INTRA PREDICTION WITH GEOMETRIC PARTITION

In one implementation, when coding a block in the intra prediction mode, the block can be split into two sub-partitions by a geometrically located straight line. Each geometric partition within the CU is intra predicted using its own intra mode with its available reference sample. One sub-partition copies and uses the intra prediction mode from the parent block, and another sub-partition uses another implicit or explicit signaled intra prediction mode. After predicting geometric partition, the sample values along the split boundary are adjusted using a blending process with adaptive weights. The geometric partition based intra prediction could be applied for one angular intra prediction mode, or only for one negative-directional intra prediction mode, or only for one specific intra prediction mode (e.g., mode 34). The transform selection or other intra coding tools (i.e., intra sub-partition) can be adapted for the geometric partition based intra prediction.

TECHNICAL FIELD

The present embodiments generally relate to a method and an apparatus for intra prediction with geometric partition in video encoding and decoding.

BACKGROUND

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

According to an embodiment, a method of video encoding or decoding is provided, comprising: splitting a block of a picture into at least two partitions by a straight line; performing intra prediction with a first intra prediction mode on a first partition of said at least two partitions to obtain prediction samples for said first partition; performing intra prediction with a second intra prediction mode on a second partition of said at least two partitions to obtain prediction samples for said second partition; and adjusting prediction sample values along said straight line using a blending process with adaptive weights.

According to another embodiment, an apparatus for video encoding or decoding is presented, comprising one or more processors, wherein said one or more processors are configured to: split a block of a picture into at least two partitions by a straight line; perform intra prediction with a first intra prediction mode on a first partition of said at least two partitions to obtain prediction samples for said first partition; perform intra prediction with a second intra prediction mode on a second partition of said at least two partitions to obtain prediction samples for said second partition; and adjust prediction sample values along said straight line using a blending process with adaptive weights.

According to another embodiment, an apparatus for video encoding or decoding is presented, comprising: means for splitting a block of a picture into at least two partitions by a straight line; means for performing intra prediction with a first intra prediction mode on a first partition of said at least two partitions to obtain prediction samples for said first partition; means for performing intra prediction with a second intra prediction mode on a second partition of said at least two partitions to obtain prediction samples for said second partition; and means for adjusting prediction sample values along said straight line using a blending process with adaptive weights.

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 above. One or more of the present embodiments also provide a computer readable storage medium having stored thereon instructions for encoding or decoding video data according to the methods described above.

One or more embodiments also provide a computer readable storage medium having stored thereon a bitstream generated according to the methods described above. One or more embodiments also provide a method and apparatus for transmitting or receiving the bitstream generated according to the methods described above.

DETAILED DESCRIPTION

FIG.1illustrates a block diagram of an example of a system in which various aspects and embodiments can be implemented. System100may 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 system100, 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 system100are distributed across multiple ICs and/or discrete components. In various embodiments, the system100is 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, the100is configured to implement one or more of the aspects described in this application.

The system100includes at least one processor110configured to execute instructions loaded therein for implementing, for example, the various aspects described in this application. Processor110may include embedded memory, input output interface, and various other circuitries as known in the art. The system100includes at least one memory120(e.g., a volatile memory device, and/or a non-volatile memory device). System100includes a storage device140, 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 device140may include an internal storage device, an attached storage device, and/or a network accessible storage device, as non-limiting examples.

System100includes an encoder/decoder module130configured, for example, to process data to provide an encoded video or decoded video, and the encoder/decoder module130may include its own processor and memory. The encoder/decoder module130represents 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 module130may be implemented as a separate element of system100or may be incorporated within processor110as a combination of hardware and software as known to those skilled in the art.

Program code to be loaded onto processor110or encoder/decoder130to perform the various aspects described in this application may be stored in storage device140and subsequently loaded onto memory120for execution by processor110. In accordance with various embodiments, one or more of processor110, memory120, storage device140, and encoder/decoder module130may 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.

In several embodiments, memory inside of the processor110and/or the encoder/decoder module130is 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 processor110or the encoder/decoder module130) is used for one or more of these functions. The external memory may be the memory120and/or the storage device140, for example, 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.

The input to the elements of system100may be provided through various input devices as indicated in block105. 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.

Additionally, the USB and/or HDMI terminals may include respective interface processors for connecting system100to other electronic devices across USB and/or HDMI connections. 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 processor110as necessary. Similarly, aspects of USB or HDMI interface processing may be implemented within separate interface ICs or within processor110as necessary. The demodulated, error corrected, and demultiplexed stream is provided to various processing elements, including, for example, processor110, and encoder/decoder130operating in combination with the memory and storage elements to process the datastream as necessary for presentation on an output device.

Various elements of system100may be provided within an integrated housing, Within the integrated housing, the various elements may be interconnected and transmit data therebetween using suitable connection arrangement115, for example, an internal bus as known in the art, including the I2C bus, wiring, and printed circuit boards.

The system100includes communication interface150that enables communication with other devices via communication channel190. The communication interface150may include, but is not limited to, a transceiver configured to transmit and to receive data over communication channel190. The communication interface150may include, but is not limited to, a modem or network card and the communication channel190may be implemented, for example, within a wired and/or a wireless medium.

Data is streamed to the system100, 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 channel190and the communications interface150which are adapted for Wi-Fi communications. The communications channel190of 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 system100using a set-top box that delivers the data over the HDMI connection of the input block105. Still other embodiments provide streamed data to the system100using the RF connection of the input block105.

The system100may provide an output signal to various output devices, including a display165, speakers175, and other peripheral devices185. The other peripheral devices185include, 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 system100. In various embodiments, control signals are communicated between the system100and the display165, speakers175, or other peripheral devices185using 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 system100via dedicated connections through respective interfaces160,170, and180. Alternatively, the output devices may be connected to system100using the communications channel190via the communications interface150. The display165and speakers175may be integrated in a single unit with the other components of system100in an electronic device, for example, a television. In various embodiments, the display interface160includes a display driver, for example, a timing controller (T Con) chip.

The display165and speaker175may alternatively be separate from one or more of the other components, for example, if the RF portion of input105is part of a separate set-top box. In various embodiments in which the display165and speakers175are external components, the output signal may be provided via dedicated output connections, including, for example, HDMI ports, USB ports, or COMP outputs.

FIG.2illustrates an example video encoder200, such as a VVC (Versatile Video Coding) encoder.FIG.2may also illustrate an encoder in which improvements are made to the VVC standard or an encoder employing technologies similar to VVC.

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.

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 pre-processing, and attached to the bitstream.

In the encoder200, 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. 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 one of 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.

The prediction residuals are then transformed (225) and quantized (230). The quantized transform coefficients, as well as motion vectors and other syntax elements, are entropy coded (245) to output a bitstream. The encoder can skip the transform and apply quantization directly to the non-transformed 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.

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) filtering to reduce encoding artifacts. The filtered image is stored at a reference picture buffer (280).

FIG.3illustrates a block diagram of an example video decoder300. In the decoder300, a bitstream is decoded by the decoder elements as described below. Video decoder300generally performs a decoding pass reciprocal to the encoding pass as described inFIG.2. The encoder200also generally performs video decoding as part of encoding video data.

In particular, the input of the decoder includes a video bitstream, which can be generated by video encoder200. The bitstream is first entropy decoded (330) to obtain transform coefficients, 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 picture buffer (380).

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 pre-encoding processing (201). The post-decoding processing can use metadata derived in the pre-encoding processing and signaled in the bitstream.

As described above, in the VVC video compression, a picture is divided into so-called Coding Tree Units (CTU), and each CTU is represented by one or more Coding Units (CUs) in the compressed domain as shown inFIG.4. Each CU is then given some Intra or Inter prediction parameters (Prediction Info).

In Intra prediction, a CU is spatially predicted from the causal neighbor CUs, i.e., the decoded CUs on the top and the left of the current CU. For that purpose, VVC uses simple spatial models called prediction modes. Based on the decoded pixel values in the top and left CUs, called reference pixels, the encoder constructs different predictions for the target block and chooses the one that leads to the best RD performance. Out of the 95 pre-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. The angular modes aim to model the directional structures of objects in a frame. Therefore, the decoded pixel values in the top and left CUs are simply repeated along the pre-defined directions to fill up the target CU.

The angular prediction modes can describe image regions containing object structures with different directionalities. The PLANAR and DC modes describe constant and gradually changing regions without any particular directionality. But inside a frame there may be blocks which contain part of an object and the background, or parts of the same or multiple objects having different directionalities. Such blocks usually cannot be inadequately described by a single angular mode or a non-angular mode (i.e., the PLANAR and DC modes). In the following, we briefly present the intra prediction and geometric partition in VVC. For easier reference, we will be using the terms “CU” and “block” interchangeably throughout the text.

Intra Prediction in VVC

The intra prediction process in VVC consists of three steps: (1) reference sample generation, (2) intra sample prediction, and (3) post-processing of predicted samples. The reference sample generation process is illustrated inFIG.5. The reference pixel values at co-ordinates (x,y) are indicated in the figure by R(x,y). For a CU of size W×H, where W and H denote the width and the height, respectively, a row of 2W decoded samples on the top is formed from the previously reconstructed top and top-right pixels to the current CU. Similarly, a column of 2H samples on the left is formed from the reconstructed left and below-left pixels. The corner pixel at the top-left position is also used to fill up the gap between the top row and the left column references.

The next step, i.e., the intra sample prediction, consists of predicting the pixels of the target CU based on the reference samples. As mentioned before, in order to predict different kinds of content efficiently, VVC supports a range of prediction modes. Planar and DC prediction modes are used to predict smooth and gradually changing regions, whereas angular prediction modes are used to capture different directional structures. VVC supports 95 directional prediction modes which are indexed from −14 to −1 and from 2 to 80. For a square CU, only prediction modes 2-66are used. These prediction modes correspond to different prediction directions from 45 degree to −135 degree in clockwise direction, as illustrated inFIG.6. The number denotes the prediction mode index associated with the corresponding direction. Modes 2 to 33 indicate horizontal predictions and modes 34 to 66 indicate vertical predictions.

The modes are defined by intraPredAngle (A), the offset of the predictor with respect to the (0, 0) position in horizontal/vertical direction as shown in Table 1. When intraPredAngle (A) equals to 0, the prediction mode might be strictly horizontal mode (mode 18) or vertical mode (mode 50); when the value of intraPredAngle (A) is negative, the prediction mode is a negative direction, i.e., a mode in the range 19-49, and when the value of intraPredAngle (A) is positive, the prediction mode is a positive direction, i.e., any of the remaining angular modes.

TABLE 1mapping between intra prediction modes and intraPredAngle (A) in VVC.mode23456789101112131415161718A322926232018161412108643210mode1920212223242526272829303132333435A−1−2−3−4−6−8−10−12−14−16−18−20−23−26−29−32−29mode3637383940414243444546474849505152A−26−23−20−18−16−14−12−10−8−6−4−3−2−1012mode5354555657585960616263646566A346810121416182023262932

After the second step, some prediction modes can lead to discontinuities along the top and left reference boundaries, hence those prediction modes include a subsequent post-processing, known as position dependent intra prediction combination (PDPC), which aims to smoothen the predicted pixel values near those boundaries.

Geometric Partition in VVC

For better alignment of inter prediction boundary with objects, in WET-P0068 (see Han Gao, et al., “CE4: CE4-1.1, CE4-1.2 and CE4-1.14: Geometric Merge Mode (GEO)”, Document JVET-P0068, 16th Meeting: Geneva, CH, 1-11 Oct. 2019), a geometric merge mode has been proposed with 32 angles and 5 distances in inter prediction for VVC. When the geometric merge mode is used, a CU is split into two partitions. Each partition in the CU is inter-predicted using its own motion parameters; only uni-prediction is allowed for each partition, that is, each partition has one motion vector and one reference index. After predicting each of the partitions, the sample values along the splitting edge are adjusted using a blending process with adaptive weights.

The split boundary can be described by angle φiand distance offset ρi. The angle φiis quantized from 0 degree to 360 degrees with a step equal to 11.25 degrees. In total 32 angles are proposed as shown inFIG.7. The description of a geometric split with angle φiand distance ρiis depicted inFIG.8. Distance ρiis quantized from the largest possible distance ρmaxwith a fixed step, which indicates a distance from the center of the block. For distance ρi=0, only the first half of the angles are available as splits are symmetric in this case. The results of geometric partitioning using angle 12 and distance between 0 and 3 is depicted inFIG.9.

As shown inFIG.10, some examples of non-rectangular partitioning in inter prediction, e.g., diagonal partitioning (1010) and general geometric partitioning (1020), are quite useful for outlining the complicate shapes of objects from the background or other objects. In VVC, only rectangular (including square) partitioning is applied on intra frames, so the objects with very different features could be contained inside one intra-coded block. If any block has changing region along certain directions and constant changing region at the same time, or if any block has more than one changing regions along different directions, they usually cannot be inadequately described by either a single corresponding angular mode, or the PLANAR or the DC mode.

For instance, if we consider a piecewise smooth image model as illustrated inFIG.11, where two different smooth regions, with different smoothness properties, are separated by an edge (1110), it is less accurate to predict both regions with a single intra prediction model. In near-edge areas, they could be continually partitioned into smaller square/rectangular blocks and coded as smaller blocks separately. However, these smaller prediction blocks with similar data might result in unnecessary overhead.

To better model such blocks, we propose intra geometric partition to be used. In particular, we propose geometric/diagonal partition based intra prediction to adapt to complicated features of natural images. Different embodiments are provided, which can include one or more of the following:1. Split an intra-predicted CU into two or more sub-partitions by a geometrically located straight line (including diagonal splitting).2. Each geometric partition within the CU, is intra-predicted using its own intra mode with its available reference samples, respectively. One sub-partition copies and uses the intra prediction mode from the parent CU; another sub-partition uses another implicit or explicit signaled intra prediction mode.3. After predicting each geometric partition, the sample values along the split boundary are adjusted using a blending process with adaptive weights.4. The geometric partition based intra prediction could be applied for one angular intra prediction mode, or only for one negative-directional intra prediction mode, or only for one specific intra prediction mode (e.g., mode 34).5. The Rate-Distortion (RD) cost of geometric partition based intra prediction could be checked after or before the optimal intra prediction mode is selected.6. Adapt the transform selection or other intra coding tools (i.e., intra sub-partition) for the geometric partition based intra prediction.

In the following, several embodiments with respect to intra geometric partition are described in detail.

Diagonal Partition Based Intra Prediction for Negative-Directional Mode

In this embodiment, after one negative-directional intra prediction mode is selected out of these defined modes for a target CU that leads to the best RD performance, this target CU could be split into two triangle-shaped partitions, using the diagonal split from the top-left position, as illustrated inFIG.12. Specifically, a sub-partition flag cu_sbp_flag is signaled for an intra CU, and diagonal partition is further applied on this intra CU if cu_sbp_flag equals to 1.

FIG.13illustrates a method (1300) of diagonal partition based intra prediction for an image block at the encoder, according to an embodiment. Method1300starts at step1305. At step1310, the most probable mode (MPM) candidate list is generated. At steps1320and1330, the encoder checks all potential intra prediction modes, by generating prediction blocks P(n) and calculating the RD cost COST(n) for each potential intra prediction mode n. The optimal intra prediction mode m (e.g., the one with smallest RD cost) is used to encode (1340) the current block. If a negative-directional intra prediction mode is selected out of these pre-defined modes (1350), the diagonal partition is checked. At step1360, a sub-partition flag cu_sbp_flag to indicate whether the block is split into two sub-partitions diagonally or not, is initialized to 0. At step1370, the block is diagonally split and the related RD cost with splitting is calculated. The RD cost with and without splitting is compared (1380). If the proposed diagonal partition based intra prediction has a smaller RD cost, diagonal partition is applied for the intra block, and the sub-partition flag cu_sbp_flag is encoded as 1 (1390). Method1300ends at step1399.

FIG.14illustrate the generation process1400of the diagonal partition based intra predicted block, according to an embodiment. Method1400can be used in step1370to apply intra diagonal partition. When this diagonal partition is used, an intra CU is split into two triangle-shaped child partitions: Partition0and Partition1(1410). Partition0is inferred to use the negative-directional intra prediction mode of the parent CU. Another child Partition1is then intra-predicted using another default or signaled intra prediction mode. By allowing two different intra prediction modes for an intra block with two regions with different smoothness properties, more accurate prediction could be expected.

A partition position flag cu_sbp_pos is signaled to indicate which child partition is Partition0(1420). As shown inFIGS.12(a) and12(b)respectively, Partition0is the region located near the left boundary when cu_sbp_pos equals to 0; on the contrary, Partition0is the region located near the above boundary when cu_sbp_pos equals to 1. In order to further improve the coding efficiency and simplify the coding process, the partition position flag cu_sbp_pos could also be implicit under some conditions as described hereinafter, and the signaling could be skipped.

The intra prediction mode of Partition0is directly copied from the current CU (1430). Depending on different design philosophies, the intra prediction mode of Partition1could either be explicitly signaled, or be implicitly signaled as a default intra prediction mode (1440).

Each child partition is intra predicted with its intra prediction mode and its available reference samples, respectively. After predicting each of the triangle partitions, the sample values along the diagonal edge/boundary are adjusted using a blending process with adaptive weighting masks or factors (1450). Further details for steps1420,1440and1450will be described below.

FIG.15illustrates a method (1500) for performing the diagonal partition based intra prediction at the decoder, according to an embodiment. Method1500starts at step1505. The intra prediction mode m for a CU is decoded at step1510. If this intra prediction mode is a negative-directional intra prediction mode (1520), a sub-partition flag cu_sbp_flag is decoded to indicate whether the block is split into two sub-partitions diagonally or not (1530). If the intra prediction mode m is not negative-directional, or cu_sbp_flag equals to 0, this CU will be intra predicted with its intra mode m (1570). If the CU is diagonal split (1535), a partition position flag cu_sbp_pos is explicitly or implicitly decoded to indicate which child partition is Partition0(1540). For Partition1, an additional intra prediction mode cu_sbp_mode is explicitly or implicitly decoded (1550), and it is used for the intra prediction of Partition1(1565); for Partition0, it is intra predicted with the intra prediction mode m, which is directly copied from its parent CU (1560). After obtaining the predicted Partition0and Partition1, they are blended to get the final predicted CU (1580). Method1500ends at step1599.

One reason for applying further diagonal partition only on negative-directional intra prediction modes is to make sure that there are reference samples available for predicting both triangle-shaped partitions. According to a variant of this embodiment, the proposed diagonal intra partition is enable only if the intra prediction mode 34 is selected.

Decision of Partition0and Partition1using a Partition Position Flag cu_sbp_pos (1420)

As described inFIG.14andFIG.15, a partition position flag cu_sbp_pos is signaled to indicate which region of the parent intra CU is intra-predicted using the intra prediction mode copied from the parent CU (Partition0). The remaining region (Partition1) is intra-predicted using another default or signaled mode.

As shown inFIG.12, Partition0is the region located near the left boundary when cu_sbp_pos equals to 0; on the contrary, Partition0is the region located near the above boundary when cu_sbp_pos equals to 1.

Rather than signalling the partition position flag cu_sbp_pos, the position of partition intra-predicted with the inferred mode (Partition0) could be implicit under some conditions to further improve the coding efficiency and simplify the coding process.

In one example, Partition0could be implicit according to the negative-directional intra prediction mode of the parent intra CU as shown inFIG.16. If the intra prediction mode of the parent intra CU belongs to horizontal negative directions (e.g., modes 19 to 33 as shown inFIG.6), Partition0is the region located near the left boundary; otherwise, for vertical negative directions (e.g., modes 34 to 49 as shown inFIG.6), Partition0is the region located near the above boundary. One reason for this implicit signaling is that if diagonal partition is further applied on an intra CU, when the intra prediction mode of this CU belongs to horizontal directions, it is likely to use the left reference array to perform horizontal intra-prediction on the region near the left boundary; and the remaining region having different changing property, could be better intra-predicted using another default or signaled mode.

The range of the horizontal and vertical negative directions defined for this implicit signaling method could be further reduced or increased. For example, only modes 19 to 26 as shown inFIG.6could be classified into horizontal negative directions to which implicit signaling of P0/P1partition will be applied, and modes 42 to 49 as shown inFIG.6are included in vertical negative directions to which implicit signaling of P0/P1partition will be applied.

Intra Prediction Mode of the Partition1cu_sbp_mode (1440)

As described inFIG.14andFIG.15, Partition0, which is a child partition of the target intra CU, is intra-predicted using the intra prediction mode from its parent CU; Partition1, which is the remaining child partition of the target intra CU, is intra-predicted using 1) either a default intra prediction mode (i.e., the DC/Horizontal/Vertical mode), 2) or a signaled intra prediction mode out of remaining pre-defined intra prediction modes.

For the sake of simplicity, Partition1could be automatically intra-predicted using DC mode. In this case, cu_sbp_mode is set as 1 (DC mode). The sample values of current Partition1are predicted by computing average of the reference samples from left or/and above neighbors; and the top-left corner reference sample will not be used for its prediction if the DC mode is applied. This implementation could address the case that a block has a changing region along certain directions and a constant changing region at the same time.

If the target intra CU is a square block, then Partition1is a symmetric triangle-shaped partition as shown inFIG.17(reference samples used for Partition1are marked in dark gray). The reference samples from both left and above neighbors along the length L are both used to compute the average according to the following equation:

where the length L equals to the width/height of the square block L=W or H. The reference samples used for the prediction of Partition1are marked in dark gray in this example.

If the target intra CU is a rectangular block, to avoid division operations for generating DC prediction, only the longer side along the left and above neighbors is used to compute the average for an asymmetric triangle-shaped partition as shown inFIG.18.

If the horizontal side of Partition1is longer, the intra-prediction p(x,y) is derived by averaging the reference samples from top neighbors along the length L according to the following equation:

Similarly, if the vertical side of Partition1is longer, the intra-prediction p(x,y) is derived by averaging the reference samples from left neighbors along the length L according to the following equation:

According to a variant for an asymmetric triangle-shaped partition, rather than using DC mode, the sample values of Partition1can be predicted by using Horizontal mode (cu_sbp_mode=18) or Vertical mode (cu_sbp_mode=50).

If the vertical side of Partition1along the left and above neighbors is longer, Horizontal mode (mode 18) is implicit as the intra prediction mode of Partition1. Intra-prediction p(x,y) is derived by copying the reference samples in horizontal direction as shown inFIG.19. Similarly, Vertical mode (mode 50) is applied if the horizontal side of Partition1along the left and above neighbors is longer, intra-prediction p(x,y) is derived by copying the reference samples in vertical direction.

According to another variant, for an asymmetric triangle-shaped partition, the sample values of current Partition1can be predicted by using one mode selected from DC and Horizontal/Vertical modes. In this case, cu_sbp_mode is signaled with one additional bit into the bitstream.

According to another variant, to predict Partition1more accurately and flexibly, Partition1could be intra-predicted using a signaled intra prediction mode cu_sbp_mode out of remaining pre-defined modes based on the optimal RD cost. In this case, if the mode cu_sbp_mode of Partition1belongs to DC or Horizontal/Vertical modes, the top-left corner reference sample will not be used for the prediction; if the mode cu_sbp_mode is one of other intra modes, the top-left corner reference sample could be used.

In order to speed up the mode selection process at the encoder, the candidate number of intra prediction modes could be limited, or another most probable mode (MPM) list could be used for the intra prediction mode cu_sbp_mode of Partition1. For example, a list with 3 MPMs is generated by considering the intra modes of the left and above neighbouring block of Partition1. Suppose the mode of the left is denoted as Left and the mode of the above block is denoted as Above.

If both modes Left and Above are available and they are different as shown inFIG.20, the MPM list is constructed as follows:If both modes Left and Above are non-angular modes:MPM list→{DC, V, H}If one of modes Left and Above is angular mode, and the other is non-angular mode:Set a mode Max as the larger mode in Left and AboveMPM list→{Max, DC, Max-1}If both modes Left and Above are angular modes:if the Partition1is a symmetric triangle-shaped partitionMPM list→{DC, Left, Above}if the Partition1is an asymmetric triangle-shaped partition, and horizontal side of Partition1along the left and above neighbors is longerMPM list→{Left, H, Above}OtherwiseMPM list→{Above, V, Left}

If only one of modes Left and Above is available, or if both modes Left and Above are angular and they are equal, the Partition1could just infer this available mode Left or Above for intra-prediction as shown inFIG.21.

A Blending Process with Adaptive Weights (1450)

After predicting each of the partitions, the sample values on the splitting edge are adjusted using a blending process of the prediction predP0(x,y) for P0and the prediction predP1(x,y) for P1with an adaptive factor W using the following equation:

where the weighting factor could be 1/2 or 3/4, or other values, as shown inFIG.22.

To further reduce the boundary effects along the two partitions P0and P1, same as Triangle Partitioning mode (TPM), the proposed diagonal partition based intra prediction can operate using two intra prediction modes m and cu_sbp_mode to produce a final predicted block predfinal(x,y) with two predictions pred0(x,y) and pred1(x,y) for the CU using the blending masks W0and W1as in the following equation:

The blending masks of the proposed intra diagonal partition W0and W1are derived from the distance between the sample position and the split boundary as shown inFIG.23. In this example, the weights {7/8, 6/8, 5/8, 4/8, 3/8, 2/8, 1/8} are used in the blending process.

According to a variant, the blending masks of the intra diagonal partition W0and W1could also be asymmetric around the diagonal edge as shown inFIG.24. The prediction pred0(x,y), wherein the intra prediction mode of the partition is inferred from the intra mode of the block, could use larger weight for blending. In this example, the weights {3/4, 2/4, 1/4} are used in the blending process.

According to another variant, the blending could only be processed at the diagonal edge with a weighting factor W using the following equation:

The weighting factor could be 1/2 or 3/4, or other values.

Geometric Partition for Negative-Directional Intra Prediction Mode

In the above, a CU is split into two parts by a diagonal line. More generally, we propose to split an intra-predicted CU into two parts by a geometrically located straight line to better align the edge/boundary of the two regions. The splitting line could be parallel to the intra prediction mode or be selected from several specific partitions; and the splitting line could start from top-left position (0, 0) or with an offset.

Similarly to the diagonal partition based intra prediction, the proposed geometric partition based intra prediction could further split an intra-predicted CU into two partitions. Then each geometric partition within the CU, Partition0and Partition1, is intra-predicted with its own intra mode using its available reference samples, respectively. After predicting each geometric partition, the sample values along the split boundary are adjusted using a blending process with adaptive weights. The weighting factors could be derived from the distance of the sample position and the split boundary.

Split Boundary Derivation for Geometric Partition Based Intra Prediction

As described above, a geometric partition can split the target CU into two parts by a splitting line that is parallel to the direction associated with the intra prediction mode of this CU.

As shown the examples inFIG.25(a), if the intra prediction mode is mode 19, then the split boundary (2510) is parallel to a horizontal negative direction; or if the intra prediction mode is mode 49 as shown inFIG.25(b), the split boundary (2520) is parallel to a vertical negative direction. If the intra prediction direction of the target intra CU is mode 34 as shown inFIG.25(c), then the diagonal splitting (2530) as described before is applied for square block; and the 45 degree splitting, which is parallel to the mode 34, is applied for rectangular block.

To further increase the splitting flexibility of the proposed intra geometric partition, the split boundary can be selected from several pre-defined partitions as shown inFIG.26. In this example, there are seven pre-defined split boundaries, each of which represents an angle between 0 and −90 degrees with 11.25 degrees steps. For this example, a syntax element cu_sbp_boundary is signaled to indicate which splitting boundary is applied.

According to another variant, the number of pre-defined split boundaries could be any other value than 7, and it could also be a different value adapting to the intra prediction mode. For example, if the intra prediction mode belongs to horizontal negative directions (modes 19 to 33 as shown inFIG.6), then only half of the split boundaries that are close to the above part inFIG.26will be applied.

Splitting Line Start Position for Geometric Partition cu_sbp_start

As mentioned above, a splitting line for geometric partition of the target CU can start from the top-left position (0, 0), or the splitting start position could be shifted with an offset to better align with the geometric edge/boundary of the two child partitions.

In this case, a syntax element cu_sbp_start is signaled to indicate where the splitting start position locates. Taken mode 34 as an example as illustrated inFIG.27, there are three pre-defined splitting start positions. The 45-degree splitting line starts from the top-left position p(0, 0) when cu_sbp_start equals to 0. If cu_sbp_start equals to 1, the 45-degree splitting line will start from the middle of the left boundary

Similarly, the 45-degree splitting line will start from the middle of the above boundary

if cu_sbp_start equals 2. More splitting start positions could be pre-defined in the similar rule. For example, the splitting start position could be located at one quarter of the left or above boundary

According to another variant, a 45-degree splitting of the target CU can start from an arbitrary position at the left or above boundary whose coordinate information is signaled into the bitstream as illustrated inFIG.28. For this variant, a syntax flag cu_sbp_start_left is signaled to indicate whether the splitting start position is located on the left boundary; and then another syntax element cu_sbp_start_offset is signaled subsequently to indicate the distance between the splitting start position and the top-left position. If cu_sbp_start_left equals to 1, the geometric partition will start from an arbitrary position at the left boundary p (0,y), and the distance y is signaled as cu_sbp_start_offset into bitstream. Similarly, the geometric partition will start from an arbitrary position at the above boundary p(x,0) if cu_sbp_start_left equals to 0, and the distance x is signaled as cu_sbp_start_offset. This variant is more flexible to align with geometric edge/boundary, while the signaling of cu_sbp_start offset could be quite costly.

Decision of Partition0and Partition1Based on the Area

Since a geometric partition can split the target CU into two asymmetric parts, Partition0could be implicit according to the area of these two child partitions of the target intra CU as shown inFIG.29. The concept of this variant is to automatically set the region with the larger area among these two child partitions as Partition0.

The reason for this proposed variant is that most areas of the target intra CU are likely to be predicted with the intra mode of this CU; only the remaining smaller areas having different changing property might need to be intra-predicted using another mode. Otherwise, this target CU should have been assigned with another intra prediction mode.

Geometric Partition Based Intra Prediction for Angular Mode

Rather than only checking geometric partition after a negative-directional intra prediction mode is selected as described above, in the third embodiment, we propose to split an intra-predicted CU into two parts by a geometrically located straight line after one angular intra prediction mode is selected and reference neighbouring samples for the two regions are available.

As a supplement to the geometric partition based intra prediction after a negative-directional intra prediction mode is selected, it is proposed that the geometric partition based intra prediction could further split an intra-predicted CU into two partitions from a top-right position with an offset, or from a bottom-left position with an offset, after a positive-directional intra prediction mode is selected. More details are described below.

Splitting Boundary and Splitting Start Position for Geometric Partition Based Positive-Directional Intra Prediction

As described above, after a negative-directional intra prediction mode is selected, a geometric partition can split the target CU into two parts by a splitting line which is parallel to the intra prediction mode of this CU.

As a supplement for positive-directional intra prediction modes, a geometric partition based intra prediction could further split an intra-predicted CU into two partitions by a splitting line which is parallel to the intra prediction mode of this CU. The splitting start position could be either from a top-right position with an offset, or from a bottom-left position with an offset.

As the examples shown inFIG.30, the splitting line could be parallel to the positive-directional intra prediction mode of the target CU. If the intra prediction mode is mode 8, which belongs to horizontal positive direction, then the splitting line is parallel to a horizontal positive direction; or if the intra prediction mode is mode 60, one of the vertical positive directional modes, the splitting line is parallel to a vertical positive direction. If the intra prediction direction of the target intra CU is mode 2 or mode 66, then the 135-degree splitting is applied.

The splitting line for the positive-directional intra prediction modes, can also be selected from several pre-defined partitions as shown in the example ofFIG.31, to further increase the splitting flexibility. There are four pre-defined splitting boundaries for horizontal/vertical positive directional modes, respectively. The four pre-defined splitting boundaries inFIG.31(a)are used for horizontal positive directional modes, each of which represents an angle between 0 and 45 degrees with 11.25 degrees steps. Another four pre-defined splitting boundaries inFIG.31(b)are used for vertical positive directional modes, which represents an angle between −45 and −90 degrees with 11.25 degrees steps. For this variant, a syntax element cu_sbp_boundary is signaled to indicate which splitting boundary is applied.

Geometric or Diagonal Partition Based Intra Prediction Checking for Angular Intra Prediction Modes before the Optimal Intra Prediction Mode is Selected

In the above, the proposed embodiments are all applied only after an angular intra prediction mode is selected via the recursive RDO search. One advantage is that it limits the search complexity for intra mode selection.

Rather than checking whether splitting the CU into two parts or not only after an angular intra prediction mode is selected, in the fourth embodiment, we propose that the geometric/diagonal intra partition could be checked for angular intra prediction mode candidates during the recursive RDO search before the best intra prediction mode is selected. That is, the RD cost of some or all angular intra prediction modes with or without using geometric/diagonal intra partition could be both calculated, and the remaining intra prediction modes will only calculate the RD cost without splitting. The final intra prediction mode is selected from all these possible situations, which could lead to the best RD performance.

A sub-partition flag cu_sbp_flag is signaled for an intra predicted CU with an angular intra prediction mode, which belongs to the modes needed to check splitting or not. The proposed geometric/diagonal intra partition is further applied on this intra CU if cu_sbp_flag equals to 1.

FIG.32illustrate a method (3200) for performing intra prediction mode searching at the encoder, according to an embodiment. Method3200starts at step3205. At step3210, the most probable mode (MPM) candidate list is generated. At steps3220, the encoder checks a potential intra prediction mode m, by generating prediction blocks P(m) and calculating the RD cost COST(m). For an intra mode that is one of candidates to check the geometric intra partition (3230), a sub-partition flag cu_sbp_flag to indicate whether the block is split into two sub-partitions diagonally or not, is initialized to 0 (3240), and the block is split into two sub-partitions and the RD cost COST(m_sbp) is calculated (3250). If the geometric partition based intra prediction has a smaller RD cost (3260), geometric partition is applied for the intra block, the sub-partition flag cu_sbp_flag is encoded as 1, and the optimal cost is updated to COST(m_sbp) (3270). At step3280, the encoder checks whether all modes are checked. If not, the control returns to step3220. Otherwise, the encoder encodes (3290) the current block using the optimal intra prediction mode. Method3200ends at step3299.

Compared to method1300, this embodiment (3200) increases the searching complexity and the signaling cost of cu_sbp_flag, while it optimizes the best intra prediction mode searching. In order to balance the complexity and coding efficiency, some searching speed-up schemes are described in the following variants.

According to a variant of this embodiment, only when one specific intra prediction mode (e.g., mode 34) is checked, the proposed geometric/diagonal intra partition can be enabled to be checked.

According to another variant of this embodiment, only when one negative-directional intra prediction mode is checked, the proposed geometric/diagonal intra partition can be enabled to be checked.

According to another variant of this embodiment, the geometric/diagonal intra partition can only be enabled to be checked, when one of its left or above neighbouring block applies the geometric/diagonal intra partition.

According to another variant of this embodiment, the geometric/diagonal intra partition can only be enabled to be checked, when both its left and above neighbouring blocks apply the geometric/diagonal intra partition.

Transform Selection for Proposed Intra Geometric/Diagonal Partition

After the final prediction signal for the whole CU is obtained by the process described above, then transform and quantization process will be applied to the whole CU as in other intra prediction modes. The transform selection could be adapted to the intra geometric partition in this embodiment.

In VVC, in addition to DCT2, a Multiple Transform Selection (MTS) scheme is used for residual coding in both intra and inter coded blocks. It uses multiple selected transforms from the DCT8/DST7. Transform and signaling mapping table is shown in Table 2. The introduction of MTS improves the efficiency of transform in VVC while the exhaustive RDO search for the optimal transform candidates brings large computational burden to the VVC encoder.

In VVC, a CU level flag is signalled to indicate whether MTS is applied or not. In the fifth embodiment, we propose that the MTS CU level flag is not signalled but inferred, when the proposed intra geometric/diagonal partition is applied.

If the difference between the intra prediction mode of the two splitting partitions (Partition0and Partition1) is smaller than a pre-defined threshold, or if Partition1is intra-predicted using the DC mode, then the MTS CU level flag is inferred as zero, where DCT2 is applied in both directions. For the remaining cases, whose contents are normally with very complex textures, only one single transform (DCT2) is not efficient to model the different statistical variations, then MTS is highly possible to apply. The MTS CU level flag is inferred as one, then two flags are directly signalled to indicate the transform type for the horizontal and vertical directions as shown in Table 3, respectively.

According to a variant of this embodiment, the transform type for an intra CU using intra geometric partition can be implicitly derived from the intra prediction mode of the target intra CU. The same logics can be implemented differently in practice by using different coding parameters.

According to a variant of the embodiments mentioned before, it could further narrow down the range of the directional intra prediction modes wherein the proposed geometric/diagonal intra partition can be enabled. For example, only after one intra prediction mode from modes 26-42 is selected, the proposed geometric/diagonal intra partition can be enabled.

According to another variant of the embodiments mentioned before, the target CU could be split into more than two parts.

According to another variant of the embodiments mentioned before, the intra sub-partition (ISP) is not allowed when the proposed geometric/diagonal intra partition is applied for the current CU.

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 first decoding 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.

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 encoder200and decoder300as shown inFIG.2andFIG.3. Moreover, the present aspects are not limited to 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.

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.

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.

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.

Note that the syntax elements as used herein are descriptive terms. As such, they do not preclude the use of other syntax element names.

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.

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.