Tile partitions with sub-tiles in video coding

An encoder (400), a decoder (450), and methods (20, 30) for partitioning a picture from a sequence of video pictures into a layout having a plurality of flexible tiles or segments is disclosed. Each tile or segment (T) comprises a single rectangular or square region. The encoder generates (22) a partition structure and encodes (28) the tiles according to the partition structure. The encoder also generates a bitstream (12) comprising a plurality of coded segments and information indicating the partition structure used to partition the picture into the plurality of flexible tiles or segments, and sends (29) the bitstream to the decoder. Upon receipt, the decoder uses the coded segments and information in the bitstream to decode (38) the plurality of coded segments.

TECHNICAL FIELD

The present disclosure relates generally to picture encoding and decoding techniques, and more particularly, to the encoding and decoding of picture segments with subsegments.

BACKGROUND

High Efficiency Video Coding (HEVC) is a block based video codec standardized by both the ITU-T and the Moving Picture Experts Group (MPEG) that utilizes both spatial and temporal prediction techniques. Spatial prediction reduces spatial redundancy and is achieved using intra (I) prediction from within the same frame of a current picture. Temporal prediction reduces temporal redundancy and is achieved using inter (P) or bi-directional inter (B) prediction on a block level using previously decoded reference pictures. Regardless of the particular type of prediction technique, however, the resultant differences between the original pixel data and the predicted pixel data, referred to as the “residual,” is transformed into the frequency domain and quantized. Quantizing the transformed residuals, the level of which is determined by the quantization parameter (QP), facilitates the control of a tradeoff between bitrate and the quality of the video.

The transformed and quantized residual is then entropy coded before being transmitted to a decoder together with the necessary prediction parameters. The prediction parameters, which are also entropy encoded, include prediction mode and motion vectors. Upon receipt, the decoder performs entropy decoding, inverse quantization, and inverse transformation to obtain the residual. The decoder then reconstructs the image from the residual using an intra-prediction or inter-prediction technique.

Both MPEG and ITU-T are working on a successor to HEVC within the Joint Video Exploratory Team (JVET). The name of this video codec is Versatile Video Coding (VCC). The HEVC video coding standard includes different tools to divide a picture (e.g., into slices or tiles). Tiles, for instance, divide a picture into rectangular spatially independent regions. However, the partitioning always spans across the entire picture, from left to right and top to bottom respectively. A picture parameter set (PPS) is used to specify the thickness of the rows and the widths of the columns in terms of coding tree units.

SUMMARY

Embodiments herein are useful for encoding and decoding a picture using a flexible partitioning, for instance, partitioning that would reduce the costs of transmission of coded segments of the picture. For example, one or more embodiments enable a partition structure where each picture segment of the partition structure comprises one or more subsegments in which each of the subsegments comprises a plurality of units. By providing a size of both the subsegment and the unit, the size of a picture segment that comprises subsegments can be expressed with less code words thereby saving costs of transmission of a corresponding coded segment.

One or more embodiments include a method of decoding a picture. The method comprises receiving a bitstream comprising a plurality of coded segments, and information associated with the plurality of coded segments. The method further comprises deriving, from the information in the bitstream, a partition structure that divides the picture into a plurality of picture segments. Each picture segment corresponds to one of the plurality of coded segments and comprises one or more subsegments. Further, at least one picture segment comprises at least two subsegments. Each subsegment of the partition structure further comprises a plurality of units and each of the plurality of units with each unit corresponding to a region of the picture. The method further comprises deriving, from the information in the bitstream, the size of a subsegment of the partition structure and the size of a unit of the partition structure. Then, the method further comprises decoding the plurality of coded segments received in the bitstream in accordance with the partition structure. The derivation of any intra prediction mode for a second unit of a given one of the picture segments depends on a derived intra prediction mode for a first unit of the given one of the picture segments and is independent of any intra prediction mode for units of any other picture segments.

One or more embodiments of the present disclosure also include a corresponding method of encoding a picture that can be used in conjunction with, or separately, from methods for decoding a picture. In one embodiment, a method of encoding a picture comprises defining a partition structure that divides a picture into picture segments. Each picture segment of the partition structure comprises one or more subsegments with at least one picture segment comprising at least two subsegements. Each subsegment of the partition structure comprises a plurality of units with each unit corresponding to a region of the picture. The method further comprises encoding the picture segments in accordance with the partition structure to generate a plurality of coded segments. Each coded segment corresponds to one of the picture segments of the partition structure. Further, each coded segment is independent such that any derivation of any intra prediction mode for a second unit of a given one of the picture segments depends on a derived intra prediction mode for a first unit of the given one of the picture segments and is independent of any intra prediction mode for units of any other picture segments. The method further comprises generating a bitstream comprising the plurality of coded segments and information indicating the partition structure used to divide the picture into the picture segments. The information indicates a size of a unit of the partition structure and a size of a subsegment of the partition structure.

One or more embodiments of the present disclosure also provide a decoder circuit configured to receive a bitstream comprising a plurality of coded segments, and information associated with the plurality of coded segments. The decoder circuit is further configured to derive, from the information in the bitstream, a partition structure that divides the picture into a plurality of picture segments, wherein each picture segment corresponds to one of the plurality of coded segments and comprises one or more subsegments, with at least one picture segment comprising at least two subsegments. Further, each subsegment comprises a plurality of units with each unit corresponding to a region of the picture. The decoder circuit is further configured to derive, from the information in the bitstream, a subsegment size of a subsegment of the partition structure, and a unit size of a unit of the partition structure. The decoder circuit is then further configured to decode the plurality of coded segments received in the bitstream in accordance with the partition structure. In these embodiments, derivation of an intra prediction mode for a second unit of a given one of the picture segments depends on a derived intra prediction mode for a first unit of the given one of the picture segments, and is independent of any intra prediction mode for units of any other picture segments.

One or more embodiments of the present disclosure also provides a decoder comprising communications circuitry configured to receive a bitstream comprising a plurality of coded segments, and information associated with the plurality of coded segments, a memory configured to store instructions, and processing circuitry operatively connected to the memory and the communications circuitry. The processing circuitry is configured to execute the instructions stored in the memory whereby the decoder is configured to derive, from the information in the bitstream, a partition structure that divides the picture into a plurality of picture segments, wherein each picture segment corresponds to one of the plurality of coded segments and comprises one or more subsegments, with at least one picture segment comprising at least two subsegments. Further, each subsegment comprises a plurality of units with each unit corresponding to a region of the picture. By executing the instructions, the processing circuitry further configures the decoder to derive, from the information in the bitstream, a subsegment size of a subsegment of the partition structure, and a unit size of a unit of the partition structure. By executing the instructions, the processing circuitry further configures the decoder to decode the plurality of coded segments received in the bitstream in accordance with the partition structure. In these embodiments, derivation of an intra prediction mode for a second unit of a given one of the picture segments depends on a derived intra prediction mode for a first unit of the given one of the picture segments, and is independent of any intra prediction mode for units of any other picture segments.

One or more embodiments of the present disclosure further provide an encoder circuit configured to define a partition structure that divides a picture into a plurality of picture segments. Each picture segment comprises one or more subsegments with at least one picture segment of the partition structure comprising at least two subsegments. Further, each subsegment comprises a plurality of units, with each unit corresponding to a region of the picture. The encoder circuit is further configured to encode the picture segments in accordance with the partition structure to generate a plurality of coded segments. Each coded segment corresponds to one of the picture segments of the partition structure. Further, each coded segment is independent such that derivation of an intra prediction mode for a second unit of a given one of the picture segments depends on a derived intra prediction mode for a first unit of the given one of the picture segments, and is independent of any intra prediction mode for units of any other picture segments. The encoder circuit is further configured to generate a bitstream comprising the plurality of coded segments and information indicating the partition structure used to divide the picture into the picture segments and transmit the bitstream. In these embodiments, the information indicates a unit size of a unit of the partition structure and a subsegment size of a subsegment of the partition structure.

One or more embodiments of the present disclosure further provide an encoder comprising a memory configured to store instructions and processing circuitry operatively connected to the memory. The processing circuitry is configured to execute the instructions stored in the memory whereby the encoder is configured to define a partition structure that divides a picture into a plurality of picture segments. Each picture segment comprises one or more subsegments with at least one picture segment of the partition structure comprising one or more subsegments. Further, each subsegment comprises a plurality of units, with each unit corresponding to a region of the picture. By executing the instructions, the processing circuit further configures the encoder to encode the picture segments in accordance with the partition structure to generate a plurality of coded segments. Each coded segment corresponds to one of the picture segments of the partition structure. Further, each coded segment is independent such that derivation of an intra prediction mode for a second unit of a given one of the picture segments depends on a derived intra prediction mode for a first unit of the given one of the picture segments, and is independent of any intra prediction mode for units of any other picture segments. By executing the instructions, the processing circuit further configures the encoder to generate a bitstream comprising the plurality of coded segments and information indicating the partition structure used to divide the picture into the picture segments and transmit the bitstream. In these embodiments, the information indicates a unit size of a unit of the partition structure and a subsegment size of a subsegment of the partition structure.

DETAILED DESCRIPTION

This application claims priority to U.S. Application No. 62/688,301, filed 21 Jun. 2018, disclosure of which is incorporated in its entirety by reference herein.

Quad Tree and Binary Tree (QTBT) Structure

As previously stated, HEVC is a block based video codec standardized by ITU-T and MPEG that utilizes both temporal and spatial prediction. HEVC uses a block structure where each top level coding block, i.e. the largest block in the coding block partitioning referred to herein as the Coding Tree Unit (CTU), can be partitioned by a Quad Tree (QT) structure. This partitioning produces coding block partitions, referred to herein as coding units (CUs). The CUs can then be further partitioned, recursively, into smaller equally sized CUs with the quad tree structure down to an 8×8 block size.

The block structure in the current version of Versatile Video Coding (VVC) is different than the block structure in HEVC. Particularly, the block structure in VVC is referred to as Quad Tree plus Binary Tree plus Ternary Tree block structure (QTBT+TT). A CU in QTBT+TT can have either square or rectangular shapes. A coding tree unit (CTU) is first partitioned by a quad tree structure as in HEVC. Then it is further partitioned with equally sized partitions either vertically or horizontally in a binary structure to form coding blocks (also referred to as coding units or CUs). A block could thus have either a square or rectangular shape.

The depth of the quad tree and the binary tree can be set by the encoder in the bitstream. An example of dividing a CTU using QTBT+TT is illustrated inFIG.1. The TT allows the CU to be divided into three partitions instead of two equally sized partitions. This increases the possibility of using a block structure that better fits the content structure in a picture.

Context Adaptive Binary Arithmetic Coding (CABAC)

Context Adaptive Binary Arithmetic Coding (CABAC) is an entropy coding tool used in HEVC and VVC. CABAC is configured to encode binary symbols, which keeps complexity low and allows modelling of probabilities for bits of a symbol that are more frequently used. The probability models are selected adaptively based on local context, since coding modes are usually well correlated locally.

Slices

The concept of slices in HEVC divides the picture into independently coded slices, where each slice is read in raster scan order in units of CTUs. Different coding types could be used for slices of the same picture. For example, a slice could either be an I-slice, P-slice or B-slice. However, the main purpose of slices is to enable resynchronization in case of a loss of data.

Tiles

The HEVC video coding standard also includes a tool called “Tiles” that divides a picture into rectangular, spatially independent regions. Using tiles, a picture in HEVC can be partitioned into rows and columns of samples where any given tile is located at an intersection of a given row and a given column.FIG.2shows an example of a tile partitioning using four (4) rows of tiles and five (5) columns of tiles resulting in a total of twenty (20) tiles for the picture. As seen inFIG.2, the tiles in HEVC are always aligned with CTU boundaries.

The tile structure is signaled in the Picture Parameter Set (PPS) by specifying the thicknesses of the rows and the widths of the columns. Individual rows and columns can have different sizes, but the partitioning always span across the entire picture, from left to right and top to bottom respectively.

The PPS syntax used for specifying the tile structure in HEVC is listed in Table 1. As seen in Table 1, a flag (i.e., the tiles_enabled_flag) indicates whether tiles are used or not used. If the tiles_enabled_flag is set, the number of tiles columns (i.e., num_tile_columns_minus1) and rows (i.e., num_tile_rows_minus1) are specified. The uniform spacing flag is a flag that specifies whether the column widths and row heights are explicitly signaled, or whether a pre-defined method to space the tile borders evenly should be used. If explicit signaling is indicated, the column widths are signaled one-by-one followed by the row heights. The column width and row height information is signaled in CTU units. Finally, the flag loop_filter_across_tiles_enabled_flag specifies whether in-loop filters across tile boundaries are turned on or off for all tile boundaries in the picture.

Similar to slices, there is no decoding dependency between tiles of the same picture. This includes intra prediction, context selection and motion vector prediction. One exception, however, is that in-loop filtering dependencies are generally allowed between tiles. Those dependencies can, however, be disabled by setting the loop_filter_across_tiles_enabled_flag appropriately.

In contrast to slices, tiles do not require as much header data. The header overhead per tile comprises the signaling of bit-stream offsets, which are present in the slice header and indicate the starting point of all tiles in a picture. A decoder decodes the starting points to enable splitting the coded picture into coded tiles in order to distribute them for parallel decoding. In HEVC, the inclusion of bit-stream offsets in the slice header is mandatory when tiles are enabled. However, the combination of tiles and slices is restricted in HEVC. Particularly, either all CTUs in a tile belong to the same slice or all CTUs belong to the same tile.

Bit-stream offsets can also allow the extraction of tiles and stitching of tiles to re-constitute the tiles into an output stream. This requires some encoder side constraints to make the tiles temporally independent. One constraint restricts motion vectors so that motion compensation for a tile only uses samples included in spatially co-located tiles of previous pictures. Another constraint restricts temporal motion vector prediction (TMVP) such that this process is made temporally independent. For complete independence, the deblocking of boundaries between tiles must be disabled via the previously described loop_filter_across_tiles_enabled_flag. However, disabling deblocking can introduce visible lines between tiles. Therefore, some implementations disable deblocking while other implementations do not.

Tiles are sometimes used for 360-degree video that is intended for consumption using head-mounted display (HMD) devices. The field-of-view when using today's HMD devices is limited to around 20% of a full sphere. This means that only 20% of the full 360-degree video is consumed by the user. Typically, the entire 360-degree video sphere is made available to the HMD device, which then crops out the part that is rendered for the user. That part, i.e. the part of the sphere the user sees, is called the viewport. A well-known optimization of resources is to make the HMD device video system aware of head movements and the direction the user is looking so that fewer resources are spent on processing video samples that are not rendered to the user. The resources can be, for example, bandwidth from a server to a client or the decoding capability of the device. For future HMD devices, where the field of view will be larger than is currently possible, a non-uniform resource allocation would still be beneficial. Particularly, the human vision system demands a higher image quality in the central vision area (about 18° horizontal view), while a lower demand is placed on the image quality in the peripheral region (about 120° or more for a comfortable horizontal view). Therefore, non-uniform resource allocation would be helpful to meet the demands of the human vision system with more resources being allocated in the central vision area as compared to the peripheral region.

Optimizing resources to the Region of Interest (RoI) is another use case for tiles. RoI can be specified in the content or extracted by methods such as eye tracking. One method of using head movements to reduce the amount of required resources is to use tiles. This method first encodes the video sequence multiple times using tiles. The tile partitioning structure in some embodiments is the same for each of these encodings, but the video sequence is encoded at different video qualities. This produces at least one high-quality encoding for the video sequence and one low-quality encoding for the video sequence. This means that for each tile at a particular point in time, there are at least one high-quality tile representation and at least one low-quality tile representation. The difference between a high-quality tile and a low-quality tile can be that the high-quality tile is encoded at a higher bitrate than the low-quality tile, or that the high-quality tile is of higher resolution than the low-quality tile.

FIGS.3A-3Billustrate an example of high-quality tiles having higher resolution than low-quality tiles. Particularly,FIG.3Aillustrates a picture that just has been decoded by a decoder. In this context, pictures showing the output from the decoder (i.e., the decoded pictures) are pictures shown in the compressed domain. In this example, all 8 tiles A-H in the compressed domain are of equal spatial size. Then, after the picture is decoded, but before the picture is rendered, the tiles A-H are scaled and arranged spatially. Pictures that have been prepared for rendering (i.e., after scaling and re-arrangement) are pictures to be shown in the output domain. The output domain, seen inFIG.3B, illustrates the picture as it is rendered or presented to the user.

As seen inFIGS.3A-3B, tiles B, D, F, and H are high-quality tiles since they are of higher resolution in the output domain. Tiles A, C, E, and G, however, are low-resolution tiles since the scaling step decreases the actual resolution.

Besides illustrating how tiles have different resolutions,FIGS.3A-3Balso illustrate that tiles in the compressed domain (FIG.3A) do not need to be spatially ordered in the same way as they are ordered in the output domain (FIG.3B). Given that the tiles are both spatially and temporally independent, the spatial placement of tiles in the compressed domain is not vital.

There are multiple elements that increase bit-costs when enabling tiles in HEVC. First, prediction across tiles is disabled, which means that motion vectors and intra modes are not predicted across tiles. The use of tiles also disables quantization parameter (QP) prediction and context selection. Second, CABAC is initialized for each tile, which means that CABAC adaptation is impaired. Third, bit-stream offsets must be signaled for each tile. Fourth, the tile partitioning structure needs to be specified in the PPS. Finally, CABAC is flushed after each tile and the coded data has to be byte-aligned.

Tiles are useful; however, there are some considerations that need to be addressed. For example, in its current form, HEVC restricts tiles to ensure that they span across the entire picture. This limits the flexibility of tiles, however. For example,FIG.4illustrates an example of a tile structure that is not supported by the current implementation of HEVC. In order to virtually support such a tile structure, one can specify the partition structure in HEVC seen inFIG.5. However, the tile structure seen inFIG.5comprises more tile boundaries. This can harm compression efficiency and can cause unnecessary tiling artifacts. Additionally, if the tiles are to be completely spatially independent, HEVC in its current form only allows for either enabling or disabling the in-loop filtering dependencies for all tile boundaries of a picture. Therefore, unnecessary in-loop filtering restrictions will occur with conventional HEVC solutions because they do not support flexible tile partitions.

Embodiments of the present disclosure address these issues by providing flexible tile partitioning methods. It will be obvious to one of ordinary skill in the art that one or more embodiments are useful for other partitioning applications (e.g., to partitioning involving slices). Thus, embodiments provide flexible partitioning of picture segments that comprise units corresponding to a region of a partitioned picture. These picture segments are therefore in one or more embodiments described as tiles and the partition structure referred to as a tiling structure, but the teachings are applicable to other types of picture segments (e.g., slices).

It should be noted that some embodiments refer to the term “unit.” A unit is, for instance, a rectangular area of a picture (e.g. coding tree units or coding units) such that a tile consists of one or multiple units. Further, some embodiments refer to the term “subsegment” or “sub-tile” or “sub-tile units” interchangeably. Each subsegment is also for instance, a rectangular area of a picture (e.g. coding tree units or coding units) such that a tile consists of one or multiple subsegments. Thus, a size of a subsegment or unit is sometimes referred to as a spatial size. In the context of a picture segment that is a tile, a subsegment is referred to as a sub-tile. One of ordinary skill in the art will appreciate that teachings applicable to a sub-tile are applicable to other subsegments and thus the term sub-tile and subsegment are used interchangeably herein. A subsegment in one or more embodiments includes a plurality of units. However, in one or more embodiments a subsegment comprises a single unit. In those cases, one of ordinary skill in the art will appreciate that teachings applicable to a unit are also applicable to a subsegment that comprises a single unit.

Methods described herein provide more freedom for an encoder to partition pictures into tiles. In one embodiment, each tile comprises a single rectangular area, and the width and height of each tile is signaled to a decoder in a bitstream. Upon receipt, the decoder is configured to decode the individual width and height values for each tile from the bitstream.

In another embodiment, the picture is divided into units, with each unit being assigned a tile identity (ID) that identifies which particular tile the unit is assigned to. Further, in one or more embodiments the picture is divided into subsegments which each comprise a unit or a plurality of units. Each subsegment is assigned a tile ID that identifies which particular tile the subsegment is assigned to. This latter embodiment is beneficial as it supports flexible tile partitions having non-rectangular tiles, as well as partitions in a checkerboard pattern.

Embodiments of the present disclosure enable the use of more flexible tile partitions for various use cases, such as 360-degree video. Further, when compared with fixed tile partitions, the bit cost associated with flexible tile partitions is lower. Specifically, the number of tiles is higher with the use of conventional tile partitioning, and each tile comes at a cost (e.g. the processing associated with CABAC flush and adaptation reset, bit-stream offsets, etc.). The flexible tile partitioning of the present embodiments, however, uses fewer tiles thereby reducing bit costs.

The present embodiments also enable flexible CPU load balancing for parallel processing where an encoder can, for example, analyze an input picture and select a tile partitioning that splits the picture into as few tiles as possible (to reduce bit cost) while filling the available number of cores equally. Moreover, embodiments of the present disclosure also facilitate increased control over in-loop filters as compared to conventional methods that to make traditional tiles more flexible.

The present embodiments are even more beneficial for demanding use cases, such as Multi-View (MV) video coding, for example, where the bandwidth or decoding capability is a major bottleneck. In such cases, the present embodiments enable better resource allocation. In applications where the region of interest (RoI) is specified, the embodiments of the present disclosure also provide a more flexible resource allocation.

The flexible tile partitioning embodiments of the present disclosure use larger tiles than do conventional tiling methods. Because of the larger tile size, the present embodiments also contribute to the quality of a picture by putting fewer limitations on the intra motion vectors. Further the flexible tile partitioning methods of the present disclosure removes unnecessary partitioning lines between tiles. Such line removal reduces the cost of filtering with respect to lines between the tiles.

Embodiments of the present disclosure partition a picture from a sequence of video pictures into tiles with a more flexible tile layout than can be provided by conventional means, e.g. HEVC. This includes configuring an encoder to generate the partition structure and tiles for encoding, as well as a decoder to decode the partition structure and tiles. In some embodiments, the flexible tile partitioning method of the present disclosure co-exists with the processing of HEVC in its conventional form. Thus, as previously described, the tiles_enabled_flag in the bitstream can be used to specify whether the traditional tile partitioning scheme should be used, or whether the flexible tile partitioning scheme according to the present embodiments should be used.

FIG.6Aillustrates an example in which a picture10is partitioned into four (4) tiles or “segments” T1, T2, T3, T4(collectively, “T”) (as shown by the solid black lines), with each tile or segment T1, T2, T3, T4comprising 16 units U. This provides a total of 64 units U for the picture10.FIG.6Billustrates picture10in which segments T1, T2, T3, T4are further partitioned into 16 subsegments (e.g., T1-1, T1-2, T1-3, T1-4) as shown by the solid black lines and dotted lines. Each subsegment comprises 4 units U.

The flexible tile structure is signaled in a bitstream. For example, the encoder signals the flexible tile structure in a parameter set such as the sequence parameter set (SPS) or picture parameter set (PPS). However, as those of ordinary skill in the art will appreciate, the present disclosure is not limited to signaling the flexible tile structure in a parameter set. Although the embodiments herein describe the tile structure as being signaled in the PPS, this is for illustrative purposes only and this information can be signaled in other ways.

General Description

FIG.7shows a bitstream12that is decoded into a picture10.FIG.8shows picture10partitioned into 64 units U.FIGS.11A-Cshows another example picture10with 64 units U shown inFIG.11Athat also uses subsegments (e.g., T1-1, T1-2, T1-3, T1-4) as shown inFIG.11B. One example of a unit U is a macroblock known from H.264. Another example of a unit U is a CTU known from HEVC.

As seen inFIG.7, bitstream12comprises one or more coded segments T and one or more code words16,17,18. The one or more codewords16comprises information that specifies the number of bytes or bits in the bitstream comprising the data needed for decoding each of the coded segments T. The one or more codewords17carry information specifying a spatial size of the units U. The one or more codewords18carry information specifying a spatial size of a subsegment. As an example, the spatial size of the units U is 64 luma samples horizontally and 64 luma samples vertically; however, as those of ordinary skill in the art will appreciate, the present disclosure is not limited to these sizes and shapes of units U. Other unit sizes and shapes, including irregularly-shaped units (e.g., units not shaped as squares), are also possible according to the embodiments of the present disclosure.

As seen inFIGS.7-8, picture10has been spatially partitioned into three segments T1, T2, T3using a partition structure. The information defining the partition structure was derived from information in the bitstream12. For example, the coded segments14from the bitstream12are decoded to derive the sample values of a corresponding decoded segment in the picture10. Additionally, the size and/or position of the coded segment14can be parsed from the one or more codewords16in the bitstream12. As previously stated, one embodiment of the present disclosure utilizes the one or more codewords16to carry information specifying the number of bytes or bits in the bitstream that comprise information needed to decode the coded segment14. Alternatively, however, embodiments of the present disclosure can utilize the one or more codewords16to carry information needed to derive the locations or positions of the coded segment14in the bitstream12.

As seen inFIG.8, the boundaries of each segment T1, T2, T3are aligned with the boundaries of the unit U. This means that each segment T1, T2, T3comprises an integer number of units U, and that each unit U is associated with one segment T. For example, as seen inFIG.8, the two rightmost segments T2, T3comprise 16 units each, while the leftmost segment T1comprises 32 units.

FIG.9illustrates an embodiment in which picture10is partitioned using the same partition structure S illustrated inFIG.8. However, for clarity's sake,FIG.9does not illustrate the units U. As seen inFIG.9, one side19-1of a first segment T1borders the sides19-2,19-3of two or more neighboring or adjacent segments T2, T3, respectively.

FIG.10illustrates picture10that has been partitioned using the same partition structure S illustrated inFIGS.8and9. As seen inFIG.10, picture10comprises three segments T1, T2, T3. All segments T1, T2, T3ofFIG.10comprise one or more units U; however, for clarity sake,FIG.10illustrates segments T1and T3as comprising a subset of the units U. Specifically, segment T1comprises unit U1while the neighboring or adjacent segment T3comprises units U2, U3. Each of the segments T1, T2, T3are independent with respect to each other. This means that the boundaries of the segments T1, T2, T3are handled similar to picture boundaries when decoding the units U. This affects the derivation process of elements during decoding such as e.g. the derivation of intra prediction modes and the derivation of quantization parameter values.

Intra prediction modes (e.g., intra (B) prediction mode and intra (P) prediction mode) are used and signaled for units that only use predictions from previously decoded samples of a current picture for sample prediction. It is common that the derivation of the intra prediction mode in a particular unit U2depends on previously derived intra prediction modes in other units U3. With segments being independent, the derivation of the intra prediction mode in a particular unit U2only depends on previously derived intra prediction modes in units U3that belong to the same current segment T3and does not depend on any intra prediction mode in any unit, such as unit U1, that is associated with a different segment, such as segment T1. Thus, the partition structure S ofFIG.10makes the intra prediction modes in the units U1in a first segment T1unavailable for the derivation of the intra prediction mode for units U2, U3in second, different segment T3.

Unit U1in the first segment T1could, however, be used for derivation of an intra prediction mode in another unit, but only if that other unit was in the same segment T1. Thus, the segment boundaries have the same or similar effect on intra mode derivation as a picture boundary for the units U2, U3in second segment T3. In other words, the derivation of an intra prediction mode for a first unit U2of a picture segment T3may depend on a derived intra prediction mode for a second unit U3of the picture segment T3.

The quantization parameter (QP) value is also well known in the art. The QP value for a unit U, for example, is typically derived using previously derived QP values. One derivation process known in the art is to first derive a predicted QP value from the bitstream12, and then add a delta QP value to the predicted QP value. Because the segments T1, T2, T3are independent of each other, the derivation of the QP value in a particular unit, such as unit U2of segment T3, for example, can only depend on previously derived QP values in units that are in the same segment T3. The derivation of a QP value in a particular given unit U2, U3does not depend on any QP values associated with any other unit that is in a different segment, such as unit U1in segment T1. This means that the partition structure S inFIG.10makes the QP values in units U1in a first segment T1unavailable for the derivation of the QP value for the units U2, U3in a second, different segment T1.

The QP value in some units U could be used for the derivation of a QP value in another unit U, but only if that other unit was in the same segment T1. Thus, the segment boundaries have the same or similar effect on QP value derivation as a picture boundary for the units U2, U3in second segment T3.

FIGS.11A-11Cillustrate the same picture10partitioned according to the partition structure S but includes the use of subsegments SS1-SS16, as shown inFIG.11B.FIG.11Aillustrates the units U. As previously described, the spatial size of the units is signaled in the bitstream12as one or more code words17.FIG.11Bshows subsegments SS1-SS16, each of which comprises an integer number of units U greater than 1. Thus, each subsegment SS1-SS16comprises at least two units U, and each unit U belongs to one subsegment only. The spatial size of the subsegments SS1-SS16is signaled in the bitstream12as one or more code words18. The spatial size of the subsegments (21) is expressed in terms of units. In the example inFIG.11B, the spatial size of the subsegments is 2 units horizontally and 2 units vertically. It should be noted that subsegments can be rectangular such that the width and height differ. The information carried in the one or more code words (24) may consist of one height value and one width value. The information can be regarded as one subsegment size (i.e. a value pair), or be regarded as independent width and height values. The code words (24) are decoded by a decoder from the bitstream12and used to derive the size of the subsegments.

FIG.11Cillustrates segments T1, T2, T3and the partition structure S of picture10. Each segment T1, T2, T3comprises at least one unit U. Segments T1, T2, T3comprise an integer number of subsegments SS1-SS16greater than 0. Thus, each segment T1, T2, T3comprises at least one subsegment SS1-SS16, and each subsegment SS1-SS16belongs to one segment T1, T2, T3only. It can be noted that if all segments T1, T2, T3comprise a single subsegment, the subsegments in one or more embodiments are redundant and need not be included in information signaled as part of the partition structure S. Therefore, subsegments are useful where there is at least one segment T1, T2, T3in a picture10that includes more than one subsegment SS1-SS16. The spatial size of the segments T1, T2, T3is expressed in terms of subsegments SS1-SS16, which means that the size of the subsegments SS1-SS16is used when deriving the spatial sizes of the segments T1, T2, T3and/or the spatial partition structure S. In the example inFIGS.11A-11C, the spatial size of two segments T2, T3is 2 subsegments horizontally and 2 subsegments vertically, and the spatial size of one segment T1is 2 subsegments horizontally and 4 subsegments vertically.

It should be noted that segments T1, T2, T3in some embodiments can be rectangular such that the width and height of the segments differ. In other embodiments, segments T1, T2, T3are non-rectangular and expressed by allocating each unit U and/or subsegment SS to one segment (e.g., subsegment T1). Such an allocation are used to represent, for example, “chessboard segments,” in which some of the segments T1, T2, T3represent white squares on a chessboard, while other segments T1, T2, T3represent the black squares on the chessboard. A segment (e.g., segment T1) can, in some embodiments, be equivalent to a tile or slice. A subsegment can, in some embodiments, be equivalent to a sub-tile.

First Embodiment

As was seen inFIG.9, a picture10or a part of a picture10in a video stream is divided into a spatial partition structure S, where the partition structure S comprises at least three segments T1, T2, T3. Each segment T1, T2, T3comprises units U, with at least one of the segments T1, T2, T3comprising two or more units U. The spatial size of the units U is signaled in, and decoded from, the one or more codewords17of the bitstream12. Additionally, the one or more code words16indicating the size or position of at least one coded segment T in the bitstream12are parsed from the bitstream12.

At least one segment T1has at least one side19-1that borders the sides19-2,19-3of two or more than one neighboring segments T2, T3. Additionally, the segments T1, T2, T3are independent with respect to each other such that the derivation of any intra prediction mode for any unit U in a first segment T1only depends on previously derived intra prediction modes in units U that are also in the first segment T1. The derivation of any intra prediction mode for a unit U in the first segment T1does not depend on any intra prediction mode in any unit U that is in a second, different segment T2. Such segments are referred to herein as “flexible tiles” or simply just “tiles” and are different from HEVC tiles which are arranged in as strict columns and rows defined with respect to a single unit. That is, with HEVC, no tiles in the picture have one side that borders more than one neighboring segment. Rather, a given side of a given tile in HEVC borders only one side of one other adjacent tile. Flexible tiles can also include optional subsegments that are the same thing as flexible sub-tiles or more simply expressed as sub-tiles. It should be noted that the present disclosure uses the terms segments, flexible tiles, and tiles interchangeably. However, within the context of these embodiments, the terms “flexible tile” and “tile” are defined to be the same as a segment, and all are different than HEVC tiles, as previously described.

FIG.12Ais a flow diagram illustrating a method20for encoding a picture or part of a picture using flexible tiles in accordance with a first embodiment of the present disclosure.

Method20is implemented at an encoder and begins with the encoder defining a flexible tile structure for the picture (box22). The flexible tile structure comprises at least one flexible tile having at least one side that borders two or more adjacent flexible tiles. The encoder then defines at least one syntax element to describe how to construct the flexible tile structure (box24). The at least one syntax element comprises one or more properties that describe the flexible tiles, or the syntax element comprises means to derive properties for the flexible tiles. Such properties comprise, for example, an identifier, a width, a height, and a position that describe how to assign and/or access the individual flexible tiles. The encoder then signals the at least one syntax element for the flexible tile structure in a video bitstream (box26), and encodes the picture or part of the picture according to the flexible tile structure into the video bitstream (box28). The flexible tile structure comprises at least one flexible tile having at least one side that borders more than one adjacent flexible tile.

FIG.12Bis a flow diagram illustrating a method30for decoding a picture or part of a picture that was encoded using flexible tiles in accordance with the first embodiment of the present disclosure.

As seen inFIG.12B, method30begins with the decoder receiving a video bitstream (box32) and parsing the at least one syntax element for the flexible tile structure from a video bitstream received from the encoder (box34). The decoder can then construct the flexible tile structure from the at least one syntax element, and assign or derive properties for the individual flexible tiles (box36). The properties include an identifier, a width, a height, and a position that can be used for assigning and/or accessing the flexible tiles. The decoder then decodes the picture or part of the picture according to the flexible tile structure (box38). As stated previously, the flexible tile structure comprises at least one flexible tile having at least one side that borders corresponding sides of more than one adjacent flexible tile.

Second Embodiment—Specifying Tile Width and Height

In a first embodiment, the flexible tiles are scanned in a pre-defined order, and a tile width and tile height are specified for each tile. In one or more embodiments, the pre-defined order is a raster scan order, e.g. along the units (e.g., the CTUs), and the tile size is specified in units. Alternatively, the pre-defined order is a raster scan order along the subsegments, and the tile size is specified in terms of subsegments.

FIG.13Ais a flow diagram illustrating a method40by which the encoder constructs and signals the flexible tiles.

As seen in method40, the encoder first determines whether the picture has one tile or multiple tiles (box42). If the picture has only one tile, the encoder sets the tile_enabled_flag equal to 0 to indicate that the picture has only a single tile (box44) and ends the process. Otherwise, if the picture has more than one flexible tile, the encoder creates an empty list of tiles (box46). Optionally, the method includes a step (box43) of determining whether subsegments are used for any of the tiles. If no, the method carries on with the encoder creating an empty list of tiles (box46). If the partition structure includes subsegments (box45), the height and or width of a sub-tile is determined and signaled in the bitstream. The subsegment height and/or width can be predefined. Following a predefined scan order, the encoder adds the flexible tile to the list, including the information identifying the height and width of the flexible tile (box48) (e.g., expressed in terms of units or subsegments if subsegments are used). The encoder continues to add flexible tiles to the list as long as the picture has more flexible tiles (box50).

FIG.13Bis a flow diagram illustrating a method60by which the decoder parses and constructs the flexible tiles.

As seen in method60, the decoder first determines whether the picture has one tile or multiple tiles (box62). If the tile_enabled_flag is set to 0, indicating that the picture has only one flexible tile with the identified width and height, the method ends. Otherwise, while there are still flexible tiles in the list (box64), the decoder parses the next tile in the list, specified by its width and length following the predefined scan order (box66). If optional subsegments are part of the partition structure and not predefined, the decoder optionally parses the sub-tile height and/or width from the bitstream. This can occur at various points in the method13B as (e.g., after it is determined there is more than one tile).

Table 2 identifies an example syntax for use in one or more embodiments of the present disclosure. The syntax could be implemented on top of the currently existing HEVC video coding specification.

In one embodiment, the syntax elements tile_width_minus1 and tile_height_minus1 are restricted such that all of the following conditions must hold prior to CTU_tile_assignment[tile_coordinate_y+y][tile_coordinate_x+x]=tile_id:1. the tile_coordinate_y+y must be less than PicHeightInCtbsY;2. tile_coordinate_x+x must be less than PicWidthInCtbsY; and3. CTU_tile_assignment[tile_coordinate_y+y][tile_coordinate_x+x] must be equal to “no tile.”

FIG.14illustrates an example tile partition where 10 tiles T0. . . T9are used to partition the picture10. The numbers seen in each tile ofFIG.14represent a tile number (i.e., an integer 0, 1, 2, . . . ) followed by the width and height of the tile in CTUs (e.g.,4x2). Using the example syntax of Table 2 above, the tile partition is expressed by a total of 63.

FIG.15shows another example tile partition a picture10having a size of 2048×1280 is partitioned into 26 tiles. As seen inFIG.15, not all tiles are the same size and shape. Rather, there is one large sized tile TLand a plurality of smaller tiles TS. The large rectangular tile TLinFIG.15is a flexible tile as it has one side that borders the sides of more than one of the smaller tiles TS. Additionally, the picture comprises 32×20=640 CTUs, each of which has a size of 64×64 luma samples. Such a tile partition is used for 360 video streaming where some smaller tiles TSare used to cover the viewport and a single large tile TLcovers the full sphere in lower resolution.

Using the example syntax in Table 2 above, the tile partition ofFIG.15can be expressed by a total of 267 bits. One bit for the tiles_enabled_flag=1 indicating that flexible tiles are used. Then the 25 smaller tiles TSare signaled with 10 bits each, and the large tile TLis signaled with 16 bits using tile_width_minus1=11 (7 bits) and tile_height_minus1=19 (9 bits). If the tile size is used, the result would be 1+tile size (5+5)+25*2+(3+5)=69 bits.

In a variant of this embodiment shown inFIGS.36A-36C, more than one sub-tile size is defined.FIG.36Ashows an example partition structure S with 5 segments or tiles T1. . . T5.FIG.36Bshows that the example partition structure S has 7 subsegments or sub-tiles SS1. . . SS7of two different types A and B.FIG.36Cshows that the example partition structure S with 5 segments or tiles T1. . . T5and 84 units or CTUs. Each sub-tile size is specified in CTU units.

For each tile, the width and height is specified in one of those previously defined sub-tile units and the information about which sub-tile size is used is sent together with the width and height of that specific tile. In the following example, illustrated inFIGS.36A-36C, two sub-tiles of different sizes are used: one of 4×4 CTUs named sub-tile A and one of 3×3 CTUs named sub-tile B. Then:height and width of tile T1is specified in sub-tile unit A as 1×1.height and width of tile T2is specified in sub-tile unit A as 1×2.height and width of tile T3is specified in sub-tile unit B as 1×1.height and width of tile T4is specified in sub-tile unit B as 1×1.height and width of tile T5is specified in sub-tile unit B as 1×2.
When a tile can be specified in more than one sub-tile unit, the sub-tile unit that gives the smaller unit count in that tile is preferably chosen by the encoder for specifying the width and height of that tile.

Third Embodiment—Specify Tile Height and Width Using Subtiles and a Flag for Tile Size Copy

In a preferred embodiment, the tile partition structure is conveyed by signaling the height and width of tiles, optionally using sub-tile units. The tiles consist of an integer number of sub-tiles where the sub-tiles consist of an integer number of coding units such as CTUs. The spatial size of the coding units or sub-tiles is signaled in the bitstream (e.g., in terms of coding units).

For example, a CTU unit size is signaled in the bitstream to have a size of 64×64 luma samples. Then a sub-tile unit size is signaled to have a width of 4 and a height of 2. That means that the sub-tile has a width of 256 luma samples and a height of 128 luma samples since the sub-tile unit size is signaled using the CTU unit size. One tile then is signaled to have a width of 2 and a height of 1, which means that the tile has a width of 512 luma samples and a height of 128 luma samples. Using a sub-tile unit is beneficial when the widths of all tiles have a greatest common divisor larger than 1 CTU width and/or when the heights of all tiles have a greatest common divisor larger than 1 CTU height. Expressing the height and width of a tile can with sub-tiles be done using smaller values which can be done using fewer bits in the bitstream.

Further in this embodiment, a flag is used to specify whether or not a current tile size is identical to the closest previously signaled tile size. If the flag has one value, the tile size for the current tile is copied from a previous tile. If the flag has another value, the height and width of the tile is explicitly signaled using the height and width of the coding unit or sub-tile. As described in more detail below, the decoder decodes this flag and, based on its value, determines whether or not the current tile size is identical to the closest previously signaled tile size.

In one variant of this embodiment, the number of tiles in the picture is decoded first. Then there is a loop over the known number of tiles for which the size is decoded. Optionally, the size of the last tile is not signaled since the size of that last tile is given by the remaining area of the picture.

In another variant of this embodiment, the number of tiles in the picture is not signaled. Instead, the decoder continues decoding tile sizes until the decoder determines that all the necessary tile sizes have been decoded. When all necessary tile sizes have been decoded, the decoder stops decoding tile sizes and proceeds decoding other data from the bitstream. One way to determine whether all necessary tile sizes have been decoded is to determine whether all CTUs in the picture are in a tile for which the tile size has been decoded. Another way to determine whether all necessary tile sizes have been decoded is to determine whether the tiles corresponding to the decoded tile sizes jointly cover the entire picture.

In yet another variant of this embodiment, an optional codeword is signaled to convey the number of times a tile is being copied to the next tile position. For example, a first tile size is equal to 128×128. Then there can be one or more codewords in the bitstream that specifies whether that tile size:

1) is not copied to any other tile, which means that the next tile size is signaled explicitly using a width and height; or

2) is copied to the next tile; or

3) is copied to the next n tiles, where the number of times to copy is signaled.

As seen inFIG.16A, a decoder is configured to decode an encoded representation of a picture of a video bitstream using one or more of the following aspects.

Particularly, as seen in method70, the decoder receives coded elements from which to derive a specific spatial size for decoding the picture (box72). The spatial size is one of units and/or sub-tiles. For instance, the decoder derives a sub-tile width and a sub-tile height to use for decoding tile sizes, where the sub-tile width and height are conveyed as two integer numbers of a derived unit size. Alternatively, the decoder is configured to use a specific spatial unit size and or sub-tile size to decode the picture. The decoder decodes more than one tile size explicitly from the video bitstream (box74). In these cases, the tile size comprises a width value that is derived from the bitstream as an integer number of the unit width or sub-tile width, as well as a height value that is derived from the bitstream as an integer number of the unit height or sub-tile height. In one embodiment, the decoder decodes at least one tile size from the video bitstream, wherein the tile size comprises a width value and a height value that is copied from a previously decoded tile size (box76). In one embodiment, the decoder is configured to decode a flag for at least one current tile (box78). In these cases, one value could indicate that the tile size of the current tile is copied from a previously decoded tile size, while another value indicates that the tile size should be explicitly decoded from information in the bitstream. In one embodiment, the decoder determines the number of tile sizes to decode by decoding a number value from the bitstream specifying the number of tile sizes (box80). In one embodiment, the decoder determines the number of tile sizes to decode by determining whether there are additional tile sizes to decode (box82). Particularly, in these cases, the decoder can determine whether all CTUs in the picture are in a tile for which a tile size has been decoded. Alternatively, the decoder can determine whether the tiles corresponding to the decoded tile sizes jointly cover the entire picture. Regardless of the particular manner in which the decoder decodes the tile sizes, the decoder configured according to this embodiment uses the decoded tile sizes to decode the encoded representation of the picture.

As seen inFIG.16B, an encoder is configured according to this embodiment to encode a representation of the picture of a video bitstream using one or more of the following aspects.

Particularly, as seen in method90, the encoder can, in one embodiment, be configured to encode a specific spatial size to use for encoding the at least one picture (box92). For instance, the spatial size is a spatial size of a unit or a sub-tile. In one or more embodiments, the encoder encodes a specific sub-tile width and/or a sub-tile height to use for encoding tile sizes by inserting one coded integer sub-tile width value and/or one coded integer sub-tile height value into the bitstream. The values are in terms of the spatial unit size. In one embodiment, the encoder can be configured to encode more than one tile size explicitly into the bitstream (box94). In these cases, the encoder can be configured to encode each tile size by inserting one coded integer tile width value indicating the width of the units or sub-tiles, and one coded integer tile height value into the bitstream indicating the height of the units or subtiles. In one embodiment, the encoder can also be configured to derive at least one tile size by setting the tile size equal to a previously derived or decoded tile size (box96). In one embodiment, the encoder can also be configured to insert a flag value for at least one current tile (box98). In these embodiments, one value could be used to specify to the decoder that the tile size of the current tile should be copied from a previously decoded tile size, or the encoder could insert the tile size explicitly into the bitstream so that the decoder can derive the tile size explicitly. In one embodiment, the encoder can also be configured to indicate the number of tile sizes that are signaled by inserting a number value specifying the number of tile sizes into the bitstream. Regardless of the particular manner in which the encoder encodes the information, the encoder is configured to use the tile sizes to encode the at least one picture into the video bitstream.

Table 3 identifies an example syntax for use in this embodiment of the present disclosure. The syntax could be implemented on top of the currently existing HEVC video coding specification.

Example Semantics for the Syntax Elements in Table 3

tiles_enabled_flag equal to 1 specifies that there is more than one tile in each picture referring to the PPS. tiles_enabled_flag equal to 0 specifies that there is only one tile in each picture referring to the PPS. It is a requirement of bitstream conformance that the tiles_enabled_flag value shall be the same for all PPSs that are activated within a CVS.
flexible_tiles_enabled_flag equal to 1 specifies that flexible tiles are used to partition pictures referring to the PPS into tiles. flexible_tiles_enabled_flag equal to 0 specifies that flexible tiles are not used to partition pictures referring to the PPS into tiles. If flexible_tiles_enabled_flag is equal to 1, the value of the variable tile_id and the value of the two-dimensional array CTU_tile_assigment are specified as follows:tile_id=0for (y=0; y<PicHeightInCtbsY; y++)for (x=0; x<PicWidthInCtbsY; x++)CTU_tile_assignment[x][y]=“no tile”
number_of_tiles_in_picture_minus2 plus 2 specifies the number of tiles in a picture. number_of_tiles_in_picture_minus2 shall be in the range of 0 to PicSizeInCtbsY-2, inclusive.
subtile_width_minus1 plus 1 specifies the width of the subtile unit in coding tree units. subtile_width_minus1 shall be in the range of 0 to PicWidthInCtbsY-1, inclusive
subtile_height_minus1 plus 1 specifies the height of the subtile unit in coding tree units. subtile_height_minus1 shall be in the range of 0 to PicHeightInCtbsY-1, inclusive
use_previous_tile_size_flag equal to 1 specifies that the size of the current tile is equal to the size of the previous tile. use_previous_tile_size_flag equal to 0 specifies that the size of the current tile is not equal to the size of the previous tile. When not present, the value of use_previous_tile_size_flag is inferred to be equal to 0.
When use_previous_tile_size_flag is equal to 1, the value of the i'th element of the arrays tile_height_minus1 and tile_width_minus1 are derived as follows:
tile_height_minus1[i]=tile_height_minus1[i−1]
tile_width_minus1[i]=tile_width_minus1[i−1]
tile_width_minus1[i] plus 1 specifies the width of the i-th tile in tile units.
tile_height_minus1[i] plus 1 specifies the height of the i-th tile in tile units.
The value of the variable tile_id and the value of the two-dimensional array CTU_tile_assigment are derived as follows:for (tile_coordinate_y=0; tile_coordinate_y<PicHeightInCtbs_Y; tile_coordinate_y++)for (tile_coordinate_x=0; tile_coordinate_x<PicWidthInCtbsY; tile_coordinate_x++)if (CTU_tile_assignment[tile_coordinate_x][tile_coordinate_y]==“no tile”) breakfor (ctu_y=0; ctu_y<(tile_height_minus1[i]+1) (subtile_height_minus1+1) && (tile_coordinatey+ctu_y)<PicHeightInCtbsY; ctu_y++)for (ctu_x=0; ctu_x<(tile_width_minus1[i]+1)*(subtile_width_minus1+1) && (tile_coordinate_x+ctu_′x)<PicWidthInCtbsY; ctu_x++)CTU_tile_assignment[tile_coordinate_x+ctu_x][tile_coordinatey+ctu_y]=tile_idtile_id++

It is a requirement of bitstream conformance that all of the following conditions be true prior to the assignment of CTU_tile_assignment[tile_coordinate_x+ctu_y][tile_coordinate_y+y] to the value tile_id:1. The value of tile_coordinate_y+ctu_y shall be less than PicHeightInCtbsY;2. The value of tile_coordinate_x+ctu_x shall be less than PicWidthInCtbsY; and3. The value of CTU_tile_assignment[tile_coordinate_x+ctu_x][tile_coordinatey+ctu_y] shall be equal to “no tile”
In one embodiment, conditions 1 and 2 above are optional.

Further, the derivation of the variable tile_id and the value of the two-dimensional array CTU_tile_assigment as specified in the semantics of tile_height_minus1[i] are executed for each value of i. This means that derivation is performed after each loop of the variable i, even in cases:1. where the use_previous tile_size_flag is not present;2. when the use_previous tile_size_flag is equal to 0; and3. when use_previous tile_size_flag is equal to 1.

Similarly, the three bitstream conformance conditions shall be true for any assignment done in any loop.

In one embodiment, it is required that the break statement is reached for each loop; for each value of i that is.

In one embodiment, it is required that after all assignments of CTU_tile_assigment, CTU_tile_assignment[x][y] shall not be equal to “no tile” for any x ranging from 0 to PicWidthInCtbsY-1 inclusive and any y ranging from 0 to PicHeightInCtbsY-1 inclusive.

Fourth Embodiment—Status Flags

In a fourth embodiment, the present disclosure provides a method for determining whether a given unit, e.g., unit U, or sub-tile, e.g. sub-tile SS, does or does not begin a new tile. For instance, the size of the sub-tile and or units is defined and then scanned in a predefined order (e.g., raster scan order) for encoding or decoding.

For example, in one embodiment, the sub-tile is defined as being the same size as a unit (e.g., a CTU) or a rectangular set of units, and then scanned in raster scan order. In raster scanning, scanning is conducted horizontally left-to-right at a steady rate, then blanks and rapidly moves back to the left, where it turns back on and sweeps out the next line such that the vertical position steadily increases. Of course one of ordinary skill in the art will appreciate that when raster scan order is described as an example in this embodiment and other embodiments, other predefined scanning orders could be used (e.g., scanning horizontally right-to-left or scanning initially vertically).

Regardless of the particular scanning order, in one or more embodiments for each sub-tile, there is a sequence of flags in the order, each one specifying whether a given sub-tile starts a new tile or not. In one or more embodiments, no flag is sent for a given sub-tile when another sub-tile positioned above and to the left belongs to the same tile. In such cases, it can be inferred that both the given sub-tile and the other sub-tile are positioned in the same tile. One or more embodiments herein are described with reference to a sub-tile. However, one of ordinary skill in the art will appreciate that the teachings are applicable to specify whether a given sub-tile or unit starts a new tile or not (e.g., in the case where a sub-tile is a unit or sub-tiles are not employed for a given picture). Similarly, one of ordinary skill in the art will appreciate that one or more embodiments specified with respect to a unit are applicable to scanning sub-tiles in a predefined order.

FIG.37graphically illustrates an example embodiment of status flags to show whether the positioning of a sub-tile being scanned indicates that the sub-tile starts a new tile, and the values (if any) to set the flag indicating whether the unit starts a new tile. A similar status flag system can be used to show whether a given unit starts a new tile or not.

For all cases shown inFIG.37, except for the ambiguous case in the last row (i.e., whether the sub-tile being scanned is in tile B, tile C, or starts a new tile), one flag and the known status of other previously scanned sub-tiles (i.e., the sub-tiles positioned to the left and top of the sub-tile being scanned, if they exist) is sufficient with which to specify the correct status of the sub-tile currently being scanned. To remove the ambiguity in the case shown in the last row ofFIG.37, the present disclosure introduces a disambiguator bit. For example, in one embodiment, all the disambiguator bits in the defined scanning order are placed into a disambiguator array. The array is then explicitly signaled to the decoder.

Table 5 provides a first example syntax for use with this embodiment. The syntax shown in Table 5 could be implemented on top of the currently existing HEVC video coding specification. Additionally, for illustrative purposes only, the size of a sub-tile is equal to the size of the CTU. Of course one of ordinary skill in the art will appreciate that a sub-tile could be several units or CTUs or the syntax could apply to a unit.

As seen inFIG.37, there are some instances where the CTU statuses remain undetermined (e.g., those in rows 2, 3, 5, 6, and 8 ofFIG.37). In these cases, the status can be determined as specified by the signaling details seen inFIG.38.

ctu_status_disambiguator_flag: This flag comprises the dismabiguator bit that is used together with ctu_status to determine the status of the CTU when needed. The length of this bitstream equals the number of disambiguitor bits in the ctu_status. The ctu_status_disambiguator_flag is set if and only if the value of the ctu_status_flag is 0. In these cases, when the disambiguator_bit=0, the ctu_status should be copied from the unit on the left. When the disambiguator_bit=1, the ctu_status should be copied from the unit above.
exist( ): This function indicates whether a tile ID exists for a neighboring block with a tile ID (tileIdAboveLeft, tileIdAbove, or tileIdLeft).
set_new_id( ): This function sets a new tile ID for the current CTU.
copy_id( ): This function copies the tile ID from a neighboring block according to the details provided inFIG.37.
assign_ctu_to_tile( ): This function assigns a given CTU to an explicit tile ID using the ctu_status and ctu_status_disambiguator flags. It also assigns the given CTU the status of a neighbor CTU positioned on the immediate left of the given CTU, immediately above the given CTU, and immediately above and to the left (i.e., diagonally) of the given CTU, if they exist.
read_more_tiles( ): This function is read in a predefined scanning order, such as raster scan order, as follows:IF there is at least one CTU in the picture that is not assigned to a tile, set the return value of read_more_tiles( ) to TRUE;ELSE set the return value of read_more_tiles( ) to FALSE

In one aspect of this embodiment, the tile ID is signaled when a new tile is signaled. The tile ID is a new tile ID or a previously defined tile ID. Signaling the tile ID in this manner allows for a more flexible tile structure with non-rectangular or discontinuous tiles.

Bit Count

FIGS.17A-17Cillustrate, according to one embodiment, the ctu_status bits (FIG.17B) for the tile partition (FIG.17A) of the picture seen inFIG.14. In this embodiment, cells that are empty indicate that no bit for that CTU is signaled in ctu_status. The “?” indicator signifies that a disambiguator bit is needed.FIG.17Cillustrates the corresponding ctu_status disamboguator bits.

Table 7 provides a second example syntax for use with this embodiment. The syntax shown in Table 7 would replace the first example syntax seen in Table 5 and could be implemented on top of the currently existing HEVC video coding specification. Additionally, for illustrative purposes only, the size of a unit is equal to the size of the CTU.

Fifth Embodiment—Copy Size Tile Mode

In a fourth embodiment, the present disclosure provides a method for scanning the tiles similar to that provided by the first embodiment above. However, this fifth embodiment includes one or more of the following additional features. In particular, this embodiment also allows:1. The use of traditional tiles in a “traditional tile mode” and the use of flexible tiles in a “flexible tile mode.” The determination of mode to operate in is signaled, for example, using a flag, such as a flexible_tiles_enabled_flag; and/or2. Sub-tiles can be used. The granularity of the sub-tiles can be signaled explicitly. Another words granularity is used herein to specify a size of the sub-tile. For instance, the size of a sub-tile is specified in an x and y coordinate system or with respect to a unit size. The granularity in one or more embodiments is explicitly signaled. Alternatively, it could be predefined.3. A particular size specification for a tile. For example, this embodiment cold specify using the size of a latest tile for a current tile, or the size of a tile to the immediate the left or above the current tile. The specified size is applied to the current tile or to all remaining tiles, for example, by signaling a mode value.

FIG.18Ais a flow diagram illustrating a method110for constructing and signaling the tiles according to this embodiment. The method110is implemented by an encoder and beings with the encoder determining whether a picture has only one tile (box112). If so, the encoder sets the tile_enabled_flag equal to 0 (box114) before ending method110. However, if the encoder determines that the picture has a plurality of tiles, the encoder creates an empty list of tiles (box116). Then, while tiles remain and following a predefined scan order (box118), the encoder sets the copy_tile_size_mode (box120). If the copy_tile_size_mode is set to 0 (box122), the encoder adds the tile, including the tile height and width, to the list of tiles (box124). The process then ends. Optionally the partition structure employs sub-tiles, which in one or more embodiments is explicitly indicated as for example in the method110ofFIG.18A. If sub-tiles are used, a flag can be used (e.g., set to 1) to signal the use of sub-tiles and the granularity can be signaled as described herein. Otherwise a flag can be set to 0 and the granularity of a unit (e.g., a CTU) is used instead. The sub-tiles indication and granularity can be signaled before or after the signaling of setting the tile_enabled_flag.

Regarding the decoder, it parses and constructs the tiles sent by the encoder. In this embodiment, the decoder perform these functions according to the method130illustrated inFIGS.18B-18C. As seen inFIG.18B, the decoder determines whether the tile_enabled_flag is set to 0 (box132). If so, the decoder will know that the picture comprises only a single tile, and therefore, method130ends. Otherwise, the decoder determines whether the flexible_tiles_enabled_flag was set to 1 (box134). If not, the decoder performs its decoding functions using traditional tiles (box136). If the decoder determines that the flexible_tiles_enabled_flag was not set to 1, however, the decoder begins processing the tiles. Optionally, the decoder also determines whether sub-tiles are employed (as by an indication described above). If sub-tiles are employed, the decoder further determines the granularity of the sub-tiles. While tiles remain to be processed (box138), the decoder parses the copy_tiles_size_mode value (box140) and uses that value to determine the width and height of the current tile (i.e., the tile currently being processed) (e.g., with respect to the granularity of the sub-tile).

Particularly, if copy_tile_size_mode is equal to 0 (box142), the decoder parses the next tile in the tile list that was created by the encoder, specified by its width and length following the predefined scan order (box144).

If copy_tile_size_mode is equal to 1 (box146), the decoder copies the width and height of the last signaled tile to the width and height of the current tile (box148).

Turning toFIG.18C, if copy_tile_size_mode is equal to 2 (box150), the decoder copies the width and height of the left tile to the width and height of the current tile (box152).

If copy_tile_size_mode is equal to 3 (box154), the decoder copies the width and height of the tile immediately above the tile currently being processed to the width and height of the tile currently being processed (box156).

If copy_tile_size_mode is equal to 4, and while not at the end of the list of tiles (box158), the decoder copies the width and height of the last signaled tile to the width and height of the current tile (box160).

If copy_tile_size_mode is equal to 5, and while not at the end of the list of tiles (box162), the decoder copies the width and height of the tile to the immediate left of the current tile, and copies those values to the width and height of the current tile (box164).

If copy_tile_size_mode is equal to 6, and while not at the end of the list of tiles (box166), the decoder copies the width and height of the tile immediately above the current tile to the width and height of the current tile (box168).

Table 8 provides example syntax for use with this embodiment. The syntax shown in Table 8 could be implemented on top of the currently existing HEVC video coding specification.

In this embodiment, the present disclosure provides a method for partitioning a picture into sub-tiles, and then scanning the sub-tiles in a pre-defined scanning order. During this process, a tile identifier (tile id) is specified for each sub-tile. The pre-defined order is in raster scan order as described above and the subtile size is equal to the size of one CTU unit or the sub-tile size is specified to be a different size (e.g., using explicit code words). Of course one of ordinary skill in the art will understand that a title identifier alternatively or additionally could also be specified for a unit.

FIGS.19A and19Bare flow diagrams illustrating methods implemented by an encoder and a decoder, respectively, to construct and signal tiles. Particularly,FIG.19Aprovides a method170by which the encoder constructs and signals the tiles. As seen in method170, the encoder can, in some embodiments, make an initial determination as to whether subtiles are used, or units are used (box171). In one embodiment, the encoder makes this determination by checking whether a sub-tile size equals a CTU size, and signals the result in a flag in the bitstream12. If subtiles are used, the granularity of the subtile can be determined (box172) (e.g., signaling a sub-tile size that does not equal a CTU in the bitstream12). Then, for each subtile in the picture (box173), the encoder signals a tile ID for the subtile in the bitstream (box175), and assigns the tile ID to the subtile (box176).

If subtiles are not used, however (box171), method170checks to see if there are more units U in picture10(box175). If not, method170ends. As long as units remain in picture10, however, method170assigns the tile IDs of tiles the units are associated with to the subtiles.

Method180seen inFIG.19Bis implemented at the decoder. Particularly, in some embodiments, the decoder may make an initial determination as to whether subtiles are used, or units are used (box181). For instance, in one embodiment, the decoder makes the determination based on a flag included in bitstream12that indicates whether a sub-tile size equals a CTU size. If the flag indicates that subtiles are used, the decoder determines the granularity of the sub-tiles (box182). For example, if the information in the bitstream12indicates that a sub-tile size does not equal a CTU size, the decoder parses a subtile size to determine the subtile granularity. Then, for each subtile in the picture (box183), the encoder signals a tile ID for the subtile in the bitstream (box185), and assigns the tile ID to the subtile (box186).

If subtiles are not used, however (box181), method180checks to see if there are more units U in picture10(box184). If not, method170ends. However, as long as units remain in picture10, method180calls for the encoder to assign the tile IDs of tiles the units are associated with to the subtiles.

An example syntax table and semantics description for embodiment 5 on top of the HEVC video coding specification may look as Table 9.

In an alternative version of the embodiment sub_tile_size_equal_to_ctu_size_flag is not present in the syntax and the sub_tile_width_minus1 and sub_tile_height_minus1 are always set. In yet another alternative version of the embodiment none of sub_tile_size_equal_to_ctu_size_flag, sub_tile_width_minus1 and sub_tile_height_minus1 are present in the syntax and the height and width of the sub tiles are set to the height and width of the CTUs, i.e. sub_tile_width_minus1 is set to 0, sub_tile_height_minus1 is set to 0, num_sub_tiles_horizontal is set to PicWidthInCtbsY and num_sub_tiles_vertical is set to PicHeightInCtbsY.

Seventh Embodiment Tile IDs Using Subtiles with Code Value for Unique Subtiles

Aspects of this embodiment can be applied to other embodiments described herein (e.g., the Fifth Embodiment) to indicate unique or independent subsegments.

First, a flag is set to determine whether flexible tiles are enabled, or whether a traditional tile structure, such as the current HEVC tile structure, is deployed.

Second, this embodiment defines a method for coding independent tiles using a fixed code value, such as code value equal to 0. A tile is defined to be independent if it has the same size as the sub-tile (e.g., a unit or CTU, or plurality of units or CTUs). This feature beneficially saves bits in cases where a given picture comprises a plurality of independent tiles.

Third, this embodiment defines a method that allows for the optional encoding of tile ids with fixed length coding. In such cases, a flag is used to signal whether fixed length coding of tile ID values is used, or whether variable length coding of tile ID values is used.

Those of ordinary skill in the art will appreciate that the present disclosure is not limited to these features and embodiments. Rather, it is possible to vary these features and how they are combined in various embodiments. Further, not all features need to be implemented. For example, using a flag to indicate flexible tiles and the method of coding independent tiles could be used together even though the capability to encode tile ids with fixed length coding is not available.

Table 10 provides example syntax for use with this embodiment. The syntax shown in Table 10 could be implemented on top of the currently existing HEVC video coding specification.

Eighth Embodiment 7—Divide into Tiles Using Quad Tree Split Flaps and Flexible Split Flaps

In an eighth embodiment of the present disclosure, the picture is divided into tiles using quad trees and/or flexible tile splitting. This process for dividing using quad trees similar to the HEVC process for splitting CUs.

For simplicity in explaining this embodiment, the sub-tile is set to the CTU size. However, those of ordinary skill in the art should readily appreciate that for other embodiments of the present disclosure, the sub-tile size in one or more embodiments is predefined or explicitly signaled in a bitstream (e.g., signaling a rectangular set of CTUs).

Additionally, not all tiles have a width and height that are a factor of 2 in terms of CTUs. In one embodiment, the splits for the quad trees use integer division if the height and width are not a factor of 2. For example, as seen inFIG.20, if the size of a given tile TAis 5×7 CTUs, a quad tree split would divide the tile into tiles T1, T2, T3, T4of sizes 2×3, 2×4, 3×3 and 3×4, respectively. Similarly, a horizontal or vertical “midway” split also uses integer divisions.

A “midway” split is defined herein such that for a given tile A having length N CTUs, where N is a positive integer larger than 1, the resulting first split tile B has the length N/2 and the second split tile C has the length N/2+N %2 where ‘/’ indicates integer division and ‘%’ denotes a modulo operator. The length of a tile could be either the width of the tile, the height of the tile or both.

For example, if a given tile has a size 1×5, that tile can be split, according to embodiments of the present disclosure, into two tiles—a first tile having a size of 1×2, and a second tile having a size of 1×3.

FIGS.21A-21Billustrate a method190implemented by an encoder to construct and signal tiles. As seen inFIG.21A, the encoder first sets a picture/slice as the only tile in a list of uncompleted tiles (box192). The encoder then creates an empty list of completed tiles (box194). The encoder then checks to determine whether any uncomplete tiles are available (box196). If not, method190ends. Otherwise, the encoder obtains the next uncomplete tile in raster scan order (box198). The encoder then determines whether the tile could be further split (box200). If not, the encoder removes the tile from the list of uncompleted tiles (box202), adds the tile including tile ID, tile position and tile dimensions to the list of completed tiles (box204). The encoder then signals the flag indicating whether the tile should be split according to a quad tree split (or a “midway” horizontal or vertical split if the height or width equals the height or width of the CTU) (box206). If the flag does indicate a split, the encoder removes the current tile from the list of uncompleted tiles (box208), and adds the split tiles to the list of uncompleted tiles (box210) before returning to determine whether any other uncompleted tiles are available (box196).

As seen inFIG.21B, if the flag does not indicate a split, the encoder signals a flag indicating whether the tile should be further split (box212). If the tile is not to be split, the encoder again determines whether any other uncompleted tiles are available (box196). If the tile should be further split, however, the encoder signals a flag indicating whether the tile should be split vertically or horizontally (box214). Note, however, that the vertical/horizontal split indication is not necessary when the tile width equals the CTU width or the tile height equals the CTU height.

The encoder then signals the particular CTU position at which to split the tile (box216) and splits the tile at that position (box218). So split, the encoder removes the current tile from the list of uncompleted tiles, and adds the split tiles to the list of uncompleted tiles (box220) before returning to determine whether any other uncompleted tiles are available (box196).

FIGS.21C-21Dillustrate a method230implemented by a decoder to parse the tiles constructed and signaled by the encoder inFIGS.21A-21B. As seen inFIG.21C, the decoder first sets a picture/slice as the only tile in a list of uncompleted tiles (box232). The decoder then creates an empty list of completed tiles (box234). The decoder then checks to determine whether any uncomplete tiles are available (box236). If not, method230ends. Otherwise, the decoder obtains the next uncomplete tile in raster scan order (box238). The decoder then determines whether the tile could be further split (box240). If not, the decoder removes the tile from the list of uncompleted tiles (box242), adds the tile including tile ID, tile position and tile dimensions to the list of completed tiles (box244) and goes back to check whether any uncomplete tiles are available. If the tile could be further split, the decoder parses the flag indicating whether the tile should be split according to a quad tree split (or a “midway” horizontal or vertical split if the height or width equals the height or width of the CTU) (box246). If the flag does indicate such a split, the decoder removes the current tile from the list of uncompleted tiles (box248), and adds the split tiles to the list of uncompleted tiles (box250) before returning to determine whether any other uncompleted tiles are available (box236).

As seen inFIG.21D, the decoder parses a flag signaled by the encoder to determine whether the tile should be further split (box252). If the tile is not to be further split, the decoder returns to determine whether any other uncompleted tiles are available (box236). If the tile should be further split, however, the decoder parses a flag indicating whether the tile should be split vertically or horizontally (box254). Note, however, that parsing this flag to determine a vertical/horizontal split is not necessary when the tile width equals the CTU width or the tile height equals the CTU height.

The decoder then parses a code word to determine the particular CTU position at which to split the tile (box256) and splits the tile at that position (box258) before returning to determine whether any other uncompleted tiles are available (box236).

Table 11 provides example syntax for use with this embodiment. The syntax shown in Table 11 could be implemented on top of the currently existing HEVC video coding specification.

According to embodiments of the present disclosure, the same tile structure can be expressed with different variations of the syntax. That is, splits can be ordered differently but still achieve the same result.

Additionally, it is often possible to assign a split_position_minus1 value that is higher than the allowed value. For example, if the tile_width is 6, then 3 bits are needed to select any of the 5 split positions of the tile. Thus in this case, 23-5=3 positions cannot be selected although it could be possible to select them without the constraint in the semantics. In one embodiment, where the number of positions is greater than 2, the additional positions are used to signal the next flags in the parsing. If at least 2 positions are available the next quadtree_split_flag, if available, is set to 0 if split_position_minus1+1=tile_width if split_in_horizontal_direction equals 0 or split_position_minus1+1=tile_height if split_in_horizontal_direction equals 1. The next quadtree_split_flag is set to 1 if split_position_minus1+1=tile_width+1 if split_in_horizontal_direction equals 0 or split_position_minus1+1=tile_height+1 if split_in_horizontal_direction equals 1.

In this embodiment, the quadtree_split_flag is not set to 1 in the tile structure of a picture like that ofFIG.14, meaning that every tile split termination costs 2 bits. In some embodiments, the order of the syntax element could be different than what is provided in the proposed tile syntax. For example, the flexible_split_flag could be put before the quadtree_split_flag, where a quad_tree_split_flag equal to 0 would terminate further splitting of the tile. In this example the tile structure ofFIG.15could be expressed using 19+10=29 bits and the tile structure ofFIG.14could be expressed using 73−19=54 bits. In one embodiment an extra flag could be signaled before parsing the tile structure indicating whether quad_tree_split_flag or flexible_split_flag should be parsed first. In yet another embodiment, the quadtree split_flag is not used. Rather, only the flexible_split_flag is used. In this aspect, after the split_in_horizontal_direction_flag, a flag “split_midways_flag” could be used to specify whether the tile should be split midways or if position split_position_minus1 should be read to do the split at that position.

Ninth Embodiment 8—Divider Lines

In this embodiment, the tile structure is described by specifying the boundaries between the tiles. Within this disclosure, the boundaries between tiles are also referred to as “divider lines.”

In one aspect of this embodiment, the divider lines are straight lines described by their start and end points. Since the start and end points are interchangeable without affecting the divider line, both are referred to hereafter as end points.

As the divider lines are either horizontal or vertical, three coordinates and a direction flag are sufficient with which to uniquely describe each line. Further, this embodiment uses one flag for signaling the group of horizontal divider lines, and one flag for signaling the group of vertical divider lines. This further reduces the total bit count.

For example, the example tile structure S illustrated inFIG.14can be expressed using 9 horizontal and vertical divider lines, as illustrated inFIGS.22A-22C. In particular,FIG.22Aillustrates an exemplary tiling structure having, in this embodiment, six (6) horizontal lines h1-h6, and three (3) vertical lines v1-v3.FIG.22Billustrates the six (6) horizontal line components, andFIG.22Cillustrates the three (3) vertical line components. This equates to 3*9=36 code words for the coordinate of the end points, and 2 flags for the groups of horizontal and vertical divider lines, respectively.

FIG.23Ais a flow diagram illustrating a method260implemented at the encoder for determining the horizontal and vertical divider lines for a given tile structure. For illustrative purposes only, method260is described in the context of the tiling structure for the picture seen inFIG.14.

As seen inFIG.23A, the encoder determines the value of the tile_enabled_flag (box262). If this flag is set to 0, it means that the picture comprises only one tile having the same width and height as the picture. Thus, the process ends.

If this flag is not set to 0, however, the encoder creates a list of all horizontal divider lines (AHDL) that form the desired tiling structure (box264), as well as a list of all vertical divider lines (AVDL) that form the desired tiling structure (box266). If the AHDL is not empty (box268), the encoder signals horizontal divider lines and sends AHDL to the decoder (box270). Similarly, if the AVDL is not empty (box272), the encoder signals the vertical divider lines and sends the AVDL to the decoder (box274). The order of creating and signaling AHDL and AVDL may be reversed without affecting the general aspects of the embodiment.

FIG.23Bis a flow diagram illustrating a method280implemented at the decoder for processing tiles having a given tile structure described with corresponding horizontal and vertical divider lines. As above, this embodiment of the decoder functionality is described in the context of the tiling structure for the picture seen inFIG.14. However, this is for ease of description only.

As seen inFIG.23B, the decoder first determines whether the tile_enabled_flag is set to 0 (box282). If this flag is set to 0, it means that the picture comprises only one tile having the same width and height as the picture. Thus, the process ends.

If the flag is not set to 0, however, the decoder creates an empty complete_tile_list (box284), parses the AHDL, and creates a list of horizontal_divider_lines (HDL) (box286). The decoder also parses the AVDL, and create a list of vertical_divider_lines (VDL) (box288). The order of parsing AHDL and AVDL may be reversed without affecting the general aspects of the embodiment.

The decoder then creates a corner_list with all the corners and their directions created by picture boundaries, HDL and VDL (box290). For example, referring to the illustration ofFIG.22, a cross point “” specifies four separate corners in the corner_list, and a T-junction “├” or “┤” or “” or “⊥” specifies two separate corner points.

Once the corner_list has been created and populated, the decoder processes that list. Particularly, as long as the corner_list is not empty (box292), the decoder obtains one or more corners in the predefined scan order. In particular the decoder picks a first corner in the scan order and locates the three corresponding corners which form a tile from the corner_list (box294). The decoder then adds the tile to the complete_tile_list (box296) and removes the four used corners from the corner_list (box298). This process continues until the corner_list is empty.

Table 12 provides example syntax for use with this embodiment. The syntax shown in Table 12 could be implemented on top of the currently existing HEVC video coding specification. Each end point in the horizontal/vertical list of divider lines, which is not located on the picture boundaries, should be located on the length of one divider line in the vertical/horizontal list (i.e., not another start or end point). This will help guaranty proper tile partitioning.

In one variant of this embodiment sub-tiles are used and the coordinates of the end points of the divider lines are given in sub-tile coordinates. The sub-tile size is predefined. Alternatively, the sub-tile size is explicitly signaled. Using the sub-tile coordinates saves bits as it creates a coarser grid where the coordinates of the end points can be expressed using smaller code words.

In yet another variant of this embodiment, a full grid is assumed. The start and end points of the divider lines that should be removed are then specified. To describe the tiling structure shown inFIG.15, this embodiment describes6divider lines that should be removed, and thus, 3*6 code words plus two code words for the group of horizontal and vertical divider lines are required to describe the partition structure.

In yet another aspect, the divider lines have bending point(s) in addition to start and end points. Bend points represent “joints.” According to the present disclosure, joints function as an end point to a previous point, and as a start point for the next point in the line description.

Multiple sets of divider lines can result in the same tiling structure. The coder optimizes for the minimum number of divider lines. In one aspect, the divider lines have parts in common. In another aspect, the divider lines do not have parts in common.

In yet another aspect, the divider lines are specified once and then copied to given position in the grid.

This embodiment of the present disclosure describes the tile structure using a set of points that are in the structure and that have specific properties. These points are referred to herein as “T-junctions.”

As previously described, the tile structure comprises a set of internal lines, also referred to as “divider lines.” Each divider line is a straight line with two end points where the divider line terminates (i.e., the divider line is not further continued). The so-called “T-junctions” are defined at the position of these end points of the divider lines. Because each divider line ends at the junction of that divider line with another perpendicular divider line (not an end point of another divider line because of rectangular tiling structure), or at the junction with a border of the picture perpendicular to the first divider line, each T-junction has one of the four possible shapes: ⊥,┤, and ├. Hence each T-junction can be fully described using the position of the junction and one of the 4 shapes, ⊥,, ┤ and ├. The three directions pointed at by each T-junction are referred to herein as the “arms” of that T-junction.

The tiling structure S of a picture10can then be determined from the set of the T-junctions by extending all three arms of all T-junctions in the picture until each arm reaches either the border of the picture or another T-junction position.

In one aspect of this embodiment, the set of T-junctions are shortened by removing redundant T-junctions. As described herein, redundant T-junctions are those which are located on the boundary of the picture and do not change the tiling structure if they are removed. An example of a tiling structure S with redundant T-junctions is seen inFIGS.24A-24C. In particular,FIG.24Aillustrates an exemplary tiling structure S.FIG.24Billustrates the set of all T-junctions, andFIG.24Billustrates the set of T-junctions after removing redundant T-junctions. Redundancy is checked on a one by one basis. This means that if a T-junction is redundant, and thus, will be removed from the set, it should not be included in redundancy check for the rest of the T-junctions in the set.

FIGS.25A-25Billustrate the example tiling structure S (FIG.25A) and a corresponding set of T-junctions that describes that specific tiling structure (FIG.25B).

FIG.26Ais a flow diagram illustrating a method300implemented at the encoder for creating the set of T-Junctions as previously described. Method300begins with the encoder selecting a tile structure and creating a set of divider lines that correspond to that tile structure (box302). The encoder then creates the set of T-junctions (STJ) (box304). In this embodiment the STJ includes information regarding the position and orientation of each T-junction. This can be accomplished using a variety of processes, however, in this embodiment; the encoder assigns one T-junction to the position of each end of a divider line in the created set, and assigns the proper direction to that T-junction based on whether the divider line is horizontal or vertical, and whether the end point is a left/right or a top/down end point.

The encoder then removes redundant T-junctions. Particularly, for each T-junction on the picture boundary (box306), the encoder determines whether there is another T-Junction in the STJ with the same column or row number, and if so, whether the direction different from the T-junction being examined (box308). If so, the encoder removes that T-junction (box310).

FIG.26Bis a flow diagram illustrating a method320implemented at the decoder for creating the set of T-Junctions as previously described. Method300begins with the decoder decoding a set or list of T-junctions (box322). Each T-junction has a position and a direction or type. There are 4 directions/types. No two T-junctions should have the same position and no T-junction should have a position outside the picture. Derive the tile partition structure. Then, the decoder derives the tile partition from the set or list of T-junctions such that each T-junction is extended from all its three arms until each arm reaches another T-junction or the picture boundary (box324).

Table 13 provides example syntax for use with this embodiment. The syntax shown in Table 13 could be implemented on top of the currently existing HEVC video coding specification.

update_complete_tile_list( ) {Using the position and direction of t_junction_list [i], extend all arms of the t_junction_list [i] until each arm reaches another T-junction or the picture boundary.if the units on the two sides of each arm are assigned to the same tile, update the complete tile list by adding the newly created tile(s).

In a variant of this embodiment sub-tiles are used and the coordinates of the T-junctions are given in sub-tile coordinates. The sub-tile size is predefined. Alternatively, the sub-tile size is explicitly signaled. Using the sub-tile coordinates saves bits as it creates a coarser grid where the coordinates of the T-junctions can be expressed using smaller code words.

This embodiment is based on the observation that the number of tiles is usually much lower than the number of blocks in the picture. This embodiment exploits this sparsity for coding the tiling structure.

In more detail, each tile can be uniquely specified with the position of its two opposite corners. For simplicity, the top left corner is considered as the start position of the tile, and the bottom right corner is considered as the end position of the tile. Corresponding start and end point coordinates of the tiles are marked in a matrix with the number of columns and rows equal to the picture width and height divided by the width and height of the picture units (e.g., sub-tiles, units or CTUs).

The matrix corresponding to start corner coordinates on the grid are considered to be a sparse matrix and are compressed using sparse matrix compression methods, such as compressed sparse column (CSC) or compressed sparse row (CSR), for example. The same applies to the end corner matrix. Another possible way to compress the sparse matrix is to compress the matrix in the same or similar way as it is done in HEVC for the matrices of coefficient significance or coefficient signs when compressing transform coefficients.FIGS.27A-27Cshow how the tiling structure is related to the matrix of start corners and the matrix of end corners. In particular,FIG.27Aillustrates picture10partitioned into an exemplary tiling structure S.FIG.27Bis a matrix of the start corners, andFIG.27Cis a matrix of end corners.

One difference from the standard CSC and CSR is that the only non-zero values in the start corner matrix and end corner matrix are equal to 1. Therefore, what is sent in the bitstream is only the position of those non-zero values.

FIG.28Ais a flow diagram illustrating a method330implemented at the encoder for creating the start and end corner matrices, as previously described. As seen inFIG.28A, method330begins with the encoder determining whether the picture has only one tile (box332). If so, the encoder sets the tile_enabled_flag equal to 0 (box334) and the process ends.

If the encoder determines that the picture has more than one tile, the encoder creates the start corners matrix and the end corners matrix by marking the start point and end point of each tile in the corresponding matrix (box336). The encoder then creates the start corners bitstream and end corner bitstreams (e.g. using CSC or CSR) (box338), and signals the compression type (CSC or CSR) by setting the sparse_matrix_compression_flag (box340).

FIG.28Bis a flow diagram illustrating a method350implemented at the decoder for processing the start and end corner matrices, as previously described. As seen inFIG.28B, method350begins with the decoder determining whether the tile_enabled_flag is set to 0 (box352). If so, the process ends because the picture includes only a single tile, which has the same width and height as the picture.

If there is more than one tile in the picture, however, the decoder creates an empty list of tiles (box354). The decoder then parses the compressed start corners and end corners from the bitstream, and expands them to the start corners matrix and end corners matrix (box356). Then, provided that there are start corners defined in the start corner matrix (box358), the decoder reads the position of the next start corner in the raster scan direction (box360).

The decoder then locates the corresponding end corner in the end corner matrix, and adds the tile (e.g. tile position and tile id, or e.g. tile position and width and height) to the complete_tile_list (box362). The decoder then sets the corresponding start and end corners in the matrix to zero and repeats the process.

As a sanity check, the decoder can determine:A number of start corners (ones in the start corner matrix) is equal to the number of tiles.A number of end corners (ones in the start corner matrix) is equal to the number of tiles.A first element of the start corner matrix is always set to 1A last element of the end corner matrix is always set to 1

Table 14 provides example syntax for use with this embodiment. The syntax shown in Table 14 could be implemented on top of the currently existing HEVC video coding specification.

update_complete_tile_list( ) {Find the first 1 value in the start_corners_matrix in the raster scan directionFind the corresponding 1 value in the end_corners_matrix.Add the tile to the complete_tile_list using the coordinates of the start corner and end corner.Set the corresponding start corner to zero in the start_corners_matrix.

An encoder400and decoder450according to one or more embodiments described herein could be implemented using a suitable combination of hardware and/or software.

Accordingly, an encoder comprising one or more processors and e.g. one or more memories is configured to carry out the methods according to the embodiments herein.FIG.30is a schematic diagram showing components of an encoder400. The processor500could be any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), application specific integrated circuit (ASIC) etc., capable of executing software instructions contained in a computer program520stored in one or more memories515. The one or more memories515is for instance, a data buffer, Read-Write Memory (RWM), Read-Only memory (ROM), random-access memory (RAM), cache memory, flash memory devices, optical storage devices, etc. The encoder could comprise an interface540with an input and/or output circuit530configured to transmit a bitstream, e.g., via transmitter or transceiver550.

As shown inFIG.31an encoder400could have units, modules, or circuits configured to carry out methods described herein. For instance, the encoder400comprises a define partition unit600that defines a partition structure that divides a picture into picture segments as described herein. The encoder400also comprises an encode unit610that encodes picture segments in accordance with the partition structure to generate a plurality of coded segments, and a generate unit620that generates a bitstream comprising the plurality of coded segments and information indicating the partition structure used to divide the picture into the plurality of picture segments.

Further, a decoder450comprising one or more processors and e.g. one or more memories is configured to carry out the methods according to the embodiments herein.FIG.32is a schematic diagram showing components of decoder450. The processor510could be any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), application specific integrated circuit (ASIC) etc., capable of executing software instructions contained in a computer program550stored in one or more memories540. The one or more memories540is for instance, a data buffer, Read-Write (RWM) Memory or Read-Only (ROM) memory. The decoder450comprises an interface545with an input and/or output circuit535configured to receive a bitstream, e.g., via receiver or transceiver560.

As shown inFIG.33, a decoder450could have units, modules, or circuits configured to carry out methods described herein. For instance, the decoder450comprises a receive unit630configured to receive a bitstream. The decoder450also comprises a derive unit640configured to derive, from the information in the bitstream, one or more of a partition structure, the size of a subsegment of the partition structure, and the size of a unit of the partition structure, and a decode unit650configured to decode the coded segments received in the bitstream in accordance with the partition structure.

Embodiments herein are described with reference to a single partition structure defined by an encoder or derived by a decoder, such as an encoder400and decoder450illustrated inFIGS.34A and34B.FIG.34Ashows an example encoder400transmitting a bitstream700, with information indicating a single partition structure730. The bitstream700comprises information indicating a size of a unit710, information indicating a size of a subsegment720, and information related to a segment740and another segment750.

FIG.34Bshows a corresponding example decoder receiving a bitstream700, with information indicating a single partition structure730. The bitstream700comprises information indicating a size of a unit710, information indicating a size of a subsegment720, and information related to a segment740and another segment750.

Of course, one of ordinary skill in the art will appreciate that an encoder400and decoder450can be used separately as shown inFIGS.34A-34Bor in conjunction to exchange a series of images as shown inFIG.29.

In such cases where a series of images are exchanged, the bitstream in one or more embodiments includes an indicator of the identity of the picture or otherwise associate a particular partition structure with a picture or multiple pictures. As shown inFIG.29, partition structure400shows divider lines420for picture segments421,422,423. A given picture segment of the partition structure comprises one or more subsegments, e.g., subsegments formed by a divider line410forming the border of a subsegment. A subsegment can comprise a plurality of units. For instance as shown inFIG.29, subsegment480comprises 4 units. An edge of a first picture segment421borders corresponding edges of adjacent picture segments422and423. As shown inFIG.29, the encoder400and decoder450encode and decode respectively a plurality of images which have the same partition structure400or different partition structures430and450. These different partition structures could be flexible partition structures as described herein. Alternatively or additionally as shown inFIG.29, the different partition structures can include as well traditional partition structures with divider lines440and460that go across the image and no subsegments.

FIG.35shows an example system or method800in which an encoder400or decoder450described herein would be useful. The system comprises a media source810(e.g., a capture, renderer or file). The media source is fed to a projection mapping820and then to a tile encoder or transcoder, e.g. an encoder400for partitioning an image of the media source as described herein. The encoded information is stored in storage830and then fed to a server840. It can be transmitted over a network850to a viewing device860. For example the image is one of video, and the viewing device860has a decoder450as described herein for decoding the partitioned image, rendering870the image and displaying the image on a display880to a user of the viewing device860. Such a viewing device can be, for example, a Head Mounted Display (HMD) device or any display device.

One or more embodiments are applicable to applications in which a viewing device860receives one or more images in connection with a particular gaze direction of the device. In one embodiment, the device has received information from the server840explaining what tile streams are available and information about each tile stream such as spatial coverage and bitrate. The gaze direction of the viewing device860is captured and the viewing device860requests a subset of tile streams from the server840. The viewing device860receives tile streams and composes a single bitstream from the tiles streams and feeds the bitstream to the decoder880. The viewing device860has the necessary information about the tile streams and knows how it composed the single bitstream. Therefore, the viewing device860is able to convert the decoded pictures into pictures that are suitable for rendering.

In another embodiment, the gaze direction of the viewing device860is captured and sent to the server840. The server840composes a single bitstream from the stored tiles streams and sends the bitstream to the decoder450. The necessary information how to convert the decoded pictures into pictures that are suitable for rendering is sent along with the bitstream. The viewing device860decodes the pictures using decoder450, converts them and renders them. Optionally, the viewing device860or client continuously sends requests for what bitrate the stream should have.

It should be noted that the embodiments in this disclosure comprise segments or tiles having a rectangular shape, where each segment or tile has four sides or edges. Additionally, each side or edge borders either other segments or tiles or the picture boundary. That is, the picture can be extended to a size equal to an integer number of units. In this disclosure, a “tile” is a type of segment, and the terms “side” and “edge” can be used interchangeably.

It should also be noted that some embodiments are described with respect to a decoder, but one or ordinary skill in the art would appreciate a corresponding encoder. Similarly some embodiments are described with respect to an encoder, but one or ordinary skill in the art would appreciate a corresponding decoder.