WO2019194501A1

WO2019194501A1 - Image coding method on basis of motion vector refinement and apparatus thereof

Info

Publication number: WO2019194501A1
Application number: PCT/KR2019/003809
Authority: WO
Inventors: 이재호
Original assignee: 엘지전자 주식회사
Priority date: 2018-04-01
Filing date: 2019-04-01
Publication date: 2019-10-10

Abstract

A picture decoding method performed by a decoding apparatus according to the present invention, comprises the steps of: obtaining, from a bitstream, information on an adaptive motion vector resolution (AMVR) flag indicating whether to apply an AMVR mode and motion prediction information including information on an AMVR mode flag indicating a type of the AMVR mode; deriving a motion vector for a current block on the basis of the value of the AMVR mode flag when the value of the AMVR flag is 1; refining the derived motion vector; deriving prediction samples for the current block on the basis of the refined motion vector; and generating reconstruction samples for the current block on the basis of the derived prediction samples.

Description

Image Coding Method and Apparatus Based on Motion Vector Refinement

The present invention relates to an image coding technique, and more particularly, to an image coding method and apparatus based on motion vector refinement in an image coding system.

Recently, the demand for high resolution and high quality images such as high definition (HD) images and ultra high definition (UHD) images is increasing in various fields. The higher the resolution and the higher quality of the image data, the more information or bit rate is transmitted than the existing image data. Therefore, the image data can be transmitted by using a medium such as a conventional wired / wireless broadband line or by using a conventional storage medium. In the case of storage, the transmission cost and the storage cost are increased.

Accordingly, a high efficiency image compression technique is required to effectively transmit, store, and reproduce high resolution, high quality image information.

An object of the present invention is to provide a method and apparatus for improving image coding efficiency.

Another technical problem of the present invention is to provide a method and apparatus for increasing the efficiency of inter prediction.

Another technical problem of the present invention is to provide a method and apparatus for increasing the efficiency of inter prediction based on AMVP (Advanced Motion Vector Prediction).

Another technical problem of the present invention is to provide a method and apparatus for increasing the efficiency of inter prediction based on adaptive motion vector resolution (AMVR).

Another technical problem of the present invention is to provide a method and apparatus for applying motion vector refinement in performing AMVR.

According to an embodiment of the present invention, a picture decoding method performed by a decoding apparatus is provided. The method includes motion prediction information including information on an AMVR flag indicating whether to apply an Adaptive Motion Vector Resolution (AMVR) mode and information on an AMVR mode flag indicating a type of the AMVR mode. ) Obtaining a motion vector for the current block based on the value of the AMVR mode flag, and refining the derived motion vector when the value of the AMVR flag is 1; And deriving prediction samples for the current block based on the refined motion vector and generating reconstructed samples for the current block based on the derived prediction samples.

According to another embodiment of the present invention, a decoding device for performing picture decoding is provided. The decoding apparatus may include motion prediction information including information on an AMVR flag indicating whether to apply an Adaptive Motion Vector Resolution (AMVR) mode and information on an AMVR mode flag indicating a type of the AMVR mode. An entropy decoding unit for obtaining information from a bitstream, and when the value of the AMVR flag is 1, a motion vector for the current block is derived based on the value of the AMVR mode flag, and the derived motion vector is refined. And a predictor that derives prediction samples for the current block based on the refined motion vector, and an adder that generates reconstruction samples for the current block based on the derived prediction samples. .

According to the present invention, the overall video / video compression efficiency can be improved.

According to the present invention, the efficiency of inter prediction can be improved.

According to the present invention, image coding efficiency can be improved by performing inter prediction based on AMVP.

According to the present invention, image coding efficiency can be improved by performing inter prediction based on AMVR.

According to the present invention, it is possible to reduce the use of bits for motion information by performing inter prediction based on AMVR.

According to the present invention, the accuracy of inter prediction can be improved by applying motion vector refinement in performing AMVR.

According to the present invention, motion vector refinement may be applied depending on conditions in performing AMVR.

According to the present invention, it is possible to select an optimal trade-off relationship between coding efficiency and complexity based on a search range of a motion vector refinement.

1 is a diagram schematically illustrating a configuration of an encoding apparatus according to an embodiment.

2 is a diagram schematically illustrating a configuration of a decoding apparatus according to an embodiment.

3 is a flowchart illustrating a method of operating a decoding apparatus, according to an exemplary embodiment.

4 is a block diagram illustrating a configuration of a decoding apparatus according to an embodiment.

As the present invention allows for various changes and numerous embodiments, particular embodiments will be illustrated in the drawings and described in detail in the written description. However, this is not intended to limit the invention to the specific embodiments. The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the spirit of the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. The terms "comprise" or "having" in this specification are intended to indicate that there is a feature, number, step, operation, component, part, or combination thereof described in the specification, and one or more other features It is to be understood that the numbers, steps, operations, components, parts or figures do not exclude in advance the presence or possibility of adding them.

On the other hand, each configuration in the drawings described in the present invention are shown independently for the convenience of description of the different characteristic functions, it does not mean that each configuration is implemented by separate hardware or separate software. For example, two or more of each configuration may be combined to form one configuration, or one configuration may be divided into a plurality of configurations. Embodiments in which each configuration is integrated and / or separated are also included in the scope of the present invention without departing from the spirit of the present invention.

The following description may be applied in the technical field dealing with video, image or video. For example, the method or embodiment disclosed in the following description may include the Versatile Video Coding (VVC) standard (ITU-T Rec. H.266), the next generation video / image coding standard after VVC, or standards prior to VVC (eg For example, it may be related to the disclosure of the High Efficiency Video Coding (HEVC) standard (ITU-T Rec. H.265, etc.).

Hereinafter, with reference to the accompanying drawings, it will be described in detail a preferred embodiment of the present invention. Hereinafter, the same reference numerals are used for the same components in the drawings, and redundant description of the same components is omitted.

In the present specification, a video may mean a series of images over time. A picture generally refers to a unit representing one image in a specific time zone, and a slice is a unit constituting a part of a picture in coding. One picture may be composed of a plurality of slices, and if necessary, the picture and the slice may be mixed with each other. Also, in some cases, an "image" may mean a concept including a still image and a video, which is a set of a series of still images over time. In addition, “video” does not necessarily mean a set of a series of still images over time, and in some embodiments, may be interpreted as a concept in which still images are included in video.

A pixel or a pel may refer to a minimum unit constituting one picture (or image). Also, 'sample' may be used as a term corresponding to a pixel. A sample may generally represent a pixel or a value of a pixel, and may only represent pixel / pixel values of the luma component, or only pixel / pixel values of the chroma component.

A unit represents the basic unit of image processing. The unit may include at least one of a specific region of the picture and information related to the region. The unit may be used interchangeably with terms such as block or area in some cases. In a general case, an M × N block may represent a set of samples or transform coefficients composed of M columns and N rows.

1 is a diagram schematically illustrating a configuration of a video encoding apparatus to which the present invention may be applied. Hereinafter, the encoding / decoding device may include a video encoding / decoding device and / or an image encoding / decoding device, and the video encoding / decoding device is used as a concept including the image encoding / decoding device, or the image encoding / decoding device is It may be used in a concept including a video encoding / decoding device.

Referring to FIG. 1, the (video) encoding apparatus 100 may include a picture partitioning module 105, a prediction module 110, a residual processing module 120, and an entropy encoding unit ( The entropy encoding module 130 may include an adder 140, a filtering module 150, and a memory 160. The residual processor 120 may include a substractor 121, a transform module 122, a quantization module 123, a rearrangement module 124, and a dequantization module 125. ) And an inverse transform module 126.

The picture divider 105 may divide the input picture into at least one processing unit.

As an example, the processing unit may be called a coding unit (CU). In this case, the coding unit may be recursively split from the largest coding unit (LCU) according to a quad-tree binary-tree (QTBT) structure. For example, one coding unit may be divided into a plurality of coding units of a deeper depth based on a quad tree structure, a binary tree structure, and / or a ternary tree structure. In this case, for example, the quad tree structure may be applied first, and the binary tree structure and the ternary tree structure may be applied later. Alternatively, the binary tree structure / tunary tree structure may be applied first. The coding procedure according to the present invention may be performed based on the final coding unit that is no longer split. In this case, the maximum coding unit may be used as the final coding unit immediately based on coding efficiency according to the image characteristic, or if necessary, the coding unit is recursively divided into coding units of lower depths and optimized. A coding unit of size may be used as the final coding unit. Here, the coding procedure may include a procedure of prediction, transform, and reconstruction, which will be described later.

As another example, the processing unit may include a coding unit (CU) prediction unit (PU) or a transform unit (TU). The coding unit may be split from the largest coding unit (LCU) into coding units of deeper depths along the quad tree structure. In this case, the maximum coding unit may be used as the final coding unit immediately based on coding efficiency according to the image characteristic, or if necessary, the coding unit is recursively divided into coding units of lower depths and optimized. A coding unit of size may be used as the final coding unit. If a smallest coding unit (SCU) is set, the coding unit may not be split into smaller coding units than the minimum coding unit. Here, the final coding unit refers to a coding unit that is the basis of partitioning or partitioning into a prediction unit or a transform unit. The prediction unit is a unit partitioning from the coding unit and may be a unit of sample prediction. In this case, the prediction unit may be divided into sub blocks. The transform unit may be divided along the quad tree structure from the coding unit, and may be a unit for deriving a transform coefficient and / or a unit for deriving a residual signal from the transform coefficient. Hereinafter, a coding unit may be called a coding block (CB), a prediction unit is a prediction block (PB), and a transform unit may be called a transform block (TB). A prediction block or prediction unit may mean a specific area in the form of a block within a picture, and may include an array of prediction samples. In addition, a transform block or a transform unit may mean a specific area in a block form within a picture, and may include an array of transform coefficients or residual samples.

The prediction unit 110 performs prediction on a block to be processed (hereinafter, may mean a current block or a residual block), and generates a predicted block including prediction samples for the current block. can do. The unit of prediction performed by the prediction unit 110 may be a coding block, a transform block, or a prediction block.

The prediction unit 110 may determine whether intra prediction or inter prediction is applied to the current block. As an example, the prediction unit 110 may determine whether intra prediction or inter prediction is applied on a CU basis.

In the case of intra prediction, the prediction unit 110 may derive a prediction sample for the current block based on reference samples outside the current block in the picture to which the current block belongs (hereinafter, referred to as the current picture). In this case, the prediction unit 110 may (i) derive the prediction sample based on the average or interpolation of neighboring reference samples of the current block, and (ii) the neighbor reference of the current block. The prediction sample may be derived based on a reference sample present in a specific (prediction) direction with respect to the prediction sample among the samples. In case of (i), it may be called non-directional mode or non-angle mode, and in case of (ii), it may be called directional mode or angular mode. In intra prediction, the prediction mode may have, for example, 33 directional prediction modes and at least two non-directional modes. The non-directional mode may include a DC prediction mode and a planner mode (Planar mode). The prediction unit 110 may determine the prediction mode applied to the current block by using the prediction mode applied to the neighboring block.

In the case of inter prediction, the prediction unit 110 may derive the prediction sample for the current block based on the sample specified by the motion vector on the reference picture. The prediction unit 110 may apply one of a skip mode, a merge mode, and a motion vector prediction (MVP) mode to derive a prediction sample for the current block. In the skip mode and the merge mode, the prediction unit 110 may use the motion information of the neighboring block as the motion information of the current block. In the skip mode, unlike the merge mode, the difference (residual) between the prediction sample and the original sample is not transmitted. In the MVP mode, the motion vector of the current block may be derived using the motion vector of the neighboring block as a motion vector predictor.

In the case of inter prediction, the neighboring block may include a spatial neighboring block existing in the current picture and a temporal neighboring block present in the reference picture. A reference picture including the temporal neighboring block may be called a collocated picture (colPic). The motion information may include a motion vector and a reference picture index. Information such as prediction mode information and motion information may be encoded (entropy) and output in the form of a bitstream.

When the motion information of the temporal neighboring block is used in the skip mode and the merge mode, the highest picture on the reference picture list may be used as the reference picture. Reference pictures included in a reference picture list may be sorted based on a difference in a picture order count (POC) between a current picture and a corresponding reference picture. The POC corresponds to the display order of the pictures and may be distinguished from the coding order.

The subtraction unit 121 generates a residual sample which is a difference between the original sample and the prediction sample. When the skip mode is applied, residual samples may not be generated as described above.

The transform unit 122 generates transform coefficients by transforming the residual sample in units of transform blocks. The transform unit 122 may perform the transform according to the size of the transform block and the prediction mode applied to the coding block or the prediction block that spatially overlaps the transform block. For example, if intra prediction is applied to the coding block or the prediction block that overlaps the transform block, and the transform block is a 4 × 4 residual array, the residual sample is configured to perform a discrete sine transform (DST) transform kernel. The residual sample may be transformed using a discrete cosine transform (DCT) transform kernel.

The quantization unit 123 may quantize the transform coefficients to generate quantized transform coefficients.

The reordering unit 124 rearranges the quantized transform coefficients. The reordering unit 124 may reorder the quantized transform coefficients in the form of a block into a one-dimensional vector form through a coefficient scanning method. Although the reordering unit 124 has been described in a separate configuration, the reordering unit 124 may be part of the quantization unit 123.

The entropy encoding unit 130 may perform entropy encoding on the quantized transform coefficients. Entropy encoding may include, for example, encoding methods such as exponential Golomb, context-adaptive variable length coding (CAVLC), context-adaptive binary arithmetic coding (CABAC), and the like. The entropy encoding unit 130 may encode information necessary for video reconstruction other than the quantized transform coefficients (for example, a value of a syntax element) together or separately according to entropy encoding or a predetermined method. The encoded information may be transmitted or stored in units of network abstraction layer (NAL) units in the form of bitstreams. The bitstream may be transmitted over a network or may be stored in a digital storage medium. The network may include a broadcasting network and / or a communication network, and the digital storage medium may include various storage media such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, and the like.

The inverse quantization unit 125 inverse quantizes the quantized values (quantized transform coefficients) in the quantization unit 123, and the inverse transformer 126 inverse transforms the inverse quantized values in the inverse quantization unit 125 to obtain a residual sample. Create

The adder 140 reconstructs the picture by combining the residual sample and the predictive sample. The residual sample and the predictive sample may be added in units of blocks to generate a reconstructed block. Although the adder 140 has been described in a separate configuration, the adder 140 may be part of the predictor 110. The adder 140 may also be called a reconstruction module or a restore block generator.

The filter unit 150 may apply a deblocking filter and / or a sample adaptive offset to the reconstructed picture. Through deblocking filtering and / or sample adaptive offset, the artifacts of the block boundaries in the reconstructed picture or the distortion in the quantization process can be corrected. The sample adaptive offset may be applied on a sample basis and may be applied after the process of deblocking filtering is completed. The filter unit 150 may apply an adaptive loop filter (ALF) to the reconstructed picture. ALF may be applied to the reconstructed picture after the deblocking filter and / or sample adaptive offset is applied.

The memory 160 may store reconstructed pictures (decoded pictures) or information necessary for encoding / decoding. Here, the reconstructed picture may be a reconstructed picture after the filtering process is completed by the filter unit 150. The stored reconstructed picture may be used as a reference picture for (inter) prediction of another picture. For example, the memory 160 may store (reference) pictures used for inter prediction. In this case, pictures used for inter prediction may be designated by a reference picture set or a reference picture list.

FIG. 2 is a diagram schematically illustrating a configuration of a video / video decoding apparatus to which the present invention can be applied. Hereinafter, the video decoding apparatus may include an image decoding apparatus.

Referring to FIG. 2, the (video) decoding apparatus 200 may include an entropy decoding module 210, a residual processing module 220, a prediction module 230, and an adder. , 240, a filtering module 250, and a memory 260. The residual processor 220 may include a rearrangement module 221, a dequantization module 222, and an inverse transform module 223. In addition, although not shown, the video decoding apparatus 200 may include a receiver that receives a bitstream including video information. The receiver may be configured as a separate module or may be included in the entropy decoding unit 210.

When the bitstream including the video / image information is input, the (video) decoding apparatus 200 may restore the video / image / picture in response to a process in which the video / image information is processed in the (video) encoding apparatus. .

For example, the video decoding apparatus 200 may perform video decoding using a processing unit applied in the video encoding apparatus. Thus, the processing unit block of video decoding may be, for example, a coding unit, and in another example, a coding unit, a prediction unit, or a transform unit. The coding unit may be split along the quad tree structure, binary tree structure and / or ternary tree structure from the largest coding unit.

The prediction unit and the transform unit may be further used in some cases, in which case the prediction block is a block derived or partitioned from the coding unit and may be a unit of sample prediction. At this point, the prediction unit may be divided into subblocks. The transform unit may be divided along the quad tree structure from the coding unit, and may be a unit for deriving a transform coefficient or a unit for deriving a residual signal from the transform coefficient.

The entropy decoding unit 210 may parse the bitstream and output information necessary for video reconstruction or picture reconstruction. For example, the entropy decoding unit 210 decodes information in a bitstream based on a coding method such as exponential Golomb coding, CAVLC, or CABAC, quantized values of syntax elements necessary for video reconstruction, and residual coefficients. Can be output.

More specifically, the CABAC entropy decoding method receives a bin corresponding to each syntax element in a bitstream, and decodes syntax element information and decoding information of neighboring and decoding target blocks or information of symbols / bins decoded in a previous step. The context model is determined using the context model, the probability of occurrence of a bin is predicted according to the determined context model, and arithmetic decoding of the bin is performed to generate a symbol corresponding to the value of each syntax element. can do. In this case, the CABAC entropy decoding method may update the context model by using the information of the decoded symbol / bin for the context model of the next symbol / bin after determining the context model.

The information related to the prediction among the information decoded by the entropy decoding unit 210 is provided to the prediction unit 230, and the residual value on which the entropy decoding has been performed by the entropy decoding unit 210, that is, the quantized transform coefficient, is used as a reordering unit ( 221 may be input.

The reordering unit 221 may rearrange the quantized transform coefficients in a two-dimensional block form. The reordering unit 221 may perform reordering in response to coefficient scanning performed by the encoding apparatus. Here, the rearrangement unit 221 has been described in a separate configuration, but the rearrangement unit 221 may be part of the inverse quantization unit 222.

The inverse quantization unit 222 may dequantize the quantized transform coefficients based on the (inverse) quantization parameter and output the transform coefficients. In this case, information for deriving a quantization parameter may be signaled from the encoding apparatus.

The inverse transform unit 223 may inversely transform transform coefficients to derive residual samples.

The prediction unit 230 may perform prediction on the current block and generate a predicted block including prediction samples for the current block. The unit of prediction performed by the prediction unit 230 may be a coding block, a transform block, or a prediction block.

The prediction unit 230 may determine whether to apply intra prediction or inter prediction based on the information about the prediction. In this case, a unit for determining which of intra prediction and inter prediction is to be applied and a unit for generating a prediction sample may be different. In addition, the unit for generating a prediction sample in inter prediction and intra prediction may also be different. For example, whether to apply inter prediction or intra prediction may be determined in units of CUs. In addition, for example, in inter prediction, a prediction mode may be determined and a prediction sample may be generated in PU units, and in intra prediction, a prediction mode may be determined in PU units and a prediction sample may be generated in TU units.

In the case of intra prediction, the prediction unit 230 may derive the prediction sample for the current block based on the neighbor reference samples in the current picture. The prediction unit 230 may derive the prediction sample for the current block by applying the directional mode or the non-directional mode based on the neighbor reference samples of the current block. In this case, the prediction mode to be applied to the current block may be determined using the intra prediction mode of the neighboring block.

In the case of inter prediction, the prediction unit 230 may derive the prediction sample for the current block based on the sample specified on the reference picture by the motion vector on the reference picture. The prediction unit 230 may apply any one of a skip mode, a merge mode, and an MVP mode to derive a prediction sample for the current block. In this case, motion information required for inter prediction of the current block provided by the video encoding apparatus, for example, information about a motion vector, a reference picture index, and the like may be obtained or derived based on the prediction information.

In the skip mode and the merge mode, the motion information of the neighboring block may be used as the motion information of the current block. In this case, the neighboring block may include a spatial neighboring block and a temporal neighboring block.

The prediction unit 230 may construct a merge candidate list using motion information of available neighboring blocks, and may use information indicated by the merge index on the merge candidate list as a motion vector of the current block. The merge index may be signaled from the encoding device. The motion information may include a motion vector and a reference picture. When the motion information of the temporal neighboring block is used in the skip mode and the merge mode, the highest picture on the reference picture list may be used as the reference picture.

In the skip mode, unlike the merge mode, the difference (residual) between the prediction sample and the original sample is not transmitted.

In the MVP mode, the motion vector of the current block may be derived using the motion vector of the neighboring block as a motion vector predictor. In this case, the neighboring block may include a spatial neighboring block and a temporal neighboring block.

For example, when the merge mode is applied, a merge candidate list may be generated by using a motion vector of a reconstructed spatial neighboring block and / or a motion vector corresponding to a Col block, which is a temporal neighboring block. In the merge mode, the motion vector of the candidate block selected from the merge candidate list is used as the motion vector of the current block. The information about the prediction may include a merge index indicating a candidate block having an optimal motion vector selected from candidate blocks included in the merge candidate list. In this case, the prediction unit 230 may derive the motion vector of the current block by using the merge index.

As another example, when the Motion Vector Prediction (MVP) mode is applied, a motion vector predictor candidate list may be generated using a motion vector of a reconstructed spatial neighboring block and / or a motion vector corresponding to a Col block, which is a temporal neighboring block. Can be. That is, the motion vector of the reconstructed spatial neighboring block and / or the Col vector, which is a temporal neighboring block, may be used as a motion vector candidate. The prediction information may include a prediction motion vector index indicating an optimal motion vector selected from the motion vector candidates included in the list. In this case, the prediction unit 230 may select the predicted motion vector of the current block from the motion vector candidates included in the motion vector candidate list using the motion vector index. The prediction unit of the encoding apparatus may obtain a motion vector difference (MVD) between the motion vector of the current block and the motion vector predictor, and may encode the output vector in a bitstream form. That is, MVD may be obtained by subtracting the motion vector predictor from the motion vector of the current block. In this case, the prediction unit 230 may obtain a motion vector difference included in the information about the prediction, and derive the motion vector of the current block by adding the motion vector difference and the motion vector predictor. The prediction unit may also obtain or derive a reference picture index or the like indicating a reference picture from the information about the prediction.

The adder 240 may reconstruct the current block or the current picture by adding the residual sample and the predictive sample. The adder 240 may reconstruct the current picture by adding the residual sample and the predictive sample in block units. Since the residual is not transmitted when the skip mode is applied, the prediction sample may be a reconstruction sample. Although the adder 240 has been described in a separate configuration, the adder 240 may be part of the predictor 230. The adder 240 may also be called a reconstruction module or a reconstruction block generator.

The filter unit 250 may apply the deblocking filtering sample adaptive offset, and / or ALF to the reconstructed picture. In this case, the sample adaptive offset may be applied in units of samples and may be applied after deblocking filtering. ALF may be applied after deblocking filtering and / or sample adaptive offset.

The memory 260 may store reconstructed pictures (decoded pictures) or information necessary for decoding. Here, the reconstructed picture may be a reconstructed picture after the filtering process is completed by the filter unit 250. For example, the memory 260 may store pictures used for inter prediction. In this case, pictures used for inter prediction may be designated by a reference picture set or a reference picture list. The reconstructed picture can be used as a reference picture for another picture. In addition, the memory 260 may output the reconstructed picture in an output order.

Meanwhile, as described above, prediction is performed in order to increase compression efficiency in performing video coding. Through this, a predicted block including prediction samples of the current block, which is a coding target block, may be generated. Wherein the predicted block comprises prediction samples in the spatial domain (or pixel domain). The predicted block is derived identically in the encoding apparatus and the decoding apparatus, and the encoding apparatus decodes information (residual information) about the residual between the original block and the predicted block, not the original sample value itself of the original block. Signaling to an apparatus may increase image coding efficiency. The decoding apparatus may derive a residual block including residual samples based on the residual information, generate the reconstructed block including reconstructed samples by adding the residual block and the predicted block, and generate reconstructed blocks. A reconstructed picture may be generated.

The residual information may be generated through a transform and quantization procedure. For example, the encoding apparatus derives a residual block between the original block and the predicted block, and performs transform procedure on residual samples (residual sample array) included in the residual block to derive transform coefficients. The quantized transform coefficients may be derived by performing a quantization procedure on the transform coefficients to signal related residual information to the decoding device (via a bitstream). Here, the residual information may include information such as value information of the quantized transform coefficients, position information, a transform scheme, a transform kernel, and a quantization parameter. The decoding apparatus may perform an inverse quantization / inverse transformation procedure and derive residual samples (or residual blocks) based on the residual information. The decoding apparatus may generate a reconstructed picture based on the predicted block and the residual block. The encoding apparatus may then dequantize / inverse transform the quantized transform coefficients for reference for inter prediction of the picture to derive a residual block, and generate a reconstructed picture based thereon.

Hereinafter, a process of applying motion vector refinement in performing the adaptive motion vector resolution (AMVR) by the decoding apparatus according to an embodiment of the present invention will be described. Inter prediction of video encoding may be performed based on motion compensation using motion information. In general, there is a method of encoding / decoding motion information, such as Advanced Motion Vector Prediction (AMVP), which directly encodes / decodes motion information, and Merge mode, which transmits only an optimal index by listing peripheral motion information of the current block. In the AMVP mode, the motion information to be encoded / decoded includes a prediction direction (bi or uni), a reference picture index, a motion vector information (for example, a motion vector predictor index, and a motion vector (MVD). In the merge mode, the motion information may include a prediction direction (bi or uni), a reference picture index, a motion vector, and the like.

In order to improve coding efficiency, motion vector information of AMVP may be expressed up to a decimal point unit. For example, motion vector information may be expressed up to a decimal point based on a quarter pixel unit, a 1/8 pixel unit, and the like. However, when the motion vector information is optimal in integer pixel units, it may be inefficient to decode / decode the motion vector information of AMVP in the unit of decimal pixels. Especially, in the case of a high quality image, the motion vector information may be encoded / decoded in the unit of decimal pixels. May be inefficient. Therefore, in order to solve the above-mentioned problem, it is possible to use adaptive motion vector resolution (AMVR) that adaptively selects the resolution of the motion vector information.

In the AMVR, a signal for determining whether to express motion information in units of fractional pixels or in units of integer pixels may be explicitly encoded / decoded. In addition, when the motion information is expressed in integer units, a signal for determining whether to express in 1 pixel unit or 4 pixel unit may be explicitly encoded / decoded. Tables 1 and 2 below show the syntax of the AMVP mode when AMVR is applied, and Table 2 shows the decoding process in that case.

TABLE 1

TABLE 2

In Table 1 and Table 2, skip_flag represents a flag for specifying whether or not a skip mode. If the value of skip_flag is 1, the skip mode is performed. If the value of skip_flag is 0, the skip mode is not performed. merge_flag represents a flag for specifying whether the merge mode or not. If the merge_flag value is 1, the merge mode is performed. If the merge_flag value is 0, the merge mode is not performed. amvr_flag represents a flag for specifying whether to be in AMVR mode or not. If the value of amvr_flag is 1, the AMVR mode is executed. If the value of amvr_flag is 0, the AMVR mode is not performed. amvr_mode represents a flag for specifying whether to perform a 1 pixel reference AMVR or a 4 pixel reference AMVR. When the value of amvr_mode is 0, the motion vector is derived (or determined) by performing AMVR on the basis of 1 pixel, and when the value of amvr_mode is 1, the motion vector is derived (or determined) on the basis of 4 pixels.

When the motion vector is determined by the AMVR, bit usage for encoding the motion vector can be saved, but the accuracy of the motion vector can be relatively low. Therefore, in the present embodiment, to compensate for this, the accuracy of the motion vector is increased by refining the motion vector. Motion vector refinement for AMVR can be based on the following three methods.

First, there is a method of using neighboring pixels of the current block. The motion vector may be determined based on the similarity between the peripheral pixels of the predetermined area of the current block and the peripheral pixels of the predefined area of the reference block. At this time, the neighboring pixels of the current block must satisfy the decodable condition.

Secondly, there is a method using the similarity of reference blocks. In bidirectional prediction, a motion vector may be determined based on the similarity between two reference blocks.

Third, there is a method using the similarity of prediction blocks. In the bidirectional prediction block, the motion vector in each direction may be determined based on the similarity with the average block of the two prediction blocks.

In one embodiment, a method of applying motion vector refinement to an AMVR may be a method of applying to an AMVR mode first, a method of applying to an AMVR mode of a second pixel, and a third of There may be a method applied to the AMVR mode in the 4-pixel unit.

In one example, motion vector refinement can be applied to the AMVR. In the AMVR mode, the motion vector refinement may be performed after encoding the motion vector. An example of the decoding process in which the motion vector refinement is applied to the AMVR is shown in Table 3 below.

TABLE 3

In another example, motion vector refinement may be applied to an AMVR mode in one pixel. Motion vector refinement has a trade-off relationship between complexity and coding efficiency. Therefore, refinement can be performed by selecting a specific mode among AMVR modes. In general, when the AMVR mode is selected, most of the 1-pixel unit AMVR mode can be selected. Therefore, in order to design the motion vector refinement process in the direction of reducing the coding complexity while maintaining the maximum coding efficiency, refinement may be performed only in the 1-pixel AMVR mode. Table 4 below shows the decoding process.

TABLE 4

In another example, motion vector refinement may be applied to a 4-pixel AMVR mode. The 4-pixel AMVR has low reliability in motion vectors when compared to the 1-pixel AMVR mode. Therefore, it can be designed to perform the motion vector refinement only in the 4-pixel AMVR mode. Table 5 shows the decoding process of the method.

TABLE 5

In one embodiment, the search range (SR) of the motion vector refinement technique for AMVR may be considered. The search range has a trade-off relationship between coding complexity and coding performance. If the search range is too large, the coding performance improvement may be small compared to the coding complexity. If the search range is too small, the absolute coding performance may be small. Therefore, the search range needs to be determined in consideration of coding complexity and efficiency.

In the case of a 1-pixel unit AMVR, an optimal motion vector exists in a range of ± 1, and a motion vector determined by the 4-pixel unit AMVR may have an optimal motion vector in a range of ± 4. Therefore, if the search range of one pixel unit AMVR is indicated as SR_1, and the search range of four pixel unit AMVR is indicated as SR_4, the search range for AMVR can be defined as follows.

[Equation 1]

-1 <SR_1 <1, -4 <SR_4 <4

Equation 1 may be expressed as in Equation 2, and m and n values may be determined in consideration of encoding complexity and encoding performance.

[Equation 2]

-1 / m <SR_1 <1 / m, -4 / n <SR_4 <4 / n

In Equation 2, m, n or 1 / m, 1 / n may be transmitted in a high level syntax such as Picture Parameter Set (PPS) or Sequence Parameter Set (SPS), or may be determined at the frame or coding block level. Can be.

Inequalities <in Equations 1 and 2 may be replaced with =. Such substitution may be effective when the reliability of the motion vector of the AMVR is low.

3 is a flowchart illustrating a method of operating a decoding apparatus according to an embodiment, and FIG. 4 is a block diagram illustrating a configuration of a decoding apparatus according to an embodiment.

The decoding apparatus according to FIGS. 3 and 4 and the operating method of the decoding apparatus may be similarly applied to the encoding apparatus according to FIG. 1.

Each step disclosed in FIG. 3 may be performed by the decoding apparatus 200 disclosed in FIG. 2. More specifically, S300 may be performed by the entropy decoding unit 210 shown in FIG. 2, S310 to S330 may be performed by the predictor 230 shown in FIG. 2, and S340 may be added as shown in FIG. 2. It may be performed by the unit 240. Therefore, detailed descriptions that overlap with the foregoing description in FIG. 2 will be omitted or simply described.

As shown in FIG. 4, the decoding apparatus according to an embodiment may include an entropy decoding unit 210, a predictor 230, and an adder 240. However, in some cases, all of the components shown in FIG. 4 may not be essential components of the decoding apparatus, and the decoding apparatus may be implemented by more or fewer components than those illustrated in FIG. 4.

In the decoding apparatus according to an embodiment, the entropy decoding unit 210, the prediction unit 230, and the adder 240 are each implemented as separate chips, or at least two or more components are implemented through one chip. May be

According to an embodiment, a decoding apparatus includes: motion prediction including information on an AMVR flag indicating whether to apply an adaptive motion vector resolution (AMVR) mode and information on an AMVR mode flag indicating a type of the AMVR mode; Motion prediction information may be obtained from the bitstream (S300). More specifically, the entropy decoding unit 210 of the decoding apparatus may provide information on an AMVR flag indicating whether to apply an Adaptive Motion Vector Resolution (AMVR) mode and information on an AMVR mode flag indicating a type of the AMVR mode. Motion prediction information including may be obtained from the bitstream.

If the value of the AMVR flag is 1, the decoding apparatus according to an embodiment may derive a motion vector for the current block based on the value of the AMVR mode flag (S310). More specifically, when the value of the AMVR flag is 1, the prediction unit 230 of the decoding apparatus may derive a motion vector for the current block based on the value of the AMVR mode flag.

In one embodiment, when the value of the AMVR mode flag is 0, the motion vector is derived based on 1 pixel unit, and when the value of the AMVR mode flag is 1, the motion vector is derived based on 4 pixel unit. Can be.

The decoding apparatus according to an embodiment may refine the derived motion vector (S320). More specifically, the prediction unit 230 of the decoding apparatus may refine the derived motion vector.

According to an embodiment, the decoding apparatus may refine the motion vector when the motion vector is derived based on the one pixel unit or the four pixel unit.

In another embodiment, the decoding apparatus may refine the motion vector when the motion vector is derived based on the one pixel unit.

According to another embodiment, the decoding apparatus may refine the motion vector when the motion vector is derived based on the 4 pixel unit.

In an embodiment, the decoding apparatus may refine the derived motion vector based on a similarity between neighboring pixels of the predefined area of the current block and neighboring pixels of the predefined area of the reference block with respect to the current block. Can be.

In another embodiment, the decoding apparatus may refine the derived motion vector based on the similarity between the bidirectional reference blocks with respect to the current block.

In another embodiment, the decoding apparatus may refine the derived motion vector based on the similarity of the average block of the bidirectional reference blocks to the current block.

In one embodiment, the decoding apparatus may refine the derived motion vector within a search range. In one example, when the motion vector is derived based on one pixel unit, the search range may be determined as a value between -1 and 1. FIG. In another example, when the motion vector is derived based on four pixel units, the search range may be determined as a value between -4 and 4. In another example, when the motion vector is derived based on one pixel unit, the search range is determined as a value between -1 / m and 1 / m, wherein the value of m is determined by the motion prediction information. May be included. In another example, when the motion vector is derived based on four pixel units, the search range is determined as a value between -4 / n and 4 / n, wherein the value of n is determined in the motion prediction information. May be included.

The decoding apparatus according to an embodiment may derive prediction samples for the current block based on the refined motion vector (S330). More specifically, the prediction unit 230 of the decoding apparatus may derive the prediction samples for the current block based on the refined motion vector.

The decoding apparatus according to an embodiment may generate reconstruction samples for the current block based on the derived prediction samples (S340). More specifically, the adder 240 of the decoding apparatus may generate reconstruction samples for the current block based on the derived prediction samples.

According to the decoding apparatus and the method of operating the decoding apparatus disclosed in Figs. Motion prediction information including information on the indicated AMVR mode flag is obtained from the bitstream (S300). When the value of the AMVR flag is 1, the current block is determined based on the value of the AMVR mode flag. Derive a motion vector for the current block (S310), refine the derived motion vector (S320), and derive prediction samples for the current block based on the refined motion vector (S330). Reconstruction samples for the current block may be generated (S340) based on prediction samples. That is, the accuracy of inter prediction can be improved by applying motion vector refinement in performing AMVR.

The above-described method according to the present invention may be implemented in software, and the encoding device and / or the decoding device according to the present invention may perform image processing of, for example, a TV, a computer, a smartphone, a set-top box, a display device, and the like. It can be included in the device.

When the embodiments of the present invention are implemented in software, the above-described method may be implemented as a module (process, function, etc.) for performing the above-described function. The module may be stored in memory and executed by a processor. The memory may be internal or external to the processor and may be coupled to the processor by various well known means. The processor may include an application-specific integrated circuit (ASIC), other chipsets, logic circuits, and / or data processing devices. The memory may include read-only memory (ROM), random access memory (RAM), flash memory, memory card, storage medium and / or other storage device.

Claims

In the picture decoding method performed by the decoding apparatus,

A bitstream of motion prediction information including information on an AMVR flag indicating whether to apply an Adaptive Motion Vector Resolution (AMVR) mode and information on an AMVR mode flag indicating a type of the AMVR mode. Obtaining from;

When the value of the AMVR flag is 1, deriving a motion vector for the current block based on the value of the AMVR mode flag;

Refining the derived motion vector;

Deriving prediction samples for the current block based on the refined motion vector; And

Generating reconstructed samples for the current block based on the derived prediction samples.
The method of claim 1,

When the value of the AMVR mode flag is 0, the motion vector is derived based on one pixel unit.

When the value of the AMVR mode flag is 1, the motion vector is derived based on 4 pixel units.

Refining the derived motion vector,

And refining the motion vector when the motion vector is derived based on the one pixel unit or the four pixel unit.
The method of claim 1,

When the value of the AMVR mode flag is 0, the motion vector is derived based on one pixel unit.

Refining the derived motion vector,

And refining the motion vector when the motion vector is derived based on the unit of one pixel.
The method of claim 1,

When the value of the AMVR mode flag is 1, the motion vector is derived based on 4 pixel units.

Refining the derived motion vector,

Refining the motion vector when the motion vector is derived based on the 4 pixel unit.
The method of claim 1,

Refining the derived motion vector,

Refining the derived motion vector based on similarity between neighboring pixels of the predefined area of the current block and neighboring pixels of the predefined area of the reference block with respect to the current block. A picture decoding method.
The method of claim 1,

Refining the derived motion vector,

Refining the derived motion vector based on the similarity between bidirectional reference blocks with respect to the current block.
The method of claim 1,

Refining the derived motion vector,

Refining the derived motion vector based on similarity of the average block of bidirectional reference blocks to the current block.
The method of claim 1,

Refining the derived motion vector,

Refine the derived motion vector within a search range,

When the motion vector is derived based on one pixel unit, the search range is determined as a value between -1 and 1.
The method of claim 1,

Refining the derived motion vector,

Refine the derived motion vector within a search range,

If the motion vector is derived based on a unit of 4 pixels, the search range is determined as a value between -4 and 4.
The method of claim 1,

Refining the derived motion vector,

Refine the derived motion vector within a search range,

When the motion vector is derived based on one pixel unit, the search range is determined as a value between -1 / m and 1 / m.

The value of m is included in the motion prediction information, picture decoding method.
The method of claim 1,

Refining the derived motion vector,

Refine the derived motion vector within a search range,

When the motion vector is derived based on four pixel units, the search range is determined as a value between -4 / n and 4 / n.

The value of n is included in the motion prediction information, picture decoding method.
In the decoding apparatus for performing picture decoding,

A bitstream of motion prediction information including information on an AMVR flag indicating whether to apply an Adaptive Motion Vector Resolution (AMVR) mode and information on an AMVR mode flag indicating a type of the AMVR mode. An entropy decoding unit obtained from the;

When the value of the AMVR flag is 1, the motion vector for the current block is derived based on the value of the AMVR mode flag, the derived motion vector is refined, and the motion vector is refined based on the refined motion vector. A prediction unit for deriving prediction samples for the current block; And

And an adder configured to generate reconstructed samples for the current block based on the derived prediction samples.