TECHNICAL FIELD

The present disclosure describes embodiments generally related to video coding.

BACKGROUND

The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.

Video coding and decoding can be performed using inter-picture prediction with motion compensation. Uncompressed digital video can include a series of pictures, each picture having a spatial dimension of, for example, 1920×1080 luminance samples and associated chrominance samples. The series of pictures can have a fixed or variable picture rate (informally also known as frame rate), of, for example 60 pictures per second or 60 Hz. Uncompressed video has significant bitrate requirements. For example, 1080p60 4:2:0 video at 8 bit per sample (1920×1080 luminance sample resolution at 60 Hz frame rate) requires close to 1.5 Gbit/s bandwidth. An hour of such video requires more than 600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction of redundancy in the input video signal, through compression. Compression can help reduce the aforementioned bandwidth or storage space requirements, in some cases by two orders of magnitude or more. Both lossless and lossy compression, as well as a combination thereof can be employed. Lossless compression refers to techniques where an exact copy of the original signal can be reconstructed from the compressed original signal. When using lossy compression, the reconstructed signal may not be identical to the original signal, but the distortion between original and reconstructed signals is small enough to make the reconstructed signal useful for the intended application. In the case of video, lossy compression is widely employed. The amount of distortion tolerated depends on the application; for example, users of certain consumer streaming applications may tolerate higher distortion than users of television distribution applications. The compression ratio achievable can reflect that: higher allowable/tolerable distortion can yield higher compression ratios.

Motion compensation can be a lossy compression technique and can relate to techniques where a block of sample data from a previously reconstructed picture or part thereof (reference picture), after being spatially shifted in a direction indicated by a motion vector (MV henceforth), is used for the prediction of a newly reconstructed picture or picture part. In some cases, the reference picture can be the same as the picture currently under reconstruction. MVs can have two dimensions X and Y, or three dimensions, the third being an indication of the reference picture in use (the latter, indirectly, can be a time dimension).

In some video compression techniques, an MV applicable to a certain area of sample data can be predicted from other MVs, for example from those related to another area of sample data spatially adjacent to the area under reconstruction, and preceding that MV in decoding order. Doing so can substantially reduce the amount of data required for coding the MV, thereby removing redundancy and increasing compression. MV prediction can work effectively, for example, because when coding an input video signal derived from a camera (known as natural video) there is a statistical likelihood that areas larger than the area to which a single MV is applicable move in a similar direction and, therefore, can in some cases be predicted using a similar motion vector derived from MVs of neighboring area. That results in the MV found for a given area to be similar or the same as the MV predicted from the surrounding MVs, and that in turn can be represented, after entropy coding, in a smaller number of bits than what would be used if coding the MV directly. In some cases, MV prediction can be an example of lossless compression of a signal (namely: the MVs) derived from the original signal (namely: the sample stream). In other cases, MV prediction itself can be lossy, for example because of rounding errors when calculating a predictor from several surrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec. H.265, “High Efficiency Video Coding”, December 2016). Out of the many MV prediction mechanisms that H.265 offers, described here is a technique henceforth referred to as “spatial merge”.

Referring toFIG. 1, a current block (101) comprises samples that have been found by the encoder during the motion search process to be predictable from a previous block of the same size that has been spatially shifted. Instead of coding that MV directly, the MV can be derived from metadata associated with one or more reference pictures, for example from the most recent (in decoding order) reference picture, using the MV associated with either one of five surrounding samples, denoted A0, A1, and B0, B1, B2(102through106, respectively). In H.265, the MV prediction can use predictors from the same reference picture that the neighboring block is using.

SUMMARY

Aspects of the disclosure provide methods and apparatuses for video encoding/decoding. In some examples, an apparatus for video decoding includes processing circuitry. The processing circuitry can decode, from a coded video bitstream, a first syntax element signaled at a first coding level. The first syntax element can indicate a maximum number of merge motion vector prediction (MVP) candidates. The first coding level can be higher than a picture parameter set (PPS) level. The processing circuitry can determine the maximum number of merge MVP candidates based on the first syntax element. The processing circuitry can reconstruct coding blocks associated with the first coding level based at least on the maximum number of merge MVP candidates.

In an embodiment, the processing circuitry can decode, from the coded video bitstream, a second syntax element, a third syntax element, and a fourth syntax element signaled at the first coding level. The second syntax element can indicate a maximum number of geometric merge mode merge candidates, the third syntax element can indicate a maximum number of subblock-based merge MVP candidates, and the fourth syntax element can indicate a maximum number of intra block copy (IBC) merge block vector prediction (BVP) candidates. The processing circuitry can determine (i) the maximum number of geometric merge mode merge candidates based on the second syntax element, (ii) the maximum number of subblock-based merge MVP candidates based on the third syntax element, and (iii) the maximum number of IBC merge BVP candidates based on the fourth syntax element.

In an embodiment, the first coding level is a sequence level and the first syntax element is signaled in a sequence parameter set (SPS).

In an embodiment, the processing circuitry can decode, from the coded video bitstream and based on a condition being satisfied, a second syntax element signaled at a second coding level. The second syntax element can indicate a maximum number of merge candidates for one of a geometric merge mode, a subblock-based merge mode, and an intra block copy (IBC) merge mode. In an example, the one of the geometric merge mode, the subblock-based merge mode, and the IBC merge mode is the geometric merge mode. The maximum number of merge candidates is a maximum number of geometric merge mode merge candidates. The second syntax element indicates the maximum number of geometric merge mode merge candidates. The condition is the geometric merge mode being enabled at the second coding level and the maximum number of merge MVP candidates being larger than or equal to 3. The processing circuitry can determine the maximum number of geometric merge mode merge candidates based on the second syntax element.

In an example, the one of the geometric merge mode, the subblock-based merge mode, and the IBC merge mode is the subblock-based merge mode. The maximum number of merge candidates is a maximum number of subblock-based merge MVP candidates. The second syntax element indicates the maximum number of subblock-based merge MVP candidates. The condition is an affine mode being enabled at the second coding level. The processing circuitry can determine the maximum number of subblock-based merge MVP candidates based on the second syntax element. In an example, the second syntax element is modified at a coding level that is lower than the second coding level. In an example, the first coding level and the second coding level are a sequence level, and the coding level that is lower than the second coding level is a picture level.

In an example, the one of the geometric merge mode, the subblock-based merge mode, and the IBC merge mode is the IBC merge mode. The maximum number of merge candidates is a maximum number of IBC merge block vector prediction (BVP) candidates. The second syntax element indicates the maximum number of IBC merge BVP candidates. The condition is the IBC merge mode being enabled at the second coding level. The processing circuitry can determine the maximum number of IBC merge BVP candidates based on the second syntax element.

In an example, the first coding level and the second coding level are a sequence level.

Aspects of the disclosure also provide a non-transitory computer-readable medium storing instructions which when executed by a computer for video decoding cause the computer to perform the method for video decoding.

