KR102183366B1 - Method for determining coding mode and apparatus thereof - Google Patents

Abstract

The video encoding method according to the present invention includes determining whether intra prediction for a current block is omitted, and performing a rate-distortion optimization process on a plurality of coding mode candidates determined according to whether intra prediction is omitted, And determining an encoding mode to be applied to the current block from among encoding mode candidates of, and performing encoding on the current block based on the determined encoding mode.

Description

Coding mode determination method and apparatus thereof {METHOD FOR DETERMINING CODING MODE AND APPARATUS THEREOF}

The present invention relates to image processing, and more particularly, to a method and apparatus for determining an encoding mode.

Recently, as broadcasting services having high definition (HD) resolution are expanded not only domestically but also globally, many users are getting accustomed to high-resolution and high-definition images, and accordingly, many organizations are spurring the development of next-generation video devices. In addition, as interest in UHD (Ultra High Definition) having a resolution of 4 or more times higher than that of HDTV in addition to HDTV has increased, a compression technology for higher resolution and high-definition images is required.

For image compression, an inter prediction technology that temporally predicts a pixel value included in a current picture from a previous and/or subsequent picture, and predicts a pixel value included in the current picture using pixel information in the current picture. An intra prediction technique, an entropy coding technique in which a short code is assigned to a symbol with a high frequency of appearance, and a long code is assigned to a symbol with a low frequency of occurrence, may be used.

An object of the present invention is to provide an image encoding method and apparatus capable of improving image encoding efficiency and reducing complexity.

An object of the present invention is to provide a method and apparatus for determining an encoding mode capable of improving image encoding efficiency and reducing complexity.

One embodiment of the present invention is a video encoding method. The method includes the steps of determining whether intra prediction for a current block is omitted, rate-distortion optimization for a plurality of coding mode candidates determined according to whether the intra prediction is omitted or not. RDO), determining an encoding mode to be applied to the current block from among the plurality of encoding mode candidates, and performing encoding on the current block based on the determined encoding mode. Here, in the determining of the encoding mode, when it is determined that the intra prediction is omitted, the remaining encoding modes other than the intra mode among the encoding modes that the current block may have may be determined as the plurality of encoding mode candidates.

Determining whether the intra prediction is omitted may include: a left neighboring block located adjacent to the left of the current block, an upper neighboring block located adjacent to an upper end of the current block, and the nearest upper right corner of the current block. It may be performed when both the encoding modes of the upper right neighboring block and the upper left neighboring block located closest to the upper left corner of the current block are inter mode.

Determining whether the intra prediction is omitted may include deriving a BAD (Boundary Absolute Difference) value for the current block, and determining that the intra prediction is omitted when the BAD value is greater than a predetermined threshold value. It may further include a step. Here, the BAD value may be derived based on pixel values of pixels located adjacent to an upper boundary and a left boundary of the current block among pixels in the current block and pixels located around the current block.

The predetermined threshold value may be a value determined experimentally and previously stored in an encoder.

The predetermined threshold value may be determined as an average value of BAD values of a plurality of neighboring blocks located around the current block.

Determining whether the intra prediction is omitted may include deriving a SNMV (Sum of Norm Motion Vector) value for the current block, and when the SNMV value is less than a predetermined threshold, the intra prediction is omitted. It may further include the step of determining. Here, the SNMV value may be derived by adding norm values of motion vectors of a plurality of neighboring blocks located around the current block.

The predetermined threshold value may be a value determined experimentally and previously stored in an encoder.

The predetermined threshold value may be determined as an average value of SNMV values of a plurality of neighboring blocks located around the current block.

Determining whether the intra prediction is omitted may include deriving a cross-correlation value for the current block, and when the cross-correlation value is less than a predetermined threshold value, the intra prediction is omitted. It may further include the step of determining to be. Here, the cross correlation value may be derived based on pixel values of pixels in the current block and pixels in a co-located block, and the co-located block is temporally encoded immediately before It may be a block that exists at the same spatially as the current block in the picture.

The predetermined threshold value may be a value determined experimentally and previously stored in an encoder.

The predetermined threshold value may be determined as an average value of cross correlation values of a plurality of neighboring blocks located around the current block.

Another embodiment of the present invention is a method of determining an encoding mode. The method includes the steps of determining whether intra prediction for a current block is omitted, and rate-distortion optimization for a plurality of coding mode candidates determined according to whether or not the intra prediction is omitted. : RDO), determining an encoding mode to be applied to the current block from among the plurality of encoding mode candidates. Here, in the determining of the encoding mode, when it is determined that the intra prediction is omitted, the remaining encoding modes other than the intra mode among the encoding modes that the current block may have may be determined as the plurality of encoding mode candidates.

Determining whether the intra prediction is omitted may include: a left neighboring block located adjacent to the left of the current block, an upper neighboring block located adjacent to an upper end of the current block, and the nearest upper right corner of the current block. It may be performed when both the encoding modes of the upper right neighboring block and the upper left neighboring block located closest to the upper left corner of the current block are inter mode.

Determining whether the intra prediction is omitted may include deriving a BAD (Boundary Absolute Difference) value for the current block, and determining that the intra prediction is omitted when the BAD value is greater than a predetermined threshold value. It may further include a step. Here, the BAD value may be derived based on pixel values of pixels located adjacent to an upper boundary and a left boundary of the current block among pixels in the current block and pixels located around the current block.

Determining whether the intra prediction is omitted may include deriving a SNMV (Sum of Norm Motion Vector) value for the current block, and when the SNMV value is less than a predetermined threshold, the intra prediction is omitted. It may further include the step of determining. Here, the SNMV value may be derived by adding norm values of motion vectors of a plurality of neighboring blocks located around the current block.

Determining whether the intra prediction is omitted may include deriving a cross-correlation value for the current block, and when the cross-correlation value is less than a predetermined threshold value, the intra prediction is omitted. It may further include the step of determining to be. Here, the cross correlation value may be derived based on pixel values of pixels in the current block and pixels in a co-located block, and the co-located block is temporally encoded immediately before It may be a block that exists at the same spatially as the current block in the picture.

Another embodiment of the present invention is a video encoding device. The apparatus includes a prediction unit configured to generate a prediction block corresponding to the current block by determining a coding mode to be applied to the current block and performing prediction on the current block based on the coding mode, and the current block and the prediction block It may include a bitstream generator that generates an encoded bitstream based on. Here, the prediction unit may determine whether intra prediction for the current block is omitted, and rate-distortion optimization for a plurality of coding mode candidates determined according to whether or not the intra prediction is omitted (Rate- By performing a Distortion Optimization: RDO) process, a coding mode to be applied to the current block may be determined from among the plurality of coding mode candidates.

When it is determined that the intra prediction is omitted, the prediction unit may determine, among the encoding modes that the current block may have, other encoding modes other than the intra mode as the plurality of encoding mode candidates.

According to the image encoding method according to the present invention, image encoding efficiency may be improved and complexity may be reduced.

According to the method of determining an encoding mode according to the present invention, image encoding efficiency can be improved and complexity can be reduced.

1 is a block diagram showing a configuration according to an embodiment of an image encoding apparatus to which the present invention is applied.
2 is a block diagram showing a configuration according to an embodiment of an image decoding apparatus to which the present invention is applied.
3 is a flowchart schematically showing an embodiment of an image encoding method to which the present invention is applied.
4 is a diagram for explaining an embodiment of a method of determining whether to apply intra mode skipping in an encoder.
FIG. 5 is a diagram for explaining another embodiment of a method for determining whether to apply an intra mode skip in an encoder.
FIG. 6 is a diagram for explaining another embodiment of a method for determining whether to apply an intra mode skip in an encoder.
7 is a flowchart schematically showing an embodiment of a method for determining an encoding mode according to the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In describing the embodiments of the present specification, if it is determined that a detailed description of a related known configuration or function may obscure the subject matter of the present specification, a detailed description thereof will be omitted.

When a component is referred to as being “connected” or “connected” to another component, it is understood that it may be directly connected or connected to the other component, but other components may exist in the middle. Should be. In addition, the description of "including" a specific configuration in the present invention does not exclude configurations other than the corresponding configuration, and means that additional configurations may be included in the scope of the implementation of the present invention or the technical idea of the present invention.

Terms such as first and second may be used to describe various components, but the components should not be limited by the terms. These terms are used only for the purpose of distinguishing one component from another component. For example, without departing from the scope of the present invention, a first element may be referred to as a second element, and similarly, a second element may be referred to as a first element.

In addition, the components shown in the embodiments of the present invention are shown independently to represent different characteristic functions, and it does not mean that each component is formed of separate hardware or a single software component. That is, each constituent part is listed and included as a respective constituent part for convenience of explanation, and at least two of the constituent parts are combined to form one constituent part, or one constituent part is divided into a plurality of constituent parts to perform a function. Integrated embodiments and separate embodiments of the components are also included in the scope of the present invention unless departing from the essence of the present invention.

In addition, some of the components are not essential components that perform essential functions in the present invention, but may be optional components only for improving performance. The present invention can be implemented by including only the components essential to implement the essence of the present invention excluding components used for performance improvement, and a structure including only essential components excluding optional components used for performance improvement Also included in the scope of the present invention.

1 is a block diagram showing a configuration according to an embodiment of an image encoding apparatus to which the present invention is applied.

Referring to FIG. 1, the image encoding apparatus 100 includes a motion prediction unit 111, a motion compensation unit 112, an intra prediction unit 120, a switch 115, a subtractor 125, and a transform unit 130. , A quantization unit 140, an entropy encoding unit 150, an inverse quantization unit 160, an inverse transform unit 170, an adder 175, a filter unit 180, and a reference picture buffer 190.

The image encoding apparatus 100 may encode an input image in an intra mode or an inter mode and output a bitstream. Intra prediction means intra prediction, and inter prediction means inter prediction. In the intra mode, the switch 115 may be switched to intra, and in the inter mode, the switch 115 may be switched to inter. The image encoding apparatus 100 may generate a prediction block for an input block of an input image, and then encode a residual between the input block and the prediction block.

In the case of the intra mode, the intra prediction unit 120 may generate a prediction block by performing spatial prediction using pixel values of an already coded block around the current block.

In the case of the inter mode, the motion estimator 111 may find a region that best matches the input block in the reference image stored in the reference picture buffer 190 in the motion prediction process to obtain a motion vector. have. The motion compensation unit 112 may generate a prediction block by performing motion compensation using a motion vector.

In the embodiment of FIG. 1, the motion prediction unit 111, the motion compensation unit 112, and the intra prediction unit 120 are shown as separate configurations, but the present invention is not limited thereto. For example, the motion prediction unit 111 and the motion compensation unit 112 may constitute one inter prediction unit, and the motion prediction unit 111, the motion compensation unit 112 and the intra prediction unit 120 We can also constitute wealth.

The subtractor 125 may generate a residual block based on a difference between the input block and the generated prediction block. The transform unit 130 may perform transform on the residual block and output a transform coefficient. In addition, the quantization unit 140 may quantize the input transform coefficient according to a quantization parameter to output a quantized coefficient.

The entropy encoding unit 150 performs entropy encoding based on values (eg, quantized coefficients) calculated by the quantization unit 140 and/or an encoding parameter value calculated during an encoding process to perform the bitstream ( bit stream).

When entropy coding is applied, a small number of bits are allocated to a symbol having a high probability of occurrence, and a large number of bits are allocated to a symbol having a low probability of occurrence, and the symbol is expressed. The size of the column can be reduced. Therefore, compression performance of image encoding may be improved through entropy encoding. The entropy encoding unit 150 may use an encoding method such as exponential golomb, Context-Adaptive Variable Length Coding (CAVLC), and Context-Adaptive Binary Arithmetic Coding (CABAC) for entropy encoding.

Since the image encoding apparatus according to the embodiment of FIG. 1 performs inter prediction encoding, that is, inter-frame prediction encoding, the currently encoded image needs to be decoded and stored in order to be used as a reference image. Accordingly, the quantized coefficients are inverse quantized by the inverse quantization unit 160 and inversely transformed by the inverse transform unit 170. The inverse quantized and inverse transformed coefficients are added to the prediction block through an adder 175 and a reconstructed block is generated.

The restoration block passes through the filter unit 180, and the filter unit 180 applies at least one of a deblocking filter, a sample adaptive offset (SAO), and an adaptive loop filter (ALF) to a restoration block or a reconstructed picture. can do. The filter unit 180 may also be referred to as an adaptive in-loop filter. The deblocking filter may remove block distortion and/or blocking artifacts generated at the boundary between blocks. SAO may add an appropriate offset value to a pixel value to compensate for a coding error. ALF may perform filtering based on a value obtained by comparing the reconstructed image and the original image, and may be performed only when high efficiency is applied. The reconstructed block that has passed through the filter unit 180 may be stored in the reference picture buffer 190.

Meanwhile, in the embodiment of FIG. 1, the subtractor 125, the transform unit 130, the quantization unit 140, and the entropy encoding unit 150 are shown as separate configurations, but this The invention is not limited thereto. Each component shown in the embodiment of FIG. 1 is arranged and included as a separate component for convenience, and at least two components of each component are combined to form one component, or one component is divided into a plurality of components to perform a function. You may. For example, in the embodiment of FIG. 1, the subtractor 125, the transform unit 130, the quantization unit 140, and the entropy encoder 150 may be regarded as constituting one bitstream generation unit. In this case, the bitstream generator may generate an encoded bitstream based on the current block and the prediction block.

2 is a block diagram showing a configuration according to an embodiment of an image decoding apparatus to which the present invention is applied.

Referring to FIG. 2, the image decoding apparatus 200 includes an entropy decoding unit 210, an inverse quantization unit 220, an inverse transform unit 230, an intra prediction unit 240, a motion compensation unit 250, and an adder ( 255), a filter unit 260, and a reference picture buffer 270.

The image decoding apparatus 200 may receive a bitstream output from an encoder, perform decoding in an intra mode or an inter mode, and output a reconstructed image, that is, a reconstructed image. In the intra mode, the switch may be switched to intra, and in the inter mode, the switch may be switched to inter. The image decoding apparatus 200 may obtain a residual block from an input bitstream, generate a prediction block, and then add the residual block and the prediction block to generate a reconstructed block, that is, a reconstructed block.

The entropy decoder 210 may entropy-decode the input bitstream according to a probability distribution to generate symbols including symbols in the form of quantized coefficients. The entropy decoding method is similar to the entropy encoding method described above.

When the entropy decoding method is applied, a small number of bits are allocated to a symbol having a high probability of occurrence and a large number of bits are allocated to a symbol having a low probability of occurrence, thereby expressing the symbol, so that the size of the bit string for each symbol is reduced. Can be reduced. Therefore, the compression performance of video decoding can be improved through the entropy decoding method.

The quantized coefficients are inverse quantized in the inverse quantization unit 220 and inversely transformed in the inverse transform unit 230, and as a result of inverse quantization/inverse transformation of the quantized coefficients, a residual block may be generated.

In the case of the intra mode, the intra prediction unit 240 may generate a prediction block by performing spatial prediction using pixel values of an already decoded block around the current block. In the inter mode, the motion compensation unit 250 may generate a prediction block by performing motion compensation using a motion vector and a reference image stored in the reference picture buffer 270.

In the embodiment of FIG. 2, the intra prediction unit 240 and the motion compensation unit 250 are shown as separate configurations, but the present invention is not limited thereto. For example, the intra prediction unit 240 and the motion compensation unit 250 may constitute one prediction unit.

The residual block and the prediction block are added through an adder 255, and the added block may pass through the filter unit 260. The filter unit 260 may apply at least one of a deblocking filter, SAO, and ALF to a reconstructed block or a reconstructed picture. The filter unit 260 may output a reconstructed image, that is, a reconstructed image. The reconstructed image may be stored in the reference picture buffer 270 and used for inter prediction.

Hereinafter, a block means a unit of image encoding and decoding. When encoding and decoding an image, an encoding or decoding unit refers to a unit divided when an image is divided and encoded or decoded. Therefore, a coding unit (CU), a prediction unit (PU: Prediction Unit), and a transformation unit (TU) are used. : It may be called a Transform Unit, a transform block, etc. One block may be further divided into sub-blocks having smaller sizes. In addition, in this specification, a current block may mean an encoding object block that is a current encoding object and/or a decoding object block that is a current decoding object.

Meanwhile, a slice on which inter prediction is performed may include a P slice and a B slice. P slice may mean a slice in which unidirectional prediction using one reference picture can be performed. For example, in a P slice, intra prediction may be performed or inter prediction using one reference picture may be performed. A B slice may mean a slice in which forward, backward, or bidirectional prediction using one or more reference pictures can be performed. For example, in a B slice, intra prediction may be performed, inter prediction may be performed using one reference picture, or inter prediction may be performed using two reference pictures.

3 is a flowchart schematically showing an embodiment of an image encoding method to which the present invention is applied.

Referring to FIG. 3, the encoder may determine an encoding mode for a current block (S310). Here, as an example, the current block may correspond to the coding unit CU.

In order to determine the encoding mode of the current block, the encoder may perform a rate-distortion optimization (RDO, hereinafter referred to as RDO) process for a plurality of encoding mode candidates that the current block may have. In this case, the encoder may determine an optimal encoding mode for the current block from among the plurality of encoding mode candidates according to a result of performing the RDO. Coding mode candidates that the current block may have may include an inter mode and an intra mode, and there may be a skip mode in which transmission of a residual signal for the current block is omitted.

In the existing video codec, encoding is performed in units of macro blocks having a size of 16x16, but in order to provide optimal compression efficiency for high-resolution video, CU (Coding Unit), PU (Prediction Unit), and TU A processing unit such as (Transform Unit) may be used. Accordingly, the optimal block size can be used in performing compression, encoding, prediction, and transformation.

At this time, for one CU having a size of 2Nx2N, a skip mode in units of 2Nx2N blocks, an inter mode in units of 2Nx2N blocks (Inter_2Nx2N), an inter mode in units of 2NxN blocks (Inter_2NxN), and an inter mode in units of Nx2N blocks (Inter_Nx2N), Intra_PCM mode, and the like may be supported. Here, the intra PCM mode may mean an encoding mode in which processes such as prediction, transformation, and quantization of the current block are omitted. In addition, when the size of the current block (eg, CU) is 8x8, an inter mode (Inter_NxN) in units of NxN blocks, an intra mode (Intra_2Nx2N) in units of NxN blocks may be further supported. The encoder may determine an optimal encoding mode to be applied to the current block (eg, CU) from among the plurality of encoding mode candidates by performing an RDO process on a plurality of encoding mode candidates that the current block may have.

When the encoding mode of the current block is determined, the encoder may perform encoding on the current block based on the determined encoding mode (S320).

Meanwhile, when the current block is a block belonging to a P slice or a B slice, a probability that an encoding mode for the current block is determined as an intra mode may be relatively lower than a probability that the current block is determined as an inter mode. However, a large amount of computation is required to perform intra prediction and/or intra encoding in an image encoding process. This is because up to 35 directional predictions can be performed in the intra coding process.

As described above, the probability of selecting an intra mode from a P slice or a B slice is low, but since the complexity of intra prediction and/or intra coding is quite large, a method of determining an encoding mode capable of efficiently determining an encoding mode is required. For this, the encoder can reduce the complexity of the encoder by terminating the encoding process for the intra mode early. That is, the encoder determines whether to perform intra prediction and/or intra coding on the current block in the P slice or B slice, that is, whether to skip the intra prediction process and/or the intra coding process on the current block. It can be determined early (for example, before performing an RDO process for a plurality of coding modes that the current block may have).

If it is determined that intra prediction is omitted, the encoder may skip the intra prediction and/or intra coding process for the current block. That is, in this case, the encoder may determine other encoding modes other than the intra mode among the encoding modes that the current block may have as encoding mode candidates for the current block. In this case, the encoder may determine an encoding mode to be applied to the current block from among the encoding mode candidates by performing an RDO process on the determined encoding mode candidate. When it is determined that intra prediction is omitted, the encoder may perform only inter prediction and/or inter encoding.

In the present specification, for convenience of description, an omission of intra prediction and/or intra encoding during a video encoding process will be referred to as'intra mode skip'. Hereinafter, embodiments of a method of determining whether an intra prediction process and/or an intra coding process is omitted for the current block (ie, a method of determining whether intra mode skip is applied to the current block) are described.

4 is a diagram for explaining an embodiment of a method of determining whether to apply intra mode skipping in an encoder. The process of determining whether to apply the intra mode skip according to the embodiment of FIG. 4 may be performed by the predictor of FIGS. 1 and 2 described above as an example.

Referring to 410 of FIG. 4, block x represents a current block to be encoded. Here, the current block may correspond to a CU as an example. In addition, 410 of FIG. 4 denotes a block (a) adjacent to the left of the current block, a block (b) adjacent to the top of the current block, a block (c) located at the upper right corner outside the current block, and an upper left corner outside the current block. Block (d) located at is shown. In FIG. 4, blocks a, b, c, and d may be referred to as neighboring blocks, and may be blocks that have already been encoded and/or decoded. Each of the neighboring blocks shown in 410 of FIG. 4 may be a CU as an example.

Hereinafter, in the present specification, the term "peripheral block" may be used as a concept including blocks a, b, c, and d shown in 410 of FIG. 4. In addition, hereinafter, in the present specification, block a, block b, block c, and block d may mean blocks at the same position as blocks a, b, c, and d shown in 410 of FIG. 4, respectively.

Meanwhile, during video encoding, the current block (x) and the neighboring blocks (a, b, c, and d) may have strong spatial correlation. Accordingly, when all of the encoding modes of neighboring blocks are determined as inter mode (and/or inter encoding mode), there is a high probability that the encoding mode of the current block is also determined as inter mode (and/or inter encoding mode). Accordingly, when the encoding modes of the neighboring blocks (a, b, c, d) are all inter mode, the encoder may determine in advance whether intra mode skip is applied to the current block, as described later at 420 of FIG. 4. have.

Reference numeral 420 of FIG. 4 illustrates locations of pixels used to determine whether intra mode skip is applied.

In the intra prediction and/or intra encoding process, prediction may be performed based on pixel values of previously reconstructed neighboring pixels located around the current block. Accordingly, if the BAD (Boundary Absolute Difference) between pixels in the current block and pixels located around the current block is large, the probability that the encoding mode of the current block is determined as the intra mode may be small. In this case, BAD may mean an absolute value of a difference between a pixel value of a pixel located in a current block and a pixel value of a neighboring pixel located around the current block.

Referring to 420 of FIG. 4, an upper left corner pixel 433 located at the top leftmost in the current block 425, an upper left peripheral pixel 436 located at the top left of the top left corner pixel, and the topmost in the current block. The upper pixels 443 located at, the upper surrounding pixels 446 located adjacent to the top of the upper pixels, the leftmost pixels 453 located at the left side of the current block, and the left side adjacent to the left side of the left pixels The surrounding pixels 456 may be used to determine whether to apply the intra mode skip. This can be expressed as in Equation 1 below.

[Equation 1]

Here,'CUsize' may indicate the size of the current block. In addition, abs(X) may represent the absolute value of X. 'Orig' may represent a pixel value of a pixel in the current block, and may correspond to any one of the upper left corner pixel 433, the upper pixels 443, and the left pixels 453. 'Recon' may represent a pixel value of the reconstructed surrounding pixel, and may correspond to any one of the upper left peripheral pixel 436, the upper peripheral pixels 446, and the left peripheral pixels 456. In addition, [y][x] may indicate the location of the upper left corner pixel 433 located at the top leftmost in the current block.

Referring to Equation 1, the encoder may obtain an absolute value of a pixel difference value between the upper left corner pixel 433 and the upper left peripheral pixel 436. In addition, the encoder may obtain an absolute value of a pixel difference value between pixels adjacent to each other among the upper pixels 443 and the upper surrounding pixels 446. In addition, the encoder may obtain an absolute value of a pixel difference value between pixels adjacent to each other among the left pixels 453 and the left surrounding pixels 456. The BAD value for the current block can be derived by adding all of the obtained absolute values.

As described above, the pixels used for deriving the BAD value are pixels located adjacent to the upper boundary and the left boundary of the current block 425 among pixels in the current block 425 and pixels located around the current block 425 It may correspond to. Accordingly, the BAD may correspond to a boundary value difference between a current block and a neighboring block.

When the BAD value derived by the above-described process is greater than a predetermined threshold value, the encoder may apply intra mode skip to the current block. That is, when the BAD value is greater than a predetermined threshold, the encoder may omit the intra prediction and/or intra encoding process for the current block, and may only perform the inter prediction and/or inter encoding process. In this case, the encoder may perform an RDO process for other modes except for the intra mode, and may determine an encoding mode to be applied to the current block from among other encoding modes excluding the intra mode.

The threshold value used to determine whether to apply the intra mode skip may be, for example, a value that is experimentally determined and previously stored in an encoder. For example, an average BAD value of blocks determined as inter-mode in the entire image or picture may be experimentally determined, and the determined average BAD value may be used as the threshold value.

As another example, the threshold value may be a value derived during an encoding process. For example, the encoder may obtain the BAD of each of the neighboring blocks a, b, c, and d and use the average value of the obtained BAD values as the threshold value.

FIG. 5 is a diagram for explaining another embodiment of a method for determining whether to apply an intra mode skip in an encoder. The process of determining whether to apply the intra mode skip according to the embodiment of FIG. 5 may be performed by the prediction unit of FIGS. 1 and 2 described above as an example.

As described above at 410 of FIG. 4, during video encoding, the current block x and the neighboring blocks a, b, c, and d may have a strong spatial correlation. Accordingly, when all of the encoding modes of neighboring blocks are determined as inter mode (and/or inter encoding mode), there is a high probability that the encoding mode of the current block is also determined as inter mode (and/or inter encoding mode). Accordingly, when all of the encoding modes of the neighboring blocks a, b, c, and d are inter mode, the encoder may determine in advance whether intra mode skip is applied to the current block according to an embodiment to be described later.

510 of FIG. 5 shows an example in which an intra coding mode is determined in a very homogeneous region of a picture. In the case of a homogeneous region, the magnitudes of residual signals derived from the inter mode and the intra mode may be similar to each other. However, in this case, since a large amount of bits may be consumed to encode and transmit a motion vector, the encoding mode of the current block may be determined as an intra mode.

The encoding modes determined for each block in the picture 510 of FIG. 5 may be P, S, I16, I4, and the like, respectively. Here, P may mean an inter mode, and S may mean a skip mode. Also, I16 may mean an intra mode in which intra prediction is performed in units of 16x16 blocks, and I4 may mean an intra mode in which intra prediction is performed in units of 4x4 blocks. However, the encoding mode shown in 510 of FIG. 5 is exemplary, and the present invention is not limited thereto.

Referring to 510 of FIG. 5, the encoding mode determined for the current block 515 may be I16. If the motion vector of the neighboring blocks 523, 526, and 529 located in the vicinity of the current block is large, even if the current block is a block located in a homogeneous region, the probability that the encoding mode for the current block is determined as the intra mode This can be high. When the motion vector of the neighboring blocks 523, 526, and 529 is large, the motion vector of the current block is also likely to be large, and thus the amount of bits required to encode the motion vector may be large from a rate-distortion perspective. Accordingly, the encoder may determine the encoding mode of the current block as the intra mode even when the sizes of residual signals derived from the inter mode and the intra mode are similar to each other.

Therefore, the encoder estimates the size of the motion vector of the current block by calculating the sum of norm motion vector (SNMV) of the neighboring blocks 523, 526, and 529 on which encoding and/or decoding is completed, and then the calculated SNMV value (current When the estimated value of the motion vector size of the block) is less than a predetermined threshold value, intra mode skipping may be applied to the current block. Here, the reconstructed neighboring blocks 523, 526, and 529 may correspond to blocks a, b, and c of 410 of FIG. 4, respectively.

Referring to 520 of FIG. 5, the neighboring blocks used for SNMV calculation are the left neighboring block 523 adjacent to the left of the current block 515, the upper neighboring block 526 adjacent to the top of the current block 515, and the current block. (515) It may be the upper right peripheral block 529 located in the upper right corner of the outside. Here, the current block 515, the left neighboring block 523, the upper neighboring block 526, and the upper right neighboring block 529 may correspond to a CU as an example.

0 displayed in the left neighboring block 523, 0 displayed in the upper neighboring block 526, and 2 displayed in the upper right neighboring block 529 may respectively represent a reference picture index. In addition, in 520 of FIG. 5, the motion vector of the left neighboring block 523 is (-2,-12), the motion vector of the upper neighboring block 526 is (12,-12), and the upper right neighboring block 529 ) May be (-32,-24). In (x,y) representing a motion vector, x may represent an x-axis component of the motion vector, and y may represent a y-axis component of the motion vector. In this case, SNMV can be derived by adding all norm values of motion vectors of the neighboring blocks 523, 526, and 529. This can be represented by the following equation (2).

[Equation 2]

Here, MV may represent a motion vector. Further, MVx may represent the x-axis component of the motion vector, and MVy may represent the y-axis component of the motion vector. In Equation 2, the SNMV value may be derived by adding all the norm values of the motion vectors of the block a, the block b, and the block c. Here, block a represents the left peripheral block 523, block b represents the upper peripheral block 526, and block c represents the upper right peripheral block 529.

When the SNMV value derived by the above-described process is smaller than a predetermined threshold value, the encoder may apply intra mode skip to the current block. That is, when the SNMV value is less than a predetermined threshold, the encoder may omit the intra prediction and/or intra encoding process for the current block, and may only perform the inter prediction and/or inter encoding process. In this case, the encoder may perform an RDO process for other modes except for the intra mode, and may determine an encoding mode to be applied to the current block from among other encoding modes excluding the intra mode.

The threshold value used to determine whether to apply the intra mode skip may be, for example, a value that is experimentally determined and previously stored in an encoder. For example, an average SNMV value of blocks determined as inter mode in the entire image or picture may be experimentally determined, and the determined SNMV average value may be used as the threshold value.

As another example, the threshold value may be a value derived during an encoding process. For example, the encoder may obtain the SNMV of each of the neighboring blocks located around the current block and use an average value of the obtained SNMV values as the threshold value. Here, as an example, the neighboring block is the left neighboring block (same as block a of 410 in FIG. 4, 523), the upper neighboring block (same as block b of 410 of FIG. 4, 526), and The same as block c of 410, 529) may be included. In addition, the neighboring block used for deriving the threshold value may further include an upper left neighboring block (same as block d of 410 of FIG. 4) located in the upper left corner outside the current block.

FIG. 6 is a diagram for explaining another embodiment of a method for determining whether to apply an intra mode skip in an encoder. The process of determining whether to apply the intra mode skip according to the embodiment of FIG. 6 may be performed by the prediction unit of FIGS. 1 and 2 described above as an example.

As described above at 410 of FIG. 4, during video encoding, the current block x and the neighboring blocks a, b, c, and d may have a strong spatial correlation. Accordingly, when all of the encoding modes of neighboring blocks are determined as inter mode (and/or inter encoding mode), there is a high probability that the encoding mode of the current block is also determined as inter mode (and/or inter encoding mode). Accordingly, when all of the encoding modes of the neighboring blocks a, b, c, and d are inter mode, the encoder may determine in advance whether intra mode skip is applied to the current block according to an embodiment to be described later.

6 shows a current picture 620 to which a current block (a block to be encoded 625) belongs, and a picture (hereinafter referred to as a previous picture, 610) that has been encoded and/or decoded immediately before the time axis. A block 615 belonging to the previous picture 610 represents a block that exists in the same spatially as the current block 625 in the previous picture 610. Hereinafter, in this specification, for convenience of description, a block existing at the same position as the current block 625 in the previous picture 610 is referred to as a co-located block 615. Here, the current block 625 and the co-located block 615 may correspond to a CU as an example.

In FIG. 6, encoding modes determined for each block may be P, S, I16, I4, and the like, respectively. Here, P may mean an inter mode, and S may mean a skip mode. Also, I16 may mean an intra mode in which intra prediction is performed in units of 16x16 blocks, and I4 may mean an intra mode in which intra prediction is performed in units of 4x4 blocks. However, the encoding mode shown in FIG. 6 is exemplary, and the present invention is not limited thereto.

Referring to FIG. 6, the encoding mode determined for the current block 625 may be I4. When the cross-correlation between the current block 625 and the co-located block 615 is large, the encoding mode of the current block 625 may have a high probability of being determined as an intra mode. Accordingly, the encoder may apply intra mode skip to the current block when the cross correlation between the current block 625 and the co-located block 615 is less than a predetermined threshold value. Equation 3 below shows an embodiment of a method for deriving a cross-correlation degree between the current block 625 and the co-located block 615.

[Equation 3]

Here,'a' may denote a pixel group in the same location block 615, and'b' may denote a pixel group in the current block 625. In addition,'a x b'may represent a group consisting of values obtained by multiplying pixels in the same location block 615 and pixels in the current block 625 by pixels existing at the same spatial location.

In addition,'E[x]' denotes the average of the group corresponding to x, and'sd[x]' denotes the standard deviation of the group corresponding to x. 'Corr[a,b]' may represent a cross-correlation degree between a group corresponding to a and a group corresponding to b.

When the cross-correlation degree value derived by the above-described process is less than a predetermined threshold value, the encoder may apply intra mode skipping to the current block. That is, when the cross-correlation value is less than a predetermined threshold, the encoder may omit the intra prediction and/or intra encoding process for the current block, and may only perform the inter prediction and/or inter encoding process. In this case, the encoder may perform an RDO process for other modes except for the intra mode, and may determine an encoding mode to be applied to the current block from among other encoding modes excluding the intra mode.

When the cross correlation value is greater than or equal to a predetermined threshold value, the encoder may perform intra prediction and/or intra encoding on the current block. Accordingly, in this case, the encoder may perform an RDO process for all encoding modes including the intra mode, and may determine an encoding mode to be applied to the current block among all encoding modes including the intra mode.

The threshold value used to determine whether to apply the intra mode skip may be, for example, a value that is experimentally determined and previously stored in an encoder. For example, an average value of a cross-correlation degree of blocks determined as inter-mode across the entire image or picture may be experimentally determined, and an average value of the determined cross-correlation degree may be used as the threshold value.

As another example, the threshold value may be a value derived during an encoding process. For example, the encoder may obtain a cross-correlation degree of each of neighboring blocks located around a current block and use an average value of the obtained cross-correlation values as the threshold value. Here, as an example, the neighboring block is a block (a) adjacent to the left of the current block (b), a block located at the upper right corner of the outside of the current block as shown in 410 of FIG. c) and a block (d) located in the upper left corner outside the current block.

7 is a flowchart schematically showing an embodiment of a method for determining an encoding mode according to the present invention. The process of determining whether to apply the intra mode skip according to the embodiment of FIG. 7 may be performed by the prediction unit of FIGS. 1 and 2 described above as an example.

In the embodiment of FIG. 7, the current block may mean an encoding object block that is a current encoding object. For example, the current block may correspond to a CU.

Referring to FIG. 7, the encoder may derive the BAD value of the current block (S710). Here, BAD may mean an absolute value of a difference between a pixel value of a pixel located in the current block and a pixel value of a neighboring pixel located around the current block. In this case, pixels used to derive the BAD value may be pixels located adjacent to the upper boundary and the left boundary of the current block. Accordingly, the BAD may correspond to a boundary value difference between a current block and a neighboring block. Since a specific embodiment of the process of deriving the BAD value has been described above in FIG. 4, a detailed description thereof will be omitted here.

Referring back to FIG. 7, the encoder may derive the SNMV value of the current block (S720). SNMV can be derived by adding all norm values of motion vectors of neighboring blocks located around the current block. Since a specific embodiment of the process of deriving the SNMV value has been described above in FIG. 5, a detailed description thereof will be omitted here.

Referring back to FIG. 7, the encoder may derive a cross correlation between the current block and the co-located block (S730). Here, the co-located block may be a block in the previous picture (or previous frame) that has been encoded and/or decoded immediately before the time axis, and is spatially at the same position as the current block within the previous picture (or previous frame) It may be an existing block. Since a specific embodiment of the process of deriving a cross-correlation degree has been described above in FIG. 6, a detailed description thereof will be omitted here.

If BAD, SNMV, and/or cross-correlation are derived, the encoder may determine whether intra mode skip is applied to the current block (ie, whether intra prediction is omitted) based on this (S740).

For example, as a first condition, the encoder may apply intra mode skip to the current block when the BAD value is greater than a predetermined first threshold value, and otherwise, may perform intra prediction on the current block. In addition, as a second condition, the encoder may apply intra mode skip to the current block when the SNMV value is less than the second predetermined threshold value, and otherwise, may perform intra prediction on the current block. And, as a third condition, the encoder may apply intra mode skip to the current block when the cross-correlation value is less than a predetermined third threshold value, and otherwise, perform intra prediction for the current block. have.

When the first condition, the second condition and/or the third condition are satisfied, the encoder may omit the intra prediction and/or intra coding process for the current block. In this case, the encoder can reduce the complexity of the encoder by early terminating the encoding process for the intra mode.

Whether to apply the intra mode skip to the current block may be determined by an'OR' operation for the first condition, the second condition, and the third condition, or the'AND' for the first condition, the second condition, and the third condition. 'Can also be determined by operation. When the'OR' operation is used, if at least one of the first condition, the second condition, and the third condition is satisfied, intra prediction for the current block may be omitted. In addition, when the'AND' operation is used, intra prediction for the current block may be omitted when all of the first condition, the second condition, and the third condition are satisfied.

In addition, in the embodiment of FIG. 7, BAD, SNMV, and cross-correlation are all calculated for the current block, but the present invention is not limited thereto. For example, the encoder may calculate only one or two of BAD, SNMV, and cross correlation. When two of the BAD, SNMV, and cross-correlation are calculated, the encoder may determine whether to apply the intra mode skip by performing an OR operation or an AND operation on them.

On the other hand, the above-described BAD derivation process (S710), SNMV derivation process (S720), cross-correlation degree derivation process (S730), and/or intra mode skip application determination process (S740) are performed by the neighboring blocks located around the current block. It may be performed only when all of the encoding modes are inter mode. This is because when the encoding modes of the neighboring blocks are all determined as inter mode, the probability that the encoding mode of the current block is also determined as inter mode is high.

Referring back to FIG. 7, the encoder may determine the encoding mode of the current block (S750 ).

If it is determined that intra prediction is omitted for the current block, the encoder may omit the intra prediction and/or intra coding process for the current block, and may only perform the inter prediction and/or inter coding process. In this case, the encoder may determine other encoding modes other than the intra mode among the encoding modes that the current block may have as encoding mode candidates for the current block. In this case, the encoder may determine a coding mode to be applied to the current block from among the coding mode candidates by performing an RDO process on the determined coding mode candidate (a coding mode other than an intra mode among coding modes that the current block may have). .

When it is determined that intra mode skip is not applied to the current block, the encoder may perform intra prediction and/or intra coding on the current block. Accordingly, in this case, the encoder may determine all encoding modes that the current block may have as encoding mode candidates of the current block, including the intra mode. In this case, the encoder may determine an encoding mode to be applied to the current block from among encoding mode candidates including the intra mode by performing an RDO process on the determined encoding mode candidate.

According to the above-described embodiments, the encoding process for the intra mode can be terminated early. Therefore, since unnecessary intra mode-related calculations can be skipped, complexity in the video encoder can be reduced.

In the above-described embodiments, the methods are described on the basis of a flowchart as a series of steps or blocks, but the present invention is not limited to the order of steps, and certain steps may occur in a different order or concurrently with other steps as described above. I can. In addition, those of ordinary skill in the art understand that the steps shown in the flowchart are not exclusive, other steps are included, or one or more steps in the flowchart may be deleted without affecting the scope of the present invention. You can understand.

The above-described embodiments include examples of various aspects. Although not all possible combinations for representing the various aspects can be described, those of ordinary skill in the art will recognize that other combinations are possible. Accordingly, the present invention will be said to cover all other replacements, modifications and changes falling within the scope of the following claims.

Claims (18)

Determining whether intra prediction for the current block is omitted;
By performing a rate-distortion optimization (RDO) process on a plurality of coding mode candidates determined according to whether the intra prediction is omitted, a coding mode to be applied to the current block is determined from among the plurality of coding mode candidates. The step of doing; And
Including the step of performing encoding on the current block based on the determined encoding mode,
In the step of determining the encoding mode,
When it is determined that the intra prediction is omitted, the remaining encoding modes other than the intra mode among the encoding modes that the current block may have are determined as the plurality of encoding mode candidates,
Determining whether the intra prediction is omitted,
Deriving a cross-correlation value for the current block; And
Further comprising determining that the intra prediction is omitted when the cross-correlation degree value is less than a predetermined threshold value,
The cross-correlation value is derived based on pixel values of pixels in the current block and pixels in a co-located block. It is a block that exists spatially at the same location as the current block,
The cross-correlation degree value is calculated using an average and a standard deviation of the current block and the co-located block. The method of claim 1,
Determining whether the intra prediction is omitted,
A left neighboring block located adjacent to the left side of the current block, an upper neighboring block located adjacent to the top of the current block, an upper right neighboring block located closest to the upper right corner of the current block, and an upper left corner of the current block. An image encoding method, characterized in that it is performed when all encoding modes of the nearest upper left neighboring block are inter mode. The method of claim 1,
Determining whether the intra prediction is omitted,
Deriving a Boundary Absolute Difference (BAD) value for the current block; And
Further comprising determining that the intra prediction is omitted when the BAD value is greater than a predetermined threshold value,
The BAD value is derived based on pixel values of pixels located adjacent to an upper boundary and a left boundary of the current block among pixels in the current block and pixels located around the current block. . The method of claim 3,
The predetermined threshold value is a value determined experimentally and stored in an encoder in advance. The method of claim 3,
The predetermined threshold value is determined as an average value of BAD values of a plurality of neighboring blocks located around the current block. The method of claim 1,
Determining whether the intra prediction is omitted,
Deriving a Sum of Norm Motion Vector (SNMV) value for the current block; And
Further comprising the step of determining that the intra prediction is omitted when the SNMV value is less than a predetermined threshold value,
The SNMV value is derived by adding norm values of motion vectors of a plurality of neighboring blocks located around the current block. The method of claim 6,
The predetermined threshold value is a value determined experimentally and stored in an encoder in advance. The method of claim 6,
The predetermined threshold value is determined as an average value of SNMV values of a plurality of neighboring blocks located around the current block. delete The method of claim 1,
The predetermined threshold value is a value determined experimentally and stored in an encoder in advance. delete delete delete delete delete delete delete delete

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