Image encoding / decoding method and device, and recording medium having bitstream stored therein

Abstract

Provided are an image encoding / decoding method and device, a recording medium having a bitstream stored therein, and a transmission method. The image decoding method comprises the steps of: deriving a transform kernel determination intra prediction mode of the current block; determining a transform kernel of the current block on the basis of the transform kernel determination intra prediction mode; and performing an inverse transformation of the current block on the basis of the transform kernel, wherein the transform kernel determination intra prediction mode can be derived by using an occurrence frequency histogram generated on the basis of an intra prediction mode of a neighboring block of the current block, and the occurrence frequency histogram can be generated by accumulating the intra prediction mode of the neighboring block.

Description

Video encoding / decoding method, device, and recording medium storing bitstream

[0001] The present invention relates to a video encoding / decoding method, device, and recording medium storing a bitstream. Specifically, the present invention relates to a video encoding / decoding method, device, and recording medium storing a bitstream based on a method for determining a transform set.

[0002] Recently, the demand for high-resolution, high-quality images, such as Ultra High Definition (UHD) images, is increasing across various application fields. Furthermore, interest in and demand for immersive media, such as Virtual Reality (VR) content, Artificial Reality (AR) content, and holograms, is also increasing. Furthermore, broadcasting of images with characteristics different from reality, such as game images, is also on the rise. As image data increases in resolution and quality, the relative amount of data increases compared to conventional image data. Therefore, transmitting image data using existing media such as wired and wireless broadband lines or storing it using existing storage media increases transmission and storage costs. To address these issues arising from the increasing resolution and quality of data, highly efficient image encoding / decoding technologies for higher-resolution and higher-quality images are required.

[0003] In the past, conversion was performed based on a conversion kernel that did not properly reflect the characteristics of the current block, which may limit coding efficiency.

[0004] The purpose of the present invention is to provide a video encoding / decoding method and device with improved encoding / decoding efficiency.

[0005] In addition, the present invention aims to provide a recording medium storing a bitstream generated by an image decoding method or device according to the present invention.

[0006] In addition, the present invention aims to provide an intra prediction mode derivation method and a transformation kernel determination method for determining a transformation kernel to solve the above problems.

[0007] A video decoding method according to one embodiment of the present invention includes the steps of deriving a transform kernel determined intra prediction mode of a current block, determining a transform kernel of the current block based on the transform kernel determined intra prediction mode, and performing an inverse transformation of the current block based on the transform kernel, wherein the transform kernel determined intra prediction mode is derived using an occurrence frequency histogram generated based on intra prediction modes of neighboring blocks of the current block, and the occurrence frequency histogram can be generated by accumulating intra prediction modes of the neighboring blocks.

[0008] In the above image decoding method, the transform kernel determination intra prediction mode can be derived as an intra prediction mode having a maximum value in the occurrence frequency histogram.

[0009] In the above image decoding method, the transformation kernel determination intra prediction mode can be derived using a predefined number of intra prediction modes among the intra prediction modes of the occurrence frequency histogram.

[0010] In the above image decoding method, the intra prediction mode of the surrounding block can be accumulated in the occurrence frequency histogram based on the size of the surrounding block.

[0011] In the above image decoding method, the step of determining the transformation kernel of the current block may include the step of determining a transformation set of the current block based on the transformation kernel determination intra prediction mode, and the step of determining a transformation kernel of the current block among transformation kernels included in the transformation set.

[0012] In the above image decoding method, the step of determining the transformation set and the step of determining a transformation kernel among the transformation kernels included in the transformation set can be performed simultaneously.

[0013] In the above image decoding method, the transformation set may be a transformation set of non-separable transformations.

[0014] In the above image decoding method, the non-separable transform may be either a non-separable first transform or a non-separable second transform.

[0015] In the above image decoding method, the surrounding blocks may include blocks spatially adjacent to the current block and blocks not spatially adjacent to the current block.

[0016] In the above image decoding method, a block that is not spatially adjacent to the current block may be included in at least one of a left area of ​​the current block, an upper area of ​​the current block, and an upper left area of ​​the current block.

[0017] A video encoding method according to one embodiment of the present invention includes the steps of deriving a transform kernel-determined intra-prediction mode of a current block, determining a transform kernel of the current block based on the transform kernel-determined intra-prediction mode, and performing a transform of the current block based on the transform kernel, wherein the transform kernel-determined intra-prediction mode is derived using an occurrence frequency histogram generated based on intra-prediction modes of neighboring blocks of the current block, and the occurrence frequency histogram can be generated by accumulating intra-prediction modes of the neighboring blocks.

[0018] A non-transitory computer-readable recording medium storing a bitstream generated by an image encoding method according to one embodiment of the present invention can store the bitstream generated by the image encoding method.

[0019] A bitstream transmission method according to one embodiment of the present invention can transmit a bitstream generated by the image encoding method.

[0020] The features briefly summarized above regarding the present disclosure are merely exemplary aspects of the detailed description of the present disclosure that follows and do not limit the scope of the present disclosure.

[0021] According to the present invention, a video encoding / decoding method and device with improved encoding / decoding efficiency can be provided.

[0022] Additionally, according to the present invention, a method for deriving an intra prediction mode for determining a transformation kernel of a current block can be provided.

[0023] Additionally, according to the present invention, a method for determining a transformation kernel for inverse transformation of a current block can be provided.

[0024] In addition, according to the present invention, it is possible to determine a suitable transformation set and transformation kernel, thereby improving transformation efficiency.

[0025] The effects that can be obtained from the present disclosure are not limited to the effects mentioned above, and other effects that are not mentioned will be clearly understood by a person having ordinary skill in the art to which the present disclosure pertains from the description below.

[0026] Figure 1 is a block diagram showing the configuration according to one embodiment of an encoding device to which the present invention is applied.

[0027] Figure 2 is a block diagram showing the configuration according to one embodiment of a decryption device to which the present invention is applied.

[0028] FIG. 3 is a diagram schematically showing a video coding system to which the present invention can be applied.

[0029] FIG. 4 is a drawing for explaining a surrounding block of a current block according to one embodiment of the present invention.

[0030] FIG. 5 is a flowchart illustrating a method for determining a transformation kernel according to one embodiment of the present invention.

[0031] FIG. 6 is a drawing exemplarily showing a content streaming system to which an embodiment according to the present invention can be applied.

[0032] A video decoding method according to one embodiment of the present invention includes the steps of deriving a transform kernel determined intra prediction mode of a current block, determining a transform kernel of the current block based on the transform kernel determined intra prediction mode, and performing an inverse transformation of the current block based on the transform kernel, wherein the transform kernel determined intra prediction mode is derived using an occurrence frequency histogram generated based on intra prediction modes of neighboring blocks of the current block, and the occurrence frequency histogram can be generated by accumulating intra prediction modes of the neighboring blocks.

[0033] The present invention is susceptible to various modifications and embodiments. Therefore, specific embodiments are illustrated in the drawings and described in detail in the detailed description. However, this is not intended to limit the present invention to specific embodiments, but rather to encompass all modifications, equivalents, and substitutes falling within the spirit and scope of the present invention. In the drawings, similar reference numerals designate the same or similar functions throughout. The shape and size of elements in the drawings may be provided by way of example only for clarity. The detailed description of the exemplary embodiments described below refers to the accompanying drawings, which illustrate specific embodiments. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments. It should be understood that the various embodiments, while different, are not necessarily mutually exclusive. For example, specific shapes, structures, and characteristics described herein may be implemented in other embodiments without departing from the spirit and scope of the present invention. Furthermore, it should be understood that the location or arrangement of individual components within each disclosed embodiment may be modified without departing from the spirit and scope of the embodiment. Accordingly, the detailed description set forth below is not intended to be taken in a limiting sense, and the scope of the exemplary embodiments, if properly described, is defined only by the appended claims, along with the full scope equivalents to which such claims are entitled.

[0034] In the present invention, terms such as first, second, etc. may be used to describe various components, but the components should not be limited by these terms. These terms are used solely to distinguish one component from another. For example, without departing from the scope of the present invention, the first component may be referred to as the second component, and similarly, the second component may also be referred to as the first component. The term "and / or" includes a combination of multiple related described items or any of multiple related described items.

[0035] The components shown in the embodiments of the present invention are independently depicted to represent different characteristic functions, and do not mean that each component is composed of separate hardware or a single software component. That is, each component is listed and included as a separate component for convenience of explanation, and at least two components among each component may be combined to form a single component, or a single component may be divided into multiple components to perform a function, and such integrated and separate embodiments of each component are also included in the scope of the present invention as long as they do not deviate from the essence of the present invention.

[0036] The terminology used herein is merely used to describe specific embodiments and is not intended to limit the present invention. The singular expression includes the plural expression unless the context clearly indicates otherwise. In addition, some components of the present invention are not essential components that perform essential functions in the present invention and may be optional components merely for performance enhancement. The present invention may be implemented by including only components essential to realizing the essence of the present invention, excluding components used only for performance enhancement, and a structure including only essential components, excluding optional components used only for performance enhancement, is also within the scope of the present invention.

[0037] In an embodiment, the term "at least one" may mean one of a number greater than or equal to 1, such as 1, 2, 3, and 4. In an embodiment, the term "a plurality of" may mean one of a number greater than or equal to 2, such as 2, 3, and 4.

[0038] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In describing the embodiments of this specification, if it is determined that a detailed description of a related known configuration or function may obscure the gist of this specification, the detailed description will be omitted. The same reference numerals will be used for identical components in the drawings, and duplicate descriptions of identical components will be omitted.

[0039] Glossary of Terms

[0040] Hereinafter, “video” may mean a single picture constituting a video, or may refer to the video itself. For example, “encoding and / or decoding of a video” may mean “encoding and / or decoding of a video,” or may mean “encoding and / or decoding of one of the videos constituting the video.”

[0041] Hereinafter, the terms "video" and "movie" may be used interchangeably and have the same meaning. Furthermore, the target image may be an encoding target image, which is the target of encoding, and / or a decoding target image, which is the target of decoding. Furthermore, the target image may be an input image input to an encoding device, or an input image input to a decoding device. Here, the target image may have the same meaning as the current image.

[0042] Hereinafter, the terms encoder and image encoding device may be used interchangeably and have the same meaning.

[0043] Hereinafter, the terms decoder and image decoding device may be used interchangeably and interchangeably.

[0044] Hereinafter, “image”, “picture”, “frame” and “screen” may be used with the same meaning and may be used interchangeably.

[0045] Hereinafter, the term "target block" may refer to an encoding target block, which is the target of encoding, and / or a decoding target block, which is the target of decoding. Furthermore, the target block may refer to a current block, which is the target of current encoding and / or decoding. For example, the terms "target block" and "current block" may be used interchangeably and have the same meaning.

[0046] Hereinafter, "block" and "unit" may be used with the same meaning and may be used interchangeably. In addition, "unit" may mean including a luminance component block and a corresponding chroma component block to distinguish it from a block. For example, a coding tree unit (CTU) may be composed of one luma component (Y) coding tree block (CTB) and two chroma component (Cb, Cr) coding tree blocks associated with it.

[0047] Hereinafter, the terms “sample,” “pixel,” and “pixel” may be used interchangeably and have the same meaning. Here, a sample may represent a basic unit that constitutes a block.

[0048] Hereinafter, “inter” and “between screens” may be used interchangeably and have the same meaning.

[0049] Hereinafter, “intra” and “within screen” may be used interchangeably and have the same meaning.

[0050]

[0051] Figure 1 is a block diagram showing the configuration according to one embodiment of an encoding device to which the present invention is applied.

[0052] The encoding device (100) may be an encoder, a video encoding device, or an image encoding device. A video may include one or more images. The encoding device (100) may sequentially encode one or more images.

[0053] Referring to FIG. 1, the encoding device (100) may include an image segmentation unit (110), an intra prediction unit (120), a motion prediction unit (121), a motion compensation unit (122), a switch (115), a subtractor (113), a transformation unit (130), a quantization unit (140), an entropy encoding unit (150), an inverse quantization unit (160), an inverse transformation unit (170), an adder (117), a filter unit (180), and a reference picture buffer (190).

[0054] Additionally, the encoding device (100) can generate a bitstream including encoded information through encoding an input image and output the generated bitstream. The generated bitstream can be stored in a computer-readable recording medium or can be streamed via a wired / wireless transmission medium.

[0055] The video segmentation unit (110) can segment the input video into various forms to increase the efficiency of video encoding / decoding. That is, the input video is composed of multiple pictures, and one picture can be hierarchically segmented and processed for compression efficiency, parallel processing, etc. For example, one picture can be segmented into one or more tiles or slices, which can then be segmented into multiple Coding Tree Units (CTUs). Alternatively, one picture can first be segmented into multiple sub-pictures defined as groups of rectangular slices, and each sub-picture can then be segmented into the tiles / slices. Here, the sub-pictures can be utilized to support the function of partially independently encoding / decoding and transmitting the picture. Since multiple sub-pictures can each be individually restored, there is an advantage of easy editing in applications that configure multi-channel input into a single picture. In addition, tiles can be segmented horizontally to generate bricks. Here, a brick can be utilized as the basic unit of intra-picture parallel processing. In addition, one CTU can be recursively split into a quadtree (QT), and the terminal node of the split can be defined as a coding unit (CU). The CU can be split into a prediction unit (PU) and a transformation unit (TU), and prediction and splitting can be performed. Meanwhile, the CU can be utilized as a prediction unit and / or a transformation unit itself. Here, for flexible splitting, each CTU can be recursively split into a multi-type tree (MTT) as well as a quadtree (QT). Splitting of a CTU into a multi-type tree can start from the terminal node of a QT, and the MTT can be composed of a binary tree (BT) and a triple tree (TT).For example, the MTT structure can be divided into vertical binary split mode (SPLIT_BT_VER), horizontal binary split mode (SPLIT_BT_HOR), vertical ternary split mode (SPLIT_TT_VER), and horizontal ternary split mode (SPLIT_TT_HOR). In addition, the minimum block size (MinQTSize) of the quad tree of the luminance block during splitting can be set to 16x16, the maximum block size (MaxBtSize) of the binary tree can be set to 128x128, and the maximum block size (MaxTtSize) of the triple tree can be set to 64x64. In addition, the minimum block size (MinBtSize) of the binary tree and the minimum block size (MinTtSize) of the triple tree can be set to 4x4, and the maximum depth (MaxMttDepth) of the multi-type tree can be set to 4. Additionally, to improve the encoding efficiency of the I slice, a dual tree can be applied that uses different CTU partition structures for luminance and chrominance components. On the other hand, in the P and B slices, the luminance and chrominance CTBs (Coding Tree Blocks) within the CTU can be partitioned into a single tree that shares the coding tree structure.

[0056] The encoding device (100) may perform encoding on the input image in intra mode and / or inter mode. Alternatively, the encoding device (100) may perform encoding on the input image in a third mode (e.g., IBC mode, Palette mode, etc.) other than the intra mode and inter mode. However, if the third mode has functional characteristics similar to the intra mode or inter mode, it may be classified as intra mode or inter mode for convenience of explanation. In the present invention, the third mode will be classified and described separately only when a specific description is required.

[0057] When the intra mode is used as the prediction mode, the switch (115) can be switched to intra, and when the inter mode is used as the prediction mode, the switch (115) can be switched to inter. Here, the intra mode can mean an intra-screen prediction mode, and the inter mode can mean an inter-screen prediction mode. The encoding device (100) can generate a prediction block for an input block of an input image. In addition, after the prediction block is generated, the encoding device (100) can encode a residual block using a residual of the input block and the prediction block. The input image can be referred to as a current image that is currently a target of encoding. The input block can be referred to as a current block that is currently a target of encoding or an encoding target block.

[0058] When the prediction mode is intra mode, the intra prediction unit (120) can use samples of blocks already encoded / decoded around the current block as reference samples. The intra prediction unit (120) can perform spatial prediction on the current block using the reference samples, and can generate prediction samples for the input block through spatial prediction. Here, intra prediction can mean prediction within the screen.

[0059] As an intra prediction method, non-directional prediction modes such as DC mode and Planar mode, as well as directional prediction modes (e.g., 65 directions) can be applied. Here, the intra prediction method can be expressed as an intra prediction mode or an intra-screen prediction mode.

[0060] When the prediction mode is inter mode, the motion prediction unit (121) can search for an area that best matches the input block from the reference image during the motion prediction process and derive a motion vector using the searched area. At this time, the area can be used as a search area. The reference image can be stored in the reference picture buffer (190). Here, when encoding / decoding for the reference image is processed, it can be stored in the reference picture buffer (190).

[0061] The motion compensation unit (122) can generate a prediction block for the current block by performing motion compensation using a motion vector. Here, inter prediction may mean inter-screen prediction or motion compensation.

[0062] The above motion prediction unit (121) and motion compensation unit (122) can generate a prediction block by applying an interpolation filter to a portion of an area within a reference image when the value of the motion vector does not have an integer value. In order to perform inter-screen prediction or motion compensation, it is possible to determine whether the motion prediction and motion compensation method of the prediction unit included in the corresponding encoding unit is one of Skip Mode, Merge Mode, Advanced Motion Vector Prediction (AMVP) mode, and Intra Block Copy (IBC) mode based on the encoding unit, and perform inter-screen prediction or motion compensation according to each mode.

[0063] In addition, based on the above inter-screen prediction method, the AFFINE mode of sub-PU based prediction, the SbTMVP (Subblock-based Temporal Motion Vector Prediction) mode, and the MMVD (Merge with MVD) mode and the GPM (Geometric Partitioning Mode) mode of PU based prediction can be applied. In addition, in order to improve the performance of each mode, the HMVP (History based MVP), the PAMVP (Pairwise Average MVP), the CIIP (Combined Intra / Inter Prediction), the AMVR (Adaptive Motion Vector Resolution), the BDOF (Bi-Directional Optical-Flow), the BCW (Bi-predictive with CU Weights), the LIC (Local Illumination Compensation), the TM (Template Matching), and the OBMC (Overlapped Block Motion Compensation) can be applied.

[0064] Among these, AFFINE mode is a technology that is used in both AMVP and MERGE modes and also has high encoding efficiency. In the existing video coding standard, since MC (Motion Compensation) is performed by considering only the parallel translation of the block, there was a disadvantage in that it could not properly compensate for motions that occur in reality, such as zoom in / out and rotation. To supplement this, a 4-parameter affine motion model using two control point motion vectors (CPMV) and a 6-parameter affine motion model using three control point motion vectors can be applied to inter prediction. Here, CPMV is a vector representing the affine motion model of one of the upper left, upper right, and lower left of the current block.

[0065] The subtractor (113) can generate a residual block using the difference between the input block and the predicted block. The residual block may also be referred to as a residual signal. The residual signal may refer to the difference between the original signal and the predicted signal. Alternatively, the residual signal may be a signal generated by transforming, quantizing, or transforming and quantizing the difference between the original signal and the predicted signal. The residual block may be a residual signal in block units.

[0066] The transform unit (130) can perform a transform on the residual block to generate a transform coefficient and output the generated transform coefficient. Here, the transform coefficient may be a coefficient value generated by performing a transform on the residual block. When the transform skip mode is applied, the transform unit (130) may also skip the transform on the residual block.

[0067] Quantized levels can be generated by applying quantization to transform coefficients or residual signals. In the following embodiments, quantized levels may also be referred to as transform coefficients.

[0068] For example, a 4x4 luminance residual block generated through intra prediction can be transformed using a basis vector based on DST (Discrete Sine Transform), and the remaining residual blocks can be transformed using a basis vector based on DCT (Discrete Cosine Transform). In addition, through RQT (Residual Quad Tree) technology, the transform block is divided into a quad tree shape for one block, and after performing transformation and quantization on each transform block divided through RQT, a coded block flag (cbf) can be transmitted to increase encoding efficiency when all coefficients become 0.

[0069] Another alternative is to apply Multiple Transform Selection (MTS) technology, which selectively performs transformation using multiple transformation bases. That is, instead of dividing CUs into TUs via RQT, a Sub-block Transform (SBT) technology can perform a function similar to TU division. Specifically, SBT is applied only to inter-screen prediction blocks, and unlike RQT, it can divide the current block into ½ or ¼ blocks vertically or horizontally, and then perform transformation on only one of the blocks. For example, in a vertically divided block, the transformation can be performed on the leftmost or rightmost block, and in a horizontally divided block, the transformation can be performed on the topmost or bottommost block.

[0070] Additionally, LFNST (Low Frequency Non-Separable Transform), a secondary transform technique that further transforms the residual signal converted to the frequency domain through DCT or DST, can be applied. LFNST additionally performs a transform on the low-frequency region of 4x4 or 8x8 in the upper left, which allows the residual coefficients to be concentrated in the upper left.

[0071] The quantization unit (140) can generate a quantized level by quantizing a transform coefficient or residual signal according to a quantization parameter (QP), and can output the generated quantized level. At this time, the quantization unit (140) can quantize the transform coefficient using a quantization matrix.

[0072] For example, a quantizer with QP values ​​of 0 to 51 can be used. Alternatively, if the image size is larger and high encoding efficiency is required, a QP of 0 to 63 can be used. In addition, a Dependent Quantization (DQ) method that uses two quantizers instead of a single quantizer can be applied. DQ performs quantization using two quantizers (e.g., Q0 and Q1), but even without signaling information about the use of a specific quantizer, the quantizer to be used for the next transform coefficient can be selected based on the current state through a state transition model.

[0073] The entropy encoding unit (150) can generate a bitstream by performing entropy encoding according to a probability distribution on values ​​produced by the quantization unit (140) or coding parameter values ​​produced during the encoding process, and can output the bitstream. The entropy encoding unit (150) can perform entropy encoding on information about image samples and information for decoding the image. For example, the information for decoding the image can include syntax elements, etc.

[0074] When entropy encoding is applied, a small number of bits are allocated to symbols with a high occurrence probability, and a large number of bits are allocated to symbols with a low occurrence probability, thereby representing the symbols, whereby the size of the bit string for the symbols to be encoded can be reduced. The entropy encoding unit (150) can use an encoding method such as exponential Golomb, Context-Adaptive Variable Length Coding (CAVLC), or Context-Adaptive Binary Arithmetic Coding (CABAC) for entropy encoding. For example, the entropy encoding unit (150) can perform entropy encoding using a Variable Length Coding / Code (VLC) table. In addition, the entropy encoding unit (150) may perform arithmetic encoding using the binarization method, probability model, and context model derived from the binarization method of the target symbol and the probability model of the target symbol / bin.

[0075] In this regard, when applying CABAC, the table probability update method can be changed to a simple formula-based table update method to reduce the size of the probability table stored in the decryption device. Furthermore, two different probability models can be used to obtain more accurate symbol probability values.

[0076] The entropy encoding unit (150) can change a two-dimensional block form coefficient into a one-dimensional vector form through a transform coefficient scanning method to encode a transform coefficient level (quantized level).

[0077] Coding parameters may include not only information (flags, indexes, etc.) encoded in an encoding device (100) and signaled to a decoding device (200), such as syntax elements, but also information derived during an encoding process or a decoding process, and may mean information necessary when encoding or decoding an image.

[0078] Here, signaling a flag or index may mean that the encoder entropy encodes the flag or index and includes it in the bitstream, and that the decoder entropy decodes the flag or index from the bitstream.

[0079] The encoded current image can be used as a reference image for other images to be processed later. Accordingly, the encoding device (100) can reconstruct or decode the encoded current image again and store the reconstructed or decoded image as a reference image in the reference picture buffer (190).

[0080] The quantized level can be dequantized in the dequantization unit (160) and inversely transformed in the inverse transformation unit (170). The dequantized and / or inversely transformed coefficients can be combined with a prediction block through an adder (117), and a reconstructed block can be generated by combining the dequantized and / or inversely transformed coefficients and the prediction block. Here, the dequantized and / or inversely transformed coefficients refer to coefficients on which at least one of dequantization and inverse transformation has been performed, and may refer to a reconstructed residual block. The dequantization unit (160) and the inverse transformation unit (170) can be performed in the reverse process of the quantization unit (140) and the transformation unit (130).

[0081] The restoration block may pass through a filter unit (180). The filter unit (180) may apply a deblocking filter, a sample adaptive offset (SAO), an adaptive loop filter (ALF), a bilateral filter (BIF), a Luma Mapping with Chroma Scaling (LMCS), etc. as a filtering technique, in whole or in part, to the restoration sample, restoration block, or restoration image. The filter unit (180) may also be referred to as an in-loop filter. In this case, the in-loop filter is also used as a name excluding LMCS.

[0082] A deblocking filter can remove block distortion that occurs at the boundaries between blocks. Whether to apply a deblocking filter to the current block can be determined based on the samples contained in several columns or rows within the block. When applying a deblocking filter to a block, different filters can be applied depending on the required deblocking filtering strength.

[0083] Sample adaptive offset can be used to compensate for encoding errors by adding an appropriate offset value to sample values. Sample adaptive offset can compensate for the offset from the original image on a sample-by-sample basis for deblocked images. This can be done by dividing the samples contained in the image into a fixed number of regions, determining the regions to be offset, and applying the offset to those regions. Alternatively, the offset can be applied by considering the edge information of each sample.

[0084] Bilateral filter (BIF) can also compensate for the offset from the original image on a sample-by-sample basis for the deblocked image.

[0085] An adaptive loop filter can perform filtering based on a comparison between a reconstructed image and the original image. By dividing the samples contained in the image into predetermined groups and determining the filter to be applied to each group, filtering can be performed differentially for each group. Information regarding whether to apply an adaptive loop filter can be signaled for each coding unit (CU), and the shape and filter coefficients of the adaptive loop filter applied to each block can vary.

[0086] In LMCS (Luma Mapping with Chroma Scaling), luma mapping (LM) refers to remapping luminance values ​​through a piece-wise linear model, and chroma scaling (CS) refers to a technique that scales the residual values ​​of chrominance components according to the average luminance value of the prediction signal. In particular, LMCS can be utilized as an HDR correction technique that reflects the characteristics of HDR (High Dynamic Range) images.

[0087] The restored block or restored image that has passed through the filter unit (180) may be stored in the reference picture buffer (190). The restored block that has passed through the filter unit (180) may be a part of the reference image. In other words, the reference image may be a restored image composed of restored blocks that have passed through the filter unit (180). The stored reference image may be used for inter-screen prediction or motion compensation thereafter.

[0088] Figure 2 is a block diagram showing the configuration according to one embodiment of a decryption device to which the present invention is applied.

[0089] The decoding device (200) may be a decoder, a video decoding device, or an image decoding device.

[0090] Referring to FIG. 2, the decoding device (200) may include an entropy decoding unit (210), an inverse quantization unit (220), an inverse transformation unit (230), an intra prediction unit (240), a motion compensation unit (250), an adder (201), a switch (203), a filter unit (260), and a reference picture buffer (270).

[0091] The decoding device (200) can receive a bitstream output from the encoding device (100). The decoding device (200) can receive a bitstream stored in a computer-readable recording medium, or a bitstream streamed through a wired / wireless transmission medium. The decoding device (200) can perform decoding on the bitstream in intra mode or inter mode. In addition, the decoding device (200) can generate a restored image or a decoded image through decoding, and can output the restored image or the decoded image.

[0092] If the prediction mode used for decryption is intra mode, the switch (203) can be switched to intra. If the prediction mode used for decryption is inter mode, the switch (203) can be switched to inter.

[0093] The decoding device (200) can decode the input bitstream to obtain a reconstructed residual block and generate a prediction block. Once the reconstructed residual block and the prediction block are obtained, the decoding device (200) can generate a reconstructed block to be decoded by adding the reconstructed residual block and the prediction block. The block to be decoded may be referred to as a current block.

[0094] The entropy decoding unit (210) can generate symbols by performing entropy decoding according to a probability distribution for the bitstream. The generated symbols may include symbols in the form of quantized levels. Here, the entropy decoding method may be the reverse process of the entropy encoding method described above.

[0095] The entropy decoding unit (210) can change a one-dimensional vector-shaped coefficient into a two-dimensional block-shaped coefficient through a transform coefficient scanning method to decode a transform coefficient level (quantized level).

[0096] The quantized level can be inversely quantized in the inverse quantization unit (220) and inversely transformed in the inverse transformation unit (230). The quantized level can be generated as a restored residual block as a result of performing inverse quantization and / or inverse transformation. At this time, the inverse quantization unit (220) can apply a quantization matrix to the quantized level. The inverse quantization unit (220) and inverse transformation unit (230) applied to the decoding device can apply the same technology as the inverse quantization unit (160) and inverse transformation unit (170) applied to the encoding device described above.

[0097] When intra mode is used, the intra prediction unit (240) can generate a predicted block by performing spatial prediction on the current block using sample values ​​of already decoded blocks surrounding the block to be decoded. The intra prediction unit (240) applied to the decoding device can apply the same technology as the intra prediction unit (120) applied to the encoding device described above.

[0098] When the inter mode is used, the motion compensation unit (250) can generate a prediction block by performing motion compensation using a motion vector and a reference image stored in the reference picture buffer (270) on the current block. The motion compensation unit (250) can generate a prediction block by applying an interpolation filter to a portion of the reference image when the value of the motion vector does not have an integer value. In order to perform motion compensation, it is possible to determine whether the motion compensation method of the prediction unit included in the corresponding encoding unit is skip mode, merge mode, AMVP mode, or current picture reference mode based on the encoding unit, and motion compensation can be performed according to each mode. The motion compensation unit (250) applied to the decoding device can apply the same technology as the motion compensation unit (122) applied to the encoding device described above.

[0099] The adder (201) can add the restored residual block and the predicted block to generate a restored block. The filter unit (260) can apply at least one of an Inverse-LMCS, a deblocking filter, a sample adaptive offset, and an adaptive loop filter to the restored block or restored image. The filter unit (260) applied to the decoding device can apply the same filtering technology as that applied to the filter unit (180) applied to the encoding device described above.

[0100] The filter unit (260) can output a restored image. The restored block or restored image can be stored in the reference picture buffer (270) and used for inter prediction. The restored block that has passed through the filter unit (260) can be a part of the reference image. In other words, the reference image can be a restored image composed of restored blocks that have passed through the filter unit (260). The stored reference image can be used for inter-screen prediction or motion compensation thereafter.

[0101] FIG. 3 is a diagram schematically showing a video coding system to which the present invention can be applied.

[0102] A video coding system according to one 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 the form of a file or streaming through a digital storage medium or a network.

[0103] An encoding device (10) according to one embodiment may include a video source generation unit (11), an encoding unit (12), and a transmission unit (13). A decoding device (20) according to one embodiment may include a reception 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 transmission unit (13) may be included in the encoding unit (12). The reception unit (21) may be included in the decoding unit (22). The rendering unit (23) may include a display unit, and the display unit may be configured as a separate device or an external component.

[0104] The video source generation unit (11) can obtain video / images through a process of capturing, synthesizing, or generating video / images. The video source generation 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 including previously captured video / images, etc. The video / image generation device can include, for example, a computer, a tablet, a smartphone, etc., and can (electronically) generate video / 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 with a process of generating related data.

[0105] The encoding unit (12) can encode the input video / image. The encoding unit (12) can perform a series of procedures such as prediction, transformation, and quantization for compression and encoding efficiency. The encoding unit (12) can output encoded data (encoded video / image information) in the form of a bitstream. The detailed configuration of the encoding unit (12) can also be configured in the same manner as the encoding device (100) of FIG. 1 described above.

[0106] The transmission unit (13) can transmit encoded video / image information or data output in the form of a bitstream to the reception unit (21) of the decoding device (20) via a digital storage medium or a network in the form of a file or streaming. The digital storage medium can include various storage media such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, etc. The transmission unit (13) can include an element for generating a media file through a predetermined file format and can include an element for transmission via a broadcasting / communication network. The reception unit (21) can extract / receive the bitstream from the storage medium or network and transmit it to the decoding unit (22).

[0107] The decoding unit (22) can decode video / image by performing a series of procedures such as inverse quantization, inverse transformation, and prediction corresponding to the operation of the encoding unit (12). The detailed configuration of the decoding unit (22) can also be configured in the same manner as the decoding device (200) of FIG. 2 described above.

[0108] The rendering unit (23) can render the decrypted video / image. The rendered video / image can be displayed through the display unit.

[0109]

[0110] In this specification, a transform kernel may refer to a transform core used when applying a transformation from the spatial domain to the frequency domain. Furthermore, a transform set may refer to a group containing transform kernels. Furthermore, a transform set may refer to a kernel cluster.

[0111] A secondary transform may refer to a transform performed based on the correlation of the primary transform coefficients generated by the primary transform after the primary transform from the spatial domain to the frequency domain has been performed. Here, the primary transform and the primary transform may have the same meaning. By performing a secondary transform that converts to a more compressed representation than the primary transform, higher compression efficiency can be achieved than when only the primary transform is performed.

[0112] The first and second transformations can be performed as separable or non-separable transformations.

[0113] A separable transformation can refer to a transformation in which vertical and horizontal transformations are performed independently. Here, a vertical transformation refers to a transformation in the vertical direction and is a transformation to which a vertical transformation kernel is applied. Furthermore, a horizontal transformation refers to a transformation in the horizontal direction and is a transformation to which a horizontal transformation kernel is applied. For example, in a separable transformation, a vertical transformation may be performed before a horizontal transformation. Alternatively, a horizontal transformation may be performed after a vertical transformation.

[0114] A non-separable transformation can mean that the transformation is performed at once, rather than in the horizontal or vertical direction, using a non-separable transformation kernel in the form of a matrix.

[0115] In separable and non-separable transformations, a basis vector or a combination of basis vectors can be used. A basis vector is a vector whose size corresponds to the pixels of the two-dimensional space where the transformation is performed, or a vector whose size corresponds to the coefficients, and can be designed by considering the overall characteristics and similarity of the pixels and / or coefficients of the two-dimensional space where the transformation is performed.

[0116] For secondary transforms such as LFNST (Low frequency non-separable transform), non-separable secondary transform kernels of various sizes can be used to improve coding efficiency and transform complexity for specific input block sizes.

[0117] In the case of NSPT (Non-separable primary transform), although it is a primary transform, similar to LFNST, non-separable primary transform kernels of various sizes can be used to improve coding efficiency and transform complexity for specific input block sizes.

[0118] MTS (Multiple transform selection) may mean using a selected transform kernel among multiple transform kernels for the transformation of the current block.

[0119] MTSS (Multiple Transform Set Selection) may mean that a transform set is selected from among multiple transform sets, and a transform kernel selected from among multiple transform kernels included in the selected transform set is used for the transformation of the current block. In this case, the transform kernel may be selected after the transform set is selected, or the selection of the transform set and the selection of the transform kernel may be performed simultaneously.

[0120]

[0121] To achieve high compression performance, it is crucial to select a transform set and / or transform kernel appropriate for the characteristics of the pixels and / or coefficients to be encoded. Conventionally, the transform set and / or transform kernel are determined based on the intra-prediction mode of the block on which the transform is performed. In such cases, information regarding the intra-prediction mode of the current block may not be beneficial to the performance of the transform technology and the coding structure. Consequently, the transform set and / or transform kernel appropriate for the characteristics of the pixels and / or coefficients to be encoded may not be selected, which may limit the transform efficiency.

[0122]

[0123] Hereinafter, the present specification provides a method for efficiently deriving a virtual intra prediction mode for determining a transformation set and / or a transformation kernel. Here, the method for deriving a virtual intra prediction mode may refer to a method for deriving an intra prediction mode used when determining a transformation set for transformation / inverse transformation of a current block among a plurality of transformation sets. Furthermore, the method may refer to a method for deriving an intra prediction mode used when determining a transformation kernel for transformation / inverse transformation of a current block among a plurality of transformation kernels.

[0124] Meanwhile, in this specification, the virtual intra prediction mode for determining a transformation set and / or a transformation kernel and the transformation kernel determination intra prediction mode may have the same meaning.

[0125] If information about the intra prediction mode of the current block does not benefit the performance and coding structure of the transform technique, a transform kernel decision intra prediction mode may be derived and used to determine the transform set and / or the transform kernel.

[0126] Additionally, a transform kernel decision intra prediction mode can be derived even if the current block is predicted in a way other than the conventional directional intra prediction mode and non-directional intra prediction mode.

[0127] Meanwhile, the other methods described above may include the Decoder Side Intra Mode Derivation (DIMD) method, the Template-based Intra Mode Derivation (TIMD) method, the Intra Template Matching Prediction (intraTMP) method, the Combined Intra-Inter Prediction (CIIP) method, the Spatial CIIP (SCIIP, Spatial Combined Intra-Intra Prediction) method, the Spatial Geometric Prediction mode (SGPM) method, the Extrapolation-based Intra Prediction (EIP) method, the Matrix-based Intra Prediction (MIP) method, the Intra Block Copy (IBC) method, the Merged Histogram of Gradients (MHog) method, and the Occurrence-Based Intra Coding (OBIC) method.

[0128] Here, the DIMD method may refer to a method in which an intra prediction mode is derived on the decoder side, and TIMD may refer to a method in which the prediction mode of the current block is derived using a reference region of a template. IntraTMP may refer to a method in which intra prediction is performed using template matching. In addition, CIIP may refer to a method in which prediction is performed by adding weights to intra prediction and inter prediction, and SCIIP may refer to the use of multiple intra prediction methods. In addition, SGPM may refer to a method in which prediction is performed by dividing a block. In addition, EIP may refer to a method in which prediction is performed using extrapolation, and MIP may refer to a method in which prediction is performed using a weight matrix. In addition, IBC may refer to a method in which prediction is performed using a block vector, and MHog may refer to a method in which gradient histograms of neighboring blocks are combined. In addition, OBIC may refer to a method in which an incidence histogram is used.

[0129] Additionally, the aforementioned other methods may include cases where inter-prediction is performed on the current block. Furthermore, the method may also include cases where two or more intra-prediction methods are fused to perform prediction on the current block. Furthermore, the method may also include cases where two or more inter-prediction methods are fused to perform prediction on the current block. Furthermore, the method may also include cases where intra-prediction and inter-prediction methods are fused to perform prediction on the current block.

[0130] Additionally, a transform kernel-determined intra-prediction mode can be derived even when the current block is divided into multiple sub-blocks. In this case, a transform kernel-determined intra-prediction mode for transforming some of the sub-blocks of the current block can be derived, or a transform kernel-determined intra-prediction mode for transforming all sub-blocks of the current block can be derived.

[0131] Meanwhile, in the present invention, the transformation set may include a transformation set of primary transformations and a transformation set of secondary transformations. In addition, the transformation set may include a transformation set of separable transformations and a transformation set of non-separable transformations.

[0132] Meanwhile, in the present invention, the transformation kernel may include a transformation kernel used for a first transformation and a transformation kernel used for a second transformation. In addition, the transformation kernel may include a transformation kernel used for a separable transformation and a transformation kernel used for a non-separable transformation.

[0133]

[0134] According to one embodiment of the present invention, a transform kernel-determined intra prediction mode can be derived using a histogram of occurrence frequencies (HoC). Therefore, when a transform kernel-determined intra prediction mode is derived using the occurrence frequency histogram, a transform kernel can be determined based on the transform kernel-determined intra prediction mode, and transformation / inverse transformation can be performed based on the transform kernel.

[0135] Meanwhile, in this embodiment, the occurrence frequency histogram may have the same meaning as the occurrence rate histogram.

[0136] Meanwhile, the method for deriving the intra prediction mode for determining the transformation kernel according to the present embodiment may be referred to as the Occurrence-Based Intra Coding (OBIC) method. Here, the OBIC method may refer to a method for deriving the intra prediction mode of the current block based on the occurrence frequency of the intra prediction mode of the surrounding blocks.

[0137] The occurrence frequency histogram can be generated based on the intra prediction modes of neighboring blocks of the current block for which transformation / inverse transformation is performed. Specifically, the occurrence frequency histogram can be generated by accumulating the intra prediction modes of the neighboring blocks.

[0138] At this time, the intra prediction modes of the surrounding blocks can be accumulated as is to generate an occurrence frequency histogram.

[0139] Alternatively, to generate an occurrence frequency histogram, intra prediction modes of surrounding blocks can be accumulated based on the size of the surrounding blocks.

[0140] For example, the intra prediction mode of a neighboring block can be accumulated in proportion to the size of the neighboring block. That is, the value obtained by multiplying the size of the neighboring block by an arbitrary real number can be accumulated in the occurrence frequency histogram. Here, the arbitrary real number can be determined by the encoder / decoder's agreement.

[0141] Meanwhile, the size of the surrounding block may be determined by at least one of the width and height of the surrounding block. For example, the size of the surrounding block may be a product of the width and height of the surrounding block. As another example, the intra prediction mode of the surrounding block may be accumulated in the occurrence frequency histogram based on at least one of the width and height of the surrounding block.

[0142] Meanwhile, the size of the surrounding block may be determined by the number of pixels contained in the surrounding block. For example, the intra prediction mode of the surrounding block may be accumulated in proportion to the number of pixels contained in the surrounding block. Specifically, the value calculated by multiplying the number of pixels contained in the surrounding block by a predetermined real number may be accumulated in the occurrence frequency histogram. Here, the predetermined real number may be defined by a convention of the encoder / decoder.

[0143] Meanwhile, the number of pixels contained in a given neighboring block and the number of samples in that neighboring block may have the same meaning. For example, the intra prediction mode of a neighboring block may be accumulated in proportion to the number of samples in that neighboring block.

[0144] Meanwhile, in the present embodiment, the adjacent blocks may include adjacent blocks to the current block and non-adjacent neighboring blocks. Here, adjacent to the current block may mean spatially adjacent to the current block.

[0145] Here, the surrounding blocks adjacent to the current block can be called adjacent blocks, and the surrounding blocks not adjacent to the current block can be called non-adjacent blocks.

[0146] Therefore, the occurrence frequency histogram can be generated using the intra prediction modes of adjacent blocks and non-adjacent blocks.

[0147] FIG. 4 is a diagram illustrating blocks surrounding a current block according to one embodiment of the present invention. Specifically, FIG. 4 is a diagram illustrating non-adjacent blocks among blocks surrounding a current block.

[0148] Referring to Fig. 4, a predetermined area around a current block (401) in a current picture (400) can be defined as a neighboring area (402). In addition, a block that is not spatially adjacent to the current block but is included in the neighboring area can be defined as a non-adjacent neighboring block (403).

[0149] In addition, the intra prediction mode for determining the transformation kernel of the current block can be derived based on a non-adjacent neighboring block (403). For example, the intra prediction modes of the non-adjacent blocks (403) can be accumulated to generate an occurrence frequency histogram, and the intra prediction mode for determining the transformation kernel can be derived based on the occurrence frequency histogram.

[0150] Meanwhile, in FIG. 4, the surrounding area including the non-adjacent block (403) is the left area, the upper left area, and the upper area of ​​the current block, but this is just one example, and the non-adjacent block (403) may be included in a preset area. In this case, the preset area may be any one of the current tile, the current slice, and the current picture including the current block.

[0151] Meanwhile, the location of the non-adjacent block in FIG. 4 is merely an example; the location of the non-adjacent block may be any location in the surrounding area. The location of the non-adjacent block may be determined by an agreement between the encoder and decoder. Furthermore, information regarding the location of the non-adjacent block may be determined by the encoder and transmitted to the decoder. Furthermore, the location of the non-adjacent block may be indicated by information regarding the current block.

[0152]

[0153] Once the occurrence frequency histogram is generated, a transform kernel decision intra prediction mode can be derived based on the generated occurrence frequency histogram.

[0154] For example, the transform kernel decision intra prediction mode can be derived as the intra prediction mode having the largest value in the occurrence frequency histogram. Here, having the largest value means having the largest amplitude in the histogram, which can mean the most accumulated.

[0155] As another example, the intra prediction mode determined by the transform kernel can be derived using the N intra prediction modes that are most accumulated among the intra prediction modes of the occurrence frequency histogram. Specifically, the intra prediction mode determined by the transform kernel can be derived from the intra prediction mode of the current block and any one of the N intra prediction modes described above. In this case, the intra prediction mode of the current block and the N intra prediction modes described above can be referred to as candidate modes. Here, N is any positive integer.

[0156] First, a rate-distortion cost (RD cost) can be calculated for each candidate mode in the encoder.

[0157] And, the transformation kernel decision intra prediction mode can be derived as the intra prediction mode with the smallest distortion among the candidate modes.

[0158] And, information about the corresponding transform kernel determined intra prediction mode can be signaled to the decoder. Here, the information about the transform kernel determined intra prediction mode can include at least one of a flag regarding whether the transform kernel determined intra prediction mode is used, a transform set index, a transform kernel index, and an index regarding the transform kernel determined intra prediction mode.

[0159] And, in the decoder, the transform set and / or transform kernel of the current block can be determined based on information about the transform kernel determination intra prediction mode.

[0160] Meanwhile, in the above example, the cost value is calculated as a bit rate-distortion cost, but this is only one example, and the cost value can be calculated by a predetermined cost function.

[0161] As another example, the transform kernel decision intra prediction mode can be derived as the intra prediction mode with the smallest assigned prediction mode number among the N intra prediction modes most accumulated in the occurrence frequency histogram, where N is any positive integer.

[0162] As another example, the intra prediction mode determined by the transform kernel can be derived as the intra prediction mode with the largest assigned prediction mode number among the N most accumulated prediction modes in the intra prediction mode of the occurrence frequency histogram, where N is any positive integer.

[0163] Meanwhile, in the above-described example, the N intra prediction modes are selected in the order of accumulation in the occurrence frequency histogram, but the transformation kernel determination intra prediction mode can be derived based on the N intra prediction modes randomly determined in the occurrence frequency histogram.

[0164] Meanwhile, in this specification, it may be synonymous that the intra prediction mode determined by the transformation kernel is derived based on the occurrence frequency histogram and that the intra prediction mode determined by the transformation kernel is determined based on the occurrence frequency histogram.

[0165] As another example, if the amplitudes of the occurrence frequency histograms are all below a specific value, the intra prediction mode determined by the transformation kernel of the current block may be determined as either the Planar mode or the DC mode. Alternatively, if the amplitudes of the occurrence frequency histograms are all below a specific value, the intra prediction mode determined by the transformation kernel of the current block may be determined as a predetermined intra prediction mode.

[0166] Here, the specific values ​​and the predetermined intra prediction mode may be determined by a promise of the encoder / decoder, or may be determined by the encoder and transmitted to the decoder.

[0167] Meanwhile, the method for deriving an intra prediction mode for determining a transformation kernel according to the present embodiment can be similarly performed when deriving an intra prediction mode used for intra prediction of a current block. In this case, the intra prediction mode used for intra prediction and the intra prediction mode used for determining a transformation set / kernel may be the same. Alternatively, the intra prediction mode used for intra prediction may be derived independently from the intra prediction mode used for determining a transformation set / kernel.

[0168]

[0169] According to another embodiment of the present invention, the transform kernel determination intra prediction mode can be derived using the gradient of surrounding samples. Specifically, the transform kernel determination intra prediction mode is derived based on the directionality information of pixels included in the surrounding samples of the current block, and the transform kernel of the current block can be determined based on the derived transform kernel determination intra prediction mode. Here, the surrounding samples of the current block can mean samples included in the surrounding area of ​​the current block.

[0170] Meanwhile, directional information can refer to information obtained by calculating the gradient of pixels. Specifically, directional information can refer to information obtained by accumulating or processing gradient information calculated by applying a filter to the corresponding pixels.

[0171] First, the gradient of pixels included in the surrounding samples of the current block can be calculated. At this time, the applied filter can be an edge detection filter such as a Sobel filter, a Roberts cross filter, a Prewitt filter, a Scharr filter, or a Laplacian filter.

[0172] And, a histogram of gradient can be generated based on the calculated gradient.

[0173] Meanwhile, the gradients of only some pixels, not all pixels contained in the surrounding samples of the current block, can be calculated. In this case, a gradient histogram can be generated based on the gradients of only some pixels.

[0174] And, the intra prediction mode for determining the transformation kernel can be derived using the gradient histogram.

[0175] For example, the transform kernel decision intra prediction mode can be derived as the intra prediction mode corresponding to the most accumulated direction in the gradient histogram. Here, the most accumulated can mean the largest amplitude in the histogram, and the largest amplitude can mean the largest value in the histogram.

[0176] As another example, the intra prediction mode determined by the transform kernel can be derived from any one of the N prediction modes corresponding to the most accumulated directionality in the gradient histogram and the prediction mode of the current block. In this case, the N prediction modes and the prediction mode of the current block can be referred to as candidate modes, where N is any positive integer.

[0177] First, a rate-distortion cost (RD cost) can be calculated for each candidate mode in the encoder.

[0178] And, the transformation kernel decision intra prediction mode can be derived as the intra prediction mode with the smallest distortion among the candidate modes.

[0179] And, information about the corresponding transform kernel determined intra prediction mode can be signaled to the decoder. Here, the information about the transform kernel determined intra prediction mode can include at least one of a flag regarding whether the transform kernel determined intra prediction mode is used, a transform set index, a transform kernel index, and an index regarding the transform kernel determined intra prediction mode.

[0180] And, in the decoder, the transform set and / or transform kernel of the current block can be determined based on information about the transform kernel determination intra prediction mode.

[0181] Meanwhile, in the above example, the cost value is calculated as a bit rate-distortion cost, but this is only one example, and the cost value can be calculated by a predetermined cost function.

[0182] As another example, the transform kernel decision intra prediction mode can be derived as the intra prediction mode with the smallest assigned prediction mode number among N prediction modes corresponding to the most accumulated direction in the gradient histogram, where N is any positive integer.

[0183] As another example, the intra prediction mode determined by the transform kernel can be derived as the intra prediction mode with the largest assigned prediction mode number among the N prediction modes corresponding to the most accumulated direction in the gradient histogram, where N is an arbitrary positive integer.

[0184] Meanwhile, in this specification, it may be synonymous that the intra prediction mode determined by transforming the kernel is derived using the gradient of the surrounding samples and that the intra prediction mode determined by transforming the kernel is determined using the gradient of the surrounding samples.

[0185] As another example, if the amplitudes of the gradient histograms are all less than or equal to a specific value, the intra prediction mode determined by the transform kernel of the current block may be determined as either the Planar mode or the DC mode. Alternatively, if the amplitudes of the gradient histograms are all less than or equal to a specific value, the intra prediction mode determined by the transform kernel of the current block may be determined as a predetermined intra prediction mode. Here, the specific value and the predetermined intra prediction mode may be determined by a convention between the encoder / decoder, or may be determined by the encoder and transmitted to the decoder.

[0186]

[0187] Meanwhile, according to another embodiment of the present invention, a gradient histogram can also be generated by a method based on the prediction mode of a surrounding block as follows. Here, a surrounding block may mean a block included in a surrounding area of ​​a current block.

[0188] First, if the prediction mode of the surrounding block is either the DIMD (Decoder side intra mode derivation) mode or the MIMD (Merged intra mode derivation) mode, the amplitude value of the histogram of the corresponding mode can be accumulated in the gradient histogram for determining the transformation kernel of the current block.

[0189] For example, if the prediction mode of a neighboring block is DIMD mode, a gradient histogram can be generated based on the directionality of the reference samples of the neighboring block. Then, the amplitude value of the gradient histogram of the mode can be accumulated in the gradient histogram for determining the transformation kernel of the current block. In this case, the amplitude value can be accumulated as is in the gradient histogram for determining the transformation kernel of the current block, or can be normalized and accumulated.

[0190] As another example, if the prediction mode of a neighboring block is MIMD mode, a merged histogram of gradients (MHog) for that mode may be generated. Specifically, depending on the intra prediction mode of the reference samples of the neighboring block, some or all of the amplitudes of the gradient histogram may be overlapped to generate the merged gradient histogram. Furthermore, the amplitude values ​​of the merged gradient histogram may be accumulated in the gradient histogram for determining the transformation kernel of the current block. In this case, the amplitude values ​​may be accumulated directly in the gradient histogram or may be normalized and then accumulated.

[0191] In addition, if the prediction mode of the surrounding block is not DIMD or MIMD mode, the amplitude value of the intra prediction mode of the surrounding block can be accumulated in the gradient histogram. In this case, the accumulation can be done by multiplying the number of pixels contained in the surrounding block by an arbitrary real number. Alternatively, the amplitude value can be accumulated by multiplying the number of pixels contained in the current block by an arbitrary real number.

[0192] In addition, if the prediction mode of a neighboring block has two or more intra prediction modes, such as SGPM mode and TIMD mode, the amplitude value of the intra prediction mode of the neighboring block can be accumulated in the gradient histogram for determining the transformation kernel of the current block. In this case, the accumulation can be performed by multiplying the number of pixels included in the neighboring block by an arbitrary real number and calculating the value. Alternatively, the accumulation can be performed by multiplying the number of pixels included in the current block by an arbitrary real number and calculating the value.

[0193] Meanwhile, arbitrary errors can be determined by the encoder / decoder's agreement, or can be determined by the encoder and signaled to the decoder.

[0194] When the amplitude values ​​are accumulated as described above to generate a gradient histogram for determining the transformation kernel, the transformation kernel determination intra prediction mode can be derived using this.

[0195] Meanwhile, the aforementioned method accumulates amplitude values ​​to generate a slope histogram for determining a transformation kernel, but the slope histogram can also be generated by performing a predetermined operation on the amplitude values. Here, the predetermined method may refer to an averaging method, etc. Alternatively, normalized amplitude values ​​may be accumulated to generate a slope histogram.

[0196]

[0197] Once the transform kernel decision intra prediction mode is derived, the transform kernel of the current block can be determined based on the derived transform kernel decision intra prediction mode. Then, the transform / inverse transform of the current block can be performed based on the determined transform kernel.

[0198] First, R intra prediction modes can be grouped and mapped to Q transform sets for each group. The mapping method of the R intra prediction modes to the transform sets can be determined by the encoder / decoder's agreement. For example, this can be performed in a manner similar to LFNST and NSPT. Here, R and Q are arbitrary positive integers.

[0199] And, the transformation set of the current block can be determined as a transformation set mapped to the transformation kernel decision intra prediction mode.

[0200] And, the transformation kernel of the current block can be determined as any one of the transformation kernels included in the determined transformation set.

[0201] For example, the transformation kernel of the current block may be determined as the transformation kernel with the smallest cost value among the transformation kernels included in the determined transformation set. Specifically, the encoder may calculate a rate-distortion cost (RD cost) for each transformation kernel, and the kernel with the smallest distortion may be selected as the optimal transformation kernel.

[0202] And, information about the optimal transformation kernel can be signaled to the decoder. Here, the information about the optimal transformation kernel can be a flag if the number of transformation kernels included in the transformation set is 2, or a kernel index if there are 3 or more. The decoder determines the transformation kernel based on the information, and inverse transformation can be performed based on the determined transformation kernel.

[0203] Meanwhile, according to the above-described embodiment, the transformation kernel with the smallest cost value among the transformation kernels included in the transformation set is determined as the transformation kernel of the current block. However, this is only one example, and the transformation kernel of the current block may be determined as any one of the transformation kernels included in the transformation set determined by any method. In this case, the arbitrary method may be determined by a promise of the encoder / decoder, or information about the determined transformation kernel may be signaled.

[0204] Meanwhile, according to the above-described embodiment, the transformation set of the current block is determined as a transformation set mapped to the transformation kernel-determined intra-prediction mode, but this is only one example, and the transformation set of the current block may be determined as either a transformation set mapped to the intra-prediction mode of the current block or a transformation set mapped to the transformation kernel-determined intra-prediction mode. In other words, information about the intra-prediction mode of the current block can be utilized.

[0205] In this case, the intra prediction mode and transformation kernel determination intra prediction mode of the current block can be referred to as a candidate mode.

[0206] First, a rate-distortion cost (RD cost) can be calculated for each candidate mode in the encoder.

[0207] And, the transformation set of the current block can be determined as a transformation set that is mapped to an intra prediction mode with small distortion among the candidate modes.

[0208] In this case, information about the determined transformation set may be signaled to the decoder. Here, the information about the determined transformation set may include at least one of a flag regarding whether to use a transformation kernel determined intra prediction mode, a transformation set index, a transformation kernel index, and an index regarding a transformation kernel determined intra prediction mode.

[0209] And, in the decoder, the transformation set of the current block can be determined based on information about the determined transformation set.

[0210] Meanwhile, in the above example, the cost value is calculated as a bit rate-distortion cost, but this is only one example, and the cost value can be calculated by a predetermined cost function.

[0211] Meanwhile, according to the above-described embodiments, the transformation kernel is determined after the transformation set is determined, but the determination of the transformation set and the transformation kernel can be performed simultaneously.

[0212]

[0213] According to another embodiment of the present invention, there may be S transform kernels without a transform set. In this case, R intra prediction modes may be grouped and mapped to S transform kernels for each group. In addition, the method by which the R intra prediction modes are mapped to the transform kernels may be determined by the agreement of the encoder / decoder. Here, R and S are arbitrary positive integers.

[0214] Then, when the transformation kernel decision intra prediction mode is derived, the transformation kernel of the current block can be determined based on the derived transformation kernel decision intra prediction mode. Specifically, the transformation kernel of the current block can be determined as a transformation kernel mapped to the transformation kernel decision intra prediction mode.

[0215] And, based on the determined transformation kernel, transformation / inverse transformation of the current block can be performed.

[0216] Meanwhile, according to the above-described embodiment, the transformation kernel of the current block is determined as a transformation kernel mapped to the transformation kernel-determined intra-prediction mode, but this is only one example, and the transformation kernel of the current block may be determined as either a transformation kernel mapped to the intra-prediction mode of the current block or a transformation kernel mapped to the transformation kernel-determined intra-prediction mode. In other words, information about the intra-prediction mode of the current block can be utilized.

[0217] In this case, the intra prediction mode and transformation kernel determination intra prediction mode of the current block can be referred to as a candidate mode.

[0218] First, a rate-distortion cost (RD cost) can be calculated for each candidate mode in the encoder.

[0219] And, the transformation kernel of the current block can be determined as a transformation kernel that is mapped to an intra prediction mode with small distortion among the candidate modes.

[0220] In this case, information about the determined transformation kernel can be signaled to the decoder. Here, the information about the determined transformation set can include at least one of a flag regarding whether to use the transformation kernel determined intra prediction mode, a transformation set index, a transformation kernel index, and an index regarding the transformation kernel determined intra prediction mode.

[0221] And, in the decoder, the transformation kernel of the current block can be determined based on information about the determined transformation kernel.

[0222] Meanwhile, in the above example, the cost value is calculated as a bit rate-distortion cost, but this is only one example, and the cost value can be calculated by a predetermined cost function.

[0223]

[0224] Fig. 5 is a flowchart illustrating a method for determining a transform kernel according to one embodiment of the present invention. The method for determining a transform kernel of Fig. 5 can be performed by an image decoding device.

[0225] The video decoding device can derive an intra prediction mode for determining a transform kernel of the current block (S500).

[0226] Meanwhile, the intra prediction mode determined by the transformation kernel can be derived using an occurrence frequency histogram generated based on the intra prediction mode of the surrounding blocks of the current block.

[0227] Meanwhile, the above occurrence frequency histogram can be generated by accumulating the intra prediction modes of the surrounding blocks.

[0228] Meanwhile, the intra prediction mode determined by the above transformation kernel can be derived as an intra prediction mode having a maximum value in the occurrence frequency histogram.

[0229] Meanwhile, the intra prediction mode determined by the above transformation kernel can be derived using a predefined number of intra prediction modes among the intra prediction modes of the occurrence frequency histogram.

[0230] Meanwhile, the above occurrence frequency histogram can be generated by accumulating the intra prediction modes of the surrounding blocks.

[0231] Meanwhile, the intra prediction mode of the surrounding block can be accumulated in the occurrence frequency histogram based on the size of the surrounding block.

[0232] And, the image decoding device can determine the transformation kernel of the current block based on the transformation kernel determination intra prediction mode (S510).

[0233] Meanwhile, the step of determining the transformation kernel of the current block may include the step of determining a transformation set of the current block based on the transformation kernel determination intra prediction mode, and the step of determining a transformation kernel of the current block among transformation kernels included in the transformation set.

[0234] Meanwhile, the step of determining the transformation set and the step of determining a transformation kernel among the transformation kernels included in the transformation set can be performed simultaneously.

[0235] Meanwhile, the above transformation set may be a transformation set of non-separable transformations.

[0236] Meanwhile, the non-separable transformation may include a non-separable first-order transformation and a non-separable second-order transformation.

[0237] Meanwhile, the surrounding blocks may include blocks spatially adjacent to the current block and blocks not spatially adjacent to the current block.

[0238] Meanwhile, a block that is not spatially adjacent to the current block may be included in at least one of the left area of ​​the current block, the upper area of ​​the current block, and the upper left area of ​​the current block.

[0239] And, the image decoding device can perform inverse transformation of the current block based on the transformation kernel (S520).

[0240] Meanwhile, the steps described in FIG. 5 can be performed in the same manner in an image encoding method. Furthermore, a bitstream can be generated by an image encoding method including the steps described in FIG. 5. The bitstream can be stored on a non-transitory computer-readable recording medium and can also be transmitted (or streamed).

[0241]

[0242] FIG. 6 is a drawing exemplarily showing a content streaming system to which an embodiment according to the present invention can be applied.

[0243] As illustrated in FIG. 6, a content streaming system to which an embodiment of the present invention is applied may largely include an encoding server, a streaming server, a web server, a media storage, a user device, and a multimedia input device.

[0244] The encoding server compresses content input from multimedia input devices such as smartphones, cameras, and CCTVs into digital data, generates a bitstream, and transmits it to the streaming server. Alternatively, if multimedia input devices such as smartphones, cameras, and CCTVs directly generate bitstreams, the encoding server may be omitted.

[0245] The above bitstream can be generated by an image encoding method and / or an image encoding device to which an embodiment of the present invention is applied, and the streaming server can temporarily store the bitstream during the process of transmitting or receiving the bitstream.

[0246] 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 an intermediary 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 multimedia data to the user. At this time, the content streaming system may include a separate control server, and in this case, the control server may control commands / responses between each device within the content streaming system.

[0247] The streaming server can receive content from a media repository and / or encoding server. For example, when receiving content from the encoding server, the content can be received in real time. In this case, to provide a smooth streaming service, the streaming server can store the bitstream for a certain period of time.

[0248] Examples of the user devices may include mobile phones, smart phones, laptop computers, digital broadcasting terminals, personal digital assistants (PDAs), portable multimedia players (PMPs), navigation devices, slate PCs, tablet PCs, ultrabooks, wearable devices (e.g., smartwatches, smart glasses, HMDs), digital TVs, desktop computers, digital signage, etc.

[0249] Each server within the above 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.

[0250]

[0251] The above embodiments can be performed in the same or corresponding manner in an encoding device and a decoding device. In addition, an image can be encoded / decoded using at least one or a combination of at least one of the above embodiments.

[0252] The order in which the above embodiments are applied may be different in the encoding device and the decoding device. Alternatively, the order in which the above embodiments are applied may be the same in the encoding device and the decoding device.

[0253] The above embodiments can be performed for each of the luminance and chrominance signals. Alternatively, the above embodiments can be performed identically for the luminance and chrominance signals.

[0254] In the above embodiments, the methods are described based on a flowchart as a series of steps or units. However, the present invention is not limited to the order of the steps, and some steps may occur in a different order or simultaneously with other steps described above. Furthermore, those skilled in the art will understand that the steps depicted in the flowchart are not exclusive, and that other steps may be included, or one or more steps in the flowchart may be deleted without affecting the scope of the present invention.

[0255] The above embodiments may be implemented in the form of program commands that can be executed by various computer components and recorded on a computer-readable recording medium. The computer-readable recording medium may include program commands, data files, data structures, etc., either singly or in combination. The program commands recorded on the computer-readable recording medium may be those specifically designed and constructed for the present invention, or may be known and usable by those skilled in the art of computer software.

[0256] The bitstream generated by the encoding method according to the above embodiment can be stored in a non-transitory computer-readable recording medium. In addition, the bitstream stored in the non-transitory computer-readable recording medium can be decoded by the decoding method according to the above embodiment.

[0257] Here, examples of computer-readable recording media include magnetic media such as hard disks, floppy disks, and magnetic tapes, optical recording media such as CD-ROMs and DVDs, magneto-optical media such as floptical disks, and hardware devices specifically configured to store and execute program instructions such as ROMs, RAMs, and flash memories. Examples of program instructions include not only machine language codes such as those generated by a compiler, but also high-level language codes that can be executed by a computer using an interpreter, etc. The hardware devices may be configured to operate as one or more software modules to perform processing according to the present invention, and vice versa.

[0258] Although the present invention has been described above with specific details such as specific components and limited examples and drawings, these are provided only to help a more general understanding of the present invention, and the present invention is not limited to the above examples, and those with ordinary knowledge in the technical field to which the present invention pertains can make various modifications and variations from this description.

[0259] Therefore, the idea of ​​the present invention should not be limited to the embodiments described above, and all things that are modified equally or equivalently to the following claims as well as the claims are considered to fall within the scope of the idea of ​​the present invention.

[0260]

[0261] The present invention can be used in a device for encoding / decoding an image and a recording medium storing a bitstream.

Claims

1. In the video decryption method, A step of deriving an intra prediction mode for determining the transformation kernel of the current block; A step of determining a transform kernel of the current block based on the intra prediction mode of the transform kernel determination; and A step of performing an inverse transformation of the current block based on the above transformation kernel, The above transformation kernel decision intra prediction mode is derived using an occurrence frequency histogram generated based on the intra prediction mode of the surrounding blocks of the current block, An image decoding method, characterized in that the above occurrence frequency histogram is generated by accumulating intra prediction modes of the surrounding blocks.

2. In paragraph 1, An image decoding method, characterized in that the above transformation kernel determination intra prediction mode is derived as an intra prediction mode having a maximum value in the occurrence frequency histogram.

3. In paragraph 1, An image decoding method, characterized in that the intra prediction mode determined by the above transformation kernel is derived using a predefined number of intra prediction modes among the intra prediction modes of the occurrence frequency histogram.

4. In paragraph 1, An image decoding method characterized in that the intra prediction mode of the above-mentioned surrounding block is accumulated in the occurrence frequency histogram based on the size of the above-mentioned surrounding block.

5. In paragraph 1, The step of determining the transformation kernel of the current block above is: determining a transform set of the current block based on the intra prediction mode determined by the transform kernel; and An image decoding method, characterized by comprising a step of determining a transformation kernel of the current block among the transformation kernels included in the transformation set.

6. In paragraph 5, An image decoding method, characterized in that the step of determining the transformation set and the step of determining a transformation kernel among the transformation kernels included in the transformation set are performed simultaneously.

7. In paragraph 5, An image decoding method, characterized in that the above transformation set is a transformation set of non-separable transformations.

8. In paragraph 7, An image decoding method, characterized in that the above non-separable transform is one of a non-separable first-order transform and a non-separable second-order transform.

9. In paragraph 1, An image decoding method, characterized in that the surrounding blocks include blocks spatially adjacent to the current block and blocks not spatially adjacent to the current block.

10. In paragraph 9, A method for decoding an image, characterized in that a block that is not spatially adjacent to the current block is included in at least one of a left area of ​​the current block, an upper area of ​​the current block, and an upper left area of ​​the current block.

11. In the video encoding method, A step of deriving an intra prediction mode for determining the transformation kernel of the current block; A step of determining a transform kernel of the current block based on the intra prediction mode of the transform kernel determination; and A step of performing transformation of the current block based on the transformation kernel, The above transformation kernel decision intra prediction mode is derived using an occurrence frequency histogram generated based on the intra prediction mode of the surrounding blocks of the current block, An image encoding method, characterized in that the above occurrence frequency histogram is generated by accumulating intra prediction modes of the surrounding blocks.

12. In a non-transitory computer-readable recording medium storing a bitstream generated by an image encoding method, The above image encoding method is, A step of deriving an intra prediction mode for determining the transformation kernel of the current block; A step of determining a transform kernel of the current block based on the intra prediction mode of the transform kernel determination; and A step of performing transformation of the current block based on the transformation kernel, The above transformation kernel decision intra prediction mode is derived using an occurrence frequency histogram generated based on the intra prediction mode of the surrounding blocks of the current block, A non-transitory computer-readable recording medium, characterized in that the above occurrence frequency histogram is generated by accumulating intra prediction modes of the surrounding blocks.

13. In a method for transmitting a bitstream generated by a video encoding method, The above transmission method includes a step of transmitting the bitstream, The above image encoding method is, A step of deriving an intra prediction mode for determining the transformation kernel of the current block; A step of determining a transform kernel of the current block based on the intra prediction mode of the transform kernel determination; and A step of performing transformation of the current block based on the transformation kernel, The above transformation kernel decision intra prediction mode is derived using an occurrence frequency histogram generated based on the intra prediction mode of the surrounding blocks of the current block, A transmission method, characterized in that the above occurrence frequency histogram is generated by accumulating intra prediction modes of the surrounding blocks.

Images