstream based on a coding method such as exponential Golomb coding, CAVLC, or CABAC, and output values ​​of syntax elements required for image restoration and quantized values ​​of transform coefficients related to the residual. More specifically, the CABAC entropy decoding method may receive bins corresponding to each syntax element from the bitstream, determine a context model using syntax element information to be decoded and decoded information of neighboring blocks and a block to be decoded, or information of a symbol / bin decoded in a previous step, predict the occurrence probability of the bin based on the determined context model, and perform arithmetic decoding of the bin to generate a symbol corresponding to the value of each syntax element. In this case, the CABAC entropy decoding method may update the context model using information of the decoded symbol / bin for the context model of the next symbol / bin after determining the context model.Among the information decoded by the entropy decoding unit 210, information related to prediction is provided to a prediction unit (inter prediction unit 260 and intra prediction unit 265), and residual values ​​entropy-decoded by the entropy decoding unit 210, i.e., quantized transform coefficients and related parameter information, may be input to the inverse quantization unit 220. Also, among the information decoded by the entropy decoding unit 210, information related to filtering may be provided to the filtering unit 240. Meanwhile, a receiving unit (not shown) for receiving a signal output from the image encoding device may be further provided as an internal / external element of the image decoding device 200, or the receiving unit may be provided as a component of the entropy decoding unit 210.

[0091] Meanwhile, the image decoding device according to the present disclosure may be called a video / image / picture decoding device. The image decoding device may include an information decoder (video / image / picture information decoder) and / or a sample decoder (video / image / picture sample decoder). The information decoder may include an entropy decoding unit 210, and the sample decoder may include at least one of an inverse quantization unit 220, an inverse transform unit 230, an adder 235, a filtering unit 240, a memory 250, an inter prediction unit 260, and an intra prediction unit 265.

[0092] The inverse quantization unit 220 may inverse quantize the quantized transform coefficients to output transform coefficients. The inverse quantization unit 220 may rearrange the quantized transform coefficients in a two-dimensional block format. In this case, the rearrangement may be performed based on a coefficient scan order performed in the image encoding device. The inverse quantization unit 220 may perform inverse quantization on the quantized transform coefficients using a quantization parameter (e.g., quantization step size information) to obtain transform coefficients.

[0093] The inverse transform unit 230 can inversely transform the transform coefficients to obtain a residual signal (residual block, residual sample array).

[0094] The prediction unit may perform prediction on a current block and generate a predicted block including a prediction sample for the current block. The prediction unit may determine whether intra prediction or inter prediction is applied to the current block based on the prediction information output from the entropy decoding unit 210, and may determine a specific intra / inter prediction mode (prediction technique).

[0095] The prediction unit can generate a prediction signal based on various prediction methods (techniques) described below, as has been described in the explanation of the prediction unit of the image encoding device 100.

[0096] The intra prediction unit 265 may predict the current block by referring to samples in the current picture. The description of the intra prediction unit 185 may be similarly applied to the intra prediction unit 265.

[0097] The inter prediction unit 260 may derive a predicted block for the current block based on a reference block (reference sample array) identified by a motion vector on a reference picture. In this case, in order to reduce the amount of motion information transmitted in the inter prediction mode, the motion information may be predicted in units of blocks, sub-blocks, or samples based on correlation of motion information between neighboring blocks and the current block. The motion information may include a motion vector and a reference picture index. The motion information may further include inter prediction direction (L0 prediction, L1 prediction, Bi prediction, etc.) information. In the case of inter prediction, the neighboring blocks may include spatial neighboring blocks present in the current picture and temporal neighboring blocks present in the reference picture. For example, the inter prediction unit 260 may configure a motion information candidate list based on the neighboring blocks, and derive a motion vector and / or a reference picture index for the current block based on the received candidate selection information. Inter prediction may be performed based on various prediction modes (techniques), and the prediction information may include information indicating a mode (technique) of inter prediction for the current block.

[0098] The adder 235 may generate a reconstructed signal (reconstructed picture, reconstructed block, reconstructed sample array) by adding the acquired residual signal to a prediction signal (predicted block, predicted sample array) output from a prediction unit (including the inter prediction unit 260 and / or the intra prediction unit 265). When there is no residual for the current block, such as when a skip mode is applied, the predicted block can be used as the reconstructed block. The description of the adder 155 may also be applied to the adder 235. The adder 235 may also be referred to as a reconstruction unit or a reconstructed block generator. The generated reconstructed signal may be used for intra prediction of the next current block in the current picture, and may also be used for inter prediction of the next picture via filtering, as described below.

[0099] The filtering unit 240 may apply filtering to the reconstructed signal to improve subjective / objective image quality. For example, the filtering unit 240 may apply various filtering methods to the reconstructed picture to generate a modified reconstructed picture, and may store the modified reconstructed picture in the memory 250, specifically, in the DPB of the memory 250. The various filtering methods may include, for example, deblocking filtering, sample adaptive offset, an adaptive loop filter, a bilateral filter, etc.

[0100] The (modified) reconstructed picture stored in the DPB of the memory 250 may be used as a reference picture in the inter prediction unit 260. The memory 250 may store motion information of a block from which motion information in the current picture is derived (or decoded) and / or motion information of a block in an already reconstructed picture. The stored motion information may be transmitted to the inter prediction unit 260 to be used as motion information of a spatial surrounding block or motion information of a temporal surrounding block. The memory 250 may store a reconstructed sample of a reconstructed block in the current picture and transmit it to the intra prediction unit 265.

[0101] In this specification, the embodiments described for the filtering unit 160, inter prediction unit 180 and intra prediction unit 185 of the image encoding device 100 can also be applied in a similar or corresponding manner to the filtering unit 240, inter prediction unit 260 and intra prediction unit 265 of the image decoding device 200, respectively.

[0102] Overview of Intra Prediction

[0103] Intra prediction according to the present disclosure will be described below.

[0104] Intra prediction may refer to a prediction that generates a prediction sample for a current block based on a reference sample in a picture to which the current block belongs (hereinafter, the current picture). When intra prediction is applied to the current block, neighboring reference samples used for intra prediction of the current block may be derived. The neighboring reference samples of the current block may include a total of 2×nH samples adjacent to the left boundary of the current block of size nW×nH and adjacent to the bottom-left, a total of 2×nW samples adjacent to the top boundary of the current block and adjacent to the top-right, and one sample adjacent to the top-left of the current block. Alternatively, the neighboring reference samples of the current block may include upper neighboring samples of multiple columns and left neighboring samples of multiple rows. In addition, the neighboring reference samples of the current block may include a total of nH samples adjacent to the right boundary of the current block of size nW×nH, a total of nW samples adjacent to the bottom boundary of the current block, and one sample adjacent to the bottom-right of the current block.

[0105] However, some of the neighboring reference samples of the current block may not yet be decoded or may not be available. In this case, the decoder may construct the neighboring reference samples used for prediction by substituting unavailable samples with available samples, or may construct the neighboring reference samples used for prediction through interpolation of available samples.

[0106] When neighboring reference samples are derived, (i) a predicted sample can be derived based on an average or an interpolation of neighboring reference samples of the current block, or (ii) the predicted sample can be derived based on a reference sample that exists in a specific (prediction) direction with respect to the predicted sample among the neighboring reference samples of the current block. Case (i) is sometimes called a non-directional mode or a non-angular mode, and case (ii) is sometimes called a directional mode or an angular mode.

[0107] In addition, the prediction sample may be generated through interpolation between a first neighboring sample located in a prediction direction of an intra prediction mode of the current block and a second neighboring sample located in the opposite direction based on a prediction target sample of the current block among the neighboring reference samples. The above case may be called linear interpolation intra prediction (LIP).

[0108] Alternatively, the chroma prediction samples may be generated based on the luma samples using a linear model, which is sometimes called the Linear Model (LM) mode.

[0109] In addition, a temporal prediction sample of the current block may be derived based on the filtered neighboring reference samples, and a prediction sample of the current block may be derived by weighting the temporal prediction sample and at least one reference sample derived according to the intra prediction mode among the existing neighboring reference samples, i.e., unfiltered neighboring reference samples. In this case, it may be called Position Dependent Intra Prediction (PDPC).

[0110] Also, a reference sample line with the highest prediction accuracy may be selected from among multiple reference sample lines surrounding the current block, and a predicted sample may be derived using a reference sample located in a prediction direction from the selected line. At this time, information on the used reference sample line (e.g., intra_luma_ref_idx) may be coded and signaled in a bitstream. In this case, it may be called multi-reference line intra prediction (MRL) or MRL-based intra prediction. If MRL is not applied, a reference sample may be derived from a reference sample line directly adjacent to the current block, and in this case, information on the reference sample line may not be signaled.

[0111] Also, the current block may be divided into vertical or horizontal sub-partitions, and intra prediction may be performed for each sub-partition based on the same intra prediction mode. In this case, the neighboring reference samples for intra prediction may be derived for each sub-partition. That is, in the encoding / decoding order, the restored samples of the previous sub-partition may be used as the neighboring reference samples for the current sub-partition. In this case, the intra prediction mode for the current block is applied to the sub-partitions in the same manner, but the neighboring reference samples may be derived and used for each sub-partition, thereby improving the intra prediction performance as the case may be. Such a prediction method may be called intra sub-partitions (ISP) or ISP-based intra prediction.

[0112] The above-mentioned intra prediction techniques may be referred to by various terms such as the intra prediction type or additional intra prediction mode, distinguished from directional or non-directional intra prediction modes. For example, the intra prediction techniques (intra prediction types or additional intra prediction modes, etc.) may include at least one of the above-mentioned LIP, LM, PDPC, MRL, and ISP. A general intra prediction method other than the specific intra prediction types such as LIP, LM, PDPC, MRL, and ISP may be referred to as a normal intra prediction type. The normal intra prediction type may be generally applied when the above-mentioned specific intra prediction types are not applied, and prediction may be performed based on the above-mentioned intra prediction modes. Meanwhile, post-processing filtering may be performed on the derived prediction samples as necessary.

[0113] Specifically, the intra prediction procedure may include an intra prediction mode / type determination step, a neighboring reference sample derivation step, and an intra prediction mode / type-based prediction sample derivation step. In addition, a post-processing filtering step may be performed on the derived prediction sample, if necessary.

[0114] Meanwhile, in addition to the above-mentioned intra prediction types, affine linear weighted intra prediction (ALWIP) can be used. The ALWIP is also called linear weighted intra prediction (LWIP) or matrix weighted intra prediction or matrix based intra prediction (MIP). When the MIP is applied to the current block, a prediction sample for the current block can be derived by i) performing a matrix-vector-multiplication procedure using neighboring reference samples that have been subjected to an averaging procedure, and iii) further performing a horizontal / vertical interpolation procedure as necessary. The intra prediction mode used for the MIP can be configured to be different from the intra prediction mode used in the above-mentioned LIP, PDPC, MRL, and ISP intra prediction, and normal intra prediction. The intra prediction mode for the MIP is also called MIP intra prediction mode, MIP prediction mode, or MIP mode. For example, the matrix and offset used in the matrix-vector multiplication can be set to be different depending on the intra prediction mode for the MIP. Here, the matrix may be called a (MIP) weight matrix, and the offset may be called a (MIP) offset vector or a (MIP) bias vector. A specific MIP method will be described later.

[0115] The procedure for block reconstruction based on intra prediction and the intra prediction unit in the encoding device will be described later with reference to FIGS.

[0116] FIG. 4 is a flow chart illustrating a video / image coding method based on intra prediction.

[0117] The encoding method of FIG. 4 may be performed by the image encoding device of FIG. 2. Specifically, step S410 may be performed by the intra prediction unit 185, and step S420 may be performed by the residual processing unit. Specifically, step S420 may be performed by the subtraction unit 115. Step S430 may be performed by the entropy encoding unit 190. The prediction information of step S430 may be derived by the intra prediction unit 185, and the residual information of step S430 may be derived by the residual processing unit. The residual information is information about the residual sample. The residual information may include information about quantized transform coefficients for the residual sample. As described above, the residual sample may be derived into transform coefficients through the transform unit 120 of the image encoding device, and the transform coefficients may be derived into quantized transform coefficients through the quantization unit 130. The information about the quantized transform coefficients may be coded in the entropy encoding unit 190 through a residual coding procedure.

[0118] The image encoding apparatus may perform intra prediction on a current block (S410). The image encoding apparatus may determine an intra prediction mode / type for the current block, derive surrounding reference samples for the current block, and then generate prediction samples in the current block based on the intra prediction mode / type and the surrounding reference samples. Here, the steps of determining the intra prediction mode / type, deriving the surrounding reference samples, and generating the prediction samples may be performed simultaneously, or one of the steps may be performed prior to the other steps.

[0119] FIG. 5 is a diagram illustrating an example configuration of the intra prediction unit 185 according to the present disclosure.

[0120] As shown in FIG 5, the intra prediction unit 185 of the image encoding device may include an intra prediction mode / type determination unit 186, a reference sample derivation unit 187, and / or a prediction sample derivation unit. The intra prediction mode / type determination unit 186 may determine an intra prediction mode / type for the current block. The reference sample derivation unit 187 may derive neighboring reference samples of the current block, and the prediction sample derivation unit 188 may derive a prediction sample of the current block. Meanwhile, although not shown, if a prediction sample filtering procedure (not shown) is performed, the intra prediction unit 185 may further include a prediction sample filter unit (not shown).

[0121] The image encoding apparatus may determine a mode / type to be applied to the current block from among a plurality of intra prediction modes / types, and may compare rate-distortion costs (RD costs) for the intra prediction modes / types to determine an optimal intra prediction mode / type for the current block.

[0122] Meanwhile, the image coding apparatus may also perform a prediction sample filtering procedure. The prediction sample filtering may be called post-filtering. The prediction sample filtering procedure may filter some or all of the prediction samples. In some cases, the prediction sample filtering procedure may be omitted.

[0123] 4, the image encoding apparatus may generate a residual sample for the current block based on a predicted sample or a filtered predicted sample (S420). The image encoding apparatus may derive the residual sample by subtracting the predicted sample from an original sample of the current block. That is, the image encoding apparatus may derive a residual sample value by subtracting a corresponding predicted sample value from an original sample value.

[0124] The image encoding apparatus may encode image information including information on the intra prediction (prediction information) and residual information on the residual sample (S430). The prediction information may include the intra prediction mode information and / or the intra prediction technique information. The image encoding apparatus may output the encoded image information in a bitstream format. The output bitstream may be transmitted to an image decoding apparatus via a storage medium or a network.

[0125] The residual information may include a residual coding syntax, which will be described later. The image encoding apparatus may transform / quantize the residual samples to derive quantized transform coefficients. The residual information may include information on the quantized transform coefficients.

[0126] Meanwhile, as described above, the image coding apparatus may generate a reconstructed picture (including reconstructed samples and reconstructed blocks). To this end, the image coding apparatus may again inverse quantize / inverse transform the quantized transform coefficients to derive (modified) residual samples. The reason for again performing inverse quantization / inverse transform after transforming / quantizing the residual samples is to derive residual samples identical to the residual samples derived in the image decoding apparatus. The image coding apparatus may generate a reconstructed block including reconstructed samples for the current block based on the predicted samples and the (modified) residual samples. A reconstructed picture for the current picture may be generated based on the reconstructed block. As described above, an in-loop filtering procedure or the like may further be applied to the reconstructed picture.

[0127] FIG. 6 is a flow chart illustrating a video / image decoding method based on intra prediction.

[0128] The image decoding apparatus can perform operations corresponding to those performed by the image encoding apparatus.

[0129] The decoding method of FIG. 6 may be performed by the image decoding apparatus of FIG. 3. Steps S610 to S630 may be performed by the intra prediction unit 265, and the prediction information of S610 and the residual information of S640 may be obtained from a bitstream by the entropy decoding unit 210. The residual processing unit of the image decoding apparatus may derive a residual sample for a current block based on the residual information (S640). Specifically, the inverse quantization unit 220 of the residual processing unit may derive a transform coefficient by performing inverse quantization based on the quantized transform coefficient derived based on the residual information, and the inverse transform unit 230 of the residual processing unit may derive a residual sample for the current block by performing inverse transform on the transform coefficient. Step S650 may be performed by the adder 235 or a reconstruction unit.

[0130] Specifically, the image decoding apparatus may derive an intra prediction mode / type for a current block based on received prediction information (intra prediction mode / type information) (S610). The image decoding apparatus may also derive neighboring reference samples for the current block (S620). The image decoding apparatus may generate prediction samples in the current block based on the intra prediction mode / type and the neighboring reference samples (S630). In this case, the image decoding apparatus may perform a prediction sample filtering procedure. Prediction sample filtering may be referred to as post-filtering. Some or all of the prediction samples may be filtered by the prediction sample filtering procedure. In some cases, the prediction sample filtering procedure may be omitted.

[0131] The image decoding apparatus may generate a residual sample for the current block based on the received residual information (S640). The image decoding apparatus may generate a reconstructed sample for the current block based on the predicted sample and the residual sample, and derive a reconstructed block including the reconstructed sample (S650). A reconstructed picture for the current picture may be generated based on the reconstructed block. As described above, an in-loop filtering procedure or the like may be further applied to the reconstructed picture.

[0132] FIG. 7 is a diagram illustrating an example configuration of the intra prediction unit 265 according to the present disclosure.

[0133] As shown in FIG. 7, the intra prediction unit 265 of the image decoding apparatus may include an intra prediction mode / type determination unit 266, a reference sample derivation unit 267, and a prediction sample derivation unit 268. The intra prediction mode / type determination unit 266 determines an intra prediction mode / type for the current block based on intra prediction mode / type information generated and signaled by the intra prediction mode / type determination unit 186 of the image encoding apparatus, and the reference sample derivation unit 266 may derive a neighboring reference sample of the current block from a restored reference area in the current picture. The prediction sample derivation unit 268 may derive a prediction sample of the current block. Meanwhile, although not shown, if the above-mentioned prediction sample filtering procedure is performed, the intra prediction unit 265 may further include a prediction sample filter unit (not shown).

[0134] The intra prediction mode information may include, for example, flag information (e.g., intra_luma_mpm_flag) indicating whether a most probable mode (MPM) is applied to the current block or a remaining mode is applied. If the MPM is applied to the current block, the intra prediction mode information may further include index information (e.g., intra_luma_mpm_idx) indicating one of the intra prediction mode candidates (MPM candidates). The intra prediction mode candidates (MPM candidates) may be composed of an MPM candidate list or an MPM list. In addition, if the MPM is not applied to the current block, the intra prediction mode information may further include remaining mode information (e.g., intra_luma_mpm_remainder) indicating one of the remaining intra prediction modes excluding the intra prediction mode candidates (MPM candidates). The image decoding apparatus may determine the intra prediction mode of the current block based on the intra prediction mode information.

[0135] Further, the intra prediction technique information may be realized in various forms. As an example, the intra prediction technique information may include intra prediction technique index information indicating one of the intra prediction techniques. As another example, the intra prediction technique information may include at least one of reference sample line information (e.g., intra_luma_ref_idx) indicating whether the MRL is applied to the current block and, if so, which reference sample line is used, ISP flag information (e.g., intra_subpartitions_mode_flag) indicating whether the ISP is applied to the current block, ISP type information (e.g., intra_subpartitions_split_flag) indicating a subpartition split type when the ISP is applied, flag information indicating whether PDPC is applied, or flag information indicating whether LIP is applied. In addition, the intra prediction type information may include an MIP flag indicating whether MIP is applied to the current block. In the present disclosure, the ISP flag information may be referred to as an ISP application indicator.

[0136] The intra prediction mode information and / or the intra prediction technique information may be encoded / decoded through a coding method described in the present disclosure. For example, the intra prediction mode information and / or the intra prediction technique information may be encoded / decoded through entropy coding (e.g., CABAC, CAVLC) based on a truncated (rice) binary code.

[0137] Meanwhile, the intra prediction modes may further include a cross-component linear model (CCLM) mode for chroma samples in addition to the PLANAR mode, DC mode, and directional intra prediction mode. The CCLM modes are divided into L_CCLM, T_CCLM, and LT_CCLM depending on whether the left sample, the top sample, or both are considered for deriving CCLM parameters, and can be applied only to chroma components.

[0138] The intra prediction modes can be indexed, for example, as shown in Table 1 below.

[0139] [Table 1]

[0140] Meanwhile, the intra prediction type (or additional intra prediction mode, etc.) may include at least one of the above-mentioned LIP, PDPC, MRL, ISP, and MIP. The intra prediction type may be indicated based on intra prediction type information, and the intra prediction type information may be realized in various forms. As an example, the intra prediction type information may include intra prediction type index information indicating one of the intra prediction types. As another example, the intra prediction type information may include at least one of reference sample line information (e.g., intra_luma_ref_idx) indicating whether the MRL is applied to the current block and, if so, which reference sample line is used, ISP flag information (e.g., intra_subpartitions_mode_flag) indicating whether the ISP is applied to the current block, ISP type information (e.g., intra_subpartitions_split_flag) indicating a subpartition split type when the ISP is applied, flag information indicating whether PDCP is applied, or flag information indicating whether LIP is applied. In addition, the intra prediction type information may include a MIP flag (which may be referred to as intra_mip_flag) indicating whether MIP is applied to the current block.

[0141] MRL (Multi-reference line) intra prediction

[0142] FIG. 8 is a diagram illustrating multiple reference lines for intra prediction according to one embodiment of the present disclosure.

[0143] Conventional intra prediction uses only the neighboring samples of the first upper line and the first left line of the current block as reference samples for intra prediction. However, in a multiple-reference line (MRL) method, intra prediction can be performed using neighboring samples located on sample lines one to three sample distances away from the upper and / or left side of the current block as reference samples. Alternatively, intra prediction using the MRL method can be performed using neighboring samples located on sample lines N sample distances away from the upper and / or left side of the current block as reference samples. FIG. 8 shows an example of the multiple reference lines, and a multiple reference line index (e.g., mrl_idx) indicates which line is used for intra prediction for the current block.

[0144] For example, the multiple reference line index may be signaled via a coding unit syntax as shown in Table 2. The multiple reference line index may be configured in the form of an intra_luma_ref_idx syntax element.

[0145] [Table 2]

[0146] intra_luma_ref_idx[x0][y0] may indicate intra reference line index IntraLumaRefLineIdx[x0][y0]. If intra_luma_ref_idx[x0][y0] is not present, its value can be inferred to be 0. intra_luma_ref_idx may be referred to as (intra) reference sample line index or mrl_idx. Intra_luma_ref_idx is also referred to as intra_luma_ref_line_idx. Table 3 below shows the value of IntraLumaRefLineIdx[x0][y0] based on intra_luma_ref_idx[x0][y0].

[0147] [Table 3]

[0148] The MRL may not be used for a block in the first line (row) in a CTU. That is, if the top boundary of the current block is the boundary of a CTU, the MRL may not be used for the current block. In Table 2, if the top boundary of the current block is the boundary of a CTU, intra_luma_ref_idx[x0][y0] does not exist and its value is inferred to be 0. Therefore, the first reference line can be used for the current block. This is to prevent extended reference lines outside the current CTU line from being used. Also, if the above-mentioned additional reference line is used, the above-mentioned PDPC may not be used.

[0149] Generalized prediction sample derivation for chroma components

[0150] When intra prediction is performed on a current block, prediction can be performed on a luma component block (luma block) and a chroma component block (chroma block) of the current block, in which case the intra prediction mode for the chroma component (chroma block) can be set separately from the intra prediction mode for the luma component (luma block).

[0151] For example, an intra-prediction mode for a chroma component may be indicated based on intra-chroma prediction mode information, and the intra-chroma prediction mode information may be signaled in the form of an Intra_chroma_pred_mode syntax element. As an example, the intra-chroma prediction mode information may indicate one of candidate modes including at least one of a Planar mode, a DC mode, a vertical mode, a Horizontal mode, a Derived Mode (DM), an L_CCLM, a T_CCLM, and a LT_CCLM mode. DM may also be referred to as a direct mode. CCLM may also be referred to as an LM.

[0152] Meanwhile, DM and CCLM are dependent intra prediction modes that predict a chroma block using information of a luma block. The DM may indicate a mode in which the same intra prediction mode as the intra prediction mode for the luma component is applied to the intra prediction mode for the chroma component. In addition, the CCLM may indicate an intra prediction mode in which a reconstructed sample of a luma block is subsampled in the process of generating a prediction block for a chroma block, and a sample derived by applying CCLM parameters (e.g., α and / or β) to the subsampled sample is used as a prediction sample for the chroma block.

[0153] Overview of CCLM (Cross-component linear model)

[0154] A CCLM mode may be applied to a current chroma block. The CCLM mode is an intra prediction mode using correlation between a luma block and a chroma block corresponding to the luma block, and may indicate a mode in which a linear model may be derived based on surrounding samples of the luma block and surrounding samples of the chroma block, and a predicted sample of the chroma block may be derived based on the linear model and a reconstructed sample of the luma block. Specifically, when the CCLM mode is applied to a current chroma block, parameters for the linear model may be derived based on surrounding samples used for intra prediction of the current chroma block and surrounding samples used for intra prediction of the current luma block.

[0155] In order to reduce the redundancy of cross-components, the present disclosure may use a CCLM prediction mode, where a chroma sample may be predicted based on the reconstructed luma sample of the same CU using a linear model. For example, the linear model may be the following Equation 1:

[0156] [Formula 1]

number

[0157] Here, pred C (i, j) may denote a predicted sample of the (i, j) coordinate of the current chroma block in the current CU, and rec' L (i, j) may indicate a reconstructed sample of the (i, j) coordinate of the current luma block in the CU. L (i, j) may denote the down-sampled reconstruction sample of the current luma block.

[0158] 9 is a diagram illustrating the locations of neighboring samples used for CCLM parameter derivation according to one embodiment of the present disclosure, showing examples of the location of the left neighboring sample, the location of the upper neighboring sample, and the samples neighboring the current block associated with the CCLM mode.

[0159] The CCLM parameters (e.g., α and / or β) may be derived using up to four adjacent chroma samples and their corresponding downsampled luma samples. Alternatively, the CCLM parameters may be derived using N adjacent chroma samples and their corresponding downsampled luma samples. If the size of the current chroma block is W×H, W′ and H′ may be set as follows: W′ may refer to the range in which the upper adjacent chroma samples used for the CCLM parameter derivation are located. H′ may refer to the range in which the left adjacent chroma samples used for the CCLM parameter derivation are located.

[0160] -When LM mode is applied, W'=W, H'=H

[0161] -When LM_A mode is applied, W'=W+H

[0162] -When LM_L mode is applied, H'=H+W

[0163] The position of the upper neighboring sample can be denoted as S[0,-1]...S[W'-1,-1], and the position of the left neighboring sample can be denoted as S[-1,0]...S[-1,H'-1]. In this case, the four samples can be selected as follows:

[0164] - When LM mode is applied and all upper and left adjacent samples are available: S[W' / 4, -1], S[3W' / 4, -1], S[-1, H' / 4], S[-1, 3H' / 4]

[0165] - When LM_A mode is applied or only upper adjacent samples are available: S[W' / 8, -1], S[3W' / 8, -1], S[5W' / 8, -1], S[7W' / 8, -1]

[0166] -If LM_L mode is applied or only the left adjacent sample is available: S[-1, H' / 8], S[-1, 3H' / 8], S[-1, 5H' / 8], S[-1, 7H' / 8]

[0167] The four adjacent luma samples at the positions selected in the above manner can be obtained by downsampling. The two smallest values ​​(X 0 A and X 1 A) and the two larger values ​​(X 0 B and X 1 B Four comparisons can be made to find y 0 A , y 1 A , y 0 B and y 1 B In this case, X a , X b , Y a and Y b can be derived by the following equation 2.

[0168] [Formula 2]

number

[0169] The CCLM parameters α and β can be obtained by the following Equation 3.

[0170] [Formula 3]

number

[0171] An embodiment of the present disclosure can derive CCLM parameters using upper and left adjacent samples, which can also be used in the other two LM modes (LM_A mode and LM_L mode). According to an embodiment of the present disclosure, in LM_A mode, only the upper adjacent sample can be used to derive CCLM parameters. In this case, the upper adjacent sample can be extended to a sample at a W+H position to obtain more samples. In contrast, in LM_L mode, only the left adjacent sample can be used to derive CCLM parameters. In that case, the left adjacent sample can be extended to a sample at a H+W position to obtain more samples. As another example, in a non-square block, the upper adjacent sample can be extended to a W+W position, and the left adjacent sample can be extended to a H+H position.

[0172] To match the positions of the chroma samples for a 4:2:0 video sequence, two types of downsampling filters can be applied to the luma samples to achieve a 2:1 downsampling ratio in both the horizontal and vertical directions. The downsampling filters can be specified by an SPS level flag. The downsampling filters can be applied to obtain a luma sample corresponding to a chroma sample at a given position (i, j). The two downsampling filters can be represented by the following Equations 4 and 5.

[0173] [Formula 4]

number

[0174] [Formula 5]

number

[0175] If the current luma block boundary coincides with the CTU boundary, only one luma reference line can be used to generate the downsampled luma sample.

[0176] The derivation of the parameters may be performed as part of the image decoding process and not only in the encoder search operation, and as a result, no syntax may be used to convey the α and / or β values ​​to the image decoding device.

[0177] According to the present disclosure, a total of eight intra prediction modes may be used in the intra chrominance prediction mode encoding process. The eight intra prediction modes may include five existing intra prediction modes and three CCLM modes (CCLM, LM_A, and LM_L). The intra chrominance prediction mode signaling and deriving process will be described below with reference to Tables 4 and 5 below.

[0178] [Table 4]

[0179] [Table 5]

[0180] Intra-chroma prediction mode coding may be dependent on the intra-prediction mode of the corresponding luma block. In an I slice, a separate block division structure for luma and chroma components is activated, so one chroma block may correspond to multiple luma blocks. Therefore, in the case of a chroma DM mode, the intra-prediction mode of the corresponding luma block that covers the center position of the current chroma block may be directly applied.

[0181] Table 4 shows a matching table for deriving an intra-chroma prediction mode when CCLM is not applicable, and Table 5 shows a mapping table for deriving an intra-chroma prediction mode when CCLM is applicable. As shown in the tables, the intra-chroma prediction mode can be determined based on the intra-luma prediction mode for the luma block (e.g., when DUAL_TREE is applied) covering the center lower right sample of the current block or chroma block and the value of the signaled intra-chroma prediction mode (intra_chroma_pred_mode) information. The indexes of IntraPredModeC[xCb][yCb] derived from the above table may correspond to the indexes of the intra-prediction modes disclosed in Table 1 above.

[0182] Overview of MMLM (Multi-model LM)

[0183] The CCLM mode can be extended to the MMLM mode. For example, three MMLM modes may be added. The reconstructed adjacent samples in each MMLM mode can be classified into two groups using a threshold. The threshold can be, for example, the average value of the reconstructed luma samples. A linear model for each group can be derived using the Linear-Mean-Square (LMS) method. The LMS method can also be used to derive a linear model for the CCLM mode.

[0184] With MMLM, there may be more than one linear model between luma samples and chroma samples in a CU. In this method, adjacent luma samples and adjacent chroma samples of the current block may be classified into several groups. Each group may be used as a training set for deriving a linear model. That is, CCLM parameters (e.g., α and / or β) may be derived for each group. Samples in the current luma block may be classified in the same manner as for classifying adjacent luma samples.

[0185] In this method, adjacent samples can be classified into M groups, where M can be 2 or 3. When M is 2 or 3, the MMLM method can be designed into two additional chroma prediction modes, MMLM2 and MMLM3, in addition to the original LM mode. The image coding device can select the optimal mode in the RDO process and signal the mode.

[0186] FIG. 10 is a diagram illustrating a method for classifying adjacent samples into two groups according to an embodiment of the present disclosure. FIG. 10 illustrates a case where M is 2. Referring to FIG. 10, the threshold can be calculated as the average value of the reconstructed adjacent luma samples. Rec' below the threshold L The adjacent luma samples with [x, y] are classified into group 1, and the over-threshold Rec' L Neighboring luma samples having [x, y] may be classified into group 2. The two models of group 1 and group 2 may be derived using Equation 6 below.

[0187] [Formula 6]

number

[0188] An image encoding / decoding method according to an embodiment of the present disclosure will be described in detail below.

[0189] The present disclosure relates to a CCLM method, and more particularly to a CCLM method based on multiple reference lines. The CCLM modes according to the present disclosure include an LM mode using a surrounding upper reference sample and a left reference sample, an LM_L mode using only a left reference sample, an LM_A mode using only an upper reference sample, and an MM (multi-model) LM-based 3LM mode using two or more linear model parameters (MMLM mode (using an upper reference sample and a left reference sample), MMLM_L mode (using only a left reference sample), and MMLM_A mode (using only an upper reference sample). In the present disclosure, the six LM methods are collectively referred to as a CCLM method. Also, the "reference line" referred to in the present disclosure may be referred to as a "reference sample line" or another term having an equivalent technical meaning.

[0190] The present disclosure proposes a method of using more surrounding reference samples than one reference line when generating a predicted block of a chroma image through the CCLM method. The existing CCLM method derives linear model parameters (hereinafter, referred to as "CCLM parameters") α and β values ​​using one reference line around a chroma block. The CCLM parameters can be derived by a linear model induced using four surrounding reference samples as described above. Alternatively, the CCLM parameters can be derived through a linear mean-square (LMS) model. For example, Equation 7 may be an example of deriving the CCLM parameters using the LMS model.

[0191] [Formula 7]

number

[0192] Here, Rec C (i) shows the reconstructed chroma samples around the target block (current chroma block), and Rec' L(i) denotes the downsampled luma samples around the target block (current luma block), and I denotes the total number of samples.

[0193] As mentioned above, if a single reference line around the current chroma block is used to derive the CCLM parameters, it is not easy to derive an accurate linear model, which may reduce prediction accuracy and ultimately reduce image coding efficiency.

[0194] For example, according to the present disclosure, by using two or more reference lines around the current chroma block to derive the CCLM parameters, a more sophisticated linear model can be derived, which can improve prediction accuracy and, consequently, image coding efficiency.

[0195] According to an embodiment of the present disclosure, two or more surrounding reference lines may be used to derive LMS-based CCLM parameters. According to an embodiment of the present disclosure, when deriving LMS-based CCLM parameters, the accuracy of CCLM prediction may be improved by deriving CCLM parameters using two or more surrounding reference lines of a current chroma block.

[0196] FIG. 11 is a diagram illustrating a method of selecting reference samples for deriving CCLM parameters according to one embodiment of the present disclosure.

[0197] Specifically, FIG. 11 illustrates an example of a method for selecting peripheral reference samples when deriving CCLM parameters using two peripheral reference lines of a 4×4 current chroma block. According to an embodiment of the present disclosure illustrated in FIG. 11, eight chroma samples of a first reference line and eight chroma samples of a second reference line can be used to derive CCLM parameters. In this case, downsampling of peripheral reference samples of a corresponding luma block is performed. For example, the first downsampled upper reference line and the second upper reference line of a luma block can be downsampled to obtain a first downsampled upper reference line. Also, the third upper reference line and the fourth upper reference line can be downsampled to obtain a second downsampled upper reference line. In this manner, two downsampled upper reference lines can be generated. Also, according to an embodiment of the present disclosure, the first downsampled left reference line can be obtained by downsampling the first left reference line, the second reference line, and the third reference line of a luma block. Also, the third left reference line, the fourth reference line, and the fifth reference line may be downsampled to obtain a second downsampled left reference line. Thus, two downsampled left reference lines may be generated. The luma sample downsampling equation may be an existing method as is. Alternatively, the luma sample downsampling may be performed by a method different from the existing method. The above-mentioned method may be similarly applied to all CCLM methods (LM, LM_A, LM_L, MMLM, MMLM_A, MMLM_L).

[0198] The embodiment described with reference to FIG. 11 is an example performed when the number of chroma reference lines is two. However, the embodiment can be similarly applied when the number of chroma reference lines is N. For example, when the number of chroma reference lines is N, 2N upper luma reference lines can be used to derive the CCLM parameters. In this case, the i-th upper luma reference line can be generated by down-sampled based on the 2i-1-th upper luma reference line and the 2i-th upper luma reference line among the 2N upper luma reference lines. Here, i can be an integer between 1 and N. The CCLM parameters can be derived based on the N down-sampled upper luma reference lines generated using this method. Also, when the number of chroma reference lines is N, 2N+1 left luma reference lines can be used to derive the CCLM parameters. In this case, the i-th left luma reference line can be generated by down-sampled based on the 2i-1-th left luma reference line, the 2i-th left luma reference line, and the 2i+1-th left luma reference line among the 2N+1 left luma reference lines. CCLM parameters can be derived based on N downsampled left luma reference lines generated using this method. In this way, when CCLM parameters are derived using reference samples of two (or N) downsampled reference lines obtained through downsampling, CCLM prediction performance can be improved due to improved parameter accuracy. In addition, since the number of chroma samples and luma samples for CCLM parameter derivation is doubled (or N-fold), the existing process for CCLM parameter derivation can be used as it is without modification.

[0199] FIG. 12 is a diagram for explaining a reference sample selection method for deriving a CCLM parameter in the LM_A mode according to an embodiment of the present disclosure. The LM_A mode may be a method for deriving a CCLM parameter using only an upper reference sample of a current chroma block. To derive the CCLM parameter, eight chroma samples of a second reference line may be used in addition to eight chroma samples of an existing first reference line. In this case, the eight chroma samples of the first upper reference line may be samples present within a range of a W+H position. In addition, the image decoding apparatus may downsample reference samples around a corresponding luma block to derive the CCLM parameter. For example, the first downsampled upper reference line and the second upper reference line of the luma block may be downsampled to obtain a first downsampled upper reference line. In addition, the third upper reference line and the fourth upper reference line may be downsampled to obtain a second downsampled upper reference line.

[0200] 13 and 14 show an example of a method for selecting reference samples in a non-square block according to an embodiment of the present disclosure. As shown in FIG. 13, for one of the left and top sides (e.g., the long side) of a non-square chroma block, only some of the adjacent reference samples may be selected and used to derive CCLM parameters. In this case, when two reference lines around the current chroma block are used, for example, as shown in the left diagram of FIG. 13, subsampling in the first top reference line and subsampling in the second top reference line may be the same. Or, as shown in the right diagram of FIG. 13, subsampling in the first left reference line and subsampling in the second left reference line may be the same. For a corresponding luma block, downsampling may be performed based on the subsampled position, so that a corresponding restored luma sample may be derived.

[0201] Referring to FIG 14a, for the upper side of a non-square chroma block, only some of the adjacent reference samples may be selected and used to derive CCLM parameters. In this case, when two reference lines around the current chroma block are used, the subsampling of the first upper reference line and the subsampling of the second upper reference line may be the same as shown in FIG 14a. Specifically, the samples in the second reference line may be selected at positions that are multiples of the subsampling ratio of the samples in the first reference line. For example, when the subsampling ratio is 4, the reference samples at the 0th, 4th, 8th, and 12th positions may be selected as the second reference line samples.

[0202] According to another embodiment of the present disclosure, for the upper side of a non-square chroma block, only some of the neighboring reference samples may be selected and used to derive CCLM parameters. In this case, when two reference lines around the current chroma block are used, the subsampling of the first upper reference line may be different from the subsampling of the second upper reference line, for example, as shown in FIG. 14b. That is, the subsampling of the sample in the first reference line may be performed by selecting a reference sample at a position shifted by half the subsampling ratio as the second reference line sample, rather than at a position that is a multiple of the subsampling ratio. For example, when the subsampling ratio is 4, the reference samples at the 2nd, 5th, 10th, and 14th positions may be selected as the samples of the second reference line.

[0203] The embodiment described with reference to Fig. 14 is an example in which subsampling is performed on the upper reference line. However, the embodiment can be similarly applied to the case in which subsampling is performed on the left reference line. That is, the subsampling on the first left reference line and the subsampling on the second left reference line may be the same or different.

[0204] In applying the CCLM prediction method according to an embodiment of the present disclosure, a CCLM mode may be applied differently to a current chroma block based on whether the top boundary of the current chroma block coincides with a boundary of a CTU. According to an embodiment of the present disclosure, if the top boundary of the current chroma block coincides with a boundary of a CTU, a first (upper) reference line sample may be copied to a second reference line, and CCLM parameters according to the present disclosure may be derived using the first and second reference lines.

[0205] According to another embodiment of the present disclosure, when the top boundary of the current chroma block coincides with the boundary of the CTU, the CCLM prediction method according to the present disclosure may not be used, i.e., when the top boundary of the current chroma block coincides with the boundary of the CTU, one (upper) reference line may be used to derive CCLM parameters.

[0206] According to another embodiment of the present disclosure, when the upper boundary of the current chroma block coincides with the boundary of the CTU and a CCLM prediction method using an upper reference line is applied (e.g., LM, LM_A, MMLM, MMLM_A), the CCLM prediction method proposed in the present disclosure may not be used.

[0207] According to an embodiment of the present disclosure, when deriving an LMS-based CCLM parameter, more reference samples can be used than when deriving a CCLM parameter based on a single line. By deriving a CCLM parameter using more reference samples than when deriving a CCLM parameter based on a single line, the accuracy of the CCLM parameter can be increased.

[0208] In the above embodiment, the CCLM parameters are derived based on two reference lines, but the present disclosure is not limited thereto. For example, four peripheral reference lines of the current chroma block can be used when deriving the CCLM parameters. Or, N peripheral reference lines of the current chroma block can be used when deriving the CCLM parameters. In this case, N can have a value between 2 and the CTU size. The above-mentioned two reference line-based CCLM parameter derivation method can be applied to an N reference line-based CCLM parameter derivation method. For example, when deriving the CCLM parameters using N reference lines, samples required for deriving the CCLM parameters in the N reference lines can be selected using the same method as the above-mentioned two reference line-based CCLM parameter derivation method.

[0209] According to another embodiment of the present disclosure, the CCLM parameters may be derived based on a predetermined number of reference samples. For example, as described above, the CCLM parameters may be derived based on four reference samples. The present disclosure provides various embodiments for deriving the CCLM parameters based on more reference samples. For example, as shown in FIG. 13, the CCLM parameters may be derived using reference samples selected from two or more reference lines located around the current chroma block. According to this embodiment, the accuracy of the CCLM parameters may be increased by using more reference lines than in the case of CCLM prediction based on one reference line.

[0210] For example, according to one embodiment of the present disclosure, the CCLM parameters (α and / or β) may be derived using up to 8 adjacent chroma samples and their corresponding downsampled luma samples. Alternatively, the CCLM parameters (α and / or β) may be derived using N adjacent chroma samples and their corresponding downsampled luma samples. If the size of the current block is W×H, W′ and H′ may be derived as follows: W′ may represent a range in which the upper adjacent chroma samples used for the CCLM parameter derivation are located. H′ may represent a range in which the left adjacent chroma samples used for the CCLM parameter derivation are located.

[0211] -When LM mode or MMLM mode is applied, W'=W, H'=H

[0212] When -LM_A or MMLM_A mode is applied, W'=W+H

[0213] When -LM_L mode or MMLM_L mode is applied, H'=H+W

[0214] The position of the upper adjacent sample can be expressed as S[0,-1]...S[W'-1,-1]. The position of the left adjacent sample can be expressed as S[-1,0]...S[-1,H'-1]. In this case, the four samples in the first reference line required for CCLM parameter derivation can be selected as follows:

[0215] When the -LM or MMLM mode is applied and all upper and left adjacent samples are available, S[W' / 4, -1], S[3W' / 4, -1], S[-1, H' / 4], S[-1, 3H' / 4]

[0216] - If LM_A or MMLM_A mode is applied or only upper adjacent samples are available, S[W' / 8, -1], S[3W' / 8, -1], S[5W' / 8, -1], S[7W' / 8, -1]

[0217] -If LM_L or MMLM_L mode is applied or only the left adjacent sample is available, S[-1, H' / 8], S[-1, 3H' / 8], S[-1, 5H' / 8], S[-1, 7H' / 8]

[0218] Meanwhile, the position of the adjacent sample located on the second upper reference line can be expressed as S2[0,-1]...S2[W'-1,-1]. Also, the position of the adjacent sample located on the second left reference line can be expressed as S2[-1,0]...S2[-1,H'-1]. In this case, the additional four samples required for CCLM parameter derivation can be selected as follows:

[0219] When the -LM or MMLM mode is applied and all upper and left adjacent samples are available, S2[W' / 4, -1], S2[3W' / 4, -1], S2[-1, H' / 4], S2[-1, 3H' / 4]

[0220] - If LM_A or MMLM_A mode is applied or only upper adjacent samples are available, S2[W' / 8, -1], S2[3W' / 8, -1], S2[5W' / 8, -1], S2[7W' / 8, -1]

[0221] -If LM_L or MMLM_L mode is applied or only the left adjacent sample is available, S2[-1, H' / 8], S2[-1, 3H' / 8], S2[-1, 5H' / 8], S2[-1, 7H' / 8]

[0222] After the eight reference sample pairs for deriving the CCLM parameters are selected by the above-mentioned method, the CCLM parameters can be derived by extending the existing CCLM parameter derivation method. For example, the eight reference samples can be divided into two types by comparing the magnitudes of the sample values, and the CCLM parameters can be derived based on the average value of each group. For example, the CCLM parameters can be derived using the above-mentioned Equation 3. That is, the average of the luma sample values ​​of each group is X in Equation 3. a, X b It can correspond to Y a , Y b can also be derived based on the average of the chroma sample values ​​of each group.

[0223] According to another embodiment of the present disclosure, when applying the CCLM prediction method, if the top boundary of the current chroma block coincides with the boundary of the CTU, samples in the first reference line may be copied to the second reference line, and then the CCLM prediction method according to the present disclosure may be used. According to another embodiment of the present disclosure, if the top boundary of the current chroma block coincides with the boundary of the CTU, and a CCLM prediction method using an upper reference line is applied (e.g., LM, LM_A, MMLM, MMLM_A), the CCLM prediction method proposed in the present disclosure may not be used.

[0224] According to another embodiment of the present disclosure, when deriving CCLM parameters based on LMS, more reference samples can be used than when deriving CCLM parameters based on one reference line. By deriving CCLM parameters using more reference samples than when deriving CCLM parameters based on one reference line, the accuracy of the CCLM parameters can be increased. According to one embodiment of the present disclosure, when deriving CCLM parameters, four reference lines around a chroma block can be used. For example, in this case, the CCLM parameters can be derived using Equation 7 described above. Alternatively, when deriving CCLM parameters, N reference lines around a chroma block can be used. In this case, N can have a value between 2 and the size of the CTU. The above-mentioned two reference line-based CCLM parameter derivation methods can be applied to the N reference line-based CCLM parameter derivation method. For example, when deriving CCLM parameters using N reference lines, samples required for deriving CCLM parameters in N reference lines can be selected using the same method as the above-mentioned two reference line-based CCLM parameter derivation method.

[0225] According to another embodiment of the present disclosure, a CCLM method based on multiple reference lines may be adaptively applied to the size of a current chroma block. That is, the number of chroma reference lines used to derive CCLM parameters may be determined based on a comparison between the size of the chroma block and a predetermined threshold. A specific example is as follows.

[0226] - If the width x height of a chroma block is equal to or greater than N, the multiple reference line based CCLM method is applied. Otherwise, the existing single reference line based CCLM method is applied. JPEG2024545913000014.jpg1095

[0227] - If the width of the chroma block is greater than or equal to N, the multiple reference line based LM, LM_A, MMLM, MMLM_A method is applied. Otherwise, the existing single reference line based LM, LM_A, MMLM, MMLM_A method is applied. JPEG2024545913000015.jpg981

[0228] - If the chroma block height is greater than or equal to N, the multiple reference line based LM, LM_L, MMLM, MMLM_L method is applied, otherwise the existing one reference line based LM, LM_L, MMLM, MMLM_L method is applied. JPEG2024545913000016.jpg885

[0229] - If the width x height of a chroma block is less than or equal to N, the multiple reference line based CCLM method is applied. Otherwise, the existing one reference line based CCLM method is applied. JPEG2024545913000017.jpg893

[0230] - If the width of the chroma block is less than or equal to N, the multiple reference line based LM, LM_A, MMLM, MMLM_A method is applied. Otherwise, the existing one reference line based LM, LM_A, MMLM, MMLM_A method is applied. JPEG2024545913000018.jpg1085

[0231] - If the chroma block height is less than or equal to N, the multiple reference line based LM, LM_L, MMLM, MMLM_L method is applied. Otherwise, the existing one reference line based LM, LM_L, MMLM, MMLM_L method is applied. JPEG2024545913000019.jpg892

[0232] According to the above-described method, when the number of reference samples for prediction of the current chroma block is large or small, the efficiency of CCLM prediction can be maximized by adaptively selecting the number of reference lines.

[0233] According to another embodiment of the present disclosure, it is possible to determine whether to apply a CCLM method based on multiple reference lines adaptively to the type of CCLM. Whether to apply a CCLM method based on multiple reference lines based on the type of CCLM (LM, LM_A, LM_L, MMLM, MMLM_A, MMLM_L) can be determined as follows.

[0234] - If at least one of MMLM, MMLM_A or MMLM_L is selected, a multiple reference line based CCLM method is applied.

[0235] - If at least one of LM, LM_A or LM_L is satisfied, the multiple reference line based CCLM method is applied.

[0236] If at least one of -LM or MMLM is selected, a multiple reference line based CCLM method is applied.

[0237] - If at least one of LM_A, MMLM_A, LM_L or MMLM_L is satisfied, a multiple reference line based CCLM method is applied.

[0238] -If at least one of LM_A or MMLM_A is satisfied, a multiple reference line based CCLM method is applied.

[0239] If at least one of -LM_L or MMLM_L is satisfied, a multiple reference line based CCLM method is applied.

[0240] Hereinafter, the embodiments described with reference to Figs. 15 to 18 may be performed in an image encoding device or an image decoding device. For example, the methods shown in Figs. 15 to 18 may be performed in the intra prediction unit 185 of the image encoding device 100 of Fig. 2, or the intra prediction unit 265 of the image decoding device 200 of Fig. 3. When performed in the image encoding device 100, the methods shown in Figs. 15 to 18 may be performed in step S410 of Fig. 4. When performed in the image decoding device 200, the methods shown in Figs. 15 to 18 may be performed in steps S610 to S630 of Fig. 6. Hereinafter, for convenience of explanation, it will be described that the methods shown in Figs. 15 to 18 are performed in an image decoding device. However, the methods shown in Figs. 15 to 18 may be similarly performed in an image encoding device.

[0241] FIG. 15 is a flowchart illustrating a CCLM prediction method according to one embodiment of the present disclosure.

[0242] Referring to FIG. 15, the image decoding apparatus may derive a chroma reference sample located in a periphery of a chroma block and a luma reference sample corresponding to the chroma reference sample (S1510). The chroma reference sample located in a periphery of the chroma block may be a reference sample adjacent to the chroma block. Also, the luma reference sample corresponding to the chroma reference sample may be a reference sample adjacent to the luma block. In this case, according to an embodiment of the present disclosure, the chroma reference sample may be obtained from two or more chroma reference lines adjacent to the chroma block. At least one or a combination of two or more of various embodiments of the present disclosure may be applied to derive the reference sample in step S1510.

[0243] The image decoding apparatus may derive CCLM parameters based on the chroma reference samples and the luma reference samples (S1520). That is, the CCLM parameters may be derived using the chroma reference samples and the luma reference samples derived in step S1510. According to an embodiment of the present disclosure, the CCLM parameters may be derived using the chroma reference samples and the luma reference samples derived in step S1510. In this case, the CCLM parameters may be derived using an LMS model. For example, the CCLM parameters may be derived using Equation 7 above. According to another embodiment of the present disclosure, the CCLM parameters may be derived using only a predetermined number of the chroma reference samples and the luma reference samples derived in step S1510. For example, the CCLM parameters may be derived using Equation 2 and / or Equation 3 above.

[0244] The image decoding apparatus may generate a predicted sample of the current chroma block based on the CCLM parameters (S1530). Specifically, the predicted sample of the current chroma block may be derived using the CCLM parameters derived in step S1520 and a reconstructed sample of the current luma block. Alternatively, the predicted sample of the current chroma block may be derived using the CCLM parameters derived in step S1520 and a downsampled reconstructed sample of the current luma block. According to an embodiment of the present disclosure, the predicted sample of the chroma block may be derived using Equation 1 above.

[0245] According to the method shown in Fig. 15, since two or more reference lines can be used to derive the CCLM parameters, more sophisticated CCLM parameters can be derived, which can improve the accuracy of CCLM prediction and improve coding efficiency.

[0246] FIG. 16 shows an example of how CCLM is applied when the top boundary of the current chroma block coincides with the boundary of a CTU.

[0247] The method of Fig. 16 may be performed as a previous step of step S1510 of Fig. 15. That is, the method of Fig. 16 may be a method for determining whether to use multiple reference lines, a single reference line, or a reference line of another modified type to derive CCLM parameters.

[0248] 16, the image decoding apparatus may determine whether the top boundary of a current chroma block coincides with the boundary of a CTU (S1610). If the top boundary of the chroma block coincides with the boundary of a CTU (YES in S1610), the image decoding apparatus does not apply multiple reference line based CCLM (S1620). In this case, for example, the image decoding apparatus may apply CCLM based on one reference line. Alternatively, the image decoding apparatus may copy samples present in a first reference line to a second reference line and then apply multiple reference line based CCLM according to the present disclosure.

[0249] If the top boundary of the chroma block does not coincide with the boundary of the CTU (NO in S1610), the image decoding apparatus may apply a CCLM based on multiple reference lines according to the present disclosure to predict the current chroma block (S1630). For example, if the top boundary of the chroma block does not coincide with the boundary of the CTU, the image decoding apparatus may derive CCLM parameters using an LMS model based on multiple reference lines or a predetermined number of reference samples selected from the multiple reference lines.

[0250] According to the embodiment shown in FIG. 16, various embodiments of the present disclosure can be adaptively applied depending on whether the top boundary of a chroma block coincides with the boundary of a CTU, thereby improving the efficiency of CCLM parameter derivation.

[0251] FIG. 17 is a diagram illustrating an embodiment of the present disclosure in which it is determined whether or not a multiple reference line-based CCLM is applied based on the size of a current chroma block.

[0252] Referring to FIG. 17, the image decoding apparatus may check whether the size of the current chroma block satisfies a predetermined condition (S1710). According to an embodiment of the present disclosure, the predetermined condition may be a condition for checking whether a width×height of the current chroma block is equal to or greater than N. Here, N may be at least one of 4, 8, 16, 32, 64, 128, 256, or 1024, or may include other numbers. According to another embodiment of the present disclosure, the predetermined condition may be a condition for checking whether a width×height of the current chroma block is equal to or greater than N. Here, N may be at least one of 2, 4, 8, 16, 32, 64, or 128, or may include other numbers. According to another embodiment of the present disclosure, the predetermined condition may be a condition for checking whether a height of the current chroma block is equal to or greater than N. Here, N may be at least one of 2, 4, 8, 16, 32, 64, or 128, or may include other numbers. According to another embodiment of the present disclosure, the predetermined condition may be a condition for checking whether a width×height of the current chroma block is equal to or less than N. Here, N may be at least one of 4, 8, 16, 32, 64, 128, 256, or 1024, or may include other numbers. According to another embodiment of the present disclosure, the predetermined condition may be a condition for checking whether a width of a current chroma block is less than or equal to N, where N may be at least one of 2, 4, 8, 16, 32, 64, or 128, or may include other numbers. According to another embodiment of the present disclosure, the predetermined condition may be a condition for checking whether a height of a current chroma block is less than or equal to N, where N may be at least one of 2, 4, 8, 16, 32, 64, or 128, or may include other numbers.

[0253] If the size of the current chroma block satisfies a predetermined condition (YES in S1710), CCLM based on multiple reference lines can be applied (S1720). For example, if the size of the current chroma block satisfies a predetermined condition, the image decoding apparatus can derive CCLM parameters using an LMS model based on multiple reference lines, or using a predetermined number of reference samples selected from the multiple reference lines. On the other hand, if the size of the current chroma block does not satisfy the predetermined condition (NO in S1710), CCLM based on multiple reference lines is not applied (S1730). In this case, for example, CCLM based on one reference line can be applied.

[0254] According to the embodiment shown in FIG. 17, when there are more or less samples for prediction of the current chroma block, the efficiency of CCLM prediction can be improved by adaptively selecting the number of reference lines for CCLM parameter derivation.

[0255] FIG. 18 is a diagram illustrating an embodiment of the present disclosure in which it is determined whether to apply CCLM based on a plurality of reference lines based on the type of CCLM applied to a current chroma block.

[0256] Referring to FIG. 18, the image decoding apparatus may determine whether a type of CCLM applied to a current chroma block satisfies a predetermined condition (S1810). According to an embodiment of the present disclosure, the predetermined condition may be a condition for determining whether a CCLM mode applied to a current chroma block is at least one of MMLM, MMLM_A, or MMLM_L. According to another embodiment of the present disclosure, the predetermined condition may be a condition for determining whether a CCLM mode applied to a current chroma block is at least one of LM, LM_A, or LM_L. According to another embodiment of the present disclosure, the predetermined condition may be a condition for determining whether a CCLM mode applied to a current chroma block is at least one of LM_A or MMLM_A. According to another embodiment of the present disclosure, the predetermined condition may be a condition for determining whether a CCLM mode applied to a current chroma block is at least one of LM_A, MMLM_A, LM_L, or MMLM_L. According to another embodiment of the present disclosure, the predetermined condition may be a condition for determining whether a CCLM mode applied to a current chroma block is at least one of LM_A or MMLM_A. According to another embodiment of the present disclosure, the predetermined condition may be a condition that checks whether the CCLM mode applied to the current chroma block is at least one of LM_L or MMLM_L.

[0257] If the type of CCLM applied to the current chroma block satisfies a predetermined condition (YES in S1810), a CCLM based on multiple reference lines can be applied (S1820). For example, if the type of CCLM applied to the current chroma block satisfies a predetermined condition, the image decoding apparatus can derive CCLM parameters using an LMS model based on multiple reference lines, or using a predetermined number of reference samples selected from the multiple reference lines.

[0258] On the other hand, if the type of CCLM applied to the current chroma block does not satisfy a predetermined condition (NO in S1810), the multiple reference line-based CCLM is not applied (S1830). In this case, for example, a CCLM based on one reference line can be applied.

[0259] According to the embodiment shown in FIG. 18, various embodiments according to the present disclosure can be adaptively applied according to the type of CCLM applied for prediction of the current chroma block, thereby improving the efficiency of CCLM parameter induction. For example, in the case of MMLM, MMLM_A, or MMLM_L, two or more linear models are induced, so that a sufficient number of reference samples required for deriving the linear models needs to be secured. Therefore, when the CCL applied to the current chroma block is at least one of MMLM, MMLM_A, or MMLM_L, the accuracy of the multiple linear models can be improved by applying the CCLM based on multiple reference lines according to the present disclosure, thereby improving the accuracy of prediction.

[0260] Although the exemplary method of the present disclosure is expressed as a series of operations for clarity of explanation, this is not intended to limit the order in which the steps are performed, and each step may be performed simultaneously or in a different order, if necessary. To realize the method according to the present disclosure, the steps illustrated may include other steps, some steps may be omitted and the remaining steps may be omitted, or some steps may be omitted and additional steps may be included.

[0261] In the present disclosure, an image encoding device or an image decoding device that performs a predetermined operation (step) can perform the operation (step) to check the execution conditions or circumstances of the operation (step). For example, if it is described that a predetermined operation is performed when a predetermined condition is satisfied, the image encoding device or the image decoding device can perform the predetermined operation after performing an operation to check whether the predetermined condition is satisfied or not.

[0262] The various embodiments of the present disclosure are not intended to enumerate all possible combinations, but are intended to describe representative aspects of the present disclosure, and the matters described in the various embodiments may be applied independently or in combination of two or more.

[0263] Additionally, various embodiments of the present disclosure may be implemented using hardware, firmware, software, or a combination thereof, etc. In the case of a hardware implementation, the implementation may be implemented using one or more Application Specific Integrated Circuits (ASICs), Digital Signal Processors (DSPs), Digital Signal Processing Devices (DSPDs), Programmable Logic Devices (PLDs), Field Programmable Gate Arrays (FPGAs), general processors, controllers, microcontrollers, microprocessors, etc.

[0264] In addition, the image decoding device and the image encoding device to which the embodiments of the present disclosure are applied may be included in a multimedia broadcast transmitting / receiving device, a mobile communication terminal, a home cinema video device, a digital cinema video device, a surveillance camera, a video conversation device, a real-time communication device such as a video communication, a mobile streaming device, a storage medium, a camcorder, a custom video (VoD) service providing device, an over-the-top video (OTT) device, an internet streaming service providing device, a three-dimensional (3D) video device, an image telephone video device, and a medical video device, and may be used to process a video signal or a data signal. For example, the over-the-top video (OTT) device may include a game console, a Blu-ray player, an internet-connected TV, a home theater system, a smartphone, a tablet PC, a digital video recorder (DVR), and the like.

[0265] FIG. 19 is a diagram illustrating a content streaming system to which an embodiment of the present disclosure can be applied.

[0266] As shown in FIG. 19, a content streaming system to which an embodiment of the present disclosure is applied may broadly include an encoding server, a streaming server, a Web server, a media storage, a user device, and a multimedia input device.

[0267] The encoding server compresses content input from a multimedia input device such as a smartphone, camera, camcorder, etc. into digital data to generate a bitstream and transmits the bitstream to the streaming server. As another example, if a multimedia input device such as a smartphone, camera, video camera, etc. directly generates a bitstream, the server can be omitted.

[0268] The bitstream may be generated by an image encoding method and / or image encoding device to which an embodiment of the present disclosure is applied, and the streaming server may temporarily store the bitstream in the process of transmitting or receiving the bitstream.

[0269] The streaming server transmits multimedia data to a user device based on a user request via a web server, and the web server can act as a medium to inform the user of available services. When a user requests a desired service from the web server, the web server transmits the request to the streaming server, and the streaming server can transmit the multimedia data to the user. At this time, the content streaming system can include a separate control server, and in this case, the control server can control commands / responses between devices in the content streaming system.

[0270] The streaming server may receive the content from a media storage and / or an encoding server. For example, when receiving the content from the encoding server, the content may be received in real time. In this case, the streaming server may store the bitstream for a certain period of time in order to provide a smooth streaming service.

[0271] Examples of the user devices include mobile phones, smart phones, laptop computers, digital broadcasting terminals, personal digital assistants (PDAs), portable multimedia players (PMPs), navigation systems, slate PCs, tablet PCs, ultrabooks, wearable devices such as smartwatches, smart glass, head mounted displays (HMDs), digital TVs, desktop computers, and digital signage.

[0272] Each server in the content streaming system can be operated as a distributed server, in which case data received from each server can be processed in a distributed manner.

[0273] The scope of the present disclosure includes software or machine-executable commands (e.g., operating systems, applications, firmware, programs, etc.) that cause operations according to the methods of the various embodiments to be performed on a device or computer, and non-transitory computer-readable medium on which such software or commands can be stored and executed on a device or computer. [Industrial Applicability]

[0274] The embodiments of the present disclosure can be used to encode / decode images.

Claims

1. An image decoding method performed by an image decoding device, comprising: The image decoding method includes: deriving chroma reference samples located around the chroma block and luma reference samples corresponding to the chroma reference samples based on the intra prediction mode of the chroma block being a cross-component linear model (CCLM) mode; deriving CCLM parameters based on the chroma reference sample and the luma reference sample; generating predicted samples for the chroma blocks based on the CCLM parameters; The chroma reference samples are obtained from two or more chroma reference lines adjacent to the chroma block.

2. The step of deriving the CCLM parameters comprises:

2. The image decoding method according to claim 1, wherein the decoding is performed based on a Linear-Mean-Square (LMS) scheme using the chroma reference samples present in two or more of the chroma reference lines adjacent to the chroma block.

3. The step of deriving the CCLM parameters comprises: The image decoding method according to claim 1 , further comprising the step of: using a predetermined number of the chroma reference samples present in two or more of the chroma reference lines adjacent to the chroma block.

4. 2. The image decoding method of claim 1, wherein a subsampling method for a first reference line adjacent to the chroma block and a subsampling method for a second reference line adjacent to the chroma block are identical based on the chroma block being a non-square block.

5. 2. The image decoding method of claim 1, wherein a subsampling method for a first reference line adjacent to the chroma block is different from a subsampling method for a second reference line adjacent to the chroma block based on the chroma block being a non-square block.

6. The image decoding method of claim 1 , further comprising: determining a number of the chroma reference lines for deriving the chroma reference sample based on whether an upper boundary of the chroma block coincides with a boundary of a CTU.

7. generating a downsampled i-th upper luma reference line based on a 2i-1-th upper luma reference line and a 2i-th upper luma reference line among the 2N plurality of upper luma reference lines, where i is an integer from 1 to N; The image decoding method of claim 1 , wherein the luma reference sample is obtained based on the downsampled N upper luma reference lines.

8. generating a downsampled i-th left luma reference line based on a 2i-1-th left luma reference line, a 2i-th left luma reference line, and a 2i+1-th left luma reference line among the 2N+1 left luma reference lines, where the number of the two or more chroma reference lines is N, where i is an integer from 1 to N; The image decoding method of claim 1 , wherein the luma reference sample is obtained based on the downsampled N left luma reference lines.

9. 2. The method of claim 1, further comprising: determining a number of chroma reference lines used to derive the CCLM parameters based on a comparison of the chroma block size with a predetermined threshold.

10. The image decoding method of claim 1 , wherein a number of chroma reference lines used to derive the CCLM parameters is determined to be different based on a type of CCLM applied to the chroma block.

11. The step of deriving the CCLM parameters comprises: classifying the luma reference samples and the chroma reference samples into two or more groups based on a predetermined threshold; The image decoding method of claim 1 , further comprising: deriving CCLM parameters for each of the classified groups.

12. An image coding method performed by an image coding device, comprising: The encoding method comprises: deriving chroma reference samples located in the periphery of the chroma block and luma reference samples corresponding to the chroma reference samples based on the intra prediction mode of the chroma block being a cross-component linear model (CCLM) mode; deriving CCLM parameters based on the chroma reference sample and the luma reference sample; generating predicted samples for the chroma blocks based on the CCLM parameters; 11. A method for encoding an image, wherein the chroma reference samples are obtained from two or more chroma reference lines adjacent to the chroma block.

13. A computer-readable recording medium storing a bitstream generated by an image encoding method, comprising: The image encoding method includes: deriving chroma reference samples located in the periphery of the chroma block and luma reference samples corresponding to the chroma reference samples based on the intra prediction mode of the chroma block being a cross-component linear model (CCLM) mode; deriving CCLM parameters based on the chroma reference sample and the luma reference sample; generating predicted samples for the chroma blocks based on the CCLM parameters; The chroma reference samples are obtained from two or more chroma reference lines adjacent to the chroma block.

14. 1. A method for transmitting a bitstream generated by an image coding method, comprising: The image encoding method includes: deriving chroma reference samples located in the periphery of the chroma block and luma reference samples corresponding to the chroma reference samples based on the intra prediction mode of the chroma block being a cross-component linear model (CCLM) mode; deriving CCLM parameters based on the chroma reference sample and the luma reference sample; generating predicted samples for the chroma blocks based on the CCLM parameters; The chroma reference samples are obtained from two or more chroma reference lines adjacent to the chroma block.

Images