Patent Publication Number: US-2020296360-A1

Title: Inter merge mode and intra block copy merge mode in the same shared merge list area

Description:
This application claims the benefit of U.S. Provisional Application No. 62/818,022, filed Mar. 13, 2019, which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to video encoding and video decoding. 
     BACKGROUND 
     Digital video capabilities can be incorporated into a wide range of devices, including digital televisions, digital direct broadcast systems, wireless broadcast systems, personal digital assistants (PDAs), laptop or desktop computers, tablet computers, e-book readers, digital cameras, digital recording devices, digital media players, video gaming devices, video game consoles, cellular or satellite radio telephones, so-called “smart phones,” video teleconferencing devices, video streaming devices, and the like. Digital video devices implement video coding techniques, such as those described in the standards defined by MPEG-2, MPEG-4, ITU-T H.263, ITU-T H.264/MPEG-4, Part 10, Advanced Video Coding (AVC), ITU-T H.265/High Efficiency Video Coding (HEVC), and extensions of such standards. The video devices may transmit, receive, encode, decode, and/or store digital video information more efficiently by implementing such video coding techniques. 
     Video coding techniques include spatial (intra-picture) prediction and/or temporal (inter-picture) prediction to reduce or remove redundancy inherent in video sequences. For block-based video coding, a video slice (e.g., a video picture or a portion of a video picture) may be partitioned into video blocks, which may also be referred to as coding tree units (CTUs), coding units (CUs) and/or coding nodes. Video blocks in an intra-coded (I) slice of a picture are encoded using spatial prediction with respect to reference samples in neighboring blocks in the same picture. Video blocks in an inter-coded (P or B) slice of a picture may use spatial prediction with respect to reference samples in neighboring blocks in the same picture or temporal prediction with respect to reference samples in other reference pictures. Pictures may be referred to as frames, and reference pictures may be referred to as reference frames. 
     SUMMARY 
     In general, this disclosure describes techniques related to intra block copy (IBC) mode and shared motion vector predictor list design. The techniques of this disclosure may, for certain processing areas of video data, prevent a video coder (e.g., a video encoder or a video decoder) from generating separate merge candidate lists for IBC merge/list mode and inter merge/list mode. Instead, when a processing area meets a criterion, if a block in the processing area is coded using IBC merge/skip mode, the video coder may disable use of inter merge/skip mode for coding any of the remaining blocks in the processing area. Similarly, when a processing area meets a criterion, if a block in the processing area is coded using inter merge/skip mode, the video coder may disable use of IBC merge/skip mode for coding any of the remaining blocks in the processing area. 
     The techniques of this disclosure may be applied to any of the existing video codecs, such as HEVC (High Efficiency Video Coding) or be an efficient coding tool in any future video coding standards (e.g., Versatile Video Coding (VVC)). JEM (Joint Exploration Model) techniques related to this disclosure are discussed, although it will be understood that the techniques of this disclosure are not limited to JEM and may also be applicable to other existing and/or future-arising standards, such as VVC. 
     In one example, a method for coding video data includes determining that a first block of video data in a processing area is coded using a first prediction mode. The method further includes determining whether a characteristic of the processing area meets a criterion. The method further includes in response to determining that the characteristic of the processing area meets the criterion, determining, based at least in part on the first prediction mode used to code the first block of video data in the processing area, whether to use a second prediction mode to code a current block of video data in the processing area. The method further includes in response to determining not to use the second prediction mode to code the current block of video data, coding the current block of video data using a default prediction mode. 
     In another example, a device for coding video data includes a memory configured to store video data. The device further includes processing circuitry in communication with the memory, the processing circuitry being configured to: determine that a first block of video data in a processing area is coded using a first prediction mode; determine whether a characteristic of the processing area meets a criterion; in response to determining that the characteristic of the processing area meets the criterion, determine, based at least in part on the first prediction mode used to code the first block of video data in the processing area, whether to use a second prediction mode to code a current block of video data in the processing area; and in response to determining not to use the second prediction mode to code the current block of video data, code the current block of video data using a default prediction mode. 
     In another example, an apparatus for coding video data includes means for determining that a first block of video data in a processing area is coded using a first prediction mode; means for determining whether a characteristic of the processing area meets a criterion; means for, in response to determining that the characteristic of the processing area meets the criterion, determining, based at least in part on the first prediction mode used to code the first block of video data in the processing area, whether to use a second prediction mode to code a current block of video data in the processing area; and means for, in response to determining not to use the second prediction mode to code the current block of video data, coding the current block of video data using a default prediction mode. 
     In another example, a computer-readable storage medium is encoded with instructions that, when executed, cause a programmable processor to: determine that a first block of video data in a processing area is coded using a first prediction mode; determine whether a characteristic of the processing area meets a criterion; in response to determining that the characteristic of the processing area meets the criterion, determine, based at least in part on the first prediction mode used to code the first block of video data in the processing area, whether to use a second prediction mode to code a current block of video data in the processing area; and in response to determining not to use the second prediction mode to code the current block of video data, code the current block of video data using a default prediction mode. 
     The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description, drawings, and claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating an example video encoding and decoding system that may perform the techniques of this disclosure. 
         FIGS. 2A and 2B  are conceptual diagrams illustrating an example quadtree binary tree (QTBT) structure, and a corresponding coding tree unit (CTU). 
         FIGS. 3A and 3B  are conceptual diagrams illustrating spatial neighboring candidates in HEVC. 
         FIGS. 4A and 4B  are conceptual diagrams illustrating example temporal motion vector predictor (TMVP) candidates and motion vector (MV) scaling. 
         FIG. 5  illustrates an example of an intra block copy (IBC) coding process, in accordance with one or more techniques of this disclosure. 
         FIG. 6  illustrates examples of merge sharing nodes. 
         FIG. 7  is a block diagram illustrating an example video encoder that may perform the techniques of this disclosure. 
         FIG. 8  is a block diagram illustrating an example video decoder that may perform the techniques of this disclosure. 
         FIG. 9  is a flowchart illustrating an example method for encoding a current block. 
         FIG. 10  is a flowchart illustrating an example method for decoding a current block. 
         FIG. 11  is a flow diagram illustrating a method of coding video data according to the techniques of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure describes techniques related to intra block copy (IBC) mode and shared motion vector predictor list design. The techniques of this disclosure may, for certain processing areas of video data, prevent a video coder (e.g., a video encoder or a video decoder) from using both IBC merge/skip mode and inter merge/skip mode to code blocks within the same processing area. Instead, when a processing area meets a criterion, if a block in the processing area is coded using IBC merge/skip mode, the video coder may disable use of inter merge/skip mode for coding any of the remaining blocks in the processing area. Similarly, when a processing area meets a criterion, if a block in the processing area is coded using inter merge/skip mode, the video coder may disable use of IBC merge/skip mode for coding any of the remaining blocks in the processing area. 
     By preventing the video coder from using both IBC merge/skip mode and inter merge/skip mode to code blocks within the same processing area if the processing area meets a criterion, aspects of the present disclosure may potentially reduce the amount of processing required by the video coder to code blocks in such processing areas. For example, by preventing the video coder from using both IBC merge/skip mode and inter merge/skip mode to code blocks within the same processing area, aspects of the present disclosure may enable the video coder to refrain from generating separate merge candidate lists for IBC merge/list mode and inter merge/list mode, thereby reducing the amount of processing performed by the video coder to code blocks within the processing area. The techniques of this disclosure may be applied to any of the existing video codecs, such as High Efficiency Video Coding (HEVC), current drafts of Versatile Video Coding (VVC), or be an efficient coding tool in any future video coding standards. 
       FIG. 1  is a block diagram illustrating an example video encoding and decoding system  100  that may perform the techniques of this disclosure. The techniques of this disclosure are generally directed to coding (encoding and/or decoding) video data. In general, video data includes any data for processing a video. Thus, video data may include raw, unencoded video, encoded video, decoded (e.g., reconstructed) video, and video metadata, such as signaling data. 
     As shown in  FIG. 1 , system  100  includes a source device  102  that provides encoded video data to be decoded and displayed by a destination device  116 , in this example. In particular, source device  102  provides the video data to destination device  116  via a computer-readable medium  110 . Source device  102  and destination device  116  may comprise any of a wide range of devices, including desktop computers, notebook (i.e., laptop) computers, tablet computers, set-top boxes, telephone handsets such as smartphones, televisions, cameras, display devices, digital media players, video gaming consoles, video streaming device, or the like. In some cases, source device  102  and destination device  116  may be equipped for wireless communication, and thus may be referred to as wireless communication devices. 
     In the example of  FIG. 1 , source device  102  includes video source  104 , memory  106 , video encoder  200 , and output interface  108 . Destination device  116  includes input interface  122 , video decoder  300 , memory  120 , and display device  118 . In accordance with this disclosure, video encoder  200  of source device  102  and video decoder  300  of destination device  116  may be configured to apply the techniques for of the present disclosure to determine that a first block of video data in a processing area is coded using a first prediction mode, determine whether a characteristic of the processing area meets a criterion, in response to determining that the characteristic of the processing area meets the criterion, determine, based at least in part on the first prediction mode used to code the first block of video data in the processing area, whether to use a second prediction mode to code a current block of video data in the processing area, and in response to determining not to use the second prediction mode to code the current block of video data, code the current block of video data using a default prediction mode. Thus, source device  102  represents an example of a video encoding device, while destination device  116  represents an example of a video decoding device. In other examples, a source device and a destination device may include other components or arrangements. For example, source device  102  may receive video data from an external video source, such as an external camera. Likewise, destination device  116  may interface with an external display device, rather than include an integrated display device. 
     System  100  as shown in  FIG. 1  is merely one example. In general, any digital video encoding and/or decoding device may perform techniques for determining that a first block of video data in a processing area is coded using a first prediction mode, determining whether a characteristic of the processing area meets a criterion, in response to determining that the characteristic of the processing area meets the criterion, determining, based at least in part on the first prediction mode used to code the first block of video data in the processing area, whether to use a second prediction mode to code a current block of video data in the processing area, and in response to determining not to use the second prediction mode to code the current block of video data, coding the current block of video data using a default prediction mode. Source device  102  and destination device  116  are merely examples of such coding devices in which source device  102  generates coded video data for transmission to destination device  116 . This disclosure refers to a “coding” device as a device that performs coding (encoding and/or decoding) of data. Thus, video encoder  200  and video decoder  300  represent examples of coding devices, in particular, a video encoder and a video decoder, respectively. In some examples, source device  102  and destination device  116  may operate in a substantially symmetrical manner such that each of source device  102  and destination device  116  includes video encoding and decoding components. Hence, system  100  may support one-way or two-way video transmission between source device  102  and destination device  116 , e.g., for video streaming, video playback, video broadcasting, or video telephony. 
     In general, video source  104  represents a source of video data (i.e., raw, unencoded video data) and provides a sequential series of pictures (also referred to as “frames”) of the video data to video encoder  200 , which encodes data for the pictures. Video source  104  of source device  102  may include a video capture device, such as a video camera, a video archive containing previously captured raw video, and/or a video feed interface to receive video from a video content provider. As a further alternative, video source  104  may generate computer graphics-based data as the source video, or a combination of live video, archived video, and computer-generated video. In each case, video encoder  200  encodes the captured, pre-captured, or computer-generated video data. Video encoder  200  may rearrange the pictures from the received order (sometimes referred to as “display order”) into a coding order for coding. Video encoder  200  may generate a bitstream including encoded video data. Source device  102  may then output the encoded video data via output interface  108  onto computer-readable medium  110  for reception and/or retrieval by, e.g., input interface  122  of destination device  116 . 
     Memory  106  of source device  102  and memory  120  of destination device  116  represent general purpose memories. In some examples, memories  106 ,  120  may store raw video data, e.g., raw video from video source  104  and raw, decoded video data from video decoder  300 . Additionally or alternatively, memories  106 ,  120  may store software instructions executable by, e.g., video encoder  200  and video decoder  300 , respectively. Although memory  106  and memory  120  are shown separately from video encoder  200  and video decoder  300  in this example, it should be understood that video encoder  200  and video decoder  300  may also include internal memories for functionally similar or equivalent purposes. Furthermore, memories  106 ,  120  may store encoded video data, e.g., output from video encoder  200  and input to video decoder  300 . In some examples, portions of memories  106 ,  120  may be allocated as one or more video buffers, e.g., to store raw, decoded, and/or encoded video data. 
     Computer-readable medium  110  may represent any type of medium or device capable of transporting the encoded video data from source device  102  to destination device  116 . In one example, computer-readable medium  110  represents a communication medium to enable source device  102  to transmit encoded video data directly to destination device  116  in real-time, e.g., via a radio frequency network or computer-based network. Output interface  108  may modulate a transmission signal including the encoded video data, and input interface  122  may demodulate the received transmission signal, according to a communication standard, such as a wireless communication protocol. The communication medium may comprise any wireless or wired communication medium, such as a radio frequency (RF) spectrum or one or more physical transmission lines. The communication medium may form part of a packet-based network, such as a local area network, a wide-area network, or a global network such as the Internet. The communication medium may include routers, switches, base stations, or any other equipment that may be useful to facilitate communication from source device  102  to destination device  116 . 
     In some examples, source device  102  may output encoded data from output interface  108  to storage device  112 . Similarly, destination device  116  may access encoded data from storage device  112  via input interface  122 . Storage device  112  may include any of a variety of distributed or locally accessed data storage media such as a hard drive, Blu-ray discs, DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or any other suitable digital storage media for storing encoded video data. 
     In some examples, source device  102  may output encoded video data to file server  114  or another intermediate storage device that may store the encoded video generated by source device  102 . Destination device  116  may access stored video data from file server  114  via streaming or download. File server  114  may be any type of server device capable of storing encoded video data and transmitting that encoded video data to the destination device  116 . File server  114  may represent a web server (e.g., for a website), a File Transfer Protocol (FTP) server, a content delivery network device, or a network attached storage (NAS) device. Destination device  116  may access encoded video data from file server  114  through any standard data connection, including an Internet connection. This may include a wireless channel (e.g., a Wi-Fi connection), a wired connection (e.g., digital subscriber line (DSL), cable modem, etc.), or a combination of both that is suitable for accessing encoded video data stored on file server  114 . File server  114  and input interface  122  may be configured to operate according to a streaming transmission protocol, a download transmission protocol, or a combination thereof. 
     Output interface  108  and input interface  122  may represent wireless transmitters/receivers, modems, wired networking components (e.g., Ethernet cards), wireless communication components that operate according to any of a variety of IEEE 802.11 standards, or other physical components. In examples where output interface  108  and input interface  122  comprise wireless components, output interface  108  and input interface  122  may be configured to transfer data, such as encoded video data, according to a cellular communication standard, such as 4G, 4G-LTE (Long-Term Evolution), LTE Advanced, 5G, or the like. In some examples where output interface  108  comprises a wireless transmitter, output interface  108  and input interface  122  may be configured to transfer data, such as encoded video data, according to other wireless standards, such as an IEEE 802.11 specification, an IEEE 802.15 specification (e.g., ZigBee™), a Bluetooth™ standard, or the like. In some examples, source device  102  and/or destination device  116  may include respective system-on-a-chip (SoC) devices. For example, source device  102  may include an SoC device to perform the functionality attributed to video encoder  200  and/or output interface  108 , and destination device  116  may include an SoC device to perform the functionality attributed to video decoder  300  and/or input interface  122 . 
     The techniques of this disclosure may be applied to video coding in support of any of a variety of multimedia applications, such as over-the-air television broadcasts, cable television transmissions, satellite television transmissions, Internet streaming video transmissions, such as dynamic adaptive streaming over HTTP (DASH), digital video that is encoded onto a data storage medium, decoding of digital video stored on a data storage medium, or other applications. 
     Input interface  122  of destination device  116  receives an encoded video bitstream from computer-readable medium  110  (e.g., a communication medium, storage device  112 , file server  114 , or the like). The encoded video bitstream may include signaling information defined by video encoder  200 , which is also used by video decoder  300 , such as syntax elements having values that describe characteristics and/or processing of video blocks or other coded units (e.g., slices, pictures, groups of pictures, sequences, or the like). Display device  118  displays decoded pictures of the decoded video data to a user. Display device  118  may represent any of a variety of display devices such as a cathode ray tube (CRT), a liquid crystal display (LCD), a plasma display, an organic light emitting diode (OLED) display, or another type of display device. 
     Although not shown in  FIG. 1 , in some examples, video encoder  200  and video decoder  300  may each be integrated with an audio encoder and/or audio decoder, and may include appropriate MUX-DEMUX units, or other hardware and/or software, to handle multiplexed streams including both audio and video in a common data stream. If applicable, MUX-DEMUX units may conform to the ITU H.223 multiplexer protocol, or other protocols such as the user datagram protocol (UDP). 
     Video encoder  200  and video decoder  300  each may be implemented as any of a variety of suitable encoder and/or decoder circuitry, such as one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), discrete logic, software, hardware, firmware or any combinations thereof. When the techniques are implemented partially in software, a device may store instructions for the software in a suitable, non-transitory computer-readable medium and execute the instructions in hardware using one or more processors to perform the techniques of this disclosure. Each of video encoder  200  and video decoder  300  may be included in one or more encoders or decoders, either of which may be integrated as part of a combined encoder/decoder (CODEC) in a respective device. A device including video encoder  200  and/or video decoder  300  may comprise an integrated circuit, a microprocessor, and/or a wireless communication device, such as a cellular telephone. 
     Video encoder  200  and video decoder  300  may operate according to a video coding standard, such as ITU-T H.265, also referred to as High Efficiency Video Coding (HEVC) or extensions thereto, such as the multi-view and/or scalable video coding extensions. Alternatively, video encoder  200  and video decoder  300  may operate according to other proprietary or industry standards, such as the Joint Exploration Test Model (JEM) or ITU-T H.266, also referred to as Versatile Video Coding (VVC). A recent draft of the VVC standard is described in Bross, et al. “Versatile Video Coding (Draft 7),” Joint Video Experts Team (WET) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 16th Meeting: Geneva, CH, 1-11 Oct. 2019, JVET-P2001-v9 (hereinafter “VVC Draft 7”). The techniques of this disclosure, however, are not limited to any particular coding standard. 
     In general, video encoder  200  and video decoder  300  may perform block-based coding of pictures. The term “block” generally refers to a structure including data to be processed (e.g., encoded, decoded, or otherwise used in the encoding and/or decoding process). For example, a block may include a two-dimensional matrix of samples of luminance and/or chrominance data. In general, video encoder  200  and video decoder  300  may code video data represented in a YUV (e.g., Y, Cb, Cr) format. That is, rather than coding red, green, and blue (RGB) data for samples of a picture, video encoder  200  and video decoder  300  may code luminance and chrominance components, where the chrominance components may include both red hue and blue hue chrominance components. In some examples, video encoder  200  converts received RGB formatted data to a YUV representation prior to encoding, and video decoder  300  converts the YUV representation to the RGB format. Alternatively, pre- and post-processing units (not shown) may perform these conversions. 
     This disclosure may generally refer to coding (e.g., encoding and decoding) of pictures to include the process of encoding or decoding data of the picture. Similarly, this disclosure may refer to coding of blocks of a picture to include the process of encoding or decoding data for the blocks, e.g., prediction and/or residual coding. An encoded video bitstream generally includes a series of values for syntax elements representative of coding decisions (e.g., coding modes) and partitioning of pictures into blocks. Thus, references to coding a picture or a block should generally be understood as coding values for syntax elements forming the picture or block. 
     HEVC defines various blocks, including coding units (CUs), prediction units (PUs), and transform units (TUs). According to HEVC, a video coder (such as video encoder  200 ) partitions a coding tree unit (CTU), also called a coding tree block (CTB), into CUs according to a quadtree structure. That is, the video coder partitions CTUs and CUs into four equal, non-overlapping squares, and each node of the quadtree has either zero or four child nodes. Nodes without child nodes may be referred to as “leaf nodes,” and CUs of such leaf nodes may include one or more PUs and/or one or more TUs. The video coder may further partition PUs and TUs. For example, in HEVC, a residual quadtree (RQT) represents partitioning of TUs. In HEVC, PUs represent inter-prediction data, while TUs represent residual data. CUs that are intra-predicted include intra-prediction information, such as an intra-mode indication. 
     As another example, video encoder  200  and video decoder  300  may be configured to operate according to JEM or VVC. According to JEM or VVC, a video coder (such as video encoder  200 ) partitions a picture into a plurality of coding tree units (CTUs). Video encoder  200  may partition a CTU according to a tree structure, such as a quadtree-binary tree (QTBT) structure or Multi-Type Tree (MTT) structure. The QTBT structure removes the concepts of multiple partition types, such as the separation between CUs, PUs, and TUs of HEVC. A QTBT structure includes two levels: a first level partitioned according to quadtree partitioning, and a second level partitioned according to binary tree partitioning. A root node of the QTBT structure corresponds to a CTU. Leaf nodes of the binary trees correspond to CUs. 
     In an MTT partitioning structure, blocks may be partitioned using a quadtree (QT) partition, a binary tree (BT) partition, and one or more types of triple tree (TT) (also called ternary tree (TT)) partitions. A triple or ternary tree partition is a partition where a block is split into three sub-blocks. In some examples, a triple or ternary tree partition divides a block into three sub-blocks without dividing the original block through the center. The partitioning types in MTT (e.g., QT, BT, and TT), may be symmetrical or asymmetrical. 
     In some examples, video encoder  200  and video decoder  300  may use a single QTBT or MTT structure to represent each of the luminance and chrominance components, while in other examples, video encoder  200  and video decoder  300  may use two or more QTBT or MTT structures, such as one QTBT/MTT structure for the luminance component and another QTBT/MTT structure for both chrominance components (or two QTBT/MTT structures for respective chrominance components). 
     Video encoder  200  and video decoder  300  may be configured to use quadtree partitioning per HEVC, QTBT partitioning, MTT partitioning, or other partitioning structures. For purposes of explanation, the description of the techniques of this disclosure is presented with respect to QTBT partitioning. However, it should be understood that the techniques of this disclosure may also be applied to video coders configured to use quadtree partitioning, or other types of partitioning as well. 
     The blocks (e.g., CTUs or CUs) may be grouped in various ways in a picture. As one example, a brick may refer to a rectangular region of CTU rows within a particular tile in a picture. A tile may be a rectangular region of CTUs within a particular tile column and a particular tile row in a picture. A tile column refers to a rectangular region of CTUs having a height equal to the height of the picture and a width specified by syntax elements (e.g., such as in a picture parameter set). A tile row refers to a rectangular region of CTUs having a height specified by syntax elements (e.g., such as in a picture parameter set) and a width equal to the width of the picture. 
     In some examples, a tile may be partitioned into multiple bricks, each of which may include one or more CTU rows within the tile. A tile that is not partitioned into multiple bricks may also be referred to as a brick. However, a brick that is a true subset of a tile may not be referred to as a tile. 
     The bricks in a picture may also be arranged in a slice. A slice may be an integer number of bricks of a picture that may be exclusively contained in a single network abstraction layer (NAL) unit. In some examples, a slice includes either a number of complete tiles or only a consecutive sequence of complete bricks of one tile. 
     This disclosure may use “N×N” and “N by N” interchangeably to refer to the sample dimensions of a block (such as a CU or other video block) in terms of vertical and horizontal dimensions, e.g., 16×16 samples or 16 by 16 samples. In general, a 16×16 CU will have 16 samples in a vertical direction (y=16) and 16 samples in a horizontal direction (x=16). Likewise, an N×N CU generally has N samples in a vertical direction and N samples in a horizontal direction, where N represents a nonnegative integer value. The samples in a CU may be arranged in rows and columns. Moreover, CUs need not necessarily have the same number of samples in the horizontal direction as in the vertical direction. For example, CUs may comprise N×M samples, where M is not necessarily equal to N. 
     Video encoder  200  encodes video data for CUs representing prediction and/or residual information, and other information. The prediction information indicates how the CU is to be predicted in order to form a prediction block for the CU. The residual information generally represents sample-by-sample differences between samples of the CU prior to encoding and the prediction block. 
     To predict a CU, video encoder  200  may generally form a prediction block for the CU through inter-prediction or intra-prediction. Inter-prediction generally refers to predicting the CU from data of a previously coded picture, whereas intra-prediction generally refers to predicting the CU from previously coded data of the same picture. To perform inter-prediction, video encoder  200  may generate the prediction block using one or more motion vectors. Video encoder  200  may generally perform a motion search to identify a reference block that closely matches the CU, e.g., in terms of differences between the CU and the reference block. Video encoder  200  may calculate a difference metric using a sum of absolute difference (SAD), sum of squared differences (SSD), mean absolute difference (MAD), mean squared differences (MSD), or other such difference calculations to determine whether a reference block closely matches the current CU. In some examples, video encoder  200  may predict the current CU using uni-directional prediction or bi-directional prediction. 
     Some examples of JEM and VVC also provide an affine motion compensation mode, which may be considered an inter-prediction mode. In affine motion compensation mode, video encoder  200  may determine two or more motion vectors that represent non-translational motion, such as zoom in or out, rotation, perspective motion, or other irregular motion types. 
     As mentioned above, a video coder (e.g., video encoder  200  or video decoder  300 ) may apply inter prediction to generate a prediction block for a video block of a current picture. For instance, the video coder may apply inter prediction to generate a prediction block for a prediction block of a CU. If the video coder applies inter prediction to generate a prediction block, the video coder generates the prediction block based on decoded samples of one or more reference pictures. Typically, the reference pictures are pictures other than the current picture. In some video coding specifications, a video coder may also treat the current picture itself as a reference picture. The video coder may determine one or more reference picture lists. Each of the reference picture lists includes zero or more reference pictures. One of the reference picture lists may be referred to as Reference Picture List 0 (RefPicList0) and another reference picture list may be referred to as Reference Picture list 1 (RefPicList1). 
     The video coder may apply uni-directional inter prediction or bi-directional inter prediction to generate a prediction block. When the video coder applies uni-directional inter prediction to generate a prediction block for a video block, the video coder determines a single reference block for the video block based on a samples of a single reference picture. The reference block may be a block of samples that is similar to the prediction block. Furthermore, when the video coder applies uni-directional inter prediction, the video coder may set the prediction block equal to the reference block. When the video coder applies bi-directional inter prediction to generate a prediction block for a video block, the video coder determines two reference blocks for the video block. In some examples, the two reference blocks are in reference pictures in different reference picture lists. Additionally, when the video coder applies bi-direction inter-prediction, the video coder may determine the prediction block based on the two reference blocks. For instance, the video coder may determine the prediction block such that each sample of the prediction block is a weighted average of corresponding samples of the two reference blocks. Reference list indicators may be used to indicate which of the reference picture lists include reference pictures used for determining reference blocks. 
     As mentioned above, a video coder may determine a reference block based on samples of a reference picture. In some examples, the video coder may determine the reference block such that each sample of the reference block is equal to a sample of the reference picture. In some examples, as part of determining a reference block, the video coder may interpolate samples of the reference block from samples of the reference picture. For example, the video coder may determine that a sample of the prediction block is a weighted average of two or more samples of the reference picture. 
     In some examples, when video encoder  200  performs uni-directional inter prediction for a current block of a current picture, video encoder  200  identifies a reference block within one or more reference pictures in one of the reference picture lists. For instance, video encoder  200  may search for a reference block within the one or more reference pictures in the reference picture list. In some examples, video encoder  200  uses a mean squared error or other metric to determine the similarity between the reference block and the current block. Furthermore, video encoder  200  may determine motion parameters for the current block. The motion parameters for the current block may include a motion vector and a reference index. The motion vector may indicate a spatial displacement between a position of the current block within the current picture and a position of the reference block within the reference picture. The reference index indicates a position within the reference picture list of the reference frame that contains the reference picture list. The prediction block for the current block may be equal to the reference block. 
     When video encoder  200  performs bi-directional inter prediction for a current block of a current picture, video encoder  200  may identify a first reference block within reference pictures in a first reference picture list (“list 0”) and may identify a second reference block within reference pictures in a second reference picture list (“list 1”). For instance, video encoder  200  may search for the first and second reference blocks within the reference pictures in the first and second reference picture lists, respectively. Video encoder  200  may generate, based at least in part on the first and the second reference blocks, the prediction block for the current block. In addition, video encoder  200  may generate a first motion vector that indicates a spatial displacement between the current block and the first reference block. Video encoder  200  may also generate a first reference index that identifies a location within the first reference picture list of the reference picture that contains the first reference block. Furthermore, video encoder  200  may generate a second motion vector that indicates a spatial displacement between the current block and the second reference block. Video encoder  200  may also generate a second reference index that identifies a location within the second reference picture list of the reference picture that includes the second reference block. 
     When video encoder  200  performs uni-directional inter prediction on a current block, video decoder  300  may use the motion parameters of the current block to identify the reference block of the current block. Video decoder  300  may then generate the prediction block of the current block based on the reference block. When video encoder  200  performs bi-directional inter prediction to determine a prediction block for a current block, video decoder  300  may use the motion parameters of the current block to determine two reference blocks. Video decoder  300  may generate the prediction block of the current block based on the two reference samples of the current block. 
     Video encoder  200  may signal motion parameters of a block in various ways. Such motion parameters may include motion vectors, reference indexes, reference picture list indicators, and/or other data related to motion. In some examples, video encoder  200  and video decoder  300  may use motion prediction to reduce the amount of data used for signaling motion parameters. Motion prediction may comprise the determination of motion parameters of a block (e.g., a PU, a CU, etc.) based on motion parameters of one or more other blocks. There are various types of motion prediction. For instance, merge mode and advanced motion vector prediction (AMVP) mode are two types of motion prediction. 
     In merge mode, video encoder  200  generates a candidate list. The candidate list includes a set of candidates that indicate the motion parameters of one or more source blocks. The source blocks may spatially or temporally neighbor a current block. Furthermore, in merge mode, video encoder  200  may select a candidate from the candidate list and may use the motion parameters indicated by the selected candidate as the motion parameters of the current block. Video encoder  200  may signal the position in the candidate list of the selected candidate. Video decoder  300  may determine, based on information obtained from a bitstream, the index into the candidate list. In addition, video decoder  300  may generate the same candidate list and may determine, based on the index, the selected candidate. Video decoder  300  may then use the motion parameters of the selected candidate to generate a prediction block for the current block. 
     Skip mode is similar to merge mode. In skip mode, video encoder  200  and video decoder  300  generate and use a candidate list in the same way that video encoder  200  and video decoder  300  use the candidate list in merge mode. However, when video encoder  200  signals the motion parameters of a current block using skip mode, video encoder  200  does not signal any residual data for the current block. Accordingly, video decoder  300  may determine a prediction block for the current block based on one or more reference blocks indicated by the motion parameters of a selected candidate in the candidate list. Video decoder  300  may then reconstruct samples in a coding block of the current block such that the reconstructed samples are equal to corresponding samples in the prediction block of the current block. In some examples, merge mode and skip mode may be referred to as a merge/skip mode. Further, in some examples, a candidate list for merge mode and/or skip mode may be referred to as a merge candidate list, a merge/skip candidate list, a merge/skip list, and the like. 
     AMVP mode is similar to merge mode in that video encoder  200  may generate a candidate list for a current block and may select a candidate from the candidate list. However, for each respective reference block used in determining a prediction block for the current block, video encoder  200  may signal a respective motion vector difference (MVD) for the current block, a respective reference index for the current block, and a respective candidate index indicating a selected candidate in the candidate list. An MVD for a block may indicate a difference between a motion vector of the block and a motion vector of the selected candidate. The reference index for the current block indicates a reference picture from which a reference block is determined. 
     Furthermore, when AMVP mode is used, for each respective reference block used in determining a prediction block for the current block, video decoder  300  may determine a MVD for the current block, a reference index for the current block, and a candidate index and a motion vector prediction (MVP) flag. Video decoder  300  may generate the same candidate list and may determine, based on the candidate index, a selected candidate in the candidate list. As before, this candidate list may include motion vectors of neighboring blocks that are associated with the same reference index as well as a temporal motion vector predictor which is derived based on the motion parameters of the neighboring block of the co-located block in a temporal reference picture. Video decoder  300  may recover a motion vector of the current block by adding the MVD to the motion vector indicated by the selected AMVP candidate. That is, video decoder  300  may determine, based on a motion vector indicated by the selected AMVP candidate and the MVD, the motion vector of the current block. Video decoder  300  may then use the recovered motion vector or motion vectors of the current block to generate prediction blocks for the current block. 
     When a video coder (e.g., video encoder  200  or video decoder  300 ) generates an AMVP candidate list for a current block, the video coder may derive one or more AMVP candidates based on the motion parameters of reference blocks (e.g., spatially-neighboring blocks) that contain locations that spatially neighbor the current PU and one or more AMVP candidates based on motion parameters of PUs that temporally neighbor the current PU. The candidate list may include motion vectors of reference blocks that are associated with the same reference index as well as a temporal motion vector predictor which is derived based on the motion parameters (i.e., motion parameters) of the neighboring block of the co-located block in a temporal reference picture. A candidate in a merge candidate list or an AMVP candidate list that is based on the motion parameters of a reference block that temporally neighbors a current block. This disclosure may use the term “temporal motion vector predictor” to refer to a block that is in a different time instance than the current block and is used for motion vector prediction. 
     To perform intra-prediction, video encoder  200  may select an intra-prediction mode to generate the prediction block. Some examples of JEM and VVC provide sixty-seven intra-prediction modes, including various directional modes, as well as planar mode and DC mode. In general, video encoder  200  selects an intra-prediction mode that describes neighboring samples to a current block (e.g., a block of a CU) from which to predict samples of the current block. Such samples may generally be above, above and to the left, or to the left of the current block in the same picture as the current block, assuming video encoder  200  codes CTUs and CUs in raster scan order (left to right, top to bottom). 
     Intra block copy (IBC) generally refers to predicting the CU from data from a previously coded area of the current picture of the CU. To perform intra block copy, video encoder  200  may generate the prediction block using one or more motion vectors. Video encoder  200  may generally perform a motion search to identify a reference block in the current picture that closely matches the CU, e.g., in terms of differences between the CU and the reference block. Video encoder  200  may calculate a difference metric using a sum of absolute difference (SAD), sum of squared differences (SSD), mean absolute difference (MAD), mean squared differences (MSD), or other such difference calculations to determine whether a reference block closely matches the current CU. Similar to inter-prediction, motion parameters for IBC may be signaled via merge mode, mode, and/or AMVP mode similar to the techniques described above. 
     Video encoder  200  encodes data representing the prediction mode for a current block. For example, for inter-prediction modes, video encoder  200  may encode data representing which of the various available inter-prediction modes is used, as well as motion information for the corresponding mode. For uni-directional or bi-directional inter-prediction, for example, video encoder  200  may encode motion vectors using AMVP or merge mode. Video encoder  200  may use similar modes to encode motion vectors for affine motion compensation mode. 
     Following prediction, such as intra-prediction, IBC, or inter-prediction of a block, video encoder  200  may calculate residual data for the block. The residual data, such as a residual block, represents sample by sample differences between the block and a prediction block for the block, formed using the corresponding prediction mode. Video encoder  200  may apply one or more transforms to the residual block, to produce transformed data in a transform domain instead of the sample domain. For example, video encoder  200  may apply a discrete cosine transform (DCT), an integer transform, a wavelet transform, or a conceptually similar transform to residual video data. Additionally, video encoder  200  may apply a secondary transform following the first transform, such as a mode-dependent non-separable secondary transform (MDNSST), a signal dependent transform, a Karhunen-Loeve transform (KLT), or the like. Video encoder  200  produces transform coefficients following application of the one or more transforms. 
     As noted above, following any transforms to produce transform coefficients, video encoder  200  may perform quantization of the transform coefficients. Quantization generally refers to a process in which transform coefficients are quantized to possibly reduce the amount of data used to represent the transform coefficients, providing further compression. By performing the quantization process, video encoder  200  may reduce the bit depth associated with some or all of the transform coefficients. For example, video encoder  200  may round an n-bit value down to an m-bit value during quantization, where n is greater than m. In some examples, to perform quantization, video encoder  200  may perform a bitwise right-shift of the value to be quantized. 
     Following quantization, video encoder  200  may scan the transform coefficients, producing a one-dimensional vector from the two-dimensional matrix including the quantized transform coefficients. The scan may be designed to place higher energy (and therefore lower frequency) transform coefficients at the front of the vector and to place lower energy (and therefore higher frequency) transform coefficients at the back of the vector. In some examples, video encoder  200  may utilize a predefined scan order to scan the quantized transform coefficients to produce a serialized vector, and then entropy encode the quantized transform coefficients of the vector. In other examples, video encoder  200  may perform an adaptive scan. After scanning the quantized transform coefficients to form the one-dimensional vector, video encoder  200  may entropy encode the one-dimensional vector, e.g., according to context-adaptive binary arithmetic coding (CABAC). Video encoder  200  may also entropy encode values for syntax elements describing metadata associated with the encoded video data for use by video decoder  300  in decoding the video data. 
     To perform CABAC, video encoder  200  may assign a context within a context model to a symbol to be transmitted. The context may relate to, for example, whether neighboring values of the symbol are zero-valued or not. The probability determination may be based on a context assigned to the symbol. 
     Video encoder  200  may further generate syntax data, such as block-based syntax data, picture-based syntax data, and sequence-based syntax data, to video decoder  300 , e.g., in a picture header, a block header, a slice header, or other syntax data, such as a sequence parameter set (SPS), picture parameter set (PPS), or video parameter set (VPS). Video decoder  300  may likewise decode such syntax data to determine how to decode corresponding video data. 
     In this manner, video encoder  200  may generate a bitstream including encoded video data, e.g., syntax elements describing partitioning of a picture into blocks (e.g., CUs) and prediction and/or residual information for the blocks. Ultimately, video decoder  300  may receive the bitstream and decode the encoded video data. 
     In general, video decoder  300  performs a reciprocal process to that performed by video encoder  200  to decode the encoded video data of the bitstream. For example, video decoder  300  may decode values for syntax elements of the bitstream using CABAC in a manner substantially similar to, albeit reciprocal to, the CABAC encoding process of video encoder  200 . The syntax elements may define partitioning information of a picture into CTUs, and partitioning of each CTU according to a corresponding partition structure, such as a QTBT structure, to define CUs of the CTU. The syntax elements may further define prediction and residual information for blocks (e.g., CUs) of video data. 
     The residual information may be represented by, for example, quantized transform coefficients. Video decoder  300  may inverse quantize and inverse transform the quantized transform coefficients of a block to reproduce a residual block for the block. Video decoder  300  uses a signaled prediction mode (intra- or inter-prediction) and related prediction information (e.g., motion information for inter-prediction) to form a prediction block for the block. Video decoder  300  may then combine the prediction block and the residual block (on a sample-by-sample basis) to reproduce the original block. Video decoder  300  may perform additional processing, such as performing a deblocking process to reduce visual artifacts along boundaries of the block. 
     In accordance with the techniques of this disclosure, Video encoder  200  and/or video decoder  300  may determine that a first block of video data in a processing area is coded using a first prediction mode, determine whether a characteristic of the processing area meets a criterion, in response to determining that the characteristic of the processing area meets the criterion, determine, based at least in part on the first prediction mode used to code the first block of video data in the processing area, whether to use a second prediction mode to code a current block of video data in the processing area, and in response to determining not to use the second prediction mode to code the current block of video data, code the current block of video data using a default prediction mode 
     This disclosure may generally refer to “signaling” certain information, such as syntax elements. The term “signaling” may generally refer to the communication of values for syntax elements and/or other data used to decode encoded video data. That is, video encoder  200  may signal values for syntax elements in the bitstream. In general, signaling refers to generating a value in the bitstream. As noted above, source device  102  may transport the bitstream to destination device  116  substantially in real time, or not in real time, such as might occur when storing syntax elements to storage device  112  for later retrieval by destination device  116 . 
       FIGS. 2A and 2B  are conceptual diagrams illustrating an example quadtree binary tree (QTBT) structure  130 , and a corresponding CTU  132 . The solid lines represent quadtree splitting, and dotted lines indicate binary tree splitting. In each split (i.e., non-leaf) node of the binary tree, one flag is signaled to indicate which splitting type (i.e., horizontal or vertical) is used, where 0 indicates horizontal splitting and 1 indicates vertical splitting in this example. For the quadtree splitting, there is no need to indicate the splitting type, because quadtree nodes split a block horizontally and vertically into 4 sub-blocks with equal size. Accordingly, video encoder  200  may encode, and video decoder  300  may decode, syntax elements (such as splitting information) for a region tree level of QTBT structure  130  (i.e., the solid lines) and syntax elements (such as splitting information) for a prediction tree level of QTBT structure  130  (i.e., the dashed lines). Video encoder  200  may encode, and video decoder  300  may decode, video data, such as prediction and transform data, for CUs represented by terminal leaf nodes of QTBT structure  130 . Nodes  142 ,  144 ,  146  and  148  will be discussed later below. 
     In general, CTU  132  of  FIG. 2B  may be associated with parameters defining sizes of blocks corresponding to nodes of QTBT structure  130  at the first and second levels. These parameters may include a CTU size (representing a size of CTU  132  in samples), a minimum quadtree size (MinQTSize, representing a minimum allowed quadtree leaf node size), a maximum binary tree size (MaxBTSize, representing a maximum allowed binary tree root node size), a maximum binary tree depth (MaxBTDepth, representing a maximum allowed binary tree depth), and a minimum binary tree size (MinBTSize, representing the minimum allowed binary tree leaf node size). 
     The root node of a QTBT structure corresponding to a CTU may have four child nodes at the first level of the QTBT structure, each of which may be partitioned according to quadtree partitioning. That is, nodes of the first level are either leaf nodes (having no child nodes) or have four child nodes. The example of QTBT structure  130  represents such nodes as including the parent node and child nodes having solid lines for branches. If nodes of the first level are not larger than the maximum allowed binary tree root node size (MaxBTSize), then the nodes can be further partitioned by respective binary trees. The binary tree splitting of one node can be iterated until the nodes resulting from the split reach the minimum allowed binary tree leaf node size (MinBTSize) or the maximum allowed binary tree depth (MaxBTDepth). The example of QTBT structure  130  represents such nodes as having dashed lines for branches. The binary tree leaf node is referred to as a CU, which is used for prediction (e.g., intra-picture or inter-picture prediction) and transform, without any further partitioning. As discussed above, CUs may also be referred to as “video blocks” or “blocks.” 
     In one example of the QTBT partitioning structure, the CTU size is set as 128×128 (luma samples and two corresponding 64×64 chroma samples), the MinQTSize is set as 16×16, the MaxBTSize is set as 64×64, the MinBTSize (for both width and height) is set as 4, and the MaxBTDepth is set as 4. The quadtree partitioning is applied to the CTU first to generate quad-tree leaf nodes. The quadtree leaf nodes may have a size from 16×16 (i.e., the MinQTSize) to 128×128 (i.e., the CTU size). If the leaf quadtree node is 128×128, the leaf quadtree node will not be further split by the binary tree, because the size exceeds the MaxBTSize (i.e., 64×64, in this example). Otherwise, the leaf quadtree node will be further partitioned by the binary tree. Therefore, the quadtree leaf node is also the root node for the binary tree and has the binary tree depth as 0. When the binary tree depth reaches MaxBTDepth (4, in this example), no further splitting is permitted. The binary tree node having a width equal to MinBTSize (4, in this example) implies no further horizontal splitting is permitted. Similarly, a binary tree node having a height equal to MinBTSize implies no further vertical splitting is permitted for that binary tree node. As noted above, leaf nodes of the binary tree are referred to as CUs, and are further processed according to prediction and transform without further partitioning. 
     Aspects of CU structure and motion vector prediction in HEVC are described in the following paragraphs. In HEVC, the largest coding unit in a slice is called a CTB or CTU. In HEVC, a CTB contains a quad-tree the nodes of which are CUs. The size of a CTB may range from 16×16 to 64×64 in the HEVC main profile (although technically 8×8 CTB sizes can be supported). The size of a CU may range from being the same size of a CTB to being as small as 8×8. Each CU is coded with one mode, such as inter prediction or intra prediction. When a CU is inter coded, the CU may be further partitioned into 2 or 4 prediction units (PUs) or become just one PU when further partition does not apply. When two PUs are present in one CU, they can be half size rectangles or two rectangle size with quarter (¼) or three-quarter (¾) size of the CU. When the CU is inter coded, one set of motion information is present for each PU. In addition, each PU is coded with a unique inter prediction mode to derive the set of motion information. 
     Aspects of motion vector prediction in HEVC are discussed in the following paragraphs. In the HEVC standard, there are two inter prediction modes, named merge (skip is considered as a special case of merge) and advanced motion vector prediction (AMVP) modes respectively for a PU. In either AMVP or merge mode, a motion vector (MV) candidate list is maintained for multiple motion vector predictors. The motion vector(s), as well as reference indices in the merge mode, of the current PU are generated by taking one candidate from the MV candidate list. In the case of merge mode, the MV candidate list may be referred to as a “merge candidate list” and candidates in a merge candidate list may be referred to as “merge candidates.” Similarly, in the case of AMVP mode, the MV candidate list may be referred to as an “AMVP candidate list” and candidates in an AMVP candidate list may be referred to as “AMVP candidates.” In some instances, this disclosure may simply refer to an MV candidate list (e.g., a merge candidate list or an AMVP candidate list) as a “candidate list.” Furthermore, this disclosure may use the term “MV candidate” to refer to either a merge candidate or an AMVP candidate 
     In HEVC and certain other video coding standards, the MV candidate list may contain up to five (5) candidates for the merge mode and only two (2) candidates for the AMVP mode. A merge candidate may contain a set of motion information, e.g., motion vectors corresponding to both reference picture lists (list 0 and list 1) and the reference indices. In some examples, reference picture lists may also be referred to as “reference lists.” If a merge candidate is identified by a merge index, the reference pictures are used for the prediction of the current blocks, as well as the associated motion vectors are determined. However, under AMVP mode for each potential prediction direction from either list 0 or list 1, a reference index needs to be explicitly signaled, together with an MVP index to the MV candidate list since the AMVP candidate contains only a motion vector. In AMVP mode, the predicted motion vectors can be further refined. 
     As can be seen above, a merge candidate corresponds to a full set of motion information while an AMVP candidate contains just one motion vector for a specific prediction direction and reference index. The candidates for both modes are derived similarly from the same spatial and temporal neighboring blocks. 
       FIGS. 3A and 3B  are conceptual diagrams illustrating spatial neighboring candidates in HEVC. Spatial MV candidates are derived from the neighboring blocks shown on  FIGS. 3A and 3B , for a specific PU (PU 0 ), although the methods of generating the candidates from the blocks differ for merge and AMVP modes. 
     In merge mode, up to four spatial MV candidates can be derived with the orders shown in  FIG. 3A  with numbers, and the order is the following: left (0, A 1 ), above (1, B 1 ), above-right (2, B 0 ), below-left (3, A 0 ), and above left (4, B 2 ), as shown in  FIG. 3A . That is, in  FIG. 3A , block  150  includes PU 0   154 A and PU 1   154 B. When a video coder is to code motion information for PU 0   154 A using merge mode, the video coder adds motion information from spatial neighboring blocks  158 A,  158 B,  158 C,  158 D, and  158 E to a candidate list, in that order. Spatial neighboring blocks  158 A,  158 B,  158 C,  158 D, and  158 E may also be referred to as, respectively, blocks A 1 , B 1 , B 0 , A 0 , and B 2 , as in HEVC. 
     In AMVP mode, the spatial neighboring blocks are divided into two groups: a left group including blocks  0  and  1 , and an above group including blocks  2 ,  3 , and  4  as shown on  FIG. 3B . These spatial neighboring blocks are labeled, respectively, as blocks  160 A,  160 B,  160 C,  160 D, and  160 E in  FIG. 3B . In particular, in  FIG. 3B , block  152  includes PU 0   156 A and PU 1   156 B, and blocks  160 A,  160 B,  160 C,  160 D, and  160 E represent spatial neighbors to PU 0   156 A. For each group, the potential candidate in a neighboring block referring to the same reference picture as that indicated by the signaled reference index has the highest priority to be chosen to form a final candidate of the group. It is possible that all spatial neighboring blocks do not contain a motion vector pointing to the same reference picture. Therefore, if such a candidate cannot be found, the video coder may scale the first available candidate to form the final candidate; thus, the temporal distance differences can be compensated. 
       FIGS. 4A and 4B  are conceptual diagrams illustrating temporal motion vector prediction (TMVP) candidates in HEVC. In particular,  FIG. 4A  illustrates an example CU  170  including PU 0   172 A and PU  1   172 B. PU 0   172 A includes a center block  176  for PU  172 A and a bottom-right block  174  to PU 0   172 A.  FIG. 4A  also shows an external block  178  for which motion information may be predicted from motion information of PU 0   172 A, as discussed below.  FIG. 4B  illustrates a current picture  180  including a current block  188  for which motion information is to be predicted. In particular,  FIG. 4B  illustrates a co-located picture  184  to current picture  180  (including co-located block  190  to current block  188 ), a current reference picture  182 , and a co-located reference picture  186 . Co-located block  190  is predicted using motion vector  194 , which is used as a temporal motion vector predictor (TMVP) candidate  192  for motion information of current block  188 . 
     A video coder, such as video encoder  200  or video decoder  300 , may add a TMVP candidate, such as TMVP candidate  192 , into the MV candidate list after any spatial motion vector candidates if TMVP is enabled and the TMVP candidate is available. The process of motion vector derivation for TMVP candidate is the same for both merge and AMVP modes; however, the target reference index for the TMVP candidate in the merge mode is set to 0, according to HEVC. 
     The primary block location for TMVP candidate derivation is the bottom right block outside of the co-located PU, as shown in  FIG. 4A  as bottom right block  174  to PU 0   172 A, to compensate the bias to the above and left blocks used to generate spatial neighboring candidates. However, if bottom right block  174  is located outside of the current CTB row or motion information is not available for bottom right block  174 , the block is substituted with center block  176  of the PU as shown in  FIG. 4A . 
     As shown in  FIG. 4B , motion vector for TMVP candidate  192  is derived from co-located block  190  of the co-located picture  184 , as indicated in the slice level information. The motion vector for the co-located PU is referred to as a “co-located MV” or a “co-located MV.” Similar to temporal direct mode in AVC, in order to derive a motion vector of the TMVP candidate, the co-located MV may have to be scaled to compensate for the temporal distance differences, as shown in  FIG. 4B . 
     Similar to temporal direct mode in AVC, a motion vector of the TMVP candidate may be subject to motion vector scaling, which is performed to compensate picture order count (POC) distance differences, as shown in  FIGS. 2A and 2B . For instance, a motion vector of the TMVP candidate may be scaled to compensate POC distance differences between current picture  180  and current reference picture  182 , and co-located picture  184  and co-located reference picture  186 . That is, motion vector  194  may be scaled to produce TMVP candidate  192 , based on these POC differences. 
     Other aspects of motion prediction in HEVC are described in the following paragraphs. Several aspects of merge and AMVP modes are described as follows. One such aspect is motion vector scaling that may be performed by a video coder, such as video encoder  200  and video decoder  300 . It is assumed that the value of motion vectors is proportional to the distance of pictures in the presentation time. A motion vector associates two pictures, the reference picture, and the picture containing the motion vector (namely the “containing” picture). When a motion vector is utilized to predict the other motion vector, the distance of the containing picture and the reference picture is calculated based on the Picture Order Count (POC) values. 
     For a motion vector to be predicted, both the motion vector&#39;s associated containing picture and reference picture may be different. Therefore a new distance (based on POC) may be calculated. Video encoder  200  and video decoder  300  may scale the motion vector based on these two POC distances. For a spatial neighboring candidate, the containing pictures for the two motion vectors are the same, while the reference pictures are different. In HEVC, motion vector scaling applies to both TMVP and AMVP for spatial and temporal neighboring candidates. 
     In another example, video encoder  200  and video decoder  300  may perform artificial motion vector candidate generation. If a motion vector candidate list is not complete, artificial motion vector candidates are generated and inserted at the end of the MV candidate list until the MV candidate list has all MV candidates. In merge mode, there are two types of artificial MV candidates: combined candidates derived only for B-slices (bi-predictively coded slices) and zero candidates used if the first type does not provide enough artificial candidates. In a B-slice, video blocks may be coded using intra prediction, uni-directional inter prediction, bi-directional inter prediction, and/or other coding modes. A zero candidate is a candidate that specifies motion vectors with 0 magnitude. For each pair of candidates that is already in the candidate list and has the necessary motion information, video encoder  200  and video decoder  300  may derive bi-directional combined motion vector candidates by a combination of the motion vector of the first candidate referring to a picture in the list 0 and the motion vector of a second candidate referring to a picture in the list 1. 
     In another example, video encoder  200  and video decoder  300  may perform a pruning process for candidate insertion. Candidates from different blocks may happen to be the same, which decreases the efficiency of a merge mode candidate list or AMVP mode candidate list. Accordingly, video encoder and video decoder  300  may apply a pruning process to address this problem. The pruning process compares one candidate against the other candidates in a current candidate list to avoid inserting an identical candidate, to a certain extent. To reduce the complexity, video encoder  200  and video decoder  300  may apply the pruning process to a limited number of candidates instead of comparing each potential candidate with all the other existing candidates. 
     In yet another example, video encoder  200  and video decoder  300  may perform an enhanced motion vector prediction process, such as those described below. In the development of Versatile Video Coding (VVC), there are several inter coding tools which derive or refine the candidate list of motion vector prediction or merge prediction for the current block. Several of these approaches are described below. These approaches include history-based motion vector prediction, pairwise average candidates, and merge list in VTM3.0. 
     History-based motion vector prediction (HMVP) (e.g., as described in JVET-K0104, available at phenix.it-sudparis.eu/jvet/doc_end_user/documents/11_Ljubljana/wg11/JVET-K0104-v5.zip) is a history-based method in which a video coder, such as video encoder  200  and video decoder  300 , may determine a MV predictor for each block from a list of previously-decoded MVs in addition to MVs in immediately adjacent causal neighboring motion fields. The immediately adjacent causal neighboring motion fields are motion fields of locations that are immediately adjacent to a current block and occur prior to the current block in decoding order. In HMVP, a table is maintained for previously decoded motion vectors as HMVP candidates. 
     Video encoder  200  and video decoder  300  may maintain a table with multiple HMVP candidates during the encoding/decoding process. To maintain the table, video encoder  200  and video decoder  300  may add HMVP candidates to the table as well as remove HMVP candidates from the table. Video encoder  200  and video decoder  300  may be configured to empty the table (e.g., remove all of the HMVP candidates) when a new slice is encountered. Video encoder  200  and video decoder  300  may be configured such that, whenever there is an inter-coded block, video encoder  200  and video decoder  300  may insert the associated motion information into the table in a first-in-first-out (FIFO) fashion as a new HMVP candidate. Then, video encoder  200  and video decoder  300  may be configured to apply a constraint FIFO rule. When inserting a HMVP candidate to the table, video encoder  200  and video decoder  300  may first apply a redundancy check (e.g., pruning) to determine whether there is an identical HMVP candidate in the table. If found, video encoder  200  and video decoder  300  may remove that particular HMVP candidate from the table and may move all the HMVP candidates after that candidate. For example, if the removed HMVP candidate was in the first slot in the FIFO, when the removed HMVP candidate was removed, video encoder  200  and video decoder  300  move each of the other HMVP candidates forward one position in the table. 
     Video encoder  200  and video decoder  300  may be configured to use HMVP candidates in the merge candidate list construction process. For example, video encoder  200  and video decoder  300  may be configured to insert all HMVP candidates from the last entry to the first entry in the table after the TMVP candidate. Video encoder  200  and video decoder  300  may be configured to apply pruning on the HMVP candidates. In some examples, once the total number of available merge candidates reaches the signaled or predetermined maximum number of allowed merge candidates, video encoder  200  and video decoder  300  may terminate the merge candidate list construction process. 
     Similarly, video encoder  200  and video decoder  300  may be configured to also use HMVP candidates in the AMVP candidate list construction process. Video encoder  200  and video decoder  300  may be configured to insert the motion vectors of the last K HMVP candidates in the table after the TMVP candidate. Video encoder  200  and video decoder  300  may be configured to use only HMVP candidates with the same reference picture as an AMVP target reference picture (i.e., a reference picture in an AMVP reference picture list selected for use with the current block) d to construct the AMVP candidate list. Video encoder  200  and video decoder  300  may be configured to apply pruning on the HMVP candidates. 
     Pairwise average candidates are another enhancement to motion vector prediction. Pairwise average candidates are used in VTM3.0. Pairwise average candidates are generated by averaging predefined pairs of candidates in the current merge candidate list (includes spatial candidates, TMVP, and HMVP), and the predefined pairs are defined as {(0, 1), (0, 2), (1, 2), (0, 3), (1, 3), (2, 3)}, where the numbers denote the merge indices to the merge candidate list. Video encoder  200  and/or video decoder  300  may calculate the averaged motion vectors are calculated separately for each reference list (i.e., reference picture list), such as in the example of bi-prediction. For example, video encoder  200  and/or video decoder  300  may take a merge candidate in the current merge candidate list at merge index 0 and average that merge candidate with the merge candidate in the current merge candidate list at merge index 1. Video encoder  200  and/or video decoder  300  may average the other defined pairs noted above. If both motion vectors are available in one reference list, video encoder  200  and/or video decoder  300  may average these two motion vectors even when they point to different reference pictures. If only one motion vector is available in the reference list, video encoder  200  and/or video decoder  300  may use the one available motion vector directly, in other words, without averaging the available motion vector with another motion vector. If no motion vector is available, video encoder  200  and/or video decoder  300  may keep this list illegal. The pairwise average candidates may replace the combined candidates of the HEVC standard. 
     In VTM4.0, for normal inter merge mode, the size of the merge candidate list is six (6) and the order of the merge candidate list may be as follows:
         1. Spatial candidates for blocks A 1 , B 1 , B 0  and A 0 .   2. If number of candidates less than four (4), add the spatial candidate for block B 2  to the list.   3. TMVP candidate.   4. HMVP candidates (cannot be the last candidate in the list).   5. Pairwise candidates.   6. Zero motion vector candidates.       

     In VTM4.0, for intra block copy (IBC) merge mode, the size of the merge candidate list is six (6) and the order of the merge candidate list may be as follows:
         1. Spatial candidates for blocks A 1 , B 1 , B 0  and A 0 .   2. If the number of candidates is fewer than four (4), add the spatial candidate for block B 2 .   3. HMVP candidates (cannot be the last candidate in the list).   4. Pairwise candidates.       

     For IBC merge mode, if the candidates are legal, the video coder may add the candidates into the merge candidate list. Legal candidate are candidates coded in IBC mode and satisfy the following conditions: the spatial motion vector predictor candidate for block B 1  is pruned by the spatial motion vector predictor candidate for block A 1  by comparing the spatial motion vector predictor candidate of block B 1  with the spatial motion vector predictor candidate of block A 1 . If the spatial motion vector predictor candidate for block B 1  is different from the spatial motion vector predictor candidate for block A 1 , the spatial motion vector predictor candidate for block B 1  is added to the merge/skip list along with the spatial motion vector predictor candidate for block A 1 . In a similar fashion, the spatial motion vector predictor candidate for block B 0  is pruned by the spatial motion vector predictor candidate for block B 1 , and the spatial motion vector predictor candidate for block A 0  is pruned by the spatial motion vector predictor candidate for block A 1 . If the number of candidates resulting from the pruning process is fewer than four (4), the spatial motion vector predictor candidate for block B 2  is added to the merge/skip list, subject to pruning by the spatial motion vector predictor candidates for blocks A 1  and B 1 ; The first two HMVP candidates are similarly pruned by the spatial motion vector predictor candidates for blocks A 1  and B 1 ; No pruning is performed on pairwise candidates. 
     In the most recent draft, for IBC merge mode, pairwise candidates have been removed from the merge candidate list. In addition, spatial candidates for blocks A 0  and B 0  have also been removed from the merge candidate list. The size of the merge candidate list has also been modified to five (5). Thus, the order of the merge candidate list may be as follows:
         1. Spatial candidates for blocks A 1 , B 1 .   2. HMVP candidates (cannot be the last candidate in the list).       

     If the number of candidates is fewer than five (5), zero motion vectors (motion vectors having a value of zero) are added to the end of the merge candidate list. The spatial motion vector predictor candidate for B 2  and the first two HMVP candidates may each be subject to pruning by the spatial motion vector candidates for blocks A 1  and B 1 , similar to the previously described pruning process. 
     Various examples of screen content coding (SCC) tools are described below, in particular intra block copy (IBC). While the coding tools described below (e.g., intra block copy (IBC), independent IBC mode, and shared merging candidates list) may be used in the context of SCC, video encoder  200  and/or video decoder  300  may, in some examples, also use these coding tools outside the context of SCC. Intra block copy (IBC) is sometimes referred to as current picture referencing (CPR). In IBC, a motion vector refers to already-reconstructed reference samples in the current picture. In some examples, such a motion vector is also referred to as a block vector. IBC was supported in HEVC screen content coding extension (HEVC SCC). Video encoder  200  may signal an IBC-coded CU as an inter coded block. Currently, in HEVC, the luma motion (or block) vector of an IBC-coded CU must be in integer precision. For instance, video encoder  200  and/or video decoder  300  may clip luma motion vectors to integer precision. In some examples, video encoder  200  and/or video decoder  300  may also clip chroma motion vectors to integer precision. In other video coding standards, a luma motion vector and/or a chroma motion vector of an IBC-coded CU may use sub-pel precision. 
     When combined with AMVR, the IBC mode can switch between 1-pel and 4-pel motion vector precisions. Video encoder  200  and video decoder  300  may place the current picture at the end of reference picture list L0. To reduce memory consumption and decoder complexity, the version of IBC in VTM-3.0 allows video decoder  300  to use only the reconstructed portion of the current CTU. The restriction of allowing video decoder  300  to use only the reconstructed portion of the current CTU may allow for video decoder  300  to implement the IBC mode using local on-chip memory for hardware implementations. While this disclosure describes the reconstruction-based aspects of IBC as being performed by video decoder  300 , it will be appreciated that video encoder  200  may also implement these aspects of IBC using a decoding loop or reconstruction loop. 
     At the encoder side, video encoder  200  may perform hash-based motion estimation for IBC. Video encoder  200  may perform a rate distortion (RD) check for blocks with either width or height no larger than sixteen (16) luma samples. For a non-merge mode, video encoder  200  may perform the block vector search using a hash-based search first. For example, video encoder  200  may apply a hash transform to blocks of video data. Video encoder  200  then may search for blocks with the same or similar hash values as the current block. If hash search does not return a valid candidate, video encoder  200  may perform a block matching based local search. 
     Another example of an SCC tool is independent IBC mode. In VTM4.0, video encoder  200  may signal IBC mode with a block-level flag and can signal an IBC mode as IBC AMVP mode or IBC skip/merge mode. The version of IBC mode applied in VTM4.0 may be referred to as independent IBC mode. According to VTM4.0, IBC mode is treated as a third prediction mode in addition to intra prediction mode and inter prediction mode. In the IBC mode of VTM4.0 (i.e., independent IBC mode), the current picture is no longer included as one of the reference pictures in reference picture list 0. Further, the derivation process of motion vectors for IBC mode excludes all neighboring blocks in inter mode and vice versa. In other words, if a current block is an IBC mode block, motion vectors from neighboring inter mode blocks may not be motion vector prediction candidates for the current block and if the current block is an inter mode block, motion vectors from neighboring IBC mode blocks may not be motion vector prediction candidates for the current block. Bitstream conformance checks are also no longer needed at video encoder  200 , and video encoder  200  may remove redundant mode signaling. 
       FIG. 5  illustrates an example of an intra block copy (IBC) coding process, in accordance with one or more techniques of this disclosure. According to one example IBC coding process, video encoder  200  may select, for a current block, a predictor video block, e.g., from a set of previously coded and reconstructed blocks of video data located in the current picture. In the example of  FIG. 5 , area  195  includes the set of previously coded and reconstructed video blocks of the current picture that can be referenced by current block  197 . The blocks in the area  195  may represent blocks that have been decoded and reconstructed by video decoder  300  and stored in decoded picture buffer  314 , or blocks that have been decoded and reconstructed in the reconstruction loop of video encoder  200  and stored in decoded picture buffer  218 . Current block  197  represents a current block of video data to be coded. Prediction block  198  represents a reconstructed video block, in the same picture as current block  197 , which is used for IBC prediction of current block  197 . 
     In the example IBC process, video encoder  200  may determine and encode motion vector  196 , which indicates the position of prediction block  198  relative to current block  197 , together with the residue signal. For instance, as illustrated by  FIG. 5 , motion vector  196  may indicate the position of the upper-left corner of prediction block  198  relative to the upper-left corner of current block  197 . Motion vector  196  may also be referred to as an offset vector, displacement vector, or block vector (BV). Video decoder  300  may utilize the encoded information for decoding the current block. 
     As discussed above, in IBC mode, the reference area (e.g., area  195 ) may be restricted to reconstructed samples of a current picture that is being predicted. In other examples, the reference area may be further restricted, such as to a slice, a tile, a CTU, a parallel processing unit, and the like, of the current picture. 
     Another example of a screen content coding tool is a shared merge candidate list. A merge candidate list may be a list of merge candidates for inter mode coding and a shared merge candidate list is a list of merge candidates that is shared by multiple blocks within a shared list region (also referred to as a shared merge list area). A shared merge candidate list algorithm was adopted in VTM4.0. The shared merge candidate list algorithm represents a design that is friendly to parallel processing with respect to video encoder  200  and video decoder  300 . According to the shared merge candidate list algorithm, the same merge candidate list is shared for all leaf CUs (such as skip or merge coded CUs) of one ancestor node in a CU split tree. The leaf CUs of one ancestor node may be referred to, for example, as a shared list region or a shared merge list area. Sharing the same merge candidate list for small skip or merge coded CUs of an ancestor node may enable parallel processing of the small skip/merge-coded CUs, such as because video encoder  200  and video decoder  300  generates a single merge candidate list that is shared by the small skip/merge-coded CUs. The ancestor node is named “merge sharing node.” For example, referring back to  FIG. 2A , ancestor node  142  may be a merge sharing node and leaf nodes  144 ,  146  and  148  may share a merging candidates list. 
       FIG. 6  is a conceptual diagram illustrating examples of merge sharing nodes. Video encoder  200  and video decoder  300  may generate the shared merge candidate list at the merge sharing node by treating the merge sharing node as a leaf CU. Video decoder  300  may decide the merge sharing node for each CU inside a CTU during parsing stage of decoding (or video encoder  200  may do so via a decoding loop). Moreover, the merge sharing node is an ancestor node of leaf CUs which satisfies the following 2 criteria: the merge sharing node size is equal to or larger than a size threshold and, in the merge sharing node, one of the child CU sizes is smaller than the size threshold. 
     In VTM4.0, IBC merge/skip mode and inter merge/skip mode use two independent merge candidate lists. IBC merge/skip mode is also referred to as IBC merge mode as skip mode is a special case of merge mode. Similarly, inter merge/skip mode is also referred to herein as inter merge/skip mode. IBC merge mode refers to coding a block using IBC mode in which the motion parameters of the block are signaled using a merge candidate list. Similarly, inter merge mode refers to coding a block using inter-prediction mode in which the motion parameters of the block are signaled using a merge candidate list. Sharing a merge candidate list for inter merge mode and IBC merge mode was introduced in VTM4.0 and VTM4.0.1 in order to process small merge/skip CUs in parallel. By sharing a merge candidate list for both inter merge mode and IBC merge mode, video encoder  200  and video decoder  300  may generate a single merge candidate list that can be used for both inter merge mode and IBC merge mode rather than two separate single merge candidate lists for inter merge mode and IBC merge mode. 
     However, in the worst case, there may still be situations where video encoder  200  and video decoder  300  generates two merge candidate list for CUs (e.g., 4×4 CUs) in a shared list region (also referred to as a shared merge list area): one merge candidate list for inter merge mode and one merge candidate list for IBC merge mode, where the shared list region may be the leaf CUs of a common ancestor node. For example, in  FIG. 6 , if the merge sharing node size is smaller than the size threshold, then the leaf CUs of the merge sharing node may share a first merge candidate list for inter merge mode and may also share a second merge candidate list for IBC merge mode. Since these two merge candidate lists can be generated in parallel, the cycle budget for a shared list region can still be met. However, it may still be desirable to simplify the techniques for coding blocks in a shared list region (e.g., leaf CUs with a common ancestor node) to reduce the area size for hardware implementations (e.g., processing circuitry) of techniques for coding blocks in a shared list region and to reduce the processing cycles for software implementations of techniques for coding blocks in a shared list region. 
     In accordance with aspects of the present disclosure, video encoder  200  and video decoder  300  may determine whether a characteristic of a processing area meets a criterion. A processing area is a two-dimensional area of video data (e.g., a two-dimensional area of a picture in the video data) being processed. Examples of a processing area may include a CU or a plurality of CUs. In some examples, a processing area refers to the shared merge list area or the shared list region discussed above. In some examples, a processing area may correspond to a node in a partitioning tree of a CTU, such as ancestor node  142  of  FIG. 2A . For example, the processing area may encompass all leaf CUs of a common ancestor node, as described with respect to  FIG. 6 . 
     The criterion may be associated with whether video encoder  200  and video decoder  300  are able to use a shared merge candidate list for coding blocks within the processing area using inter merge mode and IBC merge mode. For example, if a characteristic of a processing area meets a criterion, video encoder  200  and video decoder  300  may determine that video encoder  200  and video decoder  300  are not able to use a shared merge candidate list for coding blocks within the processing area using inter merge mode and IBC merge mode. Instead, video encoder  200  and video decoder  300  may have to generate two separate merge candidate lists: one for coding blocks using inter merge mode and one for coding blocks using IBC merge mode. Generating separate merge candidate lists for IBC merge mode and inter merge mode may potentially increase the number of processing cycles and/or the size and complexity of hardware implementations to code blocks of the processing area as compared to using a shared merge candidate list for coding blocks of the processing area using IBC merge mode and inter merge mode. 
     To address this potential issue, if video encoder  200  and/or video decoder  300  determine that the characteristic of a processing area meets the criterion such that video encoder and video decoder  300  may not use a shared merge candidate list for coding blocks within the processing area using inter merge mode and an IBC merge mode, video encoder  200  and video decoder  300  may, when coding blocks within the same processing area, refrain from using both inter merge mode and IBC merge mode to code blocks within the processing area. For example, if a block in the processing area is coded using IBC merge mode, video encoder  200  and video decoder  300  may refrain from using inter merge mode to code any of the remaining blocks in the processing area. Similarly, if a block in the processing area is coded using inter merge mode, video encoder  200  and video decoder  300  may refrain from using IBC merge mode to code any of the remaining blocks in the processing area. 
     In one example, if one coding block in this processing area is IBC merge mode, the rest of the blocks of this processing area cannot use inter merge mode. Instead, a default prediction mode is used in place of inter merge mode. The rest of the blocks of this processing area can select this default prediction mode or other prediction modes except inter merge mode. For example, in the decoder side, given that the prediction mode of the first coding block is IBC merge mode, if the decoded prediction mode of any of the rest of the blocks is inter merge mode, those blocks are coded using a default prediction mode. 
     In another example, if one coding block in this processing area is inter merge mode, the rest of the blocks of this processing area cannot use IBC merge mode. Instead, a default prediction mode is used in place of IBC merge mode. The rest of the blocks of this processing area can select this default prediction mode or other prediction modes except IBC merge mode. For example, in the decoder side, given that the prediction mode of the first coding block is inter merge mode, if the decode prediction mode of any of the rest of the blocks is IBC merge mode, those blocks are coded using a default prediction mode. 
     By refraining from using both inter merge mode and IBC merge mode to code blocks within the same processing area, video encoder  200  and video decoder  300  may avoid situations where it generates two merge/skip candidate lists for coding blocks within the processing area: one for coding blocks using IBC merge mode and another one for coding blocks using inter merge mode, such as in the examples described above. In this way, aspects of the present disclosure may reduce the area size for hardware implementations and/or reduce the processing cycles for software implementations of techniques for coding blocks in a shared list region, thereby providing a technical solution to the potential issue of generating separate merge candidate lists for IBC merge mode and inter merge mode in a shared list region. 
     In one example, to refrain from using both inter merge mode and IBC merge mode to code blocks within a single processing area having a characteristic that meets a criterion, video encoder  200  and video decoder  300  may, when determining whether to code a current block of video data in a processing area using one of: inter merge mode or IBC merge mode, whether a previous block of video data in the processing area was coded by video encoder  200  and video decoder  300  using the other one of: inter merge mode or IBC merge mode. 
     Thus, when determining whether to code a current block of video data in a processing area using IBC merge mode, whether a previous block of video data in the processing area was coded by video encoder  200  and video decoder  300  using inter merge mode. If video encoder  200  and video decoder  300  determines that a previous block of video data in the processing area was coded using inter merge mode, video encoder  200  and video decoder  300  may disable use of IBC merge mode to code all remaining blocks of video data in the processing area, including disabling use of IBC merge mode to code the current block of video data in the processing area. 
     Conversely, when determining whether to code a current block of video data in a processing area using inter merge mode, whether a previous block of video data in the processing area was coded by video encoder  200  and video decoder  300  using IBC merge mode. If video encoder  200  and video decoder  300  determines that a previous block of video data in the processing area was coded using IBC merge mode, video encoder  200  and video decoder  300  may disable use of inter merge mode to code all remaining blocks of video data in the processing area, including disabling use of inter merge mode to code the current block of video data in the processing area. 
     In one example, video encoder  200  and video decoder  300  may determine that a first block of video data in a processing area is coded using a first prediction mode, where the first prediction mode is one of: an inter merge mode or an IBC merge mode. A first block of video data in the processing area may not necessarily be the first of a sequence of blocks of video data in the processing area encountered by video encoder  200  and video decoder  300 , but may instead denote a block of video data in the processing area that was coded by video encoder  200  and video decoder  300  prior to coding a current block of video data in the processing area. 
     If video encoder  200  and/or video decoder  300  determine that the characteristic of a processing area meets the criterion, video encoder  200  and video decoder  300  may determine, based at least in part on the first block of video data in the processing area being coded using the first prediction mode, whether a current block of video data in the video area can be coded using a second prediction mode. In particular, because the first block of video data in the processing area is coded using a first prediction mode that is one of: an inter merge mode or an IBC merge mode, video encoder  200  and video decoder  300  may not be able to code the current block of video data in the processing area using a second prediction mode if the second prediction mode one of: an inter merge mode or an IBC merge mode that is different from the first prediction mode used to code the first block of video data. If the first prediction mode used to code the first block of video data in the processing area is IBC merge mode, then video encoder  200  and video decoder  300  may determine not to use the second prediction mode to code the current block of video data if the second prediction mode is inter merge mode. Conversely, if the first prediction mode used to code the first block of video data in the processing area is inter merge mode then video encoder  200  and video decoder  300  may determine not to use the second prediction mode to code the current block of video data if the second prediction mode is IBC merge mode. In this way, only one of inter merge mode or IBC merge mode, but not both prediction modes, are available for use by video encoder  200  and video decoder  300  to decode the blocks of a single processing area. 
     In some examples, video encoder  200  and video decoder  300  may not restrict the use of prediction modes other than inter merge mode and IBC merge mode. Thus, if encoder  200  and/or video decoder  300  determines that the characteristic of a processing area meets a criterion and if a first block of video data in the processing area is coded using inter merge mode, the remaining blocks of video data in the processing area may be coded using any suitable prediction mode, including inter merge mode, except for IBC merge mode. Similarly, if encoder  200  and/or video decoder  300  determines that the characteristic of a processing area meets a criterion and if a first block of video data in the processing area is coded using IBC merge mode, the remaining blocks of video data in the processing area may be coded using any suitable prediction mode, including IBC merge mode, except for inter merge mode. 
     In some examples, video encoder  200  and video decoder  300  may determine whether a current block of video data in the video area can be coded using a second prediction mode in response to the current block of video data in the processing area being encoded using the second prediction code. In other words, video encoder  200  and video decoder  300  may determine whether it can decode a current block of video data using the same prediction mode that was used to encode the current block of video data. In particular, when encoder  200  and/or video decoder  300  encounters the current block of video data that is encoded using the second prediction mode, video encoder  200  and video decoder  300  may determine whether video encoder  200  and video decoder  300  can use the second prediction mode to generate a prediction block for the current block of video data. 
     Video encoder  200  and video decoder  300  may code the current block of video data. Coding the current block of video data may include generating the prediction block for the current block of video data. Video encoder  200  and video decoder  300  may, in response to determining not to use the second prediction mode to code the current block of video data, code the current block of video data using a default prediction mode. A default prediction mode can be any suitable prediction mode for predicting a block of data other than the second prediction mode (e.g., one of the inter merge mode or the IBC merge mode) that video encoder  200  and video decoder  300  has determined not to use for coding the second block. 
     In some examples, video encoder  200  and video decoder  300  may use a default prediction mode to code a current block of video data by using one or more default sample values to generate a prediction block for the current block. In some examples, the one or more default sample value can be predefined at the encoder side (e.g., in video encoder  200 ) and/or at the decoder side (e.g., video decoder  300 ), or may be set as a value signaled from the video encoder to the video decoder at sequence level, picture level, slice level, or block level. For instance, this value can be signaled in an SPS, PPS, slice header, CTU, or CU. 
     In some examples, the one or more default sample values can depend on the input bit depth of the sample, the internal bit depth of the samples of the prediction block for the current block or video data, or may be calculated based on previously decoded samples. For example, in the following equation, where the internal bit depth of samples in the prediction block is represented by bitDepth i , and where i represents components of luma, and chroma, video encoder  200  and video decoder  300  may compute a default sample value for each sample N of the prediction block for the current block of video data as N i =1&lt;&lt;(bitDepth i −1). In one specific example using the equation, in VTM 4.0, if the internal bit depth is 10, then the default sample value is equal to 512=1&lt;&lt;(10−1). Video encoder  200  and video decoder  300  may reconstruct the current block of video data based at least in part on the prediction block, in accordance with various techniques disclosed in the present disclosure. 
     In some examples, video encoder  200  and video decoder  300  may use a default prediction mode to code a current block of video data by using an intra prediction mode to code the current block. In some examples, video encoder  200  and video decoder  300  may use a default intra prediction mode to code the current block, such as DC mode, planar mode, or other intra prediction modes. In some examples, the default intra prediction mode can be predefined both on the encoder side (e.g., video encoder  200 ) and on the decoder side (e.g., video decoder  300 ), or may be set as a value signaled from video encoder  200  to video decoder  300  at a sequence level, a picture level, a slice level, or a block level. For instance, this value can be signaled in an SPS, PPS, slice header, CTU, or CU. In some examples, the intra prediction mode can be DC, Planar, Vertical, Horizontal, or any of the other intra prediction modes. 
     In some examples, instead of using a default intra prediction mode as the default prediction mode, video encoder  200  and video decoder  300  can select an intra prediction mode from a set of available intra prediction modes, and signal the selected intra prediction mode. The set of intra prediction modes can be predefined in both encoder side and decoder side, or may be set as a value signaled from the encoder to the decoder at a sequence level, a picture level, a slice level, or a block level. For instance, this value can be signaled in an SPS, PPS, slice header, or CU. The intra prediction mode can be DC, Planar, Vertical, Horizontal, or any of the other intra prediction modes. 
     As described above, video encoder  200  and video decoder  300  may determine whether the characteristic of a processing area meets a criterion in order to determine whether video encoder  200  and video decoder  300  refrains from using both IBC merge mode and inter merge mode to code blocks of video data in the processing area. In some examples, if video encoder  200  and/or video decoder  300  determines that the characteristic of a processing area does not meet the criterion, then video encoder  200  and video decoder  300  may not restrict the use of IBC merge mode and inter merge mode to code the blocks in the processing area. For example, if video encoder  200  and/or video decoder  300  determines that the characteristic of a processing area does not meet the criterion video encoder  200  and video decoder  300  may code a first block of video data in the processing area using IBC merge mode and may code one or more remaining blocks of video data in the processing area using inter merge mode. Similarly, if video encoder  200  and/or video decoder  300  determines that the characteristic of a processing area does not meet the criterion video encoder  200  and video decoder  300  may code a first block of video data in the processing area using inter merge mode and may code one or more other blocks of video data in the processing area using IBC merge mode. 
     As discussed above, in some examples, if a merge sharing node size is smaller than a size threshold, then the leaf CUs of a merge sharing node may not be able to share a single merge candidate list for inter merge mode and IBC merge mode. Instead, video encoder  200  and video decoder  300  may generate separate merge candidate lists for coding blocks in the leaf CUs using inter merge mode and IBC merge mode. In particular, according to one technique of this disclosure, when the processing area is equal to or less than a threshold N of the shared list region, IBC merge mode and inter merge mode cannot be used together by the blocks in this processing area. 
     As such, in one example, determining whether a characteristic of the processing area meets a criterion is based at least in part on the size of the processing area. In one example, determining whether a characteristic of the processing area meets a criterion includes comparing the size of the processing area to a threshold N. In one example, comparing the size of the processing area to a threshold N includes determining whether the size of the processing area is less than equal to N. In this example, if the size of the processing area is equal to or less than a threshold N of the shared list region, video encoder  200  and video decoder  300  may determine that IBC merge mode and inter merge mode cannot be used together by the blocks in this processing area. In another example, comparing the size of the processing area with a threshold N includes determining whether the size of the processing area is less than N. In this example, if the processing area is less than a threshold N of the shared list region, video encoder  200  and video decoder  300  may determine that IBC merge mode and inter merge mode cannot be used together by the blocks in this processing area. 
     In some examples, the threshold N may specify the number of samples included in the processing area (e.g., 16 samples, 32 samples, or 64 samples). Thus, determining whether the size of the processing area is less than or equal to a threshold N comprises determining whether the number of samples in the processing area is less than or equal to the number of samples specified by the threshold N. In other examples, the threshold N may specify one or more dimensions (i.e., width and height) of the processing area in samples (e.g., 4×4, 4×8, 8×8, 8×4, and other dimensions), and video encoder  200  and video decoder  300  may determine whether the characteristic of the processing area meets the criterion by determining whether the processing area has one or more dimensions specified by threshold N (e.g., if the width of the processing area is the same as the specified width, if the height of the processing area is the same as the specified width, or if the width and the height of the processing area is the same as the specified width and height). Thus, determining whether the size of the processing area is less than or equal to a threshold N may comprise determining whether the processing area has one or more specified dimensions, such as determining whether the dimensions of the processing area matches the one or more dimensions specified by the threshold N. 
     In another example, determining whether a characteristic of the processing area meets a criterion includes determining whether a characteristic of a current block of video data in the processing area being coded by video encoder  200  and video decoder  300  meets a criterion. A current block of video data in the processing area may be a current block of video data for which video encoder  200  and video decoder  300  may enable or disable using IBC merge mode or inter merge mode to code the current block, and video encoder  200 . In one example, the criterion may specify one or more dimensions (i.e., width and height) of the current block of video data in samples (e.g., 4×4, 4×8, 8×8, 8×4, 8×16, and other dimensions). Thus, determining whether a characteristic of a current block of video data in the processing area being coded by video encoder  200  and video decoder  300  meets a criterion may include determining whether the current block of video data has the one or more dimensions specified by the criterion. For example, video encoder  200  and video decoder  300  may determine that the current block of video data meets the criterion if the current block of video data has one or more dimensions specified by the criterion, such as if the width and height of the current block of video data matches the width and height specified by the criterion, or if one of the width or the height of the current block of video data matches the width or the height specified by the criterion. 
     In another example, determining whether a characteristic of the processing area meets a criterion includes determining whether the current block of video data is located at a specified location of the current picture. Video encoder  200  and video decoder  300  may determine locations or regions within a current picture (e.g., a frame of video data) in which IBC merge mode and/or inter merge mode cannot be used together. Thus, if the criterion specifies one or more locations in a current picture, video encoder  200  and video decoder  300  may determine whether the current block of video data is within the one or more locations specified by the criterion. If video encoder  200  and video decoder  300  determines that the location of the current block of video data is within the one or more locations specified by the criterion, video encoder  200  and video decoder  300  may determine that the characteristic of the processing area meets the criterion. There may be alternative definitions of a criterion or a threshold to which the techniques of this disclosure would equally be applicable. 
     In the techniques disclosed herein for comparing a characteristic of a processing area to a criterion, the criterion such as the threshold N can be predefined in both encoder side and decoder side, or may be set as a value signaled from video encoder  200  to video decoder  300  at a sequence level, a picture level, a slice level, or a block level. For instance, value of the criterion can be signaled in an SPS, PPS, slice header, CTU, or CU. 
       FIG. 7  is a block diagram illustrating an example video encoder  200  that may perform the techniques of this disclosure.  FIG. 7  is provided for purposes of explanation and should not be considered limiting of the techniques as broadly exemplified and described in this disclosure. For purposes of explanation, this disclosure describes video encoder  200  according to the techniques of JEM, VVC (ITU-T H.266, under development), and HEVC (ITU-T H.265). However, the techniques of this disclosure may be performed by video encoding devices that are configured to other video coding standards. 
     In the example of  FIG. 7 , video encoder  200  includes video data memory  230 , mode selection unit  202 , residual generation unit  204 , transform processing unit  206 , quantization unit  208 , inverse quantization unit  210 , inverse transform processing unit  212 , reconstruction unit  214 , filter unit  216 , decoded picture buffer (DPB)  218 , and entropy encoding unit  220 . Any or all of video data memory  230 , mode selection unit  202 , residual generation unit  204 , transform processing unit  206 , quantization unit  208 , inverse quantization unit  210 , inverse transform processing unit  212 , reconstruction unit  214 , filter unit  216 , DPB  218 , and entropy encoding unit  220  may be implemented in one or more processors or in processing circuitry. For instance, the units of video encoder  200  may be implemented as one or more circuits or logic elements as part of hardware circuitry, or as part of a processor, ASIC, of FPGA. Moreover, video encoder  200  may include additional or alternative processors or processing circuitry to perform these and other functions. 
     Video data memory  230  may store video data to be encoded by the components of video encoder  200 . Video encoder  200  may receive the video data stored in video data memory  230  from, for example, video source  104  ( FIG. 1 ). DPB  218  may act as a reference picture memory that stores reference video data for use in prediction of subsequent video data by video encoder  200 . Video data memory  230  and DPB  218  may be formed by any of a variety of memory devices, such as dynamic random access memory (DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM), resistive RAM (RRAM), or other types of memory devices. Video data memory  230  and DPB  218  may be provided by the same memory device or separate memory devices. In various examples, video data memory  230  may be on-chip with other components of video encoder  200 , as illustrated, or off-chip relative to those components. 
     In this disclosure, reference to video data memory  230  should not be interpreted as being limited to memory internal to video encoder  200 , unless specifically described as such, or memory external to video encoder  200 , unless specifically described as such. Rather, reference to video data memory  230  should be understood as reference memory that stores video data that video encoder  200  receives for encoding (e.g., video data for a current block that is to be encoded). Memory  106  of  FIG. 1  may also provide temporary storage of outputs from the various units of video encoder  200 . 
     The various units of  FIG. 7  are illustrated to assist with understanding the operations performed by video encoder  200 . The units may be implemented as fixed-function circuits, programmable circuits, or a combination thereof. Fixed-function circuits refer to circuits that provide particular functionality, and are preset on the operations that can be performed. Programmable circuits refer to circuits that can be programmed to perform various tasks, and provide flexible functionality in the operations that can be performed. For instance, programmable circuits may execute software or firmware that cause the programmable circuits to operate in the manner defined by instructions of the software or firmware. Fixed-function circuits may execute software instructions (e.g., to receive parameters or output parameters), but the types of operations that the fixed-function circuits perform are generally immutable. In some examples, one or more of the units may be distinct circuit blocks (fixed-function or programmable), and in some examples, one or more of the units may be integrated circuits. 
     Video encoder  200  may include arithmetic logic units (ALUs), elementary function units (EFUs), digital circuits, analog circuits, and/or programmable cores, formed from programmable circuits. In examples where the operations of video encoder  200  are performed using software executed by the programmable circuits, memory  106  ( FIG. 1 ) may store the instructions (e.g., object code) of the software that video encoder  200  receives and executes, or another memory within video encoder  200  (not shown) may store such instructions. 
     Video data memory  230  is configured to store received video data. Video encoder  200  may retrieve a picture of the video data from video data memory  230  and provide the video data to residual generation unit  204  and mode selection unit  202 . Video data in video data memory  230  may be raw video data that is to be encoded. 
     Mode selection unit  202  includes a motion estimation unit  222 , motion compensation unit  224 , and an intra-prediction unit  226 . Mode selection unit  202  may include additional functional units to perform video prediction in accordance with other prediction modes. As examples, mode selection unit  202  may include a palette unit, an intra-block copy unit (which may be part of motion estimation unit  222  and/or motion compensation unit  224 ), an affine unit, a linear model (LM) unit, or the like. 
     Mode selection unit  202  generally coordinates multiple encoding passes to test combinations of encoding parameters and resulting rate-distortion values for such combinations. The encoding parameters may include partitioning of CTUs into CUs, prediction modes for the CUs, transform types for residual data of the CUs, quantization parameters for residual data of the CUs, and so on. Mode selection unit  202  may ultimately select the combination of encoding parameters having rate-distortion values that are better than the other tested combinations. 
     Video encoder  200  may partition a picture retrieved from video data memory  230  into a series of CTUs, and encapsulate one or more CTUs within a slice. Mode selection unit  202  may partition a CTU of the picture in accordance with a tree structure, such as the QTBT structure or the quad-tree structure of HEVC described above. As described above, video encoder  200  may form one or more CUs from partitioning a CTU according to the tree structure. Such a CU may also be referred to generally as a “video block” or “block.” 
     In general, mode selection unit  202  also controls the components thereof (e.g., motion estimation unit  222 , motion compensation unit  224 , and intra-prediction unit  226 ) to generate a prediction block for a current block (e.g., a current CU, or in HEVC, the overlapping portion of a PU and a TU). For example, mode selection unit  202  may determine that a first block of video data in a processing area is coded using a first prediction mode. Mode selection unit  202  may determine whether a characteristic of the processing area meets a criterion. Mode selection unit  202  may, in response to determining that the characteristic of the processing area meets the criterion, determine, based at least in part on the first prediction mode used to code the first block of video data in the processing area, whether to use a second prediction mode to code a current block of video data in the processing area. Video encoder  200  may, in response to determining not to use the second prediction mode to code the current block of video data, code the current block of video data using a default prediction mode. In this way, if mode selection unit  202  codes a block of video data in a processing area using IBC skip/merge mode, mode selection unit  202  may refrain from coding any of the other blocks in the same processing area using inter skip/merge mode. Similarly if mode selection unit  202  codes a block of video data in a processing area using inter skip/merge mode, mode selection unit  202  may refrain from coding any of the other blocks in the same processing area using IBC skip/merge mode. 
     For inter-prediction of a current block, motion estimation unit  222  may perform a motion search to identify one or more closely matching reference blocks in one or more reference pictures (e.g., one or more previously coded pictures stored in DPB  218 ). In particular, motion estimation unit  222  may calculate a value representative of how similar a potential reference block is to the current block, e.g., according to sum of absolute difference (SAD), sum of squared differences (SSD), mean absolute difference (MAD), mean squared differences (MSD), or the like. Motion estimation unit  222  may generally perform these calculations using sample-by-sample differences between the current block and the reference block being considered. Motion estimation unit  222  may identify a reference block having a lowest value resulting from these calculations, indicating a reference block that most closely matches the current block. 
     Motion estimation unit  222  may form one or more motion vectors (MVs) that defines the positions of the reference blocks in the reference pictures relative to the position of the current block in a current picture. Motion estimation unit  222  may then provide the motion vectors to motion compensation unit  224 . For example, for uni-directional inter-prediction, motion estimation unit  222  may provide a single motion vector, whereas for bi-directional inter-prediction, motion estimation unit  222  may provide two motion vectors. Motion compensation unit  224  may then generate a prediction block using the motion vectors. For example, motion compensation unit  224  may retrieve data of the reference block using the motion vector. As another example, if the motion vector has fractional sample precision, motion compensation unit  224  may interpolate values for the prediction block according to one or more interpolation filters. Moreover, for bi-directional inter-prediction, motion compensation unit  224  may retrieve data for two reference blocks identified by respective motion vectors and combine the retrieved data, e.g., through sample-by-sample averaging or weighted averaging. 
     As another example, for intra-prediction, or intra-prediction coding, intra-prediction unit  226  may generate the prediction block from samples neighboring the current block. For example, for directional modes, intra-prediction unit  226  may generally mathematically combine values of neighboring samples and populate these calculated values in the defined direction across the current block to produce the prediction block. As another example, for DC mode, intra-prediction unit  226  may calculate an average of the neighboring samples to the current block and generate the prediction block to include this resulting average for each sample of the prediction block. 
     Mode selection unit  202  provides the prediction block to residual generation unit  204 . Residual generation unit  204  receives a raw, unencoded version of the current block from video data memory  230  and the prediction block from mode selection unit  202 . Residual generation unit  204  calculates sample-by-sample differences between the current block and the prediction block. The resulting sample-by-sample differences define a residual block for the current block. In some examples, residual generation unit  204  may also determine differences between sample values in the residual block to generate a residual block using residual differential pulse code modulation (RDPCM). In some examples, residual generation unit  204  may be formed using one or more subtractor circuits that perform binary subtraction. 
     In examples where mode selection unit  202  partitions CUs into PUs, each PU may be associated with a luma prediction unit and corresponding chroma prediction units. Video encoder  200  and video decoder  300  may support PUs having various sizes. As indicated above, the size of a CU may refer to the size of the luma coding block of the CU and the size of a PU may refer to the size of a luma prediction unit of the PU. Assuming that the size of a particular CU is 2N×2N, video encoder  200  may support PU sizes of 2N×2N or N×N for intra prediction, and symmetric PU sizes of 2N×2N, 2N×N, N×2N, N×N, or similar for inter prediction. Video encoder  200  and video decoder  300  may also support asymmetric partitioning for PU sizes of 2N×nU, 2N×nD, nL×2N, and nRx2N for inter prediction. 
     In examples where mode selection unit  202  does not further partition a CU into PUs, each CU may be associated with a luma coding block and corresponding chroma coding blocks. As above, the size of a CU may refer to the size of the luma coding block of the CU. The video encoder  200  and video decoder  300  may support CU sizes of 2N×2N, 2N×N, or N×2N. 
     For other video coding techniques such as an intra-block copy mode coding, an affine-mode coding, and linear model (LM) mode coding, as few examples, mode selection unit  202 , via respective units associated with the coding techniques, generates a prediction block for the current block being encoded. In some examples, such as palette mode coding, mode selection unit  202  may not generate a prediction block, and instead generate syntax elements that indicate the manner in which to reconstruct the block based on a selected palette. In such modes, mode selection unit  202  may provide these syntax elements to entropy encoding unit  220  to be encoded. 
     As described above, residual generation unit  204  receives the video data for the current block and the corresponding prediction block. Residual generation unit  204  then generates a residual block for the current block. To generate the residual block, residual generation unit  204  calculates sample-by-sample differences between the prediction block and the current block. 
     Transform processing unit  206  applies one or more transforms to the residual block to generate a block of transform coefficients (referred to herein as a “transform coefficient block”). Transform processing unit  206  may apply various transforms to a residual block to form the transform coefficient block. For example, transform processing unit  206  may apply a discrete cosine transform (DCT), a directional transform, a Karhunen-Loeve transform (KLT), or a conceptually similar transform to a residual block. In some examples, transform processing unit  206  may perform multiple transforms to a residual block, e.g., a primary transform and a secondary transform, such as a rotational transform. In some examples, transform processing unit  206  does not apply transforms to a residual block. 
     Quantization unit  208  may quantize the transform coefficients in a transform coefficient block, to produce a quantized transform coefficient block. Quantization unit  208  may quantize transform coefficients of a transform coefficient block according to a quantization parameter (QP) value associated with the current block. Video encoder  200  (e.g., via mode selection unit  202 ) may adjust the degree of quantization applied to the transform coefficient blocks associated with the current block by adjusting the QP value associated with the CU. Quantization may introduce loss of information, and thus, quantized transform coefficients may have lower precision than the original transform coefficients produced by transform processing unit  206 . 
     Inverse quantization unit  210  and inverse transform processing unit  212  may apply inverse quantization and inverse transforms to a quantized transform coefficient block, respectively, to reconstruct a residual block from the transform coefficient block. Reconstruction unit  214  may produce a reconstructed block corresponding to the current block (albeit potentially with some degree of distortion) based on the reconstructed residual block and a prediction block generated by mode selection unit  202 . For example, reconstruction unit  214  may add samples of the reconstructed residual block to corresponding samples from the prediction block generated by mode selection unit  202  to produce the reconstructed block. 
     Filter unit  216  may perform one or more filter operations on reconstructed blocks. For example, filter unit  216  may perform deblocking operations to reduce blockiness artifacts along edges of CUs. Operations of filter unit  216  may be skipped, in some examples. 
     Video encoder  200  stores reconstructed blocks in DPB  218 . For instance, in examples where operations of filter unit  216  are not needed, reconstruction unit  214  may store reconstructed blocks to DPB  218 . In examples where operations of filter unit  216  are needed, filter unit  216  may store the filtered reconstructed blocks to DPB  218 . Motion estimation unit  222  and motion compensation unit  224  may retrieve a reference picture from DPB  218 , formed from the reconstructed (and potentially filtered) blocks, to inter-predict blocks of subsequently encoded pictures. In addition, intra-prediction unit  226  may use reconstructed blocks in DPB  218  of a current picture to intra-predict other blocks in the current picture. 
     In general, entropy encoding unit  220  may entropy encode syntax elements received from other functional components of video encoder  200 . For example, entropy encoding unit  220  may entropy encode quantized transform coefficient blocks from quantization unit  208 . As another example, entropy encoding unit  220  may entropy encode prediction syntax elements (e.g., motion information for inter-prediction or intra-mode information for intra-prediction) from mode selection unit  202 . Entropy encoding unit  220  may perform one or more entropy encoding operations on the syntax elements, which are another example of video data, to generate entropy-encoded data. For example, entropy encoding unit  220  may perform a context-adaptive variable length coding (CAVLC) operation, a CABAC operation, a variable-to-variable (V2V) length coding operation, a syntax-based context-adaptive binary arithmetic coding (SBAC) operation, a Probability Interval Partitioning Entropy (PIPE) coding operation, an Exponential-Golomb encoding operation, or another type of entropy encoding operation on the data. In some examples, entropy encoding unit  220  may operate in bypass mode where syntax elements are not entropy encoded. 
     Video encoder  200  may output a bitstream that includes the entropy encoded syntax elements needed to reconstruct blocks of a slice or picture. In particular, entropy encoding unit  220  may output the bitstream. 
     The operations described above are described with respect to a block. Such description should be understood as being operations for a luma coding block and/or chroma coding blocks. As described above, in some examples, the luma coding block and chroma coding blocks are luma and chroma components of a CU. In some examples, the luma coding block and the chroma coding blocks are luma and chroma components of a PU. 
     In some examples, operations performed with respect to a luma coding block need not be repeated for the chroma coding blocks. As one example, operations to identify a motion vector (MV) and reference picture for a luma coding block need not be repeated for identifying a MV and reference picture for the chroma blocks. Rather, the MV for the luma coding block may be scaled to determine the MV for the chroma blocks, and the reference picture may be the same. As another example, the intra-prediction process may be the same for the luma coding block and the chroma coding blocks. 
     Video encoder  200  represents an example of a device configured to encode video data including a memory configured to store video data, and one or more processing units implemented in circuitry and configured to determine that a first block of video data in a processing area is coded using a first prediction mode, determine whether a characteristic of the processing area meets a criterion, in response to determining that the characteristic of the processing area meets the criterion, determine, based at least in part on the first prediction mode used to code the first block of video data in the processing area, whether to use a second prediction mode to code a current block of video data in the processing area, and in response to determining not to use the second prediction mode to code the current block of video data, code the current block of video data using a default prediction mode. 
       FIG. 8  is a block diagram illustrating an example video decoder  300  that may perform the techniques of this disclosure.  FIG. 8  is provided for purposes of explanation and is not limiting on the techniques as broadly exemplified and described in this disclosure. For purposes of explanation, this disclosure describes video decoder  300  according to the techniques of JEM, VVC (ITU-T H.266, under development), and HEVC (ITU-T H.265). However, the techniques of this disclosure may be performed by video coding devices that are configured to other video coding standards. 
     In the example of  FIG. 8 , video decoder  300  includes coded picture buffer (CPB) memory  320 , entropy decoding unit  302 , prediction processing unit  304 , inverse quantization unit  306 , inverse transform processing unit  308 , reconstruction unit  310 , filter unit  312 , and decoded picture buffer (DPB)  314 . Any or all of CPB memory  320 , entropy decoding unit  302 , prediction processing unit  304 , inverse quantization unit  306 , inverse transform processing unit  308 , reconstruction unit  310 , filter unit  312 , and DPB  314  may be implemented in one or more processors or in processing circuitry. For instance, the units of video decoder  300  may be implemented as one or more circuits or logic elements as part of hardware circuitry, or as part of a processor, ASIC, of FPGA. Moreover, video decoder  300  may include additional or alternative processors or processing circuitry to perform these and other functions. 
     Prediction processing unit  304  includes motion compensation unit  316  and intra-prediction unit  318 . Prediction processing unit  304  may include additional units to perform prediction in accordance with other prediction modes. As examples, prediction processing unit  304  may include a palette unit, an intra-block copy unit (which may form part of motion compensation unit  316 ), an affine unit, a linear model (LM) unit, or the like. In other examples, video decoder  300  may include more, fewer, or different functional components. 
     CPB memory  320  may store video data, such as an encoded video bitstream, to be decoded by the components of video decoder  300 . The video data stored in CPB memory  320  may be obtained, for example, from computer-readable medium  110  ( FIG. 1 ). CPB memory  320  may include a CPB that stores encoded video data (e.g., syntax elements) from an encoded video bitstream. Also, CPB memory  320  may store video data other than syntax elements of a coded picture, such as temporary data representing outputs from the various units of video decoder  300 . DPB  314  generally stores decoded pictures, which video decoder  300  may output and/or use as reference video data when decoding subsequent data or pictures of the encoded video bitstream. CPB memory  320  and DPB  314  may be formed by any of a variety of memory devices, such as DRAM, including SDRAM, MRAM, RRAM, or other types of memory devices. CPB memory  320  and DPB  314  may be provided by the same memory device or separate memory devices. In various examples, CPB memory  320  may be on-chip with other components of video decoder  300 , or off-chip relative to those components. 
     Additionally or alternatively, in some examples, video decoder  300  may retrieve coded video data from memory  120  ( FIG. 1 ). That is, memory  120  may store data as discussed above with CPB memory  320 . Likewise, memory  120  may store instructions to be executed by video decoder  300 , when some or all of the functionality of video decoder  300  is implemented in software to be executed by processing circuitry of video decoder  300 . 
     The various units shown in  FIG. 8  are illustrated to assist with understanding the operations performed by video decoder  300 . The units may be implemented as fixed-function circuits, programmable circuits, or a combination thereof. Similar to  FIG. 7 , fixed-function circuits refer to circuits that provide particular functionality, and are preset on the operations that can be performed. Programmable circuits refer to circuits that can be programmed to perform various tasks, and provide flexible functionality in the operations that can be performed. For instance, programmable circuits may execute software or firmware that cause the programmable circuits to operate in the manner defined by instructions of the software or firmware. Fixed-function circuits may execute software instructions (e.g., to receive parameters or output parameters), but the types of operations that the fixed-function circuits perform are generally immutable. In some examples, one or more of the units may be distinct circuit blocks (fixed-function or programmable), and in some examples, one or more of the units may be integrated circuits. 
     Video decoder  300  may include ALUs, EFUs, digital circuits, analog circuits, and/or programmable cores formed from programmable circuits. In examples where the operations of video decoder  300  are performed by software executing on the programmable circuits, on-chip or off-chip memory may store instructions (e.g., object code) of the software that video decoder  300  receives and executes. 
     Entropy decoding unit  302  may receive encoded video data from the CPB and entropy decode the video data to reproduce syntax elements. Prediction processing unit  304 , inverse quantization unit  306 , inverse transform processing unit  308 , reconstruction unit  310 , and filter unit  312  may generate decoded video data based on the syntax elements extracted from the bitstream. 
     In general, video decoder  300  reconstructs a picture on a block-by-block basis. Video decoder  300  may perform a reconstruction operation on each block individually (where the block currently being reconstructed, i.e., decoded, may be referred to as a “current block”). 
     Entropy decoding unit  302  may entropy decode syntax elements defining quantized transform coefficients of a quantized transform coefficient block, as well as transform information, such as a quantization parameter (QP) and/or transform mode indication(s). Inverse quantization unit  306  may use the QP associated with the quantized transform coefficient block to determine a degree of quantization and, likewise, a degree of inverse quantization for inverse quantization unit  306  to apply. Inverse quantization unit  306  may, for example, perform a bitwise left-shift operation to inverse quantize the quantized transform coefficients. Inverse quantization unit  306  may thereby form a transform coefficient block including transform coefficients. 
     After inverse quantization unit  306  forms the transform coefficient block, inverse transform processing unit  308  may apply one or more inverse transforms to the transform coefficient block to generate a residual block associated with the current block. For example, inverse transform processing unit  308  may apply an inverse DCT, an inverse integer transform, an inverse Karhunen-Loeve transform (KLT), an inverse rotational transform, an inverse directional transform, or another inverse transform to the transform coefficient block. 
     Furthermore, prediction processing unit  304  generates a prediction block according to prediction information syntax elements that were entropy decoded by entropy decoding unit  302 . For example, if the prediction information syntax elements indicate that the current block is inter-predicted, motion compensation unit  316  may generate the prediction block. In this case, the prediction information syntax elements may indicate a reference picture in DPB  314  from which to retrieve a reference block, as well as a motion vector identifying a location of the reference block in the reference picture relative to the location of the current block in the current picture. Motion compensation unit  316  may generally perform the inter-prediction process in a manner that is substantially similar to that described with respect to motion compensation unit  224  ( FIG. 7 ). 
     As another example, if the prediction information syntax elements indicate that the current block is intra-predicted, intra-prediction unit  318  may generate the prediction block according to an intra-prediction mode indicated by the prediction information syntax elements. Again, intra-prediction unit  318  may generally perform the intra-prediction process in a manner that is substantially similar to that described with respect to intra-prediction unit  226  ( FIG. 7 ). Intra-prediction unit  318  may retrieve data of neighboring samples to the current block from DPB  314 . 
     Prediction processing unit  304  may perform the techniques of the present disclosure. For example, prediction processing unit  304  may determine that a first block of video data in a processing area is coded using a first prediction mode. Prediction processing unit  304  may determine whether a characteristic of the processing area meets a criterion. Prediction processing unit  304  may, in response to determining that the characteristic of the processing area meets the criterion, determine, based at least in part on the first prediction mode used to code the first block of video data in the processing area, whether to use a second prediction mode to code a current block of video data in the processing area. Video decoder  300  may, in response to determining not to use the second prediction mode to code the current block of video data, code the current block of video data using a default prediction mode. For example, video decoder  300  may reconstruct the current block using a default prediction mode instead of the second prediction mode. 
     In this way, if prediction processing unit  304  codes a block of video data in a processing area using IBC skip/merge mode, prediction processing unit  304  may refrain from coding any of the other blocks in the same processing area using inter skip/merge mode. Similarly if prediction processing unit  304  codes a block of video data in a processing area using inter skip/merge mode, prediction processing unit  304  may refrain from coding any of the other blocks in the same processing area using IBC skip/merge mode. Thus, even if a current block is encoded using one of IBC skip/merge mode or inter skip/merge mode, prediction processing unit  304  may determine that the current block cannot be decoded using the prediction mode used to encode the current block, and video decoder  300  may instead reconstruct the current block using a default prediction mode. 
     Reconstruction unit  310  may reconstruct the current block using the prediction block and the residual block. For example, reconstruction unit  310  may add samples of the residual block to corresponding samples of the prediction block to reconstruct the current block. 
     Filter unit  312  may perform one or more filter operations on reconstructed blocks. For example, filter unit  312  may perform deblocking operations to reduce blockiness artifacts along edges of the reconstructed blocks. Operations of filter unit  312  are not necessarily performed in all examples. 
     Video decoder  300  may store the reconstructed blocks in DPB  314 . For instance, in examples where operations of filter unit  312  are not performed, reconstruction unit  310  may store reconstructed blocks to DPB  314 . In examples where operations of filter unit  312  are performed, filter unit  312  may store the filtered reconstructed blocks to DPB  314 . As discussed above, DPB  314  may provide reference information, such as samples of a current picture for intra-prediction and previously decoded pictures for subsequent motion compensation, to prediction processing unit  304 . Moreover, video decoder  300  may output decoded pictures (e.g., decoded video) from DPB  314  for subsequent presentation on a display device, such as display device  118  of  FIG. 1 . 
     In this manner, video decoder  300  represents an example of a video decoding device including a memory configured to store video data, and one or more processing units implemented in circuitry and configured to determine that a first block of video data in a processing area is coded using a first prediction mode, determine whether a characteristic of the processing area meets a criterion, in response to determining that the characteristic of the processing area meets the criterion, determine, based at least in part on the first prediction mode used to code the first block of video data in the processing area, whether to use a second prediction mode to code a current block of video data in the processing area, and in response to determining not to use the second prediction mode to code the current block of video data, code the current block of video data using a default prediction mode. 
       FIG. 9  is a flowchart illustrating an example method for encoding a current block. The current block may comprise a current CU. Although described with respect to video encoder  200  ( FIGS. 1 and 7 ), it should be understood that other devices may be configured to perform a method similar to that of  FIG. 9 . 
     In this example, video encoder  200  initially predicts the current block ( 350 ). For example, video encoder  200  may form a prediction block for the current block. As part of predicting the current block, video encoder  200  may perform any of the techniques of this disclosure described above. For example, mode selection unit  202  may determine that a first block of video data in a processing area is coded using a first prediction mode. Mode selection unit  202  may determine whether a characteristic of the processing area meets a criterion. Mode selection unit  202  may, in response to determining that the characteristic of the processing area meets the criterion, determine, based at least in part on the first prediction mode used to code the first block of video data in the processing area, whether to use a second prediction mode to code a current block of video data in the processing area. 
     Video encoder  200  may then calculate a residual block for the current block ( 352 ). To calculate the residual block, video encoder  200  may calculate a difference between the original, unencoded block and the prediction block for the current block. Video encoder  200  may then transform the residual block and quantize transform coefficients of the residual block ( 354 ). Next, video encoder  200  may scan the quantized transform coefficients of the residual block ( 356 ). During the scan, or following the scan, video encoder  200  may entropy encode the transform coefficients ( 358 ). For example, video encoder  200  may encode the transform coefficients using CAVLC or CABAC. Video encoder  200  may then output the entropy encoded data of the block ( 360 ). 
       FIG. 10  is a flowchart illustrating an example method for decoding a current block of video data. The current block may comprise a current CU. Although described with respect to video decoder  300  ( FIGS. 1 and 8 ), it should be understood that other devices may be configured to perform a method similar to that of  FIG. 10 . 
     Video decoder  300  may receive entropy encoded data for the current block, such as entropy encoded prediction information and entropy encoded data for coefficients of a residual block corresponding to the current block ( 370 ). Video decoder  300  may entropy decode the entropy encoded data to determine prediction information for the current block and to reproduce coefficients of the residual block ( 372 ). Video decoder  300  may predict the current block ( 374 ), e.g., using an intra- or inter-prediction mode as indicated by the prediction information for the current block, to calculate a prediction block for the current block. As part of predicting the current block, video decoder  300  may use any of the techniques of this disclosure described above. For example, prediction processing unit  304  may determine that a first block of video data in a processing area is coded using a first prediction mode. Prediction processing unit  304  may determine whether a characteristic of the processing area meets a criterion. Prediction processing unit  304  may, in response to determining that the characteristic of the processing area meets the criterion, determine, based at least in part on the first prediction mode used to code the first block of video data in the processing area, whether to use a second prediction mode to code a current block of video data in the processing area. Video decoder  300  may, in response to determining not to use the second prediction mode to code the current block of video data, code the current block of video data using a default prediction mode. 
     Video decoder  300  may then inverse scan the reproduced coefficients ( 376 ), to create a block of quantized transform coefficients. Video decoder  300  may then inverse quantize and inverse transform the transform coefficients to produce a residual block ( 378 ). Video decoder  300  may ultimately decode the current block by combining the prediction block and the residual block ( 380 ). 
       FIG. 11  is a flow diagram illustrating a method of coding video data according to the techniques of the present disclosure. Although described with respect to video encoder  200  ( FIGS. 1 and 7 ) and video decoder  300  ( FIGS. 1 and 8 ), it should be understood that other devices may be configured to perform a method similar to that of  FIG. 10 . 
     As shown in  FIG. 11 , video encoder  200  (e.g., mode selection unit  202 ) and/or video decoder  300  (e.g., prediction processing unit  304 ) may determine that a first block of video data in a processing area is coded using a first prediction mode ( 400 ). For example, video encoder  200  and/or video decoder  300  may determine that the first prediction mode is one of: an inter merge mode or an IBC merge mode. In some examples, the processing area may be all leaf coding units (CUs) of an ancestor node. 
     Video encoder  200  (e.g., mode selection unit  202 ) and/or video decoder  300  (e.g., prediction processing unit  304 ) may determine whether a characteristic of the processing area meets a criterion ( 402 ). In some examples, to determine whether the characteristic of the processing area meets the criterion, video encoder  200  and/or video decoder  300  may compare a size of the processing area to a threshold. In some examples, to compare the size of the processing area to the threshold, video encoder  200  and/or video decoder  300  may determine whether the size of the processing area is less than or equal to the threshold. 
     In some examples, to determine whether the characteristic of the processing area meets the criterion, video encoder  200  and/or video decoder  300  may determine whether a characteristic of the current block of video data meets the criterion. In some examples, to determine whether the characteristic of the current block of video data meets the criterion, video encoder  200  and/or video decoder  300  may determine whether the current block of video data has one or more specified dimensions. In some examples, to determine whether the characteristic of the current block of video data meets the criterion, video encoder  200  and/or video decoder  300  may determine whether the current block of video data is at a specified location in a current picture. 
     Video encoder  200  (e.g., mode selection unit  202 ) and/or video decoder  300  (e.g., prediction processing unit  304 ) may, in response to determining that the characteristic of the processing area meets the criterion, determine, based at least in part on the first prediction mode used to code the first block of video data in the processing area, whether to use a second prediction mode to code a current block of video data in the processing area ( 404 ). For example, to determine whether to code a current block of video data in the processing area using the second prediction mode, video encoder  200  and/or video decoder  300  may determine whether to use the second prediction mode to generate a prediction block for the current block of video data in the processing area, the current block of video data being encoded using the second prediction mode 
     In some examples, video encoder  200  and/or video decoder  300  may, in response to determining that the first prediction mode is one of: the inter merge mode or the IBC merge mode, determine whether the second prediction mode is one of: the inter merge mode or the IBC merge mode that is different from the first prediction mode, and may, in response to determining that the second prediction mode is one of: the inter merge mode or the IBC merge mode that is different from the first prediction mode, determine not to use the second prediction mode to code the current block of video data. 
     In some examples, video encoder  200  and/or video decoder  300  may, in response to determining that the first block of video data is coded using one of: the inter merge mode or the IBC merge mode and that the second prediction mode is one of: the inter merge mode or the IBC merge mode that is different from the first prediction mode, refrain from generating a merge candidate list for the second prediction mode for processing area. 
     Video encoder  200  (e.g., mode selection unit  202 ) and/or video decoder  300  (e.g., prediction processing unit  304 ) may in response to determining not to use the second prediction mode to code the current block of video data, code the current block of video data using a default prediction mode ( 406 ). In some examples, to code the current block of video data using the default prediction mode, video encoder  200  and/or video decoder  300  may generate the prediction block for the current block of video data using the default prediction mode. 
     In some examples, to code the current block of video data using the default prediction mode, video encoder  200  and/or video decoder  300  may code the current block of video data using an intra prediction mode. In some examples, to code the current block of video data using the default prediction mode, video encoder  200  and/or video decoder  300  may generate a prediction block for the current block of video data using one or more default values and may reconstruct the current block of video data based at least in part on the prediction block. 
     In some examples, video encoder  200  and/or video decoder  300  may be one or more of a camera, a computer, a mobile device, a broadcast receiver device, or a set-top box. In some examples, video decoder  300  may be a wireless communication device that includes a receiver configured to receive encoded data. In some examples, the wireless communication device may be a telephone handset, and the receive may be configured to demodulate, according to a wireless communication standard, a signal comprising the encoded video data. In some examples, video decoder  300  may include a display that is configured to display a picture that includes decoded video data. 
     Illustrative examples of the disclosure include: 
     Example 1 
     A method of decoding video data, the method comprising: generating a candidate list for a current block of video data, wherein the candidate list includes a plurality of entries, each entry having associated motion information; receiving an index identifying an entry of the plurality of entries; and decoding the current block using motion information of the identified entry. 
     Example 2 
     The method according to Example 1, wherein the current block belongs to a processing area, and wherein the processing area is less than a threshold size. 
     Example 3 
     The method according to Example 2, further comprising: in response to determining that the current block belongs to the processing area that is less than the threshold size, determining that intra block copy (IBC) merge/skip mode is disabled. 
     Example 4 
     The method according to Example 3, further comprising: in response to determining that IBC merge/skip mode is disabled, decoding the current block in a default mode. 
     Example 5 
     The method according to Example 2, further comprising: in response to determining that the current block belongs to the processing area that is less than the threshold size, determining that inter merge/skip mode is disabled. 
     Example 6 
     The method according to Example 5, further comprising: in response to determining that inter merge/skip mode is disabled, decoding the current block in a default mode. 
     Example 7 
     The method according to Example 2, further comprising: in response to determining that a first block is coded in IBC merge/skip mode, determining that inter merge/skip mode is disabled for the current block, wherein the first block belongs to the processing area. 
     Example 8 
     The method according to Example 7, further comprising: in response to determining that inter merge/skip mode is disabled for the current block, decoding the current block using a default mode. 
     Example 9 
     The method according to Example 2, further comprising: in response to determining that a first block is coded in inter merge/skip mode, determining that IBC merge/skip mode is disabled for the current block, wherein the first block belongs to the processing area. 
     Example 10 
     The method according to Example 9, further comprising: in response to determining that intra merge/skip mode is disabled for the current block, decoding the current block using a default mode. 
     Example 11 
     A device for decoding video data, the device comprising: a memory configured to store video data; and one or more processors configured to decode the video data using any technique described in Examples 1-10 and/or any technique described in this disclosure. 
     Example 12 
     The device according to Example 11, wherein the device comprises a wireless communication device, further comprising a receiver configured to receive encoded video data. 
     Example 13 
     The device according to Example 12, wherein the wireless communication device comprises a telephone handset and wherein the receiver is configured to demodulate, according to a wireless communication standard, a signal comprising the encoded video data. 
     Example 14 
     A computer readable storage medium storing instructions that when executed by one or more processors cause the one or more processors to decode the video data using any technique described in Examples 1-10 and/or any technique described in this disclosure. 
     Example 15 
     An apparatus for decoding video data, the apparatus comprising: means for decoding the video data using any technique described in Examples 1-10; and/or means for decoding the video data using any technique described in this disclosure. 
     Example 16 
     A method of encoding video data, the method comprising: generating a candidate list for a current block of video data, wherein the candidate list includes a plurality of entries, each entry having associate motion information; encoding the current block using motion information of an entry from the plurality of entries. 
     Example 17 
     The method according to Example 16, wherein the current block belongs to a processing area, and wherein the processing area is less than a threshold size. 
     Example 18 
     The method according to Example 17, further comprising: in response to determining that the current block belongs to the processing area that is less than the threshold size, determining that intra block copy (IBC) merge/skip mode is disabled. 
     Example 19 
     The method according to Example 3, further comprising: in response to determining that IBC merge/skip mode is disabled, determining a default mode for the current block. 
     Example 20 
     The method according to Example 17, further comprising: in response to determining that the current block belongs to the processing area that is less than the threshold size, determining that inter merge/skip mode is disabled. 
     Example 21 
     The method according to Example 20, further comprising: in response to determining that inter merge/skip mode is disabled, determining a default mode for the current block. 
     Example 22 
     A device for encoding video data, the device comprising: a memory configured to store video data; and one or more processors configured to encode the video data using any technique described in Examples 16-21 and/or any technique described in this disclosure. 
     Example 23 
     The device according to Example 22, wherein the device comprises a wireless communication device, further comprising a transmitter configured to transmit encoded video data. 
     Example 24 
     The device according to Example 23, wherein the wireless communication device comprises a telephone handset and wherein the transmitter is configured to modulate, according to a wireless communication standard, a signal comprising the encoded video data. 
     Example 25 
     A computer readable storage medium storing instructions that when executed by one or more processors cause the one or more processors to encode the video data using any technique described in Examples 16-21 and/or any technique described in this disclosure. 
     Example 26 
     An apparatus for decoding video data, the apparatus comprising: means for encoding the video data using any technique described in Examples 16-21; and/or means for encoding the video data using any technique described in this disclosure. 
     It is to be recognized that depending on the example, certain acts or events of any of the techniques described herein can be performed in a different sequence, may be added, merged, or left out altogether (e.g., not all described acts or events are necessary for the practice of the techniques). Moreover, in certain examples, acts or events may be performed concurrently, e.g., through multi-threaded processing, interrupt processing, or multiple processors, rather than sequentially. 
     In one or more examples, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media, or communication media including any medium that facilitates transfer of a computer program from one place to another, e.g., according to a communication protocol. In this manner, computer-readable media generally may correspond to (1) tangible computer-readable storage media which is non-transitory or (2) a communication medium such as a signal or carrier wave. Data storage media may be any available media that can be accessed by one or more computers or one or more processors to retrieve instructions, code and/or data structures for implementation of the techniques described in this disclosure. A computer program product may include a computer-readable medium. 
     By way of example, and not limitation, such computer-readable storage media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage, or other magnetic storage devices, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer. Also, any connection is properly termed a computer-readable medium. For example, if instructions are transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. It should be understood, however, that computer-readable storage media and data storage media do not include connections, carrier waves, signals, or other transitory media, but are instead directed to non-transitory, tangible storage media. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc, where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. 
     Instructions may be executed by one or more processors, such as one or more DSPs, general purpose microprocessors, ASICs, FPGAs, or other equivalent integrated or discrete logic circuitry. Accordingly, the terms “processor” and “processing circuitry,” as used herein may refer to any of the foregoing structures or any other structure suitable for implementation of the techniques described herein. In addition, in some aspects, the functionality described herein may be provided within dedicated hardware and/or software modules configured for encoding and decoding, or incorporated in a combined codec. Also, the techniques could be fully implemented in one or more circuits or logic elements. 
     The techniques of this disclosure may be implemented in a wide variety of devices or apparatuses, including a wireless handset, an integrated circuit (IC) or a set of ICs (e.g., a chip set). Various components, modules, or units are described in this disclosure to emphasize functional aspects of devices configured to perform the disclosed techniques, but do not necessarily require realization by different hardware units. Rather, as described above, various units may be combined in a codec hardware unit or provided by a collection of interoperative hardware units, including one or more processors as described above, in conjunction with suitable software and/or firmware. 
     Various examples have been described. These and other examples are within the scope of the following claims.