KR20130085376A - Method for determining coding mode and apparatus thereof - Google Patents

Abstract

PURPOSE: A coding mode determining method and an apparatus thereof are provided to skip an intra prediction process or an intra coding process about a current block according to the satisfaction of a condition, thereby reducing the complexity of an encoding device. CONSTITUTION: An encoding device determines whether intra prediction about a current block is skipped or not. In case the intra prediction is determined to be skipped, the encoding device determines coding modes except an intra mode from all coding modes of the current block as multiple coding mode candidates. The encoding device determines a corresponding coding mode among the coding mode candidates and encodes the current block by performing a rate distortion optimization (RDO) process about the coding mode candidates (S310,S320). [Reference numerals] (S310) Determine a decoding module of a current block; (S320) Conduct a decoding of the current block

Description

Method and apparatus for determining encoding mode {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, broadcasting service having high definition (HD) resolution has been expanded not only in domestic but also in the world, so that many users are accustomed to high-resolution and high-definition video, and thus many organizations are spurring development of next generation video equipment. In addition, with the increase of interest in UHD (Ultra High Definition) having resolution more than 4 times of HDTV in addition to HDTV, a compression technique for a higher resolution and a higher image quality is required.

An inter prediction technique for predicting pixel values included in the current picture from temporally preceding and / or following pictures for image compression, prediction of pixel values 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 having a high appearance frequency and a long code is assigned to a symbol having a low appearance frequency.

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 an encoding mode determining method and apparatus capable of improving image encoding efficiency and reducing complexity.

One embodiment of the present invention is a video encoding method. The method includes determining whether intra prediction for a current block is omitted, and rate-distortion optimization for a plurality of encoding mode candidates determined according to whether intra prediction is omitted. Performing an RDO) process, determining an encoding mode to be applied to the current block among the plurality of encoding mode candidates, and performing encoding on the current block based on the determined encoding mode. In the determining of the encoding mode, when it is determined that the intra prediction is omitted, the 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.

The determining of whether or not the intra prediction is omitted may include: a left peripheral block located adjacent to the left side of the current block, a top peripheral block located adjacent to the top of the current block, and the nearest right corner of the current block; The encoding mode of the upper right peripheral block and the upper left peripheral block nearest to the upper left corner of the current block may be performed when the inter mode is used.

Determining whether or not the intra prediction is omitted may include: deriving a Boundary Absolute Difference (BAD) value for the current block and determining that the intra prediction is omitted when the BAD value is larger than a predetermined threshold value. It may further comprise 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 may be a value determined experimentally and stored in the encoder in advance.

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 or not the intra prediction is omitted may include deriving a sum of norm motion vector (SNMV) value for the current block and if the SNMV value is smaller than a predetermined threshold, the intra prediction is omitted. The method may further include determining. Here, the SNMV value may be derived by adding a norm of motion vectors of a plurality of neighboring blocks located around the current block.

The predetermined threshold may be a value determined experimentally and stored in the encoder in advance.

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 or not the intra prediction is omitted, deriving a cross-correlation value for the current block and if the cross correlation value is less than a predetermined threshold, the intra prediction is omitted. The method may further include determining to be. 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 may have been encoded just before. The block may be a block existing at the same position as the current block in the picture.

The predetermined threshold may be a value determined experimentally and stored in the encoder in advance.

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 coding mode determination method. The method includes 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 intra prediction is omitted. And determining an encoding mode to be applied to the current block among the plurality of encoding mode candidates. In the determining of the encoding mode, when it is determined that the intra prediction is omitted, the 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.

The determining of whether or not the intra prediction is omitted may include: a left peripheral block located adjacent to the left side of the current block, a top peripheral block located adjacent to the top of the current block, and the nearest right corner of the current block; The encoding mode of the upper right peripheral block and the upper left peripheral block nearest to the upper left corner of the current block may be performed when the inter mode is used.

Determining whether or not the intra prediction is omitted may include: deriving a Boundary Absolute Difference (BAD) value for the current block and determining that the intra prediction is omitted when the BAD value is larger than a predetermined threshold value. It may further comprise 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 or not the intra prediction is omitted may include deriving a sum of norm motion vector (SNMV) value for the current block and if the SNMV value is smaller than a predetermined threshold, the intra prediction is omitted. The method may further include determining. Here, the SNMV value may be derived by adding a norm of motion vectors of a plurality of neighboring blocks located around the current block.

Determining whether or not the intra prediction is omitted, deriving a cross-correlation value for the current block and if the cross correlation value is less than a predetermined threshold, the intra prediction is omitted. The method may further include determining to be. 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 may have been encoded just before. The block may be a block existing at the same position as the current block in the picture.

Yet another embodiment of the present invention is a video encoding apparatus. The apparatus may include a prediction unit configured to determine a coding mode to be applied to a current block and to generate a prediction block corresponding to the current block by 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 for generating an encoded bitstream based on the. Here, the prediction unit may determine whether intra prediction for the current block is omitted, and rate-distortion optimization (Rate−) for a plurality of encoding mode candidates determined according to whether the intra prediction is omitted. By performing a Distortion Optimization (RDO) process, an encoding mode to be applied to the current block may be determined from among the plurality of encoding mode candidates.

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

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

1 is a block diagram illustrating a configuration of an image encoding apparatus according to an embodiment of the present invention.
2 is a block diagram illustrating a configuration of an image decoding apparatus according to an embodiment of the present invention.
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 describing an embodiment of a method of determining whether intra mode skip is applied in an encoder.
FIG. 5 illustrates another embodiment of a method of determining whether intra mode skip is applied in an encoder.
FIG. 6 is a diagram for describing another embodiment of a method of determining whether intra mode skip is applied by an encoder.
7 is a flowchart schematically illustrating an embodiment of a coding mode determination method according to the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following description of the embodiments of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present disclosure rather unclear.

It is to be understood that when an element is referred to as being "connected" or "connected" to another element, it may be directly connected or connected to the other element, . In addition, the description of "including" a specific configuration in the present invention does not exclude a configuration other than the configuration, and means that additional configurations can be included in the practice of the present invention or the technical scope of the present invention.

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

In addition, the components shown in the embodiments of the present invention are shown independently to represent different characteristic functions, which does not mean that each component is composed of separate hardware or software constituent units. That is, each constituent unit is included in each constituent unit for convenience of explanation, and at least two constituent units of the constituent units may be combined to form one constituent unit, or one constituent unit may be divided into a plurality of constituent units to perform a function. The integrated embodiments and separate embodiments of the components are also included within the scope of the present invention, unless they depart from the essence of the present invention.

In addition, some of the components are not essential components to perform essential functions in the present invention, but may be optional components only to improve performance. The present invention can be implemented only with components essential for realizing the essence of the present invention, except for the components used for the performance improvement, and can be implemented by only including the essential components except the optional components used for performance improvement Are also included in the scope of the present invention.

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

Referring to FIG. 1, the image encoding apparatus 100 may include a motion predictor 111, a motion compensator 112, an intra predictor 120, a switch 115, a subtractor 125, and a converter 130. The quantizer 140 may include an entropy encoder 150, an inverse quantizer 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 bit stream. Intra prediction is intra prediction, and inter prediction is inter prediction. In the intra mode, the switch 115 is switched to the intra mode, and in the inter mode, the switch 115 can be switched to the inter mode. The image encoding apparatus 100 may generate a prediction block for an input block of an input image, and then may code a residual between the input block and the prediction block.

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

In the inter mode, the motion estimator 111 can obtain a motion vector by searching an area of the reference picture stored in the reference picture buffer 190 that is best matched with the input block in the motion prediction process 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 predictor 111, the motion compensator 112, and the intra predictor 120 are illustrated in 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 may generate one prediction And the like.

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

The entropy encoding unit 150 performs entropy encoding on the basis of values (e.g., quantized coefficients) calculated in the quantization unit 140 and / or encoding parameter values calculated in the encoding process, bit stream.

When entropy encoding 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 symbols, The size of the column can be reduced. Therefore, the compression performance of the image encoding can be enhanced through the 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 predictive encoding, the currently encoded image needs to be decoded and stored for use as a reference image. Accordingly, the quantized coefficients are inversely quantized in the inverse quantization unit 160 and inversely transformed in the inverse transformation unit 170. The inverse quantized and inverse transformed coefficients are added to the prediction block through the adder 175 and a reconstruction 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) can do. The filter unit 180 may be referred to as an adaptive in-loop filter. The deblocking filter may remove block distortion and / or blocking artifacts that have occurred at the boundary between the blocks. The SAO may add a proper offset value to the pixel value to compensate for coding errors. The ALF may perform filtering based on a comparison between 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 illustrated in separate configurations, but this is for convenience of description. The invention is not limited to this. Each component shown in the embodiment of FIG. 1 is listed as a 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 transformer 130, the quantizer 140, and the entropy encoder 150 may constitute one bitstream generator. 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 illustrating a configuration of an image decoding apparatus according to an embodiment of the present invention.

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, an adder 255, a filter unit 260, and a reference picture buffer 270.

The video decoding apparatus 200 receives the bit stream output from the encoder and decodes the video stream into the intra mode or the inter mode, and outputs the reconstructed video, that is, the reconstructed video. In the intra mode, the switch is switched to the intra mode, and in the inter mode, the switch can be switched to the inter mode. The image decoding apparatus 200 may obtain a residual block from the input bitstream, generate a prediction block, and then add the residual block and the prediction block to generate a reconstructed block, that is, a reconstruction block.

The entropy decoding unit 210 may entropy-decode the input bitstream according to a probability distribution to generate symbols including a symbol of a quantized coefficient type. 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 assigned to a symbol having a high probability of occurrence, and a large number of bits are assigned to a symbol having a low probability of occurrence, so that the size of a bit string for each symbol is Can be reduced. Therefore, the compression performance of the image decoding can be enhanced through the entropy decoding method.

The quantized coefficient is inversely quantized by the inverse quantizer 220 and inversely transformed by the inverse transformer 230, and as a result of the inverse quantization / inverse transformation of the quantized coefficient, a residual block may be generated.

In the intra mode, the intraprediction unit 240 may generate a prediction block by performing spatial prediction using the pixel value of the 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 predictor 240 and the motion compensator 250 are illustrated in separate configurations, but the present invention is not limited thereto. For example, the intraprediction unit 240 and the motion compensation unit 250 may constitute one prediction unit.

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

Hereinafter, a block refers to a unit of image encoding and decoding. The coding or decoding unit in the image coding and decoding means a divided unit when the image is divided and encoded or decoded. Therefore, a coding unit (CU), a prediction unit (PU), a conversion unit (TU) : Transform Unit), a transform block, and the like. One block may be further subdivided into sub-blocks of smaller size. In addition, in the present specification, the current block may mean an encoding target block that is currently encoding target and / or a decoding target block that is currently decoding target.

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

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 the current block (S310). Here, as an example, the current block may correspond to the coding unit CU.

In order to determine an encoding mode of the current block, the encoder may perform a rate-distortion optimization (RDO) process on 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 among the plurality of encoding mode candidates according to the result of performing the RDO. The encoding mode candidates that the current block may have include an inter mode and an intra mode, and may also include a skip mode in which the transmission of the 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 16 × 16. However, in order to provide optimal compression efficiency for high resolution video, a coding unit (CU), a prediction unit (PU), and a TU are provided. Processing units such as (Transform Unit) may be used. Therefore, an optimal block size may be used in performing compression, encoding, prediction, and transformation.

In this case, for one CU having a size of 2Nx2N, a skip mode of 2Nx2N blocks, an inter mode of 2Nx2N blocks (Inter_2Nx2N), an inter mode of 2NxN blocks, Inter_2NxN, and an inter mode of Nx2N blocks (Inter_Nx2N), intra PCM mode, and the like may be supported. In this case, the intra PCM mode may refer to an encoding mode in which processes such as prediction, transform, and quantization of a current block are omitted. In addition, when the size of the current block (eg, CU) is 8x8, inter mode Inter_NxN in units of NxN blocks, intra mode N_2N in units of NxN blocks, and the like may be further supported. The encoder may determine an optimal encoding mode to be applied to the current block (eg, CU) 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, the probability that the encoding mode for the current block is determined to be an intra mode may be relatively lower than the probability that the current block is determined to be an inter mode. However, a large amount of computation is required for performing intra prediction and / or intra encoding in the image encoding process. This is because up to 35 directional predictions may be performed in the intra encoding process.

As described above, since the probability of selecting an intra mode in a P slice or a B slice is low, but the complexity of intra prediction and / or intra encoding is considerably large, an encoding mode determination method capable of efficiently determining an encoding mode is required. To this end, 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 encoding on the current block in the P slice or the B slice, that is, whether to skip the intra prediction process and / or the intra encoding process on the current block. The decision may be made early (eg, before performing an RDO process for a plurality of encoding modes that the current block may have).

If it is determined that intra prediction is omitted, the encoder may omit the intra prediction and / or intra encoding process for the current block. That is, in this case, the encoder may determine a remaining encoding mode except the intra mode among the encoding modes that the current block may have as the encoding mode candidate of the current block. In this case, the encoder may determine an encoding mode to be applied to the current block among the encoding mode candidates by performing an RDO process on the determined encoding mode candidate. If 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, it is referred to as 'intra mode skip' that intra prediction and / or intra encoding are omitted in a video encoding process. Hereinafter, embodiments of a method of determining whether an intra prediction process and / or an intra encoding process are omitted for a current block (that is, a method of determining whether intra mode skip is applied to the current block) are described.

4 is a diagram for describing an embodiment of a method of determining whether intra mode skip is applied in an encoder. The process of determining whether to apply intra mode skip according to the embodiment of FIG. 4 may be performed by the prediction unit of FIGS. 1 and 2 described above.

Referring to 410 of FIG. 4, block x represents a current block to be encoded. Here, the current block may correspond to a CU, for example. In addition, 410 of FIG. 4 shows a block (a) adjacent to the left side 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. A 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 illustrated at 410 of FIG. 4 may be a CU, for example.

Hereinafter, the term 'peripheral block' herein may be used as a concept including block a, block b, block c, and block d illustrated in 410 of FIG. 4. Also, hereinafter, blocks a, b, c and d may refer to blocks at the same positions as blocks a, b, c and d shown in 410 of FIG. 4.

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

420 of FIG. 4 shows the location of the pixels used to determine whether intra mode skip is applied.

In intra prediction and / or intra encoding, prediction may be performed based on pixel values of neighboring pixels that are already reconstructed around the current block. Therefore, if the boundary absorptive difference (BAD) between pixels in the current block and pixels located around the current block is large, a probability that the encoding mode of the current block is determined to be an intra mode may be small. In this case, the BAD may mean an absolute value of the difference between the pixel value of the pixel located in the current block and the pixel value of the neighboring pixel located around the current block.

Referring to 420 of FIG. 4, an upper left corner pixel 433 located at an upper left corner in the current block 425, an upper left peripheral pixel 436 located at an upper left corner of the upper left corner pixel, and an uppermost corner in the current block. Top pixels 443 located at, top peripheral pixels 446 located adjacent to the top of the top pixels, leftmost pixels 453 located at the leftmost in the current block, and left located adjacent to the left of the left pixels Peripheral pixels 456 may be used to determine whether intra mode skip is applied. This may be represented as in Equation 1 below.

[Equation 1]

Here, 'CUsize' may indicate the size of the current block. Also, abs (X) may represent the absolute value of X. 'Orig' may indicate a pixel value of a pixel in the current block and may correspond to any one of an upper left corner pixel 433, an upper pixel 443, and a left pixel 453. 'Recon' may indicate a pixel value of the reconstructed peripheral 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. Further, [y] [x] may indicate the position of the top left corner pixel 433 located at the top left 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 peripheral pixels 446. The encoder may obtain an absolute value of pixel difference values between pixels adjacent to each other among the left pixels 453 and the left peripheral pixels 456. The BAD value for the current block can be derived by adding up all the absolute values obtained above.

As described above, the pixels used to derive the BAD value are pixels located near the top boundary and the left boundary of the current block 425 among the pixels in the current block 425 and the pixels located around the current block 425. It may correspond to. Therefore, the BAD may correspond to a boundary value difference between the current block and the neighboring block.

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

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

As another example, the threshold value may be a value derived in an encoding process. For example, the encoder may obtain a 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.

FIG. 5 illustrates another embodiment of a method of determining whether intra mode skip is applied in an encoder. An intra mode skip application determination process according to the embodiment of FIG. 5 may be performed by the prediction unit of FIGS. 1 and 2 described above.

As described above with reference to 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. Therefore, when all encoding modes of the neighboring blocks are determined to be inter mode (and / or inter encoding mode), there is a high probability that the encoding mode of the current block is also determined to be inter mode (and / or inter encoding mode). Accordingly, when the encoding modes of the neighboring blocks a, b, c, and d are all inter modes, 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 homogeneous region, the magnitudes of the residual signals derived in 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 the 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. In addition, I16 may mean an intra mode in which intra prediction is performed in units of 16 × 16 blocks, and I4 may mean an intra mode in which intra prediction is performed in units of 4 × 4 blocks. However, the encoding mode illustrated 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 vectors of the neighboring blocks 523, 526, and 529 located in the vicinity of the current block are large, the coding mode for the current block is determined to be intra mode even if the current block is a block located within the homogeneous region. This can be high. If the motion vectors of the neighboring blocks 523, 526, and 529 are large, the motion vector of the current block has a high probability, and therefore, the amount of bits required for encoding the motion vector from the rate-distortion point of view may be large. Accordingly, the encoder may determine the encoding mode of the current block as the intra mode even when the magnitudes of the residual signals derived in the inter mode and the intra mode are similar to each other.

Accordingly, the encoder estimates the magnitude 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 that have been encoded and / or decoded, and then calculates the calculated SNMV value (the current When the estimated value of the motion vector magnitude of the block is smaller than a predetermined threshold value, intra mode skip may be applied to the current block. Here, the restored 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, a neighboring block used for SNMV calculation includes a left neighboring block 523 adjacent to the left side of the current block 515, a top neighboring block 526 adjacent to the top of the current block 515, and a current block. 515 may be a top right peripheral block 529 located at the top right corner of the exterior. Here, the current block 515, the left peripheral block 523, the upper peripheral block 526, and the upper right peripheral block 529 may correspond to CUs, for example.

0 indicated in the left peripheral block 523, 0 indicated in the upper peripheral block 526 and 2 indicated in the upper right peripheral block 529 may indicate a reference picture index, respectively. In addition, in 520 of FIG. 5, the motion vector of the left peripheral block 523 is (-2, -12), the motion vector of the upper peripheral block 526 is (12, -12), and the right upper peripheral 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, the SNMV may be derived by adding all norm values of the motion vectors of the neighboring blocks 523, 526, and 529. This can be represented by the following equation (2).

&Quot; (2) "

Here, MV may represent a motion vector. In addition, 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 can be derived by adding all norm values of the motion vectors of blocks a, b, and c. Here, block a may represent a left peripheral block 523, block b may represent an upper peripheral block 526, and block c may represent an upper right peripheral block 529.

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

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

As another example, the threshold value may be a value derived in an encoding process. For example, the encoder can obtain the SNMV of each of the neighboring blocks located around the current block and use the average value of the obtained SNMV values as the threshold. Here, the peripheral block is, for example, the left peripheral block (equivalent to block a of 410 of FIG. 4, 523), the upper peripheral block (equivalent to block b of 410 of FIG. 4, 526), and the upper right peripheral block of FIG. 4. Same as block c of 410, which may include 529. In addition, the neighboring block used for deriving the threshold value may further include an upper left peripheral 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 describing another embodiment of a method of determining whether intra mode skip is applied by an encoder. An intra mode skip application determination process according to the embodiment of FIG. 6 may be performed by the prediction unit of FIGS. 1 and 2 described above.

As described above with reference to 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. Therefore, when all encoding modes of the neighboring blocks are determined to be inter mode (and / or inter encoding mode), there is a high probability that the encoding mode of the current block is also determined to be inter mode (and / or inter encoding mode). Accordingly, when the encoding modes of the neighboring blocks a, b, c, and d are all inter modes, the encoder may determine in advance whether intra mode skip is applied to the current block according to an embodiment to be described later.

FIG. 6 illustrates a current picture 620 to which a current block (coding target block 625) belongs and a picture (hereinafter, referred to as a previous picture) 610 that has been encoded and / or decoded just before the time axis. Block 615 belonging to the previous picture 610 represents a block existing at the same position as the current block 625 within the previous picture 610. Hereinafter, 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 same position block 615 may correspond to a CU, for example.

In FIG. 6, the encoding modes determined for each block may be P, S, I16, I4, and the like. Here, P may mean an inter mode, and S may mean a skip mode. In addition, I16 may mean an intra mode in which intra prediction is performed in units of 16 × 16 blocks, and I4 may mean an intra mode in which intra prediction is performed in units of 4 × 4 blocks. However, the encoding mode illustrated 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 same 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 for the current block when the cross correlation between the current block 625 and the co-located block 615 is smaller than a predetermined threshold value. Equation 3 below shows an embodiment of a method for deriving a cross correlation between the current block 625 and the same position block 615.

&Quot; (3) "

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

In addition, "E [x]" represents the mean of the group corresponding to x, and "sd [x]" represents the standard deviation of the group corresponding to x. 'Corr [a, b]' may indicate a cross correlation between a group corresponding to a and a group corresponding to b.

When the cross correlation value derived by the above process is smaller than a predetermined threshold value, the encoder may apply intra mode skip for the current block. That is, when the cross correlation value is smaller than a predetermined threshold value, the encoder may omit the intra prediction and / or intra encoding process for the current block and perform only the inter prediction and / or inter encoding process. In this case, the encoder may perform an RDO process on the remaining modes except the intra mode, and may determine an encoding mode to be applied to the current block among the remaining coding modes except the intra mode.

If 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. Therefore, in this case, the encoder may perform an RDO process for all encoding modes including an intra mode, and may determine an encoding mode to be applied to the current block among all encoding modes including an intra mode.

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

As another example, the threshold value may be a value derived in an encoding process. For example, the encoder can obtain a cross correlation of each of the neighboring blocks located around the current block and use the average value of the obtained cross correlation values as the threshold. Here, as an example, as shown in 410 of FIG. 4, the neighboring block includes a block adjacent to the left side of the current block (a), a block adjacent to the top of the current block (b), and a block located at the upper right corner outside the current block ( c) and block (d) located at the upper left corner outside the current block.

7 is a flowchart schematically illustrating an embodiment of a coding mode determination method according to the present invention. An intra mode skip application determination process according to the embodiment of FIG. 7 may be performed by the prediction unit of FIGS. 1 and 2 described above.

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

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

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 the motion vectors of neighboring blocks located in the periphery of the current block. Since a specific embodiment of the SNMV value derivation process has been described above with reference to FIG. 5, a detailed description thereof will be omitted herein.

Referring back to FIG. 7, the encoder may derive a cross correlation between the current block and the same block (S730). Here, the co-located block may be a block in a previous picture (or previous frame) in which encoding and / or decoding is completed immediately before on the time axis, and is located at the same spatial position as the current block in the previous picture (or previous frame). It may be an existing block. Since a specific embodiment of the cross-correlation derivation process is described above with reference to FIG. 6, a detailed description thereof will be omitted herein.

Once the BAD, SNMV, and / or cross correlation is 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 for the current block when the BAD value is larger than a predetermined first threshold value, or 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 smaller than a predetermined second threshold value, or may perform intra prediction on the current block. In addition, as a third condition, the encoder may apply intra mode skip to the current block when the cross correlation value is smaller than a predetermined third threshold value, and otherwise, perform an intra prediction on the current block. have.

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

Whether intra mode skip is applied to the current block may be determined by an 'OR' operation on the first condition, the second condition, and the third condition, and an 'AND' on the first condition, the second condition, and the third condition. May be determined by a 'operation. When the 'OR' operation is used, intra prediction for the current block may be omitted when one or more of the first condition, the second condition, and the third condition are satisfied. In addition, when the 'AND' operation is used, the 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, although the BAD, SNMV and cross correlation are all calculated for the current block in the embodiment of FIG. 7, 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 BAD, SNMV, and cross correlation are calculated, the encoder may determine whether to apply intra mode skip by performing an OR operation or an AND operation on this.

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

Referring to FIG. 7 again, the encoder may determine an 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 encoding process for the current block, and may perform only the inter prediction and / or inter encoding process. In this case, the encoder may determine a coding mode candidate of the current block as the coding mode candidate of the current block except the intra mode among the coding modes that the current block may have. In this case, the encoder may determine an encoding mode to be applied to the current block among the encoding mode candidates by performing an RDO process on the determined encoding mode candidate (the encoding mode except the intra mode among the encoding modes that the current block may have). .

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

According to the embodiments described above, the encoding process for the intra mode may be terminated early. Thus, since unnecessary intra mode related calculations can be skipped, the 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 the steps, and some steps may occur in different orders or simultaneously . It will also be understood by those skilled in the art that the steps depicted in the flowchart illustrations are not exclusive, that other steps may be included, or that one or more steps in the flowchart may be deleted without affecting the scope of the present invention. You will understand.

The above-described embodiments include examples of various aspects. While it is not possible to describe every possible combination for expressing various aspects, one of ordinary skill in the art will recognize that other combinations are possible. Accordingly, it is intended that the invention include all alternatives, modifications and variations that fall 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 intra prediction is omitted, a coding mode to be applied to the current block is determined among the plurality of coding mode candidates. Making; And
Performing encoding on the current block based on the determined encoding mode,
In the determining of the encoding mode,
And when it is determined that the intra prediction is omitted, the encoding mode other than the intra mode among the encoding modes that the current block may have is determined as the plurality of encoding mode candidates. The method of claim 1,
Determining whether or not the intra prediction is omitted,
A left peripheral block located adjacent to the left side of the current block, an upper peripheral block located adjacent to the top of the current block, a right upper peripheral block closest to the upper right corner of the current block and an upper left corner of the current block The image encoding method of claim 1, wherein the encoding mode is performed when the encoding modes of the closest upper left neighboring blocks are all inter modes. The method of claim 1,
Determining whether or not the intra prediction is omitted,
Deriving a Boundary Absolute Difference (BAD) value for the current block; And
Determining that the intra prediction is omitted if the BAD value is greater than a predetermined threshold,
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,
And the predetermined threshold value is an experimentally determined value which is previously stored in an encoder. 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 or not the intra prediction is omitted,
Deriving a sum of norm motion vector (SNMV) value for the current block; And
Determining that the intra prediction is omitted if the SNMV value is less than a predetermined threshold,
The SNMV value is derived by adding a norm of motion vectors of a plurality of neighboring blocks located around the current block. The method according to claim 6,
And the predetermined threshold value is an experimentally determined value which is previously stored in an encoder. The method according to 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. The method of claim 1,
Determining whether or not the intra prediction is omitted,
Deriving a cross-correlation value for the current block; And
Determining that the intra prediction is omitted if the cross correlation 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, wherein the co-located block is located within a picture that has been coded just before in time. And a block existing at the same position as the current block. The method of claim 9,
And the predetermined threshold value is an experimentally determined value which is previously stored in an encoder. The method of claim 9,
The predetermined threshold value is determined as an average value of cross correlation values of a plurality of neighboring blocks located around the current block. Determining whether intra prediction for the current block is omitted; And
By performing a rate-distortion optimization (RDO) process on a plurality of coding mode candidates determined according to whether intra prediction is omitted, a coding mode to be applied to the current block is determined among the plurality of coding mode candidates. Including the steps of:
In the determining of the encoding mode,
And when it is determined that the intra prediction is omitted, 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. 13. The method of claim 12,
Determining whether or not the intra prediction is omitted,
A left peripheral block located adjacent to the left side of the current block, an upper peripheral block located adjacent to the top of the current block, a right upper peripheral block closest to the upper right corner of the current block and an upper left corner of the current block The encoding mode determination method, wherein the encoding mode is performed when the encoding modes of the closest upper left neighboring blocks are all inter modes. 13. The method of claim 12,
Determining whether or not the intra prediction is omitted,
Deriving a Boundary Absolute Difference (BAD) value for the current block; And
Determining that the intra prediction is omitted if the BAD value is greater than a predetermined threshold,
The BAD value is determined based on a pixel value 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. Way. 13. The method of claim 12,
Determining whether or not the intra prediction is omitted,
Deriving a sum of norm motion vector (SNMV) value for the current block; And
Determining that the intra prediction is omitted if the SNMV value is less than a predetermined threshold,
The SNMV value is derived by adding a norm of motion vectors of a plurality of neighboring blocks located around the current block. 13. The method of claim 12,
Determining whether or not the intra prediction is omitted,
Deriving a cross-correlation value for the current block; And
Determining that the intra prediction is omitted if the cross correlation 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, wherein the co-located block is located within a picture that has been coded just before in time. Encoding mode determination method characterized in that the block exists in the same position spatially with the current block. A prediction unit to determine a coding mode to be applied to a current block and to generate a prediction block corresponding to the current block by performing prediction on the current block based on the coding mode; And
A bitstream generator configured to generate a coded bitstream based on the current block and the prediction block,
The predicting unit,
It is determined whether intra prediction for the current block is omitted, and a rate-distortion optimization (RDO) process is performed for a plurality of encoding mode candidates determined according to whether intra prediction is omitted. And determining an encoding mode to be applied to the current block among the plurality of encoding mode candidates. 18. The method of claim 17,
The predicting unit,
And if it is determined that the intra prediction is omitted, the plurality of encoding mode candidates are selected from the encoding modes other than the intra mode among the encoding modes that the current block may have.

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