KR20130027975A

KR20130027975A - Method for coding/decoding of intra prediction mode and apparatus thereof

Info

Publication number: KR20130027975A
Application number: KR1020110134968A
Authority: KR
Inventors: 이배근; 권재철
Original assignee: 주식회사 케이티
Priority date: 2011-09-08
Filing date: 2011-12-14
Publication date: 2013-03-18

Abstract

PURPOSE: An intra-prediction mode encoding or decoding method and a device thereof are provided to improve image pressure efficiency by adding an MPM(Most Probable Mode) candidate to an MPM list by selecting the MPM candidate. CONSTITUTION: A decoder extracts a DiffDir value(S610). The decoder generates an MPM list based on the DiffDir(S620). The decoder decodes a prev_intra_pred_flag(S630). When the decoded the prev_intra_pred_flag is 1, the decoder decodes mpm_idx and extracts a current prediction mode by using the mpm_idx and the MPM list(S640,S650). When the decoded prev_intra_pred_flag value is not 1, the decoder decodes a rem_intra_pred_mode(S660). [Reference numerals] (AA) Start; (BB) End; (S610) Calculating DiffDir; (S620) Generating an MPM list; (S630) Decoding prev_intra_pred_flag; (S640) Is prev-intra_pred_flag 1?; (S650) Decoding mpm_idx; (S660) Decoding rem_intra_pred_mode

Description

Intra prediction mode encoding / decoding method and apparatus {METHOD FOR CODING / DECODING OF INTRA PREDICTION MODE AND APPARATUS THEREOF}

The present invention relates to image processing, and more particularly, to an intra prediction mode encoding / decoding method and apparatus.

Recently, the demand for high resolution and high quality images such as high definition (HD) images and ultra high definition (UHD) images is increasing in various fields.

In order to provide a high resolution and high quality image, the data amount of the image data is increased. Therefore, the transmission cost and the storage cost of the image data for providing a high resolution and high quality image are increased as compared with the conventional image data processing method. High-efficiency image compression techniques can be utilized to solve such problems as image data becomes high-resolution and high-quality.

A technique for compressing image data. An inter prediction method, which predicts a pixel value included in a current picture from another picture, is referred to as an inter prediction, and a pixel value of the current picture is information of another pixel of the current picture. Various techniques are used, such as intra prediction (Intra Prediction) method, which is predicted by using a signal, and an entropy encoding method that performs encoding by assigning a shorter code to a signal having a higher frequency of occurrence or occurrence. .

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

Another object of the present invention is to provide an image decoding method and apparatus for improving image encoding efficiency.

Another technical problem of the present invention is to provide an intra prediction mode encoding method and apparatus for improving image encoding efficiency.

Another technical problem of the present invention is to provide an intra prediction mode decoding method and apparatus for improving image encoding efficiency.

1. An embodiment of the present invention is a video decoding method. The method includes generating a Most Probable Mode (MPM) list including MPM candidates using a top periphery prediction mode and a left periphery prediction mode and deriving an intra prediction mode of the current block using the MPM list. The upper peripheral prediction mode is a prediction mode of a neighboring block adjacent to the top of the current block, and the left peripheral prediction mode is a prediction mode of a neighboring block adjacent to the left of the current block.

2. The method may further comprise generating an MPRM list comprising Most Probable Remaining Mode (MPRM) candidates, using intra prediction modes other than the MPM candidates, and using the MPM list to the current block. In the deriving of the intra prediction mode, the intra prediction mode of the current block may be derived using the MPM list and the MPRM list, and the MPRM candidates may be formed by the angle of the upper edge prediction mode or the angle of the left edge prediction mode. It may be a prediction mode having an angle most similar to.

3. In 2, when the angle difference between the upper peripheral prediction mode and the left peripheral prediction mode is greater than or equal to a predetermined first threshold, one of the MPRM candidates is determined by the angle of the upper peripheral prediction mode and the angle of the left peripheral prediction mode. It may be a prediction mode having an angle corresponding to the middle.

4. In 2, when the angle difference between the upper edge prediction mode and the left edge prediction mode is greater than or equal to a second predetermined threshold, the method may not generate the MPRM list.

5. In 1, the number of the MPM candidates included in the MPM list may be determined according to an angle difference between the upper peripheral prediction mode and the left peripheral prediction mode.

When the angle difference between the upper edge prediction mode and the left edge prediction mode is greater than or equal to a third threshold value in 6. 5, the number of the MPM candidates included in the MPM list may be one or two.

7. When the angle difference between the upper edge prediction mode and the left edge prediction mode is greater than or equal to a fourth threshold, in step 5, one of the MPM candidates included in the MPM list includes the angle of the upper edge prediction mode and the left side. The prediction mode may have a first angle corresponding to the middle of the angle of the surrounding prediction mode, and the other may be a prediction mode having a second angle perpendicular to the first angle.

8. In 1, the number of the MPM candidates included in the MPM list may be a predetermined fixed number.

9. In 8, when the angle difference between the upper peripheral prediction mode and the left peripheral prediction mode is greater than or equal to a predetermined fifth threshold value, one of MPM candidates that are not the same as the upper peripheral prediction mode and the left peripheral prediction mode is the above. It may be a prediction mode having a third angle corresponding to the middle of the angle of the upper peripheral prediction mode and the angle of the left peripheral prediction mode.

In 10. 8, when the angle difference between the upper peripheral prediction mode and the left peripheral prediction mode is equal to or greater than a predetermined sixth threshold value, one of the MPM candidates that are not the same as the upper peripheral prediction mode and the left peripheral prediction mode is determined. The prediction mode may have a third angle corresponding to the middle of the angle of the upper peripheral prediction mode and the angle of the left peripheral prediction mode, and the other may be a prediction mode having a fourth angle perpendicular to the third angle.

In 11. 8., if there is no MPM candidate in the MPM list that is the same as the intra prediction mode of the current block, in the step of deriving the intra prediction mode of the current block by using the MPM list, the remaining mode transmitted from the encoder (remaining mode) and the MPM list to derive an intra prediction mode of the current block, wherein the angle of the intra prediction mode of the current block is equal to the angle of the upper peripheral prediction mode or the angle of the left peripheral prediction mode. The closer it is, the shorter codeword may be assigned to the remaining mode corresponding to the intra prediction mode of the current block.

In 12.8, a predetermined number of MPM candidates among the MPM candidates that are not the same as the upper peripheral prediction mode and the left peripheral prediction mode, determine an angle between the angle of the upper peripheral prediction mode and the angle of the left peripheral prediction mode. Among the prediction modes having, the allocated mode values may be prediction modes selected in a small order.

13. Another embodiment of the present invention is a video decoding apparatus. The apparatus generates a Most Probable Mode (MPM) list including MPM candidates using a top neighbor prediction mode and a left neighbor prediction mode, and derives an intra prediction mode for deriving an intra prediction mode of a current block using the MPM list. And an intra prediction unit configured to generate a prediction block for the current block by performing prediction on the current block by using the derived intra prediction mode, wherein the upper peripheral prediction mode includes a peripheral adjacent to the upper end of the current block. A prediction mode of a block, and the left neighbor prediction mode is a prediction mode of a neighboring block adjacent to the left of the current block.

14. Another embodiment of the present invention is an intra prediction mode decoding method. The method includes generating a Most Probable Mode (MPM) list including MPM candidates using a top periphery prediction mode and a left periphery prediction mode and deriving an intra prediction mode of the current block using the MPM list. The upper peripheral prediction mode is a prediction mode of a neighboring block adjacent to the top of the current block, and the left peripheral prediction mode is a prediction mode of a neighboring block adjacent to the left of the current block.

According to the image encoding method according to the present invention, image encoding efficiency can be improved.

According to the image decoding method according to the present invention, image encoding efficiency can be improved.

According to the intra prediction mode encoding method according to the present invention, image encoding efficiency may be improved.

According to the intra prediction mode decoding method according to the present invention, image coding efficiency may be improved.

1 is a block diagram showing a configuration of a video encoder according to an embodiment of the present invention.
2 is a block diagram illustrating a configuration of a video decoder according to an embodiment.
3 is a conceptual diagram illustrating an embodiment of a luma intra prediction mode encoding method.
4 is a conceptual diagram illustrating another embodiment of a luma intra prediction mode encoding method.
5 is a conceptual diagram schematically illustrating an embodiment of a method for selecting an MPRM candidate when an angle difference between a prediction mode of an upper neighboring block and a prediction mode of a left neighboring block is greater than or equal to a predetermined threshold.
6 is a flowchart schematically illustrating an embodiment of a luma intra prediction mode decoding method using an angle difference between a prediction mode of an upper neighboring block and a prediction mode of a left neighboring block.
FIG. 7 is a conceptual diagram schematically illustrating an embodiment of a method of generating an MPM list according to the number of MPM candidates used in the embodiment of FIG. 6.
FIG. 8 is a conceptual diagram schematically illustrating another embodiment of a method of deriving MPM candidates included in an MPM list when the DiffDir value is greater than or equal to a predetermined threshold in the embodiment of FIG. 6.
9 is a conceptual diagram illustrating an embodiment of a method of selecting an MPM candidate for a luma component when a predetermined fixed number of MPM candidates is used.
10 is a conceptual diagram illustrating an embodiment of a method of selecting an MPM candidate for a chroma component when a predetermined fixed number of MPM candidates is used.
11 is a flowchart schematically illustrating an encoding method in an intra prediction mode according to an embodiment of the present invention.
12 is a conceptual diagram schematically illustrating an embodiment of a prediction direction of an intra prediction mode for a luma component and a mode value assigned to each prediction direction.
13 is a flowchart schematically illustrating a decoding method in an intra prediction mode according to an embodiment of the present invention.

Each of the components in the drawings described herein are shown independently for the convenience of description regarding different characteristic functions in the image encoder / decoder, and it is understood that each of the components is implemented in separate hardware or separate software. It does not mean. For example, two or more of each configuration may be combined to form one configuration, or one configuration may be divided into a plurality of configurations. Embodiments in which each configuration is integrated and / or separated are also included in the scope of the present invention unless they depart from the essence of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Hereinafter, the same reference numerals will be used for the same constituent elements in the drawings, and redundant explanations for the same constituent elements will be omitted.

1 is a block diagram illustrating a configuration of a video encoder according to an embodiment of the present invention. Referring to FIG. 1, the video encoder includes a picture splitter 110, an inter predictor 120, an intra predictor 125, a transformer 130, a quantizer 135, an inverse quantizer 140, An inverse transform unit 145, a filter unit 150, a memory 155, a reordering unit 160, and an entropy encoding unit 165 are included.

The picture division unit 110 may divide the inputted current picture into one or more coding units. A CU (CU) is a unit which is encoded by a video encoder. The CU is hierarchically divided by a depth information based on a quad tree structure, . CUs can have various sizes such as 8 × 8, 16 × 16, 32 × 32, and 64 × 64. The largest sized CU is called a Large Coding Unit (LCU), and the smallest sized CU is called a Smallest Coding Unit (SCU).

The picture division unit 110 may divide the CU and generate a prediction unit (PU) and a transform unit (TU). The PU is a block smaller than or equal to the CU, not necessarily square, and may be a rectangular block. In general, intra prediction may be performed in blocks of 2N * 2N or N * N size. Where N represents the number of pixels as a natural number, and 2N * 2N and N * N represent the size of the PU.

In the inter prediction mode, the inter prediction unit 120 may perform motion estimation (ME) and motion compensation (MC). The inter prediction unit 120 generates a prediction block based on at least one picture information of a previous picture or a subsequent picture of the current picture.

The inter prediction unit 120 performs motion estimation based on the divided prediction target block and at least one reference block stored in the memory unit 155. The inter prediction unit 120 generates motion information including a motion vector (MV), a reference block index, a prediction mode, and the like as a result of the motion estimation.

In addition, the inter prediction unit 120 performs motion compensation using the motion information and the reference block. In this case, the inter prediction unit 120 generates and outputs a prediction block corresponding to the input block from the reference block.

In the intra prediction mode, the intra prediction unit 125 may generate a prediction block based on pixel information in the current picture. In the intraprediction mode, the intra prediction unit 125 may perform prediction on a current block based on a prediction target block and a reconstructed block reconstructed after being previously transformed and quantized. The reconstruction block may be a reconstructed image before passing through the deblocking filter unit.

The residual block is generated by the difference between the prediction target block and the prediction block generated in the inter or intra prediction mode.

The transformer 130 performs transform on the residual block for each TU to generate transform coefficients.

The TU can have a tree structure within a range of maximum size and minimum size. Whether a current block is divided into sub-blocks per TU may be indicated through a flag. The transform unit 130 may perform transformation using a discrete cosine transform (DCT) and / or a discrete sine transform (DST).

The quantizer 135 may quantize the values converted by the transformer 130. The quantization factor may vary depending on the block or the importance of the image. The quantized transform coefficient values may be provided to the reordering unit 160 and the inverse quantization unit 140.

The reordering unit 160 may align the transform coefficients of the quantized 2D block type into the transform coefficients of the 1D vector form through a scan in order to increase the efficiency of entropy encoding. At this time, the reordering unit 160 may increase the entropy encoding efficiency by changing the scan order based on the probabilistic statistics.

The entropy encoder 165 may entropy encode the values obtained by the reordering unit 160. The encoded information forms a compressed bit stream and may be transmitted or stored through a network abstraction layer (NAL).

The inverse quantizer 140 inverse quantizes the transform coefficients quantized by the quantizer 135, and the inverse transformer 145 inverse transforms the inverse quantized transform coefficients to generate a reconstructed residual block. The reconstructed residual block may be combined with the predicted block generated by the inter predictor 120 or the intra predictor 125 to generate a reconstructed block. The reconstruction block is provided to the intra predictor 125 and the filter 150.

The filter unit 150 may apply a deblocking filter, an adaptive loop filter (ALF), a sample adaptive offset (SAO), and the like to the restored residual block. The deblocking filter filters the reconstructed blocks to remove distortions between block boundaries that occur during encoding and decoding. The ALF performs filtering to minimize the error between the predicted block and the last reconstructed block. The ALF performs filtering based on a value obtained by comparing a reconstructed block filtered through a deblocking filter with a current predicted block, and the filter coefficient information of the ALF may be carried in a slice header and transmitted from the encoder to the decoder. have. SAO is a loop filter process that restores the offset difference from the original image on a pixel-by-pixel basis for the residual block to which the deblocking filter is applied. Offsets applied through SAO include a band offset and an edge offset. The band offset divides the pixel into 32 bands according to intensity, and applies the offset by dividing the 32 bands into two band groups of 16 bands at the edge and 16 bands at the center. The edge offset classifies the direction of the edge and the intensity of the surrounding pixels for each pixel and applies the offset.

The memory 155 may store a final reconstructed block that has passed through the filter unit 150, and the stored final reconstructed block may be provided to the inter predictor 120 that performs inter prediction.

2 is a block diagram illustrating a configuration of a video decoder according to an embodiment. Referring to FIG. 2, the video decoder includes an entropy decoder 210, a reordering unit 215, an inverse quantizer 220, an inverse transform unit 225, an inter predictor 230, an intra predictor 235, and a filter. The unit 240 and the memory 245 are included.

The entropy decoder 210 receives the compressed bit stream from the NAL. The entropy decoder 210 entropy decodes the received bit stream, and entropy decodes the prediction mode, motion vector information, and the like when the bit stream is included in the bit stream.

The entropy decoded transform coefficient or residual signal is provided to the reordering unit 215. The reordering unit 215 may inverse scan the decoded transform coefficients or the residual signal to generate transform coefficients in the form of a 2D block.

The inverse quantization unit 220 can dequantize the rearranged transform coefficients. The inverse transform unit 225 may inverse transform the inverse quantized transform coefficients to generate a residual block.

The residual block may be combined with the prediction block generated by the inter predictor 230 or the intra predictor 235 to generate a reconstructed block. The reconstruction block may be provided to the intra predictor 235 and the filter 240. The operations of the inter predictor 230 and the intra predictor 235 may be the same as the operations of the inter predictor 120 and the intra predictor 125 in the video encoder, respectively.

The filter unit 240 may apply a deblocking filter, ALF, SAO, etc. to the reconstruction block. The deblocking filter may filter the restoration block to remove distortion between block boundaries that occurs during the encoding and decoding processes. The ALF may perform filtering on the deblocking filtered reconstructed block to minimize an error between the prediction target block and the last reconstructed block. In addition, the SAO is applied to the deblocking filtered reconstructed block on a pixel basis to reduce the difference from the original image.

The memory 245 may store a final reconstructed block obtained through the filter unit 240, and the stored final reconstructed block may be provided to the inter predictor 230 that performs inter prediction.

Intra prediction may be performed according to the intra prediction mode of the current block. The number of intra prediction modes that the current block may have may be a fixed value, and the fixed value may be, for example, 9, 35, or 36, depending on implementation and / or needs. The encoder and the decoder may increase image compression efficiency by performing intra prediction using the intra prediction mode as described above.

As the number of intra prediction modes that a current block can have increases, an efficient intra prediction mode encoding / decoding method is required. Since the prediction mode of the current block has a high probability of being the same as the prediction mode of the neighboring block, the prediction mode of the current block may be encoded using the prediction mode of the neighboring block adjacent to the current block. At this time, Most Probable Mode (MPM) for the current block may be used.

3 is a conceptual diagram illustrating an embodiment of a luma intra prediction mode encoding method. Hereinafter, the neighboring block adjacent to the top of the current block is referred to as the upper neighboring block, and the neighboring block adjacent to the left of the current block is referred to as the left neighboring block. In addition, hereinafter, the intra prediction mode of the current block is called a current prediction mode.

Referring to 310 of FIG. 3, the encoder may construct an MPM list for the current block C using the prediction mode of the upper neighboring block A and the left neighboring block L. FIG. In an embodiment, the MPM list may include the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block as MPM candidates.

Referring to 320 of FIG. 3, the encoder may determine whether there is a candidate equal to the intra prediction mode of the current block among MPM candidates in the MPM list. In this case, the encoder may encode and transmit a flag indicating whether there is a same candidate as the intra prediction mode of the current block among the MPM candidates in the MPM list to the decoder. Hereinafter, the flag is referred to as an MPM flag. In one embodiment, the MPM flag may be prev_pred_mode_flag. When there is a same candidate as the intra prediction mode of the current block among the MPM candidates in the MPM list, the value of the MPM flag may be one.

Referring to 330 of FIG. 3, when there is a candidate among the MPM candidates in the MPM list that is identical to the intra prediction mode of the current block (prediction mode of C = one of the MPM candidates), for example, when prev_pred_mode_flag is 1, the encoder It may be determined whether the number of candidates is two. When the number of MPM candidates is one, the encoder may not transmit additional information other than the MPM flag to the decoder. When the number of MPM candidates is two, the encoder may encode and transmit an index indicating which MPM candidate is the same as the prediction mode of the current block to the decoder. Hereinafter, the index is referred to as an MPM index. In one embodiment, the MPM index may be mpm_idx.

Referring to 340 of FIG. 3, when none of the MPM candidates in the MPM list is the same as the intra prediction mode of the current block (prediction mode of C! = MPM), for example, when prev_pred_mode_flag is 0, the encoder is in the remaining mode. (remaining mode) can be encoded and transmitted to the decoder. The encoder may use a fixed length coding (FLC) scheme as a coding scheme for the remaining mode. The encoder may generate the remaining mode by rearranging the prediction mode of the current block except for the mode values of the MPM candidate. In one embodiment, the remaining mode may be represented by rem_intra_pred_mode.

Referring to 350 of FIG. 3, when the mode value of the current prediction mode is 2, the encoder may encode and transmit a flag indicating whether the current prediction mode is a DC mode or a planar mode to the decoder. In one embodiment, the flag may be planarFlag.

The decoder may construct an MPM list for the current block by using the prediction mode of the upper neighboring block and the left neighboring block. The MPM list construction method may be the same as the MPM list construction method in the above-described encoder.

The decoder may determine whether there is a candidate equal to the intra prediction mode of the current block among the MPM candidates in the MPM list through the MPM flag, for example, prev_pred_mode_flag.

When the same candidate as the intra prediction mode of the current block is present among the MPM candidates in the MPM list, the MPM flag, for example, mpm_idx, may determine which MPM candidate is the same as the prediction mode of the current block. If the prediction mode value of the current block is 2, according to an embodiment, the decoder may determine whether the prediction mode of the current block is a DC mode or a planner mode through the planarFlag.

If none of the MPM candidates in the MPM list have the same candidate as the intra prediction mode of the current block, the decoder may derive the prediction mode of the current block by using the remaining mode and the MPM candidates. If the prediction mode value of the current block is 2, according to an embodiment, the decoder may determine whether the prediction mode of the current block is a DC mode or a planner mode through the planarFlag.

4 is a conceptual diagram illustrating another embodiment of a luma intra prediction mode encoding method.

In the intra prediction mode encoding method according to the embodiment of FIG. 3, when the current prediction mode is the same as one of the MPM candidates, the compression efficiency may be improved. However, if there are no candidates identical to the current prediction mode among the MPM candidates, a large number of bits are required to encode the remaining mode, so that a block having a prediction mode that is not the same as any of the MPM candidates is used. The larger the compression efficiency can be lowered.

Therefore, in order to perform efficient encoding even when the current prediction mode is not the same as any of the MPM candidates in the MPM list, the encoder is likely to be equal to the current prediction mode probabilistic among intra prediction modes except the MPM candidates. The mode may be used to construct a Most Probable Remaining Mode (MPRM) list. When the current prediction mode is the same as one of the MPRM candidates included in the MPRM list, the encoder may transmit information about the current prediction mode to the decoder using the MPRM flag and the MPRM index. If the current prediction mode is not the same as one of the MPRM candidates included in the MPRM list, the encoder may encode a remaining mode for the current prediction mode and transmit it to the decoder. At this time, in one embodiment, the encoder may use a fixed length encoding method as an encoding method for the remaining mode.

According to the intra prediction mode encoding method using the above-described MPRM, image compression efficiency may be improved even when there is no candidate among the MPM candidates equal to the current prediction mode.

Referring to 410 of FIG. 4, the encoder may construct an MPM list for the current block C using the prediction mode of the upper neighboring block A and the prediction mode of the left neighboring block L. FIG. In an embodiment, the MPM list may include the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block as MPM candidates.

In addition to the MPM list, the encoder may construct an MPRM list including the MPRM candidates. The number of MPRM candidates included in the MPRM list may be a predetermined fixed value, for example, three.

The encoder may select Most Probable Remaining Modes (MPRMs) corresponding to the left neighboring block based on the intra prediction mode of the left neighboring block and the predefined table information. For example, the number of MPRMs corresponding to the left neighboring block may be three, and the MPRMs may be represented by L ₁ , L ₂ , and L ₃ , respectively. The encoder may select MPRMs corresponding to the upper neighboring block based on the intra prediction mode of the upper neighboring block and predefined table information. For example, the number of MPRMs corresponding to the upper peripheral block may be three, and the MPRMs may be represented by A ₁ , A ₂ , and A ₃ , respectively.

The encoder constructs an MPRM list by selecting a predetermined number of MPRM candidates in order of decreasing mode value except for overlapping among the selected MPRMs (L ₁ , L ₂ , L ₃ , A ₁ , A ₂ , A ₃ ). can do. The predetermined number may be 3 in one embodiment.

Referring to 420 of FIG. 4, the encoder may determine whether there is a candidate equal to the intra prediction mode of the current block among the MPM candidates in the MPM list. In this case, the encoder may encode and transmit a flag indicating whether there is a same candidate as the intra prediction mode of the current block among the MPM candidates. Hereinafter, the flag is referred to as an MPM flag. In one embodiment, the MPM flag may be prev_pred_mode_flag. When there is a same candidate as the intra prediction mode of the current block among the MPM candidates in the MPM list, the value of the MPM flag may be one.

Referring to 430 of FIG. 4, when there are the same candidates as the intra prediction mode of the current block among the MPM candidates in the MPM list (the prediction mode of C = one of the MPM candidates), for example, when prev_pred_mode_flag is 1, the encoder It may be determined whether the number of candidates is two. When the number of MPM candidates is one, the encoder may not transmit additional information other than the MPM flag to the decoder. When the number of MPM candidates is two, the encoder may encode and transmit an index indicating which MPM candidate is the same as the prediction mode of the current block to the decoder. Hereinafter, the index is referred to as an MPM index. In one embodiment, the MPM index may be mpm_idx.

If the mode value of the current prediction mode is 2, the encoder may encode and transmit a flag indicating whether the current prediction mode is a DC mode or a planar mode to the decoder. In one embodiment, the flag may be planarFlag.

Referring to 440 of FIG. 4, when none of the MPM candidates in the MPM list is the same as the intra prediction mode of the current block (prediction mode of C! = MPM), for example, when prev_pred_mode_flag is 0, the encoder is included in the MPRM list. It may be determined whether any of the MPRM candidates are the same as the intra prediction mode of the current block. In this case, the encoder may encode and transmit a flag indicating whether there is a same candidate as the intra prediction mode of the current block among the MPRM candidates in the MPRM list to the decoder. Hereinafter, the flag is referred to as an MPRM flag. In one embodiment, the MPRM flag may be mprm_pred_flag. When there is a same candidate as the intra prediction mode of the current block among the MPRM candidates in the MPRM list, the value of the MPRM flag may be one.

Referring to 450 of FIG. 4, when there is a candidate among the MPRM candidates in the MPRM list that is identical to the intra prediction mode of the current block (prediction mode of C = one of the MPRM candidates), for example, when mprm_pred_flag is 1, the encoder An index indicating whether the prediction mode of the block is the same as which MPRM candidate may be encoded and transmitted to the decoder. Hereinafter, the index is referred to as an MPRM index. In one embodiment, the MPRM index may be mprm_idx.

Referring to 460 of FIG. 4, when none of the MPRM candidates in the MPRM list is the same as the intra prediction mode of the current block (prediction mode of C! = MPRM), for example, when mprm_pred_flag is 0, the encoder is in a remaining mode. (remaining mode) can be encoded and transmitted to the decoder. The encoder may use a fixed length encoding scheme as an encoding scheme for the remaining mode. The encoder may generate the remaining mode by rearranging the prediction mode of the current block except for the mode values of the MPM candidate and the mode values of the MPRM candidate, and in one embodiment, the remaining mode may be represented by rem_intra_pred_mode. .

The decoder may construct an MPM list and an MPRM list for the current block using the prediction mode of the upper neighboring block and the left neighboring block. The MPM list construction method and the MPRM list construction method may be the same as the MPM list construction method in the above-described encoder.

The decoder may determine whether there is a candidate equal to the intra prediction mode of the current block among the MPM candidates through the MPM flag, for example, prev_pred_mode_flag.

When the same candidate as the intra prediction mode of the current block is present among the MPM candidates, it is possible to determine which MPM candidate is the same as the prediction mode of the current block through the MPM index, for example, mpm_idx. If the prediction mode value of the current block is 2, according to an embodiment, the decoder may determine whether the prediction mode of the current block is a DC mode or a planner mode through the planarFlag.

If none of the MPM candidates has the same candidate as the intra prediction mode of the current block, the decoder determines whether there is a same candidate as the intra prediction mode of the current block among the MPRM candidates in the MPRM list through the MPRM flag, for example, mprm_pred_flag. You can judge.

If the same candidate as the intra prediction mode of the current block is present among the MPRM candidates in the MPRM list, the MPRM index, for example, mprm_idx, may determine which MPRM candidate is the same as the prediction mode of the current block.

If none of the MPRM candidates in the MPRM list are the same as the intra prediction mode of the current block, the decoder may derive the prediction mode of the current block by using the remaining mode, the MPM candidates, and the MPRM candidates. If the prediction mode value of the current block is 2, according to an embodiment, the decoder may determine whether the prediction mode of the current block is a DC mode or a planner mode through the planarFlag.

In the above-described embodiment of FIG. 4, the encoder and the decoder may select MPRM candidates based on predefined table information, and construct an MPRM list using the selected MPRM candidates. However, the MPRM candidate selection method is not limited to the above-described method.

In an embodiment, the encoder and the decoder may select a prediction mode having an angle most similar to that of the prediction mode of the neighboring block as the MPRM candidate. Hereinafter, a prediction mode having an angle similar to that of the current prediction mode may be called a prediction mode close to the current prediction mode, and a prediction mode having a large angular difference from the current prediction mode may be called a prediction mode far from the current prediction mode. .

Meanwhile, when the prediction modes included in the Unified Directional Intra (UDI) mode are listed in the angular order of each prediction mode in the clockwise direction, the prediction mode existing on the left side of the current prediction mode may be called the left prediction mode for the current prediction mode. And a prediction mode existing on the right side of the current prediction mode may be called a right prediction mode with respect to the current prediction mode. Hereinafter, when the prediction modes included in the UDI mode are listed in the angular order of each prediction mode in the clockwise direction, the prediction mode existing between or between the two prediction modes may be referred to as a prediction mode located between or between the two prediction modes. .

For example, the encoder and the decoder may select a planar mode as the first MPRM candidate corresponding to each neighboring block, and select the prediction modes closest to the intra prediction mode of each neighboring block as the remaining MPRM candidates. Can be. This can be represented as follows.

MPRM _A = {Planar, Plus_ModeA, Minus_ModeA}

MPRM _L = {Planar, Plus_ModeL, Minus_ModeL}

Here, A may represent a top peripheral block, L may represent a left peripheral block, and Planar may represent a planner mode. Plus_ModeA may represent the right prediction mode closest to the prediction mode of the upper neighboring block, and Minus_ModeA may represent the left prediction mode closest to the prediction mode of the upper neighboring block. Plus_ModeL may represent the right prediction mode closest to the prediction mode of the left neighboring block, and Minus_ModeL may represent the left prediction mode closest to the prediction mode of the left neighboring block. For example, if the right prediction mode closest to prediction mode 0 is 22 and the left prediction mode closest to prediction mode 0 is 21, if the prediction mode value of the upper peripheral block is 0, Plus_ModeA is 22 and Minus_ModeA is 21 days. Can be.

The encoder and the decoder may configure three MPRM lists by selecting three of the six prediction modes. For example, the encoder selects the planner mode as the first MPRM candidate, and selects two prediction modes in the order of the lowest modes except the planner mode among the six selected prediction modes as the remaining two MPRM candidates. Lists can be constructed.

In another embodiment, the encoder and the decoder may be configured to determine the prediction modes closest to the two prediction modes and the angles of the two prediction modes when the difference between the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block is greater than or equal to a predetermined threshold. A prediction mode having an angle corresponding to a median of may be selected as an MPRM candidate. Hereinafter, a prediction mode having an angle corresponding to the middle value of the angles of the two prediction modes is called a prediction mode located in the middle of the two prediction modes.

5 is a conceptual diagram schematically illustrating an embodiment of a method for selecting an MPRM candidate when an angle difference between a prediction mode of an upper neighboring block and a prediction mode of a left neighboring block is greater than or equal to a predetermined threshold.

In the embodiment of FIG. 5, it is assumed that the prediction mode of the upper neighboring block is 11 and the prediction mode of the left neighboring block is 3. At this time, five angular modes may exist between the two prediction modes. Here, assume that the predetermined threshold is 5 or less.

Referring to FIG. 5, since the right prediction mode located closest to the prediction mode 3 of the left neighboring block is 18, the prediction mode 18 may be used as an MPRM candidate. In addition, since the left prediction mode located closest to the prediction mode 11 of the upper neighboring block is 20, the prediction mode 20 may be used as the MPRM candidate. In addition, the prediction mode located in the middle of the two prediction modes may be 19. At this time, the MPRM list may be configured as {mode 18, mode 20, mode 19}.

In another embodiment, the encoder and the decoder may encode / encode the intra prediction mode in the same manner as in the embodiment of FIG. Can be decrypted That is, the encoder and the decoder may use the fixed length coding (FLC) method for the remaining mode without using the MPRM even when the same candidate as the current prediction mode does not exist among the MPM candidates included in the MPM list.

6 is a flowchart schematically illustrating an embodiment of a luma intra prediction mode decoding method using an angle difference between a prediction mode of an upper neighboring block and a prediction mode of a left neighboring block. Hereinafter, when the prediction modes included in the UDI mode are arranged in the angular order in the clockwise direction, the prediction mode existing between the two prediction modes may be called a prediction mode located between the two prediction modes.

In the embodiment of FIG. 6, DiffDir is a block of the upper peripheral block when the prediction modes included in the Unified Directional Intra (UDI) mode are listed in the angular order of each prediction mode, and the indexes are assigned to the listed prediction modes in ascending order from the front. It may mean a difference between an index allocated to the prediction mode and an index assigned to the prediction mode of the left neighboring block. DiffDir may represent an angular difference between the two prediction modes and may indicate the number of prediction modes located between the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block. In the following embodiments, DiffDir may have the same meaning as in the embodiment of FIG. 6.

prev_intra_pred_flag means an MPM flag, mpm_idx means an MPM index, and rem_intra_pred_mode may mean a remaining mode.

When the angle difference between the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block is small, the current prediction mode may have a high probability of being located between the two prediction modes. Accordingly, the decoder can increase image compression efficiency by selecting prediction modes located close to the prediction mode of the neighboring block as MPM candidates and adding them to the MPM list. In addition, the decoder may set the number of MPM candidates added to the MPM list differently according to an angle difference between the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block.

Referring to FIG. 6, the decoder may derive a DiffDir (S610). Hereinafter, an embodiment of the DiffDir value derivation method is described.

Here, DirIdx17 and DirIdx34 are mapping tables in which indices are assigned in angular order to prediction modes included in the Unified Directional Intra (UDI) mode. DirIdxX may mean a mapping table used when the number of prediction modes that a current block may have is X. X is not limited to 17, 34, and may be 18 instead of 17 and 35 instead of 34.

leftMode refers to the prediction mode of the left neighboring block, and aboveMode refers to the prediction mode of the upper neighboring block. In addition, ABS (X) means the absolute value of X.

If leftMode and aboveMode are both DC modes, the DiffDir value may be -2. This can be represented as follows.

If both of leftMode and aboveMode are DC, DiffDir = -2

If one of leftMode and aboveMode is DC mode, the DiffDir value may be -1. This can be represented as follows.

Else if either leftMode or aboveMode are DC, DiffDir = -1

If both leftMode and aboveMode are not the DC mode and the size of the current prediction unit is 4x4, the DiffDir value may be calculated by the following procedure.

Else if current prediction unit is 4x4

DiffDir = ABS (DirIdx17 [leftMode]-DirIdx17 [aboveMode])

if DiffDir> 8, DiffDir = 16-DiffDir

If both leftMode and aboveMode are not the DC mode and the size of the current prediction unit is 8x8, 16x16, 32x32, the DiffDir value may be calculated by the following procedure.

Otherwise (current prediction unit is 8x8 or 16x16 or 32x32)

DiffDir = ABS (DirIdx34 [leftMode]-DirIdx34 [aboveMode])

if DiffDir> 16, DiffDir = 33- DiffDir

Referring to FIG. 6 again, the decoder may generate an MPM list based on the DiffDir value (S620). The number of MPM candidates included in the MPM list may vary according to an angle difference between prediction modes of neighboring blocks. Table 1 below shows an example of the number of MPM candidates according to DiffDir values.

[Table 1]

Referring to Table 1, for example, when the DiffDir value is 2, the number of MPM candidates for an 8x8 prediction unit may be three. A method of generating an MPM list according to the number of MPM candidates will be described later in the embodiment of FIG. 7.

Referring to FIG. 6 again, the decoder may decode prev_intra_pred_flag (S630). The decoder may determine whether the decoded prev_intra_pred_flag value is 0 or 1 (S640). If prev_intra_pred_flag is 1, the decoder may decode mpm_idx and derive the current prediction mode using the mpm_idx and the MPM list (S650). If prev_intra_pred_flag is 1, the decoder may decode rem_intra_pred_mode (S660). The decoder can derive the current prediction mode using the decoded rem_intra_pred_mode. In this case, the derivation method of the current prediction mode may vary depending on the number of MPM candidates.

The MPM list generation method described above in the embodiment of FIG. 6 may be equally applied to an encoder.

FIG. 7 is a conceptual diagram schematically illustrating an embodiment of a method of generating an MPM list according to the number of MPM candidates used in the embodiment of FIG. 6. In the embodiment of FIG. 7, a prediction mode located between the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block may be selected as the MPM candidate.

710 of FIG. 7 illustrates an embodiment of a method of generating an MPM list when the number of MPM candidates is three. It is assumed that the prediction modes of the top neighbor block and the left neighbor block are 4 and 0, respectively.

The encoder and the decoder may select the prediction mode 4 of the upper neighboring block and the prediction mode 0 of the left neighboring block as MPM candidates. Since the prediction mode 11 is the left prediction mode closest to the prediction mode of the upper neighboring block and the right prediction mode closest to the prediction mode of the left neighboring block, the encoder and the decoder may select the prediction mode 11 as the MPM candidate. In this case, the generated MPM list may include prediction mode 0, prediction mode 4, and prediction mode 11 as MPM candidates.

720 of FIG. 7 illustrates an embodiment of a method of generating an MPM list when the number of MPM candidates is five. It is assumed that the prediction modes of the top neighbor block and the left neighbor block are 3 and 0, respectively.

The encoder and the decoder may select the prediction mode 3 of the upper neighboring block and the prediction mode 0 of the left neighboring block as MPM candidates. The encoder and the decoder may select the left prediction mode 10 located closest to the prediction mode of the upper neighboring block as the MPM candidate. In addition, the encoder and the decoder may select the right prediction mode 11 closest to the prediction mode of the left neighboring block as the MPM candidate. In addition, the encoder and the decoder may select the DC mode as the MPM candidate. At this time, the generated MPM list may include prediction modes 0, 3, 10, 11, and DC as MPM candidates.

730 of FIG. 7 illustrates an embodiment of a method of generating an MPM list when the number of MPM candidates is seven. It is assumed that the prediction modes of the top neighbor block and the left neighbor block are 3 and 0, respectively.

The encoder and the decoder may select the prediction mode 3 of the upper neighboring block and the prediction mode 0 of the left neighboring block as MPM candidates. The encoder and the decoder may select prediction modes 26 and 18 located closest to the prediction mode of the upper neighboring block as the MPM candidates. In addition, the encoder and the decoder may select the prediction modes 21 and 22 located closest to the prediction mode 0 of the left neighboring block as the MPM candidate. In addition, the encoder and the decoder may select the DC mode as the MPM candidate. In this case, the generated MPM list may include prediction modes 0, 3, 26, 18, 21, 22, and DC as MPM candidates.

In the image decoding method according to the embodiment of FIG. 6, the number of MPM candidates in the MPM list and the method of deriving the MPM candidates may vary according to DiffDir values, that is, angle differences between prediction modes of neighboring blocks. However, if the DiffDir value is greater than or equal to a predetermined value, the number of MPM candidates to be selected increases, so that coding efficiency may be relatively low. Accordingly, a method for preventing such coding efficiency degradation may be provided.

In one embodiment, when the DiffDir value is greater than or equal to a predetermined threshold in the embodiment of FIG. 6, that is, when the angular difference between prediction modes of neighboring blocks is greater than or equal to a predetermined value, the encoder and the decoder are the same as in the embodiment of FIG. 3. The intra prediction mode can be encoded / decoded with. For example, when the DiffDir value is greater than or equal to a predetermined threshold, the encoder and the decoder may perform intra prediction mode encoding / decoding using only one or two MPM candidates.

In another embodiment, when the DiffDir value is greater than or equal to a predetermined threshold in the embodiment of FIG. 6, that is, when an angular difference between prediction modes of neighboring blocks is greater than or equal to a predetermined threshold, the encoder and the decoder are different from the prediction mode of the neighboring block. A large prediction mode may be selected as the MPM candidate.

FIG. 8 is a conceptual diagram schematically illustrating another embodiment of a method of deriving MPM candidates included in an MPM list when the DiffDir value is greater than or equal to a predetermined threshold in the embodiment of FIG. 6. In the embodiment of FIG. 8, it is assumed that the prediction mode of the upper neighboring block is 3, the prediction mode of the left neighboring block is 0, and the number of MPM candidates in the MPM list is seven.

810 of FIG. 8 schematically illustrates an embodiment of an MPM candidate derivation method according to the embodiments of FIGS. 6 and 7. Referring to 810 of FIG. 8, in the embodiments of FIGS. 6 and 7, the prediction modes 3 of the upper neighboring block, the prediction mode of the left neighboring block 0, and the prediction modes closest to the prediction mode of the upper neighboring block are shown. (18, 26), the prediction modes 21, 22 and the DC mode closest to the prediction mode of the upper neighboring block may be selected as MPM candidates.

820 of FIG. 8 schematically illustrates another embodiment of the MPM candidate derivation method when the DiffDir value is greater than or equal to a predetermined threshold in the embodiment of FIG. 6.

Referring to 820 of FIG. 8, the encoder and the decoder may select the prediction mode 3 of the upper neighboring block and the prediction mode 0 of the left neighboring block as MPM candidates. The encoder and the decoder may select the left prediction mode 18 located closest to the prediction mode of the upper neighboring block as the MPM candidate. In addition, the encoder and the decoder may select the right prediction mode 21 located closest to the prediction mode of the left neighboring block as the MPM candidate.

In 820 of FIG. 8, the encoder and the decoder may further select a prediction mode 4 located in the middle of the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block as an MPM candidate, and the selected prediction mode 4 may be selected as the MPM candidate. The prediction mode 8 with the vertical angle may be selected as the MPM candidate.

In this case, the generated MPM list may include prediction modes 0, 3, 18, 21, 4, 8, and DC as MPM candidates.

Meanwhile, in the above-described embodiment of FIG. 3, MPM candidates may be selected in consideration of the availability of neighboring blocks and the sameness between prediction modes of neighboring blocks, so that the number of MPM candidates included in the MPM list is 1. Dogs or two.

However, in the intra prediction mode encoding method according to the embodiment of FIG. 3, the VLC table used for entropy encoding / decoding may vary according to the number of MPM candidates, thereby reducing the coding efficiency. Accordingly, an intra prediction mode encoding method using a predetermined fixed number of MPM candidates may be provided.

In the intra prediction mode encoding method using the MPM, the compression efficiency may be improved when the current prediction mode is the same as one of the MPM candidates. However, if there are no candidates identical to the current prediction mode among the MPM candidates, a large number of bits are required to encode the remaining mode, so that a block having a prediction mode that is not the same as any of the MPM candidates is used. The larger the compression efficiency can be lowered. Accordingly, an intra prediction mode encoding method using more MPM candidates than the number of MPM candidates used in the embodiment of FIG. 3 may be provided. In an embodiment, in the intra prediction mode encoding method using a predetermined fixed number of MPM candidates, the predetermined fixed number may be four.

9 is a conceptual diagram illustrating an embodiment of a method of selecting an MPM candidate for a luma component when a predetermined fixed number of MPM candidates is used.

If four MPM candidates are used, additional MPM candidates may be needed in addition to the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block. 9 illustrates a method of selecting the additional MPM candidate.

In the embodiment of FIG. 9, the prediction mode 10 is a prediction mode of the upper neighboring block or the prediction mode of the left neighboring block, and is assumed to be one of the MPM candidates for the luma component. The encoder and the decoder may select prediction modes having the smallest angular difference from the MPM candidate, that is, prediction modes closest to the MPM candidate as additional MPM candidates.

For example, when the number of prediction modes that a current block can have is 34 or 35, the encoder and the decoder may select prediction mode 18 and / or prediction mode 19 having the smallest angular difference from the MPM candidate as additional MPM candidates. . When the number of prediction modes that the current block may have is 17 or 18, prediction mode 18 and prediction mode 19 may be modes not used for intra prediction. In this case, the encoder and the decoder may select prediction mode 3 and / or prediction mode 4 having the smallest angular difference from the MPM candidate as additional MPM candidates.

10 is a conceptual diagram illustrating an embodiment of a method of selecting an MPM candidate for a chroma component when a predetermined fixed number of MPM candidates is used.

In the embodiment of FIG. 10, it is assumed that prediction mode 10 is the actual intra prediction mode for the luma component. Hereinafter, the actual intra prediction mode for the luma component is called a luma prediction mode.

The encoder and the decoder may select prediction modes having the smallest angular difference from the luma prediction mode, that is, prediction modes closest to the luma prediction mode as MPM candidates. For example, when the number of prediction modes that a current block can have is 34 or 35, the encoder and the decoder may select prediction mode 18 and / or prediction mode 19 having the smallest angular difference from the luma prediction mode as additional MPM candidates. have. When the number of prediction modes that the current block may have is 17 or 18, prediction mode 18 and prediction mode 19 may be modes not used for intra prediction. In this case, the encoder and the decoder may select the prediction mode 3 and / or the prediction mode 4 having the smallest angular difference from the luma prediction mode as additional MPM candidates.

The encoder and the decoder may select a prediction mode having a large angle difference from the luma prediction mode, that is, a prediction mode located far from the luma prediction mode as the MPM candidate. In an embodiment, the prediction mode having a large angle difference from the luma prediction mode may mean a prediction mode having an angle perpendicular to the luma prediction mode. For example, the encoder and the decoder may select prediction mode 8 as an MPM candidate when the luma prediction mode is 10.

In the above-described embodiment of FIGS. 9 and 10, the number of MPM candidates may be extended based on the fact that the current prediction mode has a high probability of being close to the prediction mode of the neighboring block. For example, the added MPM candidate may be a prediction mode having a small angle difference from the prediction mode of the neighboring block.

When the angle difference between the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block is small, that is, the DiffDir value is small, the current prediction mode may be located close to the two prediction modes. However, when the angular difference between the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block is large, that is, when the DiffDir value is large, the probability that the current prediction mode is located close to the two prediction modes may be low.

Therefore, when the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block are not the same and the angular difference between the two prediction modes is large, an additional MPM candidate may be selected differently.

In an embodiment, when the angular difference between the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block is large, the encoder and the decoder may select a prediction mode located between the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block. Can be added as an MPM candidate. In addition, when the angular difference between the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block is large, the encoder and the decoder may be located between the prediction mode of the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block. A prediction mode having an angle perpendicular to the angle of the located prediction mode may be added as an MPM candidate.

Hereinafter, an embodiment of a process of selecting a prediction mode located in the middle of the two prediction modes as an additional MPM candidate is described. Here, it is assumed that the number of MPM candidates is fixed to four.

If all neighboring blocks are not valid, prediction modes 2, 0, 1, and 34 may be selected as four MPM candidates. Here, 34 may mean a planar mode. This can be represented as follows.

When the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block are the same, the prediction mode may be selected as one of the MPM candidates, and two prediction modes closest to the selected MPM candidate may be selected as the MPM candidates. This can be represented as follows.

When the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block are not the same, both prediction modes may be selected as MPM candidates.

In this case, if the number of intra prediction modes that the current prediction unit may have is four, two MPM candidates may be additionally selected using a predetermined table. This can be represented as follows.

If one of the two prediction modes selected as the MPM candidate is 34, and the DiffDir value is less than or equal to the predetermined threshold K, then the prediction mode closest to the non-34 prediction mode as the additional MPM candidate is selected as the MPM candidate. Can be selected. When one of the two prediction modes selected as the MPM candidate is 34, if the DiffDir value is greater than the predetermined threshold K, the prediction mode located in the middle of the two prediction modes selected as the MPM candidate may be selected as the additional MPM candidate. This may be represented as follows, and a portion marked with a strikethrough line (a line across the middle of a character or sentence) means a portion not used in the present embodiment.

Here, DirIdx34, InvPredMode, PredModeMinus1_35, and PredModePlus1_35 may be derived by the following Tables 2, 3, and 4.

[Table 2]

[Table 3]

[Table 4]

Here, Table 2 is a mapping table in which an index is assigned according to the angular order to prediction modes included in the Unified Directional Intra (UDI) mode. Table 3 is a mapping table showing the mapping relationships shown in Table 2 in reverse.

In Table 4, PredModeMinus1_35 refers to the left prediction mode nearest to intraPredMode, and PredModePlus1_35 refers to the right prediction mode nearest to intraPredMode.

If both prediction modes selected as the MPM candidate are not 34, the prediction mode closest to the two prediction modes selected as the MPM candidate may be selected as the additional MPM candidate. This can be represented as follows.

In the above-described embodiment, candModeList [0] means a prediction mode having a smaller mode value among the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block, and candModeList [1] is the prediction mode of the upper neighboring block and the left. It may mean a prediction mode having a larger mode value among the prediction modes of the neighboring blocks. Accordingly, when the maximum value of the mode values of the prediction mode that the current prediction mode may have is 34 and the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block are different, candModeList [0] may have a value of 34. Can't.

Therefore, in the above-described embodiment, the MPM candidate selection process when the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block are different from each other and candModeList [0] is 34 may not be necessary. The MPM candidate selection process when candModeList [0] is 34 may be removed as follows. Here, it is assumed that MPM candidates added in addition to the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block are the prediction modes located closest to the prediction mode of the neighboring block.

9 and 10 described above, a predetermined fixed number of MPMs may be used, but the predetermined fixed number is not limited to four. Accordingly, the encoder and the decoder may construct an MPM list by adding N prediction modes having a small mode value as MPM candidates among prediction modes positioned between the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block.

In the above-described embodiment of FIGS. 9 and 10, when the angle difference between the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block is small, the current prediction mode is likely to be located close to the two prediction modes. In this case, even when there is no same candidate as the current prediction mode among the MPM candidates in the MPM list, the current prediction mode may have a high probability of being located close to the two prediction modes. If the same candidate mode as the current prediction mode does not exist among the MPM candidates in the MPM list, the remaining mode may be encoded and transmitted. Thus, a method of allocating a codeword to the remaining mode in consideration of the probability may be provided.

In an embodiment, as the current prediction mode is located closer to the prediction mode of the upper neighboring block and / or the prediction mode of the left neighboring block, a shorter codeword may be allocated to the remaining mode corresponding to the current prediction mode.

11 is a flowchart schematically illustrating an encoding method in an intra prediction mode according to an embodiment of the present invention.

Referring to FIG. 11, the encoder generates a reference pixel for intra prediction (S1110). In this case, the encoder may use pixels on the rightmost vertical line of the left block adjacent to the current block and pixels on the bottom horizontal line of the upper block adjacent to the current block to generate the reference pixel.

The reference pixel may be used as it is or may be used after being filtered through an adaptive intra smoothing filter (AIS). The filter coefficient used when the reference pixel is AIS filtered may be one of [1,2,1], [1,1,4,1,1], and a filter using the latter filter coefficient may be more steep ( sharp interface). The encoder may send or signal information on whether an AIS filter is used and / or information on which filter is used to the decoder.

The encoder determines an intra prediction mode for the current block (S1120). Intra prediction mode determination may be performed for each prediction unit. As an example of determining the prediction mode, an optimal prediction mode may be determined in consideration of the relationship between the required bit rate and the amount of distortion. At this time, the prediction mode determination method may vary depending on whether the RDO (Rate Distortion Optimization) is ON or not.

The intra prediction mode for the current block may be determined among a plurality of intra prediction modes in which different mode values are assigned to each. The number of intra prediction modes for the luma component according to the PU size may be as shown in Table 5 below.

[Table 5]

12 is a conceptual diagram schematically illustrating an embodiment of a prediction direction of an intra prediction mode for a luma component and a mode value assigned to each prediction direction. 12 illustrates a plurality of intra prediction modes, each intra prediction mode having a different prediction direction. In addition, the number assigned to each intra prediction mode is called a mode value.

Referring to FIG. 12, when the mode value is 0, prediction may be performed in the vertical direction using pixel values of an adjacent block, and when the mode value is 1, prediction may be performed in the horizontal direction. When the mode value is 2, it may be called a DC mode, and a prediction block may be generated by an average of pixel values in the current block. In addition, in the remaining modes, prediction may be performed using adjacent block pixel values according to the corresponding angle.

When the prediction block is generated in the DC mode, the encoder may filter the pixels of the top line and the pixels of the leftmost line in the prediction block to increase the prediction efficiency. In this case, the encoder may not filter the remaining pixels in the prediction block. The strength of the filtering may be greater as the size of the current block is smaller.

The encoder may determine not only the prediction mode of the luma component but also the prediction mode of the chroma component. The prediction mode of the chroma component may include, for example, a DM mode, an LM mode, a vertical mode, a horizontal mode, a DC mode, and the like. In the DM mode, the prediction mode value of the chroma component may be the same as the prediction mode value of the luma component. In LM mode, a prediction block for chroma components may be generated using a linear relationship between luma components and chroma components.

The encoder may encode the prediction mode information on the current block and transmit the encoded information to the decoder. The encoder may define a predetermined syntax element to transmit the intra prediction mode information to the decoder, and the defined syntax element values may be entropy encoded by the mapping table and transmitted to the decoder.

The prediction mode of the current block may have a high correlation with the prediction mode of a neighboring block adjacent to the current block. Therefore, the encoder can reduce the amount of bits transmitted to the decoder by encoding the prediction mode of the current block using the prediction mode of the neighboring block. The encoder may determine the Most Probable Mode (MPM) of the current block and may encode the prediction mode of the current block by using the MPM.

The MPM determination method may be the same as the MPM determination method described in the embodiment of FIGS. 3 to 10 described above. For example, two or more MPM candidates may be included in the MPM list, and two or more MPM candidates may be used for intra prediction mode encoding. In addition, the MPM candidates may be determined differently according to an angle difference between the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block. According to an embodiment, when the angle difference between the prediction mode of the upper neighboring block and the prediction mode of the left neighboring block is large, the encoder and the decoder are not the prediction mode close to the prediction mode of the neighboring block, but the prediction mode located in the middle of the two prediction modes. May be determined as an MPM candidate.

Referring back to FIG. 11, the encoder performs intra prediction using the intra prediction mode of the reference pixel and the current block (S1130). The encoder may generate a prediction block for the current block by performing intra prediction.

The encoder generates a bit stream transmitted to the decoder (S1140).

The encoder may generate a residual signal, that is, a residual block by differentially dividing the pixel value of the current block and the pixel value of the prediction block on a pixel basis. The encoder can transform code the residual signal by applying a transform kernel, and the size of the transform code kernel can be 2 * 2, 4 * 4, 8 * 8, 16 * 16, 32 * 32, or 64 * 64. have. In one embodiment, the transform coefficient C for the n * n block may be calculated by Equation 1 below.

[Equation 1]

C (n, n) = T (n, n) x B (n, n) x T (n, n) ^T

Where C (n, n) is a matrix of n * n transform coefficients, T (n, n) is an n * n transform kernel matrix, and B (n, n) is n * n magnitude Matrix for the residual block.

The encoder can quantize the generated transform coefficients.

Which of the residual block and the transform coefficient is transmitted may be determined through the RDO. If the prediction is good, the residual block, that is, the residual signal, may be transmitted as it is without transform coding. The encoder can compare the cost function before and after the transcoding and can select a method in which the cost is minimized. In this case, the encoder may transmit information on the type (residual signal or transform coefficient) of a signal transmitted for the current block to the decoding apparatus.

The encoder may scan the transform coefficients and entropy encode the scanned transform coefficients and the intra prediction mode of the current block. The encoded information forms a compressed bit stream and may be transmitted or stored through a network abstraction layer (NAL).

13 is a flowchart schematically illustrating a decoding method in an intra prediction mode according to an embodiment of the present invention.

Referring to FIG. 13, the decoder derives a residual block and an intra prediction mode from the received bit stream (S1310).

The decoder may entropy decode the received bit stream. The decoder may derive the intra prediction mode of the current block from a variable length coding (VLC) table. In this case, the decoder may derive the intra prediction mode of the current block by using the MPM.

The decoder may derive block type information from the VLC table. The decoder may obtain information on whether the received signal for the current block is a residual signal or a transform coefficient, and obtain a transform signal in the form of a residual signal or a one-dimensional vector with respect to the current block. If the received bit stream includes side information necessary for decoding, they may be entropy decoded together.

The decoder may inversely scan the entropy decoded residual signal or transform coefficient to generate a two-dimensional block. In this case, a residual block may be generated in the case of the residual signal, and a transform coefficient in the form of a 2D block may be generated in the case of the transform coefficient. When the transform coefficients are generated, the decoder may perform inverse quantization. The decoder may inverse transform the inverse quantized transform coefficients to generate a residual block. The inverse transformation process can be represented by the following equation (2).

&Quot; (2) "

B (n, n) = T (n, n) x C (n, n) x T (n, n) ^T

or

B (m, n) = T (m, m) x C (m, n) x T (n, n) ^T

Referring back to FIG. 13, the decoder generates a reference pixel for intra prediction (S1320).

The decoder may receive information on whether an AIS filter is used and / or information on which filter is used from the encoder and generate a reference pixel for the current block with reference to the information. At this time, as in the encoding step, the decoder generates pixels on the rightmost vertical line of the left block, which is already decoded and reconstructed adjacent to the current block, and pixels on the bottom horizontal line of the top block, which is adjacent to the current block, to generate the reference pixel. Can be used. When AIS filtering is applied at the encoder side when generating the reference pixel, AIS (Adaptive Intra Smoothing) filtering may also be performed at the decoder side. Also, the decoder may select one filter coefficient among [1, 2, 1] and [1, 1, 4, 1, 1] using the filter type information.

The decoder performs intra prediction using the intra prediction mode of the reference pixel and the current block (S1330). The decoder may generate a prediction block for the current block by performing intra prediction.

The decoder generates a reconstruction block by adding the pixel value of the prediction block and the pixel value of the residual block in units of pixels (S1340).

In the above-described embodiment, the methods are described based on a flowchart or 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 be in a different order or simultaneously from the other steps as described above. Can occur in parallel. It will also be understood by those skilled in the art that the steps depicted in the flowchart are not exclusive, that other steps may be included, or that one or more steps in the flowchart or flowchart may be eliminated without affecting the scope of the present invention.

While the invention has been described above with reference to embodiments, the embodiments described above include illustrations of various aspects. Although not all possible combinations may be described to represent the various aspects, one of ordinary skill in the art would understand that various changes or modifications to the present invention are possible without departing from the spirit of the present invention. There will be.

Claims

Generating a list of Most Probable Modes (MPMs) including MPM candidates using the upper peripheral prediction mode and the left peripheral prediction mode; And
Deriving an intra prediction mode of a current block by using the MPM list,
The upper peripheral prediction mode is a prediction mode of a neighboring block adjacent to the top of the current block, and the left peripheral prediction mode is a prediction mode of a neighboring block adjacent to the left of the current block.

The method of claim 1, further comprising generating an MPRM list including Most Probable Remaining Mode (MPRM) candidates using intra prediction modes other than the MPM candidates,
In the deriving the intra prediction mode of the current block by using the MPM list, the intra prediction mode of the current block is derived using the MPM list and the MPRM list,
The MPRM candidates are prediction modes having an angle most similar to the angle of the upper peripheral prediction mode or the angle of the left peripheral prediction mode.

3. The method of claim 2, wherein when the angle difference between the upper peripheral prediction mode and the left peripheral prediction mode is greater than or equal to a predetermined first threshold, one of the MPRM candidates includes an angle of the upper peripheral prediction mode and an angle of the left peripheral prediction mode. The image decoding method is a prediction mode having an angle corresponding to the middle of the.

The image decoding method of claim 2, wherein the MPRM list is not generated when an angle difference between the upper peripheral prediction mode and the left peripheral prediction mode is greater than or equal to a second predetermined threshold.

The image decoding method of claim 1, wherein the number of the MPM candidates included in the MPM list is determined according to an angle difference between the upper peripheral prediction mode and the left peripheral prediction mode.

The method of claim 5, wherein the number of MPM candidates included in the MPM list is one or two when the angle difference between the upper peripheral prediction mode and the left peripheral prediction mode is greater than or equal to a third threshold.

The method according to claim 5, wherein when the angle difference between the upper peripheral prediction mode and the left peripheral prediction mode is more than a predetermined fourth threshold,
One of the MPM candidates included in the MPM list is a prediction mode having a first angle corresponding to the middle of the angle of the upper peripheral prediction mode and the angle of the left peripheral prediction mode and the other is perpendicular to the first angle. The image decoding method is a prediction mode having a second angle.

The method of claim 1, wherein the number of the MPM candidates included in the MPM list is a predetermined fixed number.

The method of claim 8, wherein when the angle difference between the upper peripheral prediction mode and the left peripheral prediction mode is equal to or greater than a predetermined fifth threshold,
One of the MPM candidates that are not the same as the upper peripheral prediction mode and the left peripheral prediction mode is a prediction mode having a third angle corresponding to the middle of the angle of the upper peripheral prediction mode and the angle of the left peripheral prediction mode. Decryption method.

The method of claim 8, wherein when the angle difference between the upper peripheral prediction mode and the left peripheral prediction mode is equal to or greater than a predetermined sixth threshold value,
One of the MPM candidates that are not the same as the upper peripheral prediction mode and the left peripheral prediction mode is a prediction mode having a third angle corresponding to the middle of the angle of the upper peripheral prediction mode and the angle of the left peripheral prediction mode. One is a prediction mode having a fourth angle perpendicular to the third angle.

The method of claim 8, wherein when there is no MPM candidate that is identical to an intra prediction mode of the current block in the MPM list,
In the deriving of the intra prediction mode of the current block by using the MPM list, a remaining mode transmitted from an encoder and the intra prediction mode of the current block are derived by using the MPM list.
The closer the angle of the intra prediction mode of the current block is to the angle of the upper peripheral prediction mode or the angle of the left peripheral prediction mode, the shorter codeword is assigned to the remaining mode corresponding to the intra prediction mode of the current block. Image Decoding Method.

The method of claim 8, wherein a predetermined number of MPM candidates among the MPM candidates that are not the same as the upper peripheral prediction mode and the left peripheral prediction mode are configured to determine an angle between the angle of the upper peripheral prediction mode and the angle of the left peripheral prediction mode. The image decoding method of claim 1, wherein the assigned mode values are selected from among the prediction modes.

An intra prediction mode derivation unit configured to generate a Most Probable Mode (MPM) list including MPM candidates using an upper peripheral prediction mode and a left peripheral prediction mode, and derive an intra prediction mode of a current block using the MPM list; And
An intra prediction unit configured to generate a prediction block for the current block by performing prediction on the current block by using the derived intra prediction mode;
And the upper peripheral prediction mode is a prediction mode of a neighboring block adjacent to the top of the current block, and the left peripheral prediction mode is a prediction mode of a neighboring block adjacent to the left of the current block.

Generating a Most Probable Mode (MPM) list including MPM candidates using a top periphery prediction mode and a left periphery prediction mode; And
Deriving an intra prediction mode of a current block by using the MPM list,
And the upper peripheral prediction mode is a prediction mode of a neighboring block adjacent to the top of the current block, and the left peripheral prediction mode is a prediction mode of a neighboring block adjacent to the left of the current block.