KR20180011285A - Method of encoding intra mode by choosing most probable mode with high hit rate and apparatus for the same, and method of decoding and apparatus for the same - Google Patents

Abstract

The present invention, when encoding mode information of a block determined according to an intra prediction mode, selects at least one most probable mode (MPM) candidate group through selection conditions more definite than a conventional method and enhances encoding efficiency, wherein the MPM has the most probability for being selected as an optimal mode among intra prediction modes of neighboring blocks of the block to be currently encoded. The present invention, if one or more MPM candidate groups are selected through definite conditions, has higher probability that the intra prediction mode of the block to be currently encoded is the same as the MPM and if the probability that the intra prediction mode of the current block is the same as the MPM gets higher, shortens length of codes to enhance compression efficiency. The definite conditions mean that criteria for determining how well the neighboring blocks are predicted are presented and the prediction modes well predicted are selected first as the MPM candidate groups in order to enhance hit rate. In addition, the prediction criteria above mentioned can be applied to two, a fixated number, and patterns of the neighboring blocks to be referred to. Besides, the present invention applies the above mentioned prediction criteria to the neighboring blocks positioned in various directions because the prediction has various directionality to set the MPM candidate groups, thereby enhancing the hit rate. An image encoding apparatus detects intra prediction accuracy of the neighboring block encoded prior to a current block of a prediction unit of an input image (S610). The apparatus determines at least one MPM of the current block based on the detected intra prediction accuracy (S620). The device performs intra prediction on the current block based on the determined MPM (S630).

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for encoding an image using an MPM having a high hit ratio and a method and an apparatus for decoding an image using the MPM,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for encoding an image, and a decoding method and apparatus. More particularly, the present invention relates to a method and apparatus for encoding an image through intraprediction, and a decoding method and apparatus.

In an image compression method such as H.264 / AVC (Advanced Video Coding), a picture is divided into a plurality of macro blocks in order to encode an image, and each intra- The block can be encoded.

Intra prediction is performed by calculating a predicted value for a current block to be encoded referring to pixel values spatially adjacent to a current block to be encoded in a current picture and then calculating a difference between a predicted value and a pixel value of the current block to generate a residual image, The residual image is coded by a discrete cosine transform (DCT) and quantization. Here, there are various intraprediction modes for predicting the pixel values of the current block. The intraprediction modes include a 4x4 intra prediction mode, an 8x8 intra prediction mode, a 16x16 intra prediction mode and a chrominance intra prediction mode . In the 4x4 intra prediction mode of H.264 / AVC, it can have nine prediction modes for intra prediction.

The nine intra prediction modes have mode numbers of 0 to 8, respectively, depending on the frequency of statistical use. That is, the frequency of the 0th mode is statistically used the most, and the frequency of the 8th mode is the least.

In order to predict in various directions in the intra-prediction mode, the number of prediction modes is increased, and many bits are required to encode the prediction mode. In order to efficiently encode the intra prediction mode, in the H.264 / AVC scheme, the mode with the highest probability of selecting the optimal mode among the prediction modes of the neighboring blocks of the current block to be encoded is called the Most Probable Mode , MPM), and to improve the coding efficiency through the encoding process.

1 is a conceptual diagram showing the positions of a current block and a neighboring block.

Referring to FIG. 1, in the H.264 / AVC, information on the intra prediction direction of the current block 100 includes information of an upper block 101 adjacent to the upper portion of the current block 100 and a lower block 101 adjacent to the left side of the current block 100 And is encoded with reference to the intra prediction direction. The upper block 101 and the left block 103 are coded prior to the encoding of the current block 100 and each have information about the intra prediction direction and the decoding is performed before the current block is decoded on the receiving side It is possible to refer to the information about the intra prediction direction.

First, the intra prediction mode number of the current block 100 is determined by the rate-distortion (RD) cost. Next, the minimum value among the intra prediction mode numbers of the upper block 101 and the left block 103 is defined as MPM and compared with the intra prediction mode number of the current block 100. Since the intra prediction mode number is assigned a number from 0 to 8 according to the statistical frequency of use, the probability that the minimum intra prediction mode number of the neighboring block matches the intra prediction mode number of the current block is high.

2 is a conceptual diagram illustrating a process of intraprediction encoding an image using an MPM.

As shown in FIG. 2, the intraprediction encoding process selects an MPM using a prediction mode of a neighboring block and compares the MPM with the optimal mode determined in the current block to encode the intra prediction mode information. That is, the maximum probability mode (MPM) selection unit 210 selects the MPM based on the prediction mode of the neighboring blocks, and the current block space prediction unit 220 performs intra prediction on the current block 100, The distortion optimization unit 230 determines the optimal mode of the current block by the rate-distortion cost for the current block 100. [ The entropy encoding unit 240 encodes the intra prediction mode information based on the optimum mode of the current block and the highest probability mode (MPM) to generate a bit stream.

3 is a flowchart of an intra prediction mode encoding method using one MPM of the H.264 / AVC scheme.

As shown in FIG. 3, only one MPM is used in H.264 / AVC, and the mode number of the prediction mode of the left block and the upper block is determined to be a small mode in the MPM. The reason why the mode number is determined to be small is that the mode number is recorded in the order of higher probability of occurrence due to the statistical frequency of use, so that the smaller the mode number is, the higher the probability of occurrence is.

Referring to FIG. 3, the entropy encoding process of the intra-prediction mode using the MPM in H.264 / AVC will be described. The MPM in which the mode number of the neighboring blocks is small, (S310). When the current block optimal mode is equal to the MPM, the optimal mode is encoded to " 1 " (S330), which means it is the same as the highest probability mode. If the optimum mode of the current block differs from that of the MPM, " 0 ", which is different from MPM, is inserted in the priority bit string, and then the optimum mode is directly entropy-coded (S320). This technique can determine an MPM suitable for the structure around the current block based on the fact that the intra prediction mode of each block expresses the directionality of an edge present in the corresponding block. However, if the intra prediction mode of the current block is determined If it does not match the MPM, there will be no compression of information at all.

Meanwhile, in the next generation high efficiency video coding (HEVC) scheme, which is currently under standardization, the direction of the intra prediction mode is set to be higher than that of the H.264 / AVC scheme in which the intra prediction mode has a direction of 9 The coding efficiency of the intra prediction mode becomes more important. Accordingly, in the HEVC scheme, a method of increasing the number of MPM candidates to two fixed number instead of one to increase the probability that the intra-prediction mode of the current block matches the MPM has been proposed.

4 is a flowchart showing a method of encoding an intra prediction mode using two MPMs in the HEVC scheme.

As shown in FIG. 4, the overall process is the same as the H.264 / AVC described above, but the MPM candidates are set to two instead of one to increase the probability that the mode of the current block and the MPM are the same. The first candidate (MPM [0]) of the MPM is set to have a smaller mode number among the prediction modes of the left block and the upper block, such as H.264 / AVC, and the second candidate MPM [1] . By considering both the prediction mode of the left block and the prediction of the upper block, the number of cases in which the current block and the MPM are equal to each other is increased. The higher the probability that the two modes coincide, the shorter the length of the bit stream, and therefore the compression efficiency can be improved.

Referring to FIG. 4, the entropy encoding process of the intra-prediction mode using the MPM in the HEVC scheme will be described. The entropy encoding process of the MPM [0] (S410). If the current block optimal mode is the same as MPM [0], it is coded to "10" which means the same as MPM [0] (S420), and is transmitted to the decoder. If MPM [0] is different from the optimal mode of the current block, MPM [1], which is a mode having a larger mode number among neighboring blocks, is compared with the optimum mode determined by the rate-distortion optimization unit in the current block ) If the current block optimal mode is the same as MPM [1], it is encoded to "11" which is the same as MPM [1] (S440) and transmitted to the decoder. If the current block optimal mode is different from MPM [1], "0", which is different from MPM [1], is inserted in the priority bit string, and then the optimum mode is directly entropy coded (S450).

However, there is a drawback to setting the MPM candidates based on the mode numbers sorted based on the overall statistical characteristics. This is because the prediction mode, which occurs frequently when the occurrence frequency is on average, does not always coincide with the prediction mode of the current block. For example, if the left block is a vertical direction with a higher average frequency of occurrence and the upper block is a horizontal direction with a lower occurrence frequency, the current block is not always the vertical direction prediction mode.

Korean Patent Laid-Open No. 10-2009-0052706 ("Method for Determining Intra Mode of Macroblock in Image Coding ", published by Hanyang University Industry & University Collaboration Foundation, May 25, 2009)

In order to solve the above-mentioned problems, the present invention does not set the minimum mode number to the MPM candidate group but detects whether the prediction mode of the neighboring block is accurately predicted and sets the correctly predicted prediction mode to one or more MPM candidate groups We want to increase the probability that the MPM and the optimal prediction mode of the current block are the same. In order to select a more precisely predicted neighboring block, the size of the transform coefficient of the residual image of the neighboring block, the number of non-zero transform coefficients, the coded block pattern (CBP) or the coded block flag ).

An object of the present invention is to provide an apparatus and method for encoding an image through an MPM having a higher hit rate by considering prediction accuracy of neighboring blocks.

It is another object of the present invention to provide an apparatus and method for decoding an image through an MPM having a higher hit rate by considering prediction accuracy of a neighboring block.

According to an aspect of the present invention, there is provided a method of encoding an image, the method comprising: detecting intra prediction accuracy of a neighboring block encoded before a current block of a prediction unit of an input image; Determining an MPM (Most Probable Mode) of at least one current block based on intra prediction accuracy of the neighboring block; And intra-prediction of the current block based on the MPM.

According to another aspect of the present invention, there is provided an apparatus for encoding an image, the apparatus comprising: an accuracy detector for detecting intra prediction accuracy of a neighboring block encoded before a current block of a prediction unit of an input image; An MPM determining unit for determining an MPM (Most Probable Mode) of at least one current block based on intra prediction accuracy of the neighboring blocks; And a current block intra predictor for intra-predicting the current block based on the MPM.

According to another aspect of the present invention, there is provided a method of decoding an image, the method comprising: entropy decoding a received bitstream to inversely quantize and inverse-transform residual values to recover residual values; Detecting an intra prediction prediction accuracy of the block; Determining an MPM (Most Probable Mode) of at least one current block based on intra prediction accuracy of the neighboring block; And intra-prediction of the current block based on the MPM. Here, the step of detecting the accuracy may include detecting a number of non-zero transform coefficients among the transform coefficients of the residual image of the neighboring block, and the step of determining the MPM may include determining a number of non-zero transform coefficients, The peripheral block is determined as an MPM. The step of detecting the accuracy may include detecting an average value of the transform coefficients of the residual image of the neighboring block, and the step of determining the MPM may include determining the neighboring block as an MPM if the average value of the transform coefficients is less than or equal to a preset threshold value And then determining whether or not the result of the determination is positive. Alternatively, the step of detecting the accuracy may include generating a coded block flag (CBF) indicating whether there is a non-zero transform coefficient among the transform coefficients of the residual image of the neighboring block, and determining the MPM May determine that the neighboring block is an MPM when the CBF indicates that there is no non-zero transform coefficient. Here, the neighboring block may include a left block and an upper block of a current block, and the neighboring block may include a left block, a left upper block, an upper block, a right upper block, and a lower left block of the current block .

According to another aspect of the present invention, there is provided an apparatus for decoding an image, the apparatus comprising: an entropy decoding unit for entropy decoding a received bitstream to inversely quantize and inverse-transform residual values to recover a residual value; An accuracy detecting unit detecting an intra prediction prediction accuracy of a block; An MPM determining unit for determining an MPM (Most Probable Mode) of at least one current block based on intra prediction accuracy of the neighboring blocks; And a current block intra predictor for intra-predicting the current block based on the MPM. Here, the accuracy detecting unit detects the number of non-zero transform coefficients among the transform coefficients of the residual image of the neighboring block, and the MPM determining unit determines the neighboring block if the number of non-zero transform coefficients is equal to or less than a preset threshold value MPM. ≪ / RTI > The accuracy detector may detect an average value of the transform coefficients of the residual image of the neighboring block, and the MPM determining unit may determine the neighboring block as an MPM if the average value of the transform coefficients is equal to or less than a preset threshold value . Alternatively, the accuracy detector may generate a coded block flag (CBF) indicating whether a non-zero transform coefficient is present in the transform coefficients of the residual image of the neighboring block, and the MPM determining unit may determine whether the CBF is 0 And determines that the neighboring block is an MPM when it indicates that there is no transform coefficient. Here, the neighboring block may include a left block and an upper block of a current block, and the neighboring block may include a left block, a left upper block, an upper block, a right upper block, and a lower left block of the current block .

According to the intra prediction mode encoding method and apparatus and the decoding method and apparatus using the MPM having a high hit rate, the image can be encoded and decoded through the MPM having a higher hit ratio by considering the prediction accuracy of the neighboring blocks.

Therefore, by presenting the MPM candidates with higher hit rate through this method, the probability that the prediction mode of the current block and the MPM are the same is increased. As the probability increases, the intraprediction mode encoding using the MPM becomes more frequent. As a result, the bit rate of the entire intra prediction mode is reduced, and the overall compression efficiency can be increased.

1 is a conceptual diagram showing the positions of a current block and a neighboring block.
2 is a conceptual diagram illustrating a process of intraprediction encoding an image using an MPM.
3 is a flowchart of an intra prediction mode encoding method using one MPM of the H.264 / AVC scheme.
4 is a flowchart of an intra prediction mode coding method using two MPMs of the HEVC scheme.
5 is an exemplary diagram of 35 intra prediction modes according to the HEVC scheme.
6 is a flowchart of a method of encoding an image according to an embodiment of the present invention.
7 is an exemplary diagram of a peripheral block according to an embodiment of the present invention.
8 is an exemplary view of a peripheral block according to another embodiment of the present invention.
9 is a block diagram of an apparatus for encoding an image according to an embodiment of the present invention.
10 is a flowchart of a method of decoding an image according to an embodiment of the present invention.
11 is a block diagram of an apparatus for decoding an image according to an embodiment of the present invention.
12 is a block diagram of an apparatus for encoding an image according to an embodiment of the present invention.
13 is a block diagram of an apparatus for decoding an image according to an embodiment of the present invention.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail.

It should be understood, however, that the invention is not intended to be limited to the particular embodiments, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.

In the following description of the embodiments of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present disclosure rather unclear.

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

It is to be understood that when an element is referred to as being "connected" or "connected" to another element, it may be directly connected or connected to the other element, . On the other hand, when an element is referred to as being "directly connected" or "directly connected" to another element, it should be understood that there are no other elements in between.

The terminology used in this application is used only to describe a specific embodiment and is not intended to limit the invention. The singular expressions include plural expressions unless the context clearly dictates otherwise. In the present application, the terms "comprises" or "having" and the like are used to specify that there is a feature, a number, a step, an operation, an element, a component or a combination thereof described in the specification, But do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, or combinations thereof.

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

Unless defined otherwise, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Terms such as those defined in commonly used dictionaries should be interpreted as having a meaning consistent with the meaning in the context of the relevant art and are to be interpreted in an ideal or overly formal sense unless explicitly defined in the present application Do not.

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

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. The following embodiments are intended to illustrate specific embodiments for implementing the intra-prediction mode encoding method and the technical idea of the apparatus according to the present invention, and the technical scope of the present invention is limited or limited by the embodiments It is not.

5 is an exemplary diagram of 35 intra prediction modes according to the HEVC scheme.

As shown in FIG. 5, in the next generation video codec HEVC scheme, it is important to determine the MPM having the highest hit rate among the 35 modes while extending the direction of the intra prediction to a maximum of 35 directions. In order to solve the above technical problem, based on local statistical characteristics, based on local statistical characteristics, at least one MPM having a high probability of being the same as an optimal intra prediction mode for the current block, Thereby enhancing the coding efficiency of the prediction mode. In addition, when referencing a neighboring block, only the left block and the upper block may be referred to. In addition, when the neighboring block has various directions, since the correlation with neighboring blocks at different positions may be high, The upper block, lower left block, upper right block can also be referenced when determining the MPM.

Coding method

6 is a flowchart of a method of encoding an image according to an embodiment of the present invention. First, the criteria for setting the MPM with a high hit rate will be described. In order to increase the hit rate of the MPM, the MPM is set based on how accurately the neighboring blocks of the current block are predicted in the corresponding prediction mode. In the present invention, instead of setting the prediction mode having a high frequency of occurrence based on the overall statistical characteristic as an existing method to the MPM first, it is possible to predict more precisely by considering how well the neighboring blocks are predicted using the corresponding mode It is desirable to set the corresponding mode of the neighboring blocks to at least one MPM in order to increase the hit ratio.

6, the method of encoding an image according to an exemplary embodiment of the present invention includes detecting (S610) intra prediction accuracy of a previously encoded neighboring block of a current block of a prediction unit of an input image (S610) (S620) of determining at least one MPM (Most Probable Mode) of the current block based on the intra prediction accuracy of the block, and intra-prediction (S630) of the current block based on the MPM .

Herein, various criteria may be applied to a criterion for determining the degree of prediction of a neighboring block using the corresponding mode, that is, a step S610 of detecting intra prediction accuracy of the neighboring block.

Intra prediction is performed by calculating a prediction value for a block to be coded by referring to pixel values spatially adjacent to a current block to be coded in a current picture and then calculating a difference between a predicted value and a pixel value of the original image to generate a residual image, The residual image is coded through discrete cosine transform (DCT) and quantization.

Therefore, if the prediction accuracy is high, the value of the residual image obtained by predicting each neighboring block through the corresponding intra prediction mode and calculating the difference between the original image and the original image will be almost zero. If the value of the residual image is less than or equal to a predetermined threshold value, the prediction can be performed well, and the reference value can be a criterion for determining that the intra prediction accuracy is high.

Since the value that can be known on the decoder side is the transform coefficient generated by transforming the residual image and the prediction mode, the number of non-zero coefficients among the transform coefficients of the residual image of the neighboring block is detected, If it is determined to be equal to or less than the predetermined threshold value, it is determined that the prediction is good, and the corresponding neighboring block can be first set to the MPM.

More specifically, the step of detecting the accuracy (S610) may detect the number of non-zero transform coefficients among the transform coefficients of the residual image of the neighboring block, and the step of determining the MPM (S620) And the number of non-conversion coefficients is equal to or less than a preset threshold value, the neighboring block may be determined as an MPM.

As a second criterion, if the average value of the transform coefficients of the residual image of the neighboring block is below the threshold value, it can be a good predictor. In this case, the neighboring block can be set to the MPM first.

More specifically, the step of detecting the accuracy (S610) may include detecting an average value of the transform coefficients of the residual image of the neighboring block, and determining the MPM (S620) The neighboring block can be determined as an MPM.

As a third criterion, it can be judged simply by using a coded block pattern (CBP) or a coded block flag (CBF). CBP and CBF are flags indicating whether non-zero coefficients exist in the transform coefficients of the residual image. For example, if CBP and CBF are nonzero, they are set to 1. If all transform coefficients are 0, they can be set to 0 have. When CBP or CBF of the neighboring block is 0, CBP or CBF may be used to set the neighboring block to MPM.

More specifically, the step of detecting the accuracy (S610) may include generating a coded block flag (CBF) indicating whether a non-zero transform coefficient exists in the transform coefficients of the residual image of the neighboring block, The step of determining MPM (S620) may determine the neighboring block as an MPM when the CBF indicates that there is no non-zero transform coefficient.

You can define various patterns of the peripheral blocks to be selected by the MPM. FIG. 7 is a diagram illustrating a peripheral block according to an embodiment of the present invention, and FIG. 8 is an exemplary view of a peripheral block according to another embodiment of the present invention.

As shown in FIG. 7, MPMs having higher hit ratios can be set by applying the above-described criteria to the prediction modes of the left block 701 and the upper block 702, as in the conventional method. 8, when referring to the prediction mode of the neighboring block to determine the MPM, the block A (801), the block B (802) as well as the upper left block (E, 805) The MPM can be determined based on the intra prediction accuracy described above with reference to the prediction modes of the left lower block (C 803) and the upper right block (D 804).

That is, the neighboring block includes a left block and an upper block of the current block, and the neighboring block includes a left block, a left upper block, an upper block, a right upper block, and a left lower block of the current block . The number of MPMs is maximum 2 when the pattern of FIG. 7 is used, and up to 5 MPMs when the pattern of FIG. 8 is used can be determined according to the above-mentioned criteria.

In case of the HEVC method, intraprediction can be performed using an extended macro block size of 32x32 pixels or more in order to apply to a high resolution having a resolution of HD (High Definition) or higher. In addition, a cyclic coding unit (CU) may be used, which may have a variable size hirachy structure.

That is, since the size of a block which is a coding unit varies according to a partial characteristic of a picture, a plurality of left blocks 701 of the current block 700 may be plural. Accordingly, in the intra-mode encoding according to the embodiment of the present invention, the left block 701 may be one of blocks adjacent to the left of the current block 700, A block located at the bottom of the blocks adjacent to the left side may be regarded as a left block.

Likewise, the top block 702 of the current block 700 may also be plural. Accordingly, in the intra-mode encoding according to the embodiment of the present invention, the upper block 702 may be one of blocks adjacent to the upper part of the current block 700, A block located at the leftmost of the blocks adjacent to the upper portion may be regarded as an upper block.

In some cases, the accuracy of the intra prediction may be detected and determined for all of the plurality of left blocks or all of the upper blocks. The maximum number of MPMs may be equal to the total number of neighboring blocks.

When there are a plurality of blocks that satisfy the above-described condition among the neighboring blocks, a plurality of MPMs may exist. The MPM may include a number of indexes corresponding to the number of neighboring blocks satisfying the above-mentioned condition. When the intra prediction accuracy of the neighboring blocks has priority among the respective blocks, the step of determining the MPM (S620) may allocate the prediction direction of the neighboring blocks to the lower index among the indexes in order of the accuracy . Here, the index is used to distinguish each of the prediction modes of the neighboring blocks when there are a plurality of neighboring blocks satisfying the above condition. For example, the index is displayed as MPM [0], MPM [1] and MPM [2] can do. Here, an index displayed using a lower number such as MPM [0] may be referred to as a lower index. Thus, for example, if neighboring blocks A, B and C have high accuracy in order, the block A to MPM [0], the block B to MPM [1], the block C to MPM [ 2].

The encoding process after the selection of the MPM according to an embodiment of the present invention is as described above with reference to FIG. 3 to FIG.

That is, if the intra prediction mode number of the current block coincides with the MPM, the intra prediction direction for the current block is encoded as '1', which means that the intra prediction direction is the same as the MPM, and inserted into the bit stream.

Next, when there are several MPMs, an index indicating the intra-prediction mode of the current block matches with which MPM is encoded and transmitted as binary numbers. For example, in the case of HEVC intra coding, a flag of 1 bit, which is represented by 0 or 1 indicating the MPM index, is further encoded and transmitted when two MPMs are used. When there are four MPMs, a 2-bit flag indicating 00, 01, 10, or 11 indicating the MPM index is additionally encoded and transmitted.

Since the probability that the intra-prediction mode of the current block coincides with the MPM is high, the information about the intra-prediction direction of the current block also has a high probability of being represented by one bit, so that information is compressed accordingly. The MPM can be obtained by referring to the intra prediction mode number of the upper block and the left block on the receiving side even if the intra prediction direction of the current block is not directly transmitted. Therefore, it is possible to perform decoding in the intra prediction direction indicated by the MPM.

In contrast, if the intra prediction mode number of the current block does not match the MPM, '0', which means that the intra prediction direction for the current block is different from the MPM, is inserted in the bitstream. Next, among the 35 4x4 intra prediction modes, the remaining 34 (35-1) intra prediction modes excluding the MPM are divided into binary numbers of 5 to 6 bits. That is, the intra prediction mode is represented by 5 bits until the intra prediction mode is 32, and the intra prediction mode is represented by 6 bits after the intra prediction mode is 33.

'XXXXX' or 'XXXXXX' corresponding to the intra prediction mode number of the current block is inserted into the bit stream and is transmitted. Therefore, the information on the intra prediction direction of the current block is expressed by a total of 6 to 7 bits.

Encoding device

9 is a block diagram of an intra prediction mode encoding apparatus according to an embodiment of the present invention.

9, an intra prediction mode encoding apparatus 900 for an image according to an exemplary embodiment of the present invention includes an accuracy detecting unit 910 for detecting intraprediction accuracy of a previously encoded neighboring block of a current block, An MPM decision unit 930 for determining at least one MPM (Most Probable Mode) of the current block based on intra prediction accuracy of a neighboring block, and a current block intra prediction unit for intra-prediction of the current block based on the MPM 950).

The accuracy detector 910 detects the number of non-zero transform coefficients among the transform coefficients of the residual image of the neighboring block, and the MPM determining unit 930 determines whether the number of non-zero transform coefficients is less than or equal to a predetermined threshold And determines the peripheral block as an MPM.

In addition, the accuracy detector 910 detects the magnitude of the transform coefficient of the residual image of the neighboring block, and when the magnitude of the transform coefficient is equal to or less than a preset threshold value, As a result of the determination.

Alternatively, the accuracy detector 910 may generate a coded block flag (CBF) indicating whether a non-zero transform coefficient exists among the transform coefficients of the residual image of the neighboring block, and the MPM determining unit 930 ) Determines that the neighboring block is an MPM when the CBF indicates that there is no non-zero transform coefficient.

Here, the neighboring block may include a left block and an upper block of a current block, and the neighboring block may include a left block, a left upper block, an upper block, a right upper block, and a lower left block of the current block .

The concrete operation of the intra-prediction mode encoding apparatus 900 according to an embodiment of the present invention follows the intra-prediction mode encoding method according to an embodiment of the present invention described above.

12 is a block diagram of an apparatus for encoding an image according to an embodiment of the present invention.

12, the image encoding apparatus includes an encoder 630 and the encoder 630 includes an inter-picture predicting unit 632, an intra-prediction unit 635, a subtractor 637, a transform unit 639, An inverse quantization unit 645, an inverse transform unit 647, an adder 649, and a frame buffer 651. The input unit 641, the entropy encoding unit 643, the inverse quantization unit 645, The inter-picture predicting unit 632 includes a motion predicting unit 631 and a motion compensating unit 633.

The input image is divided into a recursive coding unit (CU), and the coding unit performs inter picture prediction in the inter picture prediction unit 632 or in-picture prediction in the intra prediction unit 635 And is divided into Prediction Unit (PU) units. The coding unit (CU) is hierarchically partitioned. A smallest CU (SCU) can have a minimum size of 8x8 pixels, and an LCU (Largest CU) can have a size of 64x64 pixels. Information related to the SCU and LCU can be signaled to the decoder as a sequence parameter set (SPS).

The inter-picture predicting unit 632 generates a motion vector by estimating a motion of the supplied predictive unit to be currently coded.

The motion predicting unit 631 searches the provided current prediction unit for an area similar to the current block from the previous N-1th reference picture and / or the (N + 1) th reference picture of the Nth current picture to be coded, .

The motion compensation unit 633 generates a prediction block (or a predicted prediction unit) obtained by performing motion compensation using the motion vector generated from the motion prediction unit 631 and the reference picture.

The intraprediction unit 635 performs intraprediction according to the intra mode from the already-coded pixel values around the current prediction unit in the current picture. The intra prediction unit 635 determines the MPM and intra-prediction of the current block based on the MPM.

The subtractor 637 subtracts the current block (or the current prediction unit) from the prediction block (or the predicted prediction unit) provided by the motion compensation unit 633 to generate a residual value and outputs the residual value to the transform unit 639 and the quantization unit 641 ) Performs DCT (Discrete Cosine Transform) conversion and quantizes the residue.

The entropy encoding unit 643 entropy-codes the quantized DCT coefficients, motion parameters, information related to the SCU and the LCU, and generates a bitstream.

The inverse quantization unit 645 and the inverse transformation unit 647 dequantize and inverse-transform the quantized data through the quantization unit 641. [ The adder 649 adds the inversely converted data and the predicted prediction unit provided by the motion compensator 633 to reconstruct the image and provides the reconstructed image to the frame buffer 651. The reconstructed image is stored in the frame buffer 651. [

Decryption method

10 is a flowchart of a method of decoding an image according to an embodiment of the present invention.

As shown in FIG. 10, in the method of decoding an image according to an embodiment of the present invention, entropy decoding is performed on a received bitstream to inversely quantize and inverse transform residual values to restore residual values, (S1020) of determining at least one current block MPM (Most Probable Mode) based on the intra prediction accuracy of the neighboring block (S1020) And intra-prediction of the block (S1030).

That is, the MPM of the current block to be decoded is determined through the same process as the above-described encoding method. Therefore, the step of detecting the accuracy (S1010) and the step of determining the MPM (S1020) can detect the accuracy and determine the MPM like the above-described encoding method.

Thereafter, since the decoding process proceeds differently depending on whether the intra-prediction direction of the current block and the MPM are different, the MPM flag for distinguishing the decoding is decoded from the bitstream.

If the intra-prediction mode number of the current block coincides with the MPM in the encoding process, the MPM flag is encoded into '1', which means that the intra-prediction direction for the current block is the same as the MPM, and inserted into the bitstream. If there is at least one MPM, an MPM index indicating the intra-prediction mode of the current block is encoded and inserted after the MPM flag. For example, if there are four MPMs, the MPM index is encoded with two bits. Conversely, if the intra prediction mode number of the current block does not match the MPM, the MPM flag is encoded as '0' and inserted into the bitstream. The intraprediction mode of the current block is encoded with a codeword of a predetermined binary number.

In the decoding process, if the MPM flag is '1', it is found that the intra-prediction mode of the current block is equal to the MPM. And decodes the MPM index. For example, when there are four MPMs, the MPM indexes are encoded with the two-bit flags 00, 01, 10, and 11. Therefore, in the decoding process, the MPM indexes are represented by decimal numbers and the MPM indexes are 0, 1, The encoded intra prediction mode can be decoded.

In the decoding process, if the MPM flag is '0', it is known that the intra-prediction mode of the current block is different from the MPM. Among the 35 4 × 4 intra prediction modes of the HEVC, the remaining 34 intraprediction modes excluding the MPM are divided into 5-bit to 6-bit binary numbers, and five bits (XXXXX ) Or 6 bits (XXXXXX) is transmitted as a bitstream. Therefore, in the decoding process, the intra prediction mode encoded with the mode number obtained by converting the information of 5 to 6 bits encoded with the binary number into the decimal number can be decoded.

When the intra prediction mode is decoded, the current block is intra-predicted based on the intra prediction mode (S1030).

Decryption device

11 is a block diagram of an intra predictor included in an apparatus for decoding an image according to an embodiment of the present invention.

11, an intra predictor 1100 included in an apparatus for decoding an image according to an exemplary embodiment of the present invention includes an accuracy detector 1110 for detecting an intra prediction accuracy of a previously decoded neighboring block of a current block, An MPM decision unit 1130 for determining an MPM (Most Probable Mode) of at least one current block based on the intra prediction accuracy of the neighboring blocks, and a current block intra prediction unit for intra- And a prediction unit 1150. [0086]

The concrete operation of the intra-prediction mode decoding apparatus according to an embodiment of the present invention is based on the image decoding method described above.

13 is a block diagram of an apparatus for decoding an image according to an embodiment of the present invention.

13, the decoding apparatus according to an embodiment of the present invention includes an entropy decoding unit 831, an inverse quantization unit 833, an inverse transform unit 835, a motion compensation unit 837, an intra prediction unit 839, A frame buffer 841, and an adder 843. [

The entropy decoding unit 831 receives the compressed bitstream and performs entropy decoding to generate quantized coefficients. The inverse quantization unit 833 and the inverse transformation unit 835 perform inverse quantization and inverse transformation on the quantized coefficients to recover the residual value.

The motion compensation unit 837 performs motion compensation using the information decoded from the bit stream by the entropy decoding unit 831 to generate a predicted prediction unit.

The intraprediction unit 839 determines the MPM of the current block and selects the intra mode based on the MPM as described above to select the current prediction unit from the already coded pixel values around the current prediction unit in the current picture according to the intra mode And performs intra prediction.

The addition unit 843 reconstructs the image by adding the residual value provided by the inverse transform unit 835 and the predicted prediction unit provided by the motion compensation unit 837 and provides the restored image to the frame buffer 841. The frame buffer 841 And stores the reconstructed image. That is, the decoder performs a decoding operation by adding a compressed prediction error (residual value provided by the inverse transform unit) to the prediction unit.

100: current block
101: upper block
103: Left block
900: Intra prediction unit
910: Accuracy detector
930: MPM decision unit
950: current block intra prediction unit
1100: Intra prediction unit
1110: Accuracy detector
1130: MPM decision unit
1150: current block intra prediction unit

Claims (6)

In the image decoding method,
Obtaining a residual value corresponding to a current block from a bitstream;
Determining a MPM (Most Probable Mode) candidate of the current block; Here, the MPM candidate group includes a plurality of MPM candidates,
Determining an intra prediction mode of the current block based on the determined MPM candidate group;
Performing intra prediction on the basis of the intra prediction mode of the current block to obtain a predicted value of the current block; And
And restoring the current block using the obtained residual value and the predicted value,
Wherein the number of MPM candidates belonging to the MPM candidate group is three or more. The method according to claim 1,
Further comprising decrypting the MPM flag from the bitstream,
Wherein the intra prediction mode of the current block is determined based on the MPM flag and the MPM candidate group. 3. The method of claim 2,
When the intra prediction mode of the current block is identical to any one of the plurality of MPM candidates,
And if the intra prediction mode of the current block is not the same as the plurality of MPM candidates, The method of claim 3,
And decoding residual mode information from the bitstream if the value of the decoded MPM flag is zero. 5. The method of claim 4,
Wherein the residual mode information is used to determine an intra prediction mode of the current block. 6. The method of claim 5,
Wherein the residual mode information is a mode encoded using a residual intra prediction mode excluding the plurality of MPM candidates.

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