KR20210153547A - method and apparatus for encoding/decoding a VIDEO SIGNAL, and a recording medium storing a bitstream - Google Patents

Abstract

An object of the present disclosure is to provide a method and device for efficiently performing intra-prediction in encoding/decoding a video signal. According to the present invention, an image decoding method comprises: a step of determining a reference pixel line of a current block; a step of determining an intra-prediction mode of the current block; and a step of deriving a prediction sample of the current block based on a reference pixel belonging to a reference pixel line and the intra-prediction mode. In this case, a prediction sample is determined based on a prediction angle of the intra-prediction mode and an inverse-angle variable derived based on the prediction angle. When the predicted angle has a predefined value, the inverse-angle variable has a default value.

Description

A method and apparatus for encoding/decoding a video signal, and a recording medium storing a bitstream

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

Recently, the demand for high-resolution and high-quality images such as HD (High Definition) images and UHD (Ultra High Definition) images is increasing in various application fields. As the image data becomes higher resolution and higher quality, the amount of data increases relatively compared to the existing image data. The storage cost will increase. High-efficiency image compression techniques can be used to solve these problems that occur as image data becomes high-resolution and high-quality.

Interprediction technology that predicts pixel values included in the current picture from pictures before or after the current picture as an image compression technology, and intra prediction technology that predicts pixel values included in the current picture using pixel information in the current picture, appeared Various technologies exist, such as entropy encoding technology in which a short code is assigned to a value with a high frequency and a long code is assigned to a value with a low frequency of appearance.

Meanwhile, as the demand for high-resolution images increases, the demand for stereoscopic image content as a new image service is also increasing. A video compression technology for effectively providing high-resolution and ultra-high-resolution stereoscopic image content is being discussed.

An object of the present disclosure is to provide a method and apparatus for efficiently performing intra prediction in encoding/decoding a video signal.

The technical problems to be achieved in the present disclosure are not limited to the technical problems mentioned above, and other technical problems not mentioned will be clearly understood by those of ordinary skill in the art to which the present disclosure belongs from the description below. will be able

In an image decoding method according to the present disclosure, determining a reference pixel line of a current block, determining an intra prediction mode of the current block, and the reference pixel belonging to the reference pixel line and the intra prediction mode based on the and deriving a prediction sample of the current block. In this case, the prediction sample is determined based on the prediction angle of the intra prediction mode and an inverse-angle variable derived based on the prediction angle, and when the prediction angle has a predefined value, the inverse-angle variable may have a default value.

An image encoding method according to the present disclosure includes determining a reference pixel line of a current block, determining an intra prediction mode of the current block, and based on a reference pixel belonging to the reference pixel line and the intra prediction mode and deriving a prediction sample of the current block. In this case, the prediction sample is determined based on the prediction angle of the intra prediction mode and an inverse-angle variable derived based on the prediction angle, and when the prediction angle has a predefined value, the inverse-angle variable may have a default value.

In the image decoding/encoding method according to the present disclosure, reference pixels belonging to the reference pixel line may be arranged in a 1D array based on the inverse-angle variable.

In the image decoding/encoding method according to the present disclosure, based on whether the value of the prediction angle belongs to a predefined value or a predefined range, it may be determined whether to update the inverse-angle variable.

In the image decoding/encoding method according to the present disclosure, when it is determined not to update the inverse-angle variable, the inverse-angle variable may maintain the default value.

In the image decoding/encoding method according to the present disclosure, the default value may be 0.

According to the present disclosure, encoding/decoding efficiency can be improved by providing an improved intra prediction method.

The effects obtainable in the present disclosure are not limited to the above-mentioned effects, and other effects not mentioned may be clearly understood by those of ordinary skill in the art to which the present disclosure belongs from the description below. will be.

1 is a block diagram illustrating an image encoding apparatus according to an embodiment of the present disclosure.
2 is a block diagram illustrating an image decoding apparatus according to an embodiment of the present disclosure.
3 illustrates an intra prediction method according to the present disclosure.
4 shows a pre-defined intra prediction mode.

Since the present disclosure can make various changes and can have various embodiments, specific embodiments are illustrated in the drawings and described in detail in the detailed description. However, this is not intended to limit the present disclosure to specific embodiments, and it should be understood to include all modifications, equivalents and substitutes included in the spirit and scope of the present disclosure. In describing each figure, like reference numerals have been used for like elements.

Terms such as first, second, etc. may be used to describe various elements, but the elements should not be limited by the terms. The above terms are used only for the purpose of distinguishing one component from another. For example, without departing from the scope of the present disclosure, a first component may be referred to as a second component, and similarly, a second component may also be referred to as a first component. and/or includes a combination of a plurality of related listed items or any of a plurality of related listed items.

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

The terminology used in the present application is used only to describe specific embodiments, and is not intended to limit the present disclosure. The singular expression includes the plural expression unless the context clearly dictates otherwise. In the present application, terms such as “comprise” or “have” are intended to designate that a feature, number, step, operation, component, part, or combination thereof described in the specification exists, but one or more other features It should be understood that this does not preclude the existence or addition of numbers, steps, operations, components, parts, or combinations thereof.

Hereinafter, preferred embodiments of the present disclosure will be described in more detail with reference to the accompanying drawings. Hereinafter, the same reference numerals are used for the same components in the drawings, and repeated descriptions of the same components are omitted.

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

Referring to FIG. 1 , the image encoding apparatus 100 includes a picture division unit 110 , prediction units 120 and 125 , a transform unit 130 , a quantization unit 135 , a rearrangement unit 160 , and an entropy encoding unit ( 165 ), an inverse quantization unit 140 , an inverse transform unit 145 , a filter unit 150 , and a memory 155 .

Each of the constituent units shown in FIG. 1 is independently illustrated to represent different characteristic functions in the image encoding apparatus, and does not mean that each constituent unit is composed of separate hardware or one software constituent unit. That is, each component is listed as each component for convenience of description, and at least two components of each component are combined to form one component, or one component can be divided into a plurality of components to perform a function, and each of these components Integrated embodiments and separate embodiments of components are also included in the scope of the present disclosure without departing from the essence of the present disclosure.

In addition, some of the components are not essential components to perform an essential function in the present disclosure, but may be optional components for merely improving performance. The present disclosure may be implemented by including only essential components to implement the essence of the present disclosure except for 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 disclosure.

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

For example, one picture may be divided into a plurality of coding units. In order to split a coding unit in a picture, a recursive tree structure such as a quad tree structure may be used. A coding in which one image or a largest coding unit is used as a root and is divided into other coding units. A unit may be divided having as many child nodes as the number of divided coding units. A coding unit that is no longer split according to certain restrictions becomes a leaf node. That is, if it is assumed that only square splitting is possible for one coding unit, one coding unit may be split into up to four different coding units.

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

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

When generating a prediction unit for performing intra prediction based on a coding unit, if it is not the smallest coding unit, intra prediction may be performed without dividing the prediction unit into a plurality of NxN prediction units.

The prediction units 120 and 125 may include an inter prediction unit 120 performing inter prediction and an intra prediction unit 125 performing intra prediction. Whether to use inter prediction or to perform intra prediction for a prediction unit may be determined, and specific information (eg, intra prediction mode, motion vector, reference picture, etc.) according to each prediction method may be determined. In this case, a processing unit in which prediction is performed and a processing unit in which a prediction method and specific content are determined may be different. For example, a prediction method and a prediction mode may be determined in a prediction unit, and prediction may be performed in a transformation unit. A residual value (residual block) between the generated prediction block and the original block may be input to the transform unit 130 . Also, prediction mode information, motion vector information, etc. used for prediction may be encoded by the entropy encoder 165 together with the residual value and transmitted to the decoding apparatus. When a specific encoding mode is used, the original block may be encoded and transmitted to the decoder without generating the prediction block through the predictors 120 and 125 .

The inter prediction unit 120 may predict a prediction unit based on information on at least one of a picture before or after the current picture, and in some cases, prediction based on information of a partial region in the current picture that has been encoded Units can also be predicted. The inter prediction unit 120 may include a reference picture interpolator, a motion prediction unit, and a motion compensator.

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

The motion prediction unit may perform motion prediction based on the reference picture interpolated by the reference picture interpolator. As a method for calculating the motion vector, various methods such as Full search-based Block Matching Algorithm (FBMA), Three Step Search (TSS), and New Three-Step Search Algorithm (NTS) may be used. The motion vector may have a motion vector value of 1/2 or 1/4 pixel unit based on the interpolated pixel. The motion prediction unit may predict the current prediction unit by using a different motion prediction method. Various methods, such as a skip method, a merge method, an AMVP (Advanced Motion Vector Prediction) method, an intra block copy method, etc., may be used as the motion prediction method.

The intra prediction unit 125 may generate a prediction unit based on reference pixel information around the current block, which is pixel information in the current picture. When the neighboring block of the current prediction unit is a block on which inter prediction is performed, and thus the reference pixel is a pixel on which inter prediction is performed, the reference pixel included in the block on which the inter prediction is performed is a reference pixel of the block on which the intra prediction is performed. information can be used instead. That is, when the reference pixel is not available, the unavailable reference pixel information may be replaced with at least one reference pixel among the available reference pixels.

In intra prediction, the prediction mode may have a directional prediction mode in which reference pixel information is used according to a prediction direction and a non-directional mode in which directional information is not used when prediction is performed. A mode for predicting luminance information and a mode for predicting chrominance information may be different, and intra prediction mode information used for predicting luminance information or predicted luminance signal information may be utilized to predict chrominance information.

When intra prediction is performed, if the size of the prediction unit and the size of the transformation unit are the same, intra prediction for the prediction unit based on the pixel present on the left side, the pixel present on the upper left side, and the pixel present on the upper side of the prediction unit can be performed. However, when the size of the prediction unit is different from the size of the transformation unit when intra prediction is performed, intra prediction may be performed using a reference pixel based on the transformation unit. In addition, intra prediction using NxN splitting may be used only for the smallest coding unit.

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

In addition, a residual block including residual information that is a difference value from the original block of the prediction unit and the prediction unit in which prediction is performed based on the prediction unit generated by the prediction units 120 and 125 may be generated. The generated residual block may be input to the transform unit 130 .

The transform unit 130 converts the original block and the residual block including residual information of the prediction units generated by the prediction units 120 and 125 to DCT (Discrete Cosine Transform), DST (Discrete Sine Transform), and KLT It can be converted using the same conversion method. Whether to apply DCT, DST, or KLT to transform the residual block may be determined based on intra prediction mode information of a prediction unit used to generate the residual block.

The quantization unit 135 may quantize values transformed in the frequency domain by the transform unit 130 . The quantization coefficient may vary according to blocks or the importance of an image. The value calculated by the quantization unit 135 may be provided to the inverse quantization unit 140 and the rearrangement unit 160 .

The rearrangement unit 160 may rearrange the coefficient values on the quantized residual values.

The rearrangement unit 160 may change the two-dimensional block form coefficient into a one-dimensional vector form through a coefficient scanning method. For example, the rearranging unit 160 may use a Zig-Zag Scan method to scan from DC coefficients to coefficients in a high frequency region, and may change them into a one-dimensional vector form. A vertical scan for scanning a two-dimensional block shape coefficient in a column direction and a horizontal scan for scanning a two-dimensional block shape coefficient in a row direction may be used instead of the zig-zag scan according to the size of the transform unit and the intra prediction mode. That is, it may be determined whether any of the zig-zag scan, the vertical scan, and the horizontal scan is used according to the size of the transform unit and the intra prediction mode.

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

The entropy encoder 165 receives the residual value coefficient information, block type information, prediction mode information, division unit information, prediction unit information and transmission unit information, motion of the coding unit from the reordering unit 160 and the prediction units 120 and 125 . Various information such as vector information, reference frame information, interpolation information of a block, and filtering information may be encoded.

The entropy encoder 165 may entropy-encode the coefficient values of the coding units input from the reordering unit 160 .

The inverse quantizer 140 and the inverse transform unit 145 inversely quantize the values quantized by the quantizer 135 and inversely transform the values transformed by the transform unit 130 . The residual values generated by the inverse quantizer 140 and the inverse transform unit 145 are combined with the prediction units predicted through the motion estimation unit, the motion compensator, and the intra prediction unit included in the prediction units 120 and 125 and restored. You can create a Reconstructed Block.

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

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

The offset correcting unit may correct an offset from the original image in units of pixels with respect to the image on which the deblocking has been performed. In order to perform offset correction on a specific picture, a method of dividing pixels included in an image into a certain number of regions, determining the region to be offset and applying the offset to the region, or taking edge information of each pixel into account can be used to apply

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

The memory 155 may store the reconstructed block or picture calculated through the filter unit 150 , and the stored reconstructed block or picture may be provided to the predictors 120 and 125 when inter prediction is performed.

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

Referring to FIG. 2 , the image decoding apparatus 200 includes an entropy decoding unit 210, a reordering unit 215, an inverse quantization unit 220, an inverse transform unit 225, prediction units 230 and 235, and a filter unit ( 240) and a memory 245 may be included.

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

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

The entropy decoding unit 210 may decode information related to intra prediction and inter prediction performed by the encoding apparatus.

The reordering unit 215 may perform rearrangement based on a method of rearranging the entropy-decoded bitstream by the entropy decoding unit 210 by the encoder. Coefficients expressed in the form of a one-dimensional vector may be restored and rearranged as coefficients in the form of a two-dimensional block. The reordering unit 215 may receive information related to the coefficient scanning performed by the encoder and perform the rearrangement by performing a reverse scanning method based on the scanning order performed by the corresponding encoder.

The inverse quantizer 220 may perform inverse quantization based on the quantization parameter provided by the encoding apparatus and the reordered coefficient values of the blocks.

The inverse transform unit 225 may perform inverse transforms, ie, inverse DCT, inverse DST, and inverse KLT, on the transforms performed by the transform unit, ie, DCT, DST, and KLT, on the quantization result performed by the image encoding apparatus. Inverse transform may be performed based on a transmission unit determined by the image encoding apparatus. The inverse transform unit 225 of the image decoding apparatus may selectively perform a transformation technique (eg, DCT, DST, KLT) according to a plurality of pieces of information such as a prediction method, a size of a current block, and a prediction direction.

The prediction units 230 and 235 may generate a prediction block based on the prediction block generation related information provided from the entropy decoding unit 210 and previously decoded block or picture information provided from the memory 245 .

As described above, when intra prediction is performed in the same manner as in the operation of the image encoding apparatus, when the size of the prediction unit and the size of the transformation unit are the same, the pixel present at the left side of the prediction unit, the pixel present at the upper left side, and the upper side Intra prediction is performed on the prediction unit based on existing pixels, but when the size of the prediction unit and the size of the transformation unit are different when performing intra prediction, intra prediction is performed using the reference pixel based on the transformation unit can do. Also, intra prediction using NxN splitting may be used only for the smallest coding unit.

The prediction units 230 and 235 may include a prediction unit determiner, an inter prediction unit, and an intra prediction unit. The prediction unit determining unit receives various information such as prediction unit information input from the entropy decoder 210, prediction mode information of the intra prediction method, and motion prediction related information of the inter prediction method, and divides the prediction unit from the current coding unit, and predicts It may be determined whether the unit performs inter prediction or intra prediction. The inter prediction unit 230 uses information required for inter prediction of the current prediction unit provided from the image encoding apparatus based on the information included in at least one picture before or after the current picture including the current prediction unit. Inter prediction may be performed on the prediction unit. Alternatively, inter prediction may be performed based on information on a pre-restored partial region in the current picture including the current prediction unit.

In order to perform inter prediction, a motion prediction method of a prediction unit included in a corresponding coding unit based on a coding unit is selected from among skip mode, merge mode, AMVP mode, and intra block copy mode. You can decide which way to go.

The intra prediction unit 235 may generate a prediction block based on pixel information in the current picture. When the prediction unit is a prediction unit on which intra prediction is performed, intra prediction may be performed based on intra prediction mode information of the prediction unit provided by the image encoding apparatus. The intra prediction unit 235 may include an adaptive intra smoothing (AIS) filter, a reference pixel interpolator, and a DC filter. The AIS filter is a part that performs filtering on the reference pixel of the current block, and can be applied by determining whether to apply the filter according to the prediction mode of the current prediction unit. AIS filtering may be performed on the reference pixel of the current block by using the prediction mode and AIS filter information of the prediction unit provided by the image encoding apparatus. When the prediction mode of the current block is a mode in which AIS filtering is not performed, the AIS filter may not be applied.

When the prediction mode of the prediction unit is a prediction unit in which intra prediction is performed based on a pixel value obtained by interpolating the reference pixel, the reference pixel interpolator may interpolate the reference pixel to generate a reference pixel of a pixel unit having an integer value or less. When the prediction mode of the current prediction unit is a prediction mode that generates a prediction block without interpolating the reference pixel, the reference pixel may not be interpolated. The DC filter may generate a prediction block through filtering when the prediction mode of the current block is the DC mode.

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

Information on whether a deblocking filter is applied to a corresponding block or picture and information on whether a strong filter or a weak filter is applied when the deblocking filter is applied may be provided from the image encoding apparatus. The deblocking filter of the image decoding apparatus may receive deblocking filter-related information provided from the image encoding apparatus, and the image decoding apparatus may perform deblocking filtering on the corresponding block.

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

ALF may be applied to a coding unit based on information on whether ALF is applied, ALF coefficient information, etc. provided from the encoding apparatus. Such ALF information may be provided by being included in a specific parameter set.

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

As described above, hereinafter, in the embodiments of the present disclosure, a coding unit is used as a term for a coding unit for convenience of description, but may also be a unit for performing decoding as well as coding.

In addition, the current block denotes an encoding/decoding target block, and depending on the encoding/decoding step, a coding tree block (or coding tree unit), a coding block (or a coding unit), a transform block (or a transform unit), or a prediction block (or prediction unit) and the like. In this specification, a 'unit' may indicate a basic unit for performing a specific encoding/decoding process, and a 'block' may indicate a pixel array of a predetermined size. Unless otherwise specified, 'block' and 'unit' may be used interchangeably. For example, in the embodiments to be described later, it may be understood that the coding block (coding block) and the coding unit (coding unit) have the same meaning.

3 illustrates an intra prediction method according to the present disclosure.

Referring to FIG. 3 , a reference pixel line for intra prediction of a currently encoded/decoded block (hereinafter, referred to as a current block) may be determined ( S300 ).

The reference pixel line according to the present disclosure may belong to a neighboring block adjacent to the current block. The neighboring block may include at least one of a left block, an upper block, an upper left block, a lower left block, an upper right block, a right block, a lower right block, and a lower block. Also, the reference pixel line may belong to a spatial block belonging to the same picture as the current block but not adjacent to the current block, or may belong to a temporal block belonging to a picture different from the current block. Here, the temporal block may be a block at the same location as a neighboring block of the aforementioned current block.

The reference pixel line may be limited to belong only to a block pre-encoded/decoded before the current block, or may belong to a block encoded/decoded after the current block. In this case, a separate padding process may be accompanied.

The reference pixel line may be determined as any one of a plurality of reference pixel line candidates. The plurality of reference pixel line candidates include a first reference pixel line adjacent to the current block, a second reference pixel line adjacent to the first reference pixel line, ... , at least one of an nth reference pixel line adjacent to the (n−1)th reference pixel line. Here, n may be an integer of 2, 3, 4, or more.

For example, the plurality of reference pixel line candidates may include first to fourth reference pixel lines, only first to third reference pixel lines, or only first and third reference pixel lines. .

Any one of the above-described first to nth reference pixel lines may be selectively used, and for this purpose, information for specifying a position of the reference pixel line may be used. The information may be signaled by the encoding device. For example, index information indicating an index of one of a plurality of reference pixel lines may be signaled through a bitstream. Based on the index information, the index refIdx of the reference pixel line of the current block may be determined. The value of the index refIdx of the reference pixel line being 0 indicates the reference pixel line adjacent to the current block. The value of the index refIdx of the reference pixel line of n indicates a reference pixel line separated by n lines from the adjacent reference pixel line.

Alternatively, the index refIdx of each reference pixel line may be determined in consideration of proximity to the current block. For example, when one of the three reference pixel lines is selected, the index of the reference pixel line closest to the current block among the three reference pixel lines is set to 0, and the index of the reference pixel line farthest from the current block is set. can be set to 2. An index of the reference pixel line between the two reference pixels may be set to 1.

Alternatively, the reference pixel line of the current block may be determined based on a predetermined encoding parameter.

The encoding parameter may include at least one of a block size, a shape, a position, a component type (Y/Cb/Cr), a prediction mode (intra/inter), an intra prediction mode, or a non-directional mode. Here, a block may mean a current block and/or a neighboring block, or a coding block, a prediction block, and/or a transform block. In addition, the number of reference pixel line candidates available for the current block may also be variably determined based on the above-described encoding parameter, and information for specifying the number may be separately signaled.

Referring to FIG. 3 , an intra prediction mode of the current block may be determined ( S310 ).

The intra prediction mode of the current block may be determined as any one of intra prediction modes pre-defined in the encoding/decoding apparatus. The pre-defined intra prediction mode may be composed of two non-directional modes and 65 directional modes, as shown in FIG. 4 .

The directional mode includes at least one of a planar mode or a DC mode, and the non-directional mode includes a mode (eg, vertical mode, horizontal mode, diagonal mode, etc.) having a predetermined angle/direction. can

Referring to FIG. 3 , intra prediction of the current block may be performed based on the determined reference pixel line and the intra prediction mode ( S320 ).

That is, a pixel corresponding to the determined intra prediction mode among pixels of the reference pixel line is specified as a reference pixel, and a prediction pixel can be derived using the specified reference pixel.

A prediction pixel of the current block may be derived with reference to a reference pixel. In this case, the reference pixel referenced for deriving the prediction pixel may be determined based on the direction of the intra prediction mode.

5 is an example of a case in which a directional intra prediction mode having an index of 58 is selected.

In order to derive the prediction pixel of the current block, a reference pixel around the current block may be determined based on the direction of the intra prediction mode.

In this case, when the position specified by the direction of the intra prediction mode is not an integer position, a fractional position reference pixel may be generated by interpolating the reference pixels. The value of the generated fractional position reference pixel is set as the predicted pixel of the corresponding position.

The type of interpolation filter used to generate the fractional position reference pixel may be selected from among a plurality of interpolation filter types. The plurality of interpolation filter types may include a cubic convolution filter and a Gaussian filter.

In order to select one of a plurality of interpolation filter types, internal variables in the encoder and the decoder may be defined. The internal variable may be a flag filterFlag that specifies one of two filter type candidates or an index filterIdx that specifies one of a plurality of filter type candidates. For example, a value of the internal variable filterFlag of 0 indicates that a cubic convolution filter is selected, and a value of 1 of the internal variable filterFlag indicates that a Gaussian filter is selected.

In the embodiment to be described later, it is assumed that the internal variable is a flag filterFlag indicating one of two filter type candidates.

The internal variable filterFlag may be derived based on at least one of whether a smoothing filter is applied to a reference pixel, an index of a reference pixel line, an intra prediction mode, or a block size.

For example, when a smoothing filter is applied to the reference pixel or when the index refIdx of the reference pixel line is not 0, the value of the internal variable filterFlag may be set to 1.

If the above conditions are not satisfied, the value of the internal variable filterFlag may be derived in consideration of the intra prediction mode of the current block and the size of the current block. First, a variable minDistVerHor indicating a difference value between the intra prediction mode of the current block and the horizontal intra prediction mode or the vertical intra prediction mode may be derived. Specifically, the minimum value of the difference between the index of the intra prediction mode of the current block and the index of the vertical intra prediction mode or the difference between the index of the intra prediction mode of the current block and the index of the horizontal intra prediction mode is defined as the minimum value of the variable minDistVerHor. It can be set to a value. That is, the value of the variable mistDistVerHor can be derived as in Equation 1 below.

In Equation 1, the variable CurrIntraPredMode indicates the index of the intra prediction mode of the current block. 50 is an index of a vertical intra prediction mode, and 18 is an index of a horizontal intra prediction mode. Min(A,B) is a function that outputs the smaller of A and B, and abs(X) is a function that outputs the absolute value of X.

By comparing the variable minDistVerHor with the threshold, the value of the variable filterFlag can be derived.

In this case, the threshold value may be adaptively determined based on the size of the current block. As an example, Table 1 shows a mapping relationship between a size nTbS of a current block and a threshold intraHorVerDistThres.

nTbS=2 nTbS=3 nTbS=4 nTbS=5 nTbS=6 nTbS=7 intraHorVerDIstThres[nTbS] 24 14 2 0 0 0

The variable nTbS indicating the size of the current block may be derived based on at least one of a width or a height of the current block. Equation 2 shows an example of deriving the variable nTbS.

In Equation 2, the variable nTbW and the variable nTbH represent the width and height of the current block, respectively.

When the variable minDistVerHor is greater than the variable intraHorVerDistThres[nTbS], the value of the variable filterFlag may be set to 1. On the other hand, when the variable minDistVerHor is smaller than the variable intraHorVerDistThres[nTbS], the value of the variable filterFlag may be set to 0.

Alternatively, the value of the variable filterFlag may be derived based on information signaled through the bitstream (eg, a filter type flag).

A filter type may be independently determined for each color component. That is, the filter type of the luminance component and the filter type of the chrominance component may be independently determined.

Alternatively, it may be set to use the same type of interpolation filter as the luminance component for the chrominance component.

Alternatively, the filter type of the color difference component may be set to be the same as that of the luminance component, but filter characteristics for each component may be set differently. Here, the filter characteristic indicates at least one of the number of filter taps and filter coefficients.

Alternatively, for the luminance component, one of a 4-tap Gaussian filter and a cubic convolution filter is selected and used, but for the color component, a 2-tap filter whose weight is determined based on fractional pixel positions may be used.

A prediction angle may be different according to the intra prediction mode of the current block. As an example, Table 2 shows prediction angles for each intra prediction mode.

In Table 2, a variable predModeIntra indicates an index of an intra prediction mode, and a variable intraPredAngle indicates a prediction angle.

Based on the intra prediction angle, a variable invAngle for specifying a reference pixel may be derived. As an example, the variable invAngle may be derived based on Equation 3 below.

The variable invAngle can also be called an inverse angle variable. According to the value of the variable invAngle, it may be determined whether to arrange 2D reference sample arrays in 1D form, whether to rearrange left reference samples in a horizontal direction, or whether to rearrange upper reference samples in a vertical direction.

In addition to the above variables, a reference pixel array ref[x] generated by arranging reference pixels in a 1D form, a variable iIdx for specifying a reference pixel in the reference pixel array, and a position of the reference pixel (eg, an integer position or a fractional position) A prediction pixel may be derived by further derivating at least one of the variables iFact for specifying .

In this case, the variables are, in cases where the intra prediction mode of the current block is greater than or equal to the upper-left diagonal mode (that is, the intra prediction mode having the index 34), and when it is not (ie, the index of the intra prediction mode is less than 34), The induction method may be different. Accordingly, a method of deriving a prediction pixel will be described by dividing the case in which the intra prediction mode of the current block is greater than or equal to the upper left diagonal direction mode and the case in which the intra prediction mode is not.

First, when the index of the intra prediction mode of the current block is greater than or equal to the upper-left diagonal mode (ie, when predModeIntra is greater than or equal to 34), a method of deriving a prediction pixel will be described.

Reference pixels belonging to the reference pixel line may be stored in the reference pixel buffer ref[x] as shown in Equation 4 below.

In Equation 4, ref[x] represents the value of the restored pixel stored at the x position in the reference pixel buffer.

In Equation 4, p[a][b] represents a restored pixel at the position (a, b) or a restored pixel to which a smoothing filter is applied. refIdx indicates the index of the reference pixel line. For reference pixels positioned at the top of the current block, a may have a value between (-1-refIdx) and (refW-1), and b may have a value of (-1-refIdx). refW indicates the number of horizontal reference pixels, and may be set to 2nTbW or (nTbW + nTbH). For reference pixels located on the left side of the current block, a may have a value of (-1-refIdx), and b may have a value between (-1-refIdx) and (refH-1). refH indicates the number of vertical reference pixels, and may be set to 2nTbH or (nTbW + nTbH).

That is, reference pixels arranged in a 2D form may be rearranged in a 1D reference pixel buffer.

Based on at least one of the prediction angle intraPredAngle or the variable invAngle, the reference pixel used to derive the prediction pixel at the (x, y) position may be specified. For example, when the value of the variable intraPredAngle is less than 0, the reference pixel ref[x,] used to derive the prediction pixel at the (x, y) position may be derived based on Equation 5 below.

On the other hand, when the value of the prediction angle intraPredAngle is 0 or more, the reference pixel ref[x] used to derive the prediction pixel at the (x, y) position may be derived based on Equation 6 below.

In this case, the values of the reference pixel buffer ref[refW+refIdx+x] whose value of x is between 1 and (Max(1, nTbW/nTbH)*refIdx+1) are all p[-1+refW][-1- refIdx　].

Next, a variable iIdx for specifying the reference pixel in the reference pixel buffer and a variable iFact indicating the position of the fractional pixel can be derived. As an example, Equations 7 and 8 show examples of deriving the above two variables.

When an interpolation filter is used, a prediction sample may be derived by interpolating reference pixels according to the selected interpolation filter type. In this case, filter coefficients may be set differently according to color components.

As an example, for the luminance component, the filter coefficient fT[j] may be determined as in Equation 9 below.

When a 4-tap interpolation filter is used, j may be set to a value between 0 and 3. fG represents the filter coefficients of the Gaussian filter, and fC represents the filter coefficients of the cubic convolution filter.

Filter coefficients of each filter type may be pre-stored in the encoder and the decoder. As an example, Table 3 shows filter coefficients of each filter type.

When the filter coefficients are determined, a prediction pixel may be derived based on Equation 10 below.

For the chrominance component, whether to use the interpolation filter may be determined based on the variable iFact. As an example, when the variable iFact indicates an integer position (ie, when the value of iFact is 0), a prediction pixel may be derived based on Equation 11 below.

On the other hand, when the variable iFact indicates a fractional position (ie, the value of iFact is not 0), a prediction pixel may be derived based on Equation 12 below.

Next, when the intra prediction mode of the current block is smaller than the index in the upper left diagonal direction (ie, when predModeIntra is smaller than 34), a prediction pixel derivation method will be described. In this case, the reference pixel buffer can be defined as follows.

In this case, when the variable intraPredAngle representing the prediction angle is less than 0, the reference pixel buffer ref[x] may be expanded as in Equation 14.

On the other hand, when the variable intraPredAngle representing the prediction angle is greater than 0, the reference pixel buffer ref[x] may be expanded as in Equation 15.

In this case, the values of the reference pixel buffer ref[refH+refIdx+x] with the value of x between 1 and (Max(1, nTbH/nTbW)*refIdx+1) are all p[-1-refIdx][-1+ refH].

And, based on Equations 16 and 17, the variable iIdx and the variable iFact may be derived.

When an interpolation filter is used, a prediction sample may be derived by interpolating reference pixels according to the selected interpolation filter type. In this case, filter coefficients may be set differently according to color components.

As an example, for the luminance component, as in the example of Equation 8, the filter coefficient fT[j] may be determined.

When the filter coefficients are determined, a prediction pixel may be derived based on Equation 17 below.

For the chrominance component, whether to use the interpolation filter may be determined based on the variable iFact. As an example, when the variable iFact indicates an integer position (ie, when the value of iFact is 0), a prediction pixel may be derived based on Equation 19 below.

On the other hand, when the variable iFact indicates a fractional position (ie, the value of iFact is not 0), a prediction pixel may be derived based on Equation 20 below.

In the above-described example, it is exemplified that the variable invAngle for specifying the storage location of the reference pixel in the reference pixel buffer is derived based on the variable intraPredAngle indicating the prediction angle. However, in the present disclosure, for a specific intra prediction mode, the process of deriving the variable invAngle may be omitted. For example, when the variable intraPredAngle has a preset value or falls within a preset range, the derivation process (eg, Equation 3) of the variable invAngle may be omitted. Here, the preset value is an integer such as 0, 1, or 2, and the predefined range may be set to 0, -1 to 1, or -2 to 2, or the like.

That is, Equation 3 described above may be performed only when the variable intraPredAngle does not have a preset value or when the absolute value of the variable intraPredAngle is greater than a preset constant value.

If the derivation process of the variable invAngle is omitted, the procedure using the variable invAngle is set not to be performed in the prediction sample derivation procedure, or the variable invAngle is set to a default value (eg, 0) to perform the procedure using the variable invAngle. have.

As another example, after initializing the variable invAngle to a default value, it may be determined whether to update the value of the variable invAngle based on at least one of the index PredModeIntra of the intra prediction mode and the prediction angle intraPredAngle of the intra prediction mode. In this case, the default value of the variable invAngle may be an integer such as 0, 1, or 2.

For example, when the variable intraPredAngle has a preset value or falls within a preset range, the value of the variable invAngle may not be updated. On the other hand, only when the variable intraPredAngle is not a preset value or the absolute value of the variable intraPredAngle is greater than the preset value, the value of the variable invAngle may be updated. Here, the preset value is an integer such as 0, 1, or 2, and the predefined range may be set to 0, -1 to 1, or -2 to 2, or the like.

Applying the decoding process or the embodiments described based on the encoding process to the encoding process or the decoding process is included in the scope of the present disclosure. It is also within the scope of the present disclosure to change the embodiments described in a certain order in an order different from that described.

Although the above-described embodiment has been described based on a series of steps or a flowchart, this does not limit the time-series order of the invention, and may be performed simultaneously or in a different order, if necessary. In addition, each of the components (eg, unit, module, etc.) constituting the block diagram in the above-described embodiment may be implemented as a hardware device or software, or a plurality of components may be combined to form one hardware device or software. may be implemented. The above-described embodiment may be implemented in the form of program instructions that can be executed through various computer components and recorded in a computer-readable recording medium. The computer-readable recording medium may include program instructions, data files, data structures, etc. alone or in combination. Examples of the computer-readable recording medium include a hard disk, a magnetic medium such as a floppy disk and a magnetic tape, an optical recording medium such as a CD-ROM and DVD, and a magneto-optical medium such as a floppy disk. media), and hardware devices specially configured to store and execute program instructions, such as ROM, RAM, flash memory, and the like. The hardware device may be configured to operate as one or more software modules to perform processing according to the present disclosure, and vice versa.

Claims (11)

determining a reference pixel line of the current block;
determining an intra prediction mode of the current block; and
deriving a prediction sample of the current block based on a reference pixel belonging to the reference pixel line and the intra prediction mode,
The prediction sample is determined based on a prediction angle of the intra prediction mode and an inverse-angle variable derived based on the prediction angle,
When the prediction angle has a predefined value, the inverse-angle variable has a default value. According to claim 1,
The image decoding method, characterized in that, based on the inverse-angle variable, reference pixels belonging to the reference pixel line are arranged in a 1D array. According to claim 1,
Whether to update the inverse-angle variable is determined based on whether the value of the prediction angle belongs to a predefined value or a predefined range. 4. The method of claim 3,
When it is determined not to update the inverse-angle variable, the inverse-angle variable maintains the default value. According to claim 1,
The video decoding method, characterized in that the default value is 0. determining a reference pixel line of the current block;
determining an intra prediction mode of the current block; and
deriving a prediction sample of the current block based on a reference pixel belonging to the reference pixel line and the intra prediction mode,
The prediction sample is determined based on a prediction angle of the intra prediction mode and an inverse-angle variable derived based on the prediction angle,
When the prediction angle has a predefined value, the inverse-angle variable has a default value. 7. The method of claim 6,
based on the inverse-angle variable, the reference pixels belonging to the reference pixel line are arranged in a 1D array. 7. The method of claim 6,
Whether to update the inverse-angle variable is determined based on whether the value of the prediction angle belongs to a predefined value or a predefined range. 9. The method of claim 8,
When it is determined not to update the inverse-angle variable, the inverse-angle variable maintains the default value. 7. The method of claim 6,
The video encoding method, characterized in that the default value is 0. determining a reference pixel line of the current block;
determining an intra prediction mode of the current block; and
deriving a prediction sample of the current block based on a reference pixel belonging to the reference pixel line and the intra prediction mode,
The prediction sample is determined based on a prediction angle of the intra prediction mode and an inverse-angle variable derived based on the prediction angle,
When the predicted angle has a predefined value, the inverse-angle variable has a default value.

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