KR102294016B1 - Video encoding and decoding method using deblocking fitering with transform skip and apparatus using the same - Google Patents

Abstract

An image decoding method according to the present invention includes decoding filtering information indicating whether to perform deblocking filtering with reference to a transform skip mode of a filtering target block on which deblocking filtering is to be performed, a reconstructed picture based on a prediction signal and a residual signal and performing deblocking filtering with reference to transform skip modes of two filtering target blocks forming a filtering boundary on which deblocking filtering is to be performed in the reconstructed picture based on the filtering information.

Description

Video encoding/decoding method using deblocking filtering referring to transformation skipping and apparatus using the same

The present invention relates to encoding and decoding processing of an image, and more particularly, to a deblocking filtering method and apparatus with reference to a transform skip block when compressing an image.

Recently, as broadcasting services having high definition (HD) resolution are expanding not only in Korea but also worldwide, many users are getting used to high-resolution and high-definition images, and accordingly, many organizations are spurring the development of next-generation imaging devices. In addition, as interest in Ultra High Definition (UHD) having a resolution four times higher than that of HDTV is increasing along with HDTV, a compression technology for higher resolution and high-definition images is required.

For image compression, inter (in-screen) prediction technology that predicts pixel values included in the current picture from temporally previous and/or later pictures, pixel values included in the current picture using pixel information in the current picture An intra (inter-screen) prediction technique for predicting , an entropy coding technique for allocating a short code to a symbol with a high frequency of occurrence and a long code for assigning a long code to a symbol with a low frequency of occurrence may be used.

The present invention provides a method and apparatus for performing deblocking filtering with reference to a transform skip mode of a block on which filtering is to be performed.

The present invention provides a method of signaling information indicating whether to perform deblocking filtering with reference to a transform skip mode.

The present invention provides a method of signaling information indicating whether to use the function itself for performing deblocking filtering with reference to the transform skip mode.

According to an embodiment of the present invention, an image decoding method is provided. The image decoding method includes decoding filtering information indicating whether to perform deblocking filtering with reference to a transform skip mode of a filtering target block on which deblocking filtering is to be performed, generating a reconstructed picture based on a prediction signal and a residual signal and performing deblocking filtering with reference to a transform skip mode of two filtering target blocks forming a filtering boundary on which deblocking filtering is to be performed in the reconstructed picture based on the filtering information.

According to the present invention, when a transform skip block exists among blocks forming a boundary of a block for which deblocking filtering is to be performed, deblocking filtering more suitable for the signal properties of the transform skip block can be performed, thereby improving image quality can do it

According to the present invention, encoding efficiency can be improved by performing deblocking filtering with reference to whether or not transform is omitted, and in addition, subjective image quality of an image can be improved. In particular, it can be effective for artificial images created by a computer.

1 is a block diagram illustrating a configuration of an image encoding apparatus to which the present invention is applied according to an embodiment.
2 is a block diagram illustrating a configuration of an image decoding apparatus to which the present invention is applied according to an embodiment.
3 is a flowchart illustrating a conventional deblocking filtering method.
4 is a diagram illustrating pixels referenced when determining whether to perform deblocking filtering in blocks p and q.
5 is a diagram illustrating pixels referred to when determining a type of a filter in blocks p and q.
6 shows a pixel range to which strong filtering is applied for any one row in blocks p and q.
7 shows a pixel range to which weak filtering is applied for any one row in blocks p and q.
8 is a flowchart illustrating an image decoding method to which deblocking filtering referring to a transform skip mode is applied according to an embodiment of the present invention.
9 is a flowchart illustrating a method of performing deblocking filtering with reference to a transform skip mode according to an embodiment of the present invention.
10 illustrates pixels referred to when determining a filtering type in blocks p and q.
11 shows pixel positions in blocks p and q based on a filtering boundary.
12 is a flowchart illustrating an image encoding method to which deblocking filtering referring to a transform skip mode is applied according to an embodiment of the present invention.

EMBODIMENT OF THE INVENTION Hereinafter, embodiment of this invention is described concretely with reference to drawings. In describing the embodiments of the present specification, if it is determined that a detailed description of a related known configuration or function may obscure the gist of the present specification, the detailed description thereof will be omitted.

When it is said that a component is “connected” or “connected” to another component, it may be directly connected or connected to the other component, but it is understood that other components may exist in between. it should be In addition, the content described as “including” a specific configuration in the present invention does not exclude configurations other than the corresponding configuration, and means that additional configurations may be included in the practice of the present invention or the scope of the technical spirit of the present invention.

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 invention, 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.

In addition, the components shown in the embodiment of the present invention are shown independently to represent different characteristic functions, and it does not mean that each component is composed of separate hardware or a single software component. 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 Integrated embodiments and separate embodiments of components are also included in the scope of the present invention without departing from the essence of the present invention.

In addition, some of the components are not essential components for performing essential functions in the present invention, but may be optional components for merely improving performance. The present invention can be implemented by including only essential components to implement the essence of the present invention, 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 invention.

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

Referring to FIG. 1 , the image encoding apparatus 100 includes an inter prediction unit 110 , an intra prediction unit 120 , a switch 115 , a subtractor 125 , a transform unit 130 , a quantization unit 140 , and entropy. It includes an 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 bitstream.

In the intra mode, the switch 115 may be switched to the intra mode, and in the inter mode, the switch 115 may be switched to the inter mode. Intra prediction refers to intra prediction, and inter prediction refers to inter prediction. The image encoding apparatus 100 may generate a prediction block for an input block of an input image, and then encode a residual between the input block and the prediction block. In this case, the input image may mean an original picture.

In the intra mode, the intra predictor 120 may use a sample value of an already encoded/decoded block around the current block as a reference sample. The intra prediction unit 120 may perform spatial prediction using a reference sample and generate prediction samples for the current block.

In the inter mode, the inter prediction unit 110 selects a motion vector specifying a reference block having the smallest difference from the input block (current block) in the reference picture stored in the reference picture buffer 190 during the motion prediction process. can be saved The inter prediction unit 110 may generate a prediction block for the current block by performing motion compensation using the motion vector and the reference picture stored in the reference picture buffer 190 .

The subtractor 125 may generate a residual block by the difference between the input block and the generated prediction block.

The transform unit 130 may perform transform on the residual block to output a transform coefficient. Here, the transform coefficient may mean a coefficient value generated by performing transform on the residual block and/or the residual signal. Hereinafter, in the present specification, a quantized transform coefficient level generated by applying quantization to a transform coefficient may also be referred to as a transform coefficient.

When a transform skip mode is applied, the transform unit 130 may skip transform on the residual block.

The quantization unit 140 quantizes the input transform coefficient according to a quantization parameter (QP, or quantization parameter) to output a quantized coefficient. The quantized coefficient may be referred to as a quantized transform coefficient level. In this case, the quantization unit 140 may quantize the input transform coefficient using a quantization matrix.

The entropy encoder 150 may entropy-encode the values calculated by the quantizer 140 or the encoding parameter values calculated during the encoding process according to a probability distribution to output a bitstream. The entropy encoder 150 may entropy-encode information for video decoding (eg, a syntax element (SE), etc.) in addition to pixel information of the video.

The encoding parameter is information necessary for encoding and decoding, and may include information that is encoded in the encoding apparatus and transmitted to the decoding apparatus, such as a syntax element, as well as information that can be inferred during encoding or decoding.

For example, the coding parameter includes values such as intra/inter prediction mode, motion/motion vector, reference image index, coding block pattern, presence/absence of residual signal, transform coefficient, quantized transform coefficient, quantization parameter, block size, block partition information, etc. or statistics.

The residual signal may mean a difference between the original signal and the predicted signal, and a signal in which the difference between the original signal and the predicted signal is transformed or a signal in which the difference between the original signal and the predicted signal is transformed and quantized. may mean The residual signal may be referred to as a residual block in block units.

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 to express the symbol, so that the size of the bit string for the symbols to be encoded is increased. can be reduced. Accordingly, compression performance of image encoding may be improved through entropy encoding.

The entropy encoder 150 may use an encoding method such as exponential golomb, Context-Adaptive Variable Length Coding (CAVLC), or Context-Adaptive Binary Arithmetic Coding (CABAC) for entropy encoding. For example, the entropy encoder 150 may perform entropy encoding using a variable length coding (VLC) table. In addition, the entropy encoder 150 derives a binarization method of a target symbol and a probability model of a target symbol/bin, and then performs entropy encoding using the derived binarization method or probability model. You may.

Since the image encoding apparatus 100 according to the embodiment of FIG. 1 performs inter prediction encoding, that is, inter prediction encoding, the currently encoded image needs to be decoded and stored to be used as a reference image. Accordingly, the quantized coefficient may be inverse quantized by the inverse quantizer 160 and inversely transformed by the inverse transform unit 170 . The inverse-quantized and inverse-transformed coefficients are added to the prediction block through the adder 175, and a reconstructed block is generated.

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

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

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

The image decoding apparatus 200 may receive a bitstream output from the encoder, perform decoding in an intra mode or an inter mode, and output a reconstructed image, that is, a reconstructed image.

In the case of the intra mode, the switch may be switched to the intra mode, and in the case of the inter mode, the switch may be switched to the inter mode.

The image decoding apparatus 200 obtains a reconstructed residual block from the received bitstream, generates a prediction block, and then adds the reconstructed residual block and the prediction block to generate a reconstructed block, that is, a reconstructed block. .

The entropy decoding unit 210 may entropy-decode the input bitstream according to a probability distribution and output information such as a quantized coefficient and a syntax element. The entropy decoding method is the reverse of the entropy encoding method described above.

The quantized coefficients are inverse quantized by the inverse quantization unit 220 and inversely transformed by the inverse transform unit 230 . As a result of inverse quantization/inverse transformation of the quantized coefficient, a reconstructed residual block may be generated. In this case, the inverse quantizer 220 may apply a quantization matrix to the quantized coefficients. For some blocks, the inverse transform unit 230 may be configured to omit the inverse transform. Also, the inverse quantization unit 220 may be configured to be omitted for some blocks.

In the intra mode, the intra prediction unit 240 may perform spatial prediction using sample values of an already decoded block around the current block, and generate prediction samples for the current block.

In the inter mode, the inter prediction unit 250 may generate a prediction block for the current block by performing motion compensation using a motion vector and a reference picture stored in the reference picture buffer 270 .

The reconstructed residual block and the prediction block are added in an adder 255 to generate a reconstructed block. In other words, the residual sample and the prediction sample are added to generate a reconstructed sample or a reconstructed picture.

The reconstructed picture is filtered by the filter unit 260 . The filter unit 260 may apply at least one of a deblocking filter, SAO, and ALF to a reconstructed block or a reconstructed picture. The filter unit 260 outputs a reconstructed picture that has been reconstructed or filtered. The reconstructed image may be stored in the reference picture buffer 270 and used for inter prediction.

Also, the image decoding apparatus 200 may further include a parsing unit (not shown) that parses information related to the encoded image included in the bitstream. The parsing unit may include the entropy decoding unit 210 or may be included in the entropy decoding unit 210 . This parsing unit may also be implemented as one component of the decoding unit.

On the other hand, in block-based video compression techniques such as H.264 or high efficiency video coding (HEVC), deblocking filtering is performed to remove or alleviate blocking artifacts occurring at the boundary of blocks. . In the case of HEVC, in the process of performing deblocking filtering, a boundary strength (Bs) is determined according to whether a block is a block coded in an intra prediction mode or a block coded in an inter prediction mode, and pixel values of the block boundary are determined. Refer to it to determine the filter type (strong/weak filter). However, according to the current technology, the coding mode of the residual signal (ie, whether transform is applied or whether transform is applied) is not considered in the deblocking filtering process. omitted) cannot be considered. In this case, a problem of image quality deterioration or compression rate deterioration occurs.

In HEVC, there is a transform skip mode as a coding mode of the residual signal. According to the HEVC version 1 technology, the transform skip mode is a mode for encoding a 4x4 TU (Transform Unit) by omitting transform of the residual signal and performing quantization. In addition, in the case of the HEVC Range Extensions technique, such a transform skip mode may be applied up to the maximum block size in which transform omitting indicated by the encoder is possible.

The transform skip mode may be mainly used when omitting transform and performing quantization can bring benefits in terms of compression ratio and image quality, such as when the spatial gradient of the residual signal is very large or has a strong edge component.

Since the block in which the transform skip mode is used has a very large spatial variation of the residual signal value, it has a characteristic significantly different from that of a block in which a general transform is performed. Accordingly, there is a high possibility that blocking degradation occurs at a boundary between blocks having such heterogeneous characteristics or at a boundary with blocks having such heterogeneous characteristics. However, according to the current technology such as HEVC (version 1), the deblocking filtering process does not consider the transform skip mode at all, and only refers to some pixels among the pixels constituting the block boundary to select the filtering strength and type. . It is difficult to represent the characteristic of a transform skip block having a large degree of change with only some pixels on the block boundary. Therefore, like the existing deblocking filtering method, if transform omission is not considered when performing deblocking filtering, filtering strength may be excessive or insufficient.

Hereinafter, in the present invention, when a transform skip block exists, deblocking filtering more suitable for signal properties when a transform skip block exists with reference to whether or not transform or transform skip of blocks forming a block boundary to be performed deblocking filtering provides a way to do it. According to the present invention, encoding efficiency can be increased by performing deblocking filtering with reference to whether or not transform is omitted, and subjective image quality can also be improved. This effect is especially noticeable in the case of artificial images created by a computer.

3 is a flowchart illustrating a conventional deblocking filtering method.

In the current HEVC version 1, deblocking filtering is performed at the block boundary in the reconstructed picture (before deblocking filtering) during encoding/decoding. In this case, in the reconstructed picture, first, filtering is performed in the horizontal direction on the vertical boundary (vertical edge boundary) of the block, and then filtering is performed in the vertical direction on the horizontal boundary (horizontal edge boundary) of the block.

The method of FIG. 3 shows a deblocking filtering process for a luma component. Also, the method of FIG. 3 may be performed by the encoding apparatus and the decoding apparatus, and for convenience of description, it will be described as being performed by the decoding apparatus.

Referring to FIG. 3 , the decoding apparatus determines whether the boundary of the 8x8 block in the reconstructed picture is a PU (Prediction Unit) or TU (Transform Unit) boundary positioned at the 8x8 block boundary as a unit ( S300).

If there is no PU or TU boundary, deblocking filtering is not performed on the 8x8 block boundary.

In the case of an 8x8 block boundary and a PU or TU boundary, the decoding apparatus calculates a boundary strength (Boundary Strength, hereinafter referred to as Bs) at a filtering boundary on which deblocking filtering is to be performed (S310).

The Bs value is calculated in units of 4 rows or 4 columns at the filtering boundary, and the calculation process is as follows. Here, the two blocks forming the filtering boundary are referred to as p and q. For example, when deblocking filtering is performed on a horizontal boundary, a block located above the horizontal boundary becomes p, and a block located below the horizontal boundary becomes q. When deblocking filtering is performed on a vertical boundary, a block located on the left side of the vertical boundary becomes p, and a block located on the right side becomes q.

[Bs calculation process]

To summarize the Bs calculation process, when the p or q block is coded in the intra prediction mode, the Bs value becomes 2, and when the p and q blocks are coded in the inter prediction mode, the Bs value becomes 1 or 0.

_{The decoding apparatus calculates threshold values β and t c} when the Bs value is greater than 0 ( S320 ).

If the Bs value is 0, the threshold value calculation is omitted and deblocking filtering is not performed.

Here, β and t _c are values used for 1) determining whether to perform filtering (filtering on/off determination), 2) selecting strong/weak filters, and 3) weak filtering.

β and t _c are determined with reference to Table 1 below defined in HEVC based on the quantization parameters (Quantization Parameter, hereinafter referred to as QP) of the p and q blocks. Table 1 is a table predefined in the encoder and decoder used to calculate _{β and t c .}

[Table 1]

The decoding apparatus may calculate the Q value by using the QP values of the p and q blocks, calculate β' and t _c ' corresponding to the calculated Q values from Table 1, and determine the final β and t _{c .}

The Q value used to determine the β value may be calculated as in Equation 1 below.

[Equation 1]

In Equation 1, qP _L is an average value of QP values of blocks p and q, and slice_beta_offset_div2 may be a value signaled from an encoder.

The _{Q value used to determine the t c} value may be calculated as in Equation 2 below.

[Equation 2]

In Equation 2, qP _L is an average value of QP values of p and q blocks, and slice_tc_offset_div2 may be a value signaled from an encoder.

The decoding apparatus determines whether to perform deblocking filtering (filtering on/off) based on the _{β and t c values (S330).}

In this case, whether to perform deblocking filtering is determined in units of 4 rows (or 4 columns). That is, the decoding device calculates a value calculated from pixels located in the first and fourth rows (in the case of a vertical boundary) or in the first and fourth columns (in the case of a horizontal boundary) at the filtering boundary where the Bs value is greater than 0. can be compared with the β value to determine whether to perform filtering.

The following conditional expression 1 shows conditions for determining whether to perform deblocking filtering, and FIG. 4 is a diagram illustrating pixels referenced when determining whether to perform deblocking filtering in blocks p and q.

[Conditional Expression 1]

The decoding apparatus determines to perform deblocking filtering (filtering on) when both conditions 1 and 2 of Conditional Expression 1 are satisfied.

If it is determined to perform deblocking filtering in step S330, the decoding apparatus determines a filter type (strong filter/weak filter) based on _{β and t c values (S340).}

In this case, the type of filter (whether strong filtering or weak filtering is applied) is determined in units of 4 rows (or 4 columns). That is, the decoding device determines the strength of filtering by using pixels located in the first and fourth rows (in the case of a vertical boundary) or in the first and fourth columns (in the case of a horizontal boundary) and β and t _{c values at the filtering boundary.} can decide

The following conditional expression 2 shows conditions for determining the type of filter, and FIG. 5 is a diagram showing pixels referred to when determining the type of filter in the p and q blocks.

[Conditional Expression 2]

The decoding apparatus selects a strong filter when all conditions 1 to 6 of Conditional Expression 2 are satisfied, and selects a weak filter if not.

Referring to Conditional Expression 2, Conditions 1 to 3 of Conditional Expression 2 are conditions for the first row in blocks p and q in FIG. 5, and Conditions 4 to 6 of Condition Expression 2 are the fourth rows in blocks p and q in FIG. 5 are the conditions for

When the strong filter is selected in step S340, the decoding apparatus performs strong filtering in units of each row or each column (S350).

6 shows a pixel range to which strong filtering is applied for any one row in blocks p and q.

As shown in FIG. 6 , when a strong filter is selected at the filtering boundary, the decoding apparatus may perform filtering on three pixels in the p block and three pixels in the q block based on the filtering boundary.

A calculation process related to strong filtering is shown in Equation 3 below, where p' and q' denote pixel values to which strong filtering is applied, and p and q denote pixel values before filtering is applied.

[Equation 3]

When the weak filter is selected in step S340, the decoding apparatus performs weak filtering in units of each row or each column (S360).

7 shows a pixel range to which weak filtering is applied for any one row in blocks p and q.

7 , when a weak filter is selected at the filtering boundary, the decoding apparatus may perform filtering of up to two pixels in each of the p and q blocks based on the filtering boundary.

Unlike strong filtering, weak filtering includes a process of re-determining whether to perform filtering in units of each row or each column.

The following conditional expression 3 shows a condition for determining whether to perform weak filtering.

[Conditional Expression 3]

The decoding apparatus determines to perform weak filtering when the condition of Conditional Expression 3 is satisfied in units of each row or each column, otherwise, weak filtering is not performed on the corresponding row or corresponding column.

When the condition of Equation 3 is satisfied, the decoding apparatus first performs weak filtering on pixel positions p0' and q0', which are first pixels in blocks p and q, based on the filtering boundary. The calculation process of weak filtering for the pixel positions p0' and q0' is expressed in Equation 4 below.

[Equation 4]

Next, the decoding apparatus determines whether to perform filtering on each of the pixels p1' and q1', which are the second pixels in the p and q blocks, based on the filtering boundary, and according to the determined result, each of the pixels p1' and q1' Weak filtering is performed on

The following conditional expression 4 shows a condition for determining whether to perform filtering on the p1' pixel. In this case, it is possible to determine whether to perform filtering by referring to pixel values in the first row (or first column) and fourth row (or fourth column) in the p and q blocks.

[Conditional Expression 4]

When condition 4 is satisfied, the pixel value p1' may be calculated as in Equation 5 below.

[Equation 5]

The following conditional expression 5 shows a condition for determining whether to perform filtering on the q1' pixel. In this case, it is possible to determine whether to perform filtering by referring to pixel values in the first row (or first column) and fourth row (or fourth column) in the p and q blocks.

[Conditional Expression 5]

When conditional expression 5 is satisfied, the pixel value q1' may be calculated as in Equation 6 below.

[Equation 6]

Although the conditional expressions and the above equations and FIGS. 4 to 7 have been described with reference to the pixel positions within the blocks p and q forming the vertical boundary, even when filtering on the horizontal boundary is performed, the upper block is referred to as p, and the lower block is set to p. can be applied in the same way by designating q as .

As described above with reference to FIG. 3 , in the existing deblocking filtering process, it does not consider whether to omit transformation of a block forming a boundary for performing deblocking filtering. As described above, since the residual signal between the transformed block and the non-transformed transform skip block has very different characteristics, blocking degradation is highly likely to occur at the boundary between these blocks. Accordingly, in the present invention, a method of performing deblocking filtering in consideration of whether or not transform is omitted will be described.

According to the present invention, an encoder performs deblocking filtering referring to whether transform skip of a block to be subjected to deblocking filtering is performed to encode information (eg, a flag) indicating whether encoding is performed, and signal to the decoder. In addition, the encoder performs deblocking filtering referring to whether or not transformation of a block to be subjected to deblocking filtering is omitted to perform encoding.

According to the present invention, the decoder decodes (parses) information (eg, a flag) indicating whether to perform deblocking filtering with reference to whether transformation of a block on which deblocking filtering is to be performed is omitted. In addition, the decoder performs deblocking filtering by referring to whether or not transformation of a block on which deblocking filtering is to be performed is omitted.

In this case, information indicating whether to perform deblocking filtering with reference to whether transformation of a block to be subjected to deblocking filtering is omitted may be expressed as a flag value. For example, if the flag value is 1, the decoder performs the deblocking filtering process. Deblocking filtering is performed with reference to whether transform is omitted, and when the flag value is 0, the decoder may perform deblocking filtering that does not refer to whether or not transform is omitted in the deblocking filtering process.

Information indicating whether to perform deblocking filtering with reference to whether transformation of a block to be subjected to deblocking filtering is omitted may be signaled in units of slice segment headers.

In addition, according to the present invention, information (eg, flag) indicating whether to use the function itself of "performing deblocking filtering with reference to whether or not transformation of a block to be subjected to deblocking filtering is omitted" can be signaled, Such information may be signaled through a syntax transmitted from a layer higher than the slice segment header. For example, it may be signaled through a sequence parameter set (SPS), a picture parameter set (PPS), an extended sequence parameter set (SPS_extension), an extended picture parameter set (PPS_extension), and the like.

8 is a flowchart illustrating an image decoding method to which deblocking filtering referring to a transform skip mode is applied according to an embodiment of the present invention. The method of FIG. 8 may be performed by the above-described image decoding apparatus of FIG. 2 .

Referring to FIG. 8 , the decoding apparatus decodes (parses) filtering information indicating whether to perform deblocking filtering with reference to a transform skip mode of a filtering target block on which deblocking filtering is to be performed (S800).

The filtering information is signaled from the encoder and may be information expressed as a flag. For example, if the flag value is 1, it indicates that deblocking filtering is performed with reference to the transform skip mode of the filtering target block, and if the flag value is 0, deblocking filtering that does not refer to the transform skip mode of the filtering target block is performed can instruct you to do

The filtering information may be signaled through a slice segment header, SPS, PPS, or the like. A method of signaling the filtering information will be described later.

The decoding apparatus generates a reconstructed picture based on the prediction signal and the residual signal (S810).

The decoding apparatus generates a prediction signal (prediction sample) by performing intra prediction or inter prediction according to the prediction mode of the block in which prediction is to be performed, and inversely quantizes/inverse-transforms the quantized coefficient to generate a residual signal (residual sample). , a reconstructed picture may be obtained by generating a reconstructed signal (reconstructed sample) by adding the prediction signal and the residual signal.

The decoding apparatus performs deblocking filtering with reference to the transform skip mode of two filtering target blocks forming a filtering boundary on which deblocking filtering is to be performed in the reconstructed picture based on the filtering information (S820).

That is, when the filtering information indicates that deblocking filtering is performed with reference to the transform skip mode of the filtering target block, deblocking filtering with reference to the transform skip mode is performed. A method of performing deblocking filtering with reference to the transform skip mode will be described in detail with reference to FIG. 9 .

On the other hand, the encoding device refers to the transform skip mode and indicates whether to use the function for performing deblocking filtering itself through a slice segment header, SPS, PPS, SPS_entension, PPS_extension, etc. Can be signaled. The decoding apparatus may receive the filtering usage information from the encoding apparatus and decode it.

If the filtering use information indicates that the function of performing deblocking filtering is used by referring to the transform skip mode, the decoding apparatus performs deblocking filtering with reference to the transform skip mode based on the filtering information. . Otherwise, the decoding apparatus performs a (conventional) deblocking filtering method that does not refer to the transform skip mode.

9 is a flowchart illustrating a method of performing deblocking filtering with reference to a transform skip mode according to an embodiment of the present invention. The method of FIG. 9 may be performed by the above-described image decoding apparatus of FIG. 2 , and more specifically, may be performed by the filter unit of FIG. 2 .

In this embodiment, a deblocking filtering process for a luma component is shown. According to another embodiment, this deblocking filtering process may be applied to a color (chroma) component or components in other color spaces including RGB, XYZ, or YCoCg.

As described above, in the deblocking filtering (before deblocking filtering), filtering is first performed in the horizontal direction on the vertical boundary (vertical edge boundary) of the block in the reconstructed picture, and then on the horizontal boundary (horizontal edge boundary) of the block. Filtering is performed in the vertical direction.

In this embodiment, for convenience of description, a process of performing deblocking filtering based on a vertical edge boundary is described, but this can be applied to a horizontal edge boundary in the same way.

9, the decoding apparatus determines whether the boundary of the 8x8 block in the reconstructed picture is a PU (Prediction Unit) or TU (Transform Unit) boundary positioned at the 8x8 block boundary as a unit ( S900).

If there is no PU or TU boundary, deblocking filtering is not performed on the 8x8 block boundary.

In the case of an 8x8 block boundary and a PU or TU boundary, the decoding apparatus determines the boundary strength (hereinafter referred to as Bs) with reference to the transform skip mode of two filtering target blocks forming the filtering boundary on which deblocking filtering is to be performed. (S910).

Here, two filtering target blocks forming a filtering boundary are referred to as a p block and a q block. For example, when deblocking filtering is performed on a horizontal boundary, a block located above the horizontal boundary becomes p, and a block located below the horizontal boundary becomes q. When deblocking filtering is performed on a vertical boundary, a block located on the left side of the vertical boundary becomes p, and a block located on the right side becomes q.

When at least one of the p-block and q-block constituting the filtering boundary is a block to which the transform skip mode is applied, that is, when at least one of the p block and the q block has the transform omitted, the decoding apparatus performs the following four boundary strengths (Bs). ) among the determination methods, Bs may be determined using at least one method predefined in the encoder/decoder. Meanwhile, in the following implementation description, the case where the range of the Bs value is limited to [0,2] has been specifically described for convenience, but the present invention provides the same idea when the maximum value of Bs is a value other than 2 (let's call it Bs_max). can be applied as

1) If the p or q block is an intra block (a block coded in the intra prediction mode), Bs is set to 1.

2) A value obtained by subtracting 1 from the Bs value calculated by the conventional Bs determination method (refer to the Bs calculation process of FIG. 3 ) is set as the final Bs. However, if the final Bs value is negative, the final Bs value is set to 0. This can be expressed as Equation 7 below.

[Equation 7]

3) A value obtained by adding 1 to the Bs value calculated by the existing Bs determination method (refer to the Bs calculation process of FIG. 3 ) is set as the final Bs. However, when the final Bs value is greater than Bs_max, the final Bs value is set as the Bs_max value. This can be expressed as in Equation 8 (the following Equation specifically shows a case where Bs_max =2. If Bs_max is not 2, just replace 2 with Bs_max in the following Equation).

[Equation 8]

4) Bs can be determined as a specific value within the range of [0, B_max]. Here's a closer look at this:

4-1) Bs can always be determined to be 0. This may be regarded as not performing deblocking filtering on the corresponding block boundary.

4-2) Bs can always be determined as the Bs_max value (eg, when Bs_max = 2, Bs = 2 is set). This can be considered as performing the strongest deblocking filtering on the corresponding block boundary.

4-3) Bs can always be determined to be 1.

In addition, when the p or q block is an intra block (a block coded in the intra prediction mode), it may be implemented to add a predetermined value (eg, a value of 1) to Bs determined by an additive method. .

On the other hand, in another implementation, in the case of a block to which the transform skip mode is applied to both the p block and the q block constituting the filtering boundary, the decoding apparatus uses a predefined method among the four boundary strength (Bs) determination methods described above in the encoder/decoder. It may be realized to determine Bs using at least one method.

When both the p-block and the q-block constituting the filtering boundary are not blocks to which the transform skip mode is applied, the decoding apparatus determines Bs according to a conventional method. The existing method has been described in the embodiment of FIG. 3 .

The decoding apparatus determines whether to perform deblocking filtering at a filtering boundary corresponding to a case in which the Bs value is greater than 0 (S920). At this time, it is determined whether deblocking filtering is performed by referring to the transform skip mode of two filtering target blocks forming the filtering boundary.

More specifically, first, the decoding apparatus calculates _{variables β and t c used to determine whether to perform deblocking filtering.}

When at least one of the p block and the q block forming the filtering boundary is a block to which the transform skip mode is applied, the decoding apparatus may calculate the _{variables β and t c as follows.}

1) When using the same offset when calculating the values of _{β and t c}

The offset is _{information necessary for calculating the Q value used to determine the β and t c} values, and is a value signaled through an upper layer (such as a slice segment header). For example, it may be signaled by a slice_deblock_transformskip_offset syntax element.

The Q value used to determine the β value may be calculated as in Equation 9 below.

[Equation 9]

The _{Q value used to determine the t c} value may be calculated as in Equation 10 below.

[Equation 10]

2) When using different offsets when calculating the values of _{β and t c}

The offset used to determine the β value may be slice_beta_transformskip_offset, and the offset used to determine the _{t c value may be slice_tc_transformskip_offset.} The offset is a value signaled through an upper layer (such as a slice segment header).

The Q value used to determine the β value may be calculated as in Equation 11 below.

[Equation 11]

The _{Q value used to determine the t c} value may be calculated as in Equation 12 below.

[Equation 12]

The decoding apparatus calculates the Q value as described above, and calculates _{β' and t c} ' from a predefined table (see Table 1) to determine _{the β and t c values by inputting the calculated Q value as an input.} Based on the obtained β', t _c ', the final β, t _c values can be determined.

The offset used when calculating the values of variables β and t _c can be implemented as follows depending on the method of transmission from the encoder to the decoder. For example, offset values such as slice_deblock_transformskip_offset, slice_beta_transformskip_offset, and slice_tc_transformskip_offset may be signaled as they are. Alternatively, the encoder transmits a value obtained by performing a right shift operation (>>1) on each offset value, and the decoder performs a left shift operation (<<1) on the transmitted offset value before use. have.

When both the p block and the q block constituting the filtering boundary are not blocks to which the transform skip mode is applied, the decoding apparatus determines _{β and t c according to a conventional method.} The existing method has been described in the embodiment of FIG. 3 .

The decoding apparatus may determine whether to perform deblocking filtering based on the calculated β and t _{c values.} Since the process of determining whether to perform deblocking filtering based on the β and t _c values has been described above in the embodiment of FIG. 3 , a detailed description thereof will be omitted.

When it is determined to perform deblocking filtering, the decoding device determines the filtering type (strong filtering/weak filtering) on a line-by-line basis (row unit for vertical boundary/column unit for horizontal boundary), and performs filtering according to the determined filtering type do (S930).

When at least one of blocks p and q forming the filtering boundary is a block to which the transform skip mode is applied, the decoding apparatus performs the following process.

1) The decoding apparatus may determine whether to apply strong filtering or weak filtering in units of lines, unlike the existing block units.

The following conditional expression 6 shows conditions for determining the filtering type, and FIG. 10 shows pixels referenced when determining the filtering type in blocks p and q. As shown in FIG. 10 , when determining a filtering type according to an embodiment of the present invention, pixels in p and q blocks are referred to in units of lines.

[Conditional Expression 6]

The decoding apparatus applies strong filtering when all conditions 1 to 3 of Conditional Expression 6 are satisfied, and applies weak filtering if not.

2-1) When it is determined to apply strong filtering, the decoding apparatus may perform strong filtering on pixels of a corresponding line. In this case, strong filtering may be performed in the same manner as in the existing method, and the strong filtering operation may be performed as in Equation 3 of FIG. 3 .

2-2) When it is determined to apply the weak filtering, the decoding apparatus may perform filtering on the first pixel of the p and q blocks based on the filtering boundary in the same manner as in the existing weak filtering. The existing weak filtering operation process may be performed as in Equation 4 of FIG. 3 .

When weak filtering is performed on the first pixel of the p and q blocks based on the filtering boundary, the decoding apparatus may determine whether to perform the weak filtering on the second pixel of the p and q blocks based on the filtering boundary. At this time, unlike the conventional method of determining whether to perform weak filtering by referring to the first and last lines in the p and q blocks, the present invention refers to only the pixel values of the corresponding lines for the second pixel of the p and q blocks. You can decide whether to filter or not.

11 shows pixel positions in p and q blocks based on a filtering boundary.

Referring to FIG. 11 , the first pixels in the blocks p and q based on the filtering boundary may be p0' and q0'. Based on the filtering boundary, the second pixels in the blocks p and q may be p1' and q1'.

The following conditional expression 7 shows a condition for determining whether to perform filtering on the first pixel p1' in the p block, and when conditional expression 7 is satisfied, the pixel value of p1' is calculated using the above-mentioned Equation 5 .

[Conditional Expression 7]

The following conditional expression 8 represents a condition for determining whether to perform filtering on the first pixel q1' in the q block, and when conditional expression 8 is satisfied, the pixel value of q1' is calculated using the above-mentioned Equation 6 .

[Conditional Expression 8]

When both the p-block and the q-block constituting the filtering boundary are not blocks to which the transform skip mode is applied, the decoding apparatus may determine a filtering type according to an existing method and perform filtering. The conventional method has been described in the embodiment of FIG. 3 .

Although the above-described embodiment of FIG. 9 has been described as being performed by the decoding apparatus for convenience of description, the above-described encoding apparatus of FIG. 1 , and more specifically, the same may be performed in the filter unit of the encoding apparatus of FIG. 1 .

Meanwhile, according to another embodiment of the present invention, it may be determined whether to perform deblocking filtering with reference to the transform skip mode based on the size of the transform skip TU.

When transform skip is applied to the p or q block and the size of the transform skip TU is smaller than a predefined threshold, deblocking filtering may be always performed in the existing method as in the embodiment of FIG. 3 . Since the effect of the present invention is large when the size of the transform skip TU is large, when the size of the transform skip TU of the p or q block is larger than a predefined threshold, deblocking referring to the transform skip mode as in the embodiment of FIG. 9 . Filtering can be done.

In this case, when the transform skip mode is applied to both the p and q blocks, a TU having a larger size among the TUs of the p and q blocks may be determined as the size of the transform skip TU and compared with a predefined threshold value. Alternatively, a TU having a smaller size among the TUs of the p and q blocks may be determined as the size of the transform skip TU and compared with a predefined threshold value.

The predefined threshold may be a value signaled from a higher layer (eg, a slice segment header, etc.).

In addition, the size of a TU for which the p or q block is omitted for transformation refers to the size of the TU to which the corresponding p or q block belongs.

Hereinafter, filtering information indicating whether to perform deblocking filtering with reference to the transform skip mode according to the present invention, and filtering use information indicating whether to use the function for performing deblocking filtering with reference to the transform skip mode A signaling method will be described.

Table 2 shows signaling of whether to use filtering information indicating whether to use the function itself for performing deblocking filtering with reference to the transform skip mode in the PPS.

[Table 2]

Referring to Table 2, when the value of deblocking_filter_refer_transform_skip_enabled_flag is 1, the slice referring to the corresponding PPS indicates that the function of performing deblocking filtering is used by referring to the transform skip mode.

When the deblocking_filter_refer_transform_skip_enabled_flag value is 0, it indicates that the (conventional) deblocking filtering method that does not refer to the transform skip mode is used in all slices referring to the corresponding PPS.

Table 3 shows signaling of filtering information indicating whether to perform deblocking filtering by referring to the transform skip mode in the slice segment header.

[Table 3]

Referring to Table 3, when the value of deblocking_filter_refer_transform_skip_flag is 1, it indicates that deblocking filtering is performed with reference to the transform skip mode when deblocking within a corresponding slice is performed.

When the value of deblocking_filter_refer_transform_skip_flag is 0, it indicates that deblocking filtering that does not refer to the transform skip mode is performed when deblocking within a corresponding slice is performed.

In the descriptions of Tables 2 and 3 above, the transmission of the PPS level has been described using picture_parameter_set_rbsp() as an example for a detailed description, but in actual implementation, it may be at a different level, and may also be an extended_picture_parameter_set_rbsp() which is an extension of the PPS. have. In addition, the process of referring to the deblocking_filter_refer_transform_skip_flag value in Table 3 may be performed only in the case of extension.

12 is a flowchart illustrating an image encoding method to which deblocking filtering referring to a transform skip mode is applied according to an embodiment of the present invention. The method of FIG. 12 may be performed by the above-described image encoding apparatus of FIG. 1 .

Referring to FIG. 12 , the encoding apparatus generates a reconstructed picture based on a prediction signal and a residual signal ( S1200 ).

The encoding apparatus generates a prediction signal (prediction sample) by performing intra prediction or inter prediction according to the prediction mode of the block in which prediction is to be performed, and generates a residual signal (residual sample) by inverse quantizing/inverse transforming the quantized coefficients. , a reconstructed picture may be obtained by generating a reconstructed signal (reconstructed sample) by adding the prediction signal and the residual signal.

The encoding apparatus performs deblocking filtering with reference to transform skip modes of two filtering target blocks forming a filtering boundary on which deblocking filtering is to be performed in the reconstructed picture (S1210).

Since the method of performing deblocking filtering with reference to the transform skip mode has been described in detail with reference to FIG. 9 , a description thereof will be omitted herein.

The encoding apparatus encodes filtering information indicating whether to perform deblocking filtering with reference to a transform skip mode of a filtering target block on which deblocking filtering is to be performed (S1220).

The encoded filtering information may be signaled to the decoding apparatus through a slice segment header, PPS, SPS, or the like.

The method according to the present invention described above may be produced as a program to be executed by a computer and stored in a computer-readable recording medium. Examples of the computer-readable recording medium include ROM, RAM, CD-ROM, magnetic tape. , a floppy disk, an optical data storage device, and the like, and also includes those implemented in the form of a carrier wave (eg, transmission through the Internet).

The computer-readable recording medium is distributed in network-connected computer systems, so that the computer-readable code can be stored and executed in a distributed manner. In addition, functional programs, codes, and code segments for implementing the method can be easily inferred by programmers in the art to which the present invention pertains.

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 a different order or at the same time as other steps as described above. can In addition, those of ordinary skill in the art will recognize that the steps shown in the flowchart are not exclusive, 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 description is merely illustrative of the technical spirit of the present invention, and various modifications and variations will be possible without departing from the essential characteristics of the present invention by those skilled in the art to which the present invention pertains. Therefore, the embodiments disclosed in the present invention are not intended to limit the technical spirit of the present invention, but to explain, and the scope of the technical spirit of the present invention is not limited by these embodiments. The protection scope of the present invention should be construed by the claims, and all technical ideas within the scope equivalent thereto should be construed as being included in the scope of the present invention.

Claims (11)

In the video decoding method,
decoding filtering information indicating whether to perform deblocking filtering with reference to a transform skip mode of a filtering target block on which deblocking filtering is to be performed;
generating a reconstructed picture based on the prediction signal and the residual signal; and
and performing deblocking filtering with reference to transform skip modes of two filtering target blocks forming a filtering boundary on which deblocking filtering is to be performed in the reconstructed picture based on the filtering information. According to claim 1,
The filtering information is signaled in a sequence parameter set, an extended sequence parameter set, a picture parameter set, an extended picture parameter set, or a slice header. According to claim 1,
The method further includes decoding information on whether to use filtering indicating whether to use a function for performing deblocking filtering with reference to a transform skip mode of the filtering target block,
When the filtering usage information indicates that the function is used, deblocking filtering is performed on the reconstructed picture based on the filtering information. According to claim 1,
Deblocking filtering is performed with reference to the transform skip mode when the size of at least one transform block among the two filtering target blocks forming the filtering boundary is equal to or greater than a predefined threshold. According to claim 1,
When the transform skip mode is applied to at least one of the two filtering object blocks forming the filtering boundary, determining the boundary strength (BS) based on the prediction mode of the two filtering object blocks forming the filtering boundary A video decoding method characterized in that. 6. The method of claim 5,
When at least one of the two filtering blocks constituting the filtering boundary is predicted in the intra mode, the boundary strength is set to 1. According to claim 1,
When the transform skip mode is applied to at least one of the two filtering target blocks forming the filtering boundary, the method further comprises parsing offset information on variables β and tc used in the deblocking filtering process,
The step of performing the deblocking filtering comprises:
and calculating the variables β and tc based on the offset information. 8. The method of claim 7,
An image decoding method, characterized in that the variable β and the offset with respect to the variable tc are the same. 8. The method of claim 7,
The video decoding method according to claim 1, wherein offsets for the variable β and the variable tc are different from each other. According to claim 1,
The step of performing the deblocking filtering comprises:
determining a boundary strength (BS) with reference to a transform skip mode of two filtering target blocks constituting the filtering boundary;
if the boundary strength is greater than 0, determining whether to perform deblocking filtering using variables β and tc calculated with reference to transform skip modes of two filtering target blocks constituting the filtering boundary;
When it is determined to perform the deblocking filtering and a transform skip mode is applied to at least one of the two filtering target blocks forming the filtering boundary, determining a filtering type for an individual line perpendicular or horizontal to the filtering boundary An image decoding method comprising a. 11. The method of claim 10,
When the filtering type is determined to be weak filtering, the pixel value at the second position from the filtering boundary is determined by referring only to the pixel value on the same line as the pixel at the second position.

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