Adaptive Offset Multiple Reference Line Coding

A coder determines a reference line offset based on at least one of: a property of a block of a video, or a property of reference lines of the block. The coder determines a reference line based on a reference line index and the reference line offset. The coder predicts a value of a sample in the block based on the reference line and an intra prediction mode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG.1illustrates an exemplary video coding/decoding system in which embodiments of the present disclosure may be implemented.

FIG.2illustrates an exemplary encoder in which embodiments of the present disclosure may be implemented.

FIG.3illustrates an exemplary decoder in which embodiments of the present disclosure may be implemented.

FIG.4illustrates an example quadtree partitioning of a coding tree block (CTB) in accordance with embodiments of the present disclosure.

FIG.5illustrates a corresponding quadtree of the example quadtree partitioning of the CTB inFIG.4in accordance with embodiments of the present disclosure.

FIG.6illustrates example binary and ternary tree partitions in accordance with embodiments of the present disclosure.

FIG.7illustrates an example quadtree+multi-type tree partitioning of a CTB in accordance with embodiments of the present disclosure.

FIG.8illustrates a corresponding quadtree+multi-type tree of the example quadtree+multi-type tree partitioning of the CTB inFIG.7in accordance with embodiments of the present disclosure.

FIG.9illustrates an example set of reference samples determined for intra prediction of a current block being encoded or decoded in accordance with embodiments of the present disclosure.

FIG.10Aillustrates the35intra prediction modes supported by HEVC in accordance with embodiments of the present disclosure.

FIG.10Billustrates the67intra prediction modes supported by HEVC in accordance with embodiments of the present disclosure.

FIG.11illustrates the current block and reference samples fromFIG.9in a two-dimensional x, y plane in accordance with embodiments of the present disclosure.

FIG.12illustrates an example angular mode prediction of the current block fromFIG.9in accordance with embodiments of the present disclosure.

FIG.13Aillustrates an example of inter prediction performed for a current block in a current picture being encoded in accordance with embodiments of the present disclosure.

FIG.13Billustrates an example horizontal component and vertical component of a motion vector in accordance with embodiments of the present disclosure.

FIG.14illustrates an example of bi-prediction, performed for a current block in accordance with embodiments of the present disclosure.

FIG.15Aillustrates an example location of five spatial candidate neighboring blocks relative to a current block being coded in accordance with embodiments of the present disclosure.

FIG.15Billustrates an example location of two temporal, co-located blocks relative to a current block being coded in accordance with embodiments of the present disclosure.

FIG.16illustrates an example of IBC applied for screen content in accordance with embodiments of the present disclosure.

FIG.17illustrates an example set of multiple reference lines determined for multiple reference line (MRL) coding of a current block in accordance with embodiments of the present disclosure.

FIG.18illustrates an example intra prediction performed for a current block being encoded in accordance with embodiments of the present disclosure.

FIG.19illustrates one example measure of correlation between reference lines of a current block in a picture that may be used to dynamically adapt the reference line set used to perform intra prediction in accordance with embodiments of the present disclosure.

FIG.20illustrates one example determination of a reference line offset and the application of reference line offset to determine a set of reference lines for performing intra prediction of a current block in accordance with embodiments of the present disclosure.

FIG.21illustrates another example determination of a reference line offset and the application of reference line offset to determine a set of reference lines for performing intra prediction of a current block in accordance with embodiments of the present disclosure.

FIG.22illustrates a flowchart of a method for determining a prediction of a sample based on a reference line offset in accordance with embodiments of the present disclosure.

FIG.23illustrates a block diagram of AOM's video codec in accordance with embodiments of the present disclosure.

FIG.24illustrates a block diagram of an example computer system in which embodiments of the present disclosure may be implemented.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth in order to provide a thorough understanding of the disclosure. However, it will be apparent to those skilled in the art that the disclosure, including structures, systems, and methods, may be practiced without these specific details. The description and representation herein are the common means used by those experienced or skilled in the art to most effectively convey the substance of their work to others skilled in the art. In other instances, well-known methods, procedures, components, and circuitry have not been described in detail to avoid unnecessarily obscuring aspects of the disclosure.

Information, arguments, parameters, data, etc. may be passed, forwarded, or transmitted via any suitable means including memory sharing, message passing, token passing, network transmission, or the like.

Furthermore, embodiments may be implemented by hardware, software, firmware, middleware, microcode, hardware description languages, or any combination thereof. When implemented in software, firmware, middleware or microcode, the program code or code segments to perform the necessary tasks (e.g., a computer-program product) may be stored in a computer-readable or machine-readable medium. A processor(s) may perform the necessary tasks.

Representing a video sequence in digital form may require a large number of bits. The data size of a video sequence in digital form may be too large for storage and/or transmission in many applications. Video encoding may be used to compress the size of a video sequence to provide for more efficient storage and/or transmission. Video decoding may be used to decompress a compressed video sequence for display and/or other forms of consumption.

FIG.1illustrates an exemplary video coding/decoding system100in which embodiments of the present disclosure may be implemented. Video coding/decoding system100comprises a source device102, a transmission medium104, and a destination device106. Source device102encodes a video sequence108into a bitstream110for more efficient storage and/or transmission. Source device102may store and/or transmit bitstream110to destination device106via transmission medium104. Destination device106decodes bitstream110to display video sequence108. Destination device106may receive bitstream110from source device102via transmission medium104. Source device102and destination device106may be any one of a number of different devices, including a desktop computer, laptop computer, tablet computer, smart phone, wearable device, television, camera, video gaming console, set-top box, or video streaming device.

To encode video sequence108into bitstream110, source device102may comprise a video source112, an encoder114, and an output interface116. Video source112may provide or generate video sequence108from a capture of a natural scene and/or a synthetically generated scene. A synthetically generated scene may be a scene comprising computer generated graphics or screen content. Video source112may comprise a video capture device (e.g., a video camera), a video archive comprising previously captured natural scenes and/or synthetically generated scenes, a video feed interface to receive captured natural scenes and/or synthetically generated scenes from a video content provider, and/or a processor to generate synthetic scenes.

A shown inFIG.1, a video sequence, such as video sequence108, may comprise a series of pictures (also referred to as frames). A video sequence may achieve the impression of motion when a constant or variable time is used to successively present pictures of the video sequence. A picture may comprise one or more sample arrays of intensity values. The intensity values may be taken at a series of regularly spaced locations within a picture. A color picture typically comprises a luminance sample array and two chrominance sample arrays. The luminance sample array may comprise intensity values representing the brightness (or luma component, Y) of a picture. The chrominance sample arrays may comprise intensity values that respectively represent the blue and red components of a picture (or chroma components, Cb and Cr) separate from the brightness. Other color picture sample arrays are possible based on different color schemes (e.g., an RGB color scheme). For color pictures, a pixel may refer to all three intensity values for a given location in the three sample arrays used to represent color pictures. A monochrome picture comprises a single, luminance sample array. For monochrome pictures, a pixel may refer to the intensity value at a given location in the single, luminance sample array used to represent monochrome pictures.

Encoder114may encode video sequence108into bitstream110. To encode video sequence108, encoder114may apply one or more prediction techniques to reduce redundant information in video sequence108. Redundant information is information that may be predicted at a decoder and therefore may not be needed to be transmitted to the decoder for accurate decoding of the video sequence. For example, encoder114may apply spatial prediction (e.g., intra-frame or intra prediction), temporal prediction (e.g., inter-frame prediction or inter prediction), inter-layer prediction, and/or other prediction techniques to reduce redundant information in video sequence108. Before applying the one or more prediction techniques, encoder114may partition pictures of video sequence108into rectangular regions referred to as blocks. Encoder114may then encode a block using one or more of the prediction techniques.

For temporal prediction, encoder114may search for a block similar to the block being encoded in another picture (also referred to as a reference picture) of video sequence108. The block determined during the search (also referred to as a prediction block) may then be used to predict the block being encoded. For spatial prediction, encoder114may form a prediction block based on data from reconstructed neighboring samples of the block to be encoded within the same picture of video sequence108. A reconstructed sample refers to a sample that was encoded and then decoded. Encoder114may determine a prediction error (also referred to as a residual) based on the difference between a block being encoded and a prediction block. The prediction error may represent non-redundant information that may be transmitted to a decoder for accurate decoding of a video sequence.

Encoder114may apply a transform to the prediction error (e.g. a discrete cosine transform (DCT)) to generate transform coefficients. Encoder114may form bitstream110based on the transform coefficients and other information used to determine prediction blocks (e.g., prediction types, motion vectors, and prediction modes). In some examples, encoder114may perform one or more of quantization and entropy coding of the transform coefficients and/or the other information used to determine prediction blocks before forming bitstream110to further reduce the number of bits needed to store and/or transmit video sequence108.

Output interface116may be configured to write and/or store bitstream110onto transmission medium104for transmission to destination device106. In addition or alternatively, output interface116may be configured to transmit, upload, and/or stream bitstream110to destination device106via transmission medium104. Output interface116may comprise a wired and/or wireless transmitter configured to transmit, upload, and/or stream bitstream110according to one or more proprietary and/or standardized communication protocols, such as Digital Video Broadcasting (DVB) standards, Advanced Television Systems Committee (ATSC) standards, Integrated Services Digital Broadcasting (ISDB) standards, Data Over Cable Service Interface Specification (DOCSIS) standards, 3rd Generation Partnership Project (3GPP) standards, Institute of Electrical and Electronics Engineers (IEEE) standards, Internet Protocol (IP) standards, and Wireless Application Protocol (WAP) standards.

Transmission medium104may comprise a wireless, wired, and/or computer readable medium. For example, transmission medium104may comprise one or more wires, cables, air interfaces, optical discs, flash memory, and/or magnetic memory. In addition or alternatively, transmission medium104may comprise one more networks (e.g., the Internet) or file servers configured to store and/or transmit encoded video data.

To decode bitstream110into video sequence108for display, destination device106may comprise an input interface118, a decoder120, and a video display122. Input interface118may be configured to read bitstream110stored on transmission medium104by source device102. In addition or alternatively, input interface118may be configured to receive, download, and/or stream bitstream110from source device102via transmission medium104. Input interface118may comprise a wired and/or wireless receiver configured to receive, download, and/or stream bitstream110according to one or more proprietary and/or standardized communication protocols, such as those mentioned above.

Decoder120may decode video sequence108from encoded bitstream110. To decode video sequence108, decoder120may generate prediction blocks for pictures of video sequence108in a similar manner as encoder114and determine prediction errors for the blocks. Decoder120may generate the prediction blocks using prediction types, prediction modes, and/or motion vectors received in bitstream110and determine the prediction errors using transform coefficients also received in bitstream110. Decoder120may determine the prediction errors by weighting transform basis functions using the transform coefficients. Decoder120may combine the prediction blocks and prediction errors to decode video sequence108. In some examples, decoder120may decode a video sequence that approximates video sequence108due to, for example, lossy compression of video sequence108by encoder114and/or errors introduced into encoded bitstream110during transmission to destination device106.

Video display122may display video sequence108to a user. Video display122may comprise a cathode rate tube (CRT) display, liquid crystal display (LCD), a plasma display, light emitting diode (LED) display, or any other display device suitable for displaying video sequence108.

It should be noted that video encoding/decoding system100is presented by way of example and not limitation. In the example ofFIG.1, video encoding/decoding system100may have other components and/or arrangements. For example, video source112may be external to source device102. Similarly, video display122may be external to destination device106or omitted altogether where video sequence is intended for consumption by a machine and/or storage device. In another example, source device102may further comprise a video decoder and destination device106may comprise a video encoder. In such an example, source device102may be configured to further receive an encoded bit stream from destination device106to support two-way video transmission between the devices.

In the example ofFIG.1, encoder114and decoder120may operate according to any one of a number of proprietary or industry video coding standards. For example, encoder114and decoder120may operate according to one or more of International Telecommunications Union Telecommunication Standardization Sector (ITU-T) H.263, ITU-T H.264 and Moving Picture Expert Group (MPEG)-4 Visual (also known as Advanced Video Coding (AVC)), ITU-T H.265 and MPEG-H Part2(also known as High Efficiency Video Coding (HEVC), ITU-T H.265 and MPEG-I Part3(also known as Versatile Video Coding (VVC)), the WebM VP8 and VP9 codecs, and AOMedia Video1(AV1).

FIG.2illustrates an exemplary encoder200in which embodiments of the present disclosure may be implemented. Encoder200encodes a video sequence202into a bitstream204for more efficient storage and/or transmission. Encoder200may be implemented in video coding/decoding system100inFIG.1or in any one of a number of different devices, including a desktop computer, laptop computer, tablet computer, smart phone, wearable device, television, camera, video gaming console, set-top box, or video streaming device. Encoder200comprises an inter prediction unit206, an intra prediction unit208, combiners210and212, a transform and quantization unit (TR+Q) unit214, an inverse transform and quantization unit (iTR+iQ)216, entropy coding unit218, one or more filters220, and a buffer222.

Encoder200may partition the pictures of video sequence202into blocks and encode video sequence202on a block-by-block basis. Encoder200may perform a prediction technique on a block being encoded using either inter prediction unit206or intra prediction unit208. Inter prediction unit206may perform inter prediction by searching for a block similar to the block being encoded in another, reconstructed picture (also referred to as a reference picture) of video sequence202. A reconstructed picture refers to a picture that was encoded and then decoded. The block determined during the search (also referred to as a prediction block) may then be used to predict the block being encoded to remove redundant information. Inter prediction unit206may exploit temporal redundancy or similarities in scene content from picture to picture in video sequence202to determine the prediction block. For example, scene content between pictures of video sequence202may be similar except for differences due to motion or affine transformation of the screen content over time.

Intra prediction unit208may perform intra prediction by forming a prediction block based on data from reconstructed neighboring samples of the block to be encoded within the same picture of video sequence202. A reconstructed sample refers to a sample that was encoded and then decoded. Intra prediction unit208may exploit spatial redundancy or similarities in scene content within a picture of video sequence202to determine the prediction block. For example, the texture of a region of scene content in a picture may be similar to the texture in the immediate surrounding area of the region of the scene content in the same picture.

After prediction, combiner210may determine a prediction error (also referred to as a residual) based on the difference between the block being encoded and the prediction block. The prediction error may represent non-redundant information that may be transmitted to a decoder for accurate decoding of a video sequence.

Transform and quantization unit214may transform and quantize the prediction error. Transform and quantization unit214may transform the prediction error into transform coefficients by applying, for example, a DCT to reduce correlated information in the prediction error. Transform and quantization unit214may quantize the coefficients by mapping data of the transform coefficients to a predefined set of representative values. Transform and quantization unit214may quantize the coefficients to reduce irrelevant information in bitstream204. Irrelevant information is information that may be removed from the coefficients without producing visible and/or perceptible distortion in video sequence202after decoding.

Entropy coding unit218may apply one or more entropy coding methods to the quantized transform coefficients to further reduce the bit rate. For example, entropy coding unit218may apply context adaptive variable length coding (CAVLC), context adaptive binary arithmetic coding (CABAC), and syntax-based context-based binary arithmetic coding (SBAC). The entropy coded coefficients are packed to form bitstream204.

Inverse transform and quantization unit216may inverse quantize and inverse transform the quantized transform coefficients to determine a reconstructed prediction error. Combiner212may combine the reconstructed prediction error with the prediction block to form a reconstructed block. Filter(s)220may filter the reconstructed block using, for example, a deblocking filter and/or a sample-adaptive offset (SAO) filter. Buffer222may store the reconstructed block for prediction of one or more other blocks in the same and/or different picture of video sequence202.

Although not shown inFIG.2, encoder200further comprises an encoder control unit configured to control one or more of the units of encoder200shown inFIG.2. The encoder control unit may control the one or more units of encoder200such that bitstream204is generated in conformance with the requirements of any one of a number of proprietary or industry video coding standards. For example, The encoder control unit may control the one or more units of encoder200such that bitstream204is generated in conformance with one or more of ITU-T H.263, AVC, HEVC, VVC, VP8, VP9, and AV1 video coding standards.

Within the constraints of a proprietary or industry video coding standard, the encoder control unit may attempt to minimize or reduce the bitrate of bitstream204and maximize or increase the reconstructed video quality. For example, the encoder control unit may attempt to minimize or reduce the bitrate of bitstream204given a level that the reconstructed video quality may not fall below, or attempt to maximize or increase the reconstructed video quality given a level that the bit rate of bitstream204may not exceed. The encoder control unit may determine/control one or more of: partitioning of the pictures of video sequence202into blocks, whether a block is inter predicted by inter prediction unit206or intra predicted by intra prediction unit208, a motion vector for inter prediction of a block, an intra prediction mode among a plurality of intra prediction modes for intra prediction of a block, filtering performed by filter(s)220, and one or more transform types and/or quantization parameters applied by transform and quantization unit214. The encoder control unit may determine/control the above based on how the determination/control effects a rate-distortion measure for a block or picture being encoded. The encoder control unit may determine/control the above to reduce the rate-distortion measure for a block or picture being encoded.

After being determined, the prediction type used to encode a block (intra or inter prediction), prediction information of the block (intra prediction mode if intra predicted, motion vector, etc.), and transform and quantization parameters, may be sent to entropy coding unit218to be further compressed to reduce the bit rate. The prediction type, prediction information, and transform and quantization parameters may be packed with the prediction error to form bitstream204.

It should be noted that encoder200is presented by way of example and not limitation. In other examples, encoder200may have other components and/or arrangements. For example, one or more of the components shown inFIG.2may be optionally included in encoder200, such as entropy coding unit218and filters(s)220.

FIG.3illustrates an exemplary decoder300in which embodiments of the present disclosure may be implemented. Decoder300decodes an bitstream302into a decoded video sequence for display and/or some other form of consumption. Decoder300may be implemented in video coding/decoding system100inFIG.1or in any one of a number of different devices, including a desktop computer, laptop computer, tablet computer, smart phone, wearable device, television, camera, video gaming console, set-top box, or video streaming device. Decoder300comprises an entropy decoding unit306, an inverse transform and quantization (iTR+iQ) unit308, a combiner310, one or more filters312, a buffer314, an inter prediction unit316, and an intra prediction unit318.

Although not shown inFIG.3, decoder300further comprises a decoder control unit configured to control one or more of the units of decoder300shown inFIG.3. The decoder control unit may control the one or more units of decoder300such that bitstream302is decoded in conformance with the requirements of any one of a number of proprietary or industry video coding standards. For example, The decoder control unit may control the one or more units of decoder300such that bitstream302is decoded in conformance with one or more of ITU-T H.263, AVC, HEVC, VVC, VP8, VP9, and AV1 video coding standards.

The decoder control unit may determine/control one or more of: whether a block is inter predicted by inter prediction unit316or intra predicted by intra prediction unit318, a motion vector for inter prediction of a block, an intra prediction mode among a plurality of intra prediction modes for intra prediction of a block, filtering performed by filter(s)312, and one or more inverse transform types and/or inverse quantization parameters to be applied by inverse transform and quantization unit308. One or more of the control parameters used by the decoder control unit may be packed in bitstream302.

Entropy decoding unit306may entropy decode the bitstream302. Inverse transform and quantization unit308may inverse quantize and inverse transform the quantized transform coefficients to determine a decoded prediction error. Combiner310may combine the decoded prediction error with a prediction block to form a decoded block. The prediction block may be generated by inter prediction unit318or inter prediction unit316as described above with respect to encoder200inFIG.2. Filter(s)312may filter the decoded block using, for example, a deblocking filter and/or a sample-adaptive offset (SAO) filter. Buffer314may store the decoded block for prediction of one or more other blocks in the same and/or different picture of the video sequence in bitstream302. Decoded video sequence304may be output from filter(s)312as shown inFIG.3.

It should be noted that decoder300is presented by way of example and not limitation. In other examples, decoder300may have other components and/or arrangements. For example, one or more of the components shown inFIG.3may be optionally included in decoder300, such as entropy decoding unit306and filters(s)312.

It should be further noted that, although not shown inFIGS.2and3, each of encoder200and decoder300may further comprise an intra block copy unit in addition to inter prediction and intra prediction units. The intra block copy unit may perform similar to an inter prediction unit but predict blocks within the same picture. For example, the intra block copy unit may exploit repeated patterns that appear in screen content. Screen content may include, for example, computer generated text, graphics, and animation.

As mentioned above, video encoding and decoding may be performed on a block-by-block basis. The process of partitioning a picture into blocks may be adaptive based on the content of the picture. For example, larger block partitions may be used in areas of a picture with higher levels of homogeneity to improve coding efficiency.

In HEVC, a picture may be partitioned into non-overlapping square blocks, referred to as coding tree blocks (CTBs), comprising samples of a sample array. A CTB may have a size of 2nx2nsamples, where n may be specified by a parameter of the encoding system. For example, n may be 4, 5, or 6. A CTB may be further partitioned by a recursive quadtree partitioning into coding blocks (CBs) of half vertical and half horizontal size. The CTB forms the root of the quadtree. A CB that is not split further as part of the recursive quadtree partitioning may be referred to as a leaf-CB of the quadtree and otherwise as a non-leaf CB of the quadtree. A CB may have a minimum size specified by a parameter of the encoding system. For example, a CB may have a minimum size of 4×4, 8×8, 16×16, 32×32, or 64×64 samples.

For inter and intra prediction, a CB may be further partitioned into one or more prediction blocks (PBs) for performing inter and intra prediction. A PB may be a rectangular block of samples on which the same prediction type/mode may be applied. For transformations, a CB may be partitioned into one or more transform blocks (TBs). A TB may be a rectangular block of samples that may determine an applied transform size.

FIG.4illustrates an example quadtree partitioning of a CTB400.FIG.5illustrates a corresponding quadtree500of the example quadtree partitioning of CTB400inFIG.4. As shown inFIGS.4and5, CTB400is first partitioned into four CBs of half vertical and half horizontal size. Three of the resulting CBs of the first level partitioning of CTB400are leaf-CBs. The three leaf CBs of the first level partitioning of CTB400are respectively labeled7,8, and9inFIGS.4and5. The non-leaf CB of the first level partitioning of CTB400is partitioned into four sub-CBs of half vertical and half horizontal size. Three of the resulting sub-CBs of the second level partitioning of CTB400are leaf CBs. The three leaf CBs of the second level partitioning of CTB400are respectively labeled 0, 5, and 6 inFIGS.4and5. Finally, the non-leaf CB of the second level partitioning of CTB400is partitioned into four leaf CBs of half vertical and half horizontal size. The four leaf CBs are respectively labeled 1, 2, 3, and 4 inFIGS.4and5.

Altogether, CTB400is partitioned into 10 leaf CBs respectively labeled 0-9. The resulting quadtree partitioning of CTB400may be scanned using a z-scan (left-to-right, top-to-bottom) to form the sequence order for encoding/decoding the CB leaf nodes. The numeric label of each CB leaf node inFIGS.4and5may correspond to the sequence order for encoding/decoding, with CB leaf node 0 encoded/decoded first and CB leaf node 9 encoded/decoded last. Although not shown inFIGS.4and5, it should be noted that each CB leaf node may comprise one or more PBs and TBs.

In VVC, a picture may be partitioned in a similar manner as in HEVG. A picture may be first partitioned into non-overlapping square CTBs. The CTBs may then be partitioned by a recursive quadtree partitioning into CBs of half vertical and half horizontal size. In VVC, a quadtree leaf node may be further partitioned by a binary tree or ternary tree partitioning into CBs of unequal sizes.FIG.6illustrates example binary and ternary tree partitions. A binary tree partition may divide a parent block in half in either the vertical direction602or horizontal direction604. The resulting partitions may be half in size as compared to the parent block. A ternary tree partition may divide a parent block into three parts in either the vertical direction606or horizontal direction608. The middle partition may be twice as large as the other two end partitions in a ternary tree partition.

Because of the addition of binary and ternary tree partitioning, in VVC the block partitioning strategy may be referred to as quadtree+multi-type tree partitioning.FIG.7illustrates an example quadtree+multi-type tree partitioning of a CTB700.FIG.8illustrates a corresponding quadtree+multi-type tree800of the example quadtree+multi-type tree partitioning of CTB700inFIG.7. In bothFIGS.7and8, quadtree splits are shown in solid lines and multi-type tree splits are shown in dashed lines. For ease of explanation, CTB700is shown with the same quadtree partitioning as CTB400described inFIG.4. Therefore, description of the quadtree partitioning of CTB700is omitted. The description of the additional multi-type tree partitions of CTB700is made relative to three leaf-CBs shown inFIG.4that have been further partitioned using one or more binary and ternary tree partitions. The three leaf-CBs inFIG.4that are shown inFIG.7as being further partitioned are leaf-CBs5,8, and9.

Starting with leaf-CB5inFIG.4,FIG.7shows this leaf-CB partitioned into two CBs based on a vertical binary tree partitioning. The two resulting CBs are leaf-CBs respectively labeled 5 and 6 inFIGS.7and8. With respect to leaf-CB8inFIG.4,FIG.7shows this leaf-CB partitioned into three CBs based on a vertical ternary tree partition. Two of the three resulting CBs are leaf-CBs respectively labeled 9 and 14 inFIGS.7and8. The remaining, non-leaf CB is partitioned first into two CBs based on a horizontal binary tree partition, one of which is a leaf-CB labeled 10 and the other of which is further partitioned into three CBs based on a vertical ternary tree partition. The resulting three CBs are leaf-CBs respectively labeled 11, 12, and 13 inFIGS.7and8. Finally, with respect to leaf-CB9inFIG.4,FIG.7shows this leaf-CB partitioned into three CBs based on a horizontal ternary tree partition. Two of the three CBs are leaf-CBs respectively labeled 15 and 19 inFIGS.7and8. The remaining, non-leaf CB is partitioned into three CBs based on another horizontal ternary tree partition. The resulting three CBs are all leaf-CBs respectively labeled 16, 17, and 18 inFIGS.7and8.

Altogether, CTB700is partitioned into 20 leaf CBs respectively labeled 0-19. The resulting quadtree+multi-type tree partitioning of CTB700may be scanned using a z-scan (left-to-right, top-to-bottom) to form the sequence order for encoding/decoding the CB leaf nodes. The numeric label of each CB leaf node inFIGS.7and8may correspond to the sequence order for encoding/decoding, with CB leaf node 0 encoded/decoded first and CB leaf node 19 encoded/decoded last. Although not shown inFIGS.7and8, it should be noted that each CB leaf node may comprise one or more PBs and TBs.

In addition to specifying various blocks (e.g., CTB, CB, PB, TB), HEVC and VVC further define various units. While blocks may comprise a rectangular area of samples in a sample array, units may comprise the collocated blocks of samples from the different sample arrays (e.g., luma and chroma sample arrays) that form a picture as well as syntax elements and prediction data of the blocks. A coding tree unit (CTU) may comprise the collocated CTBs of the different sample arrays and may form a complete entity in an encoded bit stream. A coding unit (CU) may comprise the collocated CBs of the different sample arrays and syntax structures used to code the samples of the CBs. A prediction unit (PU) may comprise the collocated PBs of the different sample arrays and syntax elements used to predict the PBs. A transform unit (TU) may comprise TBs of the different samples arrays and syntax elements used to transform the TBs.

It should be noted that the term block may be used to refer to any of a CTB, CB, PB, TB, CTU, CU, PU, or TU in the context of HEVC and VVC. It should be further noted that the term block may be used to refer to similar data structures in the context of other video coding standards. For example, the term block may refer to a macroblock in AVC, a macroblock or sub-block in VP8, a superblock or sub-block in VP9, or a superblock or sub-block in AV1.

In intra prediction, samples of a block to be encoded (also referred to as the current block) may be predicted from samples of the column immediately adjacent to the left-most column of the current block and samples of the row immediately adjacent to the top-most row of the current block. The samples from the immediately adjacent column and row may be jointly referred to as reference samples. Each sample of the current block may be predicted by projecting the position of the sample in the current block in a given direction (also referred to as an intra prediction mode) to a point along the reference samples. The sample may be predicted by interpolating between the two closest reference samples of the projection point if the projection does not fall directly on a reference sample. A prediction error (also referred to as a residual) may be determined for the current block based on differences between the predicted sample values and the original sample values of the current block.

At an encoder, this process of predicting samples and determining a prediction error based on a difference between the predicted samples and original samples may be performed for a plurality of different intra prediction modes, including non-directional intra prediction modes. The encoder may select one of the plurality of intra prediction modes and its corresponding prediction error to encode the current block. The encoder may send an indication of the selected prediction mode and its corresponding prediction error to a decoder for decoding of the current block. The decoder may decode the current block by predicting the samples of the current block using the intra prediction mode indicated by the encoder and combining the predicted samples with the prediction error.

FIG.9illustrates an example set of reference samples902determined for intra prediction of a current block904being encoded or decoded. InFIG.9, current block904corresponds to block3of partitioned CTB700inFIG.7. As explained above, the numeric labels 0-19 of the blocks of partitioned CTB700may correspond to the sequence order for encoding/decoding the blocks and are used as such in the example ofFIG.9.

Given current block904is of w×h samples in size, reference samples902may extend over 2w samples of the row immediately adjacent to the top-most row of current block904,2hsamples of the column immediately adjacent to the left-most column of current block904, and the top left neighboring corner sample to current block904. In the example ofFIG.9, current block904is square, so w=h=s. For constructing the set of reference samples902, available samples from neighboring blocks of current block904may be used. Samples may not be available for constructing the set of reference samples902if, for example, the samples would lie outside the picture of the current block, the samples are part of a different slice of the current block (where the concept of slices are used), and/or the samples belong to blocks that have been inter coded and constrained intra prediction is indicated. When constrained intra prediction is indicated, intra prediction may not be dependent on inter predicted blocks.

In addition to the above, samples that may not be available for constructing the set of reference samples902include samples in blocks that have not already been encoded and reconstructed at an encoder or decoded at a decoder based on the sequence order for encoding/decoding. This restriction may allow identical prediction results to be determined at both the encoder and decoder. InFIG.9, samples from neighboring blocks0,1, and2may be available to construct reference samples902given that these blocks are encoded and reconstructed at an encoder and decoded at a decoder prior to coding of current block904. This assumes there are no other issues, such as those mentioned above, preventing the availability of samples from neighboring blocks0,1, and2. However, the portion of reference samples902from neighboring block6may not be available due to the sequence order for encoding/decoding.

Unavailable ones of reference samples902may be filled with available ones of reference samples902. For example, an unavailable reference sample may be filled with a nearest available reference sample determined by moving in a clock-wise direction through reference samples902from the position of the unavailable reference. If no reference samples are available, reference samples902may be filled with the mid-value of the dynamic range of the picture being coded.

It should be noted that reference samples902may be filtered based on the size of current block904being coded and an applied intra prediction mode. It should be further noted thatFIG.9illustrates only one exemplary determination of reference samples for intra prediction of a block. In some proprietary and industry video coding standards, reference samples may be determined in a different manner than discussed above. For example, multiple reference lines may be used in other instances, such as used in VVC.

After reference samples902are determined and optionally filtered, samples of current block904may be intra predicted based on reference samples902. Most encoders/decoders support a plurality of intra prediction modes in accordance with one or more video coding standards. For example, HEVC supports35intra prediction modes, including a planar mode, a DC mode, and 33 angular modes. VVC supports67intra prediction modes, including a planar mode, a DC mode, and 65 angular modes. Planar and DC modes may be used to predict smooth and gradually changing regions of a picture. Angular modes may be used to predict directional structures in regions of a picture.

FIG.10Aillustrates the35intra prediction modes supported by HEVC. The35intra prediction modes are identified by indices 0 to 34. Prediction mode 0 corresponds to planar mode. Prediction mode 1 corresponds to DC mode. Prediction modes 2-34 correspond to angular modes. Prediction modes 2-18 may be referred to as horizontal prediction modes because the principal source of prediction is in the horizontal direction. Prediction modes 19-34 may be referred to as vertical prediction modes because the principal source of prediction is in the vertical direction.

FIG.10Billustrates the67intra prediction modes supported by VVC. The67intra prediction modes are identified by indices 0 to 66. Prediction mode 0 corresponds to planar mode. Prediction mode 1 corresponds to DC mode. Prediction modes 2-66 correspond to angular modes. Prediction modes 2-34 may be referred to as horizontal prediction modes because the principal source of prediction is in the horizontal direction. Prediction modes 35-66 may be referred to as vertical prediction modes because the principal source of prediction is in the vertical direction.

Because blocks in VVC may be non-square, some of the intra prediction modes illustrated inFIG.10Bmay be adaptively replaced by wide-angle directions.

To further describe the application of intra prediction modes to determine a prediction of a current block, reference is made toFIGS.11and12. InFIG.11, current block904and reference samples902fromFIG.9are shown in a two-dimensional x, y plane, where a sample may be referenced as p[x][y]. In order to simplify the prediction process, reference samples902may be placed in two, one-dimensional arrays. Reference samples902above current block904may be placed in the one-dimensional array ref1[x]:

Reference samples902to the left of current block904may be placed in the one-dimensional array ref2[x]:

For planar mode, a sample at location [x][y] in current block904may be predicted by calculating the mean of two interpolated values. The first of the two interpolated values may be based on a horizontal linear interpolation at location [x][y] in current block904. The second of the two interpolated values may be based on a vertical linear interpolation at location [x][y] in current block904. The predicted sample p[x][y] in current block904may be calculated as

may be the horizontal linear interpolation at location [x][y] in current block904and

may be the vertical linear interpolation at location [x][y] in current block904.

For DC mode, a sample at location [x][y] in current block904may be predicted by the mean of the reference samples902. The predicted value sample p[x][y] in current block904may be calculated as

For angular modes, a sample at location [x][y] in current block904may be predicted by projecting the location [x][y] in a direction specified by a given angular mode to a point on the horizontal or vertical line of samples comprising reference samples902. The sample at location [x][y] may be predicted by interpolating between the two closest reference samples of the projection point if the projection does not fall directly on a reference sample. The direction specified by the angular mode may be given by an angle9defined relative to the y-axis for vertical prediction modes (e.g., modes 19-34 in HEVC and modes 35-66 in VVC) and relative to the x-axis for horizontal prediction modes (e.g., modes 2-18 in HEVC and modes 2-34 in VVC).

FIG.12illustrates a prediction of a sample at location [x][y] in current block904for a vertical prediction mode 906 given by an angle9. For vertical prediction modes, the location [x][y] in current block904is projected to a point (referred to herein as the “projection point”) on the horizontal line of reference samples ref1[x]. Reference samples902are only partially shown inFIG.12for ease of illustration. Because the projection point falls at a fractional sample position between two reference samples in the example ofFIG.12, the predicted sample p[x][y] in current block904may be calculated by linearly interpolating between the two reference samples as follows

where iiis the integer part of the horizontal displacement of the projection point relative to the location [x][y] and may calculated as a function of the tangent of the angle T of the vertical prediction mode 906 as follows

and ifis the fractional part of the horizontal displacement of the projection point relative to the location [x][y] and may be calculated as

where [·] is the integer floor.

For horizontal prediction modes, the position [x][y] of a sample in current block904may be projected onto the vertical line of reference samples ref2[y]. Sample prediction for horizontal prediction modes is given by:

where iiis the integer part of the vertical displacement of the projection point relative to the location [x][y] and may be calculated as a function of the tangent of the angle p of the horizontal prediction mode as follows

and ifis the fractional part of the vertical displacement of the projection point relative to the location [x][y] and may be calculated as

where [·] is the integer floor.

The interpolation functions of (7) and (10) may be implemented by an encoder or decoder, such as encoder200inFIG.2or decoder300inFIG.3, as a set of two-tap finite impulse response (FIR) filters. The coefficients of the two-tap FIR filters may be respectively given by (1−if) and if. In the above angular intra prediction examples, the predicted sample p[x][y] may be calculated with some predefined level of sample accuracy, such as 1/32 sample accuracy. For 1/32 sample accuracy, the set of two-tap FIR interpolation filters may comprise up to 32 different two-tap FIR interpolation filters—one for each of the32possible values of the fractional part of the projected displacement if. In other examples, different levels of sample accuracy may be used.

In an embodiment, the two-tap interpolation FIR filter may be used for predicting chroma samples. For luma samples, a different interpolation technique may be used. For example, for luma samples a four-tap FIR filter may be used to determine a predicted value of a luma sample. For example, the four tap FIR filter may have coefficients determined based on if, similar to the two-tap FIR filter. For 1/32 sample accuracy, a set of 32 different four-tap FIR filters may comprise up to 32 different four-tap FIR filters—one for each of the32possible values of the fractional part of the projected displacement if. In other examples, different levels of sample accuracy may be used. The set of four-tap FIR filters may be stored in a look-up table (LUT) and referenced based on if. The value of the predicted sample p[x][y], for vertical prediction modes, may be determined based on the four-tap FIR filter as follows:

where ft[i], i=0 . . . 0.3, are the filter coefficients. The value of the predicted sample p[x][y], for horizontal prediction modes, may be determined based on the four-tap FIR filter as follows:

It should be noted that supplementary reference samples may be constructed for the case where the position [x][y] of a sample in current block904to be predicted is projected to a negative x coordinate, which happens with negative vertical prediction angles9. The supplementary reference samples may be constructed by projecting the reference samples in ref2[y] in the vertical line of reference samples902to the horizontal line of reference samples902using the negative vertical prediction angle9. Supplemental reference samples may be similarly for the case where the position [x][y] of a sample in current block904to be predicted is projected to a negative y coordinate, which happens with negative horizontal prediction angles9. The supplementary reference samples may be constructed by projecting the reference samples in ref1[x] on the horizontal line of reference samples902to the vertical line of reference samples902using the negative horizontal prediction angle9.

An encoder may predict the samples of a current block being encoded, such as current block904, for a plurality of intra prediction modes as explained above. For example, the encoder may predict the samples of the current block for each of the35intra prediction modes in HEVC or67intra prediction modes in VVC. For each intra prediction mode applied, the encoder may determine a prediction error for the current block based on a difference (e.g., sum of squared differences (SSD), sum of absolute differences (SAD), or sum of absolute transformed differences (SATD)) between the prediction samples determined for the intra prediction mode and the original samples of the current block.

The encoder may select one of the intra prediction modes to encode the current block based on the determined prediction errors. For example, the encoder may select an intra prediction mode that results in the smallest prediction error for the current block. In another example, the encoder may select the intra prediction mode to encode the current block based on a rate-distortion measure (e.g., Lagrangian rate-distortion cost) determined using the prediction errors. The encoder may send an indication of the selected intra prediction mode and its corresponding prediction error to a decoder for decoding of the current block.

Similar to an encoder, a decoder may predict the samples of a current block being decoded, such as current block904, for an intra prediction modes as explained above. For example, the decoder may receive an indication of an angular intra prediction mode from an encoder for a block. The decoder may construct a set of reference samples and perform intra prediction based on the angular intra prediction mode indicated by the encoder for the block in a similar manner as discussed above for the encoder. The decoder would add the predicted values of the samples of the block to a residual of the block to reconstruct the block. In another embodiment, the decoder may not receive an indication of an angular intra prediction mode from an encoder for a block. Instead, the decoder may determine an intra prediction mode through other, decoder-side means.

Although the description above was primarily made with respect to intra prediction modes in HEVC and VVC, it will be understood that the techniques of the present disclosure described above and further below may be applied to other intra prediction modes, including those of other video coding standards like VP8, VP9, AV1, and the like.

As explained above, intra prediction may exploit correlations between spatially neighboring samples in the same picture of a video sequence to perform video compression. Inter prediction is another coding tool that may be used to exploit correlations in the time domain between blocks of samples in different pictures of the video sequence to perform video compression. In general, an object may be seen across multiple pictures of a video sequence. The object may move (e.g., by some translation and/or affine motion) or remain stationary across the multiple pictures. A current block of samples in a current picture being encoded may therefore have a corresponding block of samples in a previously decoded picture that accurately predicts the current block of samples. The corresponding block of samples may be displaced from the current block of samples due to movement of an object, represented in both blocks, across the respective pictures of the blocks. The previously decoded picture may be referred to as a reference picture and the corresponding block of samples in the reference picture may be referred to as a reference block or motion compensated prediction. An encoder may use a block matching technique to estimate the displacement (or motion) and determine the reference block in the reference picture.

Similar to intra prediction, once a prediction for a current block is determined and/or generated using inter prediction, an encoder may determine a difference between the current block and the prediction. The difference may be referred to as a prediction error or residual. The encoder may then store and/or signal in a bitstream the prediction error and other related prediction information for decoding or other forms of consumption. A decoder may decode the current block by predicting the samples of the current block using the prediction information and combining the predicted samples with the prediction error.

FIG.13Aillustrates an example of inter prediction performed for a current block1300in a current picture1302being encoded. An encoder, such as encoder200inFIG.2, may perform inter prediction to determine and/or generate a reference block1304in a reference picture1306to predict current block1300. Reference pictures, like reference picture1306, are prior decoded pictures available at the encoder and decoder. Availability of a prior decoded picture may depend on whether the prior decoded picture is available in a decoded picture buffer at the time current block1300is being encoded or decoded. The encoder may, for example, search one or more reference pictures for a reference block that is similar to current block1300. The encoder may determine a “best matching” reference block from the blocks tested during the searching process as reference block1304. The encoder may determine that reference block1304is the best matching reference block based on one or more cost criterion, such as a rate-distortion criterion (e.g., Lagrangian rate-distortion cost). The one or more cost criterion may be based on, for example, a difference (e.g., sum of squared differences (SSD), sum of absolute differences (SAD), or sum of absolute transformed differences (SATD)) between the prediction samples of reference block1304and the original samples of current block1300.

The encoder may search for reference block1304within a search range1308. Search range1308may be positioned around the collocated position (or block)1310of current block1300in reference picture1306. In some instances, search range1308may at least partially extend outside of reference picture1306. When extending outside of reference picture1306, constant boundary extension may be used such that the values of the samples in the row or column of reference picture1306, immediately adjacent to the portion of search range1308extending outside of reference picture1306, are used for the “sample” locations outside of reference picture1306. All or a subset of potential positions within search range1308may be searched for reference block1304. The encoder may utilize any one of a number of different search implementations to determine and/or generate reference block1304. For example, the encoder may determine a set of a candidate search positions based on motion information of neighboring blocks to current block1300.

One or more reference pictures may be searched by the encoder during inter prediction to determine and/or generate the best matching reference block. The reference pictures searched by the encoder may be included in one or more reference picture lists. For example, in HEVC and VVC, two reference picture lists may be used, a reference picture list0and a reference picture list1. A reference picture list may include one or more pictures. Reference picture1306of reference block1304may be indicated by a reference index pointing into a reference picture list comprising reference picture1306.

The displacement between reference block1304and current block1300may be interpreted as an estimate of the motion between reference block1304and current block1300across their respective pictures. The displacement may be represented by a motion vector1312. For example, motion vector1312may be indicated by a horizontal component (MVx) and a vertical component (MVy) relative to the position of current block1300.FIG.13Billustrates the horizontal component and vertical component of motion vector1312. A motion vector, such as motion vector1312, may have fractional or integer resolution. A motion vector with fractional resolution may point between two samples in a reference picture to provide a better estimation of the motion of current block1300. For example, a motion vector may have ½, ¼, ⅛, 1/16, or 1/32 fractional sample resolution. When a motion vector points to a non-integer sample value in the reference picture, interpolation between samples at integer positions may be used to generate the reference block and its corresponding samples at fractional positions. The interpolation may be performed by a filter with two or more taps.

Once reference block1304is determined and/or generated for current block1300using inter prediction, the encoder may determine a difference (e.g., a corresponding sample-by-sample difference) between reference block1304and current block1300. The difference may be referred to as a prediction error or residual. The encoder may then store and/or signal in a bitstream the prediction error and the related motion information for decoding or other forms of consumption. The motion information may include motion vector1312and a reference index pointing into a reference picture list comprising reference picture1306. In other instances, the motion information may include an indication of motion vector1312and an indication of the reference index pointing into the reference picture list comprising reference picture1306. A decoder may decode current block1300by determining and/or generating reference block1304, which forms the prediction of current block1300, using the motion information and combining the prediction with the prediction error.

InFIG.13A, inter prediction is performed using one reference picture1306as the source of the prediction for current block1300. Because the prediction for current block1300comes from a single picture, this type of inter prediction is referred to as uni-prediction.FIG.14illustrates another type of inter prediction, referred to as bi-prediction, performed for a current block1400. In bi-prediction, the source of the prediction for a current block1400comes from two pictures. Bi-prediction may be useful, for example, where the video sequence comprises fast motion, camera panning or zooming, or scene changes. Bi-prediction may also be useful to capture fade outs of one scene or fade outs from one scene to another, where two pictures are effectively displayed simultaneously with different levels of intensity.

Whether uni-prediction or both uni-prediction and bi-prediction are available for performing inter prediction may depend on a slice type of current block1400. For P slices, only uni-prediction may be available for performing inter prediction. For B slices, either uni-prediction or bi-prediction may be used. When uni-prediction is performed, an encoder may determine and/or generate a reference block for predicting current block1400from reference picture list0. When bi-prediction is performed, an encoder may determine and/or generate a first reference block for predicting current block1400from reference picture list0and determine and/or generate a second reference block for predicting current block1400from reference picture list1.

InFIG.14, inter-prediction is performed using bi-prediction, where two reference blocks1402and1404are used to predict current block1400. Reference block1402may be in a reference picture of one of reference picture list0or1, and reference block1404may be in a reference picture of the other one of reference picture list0or1. As shown inFIG.14, reference block1402is in a picture that precedes the current picture of current block1400in terms of picture order count (POC), and reference block1402is in a picture that proceeds the current picture of current block1400in terms of POC. In other examples, the reference pictures may both precede or procced the current picture in terms of POC. POC is the order in which pictures are output from, for example, a decoded picture buffer and is the order in which pictures are generally intended to be displayed. However, it should be noted that pictures that are output are not necessarily displayed but may undergo different processing or consumption, such as transcoding. In other examples, the two reference blocks determined and/or generated using bi-prediction may come from the same reference picture. In such an instance, the reference picture may be included in both reference picture list0and reference picture list1.

A configurable weight and offset value may be applied to the one or more inter prediction reference blocks. An encoder may enable the use of weighted prediction using a flag in a picture parameter set (PPS) and signal the weighting and offset parameters in the slice segment header for the current block. Different weight and offset parameters may be signaled for luma and chroma components.

Once reference blocks1402and1404are determined and/or generated for current block1400using inter prediction, the encoder may determine a difference between current block1400and each of reference blocks1402and1404. The differences may be referred to as prediction errors or residuals. The encoder may then store and/or signal in a bitstream the prediction errors and their respective related motion information for decoding or other forms of consumption. The motion information for reference block1402may include motion vector1406and the reference index pointing into the reference picture list comprising the reference picture of reference block1402. In other instances, the motion information for reference block1402may include an indication of motion vector1406and an indication of the reference index pointing into the reference picture list comprising the reference picture of reference block1402. The motion information for reference block1404may include motion vector1408and the reference index pointing into the reference picture list comprising the reference picture of reference block1404. In other instances, the motion information for reference block1404may include an indication of motion vector1408and an indication of the reference index pointing into the reference picture list comprising the reference picture of reference block1404. A decoder may decode current block1400by determining and/or generating reference blocks1402and1404, which together form the prediction of current block1400, using their respective motion information and combining the predictions with the prediction errors.

In HEVC, VVC, and other video compression schemes, motion information may be predictively coded before being stored or signaled in a bit stream. The motion information for a current block may be predictively coded based on the motion information of neighboring blocks of the current block. In general, the motion information of the neighboring blocks is often correlated with the motion information of the current block because the motion of an object represented in the current block is often the same or similar to the motion of objects in the neighboring blocks. Two of the motion information prediction techniques in HEVC and VVC include advanced motion vector prediction (AMVP) and inter prediction block merging.

An encoder, such as encoder200inFIG.2, may code a motion vector using the AMVP tool as a difference between the motion vector of a current block being coded and a motion vector predictor (MVP). An encoder may select the MVP from a list of candidate MVPs. The candidate MVPs may come from previously decoded motion vectors of neighboring blocks in the current picture of the current block or blocks at or near the collocated position of the current block in other reference pictures. Both the encoder and decoder may generate or determine the list of candidate MVPs.

After the encoder selects an MVP from the list of candidate MVPs, the encoder may signal, in a bitstream, an indication of the selected MVP and a motion vector difference (MVD). The encoder may indicate the selected MVP in the bitstream by an index pointing into the list of candidate MVPs. The MVD may be calculated based on the difference between the motion vector of the current block and the selected MVP. For example, for a motion vector represented by a horizontal component (MVx) and a vertical displacement (MVy) relative to the position of the current block being coded, the MVD may be represented by two components calculated as follows:

where MVDxand MVDyrespectively represent the horizontal and vertical components of the MVD, and MVPxand MVPyrespectively represent the horizontal and vertical components of the MVP. A decoder, such as decoder300inFIG.3, may decode the motion vector by adding the MVD to the MVP indicated in the bitstream. The decoder may then decode the current block by determining and/or generating the reference block, which forms the prediction of the current block, using the decoded motion vector and combining the prediction with the prediction error.

In HEVC and VVC, the list of candidate MVPs for AMVP may comprise two candidates referred to as candidates A and B. Candidates A and B may include up to two spatial candidate MVPs derived from five spatial neighboring blocks of the current block being coded, one temporal candidate MVP derived from two temporal, co-located blocks when both spatial candidate MVPs are not available or are identical, or zero motion vectors when the spatial, temporal, or both candidates are not available.FIG.15Aillustrates the location of the five spatial candidate neighboring blocks relative to a current block1500being encoded. The five spatial candidate neighboring blocks are respectively denoted A0, A1, B0, B1, and B2.FIG.15Billustrates the location of the two temporal, co-located blocks relative to current block1500being coded. The two temporal, co-located blocks are denoted C0and C1and are included in a reference picture that is different from the current picture of current block1500.

An encoder, such as encoder200inFIG.2, may code a motion vector using the inter prediction block merging tool also referred to as merge mode. Using merge mode, the encoder may reuse the same motion information of a neighboring block for inter prediction of a current block. Because the same motion information of a neighboring block is used, no MVD needs to be signaled and the signaling overhead for signaling the motion information of the current block may be small in size. Similar to AMVP, both the encoder and decoder may generate a candidate list of motion information from neighboring blocks of the current block. The encoder may then determine to use (or inherit) the motion information of one neighboring block's motion information in the candidate list for predicting the motion information of the current block being coded. The encoder may signal, in the bit stream, an indication of the determined motion information from the candidate list. For example, the encoder may signal an index pointing into the list of candidate motion information to indicate the determined motion information.

In HEVC and VVC, the list of candidate motion information for merge mode may comprise up to four spatial merge candidates that are derived from the five spatial neighboring blocks used in AMVP as shown inFIG.15A, one temporal merge candidate derived from two temporal, co-located blocks used in AMVP as shown inFIG.15B, and additional merge candidates including bi-predictive candidates and zero motion vector candidates.

It should be noted that inter prediction may be performed in other ways and variants than those described above. For example, motion information prediction techniques other than AMVP and merge mode are possible. In addition, although the description above was primarily made with respect to inter prediction modes in HEVC and VVC, it will be understood that the techniques of the present disclosure described above and further below may be applied to other inter prediction modes, including those of other video coding standards like VP8, VP9, AV1, and the like. In addition, history based motion vector prediction (HMVP), combined intralinter prediction mode (ClIP), and merge mode with motion vector difference (MMVD) as described in VVC may also be performed and are within the scope of the present disclosure.

In inter prediction, a block matching technique may be applied to determine a reference block in a different picture than the current block being encoded. Block matching techniques have also been applied to determine a reference block in the same picture as a current block being encoded. However, it has been determined that for camera-captured videos, a reference block in the same picture as the current block determined using block matching may often not accurately predict the current block. For screen content video this is generally not the case. Screen content video may include, for example, computer generated text, graphics, and animation. Within screen content, there is often repeated patterns (e.g., repeated patterns of text and graphics) within the same picture. Therefore, a block matching technique applied to determine a reference block in the same picture as a current block being encoded may provide efficient compression for screen content video.

HEVC and VVC both include a prediction technique to exploit the correlation between blocks of samples within the same picture of screen content video. This technique is referred to as intra block (IBC) or current picture referencing (CPR). Similar to inter prediction, an encoder may apply a block matching technique to determine a displacement vector (referred to as a block vector (BV)) that indicates the relative displacement from the current block to a reference block (or intra block compensated prediction) that “best matches” the current block. The encoder may determine the best matching reference block from blocks tested during a searching process similar to inter prediction. The encoder may determine that a reference block is the best matching reference block based on one or more cost criterion, such as a rate-distortion criterion (e.g., Lagrangian rate-distortion cost). The one or more cost criterion may be based on, for example, a difference (e.g., sum of squared differences (SSD), sum of absolute differences (SAD), sum of absolute transformed differences (SATD), or difference determined based on a hash function) between the prediction samples of the reference block and the original samples of the current block. A reference block may correspond to prior decoded blocks of samples of the current picture. The reference block may comprise decoded blocks of samples of the current picture prior to being processed by in-loop filtering operations, like deblocking or SAO filtering.FIG.16illustrates an example of IBC applied for screen content. The rectangular portions with arrows beginning at their boundaries are current blocks being encoded and the rectangular portions that the arrows point to are the reference blocks for predicting the current blocks.

Once a reference block is determined and/or generated for a current block using IBC, the encoder may determine a difference (e.g., a corresponding sample-by-sample difference) between the reference block and the current block. The difference may be referred to as a prediction error or residual. The encoder may then store and/or signal in a bitstream the prediction error and the related prediction information for decoding or other forms of consumption. The prediction information may include a BV. In other instances, the prediction information may include an indication of the BV. A decoder, such as decoder300inFIG.3, may decode the current block by determining and/or generating the reference block, which forms the prediction of the current block, using the prediction information and combining the prediction with the prediction error.

In HEVC, VVC, and other video compression schemes, a BV may be predictively coded before being stored or signaled in a bit stream. The BV for a current block may be predictively coded based on the BV of neighboring blocks of the current block. For example, an encoder may predictively code a BV using the merge mode as explained above for inter prediction or a similar technique as AMVP also explained above for inter prediction. The technique similar to AMVP may be referred to as BV prediction and difference coding.

For BV prediction and difference coding, an encoder, such as encoder200inFIG.2, may code a BV as a difference between the BV of a current block being coded and a BV predictor (BVP). An encoder may select the BVP from a list of candidate BVPs. The candidate BVPs may come from previously decoded BVs of neighboring blocks of the current block in the current picture. Both the encoder and decoder may generate or determine the list of candidate BVPs.

After the encoder selects a BVP from the list of candidate BVPs, the encoder may signal, in a bitstream, an indication of the selected BVP and a BV difference (BVD). The encoder may indicate the selected BVP in the bitstream by an index pointing into the list of candidate BVPs. The BVD may be calculated based on the difference between the BV of the current block and the selected BVP. For example, for a BV represented by a horizontal component (BVx) and a vertical component (BVy) relative to the position of the current block being coded, the BVD may represented by two components calculated as follows:

where BVDxand BVDyrespectively represent the horizontal and vertical components of the BVD, and BVPxand BVPyrespectively represent the horizontal and vertical components of the BVP. A decoder, such as decoder300inFIG.3, may decode the BV by adding the BVD to the BVP indicated in the bitstream. The decoder may then decode the current block by determining and/or generating the reference block, which forms the prediction of the current block, using the decoded BV and combining the prediction with the prediction error.

In HEVC and VVC, the list of candidate BVPs may comprise two candidates referred to as candidates A and B. Candidates A and B may include up to two spatial candidate BVPs derived from five spatial neighboring blocks of the current block being encoded, or one or more of the last two coded BVs when spatial neighboring candidates are not available (e.g., because they are coded in intra or inter mode). The location of the five spatial candidate neighboring blocks relative to a current block being encoded using IBC are the same as those shown inFIG.15Afor inter prediction. The five spatial candidate neighboring blocks are respectively denoted A0, A1, B0, B1, and B2.

In the above description ofFIGS.9-12, intra prediction was used to determine a predicted value of a sample based on a reference line directly adjacent to the current block of the sample. In VVC, multiple reference line (MRL) coding was introduced to allow not only the reference line directly adjacent to a current block being encoded or decoded to be used for intra prediction but also non-directly adjacent reference lines. MRL coding was introduced in VVC because the reference line directly adjacent to the current block being encoded or decoded may not be strongly correlated with the samples of the current block due to, for example, a discontinuity in the content of the directly adjacent reference line. In such a case, a non-directly adjacent reference line may be used to perform intra prediction of a current block to reduce the prediction error.

FIG.17illustrates an example set of multiple reference lines0-2determined for MRL coding of a current block1702in accordance with embodiments of the present disclosure. When referring to a reference line, such as reference line0or1, the number of the reference line may indicate an index of the reference line. For example, reference line0has an index of 0, and reference line1has an index of 1. In the example ofFIG.17, current block1702is w×h samples in size.

Reference line0, which is the reference line directly adjacent to current block1702, comprises a sample segment E with2wsamples in the row immediately adjacent to the top-most row of current block1702, a sample segment B with2hsamples in the column immediately adjacent to the left-most column of current block1702, and the top-left directly neighboring corner sample to current block1702shown with hatching. Reference line0has a sample length of L=2w+2h+1 and is marked with a dashed line.

Reference line n (n>0), which refers to the reference line removed from current block1702by n samples, comprises a sample segment E with2wsamples in the row n-samples above the top-most row of current block1702, a sample segment B with2hsamples in the column n-samples to the left of the left-most column of current block1702, sample segments D and F with n samples respectively to the left and right of segment E, sample segments A and C with n samples respectively below and above segment B, and a neighboring corner sample above segment C and to the left of segment D shown with hatching. Reference line n has a sample length of L=2w+2h+1+4n. Each reference line is marked with a dashed line.

The samples in the reference lines may come from reconstructed samples of neighboring blocks of current block1702. In one embodiment, the samples of segments A and F may, instead of coming from reconstructed samples, be padded with the closest samples available from segments B and E, respectively.

FIG.17provides only one example of multiple reference lines for coding of current block1702. In other examples, the multiple reference lines may have more or less reference samples than shown inFIG.17(e.g., for wide angle prediction). In addition, in other examples, more, less, or different reference lines may be constructed and used for intra prediction of current block1702. For example, the set of discontinuous reference lines 0, 1, and 3 (where reference line 3 is three samples removed from current block1702) may be used for intra prediction of current block1702. Reference line 2 may not be used because reference line 2 is often highly correlated with reference line 1 adjacent to reference line 2. The often high correlation of reference line 2 to reference line 1 already used for intra prediction of current block1702limits the ability of reference line 2 to improve intra prediction. Therefore, reference line 2 may be excluded and the set of discontinuous reference lines 0, 1, and 3 may be used instead.

Although a set of discontinuous reference lines, such as reference lines 0, 1, and 3 may be used to perform intra prediction of blocks within a sequence of pictures in existing technologies, a static set of discontinuous reference lines may not improve coding gain in all instances and, in some instances, actually decrease coding gain. For example, in some instances, reference line 2 may not be highly correlated with reference line 1 and may be used to provide a more accurate prediction of samples of a current block than reference line 3. This situation may occur frequently given that reference lines located in closer proximity to the current block being coded often provided better prediction results than reference lines farther away. In other instances, reference line 3 may also be highly correlated with reference lines 1 and 2. Therefore, replacement of reference line 2 by reference line 3 may provide little, if any, coding gain relative to reference line 1. Instead, a more distant reference line from the current block being coded than reference line 3 may provide better prediction results.

Embodiments of the present disclosure are directed to methods and apparatuses for dynamically adapting a reference line set used to perform intra prediction of a current block being encoded or decoded. Embodiments of the present disclosure may dynamically adapt the reference line set based on a measure of correlation between reference lines of the current block. For higher levels of correlation between reference lines of the current block, embodiments of the present disclosure may use a larger reference line offset between reference lines in the set of reference lines. Embodiments of the present disclosure may determine the correlation between reference lines of the current block based on at least one of: a property of the block being coded (e.g., a size of the block or a resolution of the picture of the block), or a property of reference lines of the block being coded. Because the reference line offset is determined based on a measure of correlation between reference lines of the current block and not a static reference line set configuration (e.g., reference lines 0, 1, 2 or reference lines 0, 1, 3), the set of reference lines used to perform intra prediction of the current block may result in more accurate predictions of the samples of the current block. These and other features of the present disclosure are described further below.

FIG.18illustrates an example intra prediction performed for a current block1802being encoded in accordance with embodiments of the present disclosure. Current block1802may be part of a picture, or sequence of pictures, representing a natural scene and/or a synthetically generated scene. When referring to a reference line, such as reference line 0 or 1, the number of the reference line may indicate an index of the reference line. For example, reference line 0 has an index of 0, and reference line 1 has an index of 1. The intra prediction ofFIG.18may be performed by an encoder, such as encoder200inFIG.2. In the example ofFIG.18, current block1802is w×h samples in size.

The encoder may determine a set of reference lines 0-2 for intra prediction of current block1802. Reference line 0, which is the reference line directly adjacent to current block1802, comprises a sample segment E with 2 w samples in the row immediately adjacent to the top-most row of current block1802, a sample segment B with 2h samples in the column immediately adjacent to the left-most column of current block1802, and the top-left directly neighboring corner sample to current block1802shown with hatching. Reference line 0 has a sample length of L=2w+2h+1 and is marked with a dashed line.

Reference line n (n>0), which refers to the reference line removed from current block1802by n samples, comprises a sample segment E with 2w samples in the row n-samples above the top-most row of current block1802, a sample segment B with 2h samples in the column n-samples to the left of the left-most column of current block1802, sample segments D and F with n samples respectively to the left and right of segment E, sample segments A and C with n samples respectively below and above segment B, and a neighboring corner sample above segment C and to the left of segment D shown with hatching. Reference line n has a sample length of L=2w+2h+1+4n. Each reference line is marked with a dashed line.

The samples in the reference lines may come from reconstructed samples of neighboring blocks of current block1802. In one embodiment, the samples of segments A and F may, instead of coming from reconstructed samples, be padded with the closest samples available from segments B and E, respectively.

The encoder may predict each sample of current block1802by projecting the location of the sample in current block1802in a given direction specified by an intra prediction mode to a point (referred to herein as a “projection point”) on the horizontal or vertical line of one of reference lines 0-2.FIG.18illustrates an example projection of a location [x][y] of a sample in current block1802. The encoder may project the location [x][y] in a direction1806specified by a vertical intra prediction mode. Direction1806may be given by an angle9defined relative to the y-axis for the vertical prediction mode. In other examples, a horizontal intra prediction mode may be used, in which case the angle9may be defined relative to the x-axis. The location [x][y] may be projected to a projection point1808on the horizontal line of reference line 0, a projection point1810on the horizontal line of reference line 1, or a projection point1812of the horizontal line reference line 2. If the projection point falls directly on a reference sample of the reference line to which it was projected, the value of the reference sample may be used as the predicted value for the sample. If the projection point falls at a fractional sample position between two reference samples on the reference line to which it was projected, the encoder may apply an interpolation filter to one or more of the reference samples available at integer sample positions on each side of the projection point. The interpolation filter may filter the reference samples to interpolate a value at the fractional sample position of the projection point. For example, a two-tap interpolation filter may be used for chroma samples and a four-tap interpolation filter may be used for luma samples. The encoder may determine the predicted sample value based on the interpolated value. The encoder may determine a prediction error for current block1802based on a difference (e.g., sum of squared differences (SSD), sum of absolute differences (SAD), or sum of absolute transformed differences (SATD)) between the predicted samples, determined for the given intra prediction mode and reference line as explained above, and the original samples of current block1802.

The encoder may perform this process of predicting samples and determining a prediction error, based on a difference between the predicted samples and original samples, for a plurality of different combinations of intra prediction modes and reference lines. For example, the encoder may perform this process for each combination of the67intra prediction modes in VVC, including non-angular (e.g., DC and planar) intra prediction modes, and reference lines 0-2. With 67 intra prediction modes and 3 reference lines, there are a total of 67*3 or 201 different intra prediction mode and reference line combinations. Thus, the encoder will determine a total of 67*3 or 201 predictions and prediction errors of current block1802if all such combinations are tested.

The encoder may select an intra prediction mode and reference line combination to encode current block1802based on the determined prediction errors. For example, the encoder may select an intra prediction mode and reference line combination that results in the smallest prediction error for current block1802. In another example, the encoder may select the intra prediction mode and reference line combination to encode current block1802based on a rate-distortion measure determined using the prediction errors. The encoder may send an indication of a selected intra prediction mode, a selected reference line, and a corresponding prediction error for the selected intra prediction mode and selected reference line combination to a decoder for decoding of current block1802. The encoder may send the indication of the selected reference line by signaling the index of the reference line. For example, the encoder may send index0in the bitstream for current block1802based on the encoder selecting reference line 0 as the selected reference line.

Angular intra prediction using multiple reference lines may be performed in the same manner as described above with respect toFIG.12except that the equations described above with respect toFIG.12may be generalized for any reference line. For example, equations (1) and (2), which are used to simplify the prediction process by placing the reference samples in two, one-dimensional arrays, may be rewritten as:

where refldx is the reference line index of the reference line used for intra prediction. In addition, equations (8) and (9) for determining the integer and fractional part of the displacement for vertical intra prediction modes may be rewritten as:

and equations (10) and (11) for determining the integer and fractional part of the displacement for horizontal intra prediction modes may be rewritten as:

Equations 19-24 provide one exemplary method for performing angular intra prediction using multiple reference lines. In other examples, different equations may be used to perform angular intra prediction.

It should be noted that, although the example intra prediction ofFIG.18was described above as being performed at an encoder, a decoder may also perform a similar intra prediction. For example, a decoder may receive, from an encoder for a block, an indication of an intra prediction mode, an indication of a reference line from a plurality of reference lines (e.g., reference lines 0-2 inFIG.18or those tested by the encoder during intra prediction), and a residual. The decoder may construct the indicated reference line and perform intra prediction based on the indicated intra prediction mode for the block in a similar manner as discussed above for the encoder. The decoder may add the predicted values of the samples of the block to the residual to reconstruct the block. In an embodiment, the decoder may not receive one or both of the indication of the intra prediction mode and the indication of the reference line from the encoder for the block. Instead, the decoder may determine one or both of the intra prediction mode and reference line through other means, such as decoder side means.

Although the use of multiple reference lines may increase coding gain, the complexity of the encoder also increases because the additional lines need to be tested during intra prediction. To reduce encoder complexity, the additional reference lines may be only tested for a subset of the intra prediction modes, such as those in a most probable mode (MPM) list. The MPM list may be adaptively generated for current block1802based on, for example, the availability and indices of intra prediction modes of the top and left neighboring blocks of current block1802. The MPM list may be constructed in accordance with HEVC, VVC, or any other video coding standard. Moreover, the planar and DC intra prediction modes may be only applied to the reference line directly adjacent to current block1802(i.e., reference line 0) to further reduce encoder complexity. In such a case, a separate angular mode only MPM list may be constructed that includes only angular intra prediction modes. The additional reference lines may be only tested for the intra prediction modes in the angular mode only MPM list when such a list is used.

In an embodiment, instead of using a static configuration of reference lines, such as reference lines 0-2 as shown inFIG.18or a static discontinuous configuration of reference lines, such as reference lines 0, 1, and 3, an encoder or decoder may dynamically adapt the reference line set used to perform intra prediction of a current block being encoded or decoded. In an embodiment, the encoder or decoder may dynamically adapt the reference line set using a reference line offset. The reference line offset may be determined based on a measure of correlation between reference lines of the current block. For higher levels of correlation between reference lines of the current block, the encoder or decoder may use a larger reference line offset between reference lines in the set of reference lines. The encoder or decoder may determine the correlation between reference lines of the current block based on at least one of a property of the current block being encoded or decoded (e.g., a size of the current block or a resolution of the picture of the current block) or a property of reference lines of the current block. Because the reference line offset is determined based on a measure of correlation between reference lines of the current block, the set of reference lines used to perform intra prediction of the current block may result in more accurate predictions of the samples of the current block.

FIG.19illustrates one example measure of correlation between reference lines of a current block1902in a picture1904that may be used to dynamically adapt the reference line set used to perform intra prediction in accordance with embodiments of the present disclosure.

The example measure of correlation inFIG.19is specifically determined based on a resolution of picture1904. In an embodiment, resolution refers to a number of samples in a picture, such as luma samples. In another embodiment, resolution refers to a number of pixels in a picture. Resolution may be represented based on the number of horizontal samples or pixels and the number of vertical samples or pixels. For example, a picture with a resolution of 1920×1080 samples may have 1920 samples or pixels in the vertical direction and 1080 samples or pixels in the horizontal direction. InFIG.19, four different example resolutions of picture1904are illustrated—picture1904-A with a resolution of 480p or 720×480, picture1904-B with a resolution of 720p or 1280×720, picture1904-C with a resolution of 1080i/p or 1920×1080, and 1904-D with a resolution of 2160p (4K) or 3840×2160. Within each example picture1904A-D, a same size and similarly positioned current block1902is illustrated—current block1902-A within picture1904-A, current block1902-B within picture1904-B, current block1902-C within picture1904-C, and current block1902-D within picture1904-D.

As can be seen fromFIG.19, as the resolution of the picture increases, the amount of spatial correlation between neighboring samples within a same size current block increases in general. For example, the spatial correlation between neighboring samples of current block1902-D in highest-resolution picture1904-D is greater than the spatial correlation between neighboring samples of current block1902-A in lowest resolution picture1904-A. Current block1902-D includes samples with very similar color and content, whereas current block1902-A includes samples with a wider variety of colors and content. In addition, neighboring samples (e.g., samples above and to the left) of current block1902-D have a higher spatial correlation with the samples of current block1902-D than neighboring samples of current block1902-A do with samples of current block1902-A. This general trend of increasing spatial correlation between neighboring samples for a given block size as resolution of the picture of the block increases holds true for current blocks1902-B and 1902-C as well.

Accordingly, in an embodiment, an encoder or decoder, such as encoder200inFIG.2or decoder300inFIG.3, may determine a resolution of a picture of a current block being encoded or decoded and use the resolution to determine a spatial correlation between reference lines. For example, the encoder or decoder may determine, for a given size of the current block or independent from the given size of the current block, an increasing value of the reference line offset as the resolution of the picture of the current block increases. For example, the encoder or decoder may determine, for a given size of the current block or independent from the given size of the current block, an increasing value of the reference line offset as the resolution of the picture increases above one or more thresholds. For example, the encoder or decoder may use two thresholds of different resolution values. If the resolution of the picture of the current block is below a first one of the thresholds, the encoder or decoder may use a first value for the reference line offset. If the resolution of the picture of the current block exceeds the first one of the thresholds but is less than the second one of the thresholds, the encoder or decoder may use a second value for the reference line offset that is greater than the first value for the reference line offset. Finally, if the resolution of the picture of the current block exceeds the second one of the thresholds, the encoder or decoder may use a third value for the reference line offset that is greater than the second value for the reference line offset. In an embodiment, one or more of the following resolutions, or values, may be used as a threshold: the value of the product of 720×480, the value of the product of 1280×720, the value of the product of 1920×1080, and the value of the product of 3840×2160. In another embodiment, one of the vertical or horizontal components of a resolution may be used as a threshold. For example, for a resolution of 720×480, either720or480may be used as a threshold. In another embodiment, an index indicating a resolution may be used as a threshold. For example, an index1may be used to indicate 720×480, an index2may be used to indicate 1280×720, an index3may be used to indicate 1920×1080, and an index4may be used indicate 3840×2160. The encoder or decoder may use the reference line offset to determine a position of a reference line or distance between reference lines within the set of reference lines used to perform intra prediction of the current block.

In addition, or as an alternative to the above resolution approach, the encoder or decoder may use a size of a current block as another example measure of correlation between reference lines. In general, an encoder may determine larger block sizes, during partitioning of a picture into blocks, for areas of the picture with high spatial correlation between samples. The encoder or decoder may therefore use the size of the current block to dynamically adapt the reference line set used to perform intra prediction of the current block. The size of the current block may be indicated based on the height, width, or both the height and width of the current block. For example, the size of the current block may be indicated based on (log2w+log2h)/2, where w is the width of the current block in terms of samples and h is the height of the current block in terms of samples.

Accordingly, in an embodiment, an encoder or decoder, such as encoder200inFIG.2or decoder300inFIG.3, may use a size of a current block being encoded or decoded to determine a spatial correlation between reference lines. For example, the encoder or decoder may determine, for a given picture resolution or independent from the given picture resolution, an increasing value of the reference line offset as the size of the current block increases. For example, the encoder or decoder may determine, an increasing value of the reference line offset as the size of the current block increases above one or more thresholds. For example, the encoder or decoder may use two thresholds of different block size values. If the size of the current block is below a first one of the thresholds, the encoder or decoder may use a first value for the reference line offset. If the size of the current block exceeds the first one of the thresholds but is less than the second one of the thresholds, the encoder or decoder may use a second value for the reference line offset that is greater than the first value for the reference line offset. Finally, if the size of the current block exceeds the second one of the thresholds, the encoder or decoder may use a third value for the reference line offset that is greater than the second value for the reference line offset. In another embodiment, an index indicating a size of a current block may be used as a threshold. For example, an index1may be used to indicate a first block size, an index2may be used to indicate a second block size, etc. The encoder or decoder may use the reference line offset to determine a position of a reference line or distance between reference lines within the set of reference lines used to perform intra prediction of the current block.

In yet another embodiment, the encoder or decoder may determine a difference between reference lines of a current block (e.g., a difference between reference lines 0 and 1, 1 and 2, or 0 and 2) as another example measure of correlation between reference lines. For example, the encoder or decoder may calculate the difference between respective reference samples of different reference lines of the current block. In an embodiment, determining a correlation between the reference lines comprises obtaining a sum of absolute differences between the samples of reference lines. In an embodiment, the differences are obtained for pairs of two closest reference lines; for example, line 0 and line1, line 1 and line 2, line 2 and line 3, etc. In an embodiment, the differences are obtained for the projection points (1808, 1810 and 1812 shown inFIG.18), the values in projection points are obtained from the samples of reference lines by interpolation. In an embodiment, determining a correlation between the reference lines comprises obtaining a sum of absolute transformed differences, the differences are calculated as disclosed above. In an embodiment, determining a correlation between the reference lines comprises obtaining a sum of squared differences, the differences are obtained as disclosed above. The encoder or decoder may compare the difference to a threshold. If the difference exceeds the threshold, the encoder or decoder may use a first reference line offset. If the difference is less than the threshold, the encoder or decoder may use a second reference line offset that is greater than the first reference line offset.

FIG.20illustrates one example determination of a reference line offset2002and the application of reference line offset2002to determine a set of reference lines for performing intra prediction of current block1802inFIG.18in accordance with embodiments of the present disclosure. The determination of reference line offset2002and the application of reference line offset2002to determine a set of reference lines for performing intra prediction of current block1802may be performed by an encoder, such as encoder200inFIG.2, or the encoder described above with respect toFIG.18.

InFIG.20, the encoder may use reference line offset2002to adjust a most distance reference line from current block1802. More specifically, the encoder may use a set of reference lines comprising reference lines 0, 1, and N, where N is greater than 1 and determined based on reference line offset2002, to perform intra prediction of current block1802as described above with respect toFIG.18. In an embodiment, reference line offset2002may indicate a distance (e.g., in terms of number of samples) between reference line 1 and reference line N. For example, reference line offset2002may indicate a value between 0 and P, where P is an integer value. Reference line N may be determined based on the summation of2plus the value indicated by reference line offset2002. For example, if reference line offset2002indicates a value equal to 0, reference line N is equal to 2. In this instance, the encoder would use the same set of contiguous reference lines 0-2 to perform intra prediction of current block1802as shown inFIG.18and described above. In another example, if reference line offset2002indicates a value equal to 1, reference line N is equal to 3. In this instance, the encoder would use the set reference lines 0, 1, and 3 to perform intra prediction of current block1802. In yet another example, if reference line offset2002indicates a value equal to 2, reference line N is equal to 4. In this instance, the encoder would use the set of reference lines 0, 1, and 4 to perform intra prediction of current block1802.

In another embodiment, reference line N may be determined based on the product of2and the value indicated by reference line offset2002. For example, if reference line offset2002indicates a value equal to 1, reference line N is equal to 2. In this instance, the encoder would use the same set of contiguous reference lines 0-2 to perform intra prediction of current block1802as shown inFIG.18and described above. In another example, if reference line offset2002indicates a value equal to 2, reference line N is equal to 4. In this instance, the encoder would use the set reference lines 0, 1, and 4 to perform intra prediction of current block1802.

A reference line offset2002may be determined based on a resolution of a picture of current block1802and/or a size of current block1802as discussed above. For example,FIG.20illustrates a plot of reference line offset2002versus the resolution of the picture of current block1802or the size of current block1802. As shown in the plot, for a resolution of the picture of current block1802or a size of current block1802below a first threshold value T1, reference line offset2002may have a value O1 or may indicate a value O1. For a resolution of the picture of current block1802or a size of current block1802above the first threshold value T1 and below the second threshold value T2, reference line offset2002may have a value O2 or may indicate a value O2. The value of O2 is greater than O1. For a resolution of the picture of current block1802or a size of current block1802above the second threshold value T2 and below the third threshold value T3, reference line offset2002may have a value O3 or may indicate a value O3. The value of O3 is greater than O2. For a resolution of the picture of current block1802or a size of current block1802above the third threshold value, reference line offset2002may have a value O3 or may indicate a value O3. The value of O3 is greater than O2. It should be noted that the plot illustrated inFIG.20is provided by way of example and not limitation. In other embodiments, more or less threshold values and reference line offset values may be used. For example, a single threshold T1 and two reference line offset values O1 and O2 may be used in other embodiments.

In the instance where the encoder uses reference line N to encode current block1802, the encoder may signal, in a bitstream, an indication that reference line N is the selected reference line to a decoder. For example, the encoder may signal, in the bitstream, a reference line index2to indicate that reference line N is the selected reference line. For example, the encoder may signal, in the bit stream, a lowest possible reference line index of reference line N (which is 2 in the example ofFIG.20) to indicate that reference line N is the selected reference line. The encoder may further signal reference line offset2002in the bitstream. In another embodiment, the encoder may not signal reference line offset2002in the bitstream. Instead, the decoder may independently determine reference line offset2002in the same manner as described above with respect to the encoder. For example, when the encoder signals an indication that reference line N is the selected reference line to the decoder, the decoder may determine reference line offset2002in the same manner as described above with respect to the encoder. The decoder may further determine the value of N using reference line offset2002in the same manner as discussed above with respect to the encoder. The encoder may further signal, in the bitstream, the selected intra prediction mode used to encode current block1802and/or the residual of the intra prediction of current block1802to the decoder.

FIG.21illustrates another example determination of a reference line offset2102and the application of reference line offset2102to determine a set of reference lines for performing intra prediction of current block1802inFIG.18in accordance with embodiments of the present disclosure. The determination of reference line offset2102and the application of reference line offset2102to determine a set of reference lines for performing intra prediction of current block1802may be performed by an encoder, such as encoder200inFIG.2, or the encoder described above with respect toFIG.18.

InFIG.21, the encoder may use reference line offset2102to adjust a distance between reference lines for encoding current block1802. More specifically, the encoder may use a set of reference lines comprising reference lines 0, N, and M to perform intra prediction of current block1802as described above with respect toFIG.18. In an embodiment, reference line offset2102may indicate a distance (e.g., in terms of number of samples) between reference line 0 and reference line N and a distance between reference line N and reference line M, where 0<N<M. For example, reference line offset2102may indicate a value between 0 and P, where P is an integer value. Reference line N may be determined based on the summation of1and the value indicated by reference line offset2102.

Reference line M may be determined based on the summation of1, N, and the value indicated by reference line offset2102. For example, if reference line offset2102indicates a value equal to 0, reference line N is equal to 1 and reference line M is equal to 2. In this instance, the encoder would use the same set of contiguous reference lines 0-2 to perform intra prediction of current block1802as shown inFIG.18and described above. In another example, if reference line offset2102indicates a value equal to 1, reference line N is equal to 2 and reference line M is equal 4. In this instance, the encoder would use the set reference lines 0, 2, and 4 to perform intra prediction of current block1802. In yet another example, if reference line offset2102indicates a value equal to 2, reference line N is equal to 3 and reference line M is equal to 6. In this instance, the encoder would use the set of reference lines 0, 3, and 6 to perform intra prediction of current block1802.

In another embodiment, reference line N may be determined based on the product of1and the value indicated by reference line offset2102and reference line M may be determined based on2and the value indicated by reference line offset2102. For example, if reference line offset2102indicates a value equal to 1, reference line N is equal to 1 and reference line M is equal to 2. In this instance, the encoder would use the same set of contiguous reference lines 0-2 to perform intra prediction of current block1802as shown inFIG.18and described above. In another example, if reference line offset2102indicates a value equal to 2, reference line N is equal to 2 and reference line M is equal to 4. In this instance, the encoder would use the set reference lines 0, 2, and 4 to perform intra prediction of current block1802. In another example, if reference line offset2102indicates a value equal to 4, reference line N is equal to 4 and reference line M is equal to 8. In this instance, the encoder would use the set reference lines 0, 4, and 8 to perform intra prediction of current block1802.

A reference line offset2102may be determined based on a resolution of a picture of current block1802and/or a size of current block1802as discussed above. For example,FIG.21illustrates a plot of reference line offset2102versus the resolution of the picture of current block1802or the size of current block1802. As shown in the plot, for a resolution of the picture of current block1802or a size of current block1802below a first threshold value T1, reference line offset2102may have a value O1 or may indicate a value O1. For a resolution of the picture of current block1802or a size of current block1802above the first threshold value T1 and below the second threshold value T2, reference line offset2102may have a value O2 or may indicate a value O2. The value of O2 is greater than O1. For a resolution of the picture of current block1802or a size of current block1802above the second threshold value T2 and below the third threshold value T3, reference line offset2102may have a value O3 or may indicate a value O3. The value of O3 is greater than O2. For a resolution of the picture of current block1802or a size of current block1802above the third threshold value, reference line offset2102may have a value O3 or may indicate a value O3. The value of O3 is greater than O2. It should be noted that the plot illustrated inFIG.21is provided by way of example and not limitation. In other embodiments, more or less threshold values and reference line offset values may be used. For example, a single threshold T1 and two reference line offset values O1 and O2 may be used in other embodiments.

In the instance where the encoder uses reference line N or M to encode current block1802, the encoder may signal, in a bitstream, an indication that reference line N or M is the selected reference line to a decoder. For example, the encoder may signal, in the bitstream, a reference line index2to indicate that reference line N is the selected reference line or a reference line index3to indicate that reference line M is the selected reference line. For example, the encoder may signal, in the bit stream, a lowest possible reference line index of reference line N (which is 2 in the example ofFIG.21) to indicate that reference line N is the selected reference line or a lowest possible reference line index of reference line M (which is 3 in the exampleFIG.21) to indicate that reference line M is the selected reference line. The encoder may further signal reference line offset2102in the bitstream. In another embodiment, the encoder may not signal reference line offset2102in the bitstream. Instead, the decoder may independently determine reference line offset2102in the same manner as described above with respect to the encoder. For example, when the encoder signals an indication that reference line N or reference line M is the selected reference line to the decoder, the decoder may determine reference line offset2102in the same manner as described above with respect to the encoder. The decoder may further determine the value of N or M using reference line offset2102in the same manner as discussed above with respect to the encoder. The encoder may further signal, in the bitstream, the selected intra prediction mode used to encode current block1802and/or the residual of the intra prediction of current block1802to the decoder.

FIG.22illustrates a flowchart2200of a method for determining a prediction of a sample based on a reference line offset in accordance with embodiments of the present disclosure. The method of flowchart2200may be implemented by an encoder or a decoder, such as encoder200inFIG.2or decoder300inFIG.3.

The method of flowchart2200begins at2202. At2202, a reference line offset is determined based on at least one of: a property of a block, or a property of reference lines of the block.

In an embodiment, the determining the reference line offset comprises determining a correlation between the reference lines based on at least one of: the property of the block; or the property of reference lines of the block.

In an embodiment, the property of the reference lines is a difference between the reference lines.

In an embodiment, determining a correlation between the reference lines comprises obtaining a sum of absolute differences between the samples of reference lines. In an embodiment, the differences are obtained for pairs of two closest reference lines: line 0 and line1, line 1 and line 2, line 2 and line 3, etc. In an embodiment, the differences are obtained for the projection points (1808, 1810 and 1812 shown inFIG.18), the values in projection points are obtained from the samples of reference lines by interpolation.

In an embodiment, determining a correlation between the reference lines comprises obtaining a sum of absolute transformed differences, the differences are calculated as disclosed above. In an embodiment, determining a correlation between the reference lines comprises obtaining a sum of squared differences, the differences are obtained as disclosed above.

In an embodiment, the property of the block is at least one of: a resolution of a picture of the block; or size of a block.

In an embodiment, the reference line offset is determined based on both the resolution and the size of the block.

In an embodiment, for a given value of the resolution, the reference line offset increases as the size of the block increases. In an embodiment, for the given value of the resolution, the reference line offset increases as the size of the block increases above a threshold. In an embodiment, the size of the block is determined based on at least one of a height of the block or a width of the block.

In an embodiment, for a given value of the size of the block, the reference line offset increases as the resolution increases. In an embodiment, for the given value of the size of the block, the reference line offset increases as the resolution increases above a threshold.

In an embodiment, the determining the reference line offset comprises selecting the reference line offset, from a plurality of reference line offset values, based on at least one of: the resolution; or the size of the block.

In an embodiment, the determining the reference line offset comprises: comparing the resolution to a threshold; and based on the comparing, selecting either a first or second value for the reference line offset. In an embodiment, the threshold is equal to 1920*1080. In an embodiment, the reference line offset is equal to 4 based on the resolution being greater than the threshold. In an embodiment, the reference line offset is equal to 2 based on the resolution being equal to the threshold. In an embodiment, the reference line offset is equal to 1 based on the resolution being less than the threshold. In an embodiment, the resolution is determined based on a number of samples in the picture of the block. In an embodiment, the resolution is indicated in a picture parameter set received in a bitstream. In an embodiment, the samples are luma samples.

At2204, a reference line is determined based on a reference line index and the reference line offset. In an embodiment, the reference line is determined based on a product of the reference line index and the reference line offset.

At2206, a location of a sample in the block is projected, in an angular direction, to a point on the reference line. In another embodiment, for non-angular intra prediction modes, a value of a sample in the block may predicted based on the reference line and a non-angular intra prediction mode, such as DC or planar.

At2208, a value of the sample is predicted based on the point.

In an embodiment, the method of flowchart2200may further comprise signaling the reference line index in a bit stream.

In an embodiment, the method of flowchart2200may further comprise signaling a mode of the angular direction in the bit stream. In an embodiment, the mode is included in a most probable mode list determined for the block.

In an embodiment, the method of flowchart2200may further comprise signaling an indication that the most probable list is used to code the block. In an embodiment, the indication is the reference line index. In an embodiment, the most probable mode list is an angular-only most probable mode list.

In an embodiment, the method of flowchart2200may further comprise receiving the reference line index in a bitstream.

Similarly to VVC/H.266, MRLP is considered a candidate for adopting it into the AOM's AV2 codec. There, it is known as “Multiple Reference Line Selection for intra prediction (MRLS)* uses up to 4 reference lines. Like done in VVC, MRLS is only applied to the luma component, since chroma texture is relatively smooth. In contrast to VVC where MRLP is enabled for DC mode, MRLS is disabled for any non-directional intra prediction mode. Similarly to VVC/H.266, reference line selection is signaled into the bitstream. Thus, the above mentioned embodiments are applicable in case of the new AOM's video codec as well shown inFIG.23. In particular, described embodiments are applicable within steps “Intra prediction”, “Entropy coding” and “Entropy decoding”.

Embodiments of the present disclosure may be implemented in hardware using analog and/or digital circuits, in software, through the execution of instructions by one or more general purpose or special-purpose processors, or as a combination of hardware and software. Consequently, embodiments of the disclosure may be implemented in the environment of a computer system or other processing system. An example of such a computer system2400is shown inFIG.24. Blocks depicted in the figures above, such as the blocks inFIGS.1,2, and3, may execute on one or more computer systems2400. Furthermore, each of the steps of the flowcharts depicted in this disclosure may be implemented on one or more computer systems2400.

Computer system2400includes one or more processors, such as processor2404. Processor2404may be, for example, a special purpose processor, general purpose processor, microprocessor, or digital signal processor. Processor2404may be connected to a communication infrastructure2402(for example, a bus or network). Computer system2400may also include a main memory2406, such as random access memory (RAM), and may also include a secondary memory2408.

Secondary memory2408may include, for example, a hard disk drive2410and/or a removable storage drive2412, representing a magnetic tape drive, an optical disk drive, or the like. Removable storage drive2412may read from and/or write to a removable storage unit2416in a well-known manner. Removable storage unit2416represents a magnetic tape, optical disk, or the like, which is read by and written to by removable storage drive2412. As will be appreciated by persons skilled in the relevant art(s), removable storage unit2416includes a computer usable storage medium having stored therein computer software and/or data.

In alternative implementations, secondary memory2408may include other similar means for allowing computer programs or other instructions to be loaded into computer system2400. Such means may include, for example, a removable storage unit2418and an interface2414. Examples of such means may include a program cartridge and cartridge interface (such as that found in video game devices), a removable memory chip (such as an EPROM or PROM) and associated socket, a thumb drive and USB port, and other removable storage units2418and interfaces2414which allow software and data to be transferred from removable storage unit2418to computer system2400.

Computer system2400may also include a communications interface2420. Communications interface2420allows software and data to be transferred between computer system2400and external devices. Examples of communications interface2420may include a modem, a network interface (such as an Ethernet card), a communications port, etc. Software and data transferred via communications interface2420are in the form of signals which may be electronic, electromagnetic, optical, or other signals capable of being received by communications interface2420. These signals are provided to communications interface2420via a communications path2422.

Communications path2422carries signals and may be implemented using wire or cable, fiber optics, a phone line, a cellular phone link, an RF link, and other communications channels.

As used herein, the terms “computer program medium” and “computer readable medium” are used to refer to tangible storage media, such as removable storage units2416and2418or a hard disk installed in hard disk drive2410. These computer program products are means for providing software to computer system2400. Computer programs (also called computer control logic) may be stored in main memory2406and/or secondary memory2408.

Computer programs may also be received via communications interface2420. Such computer programs, when executed, enable the computer system2400to implement the present disclosure as discussed herein. In particular, the computer programs, when executed, enable processor2404to implement the processes of the present disclosure, such as any of the methods described herein. Accordingly, such computer programs represent controllers of the computer system2400.

In another embodiment, features of the disclosure may be implemented in hardware using, for example, hardware components such as application-specific integrated circuits (ASICs) and gate arrays. Implementation of a hardware state machine to perform the functions described herein will also be apparent to persons skilled in the art.