Coset coding of intra prediction mode

Abstract

In one implementation, we propose to improve the intra prediction mode coding using coset decomposition. Of the 67 intra prediction modes, 64 prediction modes are selected for coset decomposition leaving the remaining three in a reduced MPM (Most Probable Mode) list. The 64 modes are decomposed into cosets, where the number of cosets and the number of modes in a coset are decided by the size of the target block and / or the statistics of the intra prediction modes of neighboring blocks. The intra prediction modes in a coset are arranged in decreasing order of likelihood. The intra prediction mode of a target block is encoded as the coset index and the member index in the coset. The coset leader, that is, the first mode in the coset, is encoded with bit 0 whereas the remaining modes are encoded with a fixed or a variable length code.

Description

COSET CODING OF INTRA PREDICTION MODETECHNICAL FIELD[1] The present embodiments generally relate to a method and an apparatus for intra prediction in video encoding and decoding.BACKGROUND[2] To achieve high compression efficiency, image and video coding schemes usually employ prediction and transform to leverage spatial and temporal redundancy in the video content. Generally, intra or inter prediction is used to exploit the intra or inter picture correlation, then the differences between the original block and the predicted block, often denoted as prediction errors or prediction residuals, are transformed, quantized, and entropy coded. To reconstruct the video, the compressed data are decoded by inverse processes corresponding to the entropy coding, quantization, transform, and prediction.SUMMARY[3] According to an embodiment, a method of video decoding is presented, comprising: obtaining a set of intra prediction mode candidates for a block of a picture; decomposing remaining intra prediction modes into a plurality of cosets of intra prediction modes, said remaining intra prediction modes excluding said set of intra prediction mode candidates; decoding a coset index; decoding an intra prediction mode for said block from a coset associated with said decoded coset index; and decoding said block based on said intra prediction mode.[4] According to another embodiment, an apparatus of video decoding, comprising at least one memory and one or more processors, wherein said one or more processors are configured to: obtain a set of intra prediction mode candidates for a block of a picture; decompose remaining intra prediction modes into a plurality of cosets of intra prediction modes, said remaining intra prediction modes excluding said set of intra prediction mode candidates; decode a coset index; decode an intra prediction mode for said block from a coset associated with said decoded coset index; and decode said block based on said intra prediction mode.[5] In one embodiment, said intra prediction mode for said block is decoded from a coset associated with said decoded coset index responsive to decoding a member index of said intra prediction mode in said coset. i[6] In one embodiment, said decoding an intra prediction mode for said block from a coset associated with said decoded coset index comprises determining the first mode in said coset as said intra prediction mode.[7] According to an embodiment, a method of video encoding is presented, comprising: obtaining an intra prediction mode for a block of a picture; obtaining a set of intra prediction mode candidates for said block of said picture; decomposing remaining intra prediction modes into a plurality of cosets of intra prediction modes, said remaining intra prediction modes excluding said set of intra prediction mode candidates; determining a coset to which an intra prediction mode of said block belongs; encoding a coset index associated with said coset; and encoding said block based on said intra prediction mode.[8] According to another embodiment, an apparatus of video encoding is presented, comprising at least one memory and one or more processors, wherein said one or more processors are configure to: obtain an intra prediction mode for a block of a picture; obtain a set of intra prediction mode candidates for said block of said picture; decompose remaining intra prediction modes into a plurality of cosets of intra prediction modes, said remaining intra prediction modes excluding said set of intra prediction mode candidates; determine a coset to which an intra prediction mode of said block belongs; encode a coset index associated with said coset; and encode said block based on said intra prediction mode.[9] In one embodiment, a member index of said intra prediction mode in said coset is encoded.

[0010] In one embodiment, said obtaining an intra prediction mode for said block comprises checking the first intra prediction mode in each of said plurality of cosets; and selecting said intra prediction mode for said block from said first intra prediction modes.

[0011] In one embodiment, said set of intra prediction mode candidates corresponds to a MPM (Most Probable Mode) list.

[0012] In one embodiment, the number of said plurality of cosets is adaptive to a block size of said block. For example, said number of said plurality of cosets is larger for a larger block size.

[0013] In one embodiment, the number of said plurality of cosets is based on prediction modes of neighboring blocks of said block. For example, said number of said plurality of cosets is smaller when a prediction mode for a left neighboring block and a prediction mode for an above mode neighboring block are angular and same.

[0014] In one embodiment, said obtaining a set of intra prediction mode candidates for a block of a picture comprises obtaining a left mode indicating an intra prediction mode of a left neighboring block and a top mode indicating an intra prediction mode of an above neighboring block; determining that only one of said left mode and said top mode is angular; including a DC mode in said set of intra prediction mode candidates for said block; and including said left mode or top mode which is angular in said set of intra prediction mode candidates for said block.

[0015] In one embodiment, said obtaining a set of intra prediction mode candidates for a block of a picture comprises obtaining a left mode indicating an intra prediction mode of a left neighboring block and a top mode indicating an intra prediction mode of an above neighboring block; determining that said left mode and said top mode are angular and different; and including said left mode and said top mode in said set of intra prediction mode candidates for said block.

[0016] In one embodiment, said set of intra prediction mode candidates includes three intra prediction modes.

[0017] In one embodiment, intra prediction modes in a coset are ordered. For example, said intra prediction modes in a coset are ordered based on one of: (1) a minimum distance of an intra prediction mode from one or more angular modes in said set of intra prediction mode candidates, (2) a difference score between a prediction using an intra prediction mode and a template, and (3) a distance of an intra prediction mode from a DIMD mode or the first TIMD mode.

[0018] One or more embodiments also provide a computer program comprising instructions which when executed by one or more processors cause the one or more processors to perform the encoding method or decoding method according to any of the embodiments described herein. One or more of the present embodiments also provide a computer readable storage medium having stored thereon instructions for video encoding or decoding according to the methods described herein.

[0019] One or more embodiments also provide a computer readable storage medium having stored thereon video data generated according to the methods described above. One or more embodiments also provide a method and apparatus for transmitting or receiving the video data generated according to the methods described herein.BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 illustrates a block diagram of a system within which aspects of the present embodiments may be implemented.

[0021] FIG. 2 illustrates a block diagram of an embodiment of a video encoder.

[0022] FIG. 3 illustrates a block diagram of an embodiment of a video decoder.

[0023] FIG. 4 illustrates the intra prediction modes in VVC.

[0024] FIG. 5 illustrates a method of constructing a reduced MPM list consisting of three intra prediction modes, according to an embodiment.

[0025] FIG. 6 illustrates a method to determine the number of cosets based on the statistics of the intra prediction modes of the top and the left neighbor blocks, according to an embodiment.

[0026] FIG. 7 illustrates a method to determine the number of cosets based on the statistics of the intra prediction modes of the top and the left neighbor blocks and the block size constraint, according to an embodiment.

[0027] FIG. 8 illustrates the encoding of an intra prediction mode, according to an embodiment.

[0028] FIG. 9 illustrates a method of decoding intra prediction modes with coset decomposition, according to an embodiment.DETAILED DESCRIPTION

[0029] FIG. 1 illustrates a block diagram of an example of a system in which various aspects and embodiments can be implemented. System 100 may be embodied as a device including the various components described below and is configured to perform one or more of the aspects described in this application. Examples of such devices, include, but are not limited to, various electronic devices such as personal computers, laptop computers, smartphones, tablet computers, digital multimedia set top boxes, digital television receivers, personal video recording systems, connected home appliances, and servers. Elements of system 100, singly or in combination, may be embodied in a single integrated circuit, multiple ICs, and / or discrete components. For example, in at least one embodiment, the processing and encoder / decoder elements of system 100 are distributed across multiple ICs and / or discrete components. In various embodiments, the system 100 is communicatively coupled to other systems, or to other electronic devices, via, for example, a communications bus or through dedicated input and / or output ports. In various embodiments, the system 100 is configured to implement one or more of the aspects described in this application.

[0030] The system 100 includes at least one processor 110 configured to execute instructions loaded therein for implementing, for example, the various aspects described in this application. Processor 110 may include embedded memory, input output interface, and various other circuitries as known in the art. The system 100 includes at least one memory 120 (e.g., a volatile memory device, and / or a non-volatile memory device). System 100 includes a storage device 140, which may include non-volatile memory and / or volatile memory, including, but not limited to, EEPROM, ROM, PROM, RAM, DRAM, SRAM, flash, magnetic disk drive, and / or optical disk drive. The storage device 140 may include an internal storage device, an attached storage device, and / or a network accessible storage device, as non-limiting examples.

[0031] System 100 includes an encoder / decoder module 130 configured, for example, to process data to provide an encoded video or decoded video, and the encoder / decoder module 130 may include its own processor and memory. The encoder / decoder module 130 represents module(s) that may be included in a device to perform the encoding and / or decoding functions. As is known, a device may include one or both of the encoding and decoding modules. Additionally, encoder / decoder module 130 may be implemented as a separate element of system 100 or may be incorporated within processor 110 as a combination of hardware and software as known to those skilled in the art.

[0032] Program code to be loaded onto processor 110 or encoder / decoder 130 to perform the various aspects described in this application may be stored in storage device 140 and subsequently loaded onto memory 120 for execution by processor 110. In accordance with various embodiments, one or more of processor 110, memory 120, storage device 140, and encoder / decoder module 130 may store one or more of various items during the performance of the processes described in this application. Such stored items may include, but are not limited to, the input video, the decoded video or portions of the decoded video, the bitstream, matrices, variables, and intermediate or final results from the processing of equations, formulas, operations, and operational logic.

[0033] In several embodiments, memory inside of the processor 110 and / or the encoder / decoder module 130 is used to store instructions and to provide working memory for processing that is needed during encoding or decoding. In other embodiments, however, a memory external to the processing device (for example, the processing device may be either the processor 110 or the encoder / decoder module 130) is used for one or more of these functions. The external memory may be the memory 120 and / or the storage device 140, forexample, a dynamic volatile memory and / or a non-volatile flash memory. In several embodiments, an external non-volatile flash memory is used to store the operating system of a television. In at least one embodiment, a fast external dynamic volatile memory such as a RAM is used as working memory for video coding and decoding operations, such as for MPEG-2, HEVC, or VVC.

[0034] The input to the elements of system 100 may be provided through various input devices as indicated in block 105. Such input devices include, but are not limited to, (i) an RF portion that receives an RF signal transmitted, for example, over the air by a broadcaster, (ii) a Composite input terminal, (iii) a USB input terminal, and / or (iv) an HDMI input terminal.

[0035] In various embodiments, the input devices of block 105 have associated respective input processing elements as known in the art. For example, the RF portion may be associated with elements suitable for (i) selecting a desired frequency (also referred to as selecting a signal, or band-limiting a signal to a band of frequencies), (ii) down converting the selected signal, (iii) band-limiting again to a narrower band of frequencies to select (for example) a signal frequency band which may be referred to as a channel in certain embodiments, (iv) demodulating the down converted and band-limited signal, (v) performing error correction, and (vi) demultiplexing to select the desired stream of data packets. The RF portion of various embodiments includes one or more elements to perform these functions, for example, frequency selectors, signal selectors, band-limiters, channel selectors, filters, downconverters, demodulators, error correctors, and demultiplexers. The RF portion may include a tuner that performs various of these functions, including, for example, down converting the received signal to a lower frequency (for example, an intermediate frequency or a near-baseband frequency) or to baseband. In one set-top box embodiment, the RF portion and its associated input processing element receives an RF signal transmitted over a wired (for example, cable) medium, and performs frequency selection by filtering, down converting, and filtering again to a desired frequency band. Various embodiments rearrange the order of the above-described (and other) elements, remove some of these elements, and / or add other elements performing similar or different functions. Adding elements may include inserting elements in between existing elements, for example, inserting amplifiers and an analog-to-digital converter. In various embodiments, the RF portion includes an antenna.

[0036] Additionally, the USB and / or HDMI terminals may include respective interface processors for connecting system 100 to other electronic devices across USB and / or HDMIconnections. It is to be understood that various aspects of input processing, for example, Reed- Solomon error correction, may be implemented, for example, within a separate input processing IC or within processor 110 as necessary. Similarly, aspects of USB or HDMI interface processing may be implemented within separate interface ICs or within processor 110 as necessary. The demodulated, error corrected, and demultiplexed stream is provided to various processing elements, including, for example, processor 110, and encoder / decoder 130 operating in combination with the memory and storage elements to process the datastream as necessary for presentation on an output device.

[0037] Various elements of system 100 may be provided within an integrated housing, Within the integrated housing, the various elements may be interconnected and transmit data therebetween using suitable connection arrangement 115, for example, an internal bus as known in the art, including the I2C bus, wiring, and printed circuit boards.

[0038] The system 100 includes communication interface 150 that enables communication with other devices via communication channel 190. The communication interface 150 may include, but is not limited to, a transceiver configured to transmit and to receive data over communication channel 190. The communication interface 150 may include, but is not limited to, a modem or network card and the communication channel 190 may be implemented, for example, within a wired and / or a wireless medium.

[0039] Data is streamed to the system 100, in various embodiments, using a Wi-Fi network such as IEEE 802. 11. The Wi-Fi signal of these embodiments is received over the communications channel 190 and the communications interface 150 which are adapted for WiFi communications. The communications channel 190 of these embodiments is typically connected to an access point or router that provides access to outside networks including the Internet for allowing streaming applications and other over-the-top communications. Other embodiments provide streamed data to the system 100 using a set-top box that delivers the data over the HDMI connection of the input block 105. Still other embodiments provide streamed data to the system 100 using the RF connection of the input block 105.

[0040] The system 100 may provide an output signal to various output devices, including a display 165, speakers 175, and other peripheral devices 185. The other peripheral devices 185 include, in various examples of embodiments, one or more of a stand-alone DVR, a disk player, a stereo system, a lighting system, and other devices that provide a function based on the output of the system 100. In various embodiments, control signals are communicated between thesystem 100 and the display 165, speakers 175, or other peripheral devices 185 using signaling such as AV. Link, CEC, or other communications protocols that enable device-to-device control with or without user intervention. The output devices may be communicatively coupled to system 100 via dedicated connections through respective interfaces 160, 170, and 180. Alternatively, the output devices may be connected to system 100 using the communications channel 190 via the communications interface 150. The display 165 and speakers 175 may be integrated in a single unit with the other components of system 100 in an electronic device, for example, a television. In various embodiments, the display interface 160 includes a display driver, for example, a timing controller (T Con) chip.

[0041] The display 165 and speaker 175 may alternatively be separate from one or more of the other components, for example, if the RF portion of input 105 is part of a separate set-top box. In various embodiments in which the display 165 and speakers 175 are external components, the output signal may be provided via dedicated output connections, including, for example, HDMI ports, USB ports, or COMP outputs.

[0042] FIG. 2 illustrates an example video encoder 200, such as a VVC (Versatile Video Coding) encoder. FIG. 2 may also illustrate an encoder in which improvements are made to the VVC standard or an encoder employing technologies similar to VVC.

[0043] In the present application, the terms “reconstructed” and “decoded” may be used interchangeably, the terms “encoded” or “coded” may be used interchangeably, and the terms “image,” “picture” and “frame” may be used interchangeably. Usually, but not necessarily, the term “reconstructed” is used at the encoder side while “decoded” is used at the decoder side.

[0044] Before being encoded, the video sequence may go through pre-encoding processing (201), for example, applying a color transform to the input color picture (e.g., conversion from RGB 4:4:4 to YCbCr 4:2:0), or performing a remapping of the input picture components in order to get a signal distribution more resilient to compression (for instance using a histogram equalization of one of the color components). Metadata can be associated with the preprocessing, and attached to the bitstream.

[0045] In the encoder 200, a picture is encoded by the encoder elements as described below. The picture to be encoded is partitioned (202) and processed in units of, for example, CUs (Coding Units). Each unit is encoded using, for example, either an intra or inter mode. When a unit is encoded in an intra mode, it performs intra prediction (260). In an inter mode, motion estimation (275) and compensation (270) are performed. The encoder decides (205) which oneof the intra mode or inter mode to use for encoding the unit, and indicates the intra / inter decision by, for example, a prediction mode flag. Prediction residuals are calculated, for example, by subtracting (210) the predicted block from the original image block.

[0046] The prediction residuals are then transformed (225) and quantized (230). The quantized transform coefficients, as well as motion vectors and other syntax elements such as the picture partitioning information, are entropy coded (245) to output a bitstream. As a non-limiting example, context-based adaptive binary arithmetic coding (CABAC) can be used to encode syntax elements into the bitstream.

[0047] The encoder can skip the transform and apply quantization directly to the nontransformed residual signal. The encoder can bypass both transform and quantization, i.e., the residual is coded directly without the application of the transform or quantization processes.

[0048] The encoder decodes an encoded block to provide a reference for further predictions. The quantized transform coefficients are de-quantized (240) and inverse transformed (250) to decode prediction residuals. Combining (255) the decoded prediction residuals and the predicted block, an image block is reconstructed. In-loop filters (265) are applied to the reconstructed picture to perform, for example, deblocking / SAO (Sample Adaptive Offset) / ALF (Adaptive Loop Filter) filtering to reduce encoding artifacts. The filtered image is stored in a reference picture buffer (280).

[0049] FIG. 3 illustrates a block diagram of an example video decoder 300. In the decoder 300, a bitstream is decoded by the decoder elements as described below. Video decoder 300 generally performs a decoding pass reciprocal to the encoding pass as described in FIG. 2. The encoder 200 also generally performs video decoding as part of encoding video data.

[0050] In particular, the input of the decoder includes a video bitstream, which can be generated by video encoder 200. The bitstream is first entropy decoded (330) to obtain transform coefficients, prediction modes, motion vectors, and other coded information. The picture partition information indicates how the picture is partitioned. The decoder may therefore divide (335) the picture according to the decoded picture partitioning information. The transform coefficients are de-quantized (340) and inverse transformed (350) to decode the prediction residuals. Combining (355) the decoded prediction residuals and the predicted block, an image block is reconstructed. The predicted block can be obtained (370) from intra prediction (360) or motion-compensated prediction (i.e., inter prediction) (375). In-loop filters (365) are applied to the reconstructed image. The filtered image is stored at a reference picturebuffer (380). Note that, for a given picture, the contents of the reference picture buffer 380 on the decoder 300 side is identical to the contents of the reference picture buffer 280 on the encoder 200 side for the same picture.

[0051] The decoded picture can further go through post-decoding processing (385), for example, an inverse color transform (e.g., conversion from YCbCr 4:2:0 to RGB 4:4:4) or an inverse remapping performing the inverse of the remapping process performed in the preencoding processing (201). The post-decoding processing can use metadata derived in the preencoding processing and signaled in the bitstream.

[0052] Intra prediction mode coding in VVC

[0053] There exist different intra prediction modes in VVC. Planar and DC prediction modes are used to predict smooth and gradually changing regions, whereas 65 angular (angle defined from 45 degrees to -135 degrees in clockwise direction) prediction modes are used to capture different directional structures, as shown in FIG. 4. For rectangular blocks there are also some wide angular modes beyond this angle range depending on the shape of a target block. The wide angular modes replace equal number of regular angular modes in opposite directions, and the total number of angular modes still remains 65 for all blocks. For the encoding of the prediction mode, however, only the indices of the 65 regular angular modes are used.

[0054] Since there are multiple intra prediction modes available, the decoder needs the mode information to form the prediction for an intra-coded CU. In VVC, the intra mode may be predicted from a list of six Most Probable Modes (MPMs). In particular, the MPM list is constructed based on intra modes of the reconstructed left and the reconstructed above neighboring blocks (if coded in intra), mode indices at offsets of ±1 and ±2 from these two modes, and on some default intra modes. If the mode is one among the MPMs, its index is encoded with CABAC, otherwise the mode is one of the 61 non-MPM modes and a Truncated Binary Code (TBC) is used.

[0055] Intra prediction mode coding in ECM

[0056] In ECM, the set of 67 core intra prediction modes is inherited from VVC. ECM-7.0 (Enhanced Compression Model 7.0) uses a first (primary) MPM list (with 6 MPMs) and a secondary MPM list (with 16 MPMs). The first MPM list is constructed by sequentially adding candidate intra prediction mode indices based on the intra prediction mode indices used next to the current luminance coding block, with the first MPM index being reserved for the Planar mode. The neighboring indices added are the left neighbor, the above one, the bottom-left, ioabove right and above left. The secondary MPM list is constructed by first adding the indices of the first and second DIMD (Decoder-side Intra Mode Derivation) modes of the current luminance coding block, then adding incremented and decremented indices of the first angular MPMs (mpm[l]+l, mpm[l]-l, mpm[l]+2, mpm[l]-2, mpm[l]+3, mpm[l]-3, mpm[l]+4, mpm[l]-4, mpm[2]+l, mpm[2]-l, mpm[2]+2, mpm[2]-2, mpm[2]+3, mpm[2]-3, mpm[2]+4, mpm[2]-4, etc.) in such a way that no redundant mode index is present in the MPM list, neither in the primary MPM list nor in the secondary list. If the selected intra prediction mode does not belong to the first and secondary MPM lists, then the remaining Intra Prediction Modes (IPMs) are coded using a truncated binary encoding for 45 symbols.

[0057] In VVC and ECM, encoding of a frame of video sequence is based on a quad-tree (QT) / binary tree (BT) / triple tree (TT) block structure. A frame is divided into non-overlapping square coding tree units (CTUs) which all undergo QT / BT / TT based splitting to multiple coding units (CUs) based on rate-distortion criteria. In Intra prediction, a CU is spatially predicted from the causal neighbor CUs, i.e., the decoded CUs on the top and the left, using different prediction modes. Out of the 95 defined modes, one is a planar mode (indexed as mode 0), one is a DC mode (indexed as mode 1) and the remaining 93 (indexed as mode - 14, ... , -1, 2, ... , 80) are angular modes. Out of the 93 angular modes, only 65 adjacent modes are selected for any target CU depending on its shape. The encoder checks with the available prediction modes along with other prediction methods such as Decoder Side Intra Mode Derivation (DIMD), Template Based Intra Mode Derivation and Fusion (TIMD), Matrix Intra Prediction (MIP), Spatial Geometric Partition Mode (SGPM), etc., and selects the best in the rate-distortion sense. The selected intra prediction mode is encoded as one or several syntax elements so that the decoder obtains the same prediction as the encoder.

[0058] For the encoding of intra prediction mode, both VVC and ECM employ a method based on most probable modes (MPM) list, which is a list of modes constructed using the prediction modes of the neighbor blocks, modes with fixed offsets from the neighbor blocks’ modes, and some fixed modes such as the PLANAR mode, the DC mode, the strictly vertical mode and the strictly horizontal mode. In VVC, for encoding of the intra prediction mode of a target block, one MPM list is constructed with six candidate modes. The MPMflag denotes if the intra prediction mode of a target block belongs to the MPM list. If the MPMflag has value 1, then the index of the candidate mode in the MPM list is encoded using a variable length code, otherwise the candidate mode in a list of remaining modes is truncated binary encoded. In ECM, in addition to the primary MPM list, there is a secondary MPM list containing 16prediction modes. If the primary MPM list flag (pMPMflag) is false, then the secondary MPM list is checked for the intra prediction mode. A secondary MPM flag (sMPMflag) indicates if the intra prediction mode belongs to the secondary list. If the flag is true, the index of the candidate mode in the secondary MPM list is binary encoded, otherwise the candidate mode index in a list of remaining modes is truncated binary encoded.

[0059] The concept of most probable modes is based on the idea that the prediction mode of a target block is very likely equal or close to one of the neighbor modes. However, the construction method of the MPM lists in ECM is very complex. If the intra prediction mode of a target block is not correlated with that of any of its neighbor blocks, which can happen at the boundary of objects, or in regions having abrupt luminance or directionality changes, then the prediction mode would belong to the list of remaining modes resulting in inefficient mode coding. In this context, we propose a new encoding procedure based on coset decomposition.

[0060] Coset decomposition of prediction modes

[0061] First we construct a reduced MPM list, for example, consisting of the first three candidate modes in the MPM list of VVC or the primary MPM list of ECM, in the same order. The remaining 64 modes are used in coset decomposition. As the planar mode (indexed as mode 0) is the first mode in the MPM list, the reduced MPM list will also contain the planar mode as the first candidate.

[0062] Alternatively, a reduced MPM list consisting of three intra prediction modes can be constructed as illustrated in FIG. 5. Let mpm O, mpm l, mpm_2 denote the three modes in the reduced MPM list. Let leftMode and topMode denote the prediction modes of the left and top neighbor blocks, respectively, and PLANAR IDX, DC IDX, VER IDX denote the PLANAR, DC and strictly vertical mode, respectively. mpm_0 = PLANAR IDX (mode 0) (510)If both leftMode and topMode are non-angular (520, that is, each of them is either PLANAR or DC) mpm l = DC IDX (mode 1) (530) mpm_2 = VER IDX (mode 50) (530)Else, if one of the two modes is angular and the other is non-angular (540), then mpm l = leftMode or topMode whichever is angular (550)mpm_2 = DC IDX (550)Else, if leftMode and topMode are angular modes and if they are unequal (560), then, if W > H (570) mpm l = leftMode (580) mpm_2 = topMode (580) else mpm l = topMode (585) mpm_2 = leftMode (585) where W and H denote the width and height of a target block, respectively.Else, (leftMode and topMode are angular modes and they are equal) mpm l = leftMode (590)„ fleftMode + 1 ifleftMode < maxMode (590) mpm_2 = 1 t minMode ifleftMode = maxMode where minMode and maxMode denote the minimum and maximum angular mode indices allowed, respectively. Considering the 65 angular modes in VVC or ECM, minMode is 2 and maxMode is 66. Here, mpm_2 represents the mode next to the leftMode in the clockwise direction.

[0063] The set of 64 intra prediction modes is split into K cosets where K can be, for example, 4, 8, or 16. In one embodiment, the number of cosets, K. depends on the target block size. Each coset thus contains (64 / X) modes. The 64 modes are first re-indexed as 0, 1, ... , 63 in increasing order of their mode index m, where m denotes the actual mode index number (e.g., as shown in FIG. 4). In the first step, the K cosets are constructed. The i th coset, denoted as , contains the following modes:

[0064] Table 1 illustrates the cosets when K = 8.Table 1

[0065] It is easy to prove that Cois a subgroup of the parent group consisting of 64 modes under addition modulo 64 operation, and , i = 1, 2, ... , K — 1 are cosets of the subgroup Co.

[0066] In the second step, the modes in the cosets are ordered, for example, according to a measure where the measure could be the minimum distance of the mode from the angular modes in the MPM list. In this case the modes are arranged in the increasing order of minimum distances. Another measure could be the SAD or SATD score between the prediction using the candidate mode and the reconstruction of a template and the template, where the template could be appropriately chosen on the top and the left side of the target block. In this case the modes are arranged in the increasing order of SAD or SATD score. Yet another measure could be the distance of a mode from the DIMD mode or the first TIMD mode.

[0067] If Dt j denotes the measure of mode j in coset , then, after ordering of modes in the coset,

[0068] Thus, a coset leader, that is, the first element in a coset, has the minimum measure. Each coset is assigned a score which is the score of its coset leader. Thus if DLrepresents the score of the coset Q, then Dt= Di 0.

[0069] There can be different ways to decide the number of cosets, that is, the parameter K. One way is to decide it depending on the target block size. Generally, we consider that the larger the block is, it is more likely to have a higher number of different prediction modes. An example construction is as follows:K = 16, if W > 32 and H > 32;Else K = 8, if W > 8 and H > 8;Else K = 4, where W and H denote the width and the height of the target block, respectively.

[0070] Another example construction is the following:K = 1 << (1 + scale) where scale = (log2(W) + log2 H)) » 2.

[0071] For this example, the K values for different block sizes are given in Table 2, where the maximum block size is 128x128 and the minimum block size is 4x4.Table 2

[0072] Another way to determine the number of cosets can be based on the statistics of the prediction modes of the top and the left neighbor blocks, as illustrated in FIG. 6. For example, if the prediction mode of the left and top neighbor blocks are both angular and equal (610), then the number of cosets can be set to 4 (620); otherwise if they are angular and unequal (630), then the number of cosets can be set to 8 (640) since there are a high number of possible different prediction modes when the neighboring modes are different. In other cases (that is, when at least one neighbor block prediction mode is non-angular), the number of cosets can be set to 4 or 8 (650).

[0073] This method can also be combined with the block size constraint, as illustrated in FIG. 7. For example, if the prediction mode of the left and top neighbor blocks are both angular and equal (710), then the number of cosets can be set to 4 (730) or 8 (735) depending on if the block width is smaller than 16 and block height is smaller than 16 (720), or otherwise. Otherwise if they are angular and unequal (740), then the number of cosets can be set to 8 (760) or 16 (765) depending on if the width is smaller than 16 and height is smaller than 16 (750), or otherwise. In other cases, the number of cosets can be set to 4 (780) or 8 (785) depending on the block size (770). Once the number of cosets is determined, the cosets can be constructed as Eq. (1).

[0074] Coset encoding of prediction modes

[0075] The encoding of a prediction mode is done as illustrated in FIG. 8, according to an embodiment. The encoder first checks the reduced MPM list. If the intra prediction mode belongs to the list (810), then the MPMflag is encoded (820) as 1 and the index of the mode in the list is encoded (830) using a unary code for three candidates, which can be 0, 10, and 11. If the prediction mode does not belong to the reduced MPM list, then the MPMflag is encoded (840) as 0. The coset containing the intra prediction mode is identified (850). The mode is encoded as a pair consisting of the coset index (860) and the index of the intra prediction mode inside the coset (870). The cosets constructed above have candidate modes uniformly spaced apart, and the members in adjacent cosets (i.e., cosets with adjacent indices) have adjacent modes. Therefore the cosets are likely to have similar probabilities and the coset members in each coset can have varying probabilities. Based on this intuition, we propose to encode the coset index with a fixed length code and the member index in the coset using a variable length code.

[0076] To encode one of K cosets using a fixed-length code, where K is 4, 8 or 16, n = log2(K bits are needed. To encode (64 / / C) ( = 64 » n) modes in one coset, we can use a unary code, or a combination of fixed-length code and a unary code. In the later method, we can encode the first L, where L is a power of 2 but less than or equal to (32 / / C), with a unary code and the remaining modes in the coset with a truncated binary code. For example, with 4 cosets K = 4), each coset contains 16 candidate modes. The first 4 modes in a coset can be encoded with a unary code and the remaining 12 modes can be encoded with a truncated binary code.

[0077] In a variation of the above encoding method, the MPM list used in VV C, or the primary MPM list used in ECM, is used instead of a reduced MPM list. In this case, some of the cosets will contain a mode that belongs to the MPM list. This mode is removed from encoding. The coset encoding method as explained above remains the same. The encoding of the candidate index will be slightly changed based on the number of candidates in a coset.

[0078] In the above proposal, we have used 67 intra prediction modes as used in VVC and ECM. But the same idea can be applied in the case of other standards such as HEVC where 35 prediction modes are used. In that case the coset construction can use a total of 32 modes along with the HEVC MPM list, which consists of three prediction modes. The number of cosets can be restricted to 4 or 8 depending on the block size. The idea also can be applied if the number of prediction modes is increased, for example, to 131, beyond what is now used inVVC and ECM.

[0079] We assume a video codec that includes intra prediction with block based CU partitions and various angular and non-angular prediction modes, such as one based on HEVC, VVC, or ECM standard, etc. The encoding of the intra prediction mode of a Luma block is done using the proposed coset coding method with a reduced MPM list of 3 prediction modes or the MPM list used in VVC or ECM that contains 6 prediction modes. Various embodiments are possible based on the method of determining the number of cosets and the encoding of the prediction mode using the cosets.

[0080] The decoder performs the coset decoding by having the same coset decomposition of modes along with the reduced MPM list. FIG. 9 illustrates a method of decoding intra prediction modes with coset decomposition, according to an embodiment. As the encoder, the decoder first decodes (910) the MPMflag to determine (920) if the mode belongs to the MPM list or not. If the MPMflag is 1, then it decodes (930) the index of the mode inside the MPM list. Otherwise it performs coset decoding. First it decodes (940) the coset index, and then decodes (950) the prediction mode inside that coset. The intra prediction mode can be determined based on the coset index and the member index.

[0081] In a related embodiment, the encoder can encode only the index of a coset. In this case the encoder will check for the RD performance only with the coset leaders, or any other members which can be implicitly determined (for example, the encoder encodes only the coset index. The mode is implicitly determined as the mode in the coset that is closest to the DIMD mode, or the first TIMD mode, etc). The decoder will decode only the coset index and implicitly determine the member mode in the coset in the same manner as the encoder implicitly determines the member mode in the coset.

[0082] The proposed coset coding of intra prediction mode is simple to design. Thus, it may be much faster to implement as compared to the mode coding in ECM.

[0083] Various methods are described herein, and each of the methods comprises one or more steps or actions for achieving the described method. Unless a specific order of steps or actions is required for proper operation of the method, the order and / or use of specific steps and / or actions may be modified or combined. Additionally, terms such as “first”, “second”, etc. may be used in various embodiments to modify an element, component, step, operation, etc., for example, a “first decoding” and a “second decoding”. Use of such terms does not imply an ordering to the modified operations unless specifically required. So, in this example, the firstdecoding need not be performed before the second decoding, and may occur, for example, before, during, or in an overlapping time period with the second decoding.

[0084] Various methods and other aspects described in this application can be used to modify modules, for example, the intra prediction modules (260, 360), of a video encoder 200 and decoder 300 as shown in FIG. 2 and FIG. 3. Moreover, the present aspects are not limited to ECM, VVC or HEVC, and can be applied, for example, to other standards and recommendations, and extensions of any such standards and recommendations. Unless indicated otherwise, or technically precluded, the aspects described in this application can be used individually or in combination.

[0085] Various numeric values are used in the present application. The specific values are for example purposes and the aspects described are not limited to these specific values.

[0086] Various implementations involve decoding. “Decoding,” as used in this application, may encompass all or part of the processes performed, for example, on a received encoded sequence in order to produce a final output suitable for display. In various embodiments, such processes include one or more of the processes typically performed by a decoder, for example, entropy decoding, inverse quantization, inverse transformation, and differential decoding. Whether the phrase “decoding process” is intended to refer specifically to a subset of operations or generally to the broader decoding process will be clear based on the context of the specific descriptions and is believed to be well understood by those skilled in the art.

[0087] Various implementations involve encoding. In an analogous way to the above discussion about “decoding”, “encoding” as used in this application may encompass all or part of the processes performed, for example, on an input video sequence in order to produce an encoded bitstream.

[0088] The implementations and aspects described herein may be implemented in, for example, a method or a process, an apparatus, a software program, a data stream, or a signal. Even if only discussed in the context of a single form of implementation (for example, discussed only as a method), the implementation of features discussed may also be implemented in other forms (for example, an apparatus or program). An apparatus may be implemented in, for example, appropriate hardware, software, and firmware. The methods may be implemented in, for example, an apparatus, for example, a processor, which refers to processing devices in general, including, for example, a computer, a microprocessor, an integrated circuit, or a programmable logic device. Processors also include communication devices, for example, computers, cellphones, portable / personal digital assistants (“PDAs”), and other devices that facilitate communication of information between end-users.

[0089] Reference to “one embodiment” or “an embodiment” or “one implementation” or “an implementation”, as well as other variations thereof, means that a particular feature, structure, characteristic, and so forth described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrase “in one embodiment” or “in an embodiment” or “in one implementation” or “in an implementation”, as well any other variations, appearing in various places throughout this application are not necessarily all referring to the same embodiment.

[0090] Additionally, this application may refer to “determining” various pieces of information. Determining the information may include one or more of, for example, estimating the information, calculating the information, predicting the information, or retrieving the information from memory.

[0091] Further, this application may refer to “accessing” various pieces of information. Accessing the information may include one or more of, for example, receiving the information, retrieving the information (for example, from memory), storing the information, moving the information, copying the information, calculating the information, determining the information, predicting the information, or estimating the information.

[0092] Additionally, this application may refer to “receiving” various pieces of information. Receiving is, as with “accessing”, intended to be a broad term. Receiving the information may include one or more of, for example, accessing the information, or retrieving the information (for example, from memory). Further, “receiving” is typically involved, in one way or another, during operations, for example, storing the information, processing the information, transmitting the information, moving the information, copying the information, erasing the information, calculating the information, determining the information, predicting the information, or estimating the information.

[0093] It is to be appreciated that the use of any of the following“and / or”, and “at least one of’, for example, in the cases of “A / B”, “A and / or B” and “at least one of A and B”, is intended to encompass the selection of the first listed option (A) only, or the selection of the second listed option (B) only, or the selection of both options (A and B). As a further example, in the cases of “A, B, and / or C” and “at least one of A, B, and C”, such phrasing is intended to encompass the selection of the first listed option (A) only, or the selection of the second listedoption (B) only, or the selection of the third listed option (C) only, or the selection of the first and the second listed options (A and B) only, or the selection of the first and third listed options (A and C) only, or the selection of the second and third listed options (B and C) only, or the selection of all three options (A and B and C). This may be extended, as is clear to one of ordinary skill in this and related arts, for as many items as are listed.

[0094] Also, as used herein, the word “signal” refers to, among other things, indicating something to a corresponding decoder. For example, in certain embodiments the encoder signals a quantization matrix for de-quantization. In this way, in an embodiment the same parameter is used at both the encoder side and the decoder side. Thus, for example, an encoder can transmit (explicit signaling) a particular parameter to the decoder so that the decoder can use the same particular parameter. Conversely, if the decoder already has the particular parameter as well as others, then signaling can be used without transmitting (implicit signaling) to simply allow the decoder to know and select the particular parameter. By avoiding transmission of any actual functions, a bit savings is realized in various embodiments. It is to be appreciated that signaling can be accomplished in a variety of ways. For example, one or more syntax elements, flags, and so forth are used to signal information to a corresponding decoder in various embodiments. While the preceding relates to the verb form of the word “signal”, the word “signal” can also be used herein as a noun.

[0095] As will be evident to one of ordinary skill in the art, implementations may produce a variety of signals formatted to carry information that may be, for example, stored or transmitted. The information may include, for example, instructions for performing a method, or data produced by one of the described implementations. For example, a signal may be formatted to carry the bitstream of a described embodiment. Such a signal may be formatted, for example, as an electromagnetic wave (for example, using a radio frequency portion of spectrum) or as a baseband signal. The formatting may include, for example, encoding a data stream and modulating a carrier with the encoded data stream. The information that the signal carries may be, for example, analog or digital information. The signal may be transmitted over a variety of different wired or wireless links, as is known. The signal may be stored on a processor-readable medium.

Claims

CLAIMS1. A method of video decoding, comprising: obtaining a set of intra prediction mode candidates for a block of a picture; decomposing remaining intra prediction modes into a plurality of cosets of intra prediction modes, said remaining intra prediction modes excluding said set of intra prediction mode candidates; decoding a coset index; decoding an intra prediction mode for said block from a coset associated with said decoded coset index; and decoding said block based on said intra prediction mode.

2. An apparatus of video decoding, comprising at least one memory and one or more processors, wherein said one or more processors are configured to: obtain a set of intra prediction mode candidates for a block of a picture; decompose remaining intra prediction modes into a plurality of cosets of intra prediction modes, said remaining intra prediction modes excluding said set of intra prediction mode candidates; decode a coset index; decode an intra prediction mode for said block from a coset associated with said decoded coset index; and decode said block based on said intra prediction mode.

3. The method of claim 1, or the apparatus of claim 2, wherein said intra prediction mode for said block is decoded from a coset associated with said decoded coset index responsive to decoding a member index of said intra prediction mode in said coset.

4. The method of claim 1 or 3, wherein said decoding an intra prediction mode for said block from a coset associated with said decoded coset index comprises, or the apparatus of claim 2 or 3, wherein said one or more processors are configured to perform: determining the first mode in said coset as said intra prediction mode.

5. A method of video encoding, comprising: obtaining an intra prediction mode for a block of a picture; obtaining a set of intra prediction mode candidates for said block of said picture;decomposing remaining intra prediction modes into a plurality of cosets of intra prediction modes, said remaining intra prediction modes excluding said set of intra prediction mode candidates; determining a coset to which an intra prediction mode of said block belongs; encoding a coset index associated with said coset; and encoding said block based on said intra prediction mode.

6. An apparatus of video encoding, comprising at least one memory and one or more processors, wherein said one or more processors are configure to: obtain an intra prediction mode for a block of a picture; obtain a set of intra prediction mode candidates for said block of said picture; decompose remaining intra prediction modes into a plurality of cosets of intra prediction modes, said remaining intra prediction modes excluding said set of intra prediction mode candidates; determine a coset to which an intra prediction mode of said block belongs; encode a coset index associated with said coset; and encode said block based on said intra prediction mode.

7. The method of claim 5, further comprising, or the apparatus of claim 6, wherein said one or more processors are configured to perform: encoding a member index of said intra prediction mode in said coset.

8. The method of claim 5 or 7, wherein said obtaining an intra prediction mode for said block comprises, or the apparatus of claim 6 or 7, wherein said one or more processors are configured to perform: checking the first intra prediction mode in each of said plurality of cosets; and selecting said intra prediction mode for said block from said first intra prediction modes.

9. The method of any one of claims 1, 3-5, 7 and 8, or the apparatus of any one of claims 2-4 and 6-8, wherein said set of intra prediction mode candidates corresponds to a MPM (Most Probable Mode) list.

10. The method of any one of claims 1, 3-5 and 7-9, or the apparatus of any one of claims 2-4 and 6-9, wherein a number of said plurality of cosets is adaptive to a block size of said block.

11. The method of claim 10, or the apparatus of claim 10, wherein said number of said plurality of cosets is larger for a larger block size.

12. The method of any one of claims 1, 3-5 and 7-11, or the apparatus of any one of claims 2-4 and 6-11, wherein a number of said plurality of cosets is based on prediction modes of neighboring blocks of said block.

13. The method of claim 12, or the apparatus of claim 12, wherein said number of said plurality of cosets is smaller when a prediction mode for a left neighboring block and a prediction mode for an above mode neighboring block are angular and same.

14. The method of any one of claims 1, 3-5 and 7-13, wherein said obtaining a set of intra prediction mode candidates for a block of a picture comprises, or the apparatus of any one of claims 2-4 and 6-13, wherein said one or more processors are configured to perform: obtaining a left mode indicating an intra prediction mode of a left neighboring block and a top mode indicating an intra prediction mode of an above neighboring block; determining that only one of said left mode and said top mode is angular; including a DC mode in said set of intra prediction mode candidates for said block; and including said left mode or top mode which is angular in said set of intra prediction mode candidates for said block.

15. The method of any one of claims 1, 3-5 and 7-14, wherein said obtaining a set of intra prediction mode candidates for a block of a picture comprises, or the apparatus of any one of claims 2-4 and 6-14, wherein said one or more processors are configured to perform: obtaining a left mode indicating an intra prediction mode of a left neighboring block and a top mode indicating an intra prediction mode of an above neighboring block; determining that said left mode and said top mode are angular and different; and including said left mode and said top mode in said set of intra prediction mode candidates for said block.

16. The method of any one of claims 1, 3-5 and 7-15, or the apparatus of any one of claims 2-4 and 6-15, wherein said set of intra prediction mode candidates includes three intra prediction modes.

17. The method of any one of claims 1, 3-5 and 7-16, or the apparatus of any one of claims 2-4 and 6-16, wherein intra prediction modes in a coset are ordered.

18. The method of claim 17, or the apparatus of claim 17, wherein said intra prediction modes in a coset are ordered based on one of:(1) a minimum distance of an intra prediction mode from one or more angular modes in said set of intra prediction mode candidates,(2) a difference score between a prediction using an intra prediction mode and a template, and (3) a distance of an intra prediction mode from a DIMD mode or the first TIMD mode.

19. A signal comprising a bitstream, formed by performing the method of any one of claims 5 and 7-18.

20. A computer readable storage medium having stored thereon instructions for encoding or decoding a video according to the method of any one of claims 1, 3-5 and 7-18.

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