OA18378A - Contexts for large coding tree units. - Google Patents

Abstract

In an example, a method of coding video data may include selecting, as part of an entropy coding process, a context for a syntax element associated with a current coding unit (CU) based on information relating to CU depth of one or more neighboring CUs relative to the current CU. The syntax element may represent whether the current CU is split into sub-CUs. The method may include entropy coding the syntax element based on the selected context.

Description

[0001] This application daims the benefit of U.S. Provisional Patent Application No. 62/108,465 filed on January 27,2015, which is hereby incorporated by reference herein 5 in its entirety.

TECHNICAL FIELD [0002] This disclosure relates to video coding.

BACKGROUND [0003] Digital video capabilities can be incorporated into a wide range of devices, including digital télévisions, digital direct broadeast Systems, wireless broadeast Systems, personal digital assistants (PDAs), laptop or desktop computers, tablet computers, e-book readers, digital caméras, digital recording devices, digital media players, video gaming devices, video game consoles, cellular or satellite radio téléphonés, so-called “smart phones,” video teleconferencing devices, video streaming devices, and the like. Digital video devices implement video compression 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, décodé, and/or store digital video information more efficiently by implementing such video compression techniques.

[0004] Video compression techniques include spatial (intra-picture) prédiction and/or temporal (inter-picture) prédiction to reduce or remove redundancy inhérent in video sequences. For block-based video coding, a video slice (i.e., a video frame or a portion of a video frame) may be partitioned into video blocks. Video blocks in an intra-coded (I) slice of a picture are encoded using spatial prédiction 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 prédiction with respect to reference samples in neighboring blocks in the same picture or temporal prédiction with respect to reference samples in other reference pictures. Pictures may be referred to as fiâmes, and reference pictures may be referred to as reference frames.

[0005] Spatial or temporal prédiction results in a prédictive block for a block to be coded. Residual data represents pixel différences between the original block to be

coded and the prédictive block. An inter-coded block is encoded according to a motion vector that points to a block of reference samples forming the prédictive block, and the residual data indicates the différence between the coded block and the prédictive block. An intra-coded block is encoded according to an intra-coding mode and the residual 5 data. For further compression, the residual data may be transformed from the pixel domain to a transform domain, resulting in residual coefficients, which then may be quantized. The quantized coefficients, initially arranged in a two-dimensional array, may be scanned in order to produce a one-dimensional vector of coefficients, and entropy coding may be applied to achieve even more compression.

SUMMARY [0006] Techniques of this disclosure relate to video coding. For example, the techniques described in this disclosure may include one or more techniques for encoding or decoding a block of video data by performing an entropy coding process.

For example, one or more techniques described herein may include performing contextadaptive binary arithmetic coding (CABAC) context modeling of a syntax element associated with a coding unit (CU) based on at least one of information of a spatiallyneighboring CU relative to the CU or a temporally-neighboring CU relative to the CU. [0007] In one example, this disclosure describes a method of coding video data comprising selectîng, as part of an entropy coding process, a context for a syntax element associated with a current coding unit (CU) based on information relating to CU depth of one or more neighboring CUs relative to the current CU, wherein the syntax element represents whether the current CU Is split into sub-CUs; and entropy coding the syntax element based on the selected context.

[0008] In another example, this disclosure describes a device for coding video data comprising a memory configured to store the video data; and a video coder in communication with the memory, wherein the video coder is configured to: select, as part of an entropy coding process, a context for a syntax element associated with a current coding unit (CU) stored in the memory based on information relating to CU depth of one or more neighboring CUs relative to the cunent CU, wherein the syntax element represents whether the cunent CU is split into sub-CUs; and entropy code the syntax element based on the selected context.

[0009] In another example, this disclosure describes an apparatus for coding video data comprising means for selecting, as part of an entropy coding process, a context for a syntax element associated with a current coding unit (CU) based on information relating to CU depth of one or more neighboring CUs relative to the current CU, wherein the syntax element represents whether the current CU is split into sub-CUs; and means for entropy coding the syntax element based on the selected context.

[0010] In another example, this disclosure describes a non-transitory computer-readable storage medium having instructions stored thereon that, when executed, cause one or more processors to select, as part of an entropy coding process, a context for a syntax 10 element associated with a current coding unit (CU) based on information relating to CU depth of one or more neighboring CUs relative to the current CU, wherein the syntax element represents whether the current CU is split into sub-CUs; and entropy code the syntax element based on the selected context.

[0011] The details of one or more examples of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS [0012] FIG. 1 is a block diagram illustrating an example video coding system that may utilize the techniques described in this disclosure.

[0013] FIG. 2 is a block diagram illustrating an example video encoder that may implement the techniques described in this disclosure.

[0014] FIG. 3 is a block diagram illustrating an example video décoder that may implement the techniques described in this disclosure.

[0015] FIG. 4 is a flowchart illustrating an example process for coding video data consistent with techniques of this disclosure.

[0016] FIG. 5 is a flowchart illustrating an example process for decoding video data consistent with techniques ofthis disclosure.

[0017] FIG. 6 is a flowchart illustrating an example process for encoding video data consistent with techniques ofthis disclosure.

DETAILED DESCRIPTION [0018] The techniques ofthis dîsclosure generally relate to video coding and compression. More particularly, this dîsclosure relates to the signaling of syntax éléments in block based video coding when, for example, the coding of large coding tree units (CTUs) is supported. For example, this dîsclosure describes techniques for coding syntax éléments related to CTUs. As another example, this dîsclosure describes techniques for entropy coding syntax éléments. As another example, this dîsclosure describes techniques relating to binarization, context modeling, and arithmetic coding of syntax éléments. As yet another example, this dîsclosure describes techniques relating to binarization, context modeling, and arithmetic coding of syntax éléments using an entropy coding process, such as context-adaptive binary arithmetic coding (CABAC). The techniques of this dîsclosure may be applied to an existing video codée or video coding standard (e.g., ITU-T H.265, HEVC), or may be applied in a future video codée or video coding standard.

[0019] Context-adaptive binary arithmetic coding (CABAC) is a method of entropy coding first introduced in H.264/AVC and now used in the newest HEVC standard. CABAC generally involves three main functions: binarization, context modeling, and arithmetic coding. Binarization maps syntax éléments to binary symbols (bins), which may be called bin strings. Context modeling estimâtes the probability ofthe bins. As part of the context modeling process, a video coder selects or a context model for each bin. The context model represents the probability. Different context models may be selected for different bins. Arithmetic coding compresses the bins to bits based on the estimated probability. Otherwise stated, arithmetic coding compresses the bins to bits based on the selected context model corresponding to each bin.

[0020] The techniques of this dîsclosure may improve the bitstream efficiency of coding syntax éléments. For example, the techniques of this dîsclosure may improve the bitstream efficiency of coding syntax éléments related to CTUs and coding units (CUs). As another example, the techniques of this dîsclosure may particularly improve CABAC coding efficiency. As another example, the techniques of this dîsclosure may particularly improve CABAC coding efficiency of syntax éléments related to CTUs and CUs when, for example, large CTU sizes are enabled.

[0021] FIG. 1 is a block diagram illustrating an example video coding System 10 that may utilize the techniques ofthis dîsclosure. As used herein, the term “video coder” refers generically to both video encoders and video decoders. In this dîsclosure, the

term “video coding” may refer generically to video encoding or video decoding. Similarly, the term “coding” may refer generically to encoding or decoding. For example, reference to entropy coding may refer to entropy encoding or entropy decoding. Video encoder 20 and video décoder 30 of video coding System 10 represent 5 examples of devices that may be configured to perform one or more techniques of this disclosure. For example, video encoder 20 and video décoder 30 may be configured to perform techniques for coding data related to CTUs and CUs in accordance with various examples described in this disclosure. As one example, video encoder 20 and video décoder 30 may be configured to limit CTU size and/or perform CABAC context modeling for syntax éléments related to CTUs and/or CUs.

[0022] Video encoder 20 and/or video décoder 30 may be configured to operate according to the techniques described in this disclosure. Video décoder 30 may be configured to perform a process generally reciprocal to that of video encoder 20 described herein. Similarly, video encoder 20 may be configured to perform a process 15 generally reciprocal to that of video décoder 30 described herein.

[0023] The techniques described herein may be performed by video encoder 20 and/or video décoder 30, which are respectively illustrated in FIGS. 1-3. It should be understood that ail ofthe techniques described herein may be used individually or in any combination. For example, video encoder 20 and/or one or more components thereof and video décoder 30 and/or one or more components thereof may perform the techniques described in this disclosure in any combination. As another example, video encoder 20 and/or video décoder 30 may be configured to select, as part of an entropy coding process, a context for a syntax element associated with a current coding unit (CU) based on information relating to CU depth of one or more neighboring CUs relative to the current CU. The syntax element may represent whether the current CU is split into sub-CUs. In this example, video encoder 20 and/or video décoder 30 may be configured to entropy code the syntax element based on the selected context. As used herein, CU depth may refer to quad-tree depth of a CU. For example, the CU depth of a current CU may refer to the quad-tree depth of the current CU, and the CU depth of a neighboring block (e.g., neighboring CU) may refer to the quad-tree depth of the neighboring block. As another example, video encoder 20 and/or video décoder 30 respectively shown in FIGS. 1-3 may be configured to respectively perform one or more steps in any combination shown in FIGS. 4-6 and/or any step described with respect to FIGS. 4-6 that may not be depicted.

[0024] As shown in FIG. 1, video coding system 10 includes a source device 12 and a destination device 14. Source device 12 generates encoded video data. Accordingly, source device 12 may be referred to as a video encoding device or a video encoding apparatus. Destination device 14 may décodé the encoded video data generated by source device 12. Accordingly, destination device 14 may be referred to as a video decoding device or a video decoding apparatus. Source device 12 and destination device 14 may be examples of video coding devices or video coding apparatuses. [0025] Source device 12 and destination device 14 may comprise a wide range of devices, including desktop computers, mobile computing devices, notebook (e.g., laptop) computers, tablet computers, set-top boxes, téléphoné handsets such as so-called “smart” phones, télévisions, caméras, display devices, digital media players, video gaming consoles, in-car computers, or the like.

[0026] Destination device 14 may receive encoded video data from source device 12 via a channel 16. Channel 16 may comprise any type of medium and/or one or more media 15 or devices capable of moving the encoded video data from source device 12 to destination device 14. In one example, channel 16 may comprise one or more communication media that enable source device 12 to transmit encoded video data directly to destination device 14 in real-time. In this example, source device 12 may modulate the encoded video data according to a communication standard, such as a 20 wireless communication protocol, and may transmit the modulated video data to destination device 14. The one or more communication media may include wireless and/or wired communication media, such as a radio frequency (RF) spectrum or one or more physical transmission lines. The one or more communication media may form part of a packet-based network, such as a local area network, a wide-area network, or a global network (e.g., the Internet). The one or more communication media may include routers, switches, base stations, or other equipment that facilitate communication from source device 12 to destination device 14.

[0027] In another example, channel 16 may include a storage medium that stores encoded video data generated by source device 12. In this example, destination device 30 14 may access the storage medium, e.g., via disk access or card access. The storage medium may include a variety of locally-accessed data storage media such as Blu-ray dises, DVDs, CD-ROMs, flash memory, or other suitable digital storage media for storing encoded video data.

[0028] In a further example, channel 16 may include a file server or another intermediate storage device that stores encoded video data generated by source device

12. In this example, destination device 14 may access encoded video data stored at the file server or other intermediate storage device via streaming or download. The file server may be a type of server capable of storing encoded video data and transmitting the encoded video data to destination device 14. Example file servers include web servers (e.g., for a website), file transfer protocol (FTP) servers, network attached storage (NAS) devices, and local disk drives.

[0029] Destination device 14 may access the encoded video data through a standard data connection, such as an Internet connection. Example types of data connections may include wireless channels (e.g., Wi-Fi connections), wired connections (e.g., DSL, cable modem, etc.), or combinations of both that are suitable for accessing encoded video data stored on a file server. The transmission of encoded video data from the file server may be a streaming transmission, a download transmission, or a combination of 15 both.

[0030] The techniques of this disclosure are not limited to wireless applications or settings. 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 télévision broadcasts, cable télévision transmissions, satellite télévision transmissions, streaming video transmissions, e.g., via the Internet, encoding of video data for storage on a data storage medium, decoding of encoded video data stored on a data storage medium, or other applications. In some examples, video coding System 10 may be configured to support one-way or two-way video transmission to support applications such as video streaming, video playback, video broadcasting, and/or video telephony.

[0031] Video coding System 10 illustrated in FIG. 1 is merely an example and the techniques of this disclosure may apply to video coding settings (e.g., video encoding or video decoding) that do not necessarily include any data communication between the encoding and decoding devices. In other examples, data is retrieved from a local memory, streamed over a network, or the like. A video encoding device may encode and store data to memory, and/or a video decoding device may retrieve and décodé data from memory. In many examples, the encoding and decoding is performed by devices that do not communicate with one another, but simply encode data to memory and/or retrieve and décodé data from memory.

[0032] In the example of FIG. 1, source device 12 includes a video source 18, a video encoder 20, and an output interface 22. In some examples, output interface 22 may include a modulator/demodulator (modem) and/or a transmitter. Video source 18 may include a video capture device, e.g., a video caméra, a video archive containing previously-captured video data, a video feed interface to receive video data from a video content provider, and/or a computer graphies system for generating video data, or a combination of such sources of video data.

[0033] Video encoder 20 may encode video data from video source 18. In some examples, source device 12 directly transmits the encoded video data to destination device 14 via output interface 22. In other examples, the encoded video data may also be stored onto a storage medium or a file server for later access by destination device 14 for decoding and/or playback.

[0034] In the example of FIG. 1, destination device 14 includes an input interface 28, a video décoder 30, and a display device 32. In some examples, input interface 28 includes a receiver and/or a modem. Input interface 28 may receive encoded video data over channel 16. The encoded video data communicated over channel 16, or provided by storage device 26, may include a variety of syntax éléments generated by video encoder 20 for use by a video décoder, such as video décoder 30, in decoding the video data. Such syntax éléments may be included with the encoded video data transmitted on a communication medium, stored on a storage medium, or stored a file server. [0035] Display device 32 may be integrated with or may be extemal to destination device 14. In general, display device 32 displays decoded video data. Display device 32 may comprise 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.

[0036] Video encoder 20 and video décoder 30 each may be împlemented as any of a variety of suitable circuitry, such as one or more microprocessors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gâte arrays (FPGAs), discrète logic, hardware, or any combinations thereof. Ifthe techniques are împlemented partially in software, a device may store instructions for the software in a suitable, non-transitory computer-readable storage medium and may execute the instructions in hardware using one or more processors to perform the techniques of this disclosure. Any of the foregoing (including hardware, software, a combination of hardware and software, etc.) may be considered to be one or more

processors. Each of video encoder 20 and video décoder 30 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.

[0037] This disclosure may generally refer to video encoder 20 “signaling” or “transmitting” certain information to another device, such as video décoder 30. The term “signaling” or “transmitting” may generally refer to the communication of syntax éléments and/or other data used to décodé the compressed video data. Such communication may occur in real- or near-real-time. Altemately, such communication may occur over a span of time, such as might occur when storing syntax éléments to a computer-readable storage medium in an encoded bitstream at the time of encoding, which then may be retrieved by a decoding device at any time after being stored to this medium. Thus, while video décoder 30 may be referred to as “receiving” certain information, the receiving of information does not necessarily occur in real- or nearreal-time and may be retrieved from a medium at some time after storage.

[0038] In some examples, video encoder 20 and video décoder 30 operate according to a video compression standard, such as HEVC standard mentioned above, and described in HEVC Draft 10. In addition to the base HEVC standard, there are ongoing efforts to produce scalable video coding, multiview video coding, and 3D coding extensions for HEVC.

[0039] In HEVC and other video coding standards, a video sequence typically includes a sériés of pictures. Pictures may also be referred to as “frames.” A picture may include three sample arrays, denoted Sl, Scb and Scr. Sl is a two-dimensional array (i.e., a block) of luma samples. Scb is a two-dimensional array of Cb chrominance samples. Scr is a two-dimensional array of Cr chrominance samples. Chrominance samples may also be referred to herein as “chroma” samples. In other instances, a picture may be monochrome and may only include an array of luma samples. [0040] To generate an encoded représentation of a picture, video encoder 20 may generate a set of CTUs. Each of the CTUs may be a coding tree block of luma samples, two corresponding coding tree blocks of chroma samples, and syntax structures used to code the samples ofthe coding tree blocks. A coding tree block may be an NxN block of samples. A CTU may also be referred to as a “tree block” or a “largest coding unit” (LCU). The CTUs of HEVC may be broadly analogous to the macroblocks of other standards, such as H.264/AVC. However, a CTU is not necessarily limited to a particular size and may include one or more CUs. In monochrome pictures or pictures

having three separate color planes, a CU may comprise a single coding block and syntax structures used to code the samples of the coding block. A slice may include an integer number of CTUs ordered consecutively in the rester scan.

[0041] In some examples, a CU may include a coding node and one or more prédiction units (PUs) and/or transform units (TUs) associated with the coding node. The size of the CU may correspond to a size of the coding node and may be square in shape. The size of the CU may range from, for example, 8x8 pixels up to the size of the tree block with a maximum of64x64 pixels or greater. Each CU may contain one or more PUs and one or more TUs. Syntax data associated with a CU may describe, for example, partitioning of the CU into one or more PUs. Partitioning modes may differ between whether the CU is skip or direct mode encoded, intra-prediction mode encoded, or interprediction mode encoded. PUs may be partitioned to be square or non-square in shape. Syntax data associated with a CU may also describe, for example, partitioning of the CU into one or more TUs according to a quadtree. A TU can be square or non-square in shape.

[0042] In general, a PU may include data related to the prédiction process. For example, when a PU is intra-mode encoded, the PU may include data describing an intra-prediction mode for the PU. As another example, when a PU is inter-mode encoded, the PU may include data defining a motion vector for the PU. The data defining the motion vector for a PU may describe, for example, a horizontal component of the motion vector, a vertical component of the motion vector, a resolution for the motion vector (e.g., one-quarter pixel précision or one-eighth pixel précision), a reference picture to which the motion vector points, and/or a reference picture list (e.g., List 0, List 1, or List C) for the motion vector.

[0043] In general, a TU may be used for a transform and quantization processes. A given CU having one or more PUs may also include one or more transform units (TUs). Following prédiction, video encoder 20 may calculate residual values corresponding to the PU. The residual values may comprise pixel différence values that may be transformed into transform coefficients, quantized, and scanned using the TUs to produce serialized transform coefficients for entropy coding.

[0044] This disclosure may use the term “video unit,” “video block,” “coding block,” or “block” to refer to one or more sample blocks and syntax structures used to code samples of the one or more blocks of samples. Example types of video units or blocks may include coding tree units (CTUs), coding units (CUs), prédiction units (PUs),

transform units (TUs), macroblocks (MBs), macroblock partitions, and so on. In some contexts, discussion of PUs may be interchanged with discussion of macroblocks or macroblock partitions.

[0045] In some examples, a set of motion information may be available for each block of video data. The set of motion information may include motion information for forward and backward prédiction directions. Forward and backward prédiction directions may be two prédiction directions of a bi-directional prédiction mode. Forward and backward prédiction directions may be one of two prédiction directions of a uni-directional prédiction mode. The terms “forward” and “backward” do not necessarily hâve a geometry meaning, instead they correspond to, for example, reference picture list 0 (RefPicListO) and reference picture list 1 (RefPicListl) of a current picture. When only one reference picture list is available for a picture or slice, only RefPicListO may be available and the motion information of each block of the picture or slice may be forward.

[0046] For each prédiction direction (e.g., forward or backward), the motion information may contain a prédiction direction, a reference index, and a motion vector. In some examples, for simplicity, a motion vector itself may be referred to in a way that it is assumed that it has an associated reference index. A reference index is used to identify a reference picture in the current reference picture list (e.g., RefPicListO or

RefPicListl). A motion vector may hâve a horizontal and a vertical component. [0047] Video blocks described herein may hâve fixed or varying sizes, and may differ in size according to a specified coding standard. As an example, the size of a partïcular CU may be 2Nx2N. In such an example, video encoder 20 may be configured to perform intra-prediction for PUs having sizes of2Nx2N orNxN, and may be configured to perform inter-prediction for PUs having sizes of 2Nx2N, 2NxN, Nx2N, or NxN. In other examples, the available sizes of a PU may be the same or different.

[0048] In this disclosure, “NxN” and “N by N” may be used interchangeably to refer to the pixel dimensions of a video block in terms of vertical and horizontal dimensions, e.g., 16x16 pixels or 16 by 16 pixels. In general, a 16x16 block will hâve 16 pixels in a vertical direction (y = 16) and 16 pixels in a horizontal direction (x = 16). Likewise, an NxN block generally has N pixels in a vertical direction and N pixels in a horizontal direction, where N represents a positive integer value. The pixels in a block may be arranged in rows and columns. Moreover, blocks need not necessarily hâve the same number of pixels in the horizontal direction as in the vertical direction. For example,

blocks may comprise NxM pixels, where M is or is not equal to N and where M is a positive integer value.

[0049] Referring to FIG. 1, to generate a coded CTU, video encoder 20 may recursively perform quad-tree partitioning on the coding tree blocks of a CTU to divide the coding tree blocks into coding blocks, hence the name “coding tree units.” A coding block may be an NxN (or NxM) block of samples. A CU may be a coding block of luma samples and two conesponding coding blocks of chroma samples of a picture that has a luma sample array, a Cb sample array and a Cr sample array, and syntax structures used to code the samples of the coding blocks. Video encoder 20 may partition a coding block 10 ofa CU into one or more PUs. A PU may be a square or non-square block of samples on which the same prédiction technique is applied. A PU of a CU may be a prédiction block of luma samples, two conesponding prédiction blocks of chroma samples of a picture, and syntax structures used to predict the prédiction block samples. Video encoder 20 may generate prédictive luma, Cb, and Cr blocks for luma, Cb, and Cr prédiction blocks of each PU of the CU.

[0050] Video encoder 20 may use intra prédiction or inter prédiction to generate the prédictive blocks for a PU. If video encoder 20 uses intra prédiction to generate the prédictive blocks of a PU, video encoder 20 may generate the prédictive blocks of the PU based on decoded samples of the picture associated with the PU.

[0051] If video encoder 20 uses inter prédiction to generate the prédictive blocks of a

PU, video encoder 20 may generate the prédictive blocks of the PU based on decoded samples of one or more pictures other than the picture associated with the PU. Video encoder 20 may use uni-prediction or bi-prediction to generate the prédictive blocks of a PU. When video encoder 20 uses uni-prediction to generate the prédictive blocks for a

PU, the PU may hâve a single motion vector (MV). When video encoder 20 uses biprediction to generate the prédictive blocks for a PU, the PU may hâve two MVs. [0052] After video encoder 20 generates prédictive blocks (e.g., prédictive luma, Cb and Cr blocks) for one or more PUs of a CU, video encoder 20 may generate residual blocks for the CU. Each sample in a residual block of the CU may indicate a différence between a sample in a prédictive block of a PU of the CU and a corresponding sample in a coding block of the CU. For example, video encoder 20 may generate a luma residual block for the CU. Each sample in the CU’s luma residual block indicates a différence between a luma sample in one of the CU’s prédictive luma blocks and a corresponding sample in the CU’s original luma coding block. In addition, video

encoder 20 may generate a Cb residual block for the CU. Each sample in the CU’s Cb residual block may indicate a différence between a Cb sample in one of the CU’s prédictive Cb blocks and a corresponding sample in the CU’s original Cb coding block. Video encoder 20 may also generate a Cr residual block for the CU. Each sample in the

CU’s Cr residual block may indicate a différence between a Cr sample in one of the CU’s prédictive Cr blocks and a corresponding sample in the CU’s original Cr coding block.

[0053] Video encoder 20 may use quad-tree partitioning to décomposé the residual blocks (e.g., luma, Cb and Cr residual blocks) of a CU into one or more transform blocks (e.g., luma, Cb and Cr transform blocks). As used herein, CU depth may refer to quad-tree depth of a CU. For example, the CU depth of a current CU may refer to the quad-tree depth of the current CU, and the CU depth of a neighboring block (e.g., neighboring CU) may refer to the quad-tree depth of the neighboring block. A transform block may be a block of samples on which the same transform is applied. A transform unit (TU) of a CU may be a transform block of luma samples, two corresponding transform blocks of chroma samples, and syntax structures used to transform the transform block samples. Thus, each TU of a CU may be associated with a luma transform block, a Cb transform block, and a Cr transform block. The luma transform block associated with the TU may be a sub-block of the CU’s luma residual block. The Cb transform block may be a sub-block ofthe CU’s Cb residual block. The Cr transform block may be a sub-block of the CU’s Cr residual block.

[0054] Video encoder 20 may apply one or more transforms to a transform block to generate a coefficient block for a TU. A coefficient block may be a two-dimensional array of transform coefficients. A transform coefficient may be a scalar quantity. For example, video encoder 20 may apply one or more transforms to a luma transform block of a TU to generate a luma coefficient block for the TU. Video encoder 20 may apply one or more transforms to a Cb transform block of a TU to generate a Cb coefficient block for the TU. Video encoder 20 may apply one or more transforms to a Cr transform block of a TU to generate a Cr coefficient block for the TU.

[0055] In some examples, following intra-predictive or inter-predictive coding using

PUs of a CU, video encoder 20 may calculate residual data for the TUs of the CU. The PUs may comprise pixel data in the spatial domain (also referred to as the pixel domain) and the TUs may comprise coefficients in the transform domain following application of a transform, e.g., a discrète cosine transform (DCT), an integer transform, a wavelet

transform, or a conceptually similar transform to residual video data. The residual data may correspond to pixel différences between pixels of the unencoded picture and prédiction values corresponding to the PUs. Video encoder 20 may form the TUs including the residual data for the CU, and then transform the TUs to produce transform 5 coefficients for the CU.

[0056] After generating a coefficient block (e.g., a luma coefficient block, a Cb coefficient block or a Cr coefficient block), video encoder 20 may quantize the coefficients in the block. 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. After video encoder 20 quantizes a coefficient block, video encoder 20 may be configured to entropy encode syntax éléments indicating the quantized transform coefficients. For example, video encoder 20 may perform Context-Adaptive Binary Arithmetic Coding (CABAC) on the syntax éléments indicating the quantized transform coefficients. Video encoder 20 may output the entropy-encoded syntax éléments in a bitstream.

[0057] Video encoder 20 may output a bitstream that includes the entropy-encoded syntax éléments. The bitstream may include a sequence of bits that forms a représentation of coded pictures and associated data. The bitstream may comprise a sequence of network abstraction layer (NAL) units. Each of the NAL units includes a 20 NAL unit header and encapsulâtes a raw byte sequence payload (RBSP). The NAL unit header may include a syntax element that indicates a NAL unit type code. The NAL unit type code specifïed by the NAL unit header of a NAL unit indicates the type of the NAL unit. A RBSP may be a syntax structure containing an integer number of bytes that is encapsulated within a NAL unit. In some instances, an RBSP includes zéro bits.

[0058] Different types of NAL units may encapsulate different types of RBSPs. For example, a first type of NAL unit may encapsulate an RBSP for a picture parameter set (PPS), a second type of NAL unit may encapsulate an RBSP for a coded slice, a third type of NAL unit may encapsulate an RBSP for SEI, and so on. NAL units that encapsulate RBSPs for video coding data (as opposed to RBSPs for parameter sets and

SEI messages) may be referred to as video coding layer (VCL) NAL units.

[0059] Video décoder 30 may receive a bitstream generated by video encoder 20. In addition, video décoder 30 may parse the bitstream to décodé syntax éléments from the bitstream. Video décoder 30 may reconstruct the pictures of the video data based at least in part on the syntax éléments decoded from the bitstream. The process to

reconstruct the video data may be generally reciprocal to the process performed by video encoder 20. For instance, video décoder 30 may use MVs of PUs to détermine prédictive blocks for the inter-predicted PUs of a current CU. Likewise, video décoder 30 may generate intra-predicted blocks for PU’s of a current CU. In addition, video 5 décoder 30 may inverse quantize transform coefficient blocks associated with TUs of the current CU. Video décoder 30 may perform inverse transforms on the transform coefficient blocks to reconstruct transform blocks associated with the TUs of the current CU. Video décoder 30 may reconstruct the coding blocks of the current CU by adding the samples of the prédictive blocks for PUs of the current CU to corresponding residual 10 values obtained from inverse quantization and inverse transformation of the transform blocks of the TUs of the current CU. By reconstructing the coding blocks for each CU of a picture, video décoder 30 may reconstruct the picture.

[0060] In some examples, video encoder 20 may utilize a predefmed scan order (e.g., horizontal, vertical, or any other scan order) to scan the quantized transform coefficients 15 to produce a serialized vector that can be entropy encoded. In other examples, video encoder 20 may perform an adaptive scan. After scanning the quantized transform coefficients to form a one-dimensional vector, video encoder 20 may entropy encode the one-dimensional vector, e.g., according to context adaptive variable length coding (CAVLC), context adaptive binary arithmetic coding (CABAC), syntax-based context20 adaptive binary arithmetic coding (SBAC), Probability Interval Partitioning Entropy (PIPE) coding or another entropy encoding methodology. Video encoder 20 may also entropy encode syntax éléments associated with the encoded video data for use by video décoder 30 in decoding the video data.

[0061] To perform CABAC, video encoder 20 may assign a context within a context 25 model to a symbol to be transmitted. The context may relate to, for example, whether neighboring values ofthe symbol are non-zero or not. To perform CAVLC, video encoder 20 may select a variable length code for a symbol to be transmitted. Codewords in VLC may be constructed such that relatively shorter codes correspond to more probable symbols, while longer codes correspond to less probable symbols. In 30 this way, the use of VLC may achieve a bit savings over, for example, using equallength codewords for each symbol to be transmitted. The probability détermination may be based on a context assigned to the symbol.

[0062] Video encoder 20 and/or video décoder 30 may be configured to operate according to the techniques described in this disclosure. In general, video encoder 20

and/or video décoder 30 may be configured to perform one or more techniques described herein in any combination. In some examples, video encoder 20 and video décoder 30 may be configured to perform the techniques of this disclosure related to coding information associated with CTUs and CUs. For example, video encoder 20 and video décoder 30 may be configured to perform context-adaptive binary arithmetic coding (CABAC) context modeling of a syntax element associated with a coding unit (CU) based on at least one of information of a spatially-neighboring CU relative to the CU or a temporally-neighboring CU relative to the CU, and CABAC code the syntax element based on the modeled context. In another example, video encoder 20 and video 10 décoder 30 may be configured to: in response to determinîng that a CTU size is greater than 64x64, détermine a group of constraints comprising: a maximum CU depth and a minimum CU depth of the CTU, and code the CTU based on at least some of the group of constraints.

[0063] It is understood that techniques of this disclosure may utilize HEVC terminology 15 or other video standard terminology for ease of explanation. However, it is understood that the techniques of this disclosure are not limited to HEVC or other video standards. The techniques of this disclosure may be împlemented in successor standards to HEVC and its extensions as well as other video standards, whether past, présent, or future. [0064] FIG. 2 is a block diagram illustratîng an example video encoder 20 that may implement the techniques of this disclosure. FIG. 2 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 20 in the context of HEVC coding. However, the techniques of this disclosure may be applicable to other coding standards or methods.

[0065] Video encoder 20 represents an example of a device that may be configured to perform techniques related to coding CTUs and CUs, including CABAC coding of CTUs and CUs in accordance with various examples described in this disclosure. [0066] For example, video encoder 20 may be configured to perform context-adaptive binary arithmetic coding (CABAC) context modeling of a syntax element associated with a coding unit (CU) based on at least one of information of a spatially-neighboring CU relative to the CU or a temporally-neighboring CU relative to the CU, and CABAC encode the syntax element based on the modeled context.

[0067] In another example, video encoder 20 may be configured to: in response to determinîng that a CTU size is greater than 64x64, détermine a group of constraints

comprising: a maximum CU depth and a minimum CU depth of the CTU, and encode the CTU based on at least some of the group of constraints.

[0068] In the example of FIG. 2, video encoder 20 includes a prédiction processing unit 100, video data memory 101, a residual génération unit 102, a transform processing unit 5 104, a quantization unit 106, an inverse quantization unit 108, an inverse transform processing unit 110, a reconstruction unit 112, a filter unit 114, a decoded picture buffer 116, and an entropy encoding unit 118. Prédiction processing unit 100 includes an inter-prediction processing unit 120 and an intra-prediction processing unit 126. Interprediction processing unit 120 includes a motion estimation unit and a motion compensation unit (not shown). In other examples, video encoder 20 may include more, fewer, or different functional components.

[0069] Video data memory 101 may store video data to be encoded by the components of video encoder 20. The video data stored in video data memory 101 may be obtained, for example, from video source 18. Decoded picture buffer 116 may be a reference picture memory that stores reference video data for use in encoding video data by video encoder 20, e.g., in intra- or inter-coding modes. Video data memory 101 and decoded picture buffer 116 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), résistive RAM (RRAM), or other types of memory devices. Video data memory 101 and decoded picture buffer 116 may be provided by the same memory device or separate memory devices. In various examples, video data memory 101 may be on-chip with other components of video encoder 20, or off-chip relative to those components.

[0070] Video encoder 20 may receive video data. Video encoder 20 may encode each 25 CTU in a slice of a picture of the video data. Each of the CTUs may be associated with equally-sized luma coding tree blocks (CTBs) and corresponding CTBs ofthe picture.

As part of encoding a CTU, prédiction processing unit 100 may perform quad-tree partitioning to divide the CTBs of the CTU into progressively-smaller blocks. The smaller block may be coding blocks of CUs. For example, prédiction processing unit 30 100 may partition a CTB associated with a CTU into four equally-sized sub-blocks, partition one or more of the sub-blocks into four equally-sized sub-sub-blocks, and so on.

[0071] Video encoder 20 may encode CUs of a CTU to generate encoded représentations of the CUs (i.e., coded CUs). As part of encoding a CU, prédiction

processing unit 100 may partition the coding blocks associated with the CU among one or more PUs of the CU. Thus, each PU may be associated with a luma prédiction block and corresponding chroma prédiction blocks. Video encoder 20 and video décoder 30 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 prédiction block of the PU. Assuming that the size of a particular CU is 2Nx2N, video encoder 20 and video décoder 30 may support PU sizes of 2Nx2N or NxN for intra prédiction, and symmetric PU sizes of 2Nx2N, 2NxN, Nx2N, NxN, or similar for inter prédiction. Video encoder 20 and video décoder 30 may also support asymmetric partitioning for PU sizes of 2NxnU, 2NxnD, nLx2N, and nRx2N for inter prédiction.

[0072] Inter-prediction processing unit 120 may generate prédictive data for a PU by performing inter prédiction on each PU of a CU. The prédictive data for the PU may include prédictive blocks of the PU and motion information for the PU. Inter-prediction unit 121 may perform different operations for a PU of a CU depending on whether the PU is in an I slice, a P slice, or a B slice. In an I slice, ail PUs are intra predicted. Hence, if the PU is in an I slice, inter-prediction unit 121 does not perform inter prédiction on the PU. Thus, for blocks encoded in I-mode, the predicted block is formed using spatial prédiction from previously-encoded neighboring blocks within the same frame.

[0073] If a PU is in a P slice, the motion estimation unit of inter-prediction processing unit 120 may search the reference pictures in a list of reference pictures (e.g., “RefPicListO”) for a reference région for the PU. The reference région for the PU may be a région, within a reference picture, that contains sample blocks that most closely corresponds to the sample blocks of the PU. The motion estimation unit of interprediction processing unit 120 may generate a reference index that indicates a position in RefPicListO of the reference picture containing the reference région for the PU. In addition, the motion estimation unit may generate an MV that indicates a spatial displacement between a coding block of the PU and a reference location associated with the reference région. For instance, the MV may be a two-dimensional vector that provides an offset from the coordinates in the current decoded picture to coordinates in a reference picture. The motion estimation unit may output the reference index and the MV as the motion information of the PU. The motion compensation unit of interprediction processing unit 120 may generate the prédictive blocks ofthe PU based on

actual or interpolated samples at the reference location indicated by the motion vector of the PU.

[0074] If a PU is in a B slice, the motion estimation unit may perform uni-prediction or bi-prediction for the PU. To perform uni-prediction for the PU, the motion estimation 5 unit may search the reference pictures of RefPicListO or a second reference picture list (“RefPicListl”) for a reference région for the PU. The motion estimation unit may output, as the motion information of the PU, a reference index that indicates a position in RefPicListO or RefPicListl ofthe reference picture that contains the reference région, an MV that indicates a spatial displacement between a prédiction block ofthe PU and a 10 reference location associated with the reference région, and one or more prédiction direction indîcators that indicate whether the reference picture is in RefPicListO or RefPicListl. The motion compensation unit of inter-prediction processîng unit 120 may generate the prédictive blocks of the PU based at least in part on actual or interpolated samples at the reference région indicated by the motion vector of the PU.

[0075] To perform bi-directional inter prédiction for a PU, the motion estimation unit may search the reference pictures in RefPicListO for a reference région for the PU and may also search the reference pictures in RefPicListl for another reference région for the PU. The motion estimation unit may generate reference picture indexes that indicate positions in RefPicListO and RefPicListl of the reference pictures that contain the reference régions. In addition, the motion estimation unit may generate MVs that indicate spatial displacements between the reference location associated with the reference régions and a sample block ofthe PU. The motion information ofthe PU may include the reference indexes and the MVs of the PU. The motion compensation unit may generate the prédictive blocks of the PU based at least in part on actual or interpolated samples at the reference régions indicated by the motion vectors of the PU. [0076] Intra-prediction processîng unit 126 may generate prédictive data for a PU by performing intra prédiction on the PU. The prédictive data for the PU may include prédictive blocks for the PU and various syntax éléments. Intra-prediction processîng unit 126 may perform intra prédiction on PUs in I slices, P slices, and B slices.

[0077] To perform intra prédiction on a PU, intra-prediction processîng unit 126 may use multiple intra prédiction modes to generate multiple sets of prédictive data for the PU. Intra-prediction processîng unit 126 may use samples from sample blocks of neighboring PUs to generate a prédictive block for a PU. The neighboring PUs may be above, above and to the right, above and to the left, or to the left of the PU, assuming a

left-to-right, top-to-bottom encoding order for PUs, CUs, and CTUs. Intra-prediction processing unit 126 may use various numbers of intra prédiction modes, e.g., 33 directional intra prédiction modes. In some examples, the number of intra prédiction modes may dépend on the size of the région associated with the PU.

[0078] Prédiction processing unit 100 may select the prédictive data for PUs of a CU from among the prédictive data generated by inter-prediction processing unit 120 for the PUs or the prédictive data generated by intra-prediction processing unit 126 for the PUs. In some examples, prédiction processing unit 100 selects the prédictive data for the PUs of the CU based on rate/distortion metrics of the sets of prédictive data. The prédictive blocks of the selected prédictive data may be referred to herein as the selected prédictive blocks.

[0079] Residual génération unit 102 may generate, based on the luma, Cb and Cr coding block of a CU and the selected prédictive luma, Cb and Cr blocks of the PUs of the CU, a luma, Cb and Cr residual blocks ofthe CU. For instance, residual génération unit 102 15 may generate the residual blocks of the CU such that each sample in the residual blocks has a value equal to a différence between a sample in a coding block ofthe CU and a corresponding sample in a corresponding selected prédictive block of a PU of the CU. [0080] Transform processing unit 104 may perform quad-tree partitioning to partition the residual blocks associated with a CU into transform blocks associated with TUs of 20 the CU. Thus, in some examples, a TU may be associated with a luma transform block and two chroma transform blocks. The sizes and positions ofthe luma and chroma transform blocks of TUs of a CU may or may not be based on the sizes and positions of prédiction blocks of the PUs of the CU. A quad-tree structure known as a “residual quad-tree” (RQT) may include nodes associated with each of the régions. The TUs of a 25 CU may correspond to leaf nodes of the RQT.

[0081] Transform processing unit 104 may generate transform coefficient blocks for each TU of a CU by applying one or more transforms to the transform blocks of the TU. Transform processing unit 104 may apply various transforms to a transform block associated with a TU. For example, transform processing unit 104 may apply a discrète 30 cosine transform (DCT), a directional transform, or a conceptually similar transform to a transform block. In some examples, transform processing unit 104 does not apply transforms to a transform block. In such examples, the transform block may be treated as a transform coefficient block.

[0082] Quantization unit 106 may quantize the transform coefficients in a coefficient block. The quantization process may reduce the bit depth associated with some or ail of the transform coefficients. For example, an H-bit transform coefficient may be rounded down to an m-bit transform coefficient during quantization, where n is greater than m.

Quantization unit 106 may quantize a coefficient block associated with a TU of a CU based on a quantization parameter (QP) value associated with the CU. Video encoder 20 may adjust the degree of quantization applied to the coefficient blocks associated with a CU by adjusting the QP value associated with the CU. Quantization may introduce loss of information, thus quantized transform coefficients may hâve lower précision than the original ones.

[0083] Inverse quantization unit 108 and inverse transform processing unit 110 may apply inverse quantization and inverse transforms to a coefficient block, respectively, to reconstruct a residual block from the coefficient block. Reconstruction unit 112 may add the reconstructed residual block to corresponding samples from one or more prédictive blocks generated by prédiction processing unit 100 to produce a reconstructed transform block associated with a TU. By reconstructing transform blocks for each TU of a CU in this way, video encoder 20 may reconstruct the coding blocks of the CU. [0084] Filter unit 114 may perform one or more deblocking operations to reduce blocking artifacts in the coding blocks associated with a CU. Decoded picture buffer

116 may store the reconstructed coding blocks after filter unit 114 perforais the one or more deblocking operations on the reconstructed coding blocks. Inter-prediction processing unit 120 may use a reference picture that contains the reconstructed coding blocks to perform inter prédiction on PUs of other pictures. In addition, intra-prediction processing unit 126 may use reconstructed coding blocks in decoded picture buffer 116 to perform intra prédiction on other PUs in the same picture as the CU.

[0085] Entropy encoding unit 118 may receive data from other functional components of video encoder 20. For example, entropy encoding unit 118 may receive coefficient • blocks from quantization unit 106 and may receive syntax éléments from prédiction processing unit 100. Such syntax éléments may include syntax éléments related to the 30 size and partitioning of CTUs and CUs. Entropy encoding unit 118 may perform one or more entropy encoding operations on the data to generate entropy-encoded data. For example, entropy encoding unit 118 may perform a context-adaptive coding operation, such as a CABAC operation, 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. Video encoder 20 may output a bitstream that includes entropy-encoded data generated by entropy encoding unit 118. For instance, the bitstream may include data that represents a RQT for a CU.

[0086] Some techniques of this disclosure are related to CABAC coding of information related to, for example, CTUs and CUs. CABAC coding, which entropy encoding unit 118 performs, generally involves three main fonctions: binarization, context modeling, 10 and arithmetic coding. Binarization maps syntax éléments to binary symbols (bins), which may be called bin strings. Context modeling estimâtes the probability ofthe bins. As part of the context modeling process, a video coder selects or a context model for each bin. The context model represents the probability. Different context models may be selected for different bins. Arithmetic coding compresses the bins to bits based on 15 the estimated probability. Otherwise stated, arithmetic coding compresses the bins to bits based on the selected context model corresponding to each bin.

[0087] In some examples, binarization may refer to a variety of binarization processes, such as unary, truncated unary, kth-order exponential-Golomb (EGk), and fixed length

.. binarizations. During context modeling, entropy encoding unit 118 dérivés a probability 20 estimation (e.g., a context model) for each bin in a process referred to as context modeling. Context modeling process may be highly adaptive.

[0088] Entropy encoding unit 118 may use different context models for different bins. Entropy encoding unit 118 may update the probability ofthe context models based on the values of previously coded bins. Bins with similar distributions often share the same 25 context model. Entropy encoding unit 118 may select the context model for each bin based on the type of syntax element, bin position in syntax element (binldx), luma/chroma, neighboring information, and other syntax éléments.

[0089] During entropy coding, an entropy encoder or entropy décoder (e.g., entropy encoding unit 118 and entropy decoding unit 150) may place bits of a symbol to be 30 encoded or décodé into one or more bins. In some examples, the bins indicate whether a value of a symbol is equal to zéro. The entropy encoder or entropy décoder may use the values ofthe bins to adjust the entropy coding process. In other examples, an entropy encoder or entropy décoder may also use bins to indicate whether a value is greater than a spécifie value, e.g., greater than zéro, greater than one, etc.

[0090] Entropy encoding unit 118 may be configured to perform a process referred to as a “context switch.” For example, entropy encoding unit 118 may be configured to perform a context-switch after encoding each bin. In some examples, entropy encoding unit 118 stores probabîlity models (e.g., context models) as 7-bit entries (6 bits for the probabîlity state and 1 bit for the most probable symbol (MPS)) in context memory. In such examples, entropy encoding unit 118 addresses the probabîlity models (e.g., context models) using the context index computed by context sélection logic. [0091] Before encoding video data (e.g., a slice of video data), entropy encoding unit 118 initializes the probabîlity models (e.g., context models) based on some pre10 defined values. For example, given an input quantizatîon parameter denoted as “qp,” and a pre-defined value denoted as “initVal,” entropy encoding unit 118 may dérivé the 7-bit entry of the probabîlity model (denoted by the values “state” and “MPS”) as follows:

qp =Clip3(0, 51, qp);

slope = ( initVal »4)*5 - 45;

offset = ((initVal & 15)«3)-16;

initState= min( max( 1,((( slope * qp ) » 4 ) + offset ) ), 126 );

MPS = (initState >= 64 );

state = ( (mpState? (initState - 64) : (63 - initState)) «1) + mpState;

[0092] Entropy encoding unit 118 may be configured to perform arithmetic coding based on, for example, a recursive interval division process. As part of the arithmetic coding process, entropy encoding unit 118 may be configured to divide a range into two subintervals based on the probabîlity of the bin. In some examples, the range may initially range from 0 to 1. Entropy encoding unit 118 may be configured to select one of the two subintervals based on an offset that is converted to a binary fraction. The offset is derived from the encoded bin, and the selected subinterval is indicated by the value of the bin.

[0093] Entropy encoding unit 118 may be configured to update the range after every bin, such that the updated range equals the selected subinterval. Entropy encoding unit 118 may be configured to repeat the interval division process. The range and offset hâve limited bit précision, so entropy encoding unit 118 may be configured to perform renormalization whenever the range falls below a certain value to prevent underflow.

[0094] Entropy encoding unit 118 may perform renormalization after each bin is encoded. Similarly, entropy decoding unit 150 may perform renormalization after each bin is encoded. Entropy encoding unit 118 may perform arithmetic coding using an estimated probability (context-coded), or assuming equal probability of 0.5 (bypass coded). For bypass coded bins, entropy encoding unit 118 may perform the division of the range into subintervals by using an arithmetic shift, whereas a lookup table may be used for the context-coded bins.

[0095] During motion compensation and motion estimation, prédiction processing unit 100 détermines whether a CU should be divided into sub-blocks (e.g., prédiction units (PUs) or smaller). For example, prédiction processing unit 100 may perform a variety of encoding passes during which coding tree units (CTUs) are split in different ways, and then rate-distortion characteristics of each pass may be compared to other passes to détermine which partitioning scheme for a CTU results in the best rate-distortion metrics. The value of the split_cu_flag syntax element (a flag) indicates whether a CU is split into coding units with half horizontal and vertical sizes relative to a parent CU. For each slice type (e.g., I, P, and B slices), entropy encoding unit 118 may be configured to code the split_cu_flag syntax element using CABAC with three contexts based on the information of spatially neighboring slices.

[0096] As defined in the HEVC spécification, entropy encoding unit 118 may be configured to dérivé the context index offset (e.g., ctxlnc) based on the availability of the block to the left and the block above the current block (e.g., availableL which spécifiés the availability of the block located directly to the left of the current block, or availableA which spécifiés the availability of the block located directly above the current block),and split information (i.e., CtDepth[ xNbL/A ][ yNbL/A ] > cqtDepth) of left/above-neighboring blocks. Table 9-38, copied from the HEVC standard, describes the dérivation of the ctxlnc syntax element as follows.

Syntax element	condL	condA	ctxlnc
split_cu_flag[ x0][y0]	CtDepth[xNbL][yNbL ] > cqtDepth	CtDepth[ xNbA ] [ yNbA ] > cqtDepth	(condL && availableL ) + (condA && availableA )

Table 9-38 - Spécification of ctxlnc using left and above syntax éléments

[0097] Another syntax element, inter_pred_îdc, indicates whether listO, listl, or biprediction is used for the current prédiction unit. The syntax element has up to two bins, both of which are CABAC context coded. The binarized bin string is defined according to Table 9-36, copied from the HEVC standard, as follows where nPbW and nPbH represent the current luma prédiction block width and height, respectively:

Value of inter_pred_idc	Bin string	Bin string
(nPbW + nPbH) != 12	(nPbW + nPbH) == 12
0	00	0
1	01	1
2	1	-

Table 9-36: Dérivation of interjpred_ic [0098] For each inter-coded slide (e.g., P or B slice), entropy encoding unit 118 may be configured to perform the above context sélection dérivation of the binarized bin string based on determining whether ( nPbW + nPbH ) != 12 or whether ( nPbW + nPbH ) != 12. For example, if ( nPbW + nPbH ) is not equal to 12, entropy encoding unit 118 may be configured to code the first bin using four contexts (e.g., 0,1, 2, and 3) and code the second bin with one context. In some examples, the context sélection ofthe first bin is according to the CU depth ofthe current block. In HEVC,

CU depth is in the range of 0 to 3, inclusive. As another example, if (nPbW + nPbH ) is equal to 12, entropy encoding unit 118 may be configured to code the first bin and the second bin with the same context as the context used by the second bin in the above example.

[0099] While entropy encoding unit 118 and entropy decoding unit 150 may be configured to perform one or more aspects of entropy coding in accordance with HEVC, as described above for example, there are a number of issues with the context modeling in HEVC. Context modeling in HEVC is specially designed for a CTU no larger than 64x64. Accordingly, when larger CTU sizes (e.g., 128x128,256x256, or even larger) are used, directly reusing current context modeling processes, such as those described by HEVC, are less efficient or resuit in parsing issues.

[0100] As one example, when a CTU is larger than 64x64, directly reusing the current context modeling processes of split_cu_flag as defined by HEVC, is not efficient. As another example, when a CTU is larger than 64x64, the maximal CU depth is actually

increased so that more contexts are needed for inter_pred_idc. However, additional contexts do not necessarily bring additional coding efficiency. Accordingly, some techniques in this disclosure relate to limiting the number of contexts, whereas other techniques in this disclosure relate to increasing the number of contexts.

[0101] This disclosure describes a number of different techniques, which are described in greater detail below, and which may overcome the issues associated with entropy coding of CTUs, as discussed above. These techniques may be used alone or in any combination. The following techniques may be applicable for coding large CTUs, as weil as for coding 64x64 or smaller CTUs. The following techniques may be applied individually or in any combination.

[0102] In one example in accordance with the techniques ofthis disclosure, entropy encoding unit 118 may take into account the maximal and/or minimal CU depth information of the spatial and/or temporal neighbors of the current CU when performing the context modeling of split_cu_flag and/or a syntax element with the same semantics.

[0103] In this example, entropy encoding unit 118 may be configured to détermine the possibility (e.g., probability or likelihood) of whether a CU is further split into sub-CUs based on the maximal and/or minimal CU depths or CU sizes of the spatial and/or temporal neighbors of the current CU, meaning the CU depths corresponding to ail of the spatial and/or temporal neighbors ofthe current CU in some examples, and, one or more CU depths corresponding to one or more spatial and/or temporal neighbors of the current CU in other examples. Based on determining that the CU depth of the current CU is smaller than the minimal CU depth of one or more spatial and/or temporal neighbors of the current CU (e.g., ail such neighbors), entropy encoding unit 118 may be configured to détermine that it is very likely that the CU is to be split further. Based on determining that the CU depth of the current CU is larger than the maximal CU depth of one or more spatial and/or temporal neighbors of the current CU (e.g., ail such neighbors), entropy encoding unit 118 may be configured to détermine that it is very unlikely that the CU is to be split further.

[0104] In one example, entropy encoding unit 118 may be configured to use one dedicated context model to code the split_cu_flag syntax element upon determining that the current CU is very likely to be further split (e.g., when the CU depth of the current CU is smaller than the minimal CU depth of the spatial neighbors and/or temporal neighbors of the current CU). Altematively or additionally, entropy encoding unit 118 may be configured to set the initial probability ofthe bin value equal to T’ associated

with the context model to 1 or a value very close to 1. For example, with the initialization method introduced above, entropy encoding unit 118 may be configured to use a value of “0” (initVal=0) to initialize a context model to “very unlikely” (initState=l), and value “255” to initialize a context to “very likely” (initState=126).

[0105] In another example, entropy encoding unit 118 may be configured to use one dedicated context model to code the split_cu_flag syntax element upon determining that the current CU is very unlikely to be further split (e.g., when the CU depth of the current CU is larger than the maximum CU depth of the spatial neighbors and/or temporal neighbors of the current CU). Altematively or additionally, entropy encoding unit 118 may be configured to set the initial probabilîty of bin value equal to ‘1’ associated with the context equal to 0 or a value very close to 0.

[0106] In some examples in accordance with the techniques of this disclosure, entropy encoding unit 118 may be configured to détermine the context index offset based on uniform or non-uniform quantization of CU depth of the current CU for the inter_pred_idc syntax element rather than setting the context index offset equal to the CU depth. For example, entropy encoding unit 118 may be configured to set the context index offset to: min (CU depth, M), where Ai is a positive integer value and the min function retums the minimum value of two inputs. In some examples, entropy encoding unit 118 may set M equal to 3. Since each context index offset corresponds to a context, entropy encoding unit 118 may be configured to limit the number of contexts used for entropy encoding a CTU larger than 64x64 by limiting the context index offset as described above. For example, where M is 3, the number of contexts is limited to four. As another example, where M is 5, the number of contexts is limited to six.

[0107] The maximum CU size (the size of the CTU) and maximum CU split are usually signaled in a sequence parameter set (SPS) or picture parameter set (PPS). Video encoder 20 or video décoder 30 détermines the CU size for a spécifie bitstream accordingly. In some examples in accordance with the techniques of this disclosure in which a CTU larger than 64x64 is enabled, video encoder 20 may subject the maximum and minimum CU sizes to additional constraints such that the CU depth is restricted with a certain range. Various examples of the CTU and CU size restrictions are now described. In some examples, one or more of the restrictions/constraints described below may only apply to inter coded CUs.

[0108] In one example, video encoder 20 may be configured to set the maximum CU size to be less than or equal to a size that is smaller than the allowed maximum CTU

size. For example, the maximum CU size may be pre-defined, such as 64x64. By setting the maximum CU size less than or equal to a size that is smaller than the maximum CTU size, some modules of a video codée, such as adaptive loop filters, and sample adaptive offset processes, can be appiied at the CTU level.

[0109] In another example, video encoder 20 may be configured to signal the maximum

CU size together with CTU size and maximum CU depth in a PPS or any other high level syntax parameter set.

[0110] In another example, when the current CU has a size larger than the maximum CU size, video encoder 20 may be configured to not signal the split_cu_flag syntax element of the current CU. In such an example, video décoder 30 may be configured to infer split_cu_flag to be equal to 1.

[OUI] Various examples of constraints on maximum CU size in accordance with the techniques of this disclosure are now described. In one example, video encoder 20 and video décoder 30 may restrict the CU depth to be smaller than M, where M is equal to 4 15 in some examples. In another example, video encoder 20 and video décoder 30 may restrict the CU size to be no smaller than (1« (N-M)) where the largest CTU size is equal to (1«N), “«” is an arithmetic left shift, and M indicates the max CU depth (e.g., how many times a CTU can be further split). For example, M=4 means a CTU can be split 4 times. In one example, M may be set equal to 4. For example, a 64x64 20 CU having a CU depth of 4 may be first split into two 32x32 blocks, which may then be split into 16x16 blocks, which may then be split into 8x8 blocks, which may then be split into 4x4 blocks with the 4x4 blocks. This example demonstrates four exemplary CU depths (first 32x32, second at 16x16, third at 8x8, and fourth at 4x4). In some examples, the CU size constraint may be appiied only to inter-coded CUs. In some examples, the maximum CU depth or minimum CU size for intra- and inter-coded CUs may be different.

[0112] In various examples in accordance with the techniques of this disclosure, entropy encoding unit 118 may add two addîtional contexts for coding the split_cu_flag or a syntax element with the same semantics. The first addîtional context indicates that 30 a further split of the current CU has a high possibility (e.g., very likeiy). The second addîtional context indicates that a further split of the current CU has a low possibility (e.g., very unlikely). In these examples, entropy encoding unit 118 of video encoder 20 may be configured to define the maximal and minimal depths of the spatially neighboring (e.g., left, bottom-left, top and top-right) CUs of the current CU as

max_depth and min_depth, respectively. Entropy encoding unit 118 may be configured to select the CABAC contexts for coding split_cu_flag according to the following pseudocode:

if (cur_cu_depth < max(0, min_depth-l) ) {Select the context for further split with high possibility} else if (cur_cu_depth > min(g_max_cu_depth, max_depth+l)) {Select the context for further split with low possibility} else {Use the context sélection of split_cu_flag in HEVC} [0113] In the above pseudocode, cur_cu_depth indicates the CU depth ofthe current CU, and g_max_cu_depth dénotés the maximal CU depth allowed in the bitstream. In one example, when a spatially neighboring CU is not available, video encoder 20 may be configured to set the neighboring CU depth equal to 0 when calculating min_depth, 15 and may be configured to set the neighboring CU depth equal to g_max_cu_depth when calculating max_depth. In examples involving the two additional contexts, video encoder 20 may be configured to set the initial value (e.g., initVal described above) for the context of further split with high possibility to 255. In such examples, video encoder 20 may set the initial value (e.g., initVal described above) for the context of 20 further split with low possibility to 0.

[0114] For CTUs larger than 64x64, the CU depth may be larger than 3. For example, when a CTU has size 256x256, CU depth may be 0,1,2,3,4, or 5. In some examples, entropy encoding unit 118 may still use four contexts (e.g., 0,1,2, and 3) for the first bin of inter_pred_idc. Entropy encoding unit 118 may then select the context using 25 min(3, cur_cu_depth) where cur_cu_depth indicates the CU depth of the current CU.

[0115] FIG. 3 is a block diagram illustrating an example video décoder 30 that is configured to implement the techniques of this disclosure. FIG. 3 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 30 video décoder 30 in the context of HEVC coding. However, the techniques of this disclosure may be applicable to other coding standards or methods.

[0116] Video décoder 30 represents an example of a device that may be configured to perform techniques for efïïciently coding CTUs and CUs in accordance with various examples described in this disclosure. For example, video décoder 30 may be

configured to perform context-adaptive binary arithmetic coding (CABAC) context modeling of a syntax element associated with a coding unit (CU) based on at least one of information of a spatially-neighboring CU relative to the CU or a temporallyneighboring CU relative to the CU, and CABAC décodé the syntax element based on 5 the modeled context.

[0117] In another example, video décoder 30 may be configured to: in response to determining that a CTU size is greater than 64x64, détermine a group of constraints comprising: a maximum CU depth and a minimum CU depth of the CTU, and décodé the CTU based on at least some of the group of constraints.

[0118] In the example of FIG. 3, video décoder 30 includes an entropy decoding unit

150, video data memory 151, a prédiction processing unit 152, an inverse quantization unit 154, an inverse transform processing unit 156, a reconstruction unit 158, a filter unit 160, and a decoded picture buffer 162. Prédiction processing unit 152 includes a motion compensation unit 164 and an intra-prediction processing unit 166. In other examples, video décoder 30 may include more, fewer, or different functional components.

[0119] Video data memory 151 may store video data, such as an encoded video bitstream, to be decoded by the components of video décoder 30. The video data stored in video data memory 151 may be obtained, for example, from a computer-readable medium, e.g., from a local video source, such as a caméra, via wired or wireless network communication of video data, or by accessing physical data storage media. Video data memory 151 may form a coded picture buffer (CPB) that stores encoded video data from an encoded video bitstream. Decoded picture buffer 162 may be a reference picture memory that stores reference video data for use in decoding video data by video décoder 30, e.g., in intra- or inter-coding modes. Video data memory 151 and decoded picture buffer 162 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), résistive RAM (RRAM), or other types of memory devices. Video data memory 151 and decoded picture buffer 162 may be provided by the same memory device or separate memory devices. In various examples, video data memory 151 may be on-chip with other components of video décoder 30, or off-chip relative to those components.

[0120] A coded picture buffer (CPB) may receive and store encoded video data (e.g., NAL units) of a bitstream. Entropy decoding unit 150 may receive encoded video data

(e.g., NAL units) from the CPB and parse the NAL units to décodé syntax éléments. Entropy decoding unit 150 may entropy décodé entropy-encoded syntax éléments in the NAL units. Prédiction processing unit 152, inverse quantization unit 154, inverse transform processing unit 156, reconstruction unit 158, and filter unit 160 may generate decoded video data based on the syntax éléments extracted from the bitstream. Entropy decoding unit 150 may perform a process generally reciprocal to that of entropy encoding unit 118 as described above.

[0121] The NAL units of the bitstream may include coded slice NAL units. As part of decoding the bitstream, entropy decoding unit 150 may extract and entropy décodé syntax éléments from the coded slice NAL units. Each of the coded slices may include a slice header and slice data. The slice header may contain syntax éléments pertaining to a slice. The syntax éléments in the slice header may include a syntax element that identifies a PPS associated with a picture that contains the slice.

[0122] In addition to decoding syntax éléments from the bitstream, video décoder 30 may perform a reconstruction operation on a non-partitioned CU. To perform the reconstruction operation on a non-partitioned CU, video décoder 30 may perform a reconstruction operation on each TU of the CU. By performing the reconstruction operation for each TU of the CU, video décoder 30 may reconstruct residual blocks of the CU.

[0123] As part of performing a reconstruction operation on a TU of a CU, inverse quantization unit 154 may inverse quantize, i.e., de-quantize, coefficient blocks associated with the TU. Inverse quantization unit 154 may use a QP value associated with the CU ofthe TU to détermine a degree ofquantization and, likewise, a degree of inverse quantization for inverse quantization unit 154 to apply. That is, the compression ratio, i.e., the ratio of the number of bits used to represent original sequence and the compressed one, may be controlled by adjusting the value ofthe QP used when quantizing transform coefficients. The compression ratio may also dépend on the method of entropy coding employed.

[0124] After inverse quantization unit 154 inverse quantizes a coefficient block, inverse transform processing unit 156 may apply one or more inverse transforms to the coefficient block in order to generate a residual block associated with the TU. For example, inverse transform processing unit 156 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 coefficient block.

[0125] If a PU is encoded using intra prédiction, intra-prediction processîng unit 166 may perform intra prédiction to generate prédictive blocks for the PU. Intra-prediction processîng unit 166 may use an intra prédiction mode to generate the prédictive luma, Cb and Cr blocks for the PU based on the prédiction blocks of spatially-neighboring PUs. Intra-prediction processîng unit 166 may détermine the intra prédiction mode for the PU based on one or more syntax éléments decoded from the bitstream. [0126] Prédiction processîng unit 152 may construct a first reference picture list (RefPicListO) and a second reference picture list (RefPicListl) based on syntax éléments extracted from the bitstream. Furthermore, if a PU is encoded using inter prédiction, entropy decoding unit 150 may extract motion information for the PU. Motion compensation unit 164 may détermine, based on the motion information ofthe PU, one or more reference régions for the PU. Motion compensation unit 164 may generate, based on samples blocks at the one or more reference blocks for the PU, prédictive luma, Cb and Cr blocks for the PU.

[0127] Reconstruction unit 158 may use the luma, Cb and Cr transform blocks associated with TUs of a CU and the prédictive luma, Cb and Cr blocks of the PUs of the CU, i.e., either intra-prediction data or inter-prediction data, as applicable, to reconstruct the luma, Cb and Cr coding blocks of the CU. For example, reconstruction unit 158 may add samples ofthe luma, Cb and Cr transform blocks to corresponding samples of the prédictive luma, Cb and Cr blocks to reconstruct the luma, Cb and Cr coding blocks of the CU.

[0128] Video décoder 30 may be configured to perform a process generally reciprocal to that of video encoder 20 described herein. Similarly, video encoder 20 may be configured to perform a process generally reciprocal to that of video décoder 30 described herein. For example, disclosure that video décoder 30 may be configured to décodé an encoded syntax element in a bitstream likewise necessarily discloses that video encoder 20 may be configured to encode the syntax element into the bitstream.

[0129] As another example, entropy decoding unit 150 may be configured to perform a process generally reciprocal to that of entropy encoding unit 118 described herein. According to aspects of this disclosure, entropy decoding unit 150 may be configured to entropy décodé any code words generated by entropy encoding unit 118.

[0130] Filter unit 160 may perform a deblocking operation to reduce blocking artifacts associated with the luma, Cb and Cr coding blocks of the CU. Video décoder 30 may store the luma, Cb and Cr coding blocks of the CU in decoded picture buffer 162. Decoded picture buffer 162 may provide reference pictures for subséquent motion compensation, intra prédiction, and présentation on a display device, such as display device 32 of FIG. 1. For instance, video décoder 30 may perform, based on the luma, Cb, and Cr blocks in decoded picture buffer 162, intra prédiction or inter prédiction operations on PUs of other CUs.

[0131] FIG. 4 is a flowchart illustrating an example process for coding (e.g., encoding and/or decoding) video data consistent with techniques of this dîsclosure. The process of FIG. 4 is generally described as being performed by a video coder for purposes of illustration, although a variety of other processors may also carry out the process shown in FIG. 4. As used herein, a video coder may refer to a video encoder and/or a video décoder, such as video encoder 20 and/or a video décoder 30 described with respect to

FIGS. 1-3, for example. In some examples, video data memory 101, prédiction processing unit 100, and/or entropy encoding unit 118 may perform one or more processes shown in FIG. 4. In some examples, video data memory 151, entropy decoding unit 150, and/or prédiction processing unit 152 may perform one or more processes shown in FIG. 4.

[0132] In the example of FIG. 4, a video coder (e.g., video encoder 20 and/or video décoder 30) may be configured to select, as part of an entropy coding process, a context for a syntax element associated with a current coding unit (CU) based on information relating to CU depth of one or more neighboring CUs relative to the current CU (200). In some examples, the video coder may be configured to select the context for the syntax element based on information relating to CU depth of one or more spatially neighboring CUs relative to the current CU. In other examples, the video coder may be configured to select the context for the syntax element based on information relating to CU depth of one or more temporally neighboring CUs relative to the current CU. For example, one or more neighboring CUs relative to the current CU may be a spatial neighbor and/or one or more neighboring CUs relative to the current CU may be a temporal neighbor.

[0133] In some examples, the syntax element may represent whether the current CU is split into sub-CUs. In such examples, the syntax element may be the split_cu_flag

syntax element. In some examples, a CTU associated with the current CU may hâve a size larger than 64x64.

[0134] In some examples, one or more CU depths relating to the one or more neighboring CUs may be equal and/or not equal to a CU depth of the current CU. For 5 example, one or more CU depths relating to the one or more spatially-neighboring CUs may be equal to and/or not equal to a CU depth of the current CU. As another example, one or more CU depths relating to the one or more temporally-neighboring CUs may be equal to and/or not equal to a CU depth of the current CU.

[0135] The video coder may be configured to entropy code the syntax element based on 10 the selected context (202). In some examples, the entropy coding process may comprise a context-adaptive binary arithmetic coding (CABAC) process, and the entropy coding may comprise CABAC coding. In such examples, CABAC coding may comprise CABAC encoding or CABAC decoding.

[0136] In some examples, the video coder may be configured to détermine whether a

CU depth of the current CU is smaller than a minimal CU depth of the one or more neighboring CUs. The video coder may be configured to select the context based on a détermination that the CU depth of the current CU is smaller than the minimal CU depth of the one or more neighboring CUs. The video coder may be configured to set an initial value corresponding to the selected context equal to 255 based on the détermination that the CU depth of the current CU is smaller than the minimal CU depth of the one or more neighboring CUs. The selected context may correspond to a probability value equal to one for the syntax element when the CU depth of the current CU is smaller than the minimal CU depth of the one or more neighboring CUs. In such examples, the selected context may comprise one of a dedicated context or an adaptive context with an initial value that is equal to or very close to one.

[0137] In some examples, the video coder may be configured to détermine whether a CU depth ofthe current CU is greater than a maximum CU depth ofthe one or more neighboring CUs. The video coder may be configured to select the context based on a détermination that the CU depth of the current CU is greater than the maximum CU 30 depth of the one or more neighboring CUs. The video coder may be configured to set an initial value corresponding to the selected context equal to zéro based on the détermination that the CU depth of the current CU is greater than the maximum CU depth of the one or more neighboring CUs. The selected context may correspond to a probability value equal to zéro for the syntax element when the CU depth of the current

CU is greater than the maximum CU depth of the one or more neighboring CUs. In such examples, the selected context may comprise one of a dedicated context or an adaptive context with an initial value that is equal to or very close to zéro.

[0138] In other examples, the video coder may be configured to select, as part of an 5 entropy coding process, a context for a syntax element associated with a current coding unit (CU) based on at least one of information relating to CU depth of one or more spatially-neighboring CUs relative to the current CU or information relating to CU depth of one or more temporally-neighboring CUs relative to the current CU. In such examples, the video coder may be configured to détermine whether a CU depth of the 10 current CU is smaller than a minimal CU depth of at least one of the one or more spatially-neighboring CUs or the one or more temporally-neighboring CUs. The video coder may be configured to select the context based on a détermination that the CU depth of the current CU is smaller than the minimal CU depth of the at least one of one or more spatially-neighboring CUs or one or more temporally-neighboring CUs. The 15 video coder may be configured to set an initial value corresponding to the selected context equal to 255 based on the détermination that the CU depth of the current CU is smaller than the minimal CU depth of the at least one of one or more spatiallyneighboring CUs or one or more temporally-neighboring CUs.

[0139] In some examples, the selected context may correspond to a probability value 20 equal to or very close to one for the syntax element when the CU depth of the current

CU is smaller than the minimal CU depth of the at least one of one or more spatiallyneighboring CUs or one or more temporally-neighboring CUs. In such examples, the selected context may comprise one of a dedicated context or an adaptive context with an initial value that is equal to or very close to one. In other examples, the selected context 25 may correspond to a probability value equal to or very close zéro for the syntax element when the CU depth of the current CU is greater than the maximum CU depth of the at least one of one or more spatially-neighboring CUs or one or more temporallyneighboring CUs. In such examples, the selected context may comprise one of a dedicated context or an adaptive context with an initial value that is equal to or very 30 close to zéro.

[0140] In some examples, the video coder may be configured to détermine whether a CU depth of the current CU is greater than a maximum CU depth of at least one of the one or more spatially-neighboring CUs or the one or more temporally-neighboring CUs. The video coder may be configured to select the context based on a détermination that

the CU depth of the current CU is greater than the maximum CU depth of the at least one of one or more spatially-neighboring CUs or one or more temporally-neighboring CUs. The video coder may be configured to set an initial value corresponding to the selected context equal to zéro based on the détermination that the CU depth of the 5 current CU is greater than the maximum CU depth of the at least one of one or more spatially-neighboring CUs or one or more temporally-neighboring CUs.

[0141] In some examples, the video coder may be configured to select context from a group of at least four possible contexts. In such examples, the group of at least four possible contexts comprises six possible contexts.

[0142] FIG. 5 is a flowchart illustratîng an example process for decoding video data consistent with techniques of this disclosure. The process of FIG. 5 is generally described as being performed by a video décoder (e.g., video décoder 30) for purposes of illustration, although a variety of other processors may also carry out the process shown in FIG. 5. In some examples, video data memory 151, entropy decoding unit

150, and/or prédiction processing unit 152 may perform one or more processes shown in

FIG. 5.

[0143] In the example of FIG. 5, video décoder 30 may be configured to receive a coding tree unit (CTU) representing a current CU (210). Video décoder 30 may be configured to receive a syntax element for the current CU indicating whether the current 20 CU is split into a plurality of sub-CUs (212). In some examples, the syntax element may be a split_cu_flag syntax element. Video décoder 30 may be configured to détermine a context for entropy decoding the received syntax element based on at least one of information relating to CU depth of one or more spatially-neighboring CUs relative to the current CU or information relating to CU depth of one or more temporally-neighboring CUs relative to the current CU (214). Video décoder 30 may be configured to entropy décodé the syntax element using the determined context (216).

[0144] Video décoder 30 may be configured to détermine, based on the decoded syntax element, whether the current CU is split into a plurality of sub-CUs (218). Ifthe current CU is split into a plurality of CUs, video décoder 30 may be configured to receive a 30 syntax element for each sub-CU indicating whether each sub-CU is further split into a plurality of sub-CUs (219). Video décoder 30 may be configured to détermine a context for each syntax element for each sub-CU, and may be configured to entropy décodé each syntax element for each sub-CU using each determined context for each syntax element as described with respect to blocks 214 and 216.

[0145] If video décoder 30 détermines at block 218 that the current CU (or sub-CU) is not further split into a pluralîty of sub-CUs, then video décoder 30 may be configured to entropy décodé prédiction information and residual information (220). Video décoder 30 may be configured to form each CU and/or sub-CU (e.g., PU) using the decoded prédiction information (222). Video décoder 30 may be configured to form transform unit(s) for the CU(s) and/or sub-CU(s) using the decoded residual information (224). Video décoder 30 may be configured to décodé each CU by combining respective subblocks (e.g., PUs) and TUs (226).

[0146] FIG. 6 is a flowchart illustrating an example process for encoding video data consistent with techniques ofthis disclosure. The process ofFIG. 6 is generally described as being performed by a video encoder (e.g., video encoder 20) for purposes of illustration, although a variety of other processors may also carry out the process shown in FIG. 6. In some examples, video data memory 101, prédiction processing unit 100, and/or entropy encoding unit 118 may perform one or more processes shown in

FIG. 6.

[0147] In the example of FIG. 6, video encoder 20 may be configured to split (e.g., partition) a CTU into a pluralîty of CUs (240). Video encoder 20 may be configured to assign a value to a syntax element for each CU indicating whether each CU is further split (242). In some examples, the syntax element for each CU may be a split_cu_flag syntax element. Video encoder 20 may be configured to détermine a context for entropy encoding each syntax element for each CU based on at least one of information relating to CU depth of one or more spatially-neighboring CUs relative to the current CU (e.g., the CU with which the syntax element is associated) or information relating to CU depth of one or more temporally-neighboring CUs relative to the current CU (244).

Video encoder 20 may be configured to entropy encode each syntax element for each CU using the determined context for each syntax element for each CU (246). [0148] Video encoder 20 may be configured to form prédiction information and residual information for each CU and each sub-CU (e.g., PU) (248). Video encoder 20 may be configured to encode prédiction information and residual information for each CU and each sub-CU (250).

[0149] Example 1 : a method of processing video data according to any of the techniques or any combination or permutation of the techniques disclosed herein. [0150] Example 2: a method of coding video data, the method comprising: performing context-adaptive binary arithmetic coding (CABAC) context modeling of a

syntax element associated with a coding unit (CU) based on at least one of information of a spatially-neighboring CU relative to the CU or a temporally-neighboring CU relative to the CU and CABAC coding the syntax element based on the modeled context.

[0151] Example 3: the method of example 2, wherein CABAC coding comprises

CABAC encoding.

[0152] Example 4: the method of example 2, wherein CABAC coding comprises CABAC decoding.

[0153] Example 5: The method of any of examples 2-4, wherein the syntax element represents whether the CU is split into sub-CUs, and wherein CABAC coding comprises CABAC coding the syntax element based on a détermination that the CU is likely to be split when a current CU depth is smaller than a minimal CU depth for the spatially-neighboring CU or the temporally-neighboring CU.

[0154] Example 6: The method of any of examples 2-5, wherein the syntax element represents whether the CU is split into sub-CUs, wherein CABAC coding comprises CABAC coding the syntax element using a context model that assigns a probabilîty value equal to or very close to one for the syntax element when a current CU depth for the CU is smaller than a minimal CU depth for the spatially-neighboring CU or the temporally-neighboring CU, and wherein the context model comprises one of a dedicated context model or an adaptive context model with an initial value that is equal to or very close to one.

[0155] Example 7: The method of any of examples 2-6, wherein the syntax element represents whether the CU is split into sub-CUs, and wherein CABAC coding comprises CABAC coding the syntax element based on a détermination that the CU is not likely to be split when a current CU depth is greater than a maximum CU depth for the spatially-neighboring CU or the temporally-neighboring CU.

[0156] Example 8: The method of any of examples 2-7, wherein the syntax element represents whether the CU is split into sub-CUs, wherein CABAC coding comprises CABAC coding the syntax element using a context model that assigns a probabilîty value equal or very close to zéro for the syntax element when a current CU depth for the CU is larger than a maximal CU depth for the spatially-neighboring CU or the temporally-neighboring CU, and wherein the context model comprises one of a dedicated context model or an adaptive context model with an initial value that is equal to or very close to zéro.

[0157] Example 9: the method of any of examples 2-8, wherein the syntax element comprises the split_cu_flag syntax element.

[0158] Example 10: in response to determining that CU is not very likely to be split and in response to determining that the CU is not very unlikely to be split, performing the context modeling of the syntax element using context sélection of the split_cu_flag as defined in HEVC.

[0159] Example 11 : method of any of examples 2-10, further comprising, in response to determining the CU is likely to be split, setting a context associated with the CU being further split equal to 255, and in response to determining that the CU is not likely 10 to be split, setting a context associated with the CU being unlikely to be split equal to 0.

[0160] Example 12: The method of any of examples 2-11, wherein a CTU associated with the CU has a size larger than 64x64, the method further comprising: using one of four possible contexts to model the syntax element.

[0161] Example 13: The method of example 12, further comprising selecting a context for CABAC coding the syntax element according to: min(3, cur_cu_depth), wherein cur_cu_depth indicates a current depth ofthe CU, and wherein min is a fonction that indicates a minimum of 3 and cur_cu_depth.

[0162] Example 14: The method of example 2, wherein the syntax element comprises inter_pred_idc, the method forther comprising: determining a context index for inter_pred_idc based on whether quantization for the CU is uniform or non-uniform. [0163] Example 15: The method of example 14, wherein the context index is equal to: min (CU depth, M), wherein CU depth is a depth ofthe CU, wherein M is a positive integer value, and wherein min is a fonction that retums a minimum of M and the CU depth.

[0164] Example 16: the method of example 15, wherein M is equal to 3.

[0165] Example 17: A method of coding video data, the method comprising: in response to determining that a CTU size is greater than 64x64, determining a group of constraints comprising: a maximum CU depth and a minimum CU depth ofthe CTU, and coding the CTU based on at least some of the group of constraints.

[0166] Example 18: The method of example 17, wherein the maximum CU depth is constrained to a size of 64x64.

[0167] Example 19: the method of any of examples 17-18, forther comprising: coding a maximum CU size ofthe CTU, the CTU size, and the maximum CU depth in a parameter set.

[0168] Example 20: The method of any of examples 17-19, further comprising: in response to determining that a CU of the CTU has a size larger than a maximum CU size, inferring that a value of split_cu_flag is equal to one without coding the split_cu_flag element.

[0169] Example 21: The method of any of examples 17-20, wherein the maximum CU depth is less than a variable M.

[0170] Example 22: The method of any of examples 17-21, further comprising: restricting a CU size of the CTU to be no smaller than (1« (N-M)), wherein the maximum CTU size is equal to (1«N), wherein N and M are integer values.

[0171] Example 23: The method of any of examples 17-22, wherein the constrained values apply only to inter-predicted CUs of the CTU.

[0172] Example 24: The method of any of examples 17-22, wherein at least one of the maximum CU size or the minimum CU is different for an intra-coded CU of the CTU, and an inter-coded CU of the CTU.

[0173] Example 25: The method of example 21, wherein M is equal to 4.

[0174] It should be understood that ail of the techniques described herein may be used individually or in combination. This disclosure includes several signaling methods which may change depending on certain factors such as block size, palette size, slice type etc. Such variation in signaling or inferring the syntax éléments may be known to 20 the encoder and décoder a-priori or may be signaled explicitly in the video parameter set (VPS), sequence parameter set (SPS), picture parameter set (PPS), slice header, at a tile level or elsewhere.

[0175] It is to be recognized that depending on the example, certain acts or events of any ofthe techniques described herein can be performed in a different sequence, may be 25 added, merged, or left out altogether (e.g., not ail 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 addition, while certain aspects of this disclosure are described as being performed by a single module or unit 30 for purposes of clarity, it should be understood that the techniques of this disclosure may be performed by a combination of units or modules associated with a video coder.

[0176] Certain aspects of this disclosure hâve been described with respect to the developing HEVC standard for purposes of illustration. However, the techniques

described in this disclosure may be usefiil for other video coding processes, including other standard or proprietary video coding processes not yet developed.

[0177] The techniques described above may be performed by video encoder 20 (FIGS.

and 2) and/or video décoder 30 (FIGS. 1 and 3), both of which may be generally referred to as a video coder. Likewise, video coding may refer to video encoding or video decoding, as applicable.

[0178] It should be understood that ail ofthe techniques described herein may be used individually or in combination. For example, video encoder 20 and/or one or more components thereof and video décoder 30 and/or one or more components thereof may perform the techniques described in this disclosure in any combination. As another example, the techniques described herein may be performed by video encoder 20 (FIGS. 1 and 2) and/or video décoder 30 (FIGS. 1 and 3), both of which may be generally referred to as a video coder. Likewise, video coding may refer to video encoding or video decoding, as applicable.

[0179] 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 ail 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 addition, while certain aspects of this disclosure are described as being performed by a single module or unit for purposes of clarity, it should be understood that the techniques of this disclosure may be performed by a combination of units or modules associated with a video coder. [0180] Certain aspects of this disclosure hâve been described with respect to one or more video coding standards—whether released, under development, or the like—for purposes of illustration. However, the techniques described in this disclosure may be usefiil for other video coding processes, including other standard or proprietary video coding processes not yet developed.

[0181] In accordance with this disclosure, the term “or” may be interrupted as “and/or” where context does not dictate otherwîse. Additionally, while phrases such as “one or more” or “at least one” or the like may hâve been used for some features disclosed herein but not others; the features for which such language was not used may be interpreted to hâve such a meaning implied where context does not dictate otherwîse.

[0182] While particular combinations of various aspects of the techniques are described above, these combinations are provided merely to illustrate examples of the techniques described in this disclosure. Accordingly, the techniques of this disclosure should not be Iimited to these example combinations and may encompass any conceivable combination of the various aspects of the techniques described in this disclosure. [0183] In one or more examples, the fonctions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the fonctions may be stored on or transmitted over, as one or more instructions or code, a computer-readable medium and executed by a hardware-based processing unit.

o 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 facilitâtes transfer of a computer program from one place to another, e.g., according to a communication protocol. In this manner, computerreadable 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 implémentation of the techniques described in this disclosure. A computer program product may include a computer-readable medium.

[0184] 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 définition of medium. It should be understood, however, that computer-readable storage media and data storage media do not include connections, carrier waves, signais, or other transient media, but are instead directed to non-transient, tangible storage media. Disk and dise, as used herein, includes compact dise (CD), laser dise, optical dise, digital versatile dise (DVD), floppy disk and Blu-ray dise, where disks usually reproduce data magnetically,

while dises reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media.

[0185] Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application spécifie integrated circuits (ASICs), field programmable gâte arrays (FPGAs), or other équivalent integrated or discrète logic circuitry. Accordingly, the term “processor,” as used herein may refer to any of the foregoing structure or any other structure suitable for implémentation of the techniques described herein. In addition, in some aspects, the fùnctionality described herein may be provided within dedicated hardware and/or software modules configured for encoding and decoding, or incorporated in a combined codée. Also, the techniques could be fùlly implemented in one or more circuits or logic éléments.

[0186] 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 codée 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. [0187] Various examples hâve been described herein. Any combination ofthe described Systems, operations, fùnctions, or examples is contemplated. These and other examples are within the scope of the following claims.

Claims (13)

1. Claims

   1. A method of coding video data, the method comprising:

   determining whether a coding unit (CU) depth of a current CU is smaller than a

   5 minimal CU depth of one or more neighboring CUs, or whether the CU depth of the current CU is greater than a maximum CU depth of the one or more neighboring CUs;

   selecting, as part of an entropy coding process, a context for a syntax element associated with the current CU based on information relating to CU depth of the one or more neighboring CUs relative to the current CU, wherein information relating to CU

   10 depth of the one or more neighboring CUs relative to the current CU includes whether the CU depth of the current CU is smaller than the minimal CU depth of the one or more neighboring CUs or whether the CU depth of the current CU is greater than the maximum CU depth of the one or more neighboring CUs, wherein the syntax element represents whether the current CU is split into sub-CUs; and

   15 entropy coding the syntax element based on the selected context.
2. 2. The method of claim 1, wherein the entropy coding process comprises a contextadaptive binary arithmetic coding (CABAC) process, and wherein entropy coding comprises CABAC coding.
3. 3. The method of claim 2, wherein CABAC coding comprises CABAC encoding or CABAC decodîng.
4. 4. The method of claim 1, wherein one or more CU depths relating to the one or

   25 more neighboring CUs are equal to a CU depth of the current CU.
5. 5. The method of claim 1, wherein one or more CU depths relating to the one or more neighboring CUs are not equal to a CU depth of the current CU.

   30
6. 6. The method of claim 1, further comprising:

   setting an initial value corresponding to the selected context equal to 255 when the CU depth of the current CU is smaller than the minimal CU depth of the one or more neighboring CUs.

   «
7. 7. The method of claim 1, wherein the selected context corresponds to a probability value equal to one for the syntax element when the CU depth of the current CU is smaller than the minimal CU depth of the one or more neighboring CUs.
8. 8. The method of claim 7, wherein the selected context comprises one of a dedicated context or an adaptive context with an initial value that is equal to one.
9. 9. The method of claim 1, further comprising:
10. 10 setting an initial value corresponding to the selected context equal to zéro when the CU depth of the current CU is greater than the maximum CU depth of the one or more neighboring CUs.

    10. The method of claim 1, wherein the selected context corresponds to a probability

    15 value equal to zéro for the syntax element when the CU depth of the current CU is greater than the maximum CU depth of the one or more neighboring CUs.
11. 11. The method of claim 1, wherein selectîng the context for the syntax element is based on information relating to CU depth of one or more spatially neighboring CUs

    20 relative to the current CU.
12. 12. The method of claim 1, wherein selectîng the context for the syntax element is based on information relating to CU depth of one or more temporally neighboring CUs relative to the current CU.
13. 13. A device for coding video data, the device comprising:

    a memory configured to store the video data; and a video coder in communication with the memory, wherein the video coder is configured to:

    30 détermine whether a coding unit (CU) depth of a current CU is smaller than a minimal CU depth of one or more neighboring CUs, or whether the CU depth of the current CU is greater than a maximum CU depth of the one or more neighboring CUs;

    select, as part of an entropy coding process, a context for a syntax element associated with the current CU stored in the memory based on information relating to CU depth of the one or more neighboring CUs relative to the current CU, wherein information relating to CU depth of the one or more

    5 neighboring CUs relative to the current CU includes whether the CU depth of the current CU is smaller than the minimal CU depth of the one or more neighboring CUs or whether the CU depth of the current CU is greater than the maximum CU depth of the one or more neighboring CUs, wherein the syntax element represents whether the current CU is split into sub-CUs; and entropy code the syntax element based on the selected context.