Patent Description:
JVET-L0132 proposes a <NUM>-point multi transform selection (MTS) technique based on skipping high frequency coefficients (RMTS32). On top of existing zero-out that up-to top-left 32x32 region is kept, only left (top) half of coefficients are kept if block width (height) is greater than or equal to <NUM> when MTS flag is <NUM>. Because this zero-out is applied to horizontal and vertical directions independently, RMTS32 is applied to all block shapes. For the region which must be zero, residual coding is skipped in such a way that the associated subblock flags are implicitly derived to be <NUM>, and truncated unary binarization of last coefficient position is adjusted considering maximum feasible position.

JVET-L0358 proposes that for an inter-predicted CU with root_cbf equal to <NUM>, a sub-block transform, known as spatially varying transform (SVT) in JVET-K0139, may be used (signaled by a flag) to transform and code only a sub-part of the residual block. A CU is tiled into two TUs, one TU has residual (named residual TU) and the other is inferred to be of zero residual. There are two TU tiling types (denoted as SVT-V and SVT-H), each with two residual TU positions. For SVT-V (or SVT-H), the residual TU width (or height) equals to half of the CU width (or height) or <NUM>/<NUM> of the CU width (or height), signaled by another flag.

Advantageous embodiments are subject to the appended dependent claims. In the following, each of the described methods, apparatuses, systems, examples and aspects, which does not fully correspond to the invention as defined in the appended claims, is thus not according to the invention and is, as well as the whole following description, present for illustration purposes only or to highlight specific aspects or features of the appended claims.

In some examples, this disclosure describes a transform coefficient scan technique and a last non-zero coefficient position coding technique for large transforms that use zero-out techniques for high frequency coefficients. When using a zero-out technique, a video coder may be configured to set all transform coefficients in a certain region of a transform unit to have a value of zero. For example, for a 64x64 transform unit, the video coder may keep the values in a 32x32 region in the upper left corner of the transform unit (e.g., the lowest frequency components) with no change. However, the video coder will set all transform coefficients outside this region (i.e., the zero-out region) to have a value of zero.

In one example, this disclosure describes a scanning technique that includes only scanning transform coefficients in a transform unit that are outside the zero-out region (i.e., that are in the non zero-out region). In this way, scanning of transform coefficients known to have a value of zero may be avoided, thus increasing coder throughput.

In addition, this disclosure describes techniques of coding a position of a last non-zero transform coefficient in a transform unit that has a zero-out region.

For example, this disclosure describes techniques where a video coder determines a context (e.g., a probability model) for coding a syntax element that indicates a position of a last non-zero transform coefficient. In one example, the context is determined based on the entire size of the transform unit, and not based on the size of non zero-out region of the transform unit.

Even though the position of the last non-zero transform coefficient is guaranteed to be in the non zero-out region (e.g., the upper-left 32x32 region of a 64x64 transform unit), the context of coding the position of the last non-zero transform coefficient is more closely related to statistics of a 64x64 transform unit, not a 32x32 transform unit. Thus, coding efficiency of the position of the last non-zero transform coefficient may be increased.

<FIG> is a block diagram illustrating an example video encoding and decoding system <NUM> that may perform the techniques of this disclosure for scanning and coding zero-out transform units. The techniques of this disclosure are generally directed to coding (encoding and/or decoding) video data. In general, video data includes any data for processing a video. Thus, video data may include raw, uncoded video, encoded video, decoded (e.g., reconstructed) video, and video metadata, such as signaling data.

As shown in <FIG>, system <NUM> includes a source device <NUM> that provides encoded video data to be decoded and displayed by a destination device <NUM>, in this example. In particular, source device <NUM> provides the video data to destination device <NUM> via a computer-readable medium <NUM>. Source device <NUM> and destination device <NUM> may include any of a wide range of devices, including desktop computers, notebook (i.e., laptop) computers, tablet computers, set-top boxes, telephone handsets such as smart phones, televisions, cameras, display devices, digital media players, video gaming consoles, video streaming devices, or the like. In some cases, source device <NUM> and destination device <NUM> may be equipped for wireless communication, and thus may be referred to as wireless communication devices.

In the example of <FIG>, source device <NUM> includes video source <NUM>, memory <NUM>, video encoder <NUM>, and output interface <NUM>. Destination device <NUM> includes input interface <NUM>, video decoder <NUM>, memory <NUM>, and display device <NUM>. In accordance with this disclosure, video encoder <NUM> of source device <NUM> and video decoder <NUM> of destination device <NUM> may be configured to apply the techniques for transform coefficient coding. Thus, source device <NUM> represents an example of a video encoding device, while destination device <NUM> represents an example of a video decoding device. In other examples, a source device and a destination device may include other components or arrangements. For example, source device <NUM> may receive video data from an external video source, such as an external camera. Likewise, destination device <NUM> may interface with an external display device, rather than include an integrated display device.

System <NUM> as shown in <FIG> is merely one example. In general, any digital video encoding and/or decoding device may perform techniques for transform coefficient coding. Source device <NUM> and destination device <NUM> are merely examples of such coding devices in which source device <NUM> generates coded video data for transmission to destination device <NUM>. This disclosure refers to a "coding" device as a device that performs coding (encoding and/or decoding) of data. Thus, video encoder <NUM> and video decoder <NUM> represent examples of coding devices, in particular, a video encoder and a video decoder, respectively. In some examples, source device <NUM> and destination device <NUM> may operate in a substantially symmetrical manner such that each of source device <NUM> and destination device <NUM> include video encoding and decoding components. Hence, system <NUM> may support one-way or two-way video transmission between source device <NUM> and destination device <NUM>, e.g., for video streaming, video playback, video broadcasting, or video telephony.

In general, video source <NUM> represents a source of video data (i.e., raw, uncoded video data) and provides a sequential series of pictures (also referred to as "frames") of the video data to video encoder <NUM>, which encodes data for the pictures. Video source <NUM> of source device <NUM> may include a video capture device, such as a video camera, a video archive containing previously captured raw video, and/or a video feed interface to receive video from a video content provider. As a further alternative, video source <NUM> may generate computer graphics-based data as the source video, or a combination of live video, archived video, and computer-generated video. In each case, video encoder <NUM> encodes the captured, pre-captured, or computer-generated video data. Video encoder <NUM> may rearrange the pictures from the received order (sometimes referred to as "display order") into a coding order for coding. Video encoder <NUM> may generate a bitstream including encoded video data. Source device <NUM> may then output the encoded video data via output interface <NUM> onto computer-readable medium <NUM> for reception and/or retrieval by, e.g., input interface <NUM> of destination device <NUM>.

Memory <NUM> of source device <NUM> and memory <NUM> of destination device <NUM> represent general purpose memories. In some examples, memories <NUM>, <NUM> may store raw video data, e.g., raw video from video source <NUM> and raw, decoded video data from video decoder <NUM>. Additionally or alternatively, memories <NUM>, <NUM> may store software instructions executable by, e.g., video encoder <NUM> and video decoder <NUM>, respectively. Although memories <NUM>, <NUM> are shown separately from video encoder <NUM> and video decoder <NUM> in this example, it should be understood that video encoder <NUM> and video decoder <NUM> may also include internal memories for functionally similar or equivalent purposes. Furthermore, memories <NUM>, <NUM> may store encoded video data, e.g., output from video encoder <NUM> and input to video decoder <NUM>. In some examples, portions of memories <NUM>, <NUM> may be allocated as one or more video buffers, e.g., to store raw, decoded, and/or encoded video data.

Computer-readable medium <NUM> may represent any type of medium or device capable of transporting the encoded video data from source device <NUM> to destination device <NUM>. In some examples, computer-readable medium <NUM> represents a communication medium to enable source device <NUM> to transmit encoded video data directly to destination device <NUM> in real-time, e.g., via a radio frequency network or computer-based network. Output interface <NUM> may modulate a transmission signal including the encoded video data, and input interface <NUM> may demodulate the received transmission signal, according to a communication standard, such as a wireless communication protocol. The communication medium may include any wireless or wired communication medium, such as a radio frequency (RF) spectrum or one or more physical transmission lines. The communication medium may form part of a packet-based network, such as a local area network, a wide area network, or a global network such as the Internet. The communication medium may include routers, switches, base stations, or any other equipment that may be useful to facilitate communication from source device <NUM> to destination device <NUM>.

In some examples, computer-readable medium <NUM> may include storage device <NUM>. Source device <NUM> may output encoded data from output interface <NUM> to storage device <NUM>. Similarly, destination device <NUM> may access encoded data from storage device <NUM> via input interface <NUM>. Storage device <NUM> may include any of a variety of distributed or locally accessed data storage media such as hard drives, Blu-ray discs, DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or any other suitable digital storage media for storing encoded video data.

In some examples, computer-readable medium <NUM> may include file server <NUM> or another intermediate storage device that may store the encoded video data generated by source device <NUM>. Source device <NUM> may output encoded video data to file server <NUM>. Destination device <NUM> may access stored video data from file server <NUM> via streaming or download. File server <NUM> may be any type of server device capable of storing encoded video data and transmitting that encoded video data to the destination device <NUM>. File server <NUM> may represent a web server (e.g., for a website), a File Transfer Protocol (FTP) server, a content delivery network device, or a network attached storage (NAS) device. Destination device <NUM> may access encoded video data from file server <NUM> through any standard data connection, including an Internet connection. This may include a wireless channel (e.g., a Wi-Fi connection), a wired connection (e.g., digital subscriber line (DSL), cable modem, etc.), or a combination of both that is suitable for accessing encoded video data stored on file server <NUM>. File server <NUM> and input interface <NUM> may be configured to operate according to a streaming transmission protocol, a download transmission protocol, or a combination thereof.

Output interface <NUM> and input interface <NUM> may represent wireless transmitters/receivers, modems, wired networking components (e.g., Ethernet cards), wireless communication components that operate according to any of a variety of IEEE <NUM> standards, or other physical components. In examples where output interface <NUM> and input interface <NUM> include wireless components, output interface <NUM> and input interface <NUM> may be configured to transfer data, such as encoded video data, according to a cellular communication standard, such as <NUM>, <NUM>-LTE (Long-Term Evolution), LTE Advanced, <NUM>, or the like. In some examples where output interface <NUM> and input interface <NUM> include a wireless transmitter and/or wireless receiver, output interface <NUM> and input interface <NUM> may be configured to transfer data, such as encoded video data, according to other wireless standards, such as an IEEE <NUM> specification, an IEEE <NUM> specification (e.g., ZigBee™), a Bluetooth™ standard, or the like. In some examples, source device <NUM> and/or destination device <NUM> may include respective system-on-a-chip (SoC) devices. For example, source device <NUM> may include an SoC device to perform the functionality attributed to video encoder <NUM> and/or output interface <NUM>, and destination device <NUM> may include an SoC device to perform the functionality attributed to video decoder <NUM> and/or input interface <NUM>.

Input interface <NUM> of destination device <NUM> receives an encoded video bitstream from computer-readable medium <NUM> (e.g., a communication medium, storage device <NUM>, file server <NUM>, or the like). The encoded video bitstream from computer-readable medium <NUM> may include signaling information defined by video encoder <NUM>, which is also used by video decoder <NUM>, such as syntax elements having values that describe characteristics and/or processing of video blocks or other coded units (e.g., slices, pictures, groups of pictures, sequences, or the like). Display device <NUM> displays decoded pictures of the decoded video data to a user. Display device <NUM> may represent any of a variety of display devices such as a cathode ray tube (CRT), a liquid crystal display (LCD), a plasma display, an organic light emitting diode (OLED) display, or another type of display device.

A device including video encoder <NUM> and/or video decoder <NUM> may include an integrated circuit, a microprocessor, and/or a wireless communication device, such as a cellular telephone.

In general, this disclosure describes techniques for transform coefficient coding for video coding. In some examples, this disclosure describes a transform coefficient scan technique and last non-zero coefficient position coding for large transforms that use zero-out techniques for high frequency coefficients for complexity reduction. That is, in some examples, a video coder (e.g., video encoder <NUM> and/or video decoder <NUM>) may be configured to set all transform coefficients in a certain region of a transform unit to have a value of zero. For example, for a 64x64 transform unit, the video coder may keep the values in a 32x32 region in the upper left corner of the transform unit (e.g., the lowest frequency components) as is. However, the video coder will set all transform coefficients outside this region (i.e., the zero-out region) to have a value of zero.

In one example, this disclosure describes a scanning technique that includes only scanning transform coefficients in a transform unit that are outside the zero-out region. In this way, scanning of transform coefficients known to have a value of zero may be avoided, thus increasing coder throughput.

In addition, this disclosure describes techniques of coding a position of a last non-zero transform coefficient in a transform unit that has a zero-out region. For example, this disclosure describes techniques where a video coder determines a context (e.g., a probability model) for coding a syntax element that indicates a position of a last non-zero transform coefficient. In one example, the context is determined based on the entire size of the transform unit, and not based on the size of non zero-out region of the transform unit. Even though the position of the last non-zero transform coefficient is guaranteed to be in the non zero-out region (e.g., the upper-left 32x32 region of a 64x64 transform coefficient), the context of coding the position of the last non-zero transform coefficient is more closely related to statistics of a 64x64 transform unit, not a 32x32 transform unit. Thus, coding efficiency of the position of the last non-zero transform coefficient may be increased.

As will be explained in more detail below, video encoder <NUM> and video decoder <NUM> may be configured to determine a first region of a transform unit in which transform coefficients are subject to zero-out, determine a second region of the transform unit in which transform coefficients are not subject to zero-out, and scan only the second region of the transform unit.

Video encoder <NUM> and video decoder <NUM> may operate according to a video coding standard, such as ITU-T H. <NUM>, also referred to as High Efficiency Video Coding (HEVC) or extensions thereto, such as the multi-view and/or scalable video coding extensions. Alternatively, video encoder <NUM> and video decoder <NUM> may operate according to other proprietary or industry standards, such as the Joint Exploration Test Model (JEM) and/or H. <NUM>/VVC (Versatile Video Coding). The techniques of this disclosure, however, are not limited to any particular coding standard. The techniques of this disclosure may be applicable to encoding and decoding a variety of types of video data. In particular, video data in which blocks of data are predicted, a difference is calculated, and the difference or residual is transformed using one of a number of transforms to produce transform coefficients. The transform coefficients are then scanned. The video coding may involve such techniques according to a variety of different standards.

An early draft for new video coding standard, referred to as the H. <NUM>/Versatile Video Coding (VVC) standard, is available in the document <NPL>, and its algorithm description is available in the document <NPL>.

As another example, video encoder <NUM> and video decoder <NUM> may be configured to operate according to JEM or VVC. According to JEM or VVC, a video coder (such as video encoder <NUM>) partitions a picture into a plurality of coding tree units (CTUs). Video encoder <NUM> may partition a CTU according to a tree structure, such as a quadtree-binary tree (QTBT) structure or Multi-Type Tree (MTT) structure. The QTBT structure removes the concepts of multiple partition types, such as the separation between CUs, PUs, and TUs of HEVC. A QTBT structure includes two levels: a first level partitioned according to quadtree partitioning, and a second level partitioned according to binary tree partitioning. A root node of the QTBT structure corresponds to a CTU. Leaf nodes of the binary trees correspond to coding units (CUs).

In an MTT partitioning structure, blocks may be partitioned using a quadtree (QT) partition, a binary tree (BT) partition, and/or one or more types of triple tree (TT) (also called ternary tree (TT)) partitions. A triple or ternary tree partition is a partition where a block is split into three sub-blocks. In some examples, a triple or ternary tree partition divides a block into three sub-blocks without dividing the original block through the center. The partitioning types in MTT (e.g., QT, BT, and TT) may be symmetrical or asymmetrical.

However, it should be understood that the techniques of this disclosure may also be applied to video coders configured to use quadtree partitioning, MTT partitioning, or other types of partitioning as well.

The blocks (e.g., CTUs or CUs) may be grouped in various ways in a picture. As one example, a brick may refer to a rectangular region of CTU rows within a particular tile in a picture. A tile may be a rectangular region of CTUs within a particular tile column and a particular tile row in a picture. A tile column refers to a rectangular region of CTUs having a height equal to the height of the picture and a width specified by syntax elements (e.g., such as in a picture parameter set). A tile row refers to a rectangular region of CTUs having a height specified by syntax elements (e.g., such as in a picture parameter set) and a width equal to the width of the picture.

In some examples, a tile may be partitioned into multiple bricks, each of which may include one or more CTU rows within the tile. A tile that is not partitioned into multiple bricks may also be referred to as a brick. However, a brick that is a true subset of a tile may not be referred to as a tile.

The bricks in a picture may also be arranged in a slice. A slice may be an integer number of bricks of a picture that may be exclusively contained in a single network abstraction layer (NAL) unit. In some examples, a slice includes either a number of complete tiles or only a consecutive sequence of complete bricks of one tile.

Some examples of JEM and VVC also provide an affine motion compensation mode, which may be considered an inter-prediction mode. In affine motion compensation mode, video encoder <NUM> may determine two or more motion vectors that represent non-translational motion, such as zoom in or out, rotation, perspective motion, or other irregular motion types.

To perform intra-prediction, video encoder <NUM> may select an intra-prediction mode to generate the prediction block. Some examples of JEM and VVC provide sixty-seven intra-prediction modes, including various directional modes, as well as planar mode and DC mode. In general, video encoder <NUM> selects an intra-prediction mode that describes neighboring samples to a current block (e.g., a block of a CU) from which to predict samples of the current block. Such samples may generally be above, above and to the left, or to the left of the current block in the same picture as the current block, assuming video encoder <NUM> codes CTUs and CUs in raster scan order (left to right, top to bottom).

For uni-directional or bi-directional inter-prediction, for example, video encoder <NUM> may encode motion vectors using an advanced motion vector prediction (AMVP) mode or a merge mode.

As noted above, following any transforms to produce transform coefficients, video encoder <NUM> may perform quantization of the transform coefficients. Quantization generally refers to a process in which transform coefficients are quantized to possibly reduce the amount of data used to represent the transform coefficients, providing further compression. By performing the quantization process, video encoder <NUM> may reduce the bit depth associated with some or all of the transform coefficients. For example, video encoder <NUM> may round an n-bit value down to an m-bit value during quantization, where n is greater than m. In some examples, to perform quantization, video encoder <NUM> may perform a bitwise right-shift of the value to be quantized.

Following quantization, video encoder <NUM> may scan the transform coefficients, producing a one-dimensional vector from the two-dimensional matrix including the quantized transform coefficients. The scan may be designed to place higher energy (and therefore lower frequency) transform coefficients at the front of the vector and to place lower energy (and therefore higher frequency) transform coefficients at the back of the vector. In some examples, video encoder <NUM> may utilize a predefined scan order to scan the quantized transform coefficients to produce a serialized vector, and then entropy encode the quantized transform coefficients of the vector. In other examples, video encoder <NUM> may perform an adaptive scan. After scanning the quantized transform coefficients to form the one-dimensional vector, video encoder <NUM> may entropy encode the one-dimensional vector, e.g., according to context-adaptive binary arithmetic coding (CABAC). Video encoder <NUM> may also entropy encode values for syntax elements describing metadata associated with the encoded video data for use by video decoder <NUM> in decoding the video data.

In general, video decoder <NUM> performs a reciprocal process to that performed by video encoder <NUM> to decode the encoded video data of the bitstream. For example, video decoder <NUM> may decode values for syntax elements of the bitstream using CABAC in a manner substantially similar to, albeit reciprocal to, the CABAC encoding process of video encoder <NUM>. The syntax elements may define partitioning information of a picture into CTUs, and partitioning of each CTU according to a corresponding partition structure, such as a QTBT structure, to define CUs of the CTU. The syntax elements may further define prediction and residual information for blocks (e.g., CUs) of video data.

This disclosure may generally refer to "signaling" certain information, such as syntax elements. The term "signaling" may generally refer to the communication of values of syntax elements and/or other data used to decode encoded video data. That is, video encoder <NUM> may signal values for syntax elements in the bitstream. In general, signaling refers to generating a value in the bitstream. As noted above, source device <NUM> may transport the bitstream to destination device <NUM> substantially in real time, or not in real time, such as might occur when storing syntax elements to storage device <NUM> for later retrieval by destination device <NUM>.

<FIG> are conceptual diagrams illustrating an example quadtree binary tree (QTBT) structure <NUM>, and a corresponding coding tree unit (CTU) <NUM>. The solid lines represent quadtree splitting, and dotted lines indicate binary tree splitting. In each split (i.e., non-leaf) node of the binary tree, one flag is signaled to indicate which splitting type (i.e., horizontal or vertical) is used, where <NUM> indicates horizontal splitting and <NUM> indicates vertical splitting in this example. For the quadtree splitting, there is no need to indicate the splitting type, because quadtree nodes split a block horizontally and vertically into <NUM> sub-blocks with equal size. Accordingly, video encoder <NUM> may encode, and video decoder <NUM> may decode, syntax elements (such as splitting information) for a region tree level of QTBT structure <NUM> (i.e., the solid lines) and syntax elements (such as splitting information) for a prediction tree level of QTBT structure <NUM> (i.e., the dashed lines). Video encoder <NUM> may encode, and video decoder <NUM> may decode, video data, such as prediction and transform data, for CUs represented by terminal leaf nodes of QTBT structure <NUM>.

As stated above, the first level may be partitioned according to quadtree partitioning, and the second level may be partitioned according to binary tree partitioning.

In one example of the QTBT partitioning structure, the CTU size is set as 128x128 (luma samples and two corresponding 64x64 chroma samples), the MinQTSize is set as 16x16, the MaxBTSize is set as 64x64, the MinBTSize (for both width and height) is set as <NUM>, and the MaxBTDepth is set as <NUM>. The quadtree partitioning is applied to the CTU first to generate quad-tree leaf nodes. The quadtree leaf nodes may have a size from 16x16 (i.e., the MinQTSize) to 128x128 (i.e., the CTU size). If the leaf quadtree node is 128x128, the leaf quadtree node will not be further split by the binary tree, because the size exceeds the MaxBTSize (i.e., 64x64, in this example). Otherwise, the leaf quadtree node will be further partitioned by the binary tree. Therefore, the quadtree leaf node is also the root node for the binary tree and has the binary tree depth as <NUM>. When the binary tree depth reaches MaxBTDepth (<NUM>, in this example), no further splitting is permitted. When the binary tree node has a width equal to MinBTSize (<NUM>, in this example), it implies no further horizontal splitting is permitted. Similarly, a binary tree node having a height equal to MinBTSize implies no further vertical splitting is permitted for that binary tree node. As noted above, leaf nodes of the binary tree are referred to as CUs, and are further processed according to prediction and transform without further partitioning.

<FIG> is a block diagram illustrating an example video encoder <NUM> that may perform the techniques of this disclosure. <FIG> 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 <NUM> in the context of video coding standards such as the HEVC video coding standard and the H. <NUM>/VVC video coding standard in development. However, the techniques of this disclosure are not limited to these video coding standards, and are applicable generally to video encoding and decoding.

In the example of <FIG>, video encoder <NUM> includes video data memory <NUM>, mode selection unit <NUM>, residual generation unit <NUM>, transform processing unit <NUM>, quantization unit <NUM>, inverse quantization unit <NUM>, inverse transform processing unit <NUM>, reconstruction unit <NUM>, filter unit <NUM>, decoded picture buffer (DPB) <NUM>, and entropy encoding unit <NUM>.

In this disclosure, reference to video data memory <NUM> should not be interpreted as being limited to a memory internal to video encoder <NUM>, unless specifically described as such, or a memory external to video encoder <NUM>, unless specifically described as such. Rather, reference to video data memory <NUM> should be understood as a reference memory that stores video data that video encoder <NUM> receives for encoding (e.g., video data for a current block that is to be encoded).

The various units of <FIG> are illustrated to assist with understanding the operations performed by video encoder <NUM>. The units may be implemented as fixed-function circuits, programmable circuits, or a combination thereof. Fixed-function circuits refer to circuits that provide particular functionality, and are preset on the operations that can be performed. Programmable circuits refer to circuits that can be programmed to perform various tasks, and provide flexible functionality in the operations that can be performed. For instance, programmable circuits may execute software or firmware that causes the programmable circuits to operate in the manner defined by instructions of the software or firmware. Fixed-function circuits may execute software instructions (e.g., to receive parameters or output parameters), but the types of operations that the fixed-function circuits perform are generally immutable. In some examples, one or more of the units may be distinct circuit blocks (fixed-function or programmable), and in some examples, the one or more units may be integrated circuits.

Video encoder <NUM> may include arithmetic logic units (ALUs), elementary function units (EFUs), digital circuits, analog circuits, and/or programmable cores, formed from programmable circuits. In examples where the operations of video encoder <NUM> are performed using software executed by the programmable circuits, memory <NUM> (<FIG>) may store the instructions (e.g., object code) of the software that video encoder <NUM> receives and executes, or another memory within video encoder <NUM> (not shown) may store such instructions.

Mode selection unit <NUM> includes a motion estimation unit <NUM>, motion compensation unit <NUM>, and an intra-prediction unit <NUM>. Mode selection unit <NUM> may include additional functional units to perform video prediction in accordance with other prediction modes. As examples, mode selection unit <NUM> may include a palette unit, an intra-block copy unit (which may be part of motion estimation unit <NUM> and/or motion compensation unit <NUM>), an affine unit, a linear model (LM) unit, or the like.

Video encoder <NUM> may partition a picture retrieved from video data memory <NUM> into a series of CTUs, and encapsulate one or more CTUs within a slice. Mode selection unit <NUM> may partition a CTU of the picture in accordance with a tree structure, such as the QTBT structure, MTT structure, or the quad-tree structure of HEVC described above. As described above, video encoder <NUM> may form one or more CUs from partitioning a CTU according to the tree structure. Such a CU may also be referred to generally as a "video block" or "block.

Motion estimation unit <NUM> may form one or more motion vectors (MVs) that define the positions of the reference blocks in the reference pictures relative to the position of the current block in a current picture. Motion estimation unit <NUM> may then provide the motion vectors to motion compensation unit <NUM>. For example, for uni-directional inter-prediction, motion estimation unit <NUM> may provide a single motion vector, whereas for bi-directional inter-prediction, motion estimation unit <NUM> may provide two motion vectors. Motion compensation unit <NUM> may then generate a prediction block using the motion vectors. For example, motion compensation unit <NUM> may retrieve data of the reference block using the motion vector. As another example, if the motion vector has fractional sample precision, motion compensation unit <NUM> may interpolate values for the prediction block according to one or more interpolation filters. Moreover, for bi-directional inter-prediction, motion compensation unit <NUM> may retrieve data for two reference blocks identified by respective motion vectors and combine the retrieved data, e.g., through sample-by-sample averaging or weighted averaging.

Mode selection unit <NUM> provides the prediction block to residual generation unit <NUM>. Residual generation unit <NUM> receives a raw, uncoded version of the current block from video data memory <NUM> and the prediction block from mode selection unit <NUM>. Residual generation unit <NUM> calculates sample-by-sample differences between the current block and the prediction block. The resulting sample-by-sample differences define a residual block for the current block. In some examples, residual generation unit <NUM> may also determine differences between sample values in the residual block to generate a residual block using residual differential pulse code modulation (RDPCM). In some examples, residual generation unit <NUM> may be formed using one or more subtractor circuits that perform binary subtraction.

In examples where mode selection unit <NUM> does not further partition a CU into PUs, each CU may be associated with a luma coding block and corresponding chroma coding blocks. As above, the size of a CU may refer to the size of the luma coding block of the CU. The video encoder <NUM> and video decoder <NUM> may support CU sizes of 2Nx2N, 2NxN, or Nx2N.

For other video coding techniques such as an intra-block copy mode coding, an affine-mode coding, and linear model (LM) mode coding, as a few examples, mode selection unit <NUM>, via respective units associated with the coding techniques, generates a prediction block for the current block being encoded. In some examples, such as palette mode coding, mode selection unit <NUM> may not generate a prediction block, and instead generate syntax elements that indicate the manner in which to reconstruct the block based on a selected palette. In such modes, mode selection unit <NUM> may provide these syntax elements to entropy encoding unit <NUM> to be encoded.

Quantization unit <NUM> may quantize the transform coefficients in a transform coefficient block, to produce a quantized transform coefficient block. Quantization unit <NUM> may quantize transform coefficients of a transform coefficient block according to a quantization parameter (QP) value associated with the current block. Video encoder <NUM> (e.g., via mode selection unit <NUM>) may adjust the degree of quantization applied to the transform coefficient blocks associated with the current block by adjusting the QP value associated with the CU. Quantization may introduce loss of information, and thus, quantized transform coefficients may have lower precision than the original transform coefficients produced by transform processing unit <NUM>.

Filter unit <NUM> may perform one or more filter operations on reconstructed blocks. For example, filter unit <NUM> may perform deblocking operations to reduce blockiness artifacts along edges of CUs. Operations of filter unit <NUM> may be skipped, in some examples.

Video encoder <NUM> stores reconstructed blocks in DPB <NUM>. For instance, in examples where operations of filter unit <NUM> are not needed, reconstruction unit <NUM> may store reconstructed blocks to DPB <NUM>. In examples where operations of filter unit <NUM> are needed, filter unit <NUM> may store the filtered reconstructed blocks to DPB <NUM>. Motion estimation unit <NUM> and motion compensation unit <NUM> may retrieve a reference picture from DPB <NUM>, formed from the reconstructed (and potentially filtered) blocks, to inter-predict blocks of subsequently encoded pictures. In addition, intra-prediction unit <NUM> may use reconstructed blocks in DPB <NUM> of a current picture to intra-predict other blocks in the current picture.

When entropy encoding syntax elements representing transform coefficients, entropy encoding unit <NUM> may be configured to scan through the transform coefficients in a predetermined fashion. In accordance with examples of this disclosure, as will be described in more detail below, entropy encoding unit <NUM> may be configured to perform a transform coefficient scan technique and last non-zero coefficient position coding technique for large transforms that use zero-out techniques for high frequency coefficients for complexity reduction. That is, in some examples, entropy encoding unit <NUM> and/or transform processing unit <NUM> may be configured to set all transform coefficients in a certain region of a transform unit to have a value of zero. For example, for a 64x64 transform unit, entropy encoding unit <NUM> and/or transform processing unit <NUM> may keep the values in a 32x32 region in the upper left corner of the transform unit (e.g., the lowest frequency components) as is. However, entropy encoding unit <NUM> and/or transform processing unit <NUM> will set all transform coefficients outside this region (i.e., the zero-out region) to have a value of zero.

In one example, this disclosure describes a scanning technique where entropy encoding unit <NUM> is configured to only scan transform coefficients in a transform unit that are outside the zero-out region. In this way, scanning of transform coefficients known to have a value of zero may be avoided, thus increasing coder throughput.

In addition, this disclosure describes techniques of encoding a position of a last non-zero transform coefficient in a transform unit that has a zero-out region. For example, this disclosure describes techniques where entropy encoding unit <NUM> determines a context (e.g., a probability model) for encoding a syntax element that indicates a position of a last non-zero transform coefficient. In one example, entropy encoding unit <NUM> determines the context based on the entire size of the transform unit, and not based on the size of non zero-out region of the transform unit. Even though the position of the last non-zero transform coefficient is guaranteed to be in the non zero-out region (e.g., the upper-left 32x32 region of a 64x64 transform coefficient), the context of encoding the position of the last non-zero transform coefficient is more closely related to statistics of a 64x64 transform unit, not a 32x32 transform unit. Thus, encoding efficiency of the position of the last non-zero transform coefficient may be increased.

As will be explained in more detail below, in one example of the disclosure, entropy encoding unit <NUM> may be configured to determine a first region of a transform unit in which transform coefficients are subject to zero-out, determine a second region of the transform unit in which transform coefficients are not subject to zero-out, and scan only the second region of the transform unit.

In some examples, operations performed with respect to a luma coding block need not be repeated for the chroma coding blocks. As one example, operations to identify a motion vector (MV) and reference picture for a luma coding block need not be repeated for identifying a MV and reference picture for the chroma blocks. Rather, the MV for the luma coding block may be scaled to determine the MV for the chroma blocks, and the reference picture may be the same. As another example, the intra-prediction process may be the same for the luma coding blocks and the chroma coding blocks.

<FIG> is a block diagram illustrating an example video decoder <NUM> that may perform the techniques of this disclosure. <FIG> is provided for purposes of explanation and is not limiting on the techniques as broadly exemplified and described in this disclosure. For purposes of explanation, this disclosure describes video decoder <NUM> according to the techniques of JEM and HEVC. However, the techniques of this disclosure may be performed by video coding devices that are configured according to other video coding standards.

In the example of <FIG>, video decoder <NUM> includes coded picture buffer (CPB) memory <NUM>, entropy decoding unit <NUM>, prediction processing unit <NUM>, inverse quantization unit <NUM>, inverse transform processing unit <NUM>, reconstruction unit <NUM>, filter unit <NUM>, and decoded picture buffer (DPB) <NUM>. Prediction processing unit <NUM> includes motion compensation unit <NUM> and intra-prediction unit <NUM>. Prediction processing unit <NUM> may include additional units to perform prediction in accordance with other prediction modes. As examples, prediction processing unit <NUM> may include a palette unit, an intra-block copy unit (which may form part of motion compensation unit <NUM>), an affine unit, a linear model (LM) unit, or the like. In other examples, video decoder <NUM> may include more, fewer, or different functional components.

CPB memory <NUM> may store video data, such as an encoded video bitstream, to be decoded by the components of video decoder <NUM>. The video data stored in CPB memory <NUM> may be obtained, for example, from computer-readable medium <NUM> (<FIG>). CPB memory <NUM> may include a CPB that stores encoded video data (e.g., syntax elements) from an encoded video bitstream. Also, CPB memory <NUM> may store video data other than syntax elements of a coded picture, such as temporary data representing outputs from the various units of video decoder <NUM>. DPB <NUM> generally stores decoded pictures, which video decoder <NUM> may output and/or use as reference video data when decoding subsequent data or pictures of the encoded video bitstream. CPB memory <NUM> and DPB <NUM> may be formed by any of a variety of memory devices, such as dynamic random access memory (DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM), resistive RAM (RRAM), or other types of memory devices. CPB memory <NUM> and DPB <NUM> may be provided by the same memory device or separate memory devices. In various examples, CPB memory <NUM> may be on-chip with other components of video decoder <NUM>, or off-chip relative to those components.

Additionally or alternatively, in some examples, video decoder <NUM> may retrieve coded video data from memory <NUM> (<FIG>). That is, memory <NUM> may store data as discussed above with CPB memory <NUM>. Likewise, memory <NUM> may store instructions to be executed by video decoder <NUM>, when some or all of the functionality of video decoder <NUM> is implemented in software to be executed by processing circuitry of video decoder <NUM>.

The various units shown in <FIG> are illustrated to assist with understanding the operations performed by video decoder <NUM>. The units may be implemented as fixed-function circuits, programmable circuits, or a combination thereof. Similar to <FIG>, fixed-function circuits refer to circuits that provide particular functionality, and are preset on the operations that can be performed. Programmable circuits refer to circuits that can be programmed to perform various tasks, and provide flexible functionality in the operations that can be performed. For instance, programmable circuits may execute software or firmware that causes the programmable circuits to operate in the manner defined by instructions of the software or firmware. Fixed-function circuits may execute software instructions (e.g., to receive parameters or output parameters), but the types of operations that the fixed-function circuits perform are generally immutable. In some examples, one or more of the units may be distinct circuit blocks (fixed-function or programmable), and in some examples, the one or more units may be integrated circuits.

When entropy decoding encoded syntax elements representing transform coefficients of a transform block, entropy decoding unit <NUM> may be configured to scan through the transform block in a predetermined fashion. In accordance with examples of this disclosure, as will be described in more detail below, entropy decoding unit <NUM> may be configured to perform a transform coefficient scan technique and last non-zero coefficient position decoding technique for large transforms that use zero-out techniques for high frequency coefficients for complexity reduction. That is, in some examples, entropy decoding unit <NUM> and/or inverse transform processing unit <NUM> may be configured to set all transform coefficients in a certain region of a transform unit to have a value of zero. For example, for a 64x64 transform unit, entropy decoding unit <NUM> and/or inverse transform processing unit <NUM> may keep the values in a 32x32 region in the upper left corner of the transform unit (e.g., the lowest frequency components) as is. However, entropy decoding unit <NUM> and/or inverse transform processing unit <NUM> will set all transform coefficients outside this region (i.e., the zero-out region) to have a value of zero.

In one example, this disclosure describes a scanning technique where entropy decoding unit <NUM> is configured to only scan transform coefficients in a transform unit that are outside the zero-out region. In this way, scanning of transform coefficients known to have a value of zero may be avoided, thus increasing coder throughput.

In addition, this disclosure describes techniques of decoding one or more syntax elements indicating a position of a last non-zero transform coefficient in a transform unit that has a zero-out region. For example, this disclosure describes techniques where entropy decoding unit <NUM> determines a context (e.g., a probability model) for decoding one or more syntax elements that indicates a position of a last non-zero transform coefficient. In one example, entropy decoding unit <NUM> determines the context based on the entire size of the transform unit, and not based on the size of non zero-out region of the transform unit. Even though the position of the last non-zero transform coefficient is guaranteed to be in the non zero-out region (e.g., the upper-left 32x32 region of a 64x64 transform coefficient), the context of decoding one or more syntax elements indicating a position of the last non-zero transform coefficient is more closely related to statistics of a 64x64 transform unit, not a 32x32 transform unit. Thus, decoding efficiency of the position of the last non-zero transform coefficient may be increased.

As will be explained in more detail below, in one example of the disclosure, entropy decoding unit <NUM> may be configured to determine a first region of a transform unit in which transform coefficients are subject to zero-out, determine a second region of the transform unit in which transform coefficients are not subject to zero-out, and scan only the second region of the transform unit.

After inverse quantization unit <NUM> forms the transform coefficient block, inverse transform processing unit <NUM> may apply one or more inverse transforms to the transform coefficient block to generate a residual block associated with the current block. For example, inverse transform processing unit <NUM> may apply an inverse DCT, an inverse integer transform, an inverse Karhunen-Loeve transform (KLT), an inverse rotational transform, an inverse directional transform, or another inverse transform to the transform coefficient block.

Filter unit <NUM> may perform one or more filter operations on reconstructed blocks. For example, filter unit <NUM> may perform deblocking operations to reduce blockiness artifacts along edges of the reconstructed blocks. Operations of filter unit <NUM> are not necessarily performed in all examples.

Video decoder <NUM> may store the reconstructed blocks in DPB <NUM>. In examples where operations of filter unit <NUM> are not needed, reconstruction unit <NUM> may store reconstructed blocks to DPB <NUM>. In examples where operations of filter unit <NUM> are needed, filter unit <NUM> may store the filtered reconstructed blocks to DPB <NUM>. As discussed above, DPB <NUM> may provide reference information, such as samples of a current picture for intra-prediction and previously decoded pictures for subsequent motion compensation, to prediction processing unit <NUM>. Moreover, video decoder <NUM> may output decoded pictures (e.g., decoded video) from DPB <NUM> for subsequent presentation on a display device, such as display device <NUM> of <FIG>.

In <NPL>, the largest transform unit size is 64x64. In order to reduce the implementation complexity of transforms, VVC Draft <NUM> specifies that high frequency coefficients beyond <NUM> rows and columns (i.e., beyond the first <NUM> rows and columns of transform coefficients in a transform block starting from a DC coefficient) are set to zero (i.e., are zeroed out), keeping only the coefficients in the lower MxN region (M<=<NUM>, N<=<NUM>), where M is the width of the region in samples and N is the height of the region is samples.

In VVC Draft <NUM>, the coefficient coding module of video encoder <NUM> and video decoder <NUM> (e.g., entropy encoding unit <NUM> and entropy decoding unit <NUM>) ignores the fact that zero-out happens and treats the large transforms (where zero-out is used) as normal (i.e., non zero-out) transforms. Basically, video encoder <NUM> and video decoder <NUM> may unnecessarily handle coding of already known zero-out coefficients (i.e., coefficients whose value are set to zero due to the zero-out process described above). This disclosure addresses this issue by modifying the coefficient scan for zero-out transform coefficients and by modifying the last non-zero coefficient position coding method.

In VVC, coefficients are scanned from last to first, coefficient group (CG) by coefficient group. Each coefficient group may include a predetermined number of transform coefficients. Coefficients are also scanned within a coefficient group in a reverse diagonal scan (lower left to top right direction). In some examples, video encoder <NUM> and video decoder <NUM> also scan coefficient groups in a reverse diagonal scan (e.g., lower left to top right direction) as shown for blocks <NUM> (64x64 transform unit) and <NUM> (32x64 transform unit) in <FIG> is a conceptual diagram illustrating example coefficient group scans. The same scans may be used for transform units. Of course, other scan directions may be used.

As can been seen from blocks <NUM> and block <NUM>, all coefficients are scanned, regardless of whether or not they have been subject to zero-out. For example, in block <NUM>, video encoder <NUM> and video decoder <NUM> would scan both the non zero-out region <NUM> and the zero-out region <NUM>. Likewise, for block <NUM>, video encoder <NUM> and video decoder <NUM> would scan both non zero-out region <NUM> and zero-out region <NUM>. Even if the scan starts from the position of the last non-zero transform coefficient, which would be in a non zero-out region, the scans shown for blocks <NUM> and <NUM> would still proceed into zero-out regions for many cases. In HEVC and VVC, for transform coefficient coding, the term "last" means the last non-zero coefficient present along the scan path going from DC coefficient (or lowest frequency coefficient) to the higher frequency coefficients. The term "first" means the DC or the lowest frequency coefficient within a coefficient group (scan within a CG) or transform unit (CG scan within a TU). Coefficient groups may be of size 4x4 or 2x2.

According to the techniques of this disclosure, in a new coefficient group scan, the scan region is only kept in the non zero-out region, as shown in blocks <NUM>, <NUM>, <NUM>, and <NUM>. Block <NUM> is a 64x64 transform unit with a scan order that proceeds to the upper right. Block <NUM> includes a 32x32 non zero-out region <NUM>. The remaining area of block <NUM> is the zero-out region <NUM>. As described above, all transform coefficients in the zero-out regions are set to have a value of <NUM>.

Block <NUM> is a 64x64 transform unit with a scan order that proceeds to the lower left. Block <NUM> includes a 32x32 non zero-out region <NUM>. The remaining area of block <NUM> is the zero-out region <NUM>.

Block <NUM> is a 32x64 transform unit with a scan order that proceeds to the upper right. Block <NUM> includes a 32x32 non zero-out region <NUM>. The remaining area of block <NUM> is the zero-out region <NUM>.

Block <NUM> is a 32x64 transform unit with a scan order that proceeds to the lower left. Block <NUM> includes a 32x32 non zero-out region <NUM>. The remaining area of block <NUM> is the zero-out region <NUM>.

As can be seen from blocks <NUM>, <NUM>, <NUM>, and <NUM> of <FIG>, regardless of the dimensions of the transform units and the scan direction, video encoder <NUM> and video decoder <NUM> are configured to only perform a scan within the non zero-out regions (e.g., regions <NUM>, <NUM>, <NUM>, and <NUM>) of each respective transform unit.

That is, video encoder <NUM> and video decoder <NUM> may be configured to determine which transform coefficients in a block/CU/TU have been subjected to the zero-out process. If a transform coefficient has been subject to the zero-out process, video encoder <NUM> and video decoder <NUM> will not scan the transform coefficient in the zero-out regions. If a transform coefficient has not been subject to the zero-out process (e.g., is in a non zero-out region), video encoder <NUM> and video decoder <NUM> do scan that transform coefficient. This scan method eliminates scanning of zero-out coefficients. In some examples, the scan within a CG is not altered.

Accordingly, in one example of the disclosure, video encoder <NUM> and video decoder <NUM> are configured to determine a first region of a transform unit (zero-out region) in which transform coefficients are subject to zero-out. Video encoder <NUM> and video decoder <NUM> may be further configured to determine a second region of the transform unit (non zero-out region) in which transform coefficients are not subject to zero-out. Video encoder <NUM> and video decoder <NUM> may then scan only the second region of the transform unit (non zero-out region).

For coding of a last non-zero coefficient position in the X and Y directions (e.g., an (X,Y) coordinate in the transform unit), VVC employs a similar scheme to HEVC, except that VVC extends the scheme to larger transform units. In HEVC, the largest transform unit is 32x32. The last non-zero coefficient position may be coded as a truncated unary coded prefix code (e.g., CABAC regular mode) followed by a fixed length coded suffix code (bypass coded).

For coding of the last non-zero coefficient position of zero-out transform units (e.g., larger transform units subject to zero-out), video encoder <NUM> and video decoder <NUM> may be configured to set the maximum transform unit dimension to a zero-out threshold (e.g., <NUM>) instead of <NUM>, thus shortening the prefix code length for last coefficient position to the range <NUM>-<NUM>. Basically, the coding of the last non-zero coefficient position in transform units of dimension larger than <NUM> (e.g., one example zero-out threshold) would follow the rule for coding of the last coefficient position in transform units of dimension <NUM> (e.g., 32x32) instead of <NUM> (e.g., 64x64). Of course, the threshold for apply zero-out to transform coefficients may be variable and may take on values other than <NUM>.

As such, in accordance with the techniques of this disclosure, video encoder <NUM> and video decoder <NUM> may be configured to encode/decode a position of a last non-zero coefficient in the transform unit based on a threshold for performing a zero-out process. In one example, to encode/decode the position of the last non-zero coefficient in the transform unit based on the threshold for performing the zero-out process, video encoder <NUM> and video decoder <NUM> may be configured to encode/decode a prefix code that represents a first portion of the position of the last non-zero coefficient based on the threshold for performing the zero-out process, and encode/decode a suffix code that represents a second portion of the position of the last non-zero coefficient.

In another example for coding the last non-zero coefficient position of a zeroed out transform unit that is maxed out at a specified dimension (e.g., at <NUM> width or height), video encoder <NUM> and video decoder <NUM> may determine the entropy coding context used for coding the last non-zero coefficient position as the context for coding the last non-zero coefficient position for <NUM> dimension blocks (e.g., 64x64). That is, video encoder <NUM> and video decoder <NUM> may determine the context used for coding the position of the last non-zero coefficient based on the size of the transform unit, rather than the size of the non zero-out region. Even though the position of the last non-zero transform coefficient is guaranteed to be in the non zero-out region (e.g., the upper-left 32x32 region of a 64x64 transform unit), the context for coding the position of the last non-zero transform coefficient is more closely related to statistics of a 64x64 transform unit, not a 32x32 transform unit. Thus, coding efficiency of the position of the last non-zero transform coefficient may be increased. Accordingly, in one example of the disclosure, video encoder <NUM> and video decoder <NUM> may be configured to determine a context for entropy coding the position of a last non-zero coefficient based on an entire size of the transform unit.

The context for the last non-zero coefficient position for a regular (non zero-out) 32x32 block and the context for the last non-zero coefficient position of a 32x32 region of a zeroed out 64x64 block may still be separate context sets in that instance and similarly for other greater than <NUM> dimension transform blocks (e.g., transform blocks with a height or width of <NUM>). In other examples, the context for coding of last non-zero coefficient position can be shared between the max dimension <NUM> blocks (e.g., transform blocks with a height or width at a maximum of <NUM>) and max dimension <NUM> blocks (e.g., transform blocks with a height or width at a maximum of <NUM>). For example, the context for the last non-zero coefficient position for a regular (non zero-out) 32x32 block and the context for the last non-zero coefficient position of a 32x32 region of a zeroed out 64x64 block may share the same context set.

In another example, as another alternative to scanning only non zero-out regions, the large scans can be kept as they are, but video encoder <NUM> and video decoder <NUM> may be configured to enforce that the encoded bitstream has only <NUM> coefficient levels in the zeroed out region of the large inverse transforms. In this case the last non-zero coefficient position would be enforced to be in the upper left non zero-out region and still be limited to max non-zero size of <NUM>, as described above.

One or more techniques above can be extended to any of NxM, MxN, and NxN transform sizes, where N is the transform dimension where zero-out is applied from N/<NUM> to N-<NUM> for x dimension (i.e., horizontal dimension) ory dimension (i.e., vertical dimension), or both x and y dimensions, respectively. Only the coefficients in the non zero-out region would be scanned, and the last non-zero coefficient position coding would be for the size of the non zero-out region. However, as described above, the contexts used would be the contexts for the full size transform unit. For example, if zero-out is applied for 32x32 transforms, only the lower frequency 16x16 region (e.g., the upper left 16x16 region of the 32x32 transform unit) would be kept, and any coefficient outside of that region would be zeroed out. Video encoder <NUM> and video decoder <NUM> would only scan the 16x16 non zero-out region and the last non-zero coefficient position coding would use binarization for a 16x16 transform unit. However, video encoder <NUM> and video decoder <NUM> would determine CABAC contexts for the position of the last non-zero coefficient based on the 32x32 transform unit example.

Accordingly, in other examples of the disclosure, to determine the first region of the transform unit in which transform coefficients are subject to zero-out, video encoder <NUM> and video decoder <NUM> may be configured to determine the first region of the transform unit in which transform coefficients are subject to zero-out based on positions of the transform coefficients in the transform unit.

In another example of the disclosure, to determine the first region of the transform unit in which transform coefficients are subject to zero-out based on positions of the transform coefficients in the transform unit, video encoder <NUM> and video decoder <NUM> may be configured to determine that the first region includes transform coefficients in the transform unit that are beyond the first <NUM> rows or columns of the transform unit are subject to zero-out.

In another example of the disclosure, to scan only the second region (non zero-out region) of the transform unit, video encoder <NUM> and video decoder <NUM> may be configured to scan only the second region of the transform unit starting at the position of the last non-zero coefficient.

In another example of the disclosure, to scan only the second region of the transform unit, video encoder <NUM> and video decoder <NUM> may be configured to scan only the second region of the transform unit coefficient group by coefficient group.

<FIG> is a flowchart illustrating an example method for encoding a current block. The current block may include a current CU. Although described with respect to video encoder <NUM> (<FIG> and <FIG>), it should be understood that other devices may be configured to perform a method similar to that of <FIG>.

In this example, video encoder <NUM> initially predicts the current block (<NUM>). For example, video encoder <NUM> may form a prediction block for the current block. Video encoder <NUM> may then calculate a residual block for the current block (<NUM>). To calculate the residual block, video encoder <NUM> may calculate a difference between the original, uncoded block and the prediction block for the current block. Video encoder <NUM> may then transform and quantize coefficients of the residual block (<NUM>). Next, video encoder <NUM> may scan the quantized transform coefficients of the residual block (<NUM>).

In accordance with the techniques of this disclosure, to scan the transform coefficients, video encoder <NUM> may be configured to determine a first region of a transform unit in which transform coefficients are subject to zero-out (<NUM>), determine a second region of the transform unit in which transform coefficients are not subject to zero-out (<NUM>), and scan only the second region of the transform unit (<NUM>). During the scan, or following the scan, video encoder <NUM> may entropy encode the coefficients (<NUM>). Video encoder <NUM> may also entropy encode prediction information. In accordance with the techniques of this disclosure, video encoder <NUM> may be configured to determine a context for entropy encoding the position of a last non-zero coefficient based on an entire size of the transform unit. Video encoder <NUM> may encode the coefficients using CAVLC or CABAC. Video encoder <NUM> may then output the entropy coded data of the current block (<NUM>). For example, video encoder <NUM> may output entropy coded data for transform coefficients of a residual block corresponding to the current block.

<FIG> is a flowchart illustrating an example method for decoding a current block of video data. The current block may include a current CU. Although described with respect to video decoder <NUM> (<FIG> and <FIG>), it should be understood that other devices may be configured to perform a method similar to that of <FIG>.

Video decoder <NUM> may receive entropy coded data for the current block, such as entropy coded prediction information and entropy coded data for coefficients of a residual block corresponding to the current block (<NUM>). Video decoder <NUM> may entropy decode the entropy coded data to determine prediction information for the current block and to reproduce coefficients of the residual block (<NUM>). In accordance with the techniques of this disclosure, to reproduce the transform coefficients, video decoder <NUM> may be configured to determine a first region of a transform unit in which transform coefficients are subject to zero-out (<NUM>), determine a second region of the transform unit in which transform coefficients are not subject to zero-out (<NUM>), and scan only the second region of the transform unit (<NUM>). In accordance with the techniques of this disclosure, video decoder <NUM> may be further configured to determine a context for entropy decoding one or more syntax elements indicating a position of a last non-zero coefficient based on an entire size of the transform unit.

Video decoder <NUM> may predict the current block (<NUM>), e.g., using an intra- or inter-prediction mode as indicated by the prediction information for the current block, to calculate a prediction block for the current block. Video decoder <NUM> may then inverse scan the reproduced coefficients (<NUM>), to create a block of quantized transform coefficients. Video decoder <NUM> may then inverse quantize and inverse transform the coefficients to produce a residual block (<NUM>). Video decoder <NUM> may ultimately decode the current block by combining the prediction block and the residual block (<NUM>).

By way of example, and not limitation, such computer-readable storage media can include one or more of 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. Disk and disc, as used herein, include compact discs (CDs), laser discs, optical discs, digital versatile discs (DVDs), floppy disks and Blu-ray discs, where disks usually reproduce data magnetically, while discs reproduce data optically with lasers.

Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term "processor," as used herein may refer to any of the foregoing structures or any other structure suitable for implementation of the techniques described herein.

Claim 1:
A method of decoding video data, the method comprising:
decoding a prefix code that represents a first portion of the position of the last non-zero coefficient in an X direction based on a threshold for performing the zero-out process, wherein the threshold is <NUM>;
decoding a suffix code that represents a second portion of the position of the last non-zero coefficient in the X direction;
decoding a prefix code that represents a first portion of the position of the last non-zero coefficient in an Y direction based on said the threshold for performing the zero-out process;
decoding a suffix code that represents a second portion of the position of the last non-zero coefficient in the Y direction;
determining (<NUM>) a first region of a transform unit in which transform coefficients are subject to zero-out based on the positions of the last non-zero transform coefficients in the X direction and in the Y direction in the transform unit, wherein a maximum dimension of the transform unit is <NUM>;
determining (<NUM>) a second region of the transform unit in which transform coefficients are not subject to zero-out; and
scanning (<NUM>) only the second region of the transform unit starting at the position of the last non-zero coefficient.