Patent Publication Number: US-2020296405-A1

Title: Affine motion compensation refinement using optical flow

Description:
This application claims the benefit of U.S. Provisional Patent Application 62/818,619, filed Mar. 14, 2019, the entire content of which is hereby incorporated by reference. 
    
    
     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 for inter prediction in video codecs, and more specifically, processes related to affine motion prediction. In examples of the disclosure, a video coder (e.g., a video encoder or video decoder) may be configured to predict a subblock of a block of video data using affine motion compensation. The video coder may determine spatial gradient information for a sample of the subblock of the block of the video data. Spatial gradient information may represent a change in predicted values for the sample. For example, the video coder may generate the spatial gradient information using a 3-tap filter (e.g., [−1, 0, 1]) to the subblock at a location of the sample. 
     The video coder may determine a difference motion vector representing a difference between a pixel motion vector for the sample and a subblock motion vector for the subblock. To determine the difference motion vector, the video coder may constrain a horizontal component of the difference motion vector and a vertical component of the difference motion vector to be less than half a pixel of the video data. Constraining the difference motion vector may help to prevent a motion vector buffer overflow. A motion vector overflow may refer to instances where a motion vector indicates a bit depth value that exceeds an allowed bit depth (e.g., 18-bit) of motion field storage. The motion vector overflow may result in a stored bit depth value that indicates an incorrect bit depth, which may result in inaccurate predicted sample values. As such, the video coder may be configured to constrain the difference motion vector to help to minimize motion vector overflow, which may help to improve a prediction accuracy of the video data. 
     Additionally, or alternatively, rather than determining a difference motion vector for each sample of a block of video data, a video coder (e.g., a video encoder or video decoder) may determine a difference motion vector for each subblock (e.g., a 2×2 subblock of a 4×4 subblock of a block of video data) of the block of the video data. By determining the difference motion vector for each subblock instead of each sample, a motion vector may be determined with lower signaling overhead, thus potentially increasing coding efficiency. The techniques of this disclosure may also provide for a reduction in implementation complexity with little to no loss in prediction accuracy. 
     In one example, a method for decoding video data includes: predicting, by one or more processors implemented in circuitry, a subblock of a block of the video data using affine motion compensation; determining, by the one or more processors, spatial gradient information for a sample of the subblock of the block of the video data; determining, by the one or more processors, a difference motion vector indicating a difference between a pixel motion vector for the sample and a subblock motion vector for the subblock, wherein determining the difference motion vector comprises constraining an absolute value of a horizontal component of the difference motion vector to be less than half of a pixel of the video data and constraining an absolute value of a vertical component of the difference motion vector to be less than half of the pixel of the video data; determining, by the one or more processors, subblock prediction information for the sample based on the spatial gradient information and the difference motion vector; determining, by the one or more processors, a predicted block for the block of the video data based on the subblock prediction information; decoding, by the one or more processors, a residual block for the block of the video data; and combining, by the one or more processors, the predicted block and the residual block to decode the block of the video data. 
     In another example, a method for encoding video data includes: predicting, by one or more processors implemented in circuitry, a subblock of a block of the video data using affine motion compensation; determining, by the one or more processors, spatial gradient information for a sample of the subblock of the block of the video data; determining, by the one or more processors, a difference motion vector indicating a difference between a pixel motion vector for the sample and a subblock motion vector for the subblock, wherein determining the difference motion vector comprises constraining an absolute value of a horizontal component of the difference motion vector to be less than half of a pixel of the video data and constraining an absolute value of a vertical component of the difference motion vector to be less than half of the pixel of the video data; determining, by the one or more processors, subblock prediction information for the sample based on the spatial gradient information and the difference motion vector; determining, by the one or more processors, a predicted block for the block of the video data based on the subblock prediction information; generating, by the one or more processors, a residual block for the block of the video data based on differences between the block of the video data and the predicted block; and encoding, by the one or more processors, the residual block. 
     In one example, an apparatus configured to decode video data includes a memory configured to store a block of the video data and one or more processors implemented in circuitry and in communication with the memory, the one or more processors configured to: predict a subblock of the block of the video data using affine motion compensation; determine spatial gradient information for a sample of the subblock of the block of the video data; determine a difference motion vector indicating a difference between a pixel motion vector computed for the sample and a subblock motion vector for the subblock, wherein, to determine the difference motion vector, the one or more processors are configured to constrain an absolute value of a horizontal component of the difference motion vector to be less than half of a pixel of the video data and to constrain an absolute value of a vertical component of the difference motion vector to be less than half of the pixel of the video data; determine subblock prediction information for the sample of the subblock based on the spatial gradient information and the difference motion vector; determine a predicted block for the block of the video data based on the subblock prediction information; decode a residual block for the block of the video data; and combine the predicted block and the residual block to decode the block of the video data. 
     In another example, an apparatus configured to encode video data includes a memory configured to store a block of the video data and one or more processors implemented in circuitry and in communication with the memory, the one or more processors configured to: predict a subblock of the block of the video data using affine motion compensation; determine spatial gradient information for a sample of the subblock of the block of the video data; determine a difference motion vector indicating a difference between a pixel motion vector computed for the sample and a subblock motion vector for the subblock, wherein, to determine the difference motion vector, the one or more processors are configured to constrain an absolute value of a horizontal component of the difference motion vector to be less than half of a pixel of the video data and to constrain an absolute value of a vertical component of the difference motion vector to be less than half of the pixel of the video data; determine subblock prediction information for the sample of the subblock based on the spatial gradient information and the difference motion vector; determine a predicted block for the block of the video data based on the subblock prediction information; generate a residual block for the block of the video data based on differences between the block of the video data and the predicted block; and encode the residual block. 
     In one example, a non-transitory computer-readable storage medium stores instructions that, when executed, cause one or more processors of a device configured to decode video data to: predict a subblock of the block of the video data using affine motion compensation; determine spatial gradient information for a sample of the subblock of the block of the video data; determine a difference motion vector indicating a difference between a pixel motion vector computed for the sample and a subblock motion vector for the subblock, wherein, to determine the difference motion vector, the instructions cause the one or more processors to constrain an absolute value of a horizontal component of the difference motion vector to be less than half of a pixel of the video data and to constrain an absolute value of a vertical component of the difference motion vector to be less than half of the pixel of the video data; determine subblock prediction information for the sample of the subblock based on the spatial gradient information and the difference motion vector; determine a predicted block for the block of the video data based on the subblock prediction information; decode a residual block for the block of the video data; and combine the predicted block and the residual block to decode the block of the video data. 
     In another example, a non-transitory computer-readable storage medium stores instructions that, when executed, cause one or more processors of a device configured to encode video data to: predict a subblock of the block of the video data using affine motion compensation; determine spatial gradient information for a sample of the subblock of the block of the video data; determine a difference motion vector indicating a difference between a pixel motion vector computed for the sample and a subblock motion vector for the subblock, wherein, to determine the difference motion vector, the instructions cause the one or more processors to constrain an absolute value of a horizontal component of the difference motion vector to be less than half of a pixel of the video data and to constrain an absolute value of a vertical component of the difference motion vector to be less than half of the pixel of the video data; determine subblock prediction information for the sample of the subblock based on the spatial gradient information and the difference motion vector; determine a predicted block for the block of the video data based on the subblock prediction information; generate a residual block for the block of the video data based on differences between the block of the video data and the predicted block; and encode the residual block. 
     In one example, an apparatus configured to decode video data comprises: means for predicting a subblock of a block of the video data using affine motion compensation; means for determining spatial gradient information for a sample of the subblock of the block of the video data; means for determining a difference motion vector indicating a difference between a pixel motion vector for the sample and a subblock motion vector for the subblock, wherein the means for determining the difference motion vector comprises means for constraining an absolute value of a horizontal component of the difference motion vector to be less than half of a pixel of the video data and means for constraining an absolute value of a vertical component of the difference motion vector to be less than half of the pixel of the video data; means for determining subblock prediction information for the sample based on the spatial gradient information and the difference motion vector; means for determining a predicted block for the block of the video data based on the subblock prediction information; means for decoding a residual block for the block of the video data; and means for combining the predicted block and the residual block to decode the block of the video data. 
     In another example, an apparatus configured to encode video data comprises: means for predicting a subblock of a block of the video data using affine motion compensation; means for determining spatial gradient information for a sample of the subblock of the block of the video data; means for determining a difference motion vector indicating a difference between a pixel motion vector for the sample and a subblock motion vector for the subblock, wherein the means for determining the difference motion vector comprises means for constraining an absolute value of a horizontal component of the difference motion vector to be less than half of a pixel of the video data and means for constraining an absolute value of a vertical component of the difference motion vector to be less than half of the pixel of the video data; means for determining subblock prediction information for the sample based on the spatial gradient information and the difference motion vector; means for determining a predicted block for the block of the video data based on the subblock prediction information; means for decoding a residual block for the block of the video data; and means for combining the predicted block and the residual block to decode the block of the video data. 
     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). 
         FIG. 3  is a block diagram illustrating an example video encoder that may perform the techniques of this disclosure. 
         FIG. 4  is a block diagram illustrating an example video decoder that may perform the techniques of this disclosure. 
         FIG. 5  is a conceptual diagram illustrating an example of a block with three control-points, in accordance with techniques described herein. 
         FIG. 6  is a conceptual diagram illustrating an example of a block with four control-points, in accordance with techniques described herein. 
         FIG. 7  is a conceptual diagram illustrating an example prediction refinement with optical flow (PROF), in accordance with techniques described herein. 
         FIG. 8  is a flowchart illustrating an example method for encoding a current block, in accordance with techniques described herein. 
         FIG. 9  is a flowchart illustrating an example method for decoding a current block of video data, in accordance with techniques described herein. 
         FIG. 10  is a flowchart illustrating an example method for determining subblock prediction information constraining a difference motion vector, in accordance with techniques described herein. 
         FIG. 11  is a flowchart illustrating an example method for determining subblock prediction information using a difference motion vector for a subblock, in accordance with techniques described herein. 
     
    
    
     DETAILED DESCRIPTION 
     To account for affine motion (e.g., a rotation), a video coder (e.g., a video encoder or a video decoder) may be configured to predict a subblock of a block of video data using affine motion compensation. Moreover, the video coder may perform prediction refinement with optical flow (PROF) to account for a position of each sample within each subblock. For example, the video coder may determine spatial gradient information for a sample of the subblock of the block of the video data. Spatial gradient information may represent a change in predicted values for the sample. For instance, the video coder may generate the spatial gradient information using a 3-tap filter (e.g., [−1, 0, 1]) to the subblock at a location of the sample. In this example, the video coder may determine a difference motion vector representing a difference between a pixel motion vector computed for the sample and a subblock motion vector for the subblock. The video coder may multiply a horizontal component of the spatial gradient information and a horizontal component of the difference motion vector and multiply a vertical component of the spatial gradient information and a vertical component of the difference motion vector to generate refinement information. The video coder may add the refinement information to a predicted sample value of the subblock determined using affine motion compensation to generate subblock prediction information for the sample. In this way, the video coder may improve the affine motion compensation by adding an offset to each sample. 
     However, when applying prediction refinement with optical flow, a video coder (e.g, a video encoder or a video decoder) may generate a bit depth value for a sample that exceeds an allowed bit depth (e.g., 18-bit) of motion field storage. The motion vector overflow may result in a stored bit depth value that indicates an incorrect bit depth, which may result in inaccurate predicted sample values. As such, the video coder may inaccurately represent the bit depth value in storage, which may reduce a prediction accuracy of the video data. 
     In accordance with the techniques of the disclosure, a video coder may constrain a horizontal component of the difference motion vector and a vertical component of the difference motion vector, for example, to be less than half a pixel of the video data. Constraining the difference motion vector may help to prevent a motion vector buffer overflow. In this way, the video coder may minimize motion vector overflow, which may help to improve a prediction accuracy of the video data. 
     Additionally, or alternatively, rather than determining a difference motion vector for each sample of a block of video data, a video coder (e.g., a video encoder or video decoder) may determine a difference motion vector for each subblock (e.g., a 2×2 subblock of a 4×4 subblock of a block of video data) of the block of the video data. By determining the difference motion vector for each subblock instead of each sample, a motion vector may be determined with lower signaling overhead, thus potentially increasing coding efficiency. The techniques of this disclosure may also provide for a reduction in implementation complexity with little to no loss in prediction accuracy. 
       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, uncoded 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, mobile devices (e.g., telephone handsets such smartphones), televisions, cameras, display devices, digital media players, video gaming consoles, video streaming device, broadcast receiver devices, 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 determine subblock prediction information using a difference motion vector constraint and/or determine subblock prediction information using a difference motion vector for a subblock. 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 including 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 subblock prediction information using a difference motion vector constraint and/or determining subblock prediction information using a difference motion vector for a subblock. 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  include 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, uncoded 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 example, 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, computer-readable medium  110  may include storage device  112 . 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, computer-readable medium  110  may include file server  114  or another intermediate storage device that may store the encoded video data generated by source device  102 . 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., a 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/receiver, 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 other cellular communication standards. 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 , etc.). 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 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 8),” Joint Video Experts Team (JVET) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 17 th  Meeting: Brussels, BE, 7-17 Jan. 2020, JVET-Q2001-vA (hereinafter “VVC Draft 8”). 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-processors (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) 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 coding units (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) partitions. A triple tree partition is a partition where a block is split into three subblocks. In some examples, a triple tree partition divides a block into three subblocks 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. 
     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 has 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. 
     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). 
     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 advanced motion vector prediction (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 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 for partitioning 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. 
     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. 
     To account for affine motion (e.g., a rotation), a video coder (e.g., video encoder  200  or video decoder  300 ) may be configured to predict a subblock of a block of video data using affine motion compensation. Moreover, the video coder may perform prediction refinement with optical flow (PROF) to account for a position of each sample within each subblock. For example, the video coder may determine spatial gradient information for a sample of the subblock of the block of the video data. In this example, the video coder may determine a difference motion vector representing a difference between a pixel motion vector for the sample and a subblock motion vector for the subblock. The video coder may multiply a horizontal component of the spatial gradient information and a horizontal component of the difference motion vector and multiply a vertical component of the spatial gradient information and a vertical component of the difference motion vector to generate refinement information. The video coder may add the refinement information to a predicted sample value of the subblock to generate subblock prediction information for the sample. In this way, the video coder may improve the affine motion compensation by adding an offset to each sample. 
     However, when applying prediction refinement with optical flow, a video coder (e.g, video encoder  200  or video decoder  300 ) may generate a bit depth value for a sample that exceeds an allowed bit depth (e.g., 18-bit) of motion field storage. As such, the video coder may inaccurately represent the bit depth value in storage, which may reduce a prediction accuracy of the video data. 
     In accordance with the techniques of the disclosure, a video coder (e.g., video encoder  200  or video decoder  300 ) may constrain a horizontal component of the difference motion vector and a vertical component of the difference motion vector, for example, to be less than half a pixel of the video data. Constraining the difference motion vector may help to prevent a motion vector buffer overflow. In this way, the video coder may minimize motion vector overflow, which may help to improve a prediction accuracy of the video data. 
     Additionally, or alternatively, rather than determining a difference motion vector for each sample of a block of video data, a video coder (e.g., video encoder  200  or video decoder  300 ) may determine a difference motion vector for each group of samples (e.g., a 2×2 subblock of a 4×4 subblock of a block of video data) of the block of the video data. By determining the difference motion vector for each group of samples instead of each sample, a motion vector may be determined with lower signaling overhead, thus potentially increasing coding efficiency. 
     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 diagram illustrating an example quadtree binary tree (QTBT) structure  130 , and a corresponding coding tree unit (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, since quadtree nodes split a block horizontally and vertically into 4 subblocks 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 (i.e., the first level) of QTBT structure  130  (i.e., the solid lines) and syntax elements (such as splitting information) for a prediction tree level (i.e., the second 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 . 
     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), they 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 coding unit (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 quadtree leaf node is 128×128, the quadtree leaf node is not be further split by the binary tree, since the size exceeds the MaxBTSize (i.e., 64×64, in this example). Otherwise, the quadtree leaf node is 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. When the binary tree node has width equal to MinBTSize (4, in this example), it implies that no further vertical splitting is permitted. Similarly, a binary tree node having a height equal to MinBTSize implies that no further horizontal 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. 
       FIG. 3  is a block diagram illustrating an example video encoder  200  that may perform the techniques of this disclosure.  FIG. 3  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  in the context of video coding standards such as the HEVC video coding standard and the H.266 video coding standard in development. However, the techniques of this disclosure are not limited to these video coding standards, and are applicable generally to video encoding and decoding. 
     In the example of  FIG. 3 , 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. 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. 3  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, the one or more 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 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 , affine motion unit  223 , 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 ), 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 , affine motion unit  223 , 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 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. 
     Affine motion unit  223  may be configured to perform affine motion compensation and/or optical flow compensation. Affine motion unit  223  may be configured to constrain a magnitude of a difference motion vector. For example, affine motion unit  223  may predict a subblock of a block of video data using subblock affine motion compensation and determine spatial gradient information for a sample of the subblock of the block of the video data. In this example, affine motion unit  223  may determine a difference motion vector representing a difference between a pixel motion vector for the sample and a subblock motion vector for the subblock. To determine the difference motion vector, affine motion unit  223  may constrain an absolute value of a horizontal component of the difference motion vector to be less than a horizontal constraint (e.g., half of a pixel of video data) and constrain an absolute value of a vertical component of the difference motion vector to a be less than a vertical constraint (e.g., half of a pixel of video data). Affine motion unit  223  may determine subblock prediction information for the sample of the subblock based on the spatial gradient information and the difference motion vector. Motion compensation unit  224  may determine a predicted block for the block of the video data based on the subblock prediction information. Residual generation unit  204  may generate a residual block for the block of the video data based on differences between the block of the video data and the predicted block output by motion compensation unit  224 . Entropy encoding unit  220 , with transform processing unit  206  and quantization unit  208 , may encode the residual 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 nR×2N for inter prediction. 
     In examples where mode selection unit 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 a 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 syntax elements representing 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 syntax elements, such as the entropy encoded syntax elements, needed to reconstruct blocks of a slice or picture. For instance, in some examples, entropy encoding unit  220  outputs 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 an 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 blocks and the chroma coding blocks. 
     Video encoder  200  represents an example of an apparatus configured to encode video data including a memory (e.g., video data memory  230 ) configured to store video data, and one or more processors implemented in circuitry. Affine motion unit  223  may be configured to predict a subblock of a block of video data using subblock affine motion compensation and determine spatial gradient information for a sample of the subblock of the block of the video data. Affine motion unit  223  may be configured to determine a difference motion vector indicating a difference between a pixel motion vector computed for the sample and a subblock motion vector for the subblock. To determine the difference motion vector, affine motion unit  223  may be configured to constrain an absolute value of a horizontal component of the difference motion vector to be less than half of a pixel of the video data and constrain an absolute value of a vertical component of the difference motion vector to a be less than half of a pixel of the video data. Motion compensation unit  224  may determine a predicted block for the block of the video data based on the subblock prediction information. Residual generation unit  204  may generate a residual block for the block of the video data based on differences between the block of the video data and the predicted block output by motion compensation unit  224 . Entropy encoding unit  220 , with transform processing unit  206  and quantization unit  208 , may encode the residual block. 
     In some examples, 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 processors implemented in circuitry. Affine motion unit  223  may be configured to predict a subblock (e.g., a 4×4 subblock) of a block of video data using affine motion compensation. Affine motion unit  223  may determine spatial gradient information for each sample of the 4×4 subblock of the block of the video data. Affine motion unit  223  may determine a difference motion vector for each group of samples (e.g., each 2×2 subblock of the 4×4 subblock). Affine motion unit  223  may determine subblock prediction information for each sample of the subblock based on the spatial gradient information for a respective sample and the difference motion vector for the group of samples that includes the respective sample. Motion compensation unit  224  may determine a predicted block for the block of the video data based on the subblock prediction information. Residual generation unit  204  may generate a residual block for the block of the video data based on differences between the block of the video data and the predicted block output by motion compensation unit  224 . Entropy encoding unit  220 , with transform processing unit  206  and quantization unit  208 , may encode the residual block. 
       FIG. 4  is a block diagram illustrating an example video decoder  300  that may perform the techniques of this disclosure.  FIG. 4  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, and HEVC. 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. 4 , 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. 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 , affine motion unit  317 , 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 ), 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 dynamic random access memory (DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM), resistive RAM (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. 4  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. 3 , 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, the one or more 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. 3 ). 
     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. 3 ). Intra-prediction unit  318  may retrieve data of neighboring samples to the current block from DPB  314 . 
     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. 
     Affine motion unit  317  may be configured to perform affine motion compensation and/or optical flow compensation. Affine motion unit  317  may be configured to constrain a magnitude of a difference motion vector representing a difference between a pixel motion vector for the sample and a subblock motion vector for the subblock. For example, affine motion unit  317  may predict a subblock of a block of video data using subblock affine motion compensation and determine spatial gradient information for a sample of the subblock of the block of the video data. In this example, affine motion unit  317  may determine a difference motion vector representing a difference between a pixel motion vector for the sample and a subblock motion vector for the subblock. To determine the difference motion vector, affine motion unit  317  may constrain an absolute value of a horizontal component of the difference motion vector to be less than a horizontal constraint (e.g., half of a pixel of video data) and constrain an absolute value of a vertical component of the difference motion vector to a be less than a vertical constraint (e.g., half of a pixel of video data). Affine motion unit  317  may determine subblock prediction information for the sample of the subblock based on the spatial gradient information and the difference motion vector. Motion compensation unit  316  may determine a predicted block for the block of the video data based on the subblock prediction information. Entropy decoding unit  302 , with inverse quantization unit  306  and inverse transform processing unit  308 , may decode a residual block for the block of the video data. Reconstruction unit  310  may combine the predicted block and the residual block to decode the block of the video data. 
     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 from DPB 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 (e.g., DPB  314 , CPB memory  320 , etc.) configured to store video data, and one or more processors implemented in circuitry. Affine motion unit  317  may be configured to predict a subblock of a block of video data using subblock affine motion compensation and determine spatial gradient information for a sample of the subblock of the block of the video data. Affine motion unit  317  may be configured to determine a difference motion vector representing a difference between a pixel motion vector for the sample and a subblock motion vector for the subblock. To determine the difference motion vector, affine motion unit  317  may be configured to constrain an absolute value of a horizontal component of the difference motion vector to be less than a horizontal constraint (e.g., half of a pixel of video data) and constrain an absolute value of a vertical component of the difference motion vector to a be less than a vertical constraint (e.g., half of a pixel of video data). Affine motion unit  317  may be configured to determine subblock prediction information for the sample of the subblock based on the spatial gradient information and the difference motion vector. Motion compensation unit  316  may determine a predicted block for the block of the video data based on the subblock prediction information. Entropy decoding unit  302 , with inverse quantization unit  306  and inverse transform processing unit  308 , may decode a residual block for the block of the video data. Reconstruction unit  310  may combine the predicted block and the residual block to decode the block of the video data. 
     Video decoder  300  may represent an example of a video decoding device including a memory configured to store video data, and one or more processors implemented in circuitry. Affine motion unit  317  may be configured to predict a subblock (e.g., a 4×4 subblock) of a block of video data using affine motion compensation. Affine motion unit  317  may determine spatial gradient information for each sample of the subblock of the block of the video data. Affine motion unit  317  may determine a difference motion vector for each group of samples of the subblock. Affine motion unit  317  may determine subblock prediction information for each sample of the subblock based on the spatial gradient information for a respective sample and the difference motion vector for the group of samples that includes the respective sample. Motion compensation unit  316  may determine a predicted block for the block of the video data based on the subblock prediction information. Entropy decoding unit  302 , with inverse quantization unit  306  and inverse transform processing unit  308 , may decode a residual block for the block of the video data. Reconstruction unit  310  may combine the predicted block and the residual block to decode the block of the video data. 
       FIG. 5  is a conceptual diagram illustrating an example of a block  500  with three control-points. An affine motion model can be described as 
     
       
         
           
             
               
                 
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     wherein (v x , v y ) is the motion vector at the coordinate (x, y), and a, b, c, d, e, and f are the six parameters. In this example, the affine motion model is a 6-parameters affine motion model. In a video coder (e.g., video encoder  200  and/or video decoder  300 ), a picture is partitioned into blocks for block-based coding. The affine motion model for a block  500  can also be described by the 3 motion vectors (MVs) {right arrow over (v)} 0 =(v 0x , v 0y ), {right arrow over (v)} 1 =(v 1x , v 1y ), and {right arrow over (v)} 2 =(v 2x , v 2y ) at 3 different locations that are not in the same line. The 3 locations are usually referred to as control-points, the 3 motion vectors are referred to as control-point motion vectors (CPMVs). In the case when the 3 control-points are at the 3 corners of block  500  as shown in  FIG. 5 . The affine motion of  FIG. 5  may be described as follows. 
     
       
         
           
             
               
                 
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     wherein blkW and blkH are the width (‘w’) and height (‘h’) of block  500 . 
     A simplified 4-parameters affine model (e.g., for zoom and rotational motion) is described as follows. 
     
       
         
           
             
               
                 
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       FIG. 6  is a conceptual diagram illustrating an example of a block  510  with four control-points. Similarly, the simplified 4-parameters affine model for block  510  can be described by 2 CPMVs {right arrow over (v)} 0 =(v 0x , v 0y ) and {right arrow over (v)} 1 =(v 1x , v 1y ) at the 2 corners of the block. A video coder (e.g., video encoder  200  or video decoder  300 ) may be configured to use any combination of CPMVs {right arrow over (v)} 0 =(v 0x , v 0y ), {right arrow over (v)} 1 =(v 1x , v 1y ), and {right arrow over (v)} 3 =(v 3x , v 3y ). The motion field is then described as follows. 
     
       
         
           
             
               
                 
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     where blkW and blkH are the width (‘w’) and height (‘h’) of block  510 . 
     Given an affine motion model for a block, a video coder (e.g., video encoder  200  or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to derive different motion vectors for each pixel in the block. Therefore, the video coder may be configured to perform motion compensation in a pixel-by-pixel basis. However, to reduce the complexity, the video coder may be configured to partition the block into multiple subblocks (e.g., that have smaller block size) and each subblock is associated with one motion vector for motion compensation. The video coder may be configured to derive the motion vector for each subblock using a representative coordinate of the subblock, such as for example, a center position of a respective subblock. In some examples, the video coder may be configured to partition the block into non-overlapping subblocks. The block width is blkW, block height is blkH, the subblock width is sbW and subblock height is sbH, then there are blkH/sbH rows of subblocks and blkW/sbW subblocks in each row. For a six-parameter affine motion model, the video coder may be configured to derive the motion vector for the subblock (referred to as subblock MV) at i th  row (0&lt;=i&lt;blkW/sbW) and jai (0&lt;=j&lt;blkH/sbH) column as follows. 
     
       
         
           
             
               
                 
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                   EQUATION 
                    
                   
                       
                   
                    
                   5 
                 
               
             
           
         
       
     
       FIG. 7  is a conceptual diagram illustrating an example prediction refinement with optical flow (PROF), in accordance with techniques described herein. In Luo, et al. “CE2-related: Prediction refinement with optical flow for affine mode Associated Resources,” Joint Video Experts Team (JVET) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 14th Meeting: Geneva, CH, 19-27 Mar. 2019, JVET-N023641 (hereinafter “WET-N0236”), an example prediction refinement with optical flow (PROF) process is described to improve the affine motion compensation by adding an offset to each sample. A video coder (e.g., video encoder  200  or video decoder  300 ) may be configured to use the example PROF described in WET-N0236 or other optical flow processes. The video coder may be configured to derive an offset by optical flow as follows in the following example. 
     After the subblock based affine motion compensation is performed, a video coder (e.g., video encoder  200  and/or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to refine the luma prediction sample by, for example, adding a difference derived by the optical flow equation. An example optical flow process is described as following four steps. 
     Step  1 ) The subblock-based affine motion compensation is performed to generate subblock prediction I(i, j). Said differently, a video coder (e.g., video encoder  200  or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to predict a subblock  522  of a block  520  of video data using affine motion compensation. 
     Step  2 ) The spatial gradients g x (i, j) and g y (i, j) of the subblock prediction are calculated at each sample location using a 3-tap filter [−1, 0, 1]. Said differently, a video coder (e.g., video encoder  200  or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to determine spatial gradient information of a sample of subblock  522  of block  520 . For example, the video coder may be configured to apply a filter (e.g., a 3-tap filter) at the sample location to determine the spatial gradient information. Said differently, the video coder may be configured to apply a 3-tap filter to the subblock at a location of the sample. For example, the video coder may be configured to apply a 3-tap filter to the subblock at a location (i, j) of the sample by calculating Equation 6. 
         g   x ( i,j )= I ( i+ 1, j )− I ( i −1, j )
 
         g   y ( i,j )= I ( i,j+ 1)− I ( i,j −1)  EQUATION 6
 
     where (i, j) corresponds to a location of the sample, g x  (i,j) is a horizontal component of the spatial gradient information, g y  (i,j) is the vertical component of the spatial gradient information, I(i+1,j) is a predicted sample value for a right sample and from predicting the subblock using affine motion compensation, I(i−1,j) is a predicted sample value for a left sample and from predicting the subblock using affine motion compensation, I(i,j+1) is a predicted sample value for a bottom sample and from predicting the subblock using affine motion compensation, and I(i,j−1) is a predicted sample value for a top sample and from predicting the subblock using affine motion compensation. 
     A video coder (e.g., video encoder  200  or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may extend the subblock prediction by one pixel on each side for the gradient calculation of equation 6. To reduce the memory bandwidth and complexity, the video coder may copy the pixels on the extended borders from the nearest integer pixel position in the reference picture. Therefore, the video coder may avoid additional interpolation for padding region. 
     Step  3 ) The luma prediction refinement is calculated by the optical flow equation. Said differently, a video coder (e.g., video encoder  200  or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may calculate refinement information using Equation 7. 
       Δ I ( i,j )= g   x ( i,j )*Δ v   x ( i,j )+ g   y ( i,j )*Δ v   y ( i,j )   EQUATION 7
 
     where the ΔV (i, j) is the difference between pixel MV computed for sample location (i, j), denoted by V(i, j), and the subblock MV of the subblock to which pixel (i, j) belongs, denoted by V SB , as shown in  FIG. 7 . Said differently, a video coder (e.g., video encoder  200  or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to determine a difference motion vector  526  and determine subblock prediction information based on the spatial gradient information and the difference motion vector. For example, the video coder may multiply a horizontal component of the spatial gradient information (e.g., g x (i, j)) and the horizontal component of the difference motion vector (e.g., Δv x (i, j)). The video coder may multiply a vertical component of the spatial gradient information (e.g., g y (i, j)) and the vertical component of the difference motion vector (e.g., Δv y (i, j)). 
     Because the affine model parameters and the pixel (e.g., sample) location relative to the subblock center are not changed from subblock to subblock, a video coder (e.g, video encoder  200  and/or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to calculate a difference motion vector  526  (e.g., Δv(i, j)) for a first subblock  522 , and reuse the difference motion vectors (e.g., Δv(i, j)) for subblocks  522 - 528  in the same CU. Video encoder  200  and/or video decoder  300  may be configured to derive difference motion vector  526  (e.g., Δv(i, j)) by the following equations, where x and y are horizontal and vertical offsets, respectively, from the pixel location to the center of the subblock. 
     
       
         
           
             
               
                 
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                   EQUATION 
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                   8 
                 
               
             
           
         
       
     
     For 4-parameter affine model, 
     
       
         
           
             
               
                 
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                   EQUATION 
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                   9 
                 
               
             
           
         
       
     
     For 6-parameter affine model, 
     
       
         
           
             
               
                 
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                   EQUATION 
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                   10 
                 
               
             
           
         
       
     
     where (v 0x , v 0y ), (v 1x , v 1y ), (v 2x , v 2y ) are the top-left, top-right and bottom-left control point motion vectors, w and h are the width and height of the CU. 
     That is, a video coder (e.g, video encoder  200  and/or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may apply equation 9 to derive affine motion parameters c, d, e, and f when the video coder predicts subblock  522  of block  520  of video data using 4-parameter affine motion compensation. In this example, the video coder may derive difference motion vector  526  (e.g., Δv(i, j)) using the derived affine motion parameters c, d, e, and f with Equation 8. 
     Video coder (e.g, video encoder  200  and/or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may apply equation 10 to derive affine motion parameters c, d, e, and f when the video coder predicts subblock  522  of block  520  of video data using 6-parameter affine motion compensation. In this example, the video coder may derive difference motion vector  526  (e.g., Δv(i, j)) using the derived affine motion parameters c, d, e, and f with Equation 8. 
     Step  4 ) The luma prediction refinement is added to the subblock prediction I(i, j). Said differently, a video coder (e.g, video encoder  200  and/or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to add refinement information to a predicted sample value for the sample and from predicting the subblock using affine motion compensation to generate subblock prediction information for the sample. The video coder may generate the final prediction I′ (e.g., the subblock prediction information for the sample) by calculating equation 11. 
         I ′( i,j )= I ( i,j )+Δ I ( i,j )   EQUATION 11
 
     Said differently, for example, a video coder (e.g., video encoder  200  or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to predict subblock  522  of block  520  of video data using affine motion compensation. In this example, the video coder may be configured to determine spatial gradient information (e.g., g x (i, j) and g y (i, j)) for a sample of subblock  522  of block  520 . In this example, the video coder may be configured to determine a difference motion vector indicating a difference between a pixel motion vector computed for the sample and a subblock motion vector for the subblock (Δv x (i, j) and Δv y (i, j)). The video coder may be configured to determine subblock prediction information for the sample of subblock  522  based on the spatial gradient information and the difference motion vector. For example, the video coder may be configured to calculate ΔI(i, j) for the sample of subblock  522  based on the spatial gradient information and the difference motion vector. For instance, the video coder may calculate Equation 7. More specifically, the video coder may determine the difference motion vector using Equation 8 and 9 for a 4-parameter affine model and/or determine the difference motion vector using Equation 8 and 10 for a 6-parameter affine model. In this example, the video coder may be configured to determine subblock prediction information for the sample of subblock  522  by calculating equation 11. 
     To generate spatial gradient information, a video coder (e.g., video encoder  200  or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to fetch 11×11 integer reference samples before interpolation for the interpolation of 4×4 subblock. In this example, the video coder may generate 13×13 reference samples by extending the 11×11 block with the border copied from adjacent inner samples. The video coder may use the a 13×13 reference samples for the interpolation of 6×6 prediction, which is used to calculate the 4×4 spatial gradient information. 
     A video coder (e.g., video encoder  200  and/or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to apply any combination of the simplifications for prediction refinement with optical flow as follows.
         1. Calculate Δv(x, y) on a 2×2 basis. For example, a video coder (e.g., video encoder  200  and/or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to calculate a difference motion vector (e.g., Δv(x, y)) in 2×2 basis or another basis (e.g., 4×4).   2. Perform a gradient calculation using only the prediction samples within each 4×4 to allow parallel processing. For example, a video coder (e.g., video encoder  200  and/or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to perform a gradient calculation (e.g., calculate g x (i, j) and/or g y (i, j)) using only the prediction samples within each subblock (e.g., a 4×4 subblock) to allow parallel processing.   3. Apply a constraint on Δv(x, y) to disable prediction refinement with optical flow for a large Δv(x, y) and to prevent overflow in prediction refinement with optical flow. For example, a video coder (e.g., video encoder  200  and/or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to apply a constraint on the difference motion vector (e.g., Δv(x, y)) to disable prediction refinement with optical flow for a relatively large difference motion vector (e.g., a Δv(x, y) that exceeds a threshold) and/or to prevent overflow in prediction refinement with optical flow. In some examples, the video coder may clip the difference motion vector (e.g., Δv(x, y)) used for prediction refinement to prevent overflow in prediction refinement. For instance, if the absolute value of one component (e.g., x/y) of the difference motion vector (e.g., Δv(x, y)) is larger than a threshold, the video coder may replace the absolute value of the one component (e.g., x/y) of the difference motion vector (e.g., Δv(x, y)) value by the threshold with the same sign of original value.   4. Only apply prediction refinement with optical flow to uni-prediction. For example, a video coder (e.g., video encoder  200  and/or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may apply prediction refinement with optical flow in response to predicting a subblock using uni-prediction and refrain from applying prediction refinement with optical flow (e.g., disabling prediction refinement with optical flow) in response to predicting a subblock using bi-prediction.       

     A video coder (e.g., video encoder  200  and/or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to apply any combination of the simplifications described herein for affine motion compensation refinement using prediction refinement with optical flow. Techniques of this disclosure may be described for a 4×4 subblock, however extension to some other subblock size may be used in other examples. For example, techniques for examples using 4×4 subblocks may apply to 8×8 subblocks. The simplification of a motion vector calculation may be on a 4×4 basis, a 2×2 basis, or another basis. 
     A video coder (e.g., video encoder  200  and/or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to calculate the difference motion vector (e.g., Δv(x, y), Δv(i, j), etc.) in a 2×2 subblock basis instead for each sample of the 2×2 subblock. For example, the video coder may be configured to calculate, for each 2×2 subblock of a block, a difference (Δy(x, y)) between: (i) a pixel motion vector for a pixel at a horizontal offset (x) from a center of the subblock and a vertical offset (y) from a center of the subblock y; and (ii) a subblock MV of a subblock to which the pixel belongs. In other words, for example, samples within 2×2 may share the same motion vector. In each 4×4 subblock, the video coder may be configured to calculate the difference motion vector (e.g., Δv(x, y)) for the 2×2 subblocks within the 4×4 using one or more of Equations 12-14. 
     Top-left 2×2: 
     
       
         
           
             
               
                 
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                   12 
                 
               
             
           
         
       
     
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                   13 
                 
               
             
           
         
       
     
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                   13 
                 
               
             
           
         
       
     
     Bottom-right 2×2: 
     
       
         
           
             
               
                 
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                   EQUATION 
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                   14 
                 
               
             
           
         
       
     
     Said differently, a video coder (e.g., video encoder  200  and/or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may predict a subblock (e.g., a 4×4 subblock) of a block of video data using affine motion compensation. The video coder may determine spatial gradient information for each sample of the subblock of the block of the video data. The video coder may determine a difference motion vector for each group of samples (e.g., a 2×2 subblock) of the subblock. The video coder may determine subblock prediction information for each sample of the subblock based on the spatial gradient information for a respective sample and the difference motion vector for the group of samples that includes the respective sample. 
     A video coder (e.g., video encoder  200  and/or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to calculate the gradient of each 4×4 subblock without referring prediction samples outside the 4×4. Said differently, the video coder may be configured to calculate spatial gradient information for each sample of a subblock without referring prediction samples outside the subblock. In this way, the video coder may be configured to process each 4×4 subblock in parallel. The 4×4 subblock prediction is extended by one sample on each side for the gradient calculation, the video coder may be configured to copy the samples on the extended borders from the nearest prediction sample in the 4×4 subblock. 
     A video coder (e.g., video encoder  200  or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may perform a gradient calculation as follows. In this example, P(i, j) denotes the sample value at coordinate (i, j) in the 4×4 subblock, g x (i, j) denotes spatial gradient information in the x direction (e.g., a horizontal component), and g y (i, j) denotes the spatial gradient information in the y direction (e.g., a vertical component). 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             
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                       0 
                     
                     , 
                     1 
                     , 
                     2 
                     , 
                     3 
                   
                    
                   
                     
 
                   
                    
                   
                     
                       
                         
                           g 
                           y 
                         
                          
                         
                           ( 
                           
                             3 
                             , 
                             j 
                           
                           ) 
                         
                       
                       = 
                       
                         
                           
                             P 
                              
                             
                               ( 
                               
                                 3 
                                 , 
                                 j 
                               
                               ) 
                             
                           
                           - 
                           
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                              
                             
                               ( 
                               
                                 2 
                                 , 
                                 j 
                               
                               ) 
                             
                           
                         
                         2 
                       
                     
                     , 
                     
                       j 
                       = 
                       0 
                     
                     , 
                     1 
                     , 
                     2 
                     , 
                     3 
                   
                    
                   
                     
 
                   
                    
                   
                     
                       
                         
                           g 
                           x 
                         
                          
                         
                           ( 
                           
                             i 
                             , 
                             j 
                           
                           ) 
                         
                       
                       = 
                       
                         
                           
                             P 
                              
                             
                               ( 
                               
                                 i 
                                 , 
                                 
                                   j 
                                   + 
                                   1 
                                 
                               
                               ) 
                             
                           
                           - 
                           
                             P 
                              
                             
                               ( 
                               
                                 i 
                                 , 
                                 
                                   j 
                                   - 
                                   1 
                                 
                               
                               ) 
                             
                           
                         
                         2 
                       
                     
                     , 
                     
                       j 
                       = 
                       1 
                     
                     , 
                     
                       2 
                        
                       
                           
                       
                        
                       and 
                     
                   
                    
                   
                      
                   
                    
                   
                     
                       i 
                       = 
                       0 
                     
                     , 
                     1 
                     , 
                     2 
                     , 
                     3 
                   
                    
                   
                     
 
                   
                    
                   
                     
                       
                         
                           g 
                           y 
                         
                          
                         
                           ( 
                           
                             i 
                             , 
                             j 
                           
                           ) 
                         
                       
                       = 
                       
                         
                           
                             P 
                              
                             
                               ( 
                               
                                 
                                   i 
                                   + 
                                   1 
                                 
                                 , 
                                 j 
                               
                               ) 
                             
                           
                           - 
                           
                             P 
                              
                             
                               ( 
                               
                                 
                                   i 
                                   - 
                                   1 
                                 
                                 , 
                                 j 
                               
                               ) 
                             
                           
                         
                         2 
                       
                     
                     , 
                     
                       
 
                     
                      
                     
                       j 
                       = 
                       0 
                     
                     , 
                     1 
                     , 
                     2 
                     , 
                     
                       
                         3 
                          
                         
                             
                         
                          
                         and 
                          
                         
                             
                         
                          
                         i 
                       
                       = 
                       1 
                     
                     , 
                     2 
                   
                 
               
               
                 
                   EQUATION 
                    
                   
                       
                   
                    
                   15 
                 
               
             
           
         
       
     
     Said differently, with the x coordinate corresponding to a horizontal direction and the y coordinate corresponding to a vertical direction the gradient calculation is described as follows. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           g 
                           y 
                         
                          
                         
                           ( 
                           
                             i 
                             , 
                             
                                 
                             
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                             0 
                           
                           ) 
                         
                       
                       = 
                       
                         
                           
                             P 
                              
                             
                               ( 
                               
                                 i 
                                 , 
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                               ) 
                             
                           
                           - 
                           
                             P 
                              
                             
                               ( 
                               
                                 i 
                                 , 
                                 0 
                               
                               ) 
                             
                           
                         
                         2 
                       
                     
                     , 
                     
                       i 
                       = 
                       0 
                     
                     , 
                     1 
                     , 
                     2 
                     , 
                     3 
                   
                    
                   
                     
 
                   
                    
                   
                     
                       
                         
                           g 
                           y 
                         
                          
                         
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                               ( 
                               
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                       0 
                     
                     , 
                     1 
                     , 
                     2 
                     , 
                     3 
                   
                    
                   
                     
 
                   
                    
                   
                     
                       
                         
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                           x 
                         
                          
                         
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                             0 
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                           ) 
                         
                       
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                         2 
                       
                     
                     , 
                     
                       j 
                       = 
                       0 
                     
                     , 
                     1 
                     , 
                     2 
                     , 
                     3 
                   
                    
                   
                     
 
                   
                    
                   
                     
                       
                         
                           g 
                           x 
                         
                          
                         
                           ( 
                           
                             3 
                             , 
                             j 
                           
                           ) 
                         
                       
                       = 
                       
                         
                           
                             P 
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                               ( 
                               
                                 3 
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                                 j 
                               
                               ) 
                             
                           
                           - 
                           
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                               ( 
                               
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                     , 
                     
                         
                     
                      
                     
                       j 
                       = 
                       0 
                     
                     , 
                     1 
                     , 
                     2 
                     , 
                     3 
                   
                    
                   
                     
 
                   
                    
                   
                     
                       
                         
                           g 
                           y 
                         
                          
                         
                           ( 
                           
                             i 
                             , 
                             j 
                           
                           ) 
                         
                       
                       = 
                       
                         
                           
                             P 
                              
                             
                               ( 
                               
                                 i 
                                 , 
                                 
                                   j 
                                   + 
                                   1 
                                 
                               
                               ) 
                             
                           
                           - 
                           
                             P 
                              
                             
                               ( 
                               
                                 i 
                                 , 
                                 
                                   j 
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                               ) 
                             
                           
                         
                         2 
                       
                     
                     , 
                     
                       j 
                       = 
                       1 
                     
                     , 
                     
                       2 
                        
                       
                           
                       
                        
                       and 
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       i 
                       = 
                       0 
                     
                     , 
                     1 
                     , 
                     2 
                     , 
                     3 
                   
                    
                   
                     
 
                   
                    
                   
                     
                       
                         
                           g 
                           x 
                         
                          
                         
                           ( 
                           
                             i 
                             , 
                             j 
                           
                           ) 
                         
                       
                       = 
                       
                         
                           
                             P 
                              
                             
                               ( 
                               
                                 
                                   i 
                                   + 
                                   1 
                                 
                                 , 
                                 j 
                               
                               ) 
                             
                           
                           - 
                           
                             P 
                              
                             
                               ( 
                               
                                 
                                   i 
                                   - 
                                   1 
                                 
                                 , 
                                 j 
                               
                               ) 
                             
                           
                         
                         2 
                       
                     
                     , 
                     
                       
 
                     
                      
                     
                       j 
                       = 
                       0 
                     
                     , 
                     1 
                     , 
                     2 
                     , 
                     
                       
                         3 
                          
                         
                             
                         
                          
                         and 
                          
                         
                             
                         
                          
                         i 
                       
                       = 
                       1 
                     
                     , 
                     2 
                   
                 
               
               
                 
                   EQUATION 
                    
                   
                       
                   
                    
                   16 
                 
               
             
           
         
       
     
     A video coder (e.g., video encoder  200  or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to calculate the refined prediction sample using Equation 17. 
         I ′( i,j )= I ( i,j )+Δ I ( i,j )= I ( i,j )+ g   x ( i,j )*Δ v   x ( i,j )+ g   y ( i,j )*Δ v   y ( i,j )   EQUATION 17
 
     where (i, j) corresponds to a location of the sample, I′(i, j) is the subblock prediction information for the sample, I(i, j) is a predicted sample value for the sample and from predicting the subblock using affine motion compensation, g x (i, j) is a horizontal component of the spatial gradient information, g y (i, j) is the vertical component of the spatial gradient information, Δv x (i, j) is the horizontal component of the difference motion vector, and Δv y (i, j) is the vertical component of the difference motion vector. 
     Said differently, to determine the subblock prediction information (e.g., I(i, j)+ΔI(i, j)), a video coder (e.g., video encoder  200  or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to multiply a horizontal component of the spatial gradient information (e.g., g x (i, j)) and a horizontal component of the difference motion vector (e.g., Δv x (i, j)). The video coder may be configured to multiply a vertical component of the spatial gradient information (e.g., g y (i, j)) and a vertical component of the difference motion vector (e.g., Δv y (i, j)). 
     To prevent overflow in the above calculation, a video coder (e.g., video encoder  200  or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to constrain the absolute value of each component of the difference motion vector (e.g., Δv(x, y)) to be less than a threshold TH, such that the calculation of the above equation may be performed within a certain bit depth. The video coder may be configured to determine the threshold TH based on the bit depth of I(i, j) and/or the maximum internal bit depth for the calculation. For instance, to constrain the absolute value of a component, the video coder may be configured to clip the result of the calculation for the component to the bit depth of I(i, j). For example, if the bit depth of I(i, j) is 16, and the maximum internal bitdepth for calculation is 32, then the video coder may set the bit depth of each component of Δv(x, y) to 32−16−1=15. In some examples, the video coder may scale each component of Δv(x, y) by the maximum value of width and height to provide better accuracy. Each component may represent units of subpel, ¼ or 1/16 for example. 
     A video coder (e.g., video encoder  200  or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to apply a constraint on Δv(x, y) to avoid a large offset. For example, the video coder may be configured to constrain the absolute value of each component of Δv(x, y) to be less than one pixel, half pixel, a quarter pixel, or another threshold. 
     Said differently, for example, a video coder (e.g., video encoder  200  or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to determine a difference motion vector (Δv(x, y), ΔV(i, j), etc.) indicating a difference between a pixel motion vector computed for the sample and a subblock motion vector for the subblock. To determine the difference motion vector, the video coder may be configured to constrain an absolute value of a horizontal component of the difference motion vector (e.g., Δv x (i, j)) to be less than a horizontal constraint (e.g., one pixel, half pixel, a quarter pixel, etc.) and constrain an absolute value of a vertical component of the difference motion vector (e.g., Δv y (i, j)) to a be less than a vertical constraint (e.g., one pixel, half pixel, a quarter pixel, etc.). The vertical constraint may be half a pixel of the block of the video data. The horizontal constraint may be half a pixel of the block of the video data. 
     To reduce the worst-case complexity, a video coder (e.g., video encoder  200  and/or video decoder  300 ) may be configured to constrain the prediction refinement with optical flow to apply to uni-prediction only. For bi-prediction affine, the video coder may be configured to disable prediction refinement with optical flow. 
     Said differently, a video coder (e.g., video encoder  200  or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may be configured to predict a subblock of a block of video data using uni-prediction affine motion compensation predict the subblock. In this example, the video coder may be configured to determine spatial gradient information, determine a difference motion vector, and/or determine subblock prediction information based on whether the subblock of the block of the video data is predicted using uni-prediction affine motion compensation or bi-prediction affine motion compensation. In response to predicting the subblock of a block of video data using bi-prediction affine motion compensation, the video coder may disable prediction refinement with optical flow. For example, the video coder may, in response to predicting the subblock of a block of video data using bi-prediction affine motion compensation, refrain from determining spatial gradient information, refrain from determining a difference motion vector, and refrain from determining subblock prediction information. 
       FIG. 8  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 3 ), other devices may be configured to perform a method similar to that of  FIG. 8 . 
     In the example of  FIG. 8 , mode selection unit  202  predicts the current block ( 802 ). For example, affine motion unit  223  and/or motion compensation unit  224  may form a prediction block for the current block using one or more techniques described herein for simplification of prediction refinement with optical flow and/or constraining a difference motion vector. Residual generation unit  204  may calculate a residual block for the current block ( 804 ). To calculate the residual block, residual generation unit  204  may calculate a difference between the original, uncoded block and the prediction block for the current block. In this way, video encoder  200  (e.g., residual generation unit  204  of video encoder  200 ) may generate a residual block for the block of the video data based on differences between the block of the video data and the predicted block. Transform processing unit  206  may transform coefficients of the residual block and quantization unit  208  may quantize transform coefficients of the residual block ( 806 ). Entropy encoding unit  220  may scan the quantized transform coefficients of the residual block ( 808 ). During the scan, or following the scan, entropy encoding unit  220  may entropy encode the transform coefficients ( 810 ). For example, entropy encoding unit  220  may encode the transform coefficients using CAVLC or CABAC. In this way, video encoder  200  (e.g., entropy encoding unit  220  of video encoder  200 ) may encode the residual block. Entropy encoding unit  220  may then output the entropy encoded data of the block ( 812 ). 
       FIG. 9  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 4 ), other devices may be configured to perform a method similar to that of  FIG. 9 . 
     In the example of  FIG. 9 , entropy decoding unit  302  may receive entropy coded data for the current block, such as entropy coded prediction information and entropy coded data for transform coefficients of a residual block corresponding to the current block ( 850 ). Entropy decoding unit  302  may entropy decode the entropy encoded data to determine prediction information for the current block and to reproduce transform coefficients of the residual block ( 852 ). Prediction processing unit  304  may predict the current block ( 854 ), e.g., using an intra- or inter-prediction mode as indicated by the prediction information for the current block, to calculate a predicted block for the current block. In some examples, affine motion unit  317  and/or motion compensation unit  316  may predict the current block using one or more techniques described herein for simplification of PROF and/or constraining a difference motion vector. Entropy decoding unit  302  may then inverse scan the reproduced transform coefficients ( 856 ), to create a block of quantized transform coefficients. Inverse quantization unit  306  may then inverse quantize the transform coefficients and inverse transform processing unit  308  may inverse transform the transform coefficients to produce a residual block ( 858 ). In this way, video decoder  300  (e.g., entropy decoding unit  302 , inverse quantization unit  306 , and inverse transform processing unit  308  of video decoder  300 ) may decode a residual block for the block of the video data. Reconstruction unit  310  may ultimately decode the current block by combining the predicted block and the residual block ( 860 ). 
       FIG. 10  is a flowchart illustrating an example method for determining subblock prediction information using a difference motion vector constraint, in accordance with techniques described herein. The block may comprise a CU. Although described with respect to video encoder  200  ( FIGS. 1 and 3 ) and/or video decoder  300  ( FIGS. 1 and 4 ), other devices may be configured to perform a method similar to that of  FIG. 10 .  FIG. 10  may be performed during step  802  of  FIG. 8  and/or during step  854  of  FIG. 9 . 
     In the example of  FIG. 10 , a video coder (e.g., video encoder  200  or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may predict a subblock of the block of the video data using affine motion compensation ( 902 ). The video coder may determine spatial gradient information for a sample of the subblock of the block of the video data ( 904 ). For instance, the video coder may generate the spatial gradient information using a 3-tap filter (e.g., [−1, 0, 1]) to the subblock at a location of the sample. The video coder may generate the spatial gradient information using Equations 15 and/or 16. 
     The video coder may determine a difference motion vector, where a difference motion vector is constrained in absolute value to be less than half a pixel of the video data ( 906 ). For example, the video coder may determine the difference motion vector using Equation 8 and 9 for a 4-parameter affine model and/or determine the difference motion vector using Equation 8 and 10 for a 6-parameter affine model. In this example, the video coder may constrain an absolute value of a horizontal component of the difference motion vector to be less than half a pixel of the video data and constrain an absolute value of a vertical component of the difference motion vector to a be less than half a pixel of the video data. The video coder may determine subblock prediction information for the sample of the subblock based on the spatial gradient information and the difference motion vector ( 908 ). For example, the video coder may calculate refinement information using Equation 7 and calculate the subblock prediction information for the sample of the subblock using Equation 11. 
       FIG. 11  is a flowchart illustrating an example process for determining subblock prediction information using a difference motion vector for a subblock, in accordance with techniques described herein. The block may comprise a current CU. Although described with respect to video encoder  200  ( FIGS. 1 and 3 ) and/or video decoder  300  ( FIGS. 1 and 4 ), other devices may be configured to perform a method similar to that of  FIG. 10 .  FIG. 11  may be performed during step  802  of  FIG. 8  and/or during step  854  of  FIG. 9 . 
     A video coder (e.g., video encoder  200  or video decoder  300 , or in some examples, affine motion unit  223  of video encoder  200  or affine motion unit  317  of video decoder  300 ) may predict a subblock of a block of video data using affine motion compensation ( 922 ). For example, the video coder may predict a 4×4 subblock of a block of video data using affine motion compensation. The video coder may determine spatial gradient information for each sample of the subblock of the block of the video data ( 924 ). For example, the video coder may determine spatial gradient information for each sample of the 4×4 subblock of the block of the video data (e.g., using Equation 6, Equation 15, and/or Equation 16). The video coder may determine the spatial gradient information using only samples within the 4×4 subblock (e.g., using Equation 15 and/or Equation 16). The video coder may determine a difference motion vector for each group of samples of the subblock ( 926 ). For example, the video coder may determine a difference motion vector for each 2×2 subblock of the 4×4 subblock. For instance, the video coder may determine a difference motion vector for each 2×2 subblock of the 4×4 subblock using using one or more of Equations 12-14. 
     The video coder may determine subblock prediction information for each sample of the subblock based on the spatial gradient information for a respective sample and the difference motion vector for the group of samples that includes the respective sample ( 928 ). For example, the video coder may calculate refinement information using Equation 7 and calculate the subblock prediction information for the sample of the subblock using Equation 11. 
     A non-limiting illustrative list of examples of the disclosure are described below. 
     Example 1 
     A method of processing video data, the method comprising: for each 2×2 subblock of a block, calculating a difference (Δv(x, y)) between (i) a pixel Motion Vector (MV) for a pixel at a horizontal offset (x) from a center of the subblock and a vertical offset (y) from a center of the subblock y and (ii) a subblock MV of a subblock to which the pixel belongs; and generating prediction information based on the difference. 
     Example 2 
     The method of example 1, wherein the block is a 4×4 block and wherein calculating the difference comprises calculating: For a top-left 2×2 subblock: 
     
       
         
           
             { 
             
               
                 
                   
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           
                             v 
                             x 
                           
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                             ( 
                             
                               x 
                               , 
                               
                                   
                               
                                
                               y 
                             
                             ) 
                           
                         
                       
                       = 
                       
                         
                           - 
                           c 
                         
                         - 
                         d 
                       
                     
                   
                 
                 
                   
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           
                             v 
                             y 
                           
                            
                           
                             ( 
                             
                               x 
                               , 
                               
                                   
                               
                                
                               y 
                             
                             ) 
                           
                         
                       
                       = 
                       
                         
                           - 
                           e 
                         
                         - 
                         f 
                       
                     
                   
                 
               
                 
             
           
         
       
     
     For a top-right 2×2 subblock: 
     
       
         
           
             { 
             
               
                 
                   
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           
                             v 
                             x 
                           
                            
                           
                             ( 
                             
                               x 
                               , 
                               
                                   
                               
                                
                               y 
                             
                             ) 
                           
                         
                       
                       = 
                       
                         c 
                         - 
                         d 
                       
                     
                   
                 
                 
                   
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           
                             v 
                             y 
                           
                            
                           
                             ( 
                             
                               x 
                               , 
                               
                                   
                               
                                
                               y 
                             
                             ) 
                           
                         
                       
                       = 
                       
                         e 
                         - 
                         f 
                       
                     
                   
                 
               
                 
             
           
         
       
     
     For a bottom-left 2×2 subblock: 
     
       
         
           
             { 
             
               
                 
                   
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           
                             v 
                             x 
                           
                            
                           
                             ( 
                             
                               x 
                               , 
                               
                                   
                               
                                
                               y 
                             
                             ) 
                           
                         
                       
                       = 
                       
                         
                           - 
                           c 
                         
                         + 
                         d 
                       
                     
                   
                 
                 
                   
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           
                             v 
                             y 
                           
                            
                           
                             ( 
                             
                               x 
                               , 
                               
                                   
                               
                                
                               y 
                             
                             ) 
                           
                         
                       
                       = 
                       
                         
                           - 
                           e 
                         
                         + 
                         f 
                       
                     
                   
                 
               
                 
             
           
         
       
     
     For a bottom-right 2×2 subblock: 
     
       
         
           
             { 
             
               
                 
                   
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           
                             v 
                             x 
                           
                            
                           
                             ( 
                             
                               x 
                               , 
                               
                                   
                               
                                
                               y 
                             
                             ) 
                           
                         
                       
                       = 
                       
                         c 
                         + 
                         d 
                       
                     
                   
                 
                 
                   
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           
                             v 
                             y 
                           
                            
                           
                             ( 
                             
                               x 
                               , 
                               
                                   
                               
                                
                               y 
                             
                             ) 
                           
                         
                       
                       = 
                       
                         e 
                         + 
                         f 
                       
                     
                   
                 
               
                 
             
           
         
       
     
     wherein c, d, e, f are parameters of a affine motion model, Δv x (x, y) and Δv y (x, y) are horizontal and vertical components of Δv(x, y). 
     Example 3 
     The method of any combination of examples 1-2, comprising: performing a gradient calculation using only prediction samples within each 4×4 subblock to allow parallel processing, wherein generating the prediction information is based on the gradient calculation. 
     Example 4 
     The method of example 3, wherein the block is a 4×4 block and wherein performing the gradient calculation comprises calculating: 
     
       
         
           
             
               
                 
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                   x 
                 
                  
                 
                   ( 
                   
                     i 
                     , 
                     
                         
                     
                      
                     0 
                   
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               = 
               
                 
                   
                     P 
                      
                     
                       ( 
                       
                         i 
                         , 
                         1 
                       
                       ) 
                     
                   
                   - 
                   
                     P 
                      
                     
                       ( 
                       
                         i 
                         , 
                         0 
                       
                       ) 
                     
                   
                 
                 2 
               
             
              
             
                 
             
             , 
             
                 
             
              
             
               i 
               = 
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             , 
             1 
             , 
             2 
             , 
             3 
           
         
       
       
         
           
             
               
                 
                   g 
                   x 
                 
                  
                 
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     wherein P(i, j) is the sample value at coordinate (0) in the 4×4 subblock, the gradient in the x direction is g x (i, j), and the gradient in the y direction is g y (i, j). 
     Example 5 
     The method of any combination of example 1-4, comprising: constraining the difference between pixel MVs an Δv(x, y) to disable prediction refinement with optical flow (PROF) for large Δv(x, y), and to prevent overflow in PROF. 
     Example 6 
     The method of example 5, wherein constraining the difference comprises calculating: 
         I ( i,j )+Δ I ( i,j )= I ( i,j )+ g   x ( 1   ,j )*Δ v   x ( i,j )+ g   y ( i,j )*Δ v   y ( i,j ),
 
     wherein I(i, j) is subblock prediction, ΔI(i, j) is a luma prediction refinement, a gradient in the x direction is g x (i, j), a gradient in the y direction is g y  (i, j), Δv x (i, j) and Δv y (i, j) are horizontal and vertical components of Δv(i, j), which is the difference between pixel MV computed for sample location (i, j). 
     Example 7 
     The method of any combination of examples 1-6, comprising: only applying prediction refinement with optical flow (PROF) to uni-prediction. 
     Example 8 
     A device for coding video data, the device comprising one or more means for performing the method of any combination of examples 1-7. 
     Example 9 
     The device of example 8, wherein the one or more means comprise one or more processors implemented in circuitry. 
     Example 10 
     The device of any combination of examples 8-9, further comprising a memory to store the video data. 
     Example 11 
     The device of any combination of examples 8-9, further comprising a display configured to display decoded video data. 
     Example 12 
     The device of any combination of examples 8-11, wherein the device comprises one or more of a camera, a computer, a mobile device, a broadcast receiver device, or a set-top box. 
     Example 13 
     The device of any combination of examples 8-12, wherein the device comprises a video decoder. 
     Example 14 
     The device of any combination of examples 8-13, wherein the device comprises a video encoder. 
     Example 15 
     A computer-readable storage medium having stored thereon instructions that, when executed, cause one or more processors to perform the method of any combination of examples 1-7. 
     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 digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable gate arrays (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.