WO2017047900A1

WO2017047900A1 - Method for processing image on basis of inter prediction mode, and device therefor

Info

Publication number: WO2017047900A1
Application number: PCT/KR2016/004599
Authority: WO
Inventors: 서정동; 임재현; 박내리
Original assignee: 엘지전자(주)
Priority date: 2015-09-18
Filing date: 2016-05-02
Publication date: 2017-03-23

Abstract

A method for processing an image on the basis of an inter prediction mode, and a device therefor are disclosed. Particularly, the method for processing an image on the basis of inter prediction can comprise the steps of: deriving a reference index and a motion vector of a current block; selecting a reference picture for the current block by using the reference index within a reference picture buffer for storing a reconstructed picture of which spatial resolution is reduced by a predetermined reduction ratio; adjusting the selected reference picture so as to increase the spatial resolution of the selected reference picture while taking into consideration the reduction ratio; and generating a prediction block for the current block, by using the motion vector, on the basis of the reference picture of which the spatial resolution is increased.

Description

Inter prediction mode based image processing method and apparatus therefor

The present invention relates to a still image or moving image processing method, and more particularly, to a method for encoding / decoding a still image or moving image based on an inter prediction mode and an apparatus supporting the same.

Compression coding refers to a series of signal processing techniques for transmitting digitized information through a communication line or for storing in a form suitable for a storage medium. Media such as an image, an image, an audio, and the like may be a target of compression encoding. In particular, a technique of performing compression encoding on an image is called video image compression.

Next-generation video content will be characterized by high spatial resolution, high frame rate and high dimensionality of scene representation. Processing such content would result in a tremendous increase in terms of memory storage, memory access rate, and processing power.

Accordingly, there is a need to design coding tools for more efficiently processing next generation video content.

In conventional compression techniques of still or moving images, inter prediction is performed through motion prediction and motion compensation. However, in order to compensate for the motion, the reference picture moves by the motion vector to refer to the pixel information of the corresponding region. In this case, when the size of the reference picture is large, the data bandwidth increases. In particular, as the resolution of the input image increases from 4K (kilo pixel) to 8K, there is a problem in that the bandwidth of data for motion compensation is greatly increased.

In order to solve the above problems, the present invention proposes a method of reducing and storing the size of a reference picture.

In addition, the present invention proposes a method of re-enlarging a reference picture according to a reduction factor in a motion compensation process.

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.

An aspect of the present invention provides a method of processing an image based on inter prediction, comprising: deriving a motion vector and a reference index of a current block at a predetermined reduction ratio; Selecting a reference picture for the current block using the reference index in a reference picture buffer storing a reconstructed picture having a reduced spatial resolution; Adjusting the selected reference picture to increase the spatial resolution of a reference picture, and generating a prediction block for the current block using the motion vector based on the reference picture with the expanded spatial resolution. .

An aspect of the present invention is a device for processing an image based on inter prediction, comprising: a step motion parameter deriving unit for deriving a motion vector and a reference index of a current block A reference picture selection unit for selecting a reference picture for the current block by using the reference index in a reference picture buffer storing a reconstructed picture having a reduced spatial resolution at a reduction ratio of Generates a prediction block for the current block using the motion vector based on an image adjusting unit for adjusting the selected reference picture to increase the spatial resolution of the selected reference picture in consideration of a reduction ratio and the reference picture on which the interpolation filtering is performed. It may include a prediction block generator.

Preferably, interpolation filtering may be performed such that the selected reference picture has a spatial resolution increased by an inverse of the reduction ratio.

Preferably, when a motion vector in units of 1 / N pixels is supported, the interpolation filtering may be performed such that the spatial resolution of the selected reference picture is increased by N times the inverse of the reduction ratio.

Preferably, the selected reference picture can be enlarged to increase the spatial resolution by the inverse of the reduction ratio.

Preferably, when a motion vector in units of 1 / N pixels is supported, interpolation filtering may be performed on the enlarged reference picture so that the spatial resolution is enlarged by N times.

Preferably, the reduction ratio may be fixed in advance, or may be signaled from an encoder in units of sequence, picture, slice, and block.

According to an embodiment of the present invention, the storage space of the reference picture can be reduced by reducing the spatial resolution of the reference picture.

In addition, according to an embodiment of the present invention, it is possible to reduce the data bandwidth of motion compensation that frequently occurs during video encoding / decoding.

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

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, included as part of the detailed description in order to provide a thorough understanding of the present invention, provide embodiments of the present invention and together with the description, describe the technical features of the present invention.

1 is a schematic block diagram of an encoder in which encoding of a still image or video signal is performed according to an embodiment to which the present invention is applied.

2 is a schematic block diagram of a decoder in which encoding of a still image or video signal is performed according to an embodiment to which the present invention is applied.

3 is a diagram for describing a partition structure of a coding unit that may be applied to the present invention.

4 is a diagram for explaining a prediction unit applicable to the present invention.

5 is a diagram illustrating a direction of inter prediction as an embodiment to which the present invention may be applied.

6 illustrates integer and fractional sample positions for quarter sample interpolation, as an embodiment to which the present invention may be applied.

7 illustrates a position of a spatial candidate as an embodiment to which the present invention may be applied.

8 is a diagram illustrating an inter prediction method as an embodiment to which the present invention is applied.

9 is a diagram illustrating a motion compensation process as an embodiment to which the present invention may be applied.

10 is a diagram illustrating a block diagram of an encoder according to an embodiment of the present invention.

11 is a diagram illustrating a block diagram of a decoder according to an embodiment of the present invention.

12 is a diagram illustrating a process of reducing and storing a picture reconstructed in a reference picture buffer according to an embodiment of the present invention.

13 is a diagram illustrating an inter prediction based image processing method according to an embodiment of the present invention.

14 is a diagram illustrating a block diagram of an encoder according to an embodiment of the present invention.

15 is a diagram illustrating a block diagram of a decoder according to an embodiment of the present invention.

16 is a diagram illustrating an inter prediction based image processing method according to an embodiment of the present invention.

17 is a diagram illustrating an inter prediction unit according to an embodiment of the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The detailed description, which will be given below with reference to the accompanying drawings, is intended to explain exemplary embodiments of the present invention and is not intended to represent the only embodiments in which the present invention may be practiced. The following detailed description includes specific details in order to provide a thorough understanding of the present invention. However, one of ordinary skill in the art appreciates that the present invention may be practiced without these specific details.

In some instances, well-known structures and devices may be omitted or shown in block diagram form centering on the core functions of the structures and devices in order to avoid obscuring the concepts of the present invention.

In addition, the terminology used in the present invention was selected as a general term widely used as possible now, in a specific case will be described using terms arbitrarily selected by the applicant. In such a case, since the meaning is clearly described in the detailed description of the part, it should not be interpreted simply by the name of the term used in the description of the present invention, and it should be understood that the meaning of the term should be interpreted. .

Specific terms used in the following description are provided to help the understanding of the present invention, and the use of such specific terms may be changed to other forms without departing from the technical spirit of the present invention. For example, signals, data, samples, pictures, frames, blocks, etc. may be appropriately replaced and interpreted in each coding process.

Hereinafter, in the present specification, the 'processing unit' refers to a unit in which a process of encoding / decoding such as prediction, transformation, and / or quantization is performed. Hereinafter, for convenience of description, the processing unit may be referred to as a 'processing block' or 'block'.

The processing unit may be interpreted to include a unit for the luma component and a unit for the chroma component. For example, the processing unit may correspond to a Coding Tree Unit (CTU), a Coding Unit (CU), a Prediction Unit (PU), or a Transform Unit (TU).

In addition, the processing unit may be interpreted as a unit for a luma component or a unit for a chroma component. For example, the processing unit may be a coding tree block (CTB), a coding block (CB), a prediction block (PU), or a transform block (TB) for a luma component. May correspond to. Or, it may correspond to a coding tree block (CTB), a coding block (CB), a prediction block (PU), or a transform block (TB) for a chroma component. In addition, the present invention is not limited thereto, and the processing unit may be interpreted to include a unit for a luma component and a unit for a chroma component.

In addition, the processing unit is not necessarily limited to square blocks, but may also be configured in a polygonal form having three or more vertices.

Referring to FIG. 1, the encoder 100 may include an image divider 110, a subtractor 115, a transform unit 120, a quantizer 130, an inverse quantizer 140, an inverse transform unit 150, and a filtering unit. 160, a decoded picture buffer (DPB) 170, a predictor 180, and an entropy encoder 190. The predictor 180 may include an inter predictor 181 and an intra predictor 182.

The image divider 110 divides an input video signal (or a picture or a frame) input to the encoder 100 into one or more processing units.

The subtractor 115 subtracts the difference from the prediction signal (or prediction block) output from the prediction unit 180 (that is, the inter prediction unit 181 or the intra prediction unit 182) in the input image signal. Generate a residual signal (or difference block). The generated difference signal (or difference block) is transmitted to the converter 120.

The transform unit 120 may convert a differential signal (or a differential block) into a transform scheme (eg, a discrete cosine transform (DCT), a discrete sine transform (DST), a graph-based transform (GBT), and a karhunen-loeve transform (KLT)). Etc.) to generate transform coefficients. In this case, the transform unit 120 may generate transform coefficients by performing a transform using a transform mode determined according to the prediction mode applied to the difference block and the size of the difference block.

The quantization unit 130 quantizes the transform coefficients and transmits the transform coefficients to the entropy encoding unit 190, and the entropy encoding unit 190 entropy codes the quantized signals and outputs them as bit streams.

Meanwhile, the quantized signal output from the quantization unit 130 may be used to generate a prediction signal. For example, the quantized signal may recover the differential signal by applying inverse quantization and inverse transformation through an inverse quantization unit 140 and an inverse transformation unit 150 in a loop. A reconstructed signal may be generated by adding the reconstructed difference signal to a prediction signal output from the inter predictor 181 or the intra predictor 182.

Meanwhile, in the compression process as described above, adjacent blocks are quantized by different quantization parameters, thereby causing deterioration of the block boundary. This phenomenon is called blocking artifacts, which is one of the important factors in evaluating image quality. In order to reduce such deterioration, a filtering process may be performed. Through this filtering process, the image quality can be improved by removing the blocking degradation and reducing the error of the current picture.

The filtering unit 160 applies filtering to the reconstruction signal and outputs it to the reproduction apparatus or transmits it to the decoded picture buffer 170. The filtered signal transmitted to the decoded picture buffer 170 may be used as the reference picture in the inter prediction unit 181. As such, by using the filtered picture as a reference picture in the inter prediction mode, not only image quality but also encoding efficiency may be improved.

The decoded picture buffer 170 may store the filtered picture for use as a reference picture in the inter prediction unit 181.

The inter prediction unit 181 performs temporal prediction and / or spatial prediction to remove temporal redundancy and / or spatial redundancy with reference to a reconstructed picture.

Here, since the reference picture used to perform the prediction is a transformed signal that has been quantized and dequantized in units of blocks at the time of encoding / decoding in the previous time, blocking artifacts or ringing artifacts may exist. have.

Accordingly, the inter prediction unit 181 may interpolate the signals between pixels in sub-pixel units by applying a lowpass filter to solve performance degradation due to discontinuity or quantization of such signals. Herein, the sub-pixel refers to a virtual pixel generated by applying an interpolation filter, and the integer pixel refers to an actual pixel existing in the reconstructed picture. As the interpolation method, linear interpolation, bi-linear interpolation, wiener filter, or the like may be applied.

The interpolation filter may be applied to a reconstructed picture to improve the precision of prediction. For example, the inter prediction unit 181 generates an interpolation pixel by applying an interpolation filter to integer pixels, and uses an interpolated block composed of interpolated pixels as a prediction block. You can make predictions.

The intra predictor 182 predicts the current block by referring to samples in the vicinity of the block to which the current encoding is to be performed. The intra prediction unit 182 may perform the following process to perform intra prediction. First, reference samples necessary for generating a prediction signal may be prepared. The prediction signal may be generated using the prepared reference sample. Then, the prediction mode is encoded. In this case, the reference sample may be prepared through reference sample padding and / or reference sample filtering. Since the reference sample has been predicted and reconstructed, there may be a quantization error. Accordingly, the reference sample filtering process may be performed for each prediction mode used for intra prediction to reduce such an error.

The prediction signal (or prediction block) generated by the inter prediction unit 181 or the intra prediction unit 182 is used to generate a reconstruction signal (or reconstruction block) or a differential signal (or differential block). It can be used to generate.

Referring to FIG. 2, the decoder 200 includes an entropy decoder 210, an inverse quantizer 220, an inverse transformer 230, an adder 235, a filter 240, and a decoded picture buffer (DPB). Buffer Unit (250), the prediction unit 260 may be configured. The predictor 260 may include an inter predictor 261 and an intra predictor 262.

The reconstructed video signal output through the decoder 200 may be reproduced through the reproducing apparatus.

The decoder 200 receives a signal (ie, a bit stream) output from the encoder 100 of FIG. 1, and the received signal is entropy decoded through the entropy decoding unit 210.

The inverse quantization unit 220 obtains a transform coefficient from the entropy decoded signal using the quantization step size information.

The inverse transform unit 230 applies an inverse transform scheme to inverse transform the transform coefficients to obtain a residual signal (or a differential block).

The adder 235 outputs the obtained difference signal (or difference block) from the prediction unit 260 (that is, the prediction signal (or prediction block) output from the inter prediction unit 261 or the intra prediction unit 262. ) Generates a reconstructed signal (or a reconstruction block).

The filtering unit 240 applies filtering to the reconstructed signal (or the reconstructed block) and outputs the filtering to the reproducing apparatus or transmits the filtered signal to the decoded picture buffer unit 250. The filtered signal transmitted to the decoded picture buffer unit 250 may be used as the reference picture in the inter predictor 261.

In the present specification, the embodiments described by the filtering unit 160, the inter prediction unit 181, and the intra prediction unit 182 of the encoder 100 are respectively the filtering unit 240, the inter prediction unit 261, and the decoder of the decoder. The same may be applied to the intra predictor 262.

Processing unit split structure

In general, a still image or video compression technique (eg, HEVC) uses a block-based image compression method. The block-based image compression method is a method of processing an image by dividing the image into specific block units, and may reduce memory usage and calculation amount.

The encoder splits one image (or picture) into units of a coding tree unit (CTU) in a rectangular shape. In addition, one CTU is sequentially encoded according to a raster scan order.

In HEVC, the size of the CTU may be set to any one of 64 × 64, 32 × 32, and 16 × 16. The encoder may select and use the size of the CTU according to the resolution of the input video or the characteristics of the input video. The CTU includes a coding tree block (CTB) for luma components and a CTB for two chroma components corresponding thereto.

One CTU may be divided into a quad-tree structure. That is, one CTU has a square shape and is divided into four units having a half horizontal size and a half vertical size to generate a coding unit (CU). have. This partitioning of the quad-tree structure can be performed recursively. That is, a CU is hierarchically divided into quad-tree structures from one CTU.

CU refers to a basic unit of coding in which an input image is processed, for example, intra / inter prediction is performed. The CU includes a coding block (CB) for a luma component and a CB for two chroma components corresponding thereto. In HEVC, the size of a CU may be set to any one of 64 × 64, 32 × 32, 16 × 16, and 8 × 8.

Referring to FIG. 3, the root node of the quad-tree is associated with the CTU. The quad-tree is split until it reaches a leaf node, which corresponds to a CU.

More specifically, the CTU corresponds to a root node and has a smallest depth (ie, depth = 0). The CTU may not be divided according to the characteristics of the input image. In this case, the CTU corresponds to a CU.

The CTU may be divided into quad tree shapes, resulting in lower nodes having a depth of 1 (depth = 1). In addition, a node that is no longer divided (ie, a leaf node) in a lower node having a depth of 1 corresponds to a CU. For example, in FIG. 3 (b), CU (a), CU (b), and CU (j) corresponding to nodes a, b, and j are divided once in the CTU and have a depth of one.

At least one of the nodes having a depth of 1 may be split into a quad tree again, resulting in lower nodes having a depth of 1 (ie, depth = 2). In addition, a node (ie, a leaf node) that is no longer divided in a lower node having a depth of 2 corresponds to a CU. For example, in FIG. 3 (b), CU (c), CU (h) and CU (i) corresponding to nodes c, h and i are divided twice in the CTU and have a depth of two.

In addition, at least one of the nodes having a depth of 2 may be divided into quad tree shapes, resulting in lower nodes having a depth of 3 (ie, depth = 3). And, a node that is no longer partitioned (ie, a leaf node) in a lower node having a depth of 3 corresponds to a CU. For example, in FIG. 3 (b), CU (d), CU (e), CU (f), and CU (g) corresponding to nodes d, e, f, and g are divided three times in the CTU, Has depth.

In the encoder, the maximum size or the minimum size of the CU may be determined according to characteristics (eg, resolution) of the video image or in consideration of encoding efficiency. Information about this or information capable of deriving the information may be included in the bitstream. A CU having a maximum size may be referred to as a largest coding unit (LCU), and a CU having a minimum size may be referred to as a smallest coding unit (SCU).

In addition, a CU having a tree structure may be hierarchically divided with predetermined maximum depth information (or maximum level information). Each partitioned CU may have depth information. Since the depth information indicates the number and / or degree of division of the CU, the depth information may include information about the size of the CU.

Since the LCU is divided into quad tree shapes, the size of the SCU can be obtained by using the size and maximum depth information of the LCU. Or conversely, using the size of the SCU and the maximum depth information of the tree, the size of the LCU can be obtained.

For one CU, information indicating whether the corresponding CU is split (for example, a split CU flag split_cu_flag) may be transmitted to the decoder. This split mode is included in all CUs except the SCU. For example, if the flag indicating whether to split or not is '1', the CU is divided into 4 CUs again. If the flag indicating whether to split or not is '0', the CU is not divided further. Processing may be performed.

As described above, a CU is a basic unit of coding in which intra prediction or inter prediction is performed. HEVC divides a CU into prediction units (PUs) in order to code an input image more effectively.

The PU is a basic unit for generating a prediction block, and may generate different prediction blocks in PU units within one CU. However, PUs belonging to one CU are not mixed with intra prediction and inter prediction, and PUs belonging to one CU are coded by the same prediction method (ie, intra prediction or inter prediction).

The PU is not divided into quad-tree structures, but is divided once in a predetermined form in one CU. This will be described with reference to the drawings below.

The PU is divided differently according to whether an intra prediction mode or an inter prediction mode is used as a coding mode of a CU to which the PU belongs.

FIG. 4A illustrates a PU when an intra prediction mode is used, and FIG. 4B illustrates a PU when an inter prediction mode is used.

Referring to FIG. 4 (a), assuming that a size of one CU is 2N × 2N (N = 4,8,16,32), one CU has two types (ie, 2N × 2N or N). XN).

Here, when divided into 2N × 2N type PU, it means that only one PU exists in one CU.

On the other hand, when divided into N × N type PU, one CU is divided into four PUs, and different prediction blocks are generated for each PU unit. However, the division of the PU may be performed only when the size of the CB for the luminance component of the CU is the minimum size (that is, the CU is the SCU).

Referring to FIG. 4 (b), assuming that a size of one CU is 2N × 2N (N = 4,8,16,32), one CU has 8 PU types (ie, 2N × 2N). , N × N, 2N × N, N × 2N, nL × 2N, nR × 2N, 2N × nU, 2N × nD).

Similar to intra prediction, PU partitioning in the form of N × N may be performed only when the size of the CB for the luminance component of the CU is the minimum size (that is, the CU is the SCU).

In inter prediction, 2N × N splitting in the horizontal direction and N × 2N splitting in the vertical direction are supported.

In addition, it supports PU partitions of nL × 2N, nR × 2N, 2N × nU, and 2N × nD types, which are Asymmetric Motion Partition (AMP). Here, 'n' means a 1/4 value of 2N. However, AMP cannot be used when the CU to which the PU belongs is a CU of the minimum size.

In order to efficiently encode an input image within one CTU, an optimal partitioning structure of a coding unit (CU), a prediction unit (PU), and a transformation unit (TU) is subjected to the following process to perform a minimum rate-distortion. It can be determined based on the value. For example, looking at the optimal CU partitioning process in 64 × 64 CTU, rate-distortion cost can be calculated while partitioning from a 64 × 64 CU to an 8 × 8 CU. The specific process is as follows.

1) The partition structure of the optimal PU and TU that generates the minimum rate-distortion value is determined by performing inter / intra prediction, transform / quantization, inverse quantization / inverse transform, and entropy encoding for a 64 × 64 CU.

2) Divide the 64 × 64 CU into four 32 × 32 CUs and determine the optimal PU and TU partitioning structure that generates the minimum rate-distortion value for each 32 × 32 CU.

3) The 32 × 32 CU is subdivided into four 16 × 16 CUs, and a partition structure of an optimal PU and TU that generates a minimum rate-distortion value for each 16 × 16 CU is determined.

4) Subdivide the 16 × 16 CU into four 8 × 8 CUs and determine the optimal PU and TU partitioning structure that generates the minimum rate-distortion value for each 8 × 8 CU.

5) 16 × 16 blocks by comparing the sum of the rate-distortion values of the 16 × 16 CUs calculated in 3) above with the rate-distortion values of the four 8 × 8 CUs calculated in 4) above. Determine the partition structure of the optimal CU within. This process is similarly performed for the remaining three 16 × 16 CUs.

6) 32 × 32 block by comparing the sum of the rate-distortion values of the 32 × 32 CUs calculated in 2) above with the rate-distortion values of the four 16 × 16 CUs obtained in 5) above. Determine the partition structure of the optimal CU within. Do this for the remaining three 32x32 CUs.

7) Finally, compare the sum of the rate-distortion values of the 64 × 64 CUs calculated in step 1) with the rate-distortion values of the four 32 × 32 CUs obtained in step 6). The partition structure of the optimal CU is determined within the x64 block.

In the intra prediction mode, a prediction mode is selected in units of PUs, and prediction and reconstruction are performed in units of actual TUs for the selected prediction mode.

TU means a basic unit in which actual prediction and reconstruction are performed. The TU includes a transform block (TB) for a luma component and a TB for two chroma components corresponding thereto.

In the example of FIG. 3, as one CTU is divided into quad-tree structures to generate CUs, the TUs are hierarchically divided into quad-tree structures from one CU to be coded.

Since the TU is divided into quad-tree structures, the TU divided from the CU can be further divided into smaller lower TUs. In HEVC, the size of the TU may be set to any one of 32 × 32, 16 × 16, 8 × 8, and 4 × 4.

Referring again to FIG. 3, it is assumed that a root node of the quad-tree is associated with a CU. The quad-tree is split until it reaches a leaf node, which corresponds to a TU.

In more detail, a CU corresponds to a root node and has a smallest depth (that is, depth = 0). The CU may not be divided according to the characteristics of the input image. In this case, the CU corresponds to a TU.

The CU may be divided into quad tree shapes, resulting in lower nodes having a depth of 1 (depth = 1). In addition, a node (ie, a leaf node) that is no longer divided in a lower node having a depth of 1 corresponds to a TU. For example, in FIG. 3B, TU (a), TU (b), and TU (j) corresponding to nodes a, b, and j are divided once in a CU and have a depth of 1. FIG.

At least one of the nodes having a depth of 1 may be split into a quad tree again, resulting in lower nodes having a depth of 1 (ie, depth = 2). In addition, a node (ie, a leaf node) that is no longer divided in a lower node having a depth of 2 corresponds to a TU. For example, in FIG. 3B, TU (c), TU (h), and TU (i) corresponding to nodes c, h, and i are divided twice in a CU and have a depth of two.

In addition, at least one of the nodes having a depth of 2 may be divided into quad tree shapes, resulting in lower nodes having a depth of 3 (ie, depth = 3). And, a node that is no longer partitioned (ie, a leaf node) in a lower node having a depth of 3 corresponds to a CU. For example, in FIG. 3 (b), TU (d), TU (e), TU (f), and TU (g) corresponding to nodes d, e, f, and g are divided three times in a CU. Has depth.

A TU having a tree structure may be hierarchically divided with predetermined maximum depth information (or maximum level information). Each divided TU may have depth information. Since the depth information indicates the number and / or degree of division of the TU, it may include information about the size of the TU.

For one TU, information indicating whether the corresponding TU is split (for example, split TU flag split_transform_flag) may be delivered to the decoder. This partitioning information is included in all TUs except the smallest TU. For example, if the value of the flag indicating whether to split is '1', the corresponding TU is divided into four TUs again. If the value of the flag indicating whether to split is '0', the corresponding TU is no longer divided.

Prediction

The decoded portion of the current picture or other pictures in which the current processing unit is included may be used to reconstruct the current processing unit in which decoding is performed.

Intra picture or I picture (slice) using only the current picture for reconstruction, i.e. performing only intra-picture prediction, and picture (slice) using up to one motion vector and reference index to predict each unit A picture using a predictive picture or a P picture (slice), up to two motion vectors, and a reference index (slice) may be referred to as a bi-predictive picture or a B picture (slice).

Intra prediction means a prediction method that derives the current processing block from data elements (eg, sample values, etc.) of the same decoded picture (or slice). That is, a method of predicting pixel values of the current processing block by referring to reconstructed regions in the current picture.

Hereinafter, the inter prediction will be described in more detail.

Inter Inter prediction (or inter screen prediction)

Inter prediction means a prediction method of deriving a current processing block based on data elements (eg, sample values or motion vectors, etc.) of pictures other than the current picture. That is, a method of predicting pixel values of the current processing block by referring to reconstructed regions in other reconstructed pictures other than the current picture.

Inter prediction (or inter picture prediction) is a technique for removing redundancy existing between pictures, and is mostly performed through motion estimation and motion compensation.

Referring to FIG. 5, inter prediction includes uni-directional prediction that uses only one past picture or a future picture as a reference picture on a time axis with respect to one block, and bidirectional prediction that simultaneously refers to past and future pictures. Bi-directional prediction).

Also, uni-directional prediction includes forward direction prediction using one reference picture that is displayed (or output) before the current picture in time and 1 displayed (or output) after the current picture in time. It can be divided into backward direction prediction using two reference pictures.

The motion parameter (or information) used to specify which reference region (or reference block) is used to predict the current block in the inter prediction process (i.e., unidirectional or bidirectional prediction) is an inter prediction mode (where The inter prediction mode may indicate a reference direction (ie, unidirectional or bidirectional) and a reference list (ie, L0, L1, or bidirectional), a reference index (or reference picture index or reference list index), Contains motion vector information. The motion vector information may include a motion vector, a motion vector prediction (MVP), or a motion vector difference (MVD). The motion vector difference value means a difference value between the motion vector and the motion vector prediction value.

For unidirectional prediction, motion parameters for one direction are used. That is, one motion parameter may be needed to specify the reference region (or reference block).

Bidirectional prediction uses motion parameters for both directions. In the bidirectional prediction scheme, up to two reference regions may be used. The two reference regions may exist in the same reference picture or may exist in different pictures, respectively. That is, up to two motion parameters may be used in the bidirectional prediction scheme, and the two motion vectors may have the same reference picture index or different reference picture indexes. In this case, all of the reference pictures may be displayed (or output) before or after the current picture in time.

The encoder performs motion estimation to find the reference region most similar to the current processing block from the reference pictures in the inter prediction process. In addition, the encoder may provide a decoder with a motion parameter for the reference region.

The encoder / decoder may obtain a reference region of the current processing block using the motion parameter. The reference region exists in a reference picture having the reference index. In addition, the pixel value or interpolated value of the reference region specified by the motion vector may be used as a predictor of the current processing block. In other words, using motion information, motion compensation is performed to predict an image of a current processing block from a previously decoded picture.

In order to reduce the amount of transmission associated with the motion vector information, a method of acquiring a motion vector prediction value mvp using motion information of previously coded blocks and transmitting only a difference value mvd thereof may be used. That is, the decoder obtains a motion vector prediction value of the current processing block using motion information of other decoded blocks, and obtains a motion vector value for the current processing block using the difference value transmitted from the encoder. In obtaining the motion vector prediction value, the decoder may obtain various motion vector candidate values by using motion information of other blocks that are already decoded, and obtain one of them as the motion vector prediction value.

Reference picture set and reference picture list

In order to manage multiple reference pictures, a set of previously decoded pictures are stored in a decoded picture buffer (DPB) for decoding the remaining pictures.

The reconstructed picture used for inter prediction among the reconstructed pictures stored in the DPB is referred to as a reference picture. In other words, a reference picture refers to a picture including a sample that can be used for inter prediction in a decoding process of a next picture in decoding order.

A reference picture set (RPS) refers to a set of reference pictures associated with a picture, and is composed of all pictures previously associated in decoding order. The reference picture set can be used for inter prediction of an associated picture or of a picture that follows an associated picture in decoding order. That is, reference pictures maintained in the decoded picture buffer DPB may be referred to as a reference picture set. The encoder can provide the decoder with reference picture set information in a sequence parameter set (SPS) (ie, a syntax structure composed of syntax elements) or each slice header.

A reference picture list refers to a list of reference pictures used for inter prediction of a P picture (or slice) or a B picture (or slice). Here, the reference picture list may be divided into two reference picture lists, and may be referred to as reference picture list 0 (or L0) and reference picture list 1 (or L1), respectively. Also, a reference picture belonging to reference picture list 0 may be referred to as reference picture 0 (or L0 reference picture), and a reference picture belonging to reference picture list 1 may be referred to as reference picture 1 (or L1 reference picture).

In the decoding process of P pictures (or slices), one reference picture list (i.e., reference picture list 0) is used, and in the decoding process of B pictures (or slices), two reference picture lists (i.e., reference) Picture list 0 and reference picture list 1) may be used. Such information for distinguishing a reference picture list for each reference picture may be provided to the decoder through reference picture set information. The decoder adds the reference picture to the reference picture list 0 or the reference picture list 1 based on the reference picture set information.

A reference picture index (or reference index) is used to identify any one specific reference picture in the reference picture list.

Fractional sample interpolation

A sample of the prediction block for the inter predicted current processing block is obtained from sample values of the corresponding reference region in the reference picture identified by the reference picture index. Here, the corresponding reference region in the reference picture represents the region of the position indicated by the horizontal component and the vertical component of the motion vector. Fractional sample interpolation is used to generate predictive samples for noninteger sample coordinates, except when the motion vector has an integer value. For example, a motion vector of one quarter of the distance between samples may be supported.

For HEVC, fractional sample interpolation of luminance components applies an 8-tap filter in the horizontal and vertical directions, respectively. In addition, fractional sample interpolation of the color difference component applies a 4-tap filter in the horizontal direction and the vertical direction, respectively.

Referring to FIG. 6, the shaded block in which the upper-case letter (A_i, j) is written indicates the integer sample position, and the shaded block in which the lower-case letter (x_i, j) is written is the fractional sample position. Indicates.

Fractional samples are generated by applying interpolation filters to integer sample values in the horizontal and vertical directions, respectively. For example, in the horizontal direction, an 8-tap filter may be applied to four integer sample values on the left side and four integer sample values on the right side based on the fractional sample to be generated.

Inter prediction mode

In HEVC, a merge mode and advanced motion vector prediction (AMVP) may be used to reduce the amount of motion information.

1) Merge Mode

Merge mode refers to a method of deriving a motion parameter (or information) from a neighboring block spatially or temporally.

The set of candidates available in merge mode is composed of spatial neighbor candidates, temporal candidates and generated candidates.

Referring to FIG. 7A, it is determined whether each spatial candidate block is available according to the order of {A1, B1, B0, A0, B2}. In this case, when the candidate block is encoded in the intra prediction mode and there is no motion information, or when the candidate block is located outside the current picture (or slice), the candidate block is not available.

After determining the validity of the spatial candidate, the spatial merge candidate can be constructed by excluding unnecessary candidate blocks from candidate blocks of the current processing block. For example, when the candidate block of the current prediction block is the first prediction block in the same coding block, the candidate block having the same motion information may be excluded except for the corresponding candidate block.

When the spatial merge candidate configuration is completed, the temporal merge candidate configuration process is performed in the order of {T0, T1}.

In the temporal candidate configuration, when the right bottom block T0 of the collocated block of the reference picture is available, the block is configured as a temporal merge candidate. The colocated block refers to a block existing at a position corresponding to the current processing block in the selected reference picture. On the other hand, otherwise, the block T1 located at the center of the collocated block is configured as a temporal merge candidate.

The maximum number of merge candidates may be specified in the slice header. If the number of merge candidates is larger than the maximum number, the number of spatial candidates and temporal candidates smaller than the maximum number is maintained. Otherwise, the number of merge candidates is generated by combining the candidates added so far until the maximum number of candidates becomes the maximum (ie, combined bi-predictive merging candidates). .

The encoder constructs a merge candidate list in the above manner and performs motion estimation to merge candidate block information selected from the merge candidate list into a merge index (for example, merge_idx [x0] [y0] '). Signal to the decoder. In FIG. 7B, the B1 block is selected from the merge candidate list. In this case, “index 1” may be signaled to the decoder as a merge index.

The decoder constructs a merge candidate list similarly to the encoder, and derives the motion information of the current block from the motion information of the candidate block corresponding to the merge index received from the encoder in the merge candidate list. The decoder generates a prediction block for the current processing block based on the derived motion information (ie, motion compensation).

2) Advanced Motion Vector Prediction (AMVP) Mode

The AMVP mode refers to a method of deriving a motion vector prediction value from neighboring blocks. Thus, horizontal and vertical motion vector difference (MVD), reference index, and inter prediction modes are signaled to the decoder. The horizontal and vertical motion vector values are calculated using the derived motion vector prediction value and the motion vector difference (MVD) provided from the encoder.

That is, the encoder constructs a motion vector predictor candidate list and performs motion estimation to perform a motion estimation flag (ie, candidate block information) selected from the motion vector predictor candidate list (for example, mvp_lX_flag [x0] [y0). ] ') Is signaled to the decoder. The decoder constructs a motion vector predictor candidate list similarly to the encoder, and derives a motion vector predictor of the current processing block using the motion information of the candidate block indicated by the motion reference flag received from the encoder in the motion vector predictor candidate list. The decoder obtains a motion vector value for the current processing block by using the derived motion vector prediction value and the motion vector difference value transmitted from the encoder. The decoder generates a prediction block for the current processing block based on the derived motion information (ie, motion compensation).

In the AMVP mode, two spatial motion candidates are selected from among the five available candidates in FIG. 7. The first spatial motion candidate is selected from the set of {A0, A1} located on the left side, and the second spatial motion candidate is selected from the set of {B0, B1, B2} located above. At this time, when the reference index of the neighboring candidate block is not the same as the current prediction block, the motion vector is scaled.

If the number of candidates selected as a result of the search for the spatial motion candidate is two, the candidate configuration is terminated, but if less than two, the temporal motion candidate is added.

Referring to FIG. 8, a decoder (in particular, the inter prediction unit 261 of the decoder in FIG. 2) decodes a motion parameter for a processing block (eg, a prediction unit) (S801).

For example, if the processing block has a merge mode applied, the decoder may decode the merge index signaled from the encoder. The motion parameter of the current processing block can be derived from the motion parameter of the candidate block indicated by the merge index.

In addition, when the processing block is applied with the AMVP mode, the decoder may decode horizontal and vertical motion vector difference (MVD), reference index, and inter prediction mode signaled from the encoder. The motion vector prediction value may be derived from the motion parameter of the candidate block indicated by the motion reference flag, and the motion vector value of the current processing block may be derived using the motion vector prediction value and the received motion vector difference value.

The decoder performs motion compensation on the prediction unit by using the decoded motion parameter (or information) (S802).

That is, the encoder / decoder performs motion compensation for predicting an image of the current unit from a previously decoded picture by using the decoded motion parameter.

9 illustrates a case in which a motion parameter for a current block to be encoded in a current picture is unidirectional prediction, a second picture in LIST0, LIST0, and a motion vector (-a, b). do.

In this case, as shown in FIG. 9, the current block is predicted using values (ie, sample values of a reference block) at positions apart from the current block by (-a, b) in the second picture of LIST0.

In the case of bidirectional prediction, another reference list (eg, LIST1), a reference index, and a motion vector difference value are transmitted so that the decoder derives two reference blocks and predicts the current block value based on the reference block.

Inter Prediction-based Image Processing Method

Referring to FIG. 1 again, an image encoded and reconstructed by an encoder is stored in a decoded picture buffer (DPB) 170. These images are used in the motion prediction and motion compensation process of inter picture prediction.

Motion prediction refers to a process of finding a block that is most similar to the current block to be encoded in the reference picture, and motion compensation refers to a process of generating values close to the current block by referring to the most similar block.

Motion prediction and motion compensation are performed using a motion vector, which is a position difference between a current block and a reference block. The resolution of the motion vector supports 1/2 pixel, 1/4 pixel, etc. for accurate motion compensation.

In order to perform motion prediction / compensation using a motion vector of a pixel other than an integer unit, interpolation filtering, which is a process of generating a subpixel using an integer unit pixel, may be performed. Interpolation filtering is performed by the inter prediction unit 181 in FIG. 1.

Referring again to FIG. 2, the decoder performs motion compensation in the inter prediction unit 261, and interpolation filtering is applied in this process.

As such, inter picture prediction (or inter prediction) is performed through motion estimation and motion compensation in the encoding / decoding process of a video. In order to compensate for the motion, the reference picture moves by the motion vector to refer to the pixel information of the corresponding area. When the size of the reference picture is large, the bandwidth of the data increases.

Accordingly, in order to reduce the bandwidth of the data, in the present invention, the reference picture is reduced in size and stored in the buffer, and before the interpolation filter of the motion compensation process is performed, the reduced reference picture stored in the buffer is reduced in scale. We propose a method of performing motion compensation by zooming in to fit.

In the present specification, shrinking an image means a process of reducing spatial resolution, and enlarging an image means a process of increasing a spatial resolution.

For example, in order to reduce or enlarge an image, filtering in the form of a finite impulse response (FIR) may be used. Reducing an image using an FIR filter may have advantages in terms of implementation, similar to an interpolation filter used in motion compensation. In addition, non-linear filtering may be used for gain in the process of reducing / enlarging an image. However, the present invention is not limited thereto, and various methods of reducing / enlarging images may be used.

Example One

In this embodiment, a method of reducing and storing an image in a reference picture buffer (RPB) and performing interpolation filtering and reconstruction at the same time in the interpolation filtering step of motion compensation is proposed.

Referring to FIG. 10, the encoder includes an image splitter 1010, a subtractor 1015, a transform unit 1020, a quantizer 1030, an inverse quantizer 1040, an inverse transform unit 1050, and a filtering unit 1060. The image reduction unit 1065, the reference picture buffer (RPB) 1070, the inter prediction unit 1081, the intra prediction unit 1082, and the entropy encoding unit 1090 may be configured.

In comparison with the example of the encoder of FIG. 1, the RPB 1070 and the image reduction unit 1065 may be further included.

In addition, in the example of FIG. 10, the DPB is not included in the encoder. However, when the reconstructed image is also output in the encoder, the reconstructed picture output by the filtering unit 1060 is stored in the DPB, Accordingly, the reconstructed picture may be output.

Hereinafter, description will be given focusing on portions that differ from the description of FIG. 1.

The image reduction unit 1065 receives a reconstructed picture (or reconstruction signal / reconstruction block) to which the filtering is applied by the filtering unit 1060, reduces the picture, and displays the reduced image as an RPB ( 1070). The filtered reduced picture transmitted to the RPB 1070 may be used as the reference picture in the inter prediction unit 1081.

In more detail, the size of the RPB storage space may vary according to the ratio of reducing the image. If the image reduction occurs too large, it is preferable to set an appropriate reduction ratio since the reconstructed image may be damaged after the reduction and the performance of the motion compensation may be greatly degraded.

If the transmitting end (i.e., encoder) and the receiving end (i.e., decoder) apply the same way by fixing the reduction ratio of the picture, the encoder sends additional information (i.e., reduction ratio of the picture) to the decoder. May not transmit. In this case, the image reduction unit 1065 may reduce a picture at a reduction ratio of a predetermined picture.

If the picture reduction ratio is set in units of entire sequences (ie, a plurality of pictures) or in units of pictures according to the characteristics of the picture, the encoder transmits the picture reduction ratio to the decoder in the sequence header or the picture header. It may be. In this case, the image reduction unit 1065 may determine a reduction ratio of a picture on a picture basis and a sequence basis, and reduce the picture at a reduction ratio of the determined picture.

The RPB 1070 may store the filtered reduced reconstructed picture for use as a reference picture in the inter prediction unit 1081. As such, by reducing and storing the reconstructed picture, the RPB 1070 may reduce the size of the RPB storage space and may reduce the data transmission bandwidth generated when referring to motion compensation.

In addition, a method of reducing an image may be selected according to a situation from various techniques available for reducing the image. In this case, the encoder may select a reduction technique of an image in sequence units, picture units, slice units, or blocks (eg, prediction blocks or coding blocks) and transmit the same to the decoder.

The inter prediction unit 1081 performs temporal prediction and / or spatial prediction to remove temporal redundancy and / or spatial redundancy with reference to a reconstructed picture.

The inter prediction unit 1081 may perform the above-described inter prediction process. In particular, the inter prediction unit 1081 according to the present invention may use an interpolation filtering process to use the reduced reconstructed picture in motion prediction and motion compensation. 6 may be performed to return the reduced reconstructed picture to its original size.

In this case, assuming that a motion vector in units of 1 / N pixels (for example, 1/4 pixels) is supported, when the motion vector in units of integer pixels is applied to a block to be currently encoded, the inter prediction unit 1081 May perform interpolation filtering to enlarge the spatial resolution by the inverse of the reduction factor of the picture. Alternatively, when a motion vector in units of 1 / N pixels (for example, 1/4 pixels) is applied to a block to be currently encoded, the inter prediction unit 1081 may N times (for example, an inverse of the reduction factor of the picture). , 4x) to perform interpolation filtering to enlarge the spatial resolution.

For example, when the reduction ratio of the image is 1/2, when the motion vector of the integer unit is used in the block to be currently encoded, the motion compensation is performed when the compensation filtering is performed to enlarge only twice the image stored in the RPB. Can be. On the other hand, when a motion vector of 1/4 pixel unit is used, motion compensation may be performed only when compensation filtering is performed to enlarge an image stored in the RPB by 8 times (that is, 4 times the inverse of 1/2).

Referring to FIG. 11, the decoder includes an entropy decoding unit 1110, an inverse quantization unit 1120, an inverse transform unit 1130, an adder 1135, a filtering unit 1140, and a decoded picture buffer unit (DPB). 1150, an inter prediction unit 1161, an intra prediction unit 1162, an image reduction unit 1170, and an RPB 1180.

Compared with the example of the decoder of FIG. 2, the image reduction unit 1170 and the RPB 1180 may be further included. A description will be given focusing on the difference from the description of FIG. 2.

The image reduction unit 1170 receives a reconstructed picture (or a reconstruction signal / reconstruction block) to which the filtering is applied by the filtering unit 1140, reduces the picture, and displays the reduced image as an RPB ( 1180). The filtered reduced picture transmitted to the RPB 1180 may be used as a reference picture in the inter prediction unit 1161.

As described above, when the encoder and the decoder are applied in the same manner by fixing the reduction ratio of the picture, the image reduction unit 1170 may reduce the picture at the reduction ratio of the predetermined picture.

In addition, when the encoder transmits a reduction ratio of a picture in a picture unit or a sequence unit to the decoder, the image reduction unit 1170 may reduce the picture by the reduction ratio received from the encoder.

In addition, a method of reducing an image may be selected according to a situation from various techniques available for reducing the image. In this case, the image reduction unit 1170 selects a reduction method of an image in units of a sequence, a picture, a slice, or a block (for example, a prediction block or a coding block) from an encoder, and uses the reduction technique of the selected image. The picture can be reduced.

The RPB 1180 may store the filtered reduced reconstructed picture for use as a reference picture in the inter prediction unit 1161.

The inter prediction unit 1161 performs temporal prediction and / or spatial prediction to remove temporal redundancy and / or spatial redundancy with reference to a reconstructed picture.

The inter prediction unit 1161 may perform the above-described inter prediction process. In particular, the inter prediction unit 1161 according to the present invention may use an interpolation filtering process to use the reduced reconstructed picture in motion prediction and motion compensation. 6 may be performed to return the reduced reconstructed picture to its original size.

In this case, assuming that a motion vector in units of 1 / N pixels (for example, 1/4 pixels) is supported, when the motion vector in units of integer pixels is applied to a block to be currently encoded, the inter prediction unit 1161 may be used. May perform interpolation filtering to enlarge by the inverse of the reduction magnification of the picture. On the other hand, when a motion vector in units of 1 / N pixels (for example, 1/4 pixels) is applied to a block to be currently encoded, the inter prediction unit 1161 is N times (for example, an inverse of the reduction factor of the picture). , 4x) can perform the same interpolation filtering to obtain one image.

Referring to FIG. 12, the encoder / decoder reconstructs an encoded picture (S1201).

For example, an inter prediction unit (1081 in FIG. 10, 1161 in FIG. 11) or an intra predictor (1082 in FIG. 10, and FIG. 11) may be used to output a difference block output from the inverse return unit (1050 in FIG. 10 and 1130 in FIG. The reconstructed block may be generated by adding to the predicted block output from 1162 at.

A plurality of reconstructed blocks generated as described above may be collected to generate a reconstructed picture. This process may be performed by the image reduction unit 1010 in FIG. 10 and 1170 in FIG. 11.

The encoder / decoder resizes the reconstructed picture (S1202).

For example, the image reduction unit 1065 in FIG. 10 and 1170 in FIG. 11 may reduce the reconstructed picture at a specific ratio.

As described above, when the encoder and the decoder apply the same reduction ratio of the fixed picture, the encoder may not transmit additional side information (that is, the reduction ratio of the picture) to the decoder.

On the other hand, the encoder may determine the reduction ratio of the picture in picture units or in the sequence unit, and signal the reduced ratio of the determined picture to the decoder. In this case, when the process of FIG. 12 is performed on the encoder side, the step of determining the reduction ratio of the picture by the encoder (particularly, the image reduction unit 1065) may be further included before step S1202.

In this case, the encoder / decoder (particularly, the filtering unit 1060 in FIG. 10 and 1140 in FIG. 11) may apply filtering to the reconstructed picture and reduce the picture to which the filtering is applied. In this case, the step of performing filtering on the reconstructed picture may be further included before the step S1202.

The encoder / decoder stores the reduced reconstructed picture in the RPB (S1203).

For example, the RPB (1070 in FIG. 10 and 1180 in FIG. 11) may store the reduced reconstructed picture.

As described above, when filtering is applied to the reconstructed picture, the encoder / decoder may store the reduced reconstructed picture to which the filtering is applied to the RPB.

Referring to FIG. 13, the encoder / decoder selects a reference picture for the current block (S1301).

As described above, the reference picture buffer in the encoder / decoder may store a reconstructed picture having a reduced spatial resolution at a predetermined reduction ratio. Accordingly, the encoder / decoder may select the reference picture using the reference index in the reference picture buffer that stores the reduced reconstructed picture.

In this case, the decoder may derive a motion vector and a reference index of the current block and then select a reference picture using the reference index.

In detail, as described above, when the merge mode is applied to the current block, the decoder may decode the merge index signaled from the encoder. The motion parameter of the current block can be derived from the motion parameter of the candidate block indicated by the merge index.

In addition, when the current block is applied with the AMVP mode, the decoder may decode the horizontal and vertical motion vector difference (MVD), reference index, and inter prediction mode signaled from the encoder. The motion vector prediction value may be derived from the motion parameter of the candidate block indicated by the motion reference flag, and the motion vector value of the current processing block may be derived using the motion vector prediction value and the received motion vector difference value.

The encoder / decoder performs interpolation filtering on the selected reference picture (S1302).

If a motion vector in units of 1 / N pixels is supported, interpolation filtering may be performed on the reference picture so that the spatial resolution of the reference picture selected in step S1301 is increased by N times the inverse of the reduction ratio.

Otherwise, in other words, when only a motion vector of an integer pixel unit is supported, interpolation filtering may be performed on the reference picture so that the spatial resolution of the reference picture selected in step S1301 is increased by the inverse of the reduction ratio.

The encoder / decoder generates a prediction block for the current block (S1303).

That is, the encoder / decoder performs motion compensation that predicts an image of the current block from a previously decoded picture by using a motion parameter of the current block. In other words, the encoder / decoder predicts the current block based on the sample value of the region specified using the motion vector in the reference picture selected by using the reference index (that is, the reference picture to which interpolation filtering is applied in step 1302). Create a block.

In the present embodiment, since no filtering or expansion of the picture (that is, increase in spatial resolution) is required in addition to interpolation filtering, the entire encoding / decoding algorithm can be prevented from being complicated.

Example 2

In this embodiment, a method of reducing an image and storing the same in a reference picture buffer (RPB) and enlarging the image before performing inter-picture prediction using a reference picture for motion prediction and motion compensation is proposed.

Referring to FIG. 14, the encoder includes an image splitter 1410, a subtractor 1415, a transformer 1420, a quantizer 1430, an inverse quantizer 1440, an inverse transformer 1450, and a filter 1460. And an image reduction unit 1465, a reference picture buffer (RPB) 1470, an image enlargement unit 1475, an inter prediction unit 1481, an intra prediction unit 1462, and an entropy encoding unit 1490. have.

Compared with the example of the encoder of the first embodiment, the image enlarger 1475 may be further included.

In addition, in the example of FIG. 14, the DPB is not included in the encoder. However, when the reconstructed image is also output in the encoder, the reconstructed picture output by the filtering unit 1460 is stored in the DPB, Accordingly, the reconstructed picture may be output.

The image reduction unit 1465 receives a reconstructed picture (or a reconstruction signal / reconstruction block) to which the filtering is applied by the filtering unit 1460, reduces the picture, and displays the reduced image as an RPB ( 1470). The filtered reduced picture transmitted to the RPB 1470 may be used as the reference picture in the inter prediction unit 1401.

If the transmitting end (i.e., encoder) and the receiving end (i.e., decoder) apply the same way by fixing the reduction ratio of the picture, the encoder sends additional information (i.e., reduction ratio of the picture) to the decoder. May not transmit. In this case, the image reduction unit 1465 may reduce the picture at a reduction ratio of a predetermined picture.

If the picture reduction ratio is set in units of entire sequences (ie, a plurality of pictures) or in units of pictures according to the characteristics of the picture, the encoder transmits the picture reduction ratio to the decoder in the sequence header or the picture header. It may be. In this case, the image reduction unit 1465 may determine the reduction ratio of the picture in picture units and in the sequence unit, and reduce the picture at the reduction ratio of the determined picture.

The RPB 1470 may store the filtered reduced reconstructed picture for use as a reference picture in the inter predictor 1481. As such, by reducing and storing the reconstructed picture, the RPB 1470 may reduce the size of the RPB storage space and may reduce the data transmission bandwidth generated when referring to motion compensation.

When the image enlarger 1475 reads the reduced reconstructed picture stored in the RPB 1470 as the reference picture for the block to be currently decoded by the inter prediction unit 1471, the image enlarger 1475 spatially relies on the inverse of the reduction ratio of the picture. The resolution may be enlarged and transmitted to the inter prediction unit 1401.

As described above, FIR type filtering (or similar type of filtering) may be performed to enlarge an image. In addition, non-linear filtering may also be applied, and deterioration of a reference picture due to reduction / expansion of an image may be reduced by applying non-linear filtering.

The method of reducing / enlarging the image may be selected according to the situation from among various techniques available for reducing / enlarging the image. In this case, the encoder may select a reduction / enlargement scheme of an image in sequence units, picture units, slice units, or blocks (eg, prediction blocks or coding blocks) and transmit them to the decoder.

The inter predictor 1481 performs temporal prediction and / or spatial prediction to remove temporal redundancy and / or spatial redundancy with reference to a reconstructed picture.

The inter predictor 1481 may perform the above-described inter prediction process. In particular, the inter predictor 1481 according to the present invention may perform the reference picture enlarged by the image enlarger 1475 (that is, the spatial information before being reduced). Motion prediction and motion compensation may be performed using a reference picture having a resolution.

In this case, the inter prediction unit 1481 may perform an interpolation filtering process (refer to FIG. 6) on the reference picture enlarged by the image enlarger 1475 to support a motion vector in units of fractional samples.

In this case, assuming that a motion vector in units of 1 / N pixels (for example, 1/4 pixels) is supported, motion in units of 1 / N pixels (for example, 1/4 pixels) is applied to a block to be currently encoded. When the vector is applied, the inter predictor 1481 may perform interpolation filtering to increase the spatial resolution of N times (eg, 4 times).

Referring to the encoder configuration of FIG. 14, the same method as the existing encoder of FIG. 1 is applied except that an enlarged or reduced image is inserted in a process of storing a reduced reconstructed picture in the RPB and a process of reading a reference picture. Can be configured. In this case, the existing encoder module can be used as it is.

Referring to FIG. 15, the decoder includes an entropy decoding unit 1510, an inverse quantization unit 1520, an inverse transform unit 1530, an adder 1535, a filtering unit 1540, a decoded picture buffer (DPB, 1550), and inter prediction. The unit 1561, an intra predictor 1562, an image reduction unit 1570, an RPB 1580, and an image expansion unit 1585 may be configured.

Compared with the example of the decoder of the first embodiment, the image enlarger 1585 may be further included. A description will be given focusing on the difference from the description of FIG. 2.

The image reduction unit 1570 receives a reconstructed picture (or a reconstruction signal / reconstruction block) to which the filtering is applied by the filtering unit 1540, reduces the picture, and displays the reduced image as an RPB ( 1580). The filtered reduced picture transmitted to the RPB 1580 may be used as a reference picture in the inter prediction unit 1561.

As described above, when the encoder and the decoder are applied in the same manner by fixing the reduction ratio of the picture, the image reduction unit 1570 may reduce the picture at the reduction ratio of the predetermined picture.

In addition, when the encoder transmits a reduction ratio of a picture in a picture unit or a sequence unit to the decoder, the image reduction unit 1570 may reduce the picture by the reduction ratio received from the encoder.

The RPB 1580 may store the filtered reduced reconstructed picture for use as a reference picture in the inter prediction unit 1561.

When the image enlarger 1585 reads the reduced reconstructed picture stored in the RPB 1580 by the inter prediction unit 1561 as a reference picture for the block to be currently decoded, the image enlarger 1585 is spatially equal to the inverse of the reduction ratio of the picture. The resolution may be enlarged and transmitted to the inter predictor 1561.

In addition, a method of reducing / enlarging an image may be selected according to a situation from various various techniques available for reducing / enlarging an image. In this case, the image reduction unit 1570 / image expansion unit 1585 selects a reduction / enlarging method of an image in sequence units, picture units, slice units, or blocks (eg, prediction blocks or coding blocks) from an encoder. The picture may be reduced or enlarged using a reduction / enlargement technique of the selected image.

The inter prediction unit 1561 performs temporal prediction and / or spatial prediction to remove temporal redundancy and / or spatial redundancy with reference to a reconstructed picture.

The inter prediction unit 1561 may perform the above-described inter prediction process. In particular, the inter prediction unit 1561 according to the present invention may have a reference picture enlarged by the image enlarger 1475 (that is, the spatial information before being reduced). Motion prediction and motion compensation may be performed using a reference picture having a resolution.

In the present embodiment, the process of reducing and storing the reconstructed picture in the reference picture buffer may be performed in the same manner as the process of FIG. 12 according to the first embodiment described above, and thus the description thereof is omitted.

Referring to FIG. 16, the encoder / decoder selects a reference picture for the current block (S1601).

The encoder / decoder enlarges the selected reference picture (S1602).

In this case, the encoder / decoder may enlarge the selected reference picture by the inverse of the reduced ratio.

If a motion vector in units of 1 / N pixels is supported, interpolation filtering may be performed on the reference picture so that the spatial resolution of the reference picture enlarged in step S1602 is increased by N times. In this case, interpolation filtering may be further added to the enlarged reference picture after step S1602.

The encoder / decoder generates a prediction block for the current block (S1603).

That is, the encoder / decoder performs motion compensation that predicts an image of the current block from a previously decoded picture by using a motion parameter of the current block. In other words, the encoder / decoder uses a motion index within a reference picture selected by using a reference index (that is, a reference picture enlarged by step 1602) to use a motion vector to determine a prediction block for a current block based on a sample value of a region specified. Create

Meanwhile, in the examples of FIGS. 10, 11, 14, and 15 according to the first and second exemplary embodiments, an image reduction unit for performing image reduction is included as an additional component in an encoder and / or a decoder. However, the present invention is not limited thereto. That is, the operation performed by the image reduction unit described above may be performed in the same manner in the RPB. In this case, the image reduction unit may not be included as an additional component in the encoder and / or the decoder.

In addition, in the example of FIGS. 14 and 15 according to the second embodiment, a case in which an image enlargement unit which enlarges an image is included as a separate component in an encoder and / or a decoder, the present invention is limited thereto. no. That is, the operation performed by the image enlarger described above may be performed by the inter predictor in this case, and in this case, the image enlarger may not be included as an additional component in the encoder and / or the decoder.

In FIG. 17, the inter prediction unit is illustrated as one block for convenience of description, but the inter prediction unit may be implemented in a configuration included in the encoder and / or the decoder.

Referring to FIG. 17, the inter prediction unit implements the functions, processes, and / or methods proposed in FIGS. 5 to 16. In detail, the inter predictor may include a motion parameter derivator 1701, a reference picture selector 1702, an image adjuster 1703, and a predictive block generator 1704.

The motion parameter derivation unit 1701 may derive a motion vector and a reference index of the current block.

More specifically, as described above, when the merge mode is applied to the current block, the decoder may decode the merge index signaled from the encoder. The motion parameter of the current block can be derived from the motion parameter of the candidate block indicated by the merge index.

The reference picture selector 1702 may select the reference picture by using the reference index derived by the motion parameter derivation unit 1701.

In particular, the reference picture selector 1702 may select a reference picture in a reference picture buffer that stores a reconstructed picture whose spatial resolution is reduced by a predetermined reduction ratio.

The image adjusting unit 1703 may adjust the selected reference picture to increase the spatial resolution of the selected reference picture in consideration of the reduction ratio of the selected reference picture.

That is, the image adjusting unit 1703 may adjust the selected reference picture and / or enlarge the selected reference picture by applying interpolation filtering to increase the spatial resolution of the selected reference picture.

According to the first embodiment, when the motion vector in an integer unit is supported, the image adjuster 1703 may perform interpolation filtering on the selected reference picture so that the selected reference picture increases the spatial resolution by the inverse of the reduction ratio. Alternatively, when a motion vector in units of 1 / N pixels is supported, interpolation filtering may be performed on the selected reference picture so that the spatial resolution becomes higher by N times the inverse of the reduction ratio.

Alternatively, according to the second embodiment (in particular, when the image enlargement unit is not included as a separate component in the encoder and / or decoder and the image enlargement is performed in the inter prediction unit), the image adjusting unit 1703 moves in integer units. If the vector is supported, the selected reference picture can be enlarged (ie, the reverse process of the reduction) such that the spatial resolution is increased by the inverse of the reduction ratio. In addition, when a motion vector in units of 1 / N pixels is supported, interpolation filtering may be performed on the enlarged reference picture so that the enlarged reference picture increases the spatial resolution by N times.

The prediction block generator 1704 may generate a prediction block for the current block.

That is, the prediction block generator 1704 may present the current block based on the sample value of the region specified by the motion vector in the reference picture selected by using the reference index (that is, the reference picture adjusted by the image adjuster 1703). Generate a predictive block for the block.

Meanwhile, as described above with reference to FIGS. 11 and 15, since the decoded picture is reduced and stored in the RPB, the reduced decoded picture may be used as a reference picture, but may not be used as a picture for output. .

Therefore, the decoder may need a buffer for outputting the reconstructed image in addition to the RPB. In the present embodiment, this buffer is referred to as a DPB. However, this is merely an example and may be referred to as another name that performs a function of a buffer available for output of the reconstructed image.

Therefore, when the present embodiment is applied to the decoder, the DPB 1150 as shown in FIG. 11 or the DPB 1550 as shown in FIG. 15 may be further included separately from the RPB. On the other hand, when the present embodiment is applied to the encoder, although not illustrated in FIG. 10 and FIG. 14, the DPB may be included in the encoder separately from the RPB when the reconstructed image is output in the encoder.

The difference between the existing DPB (see FIGS. 1 and 2 above) and the DPB of the present embodiment (see FIGS. 11 and 15) is that the RPB replaces the function of managing the reconstructed picture for the reference picture, thereby decoded video. It is only necessary to perform the function of printing according to the picture order count (POC). Here, the POC indicates a position (time point) at which the picture is output as a variable for identifying each picture.

The operations of the RPB and the DPB proposed in this embodiment can be controlled according to a reference picture set (RPS) for the DPB of the existing encoder. Here, the RPS is a set of reference pictures associated with the current picture, and includes all reference pictures that can be used for inter prediction of the current picture or a picture after the current picture according to decoding order.

First, referring to the RPB, the RPB may transmit a reference picture defined in the RPS to the inter prediction unit for decoding the current picture. If the current picture is not a reference object but can be used later in decoding order, the picture may be stored in the RPB. In addition to the current picture, a picture not used in subsequent decoding may be deleted from the RPB according to the decoding order. After the current picture is decoded and reduced by the image reduction unit, the current picture may be inserted (stored) in the RPB.

Next, referring to the DPB, a picture that is not a reference object of the current picture but may be used later according to the decoding order may be stored in the DPB. If not only the current picture but also a picture not used for later decoding according to the decoding order, the picture may be output from the DPB in the order of POC. After the current picture is decoded, it may be inserted (stored) in the DPB.

In the above example, the role played by the existing DPB is divided into RPB and DPB, but in another embodiment, unlike the existing DPB, since the DPB does not need to consider a process for reference pictures, it may be performed as follows. have.

Among the pictures stored in the DPB, pictures corresponding to the current sequence may be output in the order of the 0 th POC. If there is no picture corresponding to the next POC order in the DPB, the DPB may stop the output process. After the current picture is decoded, it may be inserted (stored) in the DPB.

The operation of the DPB described above may be performed regardless of the RPS, and the storage space configuring the respective buffers may be saved.

More specifically, when the decoded picture is reduced and stored in the RPB through the present embodiment, the bandwidth of the data transmission interval existing between the current block and the RPB where motion compensation is performed may be reduced.

For example, a 4K image (3840 × 2160) has a 10-bit bit depth, 4: 4: 4 color format, and frames per second (fps) at 60 Hz (i.e. 1 When decoding to 60 frames per second), the approximate bandwidth of the data transmission interval existing between the current block where the motion compensation is performed and the existing DPB is expressed by Equation 1 below.

Equation 1 assumes two reference pictures for 10 bits depth, three color components, 60 frames per second, and bidirectional prediction at a 3840 × 2160 resolution. Α represents a variable for additional padding for filtering the corresponding block in the filtering process.

In this case, data of about 27.81 Gbits per second should be transmitted. When the resolution of the reconstructed picture is reduced by 1/2 in the horizontal and vertical directions by applying the present embodiment, the bandwidth is reduced as shown in Equation 2 below.

In Equation 2, β represents a variable for additional padding for filtering the block in the filtering process.

As shown in Equation 2, when the resolution of the reconstructed picture is reduced by 1/2 in the horizontal and vertical directions and the 3840 × 2160 resolution is reduced to 1920 × 1080 resolution, data of about 6.95 Gbits per second may be transmitted. Therefore, the bandwidth is reduced to about 1/4 level, and β, which is considered to be padding, also has a value smaller than α as the resolution of the picture is reduced, so that the overall bandwidth can be reduced to less than 1/4. .

The embodiments described above are the components and features of the present invention are combined in a predetermined form. Each component or feature is to be considered optional unless stated otherwise. Each component or feature may be embodied in a form that is not combined with other components or features. It is also possible to combine some of the components and / or features to form an embodiment of the invention. The order of the operations described in the embodiments of the present invention may be changed. Some components or features of one embodiment may be included in another embodiment or may be replaced with corresponding components or features of another embodiment. It is obvious that the claims may be combined to form an embodiment by combining claims that do not have an explicit citation relationship in the claims or as new claims by post-application correction.

Embodiments according to the present invention may be implemented by various means, for example, hardware, firmware, software, or a combination thereof. In the case of a hardware implementation, an embodiment of the present invention may include one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), FPGAs ( field programmable gate arrays), processors, controllers, microcontrollers, microprocessors, and the like.

In the case of implementation by firmware or software, an embodiment of the present invention may be implemented in the form of a module, procedure, function, etc. that performs the functions or operations described above. The software code may be stored in memory and driven by the processor. The memory may be located inside or outside the processor, and may exchange data with the processor by various known means.

It will be apparent to those skilled in the art that the present invention may be embodied in other specific forms without departing from the essential features of the present invention. Accordingly, the above detailed description should not be construed as limiting in all aspects and should be considered as illustrative. The scope of the invention should be determined by reasonable interpretation of the appended claims, and all changes within the equivalent scope of the invention are included in the scope of the invention.

As mentioned above, preferred embodiments of the present invention are disclosed for purposes of illustration, and those skilled in the art can improve and change various other embodiments within the spirit and technical scope of the present invention disclosed in the appended claims below. , Replacement or addition would be possible.

Claims

In the method for processing an image based on inter prediction,

Deriving a motion vector and a reference index of the current block;

Selecting a reference picture for the current block using the reference index in a reference picture buffer which stores a reconstructed picture whose spatial resolution is reduced at a predetermined reduction ratio;

Adjusting the selected reference picture to increase the spatial resolution of the selected reference picture in consideration of the reduction ratio; And

And generating a prediction block for the current block using the motion vector based on the reference picture having the spatial resolution increased.
The method of claim 1,

The inter prediction mode based image processing method of performing the interpolation filtering so that the selected reference picture increases the spatial resolution by the inverse of the reduction ratio.
The method of claim 2,

And interpolation filtering is performed such that the spatial resolution of the selected reference picture is increased by N times the inverse of the reduction ratio when the motion vector in units of 1 / N pixels is supported.
The method of claim 1,

And the selected reference picture is enlarged to increase spatial resolution by the inverse of the reduction ratio.
The method of claim 4, wherein

The inter prediction mode based image processing method of performing interpolation filtering on the enlarged reference picture so that the spatial resolution is increased by N times when a motion vector of 1 / N pixel unit is supported.
The method of claim 1,

The reduction ratio may be fixed in advance or signaled in units of a sequence, a picture, a slice, and a block from an encoder.
An apparatus for processing an image based on inter prediction,

A step motion parameter derivation unit for deriving a motion vector and a reference index of the current block;

A reference picture selection unit for selecting a reference picture for the current block by using the reference index in a reference picture buffer storing a reconstructed picture whose spatial resolution is reduced by a predetermined reduction ratio;

An image adjusting unit adjusting the selected reference picture to increase the spatial resolution of the selected reference picture in consideration of the reduction ratio; And

And a prediction block generator for generating a prediction block for the current block using the motion vector based on the reference picture on which the interpolation filtering is performed.
The method of claim 7, wherein

And interpolation filtering is performed such that the selected reference picture has a high spatial resolution by an inverse of the reduction ratio.
The method of claim 8,

And interpolation filtering is performed such that the spatial resolution of the selected reference picture is increased by N times the inverse of the reduction ratio when the motion vector in units of 1 / N pixels is supported.
The method of claim 7, wherein

And the selected reference picture is enlarged to increase spatial resolution by an inverse of the reduction ratio.
The method of claim 10,

An interpolation filtering is performed on the enlarged reference picture so that the spatial resolution is enlarged by N times when a motion vector in units of 1 / N pixels is supported.
The method of claim 7, wherein

The reduction ratio is fixed in advance or signaled in units of sequence, picture, slice, block from an encoder.