Patent Publication Number: US-11394983-B2

Title: Method and apparatus for merge mode with additional middle candidates in video coding

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 16/219,533 filed on Dec. 13, 2018, which claims priority under 35 U.S.C. § 119 from U.S. Provisional Application No. 62/680,486 filed on Jun. 4, 2018 in the U.S. Patent &amp; Trademark Office, the disclosures of which are incorporated herein by reference in their entireties. 
    
    
     FIELD 
     The present disclosure is directed to inter-picture prediction coding, and is applicable for merge mode. A merge candidates list is generated using extra spatial or temporal merge candidates. The generation of a merge candidate list is modified. A signaling scheme of the merge index is also proposed. 
     BACKGROUND 
     In High Efficiency Video Coding (HEVC), a merge mode for inter-picture prediction is introduced. A merge candidate list of candidate motion parameters from neighboring blocks is generated. Then, an index is signaled which identifies the candidates to be used. Merge mode also allows for temporal prediction by including in the list a candidate obtained from previously coded pictures. Referring to  FIG. 1 , in HEVC, a merge candidates list for a current block ( 100 ) is generated based on one or more spatial merge candidates ( 101 ), ( 102 ), ( 103 ), ( 104 ), and/or ( 105 ), one temporal merge candidate derived from two temporal co-located blocks, and/or additional merge candidates including combined bi-predictive candidates and zero motion vector candidates. 
     In HEVC, a skip mode is used to indicate for a block that the motion data is inferred instead of explicitly signaled and that the prediction residual is zero, i.e., no transform coefficients are transmitted. In HEVC, at the beginning of each coding unit (CU) in an inter-picture prediction slice, a skip_flag is signaled that implies the following: the CU only contains one prediction unit (PU) (e.g., 2N×2N), the merge mode is used to derive the motion data, and/or no residual data is present in the bitstream. 
     In Joint Exploration Model 7 (JEM 7), which is the test model software studied by Joint Video Exploration Team (WET), some new merge candidates are introduced. The sub-CU modes are enabled as additional merge candidates, and there is no additional syntax element required to signal the modes. Two additional merge candidates are added to the merge candidates list of each CU to represent the alternative-temporal motion vector prediction (ATMVP) mode and the spatial-temporal motion vector prediction (STMVP) mode. 
     Up to seven merge candidates are used, if the sequence parameter set indicates that ATMVP mode and STMVP mode are enabled. The encoding logic of the additional merge candidates is the same as for the merge candidates in HEVC, which means, for each CU in predicted (P) or bi-directional predicted (B) slice, two more rate-distortion (RD) checks are needed for the two additional merge candidates. In JEM 7, the order of the inserted merge candidates is A, B, C, D, ATMVP, STMVP, E (when the merge candidates in the list are less than 6), temporal motion vector prediction (TMVP), combined bi-predictive candidates and zero motion vector candidates. 
     In JEM 7, all bins of merge index are context coded by context-adaptive binary arithmetic coding (CABAC). While in HEVC, only the first bin is context coded and the remaining bins are context by-pass coded. In the JEM, the maximum number of merge candidates are 7. 
       FIG. 2  illustrates an example of the merge candidate list generation. For example, the scheme searches the candidate motion vectors from previously coded blocks, with a step size of 8×8 blocks. It defines the nearest spatial neighbors of a current block ( 200 ), i.e., immediate top row ( 201 ), left column ( 202 ), and top-right corner ( 203 ), as category 1. Other neighbors ( 204 ), ( 205 ), such as the outer regions (maximum three 8×8 blocks away from the current block boundary) and the collocated blocks in the previously coded frame are classified as category 2. The neighboring blocks that are predicted from different reference frames or are intra coded are pruned from the list. The remaining reference blocks are then each assigned a weight. The weight is related to the distance to the current block. 
     In an extended merge mode, the additional merge candidates will be a direct extension of the NEXT merge candidates. The left, above, left bottom, above right, and top left candidates that are not immediately next to the current block are checked. The detailed positions that are checked are shown in  FIG. 1 . A maximum number of merge candidates might be 10, as an example. 
       FIG. 3  illustrates a merge candidate from an outer region. For example, as shown in  FIG. 3 , the top left corner of the reference block has an offset of (−96, −96) to the current block. As shown by candidates ( 301 ), ( 302 ), and ( 303 ), each candidate B (i, j) or C (i, j) has an offset of 16 in the vertical direction compared to its previous B or C candidates. As shown by candidates ( 304 ), ( 305 ), and ( 306 ), each candidate A (i, j) or D (i, j) has an offset of 16 in the horizontal direction compared to its previous A or D candidates. As shown by candidates ( 307 ), ( 308 ), and ( 309 ), each candidate E (i, j) has an offset of 16 in both the horizontal direction and the vertical direction as compared to its previous E candidates. The candidates are checked from inside to the outside, and the order of the candidates is A (i, j), B (i, j), C (i, j), D (i, j), and E (i, j). 
     SUMMARY 
     According to an aspect of the disclosure, a method for generating a merge candidate list for encoding or decoding of a video sequence comprises determining a first candidate block that includes a vertical position above a top edge of a current block, and that includes a horizontal position adjacent to a horizontal center of the current block; determining a second candidate block that includes a horizontal position that is located to a left side of a left edge of the current block and a vertical position adjacent to a vertical center of the current block; and generating the merge candidate list using the first candidate block and the second candidate block to permit the encoding or the decoding of the video sequence. 
     According to an aspect of the disclosure, a device for generating a merge candidate list for encoding or decoding of a video sequence at least one memory configured to store program code; and at least one processor configured to read the program code and operate as instructed by the program code, the program code including: first determining code configured to cause the at least one processor to determine a first candidate block that includes a vertical position above a top edge of a current block, and that includes a horizontal position adjacent to a horizontal center of the current block; second determining code configured to cause the at least one processor to determine a second candidate block that includes a horizontal position that is located to a left side of a left edge of the current block and a vertical position adjacent to a vertical center of the current block; and generating code configured to cause the at least one processor to generate the merge candidate list using the first candidate block and the second candidate block to permit the encoding or the decoding of the video sequence. 
     According to an aspect of the disclosure, a non-transitory computer-readable medium stores instructions, the instructions comprising: one or more instructions that, when executed by one or more processors of a device, cause the one or more processors to: determine a first candidate block that includes a vertical position above a top edge of a current block, and that includes a horizontal position adjacent to a horizontal center of the current block; determine a second candidate block that includes a horizontal position that is located to a left side of a left edge of the current block and a vertical position adjacent to a vertical center of the current block; and generate a merge candidate list, for encoding or decoding of a video sequence, using the first candidate block and the second candidate block to permit the encoding or the decoding of the video sequence. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features, the nature, and various advantages of the disclosed subject matter will be more apparent from the following detailed description and the accompanying drawings in which: 
         FIG. 1  is a diagram of spatial merge candidates in accordance with an embodiment; 
         FIG. 2  is a diagram of merge candidate list generation in accordance with an embodiment; 
         FIG. 3  is a diagram of merge candidates from an outer region in accordance with an embodiment; 
         FIG. 4  is a simplified block diagram of a communication system in accordance with an embodiment; 
         FIG. 5  is a diagram of a streaming environment in accordance with an embodiment; 
         FIG. 6  is a block diagram of a video decoder in accordance with an embodiment; 
         FIG. 7  is a block diagram of a video encoder in accordance with an embodiment; 
         FIG. 8  is a flowchart of an example process for generating a merge candidate list in accordance with an embodiment; 
         FIG. 9  is a diagram illustrating the addition of a current block middle candidate to a merge candidate list; 
         FIG. 10  is a diagram illustrating the addition of a current block middle candidate to a merge candidate list 
         FIG. 11  is a diagram illustrating the addition of a middle candidate in accordance with an embodiment; and 
         FIG. 12  is a diagram of a computer system in accordance with an embodiment. 
     
    
    
     PROBLEM TO BE SOLVED 
     Some implementations described herein permit improved merge candidate list generation that results in bit rate reduction, performance increase, among other technical benefits. 
     DETAILED DESCRIPTION 
       FIG. 4  illustrates a simplified block diagram of a communication system ( 400 ) according to an embodiment of the present disclosure. The communication system ( 400 ) may include at least two terminals ( 410 - 420 ) interconnected via a network ( 450 ). For unidirectional transmission of data, a first terminal ( 410 ) may code video data at a local location for transmission to the other terminal ( 420 ) via the network ( 450 ). The second terminal ( 420 ) may receive the coded video data of the other terminal from the network ( 450 ), decode the coded data and display the recovered video data. Unidirectional data transmission may be common in media serving applications and the like. 
       FIG. 4  illustrates a second pair of terminals ( 430 ,  440 ) provided to support bidirectional transmission of coded video that may occur, for example, during videoconferencing. For bidirectional transmission of data, each terminal ( 430 ,  440 ) may code video data captured at a local location for transmission to the other terminal via the network ( 450 ). Each terminal ( 430 ,  440 ) also may receive the coded video data transmitted by the other terminal, may decode the coded data and may display the recovered video data at a local display device. 
     In  FIG. 4 , the terminals ( 410 - 440 ) may be illustrated as servers, personal computers and smart phones but the principles of the present disclosure are not so limited. Embodiments of the present disclosure find application with laptop computers, tablet computers, media players and/or dedicated video conferencing equipment. The network ( 450 ) represents any number of networks that convey coded video data among the terminals ( 410 - 440 ), including for example wireline and/or wireless communication networks. The communication network ( 450 ) may exchange data in circuit-switched and/or packet-switched channels. Representative networks include telecommunications networks, local area networks, wide area networks and/or the Internet. For the purposes of the present discussion, the architecture and topology of the network ( 450 ) may be immaterial to the operation of the present disclosure unless explained herein below. 
       FIG. 5  illustrates, as an example for an application for the disclosed subject matter, the placement of a video encoder and decoder in a streaming environment. The disclosed subject matter can be equally applicable to other video enabled applications, including, for example, video conferencing, digital TV, storing of compressed video on digital media including CD, DVD, memory stick and the like, and so on. 
     A streaming system may include a capture subsystem ( 513 ), that can include a video source ( 501 ), for example a digital camera, creating, for example, an uncompressed video sample stream ( 502 ). That sample stream ( 502 ), depicted as a bold line to emphasize a high data volume when compared to encoded video bitstreams, can be processed by an encoder ( 503 ) coupled to the camera ( 501 ). The encoder ( 503 ) can include hardware, software, or a combination thereof to enable or implement aspects of the disclosed subject matter as described in more detail below. The encoded video bitstream ( 504 ), depicted as a thin line to emphasize the lower data volume when compared to the sample stream, can be stored on a streaming server ( 505 ) for future use. One or more streaming clients ( 506 ,  508 ) can access the streaming server ( 505 ) to retrieve copies ( 507 ,  509 ) of the encoded video bitstream ( 504 ). A client ( 506 ) can include a video decoder ( 510 ) which decodes the incoming copy of the encoded video bitstream ( 507 ) and creates an outgoing video sample stream ( 511 ) that can be rendered on a display ( 512 ) or other rendering device (not depicted). In some streaming systems, the video bitstreams ( 504 ,  507 ,  509 ) can be encoded according to certain video coding/compression standards. Examples of those standards include ITU-T Recommendation H.265. Under development is a video coding standard informally known as Versatile Video Coding (VVC). The disclosed subject matter may be used in the context of VVC. 
       FIG. 6  may be a functional block diagram of a video decoder ( 510 ) according to an embodiment of the present invention. 
     A receiver ( 610 ) may receive one or more codec video sequences to be decoded by the decoder ( 510 ); in the same or another embodiment, one coded video sequence at a time, where the decoding of each coded video sequence is independent from other coded video sequences. The coded video sequence may be received from a channel ( 612 ), which may be a hardware/software link to a storage device which stores the encoded video data. The receiver ( 610 ) may receive the encoded video data with other data, for example, coded audio data and/or ancillary data streams, that may be forwarded to their respective using entities (not depicted). The receiver ( 610 ) may separate the coded video sequence from the other data. To combat network jitter, a buffer memory ( 615 ) may be coupled in between receiver ( 610 ) and entropy decoder/parser ( 620 ) (“parser” henceforth). When receiver ( 610 ) is receiving data from a store/forward device of sufficient bandwidth and controllability, or from an isosychronous network, the buffer ( 615 ) may not be needed, or can be small. For use on best effort packet networks such as the Internet, the buffer ( 615 ) may be required, can be comparatively large and can advantageously of adaptive size. 
     The video decoder ( 510 ) may include a parser ( 620 ) to reconstruct symbols ( 621 ) from the entropy coded video sequence. Categories of those symbols include information used to manage operation of the decoder ( 510 ), and potentially information to control a rendering device such as a display ( 512 ) that is not an integral part of the decoder but can be coupled to it, as was shown in  FIG. 5 . The control information for the rendering device(s) may be in the form of Supplementary Enhancement Information (SEI) messages or Video Usability Information (VUI) parameter set fragments (not depicted). The parser ( 620 ) may parse/entropy-decode the coded video sequence received. The coding of the coded video sequence can be in accordance with a video coding technology or standard, and can follow principles well known to a person skilled in the art, including variable length coding, Huffman coding, arithmetic coding with or without context sensitivity, and so forth. The parser ( 620 ) may extract from the coded video sequence, a set of subgroup parameters for at least one of the subgroups of pixels in the video decoder, based upon at least one parameters corresponding to the group. Subgroups can include Groups of Pictures (GOPs), pictures, tiles, slices, macroblocks, Coding Units (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) and so forth. The entropy decoder/parser may also extract from the coded video sequence information such as transform coefficients, quantizer parameter (QP) values, motion vectors, and so forth. 
     The parser ( 620 ) may perform entropy decoding/parsing operation on the video sequence received from the buffer ( 615 ), so to create symbols ( 621 ). The parser ( 620 ) may receive encoded data, and selectively decode particular symbols ( 621 ). Further, the parser ( 620 ) may determine whether the particular symbols ( 621 ) are to be provided to a Motion Compensation Prediction unit ( 653 ), a scaler/inverse transform unit ( 651 ), an Intra Prediction Unit ( 652 ), or a loop filter ( 656 ). 
     Reconstruction of the symbols ( 621 ) can involve multiple different units depending on the type of the coded video picture or parts thereof (such as: inter and intra picture, inter and intra block), and other factors. Which units are involved, and how, can be controlled by the subgroup control information that was parsed from the coded video sequence by the parser ( 620 ). The flow of such subgroup control information between the parser ( 620 ) and the multiple units below is not depicted for clarity. 
     Beyond the functional blocks already mentioned, decoder ( 510 ) can be conceptually subdivided into a number of functional units as described below. In a practical implementation operating under commercial constraints, many of these units interact closely with each other and can, at least partly, be integrated into each other. However, for the purpose of describing the disclosed subject matter, the conceptual subdivision into the functional units below is appropriate. 
     A first unit is the scaler/inverse transform unit ( 651 ). The scaler/inverse transform unit ( 651 ) receives quantized transform coefficient as well as control information, including which transform to use, block size, quantization factor, quantization scaling matrices, etc. as symbol(s) ( 621 ) from the parser ( 620 ). It can output blocks comprising sample values, that can be input into aggregator ( 655 ). 
     In some cases, the output samples of the scaler/inverse transform ( 651 ) can pertain to an intra coded block; that is: a block that is not using predictive information from previously reconstructed pictures, but can use predictive information from previously reconstructed parts of the current picture. Such predictive information can be provided by an intra picture prediction unit ( 652 ). In some cases, the intra picture prediction unit ( 652 ) generates a block of the same size and shape of the block under reconstruction, using surrounding already reconstructed information fetched from the current (partly reconstructed) picture ( 656 ). The aggregator ( 655 ), in some cases, adds, on a per sample basis, the prediction information the intra prediction unit ( 652 ) has generated to the output sample information as provided by the scaler/inverse transform unit ( 651 ). 
     In other cases, the output samples of the scaler/inverse transform unit ( 651 ) can pertain to an inter coded, and potentially motion compensated block. In such a case, a Motion Compensation Prediction unit ( 653 ) can access reference picture memory ( 657 ) to fetch samples used for prediction. After motion compensating the fetched samples in accordance with the symbols ( 621 ) pertaining to the block, these samples can be added by the aggregator ( 655 ) to the output of the scaler/inverse transform unit (in this case called the residual samples or residual signal) so to generate output sample information. The addresses within the reference picture memory form where the motion compensation unit fetches prediction samples can be controlled by motion vectors, available to the motion compensation unit in the form of symbols ( 621 ) that can have, for example X, Y, and reference picture components. Motion compensation also can include interpolation of sample values as fetched from the reference picture memory when sub-sample exact motion vectors are in use, motion vector prediction mechanisms, and so forth. 
     The output samples of the aggregator ( 655 ) can be subject to various loop filtering techniques in the loop filter unit ( 656 ). Video compression technologies can include in-loop filter technologies that are controlled by parameters included in the coded video bitstream and made available to the loop filter unit ( 656 ) as symbols ( 621 ) from the parser ( 620 ), but can also be responsive to meta-information obtained during the decoding of previous (in decoding order) parts of the coded picture or coded video sequence, as well as responsive to previously reconstructed and loop-filtered sample values. 
     The output of the loop filter unit ( 656 ) can be a sample stream that can be output to the render device ( 512 ) as well as stored in the reference picture memory ( 656 ) for use in future inter-picture prediction. 
     Certain coded pictures, once fully reconstructed, can be used as reference pictures for future prediction. Once a coded picture is fully reconstructed and the coded picture has been identified as a reference picture (by, for example, parser ( 620 )), the current reference picture ( 656 ) can become part of the reference picture buffer ( 657 ), and a fresh current picture memory can be reallocated before commencing the reconstruction of the following coded picture. 
     The video decoder ( 510 ) may perform decoding operations according to a predetermined video compression technology that may be documented in a standard, such as ITU-T Rec. H.265. The coded video sequence may conform to a syntax specified by the video compression technology or standard being used, in the sense that it adheres to the syntax of the video compression technology or standard, as specified in the video compression technology document or standard and specifically in the profiles document therein. Also necessary for compliance can be that the complexity of the coded video sequence is within bounds as defined by the level of the video compression technology or standard. In some cases, levels restrict the maximum picture size, maximum frame rate, maximum reconstruction sample rate (measured in, for example megasamples per second), maximum reference picture size, and so on. Limits set by levels can, in some cases, be further restricted through Hypothetical Reference Decoder (HRD) specifications and metadata for HRD buffer management signaled in the coded video sequence. 
     In an embodiment, the receiver ( 610 ) may receive additional (redundant) data with the encoded video. The additional data may be included as part of the coded video sequence(s). The additional data may be used by the video decoder ( 510 ) to properly decode the data and/or to more accurately reconstruct the original video data. Additional data can be in the form of, for example, temporal, spatial, or signal-to-noise ratio (SNR) enhancement layers, redundant slices, redundant pictures, forward error correction codes, and so on. 
       FIG. 7  may be a functional block diagram of a video encoder ( 503 ) according to an embodiment of the present disclosure. 
     The encoder ( 503 ) may receive video samples from a video source ( 501 ) (that is not part of the encoder) that may capture video image(s) to be coded by the encoder ( 503 ). 
     The video source ( 501 ) may provide the source video sequence to be coded by the encoder ( 503 ) in the form of a digital video sample stream that can be of any suitable bit depth (for example: 8 bit, 10 bit, 12 bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . ) and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb 4:4:4). In a media serving system, the video source ( 501 ) may be a storage device storing previously prepared video. In a videoconferencing system, the video source ( 503 ) may be a camera that captures local image information as a video sequence. Video data may be provided as a plurality of individual pictures that impart motion when viewed in sequence. The pictures themselves may be organized as a spatial array of pixels, wherein each pixel can comprise one or more samples depending on the sampling structure, color space, etc. in use. A person skilled in the art can readily understand the relationship between pixels and samples. The description below focuses on samples. 
     According to an embodiment, the encoder ( 503 ) may code and compress the pictures of the source video sequence into a coded video sequence ( 743 ) in real time or under any other time constraints as required by the application. Enforcing appropriate coding speed is one function of Controller ( 750 ). Controller controls other functional units as described below and is functionally coupled to these units. The coupling is not depicted for clarity. Parameters set by controller can include rate control related parameters (picture skip, quantizer, lambda value of rate-distortion optimization techniques, . . . ), picture size, group of pictures (GOP) layout, maximum motion vector search range, and so forth. A person skilled in the art can readily identify other functions of controller ( 750 ) as they may pertain to video encoder ( 503 ) optimized for a certain system design. 
     Some video encoders operate in what a person skilled in the art readily recognizes as a “coding loop.” As an oversimplified description, a coding loop can consist of the encoding part of an encoder ( 730 ) (“source coder” henceforth) (responsible for creating symbols based on an input picture to be coded, and a reference picture(s)), and a (local) decoder ( 733 ) embedded in the encoder ( 503 ) that reconstructs the symbols to create the sample data that a (remote) decoder also would create (as any compression between symbols and coded video bitstream is lossless in the video compression technologies considered in the disclosed subject matter). That reconstructed sample stream is input to the reference picture memory ( 734 ). As the decoding of a symbol stream leads to bit-exact results independent of decoder location (local or remote), the reference picture buffer content is also bit exact between local encoder and remote encoder. In other words, the prediction part of an encoder “sees” as reference picture samples exactly the same sample values as a decoder would “see” when using prediction during decoding. This fundamental principle of reference picture synchronicity (and resulting drift, if synchronicity cannot be maintained, for example because of channel errors) is well known to a person skilled in the art. 
     The operation of the “local” decoder ( 733 ) can be the same as of a “remote” decoder ( 510 ), which has already been described in detail above in conjunction with  FIG. 6 . Briefly referring also to  FIG. 5 , however, as symbols are available and en/decoding of symbols to a coded video sequence by entropy coder ( 745 ) and parser ( 620 ) can be lossless, the entropy decoding parts of decoder ( 510 ), including channel ( 612 ), receiver ( 610 ), buffer ( 615 ), and parser ( 620 ) may not be fully implemented in local decoder ( 733 ). 
     An observation that can be made at this point is that any decoder technology except the parsing/entropy decoding that is present in a decoder also necessarily needs to be present, in substantially identical functional form, in a corresponding encoder. The description of encoder technologies can be abbreviated as they are the inverse of the comprehensively described decoder technologies. Only in certain areas a more detail description is required and provided below. 
     As part of its operation, the source coder ( 730 ) may perform motion compensated predictive coding, which codes an input frame predictively with reference to one or more previously-coded frames from the video sequence that were designated as “reference frames.” In this manner, the coding engine ( 732 ) codes differences between pixel blocks of an input frame and pixel blocks of reference frame(s) that may be selected as prediction reference(s) to the input frame. 
     The local video decoder ( 733 ) may decode coded video data of frames that may be designated as reference frames, based on symbols created by the source coder ( 730 ). Operations of the coding engine ( 732 ) may advantageously be lossy processes. When the coded video data may be decoded at a video decoder (not shown in  FIG. 6 ), the reconstructed video sequence typically may be a replica of the source video sequence with some errors. The local video decoder ( 733 ) replicates decoding processes that may be performed by the video decoder on reference frames and may cause reconstructed reference frames to be stored in the reference picture cache ( 734 ). In this manner, the encoder ( 503 ) may store copies of reconstructed reference frames locally that have common content as the reconstructed reference frames that will be obtained by a far-end video decoder (absent transmission errors). 
     The predictor ( 735 ) may perform prediction searches for the coding engine ( 732 ). That is, for a new frame to be coded, the predictor ( 735 ) may search the reference picture memory ( 734 ) for sample data (as candidate reference pixel blocks) or certain metadata such as reference picture motion vectors, block shapes, and so on, that may serve as an appropriate prediction reference for the new pictures. The predictor ( 735 ) may operate on a sample block-by-pixel block basis to find appropriate prediction references. In some cases, as determined by search results obtained by the predictor ( 735 ), an input picture may have prediction references drawn from multiple reference pictures stored in the reference picture memory ( 734 ). 
     The controller ( 750 ) may manage coding operations of the video coder ( 730 ), including, for example, setting of parameters and subgroup parameters used for encoding the video data. 
     Output of all aforementioned functional units may be subjected to entropy coding in the entropy coder ( 745 ). The entropy coder translates the symbols as generated by the various functional units into a coded video sequence, by loss-less compressing the symbols according to technologies known to a person skilled in the art as, for example Huffman coding, variable length coding, arithmetic coding, and so forth. 
     The transmitter ( 740 ) may buffer the coded video sequence(s) as created by the entropy coder ( 745 ) to prepare it for transmission via a communication channel ( 760 ), which may be a hardware/software link to a storage device which would store the encoded video data. The transmitter ( 740 ) may merge coded video data from the video coder ( 730 ) with other data to be transmitted, for example, coded audio data and/or ancillary data streams (sources not shown). 
     The controller ( 750 ) may manage operation of the encoder ( 503 ). During coding, the controller ( 750 ) may assign to each coded picture a certain coded picture type, which may affect the coding techniques that may be applied to the respective picture. For example, pictures often may be assigned as one of the following frame types: 
     An Intra Picture (I picture) may be one that may be coded and decoded without using any other frame in the sequence as a source of prediction. Some video codecs allow for different types of Intra pictures, including, for example Independent Decoder Refresh Pictures. A person skilled in the art is aware of those variants of I pictures and their respective applications and features. 
     A Predictive picture (P picture) may be one that may be coded and decoded using intra prediction or inter prediction using at most one motion vector and reference index to predict the sample values of each block. 
     A Bi-directionally Predictive Picture (B Picture) may be one that may be coded and decoded using intra prediction or inter prediction using at most two motion vectors and reference indices to predict the sample values of each block. Similarly, multiple-predictive pictures can use more than two reference pictures and associated metadata for the reconstruction of a single block. 
     Source pictures commonly may be subdivided spatially into a plurality of sample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 samples each) and coded on a block-by-block basis. Blocks may be coded predictively with reference to other (already coded) blocks as determined by the coding assignment applied to the blocks&#39; respective pictures. For example, blocks of I pictures may be coded non-predictively or they may be coded predictively with reference to already coded blocks of the same picture (spatial prediction or intra prediction). Pixel blocks of P pictures may be coded non-predictively, via spatial prediction or via temporal prediction with reference to one previously coded reference pictures. Blocks of B pictures may be coded non-predictively, via spatial prediction or via temporal prediction with reference to one or two previously coded reference pictures. 
     The video coder ( 503 ) may perform coding operations according to a predetermined video coding technology or standard, such as ITU-T Rec. H.265. In its operation, the video coder ( 503 ) may perform various compression operations, including predictive coding operations that exploit temporal and spatial redundancies in the input video sequence. The coded video data, therefore, may conform to a syntax specified by the video coding technology or standard being used. 
     In an embodiment, the transmitter ( 740 ) may transmit additional data with the encoded video. The video coder ( 730 ) may include such data as part of the coded video sequence. Additional data may comprise temporal/spatial/SNR enhancement layers, other forms of redundant data such as redundant pictures and slices, Supplementary Enhancement Information (SEI) messages, Visual Usability Information (VUI) parameter set fragments, and so on. 
     The present disclosure is directed to several methods for identifying motion vector predictors for inter-picture prediction coding. The techniques proposed in this disclosure can be applied to both merge mode or motion vector prediction with difference coding. In the following, merge mode is used for detailed discussion. It is proposed to add the middle merge candidates in the extended merge mode as well. The proposed methods in this disclosure can be extended to any video coding method that uses the merge concept. Since skip mode will use merge mode to derive the motion information, the techniques in this disclosure can also apply to skip mode. 
       FIG. 8  is a flowchart of an example process  800  for generating a merge candidate list using middle candidates. In some implementations, one or more process blocks of  FIG. 8  may be performed by encoder  503 . In some implementations, one or more process blocks of  FIG. 8  may be performed by another device or a group of devices separate from or including encoder  503 , such as decoder  510 . 
     As shown in  FIG. 8 , process  800  may include determining a first candidate block that includes a vertical position above a top edge of a current block, and that includes a horizontal position adjacent to a horizontal center of the current block (block  810 ). 
     As further shown in  FIG. 8 , process  800  may include determining a second candidate block that includes a horizontal position that is located to a left side of a left edge of the current block and a vertical position adjacent to a vertical center of the current block (block  820 ). 
     As further shown in  FIG. 8 , process  800  may include generating the merge candidate list using the first candidate block and the second candidate block to permit the encoding or the decoding of the video sequence (block  830 ). 
     Although  FIG. 8  shows example blocks of process  800 , in some implementations, process  800  may include additional blocks, fewer blocks, different blocks, or differently arranged blocks than those depicted in  FIG. 8 . Additionally, or alternatively, two or more of the blocks of process  800  may be performed in parallel. 
       FIG. 9  is a diagram illustrating the addition of a current block middle candidate to a merge candidate list. For example, the middle candidates of the current block ( 900 ) are included in a merge candidate list during generation of the merge candidate list. As shown, the candidates ( 901 ), ( 902 ), and ( 903 ) located at the middle relative to the top edge of the current block ( 900 ) are included in the merge candidate list during generation of the merge candidate list. Additionally, or alternatively, the candidates ( 904 ), ( 905 ), and ( 906 ) located at the middle relative to the left edge of the current block ( 900 ) are included in the merge candidate list during generation of the merge candidate list. 
     As an example, let W and H denote the width and height of the current block ( 900 ), respectively; let the top left pixel of the current block have a coordinate of (0, 0); and let G denote the grid size, so that the non-adjacent candidates are G pixels apart horizontally and/or vertically in each category. 
     The coordinate of the bottom left pixel of the n-th (initialized at zero, which is the one closest to the top edge) middle candidates ( 901 ), ( 902 ), and ( 903 ) above the top edge of current block ( 900 ) may be denoted by as follows: 
     
       
         
           
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     The coordinate of the top right pixel of the n-th (initialized from zero, which is the one closest to the left edge) middle candidates ( 901 ), ( 902 ), and ( 903 ) above the top edge of current block may be denoted as follows: 
     
       
         
           
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                 - 
                 1 
               
               , 
               
                 H 
                 2 
               
               , 
             
             ) 
           
         
       
     
     These middle candidate can be included in the front of the merge candidate list during merge candidate list generation. In one embodiment, the order is F(i, j), G(i, j), A(i, j), B(i, j), C(i, j), D(i, j), E(i, j) when including candidates in the merge candidate list. As an alternative, these middle candidates can be included in the end of merge candidate list. As a non-limiting example, the order might be A(i, j), B(i, j), C(i, j), D(i, j), E(i, j), F(i, j), G(i, j) when generating the merge candidate list. 
     In another embodiment, the two nearest neighboring middle candidates are not added. The middle candidates that are not adjacent to the current block ( 900 ) are inserted into the merge candidate list. 
       FIG. 10  is a diagram illustrating the addition of a current block middle candidate to a merge candidate list. As shown in  FIG. 10 , the middle candidates of the above and above left are included in the merge candidate list during merge candidate list generation. The middle of the above left and the left candidates are also included in the merge candidate list. For example, as shown in  FIG. 10 , the candidates ( 1001 ), ( 1002 ), and ( 1003 ) located at the middle relative to the top edge of the current block ( 1000 ) are included in the merge candidate list during merge candidate list generation. Further, and as shown in  FIG. 10 , the candidates ( 1004 ), ( 1005 ), and ( 1006 ) located at the middle relative to the left edge of the current block ( 1000 ) are included in the merge candidate list during merge candidate list generation. 
     For example, let W and H denote the width and height of the current block ( 1000 ), respectively; let the top left pixel of the current block have a coordinate of (0, 0); and let G denote the grid size, so that the non-adjacent candidates are G pixels apart horizontally and/or vertically in each category. 
     The coordinate of the bottom left pixel of the n-th (initialized at zero, which is the one closest to the top edge) middle candidates above the top edge of current block ( 1000 ) may be denoted as follows: 
     
       
         
           
             ( 
             
               
                 
                   W 
                   - 
                   
                     n 
                     · 
                     G 
                   
                 
                 2 
               
               , 
               
                 
                   
                     - 
                     n 
                   
                   · 
                   G 
                 
                 - 
                 1 
               
             
             ) 
           
         
       
     
     The coordinate of the top right pixel of the n-th (initialized at zero, which is the one closest to the left edge) middle candidates above the top edge of current block may be denoted as follows: 
     
       
         
           
             ( 
             
               
                 
                   
                     - 
                     n 
                   
                   · 
                   G 
                 
                 - 
                 1 
               
               , 
               
                 
                   H 
                   - 
                   
                     n 
                     · 
                     G 
                   
                 
                 2 
               
               , 
             
             ) 
           
         
       
     
     The middle candidates can be included in the front of the merge candidate list. As a non-limiting example, the order is F(i, j), G(i, j), A(i, j), B(i, j), C(i, j), D(i, j), E(i, j) when generating the merge candidate list. Alternatively, these middle candidates can be included in the end of merge candidate list. For example, the order might be A(i, j), B(i, j), C(i, j), D(i, j), E(i, j), F(i, j), G(i, j), as non-limiting example. 
     In another embodiment, the two nearest neighboring middle candidates are not added. The middle candidates that are not adjacent to the current block are inserted into the merge candidate list. 
     In another embodiment, the non-adjacent above candidates or the non-adjacent left candidates can be removed from the merge candidate list. When adding the middle candidate in the above area, the above non-adjacent candidate can be replaced. When adding the middle candidate in the left area, the left non-adjacent candidate can be replaced. 
     In another embodiment, the above right or the bottom left non-adjacent candidate can be replaced by the middle candidates. 
     The locations of middle candidates on the top or to the left are determined by calculating the middle points of the reference left and right positions. After getting such a middle point, the selection of a reference candidate location for the middle candidate can be either at the left-hand side or right-hand side of the middle point. For example, as shown in  FIG. 11 , A0 (N/2) and A0(N/2+1) are two neighboring blocks next to the middle point on top of current block. Either of the two can be regarded as a middle candidate. Similarly, the middle candidates can be selected from either immediately above or below the middle point to the left. 
       FIG. 11  illustrates a current block and its surrounding neighboring sub-blocks as spatial merge candidates. A00: the top-left sub-block of current block; A0n: the above sub-blocks of current block (n=1˜(N+M)); Lm0: the left sub-blocks of current block (m=1˜(M+N)). 
     The techniques described above, can be implemented as computer software using computer-readable instructions and physically stored in one or more computer-readable media. For example,  FIG. 12  shows a computer system  1200  suitable for implementing certain embodiments of the disclosed subject matter. 
     The computer software can be coded using any suitable machine code or computer language, that may be subject to assembly, compilation, linking, or like mechanisms to create code comprising instructions that can be executed directly, or through interpretation, micro-code execution, and the like, by computer central processing units (CPUs), Graphics Processing Units (GPUs), and the like. 
     The instructions can be executed on various types of computers or components thereof, including, for example, personal computers, tablet computers, servers, smartphones, gaming devices, internet of things devices, and the like. 
     The components shown in  FIG. 12  for computer system  1200  are exemplary in nature and are not intended to suggest any limitation as to the scope of use or functionality of the computer software implementing embodiments of the present disclosure. Neither should the configuration of components be interpreted as having any dependency or requirement relating to any one or combination of components illustrated in the exemplary embodiment of a computer system  1200 . 
     Computer system  1200  may include certain human interface input devices. Such a human interface input device may be responsive to input by one or more human users through, for example, tactile input (such as: keystrokes, swipes, data glove movements), audio input (such as: voice, clapping), visual input (such as: gestures), olfactory input (not depicted). The human interface devices can also be used to capture certain media not necessarily directly related to conscious input by a human, such as audio (such as: speech, music, ambient sound), images (such as: scanned images, photographic images obtain from a still image camera), video (such as two-dimensional video, three-dimensional video including stereoscopic video). 
     Input human interface devices may include one or more of (only one of each depicted): keyboard  1201 , mouse  1202 , trackpad  1203 , touch screen  1210 , data-glove  1204 , joystick  1205 , microphone  1206 , scanner  1207 , camera  1208 . 
     Computer system  1200  may also include certain human interface output devices. Such human interface output devices may be stimulating the senses of one or more human users through, for example, tactile output, sound, light, and smell/taste. Such human interface output devices may include tactile output devices (for example tactile feedback by the touch-screen  1210 , data-glove  1204 , or joystick  1205 , but there can also be tactile feedback devices that do not serve as input devices), audio output devices (such as: speakers  1209 , headphones (not depicted)), visual output devices (such as screens  1210  to include cathode ray tube (CRT) screens, liquid-crystal display (LCD) screens, plasma screens, organic light-emitting diode (OLED) screens, each with or without touch-screen input capability, each with or without tactile feedback capability—some of which may be capable to output two dimensional visual output or more than three dimensional output through means such as stereographic output; virtual-reality glasses (not depicted), holographic displays and smoke tanks (not depicted)), and printers (not depicted). 
     Computer system  1200  can also include human accessible storage devices and their associated media such as optical media including CD/DVD ROM/RW  1220  with CD/DVD or the like media  1221 , thumb-drive  1222 , removable hard drive or solid state drive  1223 , legacy magnetic media such as tape and floppy disc (not depicted), specialized ROM/ASIC/PLD based devices such as security dongles (not depicted), and the like. 
     Those skilled in the art should also understand that term “computer readable media” as used in connection with the presently disclosed subject matter does not encompass transmission media, carrier waves, or other transitory signals. 
     Computer system  1200  can also include interface(s) to one or more communication networks. Networks can for example be wireless, wireline, optical. Networks can further be local, wide-area, metropolitan, vehicular and industrial, real-time, delay-tolerant, and so on. Examples of networks include local area networks such as Ethernet, wireless LANs, cellular networks to include global systems for mobile communications (GSM), third generation (3G), fourth generation (4G), fifth generation (5G), Long-Term Evolution (LTE), and the like, TV wireline or wireless wide area digital networks to include cable TV, satellite TV, and terrestrial broadcast TV, vehicular and industrial to include CANBus, and so forth. Certain networks commonly require external network interface adapters that attached to certain general purpose data ports or peripheral buses ( 1249 ) (such as, for example universal serial bus (USB) ports of the computer system  1200 ; others are commonly integrated into the core of the computer system  1200  by attachment to a system bus as described below (for example Ethernet interface into a PC computer system or cellular network interface into a smartphone computer system). Using any of these networks, computer system  1200  can communicate with other entities. Such communication can be uni-directional, receive only (for example, broadcast TV), uni-directional send-only (for example CANbus to certain CANbus devices), or bi-directional, for example to other computer systems using local or wide area digital networks. Certain protocols and protocol stacks can be used on each of those networks and network interfaces as described above. 
     Aforementioned human interface devices, human-accessible storage devices, and network interfaces can be attached to a core  1240  of the computer system  1200 . 
     The core  1240  can include one or more Central Processing Units (CPU)  1241 , Graphics Processing Units (GPU)  1242 , specialized programmable processing units in the form of Field Programmable Gate Areas (FPGA)  1243 , hardware accelerators for certain tasks  1244 , and so forth. These devices, along with Read-only memory (ROM)  1245 , Random-access memory (RAM)  1246 , internal mass storage such as internal non-user accessible hard drives, solid-state drives (SSDs), and the like  1247 , may be connected through a system bus  1248 . In some computer systems, the system bus  1248  can be accessible in the form of one or more physical plugs to enable extensions by additional CPUs, GPU, and the like. The peripheral devices can be attached either directly to the core&#39;s system bus  1248 , or through a peripheral bus  1249 . Architectures for a peripheral bus include peripheral component interconnect (PCI), USB, and the like. 
     CPUs  1241 , GPUs  1242 , FPGAs  1243 , and accelerators  1244  can execute certain instructions that, in combination, can make up the aforementioned computer code. That computer code can be stored in ROM  1245  or RAM  1246 . Transitional data can be also be stored in RAM  1246 , whereas permanent data can be stored for example, in the internal mass storage  1247 . Fast storage and retrieve to any of the memory devices can be enabled through the use of cache memory, that can be closely associated with one or more CPU  1241 , GPU  1242 , mass storage  1247 , ROM  1245 , RAM  1246 , and the like. 
     The computer readable media can have computer code thereon for performing various computer-implemented operations. The media and computer code can be those specially designed and constructed for the purposes of the present disclosure, or they can be of the kind well known and available to those having skill in the computer software arts. 
     As an example and not by way of limitation, the computer system having architecture  1200 , and specifically the core  1240  can provide functionality as a result of processor(s) (including CPUs, GPUs, FPGA, accelerators, and the like) executing software embodied in one or more tangible, computer-readable media. Such computer-readable media can be media associated with user-accessible mass storage as introduced above, as well as certain storage of the core  1240  that are of non-transitory nature, such as core-internal mass storage  1247  or ROM  1245 . The software implementing various embodiments of the present disclosure can be stored in such devices and executed by core  1240 . A computer-readable medium can include one or more memory devices or chips, according to particular needs. The software can cause the core  1240  and specifically the processors therein (including CPU, GPU, FPGA, and the like) to execute particular processes or particular parts of particular processes described herein, including defining data structures stored in RAM  1246  and modifying such data structures according to the processes defined by the software. In addition or as an alternative, the computer system can provide functionality as a result of logic hardwired or otherwise embodied in a circuit (for example: accelerator  1244 ), which can operate in place of or together with software to execute particular processes or particular parts of particular processes described herein. Reference to software can encompass logic, and vice versa, where appropriate. Reference to a computer-readable media can encompass a circuit (such as an integrated circuit (IC)) storing software for execution, a circuit embodying logic for execution, or both, where appropriate. The present disclosure encompasses any suitable combination of hardware and software. 
     While this disclosure has described several exemplary embodiments, there are alterations, permutations, and various substitute equivalents, which fall within the scope of the disclosure. It will thus be appreciated that those skilled in the art will be able to devise numerous systems and methods which, although not explicitly shown or described herein, embody the principles of the disclosure and are thus within the spirit and scope thereof 
     Acronyms: 
     High Efficiency Video Coding (HEVC) 
     Coding unit (CU) 
     Prediction unit (PU) 
     Joint Exploration Model 7 (JEM 7) 
     Joint Video Exploration Team (WET) 
     Alternative-temporal motion vector prediction (ATMVP) 
     Spatial-temporal motion vector prediction (STMVP) 
     Predicted (P) slice 
     Bi-directional predicted (B) slice 
     Rate-distortion (RD) 
     Temporal motion vector prediction (TMVP) 
     Context-adaptive binary arithmetic coding (CABAC) 
     Versatile Video Coding (VVC) 
     Supplementary Enhancement Information (SEI)