Patent Description:
The amount of video data needed to depict even a relatively short video can be substantial, which may result in difficulties when the data is to be streamed or otherwise communicated across a communications network with limited bandwidth capacity. Thus, video data is generally compressed before being communicated across modem day telecommunications networks. The size of a video could also be an issue when the video is stored on a storage device because memory resources may be limited Video compression devices often use software and/or hardware at the source to code the video data prior to transmission or storage, thereby decreasing the quantity of data needed to represent digital video images. The compressed data is then received at the destination by a video decompression device that decodes the video data. With limited network resources and ever increasing demands of higher video quality, improved compression and decompression techniques that improve compression ratio with little to no sacrifice in image quality are desirable. <NPL>, teach a technique of inter prediction referencing cross random access points wherein library pictures are employed that can be used by more than one random access segment.

A first aspect relates to a method of decoding implemented by a video decoder. The method includes receiving, by the video decoder, a bitstream containing an external decoder refresh (EDR) picture and a list of pictures for the EDR picture, wherein the list of pictures lists in increasing decoding order pictures referred to by entries in a first reference picture list, pictures referred to by entries in a second reference picture list, and external pictures, wherein the external pictures are pictures that were decoded prior to decoding the EDR picture and wherein a difference between picture order count values for any two consecutive pictures in the list of pictures is greater than one half of a negative of maximum picture order count least significant bits and less than one half of the maximum picture order count least significant bits; obtaining, by the video decoder, one of the external pictures referred to in the list of pictures; and decoding, by the video decoder, the EDR picture using the external reference picture that was obtained.

The method provides techniques that restrict a difference between the picture order count values for any two consecutive pictures in a list of pictures to a limited range when providing random access using an external decoder refresh (EDR) picture. By restricting the difference between picture order count values for any two consecutive pictures in a list of pictures to the limited range, the number of bits needed for signaling the picture order count values is reduced, which improves coding efficiency. Thus, usage of the processor, memory, and/or network resources may be reduced at both the encoder and the decoder. Thus, the coder / decoder (a. , "codec") in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed.

Optionally, in any of the preceding aspects, another implementation of the aspect provides that the one of the external pictures is a reference EDR picture.

Optionally, in any of the preceding aspects, another implementation of the aspect provides the one of the external pictures is a reference intra random access point (IRAP) picture.

Optionally, in any of the preceding aspects, another implementation of the aspect provides that the one of the external pictures is obtained from a second bitstream.

Optionally, in any of the preceding aspects, another implementation of the aspect provides that the picture order count values are picture order count (POC) least significant bits (LSBs).

Optionally, in any of the preceding aspects, another implementation of the aspect provides the list of pictures containing the picture order count values are signaled in a slice header of the bitstream.

A second aspect relates to a method of encoding implemented by a video encoder. The method includes generating, by the video encoder, an external decoder refresh (EDR) picture and a list of pictures for the EDR picture, wherein the list of pictures lists in increasing decoding order pictures referred to by entries in a first reference picture list, pictures referred to by entries in a second reference picture list, and external pictures in increasing decoding order, wherein the external pictures are pictures that were decoded prior to decoding the EDR picture and wherein a difference between picture order count values for any two consecutive pictures in the list of pictures is greater than one half of a negative of maximum picture order count least significant bits and less than one half of the maximum picture order count least significant bits; encoding, by the video encoder, the EDR picture and the list of pictures for the EDR picture into a bitstream; and storing, by the video encoder, the bitstream for transmission toward a video decoder.

Optionally, in any of the preceding aspects, another implementation of the aspect provides that the one of the external pictures is a reference intra random access point (IRAP) picture.

Optionally, in any of the preceding aspects, another implementation of the aspect provides encoding the list of pictures containing the picture order count values in a slice header of the bitstream.

A third aspect relates to a decoding device. The decoding device includes a receiver configured to receive a coded video bitstream; a memory coupled to the receiver, the memory storing instructions; and a processor coupled to the memory, the processor configured to execute the instructions to cause the decoding device to: receive a bitstream containing an external decoder refresh (EDR) picture and a list of pictures for the EDR picture, wherein the list of pictures lists pictures referred to by entries in a first reference picture list, the pictures referred to by the entries in a second reference picture list, and external pictures in increasing decoding order, and wherein a difference between picture order count values for any two consecutive pictures in the list of pictures is greater than one half of a negative of maximum picture order count least significant bits and less than one half of the maximum picture order count least significant bits; obtain one of the external pictures referred to in the list of pictures; and decode the EDR picture using the external reference picture that was obtained.

The decoding device provides techniques that restrict a difference between the picture order count values for any two consecutive pictures in a list of pictures to a limited range when providing random access using an external decoder refresh (EDR) picture. By restricting the difference between picture order count values for any two consecutive pictures in a list of pictures to the limited range, the number of bits needed for signaling the picture order count values is reduced, which improves coding efficiency. Thus, usage of the processor, memory, and/or network resources may be reduced at both the encoder and the decoder. Thus, the coder / decoder (a. , "codec") in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed.

Optionally, in any of the preceding aspects, another implementation of the aspect provides that one of the external pictures is a reference EDR picture or a reference intra random access point (IRAP) picture.

Optionally, in any of the preceding aspects, another implementation of the aspect provides the picture order count values are picture order count (POC) least significant bits (LSBs).

Optionally, in any of the preceding aspects, another implementation of the aspect provides a display configured to display an image as generated based on the EDR picture.

A fourth aspect relates to an encoding device. The encoding device includes a memory containing instructions; a processor coupled to the memory, the processor configured to implement the instructions to cause the encoding device to: generate an external decoder refresh (EDR) picture and a list of pictures for the EDR picture, wherein the list of pictures lists pictures referred to by entries in a first reference picture list, the pictures referred to by the entries in a second reference picture list, and external pictures in increasing decoding order, and wherein a difference between picture order count values for any two consecutive pictures in the list of pictures is greater than one half of a negative of maximum picture order count least significant bits and less than one half of the maximum picture order count least significant bits; and encode the EDR picture and the list of pictures for the EDR picture into a bitstream; and a transmitter coupled to the processor, the transmitter configured to transmit the bitstream toward a video decoder.

The encoding device provides techniques that restrict a difference between the picture order count values for any two consecutive pictures in a list of pictures to a limited range when providing random access using an external decoder refresh (EDR) picture. By restricting the difference between picture order count values for any two consecutive pictures in a list of pictures to the limited range, the number of bits needed for signaling the picture order count values is reduced, which improves coding efficiency. Thus, usage of the processor, memory, and/or network resources may be reduced at both the encoder and the decoder. Thus, the coder / decoder (a. , "codec") in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed.

Optionally, in any of the preceding aspects, another implementation of the aspect provides that the one of the external pictures is a reference EDR picture or a reference intra random access point (IRAP) picture.

Optionally, in any of the preceding aspects, another implementation of the aspect provides that the memory stores the bitstream prior to the transmitter transmitting the bitstream toward the video decoder.

A seventh aspect relates to a means for coding. The means for coding comprises receiving means configured to receive a picture to encode or to receive a bitstream to decode; transmission means coupled to the receiving means, the transmission means configured to transmit the bitstream to a decoding means or to transmit a decoded image to a display means; storage means coupled to at least one of the receiving means or the transmission means, the storage means configured to store instructions; and processing means coupled to the storage means, the processing means configured to execute the instructions stored in the storage means to perform any of the methods disclosed herein.

The means for coding provides techniques that restrict a difference between the picture order count values for any two consecutive pictures in a list of pictures to a limited range when providing random access using an external decoder refresh (EDR) picture. By restricting the difference between picture order count values for any two consecutive pictures in a list of pictures to the limited range, the number of bits needed for signaling the picture order count values is reduced, which improves coding efficiency. Thus, usage of the processor, memory, and/or network resources may be reduced at both the encoder and the decoder. Thus, the coder / decoder (a. , "codec") in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed.

The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the exemplary designs and implementations illustrated and described herein.

The following terms are defined as follows unless used in a contrary context herein. Specifically, the following definitions are intended to provide additional clarity to the present disclosure. However, terms may be described differently in different contexts. Accordingly, the following definitions should be considered as a supplement and should not be considered to limit any other definitions of descriptions provided for such terms herein.

A bitstream is a sequence of bits including video data that is compressed for transmission between an encoder and a decoder. An encoder is a device that is configured to employ encoding processes to compress video data into a bitstream. A decoder is a device that is configured to employ decoding processes to reconstruct video data from a bitstream for display. A picture is an array of luma samples and/or an array of chroma samples that create a frame or a field thereof. A picture that is being encoded or decoded can be referred to as a current picture for clarity of discussion. A reference picture is a picture that contains reference samples that can be used when coding other pictures by reference according to inter-prediction and/or inter-layer prediction. A reference picture list is a list of reference pictures used for inter-prediction and/or inter-layer prediction Some video coding systems utilize two reference picture lists, which can be denoted as reference picture list one and reference picture list zero. A reference picture list structure is an addressable syntax structure that contains multiple reference picture lists. Inter-prediction is a mechanism of coding samples of a current picture by reference to indicated samples in a reference picture that is different from the current picture where the reference picture and the current picture are in the same layer. A reference picture list structure entry is an addressable location in a reference picture list structure that indicates a reference picture associated with a reference picture list. A slice header is a part of a coded slice containing data elements pertaining to all video data within a file represented in the slice. A sequence parameter set (SPS) is a parameter set that contains data related to a sequence of pictures. An access unit (AU) is a set of one or more coded pictures associated with the same display time (e.g., the same picture order count) for output from a decoded picture buffer (DPB) (e.g., for display to a user). A decoded video sequence is a sequence of pictures that have been reconstructed by a decoder in preparation for display to a user. An intra random access point (IRAP) picture provides a point in a bitstream where decoding can begin. For example, decoding can begin at an IRAP picture so that pictures following the IRAP picture in output order, inclusive, can be output even if all pictures that precede the IRAP picture in decoding order are discarded from the bitstream (e.g., due to bitstream splicing, or the like). Because it is possible to start decoding at an IRAP picture, an IRAP picture is not dependent on any other picture in the bitstream. Decoding order is the order in which pictures are decoded. List of pictures is a listing of pictures. An external picture, also known as an earlier picture, is a picture that precedes an IRAP picture in decoding order. Picture order count (POC) determines the display (output) order of decoded frames or pictures. That is, a POC comprises a variable that is associated with each picture, uniquely identifies the associated picture among all pictures in the coded layer video sequence (CLVS), and indicates when the associated picture is to be output from the DPB. The POC least significant bits (LSBs) are the lowest bits in the picture order count variable, and the POC most significant bits (MSBs) are the highest bits in the picture order count variable. The maximum POC is greatest value that the variable for the POC can have. The maximum POC LBS are the lowest bits of the maximum POC.

External decoding refresh (EDR), also referred to as cross RAP reference (CRR), allows random access point pictures to be inter coded instead of intra coded. The basic idea of the EDR approach is as follows. Instead of coding random access points (except for the very first one in the bitstream) as intra-coded IRAP pictures, they are coded using inter prediction to circumvent the unavailability of the earlier pictures than an IRAP picture. Such pictures are referred to as EDR pictures. The trick is to provide a limited number of the earlier pictures, typically representing different scenes of the video content, through a separate video bitstream, which can be referred to as an external means. Such earlier pictures are referred to as the external pictures. Consequently, each external picture can be used for inter prediction referencing by pictures across the random access points. The coding efficiency gain basically comes from inter coding of the random access points. When a picture uses an EDR picture for reference, that EDR picture may be referred to as a reference EDR picture.

The following acronyms are used herein, Adaptive Loop Filter (ALF), Coding Tree Block (CTB), Coding Tree Unit (CTU), Coding Unit (CU), Coded Video Sequence (CVS), decoded picture buffer (DPB), External Decoding Refresh (EDR), Group Of Pictures (GOP), Joint Video Experts Team (JVET), Motion-Constrained Tile Set (MCTS), Most Significant Bit(s) (MSB), Maximum Transfer Unit (MTU), Network Abstration Layer (NAL), Picture Order Count (POC), Random Access Point (RAP), Raw Byte Sequence Payload (RBSP), Sample Adaptive Offset (SAO), Supplemental Enhancement Information (SEI), Sequence Parameter Set (SPS), Temporal Motion Vector Prediction (TMVP), Versatile Video Coding (WC), and Working Draft (WD).

<FIG> is a flowchart of an example operating method <NUM> of coding a video signal. Specifically, a video signal is encoded at an encoder. The encoding process compresses the video signal by employing various mechanisms to reduce the video file size. A smaller file size allows the compressed video file to be transmitted toward a user, while reducing associated bandwidth overhead. The decoder then decodes the compressed video file to reconstruct the original video signal for display to an end user. The decoding process generally mirrors the encoding process to allow the decoder to consistently reconstruct the video signal.

At step <NUM>, the video signal is input into the encoder. For example, the video signal may be an uncompressed video file stored in memory. As another example, the video file may be captured by a video capture device, such as a video camera, and encoded to support live streaming of the video. The video file may include both an audio component and a video component. The video component contains a series of image frames that, when viewed in a sequence, gives the visual impression of motion. The frames contain pixels that are expressed in terms of light, referred to herein as luma components (or luma samples), and color, which is referred to as chroma components (or color samples). In some examples, the frames may also contain depth values to support three dimensional viewing.

At step <NUM>, the video is partitioned into blocks. Partitioning includes subdividing the pixels in each frame into square and/or rectangular blocks for compression. For example, in High Efficiency Video Coding (HEVC) (also known as H. <NUM> and MPEG-H Part <NUM>) the frame can first be divided into coding tree units (CTUs), which are blocks of a predefined size (e.g., sixty-four pixels by sixty-four pixels). The CTUs contain both luma and chroma samples. Coding trees may be employed to divide the CTUs into blocks and then recursively subdivide the blocks until configurations are achieved that support further encoding. For example, luma components of a frame may be subdivided until the individual blocks contain relatively homogenous lighting values. Further, chroma components of a frame may be subdivided until the individual blocks contain relatively homogenous color values. Accordingly, partitioning mechanisms vary depending on the content of the video frames.

At step <NUM>, various compression mechanisms are employed to compress the image blocks partitioned at step <NUM>. For example, inter-prediction and/or intra-prediction may be employed. Inter-prediction is designed to take advantage of the fact that objects in a common scene tend to appear in successive frames. Accordingly, a block depicting an object in a reference frame need not be repeatedly described in adjacent frames. Specifically, an object, such as a table, may remain in a constant position over multiple frames. Hence the table is described once and adjacent frames can refer back to the reference frame. Pattern matching mechanisms may be employed to match objects over multiple frames. Further, moving objects may be represented across multiple frames, for example due to object movement or camera movement. As a particular example, a video may show an automobile that moves across the screen over multiple frames. Motion vectors can be employed to describe such movement. A motion vector is a two-dimensional vector that provides an offset from the coordinates of an object in a frame to the coordinates of the object in a reference frame. As such, inter-prediction can encode an image block in a current frame as a set of motion vectors indicating an offset from a corresponding block in a reference frame.

Intra-prediction encodes blocks in a common frame. Intra-prediction takes advantage of the fact that luma and chroma components tend to cluster in a frame. For example, a patch of green in a portion of a tree tends to be positioned adjacent to similar patches of green. Intra-prediction employs multiple directional prediction modes (e.g., thirty-three in HEVC), a planar mode, and a direct current (DC) mode. The directional modes indicate that a current block is similar/the same as samples of a neighbor block in a corresponding direction. Planar mode indicates that a series of blocks along a row/column (e.g., a plane) can be interpolated based on neighbor blocks at the edges of the row. Planar mode, in effect, indicates a smooth transition of light/color across a row/column by employing a relatively constant slope in changing values. DC mode is employed for boundary smoothing and indicates that a block is similar/the same as an average value associated with samples of all the neighbor blocks associated with the angular directions of the directional prediction modes. Accordingly, intra-prediction blocks can represent image blocks as various relational prediction mode values instead of the actual values. Further, inter-prediction blocks can represent image blocks as motion vector values instead of the actual values. In either case, the prediction blocks may not exactly represent the image blocks in some cases. Any differences are stored in residual blocks. Transforms may be applied to the residual blocks to further compress the file.

At step <NUM>, various filtering techniques may be applied. In HEVC, the filters are applied according to an in-loop filtering scheme. The block based prediction discussed above may result in the creation of blocky images at the decoder. Further, the block based prediction scheme may encode a block and then reconstruct the encoded block for later use as a reference block. The in-loop filtering scheme iteratively applies noise suppression filters, de-blocking filters, adaptive loop filters, and sample adaptive offset (SAO) filters to the blocks/frames. These filters mitigate such blocking artifacts so that the encoded file can be accurately reconstructed. Further, these filters mitigate artifacts in the reconstructed reference blocks so that artifacts are less likely to create additional artifacts in subsequent blocks that are encoded based on the reconstructed reference blocks.

Once the video signal has been partitioned, compressed, and filtered, the resulting data is encoded in a bitstream at step <NUM>. The bitstream includes the data discussed above as well as any signaling data desired to support proper video signal reconstruction at the decoder. For example, such data may include partition data, prediction data, residual blocks, and various flags providing coding instructions to the decoder. The bitstream may be stored in memory for transmission toward a decoder upon request. The bitstream may also be broadcast and/or multicast toward a plurality of decoders. The creation of the bitstream is an iterative process. Accordingly, steps <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> may occur continuously and/or simultaneously over many frames and blocks. The order shown in <FIG> is presented for clarity and ease of discussion, and is not intended to limit the video coding process to a particular order.

The decoder receives the bitstream and begins the decoding process at step <NUM>. Specifically, the decoder employs an entropy decoding scheme to convert the bitstream into corresponding syntax and video data. The decoder employs the syntax data from the bitstream to determine the partitions for the frames at step <NUM>. The partitioning should match the results of block partitioning at step <NUM>. Entropy encoding/decoding as employed in step <NUM> is now described. The encoder makes many choices during the compression process, such as selecting block partitioning schemes from several possible choices based on the spatial positioning of values in the input image(s). Signaling the exact choices may employ a large number of bins. As used herein, a bin is a binary value that is treated as a variable (e.g., a bit value that may vary depending on context). Entropy coding allows the encoder to discard any options that are clearly not viable for a particular case, leaving a set of allowable options. Each allowable option is then assigned a code word. The length of the code words is based on the number of allowable options (e.g., one bin for two options, two bins for three to four options, etc.) The encoder then encodes the code word for the selected option. This scheme reduces the size of the code words as the code words are as big as desired to uniquely indicate a selection from a small sub-set of allowable options as opposed to uniquely indicating the selection from a potentially large set of all possible options. The decoder then decodes the selection by determining the set of allowable options in a similar manner to the encoder. By determining the set of allowable options, the decoder can read the code word and determine the selection made by the encoder.

At step <NUM>, the decoder performs block decoding. Specifically, the decoder employs reverse transforms to generate residual blocks. Then the decoder employs the residual blocks and corresponding prediction blocks to reconstruct the image blocks according to the partitioning. The prediction blocks may include both intra-prediction blocks and inter-prediction blocks as generated at the encoder at step <NUM>. The reconstructed image blocks are then positioned into frames of a reconstructed video signal according to the partitioning data determined at step <NUM>. Syntax for step <NUM> may also be signaled in the bitstream via entropy coding as discussed above.

At step <NUM>, filtering is performed on the frames of the reconstructed video signal in a manner similar to step <NUM> at the encoder. For example, noise suppression filters, de-blocking filters, adaptive loop filters, and SAO filters may be applied to the frames to remove blocking artifacts. Once the frames are filtered, the video signal can be output to a display at step <NUM> for viewing by an end user.

<FIG> is a schematic diagram of an example coding and decoding (codec) system <NUM> for video coding. Specifically, codec system <NUM> provides functionality to support the implementation of operating method <NUM>. Codec system <NUM> is generalized to depict components employed in both an encoder and a decoder. Codec system <NUM> receives and partitions a video signal as discussed with respect to steps <NUM> and <NUM> in operating method <NUM>, which results in a partitioned video signal <NUM>. Codec system <NUM> then compresses the partitioned video signal <NUM> into a coded bitstream when acting as an encoder as discussed with respect to steps <NUM>, <NUM>, and <NUM> in method <NUM>. When acting as a decoder, codec system <NUM> generates an output video signal from the bitstream as discussed with respect to steps <NUM>, <NUM>, <NUM>, and <NUM> in operating method <NUM>. The codec system <NUM> includes a general coder control component <NUM>, a transform scaling and quantization component <NUM>, an intra-picture estimation component <NUM>, an intra-picture prediction component <NUM>, a motion compensation component <NUM>, a motion estimation component <NUM>, a scaling and inverse transform component <NUM>, a filter control analysis component <NUM>, an in-loop filters component <NUM>, a decoded picture buffer component <NUM>, and a header formatting and context adaptive binary arithmetic coding (CABAC) component <NUM>. Such components are coupled as shown. In <FIG>, black lines indicate movement of data to be encoded/decoded while dashed lines indicate movement of control data that controls the operation of other components. The components of codec system <NUM> may all be present in the encoder. The decoder may include a subset of the components of codec system <NUM>. For example, the decoder may include the intra-picture prediction component <NUM>, the motion compensation component <NUM>, the scaling and inverse transform component <NUM>, the in-loop filters component <NUM>, and the decoded picture buffer component <NUM>. These components are now described.

The partitioned video signal <NUM> is a captured video sequence that has been partitioned into blocks of pixels by a coding tree. A coding tree employs various split modes to subdivide a block of pixels into smaller blocks of pixels. These blocks can then be further subdivided into smaller blocks. The blocks may be referred to as nodes on the coding tree. Larger parent nodes are split into smaller child nodes. The number of times a node is subdivided is referred to as the depth of the node/coding tree. The divided blocks can be included in coding units (CUs) in some cases. For example, a CU can be a sub-portion of a CTU that contains a luma block, red difference chroma (Cr) block(s), and a blue difference chroma (Cb) block(s) along with corresponding syntax instructions for the CU. The split modes may include a binary tree (BT), triple tree (TT), and a quad tree (QT) employed to partition a node into two, three, or four child nodes, respectively, of varying shapes depending on the split modes employed. The partitioned video signal <NUM> is forwarded to the general coder control component <NUM>, the transform scaling and quantization component <NUM>, the intra-picture estimation component <NUM>, the filter control analysis component <NUM>, and the motion estimation component <NUM> for compression.

The general coder control component <NUM> is configured to make decisions related to coding of the images of the video sequence into the bitstream according to application constraints. For example, the general coder control component <NUM> manages optimization of bitrate/bitstream size versus reconstruction quality. Such decisions may be made based on storage space/bandwidth availability and image resolution requests. The general coder control component <NUM> also manages buffer utilization in light of transmission speed to mitigate buffer underrun and overrun issues. To manage these issues, the general coder control component <NUM> manages partitioning, prediction, and filtering by the other components. For example, the general coder control component <NUM> may dynamically increase compression complexity to increase resolution and increase bandwidth usage or decrease compression complexity to decrease resolution and bandwidth usage. Hence, the general coder control component <NUM> controls the other components of codec system <NUM> to balance video signal reconstruction quality with bit rate concerns. The general coder control component <NUM> creates control data, which controls the operation of the other components. The control data is also forwarded to the header formatting and CABAC component <NUM> to be encoded in the bitstream to signal parameters for decoding at the decoder.

The partitioned video signal <NUM> is also sent to the motion estimation component <NUM> and the motion compensation component <NUM> for inter-prediction. A frame or slice of the partitioned video signal <NUM> may be divided into multiple video blocks. Motion estimation component <NUM> and the motion compensation component <NUM> perform inter-predictive coding of the received video block relative to one or more blocks in one or more reference frames to provide temporal prediction. Codec system <NUM> may perform multiple coding passes, e.g., to select an appropriate coding mode for each block of video data.

Motion estimation component <NUM> and motion compensation component <NUM> may be highly integrated, but are illustrated separately for conceptual purposes. Motion estimation, performed by motion estimation component <NUM>, is the process of generating motion vectors, which estimate motion for video blocks. A motion vector, for example, may indicate the displacement of a coded object relative to a predictive block. A predictive block is a block that is found to closely match the block to be coded, in terms of pixel difference. A predictive block may also be referred to as a reference block. Such pixel difference may be determined by sum of absolute difference (SAD), sum of square difference (SSD), or other difference metrics. HEVC employs several coded objects including a CTU, coding tree blocks (CTBs), and CUs. For example, a CTU can be divided into CTBs, which can then be divided into CBs for inclusion in CUs. A CU can be encoded as a prediction unit (PU) containing prediction data and/or a transform unit (TU) containing transformed residual data for the CU. The motion estimation component <NUM> generates motion vectors, PUs, and TUs by using a rate-distortion analysis as part of a rate distortion optimization process. For example, the motion estimation component <NUM> may determine multiple reference blocks, multiple motion vectors, etc. for a current block/frame, and may select the reference blocks, motion vectors, etc. having the best rate-distortion characteristics. The best rate-distortion characteristics balance both quality of video reconstruction (e.g., amount of data loss by compression) with coding efficiency (e.g., size of the final encoding).

In some examples, codec system <NUM> may calculate values for sub-integer pixel positions of reference pictures stored in decoded picture buffer component <NUM>. For example, video codec system <NUM> may interpolate values of one-quarter pixel positions, one-eighth pixel positions, or other fractional pixel positions of the reference picture. Therefore, motion estimation component <NUM> may perform a motion search relative to the full pixel positions and fractional pixel positions and output a motion vector with fractional pixel precision. The motion estimation component <NUM> calculates a motion vector for a PU of a video block in an inter-coded slice by comparing the position of the PU to the position of a predictive block of a reference picture. Motion estimation component <NUM> outputs the calculated motion vector as motion data to header formatting and CABAC component <NUM> for encoding and motion to the motion compensation component <NUM>.

Motion compensation, performed by motion compensation component <NUM>, may involve fetching or generating the predictive block based on the motion vector determined by motion estimation component <NUM>. Again, motion estimation component <NUM> and motion compensation component <NUM> may be functionally integrated, in some examples. Upon receiving the motion vector for the PU of the current video block, motion compensation component <NUM> may locate the predictive block to which the motion vector points. A residual video block is then formed by subtracting pixel values of the predictive block from the pixel values of the current video block being coded, forming pixel difference values. In general, motion estimation component <NUM> performs motion estimation relative to luma components, and motion compensation component <NUM> uses motion vectors calculated based on the luma components for both chroma components and luma components. The predictive block and residual block are forwarded to transform scaling and quantization component <NUM>.

The partitioned video signal <NUM> is also sent to intra-picture estimation component <NUM> and intra-picture prediction component <NUM>. As with motion estimation component <NUM> and motion compensation component <NUM>, intra-picture estimation component <NUM> and intra-picture prediction component <NUM> may be highly integrated, but are illustrated separately for conceptual purposes. The intra-picture estimation component <NUM> and intra-picture prediction component <NUM> intra-predict a current block relative to blocks in a current frame, as an alternative to the inter-prediction performed by motion estimation component <NUM> and motion compensation component <NUM> between frames, as described above. In particular, the intra-picture estimation component <NUM> determines an intra-prediction mode to use to encode a current block. In some examples, intrapicture estimation component <NUM> selects an appropriate intra-prediction mode to encode a current block from multiple tested intra-prediction modes. The selected intra-prediction modes are then forwarded to the header formatting and CABAC component <NUM> for encoding.

For example, the intra-picture estimation component <NUM> calculates rate-distortion values using a rate-distortion analysis for the various tested intra-prediction modes, and selects the intra-prediction mode having the best rate-distortion characteristics among the tested modes. Rate-distortion analysis generally determines an amount of distortion (or error) between an encoded block and an original unencoded block that was encoded to produce the encoded block, as well as a bitrate (e.g., a number of bits) used to produce the encoded block. The intra-picture estimation component <NUM> calculates ratios from the distortions and rates for the various encoded blocks to determine which intra-prediction mode exhibits the best rate-distortion value for the block. In addition, intra-picture estimation component <NUM> may be configured to code depth blocks of a depth map using a depth modeling mode (DMM) based on rate-distortion optimization (RDO).

The intra-picture prediction component <NUM> may generate a residual block from the predictive block based on the selected intra-prediction modes determined by intra-picture estimation component <NUM> when implemented on an encoder or read the residual block from the bitstream when implemented on a decoder. The residual block includes the difference in values between the predictive block and the original block, represented as a matrix. The residual block is then forwarded to the transform scaling and quantization component <NUM>. The intra-picture estimation component <NUM> and the intra-picture prediction component <NUM> may operate on both luma and chroma components.

The transform scaling and quantization component <NUM> is configured to further compress the residual block. The transform scaling and quantization component <NUM> applies a transform, such as a discrete cosine transform (DCT), a discrete sine transform (DST), or a conceptually similar transform, to the residual block, producing a video block comprising residual transform coefficient values. Wavelet transforms, integer transforms, sub-band transforms or other types of transforms could also be used. The transform may convert the residual information from a pixel value domain to a transform domain, such as a frequency domain. The transform scaling and quantization component <NUM> is also configured to scale the transformed residual information, for example based on frequency. Such scaling involves applying a scale factor to the residual information so that different frequency information is quantized at different granularities, which may affect final visual quality of the reconstructed video. The transform scaling and quantization component <NUM> is also configured to quantize the transform coefficients to further reduce bit rate. The quantization process may reduce the bit depth associated with some or all of the coefficients. The degree of quantization may be modified by adjusting a quantization parameter. In some examples, the transform scaling and quantization component <NUM> may then perform a scan of the matrix including the quantized transform coefficients. The quantized transform coefficients are forwarded to the header formatting and CABAC component <NUM> to be encoded in the bitstream.

The scaling and inverse transform component <NUM> applies a reverse operation of the transform scaling and quantization component <NUM> to support motion estimation. The scaling and inverse transform component <NUM> applies inverse scaling, transformation, and/or quantization to reconstruct the residual block in the pixel domain, e.g., for later use as a reference block which may become a predictive block for another current block. The motion estimation component <NUM> and/or motion compensation component <NUM> may calculate a reference block by adding the residual block back to a corresponding predictive block for use in motion estimation of a later block/frame. Filters are applied to the reconstructed reference blocks to mitigate artifacts created during scaling, quantization, and transform. Such artifacts could otherwise cause inaccurate prediction (and create additional artifacts) when subsequent blocks are predicted.

The filter control analysis component <NUM> and the in-loop filters component <NUM> apply the filters to the residual blocks and/or to reconstructed image blocks. For example, the transformed residual block from the scaling and inverse transform component <NUM> may be combined with a corresponding prediction block from intra-picture prediction component <NUM> and/or motion compensation component <NUM> to reconstruct the original image block. The filters may then be applied to the reconstructed image block. In some examples, the filters may instead be applied to the residual blocks. As with other components in <FIG>, the filter control analysis component <NUM> and the in-loop filters component <NUM> are highly integrated and may be implemented together, but are depicted separately for conceptual purposes. Filters applied to the reconstructed reference blocks are applied to particular spatial regions and include multiple parameters to adjust how such filters are applied. The filter control analysis component <NUM> analyzes the reconstructed reference blocks to determine where such filters should be applied and sets corresponding parameters. Such data is forwarded to the header formatting and CABAC component <NUM> as filter control data for encoding. The in-loop filters component <NUM> applies such filters based on the filter control data. The filters may include a deblocking filter, a noise suppression filter, a SAO filter, and an adaptive loop filter. Such filters may be applied in the spatial/pixel domain (e.g., on a reconstructed pixel block) or in the frequency domain, depending on the example.

When operating as an encoder, the filtered reconstructed image block, residual block, and/or prediction block are stored in the decoded picture buffer component <NUM> for later use in motion estimation as discussed above. When operating as a decoder, the decoded picture buffer component <NUM> stores and forwards the reconstructed and filtered blocks toward a display as part of an output video signal. The decoded picture buffer component <NUM> may be any memory device capable of storing prediction blocks, residual blocks, and/or reconstructed image blocks.

The header formatting and CABAC component <NUM> receives the data from the various components of codec system <NUM> and encodes such data into a coded bitstream for transmission toward a decoder. Specifically, the header formatting and CABAC component <NUM> generates various headers to encode control data, such as general control data and filter control data. Further, prediction data, including intra-prediction and motion data, as well as residual data in the form of quantized transform coefficient data are all encoded in the bitstream. The final bitstream includes all information desired by the decoder to reconstruct the original partitioned video signal <NUM>. Such information may also include intra-prediction mode index tables (also referred to as codeword mapping tables), definitions of encoding contexts for various blocks, indications of most probable intra-prediction modes, an indication of partition information, etc. Such data may be encoded by employing entropy coding. For example, the information may be encoded by employing context adaptive variable length coding (CAVLC), CABAC, syntax-based context-adaptive binary arithmetic coding (SBAC), probability interval partitioning entropy (PIPE) coding, or another entropy coding technique. Following the entropy coding, the coded bitstream may be transmitted to another device (e.g., avideo decoder) or archived for later transmission or retrieval.

<FIG> is ablock diagram illustrating an example video encoder <NUM>. Video encoder <NUM> may be employed to implement the encoding functions of codec system <NUM> and/or implement steps <NUM>, <NUM>, <NUM>, <NUM>, and/or <NUM> of operating method <NUM>. Encoder <NUM> partitions an input video signal, resulting in a partitioned video signal <NUM>, which is substantially similar to the partitioned video signal <NUM>. The partitioned video signal <NUM> is then compressed and encoded into a bitstream by components of encoder <NUM>.

Specifically, the partitioned video signal <NUM> is forwarded to an intra-picture prediction component <NUM> for intra-prediction The intra-picture prediction component <NUM> may be substantially similar to intra-picture estimation component <NUM> and intra-picture prediction component <NUM>. The partitioned video signal <NUM> is also forwarded to a motion compensation component <NUM> for inter-prediction based on reference blocks in a decoded picture buffer component <NUM>. The motion compensation component <NUM> may be substantially similar to motion estimation component <NUM> and motion compensation component <NUM>. The prediction blocks and residual blocks from the intra-picture prediction component <NUM> and the motion compensation component <NUM> are forwarded to a transform and quantization component <NUM> for transform and quantization of the residual blocks. The transform and quantization component <NUM> may be substantially similar to the transform scaling and quantization component <NUM>. The transformed and quantized residual blocks and the corresponding prediction blocks (along with associated control data) are forwarded to an entropy coding component <NUM> for coding into a bitstream. The entropy coding component <NUM> may be substantially similar to the header formatting and CABAC component <NUM>.

The transformed and quantized residual blocks and/or the corresponding prediction blocks are also forwarded from the transform and quantization component <NUM> to an inverse transform and quantization component <NUM> for reconstruction into reference blocks for use by the motion compensation component <NUM>. The inverse transform and quantization component <NUM> may be substantially similar to the scaling and inverse transform component <NUM>. In-loop filters in an in-loop filters component <NUM> are also applied to the residual blocks and/or reconstructed reference blocks, depending on the example. The in-loop filters component <NUM> may be substantially similar to the filter control analysis component <NUM> and the in-loop filters component <NUM>. The in-loop filters component <NUM> may include multiple filters as discussed with respect to in-loop filters component <NUM>. The filtered blocks are then stored in a decoded picture buffer component <NUM> for use as reference blocks by the motion compensation component <NUM>. The decoded picture buffer component <NUM> may be substantially similar to the decoded picture buffer component <NUM>.

<FIG> is a block diagram illustrating an example video decoder <NUM>. Video decoder <NUM> may be employed to implement the decoding functions of codec system <NUM> and/or implement steps <NUM>, <NUM>, <NUM>, and/or <NUM> of operating method <NUM>. Decoder <NUM> receives a bitstream, for example from an encoder <NUM>, and generates a reconstructed output video signal based on the bitstream for display to an end user.

The bitstream is received by an entropy decoding component <NUM>. The entropy decoding component <NUM> is configured to implement an entropy decoding scheme, such as CAVLC, CABAC, SBAC, PIPE coding, or other entropy coding techniques. For example, the entropy decoding component <NUM> may employ header information to provide a context to interpret additional data encoded as codewords in the bitstream. The decoded information includes any desired information to decode the video signal, such as general control data, filter control data, partition information, motion data, prediction data, and quantized transform coefficients from residual blocks. The quantized transform coefficients are forwarded to an inverse transform and quantization component <NUM> for reconstruction into residual blocks. The inverse transform and quantization component <NUM> may be similar to inverse transform and quantization component <NUM>.

The reconstructed residual blocks and/or prediction blocks are forwarded to intra-picture prediction component <NUM> for reconstruction into image blocks based on intra-prediction operations. The intra-picture prediction component <NUM> may be similar to intra-picture estimation component <NUM> and an intra-picture prediction component <NUM>. Specifically, the intra-picture prediction component <NUM> employs prediction modes to locate a reference block in the frame and applies a residual block to the result to reconstruct intra-predicted image blocks. The reconstructed intra-predicted image blocks and/or the residual blocks and corresponding inter-prediction data are forwarded to a decoded picture buffer component <NUM> via an in-loop filters component <NUM>, which may be substantially similar to decoded picture buffer component <NUM> and in-loop filters component <NUM>, respectively. The in-loop filters component <NUM> filters the reconstructed image blocks, residual blocks and/or prediction blocks, and such information is stored in the decoded picture buffer component <NUM>. Reconstructed image blocks from decoded picture buffer component <NUM> are forwarded to a motion compensation component <NUM> for inter-prediction. The motion compensation component <NUM> may be substantially similar to motion estimation component <NUM> and/or motion compensation component <NUM>. Specifically, the motion compensation component <NUM> employs motion vectors from a reference block to generate a prediction block and applies a residual block to the result to reconstruct an image block. The resulting reconstructed blocks may also be forwarded via the in-loop filters component <NUM> to the decoded picture buffer component <NUM>. The decoded picture buffer component <NUM> continues to store additional reconstructed image blocks, which can be reconstructed into frames via the partition information. Such frames may also be placed in a sequence. The sequence is output toward a display as a reconstructed output video signal.

Keeping the above in mind, video compression techniques perform spatial (intra-picture) prediction and/or temporal (inter-picture) prediction to reduce or remove redundancy inherent in video sequences. For block-based video coding, a video slice (i.e., a video picture or a portion of a video picture) may be partitioned into video blocks, which may also be referred to as treeblocks, coding tree blocks (CTBs), coding tree units (CTUs), coding units (CUs) and/or coding nodes.

Spatial or temporal prediction results in a predictive block for a block to be coded. Residual data represents pixel differences between the original block to be coded and the predictive block. An inter-coded block is encoded according to a motion vector that points to a block of reference samples forming the predictive block, and the residual data indicating the difference between the coded block and the predictive block. An intra-coded block is encoded according to an intra-coding mode and the residual data. For further compression, the residual data may be transformed from the pixel domain to a transform domain, resulting in residual transform coefficients, which then may be quantized. The quantized transform coefficients, initially arranged in a two-dimensional array, may be scanned in order to produce a one-dimensional vector of transform coefficients, and entropy coding may be applied to achieve even more compression.

Image and video compression has experienced rapid growth, leading to various coding standards. Such video coding standards include ITU-T H. <NUM>, International Organization for Standardization/International Electrotechnical Commission (ISO/IEC) MPEG-<NUM> Part <NUM>, ITU-T H. <NUM> or ISO/IEC MPEG-<NUM> Part <NUM>, ITU-T H. <NUM>, ISO/IEC MPEG-<NUM> Part <NUM>, Advanced Video Coding (AVC), also known as ITU-T H. <NUM> or ISO/IEC MPEG-<NUM> Part <NUM>, and High Efficiency Video Coding (HEVC), also known as ITU-T H. <NUM> or MPEG-H Part <NUM>. AVC includes extensions such as Scalable Video Coding (SVC), Multiview Video Coding (MVC) and Multiview Video Coding plus Depth (MVC+D), and 3D AVC (3D-AVC). HEVC includes extensions such as Scalable HEVC (SHVC), Multiview HEVC (MV-HEVC), and 3D HEVC (3D-HEVC).

There is also a new video coding standard, named Versatile Video Coding (VVC), being developed by the joint video experts team (JVET) of ITU-T and ISO/IEC. While the VVC standard has several working drafts, one Working Draft (WD) of VVC in particular, namely <NPL> (WC Draft <NUM>) is referenced herein.

The description of the techniques disclosed herein are based on the under-development video coding standard Versatile Video Coding (VVC) by the joint video experts team (JVET) of ITU-T and ISO/IEC. However, the techniques also apply to other video codec specifications.

<FIG> is a representation <NUM> of a relationship between an intra random access point (IRAP) picture <NUM> relative to leading pictures <NUM> and trailing pictures <NUM> in a decoding order <NUM> and a presentation order <NUM>. In an embodiment, the IRAP picture <NUM> is referred to as a clean random access (CRA) picture or as an instantaneous decoder refresh (IDR) picture with random access decodable (RADL) picture. In HEVC, IDR pictures, CRA pictures, and Broken Link Access (BLA) pictures are all considered IRAP pictures <NUM>. For VVC, during the<NPL>, it was agreed to have both IDR and CRA pictures as IRAP pictures. In an embodiment, Broken Link Access (BLA) and Gradual Decoder Refresh (GDR) pictures may also be considered to be IRAP pictures. The decoding process for a coded video sequence always starts at an IRAP.

As shown in <FIG>, the leading pictures <NUM> (e.g., pictures <NUM> and <NUM>) follow the IRAP picture <NUM> in the decoding order <NUM>, but precede the IRAP picture <NUM> in the presentation order <NUM>. The trailing picture <NUM> follows the IRAP picture <NUM> in both the decoding order <NUM> and in the presentation order <NUM> (a. , output order). While two leading pictures <NUM> and one trailing picture <NUM> are depicted in <FIG>, those skilled in the art will appreciate that more or fewer leading pictures <NUM> and/or trailing pictures <NUM> may be present in the decoding order <NUM> and the presentation order <NUM> in practical applications.

The leading pictures <NUM> in <FIG> have been divided into two types, namely random access skipped leading (RASL) and RADL. When decoding starts with the IRAP picture <NUM> (e.g., picture <NUM>), the RADL picture (e.g., picture <NUM>) can be properly decoded; however, the RASL picture (e.g., picture <NUM>) cannot be properly decoded. Thus, the RASL picture is discarded. In light of the distinction between RADL and RASL pictures, the type of leading picture <NUM> associated with the IRAP picture <NUM> should be identified as either RADL or RASL for efficient and proper coding. In HEVC, when RASL and RADL pictures are present, it is constrained that for RASL and RADL pictures that are associated with the same IRAP picture <NUM>, the RASL pictures shall precede the RADL pictures in presentation order <NUM>.

An IRAP picture <NUM> provides the following two important functionalities / benefits. Firstly, the presence of an IRAP picture <NUM> indicates that the decoding process can start from that picture. This functionality allows a random access feature in which the decoding process starts at that position in the bitstream, not necessarily the beginning of the bitstream, as long as an IRAP picture <NUM> is present at that position. Secondly, the presence of an IRAP picture <NUM> refreshes the decoding process such that a coded picture starting at the IRAP picture <NUM>, excluding RASL pictures, are coded without any reference to previous pictures. Having an IRAP picture <NUM> present in the bitstream consequently would stop any error that may happen during decoding of coded pictures prior to the IRAP picture <NUM> to propagate to the IRAP picture <NUM> and those pictures that follow the IRAP picture <NUM> in decoding order <NUM>.

While IRAP pictures <NUM> provide important functionalities, they come with a penalty to the compression efficiency. The presence of an IRAP picture <NUM> causes a surge in bitrate. This penalty to the compression efficiency is due to two reasons. Firstly, as an IRAP picture <NUM> is an intra-predicted picture, the picture itself would require relatively more bits to represent when compared to other pictures (e.g., leading pictures <NUM>, trailing pictures <NUM>) that are inter-predicted pictures. Secondly, because the presence of an IRAP picture <NUM> breaks temporal prediction (this is because the decoder would refresh the decoding process, in which one of the actions of the decoding process for this is to remove previous reference pictures in the decoded picture buffer (DPB)), the IRAP picture <NUM> causes the coding of pictures that follow the IRAP picture <NUM> in decoding order <NUM> to be less efficient (i.e., needs more bits to represent) because they have less reference pictures for their inter-prediction coding.

Among the picture types that are considered IRAP pictures <NUM>, the IDR picture in HEVC has different signaling and derivation when compared to other picture types. Some of the differences are as follows.

For signaling and derivation of a picture order count (POC) value of an IDR picture, the most significant bit (MSB) part of the POC is not derived from the previous key picture but simply set to be equal to <NUM>.

For signaling information needed for reference picture management, the slice header of an IDR picture does not contain information needed to be signaled to assist reference picture management. For other picture types (i.e., CRA, Trailing, temporal sub-layer access (TSA), etc.), information such as the reference picture set (RPS) described below or other forms of similar information (e.g., reference picture lists) are needed for the reference pictures marking process (i.e., the process to determine the status of reference pictures in the decoded picture buffer (DPB), either used for reference or unused for reference). However, for the IDR picture, such information does not need to be signaled because the presence of IDR indicates that the decoding process shall simply mark all reference pictures in the DPB as unused for reference.

The latest draft specification of VVC supports two types of IRAP pictures, namely IDR pictures and CRA pictures.

In HEVC and VVC, IRAP pictures and leading pictures are given different NAL unit types so that they can be easily identified by system level applications. For example, a video splicer needs to understand coded picture types without having to understand too much detail of the syntax element in the coded bitstream, particularly to identify IRAP pictures from non-IRAP pictures and to identify leading pictures, including determining RASL and RADL pictures, from trailing pictures. Trailing pictures are those pictures that are associated with an IRAP picture and follow the IRAP picture in output order. A picture associated with a particular IRAP picture is such a picture that follows the particular IRAP picture in decoding order and precedes any other IRAP picture in decoding order. For this, giving IRAP and leading pictures their own NAL unit type helps such applications.

A so-called external decoding refresh (EDR) based video coding approach is described in the document JVET-<NUM> (publically available herein: http://phenix. fr/jvet/doc_end_user/documents/15_Gothenburg/wgll/JVET-O0149-vl. As reported in JVET-O0149, the approach can provide significant video compression gain of up to about thirty-one percent (<NUM>%) for certain video content.

In AVC, HEVC, and the current design of VVC, an IRAP picture is not only coded as an intra picture, but also the decoding of an IRAP picture when random accessing from the IRAP picture would flush the DPB. Consequently, pictures earlier than the IRAP would not be available for inter-predication reference by the IRAP picture and pictures following the IRAP picture in decoding order. To circumvent the unavailability of the earlier pictures, referred to as the external pictures in JVET-O0149, the approach in JVET-O0149 employs a trick by providing the earlier pictures through a separate video bitstream, which can be referred to as an external means. As such, each external picture can be used for inter prediction reference by pictures starting from the random accessible picture that would conventionally be coded as an IRAP picture.

The EDR design in JVET-O0149 is summarized as follows:.

The problems with the existing designs are discussed.

In the EDR design in JVET-O0149, for each EDR picture, the POC MSB values of the external pictures and the EDR picture itself are signaled in the slice headers of the EDR picture. However, it is possible to use more bits for the POC least significant bits (LSBs) signaled in each slice header, such that when decoding the bitstream when random accessing from an EDR picture, i.e., the bitstream consisting of the external pictures, the EDR picture, and all the following pictures, the POC values of the all the pictures, particularly the POC values of the external pictures and the EDR picture, can be derived based on the POC LSBs signaled in each slice header using the usual POC derivation process. This way, signaling of the POC MSB values of the external pictures and the EDR picture itself can be avoided.

Disclosed herein are techniques that restrict a difference between the picture order count values for any two consecutive pictures in a list of pictures to a limited range when providing random access using an external decoder refresh (EDR) picture. By restricting the difference between picture order count values for any two consecutive pictures in a list of pictures to the limited range, the number of bits needed for signaling the picture order count values is reduced, which improves coding efficiency. Thus, usage of the processor, memory, and/or network resources may be reduced at both the encoder and the decoder. Thus, the coder / decoder (a. , "codec") in video coding is improved relative to current codecs. As a practical matter, the improved video coding process offers the user a better user experience when videos are sent, received, and/or viewed.

<FIG> is a schematic diagram illustrating an example of unidirectional inter prediction <NUM>. Unidirectional inter prediction <NUM> can be employed to determine motion vectors for encoded and/or decoded blocks created when partitioning a picture.

Unidirectional inter prediction <NUM> employs a reference frame <NUM> with a reference block <NUM> to predict a current block <NUM> in a current frame <NUM>. The reference frame <NUM> may be temporally positioned after the current frame <NUM> as shown (e.g., as a subsequent reference frame), but may also be temporally positioned before the current frame <NUM> (e.g., as a preceding reference frame) in some examples. The current frame <NUM> is an example frame/picture being encoded/decoded at a particular time. The current frame <NUM> contains an object in the current block <NUM> that matches an object in the reference block <NUM> of the reference frame <NUM>. The reference frame <NUM> is a frame that is employed as a reference for encoding a current frame <NUM>, and a reference block <NUM> is a block in the reference frame <NUM> that contains an object also contained in the current block <NUM> of the current frame <NUM>.

The current block <NUM> is any coding unit that is being encoded/decoded at a specified point in the coding process. The current block <NUM> may be an entire partitioned block, or may be a sub-block when employing affine inter prediction mode. The current frame <NUM> is separated from the reference frame <NUM> by some temporal distance (TD) <NUM>. The TD <NUM> indicates an amount of time between the current frame <NUM> and the reference frame <NUM> in a video sequence, and may be measured in units of frames. The prediction information for the current block <NUM> may reference the reference frame <NUM> and/or reference block <NUM> by a reference index indicating the direction and temporal distance between the frames. Over the time period represented by the TD <NUM>, the object in the current block <NUM> moves from a position in the current frame <NUM> to another position in the reference frame <NUM> (e.g., the position of the reference block <NUM>). For example, the object may move along a motion trajectory <NUM>, which is a direction of movement of an object over time. A motion vector <NUM> describes the direction and magnitude of the movement of the object along the motion trajectory <NUM> over the TD <NUM>. Accordingly, an encoded motion vector <NUM>, a reference block <NUM>, and a residual including the difference between the current block <NUM> and the reference block <NUM> provides information sufficient to reconstruct a current block <NUM> and position the current block <NUM> in the current frame <NUM>.

<FIG> is a schematic diagram illustrating an example of bidirectional inter prediction <NUM>. Bidirectional inter prediction <NUM> can be employed to determine motion vectors for encoded and/or decoded blocks created when partitioning a picture.

Bidirectional inter prediction <NUM> is similar to unidirectional inter prediction <NUM>, but employs a pair of reference frames to predict a current block <NUM> in a current frame <NUM>. Hence current frame <NUM> and current block <NUM> are substantially similar to current frame <NUM> and current block <NUM>, respectively. The current frame <NUM> is temporally positioned between a preceding reference frame <NUM>, which occurs before the current frame <NUM> in the video sequence, and a subsequent reference frame <NUM>, which occurs after the current frame <NUM> in the video sequence. Preceding reference frame <NUM> and subsequent reference frame <NUM> are otherwise substantially similar to reference frame <NUM>.

The current block <NUM> is matched to a preceding reference block <NUM> in the preceding reference frame <NUM> and to a subsequent reference block <NUM> in the subsequent reference frame <NUM>. Such a match indicates that, over the course of the video sequence, an object moves from a position at the preceding reference block <NUM> to a position at the sub sequent reference block <NUM> along a motion trajectory <NUM> and via the current block <NUM>. The current frame <NUM> is separated from the preceding reference frame <NUM> by some preceding temporal distance (TD0) <NUM> and separated from the subsequent reference frame <NUM> by some subsequent temporal distance (TD1) <NUM>. The TD0 <NUM> indicates an amount of time between the preceding reference frame <NUM> and the current frame <NUM> in the video sequence in units of frames. The TD1 <NUM> indicates an amount of time between the current frame <NUM> and the subsequent reference frame <NUM> in the video sequence in units of frame. Hence, the object moves from the preceding reference block <NUM> to the current block <NUM> along the motion trajectory <NUM> over a time period indicated by TD0 <NUM>. The object also moves from the current block <NUM> to the subsequent reference block <NUM> along the motion trajectory <NUM> over a time period indicated by TD1 <NUM>. The prediction information for the current block <NUM> may reference the preceding reference frame <NUM> and/or preceding reference block <NUM> and the subsequent reference frame <NUM> and/or subsequent reference block <NUM> by a pair of reference indices indicating the direction and temporal distance between the frames.

A preceding motion vector (MV0) <NUM> describes the direction and magnitude of the movement of the object along the motion trajectory <NUM> over the TD0 <NUM> (e.g., between the preceding reference frame <NUM> and the current frame <NUM>). A subsequent motion vector (MV1) <NUM> describes the direction and magnitude of the movement of the object along the motion trajectory <NUM> over the TD1 <NUM> (e.g., between the current frame <NUM> and the subsequent reference frame <NUM>). As such, in bidirectional inter prediction <NUM>, the current block <NUM> can be coded and reconstructed by employing the preceding reference block <NUM> and/or the subsequent reference block <NUM>, MV0 <NUM>, and MV1 <NUM>.

In an embodiment, inter prediction and/or bidirectional inter prediction may be carried out on a sample-by-sample (e.g., pixel-by-pixel) basis instead of on a block-by-block basis. That is, a motion vector pointing to each sample in the preceding reference block <NUM> and/or the subsequent reference block <NUM> can be determined for each sample in the current block <NUM>. In such embodiments, the motion vector <NUM> and the motion vector <NUM> depicted in <FIG> represent a plurality of motion vectors corresponding to the plurality of samples in the current block <NUM>, the preceding reference block <NUM>, and the sub sequent reference block <NUM>.

In both merge mode and advanced motion vector prediction (AMVP) mode, a candidate list is generated by adding candidate motion vectors to a candidate list in an order defined by a candidate list determination pattern. Such candidate motion vectors may include motion vectors according to unidirectional inter prediction <NUM>, bidirectional inter prediction <NUM>, or combinations thereof. Specifically, motion vectors are generated for neighboring blocks when such blocks are encoded. Such motion vectors are added to a candidate list for the current block, and the motion vector for the current block is selected from the candidate list. The motion vector can then be signaled as the index of the selected motion vector in the candidate list. The decoder can construct the candidate list using the same process as the encoder, and can determine the selected motion vector from the candidate list based on the signaled index. Hence, the candidate motion vectors include motion vectors generated according to unidirectional inter prediction <NUM> and/or bidirectional inter prediction <NUM>, depending on which approach is used when such neighboring blocks are encoded.

<FIG> is a schematic diagram illustrating an example reference picture list structure <NUM>. A reference picture list structure <NUM> can be employed to store indications of reference pictures and/or inter-layer reference pictures used in unidirectional inter-prediction <NUM> and/or bidirectional inter-prediction <NUM>. Hence, the reference picture list structure <NUM> can be employed by a codec system <NUM>, an encoder <NUM>, and/or a decoder <NUM> when performing method <NUM>.

Reference picture list structure <NUM>, which is also known as an RPL structure, is an addressable syntax structure that contains multiple reference picture lists, such as RPL <NUM><NUM> and RPL <NUM><NUM>. The reference picture list structure <NUM> may be stored in a SPS, a picture header, and/or a slice header of a bitstream, depending on the example. A reference picture list, such as RPL <NUM><NUM> and RPL <NUM><NUM>, is a list of reference pictures used for inter-prediction and/or inter-layer prediction. Specifically, reference pictures used by unidirectional inter prediction <NUM> are stored in RPL <NUM><NUM> and reference pictures used by bidirectional inter prediction <NUM> are stored in both RPL <NUM><NUM> and RPL <NUM><NUM>. For example, bidirectional inter prediction <NUM> may use one reference picture from RPL <NUM><NUM> and one reference picture from RPL <NUM><NUM>. RPL <NUM><NUM> and RPL <NUM><NUM> may each include a plurality of entries <NUM>. A reference picture list structure entry <NUM> is an addressable location in a reference picture list structure <NUM> that indicates a reference picture associated with a reference picture list, such as RPL <NUM><NUM> and/or RPL <NUM><NUM>.

In a specific example, the reference picture list structure <NUM> can be denoted as ref_pic_list_ struct( listIdx, rplsIdx ) where listldx <NUM> identifies a reference picture list RPL <NUM><NUM> and/or RPL <NUM><NUM> and rplsIdx <NUM> identifies an entry <NUM> in the reference picture list. Accordingly, ref_pic_list_struct is a syntax structure that returns the entry <NUM> based on listIdx <NUM> and rplsIdx <NUM>. An encoder can encode a portion of the reference picture list structure <NUM> for each non-intra-coded slice in a video sequence. A decoder can then resolve the corresponding portion of the reference picture list structure <NUM> before decoding each non-intra-coded slice in a coded video sequence. In an embodiment, the reference picture lists discussed herein are coded, constructed, derived, or otherwise obtained by the encoder or decoder using information stored in the encoder or decoder, obtained at least in part from the bitstream, and so on.

<FIG> illustrates a video bitstream <NUM> configured to implement an external decoding refresh (EDR) technique <NUM>. As used herein the video bitstream <NUM> may also be referred to as a coded video bitstream, a bitstream, or variations thereof. As shown in <FIG>, the bitstream <NUM> comprises a sequence parameter set (SPS) <NUM>, a picture parameter set (PPS) <NUM>, a slice header <NUM>, and image data <NUM>.

The SPS <NUM> contains data that is common to all the pictures in a sequence of pictures (SOP). In contrast, the PPS <NUM> contains data that is common to the entire picture. The slice header <NUM> contains information about the current slice such as, for example, the slice type, which of the reference pictures will be used, and so on. The SPS <NUM> and the PPS <NUM> may be generically referred to as a parameter set. The SPS <NUM>, the PPS <NUM>, and the slice header <NUM> are types of Network Abstraction Layer (NAL) units. A NAL unit is a syntax structure containing an indication of the type of data to follow (e.g., coded video data). NAL units are classified into video coding layer (VCL) and non-VCL NAL units. The VCL NAL units contain the data that represents the values of the samples in the video pictures, and the non-VCL NAL units contain any associated additional information such as parameter sets (important header data that can apply to a large number of VCL NAL units) and supplemental enhancement information (timing information and other supplemental data that may enhance usability of the decoded video signal but are not necessary for decoding the values of the samples in the video pictures). Those skilled in the art will appreciate that the bitstream <NUM> may contain other parameters and information in practical applications.

The image data <NUM> of <FIG> comprises data associated with the images or video being encoded or decoded. The image data <NUM> may be simply referred to as the payload or data being carried in the bitstream <NUM>. In an embodiment, the image data <NUM> comprises the CVS <NUM> (or CLVS) containing an EDR picture <NUM>, one or more trailing pictures <NUM>, and an end of sequence picture <NUM>. In an embodiment, the EDR picture <NUM> is referred to as a CVS starting (CVSS) picture. The CVS <NUM> is a coded video sequence for every coded layer video sequence (CLVS) in the video bitstream <NUM>. Notably, the CVS and the CLVS are the same when the video bitstream <NUM> includes a single layer. The CVS and the CLVS are only different when the video bitstream <NUM> includes multiple layers.

The CVS <NUM> is a series of pictures (or portions thereof) starting with the EDR picture <NUM> and includes all pictures (or portions thereof) up to, but not including, the next EDR picture or until the end of the bitstream. In an embodiment, a decoding order begins with the EDR picture <NUM>, continues with the trailing pictures <NUM>, and then proceeds to the end of sequence picture <NUM>.

External decoding refresh (EDR), which is also referred to as cross RAP reference (CRR), allows random access point pictures, such as EDR picture <NUM>, to be inter coded instead of intra coded. The basic idea of the EDR approach is as follows. Instead of coding the EDR picture <NUM> as an intra-coded intra random access point (IRAP) picture, the EDR picture <NUM> is coded using inter prediction using earlier pictures (ak. , external pictures) provided by way of a second or separate video bitstream, which will be more fully discussed below. Consequently, each earlier picture can be used for inter prediction referencing by pictures across the random access points. The coding efficiency gain comes from inter coding of the random access points.

As shown in <FIG>, slices of the EDR picture <NUM>, the trailing pictures <NUM>, and the end of sequence picture <NUM> in the CVS <NUM> are each contained within their own VCL NAL unit <NUM>. The set of VCL NAL units <NUM> in the CVS <NUM> may be referred to as an access unit.

<FIG> illustrates a second video bitstream <NUM> configured to carry image data <NUM> comprising external pictures <NUM>. In an embodiment, the second bitstream <NUM> may be similar to the bitstream <NUM> in <FIG>. Therefore, a full description of the second bitstream <NUM> is intentionally omitted for the sake of brevity. In an embodiment, the second video bitstream <NUM> is separate from the video bitstream <NUM> in <FIG>. In an embodiment, the second video bitstream <NUM> may be transmitted and/or received at different times or separately from the bitstream <NUM>. To distinguish the second video bitstream <NUM> from the video bitstream <NUM> in <FIG>, the second video bitstream <NUM> may be referred to as a separate video bitstream, an external video bitstream, or a secondary video bitstream.

The external pictures <NUM> carried in the second video bitstream <NUM> may be referred to as earlier pictures. In an embodiment, the external pictures <NUM> are pictures that were decoded or encountered prior to the process of decoding the EDR picture <NUM> in <FIG>. In an embodiment, one or more of the external pictures <NUM> is another EDR picture. That is, one or more of the external pictures <NUM> in <FIG> may be an EDR picture other than the EDR picture <NUM> in <FIG>. In an embodiment, one or more of the external pictures <NUM> are IRAP pictures. As noted above, one of the external pictures <NUM> in <FIG> may be used as a reference picture to inter code or inter predict the EDR picture <NUM> in <FIG> as described herein.

<FIG> is an embodiment of a list of pictures <NUM> that may be utilized to code an EDR picture (EDR picture <NUM>) using one of the external pictures <NUM> of <FIG>. In an embodiment, the list of pictures <NUM> is signaled in a slice header (e.g., slice header <NUM>) of the bitstream. As shown in <FIG>, the list of pictures <NUM> lists a plurality of pictures <NUM> in increasing decoding order <NUM>. In an embodiment, the pictures <NUM> lists pictures referred to by entries (e.g., entries <NUM>) in a first reference picture list (e.g., RPL <NUM><NUM>), entries (e.g., entries <NUM>) in a second reference picture list (e.g., RPL <NUM><NUM>), and external pictures (e.g., the external pictures <NUM>). In <FIG>, the pictures <NUM> have been denoted Picture <NUM> to Picture N, where N is any integer. Each of the pictures <NUM> has a corresponding picture order count value <NUM>. In an embodiment, the picture order count values <NUM> are picture order count (POC) least significant bits (LSBs).

In the list of pictures <NUM>, a difference between the picture order count values of any two consecutive pictures (e.g., between Picture <NUM> and Picture <NUM>, between Picture <NUM> and Picture <NUM>, etc.) is within a specified or predetermined range. In an embodiment, the difference between the picture order count values of any two consecutive pictures is within a negative of maximum picture order count least significant bits and less than one half of the maximum picture order count least significant bits. That is, the difference is between:
-MaxPicOrderCntLsb / <NUM> and less than MaxPicOrderCntLsb / <NUM>. where the MaxPicOrderCountLsb represents the maximum picture order count LSB.

In an embodiment, for each EDR picture in the bitstream, there shall be zero long term reference picture (LTRP) entries in RefPicList[ <NUM> ] and zero LTRP entries in RefPicList[ <NUM> ].

With either of the above constraints, the POC MSB values of the external pictures and the EDR picture itself can be avoided, at the cost of the need of more bits for signaling of the POC LSBs in each slice header, to enable either of the constraints to be complied, while still allowing the distance in POC between an external picture and the EDR picture to be far away enough for high coding efficiency.

The following decoding process is specified when random accessing from an EDR picture.

A number of access units, where the number is greater than <NUM>, each containing an external picture provided for the EDR picture by an external means are placed before the EDR picture, such that the bitstream BitstreamToDecode comprises the external pictures, the EDR picture, and all the pictures that follow the EDR picture in decoding order. The decoding order of the external pictures in BitstreamToDecode shall be in the order as they are provided by the external means. The following constraints shall apply for each external picture.

The VCL NAL units of each external picture shall not be equal to RASL_NUT or RADL NUT.

Each external picture shall have TemporalId equal to <NUM> and non_reference_picture_flag equal to <NUM>.

Optionally, each external picture is intra coded, i.e., it shall not refer to any pictures other than itself for inter prediction in its decoding process. Alternatively, only the first external picture is intra coded. An external picture that is not an intra coded picture shall only refer to other external pictures for the same EDR picture that precede it in decoding order.

The bitstream BitstreamToDecode is decoded, picture by picture, wherein the first picture in BitstreamToDecode is considered as a CLVSS picture and the first non-external picture (i.e., the EDR picture) in BitstreamToDecode is considered as a non-CLVSS picture, and the decoding of each picture is specified as follows.

The decoding of NAL units is specified in clause <NUM> of the current VVC specification.

The processes in clause <NUM> of the current VVC specification specify the following decoding processes using syntax elements in the slice header layer and above.

Variables and functions relating to picture order count are derived as specified in clause <NUM>. <NUM> of the current VVC specification. This needs to be invoked only for the first slice of a picture.

At the beginning of the decoding process for each slice of a non-IDR picture, the decoding process for reference picture lists construction specified in clause <NUM>. <NUM> of the current VVC specification is invoked for derivation of reference picture list <NUM> (RefPicList[ <NUM> ]) and reference picture list <NUM> (RefPicList[ <NUM> ]).

The decoding process for reference picture marking in clause <NUM>. <NUM> of the current VVC specification is invoked, wherein reference pictures may be marked as "unused for reference" or "used for long-term reference". This needs to be invoked only for the first slice of a picture.

When the current picture is a CRA picture with NoIncorrectPicOutputFlag equal to <NUM> or GDR picture with NoIncorrectPicOutputFlag equal to <NUM>, the decoding process for generating unavailable reference pictures specified in subclause <NUM>. <NUM> of the current VVC specification is invoked, which needs to be invoked only for the first slice of a picture.

If one of the following conditions is true, PictureOutputFlag is set equal to <NUM>:
The current picture is a RASL picture and NoIncorrectPicOutputFlag of the associated IRAP or EDR picture is equal to <NUM>.

The current picture is an external picture.

Otherwise, PictureOutputFlag is set equal to pic_output_flag.

The processes in clauses <NUM>, <NUM>, <NUM>, <NUM> and <NUM> of the current VVC specification specify decoding processes using syntax elements in all syntax structure layers. It is a requirement of bitstream conformance that the coded slices of the picture shall contain slice data for every CTU of the picture, such that the division of the picture into slices, and the division of the slices into CTUs each forms a partitioning of the picture.

After all slices of the current picture have been decoded, the current decoded picture is marked as "used for short-term reference," and each ILRP entry in RefPicList[ <NUM> ] or RefPicList[ <NUM> ] is marked as "used for short-term reference.

A buffering period SEI message is allowed be present for an EDR access unit, such that the hypothetical reference decoder (HRD) conformance can be specified for bitstreams starting with an EDR access unit.

<FIG> is an embodiment of a method <NUM> of decoding implemented by a video decoder (e.g., video decoder <NUM>). The method <NUM> may be performed after the decoded bitstream has been directly or indirectly received from a video encoder (e.g., video encoder <NUM>). The method <NUM> improves the decoding process by restricting a difference between the picture order count values for any two consecutive pictures in a list of pictures to a limited range when providing random access using an external decoder refresh (EDR) picture. By restricting the difference between picture order count values for any two consecutive pictures in a list of pictures to the limited range, the number of bits needed for signaling the picture order count values is reduced, which improves coding efficiency. Therefore, as a practical matter, the performance of a codec is improved, which leads to abetter user experience.

In block <NUM>, the video decoder receives a bitstream containing an external decoder refresh (EDR) picture and a list of pictures for the EDR picture. The list of pictures lists pictures referred to by entries in a first reference picture list, the pictures referred to by the entries in a second reference picture list, and external pictures in increasing decoding order. A difference between picture order count values for any two consecutive pictures in the list of pictures is greater than one half of a negative of maximum picture order count least significant bits and less than one half of the maximum picture order count least significant bits.

In an embodiment, one of the external pictures is a reference EDR picture. In an embodiment, one of the external pictures is a reference intra random access point (IRAP) picture. In an embodiment, the picture order count values are picture order count (POC) least significant bits (LSBs). In an embodiment, the list of pictures containing the picture order count values are signaled in a slice header of the bitstream. However, the list of pictures may be included elsewhere in the bitstream in other embodiments.

In block <NUM>, the video decoder obtains one of the external pictures referred to in the list of pictures. In an embodiment, the external picture <NUM> is obtained from the second bitstream <NUM>.

In block <NUM>, the video decoder decodes the EDR picture using the external reference picture that was obtained. In an embodiment, an image generated based on the EDR picture is displayed for a user of an electronic device (e.g., a smart phone, tablet, laptop, personal computer, etc.).

<FIG> is an embodiment of a method <NUM> of encoding implemented by a video encoder (e.g., video encoder <NUM>). The method <NUM> may be performed when a picture (e.g., from a video) is to be encoded into a video bitstream and then transmitted toward a video decoder (e.g., video decoder <NUM>). The method <NUM> improves the encoding process by restricting a difference between the picture order count values for any two consecutive pictures in a list of pictures to a limited range when providing random access using an external decoder refresh (EDR) picture. By restricting the difference between picture order count values for any two consecutive pictures in a list of pictures to the limited range, the number of bits needed for signaling the picture order count values is reduced, which improves coding efficiency. Therefore, as a practical matter, the performance of a codec is improved, which leads to a better user experience.

In block <NUM>, the video encoder generates an external decoder refresh (EDR) picture and a list of pictures for the EDR picture. The list of pictures lists pictures referred to by entries in a first reference picture list, the pictures referred to by the entries in a second reference picture list, and external pictures in increasing decoding order. A difference between picture order count values for any two consecutive pictures in the list of pictures is greater than one half of a negative of maximum picture order count least significant bits and less than one half of the maximum picture order count least significant bits.

In an embodiment, one of the external pictures is a reference EDR picture. In an embodiment, one of the external pictures is a reference intra random access point (IRAP) picture. In an embodiment, the picture order count values are picture order count (POC) least significant bits (LSBs).

In block <NUM>, the video encoder encodes the EDR picture and the list of pictures for the EDR picture into a bitstream. In an embodiment, the list of pictures containing the picture order count values are encoded in a slice header of the bitstream. However, the list of pictures may be encoded elsewhere in the bitstream in other embodiments.

In block <NUM>, the video encoder stores the video bitstream for transmission toward the video decoder. In an embodiment, the video coder transmits the video bitstream toward the video decoder.

The following syntax and semantics may be employed to implement the embodiments disclosed herein. The following description is relative to the basis text, which is the latest VVC draft specification. In other words, only the delta is described, while the text in the basis text that are not mentioned below apply as they are. Updated text relative to the basis text is shown in italics.

The sequence parameter set RBSP syntax is provided.

The general slice header syntax is provided.

NOTE <NUM> - A clean random access (CRA) picture may have associated RASL or RADL pictures present in the bitstream.

NOTE <NUM> - An instantaneous decoding refresh (IDR) picture having nal_unit_type equal to IDR N LP does not have associated leading pictures present in the bitstream. An IDR picture having nal unit type equal to IDR_W_RADL does not have associated RASL pictures present in the bitstream, but may have associated RADL pictures in the bitstream.

NOTE <NUM> - An external decoding refresh (EDR) picture may have associated RASL or RADL pictures present in the bitstream.

The value of nal_unit_type shall be the same for all coded sice NAL units of a picture. A picture or a layer access unit is referred to as having the same NAL unit type as the coded slice NAL units of the picture or layer access unit.

For a single-layer bitstream, the following constraints apply:.

NOTE - It is possible to perform random access at the position of an IRAP access unit by discarding all access units before the IRAP access unit (and to correctly decode the IRAP picture and all the subsequent non-RASL pictures in decoding order), provided each parameter set is available (either in the bitstream or by external means not specified in this Specification) when it is referred.

nuh_temporal_id_plus1 minus <NUM> specifies a temporal identifier for the NAL unit.

The value of nuh_temporal_id_plus1 shall not be equal to <NUM>.

The variable TemporalId is derived as follows.

When nal_unit_type is in the range of IDR W RADL to GDR NUT, inclusive, TemporalId shall be equal to <NUM>.

When nal_unit_type is equal to STSA_NUT, TemporalId shall not be equal to <NUM>.

The value of TemporalId shall be the same for all VCL NAL units of a layer access unit. The value of TemporalId of a coded picture or a layer access unit is the value of the Temporalld of the VCL NAL units of the coded picture or the layer access unit. The value of Temporalld of a sub-layer representation is the greatest value of Temporalld of all VCL NAL units in the sub-layer representation.

The value of TemporalId for non-VCL NAL units is constrained as follows:.

NOTE <NUM> - When the NAL unit is a non-VCL NAL unit, the value of TemporalId is equal to the minimum value of the TemporalId values of all layer access units to which the non-VCL NAL unit applies. When nal unit_type is equal to PPS_NUT or APS_NUT, TemporalId may be greater than or equal to the TemporalId of the containing layer access unit, as all PPSs and APSs may be included in the beginning of a bitstream, wherein the first coded picture has TemporalId equal to <NUM>. When nal_unit_type is equal to PREFIX_SEI_NUT or SUFFIX_SEI_NUT, TemporalId may be greater than or equal to the Temporalld of the containing layer access unit, as an SEI NAL unit may contain information that applies to a bitstream subset that includes layer access units for which the TemporalId values are greater than the TemporalId of the layer access unit containing the SEI NAL unit.

The order of access units and association to CVSs is discussed.

A bitstream conforming to this Specification consists of one or more CVSs.

A CVS consists of one or more access units. The order of NAL units and coded pictures and their association to access units is described in clause <NUM>.

The first access unit of a CVS is a CVSS access unit, wherein each present layer access unit is a CLVSS layer access unit, which is either an IRAP layer access unit with NoIncorrectPicOutputFlag equal to <NUM>, an EDR layer access unit with NoIncorrectPicOutputFlag equal to <NUM>, or a GDR layer access unit with NoIncorrectPicOutputFlag equal to <NUM>.

It is a requirement of bitstream conformance that, when present each layer access unit in the next access unit after an access unit that contains an end of sequence NAL unit or an end of bitstream NAL unit shall be an IRAP layer access unit, which may be an IDR layer access unit or a CRA layer access unit, an EDR layer access unit, or a GDR layer access unit.

The sequence parameter set RBSP semantics are discussed.

The end of sequence RBSP semantics are discussed.

When present, the end of sequence RBSP specifies that the current access unit is the last access unit in the coded video sequence in decoding order and the next subsequent access unit in the bitstream in decoding order (if any) is an IRAP, EDR, or GDR access unit. The syntax content of the SODB and RBSP for the end of sequence RBSP are empty.

The general decoding process is discussed.

Input to this process is a bitstream. Output of this process is a list of decoded pictures.

The decoding process is specified such that all decoders that conform to a specified profile and level will produce numerically identical cropped decoded output pictures when invoking the decoding process associated with that profile for a bitstream conforming to that profile and level. Any decoding process that produces identical cropped decoded output pictures to those produced by the process described herein (with the correct output order or output timing, as specified) conforms to the decoding process requirements of this Specification.

For each IRAP picture in the bitstream, the following applies:.

For each EDR picture in the bitstream, the following applies:.

For each GDR picture in the bitstream, the following applies:.

NOTE - The above operations for IRAP, EDR, and GDR pictures, are needed for identification of the CVSs in the bitstream.

For each CVS in the bitstream, the list TargetLayerIdList, which identifies the list of target layers to be decoded, and the variable HighestTid, which identifies the highest temporal sub-layer to be decoded, are specified as follows:.

The variable DecodingUnitHrdFlag is specified as follows:.

For each CVS in the bitsstream, the sub-bitstream extraction process as specified in clause <NUM> is applied with the CVS, TargetLayerIdList, and HighestTid as inputs, and the output is assigned to a bitstream referred to as CvsToDecode. After that, the instances of CvsToDecode of all the CVSs are concatenated, in decoding order, and the result is assigned to the bitstream BitstreamToDecode.

The variable ExternalPicsProvidedFlag is set equal to <NUM>.

The decoding process for a coded picture is repeatedly invoked for each coded picture in BitstreamToDecode in decoding order.

The decoding process for a coded picture is discussed.

The decoding processes specified in this clause apply to each coded picture, referred to as the current picture and denoted by the variable CurrPic, in BitstreamToDecode.

Depending on the value of chroma format idc, the number of sample arrays of the current picture is as follows:.

The decoding process for the current picture takes as inputs the syntax elements and upper-case variables from clause <NUM>. When interpreting the semantics of each syntax element in each NAL unit, and in the remaining parts of clause <NUM>, the term "the bitstream" (or part thereof, e.g., a CVS of the bitstream) refers to BitstreamToDecode (or part thereof).

When ExternalPicsProvidedFlag is equal to <NUM> and the current picture is an EDR picture with NoIncorrectPicOutputFlag equal to <NUM>, the following ordered steps apply:.

Depending on the value of separate colour_plane flag, the decoding process is structured as follows:.

NOTE - The variable ChromaArrayType is derived as equal to <NUM> when separate_colour_plane_flag is equal to <NUM> and chroma_format_idc is equal to <NUM>. In the decoding process, the value of this variable is evaluated resulting in operations identical to that of monochrome pictures (when chroma format idc is equal to <NUM>).

The decoding process operates as follows for the current picture CurrPic:.

When gdr enabled flag is equal to <NUM> and PicOrderCntVal of the current picture is greater than or equal to RpPicOrderCntVal of the previous GDR picture in decoding order for which there is no IRAP picure between the current picture and the previous GDR picture in decoding order, it is a requirement of bitstream conformance that the current and subsequent decoded pictures shall be an exact match to the pictures produced by starting the decoding process at the previous IRAP picture preceding the current picture in decoding order.

The general decoding process for generating unavailable reference pictures is discussed.

This process is invoked once per coded picture when the current picture is a CRA picture with NoIncorrectPicOutputFlag equal to <NUM>, an EDR picture with NoIncorrectPicOutputFlag equal to <NUM>, or a GDR picture with NoIncorrectPicOutputFlag equal to <NUM>.

A bitstream of coded data conforming to this Specification shall fulfil all requirements specified in this clause.

The bitstream shall be constructed according to the syntax, semantics and constraints specified in this Specification outside of this annex.

The first coded picture in a bitstream shall be an IRAP picture (i.e., an IDR picture or a CRA picture), an EDR picture, or a GDR picture.

Buffering period SEI message semantics are discussed.

A buffering period SEI message provides initial CPB removal delay and initial CPB removal delay offset information for initialization of the HRD at the position of the associated access unit in decoding order.

When the buffering period SEI message is present, a picture is said to be a notDiscardablePic picture when the picture has TemporalId equal to <NUM> and is not a RASL or RADL picture.

When the current picture is not the first picture in the bitstream in decoding order, let prevNonDiscardablePic be the preceding picture in decoding order with TemporalId equal to <NUM> that is not a RASL or RADL picture.

The presence of buffering period SEI messages is specified as follows:.

cpb_removal_delay_delta_minus1 plus <NUM>, when the current picture is not the first picture in the bitstream in decoding order, specifies a CPB removal delay increment value relative to the nominal CPB removal time of the picture prevNonDiscardablePic. The lenght of this syntax element is cpb_removal_delay_length_minus1 + <NUM> bits.

When the current picture contains a buffering period SEI message and concatenation_flag is equal to <NUM> and the current picture is not the first picture in the bitstream in decoding order, it is a requirement of bitstream conformance that the following constraint applies:.

NOTE <NUM> - When the current picture contains a buffering period SEI message and concatenation flag is equal to <NUM>, the cpb_removal_delay_minus1 for the current picture is not used. The above-specified constraint can, under some circumstances, make it possible to splice bitstreams (that use suitably-designed referencing structures) by simply changing the value of concatenation_flag from <NUM> to <NUM> in the buffering period SEI message for an IRAP, EDR, or GDR picture at the splicing point. When concatenation_flag is equal to <NUM>, the above-specified constraint enables the decoder to check whether the constraint is satisfied as a way to detect the loss of the picture prevNonDiscardablePic.

<FIG> is a schematic diagram of a video coding device <NUM> (e.g., a video encoder <NUM> or a video decoder <NUM>) according to an embodiment of the disclosure. The video coding device <NUM> is suitable for implementing the disclosed embodiments as described herein. The video coding device <NUM> comprises ingress ports <NUM> and receiver units (Rx) <NUM> for receiving data; a processor, logic unit, or central processing unit (CPU) <NUM> to process the data; transmitter units (Tx) <NUM> and egress ports <NUM> for transmitting the data; and a memory <NUM> for storing the data. The video coding device <NUM> may also comprise optical-to-electrical (OE) components and electrical-to-optical (EO) components coupled to the ingress ports <NUM>, the receiver units <NUM>, the transmitter units <NUM>, and the egress ports <NUM> for egress or ingress of optical or electrical signals.

The processor <NUM> is implemented by hardware and software. The processor <NUM> may be implemented as one or more CPU chips, cores (e.g., as a multi-core processor), field-programmable gate arrays (FPGAs), application specific integrated circuits (ASICs), and digital signal processors (DSPs). The processor <NUM> is in communication with the ingress ports <NUM>, receiver units <NUM>, transmitter units <NUM>, egress ports <NUM>, and memory <NUM>. The processor <NUM> comprises a coding module <NUM>. The coding module <NUM> implements the disclosed embodiments described above. For instance, the coding module <NUM> implements, processes, prepares, or provides the various codec functions. The inclusion of the coding module <NUM> therefore provides a substantial improvement to the functionality of the video coding device <NUM> and effects a transformation of the video coding device <NUM> to a different state. Alternatively, the coding module <NUM> is implemented as instructions stored in the memory <NUM> and executed by the processor <NUM>.

The video coding device <NUM> may also include input and/or output (I/O) devices <NUM> for communicating data to and from a user. The I/O devices <NUM> may include output devices such as a display for displaying video data, speakers for outputting audio data, etc. The I/O devices <NUM> may also include input devices, such as a keyboard, mouse, trackball, etc., and/or corresponding interfaces for interacting with such output devices.

The memory <NUM> comprises one or more disks, tape drives, and solid-state drives and may be used as an over-flow data storage device, to store programs when such programs are selected for execution, and to store instructions and data that are read during program execution. The memory <NUM> may be volatile and/or non-volatile and may be read-only memory (ROM), random access memory (RAM), ternary content-addressable memory (TCAM), and/or static random-access memory (SRAM).

<FIG> is a schematic diagram of an embodiment of a means for coding <NUM>. In an embodiment, the means for coding <NUM> is implemented in a video coding device <NUM> (e.g., a video encoder <NUM> or a video decoder <NUM>). The video coding device <NUM> includes receiving means <NUM>. The receiving means <NUM> is configured to receive a picture to encode or to receive a bitstream to decode. The video coding device <NUM> includes transmission means <NUM> coupled to the receiving means <NUM>. The transmission means <NUM> is configured to transmit the bitstream to a decoder or to transmit a decoded image to a display means (e.g., one of the I/O devices <NUM>).

The video coding device <NUM> includes a storage means <NUM>. The storage means <NUM> is coupled to at least one of the receiving means <NUM> or the transmission means <NUM>. The storage means <NUM> is configured to store instructions. The video coding device <NUM> also includes processing means <NUM>. The processing means <NUM> is coupled to the storage means <NUM>. The processing means <NUM> is configured to execute the instructions stored in the storage means <NUM> to perform the methods disclosed herein.

It should also be understood that the steps of the exemplary methods set forth herein are not necessarily required to be performed in the order described, and the order of the steps of such methods should be understood to be merely exemplary. Likewise, additional steps may be included in such methods, and certain steps may be omitted or combined, in methods consistent with various embodiments of the present disclosure.

While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods might be embodied in many other specific forms without departing from the scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented.

Claim 1:
A method of decoding implemented by a video decoder, comprising:
receiving (<NUM>), by the video decoder, a bitstream containing an external decoder refresh, EDR, picture and a list of pictures for the EDR picture, wherein the list of pictures lists in increasing decoding order pictures referred to by entries in a first reference picture list, pictures referred to by entries in a second reference picture list, and external pictures, wherein the external pictures are pictures that were decoded prior to decoding the EDR picture and wherein a difference between picture order count values for any two consecutive pictures in the list of pictures is greater than one half of a negative of maximum picture order count least significant bits and less than one half of the maximum picture order count least significant bits;
obtaining (<NUM>), by the video decoder, one of the external pictures referred to in the list of pictures; and
decoding (<NUM>), by the video decoder, the EDR picture using the external reference picture that was obtained.