WO2018124821A1

WO2018124821A1 - Method and apparatus for processing video signals

Info

Publication number: WO2018124821A1
Application number: PCT/KR2017/015750
Authority: WO
Inventors: 이배근
Original assignee: 주식회사 케이티
Priority date: 2017-01-02
Filing date: 2017-12-29
Publication date: 2018-07-05
Also published as: KR20180080119A

Abstract

The image decoding method according to the present invention may comprise the steps of: obtaining motion information on a current block; specifying a reference face of a current block in a reference picture on the basis of the motion information; determining whether a current face, including the current block, and the reference face have the same direction; and performing motion compensation on the current block on the basis of the determination result.

Description

Video signal processing method and apparatus

The present invention relates to a video signal processing method and apparatus.

Recently, the demand for high resolution and high quality images such as high definition (HD) and ultra high definition (UHD) images is increasing in various applications. As the video data becomes higher resolution and higher quality, the amount of data increases relative to the existing video data. Therefore, when the video data is transmitted or stored using a medium such as a conventional wired / wireless broadband line, The storage cost will increase. High efficiency image compression techniques can be used to solve these problems caused by high resolution and high quality image data.

An inter-screen prediction technique for predicting pixel values included in the current picture from a picture before or after the current picture using an image compression technique, an intra prediction technique for predicting pixel values included in a current picture using pixel information in the current picture, Various techniques exist, such as an entropy encoding technique for allocating a short code to a high frequency of appearance and a long code to a low frequency of appearance, and the image data can be effectively compressed and transmitted or stored.

Meanwhile, as the demand for high resolution video increases, the demand for stereoscopic video content also increases as a new video service. There is a discussion about a video compression technology for effectively providing high resolution and ultra high resolution stereoscopic image contents.

An object of the present invention is to provide a method and apparatus capable of effectively encoding / decoding a 360 degree projection image in encoding / decoding a video signal.

An object of the present invention is to provide a method and apparatus for variably determining a motion compensation method of an encoding / decoding target block according to a position of a face including a reference block in encoding / decoding a video signal.

An object of the present invention is to provide a method and apparatus for improving encoding / decoding efficiency by unifying at least one of a magnitude, a shape, or a phase between a current face and a reference face in encoding / decoding a video signal.

The technical problems to be achieved in the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned above will be clearly understood by those skilled in the art from the following description. Could be.

The video signal decoding method and apparatus according to the present invention obtain motion information on a current block, specify a reference face of a current block in a reference picture based on the motion information, and determine a current face including the current block. It may be determined whether the reference faces have the same direction, and based on the determination result, motion compensation for the current block may be performed.

The video signal encoding method and apparatus according to the present invention obtain motion information on a current block, specify a reference face of a current block in a reference picture based on the motion information, and determine a current face including the current block. It may be determined whether the reference faces have the same direction, and based on the determination result, motion compensation for the current block may be performed.

In the video signal encoding / decoding method and apparatus according to the present invention, when the reference face has a direction different from the current face, the motion compensation rotates the reference face to have the same direction as the current face, The method may include specifying a reference block of the current block from the rotated reference face and generating a prediction block of the current block using the reference block.

In the video signal encoding / decoding method and apparatus according to the present invention, when the reference face has a direction different from the current face, performing the motion compensation specifies a reference block of the current block in the reference face and And rotating the reference block to have the same direction as the current face and generating a prediction block of the current block using the rotated reference block.

In the video signal encoding / decoding method and apparatus according to the present invention, whether the current face and the face have the same direction may be determined based on rotation information of at least one of the current face or the reference face.

In the video signal encoding / decoding method and apparatus according to the present invention, the rotation information may include whether the current face or the reference face is rotated, the rotation direction of the current face or the reference face, or the current face or the reference face. It may represent at least one of the rotation angle of.

The features briefly summarized above with respect to the present invention are merely exemplary aspects of the detailed description of the invention that follows, and do not limit the scope of the invention.

According to the present invention, it is possible to effectively encode / decode a 360 degree projection image.

According to the present invention, a motion compensation method of an encoding / decoding target block can be variably determined according to a position of a face including a reference block.

According to the present invention, the encoding / decoding efficiency can be improved by unifying at least one of the magnitude, shape, or phase between the current face and the reference face.

The effects obtainable in the present invention are not limited to the above-mentioned effects, and other effects not mentioned above may be clearly understood by those skilled in the art from the following description. will be.

1 is a block diagram illustrating an image encoding apparatus according to an embodiment of the present invention.

2 is a block diagram illustrating an image decoding apparatus according to an embodiment of the present invention.

3 is a diagram illustrating a partition mode that can be applied to a coding block when the coding block is encoded by inter-screen prediction.

4 to 6 are diagrams illustrating a camera apparatus for generating a panoramic image.

7 is a diagram schematically illustrating a process of encoding / decoding and rendering of 360 degree video.

8 shows an isotropic rectangular diagram method in a 2D projection method.

9 illustrates a cube projection method of the 2D projection method.

10 shows a icosahedron projection method among 2D projection methods.

11 shows an octahedral projection method among 2D projection methods.

12 illustrates a truncated pyramid projection conversion method among 2D projection methods.

FIG. 13 is a diagram for explaining the conversion between face 2D coordinates and 3D coordinates.

14 is a flowchart illustrating an inter prediction method of a 2D image according to an embodiment to which the present invention is applied.

15 is a diagram illustrating a process of deriving motion information of a current block when a merge mode is applied to the current block.

16 is a diagram illustrating a process of deriving motion information of a current block when an AMVP mode is applied to the current block.

17 is a diagram illustrating a position of a reference block used to derive a prediction block of the current block.

FIG. 18 is a diagram illustrating an example in which a face including a reference block is identified by a reference face index in a 360 degree projection image based on TPP.

19 is a diagram illustrating a motion vector when the current block and the reference block belong to the same face.

20 is a diagram illustrating a motion vector when the current block and the reference block belong to different faces.

21 is a diagram illustrating an example of modifying a reference face to match the current face.

22 is a diagram illustrating a method of performing inter prediction of a current block in a 360 degree projection image according to the present invention.

23 is a diagram illustrating an example of generating a reference block based on a sample belonging to a reference face.

24 is a diagram illustrating an example of generating a motion compensation reference face by modifying a second face adjacent to a first face including a reference point of a reference block.

FIG. 25 is a diagram illustrating an example of determining whether a reference face is available based on proximity between faces. FIG.

FIG. 26 is a diagram illustrating a 360 degree projection image in a cube map format. FIG.

27 is a diagram illustrating a method of performing motion compensation in consideration of directionality between faces.

28 is a diagram illustrating an example of performing motion compensation when the current face and the reference face have different directions.

As the 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 present invention to specific embodiments, it should be understood to include all modifications, equivalents, and substitutes included in the spirit and scope of the present invention. In describing the drawings, similar reference numerals are used for similar elements.

Terms such as first and second may be used to describe various components, but the components should not be limited by the terms. The terms are used only for the purpose of distinguishing 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 a plurality of related items or any item of a plurality of related items.

When a component is referred to as being "connected" or "connected" to another component, it may be directly connected to or connected to that other component, but it may be understood that other components may exist in the middle. Should be. On the other hand, when a component is referred to as being "directly connected" or "directly connected" to another component, it should be understood that there is no other component in between.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this application, the terms "comprise" or "have" 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 present invention does not exclude the possibility of the presence or the addition of numbers, steps, operations, components, components, or a combination thereof.

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 duplicate descriptions of the same components are omitted.

Referring to FIG. 1, the image encoding apparatus 100 may include a picture splitter 110, a

predictor

120 and 125, a transformer 130, a quantizer 135, a realigner 160, and an entropy encoder. 165, an inverse quantizer 140, an inverse transformer 145, a filter 150, and a memory 155.

Each of the components shown in FIG. 1 is independently illustrated to represent different characteristic functions in the image encoding apparatus, and does not mean that each of the components is made of separate hardware or one software component unit. In other words, each component is included in each component for convenience of description, and at least two of the components may be combined into one component, or one component may be divided into a plurality of components to perform a function. Integrated and separate embodiments of the components are also included within the scope of the present invention without departing from the spirit of the invention.

In addition, some of the components may not be essential components for performing essential functions in the present invention, but may be optional components for improving performance. The present invention can be implemented including only the components essential for implementing the essentials of the present invention except for the components used for improving performance, and the structure including only the essential components except for the optional components used for improving performance. Also included in the scope of the present invention.

The picture dividing unit 110 may divide the input picture into at least one processing unit. In this case, the processing unit may be a prediction unit (PU), a transform unit (TU), or a coding unit (CU). The picture dividing unit 110 divides one picture into a combination of a plurality of coding units, prediction units, and transformation units, and combines one coding unit, prediction unit, and transformation unit on a predetermined basis (eg, a cost function). You can select to encode the picture.

For example, one picture may be divided into a plurality of coding units. In order to split a coding unit in a picture, a recursive tree structure such as a quad tree structure may be used, and coding is divided into other coding units by using one image or a largest coding unit as a root. The unit may be split with as many child nodes as the number of split coding units. Coding units that are no longer split according to certain restrictions become leaf nodes. That is, when it is assumed that only square division is possible for one coding unit, one coding unit may be split into at most four other coding units.

Hereinafter, in an embodiment of the present invention, a coding unit may be used as a unit for encoding or may be used as a unit for decoding.

The prediction unit may be split in the form of at least one square or rectangle having the same size in one coding unit, or the prediction unit of any one of the prediction units split in one coding unit is different from one another. It may be divided to have a different shape and / or size than the unit.

When generating the prediction unit that performs the intra prediction based on the coding unit, when the prediction unit is not the minimum coding unit, the intra prediction may be performed without splitting into a plurality of prediction units NxN.

The

predictors

120 and 125 may include an inter predictor 120 that performs inter prediction and an intra predictor 125 that performs intra prediction. Whether to use inter prediction or intra prediction on the prediction unit may be determined, and specific information (eg, an intra prediction mode, a motion vector, a reference picture, etc.) according to each prediction method may be determined. In this case, the processing unit in which the prediction is performed may differ from the processing unit in which the prediction method and the details are determined. For example, the method of prediction and the prediction mode may be determined in the prediction unit, and the prediction may be performed in the transform unit. The residual value (residual block) between the generated prediction block and the original block may be input to the transformer 130. In addition, prediction mode information and motion vector information used for prediction may be encoded by the entropy encoder 165 together with the residual value and transmitted to the decoder. When a specific encoding mode is used, the original block may be encoded as it is and transmitted to the decoder without generating the prediction block through the

prediction units

120 and 125.

The inter prediction unit 120 may predict the prediction unit based on the information of at least one of the previous picture or the next picture of the current picture. In some cases, the inter prediction unit 120 may predict the prediction unit based on the information of the partial region in which the encoding is completed in the current picture. You can also predict units. The inter predictor 120 may include a reference picture interpolator, a motion predictor, and a motion compensator.

The reference picture interpolator may receive reference picture information from the memory 155 and generate pixel information of an integer pixel or less in the reference picture. In the case of luminance pixels, a DCT based 8-tap interpolation filter having different filter coefficients may be used to generate pixel information of integer pixels or less in units of 1/4 pixels. In the case of a chrominance signal, a DCT-based interpolation filter having different filter coefficients may be used to generate pixel information of an integer pixel or less in units of 1/8 pixels.

The motion predictor may perform motion prediction based on the reference picture interpolated by the reference picture interpolator. As a method for calculating a motion vector, various methods such as full search-based block matching algorithm (FBMA), three step search (TSS), and new three-step search algorithm (NTS) may be used. The motion vector may have a motion vector value of 1/2 or 1/4 pixel units based on the interpolated pixels. The motion prediction unit may predict the current prediction unit by using a different motion prediction method. As the motion prediction method, various methods such as a skip method, a merge method, an advanced motion vector prediction (AMVP) method, an intra block copy method, and the like may be used.

The intra predictor 125 may generate a prediction unit based on reference pixel information around the current block, which is pixel information in the current picture. If the neighboring block of the current prediction unit is a block that has performed inter prediction, and the reference pixel is a pixel that has performed inter prediction, the reference pixel of the block that has performed intra prediction around the reference pixel included in the block where the inter prediction has been performed Can be used as a substitute for information. That is, when the reference pixel is not available, the unavailable reference pixel information may be replaced with at least one reference pixel among the available reference pixels.

In intra prediction, a prediction mode may have a directional prediction mode using reference pixel information according to a prediction direction, and a non-directional mode using no directional information when performing prediction. The mode for predicting the luminance information and the mode for predicting the color difference information may be different, and the intra prediction mode information or the predicted luminance signal information used for predicting the luminance information may be utilized to predict the color difference information.

When performing intra prediction, if the size of the prediction unit and the size of the transform unit are the same, the intra prediction on the prediction unit is performed based on the pixels on the left of the prediction unit, the pixels on the upper left, and the pixels on the top. Can be performed. However, when performing intra prediction, if the size of the prediction unit is different from that of the transform unit, intra prediction may be performed using a reference pixel based on the transform unit. In addition, intra prediction using NxN division may be used only for a minimum coding unit.

The intra prediction method may generate a prediction block after applying an adaptive intra smoothing (AIS) filter to a reference pixel according to a prediction mode. The type of AIS filter applied to the reference pixel may be different. In order to perform the intra prediction method, the intra prediction mode of the current prediction unit may be predicted from the intra prediction mode of the prediction unit existing around the current prediction unit. When the prediction mode of the current prediction unit is predicted by using the mode information predicted from the neighboring prediction unit, if the intra prediction mode of the current prediction unit and the neighboring prediction unit is the same, the current prediction unit and the neighboring prediction unit using the predetermined flag information If the prediction modes of the current prediction unit and the neighboring prediction unit are different, entropy encoding may be performed to encode the prediction mode information of the current block.

Also, a residual block may include a prediction unit performing prediction based on the prediction units generated by the

prediction units

120 and 125 and residual information including residual information that is a difference from an original block of the prediction unit. The generated residual block may be input to the transformer 130.

The transform unit 130 converts the residual block including residual information of the original block and the prediction unit generated by the

prediction units

120 and 125 into a discrete cosine transform (DCT), a discrete sine transform (DST), and a KLT. You can convert using the same conversion method. Whether to apply DCT, DST, or KLT to transform the residual block may be determined based on intra prediction mode information of the prediction unit used to generate the residual block.

The quantization unit 135 may quantize the values converted by the transformer 130 into the frequency domain. The quantization coefficient may change depending on the block or the importance of the image. The value calculated by the quantization unit 135 may be provided to the inverse quantization unit 140 and the reordering unit 160.

The reordering unit 160 may reorder coefficient values with respect to the quantized residual value.

The reordering unit 160 may change the two-dimensional block shape coefficients into a one-dimensional vector form through a coefficient scanning method. For example, the reordering unit 160 may scan from DC coefficients to coefficients in the high frequency region by using a Zig-Zag scan method and change them into one-dimensional vectors. Depending on the size of the transform unit and the intra prediction mode, a vertical scan that scans two-dimensional block shape coefficients in a column direction instead of a zig-zag scan may be used, and a horizontal scan that scans two-dimensional block shape coefficients in a row direction. That is, according to the size of the transform unit and the intra prediction mode, it is possible to determine which scan method among the zig-zag scan, the vertical scan, and the horizontal scan is used.

The entropy encoder 165 may perform entropy encoding based on the values calculated by the reordering unit 160. Entropy encoding may use various encoding methods such as, for example, Exponential Golomb, Context-Adaptive Variable Length Coding (CAVLC), and Context-Adaptive Binary Arithmetic Coding (CABAC).

The entropy encoder 165 receives residual value coefficient information, block type information, prediction mode information, partition unit information, prediction unit information, transmission unit information, and motion of the coding unit from the reordering unit 160 and the

prediction units

120 and 125. Various information such as vector information, reference frame information, interpolation information of a block, and filtering information can be encoded.

The entropy encoder 165 may entropy encode a coefficient value of a coding unit input from the reordering unit 160.

The inverse quantizer 140 and the inverse transformer 145 inverse quantize the quantized values in the quantizer 135 and inversely transform the transformed values in the transformer 130. The residual value generated by the inverse quantizer 140 and the inverse transformer 145 is reconstructed by combining the prediction units predicted by the motion estimator, the motion compensator, and the intra predictor included in the

predictors

120 and 125. You can create a Reconstructed Block.

The filter unit 150 may include at least one of a deblocking filter, an offset correction unit, and an adaptive loop filter (ALF).

The deblocking filter may remove block distortion caused by boundaries between blocks in the reconstructed picture. In order to determine whether to perform deblocking, it may be determined whether to apply a deblocking filter to the current block based on the pixels included in several columns or rows included in the block. When the deblocking filter is applied to the block, a strong filter or a weak filter may be applied according to the required deblocking filtering strength. In addition, in applying the deblocking filter, horizontal filtering and vertical filtering may be performed in parallel when vertical filtering and horizontal filtering are performed.

The offset correction unit may correct the offset with respect to the original image on a pixel-by-pixel basis for the deblocking image. In order to perform offset correction for a specific picture, the pixels included in the image are divided into a predetermined number of areas, and then, an area to be offset is determined, an offset is applied to the corresponding area, or offset considering the edge information of each pixel. You can use this method.

Adaptive Loop Filtering (ALF) may be performed based on a value obtained by comparing the filtered reconstructed image with the original image. After dividing the pixels included in the image into a predetermined group, one filter to be applied to the group may be determined and filtering may be performed for each group. For information related to whether to apply ALF, a luminance signal may be transmitted for each coding unit (CU), and the shape and filter coefficient of an ALF filter to be applied may vary according to each block. In addition, regardless of the characteristics of the block to be applied, the same type (fixed form) of the ALF filter may be applied.

The memory 155 may store the reconstructed block or picture calculated by the filter unit 150, and the stored reconstructed block or picture may be provided to the

predictors

120 and 125 when performing inter prediction.

Referring to FIG. 2, the image decoder 200 includes an entropy decoder 210, a reordering unit 215, an inverse quantizer 220, an inverse transformer 225, a

predictor

230, 235, and a filter unit ( 240, a memory 245 may be included.

When an image bitstream is input from the image encoder, the input bitstream may be decoded by a procedure opposite to that of the image encoder.

The entropy decoder 210 may perform entropy decoding in a procedure opposite to that of the entropy encoding performed by the entropy encoder of the image encoder. For example, various methods such as Exponential Golomb, Context-Adaptive Variable Length Coding (CAVLC), and Context-Adaptive Binary Arithmetic Coding (CABAC) may be applied to the method performed by the image encoder.

The entropy decoder 210 may decode information related to intra prediction and inter prediction performed by the encoder.

The reordering unit 215 may reorder the entropy decoded bitstream by the entropy decoding unit 210 based on a method of rearranging the bitstream. Coefficients expressed in the form of a one-dimensional vector may be reconstructed by reconstructing the coefficients in a two-dimensional block form. The reordering unit 215 may be realigned by receiving information related to coefficient scanning performed by the encoder and performing reverse scanning based on the scanning order performed by the corresponding encoder.

The inverse quantization unit 220 may perform inverse quantization based on the quantization parameter provided by the encoder and the coefficient values of the rearranged block.

The inverse transform unit 225 may perform an inverse transform, i.e., an inverse DCT, an inverse DST, and an inverse KLT, for a quantization result performed by the image encoder, that is, a DCT, DST, and KLT. Inverse transformation may be performed based on a transmission unit determined by the image encoder. The inverse transform unit 225 of the image decoder may selectively perform a transform scheme (eg, DCT, DST, KLT) according to a plurality of pieces of information such as a prediction method, a size of a current block, and a prediction direction.

The

prediction units

230 and 235 may generate the prediction block based on the prediction block generation related information provided by the entropy decoder 210 and previously decoded blocks or picture information provided by the memory 245.

As described above, when performing the intra prediction in the same manner as the operation in the image encoder, when the size of the prediction unit and the size of the transformation unit are the same, the pixel present on the left side, the pixel present on the upper left side, and the upper part exist Intra prediction is performed on a prediction unit based on a pixel, but when intra prediction is performed, when the size of the prediction unit and the size of the transformation unit are different, intra prediction may be performed using a reference pixel based on the transformation unit. Can be. In addition, intra prediction using NxN division may be used only for a minimum coding unit.

The

predictors

230 and 235 may include a prediction unit determiner, an inter predictor, and an intra predictor. The prediction unit determiner receives various information such as prediction unit information input from the entropy decoder 210, prediction mode information of the intra prediction method, and motion prediction related information of the inter prediction method, and distinguishes the prediction unit from the current coding unit, and predicts It may be determined whether the unit performs inter prediction or intra prediction. The inter prediction unit 230 predicts the current prediction based on information included in at least one of a previous picture or a subsequent picture of the current picture including the current prediction unit by using information required for inter prediction of the current prediction unit provided by the image encoder. Inter prediction may be performed on a unit. Alternatively, inter prediction may be performed based on information of some regions pre-restored in the current picture including the current prediction unit.

In order to perform inter prediction, a motion prediction method of a prediction unit included in a coding unit based on a coding unit includes a skip mode, a merge mode, an AMVP mode, and an intra block copy mode. It can be determined whether or not it is a method.

The intra predictor 235 may generate a prediction block based on pixel information in the current picture. When the prediction unit is a prediction unit that has performed intra prediction, intra prediction may be performed based on intra prediction mode information of the prediction unit provided by the image encoder. The intra predictor 235 may include an adaptive intra smoothing (AIS) filter, a reference pixel interpolator, and a DC filter. The AIS filter is a part of filtering the reference pixel of the current block and determines whether to apply the filter according to the prediction mode of the current prediction unit. AIS filtering may be performed on the reference pixel of the current block by using the prediction mode and the AIS filter information of the prediction unit provided by the image encoder. If the prediction mode of the current block is a mode that does not perform AIS filtering, the AIS filter may not be applied.

When the prediction mode of the prediction unit is a prediction unit that performs intra prediction based on a pixel value interpolating the reference pixel, the reference pixel interpolator may generate a reference pixel having an integer value or less by interpolating the reference pixel. If the prediction mode of the current prediction unit is a prediction mode for generating a prediction block without interpolating the reference pixel, the reference pixel may not be interpolated. The DC filter may generate the prediction block through filtering when the prediction mode of the current block is the DC mode.

The reconstructed block or picture may be provided to the filter unit 240. The filter unit 240 may include a deblocking filter, an offset correction unit, and an ALF.

Information about whether a deblocking filter is applied to a corresponding block or picture, and when the deblocking filter is applied to the corresponding block or picture, may be provided with information about whether a strong filter or a weak filter is applied. In the deblocking filter of the image decoder, the deblocking filter related information provided by the image encoder may be provided and the deblocking filtering of the corresponding block may be performed in the image decoder.

The offset correction unit may perform offset correction on the reconstructed image based on the type of offset correction and offset value information applied to the image during encoding.

The ALF may be applied to a coding unit based on ALF application information, ALF coefficient information, and the like provided from the encoder. Such ALF information may be provided included in a specific parameter set.

The memory 245 may store the reconstructed picture or block to use as a reference picture or reference block, and may provide the reconstructed picture to the output unit.

As described above, in the embodiment of the present invention, a coding unit is used as a coding unit for convenience of description, but may also be a unit for performing decoding as well as encoding.

In addition, the current block represents a block to be encoded / decoded, and according to the encoding / decoding step, a coding tree block (or a coding tree unit), an encoding block (or a coding unit), a transform block (or a transform unit), or a prediction block. (Or prediction unit) or the like. In the present specification, 'unit' may indicate a basic unit for performing a specific encoding / decoding process, and 'block' may indicate a sample array having a predetermined size. Unless otherwise specified, 'block' and 'unit' may be used interchangeably. For example, in the embodiments described below, the coding block (coding block) and the coding unit (coding unit) may be understood to have the same meaning.

One picture may be divided into square or non-square basic blocks and encoded / decoded. In this case, the basic block may be referred to as a coding tree unit. A coding tree unit may be defined as the largest coding unit allowed in a sequence or slice. Information regarding whether the coding tree unit is square or non-square or the size of the coding tree unit may be signaled through a sequence parameter set, a picture parameter set or a slice header. The coding tree unit may be divided into smaller sized partitions. In this case, when the partition generated by dividing the coding tree unit is called depth 1, the partition generated by dividing the partition having depth 1 may be defined as depth 2. That is, a partition generated by dividing a partition that is a depth k in a coding tree unit may be defined as having a depth k + 1.

A partition of any size generated as the coding tree unit is split may be defined as a coding unit. The coding unit may be split recursively or split into basic units for performing prediction, quantization, transform, or in-loop filtering. For example, an arbitrary size partition generated as a coding unit is divided may be defined as a coding unit or a transform unit or a prediction unit that is a basic unit for performing prediction, quantization, transform, or in-loop filtering.

Alternatively, when the coding block is determined, a prediction block having the same size as the coding block or a size smaller than the coding block may be determined through prediction division of the coding block. Predictive partitioning of a coding block may be performed by a partition mode (Part_mode) indicating a partition type of a coding block. The size or shape of the prediction block may be determined according to the partition mode of the coding block. The division type of the coding block may be determined through information specifying any one of partition candidates. In this case, the partition candidates available to the coding block may include an asymmetric partition shape (eg, nLx2N, nRx2N, 2NxnU, 2NxnD) according to the size, shape, or coding mode of the coding block. As an example, a partition candidate available to a coding block may be determined according to an encoding mode of the current block. As an example, FIG. 3 is a diagram illustrating a partition mode that may be applied to a coding block when the coding block is encoded by inter prediction.

When the coding block is encoded by inter prediction, any one of eight partition modes may be applied to the coding block, as shown in the example illustrated in FIG. 3.

On the other hand, when a coding block is encoded by intra prediction, partition mode PART_2Nx2N or PART_NxN may be applied to the coding block.

PART_NxN may be applied when the coding block has a minimum size. Here, the minimum size of the coding block may be predefined in the encoder and the decoder. Alternatively, information about the minimum size of the coding block may be signaled through the bitstream. As an example, the minimum size of the coding block is signaled through the slice header, and accordingly, the minimum size of the coding block may be defined for each slice.

As another example, the partition candidates available to the coding block may be determined differently according to at least one of the size or shape of the coding block. For example, the number or type of partition candidates that a coding block may use may be differently determined according to at least one of the size or shape of the coding block.

Alternatively, the type or number of asymmetric partition candidates among partition candidates available to the coding block may be limited according to the size or shape of the coding block. As an example, the number or type of asymmetric partition candidates that a coding block may use may be differently determined according to at least one of the size or shape of the coding block.

In general, the size of the prediction block may have a size of 64x64 to 4x4. However, when the coding block is encoded by inter prediction, when the motion compensation is performed, the prediction block may not have a 4x4 size in order to reduce the memory bandwidth.

The field of view of the camera limits the field of view of the video taken by the camera. In order to overcome this problem, a plurality of cameras may be used to capture an image, and the captured image may be stitched to configure one video or one bitstream. For example, FIGS. 4 to 6 illustrate an example of capturing up, down, left, and right sides simultaneously using a plurality of cameras. As such, a video generated by stitching a plurality of videos may be referred to as a panoramic video. In particular, an image having a degree of freedom of 360 degrees based on a predetermined central axis may be referred to as a 360 degree video.

The camera structure (or camera arrangement) for acquiring 360-degree video has a circular arrangement, as in the example shown in FIG. 4, or a one-dimensional vertical / horizontal arrangement, as in the example shown in FIG. 5A. Alternatively, as in the example shown in FIG. 5B, a two-dimensional arrangement (that is, a mixture of vertical and horizontal arrangements) may be used. Alternatively, as shown in FIG. 6, a spherical device may be equipped with a plurality of cameras.

An embodiment to be described later will be described based on 360 degree video, but it will be included in the technical scope of the present invention to apply the embodiment to be described later to a panoramic video which is not a 360 degree video.

In order to encode / decode a 360 degree video using the encoder / decoder of Figs. 1 and 2, it is necessary to convert the 360 degree video into a 2D video. That is, after converting image information in a 3D space into a 2D form, 2D projection may be performed, and encoding / decoding of the converted image may be performed. By performing inverse projection on a 2D image having been encoded / decoded, an image having a degree of freedom of 360 degrees can be viewed in up, down, left, and right directions.

For projection transformation of 360-degree video to 2D, isotropic projection (ERP), cube projection transformation (Cube Map Projection, CMP), isosahedral projection transformation (ISP), octahedron projection transformation (Octahedron Projection, OHP), Truncated Pyramid Projection (TPP), Sharp Segment Projection (SSP), or rotated sphere projection (RSP) may be used.

8 shows an isotropic rectangular diagram method in a 2D projection method.

Isometric is a method of projecting a pixel corresponding to a sphere in a 2: 1 ratio rectangle, which is the most commonly used 2D transformation technique. With isotonicity, the actual length of the sphere corresponding to the unit length on the 2D plane becomes shorter toward the pole of the sphere. For example, the coordinates of both ends of the unit length on the 2D plane may correspond to a distance difference of 20 cm near the equator of the sphere, while corresponding to a distance difference of 5 cm near the pole of the sphere. As a result, the isotropic rectangular method has a disadvantage in that the image distortion is large and coding efficiency is lowered near the poles of the sphere.

9 illustrates a cube projection method of the 2D projection method.

The cube projection method is to approximate 3D data to correspond to a cube, and then convert the cube into 2D. When projecting 3D data into a cube, one face (or plane) is configured to be adjacent to four faces. The continuity between the faces is high, and the cube projection method has an advantage of higher coding efficiency than the isotonic diagram method. After projection-converting the 3D data into 2D, encoding / decoding may be performed by rearranging the 2D projection-converted image into a quadrangle form. Reordering the 2D projection-converted image into a quadrangular shape may be referred to as frame reordering or frame packing.

10 shows a icosahedron projection method among 2D projection methods.

The icosahedron projection method is a method of approximating 3D data to correspond to the icosahedron and converting it into 2D. The icosahedral projection method is characterized by strong continuity between faces. In addition, frame packing for rearranging the 2D projection-converted image may be performed.

11 shows an octahedral projection method among 2D projection methods.

The octahedron projection method is a method of approximating 3D data so as to correspond to an octahedron, and converting it into 2D. The octahedral projection method is characterized by strong continuity between faces. In addition, frame packing for rearranging the 2D projection-converted image may be performed.

The truncated pyramid projection transformation method is a method of approximating 3D data to correspond to the truncated pyramid, and converting it into 2D. Under the truncated pyramid projection transformation method, frame packing may be performed such that a face at a particular point in time has a different size than a neighboring face. For example, as in the example shown in FIG. 12, the front face may have a larger size than the side face and the back face. When the truncated pyramid projection transformation method is used, image data of a specific viewpoint is large, and thus the encoding / decoding efficiency of the specific viewpoint is higher than that of other viewpoints.

SSP divides the sphere into high latitudes, low latitudes, and mid-latitudes, mapping two north-south high latitudes into two circles, and mapping mid-latitudes into rectangles, such as ERP.

RSP represents a method of mapping a sphere into two ellipses surrounding a tennis ball.

In the following embodiment, a 2D image constructed using 2D projection transformation will be referred to as a 360 degree projection image. In addition, in the embodiments described below, although the embodiments are described based on a specific projection method, the embodiments described below may be extended to a projection method other than the described projection method.

Each sample of the 360 degree projection image may be identified by face 2D coordinates. The face 2D coordinates may include an index f for identifying the face where the sample is located, a coordinate (m, n) representing a sample grid in a 360 degree projection image.

Through the transformation between the face 2D coordinates and the 3D coordinates, 2D projection transformation and image rendering may be performed. As an example, FIG. 13 is a diagram illustrating a conversion between a face 2D coordinate and a 3D coordinate. When a 360 degree projection image is generated based on the ERP, conversion between three-dimensional coordinates (x, y, z) and face 2D coordinates (f, m, n) may be performed using Equations 1 to 4 below. have.

The current picture in the 360 degree projection image may include at least one or more faces. In this case, the number of faces may be 1, 2, 3, 4 or more natural numbers, depending on the projection method. Among the face 2D coordinates, f may be set to a value equal to or smaller than the number of faces. The current picture may include at least one or more faces having the same temporal order or output order (POC).

Alternatively, the number of faces constituting the current picture may be fixed or variable. For example, the number of faces constituting the current picture may be limited not to exceed a predetermined threshold. Here, the threshold value may be a fixed value previously promised by the encoder and the decoder. Alternatively, information about the maximum number of faces constituting one picture may be signaled through a bitstream.

The faces may be determined by partitioning the current picture using at least one of a horizontal line, a vertical line, or a diagonal line, depending on the projection method.

Each face within a picture may be assigned an index to identify each face. Each face may be parallelized, such as a tile or a slice. Accordingly, when performing intra prediction or inter prediction of the current block, neighboring blocks belonging to different faces from the current block may be determined to be unavailable.

Paces (or non-parallel regions) where parallelism is not allowed may be defined, or faces with interdependencies may be defined. For example, faces that do not allow parallel processing or faces with interdependencies may be coded / decoded sequentially instead of being parallel coded / decoded. Accordingly, even neighboring blocks belonging to different faces from the current block may be determined to be available for intra prediction or inter prediction of the current block, depending on whether parallel processing between faces or dependencies is possible.

Inter prediction in a 360 degree projection image may be performed based on motion information of a current block, such as encoding / decoding of a 2D image. For example, FIGS. 14 to 16 are flowcharts for describing an inter prediction method for a 2D image.

Referring to FIG. 14, the motion information of the current block may be determined (S1410). The motion information of the current block may include at least one of a motion vector of the current block, a reference picture index of the current block, or an inter prediction direction of the current block.

The motion information of the current block may be obtained based on at least one of information signaled through a bitstream or motion information of a neighboring block neighboring the current block.

When the merge mode is applied to the current block, a spatial merge candidate may be derived from the spatial neighboring block of the current block (S1510). The spatial neighboring block may include at least one of a block adjacent to a top, left, or corner of the current block (eg, at least one of a top left corner, a top right corner, or a bottom left corner).

The motion information of the spatial merge candidate may be set to be the same as the motion information of the spatial neighboring block.

A temporal merge candidate may be derived from a temporal neighboring block of the current block (S1520). A temporal neighboring block may mean a co-located block included in a collocated picture. The collocated picture has a different temporal order (Picture Order Count, POC) than the current picture containing the current block. The collocated picture may be determined by a picture having a predefined index in the reference picture list or by an index signaled from the bitstream. The temporal neighboring block may be determined as any block in the block having the same position and size as the current block in the collocated picture or a block adjacent to a block having the same position and size as the current block. For example, at least one of a block including a center coordinate of a block having the same position and size as the current block in the collocated picture, or a block adjacent to a lower right boundary of the block may be determined as a temporal neighboring block.

The motion information of the temporal merge candidate may be determined based on the motion information of the temporal neighboring block. For example, the motion vector of the temporal merge candidate may be determined based on the motion vector of the temporal neighboring block. In addition, the inter prediction direction of the temporal merge candidate may be set to be the same as the inter prediction direction of the temporal neighboring block. However, the reference picture index of the temporal merge candidate may have a fixed value. For example, the reference picture index of the temporal merge candidate may be set to '0'.

Thereafter, a merge candidate list including a spatial merge candidate and a temporal merge candidate may be generated (S1530). If the number of merge candidates included in the merge candidate list is smaller than the maximum merge candidate number, a merge candidate having a combination of two or more merge candidates or a merge candidate having a (0,0) zero motion vector It may be included in the merge candidate list.

When the merge candidate list is generated, at least one of the merge candidates included in the merge candidate list may be specified based on the merge candidate index (S1540).

The motion information of the current block may be set to be the same as the motion information of the merge candidate specified by the merge candidate index (S1550). For example, when the spatial merge candidate is selected by the merge candidate index, the motion information of the current block may be set to be the same as the motion information of the spatial neighboring block. Alternatively, when the temporal merge candidate is selected by the merge candidate index, the motion information of the current block may be set to be the same as the motion information of the temporal neighboring block.

When the AMVP mode is applied to the current block, at least one of the inter prediction direction or the reference picture index of the current block may be decoded from the bitstream (S1610). That is, when the AMVP mode is applied, at least one of the inter prediction direction or the reference picture index of the current block may be determined based on information encoded through the bitstream.

A spatial motion vector candidate may be determined based on the motion vector of the spatial neighboring block of the current block (S1620). The spatial motion vector candidate may include at least one of a first spatial motion vector candidate derived from an upper neighboring block of the current block and a second spatial motion vector candidate derived from a left neighboring block of the current block. Here, the upper neighboring block includes at least one of the blocks adjacent to the upper or upper right corner of the current block, and the left neighboring block of the current block includes at least one of the blocks adjacent to the left or lower left corner of the current block. Can be. The block adjacent to the upper left corner of the current block may be treated as the upper neighboring block, or may be treated as the left neighboring block.

If the reference picture is different between the current block and the spatial neighboring block, the spatial motion vector may be obtained by scaling the motion vector of the spatial neighboring block.

A temporal motion vector candidate may be determined based on the motion vector of the temporal neighboring block of the current block (S1630). If the reference picture is different between the current block and the temporal neighboring block, the temporal motion vector may be obtained by scaling the motion vector of the temporal neighboring block.

A motion vector candidate list including a spatial motion vector candidate and a temporal motion vector candidate may be generated (S1640).

When the motion vector candidate list is generated, at least one of the motion vector candidates included in the motion vector candidate list may be specified based on information for specifying at least one of the motion vector candidate lists (S1650).

The motion vector candidate specified by the information may be set as a motion vector prediction value of the current block, and the motion vector difference value is added to the motion vector prediction value to obtain a motion vector of the current block (S1660). In this case, the motion vector difference value may be parsed through the bitstream.

When motion information of the current block is obtained, motion compensation for the current block may be performed based on the obtained motion information (S1420). In detail, motion compensation for the current block may be performed based on the inter prediction direction, the reference picture index, and the motion vector of the current block.

As described with reference to FIGS. 14 through 16, inter prediction of a 360 degree projection image may be performed in units of blocks, and may be performed based on motion information of a current block. For example, when performing inter prediction on a 360 degree projection image, the prediction block of the current block to be encoded / decoded in the current picture may be derived from a region most similar to the prediction block in the reference picture. In this case, the reference block in the reference picture used to derive the prediction block of the current block may be located at the same face or at a different face than the current block.

As in the example shown in FIG. 17, the reference block in the reference picture used to derive the predictive block of the current block may exist at the same face as the current block in the current picture (see (b)), or the current block in the current picture. May be at a different face than (see (c)). Or, the reference block may span two or more faces (see (a)).

The reference picture including the reference block may be a picture different from the current picture in a temporal order or output order (POC).

Alternatively, the current picture may be used as a reference picture. For example, a block encoded / decoded before a current block in a current picture including the current block may be set as a reference block of the current block.

As in the illustrated example, the predictive block of the current block may be derived from a reference block included in the same face as the current block or a reference block included in a different face from the current block. In this case, the position of the reference block may be specified through a motion vector between the reference block from the corresponding position block corresponding to the position of the current block in the reference picture.

As another example, in order to reduce the amount of data required to encode / decode a motion vector, the current block using information for specifying a face including a reference block and / or a motion vector for specifying a position of a reference block within the face. It is also possible to perform the motion compensation of. A face including a reference block in the reference picture may be referred to as a 'reference face'.

The information for specifying a face including a reference block includes information indicating whether the reference block belongs to the same face as the current block and / or information for identifying a face including the reference block (eg, a reference face index). It may include at least one. For example, a 1-bit flag can be used to determine whether the reference block belongs to the same face as the current block. In addition, a reference face index may be used to specify a face including a reference block in the reference picture.

As in the example illustrated in FIG. 18, a reference face index 'mc_face_idx' (or 'ref_face_idx') for identifying a face including a reference block may be defined. The reference face index may be encoded / decoded through the bitstream.

As another example, a reference face index may be derived from a neighboring block neighboring the current block. For example, under merge mode, the reference face index of the current block may be derived from the merge candidate merged with the current block. In contrast, under the AMVP mode, the face index of the current block may be encoded / decoded through the bitstream.

If the reference block spans a boundary between two faces, the reference face index may specify the face that contains the reference position of the reference block. Here, the reference position may include the position of a specific corner (eg, the upper left sample) in the reference block or the center point of the reference block.

The position of the reference block in the face may be specified based on the vector value from the reference position of the reference face to the reference position of the reference block. Here, the reference position of the reference face may be a position of a specific corner (eg, the position of the upper left reference sample) or the center point of the face.

Alternatively, the reference position of the reference face may be variably determined according to the index of the face including the current block (ie, the current face index), the reference face index, the relative position between the current face and the reference face, or the position of the current block in the face. have. For example, when the current block exists at a first position in the first face, a second position corresponding to the first position in the reference face may be determined as the reference position. As another example, if the current face is to the left of the reference face, the reference position of the reference face is set to the upper left corner, and if the current face is to the top of the reference face, the reference position of the reference face is set to the top center. Can be. The motion vector from the reference position in the face to the reference block may be referred to as a face vector.

Whether the motion vector is a face vector may be determined based on whether the current face and the reference face are the same (ie, whether the current face index and the reference face index are the same). For example, when the current face index and the reference face index are the same, the motion vector may indicate a vector from the current block to the reference block. On the other hand, when the current face index and the reference face index are different, the motion vector may indicate a vector from the reference position in the reference face to the reference block.

Alternatively, information indicating whether a motion vector is a face vector may be encoded / decoded through a bitstream.

The motion vector (eg, the face vector or non-face vector) of the current block may be encoded / decoded through the bitstream. For example, the motion vector value may be encoded / decoded through the bitstream as it is.

Alternatively, according to the inter prediction mode of the current block, the motion vector may be encoded / decoded through a bitstream or a motion vector of the current block may be derived from a neighboring block. For example, when the inter prediction mode of the current block is an AMVP mode, the motion vector of the current block may be encoded / decoded using differential coding. Here, differential coding refers to encoding / decoding a difference between a motion vector of a current block and a motion vector prediction value through a bitstream. The motion vector prediction value may be derived from the spatial / temporal neighboring block of the current block. Alternatively, the motion vector of the current block may be derived in the same manner as the spatial / temporal neighboring block of the current block. On the other hand, when the inter prediction mode of the current block is the merge mode, the motion vector of the current block may be set to be the same as the motion vector of the spatial / temporal neighboring block of the current block.

When the motion vector of the current block is different from the motion vector of the neighboring block, the motion vector of the current block may be derived according to the motion vector of the current block. For example, if the motion vector of the current block is a non-face vector, while the motion vector of the neighboring block is a face vector, the neighboring block, using the vector between the neighboring block and the reference face reference point of the neighboring block and the face vector of the neighboring block, The face vector of can be converted to a non-face vector. According to the inter prediction mode of the current block, the motion vector of the current block may be derived based on the transformed non-face vector of the neighboring block.

As another example, the encoding / decoding method of the motion vector of the current block may be differently determined according to whether the motion vector of the current block is a face vector or a non-face vector. For example, when the motion vector of the current block is a non-face vector, the motion vector of the current block is derived using the motion vector of the neighboring block, whereas if the motion vector of the current block is a face vector, the face vector value is bit-changed as it is. It may also be encoded / decoded through a stream.

As described through the above example, in the 360 degree projection image, motion compensation of the current block may be performed through a reference block belonging to a different face from the current block. However, when at least one of a phase, a size, or a shape of a face including a current block and a face including a reference block is different, it is difficult to search for a reference block that matches the prediction block of the current block within the reference face. For example, under TPP, it is difficult to have similarities between blocks belonging to the front face and blocks belonging to the right face because the front face and the right face have different sizes and shapes. Accordingly, when it is desired to perform motion estimation or motion compensation from a reference face having a phase, magnitude or shape different from that of the current face, a transformation that matches the phase, magnitude or shape of the current face and the reference face needs to be performed.

Hereinafter, a method of performing inter prediction according to whether a current block and a reference block belong to the same face (or whether they belong to a mutually corresponding face) will be described in more detail.

If the face containing the current block and the reference block are the same (that is, the current face index and the reference face index are the same), the motion vector is the coordinate difference between the starting point between the current block and the starting point of the reference block, as in a 2D image. Can be used as a motion vector.

If the face that contains the current block and the reference block is different (that is, the current face index and the reference face index are different), and at least one of the size, shape, or phase between the current face and the reference face is different, the face to which the reference block belongs. May be modified according to the size, shape, or phase of the face to which the prediction block belongs. For example, the reference face may be transformed using at least one of phase warping, interpolation and / or padding. As an example, FIG. 21 is a diagram illustrating an example of modifying a reference face to a current face. If the size and / or shape of the current face and the reference face are different, apply phase transformation, padding or interpolation to the reference face, as in the example shown in FIG. 21, to make the reference face the same size and / or shape as the current face. It can be modified to have a. In modifying the reference face, at least one of phase modification, padding, and / or interpolation may be omitted, and modification of the reference face may be performed in a different order than that shown in FIG. 21.

A reference face modified for the current face may be referred to as a reference face for motion compensation.

The motion compensation reference face can be interpolated with a predefined precision (eg, quarter pel or integer pel, etc.). A block most similar to the prediction block of the current block in the interpolated motion compensation reference face may be generated as the prediction block of the current block. As in the example shown in FIG. 20, the motion vector of the current block may indicate a coordinate difference between the start position of the current block and the start position of the reference block (ie, encoding / decoding a non-face vector). Although not shown, it is also possible to set the coordinate difference between the reference position in the motion compensation reference face to the start position of the reference block as the motion vector of the current block (ie, encode / decode the face vector).

In FIG. 20 and FIG. 21, the reference face is modified to match the phase, size, or shape of the current face. Conversely, it is also possible to derive the motion vector of the current block by modifying the current face to match the phase, size or shape of the reference face.

As in the above example, if at least one of the phase, magnitude, or shape between the current face and the reference face is different, the inter prediction is performed by modifying at least one of the phase, magnitude, or shape between the current face and the reference face. Can be.

Referring to FIG. 22, first, information about a reference face may be decoded from a bitstream (S2210). When the information about the reference face is decoded, it may be determined whether the current block and the reference block belong to the same face based on the decoded information (S2220).

The information about the reference face may include at least one of whether the current block and the reference block belong to the same face or a reference face index.

For example, 'isSameFaceFlag' indicating whether a face to which a current block belongs and a face to which a reference block belongs to each other or whether the current face index and the reference face index are the same may be signaled through the bitstream. A value of isSameFaceFlag of 1 may mean that the current face index and the reference face index have the same value, or that the face to which the current block belongs and the face to which the reference block belongs correspond to each other. On the other hand, a value of isSameFaceFlag of 0 may mean that the current face and the reference face index have different values or that the face to which the current block belongs and the face to which the reference block belongs do not correspond to each other.

The reference face index may be signaled only when the value of isSameFaceFlag is 0. Alternatively, signaling of the isSameFaceFlag may be omitted and the reference face index may be signaled essentially. If signaling of isSameFaceFlag is omitted, it may be determined whether the current block and the reference block belong to the same face by comparing the current face index and the reference face index.

If it is determined that the current block and the reference block are included in the same face, a motion vector indicating a coordinate difference between the current block and the reference block in the reference face is obtained (S2230), and motion compensation is performed using the obtained motion vector. It may be performed (S2240).

On the other hand, if it is determined that the current block and the reference block are included in different faces, a motion compensation reference face obtained by modifying at least one of a phase, a size, or a shape of the reference face to the current face may be generated (S2250). When the motion vector reference face is generated, a motion vector indicating a coordinate difference between the current block and the reference block in the motion compensation reference face may be obtained, and motion compensation may be performed using the obtained motion vector.

Even if the current block and the reference block belong to different faces, the process of generating a motion vector reference face may be omitted if the phase, magnitude, or shape between the current face and the reference face are the same.

As another example, it may be determined whether to modify the reference face based on whether the reference block belongs to a specific face. For example, in a 360 degree projection image based on TPP, a flag indicating whether a reference block exists at the front face may be signaled. isRefInFrontFlag indicates whether the reference block exists at the front face. If the value is 1, the start point of the reference block exists at the front face. If the value is 0, the start point of the reference block is right, left, top, It can be present at the bottom or back face. If both the current block and the reference block belong to the front face, or if both the current block and the reference block do not belong to the front face, the process of generating the motion compensation reference face may be omitted. On the other hand, when one of the current block and the reference block belongs to the front side and the other does not belong to the front side, the motion compensation reference face may be generated and the reference block of the current block may be specified in the generated motion compensation reference face.

The motion compensation of the current block in the 360 degree projection image may be limited to using only reference blocks belonging to the same face as the current block. Motion estimation and motion compensation for the current block may be performed on a reference block belonging to the same face as the current block. For example, as in the example shown in (c) of FIG. 17, motion compensation may not be performed, such as when the current block and the reference block belong to different faces. The face to which the reference block belongs may be determined based on the position of the reference point of the reference block. Here, the reference point of the reference block may be a corner sample, a center point, or the like of the reference block. For example, even when the reference block is located over the boundary of two faces, if the reference point of the reference block belongs to the same face as the current face, it may be determined that the reference block belongs to the same face as the current block.

Whether motion compensation may be performed using a reference block belonging to a different face from the current block may be adaptively determined based on the projection method, the size / shape of the face, the size difference between the faces, and the like. Alternatively, information (eg, a flag) indicating whether motion compensation can be performed using a reference block belonging to a different face from the current block may be signaled through the bitstream.

The motion compensation of the current block may be performed based on a reference block generated by interpolation, padding, or phase shifting of pixels belonging to the reference face corresponding to the current face. For example, if a reference block spans two or more faces, and a reference point of the reference block belongs to a reference face corresponding to the current face, the reference block corresponds to a reference face corresponding to the current face (hereinafter referred to as a first face). It may include a first region belonging to and a second region belonging to a reference face outside the current face (hereinafter referred to as a second face).

In this case, the pixels of the second region may be generated by copying or interpolating the samples included in the first face, or a predetermined filter may be applied to the samples and / or the pixels of the second face. You can also create pixels. The predetermined filter may mean a weighted filter, an average filter, an interpolation filter, or the like. The pixel region to which the filter is applied may be all regions belonging to the first face and / or the second face, or may be some regions. Here, the partial region may be the first region and the second region, or may be a region of a size / type predefined by the encoder / decoder. The filter may be applied to one or more pixels adjacent to the boundary of the first and second faces.

As in the example shown in FIG. 23, a sample included in a boundary of a reference face (first face) corresponding to a current face is padded (or copied) and / or interpolated, or a sample included in a first face, Based on the reference block generated by applying a filter between samples included in the second face adjacent to the first face, motion compensation for the current block may be performed.

In the example illustrated in FIG. 23, a padding area is generated by padding a sample included in a front face to which a reference point of a reference block belongs, and using the sample included in the padding area, motion compensation is performed for the current block. It became.

Alternatively, after generating a motion reference compensation reference face in which all or part of the second face is phase-shifted using the value of the first face, motion compensation for the current block may be performed using the generated motion compensation reference face. have.

As in the example shown in FIG. 24, by performing at least one of deformation, interpolation, or padding on all or a portion of the second face that includes some region of the reference block but does not include a reference point of the reference block, A motion compensation reference face may be generated. Accordingly, motion compensation for the current block may be performed using a sample belonging to the motion compensation reference face.

Information indicating whether to use a reference block generated based on a sample value belonging to a reference face corresponding to the current face for motion compensation may be encoded / decoded through a bitstream. The information is a 1-bit flag. For example, a flag value of 0 indicates that a reference block generated based on a sample value belonging to a reference face corresponding to the current face is not used for motion compensation. 1 may indicate that a reference block generated based on a sample value belonging to a reference face corresponding to the current face may be used for motion compensation of the current block.

Depending on the projection method of the 3D data, the size / shape between faces may be different. For example, under the TPP projection method, the front face may be larger than the residual face. Small faces have a relatively small amount of information compared to large faces. As a result, the motion vector accuracy at the small face can be increased, and the coding efficiency can be increased. That is, the motion vector precision may be adaptively determined according to the size / shape of the reference face in which the reference block is included.

For example, in a TPP-based 360 degree projection image, when the reference block belongs to the front face, motion compensation is performed using a quarter pel (1 / 4pel), while the reference block has a smaller right face and left than the front face. In the case of a face, an upper face or a lower face, motion compensation may be performed using an octopel.

Conversely, it is also possible to use smaller motion vector precision with a larger reference face, and larger motion vector precision with a smaller reference face.

The reference block of the current block may be derived from a face having a neighbor with the current face. That is, a face that does not have a neighbor with the current face cannot be used as a reference face, and blocks belonging to the face cannot be used as reference blocks of the current block. Whether the current face and the predetermined face have contiguity includes whether the current face and the reference face are spatially neighboring or whether the difference between the index of the current face and the index of the reference face is larger than a predetermined threshold. Can be determined on the basis. Here, whether the current face and the reference face are spatially neighboring may be determined based on the 3D space or may be determined based on the 2D plane. Alternatively, neighbors between faces may be predefined in the encoder and the decoder.

For example, in 3D space, a face that does not spatially neighbor a current face cannot be used as a reference face. For example, in a hexahedron, the face corresponding to the opposite face of the current face may not be used as a reference face as it is not spatially neighboring the current face.

As in the example shown in FIG. 25, when the current block is included in the front face, in 3D space, the back face opposite to the front face cannot be used as the reference face of the current block. Similarly, in performing motion compensation of a block included in the back face, the front face cannot be used as the reference face.

This can also be applied between the left and right faces and between the top and bottom faces.

If a reference block is included in a face that is not adjacent to the current face, motion compensation may be performed by using a predetermined position region in a picture different from the current picture. Here, the predetermined location area may mean a block that is the same location as the current block or a block adjacent to the same location block in a specific direction.

As another example, availability of the motion vector, the merge candidate, or the motion vector candidate may be determined according to the position of the motion vector of the current block or the reference block indicated by the merge candidate. For example, when the motion vector of the current block or the reference block indicated by the merge candidate of the current block is located at a face that does not have a neighbor with the current face, it may be determined that the motion vector or the merge candidate is unavailable. The unusable merge candidate may be excluded from the merge candidate list.

When projecting a 360 degree image onto the 2D plane, the faces may have different directions from each other, depending on the projection conversion format. For example, when a 360 degree image approximated as a polyhedron is developed in a 2D plane, in order to increase encoding / decoding efficiency, frame packing may be performed by modifying or rotating faces.

In the example shown in FIG. 26, the faces (faces 0, 1, 2) included in the bottom row are rotated 90 degrees clockwise relative to the faces (faces 3, 4, 5) included in the top row. Has been shown. As in the example shown in Fig. 26, in consideration of the spatial continuity between the faces, as the faces are rotated and arranged, the faces have different directions from each other.

When the reference face has a direction different from the current face, motion compensation for the current block may be performed by rotating the reference face or the reference block according to the directionality of the reference face. Referring to FIG. 27, a method of performing motion compensation in consideration of inter-face directionality will be described in more detail.

First, a reference face or reference block of a current block may be specified (S2710). The reference face of the current block may be specified by a reference face index or by a reference block specified by the motion vector of the current block.

When the reference face or the reference block of the current block is determined, it may be determined whether the current face and the reference face have the same direction (S2720). Whether the current face and the reference face have the same direction may be determined based on rotation information related to the rotation of the face. The rotation information may include information about whether to rotate, the rotation direction, the rotation angle, or the order / position of allocating a sample of the reference block to the prediction block. The rotation information may be encoded in the encoder and signaled through the bitstream, or may be derived based on the inter-face directionality.

Alternatively, whether the face rotates or the rotation angle may be predefined in the encoder and the decoder according to the position of the face.

When the current face and the reference face have the same direction, a prediction block for the current block may be generated using the reference block included in the reference face (S2730).

On the contrary, when the current face and the reference face have different directions, the reference face or the reference block included in the reference face may be rotated to have the same direction as the current face (S2740). In operation S2750, the prediction block for the current block may be generated using the reference block or the rotated reference block included in the rotated reference face.

If the reference face is specified by the reference face index or the motion vector of the current block, it may be determined whether the reference face has the same direction as the current face. For example, in the example illustrated in FIG. 28, since the reference face 1 is rotated 90 degrees with respect to the current face 4, it may be determined that the current face and the reference face have different directions.

When the current face and the reference face have different directions, the reference face or the reference block included in the reference face may be rotated to have the same direction as the current face. For example, after rotating the reference face according to the direction of the current face, a reference block can be obtained from the rotated reference face. When the reference block is obtained, the prediction block of the current block may be generated using the obtained reference block.

Unlike in the example shown in FIG. 28, after acquiring an understanding reference block in a motion vector, the obtained reference block may be rotated according to the direction of the current face. In this case, the predicted block of the current block may be generated using the rotated reference block.

In the example described with reference to FIGS. 27 and 28, the reference face or reference block is illustrated as being rotated in accordance with the direction of the current block. Unlike in the illustrated example, it is also possible to perform motion compensation by rotating the reference picture or by rotating the current face or the current block in the direction of the reference face. Alternatively, it is possible to rotate both the current face and the reference face according to the predefined direction.

In the above example, it is assumed that a picture composed of a plurality of faces can be used as a reference picture. As another example, each face may be used as a reference picture, or a predetermined number of face sets may be used as the reference picture. Alternatively, in a 360 degree projection image based on TPP, only the front face may be used as the reference picture, or the front face may be used as the reference picture and other sets of faces may be used as the reference picture.

Although the above-described embodiments are described based on a series of steps or flowcharts, this does not limit the time-series order of the invention and may be performed simultaneously or in a different order as necessary. In addition, in the above-described embodiment, each component (for example, a unit, a module, etc.) constituting the block diagram may be implemented as a hardware device or software, and a plurality of components are combined into one hardware device or software. It may be implemented. The above-described embodiments may be implemented in the form of program instructions that may be executed by various computer components, and may be recorded in a computer-readable recording medium. The computer-readable recording medium may include program instructions, data files, data structures, etc. alone or in combination. Examples of computer-readable recording media include magnetic media such as hard disks, floppy disks and magnetic tape, optical recording media such as CD-ROMs, DVDs, and magneto-optical media such as floptical disks. media), and hardware devices specifically configured to store and execute program instructions, such as ROM, RAM, flash memory, and the like. The hardware device may be configured to operate as one or more software modules to perform the process according to the invention, and vice versa.

The present invention can be applied to an electronic device capable of encoding / decoding an image.

Claims

Obtaining motion information for the current block;

Specifying a reference face of a current block in a reference picture based on the motion information;

Determining whether a current face including the current block and the reference face have the same direction; And

And based on the determination result, performing motion compensation on the current block.
According to claim 1,

When the reference face has a direction different from the current face, performing the motion compensation may include:

Rotating the reference face to have the same direction as the current face;

Specifying a reference block of the current block from the rotated reference face; And

Generating a prediction block of the current block by using the reference block.
According to claim 1,

When the reference face has a direction different from the current face, performing the motion compensation may include:

Specifying a reference block of the current block within the reference face;

Rotating the reference block to have the same direction as the current face; And

Generating a prediction block of the current block by using the rotated reference block.
According to claim 1,

Whether the current face and the face have the same direction is determined based on rotation information of at least one of the current face and the reference face.
The method of claim 4, wherein

The rotation information indicates whether at least one of the current face or the reference face is rotated, a rotation direction of the current face or the reference face, or a rotation angle of the current face or the reference face. Way.
Obtaining motion information for the current block;

Specifying a reference face of a current block in a reference picture based on the motion information;

Determining whether a current face including the current block and the reference face have the same direction; And

And based on the determination result, performing motion compensation on the current block.
The method of claim 6,

When the reference face has a direction different from the current face, performing the motion compensation may include:

Rotating the reference face to have the same direction as the current face;

Specifying a reference block of the current block from the rotated reference face; And

And generating a prediction block of the current block by using the reference block.
The method of claim 6,

When the reference face has a direction different from the current face, performing the motion compensation may include:

Specifying a reference block of the current block within the reference face;

Rotating the reference block to have the same direction as the current face; And

And generating a prediction block of the current block by using the rotated reference block.
The method of claim 6,

Whether the current face and the face have the same direction is determined based on rotation information of at least one of the current face or the reference face.
The method of claim 9,

The rotation information indicates whether at least one of the current face or the reference face is rotated, a rotation direction of the current face or the reference face, or a rotation angle of the current face or the reference face. Way.
Obtains motion information about a current block, specifies a reference face of a current block in a reference picture based on the motion information, determines whether a current face including the current block and the reference face have the same direction, and And an inter predictor configured to perform motion compensation on the current block based on the determination result.